The hybrid cascaded (HC) LCC-MMC HVDC scheme, wherein the LCC serves as the rectifier, and a single LCC is connected in series with multiple MMCs in parallel as the inverter, amalgamates the merits of both LCC and MMC technologies. However, the utilization of thyristors in the LCC inverter still introduces a potential risk of commutation failure (CF). To mitigate this concern and bolster the stability of the hybrid system, an analysis of the LCC CF and its recovery process is undertaken, along with a mathematical investigation of the LCC's reactive power (RP) characteristics in the inverter. Subsequently, a mitigation strategy rooted in RP feedback control (RPFC) for the subsequent CF (SCF) within the HC-HVDC system is proposed. The transient performance of the system under AC fault conditions is evaluated with the proposed control approach in comparison to conventional methods. The outcomes indicate that this proposed approach effectively harnesses the RP support capabilities of MMCs, thereby endowing the LCC with enhanced resistance to potential SCF and improved post-fault recovery performance for the overall system.

The ultra-low frequency oscillation (ULFO) problem reappears in China recently. Different from the classical low frequency oscillation (LFO), the generators in ULFO have the same mode shapes, namely oscillating in the same pace. To reveal the reason why ULFO generators do not oscillate inversely as classical LFO does, this paper investigates the mechanism of ULFO based on improved Philips-Heffron model of two-machine system. Through this model, the phase angle relationship between different generators is obtained. Based on the bode diagram, it is found that the phase angle difference of the two machines is around zero in ultra-low frequency band however the generator parameters change. On the contrary, the situation is different in classical low frequency band, which reveals the mechanism that generators in ULFO always oscillate in the same pace. Finally, the analysis above is validated through a 2-area 4-machine system and a 6-machine system established in PSASP software.

The electric–hydrogen coupled integrated energy system (EHCS) is a critical pathway for the low-carbon transition of energy systems. However, the inherent uncertainties of renewable energy sources present significant challenges to optimal energy management in the EHCS. To address these challenges, this paper proposes an energy management method for the EHCS based on an improved proximal policy optimization (IPPO) algorithm. This method aims to overcome the limitations of traditional heuristic algorithms, such as low solution accuracy, and the inefficiencies of mathematical programming methods. First, a mathematical model for the EHCS is established. Then, by introducing the Markov decision process (MDP), this mathematical model is transformed into a deep reinforcement learning framework. On this basis, the state space and action space of the system are defined, and a reward function is designed to guide the agent to learn to the optimal strategy, which takes into account the constraints of the system. Finally, the efficacy and economic viability of the proposed method are validated through numerical simulation.

The hybrid HVDC with cascaded multi‐infeed MMC inverters will be put into reality soon in China, but its MMC inverter in constant DC voltage control would become a rectifier when disturbance happens. Thus, to solve such problem and enhance the stabilities, the adaptive droop control is proposed for MMC inverters and a hierarchical coordinated adaptive strategy is designed, which are realized through three parts. The first coordinated strategy is to maintain the inverter side DC voltage in constant by shifting the MMC inverters’ droop curves, such that the operation variation will not change the DC voltage in droop control, where the power transferred by the hybrid HVDC would be varied if voltage deviation occurs; the second part is to realize MMC inverters’ accurate power control considering the practical project and power dispatch, which is achieved through the droop control reference adjustment. Based on the above coordinated strategies, the third strategy is designed to make MMC inverters and LCC rectifiers coordinate with each other for the power fluctuates and fault situations, which is used to avoid inverters’ overload operation. The simulation results validate the effectiveness of the proposed strategy finally.

The underlying cause of commutation failures in traditional line-commutated converter (LCC) high-voltage direct-current (HVDC) transmission technology lies in the sensitivity of the thyristor devices, which are prone to turning off, thereby restoring the forward circuit breaker capability. This paper presents a coordination strategy between a controllable line-commutated converter (CLCC) and a voltage-sourced converter (VSC) and delves into the fault characteristics specific to CLCC damage. Our research focuses on CLCC topology, where fully controlled devices are incorporated to manage the thyristor’s turn-off time, ensuring its successful deactivation. This approach serves as a fundamental preventative measure against commutation faults. Furthermore, we employ a coordination strategy between the VSC and the CLCC to enhance the recovery time efficiency of the AC system. This strategy is simulated and validated using PSCAD software, and the results confirm its effectiveness in fault tolerance and AC system recovery.

Objective it is known that the rectifier side control usually suffers from the time delay and long distance communication problems, which could make the damping controller to be invalid and decrease the system stability. Specifically, when the system is in asynchronous operation, the rectifier controllers cannot damp the oscillations in inverter side AC system. Thus, to design the HVDC damping controller at inverter side becomes a feasible solution to avoid these problems. Methods Based on the MMC control strategy, to design the damping controllers at inverter side, the mixed H₂/H∞ robust method with regional pole placement is investigated for the two MMC inverters in constant active power control based on the master‐slave control mode, where the robustness, control effort and the damping ratios can all be guaranteed simultaneously. During the design process, the simplified system models, namely the TLS‐ESPRIT identification algorithm, is also used for controller study. Finally, two robust controllers are obtained based on the proposed method and the inverter side damping controllers are added at the MMCs of hybrid HVDC. Results The damping controllers for hybrid HVDC with cascaded multi‐infeed MMCs can be designed at inverter side, which endows the hybrid HVDC with more control functions compared with the classical HVDC systems. The damping controllers can be added at the cascaded multi‐infeed MMC inverters in constant active power control, which can enhance the connected AC systems' small stabilities. Conclusions Based on the multi‐objective robust theory, the control effectiveness and robustness of the designed controllers can be guaranteed simultaneously, which can damp the low frequency oscillations significantly in different operating situations.