Abstract: This paper discusses the importance of reactive power compensation and the principle of reactive power compensation using parallel capacitors. It analyzes common compensation methods and the problems existing in control systems, and introduces the hardware and software design scheme of PLC reactive power compensation control system based on actual engineering projects.
Keywords: reactive power compensation; PLC; power factor; capacitor
1 Introduction
In recent years, my country's installed power capacity has increased rapidly, greatly alleviating the power supply shortage. However, with the increase in power supply, system line losses will also increase. Statistics show that reactive power losses in the power system can reach up to 20% to 30% of the total generating capacity, meaning that approximately one-quarter of the generating capacity is used to offset power losses during transmission and distribution. Therefore, a lower power factor is more detrimental to the operation of the power system, mainly for the following two reasons:
1) The rated apparent power of a generator and transformer is SN = UNIN , which represents the rated capacity of the equipment and is numerically equal to the maximum allowable power output. This is because the active power output of a generator under rated operating conditions is...
P=UIcosφ
When the load's power factor cosφ = 1, P <sub>N</sub> = S<sub> N</sub> , and its capacity is fully utilized. When the load's power factor cosφ < 1, the generator's voltage and current are not allowed to exceed their rated values. Obviously, in this case, the generator can only generate a small amount of active power, while the reactive power is large. The larger the reactive power, the larger the scale of energy exchange between the circuit and the power source, and the less fully the energy generated by the generator is utilized. At the same time, the prime mover and transformer配套 with the generator cannot be fully utilized either.
2) When a certain amount of active power is delivered to a load at a constant voltage, the lower the power factor, the greater the current in the transmission line. This not only increases the voltage drop across the line but also increases the power loss along the line.
Therefore, improving the power factor of the power grid, i.e., reactive power compensation, is of great significance to the development of the national economy.
2. Parallel capacitors for reactive power compensation
1) Compensation principle
In practical engineering, most loads are inductive, and their power factors are relatively low. Connecting capacitors in parallel with inductive loads is one of the main methods to improve the power factor.
Inductive loads have currents that lead the power supply voltage, while capacitive loads have currents that lag behind. Therefore, the leading and lagging currents can complement each other. This reduces the reactive power absorbed from the power source (or grid) before the capacitor's parallel connection point; in other words, the reactive power of the capacitive load compensates for the reactive power of the inductive load. When the grid capacity is constant, reducing reactive power significantly improves the power factor.
2) Compensation and Control Methods
Common compensation methods include: one is centralized compensation (compensation capacitors are centrally installed in substations or distribution rooms for easy centralized management); the other is a combination of centralized and decentralized compensation (part of the compensation capacitors are installed in the substation, and the other part is installed in departments or workshops with large inductive loads. This method is flexible, easy to adjust, and can reduce losses in the enterprise's power supply and distribution lines).
Common compensation control methods:
Based on the load of the electrical equipment, the capacity of the compensation capacitor is calculated, a suitable reactive power compensation device is selected, and the capacitor is manually switched on and off in stages using an AC contactor. This control method clearly cannot meet the requirements of automated industrial control.
Automatic control equipment assembled from discrete components is characterized by numerous components, bulky size, complex wiring, poor reliability, and high maintenance difficulty when malfunctions occur. Some users are forced to manually control the equipment because it cannot be repaired. In today's era of rapid scientific and technological advancements and the widespread adoption of integrated circuits and microelectronics, this approach is far from meeting the demands of modern production.
Voltage and reactive power control systems based on microcontrollers as the main control unit have seen significant development. However, microcontrollers have poor anti-interference capabilities, making it difficult to guarantee reliability in medium and high voltage reactive power compensation. On the other hand, substations with higher voltage levels have a larger radiation range and a wider impact from faults, thus requiring higher control and communication capabilities from the system.
3. Design of PLC Automatic Reactive Power Compensation System
3.1 Introduction of PLC
PLCs are a new generation of industrial control devices developed based on microcomputer technology. Their structure is basically the same as that of a microcomputer. Small PLCs are designed to replace traditional relay-based contact control systems and other sequential controllers, thus differing from the hardware of general-purpose microcomputers. They combine the advantages of relay control with the comprehensive functionality, flexibility, and versatility of computers, using computer-programmed software logic to replace relay wiring logic, making them a versatile automatic control system. They are a relatively ideal new type of industrial control device. Therefore, the reactive power automatic compensation system of the Dagang Oilfield Refinery was upgraded. Based on the original compensation equipment assembled from discrete components, a programmable logic controller (PLC) reactive power automatic compensation system was designed.
