Opportunities Exist in Developing Countries for Innovative Technologies to be applied on networks that supply electricity to large rural communities. Often, these communities are in low-density rural areas of some 70 households/sq km (180/sq mile). As an alternative to extending medium-voltage (MV) overhead lines, it is now possible to install an electronic voltage regulator (EVR) on a low-voltage (LV) overhead network to maintain voltage within statutory limits.

When a new customer or group of customers is connected to a radial LV feeder, it is likely to cause voltage-regulation problems at the remote end of the circuit. This situation is not unfamiliar in the township layouts in rural areas. It presents an ideal opportunity to install an EVR, because it achieves acceptable voltage regulation, avoids reinforcing the network and is cost beneficial. The application of this innovative technology can be used to extend low-load-density networks by providing effective voltage regulation. Because the EVR offers an instantaneous response to load and supply fluctuations, it is the only means to regulate voltage-level variations of 20% from the nominal supply voltage.

Eskom (Sandton, South Africa) selected the standard power ratings of a single-phase EVR based on the standard supply options it offers to low-usage residential customers. South Africa's Homelight tariffs are applicable to single-phase supplies in areas designated by Eskom as rural or low density. This tariff, which has different energy rates based on the supply capacity required, includes a subsidy to low-usage customers (Table 1).

The 10-A current limited supply is the basic service for the poorest sector where extension of the network is feasible. Availability of this tariff option — combined with alternative MV single-wire earth return, LV dual-phase supply technologies and power electronic voltage controllers — allows settlements of low density to be given an electricity supply by bringing the average cost per connection to within the current cost targets.

Initially, it was proposed that several small EVRs could be positioned either in the service connection box or in the electricity dispenser on a LV feeder. However, during device development, it became clear this was not feasible in terms of component cost. A single EVR positioned in line at approximately 550 m (1805 ft) was deemed a more-viable solution, as it allows the feeder to be extended typically by another 550 m.

PLACEMENT CONSIDERATIONS

The lengths of LV feeders are governed by a maximum permissible voltage drop of 10%, as specified by South Africa's National Electricity Regulator (NER) standards. To allow for the after-diversity maximum demands of between 0.55 kVA and 0.85 kVA that occur after a period of five to 15 years, the LV feeder lengths in rural areas not equipped with EVRs need to be less than 550 m. By regulating the voltage at the point where the voltage reaches its minimum allowable level of ±10% below the nominal level, the LV line length can be extended. While the line length can be doubled, the area serviced by the application of this technology can be increased by a factor of four (Fig. 1).

In practice, the benefit is less significant, because the dwellings are not always evenly distributed within an area. The absolute low-end estimate would be a 50% increase in households when regulating the LV feeders at a single point per feeder. In this way, LV regulation saves one transformer-zone MV line and the transformer cost per two installed zones.

EVR TECHNOLOGY

The EVR was developed at the University of Stellenbosch (Stellenbosch, South Africa) with direction from Eskom. It is a step-voltage regulator comprising an auto transformer with five taps on the primary switched on by five thyristor modules. The nominal rating is 5 kVA with 100% overload capability. The solid-state switches selected were designed to withstand the system's short-circuit capacity and lightning surges. The EVR is manufactured under license by SEMIKRON South Africa (Arcadia, Pretoria), and distribution of this unit is currently limited to South Africa.

The device is self-protected against overvoltage, overcurrent and overtemperature, and it coordinates with the LV feeder protection philosophy. The EVR will auto-reclose after programmed intervals during fault conditions. Its specifications are summarised in Table 2.

Other features of the device include:

  • No audible noise
  • Very low distortion
  • No moving parts (fans or contactors)
  • Environmentally sealed/protected enclosure (IP 44)
  • No maintenance.

The EVR was designed with cost and environmental considerations playing an important role in development. The enclosure contains no ventilation holes for effective screening from the physical environment. Instead, a heatsink mounted on top of the enclosure is used to extract heat generated by the auto-transformer, which is the main source of losses from the enclosure.

A data acquisition board that slots onto the controller is an additional option to the EVR for research and investigation purposes. This board stores input and output voltage, line current and system ambient temperature data in flash memory that is downloaded via a Bluetooth communication link using host software developed specifically for the device.

Since June 2003, Eskom has installed more than 150 EVRs on long feeders in rural South Africa. Fig. 2 shows a typical installation.

Application in Rural Areas

EVRs are normally installed on MV systems in areas where the load density is very low and there are no access roads for trucks to bring in transformers, or where single dwellings are situated at a distance from the main settlement as shown in Fig. 3. On this feeder, the overhead line was extended from 507 m (1663 ft) to 1040 m (3412 ft), thereby almost doubling the standard maximum LV feeder length of 550 m.

