Modelling And Small Signal Stability Analysis Based On Circuit For Renewable Energy Integrated Systems In Weakly Synchronized Grid

The increasing penetration of renewable energy resources in power systems helps relieve the both environmental and energy concerns.However,with the increasing amounts and capacity of renewable energy devices,the dynamic characteristics of power systems that dominated by synchronous machines are changed,which decreases the AC grid's voltage support and transforms the AC grids into weak grids.In a weak AC grid,there exists complex couplings among renewable energy devices and between renewable energy devices and AC grids,which may cause oscillation issues.Compared with traditional oscillation problem,the oscillations caused by the interactions among renewable energy devices and AC grid are more complicated and lack for suitable small signal stability analysis methods.To this end,this thesis propose a modelling and stability analysis and method based on complex circuit,to deal with the two main reason of the small signal stability issues:coupling of between devices and AC grid and the weak grid.The proposed method transforms the oscillations analysis of renewable energy devices integrated system into the resonance of the primal-dual complex circuit,which can used for the system's stability analysis and AC grid's strength assessment.The main contributions of this thesis are as folestablishing the equivalent primal-dual complex circuit in complex space is proposed.Then the lows.Firstly,for the single renewable energy device integrated system,the method that stability mechanism and admittance shaping method are analyzed.1.The concepts of primal component and dual component in the complex space are put forward in this thesis such that the coupled-frequency oscillation components can be described exactly,which are a pair of vectors with the same frequency and conj ugated form.On this basis,the method that establishing primal-dual complex circuit is proposed.The primal-dual complex circuit reflects the coupling of between the device and AC grid,which can used for the analysis of system's small signals stability and the responses under perturbations.Moreover,the relationship between the impedance in primal-dual complex circuit and the actual system's transfer function is analyzed.It is shown that the small-signal instability of the actual system is equivalent to the resonance of its primal-dual circuit.2.The concept of modal power is put forward for analysis and control of renewable energy devices system,which is the extension of the concept of power in the RLC circuit.The relationship between modal power and system stability is derived and the resonance mechanism of primal-dual complex circuit is analyzed.On this basis,the relationship between system stability and the converter's admittance is analyzed.It is shown that the modal power's negative real part can indicate the system instability and there is no certain relationship between the converter admittances' real part and system stability.In addition,the principles of admittance shaping of power electronic devices in terms of increasing the system's damping ratio is analyzed based on modal power.An admittance shaping method of grid-connected converters is also given via phase compensation method.Secondly,for the multi renewable energy devices integrated system,the equivalent primal-dual complex circuit is established via using phase shifter.The generalized short circuit ratio is proposed for the strength assessment of AC grid.1.The phase shifter is used in complex circuit to deal with the admittance matrices with arbitrary forms,which realizes the unified modeling of multi renewable energy devices via primal-dual complex circuit.The phase shifter branch reflects the phase shifter between primal circuit and dual circuit caused by the non-diagonal elements of renewable energy devices'admittance matrixes.Moreover,the small signal model of multi renewable energy devices system is deduced,and further the equivalent primal-dual complex circuit is established.It is shown that the oscillation of multi-machine system is equivalent to the resonance of its primal-dual complex circuit.2.From the perspective of AC grid,the concept of generalized short circuit ratio is put forward for the quantitative assessment of the weak grid's strength of multi-infeed power electronic systems.First,the explicit characteristic function of short circuit ratio(SCR)of single infeed system is deduced and the relationship between the SCR and small signal stability problem is analyzed.Then the dynamic characteristics of multi infeed system with homogeneous power electronic devices are represented by n independent single infeed systems based on the multi-machine system's primal-dual complex circuit,thus the multi-infeed system can be decomposed into several single-infeed systems.Furthermore,the concept of generalized short circuit ratio is proposed to analyze the small signal stability of multi-infeed power electronic system,which can be used to depict the strength of AC systems and unify the concept of SCR in single and multi-infeed system physically and mathematically.Moreover,the participation factors of the gSCR are derived,which the impact of device nodes on the stability of multi infeed system.