Power transformer dry type bushing embarking combined low power instrument transformers. Impact of the mutual influence on the accurancy

A set of three bushings designed for the high voltage (HV) terminals of a transformer has been tested in our Power Network Lab facility to identify the key parameters influencing the current and voltage accuracy in an outdoor switchyard situation. This test program is described in this paper.

The temperature cycle test is done to verify the accuracy of the LPIT across the full temperature range defined in the specifications of the French utility, i.e. from -25 °C to +40 °C. We observed only slight variations in the accuracy of the current and voltage measurements, with the highest variations occurring at the lowest temperatures. These variations remain compatible with the accuracy classes specified by the manufacturer.

The mutual influence test evaluates the mutual influence of the three bushing phases on the analog output signal of each sensor. This test is particularly relevant for LPITs. The standard suggests testing the LPIT in the real configuration when possible. In our case, the three bushings are placed in the same configuration that they would be installed on the “All-in-One

Transformer”. The upper part of the “All-in-One Transformer” tank is simulated by an earthed metallic tank. The mutual influence of the currents flowing in the three bushings on the accuracy of the current measurements was low and was compatible with the limits set by the standard, confirming that the measurement data was accurate. The mutual influence of the potentials applied to the high voltage terminals of the bushings on the accuracy of the voltage measurements was significantly greater than the limits set by the standard, indicating that the measurement data was not correct for the configuration that was used.

When combining the current/voltage measurements it is important to determine the sensitivity of a measurement when in the presence of both current and potential. This question is particularly relevant for the combined IT function in the tested bushings. In the test facility known as “PREMICE”, the three bushings were inserted in a current loop that matched the configuration of the “All-in-One Transformer” and energized at the working potential. The effects of potential on the current measurements and those of current on the voltage measurements appear to be consistent with the assigned current and voltage accuracy classes.

Once the IT performance of the bushings has been validated by performance testing in the laboratory, the next stage will be to install three of these bushings on a 30 MVA , 90 kV transformer which will be retained as a rental unit for industrial clients interested in a digital option.