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...

An induction regulator is essentially a constant voltage transformer, one winding of which can be moved w.r.t. the other, thereby obtaining a variable secondary voltage. The primary winding is connected across the supply while the secondary winding is connected in series with the line whose voltage is to be controlled. When the position of one winding is changed w.r.t. the other, the secondary voltage injected into the line also changes. There are two types of induction regulators viz. single phase and 3-phase. 1. SINGLE-PHASE INDUCTION REGULATOR A single phase induction regulator is illustrated in Figure1, In construction, it is similar to a single phase induction motor except that the rotor is not allowed to rotate continuously but can be adjusted in any position either manually or by a small motor. The primary winding AB is wound on the stator and is connected across the supply line. The secondary winding CD is wound on the rotor and is connected in series with the line...

Sometimes it is desired to control the voltage of a transmission line at a point far away from the main transformer. This can be conveniently achieved by the use of a booster transformer as shown in Figure 1. The secondary of the booster transformer is connected in series with the line whose voltage is to be controlled. The primary of this transformer is supplied from a regulating transformer fitted with on-load tap-changing gear. The booster transformer is connected in such a way that its secondary injects a voltage in phase with the line voltage. The voltage at AA is maintained constant by tap-changing gear in the main transformer. However, there may be considerable voltage drop between AA and BB due to fairly long feeder and tapping of loads. The voltage at BB is controlled by the use of regulating transformer and booster transformer. By changing the tapping on the regulating transformer, the magnitude of the voltage injected into the line can be varied. This permits to keep ...

Figure shows diagrammatically auto-transformer tap changing. Here, a mid-tapped auto-transformer or reactor is used. One of the lines is connected to its mid-tapping. One end, say a of this transformer is connected to a series of switches across the odd tappings and the other end b is connected to switches across even tappings. A short-circuiting switch S is connected across the auto-transformer and remains in the closed position under normal operation. In the normal operation, there is no inductive voltage drop across the auto-transformer. Referring to Figure, it is clear that with switch 5 closed, minimum secondary turns are in the circuit and hence the output voltage will be the lowest. On the other hand, the output voltage will be maximum when switch 1 is closed. Suppose now it is desired to alter the tapping point from position 5 to position 4 in order to raise the output voltage. For this purpose, short-circuiting switch S is opened, switch 4 is closed, then switch 5 is op...

The excitation control method is satisfactory only for relatively short lines. However, it is not suitable for long lines as the voltage at the alternator terminals will have to be varied too much in order that the voltage at the far end of the line may be constant. Under such situations, the problem of voltage control can be solved by employing other methods. One important method is to use tap changing transformer and is commonly employed where main transformer is necessary. In this method, a number of tappings are provided on the secondary of the transformer. The voltage drop in the line is supplied by changing the secondary EMF of the transformer through the adjustment of its number of turns. (I) OFF LOAD TAP CHANGING TRANSFORMER Figure1 shows the arrangement where a number of tappings have been provided on the secondary. As the position of the tap is varied, the effective number of secondary turns is varied and hence the output voltage of the secondary can be changed. Thus...

When the load on the supply system changes, the terminal voltage of the alternator also varies due to the changed voltage drop in the synchronous reactance of the armature. The voltage of the alternator can be kept constant by changing the field current of the alternator in accordance with the load. This is known as excitation control method. The excitation of alternator can be controlled by the use of automatic or hand operated regulator acting in the field circuit of the alternator. The first method is preferred in modern practice. There are two main types of automatic voltage regulators viz. (i) Tirril Regulator (ii) Brown-Boveri Regulator These regulators are based on the “overshooting the mark principle” to enable them to respond quickly to the rapid fluctuations of load. When the load on the alternator increases, the regulator produces an increase in excitation more than is ultimately necessary. Before the voltage has the time to increase to the value corresponding to the ...

