Research On DC Current Control Method Of VSC-HVDC Systems

The new power system with the stronger capacity of new energy consumption is the key carrier to help China achieve the strategic goals of carbon peaking and carbon neutrality.VSC-HVDC has become one of the main ways of large-scale new energy transmission due to its outstanding advantages such as rapid response,flexible control,can supply power to passive networks,and no reactive power compensation.However,new energy is random,fluctuating and intermittent,which is easy to trigger wide-band oscillations when connected to the VSC-HVDC systems,not only affecting the power delivery level,but also threatening the safe and stable operation of the whole system.It is of great significance to study the analysis and traceability of wide-band oscillations in large-scale new energy delivery through VSC-HVDC systems.The voltage and current on the AC side of the MMC converter station are coupled with the voltage and current on the DC side through the switching action of the submodule and the capacitance of the submodule,and the characteristics of the AC and DC sides affect each other.To address the problem that most of the existing research on the traceability of oscillations in VSC-HVDC systems focuses on the new energy to the AC side,ignoring the influence of the dynamic characteristics of the DC side on system oscillations,and the inability to establish an impedance model for impedance analysis of the new energy VSC-HVDC systems with rapid migration of operating balance points.this paper proposes a DC current control method for VSC-HVDC systems,which realizes the direct control of DC current,suppress abnormal fluctuation of DC current,and realize smooth control of DC side of new energy VSC-HVDC systems with rapid migration of operating balance points.This provides technical support for the analysis and traceability of wide-band oscillations in large-scale new energy delivery through VSC-HVDC systems.The specific research content and achievements of this paper are as follows:Firstly,this paper takes the new energy VSC-HVDC systems as the research object,analyzes its typical topology and basic operating principles,and clarifies the control methods and basic functions of the converter stations at the sending and receiving ends;introduces the capacitor voltage balance strategy of the MMC submodule;introduces the semi-MMC AC/DC power transmission method based on the power transmission principle of AC systems.Secondly,this paper introduces the topology of the converter station at the sending end of the new energy VSC-HVDC systems and proposes the control methods for the AC and DC sides of the the sending end converter station.The AC side adopts VF control,which achieves the grid-forming control of the sending end converter station by reasonably controlling the potential inside the MMC bridge arm and the AC side voltage of the converter station;The DC side adopts DC current control,which achieves direct control of the MMC DC outlet current based on the principle of overall energy balance of the bridge arm.Then,this paper introduces the topology of the converter station at the receiving end of the new energy VSC-HVDC systems and proposes the control methods for the DC and AC sides of the the receiving end converter station.The DC side adopts constant DC voltage control to achieve the construction of the DC voltage of the VSCHVDC systems;The AC side adopts the bridge arm current control method,which generates the d-axis reference current based on the principle of bridge arm energy balance.Combined with the calculated q-axis reference current and zero axis reference current,the bridge arm reference current is generated through coordinate transformation,and at the same time makes the bridge arm current track its reference value,achieving direct control of the MMC bridge arm current.Finally,this paper build an electromagnetic transient simulation model of the new energy VSC-HVDC systems based on the PSCAD/EMTDC platform,and the proposed method is verified from both steady-state and dynamic aspects.The results show that the proposed DC current control method for VSC-HVDC systems has good effectiveness and robustness,achieve the effect of suppressing abnormal fluctuations in DC current and achieve stable control of the DC side of the new energy VSC-HVDC systems with rapid migration of operating balance points.