In the case of manual closing to the fault line or manual opening and protection or automatic device requirements that do not allow reclosing (such as busbar, transformer protection and low-frequency load shedding action), the input switch contact of the blocking reclosing is connected, H4 Output "1", NOT gate Z4 outputs "0", the counter is cleared (CD=0) and "charging" is prohibited, NOT gate Z5 is blocked, that is, the reclosing is blocked.
(5) Principle of reclosing return
While the non-gate Z3 outputs "l" self-holding, after the reclosing reset time delay, on the one hand release the self-holding of the Z3 gate, on the other hand make H4 output "1" block the non-gate Z4 to clear the counter, This will cause the reclosing to reset. If the reclosing is successful, KCP=1, the counter starts counting again, and CD=l after 20s, thus preparing for the next reclosing.
4. Principle of logic frame retention of low-frequency load shedding program
Since the microcomputer protection relies on the supplementary software to easily realize additional functions, in addition to the reclosing function, the line protection also adds the function of automatically reducing the load of low-frequency waves. This not only achieves multi-function, but also greatly reduces the number of cables. More importantly, it realizes the resource sharing of protection and automatic devices and integrates with the substation microcomputer monitoring system to realize the comprehensive automation of the substation. The program logic block diagram of low-frequency load shedding is shown in Figure 3-4.
1. Frequency sampling
Frequency is obtained through sampling and calculation of voltage and time. Microcomputer protection can use the CPU counter to measure the average time between two voltage zero crossings, and then the system frequency value can be calculated (f=1/).
2. Investment in low-frequency load shedding function
The input of the low-frequency load shedding function requires the simultaneous input of the soft connection piece (= ON) and the LP hard connection piece, so that the AND gate & 1 can output the "1" state.
3. Operating conditions for low-frequency load shedding
Low-frequency load shedding action must meet the following three conditions:
First, the voltmeter is larger than the low voltage setting mountain. , to prevent sudden voltage drop causing malfunction of low-frequency load shedding. Second, the frequency slowly changes from greater than the set value to less than the set value. This condition simulates the frequency change pattern of the actual power system under the condition of active power shortage. If the frequency is lower than the set value when the device is put into operation, the device will not operate at this time. The third is the low-frequency load shedding time limit. The low-frequency load shedding action delay is generally set to 0.5s, but to prevent malfunction before the system's backup rotating motor capacity takes effect, it should be set to 1.5s.
4. Load feedback blocking for low frequency load shedding
When there is a large-capacity motor in the system, when the power supply loses power and the motor rotates at a low speed by inertia, it is equivalent to the low-speed operation of the generator, which will generate current and voltage feedback to the system. This kind of load feedback may cause the low-frequency load shedding device to malfunction. In order to prevent malfunction, two anti-misoperation locking measures are taken. For the system where the current distribution of the load feedback is clear, you can put the current feedback blocking = ON, and use &3 to identify that the three-phase currents are all less than (no current blocking locking value). If this condition is met, the low-frequency load shedding is blocked, but the general current The distribution is difficult to grasp, but the voltage feedback law is easier to grasp. At this time, voltage feedback can be used to lock input = OFF, = ON. According to the frequency drop speed is relatively large (greater than 3Hz/s) when load voltage feedback occurs, the frequency change rate is used to identify load feedback. When the blocking condition > is satisfied, the blocking Low frequency load shedding. When one of the two load feedback blocking conditions is met, H1 outputs a "1" state, thereby blocking the inverter Z1 and preventing malfunction of low-frequency load shedding.