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Why Current Transformer (CT) Seconday Should not be Open ?


It is very important that the secondary of Current Transformer should not be kept open. 

Either it should be shorted or most be connected in series with a low resistance coil such as current cods of watt meter, coil of ammeter etc.

If it is left open, then current through secondary becomes zero hence the ampere turns produced by secondary which generally oppose primary ampere turns becomes zero. 

As there is no counter m.m.f., unopposed primary m.m.f. (ampere turns) produce high flux in the core. 

  • This produce excessive core losses, heating the core beyond limits. 
  • Similarly heavy e.m.f.s will be induced on the primary and secondary side. This may damage the insulation of the winding. 

This is danger from the operator point of view as well. It is usual to ground the C.T. on the secondary side to avoid a danger of shock to the operator. 

Hence never open the secondary winding circuit of a current transformer while its primary winding is energized. 

Thus most of the current transformers have a short circuit link or a switch at secondary terminals. When the primary is to be energized, the short circuit link must be closed so that there is no danger of open circuit secondary. 

Ideal Transformer and Practical Transformer

Ideal Transformer

An ideal transformer is one that has
  1. no winding resistance
  2. no leakage flux i.e., the same flux links both the windings
  3. no iron losses (i.e., eddy current and hysteresis losses) in the core

Although ideal transformer cannot be physically realized, yet its study provides a very powerful tool in the analysis of a practical transformer. In fact, practical transformers have properties that approach very close to an ideal transformer.



Consider an ideal transformer on no load i.e., secondary is open-circuited as shown in figure. Under such conditions, the primary is simply a coil of pure inductance. 

When an alternating voltage V₁ is applied to the primary, it draws a small magnetizing current Iₘ which lags behind the applied voltage by 90°. This alternating current Iₘ produces an alternating flux ϕ which is proportional to and in phase with it. 

The alternating flux ϕ links both the windings and induces e.m.f. E₁ in the primary and e.m.f. E₂ in the secondary. The primary e.m.f. E₁ is, at every instant, equal to and in opposition to V₁  (Lenz’s law). Both e.m.f.s E₁ and E₂ lag behind flux ϕ by 90°. However, their magnitudes depend upon the number of primary and secondary turns.

Phasor Diagram of Ideal Transformer

The phasor diagram of an ideal transformer on no load is also shown above.

Since flux ϕ is common to both the windings, it has been taken as the reference phasor. 

The primary e.m.f. E₁ and secondary e.m.f. E₂ lag behind the flux ϕ by 90°. 

Note that E₁ and E₂ are in phase. But E₁ is equal to V₁ and 180° out of phase with it.

Practical Transformer

A practical transformer differs from the ideal transformer in many respects. The practical transformer has 
  1. iron losses 
  2. winding resistances and 
  3. magnetic leakage, giving rise to leakage reactances.

1. Iron Losses 

Since the iron core is subjected to alternating flux, there occurs eddy current and hysteresis loss in it. These two losses together are known as iron losses or core losses. 

The iron losses depend upon the supply frequency, maximum flux density in the core, volume of the core etc. 

It may be noted that magnitude of iron losses is quite small in a practical transformer.

2. Winding resistances

Since the windings consist of copper conductors, it immediately follows that both primary and secondary will have winding resistance. The primary resistance R₁ and secondary resistance R₂ act in series with the respective windings as shown in figure. 

When current flows through the windings, there will be power loss as well as a loss in voltage due to IR drop. This will affect the power factor and E₁ will be less than V₁ while V₂ will be less than E₂. 



3. Leakage reactances

Both primary and secondary currents produce flux. The flux ϕ which links both the windings is the useful flux and is called mutual flux. 

However, primary current would produce some flux ϕ which would not link the secondary winding. Similarly, secondary current would produce some flux ϕ that would not link the primary winding. 

The flux such as ϕ₁ or ϕ₂ which links only one winding is called leakage flux. The leakage flux paths are mainly through the air. The effect of these leakage fluxes would be the same as though inductive reactance were connected in series with each winding of transformer that had no leakage flux as shown in figure. 

In other words, the effect of primary leakage flux ϕ₁ is to introduce an inductive reactance X₁ in series with the primary winding as shown. Similarly, the secondary leakage flux ϕ₂ introduces an inductive reactance X in series with the secondary  winding. 

There will be no power loss due to leakage reactance. However, the presence of leakage reactance in the windings changes the power factor as well as there is voltage loss due to IX drop.




