In a three-phase AC- AC inverter , each phase consists of two sets of rectifiers, one positive and one negative, sharing the same AC input power. Therefore, if both sets of rectifiers are turned on simultaneously, a short circuit will occur, creating a large circulating current between the sets, which can burn out the thyristors and the power supply. To suppress circulating current, AC-AC inverters can be divided into two operating modes: with circulating current and without circulating current.
There is a circulating operation mode
When an AC-AC inverter operates in circulating current mode, both the positive and negative rectifiers can be turned on. When one group is in rectification mode, the other group is in inversion mode. In order to prevent the generation of a large low-frequency circulating current between the rectifier group and the inverter group, the average value of the rectified voltage output by the positive group should be equal to the average value of the inverter voltage output by the negative group. For this purpose, aN = p - aP must be made. In this way, in terms of average value, the output voltages of the two rectifier groups are equal in magnitude but opposite in direction.
Although the average output voltages of the two rectifiers are equal in magnitude and opposite in direction, their instantaneous values differ due to their opposite voltage waveforms, resulting in a voltage difference, as shown in Figure 1. As can be seen from the figure, except for aP = 0° and aN = 180°, the voltage difference is not zero in all other states. The instantaneous voltage difference is greatest when aP = 60° and aN = 120°, and the resulting circulating current is also the greatest.
(a) Positive group voltage (b) Negative group voltage (c) Inter-group voltage difference (d) Circulating current
Because the instantaneous voltage difference generates circulating current between the groups, a circulating current reactor is added between the groups in order to limit the magnitude of the circulating current in the operation mode with circulating current. In this way, the positive and negative groups can work at the same time, the load current is naturally switched, and the output current of the positive and negative groups is smoothly transitioned.
The advantage of circulating current operation is that the positive and negative rectifiers can automatically switch, making the output current of the AC-AC inverter continuous, smooth, and without dead zone, resulting in high system dynamic performance and good stability. However, its disadvantages include the addition of a circulating current reactor and circulating current, increasing the cost and losses of the device.
Non-circulating operation mode
Another operating mode for AC-AC frequency converters is the non-circulating current operation mode, which aims to eliminate the circulating current reactor, reducing system cost and circulating current losses. The working principle of non-circulating current operation is as follows: when the output current is positive, the positive rectifier group is activated, while the negative rectifier group is blocked, meaning the negative group is completely non-conductive. Conversely, when the output current is negative, the negative rectifier group is activated, while the positive group is blocked. Whether each rectifier operates in rectification or inversion mode is determined by the sign of the instantaneous voltage required at that moment.
To achieve circulating current-free operation, a relatively complex control system is required. First, the output current needs to be detected to determine its zero-crossing point (from positive to negative or vice versa) to establish the logic conditions for switching the gate signals from one rectifier group to the other. This presents challenges for the control system when the load current is discontinuous, as multiple zero-crossing conditions occur within a single output half-cycle, making true zero-crossing detection difficult. Furthermore, due to the discontinuous current, the inverter's average output voltage does not depend solely on the rectification angle α as it does when the current is continuous; the rectified voltage also depends on the degree of discontinuity of the load current.
Therefore, the advantages of the non-circulating current operation mode of AC-AC frequency converters are the absence of inter-group circulating current, low system cost, and low losses. Its disadvantages include the need to detect the zero-crossing point of the load current, increasing the complexity of the control circuit. Furthermore, in the event of control failure, it can easily cause both rectifiers to conduct simultaneously, creating a power supply short circuit between the groups. Another disadvantage is the distortion of voltage and current waveforms due to the dead time in the output current.
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