Basic Steps for Design of Circuit Breaker Types:
It is well known that a Circuit Breaker Types in an electrical power system has to perform the following four major operations.
- When the contacts are closed and the rated current passes, there should be least voltage drop across the contacts.
- When the contacts are open it should behave almost like an insulator.
- When under closed condition it is required to open its contacts, it should do so within the least possible time and without causing any dangerous disturbances like overvoltages to the system.
- When open, it should be able to close its contacts even when there is already a fault on the line.
These four major operations obviously call for many contradictory requirements to be fulfilled. Thus the problem of designing a Circuit Breaker Types has considerable difficulties to begin with. To complicate matters the phenomenon of electric arc is not uniquely explained by the available scientific data. There are several theories about the initiation of the arc and consequently as many suggested methods of extinguishing the arc. The material given in this chapter therefore gives the broad outlines of how these different contradictory requirements are taken into account and the various parts are to be designed.
Basic Steps Recommended for Design of Circuit Breaker:
The following are the main points to be considered for the design of circuit breaker:
- Finalizing the basic data and specifications.
- Choice of the Circuit Breaker Types.
- Preliminary design of the arc extinguishing system.
- Calculation of the insulating system of the circuit breaker.
- Design of independent elements in the breaker.
Under the last point listed above, the following conditions will have to be taken into account.
- Calculation of the electrodynamic forces in the current carrying
- Calculation of dimensions of the current carrying parts taking into account the temperature rise not only when carrying the rated current continuously, but also while carrying through fault current for a certain specified period.
- Design of the contacts should be such that they do not get overheated under normal working and also do not get welded together when closing on a faulted system.
- In case of air-blast Circuit Breaker Types the complete calculation of the pneumatic forces.
- The design of the mechanical system which opens and closes the contacts, etc.
Basic Data and Specification:
As far as the basic data to begin the design are concerned the following information should be available:
- Nominal voltage.
- Nominal current.
- Fault current which could be opened.
- Rupturing capacity.
- Time of operation.
- The thermal stability in other words allowable temperature rise.
- Permissible through fault current.
- Highest closing current.
- Time of closing.
- The operating cycle.
- The type of location of the installation.
Selection of Type of Circuit Breaker:
The choice of a Circuit Breaker Types for a given power system of known parameters like operating voltage and the line constants, is a complicated problem. Unless the exact requirements of the circuit breaker are clearly spelt out, the choice cannot be uniquely made. For example, in a system where automatic reclosing is not installed, the choice would naturally be very wide because all the three conventional types of the circuit breakers could perhaps be utilized in this case.
Apart from the considerations of initial cost we take into account the availability, of properly trained personnel to operate the breakers and the free supply of high grade insulating oil to the various substations whenever required. As mentioned earlier in the air-blast circuit breakers have very high rupturing capacities and can be designed to have extremely small operating times. Thus at important receiving stations and generating sets of a large interconnected power system working at voltages of 220 KV and above, the choice of air-blast circuit breakers becomes almost inevitable. However, in smaller substations working at 110 KV and below where the duty of the breaker is not very demanding, the oil circuit breakers have as much possibility as air-blast circuit breakers.
Another point to be considered is location of the circuit breaker. For example, in several Circuit Breaker Types used in connection with high voltage feeder lines, the demand may be to have totally enclosed switchgear.
In other cases for breakers used in connection with electrical furnaces the number of closing and opening operations may be much more than in the case of a circuit breaker used in a substation. Thus a proper coordination between the characteristics of the known Circuit Breaker Types and the demands of the operation in the power system have to be attempted before the type is decided finally.