Fuses

FUSESThe same function as performed by CB can also be performed bya fuse, though with lesser reliability and efficiency. Invented in1890 by Edison, fuse is the cheapest form of protection againstexcessive currents. Many improvements have been made sincethe invention of first crude model Now-a-days, several types offuses are available which find extensive use in low to moderatevoltage applications subject to condition:-

(a) where frequent operations are not expected (b) where the use of circuit breaker is uneconomical.

In this chapter, we shall confine our attention to the various typesof fuses and their applications in the fast expanding powersystem.

A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit.

WORKING

The fuse element is generally made of materials having low melting point, high conductivity and least deterioration due to oxidation e.g., silver, copper etc. It is inserted in series with the circuit to be protected. Under normal operating conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal current without overheating. However, when a short-circuit or overload occurs, the current through the fuse increases beyond its rated value. This raises the temperature and 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.

The time required to blow out the fuse depends upon the magnitude of excessive current. Thegreater the current, the smaller is the time taken by the fuse to blow out.

In other words, a fuse has inverse time-current characteristics as shown in the above Fig. Such a characteristic permits its use for over current protection.

Advantages

(i) It is the cheapest form of protection available (ii) no maintenance.(iii) Its operation is inherently completely automatic unlike acircuit breaker which requires an elaborate equipment forautomatic action.(iv) It can break heavy short-circuit currents without noise orsmoke.(v) The smaller sizes of fuse element impose a current limitingeffect under short-circuit conditions.(vi) The inverse time-current characteristic of a fuse makes itsuitable for over current protection.(vii)The minimum time of operation can be made much shorterthan with the circuit breakers.

Disadvantages

(i) Considerable time is lost in rewiring or replacing a fuse after operation.

(ii) On heavy short-circuits, *discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned.

(iii) The current-time characteristic of a fuse cannot always be co-related with that of the protected

Desirable Characteristics of Fuse Element The function of a fuse is to carry the normal current without overheating but when the current exceeds its normal value, it rapidly heats up to melting point and disconnects the circuit protected by it. In order that it may perform this function satisfactorily, the fuse element should have the following desirable characteristics :-

(i) low melting point e.g., tin, lead.(ii) high conductivity e.g., silver, copper.(iii) free from deterioration due to oxidation e.g., silver.(iv) low cost e.g., lead, tin, copper.

The above discussion reveals that no material possesses all the characteristics. For instance, lead has low melting point but it has high specific resistance and is liable to oxidation. Similarly, copperhas high conductivity and low cost but oxidises rapidly. Therefore, a compromise is made in the selection of material for a fuse.

Fuse Element Materials The most commonly used materials for fuse element are lead, tin, copper, zinc and silver. For small currents up to 10 A, tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For larger currents, copper or silver is employed. It is a usual practice to prefer to tin than the copper to protect it from oxidation. Zinc (in strip form only) is good if a fuse with considerable time-lag is required i.e., one which does not melt very quickly with a small overload. The present trend is to use silver despite its high cost due to the following reasons :-

(i) It is comparatively free from oxidation.(ii) It does not deteriorate when used in dry air.(iii) The coefficient of expansion of silver is so small that no critical fatigue occurs. Therefore, the fuse element can carry the rated current continuously for a long time.

(iv) The conductivity of silver is very high. Therefore, for a given rating of fuse element, the mass of silver metal required is smaller than that of other materials. This minimises the problem of clearing the mass of vapourised material set free on fusion and thus permits fast operating speed. (v) Due to comparatively low specific heat, silver fusible elements can be raised from normal temperature to vapourisation quicker than other fusible elements. Moreover, the resistance of silver increases abruptly as the melting temperature is reached, thus making the transition from melting to vapourisation almost instantaneous. Consequently, operation becomes very much faster at higher currents.(vi) Silver vapourises at a temperature much lower than the one at which its vapour will readily ionise. Therefore, when an arc is formed through the vapourised portion of the element, the arc path has high resistance. As a result, short-circuit current is quickly interrupted.

Important TermsThe following terms are much used in the analysis of fuses :(i) Current rating of fuse element. It is the current which the fuse element can normally carry without overheating or melting. It depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse.(ii) Fusing current. It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it. Obviously, its value will be more than the current rating of the fuse element. For a round wire, the approximate relationship between fusing current I and diameter d of the wire is

I = k d3/2

where k is a constant, called the fuse constant. Its value depends upon the metal of which the fuse element is made. Sir W.H. Preece found the value of k for different materials as given in the table below :

The fusing current depends upon the various factors such as :(a) material of fuse element(b) length – the smaller the length, the greater the current because a short fuse can easily conduct away all the heat(c) diameter(d) size and location of terminals(e) previous history(f) type of enclosure used

(iii) Fusing factor. It is the ratio of minimum fusing current to the current rating of the fuse element i.e.

Its value is always more than one. The smaller the fusing factor, the greater is the difficulty in avoiding deterioration due to

overheating and oxidation at rated carrying current. For a semi-enclosed or rewirable fuse which employs copper wire as the fuse element, the fusing factor is usually 2. Lower values of fusing factor can be employed for enclosed type cartridge fuses using silver or bimetallic elements.

iv) Prospective Current. Fig. shows how a.c. current is cut off by a fuse. The fault current would normally have a very large first loop, but it actually generates sufficient energy to melt the fuse able element well before the peak of this first loop is reached. The r.m.s. value of the first loop of fault current is known as prospective current.

