About the Transformer Impedance & Losses
Transformer impedance analysis evaluates the effect of a solar step-up transformer's percentage impedance (%Z) on fault current contribution, voltage regulation at the grid connection point, and the transformer's overload capability.
Formula
Short-circuit current (A) ≈ Rated current ÷ (%Z ÷ 100)
- Rated current
- Transformer full-load current on the HV side (A)
- %Z
- Percentage impedance (typically 4–6% for distribution transformers)
How to calculate it
- 1Lower %Z (4–5%): better voltage regulation, lower voltage drop under load, but higher fault current contribution to the grid — raises breaker duty.
- 2Higher %Z (6–8%): limits fault current contribution, at the expense of slightly higher on-load voltage drop.
- 3For solar: 5–6% is standard. Some DISCOMs specify a minimum %Z to limit fault level contribution — check the DISCOM's grid connectivity requirements.
- 4Verify that the fault current from the transformer at the LV terminals is within the rating of the DC/AC panels and cable systems on the LV side.
Worked example
A 1 MVA, 11 kV / 415 V transformer with 5% impedance. Rated LV current = 1,390 A. Maximum fault current at LV terminals = 1,390 ÷ 0.05 = 27,800 A = 27.8 kA. LV switchgear and ACDB must be rated for ≥ 28 kA fault current (use 36 kA rated MCCBs for safety margin).
What you can calculate
- Impedance %Z
- Load losses
- No-load losses
- Efficiency
Frequently asked questions
Does a solar plant's transformer impedance affect DISCOM approval?
Yes — DISCOMs conducting grid impact studies check the fault level contribution of each new solar plant. If the existing substation's switchgear is already close to its fault current rating, a new solar plant with a low-impedance transformer can push the fault level over the limit — triggering a requirement to use a higher-impedance transformer or upgrade the substation's switchgear. This is an increasingly common issue in solar-saturated grids in Rajasthan, Gujarat, and Tamil Nadu.
What is voltage regulation in a solar transformer?
Voltage regulation = (no-load secondary voltage − full-load secondary voltage) ÷ full-load secondary voltage × 100%. A transformer with 5% impedance and 0.8 PF load has a regulation of approximately 4–5%, meaning the voltage at the ACDB drops 4–5% at full output. This must be accounted for when designing cable sizes and grid voltage set-points, to ensure the export voltage stays within the DISCOM's ±6% limit.
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