EE 6702 PROTECTION AND SWITCHGEAR UNIVERSITY QUESTIONS AND ANSWERS UNIT 4. Part - A

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1 1.What is resistance switching? EE 6702 PROTECTION AND SWITCHGEAR UNIVERSITY QUESTIONS AND ANSWERS UNIT 4 Part - A It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current. 2. What do you mean by current chopping? When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping. 3. What are the methods of capacitive switching? Opening of single capacitor bank Closing of one capacitor bank against another 4. What is an arc? Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition. 5. Give the two methods of arc interruption? High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished _ Current zero method:-the arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac. 6. What is restriking voltage? It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished. 7. What is meant by recovery voltage? The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage.

2 8. What is RRRV? It is the rate of rise of restriking voltage, expressed in volts per microsecond. It is closely associated with natural frequency of oscillation. PART B 1.How will you synthesize mho relay using static phase comparator?(n/d-16) A mho Relay is a high-speed relay and is also known as the admittance relay. In this relay operating torque is obtained by the volt-amperes element and the controlling element is developed due to the voltage element. It means a mho relay is a voltage controlled directional relay. A mho relay using the induction cup structure is shown in the figure below. The operating torque is developed by the interaction of fluxes due to pole 2, 3, and 4 and the controlling torque is developed due to poles 1, 2 and 4. nd the controlling torque is developed due to poles 1, 2 and 4. If the spring controlling effect is indicated by K 3, the torque equation becomes,

3 Where Θ and τ are defined as positive when I lag behind V. At balance point, the net torque is zero, and hence the equation becomes If the spring controlled effect is neglected i.e., k 3 = 0. Operating Characteristic of Mho Relay The operating characteristic of the mho relay is shown in the figure below. The diameter of the circle is practically independent of V and I, except at a very low magnitude of the voltage and current when the spring effect is considered, which causes the diameter to decrease. The diameter of the circle is expressed by the equation as Z R = K 1 / K 2 = ohmic setting of the relay The relay operates when the impedance seen by the relay within the circle. The operating characteristic showed that circle passes through the origin, which makes the relay naturally directional. The relay because of its naturally directional characteristic requires only one pair of contacts which makes it fast tripping for fault clearance and reduces the VA burdens on the current transformer. The impedance angle of the protected line is normally 60º and 70º which is shown by line OC in the figure. The arc resistance R is represented by the length AB, which is horizontal to OC from the extremity of the chord Z. By making the τ equal to, or little less lagging than Θ, the circle is

4 made to fit around the faulty area so that the relay is insensitive to power swings and therefore particularly applicable to the protection of long or heavily loaded lines. For a given relay the τ is constant, and the admittance phasor Y will lie on the straight line. The characteristic of mho relays on the admittance diagram is, therefore, a straight line and is shown in the figure below. Mho relay is suitable for EHV/UHV heavily loaded transmission lines as its threshold characteristic in Z-plane is a circle passing through the origin, and its diameter is Z R. Because of this, the threshold characteristic is quite compact enclosing faulty area compactly and hence, there is lesser chance to operate during power swing and also it is directional. 2. Explain about numerical overcurrent relay and numerical protection of transformers (N/D-16) A numerical Relay is a computerized electronic relay, which determines its qualities by method for a pre-programmed algorithm and counts (calculations), in light of the chose settings and the deliberate current and/or voltage signals. BASIC EQUATION: The formula used to determine the Inverse time characteristic in an overcurrent relay that consent to be scientifically characterized as takes after: t[s]=kβ / [(I/I>) α -1] Where T = operating time in seconds K = time multiplier I = current value I> = set current value. The unit incorporates four determined qualities with distinctive degrees of Inverse. The level of backwards is controlled by the estimations of the Constants α and β

