T & D - 32) Arrangements of LV Utility Distribution Networks (1)

Arrangements of LV Utility Distribution Networks

Introduction

In European countries the standard 3-phase 4-wire distribution voltage level is 230/400 V. Many countries are currently converting their LV systems to the latest IEC standard of 230/400 V nominal (IEC 60038).

Medium to large-sized towns and cities have underground cable distribution systems.

MV/LV distribution substations, mutually spaced at approximately 500-600 metres, are typically equipped with:

1. A 3-or 4-way MV switchboard, often made up of incoming and outgoing load-break switches forming part of a ring main, and one or two MV circuit-breakers or combined fuse/ load-break switches for the transformer circuits
2. One or two 1,000 kVA MV/LV transformers
3. One or two (coupled) 6-or 8-way LV 3-phase 4-wire distribution fuse boards, or moulded-case circuit-breaker boards, control and protect outgoing 4-core distribution cables, generally referred to as “distributors”

The output from a transformer is connected to the LV bus bars via a load-break switch, or simply through isolating links. In densely-loaded areas, a standard size of distributor is laid to form a network, with (generally) one cable along each pavement and 4-way link boxes located in manholes at street corners, where two cables cross.

Recent trends are towards weather-proof cabinets above ground level, either against a wall, or where possible, flush-mounted in the wall. Links are inserted in such a way that distributors form radial circuits from the substation with open-ended branches (see Fig. C3).

Where a link box unites a distributor from one substation with that from a neighboring substation, the phase links are omitted or replaced by fuses, but the neutral link remains in place.

Fig. C3 : Showing one of several ways in which a LV distribution network may be arranged for radial branched-distributor operation, by removing (phase) links

This arrangement provides a very flexible system in which a complete substation can be taken out of service, while the area normally supplied from it is fed from link boxes of the surrounding substations.

Moreover, short lengths of distributor (between two link boxes) can be isolated for fault-location and repair. Where the load density requires it, the substations are more closely spaced, and transformers up to 1,500 kVA are sometimes necessary.

Other forms of urban LV network, based on free-standing LV distribution pillars, placed above ground at strategic points in the network, are widely used in areas of lower load density. This scheme exploits the principle of tapered radial distributors in which the distribution cable conductor size is reduced as the number of consumers downstream diminish with distance from the substation.

In this scheme a number of large-sectioned LV radial feeders from the distribution board in the substation supply the busbars of a distribution pillar, from which smaller distributors supply consumers immediately surrounding the pillar.

Distribution in market towns, villages and rural areas generally has, for many years, been based on bare copper conductors supported on wooden, concrete or steel poles, and supplied from pole-mounted or ground-mounted transformers.

In recent years, LV insulated conductors, twisted to form a two-core or 4-core self supporting cable for overhead use, have been developed, and are considered to be safer and visually more acceptable than bare copper lines. This is particularly so when the conductors are fixed to walls (e.g. under-eaves wiring) where they are hardly noticeable.

Improved methods using insulated twisted conductors to form a pole mounted aerial cable are now standard practice in many countriesAs a matter of interest, similar principles have been applied at higher voltages, and self supporting “bundled” insulated conductors for MV overhead installations are now available for operation at 24 kV. Where more than one substation supplies a village, arrangements are made at poles on which the LV lines from different substations meet, to interconnect corresponding phases.

North and Central American practice differs fundamentally from that in Europe, in that LV networks are practically nonexistent, and 3-phase supplies to premises in residential areas are rare.

The distribution is effectively carried out at medium voltage in a way, which again differs from standard European practices.

The MV system is, in fact, a 3-phase 4-wire system from which single-phase distribution networks (phase and neutral conductors) supply numerous single-phase transformers, the secondary windings of which are centre-tapped to produce 120/240 V single-phase 3-wire supplies.

In Europe, each utility-supply distribution substation is able to supply at LV an area corresponding to a radius of approximately 300 metres from the substation. North and Central American systems of distribution consist of a MV network from which numerous (small) MV/LV transformers each supply one or several consumers, by direct service cable (or line) from the transformer location

The central conductors provide the LV neutrals, which, together with the MV neutral conductors, are solidly earthed at intervals along their lengths. Each MV/LV transformer normally supplies one or several premises directly from the transformer position by radial service cable(s) or by overhead line(s).

Many other systems exist in these countries, but the one described appears to be the most common. Figure C4 (in next part…) shows the main features of the two systems.

