In an autotransformer where only one earth is present, the direction of the neutral current can be towards the transformer or away from the transformer. Here we compare the 3i0 going through the earth with that residual current of the line. The degree of protection is very much improved by the application of restricted earth fault protection or ref protection. Restricted earth fault protection of transformer electrical. 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. Ref ct selection, setting calculation, ref testing etc. Restricted earth fault protection in dyn transformers. Simply we can say, it is a standby protection of all other earth faults. This video provides an insight of the topics covered in the full 2 hour long electrical control and protection course which. Working with the trip characteristic curves of abb sace. Restricted earth fault protection earth fault is the unintended fault between the live conductor and the earth. But during an internal fault, the neutral current transformer only carries the unbalance fault current and operation of restricted earth fault relay. The restricted earth fault protection function is configurable independently of the others settings. Metering devices are not part of power transformer.
Scope this document introduces the application of restricted earth fault protection in ref 542plus. It need not be graded or coordinated with any other protection. Definite time character the mref provides the following settings to choose from. In solidly earthed and low impedance earthed systems, high levels of earth fault current result. English pdf brochure transformer protection ret650. Fault detection is confined to the zone between the two cts hence the name restricted earth fault. Then common terminals are connected to the secondary of a neutral current transformer or nct. Restricted earth fault protection in dyn transformers linkedin. Scheme consists out of a ring ct core balance ct around the three phase conductors, and the relay connected to the secondary. Eq 3 increases in phase current iph due to the fault are negligible, meaning that. The relay is connected to a ct in the neutral point of the transformer and residually connected cts in the phases.
It is also used with a follower timer for time delayed earth fault protection. The sel487e transformer differential relay provides threephase differential protection for transformer applications with up to five threephase restraint current inputs. Use the three independent restricted earth fault ref elements for sensitive ground fault detection in grounded wye transformer. This type of protection is prevalent in dyn group of transformers delta primary and star. The earth fault relay is placed in the residual part of the current transformers. The relay is connected to the current transformers in such a way. Sep 23, 2016 restricted earth fault ref protection is basically a differential protection.
Restricted earth fault protection philosophy, restricted earth fault, ref why ref, differential earth fault protection. Provided restricted earth fault protection is not required, the scheme shown in figure 15 has the advantage of not requiring a current transformer, with its associated mounting and cabling requirements, in the neutral earth conductor. Restricted ground fault protection can be a low impedance differential function or a high impedance differential function. Medium voltage products technical guide protection. So this ref relay will not be actuated for external earth fault. Measurement of three phase residual current is done either by using core balance current transformer cbct or three cts connected in parallel.
Restricted earth fault protectionref relay working principle. Aug 18, 2018 restricted earth fault protection philosophy, restricted earth fault, ref why ref, differential earth fault protection. One of the tests for which the low impedance function failed to operate was an earth fault on the hv bushing of the transformer with the hv circuit breaker open. To prevent this voltage from becoming too high, the relay is connected in parallel with a voltage dependent resistor. It is restricted because ait detects earth faults for a partportion of the network.
Restricted earth fault protection for a transformer with neutral connection. Pdf high impedance restricted earth fault protection. Learn the basics of differential protection of transformers duration. Standby earth fault relay operation 51n electrical4u. Restricted earthfault and residual earthfault relay spaj. Pdf performance of restricted earth fault protection. T60 restricted ground fault protection calculations 2 ge multilin for a winding fault point at 5% distance from the transformer neutral, the phasetoground primary fault current is calculated as. Restricted earth fault protection for transformer pdf. When the fault occurs, the shortcircuit currents flow through the system, and this current is returned through the earth or any electrical equipment. Protection 1 cag14 relay is applied for high impedance restricted earth fault protection of generator, transformer, reactor and bus bars. If the residual current is calculated, the first group t1t2t3 shall be specified for the inputs of the threephase current transformer on the transformer side, where the protection zone of the restricted. Partial differential protection of parallel transformers earth faults on transformer windings unrestricted earth fault protection restricted earth fault protection biased differential protection of 2 and 3 winding transformers. This scheme is called restricted earth fault ref scheme. 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 illustration shows the principal of ref protection. In this scheme, the ct secondary of each phase of an electrical power transformer are connected together as shown in the figure. T60 restricted ground fault protection calculations. Normally the setting of ref protection is kept as low as 5%. Transformer protection and transformer fault electrical4u. It can be a high impedance type as shown in figure 4 or a biased lowimpedance type. The sensitivity of ref protection is superior as compared to differential protection. Ref protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs. Restricted earth fault protection 64r working transformer. C restricted earth fault protection restricted earth fault protection application contd when an earth fault current occurs, a voltage is rapidly generated across the relay circuit. Power transformer protection relaying overcurrent, restricted earth.
