Offline Testing of Rotating Machines
Partial Discharge Testing
Partial discharge (PD) is a localised electrical breakdown phenomenon that occurs within the stator insulation system of motors and generators when the applied electrical stress exceeds the insulation's dielectric strength. PD activity often signals insulation deterioration, serving as an early warning of potential insulation failure. Unlike other insulation testing techniques, PD measurement uniquely enables the detection, localisation, and characterisation of specific insulation weaknesses, making it a critical diagnostic tool in HV asset management.
Rotating machines are particularly susceptible to PD activity due to high electrical and mechanical stresses. Timely and accurate PD analysis is essential for maintaining operational efficiency, minimising downtime, and extending equipment life.
Offline PD testing is conducted when the machine is de-energised. Individual windings are energised independently using an external high-voltage power supply. This is ideal for detailed analysis during scheduled maintenance, providing a controlled environment to isolate PD sources by raising the voltage in incremental steps to the withstand test level, while monitoring the PD activity.
Tan Delta and Power Factor Measurements
The insulation system is a vital component of rotating machines, directly influencing the lifespan of motors and generators. Weak points in the insulation may originate during manufacturing or insulation processes. Additionally, these vulnerabilities can develop over time due to operational stresses, such as contamination, or asset ageing.
Advanced diagnostic techniques like tan delta and power factor measurements enable the identification of changes within the insulation. These methods provide insight into ageing processes, structural alterations, or moisture ingress, all of which can lead to reduced insulation reliability.
Insulation Resistance and Polarisation Index Testing
Insulation Resistance (IR) and Polarisation Index (PI) testing are key diagnostic tools for maintaining the reliability and performance of high-voltage rotating machines. These tests help identify insulation issues such as moisture ingress, contamination, and ageing, which can lead to costly failures if left unaddressed.
IR testing provides a snapshot of the insulation's ability to resist leakage currents, while PI testing offers deeper insights into its long-term performance under electrical stress. Together, they deliver a clear understanding of insulation health, enabling targeted maintenance actions.
By integrating IR/PI testing into routine maintenance programmes, organisations can minimise unplanned downtime, extend equipment lifespan, and maximise the reliability of critical assets, ensuring safe and efficient operations.
Borescope Inspection
Borescope inspection is a highly effective technique for assessing the internal condition of high-voltage rotating machines without requiring extensive disassembly. Using a specialised camera (borescope) inserted into access points, this method allows for detailed visual examination of critical components, including windings, cores, and rotor assemblies.
By identifying issues such as contamination, mechanical damage, or insulation deterioration, borescope inspections provide valuable insights into the machine’s health. This non-intrusive approach supports targeted maintenance, and ensures the continued reliability of assets.
Incorporating offline testing into maintenance routines helps prevent unexpected failures, optimise performance, and extend the operational life of critical equipment.
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