Therefore, prospective current can be defined as under: It is the r.m.s. value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance.

(v) Cut-off current. It is the maximum value of fault current actually reached before the fuse melts. On the occurrence of a fault, the fault current has a very large first loop due to a fair degree of asymmetry. The heat generated is sufficient to melt the fuse element well before the peak of first loop is reached (point ‘a’ in Fig. 20.2). The current corresponding to point ‘a’ is the cut off current. The cut off value depends upon :

(a) current rating of fuse(b) value of prospective current(c) asymmetry of short-circuit current

It may be mentioned here that outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak. This gives the fuse a great advantage over a circuit breaker since the most severe thermal and electro-magnetic effects of short-circuit currents (which occur at the peak value of prospective current) are not experienced with fuses. Therefore, the circuits protected by fuses can be designed to withstand maximum current equal to the cut-off value. This consideration together with the relative cheapness of fuses allows much saving in cost.

(vi) Pre-arcing time. It is the time between the commencement of fault and the instant when cut off occurs. When a fault occurs, the fault current rises rapidly and generates heat in the fuse element. As the fault current reaches the cut off value, the fuse element melts and an arc in initiated. The time from the start of the fault to the instant the arc is initiated is known as pre-arcing time. The pre-arcing time is generally small : a typical value being 0·001second

(vii) Arcing time. This is the time between the end of pre-arcing time and the instant when the arc is extinguished.

(viii) Total operating time. It is the sum of pre-arcing and arcing times. It may be noted that operating time of a fuse is generally quite low (say 0·002 sec.) as compared to a circuit breaker (say 0·2 sec or so). This is an added advantage of a fuse over a circuit breaker. A fuse in series with a circuit breaker of low-breaking capacity is a useful and economical arrangement to provide adequate short-circuit protection. It is because the fuse will blow under fault conditions before the circuit breaker has the time to operate.

(ix) Breaking capacity. It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage

Types of Fuses Fuse is the simplest current interrupting device for protection against excessive currents. Since the invention of first fuse by Edison, several improvements have been made and now-a-days, a variety of fuses are available. Some fuses also incorporate means for extinguishing the arc that appears when the fuse element melts. In general, fuses may be classified into :

(i)Low voltages fuses (ii)(ii) High voltage fuses

It is a usual practice to provide isolating switches in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety. If such means of isolation are not available, the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse carrier is being inserted or removed.

Low Voltage Fuses Low voltage fuses can be subdivided into two classes viz., (i) semi-enclosed rewireable fuse (ii) highrupturing capacity (H.R.C.) cartridge fuse.1. Semi-enclosed rewireable fuse. Rewireable fuse (also known as kit-kat type) is used where low values of fault current are to be interrupted. It consists of (i) a base and (ii) a fuse carrier. The base is of porcelain and carries the fixed contacts to which the incoming and out going phase wires are connected. The fuse carrier is also of porcelain and holds the fuse element (tinned copper wire) between its terminals. The fuse carrier can be inserted in or taken out of the base when desired. When a fault occurs, the fuse element is blown out and the circuit is interrupted. The fuse carrier is taken out and the blown out fuse element is replaced by the new one. The fuse carrier is then reinserted in the base to restore the supply. This type of fuse has two advantages. Firstly, the detachable fuse carrier permits the replacement of fuse element without any danger of coming in contact with live parts. Secondly, the cost of replacement is negligible.

Disadvantages

(i) There is a possibility of renewal by the fuse wire of wrong size or by improper material.(ii) This type of fuse has a low-breaking capacity and hence cannot be used in circuits of high fault level.(iii) The fuse element is subjected to deterioration due to oxidation through the continuous heating up of the element. Therefore, after some time, the current rating of the fuse is decreased i.e., the fuse operates at a lower current than originally rated.(iv) The protective capacity of such a fuse is uncertain as it is affected by the ambient conditions.(v) Accurate calibration of the fuse wire is not possible because fusing current very much depends upon the length of the fuse element. Semi-enclosed rewireable fuses are made upto 500 A rated current, but their breaking capacity is low e.g., on 400 V service, the breaking capacity is about 4000 A. Therefore, the use of this type of fuses is limited to domestic and lighting loads.

High-Rupturing capacity (H.R.C.) cartridge fuse. The primary objection of low and uncertain breaking capacity of semi-enclosed rewireable fuses is overcome in H.R.C. cartridge fuse. Fig. shows the essential parts of a typical H.R.C. cartridge fuse. It consists of a heat resisting ceramic body having metal end-caps to which is welded silver current-carrying element . The space within the body surrounding the element is completely packed with filling powder ( itMay be chalk, quartz/marble dust, plaster of paris, act as quinching & cooling medium).

Therefore, it carries the normal current without overheating. When a fault occurs, the current increases and the fuse element melts before the fault current reaches its first peak. The heat produced in the process vapourises the melted silver element. The chemical reaction between the silver vapour and the filling powder results in the formation of a high resistance substance which helps in quenching the arc.

