What is the harm of transformer three-phase imbalance?
Three-phase imbalance means that inconsistent amplitude of three-phase current (or voltage) in electrical power system and that amplitude difference surpasses the specified range.
Unbalanced current of transformer refers to current difference of three-phase transformer windings. The current difference is mainly caused by different three-phase load. Asymmetry of load gives rise to asymmetry of three-phase current which flow through transformer. And current asymmetry causes asymmetry of three-phase impedance drop of transformer, thus three-phase voltage of secondary side is asymmetric, which will do harm to transformer and electric equipment.
What’s more important, in transformer of Y/Y-12 wiring, the null line will have zero-sequence current. And zero-sequence current will produce zero-sequence magnetic flow. Winding will sense the zero-sequence potential to cause displacement of neutral point. And one-phase voltage with big current will drop, whole other two-phase voltage will increase. In addition, it will also do harm to full application of power output of transformer.
Harms of three-phase imbalance
1. Increase electric energy loss of circuit. In power supply network of three-phase four-wire system, when current pass through wires of circuit, since impedance will surely cause electric energy loss, its loss is in direct proportion to square of voltage. When low voltage power network adopts the three-phase four-wire system, since there is one-phase load, imbalance of three-phase load is unavoidable. When three-phase load operates off balance, current will pass through neutral line. In this way, besides phase line having losses, neutral line will also have losses, thus increasing loss of grid lines.
2. Increase electric energy loss of distribution transformer. Distribution transformer is main power supply equipment of low voltage power network. When it operates under the condition of imbalanced three-phase load, losses will be increased since power loss of distribution transformer changes together with degree of unbalance of load.
3. Decrease of contribution of distribution transformer. During design of distribution transformer, its winding structure is designed according to load balancing operation. Performance of winding is basically the same and rated capacity of various phases is equal. Maximum contribution of distribution transformer is restricted by rated capacity of each phase. When distribution transformer operates under the condition of three-phase load imbalance, one-phase with small load will have bigger capacity than needed, thus reducing contribution of distribution transformer. And reduction degree of contribution is related to unbalance degree of three-phase load. The bigger the unbalance degree of three-phase load, the more contribution distribution transformer will reduce. Therefore, when distribution transformer operates under the condition of three-phase load imbalance, the output capacity cannot reach to the rated value, and the reserve capacity will reduce correspondingly together with overload capacity. When distribution transformer operates under the condition of overload, it is easy for heating of distribution transformer, and even causes burning of distribution transformer.
- Anti-interference effect: The isolation transformer can prevent the transmission of some harmonic waves after a Y/△ wiring connection.
- Impedance conversion effect: Increase the system impedance, thus making it easy to cooperate with the protective device.
- Stabilize the system voltage: When starting a heavy load equipment, it helps reduce the impact on the system voltage.
- Prevent system grounding: When a single-phase grounding occurs on the load side of the isolation transformer, it prevents the single-phase grounding from happening to the whole system (upstream parts of the isolation transformer).
- Reduce the short-circuit current: When there’s a short-circuit at the load side, it restricts the short-circuit current of the system.