2017 Electrical Insulation Conference (EIC), Baltimore, MD, USA, 11 - 14 June 2017
Review of Field Acceptance Hipot & PD Testing of
Medium Voltage Underground Cables
Ali Naderian Jahromi, SM IEEE
METSCO Energy Solutions Inc.
potential, in any form, to XLPE cables as it is detrimental to
the insulation and to cable life due to possible trapped space
charges. The other disadvantage of DC Hipot is that it is blind
to certain types of defects including voids. Another
disadvantage of DC Hipot is that it is not possible to do PD
measurement. Moreover, the DC stress distribution is sensitive
to temperature.
Abstract—There are some well-defined guides and standards
regarding factory testing of medium voltage shielded cables in
laboratories provided by IEEE, IEC, ANSI/ICEA, and CSA.
These guides cover AC withstand and PD testing of XLPE and
EPR cables and accessories, such as 50/60Hz AC Hipot and PD
tests which are routinely conducted on every reel of cable
produced at the factory. All terminations, splices, and separable
connectors are tested with more or less the same test method
using different test levels and timing.
On the other hand, there are utilities that have been performing
50/60Hz AC Hipot for several decays for commissioning new
cables. In the past 15 years, several other Hipot systems
including Very Low Frequency (VLF), Variable Near Power
Frequency (VNPF), and Damped AC (DAC) have been
developed and promoted by test manufacturers and test
companies worldwide. DAC is a withstand test which is
described by IEEE Std 400.4. Appendix A lists test voltages
not yet acceptable standard values due to the lack of experience
[17]. Dissipation factor measurements are also used to assess
cable condition.
There is, however, limited literature and guidance available
to clarify the requirements of onsite Hipot & PD testing. IEEE
400.2 has recommendations regarding VLF Hipot testing. IEEE
400.3 provides a general guide for onsite PD testing of
underground cables, but does not cover specifics such as the
Hipot level, timing, acceptable PD levels, PDIV, and PDEV for
each different voltage class. It is critical that owners and test
crew understand the different available options for AC Hipot
and PD testing of new and aged underground cables.
This paper reviews and compares available literature and
guides related to offline Hipot and PD testing including both VLF
and 50/60 Hz AC for Polymeric Cables. The objective is to help
end users, asset managers, consultants, and service companies to
choose the correct test method, parameters and procedures.
Keywords—AC Hipot, VLF Hipot,
Underground Cables, PDIV, PDEV.
Partial
The field Hipot test has been around for 50 years yet there
are challenges and uncertainties involved in today’s testing
practices. Although different Hipot test methods are available
and introduced by IEEE 400.2 and IEEE 400.3 for field testing,
no comprehensive guides are available which provide details
on performing and interpreting the results of Hipot and PD
tests, especially PD inception voltage (PDIV) and PD
extinction (PDEV). This paper reviews available guides for
performing offline AC Hipot and PD tests for commissioning
of underground cables. It also provides recommendations with
regards to test parameters based on industry practice and
expertise of the author.
Discharge,
I. INTRODUCTION
Standards such as IEEE, IEC, ANSI/ICEA, and CSA
recommend high voltage withstand tests to manufacturers of
polymeric cables to be done in the factory [1-4]. There are
several other IEEE and IEC standards that cover AC withstand
testing of the cable accessories including terminations,
separable connectors, and splices [5-8]. The onsite high
voltage withstand test is often called the Hipot test by
contractors, utilities, and test crews. The goal of Hipot testing
is to find any weak insulation point of the underground cable
circuit including the termination, cable, and splices. The ability
of the AC withstand tests to find installation problems will
reduce the premature failure rate which has historically
occurred in new stations. The data provided by the partial
discharge (PD) test will also serve as a useful baseline for
trending the deterioration of such equipment during service.
II.
Table 1 shows a summary of high voltage withstand and
partial discharge tests recommended by different standards for
underground cables [1-4]. Table 2 provides a summary of tests
for accessories such as terminations and splices [5-8]. U0 is the
cable rated phase-to-ground rms voltage and UC is the cable
rated voltage, where UC = 1.73U0.
