This is a unit protection scheme for one winding of the transformer. Jul 31, 2016 prominent earth fault protections are restricted earth fault, standby earth fault, sensitive earth fault protection. Analysis of an autotransformer restricted earth fault. Restricted earthfault protection in 1mrs756411 ref 542plus application and setting guide ref 542plus for this reason, it is necessary to stabilize the differential protection by means of a so called bias current. Abstractrestricted earth fault, or zerosequence differential protection, is beneficial in transformer applications. Restricted earth fault protection device for protecting e. Apr 25, 2018 restricted earth fault ref protection is basically a differential protection. Restricted earth fault ref protection is a sensitive way to protect a zone between two measuring points against earth faults. Medium voltage products technical guide protection.
To determine the fault current seen by the primaryside protective device for a fault on the secondary side, an infinite zeroimpedance source is assumed. It provides protection against the stator winding of the earth fault in the stator and does not operate in case of an external earth fault. It shall be noted that for a fault outside the transformer i. Low impedance restricted earth fault protection request pdf. What is the difference between restricted earth fault and.
This setting allows the user to program the primary current rating of the protection class current transformers on the supply phases. Mar 15, 2020 from the circuit it is also clear that in the case of earth fault on the center of the field circuit will not cause any voltage to be appeared across the relay. Restricted earth fault ref relay settings of power. Medium voltage products technical guide protection criteria. The restricted earth fault protection function is configurable independently of the others settings.
Restricted earth fault protection is designed to detect power transformer earth faults. 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. Restricted earth fault protection ref is a unit protection scheme for detecting earth faults. Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. The residual phase currents for each selected winding of a transformer are first determined and then summed to provide a value designated i x. Also, it discusses on importance of restricted earth fault protection as a one of power transformer protection functions. Oct 05, 1999 restricted earth fault protection ref has been used in the past to protect one winding on a grounded wye transformer. While the outlets main contactor is open, the efl will continually monitor the resistance of the. Rotor earth fault protection of alternator or generator. This simple method of rotor earth fault protection of alternator has a big disadvantage.
In earlier posts we have already discussed differential protection of transformer and various characteristics of differential protection. Restricted earth fault ref means an earth fault from a restrictedlocalized zone of a circuit. That means simple potentiometer methods of rotor earth fault protection, is blind to the faults at the center of the field winding. Differential protection is to detect phase faults within the transformer on both primary and secondary sides,while restricted earth fault is to detect earth faults in the zone from secondary winding to secondary cts. To prevent this voltage from becoming too high, the relay is connected in parallel with a voltage dependent resistor. Protection type mcag 14, 34 switchgear manufacturing. In this scheme, the ct secondary of each phase of an electrical power transformer are. What is the need of restricted earth fault relay despite. Jagannath abstract this paper attempts to bring out the necessity of activating directional earth fault protection in numerical distance relays and also highlights an operational problem with this. In this scheme the ct secondary of each phase of electrical power transformer are connected together as shown in the figure.
The radhd relay is used for providing a high speed restricted earth fault protection for transformers and. Scope this document introduces the application of restricted earthfault protection in ref 542plus. So this ref relay will not be actuated for external earth fault. Apr 25, 2018 sensitive earth fault protection scheme is used for the detection of earth fault. Restricted earth fault protection scheme is designed to respond to earth faults in particular zone i. In a typical utility application, transformers are connected directly to ground, but occasionally a small reactor is placed in the transformer neutral that limits ground fault current to approximately the same level as three phase faults.
Buchholz relay in transformer buchholz relay operation and principle. The ct secondaries are wired to cancel each others currents during through faults and to drive all secondary current to the relay when the fault. This paper presents the detailed engineering study conducted to design a highimpedance restricted earth fault ref scheme for four 110. For the restricted earthfault protection using the biased. Ppt on protection of power transformers linkedin slideshare. It need not be graded or coordinated with any other protection. Directional earth fault protection for transmission lines. Analysis of an autotransformer restricted earth fault application. Restricted earth fault protection biased differential protection of 2 and 3 winding transformers. Sep 06, 2016 differential protection is to detect phase faults within the transformer on both primary and secondary sides,while restricted earth fault is to detect earth faults in the zone from secondary winding to secondary cts. A directional element calculates a torque value t according to the following formula. Definite time character the mref provides the following settings to choose from. Because it does not respond to load current, it offers a significant.
