Harmonic Resonance in a Garment Factory Capacitor Bank
A garment factory in Katunayake has a 630 kVA, 11 kV/400 V Dyn11 distribution transformer supplying 420 kW of mixed load, including 12 six-pulse VFDs totalling 250 kW. The 200 kvar automatic capacitor bank has failed three times in 18 months; transformer oil temperature is 78°C; neutral current on the main LV board is 85 A although phase currents are balanced at 600 A. The utility meter records a displacement power factor of 0.96, but the consumer still receives a reactive-power penalty. A new 90 kW VFD-driven compressor is to be added. Site records show the capacitor bank is connected at the main LV busbar with no reactors, and the transformer has a K-factor rating of 1.0. You are asked to investigate before the compressor is connected.
1. Explain the capacitor failures and why the reactive-power penalty persists despite a displacement power factor of 0.96.
Read the worked answer
Six-pulse VFDs draw non-sinusoidal current rich in 5th, 7th, 11th and 13th harmonics. The 200 kvar capacitor bank at the main LV busbar forms a parallel resonant circuit with the transformer and source impedance. If the resonant frequency is near the 5th or 7th harmonic, harmonic currents circulate between capacitor and transformer, overheating the capacitor dielectric and causing repeated failures. The bank has no detuned reactors, so it is exposed to the full harmonic voltage and current.
The utility meter shows displacement PF 0.96, which considers only the fundamental phase shift. True PF is lower because distortion adds apparent power: approximately true PF = displacement PF × 1/√(1+THD_i²). With THD_i of 30%, true PF ≈ 0.96 × 0.958 ≈ 0.92, so a reactive-power penalty persists even though the capacitor bank appears to correct displacement PF. The 85 A neutral current is a separate sign of triplen harmonics from single-phase electronic loads and warns that this is not simply a fundamental-frequency problem.
2. Assess whether the transformer can accept the new 90 kW compressor and state the mitigation required.
Read the worked answer
Existing load: 420 kW at 0.96 displacement PF ≈ 437 kVA. New compressor: 90 kW at, say, 0.95 ≈ 95 kVA, giving about 532 kVA, or 84% of 630 kVA. On fundamental loading alone the transformer can carry it, but the transformer has K-factor 1.0 and oil temperature is already 78°C, so harmonic eddy losses and insulation ageing reduce effective capacity. A K-factor 1.0 unit should not be loaded to 84% with a high-distortion VFD load without a harmonic study.
Mitigation: do not add conventional capacitors. Replace the failed bank with a detuned, reactor-protected bank or an active harmonic filter; specify a 12-pulse or low-harmonic compressor drive; measure the harmonic spectrum and calculate transformer derating using IEEE C57.110. Increase the neutral bar or provide a separate neutral for single-phase loads. If the study shows the transformer is thermally limited, add forced cooling or plan replacement with a K-rated unit. Confirm capacitor switching steps and resonance after the changes.
3. Specify the power-quality survey required before and after remediation.
Read the worked answer
Before remediation, conduct a minimum seven-day power-quality survey at the transformer LV terminals, main LV board, capacitor bank terminals, and representative VFD inputs. Record three-phase voltage and current, THD, individual harmonic magnitudes and phase angles to the 25th, neutral current, displacement and true PF, demand, and flicker. Calculate the harmonic loss factor and K-factor, and perform an impedance scan with the capacitor bank in and out to identify parallel resonance. Check capacitor bank step switching and any existing detuning.
After remediation, repeat the survey at the same points and load conditions. Verify that voltage THD and individual harmonics meet the utility or IEEE 519 limits, capacitor currents are within rating, neutral current is acceptable, and transformer oil temperature has fallen. Keep a log of VFD operating combinations, because resonance may appear only at certain load patterns.