9 Components of Electrical Power System Explained!

An electrical power system is a complex network of components that work together to provide electricity to homes, businesses, and other buildings. These components include generators, transformers, transmission lines, substations, and distribution lines. Each component plays a vital role in the functioning of the power system, from generating electricity to delivering it to the end user. Understanding the different components of an electrical power system is essential for those working in the field of electrical engineering and for anyone looking to improve their understanding of how electricity is generated and distributed.

Components of an Electrical Power System

The most common components used in an electrical power system are:

1. Electric power generators

In order to visualize and discuss an entire functioning power system, it is necessary that we establish the necessary basic components of such a system. The first and most obvious is the three-phase ac synchronous generator or alternator. The alternator must be driven mechanically by some sort of prime mover. The early prime movers were primarily reciprocating engines and waterwheels. The simplest form of prime mover was the hydro station with a simple waterwheel. Once the original installation was made at a waterfall or dam, the fuel was free forever.

For this reason, hydro stations are seldom retired from service. Thousands of tiny hydro stations are still in use today, often unattended and operated by remote control. As the more readily available sources of water power were developed, the emphasis shifted to the steam turbine. Fired by fossil fuels such as coal, gas, and oil, steam turbines grew in numbers and unit size until they dominated the power generation industry. Even when nuclear fuels began to command a sizable portion of the market, the nuclear reactor and its complex heat exchanger ultimately served only to make steam for driving a conventional steam turbine. The turbines driven by steam from nuclear reactors must be larger than fossil-fired units if they are to be economically feasible because of the huge fixed costs, which are independent of capability. Direct conversion of energy into its electrical form without the rotating prime mover and alternator offers great promise for the future but is not yet economically competitive in commercial quantities.

2. Transformers

Practical design problems limit the voltage level at the terminals of the alternator to a relatively low value. The transformer is used to step the voltage up (as the current is proportionately reduced) to a much higher level so that power can be transmitted up to hundreds of miles while conductor size and losses are kept down within practical limits. Physically, a power transformer bank may consist of three single-phase transformers with appropriate electrical connections external to the cases, or a three-phase unit contained in a single tank. The latter is predominant in the larger power ratings for economic reasons. The windings usually are immersed in a special-purpose oil for insulation and cooling.

3. Transmission lines

The transmission line usually consists of three conductors either as three single wires (or as bundles of wires) and one or more neutral conductors, although it is possible sometimes to omit the neutral conductor since it carries only the unbalanced return portion of the line current. A three-phase circuit with perfectly balanced phase currents has no neutral return current. In most instances, the current is accurately balanced among the phases at transmission voltages, so that the neutral conductor may be much smaller. In some locations, the soil conditions permit an effective neutral return current through the earth. The neutral conductors have another equally important function: they are installed above the phase conductors and provide an effective electrostatic shield against lightning. Manufacturers of high-voltage equipment tend to standardize as much as possible on a few nominal voltage classes. The most common transmission line-to-line voltage classes in use within the United States are 115, 138, 230, 345, 500, and 765 kV. Developmental work is being done for utilizing voltages up to 2000 kV. Costs of line construction, switchgear, and transformers rise exponentially with voltage, leading to the use of the lowest voltage class capable of carrying the anticipated load over the required distance. However, the cost of energy loss is inversely proportional to the square of transmission voltage.

4. Circuit breakers and disconnect switches

Circuit breakers are large three-pole switches located at each end of every transmission-line section, and on either side of large transformers. The primary function of a circuit breaker is to open under the control of automatic protective relays in the event of a fault or short circuit in the protected equipment. The relays indicate the severity and probable location of the fault and may contain sufficient electromechanical or solid-state logic circuitry to decide whether the line or transformer could be re-energized safely, and initiate reclosure of its circuit breakers. In very critical, extra-high-voltage (EHV)* switchyards or substations, a small digital computer may be used to analyze fault conditions and perform logical control functions. If the fault is a transient one from which the system may recover, such as a lightning stroke, the integrity of the network is best served by restoring equipment to service automatically, preferably within a few cycles. If the fault is persistent, such as a conductor on the ground, the relays and circuit breakers will isolate the faulted section and allow the remainder of the system to continue in normal operation. The secondary function of a circuit breaker is that of a switch to be operated manually by a local or remote operator to de-energize an element of the network for maintenance. When a circuit breaker’s contacts open under load, there is a strong tendency to arc across the contact gap as it separates. Various methods are used to suppress the arc, including submersion of the contact mechanism in oil or gas such as sulfur hexafluoride (SF6). The higher voltage classes use a powerful air blast to quench the arc, using several interrupters or contact sets in series for each phase.

One side of an open-circuit breaker generally remains energized. To completely isolate (or de-energize) a circuit breaker, a disconnect switch is placed in series with the circuit breaker.

