To be presented at: V SEPOPE Recife, Brasil 19 May, 1996

ITAIPU HVDC TRANSMISSION SYSTEM10 YEARS OPERATIONAL EXPERIENCE

A. PRAÇA H. ARAKAKI S. R. ALVES K. ERIKSSON J. GRAHAM G. BILEDT

FURNAS Centrais Elétricas ABB Power Systems

Brazil Sweden

ABSTRACT

The Itaipu HVDC system has now transmitted powerfor 10 years. The scheme, designed in 1979, includesmany features new at the time and continues to be thelargest HVDC transmission, in both rating,6,300 MW, and voltage, ± 600 kV. Major operatingfeatures, present use, significant experiences andavailability performance are discussed. Since finalcommissioning it has shown excellent performancewith a forced outage unavailability below half apercent.

Key words: Power transmission, HVDC, Itaipu,

1. INTRODUCTION

This paper describes the experiences of the ItaipuHVDC system during the first ten years of operation.The present use of the transmission and the systemdynamic performance is discussed, together withsignificant events and availability performance.

The Itaipu power plant is a Brazilian Paraguayan jointventure with a total installed capacity of 12,600 MW,the majority of which is transmitted to SoutheasternBrazil, that is about 28% of the total system capacity.Due to the difference in frequencies of the twocountries, half of the generation is at 50 Hz(Paraguay) and half at 60 Hz (Brazil).

Figure 1 is a simplified one-line diagram showing theinterconnection of the Itaipu power plant withSouth/Southeast Brazil through the FURNAStransmission system.

Ivaiporã

765 kV

Itaberá

765 kV

T. Preto

765 kV

Foz

500 kV 765 kV

Itaipu60 Hz

SOUTH SYSTEM

500 kV

345 kV

Foz

500 kVIbiúna 345 kV

Itaipu50 Hz

Paraguayan System

Itaipu

500 kV

9 x700MW units

Bipole 1 +/- 600 kV9 x700MW units

Bipole 2 +/- 600 kV

So

uth

eastern S

ystem

2

Figure 1. Simplified diagram of theItaipu Transmission System

Studies indicated a hybrid solution for the powertransmission, the power from the 60 Hz generatorstransmitted by AC lines via intermediate stations andthe power from the 50 Hz generators transmitted byan HVDC system.

The Itaipu 60 Hz 765 kV HVAC transmission systemis operating over two series compensated lines withthe aid of a digital emergency control scheme (ECS)to match generation and transmission capability incase of disturbances in the electrical system.

For the 6,300 MW HVDC transmission, detailedstudies, including firm bid prices for the converterstations, showed ± 600 kV to be the optimum voltage.The subsequent performance confirms this choice of±600 kV, with the attendant economical advantages.The two bipoles are designed to operateindependently of one another under normalconditions. Figure 2 gives an outline of the maincircuit, including the staged construction of theHVDC transmission.

to Guarulhos

to Interlagos

Paraguayan System

220 kV

Itaipu500 kV 500 kV

to Campinas

345 kV

Ibiúna

sc1

to T.Preto

Stage 3

Stage 1

Stage 1

Stage 2

Foz

Stage 6

Stage 4

Stage 4

Stage 5

Foz

sc2

sc3

sc4

Stage 6

3

4

5

6

7

8

9

2

1

Stage 1

Stage 4

Southeastern system

Figure 2. Itaipu HVDC System maincircuit and evolution

.

The stations are connected over two bipolartransmission lines each approximately 800 km inlength, which follow separate rights of way, at least10 km apart, except at the approaches to the stations.Either transmission line can carry the rated output ofthe station using the facility of parallel operation.

The first stage was put into commercial operation inNovember 1984 and the last commissioning activitieswere completed in September 1990.

The Itaipu HVDC system is a landmark in HVDCtechnology with regard to several features:

• Highest power rating and highest voltage, ± 600kV

• A large step in converter rating, specially thyristorand transformer

• The use of gapless Zn0 arresters• Massive use of microprocessor-based controls• Integrated factory system test of control equipment

2. THE USE OF THE HVDC TRANSMISSION

The integrated system of South-Southeastern Brazilhad a peak demand of 33,700 MW and an annual loadof 210 TWh in 1994. Of this 35.5 TWh was suppliedfrom the Itaipu 50 Hz machines, over the HVDCsystem.

Figure 3 shows the monthly transmitted energy andthe peak power since 1991, the first full year ofcommercial operation for the complete system. TheActual Energy and Power Limits (AEPL) are the ratedpower limits minus the power of the generators notavailable; normally one machine is in maintenance.The Expected Energy Use (EEU) is AEPL times 0.65,the hydrological capacity factor.

