Power Quality's Impact on System Reliability

Power quality is very important because of its direct impact on your customer’s business. It used to be that if the power was on, your customer was in business. Today, it isn’t a question of whether the power is on; rather, it is a question of whether the power provided meets the equipment’s needs. Your customer’s business demands reliable power, communication, and control systems to get its product and services to market. Power quality problems can result in unnecessary downtime due to equipment failure, expensive repairs, damaged work in process, lost productivity, and a loss of customers. Power quality problems are best addressed during the original design and construction. However, this is not always possible, because commercial, industrial, and institutional facilities are constantly shuffling personnel, adding new processes, and bringing in the latest equipment. Because of the dynamic nature of business today, the level of power quality needed tomorrow can’t always be anticipated. As a result, power quality in a facility needs to be monitored as part of the customer’s ongoing maintenance program. When power quality problems are detected, they need to be diagnosed and corrected as soon as possible to minimize or avoid equipment damage. What is power quality? The Institute of Electrical and Electronics Engineers (IEEE) in IEEE Standard 1100-1999 defines power quality as the powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment. While this definition specifically singles out electronic equipment because of the focus of the standard, it is equally applicable to electrical equipment. Power quality impacts both electronic and electrical equipment operation and reliability. It is important to note that the IEEE’s definition of power quality does not provide specific tolerances for supply voltage magnitude, frequency, waveform, or symmetry. Instead, the IEEE defines power quality in terms of the suitability of the power for the equipment that it serves. Different equipment is able to tolerate power quality problems better than other equipment. For instance, induction motors are much more tolerant of power quality problems than sensitive electronic equipment. In addition, the IEEE definition recognizes that system loads may be a source of power quality problems for the power distribution system as well as other utilization equipment fed from the distribution system. Common power quality problems If the voltage supplied to utilization equipment was always a perfect sine wave of constant frequency and magnitude and the resulting load current tracked the voltage, there would be no power quality problems. Unfortunately, the voltage supplied to utilization equipment is not always perfect and performance problems with the utilization equipment result. Common power quality problems include: • voltage transients, • voltage sags and swells, • undervoltages and overvoltages, • outages and interruptions, • electrical noise, and • harmonics Voltage transients Voltage transients are also referred to as spikes, surges, or impulses, because they typically last less than a millisecond and have a magnitude of a thousand volts or more. Voltage transients can quickly destroy sensitive electronic equipment. Electrical equipment, such as motors and transformers, is also susceptible to voltage transient damage. Voltage transients can either shorten electrical equipment’s insulation life or result in immediate failure. The most effective defense against voltage transients is to use surge protective devices, commonly referred to as transient suppressors, surge protectors, or surge arresters. Voltage sags and swells Voltage sags and swells are momentary decreases or increases in voltage magnitude that are outside the equipment’s tolerances. Voltage sags are often caused when large loads such as motors are switched on. Similarly, voltage swells often occur due to poor voltage regulation when large motors or other loads are shut down abruptly. Voltage sags and swells can cause electronic equipment to malfunction or lock up. They can also impact voltage-sensitive manufacturing processes, such as plating operations and damage work in process. Power conditioners and uninterruptible power supplies (UPS) can be used to mitigate the impact of voltage sags and swells. Undervoltages and overvoltages Undervoltages and overvoltages are abnormally high or low voltages that last a minute or more. Undervoltages are more common than overvoltages and are caused by a variety of things, including the utility supply and overloaded circuits. Utility “brownouts” are an example of an undervoltage condition. Prolonged undervoltage conditions can seriously damage motors, relays, ballasts, transformers, and other electromechanical devices by overheating them. Undervoltages can also affect manufacturing work in process, such as timed drying using resistance heating as well as cause electronic equipment to malfunction. Sustained overvoltage in a facility can shorten insulation life in electrical equipment and reduce the operating life of incandescent lamps and other similar equipment. As with voltage sags and swells, the effects of sustained over- and overvoltages can often be mitigated using power conditioners and UPSs. Outages and interruptions Outages and interruptions are a complete power loss that can last from a few milliseconds to hours. In the past, outages and interruptions were rare and typically caused during abnormal conditions, such as major equipment failure or weather. However, this is changing in some parts of the country, where power shortages and growing demand is resulting in regular outages. These outages are impacting businesses, and customers are looking for economical ways to avoid having their businesses shut down due to a shortage of needed power. If a momentary outage can be tolerated, installing an on-site standby generator that automatically picks up the load upon loss of the utility supply can mitigate the impact of an outage or interruption. If a momentary outage cannot be tolerated, then a UPS can be installed in conjunction with on-site generation to avoid the momentary loss of power during transfer between the utility supply and the on-site generation. Electrical noise Electrical noise typically does not affect electrical equipment, but it can play havoc with electronic equipment causing data errors and other problems. Electrical noise can be caused by a variety of things, including electromagnetic interference (EMI) from arcing electrical equipment, such as dirty brushes on motors, microwave devices, and electric discharge lighting. Electrical noise typically rides on the voltage sine wave, resulting in a distorted voltage waveform. Sensitive electronic equipment can be protected from electrical noise by filters, isolation transformers, power conditioners, UPS, and other similar equipment. Harmonics Harmonics are typically caused by nonlinear loads that chop up the current waveform by conducting only during a portion of the voltage cycle. Common equipment that cause harmonics includes electric discharge lighting, electronic equipment such as personal computers that use switching power supplies, dimming systems, variable-speed motor drives, battery chargers, and others. Harmonics can cause overheating in the neutral conductors of multiwire branch circuits, feeders, panelboards, switchboards, and busways. They can also cause overheating in transformers, motors, and other electromechanical equipment. This can result in the premature failure of electrical conductors and equipment or even fire. Harmonics can also result in nuisance tripping of circuit breakers and failure of power factor correction capacitors. The effects of harmonics can be mitigated in a variety of ways, including installing oversized neutrals and using either oversized or K-rated transformers to alleviate heating. Harmonic filters, power conditioners, and other equipment can also be used to mitigate the impact of harmonics on the distribution system and utilization equipment. Solving power quality problems Power quality is relative, not absolute. This has important implications when solving your customer’s power quality problems. The power quality requirements for one piece of equipment or part of a facility may not be what is needed for other equipment or parts of a facility. There are often a variety of ways to provide the needed level of power quality, and you should recommend the simplest and most economical to your customer. As with any product or service, the cost of power delivered to a piece of equipment or system varies directly with the level of quality required. This is where the electrical contracting firm can really help its customer. By understanding the power quality needs of the customer’s equipment and systems, the electrical contracting firm can use its knowledge and experience to develop solutions that solve the customer’s power quality problems as economically as possible. Properly diagnosing the customer’s power quality problems and providing an effective and efficient solution will avert equipment failure and downtime. Acknowledgement This article is the result of ongoing research into the development of service contracting business by electrical contracting firms sponsored by the Electrical Contracting Foundation, Inc. The author would like to thank the foundation for its continuing support. GLAVINICH is Chair and Associate Professor of Architectural Engineering at The University of Kansas. He can be reached at (785) 864-3435 or tglavinich@ukans.edu.

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