Motors with powers greater than 50 HP, supplied from an earthed system, should be protected against earth faults to reduce fault damage and accident risk, especially motors not protected by differential protection. This is the ‘classic’ standby earth fault (SBEF) protection CT location and transformer connection to the LVAC switchboard for a single transformer source of supply. Figure 1-1 System considered for earth fault coordination by fuse We reserve all rights in this document and in the information therein. Field failure relay should be provided for synchronous machines larger than about 500 HP which are not provided with loss of synchronism protection, preventing overheating of the rotor after continuous operation as an induction motor. Transverse differential protection can be used against interturn faults, where the stator windings are divided into two or more circuits. It is used when it is obvious that large fault currents are the consequence of fault on the equipment being protected by the relay e.g., close-in fault on a long feeder. Omar Salah Elsayed Atwa, in Practical Power System and Protective Relays Commissioning, 2019. Auxiliary transformer LV connections: (a) Single transformer LVAC auxiliary supply source and simplistic single line diagram representation (inset); (b) substation auxiliary supply derived from duplicate transformer source with incorrect SBEF CT location; (c) substation auxiliary supply derived from duplicate transformer source with correct SBEF CT location. Earth Leakage Protection Relays (ELR series) Earth Leakage relays (ELR’s) or also known as ground fault relays are residual current monitors (RCM’s) that are used to detect potentially dangerous ground fault currents before they cause any hazard. All the protection systems mentioned above, depending on circuit conditions, trip their own unit, the banked generator-motor unit, the 400 kV circuit-breakers and send a trip signal to the starting busbar system to trip the generator-motor, if they are in a starting sequence. The Example Over current/Earth fault relay is shown in the below figure. Supplied with TecQuipment’s Protection and Relay Test Set (PSL50), the relay enables investigations into protection and monitoring of transformers, transmission lines and distribution schemes. Claire Soares, in Gas Turbines (Second Edition), 2015. Figure 10.13b. =0.1 ohm phase-to-relay cable resistances per core. The earth fault relay operates only for the internal fault of the protective zone. The NewElec GA Earth Leakage relay provides earth leakage protection also known as ground fault protection designed to protect personnel and equipment by rapid disconnection of main supply to faulted equipment a unique feature is the integrated distinguishing between Earth leakage and Earth fault routing trips to incoming circuit breaker rather than the fault level limited main circuit contactor. Figure 10.13c. away from the bus. Figure 4.6b shows such a system with a key-interlocked normally open LV busbar-section circuit breaker and the same ‘classic’ SBEF CT location. Maximum 11 kV fault current If=1,347×33/11=4,042 A (see Section 10.5.3.1). The importance of this is shown in Figure 11 from which it will be seen that if the neutral of a star-connected winding is earthed through a resistance of one per unit, an overall differential system having an effective setting of 20%will detect faults in … Balanced Earth Fault Protection The balanced earth fault protection scheme is mainly used for protection of small generator where differential and self-balanced protection systems are not applicable. In directional earth fault relay, the voltage coil is actuated by the residual voltage. As the name indicates, it's action is fast. Failure of a bearing can cause motor stalling. The extremely inverse relay used for this protection has to grade with the fuses protecting the excitation thyristors. Earth fault protection of the 18 kV winding is included in the unit transformer earth fault protection scheme. Figure 4.4.2. Synchronous machines subjected to sudden mechanical overloads should be protected against loss of synchronism, which can also be caused by a reduction of field current or supply voltage. 1.2. Traditionally, such a relay has only the pick-up setting and it does not have any TMS. However, it does have the advantage over restricted, The overcurrent fault protection relays on the primary circuit of the transformer will provide some protection to the secondary circuit, but, . These relays initiate a Unit trip. The earthing conductors must be capable of carrying maximum earth fault current without overheating or causing mechanical damage. A single auxiliary transformer is normally connected to the LVAC distribution switchboard earth and neutral busbars via links as shown in Figs. The output contacts of lock out relay are wired to the breaker trip coil circuit. Synchronous machines must be protected against this condition because it can go out of step with supply after an interruption. This will result in desensitizing the LVREF protection and approximately 43/1,000=4.3% of the transformer windings at the neutral end will not be protected. The relay is connected to secondary of a CT whose primary is connected in neutral to earth connection. For Later, Many developing countries confront widespread theft of, In India electricity theft leads to annual losses estimated at, #onest consumers, poor people, and those without, connections, who bear the burden of high tariffs, system, inefficiencies, and inadeuate and unreliable power. A single earth fault results in a rise in the voltage between system neutral and earth, which may be detected by a relay measuring the residual voltage of the system (normally zero for a perfectly balanced, healthy system). The directional relay is connected in series with the Overcurrent relay so that the trip command is only given if there is a fault and also flow of power due to the fault in the forward direction, i.e. Consider an out-of-zone fault with one CT saturated. This kind of earth fault protection is also known as unrestricted earth fault protection. 5). At 22 V the relay current is approximately 11 mA and the CT magnetizing current 32 mA. In addition it should be noted that the IFB proportion of the fault current will bypass, and not be summated by the neutral CT associated with the transformer supplying the fault. With a resistance-earthed neutral the maximum earth fault current is 1,000 A. A single earth fault in the rotor circuit will complete the circuit comprising the voltage source (ac or dc), sensitive overvoltage relay and earth fault and thus earth fault will be sensed by the relay. In Electrical Systems and Equipment (Third Edition), 1992. Directional earth fault relays sense the direction in which earth fault occurs with respect to the relay location and it operates for fault in a particular direction. By continuing you agree to the use of cookies. At the same time maloperation of transformer B protection is avoided and transformer B continues to supply the healthy busbar and associated substation LVAC loads. The major function of an earth fault relay is to operate for the leakage currents to the earth. The protection setting should take into consideration that the three CTs do not have identical characteristics and will perform differently for heavy phase-to-phase faults or for initial asymmetrical motor starting currents. The directional relay is supplied with the current as well as the voltage. A sensitive underfrequency relay is used for this condition. Bayliss CEng FIET, B.J. 4.6c involve relocating the SBEF CTs close to the transformer neutrals. Copyright © 2021 Elsevier B.V. or its licensors or contributors. IF=busbar earth fault current. Over current/Earth fault relays are the basic protection relays. A current transformer (CT) associated with secondary unrestricted earth fault protection is located on the earth side of the neutral to earth link. These are normally fitted in machines of 1000 HP and above. Assuming, for example, that because of the CT errors 1% of the phase fault current will be seen by the earth-fault relay this becomes 5..20 x 1% = 5..20% of the rated secondary current. If the busbars are sectionalised, the bus-section switch may be fitted with overcurrent or directional overcurrent protection. The substation and its environment should not be connected to metal objects such that the voltages arising within the substation under fault conditions can be transferred to a point remote from the substation. magnetising inrush giving secondary currents in one set of CTs only. An undercurrent relay in the field circuit gives satisfactory protection. 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