Ratings of Circuit Breakers in Substation Design

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Read this chapter to learn the various ratings of circuit breakers and their importance in selection of circuit breakers for specific applications. A circuit breaker is nothing but an electrical switch that can turn on/off an electric circuit or can protect the circuit when a very high current flows through it.

Why Circuit Breakers are Important?

In electrical substation design, selecting a right circuit breaker is a crucial practice, as it is important for safe and reliable operation of the system. If we select a wrong circuit breaker for our substation, then it can result in severe faults or equipment failure, personnel hazards, or permanent system damages.

Main Functions of a Circuit Breaker

The following are some key functions of a circuit breaker −

• It makes or breaks the circuit for switching purposes.
• It interrupts fault current at a faster speed to protect circuit from severe damages.
• It allows for circuit isolation during testing or maintenance.

After getting a basic overview of circuit breakers, let's now discuss its important ratings that we can generally find on its name plate.

Name Plate Information of Circuit Breaker

The name plate of a circuit breaker is nothing but a label printed on the circuit breaker body to highlight important information like specifications and characteristics of the circuit breaker. A typical name plate information of a circuit breaker is given in the following table −

We will use this name plate to understand different specification of the circuit breaker.

Ratings of Circuit Breaker

A circuit breaker has the following ratings specified on its nameplate and these ratings play an important role in right CB selection for a specific application −

Rated Voltage

It is defined as the maximum system voltage that the circuit breaker can handle without getting damaged. For a circuit breaker, the rated voltage must be greater than or equal to the system voltage of the substation, that is,

$$\mathrm{CB\:Rated\:Voltage\:\:\geq\:\:Substation\:System\:Voltage}$$

For example, if a substation has a 33 kV line or circuit, we can select a 36 kV circuit breaker to provide adequate safety margin. The higher rating circuit also provides protection from transient over-voltages and ensures dielectric reliability.

In actual practice, it is a general standard to select a circuit breaker with at least 10% higher rated voltage than the system voltage of the substation.

Rated Current

Rated current of a circuit breaker is the amount of electric current that the circuit breaker can carry without getting overheated. It is also called the continuous thermal current of the circuit breaker.

In substations, the rated current of the circuit breaker must be greater than the maximum load current of the substation equipment like transformer or feeder.

The maximum load current of the transformer can be calculated as follows −

$$\mathrm{Load\:Current\:=\:\frac{MVA\:Rating\:of\:Transformer \:\times\:1000}{\sqrt{3} \:\times\: System\:Voltage\:in\:kV}}$$

For example, if substation has a transformer of 25 MVA, 33 kV, then

$$\mathrm{I_{L} \:=\: \frac{25 \:\times\: 1000}{\sqrt{3} \:\times\: 33} \:=\: 437.39\:A}$$

As from the above name plate, the circuit breaker's rated current is 1600 A which is higher than 437.39 A.

In practical applications, the circuit breaker's rated current is taken at least 25 to 30% higher than the calculated load current for thermal safety and expansion in future.

Short Circuit Breaking Current

This specification of the circuit breaker specifies the maximum fault current that the circuit breaker safely interrupt. For a circuit breaker, the short circuit breaker current must be higher than the fault level of the system.

It can be calculated as follows −

$$\mathrm{I_{sc} \:=\: \frac{Short \:Circuit\: MVA}{\sqrt{3} \:\times\: System\:Voltage}}$$

For example, for a substation whose short circuit MVA or fault MVA is 1000 MVA at 33 kV system voltage, the short circuit breaking current will be,

$$\mathrm{I_{sc} \:=\: \frac{1000 \:\times\: 10^{6}}{\sqrt{3} \:\times\: 33 \:\times\: 10^{3}} \:=\: 17.5\:kA}$$

For the above given circuit breaker, the short circuit breaking current is 26.3 kA which is higher than 17.5 kA, hence it can be used safely.

In practical substation design, it should be recommended to add a minimum safety margin of 25% to the maximum fault current of system.

Short Circuit Duration

Short circuit duration of a circuit breaker can be defined as the time for which the circuit breaker can withstand the short circuit current before opening. This time must match or higher than the protection coordination delay or backup relay operation time of the system.

For our circuit breaker, it is 3 seconds. In actual substation design, it is recommended to select a circuit breaker with a short-time withstand of greater than or equal to 3 seconds to provide backup protection operation to the system.

Short Circuit Making Current

For a circuit breaker, the short circuit making current is defined as the maximum asymmetrical fault current that the circuit breaker can withstand at the time of closing onto a fault in the system. This current rating includes both DC offset and peak due to first half cycle.

In actual substation design, the short circuit making current is taken as 2.5 times of RMS value of short-circuit breaking current. For the above given circuit breaker, it is given 66.25 kA. But if it is not given on the nameplate, then we can obtain it by multiplying the RMS short-circuit breaking current by 2.5.

Insulation Rating

It is specified for power frequency and lightning impulse, where the power frequency rating handles AC overvoltage while the lightning impulse rating handles surges due to switching or lightning.

In the above given name plate, 70 kVp is the insulation rating for power frequency while 170 kVp is the insulation rating for lightning impulse.

In actual substation design, the insulation rating is always taken equal to or greater than 1.5 times of the phase-to-ground voltage, that is,

$$\mathrm{Insulation\: Rating \:\:\geq\:\: 1.5 \:\times\: V_{ph}}$$

Ratings Related to Auxiliary and Control Circuit

These ratings are important to ensure the compatibility of the circuit breaker's control circuit with our substation's control system. Some of the important auxiliary and control circuit ratings of a circuit breaker are explains below −

While selecting a circuit breaker for substation design, it must be checked that these specifications of the circuit always match with the substation's AC/DC auxiliary system.

Operating Sequence

This rating is used for auto-reclosing scheme of the circuit breaker in overhead lines where transient faults are common. For example, the above circuit breaker has O-0.3s-CO-3min-CO, this operating sequence indicates −

• O – circuit breaker opens
• 0.3s-CO – Reclose circuit breaker within 0.3 seconds
• 3min-CO – Re-operate circuit breaker after 3 minutes

This is all about the important ratings of a circuit breaker and their estimation for a specific application.

How to Select Circuit Breaker Rating?

Electrical design engineers can select a circuit breaker for a specific substation application by comparing its ratings with system's ratings as follows −

Conclusion

In this chapter, we covered all about the ratings of circuit breakers and selecting appropriate ratings of circuit breaker for a specific application. It is always taken care that the circuit breaker rating must fulfil the design criteria of the substation.