Judgment Methods and Countermeasures for Three-phase Unbalance (1)
Three-phase unbalance is an important indicator of power quality. Although there are many factors affecting the power system, most of the normal unbalance is due to three-phase components, line parameters or load asymmetry. Because the factor of the three-phase load is not certain, the three-phase voltage and current of the power supply point are prone to unbalance and loss of line. Not only that, it will also adversely affect the motor on the power supply point, jeopardizing the normal operation of the motor. Therefore, if the three-phase unbalance exceeds the range that the distribution network can bear, the safe operation of the overall power system will be affected.
(1) The basic concept of three-phase unbalance
Three-phase unbalance refers to the inconsistency of the three-phase current (or voltage) amplitudes in the power system, and the amplitude difference exceeds the specified range. Due to the unbalanced load added by each phase power supply, it belongs to the fundamental wave load configuration problem. The occurrence of three-phase unbalance is related to the user's load characteristics, as well as the planning and load distribution of the power system. In the power grid system, three-phase balance mainly means that the three-phase voltage phasors are equal in size, and if they are arranged in the order of A, B, and C, the angle formed between them is 2n/3.
The three-phase imbalance refers to the inconsistency of the phasor size and angle. "Three-phase voltage permissible unbalance of power quality" (GB/T15543-1995) is applicable to AC rated frequency of 50 Hz. The standard stipulates that the allowable value of unbalance in the normal operation mode of the public connection point of the power system is 2%, and it shall not exceed 4% in a short period of time.
The three-phase current unbalance calculation method generally has the following two commonly used formulas:
Unbalance %=(maximum current-minimum current)/maximum current×100%
Unbalance %=(MAX phase current-three-phase average current)/three-phase average current×100%
for example:
The three-phase currents are IA=9A IB=8A IC=4A, then the three-phase average current is 7A, and the phase current-three-phase average current is 2A, 1A, and 3A, respectively, whichever has the largest difference. Therefore, MAX (phase current - three-phase average current) = 3A, so the three-phase current unbalance degree = 3/7.
