| High voltage direct current(HVDC)transmission has obvious advantages in long distance and large capacity transmission.In order to realize the interconnection of regional power grid,HVDC has become an important part of national power grid.At the same time,the structure and operation characteristics of power grid become more complex,and effective methods are needed to ensure the stable operation.Based on the AC transmission system,the high controllability of DC transmission can improve the stability of power system.DC power modulation is a kind of auxillary control mode,which extends the control capability of DC link.When the AC system is disturbed,it regulates the DC power transmission according to the state of AC system,in order to enhance the system damping and suppress inter-area power oscillation.The main work of this paper is summarized as the following:(1)A detailed analysis of the two-terminal HVDC transmission system is presented,including mathematical model and operating characteristics in different control modes.The power flow is obtained by unified iterative method and transient behavior simulation is carried out by alternating iterative method.(2)The manifestation and mechanism of low frequency oscillation are expounded and the analysis method is discussed.The mathematical model of Prony method is derived,the selection of parameters,including the sampling frequency,the length of time and model orders,etc,are addressed.The algorithm and program is verified numerically.(3)The model of DC control system is established,including the constant current control of converter,the fixed power control,the fixed voltage control,and the fixed distinguish angle control.The influence of DC power modulation on small signal stability of power system is analyzed by linearization method,and the procedure of small signal stability analysis is described.(4)Considering a four-machine-two-area system model,the influence of DC power modulation on power system transient stability is analyzed.The calculation results show that the DC power modulation can effectively damp the power oscillation and improve the stability of the system. |
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