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MEASUREMENT OF PARTIAL DISCHARGES IN AN INDUSTRIAL HV TEST LABORATORY

R. MALEWSKI* W. MOKASKI J. WIERZBICKI

Instytut Elektrotechniki Bydgoszcz Cable Works Tradeways

Poland

Keywords: HV cable test - Partial discharge Induced disturbanceElectromagnetic shield - Shielded HV laboratory

Shielding efficiency - Conducted disturbance RF filter - Deep grounding Higher harmonics

Introduction

An acceptance test of HV cables, transformers and other

HV apparatus is carried out in industrial HV

laboratories. Some of these laboratories are designed to

perform Partial Discharge (PD) measurements, and they

have to be equipped with an electromagnetic shield that

is sometimes referred to as Faraday cage. An ideal

shield forms a continuous metal enclosure that covers

the walls, ceiling and floor of the test hall. Faraday has

discovered that an external electromagnetic field

impinging on such shield induces a current that flows in

the walls, ceiling or floor. This current prevents the

external field from penetration inside the enclosure.Radio stations broadcasting in AM mode emit the

disturbing electromagnetic field. Besides, transient

electromagnetic field can be generated by the ignition

system of internal combustion engines, by sparking

between an electric locomotive pantograph and tractionwire, by an arc welding, and by many other sources.

In practice, the electromagnetic shield cannot be

continuous and completely enclosed, since it is

composed of metal panels connected to each other at the

edge, and of doors and windows. It is imperative to

ensure a low-impedance contact between the panels, andbetween the door and its frame.

The minute current induced in the shield by e.g. a

broadcasting station can be diverted by greasy contacts

between the door and its frame, or by a layer of oxide

on the metal panels bonded together by bolts. An

electromagnetic field enters the HV test hall through

such discontinuity, or opening in the shield, and

induces disturbances in the PD measuring circuit that

acts as a large-size antenna. Such disturbances mask the

minute PD signals and impair diagnostic of the

examined HV insulation.

The shield efficiency is defined as a ratio of the electric

(or magnetic) field component outside and inside theshielded area. The higher the efficiency, the lower is

background disturbance level. Naturally, the cost of HV

test hall depends largely on the electromagnetic shield

efficiency.

An acceptable PD level is specified for each kind ofapparatus, and ranges from a few hundreds of

picocoulombs (pC) for power transformers, down to a

single pC for cross-linked polyethylene (XLPE) cables.

In the case of HV power transformers the background

disturbance should not exceed a few tens of pC,

however, to test XLPE HV cables, the backgrounddisturbance should be reduced to approximately one pC.

_______________________________________________________________________________________________*Prof. Dr-Ing. Ryszard Malewski, owicka 53 m 12, 02 535 Warszawa, Poland, E-mail: [email protected]

21, rue d'Artois, F-75008 Parishttp://www.cigre.org

Session 2002 CIGR33-303


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A well-designed electromagnetic shield can effectively

reduce disturbance inducedin the PD measuring circuit.

However, disturbances are also conductedby the supply

and control cables, or by the laboratory grounding. The

power and control cables carry mainly power-frequency

currents, whereas the conducted disturbances containhigh-frequency spectral components. Radio-frequency

(RF) filters inserted in series with the supply wires

prevent these high-frequency currents form entering the

shielded enclosure.

Besides, in an industrial environment stray currents

circulate in the upper soil layers, between different

grounding points of supply transformers, large motors

and other loads installed in different factory buildings.

A large metal enclosure of the shielded HV test hall

offers a low-impedance path for these stray currents. An

on-and-off switching of such current induces a transient

disturbance in the PD measuring circuit. To prevent the

stray current from flowing in the electromagnetic shieldit has to be isolated from the local ground and from

structural elements connected to such ground. A special

grounding rod is driven deep into the soil, and insulated

from the upper soil layers. The electromagnetic shield

has to be connected to such deep-driven grounding rod,and insulated from all locally grounded structures.

