SWITCH GEAR AND PROTECTION [ ] ACTIVE LEARNING ASSIGNMENT TOPIC : DISTANCE PROTECTION OF TRANSMISSION LINES UNIVERSITY : GUJARAT TECHNOLOGICAL UNIVERSITY COLLEGE : VADODARA INSTITUTE OF ENGINEERING DEPARTMENT : ELECTRICAL ENGINEERING [E.E.– I] SEMESTER : VII COMPILED BY : [ JESTY JOSE] [ JOBIN ABRAHAM ] GUIDED BY : ASST. PROF. NIDHI SHAH [ELECTRICAL DEPARTMENT] [ELECTRICAL DEPARTMENT] ACTIVE LEARNING ASSIGNMENT 1

 Outline  Introduction to Distance protection of transmission lines Introduction to Distance protection of transmission lines Introduction to Distance protection of transmission lines  Mho type distance relay, Mho type distance relay, Mho type distance relay,  Effect of arc resistance on reach of distance relays, Effect of arc resistance on reach of distance relays, Effect of arc resistance on reach of distance relays,  Performance of distance relay during normal load and power swing. Performance of distance relay during normal load and power swing. Performance of distance relay during normal load and power swing.  References References ACTIVE LEARNING ASSIGNMENT 2

 Introduction  Distance protection, in its basic form, is a non-unit system of protection offering considerable economic and technical advantages.  Distance protection is comparatively simple to apply and it can be fast in operation for faults located along most of a protected circuit.  It can also provide both primary and remote back-up functions in a single scheme.  In this form it is eminently suitable for application with high-speed auto- reclosing, for the protection of critical transmission lines.  Many types of distance relays are there like Impedance relay, Reactance relay, Mho or admittance relay, Ohm or angle impedance relay, offset mho relay, quadrilateral and other special characteristics relays. ACTIVE LEARNING ASSIGNMENT 3

 Mho or admittance type distance relay  The operation of distance relays is mainly based on the impedance measured at relaying point. The voltage to current ratio of the fundamental frequency component seen at the relaying point is an indicator of the systems normal condition or faulty condition.  Exactly equal to an R-X diagram except all impedances are operated on by current I. ACTIVE LEARNING ASSIGNMENT 4

 Mho or admittance type distance relay {cont.}  The mho function uses the current and voltage measured at the relay to determine if the apparent impedance plots within the mho characteristic.  The determination is made by comparing the angle between the operating quantity (IZ - V) and the polarizing quantity (V, where V = IZf).  If the angle is less than or equal to 90°, then the fault impedance Zf plots within the characteristic, and the function will produce an output.  If the angle is greater than 90°, then Zf falls outside of the characteristic and no output will be produced.  Assume that the angle of maximum reach (θ) and the angle of ZL (φ) are equal. ACTIVE LEARNING ASSIGNMENT 5

 Mho or admittance type distance relay {cont.}  On that basis, the conditions shown in Figure 2 will be obtained. The key point to note in this phasor analysis (a convenient way to view relay performance) is the magnitude of the IZ - V (Vop) phasor and its relationship to the V (Vpol) phasor.  Operation will occur whenever Vop and Vpol phasors are within 90° of each other and provided both Vop and Vpol are greater than the minimum values established by the sensitivity of the relay design. ACTIVE LEARNING ASSIGNMENT 6

 Mho or admittance type distance relay {cont.}  For the balance point fault, IZ-V is zero, therefore no operation occurs, which is expected.  For an internal fault, IZ - V and V are in phase, therefore the function operates as expected.  For the external fault, operation does not occur because IZ V and V are 180° out of phase.  Observe that for the balance point fault, the V is exactly equal to IZ.  This is true for the three-phase fault shown (also for a phase-to-phase fault) and for a phase distance function only. ACTIVE LEARNING ASSIGNMENT 7

