*ABB Switchgear AB, Dept. IT, S-771 80 LUDVIKA, Sweden

DIGITAL OPTICAL INSTRUMENT TRANSFORMERS PROMOTING NEW APPROACHES TOEHV SUBSTATION LAYOUTS.

M. ADOLFSSON*, L. BERTILSSON, M. BJARME, A. JANSSON, J. JOHANSSONABB SwitchgearLudvika, Sweden

D. ARMSTRONG, N. PILLING, G. SPENCEThe National Grid Company plc

Leatherhead, UK

1. SUMMARY

This paper describes a digital optical measuring systemfor EHV substations and discusses its properties andinfluence on substation layout.

The paper also reports from an installation where theoptical system is used in parallel to an identicalprotection system operated from conventionalinstrument transformers.

Keywords: EHV Substation, Digital, Fibreoptics, Current-transducer, Voltage-transducer.

2. OPTICAL INSTRUMENT TRANSFORMERS

Optical Instrument Transformers, OIT:s, have beenunder development for a number of years. Acommercial break through has however not beenreached. There have been problems with the technologywhich are now largely being overcome, but difficultiesremain in interfacing the output from OITs to existingmetering, protection and control systems.

Digital technology is now taking over in all fields ofsecondary equipment. Metering, protection and controlsystems are based on software in computers. Thischange in technology opens the possibility to use OITswith low level digital or analog outputs.

The safety advantages of OITs are well recognized: asignal transmission immune to electromagniticinterference, full galvanic insulation and designswithout oil or pressurized gas.

3. EHV SUBSTATION LAYOUT

The small dimensions and the low weight of OITsallows them to be handled easily and transported atlower costs, when compared to conventional ITs. Moreimportant is however the possibility to integrate theOITs in other high voltage apparatus. In this waycompletely new pieces of equipment can be formed.Optical CTs can be integrated onto circuit breakers, toget the optimal position from a functional point of view.The optical fibre used for signal transmission can eitherbe included in a separate suspension insulator orintegrated in the circuitbreaker insulator. By thisarrangement, the area of the substation can be reducedby 5 to 15 % dependent on substation layout and voltagelevel.

The optical fibres can also be included in otherequipment such as disconnector switches or supportinsulators, but the economical advantages are then lessobvious.

Integration of OITs on circuit breakers also givesconsiderable economical advantages. Civil work isreduced since no separate foundations for instrumenttransformers are required. The number of clamps andearthing connections are reduced. Costs for cabling arealso reduced since common cable trenches and terminalboxes are used.

CIGR 1996 : 34-108 O

4. DOIT - DIGITAL OPTICAL INSTRUMENTTRANSFORMER

4.1 Functional description

The DOIT system consists of DOCT:s, Digital OpticalCurrent Transducers, in the high voltage busbar forcurrent measurements, DOVT:s, Digital Optical VoltageTransducers in the switchyard for voltage measurementsand a DOIM, Digital Optical Interface Module in thecontrol room. The signal transmission betweentransducers and interface is by an optical fibre systemincluding high voltage links and fibre optic cables.

Fig 1. DOCT block diagram.

4.2 Current Transducer

In the transducer, DOCT, Digital Optical CurrentTransducer, the current is measured using a low voltagecurrent transformer with a burden resistor connectedacross the secondary terminals. The voltage is filteredand converted to digital form with an A/D-converterhaving a dynamic range of 17 bits. The design of thecurrent transformer is simple compared to aconventional CT since the burden is very small andconstant and can be optimized for the currenttransducer. The design is also very flexible, allowingrated currents from 600 to 6000 A, depending on thedetailed mechanical design. A block diagram of theDOCT is shown in figure 1.

The light for powering the electronics is received by thesame component as is used for converting the measuredvalues to light pulses. This component is a specificallydesigned gallium arsenide diode PLD,Photoluminescence Diode, which has very highefficiency in both directions.

The transducer is operated at a sampling rate of 2000Hz, using low power levels for the converter circuit.This allows reliable, low power laser diodes to be usedas source for the supply power. The electronics isencapsulated in a military grade hybrid circuit, placedinside the screened enclosure formed by the transducerhousing.

