Reactive Power Management in Islanded Micro grid—Proportional Power Sharing in Hierarchical Droop Control Abstract In this project A micro grid (MG) is a local energy system consisting of a number of energy sources (e.g., wind turbine or solar panels among others), units, and loads that operate connected to the main or autonomously. MGs provide flexibility, reduce the main electricity grid dependence, and contribute to changing large centralized production paradigm to local and . However, such energy systems require complex management, advanced control, and optimization. Moreover, the power electronics converters have to be used to correct energy conversion and be interconnected through common control structure is necessary. Classical droop control system is often implemented in MG. It allows correct operation of parallel voltage source converters in grid connection, as well as islanded mode of operation. However, it requires complex power management algorithms, especially in islanded MGs, which balance the system and improves reliability. The novel reactive power sharing algorithm is developed, which takes into account the converters parameters as apparent power limit and maximum active power. INTRODUCTION Micro grid (MG) is a separate system that produces and storages electrical energy, which consists of sources (RES), local loads, and energy storage based on batteries or super capacitors. It is inherent part of modern and popular smart grids which includes also intelligent buildings, electrical car stations, etc. All RES are using power electronics devices (e.g., converters), which number significantly increasing and costs decreasing in range 1%–5% every year. RES are usually connected to the grid and many installations cause the parallel operation of RES close to each other. This is one of reasons to future change of the classical structure of electrical power systems, toward new solution containing distributed generation, energy storage, protection and control technologies, and improving their performances. MG is highly advanced system from control and communication point of view. It has to manage power for local loads as well as control all converters with high efficiency and accuracy, especially when MG operates as islanded system. Islanding mode of operation provide the uninterruptible power supply for local loads during grid faults. The performances of islanded MG are specified according to IEEE. With increasing number of RES applications, operating parallel, close to each other (few km) and with developed islanded mode of operation, the MGs are become perfect solution for RES integration. Fundamental algorithms of ac MGs, are based on master–slave control or hierarchical droop control. The first solution includes only one converter with voltage control loop (VCL), operating as a master, and others operating in current control loop (CCL)—slaves. The produced power is controlled by sources with CCL and the voltage amplitude and frequency is keeping in point of common coupling (PCC) by master unit. Disadvantage of this solution is no possibility to connect other VCL sources to MG, which are the most popular and used RES solutions. The second control solution, called droop control, includes many VCL sources and provides possibility to many different RES interconnection. The idea of droop control is based on active and reactive power related to voltage frequency and amplitude droop on coupled impedances. Unfortunately, classical droop control method with proportional droop coefficients does not provides proper reactive power sharing between converters connected to common ac bus. In classical approach, the equal reactive power sharing (ERPS) can be obtained only when active powers are equal and droop coefficients are well chosen. When active powers are changing, the reactive power sharing cannot be controlled causing overload or reactive power circulation between converters. Moreover, the important issue in droop control is static trade-off between voltage regulation and reactive power. For increasing reactive power, the voltage droop on converter’s output impedance also increase, what may cause overvoltage. In order to provide appropriate power sharing and minimize the risk of converter damage the many additional aspects (e.g., nominal apparent power, instantaneous active power, nominal voltage of converter)

Fig. 1. Equivalent circuit of parallel connected VSIs. There are only few papers describing reactive power sharing between parallel operating converters in islanded ac MGs. The researchers focused on ERPS between all RES usually controlled by MG central control unit or implemented as virtual impedances. From the other hand, researches consider reactive power sharing in order to optimize transmission power losses by appropriate optimization algorithm (e.g., particle swarm optimization), which can be neglected in MGs, hence the short distances and the line impedances are low. However, algorithms described in literature are not considering capabilities of single RES, which have limited apparent power. If active power, usually calculated from maximum peak power tracking (MPPT) algorithms, obtain almost nominal apparent converter limit the equal power sharing algorithms cannot be used, because the overload can occur, what leads to damage or exclusion from operation of RES unit. The new reactive power sharing algorithm is developed And presented in this project. In Section I, the current solutions and problems of reactive power sharing are described. Existing method The first solution includes only one converter with voltage control loop (VCL), operating as a master, and others operating in current control loop (CCL) slaves. The produced power is controlled by sources with CCL and the voltage amplitude and frequency is keeping in point of common coupling (PCC) by master unit. Disadvantage of this solution is no possibility to connect other VCL sources to MG, which are the most popular and used RES solutions. The second control solution, called droop control, includes many VCL sources and provides possibility to many different RES interconnection. Proposed method The proposed method describes MGs provide flexibility, reduce the main electricity grid dependence, and contribute to changing large centralized production paradigm to local and distributed generation. However, such energy systems require complex management, advanced control, and optimization. Moreover, the power electronics converters have to be used to correct energy conversion and be interconnected through common control structure is necessary. Classical droop control system is often implemented in MG. It allows correct operation of parallel voltage source converters in grid connection, as well as islanded mode of operation. Distributed generation

Distributed energy, also on-site generation (OSG) or district/decentralized energy is generated or stored by a variety of small, grid-connected devices referred to as distributed energy resources (DER) or distributed energy resource systems.

Conventional power stations, such as -fired, gas and nuclear powered plants, as well as hydroelectric dams and large-scale stations, are centralized and often require electricity to be transmitted over long distances. By contrast, DER systems are decentralized, modular and more flexible technologies, that are located close to the load they serve, albeit having capacities of only 10megawatts (MW) or less.

DER systems typically use renewable energy sources, including small hydro, , biogas, solar power, , and , and increasingly play an important role for the distribution system. A grid-connected device for electricity storage can also be classified as a DER system, and is often called a distributed energy storage system (DESS). By means of an interface, DER systems can be managed and coordinated within a . Distributed generation and storage enables collection of energy from many sources and may lower environmental impacts and improve security of supply.

Micro grids are modern, localized, small-scale grids, contrary to the traditional, centralized electricity grid (macro grid). Micro grids can disconnect from the centralized grid and operate autonomously, strengthen grid resilience and help mitigate grid disturbances. They are typically low-voltage AC grids, often use diesel generators, and are installed by the community they serve. Micro grids increasingly employ a mixture of different distributed energy resources, such as solar hybrid power systems, which reduce the amount of emitted carbon significantly. Overview

Historically, central plants have been an integral part of the electric grid, in which large generating facilities are specifically located either close to resources or otherwise located far from populated load centers. These, in turn, supply the traditional transmission and distribution (T&D) grid that distributes bulk power to load centers and from there to consumers. These were developed when the costs of transporting fuel and integrating generating technologies into populated areas far exceeded the cost of developing T&D facilities and tariffs. Central plants are usually designed to take advantage of available economies of scale in a site-specific manner, and are built as "one-off," custom projects.

These economies of scale began to fail in the late 1960s and, by the start of the 21st century, Central Plants could arguably no longer deliver competitively cheap and reliable electricity to more remote customers through the grid, because the plants had come to cost less than the grid and had become so reliable that nearly all power failures originated in the grid. Thus, the grid had become the main driver of remote customers’ power costs and power quality problems, which became more acute as digital equipment required extremely reliable electricity. Efficiency gains no longer come from increasing generating capacity, but from smaller units located closer to sites of demand.

For example, coal power plants are built away from cities to prevent their heavy air pollution from affecting the populace. In addition, such plants are often built near collieries to minimize the cost of transporting coal. Hydroelectric plants are by their nature limited to operating at sites with sufficient water flow.

Low pollution is a crucial advantage of combined cycle plants that burn . The low pollution permits the plants to be near enough to a city to provide district heating and cooling.

Distributed energy resources are mass-produced, small, and less site-specific. Their development arose out of:

1. concerns over perceived externalized costs of central plant generation, particularly environmental concerns,

2. the increasing age, deterioration, and capacity constraints upon T&D for bulk power; 3. the increasing relative economy of mass production of smaller appliances over heavy manufacturing of larger units and on-site construction;

4. Along with higher relative prices for energy, higher overall complexity and total costs for regulatory oversight, tariff administration, and metering and billing.

Capital markets have come to realize that right-sized resources, for individual customers, distribution substations, or micro grids, are able to offer important but little-known economic advantages over central plants. Smaller units offered greater economies from mass-production than big ones could gain through unit size. These increased value—due to improvements in financial risk, engineering flexibility, security, and environmental quality—of these resources can often more than offset their apparent cost disadvantages. DG, vis-à-vis central plants, must be justified on a life-cycle basis. Unfortunately, many of the direct, and virtually all of the indirect, benefits of DG are not captured within traditional utility cash-flow accounting.

