Air Circuit Breaker

Normally we are not used to seeing accidental explosions or fire blazing in our home or office when the current jumps above the safe level or when there are any other faults in the electrical system. When these accidental incidents are about to happen the circuit breaker cuts the power to the electrical system. Without circuit breakers (or the alternative, fuses), household electricity would be impractical because of the potential for fires and other mayhem resulting from simple wiring problems and equipment failures. It is one of the most important safety mechanisms either in the primary source of electricity or in the electrical distribution systems from where electricity is distributed in our home or in our home.

Air circuit breaker: When the breaker opens the current flow is interrupted that causes an electrical arc to be generated. This arc is cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. When the fault condition is fixed, the contact is again closed to restore power to the interrupted circuit. When a circuit breaker uses compressed air to extinguish the arc the circuit breaker is called air circuit breaker (ACB).

Construction: Each phase of a three-phase Air Circuit Breaker (ACB) consists of 3 types of contacts, namely; main contacts which carry the load current, arcing contacts and the arcing horn.

The main contacts: Under normal operation the main contacts carry the load current. Therefore, main contact resistance must be low enough to prevent overheating, when current is flowing through it. The main contacts are therefore made up of a good conducting material such as copper, silver or copper with silver plating. Since these metals have relatively lower melting points, they can be damaged if arcing occurs. To prevent this damage, the main contacts do not make or break the current.

Arcing Contacts: Since no arcing must occur at the main contacts, arcing contacts are provided, which make or break the circuit current.

Arcing Horns: After the arc is established on the arcing contacts, it is transferred to the arcing horns during the opening of the arcing contacts. Their shape is designed to weaken or extinguish the arc.

Arc chute: The arc chute is a cooling chamber located at the top end of the breaker. It cools the hot gases which are produced when arcing occurs.

Operating mechanism: To understand the sequence of operation of various contacts in the air circuit breaker, one must realize that arcing must never occur at the main contacts.

Opening cycle: When the breaker is closed, the load current is conducted by the low resistance main contacts. But when the breaker opens, the main contacts open first and transfer the current to the arcing contacts. The arcing contacts open and an arc is established across the arcing horns. The arc rises to the top of the arcing horns and enters the arc chute where it is rapidly cooled by heat transferring mechanism in the cooling plates. Cooled gas deionizes that makes it unable to conduct electricity, and consequently arc is extinguished.

Closing cycle: In the closing cycle, the arcing contacts touch first to make the circuit again. Then the main contacts close and enable the circuit to flow electrical current.

Advantages of Air Circuit Breaker (ACB) over the other circuit breakers

* Relatively inexpensive

* Simple installation

* Simple construction

* Simple maintenance requirements

Uses:

Air Circuit Breakers (ACB) are used in both low and high voltage current. They are commonly used in electrical distribution systems and NGD for voltage up to 15 KV. Air circuit breakers (ACB) which include operating mechanisms are mainly exposed to the environment. They protect for overload, short circuit and earth fault/ground fault. Most of air circuit breakers are used in indoor type substations.

What Are Circuit Breakers?

Circuit Breakers are devices that stop the electrical current of a circuit in the event of excess or very high voltage. Circuit breakers are handy, not only because they can protect against electrical fires but also because they can be reset. When a fuse blows out, it must be replaced each time, whereas circuit breakers are easily reset when tripped.

Each appliance in your home receives electrical currents by way of electrical circuit breakers. Circuit breakers are considered to be either main or individual. The main circuit breaker controls power to the entire house while individual circuit breakers transmit power to individual appliances.

Electrical circuit breakers are probably the most important safety feature in a building. A circuit breaker measures the amount of heat produced by the current or by the magnetic field created by the small coil inside the breaker. When the current becomes too high, the breaker will cut off the circuit to stop it. Fuses serve the same function but can only be used once. Anyone who has blown a fuse can tell you it is much easier to have circuit breakers.

Electrical circuit breakers are also useful for shutting down power to the entire house. If you need to do some electrical maintenance, either for the entire structure or individual electrical wires, shutting down the main breaker can be a safety tool. Although electrical waves are invisible, we all know they have great potential danger. You should label each individual electrical circuit breaker so it is easily accessible for future use.

Direct current, or DC circuit breakers, are commonly used in automobiles. DC circuit breakers, unlike alternate current (AC) breakers, only deliver currents in one direction. These types of circuit breakers are a renewable energy delivery source, and special precautions should be taken when using them.

