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How to tackle partial discharge in high voltage motors and generators

Selecting a reputable manufacturer with a strong record of quality and expertise is the best way to ensure you avoid issues with partial discharge.

There’s an increasing trend for plants to stipulate low partial discharge (PD) when they specify high voltage motors and generators. Unfortunately, this is not an effective way of tackling PD. It leads to higher costs and overdesign, but it won’t necessarily ensure a longer lifetime or increased reliability.

In our talks with customers about high voltage motors and generators, partial discharge is often raised.  It’s a kind of very small spark that occurs due to a high electrical field, and it’s found in high voltage equipment including cables, transformers, motors and generators. PD generates a short-duration plasma burst and emits UV light, causes rapid localized temperature spikes and creates chemically aggressive substances.

These effects impact the surrounding insulation, which should therefore be designed to withstand PD.

Key roles for design, materials and workmanship

At ABB we use the Micadur® Compact Industry and Enduraseal® systems for the stator windings of our high voltage motors and generators. Under normal operating conditions the insulation system will withstand PD throughout the equipment’s lifetime, and require routine periodic maintenance to manage PD. We’ve manufactured thousands of motors and generators with these insulation systems since the mid-1970s, and in that time we haven’t had any known premature failures caused by PD.

The insulation system, together with the quality of the design and workmanship, and selection of materials, play a central part in determining the level and impact of PD activity.

As a result, the first step in tackling PD has to be taken well before the motor or generator is ordered. It involves choosing the right manufacturer. Selecting a reputable manufacturer with a solid track record will go a long way towards ensuring there will be no issues with PD.

Detecting and measuring PD

At the same time it’s worth thinking about PD measurement and level tracking.

In most cases it does make sense to measure PD, because it can reveal problems that would be difficult to detect otherwise. However, individual PD measurements are notoriously difficult to evaluate. A better option is to take periodic measurements, starting with a fingerprint measurement on the new winding, in order to obtain trend data.

Some plants prefer a more comprehensive approach vs. measuring PD alone: at ABB, we offer ABB LEAP (Life Expectancy Analysis Program), which is performed at regular intervals. ABB LEAP measures PD, along with a number of other measurements in order to evaluate the remaining lifetime of the stator winding insulation. In addition, it’s a good idea to carry out visual inspections.

Issues that need to be decided are: Will PD be measured, and if so will it be done by plant personnel or outside contractors? What method will be used? And will additional assessments and inspections be performed?

These questions need to be answered at an early stage because PD measurement requires investment and may also require special hardware.

Does it make sense to stipulate low PD levels?

For plants that are reluctant to invest in PD monitoring, an alternative is an extended-lifetime motor or generator. While dependable manufacturers ensure that their standard products are designed and built to withstand PD, enhanced products like the Insulation+ and Lifetime+ motors and generators from ABB will provide an extra ‘safety margin’ and longer service lifetimes.

By contrast, we don’t recommend specifying low PD levels during the procurement process. This inevitably leads to higher costs, and often doesn’t produce the desired results. This is because there’s no direct correlation between PD magnitude and insulation lifetime. For a particular motor or generator it may well be that electrical stress is not the dominant factor behind insulation aging. And, in any case, it doesn’t really mean anything to say that PD is “low”, because individual measurements of absolute PD values are very difficult to evaluate.

For more information take a look at our recent white paper on partial discharge in high voltage motors and generators.

partial-discharge-whitepaper

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