Load Management and Control Strategies

Bill Moran Senior Electrical Engineer Generation is broken down into “base-load” and TRC Companies Inc. “peaking” units. is mostly dispatched by Lowell MA, USA schedule using historical data; peaking units meet [email protected] instantaneous demand, maintain frequency and provide Abstract- Load control and management is a key spinning reserve. component of a microgrid. It is essential at all times to In the utility world today loads are rarely controlled, maintain the balance of generation vs. load. The except in emergencies when some distribution circuits are microgrid control system needs to continuously evaluate shed to protect the system from cascading outages. and prioritize loads in order to maintain this balance. We examine methodologies for measuring, evaluating Excess generation is sold on the wholesale market, or in prioritizing and controlling loads under all conditions limited cases used store for peaking via pumped to maximize the performance of the microgrid. hydro facilities. Strategies are presented for the classification of loads by criticality, identifying active vs. inactive loads and II. MICROGRID LOAD MANAGEMENT for maintaining near real time quantitative data for matching loads to generation. We discuss the need for So, how do we apply these concepts to a community active load control when in the microgrid is in grid microgrid? Remember a “micro”-grid has all of the paralleled operation, as well as when islanded. The need characteristics of the “big” grid on a smaller scale. Here’s for high speed control operation is explained. The role how we can apply the principles of the large grid to of the load management system in control of generation community : dispatch is also discussed. The relationship between A. Load measurement: active load management and is also examined. Real time data from and from ties to main grid, aggregated to give a clear picture of power Keywords –Power system control, Load flow control, delivered to the community microgrid. SCADA systems, Load management, Energy storage, Power generation dispatch, Microgrids, Power system B. Load management: measurements In the large scale grid, load management is primarily a last resort emergency plan. For a community microgrid, I. INTRODUCTION load management becomes a much more critical function As microgrids expand into the next generation, moving as there is a much smaller pool of generation resources to away from the smaller, more easily managed college support island operation. campus style layout into true multi-user, multiple customer Managed loads within the microgrid can vary from grids, new control strategies are required. individual customers or groups of customers to specific

Expanding a microgrid to encompass entire appliances or systems within the customers premise. Since neighborhoods, or even small towns, brings about the overall intent of the community microgrid is to provide numerous challenges: improved reliability of electric service to the customer, while improving efficiency, the selection of individual • How is the load measured on a real time basis, and managed loads must be done carefully to provide the how is it controlled? necessary system flexibility with minimal inconvenience to • How does the system operator know and control the customer. both existing and new distributed generation (DG) within the microgrid? C. Classification of loads

• What loads can be controlled? In designing the microgrid, all system loads can be • What additional generation might be required to classified as “Tier-1, Tier-2 or Tier-3”. Managed Loads can achieve island operation? be grouped as follows: • How to handle excess generation? 1) Tier-1 (must run) We can take a cue from the time tested methods that are These are loads which are not shed for any reason, used to control the “big” utility grid today: the most critical loads within the microgrid. Load is measured aggregated on a real-time basis Example: nursing facilities, hospitals, 911 dispatch centers. largely at the generation sources. Historical demand and weather data is used to predict hourly and daily variations. 2) Tier-2 (discretionary loads)

