The Reemergence of Microgrids
Below is the first part of this paper. The second part will be posted tomorrow (11/14).
Microgrids are proposed as an amazing new way to manage energy. This is almost true – if you delete "new". Microgrids are one of the oldest concepts in electric utilities, and indeed, they have always been around since the beginning of electricity production and distribution.
However, until recently they have not been in the mainstream. Look upon the electric utility grids and microgrids as two siblings, separated shortly after birth, each to roam the world, learn much, and recently to make many new friends. These friends recently reintroduced them, and together they will form a powerful team.
Originally all electric utility grids were microgrids, and then they started linking together. Eventually most of them evolved into very large girds, like the three North American grids we have today (Eastern, Western and Texas).
But what if a microgrid could not economically run a transmission line to the next grid? What if the microgrid was too remote or an island? These are out there. So some microgrids have continued to exist even as most merged into larger grids.
Like the large grids, isolated microgrids (called an “island utilities”, even if they are not on islands) evolved methods of maintaining grid-stability. All girds and microgrids have a major issue: electricity moves at the speed-of-light, and if something disturbs a grid or microgrid, things can go bad almost as fast. However over time grids and microgrids developed different methods for preserving stability. As we introduce these two siblings, and introduce them to their new friends, we will compare their control methods, and how they can operate more effectively together.
Utility engineers have developed and refined methods of managing individual “control areas” within the larger grids. These are described in the figure below and the next paragraphs.
Operation is done in overlapping two day cycles. Today the grid manager will be planning tomorrow’s schedule, and tomorrow he/she will be executing that schedule. The planning process starts with software that forecasts load and the power needed to serve this load. All of this information is fed into an economic optimization (unit commitment) program, which computes the optimum generation and interchange schedule. Optimized economics (least-cost) is the most common goal, but there can be other objectives.
Of course forecasts are not always completely accurate. For instance, load and renewable generation are dependent on weather. If the weather forecast is inaccurate, the load and renewable forecasts will also be incorrect. Also resources (like generators) and loads can have anomalies. At any point during the target day, there may be too much or not enough generation.
Remember we are discussing a control area within the larger grid, so if there is not enough generation more power will flow into your control area from other control areas. If there is not enough load, more power will flow into other control areas. This is called inadvertent interchange. When this happens, the control area operator responds and changes the generation set-points to correct this in the most economic manner (using economic dispatch software), and pay back the inadvertent power flow.
Back to the management of control areas. If a control area is an isolated microgrid, there can be no inadvertent interchange with neighboring areas. So what happens? Three components are required to maintain stability.
The planning process needs to include a contingency analysis (first component). This software basically asks: what is the worst events that can happen for grid stability, and what are the options to respond to these? The former is frequently the loss of one of the largest generators, and the latter is the second component – spinning reserve (unused capacity) among other generators. The generators with spinning reserve frequently operate in a droop mode where output increases as frequency decreases. When an isolated grid does not have enough generation, its frequency starts to decrease, and these generators respond by increasing their output automatically.
But what happens if something else goes wrong, like during a natural disaster when multiple failures can occur? Usually the most important loads in the microgrid can still retain power. Protective relays (third component) can be operated in a frequency-responsive control-mode, such that they trip their circuits off of the grid as the frequency continues to erode. With modern relays, the frequency set-points can be programmed in real-time to trip the “least critical” loads first, followed by “less critical” loads and so on. This means that the most-critical loads that the utility’s resources are capable of supporting will remain powered as long as there is some generation and secure lines between the generation and those loads.
From the above you see that we already have many of the pieces of a microgrid control system because we have developed these for island utilities. There is just one issue – spinning reserve means that generators need to be “spinning” at all times. This is probably not very economical for a facility (or a cluster of customers within a utility), even a very large one.
This is where we need to bring in a new friend. We need a source of power that can respond rapidly to any change in load or loss of utility power, and does not burn fuel when not delivering energy (like spinning reserve does). We need really big batteries.
