How Avr Operate. How To Size Up Avr.77006
How Avr Operate. How To Size Up Avr.77006
How Avr Operate. How To Size Up Avr.77006
OPERATION THEORY
A voltage regulator holds line voltage within predetermined limits and assures the proper operation of lights, appliances, and motors. To understand how a regulator operates, one must first understand how a two-winding transformer operates. Figure 1 is a basic diagram of a transformer with a 10:1 turns ratio. If the primary winding has 1000 volts applied, the secondary winding will have an output of 100 volts. These two independent windings can be connected so that their voltages may aid or oppose one another. A voltmeter connected across the output terminals will measure either the sum of the two voltages or the difference between them. The transformer becomes an auto-transformer with the ability to raise (Figure 2) or lower (Figure 3) the primary or system voltage by 10%. Since the low-voltage winding is operated at the system voltage level, adequate insulation to ground must be incorporated.
Figure 5. McGraw-Edison 32-step voltage regulator. Figure 1. Transformer with 10:1 turns ratio.
load current when 5/8% of line voltage is applied. Thus, both the interrupted voltage and the interrupted kVA have been halved, reducing the tap changer interruption duty.
Figure 7. Two movable contacts on the same stationary contact; center tap of the reactor is at the same potential.
Figure 8 shows a bridging position with one movable contact on tap 2, the other on tap 3. The bridging reactor limits the circulating current caused by the two contacts being at different positions. This difference causes the voltage change to be half the 1 1/4% tap voltage of the series winding at the center tap.
The Auto-Booster 4-step voltage regulator taps directly from one tap position to the next without a stop at the bridging position (Figure 10). For the 10% voltage regulator shown, every time a tap change occurs, a 2 1/2% voltage change is made. The 6% voltage regulator provides a 1 1/2% voltage change. Although the bridging position is not available as a steady-state condition, circuit continuity during switching can only be obtained if the bridging position is used during the transition period. Bridging resistors prevent excessively high circulating currents from flowing during the switching interval. Bridging resistors have been used extensively in Europe. The main advantages are simplicity and small size.
Figure 8. Two movable contacts on different taps; voltage change is one-half the 1 1/4% tap voltage of the series winding because of its center tap.
Some regulators, depending on the rating, use an equalizer winding to improve tap changer contact life. An equalizer winding is a 5/8% winding on the same magnetic circuit as the shunt and series windings. The equalizer winding is connected into the power circuit so that the reactor is excited at 5/8% of line voltage on both even and odd positions. Figure 9 shows an equalizer winding incorporated into the main coil of a regulator. Without an equalizer winding, the reactor on a regulator is not energized on even positions and energized at 1 1/4% of line voltage on odd positions. When an equalizer winding is used, the reactor is designed so the circulating current is maintained at 50% of rated
Figure 10. The Auto-Booster 4-step voltage regulator goes directly from one tap position to the next, without a stop at the bridging position.
TYPES OF CIRCUITS
The following types of circuits can be regu lated; A single-phase circuit (Figure 12); One phase of a three-phase wye or delta circuit (Figure 13); A three-phase, three-wire wye or delta circuit (Figures 14 and 15);* A three-phase, four-wire, multigrounded wye circuit (Figure 16). * Because of possible neutral shift with resulting overstressing of insulation and continual hunting, three regulators cannot be connected in ungrounded wye on a three-phase, three-wire circuit.
Figure 15. Regulating a three-phase, three-wire wye or delta circuit with three regulators.
Figure 16. Regulating a three-phase, four-wire, multigrounded wye circuit with three regulators.
Wye-connected regulators (Figure 17) work independently; delta-connected units are interrelated. Voltage improvement depends on the range of regulation and the voltage on one phase of the system. Figure 18 shows an open-delta connection and the interrelationship between phases. A voltage improvement of 10% in the phase with the regulator connected causes a 5% voltage improvement in the phase without a regulator. When both regulators provide a 5% voltage improvement to the third phase, all three phases are regulated to 10%.
Figure 17. Wye-connected regulators operate independently. Figure 18. Open-delta connection has interrelated phases. Figure 14. Regulating a three-phase, threewire wye or delta circuit with two regulators.
The closed-delta connection (Figure 19) causes a 10% voltage improvement in the connected, phase and a 5% improvement in the adjacent phase. As all three phases have regulators connected (Figure 20), the overall effect is to increase the range of regulation to 15%.
C. Range in kV: = range x line-to-neutral kV =0.1 x7.62 = 0.762 kV D. Regulator rated kVA = load amps x range in kV = 54.6 x 0.762 = 41.7 kVA With the selection of a standard rating, 32step regulators rated 57.2 kVA, 7620 volts, 75 amps, plus or minus 10% regulation meet the application requirements. Three regulators connected line to neutral are required. 2. Example To regulate a three-phase, three-wire circuit with a system voltage of 13800 volts and a connected load of 1250 kVA that requires a 10% voltage correction: A. Three-wire wye or delta circuit. B. Calculation to determine the size of the units: Rated load in amps = three-phase kVA x 1000 line-line volts x 1.732 = 1250 x 1000 13800 x 1.732
Figure 20. One regulator of closed-delta connectin shows range of regulator increased to 15%.
= 1.38 kV D. Regulator rated kVA = load amps x range in kV = 52.3 x 1.38 = 72.2 kVA
Standard rating, 32-step regulators rated 138 kva, 13800 volts, 100 amps, plus or minus 10% regulation meet this requirement. Two regulators connected line to line in open-delta connection are required.
Bulletin 77006 February 1993 Supersedes October 1991 File Ref: Catalog 225 1993 Cooper Power Systems, Inc. McGraw-Edison and Auto-Booster are registered trademarks of Cooper Industries.