Measurement of partial discharges in medium voltage cable lines
MEASUREMENT OF PARTIAL DISCHARGES IN MEDIUM VOLTAGE CABLE LINES
Sławomir Noske / ENERGA-OPERATOR S.A. Elbląg Branch
PARTIAL DISCHARGES
A partial discharge is limited in space to an electrical discharge that partially bridges an insulator. It is
caused by a localised concentration of voltage within an insulator or on its surface. When the local electric
field exceeds the value of inception voltage, a first seed electron starts an electron avalanche. Fig. 1 shows an
equivalent circuit with a void where a partial discharge occurs. Partial discharges vary, depending on a number
of factors which include the type of insulation as well as the location and geometry of the voids.
Cb
Cc
Fig. 1. Equivalent circuit diagram of cable line with a void in the insulation where capacities Cc, Cb represent insulation located serial and parallel
to the void.
The probability of discharges occurring may, among other things, depend on the type insulation. Partial
discharges may result from the insulation’s deteriorating state. Likewise, the degradation of insulation quality
may result from the partial discharges, i.e. insulation material degradation and partial discharges mutually affect one another. On account of the fact that partial discharges occur in association with the deteriorating state
of the insulation, they are harmful and through repeated occurrences frequently lead to power failure. They
damage insulation when, for example, high energy electrons break chemical bonds in the polymers or through
the aggressive effects of chemical decay resulting from the break up of molecules.
Abstract
Medium voltage cable lines are an important element
of electricity distribution company property. They are used
to supply electricity to highly urbanised areas, where users
are particularly sensitive to power failures and the consequences are very expensive to remedy. The management
of such networks has so far chiefly relied on the analysis
of failure frequency. Increasing failure frequency has been
regarded as the deciding factor in assessing a given power
cable line’s technical condition.
Now developments in measuring apparatus allow
for new ways of examining the exploitation of power networks, providing a greater understanding of their technical
state. Thanks to the new information, it is now possible to
improve the operation and management of cable networks.
One can now examine the technical condition of the insula-
tion of various elements within a power cable line. One of
these modern methods involves the measurement of partial
discharges.
105
106
Sławomir Noske / ENERGA-OPERATOR S.A. Elbląg Branch
Rozwój informatyczny i technologiczny w ostatnich dekadach XX wieku pozwolił na budowę
przewoźnych
urządzeńDISCHARGES
dających możliwość pomiaru wyładowań niezupełnych w eksploatowanych
MEASUREMENT
OF PARTIAL
Partial discharges were recognised as harmful to insulation already at the start of the 20th century, when
sieciach energetycznych.
high voltage devices started being used. The first industrial tests of partial discharges were carried out in 1940.
Developments
in technology
and computer science
the last decades
of the 20th(wnz)
century
the
Jednym z rozwiązań
pozwalających
mierzyćinwyładowania
niezupełne
w allowed
liniach for
kablowych
construction of devices able to measure partial discharges in active power distribution networks.
napięcia
jest for
system
pomiarowy of
OWTS-25
firmy SEBA
OWTS
obecnie
Oneśredniego
of the solutions
allowing
the measurement
partial discharges
(PD) inKMT.
medium
voltagejest
cables
is the Seba
KMT OWTS-25 system.
OWTS is currently used bySA
theOddział
Elbląg branch
of ENERGA-OPERATOR S.A.
wykorzystywany
przez ENERGA-OPERATOR
w Elblągu.
Rys. 2. Wóz pomiarowy z zainstalowanym systemem OWTS-25
Photo. 1. Cable test van using the OWTS-25 system
System ten wykorzystuje jako napięcie probiercze samogasnacą falę napięciową. Wytwarzana
jest ona poprzez ładowanie badanego kabla do pożądanego napięcia, a następnie rozładowanie go
This system uses as the test voltage a self-extinguishing voltage wave. It is produced by raising the tested
cable voltage przez
to a required
levelzaprojektowaną
and then discharging
through an air-core
coil. The voltage
frequency
specjalnie
cewkęit bezrdzeniową.