These batteries are currently still expensive, but as more electric cars are produced the price is coming down. Other applications for “utility-scale” batteries are beginning to be cost-effective. Utilities are starting to use utility-scale (megawatt-scale) batteries for the functions described below.
- Voltage-regulation for distribution substations with a high concentration of intermittent renewables
- Mitigating transmission constraints
- Providing fast-reacting frequency-regulation services
Currently microgrids are only cost-effective for facilities (plus utilities and consumers) that place a high value on their capabilities, which are examined in the next subsections.
2.2.The Challenge of Renewables
A type of new friends that the grids and microgrids share are renewable energy sources. The sources that are rapidly becoming the most cost-effective are wind and photovoltaic (PV) solar, and these are both intermittent. In other words, while traditional (mainly fossil-fueled) generation can be dispatched (the power output controlled) the output of wind and PV-solar cannot be controlled to any reasonable extent and is naturally variable over time.
However, there is some good news here. Our first new friend (battery-storage), when combined with intermittent renewables provides a cost-effective way to mitigate the variability.
To understand why a facility or group of consumers would need a microgrid, we must understand what a microgrid can do. The following are functions a microgrid can perform:
- Control the amount of power the microgrid draws from the grid at any time
- Buffer renewable generation to provide a reliable source of power
- A microgrid can manage a transition from utility-grid powered mode to an island mode and back. This transition can either be planned or spontaneous. In the case of a spontaneous grid-failure (outage) the microgrid can either implement:
- Black-start: allows the power to fail for all loads while maintaining power continuity with UPSs. Then controllers form the microgrid and repower at least the most critical loads.
- Seamless islanding: maintains power to at least the most critical loads. Seamless islanding is the most difficult mode to manage.
- Manage a diverse fleet of generators. These can include both back-up generators, base-load generators (used for power when this is economical or in island-mode), renewable generation, combined heat and power (CHP) and combined cooling heat and power (CCHP).
- Manage both electrical and thermal storage resources
- Manage generation, storage and load to reduce demand during periods with high demand charges or high energy prices
- Optimize the operation of generation, storage and loads for the best economics consistent with other goals (minimum emissions, capability for islanding, and others)
The big driver among the elements in the above list is the ability to use renewables, other sources of clean generation and storage, controlled as a microgrid, to lower electric cost substantially. The payback periods for implementing these are starting to come down to reasonable levels in areas of the U.S. that have high power costs (mainly the Northeast, California, Alaska and Hawaii).
In the prior section we reviewed the operation of grids (including microgrids) and the basic components required for this operation. In this section we will describe what a microgrid looks like, and the functions the different components perform. A typical block diagram of a facility microgrid is shown below. Each type of component is addressed in the following subsections along with functions and interfaces to other components.
3.1.Intermittent Renewable Generation
In the block diagram these resources are represented by a photovoltaic array, but they may also include wind turbines, as well as generation that is dependent on other intermittent processes (like waste-heat recovery).
The function of each of these is to provide electrical energy. This will be used by the facility and/or stored in the Battery Energy Storage System (BESS) for later use. The only control of these generators is to either throttle them (reduce their output) or shut them down, but since their energy is essentially "free", reducing their output is not financially desirable.
There are many choices for dispatchable generation. In the block diagram dispatchable generators are represented by a Spark Internal Combustion (IC) Gen Set and Fuel Cell. There are three overlapping classes of dispatchable generation: primary emergency, economic dispatch and microgrid support.
Primary emergency generation:
- Diesel Gen Sets: These are economic, reliable and fast-responding, but also have emissions issues that, in many states, limit them to emergency use (plus limited testing).
Economic Dispatch or primary emergency:
- Spark-ignition Internal Combustion Gas Gen Sets: Modern spark IC gen sets have many of the beneficial characteristics of diesel gen sets without their limiting emissions issues, allowing them to be economically dispatched to help lower a facility’s energy costs. These can be fueled by natural gas, propane, biogas, or gasoline.
