Two methods of resistance earthing are commonly used: low resistance and high resistance earthing. 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. Such faults are detected by the REF. 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. At reception of a signal and detection of a forward fault at the receiving end, an instantaneous trip is given. 1. 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. The advantage with the restricted earth fault relays is their high sensitivity. 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. Let’s explain one by one. Learn about power engineering and HV/MV/LV substations. 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. 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. The electrical equipment has two non-current carrying parts such as neutral of the system and frame of the equipment. Starting protection is provided to detect phase to phase faults (earthed or unearthed) down to as low a frequency as possible. 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. The sensitive earth-fault relay is suited for both primary and back-up earth-fault protec-tion. It helps you. specific value re gardless the fault type or location. Other than this, the considerations in respect of settings and time delays are as for solidly earthed systems. Learn about power engineering and HV/MV/LV substations. Sensitive earth fault protection using a core-balance CT is required for this scheme. The type of earth fault protection used is dependent of the system earthing principle used. Application-based three-phase fault analysis. For the tower foot resistance values from below 10 Ω up to 50 Ω have been documented. Tell us what you're thinking... we care about your opinion! To connect the metallic (conductive) Parts of an Electric appliance or installations to the earth (ground) is called Earthing or Grounding. As an example we can use the 132kV network according to Figure 1 and 2. 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. Power line carrier channel (PLCC) and application of transmission line relaying. To achieve this, it may require the use of an intentional definite time delay in the relay. Earth fault protection in a solidly (effectively) earthed high voltage power systems, Electrical engineer, programmer and founder of, Fault Resistance and Fault Current Levels. 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. In some countries a second harmonic stabilization is required for sensitive earth fault relays. 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. Using an Alstom MiCOM P241 motor protection relay, the earth fault protection settings is calculated: Table 1 – Parameters of HV induction motor. The contribution from all earthing locations gives special requirements for the protection system. 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. 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. 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. An alternative technique, avoiding the use of a stabilising resistor is to use a definite time delay characteristic. 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. For such a case a voltage can be achieved across the relay. Earth fault detection presents problems on these systems since no earth fault current flows for a single earth fault. 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). A special logic according to Figure 10 is required to prevent a unneccesary function. What do you understand about 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. 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. In a blocking scheme the directional earth fault relays are provided with a reverse locking element as a complement to the forward element. 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. Figure 3(b) illustrates correct grading. Two types of earth fault protection are commonly found – depending on the sensitivity required. 20% off code for Pro Plan: EEP2020. Induction type IDMT relay is used for this purpose. 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! 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. 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 … In the effectively earthed systems all transformers are normally connected to earth and will thus feed earth fault current to the fault. According to the construction type of transformers the following protectionsmust be provided: Oil-filled transformers with conservator 1. A fault on a circuit will generally create a sag in the voltage level. The connection of a restricted earth fault relay is shown in Figure 3. How one can protect the transformer from lightening? 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. 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. 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. 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