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This paper describes a three-phase trasformerless active filter system based on hybrid cascaded H-bridge converters with delta configuration. It is intended for dynamically compensating harmonic current of three-phase nonlinear load. This active filter is characterized by per-phase series connection of a high-voltage converter operating at 50 Hz and a low-voltage converter operating at 7.5 kHz. DC-link voltages of all converters are regulated purely by control algorithm without any auxiliary circuit. The dc voltage ratio of 2:1 is realized for the series-connected converters by injecting zero-sequence current with fundamental, third-harmonic, and fifth-harmonic components to the three clusters. An experimental prototype rated at 100 V and 1 kVA is built up in authors' lab. Experiment results confirm the validity of this dc voltage control method and harmonic current compensation capability.