Control method for multi-phase cyclo converters

用于产生额定功率为100兆瓦的电能的涡轮发电机组通常直接连接到电网。因此，涡轮在固定转速下运行，导致部分负荷时效率下降。如果能在部分负荷时调节汽轮机转速，则效率增益可达25%。矩阵或循环转换器可以提供频率-弹性耦合，并允许涡轮机的变速操作。由于高稳健性和小损耗，晶闸管变换器在非常高的额定功率范围内尤其受到青睐。不幸的是，标准的三相循环变换器只能在发电机和电源之间使用高达1:3的频率比，这里没有给出。然而，如果相位的数量增加（例如，高达N = 27），扭曲将在一个可接受的水平。这种多相环控变换器的最先进的控制方法是从标准环控变换器的控制方法中衍生出来的，并且以相当高的换相频率操作变换器，这与提高效率的目标相矛盾。为了降低换相频率的建议已经提出，但这包括其他缺点，如较大的总谐波畸变。该项目的目标是改进该变换器概念的控制，以少量换流来运行变换器，以最大限度地减少损耗，并考虑其他优化目标，如总谐波失真（THD）和变换器电压利用率。方法学上的方法是使用空间矢量调制，到目前为止还没有应用于这种变换器，可能是因为要处理大量的状态（有N = 27个相位，结果为N^3 = 19683个状态），这些状态随着时间的推移而进一步变化，而且还因为换相不能用外部换相变换器任意触发。利用模型预测控制（MPC）在转子固定参考系中实现直流控制是解决这一复杂问题的一种很有前途的方法。模型预测控制通过考虑控制误差、预测控制变量的变化率（与THD近似成正比）和换相电压的目标函数来选择最佳开关状态。通过这种方式，开关损耗有望减少约1 / 2。

Turbine generator sets for generation of electric energy with power ratings of several 100 MW are typically connected directly to the electric grid. Thus, the turbine is operated at fixed speed, resulting in a drop of efficiency at partial load. If the turbine speed could be adjusted at partial load, the gain of efficiency would be up to 25%. Matrix or cyclo converters can provide frequency-elastic coupling and would allow speed-variable operation of the turbine. Due to high robustness and small losses, thyristor converters are favoured particularly in the range of very high power rating. Unfortunately, the standard three-phase cyclo converter can only be used up to frequency ratios up to 1:3 between generator and mains which is not given here. However, if the number of phases is increased (e.g. up to N = 27), the distortions would be within an acceptable level. The state-of-the-art control method for such a multi-phase cyclo converter is derived from that of the standard cyclo converter and operates the converter with a rather high commutation frequency, which contradicts the goal of efficiency improvement. A proposal in order to reduce the commutation frequency has been presented, but this includes other drawbacks as larger total harmonic distortions.Goal of this project is to improve the control of this converter concept to operate the converter with a small number of commutations in order to minimize losses as well as to consider other optimization targets as the total harmonic distortion (THD) and the converter voltage utilization. Methodological approach is the employment of space vector modulation which so far has not been applied to such converters, likely because of the large number of states to be handled (with N = 27 phases, N^3 = 19683 states result), which are furthermore varying in time, and also because the commutations cannot be triggered arbitrarily with externally commutated converters. The implementation of a Direct Current Control in a rotor-fixed reference frame by means of Model Predictive Control (MPC) is a promising way to solve this complex task. Model Predictive Control will select the best switching state with help of an objective function that considers the control error, the rate of change of the predicted control variable (which is approximately proportional to THD) and the commutation voltage. In that way, the switching losses are expected to be reduced by about a factor of two.