DOL, Star-Delta, VFD, Soft- Starters, Auto Transformer Starters advantages and disadvantges

The advantages/disadvantages of different types of motor starters like :-

1. Direct online starter
2. Star/ delta,
3. Korndorfer Auto transformer starter
4. Primary resistance/reactance
5. Shunt capacitance
6. Slip ring
7. VFD
8. Soft Starter

DOL (Direct Online Starter) 
Advantages:-
1. Simple start up 
2. Very Low Maintenance cost.
3.  Ease of operation.
4. Very Simple Circuit
5. Very Low initial cost.
Disadvantages:-
1. Very high voltage drop during Starting.
2. Very High starting current
3. Very low starting period duration
4. Less power saving.
5. Not feasible for motors above 10 KW. 

Star/delta decrease the starting current to 1/3 but required many relays for control change between start connection then to delta. 

Slip ring starters are used for starting slip ring motors and these consists of separate slip ring for starting. Main disadvantage associated with these starters is their very high maintenance cost.


Auto transformer used for MV application by connect less voltage on motor during start up then change to nominal voltage on motor terminals after start up where reflect to decrease the starting current and decrease the voltage drop but increase the starting duration (very expensive where required 3 MVCB one for motor and one for primary of auto and one for secondary of auto).


VSD or VFD used for application where its worked with inlet guide van or valve (variable load) to make power saving.  Although they save the power but main disadvantage associated with VFD/VSD is their initial cost.
Visit for more knowledge about V/f control in VFD's:-
http://electrialstandards.blogspot.com/2016/01/vf-control-in-induction-motors-volts.html

Soft-Starter usually used for limiting the current to prescribed limit these starters limited the current as per settings in starter and thereafter soft-starter get bypassed and motor kept on running at line. Very less maintenance required in these starters.
But main disadvantage associated with soft-starters is their initial cost of soft-starters.

Power plants in electrical systems and Power stations working principle

Power Plant

Everyone electrical engineer has very much interest in power generation. Now let's study 1st what is power plant???

A power plant is that industry where power get generated. Power generation usually takes place in Megawatts. From there power is power transmission is done and provided at palaces as per requirements.

Power plants requires huge land and water that is why power plants are usually located at a far away distance from main city. For this reason, a power generating station has to not only take care of efficient generation but also the fact that the power is transmitted efficiently over the entire distance. And that’s why, the transformer switch yard to regulate transmission voltage also becomes an integral part of the power plant.

At the center of it, however, nearly all power generating stations has an A.C. generator or an alternator, which is basically a rotating machine that is equipped to convert energy from the mechanical domain (rotating turbine) into electrical domain by creating relative motion between a magnetic field and the conductors. The energy source harnessed to turn the generator shaft varies widely, and is chiefly dependent on the type of fuel used.

Types of Power Station

A power plant can be of several types depending mainly on the type of fuel used. Since for the purpose of bulk power generation, only thermal, nuclear and hydro power comes handy, therefore a power generating station can be broadly classified in the 3 above mentioned types.

1. Thermal power station
2. Hydropower stations
3. Nuclear power stations


Thermal Power Station

A thermal power station or a coal fired thermal power plant is by far, the most conventional method of generating electric power with reasonably high efficiency. It uses coal as the primary fuel to boil the water available to superheated steam for driving the steam turbine. The steam turbine is then mechanically coupled to an alternator rotor, the rotation of which results in the generation of electric power. Generally in India, bituminous coal or brown coal are used as fuel of boiler which has volatile content ranging from 8 to 33 % and ash content 5 to 16 %. To enhance the thermal efficiency of the plant, the coal is used in the boiler in its pulverized form.

In coal fired thermal power plant, steam is obtained in very high pressure inside the steam boiler by burning the pulverized coal. This steam is then super heated in the super heater to extreme high temperature. This super heated steam is then allowed to enter into the turbine, as the turbine blades are rotated by the pressure of the steam. The turbine is mechanically coupled with alternator in a way that its rotor will rotate with the rotation of turbine blades. After entering into the turbine, the steam pressure suddenly falls leading to corresponding increase in the steam volume. After having imparted energy into the turbine rotors, the steam is made to pass out of the turbine blades into the steam condenser of turbine. In the condenser, cold water at ambient temperature is circulated with the help of pump which leads to the condensation of the low pressure wet steam. Then this condensed water is further supplied to low pressure water heater where the low pressure steam increases the temperature of this feed water, it is again heated in high pressure. This outlines the basic working methodology of a thermal power plant.

