FAQ's - sebaKMT

FAQ’s
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1. Is damped AC a new method for on-site testing? 2
2. Is damped AC accepted in the standards? 2
3. Is DAC (Damped AC) an alternative for continuous AC? 2
4. Is DAC electrical stress similar to DC stress? 2
5. Is damped AC more destructive as continuous AC? 2
6. Which testing parameters influence the test effectiveness of DAC voltages? 2
7. Which standards/ norms are valid for commissioning testing of HV-cables? 3
8. Are DAC voltages applicable for withstand testing Go / No-Go? 3
9. What are major advantages of using DAC? 4
10. What are the disadvantages of using DAC? 4
11. Which standards/ norms are valid for diagnostic testing of service aged HV-cables? 4
12. What is the difference between testing and diagnosis of HV and EVH power cables? 4
13. What are the PD acceptance levels for HV-cables and accessories? 5
14. What are the maximum test voltages for commissioning testing and diagnostic
testing?
5
15. Which OWTS HV system fits to my requirements? 6
16. What is the extra value of the integrated dielectric loss measurement? 6
17. What is the maximum length I can test with the OWTS? 7
18. What is the maximum length OWTS can detect and localize PD in a power cable? 7
19. What is the frequency of DAC voltages? 7
20. Is there a possibility to mount the OWTS into a truck/ container? 7
21. What is transport capacity of the OWTS (Volume/ weight)? 8
22. Can the system be transported with airfreight? 8
23. What is the setup time for a measurement? 8
24. Can you also test in indoor substations? 8
25. What are the minimum technical requirements to perform on-site testing using
OWTS?
9
26. Do cross-bounding joints have an influence on the measurement? 9
27. What is the self generated background noise of the system? 8
28. Is the PD-measuring circuit part of the equipment? 9
29. Is the PD measurement according to the international norm IEC 60270? 9
30. What is the power demand of the OWTS system? 9
OWTS ® HV Technology
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FAQ’s
1. Is damped AC a new method for on-site testing?
NO: The damped alternating voltage (DAC) is an oscillating switching impulse
voltage (OSI) of quite low damping (

FAQ’s
c) number of DAC voltage excitations appears to be an important parameter; the
breakdown voltage decreases as the number of excitations increases;
d) The DAC voltage charging time does not seem to have an important influence on
the breakdown processes;
7. Which standards/ norms are valid for commissioning testing of HV-cables?
Looking for standards important for on-site testing we have to make a distinction
between HV and EHV cables, HV cables are standardized as cables with a rated
voltage between 30kV and 150kV, EHV for cables with a rated voltage for 150kV and
higher.
Standards for (E) HV cable design and testing are: IEC 60060-3 High Voltage test
techniques –Part 3: Definitions and requirements for on-site testing
[1] IEEE 400 Guide for Field Testing and Evaluation of the Insulation of Shielded
Power Cable Systems
[2] IEEE 400.3 Guide for PD Testing of Shielded Power Cable Systems in a Field
Environment
[3] IEC 60840 Power cables with extruded insulation and the accessories for rated
voltages above 30kV up to 150kV Test methods and requirements;
[4] IEC 62067 Power cables with extruded insulation and the accessories for rated
voltages above 150kV.
In addition with regard to PD detection the following standards are of importance
[5] IEC 60270, Partial Discharges Measurements, 2000
[6] IEC 885-3 Test methods for Partial Discharges measurements on lengths of
extruded power cable
Moreover with regard to on-site testing and diagnosis several Cigre documents have been
published in the past e.g.
[7] CIGRE Electra 176, 1998, ‘’Diagnostic Methods for HV Paper Cables and
Accessories’’
[8] CIGRE Electra 208 Electra, 2003. “Experiences in partial discharge detection of
distribution power cable systems”
[9] CIGRE Brochure 226, 2004, ‘’Knowledge rules for partial discharge diagnosis in
service’’
[10] CIGRE Electra 226, 2006, ‘’Practical aspects of the detection and location of
partial discharges in power cables’’
8. Are DAC voltages applicable for withstand testing Go / No-Go?
YES: Applying DAC withstand test the cable section can be stressed with a selected
DAC voltage stress for selected time duration and the cable section can be rejected if
a breakdown has occurred.
Due to shorter duration of the excitations and decaying characteristic of the voltage it
is not directly comparable to withstand voltage testing with continuous AC.
OWTS ® HV Technology
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FAQ’s
9. What are major advantages of using DAC?
Application of DAC voltages has the following advantages:
1. Since several years accepted for on-site testing and PD measurements of all
types and lengths of power cables.
2. Simple, lightweight, cost effective equipment with very low input power
requirements and able to stress long cable lengths.
3. With regard to partial discharges similar AC electrical stresses as during factory
testing and service operation.
4. During over-voltage application the small number of cycles reduces the risk of
damage to the cable system.
5. Analyzing the damped AC voltage attenuation provides information about
dielectric losses (tan δ, ∆ tanδ).
