Research On Dynamic Performance And Stability Improvements Of Virtual Synchronous Generator Based Power Systems

In centralized electric power generation,electric power is generated mainly by enormous synchronous generators(SGs).In the power system with huge SGs,the frequency is determined by the rotational frequency that itself depends on the prime mover power.In case of a fault or disturbance,the kinetic energy preserved in massive rotors and associated equipment is injected to the power system to maintain the energy balance.Moreover,the inertia of the rotating masses prevents sudden changes in frequency and enhances the stability of the power system.The decentralization of electric power generation increased the number of inverter-based power sources.If these inverter-based units are operated and controlled similar to SGs,a large portion of problems involved with distributed generators(DG)and microgrids such as frequency regulation,islanded operation,and parallel operation of inverter-based DGs will be solved.The Virtual Synchronous Generator(VSG)is an inverter-based generating unit that uses an SG model to emulate the dynamic behaviour of an SG with a specific value of virtual inertia.By such a scheme,the inverter-based generator can benefit the advantage of an SG in frequency stabilization,preserving its original features.In chapter one of this dissertation,renewable energy sources are discussed along with the VSG structure.Additionally,the VSG control order and VSG operating principle are discussed in chapter one.Chapter two focuses on the existing VSG topologies and their working principles.VSG application in the different types of systems,such as PV and wind system,electric vehicles,AC/DC transmission lines,and energy storage systems,are discussed in detail.Chapter three discusses the microgrid idea and VSG role in microgrid along with VSG based power system stability,namely small signal stability and transient stability.VSG control scheme has received much attention from researchers as the introduction of rotational inertia to inverters.However,maintaining power-angle stability during the transient state of low inertia microgrids consisting of VSG based DG units is a critical challenge.According to the equal-area criterion,the accelerating and decelerating area plays an important role in power system stabilization.Therefore,a new control strategy based on the equal-area criterion is proposed in chapter four.According to the proposed control technique,the auxiliary inertia is incorporated into the governor unit of VSG with an additional power loop.The auxiliary inertia term minimises the difference between governor units of conventional VSG and SG.Whereas,the additional power loop provides the auxiliary power during the transient process only when the system experiences a significant fault.Moreover,auxiliary inertia helps to minimise the convergence time and dynamic characteristics of modified VSG and provide the frequency support and additional power help to shorten the acceleration area and widen the deceleration area which improve the transient stability.Additionally,the simulation analysis of a single machine and two machines connected to a large grid shows that the proposed control technique stabilizes the VSG based power system effectively when power system experience a significant fault.The integration of multiple VSGs in power systems contains new technical challenges considering transient stability.Therefore,chapter five address the transient stability issue in multiple VSG grid,a modification is proposed in VSG control to improve the stability of standalone grids with interconnected VSGs.The proposed technique is based on the concept of the centre of inertia(COI)response of the system.Owing to swing power between VSGs,the interconnected VSGs can encounter only one frequency in the grid,whereas the significant disturbances are represented in COI frequency.In this technique,the input power of VSGs is controlled to minimize the power variations and oscillations of VSGs related to COI frequency.In order to equalize the VSG speed with COI frequency during a transient condition,a separate controller is used for each VSG.Transient stability is a critical issue in the power system,and it is necessary to analyze and evaluate the effect of VSG on the transient stability of the power system.Therefore,in chapter six,the centre of inertia(COI)concept is implemented to analyze and evaluate the integration effects of VSG consisting of an improved governor and modified power input.The impact of VSG integration is divided into SG linked parts and COI associated parts.Due to VSG integration into the power system,the significant elements which disturb the COI dynamic motion and rotor dynamics of SG are examined in detail.Different cases are considered to evaluate the effectiveness of the proposed method,i.e.VSG’s different integrating location and different power capacities.It is concluded in this research that different integrating locations of modified VSG suppress the irregular power distribution of the system,which minimise the effect on rotor speed and rotor angle of each SG regarding COI.Consequently,modified VSG is integrated into the system without removing any SG,the acceleration power of COI is the only factor that affects the dynamic motion of COI,and with suitable parameter adjustments,it implies to improve stability.Moreover,the power system experiences a relatively smooth transition with respect to the reference case when an SG replace with modified VSG.By this practice,the transient dynamics of COI are influenced,and remaining SG’s experience relatively lower fluctuations than reference cases.The chapter seven concludes the current study.