Restricted earth fault ref 64 the same philosophy as differential protection will be used in ref. Use the three independent restricted earth fault ref elements for sensitive ground fault detection in grounded wye transformer applications. A restricted earth fault ref is an earth fault fro m a restricted localised zone of a circuit. It should be noted that the fault currents in zone a is limited by the impedance of the. The scheme can also be connected as shown in figure 16 when restricted earth fault protection is needed. This scheme of restricted earth fault protection is very sensitive for internal earth fault of electrical power transformer. The protection scheme is comparatively cheaper than differential protection scheme. The saturated transformer presents a low impedance path in parallel with the relay and limits the voltage applied. Nov 17, 2019 restricted earth fault protection of transformer. Dec 01, 2011 the name restricted is derived since the objective of the protection is to detect the earth fault in the specific zone, restricted to the zone, starting from the breaker to the machine terminals. This protection scheme is known globally as restricted earth fault protection. Working with the trip characteristic curves of abb sace low voltage circuit breakers abb sace 2. Restricted earth fault ref protection is basically a differential protection. However, earth fault currents are intentionally limited.
The name restricted is derived since the objective of the protection is to detect the earth fault is restricted to the specific zone, starting from the breaker to the machine terminals. However, earth fault currents are intentionally limited to very low levels in high impedance earthed systems. This paper will summarize the key points of restricted earth fault protection. 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. This relay operates when ref delayed operation, heavy earth fault outside of the ref protective zone and all other earth faults. In case of a generator, the machine terminal is the neutral point. Pdf a new method of avoiding malfunctioning of the transformer restricted earth fault ref protection is presented. Ground fault protection of equipment shall be provided for solidly grounded wye electric services of more than 150 volts to ground but.
Both windings of a transformer can be protected separately with restricted earth fault protection, thereby providing highspeed protection against earth faults for the whole transformer with relatively simple equipment. Other applications include capacitor bank unbalance protection, generator inter turn fault protection etc. Earth fault protection an overview sciencedirect topics. The radhd relay is used for providing a high speed restricted earth fault protection for transformers and.
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. Prominent earth fault protections are restricted earth fault, standby earth fault, sensitive earth fault protection. Principle, need, construction and working of the ref protection in transformers. Restricted earth fault operation under normal conditions and by application of kirchhoffs laws the sum of currents in both current transformers. To protect the equipment and for the safety of people, fault protection devices are used in the installation.
Application of the proposed method would eliminate unnecessary operation of ref. Restricted earth fault protection philosophy electrical. Mar 31, 2015 restricted earth fault ref means an earth fault from a restricted localized zone of a circuit. Jul 31, 2016 prominent earth fault protections are restricted earth fault, standby earth fault, sensitive earth fault protection. One of the major advantages of the restricted earth fault relays is their high sensitivity. Therefore for protection of transformer from such a fault we need more sensitive protection scheme which is implemented by using restricted earth fault protection. Performance of restricted earth fault protection scheme in the presence of current transformer remanence. The radhd relay is used for providing a high speed restricted earth fault protection for transformers and reactors. This is a unit protection scheme for one winding of the transformer.
This scheme is specifically used to detect if the earth fault is within the transformer. The earth fault relay is placed in the residual part of the current transformers shown in the. Analysis of an autotransformer restricted earth fault. Pdf a new method of improving transformer restricted earth. Restricted earth fault protection of transformer explained in details. Restricted earth fault protection is also known as. Restricted earth ground fault restricted earth fault protection is included to cover a larger percentage of the transformer windings than might be possible with the main differential elements. The relay will only operate for internal earth faults since the transformer.
If cts are located on the transformer terminals only the winding is protected. Electrical safety earth fault protection tutorialspoint. Shortcircuit current calculations cooper industries. The restricted ground fault function can be used to provide differential protection for such ground faults, down to faults at 5% of the transformer winding.
In this video the philosophy behind the restricted earth fault protection has been elaborated. In this article, we will examine the theory and operation of a specific type of power transformer protection. The limit of the restricted earth fault ref protection zone is defined by the location of the cts. 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. A new method of avoiding malfunctioning of the transformer restricted earth fault ref protection is presented. Applications are not limited by ct saturation for external or internal faults. On a balanced earth fault system for example, this is when one current transformer of a group is saturated whilst the others remain unaffected. The earth fault can be dispersed by using the restricted earth fault protection scheme. Restricted earth fault protection for transformer pdf 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. Restricted earth fault ref protection is a sensitive way to protect a zone between. The scheme can also be connected as shown in figure 16 when restricted earth fault protection.
