Control of a PMSG Based Wind Turbine system with Parallel Connected DC Chopper at Grid Side Converter

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14 International Journal for Modern Trends in Science and Technology

Control of a PMSG Based Wind Turbine system with Parallel Connected DC Chopper at Grid Side Converter

Y Srilekha¹ | P Naga Sowjanya²

1PG Scholar, Department of Electrical Engineering, Sir C R Reddy College of Engineering, Eluru, Andhra Pradesh, India.

2Assistant Professor, Department of Electrical Engineering, Sir C R Reddy College of Engineering, Eluru, Andhra Pradesh, India.

To Cite this Article

Y Srilekha and P Naga Sowjanya, “Control of a PMSG Based Wind Turbine system with Parallel Connected DC Chopper at Grid Side Converter”, International Journal for Modern Trends in Science and Technology, Vol. 04, Issue 08, September 2018, pp.-14-22.

Article Info

Received on 09-July-2018, Revised on 16-Aug-2018, Accepted on 25-Sept-2018.

In this paper, a grid-connected converter for a direct-driven permanent magnet synchronous generator (D-PMSG) and an active power filter (APF), is proposed to enhance the low voltage ride through (LVRT) capability and improve power quality. During normal operation, the main grid-side converter maintains the DC-link voltage constant, whereas the auxiliary grid-side converter functions as an APF with harmonic suppression and reactive power compensation to improve the power quality. During grid faults, a hierarchical coordinated control scheme for the generator-side converter, main grid-side converter and auxiliary grid-side converter, depending on the grid voltage sags, is presented to enhance the LVRT capability of the direct-driven PMSG WT. The feasibility and the effectiveness of the proposed system’s topology and hierarchical coordinated control strategy were verified using MATLAB/Simulink simulations.

KEYWORDS: Fuzzy logic, Hybrid inverter, Distributed generation

I. INTRODUCTION

Wind turbine technology has developed rapidly over the past decade into one of the most mature renewable power generation technologies.

Compared to other wind turbine systems used for commercial power generation, the accelerated evolution of the direct-driven wind turbine (WT) with a permanent magnet synchronous generator (D-PMSG) can be attributed to its simple structure, low cost of maintenance, high conversion efficiency and high reliability [1]. Moreover, its decoupling control performance is much less sensitive to the

parameter variations of the generator. Therefore, a high-performance variable-speed generation including high efficiency and high controllability is expected by using a PMSG for a wind generation system.

At the moment, substantial documentation exists on modeling and control issues for the PMSG wind turbine [2–5]. To improve the LVRT of a direct-driven WT system, several solutions using hardware devices have been proposed in the literature [6–9]; these devices include DC choppers, energy storage systems (ESSs), static VAR compensators (SVCs), dynamic voltage restorers ABSTRACT

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International Journal for Modern Trends in Science and Technology

ISSN: 2455-3778 :: Volume: 04, Issue No: 09, September 2018

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15 International Journal for Modern Trends in Science and Technology Grid Side Converter

(DVRs) and static synchronous compensators (STATCOMs). However, the use of these devices can significantly increase system costs. Studies investigating the control strategies for direct-driven WT systems have improved the LVRT capability of these systems [10–13]. However, various issues, such as the limited reactive power support for the power grid resulting from a limited current capacity of the grid-side converter (GSC), still remain. Moreover, it is impossible to suppress the distortion of the current injected into the grid.

In this paper, a new D-PMSG configuration is proposed for small- and medium-capacity wind turbine system in microgrid connected to the distribution network, by adding an auxiliary converter in parallel with the grid-side converter (GSC); this novel system resembles the structure and operating principles of a grid-connected converter for a direct-driven PMSG WT system and an APF. During normal operation of the power grid, the auxiliary GSC function as an APF, suppressing current harmonics and compensating reactive power to improve the grid power quality. When the grid voltage dips during a grid fault, a hierarchical coordinated control strategy can be implemented through the communications among the generator-side converter, the main GSC and the auxiliary GSC to enhance the LVRT capability;

supplementary measures, such as DC unloading circuit, also can be implemented to meet the LVRT requirements imposed by grid codes. In the proposed PMSG-based WT configuration with an auxiliary GSC, various elements, such as the reactive and harmonic current compensation, LVRT and other functions, can subsequently be functionally integrated within the same system.

