The transformer works on the principle of mutual induction between the primary and secondary windings. The induction is caused by the constantly varying magnetic flux that links the two windings. The flux density in the windings is directly proportional to the induced voltage and inversely proportional to the frequency and the number of turns in the winding.

Magnetic Flux α Voltage/Frequency

Overfluxing is a dangerous situation in which the magnetic flux density increases to extremely high levels. The high flux density can induce excessive eddy currents in the windings and in other conductive parts inside the transformers. The heat generated by these eddy currents can damage the windings and the insulation. The high flux density also causes magnetostriction inside the transformer core and produces noise. The powerful magnetostrictive forces can also cause damage. The winding temperatures may also increase due to the heat produced.

The magnetic flux density is dependent on the current flowing through the primary windings in a transformer. This current is dependent on the voltage applied across the windings and the winding impedance. The impedance is dependent on the frequency of the applied voltage. If the nominal voltage is applied at a reduced frequency, the low inductive reactance will cause a higher current to flow through the windings.

Overfluxing is usually encountered in Transformers which are directly connected to the generator. It usually occurs when the generator is being started or stopped. As the rpm of the generator and consequently the frequency of the power falls, the same system voltage induces a higher magnetic flux. Modern Automatic Voltage regulators are equipped with V/Hz limiters which limit the voltage in accordance with the frequency.

Overfluxing can be prevented by the use of a Overfluxing relay. An overfluxing is an adaptation of an overvoltage relay. The PT voltage is connected across a resistor and a capacitor in series. The voltage sensing relay is connected across the capacitor. The relay operates in the event of an overfluxing and isolates the transformer

Optocoupler relays are relays in which the changeover of contacts is effected by the switching on or off of a light source which is linked to a SCR or a TRIAC. The SCR or the TRIAC is switched on or off when the LED is switched unlike conventional relays, where it is done electromagnetically.

These relays are faster than electromagnetic relays. More importantly, the provide isolation between the control and the power circuits.

These relays do not have any moving parts which can deteriorate due to arcing or operational wear. However, they are expensive over conventional relays and hence still find limited application.

Another advantage of the Solid state relay is that it can open an AC circuit when the sinewave crosses zero. This ensures that the back-emf is minimum and this ensures that there are no voltage kicks in the opposite direction when the circuit is open due to inductance. This is because the Triac or the SCR used in the solid state relays conduct current till the waveform reaches the zero point even after the optocoupler LED has been switched off. This prevents premature failure of the contacts.

The Advantages of Optocoupler relays include
  • Smaller Size
  • Faster Response time
  • Noiseless operations as there is no mechanical movement of the contacts.
  • Optocoupler relays can withstand high vibration compared to conventional relays
  • They do not generate Electromagnetic radiation as there is no coils to be energized.

However, they also have some disadvantages.
  • They are more expensive
  • They generate heat and require special heat-sinking fixtures.
  • They cannot switch on very low currents
  • When Solid state relays fail, they fail in the "closed position". In this situation, the machine which is connected will continue to be in the operating condition and there will be difficulty in isolating it. Electromagnetic relays usually fail in the "open" position.

When inductive loads such as transformers or motors are switched on, a sudden rush of current into the winding is observed. This is known as the inrush current. Typically, the inrush current is around 5 to 6 times of the rated value. This is due to the absence of the back-emf when the winding when the power is first applied. This sudden surge of current causes disturbances in the system voltage and sometimes spurious operation of relays. The high current also causes stress on the windings of the machines.

There are a number of methods to address this problem. Adding resistors in series to the winding and then gradually taking them out of circuit is one option. Another option is the use of softstarters which raise the terminal voltage of the machines gradually. Zero crossover switching is one method of addressing the issue of high inrush current when switching on inductive equipment. The method involves the use of Static Relays consisting of devices such as SCR or TRIACs. These devices switch on when the sinewave crosses the zero point so that the voltage is gradually increased. This, in some circumstances, reduce the inrush current.

However, this methods has its downsides too. There have been reports that if the zero cross over switching is carried out on a core that is already saturated from previous operation, extremely high currents can result.

Apart from inductive loads, inrush currents is also observed in resistive loads such as filament lamps. In filament lamps the resistance when the filament is in the cold, that is, switched off condition is lesser than the resistance when the lamp is in the switched on condition. This is due to the positive temperature co-efficient of resistance. When the lamp is switched on from the cold condition, there is a high surge of current which continues till the temperature of the resistance increases. Zero-crossover switching in resistive loads can ensure a smoother increase in current value to the steady-state condition.