Since all batteries have the capability of producing enormous amounts of energy when short-circuited, care should be taken to properly install the breaker, fuse and disconnects in a vehicle. Autos should have additional protection against over current for the battery and each of the branch circuits. Vehicle fires sometimes result when the owner does not take proper precautions.

DC circuit breakers often have plastic beads inside of the fuse. When the current becomes too heavy, these plastic beads can melt and fill the void inside the fuse and prevent an arc from forming. If the current is very heavy this can cause electrical fires. While DC circuit breakers are effective in preventing damage from an overload of current, they are only effective to a certain degree. Any power source using DC technology should have a backup master fuse to protect the vehicle against total short-circuit.

Electrical Testing Services in India

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Working of Relays

What is a relay?

We know that most of the high end industrial application devices have relays for their effective working. Relays are simple switches which are operated both electrically and mechanically. Relays consist of a n electromagnet and also a set of contacts. The switching mechanism is carried out with the help of the electromagnet. There are also other operating principles for its working. But they differ according to their applications. Most of the devices have the application of relays.

Why is a relay used?

The main operation of a relay comes in places where only a low-power signal can be used to control a circuit. It is also used in places where only one signal can be used to control a lot of circuits. The application of relays started during the invention of telephones. They played an important role in switching calls in telephone exchanges. They were also used in long distance telegraphy. They were used to switch the signal coming from one source to another destination. After the invention of computers they were also used to perform Boolean and other logical operations. The high end applications of relays require high power to be driven by electric motors and so on. Such relays are called contactors.

Relays are also named with designations like

·         Single Pole Single Throw (SPST) – This type of relay has a total of four terminals. Out of these two terminals can be connected or disconnected. The other two terminals are needed for the coil.

·         Single Pole Double Throw (SPDT) – This type of a relay has a total of five terminals. Out f these two are the coil terminals. A common terminal is also included which connects to either of two others.

·         Double Pole Single Throw (DPST) – This relay has a total of six terminals. These terminals are further divided into two pairs. Thus they can act as two SPST’s which are actuated by a single coil. Out of the six terminals two of them are coil terminals.

·         Double Pole Double Throw (DPDT) – This is the biggest of all. It has mainly eight relay terminals. Out of these two rows are designed to be change over terminals. They are designed to act as two SPDT relays which are actuated by a single coil.

Relay Applications

·         Relays are used to realize logic functions. They play a very important role in providing safety critical logic.

·         Relays are used to provide time delay functions. They are used to time the delay open and delay close of contacts.

·         Relays are used to control high voltage circuits with the help of low voltage signals. Similarly they are used to control high current circuits with the help of low current signals.

·         They are also used as protective relays. By this function all the faults during transmission and reception can be detected and isolated.

Power system Protection

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Electric protective devices

Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth.

From time to time, disturbances in the normal operation of a power system occur. These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of electrical service, to limit injury to people, and to limit damage to equipment when problem situations develop. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system.

Protective relays

These are compact analog or digital networks, connected to various points of an electrical system, to detect abnormal conditions occurring within their assigned areas. They initiate disconnection of the trouble area by circuit breakers. These relays range from the simple overload unit on house circuit breakers to complex systems used to protect extrahigh-voltage power transmission lines. They operate on voltage, current, current direction, power factor, power, impedance, temperature. In all cases there must be a measurable difference between the normal or tolerable operation and the intolerable or unwanted condition. System faults for which the relays respond are generally short circuits between the phase conductors, or between the phases and grounds. Some relays operate on unbalances between the phases, such as an open or reversed phase. A fault in one part of the system affects all other parts. Therefore relays and fuses throughout the power system must be coordinated to ensure the best quality of service to the loads and to avoid operation in the nonfaulted areas unless the trouble is not adequately cleared in a specified time. See Fuse (electricity), Relay

Zone protection

For the purpose of applying protection, the electric power system is divided into five major protection zones: generators; transformers; buses; transmission and distribution lines; and motors (see illustration). Each block represents a set of protective relays and associated equipment selected to initiate correction or isolation of that area for all anticipated intolerable conditions or trouble. The detection is done by protective relays with a circuit breaker used to physically disconnect the equipment.

Fault detection

Fault detection is accomplished by a number of techniques, including the detection of changes in electric current or voltage levels, power direction, ratio of voltage to current, temperature, and comparison of the electrical quantities flowing into a protected area with the quantities flowing out, also known as differential protection.

Differential protection

This is the most fundamental and widely used protection technique. The system compares currents to detect faults in a protection zone. Current transformers on either side of the protection zone reduce the primary currents to small secondary values, which are the inputs to the relay. For load through the equipment or for faults outside of the protection zone, the secondary currents from the two transformers are essentially the same, and they are directed so that the current through the relay sums to essentially zero. However, for internal trouble, the secondary currents add up to flow through the relay.