10. What are the disadvantages of using DAC?
Actually there are no serious disadvantages in using DAC voltages. A number of
limitations can be mentioned:
1. Due to short duration of the excitations and decaying characteristic of the damped
AC voltage the breakdown field strengths are not directly comparable to the
breakdown stresses of withstand voltage testing with continuous AC.
2. To obtain PD occurrence conditions (e.g. PD inception, PD level) similar to 50
(60) Hz AC voltage stresses, the DAC frequency has to be below approx. 500 Hz.
3. Use of fixed inductor at different cable capacitances results in variation in
resonance frequencies.
4. In the case of very short cable length an additional capacitive load is
recommended.
5. The damped AC voltage decay characteristic depends on the actual dielectric loss
behavior of a particular cable section.
11. Which standards/ norms are valid for diagnostic testing of service aged HVcables?
Generally taken there are no standards/ norms for diagnosing HV-cables. However
experiences from diagnostic testing in the MV-network of over 10 years have proven
the need for diagnosis. When performing diagnostic testing one can monitor the
condition of the insulation, in other words follow the ageing processes of the
insulation. In the HV-application this is very useful to monitor oil-filled, oil-pressurized
or gas pressurized cables. From this type of cable it is generally known that they
show higher dielectric losses due to ageing and are reaching the end of there
estimated life time, so what to do with these cables?
12. What is the difference between testing and diagnosis of HV and EVH power
cables?
With regard to testing the following is important:
1. To detect insulation defects in HV and EHV power cables during on-site testing
voltages higher than Uo (over-voltages) can be used.
2. Voltage tests up to e.g. 1.7Uo to demonstrate that
OWTS ® HV Technology
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FAQ’s
a. the insulation is healthy (defect-free and/or non-aged) and that insulation
can withstand a high voltage stress level,
b. insulation is aged and/or consists of insulation defects should
have lower level of withstand voltage.
With regard to diagnosis the following is important:
1. PD detection up to e.g. 1.7Uo to demonstrate that the insulation is PD free by
determining the PDIV. The margin between PDIV and the Uo is a reliability
indicator of a particular cable circuit.
2. With regard to service aged power cables the dielectric loss measurement in
function of the test voltage up to e.g. 1.7Uo is very valuable indicator about the
thermal stability of a particular cable circuit (OF cables).
13. What are the PD acceptance levels for HV-cables and accessories?
Generally speaking HV-XLPE cables and accessories must be PD-free, not only in
case of newly installed cables but also for service aged cables.
Oil filled cables should be also PD-free, but at field tests some test objects have
shown PD in terminations and cable insulation. Today no threshold values for
acceptable PD level in oil filled cables are known.
14. What are the maximum test voltages for commissioning testing and
diagnostic testing?
When testing in accordance to the IEC 60840 for cables rated up to 150kV the
specified test voltages are listed below:
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FAQ’s
For cables rated higher then 150kV the test voltage has to be selected in accordance
to IEC 62067
or with 1.7U0, depending on practical operational conditions.
15. Which OWTS HV system fits to my requirements?
Depending on the maximum test voltages of a particular cable type a system can be
selected from the table below
OWTS ® HV Technology
Applicability for testing different power cable ratting voltages
Power cable rated Uo [kV] OWTS HV 150 OWTS HV 250 OWTS HV 350
voltages [kV]
X Uo
X Uo
X Uo
66 38,1 2,8
110 63,5 1,7 2,8
132 76,2 1,4 2,3
150 86,6 1,2 2.0 2,8
220 127 0.8 1,4 1,9
330 190,5 0.9 1,3
380 219,4 1,1
16. What is the extra value of the integrated dielectric loss measurement?
The estimation of the dielectric losses which has been integrated in the OWTS
determines the condition of the insulation and is therefore for HV-cables of major
importance. Especially for HV-cables with paper insulation such as; oil filled cables,
oil-pressurized or gas-pressurized cables, the dielectric losses (absolute and tip-up)
is an important parameter.
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FAQ’s
17. What is the maximum length I can test with the OWTS?
OWTS HV technology is standard tested for capacitive loads of cable lengths up to
30km) and longer. On request larger capacitive loads can also be also energized.
Depending on the type of cable the capacitance per kilometre differs. For instance a
XLPE insulated cable with a specific capacitance of 200 nF/km can have a length of
40 km.
18. What is the maximum length OWTS can detect and localize PD in a power
cable?
A distinction has to be made between PD-detection and PD-localisation. The OWTS
is the only an integrated system available which is able to energize and detect PD
from cables up to capacitive load of cable lengths to 30km) and longer.
Using IEC60270 PD detection principles OWTS is the best system to detect on-site in
[pC] very sensitively PD in all type of power cables.
To localize PD sites in a power cable OWTS uses the time domain reflectometry
method. It is known that TDR effectiveness depends on several factors e.g.
a) HF-impedance characteristic of the particular cable system
b) PD site position in the cables
c) PD level and intensity
d) actual background noise,
e) PD level etc.