5 BASIC PRINCIPLE: In computerized transfers Digital Signals are utilized for information preparing rather than Analog signs. Simple signs are Continuous Signals and can't be prepared effectively as a result of their few constraints when contrasted with advanced signs. Computerized signs are in type of coded square heartbeats which speaks to discrete components of data. In computerized framework, the signs are in double shape just two discrete qualities alluded to as paired coefficients 0 and 1 or consistent values genuine and false. The quantity of paired digits expected to encode the different discrete components of data affects the outline of an advanced framework. The advanced framework by and large works on gatherings of 8 to 32 bits of data without a moment's delay. The scope of the computerized arrangement of encoding the data by a n bit gathering is 2 n. Thus advanced frameworks with bigger piece working gathering can prepare a more extensive scope of encoded data. The data to be handled may be literary, numerical and legitimate. It is now easier and faster when this process is done through micro processor, like now at the point when the mean estimation of two back to back half-waves is figured, the DC-segment is eliminated just about 100% with no requirement for non-linear air hole Transformers or comparable segments. Then again, the count of the mean quality devours high value, which is not needed for high short out Current levels where an instantaneous trip is called for. For this case another trip criteria is basically included. On the off chance that the Current in the first halfwave surpasses double the Setting, it is clear that the mean estimation of the two half-waves will surpass the set level and along these lines a trek can be completed quickly without the need to sit tight for the following half-wave All deliberate numerical qualities can obviously effortlessly be exchanged over the serial communication, be put away in memory banks, for later recovery when flaw reasons are being examined. The Sampling is likewise utilized as a part of another great approach to minimize the transient over-reach. At the point when the working time for a stage has passed and the trip order is to be completed, the stage will sit tight for still one single example surpassing the set level before the excursion is connected to the yield relay. In this activated state, the transfer will sit tight for a brief timeframe and if no further Samples are distinguished, the Relay will reset. This implies the Retardation time or the Transient over-shoot is very short.

6 Block diagram of numerical overcurrent relay is shown above. 3.Explain about zero current interruption theories (A/M-15) The insulating material (may be fluid or air) used in circuit breaker should serve two important functions. They are written as follows: 1. It should provide sufficient insulation between the contacts when circuit breaker opens. 2. It should extinguish the arc occurring between the contacts when circuit breaker opens. The second point needs more explanation. To understand this point let us consider a situation if there is some fault or short circuit in the system, the relay provides desired signals to the circuit breaker so as to prevent system from ongoing fault. Now when circuit breaker opens its contacts, due to this an arc is drawn. The arc is interrupted by suitable insulator and technique. Limitations of high resistance method: Arc discharge has a resistive nature due to this most of the energy is received by circuit breaker itself hence proper care should be taken during the manufacturing of circuit breaker like mechanical strength etc. Therefore this method is applied in DC power circuit breaker, low and medium AC power circuit breaker. Low resistance method is applicable only for ac circuit and it is possible there because of presence of natural zero of current. The arc gets extinguished at the natural zero of the ac wave

7 and is prevented from restricting again by rapid building of dielectric strength of the contact space. The two current zero arc interruption theories are 1. Recovery rate theory 2. Energy balance theory Recovery rate theory: Recovery theory: The rate at which the dielectric strength get recovered and then it is compared with the rate at which the restriking the voltage across the contacts rises. If the rate of rising of re-striking voltage is rapid than the dielectric strength then the space breaks down and arc persists. Since arc consist of column of ionized gases. So to extinguish the arc the ions and the electrons are required to be removed from the gap immediately after the current reaches to zero. It can be removed by recombining the ions and electrons with neutral molecules or by using the insulating medium in gap it can be swept away when the ions are removed from the gap with a rate faster than the rate of ionization then the arc get interrupted. The ionization at zero current depends on the voltage known as restriking voltage. Let us define an expression for restriking voltage. For loss-less or ideal system we have, Here, v = restriking voltage. V = value of voltage at the instant of interruption. L and C are series inductor and shunt capacitance up to fault point. Thus from above equation we can see that lower the value of product of L and C, higher the value of restriking voltage. The variation of v versus time is plotted below:

