ziegler protection1

7/27/2019 Ziegler Protection1

1/10

ELECTRA iN206 - Fvrier 2003

S TAT E O F T H E A RT PA P E R

1

Summary

Protection and substation control haveundergone dramatic changes since the adventof powerful micro-processing and digitalcommunication. Smart multi-functional andcommunicative feeder units, so called IEDs(Intelligent Electronic Devices) have replacedtraditional conglomerations of mechanicaland static panel instrumentation. Combinedprotection, monitoring and control devicesand LAN based integrated substationautomation systems are now state of the art.Modern communication technologies includ-ing the Internet are used for remote moni-

toring,setting and retrieval of load and faultdata. Higher performance at lower cost hasresulted in a fast acceptance of the new tech-nology. The trend of system integration willcontinue,driven by the cost pressure of com-petition and technological progress. Theongoing development towards totally inte-grated substations is expected to pick up speedwith the approval of the open communica-tion standard IEC61850 in the next years.

1. Introduction

Increased competition has forced utili-ties to go into cost-saving asset managementwith new risk strategy:

Plants and lines are higher loaded upto thermal and stability limits.

Existing plants are operated to theend of their life-time and not replaced ear-lier by higher rated types.

Redundancy and back-up for systemsecurity are provided only with criticalindustrial load.

Corrective event based repair hasreplaced preventive maintenance.

Considering this changed environment,power system protection and control facenew technical and economical challenges:Modern secondary systems shall enablehigher system loading at lower investmentand operation cost without compromisingsystem reliability.

Users widely dispense with special cus-tom-built solutions but aim at cost reduc-

tion by accepting standard products of globalvendors.

Manufacturers had to compensate theworld-wide price drop by cost saving. Thishas mainly been achieved by right-sizing of product ranges, standardisation of products,rationalisation ofmanufacturing and expan-sion to global markets.

In this regard, the introduction of thedigital technology has played a decisive rolebecause the price reduction at a comparablefunction range could only be achieved withthe new generation of smart highly inte-

grated IEDs.Besides the lower investment cost, the

user gets a reduction of the operation costdue to the inherent self-monitoring capa-bility (corrective instead of preventive main-tenance) and the possible remote operationand diagnosis.

In the relay business, these advantageswere obvious for the user. Therefore, thetransition to the new digital technology occurred within a decade (1985-1995).

PROTECTION AND SUBSTATION

AUTOMATIONState of the Art and Development TrendsG ERH R E RM , F E AIR IG E


2/10


No. 206 - February 2003 ELECTRA 15

In the case of substation control, costcomparison between electromechanical anddigital technology has often been discussedcontroversially. The recently practised assess-ment of total life cycle cost,however, seemsto confirm economic use in most cases. Deci-sive is the possibility to rationalise and auto-mate substation operation and to save oper-ating staff on site.This pays off in particularin industrialised countries with high per-sonnel cost.

Relaying and control IEDs also serve asdata acquisition units for power system con-trol and power quality monitoring.By usingwide area information systems, the data canbe made available to all involved partners.This is becoming more and more importantin order to satisfy the information demandin the deregulated power supply market withfree network access.

2. Recent developmentof protection and sub-station automation

After more than 20 years of develop-ment,digital protection and substation con-trol have reached a mature product state.In the mean time,some 100.000 digital relaysand some 1000 digital substation control sys-tems are in service.

As a rule, a common IED hardwareplatform is used today for relays and bay control units (figure 1). Its modular designallows adaptation of the input/output inter-face to the individual application.Separateprocessing modules are dedicated to thecommunication interfaces to cope with theincreased data rates and complex transmis-sion procedures.GPS time synchronisationof microsecond accuracy is optionally offered with the latest device generations.

Global products designed for the worldmarket meet relevant IEC as well asANSI/IEEE standard requirements and canbe adapted to the communication stan-dards used in Europe and USA. The infor-mation interface of relays can for exam-ple be delivered to IEC60870-5-103 as wellas to DNP3.0 or Modbus. Windows com-patible PC programs allow comfortablelocal or remote operation of IEDs. Unfor-tunately, there is no common operating

standard, so that the user must changebetween vendor dependent program ver-sions to address relays of different make.This also concerns communication inter-faces and protocols.

