This document discusses primary substations and bus layouts in industrial distribution systems. It provides details on 10 typical arrangements for main substations with single or multiple transformers and incoming power lines. These include configurations with circuit breakers or fuses. It also discusses considerations for reliability, flexibility and cost. Additionally, it covers various bus arrangements for single or multiple power sources, including straight, sectionalized and star/synchronizing layouts. The goal is to provide redundant power paths and allow maintenance without outages.

1. Primary substations and bus
layouts in the distribution
system of an industrial plant
By Edvard | March, 2nd 2020 | 0 comments | Save to PDF
Share!
Home / Technical Articles / Primary substations and bus layouts in the distribution system of an industrial plant
Primary Distribution Systems
The primary distribution system of an industrial plant is generally the higher
voltage portion of the system, starting with the purchased-power service and
including generators, switching equipment, circuits, and all transformers with
secondary voltages higher than 600 V. This technical article tends to explain the
main substations and typical bus arrangements in the primary distribution
system of an industrial plant.

2. Primary substations and bus layouts in the distribution system of an industrial
plant (photo credit: HS Switchgear FZCO)
But, first, let’s say a word about planning such complex distribution system for
an industrial plant. Coordinated planning guided by overall system
characteristics is the only way desired design objectives can be achieved. Such
system characteristics as cost, safety, reliability, flexibility, and simplicity should
be viewed together only, because they will be interrelated in varying ways.
Distribution system arrangements tending to favor a particular desirable
characteristic will most often tend to produce compromises in one or more other
desirable characteristics.
System cost is the characteristic that receives the most planning attention. In
comparing alternative distribution layouts it is very helpful to hear in mind
that lower cost is by no means synonymous with better value. Service
reliability is considered to be improved when the arrangements are modified in
ways that promise to reduce outage time during maintenance operations or in
the event of trouble.

3. The general idea is to provide more than one power channel around system
components that need maintenance or might fail. Increased investment for such
provisions may be money wasted unless the system is well planned in some
other respects.
The primary requirements for good service reliability are that good-quality
adequate equipment will be selected, that it will be properly installed, and that it
will be well maintained.

5. Figure 0 – Typical regular and emergency supply for large industrial plants
Table of contents:
1. Main substations
2. Bus arrangements
<="" ins="" data-adsbygoogle-status="done" style="box-sizing: border-box;
height: 250px; width: 300px; display: block;">
Main substations
Not all plants own and operate a main substation for supplying the primary
distribution system. A plant main bus serves the same purpose if the
purchased-power voltage is suitable without transformation for the plant primary
system.
The principal functions of a main substation are indicated in Figure 1, which is a
simple arrangement answering the requirements of a great many smaller plants.
Figure 1 – Typical main substation arrangement used by
industrial plants
More complicated substation arrangements result when there are two or more
incoming lines, two or more power transformers, or one of a number of other
bus arrangements: Also in plants with power generation, the substation output
may not supply a plant main bus but may be connected to a synchronizing
bus.

6. The substations in a few vary large plants with heavy loads in widely separated
areas may require transmission-voltage feeders connected to the incoming-
line bus.
Figure 2 differs from Figure 1 in using power fuses instead of a circuit
breaker in the incoming line. Circuit breakers are generally preferable, but
fuses will be useful in satisfying over-all objectives in some of the smaller and
simpler substations.
Figure 2 – Typical main substation arrangement used by
industrial plants
When substation primary fuses are used, it is better to employ solid neutral
grounding of the transformer secondary than to limit the ground-fault current in
the primary distribution system.
The remaining substation examples all show two supply lines. In these
stations it will often be necessary to accept some functional compromises in the
high-voltage switching equipment for cost reasons. When we talk about smaller
plants that are served from higher voltage systems, the main substation high-
voltage circuit-breaker equipment can be disproportionately expensive among
the other substation components.
Stated in another way, a given high-voltage breaker arrangement for a given
supply system will cost just about the same regardless of the substation
size. The discussion is intended simply to indicate what the several
arrangements offer.

