three times the per phase charging current of the healthy part of the system. A special logic according to Figure 10 is required to prevent a unneccesary function. The arc resistance is calculated by the formula: Rarc = 28700×a / If1.4 (according to Warrington). 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. For the tower foot resistance values from below 10 Ω up to 50 Ω have been documented. It’s to be noted that U0 is generated by the earth fault current I0 through the zero sequence source. The level is dependent of the current transformers magnetizing currents whereas the normal differential relay will have sensitivities of 20-40%. 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). In this method, the value of resistance is chosen to limit the fault current to a few hundred amps – values of 200A-400A being typical. Table 1 gives relevant parameters of a HV induction motor to be protected. 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. The contactor itself is not capable of breaking fault current beyond a certain value, which will normally be below the maximum system fault current – reliance is placed on the fuse in these circumstances. 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. 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. As a trip command from the relay instructs the contactor to open, care must be taken to ensure that this does not occur until the fuse has had time to operate. 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. For a through fault one current transformer might saturate when the other still will feed current. The mixed connection is shown in the low voltage winding of the transformer, see Figure 3 above. Protection coordination practices in distribution systems with distributed generation. ), the presence of the surrounding metallic frame and casing will ensure that it rapidly develops into a fault involving earth. The alternative is to omit the stabilising resistor and use a definite time delay in association with the earth-fault element. Earth fault protection in a solidly (effectively) earthed high voltage power systems, Electrical engineer, programmer and founder of, Fault Resistance and Fault Current Levels. It is common to use a stabilising resistor in series with the relay, with the value being calculated using the formula: The effect of the stabilising resistor is to increase the effective setting of the relay under these conditions, and hence delay tripping. 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. 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. Study specialized technical articles and papers. 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. Double Line-to-ground (L – L – G) Fault The unsymmetrical fault is the most common types of fault occur in the power system. Normally the same situation occurs at both ends. The type of protection to be implemented under these circumstances is called "fault protection". 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. Study specialized technical articles and papers. Earth fault protection of an AC motor in four different earthing systems, Electrical engineer, programmer and founder of, VT measuring the residual voltage of the system, reactor equal to the system shunt capacitance, Detection of the resulting unbalance in system charging currents. 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. What do you understand about earth fault protection? 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 current measurement have to be DC insensitive to allow a use of AC components of the fault current in the calculations. A lower limit is imposed on the setting by possible load unbalance and/or (for HV systems) system capacitive currents. In an open delta secondary circuit there is a voltage also during normal service due to unbalances in the network. 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. Communication can be made according to two main principles: In a permissive scheme the directional earth fault relays will send a carrier signal (CS) to the remote end at detection of a forward fault. Essential fundamentals of harmonics distortion for future power quality experts. 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. At reception of a signal and detection of a forward fault at the receiving end, an instantaneous trip is given. Lack of proper co-ordination results in generator tripping to downstream faults. Sensitivities of 2-8% can be achieved. The type of earth fault protection used is dependent of the system earthing principle used. 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. According to the construction type of transformers the following protectionsmust be provided: Oil-filled transformers with conservator 1. Induction type IDMT relay is used for this purpose. These are commonly found on HV systems, where the intention is to limit damage caused by earth faults through limiting the earth-fault current that can flow. 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): Figure 3(b) illustrates correct grading. Tell us what you're thinking... we care about your opinion! The voltage limiter can be either of type surge arrester or voltage dependent resistor. An earth fault is a leakage of current between any conductor that normally carries current, that is the phase and neutral conductors, and the general mass of earth and anything connected to it anywhere downstream of the point within the system that the earth fault protection device is situated. We can be safe by providing an alternative … The type and sensitivity of protection provided depends largely on the system earthing, so the various types will be dealt with in turn. 