The integration of renewable energy sources (RESs) into active distribution networks (ADNs) is essential for reducing carbon emissions but often results in voltage fluctuations and violations. This paper proposes a hierarchical voltage control framework that effectively coordinates diverse controllable devices with various response times in an ADN. The framework comprises three stages: day-ahead scheduling of on-load tap changer (OLTC), intra-day optimization for droop slopes and references for droop controllers in Soft Open Points (SOPs) and distributed generators (DGs), and real-time local voltage regulation. Unlike existing approaches, the proposed approach analytically establishes voltage stability constraints and incorporates them into droop slope optimization for local controllers, mitigating voltage oscillation risks. Additionally, a novel deviation-aware optimization method is developed to calculate optimal voltage references. This method treats the deviations between fixed-point voltages and their references as uncertainties and accounts for their impacts on voltage security through chance-constrained programming. Simulation results demonstrate the effectiveness of the proposed framework in improving voltage regulation performance with guaranteed stability.

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.

Regarding the issue that the short-circuit ratio (SCR) has inapplicability in evaluating the voltage support effect of voltage source converters (VSCs), this letter presents the definition and calculation method of voltage stiffness (VS) for strength evaluation of VSC-penetrated power systems. Theoretical and simulation analyses are performed to verify the effectiveness of the proposed VS.

The diode rectifier unit (DRU) based low-frequency AC transmission system is an attractive option for integrating offshore wind power due to its low construction cost and high economic efficiency. However, since DRU cannot transmit active power to offshore wind farms, an additional black start power source is required to provide the initial energy required for the wind turbines. In this paper, an auxiliary converter-based black start strategy is proposed for the above transmission system. The position and topology of the auxiliary converter are introduced, and the method for selecting its capacity is analyzed. The black start strategy of the transmission system is then designed, including the black start for the auxiliary converter and the gridforming wind turbines based on self-synchronizing control. Finally, a simulation model is developed using PSCAD/EMTDC, and the simulation results demonstrate the feasibility and effectiveness of the proposed black start strategy.

As offshore wind farms increase in scale and distance, it is crucial to identify more economical and reliable converter technologies for far-offshore wind power (OWP) high voltage direct current transmission system. In this paper, a cost-efficient hybrid converter topology is proposed based on the scheme of diode rectifier unit (DRU) and modular multilevel converter (MMC) in parallel. Compared to the DRU and MMC station-parallel scheme, the proposed hybrid converter scheme eliminates the need for the transformer in DRU and ensures that no harmonics pass through the remaining converter transformer. Furthermore, the required sub-module capacitance of the proposed scheme is smaller, which is conducive to lower costs. The mathematical model of the hybrid converter is developed, and based on this model, the working principle, along with the harmonic transfer characteristics is analyzed. An active power coordination control strategy and a harmonic suppression strategy are proposed. The PSCAD/EMTDC results for a 1000 MW-OWP integration system with the hybrid converter verifies the effectiveness of the proposed scheme. Finally, a comparison with existing schemes and a detailed economic analysis are performed to demonstrate the superior advantages of the proposed scheme.

Focusing on the three technical features related to the offshore wind turbine and the offshore transmission channel, eight schemes of offshore wind power transmission and their corresponding key technologies are discussed. The first technical feature describes the grid-forming capability of the offshore wind turbine; two types of wind turbines are discussed, namely, the grid-following wind turbine and the grid-forming wind turbine. The second technical feature describes the frequency characteristics of the output voltage and current of the offshore wind turbine, including DC, low-frequency AC, power-frequency AC and medium-frequency AC. The third technical feature describes the power transmission modes adopted by the offshore high-voltage main transmission channel, which are HVDC, high-voltage low-frequency AC and high-voltage power-frequency AC. Finally, the technical characteristics and maturity of the eight offshore wind power transmission schemes are reviewed, and a technical development direction is recommended.