The The protective circuit of an adjustable

The causes of power quality problems are generally complex and difficult to detect. Technically speaking, the ideal ac line supply by the utility system should be a pure sinewave of fundamental frequency (50/60 Hz). In addition, the peak of the voltage should be of rated value.

Unfortunately the actual ac line supply that we receive everyday departs from the ideal specifications. There are many ways in which the lack of quality power affects customers. Impulsive transients do not travel very far from their point of entry. However an impulsive transient can give rise to an oscillatory transient. The oscillatory transient can lead to transient overvoltage and consequent damage to the power line insulators. Impulsive transients are usually suppressed by surge arresters.

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Short duration voltage variations have varied effects on consumers. Voltage sags (also knows as dips) can cause loss of production in automated processes since a voltage sag can trip a motor or cause its controller to malfunction. For semiconductor manufacturing industries such a loss can be substantial. A voltage sag can also force a computer system or data processing system to crash. To prevent such a crash, an uninterruptible power supply (UPS) is often used, which, in turn, may generate harmonics. The protective circuit of an adjustable speed drive (ASD) can trip the system during a voltage swell. Also voltage swells can put stress on computers and many home appliances, thereby shortening their lives.

A temporary interruption lasting a few seconds can cause a loss of production, erasing of computer data etc. The cost of such an interruption during peak hours can be hundreds of thousands of dollars.The impact of long duration voltage variations is greater than those of short duration variations. A sustained overvoltage lasting for few hours can cause damage to household appliances without their owner knowing it, until it is too late. The undervoltage has the same effect as that of a voltage sag. In the case of a sag the termination of process is sudden.

But normal operation can be resumed after the normal voltage is restored. However in the case of a sustained undervoltage, the process cannot even be started or resumed. A sustained interruption is usually caused by faults. Since the loss to customers due to any sustained interruption can be in the order of millions of dollars, it is necessary for the utility to have a good preventive maintenance schedule and to have agreements or regulations to encourage high supply reliability. Voltage imbalance can cause temperature rise in motors and can even cause a large motor to trip. Harmonics, dc offset and notching cause waveform distortions. Harmonics can be integer multiples of fundamental frequency, fractions of the fundamental frequency (subharmonics) and at frequencies that are not integer multiples of the fundamental frequency (interharmonics).

Unwanted harmonic currents flowing through the distribution network can causes needless losses. Harmonics also can cause malfunction of ripple control or traffic control systems, losses and heating in transformers, electromagnetic interference (EMI) and interference with the communication systems. Ripple control refers to the use of a 300Hz to 2500Hz signal added to distribution lines to control switching of loads such as hot water heaters or street lighting. Interharmonic voltages can upset the operation of fluorescent lamps and television receivers. They can also produce acoustic noise in power equipment. DC offsets can cause saturation in the power transformer magnetic circuits.

A notch is a periodic transient that rides on the supply voltage. It can damage capacitive components connected in shunt due to high rate of voltage rise at the notches. Voltage flickers are caused by arc discharge lamps, arc furnaces, starting of large motors, arc welding machines etc. Voltage flickers are frequent variations in voltage that can cause the light intensity from incandescent lamps to vary. This variation is perceived as disturbing by human observers, particularly in the range of 3 to 15 times per second. The voltage flicker can have adverse effects on human health as the high frequency flickering of light bulbs, fluorescent tubes or television screen can cause strain on the eyes resulting in headaches or migraines. The voltage flicker can also reduce the life span of electronic equipment, lamps etc.

We can therefore conclude that the lack of standard quality power can cause loss of production, damage of equipment or appliances or can even be detrimental to human health. It is therefore imperative that a high standard of power quality is maintained. This book will demonstrate that the power electronic based power conditioning devices can be effectively utilized to improve the quality of power supplied to customers.


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