When Less is More--Very Low Frequency Partial Discharge Measurement

Partial Discharge Analyzer

Partial discharge measurement plays an important role in the high-voltage technology field. With this data, precise statements can be made regarding the condition of the insulation in high-voltage equipment such as cables, generators and transformers. Very low frequencies (VLF) in the range of 0.1 Hz are often used to minimize power requirements during measurement. Since most partial discharge testing devices are optimized for power system frequency or higher, however, low frequencies often lead to measurement errors. With the new MPD 600 Partial Discharge Analyzer, OMICRON offers a device that provides optimum results at even the lowest frequencies and can handle dc voltage.

Weakened Insulation Because of Partial Discharge
Partial discharge (PD) is a local discharge in an electrical insulation system. Phenomena of this kind often occur prior to complete breakdown of the insulation layer and display a tree-like structure. Since this can lead to serious and costly damage, and even cause high-voltage systems to completely fail, equipment should be tested for PD. The limits for partial discharge are laid down for the various high-voltage elements in dedicated International Electrotechnical Commission (IEC) standards. PD measurement not only indicates whether partial discharge has occurred, but also allows the precise position of the damage to be determined. This localization aspect is particularly important, especially since the high-voltage and medium-voltage cables to be tested often stretch a long distance. This allows weakened cable sections to be identified quickly using an uncomplicated process and enabling prompt replacement.

Typical Test Set-up
In a typical test set-up, the element to be tested is taken out of operation. A test voltage is applied at one end and a coupling capacitor connected in parallel. This capacitor allows PD pulses (mV) to be isolated from the high voltage (kV), enabling them to be processed by the highly sensitive measuring instrument. Measurement of high-voltage equipment takes place at its installation site, which is why the voltage source is often mobile and fitted to a truck. In addition, the required laptop is generally set up in this vehicle. The PD testing device is connected to the computer through fiber-optic cable, which guarantees the greatest possible safety for the test engineer. Any electromagnetical interference, which can have an effect on the sensitive test device, is also eliminated by this approach.

Very Low Frequency Partial Discharge Measurement
PD measurements on medium-voltage cables are often performed with a low-frequency test voltage of 0.1 Hz. This allows the power requirements to be significantly reduced. Compared with measurement at power system frequency, the reactive power that needs to be provided by the high-voltage source can be reduced by a factor of 500. This low-frequency measurement, however, also has a disadvantage. PD testing devices that are optimized for use at power system frequency have difficulty processing the measured data. This can lead to measurement errors.

Very Low Frequency Compatible
The OMICRON MPD 600 is fully compatible with VLF and can process dc voltage without complications. Although a full cycle at 0.1 Hz takes 10 seconds, the MPD 600 can measure the relevant factors with no problems. Using statistical time domain reflectography (TDR), partial discharge can be precisely localized. Most manufacturers of measuring devices, however, employ standard TDR, which limits the precision of the results because of a lack of statistical back-up.

Special Software Solution
OMICRON's software solution compares the time differences of the reflections relative to the original PD pulse while using statistical TDR. The software tool used in the MPD 600 measures several thousand of these pulses and illustrates them in a graphic display as a collection of dots. Where multiple dots overlap, the system changes them from blue through green to yellow and red in the display. Reflection points—particularly at the end of the cable and PD fault locations—can be made visible, which in turn allows them to be removed. The MPD 600 can be used to test many different components—such as transformers, cables and bushings.

For more information about OMICRON or other partial discharge measurement analyzers, click here to go to Utility Products' Buyers Guide.

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