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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 various automatic restart options are only relevant when the process tolerates a certain level of speed and torque variations. The effect of the voltage sag on the de bus voltage is the main cause of equipment tripping.

INFLUENCE OF SHORT INTERRUPTIONS ON EQUIPMENT

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 down significantly. This reduction in speed of the motors might disrupt the industrial process so much that the process control trips it.

The motor can re-accelerate when the voltage comes back, if the system is strong enough. For public distribution systems re-acceleration is seldom a problem.

Also the setting of the under voltage protection should be such that it does not trip before the voltage comes back. This calls for a coordination between the under voltage setting of the motor protection and the reclosure interval setting on the utility feeder.

Induction motors fed via contactors are disconnected automatically as the contactor drops out. Without countermeasures this would always lead to loss of the load. In some industrial processes the induction motors are automatically reconnected when the voltage comes back: either instantaneously or staged (the most important motors first, the rest later).

SYNCHRONOUS MOTORS

Synchronous motors can normally not restart on full load. They are therefore equipped with under voltage protection to prevent stalling when the voltage comes back. For synchronous motors the delay time of the under voltage protection should be less than the reclosing interval. Especially for very fast reclosure this can be a problem. We see here a situation where an interruption causes a more serious threat to the synchronous motors the faster the voltage comes back. With most other load the situation is the other way around: the shorter the interruption, the less severe it is to the load.

ADJUSTABLE SPEED DRIVES

Adjustable-speed drives are very sensitive to short interruptions, and to voltage sags. They normally trip well within I second, sometimes even within one cycle; thus even the shortest interruption will cause a loss of the load. Some of the more modern drives are able to automatically reconnect the moment the voltage comes back. But being disconnected from the supply for several seconds will often have disrupted the process behind the drive so much that reconnection does not make much sense anymore.

ELECTRONIC EQUIPMENT

Without countermeasures electronics devices will trip well within the reclosing interval. This leads to the infamous "blinking-clock syndrome": clocks of video recorders, microwave ovens, and electronic alarms start blinking when the supply is interrupted; and they keep on blinking until manually reset. An easy solution is to install a small rechargeable battery inside of the equipment, to power the internal memory during the interruption.

Computers and process control equipment have basically the same problem. But they require more than a simple battery. An uninterruptible power supply (UPS) is a much-used solution.

MONITORING OF SHORT INTERRUPTIONS

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, variation among customers.

CAUSES OF LONG INTERRUPTIONS

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 systems meshed (with redundancy), faults in transmission systems do not have much influence on the reliability of the supply, but faults in distribution systems do.

2. A protection relay intervenes incorrectly, thus causing a component outage, which might again lead to a long interruption. If the incorrect tripping (or maltrip) occurs in a part of the system without redundancy, it will always lead to an interruption. If it occurs in a part of the system with redundancy the situation is different. For a completely random maltrip, the chance that the redundant component is out of operation is rather small. Random maltrips are thus not a serious reliability concern in redundant systems. However maltrips are often not fully random, but more likely when the system is faulted. In that case there will be two trips by the protection: a correct intervention and an incorrect one. The maltrip trips the redundant component just at the moment that redundancy is needed. Fault-related maltrips are a serious concern in redundant systems.

3. Operator actions cause a component outage which can also lead to a long interruption. Some actions should be treated as a backup to the power system protection, either correct or incorrect. But an operator can also decide to switch off certain parts of the system for preventive maintenance. This is a very normal action and normally not of any concern to customers. There is in most cases at least some level of redundancy available so that the maintenance does not lead to an interruption for any of the customers. In some low voltage networks there is no redundancy present at all, which implies that preventive maintenance and repair or changes in the system can only be performed when the supply to a part of the customers is interrupted. These interruptions are called "scheduled interruptions" or "planned interruptions."

The customer can take some precautions that make the consequences of the interruption less than for a nonscheduled interruption. This of course assumes that the utility informs the customer well in advance, which is unfortunately not always the case.

SWITCHGEAR EQUIPMENTS

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 order to quench the arc that occurs on opening such a switch, special arcing horns are provided. Arcing horns are pieces of metals between which arc is formed during opening operation. As the switch opens, these horns are spread farther and farther apart. Consequently, the arc is lengthened, cooled and interrupted. Air-break switches are generally used outdoor for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder.

Figure: AIR BREAK SWITCH

(II) ISOLATOR OR DISCONNECTING SWITCH

It is essentially a knife switch and is designed to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in the line. Such switches are generally used on both sides of circuit breakers in order that repairs and replacement of circuit breakers can be made without any danger. They should never be opened until the circuit breaker in the same circuit has been opened and should always be closed before the circuit breaker is closed.

Figure: ISOLATOR OR DISCONNECTING SWITCH

(III) OIL SWITCHES

As the name implies, the contacts of such switches are opened under oil, usually transformer oil. The effect of oil is to cool and quench the arc that tends to form when the circuit is opened. These switches are used for circuits of high voltage and large current carrying capacities.

