All of the components in an electrical system are designed to operate at their rated voltages for optimum efficiency and long service. An ideal electrical system would provide constant voltage to all customers under all conditions of load. Unfortunately, because of the unpredictable dynamics of a practical system, none is ideal. Thus, it is necessary to include voltage regulator in the system to correct its performance and keep its voltage reasonably close to an ideal constant.
There are now at least four different methods for maintaining close to ideal voltage on electric power transmission and distribution systems. These include the use of step voltage regulators, transformer load-tap changers, fixed and switched capacitors, and static var (volt-amperes reactive) systems (SVS). However, single-phase step-voltage regulators are most frequently used to regulate voltage in electric power distribution systems.
There are many reasons, both technical and economic, why system voltage should be held close to its intended standard. Among them is the fact that overvoltage shortens the life of heating elements in resistive appliances, components in electronic products, and filaments in both incandescent and fluorescent lamps. Moreover, overvoltage can damage motor-driven appliances and tools.
On the other hand, undervoltage increases the time taken for the resistive elements
of appliances to heat up while also causing motors to overheat and lose efficiency. It will also reduce the performance of electronic products such as computers, radios, and TVs, and dim the illumination from luminaires.
STEP-VOLTAGE REGULATORS
Step-type voltage regulators are actually tapped autotransformers that have one winding common to both the primary and secondary circuits. The primary (exciter) winding is both magnetically and electrically connected to the secondary (series) winding. The series winding is connected in series with the load current. Both single-phase and threephase step-voltage regulators are being manufactured. They are built in both polemounted and pad-mounted styles.
A single-phase pad-mounted step-voltage regulator is shown in bellow Figure. It performs the same function as the traditional overhead-mounted tank-type and substation mounted regulators, with the convenience of pad mounting. The unit shown provides regulation in 32 voltage steps of approximately 5/8 percent each for a maximum of 10 percent regulation when used singly or in wye-connected banks.
These voltage regulators are made in ratings of 7620/7200 and 14.4 kV for 60-Hz systems. Their current ratings are from 50 to 548 A. Voltage regulation is microprocessor-controlled, and the autotransformer core and coil assembly is immersed in insulating oil. Pad-mounted voltage regulators are housed in steel enclosures with front access doors that are painted green to blend in with residential surroundings.
below Figure is a simplified wiring diagram for a single-phase step-voltage regulator. The regulator includes a series winding and a shunt winding that make up the main transformer and a bridging reactor or preventive transformer. The series winding is typically rated for 10 percent of the voltage of the shunt winding. There are usually eight taps attached to the series winding, and these are wired to segments of a dial switch assembly as individual contacts. The voltage difference between these segments is 1 1/4 percent voltage.
Contacts on the center-tapped preventive autotransformer are designed to bridge the gap between the dial switch segments to avoid momentary loss of the load. Although arcing occurs as the bridge slides, the load is not lost because one contact remains on the segment. As a result of this sliding action, load voltage become the average voltage of the taps bridged.
A reversing switch permits the polarity of the series winding to be reversed with respect to the shunt winding, permitting both plus and minus regulation. Both a voltage transformer and a current transformer provide the control signal for the regulator. Any surges propagated on the line will be shunted past the regulator by the bypass arrester. The lightning arrester also protects the regulator from overvoltage surges.
Source (S), load (L), and source-to-load (SL) or common bushings (indicated as lettered circles) permit the wiring to pass into and out of the oil-filled regulator tank. Tap-changer position indicators and control-panel enclosures are mounted outside both pole- and pad-mount regulators.
Modern regulators are controlled electronically. They include a voltage sensor that monitors regulator output and signals the control circuit and a switching section that delays and/or transmits the signal. They also include a tap-changing motor drive circuit for driving a motor, which changes the taps as necessary to correct the voltage.
Digital controls are available that display voltage, current, power factor, kW, kvar, and various other quantities. Many control systems can send this information to a remote terminal unit (RTU). Thus the regulator control becomes a sending station for a SCADA (supervisory control and data acquisition) system.