Rev 1983029 ~ =-~ Engineering studies for Itaipu convertor station design

8/12/2019 Rev 1983029 ~ =-~ Engineering studies for Itaipu convertor station design

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Engineering studies for Itaipu convertor

station design

Abs~ract: The paper presents a discussion on some of the engineering studies performed to determine the

d~s.lgn param.eters of the Itaipu ~onvertor stations. The following studies are discussed: steady-state con-

ditIOns, reactive-power compensatIOn, Insulation co-ordination and arrester protective scheme current stresses

system stability, main characteristics of the master control, AC and DC filter and DC line res;nance. For each'

of these st~dy areas, the paper gives a summary of the study methodology used, indicates the main study

results, and Includes some of the system problems encountered and the solution adopted.

Introduction

Itaipu power plant is a hydroelectric project being constructed

by Brazil and Paraguay on the Parana River, with 12600 MW

installed capacity to be generated by 9 x 823.5 MVA, 50 Hz

units and 9 x 737 MVA, 60 Hz units. In accordance with the

agreement signed by the two countries, each one will havethe right to buy one half of the power to be generated by

Itaipu and, in addition, one country will have the first priority

to buy the excess power that could not be used by the other.

Considering the terms of the agreement, that Paraguay would

only need the total of its share of the power in the far future

and that Brazilian frequency is 60 Hz, it was decided to con-

struct a hybrid AC/DC transmission system with two 600 kV

bipolar lines [1] of 795 and 8l5km, to transmit the 50Hz

generation and three 750kV AC transmission lines to transmit

the 60Hz power to Brazil [2,3].

The convertor station specifications were issued in

November 1978 and the contract was awarded to the ASEA/

PROMON Consortium in June 1979. It is the scope of this

paper to report the results of several studies performed, up tonow, comparing them with the requirements of FURNAS

specification. These studies were done in Brazil and Sweden by

the Consortium in close co-operation with FURNAS, to finalise

the equipment and control specifications for the Itaipu HVDC

convertor stations.

2 Steady-state conditions: main circuit parameters

The purpose of this study, carried out using the well known

DC formulas, was to establish the main circuit parameters of

the HYDC transmission as well as its steady-state characteristics,

related to the following operating modes:

(a) balanced bipolar

(b) monopolar with ground return

(c) reduced DC voltage(d) bipole paralleling(e) high reactive absorption

(I) reverse power.

The FURNAS specification required transmission of rated

power of 6300 MW from the rectifier AC 500 kV busbar. The

rated power at the inverter substation 345 kV AC busbar was

calculated, based on the minimum line resistance with maxi-

mum ambient temperature of 40C dry and 29C wet bulb.

Paper 2320 C (P9, Pl0), received 23rd June 1982

Mr. Peixoto is with FURNAS Centrais Eletricas SA Rua Real Grandeza

219, 13~ andar, Bl.A 22281, Rio de Janeiro, Brazil. Mr. Frontin is alsowith FURNAS Centrais El


2/10

above data and includes 0.75% measuring tolerance in the DC

current, ground return circuit of total resitance of 0.7.Q,

maximum and minimum DC current, AC voltage variation

550-475 kV at Foz do Igua,!u and 362-321 kV at Sao Roque,

the four modes of operation (a) to (d) described above,

transformer reactance tolerances, resistive voltage drop 0.4%

and constant voltage drop of the thyristor 0.21 kV.

A range of - 6 to + 20 steps of 1.25% was determined forthe rectifier and inverter station transformers. This range willcover all conditions except operation at reduced DC voltage

with extreme parameter tolerances and DC voltage higher than

525 kV at Foz. A reduced DC voltage of 80% can be obtained

under such conditions and this deviation from the specification

was accepted by FURNAS. It should be noted that to prevent

equipment operation under excessive steady-state voltage twoprotections are provided: one at a level UdioG =(l - k 6.5)

UdioL, where k> 1.2 and 6.5 = 1.25% is the tap step, whichblocks the order to change the tap to increase the commutating

voltage (UdioG equals 1.019 and 1.003 Udion for rectifier and

inverter, respectively). The second level is UdioL =0.99

Udiomax (1% is the measuring tolerance) that orders a change

of tap to decrease the commutating voltage (UdioL equals

1.034 and 1.022 times Udion for rectifier and inverter,

respectively).

