Tag: current transformer

Current transformer is mainly used to measure the current of busbar and primary side with iron core coils in AC occasion. It can detect the DC current, can be single core or dual cores, generally, it's equipped with auxiliary DC winding. Such a structure is simple, reliable and slow. Technology and design of the current transformer are complex, its common precision is 5%, it's difficult to reach 0.5% because phase position and compensation are difficult. A current transformer is an electrical component that turns large current into small current, it doesn't change the current frequency but only the size. Its secondary side current is 5A, that means the output rated current is 5A. It's mainly used for electrical control, measurement, protection and signal.

Rated primary voltage of the current transformer shall be in consistent with the rated voltage of the system. Rated primary current of the current transformer shall be 20%~120% of its normal load current. Secondary loads of the current transformer, such as instruments and relay, consume a power or impedance less than rated capacity corresponding to the selected class of accuracy, otherwise the class of accuracy will reduce. A proper class of accuracy shall be selected for the current transformer according to the requirements of measurement and protection.

The secondary side is not allowed for open circuit. Once it's open circuited, current I1 at the primary side would fully turn to magnetizing current, φm and E2 would increase sharply. As a result, the iron core would occur supersaturation magnetization, heat seriously and even burn out coils. Meanwhile, supersaturation magnetization of the magnetic circuit would increase errors. Under normal operation, secondary side of the current transformer is nearly short circuited, if it's broken suddenly, the excitation electromotive force increases sharply, the magnetic flow in the iron core shows seriously saturated flat waves. As a result, the secondary side winding would induce high spike waves when the magnetic flow passes through zero, it can be thousands or even tens of thousands of volts, thus endangering the personnel safety and insulation performance of instruments.

Current transformer consists of closed iron core and winding. It has few primary winding that are connected in series in the current circuit under measurement; however, it has many secondary winding, which are connected in series in the measuring instrument and protection circuits. The current transformer plays the role of converter and electrical isolation, it's a sensor for measuring instrument, relay protection and other secondary equipment to obtain current information of primary circuit in power system. It converts the high current to a low current by ratio, its primary end is connected with the primary system, while the secondary end is connected with the measuring instrument and relay protection.

Cause of current transformer burning up

•  Secondary open circuit of the current transformer produces high voltage to burn out the current transformer.
• Long-term use of the current transformer makes the insulation aging, local breakdown or discharge occurs and overvoltage is thereof produced, thus burning out the current transformer.
• The aluminum contact surface of current transformer’s primary connection is seriously oxidized, which results in large contact resistance. The contact resistance gives out heats and burns out the current transformer
• Long-term overload operation makes the current transformer to give out heat, it is thereof burned out.

There should be no open circuit on the secondary side of the current transformer in operation. Once the open circuit occurs on the secondary side, high temperature caused by excessive iron loss may burn out the current transformer or enhance the voltage of secondary winding, thus breaking down the insulation and leading to the danger of high voltage electric shock. Therefore, when replacing a meter such as an ammeter, active power meter and reactive power meter, please ensure the current circuit in short circuit before operation. After replacement, connect the meter in the secondary circuit and then remove the short-circuit wires while checking if the meter is normal. In case sparks appear when removing the short-circuit wires, that means the current transformer disconnects, please short the loop again and confirm the meter loop isn’t an open circuit. When removing the short-circuit wires of the current transformer, please stand on an insulated pad and consider a protector to stop the current transformer loop. When it finishes work, the protector can be put in use.