Carl N from Denver, CO. Xcel Energy territory. A plastics extrusion plant client had a power factor problem two years ago — running about 0.78 lagging. They hired an electrical contractor who installed a 400 kVAR fixed capacitor bank. The problem is the plant only needs about 250 kVAR of correction at full load. At full production the PF is a perfect 0.95. But when the plant runs at reduced capacity — nights, weekends, slow periods — the 400 kVAR bank overcorrects and pushes the PF above 1.0 into leading territory. Xcel penalizes for leading PF above 0.95 the same as they penalize for lagging below 0.90. My client traded one penalty for another.
Client installed too many capacitors — now penalized for LEADING power factor
Carl, this is a classic overcorrection problem with fixed capacitor banks. The contractor sized the bank for the worst-case lagging condition without considering the partial load scenario. The solution is either to replace the fixed bank with an automatic switching bank that adjusts correction based on load, or to add a controller that disconnects capacitor steps when the load drops.
Randy, the plant manager is not happy about spending more money to fix a fix. The original capacitor installation was $12,000. Converting to automatic switching would cost another $4,500. He wants to know if the original contractor is liable for the overcorrection.
Carl, the contractor might have liability if they did not perform a load study before sizing the bank. A proper power factor correction design includes a load profile analysis at various operating conditions — not just peak load. If they sized the bank based only on the nameplate ratings of the motors without considering variable production schedules, that is a design deficiency. Check whether the contractor provided a load study as part of their proposal.
Cliff, no load study was provided. The contractor proposal says 400 kVAR based on total connected motor horsepower. That is a rule-of-thumb approach, not engineering. The actual reactive demand at full load is 250 kVAR and at minimum load it drops to about 80 kVAR. A 400 kVAR fixed bank at 80 kVAR load gives you 320 kVAR of excess correction. That is massively leading.
Negotiated with the contractor. Showed them the leading PF penalties on the bills and the lack of a load study. They agreed to retrofit the fixed bank to automatic switching at their cost — $4,500 in labor and controls. They eat the cost as a warranty correction. Client pays nothing additional. After the retrofit, PF ranges from 0.91 to 0.95 across all loading conditions. Zero penalty in either direction.
Good resolution. The lesson for auditors: when you find a power factor penalty, do not just recommend a capacitor bank. Recommend an AUTOMATIC capacitor bank with switching steps sized to the client actual load profile at various operating conditions. Fixed banks are cheaper but they create exactly this kind of overcorrection problem on variable loads.