(1) Function of starting components
In order to improve the reliability of the protection action, the outlet of the protection device is blocked by the starting element. Only after the protection starting element is started, the outlet lock of the protection device is released. In the microcomputer protection device, the starting element is completed by software. After the starting element starts, the start flag "KST" is set to 1.
(2) Phase current difference mutation starting element
The starting element program can be completed in many ways, and the method usually used in the current system is the starting method of phase current sudden change. The specific method is to find the difference between the instantaneous value of the phase current at each sampling point and the instantaneous sampling value of the same phase in the previous power frequency cycle, that is, take the mutation value. If it is greater than the setting value, the protection will start. The mutation starting method can be expressed as:
In the formula, N is the number of sampling points per week. The program of the phase current sudden change starting method is relatively simple, and the anti-interference ability is poor, and frequent starting may easily cause false starting. In practical applications, the starting method of the phase current mutation difference is adopted, that is, the difference between the mutation values of two adjacent cycles, and the difference of the mutation value of phase A is:
This starting method can overcome the unbalanced current caused when the system frequency deviates from the rated value. In order to further improve the anti-interference ability of starting, in the program, the starting mode of sudden change of phase current difference is adopted, which can be expressed as
When the sudden change of the phase current difference of any phase is greater than the setting value four times, the protection device will start, and its logic block diagram is shown in Figure 2-7 (a).
(3) Pan-segment current starting component
The line protection of medium and low voltage substations usually adopts the starting logic of conventional protection, that is, the stage III current overset value (L3=1) that is more sensitive to faults is used to form the starting element. The program logic block diagram of the starting element is shown in Figure 2-7(b). When the microcomputer line protection has a low-frequency load shedding function, in order to meet the requirements of low-frequency load shedding, the protection should also be started when the conditions of low-frequency load shedding (LF=l) are met to ensure reliable tripping of low-frequency wave operation. The starting element should also meet the requirements of reclosing. When the reclosing "charge" is intact (CD=1) and the condition of position mismatch (KTP) is met, the starting element should start.
To ensure reliable reclosing. Therefore, if any conditions such as stage III overcurrent, low-frequency load reduction starting, and reclosing starting are met, the starting relay KST can be activated through the OR gate with memory for 10 seconds to ensure reliable starting.
When the starting element of the sampling interrupt service program is judged to be protected and started, the program is transferred to the fault handling program. After entering the fault handling program, the timing sampling of the CPU is still going on. Therefore, during the execution of the fault handling program, every sampling period T, the program will re-transfer to the sampling interrupt service routine. After the sampling calculation is completed, check whether the protection has been started. If KST=1, there is no need to enter the TV, TA self-test and protection starting program parts. Go directly to the sampling interrupt service program exit, and then return to the fault handling program.