This paper will summarize the key points of restricted earth fault protection. Then common terminals are connected to the secondary of a neutral current transformer or nct. The prior arrangement uses a directional element which measures the phase angle between the neutral current of the transformer and the residual current of the three phases of current in the one winding. In a 33kv11kv substation, we use siemens protection relay 7ut612 for transformer differential and restricted earth fault protection. Scope this document introduces the application of restricted earth fault protection in ref 542plus. Typical earth fault relay settings are 3040% of nominal fullload current or minimum earth fault current for the circuit being protected. 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. Analysis of an autotransformer restricted earth fault application normann fischer, derrick haas, and david costello, schweitzer engineering laboratories, inc. Sensitive earth fault protection scheme is used for the detection of earth fault. The ground fault current magnitude depends on the power system grounding which can vary from. For ref, the neutral point of the transformer star point is required to be earthed. The stabilising current is connected in series with the. Prominent earth fault protections are restricted earth fault, standby earth fault, sensitive earth fault protection.
Restricted earth fault protection for transformers using a. Unrestricted ef protection working on the principle of comparing the unbalance on the phases. Restricted earth fault protection of transformer electrical. This video provides an insight of the topics covered in the full 2 hour. Type mcag 14, 34 high stability circulating current relay protection the mcag relay, used with a stabilising resistor, is designed for applications where sensitive settings with stability on heavy through faults are required, and is recommended for balanced and restricted earth fault, buszone and certain forms of differential protection for. If there is an earth fault between the cts then some current will bypass the cts and the sum of currents will not be zero.
Therefore, a pickup setting of 30% 40% of in is suggested, where in is. What is ref restricted earth fault protection on transformer. The value can be calculated using the following equation. It is assumed that all phase current transformers are of the same rating. Unrestricted earthground fault protection is the zone from the load side terminals of the acb to the the final circuit. Restricted earth fault protection ref has been used in the past to protect one winding on a grounded wye transformer. Unrestricted earth fault protection electric power.
In this article, we will examine the theory and operation of a specific type of power transformer protection. Then, different recent algorithms for restricted earth fault protection from various companies are illustrated. The earth fault protection gets improved by the application of restricted earth fault relay. Restricted earthfault protection is designed to detect power transformer earth faults. Restricted earth protection which also known as differential earth protection by which a particular defined zone is protected restricted. Although this type of protection is being specified more you have to be careful regarding selectivity. The relay is designed for sensing the fault current which is 15% more than that of the rated winding current.
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. Earthfault protection in systems with isolated star point. It is restricted because ait detects earth faults for a partportion of the network. This arrangement can only sense the earth fault occurred in the any point except the center of the field winding. The ampcontrol efl protection relay provides earth fault lockout eflo and frozen contactor fc protection that is compliant to asnzs 2081. Oct 20, 2016 therefore for such type of faults restricted earth fault protection is used. This scheme of restricted earth fault protection is very sensitive for internal earth fault. Is the buried tertiary covered by differential protection. Any information regarding testing procedure for transformers restricted earth fault will be higly appreciated. Is the restricted earth fault ref sensitive to interwinding faults. The term ref protection method means not to sense any earth faults outside this restricted zone. Therefore for such type of faults restricted earth fault protection is used. An external fault in the star side will result in current flowing in the line ct of the affected phase and at the same time a balancing current flows in the nct, hence the resultant current in the relay is therefore zero. Such type of protection is provided in the large generator as an additional protection scheme.
