Frequency Stabilization with Secondary Load Controllers in Diesel Hybrid Microgrids

Overview This paper describes the uses of secondary loads in islanded diesel hybrid microgrids, in particular how high speed secondary load controllers can be used to improve frequency regulation in high renewable penetration microgrids. After defining secondary loads and describing how they are used to ensure minimum diesel loading, we describe how a secondary load controller can be implemented to achieve frequency stabilization. Results from a computer simulation are presented, followed by measured data from an actual microgrid.

Secondary Load Definition According to terminology commonly used in the microgrid context, we classify electric loads in a power system as primary and secondary loads. The primary load is simply the aggregated sum of the various consumer loads, where “consumer” means any of the end users of electric power that the system is designed to serve. The primary load is the electric load that the power system must meet at any given time. Secondary loads, sometimes referred to as controllable loads, are devices over which the system controller has discretionary control. A secondary load can be turned on and off, and it can be modulated to different power levels. Ideally, a system will employ secondary loads that convert electricity into some other useful commodity (e.g. hot water, ice, pumped water, purified water, etc.), but a secondary load can also be simply a dump load, such as an air-cooled load bank designed simply to dump excess energy to the atmosphere.

A secondary load controller (SLC) is the hardware and software that control the power consumption of the secondary load. The secondary load value (kW) is set by the secondary load controller (SLC), which in turn is set by a microgrid controller or system operator.

Simple Diesel Hybrid System First let's consider a simple wind-diesel hybrid system as shown in Figure 1. In an islanded (isolated) power system, frequency regulation requires that the system maintain an instantaneous balance of real power at all times. In this simple system, in order for frequency to remain constant, the instantaneous power output of the diesel generators plus that of the wind turbines must equal the (primary) load. The power output of the wind turbines will fluctuate, sometimes dramatically, in response to fluctuating wind speed (or solar insolation, in the case of a solar-diesel hybrid). Similarly, the primary load will vary in uncontrolled fashion as end users turn various electric loads (lights, motors, heaters, air conditioners, etc.) on and off. The variability of the renewable generation combines with the variability of the primary load in the net load, defined as the primary load minus the renewable contribution. The diesel generator(s), responsible for regulating frequency and voltage, must ramp their power up and down in an attempt to exactly follow the net load. 2

Figure 1: Simple Diesel Hybrid Power System

Diesel Hybrid System with Secondary Load Figure 2 illustrates a wind-diesel hybrid microgrid incorporating a secondary load, in this case an electric boiler. It is assumed there is a microgrid controller that sends a power command to the SLC, which the SLC uses to determine which boiler elements to turn on. Now the load on the diesel generators is the net load plus the secondary load, i.e. the sum of the primary and secondary load minus the renewable contribution. Adding a secondary load gives the system controller a means of modifying the load on the diesel generators.

An electric boiler is a good example of a productive use secondary load. The boiler can be plumbed into a hydronic heating system and displace fossil fuel that would otherwise be burned to provide space heating, process heating, or domestic hot water.

Figure 2: Diesel Hybrid Microgrid with Secondary Load Controller

Secondary Loads for Minimum Diesel Loading In the simple wind-diesel hybrid system of Figure 1, as the renewable generation increases, and/or the primary load drops, the load on the diesel generator decreases. If the net load is too low, the diesel generator will operate very lightly loaded. In the extreme case of the net loaded going negative, the diesel generator would be back-driven and would lose its ability to regulate frequency. In general, it is recommended to operate diesel generators at a minimum of 25% (with some manufacturers recommending up to 40%) of rated power. A primary purpose of secondary loads, then, is to provide a means of ensuring that the minimum diesel load requirement is met in the presence of high renewable output and/or low load conditions.

The minimum diesel load requirement is based on thermal considerations in the diesel engine. As such, the minimum diesel load does not have to be maintained on an instantaneous basis, only on a moving average basis. Consequently, the minimum diesel load function by itself does not require a very fast secondary load controller. As we will see in the next section, however, there is a great benefit to having a fast response secondary load.

Secondary Load Controller for Frequency Stabilization A diesel generator governor (speed control) maintains frequency by adjusting the amount of fuel injected into the engine cylinders, using engine speed as control feedback. Because of the time it takes to actuate the fuel injection mechanism and the time it takes the fuel to reach the cylinders, the diesel genset has difficulty regulating frequency, especially when the diesel load is changing quickly. Because of the highly variable nature of wind and solar power, including large amounts of renewable generation capacity in diesel microgrids tends to degrade power quality by destabilizing frequency. For this reason, many microgrid project developers will only allow up to 20% renewable power penetration in systems that do not include energy storage. Indeed, some people are under the impression that energy storage

Because a secondary load changes the load seen by the diesel, a suitably equipped secondary load controller can also be used to stabilize system frequency, i.e. assist the diesel generators in regulating frequency. In theory, one might do this by having a central microgrid controller measure load and renewable power, and then varying the power command sent to the SLC to smooth out the net load. In practice, because of measurement and communication delays, this approach is too slow. Instead, it is much more effective to use a high speed frequency control loop in the SLC to apply fast corrections to the SLC set point.

