Grounding transformer grounding referred to as short, the operation is characterized by normal operation of the grid no-load, overload overload. According to different filling media, the common form can be divided into two kinds of oil and dry; according to the number of phases, can be divided into three-phase and single-phase ground change. Grounding is artificial to create a neutral point, used to connect the grounding resistance. When the system occurs ground fault, positive sequence negative sequence current is high impedance, zero sequence current is low resistance so that the grounding protection and reliable operation. Proper and reasonable grounding transformer selection, for short-circuit arc, eliminate electromagnetic resonance over-voltage, to ensure safe and stable operation of the grid has an important and meaningful.
Grounding transformer selection should be considered from the following technical conditions: type, capacity, frequency, current and voltage, insulation level, temperature coefficient and overload capacity, the use of environmental conditions should focus on equipment used in the ambient temperature, altitude, temperature, contamination Grade, earthquake intensity, wind speed, humidity and other conditions.
When the system can be derived when the neutral point can be used to change single-phase ground, can not be used when the three-phase grounding leads to change.
Ground variable capacity selection
The choice of grounding transformer capacity mainly considers the type of grounding change, the properties of the neutral point connecting equipment and whether the secondary side is loaded or not. Under normal circumstances, the capacity of the neutral point connection device has been left in the calculation of a larger margin, so in the selection no longer need to increase the coefficient.
In the PV power plant, the secondary side of the grounding transformer generally has a load, so I briefly explain how to determine the capacity of the grounding transformer under secondary load.
Grounding capacity under this condition is mainly based on the transformer with arc suppression coil capacity and secondary load capacity, according to arc suppression coil capacity equal to 2 hours of rated time to calculate, when the load is an important load, but also can be based on continuous operation time determine. Arc suppression coil in accordance with the reactive power considerations (Qx), the load in accordance with the active load and reactive load to consider separately (Qf, Pf).
In the use of reverse zero sequence current active component of the ground protection, in order to improve the sensitivity of grounding protection and the accuracy of the selection, in the arc suppression coil primary or secondary side of the installation of a certain resistance of the grounding resistance, resistance in use Although active consumption, but due to the use of time is short, the incremental current is small, so there is no need to increase the capacity of the ground change.
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