Compared to conventional current transformers, theDOCTs have small dimensions and low weight. As anexample a 500 kV DOCT weights less than 100 kg.

4.3 Voltage Transducer

In the transducer, DOVT, Digital Optical VoltageTransducer, the voltage is measured using a capacitivevoltage divider, connected to high voltage and ground,with an output voltage suitable for electronic circuits.The voltage is filtered and converted to a digital formwith an A/D-converter. Identical electronic circuits areused for the voltage transducer and for the currenttransducer. A block diagram of the DOVT is shown infigure 2.

Fig 2. DOVT block diagram.

4.4 Fibre System

The fibre system of the DOIT uses standard 62.5/125micrometer fibre with ST connectors. This allows use ofmass-produced high quality components developed fortelecommunications.

For insulation of the DOCTs on line potential, fibreoptic high voltage suspension or support insulators areused. The links consist of a fibre reinforced epoxymember with a polymer outer insulation. The design hasvery high mechanical strength and low weight comparedto ceramic insulators. Polymer high voltage fibre opticlinks has been successfully used since 1985.

4.5 Digital Optical Interface Module

One DOIM, Digital Optical Interface Module, canhandle six independent current or voltage channels. Itincludes the laser diodes to power the transducers anddetector circuits to receive and decode light pulses fromthe transducers. In the DOIM there are also circuits toautomatically self-supervise signal levels, timing andparity of the received messages.

The output from the DOIM is a fully digital serialprotocol, controlled by a DSP, Digital Signal Processor,allowing flexibility to implement different protocols.

Fig 3. Digital optical interface module, as free standing unitand included in a line protection.

The DOIM can either be a selfcontained unit with itsown enclosure and power supply, connected to thestation battery or a module included in a numericalprotection or control system. When the DOIM is used asa part of a protection system, the normal inputtransformers and A/D-converter are removed andreplaced by the DOIM providing digital signals directlyto the signal processing part of the relay. A photographof the DOIM as a separate unit and included in a lineprotection is shown in figure 3.

Fig 4. Blockdiagram of field trial installation.

5. TESTS ON OPTICAL INSTRUMENTTRANSFORMERS

The technology used in optical instrument transformersis very different from that used in conventional CTs.Although some of the tests which are presently requiredfor conventional CTs are approriate to OITs, some newtests are required. The test requirements for OITs arepresently being considered by the IEC.

Accuracy, temperature rise and short circuit testsbroadly in line with IEC185 and 186 can be performed.The DOCT insulation is polymeric and tests accordingto IEC 1109 can be performed. NGC have a facility tocarry out insulator accelerated aging tests in a heavilypolluted environment at the Dungeness InsulatorTesting Station in the UK.

The part of the optical instrument transformer placed inthe control room will be in the same environment as therelays and should be tested according to IEC 255. Thesetests include extensive EMC tests, environmentaltemperature and supply voltage variation tests. Thesetests have been done on the DOIT system.

In addition to these standardized tests, there are morespecifically designed type tests to assess the correctbehaviour of the DOIT under abnormal conditions. Onetest is sensitivity to magnetic field from a short circuitcurrent in an adjacent busbar. Another test is to checkthe behaviour in case of a fibre break. If the fibretransmission is broken, the self-supervision shouldoperate and stop any false operation on a relayconnected to the optical instrument transformer. Foroptical current transformers integrated on circuitbreakers, there must not be any false signals when thecircuit breaker is operated. This means a requirement towithstand mechanical acceleration levels in the order of20 g (HPL420/31B2 circuit breaker operated off load)without affecting the output. These tests have been doneon the DOIT system.

6. TEST INSTALLATION

In order to gain experience using the DOCT in anoperational environment and driving a protective relay,collaborative field trials are necessary. One such trialhas been started with the National Grid Company plc inthe UK. In this trial two identical distance protectionsystems have been installed, one using signals fromconventional current and voltage transformers and theother with signals from DOCTs and conventional CVTs.The protection systems consists of numerical lineprotection, digital fault recorder and interface to a PCfor data collection and supervision.

A block diagram of the test installation used with theNGC is shown in figure 4. A photograph of the DOCTin the test installation is shown in figure 5.

Fig 5. DOCT installed in a substation.

During commissioning, the optical fibre system waschecked using an OTDR. A current primary injectiontest was also done to check system operation.

