Below is the overview from the white paper “SCADA Patterns & Best Practices, Utility Scale PV Solar Power Plant Control,” written by Greg Brunke, Energy Engineering Manager at NLS Engineering. Read the full white paper here. NLS Engineering specializes in solar SCADA and monitoring systems for commercial and utility projects in the United States, Canada and internationally.
Photovoltaic (PV) cells have seen popular use for decades in household electronics such as calculators, battery chargers and emergency radios. PV generation is easily expandable. Single cells produce a small amount of DC voltage at their terminals and can be connected in series and parallel until their total combined output reaches the required capacity.
With modern technology, PV generation has become so efficient it is often the most cost-effective option for utility-scale production.
In a utility scale system, PV cells numbering in the millions are methodically stacked into groups connected in both series and parallel. These ordered panels are referred to as strings. Together their combined output of DC voltage and current can be used to produce a significant amount of power.
By their very nature, PV cells produce a DC voltage. DC voltage is not compatible with the grid, and that is where inverters become an integral part of a solar power plant. The fundamental role of an inverter is to convert the DC power derived from sunlight into AC power compatible with the grid. Modern-day plant controllers are designed to collect feedback from the grid and will adjust plant output to provide voltage support and frequency support.
This quality controlled AC power output is collected and combined from many inverters. Like any other utility-scale system, the output is stepped-up and distributed through a substation.
A PV plant requires many inverters to process the output of multiple arrays. Each inverter is capable of individual control functions but must coordinate, as a unified regiment, to appear as a single source at the Point of Interconnect (POI). This becomes the fundamental role of the plant controller. A power plant controller receives input from authorised users, sensory devices, and feedback applying this constant stream of data to established system directives. The controller commands the regiment of inverters as well as supporting capacitive and inductive devices to maintain the most stable and useful output possible. This document will describe the typical design, requirements and best practices when implementing a PV Plant controller.
The power triangle
A thorough understanding of AC power fundamentals and the power triangle is essential to understanding grid-tied PV plant control. The necessity to invert DC (Direct Current) to AC (Alternating Current) presents the issue of power quality. Unlike DC (Direct Current), AC (Alternating Current) oscillates at a specified frequency causing current to switch directions regularly. Each time the direction changes, electromagnetic fields from inductors need to collapse and re-form in the opposite direction. Likewise, capacitors discharge and re-charge with an opposite polarity. This dynamic behaviour causes a phase-shift where voltage and current become out of phase from one another. In an ideal system, voltage and current would be synchronised, but in real-world applications, this is rarely the reality.
Inductive loads must establish an electromagnetic field before current flow occurs. If current and voltage waveforms were plotted together, the voltage would peak before the current. The current lags the voltage in its alternation from peak to peak. Capacitive loads cannot actualize a voltage differential until they have become saturated. Opposite to an inductive phase shift, a waveform plotting current would begin to flow before voltage begins to alternate. The current flows first, leading the voltage. The left waveform in “Figure 2 – Unsynchronized/Synchronized Waveform” is a graphical representation of a perfectly synchronised waveform. Current and voltage peak and pass the origin at the same times. The image on the right is an example of an unsynchronized wave. In this case, the voltage passes the origin before the current does. This is an example of current lagging behind voltage, caused by an inductive load.
The capacitive and inductive shifts are inversely proportional. As the effects of the properties on the phase shift counteract each other degree by degree, like integers on a scale, only the cumulative result will have an apparent effect on the phase shift. Therefore, one can be used to cancel the effects of the other.
The energy used by inductors/capacitors does not perform ‘useful work’ meaning that it does not directly contribute to the output power of the device. This component of energy is called reactive power, measured in volt-amps reactive (VARs) denoted by the letter Q. If the product of current and voltage is taken as an attempt to calculate power as one would for DC, it will include the non-useful reactive power component and not be truly representative of the ‘useful’ power. This measurement is called apparent power (S) and measured in volt-amps (VA). The component of useful energy is measured in watts (W) and described by the letter P.
The ratio of the active power (“P”, measured in watts (W), the voltage and current which performs real work) to the apparent power (“S”, measured in Volt-Amps (VA), The voltage and current applied to the circuit) depicts the efficiency of a circuit. As the phase shift shrinks, and the apparent power and the real power correlate, the ratio between them approaches 1. At a power factor of 1, 100% of the power (S) applied to a circuit is performing work (P). Realistically, there will always be a small inductive presence in a circuit, as the flow of alternating power through any conductor produces electromagnetic flux. 0.98 is a practical example of high efficiency.
The power factor relationship is commonly represented by a right-angle triangle.
As seen in “Figure 4- Power Triangle,” the angle formed between the active power (P) and apparent power (S) of a circuit is the phase angle (θ). The triangle expresses the complex magnitude of reactive power (“Q,” VARs), a measure of the power required to overcome the impedance in the system.