An improvement can be expected whenrelays will be equipped with their own Inter-net server and the operator-relay dialoguecan be performed in a simple way by usingstandard browsers. First Internet enabledIEDs are already available.

F IGURE 1 - Current relay design trend


3/10



1

C O N T I N U E D

Protective relaying

The number of functions integrated in

relays has been steadily expanded in parallelwith the increasing processing power andstorage capacity. Table 1 shows a typicalexample of relay hardware evolution.

Protection relays have developed intomulti-functional universal devices, generally designated as IEDs (Intelligent ElectronicDevices). Non-protection tasks, such asmetering,monitoring,control and automa-tion, occupy an ever increasing share of thescope of functions.

Complete protection of a power systemcomponent (transformer, line,etc.) can now

be provided by only a few highly integratedrelays. For example, the protection of a largergenerating unit only needs two or threerelays, each with about 15 protection func-tions.At the time of traditional relaying, sev-eral panels or cubicles full of black-box relayswere necessary for the same protection scope.

Basic digital protection functions havepassed innumerable lab and field tests and

are well-established in practice. In the last years, they could be further improved by applying intelligent algorithms.Examples forthis are: higher accuracy and stability in caseof disturbed measuring values (e.g. duringc.t. saturation), and better load versus fault discrimination by adaptive measuring prin-ciples and flexible shaping of characteristics.

The offer of integrated functions coversthe world-wide practice (global relay).Theuser can for example choose between defi-

nite and various inverse over-current timecurves or between quadrilateral and MHOtype impedance characteristics. He has thefreedom to configure the relay for his par-ticular application case by software para-meterisation.

This trend will surely contribute to aglobal convergence of relaying practices.

Metering and event/fault recording arenow offered as standard even with smallestrelays.

An accuracy of about 1% for metering

of current and voltage and of about 2% foractive and reactive power are usually speci-fied for relays. For the total accuracy, theerrors of c.t.s and v.t.s (up to 3% with pro-tection cores) have to be added.

The storage time for fault records is now in general at least 10s with a resolution of 600to 2 400 Hz dependent on the type of relay.

Power quality monitoring is partly cov-ered by protection relays. The offered reg-istration of voltage dips greater than 10 msand harmonics up to the 5 th or 10 th order issufficient in most application cases. Moni-toring of fast transients and higher har-monics (e.g. up to the 50 th) would requirehigher sampling rates and extended relay memories. The ongoing technical improve-ment and price reduction of hardware com-ponents will however favour a trend towardsthe integration of full scale PQ monitoringin protection relays.

Over the years, there has been a globaltrend towards combined units for protectionand control on basis of IEDs (Intelligent Elec-tronic Devices, Figure 2). The main appli-cation areas are distribution systems andindustrial networks.These universal devicesintegrate all substation secondary functionswith the exception of revenue metering.

Full scale versions include a full graphicmimic display and a key pad for supervisory

Year Memory Bus ProcessingRAM/EPROM width power

1986 64k/128k 16 0.5 MIPS

1992 256/512k 16 1.0 MIPS1999 512k/4MB 32 35.0 MIPS

(+ 4 MB D-RAMprogram memory)

T ABLE 1: Development of digital relay HW performance (example)


4/10



C O N T I N U E D

control. The devices can be used stand-aloneor serially connected to an RTU or a cen-tral control unit.

Automation functions can comfortably be designed and implemented by means of a graphic PC tool (CFC: continuous func-tion chart).

u

Simulation techniques have advanced toa degree of virtual reality. This goes in par-ticular for real-time digital simulation sys-tems (RTDS) which enable absolute practicecompatible lab testing.

However, even PC-controlled portabletest sets allow for dynamic testing under realconditions.Among other features, programadditions are offered for extended relay test-ing, for example under the condition of c.t.saturation.

