Electrical Engineers Guide

Ring Main Unit(RMU) or Ring Main Distribution System

noreply@blogger.com (Manoj Sahu) — Fri, 18 Jan 2019 00:01:00 +0530

Ring Main Unit(RMU) is metal enclosed gas insulated compact switchgear unit used in secondary distribution up to 36 kV(36000 Volt), 630A. The RMUs are made of stainless steel and are hermetically tight, welded switchgear vessels. As a result, RMUs are insensitive/less sensitive to uncontrollable environmental conditions such as humidity, dust, condensation, small animals.

Siemens RMU 8DJH 36

Switchgear vessels designed as sealed pressure systems results in maintenance-free switching devices and thanks to the use of SF6 insulation, compact dimensions are possible.

There are different combinations of modules or feeders in an RMU. These modules or feeders provides application of interest. The feeders can be Ring Main feeder, Transformer feeder, Circuit Breaker feeder, Bus Sectionalizer, Cable feeder, billing and metering feeder.

Ring main feeder or Ring feeder consist of three position switch(explained later in this article) and current transformers to measure the current incoming from ring. This feeder keeps the individual RMU in ring with larger system of RMUs.
Transformer feeder or T feeder consist of Three Position Switch and HRC (high rupturing capacity) fuse to protect the feeder during fault.
Circuit Breaker feeder or Line feeder consist of Three Position Switch and Vacuum Circuit Breaker(to disconnect the system during fault condition) and current transformer whose secondary side is connected to a protective relay which sense the fault and give Trip command to the Circuit Breaker at the instant of fault.
Cable feeder is vacant individual panel.
Billing and metering feeder are used for billing purposes as the name suggests and it consist of Potential transformer (or Voltage transformer).

Three Position Switch

Three position switching devices are of three position design, having the ability to close or open or earth the feeder. To provide better mechanical interlocking, a three position is made of single shafts can be rotated to either closed or opened the disconnector or earth the line (only one operation at any given time).
Three position:-

If the disconnector is closed (line/feeder is live) the switch can never be operated to in earth position. This is known as Service condition.
If the switch is closed in earth position, the switch can never be operated to in disconnector position. This is called Maintenance condition.
The third condition is called neutral or Test. In this condition the switch is closed neither as disconnector nor as earth.

The operation of Three position switch can be fully automated or mechanised. If it is automated it can closed/opened in either disconnector or earth position using a TNC (Trip/Neutral/Close) Switch. If it is mechanised, it can be operated using an operating lever.

Applications:-

Ring main unit is used in public and industrial energy systems of the secondary distribution level, e.g. in
• Local ring-main units, customer transfer substations and switching substations of power supply and public utilities
• Wind power plants and solar plants, hydroelectric power stations
• Water and liquid waste processing systems
• Airports, train stations, underground stations
• High-rise buildings.

Miniature/Low Oil Circuit Breaker

noreply@blogger.com (Manoj Sahu) — Sun, 25 Jun 2017 17:53:00 +0530

Miniature Oil Circuit Breaker
Photo Credit - ABB Group

In the bulk oil circuit breaker, the oil has to perform two functions, first of all it act as an arc quenching medium and secondly it insulate the live part from earth. It has been found that only small percentage of oil is actually used for arc extinction while the major part is utilized for insulation purposes. For this reason the quantity of oil in bulk oil circuit breaker reaches a very high as the system voltage increases. This not only increases. This not only increases the expenses, tank size and weight of the breaker but it also increases the fire risk and maintenance problems.
A low oil circuit breaker employs solid materials for insulation purposes and uses a small quantity of oil which just sufficient for arc extinction. Regarding quenching the arc, the oil behaves identically in bulk as well as low oil circuit breaker. By using suitable arc control devices the arc extinction can be further facilitated in an low oil circuit breaker.

Construction

Fig. Miniature/Low oil circuit breaker
Photo Credit - Circuit Globe

Above figure shows the cross section of a single phase low oil circuit breaker. There are two compartment separated from each other but both filled with oil. The upper chamber is the circuit breaking chamber while the lower one is to prevent from mixing with the other chamber. This arrangement permit two advantages. First of all, the circuit breaking chamber requires a small volume of oil which is just enough for arc extinction. Secondly, the amount of oil to be replaced is reduced as the oil in the supporting chamber does not get contaminated by the arc.

Supporting chamber

It is a porcelain chamber mounted on a metal chamber. It is filled with oil which is physically separated from the oil in the circuit breaking chamber. The oil inside the supporting chamber and annular space formed both the porcelain insulation and bakelised paper is employed for insulation purposes only.

Circuit breaking chamber

It is a porcelain enclosure mounted on the top of the supporting compartment. It is filled with oil and has the following parts:-

Upper and lower fixed contact.

Moving contact

Turbulator The moving contact is hollow and includes a cylinder which moves down over a fixed piston. The turbulator is an arc control device and has both axial and radial vents. The axial venting ensures the interruption of low currents whereas radial venting vents help in the interruption of heavy current.

Top Chamber

It is metal chamber and is mounted on the circuit breaking chamber. It provides expansion space for the oil in the circuit breaking compartment. The top chamber is also provided with a separator which prevent any loss of oil by centrifugal action caused by circuit breaker operation during fault condition.

Operation

Under normal operating conditions the moving contact remains engaged with the upper fixed contact. When fault occurs the moving contact is pulled down by tripping springs and an arc is struck. The arc energy vaporizes the oil and produces gases under high pressure. This action contains the oil to pass through a central hole in the moving contact and results in forcing series oil oil through the respective passage of turbulation. The process of turbulation is ordinary one in which the section of arc are successively quenched by the effect of separate streams of oil moving across each section in turns and bearing away in gases.

Advantages

It requires lesser quantity of oil.
The space required is small.
There is reduced risk of fire.

Disadvantages

Due to smaller quantity of oil the degree of carbonisation is increased.
There is difficulty of removing the gases from contact space in time.

