The role of the current transformer In order to ensure the safe and economic operation of the power system, the operation of the power equipment must be monitored and measured. However, the general measurement and protection device cannot directly access the high-voltage equipment, but the large current of the primary system needs to be proportional. Transform into a small current and supply it to the measuring instrument and protection device.

The function of the voltage transformer is to convert the high voltage proportionally into a standard secondary voltage of 100V or lower for protection, metering and instrumentation.

The difference between the two is that one is measuring current and the other is measuring voltage. The current transformer is connected in series in the circuit. The primary winding is less than the secondary winding, and the secondary winding cannot be opened. The voltage transformer is connected in parallel in the circuit. The primary winding has more turns than the secondary winding, and the secondary winding cannot be short-circuited.

In general, a voltage transformer is a device that converts a high voltage into a secondary standard voltage (100V) according to a certain ratio. The current transformer converts a large current or a high voltage and a large current into a secondary standard current (5A). Or 1A) equipment.

The benefits of transformation:

1. The current and voltage under high voltage cannot be directly measured. Even if there is a meter that can directly measure the current and voltage under high voltage, it is not safe. It has already led the high voltage to the electrician's eyes. It is better to manufacture the insulation, and it cannot guarantee the time. Safety.

2. When the current is too large, it is difficult to access the instrument, and the terminal of the instrument and the instrument cannot be made very large.

3. After the transformer is transformed, the secondary has become the standard current (5A or 1A) and voltage (100V), so that regardless of the secondary instrument, protection device, or energy meter, it can be standardized, which is beneficial to Standardized design, production, selection and maintenance of instruments.

At present, most of the small current grounding line selection components in the project are selected according to the protection level. When the measured current transformer is far less than the rated current value, the comprehensive error is difficult to meet the requirements. The total error of the two-stage current conversion component is the main cause of misjudgment on the site. the reason. The linear measurement range of the zero-sequence current transformer used in engineering practice exceeds the actual ground capacitance current.

Zero-sequence current transformers are used in small current grounding systems. The primary current value is small. There is almost no current on the primary side during normal operation. When the ground fault occurs, the primary current (fault current) is also small, generally at 10A. the following. If the system ground fault current is greater than 10A, the arc suppression coil should be installed to compensate according to the specified requirements. The ground fault current with the arc suppression coil compensation is smaller, generally less than 2~5A (minimum to 0.2~0.5A). Under such a small primary current, the ratio of the conventional zero-sequence current transformer and the phase angle error are very large. Therefore, each transformer manufacturer can not give a ratio to the zero-sequence current transformer, and there is no error guarantee index. From the actual primary and secondary current variation curves (variation ratio curves) of the zero-sequence current transformer, it can be known that the current-to-ratio ratio of the zero-sequence current transformer varies greatly with the primary current value, and the primary current cannot be given again when the primary current is less than 1A. A specific secondary current output value.

3. Through the above analysis, the comprehensive error of the measurement link is the main reason for the misjudgment of various small current microcomputer line selection devices. In engineering applications, the parameters should be properly matched, the comprehensive error of the measurement link can be reduced, and the small current grounding can be effectively improved. Line selection accuracy of the line selection system.

4. The effective measures taken in the project include: 1 Try to select a special zero-sequence current transformer with high accuracy. The rated primary current selection should ensure that the maximum grounding capacitance current of the system is within the linear range of the zero-sequence current transformer ( Accurate limit), the linear measurement range of the primary side current should extend down to about 0.2A to accommodate the small current grounding system grounded by the arc suppression coil. 2 Zero-sequence filter should be combined with a relatively small measuring level (preferably S-class) current transformer. The smaller ratio can make the secondary value of the capacitor current larger, which is beneficial to the detection device current converter. The current value is collected, and the S-stage makes the current transformer's measurement accurate linear range wider, which is beneficial for measuring small capacitance current.

5. For the small current grounding line selection device, the accuracy of the zero-sequence current transformer at low current has certain requirements on the device. For this reason, it is recommended that: 1 if the protection factor of the transformer is required, and at low current (0.5--- 5A) When the requirements are not high, it is recommended to select the open-type transformer 50/1A, the error must be within ±10% at 2% rated primary current. Because the open-type transformer is easy to install, it also has certain advantages in protection, and the residual magnetism is much lower than that of the closed-type transformer. 2 If the accuracy of the primary current at the small current is very high, it is recommended to use a closed-type transformer, the measurement accuracy should reach 0.5, and if necessary, the error requirement of 0.5S can also be proposed. According to the manufacturing regulations: as a metering transformer, the error can be guaranteed within a certain range at 150% rated primary current. For small current grounding line selection devices, the ratio can be determined according to actual needs. Generally recommended 50/1A, the accuracy can be 0.5 or 0.5S, and the load is 1VA.