Converter Interfaced Distributed Generation - Grid Interconnection Issues

Doctoral thesis, 2007

Distributed generation (DG) with a converter interface to the grid is found in many of the green power resources applications. In this thesis, the control of a voltage source converter (VSC), as the DG front end, is in focus regarding the power quality problems that could appear at the connection point. The aims have been set to maintain a stable operation of the DG, in case of network disturbances, and to react in a corrective way during different grid operating conditions (e.g. in case of voltage dips). For this purpose, vector current controllers have been implemented with two different line filters; namely an inductance filter (L-filter) and an inductance-capacitance-inductance filter (LCL-filter). The controllers have incorporated: one sample time delay compensation, limitation of the reference voltage to avoid saturation, an integrator anti-windup, a DC-link voltage controller, a PCC voltage regulator, and an islanding detection algorithm. The ride-through capability of the DG has been examined against a variety of possible voltage dips that could appear at the connection point. Moreover, the capability of the DG to compensate for the voltage at the connection point has been studied. Finally, the intentional islanding has been considered, where the DG is allowed to energize a part of the grid in case of the utility outage forming what is called an island. The results found are that the effect of unbalanced voltage dips on the DC-link voltage ripple is minimized if the oscillating powers, produced during that period, are supplied by the grid side instead of the DC-side. Moreover, design equations have been derived in order to calculate the maximum currents that would flow through the VSC valves during voltage dips. These equations are to be used in designing VSC’s with voltage dips ride-through capability. In addition, a neural-network based PLL, which extracts the phase angle of the fundamental component of the grid voltage, has been introduced in order to provide better performance in case of a DG with voltage compensation capability. Finally, combining the voltage regulator with the estimated frequency as a measure for islanding condition has, in this work, been found as an appropriate practice, to detect islanding, especially in the case of weak grids.