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Discrimination by time and direction: If the current coil of a wattmeter is carrying the current in a circuit and its voltage coil is across the voltage of that circuit, the instrument will deflect in one or other direction according to the direction of power flow in the circuit. This will provide a discriminative feature if contacts are added to the wattmeter, thereby making it a directional relay; the contacts being used to permit or to inhibit the operation of another relay, which is not inherently directional.

This second relay usually operates after a time lag, so that the combination of the directional element and this second relay offers discrimination by time and direction. All the relays shown are non-directional, have the same current setting, and the time lags shown. By trial of the response of the relays to faults in various positions on the ring main it will be found that proper discrimination cannot be obtained, nor can this be improved by varying the time lags from those shown in the figure.

Now turn to Fig. Trial of the effects of faults anywhere on the ring main show that a fault occurring on any section of ring main will be diseriminatively cleared by the relays, and there will be no loss of supply. In the case of time discrimination it is seen that faults near to the source, which are the more severe, are held on the system for relatively long periods. These limitations can be reduced if the relays which control circuit-breakers are made such that they will measure the distance from the circuit-breaker location to the fault.

If this distance is less than that to the next circuitbreaker out from the source, the fault is within the section controlled by the breaker concerned, and this will trip. If the distance is greater than that to the next breaker out from the source, the fault is beyond the section controlled by the breaker concerned, and it will not trip. Referring again to Fig. It will be shown later that discrimination by distance need not involve long time delays. The required measurement of distance is achieved in practice in various ways, which will be dealt with in detail in later chapters, but all these rely on the fact that the length of a circuit for a given conductor diameter and spacing determines its impedance.

Therefore the relays measure an impedance directly. Indeed, such addition may be essential to make a practical scheme out of a theoretical one. It will be clear that sometimes where discrimination by current magnitude is desired, the power system layout and impedances may not allow simple tapering of the current settings of nominally instantaneous relays.

In such cases the ability to add to the current tapering a system of time tapering, as in Section 2. The addition of time to distance discrimination is somewhat complex and will be described in Chapter 9, but Fig. The system is similar to that of Fig. I A, the stations A, B and C and the intervening lines being the same in both drawings.

Distance relays, as briefly described in Section 2. The relay at A is intended to detect faults in the feeder AB and would be set to measure the distance A to B, or more specifically the impedance ZAa. Clearly, the change in impedance seen by the relay at A for a fault either just within the protected circuit at end B or just beyond it is very small, and the relay must therefore have a reach setting which falls short of the remote end if incorrect operation on through faults is to be avoided.

If a fault should occur beyond Zl up to the circuit-breaker at B, a second time lag step about 0. The distance relays at B and C have similar time lag 'second zones' and Fig. This combination of distance relay with time features has the added advantage that it provides a second, or back-up, chance of clearing faults. It is to be noted that this back-up feature achieved by combined distance time discrimination does not require to have the long time lags of simple time discrimination. Current balance methods: A practical power system is invariably more complex than those so far considered.

This is because security of supply and the need for interconnection of generating sources results in a multi-mode network with fault infeeds at many points. As a consequence, time graded methods have great difficulty in providing either adequate discrimination or, equally important, a sufficently fast operating speed to ensure system stability. Satisfactory discrimination can be provided by a protection system which makes a comparison between currents at each end of the protected circuit and the simplest and most widely used form in that of current balance protection. This is illustrated in its basic form in Fig.

The relay is normally connected across equipotential points and therefore does not operate. For an internal fault the balance is disturbed and the out of balance current will operate the relay to trip the associated circuit breaker. The circuit illustrated in Fig. The principle can readily be extended to the protection of a multi-ended circuit e. Protected winding. Circulating balance methods of this kind are widely used for apparatus protection where current transformers are within the same substation area and interconnecting leads between CTs are short.

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This is not the case for feeder circuits and the current balance principle requires some modification to permit comparison of lower power levels over the interconnecting pilot wire circuit. This, like the scheme of Fig. Balance is therefore between circuit ends, and to accomplish this, an auxiliary circuit is required, connected as shown. In the case illustrated the current transformers are of a special type, designed to produce a secondary voltage linearly proportional to the primary current. With no fault on the feeder these secondary voltages will be equal and opposite, so that no current flows in the relays, which are in series with the pilot circuit.

A fault at F will disturb the balance between ends, the current transformer secondary voltages will no longer balance, and current will flow in the relays, which, on closing their contacts, will trip the circuit-breakers at the two ends. Under through fault conditions, a current-balance system applied to a line may be regarded in another way. The current transformer and relay at this end may be said to be 'aware' that fault current is entering the line, but the relay may be regarded as 'deciding' not to cause tripping because it is also 'aware' over the auxiliary pilot channel that exactly similar fault current is leaving the other end of the line.

