Design of digital phase failure and phase sequence protection system

1 Introduction

Phase failure and phase sequence protection is an indispensable part of the AC motor starting equipment in high-power three-phase AC motors and irreversible transmission equipment. Usually, analog phase failure and phase sequence protection relays are used. With the development of digital technology, especially microcomputer technology, digital phase failure and phase sequence protection technology has become a reality. It has the advantages of small size, low energy consumption, accuracy and reliability, and easy interface with computer control systems, and has gradually become the mainstream of three-phase AC motor starting protection.

2. Introduction to Phase Failure and Phase Sequence Protection System

2.1 Three-phase AC motor control system

The three-phase AC motor power supply control circuit is shown in Figure 2-1. Press the QA button, if the power supply A, B, C three phases are normal, the motor can start and work normally; if any phase fuse of the three-phase power supply is open or the power supply is out of phase with the original phase sequence, the control system will send out an alarm signal, cut off the CJ main circuit power supply, and the three-phase AC motor cannot start, thereby achieving the purpose of protecting the equipment.

2.2 Principle of digital phase failure and phase sequence protection technology

As shown in the circuit of Figure 2-2, the three-phase AC power source A, B, C is processed by the step-down module to output three-phase AC power A', B', C'. After passing through three groups of comparators, the output terminals a, b, and c can be obtained as shown in Figure 2-3. By reading the states of a, b, and c at the same time, a set of data can be obtained; similarly, by combining the three groups of AC power sources A, B, and C in various different states and reading the states of each point at the same time, a set of codes can also be obtained. These codes are listed in a table as shown in Table 2-1. Through the above analysis process, the codes in various states are combined and compared, and the rules between them can be found: when the phase sequence is normal (ABC), the code 110011101 appears repeatedly; when the phase is wrong, the code is 110101011; if the phase is broken, neither the code 110011101 nor the code 110101011 can be obtained. This provides a theoretical basis for us to correctly handle the broken phase and the wrong phase, so it is very convenient to use digital circuits to realize this function.

2.3 Circuit Implementation and Working Process

a. Since the whole system structure is relatively simple, this system is realized by adding the signal processing circuit (as shown in Figure 2-2) on the basis of the minimum application system of 8031 ​​single-chip microcomputer. If there are other functions in the application, it can be easily expanded.

b. Working process:

When powered on, it is initialized first. After the CPU responds to the interrupt, it samples 244 and reads the data every 3.3 ms. When it reads 110, it is put into the specified memory and then reads the secondary data continuously. The CPU compares the data sampled three times with the standard codes 110011101 and 110101011. When the sampled data matches the code 110101011, the three-phase power supply must be out of phase. If it does not match the above two standard codes, it is a phase failure. For these two situations, the CPU sends an alarm signal, cuts off the main circuit power supply, and the motor cannot start. If the sampled data is consistent with the code 110011101, it means that the three-phase power supply is normal, the CPU sends a connection signal, and the motor starts; when the motor starts normally, the sampling time can be reset according to the actual situation. The CPU can repeat sampling and judgment according to the set time or 3.3 ms to prevent phase failure and mismatch during the working process, so as to achieve the purpose of accurately and reliably protecting the equipment.

3 Conclusion

The digital phase failure and phase sequence protection technology introduced in this article has a simple principle and strong practicality. It can be easily connected and used with other digital control systems. We have successfully completed the phase failure and phase sequence protection of the microcomputer-controlled soft starter with this solution and achieved satisfactory results.