According to both IEEE and IEC standards for factory PD
testing, the voltage shall be raised to 20% above the specified
partial discharge voltage level. If partial discharge exceeds the
limit of 5 pC (IEEE) or 10 pC (IEC), the test voltage shall be
lowered to the partial discharge voltage level specified and
shall be maintained at this level for at least 1 minute. If partial
discharge readings still exceed the limit, the cable accessory
does not meet the requirements of this standard.
Traditionally, DC Hipot testing has been employed for
different types of medium voltage (5kV to 44kV) and high
voltage cables (66kV to 230kV) including XLPE, EPR, or oil
filled cables. The new industry standards such as ICEA S-94649 and ICEA S-97-682 now warn against applying a DC
978-1-5090-3967-8/17/$31.00 ©2017 IEEE
FACTORY ACCEPTANCE TEST
241
determining the economic replacement interval for deteriorated
cables. Field diagnostics are performed periodically thereafter
during service (maintenance testing) to determine if the cable
system remains reliable.
Manufacturers typically have an electromagnetically
shielded high voltage lab to shield EMI coming from the
outside, and an input filter to provide a clean power supply.
This helps to minimize the background noise usually to be in
the range of 3 pC or less. In order to satisfy the 5pC
requirement of the IEEE, the measurement background noise at
test voltage has to be 2.5 pC which is usually hard to achieve.
In this regard, the IEC requirement of 10pC is more practical.
U0
[kV]
2.9
Table 1 Factory high voltage withstand test for cables rated 5kV to 46kV,
100% insulation – 5 minutes
ICEA S-94-649
IEC 60502-2
CSA C68.3-97
UC
ICEA S-97-682
Rated
voltage
PD
PD
50/60
PD
50/60Hz
50/60Hz
[kV]
EV
Hz AC
AC
AC Hipot
[kV]
Hipot
Hipot
[kV]
[kV]
[kV]
18 (6U0)
18
10
5
11.5
15
35 (4U0)
14.4
25
52 (3.6U0)
16.2
28
56 (3.5U0)
8.7
A. 50/60Hz AC Hipot
The size of the power frequency AC Hipot equipment can
be substantially reduced by using the principle of resonance. If
the effective capacitance of the cable is resonated with an
inductor, the multiplying effect of the resonant circuit, often
called the Q factor, allows the use of a smaller test source. In
the ideal case of perfect resonance, the test source will only be
required to supply energy to balance the true resistive loss in
the inductor and cable system. In practical resonant test
systems, provision is made for varying both the magnitude of
the inductance and the system voltage. IEEE 400-2001
proposed duration of acceptance testing on a new cable system
to be 15 minutes but does not specify the waveform type or
type of AC test voltage.
20.2
35
69 (3.5U0)
26.6
46
89 (3.3U0)
5pC
at AC
Hipot
Test
Level
30
50
56
70
10
pC
@
1.7U0
-
35
35
52
52
56
56
69
69
89
89
Unfortunately, IEEE 400 does not specify any 50/60Hz test
voltage or PDEV/PDIV for a given voltage class. Historically,
Ontario Hydro has implemented guides to perform power
frequency AC Hipot and PD tests [13]. NETA proposed that
the user can choose between DC Hipot, AC Hipot, and VLF
Hipot to test underground cables. NETA does not differentiate
between XLPE, EPR, and PILC cables. NETA does not specify
the duration of the tests but it refers to ANSI/ICEA S93-639,
and ANSI/ICEA S-94-649 [12]. NETA does not specify the
PDEV for 25kV and higher-rated voltage cables. Ontario
Hydro’s recommendation is currently used by some utilities in
Canada and suggests to use 70% of the factory test levels for
field testing [13]. On the other hand, Australian electric utility
Ergon Energy refers to IEC 71-1-1993 insulation co-ordination
and has proposed to choose AC Hipot test voltage equal to the
“standard short duration power frequency withstand voltage”
specified by IEC 71-1 for each voltage class. Table 3 provides
a summary of available guides for 50/60Hz AC Hipot test. The
test voltages specified by NETA and Ergon Energy are very
close to each other and to 100% of the factory acceptance test
levels presented in Table 1.
