The loop electrical distribution system used to supply bulk loads (industrial plants and buildings)

Electrical distribution system

First, let’s say a word or two about the essentials of power distribution systems for our young electrical engineers. An electric distribution system, or distribution plant as it is sometimes called, is all of that part of an electric power system between the bulk power source or sources and the consumers’ service switches. Where the bulk power sources are located? What’s the practice?

The bulk power sources are located in or near the load area to be served by the distribution system and may be either generating stations or power substations supplied over transmission lines.

Distribution systems can, in general, be divided into six parts, namely, sub-transmission circuits, distribution substations, distribution or primary feeders, distribution transformers, secondary circuits or secondaries, and consumers’ service connections and meters or consumers’ services.

Figure 1 is a schematic diagram of a typical distribution system showing these parts. The sub-transmission circuits extend from the bulk power source or sources to the various distribution substations located in the load area. They may be radial circuits connected to a bulk power source at only one end or loop and ring circuits connected to one or more bulk power sources at both ends.

Each distribution substation normally serves its own load area, which is a subdivision of the area served by the distribution system. At the distribution substation the sub-transmission voltage is reduced for general distribution throughout the area.

The substation usually consists of the following: One or more power transformers together with the necessary voltage regulating equipment, buses, and switchgear.

Figure 1 – Typical distribution system and its components

Ok, that was just an introduction to power distribution system. Now, let’s get into the loop distribution systems, their equipment, configuration and practice.

Loop Distribution System

The loop type of distribution system is used most frequently to supply bulk loads, such as small industrial plants and medium or large commercial buildings, where continuity of service is of considerable importance. The sub-transmission circuits of the loop system should be parallel or loop circuits or a sub-transmission grid as shown in Figures 2 and 3.

Figure 2 – A parallel or loop-circuit sub-transmission layout

Figure 3 – Network or grid form of sub-transmission

These sub-transmission circuits should supply a distribution substation or substations similar to this of Figure 7 (see below). The reason for this is that as much or more reliability should be built into the system from the low-voltage bus of the distribution substation back to the bulk power source or sources as is provided by the loop-primary feeders shown in Figure 4 and Figure 5.

The use in a loop system of a radial sub-transmission circuit or circuits and a distribution substation or substations, which may not provide good service continuity, does not give a well coordinated system! This is because a fault on a sub-transmission circuit or in a distribution substation transformer results in an interruption of service to the loads supplied over the more reliable loop-primary feeders.

The sub-transmission circuits and distribution substations are often common to both radial- and loop-type distribution systems.

These transformers, which step down from the distribution to the utilization voltage, are ordinarily relatively large distribution transformers.

Figure 4 – Frequently used form of the loop primary feeder: Two sectionalizing breakers per secondary substation to isolate a faulted loop section without interrupting service to any load

Figure 5 – Frequently used form of the loop primary feeder: One sectionalizing breaker per secondary substation for use where interrupting one load when a loop section fault occurs can be tolerated

Because these secondary substations are usually considerably smaller than a distribution substation only one three-phase or one bank of single-phase transformers is used ordinarily as shown in Figures 6 (a) and 6 (b). In these secondary substations the low-voltage feeders are operated at a utilization voltage of 600 volts or below and they are commonly controlled by air circuit breakers.

These secondary feeders are usually radial circuits that run directly to large motors, to power switchgear, and to lighting panels or small lighting transformers.

Figure 6 – Distribution substations with duplicate supply circuits to eliminate service interruptions due to sub-transmission faults

The loop-primary feeder is sectionalized by a circuit breaker on each side of the points where secondary substations are connected to it. The two primary-feeder breakers and the sectionalizing breakers associated with the loop feeder are ordinarily controlled by directional-overcurrent relays or by pilot-wire relays.

Pilot-wire relaying is used where the number of secondary substations connected to the loop is such that selective timing cannot be obtained with directional-overcurrent relays. With this loop primary feeder arrangement a fault on any section of the loop is cleared by the circuit breakers at the two ends of the faulty section and service is not interrupted to any secondary substation.

Suggested Guide – Design guide for upgrading existing secondary substations to be smart and intelligent

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As a feeder fault can occur in one of the sections adjacent to the distribution substation bus the entire feeder load may have to be fed in one direction over either end section of the feeder until repairs are made. Sufficient spare capacity must be built into the loop feeder to permit operating with either end section out of service without excessive voltage drop or overheating of the feeder.

A fault in a secondary substation transformer is cleared by the circuit breaker or fuse in its primary leads and the loop feeder remains intact. If no transformer primary breaker or fuse is used such a transformer fault must be cleared by tripping the two sectionalizing breakers adjacent to the faulty transformer.

In this case the loop is opened and must remain open until the defective transformer is disconnected from the loop.

A fault on one of the radial secondary feeders from a secondary substation is cleared by the tripping of the air circuit breaker associated with the faulty feeder. This interrupts service to those loads connected to that feeder until the fault. can be located and repaired.

Figure 7 – Distribution substations with duplicate supply circuits and transformers to eliminate service interruptions due to sub-transmission or transformer faults

The investment in sectionalizing breakers and relaying may make a loop system employing primary feeders similar to that of Figure 4 more expensive than the necessary quality of service justifies. If an outage to a secondary substation can be tolerated when a primary-feeder fault occurs a loop-feeder arrangement can be used as shown in Figure 5.