It has been reported that PD measured with purely

sinusoidal voltage waveform are lower than the

respective measurements taken on the same object with

a distorted-waveform voltage. However, an industrial

plant often uses large motors with a thyristor speed-controlled drive. Such drive generates higher harmonics

of current and voltage in the supply system. A distortedsupply voltage affects the test voltage waveform, and

increases the PD level measured on the examined test

object. To reduce the test-voltage waveform distortion,

the HV laboratory should be supplied by a separatepower transformer.

To test of big EHV power transformers a very large

laboratory is required. The huge test hall has to be

protected by an electromagnetic shield that is required

to enable broad-band PD measurement. Such shield has

been designed for the world largest HV laboratories of

IREQ in Montreal [1] and EdeF in Renardieres. An

important practical experience was gathered since their

commissioning. Comments on this experience and somefindings on the shield performance may be of use for

designers of new installations, and will be presented in

this paper.

Testing HV power cables imposes the most stringent

requirements on the background disturbance level.

Design of a large laboratory that can perform

contractual and development test of XLPE cables rated

up to 400 kV represents a challenge to engineers

specialized in HV technique and in electromagnetic

compatibility. Such laboratory was recently

commissioned at Bydgoszcz Cable Works [2], and

represents the state of the art facility for quality control

of EHV power cables. This installation may serve as an

example how to protect the PD measuring circuitagainst both induced and conducted disturbances.

P D measurement during acceptance-test of power

transformers.

International Standards require PD measurement during

the induced voltage test for transformers rated at 220 kVand higher. However, many utilities specify PDacceptance levels also for windings rated at 110 kV.

Usually, the acceptance level is indicated at 500 pC, but

recently lower values (300 pC and even 200 pC) are

often required [3]. Some specifications delimit an

acceptable variation of the measured PD level during

the last 30 minutes of one-hour test duration, and suchvariation shall not exceed e.g. 30 pC. An unstable and

high level of PDs may indicate a high content of

moisture in cellulose, particles in oil and other

technological flaws that disqualify the transformer.

A reliable measurement of PDs at 100 pC level can be

performed if the background noise is restricted to 10 pCrange. The test circuit of 400 kV or 220 kV transformer

acts as a large size antenna and collects the signals

induced by stray electromagnetic field. In an unshielded

test hall such disturbance may exceed by orders of

magnitude the PDs measured in the transformerinsulation. The small, sought PD impulses are then

completely masked. To suppress the disturbance to an

acceptable level the test circuit has to be installed in an

efficient electromagnetic shield.

Design of the electromagnetic shieldDesign features of the electromagnetic shield are

presented on an example of very large (8267m, 52m

clearance to the ceiling) HV test hall equipped with anelectromagnetic shield. The shielding efficiency of the

electric and magnetic field component was measured at

690 kHz and is presented in form of equal attenuation

lines shown in Fig. 1 [4]. Experience of transformer test

laboratories indicates that the attenuation of themagnetic field component by 60 dB is in general

sufficient to reduce the PD background disturbance to

the required 10 pC level [5].

Fig.1 (after [4]). Attenuation of the electric (E) and

magnetic (H) field component inside a large

shielded HV test hall. A leakage of the disturbance

field in vicinity of the large entrance door, as well

as near the windows can be identified on this graph.

A penetration of the external disturbance field into thetest hall is mainly caused by a poor contact between the


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doors and doorframe. This is due to a thin layer of

grease or dirt on the contact surface. The current that

circulates in the electromagnetic shield walls can be

effectively prevented from flowing across the door by

such a thin layer of dirt. An impedance of the dirt layer

is high enough to stop the current, since the voltage

difference at the contacts is of the order of somemicrovolts only. Such doors behave then as a large

opening in the shield, and the external field can

penetrate into the test hall.

