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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 � measurement, the phase

shift between the voltage and the current is measured from this phase shift the Tan � is accordingly

calculated, the bigger the Tan � is, the worse the condition of the insulation is. Graphically this has

been displayed in figure 1.

Figure 1: Schematic diagram of the dielectric losses/ tan �.

Basically what happens when the Tan � increases is that the insulation resistance decreases. In other

words the insulation resistance is inversely proportional with the Tan �. 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 � 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 � 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 � 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.

VLF Tan Delta

U

IC

2IR

I

� RCI

I

C

R

ωδ

1tan ==

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 �, the so called tip-up Tan �. The tip-up Tan � is a subtraction of the Tan � at 2U0 minus the

Tan � at 1U0. Since the absolute value of the Tan � is temperature, joint/termination type and

insulation type dependent, the highest weighting factor has to be laid on the tip-up Tan �, the tip-up

Tan � 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 � has been shown as function of the voltage, from this graph

both values can be obtained, the absolute value and the tip-up Tan �. For good cables the Tan � 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 �, the Tan � 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 �) 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 � 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 �) 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 � 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

� or tip-up tan � 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

(10 Hz with accuracy limitations)

Capacitance range 2 nF ... 3 µF

Tan � Measuring range

Accuracy

Resolution

1�10-4 … 1�100

1�10-4

1�10-5

Current range MDU

TCU

1 µA ... 25 mA

1 µA ... 1 mA

Insulation resistance range 1 M� … 10 T�

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

TCU

16 h (for operation with TCU)

32 h (for operation without 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. : +49(0)9544/68-7341

Mob. : +49(0)175/9361560

Fax : +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 9433Unit 1, 176 South Creek Road, Cromer NSW 2099Email sales@sebakmtaus.com

www.sebakmtaus.com