Transformer ProtectionApplication Guide
About the Author
George Rockefeller is President of Rockefeller Associates, Inc. He has a BS in EE from Lehigh University,a MS from New Jersey Institute of Technology, and a MBA from Fairleigh DickinsonUniversity. Mr. Rockefeller is a Fellow of IEEE and Past Chairman of IEEE Power Systems RelayingCommittee. He holds nine U.S. Patents and is co-author of Applied Protective Relaying (1st Edition).
Mr. Rockefeller worked for Westinghouse Electric Corporation for twenty-one years in application andsystem design of protective relaying systems. He worked for Consolidated Edison Company for ten yearsas a System Engineer. He has also served as a private consultant since 1982.
About the Guide
This guide contains a summary of information for the protection of various types of electrical equipment.Neither Basler Electric Company nor anyone acting on its behalf makes any warranty or representation,express or implied, as to the accuracy or completeness of the information contained herein, nor assumesany responsibility or liability for the use or consequences of use of any of this information.
Original issue date 05/96Revised 05/99, John Boyle; small updatesRevised 08/03, Larry Lawhead; small updatesRevised 04/07, John Horak; extensive rewriteRevised 06/07, John Horak; minor typographical and editorial corrections
1Transformer ProtectionApplication Guide
This guide focuses primarily on application ofprotective relays for the protection of powertransformers, with an emphasis on the mostprevalent protection schemes and transformers.Principles are emphasized. Setting proceduresare only discussed in a general nature in thematerial to follow. Refer to specific instructionmanuals for your relay. The references provide asource for additional theory and applicationguidance.
The engineer must balance the expense ofapplying a particular protection scheme againstthe consequences of relying on other protectionor sacrificing the transformer. Allowing a pro-tracted fault would increase the damage to thetransformer and the possibility of tank rupturewith a consequent oil fire and consequentpersonnel safety risks. There is no rule that sayswhat specific protection scheme is appropriatefor a given transformer application. There issome tendency to tie protection schemes to theMVA and primary kV of a transformer. Whilethere is some validity to this approach, there aremany other issues to be considered. Issues tobe considered include:
The severity of personnel safety concernsand the possibility that a given protectionscheme can reduce these risks.
The danger to nearby structures and pro-cesses if a transformer fails catastrophicallyand the possibility that a given protectionscheme can reduce the possibility of such afailure.
An overall view of the economic impact of atransformer failure and what can be done toreduce the risk, including: The direct economic impact of repairing
or replacing the transformer. The indirect economic impact due to
production loss. Repair time vs. complete replacement
time. The availability of backup power feed or
emergency replacement transformers, andthe cost of each option.
The possibility that a given protectionscheme can reduce the damage andresultant repair time, or that it can changea replacement into a repair.
Some specific applications that affect protectionare: A tap changer flashover can ordinarily berepaired in the field, but if this fault is allowed toevolve into a winding fault, the transformer willneed to be shipped to a repair facility; hence,protection that can rapidly sense a tap changefault is desirable. A high magnitude through fault(external fault fed by the transformer) shakes andheats a transformer winding, and the longer thethrough fault lasts, the greater the risk of itevolving into an internal transformer fault; hence,fast clearing for close-in external faults is part ofthe transformer protection scheme. Sometransformers are considered disposable andreadily replaced, reducing the need for ad-vanced protection schemes. Transformer protec-tion commonly includes some coverage of
2external bus and cable, and faults in these zonesmay expose personnel to arc flash hazards. Slowclearing protection schemes may be unaccept-able from an arc flash exposure perspective.Fires in an indoor transformer may have high riskof catastrophic facility damage and even higherpersonnel safety risks, increasing the need foradvanced high speed protection. The proximityof flammable process chemicals increases aneed for protection schemes that reduce the riskof a tank fire. The failure of a transformer used ina large base load unit-connected generator maycause extended generation-replacement costs;even the loss of a small station service trans-former can cause a notable disruption of genera-tion and high economic consequences. Similareconomic impacts may also exist at industrialsites. Some transformers are custom designsthat may have long lead times, increasing theneed for advanced protection schemes.
