ALTERNATING CURRENT (AC) INTERRUPTION OF POWER CIRCUIT BREAKERS BASIC INFORMATION

How Power Circuit Breaker Interrupt Alternating Current?

AC interruption occurs at current zero. During the following half-cycle, the recovery voltage will build up across the circuit breaker main contacts. The typical appearance of recovery voltage will differ in inductive, resistive, and capacitive circuits (see Fig. 10-62).

  

FIGURE 10-62 Typical shape of recovery voltage on interruption: (a) induction current; (b) resistive current; (c) capacitive current.

When opening an inductive circuit, the recovery voltage will rise suddenly at a high rate because current interruption occurs at the moment of system voltage peak. This case requires fast building of dielectric strength of the open contact gap.

When interrupting resistive load, current and voltage pass through zero at about the same moment. The recovery voltage will therefore rise at a moderate rate and no particular problems are imposed on the circuit breaker.

At the moment of interruption of capacitive current, the capacitance is fully charged. The recovery voltage rises slowly during the first half cycle but continues to rise to a value twice the system voltage.

This may lead to restrikes, undesired network oscillations, and over voltages. At the moment of fault current interruption the two sections—source side (S) and line side (L)—of the network are decoupled and oscillate independently about their driving voltage.

The difference of these two transients appears across the open contacts of the breaker pole. The behavior of this transient recovery voltage is determined by the circuit parameters.

The still-moving or already fully-open breaker contacts must be able to withstand the recovery voltage.

The most severe stress for the open contact gap is the initial peak and the rate of rise (kV/#s) of the recovery voltage.

If the recovery voltage exceeds the gap insulation, the arc will restrike and current will continue until the next current zero, when interruption will again be attempted. The rate of rise of recovery voltage is a function of the constants of the circuits which supply power through the breaker.

The larger the adjacent capacitance to ground before the major inductance limiting the fault current, the slower will be the rise of the recovery voltage. Some breakers modify the recovery voltage characteristics by limiting the current, modifying its power factor, and so on.

POWER FACTOR IN AC MACHINES BASIC AND TUTORIALS

The power factor at which ac machines operate is an economically important feature because of the cost of reactive kilovoltamperes. Low power factor adversely affects system operation in three principal ways.

(1) Generators, transformers, and transmission equipment are rated in terms of kVA rather than Kw because their losses and heating are very nearly determined by voltage and current regardless of power factor.

The physical size and cost of ac apparatus are roughly proportional to kVA rating. The investment in generators, transformers, and transmission equipment for supplying a given useful amount of active power therefore is roughly inversely proportional to the power factor.

(2) Low power factor means more current and greater 12 R losses in the generating and transmitting equipment.

(3) A further disadvantage is poor voltage regulation.

Factors influencing reactive-kVA requirements in motors can be visualized readily in terms of the relationship of these requirements to the establishment of magnetic flux. As in any electromagnetic device, the resultant flux necessary for motor operation must be established by a magnetizing component of current.

It makes no difference either in the magnetic circuit or in the fundamental energy conversion process whether this magnetizing current be carried by the rotor or stator winding, just as it makes no basic difference in a transformer which winding carries the exciting current. In some cases, part of it is supplied from each winding.

If all or part of the magnetizing current is supplied by an ac winding, the input to that winding must include lagging reactive kVA, because magnetizing current lags voltage drop by 90 °. In effect, the lagging reactive kVA set up flux in the motor.

The only possible source of excitation in an induction motor is the stator input. The induction motor therefore must operate at a lagging power factor. This power factor is very low at no load and increases to about 85 to 90 percent at full load, the improvement being caused by the increased real-power requirements with increasing load.

With a synchronous motor, there are two possible sources of excitation: alternating current in the armature or direct current in the field winding. If the field current is just sufficient to supply the necessary mmf, no magnetizing-current component or reactive kVA are needed in the armature and the motor operates at unity power factor.

If the field current is less, i.e., the motor is underexcited, the deficit in mmf must be made up by the armature and the motor operates at a lagging power factor. If the field current is greater, i.e., the motor is overexcited, the excess mmf must be counterbalanced in the armature and a leading component of current is present; the motor then operates at a leading power factor.

