Restricted Earth Fault- Theory of Operation. For example, if an OCPD is only applied to the high-voltage, delta side of a delta-wye grounded transformer, the device can have a problem providing sensitive fault protection for the transformer. Three independent REF elements provide sensitive protection for faults close to the winding neutral in grounded wye-connected transformers. Ground fault protection for specific equipment such as generators, transformers or motors is worthy of several articles and will only be discussed tangentially here. The SBEF relay is sensitive to the detection of earth faults well down into the LV transformer winding. Earth fault protection must be applied where impedance earthing is employed that limits the earth fault current to less than the pick-up threshold of the overcurrent and/or differential protection for a fault located down to the bottom 5% of the stator winding from the star-point. The settings below show what is possible - a setting of 20A on the residual-connected earth fault protection, vs. a setting of 4.8A on the sensitive earth fault protection. Sensitive protection functions that respond to currents are . the phase and ground fault types. A high magnitude through fault (external fault fed by the transformer) shakes and heats a transformer winding, and the longer the through fault lasts, the greater the risk of it evolving into an internal transformer fault; hence, fast clearing for close-in external faults is part of the transformer protection scheme. Of course, in addition to these protections restricted earth fault (REF) relay is also utilised in order to have more reliable protection for power transformers. It should be noted that the fault currents in Zone A are limited by the impedance of the equipment in the zone - for transformers and generators it is very low - the fault currents can rise very fast and damage the equipment. Normally NCT comes inside of the transformer. The resistance for HRG is selected to limit fault current normally less than 10 A as defined by IEEE Std 142-2007 section 1.4.3.1. Restricted Earth Fault (REF) protection is basically a Differential Protection. Earth fault 50N 51N Protection against earth faults The following types of time from EE 464 at NSS Law College, Kottiyam, Kollam, Affiliated to Kerala University When excessive current flows in a circuit, it is necessary to trip the circuit breaker protecting that circuit. sensitive for detecting an internal phase-to-ground fault if the fault is located near the neutral point of the transformer or if the ground-fault current is limited [1]. This zone is inclusive of the Low Voltage Winding of the Transformer and the load cable to the Switchgear. Both windings of a transformer can be protected separately with restricted earth fault, thereby providing high speed protection against earth faults over virtually the whole of the transformer winding. However, for more sensitive parts of the system, to further limit line-to-ground fault current, high-resistance grounded (HRG) power transformer configuration may also be used. In a restricted earth fault protection, the CTs are connected in a fashion as shown below: During earth fault, the phase lead is earthed & so fault current flows. A high magnitude through fault (external fault fed by the transformer) shakes and heats a transformer winding, and the longer the through fault lasts, the greater the risk of it evolving into an internal transformer fault; hence, fast clearing for close-in external faults is part of the transformer protection scheme. The degree of earth fault protection is very much improved by the application of REF-systems. REF protection is installed in the secondary These are normally fitted in machines of 1000 HP and above. Answer (1 of 5): Prominent earth fault protections are Restricted earth fault, standby earth fault, sensitive earth fault protection. traditional transformer differential protection was typically not sensitive enough to detect such winding turn-to-turn faults before they developed into more serious and costly to repair ground-faults. The core-balance current transformer (or CBCT) is normally of the ring type, through the centre of which is passed cable that forms the primary winding. '499A'and '499B',a Sensitive Earth Fault (SEF) checking device designated '564ST',an independent automatic voltage regulator (AVR) relay designated '590'and transformer guard devices as fitted with the Protect transformers, buses, generators, reactors, and other apparatus with a combination of differential and overcurrent protection. 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. The core balance current transformer is the basis for sensitive ground fault protection of motors. Restricted earth fault (REF) protection is a sensitive way to protect a zone between two measuring points against earth faults. 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 sensitive earth fault protection, one dedicated Core Balance CT is used to detect any unbalance in 3 phases and its setting is so low even less than 1 % so it's called sensitive earth fault protection. Hence fault current would be low. Restricted earth fault protection is provided in power transformer for sensing internal earth fault of the transformer. The Example Over current/Earth fault relay is shown in the below figure. Protection. All phase conductors pass through the same opening in the current transformer so that it senses only zero sequence or ground current. In this . Traditionally, voltage relays have been used instead of current relays, assuming the grounding is done via a distribution transformer. However, these relays would typically provide sensitive protection for only about 95% of the stator . • Prevention of back-feeding utility source fault from industrial plant generators • Sensitive hi-speed ground protection of transformers Sensitive ground and Restricted Earth Fault (REF) protection Sensitive ground and RGF protection features provide sensitive detection of ground faults. It is usually used for resistance grounded or ungrounded systems; where first earth fault doesn't . The neutral transformer is called as NCT. 12. Restricted Earth Fault Protection (REF) For solidly grounded systems a restricted earth fault protection is typically provided as an addition to the normal transformer differential relay. 