Advantages(i) They are capable of clearing high as well as low fault currents.(ii) They do not deteriorate with age.(iii) They have high speed of operation.(iv) They provide reliable discrimination.(v) They require no maintenance.(vi) They are cheaper than other circuit interrupting devices of equal breaking capacity.(vii) They permit consistent performance. Disadvantages(i) They have to be replaced after each operation.(ii) Heat produced by the arc may affect the associated switches.

Sometime, H.R.C. cartridge fuse is provided with a tripping device. When the fuse blows out under fault conditions, the tripping device causes the circuit breaker to operate Fig shows the essential parts of a H.R.C. fuse with a tripping device. The body of the fuse is of ceramic mtrl with metallic cap rigidly fixed at each end.

H.R.C. fuse with tripping device

These are connected by a number of silver fuse elements. At one end is a plunger which under fault conditions hits the tripping mechanism of the circuit breaker and causes it to operate. The plunger is electrically connected through a fusible link, chemical charge and a tungsten wire to the other end of the cap as shown. When a fault occurs, the silver fuse elements are the first to be blown out and then current is transferred to the tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charge to be detonated. This forces the plunger to move outward to operate CB. The travel of plunger is so set that it does not come out under fault cond.

Advantages. H.R.C. fuse with a tripping device has the following advantages over a H.R.C. fuse without tripping device :(i) In case of a single phase fault on a three-phase system, the plunger operates the tripping mechanism of circuit breaker to open all the three phases and thus prevents “single phasing”.(ii) The effects of full short circuit current need not be considered in the choice of circuit breaker. This permits the use of a relatively inexpensive circuit breaker.(iii) The fuse-tripped circuit breaker is generally capable of dealing with fairly small fault currents itself. This avoids the necessity for replacing the fuse except after highest currents for which it is intended.

Low voltage H.R.C. fuses may be built with a breaking capacity of 16,000 A to 30,000 A at 440V. They are extensively used on low-voltage distribution system against over-load and short circuit conditions.

HRC FUSES

High Voltage Fuses The low-voltage fuses discussed so far have low normal current rating and breaking capacity. Therefore, they cannot be successfully used on modern high voltage circuits. Intensive research by the manufacturers and supply engineers has led to the development of high voltage fuses. Some of the high voltage fuses are :(i) Cartridge type. This is similar in general construction to the low voltage cartridge type except that special design features are incorporated. Some designs employ fuse elements wound in the form of a helix so as to avoid corona effects at higher voltages. On some designs, there are two fuse elements in parallel ; one of low R (Ag wire) and the other of high R (thungston wire). Under normal load cond, the low resistance element carries the normal current. When fault occurs, the low R element is blown out and the high R element reduces the s/c current and finally breaks the circuit. HV cartridge fuses are used upto 33 kV with breaking capacity of about 8700 A at that voltage. Rating of the order of 200 A at 6·6 kV and 11 kV and 50 A at 33 kV are also available.

(ii) Liquid type. These fuses are filled with carbon tetrachloride and have the widest range of application to h.v. systems. They may be used for circuits upto about 100 A rated current on systems upto 132 kV and may have breaking capacities of the order of 6100 A. Fig shows the essential parts of the liquid fuse. It consists of a glass tube filled with carbon tetrachloride solution and sealed at both ends with brass caps. The fuse wire is sealed at one end of the tube and the other end of the wire is held by a strong phosphor bronze spiral spring fixed at the other end of the glass tube. When the current exceeds the prescribed limit, the fuse wire is blown out. As the fuse melts, the spring retracts part of it through a baffle (or liquid director) and draws it well in to the liquid.

The small quantity of gas generated at the point of fusion forces some part of liquid into the passage through baffle and there it effectively extinguishes the arc.

(iii) Metal clad fuses. Metal clad oil-immersed fuses have been developed with the object of providing a substitute for the OCB. Such fuses can be used for very high voltage circuits and operate most satisfactorily under short-circuit conditions approaching their rated capacity.

Current Carrying Capacity of Fuse ElementThe current carrying capacity of a fuse element mainly depends on the metal used and the crosssectional area but is affected also by the length, the state of surface and the surroundings of the fuse. When the fuse element attains steady temperature,

Above expression is known as ordinary fuse law.

Example A fuse wire of circular cross-section has a radius of 0·8 mm. The wire blows off at a current of 8A. Calculate the radius of the wire that will blow off at a current of 1A.

Difference Between a Fuse and Circuit Breaker

PROTECTIVE RELAY

a power system consisting of generators,transformers, txn and distr circuits, When a failure occurs on any part of the system, it must be quickly detected and disconnectedfrom the system.

There are two principal reasons for it. (a) If the fault is not cleared quickly, it may cause unnecessary

interruption of service to the customers. (b) rapid disconnection of faulted apparatus limits the amount

of damage to it and prevents the effects of fault from spreading into the system ‘

The detection of a fault and disconnection of a faulty section or apparatus can be achieved by using fuses or relays in conjunction with circuit breakers.

A fuse performs both detection and interruption functions automatically but its use is limited for the protection of low-voltage 3·3 kV), relays and circuit breakers are employed to serve the desired function of automatic protective gear.

