Chapter 1: Distribution Network Types and Configurations Free

The main components of an electric power system include generation, transmission, and distribution networks. Distribution networks and power generation stations are connected via transmission lines. Usually, transmission lines transmit a high amount of power through high-voltage links between main load centers. A brief description of each system is given below (Mariam et al., 2013).

A high amount of electric energy is generated using bulk generation units clustered in remote areas that are away from final consumption points.

Traditionally, various technologies have been used to generate bulk electric energy, such as nuclear, natural gas, coal, hydro, etc. The existing power systems were owned by one company. However, due to lower costs and the economies of scale, they were allowed to build such bulk power plants large, and some of them are still profitable.

Transmission network includes substations, lines, and equipment to connect large power plants to load centers where the consumption of power is mostly performed in cities and industrial areas. Transmission lines in transmission networks operate at high-voltage levels (above 220 kV) that can cover long distances and transport large quantities of energy; therefore, these lines operate at high-voltage levels.

A subtransmission network is considered as an intermediate link between distribution and transmission networks. Subtransmission network lines cover shorter distances compared to those of the transmission networks and that is why they operate at a lower level, i.e., 45, 66, and 132 kV.

Voltage reduction is needed because of the voltage level differences in transmission network. Bulk load demands, such as big industries, can be directly connected to the subtransmission network.

The main part of the primary distribution network is the distribution substation that receives the energy delivered by the transmission and subtransmission networks and performs another voltage reduction. From medium voltage distribution lines, e.g., 11 and 25 kV, or distribution substation, the energy will be taken one step closer to end users; thus, bulk load demands can be connected to primary distribution networks.

Secondary distribution network includes medium voltage/low voltage (MV/LV) step-down transformers and LV lines, for example, 230 and 400 V, which deliver the power generated to LV commercial and residential consumers.

The UK's power system structure is shown in Fig. 1.1. Centralized large-scale power plants generate electric power that is connected to transmission networks at 400 and 275 kV in England and Wales and at 400, 275, and 132 kV in Scotland. The generated electric power is then transmitted to a distribution network at the grid supply point (GSP), which transforms the voltage to 132 kV from 275 and 400 kV. In England and Wales, distribution networks operate at 132 kV and in Scotland at 33 kV and deliver the generated power to lower voltage consumers. At bulk supply points (BSPs), voltage is transformed from 132 to 33 kV and at a primary substation from 33 kV to 11 and 6.6 kV. The generated power is then transformed through distribution substations into the LV levels needed to supply single-phase and three-phase consumers at 230 and 400 V, respectively. Bulk industrial and commercial users can be connected at high-voltage (HV) levels (i.e., 6.6 and 11 kV) and at extra high-voltage (EHV) levels (i.e., 132 and 33 kV) (Siemens Power Distribution & Control, Technical, 2007–2008).

End users are connected to the substations via overhead lines and underground cables. In urban areas with high load density, underground cables are used for feeders while in rural areas overhead lines are used. Several switching components are required to be installed in distribution feeders to enhance the supply security through (a) quick faults isolation, (b) the number of disconnected users’ reduction, and (c) the reduction of interruption duration.

Switches are used to interrupt the load current and reconfigure power flow in the network. However, the fault current can be interrupted by the breakers, and in order to detect faults and trigger breakers to break fault current, breakers can be combined with protection relays.

The most commonly used distribution network is the radial configuration as there are no closed loops (Mehta and Mehta, 2005; Sortomme et al., 2010; and Park et al., 2013). This is the simplest and cheapest distribution network topology; however, if a line is disconnected for any reason, all downstream lines cannot be supplied. The radial configuration comprises generators at the starting point connected to the load center via distribution transformer. A sample radial distribution network is shown in Fig. 1.2 (Taher and Afsari, 2012).

This configuration has a simple circuit protection scheme in terms of design and coordination and it is simple to find the rating requirement of the system (Bayindir et al., 2014).

Another benefit of this configuration is voltage compensation techniques, such as reactive power compensator, that can be simply implemented.

