Substation

Substation Voltage Regulation Calculator

Transformer & feeder voltage drops, tap changer settings, regulation percentage, and feeder voltage profile.

IEEE C57.12.00 IEC 60076-1 EN 50160 IEC 61000-3-6 IEEE 399
Presets
Transformer Parameters

Leave blank to assume ideal ratio

Feeder Parameters
Load Conditions

Leave blank to derive from PF

Transformer Regulation
Regulation % —
Voltage Drop (kV) —
R component (%) —
X component (%) —
Total Z % —
X/R ratio —
Tap Position
Tap Position —
Tap Step (%) —
Required Tap Correction (%) —
Feeder Voltage Drop
Voltage Drop (kV) —
Voltage Drop (%) —
Voltage at Feeder End (kV) —
End Voltage (% of nominal) —
Total System Regulation
—
Total Regulation %
EN 50160
±10% at PoC for LV; ±8% for HV planning level
IEC 61000-3-6
IEC 61000-3-6 Planning Level (±8%)
Voltage Profile
Export
Transformer Regulation ▸
ΔV% ≈ (R·cosφ + X·sinφ) × I_pu × 100

Voltage drop as percentage of base voltage, derived from transformer impedance components and load current at load angle φ. R and X are separated from Z% using the X/R ratio.

Tap Changer ▸
Tap Pos = round(ΔV_tx% / Tap Step %)

On-load tap changers (OLTC) adjust transformer turn ratio in 0.625% or 1.25% steps. Position = required compensation / step size. Typical range: ±16 steps (±10%). Off-load tap changers (ULTC) require de-energisation.

Feeder Voltage Drop ▸
ΔV = √3·I·(R·cosφ + X·sinφ) [V]

Three-phase voltage drop along a uniform feeder. R and X per km from conductor tables (IEC 60287 for XLPE, IEC 60502 for EPR). Profile shows linear voltage decrease assuming uniform load distribution.

Standards & Limits ▸
• IEEE Std C57.12.00-2015 — Transformer definitions and test code
• IEC 60076-1:2011 — Power transformers — General
• EN 50160:2010 + A3:2019 — Voltage characteristics at PoC
• IEC 61000-3-6:2008 — Planning levels for MV/HV (≤8% at HV)
• IEEE Std 399 Brown Book §8 — Voltage drop in industrial power systems
Frequently Asked Questions
What is voltage regulation? +
Voltage regulation is the difference between no-load and full-load voltage at a transformer terminal, expressed as a percentage of nominal voltage. A well-regulated transformer maintains voltage within ±10% across the full load range (EN 50160 Class A).
What is the difference between OLTC and ULTC? +
An On-Load Tap Changer (OLTC) adjusts transformer taps while the transformer is energised and under load — used for daily voltage control in substations. An Off-Load Tap Changer (ULTC) requires de-energisation and is used for seasonal adjustments only.
Why does feeder length affect voltage? +
Every meter of conductor has resistance (R) and reactance (X). As current flows through these impedances, voltage drops proportionally (V = I·Z for three-phase: ΔV = √3·I·(R·cosφ + X·sinφ)). Long rural feeders at low PF can easily drop 8–12%, requiring shunt capacitor banks at the midpoint.
What is a typical X/R ratio for a substation transformer? +
Substation transformers (10–100 MVA) typically have X/R ratios between 8 and 20, meaning the reactance dominates the impedance. This increases the reactive (VAR) component of voltage drop, which is why shunt compensation is common on long rural feeders.
How do shunt capacitors improve voltage regulation? +
Shunt capacitors supply reactive power locally (VAR injection), reducing the current I in the line and therefore reducing I·Z drop. At 11 kV, each MVAR of shunt compensation raises voltage by roughly 2% at the midpoint. Compensating to roughly 95% PF eliminates most reactive voltage drop.
What is the EN 50160 voltage limit at PoC? +
EN 50160 Class A specifies that the supply voltage magnitude should not normally exceed ±10% of nominal. The 95th percentile over one week must be within these limits. For HV planning levels (IEC 61000-3-6), the stricter ±8% applies.