Electricity networks have had to become more flexible, reacting more quickly to the changes and needs of higher performance. This demand for flexibility is especially true where multiple generation sources occur, such as the intermittent power from renewables like solar and wind energy. While modern SMART grid technology can quickly identify faults in sub-station and generating units, cable faults can still occur anywhere on the network. Quickly locating a cable fault for repair is the top priority for avoiding lengthy restoration times. In the first place, this will be especially impactful on electrical utility reliability indices. These indices include SAIDI, the System Average Interruption Duration Index, which measures the total duration of an interruption for the average customer during a certain time period, and SAIFI, the System Average Interruption Frequency Index, which is found by dividing the total number of customers interrupted by the total number of customers served.
Activities centred on cable-related maintenance include:
- Cable fault location: where a cable has failed and testing is done to locate the fault
- High voltage testing; proof testing (withstand testing) is used to determine whether the cable can withstand an applied test voltage (usually greater than the operating voltage) for a specified period of time without any breakdown of the insulation
- Cable diagnostics; predictive testing – used to recognise the aging and general quality of the cable’s overall insulation
Minimising cable failures
A cable test program encompasses high voltage testing and diagnostics. The key objective with cable testing and diagnostics is to increase system reliability by preemptively identifying defects in the cable that might result in its eventual failure. One of several additional benefits is the cost: it is more economical to fix defects early rather than repair a cable when it fails. In order to conduct a successful cable test program, the following must be identified
- The goal of the program (e.g. assessing individual circuit reliability or prioritising cable replacement as part of a system-wide upgrade programme)
- Cable types to be tested (PILC, XLPE, EPR, etc.)
- Test methods to be used (some test methods cannot be applied to all types of cables; for example, a DC hi-pot should not be performed on XLPE cable because of the further aging and or damage it can cause.)
Cable problems can be generally characterised as local or global (e.g. affecting the entire cable span). Local problems include those at terminations, joints and in the insulation itself. A local insulation problem may be caused by a cable weak spot, but can also be man-made. Insulation may also be faulty on a global scale, such as general age related degradation, water trees throughout extruded type cables, etc.
If planning system-wide cable replacement and attempting to order which cable runs to replace first, one should make a global cable insulation condition assessment. Here the focus is on cable insulation on a macro scale (e.g. how deteriorated is it?). This assessment calls for diagnostic tests. If the cable run tests well, it receives the lowest replacement priority. Should a service failure occur, the it should be repaired and put back into service. On the other hand, if the cable run tests poorly, it receives highest replacement priority. If a service failure occurs, conduct an emergency repair as necessary or ideally replace the cable immediately.
If assessing individual circuit reliability, this requires both a global insulation condition assessment (to check for a condition that affects the cable insulation throughout, such as general degradation and aging of the cable or water trees) and local condition assessment (such as a critical water tree combined with electrical tree somewhere in the insulation, or a problem with a termination or splice). Global diagnostic methods include Tan Delta measurements and IRC/RVM measurements. Local diagnostic methods include partial discharge (PD) measurements. Depending on the situation either both or only one method is needed. For quality control on newly installed cables it is only recommended to perform a local PD diagnosis. A global diagnosis is not required because the cable insulation is new.