Differential Protection. These currents reaches levels up to 20% of the load current and an unnecessary tripping from earth fault relays can therefor be achieved. This type of protection was considered unnecessary in previous days because breakdown of insulation between points in the same phase winding, contained in the same slot, and between which a potential difference exists, very rapidly changes into an earth fault, and then it is detected by either the stator differential protection or the stator earth fault protection. The VTs used must be suitable for the duty, thus 3-limb, 3-phase VTs are not suitable, and the relay usually has alarm and trip settings, each with adjustable time delays. The arc resistance is calculated by the formula: Rarc = 28700×a / If1.4 (according to Warrington). Earth fault current is always less than phase fault current in magnitude. To earth or earthing rather, means to connect the part of electrical apparatus such as metallic covering of … The connection of a restricted earth fault relay is shown in Figure 3. In the following the earth fault protection for solidly (effectively) earthed systems is covered. The other protection systems employed for protection of transformers against internal faults are Buchholz protection, core-balance leakage protection, combined leakage and overload protection, restricted earth-fault protection. This relay protects the delta or unearthed star … The relay sensitivity is decided by the total current in the circuit according to the formula: It should be remembered that the vectorial sum of the currents must be used. 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. The tower foot resistance depends on the earthing effectiveness of the towers, whether top lines are used etc. The residual current detected by the relay is the sum of the charging currents flowing in the healthy part of the system plus the healthy phase charging currents on the faulted feeder – i.e. A special application problem occurs together with directional earth fault schemes communicating in a permissive overreaching (POR) scheme. The VT must be suitable for the relay and therefore the relay manufacturer should be consulted over suitable types – some relays require that the VT must be able to carry residual flux and this rules out use of a 3-limb, 3-phase VT. A setting of 125% of the single phase capacitive charging current for the whole system is possible using this method. Restricted Earth Fault Protection of Transformer. Therefore, provision of earth fault protection is very important. 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. 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. Thus with four relays as shown in fig 17.2 complete overcurrent protection can be provided. Using an Alstom MiCOM P241 motor protection relay, the earth fault protection settings is calculated: Table 1 – Parameters of HV induction motor. A fault on a circuit will generally create a sag in the voltage level. Figure 3(a) illustrates incorrect grading of the relay with the fuse, the relay operating first for a range of fault currents in excess of the contactor breaking capacity. Study specialized technical articles and papers. The time delay used will normally have to be found by trial and error, as it must be long enough to prevent maloperation during a motor start, but short enough to provide effective protection in case of a fault. A common means of supplying a motor is via a fused contactor (Figure 2). In special applications, a situation where the fault current infeed from one end isn’t ensured during certain service conditions. It operates the CB when REF is failed to trip the circuit, Heavy earth fault outside of the REF protective Zone, and all other earth faults. Magnetising current requirements are also reduced, with only a single CT core to be magnetised instead of three, thus enabling low settings to be used. 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. Co-ordination with other devices must also be considered. The current will, for through loads and through faults, circulate in the current transformer circuits, not go through the relay. The content is copyrighted to EEP and may not be reproduced on other websites. The common principle for Restricted earth fault relays is the high impedance principle, see Figure 4. It’s to be noted that U0 is generated by the earth fault current I0 through the zero sequence source. It is advantageous to use an earth fault relay under such circumstance. 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). Stator Earth fault protection: Stator Earth fault protection of any Generator provided in the following ways. To prevent the risk of flashover in the circuit, a voltage limiter must be included. The fault resistance Rf, consists of the arc resistance and the tower foot resistance. The content is copyrighted to EEP and may not be reproduced on other websites. There are three major types of connections for 51N/50N. 20% off code for Pro Plan: EEP2020. The calculations are made with the worst situations in mind and an operating voltage UR is calculated: Due to the fast saturation very high top voltages can be achieved. 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 current measurement have to be DC insensitive to allow a use of AC components of the fault current in the calculations. The mixed connection is shown in the low voltage winding of the transformer, see Figure 3 above. Such faults are detected by the REF. 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. The voltage limiter can be either of type surge arrester or voltage dependent resistor. The type of earth fault protection used is dependent of the system earthing principle used. The relay provides a high impedance to the current. 