By HVPD UK Director, Dr Lee Renforth and Lord Group Managing Director, Trevor Lord.
In a modern electricity distribution business, condition-based asset management is one of the essential tools to enable the reliable, cost-effective life extension of existing plant and cables to be achieved. Timely location and the targeted, pinpointed replacement of specific cable sections and accessories of unsatisfactory circuits enables the effective service life of the whole network to be economically extended.
Modern condition-based asset management programs are aimed at directing limited investment to those networks with the poorest performance and the highest operational costs. With annual replacement rates of plant and cables in utility medium voltage (MV) networks around the world at an all time low (typically less than 1 per cent and in some cases as low as 0.1 per cent) an alternative to the wholesale replacement of ‘worst-performing’ circuits is necessary.
Recent developments in the field of condition monitoring of both MV electricity networks have seen the ‘coming of age’ of new, on-line partial discharge (PD) detection and monitoring technologies. The new technologies enable cost-effective, on-line PD testing and monitoring of in-service cables, switchgear and other plant without the need for an outage. PD activity is produced by incipient faults in high voltage insulation and is widely regarded as the best early indicator of insulation degradation, providing an ‘early warning’ against future insulation faults.
Studies on large utility MV networks show that the largest savings are to be made in deferring and/or targeting capital replacement programs of the cable and plant population. This is simply due to the capital cost of replacement being so large compared to the other costs, that saving even a small amount of this cost tends to dominate the others. Conversely, studies on industrial and commercial networks show that a different ‘cost driver’ normally dominates. In these cases, depending on the process which depends on the power supply, the cost of an interruption normally dominates the capital costs of replacement.
Oil platforms, industrial processes, nuclear reactors, data centres and other high-profile power supplies can have a very large cost of interruption, running into many millions of dollars per day. Such customers have less difficulty in justifying the cost of diagnostic measurements, or complete system monitoring. As a ‘rule-of-thumb’, if the cost of an outage is greater than say $300,000/day, such asset owners tend to be very focussed on the condition of their MV and HV plant!
It is recommended that any PD test and monitoring technology utilised is of portable design and is suitable for mobile use in the field for stand-alone operation with built-in mobile GPRS communications. By using portable technology, moved around the network to points of concern, it is possible for the insulation condition assessment to be achieved at a fraction of the cost of permanent PD monitoring systems.
The technology is best applied using the concept of a ‘four-phase’ condition-based asset management approach. This provides for a systematic and cost-effective framework which can identify, diagnose, locate and monitor PD activity (and therefore ‘incipient faults’) within the distribution network. In essence, this concept would see the application of low-cost portable PD survey devices to the total population of MV cables and switchgear under the care of a given asset owner.
Whilst necessarily likely to be a bit of a ‘blunt instrument’, surveys by such devices are exceptionally quick and cost effective, providing an excellent means to pre-screen all plant at modest cost. Thereupon, typically 20 per cent of the plant surveyed might require fuller diagnostic analysis and these devices are also offered in portable format but, being more sophisticated in their operation, the pre-screening concept ensures the maintenance budget is allocated appropriately to the issue and not wasted on unwarranted screening per se. Up to 10 per cent of the total plant may be of such a condition as to prompt longer-term study and qualification, whereupon a portable PD monitoring unit with wireless telemetry is installed. These are now offered with the ability to determine direction of arrival of PD pulses and aid in remote location of an issue in real time.
Finally, in about 1-5 per cent of plant, reasons of strategic significance of an asset or its condition may prompt a permanently-installed on-line monitor. The ‘holy grail’ of PD technology for many years, these are now offered and able to determine reliably the onset of concerning PD issues and raise alarms in a timely manner.