Offshore wind farms are the main trend of future wind power exploitation. The diode rectifier unit (DRU) shows great potential in offshore wind power integration due to its economy and reliability benefits, and stable AC voltage control of the offshore gird is the key technology of the DRU based scheme. In this paper, the sensitivities of the DRU based system are defined and the sensitivity calculation method is proposed. According to the sensitivity characteristics analysis, the relationship between the wind turbine (WT) active power (or reactive power) and the WT output voltage (or system frequency) is clarified. On this basis, a grid-forming control strategy for WT converters is proposed. The stable operation mechanism of the grid-forming WTs in the DRU based system is demonstrated according to the analysis of steady state operation point existence and small disturbance stability. The time domain simulations of two DRU based offshore wind farm integration systems are carried out in PSCAD/EMTDC, and the feasibility of the proposed grid-forming control is verified.

The low frequency AC (LFAC) technology is found to be a competitive solution for offshore wind farm integration. This paper proposed an LFAC system with a hybrid onshore back-to-back (BtB) converter, for which the diode-based rectifier unit (DRU) and modular multilevel converter (MMC) based inverter is adopted. Then, this paper also proposed corresponding control strategies for offshore wind turbines, where the DC link voltage control is implemented on the machine side converter, and the AC voltage of the offshore grid is controlled by the grid side converter based on the fixed reference frame. The small signal analysis for the whole system was carried out based on the equivalent circuit, and the small-signal stability is proven by calculated eigenvalues. And the system responses to typical operating conditions are investigated by time-domain simulations in PSCAD. Thus, the feasibility of the proposed scheme is verified.

In this paper, the DC short-circuit fault and corresponding clearance solutions of modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems are analyzed in detail. Firstly, the analytical expressions of DC fault currents before and after blocking the MMC are derived based on the operation circuits. Before blocking the MMC, the sub-module (SM) capacitor discharge current is the dominant component of the DC fault current. It will reach the blocking threshold value in several milliseconds. After blocking the MMC, the SM capacitor is no longer discharged. Therefore, the fault current from the AC system becomes the dominant component. Meanwhile, three DC fault clearance solutions and the corresponding characteristics are discussed in detail, including tripping AC circuit breaker, adopting the full-bridge MMC and employing the DC circuit breaker. A simulation model of the MMC-HVDC is realized in PSCAD/EMTDC and the results of the proposed analytical expressions are compared with those of the simulation. The results show that the analytical DC fault currents coincide well with the simulation results.

Modular multilevel converter (MMC) has been widely used in the multi-terminal overhead line high-voltage direct current (HVDC) system due to its outstanding performance. However, the AC side and the DC side of MMC-HVDC have a strong interaction and will be affected by each other's faults, which brings the risk of system instability. In this paper, a modified MMC named active MMC with embedded energy storage in submodules (SMs) is proposed to isolate the impact of faults as a firewall. Firstly, the topologies of the active MMC and SMs with energy storage are introduced. Then the operation principle of the active MMC under different faults is analyzed in a multi-terminal MMC-HVDC system. Thirdly, the complete control scheme of the active MMC is proposed for bidirectional fault isolation. Finally, two main application scenarios of the active MMC are simulated in a three-terminal MMC-HVDC system and a particularly designed AC/DC hybrid transmission system respectively in PSCAD/EMTDC. The simulation waveforms of the active MMC are compared with the conventional MMC. Simulation results show that the active MMC can effectively protect the AC system and DC system from each other's faults.

The modular multilevel matrix converter (M3C) is the core component in low-frequency AC (LFAC) transmission, which is a competitive scheme for offshore wind power integration. In this paper, the M3C control strategy with the reduced switching frequency SM voltage balancing method is proposed. First, based on the conventional αβ0 and dq transformations, the M3C mathematical model is derived. Then, the dual-loop control structure with outer loop and inner loop controllers commonly used in voltage source converters is applied to the M3C. The inner loop controller consists of the current tracking controller in the dq reference frame and the circulating current suppressing controller in the αβ0 reference frame; the outer loop controller is proposed for the offshore wind farm LFAC integration scenario. Additionally, according to the operating characteristics of full-bridge sub-modules (FBSMs), three characteristic variables are defined and a reduced switching frequency SM voltage balancing method based on the nearest level control (NLC) is proposed. Finally, time-domain simulations in PSCAD/EMTDC demonstrate the feasibility of the proposed control strategy.

This paper studies the definition and calculation method of power grid strength in the environment of high-proportion nonsynchronous-machine sources, focusing on the effect of nonsynchronous-machine sources on voltage support strength and frequency support strength. By dividing the nonsynchronous-machine sources into four types, the equivalent circuits of each type under normal state and fault state are derived, respectively. Based on the Thevenin equivalent impedance of the power grid and the equivalent impedance of the connected device, the definition and calculation method of voltage support strength is given, and the new meaning of single-infeed short-circuit ratio and multi-infeed short-circuit ratio in the context of high proportion nonsynchronous-machine sources is presented. Based on the initial frequency change rate and the steady-state frequency deviation of any node in the power grid under the maximum expected active power disturbance, the equivalent inertia lifting factor and steady-state frequency deviation decreasing factor are defined, respectively, to describe the contribution of nonsynchronous-machine sources to the power grid frequency support strength, and the calculation methods of the equivalent inertia lifting factor and the steady-state frequency deviation decreasing factor are given.