Therefore, this scheme cannot only enhance the LVRT capability of the wind turbine system but also improve the power quality of the power grid with relatively low cost.

II. PMSGWTCONFIGURATION WITH A DCCHOPPER

The typical structure of a direct-driven PMSG WT system is shown in Figure 1. The stator winding of the synchronous generator is connected to the grid through a fully rated back-to-back converter.

Typically, the generator-side converter controls the active and reactive power output of the PMSG. In contrast, the GSC maintains the DC-link voltage and controls the reactive power exchange between the generator and the grid, i.e., the GSC transfers the active power extracted from the wind turbine to the grid at an adjustable power factor. The DC chopper circuit, which consists of power electronic

devices and unloading resistors connected in series, is used to maintain a stable DC-link voltage during power grid faults.

Figure 1. Direct-driven PMSG-based WT system.

Proposed circuit is shown in Figure 2. In the proposed configuration, an auxiliary converter with a half capacity of the GSC, which shares the DC-link of the back-to-back converters, is connected in parallel with the GSC in a conventional PMSG WT system.

Figure 2. A direct-driven PMSG-based WT system with an auxiliary GSC.

Under normal conditions, the generator-side converter regulates the generator rotation speed by controlling the electromagnetic torque to achieve maximum power point tracking. Meanwhile, the main GSC maintains the DC-link voltage to be constant, whereas the auxiliary GSC functions as an APF to implement harmonic current compensation. During the grid voltage dips, by switching the control strategy, the generator-side converter, the GSC and the auxiliary GSC are coordinately controlled to output required active and reactive powers that follow pre-established criteria; this process of hierarchical coordinated control maintains a stable DC-link voltage and provides reactive power support to the power grid to satisfy the LVRT requirements of the grid code.

The main GSC with LC output filter only transfers fundamental power and does not provide harmonics filtering within the control bandwidth,

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whereas the auxiliary GSC with L output filter compensates harmonics. Hence, according to the parameters of the output filter, the cutoff frequency of the current controllers for the main GSC is designed to be approximate 200 Hz, whereas, to extract up to the 25th harmonics, the cutoff frequency of the current controller for the auxiliary GSC is designed to be 1.28 kHz, which is one tenth of the sampling frequency. Theoretically, the larger the capacity of the auxiliary GSC, the better the performances of the harmonics filtering and LVRT capability. However, the large capacity will increase the costs of the system. Hence, considering the tradeoff between the costs and performances, the auxiliary GSC is designed to be a half capacity of the main GSC.

III. CONVERTER CONTROL UNDER NORMAL

CONDITIONS

A. Controller Design of the Generator-Side Converter

In the WECS, usually Jh _ Jg _ max(ts, t_ s, Cdc), so the dynamic responses of the WECS can be classified into three time scales. Based on the singular perturbation theory, the slow and fast dynamics will not affect each other as they have different response time scale [14]. Therefore, the fast dynamic is supposed to be fast enough to converge to its steady state instantaneously when discussing the slow dynamic. Similarly, the slow dynamic is neglected in the study of fast dynamic.

As a result, the control of the WECS can be designed based on the following three submodels:

Fig. 2. Torque characteristics of the WT.

In order to behave as the first-order dynamic, the PMSG is controlled with vector control techniques [3]. By aligning the d-axis of the rotating reference frame on the rotor flux, the control of the generator torque and stator voltage can be decoupled. The control diagram of the generator-side converter is depicted in Fig. 3. The PI regulator of the dc-link voltage controller has upper and lower limits in order to satisfy (11). Equation (8) is the input of the vector controller and i∗ ds can be set to zero to avoid demagnetization of the permanent magnetic.