8 Now let us consider a practical system, or assume there finite loss in the system. As figure shown below, in this case the restriking voltage is damped out due to the presence of some finite resistance. Here it is assumed that the current lags behind the voltage by an angle (measured in degrees) of 90. However in practical situation angle may varies depending upon time in cycle at which the fault is occurred. Let us consider the effect of arc voltage, if arc voltage is included in the system, there is an increment in the restriking voltage. However this is offset by another effect of an arc voltage which opposes the current flow and making change in the phase of current, thus bringing it more

9 into phase with the applied voltage. Hence the current is not at its peak value when voltage passes through zero value. Energy Balance theory: When the contact of circuit breaker are about to open, restriking voltage is zero, hence generated heat would be zero and when the contacts are fully open there is infinite resistance this again make no production of heat. We can conclude from this that the maximum generated heat is lying between these two cases and can be approximated, now this theory is based on the fact that the rate of generation of heat between the the contacts of circuit breaker is lower than the rate at which heat between the contact is dissipated. Thus if it is possible to remove the generated heat by cooling, lengthening and splitting the arc at a high rate the generation, arc can be extinguished.

10 4. Explain a. interruption of capacitive currents b. current chopping (A/M-15) Current chopping: Current Chopping in circuit breaker is defined as a phenomena in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in Vacuum Circuit Breaker and Air Blast Circuit Breaker. There is no such phenomena in Oil Circuit Breaker. Current chopping is predominant while switching Shunt Reactor or unloadedtransformer. Theory of Current Chopping Generally the arc extinction in a circuit breaker take place at natural current zero. But this is true if the capacity of the breaker to extinguish the arc is varies with the level of fault current. This means that, the arc extinction capability of breaker will always ensure that arc extinction is taking place at natural current zero. Now, let us assume Air Blast Circuit Breaker. In Air Blast Circuit Breaker or Vacuum Circuit Breaker, the fault clearing capacity is fixed and independent of the fault current level. In this case, when breaker is used to break the circuit of unloaded transformer or shunt reactor, the current will be brought to zero well before the natural current zero. This is because, the breaker is interrupting only the magnetizing current which is very less compared to full load current or fault current. As the capability of breaker arc extinction is high enough, therefore the low magnetizing current will be brought to zero before the natural current zero position. This phenomena is known as Current Chopping. Let us understand current chopping in detail. Consider a shunt reactor as shown in figure below.

11 In the figure above, L is the inductance of shunt reactor, C is the capacitance of winding and R is for eddy current loss in the reactor. Breaker in the figure above is Air Blast Circuit Breaker. Shunt reactor always takes magnetizing current. This magnetizing current is, of course, low. Under normal condition, the current flowing through the reactor is I (say) and hence the stored magnetic energy in it is (LI 2 / 2). But as soon as the breaker is open, current chopping will take place and the current through the reactor becomes zero. Due to this sudden drop of current through the inductor, a high voltage will be developed across it according to Faraday s Law. Therefore, the voltage across the capacitor will also rise. Now, the question arises, where did the store energy of reactor go? The stored energy in the inductance of reactor is basically transferred to the capacitor. Therefore mathematically we can write as LI 2 / 2 = CV 2 / 2 Here V = Voltage across the capacitor Thus, V = 1 I (L/C) This is the prospective voltage across the capacitor during current chopping. Notice that this prospective voltage is above the natural voltage of the system. This means that there will be a high voltage stress on the shunt reactor during current chopping. Note that the prospective voltage V is directly proportional to the value of current chopped and the surge impedance of the reactor. Let us consider a simple example to have an idea of magnitude of prospective voltage. Let the value of L = 64 mh and C = uf then the induced voltage for a chopping current of 10 A will be V = 10x ( / )= 80 kv

12 Thus we see that, the magnitude of V is quite high. Again, if this voltage V is high enough, then it may lead to the restrike of arc in the breaker and thus current again start to flow through the circuit. Again, there will be chopping of current and but this time the level of current chopped will reduce and therefore the voltage stress on the reactor is less. Thus a number of current chopping will take place till the prospective voltage become low enough to restrike the arc. Carefully observe the figure above. In the figure you can see, 4 current chopping. In each current chopping the magnitude of current reduces. This is because of dampening effect of losses in the equipment like eddy current loss and hysteresis loss. 5. Explain in detail about rating of circuit breakers (N/D-15) The rating of a circuit breaker includes, 1. Rated short circuit breaking current. 2. Rated short circuit making current. 3. Rated operating sequence of circuit breaker. 4. Rated short time current. Short Circuit Breaking Current of Circuit Breaker

