Design of Medium voltage
Design of Medium voltage
Circuit
breakers types
We have three main types of circuit breakers according to the voltage
1. High voltage circuit breakers.
2. Medium voltage circuit breakers.
3. Low voltage circuit breakers.
We have different types of low
voltage circuit breakers:
- Air type.
- Moulded case type.
- Miniature type.
We can observe the difference between the three kinds of
the circuit breakers as follows:
- The volume of the circuit breaker is proportional depends on the
operation voltage.
-We can see that the arc control technique differes from type to another
according to the voltage.
· In case of high voltage circuit breakers we use the SF6.
· In case of medium voltage circuit breakers we use vaccum and SF6.
· In case of low voltage circuit breakers we use air.
3.3.1 Medium
voltage circuit breakers
Introduction
Medium voltage circuit breakers are
used in industrial and distribution substation power plants the earlier designs
had bulk-oil circuit breakers and later minimum oil circuit breakers but now.
The SF6 circuit breaker and vacuum circuit breaker are becoming very popular
due to their non-explosive and maintenance free performance and better
capabilities for repeated and reliable switching under various switching
condition.
Rating
Rated voltage: The maximum operating voltage
at normal conditions.
Rated breaking current: the maximum value of r.m.s symmetrical current
that the C.B can interrupt safely at a power factor of 0.2 - 0.25.
Rated making current: the maximum value of current
that can pass in the C.B at closing.
Rated short-time current: the maximum r.m.s symmetrical current that can
pass in the C.B at closing.
Rated impulse withstand voltage: the maximum impulse
voltage that insulation with stand.
Rated frequency.
Rated insulation
voltage: the designed
insulation voltage
.
Advantages
of SF6 C.B for Medium Voltage
·
Non inflammable, non poisonous extinguishing
and insulating medium.
·
The volume of SF6 gas can be checked at all
times on the manometer installed at the front.
·
Separate load time breaking current circuits.
·
Good interrupting characteristics even at low
values of current. No current chopping and no switching oven voltage.
·
Enclosed extinction chamber system.
·
The breaking chamber can be inspected and
overhauled.
·
Simple and easy to maintain spring-operation
mechanism.
·
The spring-operating mechanism can be charged
by motor or manually hand crack.
Advantages
of Vaccum C.B fur Medium Voltage
·
Non –
flammable , non-toxic.
·
Sealed
interrupter, sealed for life.
·
Very
long switching life.
Application
· Urban transformer
substations in distribution system.
· Substations and internal distribution in
large industrial plants.
·
Switchgear for pumping stations.
·
Substations for large residential buildings.
·
Substations for Air-ports, realawy stations
etc.
Total
cost
This term denotes the
cost arising from purchase, operation and servicing with respect to a specific
required switching function .it is important to take the sum of all these costs
into consideration. SF6 and Vacuum circuit breakers have very modest maintenance
requirement. Hence they are economical during their long service life (amount 20years).
Mechanical
performance
