Transcript
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ECE4334
Dr. C.Y. Evrenosoglu
ECE4334
FAULT ANALYSIS
Dr. E
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ECE4334Content
System representation Single line-to-ground fault
Line-to-line fault
Double line-to-ground fault
Balanced (3) faults
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ECE4334System representation and assumptions
The power system operates under balanced steady-state conditions before
the fault occurs. Thus, the sequence networks are uncoupled before the
fault occurs. During unsymmetrical faults they are interconnected at the
fault location.
Prefault load current is neglected. This, prefault voltage, VF at the faultpoint is close to its nominal value (i.e. it can be take to be 10). The
prefault voltage at each bus (and machine internal voltages) in the
positive-sequence network equals VF. This assumption is called flat
profile. In defining the prefault flat voltage profile, we do take into account the
connection-induced phase shifts in Y xfmr.
Transformer winding resistances and shunt admittances are ignored.
Load impedances can be ignored
Series resistance and shunt elements in lines are ignored.
Armature resistance in generators is ignored.
The resulting circuit model consists only of sources and pure reactances.Dr. C.Y. Evrenosoglu
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ECE4334Using symmetrical components
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ECE4334Using symmetrical components
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ECE4334Example 9.1
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ECE4334Example 9.1
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ECE4334Example 9.2
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At bus 2 all the voltages are 0!
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ECE4334Example 9.2
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ECE4334Single line-to-ground fault
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ECE4334Single line-to-ground fault
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ECE4334Single line-to-ground fault on the example
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Remember?
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ECE4334Single line-to-ground fault on the example
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Remember?
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ECE4334Single line-to-ground fault on the example
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ECE4334Single line-to-ground fault
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ECE4334Example 9.3
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No fault impedance
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ECE4334REMEMBER: Example 9.1
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ECE4334REMEMBER: Example 9.1
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ECE4334Back to Example 9.3
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ECE4334Example 9.3
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ECE4334Line-to-line fault
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{
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ECE4334Line-to-line fault
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ECE4334Line-to-line fault on the example
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ECE4334Line-to-line fault on the example
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ECE4334Example 9.4
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ECE4334Double line-to-ground fault
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ECE4334Double line-to-ground fault
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D bl li d f l
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ECE4334Double line-to-ground fault
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ECE4334D bl li d f l h l
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ECE4334Double line-to-ground fault on the example
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ECE4334D bl li t d f lt th l
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ECE4334Double line-to-ground fault on the example
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ECE4334E l 9 5
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ECE4334Example 9.5
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ECE4334Example 9 5 (a b)
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ECE4334Example 9.5 (a,b)
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ECE4334Example 9 5 (a b)
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ECE4334Example 9.5 (a,b)
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ECE4334Example 9 5 (c)
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ECE4334Example 9.5 (c)
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ECE4334Example 9 5 (c)
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ECE4334Example 9.5 (c)
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ECE4334Example 9 5 (c)
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C 33Example 9.5 (c)
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ECE4334Example 9 5 (c)
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Example 9.5 (c)
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ECE4334Example 9.6
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Example 9.6
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ECE4334REMEMBER: Example 9.1
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REMEMBER: Example 9.1
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ECE4334Example 9.6
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p
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ECE4334Example 9.6
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p
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ECE4334Example 9.6
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p
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ECE4334Example 9.6
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p
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ECE4334Example 9.6
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Results by ignoringthe xfmr shifts
ECE4334Generalized solution for networks
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Assume that we know the pre-fault voltages (solution
of power flow)
The general procedure is given as follows:
1. Calculate Zbus for each sequence (first calculateYbus)
2. For a fault at bus i, the Zii values are the Thvenin
equivalent impedances; the pre-fault voltage is thepositive sequence Thvenin voltage.
3. Connect and solve the Thvenin equivalent
sequence networks to determine the fault currents
in each sequence.
