Abstract
In multi-terminal dc networks or future dc grids, there is an important role for high step-ratio dc–dc conversion to interface a high-voltage network to lower voltage infeeds or offtakes. The efficiency and controllability of dc–dc conversion will be expected to be similar to modular multilevel ac–dc converters. This paper presents a modular multilevel dc–dc converter with a high step-ratio for medium-voltage and high-voltage applications. Its topology on the high-voltage side is derived from the half-bridge single-phase inverter with stacks of sub-modules (SMs) replacing each of the switch positions. A near-square-wave current operation is proposed, which achieves near-constant instantaneous power for single-phase conversion, leading to reduced stack capacitor and filter volume and also increasing the power device utilization. A controller for energy balancing and current tracking is designed. The soft-switching operation on the low-voltage side is demonstrated. The high step-ratio is accomplished by combination of inherent half-bridge ratio, SM stack modulation, and transformer turns ratio, which also offers flexibility to satisfy wide-range conversion requirements. The theoretical analysis of this converter is verified by simulation of a full-scale 40 MW, 200 kV converter with 146 SMs and also through experimental testing of a down-scaled prototype at 4.5 kW, 1.5 kV with 18 SMs.
Highlights
DC TRANSMISISON is becoming the preferred option for large-scaled renewable energy integration [1]
The rapid development of High Voltage Direct Current (HVDC) technology in last decade is facilitating the evolution of dc transmission from point-to-point connections to multi-terminal networks and dc grids [2]
There is no full-scale practical project for dc tap up to date, it has attracted much interest in recent years for both academic research and industrial development to satisfy the demand and architecture for future dc grids [10]–[16]. It could collect power from small-scale offshore wind farms (OWF) near the cable routes by tapping into the HVDC link directly [10], [11], and the low power throughput but high voltage ratio (LPHR) conversion can tap out a small fraction of the link power to service demand in remote communities with inadequate ac supplies but which are crossed by the HVDC corridors [12]–[14]
Summary
Xin Xiang, Student Member IEEE, Xiaotian Zhang, Member IEEE, Thomas Luth, Michael M. C. Merlin, Member IEEE, and Timothy C. Green, Senior Member, IEEE Abstract—In multi-terminal dc networks or future dc grids, there is an important role for high step-ratio dc-dc conversion to interface a high voltage network to lower voltage infeeds or offtakes. The efficiency and controllability of dc-dc conversion will be expected to be similar to modular multi-level ac-dc converters. This paper presents a modular multilevel dc-dc converter with a high step-ratio for medium voltage and high voltage applications. Its topology on high-voltage side is derived from the half-bridge single-phase inverter with stacks of sub-modules replacing each of the switch positions. A near-square-wave current operation is proposed which achieves near-constant instantaneous power for single-phase conversion, leading to reduced stack capacitor and filter volume and also increased the power device utilization. A controller for energy balancing and current tracking is designed. The soft-switching operation on the low-voltage side is demonstrated. The high step-ratio is accomplished by combination of inherent half-bridge ratio, sub-module stack modulation and transformer turns-ratio, which also offers flexibility to satisfy wide-range conversion requirement. The theoretical analysis of this converter is verified by simulation of a full-scale 40MW, 200 kV converter with 146 sub-modules and also through experimental testing of a down-scaled prototype at 4.5 kW, 1.5 kV with 18 sub-modules. Index Terms—Modular multilevel converter, compact volume, high step-ratio, dc grids. Control headroom Duty-cycle of the near-square-wave Sum of the energy stored in all capacitors Energy stored in the top stack and bottom stack Operation frequency, switching frequency SM sorting and selection frequency AC component of the bottom stack current Manuscript received April 18, 2017; revised August 17, 2017 and December 15; accepted December, 28, 2017. This work was supported by the Reconfigurable Distribution Networks project under EPSRC Grant EP/K036327/1 and the Delta Foundation Grant DREG2016009. There are no datasets associated with this work to report. X. Xiang, T. Luth, M. M. C. Merlin and T. C. Green are with the Electrical and Electronic Engineering Department, Imperial College iBcp iBdc iDi iH iL iN1 iN2 iTac iTcp iTdc iTs, iBs nBnSM nT, nB nTP R rS, rT To, Ts vBj vBl vCB vCT vDi vH vL vN1 vN2 vSM vTj vTl vTs, vBs δdc δSM Current through the bottom dc-link capacitor CB DC component of the bottom stack current Current through the rectifier diodes Di (i = 1, 2, 3, 4) Current on the high-voltage side Current on the low-voltage side Current on the transformer primary side Current on the transformer secondary side AC component of the top stack current Current through the top dc-link capacitor CT DC component of the top stack current Current through the top stack and bottom stack On-state SM number in the bottom stack SM total number SM number in the top stack and bottom stack On-state SM number in the top stack Power rating Overall step-ratio Stack modulation ratio, transformer turns-ratio Operation cycle, switching cycle Capacitor voltage of the jth SM in the bottom stack Voltage across the bottom arm inductor LB Voltage across the bottom dc-link capacitor CB Voltage across the top dc-link capacitor CT Voltage across the rectifier diodes Di (i = 1, 2, 3, 4) Voltage on the high-voltage side dc link Voltage on the low-voltage side dc link Voltage on the transformer primary side Voltage on the transformer secondary side Average value of the SM capacitor voltages Capacitor voltage of the jth SM in the top stack Voltage across the top arm inductor LT Voltage across the top stack and bottom stack Voltage tolerance of dc-link capacitor Voltage tolerance of SM capacitor Superscript for controller reference value
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