Short answer
Transformer reactive demand overcompensation problems appear at low load. A transformer’s reactive demand has two parts. At approximately rated voltage and frequency, the magnetising part can be screened from no-load current percentage times rating. No-load current also contains active and distortion components, so use measured fundamental reactive power where available. The leakage part rises with the square of load and equals the reactance percentage times rating at full load. In an illustrative 1,000 kVA transformer with 0.5 % no-load current and 6 % impedance, magnetising demand is about 5 kvar and full-load leakage demand about 60 kvar. A fixed 100 kvar capacitor that suits a 150 kW night load at 0.9 power factor leaves about 20 kvar leading; at a 20 kW weekend load it leaves about 80 kvar leading, raising voltage and possibly incurring tariff penalties. Fixed compensation should be matched to the magnetising need, with the rest switched with load.
Two components
| Condition | Load kvar | Transformer kvar | Net with 100 kvar |
|---|---|---|---|
| Night 150 kW | ≈ 72.6 | ≈ 6.7 | ≈ 21 leading |
| Weekend 20 kW | 15 | ≈ 5 | ≈ 80 leading |
Q_0 ≈ i_0 × S_r
Qleak ≈ u_x × (S / S_r)^2 × S_r
No-load current depends on core material, flux density and joint design; grain-oriented steel cores with step-lap joints can reduce magnetising current compared with less favourable material or joint arrangements at comparable flux density and excitation. Confirm the complete design by measurement. Leakage reactance is set by winding design.
Illustrative transformer
Illustrative example. 1,000 kVA distribution transformer; no-load current 0.5 %; reactance 6 %. Plant night load 150 kW at 0.9 power factor; weekend load 20 kW at 0.8. Fixed capacitor 100 kvar. Values chosen for demonstration.
Night: load reactive ≈ 72.6 kvar; transformer: 5 + 0.06 × 0.167² × 1,000 ≈ 6.7 kvar; total ≈ 79 kvar. Fixed 100 kvar gives ≈ 21 kvar leading.
Weekend: load reactive 15 kvar; transformer ≈ 5 kvar; total ≈ 20 kvar. Fixed 100 kvar gives ≈ 80 kvar leading.
Effects of overcompensation
- Voltage rise at the bus: ΔU ≈ Q / S_sc, about 0.5 % for 80 kvar on a 16.7 MVA bus.
- Leading power factor that some tariffs penalise.
- Resonance with the transformer at a frequency set by the bank and transformer; see harmonic amplification after one transformer is removed.
- Self-excitation risk of induction motors disconnected with capacitors attached.
Sizing fixed compensation
- Fixed part close to transformer magnetising demand, a few kvar for distribution units.
- Switched stages for load reactive demand; see capacitor-bank step size and voltage steps.
- Minimum load data from measurements, not assumptions.
- Disconnect conditions: fixed capacitors out when load falls below a threshold, or controlled by the PF controller; see power-factor controller CT location.
Core influence
Lower magnetising current can reduce constant reactive demand. No-load active loss must be compared separately using the transformer test report; lower no-load current alone does not prove a lower no-load loss. Chenfan’s core products are described on its distribution transformer cores page; actual no-load current is a property of the complete transformer design and test report.
What this article does not cover
Standby energisation hours for battery storage transformers are a separate topic and are not covered here.
Where this reasoning stops. Values are illustrative; use the transformer test report for no-load current and reactance.
Practical takeaway
Calculate transformer reactive demand from no-load current and reactance at actual load, keep fixed capacitors near the magnetising demand, switch the rest with load, and check minimum-load conditions for leading power factor and voltage rise.
Reference framework
- IEC 60076-1, Power transformers: general (no-load current, impedance).
- IEC 61921, Power capacitors: low-voltage power factor correction banks.