This paper presents a comprehensive analysis of the limitations and the key dynamics of closed-loop active power control systems for VSC-HVDC, regarding stability, performance and robustness. Detailed dynamic models are derived and the controllability and robustness issues for VSC active power control are identified. Limitations imposed by ac system strength, converter operating point and current control design on the stability and performance of the two leading active power control principles are addressed, using frequency response analysis and time domain simulations. The dynamic interactions between the active power control design and the dc voltage droop control are examined. The simulations are performed using average-value VSC models and a high-fidelity modular multilevel converter model. Impacts of the active power control design on dynamic behaviors of multi-terminal dc (MTDC) systems are investigated using a four-terminal model. This paper provides a systematic study on the key stability and performance issues associated with the active power control. Furthermore the methodology offers a framework for the analysis of more complex active power and dc voltage droop controllers for future MTDC systems.

No abstract

Modular multilevel converters (MMC) are presently the converter topology of choice for voltage-source converter high-voltage direct-current (VSC-HVDC) transmission schemes due to their very high efficiency. These converters are complex, yet fast and detailed electromagnetic transients simulation models are necessary for the research and development of these transmission schemes. Excellent work has been done in this area, though little objective comparison of the models proposed has yet been undertaken. This paper compares for the first time, the three leading techniques for producing detailed MMC VSC-HVDC models in terms of their accuracy and simulation speed for several typical simulation cases. In addition, an improved model is proposed which further improves the computational efficiency of one method. This paper concludes by presenting evidence-based recommendations for which detailed models are most suitable for which particular studies.

No abstract

No abstract

Multi-terminal voltage-sourced converters (VSC) high-voltage direct current (HVDC) transmission system is expected to play a vital role in future power systems. Compared with ac power transmission, dc transmission is more vulnerable to faults due to low dc-side impedances and sensitive power electronics in the converters. Dc protection issues must be tackled before any multi-terminal VSC-HVDC grid can be built. The multi-terminal VSC-HVDC system is studied in detail using switching models for two-level converters, detailed equivalent models for the modular multi-level converters, detailed hybrid circuit breaker switching models and frequency-dependent phase models for dc cables. Using such high-fidelity system models, a systematic study of HVDC fault protection methodologies in more detail than previous studies is conducted. This is the first comprehensive study that includes pre-emptive circuit breaker operation. The results presented in this study underline the benefits of such a detailed treatment of the breaker, and of considering it as part of a fast power electronics system rather than isolated dc equipment. The study identifies the best existing fault detection method and tests it extensively. In order to further improve postfault system recovery response, which is a key but often neglected part of previous studies, a novel bump-less transfer control has been implemented in the converters.

No abstract

In this paper, the main layers of a HVDC control architecture based on autonomous converter control have been implemented and a range of tests have been conducted to assess the system's steady-state and transient performance.The simulation results show that the control system is able to accurately control DC power flow in steady-state and to maintain grid stability for fast transient events without exceeding the dynamic operating limits.