Tan Delta Manual 2008

8/13/2019 Tan Delta Manual 2008

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Low frequency loss anglemeasuring instrument for high

voltage testing

Advanced Technology for Industry

High Voltage Stack & Control Unit- the high voltage stack is operated from

the control unit through fibre optic cables, providing total isolation from the highvoltage circuit under test.


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Danger - High VoltageThis equipment is designed to be connected to highvoltage systems, and must only be operated under thesupervision of persons accredited to operate high voltage

equipment. Lethal voltages (up to 60kV) can be found innormal operation of this equipment. Some safety notes aremade below.

1. The control unit of the equipment is isolated from the high voltage using fibre optic cables, and is safefrom any high voltages used in the high voltage unit. It is still connected to the mains, and the usual

precautions taken in operation of any mains powered equipment should be taken. There are nooperator serviceable parts inside the control unit.

2. Under no circumstances should the battery be charged or changed whilst thehigh voltage is applied to the high voltage unit. Always earth the high voltagesystem before making any connections or changes

3. Following any accident or flashover, the high voltage system should be checked that it is safe tooperate at the full rated voltage. If in doubt, return to the manufacturer for calibration and repair.

4. The high voltage unit is not designed to operate in wet or damp conditions .5. DO NOT USE IN WET CONDITIONS. 6. For safety reasons, at all times, always earth the high voltage unit when the system is not being used

for making measurements.7. Never apply more than the rated voltage to the high voltage unit. This is 60kV peak DC. (42kV rms)8. All high voltage safety rules must be adhered to when operating this equipment.


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Measurement of Loss Angle (Tan ) at low frequencies.

General

This Instrument measures the loss angle of high voltage insulation at low frequencies. In practice lowfrequency means below 0.2Hz for this device. It is designed as an add-on to a high voltage lowfrequency (VLF) power supply), and as such will measure the tan (loss angle) of the insulation incables and capacitors at low frequency and high voltage.

The instrument is designed for use with high voltage cables, to measure the tan of the insulation.This can help with the diagnosis of the cable ageing. Changes in the tan at low frequencies canindicate degradation of the insulation, which can be used to make engineering decisions about theservice life of the cables.

The instrument consists of two parts. A high voltage part must be connected to the high voltagesystem, and measures current and applied system voltage. The second part of the instrument is ananalysis unit which shows the voltage, and current (in the high voltage circuit), and computes the tan (loss angle) of the cable or capacitor.

The high voltage unit is connected to the analysis unit using only two optical fibres. Full isolation ofthe voltage is thus maintained. The length of the fibres is not limited, although a 5m pair of fibres issupplied as standard.

As the voltage can be slowly changing, the anaylsis unit is provided with an indicator, which shows theapplied voltage to the sample. A calculation of tan is carried out for each period, and displayed inreal time at the end of each period. In the medium and extended modes (set on the front panel) the unitcomputes the tan from more than one cycle of data. In all modes, the unit bleeps when a new readingis available.

Measurement Circuit.

The measurement circuit is simple and easy to connect. The circuit diagram in Figure 1, shows theVLF power supply, connections to the high voltage unit, and the cable under test.

Figure 1 Circuit diagram of measurement of tan at low frequencies

VLF HighVoltage PowerSupply

To Cableunder test

HighVoltageUnit

Control Unit

Fibre opticlinks


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The connections for the fibre optics are colour coded, so that red and blue must be connected to thesame colour on both the analysis unit and high voltage unit.

The high voltage output from the VLF generator, must be attached to the top of the high voltage unit.The output to the cable under test, is the copper tube at the side of the high voltage unit. Do not mix upthe input and output of the high voltage unit.

The base of the high voltage unit must always be earthed to the earth used for the VLF generator highvoltage power supply. This is for safety reasons, and also for correct calibration of the high voltagemeasurement.

The analysis unit will automatically adjust to the HV generator amplitude and frequency, so nooperator intervention is necessary beyond setting the analysis mode to normal, medium orextended.