Nuclear Power Station

The nuclear power generating stations are similar to the thermal stations in more ways than one. How ever, the exception here is that, radioactive elements like Uranium and thorium are used as the primary fuel in place of coal. Also in a Nuclear station the furnace and the boiler are replaced by the nuclear reactor and the heat exchanger tubes.

For the process of nuclear power generation, the radioactive fuels are made to undergo fission reaction within the nuclear reactors. The fission reaction, propagates like a controlled chain reaction and is accompanied by unprecedented amount of energy produced, which is manifested in the form of heat. This heat is then transferred to the water present in the heat exchanger tubes. As a result, super heated steam at very high temperature is produced.

Once the process of steam formation is accomplished, the remaining process is exactly similar to a thermal power plant, as this steam will further drive the turbine blades to generate electricity.

Hydro-Electric Power Station

In Hydro-electric plants the energy of the falling water is utilized to drive the turbine which in turn runs the generator to produce electricity. Rain falling upon the earth’s surface has potential energy relative to the oceans towards which it flows. This energy is converted to shaft work where the water falls through an appreciable vertical distance. The hydraulic power is therefore a naturally available renewable energy given by the eqn:

P=gρQH
Where g = acceleration due to gravity = 9.81 m/sec 2

ρ = density of water = 1000 kg/m 3

H = height of fall of water.

This power is utilized for rotating the alternator shaft, to convert it to equivalent electrical energy.

An important point to be noted is that, the hydro-electric plants are of much lower capacity compared to their thermal or nuclear counterpart. For this reason hydro plants are generally used in scheduling with thermal stations, to serve the load during peak hours. They in a way assist the thermal or the nuclear plant to deliver power efficiently during periods of peak hours.


Apart from these major types of power generations, we can resort to small scale generation techniques as well, to serve the discrete demands. These are often referred to as the alternative methods of power generation and can be classified as :-

1) Solar power generation. (making use of the available solar energy)

2) Geo-thermal power generation. (Energy available in the Earth’s crust)

3) Tidal power generation.

These alternative sources of generation has been given due importance in the last few decades owing to the depleting amount of the natural fuels available to us. In the centuries to come, a stage might be reached when several countries across the globe would run out of their entire reserve for fossil fuels. The only way forward would then lie in the mercy of these alternative sources of energy which might play an instrumental role in shaping the energy supplies of the future. For this reason these might rightfully be referred as the energy of the future.

Chemical earthing; Maintenance free earthing; Gel earthing

Chemical earthing is now becoming popular within industry as they have long life without any maintenance and also they provide good resistance value.

There isn't any different method for making Chemical earthing. There is only difference in type of treatment used in making earthing pit.

Treatment used in earthing pits also know as artificial treatment of earthing pits

Multiple rods, even in large number, may sometime fail to produce an adequately low resistance to earth. This condition arises in installations involving soils of high resistivity. The alternative is to reduce the resistivity of the soil immediately surrounding the earth electrode. To reduce the soil resistivity, it is necessary to dissolve in the moisture, normally contained in the soil, some substance which is highly conductive in its water solution. The most commonly used substances are sodium chloride ( NaCl ), also known as common salt, calcium chloride ( CaClj ), sodium carbonate ( NasCOs ), copper sulphate ( CuS04 ), salt, and soft coke, and salt and charcoal in suitable proportions.

This is treatment use in earthing pits.

But in chemical earthing pits there chemical used containing following materials:-

Chemical compound have following :-

-          Bentonite,

-          Electric Cabron

-          Copper Silecate

-          Sodium silicate

-          And CCM (Carsolinite conductive mixture)

Most commonly RIYA CGM makes the chemical for Chemical Earthing which has following characteristics:-

·         Highly efficient, more conductive and exhibits anti corrosive properties.

·         Reduces soil resisitive and lowers the ohmic  values of Electrode.

·         Absorbs Moisture from the Earth and retains for  a long period of time.

