The Great British transmission network is expecting the interconnection of several new HVDC links up to and beyond 2020. Due to this proliferation of HVDC systems, it is anticipated that in some locations two or more HVDC converter stations will be installed with only a short electrical distance between them and will form a multi-infeed (MI)-HVDC system. This is a particular concern in the North of Scotland. It has been demonstrated in previous research that controller structure, parameters and limits in HVDC links have a significant effect on the stability of mixed AC-DC systems. It is anticipated that these factors will be just as, if not more, significant in MI-HVDC systems. Therefore, it is important to assess the dynamic performance of weak AC systems (with few synchronous generation sources) like Scotland with the different controllers associated with HVDC converters by considering existing industrial practice.
This research commences with the development of a test system for the North of Scotland. The proposed test system closely resembles the key features of the North Scotland power system (and is referred to as the reduced North Scotland (RNS) system). The RNS system is modelled upon a future Scottish transmission system model as reported earlier. This work investigates the impact of MI-HVDC links on the North Scotland system for two different system generation mixes with particular interest in the electromechanical time-frame. A list of control schemes has been formulated to assess the dynamic performance of the developed realistic system under typical loading scenarios. The system performance is assessed by observing a variety of aspects of system stability by detecting the stability indices. Furthermore, sensitivity studies have been performed for the worst and the best control combinations to evaluate the impacts of variations in the control characteristics among the grid-side converters of the proposed HVDC links.
The analyses results illustrated in this report suggest the following key features of the RNS system with proposed HVDC links:
- Feed-forward type power factor control can significantly increase the damping of the inter- area mode, whereas, feedback AC voltage control applied within the proposed HVDC links significantly reduces the damping of the inter-area mode.
- The direction of power flow can be influential in determining inter-area mode damping, even when similar control structures are used. This was witnessed for feedback AC voltage control in the grid-side converters when power flow direction is changed through the Northconnect link.
- The application of a constant reactive power control on the grid-side converters of the all proposed HVDC links has the most adverse effect on the transient voltage stability while the system is importing power from the adjacent grids. Furthermore, the impacts of the control combinations on the transient voltage performance significantly depend on the strength of the system and the amount of reactive power reserve into the system.
- The application of a constant reactive power control on the grid-side converters of the all proposed HVDC links has the most adverse effect on the transient stability for all operating scenarios. Again, the impacts of control combinations on the transient stability significantly depend on the strength of the system, reactive power reserve, and power flow direction into the system.
- From the sensitivity analysis that has been conducted, it appears that there is very little impact of any small variations on the control characteristics on the dynamic performance of the system. This suggests that controller structure, rather than controller parameters, is more influential in determining the dynamic performance of MI-HVDC systems.
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