This perhaps rather fanciful approach illustrates very well the essentials of a unit protection system, which can be stated thus: i there must be fault-sensing means at each end of the protected circuit; ii there must be means of communication between the ends so that the fault-sensing means at each end is simultaneously acquainted with conditions at the other end or ends , so that its tripping function can be exercised or not as appropriate.

The most widely used protection of this kind is the phase-comparison carrier current protection system in which alternate half cycles of current at each end of the protected circuit are modulated with a carrier signal and transmitted over the power line, for comparison against the locally derived signal. The comparator will have an angular setting, i. Normally, the phase angle between the currents at the two ends for the through load or through fault condition will be such that the comparator is inoperative i. Practical schemes use highly complex electronic circuitry as will be seen in Chapter Where high speed fault clearance is necessary over the whole feeder this delayed tripping can be eliminated by the use of an instruction signal.

The most obvious way in which an instruction signal may be used is to initiate it from Zone 1 which trips its local circuit breaker directly at either end and to arrange that, on receipt of a signal at the remote end, the circuit breaker is tripped. This is known as direct intertripping. An alternative approach, shown in Fig. G a is to make the tripping conditional. In this case operation of Zone 1 at the circuit end nearest to the fault sends an 'acceleration' signal to the remote end which in effect, short circuits the time lag associated with an independent Zone 2 or Zone 3 relay.

Tripping is thus dependent upon two criteria: i receipt of a signal from the remote end of the circuit to advise that tripping has taken place there. If this method is inverted we have the arrangement shown in Fig. G b where the distance relay now has an instantaneous zone Z2 which extends beyond the protected feeder and the signal transmitted at the remote end is used to block its tripping circuit under through fault conditions.

Blocking signals are not transmitted continuously but are controlled by a high speed starting relay H. With some schemes the high speed block initiation relay may be directional and operate only for faults outside the protected feeder i. For external faults say beyond Station B. ZI ' Z2 - Instantaneous reach settings of distance relays. HSS - Highspeed starting relay used for initiation of blockingsignal. D - Timedelay network Fig. Tripping at end A is prevented by relay R which is operated, via the signalling channel from relay S at end B. End B does not wish to trip because Z2 is not operated the fault is 'behind' the relay.

Clearly, tripping via relay Z2 must be delayed sufficiently to ensure that the blocking signal is received and that relay R has operated. To ensure stability for power reversals following through fault clearance,. A similar result may be obtained by increasing the time delay directly in the Z2 relay.

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For internal faults H. For a single infeed fault H. Neither relay operates at the other end so that tripping can take place. Practical schemes are, of course, far more complex than this but the elementary principle described above shows how a signalling channel can be used to modify the basic performance of a distance protection scheme so that it provides high speed fault clearance over the whole feeder.

The foregoing methods of discrimination by location of fault are equally applicable to single-phase and polyphase systems. The majority of supply systems are three phase, and the star points of such systems are nearly always connected to earth for reasons given in Chapter 1. Statistics show that the majority of system faults originate as faults between one phase and earth, possibly developing later into faults involving one or both of the other phases. If a fault can be dealt with quickly, whilst still in the single phase to earth stage, damage may in many cases be limited, and the system little disturbed.

There are eases where the system parameters are such that the currents produced by fault conditions differ little in magnitude from load currents, so that normal current-magnitude discrimination becomes difficult or even impossible to apply. Clearly a method of discriminating between a fault and normal or overload conditions is a necessity, and such a method is available if use is made by suitable circuitry of the fact that the currents in a three phase system can be resolved into their positive, negative and zero sequence components.

The value of discriminative methods which isolate the zero-sequence component usually resides in their ability to ignore load currents and phase-tophase short circuits, thus permitting the use of earth-fault relay settings lower than load current values. This is often essential for discrimination, or even for adequate protection when earth fault currents are limited. The three ways in common use of applying zero-sequence discrimination.

Arrangement A consists of putting a suitable current-sensitive device in the connection between the power system star neutral point and earth. Any current arising from a fault to earth anywhere on the power system must return via this connection and will operate the device. This arrangement can only be used at points near an earthed neutral, and it may be necessary to eliminate the effect, in the relay, of the third harmonic current liable to be present in some neutral-point earthing conditions.

The core-balance arrangement of Fig. Here the three phase conductors are passed through the opening of a core-balance transformer. Only if zero-sequence current is flowing will any resultant e. It is important to ensure that in any application of this arrangement, no other conductor, such as a cable sheath, which may carry current is passed through the transformer; this may cause incorrect operation unless its effect is deliberately neutralised by bringing the sheath earthing conductor back through the transformer core opening.