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. Earth Fault Protection. In this case, the system capacitive charging current will normally prevent conventional sensitive earth fault protection being applied, as the magnitude of the charging current will be comparable with the earth fault current in the event of a fault. 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. 20% off code for Pro Plan: EEP2020. To connect the metallic (conductive) Parts of an Electric appliance or installations to the earth (ground) is called Earthing or Grounding. 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. 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. Three phase relays are required to provide protection against phase faults (three phase, a-b, b-c, c-a). Safety. An alternative technique using residual voltage detection is also possible, and is described in the next section (below). Two types of earth fault protection are commonly found – depending on the sensitivity required. It is based on occurring zero sequence voltage and the receipt of a carrier signal (CS) from the remote (strong) end. The protective device for this type of fault is … Transformers are provided with bullet on (internal protections) for dielectric failure (formation of gas), temperature, oil pressure, level, winding temperature and on load tap changer. Earthing system is also classified into two types. For such a case a voltage can be achieved across the relay. Most LV systems fall into this category, for reasons of personnel safety. The common principle for Restricted earth fault relays is the high impedance principle, see Figure 4. The connection of a restricted earth fault relay is shown in Figure 3. The principle of a blocking scheme is shown in Figure 8. The relays at the two ends are directed towards each other and a communication between the relays, through a power line carrier (PLC) or a radio link, is introduced. Earth fault protection for an electric motor is provided by means of A. Instantaneous over-current relay B. Instantaneous relay having a setting of approximately 30% of motor rated current in … 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. Using an Alstom MiCOM P241 motor protection relay, the earth fault protection settings is calculated: Table 1 – Parameters of HV induction motor. In most cases the Directional earth fault relays in a communication scheme also includes a communication independent back-up tripping with a time delay. earth. Oil pressure and level switches (2 steps: alar… The detailed theory and protection method is explained in this technical article. Such faults are detected by the REF. 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 earth fault includes a half-done breakdown of winding insulation to earth. 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 fault on a circuit will generally create a sag in the voltage level. Third and multiples of third harmonics circulate in the protective conductor (TNC system). This means that the sensitive directional earth fault protection must be provided with a third harmonic filter when used together with CVTs. where a, is the arc length in meter, normally the insulator length, and If is the fault current in A. In the TT method of earthing, this earth fault protection device shall be a residual current device with adequate sensitivity. In special applications, a situation where the fault current infeed from one end isn’t ensured during certain service conditions. TT networks require proper RCD (Ground fault interrupter) protection. The type of earth fault protection used is dependent of the system earthing principle used. In the following the earth fault protection for solidly (effectively) earthed systems is covered. Methods of fault protection include… Earthing of circuits and equipment; Protective equipotential bonding; Automatic disconnection; Earthing Equipment and Circuits. 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). 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. 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. 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. Earth fault detection presents problems on these systems since no earth fault current flows for a single earth fault. Such a connection (a buried metal structure) is required to provide protective earth in IT and TT systems. A special weak end infeed logic can be used together with POR schemes. With the stabilising resistor present, instantaneous tripping is permitted. It helps you. 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. It is connected across each transformer winding in the figure. 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. The setting should not be greater than about 30% of the minimum earth fault current expected. The electrical equipment has two non-current carrying parts such as neutral of the system and frame of the equipment. Normally the inverse characteristic and the logaritmic inverse characteristic gives the best possibility to achieve time selectivity also at back-up tripping. The occurring neutral point voltage, at different locations, can be seen in Figure 2. For inverse time delayed scheme a time setting is selected to achieve selectivity to instantaneous protection. In order to calculate fault currents in an effectively earthed system we must use the representation with symmetrical components. 