13 This is the maximum short circuit current which a circuit breaker can withstand before it, finally cleared by opening its contacts. When a short circuit flows through a circuit breaker, there would be thermal and mechanical stresses in the current carrying parts of the breaker. If the contact area and cross-section of the conducting parts of the circuit breaker are not sufficiently large, there may be a chance of permanent damage in insulation as well as conducting parts of the CB. As per Joule s law of heating, the rising temperature is directly proportional to square of short circuit current, contact resistance and duration of short circuit current. The short circuit current continuous to flow through circuit breaker until the short circuit is cleared by opening operation of the circuit breaker. As the thermal stress in the circuit breaker is proportional to the period of short circuit, the breaking capacity of electrical circuit breaker, depends upon the operating time. At 160 C aluminum becomes soft and losses its mechanical strength, this temperature may be taken as limit of temperature rise of breaker contacts during short circuit. Hence short circuit breaking capacity or short circuit breaking current of circuit breaker is defined as maximum current can flow through the breaker from time of occurring short circuit to the time of clearing the short circuit without any permanent damage in the CB. The value of short circuit breaking current is expressed in RMS. During short circuit, the CB is not only subjected to thermal stress, it also suffers seriously from mechanical stresses. So during determining short circuit capacity, the mechanical strength of the CB is also considered. So for choosing suitable circuit breaker it is obvious to determine the fault level at that point of the system where CB to be installed. Once the fault level of any part of electrical transmission is determined it is easy to choose the correct rated circuit breaker for this part of network. Rated Short Circuit Making Capacity The short circuit making capacity of circuit breaker is expressed in peak value not in rms value like breaking capacity. Theoretically at the instant of fault occurrence in a system, the fault current can rise to twice of its symmetrical fault level. At the instant of switching on a circuit breaker in faulty condition, of system, the short circuit portion of the system connected to the source. The first cycle of the current during a circuit is closed by circuit breaker, has maximum amplitude. This is about twice of the amplitude of symmetrical fault current waveform. The breaker s contacts have to withstand this highest value of current during the first cycle of waveform when breaker is closed under fault. On the basis of this above mentioned phenomenon, a selected breaker should be rated with short circuit making capacity. As the rated short circuit making current of circuit breaker is expressed in maximum peak value, it is always more than rated short circuit breaking current of circuit breaker. Normally value of short circuit making current is 2.5 times more than short circuit breaking current.

14 Rated Operating Sequence or Duty Cycle of Circuit Breaker This is mechanical duty requirement of circuit breaker operating mechanism. The sequence of rated operating duty of a circuit breaker has been specified as Where, O indicates opening operation of CB. CO represents closing operation time which is immediately followed by an opening operation without any intentional time delay. t' is time between two operations which is necessary to restore the initial conditions and / or to prevent undue heating of conducting parts of circuit breaker. t = 0.3 sec for circuit breaker intended for first auto re closing duty, if not otherwise specified. Suppose rated duty circle of a circuit breaker is This means, an opening operation of circuit breaker is followed by a closing operation after a time interval of 0.3 sec, and then the circuit breaker again opens without any intentional time delay. After this opening operation the CB is again closed after 3 minutes and then instantly trips without any intentional time delay. Rated Short Time Current This is the current limit which a circuit breaker can carry safely for certain specific time without any damage in it. The circuit breakers do not clear the short circuit current as soon as any fault occurs in the system. There always some intentional and an intentional time delays present between the instant of occurrence of fault and instant of clearing the fault by CB. This delay is because of time of operation of protection relays, time of operation of circuit breaker and also there may be some intentional time delay imposed in relay for proper coordination of power system protection. Even a circuit breaker fails to trip, the fault will be cleared by next higher positioned circuit breaker. In this case the fault clearing time is longer. Hence, after fault, a circuit breaker has to carry the short circuit for certain time. The summation of all time delays should not be more than 3 seconds; hence a circuit breaker should be capable of carrying a maximum faulty current for at least this short period of time. The short circuit current may have two major affects inside a circuit breaker. 1. Because of the high electric current, there may be high thermal stress in the insulation and conducting parts of CB. 2. The high short circuit current, produces significant mechanical stresses in different current carrying parts of the circuit breaker. A circuit breaker is designed to withstand these stresses. But no circuit breaker has to carry a short circuit current not more than current for a specified short period. The rated short time current of a circuit breaker is at least equal to rated short circuit breaking current of the circuit breaker. Rated Voltage of Circuit Breaker Rated voltage of circuit breaker depends upon its insulation system. For below 400 KV systems, the circuit breaker is designed to withstand 10% above the normal system voltage. For above or

