When and Why to Monitor Partial Discharge

Partial Discharge Monitoring (PDM) on transformers is not a new concept. As the most expensive asset in a substation, they are the subject of constant monitoring and analysis. So much so, that some monitoring can appear to be redundant. One such case that customers ask us about is the need for Partial Discharge Monitoring in the presence of Dissolved Gas Analysis (DGA). Both can provide early warning of issues within a transformer, and both can even provide insight as to what type of defect is causing Partial Discharge or Gassing, but how they do it, their timeliness and the insight that can be gained from each varies greatly.

What PD is and why we need to monitor for it?

PD in an electrical insulation system indicate flaws that may or may not develop into a full insulation breakdown depending on the type of insulation system and the nature of PD. Transformer International standards require electrical tests and electrical measurements to detect PD as routine tests, where the PD level is an important indicator of transformer quality.

PD usually begins within voids, cracks, bubbles, inclusions, or contamination within or along the surface of the insulation system. Since PDs are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials due to the difference in their dielectric constant.

Partial discharges within or along the surface of an insulating material are usually initiated when the voltage across a portion of the insulation system exceeds PD inception voltage, PD activity will then start in this portion of the insulation system, which may or may not occur adjacent to the conductor. PD inception voltage may be decrease over time by the aging of the insulation system, contaminants and/or reduction of the dielectric withstand value of the insulating liquid.

As mentioned above, there will be PD that may develop into a full insulation breakdown and become a major or a catastrophic failure, in some cases it will evolve so fast that no other type of monitored parameter will be able to detect it on time, but PD monitoring. The reason why it can be detected it so fast is because PD monitoring is done in real time, the moment PD appears it is detected. Those fast evolving faults may result when the bridged portion of the insulation system increases the voltage of the adjacent portions, to the point that in those it is also exceeded the PD inception voltage, bridging them as well, creating a chain effect.

PD monitoring offers rapid detection of even small changes in activity that may be precursors to failures and allows correlation with other parameters such as switching events, thermal overload etc., and usually able to localize the PD source for additional risk assessment, as well as being a good indicator as to the insulation system health on the short to medium term.

Now, what DGA is and how does it work?

Dissolved gas analysis (DGA) is a method to diagnose incipient faults of transformers through the correlation between the content, quantity and rate of production of gases dissolved in transformer oil and a particular malfunction. Extensive historical data collected by laboratory analysis over the years, allows for accurate result interpretation. Today, the method is increasingly complemented by on-line DGA monitoring of transformers.

DGA is the heart of on-line monitoring as it is a well-established method of transformer diagnosis for timely potential thermal or electrical faults detection, especially in the context of the ageing worldwide transformer fleet. DGA provides a low-cost solution for maximizing transformer life and minimizing unexpected failures and is likely to remain the “workhorse” tool for a long time, since it can detect about 70% of transformer incipient faults. But DGA results as an indirect PD measurement are slower to identify fast evolving PD due to the amount of time for the combustible gases to be distributed throughout the transformer insulating liquid and reach the sampling port of the transformer tank. Experience shows that with DGA a minimum amount of gas formation is necessary to declare a PD activity to be significant. On the other hand, there are several practical examples where no increase of combustible gases was recorded despite a PD source being detected by electric, acoustic, and electro-magnetic (UHF) methods and confirmed through disassembling the transformer. Therefore, electric, acoustic, and electro-magnetic PD pulse-based measurements are of a more instantaneous and direct nature.

As stated above Partial discharge monitoring excels on fast escalating faults, in these cases prior to accumulation and sampling of gasses at a level which would indicate an issue through DGA. Under this circumstance no other options exist to indicate an issue exists allowing the user to take action to save the asset, and more importantly preserve the safety of the people within close proximity. While this is not the most common failure mode of transformers it is frequent enough that we have many customers who implement Partial Discharge Monitoring to supplement their DGA monitors. This has resulted in saving more than a dozen high voltage transformers, saving nearly one hundred million of dollars of equipment, and drastically improving the safety record of each site.

While Partial Discharge monitoring is not a substitute for other forms of Transformer monitoring, it has carved out its place in the transformer monitoring world, covering a critical and sometimes dangerous gap between the capabilities of all other forms of transformer monitoring.

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