Note
Although leakage flux in a transformer is quite small (about 5% of ϕ)  compared to the mutual flux ϕ, yet it cannot be ignored. It is because leakage flux paths are through air of high reluctance and hence require considerable e.m.f. It may be noted that energy is conveyed from the primary winding to the secondary winding by mutual flux f which links both the windings.


Properties of Good Transformer Oil


Even though the basic functions of the oil used in transformers are 
a) heat conduction and 
b) electrical insulation, 
there are many other properties which make a particular oil eminently suitable. 

Organic oils of vegetative or animal origin are good insulators but tend to decompose giving rise to acidic by-products which attack the paper or cloth insulation around the conductors.

Mineral oils are suitable from the point of electrical properties but tend to form sludge. The properties that are required to be looked into before selecting an oil for transformer application are as follows:

1. Insulting property

This is a very important property. However most of the oils naturally fulfil this. Therefore  deterioration in insulating property due to moisture or contamination may be more relevant.

2. Viscosity

It is important as it determines the rate of flow of the fluid. Highly viscous fluids need much bigger clearances for adequate heat removal.

3. Purity

The oil must not contain impurities which are corrosive. Sulphur or its compounds as impurities cause formation of sludge and also attack metal parts.

4. Sludge formation

Thickening of oil into a semisolid form is called a sludge. Sludge formation properties have to be considered while choosing the oil as the oil slowly forms semi-solid hydrocarbons.

These impede flows and due to the acidic nature, corrode metal parts. Heat in the presence of oxygen is seen to accelerate sludge formation. 

If the hot oil is prevented from coming into contact with atmospheric air sludge formation can be greatly reduced.

5. Acidity

Oxidized oil normally produces CO₂ and acids. The cellulose which is in the paper insulation contains good amount of moisture. These form corrosive vapors. A good breather can reduce the problems due to the formation of acids.

6. Flash point And Fire point

Flash point of an oil is the temperature at which the oil ignites spontaneously. This must be as high as possible (not less than 160º C from thepoint of safety). 

Fire point is the temperature at which the oil flashes and continuously burns. This must be very high for the chosen oil (not less than 200º C).

Inhibited oils and synthetic oils are therefore used in the transformers. Inhibited oils contain additives which slow down the deterioration of properties under heat and moisture and hence the degradation of oil. 

Synthetic transformer oil like chlorinated diphenyl has excellent properties like chemical stability, non-oxidizing, good dielectric strength, moisture repellant, reduced risk due fire and explosion.

It is therefore necessary to check the quality of the oil periodically and take corrective steps to avoid major break downs in the transformer.

Percentage Impedance of Transformer and Its Calculation

What is Percentage Impedance?

The impedance of a transformer is the total opposition offered an alternating current. This may be calculated for each winding. However, a rather simple test provides a practical method of measuring the equivalent impedance of a transformer without separating the impedance of the windings. When referring to impedance of a transformer, it is the equivalent impedance that is meant. 

Definition

The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage.

It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions 

Calculation of Percentage Impedance

In order to determine equivalent impedance, one winding of the transformer is short circuited, and just enough voltage is applied to the other winding to create full load current to flow in the short circuited winding. This voltage is known as the impedance voltage.


Either winding may be short-circuited for this test, but it is usually more convenient to short circuit the low-voltage winding. 

The transformer impedance value is given on the nameplate in percent. This means that the voltage drop due to the impedance is expressed as a percent of rated voltage. 

Example Calculation

For example, if a 2,400/240-volt transformer has a measured impedance voltage of 72 volts on the high voltage windings, its impedance (Z), expressed as a percent, is:

Z%  =  (Impedance Voltage / Rated Voltage)   x  100

percent Z = (72/2400)*100 = 3 percent

This means there would be a 72-volt drop in the high-voltage winding at full load due to losses in the windings and core. Only 1 or 2% of the losses are due to the core; about 98% are due to the winding impedance. 

If the transformer were not operating at full load, the voltage drop would be less. If an actual impedance value in ohms is needed for the high-voltage side:

Z = V/I

where V is the voltage drop or, in this case, 72 volts; and I is the full load current in the primary winding. 

If the full load current is 10 amps:

Z = 72V/10A = 7.2 Ohms

Of course, one must remember that impedance is made up of both resistive and reactive components.

What are Motor Control Centres?

The terms ‘Switchgear’ and ‘Motor Control Centres’ are used in general to describe combinations of enclosures, busbars, circuit breakers, power contactors, power fuses, protective relays, controls and indicating devices.
Read more about switchgear here What is Switchgear? | Features, Components and Classification
The standards used in Europe often refer to IEC60050 for definitions of general terms. Particular IEC standards tend to give additional definitions that relate to the equipment being described.
Example:

  • IEC60439 and IEC60947 for low voltage equipment, 
  • IEC60056, IEC60298 and IEC60694 for high voltage equipment. An earlier standard IEC60277 has been withdrawn. 


