Lightening Arresters

Lightning, is a form of visible discharge of electricity between rain clouds or between a rain cloud and the earth. The electric discharge is seen in the form of a brilliant arc, sometimes several kilometres long, stretching between the discharge points. How thunderclouds become charged is not fully understood, but most thunderclouds are negatively charged at the base and positively charged at the top. However formed, the negative charge at the base of the cloud induces a positive charge on the earth beneath it, which acts as the second plate of a huge capacitor. 
When the electrical potential between two clouds or between a cloud and the earth reaches a sufficiently high value (about 10,000 V per cm or about 25,000 V per in), the air becomes ionized along a narrow path and a lightning flash results.  The possibility of discharge is high on tall trees and buildings rather than to ground. Buildings are protected from lightning by metallic lightning rods extending to the ground from a point above the highest part of the roof. The conductor has a pointed edge on one side and the other side is connected to a long thick copper strip which runs down the building. The lower end of the strip is properly earthed. When lightning strikes it hits the rod and current flows down through the copper strip. These rods form a low-resistance path for the lightning discharge and prevent it from travelling through the structure itself.

Ordinary fuses and circuit breakers are not capable of dealing with lightning-induced transients. Lightning protection equipment may shunt current, block energy from traveling down the wire, filter certain frequencies, clamp voltage levels, or perform a combination of these tasks. Voltage clamping  devices capable of handling extremely high amperages of the surge, as well as reducing the extremely fast rising edge (dv/dt and di/dt) of the transient are recommended. The grounding system must address low earth impedance as well as low resistance so as to reduce the consequences of burning . A spectral study of lightning's typical impulse reveals both a high and a low frequency content. The high frequency is associated with an extremely fast rising "front" on the order of 10 microseconds to peak current. The arrester  element consists of the main component of ZnO as it blocks are very reliable apparatus with a low failure  rate and several kinds of additives, which are mixed, granulated, formed and sintered into a complete block with electrodes on the both surfaces.  At normal voltages it act as  an insulator and will  not conduct current.  But at higher voltages caused   by lightning it becomes a  conductor. Their junctions are the  Electronic Switches(in case of microcontroller based) that turn on and off to divert the lightning around the equipment by making different paths of minimum resistance.  

  

Figure shows a typical internal view of the TNR element under a scanning electron microscope. The boundary  layer  has  a  very high impedance for a small current region, so the normal operating voltage is almost all applied to its boundary layer. 

Surge Suppressor: This is also a surge diverter,  but generally for voltages well below 1000 volts.

Negative-sequence current

In a perfectly balanced system, no negative phase sequence currents would exist. However, it is virtually impossible to acheive this perfectly balanced system in practice and so these negative phase currents need to be considered.  Line voltage imbalances caused by electrical faults or imbalanced loads lead to current imbalances in each conductor. Therefore, the magnetic coupling between windings becomes uneven. A counter rotating field (in respect to the main field) will now exist and the resultant field will cause undesirable eddy currents to flow. The consequenes of this for generators will either be a loss of torque, or depending on the load, will increase the current for the same slip speed and hence raise the temperature of the alternator. 
In a rotating machine, the negative sequence current vector rotates in the same direction as the rotor. It is the magnetic flux produced by the negative sequence current that rotates in the reverse direction of the rotor. Thus, the rotor cuts through the flux at twice the synchronous speed, and the induced current in the rotor is twice the line frequency. Regarding measurement of negative sequence, it is measured by the negative sequence filters within the relays.
The net torque is reduced and if full load is still demanded, then the motor will be forced to operate at a higher slip, thus increasing the rotor losses and heat dissipation.

EARTHING

TT network

In a TT earthing system, the protective earth connections of the consumer and supply source are provided by a local connection to earth, independent of any earth connections at the generator.

The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.

TN−S Network

A TN-S system has the supply source directly connected to earth, the installations are connected to the neutral of the supply source via the lead sheath of the supply cable, and the neutral and protective conductors(i.e. earthing) throughout the whole system performing separate functions.

TN-C-S Network

A TN-C-S system is as the TN-S but the supply cable sheath is also the neutral, i.e. it forms a combined earth/neutral conductor known as a PEN (protective earthed neutral) conductor.  

The installation earth and neutral are separate conductors.

This system is also known as PME (protective multiple earthing).

T    -  terre  (French for earth) and meaning a direct connection to earth. 

N    -  neutral

C    -  combined

S    -  separate.

