Grounding transformer is referred to as grounding change. According to filling medium, grounding change can be divided into oil type and dry type. According to the number of phases, grounding change can be divided into three-phase grounding and single-phase grounding.
The function of the grounding transformer is to provide an artificial neutral point for the neutral point ungrounded system. It is convenient to use the arc suppression coil or small resistance grounding method to reduce the capacitance current to the ground when the distribution network experiences a ground fault. , improve the power supply system reliability
1. Intermittent extinguishing and re-ignition of single-phase grounding arc will produce arc grounding over-voltage, its amplitude can reach 4U (U is the peak value of normal phase voltage) or higher, long duration, will insulate the electrical equipment Caused great harm, formed a breakdown in the weak insulation; caused significant losses.
2. The dissociation of the air caused by the continuous arc destroys the insulation of the surrounding air and is prone to phase-to-phase short circuits.
3. The generation of ferromagnetic resonance overvoltage, easy to burn out the voltage transformer and cause damage to the arrester may even cause the arrester to explode. These consequences will seriously threaten the insulation of the grid equipment and endanger the grid.
China's grounding transformers usually use Z-type wiring (or zigzag-type wiring). To save space for investment and substations, a third winding is usually added to the grounding transformer to replace the transformer used to supply power to the equipment used in the substation. According to China's "reactor" national standards, grounding transformer grounding can be divided into direct grounding; through reactors, resistors and arc suppression coil grounding. Direct grounding has not yet been used in China, but there has been a power research department to start this discussion.
The grounding transformers in foreign countries are usually connected with Z-type connection and are used for 10kV ungrounded systems, which constitutes the grounding protection of the distribution network. When a grounding fault occurs in the system, the grounding transformer presents high impedance to positive and negative sequence currents and to zero. Sequential current exhibits low impedance and reliable grounding protection