Częstotliwość
napięcia
zależnadejest od
pends on the cable’s capacitance and the coil’s inductance. In tested cables this frequency is generally within
W praktyce,
dla badanych
liniilosses
kablowych
mieści się
pojemności
kabla i indukcyjności
cewki.
the range of 200–800
Hz. Suppression
of the voltage
amplitude
is associated
with dielectric
in the tested
cable.
ona w granicach 200–800 Hz. Tłumienie zanikającej amplitudy napięcia związane jest ze
stratami dielektrycznymi w badanym kablu.
1
f 
2π LC
OWTS mierzy falę napięciową powstałą od ładunku indukowanego podczas wyładowania
OWTS measures the voltage wave from the load induced during the partial discharge. It spreads in two
niezupełnego.
napięciowa
rozchodzi
sięheading
w dwóch
Aparatura
mierzy
falęvoltage
biegnącą w
opposite directions.
The Fala
device
first measures
the wave
forkierunkach.
the cable end
connected
to it (the
wave proportional
the load
produced
during the
PD) napięciowa
and next the proporcjonalna
other wave after do
it has
rebounded
kierunku tokońca
z value
podłączoną
aparaturą
(fala
wartości
ładunku
from the other end of the cable. Measurement of the time difference between the two waves allows us to locate
powstałego
podczas
wnz),
a następnie
odbitącalibration
od drugiego
końca
kabla.toPomiar
exactly where
the partial
discharge
occurred
(Fig. 3).falę
Pre-test
allows
the device
measureróżnicy
PD loadczasu
intensity in picoCoulombs (pC). This calibration is conducted in accordance with norm IEC 60270.
między pomierzonymi falami pozwala określić miejsce wystąpienia wyładowania niezupełnego (rys.
3). Dokonana przed pomiarem kalibracja pozwala na wyskalowanie aparatury i pomiar
intensywności wyładowania wnz w pikokulombach (pC). Kalibracja dokonywana jest zgodnie z PNEN 60270 (tłumaczenie międzynarodowej normy IEC 60270).
Measurement of partial discharges in medium voltage cable lines
l
q/2
l -x
q/2

x
x

q/2
q/2

l
l


l -x
l
t
Detekcja

wnz

t
Detekcja
wnz

Fig. 2. Diagram of wave propagation resulting
from a partial discharge
Rys. 3. Schematyczny obraz rozchodzenia się fali powstałej w wyniku wyładowania
niezupełnego
t1 
t2 
x
v
l  x
Δt t 2  t1
x
l  v Δt
2
v
Analiza i diagnoza wyładowań niezupełnych
ANALYSIS AND DIAGNOSIS OF PARTIAL DISCHARGES
Analysis of OWTS test results provides information on the discharge distribution along the cable. The
Analiza wyników
basic information
includes: pomiaru dokonanych systemem OWTS dostarcza informacji o rozkładzie
• The
partial discharge
inception
voltage
wyładowań
w funkcji
długości
kabla. Do podstawowych informacji uzyskanych z analizy
• The maximal and medium value of the apparent charge with the incipient voltage of partial discharges.
należą:
‘The
PD impulse apparent charge is the load that when injected very briefly between the terminals of
a testing
causes
the same reading
on the
measuring device as the actual PD impulse. The apparent
 device
Napięcie
początkowe
wyładowań
niezupełnych
charge is expressed in picocoulombs (pC) [3]
 Wartość
średniavalue
i maksymalna
ładunku
pozornego
przy
• The maximal
and medium
of the apparent
charge with
a Uo value
testnapięciu
voltage początkowym
• PD occurrence
frequency
with a Uo value
test voltage
wyładowań
niezupełnych.
„Ładunek
pozorny impulsu wnz odpowiada ładunkowi,
• The maximal and medium value of the apparent charge with a test voltage between Uo and 2Uo
który frequency
wstrzyknięty
w bardzo
między zaciski obiektu badanego
• PD occurrence
with a test
voltagekrótkim
between czasie
Uo and 2Uo
• Graphs linearly mapping the distribution of discharges throughout the entire cable (Fig. 4)
• Anomalies in the number and level of PDs with a test voltage up to Uo or between V0 and 2Uo. This
parameter is not automatically defined in the test analysis process but by the person analysing the test
results on the basis of obtained graphs linearly mapping discharge distribution along the entire cable. Increased PD occurrence in a given fragment of a cable section that clearly stands out from the level of PD
activity in the rest of that section is considered an anomaly. In such cases the analyst defines the maximal
value of the partial discharges and the location where they occur.