2. FUSES

A fuse is a short piece of wire or thin strip which melts when excessive current flows through it for sufficient time. It is inserted in series with the circuit to be protected. Under normal operating conditions, the fuse element it at a temperature below its melting point. Therefore, it carries the normal load current without overheating. However, when a short circuit or overload occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the fuse element melts (or blows out), disconnecting the circuit protected by it. In this way, a fuse protects the machines and equipment from damage due to excessive currents. It is worthwhile to note that a fuse performs both detection and interruption functions.

3. CIRCUIT BREAKERS

A circuit breaker is an equipment which can open or close a circuit under all conditions viz. no load, full load and fault conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions.

For the latter operation, a relay circuit is used with a circuit breaker. Figure2 (i) shows the parts of a typical oil circuit breaker whereas Figure2 (ii) shows its control by a relay circuit. The circuit breaker essentially consists of moving and fixed contacts enclosed in strong metal tank and immersed in oil, known as transformer oil.

Under normal operating conditions, the contacts remain closed and the circuit breaker carries the full-load current continuously. In this condition, the EMF in the secondary winding of current transformer (CT) is insufficient to operate the trip coil of the breaker but the contacts can be opened (and hence the circuit can be opened) by manual or remote control. When a fault occurs, the resulting overcurrent in the CT primary winding increases the secondary EMF This energizes the trip coil of the breaker and moving contacts are pulled down, thus opening the contacts and hence the circuit.

The arc produced during the opening operation is quenched by the oil. It is interesting to note that relay performs the function of detecting a fault whereas the circuit breaker does the actual circuit interruption.

Figure2: (i) Parts of a typical oil circuit breaker (ii) oil circuit breaker control by a relay circuit

4. RELAYS

A relay is a device which detects the fault and supplies information to the breaker for circuit interruption. Figure2 (ii) shows a typical relay circuit. It can be divided into three parts viz.

(i) The primary winding of a current transformer (CT) which is connected in series with the circuit to be protected. The primary winding often consists of the main conductor itself.

(ii) The second circuit is the secondary winding of CT connected to the relay operating coil.

(iii) The third circuit is the tripping circuit which consists of a source of supply, trip coil of circuit breaker and the relay stationary contacts.

Under normal load conditions, the EMF of the secondary winding of CT is small and the current flowing in the relay operating coil is insufficient to close the relay contacts. This keeps the trip coil of the circuit breaker unenergized. Consequently, the contacts of the circuit breaker remain closed and it carries the normal load current. When a fault occurs, a large current flows through the primary of CT. This increases the secondary EMF and hence the current through the relay operating coil. The relay contacts are closed and the trip coil of the circuit breaker is energized to open the contacts of the circuit breaker.

ESSENTIAL FEATURES OF SWITCHGEAR

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, transformers and other equipment by the short-circuit currents. If fault is not cleared by switchgear quickly, it is likely to spread into healthy parts, thus endangering complete shutdown of the system.

(IV) PROVISION FOR MANUAL CONTROL

A switchgear must have provision for manual control. In case the electrical (or electronics) control fails, the necessary operation can be carried out through manual control.

(V) PROVISION FOR INSTRUMENTS

There must be provision for instruments which may be required. These may be in the form of ammeter or voltmeter on the unit itself or the necessary current and voltage transformers for connecting to the main switchboard or a separate instrument panel.

SWITCHGEAR

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 other equipments operate at high voltages and carry large currents. When a short circuit occurs on the system, heavy current flowing through the equipment may cause considerable damage. In order to interrupt such heavy fault currents, automatic circuit breakers (or simply circuit breakers) are used. A circuit breaker is a switchgear which can open or close an electrical circuit under both normal and abnormal conditions. Even in instances where a fuse is adequate, as regards to breaking capacity, a circuit breaker may be preferable. It is because a circuit breaker can close circuits, as well as break them without replacement and thus has wider range of use altogether than a fuse.

INDUCTION REGULATORS

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 whose voltage is to be controlled.

Figure1: Single phase induction regulator

The primary exciting current produces an alternating flux that induces an alternating voltage in the secondary winding CD. The magnitude of voltage induced in the secondary depends upon its position w.r.t. the primary winding. By adjusting the rotor to a suitable position, the secondary voltage can be varied from a maximum positive to a maximum negative value. In this way, the regulator can add or subtract from the circuit voltage according to the relative positions of the two windings. Owing to their greater flexibility, single phase regulators are frequently used for voltage control of distribution primary feeders.

2. THREE-PHASE INDUCTION REGULATOR

In construction, a 3-phase induction regulator is similar to a 3-phase induction motor with wound rotor except that the rotor is not allowed to rotate continuously but can be held in any position by means of a worm gear. The primary windings either in star or delta are wound on the stator and are connected across the supply. The secondary windings are wound on the rotor and the six terminals are brought out since these windings are to be connected in series with the line whose voltage is to be controlled.

Figure2: 3-phase induction regulator

When polyphase currents flow through the primary windings, a rotating field is set up which induces an EMF in each phase of rotor winding. As the rotor is turned, the magnitude of the rotating flux is not changed; hence the rotor EMF per phase remains constant. However, the variation of the position of the rotor will affect the phase of the rotor EMF w.r.t. the applied voltage as shown in Figure3. The input primary voltage per phase is Vp and the boost introduced by the regulator is Vr. The output voltage V is the vector sum of Vp and Vr. Three phase induction regulators are used to regulate the voltage of feeders and in connection with high voltage oil testing transformers.