7. Figure 3 – Typical main substation arrangement used by
industrial plants
Figure 3 shows a two-line single-transformer substation using two high-voltage
circuit breakers. This arrangement might be used whether the two lines are
alternate, paralleled, or part of a loop.
For a loop supply, the substitution of a circuit breaker for the transformer horn-
gap switch would avoid opening the loop by the transformer protection scheme.
The use of either two or three circuit breakers might be hard to justify in
particular cases.

8. Figure 4 – Typical main substation arrangement used by
industrial plants
For alternate-line or preferred-emergency supply, the single circuit breaker in
Figure 4 with interlocked incoming line switches has a minor deficiency in not
permitting an automatic transfer between lines.
Either Figure 3 or Figure 4 permits expansion by adding one or more
transformers to the high-voltage bus. Figure 5 is simply an extension of Figure 3
for a two-transformer substation where the two incoming lines are alternate,
paralleled, or part of a loop.
As illustrated with four high-voltage breakers, this substation arrangement can
provide an unusually high degree of service reliability, except for a high-
voltage bus fault.

9. Figure 5 – Typical main substation
arrangement used by industrial plants
For the special case of two incoming lines that may be operated in parallel
but are not a loop supply, the arrangement of Figure 6 is often a good
solution.
By omitting the high-voltage bus, and paralleling on the low-voltage side of the
transformers, a saving in high-voltage breakers and structure is
accomplished. The arrangement reduces the availability of the total
transformer capacity because each unit has a transmission line in series.
However, the station-cost reduction may be so significant for smaller
substations that load curtailment during an outage becomes an acceptable risk.
It is moreover possible to reinvest part of the circuit breaker saving in additional
size of transformer units to achieve service continuity for all the load or to
reduce the amount of load curtailment during half-capacity operation.

10. Figure 6 – Typical main substation arrangement
used by industrial plants
Referring again to Figure 6 in connection with loop supplies only, the high-
voltage part of the substation employs almost all the circuit breakers that can be
fitted into a single-bus arrangement.
However, a fifth circuit breaker could be added in the bus. With appropriate
relaying, it would ensure continuity of service through one transformer
under the condition of a high-voltage bus fault.
It is perhaps more profitable to observe how reliability and flexibility are
modified by removing circuit breakers one at a time, as illustrated in Figure 7 to
Figure 10. In the three-breaker scheme of Figure 7 the main functional
compromise is that transformer protection requires opening the loop supply.

11. Figure 7 – Typical main substation arrangement
used by industrial plants
A utility would not ordinarily consider this as a serious shortcoming, but it could
be avoided in the alternative three-breaker scheme of Figure 8. Either of these
arrangements provides service continuity through one transformer for any
single fault, including a high-voltage bus fault.
Figure 8 – Typical main substation arrangement
used by industrial plants

12. In the two-breaker scheme of Figure 9, operation of the protective relaying of
either transformer not only opens the loop but drops the whole substation
load. The loop can be reclosed and plant service can be reestablished through
the unfaulted transformer circuit by manual switching.
A permanent high-voltage bus fault must, of course, be repaired before either
circuit breaker can be reclosed.
Figure 9 – Typical main substation arrangement
used by industrial plants
In attempting to use a single breaker as shown in Figure 10, a problem is
encountered. Any high-voltage fault down to the transformers will be cleared
by the single circuit breaker and the utility as a single-line short circuit, leaving
uninterrupted plant service through one transformer.
However, there will be a level of transformer fault current below which the utility
cannot trip, and the faulty unit cannot be automatically disconnected from the
system at such a level of overcurrent except by transferred tripping of a power-
company circuit breaker using carrier or a pilot wire.

13. Figure 10 – Typical main substation
arrangement used by industrial plants
Go back to contents ↑
<="" ins="" data-adsbygoogle-status="done" style="box-sizing: border-box;
height: 250px; width: 300px; display: block;">
Bus arrangements
A bus is a junction of three or more incoming and outgoing circuits. The most
common plant bus arrangement consists of one source or supply circuit and two
or more feeder circuits. The numerous other arrangements and variations are
mainly intended to improve the service reliability through the bus to all or part of
the load during expected maintenance, or in the event of equipment failure or
source outage.
Some very complicated bus arrangements have been used in trying to
improve service reliability or continuity. Some of these arrangements are
technically unsound and will not provide actual benefits. Other arrangements
that do qualify from an engineering viewpoint are useful in meeting the rather
typical requirements in the heavy industries that handle large amounts of power
through main and sub-distribution buses.
These same bus arrangements will seldom prove acceptable for cost reasons in
medium-size and small systems even when service continuity is considered to
be unusually important.

