Control method for multi-phase cyclo converters

用于发电的额定功率为100兆瓦的汽轮发电机组通常直接接入电网。因此，涡轮机以固定的速度运行，导致在部分负载时效率下降。如果能在部分负荷下调节涡轮转速，效率提高可达25%。矩阵或循环变流器可以提供频率弹性耦合，并允许涡轮机的变速运行。晶闸管变流器由于具有较高的稳健性和较小的损耗，在功率额定值很高的范围内尤其受到青睐。遗憾的是，标准的三相交交变流器只能在发电机和电源之间使用频率比高达1：3的电源，这一点在这里没有给出。然而，如果增加相位数(例如，直到N=27)，则失真将在可接受的水平内。这种多相交-交变流器的最新控制方法是在标准交-交变流器的基础上发展而来的，其换流频率较高，这与提高效率的目标相矛盾。提出了一种降低换流频率的方案，但这也存在总谐波失真较大等缺点，本项目的目标是改进该换流器概念的控制，使换流次数较少的变流器运行，使损耗最小，同时考虑总谐波失真度(THD)和变流器电压利用率等其他优化目标。方法是采用空间矢量调制，这可能是因为要处理的状态数量很大(N=27相，N^3=19683个状态)，而且这些状态还在时间上变化，并且还因为换向不能用外部换向的转换器任意触发，所以到目前为止，空间矢量调制还没有被应用于这样的转换器。通过模型预测控制(MPC)实现转子固定坐标系下的直接电流控制是解决这一复杂任务的一种很有前途的方法。模型预测控制通过一个目标函数来选择最佳开关状态，该目标函数考虑了控制误差、预测控制变量的变化率(与THD近似成正比)和换相电压。通过这种方式，开关损耗有望减少约两倍。

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.