Grid-connected converters
Power quality issues can be divided in two categories, firstly the reliability of the utility grid and secondly the quality of the mains voltage waveform. The quality of the voltage mainly involves three phenomena: voltage harmonics, voltage dips or swells and flicker. The intense use of power electronic controlled electrical loads (e.g.~personal computers) has lead to a severe increase of current harmonics drawn from the distribution system. These current harmonics induce voltage harmonics, due to the impedance of the utility network. Voltage dips originate from fault currents in the electrical network or inrush currents of electrical motors and transformers. The flicker phenomenon can be attributed to pulsating loads (e.g.~photocopiers, microwaves etc.).
Although network reliability is concerned to have the most direct financial impact, all power quality issues cause major economical losses. For instance, poor power quality may result in the overheating of transformers, damaging of capacitor banks and malfunctioning of electronic equipment, and finally in the falling-out of electrical machinery and/or utility network elements. Due to these drawbacks, power quality issues have troubled power system engineers since the large-scale introduction of converter-fed electrical loads.
The research concerning the behavior of grid-coupled converters has been stimulated by the advent of renewable generation technologies. This research has resulted in the programming of a resistive converter with programmable damping resistance, as has been stated in [Ryckaert2006]. Due to the implementation of the programmable damping resistance as secondary control function on existing grid-coupled converters, it is possible to mitigate harmonic pollution of the distribution network. A beneficial side-effect of this implementation is the ability to damp grid voltage dips and to improve the grid-voltage dip ridethrough capability of the converters by using the proposed current control strategy.
Experimental verification of the converter behaviour
Harmonic voltage components present in the grid voltage can be reduced if power-electronic devices are programmed to behave as a resistive load for harmonic voltage components. This approach is called harmonic voltage damping. If the grid voltage is perfectly sinusoïdal, the line current is sinusoïdal as well (Figure 1a).
If the grid voltage contains harmonic voltage components, the line current absorbed by the converter is changed in order to damp the harmonic voltage components (Figure 1b).
Application of the damping control strategy yields an improved grid voltage dip ridethrough capability. The main reason for converter-connected DG-units to disconnect during voltage dips, is an excessive bus voltage, which causes a trip of the corresponding protection relay and the immediate shutdown of the converter. The bus voltage can be calculated by the charge balance of the bus capacitor, and is dependent of the power injected in the utility grid Pac and the power delivered to the converter by the DG power source Pdc. The line current of a converter controlled with a 'classical' control strategy is not changed during a voltage dip (orange line in Figure 2a). The damping control algorithm increases the current injected into the utility grid instantaneously at voltage dip initiation (blue line in Figure 2a). This results in a better balance between Pac and Pdc and less variation of the bus voltage. The bus voltage is depicted for both the 'classical' and the damping control strategy during a series of increasing grid voltage dips in Figure 2b. The voltage dip ridethrough capability of the damping control strategy is superior as compared to the 'classical' control strategy.
Figure 2a: Grid voltage (black line) and line current of the converter with damping control strategy (blue line) and line current of the converter with 'classical' control strategy (orange line) during a grid voltage dip.
Figure 2b: Bus voltage of the converter with programmable damping resistance (blue line) and the 'classical' converter (orange line) during a series of increasing grid voltage dips (gray line)
Relevant publications
B. Renders, W. Ryckaert, K. De Gussemé, K. Stockman, L. Vandevelde
"Improving the Voltage Dip Immunity of Converter-Connected Distributed Generation Units"
Elsevier Renewable Energy, accepted for publication
B. Renders, K. De Gussemé, W. Ryckaert, L. Vandevelde
"Input Impedance of Grid-Connected Converters with Programmable Harmonic Resistance"
IET Electric Power Applications, vol. 1, no. 3, pp. 355-361, May 2007.