CHAPTER 2
DISTRIBUTION SYSTEM STRUCTURE

2.1 Distribution Voltage Levels

North American and European practices are the two systems of distribution voltage levels in most use around the world. The primary and secondary voltages (MV and LV, respectively) are given in Table 2.1 for both systems. The voltage choice depends on the type of load (residential, commercial, industrial), load size and the distance at which the load is located. This is illustrated by a typical block diagram shown in Figure 2.1.

Table 2.1 Distribution Voltages for North American and European Systems

Primary distribution voltage (line-to-line)

Three-phase secondary voltage (line-to-line)

Single-phase secondary voltage (line-to-neutral)

Figure 2.1 Distribution voltages for different loads (values written between parentheses describe the European practice; otherwise the values describe the North American practice).

It is seen that the distribution voltage in the European system is higher than that in the North American system. It has both advantages and disadvantages as listed in what follows.

• Advantages: The system can carry more power for a given ampacity and has less voltage drop and less line losses for a given power flow. Consequently, the system can cover a much wider area. Because of the longer reach, the system needs fewer substations.
• Disadvantages: more customer interruptions because the circuits are longer, that is, lower level of reliability. Therefore, a major concern is to keep reliability at the desired level depending on the load category (more details are given in Section 2.2.2). From the cost point of view, the system equipment (transformers, cables, insulators etc.) is more expensive.

2.2 Distribution System Configuration

Configuration of the distribution networks follows one or a combination of the following standard systems:

• radial system where the load is supplied through one radial feeder;
• open-ring system where the load is supplied through one of two available feeders, that is, one side of the ring;
• closed-ring system where the load is supplied through the two sides of the ring simultaneously;
• dual-ring system in which the load is connected with two rings at the same time, that is, it has four incoming feeders; and
• multiradial system, which means supplying the load by more than one radial feeder.

These systems can be applied to establish the distribution network at either the MV or the LV level.

2.2.1 MV Distribution Networks

The MV distribution networks are the intermediate networks between the sources and the DPs in different zones of the load area.

The DP can be connected to the source through two in-service parallel feeders. At the same time, one feeder can operate at full load if a fault occurs in the second feeder, as shown in Figure 2.2a. For more reliability, two DPs can be connected to each other as shown in Figure 2.2b. In this case, DP2 is connected to the source through two parallel feeders while DP3 is connected to the same source through one feeder.

Figure 2.2 MV supply network to DP with parallel feeders (CB = circuit breaker with overcurrent protection; F = feeder, DP = distribution point).

Of course, it is better and more reliable to use different sources feeding the DPs. In Figure 2.3a, DP1 has two incoming feeders; the first is connected to source #1 while the second is connected to source #2. In this case, one feeder only is in service. The other feeder is a standby and feeds DP1 through automatic reserve switching (ARS). Another way is to divide the bus bar of DP2, Figure 2.3b, into two sections connected to each other by a bus-coupler circuit breaker that is normally open. Each section has an incoming feeder connected to an independent source, as shown in Figure 2.3b. Different configurations can be applied such as given in Figures 2.4a and 2.4b, respectively. A ring bus scheme, shown in Figure 2.5, can also be applied (popular in the United States). In this scheme, a fault anywhere in the ring results in two circuit breakers opening to isolate the fault. For example, when a fault occurs in source #1 circuit breakers CB1 and CB4 would operate to isolate the fault, while source #2 would feed the loads. Circuit breakers are installed with two manual isolating switches on both sides to perform maintenance safely and without service interruption. Alternatively, radial and multiradial configurations can be used as shown in Figures 2.6 and 2.7, respectively. Two additional configurations, an integrated open-ring and a radial distribution network, are shown in Figures 2.8 and 2.9, respectively.

Figure 2.3 MV supply network to DP with independent feeders (ARS = automatic reserve switching, F = feeder, CB = circuit breaker, DP = distribution point).

Figure 2.4 Combined MV supply network to DP.

Figure 2.5 Ring bus scheme.

Figure 2.6 Radial nonreserved distribution network.

Figure 2.7 Multiradial distribution network with automatic reserve switching on MV side.

Figure 2.8 Open-loop distribution network.

Figure 2.9 Radial MV network with DP (ARS on LV bus-bar of TP). TP = transformer; DP = distribution point; ARS = automatic reserve switching; SWB = switchboard.

2.2.2 LV Distribution Networks

2.2.2.1 North American System

Radial structure is the most commonly used in North America. The secondary feeders transmit power to the loads through distribution transformers (MV/LV). Single-phase power at voltage level 120/240 V is usually supplied to residences, farms, small offices and small commercial buildings (Fig. 2.10a). Three-phase power is usually supplied to large farms, as well as commercial and industrial customers. Typical voltage levels for three-phase power are 208Y/120 V, 480Y/277 V or 600Y/347 V (Fig. 2.10b).

Figure 2.10 North American MV/LV distribution transformers. (a) 120/240 V single-phase service, (b) Typical 208 V, 3-phase Y-connected service.

To increase the reliability, in particular, for LV applications with a very high load density, two or more circuits operate in parallel to transmit power into a secondary (LV) bus at which the load is connected. Each circuit includes a separate primary (MV) feeder, distribution transformer, and secondary feeder that is connected to a secondary bus through a network protector. This scheme is known as a "secondary spot network" (Fig. 2.11). If a fault occurs on a primary feeder or distribution transformer of one of the circuits, its network protector receives a reverse power from the other circuits. This reverse power causes the network protector to open and disconnect the faulty circuit from the secondary bus. The load is only interrupted in case of simultaneous failure of all primary feeders or at fault occurrence on the secondary bus. Of course, this scheme is more expensive because of the extra cost of network protectors and duplication of transformer capacity. In addition, it requires a special construction of the secondary bus to reduce the potential of arcing fault escalation as well as a probable increase of secondary equipment rating since the short-circuit current capacity increases in case of transformer parallel operation.

Figure 2.11 Secondary spot network.

2.2.2.2 European System

For European distribution systems, the choice of any of the standard structures (Section 2.2) to establish the LV distribution networks depends on the type of the majority of loads and the reliability level required supplying these loads.

For reliability level 3, the distribution transformers are supplied by two incoming feeders (one is a standby to the other) on the medium voltage side. The outgoing feeders at the low voltage side are radial feeders (cables or overhead lines) as shown in Figure 2.12. If a fault occurs on one of these outgoing feeders, the concerned load is disconnected until the fault is repaired.

Figure 2.12 Supply system to a third-category customer.

For reliability level 2, the medium-voltage side of the distribution...