Vacuum Circuit Breaker

noreply@blogger.com (Manoj Sahu) — Wed, 15 Feb 2017 15:00:00 +0530

Vacuum Circuit Breaker
Image Credit- Siemens Energy

In Vacuum circuit breakers, vacuum (degree of vacuum being in the range 10-7 to 10-5 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than obtained with other circuit breaker.

Principle:-

The production of a arc in vacuum circuit breaker and its extinction can be explained as follows;-

When the contacts of the breaker are opened in vacuum (10-7 to 10-5 torr), an arc is quickly extinguished because the metallic vapor, electrons and ions produced during arc rapidly condense on the surface of the circuit breaker contacts resulting in quick recovery of dielectric strength. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

Construction:-

Above figure shows the parts of a typical vacuum circuit breaker. It consist of fixed contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapors falling on the surface of the outer insulating cover.

Working:-

When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc are diffused in a short time and seized by the surface of moving and fixed members and shield since vacuum has very fast rate of recovery dielectric strength. The arc extinction in a vacuum breaker occurs with a short contact separation say 0.625 cm.

Advantages:-

They are compact, reliable and have longer life.
There are no fire hazards.
There is no generation of gas during and after operation.
They require little maintenance and arc quiet in operation.
They can successfully withstand lightning surges.
They have low arc energy.

SF6 Circuit Breaker

noreply@blogger.com (Manoj Sahu) — Wed, 15 Feb 2017 14:09:00 +0530

SF₆ circuit breaker

In such circuit breaker SF₆ gas is used as the arc quenching medium. SF₆ is an electro-negative gas and has strong tendency to absorb free electrons.The contacts of the breaker are open in the high pressure. Flow of SF₆ gas and arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gap to form relatively immobile ions.The loss of conducting electron in the arc quickly builds up enough insulating strength to extinguish the arc. SF₆ circuit breaker has been found to be very effective for high power and high voltage service.

Construction:-

Constructional details of SF₆ circuit breaker

Above figure shows the parts of typical SF₆ circuit breaker. It consists of fixed and moving contacts enclosed in a chamber(called arc interruption chamber) containing SF₆ gas. This chamber is connected to SF₆ gas reservoir. When the contact of breaker are opened, the valve mechanism permits a high pressure SF₆ gas from the reservoir to flow towards the arc interruption chamber. The moving contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF₆ gas to let out through these holes after flowing along and across the arc. The tips of the fixed contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF₆ gas is costly, it is reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

Working:-

In the closed position of the breaker, the contacts remain surrounded by SF₆ gas at a pressure of about 2.8 Kg/cm². When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronized with the operating of a valve which permits SF₆ gas at 14 Kg/cm² pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF₆ rapidly absorb the free electrons to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly build up high dielectric strength and causes the extinction of the arc. After the breaker operation i.e., after arc extinction valve is closed by the action of a set of spring.

Advantages :-

Due to the superior quenching property of SF₆ such circuit breaker have very short arcing time.
Since the dielectric strength of SF₆ gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike air blast circuit breaker.
The closed gas enclosure keep the interior dry so that there is no moisture problem.

Disadvantages:-

SF₆ circuit breaker are costly due to high cost of SF₆ gas.
SF₆ gas has to be reconditioned after every operation of the breaker. Hence additional equipment is required for this purpose.

There are many advantages of using SF₆ circuit breaker, only few have listed above.

Classification of Electric Drives

noreply@blogger.com (Manoj Sahu) — Wed, 8 Jun 2016 14:58:00 +0530

Group Drives

Photo Credit - http://mechaholic.in

It consists of single machine which actuates several machines or mechanism by means of one or more line shaft. Hence this is also called "line shaft drive". This line shafts are connected to multi stepped pulleys and belts that connect this pulley and shaft of the driven machine, serves to vary their speed.

Group drive is economical in consideration for the cost of motor and control gear. A single motor of large capacity costs less than that of the total cost of number of small motors for same total capacity namely, a single motor of 100KW costs less than that of ten motors of 10KW each. Since all the motors may not operate on full load at the same time, the KW rating of motor of group drive is often less than the aggregate KW output rating of the individual motor and further cause reduction in cost.

Individual Drive

Photo Credit - http://mechaholic.in

In individual drive an electric motor used for transmitting motion to various parts or mechanism belonging to single equipment. For example, such drive are used to rotate the spindle, moves the feed and with the help of gears imparts motion to lubricating and cooling pumps in lathe. In application, individual drive consist of motor which is specifically designed to form an integral part of the machine.

In individual drive, the energy is transmitted to different parts of same mechanism by means of mechanical parts like gear, pulley, etc. hence occurs some power loss.

Multimotor Drive

Photo Credit - http://vertassets.blob.core.windows.net/

The multimotor drive consist of several individual motor which serve to one of many motions or mechanism in some production unit. For example, in travelling crane, there are three motors used. One for hoisting, other for long travel motion and third for cross travel motion. Such a drive is essential in complicated metal cutting machine, paper making machine, rolling mills, rotary printing machine, etc. The use of multimotor drive is expanding in modern industries due to their advantage outweighs increase in capital cost compared to the group drive.

Passive Method of Space Heating

noreply@blogger.com (Manoj Sahu) — Tue, 7 Jun 2016 16:42:00 +0530

A schematic diagram of passive space heating designed by Professor Trombe is shown in figure. The south face of the house to be heated is provided with single or double glazing. Behind it is a thick black concrete wall which absorbs Sun radiation and serve as thermal storage. Vent A and B which can be opened and closed are provided near the top and bottom of the storage wall. The whole unit consisting of storage wall with vents and glazing is referred as Trombe Wall.

During day time both vents A and B are kept open. The air between inner glazing and wall is heated and flows into living space from top vent. Simultaneously the cool air in the room is pulled out from the living space through bottom vent. Thus a natural circulation path is set up. Some energy transfer in the living space also takes place by convection and radiation from the inner surface of the storage wall. During night energy transfer take place by convection and radiation from the inner surface.