Representation of circuit breakers, fuses, and switches in a power system for a power system analysis is a simple process. If the breaker or switch is closed, it is assumed to be a short circuit with zero impedance. If the circuit breaker or switch is open, it is represented by an open circuit or infinite impedance (or zero admittance).

5. Voltage regulators

When electric power has been transmitted into the area where it is to be used, it is necessary to transform it back down to a distribution-level voltage that can be utilized locally. The step-down transformer bank may be very similar to the step-up bank at the generating station, but of a size to fill the needs only of the immediate area. To provide a constant voltage to the customer, a voltage regulator is usually connected to the output side of the step-down transformer.

It is a special type of 1:1 transformer with several discrete taps of a fractional percent each over a voltage range of ±10%. A voltage-sensing device and automatic control circuit will position the tap contacts automatically to compensate for the low-side voltage for variations in transmission voltage. In many cases, the same effect is accomplished by incorporating the regulator and its control circuitry into the step-down transformer, resulting in a combination device called a load tap changer (LTC), and the process is known as tap changing under load (TCUL).

6. Subtransmission

Some systems have certain intermediate voltage classes which they consider sub-transmission. Probably, at the time it was installed, it was considered transmission, but with rapid system growth and a subsequent overlay of higher voltage transmission circuits, the earlier lines were tapped at intervals to serve more load centers and become local feeders. In most systems, 23, 34.5, and 69 kV are considered sub-transmission types, and on some larger systems, 138 kV may also be included in that category, depending on the application. As the frontiers of higher voltages are pushed back inexorably, succeeding higher voltage classes may be relegated to sub-transmission service.

7. Distribution systems

A low-voltage distribution system is necessary for the practical distribution of power to numerous customers in a local area. A distributing system resembles a transmission system in miniature, having lines, circuit breakers, and transformers, but at lower voltage and power levels. Electrical theory and analytical methods are identical for both since the distinction is purely arbitrary. Distribution voltages range from 2.3 to 35 kV, with 12.5 and 14.14 kV predominant. Such voltage levels are sometimes referred to as primary voltages which are then stepped down to the 240/120 V at which most customers are served. Single-phase distribution circuits are supplied through transformers, balancing the total load on each phase as nearly as possible. Three-phase distribution circuits are erected only to serve large industrial or motor loads. The ultimate transformer which steps the voltage down from distribution to customer service level may be mounted on a pole for overhead distribution systems or on a pad or in a vault for underground distribution. Such transformers usually are protected by fuses or fused cutouts.

8. Loads

Countless volumes have been written about the systems and techniques necessary for the production and delivery of electric power, but very little has been recorded about loads, for which all the other components exist. Perhaps the main reason is that loads are so varied in nature as to defy comprehensive classification. In the simplest concept, any device that utilizes electric power can be said to impose a load on the system. Viewed from the source, all loads can be classified as resistive, inductive, capacitive, or some combination of them. Loads may also be time-variant, from a slow random swing to rapid cyclic pulses which cause distracting flicker in the lights of customers nearby. The composite load on a system has a predominant resistive component and a small net inductive component. Inductive loads such as induction motors are far more prevalent than capacitive loads. Consequently, to keep the resultant current as small as possible, capacitors are usually installed in quantities adequate to balance most of the inductive current. It has been shown that the power consumed by the composite load on a power system varies with system frequency. This effect is imperceptible to the customer in the range of normal operating frequencies (±0.02 Hz), but can make an important contribution to the control of systems operating in synchronism. System load also varies through daily and annual cycles, creating difficult operating problems.

Power system loads may be represented as real power (P) and reactive power (Q) taken from the system (as in power-flow analyses), as an impedance between a system bus and ground, or as a voltage source in series with an impedance in the case of rotating machinery loading. This latter representation is important in cases where a rotating machine will contribute to the system currents during the initial stages of a system fault.

9. Capacitors

When applied to a power system for the reduction of inductive current {power factor correction), capacitors can be grouped into either transmission or distribution classes. In either case, they should be installed electrically as near to the load as possible for maximum effectiveness. When applied properly, capacitors balance out most of the inductive component of current to the load, leaving essentially a unity power factor load. The result is a reduction in the size of the conductor required to serve a given load and a reduction in I2R losses.

Static capacitors may be used at any voltage, but practical considerations impose an upper limit of a few kilovolts per capacitor. Therefore, high-voltage banks must be composed of many capacitors connected in series and parallel. High-capacity transmission capacitor banks should be protected by a high-side circuit breaker and its associated protective relays. Small distribution capacitors may be vault- or pole-top-mounted and protected by fuses.

Industrial loads occasionally require very large amounts of power factor correction, varying with time and the industrial process cycle. The synchronous condenser is ideally suited to such an application. Its contribution of either capacitive or inductive current can be controlled very rapidly over a wide range, using automatic controls to vary the excitation current. Physically, it is very similar to a synchronous generator operating at a leading power factor, except that it has no prime mover. The synchronous condenser is started as a motor and has its losses supplied by the system to which it supplies reactive power.