On The Analytical Tuning Of Controllers for Grid-Coupled Voltage Source Converter: Frequency Analysis

The grid conditions determine what the output voltage of a grid connected power converters should be. Control of the output voltage of these power converters is by this condition not possible. The power converters are however, not directly connected to the grid. The conventional method to interface these converters to the grid is through filters [1,2]. The duty of the filter is to reduce the current harmonics around the switching frequency of the converter. The current harmonics is capable of polluting the grid with the effect of disturbing any connected equipment [2,3]. In some practical applications, the converter is not just a single three-phase converter but two back-to-back converters usually interfacing two AC systems. The two converters are linked to each other through a DC-link. The converter adjacent to the grid is in this work referred to as grid-side converter (GSC). The scope of this paper is limited to the analysis of control of the filter current and the influence of the filter on the grid and the grid-side converter with the DC-link. The filter can be a simple L, LC or LCL filter. The choice of any scheme may depend on cost, size, or dynamic performance. The filter current injected into the grid becomes the control variable since the grid voltage can’t be controlled. The filter input current is usually sensed and given back as feedback to close the control loop. Hence, the transfer function which decides the closed loop performance of the filter is the transfer function between output current and input voltage of the filter for zero grid voltage [1]. Usually the current control is implemented in a dq-rotating frame in order to obtain a control with rapid dynamic response. The dq components of converter output currents are controlled using state-of-art synchronous frame PI control [4]. Generally the current control is designed on per phase basis and the decoupling terms are added to allow such design. This decreases the order of system and hence simplifies controller design [5].