Three-phase unbalance is an important indicator of power quality assessment, although there are many factors that lead to three-phase unbalance in the power system, the three-phase unbalance is mostly due to the three-phase components lack of phase operation, unreasonable line parameter configuration (single-phase load mismatch imbalance), and the system power harmonics exceed the standard.
The three-phase imbalance will directly lead to the low voltage of the heavy phase, the neutral point flow current exceeds the standard, the overvoltage operation of the light load phase, the loss of the power supply line increases, and the safe operation of the electric transformer will be affected.
The basic concept of three-phase unbalance
Three-phase unbalance means that the amplitude of the three-phase current (or voltage) in the power system is inconsistent, and the amplitude difference exceeds the specified range. Due to the unbalanced load added by each phase power supply, it belongs to the fundamental load configuration problem. The occurrence of three-phase unbalance is related to the characteristics of user load, and also to the planning and load distribution of power system. In the power grid system, the three-phase balance mainly refers to the three-phase voltage phasor is equal in size, and if arranged in the order of A, B, C, the Angle between them is 2n/3.
The three-phase unbalance refers to the inconsistency of phasor size and Angle. "Power Quality Three-phase Voltage Allowable Unbalance Degree" (GB/T15543-1995) is applicable to AC rated frequency of 50 Hz. Voltage unbalance of PCC point connection point caused by negative sequence component under normal operation of power system. The standard stipulates that the allowable value of the unbalance degree under the normal operation of the public connection point of the power system is 2%, and it shall not exceed 4% for a short time.
Ideal three-phase waveform diagram and unbalanced three-phase waveform diagram
There are two common ways to calculate the three-phase unbalance
Unbalance %= (maximum current - minimum current)/maximum current x 100%
Unbalance degree %= (MAX phase current - three-phase average current)/three-phase average current x 100%
Here's an example:
The three-phase current is respectively IA=9A IB=8A IC=4A, then the three-phase average current is 7A, the phase-current - three-phase average current is respectively 2A 1A 3A, and the difference is the largest, so MAX (phase current - three-phase average current) =3A, so the three-phase current unbalance =3/7.