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)



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


#Result:
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)


#Calculation:
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



#Result:
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)


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


#Result:
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)


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


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

Voltage across breaker contacts at chopping is 866.0 kV