Energization of a Half-Wavelength Transmission Line - Pre-Operational Transients Studies

doi:10.4236/epe.2013.54B276

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Energy and Power Engineering, 2013, 5, 1456-1462

doi:10.4236/epe.2013.54B276 Published Online July 2013 (http://www.scirp.org/journal/epe)

Energization of a Half-Wavelength Transmission Line -

Pr e-Operational Transients Studies

Camilo Machado Jr1, Marcelo Maia2, Eden Carvalho Jr3, Maria C. Tavares4,

João B. Gertrudes5, Elson C. Gomes4, Walmir Freitas4, Marcos A. Paz5, Fernando A. Moreira6,

Carlos A. Floriano1, Vanderlei G. Machado1, Aniela M. Mendes1

1ELETRONORTE -Brasília, Brazil

2CHESF, Recife, Brazil

3ENTE, São Paulo, Brazil

4University of Campinas, Campinas, Brazil

5State University of Feira de Santana, Feira de Santana, Brazil

6Federal University of Bahia, Salvador, Brazil

Email: [email protected], mj[email protected], ej[email protected],

[email protected], [email protected], [email protected]

Received April, 2013

ABSTRACT

In the present paper the main results of the pre-operational studies related to the energization maneuver of a

Half-Wavelength Transmission Line (AC-Link), are presented. The simulations were performed in PSCAD with actual

Brazilian system data. Specifically the following studies were performed: electromagnetic transient studies of energiza-

tion maneuver considering no fault and the occurrence of fault along the trunk; dynamic analysis of generator units that

will energize the AC-Link; TRV on the circuit-breaker that will energize the trunk. The main conclusion is that the

AC-Link energization experiment can be implemented without jeopardizing or causing time-life reduction of the

equipment involved in the experiment, which comprises the generator unit that will energize the trunk, the step-up

transformer, the arresters that will be kept during the experiments, as well as the circuit-breaker that will energize and

then trip the AC-Link.

Keywords: Electromagnetic Transients; Half-wavelength Transmission; AC-Link; Energization Test; Dynamic

Analysis; TRV; Faults

1. Introduction

According to Brazilian Institute of Geography and Statis-

tics (IBGE) on the last 10 years Brazilian population has

grown 1.17 % per year, totalizing 12.48 % for the last

decade. This growth aligned to the Brazilian industrial

advance has generated an increasing demand for electri-

cal energy, and hence a higher need for new energy re-

sources.

Nowadays, it is important to find renewable resources

to avoid the environment degradation. A huge hydroelec-

tric potential exists on the North of Brazil, on the Ama-

zon Region, with amounts of 100 GW not yet used, as

presented in [1].

However, this potential is located very far from the big

Brazilian load centers with distances higher than

2500 km. These long distances bring some extra difficul-

ties to transmit such huge blocks of energy.

In 2008 the Brazilian Electrical Energy Regulatory

Agency (ANEEL) proposed a so called Strategic R&D

Project to be supported by utilities and coordinated by

universities named: “Half-Wavelength transmission line

experiment". According to the call: "it is considered of

great relevance to the Brazilian Electrical System the

execution of field tests to confront with theoretical and

computational results, especially the ones related to

overvoltages derived from line energization and the vol-

tage profile along the line without reactive compensation,

for the half-wavelength transmission system." [2-5]. And

even more: "This effort has the main objective of evalu-

ating the possibility of including the line with a little

more than half-wavelength in the studies of future inte-

gration of the forthcoming hydro plants to the Brazilian

Integrated Electrical System" [6].

In 2011 the research project began, technically and fi-

nancially supported by the Brazilian Utilities: ELE-

TROBRAS/ELETRONORTE, ELETROBRAS/ CHESF

and ENTE, being executed by the following universities:

UNICAMP, UFBA and UEFS.

C. M. JR ET AL. 1457

Specifically the project main objective is the imple-

mentation of the energization maneuver in a set of Bra-

zilian system composed of 500 kV transmission lines that

when connected in series will form a 2600-km long trunk

[7]. That corresponds to a little more than half-wave-

length for 60 Hz electrical system.

The proposed circuit will use the existing interconnec-

tions North-South 1, North-South 2 and part of North-

east- Southeast, totaling 2600 km [8].