The Trombe Wall can also be used for summer ventilation by providing vent C and D near the top of glazing and north facing wall. On a hot day, vent B, C and D can be kept open while vent A would be kept closed. The heated air between glazing and wall would then flow out through vent C drawing air from the living space to replace it. This in turn would cause air to be pulled in from outside through vent D. Vent D should be located such that air pulled in through it comes from shaded and cool area.

Solar Water Heater Working (Thermosymphonic Mode)

noreply@blogger.com (Manoj Sahu) — Tue, 7 Jun 2016 15:22:00 +0530

Solar Water Heater

A Solar Water Heater(SWH) is a device which provides hot water for bathing, washing, cleaning etc. using solar energy. It is generally installed at terrace or where sunlight is available and heats water during day time which is stored in an insulated storage tank for use when required including morning. It basically heats the water using heat energy of the Sun rays. Today we are going to discuss the working of solar water heater in thermosymphonic mode.

A solar water heater is shown in above figure. It consists of a tilted liquid flat plate collector(LFPC) facing south with transparent glass covers, a separate highly insulated water storage tank and well insulated pipes connecting the two. The bottom of the tank is at least 1-2 feet the top of the collector and no auxiliary energy is required to circulate water through it. Circulation occurs through thermosymphonic mode or natural convection. As the water us heated in its passage through the collector its density decreases and hence it rises and flows into the top of the storage tank, colder water from the bottom of the tank has higher density and so tends to sink and enter the lower header of the collector for further heating.
The density differences between the hot water and cold water thus provides the driving force necessary for the circulation of water through the collector and the storage tank, Hot water is drawn off from the top of the tank as required and is replaced by cold water from the service system. As long as the Sun shines the water will quietly circulate getting warmer. To provide heat, during cloudy periods, an electrical immersion heater can be used as backup for the solar system.

Wireless Power Transfer

noreply@blogger.com (Manoj Sahu) — Mon, 11 Apr 2016 14:24:00 +0530

First demonstrated by Nicholas Tesla in the 1890s, wireless power transfer is an innovative technology that has permeated major areas in the consumer and industrial electronic market.
The various forms of WPT include solar energy, microwaves, and magnetic energy. In this article, we will focus on wireless power transfer using magnetism and induction coils. The following offers an insight into the working principle, features, and applications.

Working principle

Wireless power transfer works on the inductive power transfer principle, as found in the conventional transformers. The only difference is that while in the transformer the two coils are in very close proximity and contain a ferrite material to increase the coupling, inductive chargers have an air gap between the two coils. The process follows the following procedure:

The mains voltage is converted into alternating current, preferably, high-frequency AC

This current (the high-frequency AC) is transferred to the coil via transmitter circuit. This AC induces a magnetic field in the transmitter coil.

The induced magnetic field generates a current in the adjacent receiver coil.

However, in the earlier applications, the designers faced a challenge; the strength of a magnetic field decreases with distance. The decrease in strength is proportional to the square of the distance from the source. This made it difficult to regulate power and reduced energy efficiency. To solve this, the designers introduced resonance. You acquire resonance by multiplying the capacitance of the plates attached to the ends of the coil with the coil inductance.

The introduction of resonators with the same frequency in the sources and receiver coil respectively ensures that the two systems couple magnetically, thus allowing for higher energy transfer efficiency. This means that the power transfer happens over an air gap without the need for metal or other material connection.For this to happen, both the transmitter and the receiving coil must resonate at the same frequency. The generated AC is converted into direct current for charging the battery.However, in cases where the two objects are far apart, power transfer can still be achieved through resonating the two coils at the same frequency. This eliminates the need for perfect alignment.Greater power transfer distances can be achieved by introducing resonant repeaters between the two components.

Advantages

Allows for charging of multiple devices. This is achieved by changing the coil geometry, as well as allocating large charging surface areas such as table tops and charging benches.

High charging speeds: though at the moment wireless charging offers a slower charging rate than the wired option, advances in resonance and induction technology promises an increased charging rate and improved efficiency in the future.

Wireless power transfer allows for greater spatial freedom between the power source and the device. This means that the two do not have to be precisely aligned for power transfer.

Eliminating charging cords enables engineers to make compact and watertight devices, thus maximising on safety, and varied use such as in deep-sea applications.

Prevents corrosion and sparking by eliminating mechanical connectors and wired contacts.

Reduces costs associated with maintaining and replacing mechanical connectors.

Applications

Industrial Applications: Wireless power transfer has seen tremendous applications and value addition to industries. The primary applications include wireless sensors on rotating shafts, wireless equipment charging and powering, and safe and watertight equipment through eliminating charging cords.
Subsea applications: Though subsea vehicles can self-navigate, human assistance is still required for power supply. Due to the rough terrain, as well as the distance, cabled conductors can prove to be a challenge. WPT comes in handy in these instances.
Charging mobile devices, unmanned aircraft, home appliances and electric vehicles: The charging system the smaller gadgets comes in the form of a charging pad and power benches, where the user places the device such as a mobile phone and electric toothbrushes.
Charging and operating medical implants such as subcutaneous drug supplies, pacemakers, and other implants. WPT, especially with high resonance allows convenient continual charging of these implants without the need for frequent surgeries and the inclusion of external charging ports.
Charging wearables: The convenience of wearables lies in the mobility and convenience. Considering that the wearer has to walk around, the primary problem thus is the charging. Wireless power transfer accords the convenience of charging by eliminating the requirement for cables and connectors.

Skin Effect in Transmission Lines

noreply@blogger.com (Manoj Sahu) — Wed, 23 Mar 2016 13:09:00 +0530

The distribution of current throughout the cross section of conductor is uniform only when the steady current(D.C.) is passing through it. However, an alternating current flowing through the conductor does not distribute normally, rather it has the tendency to concentrate near the surface of the conductor as shown in figure below.

