Through the SCADA HMI, the plant control system can be set to operate in one of the three modes of automatic voltage regulation (AVR): voltage-regulation, power-factor regulation or reactive-power control. The three AVR modes are illustrated in the SCADA HMI screen shown in Figure 3 below. Note that the plant can operate in only one of the three operating modes at any time.

HMI Screen on AVR Modes

In the voltage-regulation mode, the controller maintains the specified voltage set point at the POI by regulating the reactive power that is produced by the inverters as well as other devices such as capacitor banks. In the power-factor regulation mode, the controller maintains the specified power factor. The operation of the controller is illustrated in Figure 4 below, which shows field data from a PV plant producing about 212 MW of active power at that time.

Figure 4: Dynamic Power-Factor Regulation

The figure illustrates the response of the plant when the power factor set point is changed from 0.98 to 1.0. The controller commands the inverters to change their reactive power output to meet the new power factor set point, using a closed-loop control mechanism. The figure illustrates that the inverters respond very rapidly. Within a few seconds (< 4 s) the new set point is achieved in a closed-loop control mode. More specifically, the rise time to reach 90 percent of steady-state value shown above is about 3.2 s.

Active Power Management

Through the SCADA HMI, the controller can also be set to control the active power output of the plant. When the control system detects that the active power at POI exceeds the specified set point, it calculates and sends the commands for each inverter individually to lower its output to achieve the desired set point, using a closed-loop control mechanism. In some cases the plant controller will turn off certain inverters to achieve this desired set point, since output of each inverter cannot be lowered below a certain threshold without causing a high DC voltage operating condition.

Active Power Curtailment HMI

Figure 6 below illustrates field data from a PV plant operating at around 90 MW power. The curtailment limit is initially changed from 100 MW to 82.5 MW. The plant controller turns down the inverters (and turns off some of them if required) to achieve the new set point. Note that the turndown of power is gradual to meet the specified ramp-rate limit.

Figure 6: Power Curtailment at different levels

The curtailment limit is reduced again to around 75 MW, and the controller responds as expected. When the limit is raised, the controller adjusts the output of the inverters to increase the total plant output. Finally, when the limit is raised to 100 MW, the plant is no longer curtailed since the plant is producing less than the limit.

In all the control actions, the controller’s command to each inverter is unique, given the specific conditions each inverter is experiencing. In the case of curtailment due to cloud passage, where the plant has additional generating capacity but is restrained to a specified limit and curtailment is limited to part of the plant, the controller can minimize the impact of cloud cover by increasing the output of other inverters that are not impacted. This will result in increased energy yield as illustrated in Figure 7.

Impact of Cloud Passage under Curtailment

An illustration of additional function related to active power management is shown in Figure 8 below, which shows the reaction of the plant when it is shut down and started up. In this example, the field data illustrates when one block (30 MW) that is under control of the plant controller is commanded to shut down, the active power management function reduces plant output while maintaining the required ramp rate. As mentioned earlier, some inverters are turned down while others are shut down. Note that the control is quite effective even with moderately varying irradiance conditions.

Figure 8: Plant Shut-Down and Start-up Controls

The above figure also illustrates the “plant start” command that results in the controller gradually increasing the plant output by adjusting the inverters’ output and turning on the inverters in sequence.