Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

swing bus
in power-flow studies, a bus in the power system which is assigned unknown real and reactive power so as to compensate for losses in the system.
swing equation
a nonlinear differential equation utilized in determining the dynamics of synchronous machines.
See electromechanical equation
switch
(1) a device that allows current flow when closed and provides isolation when open. The switch provides similar functions to the circuit breaker, but cannot interrupt fault currents. Some switches are capable of making and breaking load currents, while others are only able to break charging current. Switches can be either manually controlled or motor controlled. See also single-pole single-throw, single-pole double-throw, transmit/receive switch, all-optical switch.

(2) a device comprising a number of input and output ports and circuitry to switch packets from one input port to one or more output ports based on the addressing information contained in the packet header.
switchable shunt

See shunt capacitor
switchboard
literally, a large panel or board upon switches were mounted in early electrical systems.
switched reluctance machine
a doubly salient, singly excited electrical machine that contains a different number of poles on the stator and rotor. Since there are a different number of poles on the rotor and stator, only one stator phase can be aligned at a time with the rotor.

When operated as a motor, the stator phases are sequentially switched on and off to pull the rotor into alignment with them. This requires knowledge of the rotor position to properly excite the stator phases. The switched reluctance machine can also operate as a generator. In this case the stator windings are charged with a current as the rotor comes into alignment. When the current reaches a determined level, the windings are reconnected to send current out of the machine.
As the rotor is driven, the inductance drops, causing an increase in current.

This type of machine requires an external capacitor bank, switches and diodes in each phase, and a sophisticated control system to operate.
symmetrical component
the method by which unbalanced three-phase power system operation (particularly unbalanced fault performance) can be efficiently analyzed. Symmetrical components convert unbalanced line currents and voltages to three sets of balanced sequence components: positive sequence, negative sequence, and zero sequence.

The transformed phasor variables fC-0s are obtained by applying the appropriate Fortescue transformation to any multi-phase set of phasor variables. Denoted, f+s;f-s;f0s, these are the positive sequence, negative sequence, and zero sequence components, respectively. The variables are so named because any unbalanced
set of currents can be expressed (in phasor form) in terms of

1. a balanced set of currents with magnitude iCs that has a phase sequence which produces counterclockwise rotor rotation in a machine (positive sequence).

2. a balanced set of phasor currents with magnitude i-s that has a phase sequence which produces clockwise rotor rotation in a machine (negative sequence), and

3. a set of three equal phasors with magnitude f0 (zero sequence) which does not produce rotor rotation in machine.
symmetrical fault
another term for a three-phase fault, a fault in which all three conductors of a three-phase power line are short-circuited together. System faults are symmetrical and can be analyzed by using single phase circuit.
symmetrical fault current
the total current flowing to a fault less the DC offset current. In many cases, fault current calculations are expressed in terms of symmetrical amps.
synchro
also called a selsyn (for self-synchronous). An AC servo machine used in pairs primarily for remote sensing and shaft positioning applications. Its construction is essentially that of a wound-rotor induction machine with either a single-phase or 3-phase rotor winding. Various stator and rotor interconnections are possible, depending on desired function and required torque.
synchro-check relay
a device used to monitor the frequency and phase angle of the voltages across an open circuit breaker.

Synchro-check relays are commonly used to prevent breaker closing or reclosing on excessive voltage or frequency difference.
synchronization
(1) a situation when two or more processes coordinate their activities based upon a condition.

(2) the process of determining (usually channel) parameters from a received signal, for example carrier frequency offset, carrier phase, or symbol timing.
synchronizing coefficient
electrical torque component in phase with the rotor angle.
synchronizing relay
a relay that monitors the voltage across an open circuit breaker to determine the frequency and phase relationship of the voltage sources on either side of the breaker. Synchronizing relays are used on generator breakers to bring the generator to the system frequency and to match the phase angle between the generator and system prior to closing the breaker.
synchronous condenser
an unloaded, over-excited synchronous motor that is used to generate reactive power.
synchronous machine
an AC electrical machine that is capable of delivering torque only at one specific speed (ns), which is determined by the frequency of the AC system (f) and the number of poles (P) in the machine. The relationship between synchronous speed and the other variables is

ns = 120f/P
synchronous motor
an AC motor in which the average speed of normal operation is exactly proportional to the frequency to which it is connected. A synchronous motor generally has rotating field poles that are excited by DC.
synchronous operation
an operation that is synchronized to a clocking signal.
synchronous reactance
the inductive reactance of the armature windings in synchronous machines under steady-state conditions. Designated by the symbol Xs,expressed in ohms per phase, the synchronous reactance is a function of the stator inductance and the frequency of the stator currents.
synchronous reference frame
a two dimensional space that rotates at an angular velocity corresponding to the fundamental frequency of the physical stator variables (voltage, current, flux) of a system.
In electric machines/power system analysis, an orthogonal coordinate axis is established in this space upon which fictitious windings are placed. A linear transformation is derived in which the physical variables of the system (voltage, current, flux) are referred to variables of the fictitious windings. See also arbitrary reference frame, rotor reference frame
See stationary reference frame
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