Grid-Forming Control and Solar Curtailment | Evaluating Microgrid Control with Simscape Electrical, Part 2
From the series: Evaluating Microgrid Control with Simscape Electrical
Patrice Brunelle, Hydro Quebec
Graham Dudgeon, MathWorks
An AC electrical power system cannot operate in a secure and stable manner without a well-regulated voltage and a well-regulated frequency. A control system in an AC electrical power system that directly contributes to the control of voltage and/or frequency setpoints is referred to as grid-forming. You will learn how,
- Droop control is a form of grid-forming control, with the added benefit that it facilitates effective power sharing if there are two or more sources under droop control.
- How solar power output must be curtailed in the event of a load disconnection in order to maintain voltage and frequency setpoints.
Published: 12 Jun 2022
The bottom line is that an AC electrical power system cannot operate in a secure and stable manner without a well regulated voltage and a well regulated frequency. When we've established appropriate grid voltage and frequency, we say the grid is formed. On an AC system, any control system that directly contributes to the control of voltage and/or frequency is referred to as a grid-forming controller. On a DC system, any controller that directly contributes to the control of DC voltage is a grid-forming controller.
Droop control, which I will summarize in a moment, is a form of grid-forming control, which has the added benefits of facilitating effective power sharing between generation units. Here is the basic architecture for droop control in an AC system. For frequency droop, we offset the frequency reference, by subtracting active power multiplied by a droop value. For voltage droop, we offset the voltage reference, by subtracting reactive power multiplied by a droop value.
The frequency droop curve tells us what the system frequency is for any active power loading condition. For example, for a frequency droop value of 5%, if active power loading is 0.5 per unit, then the frequency will be 0.975 per unit.
The gradient of the droop curve is the droop value. The voltage droop curve tells us what the system voltage is for any reactive power loading condition. For example, for a voltage droop of 5%, if reactive power loading is 0.5 per unit, then the voltage is 0.975 per unit. Again, the gradient of the droop curve is the droop value.
If we have two sources, we can plot both frequency droop curves together, to establish reactive power loading at any frequency level. Note that, regardless of loading, the power share ratio remains the same. In the animation you're seeing here, generator two has half the droop value of generator one. And so generator two provides twice the active power.
Similar to frequency droop, if we have two sources that are under voltage droop control, we can plot both voltage curves together to establish the reactive power loading any voltage level. Note that, regardless of loading, the power share ratio remains the same. In the animation that you're seeing here, generator two has half the droop value of generator one. And so generator two provides twice the reactive power.
With a solar power plant, for a given radiance, there is a maximum power transfer that can be achieved. Maximum power transfer will occur when the internal impedance of the PV array matches the load impedance. A Maximum Power Point Tracking algorithm, or MPPT algorithm, adjusts the duty cycle of a boost converter, such that the load impedance presented to the PV array matches the internal impedance.
What we can see in this example figure is MPPT in action. There are a number of solar cell powered curves, colored blue, for a fixed temperature and varying radiance. The red line is the maximum power curve. The green dot is the power output when using a Maximum Power Point Tracking algorithm known as perturb and observe.
A perturb and observe algorithm attempts to minimize the gradient of the power output. As you can see, maximum power coincides with a power curve gradient of 0. I'll now pass back to Patrice, who will discuss a grid-forming and solar curtailment scenario in our microgrid model.
OK, in this scenario, the microgrid is islanded and the battery energy storage system operating grid-forming mode. It means that it directly controls voltage and frequency. To run the scenario in the model, we double click on TestParam block and select Test number 3 from the dropdown list, before we simulate the model. Here, we can see the simulation result for various power flows in the system.
At 0.5 second, the microgrid is islanded and the BESS control switches to grid-forming mode. At 1.5 seconds, a load shed occurs. And because we are still under MPPT control for the solar plant, the battery must absorb the excess solar power. At 2.5 seconds, the solar power controller is switched to curtailment mode, at which point the solar power is reduced to match the new loading condition, and battery active power returns to 0.
On the left, we can see how the duty cycle change from MPPT mode to curtailment mode. Note that the curtailment, the duty cycle does not need to hunt for the maximum power point, and so we do not see the fluctuation in duty cycle that are evident in the MPPT mode. On the right, we see the PV voltage and current. Note that AC voltage rises and the DC current reduces during curtailment.