Introduction to Droop Control | What Is 3-Phase Power?, Part 9
From the series: What Is 3-Phase Power?
In 3-phase electrical power systems, grid-forming controllers establish and regulate voltage and frequency. Droop control is a grid-forming control mechanism that has the added benefit of enabling precise power sharing between generators. You will learn:
- How constant frequency (isochronous) control regulates frequency to a given setpoint regardless of active power output
- How constant voltage control regulates voltage to a given setpoint regardless of reactive power output
- How droop control is a simple and elegant solution to stability problems that can arise with constant frequency and constant voltage control
- How frequency droop directly controls active power
- How voltage droop directly controls reactive power
Published: 8 Jul 2022
Hello, everyone. My name is Graham Dudgeon, and welcome to part 9 in the series of tutorials on 3-phase power. The aim of the video series is to build up our engineering knowledge on the design analysis and operation of 3-phase electrical power systems. Today, I will discuss droop control.
I'll begin by showing all the elements of a closed loop synchronous generation system. Voltage is controlled by an automatic voltage regulator, which adjusts field voltage. And frequency is controlled by a speed governor, which adjusts mechanical torque. For more information or a refresher on how synchronous generators operate, please refer to part 8 in this series.
Before we dive into droop control, I'll first set some context by discussing constant frequency and constant voltage control. Note that constant frequency control is also referred to as isochronous control. Isochronous control means a generator unit will regulate frequency to a given set point regardless of reactive power output of the generator. Constant voltage control means a generator unit will regulate terminal voltage to a given set point regardless of the reactive power output of the generator.
The speed governor is responsible for regulating frequency and an error signal that measures the difference between the desired frequency set point, and the measured frequency is used to adjust fuel flow. The speech governor is designed specifically to reduce the frequency error signal to be as close to 0 as possible. The automatic voltage regulator-- or AVR-- is responsible for regulating terminal voltage, and an error signal that measures the difference between the desired terminal voltage set point and the measure terminal voltage is used to adjust field voltage. The AVR is designed specifically to reduce the voltage error signal to be as close to 0 as possible.
Here, I'm showing the result of two simulations. In the simulation on the left, I ramp up active power from 0 to a nominal value of 1 per unit. In the simulation on the right, I ramp up reactive power from 0 to a nominal value of 1 per unit. Note that in order to maintain a constant frequency, the speed governor increases mechanical torque. And in order to maintain a constant voltage, the AVR increases field voltage.
Note that fuel voltage has a value of 1 per unit when there is no reactive power demand and increases to beyond 2 per unit at reactive power increases. These results seem to suggest we have effective generator control. We have the means to regulate both voltage and frequency to constant values, which seems appropriate for effective system operation. However, there is a catch, and that catch exposes itself when we have more than one generator connected in a system.
To explore the response of a two-generator system under different active and reactive power-loading conditions, I'm using a simulation model, where I've connected two generators in parallel and have a single active power load and a single reactive power load. Here, we can see the rotor speed responds when we increase the rotor speed set point on generator 1, from 1 per unit to 1.01 per unit.
We've created a conflicting control request. Generator 1 is trying to move to 1.01 per unit, while generator 2 is trying to stay at 1 per unit. Because the two generators are directly connected, the rotor speeds are trying to stay equal, hence the conflict. Notice that the rotor speeds of the two generators are fighting against each other, as evident from the equal and opposite oscillation.
The situation gets worse when we look at the generator active powers. Generator 1 has increased to full active power in its attempt to increase system frequency. And generator 2 has decreased to zero active power and, in fact, it seems to regenerate power in the region in its attempt to decrease system frequency. This control configuration is clearly undesirable.
The situation is similar for constant voltage control. Here, we can see the voltage response when we increase the voltage set point on generator 1 from 1 per unit to 1.01 per unit. The set points are shown as dashed lines. As with isochronous control, we've created a conflicting control request. Generator 1 is trying to move to 1.01 per unit, while generator 2 is trying to stay at 1 per unit. Because the two generators are directly connected, the voltages are overlaid in this case.
On the plot in the left, you can see that generator 1 field voltage saturates at 2.5 per unit, in this case, in its attempt to increase voltage. Generator 2 field voltage decreases in order to decrease voltage. On the right, we see that generator 1's reactive power increases, and generator 2's reactive power decreases and goes negative. Therefore, generator 1 increases inductive reactive power, and generator 2 responds by ultimately generating capacitive reactive power.