15 equal 400 KV system the insulation of circuit breaker should be capable of withstanding 5% above the normal system voltage. That means, rated voltage of circuit breaker corresponds to the highest system voltage. This is because during no load or small load condition the voltage level of power system is allowed rise up to highest voltage rating of the system. A circuit breaker is also subject to two other high voltage conditions. 1. Sudden disconnection of huge load for any other cause, the voltage imposed on the CB and also between the contacts when the CB is open, may be very high compared to higher system voltage. This voltage may be of power frequency but does not stay for very long period as this high voltage situation must be cleared by protective switchgear. But a circuit breaker may have to withstand this power frequency over voltage, during its normal life span. The Circuit Breaker must be rated for power frequency withstands voltage for a specific time only. Generally the time is 60 seconds. Making power frequency withstand capacity, more than 60 second is not economical and not practically desired as all the abnormal situations of electrical power system are definitely cleared within much smaller period than 60 seconds. 2. Like other apparatuses connected to power system, a circuit breaker may have also to face lighting impulse and switching impulses during its life span. The insulation system of CB and contact gap of an open CB have to withstand these impulse voltage waveform amplitude of this disturbance is very very high but extremely transient in nature. So a circuit breaker is designed to withstand this impulse peaky voltage for microsecond range only. 6. Explain in detail the working of numerical relay with block diagram. The block diagram of numerical relay is as follows

16

17 Basic components of numerical relay:

18 SOFTWARE OF NUMERICLA RELAY

19 Operating algorithm for numerical time-overcurrent realay 7. Explain the arc phenomena and interruption.(m/j-14) Arc Phenomena When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air or vapour acts as conductor and an arc is struck between the contacts. The Potential Difference between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists. During the arcing period, the current flowing between the contacts depends upon the arc resistance.the greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors : (i) Degree of ionisation the arc resistance increases with the decrease in the number of ionised particles between the contacts. (ii) Length of the arc the arc resistance increases with the length of the arc i.e., separation of contacts.

20 Factors responsible for maintaining ARC. Potential Difference between the contacts When the contacts have a small separation, the p.d. between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that p.d. becomes inadequate to maintain the arc. However, this method is impracticable in high voltage system where a separation of many meters may be required. Ionized particles between contacts The ionized particles between the contacts tend to maintain the arc. If the arc path is deionized, the arc extinction will be facilitated. This may be achieved by cooling the arc or by bodily removing the ionized particles from the space between the contacts. Principles of Arc Quenching (Interruption) Before discussing the methods of arc extinction, it is necessary to examine the factors responsibl e forthe maintenance of arc between the contacts. These are : (i) Potential Difference between the contacts (ii) ionized particles between contacts Taking these in turn, i) When the contacts have a small separation, the p.d. between them is sufficient to maintain the arc. One way to extinguish the arc is to separate the contacts to such a distance that p.d.beco mes inadequate to maintain the arc. However, this method is impracticable in high voltage system where a separation of many metres may be required. (ii) The ionizedparticles between the contacts tend to maintain the arc. If the arc path is deionise d,the arc extinction will be facilitated.this may be achieved by cooling the arc or by bodily removing the ionised particles from the space between the contacts. Methods of Arc Extinction There are two methods of extinguishing the arc in circuit breakers viz. 1. High resistance method. 2. Low resistance or current zero method High resistance method: n this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is

