SolarCalculatorsHT Transformer & Substation Sizing for Rooftop Solar (11kV / 33kV): CEIG Approval & DISCOM NOC Guide 2026
⚡ Essential Engineering Rule: When connecting commercial rooftop solar plants above 100 kW to an existing consumer HT substation (11 kV or 33 kV), simple low-voltage (415V) rules no longer apply. Overlooking transformer impedance (%Z), back-feeding saturation limits, directional overcurrent relays, or Chief Electrical Inspector to Government (CEIG) safety clearances can lead to DISCOM rejection, transformer burnout, or indefinite plant commissioning delays.
1. HT Evacuation Thresholds by Indian DISCOMs
State electricity regulatory commissions (SERCs) mandate strict solar connection voltage levels based on total installed capacity:
| Solar System Capacity | Interconnection Voltage Level | Metering Architecture | Statutory Approvals Needed |
|---|---|---|---|
| Up to 50 kW | 415V 3-Phase Low Tension (LT) | Whole-current or LT-CT Bi-directional meter | DISCOM AE / EE approval |
| 50 kW to 100 kW | 415V LT or 11 kV HT (State specific) | LT-CT Net Meter at main LT panel | DISCOM Executive Engineer |
| 100 kW to 1,000 kW (1 MW) | 11 kV High Tension (HT) | HT-CT/PT Bi-directional 0.2s Accuracy Meter | CEIG Drawing Approval & Safety Inspection |
| Above 1,000 kW (1 MW - 5 MW) | 11 kV or 33 kV Extra High Tension | ABT Compliant 4-quadrant HT Tri-vector meter | CEIG + STU / SLDC Telemetry (RTU) |
2. Can You Back-Feed Through an Existing Consumer Step-Down Transformer?
Most industrial facilities have an existing 11 kV / 433V step-down distribution transformer (e.g., 500 kVA, 1,000 kVA, or 1,500 kVA). When injecting surplus solar power from 415V back into the 11 kV grid, the transformer operates in reverse power flow mode (acting as a step-up transformer).
Before back-feeding through an existing step-down transformer, verify these three critical engineering factors:
- Flux Density & Core Saturation: Standard distribution transformers are designed for step-down flux distribution. Back-feeding with high harmonic contents (from inverters) can increase core eddy-current losses and cause overheating. Solar capacity must not exceed 75% to 80% of the transformer's rated kVA unless it is solar-inverter duty rated.
- Impedance (%Z) & Voltage Rise: When solar current flows backwards against the transformer impedance (typically 4.5% to 5.0%), it causes an internal voltage rise on the 415V secondary bus. If grid voltage is already sitting at 430V, the bus can rise beyond 450V, tripping solar inverters on Grid Overvoltage (OV) Error.
- Vector Group Compatibility: Standard distribution transformers are Dyn11 (Delta High Voltage, Star Low Voltage with neutral). Inverters must connect to the 415V star side with solidly grounded neutral to ensure fault loop clearing.
3. Dedicated Solar Step-Up Transformer Design Parameters
For ground-mount or large rooftop installations utilizing a dedicated solar step-up transformer (415V to 11kV or 33kV), the specification sheet must include:
| Engineering Parameter | Standard Specification Requirement | Rationale & Protection Purpose |
|---|---|---|
| Transformer Type | ONAN (Oil Natural Air Natural) Copper Wound | High thermal withstand for prolonged solar peak hours |
| Winding Configuration | Dyn11 or Ynd11 with electrostatic shield | Grounds high-frequency inverter PWM switching harmonics |
| Impedance (%Z) | 5.0% to 6.25% (as per IS 2026 / IEC 60076) | Limits short-circuit fault levels while preventing excessive voltage rise |
| Tap Changer | Off-Circuit Tap Switch (OCTC) +5% to -5% in 2.5% steps | Adjusts to fluctuating local DISCOM feeder voltage baseline |
| Core Material | CRGO (Cold Rolled Grain Oriented) M3/M4 grade | Minimizes no-load losses during night and low irradiance |
4. HT Switchgear Protection & CEIG Inspection Checklist
The Chief Electrical Inspector to Government (CEIG) requires full compliance with the Central Electricity Authority (Measures Relating to Safety and Electric Supply) Regulations:
- Vacuum Circuit Breaker (VCB): 11kV/33kV indoor or outdoor VCB panel with motor-operated spring charging, shunt trip coil, and minimum 25 kA / 3s breaking capacity.
- Numerical Protection Relays: Must include Overcurrent (50/51), Earth Fault (50N/51N), Under/Over Voltage (27/59), and Reverse Power Relay (32R) if the plant is under a Zero-Export regime.
- Dedicated Earth Pits: Minimum of 4 to 6 chemical earth pits connected in a grid loop—separate pits for transformer body, neutral, HT panel, and lightning arresters, with measured resistance ≤ 1.0 Ohm.
- Danger Signboards & Safety Equipment: BIS-approved shock-treatment charts, 33kV rated rubber insulation mats (IS 15652), fire buckets filled with dry sand, and CO2/Dry Powder fire extinguishers.