Over current protection

This must be provided on all systems to prevent abnormally high currents from overheating and causing mechanical stress on equipment. Overcurrent in a power system usually indicates that current is being diverted from its normal path by a short circuit. In low-voltage, distribution-type circuits, such as those found in homes, adequate overcurrent protection can be provided by fuses that melt when current exceeds a predetermined value.

Small thermal-type circuit breakers also provide overcurrent protection for this class of circuit. As the size of circuits and systems increases, the problems associated with interruption of large fault currents dictate the use of power circuit breakers. Normally these breakers are not equipped with elements to sense fault conditions, and therefore overcurrent relays are applied to measure the current continuously. When the current has reached a predetermined value, the relay contacts close. This actuates the trip circuit of a particular breaker, causing it to open and thus isolate the fault.

Effects of Harmonics on Power Systems

Most people don't realize that harmonics have been around a long time. Since the first AC generator went online more than 100 years ago, electrical systems have experienced harmonics. The harmonics at that time were minor and had no detrimental effects.

Basic Concept

A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field are uniform in a working AC machine, voltage waveform distortions are created, and the voltage-time relationship deviates from the pure sine function. The distortion at the point of generation is very small (about 1% to 2%), but nonetheless it exists. Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function, and by definition, the voltage distortion contains harmonics.

When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage waveform. These loads are referred to as linearloads (loads where the voltage and current follow one another without any distortion to their pure sine waves). Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors.

In contrast, some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads, and the current and voltage have waveforms that are nonsinusoidal, containing distortions, whereby the 60-Hz waveform has numerous additional waveforms superimposed upon it, creating multiple frequencies within the normal 60-Hz sine wave. The multiple frequencies are harmonics of the fundamental frequency.

Normally, current distortions produce voltage distortions. However, when there is a stiff sinusoidal voltage source (when there is a low impedance path from the power source, which has sufficient capacity so that loads placed upon it will not effect the voltage), one need not be concerned about current distortions producing voltage distortions.

Examples of nonlinear loads are battery chargers, electronic ballasts, variable frequency drives, and switching mode power supplies. As nonlinear currents flow through a facility's electrical system and the distribution-transmission lines, additional voltage distortions are produced due to the impedance associated with the electrical network. Thus, as electrical power is generated, distributed, and utilized, voltage and current waveform distortions are produced.

Power systems designed to function at the fundamental frequency, which is 60-Hz in the United States, are prone to unsatisfactory operation and, at times, failure when subjected to voltages and currents that contain substantial harmonic frequency elements. Very often, the operation of electrical equipment may seem normal, but under a certain combination of conditions, the impact of harmonics is enhanced, with damaging results.

Motors

There is an increasing use of variable frequency drives (VFDs) that power electric motors. The voltages and currents emanating from a VFD that go to a motor are rich in harmonic frequency components. Voltage supplied to a motor sets up magnetic fields in the core, which create iron losses in the magnetic frame of the motor. Hysteresis and eddy current losses are part of iron losses that are produced in the core due to the alternating magnetic field. Hysteresis losses are proportional to frequency, and eddy current losses vary as the square of the frequency. Therefore, higher frequency voltage components produce additional losses in the core of AC motors, which in turn, increase the operating temperature of the core and the windings surrounding in the core. Application of non-sinusoidal voltages to motors results in harmonic current circulation in the windings of motors. The net rms current is

Irms = √[(I1)2 + (I2)2 + (I3)2 + …], where the subscripts 1, 2, 3, etc. represent the different harmonic currents. The I2R losses in the motor windings vary as the square of the rms current. Due to skin effect, actual losses would be slightly higher than calculated values. Stray motor losses, which include winding eddy current losses, high frequency rotor and stator surface losses, and tooth pulsation losses, also increase due to harmonic voltages and currents.

The phenomenon of torsional oscillation of the motor shaft due to harmonics is not clearly understood, and this condition is often disregarded by plant personnel. Torque in AC motors is produced by the interaction between the air gap magnetic field and the rotor-induced currents. When a motor is supplied non-sinusoidal voltages and currents, the air gap magnetic fields and the rotor currents contain harmonic frequency components.