Tips for good measurements

The low frequency tan measurement is made on all the capacitance on the output side of the highvoltage unit. Hence if a connecting cable is used to connect the VLF generator to the cable under test,the connecting cable will be a part of the cable under test. The effect of this cable can be corrected forin a calculation of the loss angle. However, if the loss angle and capacitance of the connecting cable isto be eliminated, the user may wish to place the high voltage unit next to the cable under test, with noconnecting cable between the high voltage unit and the actual cable being tested.

The display of the analysis unit, shows the voltage and current measured in the high voltage circuit. Itshows the values of the current and voltage at the end of each period. Where the periods are long, thedisplay may not show the actual values of voltage and current until the end of the next period. At say0.05Hz, this could be several minutes. These periods are still longer when medium or extended modeis selected. This is the reason for the unit to bleep, indicating a new reading is available.

The loss angle is also calculated at the end of each period, and the same applies to the updating of thedisplay. Also bear in mind that following a change in frequency or magnitude setting, there will be atransient which whilst small, may still leave errors of a few percent due to this transient. Hence whenmaking accurate measurements, make sure that a number of cycles have passed, and that the reading oftan is stable before using it as an accurate value. At switch on, and after mode change, ???? appearsfor a while on the display, showing answers are not yet valid.

The high voltage unit measures voltage using a resistor. This resistor also has a capacitance associatedwith it. Whilst this capacitance is small (typically 5 10 pFarads), it can change the loss anglemeasurements for higher frequencies. There will be an error due to this capacitance, but at frequenciesof 0.1Hz and 0.05 Hz, this does not change the accuracy of the measurement. The losses andcapacitances of external cables etc. can be compensated for using calculation, knowing the value of thecapacitance and loss for each component to be included.

In the same way as small changes can be affected by the capacitance of the resistor, so to can any straycapacitance of the high voltage unit. To keep this to a minimum, do not allow any earthed metalworkto get close to the high voltage unit. If this becomes unavoidable, such as in confined spaces in alaboratory or van, then measure the effect of the stray capacitances, (i.e. measure loss angles of astandard with and without the strays present) and remove the effect by calculation.

The current is measured by a shunt, which has a value of around 900ohms. This resistor can also havean effect on the measurement of loss angle near the resolution of the device. In most cases this resistorwill represent a very small value of extra loss. Most measurements can happily ignore this smalleffect. For example, it could introduce small errors at high frequencies with very lossless cables.Again this effect could simply be compensated for using calculation, by taking into account the effectof 900ohms in series with the cable under test. The high voltage unit measures current by using tworesistor tappings to make more sensitive measurements still at high accuracy. This is automatic, and


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the range is indicated on the display. The value of the shunt resistor does not change with the changesin low and high sensitivity.

The analysis unit monitors the HV output of the VLF generator and learns the frequency by timingconsecutive positive zero crossings. At least 1kV must be available for reliable operation, and it takesseveral complete cycles following start-up before full accuracy is obtained. Hence allow several validreadings to pass before making reliable measurements following initial switch-on

The battery is a lead acid type, and can be recharged with the supplied charger. The battery life of thehigh voltage unit is designed to be at least 8 hours, to allow for a full days operation. However, theactual battery life is much larger than this, and anywhere between 20 and 40 hours continuos use can beexpected. Charging the battery must be done with the charger, and the green light shows when thecharging is complete. There is no memory effect with this battery. Take great care to change the

battery safely when the system is a part of a high voltage setup. ALWAYS EARTH THE SYSTEMBEFORE MAKING ANY ALERATIONS TO THE HV UNIT.

Features of the low frequency tan measurement

Frequency range up to 0.2Hz In line measurement of current and voltage, so no changes to VLF powersupply required.

