Dictionary of Electrical Engineering

Commonly used terms in the Electrical industry.

unbalanced operation
in an n-phase system (n> 1), a condition in which the phase voltages (currents) are either

1. not equal-amplitude sinusoids or

2. have phase angles displaced by a value other than that specified for balanced operation.

The term "unbalanced" is also used to describe a machine that has unsymmetrical phase windings.
underexcited
a condition of operating a synchronous machine, in which the current to the DC field winding is insufficient to establish the required magnetic flux in the air-gap. As a result, the machine requires reactive power from the AC system. An underexcited synchronous motor operates at a lagging power factor, as it appears as an inductive load to the AC system. An under-excited synchronous generator operates with a leading power factor, since it must deliver power to a leading (capacitive) system.
underground distribution
a class of electric power distribution work, typically used in densely-populated urban business districts, in which conductors are carried in conduits under streets between manholes and submersible distribution transformers are mounted in underground vaults.
underground residential distribution
practices involved in the underground distribution of electric power to residential subdivisions through direct-buried cables and pad mound transformers.
undervoltage
a voltage that is less than nominal for a time greater than 1 minute.
undervoltage relay
a protective relay that operates on low voltage or loss of voltage.
Underwriters Laboratory
an insurance industry testing agency that establishes standards for and conducts testing of electrical equipment.
unforced system
a dynamic system where all of the external sources of excitation are identically zero.
ungrounded system
an electrical distribution system in which there is no intentional connection between a current-carrying conductor and ground.
uninterrupted power supply (UPS)
(1) a power supply designed to charge an energy storage medium, while providing conditioned output power, during the presence of input power and to continue providing output power for a limited time when the input to the supply is removed. These power supplies are typically used in critical applications to prevent shut-down of these systems during power failures, power surges, or brownouts.

(2) a device that provides protection for critical loads against power outages, overvoltages, undervoltages, transients, and harmonic disturbances. A typical UPS is a rectifier supplied battery bank for energy storage, and a PWM inverter-filter system to convert a DC voltage to a sinusoidal AC output. UPS systems can be on-line, as shown in the figure, where the UPS inverter powers the load continuously, or off-line where the load is connected directly to the utility under normal
operation and emergency power is provided by the UPS.
unsymmetrical fault
a fault on a three-phase power line in which the fault current is not equal in all three phases, e.g., a single-line-to-ground, double-line-toground or line-to-line fault.
unsymmetrical load
a load which forces the currents in the three-phase power line which supplies it to be unequal.
up-down converter

See buck-boost transformer
up-down counter
a register that is capable of operating like a counter and can be either incremented or decremented by applying the proper electronic signals.
up-down transformer

See buck-boost transformer
utilization factor
the ratio of the maximum demand on the system vs the rated capacity of the system.
utilization voltage
the voltage across the power input terminals of a piece of electrical equipment.
V-V transformer

See open-delta transformer
vacuum capacitor
a capacitor with a vacuum between its plates.
vacuum circuit breaker
a power circuit breaker where a vacuum chamber is used as an insulating and arc clearing medium.
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