·         Eliminates the use of Sodium Chloride, Calcium Chloride, Cu sulphates and other common salts.

·         Eco friendly, safe and reliable

·         Much stretched life.

Also as per the cost aspect there isn't any difference in making these earthing pits.

These earhtings are also more reliable and required no effort for maintaining them.

Working principle of MCB; Miniature arc circuit breaker

MCBs or Miniature Circuit Breakers are electromechanical devices which protect an electrical circuit from an over-current. 

Where Miniature circuit breaker means minimum arc circuit breakers.

The over-current, in an electrical circuit, may result from short circuit, overload or faulty design. A MCB is a better alternative other than Fuse since it does not require replacement once an overload is detected. Unlike fuse, an MCB can be easily reset and thus offers improved operational safety and greater convenience without incurring large operating cost.

MCB is most widely used for industrial and domestic applications i.e. for lighting loads and small motor loads.

MCB or Miniature Circuit Breakers

The working principal of MCB is simple. An MCB functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. In simple terms MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit. Generally MCB are designed to protect against over current and over temperature faults.

There are two contacts one is fixed and the other moveable. When the current exceeds the predefined limit a solenoid forces the moveable contact to open (i.e., disconnect from the fixed contact) and the MCB turns off thereby stopping the current to flow in the circuit. In order to restart the flow of current the MCB is manually turned on. This mechanism is used to protect from the faults arising due to over current or over load.

To protect against fault arising due to over heating or increase in temperature a bi-metallic strip is used. MCBs are generally designed to trip within 2.5 millisecond when an over current fault arises. In case of temperature rise or over heating it may take 2 seconds to 2 minutes for the MCB to trip.

This article covers the insight of a single pole MCB commonly used in the house hold. The following image shows the different internal parts of an MCB with top casing removed. The subsequent sections will examine each part and its function.

Internal Parts of MCB

Internal parts of MCB is as shown below:-

Above pic is self explanatory and very helping in understanding working principle of MCB.

Vacuum Circuit breaker working principle and advantages of VCB

As discussed earlier circuit breakers working principle of type of arc quenching medium during breaking of circuit breaker.  In case of Vacuum circuit breakers arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of circuit breaker contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter.

The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar.

The material used for electric current carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. It requires minimum maintenance compared to other circuit breaker technologies.

Advantages of Vacuum Circuit Breaker or VCB

1.Service life of vacuum circuit breaker is much longer than other types of circuit breakers

2.There is no chance of fire hazard as oil circuit breaker. 

3.It is much environment friendly than SF6 Circuit breaker. Beside of that contraction of VCB is much user friendly. 

4.Replacement of vacuum interrupter (VI) is much convenient.

Operation of Vacuum Circuit Breaker

The main aim of any circuit breaker is to quench arc during electric current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after electric current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the electric current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next electric current zero.

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Vacuum interrupter

At electric current zero this vacuum arc is extinguished and the conducting metal vapor is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of contact surface, for next cycle of current. That means, the arc cannot be reestablished again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after electric current zero.

Vacuum circuit breaker is shown as above

There are two types of arc shapes. For interrupting electric current up to 10 kA, the arc remains diffused and the form of vapor discharge and cover the entire contact surface. Above 10 kA the diffused arc is constricted considerably by its own magnetic field and it contracts. The phenomenon gives rise over heating of contact at its center. In order to prevent this, the design of the contacts should be such that the arc does not remain stationary but keeps travelling by its own magnetic field. Specially designed contact shape of vacuum circuit breaker make the constricted stationary arc travel along the surface of the contacts, thereby causing minimum and uniform contact erosion.

