Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

direct axis transient
reactance a value that characterizes the equivalent reactance of the d-axis windings of the synchronous machine between the initial time following a system disturbance (subtransient interval) and the steady state. This reactance cannot be directly mathematically related to the d-axis operational impedance. However, in models in which the rotor windings are represented as lumped parameter circuits, the d-axis transient reactance is expressed in closed
form as the sum of the stator winding leakage reactance, and the parallel combination of the d-axis magnetizing reactance and the field winding leakage reactance.
direct-axis subtransient open-circuit time constant
a constant that characterizes the initial decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that immediately following a disturbance, during which the effects of all amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient open-circuit time constant of a machine with a single d-axis amortisseur windings is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is very small and the detailed expression simplified.
direct-axis subtransient reactance
the high-frequency asymptote of the d-axis operational impedance of a synchronous machine. This value characterizes the equivalent reactance of the d axis of the machine during the initial time following a system disturbance. In models in which the rotor windings are represented as lumped parameter circuits, the d-axis subtransient reactance is expressed in closed form as the sum of the stator winding leakage reactance, and the parallel combination of the d-axis magnetizing reactance and the d-axis rotor leakage reactances.
direct-axis subtransient short-circuit time constant
a constant that characterizes the initial decay of transients in the d-axis variables of the synchronous machine with the stator windings short-circuited. The interval characterized is that immediately following a disturbance, during which the effects of amortisseur windings are considered. A detailed (derived) closed-form expression for the subtransient short-circuit time constant of a machine with a single d-axis amortisseur winding is obtained by taking the reciprocal of the largest root of the numerator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the field winding resistance is small and the detailed expression simplified.
direct-axis transient open-circuit time constant
a constant that characterizes the decay of transients in the d-axis variables of the synchronous machine with the stator windings open-circuited. The interval characterized is that following the subtransient interval, but prior to steady-state, during which the effects of the amortisseur windings are small (possibly negligible). A detailed (derived) closed-form expression for the transient open-circuit time constant of a machine with a single d-axis amortisseur winding is obtained by taking the reciprocal of the smallest root of the denominator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the amortisseur winding resistance is infinite and the detailed expression simplified.
direct-axis transient short-circuit time constant
a constant that characterizes the decay of transients in the d-axis variables of the synchronous machine with the stator windings short-circuited. The interval characterized is that following the subtransient interval, but prior to steady-state, in which the effects of the amortisseur windings are small (possibly negligible). A detailed (derived) closed-form expression for the short-circuit transient time constant is obtained by taking the reciprocal of the smallest root of the numerator of the d-axis operational impedance. An approximate (standard) value is often used, in which it is assumed the amortisseur winding resistance is infinite and the detailed expression simplified.
impulsive transient
a rapid frequency variation of voltage or current during steady-state operation in which the polarity is mostly unidirectional.
subtransient current
the fault current that flows during the subtransient period when the generator and motor apparent impedances are their respective subtransient impedances.
subtransient impedance
the series impedance that a generator or motor exhibits during the subtransient period, typically the first few cycles of a fault. Subtransient impedances are generally used in calculating fault currents for determining instantaneous relay settings.
subtransient open-circuit time constant
See quadrature-axis transient open-circuit time constant and direct-axis subtransient open-circuit time constant.
transient
(1) the behavior exhibited by a linear system that is operating in steady state in moving from one steady state to another. For stable systems, the transient will decay while for unstable system it will not, and thus the latter never reach another steady-state operation. See also settling time and time constant.

(2) any signal or condition that exists only for a short time.

(3) an electrical disturbance, usually on a power line.

(4) refers to momentary overvoltages or voltage reductions in an electric power system due to lightning, line switching, motor starting, and other temporary phenomena.
transient current
the fault current that flows during the transient period when the machine apparent impedance is the transient impedance.
transient fault
a fault that can appear (e.g., caused by electrical noise) and disappear within some short period of time.
transient impedance
the series impedance that a generator or motor exhibits following the subtransient period but prior to the steady-state situation.
transient open-circuit time constant
See quadrature-axis transient open-circuit time constant and direct-axis transient open-circuit time constant.
transient operation
a power system operating under abnormal conditions because of a disturbance.
transient reactance
the reactance offered for the transient currents in synchronous machines. Referred to by the symbol Xs, the transient reactance is a function of the stator frequency and the transient inductance. Xs is comparatively smaller in comparison to the steady-state inductive reactance of the machine.
transient short-circuit
time constant pass and stop bands are critical. A longer filter length generally implies that the filter can have a steeper transition band to a similarly shaped shorter length filter.
transient stability
the ability of a power system to remain stable following a system disturbance.
transient suppressor
a device connected to a piece of sensitive electrical equipment to reduce the amplitude of transient voltage excursions, thus protecting the equipment.
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