In Fig. Only zero-sequence current in the main circuit will cause current. Protection principles and components in this 'fourth wire', and therefore tend to operate the relay. The use of this arrangement involves careful choice of relay burden and setting, but these points are dealt with in a later chapter. Negative-phase sequence networks: Negative-sequence discrimination is sometimes necessary in unusual cases to secure otherwise unattainable discrimination.

A typical circuit for measuring negative phase sequence current is shown in Fig. This is considered in more detail in Chapter Positive-phase sequence networks: It will be clear that it is possible to construct the inverse of negative-sequence networks which will be sensitive only to the positive-sequence components of the currents in a three-phase circuit, and protective devices based on such positive-sequence networks would not respond to negative-sequence conditions.

They are used mainly to derive a reliable comparison quantity for phase comparison discriminating systems,. Protectipri onncipland escomponenPz 33 2. Similarly, by combining fault type discrimination systems with systems discriminative to location of fault, the effectiveness of the protection can be greatly improved, Indeed, most of the practical protection schemes in later chapters are the result of such combinations.

A common combination is that of a zero-sequence device with current magnitude and time discrimination; and this produces the well known 'two-overcurrent and earth-fault' arrangement illustrated in Fig. Using two over-current, relays and one zero-sequence relay, all with inherent time lag characteristics, this relatively simple arrangement will permit current magnitude discrimination for phase-to-phase and earth faults, with different time and current settings for the two types of fault.

Most of the protective systems to be dealt with later give different settings for phase and earth faults. They include the Translay, Solkor and phase comparison systems. Distance relays invariably make a separate measurement for phase and earth faults using different relay connections in each case. With the longitudinal, differential comparison, discriminative methods, where an auxiliary channel is used to convey from one end to the other detailed 'informa. With a direct-wire channel this would involve high costs, and in the case of voice-frequency, carrier, or radio link channels, even when three channels are technically feasible, the cost and complexity are considerable.

The difficulty can be overcome by providing at each end of the auxiliary channel a means of deriving a single-phase quantity which under both normal and fault conditions will be representative of the three-phase conditions, thereby permitting comparison over a single channel. It consists essentially of a summation transformer an auxiliary current transformer with a tapped primary winding connected to the three current transformers in the manner shown, together with a secondary winding suitable for the required duty.

The output of the summation transformer for any given set of phase currents, I R, 11, and 16 , is dependent upon the relative numbers of turns on the three sections of the primary winding and is proportional to a fLxed combination of the three phase currents. Thus, for the typical case shown the output is seen to be proportional to.

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Table 2. It will be evident, therefore, that the output obtained for a given type of fault and given phase-current magnitude is dependent on the particular phase or phases involved in the fault condition. The outputs given in the table ignore the effects of currents which might be present in the phases not involved in the fault condition. The design of a summation transformer for a given application must be such as will ensure a satisfactory output under all the conditions to be catered for in service. It will be noted that the blue-to-neutral section of the summation transformer primary winding carries the c.

Hence, by giving a relatively large number of turns to the blue-to-neutral section of the winding, the sensitivity of the device to earth-faults, as compared with phase faults, can be increased as required. This feature is important in applications involving limited earth-fault current.

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The relative number of turns on to the blue-to-neutral section of the primary winding, compared with the red-to-yellow and yellow-to-blue sections, is also important in that insufficient turns on the blue-to-neutral section can result in a blue-phase earth-fault condition being masked by the currents flowing in the healthy red and yellow phases. Considered in more general terms, a single phase relaying quantity may be derived from any combination of sequence currents.

The advantage of using phase sequence components lies in the fact that, when the whole range of fault currents is considered, it is found that currents in non-faulted phases can give rise to low or misleading outputs from the single phase quantity. The use of a controlled proportion of sequence quantities permits these 'blind spots' to be avoided, and gives a control of output levels for different types of fault.

B illustrates the general principle of phase sequence network combinations showing the phase. M, N, P - Proportions of! O, I I and 12 used for comparison signal. It will be seen in Chapter I 0 that a relaying quantity of Nit - PI2, where PIN is large gives a reliable output in both phase and magnitude for all types of system fault condition. This Section gives an introduction to the wide range of components used to build up a protective system. Extensive description is not intended, as later chapters will deal more fully with most of the items.

