Elsevier

Renewable Energy

Volume 107, July 2017, Pages 181-193
Renewable Energy

Power control for wind power generation and current harmonic filtering with doubly fed induction generator

https://doi.org/10.1016/j.renene.2017.01.059Get rights and content

Highlights

  • We modeled and simulated the system DFIG/APF with filtering of grid harmonic currents by GSC.

  • The control of powers of the DFIG is through stator flux oriented control.

  • Algorithms are proposed for obtaining control of power and harmonic filtering.

  • The harmonic filtering incorporated to the DFIG improves power quality.

  • The strategy was verified experimentally checking the efficiency of the technique.

Abstract

This paper describes a wind power system which controls the active and reactive generated powers as well as it performs the function of filtering the harmonic components of the grid currents. From the grid side converter, the harmonic filtering is achieved by an algorithm proposed by compensation of harmonics. This technique ensures the improvement of power quality. The machine side converter controls the active and reactive powers that are delivered to the electric grid by the stator flux oriented control. The design methodology of the controllers used is presented. This paper is distinguished by three key contributions. The first contribution of this article is the tutorial character, it should assist in the development of future work. The second is the analysis of the harmonic filtering behavior for some operating points of the DFIG/APF system. The third is the application of the precise model of the DC link voltage dynamics, allowing verifying the stability of the system control for each DFIG/APF (active power filter) operating point. Simulation and experimental results confirm the effectiveness of the proposed research.

Introduction

Energy quality is an important aspect not only of wind power installation, but of all that use electronic power converters connected to the grid. The increase in applications of electronic devices such as variable speed drives, computer power source, among other things, they result in harmonic injection of current on the grid. This harmonic pollution distorts voltage and current waveforms in the grid with the presence of harmonic components, providing low power factor, possible warming, reactive power fluctuating, flicker, swell, among others.

Active power filter is a solution for reducing harmonics of electric current. The active power filter (APF) detects the harmonic electric current of nonlinear load and injects a compensation of electrical current to mitigate the harmonic components that go into the grid [1].

Applications on APF have been performed by means of changes in DFIG (Doubly Fed Induction Generator) converter control, which can improve the quality of electric power supplied and compensate the most harmonic currents [1], [2], [3], [4], [5], [6], [7]. Reference [2] proposes a control strategy by rotor side converter (RSC) that achieves reactive compensation and active filtering of harmonics grid currents of 5th. and 7th. orders. While [3] shows a similar system in Ref. [2], however the system manages the priority between the maximum power point tracking (MPPT) and improvement at power quality.

The works [1], [4], [5], [6] propose a system that controls the active and reactive powers and performs compensation of harmonic current, by modifying the RSC control using the sliding mode controller type. In these studies [1], [6], the current references for harmonic compensation are determined from the load current, calculated by the instantaneous power PQ theory [8].

Paper [4] proposes a wind power system and the mitigation of grid harmonic currents, using current controllers by hysteresis with constant switching frequency. It was obtained THD bigger than in Ref. [5], that used current controller by hysteresis with variable switching frequency. The authors, in Ref. [7], show the DFIG running with control delivery power to the electric grid, by using sensorless vector control, MPPT and mitigating grid harmonics currents.

Research on mitigation of grid harmonic currents from RSC control [1], [2], [3], [4], [5], [6], [7], using the harmonic currents injection in DFIG, and the electric machine is not designed for it, which increases losses and leads to a not adequate operation, which can cause a reduction in its useful life.

Recently, other studies have implemented the current harmonic compensation based on grid side converter (GSC) [9], [10], [11]. In Ref. [10], the currents reference for harmonic current compensation are determined from the load current, calculated by the instantaneous power PQ theory [8]. In Refs. [10], [11], the reference currents are determined using synchronous reference Frame (SRF) theory is based on the transformation of currents in synchronously rotating dq frame [12].

In the literature, most of the studies addressing the simulation results. Few studies present the experimental results [7], [11]. The authors in Ref. [7] implement the functions of the system DFIG/APF using an experimental setup developed with microcontroller to include the filtering function from RSC control. However, the authors did not determine the THD of grid current, making the strategy analysis difficult. In Ref. [11], the operation of DFIG/APF system is implemented on the dSPACE system (DS1103). It is sold on the market and has a high cost. The authors present the simulation and experimental results of THD of grid current before and after filtering through GSC control, using a control technique with current controllers hysteresis.

The researches do not discuss the behavior filtering for the operation of the DFIG with grid current harmonics filtering in the literature, either for RSC control or for GSC control.

In this research, we propose a control strategy for a wind-power system with DFIG, shown in Fig. 1. Besides the power control, the proposed system improves the energy quality by using the converter connected to the grid to perform active filtering in the point of common coupling (PCC) of the electric grid in the presence of three-phase full bridge rectifier with LiL feeding a resistive load. The control of active and reactive powers of the generator is accomplished through field vector control by stator. The active filtering function is performed by the grid side converter, using dq reference frame. The power control and filtering function occur simultaneously. In harmonic identification, the extraction of the fundamental component is based on the SRF theory. Also, this paper presents a design methodology of the controllers employed in these techniques. The proposed control of DFIG/APF improves the electric power quality in the electric grid. Simulation and experimental results are presented to demonstrate the idea of this study.

