Deviation of short-circuit impedance from specified design value
For the vast territory of the former USSR, hundreds and thousands of HV and EHV power transformers were manufactured, which in the 1980s were produced mainly in Zaporozhye (Zaporozhye Transformer Plant ZTZ, now ZTR), and to a fewer extent in Moscow and Tolyatti. At this time, ZTZ also significantly increased the production of transformers for foreign customers. Transformers have been supplied to many countries in Europe, Asia, Africa, South America. For example, 67 units of the identical autotransformer 267 MVA 500/230 kV were supplied to China, 39 units of autotransformers 200 MVA 220/110 kV were supplied to Bulgaria, etc.
This mass production of transformers has led to the use of standard methods for controlling technological processes in a series production environment. The study of the scatter of the characteristics of transformers measured at the test station by statistical methods made it possible to determine whether the deviation of the measured value of a particular transformer from the calculated value is random and acceptable or indicates a defect (for example, a significant increase in no-load current indicates poor assembly of core corners). The study also identifies weak links in production, including by analysing the measurement results (for example, no-load current and no-load losses at low voltage 380 V) after individual operations and technological cycles. In addition, the study clarifies if calculation methodology is reasonable and if the distribution of tolerances between calculation and production is within the expected ranges and if there is a need for the improvement of the design methodology. And finally, the results of the study can be used to compare the production technology levels of different transformer manufacturers (for example, in the 1980s, 110 kV transformers were produced at five factories).