On-Line Partial Discharge (PD) Testing of Rotating HV Machines

The OSM-Longshot™ PD Spot Tester/Monitor	PD Sensor Options For Rotating HV Machines	Portable HFCT Connection in 6.6kV Generator Cable Box

The OSM-Longshot™ System from IPEC High Voltage has been applied successfully to the On-Line Partial Discharge (PD) Testing of Rotating High Voltage Machine Insulation. On-line PD testing is now widely accepted in the industry as the best way to assess the condition of in-service HV motors and generators.

To meet this market demand, the OSM-Longshot™ system and PDGold© software includes a dedicated 'Machines Software Module©' which has been specifically designed for the PD Testing of HV motors and generators. The technology provides the user with the ability to carry out PD testing of HV machine insulation without the need for an outage.

• PD measurements can be made On-Line using splitcore, High Frequency Current Transformers sensors around the supply cable.
• PD measurements in picoCoulombs (pC's).
• PD activity is presented in simple colour codes based on the PD 'criticality' (PD magnitude x no. of pulses per power cycle).
• Automatic diagnostic of whether the discharge is in the Slot Section or the End Windings.
• The OSM-Longshot™ Unit can also be used to test for PD activity in the HV switchgear and cables connected to the HV Machine.

A number of PD sensor options exist for the measurement of PD in HV Machine insulation, including built-in High Voltage Capacitive Couplers, permanent High Frequency Current Transformers (HFCT's) and Portable, Split-Core HFCT's.

It is now becoming increasingly common for the owners of large HV machines to fit permanent HFCT's or 'HV Capacitive Couplers' on each phase of the machine. The alternative to this for smaller, lower power machines is to use portable, split-core HFCT's attached around the supply cables to the machine. IPEC HV supply split-core, Rogowski Coil HFCT's and high current, ferrite HFCT's for these applications.

For HV motors & generators of voltage rating 11kV and above it is usual to have individual cables for each phase and it is therefore possible to make a separate measurement for PD on each phase.

With an HFCT sensor connected around each phase of the supply cable (see diagrams opposite) it is possible to identify discharges involving both phase-to-phase insulation and phase-to-ground insulation.

With motors in the voltage range of 3.3 - 6.6kV it is normal to have 3-core, 'belted' supply cables and the On-Line PD Test is made with the HFCT transducer fitted around all three cores of the cable. In this case the OSM-Longshot™ unit will see only the phase-toground discharge pulses. Whilst this does not allow for the detection of phase-to-phase discharge pulses it does allow for measurement of the overall PD magnitudes as part of a 'PD Trending' analysis.

PDGold© Software showing 3x phases of PD data

The PDGold© Machines Module automatically detects whether the PD is 'Slot-Type' or 'End Winding-Type'.

The magnitude of these two categories of pulses are measured in pC's for each phase and presented on the user interface screen shown on the right.

Depending on the results of these calculations, the PD activity is rated in six scales spanning the various insulation condition categories.

Table 1 above shows the Guideline PD Levels vs Condition for HV Machines in the voltage range 3.3kV to 13.8kV. These are coloured from Grey (Low PD - New/Excellent condition) to Red (High PD - Problem with insulation, rewind/repair required).

Correct Placement of Permanent HFCT Inside Cable Box	Correct Placement of Portable HFCT Outside the Cable Box	Permanent HFCT Installation in HV Generator

IPEC HV supply a wide Range of PD Sensors and accessories including:

• Permanent Rogowski Coil and Ferrite HFCT's
• Portable, Split-Core, Rogowski coil and Ferrite Portable HFCT's
• High Voltage coupling capacitors