These standards tend to prefer the general terms ‘switchgear’ and ‘controlgear’. Controlgear may be used in the same context as ‘motor control centres’ which is a more popular and specific term used in the oil industry.

In general switchgear may be more closely associated with switchboards that contain circuit breaker or contactor cubicles for power distribution to other switchboards and motor control centres, and which receive their power from generators or incoming lines or cables. 

Motor control centres tend to be assemblies that contain outgoing cubicles specifically for supplying and controlling power to motors. 
Motor Control Center Panels


However, motor control centres may contain outgoing cubicles for interconnection to other  switchboards or motor control centres, and circuit breakers for their incomers and busbar sectioning. 

Switchboards may be a combination of switchgear and motor control centres. For example
a main high voltage switchboard for an offshore platform will have switchgear for the generators, busbar sectioning and outgoing transformer feeders. It will have motor control centre cubicles for the high voltage motors. 

IEC60439 applies to low voltage equipment that is described as ‘factory built  assemblies’, or FBAs, of switchgear and controlgear.

Switchgear tends to be operated infrequently, whereas motor control centres operate frequently as required by the process that uses the motor. 

Apart from the incomers and busbar section circuit breakers, the motor control centres are designed with contactors and fuses (or some types of moulded case circuit breakers in low voltage equipment) that will interrupt fault currents within a fraction of a cycle of AC current. Circuit breakers need several cycles of fault current to flow before interruption is complete. Consequently the components within a circuit breaker must withstand the higher forces and heat produced when several complete cycles of fault current flow.

Switchgear is available up to at least 400 kV, whereas motor control centres are only designed for voltages up to approximately 15 kV because this is the normal limit for high voltage motors.

Types of Gas Turbine Engines

For an individual generator that is rated above 1000 kW, and is to be used in the oil industry, it is usual practice to use a gas turbine as the driving machine (also called the prime mover). Below 1000 kW a diesel engine is normally preferred, usually because it is an emergency generator running on diesel oil fuel.

There are different different types of gas turbine driven generators. Gas turbines can be classified in several ways, common forms are:-
  • Aero-derivative gas turbines.
  • Light industrial gas turbines.
  • Heavy industrial gas turbines.

Aero-derivative Gas Turbines

Aircraft engines are used as ‘gas generators’, i.e. as a source of hot, high velocity gas. This gas is then directed into a power turbine, which is placed close up to the exhaust of the gas generator. The power turbine drives the generator.
Aero Derivative Gas Turbine 


Advantages of Aero derivative Gas Turbines

The benefits of this arrangement are:-
  • Easy maintenance since the gas generator can be removed as a single, simple module. This can be achieved very quickly when compared with other systems.
  • High power-to-weight ratio, which is very beneficial in an offshore situation.
  • Can be easily designed for single lift modular installations.
  • Easy to operate.
  • They use the minimum of floor area.

Disadvantages of Aero derivative Gas Turbines

The main disadvantages of Aero derivative Gas Turbines  are:-
  • Relatively high costs of maintenance due to short running times between overhauls.
  • Fuel economy is usually lower than other types of gas turbines.
  • The gas generators are expensive to replace.

Aero-derivative generators are available in single unit form for power outputs from about 8 MW up to about 25 MW. These outputs fall conveniently into the typical power outputs required in the oil and gas production industry, such as those on offshore platforms.

Light Industrial Gas Turbines

Some manufacturers utilize certain of the advantages of the aero-derivative machines, i.e. high power-to-weight ratio and easy maintenance. 

The high power-to-weight ratios are achieved by running the machines with high combustion and exhaust temperatures and by operating the primary air compressors at reasonably high compression ratios i.e. above 7. 

A minimum of metal is used and so a more frequent maintenance programme is needed. Easier maintenance is achieved by designing the combustion chambers, the gas generator and compressor turbine section to be easily removable as a single modular type of unit. 

The ratings of machines in this category are limited to about 10 MW.

Heavy Industrial Gas Turbines

Heavy industrial gas turbines are usually to be found in refineries, chemical plants and power utilities.

They are chosen mainly because of their long and reliable running times between major maintenance overhauls. 

