| To achieve the goal of energy conservation and emission reduction,China urgently needs to build a clean,low-carbon,safe and efficient energy system,realize"clean substitution" from the energy production side,and build a new power system with new energy as the main body.With a large number of new energy power generation equipment,electric vehicles and other two-way loads connected,the penetration rate of power electronic converters in modern power supply systems continues to increase,making the problems of low inertia and under damping of the system prominent.In order to enhance the inertia and damping of power system,virtual synchronous machine(VSM),which simulates the dynamic characteristics of synchronous motor,has become a research hotspot of renewable energy and controllable load grid connection technology represented by electric vehicles.The"source-network-load" system led by virtual synchronous machine provides an effective solution to the stability problem of modern power system.However,the input and output impedance mismatch of the "source-load" virtual synchronous machine is easy to cause the problem of interactive oscillation,which restricts the popularization and application of the "source-load" virtual synchronous machine system.The controller of the "source-load" virtual synchronous machine is complex and there are "source-load" multi-frequency coupling characteristics and harmonic current interference,resulting in difficulties in modeling the wideband impedance of the "source-load" virtual synchronous machine system,unclear interactive oscillation mechanism,complex system stability evaluation process and lack of suitable oscillation suppression methods.Supported by the National Natural Science Foundation of China,this thesis studies the wideband impedance modeling,interactive oscillation mechanism,stability criteria and oscillation suppression methods of the "source-load" virtual synchronous machine system,and provides a theoretical basis and practical guidance for the construction of a new power system with the renewable energy as the main body.The main innovations of this thesis are given as follows:(1)Considering the influence of nonlinear control delay and sampling filter,a wideband dq-frame impedance model of "source load" virtual synchronous machine is established.The impedance characteristics of virtual synchronous generator(VSG),load side virtual synchronous machine(LVSM)and traditional three-phase PWM rectifier are analyzed and compared.It is found that the dd-channel impedance of the traditional three-phase PWM rectifier presents a constant negative damping characteristic in the low frequency range,while the dd-channel impedance of the LVSM presents a V-shaped negative damping characteristic in the low-frequency range,and the diagonal impedance of the VSG does not show a negative damping characteristic.Considering the multi-frequency coupling characteristics of"source-load" virtual synchronous machine and the influence of harmonic current,the harmonic-domain transfer function-based αβ-frame Multiple-Input Multiple-Output(MIMO)impedance model of "source-load" virtual synchronous machine system is established.It is found that the αβ-frame MIMO impedance models are symmetrical.The accuracy of the impedance model is verified by the impedance measurement results.(2)The stability of the load virtual synchronous machine and the traditional three-phase PWM rectifier under the weak grid condition is compared and analyzed.The dd-channel impedance of the LVSM is easy to oscillate with the inductive weak grid in the frequency range below 10Hz,while the dd-channel impedance of the traditional three-phase PWM rectifier presents an impedance peak in the medium frequency band(60Hz-100Hz),which is easy to oscillate with the inductive weak grid.The dominant factor affecting the stability margin of the "source-load" virtual synchronous machine system is proved:the dd-channel impedance of the "source load" virtual synchronous machine is easy to intersect in the low frequency band,which does not meet the generalized Nyquist stability criterion,and then leads to system instability.Increasing the proportional and integral gains of the AC voltage controller of the VSG or reducing the proportional and integral gains of the DC-link voltage controller of the LVSM can enhance the system stability.The correctness of the interactive oscillation mechanism analysis of the "source-load" virtual synchronous machine system is verified by the simulation and experimental results.(3)To solve the problems of the complex impedance measurement of the traditional MIMO impedance analysis method when used in the practical engineering,a simplified stability criterion for the multi-frequency coupled three-phase AC system based on the measured voltage ratio is proposed.Considering the MIMO characteristics caused by the multi-frequency coupling of the three-phase AC system,it is equivalent to a single input single output(SISO)system in different frame.It is found that the measured voltage ratio on the source-load side is equal to the SISO impedance ratio.Furthermore,the equivalence between the simplified stability criterion in different frame and the traditional MIMO impedance stability criterion is proved by mathematical derivation.The effectiveness of the proposed simplified stability criterion for the multi-frequency coupled three-phase AC systems based on measured voltage ratio is verified by simulation and experimental results.(4)Aiming at the problem of interactive oscillation in the "source-load" virtual synchronous machine system,the voltage differential feedback and inductance current feedforward control methods of VSG are proposed,which enhances the positive damping of VSG and reduces its impedance amplitude.On the basis of ensuring that the dynamic characteristics of VSG voltage remain unchanged,the interactive oscillation between the VSG and LVSM is suppressed.Considering the "black box" of the controller of the "source-load" virtual synchronous machine,a system damping enhancement method based on the virtual resistance and capacitance remodeling of supercapacitors+static synchronous compensator(SCSTATCOM)is proposed.The dq-frame impedance of the proposed SCSTATCOM shows positive damping characteristics in the wideband-frequency range and the amplitude is small.After being connected to the "source load" virtual synchronous machine system,the equivalent source impedance of the system is significantly reduced,making the system source subsystem closer to the characteristics of the ideal voltage source and suppressing the interactive oscillation between the VSG and LVSM.Finally,the proposed methods are verified by simulation and experimental results. |
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