The shield has been construed from 44 m steel panelsconnected by welding at spots spaced by approximately

0.3 m. The steel panels covered the four walls and

ceiling, and the whole shield was welded at spots spaced

by the same distance to the copper mesh that covers thewhole laboratory floor. A conceptual drawing of the

current flow in the electromagnetic shield is shown in

Fig. 2, assuming a vertically directed magnetic field (H)

vector.

Fig. 2. The drawing shows an exaggerated effect of the

disturbance field penetration between the spotconnections of the adjacent metal panels. An

external electromagnetic field induces a current that

flows in the shield walls, and prevents the applied

field from penetration into the shielded area. Any

disturbance or attenuation of the induced currentflow affects the shielding efficiency and allows the

field to leak-in.

The disturbance field can penetrate into the shielded

area due to the non-continuous, spot connection of the

adjacent panels. However, such the penetration depth

may reach the distance equal to the spacing between the

welded points, if the wavelength of applied

electromagnetic field is comparable to this spacing. Inreality, the PD measurements in power transformers are

performed in the frequency range up to 1 MHz, and the

shielding efficiency beyond this frequency is of nopractical concern.

It has been observed in one of the test laboratories that a

high efficiency electromagnetic shield has deteriorated

after years of service. The shield was made of

overlapping copper panels simply nailed together along

the edge to ensure a good contact between adjacentpanels. The shield worked fine as long the copper

surface was free from oxidation. However, under the

action of atmospheric humidity a layer of oxide has

developed and the contact resistance increased to theextent to impair circulation of induced current. At this

moment, the electromagnetic shield became transparent

to the external electromagnetic field, and thebackground noise level increased by orders of

magnitude. Although trivial, this example confirms the

necessity of welded or soldered contacts between the

panels that form an electromagnetic shield.

Some manufacturers use a moving-bridge (crane) totransport their transformers from the production hall tothe HV test area. The moving-bridge rails and supply

wires enter into the shielded test hall and reduce its

shielding efficiency. More modern plants use air

cushions to move the transformers, and then the

shielded test hall can be effectively closed.

Due consideration should be given to the cost of anelectromagnetic shield of a HV test hall large that is

enough to accommodate 400 kV power transformer

with the necessary clearance to the ceiling and walls.

Some smaller manufacturers cannot afford such

investment, and try to perform the induced voltage test

during night hours, when most of the disturbancesources are not active. However, the presence and

intensity of disturbances are random and unpredictable.

A large manufacturer is bound to deliver transformers

according to a tight schedule and cannot afford the

suitable time testing.

PD measurement during acceptance test of cross-

linked polyethylene(XLPE)power cablesHV cross-linked polyethylene power cables are the most

difficult test objects for PD measurements, due to a very

low level of acceptable PDs. Usually the test

specification allows one, two or at most five pC. Thebackground disturbance shall be less than 1 pC, despite

a large physical size of test circuit. For instance, the 400kV rated cable pothead assembled on a supporting

structure may be as high as 10 m. Even modest

electromagnetic disturbance field will induce a strong

signal in such antenna. An electromagnetic shield of theHV test hall shall have a high efficiency, exceeding 100

dB in the frequency range extending to the PD

measuring instrument upper frequency limit, (usually

250 kHz). It is rather difficult, if not impossible, to

reach so high attenuation in an electromagnetic shield

improvised from metal panels welded at spots [6].

At Bydgoszcz Cable Works two professional, shielded

rooms have been installed, with specially designed

seams and joints [7]. Each Faraday cage has the floorsize 2516 m and 13 m clearance to the ceiling. These

are primarily used for PD measurements on XLPE

cables up to 400 kV rated voltage. Besides, lightning

and switching impulse, HVDC and power frequency

voltage withstand tests are carried out in these two

shielded laboratories.

This shielded enclosure complies with the International

[8] and American National [9] standard requirements.

The attenuation characteristic compares favorably to the

US military and National Security Agency specification

for shielding efficiency [10].

