How to connect CT? CT wiring diagram

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.