The power factor triangle is an excellent tool for understanding the relationships between active power, reactive power, apparent power, phase angle and power factor. Power factor, the percent efficiency of real power, is the cosine of the phase angle.
Most modern inverters are capable of simultaneously controlling both active power and reactive power individually while the total output does not exceed the apparent power rating of the inverter. Controlling Reactive Power(Q) of a PV plant is an important system directive which is comprehensively addressed in Automatic Voltage Regulation (AVR).
Control system architecture
An optimal PV plant appears to the grid as a single unified source of power while maximising active power output and providing grid support. This is accomplished by balancing two modes of operation: Active Power Control (APC), and Automatic Voltage Regulation (AVR). Co-ordination between generator owners and system planners is crucial to a balanced grid. It is important to regulate the allowable amount of power at the Point of Interconnect (POI). Active Power Control (APC) limits generation at the POI to predetermined setpoints. This purposeful limitation is called Curtailment. Grid support through Automatic Voltage Regulation (AVR) is done by regulating reactive power.
Figure 5 shows a simplified control loop for plant control. The source of the plant-level setpoint may be one of the multiple authorised users. Local or remote, a plant operator or a collaborative body (ex. system operator), one source is selected at a time to direct the plant controller. Changes in setpoint from a user are passed to the PID controller through a velocity limiter.
The PID controller constantly compares the current setpoint with the actual value as it appears on the grid. This is what is referred to as a closed-loop feedback. Discrepancies between the current setpoint and instantaneous output at the POI manifest as an error which is used to correct the output. If the change in setpoint is too extreme, the output will overshoot the setpoint before it has the time to correct itself. To prevent operator induced overshoot or transients from setpoint changes, a velocity limiter transitions to the new setpoint gradually.
Each inverter is equipped with a controller that provides localised closed-loop control based on its commanded setpoints. The local inverter’s controller is constantly communicating with the plant controller. If an inverter stops communicating with the plant controller, it would appear offline or unavailable and omitted from plant control scheme. An inverter may still run and produce power even if it is unable to communicate with the controller. This emphasises the importance of a closed loop control scheme. Closed-loop feedback at the POI detects the unknown contribution and scales back the commanded output to compensate.
Unlike traditional generators, the prime mover or fuel source is often uncontrollable for renewable resources. With PV generation, weather and sunlight conditions cannot be controlled. Irradiance may be too low or irregular for the plant to meet the demand. Similarly, partial cloud cover can affect different areas of the plant disproportionally causing the output to fluctuate. The closed loop nature of the PID allows the plant controller to provide partial compensation by increasing the requested setpoint to all inverters. Inverters which are affected by cloud cover will only be able to produce the power available from their limited irradiance, but other inverters can produce more than their fair share to compensate.
To prevent overshoot during a rapid increase in irradiance, anti-windup techniques are used to limit PID output during low irradiance. Additional techniques such as inverter ramp-rate control can be used to control changes in plant output.
The center of the plant controller is a PLC that constantly monitors data from all areas of the plant, compares real-time data with operator instructions and relays commands back to plant devices. The controller aggregates information from the POI (Point of Interconnect), relays and meters, meteorological stations (MET stations), capacitor banks, and every individual inverter. The information is used by the controller in conjunction with instructions from the operator to command plant level changes to setpoints.
Occasionally, it becomes necessary to isolate a single inverter from the collective service. A plant controller should have the ability accommodate this request by placing an inverter into a PLC-Manual Control mode. Once in ‘PLC-manual’, the inverter will be separate from the control scheme; it will no longer be regulated by the main control loop.
The inverter’s output will remain included in the curtailment calculations at the POI (Point of Interconnect).
As soon as the mode is changed from automatic to manual, the inverter should maintain its last set of setpoints received in automatic. This provides a smooth changeover between manual and automatic modes. The operator can then specify setpoints on the screen to be written directly to the inverter. The setpoints are checked to ensure that they do not exceed the capability of the inverter. While in manual, an inverter can be shut down and taken out of service.
The plant control scheme should consider KW and KVAR contribution from an inverter which is running in manual mode and be versatile and adaptive enough to compensate for lost control by shifting the remaining load to the available inverters. Despite the best efforts of the controller, if too many inverters are placed in manual the controller will not have the resources to compensate and the operator will lose consistent control.
If an inverter needs to be shut down and taken out of service, it should be done so from the HMI. It is not ideal to place an inverter into manual or shut it down at the inverter because it will simply appear to the controller as though it has lost communication with that inverter. Place the inverter into PLC-manual, and then issue a stop command. This will incrementally reduce the output of the inverter until it is low enough to be brought to a full stop. Once the inverter is shut down, follow proper shutdown and lockout procedures from the manufacturer.