The relays detect the fault and supply information to the circuit breaker which performs the function of circuit interruption.

Protective Relays

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system. The relays detect the abnormal cond in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faulty circuit.

A typical relay circuit is shown in Fig. This diagram shows one phase of 3-phase system for simplicity. The relay circuit connections can be divided into three parts viz.(i) First part is the primary winding of a current transformer (C.T.) which is connected in series with the line to be protected.(ii) Second part consists of sec winding of C.T. and the relay op coil.(iii) Third part is the tripping circuit which may be either a.c.or d.c. It consists of a source of supply, the trip coil of the circuitbreaker and the relay stationary contacts. When a short circuit occurs at point F on the transmission line, the current flowing in the line increases to an enormous value. This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts.

Fundamental Requirements of Protective RelayingThe principal function of protective relaying is to cause the prompt removal from service of any element of the power system when it starts to operate in an abnormal manner or interfere with theeffective opof the rest of the system. In order that protective relay system may perform this function satisfactorily, it should have the following qualities :(i) selectivity (ii) speed (iii) sensitivity(iv) Reliability (v) simplicity (vi) economy(i) Selectivity. It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system. A well designed and efficient relay system should be selective i.e. it should be able to detect the point at which the fault occurs and cause the opening of the circuit breakers closest to the fault withminimum or no damage to the system.

This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the circuit eqpt from damage and normal working of the healthy portion of the system. Fundamental Requirements of Protective Relaying

The principal function of protective relaying is to cause the prompt removal from service of any element of the power system when it starts to operate in an abnormal manner or interfere with the eff op of the rest of the system. In order that protective relay system may perform this function satisfactorily, it should have the following qualities :(i)Selectivity (ii) speed (iii) sensitivity

(iv) reliability (v) simplicity (vi) economy

This can be illustrated diagram SHOWING portion of a typical power sys. It may be seen that cb are loc in the connections to each power sys element in order to make it possible to disconnectonly the faulty section. Thus, if a fault occurs at bus-bars on the last zone, then only breakers nearest to the fault viz. 10, 11, 12.

should open. In fact, opening of any other breaker to clear the fault will lead to a greater part of the system being disconnected.

In order to provide selectivity to the system, it is a usual practice to divide the entire system into several protection zones. When a fault occurs in a given zone, then only the circuit breakers withinthat zone will be opened. This will isolate only the faulty circuit orapparatus, leaving the healthy circuits intact. The system can be divided into the following protection zones :(a) generators (b) low-tension switchgear (c) transformers(d) high-tension switchgear (e) transmission lines It may be seen in above Fig a certain amount of overlap between the adjacent protection zones. For a failure within the region where two adjacent zones overlap, more breakers will be opened than the minimum necessary to disconnect the faulty section. But if there were no overlap, a failure in the region between zones would not lie in either region and, therefore, no breaker would be opened. For this reason, a certain amount of overlap* is provided between the adjacent zones.

Speed. The relay system should disconnect the faulty section as fast as possible for the following reasons :(a) Electrical apparatus may be damaged if they are made to carry the fault currents for a long time.(b) A failure on the system leads to reduction in the system voltage. If the faulty section is not disconnected quickly, then the low voltage created by the fault may shut down consumers’motors and the generators on the system may become unstable.(c) The high speed relay system decreases the possibility of devp of one type of fault into the other more severe type.Sensitivity. It is the ability of the relay system to operate with low value of actuating quantities.Sensitivity of a relay is a function of the volt-amperes input to the coil of the relay necessary tocause its op. The smaller the volt-ampere input required to cause relay to op, the more sensitive is the relay. Thus, a 1 VA relay is more sensitive than a 3 VA relay. It is desirable that relaysystem should be sensitive so that it op with low values of volt-ampere input.

Basic RelaysMost of the relays used in the power system operate by virtue of the current and/or voltage supplied by current and voltage transformers connected in various combinations to the system element that is to be protected. Through the individual or relative changes in these two quantities, faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates the trip circuit which results in the opening of the circuit breaker and hence in the disconnection of the faulty circuit.Most of the relays in service on electric power system today are of electro-mechanical type. They work on the following two main operating principles :(i) Electromagnetic attraction (ii) Electromagnetic inductionElectromagnetic Attraction Relays Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by d.c. or a.c.quantities. The important types of electromagnetic attraction relays are:-

(i) Attracted armature type relay. Fig. shows the schematic arng of an attracted armature type relay. It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature. The armature is balanced by a counterweight & carries a pair of spring contact fingers at its free end. Under normal op cond the current through relay coil C is such that counter wt holds the armature in the posn shown

However, when a short-circuit occurs, the current through the relay coil incr sufficiently and the relay armature is attracted upwards. The contacts on the relay armature bridge a pair of stationary contacts attached to the relay frame. This completes the trip circuit which results in the opening of the circuit breaker

and, therefore, in the disconnection of the faulty circuit. The minimum current at which the relay armature is attracted to close the trip circuit is called pickup current. It is a usual practice to provide a number of tapings on the relay coil so that the number of turns in use and hence the setting value at which the relay operates can be varied.