In the case of circuit-breakers only those features concerned with protection are described. Originally, in telegraphy, a relay had a coil which was energised by a weak, received signal current, and this coil, attracting an armature, closed a contact in the line ahead, giving a 'relayed' signal of renewed 'strength'. Relays, which are at the heart of protective schemes, are now of many diverse designs, each aimed at achieving particular results. They are not nowadays exclusively electromagnetic in principle, and may often be solid state devices. Relays associated with protection are divisible into two main groups.

The first are relays designed to detect and to measure abnormal conditions, and having achieved this, to close contacts in an auxiliary circuit to cause some other function to take place. This type of relay is often called a 'comparator' as its function is to compare electrical quantities. The second group are auxiliary relays, designed to be connected in the auxiliary ckcuits controlled by the measuring relay contacts, and to close or open further contacts usually in much heavier current circuits.

This second group is also called 'all or nothing' relays. It will be clear from what has been said already that protection is concerned with the detection and measurement of fault currents in various system components. This measurement would be dangerous and expensive, and indeed impossible, to achieve if the actual load and fault currents, often very large, and at a very high voltage, had to be taken through the measuring relays.


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A practical way of overcoming these difficulties is to use current transformers. These make available from thek secondary windings currents of manageable proportions which are replicas within limits which will be discussed in Chapter 4 of the currents in the primary windings which carry the actual power system currents under normal and fault conditions. The construction of current transformers in general follows the pattern of having a core of ferrous material which is magnetised by current in the primary winding insulation appropriate for the system voltage is used between primary winding and core and secondary winding - inducing a current in a secondary winding having an appropriate number of turns.

Current transformers for protection are essentially similar to those used for the operation of ammeters, watthourmeters and other instruments. In the interests of. Current transformers for measurement are not usually required to function that is, the secondary current is not required to be a replica of the primary current at current in excess of normal load, but for protection, because it is concerned with fault conditions, the current transformers must reproduce with acceptable accuracy the primary conditions on the secondary Sides, both in magnitude and phase, up to much higher currents.

This is dealt with more fully in later chapters. In addition to these main current transformers, use is frequently made in protection of 'auxiliary' or 'interposing' current transformers as a convenient means of performing such functions as summing the secondary currents in more than one circuit. In high voltage systems that is, exceeding V it is not practicable to connect the voltage coils of protective devices direct to the system. It is necessary to transform the voltage down to a manageable value and also to insulate the protective equipment from the power system. Voltage transforming devices are designed to serve these purposes.

For protection they do not differ appreciably from those commonly used for measurement; in fact, the same device usually serves both purposes, but often an additional secondary winding is needed for protection. The voltage on the secondary relay side is usually V between phases and For both measurement and protection the secondary voltage must be an exact reproduction in magnitude and phase of the primary voltage; but for protection the range of variation over which exact reproduction is needed is much wider than it need be for instruments.

The term 'voltage transforming device' embraces all the means for deriving a supply from a high voltage source for measurement and protection purposes. It includes specifically: i wound electromagnetic type voltage transformers ii capacitor voltage devices, which incorporate an electromagnetic unit. The wound-type voltage transformer is virtually a small power transformer, from which it differs little in design and in appearance, especially in the threephase pattern. At voltages higher than 33 kV it is usually made in single-phase units, to maintain phase segregation and because many forms of protection require a voltage transformer capable of reproducing the primary phase-toearth voltage.

This can be done very conveniently by combining three singlephase units, whereas special construction, that is a shell-type core or equivalent would be required in a three-phase type. In the wound-type, outputs of up to VA, which is adequate for all ordinary protection and measurement purposes, are easily obtained. In this country, for safety reasons, voltage transformers at kV and in some cases at lower voltages are fitted with Buchholz protectors.

This complication is avoided in the capacitor voltage device, and partly for this reason the latter type replaces the wound-type at kV and above. In capacitor voltage devices the wound primary is replaced by a capacitor divider, but the secondary voltage is taken from the secondary winding of a conventional wound-type transformer with its primary winding connected between a tapping on the main primary capacitor and earth. The tapping is at a voltage of about 12 kV measured with the wound transformer primary disconnected. The lower earthed capacitor, if necessary, with a capacitor across it, and a reactor in series with it, constitutes a tuned circuit which corrects the phase angle error displacement of the secondary voltage at system frequency.

In the United Kingdom, the capacitor voltage device has two important advantages; it obviates the need for a Buchholz protector, as safety considerations are adequately met without a Buchholz protector there being neither oil in quantity, nor a high voltage winding ; and secondly, the h. The output of a capacitor voltage device is determined largely by the value of capacitance adopted. For practical reasons this limits the output with reasonable accuracy to about VA.

