Testing EHV Secondary Arcs

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Paper acceptcd for prcscntation at PPT 2001

2001

IEEE Porto Power Tech Conference

I O t h

-13‘”

September, Porto, Portugal

Testing EHV Secondary Arcs

G

Ban, Fellow IEEE

L.

Prikler, Member IEEE

G

Banfai, Member IEEE

Abstract--The protection of the environment requires

compact EHV tower constructions. This trend results in

decreasing the clearances and phase-to-phase distances. I t is

expectable, that both the number of faults and the amplitude

of the secondary arc current

will

grow in the future.

All

these

make necessary to improve the efficiency of single-phase

reclosing. Secondary arc duration values gained at field tests

show a big spread and therefore they do not provide a reliable

basis for selecting a suitable dead time. Analyzing the main

processes affecting on the secondary arc duration, authors

conclude that traveling wave phenomena arising in the

intermittent period of secondary arcing have a determining

effect on the arc duration. Considering the difficulties

connected with field tests and the big influence of the weather

conditions

on

the secondary arc duration, authors propose to

complete laboratory test circuits by such components, which

effectuate the simulation of the wave processes.

A

method for

the detection of the secondary arc extinction of good

experiences

on 750

and

400

kV lines is mentioned, which offers

a possibility to prevent inefficient reclosures.

Index

Terms--Compacting power lines, dead time, EHV line

faults, field test, laboratory test, secondary arc, shunt reactor,

single-phase reclosing, traveling wave process.

I. INTRODUCTION

integrate transmission lines into the environment, the

T

imensions of the towers, the clearances, phase-to-

phase spacing are to be reduced. Besides it is difficult to find

a new right-of-way. To operate lines on a higher voltage

level, than their original nominal voltage offers a possibility

to overcome this problem. Transmission lines of lower

voltage level contain single phase-conductors. When

uprating lines, they have to be replaced by bundle

conductors

or

the number of subconductors

has to

be

increased. All these solutions result in decreasing the

clearance and increasing the phase-to-phase capacitance. It

is expected, that the number of faults due to lightning

strokes will increase, if the dimensions of the towers will be

reduced in proportion of the possible rate of switching

overvoltages limitation.

The activity reported in

the

paper has been partly supported by the

Hungarian Research Fund under contract

No. OTKA

T

026054

Gabor Ban, Laszl6 Prikler and Gyorgy BBnfai are with the Department of

Electric Power Systems, Budapest University

of

Technology and Economics,

Egry J. U. 18, H-1 111 Budapest, Hungary.

Authors’ e-mail addresses: [email protected], [email protected]

Single phase reclosing seems to be an efficient tool to

compensate the expectable growth in the number of EHV

line faults, since they are single phase-to-ground ones and

not permanent in the overwhelming majority of cases.

Stability requirements determine the upper limit of the dead

time; generally dead times exceeding

1.5 -

2

s

are not

permissible. When tripping the faulty phase, a secondary arc

forms in the hot plasma remaining from the primary arc.

The secondary arc is sustained by the capacitive and

inductive coupling between the healthy and faulty phase

conductors. The amplitude of the continuous secondary

current lies in the range of

10

Amps. The secondary arc

extinguishes spontaneously as a rule, however its duration

depends on many factors. Long duration secondary arcs

endanger the efficiency of single-phase reclosing, if the

secondary arc does not extinguish during the dead time, the

reclosure is followed by three-phase tripping the line.

Consequently, the lower limit of the dead time to be adjusted

in the reclosing automation is prescribed by the expectable

duration of the secondary arcs. Considering the increased

phase-to-phase capacitance of compact and uprated lines,

higher secondary arc currents have to be expected, that will

result in longer secondary arc duration.

Numerous tests have been carried out on real lines and in

laboratories to get a picture about the expectable secondary

arc duration. However, the results published in technical

papers show a significant spread. To prescribe the lower

limit of the dead time starting from the longest secondary

arc extinction time experienced does not seem economical.

On the other hand, utilities want to keep the same reliability

level for compact lines with reduced clearances and

increased phase-to-phase capacitance, as for the

conservative line constructions. It is easy to see, that exact

and reliable data about the expectable secondary arcing

times are necessary for the new tower constructions and it is

recommendable to reconsider the technique of secondary arc

testing.

Authors participated in field testing of a

420

kV

interconnecting line of

3 10

km length, without shunt

compensation, in November and December

1999.

Seven

staged faults were carried out, aiming to predict the

expectable secondary arcing times. The shortest extinction

time recorded during the tests reached

0.05

s and the longest

4 s.

The secondary arc did not extinguish during 27

s

on one

test, so the line was tripped. The

T, = 4 s

and T, >

27

0-7803-7139-9/01/ 10.00 02001

IEEE

mailto:[email protected]:[email protected]


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extinction times have been measured at calmness and

T,= 0.05 s 0.69 s values at small wind velocity 3

-

4 d s .

