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Turbo-MMC: Minimizing the Submodule Capacitor Size in Modular Multilevel Converters with a Matrix Charge Balancer Yenan Chen, Youssef Elasser, Ping Wang, Jaeil Baek, MinjieChen Power Electronics Research Lab, Princeton University Background & Motivation Control Strategy for Turbo Operation Turbo MMC with a Matrix Charge Balancer ➢ Submodule Capacitors in Modular Multilevel Converters • Buffer pulsating power at low frequencies • The voltage ripple of submodule capacitors: V pp 1/f o and I o ➢ Modular Multilevel Converter based Motor Drive • Design submodule capacitor for maximum I o and lowest f o . • Capacitors take more than 50% of total size and weight. Pulsating Power in MMC Simulation and Experiments Fig. 3: Three ways of balancing the pulsating power. Simulation Parameters V dc 700V P o 6kW f mmc 10kHz f mab 100kHz L arm 4mH C sm 1mF L b 5H C b 0.1mF SM Number 4 Fig. 1: Typical startup process of a 3- ac induction motor: (a) phase current amplitude (b) phase current waveform and motor speed. Fig. 2: Schematic of a 3- MMC and Voltage ripple and pulsating power of the submodule capacitors in Phase A. SM SM SM SM SM SM SM SM SM SM SM SM SM SM SM SM SM SM 0.5V dc 0.5V dc L arm L arm L arm L arm L arm L arm L o R o Phase A Phase B Phase C i oa i ob i oc i ua i la P UA1 N UA1 P UA2 N UA2 P UAn N UAn P LA1 N LA1 P LA2 N LA2 P LAn N LAn P UB1 N UB1 P UB2 N UB2 P UBn N UBn P LA1 N LA1 P LA2 N LA2 P LAn N LAn P UC1 N UC1 P UC2 N UC2 P UCn N UCn P LC1 N LC1 P LC2 N LC2 P LCn N LCn E ua v cua v cla p ua p la 0 0 i oa i ob i oc ft Voltage ripple: v cua , v cla Ac circulating current: i za Pulsating power: p ua , p la Output current: i oa ➢ Pulsating power = ac output power + circulating power ➢ Reduce the voltage ripple in SM capacitors: • Sophisticated control strategy for power management • Active pulsating power balancing Matrix Power Balancing p ua p ua p la p la p ub p ub p lb p lb p uc p uc p lc p lc Phase A Phase B Phase C Upper Arm Lower Arm Vertical Power Balancing Horizontal Power Balancing Fig. 5: A matrix charge balancer implemented as a multi-active-bridge converter with a single magnetic core. 0 ft 100 0 -100 p.u. [%] 0 0 -0 p.u. [%] 0.0 0 -0.0 p.u. [%] E v E h E m p v p h2 p m p h1 Fig. 4: Percentage of residual unbalanced pulsating power. ➢ Vertical: partially balance power ➢ Horizontal: completely balance the output power, partially balance the circulating power ➢ Matrix: completely balance power ➢ Balance energy among submodule capacitors Vertical Horizontal Matrix MMC Central Controller v sm MCB Port + _ MMC SM H i G PI_i Φ i v ref Delay Global Synchronize Clock Global Enable Signal Distributed Phase Shift Controller Fig. 7: Distributed phase-shift control with a ripple voltage hysteresis for submodule capacitor voltage. V #1 V #j V #k N w :1 1:N w 1:N w L 1j L jk L 1k V #n N w :1 Fig. 6: Simplified ac power flow model of a matrix charge balancer. ➢ Power flow controlled by modulating the phase-shift: = 2 2 ≠ Φ −Φ 1− Φ −Φ v 0 V pp /2 — V pp /2 V h V h ➢ Deliver energy from/to a submodule capacitor by closed-loop control with hysteresis in voltage ripple Fig. 8: Simulated capacitor voltage ripple of the Turbo-MMC in five operating modes: (1) 0.0s -- 0.3s, low frequency (10 Hz) startup with Turbo mode ON, 20% load (1200 W); (2) 0.3s -- 0.6s, light load operation with Turbo mode OFF; (3) 0.6s -- 0.9s, output frequency increased to 50Hz with Turbo mode OFF; (4) 0.9s -- 1.2s, output load steps up to 100% (6000 W); (5) 1.2s -- 1.5s, high frequency heavy load operation with Turbo mode ON. Experiment Parameters V dc 24V P o 30W f mmc 50kHz f mab 200kHz L arm 7mH C sm 1mF L b 0.1H C b 0.1mF SM Number 4 Fig. 9: A Turbo-MMC prototype Fig. 10: Measured waveforms of the prototype Turbo-MMC with f o = 50 Hz. High voltage ripple even causes arm inductor saturation without Turbo operation. ➢ Summary: Turbo-MMC operation for startup process can 1. Reduce the submodule capacitor and arm inductor size. 2. Reduce the voltage ripple in submodule capacitors for low frequency and high power operation; ➢ Power flow controlled by time domain multiplexing. 3Φ - MMC Matrix Charge Balancer Half-bridge Module Branch Inductor L b 12-Winding Transformer One Port of MCB
