As a key device to facilitate power flow in the multi-terminal direct current (MTDC) transmission system, DC power flow controller (DCPFC) can effectively expand the power flow regulation area by implementing its multiport output functionality. In order to solve the coupling issue between the transmission line current and the capacitor voltage of the inter-line multiport DCPFC (M-DCPFC), an improved M-DCPFC with a partial power rated auxiliary DC/DC converter unit is proposed in this paper. It provides the advantages of power flow reversal, high adjustment flexibility and low port-expansion cost. Meanwhile, a generic control strategy is developed for the M-DCPFC with random number of the ports. Moreover, from the perspective of the transmission system, the proposed M-DCPFC is modeled as the equivalent series voltage source. Its power flow adjustment capability and its impact on the local voltage source converter (VSC) stations are investigated in detail in this paper. Lastly, to validate the effectiveness of the proposed M-DCPFC, it is first tested in the simulations of the MATLAB/Simulink and then deployed on an experiment platform. Both simulation and experimental results show that the proposed M-DCPFC can achieve stable power flow control in different conditions.

Multi-port DC power flow controller (M-DCPFC) has become one of the most attractive solutions to regulate the power flow in the modular multi-level converter based multi-terminal DC (MMC-MTDC) transmission system. However, the current studies on the M-DCPFC are mainly focus on the power flow control under the normal operation in the transmission system. To eliminate the impact of the power oscillation which frequently happens because of the unbalanced AC grid, this paper proposes an integration to calibrate the active damping control into the M-DCPFC. It introduces the complementary virtual damping to the transmission lines in order to reduce the DC current fluctuations, which suppresses the DC side power oscillations. In this paper, the influence of the unbalanced AC grid on the MMC-MTDC transmission system is investigated and the active damping control of M-DCPFC is presented in detail. A four-terminal MMC-MTDC transmission system is developed as the test platform in the real-time model-in-the-loop (MIL) simulation. The proposed active damping control is validated and shows its effectiveness under different unbalance conditions.

A modular multilevel converter (MMC) is considered to be a promising topology for medium- or high-power applications. However, a significantly increased amount of sub-modules (SMs) in each arm also increase the risk of failures. Focusing on the fault-tolerant operation issue for the MMC under SM faults, the operation characteristics of MMC with different numbers of faulty SMs in the arms are analyzed and summarized in this paper. Based on the characteristics, a novel circulating current-suppressing (CCS) fault-tolerant control strategy comprised of a basic control unit (BCU) and virtual resistance compensation control unit (VRCCU) in two parts is proposed, which has three main features: (i) it can suppress the multi-different frequency components of the circulating current under different SM fault types simultaneously; (ii) it can help fast limiting of the transient fault current caused at the faulty SM bypassed moment; and (iii) it does not need extra communication systems to acquire the information of the number of faulty SMs. Moreover, by analyzing the stability performance of the proposed controller using the Root-Locus criterion, the election principle of the value of virtual resistance is revealed. Finally, the efficiency of the control strategy is confirmed with the simulation and experiment studies under different fault conditions.

A DC power flow controller (DCPFC) can help to facilitate power flow routing in the multi-terminal high-voltage direct current (HVDC) transmission system. Realizing its multi-port output can effectively improve the device regulate range and capability. Based on analysis of the traditional multi-port interline DC power flow controller (MI-DCPFC), this paper presents a switches reduced topology of MI-DCPFC. In addition, for solving the problem of coupling of the port-output voltage of the traditional MI-DCPFC, a novel control strategy based on carrier phase shifting pulse width modulation (CPS-PWM) is proposed. It implements the decoupling of the port-output voltage of MI-DCPFC, which can ensure completely independent tracking of the power flow regulating commands for different controlled lines. Moreover, key relationships between the system state variables are also analyzed and detailed in this study. Finally, the performance of the proposed controller and control strategy are confirmed with the simulation and experiment studies under different conditions.

Each arm of modular multilevel converter (MMC) consists of a large number of sub-module (SM) units. However, it also increases the probability of SM failure during the long-term system operation. Focusing on the fault-tolerant operation issue for the MMC under SM faults, the traditional zero-sequence voltage injection fault-tolerant control algorithm is analyzed detailed and its disadvantages are concluded. Based on this, a novel fault-tolerant control strategy based on phase disposition pulse-width modulation (PD-PWM) is proposed in this paper, which has three main benefits: (i) it has carrier and modulation wave dual correction mechanism, which control ability is more higher and flexible; (ii) it only needs to inject zero-sequence voltage in half a cycle of the modulation wave, which simplifies the complexity of traditional zero-sequence voltage injection control algorithms and much easier for implement; (iii) furthermore, the zero-sequence voltage can even be avoided injecting under the symmetrical fault conditions. Finally, the effectiveness of the proposed control strategy is verified with the simulation and experiment studies under different fault conditions.