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Passive Techniques for Reducing Input Current Harmonics
Events over the last several years have focused attention on certain types of loads on the electrical system that result in power quality problems for the user and utility alike. Equipment which has become common place in most facilities including computer power supplies, solid state lighting ballast, adjustable speed drives (ASDs), and un-interruptible power supplies (UPSs) are examples of non-linear loads. Adjustable speed drives are also known as Variable Frequency Drives (VFDs) and are used extensively in the HVAC systems and in numerous industrial applications to control the speed and torque of electric motors. The number of VFDs and their power rating has increased significantly in the past decade. Hence, their contribution to the total electrical load of a power system is significant and cannot be neglected.
Non-linear loads are loads in which the current waveform does not have a linear relationship with the voltage waveform. In other words, if the input voltage to the load is sinusoidal and the current is non-sinusoidal then such loads will be classified as non-linear loads because of the non-linear relationship between voltage and current. Non-linear loads generate voltage and current harmonics, which can have adverse effects on equipment that are used to deliver electrical energy. Examples of power delivery equipment include power system transformers, feeders, circuit breakers, etc. Power delivery equipment is subject to higher heating losses due to harmonic currents consumed by non-linear loads. Harmonics can have a detrimental effect on emergency or standby power generators, telephones and other sensitive electrical equipment.
When reactive power compensation in the form of passive power factor improving capacitors are used with non-linear loads, resonance conditions can occur that may result in even higher levels of harmonic voltage and current distortion thereby causing equipment failure, disruption of power service, and fire hazards in extreme conditions.
The electrical environment has absorbed most of these problems in the past. However, the problem has now reached a magnitude where Europe, the US, and other countries have proposed standards to engineer systems responsibly, considering the electrical environment. IEEE 519-1992 and EN61000-3-2 have evolved to become a common requirement cited when specifying equipment on newly engineered projects. Various harmonic filtering techniques have been developed to meet these specifications. The present IEEE 519-1992 document establishes acceptable levels of harmonics (voltage and current) that can be introduced into the incoming feeders by commercial and industrial users. Where there may have been little cooperation previously from manufacturers to meet such specifications, the adoption of IEEE 519-1992 and other similar world standards now attract the attention of everyone.
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