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 alternative is to omit the stabilising resistor and use a definite time delay in association with the earth-fault element. Distance (impedance relay) Distance protection detects both voltage and current. The type of earth fault protection used is dependent of the system earthing principle used. The earth fault can be dispersed by using the restricted earth fault protection scheme. By inserting varying fault resistances one can get the fault current level. The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding. Therefore, relay connected for earth fault protection is different from those for phase to phase fault protection. With a residual connection of line CT’s, the minimum sensitivity possible is about 10% of CT rated primary current, due to the possibility of CT saturation during starting. Buccholz relay for dielectric failure (2 steps: alarm and trip) 2. Learn about power engineering and HV/MV/LV substations. With an I, Restricted earth fault relays are also very quick, The maximum operating voltage have to be calculated (, A blocking of the earth fault scheme at distance protection operation is often used, Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! https://www.tutorialspoint.com/.../electrical_safety_earth_fault_protection.htm 1. If a more sensitive relay setting is required, it is necessary to use a core-balance CT (CBCT). Lack of proper co-ordination results in generator tripping to downstream faults. Manufacturers of generator generally set the typical or minimum value. The logic for an earth fault weak-end infeed function includes a check of occurring UN voltage at carrier receipt and the breaker is tripped even if no operation of directional earth fault (DEF) relay is achieved due to a to weak source. A special inverse characteristic with a logaritmic curve has been developed to be suitable for these applications. The fault statistic shows that earth faults are the dominating fault type and therefore the earth fault protection is of main importance in a network. 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). The occurring neutral point voltage, at different locations, can be seen in Figure 2. As an example we can use the 132kV network according to Figure 1 and 2. However, to mitigate the risk of broken neutrals, special cable types and many connections to earth are needed. Inverse or definite time delay can be used. In solidly earthed system How can we interpret the fault current and motor starting current if we use a Stabilizing resistor in series with Residual relay.??? In the effectively earthed systems all transformers are normally connected to earth and will thus feed earth fault current to the fault. At reception of a signal and detection of a forward fault at the receiving end, an instantaneous trip is given. The turns ratio of the CT is no longer related to the normal line current expected to flow, so can be chosen to optimize the pick-up current required. The reversed element is set to be more sensitive than the forward element and will, when a reverse fault is detected, send a carrier signal (CS), to the remote end. Reference // Network Protection & Automation Guide by Alstom Grid, Wonderful earth fault protection of an ac motor in 4 different earthing systems, Thanks Tell me about lndusrial areas good earthing systems. IEEE/IAS – Atlanta Chapter March 2010 8 Modified Differential Ground Fault Protection Systems A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. 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. It is assumed that no CBCT is fitted. This gives a long delay compared to normal inrush times and the inverse characteristic will then match the decay of the inrush current and keep the relay away from unwanted functions. The principle is the same as already detailed, except that the voltage is phase shifted by +90ºC instead of -90ºC. For solidly earthed systems a restricted earth fault protection is often provided as a complement to the normal transformer differential relay. The time delay used is not critical but must be fast enough to disconnect equipment rapidly in the event of a second earth fault occurring immediately after the first. Most LV systems fall into this category, for reasons of personnel safety. Tell us what you're thinking... we care about your opinion! This section gives examples of the protection of HV and LV induction motors. 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. If it is possible to set the relay to a value between the charging current on the feeder being protected and the charging current for the rest of the system, the directional facility is not required and the VT can be dispensed with. To provide detection of high resistance earth faults in an effectively earthed network it’s common to use a directional comparison scheme with directional earth fault relays at both ends of the power line. three times the per phase charging current of the healthy part of the system. Generator phase fault protection Generator over current protection should be delayed and co- ordinated with downstream faults. The second harmonic stabilization can then block the earth fault relay, during the inrush and prevent the risk of an unnecessary operation. An earth fault usually involves a partial breakdown of winding insulation to earth Under this circumstance it is profitable to employ an earth fault relay One method of protection is core balance protection . Whatever the initial form of the fault (phase phase, etc.) In such cases the inrush can cross the corner with minimum current before the time elapses and an unwanted function can occur. 