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Voltage Transformer Connections

There are many connective modes of voltage transformer, which are common in the following ways:

  • A single- phase voltage transformer is used to measure the connective mode of a certain relative voltage or interphase voltage.
  • An incomplete star with two single-phase transformers, also called a V-V connection, is used to measure the voltage between each phase.  However, the relative voltage cannot be measured, and it is widely used in the electric power grid that is not grounded in the neutral point  of  20 KV or grounded in the arc coil.
  • Use three single-phase three-winding voltage transformers to form YN, YN, d0 or YN, y, d0, it is widely used in 3-220 kv system, the second  windings are used to measure the phase voltage and relative voltage, auxiliary secondary windings are used to connect the open triangles,  for the access to AC power insulated monitoring meters and relays. A three-phase five-column voltage transformer is used to replace the  wiring of the three single-phase three-winding voltage transformer. Besides the core, the form is basically the same as FIG. 3, which is  generally used only for 3~15 KV system.
  • The wiring form of capacitive voltage transformer.
In a system that is not grounded or grounded in an arc coil, the PT windings must be  connected to a star grounding in order to measure the relative voltage.

In the 3~60 KV power grid, three single-phase three-winding voltage transformers or a three-phase five-column voltage transformer are usually used. It must be pointed out that this kind of measurement can not be done with a three-phase three-column voltage transformer. When a single-phase grounding fault occurs in the system, zero sequence current will be passed in the three-phase of the transformer to produce zero sequence magnetic flux with equal size and same phase. In a three-phase three-column transformer, the zero sequence magnetic flux can only form a closed magnetic path through a large magnetoresistive air gap and an iron shell. The zero sequence current is very large, which causes the transformer winding to overheat or even damage the equipment. In the three-phase five-column voltage transformer, the zero sequence magnetic flux can form the loop through the iron core on both sides, and the magnetoresistance is small. So the zero sequence current is not high, and it does not cause damage to the transformer.