Table 2 Factory high voltage withstand test for accessories rated 5kV to 46kV
U0
[kV]
UC
Rated
voltage
[kV]
IEEE 48
Terminations &
IEEE 404 joints
PD
50/60Hz
AC Hipot
[kV1min]
2.9
5
18
8.7
15
35
14.4
25
52
16.2
28
-
20.2
35
69
26.6
46
80
5pC
at
1.5U0
IEC 60502-4
Both terminations
& joints
PD
50/60Hz
AC
Hipot
[kV]
IEEE 386
Separable
connectors
PD
50/60 Hz
EV
AC
[kV]
Hipot
[kV 1min]
-
4.5 U0 5 min
and
4 U0 -15
min
34
10pC
@
1.7U0
40
50
3pC
@
1.3U0
50
-
III.
Table 3 Field 50/60Hz AC Hipot test levels for cables rated 5kV to 46kV,
100% insulation
Ontario Hydro
Ergon EnergyNETA [12]
UC
U0
[13]
Australia [14]
Rated
voltage
[kV]
PD EV
PD
AC
AC
PD
AC
[kV]
[kV]
Hipot
Hipot
EV
Hipot
[kV]
[kV]
[kV]*
[kV*
5min]
9
18/28
2.9
5
4.5
20
4
ONSITE HIPOT TEST
Although medium voltage cables are carefully tested by the
manufacturer, defects such as damage during shipment,
storage, incorrect cable installation, and defects in accessories
may occur. Field testing of completed circuit installation
including joints and terminations prior to being placed in
service provides a piece of mind to utilities, asset owners, and
others involved in a new installation. The field Hipot and PD
test is intended to assure owners that the cable and accessories
were not damaged during transportation, handling, and
installation and the circuit is ready for energization.
Additionally, users find that with time, these cable systems
degrade and service failures become troublesome [10]. The
desire to reduce or eliminate cable failure has led cable users to
perform periodic tests after some time in service. As well,
cable users need special diagnostic tests as an aid in
8.7
15
19
14
33
14.4
25
17.5*
24
50
-
35/44
11
52
-
16.2
28
39*
26
-
56
-
20.2
35
48*
33
70
69
-
26.6
46
62*
44
90
89
-
* calculated based on 70% factory test levels provided in Table 1
** calculated based on 95% UC
TXU Electric and Gas has implemented 60Hz AC Hipot at
242
3Uo [15]. This utility suggested that 60 Hz testing may be
used successfully to determine the condition of cable and
accessories, but that the cable is more susceptible to failure
than the accessories [15].
-
B. VLF Hipot
Using very low frequency (VLF) high voltage sources has
some advantages for cable testing mainly due to weight and
cost reduction. The power requirement of the capacitive load
drops by a factor of 500 when comparing the commonly used
0.1Hz with the 50/60Hz AC Hipot test. Two common VLF
waveforms are as follows:
-
-
-
IEC 60840 provides a guideline for field AC Hipot testing
of cables 30kV to 150kV [11] which allows using a NPF
power supply with frequency of 20Hz to 300Hz. The field
Hipot test level for 46kV class cables is 52kV for 1 hour.
Allowable PDEV is 39kV. The suggested Hipot test voltage of
1.7U0 and PDEV of 1.5U0 is in line with the IEC 60502-2
factory test levels.
Cosine-rectangular VLF AC voltage waveform
Sinusoidal VLF AC voltage waveform
IEEE 400.2 is the guide for VLF Hipot testing of medium
voltage underground shielded cables [11]. It introduces three
testing voltage levels (installation, acceptance, and
maintenance). The suggested “acceptance test voltage” is based
on the most used practices of 2U0 to 3U0, for cables rated
between 5kV and 69kV. The recommended maintenance test
level is 75% of the acceptance test level. The IEEE 400.2
recommended minimum testing time for an acceptance VLF
withstand test on new cable circuits is 60 minutes. The test
time can be reduced to 30 minutes if the monitored
characteristic such as voltage, leakage current, and PD remains
stable and no failure occurs [11]. A 2006 CIGRE study showed
that the VLF withstand test is the most preferred Hipot test
method in USA [9].