BOOSTER TRANSFORMER

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 the voltage at BB to the desired value. This method of voltage control has three disadvantages.

It is more expensive than the on-load tap-changing transformer.
It is less efficient owing to losses in the booster and
More floor space is required.

Figure: Three-phase booster transformer

AUTO TRANSFORMER TAP CHANGING

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 opened and finally short-circuiting switch is closed. In this way, tapping can be changed without interrupting the supply.

It is worthwhile to describe the electrical phenomenon occurring during the tap changing. When the short-circuiting switch is opened, the load current flows through one-half of the reactor coil so that there is a voltage drop across the reactor. When switch 4 is closed, the turns between points 4 and 5 are connected through the whole reactor winding. A circulating current flows through this local circuit but it is limited to a low value due to high reactance of the reactor.

TAP CHANGING TRANSFORMERS

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 referring to Figure1, when the movable arm makes contact with stud1, the secondary voltage is minimum and when with stud 5, it is maximum. During the period of light load, the voltage across the primary is not much below the alternator voltage and the movable arm is placed on stud 1. When the load increases, the voltage across the primary drops, but the secondary voltage can be kept at the previous value by placing the movable arm on to a higher stud. Whenever a tapping is to be changed in this type of transformer, the load is kept off and hence the name off load tap-changing transformer.

The principal disadvantage of the circuit arrangement shown in Figure1 is that it cannot be used for tap-changing on load. Suppose for a moment that tapping is changed from position 1 to position 2 when the transformer is supplying load. If contact with stud 1 is broken before contact with stud 2 is made, there is break in the circuit and arcing results. On the other hand, if contact with stud 2 is made before contact with stud 1 is broken, the coils connected between these two tappings are short-circuited and carry damaging heavy currents. For this reason, the above circuit arrangement cannot be used for tap-changing on load.

(II) ON LOAD TAP CHANGING TRANSFORMER

In supply system, tap-changing has normally to be performed on load so that there is no interruption to supply. Figure2 shows diagrammatically one type of on-load tap-changing transformer. The secondary consists of two equal parallel windings which have similar tappings 1a to 5a and 1b to 5b. In the normal working conditions, switches a, b and tappings with the same number remain closed and each secondary winding carries one-half of the total current. Referring to Figure2, the secondary voltage will be maximum when switches a, b and 5a, 5b are closed. However, the secondary voltage will be minimum when switches a, b and 1a, 1b are closed.

Suppose that the transformer is working with tapping position at 4a, 4b and it is desired to alter its position to 5a, 5b. For this purpose, one of the switches a and b, say a, is opened. This takes the secondary winding controlled by switch a out of the circuit. Now, the secondary winding controlled by switch b carries the total current which is twice its rated capacity. Then the tapping on the disconnected winding is changed to 5a and switch a is closed. After this, switch b is opened to disconnect its winding, tapping position on this winding is changed to 5b and then switch b is closed. In this way, tapping position is changed without interrupting the supply. This method has the following disadvantages

(i) During switching, the impedance of transformer is increased and there will be a voltage surge.

(ii) There are twice as many tappings as the voltage steps.

EXCITATION CONTROL

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 increased excitation, the regulator reduces the excitation to the proper value.

BROWN BOVERI REGULATOR

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 terminals through resistances U and U’ while a resistance R is inserted in the circuit of winding B. The ratio of resistance to reactance of the two windings are suitably adjusted so as to create a phase difference of currents in the two windings. Due to the phase difference of currents in the two windings, rotating magnetic field is set up. This produces electromagnetic torque on the thin aluminum drum C carried by steel spindle; the latter being supported at both ends by jewel bearings. The torque on drum C varies with the terminal voltage of the alternator. The variable resistance U’ can also vary the torque on the drum. If the resistance is increased, the torque is decreased and vice versa. Therefore, the variable resistance U’ provides a means by which the regulator may be set to operate at the desired voltage.

(II) MECHANICAL CONTROL TORQUE: The electric torque produced by the current in the split phase winding is opposed by a combination of two springs (main spring and auxiliary spring) which produce a constant mechanical torque irrespective of the position of the drum. Under steady deflected state, mechanical torque is equal and opposite of the electric torque.

(III) OPERATING SYSTEM: It consists of a field rheostat with contact device. The rheostat consists of a pair of resistance elements connected to the stationary contact blocks C_B. These two resistance sectors R are connected in series with each other and then in series with the field circuit of the exciter. On the inside surface of the contact blocks roll the contact sectors C_S. When the terminal voltage of the alternator changes, the electric torque acts on the drum. This causes the contact sectors to roll over the contact blocks, cutting in or cutting out rheostat resistance in the exciter field circuit.

(IV) DAMPING TORQUE: The regulator is made stable by damping mechanism which consists of an aluminum disc O rotating between two permanent magnets m. The disc is geared to the rack of an aluminum sector P and is fastened to the aluminum drum C by means of a flexible spring S acting as the recall spring. If there is a change in the alternator voltage, the eddy currents induced in the disc O produce the necessary damping torque to resist quick response of the moving system.