Because magnetizing current must be supplied to inductive loads such as transformers and induction motors, the ability of overexcited synchronous motors to supply lagging current is a highly desirable feature which may have considerable economic importance. In effect, overexcited synchronous motors act as generators of lagging reactive kilovoltamperes and thereby relieve the power source of the necessity for supplying this component.

They thus may perform the same function as a local capacitor installation. Sometimes unloaded synchronous machines are installed in power systems solely for power-factor correction or for control of reactive-kVA flow. Such machines, called synchronous condensers, may be more economical in the larger sizes than static capacitors.

Both synchronous and induction machines may become self-excited when a sufficiently heavy capacitive load is present in their stator circuits. The capacitive current then furnishes the excitation and may cause serious overvoltage or excessive transient torques.

Because of the inherent capacitance of transmission lines, the problem may arise when synchronous generators are energizing long unloaded or lightly loaded lines. The use of shunt reactors at the sending end of the line to compensate the capacitive current is sometimes necessary.

For induction motors, it is normal practice to avoid self-excitation by limiting the size of any parallel capacitor when the motor and capacitor are switched as a unit.

BEARINGS OF ALTERNATING CURRENT GENERATORS BASIC INFORMATION

Although, antifriction bearings are occasionally used on alternators of smaller ratings, the great majority are furnished with oil-lubricated babbitted bearings. For horizontal shafts at small ratings ring oiled bearings are used, but at higher ratings recirculation of externally cooled oil is used.

ARMATURE AND FIELD WINDING INSULATION OF ALTERNATING CURRENT (AC) GENERATORS

Electrical insulation is used to isolate field conductors from each other and ground, and in the armature winding to isolate strands and turns from each other and the whole winding from ground. Proper

application of electrical insulation constitutes much of the art of ac machine design, particularly in

the larger generator sizes.

Armature-Winding Insulation

Armature voltages range from about 220 V to about 27 kV. With such a wide range, different techniques are employed. In the armature, insulation is for strands, turns, and ground wall.

Strand insulation is required to prevent circulating currents within a conductor bar. The voltage levels are not high so mechanical integrity is the important feature of strand insulation. This is usually a layer of served fabric or film coating.

Turn insulation is used in multiturn coils, generally applicable only in small-size generators. This insulation is required to withstand turn-turn voltage, although in some cases large transient spikes of voltage may be incident on the winding.

Ground wall insulation must withstand full voltage to ground. Typically, the whole of an armature winding is insulated for full voltage, even though some of the coils, located near the neutral end of the winding, see lesser voltage.

In high-voltage armatures (above ~5 kV), some measures must be taken to control the effects of corona and partial discharge. In the slot portion of the coil, it is necessary to prevent discharges due to capacitive coupling through the insulation, from the surface of the insulation to the grounded stator core.

These discharges are prevented by coating the outer surface of the insulated conductor with a conductive (sometimes called semiconducting) coating (paint or tape). To prevent discharges along the surface of the conductors in the end windings, those sections are sometimes coated with very weakly conducting coatings that are called grading coatings (paint of tape).

It is important to prevent electrical discharges in the vicinity of the winding because such discharges through air and in the presence of any water vapor will produce nitrous and nitric acid and ozone, substances corrosive to the materials of the winding.

Field-Winding Insulation

Field windings operate at much lower voltages (usually less than ~800 V). Some transient conditions, such as interruption of field current, can lead to much higher voltages.

Field windings are subject to the centrifugal forces due to rotation, and this presents special challenges. Dimensional stability is required of the field winding to prevent dynamic rotor imbalance.

It is also necessary, in larger machines, to allow the field winding to expand thermally with respect to the rotor steel. The resulting “creepage” surfaces must allow slip in the axial direction but not movement in the other directions.

AC GENERATORS COOLING SYSTEM BASIC AND TUTORIALS

Dissipation in generators appears as heat which must be removed. This heat appears in the armature conductors, field-winding conductors, stator core, rotor surface, and other structural elements of the machine. Cooling of armature and field conductors may be direct or indirect; the difference is direct contact of the cooling medium with the conductor or contact through electrical insulation.

Cooling Media

Alternating-current generators may be cooled by air, hydrogen, water, or (very infrequently) oil. In large machines, no matter what the cooling medium, heat is transferred to water in heat exchangers that are located within the machine case.

Smaller machines are cooled by air. Recently, there has been a trend toward air-cooling larger machines.