They are also capable of burning most types of liquid and gaseous fuel, even the heavier crude oils. They also tend to tolerate a higher level of impurities in the fuels. 
Heavy Industrial Gas Turbines
Heavy industrial machines are unsuitable for offshore applications because:-
  • Their poor power-to-weight ratio means that the structures supporting them would need to be much larger and stronger.
  • Maintenance shutdown time is usually much longer and is inconvenient because the machine must be disassembled into many separate components. A modular concept is not possible in the design of these heavy industrial machines.
  • The thermodynamic performance is usually poorer than that of the light and medium machines. This is partly due to the need for low compression ratios in the compressor.
They do, however, lend themselves to various methods of heat energy recovery e.g. exhaust heat exchangers, recuperators on the inlet air.

Fuel for Gas Turbines

The fuels usually consumed in gas turbines are either in liquid or dry gas forms and, in most cases, are hydrocarbons. 

In special cases non-hydrocarbon fuels may be used, but the machines may then need to be specially modified to handle the combustion temperatures and the chemical composition of the fuel and its combustion products.

Gas turbine internal components such as blades, vanes, combustors, seals and fuel gas valves are sensitive to corrosive components present in the fuel or its combustion products such as carbon dioxide, sulphur, sodium or alkali contaminants.

The fuel can generally be divided into several classifications:-
• Low heating value gas.
• Natural gas.
• High heating value gas.
• Distillate oils.
• Crude oil.
• Residual oil.

Why we need Power System Protection?

It is fair to say that without discriminative protection it would be impossible to operate a modern power system. 

The protection is needed to remove as speedily as possible any element of the power system in which a fault has developed. 

So long as the fault remains connected, the whole system may be in danger from three main effects of the fault, namely:


  • it is likely to cause the individual generators in a power station or groups of generators in different stations, to lose synchronism and fall out of step with a consequent splitting of the system
  • a risk of damage to the affected plant
  • a risk of damage to the healthy plant.

There is another effect, not necessarily dangerous to the system, but important from the consumer's viewpoint, namely, a risk of synchronous motors in large industrial premises falling out of step and tripping out, with the serious consequences that entails loss of production and interruption of vital processes.

It is the function of the protective equipment, in association with the circuit breakers, to avert these effects. This is wholly true of large h.v. networks, or transmission systems. 

In the lower-voltage distribution systems, the primary function of protection is to maintain continuity of supply. This, in effect, is achieved incidentally in transmission systems if the protection operates correctly to avert the effects mentioned above; indeed it must be so because the ultimate aim is to provide 100 percent continuity of supply.

Obviously, this aim cannot be achieved by the protection alone. In addition, the power system and the distribution networks must be so designed that there are duplicate or multiple outlets from power sources to load centres (adequate generation may be taken for granted), and at least two sources of supply (feeders) to each distributing station. 

There are certain conventional ways of ensuring alternative supplies, as we shall see, but if full advantage is to be taken of their provision (always a costly matter) the protection must be highly selective in its functioning.

For this, it must possess the quality known as discrimination, by virtue of which it is able to select and to disconnect only the faulty element in the power system, leaving all others in normal operation so far as that may be possible. 

With a few exceptions, the detection and tripping of a faulty circuit is a very simple matter; the art and the skill lie in selecting the faulty one, bearing in mind that many circuits - generators, transformers, feeders - are usually affected, and in much the same way by a given fault. This accounts for the multiplicity of relay types and systems in use. 

Comparison between Induction Motor and Transformer

An induction motor may be considered to be a transformer with a rotating short circuited secondary. The stator winding corresponds to transformer primary and rotor winding to transformer secondary.

However, the following differences between the two are worth noting:

1. Air Gap

Unlike a transformer, the magnetic circuit of a 3-phase induction motor has an air gap. Therefore, the magnetizing current in a 3-phase induction motor is much larger than that of the transformer.

For example, in an induction motor, it may be as high as 30-50 % of rated current whereas it is only 1-5% of rated current in a transformer.

2. Leakage Reactance

In an induction motor, there is an air gap and the stator and rotor windings are distributed along the periphery of the air gap rather than concentrated on a core as in a transformer.

Therefore, the leakage reactances of stator and rotor windings are quite large compared to that of a transformer.

3. Mechanical and Electrical Outputs

In an induction motor, the inputs to the stator and rotor are electrical but the output from the rotor is mechanical. However, in a transformer, input as well as output is electrical.

4. Slip

The main difference between the induction motor and transformer lies in the fact that the rotor voltage and its frequency are both proportional to slip s.

