Basics of Distribution Substations For Electrical Engineers (Beginners)
Basics of Distribution Substations For Electrical Engineers (Beginners)
Basics of Distribution Substations For Electrical Engineers (Beginners)
kV (of these, 69, 115, and 138 kV are most common) for delivering electrical
energy to the various distribution substations.
Three-phase primary circuits or feeders , which typically operate in
the range of 4.16 to 34.5 kV (11 to 15 kV being most common) for supplying
the load in designated areas.
Distribution transformers rated from 10 to 2500 kVA, installed on poles,
Figure 2 Simplified diagram of a power system from generation to distribution
Power is switched from the substation transformers as shown in Figure 1 above to
separate distribution buses. In some systems the buses distribute power to two
separate sets of distribution lines at two different voltages.
Smaller transformers connected to the bus step the power down to a standard
single-phase line voltage of about 7.2 kV for residential and rural loads, while
power from larger transformers can leave in another direction at the higher threephase voltages to serve large industrial and commercial loads.
This arrangement involves one main bus with all circuits connected directly to the
bus. The reliability of this type of an arrangement is very low. When properly
protected by relaying, a single failure to the main bus or any circuit section between
its circuit breaker and the main bus will cause an outage of the entire system. In
addition, maintenance of devices on this system requires the de-energizing of the
line connected to the device.
Maintenance of the bus would require the outage of the total system, use of
standby generation, or switching to adjacent station, if available. Since the single
bus arrangement is low in reliability, it is not recommended for heavily loaded
substations or substations having a high availability requirement.
Reliability of this arrangement can be improved by the addition of a bus tiebreaker
to minimize the effect of a main bus failure.
This scheme provides a very high level of reliability by having two separate
breakers available to each circuit. In addition, with two separate buses, failure of a
single bus will not impact either line. Maintenance of a bus or a circuit breaker in
this arrangement can be accomplished without interrupting either of the circuits.
This arrangement allows various operating options as additional lines are added to
the arrangement; loading on the system can be shifted by connecting lines to only
one bus. A double bus, double breaker scheme is a high-cost arrangement, since
each line has two breakers and requires a larger area for the substation to
accommodate the additional equipment. This is especially true in a low prole
conguration.
The protection scheme is also more involved than a single bus scheme.
This scheme is arranged with all circuits connected between a main (operating) bus
and a transfer bus (also referred to as an inspection bus). Some arrangements
include a bus tie breaker that is connected between both buses with no circuits
connected to it.
Since all circuits are connected to the single, main bus, reliability of this system is
not very high. However, with the transfer bus available during maintenance, deenergizing of the circuit can be avoided. Some systems are operated with the
transfer bus normally de-energized. When maintenance work is necessary, the
transfer bus is energized by either closing the tie breaker, or when a tie breaker is
not installed, closing the switches connected to the transfer bus. With these
switches closed, the breaker to be maintained can be opened along with its
isolation switches. Then the breaker is taken out of service. The circuit breaker
remaining in service will now be connected to both circuits through the transfer bus.
This way, both circuits remain energized during maintenance. Since each circuit
may have a different circuit conguration, special relay settings may be used when
operating in this abnormal arrangement.
When a bus tie breaker is present, the bus tie breaker is the breaker used to
replace the breaker being maintained, and the other breaker is not connected to
the transfer bus. A shortcoming of this scheme is that if the main bus is taken out of
service, even though the circuits can remain energized through the transfer bus
and its associated switches, there would be no relay protection for the circuits.
Depending on the system arrangement, this concern can be minimized through the
use of circuit protection devices (reclosure or fuses) on the lines outside the
substation.
This arrangement is slightly more expensive than the single bus arrangement, but
does provide more exibility during maintenance. Protection of this scheme is
similar to that of the single bus arrangement. The area required for a low prole
substation with a main and transfer bus scheme is also greater than that of the
single bus, due to the additional switches and bus.
This scheme has two main buses connected to each line circuit breaker and a bus
tie breaker. Utilizing the bus tie breaker in the closed position allows the transfer of
line circuits from bus to bus by means of the switches. This arrangement allows the
operation of the circuits from either bus. In this arrangement, a failure on one bus
will not affect the other bus.
However, a bus tie breaker failure will cause the outage of the entire system.
Operating the bus tie breaker in the normally open position defeats the advantages
of the two main buses. It arranges the system into two single bus systems, which
as described previously, has very low reliability. Relay protection for this scheme
can be complex, depending on the system requirements, exibility, and needs.
With two buses and a bus tie available, there is some ease in doing maintenance,
but maintenance on line breakers and switches would still require outside the
substation switching to avoid outages.
In this scheme, as indicated by the name, all breakers are arranged in a ring with
circuits tapped between breakers. For a failure on a circuit, the two adjacent
breakers will trip without affecting the rest of the system. Similarly, a single bus
failure will only affect the adjacent breakers and allow the rest of the system to
remain energized. However, a breaker failure or breakers that fail to trip will require
adjacent breakers to be tripped to isolate the fault.
Maintenance on a circuit breaker in this scheme can be accomplished without
interrupting any circuit, including the two circuits adjacent to the breaker being
maintained. The breaker to be maintained is taken out of service by tripping the
breaker, then opening its isolation switches. Since the other breakers adjacent to
the breaker being maintained are in service, they will continue to supply the
circuits. In order to gain the highest reliability with a ring bus scheme, load and
source circuits should be alternated when connecting to the scheme.
Arranging the scheme in this manner will minimize the potential for the loss of the
supply to the ring bus due to a breaker failure. Relaying is more complex in this
scheme than some previously identied. Since there is only one bus in this
scheme, the area required to develop this scheme is less than some of the
previously discussed schemes. However, expansion of a ring bus is limited, due to
the practical arrangement of circuits.
6. Breaker-and-a-Half Configuration
Breaker-and-a-Half Configuration
Reliability
Cost
Available
area
.Single bus
Least area
fewer
components
.Double bus
Greater area
twice as
many
components
Low area
components
requirement
fewer
components
.Double bus,.single
breaker
Moderate
area more
components
.Ring bus
Moderate
area
increases
with number
of circuits
.Breaker anda.half