Solenoid type relay. It consists of a solenoid and movable iron plunger arranged as shown. Under normal operating conditions, the current through the relay coil C is such that it holds the plunger by gravity or spring in the position shown. However, on the occurrence of a fault, the current through the relay coil becomes more than the pickup value, causing the plunger to be attracted to the solenoid. The upward movement of the plunger closes the trip circuit, thus opening the circuit breaker and disconnecting the faulty circuit

Balanced beam type relay. Fig showsthe schematic arrangement of a balanced beam type relay. It consists of an iron armature fastened to a balance beam. Under normal operating conditions, the current through the relay coil is such that the beam is held in the horizontal position by the spring. However, when a fault occurs, the current through the relay coil becomes greater than the pickup value andthe beam is attracted to close the trip circuit. This causes the opening of the circuit breaker to isolate the faulty circuit.

Induction Relays Electro magnetic induction relays operate on the principle of induction motor and are widely used for protective relaying purposes involving a.c. quantities. They are not used with d.c. quantities owing to the principle of operation. An induction relay essentially consists of a pivoted aluminum disc

placed in two alternating magnetic fields of the same frequency but displaced in time and space. The torque is produced in the disc by the interaction of one of the magnetic fields with the currents induced in the disc by the other.To understand the production of torque in an induction relay, refer to the elementary arrangement shown in Fig. The two a.c. fluxes φ2 and φ1 differing in phase by an angle α induce e.m.f.s’ in the disc and cause the circulation of eddy currents i2 and i1 respectively. These currents lag behind their respective fluxes by 90 degree.

Referring to Fig. where the two a.c. fluxes and induced currents are shown separately for clarity, letφ1 = φ1max sin ω tφ2 = φ2max sin (ω t + α)where φ1 and φ2 are the instantaneous values of fluxes and φ2 leads φ1 by an angle α. Assuming that the paths inwhich the rotor currents flow have negligible self-inductance, therotor currents will be in phase with their voltages.

The following points may be noted from exp. (i) :(a) The greater the phase angle α between the fluxes, the greater is the net force applied to the disc. Obviously, the maximum force will be produced when the two fluxes are 90o out of phase.(b) The net force is the same at every instant. This fact does not depend upon the assumptions made in arriving at exp. (i) ie rotar current flows have negligible impedances.

(c) The direction of net force and hence the direction of motion of the disc depends upon which flux is leading.

The following three types of structures are commonly used for obtaining the phase difference in the fluxes and hence the operating torque in induction relays :(i) shaded-pole structure(ii) watthour-meter or double winding structure(iii) induction cup structure

Shaded-pole structure. The general arrangement of shaded-pole structure is shown in Fig.. It consists of a pivoted aluminium disc free to rotate in the air-gap of an electromagnet.One half of each pole of the magnet is surrounded by a copper band known as shading ring. The alternating flux φs in the shaded portion of the poles will, owing to the reaction of current induced in

ring, lag behind the flux φu in the unshaded portion by an angle α. These two a.c. fluxes differing in phase will produce the necessary torque to rotate the disc. As proved earlier, the driving torque T is

Watthour-meter structure. This structure gets its name from the fact that it is used in watthour meters. The general arrangement of this type of relay is shown in Fig. It consists of apivoted aluminum disc arranged to rotate freely between the poles of two electromagnets. The upper electro magnet carries two windings

the pirmary and the secondary. The primary winding carries therelay current i1 while the secondary winding is connected to the winding of the lower magnet. The primary current induces e.m.f. in the secondary and so circulates a current i2 in it. The flux φ2 induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind φ1 by an angle α. The two fluxes φ1and φ2 differing in phase by α will produce a driving torque on the disc proportional to φ1φ2 sin α.

An important feature of this type of relay is that its operation can be controlled by opening or closing the secondary winding circuit. If this circuit is opened, no flux can be set by the lower magnethowever great the vaule of current in the pirmary winding may be and consequently no torque will be produced. Therefore, the relay can be made inoperative by opening its secondary winding circuit.

Induction cup structure. Figshows the general arrangement of an induction cup structure. It most closely resembles an induction motor, except that the rotor iron is stationary, only the rotor conductor portion being free to rotate.The moving element is a hollow cylindrical rotor which

turns on its axis. The rotating field is produced by two pairs of coils wound on four poles as shown. The rotating field induces currents in the cup to provide the necessary driving torque. If φ1 and φ2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to φ1φ2 sin α where α is the

Is the difference between the two fluxes. A control spring and the back stop for closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation. Induction cup structures are more efficient torque producers than either the shaded-pole or the watthour meter structures. Therefore, this type of relay has very high speed and may have an operating time less then 0·1 second.

Relay Timing

An important characteristic of a relay is its time of operation. By ‘the time of operation’ is meant length of the time from the instant when the actuating element is energised to the instant when therelay contacts are closed. Sometimes, it is desirable and necessary to control the operating time of a relay. For this purpose, mechanical accessories are used with relays.

Instantaneous relay. An instantaneous relay is one in which no intentional time delay is provided. In this case, the relay contacts are closed immediately after current in the relay coil exceeds the minimum calibrated value. Fig. shows an instantaneous solenoid type of relay.

Although there will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added. The instantaneous relays have operating time less than 0·1 second. The instantaneous relay is effective only where the impedance between the relay and source is small compared to the protected section impedance. The operating time of instantaneous relay is sometimes expressed in cycles based on the power-system frequency e.g. one-cycle would be 1/50 second in a 50-cycle system.