What is skin effect

The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect.

This results in higher resistance to alternating current that to direct current and is more pronounced as frequency is increased. This is known as skin effect. It causes a larger power loss for a given rms ac than the loss when the same value of dc is flowing through the conductor. Consequently, a qualitative explanation of the phenomenon is given below.

" A conductor could be considered as composed of very thin filaments. The inner filaments carrying current gives rise to flux which links the inner filaments only when as the flux due to current carrying outer filaments enclose both the inner as well as the outer filaments. The flux linkages per ampere to inner strands is more as compared to outer strands. The inductance* of each strand will vary according to its position. Thus the strands near the center are surrounded by greater magnetic flux and hence have larger inductance than that near the surface. The high reactance of inner strands causes the alternating current to flow near the surface of conductor. This crowding of current near the conductor surface is the skin effect. "
With the increase of the frequency the non-uniformity of inductive reactance of the filaments becomes more pronounced, so also the non-uniformity of current distribution. For large solid conductors the skin effect is quite significant even at 50Hz. The analytical study of skin effect requires the use of Bessel's functions.
The skin effect depends upon the following factors:-

Nature of material
Diameter of wire- It increases with increase in diameter of wire
Frequency- It increase with increase in frequency.
Shape of wire- It is less for stranded conductor than solid conductor.

It may be noted that skin effect is negligible when the supply frequency is low (≤ 50Hz) and conductor diameter is small (< 1 cm).

Corona phenomenon in Transmission Lines

noreply@blogger.com (Manoj Sahu) — Mon, 21 Mar 2016 16:24:00 +0530

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameter, there is no apparent change in the condition of atmospheric air surrounding the conductors, if the applied voltage is low. However when the voltage on line conductor is raised beyond a certain limit, called critical disruptive voltage, the conductors are surrounded by pale violet glow together with a slight hissing noise and a smell of ozone. This phenomena is called as corona.

In short corona phenomena is the ionization of air surrounding the power conductors. Free electrons are normally present in free space because of radioactivity and cosmic rays. As the potential between the conductors is increased, the gradient around the surface of the conductor increases. Assuming that the spacing between the conductors is large as compared with the diameter of the conductors. The free electrons will move with certain velocity depending upon the field strength. These electrons will collide with the molecules of air and in case the speed is large they will dislodge electron from the air thereby the number of electrons will increase. The process of ionization is thus cumulative and ultimately forms an electron avalanche. This results in localization of air surrounding the conductor and hence corona effect is occurred.

Corona occurrence is therefore defined as a self sustained electric discharge in which the filed intensified ionization is localized only over a portion of the distance between the conductors.

The phenomena of corona is accompanied by hissing sound, production of ozone, power loss and radio interference. The higher the voltage is raised, the larger and higher the luminous envelope becomes and greater the sound, the power loss and radio noise. If the applied voltage is increased to breakdown value, a flash over will occur between the conductors due to the breakdown of air insulation.

If the conductors are polished and smoothed, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow. For a visual corona the line voltage has to be somewhat higher than critical disruptive voltage and is called visual critical voltage.

Turn ON methods of SCR/Thyristor Triggering

noreply@blogger.com (Manoj Sahu) — Wed, 2 Mar 2016 13:12:00 +0530

A thyristor can be switched from a non conducting state to a conducting state in several ways-

Forward Voltage Triggering (High Voltage)

When anode to cathode forward voltage is increased with gate circuit open, the leakage current of the thyristor increases. Due to internal current multiplication taking place inside, this current increases. As soon as the forward voltage reaches the breakover voltage (V_BO), the reverse biased junction J2 will have an avalanche breakdown.

At this voltage, thyristor changes from OFF state to ON state characterized by a low forward voltage across it with large forward current. This type of turn ON may be destructive and should be avoided.

Thermal Triggering ( Temperature Triggering)

Like any other semiconductor, the width of depletion layer of a thyristor decreases on increasing junction temperature. When the temperature of thyristor is high, there is an increase in the number of electron-hole pairs which increases the leakage current. This increase in leakage current causes increase in current amplification factor ∝₁ and ∝₂. Due to the regenerative action, ∝₁+∝₂ may tend to be unity and the thyristor may be turned ON. This is called thermal triggering of thyristor. This type of turn ON may cause thermal runaway and is normally avoided.

Light Triggering (Radiation Triggering)

If light of adequate frequency and intensity is allowed to strike the thyristor junction, then the photons will strike the electrons and increase the number of electron-hole pairs. This leads to instantaneous flow of current within the device and the triggering of the device. For light triggering to occur, the device must have high value of rate of change of voltage (dV/dt).

dV/dt Triggering

With forward voltage across the anode and cathode of a device, junction J1 and J3 are forward biased whereas the junction J2 becomes reverse biased. This reverse biased junction J2 has the characteristic of a capacitor due to charge existing across the junction. If a forward voltage is suddenly applied, a charging current will flow tending to turn ON the device. If the voltage impressed across the device is denoted by V, the charge by Q and capacitance by C_j then,

The rate of change of junction capacitance may be negligible as the junction capacitance is almost constant. If the rate of change of voltage across the device is large, the device may turn ON even though the small voltage appearing across the device is small.

Gate Triggering

This is the most commonly used method for SCR triggering. The injection of gate current by applying positive gate voltage between the gate and cathode terminals turn ON the SCR much before the specified breakover voltage. The conduction period of the SCR can be controlled by varying the gate signal within the specified value of maximum and minimum gate current. Three types of signals can be used for triggering the SCR using gate. They are either a.c. signal, d.c. signal or pulse signal.