Automatic readout of frequency and loss angle Automatically adjusts to VLF generator frequency. Magnitude and frequency display Readout of high voltage and current in real time. Connection to HV measurement module is via fibre optics. Hence no high

voltage safety hazards from the controller. Real time readout of loss angle Audible beep when new reading is available

The loss angle of high voltage insulation is measured by comparison of the phase between voltage and current waveforms at low frequency

Applications & Uses

Cable manufacture for quality assurance Testing high voltage cables in service for degradation Replacement programmes for HV cable networks Research for high voltage cable investigations. Commission testing for new and repaired cables Can be used for high voltage insulation for condition monitoring with

transformers, switchgear bushings etc.


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Specifications

VLF current Range 0 50mAFrequency Range 0.01Hz 0.2HzWorking Voltage 1kV - 40kV rmsResolution of Tan . 0.0005Absolute Accuracy 2% + 0.005 zero errorDisplay 128 x 64 Backlit Liquid crystalDisplayed data Loss angle in radians

Frequency (Hz)System Voltage magnitude (kV)Current at HV (rms)Low battery indicator

No data indicatorControls Averaging Mode Normal, Medium, Extended

(1 cycle) (5 cycles) (20 cycles)Outputs Fibre optic communication to HV unit using

5 metre duplex cable

Power requirementsControl boxHigh Voltage module

110 220V, 50 / 60Hz 70WCharging unit for battery (HV Unit)

Battery Charging

The High Voltage stack is powered from a 6Vlead acid sealed battery allowing the HV unit tofloat at the high voltage test supply with respectto ground. The battery is charged using thesmall power connector provided, The battery

should only be used with the charger andcable provided. The output voltage of thecharger is a nominal 6 Volts.Connection of the charger to the HV stackmust only be done when the equipment is notin use with the high voltage supply.To charge the battery plug the small power plug

into the socket provided on the side of the HVstack marked, this automatically disconnects theinternal circuits from the battery whilst chargingis taking place. A full overnight charge shouldgive approximately 8 hours full operation.

M&B Systems (Power Instrumentation) LTD,308/310 Slade Lane, Levenshulme

Manchester M19 2BYTel 0161 248 4090, Fax 0161 248 4099


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Tan Delta HVS Software installation and operation manual

Installation

The software is designed as a single stand alone executable file which will run under Win9X, and Win NT.The software can be run simply by double clicking on the filename (TanDelta.exe) which appears inWindows Explorer. However, it is probably best to create a new folder (eg Tan Delta measurements) andcopy the file TanDelta.exe to the folder. To run the program, double click on the file, or make a shortcutto the file. (to make shortcuts on the desktop, right click on the desktop, choose new and then shortcutand follow instructions to create the shortcut on the desktop).

Operation

When the program runs, a small window gives a choice for the COM port which can communicate with theTan Delta device. Choose COM1 unless this is not available. If the program is to be run without

communication with the measuring instrument (e.g. viewing previous data. Printing etc) then choose any port which is free.

Port Selection program window

The main window will appear as shown below. The window is shown with some data loaded, to give ademonstration of the data displays.

The operation of the program is simple and automatic. When attached with the serial cable to the TanDelta instrument, the program displays the data on the instrument as soon as it becomes available. Newdata is indicated by the instrument beeping. The program displays these data in real time.

The software will perform the following functions

Real time data displayWhen connected to the Instrument by the serial cable, the program will always show the newest data in the

boxes, when it becomes ready. The same boxes can be used to display the data in a stored record in a file. Note that the fact that the computer displays the real time data, does not mean that a record is being madeof the data. To record the data in a file, the start monitoring button should be clicked.


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Data recordingThe data can be recorded by the program. To do this, fill the edit boxes with the appropriate information (itis emphasised that this data should always be entered, as data is vital in reviewing the data later) and clickthe start Monitor button.

A file dialog box appears, and invites you to enter a filename in which the data to be recorded, will be

saved. Once this is done, the program records the real time data as a sequence of measurements. Eachmeasurement is time stamped, and so data at different voltages and frequency should simply be recordedfor each circuit. If data for separate cable phases is required, these should be stored in a separate file.