The shield walls are made of a double-layer aluminum

sheath, with plywood panels sandwiched in between toprovide a structural rigidity. Specially designed


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moldings fastened by a row of bolts join edges of

adjacent panels. Particular attention is paid to the

entrance doors that are large enough (54 m) to allow

for transportation of large and heavy HV cable drums.

An electromagnetic gasket sometimes referred to as

fingers is installed along the edge of the door frame,

and a knife edge of the doors is squeezed into thesefingers when the door is closed. The fingers are made

of a hard beryllium-copper alloy and soldered to the

doorframe. Each time the door is open and closed the

sharp and elastic fingers scratch the door edge,

remove the dirt layer and refresh the galvanic contact

between the shield and the doors.

Forced-air ventilation ducts of the shielded enclosure

are equipped with honeycomb type electromagnetic

barrier, and interference-free lighting fixtures do not

contribute to the background disturbance level.

Amplitude characteristic of the electromagnetic shield

installed in Bydgoszcz Cable Works, is shown in Fig. 3.

Fig. 3. Attenuation of the magnetic field component bythe electromagnetic shield manufactured by

Universal Shielding Corporation Company and

installed in the HV test hall of Bydgoszcz Cable

Works. Superimposed are characteristics specified

by the US Air Force and the National Security

Agency (NSA 65-6).

German Standard (VDE) specifies 2 pC as the highest

allowed PD level measured during the test of HV

cables, and Polish Standard [11] imposes 5 pC limit at

the test voltage U=2*Uo. These requirements are

routinely fulfilled during the tests of XLPE cables

manufactured by Bydgoszcz Cable Works.

High Voltage test equipment

The state of the art shielded enclosure requires equally

refined HV test equipment that provides the PD-free test

voltage. A resonant-circuit test set-up, tuned to power

frequency by reactors with a moving magnetic core was

chosen [12], to avoid intense interference produced by

semiconductor controlled, frequency-regulated resonant

test set-ups. Two regulated reactors rated at 350 kV, 30

A can be stacked up and tuned to resonance with the

examined cable section. The test voltage is supplied by

a 500 kVA regulator and an exciting transformer with

500 kV rated highest-voltage output.

The measuring instrumentation is composed from thecomputer assisted TE-571 PD detector, HV divider and

coupling capacitor, as well as auxiliary instruments, all

from the same leading manufacturer of HV test

equipment. A computerized test procedure can be

implemented owing to the integrated system of test

voltage control and measurement, as well as PD

recording. A fault location procedure is incorporated in

the PD measuring system, and a damaged insulation ofthe examined cable section can be located within a few

meter uncertainty.

Filtering out the conducted disturbances

A well-designed and installed shield suppresses the

disturbances induced by external electromagnetic field

to a negligible level. High quality HV test equipment

provides PD-free test voltage. At this stage thedominant disturbance is conducted by the supply and

control, as well as by the grounding system.

The requirement of a very low disturbance level brings

to the test hall designer attention another mechanism of

disturbance penetration into the test circuit. Namely,

disturbances conducted by the supply, control andmeasuring cables. Special filters are installed in all

circuits entering the shielded HV test hall, to attenuate

the conducted disturbing signals. The filter complexity

and cost depends on the required attenuation

characteristic and on the power frequency current that

has to pass through the filter.

A typical filter is composed of series connected reactors

and parallel capacitors. A circuit diagram and amplitude

characteristic of such filter is shown in Fig. 4.

Fig. 4. Circuit diagram of a typical RF filter employed

to supply the HV test transformer. An effect of core

saturation on the filter attenuation characteristic is

shown on the right sketch. An attenuation of the

differential and common mode disturbance is

presented conceptually on the lower sketch.

The air core reactors attenuate the differential mode

disturbance, and magnetic core reactors, with bi-filar

winding reduce the common mode disturbance. Thelatter is conducted by the phase-conductors and returnsvia the ground. In principle, the load current i50Hzshall


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not saturate the reactor core, since the bi-filar winding

compensates the magnetic flux induced by the go and

return current. However, the residual flux may saturate

the core, if the reactor rated current is exceeded. A

saturated core reduces the reactor inductance and shifts

the attenuation characteristic toward a higher frequency.

