Discussion on type selection and design of 10kV medium voltage transformer
I. Further description of YY opening voltage transformer parameter ratio. For example, the ratio label is as follows:
10 / square root 3-0. 3-0 1 /). 1 / square root 3-0. 1/3 60/60 0.2/0.5/3 p / 100 va
Phase pressure divided by the square root of 3; The wire voltage of the second first winding of the voltage transformer is 0.1kV, and the phase voltage is divided by √3. The second group line pressure 0.1kV, phase pressure divided by √3; The line voltage of the third group is 0.1kV divided by 3 (this ratio ensures that the output voltage of the secondary opening is 100V in case of single-phase grounding fault, see details below).
Yn/yn/yn/△ refers to the wiring mode of a star second 3 sets of windings, star, star, open triangle connection in turn.
Two, for the open connection ratio and secondary voltage generation description:
Under normal circumstances, there is no voltage on the open triangle. When the system is single-phase grounded, there will be no voltage on one phase of the primary winding of the voltage transformer, resulting in no voltage on the corresponding secondary winding, so there will be voltage on the open triangle. By measuring the voltage across the open triangle, it is possible to know whether the high voltage system is grounded.
If A ground fault occurs in phase C of the primary system, as shown in the above vector figure, the C phase of the primary winding of PT has no voltage, the A-phase voltage induced by PT is actually the power supply UKA (UA-UC), the B-phase voltage is actually UKB (UB-UC), that is, the phase A voltage changes into AC line voltage, and the phase B voltage is BC line voltage. A, B, and C vectors induced on the secondary side are UKA+UKB as shown in the vector calculation in the figure above. That is, the fault voltage measured by YY open PT open connection method is 3 times the normal C phase voltage and in the opposite direction. Therefore, the conversion ratio of open winding should refer to the non-open conversion ratio 3 times, so the conversion ratio is 10/√ 3:0.1/3, to ensure the output voltage of secondary fault 100V.
Vi. Points for attention in the design and assembly of voltage transformer
It is used to measure the two voltage transformers in the VV connection method. The primary side connection should meet the requirements of the local power supply office. At the primary side AB terminal, make sure that the first B terminal is short connected to the A end of the second terminal and then connected to the B phase of the power supply.
The voltage transformers with three independent YY openings shall be installed in groups according to the characteristics matching table provided by the supplier, and shall not be mixed at will, so as to meet the requirements of the national standard that the difference between products in the same group shall not be more than 30%.