Q5Power System - I

(a) Circuit Breaker and Its Types

A circuit breaker is an electromechanical switching device designed to make (close), carry, and break (interrupt) electrical current under both normal load conditions and abnormal fault conditions (such as short circuits and overloads), automatically opening the circuit when a fault is sensed by an associated protective relay, thereby isolating the faulted section and protecting equipment and personnel.

Circuit breakers are classified according to the arc-quenching medium used to extinguish the arc drawn between the separating contacts:

• Oil Circuit Breaker (OCB): contacts are immersed in insulating (transformer) oil; the arc decomposes the oil, generating hydrogen gas which cools and quenches the arc. Used historically for medium and high-voltage applications, but bulky and carries fire risk.
• Air-Blast Circuit Breaker (ABCB): a high-pressure blast of compressed air is directed across the arc at the instant of contact separation, rapidly cooling and de-ionizing the arc path. Used for high-voltage applications requiring fast interruption, but generates noise and requires a compressed-air plant.
• Sulphur Hexafluoride (SF6) Circuit Breaker: uses SF6 gas, which has excellent dielectric strength and arc-quenching properties (electronegative gas that readily captures free electrons in the arc, aiding rapid de-ionization); widely used for medium to extra-high-voltage switchgear due to compact size, minimal maintenance, and quiet, reliable operation.
• Vacuum Circuit Breaker (VCB): contacts are enclosed in a vacuum interrupter; the very high dielectric strength of vacuum and rapid diffusion of arc-generating metal vapor allow extremely fast arc extinction at natural current zero. Widely used in medium-voltage (11-33 kV) distribution systems due to minimal maintenance, long contact life and compact size.
• Air-Break Circuit Breaker: contacts separate in open air (at atmospheric pressure) with arc-chutes/splitters to elongate and cool the arc; mainly used for low-voltage applications.

In all types, the fundamental interruption process is the same: as contacts separate, an arc is drawn (since current cannot instantaneously stop in an inductive circuit); the arc-quenching medium extracts heat from the arc and increases the dielectric strength across the gap, and interruption is normally achieved at (or very near) a natural current zero-crossing, when the medium's dielectric recovery outpaces the rate of rise of the restriking (recovery) voltage across the contacts.

(b) Current Chopping

Current chopping is a phenomenon, particularly associated with air-blast and vacuum circuit breakers interrupting low-magnitude inductive currents (such as unloaded transformer magnetizing current or small reactor/motor currents), in which the powerful de-ionizing action of the arc-quenching medium forcibly extinguishes the arc before the current has naturally reached its zero crossing — effectively 'chopping' the current abruptly to zero at some non-zero instantaneous value ich.

Effect on circuit interruption: since the current in an inductor cannot change instantaneously, the sudden forced collapse of current from ich to zero attempts to release the energy stored in the circuit's inductance (½Lich²) very rapidly; this manifests as a very high transient overvoltage spike across the breaker contacts and the connected inductive equipment, given approximately by V = ich·√(L/C) (where L is the circuit inductance and C is the effective shunt capacitance of the equipment, such as a transformer's winding capacitance), since the trapped energy oscillates between L and C at the point of interruption. This overvoltage, potentially several times the normal system voltage, can severely stress and even puncture the insulation of transformers, reactors or motors being switched.

Measures taken to reduce current chopping effects: (1) using resistance switching, where a resistor is temporarily inserted in parallel with the breaker contacts during the interruption process, damping the LC oscillation and limiting the peak transient overvoltage; (2) fitting surge arresters (metal-oxide or gapped silicon-carbide arresters) across the switched equipment to clamp the overvoltage to a safe level; (3) selecting contact/electrode materials for the breaker with a higher inherent chopping current threshold characteristic suited to the specific application, or conversely materials engineered to minimize the chopping current for reactor/transformer switching duty; (4) using point-on-wave switching controllers that time the contact separation instant relative to the current waveform to minimize the chopped current magnitude; and (5) adding shunt capacitors across sensitive equipment terminals to increase the effective C in the V=ich√(L/C) relation, thereby directly reducing the resulting overvoltage for a given chopped current.

Why vacuum and air-blast breakers are particularly prone to current chopping: vacuum interrupters and air-blast breakers both have an extremely strong, rapid arc-deionizing capability by design (essential for their normal fault-clearing performance), and this same aggressive deionizing action, when interrupting a small, already near-zero inductive current, is able to force the arc to extinguish well before the current's natural zero crossing, unlike oil circuit breakers, whose comparatively less aggressive (more gradual) arc-quenching characteristic is naturally less prone to forcing a premature current zero. This is an important practical trade-off in breaker selection for reactor and unloaded-transformer switching duty, where a breaker's fault-interrupting strength must be balanced against its current-chopping tendency for these particular low-current switching scenarios.

Consequences if left unmitigated: repeated current chopping events, especially on transformer or shunt reactor switching (both are commonly and frequently switched in normal system operation), can subject the switched equipment's insulation to repetitive high-magnitude transient overvoltage stress, accelerating insulation aging and potentially causing a sudden dielectric failure over time even if no single event exceeds the equipment's rated withstand voltage; this is why current-chopping suppression measures are an essential part of switchgear application engineering wherever breakers are used to routinely switch reactive (inductive) loads such as reactors, arc furnace transformers, or unloaded/lightly-loaded power transformers.

Relation to breaker type selection in practice: because vacuum breakers are otherwise highly preferred in medium-voltage distribution (11-33 kV) for their compactness, minimal maintenance and long contact life, considerable development effort has gone into designing vacuum interrupter contact materials (such as copper-chromium alloys) specifically to give a low, well-controlled chopping current level, so that modern vacuum circuit breakers used for reactor and transformer switching duty are able to combine the general operational advantages of vacuum interruption with an acceptably low risk of damaging chopping overvoltages, illustrating how the current-chopping phenomenon directly influences both breaker design choices and the application engineering (contact material selection, surge-arrester provision) surrounding inductive-load switching in modern power systems.