Besides, the saturated reactor core contributes to thetest-voltage waveform distortion.

Distortion of the test voltage waveform by harmonics

One of practical problems to be solved by an industrial

test laboratory is to maintain a spectrally pure waveform

of the test voltage.

Usually the HV laboratory is located next to production

area, supplied from the same substation, and sometimes

even from the same transformer. A thyristor controlled

drive of large machines, and other non-linear loads,

distort the current, and in consequence the supply

voltage waveform.

This distortion is transferred to the test voltage andcorrupts PD measurements. For instance, the PD

intensity in a gas bubble trapped in solid insulation

increases with the test voltage steepness. An increasing

content of higher harmonics results in a steeper voltage

slope, and in consequence in a higher PD intensity. This

problem has been investigated [13], and a typicalexample of such distortion is shown in Fig. 5.

Fig. 5. (After [13]). An effect of the test voltage

distortion on the PD charge distribution Q=f1()

and frequency of occurrence N=f2(). On the left

side the Total Harmonic Distortion is negligible

(THD=0.7%). On the right side a high content of

fifth harmonics results in THD=10%.

To eliminate this problem at the source, Bydgoszcz

Cable Works has installed a separate power transformer

to supply the HV laboratory.

Faraday cage grounding

The electromagnetic shield is connected to a group of 5grounding rods driven to the depth of 20 m, and

insulated from the topsoil layers to the depth of 6 m.

The grounding rods are star-connected and bonded to

the shielded enclosure in one point. This joint allows for

periodic checks of the grounding resistance (that does

not exceed 1 even during the dry season,) and also for

checking the insulation between the shielded enclosure

and locally grounded objects.The upper soil layers conduct ground currents, and in

particular the higher harmonics of the power frequency

current. Such harmonics are generated by thyristor

controlled loads, such as electric motors, or rectifiers

[14].

Fig. 6. Supply circuit of thyristor controlled drives, and

the higher harmonics current flow path. The

harmonics and transient disturbance current flow

through the upper soil layers, as well as in structuralmembers of steel buildings.

It should be noted that the third and ninth harmonic

current flow by the three phase conductors in the same

phase. In consequence, these harmonic currents flow inthree-phase conductor in such a way, as if they were one

common conductor, i.e. homopolarly, as it is

schematically shown in Fig. 6.

This current returns to the supply source (power

transformer in substation) by the zero wire, and also

through the grounding system that is parallel to zero

conductor. At higher frequency the zero conductor

impedance is in general higher than that of the ground

return. For instance, the reactive impedance Xoof 300 m

section of zero wire attains Xo=2350[Hz]1[H/m]

300[m] 0.3 at the third harmonics, and at the ninth

it approaches 1.

Grounding system in general has lower impedance, andthe higher harmonics, as well as transient common

mode disturbance currents tend to flow through the

upper soil layer. Such transients are generated by arc

welding, sparking at the breaker or disconnect contacts,sparking of the electric traction locomotive and similar

events.

Any metal object buried in the ground offers even lower

impedance path and attracts the ground currents. For

instance, a steel structure of the HV hall attracts the

higher harmonic currents, as well as any metal cableshield, or a pipe buried in the ground, as it is sketched in

Fig. 7.

Random occurring disturbances sometimes recorded bythe PD measuring instrument come to the test set-up


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through the harmful coupling between the

electromagnetic shield and the factory grounding

system. Another path leads through the RF filters, if

they are saturated by an excessive load-current.

Fig. 7. Paths of the ground return current flow in the

upper layers of soil. To prevent this current fromflowing in the Faraday cage it has to be insulated

from local groundings, and connected to a deep

driven grounding rod.