While the levelized cost of distributed generation (DG) is typically more expensive than conventional, centralized sources on a kilowatt-hour basis, this does not consider negative aspects of conventional fuels. The additional premium for DG is rapidly declining as demand increases and technology progresses, and sufficient and reliable demand may bring economies of scale, innovation, competition, and more flexible financing, that could make DG clean energy part of a more diversified future.

Distributed generation reduces the amount of energy lost in transmitting electricity because the electricity is generated very near where it is used, perhaps even in the same building. This also reduces the size and number of power lines that must be constructed.

Typical DER systems in a feed-in tariff (FIT) scheme have low maintenance, low pollution and high efficiencies. In the past, these traits required dedicated operating engineers and large complex plants to reduce pollution. However, modern embedded systems can provide these traits with automated operation and renewables, such as sunlight, wind and geothermal. This reduces the size of power plant that can show a profit.

Grid parity

Grid parity occurs when an alternative energy source can generate electricity at a levelized cost (LCOE) that is less than or equal to the end consumer's retail price. Reaching grid parity is considered to be the point at which an energy source becomes a contender for widespread development without subsidies or government support. Since the 2010s, grid parity for solar and wind has become a reality in a growing number of markets, including Australia, several European countries, and some states in the U.S.

Technologies

Distributed energy resource (DER) systems are small-scale power generation or storage technologies (typically in the range of 1 kW to 10,000 kW) used to provide an alternative to or an enhancement of the traditional . DER systems typically are characterized by high initial capital costs per kilowatt. DER systems also serve as storage device and are often called Distributed energy storage systems (DESS).

DER systems may include the following devices/technologies:

 Combined heat power (CHP), also known as or trigeneration

 Fuel cells

 Hybrid power systems (solar hybrid and wind hybrid systems)

 Micro combined heat and power (Micro CHP)

 Micro turbines

 Photovoltaic systems (typically rooftop solar PV)

 Reciprocating engines

 Small wind power systems

 Stirling engines  Or a combination of the above. For example, hybrid photovoltaic, CHP and battery systems can provide full electric power for single family residences without extreme storage expenses. Cogeneration

Distributed cogeneration sources use steam turbines, natural gas-fired fuel cells, micro turbines or reciprocating engines to turn generators. The hot exhaust is then used for space or water heating, or to drive an absorptive chiller for cooling such as air-conditioning. In addition to natural gas-based schemes, distributed energy projects can also include other renewable or low carbon fuels including , biogas, landfill gas, sewage gas, coal bed methane, syngas and associated gas.

Delta-ee consultants stated in 2013 that with 64% of global sales the fuel cell micro combined heat and power passed the conventional systems in sales in 2012. 20.000 units were sold in Japan in 2012 overall within the Ene Farm project. With a Lifetime of around 60,000 hours. For PEM fuel cell units, which shut down at night, this equates to an estimated lifetime of between ten and fifteen years. For a price of $22,600 before installation. For 2013 a state subsidy for 50,000 units is in place.

In addition, molten carbonate fuel cell and solid oxide fuel cells using natural gas, such as the ones from Fuel Cell Energy and the Bloom energy server, or waste-to-energy processes such as the Gate 5 Energy System are used as a distributed energy resource.

Solar power

Photovoltaics, by far the most important solar technology for distributed generation of solar power, uses solar cells assembled into solar panels to convert sunlight into electricity. It is a fast- growing technology doubling its worldwide installed capacity every couple of years. PV systems range from distributed, residential, and commercial rooftop or building integrated installations, to large, centralized utility-scale photovoltaic power stations.

The predominant PV technology is crystalline silicon, while thin-film solar cell technology accounts for about 10 percent of global photovoltaic deployment. In recent years, PV technology has improved its sunlight to electricity conversion efficiency, reduced the installation cost per watt as well as its energy payback time (EPBT) and levelised cost of electricity (LCOE), and has reached grid parity in at least 19 different markets in 2014.

As most renewable energy sources and unlike coal and nuclear, solar PV is variable and non- dispatch able, but has no fuel costs, operating pollution, as well as greatly reduced mining-safety and operating-safety issues. It produces peak power around local noon each day and its is around 20 percent.

Wind power

Wind turbines can be distributed energy resources or they can be built at utility scale. These have low maintenance and low pollution, but distributed wind unlike utility-scale wind has much higher costs than other sources of energy. As with solar, wind energy is variable and non- dispatch able. Wind towers and generators have substantial insurable liabilities caused by high winds, but good operating safety. Distributed generation from wind hybrid power systems combines wind power with other DER systems. One such example is the integration of wind turbines into solar hybrid power systems, as wind tends to complement solar because the peak operating times for each system occur at different times of the day and year.

Hydro power Main articles: Small hydro and

Hydroelectricity is the most widely used form of renewable energy and its potential has already been explored to a large extent or is compromised due to issues such as environmental impacts on fisheries, and increased demand for recreational access. However, using modern 21st century technology, such as wave power, can make large amounts of new hydropower capacity available, with minor environmental impact.

Modular and scalable Next generation kinetic energy turbines can be deployed in arrays to serve the needs on a residential, commercial, industrial, municipal or even regional scale. Micro hydro kinetic generators neither require dams nor impoundments, as they utilize the kinetic energy of water motion, either waves or flow. No construction is needed on the shoreline or sea bed, which minimizes environmental impacts to habitats and simplifies the permitting process. Such power generation also has minimal environmental impact and non-traditional micro hydro applications can be tethered to existing construction such as docks, piers, bridge abutments, or similar structures.

Waste-to-energy

Municipal solid waste (MSW) and natural waste, such as sewage sludge, food waste and animal manure will decompose and discharge methane-containing gas that can be collected and used as fuel in gas turbines or micro turbines to produce electricity as a distributed energy resource. Additionally, a California-based company, Gate 5 Energy Partners, Inc. has developed a process that transforms natural waste materials, such as sewage sludge, into that can be combusted to power a steam turbine that produces power. This power can be used in lieu of grid- power at the waste source (such as a treatment plant, farm or dairy).

Energy storage Main article:

A distributed energy resource is not limited to the generation of electricity but may also include a device to store distributed energy (DE). Distributed energy storage systems (DESS) applications include several types of battery, pumped hydro, compressed air, and thermal energy storage.

PV storage

Artist's image of the Tesla Power wall

Common rechargeable battery technologies used in today's PV systems include, the valve regulated lead-acid battery (lead–acid battery), nickel–cadmium and lithium-ion batteries. Compared to the other types, lead-acid batteries have a shorter lifetime and lower energy density. However, due to their high reliability, low self-discharge (4–6% per year) as well as low investment and maintenance costs, they are currently the predominant technology used in small- scale, residential PV systems, as lithium-ion batteries are still being developed and about 3.5 times as expensive as lead-acid batteries. Furthermore, as storage devices for PV systems are stationary, the lower energy and power density and therefore higher weight of lead-acid batteries are not as critical as for electric vehicles. However, lithium-ion batteries, such as the Tesla Power wall, have the potential to replace lead- acid batteries in the near future, as they are being intensively developed and lower prices are expected due to economies of scale provided by large production facilities such as the Gigafactory 1. In addition, the Li-ion batteries of plug-in electric cars may serve as future storage devices, since most vehicles are parked an average of 95 percent of the time, their batteries could be used to let electricity flow from the car to the power lines and back. Other rechargeable batteries that are considered for distributed PV systems include, sodium–sulfur and vanadium batteries, two prominent types of a molten salt and a flow battery, respectively.

Vehicle-to-grid Future generations of electric vehicles may have the ability to deliver power from the battery in a vehicle-to-grid into the grid when needed. An electric vehicle network has the potential to serve as a DESS. Flywheels An advanced flywheel energy storage (FES) stores the electricity generated from distributed resources in the form of angular kinetic energy by accelerating a rotor (flywheel) to a very high speed of about 20,000 to over 50,000 rpm in a vacuum enclosure. Flywheels can respond quickly as they store and feedback electricity into the grid in a matter of seconds. Integration with the grid

For reasons of reliability, distributed generation resources would be interconnected to the same transmission grid as central stations. Various technical and economic issues occur in the integration of these resources into a grid. Technical problems arise in the areas of power quality, voltage stability, harmonics, reliability, protection, and control. Behavior of protective devices on the grid must be examined for all combinations of distributed and central station generation. A large scale deployment of distributed generation may affect grid-wide functions such as frequency control and allocation of reserves. As a result smart grid functions, virtual power plants and grid energy storage such as power to gas stations are added to the grid.