OPERATION OF BROWN BOVERI REGULATOR

Suppose that resistances U and U’ are so adjusted that terminal voltage of the alternator is normal at position 1. In this position, the electrical torque is counterbalanced by the mechanical torque and the moving system is in equilibrium. It is assumed that electrical torque rotates the shaft in a clockwise direction.

Now imagine that the terminal voltage of the alternator rises due to decrease in load on the supply system. The increase in the alternator voltage will cause an increase in electrical torque which becomes greater than the mechanical torque. This causes the drum to rotate in clockwise direction, say to position 3. As a result, more resistance is inserted in the exciter circuit, thereby decreasing the field current and hence the terminal voltage of the alternator. Meanwhile, the recall spring S is tightened and provides a counter torque forcing the contact roller back to position 2 which is the equilibrium position. The damping system prevents the oscillations of the system about the equilibrium position.

TIRRIL REGULATOR

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 exciting current is, therefore, proportional to the exciter voltage. The counterbalancing force for the exciter magnet is provided by four coil springs.

(III) AC MAGNET: It is also of solenoid type and is energized from AC busbars. It carries series as well as shunt excitation. This magnet is so adjusted that with normal load and voltage at the alternator, the pulls of the two coils are equal and opposite, thus keeping the right-hand lever in the horizontal position.

(IV) DIFFERENTIAL RELAY: It essentially consists of a U-shaped relay magnet which operates the relay contacts. The relay magnet has two identical windings wound differentially on both the limbs. These windings are connected across the exciter mains–the left hand one permanently while the right hand one has its circuit completed only when the main contacts are closed. The relay contacts are arranged to shunt the exciter-field rheostat R. A capacitor is provided across the relay contacts to reduce the sparking at the time the relay contacts are opened.

OPERATION OF TIRRIL REGULATOR:

The two control magnets (i.e. exciter magnet and AC magnet) are so adjusted that with normal load and voltage at the alternator, their pulls are equal, thus keeping the main contacts open. In this position of main contacts, the relay magnet remains energized and pulls down the armature carrying one relay contact. Consequently, relay contacts remain open and the exciter field rheostat is in the field circuit.

When the load on the alternator increases, its terminal voltage tends to fall. This causes the series excitation to predominate and the AC magnet pulls down the right-hand lever to close the main contacts. Consequently, the relay magnet is de-energized and releases the armature carrying the relay contact. The relay contacts are closed and the rheostat R in the field circuit is short circuited.

This increases the exciter-voltage and hence the excitation of the alternator. The increased excitation causes the alternator voltage to rise quickly. At the same time, the excitation of the exciter magnet is increased due to the increase in exciter voltage. Therefore, the left-hand lever is pulled down, opening the main contacts, energizing the relay magnet and putting the rheostat R again in the field circuit before the alternator voltage has time to increase too far. The reverse would happen should the load on the alternator decrease.

It is worthwhile to mention here that exciter voltage is controlled by the rapid opening and closing of the relay contacts. As the regulator is worked on the overshooting the mark principle, therefore, the terminal voltage does not remain absolutely constant but oscillates between the maximum and minimum values. In fact, the regulator is so quick acting that voltage variations never exceed ± 1%.

Figure: TIRRIL REGULATOR

VOLTAGE CONTROL

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.

The reverse would happen should the load on the system decrease. These voltage variations are undesirable and must be kept within the prescribed limits (i.e. ± 6% of the declared voltage). This is achieved by installing voltage regulating equipment at suitable places in the power system. The purpose of this chapter is to deal with important voltage control equipment and its increasing utility in this fast developing power system.

IMPORTANCE OF VOLTAGE CONTROL

When the load on the supply system changes, the voltage at the consumer’s terminals also changes. The variations of voltage at the consumer’s terminals are undesirable and must be kept within prescribed limits for the following reasons:

(i) In case of lighting load, the lamp characteristics are very sensitive to changes of voltage. For instance, if the supply voltage to an incandescent lamp decreases by 6% of rated value, then illuminating power may decrease by 20%. On the other hand, if the supply voltage is 6% above the rated value, the life of the lamp may be reduced by 50% due to rapid deterioration of the filament.

(ii) In case of power load consisting of induction motors, the voltage variations may cause erratic operation. If the supply voltage is above the normal, the motor may operate with a saturated magnetic circuit, with consequent large magnetizing current, heating and low power factor. On the other hand, if the voltage is too low, it will reduce the starting torque of the motor considerably.

(iii) Too wide variations of voltage cause excessive heating of distribution transformers. This may reduce their ratings to a considerable extent.

It is clear from the above discussion that voltage variations in a power system must be kept to minimum level in order to deliver good service to the consumers. With the trend towards larger and larger interconnected system, it has become necessary to employ appropriate methods of voltage control.

