When a solar plant connects to the Indian grid, the DISCOM protection engineer will not sign the synchronisation permit until the developer submits a relay coordination study that proves the plant protection relays operate faster than the upstream grid relays during every credible fault condition. This is not a checkbox exercise. A poorly coordinated protection scheme creates two equally bad failure modes: the plant trips for every minor grid disturbance (causing unnecessary revenue loss), or the plant relays do not trip fast enough to isolate a fault before the DISCOM upstream breaker operates (causing a sustained fault that damages transformer windings or HV cables worth crores to replace).
Direct answer. A solar relay coordination study for India follows the 4-Step Relay Coordination Methodology: fault current calculation at the point of common coupling (PCC), protection relay selection (IDMT overcurrent, directional, differential, distance), time-current grading using IEC 60255-151 IDMT curves, and coordination curve verification against the upstream DISCOM relay. CEA Connectivity Regulations 2019 specify minimum relay functions and operating times for each voltage class. The output is a relay settings report that the DISCOM protection engineer countersigns before commissioning, derived from software tools such as ETAP, ERACS, or PowerWorld.
This guide is written for Suresh and his engineering team preparing a relay coordination submission for a 25 MW solar project connecting at 33 kV in Rajasthan. The methodology applies equally to 66 kV and 132 kV interconnections, with additional relay functions required at the higher voltage classes.
What Relay Coordination Is and Why DISCOM Requires It
Relay coordination is the process of setting the operating characteristics of multiple protection relays in series so that the relay nearest to the fault operates first, isolating the minimum section of the network. All relays further from the fault provide backup, operating after a deliberate time delay if the primary relay fails.
Definition. A protection relay is a device that monitors electrical quantities (current, voltage, frequency, impedance) and issues a trip command to a circuit breaker when a measured quantity exceeds a pre-set threshold. Relay coordination ensures that, in a network with multiple relays in series, only the relay closest to the fault operates — preserving supply to the rest of the network and limiting equipment damage to the minimum necessary section.
The DISCOM requires a relay coordination study before synchronisation for two reasons. First, grid stability: an uncoordinated solar plant protection system can cause the plant to disconnect from the grid at the wrong time — either too early (nuisance tripping during minor voltage dips) or too late (allowing a fault to persist until the DISCOM upstream breaker operates, causing a wider outage). Second, equipment protection: the thermal and electromagnetic limits of transformers, cables, and switchgear are well-defined, and a relay coordination study calculates whether the protection system will isolate a fault before fault current causes irreversible thermal damage.
According to CEA Connectivity Regulations 2019, Schedule 2, the developer is responsible for completing and submitting the relay coordination study before the DISCOM issues a grid connectivity approval (GCA). Projects that proceed to energisation without a GCA face penalties under the Electricity Act 2003.
The 4-Step Relay Coordination Methodology
Heaven Designs follows a structured four-step process for every relay coordination study, regardless of project capacity or voltage level. This process ensures that the output relay settings report satisfies the DISCOM protection engineer review without requiring a second submission.
Fault Current Calculation at the PCC
Calculate the maximum and minimum three-phase and single-phase-to-earth fault currents at the point of common coupling and at every other protection relay location in the plant. Maximum fault current (using minimum grid impedance) sets pickup thresholds; minimum fault current (using maximum grid impedance) verifies that relays operate for the smallest credible fault.
Protection Relay Selection
Select the relay types, IED models, and CT/VT ratios for each protection function. At 33 kV, the standard set includes IDMT overcurrent (51), earth fault (51N), under/over-voltage (27/59), and under/over-frequency (81U/81O). At 132 kV, add distance protection (21), transformer differential (87T), and directional earth fault (67N). Document the manufacturer, model number, firmware version, and available characteristic curves for each relay.
