In power system protection, high voltage circuit breakers usually interrupt fault currents during which part of the short circuit current waveform?

Introduction / Context:
In electrical power systems, circuit breakers are critical safety devices that interrupt fault currents such as those caused by short circuits. When a short circuit occurs, the current waveform is not immediately steady; it contains decaying components and passes through different states known as sub transient, transient, and steady state. Understanding at which stage circuit breakers typically operate is important for selecting and designing protection equipment. This question focuses on that aspect.

Given Data / Assumptions:

• A short circuit occurs in an alternating current power system.
• The resulting current has an alternating component and a direct current offset component.
• The waveform evolves through sub transient, transient, and steady states as time passes.
• We are asked when circuit breakers usually interrupt the fault current.

Concept / Approach:
Immediately after a short circuit, the current rises to a high value that includes sub transient and transient effects due to machine reactances and the presence of a direct current component. The direct current component decays over a few cycles, and the current then approaches a steady state alternating value. Circuit breakers are designed to clear faults quickly, often within a few cycles of the power frequency, in order to limit damage and maintain system stability. Therefore, they typically interrupt the current while the waveform is still in its transient state, before the direct current component has completely decayed and before a steady state is reached.

Step-by-Step Solution:
Step 1: Recognise that the short circuit current at the instant of fault has a large alternating component plus a direct current offset, giving an asymmetrical waveform. Step 2: The initial very high current that decays quickly over the first few cycles is termed sub transient, and the subsequent period where the current is still higher than steady state but decaying more slowly is the transient state. Step 3: After sufficient time, the direct current offset nearly disappears and the current settles to a steady state symmetrical alternating value. Step 4: Protection relays and circuit breakers are coordinated to detect and clear faults in a small number of cycles, often well before the current reaches pure steady state but after the very first instant of sub transient behaviour. Step 5: Therefore, breakers usually operate during the transient state of short circuit current, when both alternating and residual direct current components are still present but decaying.

Verification / Alternative check:
Manufacturers specify breaking capacity for circuit breakers based on the expected current at the time of interruption, which includes the effect of transient and direct current offset. Time current characteristics of protection relays typically result in operating times of a few cycles for serious faults, which coincide with the transient period. This behaviour confirms that operation is associated with the transient state rather than waiting for steady state conditions.

Why Other Options Are Wrong:
After the direct current component has completely ceased: Waiting for the direct current component to vanish would delay fault clearing too long, increasing damage and instability. During the sub transient state only: Sub transient effects are extremely short lived, and breakers cannot usually detect and interrupt faults within this very brief window. During the steady state of the short circuit current: Clearing faults only after reaching steady state would be unsafe, as currents and mechanical forces would remain high for too long.

Common Pitfalls:
Students may assume that equipment should operate when conditions are steady because steady state is easier to analyse. In reality, protection devices must act quickly, and their operation often occurs when transients are still significant. It is important to distinguish between analytical convenience and actual operation times required for safety and reliability.

Final Answer:
Circuit breakers usually operate and interrupt fault currents during the transient state of the short circuit current.