LOCATION OF VOLTAGE CONTROL EQUIPMENT

In a modern power system, there are several elements between the generating station and the consumers. The voltage control equipment is used at more than one point in the system for two reasons. Firstly, the power network is very extensive and there is a considerable voltage drop in transmission and distribution systems. Secondly, the various circuits of the power system have dissimilar load characteristics. For these reasons, it is necessary to provide individual means of voltage control for each circuit or group of circuits. In practice, voltage control equipment is used at:

(i) Generating stations
(ii) Transformer stations
(iii) The feeders if the drop exceeds the permissible limits

METHODS OF VOLTAGE CONTROL

There are several methods of voltage control. In each method, the system voltage is changed in accordance with the load to obtain a fairly constant voltage at the consumer’s end of the system. The following are the methods of voltage control in an AC power system:

(i) By excitation control
(ii) By using tap changing transformers
(iii) Auto-transformer tap changing
(iv) Booster transformers
(v) Induction regulators
(vi) By synchronous condenser

Method (i) is used at the generating station only whereas methods (ii) to (v) can be used for transmission as well as primary distribution systems. However, methods (vi) is reserved for the voltage control of a transmission line. We shall discuss each method separately in the next sections.

GROUND DETECTORS

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 how lamps are connected to an ungrounded 3-phase system for the detection of ground fault. If ground fault occurs on any wire, the lamp connected to that wire will be dim and the lamps connected to healthy (ungrounded) wire will become brighter.

BOOSTERS

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 feeder by over compounding the generators instead of using a booster. Such a method is not practicable for feeders of different lengths because it will disturb the voltage of other feeders. The advantage of using a booster is that each feeder can be regulated independently a great advantage if the feeders are of different lengths.

PROTECTIVE DEVICES AND THEIR FUNCTIONS

By Engr. Aneel Kumar March 20, 2015 No comments:

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 the position of a number of other devices, (or to a number of predetermined conditions), in an equipment, to allow an operating sequence to proceed, to stop, or to provide a check of the position of these devices or of these conditions for any purpose.

4. MASTER CONTACTOR is a device, generally controlled by device No.1 or equivalent, and the required permissive and protective devices that serves to make and break the necessary control circuits to place an equipment into operation under the desired conditions and to take it out of operation under other or abnormal conditions.

5. STOPPING DEVICE is a control device used primarily to shut down an equipment and hold it out of operation. This device may be manually or electrically actuated, but excludes the function of electrical lockout on abnormal conditions.
6. STARTING CIRCUIT BREAKER is a device whose principal function is to connect a machine to its source of starting voltage.

7. ANODE CIRCUIT BREAKER is one used in the anode circuits of a power rectifier for the primary purpose of interrupting the rectifier circuit if an arc back should occur.

8. CONTROL POWER DISCONNECTING DEVICE is a disconnective device such as a knife switch, circuit breaker, or pull out fuse block, used for the purpose of connecting and disconnecting the source of control power to and from the control bus or equipment.

Note: Control power is considered to include auxiliary power which supplies such apparatus as small motors and heaters.

9. REVERSING DEVICE is used for the purpose of reversing a machine field or for performing any other reversing functions.

10. UNIT SEQUENCE SWITCH is used to change the sequence in which units may be placed in and out of service in multiple-unit equipments.

11. Reserved for future application.

12. OVER-SPEED DEVICE is usually a direct-connected speed switch which functions on machine over-speed.

13. SYNCHRONOUS-SPEED DEVICE, such as centrifugal-speed switch, a slip-frequency relay, a voltage relay, and undercurrent relay or any type of device, operates at approximately synchronous speed of a machine.

14. UNDER-SPEED DEVICE functions when the speed of a machine falls below a predetermined value.

15. SPEED OR FREQUENCY-MATCHING DEVICE functions to match and hold the speed or the frequency of a machine or of a system equal to, of approximately equal to, that of another machine, source or system.

16. Reserved for future application.

17. SHUNTING OR DISCHARGE SWITCH serves to open or to close a shunting circuit around any piece of apparatus (except a resistor), such as a machine field, a machine armature, a capacitor or a reactor. Note: This excludes devices which perform such shunting operations as may be necessary in the process of starting a machine by devices 6 or 42, or their equivalent, and also excludes device 73 function which serves for the switching of resistors.

18. ACCELERATING OR DECELERATING DEVICE is used to close or to cause the closing of circuits which are used to increase or to decrease the speed of a machine.

19. STARTING-TO-RUNNING TRANSITION CONTACTOR is a device which operates to initiate or cause the automatic transfer of a machine from the starting to the running power connection.

20. ELECTRICALLY OPERATED VALVE is an electrically operated, controlled or monitored valve in a fluid line. Note: The function of the valve may be indicated by the use of the suffixes.

21. DISTANCE RELAY is a device which functions when the circuit admittance, impedance, or reactance increases or decreases beyond predetermined limits.

22. EQUALIZER CIRCUIT BREAKER is a breaker which serves to control or to make and break the equalizer or the current-balancing connections for a machine field, or for regulating equipment, in a multiple-unit installation.

23. TEMPERATURE CONTROL DEVICE functions to raise or lower the temperature of a machine or other apparatus, or of any medium, where its temperature falls below, or rises above, a predetermined value. Note: An example is a thermostat which switches on a space heater in a switchgear assembly when the temperature falls to desired value as distinguished from a device which is used to provide automatic temperature regulation between close limits and would be designated as 90T.

