Anke Electrical Co., Ltd.

Abstract: Reactive power compensation in low-voltage distribution systems is a crucial aspect of electrical power quality management. In traditional reactive power compensation, the impactDue to slower response, the compensation current exhibits stepped increments, resulting in over-compensation or under-compensation, which may not always reach the theoretical level.To address this issue, a comprehensive reactive power compensation control method has been proposed.Monitor voltage, current, and power in the power system, and real-time coordinate control LC (capacitor-inductor) and SVG (static var generator).The "reactive power generator" module undergoes mixed compensation, also known as intelligent dynamic reactive power compensation, which can achieve compensation...Continuous output of current. The application of this solution in engineering practice has shown that it can effectively improve...Enhanced power supply quality and improved system power factor.

Keywords: Power Quality; Comprehensive Reactive Power Compensation Control; LC; Static Var Generator; Smart Dynamic Reactive Power Compensation

0 Introduction

With the continuous development of modern society's economy and culture, the overall electricity consumption in the ceramic production industry has surged. However, it has also brought a series of electrical quality issues. The main loads for ceramic producers include rolling molding machines, ball mills, etc., such as ball mills using variable-frequency drives. Due to the irregular shape of raw materials, the ball mills have a large surge current during operation, generating harmonics at 3, 5, and 7 times, which leads to traditional reactive power compensation not keeping up, excessive capacitor damage, and a low power factor, affecting the stability of the power system.

Overview of Compensation Without Cause

In electrical power systems, the majority of loads are inductive, such as transformers, motors, compressors, and air conditioners, which can be modeled as a series circuit of resistance R and inductance L, as shown in Figure 4 below.

Figure 4 - Equivalent Circuit of Power Supply System; Figure 5 - Improved Power Factor

Figure 5 illustrates the phase diagrams, revealing that without the capacitor C, the phase angle between voltage U and current I is φ1. After introducing capacitor C, the phase angle becomes φ2, indicating a reduction in the phase angle between voltage and current, which enhances the system's power factor. When the capacitive load releases energy, the inductive load absorbs it; conversely, when the inductive load releases energy, the capacitive load absorbs it. Energy is exchanged between the two loads, as depicted in Figure 6. This way, the reactive power absorbed by the inductive load can be compensated by the reactive power output from the capacitive load, not only improving the power factor and system voltage but also effectively reducing system energy losses.

Figure 6 - Energy exchange between capacitive and inductive loads

Analyzing from the perspective of reactive phase, the voltage and current of a purely resistive load are in phase, the voltage of an inductive load leads the current, and the voltage of a capacitive load lags behind the current, as shown in Figure 7.

Figure 7 - Reactive Phase Analysis

2. Non-penalty compensation forms

2.1 Non-Compensated LC

LC reactive compensation is a traditional capacitive compensation method that operates in parallel with the power system. It controls the switching of power capacitors for reactive compensation based on changes in the power factor of the load in the power grid. The principle is as follows: voltage and current signals are collected by a CT, and then the control unit calculates the switching plan, controlling the switching switches (such as composite switches, thyristor switches, etc.) to switch on and off the power capacitors in each group. As shown in Figure 8.

Figure 8 - The working principle of LC reactive compensation

2.2 SVG Non-Power Compensation

SVG is a type of active power factor correction equipment. It connects a three-phase bridge circuit in parallel to the power grid via reactors. According to the system's reactive power, it outputs the required capacitive or inductive fundamental frequency current through an IGBT power converter, thereby achieving dynamic reactive power compensation without overcompensation or undercompensation. The compensation is smooth, and it does not produce surges or shock to the load or the power grid. The principle is illustrated in Figure 9.

Figure 9 - Principle of SVG Reactive Compensation

2.3 Smart Dynamic Reactive Power Compensation

LC compensation fails to respond quickly in situations with rapid load changes or shock loads, often resulting in over-compensation or under-compensation. Moreover, the parallel capacitors in the LC compensation device amplify harmonic currents, which can easily cause system resonance, although it is relatively cost-effective. SVG can continuously and rapidly compensate for reactive power in both inductive and capacitive forms, preventing over-compensation and under-compensation, and it does not cause resonance with the system or load equipment. It is suitable for scenarios with rapid load changes, although its cost is relatively high.

