DC-Link Voltage Sensorless Control Technique for Single-phase Two-stage Photovoltaic Grid-connected System N.E. Zakzouk #1 , A.K. Abdelsalam #2 , A.A. Helal #3 , and B.W. Williams *4 # Electrical and Control Engineering Department Arab Academy for Science, Technology and Maritime Transport Alexandria, Egypt 1 nahlaezzeldin@hotmail.com 2 kadry_2012@yahoo.com 3 ahmedanas@aast.edu * Electronics and Electrical Engineering Department Strathclyde University Glasgow, United Kingdom 4 barry.williams@strath.ac.uk Abstract— Control techniques, applied to single-phase two- stage grid-connected photovoltaic (PV) systems, mainly achieve functions of maximum power point tracking (MPPT), voltage adjustment at inverter DC-link, and grid current control. Conventional control techniques require measurements of PV voltage and current, DC-link voltage, and grid voltage and current. Commonly, sensorless techniques are proposed to simplify system implementation and decrease its entire size and cost. However, most focus on eliminating PV voltage and/or current sensors. In this paper, a sensorless technique is proposed which keeps PV sensors, but eliminates the expensive high DC- link voltage sensor by mitigating the inverter DC-link voltage control loop. Alternatively, voltage regulation at inverter DC- link is achieved through power balance guarantee at this link. Hence, control complexity is minimized and system stability is enhanced. Moreover, the entire system implementation is simplified and its dynamic response is improved during sudden irradiance changes. Simulation work is carried out to verify the effectiveness of the proposed technique when compared to the conventional one regarding their transient and steady-state performance under varying irradiance conditions. Keywords: photovoltaic source, utility interface, voltage source inverter, energy balance at inverter DC-link, DC voltage control loop, sensorless control technique. I. INTRODUCTION Among current renewable energy resources, photovoltaic (PV) energy has gained much interest as a noise and pollution free source. Furthermore, it has the ability to be expanded and utilized in arid areas [1]. Nowadays, common distributed energy resources (DERs), particularly PV sources, are increasingly being connected to utility for best utilization of their produced electric power [2]. For PV-grid interface, a number of methods are used, among which the string inverter technology is widely used at present [3]. In this method, a number of PV modules are connected in a series arrangement; called a string, and each string has its own inverter. Thus, the MPP of each PV string is separately optimized and the PV system can be expanded easily by adding additional strings with their relative inverters [4]. For low-power (< 10 kW) applications, DERs are usually connected to the AC grid through a single-phase voltage source inverter (VSI) at low voltage (110-220V) [5]. For successful PV string-grid interface, a number of requirements arise [6, 7]; maximum power point tracking (MPPT), voltage regulation at inverter DC-link, and grid current control. To achieve the latter, two topologies exist [3]; single-stage and two-stage topologies. The single-stage topology involves a single inverter stage to achieve all the previous tasks in order to reduce component count and increase conversion efficiency [8]. However, this inverter must be carefully designed to handle the double line frequency voltage ripples that appear at its DC-link due to single-phase connection [9]. Furthermore, large electrolytic capacitors must be connected to the PV string to limit these ripples propagation in the PV power, thus reducing inverter life-time [10]. Alternatively, two-stage topology is investigated in which a power decoupling DC-DC stage is added before the inverter stage [11]. This stage decouples the energy change between the PV string and the inverter DC-link which limits the voltage ripple impact on the PV source. Moreover, transformation of PV voltage level can be achieved using this additional stage thus expanding its operating range [3]. Conventionally, maximum power point tracking (MPPT) is achieved by the DC/DC converter stage while the second inverter stage inhibits two control loops to deliver power to the grid [12-14]. The first is an outer voltage control loop at the inverter DC-link and the second is an inner current control loop which forces the inverter to produce a sinusoidal grid current at low THD and almost unity power factor. Thus, the ENERGYCON 2014  May 13-16, 2014  Dubrovnik, Croatia 978-1-4799-2449-3/14/$31.00 ©2014 IEEE 58