Reactive Power

Electrical machines work on the principle of conversion of electromagnetic  energy.A part of input energy is consumed for creating and maintaining the magneticfield.This part of the input energy cannot be converted into active energy and is returned to the electrical network on removal of the magnetic field. This power is known as “reactive” power Q.This Reactive Power has one half of the Power in the positive area and the other half in the negative area. Unlike true power, reactive power is not useful power because it is stored in the circuit itself.  This power is stored by

1)Inductors,

 Because they expand and collapse their magnetic fields in an attempt  to  keep  current  constant,   

2) Capacitors,

    Because  they  charge and  discharge  in  an attempt  to  keep voltage constant.

           

Reactive power (vars) is required to maintain the voltage to deliver active power (watts)  through transmission lines. Motor loads and other loads require reactive power to convert the flow of electrons into useful work. When  there is not enough reactive power, the voltage  sags down and it is not possible to push the power demanded by loads through the lines.

                                 

Sources Of Reactive Power:

Synchronous Generators - Synchronous machines can be made to generate or absorb reactive power depending upon the excitation (a form of generator control) applied. The output of synchronous machines is continuously variable over the operating range and automatic voltage regulators can be used to control the output so as to maintain a constant system voltage.

Synchronous Compensators - Certain smaller generators, once run up to speed and synchronised to the system, can be declutched i.e. make free to rotate and produce phase difference, from their turbine and provide reactive power without producing real power. This mode of operation is called Synchronous Compensation.

Capacitive and Inductive Compensators - These are devices that can be connected to the system to adjust voltage levels. A capacitive compensator produces an electric field thereby generating reactive power whilst an inductive compensator produces a magnetic field to absorb reactive power. Compensation devices are available as either capacitive or inductive alone or as a hybrid to provide both generation and absorption of reactive power.

Power Triangle:

·         Power dissipated by a load is referred to as true power. True power is symbolized by the letter P and is measured in the unit of Watts (W).

·         Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power. Reactive power is symbolized by the letter Q and is measured in the unit of Volt-Amps-Reactive (VAR).

·         Total power in an AC circuit, both dissipated and absorbed/returned is referred to as apparent power. Apparent power is symbolized by the letter S and is measured in the unit of Volt-Amps (VA).

CRGO steels

Cold Rolled Grain Oriented (CRGO) silicon steels are used for laminations of the Power Transformers. Cold Rolled Grain Oriented (CRGO) sheets will have superior magnetic properties in the direction of rolling. The crystals are aligned in the direction by cold rolling followed by heat treatment process. Magnetic properties of the CRGO steel Sheets are dependent on the magnetic properties of the individual crystals of the material and the direction of orientation of the crystal. The properties of the CRGO silicon steels are improved by composition, manufacturing process, heat treatment, laser irradiation etc.

When alloying, the concentration levels of carbon, sulfur, oxygen and nitrogen must be kept low, as these elements indicate the presence of carbides, sulfides, oxides and nitrides. These compounds, even in particles as small as one micrometer in diameter, increase hysteresis losses while also decreasing magnetic permeability. Larger the size of grains lesser the losses and hence grain size of CRGO steel is made larger than normal steel.

Thickness of the CRGO sheets will be of the order of 0.33mm to 0.25mm. These CRGO Steel Laminations are stacked together to form a magnetic core for the Transformer.The Commercially available CRGO steel sheets will have 3% of Silicon. Higher the Silicon content increases the resistivity and reduces the eddy current losses. But Silicon Content above 3.5% makes the CRGO silicon steel sheets brittle.

Hysteresis Loss

When a magnetic field is applied, all the grains of the magnetic material will orient in the direction of magnetizing force. In another cycle this grains will orient in opposite direction in the direction of magnetizing force. The energy required to change the orientation of the magnetic grains in the direction of the magnetic field is lost in the form of heat. This loss is called hysteresis loss which make it non reversal.

Magnetic permeability

In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. In general, permeability is not a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters.

Solenoid(Linear)

This type of solenoid is generally called a Linear Solenoid due to the linear directional movement of the plunger. Linear solenoids are available in two basic configurations called a "Pull-type" as it pulls the connected load towards itself when energised, and the "Push-type" that act in the opposite direction pushing it away from itself when energised. Both push and pull types are generally constructed the same with the difference being in the location of the return spring and design of the plunger.