T60 restricted ground fault protection calculations 2 ge multilin for a winding fault point at 5% distance from the transformer neutral, the phaseto ground primary fault current is calculated as. Abstractrestricted earth fault, or zerosequence differential protection, is beneficial in transformer applications. Restricted earth fault protection is designed to detect power transformer earth faults. Restricted earth fault protection of transformer ref protection.
The level of protection provided for transformers vary worldwide. The devices give the tripping command to break the circuit when earth fault occurs. In earlier posts we have already discussed differential protection of transformer and various characteristics of differential protection. Restricted earth fault ref protection on a transformer is a subject for which there has been little attention and, compared to other types of protection, very little literature exists.
Normally earth fault is nothing but an unintended contact between the conductor body to ground is called earth fault see here the main article. High impedance restricted earth fault protection siemens. It is a backup protection for restricted earth fault relay. Consequently ref protection is of utmost importance for generators and transformers.
What is the difference between restricted earth fault and. Transformer magnetising inrush the process of magnetizing inrush is a transient condition that primarily happens when a power transformer is energized. The fault current is restricted and the fault is dispersed by the restricted earth fault protection refp scheme. Restricted earth fault protection system explanation. In this scheme the ct secondary of each phase of electrical power transformer are connected together as shown in the figure. It is necessary that the proper impedance be used to represent the primary system. Because it does not respond to load current, it offers a significant.
The ct output should not be shared with any other electrical equipment. It also occurs, because of the insulation breakdown. In a restricted earth fault protection, the cts are connected in a fashion as shown below. A new method of improving transformer restricted earth fault. Analysis of an autotransformer restricted earth fault application normann fischer, derrick haas, and david costello, schweitzer engineering laboratories, inc. Siprotec 7ut6 the siprotec 7ut6 differential protection relays are used for fast and selective fault clearing of shortcircuits in transformers of all voltage levels and also in rotating electric machines like motors and generators, for short lines and busbars. It is not a fault condition, and hence transformer. Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. Normally used on systems where the earth fault current is limited to a very low value.
Abstract restricted earth fault, or zerosequence differential protection, is beneficial in transformer applications. An earth fault current is a current that flows to earth and has a magnitude that depends on the method of system earthing. Differential protection vs restricated earth fault protection of transformer 12232009 10. Restricted earth fault protectionsafeguarding your power. Transformer restricted earth fault protection example. This condition is applicable for both phase ct and neutral ct. Sensitive earth fault protection electrical concepts. Restricted earth fault protection is used to provide instantaneous earth fault protection to the transformer, utilizing highimpedance principal. Differential protection vs restricated earth fault. The term ref protection method means not to sense any earth faults outside this restricted zone.
In this scheme the ct secondary of each phase of electrical power transformer are connected together as shown in figure 3. For the restricted earthfault protection to operate properly, the ref 542plus shall be connected to residual and neutral current. We should use separate ct or separate protection core for restricted earth fault relay. Mar 31, 2017 this video shows the key elements of transformer earth fault protection. For all larger rated and important distribution transformers, over current protection along with restricted earth fault protection is applied. A high impedance relay is used, giving fast operation and phase fault. 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. Restricted earth fault is to detect earth faults in the zone from secondary winding to secondary cts.
The low impedance restricted earth fault function for all directly or low impedance earthed windings provides excellent stability for external faults. Ret650 transformer protection substation automation. Differential protection should be provided in the transformers rated above 5 mva. Having individual protection for each winding without the need for harmonics stabilization ensures high sensitivity and fast tripping. Restricted earthfault ref protection on a transformer is a subject for which there has been little attention and, compared to other types of protection, very little literature exists. Analysis of an autotransformer restricted earth fault application. The problem was that for a simulated hv bushing earth fault of the auto transformer the ref function did not issue a trip command to the breaker. The windings of many medium and small sized transformers are protected by restricted earth fault ref systems. Restricted earth fault protection of transformer ref. The sensitive earth fault protection scheme works by measuring the residual current across the three phases in a system. Restricted earth fault protection device for protecting e. The relay contains two energizing input circuits, that is, a differential earth fault current circuit and a residual earth fault current circuit.