Lord Consulting
Power Industry Asset Management Consultants
Lord Consulting
Power Industry Asset Management Consultants
Home » Technical Papers » On-Line vs Off-Line Partial Discharge Testing
Over the past nearly three year period that we have been discussing the physics of partial discharge, its relevance to the maintenance and reliability of power systems assets, and the evolution of the partial discharge analysis technology
Whereas extended research over some 25 years had been undertaken primarily in the UK into practical field surveying technology for PD activity in cables and switchgear especially, until very recently the equipment on offer was more research orientated than practical, being both cumbersome, complex to operate and interpret, time consuming to use and, notably, extremely expensive to acquire. Whilst on-line surveying of cables and switchgear has been a reality for some years, it existed in primitive form making the interpretation and quantification of observed data challenging and really only possible at the hands of the research engineer.
Particularly in the case of cables, quantitative PD surveys have more generally been conducted off-line using HV VLF (0.1 Hz) sources of limited voltage and power, thus imposing severe restrictions on cable lengths that could be assessed. Off-line surveys certainly have been of great significance in the PD assessment of cables and especially suited to the final investigations of circuits which have established problems. When it came to the enhanced quality and clarity of early cable PD data obtained, and the study of the influences of the effect of system voltage on the PD inception, off-line PD work has had a major part to play. Importantly too, it was off-line PD technology also that provided the genesis for the development of the first PD site location work, possible via the low residual noise level on the cable in its out of service state permitting the marking of the cable ends and thus a distance calibration for the mapping which followed.
However, off-line mapping has in recent years prompted an increasing level of Industry anxiety. Primarily, the requirement to de-energise cables to conduct the surveys has met with mounting Network concerns as to both the feasibility of doing so in a climate of enhanced asset utilisation over that of earlier years, coupled with the very high costs that the surveys attract by virtue of a very limited number of surveys being possible in one day. Such concerns have been exacerbated by the considerable cost and technical constraints of the equipment needed for off-line surveys, these limitations seriously affecting both the system voltage and the cable lengths that could be assessed. Importantly too the limitations off-line PD assessment techniques on cable, especially, fell well short of delivering the desired Network goals of a formal asset management approach to the administration of all relevant network assets.
Noting these concerns, AVO New Zealand/ LORD Consulting and its PD partners High Voltage Solutions and M&B Systems Power of the UK have invested heavily in recent years to revisit the on-line PD surveying technology to offer a result quality that would more closely, and certainly adequately, emulate the achievements of the off-line technology whilst taking advantages of the list of obvious major enticements provided by on-line surveying: huge increases in the speed of survey (down to a few minutes per cable or switchgear item); lower capital cost and bulk of equipment; the fact that an asset remains in service during its assessment; the ability of users to conduct surveys on energised equipment of any working voltage at all; and the obvious corollary of a plummeting of the survey costs per asset item!! The partnership above has now produced practical, state-of-the-art, longer-range, and modestly-priced PD surveying technology that leads the world in the quality of the quantitative assessments possible and, very importantly, the breakthrough in being able to perform on-line PD mapping even under the challenges of in-service noise and disturbances!!
Whilst the advent of the latter technology has been revolutionary, on-line cable PD mapping and monitoring has brought with it some very significant surprises in terms of the quality of the data obtained. The reasons for this come down to a composite effect of several complex physical interactions within the operating cable that are not present in its off-line state.
Looking at these, the prime effects of note are: the service factors such as operating temperature and its effect on PD via thermal expansion characteristics and material properties; mechanical loading influences; current-based effects; PDs behaving slightly differently at 50 Hz vs. 0.1 Hz; and long-term energisation effects (i.e. insulation responding differently because it has been energised for a long time, compared to initial energisation). In totality, the reality that has emerged from on-line PD assessment is that On-line testing is nearer to the "PD truth", due to the composite effect of all these 'service factors' on the phenomenon.
Indeed, the phenomenon above has been increasingly observed in PD maps (obtained off-line) that they have often shown an increased number of active PD sites than appears in reality using the latest on-line PD mapping technology. Not only that, the active sites detected by the on-line methods have tended to be an accurate reflection of the sites of concern. A typical example is illustrated in Figures 1.
It must always be remembered that no matter how good the surveying device that is used to quantify and qualify the PD activity of the day of measurement, it is purely a record of the way the asset was behaving under the performance and load conditions at the time. Whilst it is almost certainly the case that PD activity does not start and stop completely with variations in the influences on the site, it is well proven that various causes of the PD observed can be greatly affected by variations in the conditions in which the item is operating, such as system loading and environmental factors. A dramatic illustration of this effect in the case of an 11 kV feeder subject to a loading-influenced PD may be observed in Figure 2. Were the observed PD conditions of a site to be judged such as to warrant more analysis, long term trending is essential. This is only practical by on-line PD monitoring and examples of companion products, now also developed to a high level of sophistication, such as our modestly-priced and simply-used 8 channel Cable Trend product developed from a pedigree of many years of successful PD monitoring at London Power Networks, UK. Whilst the wider implementation of the technology now offered must inevitably be reflected in a much higher incidence of the prevention of circuit failures, continued developments hold the key for actual on-line fault prediction of high sophistication in the near future.
By way of conclusion, then, major recent advances in on-line monitoring technology, only available on the market since the latter part of 2001, have served to revolutionise the approach to PD surveying, quantification, qualification, and site location.
On-line PD technologies now offer asset owners an excellent quality of PD survey data with minimal inconvenience, major productivity gains, and at a much lower survey cost per item than previously attainable via off-line methods.
Figures 1a & b: Comparison of PD mapping on the same 11kV feeder via on-line methods (1a) and off-line methods (1b). Only one site is common to each set of results, increasing field evidence showing similar effects and confirming the on-line site(s) as the PD site(s) of concern
Figure 2:
On-line PD recording of an Incipient MV Fault
This characteristic is often seen on feeders being monitored. Such a condition is ideally suited to the likes of the Cable Trend monitors which include an integral common alarm facility over all 8 channels.