Abstract
This paper proposes an offset voltage control scheme for the modular multilevel converter (MMC) operated in nearest level control (NLC) to improve the total harmonic distortion (THD) of AC phase voltages. The offset (neutral-to-zero-point) voltage is adjusted so that the magnitude of each AC pole voltage maintains constant value with N + 1 level in the range of whole modulation index (MI). The validity of the proposed scheme was confirmed by computer simulations for the MMC with 22.9 kV/25 MVA and experimental works for the scaled MMC with 380 V/10 kVA. It was confirmed that the proposed control scheme can generate linearly variable AC phase voltages with improved THD in the over-modulation region as well as the normal-modulation region.
Highlights
A high voltage direct current (HVDC) system with voltage source converter has been widely operated to transmit electricity generated in the offshore wind farm to the land [1,2,3]
This paper proposes a new offset voltage control scheme for the modular multi-level converter (MMC) operated in nearest level control (NLC) to
This paper proposes offset voltage control scheme for the MMC
Summary
A high voltage direct current (HVDC) system with voltage source converter has been widely operated to transmit electricity generated in the offshore wind farm to the land [1,2,3]. The harmonic level of the output voltage is relatively low if the number of SMs is large enough. Since the HVDC system is operated with the DC voltage of 200–320 kV, the number of SMs in MMC is about 150–300 for each arm. The modulation scheme to form the output voltage normally uses the nearest level control (NLC) that generates stepped sinusoidal waveform. The modulation scheme to form the output voltage normally uses a phase shift carrier with pulse width modulation (PWM) or NLC considering the switching loss and harmonic levels [15,16,17]. PhaseThis shiftpaper carrier with pulse width modulation (PWM) scheme or NLC for considering switching proposes a new offset voltage control the MMCthe operated in loss. Its phase angle is equivalent to the lower arm voltage v jL and shifted by 180◦ from the upper arm voltage v jU
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