Zheng Xu

The flexible DC transmission scheme based on modular multilevel converters (MMCs) is currently the mainstream solution for offshore wind power delivery. With the ongoing growth in both installed capacity and the offshore distance of wind power projects, the dimensions and weight of corresponding offshore converter stations have increased substantially. These developments present significant economic constraints and engineering challenges, thereby complicating the deployment of large-scale, long-distance offshore wind energy systems. Compared with MMCs, diode rectifier units (DRUs) offer advantages such as compact size, light weight, low cost, reduced operating losses, and high reliability. Nevertheless, DRUs lack active control capability, and conventional grid following wind turbines cannot independently support the voltage of the offshore AC network, which severely limits their application in offshore wind scenarios with stringent economic requirements. Grid forming control of wind turbines is an effective approach to address this issue. Given the widespread application of doubly-fed induction generators (DFIGs) in engineering practice and their relatively low capital costs, this study investigates the implementation of grid-forming control strategies in DFIGs to address the stability challenges of DRU-based HVDC transmission systems during fault conditions. First, the mathematical model of the DFIG is established. Then, a suitable control strategy is designed to endow the turbine with certain grid forming capabilities. Finally, the developed simulation model and control strategy are verified in PSCAD/EMTDC. The results demonstrate that the proposed grid forming DFIG control strategy can maintain stable offshore AC voltage and frequency under various fault conditions, ensure continuous and reliable operation of the DRU, and achieve fault ride through. On this basis, to account for engineering practicality and cost considerations, this study further proposes a hybrid transmission scheme combining grid-following and grid-forming DFIGs. Simulation results confirm that this hybrid scheme also achieves satisfactory operational performance, while reducing the potential cost increase associated with full-scale grid-forming retrofits, it effectively ensures fault ride-through capability and system operational stability. This method provides an effective solution for low cost, highly reliable offshore wind power DC transmission.

China’s offshore wind power is rapidly developing towards the direction of “deep-water and far-shore, large-scale, and clustered”. Existing offshore wind power transmission schemes based on the MMC are technologically mature but highly expensive. Although transmission schemes based on the DRU possess economic advantages, they lack AC voltage support and reverse power flow capability. To combine the control performance of the MMC and the economic advantages of the DRU, this paper proposes a heterogeneous DC transmission system for offshore wind power based on the parallel operation of MMC-HVDC and DRU-HVDC, which can realize the clustered transmission of deep-water and far-shore wind power. First, the configuration scheme of the system is introduced, and the basic control strategy is proposed. Secondly, the small-signal model of the system is established, and the small-signal stability analysis is conducted. Then, the control strategies for the system under near-zero power conditions and AC/DC faults are proposed, respectively. Finally, the effectiveness of the proposed topology and control strategies is verified through PSCAD electromagnetic transient simulations.

To verify the harmonic amplification mechanism of wideband oscillations, this paper constructs a benchmark system. Firstly, the theoretical framework of the harmonic amplification mechanism for wideband oscillations is elaborated. Subsequently, the structure and parameters of the benchmark system are presented. Based on the s-domain nodal admittance matrix method, the resonant characteristics of the benchmark system, including resonance frequency, damping ratio, and nodal voltage mode shape, are calculated. Using electromagnetic transient simulation, the time-domain waveforms of the current, voltage, and power of the benchmark system under typical operating conditions are obtained, and harmonic decomposition of these waveforms is performed. By comparing and analyzing the harmonic components of current, voltage, and active power with the resonant characteristic quantities, the harmonic amplification mechanism of wideband oscillations is verified. The applicability of analyzing the harmonic amplification effect based on the positive-sequence network model is demonstrated. It is shown that the resonance frequency, damping ratio, nodal voltage mode shape, and resonant peak voltage are four key factors determining the harmonic amplification effect. Finally, the relationship between the frequency of the oscillatory power component and the frequency of the harmonic source is revealed.

To verify the harmonic amplification mechanism of wideband oscillations, this paper constructs a benchmark system. Firstly, the theoretical framework of the harmonic amplification mechanism for wideband oscillations is elaborated. Subsequently, the structure and parameters of the benchmark system are presented. Based on the s-domain nodal admittance matrix method, the resonant characteristics of the benchmark system, including resonance frequency, damping ratio, and nodal voltage mode shape, are calculated. Using electromagnetic transient simulation, the time-domain waveforms of current, voltage, and power of the benchmark system under typical operating conditions are obtained, and harmonic decomposition of these waveforms is performed. By comparing and analyzing the harmonic components of current, voltage, and active power with the resonant characteristic quantities, the harmonic amplification mechanism of wideband oscillations is verified. The applicability of analyzing the harmonic amplification effect based on the positive-sequence network model is demonstrated. It is shown that the resonance frequency, damping ratio, nodal voltage mode shape, and resonant peak voltage are four key factors determining the harmonic amplification effect. Finally, the relationship between the frequency of the oscillatory power component and the frequency of the harmonic source is revealed.