2. Sensitive ground fault protection includes . It should be noted that the fault currents in Zone A are limited by the impedance of the equipment in the zone - for transformers and generators it is very low - the fault currents can rise very fast and damage the equipment. Transformer Protection 1 of 12 Section I Introduction The primary objective of the Transformer Protection is to detect internal faults in the transformer with a high degree of sensitivity and cause subsequent de-energisation and, at the same time be immune to faults external to the transformer i.e. This is especially so in the case of LV switchboards with all insulated busbars. Providing protection for ground faults is a well-established need, but some users are still confused about the proper protection schemes to be used for various system configurations. • User-configurable restricted earth fault (REF) element provides sensitive ground-fault detection in grounded-wye transformers. The Restricted Earth Fault Protection is only intended to activate for an Earth Fault in a specified zone. The protection scheme is comparatively cheaper than differential protection scheme. However, these Differential protection is used to protect the transformer and generator against internal fault but for larger generators, additional protection is provided called Restricted earth fault protection. Restricted Earth Fault Protection Restricted Earth Fault Protection of Transformer. However, quite often the secondary CT is . One method of protection against earth faults in transformers is the core-balance leakage protection. Unless noted otherwise, all discussion of ground fault protection applies primarily to the coordination of distribution protective devices. for example grid side earth fault).. As you know in electrical power system, the fault occurs due to various factors such as natural disturbance, lightning on the transmission . The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding. Consequently, a separate scheme is required to detect internal earth faults. The relay contains two energizing input circuits, that is, a differential earth-fault current circuit and a residual earth-fault current circuit. 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. So this REF relay will not be actuated for . Other protective relays will be set to detect earth faults occurring outside of the zone. Sensitive Earth Fault Protection Normally used on systems where the earth fault current is limited to a very low value (Neutral earthed through resistors, Peterson coil, reactance, etc) Scheme consists out of a ring CT (Core Balance CT) around the three phase conductors, and the relay connected to the secondary side of the CT. A very low . This prevents interruptions to the power supply. A A A Restricted Earth Fault (REF) Protection Apply the REF protection feature to provide sensitive detection of internal ground faults on grounded wye-connected transformer windings and autotransformers. High resistance grounding restricts the earth fault current to less thanA. An auxiliary ac supply is connected across the primary winding of an auxiliary transformer. The modern method of providing protection against rotor earth faults includes ac injection or dc injection. Other protective relays will be set to detect earth faults occurring outside of the zone. If the relay is set too sensitive to sense such faults, then it may operate for external faults and switching surges also. Furthermore, the SEF elements of GRD140 are also applicable to the "standby earth fault protection" and the "high impedance restricted earth fault protection of . Power Transformer Faults. This scheme of REF protection is very sensitive for internal earth fault of electrical power transformer. Influence of winding connections and earthing on earth fault current magnitude. through faults. An earth fault relay, connected to the secondary winding, is energised only when there is residual current in the primary system. These protective schemes are straight-forward and dependable. Possible sources of nonfault relay current include current transformer error, harmonics, and unbalanced currents through impedances to ground, especially . This relay protects the delta or unearthed star winding of the power . Results of Repeated Faults and Mechanical Stresses Transformer Overcurrent and Mechanical Protection • Apply overcurrent protection for through-fault damage to transformers • Review IEEE thermal model • Understand how sudden pressure relays provide sensitive protection for turn-to-turn faults and how to apply them These are also known as short circuit faults and are accompanied by excessively large currents resulting in over-heating, fire, and damage to equipment. When neutral of transformer or generator solidly grounded then generator gets completely protected against earth fault. Instantaneous earth fault protection. also known as REF protection. The ∆-Y transformer is a source of ground currents for loads and faults on the transmission system, yet the generator ground fault protection is completely isolated from ground currents on the primary side of the transformer. Sensitive Earth Fault. 