107
108
Sławomir Noske / ENERGA-OPERATOR S.A. Elbląg Branch
Fig. 3. An example of measurement results. Partial discharges are approximately located between the 210th m and 230th m of the cable (the distribution
of discharges along the entire cable are divided into each phase – every point represents a single PD and the discharge value is given in pC).
Acquiring analysis results using the length function allows us to view the cable as it really is, i.e. as a line.
This is a new quality in conducting cable tests
Analysis of PD measurement results provides a unique collection of data. By interpreting this data, one
can carry out a diagnosis concerning the technical condition of specific elements within a power line.
PARTIAL DISCHARGES IN PILC CABLES
At the ENERGA-OPERTOR S.A. Elbląg Branch PILC cables account for c. 65% of its MV cables. Currently no
new PILC lines are constructed as they are being replaced by XLPE cables. Thus the purpose of PD diagnosis in
PILC cables is to assess the technical condition of the insulation and track the ageing process.
Partial discharges in PILC cables vary in number and value over time. This phenomenon may be observed
thanks to on-line PD measurements of medium voltage networks. Fig. 4 is an example of a 3D diagram using
cable length and time functions to display PD measurements taken in Holland.
Fig. 4. 3D diagram presenting partial discharge
distribution in the length function of an on-line
measurement taken over a period of 8 days (with
a clear discharge level change in the time function)
Measurement of partial discharges in medium voltage cable lines
Fig. 5. Change in the level of partial discharges depending on the load level in the cable
20,00
20,00
18,00
18,00
16,00
16,00
14,00
14,00
12,00
12,00
10,00
l [km]
[km]
Figure 5 shows the dependence between the load in the cable and the value of partial discharges. The PD
level rises when the load (pressure and temperature in the cable) falls.
When diagnosing the technical condition of PILC lines on the basis of partial discharge measurements,
one needs to take into account that:
• Partial discharges may occur in the cable insulation, but their appearance is most usually of a dispersed
nature
• The level of discharges and their intensity varies in time and is dependent on the level to which the cable
is loaded. This considerably hinders tracing changes associated with the deteriorating state of insulation.
• Practical experience has shown that the level of discharges is dependent on the quality of the cable. In the
past a country’s economic condition was undoubtedly a factor determining the quality of cables. Therefore older cables are not always characterised by a higher level of PDs (the ageing structure of a PILC
cable line is shown in Fig. 6). The example of a line comprising two cable sections from different years
(with different PD levels and intensities) is presented in Fig. 7.
10,00
8,00
8,00
6,00
6,00
4,00
4,00
2,00
2,00
0,00
19
52
19
58
1919 6
52 1
1
19 9 63
1690
19 65
1693
67
191
6966
9
1919
6971
1
19 9 73
7
192
19 75
1795
77
191
7987
9
1919
8181
1
19 9 8
8 3
194
19 85
1897
87
191
9908
9
1919
9391
1
19 9 9
9 3
196
19 95
1999
9
201 7
0929
9
2020
0501
2
20 0 0
0 3
208
05
20
07
20
09
0,00
długość kabli PILC
Fig. 6 The length of PILC cables installed in the MV network in specific years by ENERGA-Operator SA Elbląg Branch.
109
110
Sławomir Noske / ENERGA-OPERATOR S.A. Elbląg Branch
Fig. 7. Cable line comprising two cable sections (A – HAKnFtA 3x120 mm2,
built in 1979; B – HAKnFtA 3x120 mm2, built in 1984). Section A – breakdown
voltage below V0, discharge values up to 8,000 pC, voltage up to 1.7V0 only
with individual discharges of c. 25,000 pC. Section B – no discharges with V0
voltage, the discharge level exceeds 100,000 pC with test voltage values up
to 1.7V0.
