0885-8969 (c) 2020 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TEC.2020.2990866, IEEE Transactions on Energy Conversion IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 00, NO. 00, APRIL 2020 1 Enhanced SOGI Controller for Weak Grid Integrated Solar PV System Firdous Ul Nazir, Nishant Kumar Member, IEEE, Bikash C. Pal, Fellow, IEEE Bhim Singh, Fellow, IEEE, and Bijaya K. Panigrahi Senior Member, IEEE Abstract—This paper presents a two-stage three-phase solar photovoltaic (PV) system, which is controlled through a novel enhanced second order generalized integrator (ESOGI) based control technique. The proposed ESOGI is used for fundamental component extraction from nonlinear load current and distorted grid voltages. This integrator effectively and simultaneously manages to address the DC offset, inter-harmonic and integrator delay problems of the traditional SOGI. In addition, this control technique provides power factor correction, harmonic elimina- tion, and load balancing functionalities. The ESOGI controller is used to generate reference grid currents for controlling the voltage source converter (VSC), interfacing the PV panel with the grid. Extensive simulation and experimental results on a developed prototype in the laboratory, depict that the total harmonic distortion (THD) of the grid injected currents and voltages are found well under IEEE-519 standard. Index Terms—Solar photovoltaic (PV) system, maximum power point tracking (MPPT), second order generalized inte- grator (SOGI), power quality. I. I NTRODUCTION T HE decarbonisation and decentralisation of power gener- ation have spurred the growth of renewables especially solar photovoltaics (PV). These PV systems are either used in a standalone mode with appropriate battery storage facilities or as a grid integrated system [2]-[4]. However, harnessing solar energy comes with its own challenges, most important of them are discussed in [1]. The PV technology is an effective way to harness the solar energy for electricity production. The requirement of storage batteries puts the stand alone PV systems at an economical disadvantage against the grid integrated PV systems. Thus, the grid integrated systems are more prevalent especially where the grid is available [5]-[7]. The rapid proliferation of non-linear loads due to power electronic switching used in magnetic ballast, and light emit- ting diodes is adversely affecting the power quality of the utility grid. These loads inject harmonic currents into the grid. This situation is particularly challenging for the PV integration and the requirement of injecting balanced positive sequence currents into the grid needs the extraction of fundamental F. Ul Nazir and B. C. Pal are with the Electrical and Electronic Engi- neering Department, Imperial College, London SW7 2AZ, U.K. (e-mail: f.ul- nazir16@imperial.ac.uk; b.pal@imperial.ac.uk). N. Kumar is with the Electrical and Computer Engineering Depart- ment, National University of Singapore, 119077, Singapore (email: nis- hant.kumat1729@gmail.com). B. Singh and B. K. Panigrahi are with the Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi, 110016, India (email: bhimsing156@gmail.com; bkpanigrahi@ee.iitd.ac.in). This work is supported by the JUICE project under Contract EP/P003605/1 and Indo-UK project UKCERI. positive sequence component from the polluted load current. Several different types of control schemes have been proposed in the literature to meet this requirement. The noteworthy of these PV integration control schemes are synchronous frame theory [8], enhanced phase locked loop [9], instantaneous reactive power theory [10], extended observer based sliding mode control technique [11], fuzzy-logic based control [12], fourth order generalized filter [13], adaptive notch filter [14], and an adaptive control scheme [15]. The most commonly employed control technique for the control of the interfacing inverter for the grid tied PV system is the second order generalized integrator (SOGI); because of its robustness and ease of implementation [16]. The SOGI acts as a building block for the orthogonal signal generator (OSG) and hence can be either used directly for frequency estimation as a frequency locked loop [17] or indirectly through the dq frame mapping [18]. The traditional SOGI controller, however, suffers from three major disadvantages - a) the estimation is erroneous if the input signal has a DC offset, b) ripple content or inter-harmonics in the input signal also lead to wrong estimation, and c) the usage of integrators in the control structure introduces the integrator delay in the estimation pro- cess. Various solutions have been proposed in the literature to deal with these problems. Ghartemani et al. [19] addressed the DC component problem of the SOGI algorithm by introducing a new loop inside the SOGI structure. This additional loop adds another integrator to the SOGI controller and helps in the estimation of the DC offset in the input; the estimated DC value is directed towards the output for its complete cancellation in the final signal. Similarly, Golestan et al. [20] addressed this DC component challenge by proposing a novel phase locked loop (PLL) structure known as DC Immune- PLL (DCI-PLL). They have been able to successfully achieve the DC component rejection by comparing the current and the delayed value of the signal under consideration. Further, Matas et al. [21] have proposed a cascaded double SOGI approach for the DC-offset voltage distortion problem of the conven- tional SOGI. Infact this cascaded double SOGI structure is sensitive to all kinds of subharmonics present in the signal and could effectively remove all of these simultaneously. The ripples or the inter-harmonics are effectively filtered by the damped-SOGI algorithm proposed in [22]. The damped- SOGI implementation adds a feedback loop across the forward path integrator; this feedback loop introduces a damping term on the basic SOGI control, hence filtering out the ripples in the input signal. Finally, the integrator delay problem is overcome by discretizing the SOGI controller [23]. Since these Authorized licensed use limited to: Imperial College London. 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