This new quality of testing has deci-sively contributed to the upgraded perfor-mance and reliability of digital protectionsystems.

a t a

Based on the nearly complete self-mon-itoring of modern IEDs, event controlledmaintenance is propagated world-wide asa decisive contribution to cost reduction.

Theoretical studies have shown that theavailability of digital protection is even com-parable to a redundant analogue protectionscheme providing at the same time signifi-cantly higher security against false operation.Complete abolition of testing, however, ismostly not accepted as even the best self-monitoring concept cannot cover 100% of the protection scheme.

In the few publications about the cur-

rent practice, maintenance intervals of four(Germany) to six years (Japan,Sweden) wererecommended. Newer surveys indicate atrend to longer intervals, even up to 10 years.

3. Current protectionpractice

A world wide survey on reasons forblackouts and experienced protection per-

formance [1] showed the following: the judgement of protection was in general rea-sonably good. There was, however, a num-ber of maloperations of feeder protection.More attention should be paid to relay set-ting and co-ordination with overload capa-bilities of the protected plants.

The survey confirms that fast clearanceof fault, in particular on busbars, is vital toa systems ability to ride through distur-bances. Duplication of protection and

F IGURE 2 - Combined Protection and Control IED


5/10



1

C O N T I N U E D

the installation of breaker fail provision isessential on crucial busbars and on high-voltage lines since back-up operation timesoften result in system splitting and cascad-ing.

The following development trends canbe observed in the individual protectionareas:

3.1 Transmission system pro-tection

On higher voltage levels, redundant pro-tection concepts with stand alone relays havebeen kept also with the changeover to digi-tal technology. Relays with dissimilar mea-suring principles (e.g. differential and dis-tance) or relays of different make are stillpreferred. Control functions are provided by independent feeder units.

With the advent of digital wide-bandcommunication more differential protectionhas been applied at overhead lines of evenup to some 100 km length.Phase segregateddesign guarantees zone and phase selectiv-ity for all kind of short-circuits. The use isadvantageous in particular with complex lineconfigurations such as multi-circuit, multi-terminal or tapped lines.

Differential and distance protection arenow considered as ideal combination for

high voltage lines.The transfer of protectiondata via communication networks, however,requires careful planning.GPS synchronisa-tion may be necessary in critical cases.

With short lines up to some 30 km, adirect back-to-back connection of the relaysat line ends is possible provided that dedi-cated optic fibres are available.

Upgrading of fault location is a preferredsubject of ongoing studies and numerouspublications. In most cases,new or improvedmethods are discussed and proposed to com-

pensate influencing factors such as faultresistance, load transfer, parallel line cou-pling, series compensation and line charg-ing current.

Fault location based on reactance mea-surement as integral function ofdistance relayshas an accuracy of about one percent linelength under favourable conditions. Largererrors will however occur with higher faultresistances. Improvement can be expected inthe future by GPS based synchronisation of data acquisition and processing of the infor-mation from both line ends.

High accuracy is achieved by travellingwave based fault locators. ESKOM, SouthAfrica reports about +/- 150m on EHV lines[2]. Fault location is in this case estimatedby measuring the time difference of travel-ling wave propagation from the fault to bothline ends.

The use ofdigital filtering and intelligent

algorithms has dramatically upgraded trans-former differential protection performance.Stability against c.t. saturation, inrush-cur-rents and overfluxing is now much more reli-able. Integrated numerical ratio and vectorgroup adaptation belong to the standard.Relays with up to five stabilising inputs areoffered which allow to protect all kind of transformer connections.

Integrated add-on functions now reachfrom overload and overcurrent back-up to

POWER SYSTEM FAULT (LIGHTNING STRIKES AN OVERHEAD LINE ).


6/10



C O N T I N U E D

earth-fault and over-excitation protection.OLTC control and transformer monitoringare also integrated in some combined

devices.