The above mentioned phenomena mean that diagnosing the state of insulation in these cables is difficult
and requires a lot of experience. The current state of research has not yet allowed us to determine a standard
PD level beyond which the operator should decide to change an entire section in a cable network. In assessing
the technical condition of cables one should pay particular attention to anomalies. In this case anomalies are
places where there is an increased level of PDs.
Despite the lack of hard and fast rules to determine the assessment of PILC insulation, there are methods
of facilitating decisions concerning operation. Below is an example of a cable line whose section, on the basis of
PD tests carried out after a power failure, is changed. In this case it was decided that in order to prevent further
power failures the application of a cable repair joint would be inadequate.
Example 1
Fig 8 shows cable line HAKnFtA 3x120 mm2, its route map and the results of its PD analysis. With a V0 voltage, two places on the cable appear to have increased PD levels (30–50 m and 280–300 m).
Fig. 8. The distribution of
partial discharges on the
examined cable line before
the power failure.
Measurement of partial discharges in medium voltage cable lines
The line was damaged at approximately metre 290. On the basis of acquired data, it was decided that that
the whole cable section from the 260 metre cable box to the substation terminal needed to be changed. The
repairs were not limited to the application of a cable joint. The results of a PD test carried out after the repair
are presented in Fig. 9. They show that there were no discharges in the repaired section and thus potential
areas of subsequent power failure were removed.
Fig. 9. The distribution of partial
discharges after the removal of the
faulty cable section (marked red)
PARTIAL DISCHARGES IN XLPE CABLES
The insulation of XLPE cables should be free of partial discharges. Any discharges would suggest faults
in the cable itself or the fittings. The hysteresis curve of the ignition and extinction of PDs in such cables shows
that discharges may be dangerous if their voltage is higher than the working voltage. If a partial discharges
ignite with a voltage higher than the rated voltage (e.g. when there is overvoltage in the network) they have to
extinguish once working voltage is returned (i.e. the extinction voltage is lower than the ignition voltage). One
should bear in mind that PD measurements may not identify all insulation faults, which may also be caused by
water treeing. This phenomenon is particularly observable in lines using non-cross-linked polyethylene insulation. The presence of moisture speeds up the insulation’s ageing process and thus damages the cable.
Despite these difficulties, PD measurements do provide new opportunities in acquiring fuller knowledge
of the technical condition of this type of cable line. Presented below is an example of PD measurements identifying an incorrectly installed cable joint. Such a fault cannot be detected by any other measuring method.
Example 2
A 515 m line using a XRUHAKXS 120 mm2 cable and comprising two sections (360 m and 155 m) that were
connected by POLJ-24 type cable joints installed by RAYCHEM and with POLT-24 type terminals also installed by
that company. Before the line became operational voltage test and leak test measurements were carried out,
which showed everything to be in good working order. Bearing in mind that the cable installation process was
fully supervised by the regional electricity distribution services and that the fittings were mounted by trained
and highly experienced personnel, one could confidently assume that the line was indeed free of any faults.
Nevertheless, a PD test proved that a fault in fact existed: there was an increased level of discharges at
the 360th metre in the L3 phase (Fig. 10).
111
Sławomir Noske / ENERGA-OPERATOR S.A. Elbląg Branch
112
Fig. 10. The distribution of partial discharges in the 360
m joint (L3 phase) with a test voltage value up to V0 and
also up to 2Uo
Partial discharges occurred in a place where two cable sections were joined. This indicated a joint fault in
the L3 phase. Partial discharges occur with V0 voltage and this causes the danger of power failure. On the basis
of explanations provided by the manufacturers of the cable joint the most probable reason for the fault was established: the fitters had failed to apply a heat shrink cover on one of the connectors. The line was built in 2005
and for three years it was subjected to regular tests which revealed no changes (rise) in the level of discharges;
the line appeared to be undamaged. In this case one may assume that insulation degradation proceeded very
slowly. In order to confirm the PD testing results, the faulty joint was cut out and examined in a Poznań University of Technology laboratory, using James G. Biddle Co. equipment. The test findings verified those obtained
from the OWTS equipment.
