High Resistance Connections Detected by On-Line PD In Motor

Published Aug 29th, 2019


An US oil refinery

Machine Description:

20.5 MW, 13.8 kV Motor

CASE STUDY: High Resistance Connections Detected by On-Line PD In Motor

On-line PD tests were indicating high levels of activity at the zero line to ground voltage crossing (angles of 0º and 180º) on one phase (Figure 1). This can result from:

  • Degradation of the semi-conductive coating material at the slot ends, or the interface between this and the grading material in high voltage coils at or near the slot exits, or
  • Loose or oxidized connections near the line ends of the stator winding.

The motor was shut down for further investigation. Overheated bolted connections were found at the winding line and neutral ends (Figure 2 & 3). A failure in service was thus averted. Off-line testing confirmed the rest of the stator winding was in good condition.

Figure 1. Peak PD levels at 180º

Figure 2. Overheated Connections in Motor Terminals

Figure 3. Damaged Flexible Connectors

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