State-of-the-art is decentralised digitaldesign and sub-cycle operating time. Bay units are connected to the central unit viafast optical fibre links. Sophisticated algo-rithms guarantee far reaching independenceof c.t. saturation.The isolator replica is soft-ware based and can each be adapted to evencomplex bus configurations by means of the

setting program.Digital low impedance protection is now

offered even in traditionally high impedanceminded regions because high impedanceprotection can by principle not be trans-ferred to digital technology.

Function integration has further pro-ceeded. Even larger generating units can now

be protected by two or three relays with eachabout 15 protection functions (protectionof auxiliaries nor considered).

The quality (sensivity and accuracy of measurement, replica reality, etc.) of indi-vidual protection functions has been furtherimproved. In principle, however, the longtime established protection principles arefurther used.

A more recent development concernsSystem Protection Schemes (SPS) [3, 4].They operate on the basis of system wideacquired information and try to avoidpower system collapse which can occur dur-ing unstable active or reactive power con-ditions as a consequence of voltage and fre-quency drop or loss of synchronism.Normally one tries to achieve stable partialnetworks by purposeful system splitting,

load shedding and forced control of powergeneration. The SPSs are intended to oper-ate already in the initial state of instability before system control can intervene. Recent

developments include GPS-based synchro-phasor measurement for on-line systemstate monitoring.

A number of SPS systems are already inservice, mainly in Japan [3]

3.2 Distribution System Pro-tection

The current trend is towards combinedprotection and control IEDs. Driving forceis the need for cost cutting. The reduction of

the former conglomeration of black box devices to only one multifunctional relay saves on space and wiring.

Highly integrated switchgear panelsusing small scale CTs and VTs are gainingincreasing market share.

Low resistance earthed radial networksare generally protected by inverse-time OCrelays. Meshed Peterson coil earthed net-works which occur mostly in Europe are alsoequipped with distance relays.

GIS

S i

e m e n s

( G e r m a n y

)


7/10



2

C O N T I N U E D

Urban cable networks traditionally usedifferential protection. New digital relaysmust be suitable for the existing pilot wires.Therefore, proven analogue current com-parison principles are maintained, however,upgraded to digital relaying standards. Forshort cable connections also relays with dig-ital wire communication can be applied. Inthe more seldom case where optic fibre con-nections exist, relays with direct relay-to-relay OF connection can be used for dis-tances up to about 30 km.

The growing share of distributed gener-ation requires reconsideration of distribu-tion protection. In many cases changeoverto directional OC relays may be necessary to

cope with the backfeed of distributed gen-erators. A particular problem provides theloss-of-mains protection because traditionalfrequency and voltage relays may be too slow or insensitive and vector jump relays tend to

overfunction. A number of new principleshave been proposed but no satisfying solu-tion is yet available.

Fast fault finding and system restora-tion to upgrade power supply quality is get-ting more and more important. For thispurpose, the state of earth-fault and short-circuit indicators are evaluated togetherwith the distance-to-fault calculation of relays.Modern fault management includesautomatic acquisition and processing of these data. The results are then indicated inthe graphic information system of the con-trol centre. [5, 6]

In many countries, fault clearing by dis-tributed reclosers and sectionalisers is still

practised. Protection and control functionsof these devices are now also provided by digital devices. In combination with pole-top RTUs and radio connections, fast faultclearing and load restoration is also achievedin this case. [7]

Detection of high resistance faults(downed conductors) has been studied fora long time. Proposed algorithms are basedon wave shape analysis and recognition of typical arc characteristics.A recent survey comes to the conclusion that an algorithmsuitable for practical application has so farnot been found despite costly developmentefforts. [8]

4. State and trends of substation automation

Integrated protection and control firstappeared in the mid eighties and has sincethen matured to full scale substation

automation.

4.1 Recent practice

Simple systems for distribution or indus-trial networks mostly use feeder dedicatedcombined protection and control IEDs anda PC-based central unit. Alternatively,enhanced RTUs with decentralised I/Operipherals are applied.OPEN AIR SWITCHYARD

S

i e m e n s

( G e r m a n y

)


8/10



C O N T I N U E D

Ethernet is generally accepted as substa-tion LAN. Industry standards such as

Profibus and LON are successfully used inEurope while DNP3.0 and Modbus are pre-ferred in USA. Recently, Ethernet withTCP/IP has also been introduced.

