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. In addition, a fairly high bias slope is required to prevent maloperation by CT differential currents arising from. A current transformer (CT) associated with secondary unrestricted earth fault protection is located on the earth side of the neutral to earth link. 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. The earth fault relay operates only for the internal fault of the protective zone. : 4 The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, overvoltage, reverse power flow, over-frequency, and under-frequency. Thus, no CTs are required, and the technique may be useful where provision of an extensive number of core-balance CTs is impossible or difficult, due to physical constraints or on … Figure 4.4.2. Such settings protect a restricted portion of the winding. Selectivity, also known as discrimination, is one of the fundamental qualities of a protection relay. The advantages to be obtained by the use of restricted earth fault protection, discussed in previous article lead to the system being frequently used in conjunction with an overall differential system. Damper winding thermal protection may be used as an alternative to field failure protection if the machine is required to run for long periods as an induction motor. 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. 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. 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. Consider an earth fault on motor terminal, ASB. Instantaneous high-set overcurrent protection. The Example Over current/Earth fault relay is shown in the below figure. 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. ⊗=open bus-section circuit breaker. This can be included with thermal overload relays. We use cookies to help provide and enhance our service and tailor content and ads. Maximum 11 kV fault current If=1,347×33/11=4,042 A (see Section 10.5.3.1). The bias slope is chosen so that the relay is insensitive to unbalanced external lead burdens which normally give a lower ratio of differential current to through current than an internal fault. Copyright © 2021 Elsevier B.V. or its licensors or contributors. At 22 V the relay current is approximately 11 mA and the CT magnetizing current 32 mA. 2. Where the overloads period is short a loss of synchronism relay can be arranged to disconnect the field supply only, so that the machine runs as an induction motor during overload then the relay resynchronizes the machine automatically after overload periods. Relays with harmonic restraint filters can be employed in cases where the harmonic component of the load (such as with discharge lighting) is high. Such an arrangement will preserve the two healthy sections, provided the fault is not on the central section. 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. For an earth fault, IF=IFA+IFB, on the left-hand side LV switchgear busbar fed by transformer A a proportion of the fault current, IFB, may return to the neutral of transformer A via the earthing path of transformer B. This relay is not suitable for backup protection. Normally, during earth-fault conditions, very high earth-fault currents will flow in a solidly-earthed transformer. This arrangement is unsatisfactory when applied to a multi-source supply system. In a small generator, the neutral end of the three phase windings is connected internally to a … Thermal relays are employed for overloading, also a separate stalling relay is employed for motor stalling conditions. 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. The transformer windings and connections are protected by an extremely inverse relay and two high set overcurrent relays. This characteristic may be altered by adjusting the turns ratio of the operating and restraint coils. The most occurred and severe earth fault happens when insulation of winding get damaged, and it touches the core of the machine. Under normal load conditions the CT secondary currents are equal and no current flows in the relay operating coil. 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. 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. An earth fault relay is activated by the fault current flowing from line to earth, while an over-current relay is activated if the line current exceeds a certain value. 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. The relay contacts close when the ratio of this differential current to the through current exceeds the slope of the relay operating characteristic. Synchronous machines must be protected against this condition because it can go out of step with supply after an interruption. If the switchboard has three sections, directional overcurrent relays, arranged to detect fault currents flowing outwards, may be fitted to the bus-section switches. As an alternative consider a resistance-earthed neutral with earth fault current limited to 1,000 A. 4.4.2). But the use of this term is not recommended, as unrestricted earth fault protection will not detect phase-to-phase faults and therefore the term ‘back-up’ or ‘standby’ leads to some confusion. A defective bearing is indicated by temperature rise and vibration and a small rise in motor current. In the event of an earth fault the difference in potential between a person's feet (step potential) should not reach dangerous levels. The disadvantage of this connection arrangement is that the SBEF CTs will now register 3rd harmonic or out of balance load currents. Discover everything Scribd has to offer, including books and audiobooks from major publishers. The rotor is also protected against earth faults. Under internal fault conditions within the transformer LV winding or connections the CT currents will become unbalanced and the LVREF protection will operate. different CT ratios and hence saturation levels giving differential currents under through-fault conditions. An undercurrent relay in the field circuit gives satisfactory protection. Lead and CT resistances to common LVREF relay connection point=0.4 ohm neutral to relay cable resistance per core. IF=busbar earth fault current. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780080405148500197, URL: https://www.sciencedirect.com/science/article/pii/B9780128168585000186, URL: https://www.sciencedirect.com/science/article/pii/B9780080969121000101, URL: https://www.sciencedirect.com/science/article/pii/B978075064452550078X, URL: https://www.sciencedirect.com/science/article/pii/B9780123854995000224, URL: https://www.sciencedirect.com/science/article/pii/B9780123854995000248, URL: https://www.sciencedirect.com/science/article/pii/B9780080969121000046, URL: https://www.sciencedirect.com/science/article/pii/B9780124104617000080, Electrical Systems and Equipment (Third Edition), Practical Power System and Protective Relays Commissioning, DrC.R. Hence the name restricted earth fault protection. Consider the same network as shown in Fig. Unbalanced or negative-phase sequence protection should be used as rotor heating due to unbalanced currents being a function of the negative-sequence component of line currents. Consider the case of two auxiliary transformers, A and B, used to derive the substation auxiliary LVAC supply. If the earth fault current path is particularly unfavourable it is possible for the LVAC switchboard incomer from transformer B to be tripped, thereby losing the healthy side of the switchboard. And feeders from over current and earth fault overcurrent relay presented in an format! Transformers, a short chapter on the starter because the motor terminal, ASB and protective relays Commissioning 2019. An entry-level three-phase and earth earth fault relay is also known as relay is connected to secondary of CT..., very high earth-fault currents will flow in a circuit, it 's is. So we can say that RCCB does the same ‘ classic ’ SBEF CT location now 3rd. 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