As with isochronous this control, this control architecture is not appropriate for effective system operation when we have two or more generators. Droop control is a simple and elegant fix to this problem. So what do we mean by droop control? Let's consider a single generator to describe the basic concept.
With frequency droop, we add an auxiliary feedback signal that multiplies active power measurement by a frequency droop value and subtracts that from the frequency reference to form a drooped frequency reference. The drooped frequency reference is and compared with the frequency measurement, and the speed governor will bring the frequency to the drooped frequency reference value.
The figure I'm showing here shows the response of frequency to active power loading when we have a frequency droop value of 0.05, or 5%. The droop curve is shown in red, and the dashed lines indicate frequency and active power loading on a per-unit or normalized basis. As we've set the droop value to 0.05, the frequency drops from 1 per unit to 0.95 per unit, as we load the generator from 0 active power to 1-per-unit active power.
With voltage droop, we have an auxiliary feedback signal that multiplies reactive power measurement by a voltage droop value and subtract that from the voltage reference to form a drooped voltage reference. The drooped voltage reference is then compared with the voltage measurement, and the automatic voltage regulator will bring the voltage to the drooped voltage reference value.
The figure I'm showing here shows the response of voltage to reactive power loading when we have a voltage droop value of 0.01, or 1%. The droop curve is shown in red, and the dashed lines indicate voltage and reactive power loading on a per-unit basis. As we've set the droop value to 0.01, the voltage drops from one per unit to 0.99 per unit, as we load the generator from 0 reactive power to 1-per-unit reactive power.
So this is all very well. We've introduced droop control for both frequency and voltage, but how does a simple modification solve the stability issues we saw with isochronous and constant voltage control? To answer that, let's look at the response of two generators that are operating under droop control.
Here, we consider two generators under frequency droop control. In this case, I've set the frequency droop values to be different. I've set droop 2 to be half the value of droop 1. Droop 1 is set to 5%, and droop 2 is set to 2.5%. The red line is the droop 1 curve, and the blue line is the droop 2 curve. So we ramp up the total system active power load from 0 to 0.9 per unit. What we can see is that the frequency is the same for both generators, as expected, but the different frequency droop curves means that reactive power output of each generator is different.
Generator 2, G2, is providing twice reactive power of, generator one, G1. With our final loading condition of 0.9 per unit, G2 is supplying 0.6 per unit, and G1 is providing 0.3 per unit. System frequency is 0.985 per unit. The difference between droop control and isochronous control is that we are directly controlling active power when we have a droop, as there is a 1-to-1 relationship between frequency and active power output for a given generator.
This is why droop control provides a stable response, a simple, yet elegant solution to the stability problems we saw with isochronous control. Notice that half the droop provides twice reactive power. This is a fundamental observation of droop control. One final point. If both droop 1 and droop 2 were equal, then both G1 and G2 would provide equal active power.
Here, we consider two generators under voltage droop control. As with the frequency droop example, I've set the voltage droop values to be. Different I've set droop 2 to be half the value of droop 1. Droop 1 is set to 1%, and droop 2 is set to 0.5%. The red line is the droop 1 curve, and the blue line is the droop 2 curve. Total loathing is 0.9 per unit.
What we can see is that the voltage is the same for both generators, as expected, but the different voltage droop curves means that the reactive power output of each generator is different. G2 is providing twice the reactive power of G1. System frequency is 0.9975 per unit. The difference between voltage droop control and constant voltage control is that we're directly controlling reactive power when we have droop, as there is a 1-to-1 relationship between voltage and reactive power output for a given generator. Notice that, as with frequency droop, half the voltage droop provides twice the reactive power. And if both droop 1 and droop 2 were equal, the G1 and G2 would provide equal reactive power.
In summary, constant frequency, or isochronous control, means a generator unit will regulate frequency to a given set point regardless of reactive power output of the generator. Constant voltage control means a generator unit will regulate terminal voltage to a given set point regardless of the reactive power output of the generator.
For two or more generators connected in a system, isochronous control and constant voltage control can lead to instability. With frequency droop, we directly control active power, as there is a 1-to-1 relationship between frequency and reactive power output for a given generator. With voltage droop, we directly control reactive power, as there is a 1-to-1 relationship between voltage and reactive power output for a given generator. A fundamental observation of droop control for two generators is that half the droop value means twice the power. If droop values are equal, then generators provide equal power. I hope you found this information useful. Thank you for listening.