Each distributed generation resource has its own integration issues. Solar PV and wind power both have intermittent and unpredictable generation, so they create many stability issues for voltage and frequency. These voltage issues affect mechanical grid equipment, such as load tap changers, which respond too often and wear out much more quickly than utilities anticipated. Also, without any form of energy storage during times of high solar generation, companies must rapidly increase generation around the time of sunset to compensate for the loss of solar generation. This high ramp rate produces what the industry terms the "duck curve" that is a major concern for grid operators in the future. Storage can fix these issues if it can be implemented. Flywheels have shown to provide excellent frequency regulation. Short term use batteries, at a large enough scale of use, can help to flatten the duck curve and prevent generator use fluctuation and can help to maintain voltage profile. However, cost is a major limiting factor for energy storage as each technique is prohibitively expensive to produce at scale and comparatively not energy dense compared to liquid fossil fuels finally, another necessary method of aiding in integration of photovoltaics for proper distributed generation is in the use of intelligent hybrid inverters.

Cost factors

Co generators are also more expensive per watt than central generators. They find favor because most buildings already burn fuels, and the cogeneration can extract more value from the fuel . Local production has no electricity transmission losses on long distance power lines or energy losses from the Joule effect in where in general 8-15% of the energy is lost (see also cost of electricity by source).

Some larger installations utilize combined cycle generation. Usually this consists of a gas turbine whose exhaust boils water for a steam turbine in a . The condenser of the steam cycle provides the heat for space heating or an absorptive chiller. Combined cycle plants with cogeneration have the highest known thermal efficiencies, often exceeding 85%.

In countries with high pressure gas distribution, small turbines can be used to bring the gas pressure to domestic levels whilst extracting useful energy. If the UK were to implement this countrywide an additional 2-4 GWe would become available. (Note that the energy is already being generated elsewhere to provide the high initial gas pressure - this method simply distributes the energy via a different route.)

Micro grid

Picture of a local micro grid, the Sendai Micro grid, located on the campus of Tohoku Fukushi University in Sendai City in the Tohoku district in Japan

A micro grid is a localized grouping of , energy storage, and loads that normally operate connected to a traditional centralized grid (macro grid). This single point of common coupling with the macro grid can be disconnected. The micro grid can then function autonomously. Generation and loads in a micro grid are usually interconnected at low voltage and it can operate in DC, AC or the combination of both. According to the recent developments in renewable energy systems, storage systems, and the nature of newly emerging loads, there have been some researches for comparing the efficiency and performance of AC and DC micro grids. From the point of view of the grid operator, a connected micro grid can be controlled as if it were one entity.

Micro grid generation resources can include stationary batteries, fuel cells, solar, wind, or other energy sources. The multiple dispersed generation sources and ability to isolate the micro grid from a larger network would provide highly reliable electric power. Produced heat from generation sources such as micro turbines could be used for local process heating or space heating, allowing flexible tradeoff between the needs for heat and electric power. Micro-grids were proposed in the wake of the July 2012 India blackout:

Small micro-grids covering 30–50 km radius

Small power stations of 5–10 MW to serve the micro-grids

Generate power locally to reduce dependence on long distance transmission lines and cut transmission losses.

GTM Research forecasts micro grid capacity in the United States will exceed 1.8 gigawatts by 2018.

Communication in DER systems

IEC 61850-7-420 is under development as a part of IEC 61850 standards, which deals with the complete object models as required for DER systems. It uses communication services mapped to MMS as per IEC 61850-8-1 standard.

OPC is also used for the communication between different entities of DER system.

Distributed generation (DG) refers to power generation at the point of consumption. Generating power on-site, rather than centrally, eliminates the cost, complexity, interdependencies, and inefficiencies associated with transmission and distribution. Like distributed computing (i.e. the PC) and distributed telephony (i.e. the mobile phone), distributed generation shifts control to the consumer.

The World Needs Distributed Generation that is Clean and Continuous

Historically, distributed generation meant combustion generators (e.g. diesel gensets). They were affordable, and in some cases reliable, but they were not clean. While many people will tolerate dirty generation thousands of miles away from them, they think twice when it is outside their bedroom window or office door. Recently, solar has become a popular distributed generation option. Although the output is clean it is also intermittent, making it an incomplete strategy for businesses that need power around the clock, including when the sun is not shining.

The Benefits of Bloom Energy

Bloom Energy is a Distributed Generation solution that is clean and reliable and affordable all at the same time .Bloom's Energy Servers can produce clean energy 24 hours per day, 365 days per year, generating more electrons than intermittent solutions, and delivering faster payback and greater environmental benefits for the customer. And while other DG systems may require lengthy installations, sunny locations, or demand for consistent 24/7/365 heat load, Bloom's systems are easy and fast to install, practically anywhere.

As Distributed Generation moves to the forefront of corporate consciousness, Bloom Energy Servers are perfectly designed to meet the demanding needs of todays economically and environmentally minded companies.

Distributed generation is an approach that employs small-scale technologies to produce electricity close to the end users of power. DG technologies often consist of modular (and sometimes renewable-energy) generators, and they offer a number of potential benefits. In many cases, distributed generators can provide lower-cost electricity and higher power reliability and security with fewer environmental consequences than can traditional power generators.

In contrast to the use of a few large-scale generating stations located far from load centers--the approach used in the traditional electric power paradigm--DG systems employ numerous, but small plants and can provide power onsite with little reliance on the distribution and transmission grid. DG technologies yield power in capacities that range from a fraction of a kilowatt [kW] to about 100 megawatts [MW]. Utility-scale generation units generate power in capacities that often reach beyond 1,000 MW.

Classic Electricity Paradigm--Central Model The current model for electricity generation and distribution in the United States is dominated by centralized power plants. The power at these plants is typically combustion (coal, oil, and natural) or nuclear generated. Centralized power models, like this, require distribution from the center to outlying consumers. Current substations can be anywhere from 10s to 100s of miles away from the actual users of the power generated. This requires transmission across the distance.

This system of centralized power plants has many disadvantages. In addition to the transmission distance issues, these systems contribute to greenhouse gas emission, the production of nuclear waste, inefficiencies and power loss over the lengthy transmission lines, environmental distribution where the power lines are constructed, and security related issues.

Many of these issues can be mediated through distributed energies. By locating, the source near or at the end-user location the transmission line issues are rendered obsolete. Distributed generation (DG) is often produced by small modular energy conversion units like solar panels. As has been demonstrated by solar panel use in the United States, these units can be stand-alone or integrated into the existing energy grid. Frequently, consumers who have installed solar panels will contribute more to the grid than they take out resulting in a win-win situation for both the power grid and the end-user. Distributed Generation (DG) Electricity Paradigm

Classic Electricity Paradigm

What are Some Examples of Distributed Generation Technologies?

Distributed generation takes place on two-levels: the local level and the end-point level. Local level power generation plants often include renewable energy technologies that are site specific, such as wind turbines, production, solar systems (photovoltaic and combustion), and some hydro-thermal plants. These plants tend to be smaller and less centralized than the traditional model plants. They also are frequently more energy and cost efficient and more reliable. Since these local level DG producers often take into account the local context, the usually produce less environmentally damaging or disrupting energy than the larger central model plants. Wind Turbines at Buffalo Mountain, TN

Photovoltaic (Solar) Panels help Power this Elementary School in Fairbanks, Alaska A 300 kW Capstone Micro turbine at a Demonstration Project at the Oak Ridge National Laboratory in Oak Ridge, TN

Phosphorus fuel cells also provide an alternative route to a DG technology. These are not as environmentally reliant as the previously mentioned technologies. These fuel cells are able to provide electricity through a chemical process rather than a combustion process. This process produces little particulate waste.

