System Arrangements

The selection of system arrangement has a profound impact upon the reliability and maintainability of the system. Several commonly used system topologies are presented here, along with the pros and cons of each. The figures for each of these assume that the distribution and utilization voltage are the same, and that the service voltage differs from the distribution/utilization voltage. The symbology (low voltage circuit breaker, low-voltage drawout circuit breaker, medium voltage switch, medium voltage breaker) reflects the most commonly-used equipment for each arrangement. The symbology used throughout this section is shown in Symbology.

Symbology

The radial system is the simplest system topology and is shown in Radial System. It is the least expensive in terms of equipment first cost. However, it is also the least desirable since it incorporates only one utility source and the loss of the utility source, transformer, or the service or distribution equipment results in a loss of service. Further, the loads must be shut down to perform maintenance on the system. This arrangement is most used where the need for low first-cost, simplicity, and space economy outweigh the need for enhanced reliability.

Typical equipment for this system arrangement is a single unit substation consisting of a fused primary switch, a transformer of sufficient size to supply the loads, and a low-voltage switchboard.

Radial System

This arrangement is shown in Radial System with Primary Selectivity. If two utility sources are available, it provides almost the same economic advantages of the radial system in Radial System but also gives greater reliability since the loss of one utility source does not result in a loss of service (An outage occurs between the loss of the primary utility source and switching to the alternate source unless the utility allows paralleling of the two sources). The loss of the transformer or of the service or distribution equipment still results in a loss of service. System maintenance requires shut down all loads.

Radial System with Primary Selectivity

An automatic transfer scheme may optionally be provided between the two primary switches to automatically switch from an offline utility source to an available source. Most often metal-clad circuit breakers are used, rather than metal-enclosed switches. More about typical equipment application guidelines follows in a subsequent section of this guide.

The radial systems shown in Radial System and Radial System with Primary Selectivity can be expanded by the inclusion of additional transformers. Further, these transformers can be located close to the center of each group of loads to minimize voltage drop. Reliability increases with a larger number of substations since the loss of one transformer does not result in a loss of service for all the loads.

Expanded Radial System with One Utility Source and a Single Primary Feeder shows an expanded radial system utilizing multiple substations, but still with only one utility source and only one primary feeder.

Expanded Radial System with One Utility Source and a Single Primary Feeder

A more dependable and maintainable arrangement utilizing multiple primary feeders is shown in Expanded Radial system with One Utility Source and Multiple Primary Feeders. In the system shown in Expanded Radial system with One Utility Source and Multiple Primary Feeders, each unit substation is supplied by a dedicated feeder from the service entrance switchgear. Each substation is also equipped with a primary disconnect switch to allow isolation of each feeder on both ends for maintenance purposes.

Typical service entrance equipment consists of a metal-clad switchgear main circuit breaker and metal-enclosed fused feeder switches. Metal-clad circuit breakers may be used instead of metal-enclosed feeder switches if required.

Expanded Radial system with One Utility Source and Multiple Primary Feeders

Expanded Radial System with Two Utility Sources and Multiple Primary Feeders shows an expanded radial system utilizing multiple substations and two utility sources, again with metal-clad primary switchgear but with a duplex metal-enclosed switchgear for utility source selection.

Expanded Radial System with Two Utility Sources and Multiple Primary Feeders

Of the arrangements discussed this far, the arrangement of Expanded Radial System with Two Utility Sources and Multiple Primary Feeders is the most dependable; it does not rely on a single utility source for system availability, nor does the loss of one transformer or feeder cause a loss of service to the entire facility. However, the loss of a transformer or feeder results in the loss of service to a part of the facility. More dependable system arrangements are required if this is to be avoided.

The loop system arrangement is one of several arrangements that allows one system component, such as a transformer or feeder cable, to fail without causing a loss of service to a part of the facility.

Primary Loop System shows a primary loop arrangement. The advantages of this arrangement over previously mentioned arrangements are that a loss of one feeder cable does not cause one part of the facility to experience a loss of service and that one feeder cable can be maintained without causing a loss of service (An outage to part of the system is experienced after the loss of a feeder cable until the loop is switched to accommodate the loss of the cable).

