Technological development in the last decade has turned renewable energy sources into real alternatives to help reduce contaminant emissions, which is an important concern all over the world. This is a challenging scenario for Transmission System Operators (TSOs). Since renewable resources may be located far from the big consumption points, a picture of a power system with distributed generation together with transmission of large amount of power over long distances is very realistic and High Voltage Direct Current (HVDC) transmission is often being considered as a key technology to address this situation. For example, there are serious proposals for building a pan-European HVDC grid, commonly called “Supergrid”, to lead the European electric power sector into the future and, in particular, to be able to integrate large amount of offshore wind energy into the European power system. For this development, Voltage Source Converter-HVDC (VSC-HVDC) technology is, nowadays, the best alternative despite undeniable difficulties. Furthermore, the high costs of VSC-HVDC installations urges to exploit all potential applications of these systems, such as, frequency support, voltage support, angle stability improvement or Optimal Power Flow (OPF), among others. Transient stability is defined as the ability of the power system to maintain synchronism of the generators against large disturbances and is a key limiting factor when HVAC transmission of bulk power takes place. In fact, synchronism is often at risk in heavily loaded grids, as it was illustrated in a recent incident in Turkey, caused by the disconnection of a 400kV line in an already-stressed system, which, eventually, led to a blackout after 12s. Therefore, under the possibility of having heavily loaded AC/DC grids containing VSC-HVDC multi-terminal Systems, TSOs demand (1) to understand the influence of VSC-HVDC grids on transient stability and (2) to investigate the potential of coordinated control of the converters to improve it. Along these lines, this work addresses active-power control strategies in VSC-HVDC multi-terminal systems for transient stability enhancement. The main features of VSC-HVDC multi-terminal technology with a potential effect on transient stability are: • Fast control of active-power injections of the VSC converters (1-100ms) • Fast control of reactive-power injections of the VSC converters (1-100ms) • HVDC transmission (using the operating point of the HVDC system to deload critical HVAC lines) 2 Recent publications have shown that, indeed, VSC-HVDC multi-terminal systems could significantly improve transient stability by implementing suitable control strategies for the active-power injections of the VSCs. However, those proposals make use of global measurements of the speed of all generators of the system in real time to compute the supplementary set points of the controllers. This information is hard to obtain and requires a Wide Area Measurement System (WAMS) available. In this work, a new control strategy for the active-power injections in VSC-HVDC multi-terminal system is proposed for transient stability enhancement. The proposed control strategy uses only global measurements of the frequencies at each converter station of the MTDC, which are already available for synchronisation proposes. The idea consist in using the weighted-average frequency as set point of the active-power controller of each converter. Therefore, each converter will decrease (increase) its active-power injection into the AC grid if its frequency is above (below) the weighted-average frequency of the MTDC. The purpose is to slow down the generators with higher speed and to accelerate the slower ones (with respect to weighted-average frequency). The advantage of this proposal with respect to previous ones is that the data needed are much easier to obtain. Results of a case study based on the Cigré Nordic32A system have shown that the Critical Clearing Times (CCTs) of different faults can be increased significantly using the control strategy proposed.
Published: August 2016.