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Stay Online With Partial Discharge Testing

How to maintain continuous power supply while measuring for weak spots.

Fortnightly Magazine - October 2006

either are not stable enough or even more expensive.

The PD-OL measuring system uses pulse injection for synchronization. Pulses are injected at one cable end on a regular basis and measured at both cable ends. These pulses are injected with an additional coil along the measuring sensors at the cable termination. This method of synchronization, a crucial and patented element in PD-OL, is both inexpensive and very effective.

PD-OL also uses inductive sensors, which have the advantage of being cost-effective and easy to install. Such inductive sensors have certain disadvantages. In general, inductive sensors placed in a ground lead will pick up more noise/interference than do capacitive sensors. In addition, inductive sensors only can be applied on terminations where the metal screen at the cable end has a separate ground lead (a path for the PD pulse) to the general grounding system. In a termination where the metal enclosure of, for instance, the switchgear is fully bonded with the metal screen of the power cable, such an ideal location for an inductive sensor cannot be found. Since PD-OL currently is based on inductive sensors, this measuring system is optimized for terminations without full metal enclosures.

Before a PD can be measured, it is important to calibrate the system. In PD-OL a smart solution is applied. At each cable end, steep pulses are injected, and measured separately at both cable ends. Magnitude and shape of resulting signals measured with the inductive sensors depend on the impedances of the power cable under test and the connected equipment at the transformer house or substation. In this way, the transfer impedance for a pulse coming from the power cable and entering a sensor is calculated. This information is used for the actual calibration of PD-OL. Again, the pulse injection method is crucial here.

PD data acquisition is carried out at both cable ends. There the first step of the data analysis is done to extract possible PD signals from the noisy data, remove interfering signals and thereby reduce the size of the data to be communicated. The resulting data is sent by LAN, modem, and telephone connection or GPRS to a server. Here, the final PD map is made, based on PDs and their time of arrival.

The costs of the PD-OL equipment together with the interpretation of the results are only a fraction of the costs of replacing a cable connection. The costs that will be reduced are the costs related to outages. These costs usually are only partly direct costs for the utility itself. Furthermore, if the knowledge of the condition of the cable network improves (which happens when using PD-OL) it will help the utility with replacement strategies. If utilities can postpone cable replacement investments, benefits are huge compared with the costs of PD-OL.

Current Status

PD-OL has been demonstrated successfully on several sections of differing types and lengths of cable in the Netherlands, and is now at the first production stage, with approximately 70 systems on order for several utilities in Europe. The activities foreseen in the second half of 2006 are