Abstract
•This paper employs an amplitude modulation with sinusoidal plus third harmonic injection instead of trapezoidal modulation to manipulate the discharging and charging periods of the cell capacitors (during control transition and cell capacitor rebalancing modes) to:○Enable modulation index control of the CTB converter over the full range (0 to 1.155), independent of the cell capacitor voltage balancing and power factor.○Generate output ac voltage waveforms with reduced harmonic contents and low dv/dt.•Presents a method for sizing of the cell capacitors, and dedicated controllers for regulations of the cell capacitor voltages and limb currents.•Presents comprehensive simulations that validate the decoupling of the cell capacitor voltage from the modulation index and power factors, and examine the responses of the CTB converter during normal in weak and strong ac grids and ac and dc faults.•From detailed theoretical discussions and analysis and simulations, it has been concluded that the CTB converter based HVDC link offers a number of important features and trade-offs between the conventional half-bridge MMC and two-level voltage source converter such as the footprint, semiconductor loss, ac side waveform quality, and response to ac and dc network faults.
This paper employs an amplitude modulation with sinusoidal plus third harmonic injection instead of trapezoidal modulation to operate a controlled transition bridge (CTB) converter as ac/dc and dc/ac converter terminals. With such an operation, the CTB converter may require small ac filters; thus attractive for high-voltage direct current (HVDC) transmission systems. To facilitate ac voltage control over a wide range and black-start capability, the injected 3rd harmonic allows the cell capacitor voltages of the CTB converter to be regulated independent of the modulation index and power factor. The insertion of 3rd harmonic into modulating signals achieves two objectives: extends the regions around voltage zeros so that the total voltage unbalanced can be distributed between the cell capacitors, thereby exploiting the bipolar capability of the full-bridge cells in each limb; and to ensure that each limb can be clamped to the positive and negative dc rails every half fundamental period independent of the modulation index to allow recharge of the cell capacitors from the active dc link. The suitability of the CTB converter for HVDC type applications is demonstrated using a two-terminal HVDC link that employs a 21-cell CTB converter, considering normal operation and ac faults.