The harmonics are grouped into positive (+), negative (-) and zero (0) sequence components. Positive sequence harmonics (harmonic numbers 1, 4, 7, 10, 13, etc.) produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic. Negative sequence harmonics (harmonic numbers 2, 5, 8, 11, 14, etc.) develop magnetic fields and currents that rotate in a direction opposite to the positive frequency set. Zero sequence harmonics (harmonic numbers 3, 9, 15, 21, etc.) do not develop usable torque, but produce additional losses in the machine. The interaction between the positive and negative sequence magnetic fields and currents produces torsional oscillations of the motor shaft. These oscillations result in shaft vibrations. If the frequency of oscillations coincides with the natural mechanical frequency of the shaft, the vibrations are amplified and severe damage to the motor shaft may occur. It is important that for large VFD motor installations, harmonic analyses be performed to determine the levels of harmonic distortions and assess their impact on the motor.

Transformers

The harmful effects of harmonic voltages and currents on transformer performance often go unnoticed until an actual failure occurs. In some instances, transformers that have operated satisfactorily for long periods have failed in a relatively short time when plant loads were changed or a facility's electrical system was reconfigured. Changes could include installation of variable frequency drives, electronic ballasts, power factor improvement capacitors, arc furnaces, and the addition or removal of large motors.

Application of nonsinusoidal excitation voltages to transformers increase the iron lesses in the magnetic core of the transformer in much the same way as in a motor. A more serious effect of harmonic loads served by transformers is due to an increase in winding eddy current losses. Eddy currents are circulating currents in the conductors induced by the sweeping action of the leakage magnetic field on the conductors. Eddy current concentrations are higher at the ends of the transformer windings due to the crowding effect of the leakage magnetic fields at the coil extremities. The eddy current losses increase as the square of the current in the conductor and the square of its frequency. The increase in transformer eddy current loss due to harmonics has a significant effect on the operating temperature of the transformer. Transformers that are required to supply power to nonlinear loads must be derated based on the percentages of harmonic components in the load current and the rated winding eddy current loss.

One method of determining the capability of transformers to handle harmonic loads is by k factor ratings. The k factor is equal to the sum of the square of the harmonic currents multiplied by the square of the frequencies.

k = [([I.sub.1]).sup.2]([1.sup.2]) + [([I.sub.2]).sup.2]([2.sup.2]) + [([I.sub.3]).sup.2]([3.sup.2]) + . . . + [([I.sub.n]).sup.2]([n.sup.2]).

where [I.sub.1] = ratio of fundamental current to total rms current, [I.sub.2] = ratio of second harmonic current to total rms current, [I.sub.3] = ratio of third harmonic current to total rms current, etc., and 1,2,3, ... n are harmonic frequency numbers. The total rms current is the square root of the sum of square of the individual currents.

By providing additional capacity (larger-size or multiple winding conductors), k factor rated transformers are capable of safely withstanding additional winding eddy current losses equal to k times the rated eddy current loss. Also, due to the additive nature of triplen harmonic (3, 9, 15, etc.) currents flowing in the neutral conductor, k rated transformers are provided with a neutral terminal that is sized at least twice as large as the phase terminals.

Example: A transformer is required to supply a nonlinear load comprised of 200A of fundamental (60 Hz), 30A of 3rd harmonic, 48A of 5th harmonic and 79A of 7th harmonic. Find the required k factor rating of the transformer:

Total rms current, I = [square root of [([I.sub.1]).sup.2] + [([I.sub.3]).sup.2] + [([I.sub.5]).sup.2] + [([I.sub.7]).sup.2]]

Total rms current, I = [square root of [(200).sup.2] + [(30).sup.2] + [(48).sup.2] + [(79).sup.2]] = 222.4A

[I.sub.1] = 200 / 222.4 = 0.899

[I.sub.3] = 30 / 222.4 = 0.135

[I.sub.5] = 48 / 222.4 = 0.216

[I.sub.7] = 79 / 222.4 = 0.355

k = [(0.899).sup.2][(1).sup.2] + [(0.135).sup.2] [(3).sup.2] + [(0.216).sup.2]([5).sup.2] + [(0.355).sup.2][(7).sup.2] = 8.31

To address the harmonic loading in this example, you should specify a transformer capable of supplying a minimum of 222.4A with a k rating of 9. Of course, it would be best to consider possible load growth and adjust the minimum capacity accordingly.

The photo (on page 33) shows one of the things that can happen when large nonlinear loads are present in a transformer. In this case, the nonlinear loads caused a substantial temperature rise. The unit had been installed to serve an online UPS source that produced high harmonic currents in the lines coming from the transformer. The darkened areas of the coils are due to the effect of heat caused by excess eddy current losses in the transformer's windings. Very often, the damage to the coils in a transformer is not known until a failure occurs.