The second example (Fig. 4) shows two EVRs installed on two phases of a LV feeder extending to 830 m (2723 ft) and 880 m (2887 ft) on two spurs. These EVRs were installed at a position some two-thirds down the line from the source. The voltage at the output terminals of the EVRs is regulated between +5% and +10% for 95% of the time, to allow for an additional voltage drop to the end of the line. Data loggers were installed at the end of the spurs to verify the voltages at these points remain within the ±10% limit for 95% of the time and always within ±15%.

The evening peak and subsequent voltage drops shown in Fig. 5 are typical of most installations. NER standards specify that the maximum voltage deviation shall not be greater than ±15% at all times.

The input voltage drops to approximately 200 V, which is 13% below the nominal voltage of 230 V, while the output voltage is boosted by 19% to approximately 238 V, or 3.5% above the nominal voltage. This allows the line to be extended by 280 m (919 ft) longer than the normal maximum length of 550 m to 830 m. The voltage at the end of the feeder is shown in Fig. 6. On the extended-line section, the voltage drops from 238 V to 233 V, or approximately 2%, during the peak. While these voltage drops occur less than 5% of the time, the voltage can further drop to -15%, or 195.5 V, which would make a much longer extension possible.

Low housing density in rural areas results in the need for voltage regulation on LV feeders as an alternative to system development and the installation of expensive MV equipment. A reliable EVR, the subject of cooperative research and development, has been successfully produced and installed on Eskom's distribution system. This unit, which can typically supply 10 rural dwellings, compensates for the downstream voltage drop while maintaining the voltage at its input and output terminals within the NER specified limits.

EXTENDING THE DISTRIBUTION NETWORK

The authors would like to acknowledge Bernard Meyer for his contribution during the initial stages of the development of the EVR.

ACKNOWLEDGEMENT

Dr. H.J. Beukes (jbeukes@sun.ac.za) holds bachelor's and master's degrees in engineering and a Ph.D. degree in electrical and electronic engineering from the University of Stellenbosch in South Africa. Since 1997, he has had the position of extraordinary senior lecturer at the University of Stellenbosch, and since 2001, he has been a senior consultant at Eskom. His principal interests are power electronics and their applications, fields in which he has authored and coauthored more than 60 national and international papers.

Dr. Hendri Geldenhuys (Geldenhuys.Hendri@eskom.co.za) holds bachelor's and master's degrees in electrical engineering from Pretoria University (Pretoria, South Africa) and a Ph.D. degree from the University of Witwatersrand (Johannesburg, South Africa). Following a 15-year career at the CSIR South Africa in lightning engineering parameters, protection and high-voltage testing, Geldenhuys joined Eskom, working on distribution and electrification of urban and rural communities. He is a registered professional engineer in South Africa.

Rob Stephen (stepherg@eskom.co.za) has a master's degree and a MBA, and his career positions have been linked to network and component design. He specialises in planning, electrification, thermal ratings and optimisation of overhead lines. A fellow of the South African Institute for Electrical Engineers and an honorary member of CIGRÉ, Stephen served as chairman of CIGRÉ SCB2-12 (electrical aspects) for nine years and chairman of CIGRÉ SCB2 (overhead lines) from 2000 to 2004. He is responsible for Eskom Distribution's network-expansion program.

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Table 1. Supply Options to Low-Usage Residential Customers

Input
Supply power 5.1 kVA
Supply voltage 230 V (±10%)
Supply current 24.2 A
Maximum fault level(protection coordination) 500 A
Peak current 1900 A (10 msec)
Harmonic distortion introduced None
Line current surges 40 kA (8/20 µsec)
Output
Load apparent power 5 kVA
Load voltage 230 V (+4%/-10%)
Load current 21.8 A
Voltage step change (maximum) 6%
Load power factor 0.2 to 1
Overload capability at ambient temperature of 15°C to 25°C (59°F to 77°F)
400% rated power 5 sec
200% rated power 40 min
200% rated power while limiting temperature 180 min prior to temperature trip
150% rated power 120 min
150% rated power while limiting temperature continuously
Overload capability at ambient temperature of 25° to 45°C (77°F to 113°F)
400% rated power 5 sec
200% rated power Temperature limited
200% rated power while limiting temperature 180 min prior to temperature trip
150% rated power Temperature limited
150% rated power while limiting temperature continuously
Losses
No load losses 25 W
Minimum efficiency (rated power at minimum supply voltage) 98.5%

Table 2. EVR Specifications