Distributed Control of a Nanogrid Using DC Bus Signalling
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A nanogrid is a standalone hybrid renewable system that uses distributed renewable and non-renewable sources to supply power to local loads. The system is based on power electronics, with interface converters allowing the sources to supply power to the system and the loads to draw power from the system. The nanogrid is typically designed such that renewable sources supply the average load demand, while storage and non-renewable generation are used to ensure that the loads enjoy a continuous supply of power in the presence of the stochastic renewable sources. To maintain the power balance in the system while maximising use of the renewable sources, all sources in the system are scheduled according to a supply-side control law. The renewable sources are used wherever possible and the storage is operated as a slack power bus. The storage is controlled to absorb any excess power from the renewable sources and release it to the system when necessary. The non-renewable generation is only brought online when the storage and renewable sources are incapable of balancing the load demand. While the primary method for maintaining the power balance in the nanogrid is scheduling the sources according to a supply-side control law, a demand-side control law may also be used to help maintain the power balance in the system or protect the system from a complete collapse under overload conditions. A demand-side control strategy is implemented by shedding loads when the load exceeds the available generation, beginning with those loads having the lowest utilisation priority. Hybrid renewable systems are typically designed and controlled in a similar manner to the traditional ac power system, operating at 50/60~Hz, and maintaining the power balance in the system using frequency droop for power sharing and central coordination for scheduling the sources. However a nanogrid has different components compared to the ac system, employing power electronic converters to interface the sources and loads to the system. The control flexibility afforded by the use of power electronic interface converters opens the door to new transmission and control possibilities. This thesis evaluates a number of transmission options ranging from dc to high frequency ac in order to determine an operating frequency that is suitable for this niche system. A number of control topologies are also investigated to find a low cost strategy for implementing a supply-side control law. DC is selected as the operating frequency of choice largely for its simpler source interface requirements. A novel control strategy, dc bus signalling (DBS), is proposed as a means of implementing a supply-side control law. Its distributed structure maintains the modularity inherent in the distributed structure of the nanogrid. DBS uses the voltage level of the dc bus to convey system information. With a supply-side control law implemented using DBS, the source and storage interface converters operate autonomously based on the voltage level of the dc bus. The converters not only respond to the level of the dc bus, but they also change the level of the dc bus, automatically controlling other converters in the system. This thesis presents the theory of operation behind this control strategy and outlines a method for implementing a supply-side control law. A method for ensuring that the supply-side control law operates in a practical system where transmission line impedance affects the information conveyed by the dc bus is also given. For completeness, a method for implementing a demand-side control law using DBS is also presented. A simulation model of a nanogrid is presented and results are obtained to demonstrate the operation of DBS. The design of a small experimental system is also presented, and results are obtained to verify the operation of this new control strategy in a practical system. The simulation and experimental results demonstrate the feasibility of implementing supply and demand-side control laws in a nanogrid using DBS, even in the presence of transmission line impedance.