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October 2011

Partial Discharge Detection in Transformers

By Karl Haubner, High Voltage Test Application Specialist

Partial Discharge (PD) Detection is considered one of the most powerful tools to determine the condition of insulation systems. There are various options to monitor for PD activity based on the release of the energy due to PD. Conventional electrical PD measurements according to IEC60270 require the installation of a sensor to the HV circuit.

To monitor PD on transformers in the factory the PD detector is typically connected to the bushing taps. Once the transformer is placed in service this method is not always practical and in most cases will be subject to external interference, which is why other “non-conventional” methods of PD detection are desirable.

Rightfully, the most popular method to detect PD activity in a transformer is by detecting the chemical decomposition of both cellulose and oil using Dissolved Gas Analysis. However, there are sometimes situations that call for complementary techniques. Hydrogen, which is the key indicator of PD activity, can have other origins and DGA is not 100% conclusive. DGA is an integrated measure and slower to identify a rapidly changing situation. Also, direct on-line measurements can provide additional information on the type/location of PD.

Another method relies on detecting the acoustic pressure wave generated by the internal PD event using piezoelectric sensors mounted on the outside of the tank wall. This technique has poor sensitivity for PD activity from within the transformer winding and should be seen more as a location rather than a detection technique.

The demand for a more sensitive field technique has lead to the development of alternative methods. A PD event results in a transient current pulse and an electromagnetic field in very high (VHF) and ultrahigh (UHF) frequency range. These electromagnetic waves can be detected using special sensors or antennas.

The electromagnetic waves emitted by PD resonate within the transformer enclosure and can be measured everywhere in the transformer with moderate attenuation of the signal, but they are not easily detected from the outside. The transformer tank acts like a Faraday cage effectively shielding external PD signals that may corrupt the measurement. This permits much more sensitive detection of PD activity on energized transformers than other methods based on acoustics or detection of PD activity via High Frequency CT’s installed on the neutral of the transformer. To do this, an internal PD sensor is required.

There are two typical installation options for a PD sensor. The first is to permanently mount a UHF hatch type sensors on the top of the transformer. It can be incorporated during the design and manufacturing of a new transformer or installed into an existing transformer in the field by replacing an inspection cover typically installed during an outage. Alternatively, a UHF sensor can be inserted through an oil drain valve. 

 PD measurent using the Lemke UHF Drainvalve sensor + PDsmart
or for a simple survey the Lemke UHF Drainvalve + PDS100

PD measurements via the oil drain sensor can be conducted while the transformer remains energized. The output signal of the sensors can be processed in time or frequency domain. The output cannot be described in terms of apparent charge as in conventional measurements, but simultaneous measurements using both conventional IEC60270 test circuit and UHF couplers have shown sensitivities in the low pC range for the UHF couplers. Since the transformer shields the measurement from external interference interpretation is less complex. In the absence of any active PD source, in most cases, only background is detected.

Healthy transformer with no PD activity detected

Transformer with critical PD activity

Since the PD pulse characteristics are preserved using a UHF PD detector, it is sometimes possible to determine the nature of the PD using the phase resolved patterns.

The main aim of the above tests is to detect if the transformer has PD activity or not. Once the presence of PD has been confirmed and some estimate about the severity of the activity is known, PD location determination should be attempted. The output of the UHF probe can now be used as the trigger signal enhancing the accuracy of the triangulation techniques.

For the first time, PD measurements comparable in sensitivity to laboratory tests are possible in the field - even under adverse conditions and without an outage. This compliments the well known Dissolved Gas Analysis and overcomes shortcomings of the acoustic technique, which is more suitable for location determination rather than conclusive detection. The main disadvantage of the UHF method is that it is not possible to calibrate the PD in terms of apparent charge in pC.

Karl Haubner is a High Voltage Test Application Engineer in the Asia-Pacific Region. He joined Doble Engineering in 2004, previously working for the Australian utility Western Power. As the
Superintendent of the HV Test Laboratory he was responsible for all technical aspects of the test group such as development and introduction of new condition monitoring techniques on distribution, transmission and generating plants. Karl is member of Cigre D1, Australian Standards EL-007-01 committee and of VDE. He has presented several papers on
Condition Monitoring of HV assets including transformers, switchgear, cables and rotating machines.