| High proportion of power electronic equipment is one of the main characteristics of the new type power grids.While giving full play to its advantages of flexible power form transformation and rapid parameters adjustment,power electronic equipment is profoundly changing the stable operation mode of power grids and bringing new challenges,which are mainly reflected in:(1)Power electronic converters replace a large number of traditional synchronous machines,resulting in reduced inertia response and poor frequency regulation ability of the power grids;(2)The reactive power support and fault ride through ability of converters are insufficient.In the new energy power grids with DC transmission system,the transient voltage problems caused by commutation failure and DC blocking continue to occur;(3)Wide-frequency oscillation caused by the coupling of power electronic equipment on the new energy side and power grids side is complicate.With the gradual improvement of the power electronization of the new type power grids,various problems affect and promote each other,which has become a hidden danger affecting the safe and stable operation of power grids.The technical means and control methods of the traditional power systems with synchronous machines as the main generation units in response to the above challenges have been very mature.The advantages of synchronous machines,such as inertia response,reactive power regulation and excitation control,support the stability of power grids.Referring to the stability principle of traditional power grids,most of the existing studies have improved the control methods of power electronic equipment to simulate the excellent properties of synchronous machines,but the effect of synchronous machines cannot be achieved due to the limited tolerance range of power electronic equipment.Therefore,this paper puts forward the grid connection technology of motor-generator pair(MGP),tries to combine the operation characteristics of new energy converters with synchronous machines,endow the external performance of traditional power supply of new energy stations,and provide a possible solution to the stability of new type power grids.The main research contents and innovative achievements of this paper are as follows:(1)The MGP grid connected vector control method matched with the output power of new energy is proposed,and the feasibility analysis are carried out.Firstly,the physical basis of MGP is analyzed,the structure and working principle of MGP are introduced,its economy is discussed through efficiency and cost-effectiveness.and the application scenarios of MGP are further explored;Theoretically,the mathematical model of MGP is established and its power angle and power transmission characteristics are analyzed.On this basis,combined with the power prediction model of new energy units,the vector control method of new energy connected to grids through MGP is proposed;In terms of experiment,the bench of photovoltaic driven MGP connected to grids is established.In the experimental bench,the power angle characteristics of MGP and the active power vector control method are experimentally analyzed.The experimental results show that MGP can start up and be driven by the converters,and can successfully realize the basic functions of loading and grid connection;The proposed vector control method of MGP can effectively control the power transmission of MGP driven by new energy.(2)The promotion of MGP inertia support on system frequency stability is analyzed.Simulation and experimental research are carried out.In terms of the basic characteristics of MGP,the inertia support of MGP is analyzed,the frequency response model of MGP single machine is deduced,and the frequency response and disturbance suppression ability of MGP are studied;A multi-machine system frequency response(SFR)model of new energy with MGP is established.Combined with theoretical calculation and simulation model,the promotion effect of MGP on the critical permeability of new energy under the constraint of the key dynamic index of frequency response is analyzed;Finally,an experimental bench is built to analyze the inertia support and frequency response characteristics of MGP.The results show that the inertia support of MGP can reduce the power imbalance of the system and improve the frequency stability.(3)From the short circuit capacity support and reactive power regulation characteristics of MGP,the enhancement of MGP on voltage stability of new energy station is analyzed.Combined with the example calculation and simulation results,the promotion effect of MGP on the static voltage stability margin of new energy station is analyzed by P-V,VQ curve methods and short-circuit ratio index respectively;The reactive power regulation ability and fault ride through ability of new energy connected to grids via MGP are revealed.In the simulation models,the suppression effects of MGP on voltage drop and transient overvoltage in new energy power grids with DC transmission system are analyzed;The short-circuit capacity and fault ride through capability of MGP are verified in the experimental system.The results show that MGP can provide short circuit capacity after short circuit,and can help new energy successfully realize voltage fault ride through.(4)The suppression effect of MGP on the oscillation of new type power grids is explored.Based on the theoretical and experimental results,the feasibility of MGP to suppress the oscillation of power grids is analyzed from the aspects of MGP’s damping characteristics,harmonic isolation characteristics and enhancing the power grid strength;The linearized state models of singe new energy with MGP system and multi new energy system including MGP are established respectively.Combined with modal analysis method and time-domain simulation,the influences of MGP on the oscillation mode of source side new energy system and grid side power electronic converters are analyzed.The results show that MGP can improve the damping ratio corresponding to the oscillation mode of new energy,improve the power grid strength of the multi new energy system,and effectively isolate the mutual coupling of power electronic equipment on the source side and grid side,so as to suppress the oscillation of the new type power grids. |
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