The upper limit in size for air-cooled machines is, as of this writing, about 350 MVA, and may increase further. The advantage of air cooling is simplicity. The disadvantage is machine size.

Hydrogen has had wide application in cooling of larger generators. It has a high specific heat and thermal conductivity and low density, so it provides better heat transfer with lower windage losses than does air.

Hydrogen also does not support oxidation, with some advantage to insulation systems. Cooling a generator with hydrogen requires additional systems to maintain hydrogen purity and to remove hydrogen from lubricating oil and shaft seals.

Since the “explosive” range of hydrogen/ oxygen mixtures is about 5% to 75% hydrogen, if the purity of hydrogen is kept above about 95%, the cooling medium will be nonexplosive. Water is used in armature winding cooling in very large machines.

Ventilation Paths

Fans used in electric machines may be of either radial flow or axial flow, and a wide variety of cooling paths are used. Figures 7-22 and 7-23 show two possible schemes.

by punching holes in the laminations. Radial passages are formed by spacers that hold the core packets apart.

Radial passages might be about 1 cm in axial length with spacing of about 5 cm. In some cases, as shown in Fig. 7-22, axial and radial cooling passages are mixed in one machine. In some cases, the ventilating gas passes first radially inward and then radially outward.

Rotor Ventilation

As with the stator core, rotor cooling takes on a variety of forms. In some cases cooling gas passes axially from the ends of the rotor and then exits through holes in the rotor surface into the air gap, and then passes through the stator core.

In other cases, gas passes radially inward from the air gap, diagonally through the rotor, and then radially outward to the air gap. This scheme can be coordinated with cooling of the stator core.

Direct and Indirect Cooling

Direct cooling, the norm for rotor windings and widely used in stator windings, exposes the cooling medium directly to the conductors. Figure 7-24 shows hydrogen and water directly cooled conductors for both stator and rotor. In a directly gas-cooled stator, relatively large passages are built into the conductor bar.

The conductor strands are transposed around the gas passages. There is strand insulation between the conductor strands and gas passage (which is often made of stainless steel), but the gas is within the ground wall.

In a directly gas-cooled rotor the gas flow may be radial, axial, or diagonal, or some combination of all three. In a directly water-cooled stator winding, the water flow may be in direct contact with the conductors.

In some cases some or all of the conducting strands are made of hollow copper tubing. In others, stainless-steel tubes are used. Typically, water flows through the machine only one or two axial passes before being returned to the cooler.

If water cooling is used, then

1. The water is maintained at very high purity so that it has low conductivity.

2. Water is carried to the armature conductors through specially made hoses, since the conductor bars are at high potential and the water header is at ground.

3. Generally, hydrogen pressure in the machine is maintained above water pressure so that any leak will be of hydrogen into the water system, rather than water into the electrical insulation. Water-cooled field windings are relatively rare, although many have been in highly reliable service for decades in some of the world’s most powerful nuclear turbine generators.

TESTING OF AC GENERATORS BASIC AND TUTORIALS

Tests are performed on generators to establish conformance with projected performance and dynamic performance parameters. Details of such tests are contained in IEEE Standard 115, IEC 60034-2 and IEC 60034-4 standards.

Resistance
Field and armature resistances are typically small, so measurements should be made using a 4-wire technique. It is important that resistance be measured at a known temperature so that correction can be made to actual operating temperature.

Open-Circuit Saturation Curve
The generator is driven by a motor to rated speed and excitation varied to produce terminal voltage over a range, typically from perhaps 30% to 120% that of rated. Some caution is required here, particularly for large machines in which excessive flux can damage the core. Open-circuit losses may be established by this test if the drive motor is well characterized and input power is measured.

Short-Circuit Saturation Curve
This test is similar to the open-circuit test, except the armature terminals are short-circuited and excitation varied to produce armature current over some convenient range. Windage and friction losses may be inferred from power input at zero excitation.

Stray load loss may be estimated as the difference between input power at rated armature current and the sum of friction and windage and armature I2R.

Zero Power Factor Saturation Curve
For a relatively small generator, the zero power factor saturation curve can be determined by running the machine with its shaft unloaded, driven by a second generator. By adjusting the excitation on the ac generator under test and excitation on the second generator, it is possible to measure the zero power factor saturation curve.

