Chapter 19: Circuit Breakers

Example 19.1, Page Number: 483

In [1]:
from __future__ import division

#Variable declaration:
I = 1500                      #rated current of circuit breaker(A)
MVA = 1000                    #MVA rating of CB
V = 33                        #voltage rating of circuit breaker(kV)

Is = MVA*10**6/(3**0.5*33*1000)    #Rated symmetrical breaking current(A,rms)
Im = 2.55*Is                   #Rated making current(A,peak)

print "(i) Rated normal current is",I,"A"
print "(ii) Breaking capacity is",MVA,"MVA"
print "(iii) Rated symmetrical breaking current is",round(Is),"A  (peak)"
print "(iv) Rated making current is",round(Im),"A  (peak)"
print "(v) Short-time rating is",round(Is),"for 3 seconds"
print "(vi) Rated service voltage is",V,"kV (r.m.s)"
(i) Rated normal current is 1500 A
(ii) Breaking capacity is 1000 MVA
(iii) Rated symmetrical breaking current is 17495.0 A  (peak)
(iv) Rated making current is 44613.0 A  (peak)
(v) Short-time rating is 17495.0 for 3 seconds
(vi) Rated service voltage is 33 kV (r.m.s)

Example 19.2, Page Number: 484

In [1]:
from __future__ import division
import math

#Variable declaration:
f = 50                     #supply frequency(Hz)
V = 11                      #voltage rating of generator(V)
C = 0.01         #distributed capacitance upto ckt breaker b/w phase and neutral(uF)
XL = 5                   #reactance of neatral to earth(ohm)

L = round(XL/(2*math.pi*f),4)            #Inductance per phase(H)
Emax = round(2**0.5*V/3**0.5,2)     #Maximum value of recovery voltage (phase to neutral)(V
E = 2*Emax                          #Peak re-striking voltage(kV)
fn = 1/(2*3.14*(L*C*10**-6)**0.5)     #frequency of oscillations(Hz)

#Since peak re-striking voltage occurs at a time t given by
t = 1/(2*fn)                        #s
r = E/t                             #kV/sec

print "(i)  Peak re-striking voltage is",round(E,2),"kV"
print "(ii) Frequency of oscillations is",round(fn),"Hz"
print "(iii)The average rate of rise of re-striking voltage"
print "     upto the first peak is",math.floor(r/1000),"*10**3   kV/sec"
(i)  Peak re-striking voltage is 17.96 kV
(ii) Frequency of oscillations is 12628.0 Hz
(iii)The average rate of rise of re-striking voltage
     upto the first peak is 453.0 *10**3   kV/sec

Example 19.3, Page Number: 484

In [3]:
#Variable delaration:
t = 50*10**-6              #time to reach the peak re-striking voltage(s)
Vp = 100                    #the peak re-striking voltage(kV)

R = Vp/t                  #Average RRRV(kV/sec)
fn = 1/(2*t)                 #Natural frequency of oscillations(Hz)

print "Average RRRV is",R/10**6,"* 10**6   kV/sec"
print "Natural frequency of oscillations is",fn,"Hz"
Average RRRV is 2.0 * 10**6   kV/sec
Natural frequency of oscillations is 10000.0 Hz

Example 19.4, Page Number: 485

In [10]:
from __future__ import division

#Variable declaration:
Im = 11                   #magnetising current of transformer(A)
Ic = 7                    #chopped instantaneous value of current(A)
L = 35.2                  #inductance (H)
C = 0.0023                #capacitance(uF)

e = Ic*(L/(C*10**-6))**0.5         #Voltage across breaker contacts at chopping(V)

print "Voltage across breaker contacts at chopping is",round(e/1000),"kV"
Voltage across breaker contacts at chopping is 866.0 kV