The rotor flux angle θr for the frame transformation and the generator speed _ωg in (8) can be estimated based on the back electromotive force of the PMSG, which results in the sensorless implementation of the proposed scheme [25].

Fig. 3. Control diagram of the generator-side converter.

B. Controller Design of the Grid-Side Converter As the generator and grid are decoupled by the back-to-back converter, the grid disturbances would not affect the operation of the generator-side converter. The grid-side converter should take the responsibility to keep theWECS running properly even with grid faults. In order to extract the symmetrical sinusoid currents from the WECS and improve the power quality during unsymmetrical

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grid faults, the positive-sequence active and reactive power are regulated in the proposed strategy. The control diagram is illustrated in Fig.

4. The control of the grid-side converter is also based on the vector control techniques, in which the rotating reference frame is aligned to the positive-sequence grid voltage vector. The positive- and negative-sequence components of the grid voltage can be separated with the second-order generalized integrator-based phase locked loop [26]. The angle of the positive-sequence grid voltage vector θ+ is then applied to the frame transformation and control. With such a strategy, the positive-sequence active and reactive powers of the grid-side converter, Pout,pos,Qout,pos , can be controlled independently by the d-, q-axis currents id,g , iq,g. In Fig. 4, Pout,pos,Qout,pos can be calculated with the following equations:

where · represents the dot product and × denotes the cross product; the symbols with superscript “+”

represent the positivesequence variables. Since only positive-sequence powers are controlled in such a strategy, the currents injected into the grid only contain positivesequence components so that the current distortions are mitigated [27].

Fig. 4. Control diagram of the grid-side converter.

Fig. 5. Control diagram of the pitch angle.

C. Design of the Pitch Controller

The pitch control is activated once the generator speed increases above its rated value and the power extracted from the wind energy subsequently reduces. Due to the nonlinear aerodynamic characteristics of the WT, pitch angle

control is quite difficult. Conventional linear controller cannot provide satisfactory performance in a wide wind speed range [28]. Controller with gain scheduling may be effective but the gains are hard to tune. In [28] and [29], the authors proposed a robust pitch controller based on inverse-system theory. The controller is simple to implement and is robust to the system parameter deviations. Fig. 5 shows its control diagram. It consists of an inverse-system-based control block for the nominal system control and a robust compensator to mitigate the control error caused by the parameter deviations [28]. The controller is employed in the paper and the details can be found in [28].

IV. SIMULATION RESULTS

Fig : 1Simulation circuit with no chopper For SLG Fault (During 0.3 to 0.4)

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Fig 2. (a) Generator Voltage and Current, (b) Vdc and wind Speed, (c) Grid Average Voltage and Current and Active and Reactive

Power, (d) Wind Speed, Torque and current.

For LLG Fault (During 0.3 to 0.4)

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For 3-Phase to Ground Fault (During 0.3 to 0.4)

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Fig : 4 Simulation circuit with no chopper

For SLG Fault (During 0.3 to 0.4)

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For LLG Fault

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For 3 phase to ground Fault

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With Fuzzy Logic

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V. CONCLUSION

In this paper, a novel topology is developed for a direct-driven PMSG WT system, in which an auxiliary grid-side converter is paralleled with the grid-side converter to enhance low voltage ride-through capability and improve power quality.

Under normal conditions, the main GSC ensures that the WT system is smoothly connected to the grid, whereas the auxiliary GSC operates in APF mode to compensate for the harmonic current and to improve the power quality of the grid. During grid faults, a hierarchical LVRT strategy for generator-side converter, main GSC and auxiliary GSC is presented, which provides reactive support to the power grid and maintains the energy balance

of the system to achieve an LVRT requirement that complies with grid codes. The LVRT strategy exploits the converters capacity and coordinates the converters control depending on the depth of the voltage dip, thus minimizing the heat dissipation burden on the DC chopper. Compared with the conventional scheme, the proposed system combines wind power integration control, reactive and harmonic current compensation and LVRT operations to improve stability and power quality while remaining cost-effective.

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