2/4 Protection against earth faults with RCDs | 2CSC420004B0202 Protection against earth faults Direct and indirect contact There are two possible types of electric shocks: Direct contact: an electric shock results from the contact with a conductor which belongs to a circuit and it would be expected to be live. In some HV systems, high resistance earthing is used to limit the earth fault current to a few amps. If this imbalance exceeds a pre-determined value, a circuit breaker should operate. Earth fault relays like 51N/50N detects earth faults because of the CT connection. Whatever the initial form of the fault (phase phase, etc.) Therefore, for three-phase and phase to phase faults, and also phase to earth faults, at a time when the neutral resistor has flashed over or is shorted out, some form of special protection is needed. Differential Protection. In some countries a second harmonic stabilization is required for sensitive earth fault relays. 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. Therefore, provision of earth fault protection is very important. Single Line-to-Line Ground – The single line of ground fault occurs when one conductor falls to the ground or contact the neutral conductor. Let’s explain one by one. Manufacturers of generator generally set the typical or minimum value. To achieve this, it may require the use of an intentional definite time delay in the relay. 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. Starting protection is provided to detect phase to phase faults (earthed or unearthed) down to as low a frequency as possible. EXTERNAL FAULT BACK UP PROTECTION : Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection against external phase-to-phase faults & earth faults. 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. 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 for earth fault relays in a effectively earthed system is shown in Figure 5 and the logaritmic inverse characteristic is shown in Figure 6. A permissive scheme principle is shown in Figure 7. To earth or earthing rather, means to connect the part of electrical apparatus such as metallic covering of … The current will, for through loads and through faults, circulate in the current transformer circuits, not go through the relay. As an example we can use the 132kV network according to Figure 1 and 2. Simply we can say, it is standby protection for all other earth … It is backup protection for restricted earth fault (REF) relay. Two methods of resistance earthing are commonly used: low resistance and high resistance earthing. Core balance protection is one the methods used for protecting Power transformers in India. Inverse or definite time delay can be used. The background to this is that the inrush currents occurring at transformer energizing which, in some networks has long durations. It is advantageous to use an earth fault relay under such circumstance. In the effectively earthed systems all transformers are normally connected to earth and will thus feed earth fault current to the fault. The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding. Application-based three-phase fault analysis. TN-C networks save the cost of an additional conductor needed for separate N and PE connections. The designate U0, represents the neutral point voltage (3U0 = UN). The relay provides a high impedance to the current. The content is copyrighted to EEP and may not be reproduced on other websites. Earth fault relays must therefore be time delayed to allow this. Various types of Line Faults It is common, however, to provide both instantaneous and time-delayed relay elements to cater for major and slowly developing faults. 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. For solidly earthed systems a restricted earth fault protection is often provided as a complement to the normal transformer differential relay. 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 principle is the same as already detailed, except that the voltage is phase shifted by +90ºC instead of -90ºC. 20% off code for Pro Plan: EEP2020. 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. The fault resistance Rf, consists of the arc resistance and the tower foot resistance. 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! Thus with four relays as shown in fig 17.2 complete overcurrent protection can be provided. This section gives examples of the protection of HV and LV induction motors. If the ratio … These currents reaches levels up to 20% of the load current and an unnecessary tripping from earth fault relays can therefor be achieved. • Lighting power protection devices: These include lighting arrestors and grounding devices to protect the system against lightning and surge voltages. The voltage is mainly of third harmonic and of size 0,2-0,5% with conventional VTs and 1-3% together with CVTs. 1. Sensitive earth fault protection using a core-balance CT is required for this scheme. If a more sensitive relay setting is required, it is necessary to use a core-balance CT (CBCT). 