In this type of regulator, exciter field rheostat is varied continuously or in small steps instead of being first completely cut in and then completely cut out as in Tirril regulator. For this purpose, a regulating resistance is connected in series with the field circuit of the exciter. Fluctuations in the alternator voltage are detected by a control device which actuates a motor. The motor drives the regulating rheostat and cuts out or cuts in some resistance from the rheostat, thus changing the exciter and hence the alternator voltage. CONSTRUCTION OF BROWN BOVERI REGULATOR Figure 1 shows the schematic diagram of a Brown Boveri voltage regulator. It also works on the “overshooting the mark principle” and has the following four important parts: (I) CONTROL SYSTEM: The control system is built on the principle of induction motor. It consists of two windings A and B on an annular core of laminated sheet steel. The winding A is excited from two of the generator terminal...

In this type of regulator, a fixed resistance is cut in and cut out of the exciter field circuit of the alternator. This is achieved by rapidly opening and closing a shunt circuit across the exciter rheostat. For this reason, it is also known as vibrating type voltage regulator. CONSTRUCTION OF TIRRIL REGULATOR Figure shows the essential parts of a Tirril voltage regulator. A rheostat R is provided in the exciter circuit and its value is set to give the required excitation. This rheostat is put in and out of the exciter circuit by the regulator, thus varying the exciter voltage to maintain the desired voltage of the alternator. (I) MAIN CONTACT: There are two levers at the top which carry the main contacts at the facing ends. The left-hand lever is controlled by the exciter magnet whereas the right hand lever is controlled by an AC magnet known as main control magnet. (II) EXCITER MAGNET: This magnet is of the ordinary solenoid type and is connected across the exciter mains. Its exci...

WHAT IS VOLTAGE CONTROL? In a modern power system, electrical energy from the generating station is delivered to the ultimate consumers through a network of transmission and distribution. For satisfactory operation of motors, lamps and other loads, it is desirable that consumers are supplied with substantially constant voltage. Too wide variations of voltage may cause erratic operation or even malfunctioning of consumers’ appliances. To safeguard the interest of the consumers, the government has enacted a law in this regard. The statutory limit of voltage variation is ± 6% of declared voltage at consumers’ terminals. The principal cause of voltage variation at consumer’s premises is the change in load on the supply system. When the load on the system increases, the voltage at the consumer’s terminals falls due to the increased voltage drop in (i) alternator synchronous impedance (ii) transmission line (iii) transformer impedance (iv) feeders and (v) Distributors. T...

DC Ground detectors : are the devices that are used to detect/indicate the ground fault for ungrounded DC systems. When a ground fault occurs on such a system, immediate steps should be taken to clear it. If this is not done and a second ground fault happens, a short circuit occurs. Lamps are generally used for the detection of ground faults. They are connected for ungrounded 2-wire system as shown in Figure. Each lamp should have a voltage rating equal to the line voltage. The two lamps in series, being subjected to half their rated voltage, will glow dimly. If a ground fault occurs on either wires, the lamp connected to the grounded wire will not glow while the other lamp will glow brightly. AC Ground detectors : are the devices that are used to detect the ground fault for ungrounded AC systems. When a ground fault occurs on such a system, immediate steps should be taken to clear it. If this is not done and a second ground fault happens, a short circuit occurs. Figure shows ...

A booster is a DC generator whose function is to inject or add certain voltage into a circuit so as to compensate the IR drop in the feeders etc. A booster is essentially a series DC generator of large current capacity and is connected in series with the feeder whose voltage drop is to be compensated as shown in Figure. It is driven at constant speed by a shunt motor working from the bus-bars. As the booster is a series generator, therefore, voltage generated by it is directly proportional to the field current which is here the feeder current. When the feeder current increases, the voltage drop in the feeder also increases. But increased feeder current results in greater field excitation of booster which injects higher voltage into the feeder to compensate the voltage drop. For exact compensation of voltage drop, the booster must be marked on the straight or linear portion of its voltage-current characteristics. It might be suggested to compensate the voltage drop in the fe...