Another commissioning/routine test was performedusing an automatic secondary injection test set tosimulate a large number of fault cases simultaneouslychecking the operation of the relays. For this test atestmodule was used instead of the actual transducer.The testmodule consists of an identical electronic circuitas is used in the transducer, but with an input designedto take outputs from the secondary test set rather thanfrom the primary system. The operation of the two relaysystems with identical settings were tested with DOITand conventional inputs respectively. The testingshowed a very good correspondence between the twosystems. See figure 6.

Phase current and voltage rms values from both theconventionally driven and DOIT driven systems arerecorded on an hourly basis and "snap shot" waveformrecordings are taken weekly. Environmentaltemperature is also recorded and used to assist inevaluating the performance of the equipment. In case ofrelay operation, the fault recorders will record currentand voltage information from the two systems. Systemdiagnostic information can also be recorded and used inevaluation of the field trial.

All recorded information can be remotely accessed froma remote PC using a telephone modem connection.

Fig 6. Operating area of the line protection with DOIT systeminputs. Crosses give limits of operation resulting from relaytest.

7. FIELD EXPERIENCE

The field trial is expected to last at least two years but itis possible to present some data collected during the firstyear. In figure 7, the deviation in % between currentmeasured by the DOCT and the current measured by aconventional CT is plotted as a function of time. Therecorded values are rms values taken over ten periods ofthe power frequency, at intervals of 24 hours. Theconventional CT is a class X type (low reactancespecified in terms of having a high kneepoint) withaccuracy of 1% at typical load currents.

Days since May 1:st -95

00,10,20,30,40,50,60,70,80,9

11,11,21,31,41,5

0 10 20 30 40 50 60 70 80 90 100 110 120

%

Fig 7. Comparison between DOCT and conventional CT,showing difference in % as a function of time.

By picking recordings at different temperatures thetemperature dependence of the variation can beevaluated. An example of such a plot is given in figure8.

Temperature (Deg C)

00,10,20,30,40,50,60,70,80,9

11,11,21,31,41,5

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

%

Fig 8. Comparison between DOCT and conventional CTversus temperature, difference in %.

By picking recordings at different current levels thecurrent dependence of the deviation can be evaluated.An example of such a plot is given in figure 9.

% of nominal current

00,10,20,30,40,50,60,70,80,9

11,11,21,31,41,5

0 10 20 30 40 50 60 70 80 90 100

Fig 9. Comparison between DOCT and conventional CTversus current, difference in %.

There have been no primary system faults on theprotected feeder so far and thus the operation of therelay systems have been recorded. There have been nofalse operations.

At the beginning of the test period recurrent parityerrors occured on one of the channels. For the trialinstallation the DOIT system is set to shut down for anykind of malfunction, even for a single parity error,pending remote reset. Switching to a DOVT circuit in adifferent position within the panel eliminated the parityerrors. The cause of the parity errors is subject toinvestigation but is thought to be related to RFI in thecontrol room equipment.

"Snap shot" waveform recordings revealed infrequentincorrectly sampled instantaneous values of current. Thesame behaviour is also seen on rms values taken overone single period of the power frequency, whereinfrequent values show errors of around +/- 2%. This iscaused by a design defect in the DOIT circuits which,for the trial, has been compensated for by a softwarechange in the DOIM. A circuit modification on futureDOITs is expected to rectify this problem.

The DOIT system using optically powered electronicsand digital signal transmission offer several advantages:

- Few, low stressed components in the transducer gives high reliability.

- Easy installation.

- No apparent mechanical sensitivity.

- The accuracy performance is restricted to thetransducer, no additional errors from signal transmissionand processing.

The DOIT system is widely used for protection in seriescapacitor installations. The result of this trial confirmsthat the equipment fulfills the requirements for lineprotection and that a broader application in substationsis suitable.

9. REFERENCES

[1] Ahlgren L. et al A new optoelectronic measuring system for

EHV substations.Paper 23-09, 1988 CIGR Session, Paris

[2] Adolfsson M. et alEHV Series Capacitor Banks. A New Approach to Platform to Ground Signalling, Relay Protection and Supervision.IEEE Transaction on Power Delivery, Vol4, No2, April 1989