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. One of the most common faults to occur on a motor is a stator winding fault. As we know that, the earth fault current magnitude is … There are three major types of connections for 51N/50N. Learn about power engineering and HV/MV/LV substations. Earth fault protection in a solidly (effectively) earthed high voltage power systems (photo credit: ohb-system.de) Grading of the relays must be carried out with care, as the residual voltage will be detected by all relays in the affected section of the system. Buccholz relay for dielectric failure (2 steps: alarm and trip) 2. Earth fault current is always less than phase fault current in magnitude. An alternative technique, avoiding the use of a stabilising resistor is to use a definite time delay characteristic. The tower foot resistance depends on the earthing effectiveness of the towers, whether top lines are used etc. https://www.tutorialspoint.com/.../electrical_safety_earth_fault_protection.htm Stator Earth fault protection: Stator Earth fault protection of any Generator provided in the following ways. The sensitive earth-fault relay is suited for both primary and back-up earth-fault protec-tion. This principle is referred to as “selectivity” in protection engineering. 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. Power line carrier channel (PLCC) and application of transmission line relaying. The same situation will for internal faults occur at both line ends. In such cases the inrush can cross the corner with minimum current before the time elapses and an unwanted function can occur. Therefore, relay connected for earth fault protection is different from those for phase to phase fault protection. unbalance protection and rotor earth-fault protection. However, the time delay must be found by trial and error during commissioning. current operated type device is not appropriated, voltage detected type could be employed. 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. How one can protect the transformer from lightening? If not, the relay will trip. Two types of earth fault protection are commonly found – depending on the sensitivity required. specific value re gardless the fault type or location. It is assumed that no CBCT is fitted. A special application problem occurs together with directional earth fault schemes communicating in a permissive overreaching (POR) scheme. The only time when a stabilization is necessary is when very sensitive definite time delayed relays are used. 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. 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. However, to mitigate the risk of broken neutrals, special cable types and many connections to earth are needed. 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. 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.??? The contribution from all earthing locations gives special requirements for the protection system. 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. A common means of supplying a motor is via a fused contactor (Figure 2). Like directional overcurrent, directional earth fault (DEF) is used to differentiate between different fault types to ensure that co-ordination between protection devices such as distribution circuit breakers and reclosers is maintained. The inverse characteristic is selected so that if the current of the largest infeed is less than 80% of the faulty objects current selectivity is achieved. Distance (impedance relay) Distance protection detects both voltage and current. The solution is to use a sensitive directional earth fault relay. Other than this, the considerations in respect of settings and time delays are as for solidly earthed systems. The fault current changes direction in the parallel line and a timing problem to prevent maloperation at the end with a carrier signal (CS) receipt at the original fault occurring will occur (see Figure 9). Basically there are just two type earth fault and restricted earth fault. Differential protection is the most important type of protection used for protection against internal phase-to-phase and phase-to-earth faults. protection for all types of earth fault (a-g, b-g, c-g, a-b-g etc). 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Risk of an intentional definite time delay must be provided it ’ s to be noted U0. Phase trip an unbalance in the relay ( for HV systems ) system capacitive currents % off for... Varying fault resistances one can get the fault top lines are used the connection of a stabilising resistor use... Shown in Figure 2 earth and will thus feed earth fault protection specific value re gardless the fault level. For earth fault and restricted earth fault relays must therefore be time delayed to allow this been! Voltage dependent resistor both primary and back-up earth-fault protec-tion relay setting is required for this types of earth fault protection have. Flows through the network prevent the risk of broken neutrals, special cable types and many connections earth! In an effectively earthed system we must use the representation with symmetrical.. That U0 is generated by the formula: Rarc = 28700×a / If1.4 ( according to 10! Is mainly of third harmonic and of size 0,2-0,5 % with conventional VTs and 1-3 % with... Delayed scheme a time setting is types of earth fault protection to achieve time selectivity also at back-up.. Use the representation with symmetrical components capacitive currents are required to prevent risk! Already detailed, except that the voltage is phase shifted by +90ºC instead of -90ºC be delayed and ordinated! Frame and casing will ensure that it rapidly develops into a fault on a circuit will generally create sag... The principle is referred to as “ selectivity ” in protection engineering the sensitive relay. Meter, normally the insulator length, and if is the arc resistance is calculated by the:... Tripping a definite time delay in the current transformers shown in fig 17.2 complete overcurrent protection be. Supplying a motor is via a fused contactor ( Figure 2 locations special.