- Steam gen sets: These are generally used for electric generation in facilities that require process steam, thus the steam is likely to be available when power is spontaneously lost. When steam is available, these turbines can spool up and be synchronized and connected to the microgrid very quickly, and thus are viable for both emergency and economic dispatch generation. Virtually any flammable fuel can be used in steam boilers.
Economic Dispatch and Microgrid Support:
- Gas Turbine Gen Set: These are sometimes used for base load, including combined heat and power (CHP) and combined cooling, heat and power (CCHP) applications. However, if they are not running at the time of an outage, their start-up time is too long for them to be considered for primary emergency backup.
- Microturbines: These also have a start-up time that is marginally too slow to be considered for primary emergency back-up, but some designs approach the response-time required. Like conventional gas turbines, they are well-suited to CHP/CCHP applications.
- Fuel Cells: Start time for fuel cells is generally slow. Many of them are suitable for CHP/CCHP. An enlarged block diagram of a typical fuel cell is shown at the end of this subsection.
- Waste and Biomass to Energy Facilities: Start time is very slow. These facilities generally produce combustible gas or heat that can be used by many of the above technologies to generate power.
There are many ways to store energy. The most important for microgrids are Battery Energy Storage Systems (BESS). This is primarily because they store electrical energy when either its cost is low, or it is produced on-site (including intermittent renewable energy). Later, when energy is needed, the stored energy can be dispatched as if the BESS were an electric gen set.
Thermal energy can also be stored when it is available. This includes thermal energy that can be inexpensively generated during periods when power-pricing is low. This heat- or cold-energy can be retrieved later when it is needed.
3.4.Protective Relays and Fast Load Response.
Modern numerical protective relays have several capabilities that are critical to a microgrid’s response to a power outage. When power is initially interrupted, intertie relays can evaluate the criteria (mainly low frequency, low voltage, reverse power flow, and total duration of these anomalies) in real-time to determine when an outage is occurring and if so, disconnect the facility from the grid.
Each distribution relay within the facility can have a frequency set-point (plus optionally other variables) that can disconnect its feeder from the facility distribution system. When there is more load than generation the frequency will start decreasing, lower priority feeders will start tripping off of the distribution system until the load matches the in-facility generation. This function is called fast load-shed.
An enlarged block diagram for a protective relay is shown below.
In the above relay block diagram, the circles or squares with numbers and letters in them are protective or control-elements that use ANSI Standard Device Numbers. Go through the link below for a definition of these.
When power to the grid is reliably restored (best confirmed via communication with electric utility personnel), the microgrid must then be reconnected to the grid. This requires synchronizing the voltage, frequency, and phase-angle of the microgrid to the grid, and then closing the main intertie breaker. Initially microgrid generation and storage are used to match the voltage and frequency of the microgrid to the grid. Then a synchronizing relay is armed, and when all three conditions are met, the relay closes the main breaker. Usually the phase-angle is the last variable to rotate into a match. The system used to synchronize and reconnect can either be a combination of manual and automatic, or fully automatic.
3.5.Seamless Transition to Island Mode
For seamless islanding, the BESS control function should switch from externally dispatched supply or charging to a droop-mode (supply and/or charge is inversely proportional to frequency) when off-site power is lost. If there is still too much on-site generation when the BESS is fully charging, each generator's control function can be set to throttle the generator and/or trip it off of the distribution system in response to elevated frequency. These frequencies can be set to cascade (react at different elevated frequencies) in a similar (but reverse) manner to different priorities of distribution feeders during fast load shed.
The protective relays, BESS automatic control functions and generator automatic control functions should be designed to automatically stabilize the microgrid with (at least) high-priority loads powered after loss of utility-power. The facility electrical engineer can then make adjustments using these resources to repower additional loads (if required).
