How is the sequence impedance used in fault analysis?

Short Answer:

Sequence impedance is used in fault analysis to understand how current flows during unbalanced faults like line-to-ground, line-to-line, or double line-to-ground faults. It represents the system’s response in terms of positive, negative, and zero sequence components, which simplify the analysis of asymmetrical conditions.

Each type of sequence impedance is linked to a sequence network. These networks are connected in specific ways depending on the fault type. By using sequence impedance, engineers can calculate fault currents, voltages, and protection settings accurately, especially in complex, unbalanced fault scenarios.

Detailed Explanation:

Sequence impedance in fault analysis

In real power systems, faults are rarely balanced. Most faults—such as single line-to-ground (L-G), line-to-line (L-L), or double line-to-ground (L-L-G)—are unbalanced, meaning the three phases are not affected equally. Analyzing such faults using regular circuit methods is very difficult and time-consuming.

To solve this problem, engineers use symmetrical components, where the unbalanced system is broken down into three separate balanced components:

  1. Positive sequence – normal balanced condition
  2. Negative sequence – reversed phase order
  3. Zero sequence – all three phases have the same magnitude and phase angle

Each of these components flows through a different type of sequence impedance:

  • Positive sequence impedance (Z₁): Seen by balanced currents under normal conditions.
  • Negative sequence impedance (Z₂): Seen by unbalanced currents flowing in reverse order.
  • Zero sequence impedance (Z₀): Seen by currents that return through ground or neutral paths.

These three impedances are used to build equivalent sequence networks, which greatly simplify unbalanced fault analysis.

Use in different fault types

  1. Single Line-to-Ground (L-G) Fault:
    • All three sequence networks (positive, negative, and zero) are connected in series.
    • The total fault current is determined by:

If=3VZ1+Z2+Z0I_f = \frac{3V}{Z_1 + Z_2 + Z_0}If​=Z1​+Z2​+Z0​3V​

    • This is the most common fault and is highly dependent on Z₀ (zero sequence impedance).
  2. Line-to-Line (L-L) Fault:
    • Only the positive and negative sequence networks are connected in parallel.
    • Zero sequence does not play a role because there is no path for zero sequence current.
  3. Double Line-to-Ground (L-L-G) Fault:
    • All three sequence networks are connected in a complex parallel and series combination.
    • This type of fault requires all three sequence impedances for accurate analysis.
  4. Three-Phase (L-L-L) Fault:
    • Balanced fault, so only the positive sequence network is used.
    • No need for Z₂ or Z₀ in this case.

Importance of sequence impedance in fault analysis

  • Simplifies unbalanced system calculations:
    Converts complex problems into easier, balanced network equations.
  • Helps in accurate fault current calculation:
    Each impedance affects the magnitude of fault current depending on fault type.
  • Essential for protection settings:
    Relay settings and coordination depend on accurate fault current estimates, which rely on Z₁, Z₂, and Z₀ values.
  • Supports equipment rating decisions:
    Helps in choosing correctly rated breakers, transformers, and grounding systems.
  • Identifies system weaknesses:
    High zero sequence impedance may indicate poor grounding, leading to slower fault clearance.
Conclusion:

Sequence impedance plays a crucial role in fault analysis, especially for unbalanced faults. By using positive, negative, and zero sequence impedances, engineers can model complex fault conditions accurately and simplify the calculations using sequence networks. This makes fault analysis more manageable and ensures proper system protection and design in power networks.