In the next two sections we shall briefly look at the two types of ref protection and their advantages and disadvantages. A strategy for practical specifications of restricted earth fault ref relay settings of power transformers based on the high impedance principle has been described in this work. Since fault current then remains at a high value even to the last turn of the winding figure 4, virtually complete cover for earth faults is obtained. Restricted earth fault protection system explanation. This protection scheme is known globally as restricted earth fault protection. From alstom network protection and automation guide 2011 section 9. Restricted earth fault operation under normal conditions and by application of kirchhoffs laws the sum of currents in both current transformers cts equals zero. The earth fault relay must be very sensitive for sensing the fault. Differential protection vs restricated earth fault protection. Restricted earth fault protection for transformer pdf. 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. For the highimpedance type, the residual current of three.
In basic terms unrestricted earth fault protection detects all earth fault current downstream of the device. One of the major advantages of the restricted earth fault relays is their high sensitivity. In ither words you have to ensure that all devices have earthground fault protection downstream of the acb. The level of protection provided for transformers vary worldwide. Mar 31, 2015 restricted earth fault ref means an earth fault from a restricted localized zone of a circuit. In this paper the restricted earth fault relay is tested using emtpatp as well as an earth fault simulation program and gave reliable and accurate results. What is the use of ref restricted earth fault protection. It is working on the principle of balanced currents betwwen phases and nuetral.
Earthfault protection in systems with isolated star point fig. Restricted earth fault protectionsafeguarding your power. From the circuit it is also clear that in the case of earth fault on the center of the field circuit will not cause any voltage to be appeared across the relay. 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. Practical considerations for high impedance restricted. White paper double ground fault protection implementation. Sensitive earth fault protection electrical concepts. Fundamentals of shortcircuit protection for transformers. Restricted earth fault protection using an i0 input of vamp relay. The degree of protection is very much improved by the application of restricted earth fault protection or ref protection. Restrited earth fault is normally given to on star connected end of power equipment like generators,transform ers etc. Earthfaultprotectioninsubtransmissionsystems coh teslaconsultants chris. The conventional ref relay settings rely on the principle that the ref current transformers on the phase and neutral sides of the main power transformer are having identical parameters and hence the normal settings.
Restricted earth fault protection is often applied even when the neutral is solidly earthed. The neutral current for the transformer is also determined, with the neutral current being designated i y. While the outlets main contactor is open, the efl will continually monitor the resistance of the phase conductors to earth, on the load side of the contactor. In the case of a main incomer breaker it provides backup protection to the feeder breakers on the switchboard as well as the protection for the bus bar system on the switchboard. 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.
Secondary fault currents are typically in the range of 2 ka to 6 ka. It can be of the high impedance type as shown in figure1, or of the biased low impedance type. Differential protection vs restricated earth fault protection of transformer 12232009 10. Ground fault protection of equipment shall be provided for solidly grounded wye electric services of more than 150 volts to ground but not exceeding 600 volts phasetophase for each service disconnect rated amperes or more. Balanced earth fault protection of a generator working. How can we test the restricted earth fault to know that the relay is responding or not. The protection scheme is comparatively cheaper than differential protection scheme. The basic principle of the biased differential protection is shown in fig.
Restricted earth fault protection using an i0 input of. In addition, it is typically challenging for some practitioners to analyze the coverage and sensitivity of a given shortcircuit protection scheme. Such settings protect a restricted portion of the winding. This scheme is also called restricted earth fault protection scheme. Restricted earth fault protection in dyn transformers. This difficulty can be minimized by using another tap. Protection against phase to ground faults can be a difficult problem since ground fault currents vary within a large range, becoming almost negligible in some situations. In a restricted earth fault protection, the cts are connected in a fashion as shown below. Restricted earth fault ref protection is basically a differential protection. Pdf high impedance restricted earth fault protection. Directional earth fault protection for transmission lines need and operational experiences v. Restricted earth fault protection of transformer ref. 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. Nov 17, 2019 restricted earth fault protection of transformer.