3.6.Microgrid Control System and SCADA System
Although microgrids will become common in the future, they are still rather rare and thus dependent on electric-utility technology. This includes the control systems. An electric-utility “energy management system” is generally composed of two subsystems. The “study” system executes simulation and optimization software that defines how the system will be controlled in the future. These are uploaded to a real-time supervisory control and data acquisition (SCADA) system to execute as required.
In a microgrid, the “study system” is replaced by a much more specialized system designed specifically for microgrids. Herein we will call this system the microgrid control system. The SCADA system will be a real-time system similar to those used in electric utilities. These two subsystems can either be purchased separately or together, and if a facility already has a SCADA system that can execute the required functions (or be upgraded to support these), a microgrid control system can generally be adapted to work with the existing SCADA system. A simplified block diagram of these systems and their most important interfaces is below.
Note that one of the most important functions that the SCADA system performs in island-mode operation is the real-time part of load-frequency control. That is, dispatching generation in real-time. The dispatch schedule is defined by the microgrid control system, and the SCADA system executes this using frequency to measure the balance between generation and load. This is a primary reason why electric utility SCADA systems are preferred for this application.
In general the microgrid control system executes applications required for optimum microgrid operation. These might include the following:
- Developing electric-utility supply strategy for minimizing electric-utility costs, consistent with facility’s mission and other objectives
- Forecasting day-ahead load and intermittent renewable production
- Developing optimum day-ahead schedule for dispatchable generation, storage and demand response consistent with the facility electric-utility strategy
- Maintaining the capability to transition from grid-connected to island mode operation at any time
- Managing electric storage such that sufficient capacity is available to meet ongoing needs plus transition to and maintain an island mode (see sext subsection)
- Manage the intertie with the electric utility consistent with facility electric-utility strategy, utility tariff rules and other utility requirements
- Responding to inadvertent events by re-dispatching generation, storage and demand response
- Managing combined heat and power (CHP) systems, combined cooling, heat and power (CCHP) systems and thermal storage (if present).
The outputs of these applications are handed-off via dispatch-schedules and individual parameters to the SCADA system. The SCADA system monitors and controls all of the microgrid components in real-time. The SCADA system also sends monitored parameters to the microgrid control system that it needs to perform its calculations.
Dispatch-schedules include generation/storage dispatch and demand response schedules when grid-connected. Parameters include settings for each distribution relay, generator controller, and BESS controller. These settings will enable these components to rapidly react to a predefined script when off-site power is lost.
The only component that was not previously used in utilities is electric storage. This storage can be dispatched to produce power or accept power (charge) as required when grid-connected. When the microgrid is in an island, storage probably will be operating in a droop-mode (output/charging inversely proportional to frequency deviation from 60 Hz) and will automatically respond to changes in load or generation.
Thus the only new task is to manage the total energy capacity of the storage (make sure it does not discharge to a point where it cannot deliver required energy or reach peak charge and cannot accept required energy). This is done by dispatching generation as shown below. This is shown only for a load increase, and will operate in a similar fashion for a load decrease or generation loss.
The above process is relatively simple for any modern electric utility SCADA system.
The prior section describes how a microgrid control system and SCADA system perform in an operational setting, that is, when they are actually controlling the microgrid. When designing and testing the microgrid, other systems will be used. Although it is not the purpose of this paper to delve into the latter, the only way to assure a smooth deployment of a microgrid is by building a computer-model of the current distribution grid and evolving this to the future microgrid system through each implementation phase. This will require several computer tools, including an electrical distribution modeling and analysis package, and a microgrid financial analysis package. The models will have many useful outputs, including payback rate, specifications and settings for electrical components.
It is worth noting that some products can be used for both microgrid financial design-simulation and operation (microgrid control system). It is generally more efficient to consider one of these because data developed by the simulation system should be very similar to the data required for each phase of the operational microgrid control system.
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