Inverse-time relay. An inverse-time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. Fig shows the time current characteristics of an inverse current relay. At values of current less than pickup, the relay never operates. At higher values, the time of operation of the

relay decreases steadily with the increase of current. The inverse-time delay can be achieved by associating mech accessorieswith relays.

In an induction relay, the inverse-time delay can be achieved by positioning a permanent magnet (known as a drag magnet) in such a way that relay disc cuts the flux between the poles of the magnet. When the disc moves, currents set up in it produce a drag on the disc which slows its motion.

In other types of relays, the inverse time delay can be introduced by oil dashpot or a time limit fuse. Fig. shows an inverse time solenoid relay using oil dashpot. The piston inthe oil dashpot attached to moving plunger slows its upward motion. At a current valuejust equal to the pickup, the plunger moves

slowly and time delay is at a maximum. At higher values of relay current, the delay time is shortened due to greater pull on the plunger.The inverse-time characteristic can also be obtained by connecting a time-limit fuse in parallel with the trip coil terminals as shown in Fig. The shunt path formed by time-limit fuse is of negligible,

impedance as compared with the relatively high impedance of the trip coil. Therefore, so long as the fuse remains intact

it will divert practically the whole secondary current of CT from thetrip oil. When the secondary current exceeds the current carrying capacity of the fuse, the fuse will blow and the whole current will pass through the trip coil, thus opening the circuit breaker. The time lag between the incidence of excess current and the tripping of the breaker is governed by the characteristics of the fuse. Careful selection of the fuse can give the desired inverse-time characteristics, although necessity for replacement after operation is a disadvantage.Definite time lag relay. In this type of relay, there is a definite time elapse between the instant of pickup and closing of relay contacts. This particular time setting is independent of theamount of current through the relay coil ; being the same for all values of current in excess of the pickup value. It may be worth while to mention here that practically all inverse-time relays are also provided with definite minimum time feature in order that the relay may never become instantaneous in its action for very long overloads.

Important TermsIt is desirable to define and explain some important terms much used in connection with relays. Pick-up current. It is the minimum current in the relay coil at which the relay starts to operate. So long as the current in the relay is less than the pick-up value, the relay does not operateand the breaker controlled by it remains in the closed position. However, when the relay coil current is equal to or greater than the pickup value, the relay operates to energise the trip coil which opens the circuit breaker. Current setting. It is often desirable to adjust the pick-up current to any required value. This is known as current setting and is usually achieved by the use of tappings on the relay operating coil. The taps are brought out to a plug bridge as shown in Fig. The plug bridge permits to alter the number of turns on the relay coil. This changes the torque on the disc and hence the time of operation of the relay. The values assigned to each tap are expressed in terms of percentage full-load rating of C.T. with

Current setting

is associated and represents the value above which the disc commences to rotate and finally closes the trip circuit.

∴ Pick-up current = Rated secondary current of C.T. × Current setting

For example, suppose that an over current relay having current setting of 125% is connected to a supply circuit through a current transformer of 400/5. The rated secondary current of C.T is 5amp. Therefore, the pick-up value will be 25% more than 5 A i.e. 5 × 1·25 = 6·25 A. It means that with above current setting, the relay will actually operate for a relay coil current equal to or greater than 6·25 A. The current plug settings usually range from 50% to 200% in steps of 25% for overcurrent relays and 10% to 70% in steps of 10% for earth leakage relays. The desired current setting is obtained by inserting a plug between the jaws of a bridge type socket at the tap value required.

Plug-setting multiplier (P.S.M.). It is the ratio of fault current in relay coil to the pick-up current i.e.

For example, suppose that a relay is connected to a 400/5 current transformer and set at 150%. With a primary fault current of 2400 A, the plug-setting multiplier can be calculated as under :

Pick-up value = Rated secondary current of CT × Current setting = 5 × 1·5 = 7·5 A

Fault current in relay coil = 2400 x 5/400 =30 A

∴ P.S.M. = 30/7·5 = 4

Time-setting multiplier. A relay is generally provided with control to adjust the time of operation. This adjustment is known as time-setting multiplier. The time-setting dial is calibrated from 0 to 1 in steps of 0.05 sec (see Fig. 21.15). These figures are multipliers to be used to convert the time derived from time /P.S.M. curve into the actual op time.

Thus if the time setting is 0·1 and the time obtained from the time /P.S.M. curve is 3 seconds, then actual relay operating time = 3 × 0·1 = 0·3 second . For instance, in an induction relay, the timeof operation is controlled by adjusting the amount of travel of the disc from its reset position to its pickup position. This is achieved by the adjustment of the position of a movable backstop which controls the travel of the disc and thereby varies the time in which the relay will close its contacts for given values of fault current. A so-called “time dial” with an evenly divided scale provides this

adjustment. The acutal time of op is calc by multiplying the with the time obtained from time/P.S.M. curve of the relay.Time/P.S.M. CurveFig shows the curve between time of op and PSM of a typical relay.The horizontal scale is marked in terms of PSM and represents the number of time the relay current is in excess of the current setting. The vertical scale is marked in terms of the timerequired for relay op. If PSM is 10

, then the time of op (from the curve) is 3 seconds. The actual time of op is obtained by multiplying this time by the TSM. It is evident that for lower values of overcurrent, time of op varies inversely with the current but as the current approaches 20 times full-load value the op time of relay tends to become constant. This feature is necessary in order to ensure discrimination on very heavy fault currents flowing through sound feeders.