The test trunk, here called AC-Link for being an AC

point-to-point transmission, has economical advantages

when compared with AC conventional interconnection

that are heavily compensated, as this alternative needs

neither intermediate substations nor reactive compensa-

tion (series or shunt) [9]. The AC-Link is also a competi-

tive alternative when compared with HVDC Link as its

terminal substations only have ordinary AC transformers

and no harmonic filters are needed [9]. That is an impor-

tant feature as there is no Power Electronic Technology

involved [1].

For the experiment the heavily compensated line sec-

tions will have the series compensation by-passed and

the shunt compensations removed. The circuit breakers

(CB) of the intermediate substations will be locked in

closed position and the one at the remote end substation

will be locked at opened position. Only the CB at the

sending end will be in use. With this a 2600-km long

500-kV trunk is obtained. The energization maneuver

will be implemented through the 500 kV CB at the send-

ing end. All the protection schemes at intermediate sub-

stations should be out-of-service so that no CB changes

position during the test.

The main electromagnetic transient results are pre-

sented in the following sections.

2. Analyzed System Description

The transmission system used in the study is formed by

three 500 kV interconnection trunks that have similar

characteristics and will be connected to form the AC-

Link [7,8]. The characteristics of the transmission lines

are summarized in Tables 1 to 3 where the sequence

components of longitudinal and transversal parameters

per unit length were calculated assuming ideally trans-

posed lines, for 60 Hz.

These trunks have same terminal substations which

allow connecting them in series. During the test the Bra-

zilian electrical system will become practically discon-

nected, for the North and Northeast systems will remain

connected to the Southeast, South and Central systems by

only a single 500 kV interconnection trunk instead of the

three normally in service.

It is a major concern that the test has the shortest dura-

tion possible. In order to reduce the setup time for the

test and afterwards, to recompose the system, all surge

arresters at lines terminals will be kept in service. As the

transient overvoltages are expected to be very low, the

arresters will not operate during the test and their pres-

ence will not impact the validity of the test. The sin-

gle-phase diagram with the substations and lines in-

volved is depicted in Figure 1.

The energization will be performed from Serra da

Mesa substation (sending end – SE) in one shot, what

means that the entire trunk will be energized at a single

shot using a pre-insertion resistor circuit-breaker (CB).

This was identified as the most adequate procedure for

AC-Link, as the controlled switching is not effective [7].

Table 1. Longitudinal and Transversal Parameters Calcu-

lated at 60 Hz - North-South I.

Sequence Unitary

Resistance

[/km]

Unitary

Inductance

[mH/km]

Unitary

Capacitance

[F/km]

Zero 0.37138 4.11662 0.00725

Positive/Negative0.01589 0.70700 0.01612

Table 2. Longitudinal and Transversal Parameters Calcu-

lated at 60 Hz - North-South II.

Sequence Unitary

Resistance

[/km]

Unitary

Inductance

[mH/km]

Unitary

Capacitance

[F/km]

Zero 0.34822 3.74452 0.00946

Positive/Negative0.01602 0.71089 0.01634

Table 3. Longitudinal and Transversal Parameters Calcu-

lated at 60 Hz - North East-South East.

Sequence Unitary

Resistance

[/km]

Unitary

Inductance

[mH/km]

Unitary

Capacitance

[F/km]

Zero 0.34821 3.75767 0.00934

Positive/Negative0.01602 0.72403 0.01603

Figure 1. Single-phase diagram of AC-Link Test - 500 kV.

C. M. JR ET AL.

1458

The Brazilian ISO (ONS) will identify when the lines

that will form the AC-Link can be disconnected without

jeopardizing the Brazilian electrical system. The period

when these interconnections have low power flow will be

chosen to perform the test. As explained, the test is

planned to be as short as possible and it should not last

more than 2 to 3 hours, comprising the experiment setup,

the sequence of energization and finally the system res-

toration.

Traditional soil representation was applied during the

tests, specifically the soil resistivity was considered con-

stant with frequency over the entire length of the

AC-Link trunk with value of 4000 Ω.m due to high soil

resistivity in these regions [10].