IV. OFFLINE ACCEPTANCE PD TESTS
For acceptance testing of cables, it is very common to do
both Hipot and PD tests. IEEE 400.3 is the guide for partial
discharge testing of shielded power cable systems in a field
environment. Hipot and PD testing has been employed more
than before in the past 15 years. Colorado Springs Utilities,
Oklahoma Gas and Electric, PECO, Xcel Energy, TXU
Energy, BC Hydro, Hydro One, and Ontario Power Generation
have successfully implemented Hipot and PD testing programs
that help them to detect cable defects [15]. However, there are
concerns about the difficulty of PD assessment and the
accuracy of PD location needs improvement.
A. PD Limits
IEEE 400.3 proposed a minimum PDIV of 2U0 as an
example.
Regarding
PDEV,
the
only
available
recommendation is from Ontario Hydro [13] suggesting
PDEV = 0.95UC = 1.65U0. Ontario Hydro’s recommendation
for onsite maximum PD is 3pC, and IEEE 400.3 recommends
5pC. These levels of PD are typically very hard to achieve
onsite due to the power supply or background noise.
Table 4 Acceptance VLF Hipot test for cables rated 5kV to 46kV
IEEE 400.2
Utility A
NETA
UC
U0
Rated
Sine
Sine
Sine wave
Cosine
voltage
[kV]
wave
wave
[kVrms]
rectangle
[kV]
[kVrms]
[kVrms]
Wave
[kVrms]
10 (3.4U0)
2.9
5
14
10*
10
8.7
15
21 (2.4U0)
30
20
14.4
25
32 (2.2U0)
45
32*
31
16.2
28
36 (2.2U0)
51
34
34*
20.2
35
44 (2U0)
62
45*
44
26.6
46
57 (2.14Uo)
81
60
55*
It has no moving parts and no mechanical systems to
maintain. The core can be designed with a distributed gap
to reduce the size and weight.
It has typically higher Q (~80 to 150) compared to 60Hz
variable inductor systems (Q=25 to 50) resulting lower
input power.
By reducing the test frequency, a larger capacitive load can
be tested.
20
B. PD Frequency
Most of above standards refer to IEC 60270 as a classic
PD measurement protocol, with a center frequency of around
300kHz and a bandwidth of 300kHz (f = 300kHz ± 150kHz).
IEC 60270 frequency bandwidth applies to lumped
capacitances as per this standard. IEC frequency may not be
suitable for long run of cables. Higher frequencies of typically
3MHz to 20 MHz are recently used for field testing. The test
frequency shall also be selected by looking at the FFT of the
background noise to search for low ambient noise. A proper
sensitivity check and calibration is required for any selected
PD test frequency.
* calculated by extrapolation of adjacent data (lower and higher numbers)
C. Variable Frequency Hipot
Variable Frequency AC Hipot, or near power frequency
(NPF) AC Hipot, has been implemented in the past 10 years
due to the progress of power electronics devices. The test setup
has a fixed reactor fed by a variable input frequency AC/AC
converter with an output frequency between 20Hz and 300Hz.
Once the frequency of the input power is tuned to the
resonance frequency by an initial frequency sweep, the voltage
is raised to Hipot or PD test levels. The test frequency depends
on the capacitance (i.e. cable length, insulation material,
thickness, etc). The main advantages of NPF power supplies
are as follows:
C. Background Noise
PD background noise can come from different sources:
- Input power such as portable generator or power plug
- Environment EMI such as corona, working machinery at
the site, rain, and damped weather
- Testing transformer
243
Table 6 Recommended VLF Sine Hipot & PD acceptance test voltages for
polymeric cables
Air termination
Separable connectors
PDEV
AC
PDEV
Rated
AC
U0
[kV]
withstand
[kV]
voltage
withstand
1.5U0
[kV]1.3U0
[kV]
[kV] [kV]
30 min
30 min
10 min
10 min
10 (3.4U0)
5*
2.9
5
-
Background noise can be permanent or intermittent. It is
possible to reduce the power supply noise by using an input
isolating transformer as well as a high voltage filter at the
transformer output. If the background noise is still high due to
the nature of the environment, the PD measurement frequency
has to be adjusted.