16000
16000
14000
14000
12000
12000
2005-06-01
10000
10000
2005-09-26
2006-10-02
8000
8000
6000
6000
2005-06-01
2005-09-26
2006-10-02
2007-01-25
2007-01-25
2007-09-12
2007-09-12
2008-02-12
2008-02-12
4000
4000
2000
2000
0
0
PD-Uośr
PD-Uo avg
PD-Uomax
PD-Uo max
PD-2Uośr
PD-2Uo avg
PD-2Uomax
Fig. 11. Comparison of PD measurement
test results, values given in pC
PD-2Uo max
In the laboratory registered discharges with a value of several score pC (whereas in situ measurements
registered discharges with a value of several hundred pC). With such values the initial voltage turned out to be
considerably lower than V0 (4 kV).
.
Photo. 2. Partial discharge image during
tests carried out on the faulty cable joint
using James G. Biddle Co. apparatus.
Measurement of partial discharges in medium voltage cable lines
Fig. 12. Revealed fault in cable coupling: a heat shrink cover had not
been applied.
EXPECTED IMPROVEMENTS IN THE MANAGEMENT OF MV CABLE LINES
The introduction of partial discharge diagnostics as a supplement to the testing of MV cables should also
involve a change in the management of MV lines. It is important that in appraising the technical condition of
a network other information acquired while it is operating is used (see Fig. 13).
PD Diagnostics
Faliure Analisis
Technical Parameters
MV cable network mamagement
Cost
(ERP system)
Development Plan
Network System
Fig. 13. Model of the integration of data (information technology systems) for the purpose of gathering comprehensive knowledge regarding cable
networks.
A new power cable network management system will allow for a reduction of costs in maintaining this
significant part of company property by:
• Lowering operational costs as a result of a reduced number of power failures. The ability to take preventive measures (replace sections with defective insulation before power failures occur) through observation
of the ageing of particular network elements.
• Raising the quality of new cable lines. The PD measurement method allows for the location of previously
unidentifiable mistakes made in the production and fitting of cable lines (thus replacing faulty elements
before a power cable line is made operational). Awareness of greater possibilities of monitoring installation work also naturally raises its quality.
• Reducing investment and repair costs through a proper prioritising of maintenance procedures (it is not
always necessary to change a whole cable section when the replacement of some faulty elements will
do), as well as deferring such costs thanks to a better knowledge of the actual state of cable line element
insulations (by observing the cable line ageing process).
Application of the above measures will ensure medium voltage cable networks of greater reliability and
thus increase user satisfaction. By reducing the number of power failures, the quality of customer service will
be improved and consequently meet standards set not only by law but also by company policy.
113
114
Sławomir Noske / ENERGA-OPERATOR S.A. Elbląg Branch
LITERATURE
1.Gulski E., Diagnozowanie wyładowań niezupełnych w urządzeniach wysokiego napięcia w eksploatacji, Warszawa,
Oficyna Wydawnicza Politechniki Warszawskiej, 2003.
2.Guide for partial discharge measurements in compliance to IEC 60270, CIGRE Working Group D1.33, 2008.
3.Noske S., Efektywne zarządzanie siecią kablową SN, Elektro-info, 2009, nr 1–2.
4.Noske S., Wykorzystanie diagnostyki opartej o pomiar wyładowań niezupełnych do zarządzania siecią kablową
średniego napięcia, Konferencyjne Infotech 2008.
5.PN-EN 60270, Wysokonapięciowa technika probiercza. Pomiar wyładowań niezupełnych, PKN, Warszawa 2003.
6.Rakowska A., Kryteria oceny weryfikujące jakość polietylenu usieciowanego stosowanego jako izolacja kabli elektroenergetycznych, Poznań, Wydawnictwo Politechniki Poznańskiej, 2000.
7. Rakowska A., Siodła K., Noske S., Wyniki badań wyładowań niezupełnych jako źródło informacji wspomagających
zarządzanie siecią kablową średnich napięć, Przegląd Elektrotechniczny, 2008, nr 10.
8.Van der Wielen P., Steennis F.: First Field Experience of On-line Partial Discharge Monitoring of MV Cable Systems
with Location, 20th International Conference on Electricity Distribution, Prague 2009.
9.Wester F., Condition Assessment of Power Cables using Partial Discharge Diagnosis at Damped AC Voltages, Optima
Grafische Communicatie, Rotterdam 2004.