Preliminary checks before maintenance of Vacuum circuit breaker:-
Following checks must be done before doing maintenance of Vacuum circuit breaker:-

(i) Ensure that Power supply must be disconnected
(ii) By using earthing rod discharge all fixed and moving contacts.
(iii) Check externally for any damages to Circuit breaker

Maintenance of vacuum Circuit breakers:-
Regular and timely maintenance of switchgear always keeps the system healthy and increases life of switchgear. It is recommended to do maintenance related activities such as:-

(i) cleaning and lubrication of fixed and moving parts
(ii) Calibration 
(iii) Checking of vacuum by using vacuum checking kit, also known as High pot test.
(iv) Closing and opening mechanism of circuit breaker
(v) Working of motorized operations
(vi) Lubrication of spring charging mechanism
(vii) Working of all trip interlocks
(viii) Checking of working of trippings.
(ix) Working of all mechanical interlocks
(x) Check for all control wiring and all fuses 
(xi) Check for all instrument transformers
(xii) Check circuit breaker integrity by using meggar of 5000 V

Frequency of Maintenance of breaker:-
Timely maintenance must be ensured , it is always recommended that maintenance interval for vacuum circuit breaker should be Every two years or 2000 operations whichever is earlier.

Transformer Percentage impedance; Importance of Transformer impedance; Calculating Transformer Impedance

Percentage Impedance of a Transformer:-

Percentage impedance of a Transformer is donated by Z%.
The impedance of a transformer is marked on most nameplates.

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

Measuring Impedance
Transformer impedance can be measured by means of a short circuit test.  With one winding shorted, a voltage at the rated frequency is applied to the other winding sufficient to circulate full load current - see below:

The percentage impedance can then be calculated as follows:

Z%  =  Impedance Voltage   x  100
Rated Voltage



Changing the Impedance Value
The most economical arrangement of core and windings leads to a 'natural' value of impedance determined by the leakage flux.  The leakage flux is a function of winding ampere turns and the area and length of the leakage flux path.  These can be varied at the design stage by changing the volts per turn and the geometric relationship of the windings.

The Effect of Higher and Lower Impedances
The impedance of a transformer has a major effect on system fault levels.  It determines the maximum value of current that will flow under fault conditions.

It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions.  By way of example, consider a 5 MVA transformer with an impedance of 5%.  The maximum fault level available on the secondary side is:

                    5 MVA x 100/5 = 100 MVA

and from this figure the equivalent primary and secondary fault currents can be calculated.

Importance of Transformer impedance
1. A transformer with a lower impedance will lead to a higher fault level (and vice versa)
2. It helps in determines the volt drop that occurs under load - known as 'regulation'
3. It also affects load sharing when two or more transformers operate in parallel

The figure calculated above is a maximum.  In practice, the actual fault level will be reduced by the source impedance, the impedance of cables and overhead lines between the transformer and the fault, and the fault impedance itself.

 Sequence Impedance (Z1  Z2  Z0)

The calculation above deals with a balanced 3-phase fault.  Non-symmetrical faults (phase-earth, phase-phase etc) lead to more complex calculations requiring the application symmetrical component theory.  This in turn involves the use of positive, negative and zero sequence impedances (Z1,  Z2 and  Z0 respectively).

As with all passive plant, the positive and negative sequence impedances (Z1 and  Z2) of a transformer are identical.

However, the zero sequence impedance is dependent upon the path available for the flow of zero sequence current and the balancing ampere turns available within the transformer.  Generally, zero sequence current requires a delta winding, or a star connection with the star point earthed.  Any impedance in the connection between the star point and earth increases the overall zero sequence impedance.  This has the effect of reducing the zero sequence current and is a feature that is frequently put to practical use in a distribution network to control the magnitude of current that will flow under earth fault conditions.

Strain Insulators; Stay Insulator; Shackle insulators

Strain Insulator

Strain insulators working principle is same as that of suspension insulators and they have only difference is that when insulator used in vertical position then it is known as suspension insulator and when used in horizontal position then it is known as strain insulator. 

When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.

Rated System Voltage	Number of disc insulator used in strain type tension insulator string	Number of disc insulator used in suspension insulator string
33KV	3	3
66KV	5	4
132KV	9	8
220KV	15	14

Stay Insulator

 For low voltage lines, the stays are to be insulated from ground at a height. The insulator used in the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the guy-wire will not fall to the ground. Stay insulators give protection in the event of accidentally broken live wire that can accidentally energizing a stay wire and remaining in contact with line which does not trip. In such cases , the bottom portion of the stay would have no voltage due to insulation, stay insulator will normally installed in the middle of stay wire.

Shackle Insulator or Spool Insulator

The shackle insulator or spool insulator is usually used in low voltage distribution network. It can be used both in horizontal and vertical position. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose. The tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.