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.