Dr. C.Y. Evrenosoglu Thanks to Dr. Tom Overbye, University of Illinois for the content
ECE4334Generalized solution for networks
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4. Sequence voltages throughout the system can be calculated by
5. Phase values are determined from the sequence values
Dr. C.Y. Evrenosoglu Thanks to Dr. Tom Overbye, University of Illinois for the content
0
0
0
0
prefaultfIZ
= +
V V
M
M
This is solved
for each
sequencenetwork!
The entry
corresponds to the
faulted bus
ECE4334Unbalanced fault example for a network
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Pre-fault voltages are 1 pu at all buses. Pre-fault loads are ignored.
Transformer phase-shifts are ignored.
Calculate the bus voltages after a single-line-to-ground fault at bus
3.
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ECE4334Unbalanced fault example in network
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First step: Sequence networks
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ECE4334Ex.: Positive/negative sequence networks
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Form Ybus and subsequently calculate Zbus
j0.2 j0.05 j0.2j0.05j0.1
j0.1 j0.1
ECE4334Ex.: Positive/negative sequence networksF Y d b tl l l t Z
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Form Ybus and subsequently calculate Zbus
j0.2 j0.05 j0.2j0.05j0.1
j0.1 j0.1
Thvenin equivalent
impedance at bus 1
of positive & neg.
sequence networks
Thvenin equivalent
impedance at bus 2 Thvenin equivalent
impedance at bus 3
ECE4334Ex.: Zero sequence network
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Form Ybus and subsequently calculate Zbus
j0.05 j0.05 j0.05j0.05j0.3
j0.3 j0.3
ECE4334Ex.: Zero sequence network
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Form Ybus and subsequently calculate Zbus
j0.05 j0.05 j0.05j0.05j0.3
j0.3 j0.3
Thvenin equivalent
impedance at bus 1
of zero sequence
network
Thvenin equivalent
impedance at bus 2Thvenin equivalent
impedance at bus 3
ECE4334EX: Sequence networks
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For a single-line-to-ground fault at bus 3 the Thvenin equivalent
impedances are used for bus 3 and since it is a bolted fault we do not have3Zf in the final circuit.
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For SLG fault the
sequence networks
are connected in
series
ECE4334EX: Calculate the sequence currents & voltages
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Bus voltages at each bus in each sequence:
ECE4334EX: Calculate the sequence voltages
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ECE4334EX: Calculate the sequence voltages
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ECE4334EX: Calculate the phase voltages at bus 1
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ECE4334EX: Calculate the phase voltages at bus 2
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ECE4334EX: Calculate the phase voltages at bus 3
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ECE4334What about faults on lines?
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The previous analysis has assumed that thefault is at a bus. Most faults occur on
transmission lines, not at the buses.
These faults are treated by including a
dummy bus at the fault location. How the
impedance of the transmission line is thensplit depends upon the fault location.
Dr. C.Y. Evrenosoglu Thanks to Dr. Tom Overbye, University of Illinois for the content
ECE4334Power system protection
M i id i t f lt i kl
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Main idea is to remove faults as quickly as
possible while leaving as much of the system as
intact as possible
Fault sequence of events1. Fault occurs somewhere on the system, changing the
system currents and voltages
2. Current transformers (CTs) and potential transformers(PTs) sensors detect the change in currents/voltages
3. Relays use sensor input to determine whether a fault
has occurred
4. When a fault occurs the relays open the circuit
breakers to isolate fault
Dr. C.Y. Evrenosoglu Thanks to Dr. Tom Overbye, University of Illinois for the content
ECE4334Power system protection
P t ti t t b d i d ith b th
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Protection systems must be designed with both
primary protection and backup protection in
case primary protection devices fail.
In designing power system protection systemsthere are two main types of systems that need to be
considered:
1. Radial: there is a single source of power, so poweralways flows in a single direction; this is the easiest
from a protection point of view
2. Network: power can flow in either direction:protection is much more complicated
Dr. C.Y. Evrenosoglu Thanks to Dr. Tom Overbye, University of Illinois for the content
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