Electrical Performance of Dielectrics in High Voltage Transformers George Chen Energy Away Day, 22 May 2012

2 Importance of Dielectric Research Transformers are an essential and expensive part of high voltage transmission systems. Failures can occur especially for aged transformers, leading to blackout and significant economic impact. Dielectric failure accounts for 29% of the total transformer failure. High voltage dc transmission will play a key part in renewable energy and super- grid transmission. Dielectrics (oil/paper) in high voltage converter transformers experience both ac/dc voltage.

3 Reliability of HV converter transformers Space charge is the key factor that affects electrical performance of dielectrics under dc conditions. Accumulation of space charge alters electric field distribution – local electric field enhancement Conditions of oil/paper in transformers influence the amount of charge and its dynamics Status of dielectrics is related to transformer operating conditions such as loading/temperature. Space charge measurement is able to provide key information related to the status of dielectrics.

4 Space charge in dielectrics of different conditions The Tony Davies High Voltage Laboratory has developed some unique techniques that allow to measure space charge in dielectric materials. The amount of charge accumulated in the bulk dielectric increases with ageing time. Both the amount of charge and its distribution change with electrical conductivity, moisture content and temperature. The electrical breakdown strength of dielectrics deteriorates with increase in conductivity and moisture which are products of ageing. PEA System Oil/paper Aged oil/paper Oil/paper with 5% moisture

5 Industrial collaborations and beyond The research on the effect of space charge and its role in electrical performance of HV converter transformers is new but potentially important to the manufacturers and utilities. National Grid has been sponsoring further research to investigate the effect of oil conditions on charge dynamics in dielectrics under the combination of ac and dc voltages. Alstom Grid has shown a great interest in the proposed research, in hoping to improve their insulation design of HV converter transformers. We are an active member in one of CIGRE working groups to establish testing procedure for HV converter transformers. We are collaborating with Universities in China on this research.