Resistor-Thyristor Geoinduced Current Protection for Power Transformer

It is demonstrated that geomagnetic storms (GMS) can threaten electric power systems both in the Arctic and the middle latitudes of Russia due to the multi-year trend of magnetic pole relocation from Canada toward the Russian Arctic. The authors outline the key drawbacks of the well-known resistive grounding systems for power transformer neutrals that provide geoinduced current (GIC) protection. These include the need for various auxiliary electrical equipment to protect the resistor from single-phase short-circuit currents and isolate the neutral from overvoltage. Some quantitative criteria are proposed for the grounding resistor impedance to limit the GIC to levels that are safe for the power transformer and do not cause its core saturation. Some requirements for the resistor-thyristor grounding system of the power transformer neutral were put forward considering the restrictions imposed by the regulatory documents for grounding devices. The authors developed a resistor-thyristor neutral grounding system that automatically switches to the resistive grounding mode when its core is saturated due to GIC and returns to the solid-grounded mode after the GMS ends. It is shown that, when active components are represented by photothyristors with internal direct overvoltage fault protection integrated into the silicon pattern and silicon power resistors, the resistor-thyristor grounding system can be implemented without auxiliary electrical equipment. The authors calculated the electrical effects on the power components of the resistor-thyristor grounding system taking into account the rated power and voltage of the power transformer and the degree of phase voltage asymmetry.