1. Failure Statistics
Table I lists failures for six categories of faults(IEEE C37.90, Guide for Protective RelayApplications to Power Transformers, Ref. 1).Winding and tap changers account for 70% offailures. Loose connections are included as theinitiating event, as well as insulation failures. Themiscellaneous category includes CT failure,external faults, overloads, and damage inshipment. An undisclosed number of failuresstarts as incipient insulation breakdown prob-lems. These failures can be detected by sophis-ticated online monitoring devices (e.g. gas-in-oilanalyzer) before a serious event occurs.
Table I - Failure Rates, Ref. 1.
1955-1965 1975-1982 1983-1988
Percent of Percent of Percent of Number Total Number Total Number Total
Winding failures 134 51 615 55 144 37Tap changer failures 49 19 231 21 85 22Bushing failures 41 15 114 10 42 11Terminal board failures 19 7 71 6 13 3Core failures 7 3 24 2 4 1Miscellaneous failures 12 5 72 6 101 26
TOTAL 262 100 1127 100 389 100
2. Protection Example and General Concepts
The reader interested in additional information,advanced or unusual application advice, anddetailed settings guidance should refer to Ref. 1.This document includes extensive references andbibliographies. Also, Ref. 2 and 3, textbooks onprotective relaying, contain chapters on trans-former protection, and Ref. 4, another IEEEstandard, includes good overall protectionschemes where a transformer is the interfacepoint between a utility and an industrial customer.
There are three general categories of protectiverelay technology that arise in the discussions tofollow:
Electromechanical: uses magnetic fluxcreated from current and voltage to createtorques on movable disks and relays, whichis the source of the term relay. Usuallysingle device number functionality.
Solid State: uses low voltage analog signalscreated from sensed currents and voltages;uses discrete electronics and basic logiccircuits; may contain a basic microprocessorfor logic and some math. Usually single ordual device number functionality.
Numeric: a multifunction, programmablelogic relay; digitizes sensed current andvoltage, then calculates an RMS or phasorequivalent value; uses a high-end micropro-cessor. Usually incorporates many devicenumber functions.
All Basler Electric relays are solid state ornumeric.
3Table II lists some common ANSI device num-bers associated with transformer protection. Anumeric relay generally contains many imple-mentations of these devices within its program-ming, and each instance of a device is referredto, herein, as an element in the relay. Forexample, while the Basler BE1-CDS220 isprimarily a transformer differential relay (hence,includes the 87 device in elements named 87Pand 87N), it also includes nine independentimplementations of the 51 overcurrent device,called the 51P, 151P, 251P, 51Q, 151Q, 251Q,51N, 151N, and 251N elements as well as manyother device functions.
Figure 1 shows extensive use of relays thatwould be representative of a large industrialload. This will be used for discussions in some ofthe material that follows. There are two 115 kVfeeds to two 30 MVA transformers that areresistance grounded on the 13 kV side, limitingground fault current to about 400A from eachtransformer. In other applications, a reactor isused, and in some applications, the ground faultcurrent is limited to less than 10A. In a typicalutility application, transformers are connecteddirectly to ground, but occasionally a smallreactor is placed in the transformer neutral thatlimits ground fault current to approximately thesame level as three phase faults. In this examplesystem, the protection scheme describedapplies to solidly grounded (as well as imped-ance grounded) systems, except the effect ofground impedance results in the addition ofprotection functions not required on a solidlygrounded system.
The phase and ground differential (87P and 87N,Section 4) and sudden pressure relay (63,Section 6) provide the primary transformer faultprotection. The suite of overcurrent elements(51, Section 8) is generally considered backuptransformer protection, or for protection of thebus and backup protection for the feeder relays.These elements are part of the transformerprotection in that they limit the accumulateddamage that occurs from a transformer feedinghigh current into downstream faults. The 67Nrelay offers an alternative to the 87N function.Hot spot monitoring (49, Section 9) is indicated,but is likely an alarm only scheme.
If there is a possibility of over voltage on theunits due to l