00

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0

1000

2000

3000

4000

5000

6000

En

erg

y -

TW

h/m

on

th

Pe

ak

po

we

r d

uri

ng

ea

ch m

on

th -

MW

TEPL

AEPL

ENERGY

PEAKPOWER

EEU

1991 1992 1993 1994 1995

Figure 3: Itaipu HVDC Transmission:Monthly energy and peak power

It can be seen from figure 3 that the actual transmittedenergy in most years has exceeded the expectedenergy usage. This is due the considerations usedwhen deciding the energy dispatch in the Itaipu 50 Hzpower plant, that is:

- Itaipu supply to Paraguay ranges from 100 to500 MW.

- The system spinning reserve contribution forItaipu is about 350 MW.

- The South-Southeastern Brazilian electricalsystem is 95% hydraulic.

- Different hydrological conditions influence theoperation of the reservoirs. As an example therivers in the south region have relatively smallreservoirs.

- From 1992 to 1994 there were excellenthydrological conditions. The energy available wasmuch greater than the load.

- When the hydrological conditions so allow, otherpower plants closer to the load centers are chosento generate.

- Since July 1994 the energy consumption in Brazilhas experienced a high rate of increase.

Thus the HVDC system is designed to operate over alarge range to meet the full load cycle, from peak loadon a work-day to about 25% capacity at weekendlight load. A routine of converter maneuvers, shuntcapacitor switching, filter switching and high Mvarconsumption is used frequently to fulfill the operativerequirements.

3. CONTROL SYSTEMS - OPERATINGFEATURES AND EXPERIENCES

The control and protection systems for the HVDC arealmost completely based on microprocessor systems.While this may not seem very remarkable today, atthe time of design, 1979, this was a very radicaldecision. The control and protection systems arebased on an hierarchical design, see figure 4, startingat the converter level, then pole, bipole and finallystation. The aim is that any failure, main-circuit orcontrol equipment, should reduce the transmissioncapacity as little as possible.

In general, the main control functions are carried outusing the ASEA DS-8 control system, based on theIntel 8088 processor. This system provides for a largequantity of I/O, both analog and digital, as well asserial and parallel communication, being very suitablefor the tasks involved. Two control functionsrequiring higher speed and accuracy, converter firingcontrol and the current control, use a system based onthe Intel 8086 processor with specially designedhardware for the I/O functions.

At the pole and converter levels the equipment is notduplicated, but a hardware back-up with limitedcontrol possibilities is provided for the DS-8 systems.At Bipole and Station levels the DS-8 systems areduplicated, providing automatic transfer if necessary.

AC systemcontrol

Foz: Itaipu generator stationSRQ: System operation centre

Station Control

convertorcontrol

Station controldesk

Bipole controldesk

to other station

Bipole 2control

to other station

Bipole 1control

Bipole controldesk

PoleControlTele

com.

Valve

Control

Convertor Level

Pole Level

Bipole Level

Station Level

Figure 4. Control hierarchy structure

In practice, the microprocessor systems have provedvery reliable and more maintenance and interferenceproblems have been associated with the few hardwaresystems than with the computer systems.Additionally, the possibility to make fast changes,plus debugging with an on-line monitor, makes thesesystems much more convenient to use.

Due to these advantages microprocessor systems havebecome accepted as the standard for HVDC controlsand although nowadays the systems are much morepowerful, with more on-line resources, Itaipu can beseen as the first of this new generation of controlequipment.

In view of the rapid development in microprocessortechnology between 1979 and 1995, FURNAS isconsidering upgrading the control and protectionsystems, taking advantage of developments in thelatest generation of ABB equipment. The faster andmore powerful microprocessors allow an expressivereduction in the number of components, permittingcomplete digitalization and redundancy at pole,converter and valve levels. Besides the increasedreliability and availability that could be achieved byduplicating controls at converter and valve levels, asignificant reduction in routine maintenance work canalso be expected.

3.1. MAN-MACHINE INTERFACE

The majority of the control actions are made by theoperators at the inverter station and only in specialcases is this done at the rectifier station. From theregional dispatch center the following controls areavailable: bipole power ramping; synchronouscompensator set point; AC filters maneuvers. The twolast features are normally used to control the voltageat the 345 kV bus of Ibiúna.

Synchronous Power Control is the normal controlmode and it is only in this mode that all facilities areavailable, i.e. overload, frequency control and so on.The name Synchronous Power Control means that thecurrent controllers at both the rectifier and inverter aresynchronized; i.e., during ramping or other variationsin power order, the two are kept equal using thetelecommunication system. In asynchronousoperation, the power or current order at both rectifierand inverter must each be set by the operator.