The devices in switching equipment are referred to by numbers, with appropriate suffix letters when necessary, according to the functions they perform. These numbers are based on a system adopted as standard for automatic switchgear by IEEE, and incorporated in American Standard C37.2-1970. This system is used in connection diagrams, in instruction books, and in specifications. 1. MASTER ELEMENT is the initiating device, such as control switch, voltage relay, float switch, etc., which serves either directly, or through such permissive devices as protective relay system, except as specifically provided by device functions 48, 62, and 79. 2. TIME-DELAY STARTING, or closing relay is a device which functions to give a desired amount of time delay before or after any point of operation in a switching sequence or protective relay system, except as specifically provided by device functions 48, 62, and 79. 3. CHECKING OR INTERLOCKING RELAY is a device which operates in response to ...

External faults are those faults or hazards that occur outside the transformer. These hazards present stresses on the transformer that may be of concern and may shorten the transformer life. These faults include the following. • OVER LOADS Overloads cause the transformer to overheat and have the potential to cause permanent damage or loss of life to the unit. The time constant for overheating is long, however, and it may take many hours of exposure for the condition to become serious. In most cases, no protection is provided for overload, but an alarm will often be used to warn operating personnel of the condition. One cause of overload may be due to unequal load sharing of parallel transformers or unbalanced loading of three-phase banks. • OVER VOLTAGE Over-voltage can be either due to short-term transient conditions or long term power-frequency conditions. Transient over-voltages cause end-tum stresses and possible breakdown. These transients are protected against by surge...

Overheating of a synchronous generator may occur due to one of the following causes: 1. Overload 2. Failure of the ventilation or hydrogen cooling system 3. Shorted laminations in the stator iron 4. Core bolt insulation failures in the stator iron Excessive overload is not likely since the prime mover rating is usually not much greater than the generator rating. There is the possibility of overload due to high active power load coupled with high excitation. If the power factor is below rating, this will give an alarm for high excitation. Failure of the cooling system is also likely to be detected by operator alarms, The other failure, involving core failures and heating will develop slowly and must be detected by temperature measurements of some kind. Temperature detection is often accomplished using embedded thermocouples in the stator winding slots, placing the thermocouples throughout the windings in several locations. Another measurement technique is to record the inp...

One type of overvoltage in a generator is that due to transient surges caused by lightning or switching surges. These transients are protected by surge protective devices that are designed for this purpose. Power frequency over-voltages are possible if the generator controls are defective or have inadequate transient response. A defective voltage regulator, for example, can cause the exciter to ramp to its ceiling voltage. If the voltage control is performed manually, a sudden change in load will result in an increase in voltage. The loss of load may cause high voltage on units that are remotely located in the system. This is particularly true of remote hydro units since it may not be possible for the governor to close the wicket gates of large hydro units fast enough to prevent an overvoltage due to loss of load. The result is over-speed, which is associated with overvoltage. This type of overvoltage is not likely on a steam unit, since they have tighter control against over-speed and...

There are two types of backup protection that might be applied to a generator: backup of relays protecting the generator protection zone and backup of relays protecting external zones. Some types of backup protection may be graded to coordinate with both internal and external protective devices. The negative-sequence relay might be considered a form of backup protection, since most faults should be cleared by the stator differential protection with the negative sequence relay acting as backup. Balanced faults that are not cleared promptly can also cause considerable damage to a generator and backup protection is warranted. One type of such protection is to provide a distance relay that is supplied with current from a CT in the generator neutral and voltage from the generator terminals. Such a relay can recognize balanced faults both internal and external to the generator. The connection makes the relay directional from the neutral, but gives it reach in both directions from the ...

There are several different types of rotor protection, each type guarding the rotor from a particular type of fault. From this viewpoint, the protection against unbalanced loading, using negative sequence relays, can be considered a type of rotor protection since the effect of negative sequence currents is likely to result in rotor damage. 1) SHORTED FIELD WINDING PROTECTION Shorted turns in the generator field winding have the potential for distorting the field across the air gap, as illustrated in Figure 1. This is due to the unsymmetrical ampere turns of mmf in different parts of the field winding. If the air gap flux is badly distorted, there can be much distorted forces acting on the rotor, since the forces vary as the square of the flux density. Once there are unequal forces on opposite sides of the rotor, there is tendency for the rotor to warp. The unbalanced force can be very large, as much as 50 to 100 tons, tending to warp the rotor. In some cases the rotor may be d...