This paper addresses the current problem of less protection for power electronics such as PWM rectifiers. A simulation model is developed for the closed-loop PWM converter and its internal overvoltage and overcurrent are analyzed under two fault conditions, i.e., IGBT pulse fault and external short circuit. By comparing the rectifier and the inverter, we found that the PWM rectifier overvoltage is more significant. We then used this to design the corresponding trigger protection with overcurrent as the signal control pulse and circuit breaker and the matching protection for overvoltage with the lightning arrester-like principle. Our design restores 1.5 times the overvoltage of 1200V to 937V and suppresses 105A overcurrent to 846A.

Compared with the modular multilevel converter (MMC), the number of submodules of the hybrid dc converter based on the diode rectifier (DR) is greatly decreased. Therefore, the economy of offshore dc transmission is significantly increased. During the onshore grid fault, the high-voltage dc (HVDC) link voltage would increase, which cause the submodule capacitors overvoltage. In this paper, a low-voltage-ride-through (LVRT) strategy is proposed for the hybrid DC transmission system. Firstly, the characteristics of offshore ac and dc voltage during the grid fault are analysed. Under the influence of DR, the amplitude of offshore AC voltage would increase with the HVDC link voltage, which makes the offshore wind farms able to perceive the fault status without communication. Then the LVRT strategy is introduced, in which the high-voltage DC chopper of the system is not necessary. The output active power of the offshore wind farm is reduced by the grid-side converter of the wind turbines (WTs) when the fault is detected and the excess power is dissipated by the inner chopper. Finally, the validity of the proposed strategy is verified by the simulation model of the hybrid DC transmission system constructed in the MATLAB/Simulink.

The high-voltage dc (HVDC) transmission is preferred for grid-integration of large-scale long-distance offshore wind farms and practical projects based on the modular multilevel converter (MMC) for wind farms have been globally put into operation or under construction. However, the high-frequency resonance (HFR) phenomena have frequently happened in the MMCHVDC projects in recent years, e.g., 1271 Hz HFR in LuXi Project, 700 Hz and 1.8 kHz HFR in YuE Project, etc. To reveal the generation mechanisms of the HFR phenomena, a simple but accurate model is necessary. Nevertheless, the existing models of MMC are either too complicated or overly simplified, which are not appropriate to the HFR analysis. In this paper, an accurate high-frequency impedance model of MMC is proposed for the HFR analysis. The island control and gridconnection control of MMC are considered in the modelling, respectively. The developed high-frequency impedance models are compared with the detailed impedance models of MMC with different control strategies, which shows that the highfrequency impedance models are able to accurately represent all the impedance characteristics in the high-frequency range above 200 Hz. Furthermore, the impact of different control strategies on the high-frequency impedance characteristics is discussed.

With the rapid development of modular multilevel converter-based high-voltage DC (MMC-HVDC) transmission system for wind farm integration, the wideband oscillation issues have become increasingly serious. By far, most of the oscillation suppression methods are merely suitable for oscillation suppression in a specific frequency range, e.g., sub-/super-synchronous oscillation and high-frequency oscillation. In this paper, an adaptive wideband oscillation suppression method for wind farm integration through MMC-HVDC system is proposed in order to mitigate various oscillations adaptively. The wideband impedance models of the wind farm and MMC are derived analytically, which are validated by frequency scanning in simulation. Then, the effect of the proposed oscillation suppression method is theoretically analysed based on the impedance-frequency characteristics. The time-domain simulation model of wind farm integration through MMC-HVDC system is built in MATLAB/Simulink. Two oscillatory cases, i.e., sub-/super-synchronous oscillation and high-frequency oscillation, are carried out to validate the effectiveness of the proposed adaptive wideband oscillation suppression method. It is worth noting that the proposed oscillation suppression method can be applied in both controllers of wind turbine converters and wind farm side MMC.

In recent years, the great progress has been made in the development of offshore wind power in Jiangsu. The scale of offshore wind power in operation has ranked the first in China, and it is expected that the scale will reach 12870MW in the end of the "14th Five-Year Plan". With the explosion and significant growth of offshore wind power, greater challenges in terms of wind power absorption and system safety will be induced in Jiangsu power system. Simultaneously, the offshore distances of wind power exploitation are getting farther and farther. For some of the approved offshore wind power projects, they will utilize the flexible direct-current (DC) transmission technology to satisfy the demands of long geographical distances. With the widespread application of flexible DC transmission technology in planned offshore wind power projects, the controllability of wind power transmission systems can be improved, but there are also new risks are introduced, such as providing short-circuit currents to the transmission system, sub/super-synchronous oscillations induced from control systems of power electronic equipment and integrated power systems. Considering the development scale of offshore wind power, operation modes and structure characteristics in Jiangsu during "14th Five-Year Plan", influences of large-scale offshore wind turbines integrated to the transmission power system are discussed, through short-circuit currents induced, voltage stability, sub/super-synchronous oscillations. Corresponding improvement measures are put forward.