3.2. AUTOMATIC CONTROL ACTIONS

The HVDC transmission represents a rather highpercentage both for 50 Hz and 60 Hz loads. Hence itwas necessary to include transient and dynamic

automatic controls to ensure the correct performanceof the integrated systems. These controls have toattend the sometimes conflicting requirements of theSouth/Southeastern Brazilian system and theinterconnected 50 Hz Itaipu/Paraguayan system.To do that, additional power orders derived fromauxiliary controls are added to the order set by theoperators. The resulting value is checked against apower order limit, being the minimum of a set oflimitations and an automatic value depending on thenumber of 50 Hz generators in operation..The main automatically initiated features are:

• Stabilization of frequency in the rectifier end• Short-time Overload• Transfer of power between poles and bipoles• Reactive Power Modulation (Foreseen for SE

system stabilization, but now not required)

3.3. AC VOLTAGE CONTROL

The operators have the task of controlling the ACsystem voltage, assisted by "Station Control", asystem that monitors the situation and givesinformation, including minimum filter requirements,maximum permitted filter at Foz and AC bus-voltagecontrol in Ibiúna, with high and low Mvar warningset-points for the Synchronous Compensators.

At high loads, in addition to the need to supply morereactive power to the converters, in Ibiúna there isalso a need to supply the network. This can be met byadding filters above the minimum requirement and byadding shunt capacitors. The four 300 Mvarsynchronous compensators are available for finecontrol of the AC voltage. They are normallyoperated with sufficient reserve capacity to cover asingle contingency failure, i.e. trip of a filter-bank orloss of an AC line.

In Foz do Iguaçu there is a limit to the total capacityof filters which may be connected, as a function ofnumber of generators, in order to avoid a potentialself-excitation situation. The generators are designedwith a 0.85 pu power-factor and consequentlycontribute greatly to the reactive power balance.

At light loads in Ibiúna, the minimum filterrequirements may be such as to require othermeasures to absorb reactive power. For this conditionHigh Mvar Control is provided where the inverterfiring angle is forced to increase, dropping the DCsystem voltage and increasing the current. Hence thereactive power absorbed by the converters isincreased. High Mvar Control has proved to be one ofthe most useful additional facilities for the regulationof the AC voltage, minimizing switching of filters and

capacitor banks and hence reducing maintenance ofcircuit breakers.

3.4. PARALLEL OPERATION

For reasons of reliability, given the possibility of anextended transmission line outage, the HVDC systemis equipped for parallel operation at both bipoles onone bipolar transmission line. In one case when alogging truck hit an HVDC tower, causing apermanent bipolar outage, the repair of the line waspossible without any load shedding in the Braziliansouth/southeast system. In fact the transmission lineshave proven very reliable and parallel operation hasbeen used more for routine maintenance of both thetransmission line and the station line end equipment.

The actual sequences required to enter into paralleloperation are fully commanded from the stationcontrol room in Ibiúna. In general all of the operatorcontrols used in normal operation are available inparallel operation. From the operators' point of viewthe power on the parallel poles is set in exactly thesame way as in normal operation, although the polesobviously are part of different bipoles. Paralleloperation is permitted only at 600 kV, i.e. twoconverters per pole. Reduced Voltage Operation maybe ordered by the line protection when in paralleloperation. Deparalleling may be ordered by protectionoperation.

4. SYSTEM DYNAMIC EXPERIENCE

A Dynamic Performance Study (DPS) was performedduring the design stage and continued throughout thecommissioning as the various stages were put into op-eration. The fundamental objective was todemonstrate that the transmission system would meetthe required performance as defined in the technicalspecification. Exceptions to the specified performancewere sometimes found necessary to providesatisfactory performance for the integrated networks.

Operational experience acquired since the start of op-eration, as well as the normal evolution of the AC net-works resulting in a stronger inverter, indicate thatdifferent dynamic performance is possible today.Consequently FURNAS is in the process of revisingthe dynamic performance study.

There are multiple objectives for the revised study:• It is hoped that several control features, added to

achieve the specified performance can be removedor simplified.

• Relaxing slightly the allowed levels of ACharmonic distortion, (i.e. operation with less ACfiltering) more convenient reactive power controlmay be possible.

• Introducing automatic power limits based on thetopology of the inverter AC network (runbacklimiter) can maintain the inverter SCR atacceptable levels during contingencies.

• Faster AC fault and commutation failure recoveriescan reduce frequency excursions in the sending endAC system.

• Power modulation of the HVDC link may beadvantageous in enhancing the stability of theFURNAS 765 kV AC transmission system.

The tools available to perform this study today havealso evolved compared with the early 1980’s. TheHVDC simulator, with its detailed DC controls,continues to be essential. However a more completeAC network representation is now available using theRTDSTM (Real Time Digital Simulator) which isinterfaced with the HVDC Simulator.