Radial configuration is well known for its simple structure and low initial cost which is useful for low voltage generation. This configuration is also beneficial once the substation is located near the loads, which will make the analysis and operation of the system easy (Willis, 2004). One of the disadvantages of this configuration is the limited flexibility in terms of planning perspective of new generators installation and/or additional loads that will require new cables or other components installation, thus leading to an increase in the cost. In this configuration, users rely on a single feeder or distributor and any fault in the network will cause an interruption in power supply to all users connected to the feeder (Isermann, 2006).

The availability of power for each load might be lower than other configurations, which is caused due to the complexity of the maintenance of operation. In order to address this issue, an alternative route should be implemented which has redundant circuits, i.e., at least two circuits need to operate simultaneously from the sources to specific loads (Prakash et al., 2016).

Ring or loop or mesh distribution network configuration follows a ring structure starting from a generator via several loads and back to the generator. In other words, all buses in the loop configuration are connected in a way that they create a closed loop structure supplying one or more distribution transformers or load and returns to the same substation (Kaipia et al., 2006). In this configuration, a loop must meet all requirements in terms of power and voltage drop in the case when fed from only one end, not both. A one-line diagram of a meshed distribution network is shown in Fig. 1.3 (Mehta and Mehta, 2005).

Utility can supply power to the loads in any direction in this configuration; therefore, any possible faults can be isolated with no failure in load supply (Park et al., 2013).

A ring/loop distribution network with multiple loops/rings is called a multi-ring or multi-loop configuration.

In the multi-loop configuration, various paths for the power transfer might be available, which will bring substantial flexibility to the network in the case of maintenance or fault in the system. However, various or multiple paths will make the network protection complicated, where it might not be easy to determine the fault location and take proper action for minimum customer interruption (Sortomme et al., 2010; Saleh, 2014; and Haj-Ahmed and Illindala, 2015). Multiple decisions could be made to isolate a fault; however, optimal decision making will depend on the operating conditions (Plesnick et al., 2000).

Ring configuration can mainly be used in residential areas as electric current can flow in several directions, which will lead to lower power losses and improve voltage stability (Reed et al., 2012). This configuration is recognized to form a closed loop through joining buses to each other, which will lead to the creation of several protection zones in the network. Ring configuration has a better performance compared with radial configuration as it will not be affected by adding extra devices to the network, and if there is a fault in a feeder or it is under maintenance, the ring distributor can still be energized by other feeders (Kishorbha and Mangroliya, 2015). This proves that users’ power supply will not be interrupted when a feeder is not in operation. One of the main disadvantages of this configuration is the reliance on the cables that connect the components to the network. In terms of network complexity, loop configuration is a bit more complicated than radial configuration; however, its main disadvantage is the network hosting capacity and the high loop cost (Willis, 2004).

An LV distribution network is defined as a network with a maximum limit of voltage level 1 kV based on British standards (Engineering Technical Report 140, 2017). Moreover, around the world, the most common voltage levels of LV networks are within the range of 120–240 V single phase (i.e., phase to neutral), or 208–415 V three phases and four wires (3-phase 4-wire) (Schneider Electric, 2009). Based on the international standard recommendation (IEC 60038), the voltage level of a three-phase four-wire is 230/400 V (Schneider Electric, 2009). The last stage of a power system is the LV network, which connects directly to the end user customers and supply loads; thus, it has a small individual capacity with a high number of buses (Li and Crossley, 2014a). Due to the low voltage level, LV feeders’ installation and development require lower cost in comparison with the MV and HV distribution networks (Taylor, 1990; Li and Crossley, 2014b; and Al-Jaahfreh and Mokryani, 2019).

Around the world, the LV network has different structures; among them, the “European” and “American” layouts are the most widely used layouts in European countries and Central and North America (Csanyi).

“European” layout of LV networks is used by most countries in Europe. For example, the LV network in the UK is a three-phase four-wire network supplied from a three-phase MV 230/400 V distribution transformer, where 230/400 refer to a secondary voltage level, 400 V line to line, and 230 V line to neutral (nominal voltage or RMS) (Schneider Electric, 2009; and Engineering Technical Report 140, 2017). Figure 1.4 shows a typical European LV distribution network in the UK.