Will be continued…

T & D - 14) Sag and tension of transmission and distribution lines

Sag and tension of transmission and distribution line

The energized conductors of transmission and distribution lines must be placed to totally eliminate the possibility of injury to people.

Overhead conductors, however, elongate with time, temperature, and tension, thereby changing their original positions after installation. Despite the effects of weather and loading on a line, the conductors must remain at safe distances from buildings, objects, and people or vehicles passing beneath the line at all times.

To ensure this safety, the shape of the terrain along the right-of-way, the height and lateral position of the conductor support points, and the position of the conductor between support points under all wind, ice, and temperature conditions must be known.

Bare overhead transmission or distribution conductors are typically quite flexible and uniform in weight along their length. Because of these characteristics, they take the form of a catenary (Ehrenberg, 1935; Winkelmann, 1959) between support points. The shape of the catenary changes with conductor temperature, ice and wind loading, and time. To ensure adequate vertical and horizontal clearance under all weather and electrical loadings, and to ensure that the breaking strength of the conductor is not exceeded, the behavior of the conductor catenary under all conditions must be known before the line is designed.

The future behavior of the conductor is determined through calculations commonly referred to as sag-tension calculations.

Sag-tension calculations predict the behavior of conductors based on recommended tension limits under varying loading conditions. These tension limits specify certain percentages of the conductor’s rated breaking strength that are not to be exceeded upon installation or during the life of the line.

These conditions, along with the elastic and permanent elongation properties of the conductor, provide the basis for determinating the amount of resulting sag during installation and long-term operation of the line. Accurately determined initial sag limits are essential in the line design process. Final sags and tensions depend on initial installed sags and tensions and on proper handling during installation.

The final sag shape of conductors is used to select support point heights and span lengths so that the minimum clearances will be maintained over the life of the line. If the conductor is damaged or the initial sags are incorrect, the line clearances may be violated or the conductor may break during heavy ice or wind loadings.

Offshore wind farms – transmission cables

The European wind power industry is increasingly turning to the offshore wind resource, and the United States will draw on the Europeans’ experience as we begin to plan offshore wind farms. Short of generating hydrogen, or otherwise using or storing the energy offshore, it must be conducted to the on-shore load centers by submarine cables.

Offshore transmission has proved to be challenging and costly in Europe, and will present additional challenges in the US because of the lack of domestic manufacturers of high-voltage, high-capacity submarine cable, and lack of equipment for and experience in installing this type of cable.

Submarine transmission cables are common in the US for other applications, but this experience has a limited applicability to wind farms.

The offshore gas and drilling industry uses lower power levels and low (under 10 kV) to medium voltage (10-100 kV), whereas the trend in offshore wind power is toward high voltage transmission. A number of medium and high voltage transmission cables have been installed in the US to power islands but submarine transmission from generation offers different problems than transmission to a load.

For instance, windfarms usually have high reactive current demands, since most wind turbines employ induction generators. This can cause resonance with the capacitance of the cables. Economies of scale are driving up the size of offshore windfarms. Larger farms will both allow and demand more sophisticated electrical transmission systems, as wind power makes a greater impact on the onshore electrical grid. As power electronics are being developed, we may expect to see them play a greater role in offshore windfarm transmission and distribution designs, including the introduction of high voltage direct current (HVDC) transmission.

The following is a brief introduction to cable types and components as it pertains to offshore wind installations.

Insulation

Three types of cable insulation are in common use for submarine transmission for long distances (at least several kilometers.) While insulation construction and thickness vary based on voltage, all three types discussed here are used for both medium and high voltages. Insulation is characterized by their insulation material, their construction, and whether the dielectric (i.e. insulation) is lapped or extruded.

Low-pressure oil-filled (LPOF), or fluid-filled (LPFF) cables, insulated with fluid-impregnated paper, have historically been the most commonly used cables in the US for submarine AC transmission. The insulation is impregnated with synthetic oil whose pressure is typically maintained by pumping stations on either end. The pressurized fluid prevents voids from forming in the insulation when the conductor expands and contracts as the loading changes. The auxiliary pressurizing equipment represents a significant portion of the system cost. LPFF cables run the risk of fluid leakage, which is an environmental hazard.

Fluid-filled cables can be made up to about 50 km (30 mi.) in length. They are rarely used for DC applications, which are generally longer than practical for pressurizing. While LPFF cables are widely installed worldwide, the cost of the auxiliary equipment, the environmental risks, and the development of lower-cost alternatives with lower losses, have all contributed to the reduced use of LPFF cables in recent years.

