Condition Assessment of MV Motor Based on Charge Difference and Charge Ratio Analysis

Insulation resistance and polarization index tests have been conducted in utility companies as one of the major tools for evaluating the integrity of stator winding insulation of medium voltage motor. Unfortunately, the information obtained from the measurement results are the combination of the electrical problem and polarization problem in the insulation system not separated from each other. This paper presents a new approach so-called charge difference analysis (QDA) and charge ratio analysis (QRA) to identify the problem in stator winding insulation occurred from conduction mechanism or/and polarization mechanism. Moreover, proposed parameters which are QD, QG and QR have been conducted on field motors in the following case studies: the first group (A) consists of two motors of which one is new motor and the other one is severely aged motor, the second group (B) consists of two motors of which one is lightly loaded motor and the other one is thermally aged motor, and the third group (C) consists of a single motor, on which PDC measurements have been carried out after maintenance in 2014 and 2016 respectively. From experiments, it was found that proposed parameters are able to provide useful information about the condition of stator insulation, i.e. contamination or/and degradation.