In Primary Loop System metal-clad circuit breakers are used as the feeder protective devices. Fused metal-enclosed-feeder switches could be utilized for this, but take care if this is considered since the feeder fuses must be able to serve both transformers and the feeder and transformer fuses have to coordinate for maximum selectivity.

The system arrangement of Primary Loop System is designed to operate with the loop open, for example, one of the four loop switches shown would be normally-open. If closed-loop operation were required, use metal-clad circuit breakers instead to provide maximum selectivity (this arrangement is discussed further below). Momentary paralleling to allow maintenance of one section of the loop without causing an outage to one part of the facility can be accomplished with metal-enclosed loop switches, however, caution is necessary in the system design and maintenance.

Primary Loop System

Another method of allowing the system to remain in service after the loss of one component is the secondary-selective system. Secondary-selective System shows such an arrangement.

The system arrangement of Secondary-selective System has the advantage of allowing for the loss of one transformer without causing a loss of service to one part of the plant. This is a characteristic none of the previously mentioned system arrangements exhibit. The system can be run with the secondary bus tie breaker normally-open or normally-closed. If the bus tie breaker is normally-closed the loss of one transformer, if directional overcurrent relays are supplied on the transformer secondary main circuit breakers, does not cause an outage. However take care must in the system design as the available fault current at the secondary switchgear can be doubled in this case.

Typical equipment for this arrangement is low-voltage power circuit-breaker switchgear with drawout circuit breakers, both for reasons of coordination and maintenance. However, a low-voltage switchboard may be utilized if care is taken in the system design and the system coordination is achievable. For a normally-closed bus tie breaker, low-voltage power switchgear is essential since the breakers lend themselves more readily to external protective relaying.

The loss of transformer means the other transformer and its associated secondary main circuit must carry the entire load. Consider this in sizing the transformer and secondary switchgear for the effectiveness of this type of system.

Secondary-selective System

A larger-scale version of the secondary selective system is the transformer sparing scheme, shown in Transformer Sparing Scheme. This type of system allows good flexibility in switching. The system is usually operated with all of the secondary tie breakers except one (the sparing transformer secondary main/tie breaker) normally-open. The sparing transformer supplies one load bus if a transformer goes off-line or is taken off-line for maintenance. A transformer is switched out of the circuit by opening its secondary main breaker and closing the tie breaker to allow the sparing transformer to feed its loads. The sparing transformer may be allowed to feed multiple load busses if it is sized properly. Care must be used when allowing multiple transformers to be paralleled as the fault current is increased with each transformer that is paralleled, and directional relaying is required on the secondary main circuit breakers to selectively isolate an offline transformer. An electrical or key interlock scheme is required to enforce the proper operating modes of this type of system, especially since the switching is carried out over several pieces of equipment that can be in different locations. A properly designed interlocking system allows for the addition of future substations without modification of the existing interlocking.

With both types of secondary-selective system, an automatic transfer scheme may be utilized to switch between a lost transformer and an available transformer.

Transformer Sparing Scheme

A selective system arrangement may also utilize the primary system equipment. Such an arrangement is shown in Primary Selective System.

As with the secondary selective system, an automatic transfer scheme may be used to automatically perform the required transfer operations, if a utility source become unavailable. The bus tie circuit breaker may be normally-closed or normally-open, depending on utility allowances. If the bus tie circuit breaker is normally-closed, take care in the protective relaying so that a fault on one utility line does not cause the entire system to be taken off-line. The available fault current with the tie breaker normally closed increases with each utility service added to the system.

Metal-clad switchgear is most used with this type of arrangement, due to the limitations of metal-enclosed load interrupter switches.

Primary Selective System

In large municipal areas where large loads, such as high-rise buildings, must be served and a high degree of reliability, secondary network systems are often used. In a secondary network system, several utility services are paralleled at the low-voltage level, creating a highly steady system.

Network protectors are used at the transformer secondaries to isolate transformer faults which are backfed through the low-voltage system. These devices are designed to automatically isolate a faulted transformer that is backfed from the rest of the system. The transformers typically have higher-than-standard impedances to reduce the available fault current on the low-voltage network. The common secondary bus is often referred to as the “collector bus”. An example of a secondary spot-network system is shown in Secondary Spot Network.