Tan Delta program window

Drawing graphs of Tan Delta against time, Voltage and frequency

When the data is stored using the Start monitor button, all the records are kept until the Stop monitor button is clicked. The order of the data is preserved for the Tan delta versus time graphs, but the otherradiobutton options in the graphics control panel will draw the data of tan delta versus frequency, andtan delta versus voltage in the correct way whatever the order of data entry. Provided the system is in startmonitor mode, a complete set of data can be recorded as required, for different frequencies and voltages.


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Hence an entire days testing can be recorded in a single file, and the data will be correctly processed to beable to view the data in the three available ways

Tan delta versus time Tan delta versus voltage Tan delta versus frequency

In addition to the three graph types available above, when the system is set to Graphs in Time mode inthe graphics control panel, there are three options allowed in this mode. These are Tan delta versus time,Voltage versus time, and frequency verus time. These will all display the data as recorded in during themeasurements.

Printing the results

The results of the graphs, along with data recorded in the edit boxes, can be printed out on a single sheetusing the print preview option in the Graph menu. When the print preview page is viewed, click on

print to print the preview page. The device will print to the current system printer. Whatever graph iscurrently chosen will be used in the print preview page.

Copying the graphs to the clipboard for use with other programs

For using results with other programs, the graphs can be copied to the windows clipboard using the copyto clipboard item in the file menu. Simply click this menu item, and the graph will be copied. It can then

be pasted into other applications which can use windows metafile clipboard data. This gives a simple wayof copying graphs to Microsoft WORD for example.

Viewing the data records

When a set of data has been recorded, the individual records can be viewed simply by clicking the record

number slider in the cursor control panel. This allows the record data to be viewed for each individualrecord in the data set.

Entering data & comments

When recording a set of data, it is essential to put as much detail of the experimental details as possible.The data entry boxes in the Current file panel can be used for this. Fill in the data, and then click on theenter details above button. The data is not stored after simple entry of data in the edit boxes, so to dothis, the enter details above button has to be used. This method can be used to alter the data in a storedfile. However, if this is done, the file must be re-saved. (see re-save in the file menu).


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Operation of the new Tan software (01-07-05)

The new facilities allow the removal of unwanted data from the Tan v Time graph.A cursor is provided to help in identifying the points. The marked points can be

plotted or removed by using the new functions in the Select Cursor Control box.The marked points can also be unmarked if required.

The cursor is controlled by the Record Number Selection facility which also givesthe numerical value in the loss angle results table. To remove spikes from the graphtick the box marked Remove marked graph points from graphs in the GraphicsControl box. The spike is selected using the cursor control.

The graph scaling will be automatically changed to suit the new tan results.Thescaling for plotting tan is now displayed in milli-radians.

All of the data can be saved to disc, so that the files can be viewed correctly when you

recall them.

A further improvement allows the graph scaling to be manually changed by theoperator

A box is provided labeled Graph Scale, just click on the required scale which isselected from the following Auto, 10mRad, 50mRad.


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Measurement Circuit.

The measurement circuit is simple and easy to connect. The circuit diagram in Figure 1, shows the VLF power supply, connections tothe high voltage unit, and the cable under test.

Figure 1 Circuit diagram of measurement of tan at low frequencies

Specifications

VLF current Range 0 50mAFrequency Range 0.01Hz 0.2HzWorking Voltage 1kV - 40kV rmsResolution of Tan . 0.0005Absolute Accuracy 2% + 0.005 zero errorDisplay 128 x 64 Backlit Liquid crystalDisplayed data Loss angle in radians

Frequency (Hz)System Voltage magnitude (kV)Current at HV (rms)Low battery indicator

No data indicatorControls Averaging Mode Normal, Medium, Extended

(1 cycle) (5 cycles ) (20 cycles )Outputs Fibre optic communication to HV unit.

5 metre duplex cablePower requirementsControl boxHigh Voltage module

110 220V,50/ 60Hz 70WCharging unit for batteries (HV Unit)

M&B Systems Power Instrumentation LtdManchester England

VLF HighVoltage PowerSupply

To Cableunder test

HighVoltageUnit

Control Unit

Fibre opticlinks


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VLF loss factor (tan ) measuring instrument Model HVS 1000/TD

Brief operating instructionThis equipment comprises a measuring instrument and high voltage stack. The maximum working voltage is 40kV rms(60kVpeak) which must not be exceeded.

