T & D - 42) The Structure Of Power System

An interconnected power system is a complex enterprise that may be subdivided into the following major subsystems:

• Generation Subsystem
• Transmission and Subtransmission Subsystem
• Distribution Subsystem
• Utilization Subsystem

Generation Subsystem

Generation subsystem includes generators and transformers.

Generators

Three-phase ac generator from around 1895

An essential component of power systems is the three-phase ac generator known as synchronous generator or alternator. Synchronous generators have two synchronously rotating fields: One field is produced by the rotor driven at synchronous speed and excited by dc current. The other field is produced in the stator windings by the three-phase armature currents.

The dc current for the rotor windings is provided by excitation systems. In the older units, the exciters are dc generators mounted on the same shaft, providing excitation through slip rings. Current systems use ac generators with rotating rectifiers, known as brushless excitation systems. The excitation system maintains generator voltage and controls the reactive power flow. Because they lack the commutator, ac generators can generate high power at high voltage, typically 30 kV.

The source of the mechanical power, commonly known as the prime mover, may be hydraulic turbines, steam turbines whose energy comes from the burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal combustion engines burning oil.

A steam turbine used to provide electric power

Steam turbines operate at relatively high speeds of3600 or 1800 rpm. The generators to which they are coupled are cylindrical rotor, two-pole for 3600 rpm, or four-pole for 1800 rpm operation. Hydraulic turbines, particularly those operating with a low pressure, operate at low speed. Their generators are usually a salient type rotor with many poles. In a power station, several generators are operated in parallel in the power grid to provide the total power needed. They are connected at a common point called a bus.

With concerns for the environment and conservation of fossil fuels, many alternate sources are considered for employing the untapped energy sources of the sun and the earth for generation of power. Some alternate sources used are solar power, geothermal power, wind power, tidal power, and biomass.

The motivation for bulk generation of power in the future is the nuclear fusion. If nuclear fusion is harnessed economically, it would provide clean energy from an abundant source of fuel, namely water.

Transformers

High voltage transformer 40MVA

The transformer transfers power with very high efficiency from one level of voltage to another level. The power transferred to the secondary is almost the same as the primary, except for losses in the transformer.

Using a step-up transformer will reduce losses in the line, which makes the transmission of power over long distances possible.

Insulation requirements and other practical design problems limit the generated voltage to low values, usually 30 kV. Thus, step-up transformers are used for transmission of power. At the receiving end of the transmission lines step-down transformers are used to reduce the voltage to suitable values for distribution or utilization.

The electricity in an electric power system may undergo four or five transformations between generator and consumers.

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Transmission and Subtransmission Subsystem

An overhead transmission network transfers electric power from generating units to the distribution system which ultimately supplies the load.

Transmission lines also interconnect neighboring utilities which allow the economic dispatch of power within regions during normal conditions, and the transfer of power between regions during emergencies.

Standard transmission voltages are established in the United States by theAmerican National Standards Institute (ANSI). Transmission voltage lines operating at more than 60 kV are standardized at 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, 345 kV, 500 kV, and 765 kV line-to-line.

Transmission voltages above 230 kV are usually referred to as extra-high voltage (EHV).

High voltage transmission lines are terminated in substations, which are called high-voltage substations, receiving substations, or primary substations.

The function of some substations is switching circuits in and out of service; they are referred to as switching stations. At the primary substations, the voltage is stepped down to a value more suitable for the next part of the trip toward the load. Very large industrial customers may be served from the transmission system.

The portion of the transmission system that connects the high-voltage substations through step-down transformers to the distribution substations is called the subtransmission network. There is no clear distinction between transmission and subtransmission voltage levels.

Typically, the subtransmission voltage level ranges from 69 to 138 kV. Some large industrial customers may be served from the subtransmission system. Capacitor banks and reactor banks are usually installed in the substations for maintaining the transmission line voltage.

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Distribution Subsystem

The distribution system connects the distribution substations to the consumers’ service-entrance equipment. The primary distribution lines from 4 to 34.5 kV and supply the load in a well-defined geographical area.

Some small industrial customers are served directly by the primary feeders. The secondary distribution network reduces the voltage for utilization by commercial and residential consumers. Lines and cables not exceeding a few hundred feet in length then deliver power to the individual consumers.

The secondary distribution serves most of the customers at levels of 240/120 V, single-phase, three-wire; 208Y/120 V, three-phase, four-wire; or 480Y/277 V, three-phase, four-wire. The power for a typical home is derived from a transformer that reduces the primary feeder voltage to 240/120 V using a three-wire line.

Distribution systems are both overhead and underground. The growth of underground distribution has been extremely rapid and as much as 70 percent of new residential construction is via underground systems.

