The power plant controller (PPC) is tested in two stages. Cold, without exporting, the point-of-interconnection measurement, the communication with each inverter and with the control center, the signals and the logic are verified, often with simulation. Hot, with the plant connected, the loop is closed and the voltage, reactive power, power factor and active power modes, the ramps and the response to setpoints are checked, which are then demonstrated to the system operator under the grid code.
What is tested and why
The power plant controller (PPC) is what makes hundreds of inverters behave as a single machine at the point of interconnection. It reads voltage, current and power at that point, receives setpoints from the control center and distributes commands to the inverters and to the compensation equipment. What it is and what modes it has is covered in power plant controller (PPC) of a solar plant.
If the PPC fails, the plant does not pass the tests required before commercial operation, even if every inverter is fine. That is why it is best tested in stages: first its inputs and outputs, then the closed loop and, last, the formal demonstration to the system operator.
Cold tests
They are done with the plant not exporting. The goal is to reach energization knowing that the PPC measures correctly, sees every device and executes its logic.
- Point-of-interconnection measurement. Ratio and polarity of the CTs and VTs feeding the PPC’s meter or relay, sign convention of active and reactive power, and consistency with the revenue meter and with the interconnection relay. A reversed sign makes the controller correct in the wrong direction.
- Communication with the inverters. Each inverter responds, its readings have the right scaling and it accepts active and reactive power limit setpoints. If the plant uses concentrators per station, the path through them is tested too.
- Signals to the control center. Measurements, statuses, setpoints and their monitoring, with the approved naming, through point-to-point signal tests.
- Logic and simulation. Modes, priorities and limits are exercised with simulated measurements or with a factory test of the controller. For detailed plant models, grid operators typically ask for the factory acceptance test report of the generator and its controls with a comparison against the model.
- Communication failures. The link to the control center and to a group of inverters is cut in a controlled way to verify the fallback behavior defined in the engineering.
Hot tests, closed loop
With the plant connected and generating, the loop is closed and the response is tuned:
- Active power limitation and response to setpoint steps.
- Active power-frequency control, with the droop and deadband the system operator defined.
- Voltage, reactive power and power factor modes, with small steps and fast recording at the point of interconnection.
- Operational ramps, up and down.
- Compensation (capacitor banks, reactors or dynamic equipment, if present), coordinated with the controller.
These are the same quantities later demonstrated to the system operator. Arriving at that demonstration with the loop already tuned avoids repeating tests that depend on the sun and on the availability of the control center.
- PPC installed and configured
- Cold: measurement, communications and signals. Not exporting, with simulation
- Staged energization of the plant
- Hot: closed loop and tuning. Active power, frequency, voltage, reactive power, power factor and ramps
- Does it meet the times and criteria of the grid code? Yes: continue; No: back to step 4.
- Formal tests and records for the system operator
- Results in the report before commercial operation
What the grid code asks and the PPC must meet
Grid codes differ, but the tests demonstrated to the system operator usually fall in the same families, and each has a criterion that depends directly on the PPC:
| Test before the system operator | Typical criterion that depends on the PPC |
|---|---|
| Active power and frequency control | Initial response time and settling time within the limits of the grid code |
| Operational ramps | Small spread between ramps and average error against the setting within tolerance |
| Local active power setpoints | A few steps of a set percentage of rated power; the plant reaches the setpoint |
| Voltage, reactive power and power factor control modes | Initial response and settling time within limits, with no sustained oscillation |
| Local and remote voltage, reactive power and power factor setpoints | The plant reaches the system operator’s setpoint |
| Capability curve | PPC and compensation commissioned in all their modes before the test |
For ramps, test the minimum adjustable value, an intermediate value, the grid-code reference value and the maximum if it is higher, with at least five up ramps and five down ramps at each setting. The load pickup and rejection rate of new units is also usually determined after they enter operation, with several up and down steps sent from the plant control elements normally used for operation, with no injections into the internal control logic.
Setpoints from the system operator
For solar and wind plants, the operator’s requirements list the state of the frequency and voltage control functions and the monitoring of setpoints and modes. They also list the voltage control, power factor, reactive power and control mode commands that apply. Those commands arrive over the supervision link, normally IEC 60870-5-104 or the inter-control-center protocol, and the PPC must tell whether the command is local or remote. Simulated supervision tests with the system operator are done before commissioning tests begin. More in SCADA and PMU supervision of renewable plants.
Common mistakes
- Measurement from a different CT or VT than the one the interconnection relay uses, with another ratio or without the same sign reference.
- PPC behind several slow network hops, responding late. The architecture is in communications of a solar plant.
- Duplicated ramps: a ramp in the PPC and another in the inverter that limits it.
- Inverter firmware different from the certificates and models delivered.
- Untested fallback on loss of communication, discovered during a test with the system operator.
We help with your PPC
At Tiza Group we integrate and test the PPC and SCADA of solar plants: point-of-interconnection measurement, setpoints to the inverters, signals to the control center and cold and hot tests. Remote configuration and integration work is done worldwide; field testing in Colombia, or elsewhere on request. Depending on your case we recommend which tests to run and with what equipment, and include it in the quotation. Tell us about your plant.
Related guides
- Power plant controller (PPC) of a solar plant
- Communications of a solar plant
- Commissioning tests for solar plants
- Gateway and redundant links to the control center
- Tests before commercial operation
- Integrating inverters into SCADA: Modbus and SunSpec
- SCADA and PMU signals for the system operator: input tests
References
- IEC 60870-5-104, Telecontrol equipment and systems – Network access using standard transport profiles.
- The grid code and the system operator’s technical requirements applicable to the plant.
Frequently asked questions
What is a cold test of the PPC?
It is the verification of the controller without the plant exporting: point-of-interconnection measurement, communication with inverters, data concentrators and gateway, point-to-point signals, logic and behavior on communication failure. It usually relies on simulation or on a factory acceptance test of the controller.
Why does the measured ramp not match the one set in the PPC?
Because the available resource, the communication delay to each inverter, the inverters' own ramps and the active mode all play a part. Grid codes usually accept a ramp test when the spread between ramps is small and the average stays within a few percent of the setting; check the tolerance your system operator uses.
Do the system operator's setpoints reach the PPC directly?
They arrive over the plant's supervision link, normally through the gateway or the agent's control center, using IEC 60870-5-104 or the inter-control-center protocol. The list of voltage, power factor, reactive power and control mode commands that apply is defined by the system operator under the grid code.