This system was simulated using the V-CAP software, a generalized circuit analysis program for radial feeders. It has specialized tools that focus on voltage regulation and energy-loss evaluation, including voltage regulator models with tap-changing controls; shunt-capacitor models with a full range of switching controls; and load models that allow representation of load as constant impedance, constant kVA, motor load, converter, or mixed motor and impedance.

Each load can have a load profile that represents load changes over a 24-hr period in increments as small as 15 min. In addition, an annual load profile can be overlaid on the daily profile to account for seasonal load variations. The load modeling features permit a much more accurate annual loss evaluation. Regulator control modeling permits the simulation of voltage set points, time delay, bandwidth, PT and CT ratios, and maximum and minimum voltage limits. Although the program primarily is used for three-phase circuit analysis, Advance Energy modified it to represent the single-phase system with a 20-ohm ground resistance.

Advance Energy used the program for a voltage profile of its SWER system under various operating conditions. Figure 4 shows the voltage profiles scaled to a 120-V base, the two horizontal lines marking the statutory limits for the distribution transformer secondary voltage (for example, 126 V to 114 V). The theoretical system voltage profile for the total system of 394 km (245 miles), with 300 kVAR charging capacitive VARs operated at no load and with no shunt reactors, results in a voltage 25% higher than the system nominal voltage.

Shunt reactors usually are added to control the voltage. On this system, 11- to 25-kVAr units are scattered around the circuit to achieve about 90% compensation of the capacitance.

While the shunt reactors are necessary for maintaining the voltage control during light-load conditions, they continue to hold the voltage down during loaded conditions. In this example, the maximum load of 157 kVA causes voltage depression. The voltage drops below 117 V quite quickly and reaches as low as 109 V at the end of the feeder. Off-load taps at the isolation transformer are of some help but cannot be raised too high or the voltage during light loads will be excessive.

A voltage regulator can provide the control necessary to maintain voltage within statutory limits over the range of light and heavy loads. Figure 4 illustrates the effect of adding a regulator to the line. In this example, the regulator is positioned on the network where the voltage approaches the lower limit. The regulator taps up to its set-point of 122 V, and the voltage continues to fall to 115 V at the end of the feeder. This is below the acceptable primary voltage range, but setting the voltage set-point higher could help boost the voltage at the remote end. However, this would result in excessive voltages during light load.

Two solutions to the under-voltage problem are:

1. Position a second regulator down line of the first.

2. Use the line-drop compensation feature of the regulator control.

A second regulator can be added to raise the voltage once the voltage drops to the lower limit. The optimum locations for two regulators in series on a conventional feeder are approximately 20% and 50% of the entire line length from the source. On the Advanced Energy SWER system, the regulators were located at the 12% and 44% positions. In this case, the high source impedance of the SWER system required the regulation to be closer to the source.

When two regulators are used in series, it is important to adjust the controls so they coordinate tap changes. The preferred method is to adjust the time-delay settings of the farthest regulator to be greater than the sending-end regulator, allowing the sending-end regulator to respond first. The line-drop compensation feature of a regulator control also can be used to correct the undervoltage. Figure 4 shows the effect of line-drop compensation on the voltage profile — the regulator is better used and the voltage is brought within limits. At light load, the extra boost is reduced because the smaller voltage drop of the line results in a smaller subtracted voltage.

The use of shunt reactors, voltage regulators and the line-drop compensation feature effectively controls the voltage to the end of the feeder. Advance Energy did not consider either of the other available options for voltage control in this study, namely, switched shunt reactors and switched shut capacitors, because of the remoteness of the SWER system. Although these options can be effective for voltage control and for reducing losses, operations and maintenance for the multiple devices (which must be scattered over a broad area), they may make the switching of these devices impractical.

SWER systems now cover extensive areas of Australia. By 1997, New South Wales had about 35,000 km (21,700 miles) of SWER lines and many other states had incorporated SWER into their distribution networks. In addition, Canada, India, Brazil and South Africa all use the SWER design to supply electricity to thinly populated areas.

Through the Advanced Energy project and several other experiences, Australian electricity distributors have gained extensive experience in designing large-scale SWER systems. And as the project demonstrates, hardware and software is now available to ensure these networks will continue to supply economical, high-quality electrical energy into the future.

Neil Chapman is the overhead development manager for Advance Energy. He joined the electricity supply industry in 1974, since which he has held various operations and engineering positions. Chapman has the electrical trade certificate and the associate's diploma in electrical engineering from Orana Community College. He also has the graduate diploma in management from Charles Sturt University.