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
Industrial 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. The 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. The 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. These effects can also affect the surrounding structures and can result in a reduced lifetime of many materials inside a motor or generator, 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.
This 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.
The 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 epoxy-based resin. The systems undergo a global vacuum pressure impregnation (VPI) process, which helps to reduce the creation of the microscopic voids that lead to a build-up of charge. The use of mica makes the insulation resistant to PD and, under normal operating conditions, the system can withstand PD throughout the lifetime of the equipment, meaning only periodic maintenance is needed. ABB motors and generators manufactured with these materials and processes have had no premature failures caused by PD.
The normal design lifetime of an industrial high-voltage motor or generator is around 20 years or more. Some manufacturers offer special options for applications where a very long lifetime is crucial and these include ABB’s Insulation+ product designs, which have extra layers of insulation to further reduce electrical stress. Lifetime+ motors and generators offer the same benefits as Insulation+ together with reduced thermal stress.
Monitoring still important
Designing equipment to resist the effects of PD does not mean that PD monitoring should not form part of best practice when it comes to the maintenance and operation of high-voltage equipment. In some cases regular monitoring may be very important and PD measurements can, in fact, reveal problems that may otherwise be difficult to detect. PD measurements are notoriously hard to evaluate if carried out on a ‘one-off’ basis so PD testing should form part of a periodic maintenance regime that enables trends and patterns to be identified.
When it comes to monitoring, it is advisable to take a fingerprint measurement before or during the initial commissioning phase to establish a baseline condition and to determine how to carry out periodical monitoring.
However, measuring PD alone cannot provide a comprehensive picture of the health of a motor or generator and it is vital to perform a whole range of inspections and evaluation processes. For example, the ABB Life Expectancy Analysis Program (LEAP) provides regular onsite assessments of the condition of motors and generators. In addition to PD, these services can also include an analysis of polarisation-depolarisation currents, non-linear insulation behaviour, tan delta and capacitance, and other assessments. This will generate a complete picture of the state of the equipment and help maintenance with decisions regarding upgrade and replacement.
Overall, ABB advises its customers to broaden their focus beyond the specification of low PD levels when procuring their new motors and generators. This is because they face the risk of increased equipment costs and will still not achieve the reliability and long service life they seek. Instead, it recommends that PD should be accepted as a factor that can be lived with and without any negative effects as long as the insulation of the high-voltage equipment is specified correctly to withstand the effects of PD in balance with other TEAM stresses.
Jari Jäppinen is motors and generators technology manager at ABB