Abstract
In renewable energy based systems Grid-Connected Voltage Source Converters (GC-VSC) are used in many applications as grid-feeding converters, which transfer the power coming from the renewable energy sources to the grid. In some cases, the operation of GC-VSC may become unstable or uncontrollable due to, among others: a grid fault or an inappropriate current-power reference, that give rise to fast electrical transients or a saturation of the controller. In this paper, an improved control scheme is proposed to enhance the controllability of GC-VSC in all these situations. This solution consists of two parts, on the one hand a new Proportional-Resonant (PR) controller with anti-windup capability to be used as current controller, and secondly a new current/power reference modifier, which defines the suitable reactive current/power reference to keep the system stable. It is worth to mention that the proposed scheme does not need information about the grid parameters as it only uses the converter current, and the voltage at the capacitors of Inductor-Capacitor (LC) output filter.
Highlights
Two-level three-phase voltage source converters (VSCs) are widely used as grid-feeding inverters, which are controlled resembling a current source in many applications, especially in renewable energy systems, high voltage DC transmission systems and microgrids [1,2,3,4,5,6,7], among others
When an inverter works in grid-supporting mode or grid-feeding mode, the if the input of switching modulator is higher than triangle wave, the current controller becomes inverter voltage has to be higher than the PCC voltage to deliver reactive power to the grid
The difference between the controller’s output and the AC limiter (ACL) lays mainly on the feedback signal to compensate the inputs of the integrators
Summary
Two-level three-phase voltage source converters (VSCs) are widely used as grid-feeding inverters, which are controlled resembling a current source in many applications, especially in renewable energy systems, high voltage DC transmission systems and microgrids [1,2,3,4,5,6,7], among others. The most extended solutions are those based on using the following reference frames: natural abc, synchronous dq0 and stationary αβ0. In the αβ0 domain, the coordinate transformations are simple and less costly from a computational point of view Likewise, in this reference frame the cross-coupling terms between control axes are not necessary, as a difference with implementations based on dq0 axis [10]. In [23] a scheme based on a PI controller is used to reduce the reactive current set point to prevent system uncontrollability, but this is too slow and it is not applicable to transient situations. The proposed scheme includes a reference modifier located in an outer loop, and a PR controller with anti-wind capability in the inner loop.
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