Transformers#
The transformer model covers two-winding data and controls and the standard three-winding equivalent.
Two-winding model#
A two-winding transformer is a branch with series impedance, an off-nominal
turns ratio, a phase shift ANG1 in degrees, and a magnetizing admittance
branch. The effective ratio is the winding 1 value relative to the winding 2
value (WINDV1, WINDV2), so a WINDV2 other than unity shifts the ratio.
The winding 1 ratio and the impedance are interpreted according to the data
codes below and converted to per unit on the system base during loading.
Data codes#
CW selects the unit of the winding ratio fields (WINDV1, WINDV2, and
the tap limits): 1 for per unit of the connected bus base voltage, 2 for
winding voltage in kV, and 3 for per unit of the nominal winding voltage
NOMV. CZ selects the unit of R1-2 and X1-2: 1 for per unit on the
system base, 2 for per unit on the winding base SBASE1-2, and 3 for load
loss in watts together with impedance magnitude on the winding base. CM
selects the unit of MAG1 and MAG2: 1 for magnetizing admittance in per
unit on the system base, and 2 for no-load loss in watts together with
exciting current in per unit.
Impedance correction#
A transformer that names an impedance-correction table (TAB1) has its
series impedance multiplied by a factor \(F\):
The table defines \(F\) as a function of tap ratio or phase-shift angle.
Voltage-band tap control (COD1 = 1)#
The winding 1 ratio adjusts to hold the voltage of the controlled bus
(CONT1) within the band [VMI1, VMA1] in per unit. The ratio uses the
available tap positions between RMI1 and RMA1. At the solution the
controlled voltage lies within the band, or the tap rests at an end of its
range.
Reactive-flow tap control (COD1 = 2)#
The winding 1 ratio adjusts to hold the reactive power flow through the
transformer within the band [VMI1, VMA1] in Mvar. The tap chain and the
limit behavior are the same as for voltage-band control.
Phase-shift control (COD1 = 3)#
The phase-shift angle adjusts to hold the active power flow through the
transformer within the band [VMI1, VMA1] in MW, with the angle bounded by
[RMI1, RMA1] in degrees. At the solution the flow lies within the band, or
the angle rests at a limit.
Three-winding transformers#
A three-winding transformer uses its standard star equivalent. Winding status, ratio, angle, rating, and impedance correction remain associated with the applicable winding.