1. Restricted Earth Fault Protection of Transformer. Earth fault protection. fault is desirable. REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs. Differential protections. The earth fault relay is placed in the residual part of the current transformers shown in the figure below. Transformers are vital equipment in transmission and distribution network and so the protection against internal and external faults is a very important factor in the design of those networks.. Transformers faults may occur: In the dielectric (insulating) materials, namely in the oil. Of course, in addition to these protections restricted earth fault (REF) relay is also utilised in order to have more reliable protection for power transformers. 2. Sensitive detection and de- When a fault occurs very near to the neutral point of the transformer, the voltage available for driving earth fault current is small. So transformer protection scheme is required for incipient transformer faults also. When neutral of transformer or generator solidly grounded then generator gets completely protected against earth fault. earth-fault feature provides sensitive protection against earth faults for wye-connected transformers. The newer digital relays can provide a very sensitive current detection scheme as well. An earth-fault relay used for it is essentially an overcurrent relay of low setting and operates as soon as earth fault or leak develops. The single- Some Technical Data 11.10 Sensitive Ground-Fault Detection 1962 SIPROTEC 5, Transformer Differential Protection, Manual C53000-G5040-C016-B, Edition 12.2019. The Restricted Earth Fault Protection is only intended to activate for an Earth Fault in a specified zone. The sensitive earth fault (SEF) protection is applied for distribution systems earthed through high impedance, where very low levels of fault current are expected in earth faults. In the windings. This scheme is called Restricted Earth Fault (REF) scheme. The CT secondaries are wired to cancel each other's currents during through faults and to drive all secondary current to the relay when the fault is inside the protected zone. Answer: In sensitive earth fault protection, one dedicated Core Balance CT is used to detect any unbalance in 3 phases and it's setting is so low so it's called sensitive earth fault protection. NEW Certified university course: https://tugraz.at/go/LLL-eprotection*****This is a video of the course "Protection in Electrical Power Systems". The below figure shows the protection system provided in large turbo-alternators against earth-fault in the rotor. Fault detection is confined to the zone between the two CTs hence the name 'Restricted Earth Fault'. Ground-fault current is sensed using an SE-GRM . The only difference in between the Differential Protection and REF Protection is that, latter protection is more sensitive as compared to the former protection scheme. 1MRS 755627 Stabilizing Resistor in Motor Earth-Fault Protection 7 3.3. This relay can be implemented for only star connected transformer or star connected alternator. FOR GROUND FAULT PROTECTION Joe Mooney, P.E., Jackie Peer Schweitzer Engineering Laboratories, Inc. INTRODUCTION Modern digital relays provide several outstanding methods for detecting ground faults. In earlier posts we have already discussed Differential Protection of Transformer and various characteristics of Differential Protection. 14. 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. Sensitivities of 2-8% can be accomplished. Transformer Protection Relay The only compact platform with REF and up to four-terminal differential protection. This low value is limited downwards only by an eventual small amount of the false, "steady-state" negative-sequence currents. Differential Relay may not be able to sense Earth Faults in the Winding as the fault magnitude may be smaller than the Set Differential Current, Id. Used for protection of transformers and feeders from over current and earth faults. Some 11.9.2.2.3 . The new protection principle yields a very sensitive protection for low-level turn-to-turn faults. Combined harmonic blocking and restraint features provide max-imum security during transformer magnetizing inrush conditions. Calculation of the relay setting The sensitivity of the protection can be calculated as: Iprim = nI× ()s ++3 × Ie Iu (2) where: Iprim primary current at which the relay starts n CT ratio Is start current (secondary value) of the earth-fault protection Ie magnetizing current of the CT at the voltage Us If CTs are located on the transformer terminals only the winding is protected. First of all, generators are usually equipped with sensitive ground fault relay protection. for this purpose a sensitive earth leakage relay is . Ground, as defined in the NEC, is the earth. In these case REF relay is more sensitive, as it uses the current flowing through the Neutral CT. High resistance grounding enables electrical systems to continue running when one of the phases is faulted. Consequently, a separate scheme is required to detect internal earth faults. Sensitive Earth fault protection is normally used on systems where the earth fault current is limited to a very low value. The three phase side current transformer connected with the neutral current transformer as shown in the figure. • Dual-slope percentage differential protection with harmonic blocking and restraint enhances security. The combined restricted earth-fault and residual earth-fault relay SPAJ 115 C is intended to be used for the earth-fault protection of power generators, motors and transformers. The protection in either case is slightly time delayed (say by a second) before trip command is issued. Sensitivity of this protection is more than the others in case of earth fault condition. What is Standby Earth Fault Relay: A Standby earth fault relay is nothing but an earth fault protection used to protect the generator, transformer, and motor from the external earth fault (earth fault outside the zone. the phase and ground fault types. Set the differential element with either a single- or dual-slope percentage differential . 