Analysis On Dynamic Characteristics And Study On Subsynchronous Oscillation Of Power System With PMSG Integration

The integration of large-scale renewable power generation to the power system might cause a new problem of dynamic instability,the oscillation in the wind power integrated system.Several cases of oscillation have taken place in the field and have even resulted in severe operation accidents.In the engineering and academic circles,this problem has become a hot topic of special concern and an important issue demanding urgent attention.The oscillation caused by the large-scale integration of wind power into the power system is obviously different from the low frequency oscillation in the traditional AC grid.The latter is mainly concerned with the mechanical dynamic of rotor,and is a dynamic problem of the synchronized operation of the whole system generators.The former is more of an electrical oscillation under certain conditions and within a particular system,and could be the mutual oscillation between the wind farm and grid-connected system,or the oscillation triggered by the control interaction between the wind farms,or among the wind farms and other equipment such as static var compensator(SVC),high-voltage direct current(HVDC),steam turbine generators and so on.Above all,attention should be paid attention to the stability of different control loop and the control interaction between the devices in the power system with integrated wind power.Furthermore,the oscillation modes along with their mechanism,and the critical factors should be made clear,and analysis should be conducted into the dominant devices,the relevant control loops together with the propagation path of the oscillation modes.In this way,the theoretical guidance of control parameter design of wind power generator,oscillation risk assessment and stable operation of power system can be provided.With the focus on the dynamic characteristics and subsynchronous oscillation of power system with integrated direct-drive permanent magnetic synchronous generators(PMSGs),this dissertation analyzes the mechanism of the stability and the construction of the stability region of the current control loop and phase-locked loop(PLL),the dynamics characteristics of the wind farm based on the multiple aggregated PMSGs,the dynamics characteristics and the analysis method of complex power system composed of multiple wind farms and multi-type devices.Then,the eigen-mode,the mechanism and the condition of oscillation characteristics in the grid-connected PMSG system are revealed.The analysis method on the control interaction among multiple wind power generators or multiple wind farms is investigated.The main researches are as follows:(1)The working principle and the typical control of PMSG are studied specifically.The detailed mathematical models of PMSG,steam turbine generator and high voltage direct current(HVDC)transmission system are built.The method of the modeling the state-space mathematical model of the complicated grid-connected PMSGs system is proposed,which lays the theoretical foundation of the analysis on dynamic characteristics and the study on subsynchronous oscillation of power system with PMSG integration.(2)Based on the state-space model and the eigenvalue analysis,the stability and critical factors of the current control loop and PLL of the PMSG grid-side converter(GSC)are studied.The analyses on the oscillation scenario caused by the control interaction between PMSG and weak network and its oscillation mechanism are carried out respectively.After that,the stability region represented by short-circuit ratio(SCR),output power of PMSG and critical control parameters is established.The results show that the control interaction between PMSG and weak network involves two oscillation modes,namely,the current control dominant mode and the PLL dominant mode,which is related to the bandwidth design on these two controls.When the bandwidth of current control is low,the current control dominant mode will move towards the sub-/super-synchronous frequency band.Then,the current control dominant mode is exposed to the oscillation risk under the low output power of PMSG.On the other hand,when the bandwidth of PLL is high,the PLL dominant mode will be located in the subsynchronous frequency band.The PLL dominant mode is prone to instability under the high output power of PMSG.The SCR and output power have great influence on the stable region of control parameters of current control and PLL.The results of quantitative analysis on the stable region can provide the theoretical foundation of the reduction of the oscillation risk in the grid-connected PMSG system,and offer guidance to the control parameter design and optimization of PMSG under different operation conditions.(3)Considering the possible frequency resonance existing between the nominal system frequency and twice-nominal frequency of the impedance-frequency characteristics of the practical network,the stability of GSC current control loop and PLL control loop of the grid-connected PMSG system with network resonance is studied by impedance characteristics analysis.Meanwhile,the root loci and participation factors are calculated to reveal the critical factors of control loops instability problem.After that,the impacts of the network resonance on the design of the control parameters are quantitatively analyzed based on the stable region.The results reveal that the network resonance has great influence on the stability of the grid-connected PMSG system and evidently shrinks the stable region of critical parameters.Then,the SSO problem may take place even if the system strength is supposed to be high enough to maintain stability.The network resonance plays a dominant role in the SSO mode.A conclusion can be drawn that not only the SCR but also the network resonance should be properly considered to assess the oscillation risk of the grid-connected PMSG system.(4)The model of two aggregated PMSGs based wind farm integrated to power system is built firstly.Then,the oscillation characteristics of common-mode and anti-mode existing in the grid-connected PMSG system are revealed.After that,considering the difference of parameters or operation conditions,the influence of the critical parameters on the oscillation modes is investigated,and the stable region and the dominant PMSG are determined.Conclusions can be drawn that the grid-connected PMSG system has the common-mode where the PMSGs oscillate together against the network,and the anti-mode where the PMSGs oscillate against each other exists as well.The common-mode is the dominant oscillation mode,while the anti-mode will dominant the dynamic response of the PMSG system under some scenarios.Then,the oscillation component of anti-mode generates circulation among the PMSGs in the wind farm and it will not propagate to the network.Hence,the oscillation is difficult to be detected.Furthermore,there are some scenarios where both of the common-mode and anti-mode would turn unstable.The stable regions of critical parameters of PMSGs will interact with each other.Based on the eigenvalue analysis the dominant PMSG can be found,which will offer guidance to the coordinated design of control parameters among the PMSGs in the wind farm and oscillation mitigation.(5)Based on the case of complex power system,the state-space model of complex system composed of PMSGs-based wind farms,steam turbine generators and HVDC transmission system is built.According to the physical mechanism of eigenvector,a method is proposed for assessing the interaction among devices in the complex system and analyzing the oscillation path.Then,the control interaction and frequency coupling among multiple PMSG-based wind farms,and the mechanism of the influence of the PMSG-based wind farms on the stability of steam turbine generator shaft are studied.The results indicate that there are common-mode and anti-mode in the multiple wind farms integrated to power system as well.The control interaction among the wind farms may result in the instability of the anti-mode where the wind farms oscillate against each other within their local area.Based on the proposed method,the oscillation path,dominant oscillation mode along with the related wind farms are determined,and the common-mode as well as the anti-mode can be clarified.At the same time,the multi-frequency components derive from the frequency coupling and they vary with the dominant modes,which would determine the dynamic response of system.The common-mode of the wind farms might propagate to the distant area.Then,if the oscillation frequency of common-mode closes to the shaft natural frequency,the shaft would suffer from the forced torsional vibration,affecting the stable operation of steam turbine generators and power system.