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. The setting should not be greater than about 30% of the minimum earth fault current expected. specific value re gardless the fault type or location. This ensures that a single phase tripping for heavy single phase faults can be done by distance protection relay first. • Lighting power protection devices: These include lighting arrestors and grounding devices to protect the system against lightning and surge voltages. Such a connection (a buried metal structure) is required to provide protective earth in IT and TT systems. Ground Fault Protection System. A core balance CT is used in conjunction with a VT measuring the residual voltage of the system, with a relay characteristic angle setting of +45ºC. If not, the relay will trip. If the ratio … 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. Detection of earth fault and back-up tripping with maintained selectivity in a solidly (effectively) earthed system is rather complicated due to the infeed of fault current from different direction concerning all faults. TT networks require proper RCD (Ground fault interrupter) protection. A lower limit is imposed on the setting by possible load unbalance and/or (for HV systems) system capacitive currents. However, detection is still essential as overvoltages occur on sound phases and it is necessary to locate and clear the fault before a second occurs. A permissive scheme principle is shown in Figure 7. The filtering must be quite heavy to ensure correct directional measuring for 1% fundamental content also with third harmonic contents of say 3%. Starting protection is provided to detect phase to phase faults (earthed or unearthed) down to as low a frequency as possible. 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. An external fault in the star side will result in current flowing in the line current transformer of the affected phase and at the same time a balancing current flows in the neutral current transformer, hence the resultant current in the relay is therefore zero.So this REF relay will not be actuated for external earth fault. In some countries a second harmonic stabilization is required for sensitive earth fault relays. Safety. In some HV systems, high resistance earthing is used to limit the earth fault current to a few amps. This protection can be ensured by bonding and connection to earth of the accessible conductive parts and the use of an earth fault protection device. A special weak end infeed logic can be used together with POR schemes. Oil pressure and level switches (2 steps: alar… A calculation will show that values will differs from below 1 Ω for heavy faults, up to 50-400 Ω for high resistive earth faults. An earth fault includes a half-done breakdown of winding insulation to earth. Let’s explain one by one. Figure 3(b) illustrates correct grading. In the TT method of earthing, this earth fault protection device shall be a residual current device with adequate sensitivity. Application-based three-phase fault analysis. where a, is the arc length in meter, normally the insulator length, and If is the fault current in A. What do you understand about earth fault protection? Sensitivities of 2-8% can be achieved. The background to this is that the inrush currents occurring at transformer energizing which, in some networks has long durations. Various types of Line Faults It is quite common to connect the Restricted earth fault relay in the same current circuit as the transformer differential relay. Other than this, the considerations in respect of settings and time delays are as for solidly earthed systems. This principle is referred to as “selectivity” in protection engineering. For applications where a sensitivity of > 20% of motor continuous rated current is acceptable, For a core-balance CT, the sensitivity that is possible using a simple non-directional earth fault relay element is limited to, Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! An alternative technique, avoiding the use of a stabilising resistor is to use a definite time delay characteristic. Earthing system is also classified into two types. When Distance protection relays with single phase tripping and auto reclosing are used at the same line as a scheme with earth fault relays it must be ensured that the distance protection relays are allowed to give their single-phase tripping first. Grading has to be carried out with this in mind, and will generally be on a time basis for providing alarms (1st stage), with a high set definite time trip second stage to provide backup. Earth fault relays must therefore be time delayed to allow this. Sensitive earth fault protection using a core-balance CT is required for this scheme. To connect the metallic (conductive) Parts of an Electric appliance or installations to the earth (ground) is called Earthing or Grounding. The input impedance of the energizing circuit of the earth-fault relay is extremely low which means that the relay can also be energized from low output core-balance cur-rent transformers. During a single phase trip an unbalance in the complete network occurs and an earth fault currents flows through the network. Normally the inverse characteristic and the logaritmic inverse characteristic gives the best possibility to achieve time selectivity also at back-up tripping. 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. 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 … At the remote end the forward element is provided with a short time delay T0 normally set to 50-150 ms, to check if a blocking signal is received. The detailed theory and protection method is explained in this technical article. For inverse time delayed scheme a time setting is selected to achieve selectivity to instantaneous protection. Earth fault relays like 51N/50N detects earth faults because of the CT connection. 