Active Power And Frequency Control In Power Systems Incorporating Virtual Synchronous Renewables

The increasing penetration of renewable generation and the consequent displacement of traditional synchronous generators bring challenges to the frequency stability of the power systems.The renewables are commonly integrated into the grid via high-speed power electronic interfaces which are almost without inertia.Also,the renewables usually operate in the MPPT mode which do not response to the change of the frequency.And in the MPPT mode,the renewables feed power to the grid passively.Therefore,with the decreasing inertia level,frequency response resources and dispatchable sources,the active power balance and frequency stability of the power systems are facing great difficulty.Under the background,the concept and technique of virtual synchronous generator(VSG)become a hot topic targeting the deteriorating frequency stability.In this thesis,the control technology of renewable integration is investigated.On the one hand,the renewables are integrated in the active power and frequency control by emulating the frequency response capabilities of synchronous generators.On the other hand,the renewables are transformed into dispatchable sources which can be incorporated into the system economic dispatch problem.The contents of this thesis are as follows:(1)A virtual synchronous generator model which can fully emulate the characteristics of inertia response,primary frequency response,secondary frequency response,tertiary frequency regulation and ramping is established which enables the renewables to provide inertial response and participate in the active power and frequency control.First,the mechanism of frequency response is introduced.Second,the virtual synchronous generator model is established based on the second model of synchronous generators.Thirdly,by the derivation of small-signal model,the active power loop(APL)of the virtual synchronous generator is certified decoupled from the reactive power loop and the design of APL can be customized.At last,by the emulation of the frequency response and dispatchability of synchronous generators,a virtual synchronous generator model is established which can participate in the multi-temporal multi-stages of active power and frequency control in the power systems(2)In the stage of inertial response and primary frequency response,a parameter tuning method adaptive to load deviations is established.Firstly,the VSG control strategy providing the inertial response and primary frequency response is introduced.Secondly,the performance indices of frequency response are deduced through simplifications from transfer function.Thirdly,an adaptive parameter tuning algorithm is designed considering the composite influence on performance indices,the chronological sequence,and the renewable generation properties.At last,a quantitative analysis of the influence on system frequency stability by different operation strategies of wind integration.The results illustrate that the provision of IR and PFR from wind turbines supplements system inertia and primary frequency resources,which limits the rate of change of frequency and nadir under the severe condition.(3)The secondary frequency response characteristics of VSG-based inverters are analyzed in the time domain,and an adaptive parameter design algorithm is prompted according to the power system operation standards.First,the frequency response of synchronous machines,the necessity of renewables to participate system-level frequency regulation,and the VSG control strategy with frequency response capabilities are introduced.Second,the initial status of the secondary frequency response is set to be the initial disturbance and the quasi-steady-state of the primary frequency response.The relevance of control parameters and performances are derived in both stages by the maneuver of the simplification of transfer functions according to fast and slow poles.Third,considering current operation standards and local stability of renewable systems,and based on the time-domain performance expressions,a sequential structures program is proposed to calculate parameters.(4)An economic dispatch considering virtual synchronous renewables is presented.First,the necessity and control strategy of renewables participating automatic generation control are introduced.Second,the output of renewable generation is divided into the dispatchable region and the stochastic region,and the models of economic dispatch considering the operation modes of both the maximum power point tracking and the virtual synchronous renewables are established.Third,the constraints in the economic models are transformed into deterministic ones without uncertainty.Fourth,the participation factor of dispatchable generator in different operation modes are calculated.(5)A joint generation and reserve economic dispatch is prompted by transforming the renewables as virtual synchronous generators.First,the necessity of the participation of high penetration of renewable generation in the overall energy balance regulation in power systems,and the VSG control strategy are introduced.Second,according to the distribution function,the output of the renewables is divided into two regions,including the dispatchable region and the stochastic region.And the virtual synchronous renewable is dispatchable with virtual output upper limit.Based on the ricks of loss of load and renewable spillage rate,the determination of reserves considering the renewables under MPPT(maximum power point tracking)and VSG modes is presented.Third,the joint generation and reserve schedule models are established,and the object and constraints are presented for the two different modes.The research findings in this thesis could be utilized in the power systems with high penetration of renewable generation and high proportional of power electronics to supplement the inertia level,frequency response resources,and dispatchable sources.By the control strategies of virtual synchronous renewables,the frequency stability and the capability of energy balance are enhanced,and the penetration level of renewables could be increased further.