In this paper, we investigate the load frequency control (LFC) for multi-area power systems based on the sampleddata-based event-triggered communication. The controller is presented for guaranteeing the stabilization of the considered LFC model under the proposed event-triggered mechanism, which notably reduce the cost of control and communication. Lyapunov-Krasovskii functional method are employed to achieve the aforementioned control goal. Finally, a three-area networked power system is used to show the effectiveness of the proposed method.

With the growing proportion of photovoltaic (PV) generation in power systems, DC optimizers (DCOs) based distributed MPPT technologies are recently developed to mitigate the waste of solar energy due to the partial shadow in PV arrays. However, incorporating a large scale of PV capacity into power systems requires frequency regulation enhancement due to the reduced system inertia and PV power fluctuations. In this paper, a novel coordination control strategy for unit-level DCOs and grid-connected inverter is proposed to provide active frequency support from PV units. The proposed control scheme combines the f-P droop control for the power generation of PV-DCO units and the virtual inertia emulation for the DC-link capacitor of the voltage source inverter (VSI), to realize a communication-free cooperative control strategy between large-scale PV systems and grid systems. After that, a flexible power reserve for PV systems is achieved with the improvement of the rate of change of frequency (RoCoF), the frequency nadir, and the steady-state frequency deviation during under/over-frequency events in power systems. Case studies are performed to validate the effectiveness of the proposed control strategy.

The modular multilevel converter based high voltage dc (MMC-HVDC) transmission has been widely used in grid-integration of large-scale renewable power plants, especially for offshore wind farms. However, the sub/super-synchronous oscillation (SSO) and high-frequency resonance (HFR) phenomena have been reported in many MMC-HVDC projects during the last decade. To reveal the generation mechanisms of these oscillatory phenomena, the accurate ac-side impedance model of MMC is established based on harmonic state space (HSS) method in this paper, which includes all control loops and internal dynamics within MMC. The analytical impedance model is validated by frequency scanning in simulation. Based on the impedance model, a damping characteristics analysis method is proposed to extract and analyse the key influencing factors on the damping characteristics of MMC. To do that, a damping sensitivity of different parameters in main circuit and controller is defined to quantitatively assess and extract the key factors that mainly influence the damping characteristics in different frequency ranges. Subsequently, how those key factors influence the damping characteristics in different frequency ranges is then analysed using three-dimensional diagrams. Finally, the simulation results have confirmed the above theoretical analysis.

The traditional modelling method of permanent magnet synchronous generator (PMSG) relies heavily on the internal parameters of the PMSG, dynamic of which is inconsistent with that of the actual PMSG if some parameters are unknown, resulting in errors in analysis results. In order to realize the dynamic fitting of the PMSG without known parameters, and improve the accuracy of the analysis results, a data-driven modelling method of the PMSG is proposed in this paper. Here in, the full-order dynamics of the PMSG is divided as multiple subsystems, so that the long short-term memory (LSTM) method can be used to quickly and accurately simulate the dynamics of each subsystem based on a large of measured data. Finally, a power system connected with a PMSG is as an example to demonstrate that the proposed method can fit dynamics of the PMSG accurately even if the parameters are unknow.

Wideband oscillations frequently happened in practical modular multilevel converter based high-voltage dc (MMC-HVDC) transmission systems in recent years. The impedance-based analysis method is an effective tool for revealing the wideband oscillation mechanisms and designing the stabilizing controllers of MMC-HVDC systems. To do that, the wideband impedance model of MMC is prerequisite. A practical MMC-HVDC system with negative-sequence control structure is considered in this paper. The wideband impedance of the MMC is derived based on the harmonic state-space (HSS) method, which includes both the positive- and negative-sequence control strategies as well as the MMC internal dynamics. The derived wideband impedance is validated by frequency scanning in simulation. On this basis, the impact of the negative-sequence control on the impedance characteristics of MMC is analysed and the impact of different positive-negative sequence separation algorithms is also discussed.