Stability Enhancement and Direct Speed Control of DFIG Inertia Emulation Control Strategy

Abstract

With the increasing penetration of renewable energy generators in power grid, traditional vector control (VC) strategy for double fed induction generator (DFIG) is unable to provide extra active power support to grid because DIFG inertia is made decoupled from grid frequency fluctuations. To solve this problem, Virtual Synchronous Generator (VSG) control strategy as well as Inertial Synchronization Control (ISynC) strategy are proposed for DFIG rotor side converter (RSC) and grid side converter (GSC) respectively, so that DFIG rotor speed will experience an acceleration or a deceleration process to release or absorb the kinetic energy stored in DFIG wind turbines, which can prevent grid frequency from deep drop or increase. However, VSG-ISynC control strategy has its limitations in that rotor speed may lose its stability when large load is added into power system, at the same time, the secondary frequency drop is serious if rotor speed has decreased lower than the admissible minimum value. To address this issue, a modified VSG (M-VSG) control strategy is proposed by dynamically changing the P-f droop coefficient of conventional VSG control strategy, aiming to expand the stability boundary of DFIG operation. Additionally, an extra rotor speed closed loop is added into VSG control strategy, which can significantly reduce serious frequency secondary drop by controlling rotor speed directly. Simulation and hardware-in-loop (HIL) verification are both carried out in RTDS & GH Bladed co-simulation research platform to verify the effectiveness of proposed M-VSG control strategy.

Tuning of Discrete Complex Proportional Integral Current Controller for Grid-Connected Converters Based on Critical Damping

“…The reference voltage of the converter is kept constant over each sampling interval, so the converter can be assumed to be a ZOH circuit [13], [14], [17], [27], [33], [35]. As a result, the discrete-time model can be obtained through the ZOH method to (1), which can be expressed as follows:…”

“…Similarly, the root locus of current loop, which is plotted by varying the controller gain k in (8), can be obtained, as shown in Fig.5(a). The two trajectories initiated from p 1,2 ol , are noted as 1 and 2 , respectively. It can be observed that with the increasing of k, trajectories 1 and 2 meet at the point of ''A'' and then depart in the opposite direction, before which the system is over damped.…”

“…To better understand these features, the closed-loop poles p 1,2 cl can further be substituted into (11) and (14). In this manner, the plots of ξ vs. k and p z cl vs. k can be obtained, as shown in Figs.5(b) and (c), respectively, where the controller gain related to ''A'' is noted as k opt .…”

“…Grid-connected converters (GCCs) have been widely employed for power conversion in a lot of different applications, such as renewable power generation [1]- [4], active power filters [5]- [8], and rectifier systems [9], [10], just to name a few. In these scenarios, the converter presents as a controllable current source to the grid.…”

“…This coupling characteristic can further lead to interference in the active-and reactive-power regulation. Commonly, a state-feedback (SF) technique can be employed for cross coupling decoupling in this case [1]- [3], [8], [25], [28]- [30], [33]- [36]. This method is however typically sensitive to the one-step sampling delay found in a digital implemented system, resulting in reduced performance in cross coupling decoupling and system's overall stability.…”