Rather extensive discussion of this method is described in IEEE 115. For large generators for which this “back-to-back” method is not practical, the zero power factor curve is usually determined by numerical methods. Often those methods employ finite elements.

Deceleration
Deceleration may be used for determining losses if the inertia of the machine is known. Since, if the shaft of a machine is unloaded, power dissipated is

where wm is mechanical speed, deceleration through synchronous speed can give a good measure of dissipation. The test may be run with the machine operating either at open-circuit or short-circuit conditions, or at zero excitation. It is usually run from a slight overspeed.

This test can be used to determine an unknown inertia from known losses and observed deceleration.

Heat Runs
These are tests performed by operating the generator at some condition until the temperature stabilizes. Heat runs at open-circuit, short-circuit, and zero power factor may be combined to estimate temperature rise in actual operation.

In large machines, good estimates of dissipation may be made by measuring the temperature rise of coolant (e.g., water). This is an alternative or supplement to measuring input power to the drive motor or machine deceleration. wm

POLES AND FREQUENCY OF ALTERNATING CURRENT (AC) GENERATORS BASIC AND TUTORIALS

The rotor and stator (field and armature) of a synchronous machine must have the same number of poles, as the magnetic interaction is between a succession of north-south magnetic-field pole pairs. The number of pole pairs for a machine will be noted as p.

The relationship between electrical frequency fe and mechanical speed N is

fe = pN/60

where P is number of poles (not number of pole pairs).

Synchronous generators are built in two elementary forms:

• Round-rotor machines are constructed with a rotor consisting of a cylinder of magnetic steel. In modern generators, the cylinder is formed from a single forging of vacuum degassed steel. The field winding is contained in radial slots in the surface of the rotor.

Round-rotor machines usually have two or four poles as illustrated in Figs. 7-2 and 7-3 respectively. The diameter of the rotor of a typical 25-MW generator is about 700 mm. The diameter of a 2000-MW generator can approach 2 m.

Figure 7.2 Round-rotor generator with two poles.

• Salient-pole machines are constructed with a number of pole pieces mounted to a central rotor shaft. The rotor pole pieces can be solid steel or assemblies of steel plates that are bound together axially with bolts.

The diameter of the rotor can range from less than 1 m in smaller salient pole generators to nearly 20 m in the largest hydroelectric generators.

In both round-rotor and salient-pole generators, the magnetic flux passing through the rotors does not vary in time, and the magnetic flux passing through the stator core does vary periodically in time at the electrical line frequency.

Figure 7.3 4-pole generator (left is round rotor, right is salient pole.

Consequently, the rotors can be made of solid steel, but the stator cores must be made of thousands of thin layers of highly permeable electrical steel. Each layer of stator core steel is coated with a thin layer of electrical insulation.

For electric utility operation, in which generation takes place at 50 or 60 Hz, mechanical speed is inversely proportional to the number of poles. Thus, 2-pole machines, which turn at 3000 or 3600 r/min, are used for most fossil-(fuel)-fired steam turbine generators which require high shaft speeds.

Most nuclear steam turbine generators, which have a lower shaft speed requirement, employ 4-pole designs and therefore turn at 1500 or 1800 r/min. Turbine generators for both fossil and nuclear power plants are typically round-rotor designs.

Hydroelectric generators, which typically have much lower shaft speeds than turbine generators and consequently require a large number of poles, are generally built as salient-pole machines. This is true also for generators intended for operation with large reciprocating engines, such as medium-speed diesels.

TIME CONSTANTS BASIC DEFINITION AND TUTORIALS

When a capacitor is connected to a voltage source, it takes a certain length of time for the capacitor to become fully charged. If a high resistance is connected in series with the capacitor, the time for charging is increased.

For any given circuit containing capacitance and resistance only, the time in seconds required to charge the capacitor to 63.2 percent of its full charge is called the time constant for that circuit.

This same time constant applies when the capacitor is discharged through the same resistance and is the time required for the capacitor to lose 63.2 percent of its charge.

The charging and discharging of a capacitor in terms of time constants is illustrated in the graph of Fig. 3.8. It will be noted that it takes six time constants to charge the capacitor to 99.8 percent of full charge.

The discharge curve is the exact reverse of the charge curve. When the capacitor is short circuited, it will lose 63.2 percent of its charge in one time constant and almost 99.8 percent of its charge in six time constants.