 Effect of arc resistance on reach of distance relay  The critical arc location is just short of the point on a line at which a distance relay's operation changes from high-speed to intermediate time or from intermediate time to back-up time.  We are concerned with the possibility that an arc within the high-speed zone will make the relay operate in intermediate time, that an arc within the intermediate zone will make the relay operate in back-up time, or that an arc within the back-up zone will prevent relay operation completely.  In other words, the effect of an arc may be to cause a distance relay to under reach. ACTIVE LEARNING ASSIGNMENT 8

 Effect of arc resistance on reach of distance relay {cont.}  For an arc just short of the end of the first- or high- speed zone, it is the initial characteristic of the arc that concerns us.  A distance relay's first-zone unit is so fast that, if the impedance is such that the unit can operate immediately when the arc is struck, it will do so before the arc can stretch appreciably and thereby increase its resistance. Therefore, we can calculate the arc characteristic for a length equal to the distance between conductors for phase-to- phase faults, or across an insulator string for phase-to-ground faults.  On the other hand, for arcs in the intermediate-time or back-up zones, the effect of wind stretching the arc should be considered, and then the operating time for which the relay is adjusted has an important bearing on the outcome. ACTIVE LEARNING ASSIGNMENT 9

 Effect of arc resistance on reach of distance relay {cont.}  Tending to offset the longer time an arc has to stretch in the wind when it is in the intermediate or back-up zones is the fact that, the farther an arcing fault is from a relay, the less will its effect be on the relay's operation.  In other words, the more line impedance there is between the relay and the fault, the less change there will be in the total impedance when the arc resistance is added.  On the other hand, the farther away an arc is, the higher its apparent resistance will be because the current contribution from the relay end of the line will be smaller, as considered later. ACTIVE LEARNING ASSIGNMENT 10

 Effect of arc resistance on reach of distance relay {cont.}  A small reduction in the high-speed-zone reach because of an arc is objectionable, but it can be tolerated if necessary.  One can always use a reactance-type or modified- impendance- type distance relay to minimize such reduction.  3 The intermediate-zone reach must not be reduced by an arc to the point at which relays of the next line back will not be selective; of course, they too will be affected by the arc, but not so much. Reactance-type or modified- impendance-type distance relays are useful here also for assuring the minimum reduction in second-zone reach. ACTIVE LEARNING ASSIGNMENT 11

 Effect of arc resistance on reach of distance relay {cont.}  Figure 5 shows how an impedance or mho characteristic can be offset to minimize its susceptibility to an arc. One can also help the situation by making the second- zone reach as long as possible so that a certain amount of reach reduction by an arc is permissible. Conventional relays do not use the reactance unit for the back-up zone; ACTIVE LEARNING ASSIGNMENT 12

 Effect of arc resistance on reach of distance relay {cont.}  instead, they use either an impedance unit, a modified- impendance unit, or a mho unit. If failure of the back- up unit to operate because of an arc extended by the wind is a problem, the modified-impendance unit can be used or the mho–or "starting"–unit characteristic can also be shifted to make its operation less affected by arc resistance.  The low-reset characteristic of some types of distance relay is advantageous in preventing reset as the wind stretches out an arc. ACTIVE LEARNING ASSIGNMENT 13

 Effect of arc resistance on reach of distance relay {cont.}  Although an arc itself is practically all resistance, it may have a capacitive-reactance or an inductive-reactance component when viewed from the end of a line where the relays are. The impedance of an arc (ZA) has the appearance: ACTIVE LEARNING ASSIGNMENT 14

 Effect of arc resistance on reach of distance relay {cont.}  where I1 = the complex expression for the current flowing into the arc from the end of the  line where the relays under consideration are.  I2 = the complex expression for the current flowing into the arc from the other end of the line.  RA = the arc resistance with current (I1 + I2) flowing into it.  If I1 and I2 are out of phase, ZA will be a complex number. Therefore, even a reactance-type distance relay may be adversely affected by an arc. This effect is small, however, and is generally neglected. ACTIVE LEARNING ASSIGNMENT 15

