Because the reactors are fixed rather than switched, they cannot automatically regulate the voltage.
In the early 1950s, Australia saw a need to expand its electrical network to provide power to the country's agricultural areas. Because the loads were small and spread over a wide area, financial constraints demanded a network that was economical to construct and maintain, as the return on capital investment would take a lot of time. As a result, Advance Energy, New South Wales, Australia, turned to single-wire earth return (SWER) systems, which had been successfully tried in New Zealand several years earlier.
Initially, the loads nourished by the feeders were small. However, over the years, the loads grew and began creating problems for the existing SWER systems. One of the principal problems was greater voltage variation along the line. A common solution to this problem is to build a new three-phase 22-kV or 33-kV backbone feeder through the area, redesigning the existing SWER system into several smaller systems sustained by the new feeder. Although this approach is effective, it can be expensive — particularly for areas where load growth rates are inadequate to justify capital investment on a three-phase feeder.
Step voltage regulators (SVR) provide a more economical solution because they can be inserted in the SWER line to control voltage both under light load and at peak demand. Advance Energy's Deca design, a project in the western part of New South Wales, illustrates the effect of regulator application in a SWER system using the V-CAP software program for circuit analysis.
SWER systems are significantly different from the three-phase, three-wire and single-phase, two-wire systems commonly used throughout Australia. As the name implies, it is a single-wire distribution system in which all equipment is grounded to earth and the load current returns through the earth. Its loads are light and its lines are long, often causing the current to have a leading power factor.
The main features and components of the SWER system are:
System voltage. Typical SWER voltages are 12.7 kV and 19 kV, although they can range from 6.35 kV to 19.1 kV. The 12.7-kV and 19-kV levels are convenient because they are the phase-to-ground voltages of a 22-kV and 33-kV system, respectively, and allow the use of standard hardware and equipment. The industry standard for distribution voltages is ± -6% from a nominal 240 V. This standard established criteria for acceptable voltage limits in specifying and optimizing the location of voltage regulators and determining the maximum load capacity of the system. Studies allowed 2% for low-voltage regulation and assumed the use of 16- and 25-kVA, 19,000-kV/500-250-V transformers with an approximate 4% impedance and taps of ± -5%, 2.5% and 0%.
Isolation transformer. The isolation transformer isolates the earth currents (zero sequence currents) of the SWER system from the three-phase main supply feeder. This limits the exposure to telephone interference and allows the main supply feeder to maintain its sensitive earth fault detection protection. The transformer carries all the current of the system, including load current and capacitive charging current. The charging current on long feeders is significant, therefore, the impedance of the unit must be low.
Originally, the earth return current was limited to 8 A at the isolation transformer to avoid potential step-and-touch problems for livestock and humans. Although this regulation no longer strictly applies, currents still are kept relatively low. The typical isolation transformer sizes are 50 kVA, 100 kVA, 150 kVA and 300 kVA; however, 1-MVA units have been used on several occasions.
Conductor characteristics. Line length varies according to customer distribution, with an average SWER feeder length of 60 km (37 miles), although a 400-km (250-mile) SWER system is in operation in one state. Therefore, circuit losses because of the high resistance of the SWER conductors, reactive losses in the isolating transformers and resistive losses in the earthing systems can be up to 100% greater compared to those of a single-phase (two-wire) system serving similar loads.
Typically, conductors have a small diameter and high strength, and are made of aluminum/steel or steel cable. Sections at the sending end of the line often are 3/4/2.5 ACSR (34mm2 [0.053in2]) or are similar. Tee-offs and lightly loaded sections often are 3/2.75 SC/GZ (17.8mm2 [0.028in2]). These high impedance conductors in long SWER lines can cause the total impedance at the end of the feeder to be 1000 ohms.
Reactors for compensation. Charging currents are approximately 0.025 A/km for 12.7 kV and 0.04 A/km for 19-kV SWER systems. These charging currents, although similar to other types of distribution construction, are a concern because of long line lengths and high impedance of the conductors and supplying source. Shunt reactors usually are applied to the line to compensate for the charging current to reduce line losses, provide voltage control and minimize isolation transformer size. Because the reactors are fixed rather than switched, they cannot automatically regulate the voltage.
Load densities. Load densities for a SWER distribution system typically are less than 0.5 kVA per kilometer (0.31 kVA per mile) of line with a maximum demand per customer of 3.5 kVA. A large system may supply up to 80 distribution transformers with unit ratings of 5 kVA, 10 kVA and 25 kVA. The load patterns and demands vary greatly from customer to customer and from one season to another; thus, as load growth continues, SWER systems are reaching their technical capability. And with customers keenly aware of supply quality, customer complaints are increasing.
Earthing. Transformer earthing must be reliable and have low resistance, requiring frequent testing to maintain its reliability. Poor earthing systems reduce safety and supply quality. The maximum earthing resistances are:
A 5-kVA distribution transformer — 20 ohms
A 10-kVA distribution transformer — 20 ohms
A 20-kVA distribution transformer — 10 ohms
All isolation transformers — 2 ohms
Although a number of inherent disadvantages are associated with the SWER option (for example, load balance on the primary distribution line, restricted load capacity and the inability to provide a three-phase supply), there are many advantages to using SWER in sparsely settled areas, for instance:
A low capital cost — through fewer conductors, fewer pole-top fittings, graded insulation on distribution transformers, and fewer switching and protection devices. Although every new project will vary, savings of up to 30% per customer are common for long, lightly loaded feeders.
Simplicity of design, which allows for speed of construction. This particularly applies to the stringing of a single conductor.
Reduced maintenance costs, because there is only one conductor and no crossarm.
Fewer bush-fire hazards, because conductor clashing cannot occur in high winds.
As load on the SWER system increases, voltage regulation becomes an increasingly severe problem. Compared with three-phase system reinforcement, voltage regulators are an economic choice. Their capital expenditure is minimal, and they provide the control needed for adequate regulation, even at some distance from the sending end of the line.
The SVR is an autotransformer in which the series winding is tapped and equipped with a reversing switch that permits its voltage to add or subtract from the shunt-winding voltage. The voltage regulator, therefore, is able to boost or decrease (buck) the voltage on the load side as compared to the source-side voltage. A control winding senses the load-side voltage and supplies this intelligence to the control, which in turn activates an automatic tap changer on the series winding to raise or lower voltage. A line-drop compensation feature often is added to enable a constant voltage to be maintained at a load center remote from the regulator despite fluctuations in load. Figures 1 and 2 show the single-phase regulator and its schematic.
Figure 3 shows a diagram of the 19-kV SWER system used in Advance Energy's Deca project. The longest feeder is 161 km (100 miles), with the isolation transformer installed at Bindara Gate. The maximum loading for this line is 157 kVA, supplying 20 primary feeder loads via a radial network that comprises three conductor types — 3/4/2.50 ACSR (34mm2 [0.053in2]), 3/2.75 SC/AC (17.8mm2 [0.028in2]) and 3/2.75 SC/GZ (17.8mm2 [0.028in2]).
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:
Position a second regulator down line of the first.
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.