The current measurement have to be DC insensitive to allow a use of AC components of the fault current in the calculations. Manufacturers of generator generally set the typical or minimum value. The fault resistance Rf, consists of the arc resistance and the tower foot resistance. Care must be taken to ensure that the relay does not operate from the spill current resulting from unequal CT saturation during motor starting, where the high currents involved will almost certainly saturate the motor CT’s. The principle is the same as already detailed, except that the voltage is phase shifted by +90ºC instead of -90ºC. A typical setting of 30% of motor rated current is used, leading to an earth fault relay setting of: A stabilising resistor is required, calculated in accordance with Equation above to prevent maloperation due to CT spill current during starting as the CTs may saturate. In the TT method of earthing, this earth fault protection device shall be a residual current device with adequate sensitivity. TN-C networks save the cost of an additional conductor needed for separate N and PE connections. IGF 3VLN Z0 ++ +Z1 Z2 3ZG = -----IGF VLN Z1 = -----GET-8390: Sensitive Ground Fault Protection in the F60 2 GE Power Management Typical Connections for Ground Fault Protection RESIDUAL CONNECTION The ground fault elements are connected in the common neutral connection of the line current transformers. For such a case a voltage can be achieved across the relay. This section gives examples of the protection of HV and LV induction motors. It helps you. The earth fault relay is placed in the residual part of the current transformers shown in the figure below. However, to mitigate the risk of broken neutrals, special cable types and many connections to earth are needed. Basically there are just two type earth fault and restricted earth fault. Earth Fault Protection Types This is the different between types of earth fault protection Residual earth fault protection: Uses three CTs and the earth fault relay is connected in the residual point of the three CTs. Sensitive earth fault protection using a core-balance CT is required for this scheme. Thus with four relays as shown in fig 17.2 complete overcurrent protection can be provided. The second harmonic stabilization can then block the earth fault relay, during the inrush and prevent the risk of an unnecessary operation. They also enhance the protection of the power semiconductors in the event of an earth fault or short-circuit, and protect the motors from excessive rates of voltage rise … ), the presence of the surrounding metallic frame and casing will ensure that it rapidly develops into a fault involving earth. 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). Inverse or definite time delay can be used. Normally the inverse characteristic and the logaritmic inverse characteristic gives the best possibility to achieve time selectivity also at back-up tripping. The type and sensitivity of protection provided depends largely on the system earthing, so the various types will be dealt with in turn. Study specialized technical articles and papers. The voltage is mainly of third harmonic and of size 0,2-0,5% with conventional VTs and 1-3% together with CVTs. To achieve this, it may require the use of an intentional definite time delay in the relay. In plain language this means that fault protection provides us with protection against receiving an electric shock from conductive parts that are not intended to be live but have become live due to a fault. As there is no earth fault current, it is also possible to set the relay at site after deliberately applying earth faults at various parts of the system and measuring the resulting residual currents. The problem is fault current reversal which occurs when the CB at one end of the faulty line trips before the breaker at the other end. It is quite common to connect the Restricted earth fault relay in the same current circuit as the transformer differential relay. This is a ring type CT, through which all phases of the supply to the motor are passed, plus the neutral on a four-wire system. The type of earth fault protection used is dependent of the system earthing principle used. Sensitivities of 2-8% can be achieved. Figure 4 illustrates the application of a core-balance CT, including the routing of the cable sheath to ensure correct operation in case of core-sheath cable faults. According to the construction type of transformers the following protectionsmust be provided: Oil-filled transformers with conservator 1. By inserting varying fault resistances one can get the fault current level. It is common, however, to provide both instantaneous and time-delayed relay elements to cater for major and slowly developing faults. It is also necessary to ‘mirror’ the Carrier signal back so the signal is sent back on receipt if the UN voltage is low, or if the circuit breaker is open. An alternative technique, avoiding the use of a stabilising resistor is to use a definite time delay characteristic. TT networks require proper RCD (Ground fault interrupter) protection. The arc resistance is calculated by the formula: Rarc = 28700×a / If1.4 (according to Warrington). An alternative technique using residual voltage detection is also possible, and is described in the next section (below). The content is copyrighted to EEP and may not be reproduced on other websites. For applications where a sensitivity of > 20% of motor continuous rated current is acceptable, conventional earth fault protection using the residual CT connection of Figure 1 can be used. Tell us what you're thinking... we care about your opinion! The common principle for Restricted earth fault relays is the high impedance principle, see Figure 4. The relay provides a high impedance to the current. Restricted earth fault protection is a type of earth fault protection which looks for earth fault between two sets of current transformers (hence restricted to that zone). Two types of earth fault protection are commonly found – depending on the sensitivity required. There are three types of TN systems, differentiated on the basis of the arrangement of the neutral and protective earth conductors: TN-S: The protective earth conductor is separate throughout the system. To prevent the risk of flashover in the circuit, a voltage limiter must be included. 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 cost grounds. Induction type IDMT relay is used for this purpose. Co-ordination with other devices must also be considered. In other words, to connect the metallic parts of electric machinery and devices to the earth plate or earth electrode (which is buried in the moisture earth) through a thick conductor wire (which has very low resistance) for safety purpose is known as Earthing or grounding. The voltage limiter can be either of type surge arrester or voltage dependent resistor. The long durations are often achieved in weak networks. The setting voltage must be calculated from knowledge of system earthing and impedances, an example for a resistance-earthed system is shown in Figure 7. The solution is to use a sensitive directional earth fault relay. An earth fault includes a half-done breakdown of winding insulation to earth. Differential protection is the most important type of protection used for protection against internal phase-to-phase and phase-to-earth faults. Using an Alstom MiCOM P241 motor protection relay, the earth fault protection settings is calculated: Table 1 – Parameters of HV induction motor. The advantage with the restricted earth fault relays is their high sensitivity. 95% Stator Protection: Neutral Overvoltage Scheme; Overcurrent Scheme; 100% Stator Protection: 3 rd Harmonic Under-Voltage Scheme; Neutral Injection Scheme; 95% Stator Earth Fault Protection: Neutral Overvotage (59N) or Neutral Displacement Relay (64N): The sensitive earth-fault relay is suited for both primary and back-up earth-fault protec-tion. The distribution of fault currents, from the different system earthing points, can be derived from the distribution in the zero sequence network (see Figure 2 below). The earth fault protection scheme consists the earth fault relay, which gives the tripping command to the circuit breaker and hence restricted the fault current. It must also be noted that UN will be very low when sensitive earth fault relays are used in a strong network with low zero sequence source impedances. 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