Calculation of Relay Protection for Photovoltaic Power Plants

Relay protection settings for PV stations are calculated to ensure coordination, selectivity, and reliability, considering fault currents, inverter characteristics, and bidirectional power flows.Key PrinciplesRelay protection in photovoltaic stations aims to detect and isolate faults quickly while minimizing disruption to the rest of the network. The main objectives include:Selectivity: Only the faulty section is disconnected.Sensitivity: Relays detect even low-level fault currents.Coordination: Backup and primary relays operate in proper sequence to avoid maloperation .Inputs for Relay Setting CalculationsTo calculate relay settings, the following data are required:Single Line Diagram (SLD) of the PV plant and associated transformers .Transformer ratings and impedance values.Cable sizing and short-circuit capacity at different voltage levels (e.g., 33 kV MV, 800 V LV), .Inverter characteristics: Maximum output current, short-circuit current, and voltage ratings .Fault current calculations: Three-phase and single-phase-to-ground faults at various points in the network .Calculation MethodologyShort-Circuit Analysis: Determine maximum and minimum fault currents at each bus and feeder using software tools like DIgSILENT PowerFactory or EasyPower .Relay Type Selection: Common relays include:Overcurrent Relays (OCR)Directional Overcurrent Relays (DOCR)Ground Fault RelaysUnder/Over Voltage and Frequency Relays for inverter protection .Setting Pickup Current (PS) and Time Dial Setting (TDS):PS is set above the maximum load current but below the minimum fault current.TDS ensures proper coordination with upstream and downstream relays .Time-Current Characteristic (TCC) Curves: Plot TCC curves for all relays to verify coordination and avoid overlap that could cause misoperation .Adaptive Settings: For PV plants with variable generation, adaptive relay schemes can update PS and TDS dynamically based on real-time PV output to prevent maloperation .Coordination ConsiderationsBidirectional Power Flow: PV integration can reverse current flow, requiring directional relays for proper coordination .Inverter Limitations: Inverters may limit fault current, affecting relay sensitivity and coordination .Backup Protection: Ensure backup relays operate with a time delay to allow primary relays to clear faults first .Tools and SoftwareEasyPower Power Protector Module: For TCC plotting and coordination studies .DIgSILENT PowerFactory: For short-circuit and relay coordination analysis .ADMO Software: For managing relay settings, revisions, and documentation in large networks .Practical ExampleFor a 100 MW / 150 MWp solar plant:MV system: 33 kV, LV system: 800 VShort-circuit capacity: 25 kA at 33 kV, 36 kA at 800 VRelays are set for overcurrent, ground fault, and directional protection, coordinated with upstream HV fuses and LV breakers .Adaptive DOCR settings are updated based on PV output to maintain coordination under varying generation conditions .SummaryCalculating relay protection settings for PV stations involves:Collecting system and equipment data.Performing short-circuit analysis.Selecting appropriate relays and calculating PS and TDS.Plotting TCC curves for coordination.Implementing adaptive schemes for variable PV output. This ensures safe, reliable, and coordinated operation of the PV plant while protecting both the plant and the distribution network from faults .