To determine the length of a time constant in seconds for any particular capacitor-resistance circuit, it is necessary to multiply the capacitance in microfarads by the resistance in megohorns, that is,

T = CR

As an example of how the time constant may be used in determining the performance of a capacitance-resistance circuit, we shall assume that a 20-pf capacitor is connected in series with a 10,000-ohm resistor and that 110 volts is applied to the circuit at intervals off sec.

The time constant is equal to 20 x 0.01 or 0.2 sec. (Note that 10,000 ohms is equal to 0.01 megohm.)

The time interval is given as sec, hence, the number of time constants is 2.5.

If we examine a time constant chart or graph, we find that the voltage at 2.5 time constants will be approximately 92 percent of full voltage. Applying this to our problem, 92 percent of 110 volts is approximately 101 volts. Thus, we find that the capacitor in this problem will charge to approximately 10 1 volts.

BRIEF HISTORY OF THE ELECTRIC POWER SYSTEM – BASIC INFORMATION

Over the past century, the electric power industry continues to shape and contribute to the welfare, progress, and technological advances of the human race. The growth of electric energy consumption in the world has been nothing but phenomenal.

In the United States, for example, electric energy sales have grown to well over 400 times in the period between the turn of the century and the early 1970s. This growth rate was 50 times as much as the growth rate in all other energy forms used during the same period. It is estimated that the installed kW capacity per capita in the U.S. is close to 3 kW.

Edison Electric Illuminating Company of New York inaugurated the Pearl Street Station in 1881. The station had a capacity of four 250-hp boilers supplying steam to six engine-dynamo sets. Edison’s system used a 110-V dc underground distribution network with copper conductors insulated with a jute wrapping.

In 1882, the first water wheel-driven generator was installed in Appleton, Wisconsin. The low voltage of the circuits limited the service area of a central station, and consequently, central stations proliferated throughout metropolitan areas.

The invention of the transformer, then known as the “inductorium,” made ac systems possible. The first practical ac distribution system in the U.S. was installed by W. Stanley at Great Barrington, Massachusetts, in 1866 for Westinghouse, which acquired the American rights to the transformer from its British inventors Gaulard and Gibbs.

Early ac distribution utilized 1000-V overhead lines. The Nikola Tesla invention of the induction motor in 1888 helped replace dc motors and hastened the advance in use of ac systems. The first American single-phase ac system was installed in Oregon in 1889. Southern California Edison Company established the first three phase 2.3 kV system in 1893.

By 1895, Philadelphia had about twenty electric companies with distribution systems operating at 100 V and 500-V two-wire dc and 220-V three-wire dc, single-phase, two-phase, and three-phase ac, with frequencies of 60, 66, 125, and 133 cycles per second, and feeders at 1000-1200 V and 2000- 2400 V.

The subsequent consolidation of electric companies enabled the realization of economies of scale in generating facilities, the introduction of equipment standardization, and the utilization of the load diversity between areas. Generating unit sizes of up to 1300 MW are in service, an era that was started by the 1973 Cumberland Station of the Tennessee Valley Authority.

Underground distribution at voltages up to 5 kV was made possible by the development of rubber-base insulated cables and paper-insulated, leadcovered cables in the early 1900s. Since then, higher distribution voltages have been necessitated by load growth that would otherwise overload low-voltage circuits and by the requirement to transmit large blocks of power over great distances. Common distribution voltages presently are in 5-, 15-, 25-, 35-, and 69-kV voltage classes.

The growth in size of power plants and in the higher voltage equipment was accompanied by interconnections of the generating facilities. These interconnections decreased the probability of service interruptions, made the utilization of the most economical units possible, and decreased the total reserve capacity required to meet equipment-forced outages.

This was accompanied by use of sophisticated analysis tools such as the network analyzer. Central control of the interconnected systems was introduced for reasons of economy and safety. The advent of the load dispatcher heralded the dawn of power systems engineering, an exciting area that strives to provide the best system to meet the load requirements reliably, safely, and economically, utilizing state of-the-art computer facilities.

Extra higher voltage (EHV) has become dominant in electric power transmission over great distances. By 1896, an 11-kv three-phase line was transmitting 10 MW from Niagara Falls to Buffalo over a distance of 20 miles. Today, transmission voltages of 230 kV, 287 kV, 345 kV, 500 kV, 735 kV, and 765 kV are commonplace, with the first 1100-kV line already energized in the early 1990s.