One serious problem met with in the application of high speed distance protection is that the transient performance of capacitor v. These are mainly caused by the fact that the c. The output circuit to the relays is still derived from the lower capacitor but is associated with a voltage amplifier to provide the correct voltage and power levels for relay equipment. This replaces the reactor and wound electromagnetic transformer unit of the conventional c.

They are something more than mere coupling capacitors, as they include means for tuning the coupling circuits to the frequency of the injected signal. Even when the capacitor portion forms part of a capacitor voltage device, the design and the capacitance values, and also the necessary 'surge' protection, are decided more by.

In this latter event it is described as a combined h. A shows a typical circuit with a combined unit in one leg and a straight coupler in the other; hence the two 'legs' referred to. Any one or more of the legs may include a voltage transforming function, as required by the protection or other measuring devices. A also shows line traps connected in series with the h. Their purpose is to direct the h. To achieve this, the line trap is tuned by means of a parallelconnected capacitor to the frequency of the injected h.

The line trap, consisting of an air-cored coil wound with stranded conductor, or equivalent, of cross-section adequate to carry the line load and short-circuit currents, is usually mounted with its integral tuning capacitor on top of the h. To disconnect a fault from the power system, one or more circuit-breakers are required in conjunction with the protection.

It is outside the scope of this book to deal with the construction of circuitbreakers, but they all have certain features in common, some of which are of importance to the protection engineer. The design of the opening mechanism is such that when opening tripping is required a trip coil is energised, which releases energy stored in the mechanism, thus causing the main contacts to part.

The trip coil is usually energised from a battery by the closing of the protective relay contacts, either direct or via an auxiliary relay. For the smaller circuit-breakers where the operating current of the trip coil is not greater than the rating of the protective relay contacts, the protective relay would be used to energise the trip coil direct.


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  • When more than one breaker is to be tripped, or where the trip coil current is in excess of the relay-contact rating, an intermediate auxiliary relay having the necessary contact rating must be used. Trip coils may be operated by either d. In the U. Trip coils are highly inductive but when energised do not impose a very onerous duty on the relay contacts.

    Considerable damage would be done to these contacts, however, if they were arranged to break the trip coil current. In consequence, an auxiliary switch, operated by the circuit-breaker mechanical link mechanism, is connected in series with the trip coil and relay contacts. This auxiliary switch opens when the circuit-breaker opens, before the relay contacts open, and closes again as the circuit-breaker closes.

    Auxiliary switches may also be used in the protection circuits, and in alarm and indication circuits. It is important to ensure that the sequence of make or break of the auxiliary switches, in relation to the circuit-breaker and relay contacts, is correct for the duty required. Another matter which must be taken into account is the time for the breaker to open, that is the time interval between the trip coil being energised, and the arc being extinguished.

    Tl-ds time is usually between 0. Because protective equipment must at all times be ready to remove faulty elements from power systems, it follows that its reliability cannot be too strongly emphasised. This reliability will not be achieved without an absolutely reliable source of supply to operate the circuit-breaker trip coils, and the related auxiliary relays concerned with tripping.

    In generating stations and major substations, the source of tripping supplies is a nominal V lead-acid battery of suitable ampere-hour capacity. Connected in parallel with the battery is a charger. At one time the charger comprised two sections, the first being sufficient for the standing load for example indicating lamps and for the normal losses of the battery that is the trickle charge. The second section was separately switched and for use when boost charging. This type of charger has been superseded by the constant voltage charger which will automatically cater for heavy emergency drains on the battery.

    The same battery may be used for tripping and closing, and in such cases the closing requirements usually dictate the ampere-hour rating of battery required. A typical rating for a substation with kV bulk oil circuit-breakers solenoid operated is Ah. If used for tripping only the rating might be Ah. In smaller substations the tripping supply is often derived from a 30 V lead-acid or nickel-iron battery. The latter is preferred because of its reduced maintenance needs.

    Such batteries may or may not be provided with continuous charging facilities. It is usual to arrange for the whole tripping supply, including the battery and its charging means, to operate without a direct earth connection. The advantage in earthing the positive pole of a d. On the other hand, operating without an earth reduces the risk of loss of tripping supply arising from faults on the tripping circuits.

    Negative biasing of the positive pole overcomes the electrolysis hazard of the unearthed battery. The principle of negative biasing will be clear from Fig. The bias on a V battery applied to its positive pole is about 30 V, with the result that the positive pole of the battery has a voltage o f - 3 0 to earth and the negative pole has a voltage of over - V to earth. The negative biasing scheme illustrated also provides earth-fault supervision of the d. The disadvantage of the scheme is that it increases the danger of auxiliary relays mal-operating for faults at F because the driving voltage in such cases is the actual battery voltage plus the bias voltage.