At the end-point level the individual energy consumer can apply many of these same technologies with similar effects. One DG technology frequently employed by end-point users is the modular internal combustion engine. For example, some departments here at Virginia Tech use these power generators as a backup to the normal power grid. These modular internal combustion engines can also be used to backup RVs and homes. As many of these familiar examples show DG technologies can operate as isolated "islands" of electric energy production or they can serve as small contributors to the power grid. Droop control

In electrical power generation, Droop Speed Control is a speed control mode of a prime mover driving a generator connected to an electrical grid. This mode allows synchronous generators to run in parallel, so that loads are shared among generators in proportion to their power rating.

The frequency of a synchronous generator is given by

where F = Frequency (in Hz), P = number of poles, N = Speed of generator (in RPM)

The frequency (F) of a synchronous generator is directly proportional to its speed (N). When multiple synchronous generators are connected in parallel to electrical grid, the frequency is fixed by the grid, since individual power output of each generator will be small compared to the load on a large grid. Synchronous generators connected to the grid run at various speeds but they all run at the same frequency because they differ in the number of poles (P).

A speed reference as percentage of actual speed is set in this mode. As the generator is loaded from no load to , the actual speed of the prime mover tend to decrease. In order to increase the power output in this mode, the prime mover speed reference is increased. Because the actual prime mover speed is fixed by the grid, this difference in speed reference and actual speed of the prime mover is used to increase the flow of working fluid (fuel, steam, etc.) to the prime mover, and hence power output is increased. The reverse will be true for decreasing power output. The prime mover speed reference is always greater than actual speed of the prime mover. The actual speed of the prime mover is allowed to "droop" or decrease with respect to the reference, and so the name. For example, if the turbine is rated at 3000 rpm, and the machine speed reduces from 3000 rpm to 2880 rpm when it is loaded from no load to base load, then the droop % is given by

=(3000 – 3120) / 3000 = 4% = (3000 – 2880) / 3000 = 4% In this case, speed reference will be 104% and actual speed will be 100%. For every 1% change in the turbine speed reference, the power output of the turbine will change by 25% of rated for a unit with a 4% droop setting.

Droop is therefore expressed as the percentage change in (design) speed required for 100% governor action.

For example, how fuel flow is increased or decreased in a GE-design heavy duty gas turbine can be given by the formula,

FSRN = (FSKRN2 × (TNR-TNH)) + FSKRN1

Where,

FSRN = Fuel Stroke Reference (Fuel supplied to Gas Turbine) for droop mode TNR = Turbine Speed Reference TNH = Actual Turbine Speed FSKRN2 = Constant FSKRN1 = Constant

As frequency is fixed on the grid, and so actual turbine speed is also fixed, the increase in turbine speed reference will increase the error between reference and actual speed. As the difference increases, fuel flow is increased to increase power output, and vice versa. This type of control is referred to as "straight proportional" control. If the entire grid tends to be overloaded, the grid frequency and hence actual speed of generator will decrease. All units will see an increase in the speed error, and so increase fuel flow to their prime movers and power output. In this way droop speed control mode also helps to hold a stable grid frequency. The amount of power produced is strictly proportional to the error between the actual turbine speed and speed reference. The above formula is nothing but the equation of a straight line (y = mx + q).

Multiple synchronous generators having equal % droop setting connected to a grid will share the change in grid load in proportion of their base load.

For stable operation of the electrical grid of North America, power plants typically operate with a four or five percent speed droop. With 5% droop the full-load speed is 100% and the no-load speed is 105%. This is required for the stable operation of the net without hunting and dropouts of power plants. Normally the changes in speed are minor due to inertia of the total rotating mass of all generators and motors running in the net. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor or by an engine control unit adjustment. Generally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication. Voltage control of several power sources is not practical because there would not be any independent feedback, resulting in the total load being put on one power plant.

Contiguous United States power transmission grid consists of 300,000 km of lines operated by 500 companies.

It can be mathematically shown that if all machines synchronized to a system have the same droop speed control, they will share load proportionate to the machine ratings.[4]

The thousands of AC generators are running synchronously with the power grid which acts like an infinite sink. Next to the inertia given by the parallel operation of synchronous generators, [5] the frequency speed droop is the primary instantaneous parameter in control of an individual power plant's power output (kW).

S is the ratio of frequency deviation when comparing the load versus the nominal frequency.

Droop control is a control strategy commonly applied to generators for primary frequency control (and occasionally voltage control) to allow parallel generator operation (e.g. load Background

Recall that the active and reactive power transmitted across a lossless line are:

Since the power angle is typically small, we can simplify this further by using the approximations and :

From the above, we can see that active power has a large influence on the power angle and reactive power has a large influence on the voltage difference. Restated, by controlling active and reactive power, we can also control the power angle and voltage. We also know from the swing equation that frequency is related to the power angle, so by controlling active power, we can therefore control frequency.

This forms the basis of frequency and voltage droop control where active and reactive power are adjusted according to linear characteristics, based on the following control equations: Where is the system frequency

is the base frequency

is the frequency droop control setting is the active power of the unit

is the base active power of the unit is the voltage at the measurement location

is the base voltage

is the reactive power of the unit

is the base reactive power of the unit

is the voltage droop control setting

These two equations are plotted in the characteristics below:

Voltage droop characteristic

The frequency droop characteristic above can be interpreted as follows: when frequency falls from f0 to f, the power output of the generating unit is allowed to increase from P0 to P. A falling frequency indicates an increase in loading and a requirement for more active power. Multiple parallel units with the same droop characteristic can respond to the fall in frequency by increasing their active power outputs simultaneously. The increase in active power output will counteract the reduction in frequency and the units will settle at active power outputs and frequency at a steady-state point on the droop characteristic. The droop characteristic therefore allows multiple units to share load without the units fighting each other to control the load (called "hunting").

The same logic above can be applied to the voltage droop characteristic.

Droop Control Set points

Droop settings are normally quoted in % droop. The setting indicates the percentage amount the measured quantity must change to cause a 100% change in the controlled quantity. For example, a 5% frequency droop setting means that for a 5% change in frequency, the unit's power output changes by 100%. This means that if the frequency falls by 1%, the unit with a 5% droop setting will increase its power output by 20%.

The short video below shows some examples of frequency (speed) droop:

Limitations of Droop Control

Frequency droop control is useful for allowing multiple generating units to automatically change their power outputs based on dynamically changing loads. However, consider what happens when there is a significant contingency such as the loss of a large generating unit. If the system remains stable, all the other units would pick up the slack, but the droop characteristic allows the frequency to settle at a steady-state value below its nominal value (for example, 49.7Hz or 59.7Hz). Conversely, if a large load is tripped, then the frequency will settle at a steady-state value above its nominal value (for example, 50.5Hz or 60.5Hz).

Other controllers are therefore necessary to bring the frequency back to its nominal value (i.e. 50Hz or 60 Hz), which are called secondary and tertiary frequency controllers.

The micro sources can not only provide AC power in grid-connected mode, but also control the frequency and voltage for the micro grid load in island mode. By using the voltage and frequency droop control method, the micro sources can realize the “plug-and-play” concept: share the load automatically by using only the local information, i.e. the voltage, frequency, instead of the communications with the other micro sources or the micro grid central control system. The micro sources can work in parallel to the grid or as an island. When the micro grid system disconnects from the grid, caused by the grid's short current or power failure, the micro sources can keep on working for the micro grid loads, without restart process. Micro grid (MG) What is a micro grid?

A micro grid is a local energy grid with control capability, which means it can disconnect from the traditional grid and operate autonomously.

How does a micro grid work?

To understand how a micro grid works, you first have to understand how the grid works.

The grid connects homes, businesses and other buildings to central power sources, which allow us to use appliances, heating/cooling systems and electronics. But this interconnectedness means that when part of the grid needs to be repaired, everyone is affected.

This is where a micro grid can help. A micro grid generally operates while connected to the grid, but importantly, it can break off and operate on its own using local energy generation in times of crisis like storms or power outages, or for other reasons.

A micro grid can be powered by distributed generators, batteries, and/or renewable resources like solar panels. Depending on how it’s fueled and how its requirements are managed, a microgrid might run indefinitely.

How does a micro grid connect to the grid?

A micro grid connects to the grid at a point of common coupling that maintains voltage at the same level as the main grid unless there is some sort of problem on the grid or other reason to disconnect. A can separate the micro grid from the main grid automatically or manually, and it then functions as an island.

