The current transformer is used in an AC loop where the direction of the current changes over time. The polarity of the current transformer means that the polarity of the primary side is the same as the polarity of one end of the secondary side at a certain moment, that is, it is positive at the same time or negative at the same time, and the polarity is called the same polarity end or the same name end. The symbol "*", "-" or "." indicates. (It can also be understood as the direction relationship between primary current and secondary current).

According to the regulations, the primary end of the current transformer of the current transformer is marked L1, and the tail end is marked as L2; the head end of the secondary coil is marked K1, and the tail end is marked as K2. In the wiring, L1 and K1 are called the same polarity end, and L2 and K2 are also the same polarity end. The three annotation methods are shown in Figure 1.

The discrimination of the same polarity end of the current transformer is the same as the polarity determination of the coupling coil. A simpler method is to connect the primary coil with a 1.5V dry battery and connect the secondary coil with a high internal resistance, large-range DC voltmeter. When the switch is closed, if the voltmeter pointer is found to be deflected in the forward direction, it can be judged that 1 and 2 are the same polarity end. When the switch is closed, if the voltmeter pointer is found to be deflected in the reverse direction, it can be judged that 1 and 2 are not the same polarity end.



Figure 1 Three polarity labels for current transformers



Figure 2 One-phase wiring



One-phase current-protected current transformers are mainly used to measure one-phase current in a three-phase device with a symmetrical three-phase load or a small phase load balance. The wiring of the current transformer has little relationship with the polarity, but it should be noted that the secondary side should have protective grounding to prevent the current transformer from being broken down when the overcurrent occurs on the primary side, and the secondary side instrument is burned out. Electrical equipment. However, multi-point grounding is strictly prohibited. Two-point grounding secondary current forms a shunt in front of the relay, which will cause succession

The appliance has no action. Therefore, in the "Technical Regulations for Relay Protection", it is stipulated that for a protection device with several sets of current transformers connected together, it should be grounded through the terminal on the protection screen. For example, the differential protection of the transformer, and the combination of several sets of current transformers has only one independent grounding point.

2. two-phase incomplete star connection

Two-phase incomplete star connections are used in phase load balanced and unbalanced three-phase systems. As shown in Figure 3. If there is a phase second polarity, then the current flowing through 3KA is IAI e, the current difference is 3 times of the current value Ia, and the phase lags I a 300 angle. If the three relays are the same, 3KA will advance. Action, causing protection misoperation.



Figure 3 two-phase wiring



Figure 4 two-phase current difference wiring



3. two-phase current difference wiring

The current flowing through relay KA in Figure 4 is I A I e with a wiring factor of 3. If the secondary polarity of phase C is reversed, the current flowing through relay KA is I A I e . When the A and C phases are short-circuited, the primary currents I AD and I CD become equal and opposite directions. That is, I AD =- I CD , assuming that the reference direction of I AD is positive, and the current flowing to the secondary side through the relay KA is zero. This means that due to the reverse polarity of the C-phase, the relay KA may not operate when the primary side A and C phases are short-circuited.

4. three-phase full star connection

The three-phase full star connection is shown in Figure 5. It is used for current measurement of three-phase load with large phase load balance and three-phase four-wire measuring instrument with voltage of 380/220V. It monitors the asymmetry of each phase load. If any phase is reversed, it flows through neutral. The current of the line will increase. In the absence of a neutral neutral star connection, the defect is that when the load is unbalanced during operation, the secondary side neutral point displacement will occur. Figure 5 Three-phase full star connection makes the current flowing through the relay not reflect correctly. The magnitude of the phase current will also cause a malfunction.





5. Current transformer wiring for relay protection

The current transformer wiring for relay protection is usually a star connection when it is used as a protection device in a power system where the neutral point is directly grounded. In power systems where the neutral point is not directly grounded, the incomplete star connection is widely used because it allows short-time single-phase grounding operation and, in most cases, a single-phase grounding signal device. The delta connection of the protection current transformer is applied to the transformer differential protection of the Y/△ wiring.

6. the problem should be paid attention to in the operation of current transformer

(1) The current transformer shall not open the circuit on the secondary side during operation. Once the secondary side is open, the iron loss is too large, the temperature is too high to burn, or the secondary winding voltage is increased to break the insulation, and a high voltage electric shock occurs. danger. Therefore, when changing the meter, such as changing the ammeter, the active meter, the reactive meter, etc., the current loop should be shorted before the meter is exchanged.

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.