14. The highest quality of service reliability can often be obtained more
economically for smaller plants, particularly for those with load-center
systems, by over-all system arrangements that employ simpler and less costly
bus arrangements.
The double-bus arrangement shown in Figure 11 is an example of the more
complicated arrangements that is technically sound if good-quality equipment is
used, but it is very costly for the usual sizes of feeder circuits.
Figure 11 – Typical bus
arrangements used by industrial plants
The arrangement is suitable for outdoor circuit breakers, station-type cubicles,
or metal-clad construction. In metal-clad switchgear, some requirements can be
met a t lower cost by employing two positions and one circuit breaker per circuit
plus one spare removable circuit-breaker element, as illustrated for one of the
several circuits.
This variation still allows transferring any circuit or maintaining any circuit
breaker without a feeder interruption.
Figure 11 was intended to indicate a preferred physical arrangement with
companion circuit-breaker compartments in separate standard equipment facing
each other across an operating aisle. A cable connection would usually join the
circuit breakers.
Most of the more complicated arrangements have in common the general
characteristic that, individual lines can be connected to either of two buses
(often without service interruption) with good maintenance access to most of the
apparatus. Intermediate flexibility and reliability can be more economically

15. obtained for multiple-source bus arrangements by sectionalizing straight
single buses.
Figure 12 illustrates a typical two-source sectionalized-bus arrangement with a
single circuit breaker per line. Figure 12 or some variation places lines and
breakers on the same basis of availability.
Figure 12 –
Typical two-source sectionalized-bus arrangement with a single circuit breaker
per line
Where metal-clad switchgear is used in the primary system, a feeder outage for
circuit-breaker maintenance can be reduced to a matter of minutes with a spare
removable circuit breaker on hand.
In extending reliability from a main bus to a sub-bus in an important load area,
parallel feeders may be used. In the load-center system each load-center
transformer has the same availability as its primary feeder and supply breaker.
Improvement in service reliability is secured by interconnection at secondary
voltage.
When three or more sources are available at a main bus, Figure 13 is a natural
extension of Figure 12.

16. Figure 13 – Three-source sectionalized-bus arrangement with a single circuit
breaker per line
However, Figure 14 is more flexible and is usually preferred even when another
circuit breaker is needed. This arrangement may be referred to as a star bus,
but it also is sometimes called a synchronizing bus arrangement whether any
of the sources is a generator or not.
Particularly if reactors are needed to parallel the sources, Figure 14 will be
preferable to a straight bus (or a riug bus) with the current-limiting reactors
installed between each tie circuit breaker and the common bus.

17. Figure 14 – Star bus arrangement (or synchronizing bus arrangement)
The need for tie circuit breakers is obvious in some straight buses, but there are
other cases where the value may be in doubt.
Experience shows they are too often omitted where a choire rail can be made in
the planning stage. The following remarks are intended to summarize the
various ways in which bus-tie circuit breakers may be useful initially and later.

18. When two sources are used simultaneously but must not be operated in
parallel, a normally open bus-tie circuit breaker interlocked with the source
circuit breakers permits serving both bus sections from one of the sources when
the other is not available.
Reasons for not paralleling the sources might be that they are not
synchronized or have a phase-voltage difference. Another reason could be
to reduce the bus short-circuit duty either initially or in the future if the duty
might be increased beyond desired limits through additions to the source
capacity.
For alternate (or preferred-emergency) or normally paralleled sources, a single
straight bus may be used. It is preferable to use a normally closed bus-tie
circuit breaker so that one bus section can be kept available when the other is
out for maintenance or repair or to permit additions during a plant expansion.
For paralleled sources, relaying of the tie circuit breaker may be employed to
split the system so that service continuity is retained on one bus if the other bus
fails or it became necessary to back up a feeder circuit breaker on that bus.
Go back to contents ↑
Sources: Industrial Power Systems Handbook by Donald Beeman