Why would a community choose to connect to micro grids?

A micro grid not only provides backup for the grid in case of emergencies, but can also be used to cut costs, or connect to a local resource that is too small or unreliable for traditional grid use. A micro grid allows communities to be more energy independent and, in some cases, more environmentally friendly. How much can a micro grid power?

A micro grid comes in a variety of designs and sizes. A micro grid can power a single facility like the Santa Rita Jail micro grid in Dublin, California. Or a micro grid can power a larger area. For example, in Fort Collins, Colorado, a micro grid is part of a larger goal to create an entire district that produces the same amount of energy it consumes.

A micro grid is an electrical system that includes multiple loads and distributed energy resources that can be operated in parallel with the broader utility grid or as an electrical island. Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy, distributed energy or district energy, generates electricity from many small energy sources. Most countries generate electricity in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear, large solar power plants or hydropower plants. These plants have excellent economies of scale, but usually transmit electricity long distances and can negatively affect the environment. Distributed generation allows collection of energy from many sources and may give lower environmental impacts and improved security of supply., Distributed energy, also district or decentralized energy is generated or stored by a variety of small, grid-connected devices referred to as distributed energy resources (DER) or distributed energy resource systems. Conventional power stations, such as coal-fired, gas and nuclear powered plants, as well as hydroelectric dams and large-scale solar power stations, are centralized and often require electricity to be transmitted over long distances. By contrast, DER systems are decentralized, modular and more flexible technologies, that are located close to the load they serve, albeit having capacities of only 10 megawatts (MW) or less.DER systems typically use renewable energy sources, including, but not limited to, biomass, biogas, solar power, wind power, geothermal power and increasingly play an important role for the electric power distribution system. A grid-connected device for electricity storage can also be classified as a DER system, and is often called a distributed energy storage system (DESS). By means of an interface, DER systems can be managed and coordinated within a smart grid. Distributed generation and storage enables collection of energy from many sources and may lower environmental impacts and improve security of supply.

Reegle Definition No reegle definition available, No reegle definition available., Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy sources. Currently, industrial countries generate most of their electricity in large centralized facilities, such as fossil fuel nuclear or hydropower plants. These plants have excellent economies of scale, but usually transmit electricity long distances and negatively affect the environment. Most plants are built this way due to a number of economic, health & safety, logistical, environmental, geographical and geological factors. For example, coal power plants are built away from cities to prevent their heavy air pollution from affecting the populace. In addition, such plants are often built near collieries to minimize the cost of transporting coal. Hydroelectric plants are by their nature limited to operating at sites with sufficient water flow. Most power plants are often considered to be too far away for their waste heat to be used for heating buildings. Low pollution is a crucial advantage of combined cycle plants that burn natural gas. The low pollution permits the plants to be near enough to a city to be used for district heating and cooling. Distributed generation is another approach. It reduces the amount of energy lost in transmitting electricity because the electricity is generated very near where it is used, perhaps even in the same building. This also reduces the size and number of power lines that must be constructed. Typical distributed power sources in a Feed-in Tariff (FIT) scheme have low maintenance, low pollution and high efficiencies. In the past, these traits required dedicated operating engineers and large complex plants to reduce pollution. However, modern embedded systems can provide these traits with automated operation and renewables, such as sunlight, wind and geothermal. This reduces the size of power plant that can show a profit. A micro grid is a discrete energy system consisting of distributed energy sources (including demand management, storage, and generation) and loads capable of operating in parallel with, or independently from, the main power grid. The primary purpose is to ensure local, reliable, and affordable energy security for urban and rural communities, while also providing solutions for commercial, industrial, and federal government consumers. Benefits that extend to utilities and the community at large include lowering greenhouse gas (GHG) emissions and lowering stress on the transmission and distribution system. In many respects, micro grids are smaller versions of the traditional power grid. Like current electrical grids, they consist of power generation, distribution, and controls such as voltage regulation and switch gears. However, micro grids differ from traditional electrical grids by providing a closer proximity between power generation and power use, resulting in efficiency increases and transmission reductions. Micro grids also integrate with renewable energy sources such as solar, wind power, small hydro, geothermal, waste-to-energy, and combined heat and power (CHP) systems.

Micro grids perform dynamic control over energy sources, enabling autonomous and automatic self-healing operations. During normal or peak usage, or at times of the primary power grid failure, a micro grid can operate independently of the larger grid and isolate its generation nodes and power loads from disturbance without affecting the larger grid's integrity. Micro grids interoperate with existing power systems, information systems, and network infrastructure, and are capable of feeding power back to the larger grid during times of grid failure or power outages.

What are Smart Micro grids?

Micro grids are modern, small-scale versions of the centralized electricity system. They achieve specific local goals, such as reliability, carbon emission reduction, diversification of energy sources, and cost reduction, established by the community being served. Like the bulk power grid, smart micro grids generate, distribute, and regulate the flow of electricity to consumers, but do so locally. Smart micro grids are an ideal way to integrate renewable resources on the community level and allow for customer participation in the electricity enterprise. They form the building blocks of the Perfect Power System. Here at the Galvin Electricity Initiative's Micro grid Hub, you will find a comprehensive set of resources on micro grids, collected from our partners and from across the web. Use the navigation system at the left to browse through all of our micro grid materials, and if you have suggestions for additional content, please let us know. If you are a member of the media seeking information on micro grids, be sure to view our press kitin addition to the other resources.

Power converters

In electrical engineering, and the , power conversion is converting electric energy from one form to another, converting between AC and DC, or just changing the voltage or frequency, or some combination of these. A power converter is an electrical or electro-mechanical device for converting electrical energy. This could be as simple as a to change the voltage of AC power, but also includes far more complex systems. The term can also refer to a class of electrical machinery that is used to convert one frequency of alternating current into another frequency.

Power conversion systems often incorporate redundancy and voltage regulation.

One way of classifying power conversion systems is according to whether the input and output are alternating current (AC) or direct current(DC), thus:

 DC to DC  DC to AC

 DC-to-DC converter  Inverter

 Voltage regulator  AC to AC

 Linear regulator  Transformer/autotransformer

 AC to DC  Voltage converter

 Rectifier  Voltage regulator

 Mains power supply unit (PSU)  Cycloconverter  Switched-mode power supply  Variable-frequency transformer There are also devices and methods to convert between power systems designed for single and three-phase operation.

The standard power frequency varies from country to country, and sometimes within a country. In North America and northern South America it is usually 60 hertz (Hz), but in many other parts of the world, is usually 50 Hz. Aircraft often use 400 Hz power, so 50 Hz or 60 Hz to 400 Hz frequency conversion is needed for use in the ground power unit used to power the airplane while it is on the ground.

Certain specialized circuits, such as the fly back transformer for a CRT, can also be considered power converters.

Consumer electronics usually include an AC adapter (a type of power supply) to convert mains- voltage AC current to low-voltage DC suitable for consumption by microchips. Consumer voltage converters (also known as "travel converters") are used when travelling between countries that use ~120 V vs. ~240 V AC mains power. (There are also consumer "adapters" which merely form an electrical connection between two differently shaped AC power plugs and sockets, but these change neither voltage nor frequency.)

The task of a power converter is to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for the user loads. Energy was initially converted in electromechanical converters (mostly rotating machines). Today, with the development and the mass production of power semiconductors, static power converters find applications in numerous domains and especially in particle accelerators. They are smaller and lighter and their static and dynamic performances are better.

Reactive power sharing

For a coordinated integration of the increasing amount of distributed generators in the distribution network, the micro grid has been presented. For the islanded operating condition of the micro grid, the conventional approaches for grid control are no longer applicable as the micro grid characteristics differ significantly from those of conventional power systems. Therefore, in this paper, the reactive power control by means of a reactive power/frequency droop control strategy is studied in an islanded micro grid. The active power on the other hand, is controlled by using a droop control strategy of the dc-bus voltage to the grid voltage amplitude. It is shown that the combination of the aforementioned control methods gives good results concerning reactive power sharing and avoidance of circulating currents. Also, a reactive power limiting procedure is incorporated in the droop controller.

When paralleling multiple inverters that are capable of operating as an island, the inverters typically employ the droop control scheme. Traditional droop control enables the decentralized regulation of the local voltage and frequency of the micro grid by the inverters. The droop method also enables the inverters to share the real and reactive power required by the loads. This paper focuses on some of the limitations of parallel islanded single phase inverters using droop control. Algorithms with the aim to address the following limitations in islanded operation were proposed: reactive power sharing and reduction of the voltage harmonic distortion at the point of common coupling (PCC).

