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MOTORS, DRIVES & CONTROLS Specifying low PD as a critical


requirement for high-voltage motors and generators is not always the most effective way of tackling PD


UNDERSTANDING I


ndustrial plant operators are naturally concerned about the impact of partial discharge (PD) on the health and reliability of their high-voltage


(HV) motors and generators. Te PD phenomenon occurs when a very small spark is produced due to a high electrical field. PD triggers a short-lived plasma burst. PD exists in two forms: internal and external. Internal PD happens inside the insulation within microscopic air-filled voids, while external PD occurs on the surface of the insulation when the local electric field stress exceeds the threshold to ionise the air. Te phenomenon stresses the equipment’s insulation because of its rapid, localised temperature spikes and the creation of substances such as nitrogen oxides, ozone and nitric acid – all of which can be chemically aggressive against organic materials. Tese effects can also affect the surrounding structures and can result in a reduced lifetime of many materials inside a motor or generator,


54 www.engineerlive.com


Designing high-voltage motors and generators with low partial discharge (PD) can be costly and fail to deliver reliability. Jari Jäppinen explains why it is better to design equipment that fits its specific purpose


PD PHENOMENON


leading to equipment damage or even downtime of processes. Many plant operators are therefore


increasingly specifying low PD as a critical requirement for their motors and generators, believing that this will reduce the risk of failure and improve reliability. However, this is not always the most effective way of tackling PD and may only result in higher costs and over-design with no guarantee of a longer lifetime or increased reliability.


Tis conclusion is supported by


test programmes that show no direct correlation between PD magnitude and insulation lifetime. In fact, for a particular motor or a generator, it may well be that electrical stress and the resulting PD is not the main cause of insulation ageing.


LONG-LIFE EQUIPMENT TO COMBAT PD Designing high-voltage equipment with robust insulation for its specific


purpose will more effectively ensure a longer life. Any equipment design process should include specifications for hardier insulation that covers all four of the thermal, electrical, ambient and mechanical (TEAM) stresses that contribute to ageing.


Te quality of the insulation plays a


crucial role in reducing PD and related impacts. A particularly effective insulation is that found in modern vacuum-pressure impregnated epoxy-mica based systems. At ABB, this type of insulation is used for the stator windings of high-voltage motors and generators. Since the mid- 1970s, thousands of these units have been manufactured and the insulation has established an excellent track record for reliability.


INSULATION TO WITHSTAND PD Modern high-voltage insulation systems are based on a form-wound technology built on inorganic mica together with an


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