Three-phase power systems are designed to operate in a balances mode therefore most of the analysis of such systems assumes balanced conditions. For this reason, it is convenient to simplify 3-phase circuits by showing only one phase as in Fig. 19.
I do understand that in each phase or leg of the three-phase device, the phase voltage and phase current waveforms are in general out of phase, and thus, there's a non-zero single-phase reactive power in each leg of the three-phase device.
Some of the reasons for current unbalance (or imbalance) are: In three phase system, voltage unbalance occurs when phase or line voltage differ from nominal balanced condition. Normal balanced condition is when the three phase voltages are identical in magnitude and are displaced 120 degree vectorially.
The three-phase system configuration consists of three alternating currents (also known as phases) that are generated and transmitted simultaneously. These phases are referred to as Phase A, Phase B, and Phase C. Figure 15: Three-phase AC The three-phase system can be connected in two methods: Delta (Δ) and Wye (Y or Star) configurations.
There are two reasons for configuring alternating current in three phases: economy of transmission, and efficiency of power conversion in rotating machines. The choice of delta or wye connection offers one way to select different voltage levels for loads from the same three‐phase power supply.
Consider the three-phase power system depicted in Fig. 2.22. The rated powers and voltages of the system components are provided below: Generators G 1 and G 2: 500 MVA, 20 kV. Transformers T 1 and T 2: 200 MVA, 500/18 kV. Transformers T 3 and T 4: 150 MVA, 500/20 kV. Motor M: 111 MW, cos ϕ = 0.8 (inductive), 20 kV.