Larger systems typically use a specialcentral unit (server) and separate bay unitsfor control. Independent protection relaysare usually connected to the bay control unitsin this case.

The remote control function is emulatedeach in the central unit. The standardIEC60870-5-101 has in the mean time been

generally adopted for communicationbetween substation and control centre.Time accurate GPS based synchronising

is available as an option.Direct peer-to-peer communication

between bay units is offered in some cases.It can be used for control (e.g.interlocking),however not for protection because of therelatively long reaction time (some 100 ms).

Figure 3 shows the complex communi-cation world of protection and substationautomation.

The upcoming standard IEC61850 foropen communication in substations is stillin the test phase and not generally availablefor application. [9]

Some vendors in USA already offerUCA2-compatible devices according to thepreliminary standard draft.A few pilot sys-tems are in operation using standard 10Mbit/s Ethernet. It is also reported on a suc-cessful implementation ofpeer-to-peer com-munication with quarter cycle reaction time.

4.2 Internet technology

The latest trend goes to using Internettechnology in an Intranet or the Internetitself.

Several vendors already offer substa-tion automation systems with integratedInternet server. In this way, the acquireddata can be exchanged in a cost saving way in an Intranet and distributed to a widercircle of users. Classic workstations can bereplaced by normal Internet browsers.Maintenance work, for example imple-mentation of new functions, must then

only be performed at the central applica-tion server. [10]In Japan, some systems have been in ser-

vice where mini-servers are implemented inrelays and bay control units on basis of JavaVM (Java Virtual Machine) [11]

Also NGC in England has been testingapplication servers in substations.Relays andother devices are in this case connected tothe server via an Ethernet information bus.Information gathered on the SQL data bank of the server can be accessed through stan-dard browsers using ASP (Active Server

Page) procedures. [12]An American vendor is even a step ahead

and offers a monitoring system where spaceand administration of data is provided onthe vendors own server. The user must only install the Internet enabled relays and devicesin his substation and connect them to t heInternet via the local service provider.

Safety against foreign access is claimedto be guaranteed by passwords, authentica-tion procedures and firewalls. The

F IGURE 3 - Communication world of substation automation


9/10



2

C O N T I N U E D

offer aims at small users where an ownSCADA system is too expensive or not yetinstalled.

4.3 Highly integrated substa-tions

The use of electronic sensors instead of traditional current and voltage transform-ers in combination with digital protectionand control allows to design compact sub-stations.

In the distribution area, there has beena long lasting trend to highly integratedswitchgear panels. The current transform-ers are in this case designed as Rogowski coilsor closed-core low-signal transformers.Resistive or capacitive voltage dividers areused as voltage sensors.The low signal levelrequires to use shielded cables for the con-nection of the combined protection and con-trol IEDs. [13] This design approach con-siders the switchgear panel as one totally

integrated module.In high voltage, the development goes to

optic-electronic current and voltage trans-formers (acc. toFaraday and Pockels prin-ciples) and data bus systems.(Figure 4).Cur-rent and voltage sensors provided withdigital output are connected to a fast fieldbus (Fast Ethernet 100 Mbit/s or even 1Gbit/s) in the switchgear bay.

Discussion at the CIGRE conference2000 in Paris showed that the technical prob-

lems can be mastered.A number of pilot pro- jects are successful in operation.[14, 15, 16]

In general, a drastic cost reduction isexpected with this novel substation design.Broad application, however, will only takeplace when established standards (IEC61850)for open communication are available.

5. Concluding remarksModern media and cost pressure have

been the diving forces for system integrationand automation in substations. The furtherprogress in data acquisition (synchronisedsampling, higher sampling rate), processingand storage capability (doubling every 18 months as per Moores Law) will allow further upgrade of protection functions andseamless monitoring and recording of load,fault events and switchgear state. Wide-bandcommunication LANs and Internet tech-

nology (relay integrated servers and browserbased dialogue) will make the informationavailable at any place of the enterprise. Theproblem will however be to select the usefulinformation from the large amount of indi-cated and stored data. Expert systems willhave to take on this task.