Time-Current Grading
Plot the time-current characteristics of every relay in the protection chain on the same logarithmic time-current graph. Adjust the time-multiplier setting (TMS) of each relay so that the plant relay operates before the upstream DISCOM relay by a minimum coordination margin of 0.3–0.4 seconds. The IDMT curve shape — Normal Inverse, Very Inverse, or Extremely Inverse — must be selected to achieve coordination across the full fault current range.
Coordination Curve Verification
Verify the final relay settings against three tests: (a) the relay operates for the minimum fault current at the far end of the protected zone; (b) the coordination margin is maintained at every fault current from minimum to maximum; (c) the operating time of each relay does not exceed the thermal withstand time of the protected equipment. Document the verification results in tabular form for the DISCOM submission.
Fault Current Calculation at the Point of Common Coupling
The fault current calculation is the foundation of the entire relay coordination study. An error here propagates through every subsequent step, producing relay settings that are either too sensitive (causing nuisance trips) or too insensitive (missing real faults).
16–40 kA
Typical 33–132 kV bus fault level in India
CEA annual report on grid parameters, 2024
0.3 s
Minimum coordination margin (grading time)
IEEE C37.91, standard grading practice
1.1–1.5x
Solar inverter fault current output
IRENA grid integration guidelines, 2023
The fault current at the PCC has two sources: the upstream grid (the dominant source carrying 90–99% of fault current for most plant configurations) and the solar plant itself (limited by inverter current limiting to 1.1–1.5 times rated output current). For relay coordination purposes, the contribution from the solar plant is typically small enough to treat as negligible for radial feeder configurations, but it must be calculated and verified — especially for projects at the end of a weak 33 kV feeder where the grid fault current is low.
The fault current calculation follows the symmetrical components method as described in IEEE Standard C37.113 (Guide for Protective Relay Applications to Transmission Lines). The inputs required from the DISCOM are the positive-sequence impedance of the grid at the PCC, the zero-sequence impedance for earth fault calculations, and the maximum and minimum fault MVA at the PCC from the DISCOM power system model. Request these values in writing from the DISCOM protection engineer as part of the grid connectivity application.
Relay Types Used in Solar Protection
The relay types used in a solar plant protection scheme are determined by the interconnection voltage, the plant configuration, and the DISCOM specific requirements.
| Relay Function | IEEE Number | Application | Voltage Class |
|---|---|---|---|
| IDMT Overcurrent | 51 | Line feeder, transformer HV | 33 kV and above |
| Earth Fault | 51N | Line feeder, transformer neutral | 33 kV and above |
| Directional Overcurrent | 67 | Ring bus, parallel feeders | 66 kV and above |
| Directional Earth Fault | 67N | Ring bus, parallel feeders | 66 kV and above |
| Distance | 21 | Transmission line | 132 kV and above |
| Transformer Differential | 87T | Main power transformer | 33 kV and above (recommended) |
| Under/Over Voltage | 27/59 | PCC bus | All voltage classes |
| Under/Over Frequency | 81U/81O | PCC bus | All voltage classes |
| Anti-islanding | 27/59/81 combination | PCC bus | All voltage classes |
| Reverse Power | 32 | HV feeder | Net metering projects |
IDMT Overcurrent (51) setting methodology for solar plants:
The pickup current (Is) for the 51 relay must satisfy two conditions: it must be above the maximum continuous load current (including the maximum export current from the solar plant), and it must be below the minimum fault current at the far end of the protected zone.
For a 25 MW plant at 33 kV with a rated output current of 437 A at the PCC:
- Maximum load current = 437 A
- Pickup current setting minimum = 437 A x 1.2 = 524 A
- CT ratio = 600/1 A (selected above full load)
- Pickup setting in CT secondary amps = 524 / 600 = 0.87 A (set to 0.9 A)
The time-multiplier setting (TMS) is calculated by selecting the IDMT curve (Very Inverse is typical for solar feeders) and adjusting the TMS until the operating time at minimum fault current provides the required 0.3-second grading margin with the DISCOM upstream relay.
