The static characteristic

The Vd-Id characteristic of a rectifier-inverter system is shown in Figure 11-7.

Vd-Id Static Characteristic

Normally, the rectifier regulates the direct current to maintain it equal to the current reference. In abnormal cases (e.g. voltage drop in the AC power system) the alpha delay angle can reach its minimal value alpha_min and the rectifier will not be able to regulate the current. In such a case, it is said to operate in alpha_min mode.

The inverter regulates the direct voltage Vd measured at the DC line input to keep it equal to the voltage reference. In Figure 11-7 the inverter operates in voltage regulation mode. If the current drops below the current reference, the inverter goes in current-compensated mode where the voltage is regulated with a slope determined by the voltage margin Delta V and the current margin Delta I. If the current falls even lower than the (current referent margin), the inverter will operate in current regulation mode.

If the DC current increases (load increase or fault in the AC power system on the inverter side), the extinction angle gamma can reach the minimum value allowed and the inverter can lose its regulating capability. It then operates in gamma_min mode.