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978-1-5090-2157-4/16/$31.00 ©2016 IEEE Loads that can be shed for short term to reduce load aggregate can be shed to maintain system stability. Often peaks, or time shift them. These are loads that can these loads can be quantified by a one-time measurement or be shed to allow starting of additional generation. calculation and the presence of the load as “on” of “off” Example: Heating, Ventilating and Air-conditioning gives sufficient information for the system to calculate the (HVAC) equipment, hot water heating, pool filters, value of this load. washers and dryers. An example would be hot water heating. If the heating 3) Tier-3 (emergency load shed) element is “on” it is a consistent and repeatable load. Once These are loads that are to be shed only in an measured, this load will be present any time the heater is in emergency. These loads will be shed only in operation and drawing power. This load data can be sensed emergency to protect the stability of microgrid and by a simple current relay that produces a contact closure prevent a blackout. Example: residential customers, when is senses current flowing. This contact closure can commercial facilities with back-up generation. then be transmitted back to the microgrid load management Initial Tier-3 activation can remotely start system as a binary input representing a fixed amount of emergency generators prior to customer curtailment, load. thus reducing load on the microgrid system. B. Use of binary load sensing III. LOAD MANAGEMENT IN GRID Many small loads can be sensed in this way. The use of PARALLELED OPERATION binary sensing reduces the cost of data collection and can The total microgrid load can be easily measured as the be combined with remote I/O that can both sense and sum of power produced by the distributed generation, plus control these system loads. Wireless, Bluetooth I/O the amount of power imported via the grid tie(s). But, what modules, can widely dispersed within microgrid customer’s if that utility grid power is suddenly lost? How do we premises with minimal cost and intrusion. Similar assure that the microgrid can continue to operate and that technology is being integrated into consumer appliances voltage and frequency will be maintained? that will allow a greater penetration into load management for microgrids. When the microgrid is in grid paralleled operation, the on-line distributed generation is often less than the total The load management control system will maintain a load of the microgrid. This may be due to the economics of continuous tally of all (Tier-2) loads and will continuously power production, or due to maintenance on one or more update the list of what loads are available for shedding, if generators. The Load Management system must determine needed, and will maintain a list of loads that could be shed on a continuous basis, what the balance of load/generation due to system contingencies, like the loss of the utility tie, is, and even more importantly, what loads might be shed in loss of a generator etc. As system loads come on or off, the the short term to maintain stability until additional list will be updated in real time to reflect actual operating generation can be brought on-line in the island mode. To load. accomplish this, the load management system has to C. Example constantly analyze the total capacity of the currently operating generation and the microgrid load and have a pre- An example of these strategies in operation is as determined load reduction solution ready for follows: implementation if the grid connection is lost. A microgrid is operating in grid paralleled mode with a This load reduction solution will be constantly changing total load of 1,200 kW; 1,000 kW is from distributed based on the amount of load that needs to be shed, and generation resources, 200 kW is imported from the utility what loads are currently active. grid. A. Active load selection The microgrid load management system has identified a total of 600 kW of Tier-2 load that is presently in Knowing the operational status of loads is extremely operation. Of those loads, 300 kW is designated by the load important. For example, assuming that air conditioning management system for immediate load shedding in the load can be shed if necessary, and that the load reduction event of a loss of the utility power. In the interim, if any of will be “x” kilowatts, may not be true if the weather is cool, the preselected 300 kW loads become no longer active, that or if the need for load reduction occurred during a time of load will be dropped from the “preselected” list and an day when most buildings are unoccupied. In order for the equivalent active Tier-2 load will be added to the list. load management system to effectively control the load, the presence or absence of that load and the quantity of load In this example, if the utility tie is opened resulting in a must be verified. loss of the 200 kW of grid power. Immediately, that preselected 300 kW of Tier 2 load is shed, maintaining the For large varying loads such as a large central air system balance and giving the microgrid control system conditioning chiller, the actual kW of the load can be time to initiate the start-up of additional generation. metered, and the data transmitted in real time to the load Once additional generation is on-line, the previously management system. However in a community microgrid, shed 300 kW load is restored. there are often many smaller loads that individually or in 2

978-1-5090-2157-4/16/$31.00 ©2016 IEEE IV. LOAD MANAGEMENT IN ISLAND The load management system should be hosted on a MODE dedicated computer to maximize speed of operation Redundant back-up computers should be configured for Whenever the microgrid is operating in island mode, the reliability. Watchdog timer functions should be employed Load Management system will continuously update the to assure that computer functionality is monitored. selection of active Tier 2 loads to be shed, if necessary, to maintain Load/generation balance. The load management software should be maintained in an SQL database that is continuously updated as further A. Planning for multiple contingencies. described below. A set of specific queries should be A “preselected” contingency list will be maintained and automatically generated in response to the microgrid status continuously updated for one or more contingencies (grid parallel or islanded), the total system load, utility depending on the number of the DG resources, and their import level, and capacity of operating generation and capacity presently on line. Sufficient load reduction current level of spinning reserve. capacity will be calculated and available to be shed on an E. Emergency operation instantaneous basis to maintain load/generation balance in the event of an unplanned loss of one or more generators. To handle more extreme contingencies such as the loss of multiple generators, for which there is insufficient active B. Rolling curtailment for longer term events Tier-2 load that could be shed, Tier 3 loads will be selected In the event of a prolonged shortfall of generation to be shed. Tier-3 loads will be preselected and sequenced capacity, for example due to failure of a generator, the in the same manner as previously described for Tier-2. Load Management System will “cycle” loads by curtailing Only active loads will be selected. One difference additional Tier 2 loads and restoring previously shed loads. though, is that in the event of Tier 3 load shed activation, This will continue on a rolling basis to assure that all loads all buildings equipped with emergency generators would are provided with power, even if there must be a receive a start signal to immediately start those generators curtailment in overall system capacity. This is similar in and transfer their emergency systems to the standby concept to a “”. This will allow essential generators. This will maximize the use of the emergency functions such as water and space heating to occur in a generators as a means of supporting the microgrid in an limited basis even during a load shed event, thus emergency. maximizing the overall reliability of the microgrid under less than optimum conditions. All Tier-2 loads will be prevented from operating in the event that Tier-3 is activated. If sufficient generation C. Speed of operation capacity is restored, Tier-3 loads would be restored in steps Load management and in particular load shedding has to after a 5 minute delay to assure stability, followed by Tier- be implemented on a very short time line. When a need 2 loads to the extent that the on-line generation will support arises to reduce load due to an unplanned system upset, them. action must take place in as little as 3 cycles (0.05 seconds). If the load/generation balance is not restored V. MICROGRID LOAD MANAGEMENT within that time, system frequency and voltage will degrade AND INTEGRATION OF SMALL DG AND and ultimately result in a complete loss of power to the ENERGY STORAGE microgrid. A. DG resources for microgrids Conventional building energy management systems are DG resources can be divided into “base load”, designed to monitor and control for slower reacting “peaking” and “demand reduction” categories. The latter processes, HVAC and lighting control. The “scan time” of refers to small generation that is uncontrolled and serves to these systems (time to read all points on their network, and reduce load on the system when present, but is not actively issue a control command) is measured in seconds, and controlled by the system operator. An example would be cannot achieve the response time needed for load shedding residential or small commercial PV systems that supply during island operation. A dedicated control system with energy primarily to the customer, with any excess flowing high speed computing and fast operating control functions out onto the distribution grid. This is the methodology is needed. The primary load management computer should presently applied by utilities to most DG resources. They be dedicated to that task and not share processor time with are seen as reductions in demand, as opposed to generation other system control functions. that must be dispatched and controlled. To a utility grid that The communications network used for load appears “infinite” compared to the DG resource, this works measurement and load management must be carefully well. analyzed to assure the round trip data transmission time is B. Uncontrolled Generation not impacted by other network traffic. For the community microgrid, the definition of “small” D. Load management hardware and software generation will change with the size of the overall system, and the amount of uncontrolled DG in that system.