5. SIGNIFICANT OPERATING EXPERIENCES

During these ten years of operation a considerablenumber of interesting experiences have occurred,especially during the first five years which wereconcurrent with construction and commissioning. Thedivision of the work into stages which wereconstructed and put into operation sequentiallyfollowed the schedule for the generating units inItaipu and permitted continuous transmission of theavailable energy. This simultaneous commissioningand operation, while not restricting energytransmission, can be seen in the higher forced outagefigures in the first years. This section will not try tocover all events from these ten years, but concentrateon the most significant items.

5.1. HARMONIC EXPERIENCE

The service experience, both with respect to level ofharmonics and to consequent interference, has shownthe performance to be well within specifiedrequirements.

5.1.1. AC side harmonics

The method of calculating the level of AC sideharmonics is very conservative. It takes into account arange of possible resonances with the AC system aswell as worst case parameters, e.g. combinations oftolerances in transformer impedances and unbalancesin firing angles.

In practice, these factors are not as bad as assumedand the AC side harmonics are less than calculated,especially for non-characteristic harmonics, and theinterference levels well within the specified levels.

One interesting factor is that a filter is included for thenon-characteristic third and fifth harmonics. Thisfilter was included for performance reasons, whencalculating with pessimistic assumptions. In actualoperation, it has been found that the loading of thisfilter is well above the expected level. However, thisis due to fifth harmonic currents originating in the ACnetwork. The sources of these harmonics are diverse,and at the moment not completely identified, but forsure influenced by industrial loads and to a largequantity of TV sets with single pulse rectifiers.

On a weekend and during a final match of the 1995South America Football Cup, measurements taken atthe Ibiúna station demonstrated clearly the heavyinfluence of TV sets on the fifth harmonics levels.

5.1.2. PLC harmonics

The technical specification gave very strictrequirements regarding PLC interference to reduce thenoise generated by the converter stations over thefrequency range 30-500 kHz. Theoretical calculationsindicated that this may not be met over the range 30-120 kHz. However, in view of the potential costsavings, it was agreed to postpone any decision onPLC filters until measurements could be made on theoperating system.

As part of the final commissioning, measurements ofPLC noise were made at the coupling capacitors onthe AC lines. These measurements, made over the 30-500 kHz range, showed that the noise somewhatexceeded the specified level at frequencies below 100kHz, but was acceptable, especially when consideringactual and proposed PLC frequencies. Accordingly, itwas decided not to install the PLC noise filters, seereference 3.

5.2. FLASHOVERS ON 600 kV WALLBUSHINGS

Operation with 600 kV started in July 1985 in thenegative pole of Bipole 1, with no apparent problems.Less than 3 months later a flashover occurred on theDC pole bushing at Foz do Iguaçu. The bushing waschecked, but no defect found. Pollution samplesshowed a low level compared to those normally foundin connection with pollution flashovers. At the timeof the occurrence it had been raining heavily for abouthalf an hour.

By the end of the year, a total of five flashovers hadoccurred at Foz do Iguaçu and two flashovers atIbiúna, on both the positive and negative poles forfive different bushings, all under heavy rainconditions. By that time it had become clear that thiswas a general problem for the 600 kV wall bushings

and full voltage tests had been started to determinethe reason for the flashovers. The tests weresupplemented with data collection from other sitesand comprehensive meteorological observations fromthe Foz and Ibiúna stations.

It was confirmed that a rain shadow close to the walldecreased the withstand voltage considerably, seefigure 5, and that the length of the dry zone was animportant parameter for withstand strength. Aminimum withstand dry zone could be found andfigures as low as 350 kV were recorded. Observationsat site established that a rain shadow gave a dry zoneon the bushings close to the wall. Later electrical fieldmeasurements confirmed that a dry zone causes asteep increase in the electric field and that this is thecause of the flashover.

Figure 5. Rain shadow

From the investigations it became evident that it wasnecessary to either decrease the stresses over the dryzone or increase its withstand capability. The mostpromising method seemed to be to make the insulatorsurface non-wetting and thereby decrease the steepelectrical field increase in the transition between thewet and dry zones. This could be made by applicationof a hydrophobic coating on the insulator surface bysilicone grease or room temperature vulcanizedsilicone rubber (RTV).

In June 1986 silicone grease was applied to all 600kV wall bushings as a temporary measure while theinvestigations continued. Evaluation was made ofcoated insulators, both compared to other possiblecountermeasures and also between RTV and siliconegrease. No conclusive improvements could be foundfrom other measures such as alternative porcelains orbooster sheds. Both RTV and silicone grease seemedto give considerable improvements and theexperiences from the temporary silicon greaseapplication was excellent, preventing further

flashovers. Thus, further work was concentrated tothese solutions.

