The sequence impedance of a power system is crucial in understanding how the system behaves under asymmetrical fault conditions. The system’s performance can be determined by calculating the impedance offered by different elements of the power system to the flow of different phase sequence components of the current. In a power system, every component, whether it is static or rotating, has three impedance values- one for each symmetrical current value. The sequence impedance in a power system is classified into three categories- positive sequence impedance, negative sequence impedance, and zero sequence impedance.
Positive Sequence Impedance(Z1): The positive sequence impedance is the network’s impedance to the flow of positive sequence current. Positive sequence refers to electrical quantities that are numerically equal but displaced from each other by 120º. The positive sequence impedance is used to analyze the system’s behavior under normal operating conditions.
Negative Sequence Impedance(Z2): The impedance offered by an electrical network to the negative sequence current is called negative sequence impedance.The negative sequence current flows when the system experiences an unbalanced condition due to a phase loss fault, an unbalanced load, or a fault. The negative sequence impedance helps identify phase-to-phase or phase-to-ground faults.
Zero Sequence Impedance(Z0): The impedance that restricts the flow of zero sequence current is known as the zero sequence impedance. Zero-sequence current can occur due to unbalanced loads, transformer core saturation, or ground faults. The zero sequence impedance is crucial in analyzing the system’s behavior during ground faults and unbalanced conditions.
The ratio of the phase sequence voltage to the phase sequence current of the system determines the impedance of the positive, negative, and zero sequence components.
Various symmetrical components have no mutual impedance. To simplify asymmetrical fault calculations, each sequence impedance is considered separately.
Conclusion:
Understanding the behavior and performance of electrical systems under normal and unbalanced conditions is crucial for their reliable and efficient operation. Sequence impedance is a fundamental concept that provides valuable information for analyzing stability, protection system design, fault analysis, and overall system performance evaluation. Engineers can ensure the reliability and efficiency of electrical systems by accurately calculating and analyzing sequence impedance.