Considering the advantages and disadvantages of the aforementioned two compensation methods, people have designed a new type of power electronic device for reactive power compensation known as the Smart Dynamic Reactive Power Compensation Device. It employs an integrated control method for reactive power compensation, incorporating a controller to switch the SVG module and LC module. The SVG module's rapid response and accurate compensation characteristics are used to offset the slow response and stepped compensation drawbacks of the LC module, while also reducing costs compared to full SVG compensation. The compensation principle is illustrated in Figure 10. It detects the voltage and current of the compensation object, calculates the compensation current command signal through an instruction current calculation circuit, amplifies the signal through a compensation current generation circuit to produce the compensation current, and then controls the SVG module to compensate first, followed by the LC module. The output current of the SVG module is adjusted to meet the reactive power demand, thereby achieving the set power factor value.

Figure 10 - Principle of Operation for an Intelligent Dynamic Reactive Power Compensation

The compensation curves for LC (capacitive compensation) and LC+SVG (smart reactive power compensation) are shown in Figures 11 and 12 respectively.

Figure 11 - Capacitive Compensation, Figure 12 - Smart Reactive Power Compensation

3 Industrial Application Cases of Smart Reactive Power Compensation

A ceramic production enterprise in Jiangsu primarily relies on ball mills, which experience significant current surges during operation. The site's harmonics are mainly the 3rd, 5th, and 7th orders, affecting the stability of the power system and leading to a high number of damaged compensating capacitors and a low power factor. The transformer capacity in the on-site power distribution room is 1600kVA, and the original capacitor cabinet's installed capacity was 300kvar. The data on power quality before and after the application of the intelligent reactive power compensation solution are shown in Figures 14 and 15.

Figure 14 - Power Quality Data Before the Smart Reactive Power Compensation Application

Figure 15 - Power Quality Data After Smart Reactive Power Compensation Application

Based on the on-site conditions, we have implemented an Ankorui LC+SVG integrated control intelligent reactive power compensation solution. The preliminary measurement data estimates the required total cabinet capacity to be 500kvar (two units of 250kvar, main and auxiliary cabinets). Considering the on-site harmonics are primarily the 3rd, 5th, and 7th orders, the LC capacitor compensation uses a reactor with a reactance rate of 14% in series with the compensation capacitor for better harmonic suppression and capacitor protection. By coordinating the ANSVG-S-G 100kvar module with the capacitors through the controller, the reactive power output is achieved simultaneously. Comparing the data before and after treatment, the power factor has improved from 0.87 to 0.98. Since the SVG can also treat some harmonics while compensating reactive power, the current distortion rate of phase B has been reduced from the original 23.28% to 9.06%, achieving significant treatment effects. Table 1 provides the configuration scheme for the main equipment components of a 250kvar intelligent dynamic reactive power compensation device.

Table 1 - Main Equipment Component Configuration Scheme for a Single 250kvar Smart Dynamic Reactive Power Compensation Device

Conclusion

This article briefly describes Ankoray's intelligent dynamic reactive power compensation solution, which combines the advantages of LC compensation and SVG compensation. It coordinates and controls capacitors and SVG compensation modules in real-time for mixed reactive power compensation, maintaining the rapidity, continuity, and accuracy of reactive power compensation, while the cost of the solution is also acceptable to most enterprises. Through the application of engineering case studies, comparing the treatment effects before and after the implementation of the solution, it improves the power factor of the system, controls harmonics, improves the enterprise's power usage environment, reduces the high cost of power quality treatment, and is beneficial to the enterprise's operations and production, creating value for the enterprise.

Reference

GB 50227—2017 Specification for Design of Shunt Capacitor Units

[2] GB/T 15576-2020 Low-Voltage Complete Reactive Power Compensation Equipment

[3] DL/T 1216-2019 Technical Specification for Low-Voltage Static Reactive Generator

Ankorri Enterprise Microgrid Design and Application Manual, 2022.05 Edition

Wang Zhaoan. Harmonic Suppression and Reactive Power Compensation. Machine Industry Press

Huang Chun Guang. A comprehensive control method for reactive power compensation in a low-voltage distribution system. Electrical Technology - Issue 7