استراتژی کنترل توان راکتیو در سیستم های چندانتهایی VSC-HVDC
Abstract
1- Introduction
2- VSC-HVDC multi-terminal systems
3- Q control for transient stability improvement using local measurements
4- Illustrative example
5- Case study
6- Conclusion
References
Abstract
This paper proposes a new reactive-power control strategy for High Voltage Direct Current multi-terminal systems with Voltage Source Converter stations (VSC-HVDC) to improve transient stability in electric power transmission systems. The proposed algorithm uses local measurements to estimate, in each converter station, a weighted average of the frequencies seen by the VSC stations of the HVDC multi-terminal system. This estimation is carried out making use of a (small) auxiliary local active-power modulation along with the DC-side voltage droop control in the VSC stations, where the latter also uses local measurements only. The estimated frequency is used as the set point for supplementary reactive-power control at each converter station. The proposed control law has been simulated in a test system using PSS/E and the results show that it improves transient stability, significantly, producing similar results to those obtained controlling reactive power using global measurements.
Introduction
Electrical grids in Europe are expected (a) to ease the integration of local distributed generation, (b) to integrate remote renewable generation such as a large amount of offshore wind power from the North Sea and (c) to connect to neighbouring electrical systems such as the Baltic Energy Market or those in African Countries [1]. To make this possible, it is necessary to increase the transmission capability of the system, interconnect power systems of different countries (even asynchronous) and be able to transmit, efficiently, distant offshore energy to the onshore grid through submarine cables. Meanwhile, the reinforcement of the existing infrastructure with new overhead AC lines raises important environmental concern and public opposition and Transmission System Operators (TSOs) are often driven to plan for underground connections. High Voltage Direct Current (HVDC) technology has a great potential to tackle these challenges, above all if submarine or underground cables are required. HVDC based on Line Commutated Converters (LCC-HVDC) is already a mature technology while multiterminal HVDC systems based on Voltage Source Converters (VSCHVDC) have many advantages as pointed out in [2], for example. Unfortunately, the latter is less mature and more expensive than the former and it is necessary to investigate further whether its potential advantages can overweight its investment costs. When planning to stretch power systems to their transmission limits, transient stability (i.e., the ability of the generators to remain in synchronism after large disturbances [3]) is one of the most important limiting factors. Although bulk power transmission should remain the main purpose of any multi-terminal VSC-HVDC system (VSC-MTDC, for short) to be built, they are also a very attractive alternative to improve transient stability, due to the fast control of active- and reactive-power injections of the VSC stations at different (often distant) points [4,5]. In fact, previous publications have showen that transient stability could be greatly improved using VSC-HVDC point-to-point links [6–9] and VSC-HVDC multi-terminal systems [10–14]. A complete review of previous work on control strategies in VSC-MTDC systems with a detailed description of each strategy is presented in Section 1.1.