A robust adaptive control of interleaved boost converter with power factor correction in wind energy systems

Power converters are generally utilized to convert the power from the wind sources to match the load demand and grid requirement to improve the dynamic and steady-state characteristics of wind generation systems and to integrate the energy storage system to solve the challenge of the discontinuous character of the renewable energy. In the low-voltage wind energy systems, interleaved boost converters (IBC) are often used to operate high currents in the system. IBCs are extremely sensitive to the constantly changing loading conditions. These situations require a robust control operation which can ensure a sufficient performance of the IBC over a large-scale changing load. Neural networks (NN) have emerged over the years and have found applications in many engineering fields, including control. In this paper, the adaptive control of interleaved boost converter with power factor correction (PFC) is investigated for gridconnected synchronous generator of wind energy system. For this purpose, a model reference adaptive control (MRAC) based on NN is proposed. Analysis results show that the proposed control strategy for the IBCs achieves near unity power factor (PF) and low total harmonic distortion (THD) in a wide operating range.


Introduction
Wind energy has great potential to reduce energy dependence on traditional resources such as coal, oil and gas and to do it without as much damage to the environment. Wind energy systems have been the fastest growing source of electricity generation in the world since the 1990s. Today, wind energy systems that generate electricity are new and innovative. Developments in the field of aerodynamics, mechanical/electrical engineering, control technology, and electronics provide the technical basis for wind turbines commonly used today. In wind energy conversion systems, power converters are widely used. In fixed-speed wind turbines, the converters are used to reduce inrush current (difference between the peak current value and the steady-state current value) and torque oscillations during the system start-up, while in variable-speed wind turbines they are employed to control the torque/speed of the generator and the reactive/active power to the network [1,2]. Also, boost converters are often used for low and medium power wind systems of a few kilowatts to hundreds of kilowatts. As the power capacity of the wind turbines increases, regulating the voltage and the frequency in the power grid become more substantial. For this reason, it has become essential to use new power control systems as an intelligent interface between the wind turbine and the grid. The problem of harmonic current pollution in electrical distribution systems has raised a significant interest over the recent years. Special attention has been dedicated to the improvement of line current harmonics and their negative effects due to the distortion of sinusoidal waveform characteristics [3]. The attention devoted to the quality of the currents absorbed from the utility line by electronic equipment is increasing due to several reasons. In fact, when a high THD of the line current causes electromagnetic interferences problems, a low PF reduces the power available from the utility grid [4]. Research on harmonic reduction and PFC has been intensified in the early nineties. Today's harmonic reduction and PFC techniques to improve distortion are still under development. With the introduction of compulsory technical standard such as IEC 1000-3-2, more researchers from both industries and universities are focusing in the area of harmonic reduction and PFC, resulting in numerous circuit topologies and control strategies. Hence, the PFC converters are an important research area in power electronics [3]. In AC power systems, harmonics occur when the regular electrical current waveform is distorted by nonlinear loads. PFC converters have been widely used in ac-dc power conversions to achieve high input PF and low THD. There are different basic topologies used in PFC converters (boost, buck, fly back, bridgeless etc.) [4,5]. Also, there is an IBC which has some advantages. Extensive research work has been carried out on the IBCs and is available in the literature [6][7][8][9][10][11][12]. The IBC is considered as a good solution due to its merits, such as high efficiency, ripple reduction and small filter components. Hence, interleaving techniques provide high power capability, modularity and reliability [7][8][9]. The IBCs were used in various energy systems [10][11][12]. In recent years, many works have been focused on high efficiency or high voltage gain of IBCs and DC/DC boost converters [13][14]. In Ref [15] the authors used the maximum power point tracking (MPPT) to control an IBC in fuel cell electric vehicles. In Ref [16] the authors proposed a power switch failures tolerance and remedial strategy of a 4-leg floating IBC for photovoltaic/fuel cell applications. In Ref [17] the authors provided a comprehensive review of past and present converter topologies applicable to different generatorconverter combinations in wind turbine systems. PFC control strategy is typically done with two loops: an internal and fast current loop to improve THD and PF, and an external and slow voltage loop to stabilize the output voltage. The PI controller is generally used for the output voltage regulation. The current control is the most common control strategy which has different types since the primary objective of PFC is to force the input current to trace the shape of line voltage [3,4]. As compared to well-known fixed gain controllers, the MRAC has ability to adapt itself to variations of process dynamics [18,19]. MRAC has two parts, a reference model and the plant model that tracks reference model. The aim of the control strategy is to minimize the error between outputs of the plant and reference model [19]. During last decades lots of adaptation mechanisms (based on NN, fuzzy systems, PID etc.) have been proposed for MRAC [20][21][22][23][24]. The main contribution of this paper is the proposal of a new adaptive approach for current control of the IBC with PFC in wind power generation system. The output voltage of the proposed system is also regulated by PI controller.
The paper is organized as follows. In Section 2, the control system configuration of the IBC for wind energy systems are described and modeled. Section 3 is devoted to the modeling and control design of the IBC. The controller performances are illustrated through numerical simulations in Section 4. Section 5 provides the conclusion of the paper.