The following figure shows how the control loop can be implemented in the SLC.

Figure 3: Secondary Load Control Algorithms with Frequency Stabilization

The control is split into two functions, frequency stabilization, and minimum load control. A microgrid controller is responsible for determining the SLC set point which keeps the diesel above it's minimum load, while the SLC uses locally measured voltage to determine frequency and calculate a frequency control set point bias. This division of responsibility is natural, since the microgrid controller already has access to the power measurements needed to determine the secondary load set point for minimum diesel loading. Also, the microgrid controller cannot measure frequency and communicate the information as quickly to the SLC as the SLC can on its own.

Speed and power resolution (the smallest increment by which the secondary load may be varied) are important factors, because faster control loop speeds and high power resolution allow for higher control gains to be used, resulting in tighter frequency regulation. In cases where the secondary load is resistive, it can react almost instantaneously. An electric boiler, as shown in Figure 2, is an excellent choice as a high speed secondary load, because the resistive elements can be sized to provide excellent power resolution and because they can be switched very rapidly using solid-state switches. The secondary load responds to frequency variations significantly faster than the diesel generator's fuel delivery system, which is why the SLC can significantly improve diesel frequency control.

Computer Simulation To determine how effective the SLC can be at stabilizing frequency, a model was developed using SimPowerSystems (power systems dynamic analysis software), shown in Figure 4. The model was based on the wind-diesel hybrid microgrid on San Nicolas Island in California, a system designed by Sustainable Power Systems and commissioned in 2016. This microgrid incorporates a SPS Secondary Load Controller, with an air-cooled resistive dump load as the secondary load. The model consists of a 1031 kVA diesel genset serving an island load that is roughly 750 kW. There are seven 100 kW wind turbines, and a 160 kW secondary load. The simulation model incorporates the same SLC measurement and control algorithms used in the Sustainable Power Systems High Speed SLC. The secondary load was modeled as an array of resistive elements switched with solid-state relays. The microgrid controller in this model is used to calculate the base SLC set point to ensure minimum diesel loading. If addition load is not needed to keep the diesel above a minimum, a small base secondary load may still be applied in order for the SLC to have a margin of positive load available that can be quickly removed in response to a frequency drop.

Figure 4: SimPowerSystems Model

The wind data for the model was downloaded from the National Renewable Energy Laboratory (NREL) website, and is real collected data with 20Hz resolution. The wind data was collected in gusty conditions.

Figure 5 shows the modeled wind turbine output, diesel generator output, and secondary load plotted over a 30 second interval.

Figure 5: Modeled Diesel, Wind, and SLC Power during Gusty Wind Conditions

This power plot shows that the SLC is effective at smoothing the diesel load, since much of the variation seen in the wind power is not reflected in the diesel load.

To evaluate the frequency stabilizing effect of the SLC, the model was run twice, once with the SLC active, and once without it. The results, shown in figure 6, clearly show that the SLC-equipped system can maintain a tighter frequency band than the diesel can by itself. In practice, what this means is that the system can tolerate a higher penetration of renewable power without sacrificing power quality.

Figure 6: Frequency Response with and without SLC

Logged Data from Islanded Microgrid Actual logged data from the San Nicolas Island wind-diesel microgrid is shown below. Two figures are presented: Figure 7 shows the 10-minute minimum and maximum frequency values for a 24-hour period. The microgrid controller continuously measures frequency, and every 10 minutes it logs the maximum and minimum observed during that period. The width of the frequency band defined by the max and min values is a measure of the quality of frequency regulation. At 10:00 AM the frequency stabilization feature of the SLC was turned off and the band becomes wider. Figure 8 is the microgrid power measured at the same time as the frequency. It is provided to show that the increase in frequency variability is not due to more variable load, or wind conditions. Note that the primary and secondary load are plotted as negative numbers.

In this paper we are only presenting data from a brief operating period, but it should be noted that the San Nicolas Island microgrid has been running successfully with instantaneous wind power penetrations up to 70 percent with no loss in power quality relative to the previous diesel-only system.

Figure 7: 24 Hour Frequency Chart using Measured 10 Minute Maximums and Minimums

Figure 8: 24 Hour Microgrid Power Measurements

Conclusions High speed secondary load controllers can be used to stabilize frequency in diesel hybrid microgrids. This allows for higher levels of renewable energy penetration without system stability problems. The computer simulation model accurately predicts improvement in frequency regulation gained by using a Sustainable Power Systems high speed secondary load controller.

Energy storage combined with a frequency responsive power conversion system (PCS) can provide similar frequency stabilizing effects to high renewable penetration microgrids. However, energy storage is still quite expensive, and in cases where energy storage is not needed for time shifting of renewable energy, high speed secondary loads can be a very cost effective alternative, particularly in cases where a productive secondary load can be implemented (as opposed to a dump load).

Sustainable Power Systems offers its line of ControlFreqTM frequency stabilizing secondary load controllers. For more information on how they can help you increase the power quality and/or renewable energy penetration of your diesel hybrid microgrid, please contact us at www.SustainablePowerSystems.com.