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Application Note

Method statement for partial discharge measurement

Author Dr.-Ing. Kay Rethmeier | kay.rethmeier@omicron.at Date Oct, 2007

Related OMICRON Product CAL 542, MPD 600, MCU 502

Application Area Partial Discharge Measurements

Version v1.0

Document ID ANP_10036_ENU

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Content

1 Principle measurement cicuit ...............................................................................................................3

2 Connection (decoupling) possibilities ................................................................................................4

3 Test equipment ......................................................................................................................................6

4 Test procedure .......................................................................................................................................7

5 Calibration ..............................................................................................................................................8

6 Measurement ..........................................................................................................................................9

Please use this note only in combination with the related product manual which contains several important safety instructions. The user is responsible for every application that makes use of an OMICRON product. OMICRON electronics GmbH including all international branch offices is henceforth referred to as OMICRON. OMICRON 2010. All rights reserved. This application note is a publication of OMICRON.

All rights including translation reserved. Reproduction of any kind, for example, photocopying, microfilming, optical character recognition and/or storage in electronic data processing systems, requires the explicit consent of OMICRON. Reprinting, wholly or in part, is not permitted.

The product information, specifications, and technical data embodied in this application note represent the technical status at the time of writing and are subject to change without prior notice.

We have done our best to ensure that the information given in this application note is useful, accurate and entirely reliable. However, OMICRON does not assume responsibility for any inaccuracies which may be present. OMICRON translates this application note from the source language English into a number of other languages. Any translation of this document is done for local requirements, and in the event of a dispute between the English and a non-English version, the English version of this note shall govern.

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1 Principle measurement cicuit The principle measurement circuit is described in the block diagram below. The coupling capacitor Ck is only one possibility to decouple the PD signal. Other ways of decoupling are shown in chapter 2.

Figure 1: principle measurement circuit

HV filter: High voltage filter to suppress noise / interferences from the high voltage source (e.g. resonance voltage source). Recommended for sources with high interferences

Test object: object under test, e.g. cable, transformer, Ck: Coupling capacitor for decoupling the PD signal. CPL542: Measuring impedance for splitting up the voltage and the PD measurement path (incl.

additional protection circuit) MPD600: Acquisition unit (measuring channel) USB502: fiber optical controller The connection(s) between the test object and the acquisition unit MPD 600 should be as short as possible to reduce unwanted interference. The length of the connection between the MPD 600 and the USB 502 can be as long as necessary (up to 2 km) because no additional interferences will be added due to the fiber optical transmission. If more than one measurement is needed, the channels can be easily cascaded via the optical bus.

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2 Connection (decoupling) possibilities The decoupling of the PD signal can be done on different ways depending on the test object and the onsite conditions. The most common ways are shown in the pictures below:

Figure 2: connection to capacitive measurement taps of the transformer

Figure 3: connection through coupling capacitors

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Figure 4: decoupling through RFCT's at the Figure 5: decoupling through RFCT's at the cable cross bonding box cable screen connection

Figure 6: decoupling with capacitive sensor in the cable joint

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3 Test equipment Beside the decoupling equipment mentioned before, following items are necessary to perform the PD test:

Figure 7: MPD 600 test equipment

The charge range of the calibrator has to be selected in the range of the expected (allowed) charge level for the test object.

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4 Test procedure

Figure 8: Test procedure

Preparation: The prior information about the test object is necessary to select the right measurement equipment (calibrator, quadripole/measuring impedance, fiber optical cable length). The system will be set up the following way:

> Connect the fiber optics controller to the Notebook Computer using a standard USB cable (USB 2.0).

> Connect the MPD 600 Acquisition unit to the fiber optics controller using fiber optical cables.

See wiring diagram on next page for details. Please observe that TX (transmit) is always

connected to RX (receive) and vice versa.

> Connect the power supply (or battery) to the MPD 600 Acquisition unit. The red LED starts flashing indicating that the acquisition unit is ready for operation.

> Connect the external coupling unit CPL 542, also referred to as quadripole or measuring impedance.

Use two short BNC cables and connect the PD and V outputs of the coupling unit

CPL 542 to the PD and V inputs of the acquisition unit MPD 600.

