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 DFIG 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.
Highlights
With the rapid development of sustainable clean energy, the penetration of wind generators in power grid has increased significantly, which has put forward higher requirements for wind turbine generators to participate in power system frequency regulation [1]–[4]
In order to meet the requirement of double fed induction generator (DFIG) wind turbine generators (WTG) frequency regulation, many scholars have conducted comprehensive researches, aiming to control DFIG WTG like traditional synchronous generators (SG), because SGs are capable of releasing the generator kinetic energy to grid or absorbing extra energy from grid during grid disturbances [1], [7]–[15]
To conquer the problem of weak grid stability, virtual synchronous generator (VSG) control strategy is proposed in [1], [12]–[16], where swing equation of traditional SG is utilized instead of phase locked loop (PLL) to synchronize with power grid, rotor side converter (RSC) is externally embodied as a voltage source, DFIG WTG is able to actively response to frequency fluctuations
Summary
With the rapid development of sustainable clean energy, the penetration of wind generators in power grid has increased significantly, which has put forward higher requirements for wind turbine generators to participate in power system frequency regulation [1]–[4]. To conquer the problem of weak grid stability, virtual synchronous generator (VSG) control strategy is proposed in [1], [12]–[16], where swing equation of traditional SG is utilized instead of PLL to synchronize with power grid, RSC is externally embodied as a voltage source, DFIG WTG is able to actively response to frequency fluctuations. Four main types of inertia emulation control strategies are compared in [17] in terms of inertia response performance, weak grid operation stability, secondary frequency drop, and wind tower fatigue load. It is of urgent need to propose a new modified VSG control strategy to expand DFIG stability boundary under large load changes Another serious problem faced by all inertia emulation control strategies is frequency secondary drop [20], [21]. Different from frequency drop situation, no matter how serious the frequency rise is, DFIG can always find its new equilibrium operating point in frequency rise situation
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