Calculation of Relay Operating TimeIn order to calculate the actual relay operating time, the following things must be known :(a) Time/P.S.M. curve (b) Current setting(c) Time setting (d) Fault current(e) Current transformer ratioThe procedure for calculating the actual relay operating time is as follows :(i) Convert the fault current into the relay coil current by using the current transformer ratio.(ii) Express the relay current as a multiple of current setting i.e. calculate the P.S.M.(iii) From the Time/P.S.M. curve of the relay, read off the time of operation for the calculated P.S.M.(iv) Determine the actual time of operation by multiplying the above time of the relay by time setting multiplier in use.

Protection of AlternatorsThe generating units, especially the larger ones, are relatively few in number and higher in individual cost than most other eqpts. Therefore, it is desirable and necessary to provide protection tocover the wide range of faults which may occur in the modern generating plant. Some of the important faults which may occur on an alternator are :(i) failure of prime-mover (ii) failure of field(iii) overcurrent (iv) overspeed(v) overvoltage (vi) unbalanced loading

Failure of prime-mover. When input to the prime-mover fails, the alternator runs as a synchronous motor and draws some current from the supply system. This motoring conditions is known as “inverted running”.(a) In case of turbo-alternator sets, failure of steam supply may cause inverted running. If the steam supply is gradually restored, the alternator will pick up load without disturbing the system. If the

steam failure is likely to be prolonged, the machine can be safelyisolated by the control room attendant since this condition is relatively harmless. Therefore, automatic protection is not required(b) In case of hydro-generator sets, protection against inverted running is achieved by providing mechanical devices on the water-wheel. When the water flow drops to an insufficientrate to maintain electrical output, the alternator is disconnected from the system. Therefore, in this case also electrical protection is not necessary.(c) Diesel engine driven alternators, when running inverted, draw a considerable amount of power from the supply system and it is a usual practice to provide protection againstmotoring in order to avoid damage due to possible mechanical seizure. This is achieved by applying reverse power relays to the alternators which isolate the latter during their motoring action. It is essential that the reverse power relays have time-delay in operation in order to prevent inadvertent tripping during system disturbances caused by faulty synchronising and phase swinging.

Failure of field. The chances of field failure of alternators are undoubtedly very rare. Even if it does occur, no imdt damage will be caused by permitting the alternator to run without a field for a short-period. It is sufficient to rely on the control room attendant todisconnect the faulty alternator manually from the system bus-bars. Therefore, it is a universal practice not to provide †automatic protection against this contingency.Overcurrent. It occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system. Overcurrent protection for alternators is considered unnecessary because of the following reasons :(a) The modern tendency is to design alternators with very high values of internal impedance so that they will stand a complete short-circuit at their terminals for sufficient time without serious overheating. On the occurrence of an overload, the alternators can be disconnected manually.(b) The disadvantage of using overload protection for alternators is that such a protection might disconnect the alternators from the.

power plant bus on account of some momentary troubles outside the plant and, therefore, interfere with the continuity of electricservice Overspeed. The chief cause of overspeed is the sudden loss of all or the major part of load on the alternator. Modern alternators are usually provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime-mover when a dangerous over speed occurs.Over-voltage. The field excitation system of modern alternators is so designed that overvoltage conditions at normal running speeds cannot occur. However, overvoltage in an alternator occurs when speed of the prime-mover increases due to sudden loss of the alternator load Unbalanced loading. It means that there are different phase currents in the alternator. It arises from faults to earth or faults between phases on circuit external to the alternator. The unbalanced currents, if allowed to persist, may either severely burn the mech fixings of the rotor core or damage the fd winding.

Unbalanced loading.

connected in parallel. A relay is connected in parallel across the transformer secondaries. Under normal operating condition, equal current flows through the different phases of the alternator andtheir algebraic sum is zero. Therefore, sum of currents flowing in the secondaries is also zero and no current flows through the operating coil of the relay. However, if unbalancing occurs, the currents induced in the secondaries will be different and the resultant of these currents will flow through the relay. The operation of the relay will trip the circuit breaker to disconnect the alternator from the system

Fig shows schematic arng for protection of alternator against unbalanced loading. The scheme comprises three line current transformers, one mounted in each phase, having their secondaries

Stator winding faults. These faults occur mainly due to the insulation failure of the stator windings. The main types of stator winding faults, in order of importance are : (a) fault between phase and ground(b) fault between phases(c) inter-turn fault involving turns of the same phase winding

The stator winding faults are the most dangerous and are likely to cause considerable damage to the expensive machinery. Therefore, automatic protection is absolutely necessary to clear such faults in the quickest possible time in order to minimise the extent of damage If the stator winding fault is not cleared quickly, it may lead to (i) burning of stator coils(ii) burning and welding-up of stator laminations. For protection of alternators against such faults, differential method of protection (also knows as Merz-Price system) is most commonly employed due to its greater sensitivity and reliability. This system of protection is discussed in the following section

Merz-Price circulating current scheme. Identical current transformer pairs CT1 and CT2 are placed oneither side of each phase of the stator windings. The secondaries of each set of CT are connected in star, two neutral points and the terminals of two star groups being connected together by means of a 4core pilot cable. Thus there is an indp path for currents circulating in each pair of CT and the pilot P. The relay coils are connected in star, neutral point being connected to CT common neutral and outer ends one to each of the other three pilots. In order that burden on each CT is the same, relays are connected across equi potential points located at the middle of the pilot wires. The relays are generally of EM type and are arranged for instantaneous action since fault should be cleared as quickly as possible.

