Condition monitoring of power transformers is a very important task for electric utilities to ensure the reliability of their power system. Among many methods, non-destructive diagnostic methods have become more and more important for the condition monitoring of power equipment, as electric utilities have changed their maintenance philosophy toward condition-based maintenance from periodic maintenance. Currently among other chemical based non-destructive diagnoses, electrical non-destructive diagnoses based on dielectric response methods are gaining significant importance. Dielectric response measurement is a state of the art technology for power transformer condition assessment, it has strength in monitoring moisture and/or thermal ageing development in the oil-paper insulation. Return voltage measurement (RVM) and polarisation/depolarisation current (PDC) measurements are the dielectric response measurement methods in the time domain. RV and PDC measurements provide great advantages of being non-destructive, sensitive to reflect the moisture and general ageing condition of the transformer insulation, etc. The most difficult part in these diagnostic techniques is to accurately interpret the measurement results, particularly to distinguish the moisture and ageing effects in the insulation. That is, to know whether the moisture or the ageing in the insulation dominates the results from the measurement, and to provide clear physical meaning for the results interpretation.
This thesis reports the achievement in the research of developing a precise and effective method to interpret dielectric response measurement results for condition monitoring of power transformer electrical insulation. To obtain first hand data, a new computer based dielectric response measurement system has been built. Several batches of sample test objects have been prepared with a newly developed insulation paper moisture conditioning system. Dielectric response measurement data from sample test objects and power transformers have been analysed extensively. Mathematical models based on dielectric physics have been developed to support the measurement results interpretation. The progress and the outcome are described in brief as follows.
A computer based instrument has been developed to perform the dielectric response measurements on power transformers and sample test objects. This new generation dielectric response measurement system is a specially designed portable equipment to perform tests in an industrial environment. It can perform both PDC and RV measurements. In the hardware development of this system, special shielding arrangements have been made for all related conducting leads in the system to avoid the influence of substation and power plant environment noise. Comprehensive software in LabVIEW has been designed for instrument control, data acquisition, graph presentation and data analysis in this system. Dielectric response measurements have been performed with this instrument on samples that were prepared in the laboratory and full size transformers in substations. The research outcome in this part provides ideas and experiences about how a dielectric response measurement system can be developed and how to obtain consistent measurement results (the first hand data).
To obtain sufficient data to analyse the nature of the dielectric response, four batches of moisture conditioning samples were prepared using the novel moisture conditioning system with achieved paper moisture levels at 2%, 3% and 5%. Correspondingly, four batches of accelerated ageing experiments at 115°C or 95°C with above known discrete moisture levels samples were conducted.
A huge effort has been spent on data analysis of dielectric response measurement results obtained on the above samples. The findings:
• When the paper insulation has a moisture content higher than 4%, this moisture content dominates the dielectric response measurement results.
• When the moisture content is in the range of 1.5%-3.5%, both the moisture and the ageing influence the PDC and RVM results.
• Furthermore, the dryer the insulation, the more contribution the ageing makes to the PDC and RVM results.
Field tests have been carried out on eight full size power transformers (in the range of 7 MVA to 100 MVA) using the newly developed dielectric response measurement system. Measurement data has been analysed thoroughly, the findings suggest that PDC has the advantage of separately investigating the oil and paper conditions in the insulation system. The findings also suggest that the RVM results indicate the overall change of the bulk insulation.
The interpretation of dielectric response measurement results is a difficult task in the field which often leads to confusing conclusion. For a better understanding of the physical meaning of the data obtained from the field tests, extensive work has been performed in this research on mathematical modelling. Mathematical models based on dielectric physics have been developed for RVM and PDC. The models have been implemented by software written in C++ environment. The outcome of this part demonstrates that with help from the model, a greater clarity can be obtained to see what physical parameters contribute to measurement results, hence providing solid support to the results interpretation (from the points of dielectric physics to view).
This study has shown that by using dielectric response measurement along with mathematical models, it is possible to assess the electrical insulation condition of a power transformer much more accurately.