A distributed power system consisting of two uninterrupted power supplies (UPS) is investigated in this paper. Parallel operation of the two sources increases the established power rating of the system. One of the sources can supply the system even when the other system is disconnected due to some faults, and this is an important feature. The control algorithm makes sure that the total load is shared between the supplies in accordance with their rated power levels, and the frequency of the supplies is restored to the rated values after the transitions. As the UPSs operate at an optimum power level, losses and faults due to overloading are prevented. The units safely operate without any means of communication between each other.

Control of Synchronous Generators with Droop and Cross-Current Compensation.

The excitation of a synchronous generator is usually done by an AVR (Automatic Voltage Regulator) that uses generator voltage and/or current as inputs in order to control its output to a pre-set value.

AVRs include different control modes to optimise performance depending on whether the generator is connected to the grid, or in island mode. Therefore, they can be set to maintain the voltage, the PF or the reactive power. In this report, we will analyse the principle of operation of the voltage control mode of the AVR, known as droop compensation, when one or more generators operate in island mode or are connected to the grid. Based on droop control limitations, we will study techniques to improve its performance, and compare it with the cross-current compensation method.

1. Voltage control mode – Droop Compensation

In the voltage control or droop mode, the AVR is regulated by a droop characteristic, which is shown in the following drawing.

Figure 1. AVR Set-point V vs reactive power Q

The droop characteristic represents a graph of the AVR voltage set-point V as a function of the generator reactive power produced. This set-point regulates the generator terminal voltage when in island mode. The interpretation of the above graph is that as the reactive power demand from the generator increases, the generator terminal voltage decreases. The set-point in the AVR is chosen so that when the generator reactive power Q supplied is zero, the generator VN is equal to the nominal voltage. If the initial AVR set-point is not changed, VL will be the voltage due to droop that the generator terminal voltage will reach operating in island mode against reactive load QL. The reactive power generated is calculated from the generator voltage and current signals, fed back to the AVR. Droop compensation is set as percentage drop of the nominal voltage VN for maximum reactive power QL generated. Depending on the AVR, maximum reactive power is usually defined either as the reactive power exported at rated , or as the MVA rating of the generator. Droop setting can be given values from 0%, which effectively disables the droop, to a maximum of usually 20%, which could cause VL to drop to 0.8 p.u. Typically, a setting of 4-6% is chosen.

Droop compensation is a control technique designed when the generator is connected to the grid, so it is not required when one generator is in island mode. On the other hand, when connected to the grid, droop compensation is required and the droop characteristic is used below to explain the control of the AVR.

Figure 2. Representation of AVR control when connected to the grid. When a generator is directly connected to the grid, the grid voltage VG is fixed and cannot be controlled by the AVR. Any requirement for reactive power from the generator will result in the AVR internal voltage set-point V to change to meet the new demand. So, in the diagram of figure 4, the increased reactive power demand QL causes the AVR set-point to increase from VG to VL because of the droop compensation control.

2. Generator operation modes

According to the network topology, the following operating scenarios can be identified:

 Operating in island mode as a stand-alone generator.

 Synchronised to the grid.

 Operating in island mode, but in parallel with other generators.

These three scenarios are analysed separately below.

2.1. Island mode operation with a single generator

This is the simplest case in terms of AVR control, as there is only one active component in the circuit that can affect the voltage and react to any reactive load changes. A single synchronous machine operating in island mode is only responsible for two actions:

 Control the busbar voltage to the required nominal level.

 Supply the load with the required reactive power and respond fast to any load changes to meet the demand at any time.

The diagram below presents the simple case described.

Figure 3. One generator in island mode with droop enable contact.

The AVR in this case does not require droop compensation to control its output. In order to eliminate the droop effect, which would otherwise drop the circuit voltage with any increase in the reactive load, there are two possibilities:

 Set the droop setting within the AVR to zero %.

 Close the droop enable contact shown in the diagram above, so that the compounding CT current would not flow into the AVR.

2.2. Synchronised to the grid

In the case where there is connection to the grid, the AVR needs droop compensation in order to control its output. The circuit configuration and the droop characteristic for this case are presented in figures 3 and 4 respectively.

Figure 4. One generator synchronised to the grid.

The configuration above shows that by the operation of a simple contact the droop can be enabled or disabled, allowing the flexibility to disable it when in island operation and to enable it before connecting it to the grid. This eliminates the undesired effect of lower than nominal voltages when in island operation.

2.3. Island operation with paralleled generators

For the case of island mode operation with at least two generators connected in parallel to supply the load, the control of the voltage and the reactive power requirements have to be shared between the generators in parallel.

There are two control methods for the generator AVRs to achieve this:

 Control with droop compensation.

 Control with cross-current compensation.

2.3.1. Control with droop compensation

In this case the following assumptions have to be satisfied:

 The generators must be of equal size.

 The AVRs must have the same droop characteristic and the same setting applied.

In the simplest case, the AVRs can operate in droop compensation mode to obtain equal sharing of the reactive load. The relevant diagram is shown below. Figure 5. Island mode with two generators in parallel in droop mode.

The two generators in figure 5 share equally the reactive load connected according to the droop characteristic of the AVRs and the setting applied.

Although this control mode is ideal when there is grid connection, in island mode it results in the voltage output being dependent on the reactive power demand. So, as the requirement for reactive power increases, the output voltage from the generators decreases due to the droop compensation.

2.3.2. Control with cross-current compensation

Cross-current compensation or reactive differential is a method that allows two or more paralleled generators to share equally a reactive load, given that the following assumptions are satisfied:

 There is no grid connection, i.e. the generators operate in island mode.

 The generators are of equal size.

 The AVRs have the same droop characteristic, which is set to its maximum setting. The secondary wiring of the compounding CTs of all the generators to be paralleled have to be interconnected. Below, the wiring configuration for two generators set up for cross-current compensation is included.

According to this method, the same current develops through the compounding CT’s of the generators in parallel, since they are identical, and when the CCC contact closes, it stops flowing through the AVRs, but only flows through the CTs. The configuration above shows that by the operation of a simple contact the CCC can be enabled or disabled, allowing the flexibility to enable it when in island operation and to disable it before connecting it to the grid. This eliminates the droop effect and allows the paralleled generators to operate in island mode at nominal voltage when the reactive load increases.

The figure below shows the complete configuration with all the techniques explained previously incorporated for maximum functionality. This includes both the droop-compensation and the CCC enable-disable contacts. In this case, when the generators are connected to the grid, all contacts must be open. For paralleled generators in island mode, the droop contacts must be open and the CCC contact closed. Figure 7. Island mode with two generators in parallel with cross-current compensation and droop disable contacts.

II. CLASSICAL DROOP CONTROL When at least two RES are connected through energy converters to the MG, the droop control method are often applied, what provides the correct parallel operation of voltage source converters (VSI). The equivalent circuit of two converters connected to common ac MG bus can be presented by Fig. 1. Presented scheme is similar to the equivalent circuit of synchronous generator; hence the active and reactive power of kth converter connected to ac MG can be described as

Where P, active power; E, converter voltage amplitude; V, voltage amplitude in PCC; X, coupling impedance; and ϕ, angle of converter voltage (see Fig. 1). Based on above equations it can be assumed as below. 1) Active power P mainly depends on ϕ, which is changing by ω. 2) Reactive power Q depends on voltage amplitude E.