DANGER High Voltage : This equipment is designed to be connected to high voltage systems. It must be operatedonly under the supervision of persons accredited to operate high voltage networks. Always discharge and ground

the high voltage side before making any connections. Always disconnect completely from high voltage beforeattempting to change or charge the battery in the HV stack.

Ensure that the sealed lead-acid battery in the HV stack is fully charged before use. Battery life should be 8 hoursunder normal working conditions.

Position the HV stack on a level surface as close as possible to the terminal of the test object. Connect a good groundconductor from the ground terminal on the base of the HV stack to the common ground terminal on the test voltagegenerator.

Connect the high voltage output socket of the HV stack by a suitable corona-free conductor to the test object. Nominal 1 aluminum or copper tubing is ideal. Note that the connection between the HV stack and the test objectwill feature in the measurement of the overall tan value; if this connection is by cable. It should be kept as short as

possible.

Connect the high voltage output of a very low frequency (VLF) test voltage generator having a sinusoidal outputwaveform and frequency in the range 0.01-0.2Hz to the top terminal on the HV stack.

Locate the measuring instrument on a suitable bench or other surface. Connect a good ground conductor from theground terminal on the rear of the instrument direct to the ground terminal of the test voltage generator.

Connect the HV stack to the measuring instrument by means of the two supplied fiberoptic cables, observing the redand blue colour coding.

Connect the measuring instrument to a 110 / 220V * 60Hz supply using the line cord provided. Power up themeasuring instrument using the rocker switch on the rear panel. The front panel LCD display should light up andindicate the parameters that will be measured. *(Check voltage selector switch is selected to the correct voltage, this islocated on the Rear Panel of the control unit)

Set the three-position rotary switch to the right hand side of the front panel to normal medium or extended as

desired. This selects the number of measurement taken and will greatly affect the measurement time. In normalsetting one measurement of tan is made for each full test voltage cycle; in medium and extended settings theinstrument computes the tan value from 5 cycles or 20 cycles of data respectively. When a valid reading iscompleted the instrument issues a bleeping sound. The LCD display will indicate which measurement mode isselected.

Set the three position rotary switch on the HV stack to remote The red on lamp should light and the red datalamp should flicker to indicate that a data string is being sent to the stack.

The bar graph >100%100%< at the bottom of the LCD display indicates that data is being exchanged. Until a validmeasurement of tan is achieved, and after switch on or any mode change, the tan value is shown as ????. TheLCD display indicates test voltage, load current and frequency measured at the high voltage terminal. It may takeseveral minutes for the first valid measurements to be obtained after any mode change.

Before making any adjustments to the test circuit, or before charging the battery, always switch off the highvoltage test supply and allow a few seconds for the voltage to decay to zero. Then disconnect high voltage andsolidly ground all parts before handling.


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Tan Delta TestingWith The

High Voltage Inc.VLF Voltage Generator

Set up the VLF Voltage Generator and the Tan Delta test set in accordance with the instructions outlined inthe operators manuals. (See Diagram below)

Circuit diagram for measurement of tan at low frequencies

There are two options for recording data from the tan delta test set. Option one is to simply read and recordthe data points from the LCD of the tan delta control unit. The recommended option is to connect a notebook

computer to the serial port on the rear panel of the tan delta and use the included software to record all data.

Quick Basic Instructions: Turn on the Tan Delta control Switch the high voltage stack to the remote position Open the Tan Delta software on the computer and enter file details, circuit name, etc. Turn on main power of the VLF control Set the output frequency to 0.1Hz Turn on high voltage and set the output to Uo of the cable you are testing The LCD will update (beep) every cycle and you should see the live data displayed on the computer Wait for a stable Tan Delta reading. This may take up to five cycles When you receive a satisfactory reading, click the start monitor button on the computer screen You may then increase the voltage incrementally to a maximum of 2Uo At the conclusion of the test click the stop monitoring button on the computer and save the data

NOTE: You are looking for a relatively flat line graph of Tan Delta vs. Voltage, if the Tan Delta readingis increasing exponentially when you increase voltage, stop testing you may cause a failure in thetest cable. For best results increase voltage following the zero crossing (the voltmeter reads zero)after the unit beeps and the data is updated.

