Like conventional protection, microcomputer current protection is also designed as a three-stage type. Stage I is instantaneous current quick-break protection, Stage II is limited-time current quick-break protection, and Stage III is over-current protection. All three stages can be selected with directional line protection or without directional feeder protection. In order to improve the sensitivity of over-current protection and improve the reliability of the entire protection action, the current protection of the line is blocked by low voltage. Doing so seems complicated, and is usually rarely configured like this in conventional protection. However, setting low-voltage blocking for microcomputer line protection does not require adding any hardware, and is completely implemented by software (there is no low-voltage blocking program in microcomputer feeder protection). ). Since the logic programs of stage I, II, and III current protection are very similar, here we only take the logic program of stage II current protection as an example. The logic block diagram of the low-voltage locking direction limited-time current quick-break protection is shown in Figure 3-2. This picture only shows phase A, and the other two phases are exactly the same. What is shown in the figure is only the program logic relationship, not the electronic logic circuit. The program logic block diagram can be easily rewritten into a program, and it is more intuitive and concise than the program block diagram. Therefore, the principles of many current microcomputer protection products are usually represented by program logic block diagrams.
There are two types of settings in microcomputer protection, one is the switch type setting and the other is the numerical type setting. Switch-type setting values are commonly represented by setting characters. For example, d10=ON means that the limited-time current quick-break is turned on, and H90=OFF means that the I-section current protection directional element is turned off. Among them, ON and OFF are switch-type fixed values, and the corresponding inputs in the logic diagram are represented as 1 state and 0 state respectively. The numerical fixed value d00 is the speed limit judgment value. >d00 indicates an over-set value. When the value is over-set, the corresponding input is expressed as 1 state. d48 is the low voltage blocking value. When > d48, the corresponding input is expressed as "1" state; when < d48 low voltage action, the corresponding input is expressed as "0" state.
In the logic block diagram, we use electronic AND gates, OR gates, and NOT gates to represent the logical relationships of the program, but it should be noted that there are no electronic gate circuits here. According to the logical relationship of the electronic gate circuit, when the control input terminal of the NOT gate (the input terminal of the AND gate with a ○ symbol) is in the "1" state, regardless of whether the controlled input terminal is 0 or 1, the NOT gate is blocked, and its output terminal All are in the "0" state; when the NOT gate control terminal is in the "0" state, the state of the NOT gate output terminal changes with the state of the input controlled terminal, that is, the latch is released.
It can be seen from Figure 3-2 that when the low voltage lockout and TV disconnection lockout are both enabled, during normal operation > and >, or TV is disconnected for the second time, the NOT gate Z1 is blocked and outputs "0" state, and the AND gate &3 also outputs "0" state, that is, phase A II section does not operate. When < or <, both AND gate & 1 and AND gate & 2 output "0" state, and the NOT gate Z1 is unlocked. For example, DA = 1, the positive direction element operates. At this time, as long as the > and = ON conditions are met, H1 and & 5 output "1 ” state, the time-limited quick-break delay starts and after the setting time delay, the protection action sends out the F01 signal, and the trip command is issued through the OR gate H2.