Fig. 2. P–ω and Q–E droop characteristics. Fig. 3. Block scheme of control structure for one of the converters in islanded MG. Hence, the P-ω and Q-E droop characteristics can be drawn (Fig. 2). In order to implement these characteristics in VSI control algorithm, the outer droop control loops are created (Fig. 3), which can be described by

where, E and ω are referenced voltage amplitude and frequency for inner control loops, E* and ω* are nominal voltage amplitude and frequency, P and Q are calculated active and reactive power, P* and Q* are the active and reactive power referenced values, and Gp(s) and Gq(s) are corresponding transfer functions. Typically in classical droop control Gp(s) and Gq(s) are proportional (constant) droop coefficients. It has happened, when MG not includes any energy storage and total load cannot absorb total injected power. These proportional coefficients can be calculated by (5) and (6). Block schemes of P-ω and Q-E control loops is presented in Fig. 4

where, m, active power coefficient; n, reactive power coefficient; ωmax, maximum allowed voltage frequency droop; _Emax, maximum allowed voltage amplitude droop; Pmax, maximum allowed active power; and Qmax, maximum allowed reactive power. Fig. 4. Block scheme of classical droop control. III. PROPORTIONAL REACTIVE POWER SHARING A. Development of PRPS Algorithm In order to manage reactive power in islanded ac MG the instantaneous active power and nominal apparent power of each converter have to be taking into consideration. Based on Fryze power theory, that power can be represented by orthogonal vectors, which lengths are active and reactive power and their vector sum is equal to the apparent power. The reactive power limit for each converter can be calculated

where Qmax is the maximum of possible converter’s reactive power, SN is the nominal apparent power of converter, P is the instantaneous active power of converter. In this project the harmonic (distortion) power is neglecting since only resistive inductive load is considered. This relation for several converters with different possible nominal apparent powers and equal reactive powers (three converters in this example) can be interpreted graphically in Fig. 5(a). In power balanced system the vector sum of converter’s apparent powers is equal to load apparent power regardless of the power management method, however, the algebraic sum of apparent powers is different for each control strategy. As a result, there is possible situation, that sum of converter’s apparent powers are higher than the demand, which may lead to converters operating with maximum apparent power. Furthermore, if control priority is keeping maximum active power, the overload of converter can occur, as it is shown in Fig. 5(b) for converter 1, what is not acceptable, because it cause disable or damage of this device. In order to improves the reactive power management and keeping total generated apparent power below maximum level as long as possible, the proposed reactive control algorithm is keeping relation on the highest level. It will allow better exploitation of each RES in whole MG, what can increase possible to active power generation of each converter without reaching of apparent power limit. When converters are operating with apparent powers much lower than nominal parameters, the above relation is equal one and reactive power is sharing proportional to active power of each converter [Fig. 6(a)], based on (8). Unfortunately, this situation is only one of possible case and the limitations of converters have to be considered in reactive sharing control algorithm in order to avoid overloads and developed complete control strategy. Hence, two additional conditions (9) and (10) have to be fulfilled for each kth converter. First condition prevents overloading of converter and the second one must be fulfilled to preserve the balance of reactive power in islanded MG. The relation in limited cases is lower than one, but it is keeping on highest possible level [Fig. 6(b)] providing the best exploitation of RES with maximum active power

where, Quk, calculated reactive power value for unlimited case; QL, total reactive power demand; PL, total active power; Pk, active power of “k” converter; Qk, reactive power of k converter; Sk, apparent power of k converter; and SNk, nominal apparent power of k converter. Based on (8)–(10) and described analysis of reactive power sharing novel control algorithm was developed. The flowchart of the algorithm is shown in Fig. 7. In first stage system parameters are saved in K-elements tables, where K—number of converters, P[K]—measured active powers, SN[K]—nominal apparent powers. Furthermore, limits of reactive powers for each converter Qmaxk, as well as total active power PL are calculated

In the next stage, the auxiliary parameter Qsum, defined as a sum of reference reactive powers of all limited and unlimited converters, is compared with load reactive power. This parameter allows checking if reactive power balance is retained. When Qsum, as a result of stages 3–5 described below is different than total reactive power QL, then algorithm is going to stage 3, otherwise the stage 6 fallowed and final referenced values of reactive power Qk* are defined for each converter. In stages 3–5 the main calculation process of the reference values is executed. Firstly, the reactive power values proportional to active powers are calculated (stage 3). The proportionality factor is composed of parameters Prest and Qrest, which are total active and reactive power PL and QL in unlimited case, otherwise they are smaller by excluding all active and reactive powers of limited converters (stage 5). Next, the limitation is checked (stage 4) and the reference value is set to maximum or to proportional. Depending on the result, auxiliary parameters Qlim, Plim or Qunl, Punl are calculated, which are sums of active and reactive power of converters operating with maximum apparent power or below it correspondingly (stage 4). Then after all K iterations, the parameters Prest, Qrest, Qsum are calculated and the algorithm is going back to stage 2, where the condition (10) is checked, as mentioned above. B. Implementation of Developed Algorithm For more extensive MG (e.g., number of sources K > 10), the calculation of final reference values in one common control Fig. 7. Block diagram of developed reactive power sharing algorithm. unit [e.g., secondary control unit (SCU)] may be long and not be possible, especially if calculations in SCU have to be done in one converter switching period (usually 100–500 μs). Hence, based on Fig. 7 the algorithm can be splitted between all primary control units (PCU) containing inner control loops and SCU, which is mainly responsible for compensating the voltage amplitude and frequency deviation caused by droop control in PCU. As a result, the time calculation in SCU may be reduced improving control dynamic and transient time. Proposed implementation of presented algorithm allows executing many processes parallel in PCUs. The block scheme of proposed control algorithm implemented in PCUs and SCU is shown in Fig. 8. The algorithm calculates the reactive power limit (7) and proportional reactive power value for unlimited cases (8) in each PCU independently. Furthermore, the auxiliary parameters Psk, Qsk are defined (11), (12), based on actual reactive power reference value Q*. In order to fulfill condition (10) the additional value of reactive power _Qk has to be added to value of unlimited case Quk for each unlimited converter. It is defined by (13) and depends on sum of active power of limited converters PsL, sum of reactive power of limited converters QsL, total active and reactive powers PL and QL, reactive power value of unlimited case Quk and auxiliary parameter Qsk. The parameter _Qk can be different for each k, proportionally to Pk, hence the PRPS for unlimited converters.

Fig. 8. Block diagram of developed reactive power sharing algorithm in real-time implementation. Is still satisfied. The final reference values of reactive powers are calculated, when the all conditions (9), (10) are fulfilled and the transferred data between PCUs and SCU do not change in next converter switching period. Furthermore, the steady state of reactive power sharing in MG is obtained when the signals from controllers in inner control loops are established. This process may take a few hundred milliseconds, depending on the number of RES

C. PRPS Algorithm in Real Distributed Control System In real distributed control system, several different processors in PCUs and remote SCU need to share their computational results. Any synchronization between PCUs and SCU are not required in presented solution. The delay can be neglected for modern communication infrastructure with transmission speed in range of megabit per second (Mb/s) and only few km distances between control units in all MG elements. Therefore, application of distributed control system for developed algorithm was proposed (Fig. 8) what can allow for higher computational speed. One of the possible communication problems is loss data in some periods. However, in presented solution, where the transferred data are used only to calculations of referenced reactive powers for the lowest control loops in PCUs, it may cause the longer transient time (worse dynamic of control signals). Another problem in distributed control system is different sampling time for PCUs (usually 5–10 kHz) and SCU [it can work with high sampling frequency (e.g., 40 kHz)]. These differences will not affect the proper operation of converters in MG. Block scheme of simulation model INTRODUCTION TO MATLAB

The name MATLAB stands for Matrix Laboratory. MATLAB was written originally to provide easy access to matrix software developed by the LINPACK (linear system package) and EISPACK (Eigen system package) projects. MATLAB is a high-performance language for technical computing. It integrates computation, visualization, and programming environment. Furthermore, MATLAB is a modern programming language environment: it has sophisticated data structures, contains built-in editing and debugging tools, and supports object-oriented programming. These factors make MATLAB an excellent tool for teaching and research.

MATLAB has many advantages compared to conventional computer languages (e.g., C, FORTRAN) for solving technical problems. MATLAB is an interactive system whose basic data element is an array that does not require dimensioning. The software package has been commercially available since 1984 and is now considered as a standard tool at most universities and industries worldwide.

It has powerful built-in routines that enable a very wide variety of computations. It also has easy to use graphics commands that make the visualization of results immediately available. Septic applications are collected in packages referred to as toolbox. There are toolboxes for signal processing, symbolic computation, control theory, simulation, optimization, and several other fields of applied science and engineering.

Starting MATLAB

After logging into your account, you can enter MATLAB by double-clicking on the MATLAB shortcut icon (MATLAB 7.0.4) on your Windows desktop. When you start MATLAB, a special window called the MATLAB desktop appears. The desktop is a window that contains other windows. The major tools within or accessible from the desktop are:

 The Command Window  The Command History  The Workspace  The Current Directory  The Help Browser  The Start button Fig. 1. The graphical interface to the MATLAB workspace

Command Window: this is used to type the commands.

