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What are the methods of zero sequence voltage relay for motor phase loss protection

The requirement is to use a zero-sequence voltage relay for phase loss protection of the motor. In fact, there are other methods. However, these protection methods are more or less outdated. With the advancement of technology, we have entered the era of PLC and inverter.


Then, since the problem is raised, let us introduce the principle and connection method of zero sequence voltage relay for phase loss protection.


The principle of zero-sequence voltage phase loss protection is to use a three-phase balanced system grounded at the neutral point. The neutral-to-ground voltage of the star-connected motor is theoretically zero (in fact, because the three phases are not balanced, there are several volts). Voltage), when the phase loss operation occurs, the voltage of the three-phase windings of the motor stator is extremely unbalanced, so that the zero-sequence voltage of the neutral point to ground increases. When the motor is fully loaded, this zero-sequence voltage can reach 20v ~ 45v This principle of left and right, through the zero-sequence voltage relay connected in series to the neutral line of the motor to achieve the lack of phase protection of the motor. During normal operation of the motor, the neutral-sequence voltage relay does not operate because the neutral-to-ground voltage is low. When a phase loss occurs on the line, the neutral-to-earth voltage rises, causing the relay to energize and act. The contacts are disconnected to cut off the three-phase power.


What are the methods of zero sequence voltage relay for motor phase loss protection


Then, for a delta-connected motor, a star-shaped capacitor can be used, which is connected to the triangular winding, and then a neutral line is drawn out. This neutral line was originally not available, but you can make a neutral line according to your needs. Sex line. The capacity of the capacitor is preferably 2 ~ 4 microfarads. Because the actual neutral point has a few volts to the ground (3 ~ 4% of the phase voltage), the relay operating voltage can be set between 12v ~ 24v.


Generally, the protection of three-phase AC motors has a current transformer. Since it is a current transformer, it is only sensitive to the current signal, so the zero-sequence current signal is taken. If you use the voltage zero sequence signal, if you take it from the motor junction box, then each motor must have a single signal cable. This will bring additional burdens on cost, installation and maintenance. The cabinet takes the voltage signal. The cable from the control cabinet to the motor and the motor has a broken wire and a phase failure. The zero-sequence voltage signal of the control cabinet cannot be reflected. The current signal is different, because the current of the circuit breaker, contactor, thermal relay, cable, and motor is in series. The series current is equal everywhere, so the current signal you take in the control cabinet is exactly the same as the current signal at the motor. . If you can use the current artificial intelligence technology to come up with an algorithm that can indirectly protect the three-phase AC motor from phase loss with three-phase voltage fluctuation signals, I do n’t think it is necessary because the zero-sequence current signal itself is too powerful. You want to In fact, the leakage protector is also based on the protection principle of zero-sequence current. The leakage current of tens of milliamperes can be sensitively protected. The phase difference current of the motor is generally tens or even hundreds of amps, which means that when A certain motor line suddenly breaks during operation, and the current of several hundred amps suddenly becomes zero, and the voltage fluctuation caused by it is very small, and the changes of other external factors affect the power grid all the time. Voltage fluctuations and changes, so I think it is too difficult to use a voltage signal for phase loss protection of a three-phase AC motor. Even if it is, it may not be sensitive and reliable (compared with the current signal).