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 symmetrical component scheme for a 132 kV system with a fault according to Figure 1 is shown in Figure 2. The principle for earth fault relays in a effectively earthed system is shown in Figure 5 and the logaritmic inverse characteristic is shown in Figure 6. With the stabilising resistor present, instantaneous tripping is permitted. For such a case a voltage can be achieved across the relay. How one can protect the transformer from lightening? High resistance grounding restricts the earth fault current to a very less value and permits the operation of equipment. This implies that the angle between U0 and I0 is always equal to the zero sequence source angle, independent of the fault resistance and the angle between the faulty phase voltage and the line current in the faulty phase. Save the cost of an intentional definite time delayed to allow this less value and permits the of... Has two non-current carrying parts such as metallic covering of … earth fault relays are normally blocked the... 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Is copyrighted to EEP and may not be greater than about 30 % the! In special applications types of earth fault protection a situation where the fault current in the complete network occurs and an function! Countries a second harmonic stabilization is required for sensitive earth fault protection: stator earth fault relays is their sensitivity. At the receiving end, an instantaneous trip is given what you 're...... Can be safe by providing an alternative technique using residual voltage detection is also valid when communication are... Avoiding the use of AC components of the most common faults to on. Distributed generation in magnitude possible load unbalance and/or ( for HV systems ) system capacitive currents fault! For dielectric failure ( 2 steps: alarm and trip ) 2 carrier... Protection coordination practices in distribution systems with distributed generation re gardless the fault or... Where the fault current to the forward element ) is required for this.! ) earthed systems all transformers are normally blocked during the single line of Ground fault interrupter protection! 1 gives relevant parameters of a HV induction motor to be noted that U0 is generated by the:! The logaritmic inverse characteristic with a reverse locking element as a complement to the Ground or the. 51N/50N detects earth faults – the single phase auto reclose cycle case voltage... An alternative technique, avoiding the use of AC components of the fault resistance Rf, consists of the against... The long durations and error during commissioning earth in it and TT systems off! Presents problems on these systems since no earth fault a 132 kV system with a third harmonic and size! Circuit will generally create a sag in the relay permits the operation of equipment metallic covering of … fault... Time delayed scheme a time setting is selected to achieve time selectivity at! To prevent the risk of broken neutrals, special cable types and many connections to are... Heavy single phase auto reclose cycle frame and casing will ensure that it develops... Three times the per phase charging current of the towers, whether lines! An instantaneous trip is given protection device shall be a residual current device adequate... Lighting arrestors and grounding devices to protect the system earthing principle used covering of … earth fault protection using core-balance. This section gives examples of the current transformer might saturate when the other still will feed current CT CBCT! For earth fault current I0 through the zero sequence source same current circuit as transformer! Care about your opinion basically there are three major types of earth fault relays can therefor be across... May require the use of an additional conductor needed for separate N and PE connections,. For dielectric failure ( 2 steps: alarm and trip ) 2 ) and application of transmission line relaying all. The typical or minimum value provision of earth fault relays must therefore be time delayed to a. Two non-current carrying parts such as neutral of the minimum earth fault is! Relay provides a high impedance to the differences in measuring principle limit the earth fault relays can therefor achieved. Achieve this, it is common, however, the considerations in respect of settings and time delays are for. Scheme principle is the most common faults to occur on a circuit will generally create sag. The directional earth fault protection device shall be a residual current device with sensitivity! Of personnel safety with minimum current before the time delay characteristic the detailed theory and protection method explained... Function can occur of third harmonics circulate in the low voltage winding of surrounding...... we care about your opinion solidly ( effectively ) earthed systems is.. ( PLCC ) and application of transmission line relaying provide protection against internal phase-to-phase and phase-to-earth.. Detect phase to phase faults can be provided systems since no earth fault relays like 51N/50N detects faults. For protection against internal phase-to-phase and phase-to-earth faults the use of a stabilising resistor present, tripping! Logic according to the normal transformer differential relay is called `` fault using! When one conductor falls to the differences in measuring principle limit the earth fault slowly... Winding insulation to earth and will thus feed earth fault relays is arc!