Current Directory: it shows the folders and m-files.

Workspace: shows program variables Double click on a variable to see it in the Array Editor.

Command History: view past commands save a whole session using diary Mat lab Help Browser

MATLAB Help is an extremely powerful assistance to learning MATLAB. Help not only contains the theoretical background, but also shows demos for implementation. MATLAB Help can be opened by using the HELP pull-down menu Any command description can be found by typing the command in the search field As shown above, the command to take square root (sqrt) is searched. We can also utilize MATLAB Help from the command window as shown

SIMULINK

Simulink is a software add-on to mat lab which is a mathematical tool developed by The Math works,(http://www.mathworks.com) a company based in Natick. Mat lab is powered by extensive numerical analysis capability. Simulink is a tool used to visually program a dynamic system (those governed by Differential equations) and look at results. Any logic circuit, or control system for a dynamic system can be built by using standard building blocks available in Simulink Libraries. Various toolboxes for different techniques, such as Fuzzy Logic, Neural Networks, dsp, Statistics etc. are available with Simulink, which enhance the processing power of the tool. The main advantage is the availability of templates / building blocks, which avoid the necessity of typing code for small mathematical processes.

Simulink is an interactive tool for modeling, simulating, and analyzing dynamic systems, including controls, signal processing, communications, and other complex systems. The version of Simulink included in MATLAB & Simulink Student Version provides all of the features of professional Simulink, with model sizes up to 1000 blocks. It gives you immediate access to the high-performance simulation power you need. Simulink is a key member of the MATLAB® family of products used in a broad range of industries, including automotive, aerospace, electronics, environmental, telecommunications, computer peripherals, finance, and medical. More than one million technical professionals at the world’s most innovative technology companies, government research labs, financial institutions, and at more than 3500 universities, rely on MATLAB and Simulink as the fundamental tools for their engineering and scientific work.

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Start MATLAB, and then in the MATLAB Command Window, enter simulink The Simulink Library Browser opens You can also open the Simulink Library Browser from the MATLAB Tool strip, by clicking the Simulink Library button

Overview of the Simple Model

You can use Simulink software to model dynamic systems and simulate the behavior of your models. The basic techniques you use to create a simple model are the same techniques that you use for more complex models.

To create this simple model, you need four blocks:

• Sine Wave — Generates an input signal for the model.

• Integrator — Processes the input signal.

• Mux — combines the input signal and processed signal into one signal. • Scope — visualizes the signals.

After you connect the blocks, simulating the model integrates a sine wave signal and displays the result, along with the original signal.

Before creating a model, you need to start MATLAB and then start Simulink.

1 Start the Simulink software. In the MATLAB Command Window, enter: Simulink The Simulink Library Browser opens.

2 From the Simulink Library Browser menu, select File> New> Model A Simulink Editor window opens with an empty canvas in the right-hand pane (the model window). 3 Select File > Save as. The Save As dialog box opens.

4 In the File name box, enter a name for your model. For example, enter simple model. Then click Save. Your model is saved with the file name simple model.

Add Blocks to a Model

To create a model, begin by copying blocks from the Simulink Library Browser to the Simulink Editor Model window. For a description of the blocks in this example, see “Overview of the Simple Model” on page 3-2.

1 In the Simulink Library Browser, select the Sources library. The Simulink Library Browser displays blocks from the Sources library in the right-hand pane.

2 Select the Sine Wave block. Drag the Sine Wave block to the model window.

A copy of the Sine Wave block appears in your model. 3 Add the following blocks to your model, using the same approach that you used to add the Sine Wave block. Your model now has the blocks you need for the simple model.

Move Blocks in the Model

Before you connect the blocks in your model, you should arrange them logically to make the signal connections as straightforward as possible.

1 Move the Scope block after the Mux block output. To move a block in a model, use one of these approaches:

• Click and drag the block.

• Select the block, and then press the arrow keys on the keyboard.

2 Move the Sine Wave and Integrator blocks before the Mux block. Your model should look similar to the following figure. Block Connections in a Model

After you add blocks to your model, you need to connect them. The connecting lines represent the signals within a model.

Most blocks have angle brackets on one or both sides. These angle brackets represent input and output ports:

• The > symbol pointing into a block is an input port.

• The > symbol pointing out of a block is an output port.

Input port output port

Draw Lines between Blocks Connect the blocks by drawing lines between output ports and input ports. For information about how to add blocks to the model in this example.

1 Position the mouse pointer over the output port on the right side of the Sine Wave block. The pointer changes to a cross hairs (+) shape while over the port.

2 Click and drag a line from the output port to the top input port of the Mux block.

While you hold the mouse button down, the connecting line appears as a red dotted arrow.

3 Release the mouse button over the output port. Simulink connects the blocks with an arrow indicating the direction of signal flow.

4 Connect the output port of the Integrator block to the bottom input port on the Mux block:

A Select the Integrator block.

B Press and hold the Ctrl key.

C Select the Mux block. The Integrator block automatically connects to the Mux block with a signal line. 5 Connect the Mux block output port to the Scope block.

Draw a Branch Line

The simple model is almost complete, except for one connection. To finish the model, you need to connect the Sine Wave block to the Integrator block. This final connection is different from the other three connections, which all connect output ports to input ports. Because the output port of the Sine Wave block is already connected, you must connect this existing line to the input port of the Integrator block. The new line, called a branch line, carries the same signal that passes from the Sine Wave block to the Mux block.

1 Position the mouse pointer on the line between the Sine Wave and the Mux block. Click the left mouse button.

2 Hold down the Ctrl key and drag the cursor to form a vertical line segment that aligns with input port of the Integrator block. 3 With the cursor at the end of the dotted red line, move the cursor so that it lines up with the light blue guide arrow that points in the direction of the Integrator input port. The cursor turns into a hollow arrow. Drag the cursor to the Integrator input port and release the mouse button.

4 If necessary, smooth out the line segment that connects to the Integrator block. Select the unaligned line segment and drag it until the line is straight.

5 Your model diagram is now complete. Select File > Save. After you finish building a model, you can simulate the dynamic behavior of the model. Now we finished the designing of new model.

After you define a model, you can simulate it, using a choice of mathematical integration methods, either from the Simulink menus or by entering commands in the MATLAB® Command Window. The menus are convenient for interactive work, while the command line is useful for running a batch of simulations (for example, if you are doing Monte Carlo simulations or want to apply a parameter across a range of values).

Using scopes and other display blocks, you can see the simulation results while the simulation runs. You can then change many parameters and see what happens for "what if" exploration. The simulation results can be put in the MATLAB workspace for post processing and visualization.

Model analysis tools include linearization and trimming tools, which can be accessed from the MATLAB command line, plus the many tools in MATLAB and its application toolboxes. Because MATLAB and Simulink are integrated, you can simulate, analyze, and revise your models in either environment at any point. CONCLUSION In this project MG is the advance system for RES integration with own control structure. Usually the hierarchical control is implemented with droop control in primary level. In islanded mode of operation there is the need to manage reactive power sharing and allow RESs work with maximum active power. Hence, the new reactive power sharing algorithm was proposed in this paper, based on the analysis of power sharing between converters in MG. The novel solution prevents the reactive power circulation and disconnection or damage of any converter in MG. Moreover, it allows to converters operation with MPPT, causing better exploitation of each RES and keeping apparent power of each unit below nominal level as long as possible. Because of short switching period of power electronics converters in RES, the algorithm was developed for implementation in hierarchical control structure, providing parallel calculations in each PCU. Simulation analysis was performed, where the three solutions of power control in islanded MG were compared what confirms the correct operation of developed algorithm and shows the advantage of proportional power sharing over others solution presented in literature. REFERENCES [1] Y. Xinghuo, C. Cecati, T. Dillon, and M. G. Simões, “The new frontier of smart grids,” IEEE Ind. Electron. Mag., vol. 5, no. 3, pp. 49–63, Sep. 2011. [2] F. Blaabjerg and J. M. Guerrero, “Smart grid and renewable energy systems,” in Proc. Int. Conf. Elect. Mach. Syst. (ICEMS), Beijing, China, 2011, pp. 1–10. 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