| Vigorously developing clean energy to replace fossil energy and establishing a safe and efficient energy system are not only the essential requirements of energy development,but also the central work of supply side structural reform in the energy field.In recent years,with the rapid development of new energy,voltage-source converter(VSC)-DC transmission technology appears in people's vision because of its unique advantages.However,due to the distribution characteristics of renewable energy in our country,most of them is at the end of the power grid without AC voltage support.The development trend of the future VSC-HVDC projects in our country is that the renewable energy at the transmission end is an island,in this case,the converter station can not control the input power.VSC grid has the characteristics of low inertia and weak damping,and the fault development speed is fast.When the new energy cut-off command is executed,single fault may develop into chain fault,leading to the collapse of the whole VSC-HVDC system.This paper focuses on the scene of VSC transmission system connected island renewable energy,studies its fault characteristics,optimizes the existing transmission topology,and proposes a new control strategy,which can realize the fault ride through(FRT)of VSC converter station.The main contents of this paper are as follows:(1)The DC side fault characteristics of VSCconverter station are studied.For the blocking of the receiving converter station,the expression of DC voltage is derived,its dynamic process is explained,the key factors affecting DC voltage are extracted,and the capacitance value of the sub module required to reach the protection setting value is inversely deduced.In view of the blocking of the sending converter station,a reasonable assumption is made,the speed of power regulation of the converter station with constant DC voltage is simulated,and the analytical solution of the bridge arm current is obtained.It is found that the change of the bridge arm current depends on the change speed of the DC current,and the increase of the capacitance and inductance has little effect on the delay of DC current rising speed.In view of the metal return line fault,the paper analyzes the short circuit and open circuit fault characteristics of the metal return line under the symmetrical and asymmetric operation of the converter station.(2)In order to solve the problem of surplus power caused by the fault of converter station in VSC-DC transmission system connected island renewable,the idea of reserve margin of converter station is put forward.In order to transmit as much power as possible,a multi-point embedded topology strategy is proposed,which makes the converter station at the sending end have electrical connection directly and can be backup for each other.When one pole occurs fault,the surplus power is transferred to the sound pole,and the margin design criteria of the converter station is proposed to ensure that the sound pole is still in the safe and stable operation range after the failure of the converter station.(3)For the multi-point embedded topology,the corresponding coordinated control strategy is proposed,which can realize the power transfer between the converter stations at the sending end and bring the fault pole power to the perfect pole.The corresponding simulation model is established on PSCAD/EMTDC,and the different control strategies are compared.Finally,it is concluded that when two voltage control poles are adopted F-P droop control and power control poles are adopted P-F droop control,the bridge arm current will not be over-current when the single pole at the sending end is locked,and when the single pole at the receiving end is locked,the DC voltage will not reach the protection setting value before the renewable energy switch off the plant,which has good transient stability and realizes the FRT of the VSC converter station. |
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