This ensures a smoother increase in the current to the steady-state value.

AC and DC relays work on the same principle, that of, electromagnetic induction. However, there are some differences in construction. DC relays have something known as the freewheel diode which acts to discharge the emf built in the inductance when the coil is de-energized. (Click here to read about the phenomenon of freewheeling.) AC relays have cores which are laminated to prevent losses due to eddy current heating.

Another more conspicuous difference between a DC relay and an AC relay is presence of the Shading Coil. In AC relays, the alternating current supply changes direction about 100 times a second. At each instance, when the sine wave passes through zero, the current flowing through the coil becomes zero. This results in a loss of magnetism for a few milliseconds. When this happens about 100 times a second, the repeated drop and pickup of the coil produces a noise known as chattering. This also leads to the making and breaking of the relay contacts leading to disturbances in the connected electric circuits.

A shading coil is a coil with high remanence. thus when the magnetism of the coil collapses when the current becomes zero. The shading coil still retains the magnetism. Thus, ensuring that the contacts do not drop off.

Autotransformers are transformers which contain only one winding unlike two windings in the conventional transformer. The same winding, therefore, serves as the primary and secondary windings.

Autotransformers are advantageous over normal transformers as they are cheaper. Autotransformers are used generally for voltage conversion of equipments from one voltage to another such as from 110V to 220V or vice versa.

However, the autotransformer does not provide isolation between the primary and the secondary. Hence, there may a need to connect external filtering or suppression circuits. Thus, in the event of a failure of the insulation between the turns of the winding, there are chances of the primary voltage appearing on the secondary.

Another aspect which needs to be checked is the neutral point. If the neutral point is not at ground potential in the primary, the secondary neutral wil also not be at ground potential.

The autotransformer has higher voltage stability and better overload transformers than the ordinary transformers.

A Variac is a variable autotransformer with movable taps. Thus, it provides variable output voltage for a steady input voltage. The taps can be adjusted by a movable knob.

See Also:

Difference between Autotransformer, Variac and Dimmerstat

A scope meter is a handheld instrument that is used to see electrical parameters in graphical form. It can be described as a combination of a multimeter and an oscilloscope. It can be used to take measurements of the magnitude and frequency of a signal such as current and voltage.

A scope meter is an effective instrument in troubleshooting. As it displays the parameters in graphical form, transients and other momentary variations can be identified. This enables the identifications of improper and loose connections, grounding loops, etc which may be missed by indicating instruments.

Modern Scopemeters can also be connected to a computer by means of a USB port and the data collected can be transferred into the computer for analysis. The Scopemeter usually comes with software for this purpose. They also have recording and playback facility.

Since, scopemeters are handy and battery powered, they are portable and can be taken around easily unlike oscilloscopes.

image courtesy : http://www.aikencolon.com

An Autotransformer is a transformer with only one winding. See more. A variable autotransformer is an autotransformer with a sliding scale which can be used to adjust the transformer ratio. "Variac" and "Dimmerstat" are trade names of variable autotransformers. Hence, they are the same.

Freewheel or Fly back diodes are used across inductive components such as coils to prevent voltage spikes when the power is turned off to the devices.

When power to inductive loads such as coils and inductors is turned off, there is a sharp voltage spike. The direction of this voltage is opposite to the applied voltage in accordance with Lenz’s Law.

When a current flows through the coil of a relay, the coil gets electromagnetically charged. The energy is stored in the magnetic field around the coil. When the power supply to the coil is interrupted and the current in the coil tends to decrease, the magnetic field discharges causing a surge in the voltage.

The voltage, thus induced, can jump across the contacts of relays connected to the coils. The sparks and arcing produced can affect the life of the contacts. The voltage spikes can also damage electronic components like transistors which may be driving the relay coils.

Freewheel diodes are connected in reverse bias vis-à-vis the supply voltage. Hence, when the voltage spike appears in the opposite direction, they are short-circuited through the diode. The voltage spike is thus short-circuited across the coil. This protects the connected circuits.

Ordinary clampmeters used to measure AC currents work on the principle of electromagnetic induction caused by the alternating current flowing in the conductor which reverses direction causing a dynamically changing magnetic field. However, in DC conductors, the current flows in a fixed polarity. Consequently, the magnetic field around the conductor is fixed and does not change. Hence, a conventional clamp meter will register no reading.