3 Reactive-power compensation

The study of the reactive compensation to supply the demandsof the convertors and system comprised the verification of:

(a) reactive balance in steady-state operation(b) voltage variation when switching reactive devices

(c) fundamental frequency overvoltages

(d) self excitation of synchronous machines

(e) low-order harmonic resonance

(f) power recovery after system faults.

The reactive compensation in both stations is designed, accord-

ing to the specification, to allow transmission of the nominalrating of 700MW per generator with anyone switchable

element out of service, in any stage of development shown in

Table 2 and bipolar operation. The reactive compensation

should also permit the transmission of the maximum output

of 727 MW per generator with all elements in service up to a

maximum of eight generators.

For Foz do Igua,!u these requirements will be met over the

voltage range 0.95 to 1.05 p.u. at the rectifier 500 kV AC bus-

bar, although the equipments are designed to operate up to

1.10 p.u. The generators' MVAR capability at the rectifier

busbar was considered part of the reactive compensation.

For the Sao Roque station, these requirements will be met

over the voltage range 0.93-1.05 p.U. at the inverter 345 kV

AC busbar. With nominal DC infeed and at 0.95 p.u. AC

busbar voltage, there will be zero exchange of reactive powerto the receiving system and at 1.05 p.u. 300 MVAR can be

supplied to the receiving AC system. In addition, there will be

a further controlled 300 MVAR supply capacity to cover

emergency conditions.

Table 2: Stages of development

Bipole voltage

Number of

machines

kV

300

+ 600, - 300 600

600-I- 600

= 600

300+ 600, - 300 600

The reactive-power consumption of the convertors was

calculated by the Consortium considering the normal range of

DC voltage and the angles a; (12.5 -17 .5) and 'Y (17). For

this calculation the nominal value of convertor transformer

reactance was used based on the FURNAS specification. The

values of the compensation are shown in Table 3 and fulfil

the specified requirements unless the average of all transformer

reactances is above the nominal value. If the average is 5%above nominal there will be a lack of 75 and 55 MVAR at Foz

and S. Roque, respectively. As it is considered unlikely that

such deviation will occur, the amount of compensation in-

dicated in Table 3 will be retained until a check can be made on

the reactance of the convertor transformers as manufactured.

This would permit the determination of the possible need of

extra compensation, allowing this to be installed in due time.

Besides the case of bipolar operation used for dimensioning

as per the criteria specified, the other operating modes were

analysed but with all reactive devices in. The given reactive-

power compensation is adequate for the majority of cases.

However, in a few situations it will not be possible to operate

in the full range of AC voltage in the sending end.

It was required in the specification that the voltage variation

when switching the largest compensation device should belower than 5% of the preswitching value at Sao Roque. In the

proposal, the largest element size was 444 MVAR. As this

would result in voltage variation higher than 5%, a special

action in the control of 'Y was proposed to solve the problem

during the scheduled operation. When switching in a filter or

capacitor, 'Y would be rapidly increased and then returned to

its initial value slowly. When switching out, 'Y would be in-

creased slowly to allow the synchronous condenser to pick the

extra reactive power, and then 'Y would be rapidly decreased

after switching out the device. This control would not cover

unscheduled tripping. FURNAS considered this a deviation

from the specification and the size of the banks was changed

to the values of Table 3.

For Foz do Igua,!u, with the filter sizes shown in Table 3,the voltage variation when switching one filter bank will be

higher than 5% in the initial stage. To solve this problem,

FURNAS is considering the installation of a branch of each

filter bank on a separate switch bay by advancing the instal-

lation of bays that will be later utilised for complete fIlter

banks. This will permit the installation of the required filters,

while meeting the voltage-variation criterion.

Another requirement related to reactive compensation was

that, during complete DC load rejection (blocking of all con

vertor groups), the fundamental frequency overvoltage at the

AC convertor busbar should not exceed 1.4 p.u. and should be

reduced to 1.05 p.u. of the initial voltage within 5 s, without

any switching operations. For the rectifier system the voltage

at the generator terminals should not exceed 1.3 p.u. Also in

the situation of partial blocking of the convertor groups, atany stage, the fundamental frequency temporary overvoltages

with some convertors remaining in service should not exceed

the design limits of the thyristor valves. In the final instal-

lation, when blocking one bipole from an initial operation

Equipment Foz do Igua,>u

Filters 2 X 350 + 3 X 280.3(number X MV AR =1540.9

at 500 and 345 kV)

Shunt capacitor

(number X MVAR

at 345 kV)

Svnchronous com-

pensation (number X

MV ARc/MVARi)

3 X 220.8 + 279.8 -296 + 4 X 237 +296.3 = 2482.5


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~Gndition of three poles at rated load, the convertor busbar

:'undamental frequency voltage should not exceed 1.3 p.u.

