Design of grid reactive power compensation control system

    Abstract: This article introduces a design method of a power grid reactive power compensation control system composed of a single-chip microcomputer system, input/output circuit, keyboard display circuit and communication interface circuit, and details the system hardware composition and software design of this design method. scheme and its anti-interference method is given, and finally the software design flow chart of the control system is given.

    Keywords: reactive power compensation thyristor power grid

1 Introduction

In the power system, voltage is an important indicator of power quality. Ensuring that the voltage at the user's site is close to the rated value is one of the basic tasks of power system operation adjustment, and reactive power has a very close relationship with voltage. On the one hand, the change in reactive load when the voltage changes is much greater than the change in active load; on the other hand, the voltage fluctuation caused by reactive load is also much greater than the change in active load. If the reactive power supply of the power system is relatively sufficient, it can meet the need for reactive power balance at a higher level, and the system will also have a higher operating voltage level. On the contrary, insufficient reactive power will be reflected as a low operating voltage level, which may cause The voltage collapses, thereby destroying the safe operation and operational stability of the power system. Therefore, reasonable adjustment of voltage and reactive power is of extremely important significance in improving power quality, reducing network losses, and operating the stability and safety of the power grid.

The method of adjusting reactive power is to adjust the size of the grid compensation capacitor according to the size of the reactive power. Because reactive power transmission losses are large, it is not suitable for long-distance transmission. Therefore, the reactive power required by the load should be supplied locally as much as possible.

2 System hardware design

The hardware block diagram of the reactive power compensation control system of this power grid is shown in Figure 1. It mainly consists of a single-chip microcomputer system (self-reset circuit composed of 80196 single-chip microcomputer, 74LS373, and FLASH MEMERY 29C256), keyboard display circuit, input circuit, output circuit, and communication interface circuit MAX232.

2.1 Microcontroller system

The core part of the system is an 80196KB microprocessor, and 80196KB is an MCS-96 series 16-bit processor; its powerful functions, rich resources and high efficiency have laid the foundation for the rapid and real-time operation of the entire system.

2.2 Input circuit

The input circuit consists of two parts: a voltage and current phase difference detection circuit and a voltage and current effective value detection circuit. The voltage and current phase difference detection circuit is composed of PT (voltage transformer), CT (current transformer) and zero-crossing detection circuit. PT can be used to convert the high voltage on the power grid into a low voltage signal, and then convert it into a square wave signal after zero-crossing detection. CT can convert the current on the power grid into a voltage signal, and at the same time, it can also be converted into a square wave after zero-crossing detection. The CPU calculates the time difference Δt by detecting the rising edge of this square wave , and then compares it with the measured period T to obtain the phase difference φ , that is:

φ=2πΔt/T

The voltage and current effective value detection circuit is composed of PT, CT, V/F circuit and counter 8254. The V/F circuit uses an AC V/F circuit. This V/F circuit has the following advantages:

●Speed ​​up the tracking of measured changes.

●The intermediate links are reduced, thereby reducing the sources of errors and interference.

●Simplified installation and wiring.

●Reduces the hardware investment in the transmitter.

●You can make a reasonable choice between sampling speed and accuracy through software.

The 8254 chip is a counter. Through its timing counting, a series of instantaneous values ​​of voltage and current signals can be obtained, and then the effective values ​​of the voltage and current can be obtained using software and a series of algorithms. Common algorithms include full-wave Fourier algorithm, half-wave Fourier algorithm, two-point product algorithm, derivative algorithm, etc. This system uses the full-wave Fourier algorithm. Because this algorithm is more accurate than other algorithms, it requires a longer data window and a slower calculation process. Therefore, considering the speed and accuracy of the system, the author chose the algorithm with higher accuracy. High algorithm.

The idea of ​​the full-wave Fourier algorithm is based on the Fourier series, because the voltage and current signals are a periodic function and can be decomposed into infinite series of DC components, fundamental waves and sub-harmonics. That is:

    the base-to-skin current can be obtained from this:

    This system uses a 13-point algorithm, that is, N=13.

2.3 Output circuit

After the output circuit is isolated by an optocoupler, it can drive the thyristor control circuit to control the on and off of the thyristor, that is, whether the capacitor is put into use. In addition, when voltage regulation is required, the voltage boost and voltage reduction of the transformer can also be controlled.

2.4 Communication interface

The communication interface of the control system adopts the 232 interface, and its interface circuit is designed using the MAX232 dedicated 232 interface circuit chip produced by MAXIM Company. This chip has the characteristics of simple use and reliable performance.

3 Software design

The system software is written in PLM96 language, and its main program block diagram is shown in Figure 2. At the same time, the program also opens three interrupts: HIS interrupt, timer interrupt and serial interrupt. The HIS interrupt is used to accurately record the rising edge time of the voltage and current square waves, and provide real-time values ​​for the main program to calculate the phase difference in time. The scheduled interrupt is used to sample the voltage and current at regular intervals; the serial interrupt is used when the system is networked. Implement communication functions. In order to prevent these three interrupts from affecting each other, the time consumption of each interrupt subroutine should be as short as possible, and the nesting of each interrupt can also be considered. This system software also opens scheduled interrupts and HIS interrupts in serial interrupts. The real-time requirements of these two interrupts are relatively high, and the interrupt time is also very short.

Through the opening of three interrupts, the real-time performance of each system parameter can be improved, and the main program can also be made simpler and more efficient.

4 Conclusion

Since this system was put into use, it has been operating stably, saving manufacturers and users a lot of expenses, thereby increasing economic benefits, and has been well received by users and manufacturers.