VLFHigh VoltageGenerator

To Cableunder test

HighVoltageStack

Tan DeltaControl Unit

Fiberopticlinks


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Example Graphs

The above result is a representation of the results when testing moderatelyaged cable

The above result is what you may see when an Arrestor or another accessory ismistakenly left connected to the cable under test. The Tan Delta Measurementincreases exponentially above 15 kV.


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The spike shown above is the result of the Tan Delta unit auto-ranging the currentmetering circuit from low to high. The unit switches from low to high range atapproximately 1.5mA. Spikes like these are common after increasing voltage. The TanDelta unit just needs a few cycles to get back in sync. The spikes will be marked as badevents and can be edited from your final report.


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Tan Delta Error Codes

READING MODE CURRRENT

RANGE

STATUS

VALID NORMAL HIGH 0

VALID NORMAL LOW 1

NOT VALID NORMAL HIGH 2

NOT VALID NORMAL LOW 3

VALID MEDIUM HIGH 4

VALID MEDIUM LOW 5

NOT VALID MEDIUM HIGH 6

NOT VALID MEDIUM LOW 7

VALID EXTENDED HIGH 8

VALID EXTENDED LOW 9

NOT VALID EXTENDED HIGH 10

NOT VALID EXTENDED LOW 11


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TAN CABLE TESTING

OVERVIEW AND ANSWERS TOFREQUENTLY ASKED QUESTIONS

What Is Tan , Or Tan Delta?

Tan Delta, also called Loss Angle or Dissipation Factor testing, is a diagnostic method of testingcables to determine the quality of the cable insulation. This is done to try to predict theremaining life expectancy and in order to prioritize cable replacement.

How Does It Work?

If the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets, etc., the cable acts as a perfect capacitor. It is very similar to a parallel plate capacitorwith the conductor and the neutral being the two plates separated by the insulation material.

In a perfect capacitor, the voltage and current are phase shifted 90 degrees. If there areimpurities in the insulation, like those mentioned above, a resistive element is introduced into theinsulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted90 degrees. It will be something less than 90 degrees. The extent to which the phase shift is lessthan 90 degrees is indicative of the level of insulation contamination, hence quality/reliability.This Loss Angle is measured and analyzed.

Below is a representation of a cable. The tangent of the angle is measured. This will indicatethe level of resistance in the insulation. By measuring I R /IC, we can determine the quality of thecable insulation. In a perfect cable, the angle would be nearly zero. The greater the angle, theworse the cable.

I

VI

R

IC

IC

+ IR

CableCircuit

R C


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What Are Water Trees?

Water trees are small tree shaped channels found within the insulation of a cable, caused by the presence of moisture. They are very prevalent in service aged XLPE and other solid dielectriccables, like PE and EPR cables. These tree shaped moisture channels, in the presence of anelectrical field, eventually lead to the inception of partial discharge (pd), which eventually leadsto the formation of electrical trees, which grow to a point where insulation failure occurs. Thetan delta test shows the extent of water tree damage in a cable.

What Hardware Is Necessary?

The tan delta unit consists of a highvoltage divider and a fiber opticallylinked measurement box. The high voltagedivider measures the voltage and current inputto the cable, sends this information to thecontroller, which analyzes the voltage andcurrent waveforms and calculates the tan deltanumber. A connected laptop computerdisplays and stores the results.

A voltage source is needed to energize the cable. What is typically used is a Very LowFrequency (VLF) AC Hipot. The VLF equipment supplied by High Voltage Inc. is a 40 kV(peak) unit that is capable of testing from 1.1 uF of cable load at 0.1 Hz, up to 5.5uF at 0.02 Hz.Other models offer an output frequency of 0.01 Hz, used to test very long cables.


