Phase failure and phase sequence protection is an indispensable part of the starting equipment of larger power three-phase AC motors and irreversible transmission equipment. Analog phase failure and phase sequence protection relays are usually used. With digital technology, especially With the development of 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. It has gradually become the mainstream of three-phase AC motor starting protection .
2 Brief introduction of phase failure and phase sequence protection system
2. A three-phase AC motor control system
The three-phase AC motor power control loop is shown in Figure 2-1. Press the QA button, if the three-phase power supply A, B, C is normal, the motor can start to work normally; if any one-phase fuse in the three-phase power supply is open or the power supply is out of phase with the original recognized phase sequence, the control system issues Alarm signal, cut off CJ main circuit power, three-phase AC motor can not start, so as to achieve the purpose of protecting equipment.
2.2 The principle of digital phase failure and phase sequence protection technology
As shown in the circuit shown in Figure 2-2, the three-phase AC power sources A, B, and C are processed by the step-down module and output three-phase AC power A ′, B ′, and C ′. The three points b and c can be obtained. As shown in Figure 2-3, a set of data can be obtained by reading the status of the three points a, b and c at the same time; similarly, the three sets of AC power sources A, B and C are used as A combination of different states and reading the state of each point at the same time can also get a set of codes. List these codes into 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 repeats; when the phase is wrong, the code is 110101011; if broken At the same time, neither the code of 110011101 nor the code of 110101011 can be obtained, which provides a theoretical basis for us to correctly deal with the phase failure and the wrong phase, so it can be easily achieved with digital circuits.
2. 3 circuit realization and working process
a. Because the overall system structure is relatively simple, this system adds the signal processing part of the circuit based on the minimum application system of the 8031 ​​single-chip microcomputer (as shown in Figure 2-2). If there are other functions in the application, it can be very convenient To be expanded.
b. work process:
When the power is 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 reaches 110, it puts it into the designated memory and reads in the second data continuously. The data sampled three times is compared 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 at this time. If it does not match the above two standard codes, it is a phase failure. In both cases, the CPU issues an alarm signal to cut off the main circuit power 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 turn-on signal, and the motor starts; when the motor starts normally, the sampling time can be reset according to the actual situation. The CPU can press the set time or 3.3 Repeat sampling and judgment at 3 ms to prevent phase failure and wrong phase in the process of work, so as to achieve the purpose of protecting the equipment accurately and reliably.
3 Conclusion
The principle of digital phase failure and phase sequence protection introduced in this paper is simple, practical and easy to connect with other digital control systems. We successfully completed the phase failure and phase of the microcomputer controlled soft starter with this program Order protection and achieved satisfactory results.
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