For complete blocking it was not possible to keep the AC

overvoltage level lower than 1.5 p.u. at the rectifier station

unless contraints of initial voltage and tap position of the step-

up transformer were imposed. To solve this problem, the

five equivalent networks were represented on the simulator.

The equivalents 1 and 2 were represented by LR circuits wned

to 2.15 or 2 p.u. frequency, respectively. Impedance angle was

85 at 60 Hz. Equivalents 3 and 4 matched the same Z (0.:)

plot of the actual network, but tuned to 120 and 110Hz. At

the second harmonic the impedance angle is 75. Equivalent 5

------'QI : ~ " ,P L C Ii Itersf2

. . .I!J

~. . .

u 50

P

compensation could be decreased but this would result in lack

of reactive power and, consequently, operating restrictions.

However, all equipment is able to withstand this overvoltage,

and this exception is under consideration. It is important to

point out that, owing to the characteristics of the control

equipment, simultaneous blocking of two bipoles is unlikely to

occur. For internal faults in one convertor only that convertor

will be blocked. Persistent commutation failure during system

AC faults, if initiating blocking, would result in complete load

rejection at the sending end. However, in the case of Itaipu

control equipment design, for commutation failure due to

system faults, blocking is not carried out. Blocking would

occur only if the commutation failure lasts for longer than one

second. With reference to self excitation of the Itaipu gener-

ators, preliminary investigation by dynamic simulation showed

that, if the number of generators is greater than the number offilter banks at any configuration, no risk of self excitation

exists. This will be included in the supervision system. For the

inverter station, a synchronous condenser connected with one

filter or capacitor bank must of be permitted and this is

~overed by station layout and protection.

With respect to low-order harmonic resonance in the re-

:ei\'ing system, in the Consortium proposal, a preliminary

lli\'estigation was made regarding the resonant frequency of

the network in the various stages of the development of the

'X system. This was based on an LR equivalent network with

i"l:::;Jedance angle of 85 from the fundamental frequency to

:" ,e :::'::d harmonic. The number of synchronous condensers

se:e ::e~ ,esulted in a resonant frequency of the system plus

:11:e, .i::~ synchronous condensers above 2.15 times funda-

:-::e::::c:. O..1ng to changes in system configuration later on, it

'.'-.is :":s:.y:e,ed that some combination of system plus compen-

satlo" :'J'.::d be in resonance at the second harmonic. Then asim:.:::.::,:, study was conducted to evaluate this condition and

to ~J:-'.;::.:,e \\ith the results obtained during the proposal

period. To compare and analyse the effect of these conditions,

--." "'-J-electrode

to the Ii neother pole

represented the actual system with no adjustment. Fault

clearing, transformer DC saturation and valve faults (misfire

and firing through) were studied and it was concluded that

although a second harmonic resonance could be found in the

actual system, the angle of the impedance was very low,

offering a high damping of the possible oscillation. Conse-

quently, the transient performance of the actual system was

better than the simple equivalent considered in the proposalwith the same number of synchronous condensers.

It should also be noted that the number of synchronous

condensers chosen was essentially determined by the require-

ments of system recovery after faults.

4 Insulation co-ordination and arrester protective scheme

Only zinc-oxide gapless arresters are provided for the Itaipu

HYDC system (Fig. 1).

Extensive overvoltages studies were performed to evaluate

the energies and currents for which the arresters would have to

be tested, and to find the maximum current and respective

waveshape that define the protective level and, consequently,

the insulation. The studies covered fundamental frequencyovervoltages, switching-surge overvoltages and lightning surges.

