Distributed voltage control and demand response

Abstract

Future power systems will have low inertia which will lead to larger deviations and rates of change of grid frequency. Large loss of infeed (e.g. due to a fault in the DC grid) higher than the spinning reserve can be more frequent. Also, voltage constraints will result in curtailment of renewable energy in the distribution systems. This thesis investigates the characteristics and effectiveness of the use of power electronic compensators in the control of both grid voltage and frequency through aggregated demand response in an autonomous fashion without any need for communication. Not relying on communication is essential to have a plug-and-play functionality for these compensators, so no central controller is critical for the system operation. Concept of Electric Spring (ES) has been proposed recently, which is a series compensator used to decouple the non-critical load from the mains to form a Smart Load (SL). The voltage and hence the active and/or reactive power of the SL can be controlled to regulate the mains frequency and/or voltage. The classification of SLs is provided and the characteristics and capabilities of each type are demonstrated through modelling, control design and simulations. The effectiveness of SLs working in unison for voltage control is demonstrated through case studies on realistic power system models. The limitations of smart load with reactive compensation only are highlighted. To overcome these limitations, an additional shunt converter in back-to-back configuration is proposed, which increases the flexibility of the smart load without requiring any energy storage. The contribution of different types of SLs in primary frequency control is also investigated. Sensitivity analysis are included to show the effectiveness and limitations of SLs for varying load power factors, proportion of SLs, and system strengths. Due to the limitations associated with input voltage control technique used in SLs (such as load voltage tolerance and availability of non-critical loads), use of power electronic compensators in output voltage control like mid-feeder compensation (MFC) and point-of-load compensation (PoLC) are also analysed. A comparison between the MFC and PoLC option is presented in terms of their voltage control capability, required compensator capacity, network losses and PV throughput. Alongside voltage control, effectiveness of the PoLC option for exercising voltage controlled demand response is also demonstrated and compared against the MFC option.