Field tip. Always verify the relay pickup setting against the minimum fault current at the furthest point in the protected zone, not just the maximum fault current at the relay location. A relay that is set too insensitively may fail to operate for a high-impedance fault — such as a single conductor touching dry soil — even though it would operate correctly for a bolted three-phase fault.
CEA 2019 Connectivity Requirements for Relay Settings
The CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations 2019 specify mandatory relay settings for solar plants at each voltage class.
| Protection Function | CEA Setting Requirement (33 kV) | Operating Time |
|---|---|---|
| Under-voltage (27) | Trip below 0.85 pu for more than 0.2 s | Under 0.2 s |
| Over-voltage (59) | Trip above 1.15 pu for more than 0.2 s | Under 0.2 s |
| Under-frequency (81U) | Trip below 47.5 Hz for more than 0.2 s | Under 0.2 s |
| Over-frequency (81O) | Trip above 51.5 Hz for more than 0.2 s | Under 0.2 s |
| Anti-islanding | Detect islanding within 0.2 s; trip within 0.5 s | Under 0.5 s |
| IDMT Overcurrent (51) | Set above 120% of full load current | Per IDMT curve and TMS |
For LVRT (low-voltage ride-through) compliance, the plant must remain connected during voltage dips to 0.8 pu for up to 100 ms. This means the under-voltage (27) relay time delay must be set to at least 0.2 seconds to avoid tripping during normal LVRT events. The relay settings report must explicitly demonstrate this LVRT/27 relay interaction.
According to CEA Connectivity Regulations 2019, Section 7, the developer must submit the relay settings report to the DISCOM for review and approval before commissioning. The DISCOM has 30 days to respond with approval or with technical comments requiring revision.
Time-Current Grading — IDMT Curves and Coordination Margin
Time-current grading is the process of plotting the operating time of every relay in a protection chain as a function of fault current and verifying that adjacent relays are separated by the minimum coordination margin.
The standard IDMT curve equations defined in IEC 60255-151 are:
Normal Inverse (NI): t = TMS x (0.14 / ((I/Is)^0.02 - 1))
Very Inverse (VI): t = TMS x (13.5 / ((I/Is) - 1))
Extremely Inverse (EI): t = TMS x (80 / ((I/Is)^2 - 1))
Where t is the operating time in seconds, TMS is the time-multiplier setting, I is the fault current, and Is is the pickup current setting.
The coordination margin between adjacent relays must be at least 0.3 seconds at every fault current level. This margin accounts for the circuit breaker interruption time (typically 60–100 ms for modern vacuum circuit breakers at 33 kV), the relay timing tolerance (typically plus or minus 5% per IEC 60255-151), and a safety allowance for circuit breaker failure.
Watch out. Using Normal Inverse curves for solar projects with a high ratio of maximum to minimum fault current (greater than 10:1) often makes it impossible to achieve coordination at both ends of the fault current range simultaneously. In these cases, switch to Very Inverse or Extremely Inverse curves, which have a steeper slope that provides better discrimination at low fault currents without excessive operating time at high fault currents.
How to Submit Relay Setting Documents to the DISCOM
The relay settings submission to the DISCOM protection engineer must be a complete, self-contained technical document. A submission that requires the DISCOM engineer to make assumptions or request additional information will be returned, resetting the review clock.
The relay settings report must include:
- Project description page: Project name, capacity, location, voltage class, PPA reference number, developer name, and engineering consultant name.
- Single-line diagram extract: A simplified SLD showing only protection-relevant elements (transformers, circuit breakers, CTs, VTs, and relay designations). Refer to our full guide on solar HV interconnection drawings for the complete SLD specification.
- Short-circuit study results: Tables of maximum and minimum fault current (three-phase and single-phase) at every relay location, with the grid impedance data provided by the DISCOM.
