Ensuring High Quality Insulation System of Large Motors-Design & Testing Requirements

In recent past, several high voltage motor winding failures occurred within one year of commissioning in two of NTPC projects. They include bearing failures, winding cable connection failure at termination and winding insulation failures in overhang region. During review it was found that some of the important requirements, which ensures healthy machine insulation system have not been taken care during manufacturing process. The commonly observed deficiencies in manufacturing process of end winding are:

Poorly done brazing at bus connection.

Poor workmanship causing uneven air clearances.

Selection of inadequate cable and lug size, leading to heating and subsequent failure.

Pre-VPI activities done in uncontrolled (non-air conditioned) environment.

This paper discuss the type of failure occurred, analyses the root cause and mitigation measures. It gives insight to best practices being adopted in machine manufacturing and testing to ensure high quality of insulation system. The importance of minimum clearance between two adjacent coils has been discussed in the paper. The paper also suggests the minimum clearances to be maintain between insulation surface of two adjacent coils of same phase and different phases. It emphasize on use of software based advance coil taping and coil spreader machines instead of old conventional practices to ensure shape of coil as per design. The use of automated coil spreading and automatic taping machine avoids presence of wrinkles, knots and lashes, maintain adequate clearances in overhang, a very important aspect to ensure quality of capsule. The paper describes the effect of taping method on machine life. It discusses the advantages of machine taping over manual taping.

It also covers other critical design requirement in high voltage motor overhang insulation. It suggests criteria for selection of winding cable connection, termination arrangement and best practice for coil brazed joints. Some of very important tests namely Corona Imaging Test, Monitoring of cable lug temperature at termination using RTD, partial discharge test have been suggested to be conducted as `ROUTINE TESTS’. The use of infrared camera as routine test to check healthiness of winding lead termination has been suggested.