In the first group, complete and partial blocking of the con-

vertors was examined. In the second group, investigations

of transients were related to load rejection, fault clearing,

switching of transformer, filters and lines, valve faults and the

consequences on the AC side, monopolar faults on the DC

lines, ground faults on the AC phase of the valve side of the

convertor transformers, faulty closing of bypass switches,

ground faults on the DC busbar including fast surges, bipole

paralleling, DC mter switching and current extinction in one

three-pulse commutation group. In the third group, the effect

of lightning surges injected from AC and DC lines and due to

shielding failure were evaluated.

Digital computer programs were used to study fundamentalfrequency and lightning overvoltages; whereas TNA, DC

simulator and computer programs were used to study switching-

surge overvoltages. In the latter, as a general rule the TNA and


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Table 4: Arrester characteristics and stresses

6Stress Location (Fig. 1) 1A 18 2 3 4A

arrester parameters 48 Foz S. Roque

Number of colums 8 4 6 (3) 2 2 2Energy capability, MWs 8 4.2 6.5 2 3.6 5.3 3.6

Switching Maximum continuous

surge voltage, kVp 206.7 206.7 347.5 10 614 450 296

Maximum energy formstudies, MWs 14.4 4.2 6.5 2 3.6 5.3 3.6Maximum current, kA 6.3 2.1 1.8 60 O.b- 1 1waveshape, J .1 .S 1000 1000 1000 (2) 1000 45 X 90 45 X 90Protective level, (1) (1)

kV 325/312 316/302 579 110 1057 862 55220% margin

(reference) 390/374 379/362 695 132 1269 1035 663Impulse Maximum current, kA 3 3 3 11 17.5 12.8 10surge Protective level, kV (1) (1)

332/319 332/319 626 101 1380 1050 67725% margin

(reference) 415/399 415/399 783 127 1725 1312 847

(1) Lower values refer to Sao Roque station and higher ones to Foz

(2) High frequency due to filter discharge wave shape (close to lightning-surge type)

(3) Preliminary value

(4) AC/DC filter arrester specification not yet finalised (location 5 and 7)

the DC simulator were used for general investigations of worst

cases and digital programs used to make the final calculations

of energy and currents in the arrester. This procedure was

adopted because the energy given in TNA and simulator

studies is considerably lower than that obtained with digital

calculation owing to the inherent damping of the former tools.

In the digital simulation, the arresters were represented by their

average characteristic less 2.5% (manufacturing tolerance).

The protective level is taken for this same reason as plus 2.5%

above the average characteristic. The results presented in terms

of arrester stress are shown in Table 4, together with other

characteristics.

The maximum stress obtained in the studies for the 1A

arrester was 14.4 MWs during an AC phase-to-ground fault on

the valve side of the transfonner in parallel-operation mode of

convertors and a certain range of fault application instants. As

this condition has a low probability of occurrence and the

energy specified of 8 MWs covers most of the cases in parallel

operation and all other modes of operation, this risk was

accepted by FURNAS. In any case, should this high-energy

(14.4MWs) condition occur, the protective level will not be

exceeded even if the arrester fails. For the other arrester the

conditions that gave highest energy were:

arrester lB- current excitation in only one three pulse

group

arrester 2 - faulty closing of the bypass switch

arrester 3 - DC busbar fault

arrester 4A/4B - discharge DC line at 1.7 p.u.

arrester 6 - fault clearing [4] .

As required by the FURNAS specification, the insulation levels

were selected in the following way:

Non-self-restoring insulation: For the oil insulation the BIL

is equal to standard IEC value greater than 1.20/0.83 times the

switching surge protective level of the arrester. The BSL is

equal to 0.83 BIL. For the values, 15 and 20% margin are used

for switching surge and lightning/fast surges, respectively. It

should be noted that minimum values of BIL = 1425 kV and

BSL =1175 kV for the Foz transformer, and BIL =1050 kV

and BSL =870 kV for the Sao Roque transformer were

specified for AC side.

