Mechanical Voltage Regulator Characteristics – Power Quality vs Line Drop

Mechanical voltage regulators for power quality and utility line drop compensation (LDC) function in similar manners but they differ substantially in application:

Power Quality AVRUtility AVR
Type:Tap switch or magnetic inductionTap switch
Size (kVA):1 to 1,500500 to 3,500+
Nominal Voltages:+20% to -20%+10% to -10%
Ouput Voltage Range:1% to 3%0.6% to 2.5%
Output Regulation:Power Quality AVRUtility AVR
Phase:Single or threeSingle or three
Voltage Step Up/Down:YesYes
Cooling Media:AirOil
Efficiency:> 95%> 95%

The primary differences between power quality and utility applications are the range of input voltage regulation and voltage levels, but there are other, less obvious, differences including the terms used for these units.

For power quality applications the mechanical AVR goes by names such as: electromechanical voltage regulator or stabilizer, mechanical tap changing voltage regulator, tap switching voltage regulator, motorized variable transformer, motorized variac, magnetic induction voltage regulator or stabilizer, servo voltage regulator or stabilizer, motor-driven variable autotransformer and variable autotransformer.

Mechanical AVRs for power quality applications operate at low voltage to match the voltage of the equipment being protected and also have a larger input voltage range to account for voltage fluctuations from the utility as well as voltage fluctuations in the end user’s facility. For power quality applications, the mechanical tap switching or servo induction voltage regulator usually comes as a completely assembled package.

The mechanical tap changing voltage regulator used by electric utilities often goes by names such as: line drop compensator (LDC), on-load tap changer (OLTC), step voltage regulator, auto-boost regulator or mechanical tap changer.

The on-load tap changer is easily the most common type used by electric utilities. OLTCs typically differ from the power quality version in that they operate at medium to high voltage, usually have relatively finer regulating steps (5/8% per step) and the tap changing mechanisms and controls are often purchased as “add-ons” for a tapped transformer. The sensors for voltage measurement may be miles down the distribution or transmission line and relay signals to the OLTC upstream to affect voltage changes.

The utility line drop compensator typically includes sophisticated controls to delay tap changing for a variety of reasons such as: avoiding tap changing during momentary fluctuations and to coordinate tap changing with other LDCs in series downstream. Because access to an oil-filled, line drop compensator is difficult and costly, these units will have more elaborate provisions for testing, self-cleaning and adjustment. Magnetic or servo induction voltage regulators are rarely used by electric utilities.