Silicon Controlled Rectifier (SCR)

noreply@blogger.com (Manoj Sahu) — Wed, 2 Mar 2016 12:11:00 +0530

Silicon Controlled Rectifier or SCR is one of the oldest type of solid state power device. It was invented in 1975 by the General Electric Research Laboratory. It has the highest power handling capacity of all the power semiconductor device. It has four layer construction with three user accessible terminals. SCR is a latching type device that can be turned ON by the gate terminal but once turned ON, the Gate loses control on it.

Important Features

It is latching type device.
It can handle a very large power.
It is a current controlled device because the gate current controls the SCR.
It acts as an open or closed switch.
The ON state voltage drop is very low.
It can handle thousands of ampere of current.

Construction

It is a four layer PNPN device with three terminals brought out for the user, namely Anode(A), Cathode(K) and Gate(G). The Gate terminal is used in ON process. It can be split into two sections of NPN and PNP as shown below,

It has three junctions J1, J2 and J3. The anode and cathode are connected to the main power circuit. The gate terminal carries a low level gate current in the direction of gate to cathode. Normally, the gate terminal is provided at the P layer near the cathode as shown in above figure. This is known as cathode gate.

Principle of operation of SCR (Silicon Controlled Rectifier)

noreply@blogger.com (Manoj Sahu) — Tue, 9 Feb 2016 12:57:00 +0530

Silicon Controlled Rectifier

When the anode voltage voltage is made positive with respect to the cathode, the junctions J1 and J3 are forward biased but the middle junction J2 is reverse biased and only a small leakage current flows from anode to cathode due to the mobile charges. The junction J2, because of the presence of depletion layer does not allow any current to flow through the device. The leakage current is insufficient to make the device conduct. The depletion layer mostly of immovable charges does not constitute any flow of current. The SCR is then said to be in the forward blocking or OFF sate condition and the leakage current is known as OFF state current I_D.

Silicon Controlled Rectifier

When the cathode voltage is positive with respect to the anode, the middle junction J2 becomes forward biased but the two outer junctions J1 and J3 becomes reverse biased. This is like two series connected diodes with reverse voltage across them. The junction J1 and J3 do not allow any current to flow through the device. Only a very small leakage current may flow because of the drift the charges. This leakage current is again insufficient to make the device conduct. The SCR is in the reverse blocking state or OFF state and a reverse leakage current known as reverse current I_R flows through the device. The width of the depletion layer at the junction J2 decreases with increase in anode to cathode voltage (since the width is inversely proportional to the voltage). If the anode to cathode voltage V_AK is kept on increasing sufficiently to a large value, a stage comes when the depletion layer at J2 vanishes. The reverse biased junction J2 will breakdown due to the large voltage gradient across its depletion layer. This is known as avalanche breakdown and the corresponding voltage is called forward breakdown voltage V_BO.

Silicon Controlled Rectifier

Because the other junctions J1 and J3 are already forward biased, there will be a free carrier movement across all three junctions resulting in a large forward anode to cathode current through the device. Due to the flow of this anode to cathode forward current, the device is said to be in conducting state or ON state. The voltage drop would be due to the ohmic drop in the four layers and is small typically, 1V.
The anode to cathode forward current must be more than latching current I_L to maintain the required amount of carrier flow across the junction; otherwise, the device reverts to blocking state as the anode to cathode voltage is reduced.

Latching Current (I_L):-

It is the minimum anode to cathode current that must flow through SCR to maintain the device in the ON state immediately after it has been turned ON and the gate signal has been removed.

Once an SCR conducts, it behaves like a conducting diode and there is no control over the device. The device continues to conduct because there is no depletion layer on the junction J2 due to free movements of carriers. However, if the forward anode current is reduced below a level known as holding current IH, a depletion region develops around junction J2 due to the reduced number of carriers and SCR is in the blocking state.

Holding Current (I_H):-

It represents the minimum current that can flow through SCR and still "hold" it in the ON state. The accompanying voltage is termed as V_H. If the forward anode current is reduced below holding current, SCR will be turned OFF. The holding current is defined for zero gate current (IG = 0).

Note:- The ON state of SCR is known as firing or triggering.

Classification of Overhead Transmission Line

noreply@blogger.com (Manoj Sahu) — Tue, 2 Feb 2016 12:06:00 +0530

A transmission line has four constants R, L, C and shunt conductance. But generally, three constants R, L and C are considered and they are uniform along the whole length of line. The fourth constant shunt conductance between conductors or between conductor and ground and accounts for the leakage current at the insulators. It is very small in case of overhead lines and may be assumed zero. The capacitance existing between conductors for 1φ line or 3φ line forms a shunt path throughout the length of line. Therefore capacitance effects introduce complication in transmission line calculation. Depending upon the manner in which capacitance is taken into account, the overhead transmission line are classified as,

Short transmission lines
Medium transmission lines
Long transmission lines

Short transmission lines

A short transmission line is one in which the line voltage is comparatively low (< 20kV) and the length of an overhead transmission line is upto about 50km. Due to smaller length and lower voltages the capacitance effects are small and hence can be neglected. Hence, whenever studying the performance of a short tranmssion line only resistance and inductance of the line are taken into consideration.

Medium transmission lines

The transmission line having length of an overhead transmission line in the range 50-150 km and the line voltage is moderately high (> 20 kV < 100kV) is considered as a medium transmission line. Since the line is having sufficient length and line voltage, the capacitance effects are taken into consideration. For the puropose of calcuklations, the distributed capacitance of the line is divided and lumped in the form of condensers shunted across the line at one or more points.

Long Transmission Line

When the length of an overhead line is more than 150 km and the line voltage is very high (>100 kV), it is considered as long transmission line. For the treatment of such line, the line constants are considered uniformly distributed over the whole length of the line and rigorous methods are employed for solution.