Therefore it is not possible to give a fixed number of max. length to localize PD sites.
It is advisable in the case of long cables to perform PD detection form both cable
ends.
19. What is the frequency of DAC voltages?
OWTS uses large PD-free and low losses inductors specially developed for testing
power cables. Depending on the type of OWTS system the inductors may vary
between 4H and 8H. As a result the frequency of the DAC wave will be between
20Hz and 300Hz, which is the normalized frequency according to the IEC standards
IEC 62067 and IEC 60840. The particular DAC frequency is depending on the length
of the cable section under test. In particular after charging the cable section up to
selected voltage level the LC circuit as obtained from the cable capacitance and the
external inductance (7H) As a result system produces sinusoidal damped AC
voltages in the range of 20Hz-300Hz, depending on cable capacitance.
Examples of typical damped ac voltage frequencies for different lengths of two typical 150kv
power cables
Length [km] XLPE (C=154pF/m) [Hz] Oil filled (C=373pF/m) [Hz]
0.25 300 194
0.5 213 137
1 151 97
2 107 69
4 76 49
8 53 34
16 38 24
20 34 22
20. Is there a possibility to mount the OWTS into a truck/ container?
YES: The OWTS has been originally designed to be transportable, that’s why the
components have been packed in transportation boxes. However mounting the
OWTS 150kV in a container must be possible.
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OWTS ® HV Technology

FAQ’s
For the other two systems (HV250 and HV350) this will bring implications since the
height of the system + clearance will be more then 4m, which makes transportation
on road or by rail problematic.
The systems can be mounted on a trailer of sufficient length. In this case extra
measures have to be taken to support the individual components during transport.
21. What is transport capacity of the OWTS (Volume/ weight)?
In the table listed below the total volume of each system has been given. When
compared with a resonance test set this is almost nothing.
OWTS HV 150kV 10 m 3
OWTS HV 250kV 12 m 3
OWTS HV 350kV 20 m 3
E.g. for the OWTS HV 150 system for transportation purpose the minimal cubic size
must be 10m3 Vans like: Mercedes Sprinter 311, Opel Movano L2H2
Component Height (cm) Width (cm) Length (cm) Net weight
3 x coil box 65 75 75 45kg
Bas unit box 91 130 85 35kg
HV source box 137 105 75 85kg
PD divider box 115 70 70 45kg
22. Can the system be transported with airfreight?
YES: all system of the OWTS technology can be transported with airfreight.
Since the weight and size of the equipment are both relatively small it is possible to
transport the system by airfreight. Something that isn’t possible with a resonance test
set).
23. What is the setup time for a measurement?
The setup of the system is one of the most important aspects of the entire
measurement and must therefore be done carefully. Generally it takes up to 2 hours
to setup the system, required that all other work like de-energizing the cable and
making the termination free for connecting the OWTS on it, is performed on forehand.
E.g. Un-packing/packing OWTS HV 150 components and connection to the test
object: approx. 2.0 hrs.
Measuring (one phase) depending on max test voltage applied, e.g. starting from 10
kV (peak) to 150 kV (peak) with steps 10kV, testing/measurement takes: approx.
0.5hrs/phase. Changing between particular cable phases approx. 2x 0.5hrs
Total duration: Depending on cable section detailed information, logistics and on-site
local situation and the amount of diagnostic data to be collected for 3-phase cable
approx. 5-8 hrs.
24. Can you also test in indoor substations?
YES: One of the big advantages of the OWTSHV Technology is the compactness of
the system. Since the dimensions of the OWTS HV series are relatively small, it is
absolutely no problem to measure indoor. Each component has been packed in a
portable transportation box on wheels which makes it possible to easily transport the
equipment to the measurement side.
OWTS ® HV Technology
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FAQ’s
25. What are the minimum technical requirements to perform on-site testing
using OWTS?
There are no special or extraordinary requirements regarding testing using OWTS.
Preparing the on-site test the following has to be considered:
1. At the location a power supply of 220Vac 50(60Hz and rated for at least 5kW has
to be available.
2. The access to the test object (connection point between the OWTS system and
the cable terminations) has to be below 7m height.
3. The test object has fully be disconnected from the network and other components
e.g. transformers, surge arrestors etc.
4. In the case of GIS connection the specific tools (inserts, bushings, etc) needed to
connect the test system has to be provided and installed by the customer.
5. Tent of other solution needed to protect the test system against the rainwater.
6. In case of an higher position of the termination (more than 5-7m) a special pd free
connecting cable is required.
26. Do cross- bonding joints have an influence on the measurement?
YES: Cross-bonding joints have an influence on the signal quality of the travelled PD
pulses. Cross links must therefore be removed and sheaths must be connected
normal during PD-testing.
27. What is the self generated background noise of the system?
Since the voltage source is disabled during the PD measuring, DAC principle, the self
generated background-noise of the system can be assumed as