    It is possible to overcome this danger by ensuring that all auxiliary relays are designed not to operate under such conditions. Where negative biasing is not employed, adequate supervision of the d. Where there are only one or two circuit-breakers to be tripped at any given location this can be achieved by deriving the tripping from an electrolytic capacitor normally kept energised from the local a. So important is the reliability of the tripping supply that arrangements are often made to supervise it so as to obtain early warning of failure.

    In one scheme, a separate 'trip supply healthy' indicating lamp is provided, either one per panel, or one per section of switchgear. In another, the circuit of the normal green 'breaker closed' lamp fulfds the same duty. The most comprehensive schemes employ relays of very high resistance with back contacts, that is those closing when the relay is de-energised, completing an alarm circuit. In unattended substations, the trip circuit supervision relay is arranged to give a 'trip-circuit faulty' alarm at the nearest attended point. Ultimately, however, where the consequences of a failure of tripping are severe, as on the British Supergrid kV and kV systems, arrangements must be made for duplication of relaying equipment, tripping relays, circuit-breaker trip coils and, of course, tripping batteries so that two quite independent lines of tripping are available.

    Another form of auxiliary supply that should be mentioned is used for those forms of inter-tripping in which a relay at one point trips a local breaker and must also trip a remote one in order to remove a fault. This requires a pilot circuit, and in some arrangements such an inter-tripping circuit would result in paralleling the tripping supplies at the two separated stations. This is undesirable, because system earth-faults can impress high voltages momentarily between station earths, and if the tripping supplies are paralleled by a pilot circuit, the whole of the tripping circuits may thereby be subjected to a considerable or even hazardous voltage stress to earth, with consequent risk of breakdown.

    In such circumstances, therefore, the inter-trip circuit supply may be derived from a small independent unearthed and insulated battery. These components play an essential part in protection, and although seemingly 'pedestrian' in character, they must be carefully chosen, carefully installed, and well looked after if they are not to be sources of weakness in the maintenance of the reliability of the protection. The first is in connection with a.

    These fuses have the primary function of protecting the source of supply and the wiring from faults, and a secondary one of acting as a convenient means of isolating subcircuits when testing or other work is in progress on a relay panel. The form of fuse and its holder must be of an intrinsically safe design and approved for the purpose.

    The second use of fuses is in connection with tripping supply wiring, and here their function is similar to those just mentioned for a. Again, reliability is the prime requirement. Rewireable type fuses were used for many years for the above functions, because they were more reliable than the early type of HRC cartridge fuse. The latter type, however, has now replaced the rewireable fuse in modern installations.

    Small wiring: 'Small wiring' is a term which covers the very large number of connections needed between relays, current and voltage transformers, trip coils and auxiliary circuits. Panel wiring must be of high quality, and it must be so installed that it is not unduly strained and no risk of damage exists. Particular care should be given to points where the wiring passes across hinges to swing panels. Included in the term 'small wiring' will be multicore cable runs, and these must also be protected from mechanical damage and the risk of penetration of moisture.

    Terminals: Of the very large number of terminations, every one needs to be numbered for identification, to avoid inadvertent earthing or energisation which could have disastrous results. Terminals must be mechanically strong enough not to shear, or to be otherwise damaged, when the nuts securing the terminations are being tightened. These requirements can be met by the use of 6, 5, 4 and 3 nun terminals. Relays which are traditionally panel mounted and back connected often have many terminals. It is present practice to provide terminals at points where wires enter and leave relay panels, at the ends of multicore cables, and.

    All this seeming complexity has been found necessary for the tidy and safe installation and maintenance of this small wiring. The terminals themselves are usually grouped on standardised moulded terminal boards, the moulding being arranged to provide a barrier between terminals which are usually about 30 mm apart, so that the risk of inadvertent contact when working on the terminalboard with spanners is minimised. Insulation type moulded blocks are also used. Test links: a valuable feature found necessary for expeditious and troublefree routing testing of protective gear is the provision of test links designed to permit the insertion of injection sets portable adjustable sources of test current into current circuits without risk of open-circuiting current transformers, which, besides, being dangerous on account of the high voltages which such open-circuiting can cause, also involves a risk of damage to the current transformers themselves, and of causing inadvertent operation of zero-sequence relays.

    A shows a typical test link board. It will be seen that by manipulating the double links in turn the current transformer can be short-circuited and the test set put safely into the circuit. The covers of such test link boards should preferably be sealed against unauthorised interference, and should be clearly labelled to indicate the circuit with which they are connected. An alternative approach uses test blocks which perform a similar function but which also include 'built in' safeguards against wrong use.

