Asymmetrical three-phase circuit

If there is a three-phase asymmetric power supply or a three-phase asymmetric load in a three-phase circuit, it becomes an asymmetric three-phase circuit. Asymmetric three-phase circuits do not have the characteristics described in the previous section and cannot be calculated using single-phase diagrams. In general, asymmetric three-phase circuits can be regarded as complex AC circuits and can be analyzed and calculated using general complex AC circuit methods, or other methods such as the symmetrical component method. This section only uses a simple example to analyze the basic concepts of asymmetric three-phase circuits.

The three-phase circuit shown in Figure 4-4-1 assumes that ,, are a set of three-phase symmetrical power supplies, and the load impedances ZA , _{ZB ,} and ZC are not equal, so it is an asymmetrical three- _{phase circuit. If a three-phase four-wire power supply is used, and the neutral line impedance can be ignored, it can be seen} from the figure that the load phase voltage is equal to the corresponding power supply phase voltage. Therefore, the current of each phase can be obtained as

Since the load is asymmetrical, the three-phase load current is also asymmetrical. The line current is generally not zero.

Figure 4-4-1

When the neutral line is disconnected (three-phase three-wire power supply), the voltage between the neutral point N and can be calculated as

At this time, even if the power supply voltage is symmetrical, the voltage between the two neutral points is not zero, and the neutral point is not equipotential. This phenomenon is called load neutral point displacement. Figure 4-4-1b shows the phase diagram of the power supply and load voltages after the neutral line is disconnected. In the figure, the phase represents the magnitude of the load neutral point displacement. Obviously, when the midpoint displacement is large, it is bound to cause some phase voltages of the load to be too high, while some phase voltages are very low. Therefore, when the midpoint is displaced, a phase load may be damaged due to overvoltage, while another phase load may not work properly due to undervoltage. Therefore, in a three-phase power supply system, it is always best to distribute the loads of each phase symmetrically. Especially in civil low-voltage power grids, due to the existence of a large number of single-phase loads (such as lighting equipment, household appliances, etc.), and the load power consumption often changes, it is impossible to make the three phases completely symmetrical, so a three-phase four-wire system is generally used. No fuses or switches are installed on the neutral line, so that the load voltages of each phase are close to the symmetrical power supply voltage.