In this layout, each MV/LV distribution substation can supply numerous three-phase four-wire LV feeders. Moreover, the LV feeder can carry the power efficiently up to approximately 300 m (Taylor, 1990). In other words, at a low voltage level (400 V), each substation can supply an area corresponding to a radius of 300 m from the substation, which makes it suitable for high load density areas (Al-Jaahfreh and Mokryani, 2019).

The LV feeders can be underground cables or overhead lines extended from the secondary distribution substation. Most LV feeders in the UK are designed as multi-phase feeders, which consist of four wires (three phases and neutral) (Taylor, 1990). Table 1.1 summarizes the catachrestics of LV feeders in the UK in both urban and rural areas (Al-Jaahfreh and Mokryani, 2019).

Figure 1.5 shows the American LV distribution networks layout, which is completely different from the European layout, where the three-phase LV network is practically non-existent (Schneider Electric, 2009; and Al-Jaahfreh and Mokryani, 2019). The MV network supplies numerous single-phase transformers that are connected through several single-phase primary laterals (Navarro-Espinosa et al., 2014). The secondary windings of a single-phase transformer are center-tapped to produce a single-phase three-wire supply, 120 V (line to neutral), and 240 V (line to line).

As a result, the capacity rating of the single-phase MV/LV secondary transformer is much smaller than those in the European network and the LV feeders are minimized (Al-Jaahfreh and Mokryani, 2019). The main advantage of this layout is that the load density supplied by each substation and its installation capital cost is lower than that in the European layout. However, the LV level (120 V) at the secondary side of the single-phase transformers is about half of the European single-phase secondary voltage (240 V), which results in some technical issues. For instance, it limited the extension of the single-phase feeder, which only has the ability to carry the power efficiently up to 60 m from the substation (Navarro-Espinosa et al., 2014). In addition, the power losses and voltage drop in the single-phase LV feeder are much higher than that in the three-phase LV feeders (Li and Crossley, 2014a).

Both “European” and “American” layouts are widely adopted in many countries around the world outside Europe, Central America, and North America (Navarro-Espinosa et al., 2014). Moreover, in some regions, the distribution network layout is a mixture of the European and American ones (Navarro-Espinosa et al., 2014). Table 1.2 and Figs. 1.6 and 1.7 provide a summary of the voltage level of LV distribution networks and their associated layout in different countries around the world (Schneider Electric, 2009). It is seen that the European layout is the most adopted layout. However, the American layout is adopted in North America, Latin America, and a few countries in Asia and the Middle East, such as Saudi Arabia. Moreover, some countries mixed the European and American layouts, such as Iran and South Korea (Schneider Electric, 2009).

The most common LV networks are radial networks due to the simplicity of analysis and protection system design (Taylor, 1990). However, due to the advantages of the loop or ring or mesh configuration to mitigate some of the technical issues, such as voltage variations and reverse power flow, the use of mesh configuration is becoming more common (Csanyi; Navarro-Espinosa et al., 2014; Aydin et al., 2015; and Al-Jaahfreh and Mokryani, 2019). As mentioned in Sec. 1.4, most LV networks are following the European layout. So, the three-phase 400 LV secondary circuit is designed based on the circuit diagram as shown in Fig. 1.6(a), which is a three-phase four-wire circuit with underground cables or overhead lines suspended from concrete, metal, or wooden poles (Lee Willis, 2004). Due to the high load density in urban areas, the underground cables are mainly used in LV networks (Al-Jaahfreh and Mokryani, 2019). Therefore, interconnecting the surrounding substations to each other is called a looped or meshed or ring network arrangement as those shown in Fig. 1.8(a) (Lee Willis, 2004; and Navarro-Espinosa and Ochoa, 2016). Also, loop or ring topology has proven its ability to improve the system hosting capacity for Distributed Generators (DGs), such as photovoltaic systems, which helps us to mitigate the technical issues, such as voltage rise and reverse power flow (Lee Willis, 2004). However, in rural areas, the radial arrangement could be the most economical one with a simple protection system, as shown in Fig. 1.8(b) (Al-Jaahfreh and Mokryani, 2019). Despite the fact that the radial topology is widely used in 400 V three-phase four-wire LV networks, it has the lowest level of supply security and reliability, with the absence of flexibility.

