Power Factor Correction Solutions
Examining Supply and Demand
By Michael E. Havens
Utility companies commonly charge forpeak electrical demand to recoup a portionof the capital investment made intheir distribution network. Each month, customerspay a demand charge for their peakoperating loads, for which the utility must sizeits distribution equipment to reliably and consistentlyprovide for customers. Most utilities offertheir customers incentives to sustain a highlevel of electrical efficiency, which is measuredby power factor.
Power factor correction (PFC) devices improve overall electrical efficiency upstream of their point of connection in the network. They can be used to minimize but not eliminate utility demand charges. Capital investment in PFC equipment can provide a return on investment dependent on the utility’s demand rate structure. Demand-charge savings are one of the benefits of adding capacitor-correction systems. Other benefits include:
• Transformer and distribution network offloading. Improving power factor reduces KVA loading of the distribution network. Additional process equipment may eliminate incremental investment in transformers and distribution equipment.
• Ensured compliance with harmonic standards. The utility can enforce standards limiting a customer’s harmonic pollution of the grid system.
• Improved reliability of the distribution network and process equipment. Harmonics generated by nonlinear loads can harm sensitive process equipment and lead to downtime. The distribution network may encounter nuisance breaker or fuse operation, as well as overheating and insulation break down within motors, transformers, and conductors.
Effective devices are available to improve customer power factor, and nearly all add capacitance to the electrical distribution system. This is accomplished by either hard wiring to the system or by automatically switching capacitor banks in and out of it during customer power factor fluctuation. These devices include:
• Fixed capacitors. These are well-suited for power-correction applications in which the load does not change or the capacitor is switched with the load. They work best when no harmonic currents or voltages are available.
• Automatic capacitor banks. This type corrects the power factor at the main service entrance when plant loading constantly changes. A microprocessor-based reactive power controller measures plant power factor with a single current transformer. The unit is located on the bus or cable at the main service entrance terminal compartment and switches capacitor stages in and out of service to track a user-selected target power factor. Automatic capacitor banks are designed for use in electrical distribution systems with little or no current or voltage harmonics.
• Anti-resonant automatic capacitor banks. Harmonics in an electrical distribution system are a natural byproduct of nonlinear loads. These include variable frequency drives, motor soft starters, welders, uninterruptible power supplies (UPS), robotics, programmable logic controllers, and other electronic devices. Harmonics produce higher-than-60Hz currents and voltages into the system. Capacitors provide a low impedance path and will actually absorb these harmonic energies.
Combinations of capacitors and system inductances such as motors and transformers can form series and parallel-tuned circuits that resonate at certain frequencies.
Harmonics produced by nonlinear loads can excite a standard capacitor bank into resonance that magnifies currents and voltages and can result in system-wide damage and equipment failure. The anti-resonant capacitor bank’s primary function is power factor correction. Iron core reactors are added in series with the capacitor modules and will tune the bank below the first dominant harmonic, usually the fifth or 300Hz. Below that tuning point, the system appears to be capacitive and will correct power factor; above the tuning point, it appears inductive and minimizes resonance.
• Transient-free automatic capacitor banks (TFACB). This type is preferred for systems in which equipment is extremely sensitive to supply voltage variations. These banks are typically switched with electromechanical contactors that generate voltage transients on the distribution system when capacitor stages are switched on and off, even when current limiting or tuning reactors are used. Transients, however, can impair the operation of sensitive equipment, including programmable logic controllers, variable speed drives, computers, and UPS systems. In sensitive networks such as hospitals, data processing centers, airports, and many manufacturing environments, any transient, however slight, may be unacceptable. TFACB electronicswitching elements energize capacitor stages; switching occurs when capacitor residual voltage matches network voltage and produces reactive compensation without generating voltage transients.
Electrical switchgear manufacturer Square D, Co., and Parsons Electric LLC can facilitate the application of capacitors in power factor correction and harmonic-rich environments. Both firms can assess the likelihood of application problems, suggest solutions, and implement system studies, custom engineering, installation, and commissioning as required.
For more information, contact Michael E. Havens, PE, Senior Account Executive of Square D, Co., at (972) 471-6100.
