Research On Improving DC Feed Power For "Strong DC And Weak AC" Receiving-end Grid

In the transition period of UHV power grid construction,the power grid "strong DC and weak AC" form is remarkable,and the voltage stability problem of weakly-end power system is serious.This paper focuses on the voltage instability problem of"strong DC and weak AC" system and analyzed the hidden dangers in the DC process which mainly include the deterioration of the voltage regulation capability caused by the large-scale DC infeed instead of the conventional unit;the voltage drop in the near-zone caused by high power transmission from the DC concentrated point and the lack of reactive power support caused by the commutation failure;dynamic load instability and even voltage instability problems caused by the high ratio of motor load.In order to solve the above problems,three strategies are proposed in this paper:(1)The first is a generator scheduling optimization strategy for weak AC power grid:firstly,we construct a branch vulnerability assessment model to select serious branch breaking fault set and then,the bus voltage sag index is constructed to identify the set of nodes whose voltage stability is weak and finally,the electrical distance index is constructed to get the generator scheduling optimization sequencing strategy.When the power system needs to adjust the generator scheduling,preferentially shutting down the later units and retaining the front units as far as possible can effectively improve voltage support capacity of the generators.(2)The second is reactive power compensation strategy:Optimizing the switching of capacitors in the DC near-substation which is based on coupling degree index about the two-node voltage sensitivity relationship and voltage support strength index about the converter station voltage lifting capacity of unit reactive power compensation to select the alternative input substation and sort.Only putting the capacitors in the front substation as far as possible and installing large-capacity phase modifier in the converter station to make centralized reactive power compensation can improve the DC voltage stability level.(3)The third is load model allocation strategy:Strengthen the research on load model modeling and rationally allocate the various load ratios of each substation to correct the proportion of induction motor load in aggregate load,which can improve the slip recovery characteristics of induction motor,the voltage recovery capability,and the safe feeding of DC power level to the receiving grid.