24. Reserved for future application.

25. SYNCHRONIZING OR SYNCHRONISM-CHECK DEVICE operates when two ac circuits are within the desired limits of frequency, phase angle or voltage, to permit or to cause the paralleling of these two circuits.

26. APPARATUS THERMAL DEVICE functions when the temperature of the shunt field or the armortisseur winding of a machine, or that of a load limiting or load shifting resistor or of a liquid or other medium exceeds a predetermined value; or if the temperature of the protected apparatus, such as a power rectifier, or of any medium decreases below a predetermined value.

27. UNDER VOLTAGE RELAY is a device which functions on a given value of under voltage.

28. FLAME DETECTOR is a device that monitors the presence of the pilot or main flame in such apparatus as a gas turbine or a steam boiler.

29. ISOLATING CONTACTOR is used expressly for disconnecting one circuit from another for the purposes of emergency operation, maintenance, or test.

30. ANNUNCIATOR RELAY is a non-automatically reset device that gives a number of separate visual indications upon the functioning of protective devices, and which may also be arranged to perform a lockout function.

31. SEPARATE EXCITATION DEVICE connects a circuit such as the shunt field of a synchronous converter, to a source of separate excitation during the starting sequence; or one which energizes the excitation and ignition circuits of a power rectifier.

32. DIRECTIONAL POWER RELAY is one which functions on a desired value of power flow in a given direction, or upon reverse power resulting from arc back in the anode or cathode circuits of a power rectifier.

33. POSITION SWITCH makes or breaks contact when the main device or piece of apparatus, which has no device function number, reaches a given position.

34. MASTER SEQUENCE DEVICE is a device such as a motor-operated multi-contact switch, or the equivalent, or a programming device, such as a computer, that establishes or determines the operating sequence of the major devices in an equipment during starting and stopping or during other sequential switching operations.

35. BRUSH-OPERATING, OR SLIP-RING-SHORT-CIRCUITING, DEVICE is used for raising, lowering, or shifting the brushes of a machine, or for short-circuiting its slip rings, or for engaging or disengaging the contacts of a mechanical rectifier.

36. POLARITY OR POLARIZING VOLTAGE DEVICE operates or permits the operation of another device on a predetermined polarity only or verifies the presence of a polarizing voltage in an equipment.

37. UNDERCURRENT OR UNDER POWER RELAY functions when the current or power flow decreases below a predetermined value.

38. BEARING PROTECTIVE DEVICE functions when the current or power flow decreases below a pre-determined value.

39. MECHANICAL CONDITION MONITOR is a device that functions upon the occurrence of an abnormal mechanical condition, such as excessive vibration, eccentricity, expansion, shock, tilting, or seal failure.

40. FIELD RELAY functions on a given or abnormally low value or failure of machine field current, or on an excessive value of the reactive component of armature current in an AC machine indicating abnormally low field excitation.

41. FIELD CIRCUIT BREAKER is a device which functions to apply, or to remove, the field excitation of a machine.

42. RUNNING CIRCUIT BREAKER is a device whose principal function is to connect a machine to its source of running or operating voltage. This function may also be used for a device, such as a contactor, that is used in series with a circuit breaker or other fault protecting means, primarily for frequent opening and closing of the circuit.

43. MANUAL TRANSFER OR SELECTOR DEVICE transfers the control circuits so as to modify the plan of operation of the switching equipment of some of the devices.

44. UNIT-SEQUENCE STARTING RELAY is a device which functions to start the next available unit in a multiple-unit equipment on the failure or on the non-availability of the normally preceding unit.

45. ATMOSPHERIC CONDITION MONITOR is a device that functions upon the occurrence of an abnormal atmospheric condition, such as damaging fumes, explosive mixtures, smoke, or fire.

46. REVERSE-PHASE, OR PHASE-BALANCE, CURRENT RELAY is a relay which functions when the polyphase currents are of reverse-phase sequence, or when the polyphase currents are unbalanced or contain negative phase-sequence components above a given amount.

47. PHASE-SEQUENCE VOLTAGE RELAY functions upon a predetermined value of poly-phase voltage in the desired phase sequence.

48. INCOMPLETE SEQUENCE RELAY is a relay that generally returns the equipment to the normal, or off, position and locks it out if the normal starting, operating, or stopping sequence is not properly completed within a predetermined time. If the device is used for alarm purposes only, it should preferably be designated as 48A (alarm).

49. MACHINE, OR TRANSFORMER, THERMAL RELAY is a relay that functions when the temperature of a machine armature, or other load carrying winding or element of a machine, or the temperature of a power rectifier or power transformer (including a power rectifier transformer) exceeds a predetermined value.

50. INSTANTANEOUS OVERCURRENT, OR RATE-OF-RISE RELAY is a relay that functions instantaneously on an excessive value of current, or on an excessive rate of current rise, thus indicating a fault in the apparatus or circuit being protected.

51. AC TIME OVERCURRENT RELAY is a relay with either a definite or inverse time characteristic that functions when the current in an AC circuit exceeds a predetermined value.