System configuration
Wind turbine systems capture the power from the wind by means of aerodynamically construction designed blades and convert mechanical energy to electrical energy. It is significant to be able to limit and control the converted mechanical power at higher wind speed. In wind energy conversion systems, a gear-box and a standard fixed speed generator are used to convert the low-speed, high-torque power to electrical power. It is necessary to step up the voltage using a DC/DC boost converter. A boost converter can be inserted between the synchronous generator and grid. Typically, A variable-speed wind energy power system with DC/DC boost converter is shown in Figure 1 [1]. According to the changing of the input direction of wind energy sources, the output voltage can be changed. To transfer the electrical power from wind energy sources to conventional 380Vrms AC electrical distribution systems, the converters are controlled by interleaved switching signals, which have the same phase shift and same switching frequency. Interleaving techniques have many features such as higher efficiency and reduced input and output ripple, are also realized in the boost topology. The interleaving technique consists in the parallel interconnection of a determined number of identical converter cells, whose control signals are strategically phase shifted in each switching time [1,27]. In small wind turbines which are generally defined as producing no more than 100 kW of electricity, interleaved boost converters are mostly used to handle high currents in the system. The overall system is comprised of an uncontrolled diode rectifier followed by two boost cells and control blocks. Fig. 2 shows the system. According to the duty cycle status, in continuous conduction mode of the currents IL1 and IL2, the converter has four possible stages of operation. A diagram indicating the conduction path of the IBC during stages is shown in Fig. 2.  (1) and can be represented as given in Fig. 3 for an input line voltage of 220 Vrms, 50 Hz line frequency (fs) and an output voltage (Vo) 400 V. Ir is defined as below, also.
Two modes of operation can be characterized depending on relative amplitude of Vr(t) and Vo. When Vr(t)<Vo/2 (d(t)>0.5), just the stages (a), (b) and (c) occur. When Vr(t)>Vo/2 (d(t)<0.5), just the stages (b), (c) and (d) occur. The switching instants are sequentially phase shifted by equal fractions of a switching period. The second pulse width modulation (PWM) signal (d2) is only phase shifted ( Fig.4 and Fig. 5). This arrangement reduces the input current ripple amplitude and raises the effective ripple frequency of the overall converter without increasing switching losses or voltage and current stresses of any component [7,9].

Modeling and control design of the interleaved boost converter
The proposed current controller is a MRAC based on NN. It has two main parts. There are a reference model part and NN part. Reference model is linear stable system to be imitated by NN and nonlinear converter system. NN part is radial bases function network (RBFN). RBFN supplies Nf signal to compensate error between reference model and nonlinear converter system. On the other hand Nf signal compensates nonlinearity of the plant. Sigmamodification-type updating law and RBFN used to implement nonlinear part of controller system [24,25]. So, it is a strong adaptive controller for nonlinear systems.
Nf is output and w is the weight vector of the RBFN. Control strategy would compensate nonlinearities of the plant in the course of time thanks to the fast adaptation ability of RBFN. Studies [25,26] cover detailed explanation and stability analysis for this kind of controllers. Reference model is a very simple system. It has first order transfer function.
( ) In this study, the converter system is the IBC with PFC. c(t), r(t) and u(t) stand for Ir, Iref and d(t), respectively.

Performance analysis
In this section, simulations are carried out to verify the theoretical analysis given in the previous section. Analysis studies are performed and presented by the Matlab®/Simulink. The topology of the proposed converter and the controllers are set up as shown in Fig. 6. To simplify the analysis, all the components are assumed ideal.
The performance of the proposed rectifier in Fig. 7 is evaluated on a 50 kHz, 1.5 kW prototype circuit that is designed to operate from a universal AC-line input (85-264 Vrms) with a 400 V output. A closed-loop control circuit is used to drive the two active switches and regulate the output voltage. Each duty cycle PWM signal is controlled through 1800 phase shift, and two-channel PWMs can be stably generated and operated. The performance of the proposed method is tested by observing the input current. Fig. 8 shows the waveforms of the input voltage, input current, and output voltage. The output voltage is 400 V, as expected. The input current is sinusoidal and in phase with the input voltage. The measured PF is 0.99, and THD is 3.2%. Model parameters and results at nominal power are showed in Table 1.
The waveforms of voltage and current of the shared active switches are shown in Fig. 8, 9. The result shows that input current of the converter is converged to the reference value. Fig. 10 shows the harmonic spectrum of the input line current. The harmonic current limits are met for all specified operating conditions.

Conclusion
The power of the wind generator describes the features of the converters used in the extraction of wind electric power. The designs of converters are based on the requirements of the generation source and the electric load. In wind conversion systems, an optimal control may improve the efficiency of the generation system and extend the life of its elements. In this study, the MRAC based on NN current control for wind power systems has been proposed to improve the robustness, stability and dynamic characteristic of the IBC with PFC. For the specific purpose, a comprehensive analysis has been conducted. Basically, the controller inspects the state of the converter and generates the control variable that provides adaptive control. Analysis results show that the proposed method can achieve sinusoidal line current, unity PF with low THD and voltage stability under both the steady and transient states. Although the controller that proposed in our study is applied to a specific converter, this control technique can definitely be generalized to other power converters in wind conversion systems.