Analysis of Power Transformers under DC/GIC Bias

Power transformers are key components in the interconnected bulk power transmission grid. Moreover, to ensure the reliable and stable operation of the power grid, the interaction of the transformers and the power grid during normal and abnormal operation conditions were studied. To study abnormal operation conditions of power transformers it may be necessary to include the non-linear hysteresis characteristics of the transformer cores in electromagnetic transient studies. The modelling of the hysteresis characteristics of the transformer cores requires detailed information about the transformer core design and material. If this information is not available, it is challenging to establish an adequate electromagnetic transformer model. Especially during deep saturation conditions, typically near two Tesla for gain-oriented steels, an accurate modelling of the hysteresis characteristics can be essential for the calculated phase currents. Such saturation conditions could be caused by geomagnetically induced currents (GIC's) or direct current (DC) bias caused e. g. by power electronic devices. This work is a follow-up investigation, motivated by increased transformer sound, which could be traced back to GIC s in the high and extra high-voltage transmission grid. This work presents a measurement based modelling approach to establish electromagnetic topology models of power transformers, including the transformer’s core hysteresis characteristics. First the AC saturation test was developed with the idea to saturate the outer two legs of a three-phase transformer core by two elevated 180° phase-shifted single-phase voltages. The AC saturation test was successfully used to parametrise the hysteresis model of two transformer topology models, using the inductance-reluctance and the capacitance-permeance analogy. Because the AC saturation test requires a sufficiently large power source, it was further developed to the DC hysteresis test. Instead of using a 50/60 Hz sinusoidal voltage, a DC with reversal polarity was used. The DC hysteresis test was also successfully used to parametrise the transformer hysteresis models. The implementation of the DC hysteresis test in a portable transformer test allows to conduct this test in the laboratory and in the field. Together with the principle of variable core gap inductance the transformer topology models of a 50 kVA reveal a high accuracy of the calculated and measured current waveforms during the AC saturation and the standard no-load test, as well as the corresponding power demand. For the measurement of transformer neutral point currents, including geomagnetically induced currents (GICs), an existing measurement system was further developed to minimise the constraints of the monitoring system on grid operations. The utilisation of a split-core current transducer around the earthing switch, together with a software-supported correction of the offset drift, reveals a low long-term offset drift of the measured transformer neutral point current. In addition to the measurement of the transformer neutral point current, the measurement system was extended to monitor a direct current compensation (DCC) system, installed in several transformers in the transmission grid. The analysis of the DCC measurements, which allows a calculation of the DC per phase, reveals an equal distribution of the DC between the high-voltage phases and the capability of the system to minimise the effects of GICs in transformers.