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Load Subsystems

Power systems loads are divided into industrial, commercial, and residential.

Heavy-Duty Single-Phase Capacitor Start And Run Induction Motor

Industrial loads are composite loads, and induction motorsform a high proportion of these loads. These composite loads are functions of voltage and frequency and form a major part of the system load.

Commercial and residential loads consist largely of lighting,heating, and cooking. These loads are independent of frequency and consume negligibly small reactive power. The load varies throughout the day, and power must be available to consumers on demand.

The daily-load curve of a utility is a composite of demands made by various classes of users.

The greatest value of load during a 24-hr period is called the peak or maximum demand. To assess the usefulness of the generating plant the load factoris defined. The load factor is the ratio of average load over a designated period of time to the peak load occurring in that period. Load factors may be given for a day, a month, or a year.

The yearly, or annual load factor is the most useful since a year represents a full cycle of time.

The daily load factor is:

Daily L.F. = average load / peak load            (1.1)

Multiplying the numerator and denominator of (1.1) by a time period of 24 hr, we obtain:

Daily L.F. = average load x 24 hr / peak load

Daily L.F. = energy consumed during 24 hr / peak load x 24 hr          (1.2)

The annual load factor is:

Annual L. F. =  total annual energy / peak load x 8760 hr          (1.3)

Generally there is diversity in the peak load between different classes of loads, which improves the overall system load factor.

In order for a power plant to operate economically, it must have a high system load factor. Today’s typical system load factors are in the range of 55 to 70 percent. Load-forecasting at all levels is an important function in the operation, operational planning, and planning of an electric power system. Other devices and systems are required for the satisfactory operation and protection of a power system.

Some of the protective devices directly connected to the circuits are called switchgear. They include instrument transformers, circuit breakers, disconnect switches, fuses and lightning arresters. These devices are necessary to deenergize either for normal operation or on the occurrence of faults.

The associated control equipment and protective relays are placed on switchboards in control houses.

Reference: Electrical Energy Systems by Mohamed E. El-Hawary (Dalhousie University)

T & D - 41) Primary Distribution Voltage Levels

Primary Distribution Voltage Levels

Most distribution voltages are between 4 and 35 kV. In this article, unless otherwise specified, voltages are given as line-to-line voltages; this follows normal industry practice, but it is sometimes a source of confusion.

The four major voltage classes are 5, 15, 25, and 35 kV. A voltage class is a term applied to a set of distribution voltages and the equipment common to them; it is not the actual system voltage.

For example, a 15-kV insulator is suitable for application on any 15-kV class voltage, including 12.47 kV, 13.2 kV, and 13.8 kV. Cables, terminations, insulators, bushings, reclosers, and cutouts all have a voltage class rating. Only voltage-sensitive equipment like surge arresters, capacitors, and transformers have voltage ratings dependent on the actual system voltage.

Utilities most widely use the 15-kV voltages as shown by the survey results of North American utilities in Figure 1. The most common 15-kV voltage is 12.47 kV, which has a line-to-ground voltage of 7.2 kV.

The dividing line between distribution and subtransmission is often gray. Some lines act as both subtransmission and distribution circuits. A 34.5-kV circuit may feed a few 12.5-kV distribution substations, but it may also serve some load directly.

Some utilities would refer to this as subtransmission, others as distribution.

Figure 1 - Usage of different distribution voltage classes (n = 107). (Data from IEEE Working Group on Distribution Protection, 1995)

The last half of the 20th century saw a move to higher voltage primary distribution systems. Higher-voltage distribution systems have advantages and disadvantages (see Advantages and disadvantages of higher voltage distribution below).

The great advantage of higher voltage systems is that they carry more power for a given current.

Higher Voltage Distribution

Advantages

Voltage drop - A higher-voltage circuit has less voltage drop for a given power flow.
Capacity - A higher-voltage system can carry more power for a given capacity.
Losses - For a given level of power flow, a higher-voltage system has fewer line losses.
Reach - With less voltage drop and more capacity, higher voltage circuits can cover a much wider area.
Fewer substations - Because of longer reach, higher-voltage distribution systems need fewer substations.

Disadvantages

Reliability - An important disadvantage of higher voltages: longer circuits mean more customer interruptions.
Crew safety and acceptance - Crews do not like working on higher-voltage distribution systems.
Equipment cost - From transformers to cable to insulators, higher-voltage equipment costs more.

Information above shows maximum power levels typically supplied by various distribution voltages.

Less current means lower voltage drop, fewer losses, and more power-carrying capability. Higher voltage systems need fewer voltage regulators and capacitors for voltage support. Utilities can use smaller con-ductors on a higher voltage system or carry more power on the same size conductor.

