20-07-2013, 03:38 PM
Fault Analysis Symmetrical Components
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Fault Analysis
• The cause of electric power system faults is
insulation breakdown
• This breakdown can be due to a variety of
different factors:
• Lightning.
• wires blowing together in the wind.
• animals or plants coming in contact with the
wires.
• salt spray or pollution on insulators.
Fault Types
• There are two main types of faults
• Symmetric faults: system remains balanced; these faults
are relatively rare, but are the easiest to analyze so we’ll
consider them first.
• Un-symmetric faults: system is no longer balanced; very
common, but more difficult to analyze.
• The most common type of fault on a three phase system by
far is the single line-to-ground (SLG), followed by the line-to-
line faults (LL), double line-to-ground (DLG) faults, and
balanced three phase faults.
Fault Analysis Solution Techniques
•Circuit models used during the fault allow the network to be
represented as a linear circuit
•There are two main methods for solving for fault currents:
1.Direct method: Use pre-fault conditions to solve for the
internal machine voltages; then apply fault and solve
directly.
2.Superposition: Fault is represented by two opposing
voltage sources; solve system by superposition.
Analysis of Un-Symmetric Systems
• Except for the balanced three-phase fault, faults result in
an unbalanced system.
• The most common types of faults are single line-ground
(SLG) and line-line (LL). Other types are double line-
ground (DLG), open conductor, and balanced three phase.
• System is only unbalanced at point of fault!
• The easiest method to analyze unbalanced system
operation due to faults is through the use of Symmetrical
Components
Symmetric Components
• The key idea of symmetrical component analysis is to
decompose the system into three sequence networks.
The networks are then coupled only at the point of the
unbalance (i.e., the fault)
• The three sequence networks are known as the
• positive sequence (this is the one we’ve been using).
• negative sequence.
• zero sequence.
Grounding
• When studying unbalanced system operation how a
system is grounded can have a major impact on the fault
flows
• Ground current only impacts zero sequence system
• In previous example if load was ungrounded the zero
sequence network is (with Zn equal infinity):