When electrical current flows through a conductor it generates a magnetic field, and the direction of this magnetic field with regards to its North and South Poles is determined by the direction of the current flow within the wire. This coil of wire becomes an "Electromagnet" with its own north and south poles exactly the same as that for a permanent type magnet. The strength of this magnetic field can be increased or decreased by either controlling the amount of current flowing through the coil or by changing the number of turns or loops that the coil has. Electromechanical solenoids consist of an electromagnetically inductive coil, wound around a movable steel or iron slug also called as armature. The coil is shaped such that the armature can be moved in and out of the center, altering the coil's inductance and thereby becoming an electromagnet. The armature is used to provide a mechanical force.The magnetic field inside the solenoid is radially uniform. The magnetic field lines follow the longitudinal path of the solenoid inside, so they must go in the opposite direction outside of the solenoid so that the lines can form a loop. However, the volume outside the solenoid is much greater than the volume inside, so the density of magnetic field lines outside is greatly reduced and sometimes negligible in very large solenoids. The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil

The inductance in solonoid is  a function of the structural makeup of the solenoid, i.e. its cross-sectional area, the number of turns, and its length. Placing an iron-core inside a solenoid will increase the inductance of a solenoid in the same way that placing a dielectric in a capacitor increases its capacitance. In an iron-core solenoid some of the energy in the magnetic field being created is used to magnetize the iron and to increase  magnetic energy current supply should be increase which further increases flux which further increases inductance. As we can see that magnetic field in plunger is opposite than that of coil, and hence a inward force will be produced. In this way motion of armature can be analysed. The force applied to the armature will always move the armature in a direction that increases the coil's inductance because the magnetic field that is developed will produce a mechanical force on ferromagnetic materials, i.e. the armature (or solenoid plunger), drawing them to the densest part of the field

Eddy Current Brakes

An eddy current brake,  unlike electro-mechanical brakes, which apply mechanical pressure on two separate objects,  slow an object by creating eddy currents through electromagnetic induction which create resistance, and in turn either heat or electricity. Magnetic brakes are silent and are much smoother than friction brakes, gradually increasing the braking power so that the people on the ride do not experience rapid changes in acceleration. Eddy current brakes are made from a large electrically conducting object moving through a stationary magnetic field. The magnet can be either a permanent magnet or an electromagnet. The movement can be either in a straight line or circular. When a metallic wheel passes between the rows of magnets, eddy currents are generated. Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which is generally much less useful.  During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect since large speed produces large change in flux and hence large amount of eddy current   which is proportional to force , meaning that as the train slows the braking force is reduced, producing a smooth stopping motion. This very property, however, is also one of magnetic breaking’s disadvantages in that the eddy force itself can never completely stop a train in ideal condition.

Harmonics

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 . 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.

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  oscillations of the motor shaft.

Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetics technology designed to remove high neutral current and the most harmful harmonics from the 3^rd through 21^st. The technique used in these transformers is call "low zero phase sequencing and phase shifting". These transformers can be used to treat existing harmonics in buildings or facilities.

Electrical meters

The most common type of electricity meter is the electromechanical induction watt-hour meter. The electromechanical induction meter operates by counting the revolutions of an aluminium disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. It consumes a small amount of power. The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil. This produces eddy currents in the disc and when these currents get influenced by fluxes of each coil then a force is exerted on the disc which is  proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc which provides controlling torque. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power being used. The disc works like the odometer in a car, in order to provide measurement of the total energy used over a period of time. The aluminium disc is supported by a spindle which has a worm gear which drives the register. The register is a series of dials which record the amount of energy used. The dials may be of the pointer type where a pointer indicates each digit or of other types. The amount of energy represented by one revolution of the disc is denoted by the symbol Kh which is given in units of watt-hours per revolution. The value 7.2 is commonly seen. Using the value of Kh, one can determine their power consumption at any given time by timing the disc with a stopwatch. If the time in seconds taken by the disc to complete one revolution is t, then the power in watts is  

 For example, if Kh = 7.2, as above, and one revolution took place in 14.4 seconds, the power is 1800 watts. This method can be used to determine the power consumption of household devices by switching them on one by one.

Buchholz relay

In the field of electric power distribution and transmission, a Buchholz relay is a safety device mounted on some oil-filled  transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.

PRINCIPLE OF OPERATION

The operation of buchholz relay is as follows:

i. In case of slow developing faults within the transformer, the heat due to the fault

causes decomposition of some transformer oil in the main tank. The products of decomposition mainly contain 70 % of hydrogen gas. The hydrogen gas being light tries to go into the conservator and in the process gets trapped in the upper part of the relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close the contacts of mercury switch attached to it. This completes the alarm circuit to sound an alarm.

ii. If serious fault occur in the transformer, an enormous amount of gas is generated in

the main tank. The oil in the main tank rushes towards the conservator via the buchholz relay and in doing so it tilts the flap to close the contacts of mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer.

Construction

The cover is a weather-resistant casting of light alloy and is provided with a paint coat. Terminal box  (1), test valve (2) and test key, covered by a cap nut (3) as well as a plate for operation of the test key (4) are arranged above the cover. The terminal box has an earthing contact (5) and the electrical  connectors (6). The aluminium cap (7) seals the terminal box. If the cap is opened the contact setting

(8) can be seen. The cable may be optionally brought in through one of both cable glands (9).