98. appropriate resistance is to be inserted between the neutral point of the transformer and the earthing mat to restricted earth fault current. TechTopics No. unbalance protection and rotor earth-fault protection. Dropout The greater dropout differential (= | pickup value - dropout value |) of the following 2 criteria applies: Dropout differential derived from the parameterDropout ratio If this . EFCT Series Earth-Fault Current Transformer Required zero-sequence current transformer specifically designed for low level detection. Without REF, faults in the transformer star secondary winding need to be detected on the primary of the . For a low-voltage (wye side) line-to-ground fault, the high-side line current is only 58% of the low-voltage, per-unit fault current [Sec. This zone is inclusive of the Low Voltage Winding of the Transformer and the load cable to the Switchgear. The rotor is also protected against earth faults. Ground protection in metal-clad switchgear - ground sensor current transformers vs. residual connection of current transformers. Restricted Earth Fault Protection. Another typical type of core balance CT is the 50:0.025A ground fault CT supplied by GE for use with their Multilin 269 / 369 / 469 series of motor protection relays. a) 100% Stator Winding Ground Fault Protection Conventional protection for stator ground fault detection on high-resistance grounded systems has been discussed in the previous section. That is why you need a restricted earth fault relay with sensitive settings like 5%. (above 5MVA).Two phase fault and one ground fault relay is sufficient to provide OC protection to star delta transformer. Over current/Earth fault relays are the basic protection relays. 12. Question: Restricted Earth Fault Protection of a Three-Phase Power Transformer Describe what happens when an earth fault occurs on one of the power transformer primary windings On the Faultable Transformers, set fault switch FS1 of transformer T1 to the O position to remove the fault. The earth fault sensing is done through open delta connected VTs (residual voltage signal) and relay 59N, CT in the secondary of the distribution transformer connected to an earth fault relay (current based), 51N. A star point CT and earth fault relay are used because faults in the transformer LV winding and LV switchboard most often occur initially as earth faults. fault is desirable. The earth fault, very nearer to neutral point of transformer star winding may also be considered as an incipient fault. For a true high-resistance grounding system, line-to-ground fault current will be only a few amps. 10 EARTH FAULT PROTECTION FOR THE DELTA SIDE OF DELTA STAR TRANSFORMER : (C) OVER CURRENT PROTECTION : Over current protection is used for the purpose of providing back up protection for large transformers. . One of the major advantages of the restricted earth fault relays is their high sensitivity. Differential protection is used to protect the transformer and generator against internal fault but for larger generators, additional protection is provided called Restricted earth fault protection. Sensitive Earth Fault protection is required to either to alarm or trip the faulted circuit. The Sensitive Earth Fault protection scheme works by measuring the residual current across the three phases in a system. Standby Earth Fault Protection is a course protection used predominantly as a back up to other protection. Restricted Earth fault Protection. It provides sensitive ground-fault protection without the problems associated with nuisance tripping. The sensitive earth-fault relay is suited for both primary and back-up earth-fault protec-tion. Order the SEL-787 with the Slot E card containing the 1 A or 5 A current input for REF protection. also known as REF protection. Measurement of three phase residual current is done either by using Core Balance Current Transformer (CBCT) or three CTs connected in parallel. The CT secondaries are wired to cancel each other's currents during through faults and to drive all secondary current to the relay when the fault is inside the protected zone. This system consists of three primary conductors surrounded by the magnetic circuit of a current transformer. Sensitivity of this protection is more than the others in case of earth fault condition. All such faults, which involve around 1% short-circuited turns, can be detected. REF protection is installed in the secondary Restricted Earth Fault- Theory of Operation. The protection consists of a CT and basic relay , the CT is placed as close as possible on the earth path return to the star point of a transformer. Restricted earth fault (REF) protection is a sensitive way to protect a zone between two measuring points against earth faults. Where in Standby Earth fault protection, CT placed in neutral only and detect zero sequence current. Alternatively, such faults can be as well detected by Sudden Pressure Relay relays. Motors with powers greater than 50 HP, supplied from an earthed system, should be protected against earth faults to reduce fault damage and accident risk, especially motors not protected by differential protection.. Harmonic Blocking and Restraint. Transformer Faults and Their Protection Overcurrent Protection using Phase and Ground Relays: Overcurrents in a transformer occur due to single line ground faults and phase to phase faults. 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