Abstract
The modular multilevel DC transformer (MMDCT) is more likely to suffer from transient oscillation, which is caused by insufficient damping of the equivalent RLC circuit with smaller arm resistance and submodule capacitance and may threaten the system stability. To tackle this issue, this paper conducts an in-depth analysis of the oscillation generating mechanism and thereby proposes a transient oscillation suppression method by incorporating an oscillation suppression angle (OSA) between the upper and lower arm switching voltages, with which the direction of the circulating current could be adjusted during this overlapped period. However, the open loop control of OSA shows indistinctive improvement of the damping ratio so that a damping compensator for regulating the circulating current is utilized. Furthermore, the compensating gain is optimally designed to achieve the critical damping state based on the small-signal model viewing OSA as an input variable. Besides, the outer capacitor voltage control loop and the power control loop are incorporated to provide the circulating current reference and to track the load power demand, respectively. Extensive simulation results from a realistically designed MMDCT model in PSIM validate the feasibility and effectiveness of the proposed transient oscillation suppression method.
Highlights
As the concept of sustainable development is deeply rooted in people's minds, the development and utilization of renewable energy represented by hydropower, wind energy and solar energy has become a hot topic worldwide
The load transient at t=0.4s triggers much more serious oscillations in the aforementioned variables with higher resonance peak value. Both steady-state response and transient response are undesirable without using any damping compensation method in modular multilevel DC transformer (MMDCT) system, indicating the necessity of incorporating the proposed control method to suppress the transient oscillation in a second-order system
The study in this paper shows that MMDCT has a low
Summary
As the concept of sustainable development is deeply rooted in people's minds, the development and utilization of renewable energy represented by hydropower, wind energy and solar energy has become a hot topic worldwide. The studies in [29]-[31] only focus on the oscillation suppression in a traditional PWM modulated MMC with sinusoidal modulation, where the modulation signal can be readily augmented These strategies are inapplicable for the MMDCT scenario because the DC transformer typically adopts the square wave modulation for achieving higher DC voltage utilization and smaller lower frequency energy ripples [10][11]. Because they do not need to buffer low-frequency power ripples, the specific characteristics of the RLC series configuration in MMDCT are more worthy of attention since the submodule capacitors in MMDCT are usually designed smaller than those in traditional MMCs [18][19]. Comparative simulation results verify the correctness and effectiveness of the proposed transient oscillation suppression method
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