RTV had shown good performance in otherinstallations and had indicated longer life-times thansilicone grease, but application is more difficult and itis uncertain if it can be removed easily. During aninvestigation on test specimens on site, RTVexhibited an unexpectedly rapid loss of itshydrophobic properties. This was followed up bylaboratory experiment that showed that if exposed towater over a long time, RTV would lose itshydrophobicity. Thus it seems that RTV may not beefficient in tropical climates with heavy and abundantrain. The conclusions were that silicone grease of thetype chosen could most probably get a lifetime of 4 to5 years in a clean ambient atmosphere and thatthickness and evenness were not a problem. Even afairly thin layer would protect the hydrophobicity andthereby prevent rain flashovers. This was confirmedby an on site test program for various types ofsilicone grease and RTV, including application toenergised test insulators at 600 kV. On the wallbushings the time between exchanges was increased,for the original grease type, under close supervisionof characteristics, especially hydrophobicity, seefigure 6.

Figure 6. Foz do Iguaçu 600 kV wall bushing.Hydrophobicity test on silicone grease after 38months exposure time.

Actual experience has shown that removal andreapplication can be considerably facilitated bycontrol of the chemical composition and the use ofsimple hand tools that were developed. Now, withmore than nine years successful use of siliconegrease, the procedures developed have led to fast andwell controlled grease renewal. A minimum four-yearlifetime is proven by operation experience for theconditions prevailing at the Itaipu converter stations.

5.3. CONVERTER TRANSFORMERS

During the early years of operation significantproblems were experienced with the convertertransformers. This was distributed fairly evenly be-tween the rectifier and the inverter and between300 kV and 600 kV. The four types have the sameconceptual design, although they are not identical indetail.

The first indications were given mid-1985, after aboutsix months of operation, when gas in oil analysisshowed gassing, at a low rate, in some transformers.In October 1985 a failure occurred in one transformerat the inverter station and was traced to the AC wind-ing. Shortly after a second failure occurred in therectifier, located at the DC winding outlet. One of thefailed transformers had shown gassing, includingacetylene, while the other one did not show more gasthan had been generally expected.

The examination of the first failed unit, together withinspections in another gassing unit, revealed badcontacts between a copper plate and an aluminumplate. This was evaluated as the cause of the gassing.A program to modify the contacts was carried outduring early 1986.

After a few months operation, gas in oil was foundagain in some modified transformers. In May 1986,another transformer failed in the inverter, with thefailure in the AC winding. Special investigations wereundertaken at the factory and an extended heat runtest indicated that the gassing had its origin ininduced currents circulating in the core.

A second modification of all transformers to reducethe induced currents was undertaken and again carriedout on site and in the workshop. The site modificationwas carried out while the link was in commercialoperation and this meant that a considerable timeelapsed, even though extra spare units were deliveredto facilitate the execution of the work and to givemore security against service interruptions.

In 1989 the work to execute the modifications wasfinalised. During this time further failures occurred,some of which were after the second modification hadbeen completed.

A detailed analysis of the failures was presented in theCIGRÉ 1993 Joint Task Force 12/14, 10-01 and willnot be repeated here. A summary is given below,where it can be seen that the failures are concentratedin the first years of a transformers operation.

Years of Number ofOperation Failures

0 - 1 6

1 - 2 3 2 - 3 2

3 - 4 1 4 -10 0

The failure rate for these converter transformers hasbeen higher than one would normally expect for thistype of equipment. This can be explained as due to alarge step in converter transformer developmentespecially with regard to unit rating, coupled withsevere transport restrictions. This resulted in arelatively large number of new techniques which,although extensive prototype testing was made, werestill not fully controlled during the manufacturingprocess. However the last five years of operationmake us feel confident in the present and futureservice performance of these transformers.5.4. VALVE HALL FIRE

In May 1989 a fire broke out in Converter 5 at theFoz do Iguaçu substation. This was due to anunfortunate combination of circumstances which aresummarised here.

Information from personnel present at the time, plusevent recorder printouts and a detailed inspectionmade in the valve hall, identified the following failuresequence:

1. A water tube between two thyristor levels in valve2, close to the top of phase A quadruple valve,comes loose and water leaks at approximately 1 l/s.

2. A water leakage alarm is emitted, however theprotection system does not trip as the trip signal isinhibited due to an open by-pass valve.

3. Water flows down in quadruple valve A during atleast 8 minutes, corresponding to approximately500 liters. The conductivity of the originallydeionized water increases when flowing over thevalve modules, due to contamination with thesmall amounts of dust and dirt it picks up.

