Rural electric networks normally comply with urban standards. Here a transformer subsation is under construction in Mozambique.

 

Rural Transformation

 
 
 

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Single Phase line and Single-Phase Transformer

 

 

Rural elelectrification standards and different design options


Cost effective primary supply to rural areas is commonly at the higher end of the distribution voltage range (11kV to 34.5kV) as loads are often distributed and scattered over long distances and vast areas. Presently 11kV, 22kV, 33kV and 34.5kV are currently the voltages of choice for supply to isolated rural communities in most Sub Sahara African countries. Supply can be designed to any one of the following systems:

 

Standardized Three-phase Systems. Backbone rural feeders built as three-phase lines, commonly with aluminum conductor of 70mm2 to 150mm2 provide a technically sound and economic solution over reasonably long distances. There are four main options: a 4-wire system and three 3-wire systems.

 

The 4-wire System. This option is based on the American system with the 4th wire being an aerial ground erected 1.5 to 2meters under the phase conductors. This System enables 1-wire + ground tap off to loads away from the backbone and the utilization of low cost single MV bushing transformers. This system is also common in South American countries and the Philippines for urban and rural reticulation typically at 34.5kV and 13.8kV and in Bangladesh at 33kV for rural reticulation. Ghana operates a 34.5kV solidly grounded neutral system, which is a standard 4-wire system voltage.

 

The 3-wire Impedance Earthed System. This system is currently used in Mozambique and Uganda at 33kV. A three-phase 3-wire system of several feeders provides the backbone. It is capable of having 2-wire tap-off connections and 1-wire SWER using isolating transformers, but these options are not currently used.

 

The 3-wire Directly Earthed System. This establishes a solidly grounded three-phase primary substation point of supply, supplying the three-phase backbone. It is similar to option 1a except that 1-wire unisolated SWER tap-offs can be used to distribute to local loads. For SWER loads current return is via earth to the primary substation.

Unisolated SWER loads require balancing of loads on each feeder phase to minimize false operation of protection on other feeders and to reduce safety hazard issues. Also interference with open wire communication systems can be a significant problem. These problems are as a result of tripplen harmonics (third and its multiples) which are additive and cannot be balanced out by balancing of loads.

Adoption of this option would require a change to the current policy of earthing via a neutral reactor at primary substations as per option 1b. It is notable that South Africa is adopting this system where new primary substation points of supply are required to serve rural areas.

 

3-wire Primary Isolating Transformer System. This system is sometimes referred to as a triplex system, but maintains the integrity of the three-phase backbone for delivery of three-phase loads to those areas where required.  This system, which is being currently introduced into South Africa, enables one isolating transformer of 1MVA to 2.5MVA or more to supply a larger isolated power system area with a three-phase backbone. Laterals may be three-phase or single-phase 2-wire or 1-wire SWER without requiring additional isolating transformers for the SWER.

Careful balancing of loads is also required as for the directly earthed system, but as this system operates from one feeder from the primary substation and is often remote from the point of supply, interference to open wire communication systems, is localized.

The three-phase 4-wire system is more expensive than the three-phase 3-wire system. The 1-wire + ground system (1 insulated wire + 1 wire grounded) has a slightly lower per km cost than the 2-wire phase to phase system as no insulation is required for the ground wire. However, because of the requirement to have a three-phase 4-wire supply the overall cost to provide the lead up ground wire generally results in a slightly more expensive system. The system was developed in the USA to deliver supply to a large number of small distribution transformers required to deliver 110V-0-110V to consumers.


Single-phase Systems. Single-phase construction is appropriate for feeding smaller isolated loads. There are three different single-phase MV systems:

Phase to phase (or single-phase) is a 2-wire system utilized to provide a single-phase supply at line to line voltage. This system is commonly used worldwide and can be upgraded to three-phase very easily where the design allows for this.

Phase to ground wire is a 1-wire + ground wire system as detailed above.

Single Wire Earth Return (SWER) in which a single-phase 1-wire, derived usually from an isolating transformer, is utilized alone. The current returns through the ground. There are four main variants of this system:

Single Wire Isolated System that employs an isolating transformer at the tapping point. This form of SWER is the most widespread in use.

Single Wire Unisolated System that compliments options above. This system requires a solidly grounded earthing system at the primary substation or primary isolating transformer.

Two-wire Duplex System provides a 2-wire isolated backbone. This system provides two 19.1kV circuits in a 19.1kV-0-19.1kV format or 38.1kV line to line. 1-wire unisolated tap-offs are made from this backbone. It can be used where additional capacity is required over that deliverable by the single wire system. Harmonics, as well as fundamental ground currents are cancelled by balancing loading, thereby reducing communications interference to about 10% of a conventional SWER circuit.

Three-wire Triplex System similar to the 3-wire primary isolating transformer system outlined in option 1d, except that the three wires usually radiate in different directions after the transformer with little or no backbone component. This enables the establishment of a larger isolating transformer and by balancing the loads the earth return current is minimized.