- Relay specification table: For every relay: manufacturer, model, function number, CT/VT ratio, pickup setting, time-multiplier setting, and characteristic curve.
- Coordination curves: Logarithmic time-current plots showing all relays in the coordination chain simultaneously. Plots must be generated from study software (ETAP, ERACS, or equivalent) — hand-drawn curves are not accepted.
- IDMT curve calculations: Tabulated operating times at relevant fault current levels for each relay, showing the grading margin at each level.
- Anti-islanding verification: Calculation demonstrating that the combined action of 27/59/81U/81O relays will detect and trip for islanded operation within 0.5 seconds.
- LVRT compatibility check: Confirmation that the 27 relay time delay is consistent with the plant LVRT capability and the relay will not trip during a voltage dip the inverter is required to ride through.
See also our guide on CEA connectivity regulations for solar projects for state-specific addenda that some DISCOMs require alongside the standard CEA format. For projects requiring lenders to review the protection scheme, see our article on lenders due diligence and engineering in India.
Download a sample relay settings report
Redacted relay coordination study for a 25 MW, 33 kV solar project in Rajasthan — includes short-circuit study results, ETAP coordination curves, and DISCOM submission format.
Get the sample pack →Software Tools for Relay Coordination Studies
Three software platforms dominate the Indian market for relay coordination studies on solar projects.
ETAP (MOST COMMON IN INDIA)
- Most widely specified by Indian DISCOMs and independent engineers
- Full short-circuit, coordination, and arc flash modules integrated
- Accepted by SECI, NTPC, and most STUs as the reference tool
- Higher licence cost: ₹3–8 lakhs per year
ERACS AND POWERWORLD
- ERACS has strong relay library for IEC 60255-151 curves
- PowerWorld has better load flow capabilities for large networks
- Less common in India — DISCOM acceptance varies by state
- Lower licence cost than ETAP
The relay settings report must state the software name, version, and licence number used for the study. Some DISCOMs ask for the software model file (.etap or equivalent) alongside the PDF report so their in-house protection engineers can verify the inputs.
According to BIS (Bureau of Indian Standards), IS 16082 (the Indian standard for protection relay testing) requires all protection relays to be tested in a NABL-accredited laboratory before commissioning. The test certificates from the relay manufacturer must be included in the commissioning documentation, not just the relay settings report.
How Heaven Designs Helps with Relay Coordination
Most Indian EPC contractors complete relay coordination studies once or twice per year at most — not frequently enough to maintain in-house expertise in ETAP or the nuances of DISCOM submission formats across different states. The result is a study that gets returned by the DISCOM protection engineer with technical comments, adding 4–8 weeks to the commissioning timeline.
Heaven Designs maintains a dedicated protection engineering team that has completed relay coordination studies for projects across Rajasthan, Gujarat, Karnataka, Andhra Pradesh, Tamil Nadu, and Maharashtra. Our studies are accepted on first submission in more than 85% of cases because we maintain current knowledge of each state DISCOM protection engineering department’s documentation preferences.
- Electrical CEIG Drawings — CEIG-approval-ready protection drawings, including relay scheme drawing and relay settings report in DISCOM format.
- Solar Rooftop Detailed Engineering Design — Full IFC package from DC string to HV switchyard, with protection scheme and relay coordination study included.
- Solar Civil and Structural Engineering — Control panel room structural design to house the relay panels and SCADA equipment.
- Download a sample relay coordination study — Redacted ETAP study for a 25 MW, 33 kV project in RVPNL jurisdiction.
For projects approaching the synchronisation milestone with a pending relay setting approval, contact us for an expedited study. Heaven Designs can deliver a complete relay settings report within 5–7 working days from receipt of the DISCOM grid impedance data and the as-built SLD.
FAQ
What is relay coordination and why is it needed for solar projects?