4. A flashover occurs in the bottom valve of phase Aand gives a short circuit across that.

5. The flashover strikes the damping capacitors in thebottom valve of phase A and starts a fire in some.

6. The fire spreads upwards in the quadruple valvestructure where it increases in heat generatingcapacity and intensity, so that aluminum andplastics melt and porcelains crack.

7. The fire is discovered. The ventilation system isclosed and after some hours the fire has self-extinguished.

A full scale experiment in which the correspondingwater tube was cut open was made and the sequencerecorded. This confirmed that the trip signal was not

given due to the fact that the by-pass valve for rapidrefill was erroneously left in the open position.

Phase A quadruple valve was damaged by fire. Theother two quadruple valves were not directly damagedby fire, but rather by smoke. A large amount ofhydrochlorides from burning plastic was precipitatedon all equipment. Due to this contamination the majorpart of the equipment was replaced. Only the busbarsand some very special items were not manufacturednew. The valve hall was put back into operation inslightly less than 14 months and entered intooperation in July 1990.After the fire incident some modifications to thesystem were introduced:

• A locking key on the by-pass valve for rapid refillwas installed to avoid operation of the thyristorvalves if this valve is left open.

• A new trip function was introduced, based on thederivative of the water level.

• The start of the make-up pump was delayed 10minutes in order to ensure operation of the lowwater level trip at a leakage rate of more than 0.14l/s.

• A new frequent make-up alarm was included.• The Bipole II water connections between thyristor

levels were exchanged for the type used in BipoleI, having a superior ferrule fixing.

Later a full scale test of incipient fire detectors wasmade. Testing demonstrated the possibility to identifya pre-combustion condition by detecting submicronparticles emitted from overheated plastic material.The successful testing led to the decision of a fullscale installation of such detectors in all converters.This installation was completed in August 1994.

5.5. VALVE COOLING SYSTEM

After several years of operation, discoloration wasnoticed on the stainless steel pipe couplings used toconnect the fine water PEX pipes between modules.In some converters actual corrosion of the tubes incertain positions had taken place, but the process wasfar from uniform and not necessarily worse inconverters having longer operating times. As thiseffect occurred only at the anodic ends of the tubes aprocess of electrolytic corrosion was suspected. Infact the design allowed for some corrosion of thetubes over a 50 year period, however in many cases afaster effect was seen. This was the case even thoughthe water quality had been maintained at the lowlevels of conductivity required to limit the straycurrents in the cooling water. A series ofinvestigations both in the laboratory and at sitedetermined that the foreseen electrolytic corrosionwas accelerated by the presence of free negative ions.

The origin of these ions was probably the watertreatment resins used on the system, the ions beingreleased at the time of exchange of resins.

There were several ways to resolve this situation,however the chosen solution was to modify thecooling water pipe couplings to make them resistantto this phenomenon. In summary the stainless steelpipe couplings were replaced by plastic couplings, inwhich an inert electrode is utilised to ensuresatisfactory voltage distribution within the watercircuit. In addition to laboratory tests on the materialsused and thermo- mechanical life cycle testing on thenew couplings, a full scale dielectric test program wascarried out. Also finer water filters were introduced toreplace the original filters.The work to change the pipe couplings and also toclean the valves of corrosion products and possibleremaining catalytic agents was carried out during thefirst half of 1994. The results of monitoring themodified cooling system indicate that the solutionadopted is giving fully satisfactory performance.5.6. MAINTENANCE

Preventive maintenance is an activity having asignificant influence on the system operation, bothfrom the point of view of manpower andperformance. In order to evaluate the impact of thepreventive maintenance activities on the availabilityof the power transmission, a trial preventivemaintenance was performed. This determined theduration to be used as a maximum for the availabilityguarantee calculations.

This trial consisted of an actual maintenance atBipole, Pole and Converter levels of equipment forwhich the shutdown has an impact on the powertransmission capacity. The activities to be carried outwere defined by the Preventive Maintenance Plans forthe substation and considered necessary for a goodperformance of the HVDC system and were based onthe manufacturers recommendations, the previousexperiences of the converter station supplier and theexperience obtained by FURNAS in the operation ofthe converter station.

At converter level, the preventive maintenance of theBy Pass Breaker was found to be time-consuming andthis equipment proved critical for the converteroutage time, much more than, for example, thevalves. At Pole level the HVDC Control andProtection maintenance showed to be the most timeconsuming equipment, as expected. DisconnectingSwitches, in spite of the large number, weremaintained in only 7 hours. At Bipole level bothBipole Control and Protection and Common Yardequipment were maintained in around one and a halfhour.