The big spread of the extinction times and very long arcing

times experienced in two cases correspond to the data about

extinction times published in former papers containing

generalizing diagrams

[3].

The voltage and currents records

as well as shots made about the arc movement make possible

to investigate the secondary arcing process in details. The

purposes of this investigation are:

-

to explain the big spread of the secondary arc extinction

times,

-

to study the factors influencing on the secondary arcing

process,

to improve the technique of field tests to get more reliable

data,

- to elaborate a correct test circuit for laboratory studies,

- to find a way for establishing a dead time which is

acceptable both from stability and single phase reclosure

efficiency point of view.

Authors considered also their former experiences and results

gained at former field tests on a shunt compensated 750 kV

line of 479 kilometers length and numerous 400 kV lines.

The paper analyses the following points:

-

the shape and the amplitude of the secondary arc current,

-

the impact of the wind velocity,

-

the technique of the secondary arc initiation,

11. THESHAPE AND

THE

AMPLITUDEF

THE

SECONDARYRCCURRENT

The dependency of the secondary arc extinction time on

the sinusoidal, steady state arc current is given in the

generalizing papers. This current can be calculated using the

circuit in Fig. 1, where Ua U,, U, are the phase voltages,

Cab-the hase-to-phase, Co-thezero sequence capacitance,

a) three-phase network

b) equivalent circuit

Fig. 1.

a.

-the secondary arc current. The fault is modeled by switch

S,. The secondary current contains an induced component

as well due to the inductive coupling between the phase

conductor and an inductive one caused by the distributed

character of

C a b

and the lumped character of the fault. The

induced and the inductive components may be neglected

usually due to their small amplitude. These components play

an important role only when the capacitive component is

compensated artificially (e.g. by neutral reactors).

The predominant part of the published laboratory tests

was carried out using the circuit in Fig. l/b. However, this

circuit gives a reference about the steady state secondary

current only. As numerous records of single phase switching

tests on transmission lines show the character of secondary

arcing is intermittent during a significant part of the

process. The extinction of the secondary arc is always

preceded by an intermittent interval, containing current

impulses. The amplitude of these impulses can be much

higher than in the steady-state interval (Fig.2). This part of

the arcing process can be explained by the thermal effect of

the high current primary arc which generates a.powerfu1 air

movement resulting in a speedy elongation of the secondary

arc and producing separated from each other plasma clouds.

Wind can have the same effect on the secondary arcing.

When the transient recovery voltage, arising along the arc

channel, exceeds the critical value, sufficient to reignite the

arc, traveling waves start on the line. The superposition of

the reflected waves produces a current zero at the fault and

mostly partial arc extinction. The length

of

the current

impulses depends on the place of the fault. If the fault is in

the middle point, the impulse length is equal to the travel

time along the line

T).

For the predominant part

of

the line

length, the duration of the impulse is 2T. The shape of the

impulse and its thermal effect is depending on the line

b.

600

400

200

0

200

400

600

0 .6 0 7

0 8 0.9 1

o

1 1

Typical secondary arc voltage and the current measured)

Fig. 2


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............

............ ............ ............. ........ ........

1

4 0 0 I

0.67

0.69

0.71 0.73 0.75 0.77

Fig.

3.

Measured secondary arc voltage and current

configuration (discontinuity points) as well. The actual value

of the recovery voltage at the moment of the restrike

determines the peek value of the current impulse. The

energy of the current impulse is higher, when the restrike

occurs near to, but before the peek of the recovery voltage

(see Fig.3).

A

high current impulse containing a relatively high

amount of energy is able to re-ionize a large amount of

plasma, to shift back the process into the quasi-sinusoidal

interval and elongate the secondary arc duration. From these

effects it emerges, that traveling wave phenomena have a

big influence on intermittent arcing process and

consequently on the secondary arc duration. Thus circuits

applied for testing secondary arc in laboratory cannot

furnish correct data without modeling the wave processes

during the intermittent interval. The shape and the

amplitude of the recovery voltage arising after the partial

extinctions of the secondary arc must have also an effect on

the arc duration. These parameters also strongly depend on

the wave phenomena.

Shunt reactors connected directly to a power line affect

both on the arc current and the recovery voltage. From the

equivalent circuit created for this configuration (Fig.

4)

follows, that here the recovery voltage is of beating

character, it contains

two

frequencies: the power and the

transient frequency of the circuit. Consequently the partial

-u /2

LO

Fig. 4. Equivalent circuit for shunt compensated lines

or final arc extinctions are followed by a modulated voltage

oscillation, the amplitude of which may be rather high,

depending on the compensation degree of the line.

