Abstract
This paper discusses the control of a modular multilevel converter (MMC) used as a grid interface for the klystron modulators in the compact linear collider (CLIC). The converter has a DC side load which takes short-duration power pulses, causing high DC side power fluctuations that are not tolerable if seen by the AC grid. The DC–AC power decoupling capability of the MMC enables mitigation of the power ripple on the AC side, guaranteeing compliance with power quality requirements. However, the pulse repetition rate of the CLIC modulators is synchronized the 50-Hz AC grid and this induces permanent power imbalance in the arms of the MMC, causing voltage deviation and overmodulation unless appropriate balancing strategies are implemented. Unlike existing arm balancing methods that control 50-Hz circulating currents to balance the arm powers, the method proposed in this paper introduces an augmented modulation strategy where modulation signals are redistributed among arms based on the demand from a balancing controller. The resulting controller has lower complexity and its simple structure enables an easier design of the balancing loop, which guarantees predictable dynamics in operation. The effectiveness of the method has been demonstrated in simulation for the full-scale CLIC converter ratings and experimentally on a 7-kW MMC prototype operating with a 3.3-kA pulsed DC load.
Highlights
Future high-luminosity colliders, such as the Compact Linear Collider (CLIC), require input power supplied to the accelerating cavities via high voltage, high power pulses
The Modular Multilevel Converter (MMC) shown in Figure 1 has been selected as a suitable topology [3] due to its modularity, efficiency and high quality AC waveforms [4]
Referring to the full-scale CLIC converter in Table I, the circulating current controller in Figure 6 has been initially designed for a nominal bandwidth of 3750 rad/s and phase margin (PM) of 89 â—¦, which corresponds to kcniormc p = 13.3 V/A and kcniormc i = 532 V/A/s
Summary
Future high-luminosity colliders, such as the Compact Linear Collider (CLIC), require input power supplied to the accelerating cavities via high voltage, high power pulses. The power electronic grid-interface for the klystron modulators has to be highly efficient and reliable, capable of processing high powers and must be able to prevent propagation of the pulsed power effects from the DC side to the AC grid [1], [2]. These are periodically discharged by the high current. Vij _ C1 vij _ C 2 vij _ Cn v i A, B,C j up, dn n number of submodules per arm
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