The trend is motivated by economy of scale due to the higher transmission capacities possible, more efficient use of right-of-way, lower transmission losses, and reduced environmental impact.

In 1954, the Swedish State Power Board energized the 60-mile, 100-kV dc submarine cable utilizing U. Lamm’s Mercury Arc valves at the sending and receiving ends of the world’s first high-voltage direct current (HVDC) link connecting the Baltic island of Gotland and the Swedish mainland. Currently, numerous installations with voltages up to 800-kV dc are in operation around the world.

In North America, the majority of electricity generation is produced by investor-owned utilities with a certain portion done by federally and provincially (in Canada) owned entities. In the United States, the Federal Energy Regulatory Commission (FERC) regulates the wholesale pricing of electricity and terms and conditions of service.

The North American transmission system is interconnected into a large power grid known as the North American Power Systems Interconnection. The grid is divided into several pools. The pools consist of several neighboring utilities which operate jointly to schedule generation in a cost-effective manner.

The electric power industry is undergoing fundamental changes since the deregulation of the telecommunication, gas, and other industries. The generation business is rapidly becoming market-driven. The power industry was, until the last decade, characterized by larger, vertically integrated entities.

The advent of open transmission access has resulted in wholesale and retail markets. Utilities may be divided into power generation, transmission, and retail segments. Generating companies (GENCO) sell directly to an independent system operator (ISO). The ISO is responsible for the operation of the grid and matching demand and generation dealing with transmission companies as well (TRANSCO).

This scenario is not the only possibility, as the power industry continues to evolve to create a more competitive environment for electricity markets to promote greater efficiency. The industry now faces new challenges and problems associated with the interaction of power system entities in their efforts to make crucial technical decisions while striving to achieve the highest level of human welfare.

THREE PHASE SYSTEM AND PHASE SEQUENCE BASIC AND TUTORIALS

The major portion of all electric power presently used in generation, transmission, and distribution uses balanced three-phase systems. Three-phase operation makes more efficient use of generator copper and iron.

Power flow in single-phase circuits was shown in the previous section to be pulsating. This drawback is not present in a three-phase system. Also, three-phase motors start more conveniently and, having constant torque, run more satisfactorily than single-phase motors.

However, the complications of additional phases are not compensated for by the slight increase of operating efficiency when polyphase systems other than three-phase are used.

A balanced three-phase voltage system is composed of three single phase voltages having the same magnitude and frequency but time-displaced from one another by 120°.

Figure 2.5(a) shows a schematic representation where the three single-phase voltage sources appear in a Y connection; a Δ configuration is also possible. A phasor diagram showing each of the phase voltages is also given in Figure 2.5(b).

Phase Sequence

As the phasors revolve at the angular frequency ω with respect to the reference line in the counterclockwise (positive) direction, the positive maximum value first occurs for phase a and then in succession for phases b and c.

Stated in a different way, to an observer in the phasor space, the voltage of phase a arrives first followed by that of b and then that of c. The three-phase voltage of Figure 2.5 is then said to have the phase sequence abc (order or phase sequence or rotation are all synonymous terms).

This is important for applications, such as three-phase induction motors, where the phase sequence determines whether the motor turns clockwise or counterclockwise.

With very few exceptions, synchronous generators (commonly referred to as alternators) are three phase machines. For the production of a set of three voltages phase-displaced by 120 electrical degrees in time, it follows that a minimum of three coils phase-displaced 120 electrical degrees in space must be used.

It is convenient to consider representing each coil as a separate generator. An immediate extension of the single-phase circuits discussed above would be to carry the power from the three generators along six wires.

However, instead of having a return wire from each load to each generator, a single wire is used for the return of all three. The current in the return wire will be Ia + Ib + Ic; and for a balanced load, these will cancel out. If the load is unbalanced, the return current will still be small compared to either Ia, Ib, or Ic.

Thus the return wire could be made smaller than the other three. This connection is known as a four wire three-phase system. It is desirable for safety and system protection to have a connection from the electrical system to ground. A logical point for grounding is the generator neutral point.

STATIC CHARGE - BASIC ELECTRICAL PARAMETER INFORMATION AND TUTORIALS

What Is Static Charge?


