RUS BULLETIN 1724E-300 SUBSTATION VOLTAGE REGULATORS

RUS BULLETIN 1724E-300 SUBSTATION VOLTAGE REGULATORS

Voltage Regulators for Distribution System

General
Both three-phase and single-phase voltage regulators are used in distribution substations to regulate the loadside voltage. Substation regulators are one of the primary means, along with load-tap-changing power transformers, shunt capacitors, and distribution line regulators, for maintaining a proper level of voltage at a customer’s service entrance.

A very important function of substation voltage regulation is to correct for supply voltage variation. With the proper use of the control settings and line drop compensation, regulators can correct for load variations as well. A properly applied and controlled voltage regulator not only keeps the voltage at a customer’s service entrance within approved limits but also minimizes the range of voltage swing between light and heavy load periods. The substation regulators may be located on individual feeders or in the transformer secondary circuit for main bus regulation. Normally, the low-voltage substation bus will be regulated rather than the individual feeders. Individual feeder regulation can usually be justified only when there are extreme variations between individual distribution feeder peak load times. Very long or heavily loaded distribution feeders may require supplemental regulators strategically located out on the line to maintain voltage levels within required limits.

Types
There are two general types of voltage regulators, the induction regulator and the step-type regulator. Both types are available in single- or three-phase designs. The step-type regulator has by far the wider application in the electric distribution system. The step-voltage regulator has virtually replaced the induction-voltage regulator because it is lower in cost and equally reliable.

Step-Type Voltage Regulator:
A step-voltage regulator is a regulator having one or more windings excited from the system circuit or a separate source and one or more windings connected in series with the system circuit for adjusting the voltage, or the phase relation, or both, in steps, without interrupting the load. A step-type voltage regulator consists of an autotransformer and a load-tap-changing mechanism built into an integral unit.

As with the induction regulator, when a voltage is impressed on the primary winding, the magnetic flux linking the secondary or series winding will induce a voltage in the series winding. An automatic reversing switch is incorporated to obtain an additive or subtractive voltage from the series winding with respect to the primary voltage. Taps of the series winding are connected to an automatic tap-changing mechanism to regulate the amount of voltage change in equal steps.

The terminal designations of step-type voltage regulators are as follows:
• The terminal connected to the load is designated L.
• The terminal connected to the source is designated S.
• The common terminal is designated SL.
For three-phase regulators, these identifications are Sl, S2, S3, Ll, L2, L3, and SOLO.

Single-Phase Versus Three-Phase:
 Several factors influence the selection. For substation sizes used most frequently by rural electric systems, single-phase regulators are usually less expensive. They also do a better job of maintaining balanced phase voltages under conditions of unbalanced loading. Single-phase regulators are also more adaptable to line use because of the relative ease of pole mounting. Regulation by single-phase regulators also gives maximum reliability for the system because a regulator can be removed for maintenance or repair without the need to de-energize transformers or other regulators. Special switches are available to permit removing a regulator from service without interrupting the circuit. These should always be provided.

In large distribution substations, the choice of three-phase regulators may be based on costs or on the availability of single-phase regulators of the required size. Three-phase regulators require somewhat less space than three single-phase regulators; however, this is not generally a major factor in selection. Load-tap-changing power transformers are becoming more common in distribution substations. They consist essentially of a three-phase regulator built into a three-phase power transformer. The relative cost of this combination compared to a separate transformer and either three-phase or single-phase regulators vary depending on the size of the substation.

Aside from the base cost of the equipment, the LTC method generally will result in a saving in space, buswork, and supporting structures. Because their controls sense only one phase of a three-phase circuit and since some unbalance may be expected among the phases, the voltage correction of three-phase regulators and LTC transformers will be less precise than that of single-phase regulators. Regardless of the selection of single-phase or three-phase regulators, a spare regulator for each substation is normally not justified.

Ratings
kVA Rating: The kVA rating of a single-phase regulator is the product of its rated load amperes and its rated range of regulation in kilovolts. For polyphase regulators, this product has to be multiplied by the appropriate phase factors (1.732 for three-phase regulators). The kVA rating of a 10 percent, 7620-volt, single-phase regulator capable of carrying a rated load current of 100 amperes would be:

                                                    kVA = 7.620 x .10 x 100 = 76.2 kVA

In those cases where the range of regulation is different for the “raise” position than for the “lower” position, the larger percentage regulation is used to determine the regulator kVA rating. The ratings for regulators generally are based on operation at 60 Hz with a range of regulation of 10 percent “raise” and 10 percent “lower” without exceeding the specified temperature rise at the given operating voltage. Regulator losses decrease as the regulator moves from the extreme tap positions (boost or buck) closer to the neutral point. Since the range of regulation required need not always be a full 10 percent, this allows for an extended range of regulator operation.