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How Is The Test Performed ?

The cable to be tested must be de-energized and each end isolated. Using a VLF AC Hipot, thetest voltage is applied to the cable while the tan delta controller takes measurements. Typically,the applied test voltage is raised to 1Uo, or normal line to ground operating voltage. If the tandelta numbers indicate good cable insulation, the test voltage is raised up to 1.5 2 Uo. The tandelta numbers at the higher voltages are compared to those at lower voltages and an analysis ismade. Below is the test setup.

How Are The Test Results Interpreted, And, Is It Necessary To Have A Benchmark, OrStandard, Result To Compare To?

While it is beneficial to have a standard or previous test to compare to for trending purposes, likewith many diagnostic methods of testing, it is not necessary. The very first test on a cable yieldsvaluable information about the insulation.

If a cables insulation is perfect, the loss factor (tan delta) will not change as the applied voltageis increased. The capacitance and loss will be similar with 1 kV or 10kV applied to the cable. Ifthe cable has water tree contamination, thus introducing a resistive element to the insulation, thenthe tan delta numbers will be higher at higher voltages. Rather than a flat curve for the lossnumber versus voltage, the curve will be non linear. See the below graph.

Loss Angle Analyzer

VLF Control

V

L

F

M e a s u r i n g

U n

i t

C a b l e U n d e r T e

s t

Fiber Optic Cables

New and Aged 15 kV XLPE Cable (Nov 2000)

0

0.01

0.02

0.03

0.04

0.05

0.06

0 2.5 5 7.5 10

VLF Voltage (kV rms)

L o s s A

n g

l e ( T a n

D e

l t a

)

Aged Cable

New Cable

Measurements are voltagede endent in an old cable.

Measurements are not voltagede endent in new cable.


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it is measured in radians, not degrees as power factor is done. For slight angles, the tan deltareadings will be the same as power factor. As the angle, hence loss, increases, the tan deltanumbers and the power factor numbers will not be the same.

Why Is A VLF Hipot Used Instead Of A Regular 60 Hz Model.

Two reasons. First, to test a cable with 60 Hz power requires a very high power supply. It is not practical, nearly impossible, to test a cable of several thousand feet with a 60 Hz supply. At atypical VLF frequency of 0.1Hz, it takes 600 times less power to test the same cable compared to60 Hz.

Secondly, the magnitude of the tan delta numbers increase as the frequency decreases, makingmeasurement easier. As the below equation shows, the lower the frequency (f), the higher thetan delta number.

Tan Delta ( ) = I R / IC = 1/(2p f CR) (tan delta is measured in radians)

Are There Any Limitations To Using Tan Delta Testing?

Since we are measuring the loss angle of an insulating material, and making an analysis aboutthe test results possibly based on historical data, it is not advisable to test a cable length thatcontains more than one type of cable. Different cables have different loss characteristics. It isnot a good test to test a cable length with 300 of XLPE insulation spliced to an EPR or PILCcable. The only way in which this is meaningful is when many tests are done on the same cablelength over time and the results are carefully trended.

Concentric Neutral

Since we are measuring the loss angle between the conductor and the outer shield, the outershield must be intact. It is advisable to test the integrity of the concentric neutral before

performing the test. (This is a worthwhile test anyway for several reasons, whether or not a tandelta test is being performed.) If there are large gaps in the neutral, the tan delta numbers willnot be as meaningful if there were no gaps. There are easy ways to test the neutral integrity andwe can help with that.

How Long Does The Whole Test Take?

The test itself can take less than a minute, depending upon the settings of the instrument. It isonly necessary to capture a few cycles of the voltage and current waveform to make the analysis.At 0.1 Hz, the period of the sine wave is 10 seconds, so it takes 20 30 seconds for a reading to

be made. At .02 Hz, the period is 50 seconds, requiring perhaps 3 minutes of test time.

Tan Delta Manual 2008

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Transcript of Tan Delta Manual 2008