The sampling interrupt service program block diagram is shown in Figure 2-6. The sampling interrupt service program mainly includes three parts: sampling calculation, TV and TA disconnection self-test and protection starting components. At the same time, you can also add some programs to detect the status of the protected system according to different protection characteristics.
1. Overview of Sampling Calculation
Enter the sampling interrupt service routine and first perform sampling calculations. Before calculation, the instantaneous values of three-phase current, zero-sequence current, three-phase voltage, zero-sequence voltage and line voltage must be sampled at the same time, such as sampling at 12 points per week, and the sampling frequency is 12X50=600Hz. After sampling, according to the formula (1 -4) Calculate its instantaneous value, and then store each instantaneous value in the corresponding address unit of the random access memory RAM.
When calculating the AC effective value of each current and voltage, a group of instantaneous values of the same cycle of a certain calculated analog quantity are used for calculation, and the algorithms are various. For example, the semicircular integral method, Fourier's algorithm, etc. (see Section 2 of this chapter for details). In short, the sampling calculation of protection is to use some appropriate algorithm to calculate the effective value, phase, frequency and impedance of each phase voltage and current respectively. It can also further calculate the voltage, current and power direction of each sequence according to the needs, and store them in the designated area of RAM respectively for subsequent program calls (for logic judgment and further calculation).
The sampling calculation of microcomputer protection is the use of microcomputer capabilities for numerical calculation and storage. It realizes many functions that conventional protection cannot realize by hardware, and also makes it very simple, flexible and convenient for the protection to realize complex action characteristics, which is an important feature of the microcomputer protection software.
It should be pointed out that whether it is running or debugging the sampling channel, it must enter the sampling interrupt service program and perform sampling calculation. Therefore, in the sampling interrupt service program, after the sampling calculation is completed, it is necessary to query the current working mode.
2. Self-check for TV disconnection
Before starting the protection judgment, check whether the secondary voltage transformer TV is disconnected. In a small ground current system, you can simply check whether the TV secondary circuit is broken according to the following two criteria.
(1) The positive sequence voltage is less than 30V, and the current of any phase is greater than 0.1A.
(2) The negative sequence voltage is greater than 8V.
However, when the system fails, the positive sequence voltage will also drop, and the negative sequence voltage will increase. Therefore, when any of the above conditions is met, it must be delayed for 10s before the TV disconnection of the bus can be determined, and the abnormal operation "TV disconnection" signal will be issued. treat
The signal returns after the voltage returns to normal. During the TV disconnection period, a special flag bit DYDX is set to "l" in the software to mark the TV disconnection, and the locking and automatic reclosing are arranged through the program. At this time, the protection will decide whether to exit the voltage-related protection based on the set control word.
Not all protections have this function. For example, three-stage current protection does not require TV disconnection self-test.
3. Self-check for TA disconnection
When the TA secondary circuit is disconnected or the intermediate link of the current channel is poorly connected, some protections (such as transformer differential protection) may malfunction, so the TA secondary circuit must be monitored, and when the circuit is disconnected, the protection will be blocked and the alarm should be issued. . Since the "Y" connection is adopted for the currents introduced in each side of the transformer protection of the microcomputer, the judgment of TA disconnection becomes simple and clear, and it can be used for large grounding current systems.
Use the following two criteria for zero sequence current:
(1) If a zero-sequence current appears on the "△" side of the transformer, it is judged that this side is broken;
(2) On the "Y" connection side, compare the self-produced zero-sequence current (++) with the zero-sequence current (3) introduced by TA on the neutral point side of the transformer. If there is a differential current, it is judged that TA on this side is broken.
The specific criteria are as follows:
When a ground fault occurs in the system, the value of 3 increases. Therefore, another criterion must be added for TA disconnection. The system 3 is less than the fixed value.
In the formula, and are the two current settings for TA disconnection.
The above criteria are relatively complex, and simpler judgment methods can also be selected for medium and low voltage substations. The following are two criteria for transformer protection to use negative sequence current to determine TA disconnection:
(l) The negative sequence current generated when TA is disconnected only appears on one side of the disconnection, but in the event of a fault, negative sequence current will appear on at least both sides. (2) If the above criterion fails when the transformer is no-load, since negative sequence current only appears on the power supply side, it will be mistakenly judged that TA is disconnected. Therefore, additional conditions are required: the three phases on the low-voltage side of the step-down transformer have a certain load current.
During the TA disconnection period, the software must also set the flag bit DLDX=1 to mark the TA disconnection, and decide whether to exit the operation based on the set control word.
It should be pointed out that not all protections require TA and TV disconnection self-test. The disconnection self-test program should be designed based on the impact of TA and TV disconnection on protection.