 Effect of arc resistance on reach of distance relay {cont.}  Of more practical significance is the fact that, as shown by the equation, the arc resistance will appear to be higher than it actually is, and it may be very much higher. After the other end of the line trips, the arc resistance will be higher because the arc current will be lower. However, its appearance to the relays will no longer be magnified, because I2 will be zero. Whether its resistance will appear to the relays to be higher or lower than before will depend on the relative and actual magnitudes of the currents before and after the distant breaker opens. ACTIVE LEARNING ASSIGNMENT 16

 Power swings and its causes  In the normal operation, the electric power system maintains a dynamic and delicate balance between generation and load.  A disturbance, such as a sudden change of load, a power system fault, or a trip of a generator, may break the balance and cause oscillations in generator rotor angles.  The oscillations are due to the large inertia of the generators and the relatively slow control of input mechanical power.  During a power swing, voltages and currents in the power grid will show a certain amount of oscillations in magnitude and phase angle, which can cause unwanted operation of distance protection, relays.  Moreover, the operation of protective relays may exacerbate system stability and can lead to cascading power outages. ACTIVE LEARNING ASSIGNMENT 17

 Performance of Power swing in distance relay  As modern transmission systems become heavily loaded the benefits of series compensation for many of the grid’s transmission lines become more obvious.  Clearly, adding fixed series compensation has long been the preferred solution for optimizing performance in very large bulk transmission corridors.  Installing a capacitive reactance in series in a long (typically more than 200 km) transmission line reduces both the angular deviation and the voltage drop, which lowering losses, increases the load ability and stability of the line. ACTIVE LEARNING ASSIGNMENT 18

 Performance of Power swing in distance relay {cont.}  It also improves the voltage profile along the power corridor and optimizes power sharing between parallel circuits.  Unfortunately, the series capacitor can undermine the effectiveness of many of the protection schemes used for long distance transmission lines.  The introduction of the capacitance in series with the line reactance adds certain complexities to the effective application of impedance based distance relays.  However, series compensation increases the fault current level and may also cause generator sub- synchronous resonance. ACTIVE LEARNING ASSIGNMENT 19

 Performance of Power swing in distance relay {cont.}  Power system at steady operation maintains a balance between the generation and load. System disturbances, such as line switching due to the fault, generator disconnection, and switching ON/OFF large loads cause oscillations in rotor angles among generators and can result in severe power-flow swings.  As a consequence, the apparent impedance seen by a distance relay may fall within its operating zone.  This may be misinterpreted as a fault and the relay would trip the line unnecessarily. ACTIVE LEARNING ASSIGNMENT 20

 Book References 1. BHAVESH BHALJA, R.P. MAHESHWARI, NILESH G. CHOTANI “Protection and Switchgear”, OXFORD UNIVERSITY PRESS, Sixth impression 2016, ISBN : SUNIL S. RAO “SWITCHGEAR PROTECTION AND POWER SYSTEMS – Theory, Practice & Solved Problems”, KHANNA PUBLISHERS, Thirteenth edition, Fifteenth Reprint 2015, ISBN : J.B. Gupta “A Course in POWER SYSTEMS”, S.K. KATARIA & SONS PUBLISHERS, Eleventh Edition, Reprint, 2015, ISBN : V.K. METHA, ROHIT METHA “PRINCIPLES OF POWER SYSTEMS”, S.CHAND & Co. PVT. LTD., First Multicolour Edition, Reprint, 2014, ISBN : ACTIVE LEARNING ASSIGNMENT 21

 Website References pdf pdf pdf 2. /ger-3966.pdf /ger-3966.pdf /ger-3966.pdf 3. /art14.pdf /art14.pdf /art14.pdf _2000/GER-3743.pdf _2000/GER-3743.pdf _2000/GER-3743.pdf 5. analysis-of-power-swing-indistance-relay- quadrilateral-relay-characteristics-for- seriescompensatedtransmission-line.php?aid= analysis-of-power-swing-indistance-relay- quadrilateral-relay-characteristics-for- seriescompensatedtransmission-line.php?aid= analysis-of-power-swing-indistance-relay- quadrilateral-relay-characteristics-for- seriescompensatedtransmission-line.php?aid=43312 ACTIVE LEARNING ASSIGNMENT 22

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