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. Learn about power engineering and HV/MV/LV substations. The earth fault relays are normally blocked during the single phase auto reclose cycle. To earth or earthing rather, means to connect the part of electrical apparatus such as metallic covering of … In some HV systems, high resistance earthing is used to limit the earth fault current to a few amps. Application-based three-phase fault analysis. In a blocking scheme the directional earth fault relays are provided with a reverse locking element as a complement to the forward element. If a more sensitive relay setting is required, it is necessary to use a core-balance CT (CBCT). Protective relays are of different types like impedance relays, mho relays, etc. Thus with four relays as shown in fig 17.2 complete overcurrent protection can be provided. 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. It is connected across each transformer winding in the figure. Simply we can say, it is standby protection for all other earth … 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 … 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. Earth fault current is always less than phase fault current in magnitude. 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. These currents reaches levels up to 20% of the load current and an unnecessary tripping from earth fault relays can therefor be achieved. current operated type device is not appropriated, voltage detected type could be employed. Two types of earth fault protection are commonly found – depending on the sensitivity required. Lack of proper co-ordination results in generator tripping to downstream faults. Sensitive earth fault protection using a core-balance CT is required for this scheme. The current measurement have to be DC insensitive to allow a use of AC components of the fault current in the calculations. The electrical equipment has two non-current carrying parts such as neutral of the system and frame of the equipment. In the effectively earthed systems all transformers are normally connected to earth and will thus feed earth fault current to the fault. The mixed connection is shown in the low voltage winding of the transformer, see Figure 3 above. Learn about power engineering and HV/MV/LV substations. 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. 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. Restricted Earth Fault Protection of Transformer. If not, the relay will trip. A special application problem occurs together with directional earth fault schemes communicating in a permissive overreaching (POR) scheme. specific value re gardless the fault type or location. Two methods of resistance earthing are commonly used: low resistance and high resistance earthing. In an open delta secondary circuit there is a voltage also during normal service due to unbalances in the network. 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. Buccholz relay for dielectric failure (2 steps: alarm and trip) 2. This principle is referred to as “selectivity” in protection engineering. Power line carrier channel (PLCC) and application of transmission line relaying. 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. 20% off code for Pro Plan: EEP2020. Protection coordination practices in distribution systems with distributed generation. 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. 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. 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. However, to mitigate the risk of broken neutrals, special cable types and many connections to earth are needed. To connect the metallic (conductive) Parts of an Electric appliance or installations to the earth (ground) is called Earthing or Grounding. 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. Whatever the initial form of the fault (phase phase, etc.) To enable use together with distance protection giving single-phase tripping a definite minimum time is set (normally 0.3 sec.). The principle of a blocking scheme is shown in Figure 8. This means that the sensitive directional earth fault protection must be provided with a third harmonic filter when used together with CVTs. 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. TN-C networks save the cost of an additional conductor needed for separate N and PE connections. 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 current will, for through loads and through faults, circulate in the current transformer circuits, not go through the relay. 20% off code for Pro Plan: EEP2020. To achieve this, it may require the use of an intentional definite time delay in the relay. 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. It is quite common to connect the Restricted earth fault relay in the same current circuit as the transformer differential relay. A fault on a circuit will generally create a sag in the voltage level. 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 advantage with the restricted earth fault relays is their high sensitivity. 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. 1. The solution is to use a sensitive directional earth fault relay. 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. For solidly earthed systems a restricted earth fault protection is often provided as a complement to the normal transformer differential relay. To prevent the risk of flashover in the circuit, a voltage limiter must be included. A special logic according to Figure 10 is required to prevent a unneccesary function. The long durations are often achieved in weak networks. 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). What do you understand about earth fault protection? 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. It’s to be noted that U0 is generated by the earth fault current I0 through the zero sequence source. 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. If this imbalance exceeds a pre-determined value, a circuit breaker should operate. Oil pressure and level switches (2 steps: alar… How one can protect the transformer from lightening? Distance (impedance relay) Distance protection detects both voltage and current. Co-ordination with other devices must also be considered. Single Line-to-Line Ground – The single line of ground fault occurs when one conductor falls to the ground or contact the neutral conductor. An alternative technique, avoiding the use of a stabilising resistor is to use a definite time delay characteristic. In order to calculate fault currents in an effectively earthed system we must use the representation with symmetrical components. The arc resistance is calculated by the formula: Rarc = 28700×a / If1.4 (according to Warrington). unbalance protection and rotor earth-fault protection. Ground Fault Protection System. 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 . This will due to the differences in measuring principle limit the differential relays possibility to detect earth faults. The setting should not be greater than about 30% of the minimum earth fault current expected. 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! 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. 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. Study specialized technical articles and papers. A special weak end infeed logic can be used together with POR schemes. 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 based on occurring zero sequence voltage and the receipt of a carrier signal (CS) from the remote (strong) end. This relay protects the delta or unearthed star … Generator phase fault protection Generator over current protection should be delayed and co- ordinated with downstream faults. 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. Earthing system is also classified into two types. In special applications, a situation where the fault current infeed from one end isn’t ensured during certain service conditions. The content is copyrighted to EEP and may not be reproduced on other websites. 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. Earth fault protection in a solidly (effectively) earthed high voltage power systems, Electrical engineer, programmer and founder of, Fault Resistance and Fault Current Levels. This ensures that a single phase tripping for heavy single phase faults can be done by distance protection relay first. The principle is the same as already detailed, except that the voltage is phase shifted by +90ºC instead of -90ºC. The filtering must be quite heavy to ensure correct directional measuring for 1% fundamental content also with third harmonic contents of say 3%. Tell us what you're thinking... we care about your opinion! earth. The protective device for this type of fault is … According to the construction type of transformers the following protectionsmust be provided: Oil-filled transformers with conservator 1. Third and multiples of third harmonics circulate in the protective conductor (TNC system). 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. 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. Double Line-to-ground (L – L – G) Fault The unsymmetrical fault is the most common types of fault occur in the power system. The type of earth fault protection used is dependent of the system earthing principle used. Essential fundamentals of harmonics distortion for future power quality experts. For such a case a voltage can be achieved across the relay. The occurring neutral point voltage, at different locations, can be seen in Figure 2. 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. 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 common principle for Restricted earth fault relays is the high impedance principle, see Figure 4. 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). However, the time delay must be found by trial and error during commissioning. 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.