> Connect the coupling unit CPL 542 to a high voltage coupling capacitor (or RFCT, measuring tap,

other Sensor). Always remember to keep the cable(s) as short as possible.

> Connect the Notebook PCs power supply and switch it on. Perform test: Select an appropriate measurement frequency and bandwidth (in the range of the applicable measurement standard). This is to optimize the signal to noise ratio and to reduce external interferences. In case of high external disturbances a gaiting channel with antenna can be connected to the measuring system. The "perform test" steps are described in the next chapters. Reporting/assessment The reporting will be done based on the stored data. This can be done either with the exported graphics or with the stored data stream.

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5 Calibration After setting up the system a calibration of the complete test setup is essential:

> Once the computer has started, double-click the Omicron Icon to start the software:

> Use the mouse or keyboard to select the main menu item Mode in the top menu bar. Then

select Basic for the systems operating mode.

> Make sure that the Settings tab is selected. Find the combo box labeled Trigger source and

select unit1.1 as the trigger source. If you are performing a calibration you may also select line

as trigger source. The system then uses a light-sensitive sensor to synchronize itself to

fluorescent light sources. Do not forget to change the trigger source back to unit 1.1 before

performing the actual measurement.

> Select the fCenter (center frequency) box and enter a value of 350 kHz.

> Select a bandwidth of 300 kHz. This center frequency and bandwidth configuration sets up the

measurement system for an IEC 602 70-compliant measurement. Other settings might be

necessary to receive a better signal to noise ration

> Make sure that the Input Protection option is checked. You may uncheck this option for very

sensitive measurements but the PD input of the acquisition unit is then somewhat more sensitive

to damage by electrical surges.

> Go to the Settings tab. For Mode select Unipolar Logarithmic.

> Set QMax (maximum charge displayed) to 1 nC.

> Set QMin (minimum charge registered and displayed) to 1 pC.

> Optionally, check the Show individual PD events option. Individual PD events will then be shown

as phase and amplitude resolved spikes in real time. Now change to the calibration settings by selecting the Calibration tab.

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Figure 9: Omicron Software, Calibration Tab

> Uncheck the Enable test generator option.

> Check the External quadripole configuration option.

Connect the calibrator to the test object. Select a charge of 100 pC. Pulses should clearly extend from the background noise as spikes. If the noise floor covers PD pulses, select a higher charge on the calibrator (depending on your test object and available charge calibrator you may use another charge level).

> Enter the calibrator charge (e.g. 100 pC) into the Calibrate to field in the PD section.

> Click on the Compute button in the PD section. The display value for QIEC should now

correspond to the calibrator charge. Now the charge value is calibrated. The second step is to calibrate the test voltage value:

> Disconnect the calibrator.

> Secure the high voltage test set-up observing safety procedures.

> Turn on the high-voltage supply. Set voltage to a known level (e.g. 2 kV).

> Enter voltage level (e.g. 2 kV) into the Calibrate to field in the Voltage section.

> Click on the Compute button in the voltage section. The display value for voltage should now

correspond to the actual voltage.

6 Measurement After the calibration is finished, the PD measurement can take place. The measurement should be performed according to the applicable standard for this type of test and test object. Main issues of the standards are test voltage levels, test duration and maximum PD levels. To get a so called "pattern" of the partial discharge, the creation of a histogram is useful:

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> Start the test voltage run (of the test voltage source)

> Click on the Go button in the "Settings" tab. A histogram will be accumulated over a specified

time period. The progress bar at the bottom will indicate the progress of this operation.

> Once a histogram has been accumulated, the image may be saved to disk by right-clicking on the

scope display. A context-menu will pop up and offer to save the image to disk as a PNG file. PNG

is a popular image format providing lossless compression.

> All of the data gathered during a measurement may be recorded for later analysis by clicking on

the Record button (marked by a large red dot). The software will display the hard disk space

consumed during this operation and the remaining disk space. The stored data can be used afterwards to assess the test object, to create a test report and to give recommendations.

OMICRON is an international company serving the electrical power

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consulting and commissioning please visit our web site. www.omicron.at www.omicronusa.com