National Grid Electricity System Operator
National Grid TO Innovation Team
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Gas Distribution Networks
National Grid's technical specifications require MI cable to be tested to the
internationally accepted CIGRE test procedures. As the operating voltages of DC
cables increase cable manufacturers are progressively taking the view that the
CIGRE test voltages are too severe and unless the test voltage is reduced
(particularly during the cooling phase of heat cycling) there is an unacceptably
high risk of the cable failing the type test.
In order to achieve type registration of these cables it will be necessary for
National Grid to consider relaxing the test voltage. There is no published basis to
justify this reduction and it is difficult to assess the risk of accepting cable
systems which cannot meet the CIGRE requirements.
There is a possible mitigation strategy based on applying condition monitoring
techniques during type testing so that the test is not reliant on simple withstand
criteria. When a MI HVDC cable fails the heat cycle type test it is likely to be the
result of accumulated Partila Discharge (PD) damage. Hence PD monitoring appears to be the most
appropriate option to investigate.
PD detection in DC systems is significantly more difficult than in AC systems
because (i) the discharge repetition rate if far lower and (ii) there is no alternating
voltage to which the discharge activity is synchronised. It is therefore difficult to
distinguish between PD activity and random background noise.
Recent work at Southampton on PD from AC cable systems indicates that
clustering algorithms can be used to distinguish between PD from different
sources. It appears feasible to use this technique during DC testing to distinguish
between PD from the cable and that from the terminations or external noise
sources. The technique relies on analysing the PD signals to measuring the
energy content in a number of time and frequency windows. The multidimensional
results are converted to a pseudo 3-dimensional data set for easier
visualisation and automatic classification.
In addition to developing a procedure to detect and classify DC PD signals the
work will emphasise the need for the technique to be suitable for implementation
during DC cable type tests. This requires that PD testing can be done safely in
an industrial laboratory without impacting on the smooth running of the type test.