Protection of TransformersTransformers are static devices, totally enclosed and generally oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessitates to provide adequate automatic protection for transformers against possible faults. Small distr transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay equipment is required. However,the probability of faults on power transformers is undoubtedly more and hence automatic protection is absolutely necessary Common transformer faults. As compared with generators, in which many abnormal conditions may arise, power transformers may suffer only from :(i) open circuits (ii) overheating(iii) wdg short-circuits. earth-faults, phase-to-phase faults and inter-turn faults.

An open circuit in one phase of a 3-phase transformer may cause undesirable heating. In practice, relay protection is not provided against open circuits because this condition is relatively harmless.On the occurrence of such a fault, the transformer can bedisconnected manually from the system. Overheating of the transformer is usually caused by sustained overloads or short-circuits and very occasionally by the failure of the cooling system. The relay protection is also not providedagainst this contingency and thermal accessories are generally used to sound an alarm or control the banks of fans. Winding short-circuits (also called internal faults) on the transformer arise from deterioration of winding insulation due to overheating or mechanical injury. When an internal fault occurs, the transformer must be disconnected quickly from the system because a prolonged arc in the transformer may cause oil fire. Therefore, relay protection is absolutely necessary for internal faults.

Protection Systems for Transformers the troublesome conditions imposed by the wide variety of voltages, currents and earthing conditions are invariably associated with power transformers. Under such circumstances, alternative protectivesystems are used which in many cases are as effective as the circulating-current system. The principal relays and systems used for transformer protection are :-

(i) Buchholz devices providing protection against all kinds of incipient faults i.e. slow-developing faults such as insulation failure of windings, core heating, fall of oil level due to leaky joints etc.

(ii) Earth-fault relays providing protection against earth-faults only.

(iii) Overcurrent relays providing protection mainly against phase-to-phase faults and overloading

(iv) Differential system (or circulating-current system) providing protection against both earth and phase faults. The complete protection of transformer usually requires combination of these systems. Choice of a particular combination of systems may depend upon several factors such as (a) size of the transformer(b) type of cooling (c) location of transformer in the network (d) nature of load supplied and (e) importance of service for which transformer is required. In the following sections, above sys of protection will be discussed in detail.

Buchholz relay is a gas-actuated relay installed in oil immersed transformers for protection against all kinds of faults. Named after its inventor, Buchholz, it is used to give an alarm in case of incipient (i.e.slow-devp) faults in the transformer and to disconnect transformer from the supply in the event of severe internal faults. It is usually installedin the pipe connecting the conservator to the

the main tank as shown in Fig. It is a universal practice to use Buchholz relay on all such oil immersed transformers having ratings in*excess of 750 kV. Construction. Fig shows constructional details of a Buchholz relay. It takes the form of a domed vessel placed in the connecting pipe between the main tank and the conservator. The device has two elements. The upper element consists of a mercury type switch attached to a float. The lower element contains a mercury switch mounted on a hinged type flap located in the direct path of the

flow of oil from the transformer to the conservator. The upper element closes an alarm circuit during incipient faults whereas the lower element is arranged to trip the circuit breaker in case ofsevere internal faults. Operation. The operation of Buchholz relay is as follows :(i) In case of incipient faults withinTx, the heat due to fault causes the decomposition of some Tx oil in the main tank. The products of decomposition contain more than 70% of H2gas. Being lt gas tries to go into the conservator and in the process gets entrapped in upper part of relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pr on the float to cause it to tilt and close the contacts of mercury switch attached to it. This completes the alarm circuit to sound an alarm. (ii) If a serious fault occurs in Tx, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards conservator via relay and in doing so tilts the flap to close the contacts of Hg switch. This completes trip circuit to open CB controlling the Tx.

Advantages(i) It is the simplest form of transformer protection.(ii) It detects the incipient faults at a stage much earlier than is possible with other forms of protection. Disadvantages(i)It can only be used with oil immersed transformers equipped with conservator tanks (ii)ii) The device can detect only faults below oil level in the transformer. Therefore, separateprotection is needed for connecting cables.

TUTORIAL-2

Q1 A 3-phase transformer of 220/11,000 line volts is connected in star/delta. The CT on 220 V side have a current ratio of 600/5. What should be the CT ratio on11,000 V side ?

Q2 A 3-phase transformer having line voltage ratio of 0·4 kV/11kV is connected in star-delta and CT on 400 V side have a current ratio of 500/5. What must be the ratio of the protective transformers on the 11 kV side ?