Determination of Operating Conditions of LCC for PCOV Calculation Based on Detailed Analysis of Commutation Overshoot

The peak value of continuous operating voltage (PCOV) has a direct influence on the reference voltage of valve arrester, which appreciably influences the manufacturing cost of the line-commutated converter (LCC). Since simulations with high accuracy based on broadband models are memory intensive and time consuming, it is important to determine the operating conditions for PCOV calculation to reduce computational burden. This study establishes piecewise expressions of ideal valve voltages in different converter operation modes, which is the basis for the investigation of commutation overshoots. A simplified circuit is established to evaluate the valve voltage stress, with a modification introduced on the calculation of voltage peaks across blocking valves, which is neglected by previous literature. Then, this study proves theoretically that the 6-pulse rectifier operation mode and the long-term overload power transmission with the maximum delay angle should be the corresponding operating condition for PCOV calculation. Conclusions derived from the analytical analysis have been validated by simulations based on broadband models for practical projects.