What is meant by excitation?

noreply@blogger.com (Manoj Sahu) — Mon, 18 Jan 2016 11:32:00 +0530

"Self Excited vs Magneto". Licensed under PD-US via Wikipedia

Electric generators work on the principle of electromagnetic induction. The essential part of this principle is the magnetic field. The magnetic field is produced from a d.c. power source from an exciter that is part of the generator system. The main requirement for electricity generation as per the basic principle is a magnetic field. The generator while producing electricity also has to produce the excitation current at a constant voltage for the electrical system to work properly. Controlling the magnetic field controls the voltage output of the generator.

The rotor or field coils in a generator produce the magnetic flux that is essential to the production of the electric power. The rotor is rotating electromagnet that requires a d.c. electric power source to excite the magnetic field. This power comes from an exciter.

Exciter is a device that provides a magnetizing current for the electromagnets in a motor or generator. There are two types of exciter, static exciter and rotory exciter. Rotory exciter is an additional small generator mounted on shaft of main generator. It will supply d.c. voltage to the rotory poles through slip ring and brushes. If it is an a.c. exciter, output of a.c. exciter is rectified by rotating diodes and supply d.c. to main field poles.

What is hunting in synchronous machine?/Phase Swinging/Surging

noreply@blogger.com (Manoj Sahu) — Sat, 16 Jan 2016 09:58:00 +0530

With the extensive use of synchronous machines, the importance of thoroughly investigating the influence of electrical constants such as resistance and reactance, on hunting is obvious. Hunting is a term used to designate the oscillations of the rotating parts of machines when they are accelerated or decelerated with respect to normal speed. It is essentially a mechanical phenomenon and produces pulsations in the current, voltage and power, due to the variations of angular velocity (due to irregularity of torque) or to the electrical operation of the machines; and if the oscillations exceed a certain amount the regulation of the machines becomes unstable and they fall out of step.

When a synchronous motor is loaded to a varying load, the rotor of the motor falls back by certain angle behind the revolving magnetic field. As the load on the motor is progressively increased, this angle also increases so as to produce necessary torque required to cope up with the load. If the load is suddenly decreased, the motor is immediately pulled up or advanced to a new value corresponding to new load. But in this process the rotor overshoot, hence it is again pulled back. In this way the rotor starts oscillating about its new position of equilibrium corresponding to a new load. If the time period of this oscillation happens to be equal to natural time period of the machine, the mechanical resonance is set up.

The amplitude of these oscillations is built up to a very large value and may eventually become so great that machine is thrown out of the synchronism. This oscillations of rotor about its equilibrium position due to change in load is called hunting. To stop the build up of these oscillations, damper winding are employed which consist of short copper bars embedded in the faces of field pole of the motor. The oscillations of rotor sets up eddy current in the damper winding which flow in such a way so as to suppress the oscillations.

Figure above shows the variation in current during hunting.

Torque Equation of D.C. Motor

noreply@blogger.com (Manoj Sahu) — Mon, 11 Jan 2016 15:57:00 +0530

In D.C. machine it is seen that the machine torque is uniform for given flux per pole and armature current. Torque is defined as the product of force with its radial distance at which it acts.

T = F × r

But

F = BIL...Newton

where,

B = flux density in Wb/m2
I = current through armature conductor in Amp
L = Length of armature conductor
r = radius of armature drum.

Considering,

P = Total number of poles
Φ = Flux cut per pole in Wb
Z = Total number of armature conductor
A = Number of parallel paths
I_a = Armature current

Substituting the values of B and I we get,

Since Z are the total number of conductors, therefore total torque is given by

This is known as torque equation of D.C. motor. But P, Z and A are constant hence,

T ∝ ΦIa

Thus the torque produced by a D.C. Motor is directly proportional to main flux Φ as well as armature current Ia.

Back E.M.F.

noreply@blogger.com (Manoj Sahu) — Fri, 8 Jan 2016 15:27:00 +0530

What is Back e.m.f. ?

When the armature of a dc motor rotates under the influence of the driving torque, the armature conductor move through the mangetic field and hence e.m.f. is induced in them as in generator. The induced e.m.f. acts in opposite direction to the applied voltage V (by Lenz's Law) and is known as back e.m.f. or counter e.m.f. E_b. The value of back emf depends upon the speed of rotation of armature conductors.

Significance of Back e.m.f.

The presence of back e.m.f. makes the d.c. motor a self regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load. The back e.m.f. is always less than applied voltage.

Let,

V = Applied voltage

E_b = Back e.m.f.

R_a = Resistance of armature conductor

Then the current in the armature conductor I_a at any instant is given by,

I_a = ^{Net voltage}/_Resistance

=^V-E_b/_{R_a}

OR V = E_b + I_aR_a

When the motor is running on no load small torque is required to overcome the friction and windage losses. Therefore, the armature current is I_asmall and the back e.m.f. is nearly equal to the applied voltage. If the motor is suddenly loaded the first effect is to cause the armature to slow down. Therefore the speed at which the armature conductors move through the field is reduced and hence the back e.m.f. E_b falls. The decreased back e.m.f. E_ballows larger current to flow through the armature and larger current means increased driving torque. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load.

If the load on the motor is suddenly decreased, the driving toque is momentarily in excess of the requirement so that armature is accelerated. As the armature speed increase and causes the armature current I_a to decrease as back e.m.f. increases too. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load.

It follows, therefore, that back e.m.f. in d.c. motor regulates the flow of armature current i.e., it automatically changes the armature current to meet the load requirement.

Necessity of Connecting Alternator in Parallel

noreply@blogger.com (Manoj Sahu) — Fri, 1 Jan 2016 13:22:00 +0530

For high efficiency of operation, reliability, convenience and economy in maintenance and repair and possibility of additions to plant with the growth in load on the power station the alternators are put in parallel.

Circuit diagram for connecting alternators in parallel
Photo Credit - www.allaboutcircuits.com

Continuity of supply:-

The continuity of supply is one of the important requirement of any electrical apparatus. If one alternator fails, the continuity of supply can be maintained through other units. This will ensure uninterrupted supply to consumer.

