Medium-Voltage Distribution System Design

A. Single Bus, Figure 1.1-40

The sources (utility and/or generator(s)) are connected to a single bus. All feeders are connected to the same bus. 

This configuration is the simplest system; however, outage of the utility results in total outage. 

Normally the generator does not have adequate capacity for the entire load. A properly relayed system equipped with load shedding, automatic voltage/ frequency control may be able to maintain partial system operation. 

Any future addition of breaker sections to the bus will require a shutdown of the bus, because there is no tie breaker.

Figure 1.1-40. Single Bus

B. Single Bus with Two Sources from the Utility, Figure 1.1-41

Same as the single bus, except that two utility sources are available. This system is operated normally with the main breaker to one source open. Upon loss of the normal service, the transfer to the standby normally open (NO) breaker can be automatic or manual. Automatic transfer is preferred for rapid service restoration especially in unattended stations. 

Retransfer to the “Normal” can be closed transition subject to the approval of the utility. Closed transition momentarily (5–10 cycles) parallels both utility sources. Caution: when both sources are paralleled, the fault current available on the load side of the main device is the sum of the available fault current from each source plus the motor fault contribution. It is recommended that the short-circuit ratings of the bus, feeder breakers and all load side equipment are rated for the increased available fault current. 

If the utility requires open transfer, the disconnection of motors from the bus must be ensured by means of suitable time delay on reclosing as well as supervision of the bus voltage and its phase with respect to the incoming source voltage. 

This busing scheme does not preclude the use of cogeneration, but requires the use of sophisticated automatic synchronizing and synchronism checking controls, in addition to the previously mentioned load shedding, automatic frequency and voltage controls. 

This configuration is more expensive than the scheme shown in Figure 1.1-40, but service restoration is quicker. Again, a utility outage results in total outage to the load until transfer occurs. Extension of the bus or adding breakers requires a shutdown of the bus.

If paralleling sources, reverse current, reverse power and other appropriate relaying protection should be added as requested by the utility.

Figure 1.1-41. Single Bus with Two-Sources

C. Multiple Sources with Tie Breaker, Figure 1.1-42 and Figure 1.1-43

This configuration is similar to the configuration shown in Figure 1.1-41. It differs significantly in that both utility sources normally carry the loads and also by the incorporation of a normally open tie breaker. The outage to the system load for a utility outage is limited to half of the system. Again, the closing of the tie breaker can be manual or automatic. The statements made for the retransfer of the configuration shown in Figure 1.1-41 apply to this scheme also.

Figure 1.1-42. Two-Source Utility with Tie Breaker

If looped or primary selective distribution system for the loads is used, the buses can be extended without a shutdown by closing the tie breaker and transferring the loads to the other bus. 

This configuration is more expensive than the configuration shown in Figure 1.1-41. The system is not limited to two buses only. Another advantage is that the design may incorporate momentary paralleling of buses on retransfer after the failed line has been restored to prevent another outage. See the Caution for Figure 1.1-41, Figure 1.1-42 and Figure 1.1-43.

In Figure 1.1-43, closing of the tie breaker following the opening of a main breaker can be manual or automatic. However, because a bus can be fed through two tie breakers, the control scheme should be designed to make the selection. 

The third tie breaker allows any bus to be fed from any utility source.

Caution for Figure 1.1-41, Figure 1.1-42 and Figure 1.1-43:

If continuous paralleling of sources is planned, reverse current, reverse power and other appropriate relaying protection should be added. When both sources are paralleled for any amount of time, the fault current available on the load side of the main device is the sum of the available fault current from each source plus the motor fault contribution. It is required that bus bracing, feeder breakers and all load side equipment is rated for the increased available fault current.

Figure 1.1-43. Triple-Ended Arrangement

Summary

The medium-voltage system configurations shown are based on using metal-clad drawout switchgear. The service continuity required from electrical systems makes the use of single-source systems impractical. In the design of a modern mediumvoltage system, the engineer should:

1. Design a system as simple as possible.

2. Limit an outage to as small a portion of the system as possible.

3. Provide means for expanding the system without major shutdowns.

4. Design a protective relaying system so that only the faulted part is removed from service, and damage to it is minimized consistent with selectivity.

5. Specify and apply all equipment within its published ratings and national standards pertaining to the equipment and its installation.