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VLF Tan Delta
VLF Tan Delta
General Description
All MV and HV power cables are continuously subjected to thermal, electrical and mechanical stresses
during their service life. These stresses will lead to ageing of the insulation material, one of the most
well known ageing phenomenon is water treeing in case of PE/ XLPE cables. Ageing of the insulation
actually means that the insulations degrades/ gets older. When the insulation is degraded it does not
have the same physical properties anymore as compared with new cables, what basically means that
the breakdown-strength has been decreased and the risk of failure has been increased.
There are several solutions to determine the condition of the cable-insulation, one thing they have
common, all of them are integral solutions; up to know it is not possible to localize parts of degraded
insulation without splicing the cable. One of the solutions to condition the cable is the dielectric loss
measurement. With the dielectric loss measurement, also known under tan į measurement, the phase
shift between the voltage and the current is measured from this phase shift the Tan į is accordingly
calculated, the bigger the Tan į is, the worse the condition of the insulation is. Graphically this has
been displayed in figure 1.
U
I
2IR
į
IC
tan δ =
1
IR
=
I C ωRC
Figure 1: Schematic diagram of the dielectric losses/ tan į.
Basically what happens when the Tan į increases is that the insulation resistance decreases. In other
words the insulation resistance is inversely proportional with the Tan į. However to assess the
condition of the cable a simple resistance test will not be sufficient since most processes are voltage
type dependent (capacitive processes).
Like previously mentioned the Tan į can only be measured accurately when using true-rms sine-wave
voltages. The frequency of the voltage can range from 0,001Hz to several hundreds of Hz, however
the Tan į measurement is at a fixed frequency. For cable testing lower frequencies <1Hz are
preferred to reduce size, weight and costs of the equipment. Typical Very Low Frequencies (VLF) test
systems operate at 0.1 Hz. Measuring Tan į at very low frequencies of 0.1 Hz, is a well accepted
method to determine the ageing status of cables for more then a decade. Several studies have proven
the effectiveness of dielectric loss measurements to determine water trees inside the PE/ XLPE
insulation.
In table 1 the trending limits for homo-polymer PE/ XLPE cables are given which are stated in the
IEEE 400-2001. The assessment can be either based on the absolute value or on the differential of
the Tan į, the so called tip-up Tan į. The tip-up Tan į is a subtraction of the Tan į at 2U0 minus the
Tan į at 1U0. Since the absolute value of the Tan į is temperature, joint/termination type and
insulation type dependent, the highest weighting factor has to be laid on the tip-up Tan į, the tip-up
Tan į is less dependent on external factors like the absolute value is.
Table 1: Table of criterion for homo-polymer PE/XLPE cables taken from IEEE 400-2001 (Chapter 8.4.3).
In figure 2 the typical graphs of the Tan į has been shown as function of the voltage, from this graph
both values can be obtained, the absolute value and the tip-up Tan į. For good cables the Tan į must
remain constant with increasing voltage, like the yellow and red line in figure 2. When the insulation
ages/ degrades this will have a result on the Tan į, the Tan į will not remain constant with increasing
voltage, but will increase as well, see green, light blue and dark blue lines in figure 2.
Figure 2: U/U0 = f(Tan į) of several cable samples
ranging from new till strongly aged (JICABLE,
Versailles, June 1995, paper B.9.6.)
For homo-polymer insulation clear limits are given like stated in table 1, for co-polymer cables or
paper-mass cables however there are no such limits. In this case a comparison between the Tan į of
each phase will be useful. When one phase is different then the other phases and a PD test in that
phase is not significantly different then the other phases, one can conclude that the phase is faulty and
a VLF test must be performed to find the weak spot. For paper-mass cables or mixed cables an RVM
measurement will be decisive.
System Description
The basic version of the VLF Tan Delta consists of a tripod (voltage divider) on which the MDU
(Measurement Data Unit) will be connected. Together in combination with the required VLF SIN
source and laptop, dielectric loss (tan į) measurements can be performed without leakage current
correction. For accurate measurements however leakage current correction is recommended,
especially when measuring on older terminations or in humid environments this is an utmost. The
leakage current correction is included in the upgrade version, apart from the MDU also a TCU
(Termination Current Unit) will be part of the package.
