Abstract
A constrained optimization problem based on the Lagrange multipliers method is formulated to derive the circulating current references of modular multilevel converters (MMCs) directly in abc coordinates. The resulting analytic expressions for calculating the circulating current reference signals are designed to eliminate oscillations in the dc-side power flow, independently of the ac-side operation of the MMC. As a result of the constrained optimization, the circulating currents are shaped to optimally utilize the degrees of freedom provided by the internal energy buffering capacity of the MMC, to effectively decouple the ac-grid conditions from the dc bus. This property of the proposed control method makes it especially suitable for preventing oscillations due to unbalanced ac-grid voltage conditions from propagating into multiterminal high-voltage dc systems. It is shown that the power flow at the dc-side of the MMC will be most effectively decoupled from ac-side transients if the desired steady-state power flow is imposed by acting directly on the circulating current references instead of by acting on the ac-side current references. The operation of an MMC controlled by the proposed approach is demonstrated by simulation studies, verifying the ability to keep the dc power flow free of second harmonic oscillations, independently of the power control objectives applied for calculating the ac-side current references of the converter.
Highlights
For High Voltage dc (HVDC) transmission systems based on voltage source converter (VSC) technology, and especially for future multi-terminal systems, it will be important to avoid that power oscillations originating from the ac grid can propagate into the dc system [1], [2]
To achieve a flexible and versatile approach for controlling the Modular Multilevel Converters (MMC) during unbalanced grid conditions, this paper presents a generalization of the approach in [26] that will ensure oscillation-free power flow at the dc terminals of the converter
The applied strategy for circulating current reference calculation for the MMC is capable of preventing steady state power oscillations at the ac-side during unbalanced conditions from propagating into the dc side
Summary
For High Voltage dc (HVDC) transmission systems based on voltage source converter (VSC) technology, and especially for future multi-terminal systems, it will be important to avoid that power oscillations originating from the ac grid can propagate into the dc system [1], [2]. The circulating currents can be controlled to be constant [11], or to contain a second harmonic component to compensate for second harmonic voltage or energy oscillations in each phase of the converter [12] This flexibility can be utilized during unbalanced grid voltage conditions, as discussed in numerous recent publications [13]. The MMC can be controlled to act as a “power oscillation firewall” or an “energy buffer” during unbalanced faults, preventing power oscillations on the ac side from propagating into the dc system This can be achieved even if the ac-side currents are kept balanced during unbalanced conditions [14]–[20]
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