Self-restoring insulation: For all equipment, except the ones

mentioned above, the BSL is equal to standard lEC value

greater than 1.25 times the switching-surge protective level of

the arrester, to give an overall risk of failure not greater than

one flashover in 100 years within the substation. The BIL is

Table 5: Self-restoring insulation levelsfor Foz (1)

Sector Protected by BIL Margin BSL Margin

protected arrester

kV % kV %

DC line end 4A 1800 30 1321 25

DC switch yard 4B 1675 31 1321 25

Valve side of

DC reactor 2+2 1675 34 1448 25

Neutral busbar 3 325 221 150 36

Transformer

valve side

lower D wind 3 + 1B 750 73 533 25

lower Y wind 3+1B+1B 1050 37 928 25

upper D wind 2+1B 1300 36 1119 25

upper Y wind 2+1B+1B 1800 39 1514 25

Busbar between 12

pulse bridges 2 850 35 724 25

Across single

valve 1A/1 B 399 20 374/363 15

- 'I e busbar~:z 8: l;;uEzu 6 1550 48, 1 ~ "75 .-::::=: - ::_:=- 0 ";-:72: 73 ~ = = -


5/10

--~~'"l.

~"

Table 6: Nonselfrestoring insulation levels for Foz (1)

Sector Protected by BIL Margin BSL Margin

protected arrester

kV % kV %

DC reactor

line side 4B 1550 21 1287 21

DC reactor

valve side 2+2 1675 33 1390 20

Across DC

reactor (note 2) 1800 20 not governed

Transformer

valve side

lower D 3 + 1 B 650 50 540 27

lower Y 3+1B+1B 1175 54 975 31

upper D 2 + 1 B 1300 36 1079 20

upper Y 2+1B+1B 1800 40 1494 23

Line side

F5?zIgua


6/10

To evaluate the overcurrents on the DC side of the

smoothing reactor and line, the HVDC simulator was used to

investigate the following conditions:

DC line to ground fault

blocking of the inverter

uncontrolled rectifier and subsequent blocking of the

inverter

transition of the inverter to rectifier mode of operationparallel operation.

The maximum current peak value, 13 kA, was obtained. For

equ~~ment on the valve side of the smoothing reactor the

declslve current 25 kA was measured at a short-circuit across

convertors.

AC busbar short circuits were evaluated with conventional

procedure and the results are shown in Table 7.

6 Stability studies

During the preparation of the specification FURNAS conducted

extensive stability studies in all development stages and with a

complete representation of the system [2,3] . The recovery timeof the DC system after faults was specified as 160 ms, based on

this digital study and a preliminary simulator study. 1985 was

found to be the most critical year, because it represents the

last stage of the development of the DC system with the

weakest receiving system. In 1988 the AC part of the Itaipu

power plant is completed and the receiving system is much

stronger.

Owing to the possible difficulties of reproducing these

studies during the bid preparation, the specification asked for

a demonstration of the control behaviour in a reduced AC

system (total of 14 busbars) that has the same response of the

actual network (year 1988) in the first swing, even recognising

t~at the behaviour in the dynamic period could be completely

dlfferent. The Consortium did not present the study in the

proposal, but guaranteed a performance equivalent of the ref-

erence system to be demonstrated afterwards.

After the award of the contract, it was agreed to study theyears 1983, 1985 and 1988 (heavy and light load) with thecomplete system representation. Important conclusions related

to time to recovery and modulation of the DC system were

determined. The study concentrated on the 1985 heavy load

condition [5]; this year, with constant power mode of

control, a reduction in AC voltage will be accompanied by a

reduction in DC voltage and, consequently, a demand for

higher current. This increase in current aggravates the initial

voltage reduction owing to the increase in reactive-power requi-

rement and communication drop. The use of constant current

mode of control would be, in this situation, better for the

system, or any other measure that avoids high increases in

current or reductions in voltage (i.e. lower setting of VDCL or

increased synchronous compensation). The studies performed

by the Consortium confirmed this principle and showed thatrecovery in 400 ms is better than 160 ms. This will be achieved

by introducing a time constant of this order in the constant-

power control loop between the DC voltage signal and the

calculation of Iorder = Porder/ Ud' During the preparation ofthe specification, power modulation was found to be useful

because its action was to avoid increase in current. Studies on

modulation signals made by the Consortium found that a

modulation of 'Y derived from AC voltage at Sao Roque (see

Table 7: AC busbar short-circuit current, kArms

Station 30 10 Breaker rating

"4~ 50

= .,'

~ . - - - -

Fig. 3) w?uld give sat.isfactory performance and was preferable

for. cer.tam :easons; mc1uding the fact it does not introduce

oscillatlOns m the Paraguayan system. This modulation should

have a lower limit so that 'Ydoes not corne below 'Y . =17

to ensure a sufficient margin for commutation (see F ig ,.n3).

