Electrical Power Systems

190 Electrical Power Systems
8.2 RATED MVA INTERRUPTING CAPACITY O A CIRCUIT BREAKER
The circuit breakers rating requires the computation of rated momentary current and rated
symmetrical interrupting current computation of symmetrical short circuit current requires
subtransient reactances for synchronous machines. RMS value of momentary current is then
computed by multiplying the symmetrical momentary current by a factor of 1.60 to consider the
presence of DC off-set current.
The interrupting current of a circuit breaker is inversely proportional to the operating
voltage over a certain range, i.e.,
I ov = I r ×
V
V
r
ov
...(8.1)
Where
Iov = current at operating voltage
Ir = current at rated voltage
Vr = rated voltage
Vov = operating voltage
Note that operating voltage cannot exceed the maximum design value. Also rated interrupting
current cannot exceed the rated maximum interrupting current.
Therefore, three phase rated interrupting MVA capacity of a circuit breaker is given as
where
(MVA) rated-3f = 3 Vline ´ Iline
...(8.2)
r ric
V line r = rated line voltage (kV)
I line ric = rated interrupting current (KA)
Thus, three phase short circuit MVA to be interrupted, where
SC MVA (3f) = 3 Eo Isc × (MVA) Base ...(8.3)
where
|Eo| = prefault voltage (kV)
|ISC| = short circuit current (KA)
Note that MVArated–3f is to be more than or equal to the SC MVA (3f) required to be
interrupted. A three phase fault which is very rare gives the highest short circuit MVA and a
circuit breaker must be capable of interrupting it.
Example 8.3: Three 11.2 KV generators are interconnected as shown in ig. 8.5 by a tie-bar
through current limiting reactors. A three phase feeder is supplied from the bus bar of generator
A at a line voltage 11.2 KV. Impedance of the feeder is (0.12 + j0.24) ohm per phase. Compute the
maximum MVA that can be fed into a symmetrical short circuit at the far end of the
feeder.