As another effect of shunt reactors, a DC component

occurs in the initial part of the secondary arc current, caused

by the magnetic energy stored in the reactor at the moment

of tripping the faulty phase. The circuit in Fig. 4 simulates

this

DC

component, which affects

on

the arc duration only

when four legged shunt reactor sets are connected to the

line. Besides, the shunt reactor of the faulty phase generates

a DC component in the secondary arc during each arc

reignition. Fig. 5 shows, that the shunt reactor current

Ir)

is

opposite to the current wave

I,,

reflected at the line end, if

the arc reignites before the peek of the recovery voltage UIec.

In case of a reignition after the peek of

U,

(this happens

frequently when the gaps in the secondary arc channel

increase) the directions of I, and I,, are equal. As a result,

the first zero transition of the arc current becomes very

steep, and the arc can not extinguish in the first current

zero, the length of the restrike elongates by 10 milliseconds.

Occurring long duration restrikes can delay the final

extinction of the secondary arc for a long time. As a matter

of course the circuit shown in Fig. 4 does not simulate the

wave process.

111. THEEFFECT

F

THE WIND V E L O C I T Y

The secondary arc forms inside the plasma left by the

primary arc which is moving upwards due to its high

temperature and in horizontal direction due to the wind.

After a short while individual plasma clouds will be

separated from each other by cold channel zones due to the

elongation of the arc channel and the thermal losses.

0.10 0 . 1 2 0 . 1 4

0 . i 6

o . i s

0.50

0 52

upper curves:

I

green) and

Iarc=I,+lcw,

ower curve:

U,

Fig

5


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The secondary arc extinguishes finally, when the voltage

necessary to break down the high resistance channel zones,

exceeds the recovery voltage. So it is evident, that the wind

velocity is a determining factor of the secondary arc

extinction process. Unfortunately, most papers do not

publish the wind parameters during the tests.

At calmness no horizontal component of the arc motion,

the plasmas left by the primary arc and generated by the

secondary one move slowly, without any significant channel

elongation (Fig. 6/a). Ions generated during the intermittent

arc interval remain in the environment of the arc. These

circumstances make the self-extinction time very long. At

small wind velocities the elongation of the arc is slow. Due

to the electromagnetic field, loops occur in the arc channel

(Fig.

6h . So

parts of the arc channel, which are electrically

far from each other, may get close geometrically. If the

recovery voltage is sufficient to produce a breakdown,

bridging a significant length of the arc channel, the arcing

process may return to the condition of steady state burning,

remarkably elongating the self-extinction time.

The formation of loops is of random character; as a

consequence the spread of the self-extinction times is

relatively big at small wind velocities. There is an

interesting interaction of the loop formation and the wave

processes. Being the motion of the arc channel slower, than

increasing the recovery voltage, a breakdown between two

plasma clouds occurs mainly at the peek of the recovery

voltage. Such a breakdown results in a high current impulse

that

is

not able to extinguish at the first zero transition

producing a high energy elongated restrike (see the first and

second restrikes in Fig.

7),

which sets back the intermittent

arcing

into the

steady-state one.

In strong wind the distances between the active parts of

the inhomogeneous arc channel increase quickly.

Consequently, the re-ignition voltage does not take long to

excess the recovery voltage, resulting in a short extinction

time.

Iv . THETECHNIQUEF

THE

SECONDARYARC INITIATION

Secondary arc tests on real lines are often carried out by

omitting the primary arc; the secondary arc is initiated by a

thin wire in such cases. This method of testing is based on

the conclusion drawn from numerous field tests [6], that the

primary arc does not affect the self extinction time, as far as

the secondary arc duration excesses 0.7 s. Considering, that

the omission of the primary arc makes the tests more simple

and less dangerous for the system, and secondary arc

durations shorter than 0 7 s are allowed from stability point

of view as a rule, this way of testing seems to be attractive.

In case of a real phase-to-ground fault the secondary arc

develops inside the primary plasma, along its whole length

(Fig. 6/c). Our field test results show, that the thickness of

the wire and the way of its connection to the arcing honis

may influence on the self-extinction time, particularly at

middle and big wind velocities, when the speeds of the arc

elongation and wire evaporation are commensurable.

In such cases the arc may extinguish due to the

elongation caused by the wind before the wire evaporates

along its whole length (Fig. 6/d). The arc initiation by

means of a wire may result in shorter extinction times than

under natural conditions.

v.

DETECTION

F

THE SECONDARY

ARC

EXTINCTION

As it was mentioned before, the final extinction of the

secondary arc is preceded by an intermittent interval during

which the arc current is of impulse character. Every

impulse is initiated by a breakdown between the individual

plasma clouds, as a consequence, the breakdown is followed

by starting steep front waves along the line. Waves, reflected

on the open ends of the faulty phase conductor result in a

current transition and mostly an intermediate extinction at

the place

of

the fault.