VS.Roque

s T,

('.sT, )(l.sTZ)

K =Z36 , 4 I P u.

T) = 2.0TZ=0.0127

l!.y = 13Emax

1m;n ref=17E

This modulation solved all stability problems for all years

of the system studies and will be incorporated into the control

system at Sao Roque [5] .

7 Main characteristics of the master control

The master control functions are mainly related to the inter-

action between the AC systems and the DC link, which are

performed automatically or through action of the system

operators.

The master control is organised in an hierarchical manner:

station control, bipole control and pole control such that the

lowest level possible controls the minimum unit or block of

power with the maximum independence.

The pole control receives the power order from the bipole

control and is responsible for the various additional power

orders, such as stabilising of the 50 Hz network and 60 Hz

frequency regulation. The calculation to allow short and longtime overloads, as well as start, stop, paralleling and deparal-

leling sequences, are executed at the pole level.

The bipole control is responsible for the transmission of the

power order received from the operator to the pole control. Ir-

a loss of transmission capability occurs within one bipole, an

additional power order is transmitted to the other bipole to

compensate for this loss. The selection of the operator control

location for power control and bipole/pole functions is also

made at bipole level.

The station control is responsible for the supervision of the

number of fIlters in operation, due to the related conditions c': '

both AC and DC systems, and the reactive-power balan~e

Regarding the hardware, microprocessors will be the heart c C

the master control and a redundant microcomputer stmctu:e

will be used by the two highest levels of the control hierarc}-"

The functions performed by the microprocessors are differe;'-

in each station, mainly in the normal operation mode. T::e

lead station (Foz do Igua~u) performs almost all the funeti:"

related to the power transmission, the trail station (Sao RO':L2

receiving and processing the final current order in synchro':-is:-

with the lead station.The communication between stations will utilise 1\\,: :e-

dundant and independent channels: power-line carrie: ~:-

microwave. The received telegrams will then be C0mDJ:e:. '.

achieve a high degree of security. An error-rate l11onit:ri;:: :. -

counting feature will be supplied which will allow J :'e~:_

ance analysis of the telecommunication._ Regarding the operator, each bipole may ':'e __

trom anyone of four different locations. t\C Jt oJ'::.

IS!3.tion control room and bipolc ,>J=-~::-o~:-::~-=~


7/10

1

--:~

the master station concept is dependent on the actual control

location. The station control room at Sao Roque is to be the

master station, in normal conditions, as all control facilities

are placed there, such as mimic AC and DC panels. When Sao

Roque is acting as the master station, there is a possibility of

receiving the power order and ramp from the system operation

centre (SaC).

The normal control mode will be synchronous power

control, 'synchronous' meaning that the pole-current order

generated by the lead station is transmitted by telecom to thetrail station. In this case, all control functions summarised as

follows are available:

(a) automatic synchronisation between the current ordersto be applied at both the rectifier and inverter

(b) keeping the earth current to minimum levels regardlessof the voltage levels of the poles

(c) compensation for loss of power when the inverter takesover the current control for any reason (this is performed by

the automatic margin regulator)

(d) automatic power-loss compensation within the bipole

due to the loss of a convertor

(e) possibility to help the speed regulation of the 60 Hz

system

(I)stabilising the 50 Hz frequency due to loss of Itaipugenerators

(g) possibility of receiving automatic generation controls

signals

(h) utilisation of a continuous overload due to low ambient

conditions

(i) utilisation of a short time overload of at least 125%

during 5 s and 115% during the next 20 s following system

disturbances and possibility of paralleling.

In some few situations it is impossible to utilise the synchron-

ous power control: loss of telecommunication, loss of pole

control equipment and complete loss of communication

between stations. In these cases, the following operation

modes shall be used:

Asynchronous power control: This mode will be used when

there is a loss of the telecommunication between the pole

power controllers. All the additional inputs (i.e. frequency

stabilisation) and facilities are switched off and two operators

(one at each station) are necessary to set a new power order.

Current control: This mode is mainly utilised due to loss of

pole control equipment, and its operation procedure is similarto the power asynchronous mode. A device named current

order medmory allows this operation.

Automatic current margin control (AMC): It is intended to

be used when there is no communication (including voice)

between the two stations. In this case, one operator at the

rectifier station is able to run the system. A follow up control

is utilised at the inverter.

