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Management of power quality has been an additional responsibility of the facility managers in the recent years. Power quality standards vary with sensitivity of equipments. A three phase induction motor has good degree of tolerance to poor power quality compared to a VFD. Power Quality can get affected due to either poor quality of power supplied by electric utility or effect of certain equipments installed by the consumer. PCs & UPS, Variable Speed Drives, Soft starters, Electronic ballast & Dimming systems in lighting, DC supplies are commonplace in industry and commercial establishments. Extensive use of such and other power electronic equipments has given rise to high levels of harmonics in the electrical distribution networks.
Harmonics are responsible for distortion of AC waveform and a number of undesired events that indicate threat to power quality. Power quality management, therefore, consists in restoring degree of power quality to different equipments as per required standards. First step to PQM is power quality audit, which establishes quality of power received from electric utility as well as the degree of effect on power quality by the equipments installed. Management of power quality brings in certain benefits that are direct as well as indirect. Direct benefits include reduction in KWh consumption & KVA demand thereby reducing electrical charges. Indirect benefits include enhancement of loading capacity by reduction in loading of transformers, switchgear & cables, reduction of iron & copper losses in transformers & motors, protection to power factor correction capacitors, etc.
Harmonics are defined as the sinusoidal components of a non-linear periodic waveform with a frequency that is a whole multiple of the fundamental frequency. Putting this simply; any switching operation of the mains supply sine wave will produce lots of other sine waves at higher frequencies. These are not apparent but they are harmful. Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60hz (50hz for european power and 60hz for american power). For example, if the fundamental power frequency is 60 hz, then the 2nd harmonic is 120 hz, the 3rd is 180 hz, etc. In modern test equipment today harmonics can be measured up to the 63rd harmonic. When harmonic frequencies are prevalent, electrical power panels and transformers become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When this happens, the power panel or transformer vibrates and emits a buzzing sound for the different harmonic frequencies. Harmonic frequencies from the 3rd to the 25th are the most common range of frequencies measured in electrical distribution systems.
Harmonics are responsible for a number of undesired events and malfunctions.
• Measurement errors in analog instruments since these can not detect harmonics.
• Nuisance tripping of micro-processor based protective relays & earth leakage circuit breakers.
• Greater risk of interference with communication network.
• Overheating due to overloading of capacitors, motors, transformers, cables.
• Rise in iron losses in transformers, motors & chokes due to high frequency.
• Risk of short circuits at power factor correction capacitors due to parallel resonance.
• High currents in neutrals due to triplents.
Management of harmonics is an important aspect in power quality & begins with comprehensive analysis of harmonics and periodic monitoring of total harmonic distortion.
• Power factor correction capacitors are protected from short circuits as a result of occurrence of parallel resonance by using de-tuned filters.
• Tuned filters are used to suppress individual harmonic orders.
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