Efficiency:-

The load on the power system varies during whole day, being minimum during the late night hours. Single alternator operate most efficiently only when delivering full load, units can be added or put off depending upon the load requirement. This permit the efficient operation of the power system.

Maintenance and repair:-

It is often desirable to carry out routine maintenance and repair of one or more units. For this purpose, the desired unit can be shut down and the continuity of supply can be maintained through other units.

Load Growth:-

The load demand is increasing due to the increasing use of electrical energy. The load growth can be met by adding more units without disturbing the original installation.

Star Delta Starter

noreply@blogger.com (Manoj Sahu) — Wed, 30 Dec 2015 14:20:00 +0530

If normal supply voltage is applied to the stationary motor then a very large initial current is taken by the stator for short while. This will lead to excess copper losses in the winding which will overheat the motor. This will produce line voltage drop, that in turn, will affect the operation of other electrical equipments connected to the same line and heavy starting current may damage the motor winding. In order to avoid these problem, a starter is used to start the induction motor safely.

Star-Delta Starter

The motor is designed to operate as delta connected motor under normal conditions. The motor starts as a star connected motor and the voltage per phase is ^V_L/_√3. The starting torque is reduced as it is directly proportional to square of stator voltage and there is jerk while switching from star to delta.

Start Mode

The 3 pole 6 way switch is kept in start mode. This will connect terminal R', Y' B' of stator winding to each other. This act as star point. The supply is connected to R,Y,B terminal of the stator winding. Thus in start mode the stator winding is connected to form the star.

Run Mode

The 3 pole 6 way switch is thrown in run position once the motor accelerate. This will connect the terminals in following manner:

R→B', Y→R', B→Y'

Capacitor Start Capacitor Run Induction Motor

noreply@blogger.com (Manoj Sahu) — Tue, 29 Dec 2015 16:44:00 +0530

Capacitor Run Induction Motor-Representational Image

We know that single phase induction motor is not self starting due to the absence of rotating magnetic field at starting. In order to produce rotating field there must be some phase difference. In capacitor start capacitor run induction motor we are using two winding, one main winding and other auxiliary winding. For producing the phase difference two capacitors are used since we know that in capacitor the current leads the applied voltage by some angle. The capacitor serves to shift the phase on one of the windings so that the voltage across the winding is at 90° from the other winding, thus making the capacitor run motor a truly two-phase machine at its rated load.

Capacitor start capacitor run induction motor

Instead of one if two capacitors are used in the auxiliary winding of the motor, one of them for starting and other for running, optimum starting and running performances can be obtained. Figure shows the schematic diagram of the motor. The capacitor Cs is cut out when the motor reaches about 75% of the synchronous speed by centrifugal device. The capacitor C and auxiliary winding A retained in the circuit for running condition. A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

Torque- Speed Characteristics of Capacitor Start Capacitor Run Induction Motor

Capacitor-start/capacitor-run motors have moderate-to-high starting torque compared to other types of single-phase motors. Starting torque generally ranges from 200%-350% of normal full-load torque.

Torque- Speed Characteristic of Capacitor Start Capacitor Run Induction Motor

Their main advantage over the capacitor-start motor is their lower starting-torque that results in lower starting current.
Because of the lower starting current, they are generally used for most single-phase applications between 3-and-10 horsepower due to their lower starting current. Common applications include: larger single-phase compressors, pumps, grinders, conveyors, and larger single-phase air-conditioning compressors.

Divergence Theorem

noreply@blogger.com (Manoj Sahu) — Tue, 29 Dec 2015 14:32:00 +0530

It gives the facility to convert surface integral into its equivalent volume integral.

Representational Image
Photo Credit - www.wikipedia.com

Statement:- It states that the surface integral of a normal component of any vector function on a closed surface is equal to the volume integral of the divergence of vector function.
Proof:- The total charge enclosed by volume △V is given by

........(1)

The total outward flux coming out from the volume is △V equal to the charge enclosed.

△Ψ = △Q = charge density x volume

△Ψ = △Q =ρ_v△V = ρ_v△x△y△z.......(2)

From equation (1) and (2)

........(3)

The divergence of D is given by

........(4)

From (3) and (4)

According to Gauss's Law

ψ = Q

Universal Motors

noreply@blogger.com (Manoj Sahu) — Sun, 27 Dec 2015 15:08:00 +0530

A universal motor is defined as a motor which may be operated on either d.c. or single phase a.c. supply at very high speed in the range of 3000 to 7000 rpm or so. It has starting torque and variable speed characteristics. Universal motor are designed for weak field to minimize commutation difficulties. The stator core is laminated and high resistance brushes are used.

Universal Motor
Photo Credit www.woodgears.ca

Construction

The construction of universal motor is similar to that of DC Machine. The field poles are mounted on stator winding. Field winding is wound on the field poles. Both the stator field poles and armature is laminated to minimize the Eddy current losses while working on a.c. supply. The armature has straight or skewed slots and the commutator and brush arrangement is resting on it.

Universal Motor - Construction

Working

Such motors develop unidirectional torque regardless of whether they operate on A.C. or D.C. supply. The motor runs on the same principle as d.c. motor i.e., force between the main pole flux and the current carrying armature conductor. The universal motor produces the electric torque proportional to the quadrate of the supply current. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus, a universal motor can run both on dc and ac. So the electric torque has the same torque direction at any current polarity and in addition at AC power. The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current.Consequently, the starting torque on AC power will be lower than the starting torque on DC power. The characteristics of universal motor are very much similar to those of D.C. series motors, but the series motor develops less torque when operating from an A.C. supply than when working from an equivalent D.C. supply.

Torque-Speed Characteristics

Torque speed characteristics of Universal Motor

Application

Universal motors are used in most hand held power tools such as jigsaw, routers, drills and sanders.
They also find their use in household appliances like vacuum cleaner, drink and food mixers and sewing machine.

