Design and implementation of a power compensation system in low voltage distribution system

1. Introduction

Reactive power compensation, referred to as reactive compensation, plays a role in improving the power factor of the power grid in the electronic power supply system, reducing the loss of power supply transformers and transmission lines, improving power supply efficiency, and improving the power supply environment. Therefore, the reactive power compensation device is in an indispensable and very important position in the power supply system. Reasonable selection of compensation devices can minimize the loss of the network and improve the quality of the power grid. On the contrary, improper selection or use may cause many factors such as power supply system, voltage fluctuation, and increased harmonics. The reactive dynamic compensation device consists of a controller, a thyristor, a parallel capacitor, a reactor, a zero-crossing trigger module, and a discharge protection device. The device tracks and measures the voltage, current, reactive power and power factor of the load in real time, analyzes it through a microcomputer, calculates the reactive power and compares it with the pre-set value, automatically selects the compensation capacity that can achieve the best compensation effect and issues instructions, and the zero-crossing trigger module determines the conduction moment of the bidirectional thyristor to achieve rapid and impact-free input of the parallel capacitor bank.

The total installed capacity of Xinglongzhuang Mine Power Plant is 24MW, including two 6MW and one 12MW. According to the local grid-connected power factor requirement of 0.85-0.9, the generator power factor is also controlled between 0.85-0.9, while the power factor of the low-voltage distribution device is generally around 0.86, requiring reactive power compensation. According to the on-site conditions, it is preferred to install a centralized compensation device on the low-voltage side of the transformer, and install a reactive power compensation cabinet on each of the four sections.

2. On-site reactive load operation and determination of reactive compensation amount

It is recommended that the power factor of the four-section low-voltage busbar of Xinglongzhuang Mine Power Plant be compensated to above 0.95. The active load is relatively average, with a maximum of about 630KW, calculated as 620KW.

Compensation capacity calculation:

The compensation capacity is 164.12KVar respectively, so the total compensation is about 164.12×4=656.48 KVar.

3. Optimal selection and performance of reactive power compensation devices

Based on the principles of safety, economy, rationality, reliability and high precision, Xinglongzhuang Mine Power Plant selected QFJ-I intelligent reactive power compensation device. This device is suitable for dynamic reactive power compensation of various loads in low-voltage distribution system, and can automatically adjust the operation of reactive power compensation device. The unique reactive power and voltage joint #-shaped control strategy, the combination of common compensation and separate compensation, makes the reactive power compensation of each phase of the power grid remain in a high-precision state, achieving the best loss reduction and energy saving effect. The maintenance-free quality assurance makes it widely used in the power distribution system of 50Hz and rated voltage 400V, as dynamic reactive power compensation of various loads, to improve the power factor of the system, increase the system output, reduce network loss and improve the power quality.

The controller uses a high-performance Intel chip as the core unit. It uses the optimized Fourier algorithm and instantaneous integral comparison method to accurately calculate and display various parameters of the power grid (such as bus three-phase voltage, power factor, active power, reactive power, harmonic components, output current, compensation, etc.) and protection status. The settings and output displays are all digital quantities, with a good human-machine interface, intuitive operation, and high reliability. This controller has a variety of communication functions to meet the requirements of distribution network automation. In addition to dynamically monitoring the active power and reactive power of the power system and automatically tracking and compensating for reactive power, the controller can also monitor, count, and store 52 data such as voltage, current, power factor, active power, reactive power, and harmonic components of each phase of the power grid.

The cabinet of QFJ-I series intelligent reactive power compensation cabinet is made of high-quality stainless steel, with a coating layer on the outside. The internal design is a modular structure. The installation position of the components can be freely adjusted in a certain module in three-dimensional space according to actual requirements. The top is designed with a ventilation structure, which not only meets the protection level requirements, but also enables the air inside the device to circulate naturally, and has a good heat dissipation effect.

QFJ-I reactive power compensation cabinet has rich and complete protection functions, which can meet the reactive power compensation use in various situations. The device has overvoltage protection; undervoltage protection; overcurrent protection; harmonic protection; temperature protection; short circuit protection; lightning protection; reverse phase, phase loss protection, etc.

Technical performance of selected equipment

Rated voltage (UN): 0.40KV; Rated frequency: 50HZ; Rated compensation capacity: 180KVar;

Capacitor connection mode: △, Y0; Measurement error: Voltage: ±0.5% Current: ±1.0% Reactive power: ±1.0%;

Output circuit: 6 levels of common compensation + 2 levels of separate compensation adjustable; dynamic response time: 20ms; switching delay: 3~999 seconds adjustable.

4. Test and benefit analysis

(1) Test

After installation, the control circuit and wiring were carefully checked, and the insulation telemetry of the main circuit was performed. After no problems were found, the device parameters were set according to the product manual after power was supplied. The device was put into operation manually/automatically, and the device operated normally.

By adjusting the load size of each bus section, the data is as follows:

Active load power factor

According to the test, after the reactive power compensation device was installed on the four-section busbar, the requirements of the plan and technical agreement were met and exceeded the national regulations on the power factors of industrial and mining enterprises.

(2) Benefit Analysis

The installation of reactive power compensation equipment in Xinglongzhuang Mine Power Plant can bring significant economic benefits, which are mainly reflected in the following aspects: improving equipment output, improving power quality, reducing power loss, and tapping the potential of power generation and supply equipment.

① Reduce the losses of transformers, electrical equipment and wires by improving the power factor.

② By ensuring a good power factor value, the voltage loss in the power supply system can be reduced, the load voltage can be made more stable, and the quality of electric energy can be improved.

③ Increase the margin of the low-voltage distribution system and fully tap the potential of power generation and supply equipment.