Based on the dilemma of analyzing the resonance stability of AC power grids using impedance models, it is demonstrated that the “negative resistance” mechanism of wideband oscillation is untenable. A general method for describing power electronic devices using wideband voltage-source converter models and wideband current-source converter models is proposed, thereby representing the nonlinear characteristics of power electronic devices with harmonic voltage sources and harmonic current sources. This allows the renewable energy power system to still be described by a linear system, and interprets the mechanism of wideband oscillation as a “harmonic amplification” phenomenon caused by network resonance, thus establishing a new framework for explaining the mechanism of wideband oscillation in renewable energy power systems. Through the analysis of two basic resonant circuits, the relationship between the damping ratio of resonant modes and the harmonic amplification factor is derived, laying a theoretical foundation for the analysis and suppression of wideband oscillation based on the s-domain nodal admittance matrix method and C-type damping filters. Based on the maximum damping criterion, a design method for C-type damping filters is proposed. The designed C-type damping filters exhibit strong broadband damping effects.

Due to the negative resistance effect of power electronic devices, power systems with a high proportion of renewable energy face a significant resonance risk. To address this, this paper proposes a resonance-suppression strategy for high-penetration renewable energy systems based on an active amplitude and phase corrector (APC). Firstly, by considering its internal dynamics and complete control loops, the impedance model of the APC is derived. Next, the similarities and differences between resonance stability and harmonic resonance are analyzed using the s-domain and frequency-domain admittance matrices, concluding that resonance suppression for low-damping s-domain modes can be handled in the frequency domain. Then, a supplementary APC control strategy in the abc-frame is proposed, which improves impedance magnitude at specific frequencies while keeping the phase almost unchanged. Finally, the proposed strategy is validated through case studies on an offshore wind power system in Zhejiang Province.

Based on the dilemma of analyzing the resonance stability of AC power grids using im-pedance models, it is demonstrated that the "negative resistance" mechanism of wide-band oscillation is untenable. A general method for describing power electronic devices using wideband voltage source converter models and wideband current source convert-er models is proposed, thereby representing the nonlinear characteristics of power elec-tronic devices with harmonic voltage sources and harmonic current sources. This allows the new energy power system to still be described by a linear system, and interprets the mechanism of wideband oscillation as a "harmonic amplification" phenomenon caused by network resonance, thus establishing a new framework for explaining the mechanism of wideband oscillation in new energy power systems. Through the analysis of two basic resonant circuits, the relationship between the damping ratio of resonant modes and the harmonic amplification factor is derived, laying a theoretical foundation for the analysis and suppression of wideband oscillation based on the s-domain nodal admittance matrix method and C-type damping filters. Based on the maximum damping criterion, a design method for C-type damping filters is proposed. The designed C-type damping filters ex-hibit strong broadband damping effects.

Offshore wind power is developing rapidly. The high resonance stability risks in offshore wind power systems (OWPSs) caused by submarine cables’ capacitive effect and wind turbines’ negative resistance effect have drawn much attention. The s‐domain node admittance matrix (s‐NAM) method is suitable for analysing resonance stability in OWPSs, while its efficiency is limited by massive high‐order determinant calculations. To improve efficiency, this paper combines impedance aggregation with the s‐NAM method. First, the resonance modes and mode shapes in the impedance aggregated system are analysed, and their partial inheritabilities are discovered. Then, regional observability and linkage are defined to distinguish between global and local resonance modes. Furthermore, a resonance stability analysis strategy for OWPSs based on the s‐NAM method and impedance aggregation is proposed. With this method, the resonance stability analysis for the original complex system can be divided into the ones for several small subsystems with aggregated impedances. Finally, the case studies of a practical OWPS are conducted, whose analytical calculations and simulation verification demonstrate the feasibility of the proposed strategy.

The massive integration of renewable energy has imposed new challenges on traditional AC power networks, such as excessive short‐circuit currents, overload risks of N‐1 contingencies, and voltage/frequency instability. With the capabilities of electrical decoupling and flexible power flow control, embedded high‐voltage direct current (HVDC) systems have emerged as a key technology for enhancing the security and stability of dense AC grids. Focusing on the topological optimisation of embedded HVDC systems, this paper first analyses the coupling characteristics between HVDC systems and intensive AC grids from four technical dimensions. Then, integrating multi‐dimensional stability constraints — power flow under steady‐state and N‐1 contingency, short‐circuit current, voltage and transient stability, a multi‐dimensional stability constrained bi‐level topology optimisation model is proposed for HVDC systems embedded in dense AC grids, which incorporates upper‐level economic optimisation and lower‐level stability correction using penalty functions, addressing the limitations of traditional planning in multi‐objective coordination and computational efficiency for large‐scale grids. Finally, simulation calculations are conducted on a typical 500 kV dense AC transmission network in a provincial region. The results demonstrate the feasibility and scientific validity of the optimisation method in suppressing short‐circuit currents, optimising power flow distribution and enhancing the grid stability margin.