If f is the stator frequency, E₂ is the per phase rotor e.m.f. at standstill and X₂ is the standstill rotor reactance/phase, then at any slip s, these values are:

Rotor e.m.f./phase, E₂'  = s E₂
Rotor reactance/phase, X₂'  = sX₂
Rotor frequency, f' = sf

Overcurrent Relays for Transmission Line Protection

The use of overcurrent relays is the simplest and cheapest type of line protection. Three types of overcurrent relays are used:

  1. Inverse time delay overcurrent (TDOC) relays, 
  2. Instantaneous overcurrent relays, 
  3. Directional overcurrent relays.

In general, overcurrent relays are difficult to implement where coordination, selectivity, and speed are important. They usually require changes to their settings as the system configuration changes. 

Also, they cannot discriminate between load and fault currents. Hence, when they are implemented only for phase fault protection, they are only useful when the minimum fault current exceeds the full load current. 

However, they can effectively be used on subtransmission systems and radial distribution systems. This is due to the fact that faults on these systems usually do not affect system stability and therefore high-speed protection is not needed.

Inverse Time Delay Overcurrent Relays

The main use of TDOC relays is on a radial system where they are used for both phase and ground
protection. Basic complements of such relays are two phase and one ground relays. 

This can protect the line for all combinations of phase and ground faults using the minimum number of relays.

According to Horowitz, adding a third phase relay can provide complete backup protection, having two relays for every type of fault, and is the preferred practice. 

These relays are usually applied on subtransmission lines and industrial systems due to the low cost involved.

Instantaneous Overcurrent Relays

Since the closer the fault is to the source, the greater the fault current magnitude but the longer the
tripping time, the TDOC relay cannot be used all by itself. 

As shown in the figure, the addition of an instantaneous overcurrent relay makes such system of protection possible. 

However, there must be considerable reduction in fault current as the fault moves from the relay toward the far end of the line. In this manner, the instantaneous relay can be made to see almost up to, but not including, the next bus. 

The relay will not operate for faults beyond the end of the line but still provide high-speed protection for most of the line.

Directional Overcurrent Relays

When it is important to limit tripping for faults in only one direction in multiple-source circuits, the
use of directional overcurrent relays becomes necessary. 

The overcurrent relaying is made directional to provide relay coordination between all the relays that can see a given fault. Otherwise, the coordination is often too difficult if not impossible.

The directional relays require two inputs that are the operating current and a reference(or polarizing) quantity that does not change with fault location. For phase relays, the polarizing quantity is the system voltage at the relay location. 

For ground directional reference, the zero-sequence voltage (3Va₀ ) is used. However, it is often that the current in the neutral of a wye-connected/delta power transformer is used as a directional reference instead.

Neutral Grounding (Earthing) and Equipment Grounding


The term Grounding or Earthing refers to the connecting of a conductor to earth. The neutral points of generator and transformer are deliberately connected to the earth.

In 3 phase a.c. systems the earthing is provided at each voltage level. 

If a neutral point is not available, a special Earthing Transformer is installed to obtain the neutral point for the purpose of earthing. 

Neutral points of star connected VTs and CTs are earthed. The neutral earthing has several advantages.

Advantages of Neutral Grounding

The neutral earthing has several advantages.

  • Freedom from persistent arcing grounds. The capacitance between the line and earth gets charged from supply voltage. During the flash-over the capacitance get discharged to the earth. The supply voltage charges it again. Such alternate charging and discharging produces repeated arcs called Arcing Grounds. The neutral grounding eliminates the problem of 'arcing grounds'. 
  • The neutral grounding stabilizes the neutral point. The voltages of healthy phases with respect to neutral are stabilized by neutral earthing. 
  • The neutral earthing is useful in discharging over-voltages due to lightning to the earth.
  • Simplified design of earth fault protection. 
  • The grounded systems require relatively lower insulation levels as compared with u. grounded systems. 

The modern power systems are 3 phase a.c systems with grounded neutrals.

Equipment Grounding

The Equipment Grounding refers to the grounding of non-current carrying metal parts earth. It is used for safety of personnel. 

If a metal part is grounded, its voltage with respect earth does not rise to a dangerously high value and the danger of a severe shock to personnel avoided. 

Video : How Does a Tesla Electric Car Work?

Electric cars are making big waves in the automobile world. These noise-free, pollution-free and high-performance vehicles are expected to make their I.C. engine counterparts obsolete by 2025. 

This video will unveil the hidden technologies behind the Tesla Model S, which recently became the world’s fastest accelerating car. 
We will see how electric cars have achieved superior performance by analyzing the technology behind the induction motor, inverter, lithium ion battery power source, regenerative braking and above all, the synchronized vehicle mechanism, in a logical, step-by-step manner. 

The working and features of Tesla car is explained here with help of animation.

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