The commonly used connection modes of voltage transformer are: 
The main results are as follows: (1) single connection can be used to measure the line voltage of the system whose neutral point is not directly grounded below 35kV or the relative grounding voltage of the system with direct grounding above 110kV. 
(2) the Vamp V connection is to connect the high and low voltage windings of two fully insulated single-phase voltage transformers to phase and phase respectively to form an incomplete triangle. 
This method is often used in 35kV and below high voltage three-phase systems with ungrounded neutral points or arc suppression coils, especially in 10kV three-phase systems. 
(3) the connection form of YN, yn, d0 or YN, y, d0 consists of three single-phase three-winding voltage transformers, which are widely used in 3~220kV system. The secondary windings are used to measure the interphase voltage and the relative ground voltage, and the auxiliary secondary windings are connected into open triangles to connect the insulation monitoring instruments and relays of the AC power network. 
The wiring composed of three single-phase three-winding voltage transformers is replaced by a three-phase five-column voltage transformer. Except for the iron core, its form is basically the same as that of figure 3, and it is generally only used in 3~15kV systems. 
(4) the three-phase three-winding five-column voltage transformer, the primary winding and the main secondary winding are connected into a star, the neutral point is grounded, and the auxiliary secondary winding is connected into an open triangle. 
Therefore, the voltage transformer can measure the line voltage and the relative ground voltage, and the auxiliary secondary winding can be involved in the relay and signal indicator of AC power network insulation monitoring.