21 extinguished. It is employed only in d.c. circuit breakers and low-capacity a.c. circuit breakers. It occurs as the heat dissipation is very large Methods of increasing arc resistance. 1. Lengthening of arc. 2. Cooling of arc. 3. Reducing cross section area of arc. 4. Splitting the arc. Lengthening the arc. The resistance of the arc is directly proportional to its length. The length of the arc can be increased by increasing the gap between contacts. Cooling the arc. Cooling helps in the deionisation of the medium between the contacts. This increases the arc resistance. Efficient cooling may be obtained by a gas blast directed along the arc. Reducing X-section of the arc. If the area of X-section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having smaller area of contacts. Splitting the arc. The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts. Low resistance or Current zero method: This method is employed for arc extinction in a.c.circuits only.in this method, arc resistance is kept low until current is zero where the arc extin guishes naturally and is prevented from restriking inspite of the rising voltage across the contacts.all modern high power a.c. circuit breakers employ this method for arc extinction.in an a.c. system,c urrent drops to zero after every half-cycle.

22 At every current zero,the arcextinguishes for a brief moment.now the medium between the conta cts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltag e knownas restringvoltage If such a breakdown does occur, the arc will persist for another half cycle.if immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to re strike and the current willbe interrupted. The rapid increase of dielectric strength of the medium near current zero can be achieved by : (a) causing the ionised particles in the space between contacts to recombine into neutral molecules. (b) sweeping the ionised particles away and replacing them by unionised particles.therefore, th e real problem in a.c. arc interruption is to rapidly deionise the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking volta ge cannot breakdown the space between contacts. The deionisation of the medium can be achieved by: (i) lengthening of the gap. The dielectric strength of the medium is proportional to the lengthof the gap between contacts.th erefore, by opening the contacts rapidly, higher dielectricstrength of the medium can be achieve d. (ii) high pressure. If the pressure in the vicinity of the arc is increased, the density of the particles constituting the discharge also increases. The increased density of particles causes higher rate of desalinization and consequently the dielectric strength of the medium between contacts isincreased. (iii) cooling. Natural combination of ionized particles takes place more rapidlyif they are allowed to cool.ther efore, dielectric strength of the medium between the contacts can beincreased by cooling the arc. (iv) blast effect. If the ionized particles between the contacts are swept away and replaced by unionizedparticles, the dielectric strength of the medium can be increased considerably.thismay be achieved by a g as blast directed along the discharge or by forcing oil into the contact space.

23 8. Explain the distant protection of transmission lines. Necessity of Transmission line protection: As the length of electrical power transmission line is generally long enough and it runs through open atmosphere, the probability of occurring fault in electrical power transmission line is much higher than that of electrical power transformers and alternators. That is why a transmission line requires much more protective schemes than a transformer and an alternator. Protection of line should have some special features, such as- 1. During fault, the only circuit breaker closest to the fault point should be tripped. 2. If the circuit breaker closest the faulty point, fails to trip the circuit breaker just next to this breaker will trip as back up. 3. The operating time of relay associated with protection of line should be as minimum as possible in order to prevent unnecessary tripping of circuit breakers associated with other healthy parts of power system. There are three major categories of protection schemes for transmission lines. Two are known as distance protection schemes. These are Pilot Protection and Non-Pilot Protection. The term pilot referring to the use of a communications link between the ends of the line to be protected (allowing for instantaneous fault clearing).2 In pilot schemes, there is the advantage of knowing the conditions of the line at both ends. The third type of approach are differential in nature. Phase comparison relaying is one of these. This allows for one fairly common solution to the problem of series compensated transmission line protection which is successful in most cases. This solution does not use distance relaying principles, instead it compares the phase of the currents at both ends of the line to see if there is a fault in the middle. One of the most critical issues in power system protection of any kind is the speed with which a fault can be cleared. Due to uncertainty in impedance measurements, when protecting a transmission line with non-pilot distance protection schemes (and some types of pilot protection schemes), it is necessary to rely on stepped zones of protection. This technique protects any given section of transmission line with multiple zones. Close in faults are cleared instantaneously by zone 1 protection. This protects roughly 85-90% of the line. When a fault is at 95% of the line its location becomes uncertain, based again on accuracy of impedance measurements, whether the fault is actually on that particular section of line or on an adjacent section. Therefore, it makes sense to delay tripping of faults which are perceived by the relay to be between the zone 1 upper limit and the zone 2 upper limit ( %) of the length of the line in question. Zone three provides backup for neighboring lines. Delaying a trip on zone 2 5 and 3 faults allows time for a zone 1 reaction of the relay on the adjacent line if the fault is in fact on