A DC clampmeter works on the principle of the Hall Effect. The Hall Effect, named after Edwin Hall who discovered it 1879, states that when a conductor carrying current is placed in a magnetic field, a potential is induced across the conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. It is caused as the charge carriers, electrons or holes, experience a force known as the Lorentz force and are pushed to the sides of the conductor.

A clampmeter which works on the Hall effect has a sensor known as the Hall element. The Hall element is subjected to the magnetic field caused by the flow of current to be measured. This causes a small voltage across the Hall element. This voltage is amplified and measured.

images courtesy: commons.wikimedia.com
www.sensormag.com

The Rogowski Coil is a coil which is used to measure alternating currents and fast changing current impulses.

It is named after Walter Rogowski, a German Physicist.

The Rogowski Coil consists of a coil of wire wound in a helical manner. One end of the coil is taken through the coil itself and brought to the other side. Thus, the coil has both its ends at the same side.

The coil can be wrapped around a conductor to measure the current. The Voltage induced in the coil will be proportional to the rate of change of current in the primary conductor. The voltage thus induced can be connected to an electronic integrator which will generate a signal in accordance with the changing current signal.

The advantage of the Rogowski coil over the tradition current transformer is that it can be easily wrapped around the conductor. This permits online measurements of systems without requiring a shutdown. Besides, as the Rogowski Coil has an air core instead of a metallic core, it does not get saturated while measuring large currents.

The Rogowski coil is also immune to stray electromagnetic interference to a large extent.

Rotors used in Synchronous alternators can be classified into
 1)Salient and 2)Non-Salient Pole Rotors. 

Salient pole rotors are used in application with speeds from 100 to 1500rpm. They are alternative known as "projected pole" type of rotors. 

The poles mounted on the rotor are made of laminations made of steel. The poles are connected to the rotor shaft by means of dovetail joints. Each pole has a pole shoe around which the winding is wound. The salient pole rotor is generally used in applications where the prime mover is a hydel turbine or a combustion engine which have low or medium speeds. Salient pole rotors usually contain damper windings to prevent rotor oscillations during operation. 

Non-salient pole rotors are generally used in application which operate at higher speeds, 1500rpm and above. The prime movers in these applications are generally gas or steam turbines. 

These are sometimes known as "drum rotors". 

The rotor is a cylinder made of solid forged steel. The slots on which the windings are fixed are milled on the rotor. The number of poles is usually 2 or 4 in number. 

Since these rotors are cylindrical, the windage loss is reduced. 

The noise produced is also less. These rotors have higher axial length. These rotors do not need damper windings.

Corona is a form of Partial discharge which is luminous and occassionally visible to the Naked Eye.

Corona are caused when the air around an energized conductor gets ionized, causing a discharge. Corona are caused when defects are present in the conductors such as jagged edges or cracks which cause high local electric field. A hissing sound can also be heard during the occurence of Corona.

The Nitrogen molecules in the air get excited and result and cause ultraviolet radiation. Corona is sometimes visible as a hazy blue light around conductors, especially during the night.

Effects of Corona
  1. Corona is accompanied by the creation of ozone and nitrogen oxides. Nitrogen oxides may react with moisture in the air and form nitric acid a potentially corrosive substance.
  2. Corona can cause damage to insulators used in High voltage applications.
  3. Corona emissions are accompanied by the generation of radio waves which can interfere with commercial radio transmission. Sometimes, Corona can also result in noise which may disturb the neighborhood.
  4. It can sometimes cause carbon deposits which may later result in arcing.
Corona can also be used to identify problems. Corona in power transmission lines indicate the presence of dirt or other substances on power lines and may require cleaning.

Corona after commission an equipment may indicate improper installation.

The degree of corona will vary with the humidity. In high humidity conditions, corona can develop into a flashover which can cause trippings and damage to equipment.

Prevention of Corona
One of the means of reducing corona in transmission lines has been to increase the radius of the conductors.  This leads to decreased electrostatic field stress on the air around the conductor and prevents the initation of the corona.  ACSR conductors ( see article on ACSR conductors) which have a greater radii than steel conductors suffer less from the effects of corona.

Partial Discharge is a type of electrical discharge which occurs in insulating materials which are located between two conductors as in cables or windings. As the name suggests, the discharge is partial without completely bridging the two conductors.

Partial discharges usually occur in voids in the insulation of cables and windings as the di-electric constant of the voids are significantly lesser than the surrounding insulating media. They can also appear across bubbles in liquid insulating media such as in breakers or transformers. In gaseous media, they can appear as corona around an electrode. Corona can be seen in wet weather in power lines.