D. Onsite PD localization
One of the challenges of onsite PD measurement is
localization in the event of PD presence. To fix the problem it
is necessary to determine if the PD is in the terminations, or
splice, or inside the cable. Most PD equipment is equipped
with PD time domain reflection analysis that allows users to
estimate the location of the PD and see how far it is from the
source.
V.
8.7
15
21 (2.4U0)
13
21
11
14.4
25
32 (2.2U0)
22
25
18
16.2
28
36 (2.2U0)
24
28
21
20.2
35
44 (2U0)
30
35
26
26.6
46
57 (2.14Uo)
40
-
-
*selected equal to Uc
RECOMMENDATIONS AND CONCLUSION
In the absence of this data, it is proposed to use PDEV of
1.5U0 for air terminated cables and 1.3U0 for separable
connector terminations. More systematic research is necessary
to improve the proposed test parameters. It is expected that
IEEE 400 series standards will improve and cover the existing
gaps.
The recommendation is based on available reviewed
documents and limited experience of the author. Table 5
summarizes the recommendation for field AC Hipot and PD
acceptance tests for XLPE and EPR cables. The recommended
acceptance level for 50/60Hz AC Hipot testing is 70% of the
factory test voltage for 5 minutes. The test voltage must be
selected based on the termination type. The recommended
minimum PDIV is 2U0 proposed by IEEE 400.1. The
recommended PDEV is based on the termination PDEV
proposed by IEEE 48 for air-terminated and IEEE 386 for
separable connectors which is 1.5U0 and 1.3U0, respectively.
10pC is recommended as a limit with a maximum of 5pC of
background noise. In case of NPF variable frequency, the
same test voltage can be used (Table 5). If 5pC of background
noise is not achievable, it might be possible to select a better
PD frequency range to achieve a reasonable PD background.
VI.
[1]
[2]
[3]
[4]
[5]
[6]
The VLF sinusoidal AC withstand test voltage for air
terminated cables is selected based on IEEE 400.2. The test
voltage for cables with separable connectors in Table 6 is
selected based on the cable line-to-line voltage rating. The
proposed 30-minute test time is accepted when the monitored
PD and leakage current does not increase during the 30-minute
test. Otherwise, the VLF AC Hipot will continue for 60
minutes. Although there are several publications comparing
VLF and 50/60Hz PD data, there is not enough information
regarding VLF PDEV and PDIV.
[7]
[8]
[9]
[10]
[11]
[12]
Table 5 Recommended field AC Hipot &PD acceptance test voltages
Air termination
Separable connectors
PDEV
PDEV
AC
UC
AC
U0
[kV]
[kV]
withstand
withstand
Rated
1.5U0
1.3U0
[kV] [kV] voltage
[kV]
5min
5min
[kV]
1min
1min
5
2.9
5
12.5
5
[13]
[14]
8.7
15
24.5
13
24
11
[15]
14.4
25
36.5
22
28
18
[16]
16.2
28
39.0
24
28
21
20.2
35
48.0
30
35
26
26.6
46
62.0
40
-
35
[17]
244
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ANSI/ICEA S-94-649-2013- STANDARD FOR CONCENTRIC
NEUTRAL CABLES RATED 5 THROUGH 46 kV.
ANSI/ICEA S-97-682-2013- STANDARD FOR UTILITY SHIELDED
POWER CABLES RATED 5 THROUGH 46 kV.
IEC 60502-2-2005 Power cables with extruded insulation and their
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CSA-C68.3-97 - Shielded and Concentric Neutral Power Cables Rated
5-46 kV.
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