Abstract
Interactions between individually designed power subsystems in a cascaded system may cause instability. This paper proposes an approach, which connects a virtual impedance in parallel or series with the input impedance of the load converter so that the magnitude or phase of the load converter's input impedance is modified in a small range of frequency, to solve the instability problem of a cascaded system. The requirements on the parallel virtual impedance (PVI) and series virtual impedance (SVI) are derived, and the control strategies to implement the PVI and SVI are proposed. The comparison and general design procedure of the PVI and SVI control strategies are also discussed. Finally, considering the worst stability problem that often occurs at the system whose source converter is an $LC$ filter, two cascaded systems consisting of a source converter with an $LC$ input filter and a load converter, which is either a buck converter or a boost converter, are fabricated and tested to validate the effectiveness of the proposed control methods.
Highlights
D C distributed power systems (DPSs) have been widely used in space stations, aircraft, ships, hybrid vehicles, Manuscript received December 3, 2014; revised March 18, 2015 and May 5, 2015; accepted June 7, 2015
This paper proposes a set of virtual-impedance-based control strategies to introduce a virtual impedance to be connected in parallel or series with the load converter’s input impedance and regulate the load converter’s input impedance
The transfer function Zvir_S (s) is introduced to the control block between the input current and the input voltage to realize the series virtual impedance (SVI), and vbus_ori is the disturbance of the load converter’s input voltage with Zvir_S(s). This is the basic idea of the SVI control strategy
Summary
D C distributed power systems (DPSs) have been widely used in space stations, aircraft, ships, hybrid vehicles, Manuscript received December 3, 2014; revised March 18, 2015 and May 5, 2015; accepted June 7, 2015. Compared to the stability solutions by changing the source converter’s output impedance, the methods of regulating the load converter’s input impedance are rarely reported. If the source converter is a simple LC filter, there will be no regulation signals of the source converter, and as a result, the load converter’s input impedance cannot be regulated to stabilize the cascaded system. The remaining parts of this paper are organized as follows: In Section II, the instability reason of the cascaded system is reviewed and the stability-preferred input impedance of the load converter is discussed. D) Worst case of instability problem: According to the characteristic of Zo_S, when the source converter is an LC filter, its output impedance is higher than that of other types of source converters [26]. The worst case of instability problem of a cascaded system occurs at full load and with the source converter being an LC filter
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