Voimatel has expanded its range of services to the performance of partial discharge (PD) measurements on underground cables in electricity distribution networks. Voimatel’s Grid Services have acquired PD measurement equipment which has been used by the Pohjanmaa team since last spring. PD measurements are complementary to standard commissioning tests and are also carried out on existing cables.
Grid operators have partial PD measurements carried out on new networks being commissioned as well as on existing cables whose condition can be checked at the same time. Measurements performed on new cables make it possible to identify potential weak points that would not necessarily be detected in a standard commissioning test.
– The meter shows weak spots that may later break down or otherwise fail. Usually a failure is caused by the ingress of dust or moisture into the cable joint or terminal or because the joint is not crimped properly, says project leader trainee Janne Silvonen who is preparing his graduation thesis on partial discharge measurement.
Measurements help grid operators achieve considerable savings
PD measurements make it possible to detect a fault before it is too late. In fact, measurements are becoming increasingly common as grid operators want to improve the reliability of their networks. Normally, measurements start with the most important cables.
– If a large mill site or wind farm needs to be shut down because of a blackout, the cost to the grid operator is substantial. This can be avoided by proactive PD measurements, which makes it possible to repair the faults on time. Most grid operators are already carrying out random measurements, says project manager Ulf Hultholm.
Analysis of measurement results calls for special expertise
Measurements can be made using the old VLF technique or the new DAC technique in which a pulsed voltage is transmitted to the cable. The new technology puts less stress on the cable and speeds up the process considerably. Measuring a single cable along its entire length takes about two hours.
An analysis of the results can be commenced while the measurements are still in progress to identify clear abnormalities. A more detailed analysis is carried out when a report is run on the measurement data. The meter does not show the cause of the problem, but if partial discharges or disruptions occur in the cable, the results can pinpoint the location of the fault to an accuracy of one metre. For instance, the analysis reveals whether the problem lies in the terminal or cable joint. At the same time, the measurements indicate whether the problem is acute or whether the repair can be postponed to a later date.
– That fact that we detect discharges doesn’t mean that the cable will fail tomorrow. If the results suggest that the fault is not acute, we can recommend to the customer that a new measurement is carried out in one or two years’ time before taking the decision on further measures. Of course, our recommendations depend on the size of the discharge and its location, the voltage involved and how important the cable is, says Ulf.
– If discharges start occurring below the operating voltage, you naturally need to be more careful.At the end of the day, it is up to the grid operator to determine whether corrective action is taken immediately or whether the situation will be monitored and the measurement repeated some time later, he adds.
It is the analysis of the results that is the most challenging part of the process and takes special expertise.
Measurements are being carried out as part of Vaasan Sähköverkko’s Maalahti project – growing interest in other parts of the country
The high-quality PD measuring equipment was acquired last spring and operator training provided for two employees and three installers in Voimatel’s western district. The equipment is mounted in a van.
Currently, PD measurements are being carried out on all 20 kV cables in Vaasan Sähköverkko’s Maalahti project before commissioning. One significant partial discharge was detected by the measurements with the result that a new cable was laid to replace the existing one.
– There is also increasing interest in the measurements in other parts of the country. We’ll carry out measurements wherever there is demand for this service and provide training for additional personnel, says Jari Nurmenrinta, Regional Manager of the Western Grid District.
Project leader trainee Janne Silvonen carrying out PD measurements.