A current can only flow as long as a potential difference is sustained; in other words, the flowing charge must be replenished. Therefore, some currents have a very short duration.

For example, a lightning bolt lasts only a fraction of a second, until the charge imbalance between the clouds and the ground is neutralized.When charge accumulates in one place, it is called static charge, because it is not moving.

The reason charge remains static is that it lacks a conducting pathway that enables it to flow toward its opposite charge. When we receive a shock from static electricity—for example, by touching a doorknob—our body is providing just such a pathway.

In this example, our body is charged through friction, often on a synthetic carpet, and this charge returns to the ground via the doorknob (the carpet only gives off electrons by rubbing, but does not allow them to flow back).

As our fingers approach the doorknob, the air in between is actually ionized momentarily, producing a tiny arc that causes the painful sensation.14 Static electricity occurs mostly in dry weather, since moisture on the surface of objects makes them sufficiently conductive to prevent accumulations of charge.

However startling and uncomfortable, static electricity encountered in everyday situations is harmless because the amount of charge available is so small,15 and it is not being replenished.

This is true despite the fact that very high voltages can be involved (recall that voltage is energy per charge), but these voltages drop instantaneously as soon as the contact is made.

HOW ALTERNATING CURRENT WORKS - THE BASICS OF ALTERNATING CURRENT

The modern electric power system is an alternating current, three phase system. Electricity is generated by synchronous generators which are machines which convert the rotational energy of a shaft into electrical energy.

The energy conversion is based on a phenomenon associated with magnetism and electricity called induction. If a stationary wire loop is placed in the field of a rotating magnet, an electric current will be induced in the wire.

The rotor of an electric generator is made to look like a magnet by energizing conductors embedded in it with a source of direct current.The system that provides direct current to the rotor windings is called the excitation system.

The energized windings on the rotor are conventionally called the field or field circuit. In modern generators the direct current excitation is derived from an alternating current source that has been rectified to provide dc.

The direct current excitation establishes a magnetic field in the metal of the rotor which extends across the air gap between the rotor and the stationary part of the generator (stator or armature). Electricity is induced in coils which are placed in slots in the stator.

The voltage induced in any one coil reflects the time varying characteristic of the magnetic field, as viewed by a stationary observer, caused by the rotation of the rotor. The magnitude of the induced voltage can be adjusted up or down by changing the magnitude of the direct current flowing in the rotor.

This is done by a voltage regulator in the excitation system which monitors the voltage at the terminal of the electric generator and adjusts the field voltage up or down as required to maintain the desired generator terminal voltage.

The voltage and current have a sinusoidal shape, that is, in each cycle of 360 degrees, it starts at a zero value at zero degrees, rises to a positive maximum at 90 degrees, falls to zero at 180 degrees, continues to fall to a negative maximum at 270 degrees and returns to zero at 360 degrees, where the process repeats as shown in figure below.

This sinusoidal shape reflects the rotating pattern of the magnetic field produced on the rotor. If the stator coil is connected to an external load, current will flow. The current will also be oscillatory in nature, hence the name alternating current. The number of full cycles that occur in a set time defines the frequency of the electricity.

In the United States and many other areas of the world, the frequency is 60 hertz or cycles per second. In other areas a frequency of 50 cycles is used. The frequency is set by the number of magnetic circuits that are established on the rotor.

The frequency of the electricity produced by a particular generator is defined as: where n is the speed in revolutions per minute (rpm) and p is the number of pairs of magnetic poles. Steam turbines rotate at high speeds.

For example, if one magnetic circuit is established, that is, there two magnetic poles established (a single pair consisting of a north and a south pole), a speed of 3,600 rpm will result in a frequency of 60 hertz.

Alternately, if two magnetic circuits are established using two pairs of poles, a speed of only 1,800 rpm is needed to produce a frequency of 60 hertz. Hydraulic turbines rotate at relatively low speeds and will have many poles to produce the required frequency.

Because of the oscillatory nature of the voltage and current, an “effective” voltage and current value is defined. These effective values are considered to be equivalent to the direct current voltage and currents that would produce the same power dissipation (as heat) in a resistance.

The effective value for a sine wave is equal to 0.707 x the peak value. In the United States, for example, the oft quoted household voltage of 120 volts is an effective value and corresponds to a peak value of 169.7 volts.