Relay coordination ensures that protection relays in a solar plant operate in the correct sequence during a fault — the relay nearest the fault operates first, isolating the minimum section of the network, while relays further away provide time-delayed backup. Without coordination, the upstream DISCOM relay may operate before the plant relay, causing a wider outage and potential equipment damage. CEA Connectivity Regulations 2019 require the developer to submit a relay coordination study demonstrating correct grading before the DISCOM issues a grid connectivity approval.
What is an IDMT relay and which curve should I use for a 33 kV solar feeder?
An IDMT (Inverse Definite Minimum Time) relay is a protection relay whose operating time varies inversely with the fault current — the higher the fault current, the shorter the operating time. Three standard IDMT curve shapes are defined in IEC 60255-151: Normal Inverse, Very Inverse, and Extremely Inverse. For 33 kV solar feeders with a fault current ratio (maximum to minimum) greater than 5:1, Very Inverse curves are recommended because they provide better discrimination at intermediate fault current levels without being excessively slow at minimum fault current.
How do I get the grid impedance data needed for the relay coordination study?
Request the grid impedance data from the DISCOM protection engineer as part of the grid connectivity application. The standard request includes the positive-sequence and zero-sequence Thevenin impedances at the PCC under maximum and minimum grid strength conditions, the maximum and minimum three-phase fault MVA at the PCC, and the operating time-current characteristics of the upstream DISCOM relay at the injection substation. Most state DISCOMs in India have standard forms for this request. In Rajasthan (RVPNL), submit Form NR-1; in Gujarat (GETCO), submit Form G-7C.
How long does DISCOM relay setting approval take in Rajasthan and Gujarat?
In Rajasthan (RVPNL jurisdiction), the relay settings review process typically takes 4–8 weeks from submission of the complete report, assuming no technical comments requiring revision. In Gujarat (GETCO jurisdiction), the process takes 6–10 weeks because GETCO requires review by both the protection engineering department and the grid connectivity cell. Expedite the process by submitting the relay settings report simultaneously with the short-circuit study and the SLD — staggered submissions cause DISCOM engineers to hold the entire file until all documents are available.
Can string inverter solar plants use the same relay coordination methodology as central inverter plants?
The relay coordination methodology is the same, but the fault current contribution from string inverters is lower than from central inverters. String inverters current-limit to approximately 1.1–1.2 times rated output during a fault. This low fault current contribution makes it important to calculate the minimum fault current scenario carefully — the plant relay must be able to detect a fault even when the inverters are contributing negligible current and the grid fault current at the plant 33 kV bus is at its minimum during light-load conditions.
What happens if the DISCOM rejects the relay settings report?
The DISCOM will issue a technical comments letter specifying the reasons for rejection — typically incorrect pickup settings, insufficient coordination margin at a specific fault current level, or missing documentation such as the LVRT compatibility check. The developer must revise the relay settings report addressing every comment and resubmit the full document, not just the revised pages. The DISCOM review clock resets to 30 days on resubmission. Two rejections at the same project are rare if the study is performed by an experienced protection engineering team.
Does the relay coordination study need to be stamped by a licensed electrical engineer?
The relay settings report for a CEA Connectivity submission must be signed and certified by a qualified electrical engineer (Class A licence or equivalent) employed by or engaged by the developer. India does not use a state PE stamp system equivalent to the USA, but the CEIG may require the protection scheme drawing and relay settings summary to be countersigned by the developer licensed electrical contractor. See our detailed guide on the CEIG drawing approval process in India for state-specific requirements.
Which software does Heaven Designs use for relay coordination studies?
Heaven Designs uses ETAP (version 21 and above) as the primary tool for relay coordination studies because it is the most widely accepted platform by Indian DISCOMs and SECI independent engineers. For projects requiring PowerFactory DiGSILENT (requested by some state STUs for dynamic stability studies), Heaven Designs maintains access through its subcontract protection engineering network. All study output files and PDF reports are provided to the client — this transparency allows the DISCOM protection engineer to review the input assumptions directly if they request it.
