We're having a nightmare situation with our new capacitor bank installation here in Spokane. Avista was hitting us with power factor penalties of $1,500/month so we installed 300 kVAR of automatic capacitors. Now the PF penalties are gone but we're getting voltage distortion issues every time the caps switch. Our sensitive electronic equipment is tripping offline and we're seeing 8% THD spikes during switching events. The capacitor contractor says this is normal but it's costing us production downtime. Has anyone dealt with harmonic issues from capacitor switching? We're on Avista Schedule 25 service at 480V.
Avista capacitor switching causing harmonic issues - help needed
Janet, this is a classic problem with automatic capacitor switching in facilities with non-linear loads. The inrush current from capacitor energization can excite harmonic resonances in your electrical system. You need to look at either pre-insertion resistors for soft starting or switching to synchronous switching contactors. What type of loads are tripping offline - VFDs, computers, or other sensitive equipment?
Randy, it's mostly our CNC machines with Fanuc controls and some PLC systems. The VFDs seem to handle it better but the CNCs are very sensitive to voltage distortion. The capacitor bank uses standard contactors, not synchronous switching. Would pre-insertion resistors help with the harmonic excitation, or do we need to go full synchronous switching?
Janet, I dealt with similar issues at a Louisville facility with LG&E service. Fanuc controls are notorious for voltage distortion sensitivity. Pre-insertion resistors will help with the inrush transient but won't eliminate harmonic resonance issues. You might need series reactors on the capacitor bank to detune it from system harmonics. What's your background harmonic content before the caps switch?
Pete, we're seeing about 4% THD normally, spiking to 8% during switching. The capacitor contractor is now saying we need harmonic filters instead of just standard caps. That's going to double our cost from $35K to $70K. I'm wondering if we should have gone with this approach initially. Do you think Avista would share any responsibility for not warning us about potential harmonic issues?
Janet, utilities rarely take responsibility for harmonic issues caused by customer equipment, even if they're the ones penalizing you for poor power factor. RG&E here in Rochester takes the same hands-off approach. Your best bet is probably upgrading to detuned capacitors with 5% reactors. It's more expensive but will solve both the power factor and harmonic problems. The alternative is dealing with ongoing production losses that could cost more than the equipment upgrade.