3.2 System Hardware Design
The original reactive power automatic compensation controller, composed of discrete components, mainly consists of ten parts, including a phase angle detection circuit, an addition level conversion and delay circuit, a subtraction level conversion and delay circuit, a reversible counter, a decoder, and an output circuit. After being controlled by a PLC, the hardware block diagram of the system is shown in Figure 1.
The original system's main circuit, phase angle detection circuit, output circuit, and regulated power supply are retained. However, the control functions of hardware circuits such as the adder level conversion and delay circuit, the subtractor level conversion and delay circuit, the clock pulse generator, the reversible counter, the reset circuit, and the decoder are implemented using a PLC. The output signal of the phase angle detection circuit is too weak to drive the PLC input, so it is amplified before being used as the PLC input signal. Automatic control is implemented using PLC software according to the system's control requirements. The transistor switching circuit in the original output circuit is replaced with a PLC output relay. Due to the limitation of the PLC's output point capacity, an intermediate relay is added as part of the output circuit.
3.3 PLC Selection
The Japanese OMRON C28P was selected as the control host. Its main technical parameters and performance are as follows: storage capacity of 1194 addresses, 136 internal auxiliary relays, 160 holding relays, 48 timers/counters, input opto-isolation, output relay isolation, host I/O points expandable to 80/60 points, with basic logic instructions and a full range of dedicated instructions, and strong data processing capabilities, which can meet the requirements of the reactive power automatic compensation control system.
3.4 PLC Software Design
The control program adopts a modular and structured design with clear hierarchy and structure. The program flowchart is shown in Figure 2. The detection module continuously collects the phase angle information of the power system and compares it with the given parameters. If the requirements are not met, the compensation capacitor is promptly switched on or off to ensure that the power factor of the power system meets the set requirements.
4. Advantages of PLC Automatic Reactive Power Compensation Control System
4.1 High reliability.
Control systems assembled from discrete components consist of dozens of components in each part, with hundreds of components in the entire controller. A malfunction in any one component can cause the entire controller to fail and malfunction. However, using a PLC for control greatly simplifies the wiring, fundamentally reducing the chance of failure. Furthermore, the modular structure significantly improves the system's reliability.
4.2 Strong anti-interference ability
Control systems composed of discrete components, such as diodes, transistors, and capacitors, have certain requirements for the circuit's operating environment and temperature. PLCs, on the other hand, are designed specifically for industrial control and incorporate various anti-interference measures during their design and manufacturing process. They can work alongside high-powered equipment in harsh industrial environments, exhibiting strong anti-interference capabilities and high operational stability.
4.3 Easy to install and maintain
Compared to discrete component control systems, PLC control systems are smaller, lighter, and easier to install. This system features self-testing and monitoring functions, and can dynamically monitor the execution of the control program, facilitating on-site debugging and maintenance. Due to fewer wiring connections, maintenance is convenient and repair time is short.
5. Conclusion
Actual operation results prove that the above control system fully meets the technical requirements. Using PLC for automatic reactive power compensation control results in a simple system structure, stable operation, and high reliability. Whether from an economic or technical perspective, it is undoubtedly the best choice.
The innovation of this article lies in its use of PLC for automatic reactive power compensation, which improves the reliability and reactive power optimization of the reactive power compensation system. The system is flexible in configuration and easy to expand. It also has high reference value and a very broad prospect for the retrofitting of old reactive power compensation systems.
References:
[1] Wang Ruiyan. PLC-based integrated voltage and reactive power control system for substations. Electric Power Automation Equipment, 2003, 04: 34-36
[2] Liu Chunfang. Application and benefit analysis of low-voltage reactive power compensation. Electric Power Technology, 2002, 5: 32-35
[3] Wang Liyan. Research on reactive power optimization method of distribution network under power market conditions. Microcomputer Information, 2006, 12: 107-109
