Research On Imbalance Mechanism And Balanced Control Of Capacitor Energy For A Hybrid Modular Multilevel Converter

Due to the low cost,high efficiency and good dc short-fault ride-through(DC-FRT)capability,hybrid modular multilevel converter(MMC)containing half-bridge submodules(HBSMs)and full-bridge submodules(FBSMs)is a promising candidate in a MMC-based high-voltage dc(HVDC)system.To further increase the power transmission capability,hybrid MMC can also operate with a higher ac output voltage for a given dc-link voltage by increasing the number of FBSMs.In addition,hybrid MMC-based HVDC systems can operate at a reduced dc-link voltage to avoid flashovers under extreme atmosphere conditions.Thus,operations of hybrid MMCs can be divided into the following scenarios:normal operations(i.e.,HBSMs-based MMCs),DC-FRT operations,boosted ac output voltage operations and reduced dc-link voltage operations.When operating under the last three scenarios,due to the asymmetry operations of the upper and lower arms and the different charging and discharging characteristics of FBSMs and HBSMs,the imbalance of arm capacitor energy or the imbalance of capacitor voltage may occur.However,there are only a few studies on these two problems,and the available studies lack a depth analysis on their mechanism.In this paper,the mechanism of energy imbalance is focused.Further,advanced control methods are proposed.Details are as follows:(1)The mechanism of capacitor energy balance for the direct modulated MMC with half HBSMs and half FBSMs during normal operation is analyzed.Equivalent circuits of the fundamental-frequency component and dc component of the internal circulating currents are given.It's found that the fundamental-frequency component of the internal circulating currents can achieve the natural balance of the arm capacitor energy,and the dc component of the internal circulating currents can achieve the natural balance of the phase capacitor energy.Further,the total stored energy is balanced when the dc-link voltage is stable.When the circulating current suppression control is applied,parameter K_P damps both the fundamental-frequency and dc components,thus it can accelerate the natural balance of the arm and phase capacitor energy.(2)The phenomenon that arm energy is difficult to be balanced for the direct modulated MMC with half HBSMs and half FBSMs during riding through pole-to-pole(PTP)dc fault is analyzed,and an active damping method utilizing the circulating current controller is proposed.Further a control method to charge the HBSMs during the fault period is proposed.After the converter rides through the dc fault,capacitors of the inserted FBSMs,arm impedance and line impedance work as a two order oscillating circuit with variable coefficients for the discharge current,which make arm energy difficult to be balanced.Considerating that the dc fault current is zero under steady state during the fault period,an active damping method utilizing the circulating current controller is proposed.Besides,the general DC-FRT strategy considering the redundany of the arm voltage generation schemes under zero dc-link voltage is derived.Further an improved DC-FRT method which can charge the HBSMs during the fault period is proposed.Compared with the available DC-FRT schemes,the proposed DC-FRT scheme enhances the controllability of HBSMs'capacitor voltages,and maybe a promising scheme when converter rides through a long-term dc fault.(3)The mechanism of arm capacitor energy imbalance and the discharge problem on HBSMs'capacitors for the direct modulated MMC with half HBSMs and half FBSMs during riding through pole-to-ground(PTG)dc fault are analyzed.Further,improved methods are proposed.It's found that certain fundamental-frequency internal circulating current is excited during the fault period.Parameter K_P of the circulating current suppression controller and the arm impedance damp the above circulating current,and cause the arm energy imbalance.Capacitor voltages for the healthy pole are around their rated values,while those for the fault pole deviate from their rated values.To solve this problem,an fundamental-frequency circulating current feedforward compensation based arm energy balanced control method is proposed.In addition,due to the asymmetrical operation for the upper and lower arms,arm currents flowing in the fault pole are always negative for the sending terminal when converter operates under a high power factor.HBSMs'capacitors cannot be charged.However,the power consumption on the HBSMs discharges the capacitors,and the system cannot ride through a long-term(e.g.,several minutes or longer)reliably.To solve this problem,an improved PTG FRT with fundamental-frequency reactive circulating current(FFRCC)injection and the coordinate control of FBSMs'and HBSMs'output voltage is proposed.Based on FFRCC injection,arm currents flowing in the fault pole can be bidirectional without sacrificing the power factor at AC side.Coordinate control of FBSMs'and HBSMs'output voltages can achieve the fast and accurate charge of HBSMs'capacitors.Thus,the converter can ride-through a long-term PTG dc fault.(4)The mechanism underlying capacitor voltage imbalance in a hybrid MMC operating with boosted ac voltages and reduced dc-link voltages is analyzed in depth.First,the mechanism of capacitor voltage imbalance is explored through analyzing the charging and discharging process of HBSMs'and FBSMs'capacitors in a fundamental-frequency cycle.Further,the three main factors of modulation index,power angle and the ratio of FBSMs are summarized,and their effects on capacitor voltage imbalance are analyzed.Finally,an improved control strategy based on FFRCC injection is proposed to ensure capacitor voltage balance.Compared with the conventional solutions,this method does not reduce the power factor,increase the required number of FBSMs,or change the symmetric structure of the hybrid MMC.(5)Based on the experimental platform of three-phase FBSMs based MMC,experiments of grid connection,PTG FRT during the steady state and operations with boosted ac voltages and reduced dc-link voltages are performed,which demonstrate the validity and effectiveness of the analysis of the mechanism of arm capacitor imbalance and capacitor voltage imbalance,and the proposed control strategy,i.e.,the fundamental-frequency circulating current feedforward compensation based arm energy balanced control method,and FFRCC injection based capacitor voltage balanced control.