52. AC CIRCUIT BREAKER is a device that is used to close and interrupt an AC power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

53. EXCITER OR DC GENERATOR RELAY is a relay that forces the de machine field excitation to build up during starting or which functions when the machine voltage has built up to a given value.

54. Reserved for future application.

55. POWER FACTOR RELAY is a relay that operates when the power factor in an AC circuit rises above or below a predetermined value.

56. FIELD APPLICATION RELAY is a relay that automatically controls the application of the field excitation to an AC motor at some predetermined point in the slip cycle.

57. SHORT-CIRCUITING OR GROUNDING DEVICE is a primary circuit switching device that functions to short circuit or to ground a circuit in response to automatic or manual means.

58. RECTIFICATION FAILURE RELAY is a device that functions if one or more anodes of a power rectifier fail to fire, or to detect an arc-back or on failure of a diode to conduct or block properly.

59. OVERVOLTAGE RELAY is a relay that functions on a given value of overvoltage.

60. VOLTAGE OR CURRENT BALANCE RELAY is a relay that operates on a given difference in voltage, or current input or output of two circuits.

61. Reserved for future application.

62. TIME-DELAY STOPPING OR OPENING RELAY is a time-delay relay that serves in conjunction with the device that initiates the shutdown, stopping, or opening operation in an automatic sequence.

63. PRESSURE SWITCH is a switch which operates on given values or on a given rate of change of pressure.

64. GROUND PROTECTIVE RELAY is a relay that functions on failure of the insulation of a machine, transformer or of other apparatus to ground, or on flashover of a de machine to ground. Note: This function is assigned only to a relay which detects the flow of current from the frame of a machine or enclosing case or structure of a piece of apparatus to ground, or detects a ground on a normally ungrounded winding or circuit. It is not applied to a device connected in the secondary circuit or secondary neutral of a current transformer, or in the secondary neutral of current transformer connected in the power circuit of a normally grounded system.

65. GOVERNOR is the assembly of fluid, electrical, or mechanical control equipment used for regulating the flow of water, steam, or other medium to the prime mover for such purposes as starting, holding speed or load, or stopping.

66. NOTCHING OR JOGGING DEVICE functions to allow only a specified number of operations of a given device, or equipment, or specified number of successive operations within a given time of each other. It also functions to energize a circuit periodically or for fractions of specified time intervals, or that is used to permit intermittent acceleration or jogging of a machine at low speeds for mechanical positioning.

67. AC DIRECTIONAL OVERCURRENT RELAY is a relay that functions on a desired value of ac overcurrent flowing in a predetermined direction.

68. BLOCKING RELAY is a relay that initiates a pilot signal for blocking or tripping on external faults in a transmission line or in other apparatus under predetermined conditions, or cooperates with other devices to block tripping or to block reclosing on an out-of-step condition or on power swings.

69. PERMISSIVE CONTROL DEVICE is generally a two-position, manually operated switch that in one position permits the closing of a circuit breaker or the placing of an equipment into operation, and in the other position prevents the circuit breaker or the equipment from being operated.

70. RHEOSTAT is a variable resistance device used in an electric circuit, which is electrically operated or has other electrical accessories, such as auxiliary, position, or limit switches.

71. LEVEL SWITCH is a switch which operates on given values, or on a given rate of change, of level.

72. DC CIRCUIT BREAKER is used to close and interrupt a de power circuit under normal conditions or to interrupt this circuit under fault or emergency conditions.

73. LOAD-RESISTOR CONTACTOR is used to shunt or insert a 'step of load limiting, shifting, or indicating resistance in a power circuit, or to switch a space heater in circuit, or to switch a light, or regenerative load resistor of a power rectifier or other machine in and out of circuit.

74. ALARM RELAY is a device other than an annunciator, which is used to operate, or to operate in connections with, a visual or audible alarm.

75. POSITION CHANGING MECHANISM is a mechanism that is used for moving a main device from one position to another in an equipment; as for example, shifting a removable circuit breaker unit to and from the connected, disconnected, and test positions.

76. DC OVERCURRENT RELAY is a relay that functions when the current in a dc circuit exceeds a given value.

77. PULSE TRANSMITTER is used to generate and transmit pulses over a telemetering or pilot-wire circuit to the remote indicating or receiving device.

78. PHASE ANGLE MEASURING, OR OUT-OR-STEP PROTECTIVE RELAY is a relay that functions at a predetermined phase angle between two voltages or between two currents or between voltage and current.

79. AC RECLOSING RELAY is a relay that controls the automatic reclosing and locking out of an AC circuit interrupter.

80. FLOW SWITCH is a switch which operates on given values, or on a given rate of change, of flow.

81. FREQUENCY RELAY is a relay that responds to the frequency of an alternating electrical input quantity.

82. DC RECLOSING RELAY is a relay that controls the automatic closing and reclosing of a DC circuit interrupter, generally in response to load circuit conditions.

83. AUTOMATIC SELECTIVE CONTROL OR TRANSFER RELAY is a relay that operates to select automatically between certain sources or conditions in an equipment, or performs a transfer operation automatically.

