Offshore wind power has a broad development prospect, but with the development of offshore wind farms to the deep sea, the traditional high-voltage AC transmission has been difficult to adapt to the offshore wind power transmission distance and transmission capacity needs. A flexible DC transmission system applying modular multilevel converter is a common scheme for offshore wind power, which has been put into use in actual projects, but it is still facing the problems of high cost of offshore converter station platforms and high loss of collector systems. In order to improve the economy and reliability of the medium- and long-distance offshore wind power delivery systems, this paper proposes a diode rectifier-based medium-frequency AC pooling soft-direct low-cost delivery system for medium- and long-distance offshore wind power. Firstly, the mid-frequency equivalent model of the diode converter is established, and the influence of topology and frequency enhancement on the parameters of the main circuit equipment is analysed; then, the distribution parameters and transmission capacity of the mid-frequency cable are calculated based on the finite element modelling of the marine cable, and the transmission losses of the mid-frequency AC pooling system are then calculated, including the collector losses, converter valve losses, and transformer losses, etc. Finally, an economic analysis is carried out based on a specific example, comparing with the Jiangsu Rudong offshore wind power transmission project, in order to verify the economy of the medium-frequency AC flexible and direct transmission system of the medium- and long-distance offshore wind power using diode rectifier technology.

Real-time grid monitoring of photovoltaic (PV) systems with battery storage requires continuous access to voltage, current, and power states. Traditional simulation models obtain these responses by calculating switching events, which limits the scale of real-time simulation. This paper proposes a Multi-Feature Dynamic Reconstruction (MFDR) method that combines environmental inputs, averaged converter states, and frequency-domain electrical variables. On the DC side, PV output is calculated from irradiance and temperature, while the bidirectional battery converter is represented by a low-frequency reconstruction model. On the AC side, dynamic phasors are used to convert the grid-connected inverter into a frequency-domain Norton equivalent for reconstructing its port voltage and current responses. Controller hardware-in-the-loop tests are conducted under different operating conditions. The reported peak and normalized tracking errors of the evaluated transient quantities remain below 3%. In the four-core IEEE 118-bus case, the average per-core CPU utilization decreases from 70.43% to 41.08%, while the maximum step execution time decreases from 38 μs to 27 μs. The model with 1069 state variables also operates within the fixed 50 μs simulation step. The results show that the MFDR method reduces the computational demand of real-time grid monitoring while retaining the voltage, current, and power responses.

To establish quantitative criteria for selecting the operating frequency of diode rectifier unit (DRU)-based medium-frequency AC collection and DC transmission systems, this paper proposes a life-cycle frequency-selection method for far-offshore wind power. A power-flow model incorporating wind-turbine Q–f droop control and frequency-dependent submarine-cable parameters is developed to evaluate voltage distribution, reactive-power accumulation, power factor, and steady-state losses. An electromagnetic transient model is then used to quantify energy losses caused by single-phase and three-phase AC-side short-circuit faults. These electrical results are integrated with cable and converter investment costs in a multi-stage life-cycle economic model that accounts for equipment aging and loss growth. For the studied Rudong 500 MW case under the adopted baseline parameters, medium-frequency operation increases submarine-cable reactive-power accumulation and fault losses, while the total life-cycle cost first decreases and then increases with frequency. The minimum cost occurs at 180 Hz. The proposed framework links steady-state performance, transient fault losses, and long-term economics, providing a project-oriented basis for selecting the operating frequency of DRU-based offshore wind transmission systems.