The most common MV/LV radial distribution systems applied in buildings

Distribution of power within the building

In many cases, power is supplied by the power utility to a building at the utilization voltage. In these cases, the distribution of power within the building is achieved through the use of a simple radial distribution system.

In cases where the utility service voltage is at higher than the utilization voltage within the building, the system design engineer has a choice of a number of types of systems that may be used.

This technical article focuses on two major types of radial distribution systems while the next part will cover few practical modifications of them (read it here).

Contents:

1. Simple low voltage radial system
2. Simple medium voltage radial system
3. Loop-primary system – radial secondary system
4. Primary selective system – secondary radial system
5. Two-source primary – secondary selective system

1. Simple Low Voltage Radial System

In a conventional low-voltage radial system, the utility owns the pole-mounted or pad-mounted transformers that step their distribution voltage down from medium voltage to the utilization voltage, typically 480/277 Vac or 208/120 Vac. In these cases, the service equipment is generally a low-voltage main distribution switchgear or switchboard.

Specific requirements for service entrance equipment may be found in NEC Article 230, Services.

Each feeder is connected to the switchgear or switchboard bus through a circuit breaker or other overcurrent protective device. A relatively small number of circuits are used to distribute power to the loads.

Because the entire load is served from a single source, full advantage can be taken of the diversity among the loads.

This makes it possible for the utility to minimize the installed transformer capacity. However, if capacity requirements grow, the voltage regulation and efficiency of this system may be poor because of the low-voltage feeders and single source.

Typically, the cost of the low-voltage feeder circuits and their associated circuit breakers are high when the feeders are long and the peak demand is above 1000 kVA.

Additionally, a fault on the Service Switchgear or Switchboards low-voltage bus will cause the main overcurrent protective device to operate, interrupting service to all loads. Service cannot be restored until the necessary repairs have been made.

A fault on a low-voltage feeder circuit will interrupt service to all the loads supplied by that feeder. An engineer needs to plan ahead for these contingencies by incorporating backup power plans during the initial design of the power system.

Resiliency from storms, floods and other natural disasters can be accomplished with the addition of permanently installed standby generation, or by including a provision in the incoming Service equipment for the connection of a portable roll-up temporary generator.

Figure 2 shows a typical incoming service switchboard with the addition of a key interlocked generator breaker.

In this design, the breaker pair shares a single key that can only be used to close one breaker at a time. This arrangement ensures against paralleling with the utility but requires manual intervention in the event of an outage.

In a typical standby generation arrangement, automatic transfer switches are used to feed either Normal utility power or an alternate generator source of backup power to the critical loads. The transfer switches sense the loss of power from the Normal source and send a run command to the generator to start.

The location and type of the transfer switches depends on the Utility and the overall design intent. Transfer switches can be Service Entrance Rated and used as the main Service Disconnect feeding all the loads downstream.

See Figure 3 below.

Transfer switches can be also be incorporated into the service switchboard as an integral part of the assembly. Alternately, they can be located downstream of the incoming service and applied only to the individual loads they are feeding.

It is important to consider the grounding of the generator neutral when using automatic transfer switches in power system design. If the generator neutral is grounded at the generator, a separately derived system is created. This requires the use of four-pole transfer switches for a three-phase system.

If the three-phase generator neutral is brought back through the transfer switches and grounded at the service entrance, a three-pole transfer switch with solid neutral should be provided.

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2. Simple Medium Voltage Radial System

In those cases where the customer receives his supply from the primary system and owns the primary switch and transformer along with the secondary low-voltage switchboard or switchgear, the equipment may take the form of a separate primary switch, separate transformer, and separate low-voltage switchgear or switchboard.

This equipment may be combined in the form of an outdoor pad-mounted transformer with internal primary fused switch and secondary main breaker feeding an indoor switchboard.

Another alternative would be a secondary unit substation where the primary fused switch, transformer and secondary switchgear or switchboard are designed and installed as a close-coupled single assembly.

The transformers are usually connected to their associated load bus through a circuit breaker, as shown in Figure 4.

Each secondary unit substation is an assembled unit consisting of a three-phase, liquid-filled or air-cooled transformer, an integrally connected primary fused switch, and low voltage switchgear or switchboard with circuit breakers or fused switches.

Circuits are run to the loads from these low-voltage protective devices. Because each transformer is located within a specific load area, it must have sufficient capacity to carry the peak load of that area. Consequently, if any diversity exists among the load area, this modified primary radial system requires more transformer capacity than the basic form of the simple radial system.

However, because power is distributed to the load areas at a primary voltage, losses are reduced, voltage regulation is improved, feeder circuit costs are reduced substantially, and large low-voltage feeder circuit breakers are eliminated.

A fault on a primary feeder circuit or in one transformer will cause an outage to only those secondary loads served by that feeder or transformer.

In the case of a primary main bus fault or a utility service outage, service is interrupted to all loads until the trouble is eliminated.

Reducing the number of transformers per primary feeder by adding more primary feeder circuits will improve the flexibility and service continuity of this system – the ultimate being one secondary unit substation per primary feeder circuit.

This of course increases the investment in the system but minimizes the extent of an outage resulting from a transformer or primary feeder fault.

Primary connections from one secondary unit substation to the next secondary unit substation can be made with “double” lugs on the unit substation primary switch as shown, or with load break or non-load break separable connectors made in manholes or other locations.

Depending on the load kVA connected to each primary circuit and if no ground fault protection is desired for either the primary feeder conductors and transformers connected to that feeder or the main bus, the primary main and/or feeder breakers may be changed to primary fused switches.

In addition, if only one primary fuse on a circuit opens, the secondary loads are then single phased, causing damage to low-voltage motors.

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Substation Tour (VIDEO)

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Sources: Power system design basics – Eaton