The evaluation of the times achieved during the trialmaintenance resulted in outage to be used for thecalculations of the Availability and ReliabilityGuarantee (see Table 1).

LEVEL MAXIMUM OUTAGE TIMESBipole 2 hours per year and per bipolePole 11 hours per year and per pole of which

2 hours overlapping with bipole outageConverter 18 hours per year and converter of

which 11 hours overlapping with poleoutage

Table 1: Outage times to be used for AR Guarantee

In practice the preventive maintenance is plannedaccording to an annual time schedule, to a largeextent based on optimisation of manpower. Twoweeks before the maintenance, the central dispatch isinformed about the equipment that will be undermaintenance, so that all interconnected utilities can beinformed. However, if for some reason any equipmentis out of service, efforts are made to take advantage ofthis and preventive maintenance is performed.So far, taking into account the use of the HVDCtransmission, it has not been necessary to schedulemaintenance in the manner in which the trialmaintenance was performed. This is because it hasbeen possible to shut down converters, except in theheavy load period, without prejudicing the powertransmission, allowing more time for maintenanceand reducing total maintenance staff and overtime.Such a procedure reduces costs, although the systemavailability decreases.

One interesting result of the maintenance activities isthe very low thyristor level failure rates found. Thishas been consistently below 0.1% per year and in1995 was 0.03%, that is three failures in more than18 000 levels.

6. AVAILABILITY

Table 2 presents the availability of the Itaipu HVDCtransmission system in two different formats. Oneaccording to CIGRÉ definitions and anotheraccording to the contract for the supply of theconverter stations.

YEAR/BIPOLE

UNAVAILABILITYBY CIGRÉ (%)

AVAILABILITY(%)

Forced Scheduled

CIGRÉ Contract

88 BP1 1.5 6.9 91.6 -BP2 - - - -

89 BP1 4.6 6.9 88.5 -

BP2 15.6 7.8 76.6 -90 BP1 0.7 7.7 91.6 -

BP2 14.2 10.6 75.2 -91 BP1 0.6 12.3 87.1 -

BP2 0.4 12.9 86.7 -92 BP1 0.3 8.8 90.9 99.5

BP2 0.7 6.4 92.9 98.993 BP1 0.2 2.6 97.2 99.1

BP2 0.2 2.5 97.3 99.494 BP1 0.4 6.4 93.2 99.3

BP2 0.2 5.9 93.9 99.495 BP1 0.1 2.1 97.8 99.4

BP2 0.4 2.1 97.4 99.3

Table 2. Itaipu HVDC Transmission System BipolesAvailability

It can be seen from that table that the contractualfigures do not match the reporting to CIGRÉ as thetwo formats are different. The main differencebetween the calculations is that the report to CIGRÉdoes not take into account the actual maintenanceprocedures used at site, penalizing the availabilityfigures, while the contractual figures consider that themaintenance could be performed, if necessary,according to the trial maintenance results, see 5.6,which corresponds to an equivalent unavailability of0,28% per bipole and year.

Considering that scheduled maintenance is carried outtaking advantage of periods of low utilization, theavailability format used by CIGRÉ has beenquestioned by many utilities, including ManitobaHydro and FURNAS, at the HVDC Users Conferenceheld last September, 1995 in Winnipeg, Canada.

Another difference between CIGRÉ and contractualavailability figures is that the contractual calculationgives less than proportional weight to outage of aconverter than to outage of a complete pole,considering the design with series connectedconverters and the impact on power transmission ofhaving one converter out of operation. Also certaintype of events outside the contract such as DC line,AC network and human errors are excluded from thecalculations based on the contract.

In table 2, the unavailability figures for Bipole 2 in1989 and 1990 were affected by the valve hall fire.The 1994 scheduled unavailability for both bipolesreflect the outage time for installation of the incipientfire detection systems mentioned in 5.4 and themodification to the valve cooling circuits discussed in5.5, which were carried out together, sequentially, inall converters

Table 3 shows the main causes of disturbances for thelast eight years without giving weight to how largepart of a pole is affected. In that table all forcedoutages for the two bipoles, with a total of eightconverters per station, are summarised. It can be seenthat there is a general trend for the annual number ofconverter equipment forced outages to reduce withtime. The number of forced outages due to controland protection is significant, especially in 1989 whenmany were due to commissioning of the bipoleparalleling system together with commercialoperation. In recent years, there is a trend to decrease,except for 1992 and 1993, when they were high.These two years suffered from repetitive faults oftransient nature that were hard to trace. In particularone fault in a pole current control amplifier and aseries of similar faults in the power supplies to thevalve control cubicles are responsible for most of thecontrol faults.