    With many of the highly complex primary circuit configurations now in use comprehensive protection testing can only be performed by using 'built in' disconnection points which provide a plug connection or disconnection facility between incoming and outgoing wires on the associated terminal block. Reference has already been made to these as integral corfiponents of some forms of discriminative protection. There are four arrangements in common use. The first is privately-owned pilot wires, which consist ofmetaUic circuits, run in an auxiliary cable and often laid with the main cable to be protected.

    Alternatively, the pilot cable may be suspended from a catenary wire on wood poles. In either case the pilot cable may be exposed to high induced voltages. The requirements for protection pilots are stringent in two particular respects. First, they must not break down between cores and sheath under fault conditions and secondly, induced voltages must not appear between cores to disturb the balance of the relaying signals under through fault conditions. Either of these factors will give rise to incorrect tripping under through fault conditions.

    It follows, therefore, that a high insulation level must be provided between pilot cores and sheath and levels of 5 kV or 15 kV are standard, the higher level being used where pilots are in close proximity to the primary circuit. The use of fully transposed i. Where high core to sheath voltages are possible, it is essential to provide similar insulation levels to earth on the relay equipment at each end. This is done preferably by using a pilot isolating transformer between relays and pilot wires Fig.

    The use of spark gap or similar protectors is not possible because, on operation, they short circuit the pilot wires which then precludes correct operation of the protection. Privately-owned metallic circuits are a convenient arrangement for protection systems which require the transmission from end to end of a 'sample', both in magnitude and phase, of the current longitudinal differential protection. Circuits rented from telecommunication companies fall broadly into two categories; direct metallic circuits between ends over which some form of 50 Hz comparison may be possible, and audiofrequency communication circuits.

    The former may be used with special agreement for pilot wire differential protection whereas audiofrequency signalling is widely used for direct intertripping or distance protection signalling, and for phase comparison differential protection. Telecommunication companies impose limitations on current and voltage levels and require insulation between the secondary protective equipment and their own line capable of withstanding momentarily 15 kV for kV and higher voltage circuits, and 5 kV for 33 kV circuits, these being the values which might be impressed on the secondary equipment by the breakdown of primary insulation.

    The third type of pilot circuit makes use of the power lines being protected, by using them as a channel for a superimposed high-frequency carrier current. This carrier current is generated, and detected, by electronic means. Carrier channels are extensively used for phase comparison systems and for distance protection signalling and direct intertripping.

    The fourth type of pilot circuit is the microwave radio link. This is of value when other forms of pilot circuit are impracticable, or as emergency circuits when others fail. It can also be used to advantage where a line includes a long river crossing; in this case, not because it is necessarily superior to 'carrier' on the power line, but because the radio link will simultaneously provide multiple channels for telemetering and control which cannot always be obtained with 'carrier' channels. The equipment concerned consists of a line-of-sight radio circuit, if need be with repeater stations, working at very high or ultra-high frequency.

    As with 'carrier' there are electronic transmitters and receivers, and these are switched on as the protection and other services using the link require. The use of radio links in Britain is not at present encouraged by the Home Office, who have to agree to such use, and who allocate the frequencies to be used. Radio links, like 'carrier', are suitable for types of protection requiring end-to-end signalling, as distinct from the transmission of current 'samples', as is required, for example, with longitudinal differential protection.

    The foregoing sections have examined, in a general and discursive way, some of those basic aspects of power system protection which will be considered in greater depth in subsequent chapters. Before pursuing these matters in greater detail, however, it is helpful to summar. The basic objective of power system protection is to maintain continuity of supply which it does by rapid and discriminative isolation of faulted items of plant from the rest of the system thereby reducing damage to the plant itself and also reducing the effect of the faulted item of plant upon the rest of the system.

    The requirement of the protection equipment, therefore, is to pass an opera. For large power systems where system stability is at risk the detection must take place within one or two cycles of the inception of the fault i. In order to determine the location of the fault, the protection scheme must make a comparison of the a. This difference is known as the discrimination margin and there are many factors which tend to reduce it, and thereby to invalidate the provision of fully discriminative protection, a The fault represents a sudden discontinuity; there is, therefore, no guidance available from previous history, and relaying quantities must be measured during the transient fault period.

    These are not readily predictable. Normally, only a single phase quantity derived from the three-phase system can be transmitted, which may result in loss of discriminative information. Cg Signalling levels must be limited to values acceptable to the telephone company and others to prevent unacceptable interference. When considered in relation to the high noise levels often associated with the power system. The high fault currents produced by the power system fault return to the substation earth electrode system in close proximity to the low current secondary wiring associated with protection circuits.

