Mathematical Problems in Engineering

Volume 2016, Article ID 3925879, 10 pages

http://dx.doi.org/10.1155/2016/3925879

## A Novel Control Strategy of DFIG Based on the Optimization of Transfer Trajectory at Operation Points in the Islanded Power System

^{1}State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, Baoding 071003, China^{2}Power Dispatching Control Center, State Grid East Inner Mongolia Electric Power Company Limited, Hohhot 010020, China

Received 16 October 2015; Revised 28 December 2015; Accepted 6 January 2016

Academic Editor: Ivan Kyrchei

Copyright © 2016 Zengqiang Mi et al. 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

A novel control strategy based on the optimization of transfer trajectory at operation points for DFIG is proposed. Aim of this control strategy is to reduce the mechanical fatigue of DFIG caused by the frequent adjustment of rotating speed and pitch angle when operating in the islanded power system. Firstly, the stability of DFIG at different operation points is analyzed. Then an optimization model of transfer trajectory at operation points is established, with the minimum synthetic adjustment amount of rotating speed and pitch angle as the objective function and with the balance of active power and the stability of operation points as the constraint conditions. Secondly, the wind speed estimator is designed, and the control strategy of pitch system is improved to cooperate with the indirect stator flux orientation control technology for rotor-side inverter control. Then by the coordination control of its rotating speed and pitch angle, an operation trajectory controller is established to ensure the islanded operation of DFIG along the optimal transfer trajectory. Finally, the simulation results show that the proposed control strategy is technical feasibility with good performance.

#### 1. Introduction

The islanded operation is a significant operation mode of wind turbine, and this can widen the applied range of wind turbine [1]. It can improve the service ability, reduce the cost of power supply, and protect the environment by composing an islanded power system with wind turbine in some remote regions or islands [2, 3]. And for some areas that lack emergency power supply, their economic losses caused by the power grid failure can be reduced when the important local load is supplied by the wind turbine [4, 5]. So it is significant to make an intensive study of the control strategy for wind turbine to operate in the islanded operation mode.

The doubly fed induction generator based wind turbine (DFIG) is currently the most widely used type of unit [6], and some researches about the islanded operation control strategy of DFIG have been carried out at home and abroad. In [7], the battery energy storage system (BESS) is connected in parallel with the DC link of DFIG inverter, and the hybrid system can operate in different modes and has the ability to supply power for the load independently. The coordinate control strategies of wind turbines and energy storage systems for stabilizing the islanded power system are proposed in [8, 9]. In [10–12], with the droop control strategy introduced, the DFIG can respond to the frequency and voltage changes of the isolated power system and keep its voltage and frequency to be stable, so the DFIG can continue to supply the load around in the case of disconnection from power grid. Through applying the control strategies proposed in [7–12], the DFIG can operate in the islanded mode. However, the strategies only make the DFIG to be similar with a current source essentially, so that a large power source is needed to sustain the voltage of islanded power system. An indirect stator flux orientation (ISFO) control strategy which is suitable for the doubly fed induction generator is proposed in [13, 14]; it can enable the doubly fed induction generator with voltage source characteristics to control the frequency and amplitude of its stator voltage independently. The DFIG can obtain the ability to supply the load independently with the application of ISFO control strategy [15, 16]. Operating in the islanded mode, the frequent adjustment of rotating speed and pitch angle may produce mechanical fatigue on the DFIG and affect its service life seriously.

In this paper, a control strategy based on the optimization of transfer trajectory at operation points for a stand-alone DFIG is proposed. Through applying this control strategy to coordinately regulate the rotating speed and pitch angle, the DFIG can autonomously operate along the transfer trajectory with the minimum synthetic adjustment amount of rotating speed and pitch angle. This paper is structured as follows. In Section 2, the mathematical model of DFIG is outlined, and the optimization model of transfer trajectory at operation points is established by the stability analysis of operation points. In Section 3, the designed control strategy is described. It mainly contains the wind speed estimator, the control unit of rotor-side converter and variable pitch system, and the operation trajectory controller. Simulation results are given and discussed in Section 4. Finally, conclusions are presented in Section 5.

#### 2. Transfer Trajectory Optimization of DFIG Operation Point

##### 2.1. Mathematical Model of DFIG

The aerodynamic power captured by the turbine is as follows:where is the aerodynamic power, is the air density, is the rotor radius, is the aerodynamic power performance coefficient which is the function of tip speed ratio and pitch angle , is the wind speed, is the aerodynamic torque, and is the rotating speed of the turbine.

The aerodynamic power performance coefficient is calculated according to the following equation: where are constants related to the aerodynamic characteristics of the wind wheel.

The dynamic behavior of the drive train can be described with a two-mass model [17]:where and are the turbine and generator inertia, respectively, is damping coefficient, is the shaft elastic coefficient, is the friction coefficient of generator, is the electromagnetic torque of generator, is the generator speed, is the gearbox ratio, and is the shaft deformation angle.

The equations of doubly fed induction generator written in a synchronously rotating - reference frame are expressed as follows [13]:where , , , are the components of stator voltage and rotor voltage, respectively, , , , are the components of stator flux and rotor flux, respectively, , , , are the components of stator current and rotor current, respectively, and are the stator resistance and rotor resistance, , , and are the stator inductance, rotor inductance, and the mutual inductance, is the synchronous speed, and is the equivalent stator magnetizing current.

##### 2.2. Stability Analysis of DFIG Operation Points

Based on the historical statistics data of wind speed and load in the islanded power system, if the capacity of DFIG is configured reasonably, the DFIG can satisfy the reactive power demand of load. On this basis, the specific active power can be output by adjusting the rotating speed and pitch angle of DFIG. It is known from (1) and (2) that, for a certain wind speed, the aerodynamic power captured by the turbine is a nonlinear function of the rotating speed and pitch angle and this is shown in Figure 1.