[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Electric supply systems are to be designed and operated so that most service voltages fall within the
range A limits. User systems are to be designed and operated so that, when the service voltages are within
range A, the utilization voltages are within range A. Utilization equipment is to be designed and rated
to give fully satisfactory performance within the range A limits for utilization voltages.
Range B is provided to allow limited excursion of voltage outside the range A limits that necessarily
result from practical design and operating conditions. The supplying utility is expected to take action
within a reasonable time (e.g., 2 or 3 min) to restore utilization voltages to range A limits.
The combination of step-voltage regulators and fixed-ratio power transformers is often used in lieu
of load-tap-changing power transformers in the substation. To obtain constant voltage at some distance
(load center) from the step-voltage regulator bank, a line-drop compensation feature in the control can
be utilized.
Step-voltage regulators — single- and three-phase — are designed, manufactured, and tested in
accordance with IEEE Std. C57.15, IEEE Standard Requirements, Technology, and Test Code for Step-
Voltage Regulators. A step-voltage regulator is defined as “an induction device having one or more
windings in shunt with and excited from the primary circuit, and having one or more windings in series
between the primary circuit and the regulated circuit, all suitably adapted and arranged for the control
of the voltage, or of the phase angle, or of both, of the regulated circuit in steps by means of taps without
interrupting the load.”
The most common step-voltage regulators manufactured today are single-phase, using reactive switching
resulting in 32 5/8% voltage steps (16 boosting and 16 bucking the applied voltage), providing an
overall 10% regulation. They are oil-immersed and typically use ANSI Type II insulating oil in accordance
with ANSI/ASTM D-3487.
Although not required by the IEEE standard, which now recognizes 65C rise, most manufacturers
today design and manufacture step-voltage regulators rated 55C average winding rise over a 30C average
ambient and use a sealed-tank-type construction. The gases generated from arcing in oil, as a result of
normal operation of the load-tap changer, are vented through a pressure-relief device located on the tank
above the oil level. Thermally upgraded paper insulation designed for an average winding rise of 65C,
along with the use of a sealed-tank-type system, allows for a 12% increase in load over the nameplate
55C rise kVA rating.
Many regulators with a continuous-current rating of 668 A and below can be loaded in excess of their
rated ampere load if the range of voltage regulation is limited at a value less than the normal 10% value.
Table 2.7.2 shows the percent increase in ampere load permitted on each single-phase step-voltage
regulator when the percent regulation range is limited to discrete values less than 10%.
Some regulators have limitations in this increased current capacity due to tap-changer ampacity
limitations. In those cases, the maximum tap-changer capacity is shown on the regulator nameplate.
Limiting the percent regulation range is accomplished by setting limit switches in the position indicator
of the regulator to prevent the tap changer from traveling beyond a set position in either raise or lower
directions. It should be recognized, however, that although the regulators can be loaded by these additional
amounts without affecting the regulator’s normal coil-insulation longevity, when the percent
regulation is decreased, the life of the tap-changer contacts will be adversely affected.
TABLE 2.7.2 Increase in Ampere Load Permitted on Single-Phase Step-Voltage
Regulators for Regulation Range