Here only one sectionalizing breaker is used with each secondary substation thus reducing the number of these breakers to half of the number used in Figure 4. The sectionalizing breakers are relayed as discussed in connection with Figure 4.

When a primary feeder fault occurs the two breakers at the ends of the faulty section open as in the previous arrangement. In this case, how-ever, the secondary substation associated with the faulty section is deenergized because its transformer is tied directly to the feeder section through a disconnecting switch, a primary transformer breaker, or a fuse. Service to the deenergized substation cannot be restored until the fault has been located and repairs have been made.

Sometimes a consumer connected to the loop requires more reliable service than the arrangement of Figure 5 provides. Service to this consumer can be improved in several ways. Two sectionalizing breakers can be used, one on each side of the point where the secondary substation that serves him is connected to the loop, as shown in Figure 4.

Another way is to divide the transformer capacity of the secondary substation serving this consumer into two units, and connect one of these units or transformers to the loop feeder on each side of the single sectionalizing breaker. Each of these transformers should have sufficient capacity to supply the entire station load and is usually connected to the loop feeder through a circuit breaker or fuse.

The two transformers are ordinarily bused on the secondary side through transformer-secondary breakers. When this arrangement is used a feeder fault in either of the two loop sections immediately adjacent to the sectionalizing breaker results in the deenergization of one of the two transformers at the substation.

This is because the sectionalizing breaker at the station, the sectionalizing breaker at the far end of the faulty section, and the breaker in the secondary leads of the transformer connected to the faulty section are tripped.

When this happens the secondary substation load is fed over the good loop section which is adjacent to the open sectionalizing breaker at the station, and its associated transformer.

Figure 8 – Common arrangement of loop primary feeder for supplying distributed loads

A third way of improving service is to supply the consumer through a single-transformer substation arranged so that it can be connected to either side of its associated sectionalizing breaker by a double-throw switch or two interlocked disconnecting switches. Service will then be interrupted to the secondary substation when a fault occurs in the loop section to which the station is normally connected.

The station loads can be quickly reenergized, however, without waiting for repairs, by connecting the substation to the good section of the loop on the other side of the open sectionalizing breaker.

The above discussion of the loop system has been on the basis of supplying relatively small bulk loads from distribution substations over loop primary feeders.

The arrangement of the sub-transmission loop and its control and protection is in general similar to that discussed in connection with the loop-primary feeders of Figures 4 and 5. Any form of loop system normally provides a two-way feed to the distribution transformers or secondary substations.

In general, the service continuity and voltage regulation provided is better than when using a radial system. The amount by which the quality of service of the loop system exceeds that provided by the radial system depends upon the particular forms of the two systems being compared. Ordinarily the loop system will be more expensive than the radial system.

Also it is usually less flexible than the radial system particularly in the forms used in supplying bulk loads discussed above. This is principally because two circuits must be run to each new secondary substation location in order to connect the station onto the loop.

A common arrangement of a loop-primary feeder for supplying distributed loads is shown in Figure 8 above.

The similarity between this loop-feeder arrangement and the radial-primary feeders of Figure 9, where emergency ties are provided between adjacent feeders, is apparent. Each end of the loop-primary feeder is connected to the low-voltage bus of its distribution substation through primary-feeder breakers.

The main feeder is automatically sectionalized near its midpoint by a sectionalizing breaker controlled by overcurrent relays.

Figure 9 – Simple form of radial primary feeder with tie and sectionalizing switches to provide for quick restoration of service to customers on unfaulted feeder sections

Manual sectionalizing switches are provided at other points in the main feeder as shown in order to reduce the area that must remain without service, until repairs are made, when a fault occurs on the main feeder. Fuses are not used in the main-feeder loop.

The sectionalizing breaker could be replaced with fuses at some sacrifice in the speed of restoring service, but not more than one set of fuses should be used in the loop because they will not operate selectively for feeder faults in various locations. More than one automatic sectionalizing breaker could be used in the main loop with directional-overcurrent or pilot-wire relaying, as was done in Figures 4 and 5, to reduce the extent of the outage when a main feeder fault occurs.

This results in an interruption of service to about half the loads normally supplied over the loop-primary feeder. Service can be quickly restored to all deenergized loads except those connected to the faulty section of the feeder by opening the sectionalizing switch or switches associated with the faulty section and then reclosing one of both of the tripped breakers depending upon the location of the fault. Faults on the subfeeders and laterals are cleared by their associated primary fuses.

These fuse operations do not interrupt service to any of the feeder loads except those beyond the blown fuse on the sub-feeder and laterals.

As in the case of the loop feeders or Figures 4 and 5, this loop-primary feeder should be designed to permit its carrying all loads that can be connected to it when any section of the loop is out of service.

Suggested Reading – Technical specification of switchyard equipment in a nutshell

Technical specification of switchyard equipment in a nutshell (how-to guidelines and advice)

Sources:

1. Power System Analysis And Design By J. Duncan Glover, Mulukutla S. Sarma And Thomas J. Overbye
2. Industrial Power Systems Handbook by Donald Beeman