An Improved Transformer Protection SchemeBased on Integrated Protection System

Hao Zhang, Jinghan HeBeijing Jiaotong

Universityabl23h@hotmail.com

BinLiTianjin Universitylibin_tju@126.com

ZhiqianBoAREVA T&D Automation,

UKzhiqian.bo@areva-td.com

Abstract-- A back-up protection scheme for a distributionnetwork is described in the paper. The progresses inmicroprocessor, communication and transducer technologies haveprovided new means for the design and development of newgeneration of algorithms and devices in power system. Studies havefound that the fault generated electric quantities can not only bedetected by integrated relay to achieve the line protection but alsoused for developing transformer protection techniques. With thecooperation between different protection functions, integratedpower transformer protection can quicken up the pace of back upprotection.

Index Terms-Integrated Protection; Power transformerprotection; Substation communication system; Back-up protection;

I. INlRODUCTION

Transformer is one of the most important elements in powersystem. If the transformer is an integral part of the bulk powersystem, it will probably require more sophisticated relays interms of design and redundancy [1,2]. In distribution network,step-down transformer trip slow for external fault is achallenging problem in the area of transformer protection. Thedemand of accelerated tripping is needed, due to the deleteriouseffect of a delayed fault clearing on system performance, andhigh cost of transformer repair. In recent years, the progresses inmicroprocessor, communication and transducer technologieshave provided new means for the design and development ofnew generation of algorithms and devices in power system.Integrated Protection System has more advantages on controland protection, which can also be used to enhance thetransformer back-up protection [2-5].

This paper reviews development of Integrated ProtectionSystem and put forward strategies and measures to improve thetransformer back-up protection, which is worth of referring inpower system protection.

This work was supported in part by the National Natural ScienceFoundation of China (50677003) and AREVA T&D Automation

II. COMMUNICATION NETWORK

Based on the principle of substation automation, thetechniques for intelligent switchgear, non-conventionalinstrument transformer and online monitoring for primaryequipment are becoming more and more mature; and high-speedcommunication network based on IEC61850 standard in real-time system are being developed, which makes theimplementations of a fully Integrated Protection Systempossible.

In Integrated Protection System, the exchange of data is notonly between functional elements, but also between differentlevels of the substation functional hierarchy. It should be kept inmind that functions at different levels of the functional hierarchycan be located in the same physical device, and at the same timedifferent physical devices can exchange data at the samefunctional level.

[SWitChl-gear

Fig. I . Integrated protection system in digital substation

Fig. 1 shows an example of an Integrated Protection Systemin digital substation, in which all function devices(communication gateway, human machine interface and GPS

978-0-947649-44-9/09/$26.00 ©2009 IEEE

III. TRADITIONAL TRANSFORMER PROTECTION

A conventional differential relay derives the magnitude ofthe operating differential current using:

clock, etc.) in a substation are connected to the network. Asshown in the Fig. 1, the Interface Unit (IV) is a measurementand control unit, which interfaces to apparatus through differenttypes of transducer, such as optical combined sensors andelectronic combined sensors . The analogue and digital signalsmeasured are converted into optical format and sent to thedifferent devices through the redundant optical network.

As can be seen from Fig. 1, the functions in the substationcan be distributed between IEDs on the same, or on differentlevels of the substation functional hierarchy. IEC61850 definesthree such levels: Station ; Bay/Unit; Process

The red blocks in Fig. 2 show the connection logic for thetransformer protection. The transformer protection device,which is in the Bay/Unit level, not only obtains the informationfrom the power transformer, but also can obtain the informationof other devices in substation through the communicationnetwork . Therefore the interaction and grading between IEDs inthe substation can be achieved.

Fig. 3. Differential relay characteristic

In order to compensate these mentioned errors and to ensurestability of the relay in the case of external faults, a restrainbased percentage differential relay characteristic should bechosen.

Back-up protection against excessive overload, or persistingexternal fault, is provided by time-delay overcurrent protectionand other feeder protections. Generally, the transformer back-upprotection time-delay must coordinate with the low-sideprotections.

The principle of relay coordination can be explained byreference to FigA which shows transformer and a series of linesand the time-distance characteristics of the associated inverse-time protections. These are relay operating curves selected foreach of the relays, plotted as a function of fault location. Sincethe magnitude of the fault current decreases as the fault movesaway from the source, these curves appear 'reversed'.

As it shown in FigA , a fault at the end of the line2 should becleared by R3. Rl sees the same fault as R3 and start to countdown for closing its contacts. If the circuit breaker clears thefault, Rl will reset after some overtravel. Contact overtravel isincluded in the margin of safety. If the fault is not cleared, Rlcontinues count down to close its contacts and initiates a trip ofCB 1 at the end of its operating time.

Using these specifications, the bias current and the slope ofthe relay characteristic is selected and depicted in Fig. 3: [6,7]

I IIT

Others primaryequipment

(1)

(2)

I D =Ih -I,

IR=(Ih +1,)/2

I

TransformerFig. 2. Diagram of communication network

)(Transformer

T(s)t3

12

t1

o

(b)

(c)

F1

L

Rulel:

If (Vi, L, see vnormaly

=max {Tu} +U +V +X

Rule2 :

If (Vi, L,

&

Then

Countdown

Tjeeder-delay

These two roles are the normal protection logic. The rulestates that if the abnormal situation happens on one feeder , thefeeder protection relay and transformer relay can both see it andstart the count down for tripping.