Partial Discharge occurs usually in insulation which has been subjected to mechanical and electrostatic stress or which has been weakened by premature aging due to adverse environmental conditions. Once initiated, Partial Discharge results in the formation of electrical trees which can accelerate the failure of the cable causing a short-circuit between two conductors or an earth fault. They can appear as "tracking" or distinct pathways in the cable insulation

Partial discharge can also be caused by improperly terminated/jointed connections in HV cables and windings.

The currents which flow during Partial discharge are extremely small and last for very short periods of time of the order of nanoseconds. This makes them difficult to measure. Partial Discharges generally dissipate energy in the form of heat, light and sound.

There are a wide range of methods(Partial discharge Analtysis) to detect and monitor Partial Discharge. Acoustic sensors detect the ultrasonic frequency noises which occur when a partial discharge is taking place. There are also inductive and capacitive sensors which can detect Partial Discharges

A Flashover or an arcing fault occurs when current flows between two conductors or between the conductor and the ground or a neutral line. A sudden burst of Energy is released during an arcing fault.

The high current which flows during a flash over can cause a sudden blast of energy in the form of heat, light and sound.

Personnel who come in contact with the arc or are in the vicinity of the fault can face serious injuries such as burns, damage to sight and hearing. Sometimes, Flashovers can lead to death of personnel. The Flashover is usually accompanied by a tremendous pressure blast which may throw pieces of equipment (shrapnel) over large distances increasing the chance of injury to people nearby

Flashovers can be caused by a number of reasons such as making contact with energized equipment accidentally, dropping tools into energized equipment such as busbars, transformers when working on them. The accumulation of dirt, dust and moisture over extended periods of time can also trigger flashovers in equipment.

Flashovers can be prevented by following standard safety procedures while working on Energized equipment. Routine safety drills will also greatly reduce mistakes during work. It must be ensured that all HV equipment are insulated. The minimum line-to-line and line-to-ground clearances should be followed for all busbars. The Insulation Resistances of all cables and windings need to be checked at scheduled intervals

The relays and other protective devices should be periodically checked for correct operation.

STOCKHOLM, Sweden, Sept. 16 (UPI) — Offshore wind farms could meet up to 17 percent of Europe’s electricity needs in 2030, an industry group said.

“Offshore wind power is vital for Europe’s future,” the European Wind Energy Association said in a report published at an offshore wind industry conference in Stockholm.

Currently, 11 wind farms harness the stronger winds at sea, mostly in Britain and Scandinavia. Together, they account for only 0.2 percent of Europe’s electricity demand, but that figure could be boosted to 10 percent in 2020 and up to 17 percent in 2030, EWEA said.

Offshore wind farms with a total capacity of 100 GW are currently being developed or proposed in most countries that have an Atlantic, North Sea or Baltic Sea coastline.

“If realized, these projects would produce 10 percent of the EU’s electricity whilst avoiding 200 million tons of CO2 emissions each year,” EWEA said.

The European Union wants to boost the share of renewables in its energy mix to 20 percent, a target it aims to meet in part with the help of offshore wind.

Energy Commissioner Andris Piebalgs promised in Stockholm that Brussels is “committed to doing everything we can to support offshore wind developers and make sure their … projects come to fruition.”

The EWEA wants the EU and national governments to support the creation of a functioning North Sea grid. Wind turbines at sea produce more power because of stronger winds, but turbine maintenance and getting the electricity back on land — via seabed cables — makes the source much more expensive.

On Monday, the association presented what it called an “offshore network development plan” for the construction of a transnational offshore power grid.

“EWEA’s new offshore network plan will provide a truly pan-European electricity super highway,” Christian Kjaer, CEO of EWEA, said in a statement. “This will bring affordable electricity to consumers, reduce import dependence, cut CO2 emissions and allow Europe to access its largest domestic energy source — offshore wind.”

The EU is currently drafting its own plan for a transnational grid, and it is expected that Brussels will call on utilities to share a large part of the costs.

The industry nevertheless believes that offshore wind will see similarly high growth rates that the onshore industry did over the past decade.

“There is huge developer interest in offshore wind power,” Arthuros Zervos, president of EWEA, said in a statement. “The scale of planned projects is far greater than most people realize.”

The world’s largest offshore wind farm, the $1 billion Horns Rev 2, is due to go into operation Thursday. Developed and built by Danish utility Dong, it consists of 91 Siemens-made turbines placed around 20 miles off the Danish coast. With its total capacity of 209 MW, the farm will produce power for an estimated 200,000 households.

courtesy : ecoworld.com