Double Cage Induction Motor

noreply@blogger.com (Manoj Sahu) — Sun, 27 Dec 2015 13:00:00 +0530

Double Cage Induction motor Stator Construction

Construction

An induction motor with two cage rotor is used for high starting torque. The slotting arrangement for double cage induction motor is as shown in above figure. As the name indicate the double cage induction motor has two winding in rotor. The outer bars consists of rotor bars having low reactance and high resistance. On the other hand, the inner cage consists of rotor bars having high reactance and low resistance.

Working

At start the rotor frequency is high, the outer cage carries most of the current despite its high resistance. The inner cage has low reactance and is mostly ineffective. This gives high starting torque and low starting current. As the motor picks up the speed, the rotor frequency reduces and the inner cage carries most of the current. Under normal running condition, the outer cage and inner cage are in parallel giving low combined resistance and both the cages are active.

When the speed is normal frequency reduces and it is so small that the reactance of both the cages are practically negligible. Hence it has been made possible to construct a single machine which has high starting torque with reasonable starting current which maintains speed regulation and high efficiency.

Torque-Speed characteristics

Torque Speed Characteristic of Double Cage Induction Motor

Advantages

It has high efficiency and good speed regulation.
It gives higher starting torque.
Lower starting current and are cheaper in cost.
They are more robust and are explosion proof since the risk of sparking is eliminated by the absence of slip ring and brushes.

Construction and Working of Induction Motor

noreply@blogger.com (Manoj Sahu) — Tue, 22 Dec 2015 14:08:00 +0530

Construction:-

An induction motor consists of two main parts

Stator
Rotor

The stationary frame is called stator and the rotating armature is called rotor. The stator of induction motor is similar to that of the synchronous motor or generator. It is made of large number of stampings. These stampings are slotted in order to receive the stator windings. Figure below shows the construction of Induction motor.

Induction Motor

The functions of various parts is as follows:-

Frame:- Its function is to provide mechanical support to the entire construction. The frame also contains the stator winding of Induction motor.
Air gap:- Air gap provides the space for the rotating magnetic field between the stator and rotor.
Fan:- The fan rotates with the rotor. Its function is to cool down the motor.
Slip rings:- The rotor winding terminals are permanently connected to the slip rings(in slip ring type induction motor). The slip rings are continuously in contact with the brushes which are pressed against slip rings. External connections from the brushes are brought out,

Figure below shows the rotor and stator of induction motor.

Stator and Rotor construction of Induction motor

Rotor drum is provided with slots. The stator is stationary which can be star connected or delta connected to the 3Φ AC supply through a switch as shown in figure below.

Stator winding connections

The function of stator winding is to produce a rotating magnetic field in the air gap between the stator and rotor. It should be known that the rotor is connected to any external supply. The current flows through the rotor due to principle of induction.

Working:-

The 3Φ induction motor has 3Φ winding which is supplied by 3Φ alternating voltage and 3Φ balanced current flow in the winding. The current produce magnetic flux which is constant in magnitude and rotating at synchronous speed. The rotating magnetic filed swift pass the rotor conductors which as yet are stationary. The relative velocity between the magnetic flux and stationary rotor conductors induces emf in the rotor conductors according to Faraday's Law of Electromagnetic Induction. The induced emf is directly proportional to the relative velocity between the magnetic flux and stationary rotor conductors. The frequency of induced emf is same as the supply frequency and the direction of induced emf is given by Fleming's Right Hand Rule.

Since the rotor conductor forms a closed circuit, rotor current produces which opposes the very cause producing it according to Lenz's Law. In this case, the cause which is producing the rotor current is relative velocity between the magnetic flux and stationary rotor conductors. Hence the rotor starts running in the same direction as rotating magnetic field and always try to catch up the the speed of rotating magnetic field.

Electrical Engineers Guide

Ring Main Unit(RMU) or Ring Main Distribution System

Three Position Switch

Applications:-

Miniature/Low Oil Circuit Breaker

Construction

Operation

Advantages

Disadvantages

Vacuum Circuit Breaker

Principle:-

Construction:-

Working:-

Advantages:-

SF6 Circuit Breaker

Construction:-

Working:-

Advantages :-

Disadvantages:-

Classification of Electric Drives

Group Drives

Individual Drive

Multimotor Drive

Passive Method of Space Heating

Solar Water Heater Working (Thermosymphonic Mode)

Wireless Power Transfer

Working principle

Advantages

Applications

Skin Effect in Transmission Lines

What is skin effect

Corona phenomenon in Transmission Lines

Turn ON methods of SCR/Thyristor Triggering

Forward Voltage Triggering (High Voltage)

Thermal Triggering ( Temperature Triggering)

Light Triggering (Radiation Triggering)

dV/dt Triggering

Gate Triggering

Silicon Controlled Rectifier (SCR)

Important Features

Construction

Principle of operation of SCR (Silicon Controlled Rectifier)

Latching Current (IL):-

Holding Current (IH):-

Classification of Overhead Transmission Line

Short transmission lines

Medium transmission lines

Long Transmission Line

What is meant by excitation?

What is hunting in synchronous machine?/Phase Swinging/Surging

Torque Equation of D.C. Motor

Back E.M.F.

What is Back e.m.f. ?

Significance of Back e.m.f.

Necessity of Connecting Alternator in Parallel

Continuity of supply:-

Efficiency:-

Maintenance and repair:-

Load Growth:-

Star Delta Starter

Start Mode

Run Mode

Capacitor Start Capacitor Run Induction Motor

Torque- Speed Characteristics of Capacitor Start Capacitor Run Induction Motor

Divergence Theorem

Universal Motors

Construction

Working

Torque-Speed Characteristics

Application

Double Cage Induction Motor

Construction

Working

Torque-Speed characteristics

Construction and Working of Induction Motor

Construction:-

The functions of various parts is as follows:-

Working:-

Note:-For better understanding of the working of Induction motor, refer following videos.

Latching Current (I_L):-

Holding Current (I_H):-