3. Electromagnetic Transient Studies

For the implementation of the energization maneuver the

regular transient studies were performed in PSCAD, as

described in the following sections.

Due to the line length the overvoltages are expected to

be much lower than the ones observed in regular trans-

mission lines of few hundreds of kilometers long. This

occurs because the traveling waves are attenuated as they

travel along the line, mainly the zero sequence one.

The lines were modeled with the phase domain model

which properly represents the line longitudinal parame-

ters frequency dependence.

3.1. Line Energization Without Faults

The insulation level of the 500 kV lines and the equip-

ment connected in the test system were not surpassed

during the energization tests.

The surge arresters that were kept connected during

the simulations did not affect the AC-Link behavior as

the results were similar to the configuration when they

were considered just in the AC-Link terminals.

The overvoltage levels observed and the surge arrest-

ers energy consumption during the energization were

very low, far below the equipment limits. In Figure 2 the

line-to-ground voltage at AC-Link terminal is presented.

Figure 2. Voltage at receiving end during AC-Link

energization - Pre-switching terminal voltage of 0.9 pu and

no pre-insertion resistor.

When the number of generator units was varied for the

line energization the lower overvoltage levels were ob-

tained when more units were used. However it was pos-

sible to energize the link with any number of units avail-

able at the SE substation. The simulation results showed

that the overvoltage levels and the sustained voltage were

within the system capability.

It was also verified that it is possible to perform the

maneuver with or without the pre-insertion resistor. The

resistor is designed to remain in service for 8-10 ms and

that is not adequate to mitigate the AC-Link overvoltages

[5]. For an AC-Link the resistor should be kept for 20 ms

so the traveling waves could reach the opposite terminal

and return at least once. As this would entail important

device modification the other alternative would be the

resistor bypass. However that would affect the test setup

time.

As the overvoltages with the resistor kept for 10 ms

and without the resistor were not important the test shall

be implemented with the resistor operating as originally

designed.

3.2. Line Energization with Fault

Some simulations were performed supposing occurrence

or existence of single line to ground faults and three-

phase faults, involving or not the ground, during the

AC-Link energization test.

Although the experiment will be performed with fine

weather in the majority of the AC-Link, due to its length

there is a possibility of rain in part of the Link. However,

due to the short duration of the experiment the probabil-

ity of occurring a fault during the test is very low. Nev-

ertheless, the equipment involved in the test, namely the

generator unit, the step-up transformer, the circuit-

breaker and the line sections with their arresters should

not be damaged and should be put back into service in

the following hours after the experiment.

The faults were represented along the Link, at each

line section terminal and in the middle of each section.

When the fault caused important overvoltage this interval

was reduced. The fault was represented by a 20- resis-

tor.

The simulations consisted of energizing the line with

fault and removing the fault after 100 ms.

The voltage at SE terminal for single phase fault (SLF)

is presented in Figure 3.

The measured overvoltages due to SLF were limited to

2.0 pu along the line as the surge arresters were repre-

sented. In order to observe the transient severity the ar-

rester energy was analyzed. The energy was not impor-

tant except for faults occurring between 85 and 90 % of

the AC-Link length measured from the SE. If a fault oc-

curs in this region the arrester located at the remote end

absorbs high amount of energy. An additional arrester

C. M. JR ET AL. 1459

could be installed for the test at that location. However

the probability of occurring a fault in that specific region

during the short duration of the experiment is extremely

low. The existent relay was used in RTDS study and the

protection operation time prevented this severe arrester

energy consumption [10].

The maximum overcurrent observed in the CB oc-

curred for terminal fault and was of 2.6 kA. The highest

fault current was produced for a fault at 70 % of the Link

length reaching 2.9 kA. The transformer neutral current

reached a maximum value of 2.9 kA.

The results for isolated and grounded three-phase

faults (3LF) were similar. The overvoltages and overcur-

rents vary with the fault location, being more severe if

the fault occurs between 65% and 95 % of the AC-Link

length.

The critical regions for both SLF and 3LF are related

to the sequence impedances seen by the SE. For instance,

the SLF critical region occurs at multiples of quarter

wavelength of zero sequence seen from the terminal. The

network at this terminal will also influence the quasi-

resonance region, but in this experiment the major effect

will be the AC-Link itself as the terminal is just formed

by the generation unit and the transformer (this is an iso-

lated system).

