Microgrid Load Management and Control Strategies
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Microgrid Load Management and Control Strategies Bill Moran Senior Electrical Engineer Generation is broken down into “base-load” and TRC Companies Inc. “peaking” units. Base load 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 energy 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 microgrids: dispatch is also discussed. The relationship between A. Load measurement: active load management and energy storage is also examined. Real time data from distributed generation 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) 1 978-1-5090-2157-4/16/$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.