Breaking of a short circuit current in a direct current system

Breaking of a short circuit current

Direct current presents different problems than alternating current with aregard to the phenomena associated to the breaking of a short circuit current and interruption of other types of high value current since the arc extinction results to be particularly difficult.

As Figure 1 shows, with alternating current there is natural passage of current through zero at each half cycle, which corresponds to the quenching of the arc during the circuit opening.

With direct current (see figure 2) there is not such natural passage and therefore, to guarantee arc extinction, the current must decrease to null (forcing the current passage through zero).

Equivalent circuit

To understand the above, reference to the circuit shown in the figure shall be made:

In this case:

where:

• U is the rated voltage of the supply source L is the inductance of the circuit
• R is the resistance of the circuit
• Ua is the arc voltage.

The formula can be written also as:

To guarantee arc extinction, it is necessary that:

The extinction constant is a parameter depending on the arc characteristic and on the circuit supply voltage.

Lab testing of DC circuit breaker

The following figure shows an oscillogram relative to a short circuit test carried out in ABB SACE power testing laboratories.

Where:

• Ip – Short circuit making current
• Icn – Prospective short circuit current
• Ua – Maximum arc voltage
• Un – Network voltage
• T – Time constant
• to – Instant of beginning of short circuit
• ts – Instant of beginning of separation of the CB contacts
• ta – Instant of quenching of the fault current

When a short circuit occurs, in correspondence to the instant to, the current starts rising according to the time constant of the circuit.

The circuit breaker contacts begin separating, thus striking an arc starting from the instant ts. The current keeps on rising for a short instant also after the beginning of contact opening, and then decreases depending on the value higher and higher of the arc resistance progressively introduced in the circuit.

As the graph shows, the short circuit current represented by the red line is extinguished without abrupt interruptions which could cause high voltage peaks.

As a consequence, to obtain a gradual extinction (the graph represents the descent of Ip), it is necessary to cool and extend the arc, so that a higher and higher arc resistance is inserted in the circuit (with the consequent increase of the arc voltage Ua).

This extinction involves energetic phenomena which depend on the voltage level of the plant (Un) and lead to install circuit breakers according to connection diagrams in series to the advantage of the performances under short circuit conditions. As a matter of fact, the higher is the number of contacts opening the circuit, the higher is the breaking capacity of the circuit breaker.

This means that, when the voltage rises, it is necessary to increase the number of current interruptions in series, so that a rise in the arc voltage is obtained and consequently a number of poles for breaking operation suitable to the fault level.

To summarize

In order to guarantee breaking of a short circuit current in a DC system it is necessary to employ circuit breakers which can ensure:

1. Rapid tripping with adequate breaking capacity
2. High fault current limiting capacity
3. Overvoltage reduction effect.

Interesting video

DC circuit breaker fires

Reference // Circuit breakers for direct current applications – Technical paper by ABB