epoxy-based resin. Te 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. Te 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. Te 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. Tis 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. Tis 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.

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