24 that section of line. If it is actually at 95% of the line in question, then it will be cleared in zone 2. This delay insures proper coordination, and helps in the effort to avoid shutting down longer sections of line than are necessary to clear the fault. Non-Pilot Distance Protection of Transmission Lines The primary advantage to the use of non-pilot protection is that there is no need to construct the communications link (be it PLC, fiber optic, copper, microwave etc.). This is a tremendous cost savings to begin with as none of these methods are cheap. Copper wiring is only good for lines no longer than a couple of miles due to the expense of the high insulation copper cable and the induced current from neighboring power circuits. In an attempt to better comprehend, visualize, and diagnose the operation of impedance based relays, the R-X diagram is used. This diagram permits the use of only two quantities R and X (or Z and θ in polar form) instead of the confusing combination of E, I, and θ. Further, we are able to represent the relay characteristics as well as the system characteristics on the same diagram and quickly determine at a glance what conditions will lead to relay operation. Impedance diagram showing line and relay characteristic Once the positive sequence impedance of a fault is known, it should be quite easy to determine the location of the fault and thus make a relaying decision. The only problem which creeps into the computation is fault resistance. In the case of phase to ground faults this is a more serious problem as there is the fault arc resistance in series with tower, footing, and grounding resistances. However, all fault types are subject to the varying arc resistance phenomenon. Therefore, it is of importance to discuss the effect of this resistance on relaying 7 computations and more specifically, the characteristic of the relay itself. As far as the tower footing resistance is concerned (in phase to ground faults), it is roughly a constant between 5 and 50 ohms. This can be compensated for by adding width to the relay trip characteristic on either side of the apparent impedance representing the line. This effectively covers all possible scenarios. As far as fault arc resistance, a generally accepted formula for estimation is:2 Rarc = 76V2 / SSC In the above equation, V represents the system line to line voltage in kv, and SSC represents the short circuit kva at the fault location. A worst case can be computed for this value and added to the resistance reach in the relay trip characteristic.

25 Pilot Distance Protection of Transmission Lines. There are quite a few protection schemes which are based on communication links between the relays at the far ends of a given transmission line. This is in order to realize the benefits of having information from both ends of the line in order to make accurate relaying decisions. Most of these pilot schemes depend on a power line carrier (PLC). Pilot protection schemes can be divided into two categories, tripping and blocking. Blocking refers to the fact that the communications signal is used to block a trip. When a fault is detected and no blocking signal is present, a trip is issued. When the blocking signal is present, the other end of the transmission line is sending the signal the fault is outside of our line and the line does not trip. Conversely, a tripping scheme is one in which the presence of a communications signal indicates that a trip should be issued. Tripping is only used when an alternative communications link to the line itself is available. Blocking on the other hand is usually used only for PLC. The reason for this is simple. When a fault occurs on a transmission line which is making use of PLC, the 9 signal between the two ends of the line can become severely attenuated. Under these conditions it is desirable to initiate a trip. Additionally, if a tripping scheme is used, the signal meant to initiate the trip could be lost and the line could fail to trip. Tripping is a viable option chosen when non PLC methods of communications are used such as microwave, fiber optic, or pilot wire.2 Further, it is better to use tripping methods when possible as they are faster since there is no need for coordination delays.