    Protection equipment must, be designed to be immune from the mutual coupling effects of these currents. The impact of all these additional disturbing influences must be taken into account in the design and application of protective systems. A itemises important design considerations in meeting practical requirements for some of the more widely used forms of main protection. Subsequent chapters will describe in detail the implications of many of these. Fault calculation is the analysis of power system electrical behaviour under fault conditions, with particular reference to the effects of these conditions on the power system current and voltage values.

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    Together with other aspects of system analysis, fault calculation forms an indispensable part of the whole function and process of power system design. Correct design depends essentially on a full knowledge and understanding of system behaviour and on the ability to predict this behaviour for the complete range of possible system conditions.

    Accurate and comprehensive analysis, and the means and methods of achieving it, are therefore of essential importance in obtaining satisfactory power system performance and in ensuring the continued improvement in performance which results from the development and application of new methods and techniques. The applications of power system analysis cover the full range of possible system conditions, these being divisible into two main classes, namely conditions in which the power system is operating in a normal healthy state, and others in which it is subjected to one or more of a wide variety of possible fault conditions.

    The analysis of these conditions and their effects on the power system is of particular relevance R such considerations as: a the choice of a suitable power system arrangement, with particular reference to the configuration of the transmission or distribution network b the determination of the required load and short-circuit ratings of the power system plant c the determination of the breaking capacity required of the power system switchgear and fusegear d the design and application of equipment for the control and protection of the power system e the operation of the system, with particular reference to security of supply and economic considerations.

    The present chapter is concerned principally with the analysis of system fault conditions, these conditions being of direct and particular relevance to the design and application of power system protection. The methods of analysis employed, however, are essentially applications of general analysis and, as such have equal application to a wide range of other problems whose solution is dependent on electrical network analysis.

    In the context of electrical fault calculation, a power system fault may be defined as any condition or abnormality of the system which involves the electrical failure of primary equipment, the reference to primary as opposed to ancillary equipment implying equipment such as generators, transformers, busbars, overhead lines and cables and all other items of plant which operate at power system voltage.

    Electrical failure generally implies one or the other or both of two types of failure, namely insulation failure resulting in a short-circuit condition or conductingpath failure resulting in an open-circuit condition, the former being by far the more common type of failure. The principal types of fault are listed and classified in Table 3.

    Table 3. Typical examples are the cross-country earthfault and the open-circuit-with-earth-fault condition Winding-to-earth short-circuit Winding. Short-circuited phases: Faults of this type are caused by insulation failure between phase conductors or between phase conductors and earth, or both, the result being the short-circuiting of one or more phases to earth or to one another, or both. The full range of possible fault conditions of this type is illustrated in Fig. The three-phase fault, which may be to earth or clear of earth, is the only balanced or symmetrical short-circuit condition, the presence or absence of the earth connection being normally of little or no significance unless the fault occurs simultaneously with a second unbalanced fault involving earth.

    The threephase short-circuit is commonly used as a standard fault condition as, for example, in the determination of system fault-levels, these levels being normally quoted as three-phase short-circuit values. Open-circuited phases: This type of fault, illustrated in Fig. The more common causes of this type of fault are joint failures on overhead lines and cables, and the failure of one or more phases of a circuit-breaker or isolator to open or close. The single-phase and two-.

    Simultaneous faults: A simultaneous fault condition, sometimes termed a multiple fault condition, is defined as the simultaneous presence of two or more faults of similar or dissimilar types at the same or different points on the power system. Such conditions may result from a common cause, from different but consequential causes or, extremely rarely, from quite separate and independent causes.

    The commonest simultaneous fault condition is undoubtedly the doublecircuit overhead-line fault in which a common cause for example lightning or accidental contact results in a fault on each of the two circuits concerned. A simultaneous fault condition of particular interest is that known as the cross-country earth-fault, in which a single-phase-to-earth fault at one point in the power system occurs coincidentally with a second single-phase-toearth fault on another phase and at some other point in the system.

    This condition is most commonly experienced on impedance-earthed systems where the second earth-fault may be initiated by the increased healthy-phase voltage resulting from the neutral displacement produced by the first. As already stated, a simultaneous fault condition may consist of two different types of fault at the same point, and one example of this is the open-circuit-with-earth-fault condition in which two faults, namely a single-phase open-circuit and a single-phase-to earth fault, occur coincidentally on the same phase and at the same point in the power system.

    Such a condition can occur on an overhead line for example, due to a phase conductor breaking at a point near to a tower, the conductor on the tower side of the break being held by the suspension insulator and that on the other side falling to ground. The fault conditions described are divisible into two distinct classes, namely. Of the faults listed. Winding faults: The types of fault which can occur on machine and transformer windings are illustrated in Fig.

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