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In an effort to minimize the power disruption between a dc grid and ac grids that host power converters during ac and dc network faults, this paper proposes a novel converter station structure to improve ac and dc fault ride-through performance of the multiterminal HVdc grid. The proposed structure consists of two independent ac and dc interfacing circuits, which are a half-bridge modular multilevel converter and a cascaded H-bridge (CHB) based energy storage system. Taking the advantages of high controllability and flexibility of the independent CHB converter and ease of integrating energy modules, a decoupled power relationship between the ac and dc sides is achieved, which is important for enhancing ac and dc fault performance. Operation of the proposed converter station under normal conditions and during ac and dc faults is explained, with the control system presented. Simulation validation of the proposed structure on a three-terminal HVdc grid confirms the enhanced performance, including the continuous operation during ac and dc faults with negligible power transfer disruption. [ABSTRACT FROM AUTHOR]

In this paper, a dc–ac buck converter is proposed as a grid-side converter in high voltage dc (HVdc) transmission systems for high-power renewable energy source grid integration. The proposed architecture consists of multimodules of half-bridge voltage source converters (HB-VSCs). The dc terminals of the HB-VSCs are connected in series across the entire dc link (i.e., capacitor tapped), whereas their ac outputs are connected to multiwinding transformers to provide the three-phase terminals for ac grid integration. The proposed grid-connected capacitor-tapped multimodule converter (CT-MC) architecture, inspired from the HVdc shunt tap proposed by ABB, provides a dc–ac conversion with a relatively moderate voltage rating of semiconductor devices. It also provides operation with a lower number of semiconductor devices, gate driver circuits, voltage sensors, and lower total MVA rating of semiconductor devices (67%) compared with the conventional three-phase HB modular multilevel converter, which positively affects the system cost and reduces the computational burden of the employed controller. In this paper, the operational concept and control of the CT-MC are presented along with a capacitor voltage balancing approach. Simulation results are presented during normal and abnormal conditions to show the viability of the proposed architecture. Finally, a scaled down prototype for the CT-MC is employed to validate the converter operation. [ABSTRACT FROM AUTHOR]

In order to generate bipolar high-voltage (HV) pulses across certain load from HV dc (HVdc) power supply, a voltage source inverter (VSI) stage should be inserted between the load and the supply. In order to meet the required HV level, series connection of semiconductor devices should be employed with dynamic voltage sharing between the involved devices. Modular multilevel converter (MMC) can be used instead of the conventional two-level VSI to alleviate the complexity introduced by the deployment of series-connected devices. The voltage level of the HVdc power supply and the voltage rating of the available semiconductor devices determine the suitable number of MMC voltage levels ( $N$ ). Capacitor voltage balancing is a vital issue for proper operation of the MMC. In general, conventional sensor-based balancing techniques require a significant amount of measurements, which increases the complexity of the system. In addition, the high dv/dt during switching times causes electromagnetic interference, which may adversely affect the accuracy of the measurements. In this paper, a sensorless voltage balancing technique is proposed for the MMC-based bipolar HV pulse generator that reduces the system sensitivity, cost, and complexity. A detailed illustration of the proposed approach is presented in this paper. The simulation and experimental results are used to validate the proposed concept. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC)-based high voltage DC (HVDC) transmission is considered the future of offshore power transmission. This paper aims at providing a reliable VSC-HVDC transmission system architecture between offshore wind farms and onshore grids. In this paper, a large-capacity, low-speed flywheel energy storage system (FESS) based on a squirrel cage induction machine is applied in parallel with the VSC-HVDC at the grid side converter. The FESS is dedicated for surge power (due to power flow imbalance during fault) absorption instead of being dissipated in the form of resistive losses. Since the duration of these surges is relatively small, it has been shown that the flywheel can effectively mitigate this problem. In addition to the fault ride-through support during fault conditions, the FESS is employed for power leveling functionality during normal operation. The performance parameters of the proposed approach are investigated via both simulation and experimental results. A 132-kV, 100-MW HVDC system is simulated using MATLAB/Simulink during normal and fault conditions. The proposed architecture is substantiated experimentally through a scaled down test rig with a 2-kW FESS. [ABSTRACT FROM AUTHOR]

