Capacitor Resonance and VFD Overvoltage
A 480 V plant has a 1,000 kVA transformer with 5.75% impedance and a 600 kvar shunt capacitor bank at the main bus. A new 200 hp, 6-pulse PWM variable-frequency drive is installed on a 150 ft motor cable. Shortly after commissioning, capacitor fuses blow and the drive trips on overcurrent during acceleration. The drive carrier frequency is 4 kHz, the motor is rated 1200 V peak insulation, and the drive current limit is set to 150% of motor full-load current. The plant wants to know whether the problem is resonance, reflected-wave overvoltage, or drive settings, and what to change first.
1. Calculate the parallel resonant harmonic order of the capacitor bank and the source. Is the 6-pulse drive likely to excite it?
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Compute the parallel resonance between the 600 kvar bank and the source impedance. The capacitor reactance at 480 V is
XC = V²/Q = 480²/(600×10³) = 0.384 Ω.
The transformer impedance on its own base is Zbase = 480²/1,000,000 = 0.2304 Ω, so XT = 0.0575(0.2304) = 0.01325 Ω. The resonant harmonic order is approximately
h = √(XC/XT) = √(0.384/0.01325) = 5.38.
A 6-pulse drive produces strong 5th and 7th harmonic currents. The 5th is very close to the 5.38 resonance, so the bank can amplify 5th-harmonic current and voltage, blowing fuses and stressing the drive. This is a resonance problem, not just a drive current-limit setting.
2. Estimate the reflected-wave overvoltage on the 150 ft motor cable. Is it a credible cause of the drive trips?
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Estimate reflected-wave overvoltage separately. A 480 V PWM drive has a DC bus near 650 V, and a full reflection at an open motor terminal can approach twice the bus voltage, about 1,300 V. The 150 ft cable is long enough to produce a reflected wave if the drive rise time is short, and 1,300 V exceeds the motor's 1,200 V peak insulation rating. That is a real risk, but it does not explain capacitor fuse blowing. The drive overcurrent during acceleration may be caused by the resonance, by a current limit set too low, or by the motor cable capacitance. Check the drive's DC-bus voltage, output dv/dt, and motor terminal voltage with a high-frequency probe. If peak voltage exceeds the motor rating, add a load reactor or dv/dt filter and consider a higher-insulation motor. Do not fix the capacitor fuses by increasing fuse size.
3. Recommend the first corrective action and the checks that should accompany it.
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First, replace the plain capacitor bank with a detuned bank or remove it temporarily. A series reactor tuned below the 5th harmonic, for example to about 4.7 times fundamental, requires XL = XC/h² = 0.384/4.7² = 0.0174 Ω, which is about 4.5% reactance on the capacitor rating. A 5% reactor tunes near 4.47 and avoids the 5th resonance. The capacitor voltage rises to approximately 1/(1 - 0.05) = 1.053 times nominal, so use 525 V-rated capacitors or confirm the 480 V units are adequate. Verify fuse and breaker ratings, check harmonic distortion before and after, and coordinate with the drive manufacturer. If the drive still trips, review acceleration time, current limit, and cable length; a load reactor or dv/dt filter may be needed for the motor insulation. The capacitor change should come first because it addresses the fuse failures and the likely overcurrent cause.