For the 3LF the critical regions corresponds to multi-

ple of quarter wavelength positive sequence seen from

the SE. At these regions sustained overvoltage may be

produced and have to be mitigated promptly.

For 3LF the overvoltage increases in an accentuated

rate, as the positive sequence attenuation for 60 Hz is

very small. A common mistaken analysis seen in a large

number of researches that deal with AC-Link is the ex-

tremely high overvoltage for three-phase faults, far be-

yond the line insulation level. The simulations cannot

produce voltages along the line supposing flashovers will

not happen. There is no meaning in obtaining 4.0 pu

overvoltage as before that a flashover will occur. When

the flashover occurs the AC-Link will immediately be

removed from the quasi-resonance condition and the

Figure 3. Maximum transient overvoltages measurured at

Sending End terminal for single-phase fault.

voltages that will arise will depend on the new fault loca-

tion.

In order to mitigate the stresses produced by 3LF at

critical locations it was proposed to use a mitigation me-

thod called Reduced Insulation Distance (RID) [11]. This

method, applied for the no-load test study, consists in

reducing the insulator string length in a selected tower in

order to provoke the flashover in that specific location.

The RID was installed in a tower near Imperatriz substa-

tion (km 1306 – 40% distant from the SE).

The RID was prepared to disrupt at 1.6 pu (line to

ground), for:

- This voltage is much higher than the transient

overvoltage at this location during energization without

any faults (maximum transient value of 1.2 pu);

- During no-load steady state the sustained voltage

is very low at this location (0.54 pu);

- 3LF produces no severe overvoltages or overcur-

rents along the AC-Link when it occurs this location.

With this method the severe overvoltages/overcurrents

are not established and the severe condition is immedi-

ately eliminated. Using RID the overvoltages due to 3LF

are below withstand levels and regular thermal capability

arresters can be used. The maximum current levels ob-

tained in CB, transformer neutral and RID are, respec-

tively, 3.11 kA, 2.41 kA and 8.32 kA. The RID current

value is lower than the electric discharge current that can

reach more than 30 kA and, therefore, it will not damage

the insulator string.

In Figure 4 the maximum voltages measured at SE

during grounded 3LF are presented. In Figure 5 the

maximum currents measured at CB are during grounded

3LF are shown.

This RID will not act for SLF as the voltage at this lo-

cation will not reach 1.6 pu. It will only operate for 3LF

at the severe region. If the RID operates for a lower vol-

tage for SLF this will not be a severe condition.

Figure 4. Maximum transient overvoltages at sending

terminal for grounded 3LF along the Link with RID.

C. M. JR ET AL.

1460

3.3. Generator unit Performance During

Energization without Fault

The AC-Link can be energized from 1, 2 or 3 power

units. The requirements of the generators are greater if

only one machine is used. However the measured values

do not compromise the equipment integrity or reduces its

life-time.

In Figure 6 the active power is presented for energiza-

tion without pre-insertion resistor.

3.4. Generator unit Performance during

Energization with Faults

Simulations were performed considering the occurrence

of faults along the AC-Link during the energization Test.

If a single-phase fault occurs the stress will not dam-

age the generator unit. Specifically, if a fault occurs in

the critical region for SLF the generator will send 1.5 pu

of active power and 0.5 pu of reactive power. These val-

ues do not affect its unit due to the short duration of this

event.

In case of three-phase fault, grounded or isolated, the

stress again will not damage the generator. Specifically,

if a fault occurs in the critical region RID will operate

immediately, protecting the generator. During the tran-

sient the current in the generator will reach 3.0 pu, re-

ducing promptly to 0.7 pu. After the transient the ma-

chine injects 0.1 pu active power and 0.60 pu reactive

power.

3.5. Line Tripping without Faults

The opening of no-load AC-Link results in very low ca-

pacitive current and the voltage between the breaker

poles is not severe and attenuates very fast, as presented

in Figure 7. The results showed that it is possible to open

the AC-Link using the existing CB at Serra da Mesa sub-

station (SE). The capacitive current through CB poles

and the voltage across its terminals are much lower than

its capability.