Many of the counted outages were in fact the samefailure, but the process of ”trouble shooting” couldhave contributed to a further disturbance. This isillustrated in Table 3 by the values in parentheses,where the outages due to the same failure have beendiscounted.

This illustrates the advantage that would be obtainedby the use of duplicated control equipment, which notonly avoids the outage, but also permits on-line faulttracing, as well as routine maintenance. Such a systemwould therefore increase the availability, both forcedoutage and scheduled.

For the last four years the "equivalent weighted forcedoutage per bipole" has been monitored. This valueexcludes human errors and covers station equipmentonly.

The weighting counts one pole as a single unit,reducing the impact of converter outages to thesignificance that they have on power transmission.

DESCRIPTION YEAR

1988 1989 1990 1991 1992 1993 1994 1995

AUXILIARY SERVICES 10 5 6 5 1(1) 4(3) 1 7(5)

CONTROL AND PROTECTION 22 60 29 5 23(16) 18(12) 9 9(7)

MAIN CIRCUIT EQUIPMENT 16 7 9 5 2(2) 4(4) 4 2

TOTAL CONVERTEREQUIPMENT

48 72 44 15 26(19) 26(19) 14 18

DC LINES 20 9 36 4 10 19 20 12AC SYSTEM 2 3 - - - - - -

HUMAN ERRORS 22 30 12 5 12 7 4 4

TOTAL 92 114 92 24 48(41) 52(45) 38 34

EQUIVALENT WEIGHTEDOUTAGE PER BIPOLE

BP1/BP25.8/4.5

BP1/BP22.5/3.8

BP1/BP25.8/4.5

BP1/BP21.5/4.0

Table 3. Itaipu HVDC transmission. Total forced outages

The figures given for the dc line include transientfaults with successful restart. Many of the DC linefaults in 1990 were caused by flashovers to aeucalyptus tree and in most cases the faults did notcause a pole outage. The performance of the ± 600 kVdc lines has proven to be very good. The two linesuse 30 insulators per suspension string, giving aspecific creepage of 27.7 mm/kV and have a totallength of over 1600 km. In the last three years therehave been a total of 51 faults, of which only 12 didnot restart with success. This gives an average of twopole outages per bipole/year, which in general wererestarted manually without any problem.

7. CONCLUSION

The Itaipu HVDC Project, designed in 1979, includesmany features new at the time and continues to be thelargest HVDC transmission, in both rating,6,300 MW, and voltage, ± 600 kV. During the nowmore than ten years since first power transmission,the system has operated continuously supplying amajor part of the Brazilian load, always providingtransmission capacity for the available generation.The early years experienced significant challenges incommissioning and operating the system, as one mayexpect in such a large project. The final stage of theproject was fully commissioned in 1990, includingthe use of parallel operation. In the last five yearssince then, the average forced outage unavailabilityhas been below half a percent.

Figure 8. View of Foz do Iguaçu station

8. REFERENCES

1. V. Madzarevic, C.A.O. Peixoto, L. Hagloef:“General Description and Principal Characteristics ofthe Itaipu HVDC Transmission System” fromConference Sharing the Brazilian Experience, Paper1.1, Rio de Janeiro March 20-25, 1983

2. A.G Figueiredo, A. Praça, N.L. Shore: “MasterControl of the Itaipu HVDC Transmission System”from Conference Sharing the Brazilian Experience,Paper 1.7, Rio de Janeiro March 20-25, 1983.

3. Marly R. Bastos, Jose Torquato P. de Souza, JohnGraham, “Interferencia das Conversoras de CorrenteContinua no PLC do Sistema de Corrente Alternada.”XI SNPTEE, Rio de Janeiro, 1991.

4. W. Lampe, D. Wikström, B. Jacobsson: “FieldDistribution on an HVDC Wall Bushing duringLaboratory Rain Tests” IEEE 1991 WM. 125-5PWRD.

5. “In-service performance of HVDC ConverterTransformers and Oil-cooled Smoothing Reactors”.CIGRÉ Joint Task Force 12/14.10-01, 1993.

6. H. Arakaki, J. Lopes, A. Praça: “Itaipu HVDCTransmission. System-Analysis of Control Systemand Protection Performance after two years ofOperation”. Winnipeg, July 1987.

7. A. Farias, I. Oliveira, L. Correa et Al.: “EmergencyControl Scheme for the 765 kV AC ItaipuTransmission System Using Logical ProgrammableControllers”. CIGRÉ Latin American Conference.May 1995, Foz do Iguaçu, Brazil.

8. Delfim P. Amaral, A. M. de Freitas, John Graham.”Implantaçâo de Sistema de Detecçâo de Incendio emValvulas Tiristoras - Experiencia de Furnas”XIII SNPTEE, Florianopolis, Brazil, 1995.