Converting existing ac transmission lines to an extended multiterminal three-wire bipolar HVDC system can be considered a cost-effective way to interconnect dispersed offshore wind farms to onshore ac grids instead of building new bipolar HVDC systems. To automatically coordinate between different converters in a multiterminal HVDC (MTDC) system, droop control techniques have been adopted as an effective means without the need for fast communications between units. The droop control design is mainly dependent on the line resistances. This paper shows that the equivalent resistance of a three-wire bipolar system changes based on the operational mode. The modification to droop control design of an MTDC equipped with a three-wire bipolar system is then presented to tackle this resistance variation with the operating condition. Two types of MTDC systems are considered in this work, namely, radial parallel and meshed parallel systems. Different simulation studies have been conducted to validate the results of the presented analysis. [ABSTRACT FROM PUBLISHER]

Two-level voltage-source converters and half-bridge modular multilevel converters are among the most popular types of HVDC converters. One of their serious drawbacks is their vulnerable nature to dc-side faults, since the freewheeling diodes act as a rectifier bridge and feed the dc faults. The severity of dc-side faults can be limited by connecting double thyristor switches across the semiconductor devices. By turning them on, the ac current contribution into the dc side is eliminated and the dc-link current will freely decay to zero. The main disadvantages of this method are: high dv/dt stresses across thyristors during normal conditions, and the absence of bypassing for the freewheeling diodes during dc faults since they are sharing the fault current with thyristors. This paper proposes combining and connecting the double thyristor switches across the ac output terminals of the HVDC converter. The proposed protection scheme provides advantages, such as lower dv/dt stresses and lower voltage rating of thyristor switches in addition to providing full segregation between the converter semiconductor devices and ac grid during dc-side faults. A simulation case study has been carried out to demonstrate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Offshore wind energy is now seen as a key contributor for the future renewable energy mix. HVDC technology is among the chief technologies enabling widespread use of offshore wind. Thanks to their numerous advantages, voltage-source converter-based-HVDC (VSC-HVDC) systems are expected to be the technology of choice. Unfortunately, most of VSCs are defenseless against dc-side faults, such as two-level VSCs, and half-bridge modular multilevel converters. This paper proposes the buck-boost inverter-based-HVDC system (BBI-HVDC) as a means to overcome the limitations of the classical VSC-HVDC systems. The proposed configuration does not only provide sinusoidal three-phase voltage, but also provides complete blocking capability of ac-side contributions during a dc-side fault. The latter is achieved by simply disabling the gating signals to the switches upon fault detection. The performance of the proposed system is illustrated during normal conditions, ac-side faults, and dc-side faults. A simulation case study comparing the performance of the conventional HVDC converters with the proposed system during dc-side faults is conducted. The simulation results reveal the promising performance under normal operation as well as a significant decrease in the dc fault current due to the ac-side contribution blocking capability of the BBI-HVDC system during dc-side faults. [ABSTRACT FROM AUTHOR]

Power generation through wind is expected to play a major role in the world's future energy portfolio. Nevertheless, wind power integration remains a challenging research area due to the special characteristics of wind power generation. Specifically, offshore wind has received significant attention worldwide due to the vast generation potential available. The electrical infrastructure of offshore wind farms is thus of significant importance. The multi-terminal HVDC solution represents a preferable solution and has shown promise in solving wind farm interconnection problems. Droop control techniques have been proposed as a means to regulate the DC voltage and facilitate the automatic coordination between different converters without the need for fast communication between units. Different methodologies have been developed to select the droop gains to satisfy the system performance specifications. In this work, a control design methodology is proposed for power sharing among the multi-terminal HVDC feeders providing that the power transmission efficiency is optimized. A simulation study on a 400-kV/1000-MW four-terminal HVDC transmission topology is conducted to ensure the validity of the proposed methodology. [ABSTRACT FROM PUBLISHER]