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Power quality related problems, in particular voltage sags and brownouts, have a major negative impact on industrial productivity. This appears to be true for both industrialized as well as developing nations. There are several distinct solutions for voltage regulation including servo-stabilizers, CVT's, Ferro resonant regulators, thyristor ac regulators, tap changers and more recently, electronic regulators. This paper briefly examines commercially available technologies, and compares performance attributes of the various solutions for industrial and commercial use. Important comparison points include speed of response, types of faults handled, ruggedness, cost and efficiency.

Power Quality Issues in Industrial Plants :

As industrial plants move towards a globally competitive environment, achieving and sustaining high levels of productivity becomes a critical factor. For this to occur, it is important that processes operate essentially uninterrupted. Even in industrialized nations, one of the dominant reasons for process interruption is due to power quality problems with the incoming utility supply. In developing countries, the situation is substantially worse, as both the quality and the availability of electrical power are highly compromised. Power quality problems which compromise industrial productivity include transients, voltage sags and surges, brownouts, harmonics and power interruptions. Power interruptions have conventionally been handled through the use of a backup power source such as a generator or UPS. The UPS is a fairly expensive solution, as it relies on batteries for energy storage, and it has traditionally been applied to support electronic loads such as computers and industrial controllers. Very rarely is a UPS used to support an entire industrial process. The use of generators for backup power, particularly in the developing countries is prevalent. This is primarily because power outages are very common, and operation on generator power can keep the plant operational. It is important to note that in areas where there is an acute shortage of power, power outages are typically scheduled on a rotating basis. This allows industrial plants to coordinate their plant holidays, or to schedule operation with internal generators. This suggests that a more important problem is impaired industrial productivity due to the occurrence of random events such as transients, voltage sags and brownouts, and not due to power outages.

The primary focus of this paper is to examine problems and solutions which can reduce the susceptibility of an industrial plant to power quality related problems. Of the many problems listed above, the potentially most serious, and difficult to tackle, is the occurrence of voltage sags (typically 6-10 cycles), and brownouts. Voltage sags are inevitable on the power system, even in the most advanced utility networks. These are mostly caused by faults on the system, with line to ground faults being most frequent. In industrialized countries, it is very rare to have outages that extend significantly beyond the time required to clear the fault, and to re-close a breaker to connect the customer to a different part of the utility network. In fact, one of the more promising solutions which is being actively pursued for industrialized countries is the use of static transfer switches, where two independent power feeds are brought into the plant, and the plant supply can be rapidly switched from the faulted line to the unfaulted line so as to maintain power to the plant. It has been shown that this can handle over 95% of all voltage sag and utility fault related events. The static transfer switch is an expensive solution, unless the second feed is already present. In any case, it is important to note that this solution is not effective in cases where voltage sags and brownouts are due to acute power shortage (typical for developing countries), as all feeds will see similar under-voltage conditions.


Problems Due to Voltage Sags :

The impact of voltage sags on industrial processes has been extensively studied, and is the subject of numerous reports by utilities, industries and consultants. Short term (3-10 cycle) voltage sags are more frequent in the industrialized nations, while brownouts tend to be more prevalent in the developing countries. The biggest problem associated with voltage sags is the tripping of sensitive equipment such as motor drives. This occurs because of insufficient voltage across contactor and relay coils, and reduced dc bus voltages in adjustable speed motor drives, programmable controllers and other electronic loads. Some ASD's are designed to provide automatic restart, or in some cases ride through using load inertial energy. In such cases, the ASD's will ride through the short-term sag, unless the motor is part of a precision position controlled servo process. Short-term voltage sags have little impact on most direct line connected motor loads.

Voltage sag on typical utility line, and action of automatic voltage regulator Operation of industrial equipment under brownout conditions is much more problematic than under voltage sags. In addition to all the problems encountered for short-term sags, one has to contend with plant operation at reduced voltages, possibly for hours at a time. Severe problems of motor stalling, motor over heating, motor tripping, operation at improper load/speed points, and complete process shut down are likely scenarios.

It is clear that in developing countries in particular, any solution proposed should offer protection under both short-term sags as well as under brownout conditions.

Various solutions are presently used for correcting voltage sags and brownouts. These include tap-changing transformers with mechanical or static switches, saturable reactor regulators, phase-controlled regulators, Ferro resonant transformers, and motorized variacs.



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