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coils which are free to move with respect to each other, thereby varying the magnetic leakage
reactance of the transformer. This reactance automatically adjusts itself to a value which, when
added to the load impedance, permits a constant current to flow. Constant-current transformers
are used primarily for series street lighting circuits. Standard output ratings are 10, 15, 20, 25
and 30 kW. The standard rated output current of these transformers is 6.6 or 20 A.
3.2.11.4 Furnace Transformers. Furnace transformers supply power to various types of
electric furnaces. The secondary voltages are low, generally several hundred volts, and
occasionally less than 100 V. Sizes range from a few kVA to over 50 MVA, with secondary
currents over 60,000 A. High currents are obtained by parallel connection of many winding
sections and collected by internal bus bars.
3.2.11.5 Self-Protected Transformers. Self-protected transformers generally have an
internally mounted, thermally controlled secondary breaker for overload and short-circuit
protection; an internally mounted protection link in series with the high-voltage winding to
disconnect the transformer from the line in the event of an internal coil failure; and a lightning
arrester, or arresters, integrally mounted on the outside of the tank for overvoltage protection.
3.3 VOLTAGE REGULATORS. The function of a transmission and distribution system is to
deliver power to the user at a voltage that is within acceptable limits. It is impractical, however,
to regulate large systems solely by means of generator regulators. Other devices for correcting
the voltage, therefore, are commonly used throughout the system. These devices include
transformers with automatic load-tap-changers, synchronous condensers, switched capacitors,
and voltage regulators.
3.3.1 Definition. A voltage regulator (transformer type) is an induction device having
windings in shunt with the primary circuit and secondary windings in series with the regulated
circuit. Although similar in operation to transformer load-tap-changing equipment, voltage
regulators are distinct pieces of power system apparatus.
3.3.2 Types of Voltage Regulators. There are three types of regulators; step, induction, and
ferroresonant. The induction regulator was commonly used in the past for power circuits,
however, it has been generally replaced by the less expensive and more serviceable step
regulator. The ferroresonant transformer (FRT) has been used as a constant voltage
transformer in low power applications (less than 1 kVA to approximately 15 kVA) for many
years. With proper filtering, the FRT is now used quite often to serve sensitive electronic and
computer equipment.
3.3.2.1 Step Voltage Regulator. The step voltage regulator is an autotransformer equipped
with a mechanism capable of changing taps under load. Standard step regulators have a
regulation range of +/- 10 percent of the applied voltage. This range is usually divided into
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