Electric Power Generation, Transmission, and Distribution

The defining voltage and current equations for a regulator in the lower position are as follows:

Voltage equations Current equations

V 1

N 1

¼

V 2

N 2

N 1 I 1 ¼N 2 I 2 (21:84)

VS¼V 1 þV 2 IL¼IsI 1 (21:85)

VL¼V 1 I 2 ¼IS (21:86)

V 2 ¼

N 2

N 1

V 1 ¼

N 2

N 1

VL I 1 ¼

N 2

N 1

I 2 ¼

N 2

N 1

(IS) (21:87)

VS¼ 1 þ

N 2

N 1



Vl IL¼ 1 þ

N 2

N 1



IS (21:88)

VS¼aRVL IL¼aRIS (21:89)

aR¼ 1 þ

N 2

N 1

(21:90)

Equations (21.83)and (21.90) give the value of the effective regulator ratio as a function of the ratio of
the number of turns on the series winding (N 2 ) to the number of turns on the shunt winding (N 1 ). The
actual turns ratio of the windings is not known. However, the particular position will be known.
Equations (21.83) and (21.90) can be modified to give the effective regulator ratio as a function of
the tap position. Each tap changes the voltage by 5=8% or 0.00625 per unit. On a 120 V base, each step
change results in a change of voltage of 0.75 V. The effective regulator ratio can be given by

aR¼ 1  0 : 00625 Tap (21:91)

In Eq. (21.91), the minus sign applies to the ‘‘raise’’ position and the positive sign for the ‘‘lower’’
position.

21.1.4.3 Line Drop Compensator

The changing of taps on a regulator is controlled by the ‘‘line drop compensator.’’Figure 21.17shows a
simplified sketch of the compensator circuit and how it is connected to the circuit through a potential
transformer and a current transformer.
The purpose of the line drop compensator is to model the voltage drop of the distribution line from
the regulator to the load center. Typically, the compensator circuit is modeled on a 120 V base. This
requires the potential transformer to transform rated voltage (line-to-neutral or line-to-line) down to
120 V. The current transformer turns ratio (CTp:CTs) where the primary rating (CTp) will typically be
the rated current of the feeder. The setting that is most critical is that ofR^0 andX^0. These values must
represent the equivalent impedance from the regulator to the load center. Knowing the equivalent
impedance in Ohms from the regulator to the load center (RlineohmsandXlineohms), the required value
for the compensator settings are calibrated in volts and determined by

R^0 voltsþjX^0 volts¼ðÞRlineohmsþjXlineohms 

Ctp

Npt

V (21:92)

The value of the compensator settings in ohms is determined by

R^0 ohmsþjXohms^0 ¼

R^0 voltsþjX^0 volts

Cts

V (21:93)