Research On Several Issues In Unified Power Flow Controller With Solid-state Fault Current Limiter For Power System Application

The series converter of unified power flow controller (UPFC) may be damaged when a short circuit fault occurs in the power systems. So, a topology of UPFC with fault current limiting (UPFC-FCL) was proposed by scholars from Zhejiang University, which combined UPFC with bridge type solid state fault current limiter. In normal conditions, FCL module is equivalent to zero impedance and UPFC-FCL runs as a conventional UPFC. When a short circuit fault occurs near the device in the power systems, FCL module will transform from zero impedance into large impedance to limit the fault circuit current immediately and automatically, which can protect the series converter from being damaged by fault current. But before the thyristors of FCL turn off completely, the fault current can flow through the diode of IGBT-Module and cause the voltage rise of DC-Capacitor to exceed the voltage level of IGBT-Module, which can also damage the series converter. This paper presented a brief analysis on the mechanism of the voltage rise of DC-Capacitor when a short circuit occurs, and proposed a novel improvement topology of UPFC-FCL.The relationships between the instantaneous power injected into the end of the line by UPFC and the voltage of series converter are obtained based on the mathematical model of UPFC in synchronous dq coordinates with the theory of instantaneous power. A direct power control (DPC) strategy of UPFC with constant switching frequency is proposed, which solved the coupling problem between active power and reactive power by power feedforward decoupling. Then, the control strategies based on power near the end of the line and power near the start of the line are proposed. Compared with conventional indirect power control of UPFC based on direct current control, the proposed method realized the direct power control of UPFC, reduced the amount of PI controller, and accelerated the respond of power control.This paper proposed the detailed design of the overall structure as well as the core part of the control system. An open hierarchical control structure is applied to the control system. There are five layers in this control structure:interactive layer, communication layer, control layer, drive layer and power layer. Finally, the detailed design of the control layer is given.In this paper, man-machine interactive system based on NI Measurement Studio with Microsoft Visual Studio is proposed, which can monitor the status and adjust the parameters of UPFC-FCL when it is running. The data is transferred between man-machine interactive system and controller via USB.An380V UPFC-FCL prototype is developed. The experiment procedure and results during normal conditions and short circuit conditions are given. The results demonstrate that the topology and control strategies of UPFC are feasible and effective. At the same time, the control system and man machine inactive system are flexible and stable.