Power Ripple Control Method for Modular Multilevel Converter under Grid Imbalances

Abstract

Modular multilevel converters (MMCs) are primarily adopted for high-voltage applications, and are highly desired to be operated even under fault conditions. Researchers focused on improving current controllers to reduce the adverse effects of faults. Vector control in the DQ reference domain is generally adopted to control the MMC applications. Under unstable grid conditions, it is challenging to control double-line frequency oscillations in the DQ reference frame. Therefore, active power fluctuations are observed in the active power due to the uncontrolled AC component’s double line frequency component. This paper proposes removing the active power’s double-line frequency under unbalanced grid conditions during DQ transformation. Feedforward and feedback control methods are proposed to eliminate ripple in active power under fault conditions. An extraction method for AC components is also proposed for the power ripple control to eliminate the phase error occurring with the conventional high-pass filters. The system’s stability with the proposed controller is tested and compared with a traditional MMC controller using the Nyquist stability criterion. A real-time digital simulator (RTDS) and Xilinx Virtex 7-based FPGA were used to verify the proposed control methods under single-line-to-ground (SLG) faults.

Control Approach of Grid-Connected PV Inverter under Unbalanced Grid Conditions

“…These oscillations must be carefully managed to prevent adverse effects on the power system's performance. Applying Park's transformation to a three-phase system of AC voltages and currents, the components of grid voltages and currents within the dq frame are depicted as follows [13]:…”

“…Unbalanced conditions occur when the amplitudes of the three-phase grid voltages or currents are not equal, which can be due to various issues such as asymmetric loads or faults within the grid. Under the assumption of a zero initial phase angle for these oscillatory components, the equations for the grid voltages and currents within the dq frame can be expressed as a sum of their DC and oscillatory components as follows [13]:…”

“…The equation is constructed based on the dq transformation of grid voltages and currents derived in Equations (1) and (2). The power formula, as outlined in Equation (7), can be reformulated to express the DC and oscillatory elements as follows [13]:…”

“…However, the challenge of controlling these oscillations in the DC-link voltage under conditions of grid imbalance remains challenging. Conventional methods of vector control within the dq reference framework, typically employed for managing current in inverter applications, face difficulties in managing oscillatory components under unbalanced grid voltages [13,14]. This inadequacy necessitates the exploration of novel control strategies that can effectively address the unique challenges posed by unbalanced grid conditions.…”

Three-Phase Grid-Connected Inverter Power Control under Unbalanced Grid Conditions Using a Time-Domain Symmetrical Components Extraction Method

“…Several control techniques have been suggested under unbalanced grid voltage conditions [9][10][11][12][13][14][15]. Some approaches have extracted the symmetrical components of the unbalanced grid voltages ( positive-and negative-phase sequences) and then have tried to balance currents under unbalanced grid voltage conditions.…”