About the Protection Coordination
Protection relay coordination for solar substations ensures that when a fault occurs, the protection zone closest to the fault operates first (primary protection) and clears the fault within the fastest possible time, while upstream relays remain stable — minimising the outage to the smallest possible part of the plant.
How to calculate it
- 1Define protection zones: string level (fuse), inverter output (built-in LVRT + island detection), AC collection feeder (MCCB or feeder relay), HV transformer (differential + overcurrent), grid interconnect (distance or directional overcurrent at 33 kV+).
- 2For each zone, select a relay characteristic: Inverse Definite Minimum Time (IDMT) for overcurrent, or high-set instantaneous for close-in faults.
- 3Set time-current grading margins: 0.3–0.4 s between successive overcurrent relays for electromechanical types; 0.2–0.25 s for numerical relays (faster reset).
- 4Verify the highest-set relay still clears the minimum fault (maximum impedance path) within the protection's reach.
- 5Carry out a secondary injection test at commissioning to confirm all relays operate at the set values.
Worked example
A 10 MW, 33 kV solar plant: feeder relay (R1) set at 300 A, 0.3 s IDMT. Grid interface relay (R2) set at 500 A, 0.6 s IDMT — 0.3 s above R1. Transformer differential relay (87T) operates instantaneously for internal transformer faults. Fault at the feeder: R1 trips in 0.3 s; R2 remains stable. Fault inside the transformer: 87T trips in <0.08 s, R1 and R2 do not operate.
What you can calculate
- IDMT grading
- CTI margin
- IEC 60255
- Relay coordination
Standards & references
Frequently asked questions
Does a solar plant need island detection (anti-islanding) protection?
Yes — all solar plants connecting to the grid must have anti-islanding protection to prevent the inverter from energising a de-energised grid section during a grid outage, which could endanger DISCOM personnel working on the lines. Under-voltage, over-voltage, under-frequency, and over-frequency trip relays are mandatory at the grid interface. Many numerical relays also implement Rate-of-Change-of-Frequency (ROCOF) and vector shift schemes for faster islanding detection.
What CEA-mandated protections are required for a solar plant connecting at 33 kV?
CEA Grid Connectivity Standards require: directional overcurrent relay (or distance relay for large plants), earth fault relay, under/over voltage relay, under/over frequency relay, islanding protection (ROCOF or vector shift), synchronism check relay for auto-reclose, and circuit breaker fail (CBF) protection. The DISCOM's protection relay settings must be communicated to the plant EPC and set before synchronisation.
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