POWER AND DISTRIBUTION TRANSFORMER TERMS DEFINITION 2

load losses: Those losses that are incident to the carrying of a specified load. Load losses include I²R loss in the current carrying parts (windings, leads, busbars, bushings), eddy losses in conductors due to eddy currents and circulating currents (if any) in parallel windings or in parallel winding strands, and stray lossinduced by leakage flux in the tank, core clamps, or other structural parts. In equation form:

 PLL = I ²R+PEC+PSL

where

PLL is the load loss (W)

I²R is the loss due to current and resistance (W)

PEC is the eddy current loss (W)

PSL is the stray loss (W).

See also: no-load (excitation) losses.

no-load (excitation) losses: Those losses that are incident to the excitation of the transformer. Noload (excitation) losses include core loss, dielectric loss, conductor loss in the winding due to exciting current, and conductor loss due to circulating current in parallel windings. These losses change with the excitation voltage. See also: load losses.

open-delta connection: A connection similar to a delta-delta connection utilizing three single-phase transformers, but with one single-phase transformer removed.

partial discharge (PD): An electric discharge that only partially bridges the insulation between conductors, and which may or may not occur adjacent to a conductor.

power transformer: A transformer that transfers electric energy in any part of the circuit between the generator and the distribution primary circuits. See also: distribution transformer.

primary unit substation: A substation in which the low-voltage section is rated above 1000 V.

primary winding: The winding on the energy input side.

rated kVA of a transformer: The output that can be delivered for the time specified at rated secondary voltage and rated frequency without exceeding the specified temperature-rise limitations under prescribed conditions.

secondary unit substation: A substation in which the low-voltage section is rated 1000 V and below.

series transformer: A transformer with a “series” winding and an “exciting” winding, in which the “series” winding is placed in a series relationship in a circuit to change voltage or phase, or both, in that circuit as a result of input received from the “exciting” winding. Syn: booster transformer (IEC).

shell form transformer: A transformer in which the laminations constituting the iron core surround the windings and usually enclose the greater part of them. See also: core form transformer.

station-type transformer: A transformer designed for installation in a station or substation.

step-down transformer: A transformer in which the power transfer is from a higher voltage source circuit to a lower voltage circuit.

step-up transformer: A transformer in which the power transfer is from a lower voltage source circuit to a higher voltage circuit.

stray loss: The loss that is due to the stray leakage flux, which introduces losses in the core, clamps, tank, and other structural parts. There is no test method to determine individual winding eddy loss or to separate transformer stray loss from eddy loss. The total stray and eddy loss is determined by measuring the total load loss during the impedance test. The total stray and eddy loss is determined by subtracting the I²R loss from the load loss as follows:

PEC + PSL = PLL – I ²R

where

PEC is the eddy current loss (W)

PSL is the stray loss (W)

PLL is the load loss (W)

I²R is the loss due to current and resistance (W).

system voltage: A root-mean-square (rms) phase-to-phase power frequency voltage on a three-phase alternating-current electrical system.

temperature rise: The difference between the temperature of the part under consideration (commonly the “average winding rise” or the “hottest-spot winding rise”) and the ambient temperature.

tertiary winding: An additional winding in a transformer that can be connected to a synchronous condenser, a reactor, an auxiliary circuit, etc. For transformers with wye-connected primary and secondary windings, it may also help

a) To stabilize voltages to the neutral, when delta connected.

b) To reduce the magnitude of third harmonics when delta connected.

c) To control the value of the zero-sequence impedance.

d) To serve load.

three-phase circuit: A three-phase circuit is a combination of circuits energized by alternating electromotive forces that differ in phase by one third of a cycle, that is, 120 degrees.

total losses (transformer or regulator): The sum of the no-load and load losses, excluding losses due to accessories.

transformer: A static electric device consisting of a winding, or two or more coupled windings, with or without a magnetic core, for introducing mutual coupling between electric circuits. Transformers are extensively used in electric power systems to transfer power by electromagnetic induction between circuits at the same frequency, usually with changed values of voltage and current.

turn ratio of a current transformer: The ratio of the secondary winding turns to the primary winding turns.

two-phase circuit: A polyphase circuit of three, four, or five distinct conductors intended to be so energized that in the steady state the alternating voltages between two selected pairs of terminals of entry, other than the neutral terminal when one exists, have the same periods, are equal in amplitude, and have a phase difference of 90 degrees. When the circuit consists of five conductors, but not otherwise, one of them is a neutral conductor.

unit substation: A substation consisting primarily of one or more transformers that are mechanically and electrically connected to and coordinated in design with one or more switchgear or motor control assemblies, or combinations thereof.

voltage ratio of a transformer: The ratio of the rms terminal voltage of a higher voltage winding to the rms terminal voltage of a lower voltage winding, under specified conditions of the load.

voltage transformer: An instrument transformer intended to have its primary winding connected in shunt with a power supply circuit, the voltage of which is to be measured or controlled.

winding (of a transformer): The assembly of turns forming an electric circuit associated with one of the voltages assigned to the transformer or to the reactor. See also: coil (of a transformer), high-voltage and low-voltage windings, primary winding, secondary winding, tertiary winding, winding (IEC).

wye connection: So connected that one end of each of the windings of a polyphase transformer (or of each of the windings for the same rated voltage of single-phase transformers associated in a polyphase bank) is connected to a common point (the neutral point) and the other end to its appropriate line terminal.

Reference: : IEEE Std C57.12.80™-2002 IEEE Standard Terminology for Power and Distribution    

                    Transformers