Compensation of distortions in VSC-based DC–AC power systems using a modified vector control method

Abstract

Offshore wind farms are always connected to onshore alternating current (AC) power systems using direct current (DC) submarine cables through a DC–AC voltage source converter station. In most cases, nonlinear loads are located on the AC side of the station. The distortion caused by such loads tends to flow into the AC system due to its low impedance, negatively impacting power quality and raising the power losses. This paper presents a novel vector control method (VCM) for the DC–AC station to simultaneously deliver the power demanded by the AC side and act as a shunt active power filter to suppress distortions. In this scope, the conventional VCM is further modified by developing several control loops with proper control and power parameters to obtain a sufficiently wide bandwidth. Additionally, the performance of three different types of low pass filters (LPF) is investigated due to the significant role of LPF in the success of the proposed method. Simulation and experimental outcomes are provided to validate the effectiveness of the proposed method

Introduction

The application of renewable energy sources (RES) has been growing rapidly due to fossil fuels shortages and drawbacks. Wind turbine system (WTS), often located offshore, is one of the most critical RESes (Parker, Holliday, & Finney, 2019). Offshore WTSes are usually connected to the onshore power system via submarine DC cables since AC cables are affected by capacitive chargings (Bhuiyan & McDonald, 2018). Thus, a voltage source converter (VSC) must convert the transmitted DC power for feeding the local nonlinear loads and the AC power system onshore. Such combinations of AC and DC systems, including the DC transmission systems and power electronics interfaces, form a multi-terminal DC (MTDC) system (Du et al., 2017, Zhang et al., 2019). The control scheme of the VSC station plays a crucial role in the performance success of these systems. This control method generally comprises two layers: the outer and the inner layers (Gavriluta, Candela, Citro, Luna, & Rodriguez, 2015). Two control loops have been embedded in the outer layer to inject active and reactive powers using proportional–integral (PI) controllers. Also, this control layer can be responsible for adjusting the output voltage (Bin-Kai & Zhi-Xin, 2017). Various approaches, such as the master–slave, voltage margin, and voltage droop methods, are suggested in the literature to realize the outer layer (Dierckxsens et al., 2012, Huang et al., 2018, Rouzbehi et al., 2014). It is evident from the previous studies that the voltage droop method has a more suitable performance since it can overcome the associate weaknesses of the master–slave and voltage margin methods in terms of reliability and the DC voltage oscillations (Dierckxsens et al., 2012).

On the other hand, implementing the inner control layer can be fulfilled using various well-known techniques such as the direct control method (power-angle control), the power-synchronization control method, and the vector control method (VCM) (Asensio et al., 2019, Diaz et al., 2019, Lou et al., 2019, Naderi et al., 2019). Phase and amplitude regulation of the VSC’s AC side voltage is the main target of the direct control method. Although a VSC operated by direct control method can manage to deliver the determined active and reactive powers, this control technique has some significant drawbacks, such as lack of a current restriction capability and a limited frequency bandwidth (Fallah, Kojabadi, & Blaabjerg, 2020).

The power-synchronization control method uses power readings instead of a regular phase-locked loop (PLL) to fulfil synchronization. However, its dependency on the AC power system’s stiffness can be considered a drawback for this method (Rouzbehi et al., 2015, Satapathy et al., 2016, Yuan et al., 2018), and therefore, not preferred for the VSC stations. On the other hand, the VCM has been widely used in the realization of VSC stations. This method can control active and reactive powers independently (Han, Xiong, & Wan, 2012). The corresponding signals are transferred to the rotational reference frame (dq-frame) using the Park transformation and then are assessed to generate suitable control signals. This method also benefits from the current limiting capability and demonstrating a wide bandwidth operation. The power flow control between the DC and AC sides has been gaining more attention (Li et al., 2016, Yadav et al., 2019). Also, some studies have considered addressing the associated power quality concerns (Xu, Han, & Gole, 2017). Usually, nonlinear loads are located on the AC side of the VSC stations. The distortions caused by such loads tends to flow into the AC system due to its low impedance, negatively impacting power quality and raising the power losses. Traditionally, some bulky and costly passive filters are employed to eliminate those harmonics (Lacerda, Coury, & Monaro, 2017). From both the technical and commercial points of view, it is highly preferred to suppress the distortions by developing an efficient control method for the VSC stations. Consequently, the power quality and efficiency of the AC grid will be enhanced, reducing power losses considerably.

In general, the harmonic suppression schemes can be divided into two major categories: repetitive harmonic controllers (Escobar et al., 2013, Yang et al., 2014) and linear harmonic compensators (Blaabjerg et al., 2006, Castilla et al., 2009). Members of the former group can improve total harmonic distortion (THD) of the VSC station’s output current without contributing to the compensation of the distortions. Such methods require a complex design and implementation procedure, though. The techniques categorized in the latter one operate based on a frequency tuning scheme to reduce the closed-loop impedance of the VSC at prespecified odd harmonics. As a result, the voltage harmonics at the point of common coupling (PCC) can be suppressed considerably, which in turn leads to a reduction in current distortions caused by the local nonlinear loads. In Micallef, Apap, Spiteri-Staines, and Guerrero (2015), a control method was proposed based on the virtual admittances and impedances to share the distortions between stations and minimize voltage harmonics in the PCC. A feed-forward scheme is utilized in Li et al., 2012, Wang et al., 2010 to reduce the injected current harmonics into the grid. Although the feed-forward techniques achieve a good level of performance, the corresponding controllers can be pretty complicated. For example, in the cascaded proportional–resonant (PR) controllers with harmonic compensation capability, the resulting feed-forward transfer function becomes impractical due to its high order and complexity.

This paper initially presents the modelling and analysis of the converter and control units of a VSC station. This will demonstrate how both the bandwidth and the current limitation of VSC can vary by selecting different control parameters, revealing the potentials of developing harmonic current mitigation techniques. Then, this paper presents a novel control method by modifying the conventional VCM and adding two new control loops. These loops can significantly detect the harmonic distortions caused by local nonlinear loads . For this, the current readings for both the nonlinear load and grid are transferred to the dq domain using the Park transformation. Four low pass filters (LPF) are employed in the newly added control loops to extract the DC component of the transformed currents (Fallah et al., 2016, Hadjidemetriou et al., 2013, Modarresi et al., 2018). Then, the harmonic components are extracted by subtracting the DC terms from the total signals. Eventually, the VSC station, operated by the VCM in its inner layer, supplies currents corresponding to the extracted harmonics to suppress the current distortions on the grid side. Due to the significant role of the LPFs in the proposed method’s performance, three types of LPFs have employed and evaluated in the suggested structure. Consequently, the second-order generalized integrator-frequency locked loop (SOGI-FLL) is adopted in this paper to achieve a fast and accurate response.

The main contribution of this paper is actually the modification of the VCM for the control of DC–ACstations. The presented technique can efficiently fulfil power exchange between the DC and AC sides while addressing the power quality concerns caused by the nonlinear loads. The effectiveness of the proposed control method is verified by performing several simulation studies in MATLAB/Simulink. Additionally, an experimental setup is developed using the TMS320F2812 digital signal processor (DSP) for validating the claimed contributions.

The rest of this paper is organized as follows. In Section 2, the model of VSC stations is elaborated. The proposed control approach is presented in Section 3. In Sections 4 Performance evaluation: simulation results, 5 Performance evaluation: Experimental results, the simulation and experimental results are presented and analysed, respectively. Eventually, the article is concluded in Section 6.