Three-phase power measurement method

　　Three-phase AC circuits are divided into three-phase three-wire and three-phase four-wire systems according to the different connection methods of power supply and load, and each system has different conditions during operation. According to the characteristics of three-phase circuits, there are three measurement methods as follows.

　　① One-meter method. Use a single- phase power meter to directly measure the power of any phase in a three-phase three-wire Y-shaped symmetrical circuit, and then multiply it by 3 to get the power consumed by the three phases, as shown in Figure 1 (a)

　　Figure 1 One-meter method for measuring active power in a symmetrical three-phase circuit

　　For a three-phase three-wire fully symmetrical circuit, the measurement can be carried out according to the wiring method shown in Figure 1 (b); however, if the midpoint of the circuit under test is not convenient for wiring, or the load cannot be disconnected, the measurement should be carried out according to the circuit shown in Figure 2. In the figure, the non- generator is connected to an artificial midpoint, that is, two impedances with the same impedance value as the voltage branch are connected in a Y shape as an artificial midpoint.

　　Figure 2: One-table method circuit using artificial midpoint

　　Figure 3 Circuit for measuring power of three-phase three-wire circuit using two meters

　　② Two-meter method. In a three-phase three-wire circuit, regardless of whether the circuit is symmetrical or not, the two-meter method shown in Figure 3 can be used to measure its power (or electric energy). Its three-phase total power P is the algebraic sum of the two power meter readings Pt and P2, that is,

 P=P1+P2

　　When using the two-meter method, two points should be noted. First, when wiring, the current coils of the two power meters should be connected in series to any two lines of the circuit so that the current passing through them is the line current of the three-phase circuit. The generator must be connected to the line where the current coil is located, and the other end must be connected to the third line without the current coil. Second, the sign must be taken into account when reading. When the power factor of the load is greater than 0.5, the sum of the readings of the two power meters is the total three-phase power; when the power factor of the load is less than 0.5, the pointer of one power meter will reverse. At this time, the two terminals of the current coil of the meter should be reversed to make the pointer deflect in the positive direction. The reading of the meter should be negative, and the total three-phase power is the difference between the readings of the two meters.

　　③ Three-meter method. In a three-phase four-wire circuit, regardless of whether it is symmetrical or not, three power meters can be used to measure the power of each phase, and then the three readings are added together to get the total power of the three phases. The wiring diagram of the three-meter method for measuring the active power of a three-phase four-wire circuit is shown in Figure 4.

　　Figure 4 Wiring diagram for measuring active power in a three-phase four-wire circuit using the three-meter method