Secondary Spot Network

Essentially a loop system in which the loop is normally closed, the ring bus is a highly dependable system arrangement. A typical ring-bus system is depicted in Primary Ring Bus System.

A fault at any bus causes only the loads served by that bus to lose service. Bus differential relaying is recommended for optimum reliability with this scheme. The bus differential relaying opens both breakers feeding a bus for a fault on that bus. Metal-clad switchgear is usually used for the primary ring bus.

Although Primary Ring Bus System shows two utility sources, this system arrangement is easily expanded to incorporate additional utility sources. As with the primary-selective system with a normally-closed bus tie breaker, the available fault current is increased with each utility source added to the system.

Primary Ring Bus System

The above system arrangements are the basic building blocks of power distribution system topologies but are rarely used alone for a given system. To increase system reliability, it is usually necessary to combine two or more of these arrangements. For example, one commonly used arrangement is shown in Composite System: Primary Loop/Secondary Selective.

A fault on a primary loop cable or the loss of one transformer can be accommodated without loss of service to either load bus (but with an outage to part of the system until the system is switched to accommodate the loss). In addition, a single section of the primary loop or one transformer can be taken out of service while maintaining service to the loads.

The system of Composite System: Primary Loop/Secondary Selective can be expanded by the addition of an additional utility source and a primary bus tie breaker to form an even more dependable system, as shown in Composite System: Primary Selective/Primary Loop/Secondary Selective. With this arrangement, the loss of a single utility source, a single primary circuit breaker, a single loop feeder cable, or a single transformer can be accommodated without loss of service. And any one primary circuit breaker, any one section of the primary distribution loop, or any one transformer can be taken out of service without loss of service to the loads. However, the cost of a second utility service and two additional metal-clad breakers must be considered.

Composite System: Primary Loop/Secondary Selective

A logical expansion of this system, resulting in a further increase in system reliability, is had by replacing the primary distribution loop with dedicated feeder circuit breakers from each primary bus, as shown in Composite System: Primary Selective/Primary Loop/Secondary Selective. In this system arrangement multiple primary feeder cable losses can be accommodated without jeopardizing service to the loads (However, an outage occurs until the system is switched to accommodate the losses).

An example of an extremely dependable system arrangement is given in Primary Ring Bus, Primary Source Selective, Secondary Selective System is a re-arrangement of the primary ring-bus configuration shown in Composite System: Primary Selective/Primary Loop/Secondary Selective, along with the primary source-selective configuration shown in Composite System: Primary Loop/Secondary Selective and a variant of the transformer sparing scheme given in Transformer Sparing Scheme. This system arrangement gives good flexibility in switching for maintenance purposes, and allows any one utility, primary switchgear bus, or transformer fail without loss of service to any of the loads (again, an outage may be taken until the system is switched to accommodate the loss, depending upon the loss under consideration). It also allows any three primary feeders to be faulted without loss of service to any of the loads. Other composite arrangements are possible.

Composite System: Primary Selective/Primary Loop/Secondary Selective

Composite System: Primary Double Selective/Secondary Selective

Primary Ring Bus, Primary Source Selective, Secondary Selective System

Various system arrangements are presented in this section, starting with the least complex and progressing to a very complex, robust system arrangement. In general, as reliability increases so does complexity and cost. Economic considerations usually dictate how complex a system arrangement can be used, and thus have a great deal of impact on system reliability. Power System Arrangements Summary for the Basic Arrangements in this Section and Power System Arrangements Summary for the Composite Arrangements in this Section show the features of each system arrangement given in this section.

The formulas given in these tables are for the systems as shown in the earlier figures. They hold true for expanded versions of these system arrangements where the expansion is made symmetrically with respect to the configuration shown. They do not hold true when modifications are made to the system arrangements with respect to symmetry, with altered numbers of switching/protective devices, or for concurrent loss of different types of system components. When in doubt regarding a system which is derived from, but not identical, to the systems shown in the earlier figures, double-check these numbers.

From a maintenance perspective, the number of system elements that can be taken down for maintenance is the same as the number that can fail while maintaining service to the loads.

These tables do not attempt to address concurrent losses of different types of system components, nor are they always mean a loss of service to a particular load after a component loss while the system is being switched to an alternate configuration. However, they are a guide to the relative strengths and weaknesses of each of the system arrangements presented.