The disturbance indices for harmonic distortion and telephone

interference required in the specification were:

Foz do Igual.(u Sao Roque

lndi\'idual harmonic

::s~crtion (Dn)

T=,~:.~harmonic

::s~ = =~ion(D)

T~:~~hc:ne-int1uence

:'::c~== ( IIF I

IT :::',=:UCI (design'J'J~ec~iT,'2 )

These :'::-:-.:~sshould not be exceeded for any foreseeable

operatlr,g condition of the HYDC transmission system and

using the worst point of the envelope of the AC system

impedance, either with all filters in operation or with a com-

plete filter bank out. For the purpose of the performance

calculations, one filter bank should be assumed as comprising

at least one filter branch of each type and at least 15% of the

total filter fundamental frequency MYAR, independent

of the switching scheme.

For the purpose of the calculation each characteristic

harmonic current should be assumed as having the maximum

value possible for the complete range of firing angles, com-mutating reactances and DC currents permitted for the

operating condition considered. The noncharacteristic har-

monic currents should also be calculated pessimistically

by assuming a combination of the most onerous conditions

of variation of firing angle and commutating reactance

between phases within a bridge, between bridges in 12-pulse

pairs, between 12-pulse pairs within a pole, between poles

in a bipole, and between bipoles. The individual harmonics

should be assumed as having their maxima occurring simul-

taneously.

The filters should be considered off-tune in the calculations

and their harmonic impedances obtained by assuming a

combination of maximum system frequency deviation of

duration exceeding one minute, maxumum temperaturevariation, initial mistuning, capacitor failure to the maxi-

mum extent possible prior to the first alarm being generated

and detuning due to component ageing.

The harmonic impedance of the AC system was specified

by envelopes obtained from extensive digital computer

calculations of the system impedance, as a function of

frequency for different developments of the system and

years of development. The calculation of Dn, D and TIF

should be done using the worst point within these envelopes.

For IT calculations a table of harmonic system impedances

related to a specific year was given.

The harmonics have been calculated for one 12-pulse

bridge and for Id in the range 10-110% Idn. For the charac-

teristic harmonics, the combination of the ranges of 0;, randdx were considered. For the calculation of noncharacteristic

harmonics, the same combination of 0;and dx were considered.

The asymmetries which are responsible for the generation of

these harmonics were considered either as statistical (normal

distributions defined by media f J . - and standard deviation a) or

as systematic, as shown in Table 8.

Distribution

} l a

Firing angle

asymmetry, 0

e1 0 0.02

Transformer phasesreactance, % 0 0.33

Asymmetry between

Ll-" % 0 0.66

Negative sequence, % 0.5

Many runs were done using a Monte Carlo method and a

distribution of harmonic magnitude and angles was obtained.

A maximum value IIwas obtained for each harmonic by

II= f J . - + Kp a,where Kp is chosen for 99% confidence level. Toobtain the total harmonic due to n twelve-pulse groups two

approaches were used: In = nIl (for harmonics of order 6n +1, n odd) and In = nfJ.- + Kp a Vii(other noncharacteristic har-monics). The linear addition was considered for the 6n 1, n

odd because they are due to asymmetries between 1and Llwindings and could be systematic owing to the design of thetransformer.


8/10

For Foz do Igua~u, as a result of these calculations, a

scheme with two 350 MVAR and three 280.3 MVAR filter

banks has been proposed. The 350 MVAR banks contain one

3rd/5th double tuned branch (69.7 MVAR), one II/13th

double tuned branch (134 MVR) and one HP branch tuned

to the 24th (146.3 MVAR), whereas the 280.3 MVAR banks

contain one II/13th and one HPbrancheach. These different

branches are to be installed gradually as the number of 50 Hz

generators at Itaipu power plant increases.

The highest TIF for this scheme with one filter out is

36.9. To reduce this figure to the specified values the Con-

sortium is studying a change in the HP filters to have 3

branches tuned to the 24th harmonics and two branches

double tuned to 24th and 36th (or 48th) harmonics.