84. OPERATING MECHANISM is the complete electrical mechanism or servo-mechanism, including the operating motor, solenoids, position switches, etc., for a tap changer, induction regulator or any similar piece of apparatus which has no device function number.

85. CARRIER OR PILOT-WIRE RECEIVER RELAY is a relay that is operated or restrained by a signal used in connection with carrier-current or dc pilot-wire fault directional relaying.

86. LOCKING-OUT RELAY is an electrically operated, hand or electrically reset relay that functions to shut down and hold an equipment out of service on the occurrence of abnormal conditions.

87. DIFFERENTIAL PROTECTIVE RELAY is an electrically operated, hand or electrically reset, relay that functions to shut down and hold an equipment out of service on the occurrence of abnormal conditions.

88. AUXILIARY MOTOR OR MOTOR GENERATOR is one used for operating auxiliary equipment such as pumps, blowers, exciters, rotating magnetic amplifiers, etc.

89. LINE SWITCH is used an a disconnecting load-interrupter, or isolating switch in an AC or DC power circuit, when this device is electrically operated or has electrical accessories, such as an auxiliary switch, magnetic lock, etc.

90. REGULATING DEVICE functions to regulate a quantity, or quantities, such as voltage, current, power, at a speed, frequency, temperature, and load, at a certain value or between certain (generally close) limits for machines, tie lines or other apparatus.

91. VOLTAGE DIRECTIONAL RELAY is a relay that operates when the voltage across an open circuit break or contactor exceeds a given value in a given direction.

92. VOLTAGE AND POWER DIRECTIONAL RELAY is a relay that permits or causes the connection of two circuits when the voltage difference between them exceeds a given value in a pre-determined direction and causes these two circuits to be disconnected from each other when the power flowing between them exceeds a given value in the opposite direction.

93. FIELD CHANGING CONTACTOR functions to increase or decrease in one step the value of field excitation on a machine.

94. TRIPPING OR TRIP-FREE RELAY functions to trip a circuit breaker, contactor or equipment, or to permit immediate tripping by other devices; or to prevent immediate re closure of a circuit interrupter, in case it should open automatically even though its closing circuit is maintained closed.

TRANSFORMER EXTERNAL FAULTS

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 protective devices that are designed for this purpose. Power frequency over-voltages occur due to an emergency operating condition, such as a sudden loss of load on an isolated portion the system that causes the voltage to rise. This condition causes over-ftuxing of the transformer and an increase in stress on the winding insulation. Over-fluxing increases iron losses and may result in a large increase in exciting current. Such conditions result in rapid heating of the iron circuits of the transformer, with possible damage to core lamination insulation and even to winding insulation.

• UNDER FREQUENCY

Under frequency also is caused by a major system disturbance that causes an imbalance between generation and load. The condition is similar to overvoltage in that exciting current is greatly increased at low frequencies, causing over-fluxing of the transformer iron circuits. The transformer may be able to continue operation at either high voltage or under-frequency, but the two conditions experienced at the same time could be very serious. Usually, the ratio of voltage to frequency should not be allowed to exceed 1.1 per unit, which is usually called a "volts per hertz" limit.

• EXTERNAL SYSTEM SHORT CIRCUITS

System faults that are external to the transformer protection zone, but cause high transformer currents, can cause transformer winding damage. Large external fault currents cause high mechanical stress in the transformer windings, with the maximum stress occurring during the first cycle. This short time frame makes it almost impossible to protect the transformer from experiencing these stresses. The protection strategy for these events is, therefore, a matter of transformer design.

Most of the foregoing conditions are often ignored in specifying transformer relay protection, depending on the criticality of the transformer and its importance in the system. The exception is protection against over-fluxing, which may be provided by devices called "volts per hertz" relays, which detect either high voltage or under frequency, or both, and will disconnect the transformer if this quantity exceed a given limit, which is usually 1.1 per unit.

GENERATOR OVERHEATING PROTECTION

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 input and output cooling medium to note any marked changes in the readings. Smaller generators are often provided with "replica" type temperature estimating devices that use stator current in a heat storage enclosure to estimate actual machine temperature.

All of these devices are used to alarm the operator of possible serious problems. At unattended stations, the output of the temperature indicator may be used to shut down the unit.

GENERATOR OVERVOLTAGE PROTECTION

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 are designed to limit over-speed to low values.
Steam turbine generator units are not always provided with over-speed or overvoltage protection, but this type of protection is often recommended for hydro units or combustion turbine units. In many cases, the desired protection is provided by the voltage regulating equipment.

If not, it can be provided by overvoltage relays or over-frequency relays. Overvoltage relays should have a time delay and a pickup of about 110% of rated voltage. An instantaneous unit is sometimes provided with a pickup of 130%-150% of rated voltage. Some types are compensated for varying frequency and should be supplied from a voltage transformer that is different from that supplying the voltage regulator. Some of these relays are used to simply insert a large resistance in the exciter field circuit.

GENERATOR BACKUP PROTECTION

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 voltage transformer location, thereby sensing both generator and GSU transformer faults. This type of relay can also be set to reach through the transformer, making it operable for phase faults on the high voltage side of the transformer. Coordination is achieved through time delay.

GENERATOR ROTOR PROTECTION