Reducing costs of rural distribution networks by increasing pole-span lengths, using steel conductors and change to Single Phase Approach.


Distribution of power in rural areas could in many circumstances be significantly cheaper taking into consideration a few basic parameters. The extremely low power required (< 700 kVA), distances which are moderate (< 40 km), and no immediate need for three-phase power supply.

It is commonly known that a remarkable price reduction of rural distribution systems can be achieved if pole span widths are increased. Typically span lengths in today’s distribution networks are in the region of 100-150 m, resulting in 7-10 poles per km. However, using steel conductors very long spans, up to 400 m (2.5 poles per km), can be permitted, reducing the number of poles and pole components to 25 – 30%.

By the higher tensile strength of steel, compared to aluminum or aluminum steel reinforced (ASR) conductors, very long spans are therefore feasible from an economical point of view. Cost reductions in this case may be as much as 50% of traditional designs as the poles (pylons) are a main cost element in rural distribution networks.

If growth of electricity requires it, the capacity of the line can later on easily be upgraded by adding more poles and changing the conductor.

However, this simple scheme can also be combined with a Single Phase Earth Return (SWER) approach saving even more in up-front investment costs. In that case a minimum of pole-top hardware is needed and the overall conductor length reduced by 66%.

Long span SWER designs could ultimately reduce costs for low power rural distribution networks to 20% of “standard three phase designs”. Even in this case an easy upgrading is possible


Relative Rankings

For small loads, as is typical for many rural communities, single-phase systems are cheaper and can be more reliable than three phase systems. Higher reliability is due to fewer wires exposed to failure causes such as trees, birds or line breakage's and a reduced number of insulators. The relative merits of each system are shown below.

Table 1 - Relative Rankings of Rural Power Distribution Systems

Indices of

System Arrangement: 1 = Best, 3 = Worst

 

Three Phase   3-wire

Phase to Phase

SWER

Reliability

2

2

1

Voltage Regulation

1

3

2

Losses

1

3

2

Capital Costs

3

2

1

Ease of Construction

3

2

1

Thus application of the appropriate system arrangement can provide significant cost reductions in rural electric distribution without compromising reliability.

Design Parameters, Electrical Characteristics and Performance Standards

There are a number issues that require review to optimize rural reticulation costs. Environmental issues have the single biggest impact in terms of line costs. The key environmental issues are design wind level and maximum conductor operating temperatures. The issues on changing parameters recognize that risk and probability of events occurring must be aligned to the particular circumstances being designed.

The recommendation is that the design standards for wind and conductor temperature be reviewed to optimize rural design and hence minimize costs. Also in the area of performance standards, larger allowances for volt drop may be tolerable. The following sub-sections review the key issues.

Risk and Probability

The consideration of risk and taking a probabilistic approach has the potential to significantly reduce rural construction costs. Reliability can be defined as "the probability that a line performs a given task under a certain set of conditions, during a specified time". The compliment to reliability is risk, or the probability of failure (unreliability). IEC 826 specifies structural reliability levels in the context of a stress event being exceeded within a return period (time) of a stress condition related to an extreme weather event or a combination of events as given in the table below.

Table 2a - Event Probability

 

Return period of the Event

50 years

150 years

500 years

Probability of exceeding the event in any one year

2%

0.67%

0.2%

In 50 years

63.2%

28%

10%

Many building standards, from which often line design standards are derived, use either a 1:350 year or 1:1000 year return period. The table below sets out reliability levels suggested for distribution lines and feeders.

Table 2b - Event Probability for Distribution Lines

Return period of the Event

35 years

50 years

98 years

35 years

63.2%

50%

30%

50 years

 

63.2%

40%

To minimize risk in an urban area it would be appropriate to design to the upper end of the scale, whilst in rural areas to the lower end of the scale. The impact on design wind levels and temperatures follow.

Impact on Design Wind Loads

The primary impact in considering risk fully, is the potential reduction of cost bought about by reducing the design wind pressure. This in turn allows maximizing the spans subject to pole strengths and conductor sag. Thus in sparsely populated areas should such a risk event occur, the risk to man and beast will be very low. In the areas where people are likely to be, such as villages, then a design appropriate to urban construction should be used.

Conductor Temperature

Lowering of maximum conductor temperatures can result in a significant reduction in maximum sag. This potentially enables longer spans, subject to wind loads and pole strengths. The risk evaluation is in determining the probability of the maximum design temperature being exceeded, hence the ground clearance reducing and the co-incidence of contact by the public. The key issues in lowering design temperature areThe lowering of conductor temperature may be achieved by evaluating the probability of the maximum design temperature being exceeded. In rural design the use of mean maximum temperature is appropriate.

In rural areas conductor current flow is light compared to the conductor capacity. Therefore the contribution of conductor heating from I2R losses is minimal at 33kV or in SWER lines and can be ignored.

The timing of maximum demand will impact. If MD is in the evening when it is cooler this enhances the flexibility to reduce design temperature.