Figure 5. Maximum transient current at circuit-breaker for

grounded 3LF along the Link with RID.

Potência ativa

(

)

0.0 1.0 2.0 3.0 4.0 5.0

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

PG (pu)

PG 1

Figure 6. Active power injected by 1 generator unit during

energization without fault – Pre switching voltage: 0.9 pu –

Without pre-insertion resistor.

Figure 7. TRV of pole A during AC-Link no-load tripping

after energization without fault.

3.6. Line Tripping with Faults

The simulations consisted of energization and subsequent

tripping considering terminal fault, short-line fault and

remote fault. The faults analyzed were SLF and 3LF.

For remote faults the fault location was varied in order

to verify the behavior in critical regions. As there is no

standard for such a long trunk tripping, simulations were

performed comparing the AC-Link with the line for

which the CB was designed for terminal and short-line

faults. The actual line is 256 km long with shunt com-

pensation in both terminals.

It was observed that:

- The terminal fault current is similar to the one ob-

served for shorter lines even when shunt compensated, as

presented in Figure 8;

- The short-line-fault for AC-Link is less severe than

for similar lines of hundreds of kilometers, as in the latter

the inductance of the shunt compensation will produce

higher TRV;

- The highest current to be interrupted is not defined

by terminal fault, but rather by three-phase fault in spe-

C. M. JR ET AL. 1461

cific location which varies with the short circuit power

(Ssc) of the terminal network. These locations can be

from 60 to 85 % of the line length for low and high Ssc,

respectively, measured from the circuit breaker;

- The voltages at the circuit breaker contacts for re-

mote 3LF faults are not severe when RID mitigation

procedure is applied;

The TRV produced by remote SLF are not severe.

4. Conclusions

In the present paper electromagnetic results concerning

an energization of an AC-Link trunk formed by existing

500 kV lines are presented.

The use of Reduced Insulation Distance (RID) at a

tower near Imperatriz substation (1306 km) will mitigate

the overvoltages that are produced when a three-phase

fault occurs in a specific region (65 to 95 % of Link

length measured from SE). This method is adequate for

the energization maneuver but is not a straightforward

solution to be used during normal operation.

With RID the 3LF is not a sever condition for the

AC-Link energization. The SLF does not produce high

overvoltages, but the energy absorbed by the arresters at

the remote end can become excessive if protection does

not operate as designed [10]. In that case an additional

arrester or a higher thermal capacity arrester may be

needed.

Regarding the generator units, the requirements for

normal operation and operation under faults did not re-

sult in special requirements, and the actual machine can

cope with the experiment without damaging or reducing

its life-time.

The actual circuit-breaker designed for a highly shunt

compensated 256-km long line was capable of opening

Figure 8. TRV across pole A during AC-Link tripping after

energization under SLF terminal fault.

the no-load AC-Link and also tripping the AC-Link un-

der SLF and 3LF (terminal, short-line and remote). Even

for the critical region for SLF and 3LF the CB suffered

no special stress.

It can be stated that the AC-Link energization test can

be implemented in the analyzed system without impact-

ing the equipment involved (specifically generator unit,

transformer, circuit-breaker, line surge arresters and line

structure). There will be no damage to the devices in-

volved and their life-time will not be reduced due to the

stresses imposed by the experiment.

The HVAC-Link is a reliable and technically robust

solution for very long transmission trunks and due to its

constant terminal voltage for all load profile, it should

also be carefully analyzed for intermittent power transfer

as the ones produced by alternative energy sources, as

large solar and wind power plants.

In [11, 13] some results regarding line protection are

presented. Ongoing research on Single-Phase Auto Re-

closing will be presented in a near future. As the AC

Link cost is attractive for very long transmission, proper

technical solution can be developed if necessary to solve

specific needs.

5. Acknowledgements

This work was supported CNPq, CAPES and FAPESP in

Brazil. The results presented in this paper are partial re-

sults from the project supported by ELETROBRAS/

ELETRONORTE, ELETROBRAS/ CHESF and ENTE

as part of ANEEL (Brazilian Electrical Energy Regula-

tory Agency) strategic project 2008 (proc. 4500072477).

The project was coordinated by UNICAMP with the par-

ticipation of UFBA and UEFS.

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