The working principle and equivalent circuit of a current transformer are the same as those of a general transformer, except that the primary winding is connected in series in the circuit under test, and the number of turns is small; the secondary winding is connected to low-impedance loads such as ammeters and relay current coils, which are approximately short-circuited. The primary side current (that is, the current under test) and the secondary side current depend on the load of the line under test and have nothing to do with the secondary side load of the current transformer. When the current transformer is running, the secondary side is not allowed to open circuit, and it is not allowed to dismantle the ammeter and relay and other equipment without bypassing during operation.
Current transformer is an important equipment for power system energy measurement and protection control. It is an important part of power system energy measurement, relay protection, system diagnosis and detection analysis. The premise of economic operation.
1. Features
1. The primary winding is connected in series in the circuit, and the number of turns is small, so the current in the primary winding completely depends on the load current of the circuit under test, and has nothing to do with the size of the secondary current.
2. The impedance of the current coil of the instrument connected to the secondary winding of the current transformer is very small, so under normal circumstances, the current transformer operates in a state of almost short circuit.
2. Classification of current transformers
1. According to the purpose:
1) Measuring current transformers or measuring windings of current transformers. Within the normal operating current range, the current information of the power grid is provided to measurement, metering and other devices.
2) Protective current transformers or protective windings of current transformers. In the state of grid fault, the information of grid fault current is provided to relay protection and other devices.
2. According to the insulating medium:
1) Dry-type current transformers are wrapped by ordinary insulating materials and treated with impregnating paint. Current transformers with or without plastic shells are mostly low-voltage current transformers.
2) Casting current transformer. Current transformers cast with epoxy resin or other resin mixtures.
3) Oil-immersed current transformer. It is insulated by insulating paper and insulating oil, which is generally outdoor type. At present, it is commonly used in various voltage levels in my country.