Design of reactive power compensation control system for power grid

1 Introduction

In the power system, voltage is an important indicator to measure the quality of electric energy. Ensuring that the voltage at the user is close to the rated value is one of the basic tasks of the power system operation and adjustment, and reactive power has a very close relationship with voltage. On the one hand, when the voltage changes, the change of reactive load is much greater than the change of active load; on the other hand, the voltage fluctuation caused by reactive load is also much greater than the change of active load. If the reactive power supply of the power system is sufficient, it can meet the needs of reactive power balance at a higher level, and the system will also have a higher operating voltage level. On the contrary, insufficient reactive power is reflected in the low operating voltage level, which may cause voltage collapse, thereby destroying the safe operation and stability of the power system. Therefore, the reasonable adjustment of voltage and reactive power is of great significance in improving the quality of electric energy, 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 reactive power. Because reactive power transmission loss is large and it is not suitable for long-distance transmission, 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 the power grid is shown in Figure 1. It mainly consists of a single-chip microcomputer system (a self-reset circuit composed of an 80196 single-chip microcomputer, 74LS373, and a FLASH MEMERY 29C256), a keyboard display circuit, an input circuit, an output circuit, and a communication interface circuit MAX232.

2.1 Single-Chip Microcomputer System

The core of the system is a 80196KB microprocessor, which is a 16-bit processor of the MCS-96 series. Its powerful functions, rich resources and high efficiency lay the foundation for the rapid and real-time operation of the entire system.

2.2 Input Circuit

The input circuit consists of two parts: the voltage and current phase difference detection circuit and the voltage and current effective value detection circuit. The voltage and current phase difference detection circuit consists 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 also convert it 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 adopts AC V/F circuit, which has the following advantages:

●Speeds up the tracking of changes in the measured quantity.

●Reduces intermediate links, thereby reducing errors and interference sources.

●Simplified installation and wiring.

●Reduces the hardware investment in transmitters.

●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 voltage and current can be obtained by 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 it has higher accuracy than other algorithms, but the data window required is longer and the calculation process is slower. Therefore, considering the speed and accuracy of the system, the author selected an algorithm with higher accuracy.

The idea of ​​full-wave Fourier algorithm is based on Fourier series, because voltage and current signals are periodic functions, and thus can be decomposed into an infinite series of DC components, fundamental waves, and subharmonics. That is:

From this we can get the base-to-skin current:

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

2.3 Output Circuit

The output circuit can drive the thyristor control circuit after optical coupling isolation, thereby controlling 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 step-up and step-down of the transformer can also be controlled.

2.4 Communication Interface

The communication interface of the control system adopts 232 interface, and its interface circuit is designed with MAX232 special 232 interface circuit chip produced by MAXIM company. The 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 flowchart is shown in Figure 2. At the same time, the program also opens three interrupts: HIS interrupt, timer interrupt and serial interrupt. HIS interrupt is used to accurately record the time of the rising edge of the voltage and current square wave, and timely provide real-time values ​​for the main program to calculate the phase difference. The timing interrupt is used to sample the voltage and current at a fixed time; the serial interrupt is used to realize the communication function when the system is connected to the network. 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 the timing interrupt and HIS interrupt in the serial interrupt. The real-time requirements of these two interrupts are relatively high, and the interrupt time is also very short.

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

4 Conclusion

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