In rural areas the impact of fault current conductor heating on conductor sag is ignored.

These considerations may result in design temperature reducing from 600C to say 450C or 500C and a corresponding reduction in design maximum sag. For 33kV the minimum ground clearance internationally is 6.5m over roads and other land 5.5m, but standards could accept a lower clearance on the basis of determining an appropriate risk. This will depend primarily on the flashover clearances and the maximum height of vehicles including such issues as the populous riding on bus roof tops etc.

Coincident Maximum Wind and Ambient Temperature

Another conductor/ambient condition is the expectation of maximum design wind and the ambient temperature at which this occurs. For example, in tropical areas storms arrive in the warmer months of the year. In cold climate environments maximum winds tend to be co-incident with low temperatures. This issue has bearing on maximum conductor tensions hence structural loading. It is a minor issue in tropical environments.

Span Reduction Factor

Wind does not present a uniform wall of force. For long spans a wind load reduction factor can be applied. For example Australian Standards provide for a span factor of 1.0 at 100-meter wind spans, reducing to 0.5 at 300 meter or larger wind spans.

Standard Performance

Design standards establish acceptable volt drop tolerances to the consumer. These tolerances are typically 6% to the customers point of connection to the network plus an allowance of typically of 2.5% in the service main but may range to drops exceeding 10% at the customers installation.

In long rural feeders the constraint is not conductor rating but voltage drop in the lines in delivering a design load. Typically 3 to 5% volt drop is used in the design of rural overhead lines, however in remote rural areas up to 10% is commonly permitted. This impacts on the end consumer, as the consumer installation voltage can drop to 85% of the rated voltage. Providing motor starting is restricted during such periods most household appliances will tolerate the volt drop but lighting will become dimmer. These voltage issues only arise when the line is approaching design capacity.

Reliability of Supply

High reliability of supply cannot be provided economically in rural areas, nor is it warranted in terms of the cost of interruptions. On the other hand the cost or repairs in remote areas is high. Designs should incorporate materials of adequate durability and quality, and incorporate simple, low cost and readily available components easy to transport and erect.

The phase to phase and SWER reticulation options are inherently more reliable than a three phase system. 1-wire SWER has additional advantages over both the 3-wire and 2-wire systems in that there are no conductor clashes and right of way clearance requirement from vegetation is less exacting. Given the long line lengths it is inherent that the rural lines will be less reliable than urban lines on a per km basis. Appropriate selection of components, particularly insulator selection and provision of surge arresters on transformers will minimize the impact of outages in high lightning prone areas.

Upgradability

Smart selection of pole heights and strengths, conductor types and configuration arrangements enable a SWER constructed system line to be built in such a manner that will enable simple upgrade to a 2-wire single-phase or 3-wire three-phase system. Given that the initial capital cost of a well designed SWER line, including isolating transformer, is 25 to 30% of the initial capital of a three-phase line substantial project cost reductions are achieved. The upgrade would be at a capital cost that would only be marginally higher (<15%) overall than building as a three-phase system initially. In practice the requirement to upgrade to three phase is not likely to eventuate for a long time (10 to 30 years) after the initial installation. Discounted cashflow calculations support the case for the lower initial installation cost.

The distribution transformers would also require changing to a 33kV supply voltage. For a three-phase upgrade some additional three-phase LV distribution could be required and re-balancing of phases for all single-phase loads. Any 460V single-phase motors would need reconfiguring to 230V or replaced with three-phase units.

Similarly a 2-wire single-phase system can be designed for easy upgrade to three-phase, though in practice such an upgrade is rarely required unless a locating industry has larger three-phase motor loads. Technologies are available that enable the production of a three-phase supply from a single-phase system if motor loads exceed 22kW. The cost and serviceability of such systems requires balancing against the cost of the upgrade.

In practice only a percentage of rural supplies will require future upgrade within the life expectancy of the line. Even if the unexpected happened and some lines require upgrade within a short duration (< 5years), then the overall savings and hence economics are still favorable. These events will happen due to the enterprise of those communities that will fully grasp the opportunities of electrification. In such cases growth will justify and fund the additional capital.

Maintainability

The 2-wire and 1-wire systems have lower maintenance requirements: directly analogous to the reliability. With SWER Systems there are two issues that will impact:

  • Firstly, the maintenance requirement and stringent testing of SWER transformer earth mats.

  • Secondly, an additional item of equipment is put in the chain: the isolating transformer. Transformers are low maintenance items, however the availability of spares is a key issue to manage.

Safety and Protection Systems

A key issue with all long rural feeders is the ability to clear a fault. The fault currents are often very low. This can make protection discrimination difficult. Automatic reclosers and sectionalizers are an essential part of rural power schemes to isolate faults, particularly as many faults are of a transient nature. Though the application of such equipment raises the initial capital cost of a project, the long-term minimization of fault response costs and outage inconvenience to consumers justifies the expenditure.