Fig. 4 Relay coordination principles

IV. IMPROVEDCRITERION

Referring again to FigA (a) and to the time sequence shownin Fig.3(a) and (c). A fault named FI happened. Ifit not clearedby R3, RI need to trip. But obviously, RI does not close itscontacts before the fault is cleared by CB3 it must be set longerthan R3 operating time (U), plus CB3 clearing time (V), and afactor of safety (X). So the time delay compared withdownstream feeder is :

Rule3 :

If Tfeeder-delay 0

Then

TriPtransformer Block

Rule 3: If the feeder relay sends out a trip signal , integratedprotection system should block the transformer relay. This rulecan prevent the mal-trip of the transformer relay.

It is usual to add 0.3-0 .5 s coordinating time to the operatingtime ofR3 which is calculated at its maximum fault. The samefault current is used to determine the operating time of relay RI.What is more, if transformer connects not only one feeder , thetime delay should be plus more 0.3-0.5s than the maximal delayofall the feeders .

The long time delay will cause a worse deleterious effect onsystem. It may damage the transformer.

When applied to region of a distribution network,transformer back-up protection is implemented using IntegratedProtection System, normally one per substation.

In the Integrated Protection System, information obtainedfrom feeders through communication can be used to designmore advantage control strategies:

t:..T=U+V+X (4) Rule4 :

If Vi ,

&

Then

Rtransjormer Trip

Rule 4: If the feeder breaker can not clear the abnormalsituation, transformer protection will accelerated tripping. To besure that feeder breaker has an operating time after the tripsignal.

V . CONCLUSION

The paper proposes an advanced transformer back-puprotection criterion for application to integrated protectionsystem. The integrated Protection System is able to provide afast response, high accuracy, and is not affected by power

frequency phenomena, such as fault path impedance, powerswing and CT saturation.

With the Integrated Protection System, studies show that theproposed technique is able to offer a very fast response in thesituation of fault can't be cleared by feeder breaker. Thescheme, referred to as the communication network, isimplemented using a batch of control logics based IntegratedProtection System. The protection system provides quicken uptripping to clear the fault in the distribution network that failedto be cleared by feeder breaker.

VI. REFERENCES

[1] Sanaye-Pasand, M., Zangiabadi, M., Fereidunian, A.R., "An extendedmagnetizing inrush restraint method applied to digital differential relaysfor transformer protection," IEEE Power Engineering Society GeneralMeeting, pp.2077-2082, Jul. 2003.

[2] Zhiqian Bo, Weller, G., Lomas, T., "A new technique for transformerprotection based on transient detection"IEEE Transactions On PowerDelivery, pp.870 - 875, Jul. 2000

[3] Hagh, M.T., Valizadeh, M., "Analysis and comparative study of transientinrush current reduction methods," Power Engineering Conference, pp.287-291, Dec. 2007.

[4] Kang, Y.C., Jin, E.S., Kang, S.H., Crossley, P.A., "Compensated-currentdifferential relay for protection of transformers," Generation,Transmission and Distribution, lEE Proceedings, pp. 281-289, May. 2004.

[5] Brunke, J.H., Frohlich, K.J., "Elimination of transformer inrush currentsby controlled switching. I. Theoretical considerations," Power Delivery,IEEE Transactions on, pp. 276-285, Apr. 2001.

[6] Stanley H. Horowitz, Arun G. Phadke, Power System Relaying, vol. I.Research Studies Press Limited, 2008

[7] lC. Tan; P. A. Crossley, D.Kirschen, lGoody, l A. Downes. "An expertsystem for the back-up protection of a transmission network," IEEETransactions On Power Delivery, pp.508 - 514, April. 2000

VII. BIOGRAPHIES

Hao Zhang obtained his B.Sc. degree from Beijing Jiaotong University in2006. And he worked in the design and application of protection relays as aresearch fellow, in AREVA Company U.K. in 2008. He is currently workingtoward the Ph.D. degree in the School of Electrical Engineering, BeijingJiaotong University, Beijing, China. His research interests are integrated powersystem protection and substation communication.

Jinghan He obtained her M.Sc in 1994 in the Department of Automation,Tianjing University, China, respectively. Presently, she is employed as theProfessor in Beijing JiaoTong University, China. Her main research interestsare Protective Relaying, Fault distance measurement and Location in PowerSystem.

Bin Li obtained the B.Sc, M.Sc and Ph.D degrees from Tianjin University in1999,2002 and 2005 respectively. He was academic visitor of the University ofManchester, U.K. in 2006. And he worked in the design and application ofprotection relays and phasor measurement unit as a research fellow, in AREVACompany U.K. in 2008. Currently, he is associated professor of the School ofElectrical Engineering and its Automation, Tianjin University, Tianjin, China.His main research field is involved in the protection and control of distributedgeneration.

Zhiqian Bo obtained his B.Sc degree from the Northeastern University, Chinain 1982 and PhD degree from The Queen's University of Belfast, UK in 1988

respectively. Presently, he is with AREVA T&D Automation & InformationBusiness