Understanding Grid Compliance Studies Using PSCAD for Renewable Power Plants
Grid connection approval for renewable power plants now requires detailed technical studies. For inverter-based resources such as solar PV, wind and Battery Energy Storage Systems, developers must demonstrate that the plant can remain stable and support the grid during normal and disturbed conditions.
In This Article
PSCAD studies commonly cover:
- Harmonic performance
- DC current injection
- Flicker
- Fault ride-through
- Frequency response
- Active power control
- Unbalanced faults
- Controller stability
Why PSCAD Studies Are Required
Tools such as PSS®E use RMS-based models. RMS simulations are suitable for system-wide voltage, frequency and stability studies. They represent inverter controls through simplified, time-averaged equations. However, RMS models cannot fully capture:
- Inverter switching harmonics
- Instantaneous waveform distortion
- Fast control interaction
- Unbalanced fault behaviour
- Negative-sequence current response
- Manufacturer-specific protection logic
- Resonance between the plant and grid
PSCAD is an Electromagnetic Transient simulation tool. It represents instantaneous voltage and current waveforms using a small simulation time step. It can model inverter switching, converter controls, protection logic, plant controllers and network transients in detail. A plant may appear stable in an RMS study but show oscillations, current-limit issues or delayed recovery in an EMT study. A complete PSCAD model should represent both the renewable plant and the nearby grid network.
The model normally includes:
- OEM inverter, WTG or PCS model
- Plant Power Controller
- Inverter duty transformers
- Collector cables and overhead lines
- Main power transformers
- Reactive compensation equipment
- Harmonic filters
- Transmission line to the POI
- Grid equivalent
- Protection and control logic
The grid equivalent should use the required fault level and X/R ratio at the Point of Interconnection. Studies may also be performed under strong-grid and weak-grid conditions because inverter response depends heavily on Short Circuit Ratio. A PSCAD grid compliance assessment contains several separate studies. Each study checks a specific grid-code requirement at the POI.
1. Harmonic Analysis
Solar inverters, wind converters and BESS PCS units use electronic switching. This switching can produce harmonic currents at multiples of the fundamental frequency.
For a 50 Hz system:
- 5th harmonic is 250 Hz
- 7th harmonic is 350 Hz
- 11th harmonic is 550 Hz
- 13th harmonic is 650 Hz
Excessive harmonics may cause:
- Transformer heating
- Cable overloading
- Protection maloperation
- Capacitor stress
- Network resonance
- Poor power quality
The plant should be tested at different active power levels. When a harmonic exceeds the limit, the engineer should review the inverter data, grid impedance, cable configuration, transformer impedance and filter design. A filter should be checked across all operating cases. A solution for one harmonic order may create an issue at another order.
2. DC Injection Analysis
Inverters are designed to produce AC current. However, switching imbalance, measurement offset or controller behaviour may introduce a small DC component.
DC injection may result in:
- Transformer core saturation
- Increased losses
- Transformer heating
- Metering errors
- Protection relay errors
The DC component of current should be measured at the POI under different operating conditions. For BESS projects, both charging and discharging modes should be studied. When the value exceeds the permitted limit, the inverter controller, current measurement and output filter should be reviewed.
3. Flicker Analysis
Flicker is caused by repeated voltage fluctuations.
For renewable plants, flicker may occur due to:
- Solar irradiance variation
- Wind-speed variation
- Active power step changes
- Inverter start-up and shutdown
- Capacitor-bank switching
- Reactive power control
The POI voltage is assessed using a flicker meter.
The main indices are:
- Short-term flicker severity, Pst
- Long-term flicker severity, Plt
When flicker exceeds the limit, the plant ramp rate, PPC response, reactive power control and switching sequence may need correction.
4. Dynamic Studies
Dynamic studies verify plant behaviour during grid disturbances. Before applying any event, a no-disturbance test should be completed. During this test, voltage, active power, reactive power, frequency and controller outputs should remain stable.
Low-Voltage Ride-Through
During an LVRT study, a fault is applied at or near the POI.
The plant must:
- Remain connected
- Inject the required reactive current
- Limit converter current
- Support grid voltage
- Recover active power after fault clearance
- Return to stable operation
Both balanced and unbalanced faults may be required.
High-Voltage Ride-Through
During an HVRT study, the plant is exposed to a temporary overvoltage. The plant should remain connected, control reactive power correctly and return to normal operation after the voltage recovers.
Frequency Response
Frequency response should be tested with the frequency-control function disabled and enabled. Without frequency control, the plant should ride through the frequency variation without unstable operation. With frequency control, the plant changes active power through droop-based control.
The study should verify:
- Frequency deadband
- Droop gain
- Response time
- Active power limits
- Recovery behaviour
- PPC and inverter coordination
For BESS projects, charging and discharging modes should be checked separately.
Active Power Ramp Test
An active power command is applied to the PPC. The study checks whether the plant follows the reference without exceeding the permitted ramp rate. Both ramp-up and ramp-down conditions should be tested.
Unbalanced Fault Tests
Single-line-to-ground, line-to-line and double-line-to-ground faults are applied.
These studies check:
- Negative-sequence current response
- Phase current balance
- Converter current limitation
- Controller stability
- Protection response
- Post-fault recovery
Grid Strength Assessment
A plant may perform correctly under a strong grid but become unstable under a weak grid.
Weak-grid conditions may cause:
- Voltage oscillations
- Poor PLL response
- Slow current control
- PPC interaction
- Harmonic amplification
- Delayed fault recovery
Where required, disturbance cases should be repeated at minimum and maximum grid-strength conditions.
Final Submission Package
A complete PSCAD study package should include:
- PSCAD project files
- OEM inverter, WTG or PCS model
- PPC model
- Grid equivalent
- Transformer and line data
- User manual
- Study report
- Test-case summary
- Simulation plots
- Parameter sheets
- Model dependency files
The model folder should be self-contained. A reviewer should be able to open, compile and run the project without searching for missing files. PSCAD studies show whether the plant can remain connected, support grid voltage, respond to frequency changes and recover after disturbances. The main question is not only whether the plant can export power. It is whether the plant can operate safely and remain stable during every required grid condition.