Table 1 - Power Supplied by Each Distribution Voltage for a Current of 400 A

System Voltage (kV)	Total Power (MVA)
4.8	3.3
12.47	8.6
22.9	15.9
34.5	23.9

Utilities can run much longer distribution circuits at a higher primary voltage, which means fewer distribution substations. Some fundamental relationships are:

Power – For the same current, power changes linearly with voltage.

when I₂ = I₁

Current – For the same power, increasing the voltage decreases current linearly.

when P₂ = P₁

Voltage drop – For the same power delivered, the percentage voltage drop changes as the ratio of voltages squared. A 12.47-kV circuit has four times the percentage voltage drop as a 24.94-kV circuit carrying the same load.

when P₂ = P₁

Area coverage – For the same load density, the area covered increases linearly with voltage: A 24.94-kV system can cover twice the area of a 12.47-kV system; a 34.5-kV system can cover 2.8 times the area of a 12.47-kV system.

where:
V₁, V₂ = voltage on circuits 1 and 2
P₁, P₂ = power on circuits 1 and 2
I₁, I₂ = current on circuits 1 and 2
V_%1, V_%2 = voltage drop per unit length in percent on circuits 1 and 2
A₁, A₂ = area covered by circuits 1 and 2

The squaring effect on voltage drop is significant. It means that doubling the system voltage quadruples the load that can be supplied over the same distance (with equal percentage voltage drop); or, twice the load can be supplied over twice the distance; or, the same load can be supplied over four times the distance.

Resistive line losses are also lower on higher-voltage systems, especially in a voltage-limited circuit. Thermally limited systems have more equal losses, but even in this case higher voltage systems have fewer losses.

Line crews do not like higher voltage distribution systems as much. In addition to the widespread perception that they are not as safe, gloves are thicker, and procedures are generally more stringent. Some utilities will not glove 25- or 35-kV voltages and only use hotsticks.

The main disadvantage of higher-voltage systems is reduced reliability. Higher voltages mean longer lines and more exposure to lightning, wind, dig-ins, car crashes, and other fault causes. A 34.5-kV, 30-mi mainline is going to have many more interruptions than a 12.5-kV system with an 8-mi main-line. To maintain the same reliability as a lower voltage distribution system, a higher-voltage primary must have more switches, more automation, more tree trimming, or other reliability improvements.

Higher voltage systems also have more voltage sags and momentary interruptions. More exposure causes more momentary interruptions. Higher voltage systems have more voltage sags because faults further from the substation can pull down the station’s voltage (on a higher voltage system the line impedance is lower relative to the source impedance).

Cost comparison between circuits is difficult (see Table 2 for one utility’s cost comparison). Higher voltage equipment costs more – cables, insulators, transformers, arresters, cutouts, and so on. But higher voltage circuits can use smaller conductors. The main savings of higher-voltage distribution is fewer substations.

Higher voltage systems also have lower annual costs from losses.

Table 2 – Costs of 34.5 kV Relative to 12.5 kV

Item	Underground	Overhead
Subdivision without bulk feeders	1.25	1.13
Subdivision with bulk feeders	1.00	0.85
Bulk feeders	0.55	0.55
Commercial areas	1.05–1.25	1.05–1.25

Source: Jones, A.I., Smith, B.E., and Ward, D.J., “Considerations for Higher Voltage Distribution,” IEEE Transactions on Power Delivery, vol. 7, no. 2, pp. 782–8, April 1992.

As far as ongoing maintenance, higher voltage systems require less substation maintenance, but higher voltage systems should have more tree trimming and inspections to maintain reliability. Conversion to a higher voltage is an option for providing additional capacity in an area. Conversion to higher voltages is most beneficial when substation space is hard to find and load growth is high.

If the existing subtransmission voltage is 34.5 kV, then using that voltage for distribution is attractive; addi-tional capacity can be met by adding customers to existing 34.5-kV lines (a neutral may need to be added to the 34.5-kV subtransmission line).

Higher voltage systems are also more prone to ferroresonance. Radio inter-ference is also more common at higher voltages.

Overall, the 15-kV class voltages provide a good balance between cost, reliability, safety, and reach. Although a 15-kV circuit does not naturally provide long reach, with voltage regulators and feeder capacitors it can be stretched to reach 20 mi or more. That said, higher voltages have advantages, especially for rural lines and for high-load areas, particularly where substa-tion space is expensive.

Many utilities have multiple voltages (as shown by the survey data in Figure 1). Even one circuit may have multiple voltages. For example, a utility may install a 12.47-kV circuit in an area presently served by 4.16 kV. Some of the circuit may be converted to 12.47 kV, but much of it can be left as is and coupled through 12.47/4.16-kV step-down transformer banks.

Resource: Electric distribution equipment and systems by T. A. Short (Buy the book at Amazon)