The paper contributions are: (1) the tutorial character, assisting in the development of future researches; (2) the analysis of the harmonic filtering behavior for some operating points of the DFIG/APF system; (3) the application of the precise model of the DC link voltage dynamics, allowing evaluating the stability of the system control for each DFIG/APF operating point.

The paper is organized as follows: Section 2 describes the studied the power control of the generator. Section 3 contains the technique proposed of harmonic current filtering. Section 4 Current and dc link voltage control, 5 RSC current control contain the design methodology of the controllers. Section 6 shows simulation results and Section 7 shows experimental verifications. Section 8 presents stability analysis of the system and Section 9 presents feasible application of the wind turbine converter in range of MVA up to 15 kHz. Conclusions are summarized in Section 10.

Section snippets

Power control of the doubly fed induction generator - DFIG

The generator is controlled in the reference of synchronous rotation with the stator flux directed along the axis d. Thus, the active and the reactive powers of the stator are decoupled. The mathematical model of a DFIG in the d-q reference frame is described from (1), (2), (3), (4), (5), (6) [13],vsd=rsisd+dψsddtωeψsqvsq=rsisq+dψsqdt+ωeψsdvrd=rrird+dψrddtωslψrqvrq=rrirq+dψrqdt+ωslψrdωsl=ωeωrψsd=Lsisd+Lmirdψsq=Lsisq+Lmirqψrd=Lrird+Lmisdψrq=Lrirq+LmisqTe=3p2Lm(isqirdisdirq)where vsd, vsq and

Active power filter in the GSC

The presence of a non-linear load in the PCC should distort the grid current, then the use of active filter can reduce the distortion of the current flowing through the electric grid.

The control structure of the grid side converter is modified with the inclusion of iLhd in the loop id and iLhq in the loop iq, as shown in Fig. 4. These modifications maintain the DC link voltage and allows mitigate harmonic currents in the electric grid, as shown in the control algorithm, Fig. 4.

New reference

Current and dc link voltage control

The dynamics of the three-phase voltage source converter (VSC) connected to the grid with L filter, in Fig. 6, is represented by blocks: PI(s) controller, G(s) represents the PWM dynamic of the VSC, Gp(s) is the plant of the VSC with L filter [13].

The transfer function Gp(s) is given by (24):Gp(s)=1Ls+Rwhere: L is filter of the VSC; R is filter resistance, Gp2(s) is G(s) in cascade with Gp(s).

The controllers design of GSC and RSC is done through frequency response method, it considers the

RSC current control

The current dynamic of the voltage source converter connected to the DFIG, in Fig. 10, is represented by: PI controller, G(s) represents the PWM dynamic of the VSC and Gm(s) is DFIG plant.

The transfer function Gm(s) matches (35) [13]Gm(s)=1σLrs+Rr,where σ is 1 - (Lm2/LsLr).

As the adopted controller for RSC control is PI controller, it was used the same design criteria for DC link voltage controller [14]. The values of kp = 6.9 and Ti = 0.0028 of RSC current controller were obtained for

Simulation results

The system shown in Fig. 1 was modeled and simulated in the Matlab/Simulink® associated with SimPowerSystem toolbox to analyze the control strategy proposed by DFIG/APF system. The circuit shown consists of DFIG, converters, a balanced three-phase source, a three-phase full bridge rectifier with LiL feeding a R load and the DFIG. The parameters used in the simulation are shown in Appendix.

The stator terminals are connected to electric grid and rotor terminals are connected to grid through

Experimental results

The experimental setup has been designed and assembled and consists of: DFIG coupled to a squirrel cage induction controlled by converter, working as the wind turbine in Fig. 20.

Routines in C language were developed for the measured variables (speed, electric currents and voltages), protection, drive relays and generation of PWM pulses. A software was developed in Labview to get the data and supervises the system.

In the control of DFIG RSC, the rotor speed is set to 1700 r/min. Both the RSC and

Stability analysis of the system control

Unlike other controls in the GSC shown in the literature, it was used the precise model of the DC link voltage dynamics described in Ref. [13]. This model allows evaluating the stability of the system control for each DFIG/APF operating point.

The frequency response of the converter control depends on the system operating point for both rectifier and inverter mode. For each operating point, the DC link voltage dynamics transfer function is modified in accordance with the new active power, Pexto.

Application of the wind turbine converter in range of MVA up to 15 kHz

The use of Silicon Carbide (SiC) in the development of semiconductors is a promising technology. Commercial SiC power switches are already available in the market and allow to process large power at frequencies higher.

Simulations results implemented on SemiSEL software indicate that the use of SiC devices in wind power converters are technically feasible in applications, which present the range of MVA for the switching frequency up to 15 kHz. In order to demonstrate a comparative between losses

Conclusion

This research investigated a wind-power system with DFIG in the traditional mode of power generation (case 1), and the system working with power control and current harmonic filter grid (case 2).

Both in case 1 as in case 2, the power generation through DFIG remains the same. Thus, the technique of stator flux oriented control to power control of the generator is satisfactory.

Comparing the harmonic spectrum of the grid current before the harmonic compensation with the compensated grid current

Acknowledgements

This work was supported by FAPESP grant number 2015/03248-9.

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