ENGINEERING ARTICLES

Many adjustable-speed drives are equally sensitive to voltage sags as process control equipment discussed in the previous section. Tripping of adjustable-speed drives can occur due to several phenomena: • The drive controller or protection will detect the sudden change in operating conditions and trip the drive to prevent damage to the power electronic components. • The drop in de bus voltage which results from the sag will cause mal-operation or tripping of the drive controller or of the PWM inverter. • The increased ac currents during the sag or the post-sag over currents charging the de capacitor will cause an overcurrent trip or blowing of fuses protecting the power electronics components. • The process driven by the motor will not be able to tolerate the drop in speed or the torque variations due to the sag. After a trip some drives restart immediately when the voltage comes back; some restart after a certain delay time and others only after a manual restart. The vari...

During a short interruption the voltage is zero; thus, there is no supply of power at all to the equipment. The temporary consequences are that there is no light, which motors slow down, that screens turn blank, etc. All this only lasts for a few seconds, but the consequences can last much longer: disruption of production processes, loss of contents of computer memory, evacuation of buildings due to fire alarms going off, and sometimes damage when the voltage comes back (uncontrolled starting). For most sensitive equipment, there is no strict border between a voltage sag and an interruption: an interruption can be seen as a severe sag, i.e. one with zero remaining voltage. INDUCTION MOTORS The effect of a zero voltage on an induction motor is simple: the motor slows down. The mechanical time constant of an induction motor plus its load is in the range of 1 to 10 seconds. With dead times of several seconds, the motor has not yet come to a standstill but is likely to have slowed...

As short interruptions are due to automatic switching actions, their recording requires automatic monitoring equipment. Unlike long interruptions, a short interruption can occur without anybody noticing it. That is one of the reasons why utilities do not yet collect and publish data on short interruptions on a routine basis. One of the problems in collecting this data on a routine basis is that some kind of monitoring equipment needs to be installed on all feeders. A number of surveys have been performed to obtain statistical information about voltage magnitude variations and events. With those surveys, monitors were installed at a number of nodes spread through the system. As with long interruptions, interruption frequency and duration of interruption are normally presented as the outcome of the survey. Again like with long interruptions much more data analysis is possible, e.g, interruption frequency versus time of day or time of year, distributions for the time between events, varia...

Long interruptions are always due to component outages. Component outages are due to three different causes: I. A fault occurs in the power system which leads to an intervention by the power system protection. If the fault occurs in a part of the system which is not redundant or of which the redundant part is out of operation the intervention by the protection leads to an interruption for a number of customers or pieces of equipment. The fault is typically a short-circuit fault, but situations like overloading of transformers or under frequency may also lead to long interruptions. Although the results can be very disturbing to the affected customers, this is a correct intervention of the protection. Would the protection not intervene, the fault would most likely lead to an interruption for a much larger group of customers, as well as to serious damage to the electrical equipment. As distribution systems are often operated radially (i.e., without redundancy) and transmission ...

Switchgear covers a wide range of equipment concerned with switching and interrupting currents under both normal and abnormal conditions. It includes switches, fuses, circuit breakers, relays and other equipment. A brief account of these devices is given below. However, the reader may find the detailed discussion on them in the subsequent chapters. 1. SWITCHES A switch is a device which is used to open or close an electrical circuit in a convenient way. It can be used under full-load or no-load conditions but it cannot interrupt the fault currents. When the contacts of a switch are opened, an arc is produced in the air between the contacts. This is particularly true for circuits of high voltage and large current capacity. The switches may be classified into (i) air switches (ii) oil switches. The contacts of the former are opened in air and that of the latter are opened in oil. (I) AIR-BREAK SWITCH It is an air switch and is designed to open a circuit under load. In or...

The essential features of switchgear are: (I) COMPLETE RELIABILITY With the continued trend of interconnection and the increasing capacity of generating stations, the need for a reliable switch-gear has become of paramount importance. This is not surprising because switchgear is added to the power system to improve the reliability. When fault occurs on any part of the power system, the switchgear must operate to isolate the faulty section from the remainder circuit. (II) ABSOLUTELY CERTAIN DISCRIMINATION When fault occurs on any section of the power system, the switchgear must be able to discriminate between the faulty section and the healthy section. It should isolate the faulty section from the system without affecting the healthy section. This will ensure continuity of supply. (III) QUICK OPERATION When fault occurs on any part of the power system, the switchgear must operate quickly so that no damage is done to generators, ...

The apparatus used for switching, controlling and protecting the electrical circuits and equipment is known as switchgear. The switchgear equipment is essentially concerned with switching and interrupting currents either under normal or abnormal operating conditions. The tumbler switch with ordinary fuse is the simplest form of switchgear and is used to control and protect lights and other equipment in homes, offices etc. For circuits of higher rating, a high-rupturing capacity (H.R.C.) fuse in conjuction with a switch may serve the purpose of controlling and protecting the circuit. However, such a switchgear cannot be used profitably on high voltage system (3·3 kV) for two reasons. Firstly, when a fuse blows, it takes some time to replace it and consequently there is interruption of service to the customers. Secondly, the fuse cannot successfully interrupt large fault currents that result from the faults on high voltage system. With the advancement of power system, lines and ot...