Functionality of the VLF Tan Delta basic version is accordingly, see figure 3a; test object will be
energized with a sinusoidal VLF voltage of 0,1Hz, MDU will measure the current and voltage and from
those two obtained values it will calculate the Tan Delta, capacitance and resistance of the cable
under test. Both VLF SIN source and MDU unit are controlled by a laptop, VLF SIN source by an USB/
Ethernet connection, MDU by a wireless radiolink connection of 868 Hz for safe operation. For
accurate tan į measurements it is of importance that the MDU is close to the test object, with the
wireless radiolink connection this has been made possible; distance between MDU and the laptop can
be up to 100m.
User interface of the software of the VLF Tan Delta is Centrix based, what implies that it haves all the
advantages of the Centrix software like easy-go operation, easy protocolling and history function as
well. Test sequences are programmed in the twinkling of an eye, during the measurement phases can
be compared with each other, data can be accessed and warning messages will pop-up when the tan
į or tip-up tan į will be out of boundaries.
The TCU of the upgrade version will be connected with the MDU via fibre-optic cables, see figure 3b
and leakage current will be subtracted from the total current leading to accurate and reliable results.
Leakage currents are currents flowing along the terminations because of humidity, filthy/ polluted
terminations or old terminations.
Both MDU and TCU are battery operated and will function up to 32h. Batteries are inductively charged
via the included transport/ charging unit in a maximum of 3h (empty to full), so no need to replace
batteries. Charging is not only possible via mains voltage (90 to 240 VAC wide range input) but also
via the 12 VDC plug which is common in every auto/ test van.
a)
b)
Fig 3: Schematic diagram of the test setup with (b) and without (a) leakage current correction.
Technical Specifications:
Tan D MDU (Measurement Data Unit)
Tan D TCU (Termination Current Unit)
Charging Unit with MDU and TCU
VLF Tan Delta
Voltage range
0 … 36 kVrms
Frequency range
0,1 Hz … 1 Hz
Capacitance range
2 nF ... 3 µF
Tan į
1Â10-4 … 1Â100
(10 Hz with accuracy limitations)
Measuring range
Accuracy
1Â10-4
Resolution
1Â10-5
Current range
MDU
1 µA ... 25 mA
TCU
1 µA ... 1 mA
Insulation resistance range
1 Mȍ … 10 Tȍ
Operating temperature
-25 °C ...+ 55 °C
Power supply
MDU/TCU
Charger
Battery powered
90 V … 240 V 50/60 Hz or 12 VDC
(Charging time 3 h)
Operating time
MDU
16 h (for operation with TCU)
32 h (for operation without TCU)
TCU
24 h
Weight (total)
12,25 kg
Dimensions
Peli-case with MDU TCU
Ø: 600 mm, H: 650 mm
Tripod
Ø: 600 mm, H: 650 mm
Required:
` VLF SIN power source (VLF Sin 28kV for portable versions, VLF Sin 51kV for integrated versions).
Optional:
` Leakage current correction (TCU Unit is not delivered in basic package).
` Notebook for protocolling and communication with MDU Unit and VLF Sin source.
` OWTS-connection set for a PD-free connection on terminations.
Contact information:
Hein Putter, MSc., Dipl.-Ing.
Technical Support Testing and Diagnostics
Tel.
Mob.
Fax
: +49(0)9544/68-7341
: +49(0)175/9361560
: +49(0)9544/2273
putter.h@sebakmt.com
www.sebakmt.com
SebaKMT
Seba Dynatronic Mess- und Ortungstechnik GmbH
Dr. Herbert Iann Str. 6
96148 Baunach / Germany
Reg.Gericht (96045 Bamberg) HRB 195
Geschäftsf.: Dr. Max Iann, Dr. Frank Petzold, Thomas Clemens
Advanced Utility Solutions Pty Ltd. T/A.
Phone 02 9972 9244 Fax 02 9972 9433
Unit 1, 176 South Creek Road, Cromer NSW 2099
Email sales@sebakmtaus.com
www.sebakmtaus.com