1. The primary internal impedance of voltage transformer is small, as can be ignored, can be considered that the voltage transformer is a voltage source; and the primary internal impedance of current transformer is very large, as can be considered as a current source with infinite internal resistance.

2. The secondary side of the voltage transformer in operation shall not be short-circuited, the secondary side of the current transformer shall not open. 

3. In the general situation, the primary and secondary voltage of voltage transformer, and the primary and secondary current of current transformer are proportional and inversely proportional with their corresponding number of turns. 

4. If the installed oil-immersed transformer has been installed for a long time (110kV and above is more than half a year; 35kV and below for more than one year), the preventive test shall be carried out in accordance with the pre-test procedure.

5. Electronic instrument transformercan be divided into active electronic transformer and passive electronic transformer according to whether it need power supply for the primary sensing part.

6. Electronic transformers can fundamentally avoid the saturation due to the use of iron core  and ferromagnetic resonance and other factors of traditional transformer.

7. What are the advantages of electronic transformers?

A: Compared with traditional transformers: insulation is simple; small size, light weight; wide dynamic range, non-magnetic saturation; no resonance phenomenon of TV; the secondary output can be open; with digital output form. The higher the voltage level is, the more obvious the advantages of electronic transformers; it’s not very necessary to use electronic transformers in low and medium voltage levels. Compared with the traditional transformer which output analog quantity, the most digital feature s are: output digital signal through optical fiber at the low-voltage part eventually, and provide to the secondary devices.