Journal of Control Science and Engineering

Volume 2017 (2017), Article ID 9376735, 9 pages

https://doi.org/10.1155/2017/9376735

## Direct Power Control Strategy of PWM Rectifier Based on Improved Virtual Flux-Linkage Observer

Electrical Engineering College, Henan University of Science and Technology, Luoyang 471023, China

Correspondence should be addressed to Wenshao BU

Received 16 May 2017; Revised 13 August 2017; Accepted 27 August 2017; Published 2 October 2017

Academic Editor: Fabio Mottola

Copyright © 2017 Wenshao BU and Leilei Xu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

In order to achieve the low cost and high performance control of three-phase PWM rectifier, a direct power control (DPC) strategy based on a new-style virtual flux-linkage observer is proposed. The model of three-phase PWM rectifier and the principle of virtual flux-linkage vector control are introduced firstly. Then, in order to avoid the effect of integral initial value and cumulative deviation, three first-order low-pass filters are cascaded to replace the pure integral link; an improved virtual flux-linkage observer of three-phase power grid is presented. From the observed virtual flux-linkage, the voltage and instantaneous power of three-phase power grid are online estimated. On this basis, the power grid voltage sensorless direct power control system of three-phase PWM rectifier is designed. Simulation results have shown that, both in the rectifying state and in the inverting state, the power grid side current and the DC side voltage of three-phase PWM rectifier all can be effectively controlled; the high power factor operation of three-phase PWM rectifier is realized.

#### 1. Introduction

Compared with the conventional uncontrolled rectifier using diodes, adopting three-phase PWM rectifier, the energy can be controlled to freely flow along positive and negative directions [1, 2]. When the PWM rectifier technology is applied to back to back converter, photovoltaic grid connected inverter, power active filter, and so on, the power factor of system can be improved greatly, and the harmonic and reactive pollution to the grid can be effectively reduced [1, 3, 4]. And then, it has a broad application prospects in modern power system and equipment [5–8].

With the development of power electronics and control theory, in order to improve the control performance and reduce system cost and harmonics, numerous modern control strategies have been studied and applied in an attempt to the control system of three-phase PWM rectifier [9–15]. Hereinto, direct power control (DPC) strategy has become one of the hot research topics in recent years, because of its fast dynamic response, simple structure, and high power factor, and so on [8, 11, 12, 14, 16–18]. In general station, the control system of PWM rectifier contains more sensors and complex detection device, which not only increases the volume of the system device and improves the system cost, but also reduces the system reliability of three-phase PWM rectifier. The common used sensors in the control system of three-phase PWM rectifier include DC voltage sensor, power grid voltage sensors, and AC current sensors on power grid side. The AC current sensor is used to detect the AC current on power grid side of PWM rectifier and used for the closed-loop control of incoming line current; it plays an important role in the over-current protection of PWM rectifier system. The DC voltage sensor is used to detect the level and stability of DC bus voltage and used for the closed-loop control of DC side voltage. The AC current and DC voltage sensors play a crucial role in ensuring the control performance of three-phase PWM rectifier system. Even in the DPC control system of three-phase PWM rectifier, the incoming line AC current and the DC side voltage are also necessary to calculate instantaneous power variables [2, 8, 11, 12, 14, 19]. And then, the two sensors are generally not to be omitted in actual application.

As for the power grid voltage, it can be obtained from online identification [2, 10, 13, 15]. If the power grid voltage sensor can be saved, the system performance would not be affected by too much; meanwhile the system structure can be more conducive to simplifying, the system costs can be reduced also. The power grid side of three-phase PWM rectifier is similar to the stator structure of common three-phase AC motor driven by a three-phase inverter; the power grid can be seen as a virtual AC motor; its virtual flux-linkage can be used to estimate the power grid voltage of three-phase PWM rectifier. And then, the vector control method based on virtual flux oriented is widely used to the power grid voltage sensorless control scheme of three-phase PWM rectifier. But, in the conventional virtual flux-linkage observer, there exists a pure integral link, and then the estimation process of virtual power grid flux-linkage is inevitably affected by the initial value and the cumulative deviation of the integrator [2, 20–23]. In order to overcome the deficiencies of pure integrator, a first-order low-pass filter is adopted to replace the pure integrator; the initial value problem of pure integrator can be successfully solved [13]; however, the problem of amplitude and phase deviation is caused. About the direct power control (DPC) of three-phase PWM rectifier without power grid voltage sensor, preliminary research progress has been made [24, 25]; however, the integral initial value and cumulative deviation of pure integrator remain to be solved. In order to overcome the influence of integral initial value and cumulative deviation of pure integrator and avoid the amplitude and phase deviation caused by first-order low-pass filter, three first-order low-pass filters are cascaded to replace the pure integrator; an improved virtual flux-linkage observer is proposed in [2]; but the working principle of virtual flux oriented vector control system is not introduced in detail; moreover, the improved virtual flux-linkage observation method is not combined with the DPC system of three-phase PWM rectifier.

In this paper, based on the existing technology, the characteristic of virtual flux-linkage is further analyzed, and an improved virtual flux-linkage observation scheme is adopted; the observation scheme can effectively avoid the influence of integral initial value and cumulative deviation of pure integrator. Based on the improved virtual flux-linkage observer, the power grid voltage sensorless DPC strategy of a three-phase PWM rectifier is proposed. Simulation results have verified the validity and feasibility of the proposed control strategy.

#### 2. Power Grid Voltage Sensorless Control Principle

Figure 1 shows the circuit topology of three-phase PWM rectifier. In Figure 1: , , and are the AC side voltages; , , and are the three-phase power grid voltage, that is, the three-phase EMF of power grid; , , and are the AC side current of three-phase PWM rectifier; is the DC side voltage of three-phase PWM rectifier; is the resistance of line reactor; is the inductance of line reactor; is the capacitor on DC side; is the load current.