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978-1-5090-2157-4/16/$31.00 ©2016 IEEE For example: A community microgrid having 2000 unplanned loss of the utility, allowing sufficient time to customers, with a day time load of 4,000 Kw and 100 – 6 bring on additional generation while preventing the need to Kw residential PV systems, can safely let these systems shed any microgrid loads. operate uncontrolled during islanded events, as the E. Excess Generation remaining generation assets can compensate for their operation. The PV “looks” like fluctuations in load Excess generation for community microgrids should be demand. managed at all times. When the main grid needs capacity, any excess generation may be allowed to be exported to be However a single 1 MW PV installation, or many utilized by the grid at large. At other times, the generation smaller ones aggregated to that size, will create stability should be controlled to maintain a balance of DG problems for a 4,000 kW microgrid when islanded. It’s best generation vs. load within the microgrid. In Island mode, to develop a strategy for handling small generators (10 kW excess generation must be absorbed either by reducing the or less) based on the overall size of the microgrid, and the output of other generation, or by adding load in the form of number of that type that are or can be expected to be charging energy storage. present. The microgrid Load management system, in conjunction VI. COMMUNICATIONS with energy storage can manage these “uncontrolled DG” resources both individually and in aggregate. Large, utility Connection all of this data and control technology will require a flexible, yet highly reliable and secure scale PV installations for example, can be remotely communications system. The system must be flexible metered to give the Load Management System power enough to allow easy connectivity with mass produced output data on a real-time basis. In grid parallel mode, the consumer appliances and equipment, yet prevent cyber- load management system can allow the power to flow into the microgrid, or activate the charging mode function of the attacks from disabling community microgrids. energy storage system to store all or part of the energy Communication protocols need to be standardized and generated, or allow the excess generation to flow out onto built into DG systems to allow easy integration of the utility grid. monitoring and controls into the next generation of microgrids to reduce the cost of implementation and assure The short duration power swings produced by PV reliability. systems on cloudy days can be mitigated by the Load Management System activating power production by the energy storage to levelize the power supplied to the VII. CONCLUSION microgrid over a rolling 30 minute time frame The microgrid Load Management System plays a key Smaller PV systems would be metered at the customer role in maintaining the essential balance between load and service entrance level. Operation of the PV will look like a generation capacity during island operation, and also reduction or reversal of customer load flow. To microgrid provides pre-planned solutions for contingencies that can operating in grid paralleled mode, the effect will be overall occur during grid-paralleled operation. Preselected and load reduction when the PV is in operation. No active load reduction calculations based on constantly updated management is required. active load calculations allows the microgrid to accurately and smoothly make load reductions when events require it, C. Energy Storage with a minimum of customer inconvenience. In island mode, the Load Management System can be used Binary measurement of load status is an inexpensive and to control the charge-discharge rate of the energy storage easy alternative to analog data collection methods that system, based on the aggregate output of the large scale PV would be more expensive to implement, and require greater systems and the 30 minute rolling average of the small communications bandwidth. customer based systems. The Load Management System As microgrids grow from the single-user/campus model may also dispatch the energy storage as needed to provide additional generation as needed for dynamic load following to true multi-user community systems, a uniform approach and maintaining system frequency. to load management that integrates commercially available technology becomes increasingly essential to D. Loss of utility contingency reliable operation. In systems with energy storage, the Load Management Load management, while necessary, should be nearly System can manage the use of power produced from transparent to the end user of the power. Ideally, the storage to supply all or part of the contingency planning for microgrid load management system should have little or no loss of the utility tie during grid paralleled operation and visible impact to the customer. Emerging technologies that loss of generation during island mode. Power from storage allow for wireless monitoring and control of loads have the can be very effectively used to maintain the load/generation greatest chance for acceptance by the consumers. balance over the short term such as might occur during

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978-1-5090-2157-4/16/$31.00 ©2016 IEEE