Zero Sequence Vs. Negative Sequence

Negative and Zero Sequence

• Negative sequence calculations measure the amount of imbalance the power system is undergoing. Zero sequence, on the other hand, are calculations associated with the ground having an imbalanced existing condition. The negative and zero sequence are part of the three-phase power system that makes the system balanced due to the voltages and currents of the phases having the same amplitude and shift direction with respect to each other. When there is an imbalance, there is usually a change in either voltage, current or direction in one of the phases. The negative and zero sequence phase unbalances are used to determine if a faulty condition exists within the power system.

Positioning of Negative and Zero Sequences

• The negative and zero sequences are positioned in a specific manner to create a balance of voltage and current in the power system. For instance, a set of negative sequence voltages would be equal in magnitude and displaced at 120 degrees apart in a counter clockwise rotation sequence. A set of zero sequence voltages would be set up to be equal in magnitude and in phase with each other, with no rotation sequence.

Results in Balanced Power Systems

• Calculations of the symmetrical components in a three phase balanced power system results in positive sequence voltages only. The power systems are electricity systems that are constructed with a symmetrical geometrical shape of overhead electric lines that are symmetric with respect to the ground. The positive sequence voltages or phases are usually supplied by generators and are therefore always present. In a balanced system, the currents have equal magnitudes, which produce positive sequences only with no negative or zero sequence currents present. The negative and zero sequence currents only exhibit voltage or current when there is an unbalance in the electric lines of the power system.

Capacity of Power

• The negative sequence portion of the three phase power system reduces the capacity for voltage and current in transformers, cables and lines. Therefore, a power supply company needs to consider the negative sequence phase when figuring out what they want their maximum capacity of voltages and currents to be. When considering the negative sequence in the mix, a larger system may be necessary to handle the load and maintain the balance the power system needs.