Stator insulation faults are the second most common cause of failure in large rotating machines. Partial discharge (PD) is a reliable measurement parameter used to assess the condition of insulation in rotating machines. PD occurs in
the insulation system of rotating machines, where local electric field stress exceeds the local electrical strength. The insulation materials typically used for rotating machines are resistant to a certain level of PD. An increase of PD activity can indicate insulation degradation caused by overheating, load cycling or mechanical stress.
Successful PD measurement in stator windings is based on the separation of parallel PD sources, and the distinction between harmful PD, normal PD occurrences and external noise inevitably present in industrial surroundings. To achieve this, the following techniques are applied:
Synchronous multi-channel data acquisition Multi-spectral evaluation
Advanced noise suppression Source separation techniques:
3PARD (3-Phase Amplitude Relation Diagram)
Automated cluster separation
Depending on the accessibility of star point, measurement setup will be chosen. Figure 2 shows a basic measurement setup for an offline single channel PD measurement on the open star point of a rotating machine. The voltage
(no specific voltage source) is applied at the open star point. The measurements are performed phase by phase (Phase U1) where the unused terminals are grounded. The setup is as described in IEC 60034-27: open star point. This measurement setup aims for an assessment of winding insulation between phase and laminated core.
OMICRON even supports a setup for combined measurement system as shown in Figure 3. This extended setup combines the measurement of winding insulation capacitance, Power Factor/Dissipation Factor (PF/DF) and PD in one setup using CPC 100 + CP TD1 as voltage source and CP CR 500 for reactive power compensation with open star point (if accessible) using 3 channels. The additional BLI1 on top of the coupling capacitor is used as blocking impedance to filter undesired PD from the voltage supply (CP TD1) in the standard IEC measurement frequencies 100-500 kHz.
This measurement setup brings many advantages. It is lightweight with portable voltage source due to compensation of losses. Parallel measurement of capacitance, PF/DF and PD are possible with no additional setup. Complete information about the condition of winding insulation can also be obtained. This measurement setup can also save a lot of time.
For technical enquiries, please contact Seokhoon Hong (Regional Application Specialist – Partial Discharge) via email seokhoon.hong@omicronenergy.com or contact ENTEC A&T’s Email: support@automationandtesting.vn