The total distortion exceeded 4% in many stages, even

with all filters in (values up to 6.2), caused mainly by the 7th

harmonic, for which the individual distortion is up to 1.5%

in the final stage. A decision about the need for extra filtering

to solve this problem will be taken later on, after a recalculation

per bipole of the disturbance indices with the actual measured

values of the transformer reactances. This approach has been

suggested due to the fact that the noncharacteristic harmonics

of order 6n 1 (n odd), which are considerably influenced

by the assumption of a systematic difference between thereactances of the Y and ll.-connected transformer windings,

are responsible for a large percentage of the disturbance

indices which exceed the specified limits. Among those non-

characteristic harmonics the seventh is particularly influential.

It is foreseen that seventh harmonic branch will have to be

added to the present scheme, if additional filters are called

for by a decision based either on the already calculated

disturbance indices or on the above mentioned recalculated

values. It should be noted that an increase in MVAR is not

desirable, because of problems of generator self excitation

and dynamic overvoltages.

For Sao Roque a scheme with ten filter banks and two

shunt-capacitor banks has been presented by the Consortium.The total filtering MVAR (2482.5 MVAR) is divided in the

following independently switchable units, to be installed

gradually as the transmitted power increases:

Bank size, MVAR

220.8 11th,13th

HP

11th, 13th, 3rd/5th

3rd/5th, HP

The impedance of all types of HP arms together reaches

minimum around frequencies ranging from the 21st to the

27th harmonic. It is worth noting that the 11th and the 13th

harmonic filters are of the damped type, with a configuration

similar to that of the HP arms. This filter scheme met the

specified criteria. It should be noted that for both stations

there will be filters tuned to 3rd/5th harmonics. These

branches are needed to meet the performance requirements

and are useful to decrease the temporary overvoltages on

fault clearing, transformer energisation etc. by minimising

resonant conditions.

With reference to the IT product, the design objectives of

35000 and 25000 specified for Foz do Igu~u and Sao Roque,-2spectively, are exceeded considerably for the filtering

:~:'.e::eproposed. The maximum values of the IT product

, , ,,:',~ ,":- :~'eF:z j.J IguJ;;-uand Sao Roque were of the order, ,.: c:' ~ =:: : :' : . :es:-2c:iwly. At Foz the problems are

minimised in comparison with Sao Roque, because the soil

resistivity is low (about 500 D-m). At Sao Roque the problem

is aggravated because the soil resistivity could be in the range

1000 to 5000 D-m and the AC lines entre Sao Paulo city with

a very high density of telephone circuits. To evaluate this

problem FURNAS is conducting an extensive study that

includes harmonic penetration analysis with three-phase

representation of part of the AC network, soil resistivity

measurement for all frequencies of interest, calculation of the

induced harmonic voltage in the telephone lines as a function

of the separation between the power and telephone lines. If

high interference is confirmed in this new programme of

analysis, FURNAS is considering the conversion of the two

shunt banks into HP filters.

9 DC filter

During the short time available to perform the, studies rtquired

to determine the data to be included in the convertor speci-

fications, it was not possible to do the inductive co-ordination

study required to specify the DC filter. Based on the

experience of other projects and very preliminary studies that

consider different aspects of the inductive co-ordination

such as dangerous induced voltages, secondary interferenceand interference on controls, it was decided to ask the bidders

to quote two filter alternatives: a design to limit the harmonic

current at the terminal of the HVDC line to the values shown

in Fig. 4 (level 1) and a design to limit the currents to a mag-

nitude equal to three times these values (level 2). Rules

allowing the determination of the cost of a filter design in

between the values for levelland level 2 were also requested.

With this information in hand FURNAS could choose the final

DC filter configuration after contract award. Based on the

noise criteria of 240 mVein an open wire telephone circuit,

in bipolar operation and 10 dB higher noise level in mono-

polar operation (less than 2% of time), it was decided to

install the filter level 2 design. The detailed design of this

filter resulted in a configuration with branches tuned to 100/

300 Hz, 1200/2400 Hz, 120/600 Hz at Foz and 100/120 Hz,

720 Hz 1440/2160 Hz at Sao Roque. The design was different

for the two bipoles due to different lengths.

It should be noted that, for the calculation of the harmonic

voltage, the same presumptions related to transformer

I

I6. 0

tn 40

E~

, o r

Rev 1983029 ~ =-~ Engineering studies for Itaipu convertor station design

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Transcript of Rev 1983029 ~ =-~ Engineering studies for Itaipu convertor station design