Similar in construction are the solid, mass-impregnated paper-insulated cables, which are traditionally used for HVDC transmission. The lapped paper insulation is impregnated with a high-viscosity fluid and these cables do not have the LPOF cable’s risk of leakage.

Extruded insulation is replacing lapped installation as the favored options in many applications. Cross-linked polyethylene (XLPE, also called PEX) is lower cost than LPOF of a similar rating and has lower capacitance, leading to lower losses for AC applications. XLPE can be manufactured in longer lengths than LPFF (Gilbertson 2000.)

Until recently XLPE was not an option for DC transmission, since it broke down quickly in the presence of a DC current, but recent improvements allow its use for DC as well. Figure 1 shows an example of an XLPE cable.

Figure 1: Anatomy of a single-core XLPE cable (from ABB’s Long Island Cross Sound cable)

Another extruded insulation used in submarine cables is ethylene propylene rubber (EPR), which has similar properties to XLPE at lower voltages, but at 69 kV and above, has higher capacitance (Gilbertson, 2000). High-voltage submarine XLPE cable is not manufactured in the North or South America. LPOF cables are manufactured here but are not available in the sizes and lengths that will be required for an economically sized offshore wind farm. Currently any offshore windfarm in the US (or anywhere else in the Western Hemisphere) will have to import cables from Europe or Japan.

With cables that may weigh more than 75 kg/m (50 lbm/ft), the transportation costs will be a significant portion of the cost of the cable.

Conductors

The conductor in medium and high-voltage cables is copper, or less commonly aluminum, which has a lower current carrying capacity (ampacity) and so requires a greater diameter. Ampacity increases proportionally with the cross sectional area, which can range up to about 2000 mm2 (3 in2, i.e. 50 mm (2 in) in diameter) before the cable becomes unwieldy and the bending radius is too great. Large cables may have a bending radius as large as 6 m (20 ft).

The design amperage is a function not only of the voltage and the power to be carried, but also the cable length, insulation type, laying formation, burial depth, soil type, and electrical losses. Gilbertson (2000) offers a thorough technical reference on these subjects. The issues of length and losses are discussed in more detail below.

Number of Conductors

When possible in AC-cable applications, all three phases are bundled into one “three-core” cable. A three-core cable reduces cable and laying costs. It also produces weaker electromagnetic fields outside the cable and has lower induced current losses than three single core cables laid separately. As the load requirements and conductor diameter rise, however, a three-core cable becomes unwieldy and no longer feasible.

One advantage of single-core cables is that it is easier and cheaper to run a spare, fourth wire. Another advantage is that longer lengths can be made without splices or joints. Figure 2 shows a three-core cable.

Screening

A semi conductive screening layer, of paper or extruded polymer, is placed around the conductor to smooth the electric field and avoid concentrations of electrical stress, and also to assure a complete bond of the insulation to the conductor.

Figure 2: Three-core cable (Nexans)

Figure 1 shows screening on a single-core cable, and Figure 3 shows a three-core cable with screening on both the individual conductors and the three-core bundle.

Sheathing

Outside the screening of all the conductors is a metallic sheathing, which plays several roles. It helps to ground the cable as a whole and carries fault current if the cable is damaged. It also creates a moisture barrier. In AC cables, current will be induced in this sheath, leading to circulating sheath losses; various sheath-grounding schemes have been developed to reduce circulating currents that arise in the sheath.

Unlike other cable types, EPR insulation does not require a metal sheath.

Figure 3: Three-core cable (Okonite)

Table 1: Capacities of high voltage cable (Häusler, ABB, 2002)

System	AC 3 single-core cables		DC bipolar operation, 2 cables
Cable insulation type	XLPE polymer	LPOF: Oil- filled paper	LPOF: Oil- filled paper	Mass imp. Paper	XLPE polymer
Maximum Voltage	400 kV	500 kV	600 kV	500 kV	150 kV
Maximum Power	1200 MVA*	1500 MVA*	2400 MW	2000 MW	500 MW
Max. length, km (mi.)	100 (62)	60 (37)	80 (50)	Unlimited	Unlimited

* Losses may be excessive at these powers

Armor

An overall jacket and then armoring complete the construction. Corrosion protection will be applied to the armor; this may include a biocide to inhibit destruction by marine creatures such as marine borers that are present in Southeast US waters, and have recently been reported in the Northeast (Fox Islands, 2001).

Fiber optic cables for communications and control can be bundled into the cables. Note the bundled fiber optic line in Figure 2. Table 1 summarizes the current availability and limitations of AC & DC cables.