Functionality, performance,and opera-tion comfort of substation control will beenhanced corresponding to the current stateof media (colour graphics, images, video,

F IGURE 4 - Structure of a highly integrated substation


10/10



C O N T I N U E D

voice recognition,etc.).Wireless hand helddevices may be used for local operation andservices. There will be cross-links throughfast WAN to system control and there will bea development towards totally integratedover-all control systems. Access for operationand diagnosis will be possible from any place,even world wide via mobile communication.

Substation automation and remote con-trol will increasingly extend to the distri-bution level. The much discussed distribu-tion automation should become reality inthe foreseeable future.

The further fast proceeding system inte-gration implicates however application issuesin particular with reduced technical staff

after utility privatisation and deregulation.Users already complain about the complex-ity of presently offered systems and ask foreasy and vendor independent configuration,parameterisation and setting procedures.

It remains to be seen if applicable stan-dards and tools will be available in the nearfuture and if the promised plug and playcompatibility can be achieved.

Anyway, nomadic knowledge workerswill be around to provide adequate services.

6. Literature

[1] Mackey,M.: Summary report on sur-vey to establish protection performance dur-ing major disturbances, ELECTRA No.196,June/July 2001, pp.19-29.

[2] Gale, P.F. et al: Travelling wave faultlocator experience in ESKOMs transmissionnetwork. IEE DPSP Conference, 9-12 April2001 in Amsterdam, Conference manualpp. 327-330.

[3] CIGRE Brochure.No. 187: Systemprotection schemes in Power Networks.

[4] CIGRE Brochure No. 200: Isolationand restoration policies against power sys-tem collapse.

[5] Lehtonen, M. et al: Automatic faultmanagement in distribution networks.,CIRED 2001,Report 3.9.

[6] Roman H. und Hylla, H.: Fast faultlocating in rural MV distribution networks.CIRED 2001, Report 3.6.

[7] Roth, P.D.: Communication archi-tecture in modern distribution systems,CIRED 2001, Report 3.8.

[8] Redfern, M.A.: A review of tech-niques to detect downed conductors in over-head distribution systems. IEE DPSP Con-ference, 9-12 April 2001 in Amsterdam,Conference manual pp.169-172.

[9] Shephard, B.; Janssen, M:C:; Schu-bert, H.: Standardised communication insubstations. IEE DPSP Conference, 9-12April 2001 in Amsterdam,Conference man-ual pp. 270-274.

[10] Kirkman,R.: WWW-technology forsubstation automation. Monitor your sub-station via the Intranet/Internetdisplay sub-station data using your web browser, CIGRE2002,Report 34-211

[11] Hamamatsu, K. e t al : A new approach to the implementation of Internet-based measurement and monitoring. IEEDPSP Conference, 9-12 April 2001 in Ams-terdam, Conference manual pp. 102-105.

[12] Hughes, J.V. et al:Substation infor-mation project Field experience with Inter-net technologies. IEE DPSP Conference, 9-12 April 2001 in Amsterdam, Conferencemanual pp. 122-125.

[13] Herrmann, H.-J.; Mller, A.B.;Schmidt, J.: Optimised system operationwith low power instrument transformers.IEE DPSP Conference, 9-12 April 2001 inAmsterdam, Conference manual pp. 13-16.

[14] Gross, R.; Schmidt, J. et al: Substa-tion control and protection systems for novelsensors, CIGRE Session 2000, Report12/23/34-03.

[15] Dupraz, J.P. et al: Integration of

electronic CTs and VTs in very high volt-age substations. CIGRE Session 2000, Report12/23/34-02.

[16] Brunner, C. und Ostermeier, A.:Serial communication between processand bay level Standards and practicalexperience. CIGRE Session 2000, Report34-106. I

ziegler protection1

Documents