Conclusions

Partial discharges measured in a HV laboratory

designed to test power transformers and XLPE cables

have a distinctly different intensity. In consequence, the

background disturbance level acceptable in these

laboratories is also quite different. The most severe

requirements are specified for XLPE power cables, andthe cable test laboratory has to be equipped with a high

efficiency electromagnetic shield. An electromagnetic shield of a large HV test hall

represents a large investment for the factory, and has to

be carefully designed to attain the required, lowdisturbance level. In the case of a power transformer

factory the shield can be made of steel panels welded

together in regularly spaced spots. However, a XLPE

cable factory had to acquire a professionally made

shield, with carefully designed seams and joints.

In addition to the disturbance signals inducedby an

external electromagnetic field, disturbances may be

conducted by supply, control and measuring cables that

enter the shielded enclosure. An effective system of

filters has been installed, and every wire that penetratesthe shield passes through such filter.

Thyristor controlled drives and other non-linearloads generate higher harmonics of the current drawn

from the local supply circuit. These result in a distortion

of the supply voltage, and in consequence of the test

voltage waveform. PDs measured in HV insulation

depend on the voltage steepness, and a distorted test

voltage waveform results in an exaggerated PD

intensity. To reduce the harmonic distortion, a separate

power transformer was installed to supply the HV test

laboratory directly from the substation.

Higher harmonics and transient disturbance

currents are conducted through the factory groundingsystem. A large part of such currents circulates in the

topsoil layers. To suppress such disturbance, the

electromagnetic shield has been insulated from the

factory grounding system and connected to a group of

deep driven rods insulated from the topsoil. A specialized supplier has delivered a complete setof PD-free regulated transformer and reactors, as well as

a modern computer-assisted recording instrumentation.

These are pre-requisites to achieve the sought, very low-

level background disturbances.

References

1. G., Karady, N., Hylten-Cavallius, Electromagnetic

Shielding of HV Laboratory, (IEEE Trans. Vol.

PAS-90, No. 3, May/June 1971, p. 1400).

2. H., Wesolowski, A., Rynkowski, New HV

Laboratory of Bydgoszcz Cable Plant, (V

Conference Power and communication cablelines, Zakopane, March 1990, p. 265).

3. G., Vaillancourt, R., Malewski, D., Train,

"Comparison of Three Techniques of PartialDischarge Measurements in Power Transformers",

(IEEE Transactions, Vol. PAS-104, No. 4, 1985,

pp. 900-909).4. F., Rizk, Y., Gervais, H., Lhrmann, Performance

of Electromagnetic Shields in HV Laboratory,

(IEEE Trans. Vol. PAS-94, No. 6, 1975).

5. B., Don Russel, W.C., Kothenheimer, R.,

Malewski, Substation Electromagnetic

Interference. Part 2: Susceptibility Testing and EMI

Simulation in High Voltage Laboratories,(IEEETransactions, Vol. Vol.PAS-103, No. 7, p. 1871-

1878)

6. J., Miedziski, Electromagnetic Screeing Theory

and Practice, (The British Electrical and Allied

Industries Research Association, Report #M/T135,1959, Leatherhead, UK).

7. Shielded Enclosures, (catalogue of Universal

Shielding Corporation, US, 1996).

8. International Electrotechnical Commission, (Publ.

1000-4, -3; Publ, 461/462D).9. American National Standards Association, (Std.

C63.4).

10. Department of Defense, US (Military StandardMIL-258,).

11. Polish Standard Committee, (National Standard

PN-E-90410: 1994)12. AC Voltage Test System Model RSZ, (Haefely

catalogue #E152.50, Basel, 1994).

13. M., Florkowski, Distortion of Partial Discharge

Images caused by High Voltage Harmonics, (10th

International Symposium on HV Engineering,

Montreal, 25-29 August, 1997, tom 4, p. 95.)14. R., Redl, P., Tenti, Daan J., van Wyk, Power

Electronics Polluting Effects, (IEEE Spectrum,

May 1997, p. 33).

pd measurement

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