So

both the front and the tail of the

current impulse are relatively steep, causing two individual

transients on the line. Both transients result in a voltage

oscillation on the line ends. The frequency of the breakdown

transient depends on the situation of the fault. The frequency

of the extinction transient is depending on the line length

only. This frequency can be calculated very exactly, taking

into consideration the effect of the healthy phases on the

process. Therefore

it can be used as a signal for the

detection.

The device, elaborated in the Budapest University of

Technology and Economics for the detection of the final

extinction

of

the secondary arc has been based

on

sensing

the presence of the extinction transients by means of special

filters on the line ends. As long as the intermittent

secondary arc exists, a high voltage occurs on the outputs of

the filters. Dropping the output voltage to a low level

indicates the final extinction of the secondary arc. The ten

years experience on

75

kV and 400 kV lines was positive:

the disturbance recorders showed, that the devices operated

correctly at every fault.

The scheme has been completed by a sensor which detects

the amplitude of the recovery voltage and serves for

establishing the arc extinction in that rare cases when no

intermittent interval occurs.

Summarizing: one may choose

an

optimum dead time

from stability point of view, knowing that the above

described device blocks the reclosure when the secondary

arc does not extinguish in time.


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a)

secondary arc at calmness

b)

secondary arc in m iddle velocity wind

c)

formation

of

the secondary arc inside the primary plasma

d)

extinction of the secondary arc

before

reaching its whole length

Fig. 6 . Second ary arc tests on a 400 kV line


6/6

3. One can decide the question about the validity of tests

without primary arc and select the optimal way of arc

initiation using the test circuit mentioned above.

4. The negative consequences of the big spread of

extinction times experienced on transmission lines and in

laboratories can be reduced by applying an automation,

which establishes the moment of the final extinction of the

actual secondary arc and prevents the reclosure when it did

not extinguish in time. Such a device has been developed in

Hungary and used on 750 kV and 400 kV lines [7].

0 8

0.4

0.41

v

0.8

I I I I

I

I I

0.096 0.100 0.104 0 108 0.1 12

Fig.

8.

Two restrikes on the power line and in the laboratory test circuit

calculated). Upper curves: recovery voltage; lower curves: arc current

VI. CONCLUSIONS

1. Field tests aiming to predict the secondary arc duration

for a new line need long preparation and stress the power

system. On the other hand, a big spread characterizes the

results gained from the individual tests due to the

differences in the wind velocities, arc initiation, line

construction, shunt compensation degree etc. In order to

reduce this spread, correct recording of the wind velocity

and a detailed description of the way of arc initiation are

recommended.

2.

Systematic laboratory tests at which the wind velocity

can be controlled and all parameters can be correctly

adjusted, furnish reliable and generalizing data. Laboratory

test circuits have to simulate the wave processes during

intermittent arcing; it can be effectuated by using a simple

reference circuit of the line

[5]

that takes into consideration

the wave process both in the faulty and healthy phases. As

Fig. 8 shows, one can realize a suitable similarity of the

processes in such a way. The shapes of the current impulses

differ to a certain degree, but the integrals, being responsible

for the thermal effect, are identical.

VII. ACKNOWLEDGEMENT

Authors acknowledge the contribution of the Hungarian

Power Companies Ltd.

VIII. REFERENCES

[ l ]

Fukinishi,M. et al.: “Laboratory Study on Dead Time

of

High

Speed Reclosing of 500 kV Systems” CIGRE 1970, Rep.

[2] Haubrich, H.J. et al.: “Single-phase Auto-Reclosing in EHN

Systems” CIGRE

1974,

Rep.

31-09

[3] Rashkes, V.S.: “Generalizationof the Operation Experiences,

Connected with the Efficiency of Single-phase Reclosing,

Experimental Data about the Extinction Time of the

Secondary Arc” Elektricheskie Stancii

1989. No.3

(in

Russian)

[4] Esztergalyos, J. et al.: “Single-phase Tripping and Auto

Reclosing of Transmission Lines (IEEE Committee Report)”,

IEEE Trans. Power Deliveiy vol.

7, No 1 .

pp.

182-192,

January 1992

[5]

G. Ban,

L.

Prikler: “ReferenceCircuits for the Investigation of

Power Systems Transients” in Proc o International

Conference

on

Power Systems Transients

Lisbon, 1995,

[6] Beliakov et al.: “Investigation of Single-phase Reclosing on

750 kV shunt-compensated transmission lines”

Elektrichestvo 1981

No.7 in Russian).

[7]

Ban, G.: “Adaptive Single-Pole Reclosing Experience” Proc.

o

the 33th Session o CIGRE 1990 Gr. 34, pp. 28-29.

3 1-03

pp.5

1 56.

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