In order to increase the reliability level of two adjacent radial networks, the feeders can be interconnected via a normally open point to the supply in order to ensure the radial operation of each feeder, where the location of the normally open point can be moved following the occurrence of the fault to isolate the faulty section while maintaining the supply for the rest of the faulty feeder (Taylor, 1990). Such an arrangement is also known as ring open loop topology, as shown in Fig. 1.8(c) (Chen et al., 2004). Moreover, a parallel interconnected configuration or spot topology can be used by interconnecting two adjacent LV radial feeders supplied from two different substations, as shown in Fig. 1.8(d) (EEP). Such a configuration improves the system reliability and flexibility in the case of a maintenance event, where the loads may still be supplied by the other transformer (Schneider Electric, 2009).

Based on the above investigation, the main characteristic of the LV distribution network is listed as follows:

1. Large number of buses: the LV network is to supply many domestic consumers. For example, a part of the LV distribution network in Netherlands is shown in Fig. 1.9, which has more than 99.6% of the whole network connections (Nijhuis, 2017).

2. The network is not monitored: a significant part of the metering system, particularly the household meter, is still without a communication system with the network operators or smart meters. The advanced metering infrastructure (AMI) is still in the early stage of installation, which leads to the lack of understanding of the LV networks’ real state and, thus, a high number of uncertainties.

3. Operated in radial or weakly meshed topology: the majority of LV network configurations is radial, and this simplifies the power flow analysis.

4. High R/X ratios compared with HV and MV networks: especially in the case of underground cables, which makes resistance a very important factor in determining the voltage, where the voltage angle is approximately constant at the LV network.

5. Highly violated load pattern: the load pattern is unbalanced with a high level of uncertainty.

6. Untransposed feeders: the spacing between conductors is non-symmetrical and the transposition principle does not apply compared with HV and MV networks.

7. Bi-directional power flow: the distribution generator injected the excess generated power into the LV network, which resulted in a reverse power flow from the load side, and this raises the voltage level in the load side.

For different reasons related to the operation and planning of the LV distribution networks, they are considered as unbalanced or asymmetrical in which the voltages and current magnitudes in the three phases are also not equal, and the phase shift angle between two adjacent phases is not exactly 120° (Al-Jaahfreh and Mokryani, 2019).

Figure 1.10 presents a phasor diagram in order to compare the balanced three-phase network and several unbalanced conditions, where U_a, U_b, and U_c refer to the voltage or current phasors in the network (Beharrysingh, 2014). The main reason for voltage and current unbalance in LV distribution networks is the uneven distribution of single-phase loads among the three phases. This might either happen normally or due to the integration of a high amount of low carbon technologies (LCTs), such as residential PVs or electric vehicles EVs (Li and Crossley, 2014b).

In MV and HV distribution networks, the three-phase connected loads are balanced while most LV networks’ loads are single-phase (Taylor, 1990). Network planners have put much effort into the planning stage to connect an equal number of customers to each phase to make the load level balanced among the three phases (Al-Jaahfreh and Mokryani, 2019). However, in practice, phase load balancing cannot be easily done. To connect the single-phase loads to the three-phase four-wire LV distribution network, each load should be connected to two wires including phase and neutral. Figure 1.11 shows an example of three-phase four-wire LV feeder with an uneven distribution of single-phase loads (Beharrysingh, 2014; and Al-Jaahfreh and Mokryani, 2019).

The voltage unbalance is not significant in MV distribution networks; however, the MV network's unbalanced voltages are interpreted through the MV/LV distribution transformers’ windings (Taylor, 1990). Low Carbon Technologies (LCTs) are associated with any kind of technology installed in the electricity networks and used by end user customers to reduce carbon emission (Li and Crossley, 2014a). LCTs include renewable DGs, such as PVs, EVs, heat pumps, and Combined Heat and Power (CHP) (Chua et al., 2011). However, the integration of LCTs into LV distribution networks will increase the current and voltage imbalance levels mainly because of the uneven distribution of single-phase LCTs. High penetration of LCTs in a specific phase will result in a highly fluctuating demand profile. Also, the integration of a mix LCT technologies along LV feeders may lead to an unpredicted increase and decrease in load demand and, thus, a significant increase in the current (Mansor and Levi, 2017).