The grounding transformer is referred to as the grounding change. The operating characteristics are no-load during normal operation of the power grid and overload during short-circuit. According to different filling media, common forms can be divided into oil type and dry type; according to the number of phases, it can be divided into three-phase and single-phase grounding. Grounding is artificially creating a neutral point that is used to connect the grounding resistor. When the system has a ground fault, the positive sequence negative sequence current is high impedance, and the zero sequence current is low impedance, so that the ground protection works reliably. Correct and reasonable selection of grounding transformers is of great significance for short-circuit arc extinguishing, elimination of electromagnetic resonance overvoltage, and ensuring safe and stable operation of the power grid.

The selection of the grounding transformer should be considered from the following technical conditions: type, capacity, frequency, current and voltage, insulation level, temperature rise coefficient and overload capacity. For the environmental conditions used, the environmental temperature, altitude, temperature difference and pollution of the equipment should be paid attention to. Grade, seismic intensity, wind speed, humidity and other conditions.

When the neutral point of the system can be taken out, single-phase grounding should be adopted, and when three-phase grounding cannot be taken out.

Grounding variable capacity selection

The selection of the grounding transformer capacity mainly considers the type of grounding change, the properties of the neutral point connecting device, and whether the secondary side has a load or the like. In general, the capacity of the neutral point connection device has already left a large margin in the calculation, so it is no longer necessary to increase the coefficient when selecting the type.

In the photovoltaic power station, the secondary side of the grounding transformer is generally loaded, so the author briefly explains how the capacity of the grounding transformer is determined under the condition of secondary load.

Under these conditions, the grounding variable capacity is mainly calculated according to the capacity of the arc-extinguishing coil and the secondary load capacity of the transformer, and is calculated according to the 2-hour rated time equal to the capacity of the arc-suppression coil. When the load is an important load, it can also be based on the continuous running time. determine. The arc suppression coil is calculated according to the reactive power (Qx) and the load according to the active load and the reactive load (Qf, Pf). Calculated as follows:

In the grounding protection with the active component of the reverse zero-sequence current, in order to improve the sensitivity and accuracy of the grounding protection, a grounding resistance of a certain resistance is added to the primary or secondary side of the arc-suppression coil. Although the power is consumed, the current increase is not large due to the short use time, so there is no need to additionally increase the grounding capacity.