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Hybrid HVDC system, which comprises a line commutated converter (LCC) at rectifier side and a voltage source converter (VSC) at inverter side, is drawing wide attention due to its high reliability and economic benefit. For such a new transmission technology, the system characteristics are not comprehensively studied, and inappropriate control parameters may be coherent to system instability. This paper puts forward a novel coherence analysis procedure which analyzes system stability based on a verified hybrid HVDC small-signal model. For the first time, the procedure is proposed to capture the coherence relationships between the system characteristics and control parameters. The analysis results reveal: the circuit and control parameters can highly affect the oscillatory modes and even cause instability. Moreover, the variation of grid Short Circuit Ratio (SCR) at LCC rectifier side trades off the response speed and damping of the oscillatory modes - under low SCR, large proportional and small integral coefficients of the LCC DC current control are suggested to enhance response performance. Also, the increase of VSC DC capacitance reduces the mode response speed - for a large capacitance, large proportional and small integral coefficients of the VSC DC voltage control shall be set to guarantee system stability.

A hybrid modular multilevel converter (MMC) with reduced full-bridge (FB) submodules (SMs) is proposed, where a high voltage rating half-bridge (HB) based MMC is connected in series with a low voltage rating FB-MMC in parallel with a fault breaking circuit on its DC side. Unlike conventional hybrid MMCs with mixed HB and FB SMs, the proposed topology uses the DC capacitor in the fault breaking circuit to block DC faults, while the FB-MMC only commutates the fault current from the FB-MMC to the fault breaking circuit. Thus, the proposed converter only requires around 10%-20% FB SMs, leading to reduced capital cost and losses compared to typical hybrid MMC. The optimal ratio of the FB-MMC and HB-MMC is assessed and comparative studies show superiority of the proposed topology over other alternatives. A case study with 10% FB SMs demonstrates the validity of the proposed hybrid MMC for DC fault blocking and post-fault system restart.

This paper investigates the stability of diode rectifier based HVDC (DR-HVDC) connected offshore wind farms. The impedance model of the DR-HVDC connected offshore wind farms in dq frame is analytically developed and is validated by comparing its impedances to those obtained from frequency scanning of time domain EMT model in PSCAD/EMTDC. Based on the impedance model, the impacts of the sizes of DR-HVDC DC smoothing reactors and AC filters on offshore system stability are analyzed. It shows reduced DC smoothing reactance degrades the d-axis stability margin and can potentially lead to system instability. Higher bandwidth of offshore AC voltage control of wind turbine line side converter is proposed to minimize such instability potential. The analysis also shows that reduced AC filters size helps to increase q-axis stability margin and leads to a more stable system. Moreover, stability analysis considering the interaction between WTs is conducted and it reveals that the WT P-V control has a significant impact on the stability of the DR-HVDC connected WTs.

Parallel operation of a diode rectifier-based high-voltage direct current (DR-HVDC) and modular multilevel converter (MMC)-based HVDC (MMC-HVDC) for transmitting offshore wind energy is investigated in this article. An enhanced active power control scheme of the offshore MMC station is proposed to improve the power flow distribution between the MMC-HVDC and DR-HVDC links which are both connected to the offshore wind farm (OWF) ac network. By regulating the offshore voltage, all the wind powers are transmitted via the DR-HVDC link in low wind conditions while the offshore MMC power is controlled around zero to reduce transmission losses, considering the efficiency superiority of DR-HVDC over its MMC counterpart. When the DR-HVDC is out of service, wind energy is transferred via the MMC-HVDC and the wind turbine (WT) generated power is automatically limited by slightly increasing the offshore ac voltage to avoid potential MMC-HVDC overload. A power curtailment control is also proposed which slightly increases the dc voltage of the DR-HVDC to enable autonomous reduction of the generated wind power so as to avoid DR-HVDC overload during MMC-HVDC outage. The proposed coordinated control only uses local measurements and, without the need for communication, can seamlessly handle transitions including various faults. The proposed scheme enables fault ride-through operation and provides a high efficient solution with flexible operation for integrating large OWFs. Simulation results confirm the proposed control strategy.

This paper analyzes the control and operation of offshore wind farms connected with diode rectifier based HVDC (DR-HVDC) system. A small-signal state-space model of the offshore wind turbines (WTs) connected with DR-HVDC system is developed to design the WT Q–f droop control. The use of WT P–V and Q–f control during individual WT active power variation is clearly clarified. In order to reduce the interaction between WT active power and reactive power, an angle feedforward control is proposed where an additional phase shift is directly added to the WT output voltage based on the WT's active power output. The effectiveness of the proposed control on improving dynamic response and reducing active and reactive power interaction is verified by frequency-domain analysis and time-domain simulations in PSCAD/EMTDC.

This paper investigates the integration of large offshore wind farms using parallel HVAC and diode-rectifier based HVDC (DR-HVDC) systems. Three different operation modes, i.e. HVAC operation mode, DR-HVDC operation mode and parallel operation mode are investigated. A wind turbine control scheme including distributed control and centralised control is proposed to ensure the stable operation of the offshore wind farms under different operation modes. The proposed control requires no switching of the distributed control strategy when the operation mode is changed. Moreover, power flow between the DR-HVDC link and HVAC link under parallel operation can be well controlled with the centralised control. Simulation results in PSCAD/EMTDC verify the proposed control during transition among the three operation modes.

Distributed phase-locked loop-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier-based high-voltage-direct-current systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore ac frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore ac grid. Stability of the proposed frequency control is analyzed using the root locus method. Moreover, active dc voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through an onshore ac fault, where the onshore MMC converter quickly increases the dc voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power rebalance between the offshore and onshore sides. Thus, the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during startup, synchronization, and under onshore and offshore fault conditions.