Buchholz Relay - Construction, Working, Principle

Buchholz Relay:-

When we discuss about protection of transformer we come across buchholz relay first.What Is A Buchholz Relay?.Buchholz Relay is the major safety device of  transformer.it is pipe like construction  between the top of the transformer main tank and the conservator,employed in gas and oil operated transformer(rated >500 kVA).it prevents transformer from internal faults such as impulse breakdown of the insulating oil, insulation failure of turns,over heat of coils due to failure of windings etc.

Construction & Working of Buchholz Relay:-

Buchholz relay consists of an oil filled chamber with one inlet from transformer tank and one outlet to conservator. There are two hinged floats, one at the top of oil level and other at the bottom in the oil chamber. Each float is connected by a mercury switch. The upper float mercury switch which is connected to an alarm circuit and that on the lower float is connected to an external trip breaker. The schematic construction of a  buchholz relay is shown in the above fig.

Working Principle Of Buchholz Relay:-

Whenever there will be a minor internal fault such as impulse breakdown of the insulating oil, insulation failure of turns,core heating,over heat of coils due to failure of windings etc high amount of heat will be produced this heat heat decomposes the transformer insulating oil which results in production of gases like hydrocarbon gases, CO2 and CO.this gases occupies the area aboove the oil level which results in decrease in oil level.when oil level fall down to certain height alarm circuit will be energized.

Trip circuit will be energizes when phase to earth short circuit happens,baffle  plate or lower float will be closed.This switch energized the trip circuit of the circuit breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by inter tripping the circuit breakers associated with both LV and HV sides of the transformer. This is functioning of  buchholz relay.

Advantages & Uses Of Buchholz Relay:-

1.Buchholz relay identifies the internal faults of transformer due to heating and it helps in avoiding the major faults by simple mechanical phenomenon.

2.Severity and type of the fault can be determined without even dismantling or disturbing  the transformer.

3.If a major fault occurs, the transformer can be isolated automatically with the help of buchholz relay to prevent accidents.

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EMF Equation of Transformer

EMF Equation of Transformer :

Here is simple method to find out EMF Equation of Transformer:

Let            N1 = No. of turns in primary

                    N2 = No. of turns in secondary

                    Øm = Maximum flux in core in webers = Bm x A

                    f = Frequency of a.c. input in Hz.

The flux increases from it's zero value to maximum value Øm in one quarter of the cycle i.e. in 1/4 f second.

Therefore, r.m.s value of e.m.f./turn = 4.44 Øm volts

Now, r.m.s value of induced e.m.f in the whole primary winding 

                          = ( induced e.m.f. / turn ) x No. of primary winding

                               E1 = 4.44 f N1Øm ------------------- (i)

Similarly, r.m.s. value of e.m.f. induced in secondary is,

                               E2 = 4.44 f N2Øm ------------------- (ii) 

Voltage Transformation ratio (K)

From the above equations (i) and (ii), we get  

E2/E1 = N2/N1 =k

the constant 'K' is called voltage transformation ratio .

1. If N2>N1 i.e. K>1 ,then transformer is called STEP-UP transformer .

2. If N2<N1 i.e. K<1 ,then transformer is called STEP-DOWN transformer .

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Hydro-electric Power Station:Arrangement & Working

Definition: A power generating station which utilises the potential energy of water at a high level for the generation of electrical energy by converting potential energy to kinetic energy is known as a hydro-electric power station.

Arrangement of Hydro-electric Power Station & Working:

Hydro-electric Power Plant Consists of basic sections like:
1) Dam
2) Reservoir
3) Control Gate
4) Penstock
5) Turbines and generators
6) Power House

Dam:- A dam is a barrier which reserves the water and creates water head.reserved water will be at some height this height results in potential energy (E=mgh)of water.where h is height of the water that depends on height & storage capacity of the dam.

Reservoir:-The part of river where water will be stored is called Reservoir.

Control Gate&Penstock:Penstocks are generally made of reinforced concrete or steel to transport water from reservoir to turbine with less friction losses.control gate is used to control over the water travelling in penstock.

Turbines and generators:-Water turbines are used to convert the energy of falling water into mechanical energy.generators to produce electrical energy from rotating shaft of turbine.

Power House:-At the power house generated power from alternator will be stepped up and supplied to transmission lines.

Typical Hydro Electric Power Station Block Diagram

Choice of Site for Hydro-electric Power Stations:-

The following points should be taken into account while selecting the site for a hydro-electric power station :

(i) Availability of water & Storage of water. Since the primary requirement of a hydro-electric power station is the availability of huge quantity of water, such plants should be built at a place (e.g., river,canal) where adequate water is available at a good head not only the amount of water,storage of water at heights also important to convert potential energy of water into kinetic energy then into electrical energy.

(ii) Capital Cost and type of land. The land for the construction of the hydel power plant should be adequate to withstand the weight of heavy electrical equipment like alternators,turbines,transformers and cost of the land should be less to get profits.

(iii) Transportation facilities. The place selected for a hydro-electric (hydel power)plant should be accessible by rail and road so that necessary equipment and machinery could be easily transported.

Ideal choice of Hydro-electric Power Stations is a site at  river in hilly areas where dam can be conveniently built and large reservoirs can be obtained.

Advantages
(i) It requires no fuel as water is used for the generation of electrical energy.
(ii) It is quite neat and clean as no smoke or ash is produced.
(iii) It requires very small running charges because water is the source of energy which is available free of cost.
(iv) It is comparatively simple in construction and requires less maintenance.
(v) It does not require a long starting time like a steam power station. In fact, such plants can be put into service instantly.
(vi) It is robust and has a longer life.
(vii) Such plants serve many purposes. In addition to the generation of electrical energy, they also help in irrigation and controlling floods.
(viii) Although such plants require the attention of highly skilled persons at the time of construction, yet for operation, a few experienced persons may do the job well.
Disadvantages
(i) It involves high capital cost due to construction of dam.
(ii) There is uncertainty about the availability of huge amount of water due to dependence on weather conditions.
(iii) Skilled and experienced hands are required to build the plant.
(iv) It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.


Reference:Google Books,Hydro Power Plant PDFs Download

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Thermal Power Generation Plant or Steam Power Station

Thermal Power Generation Plant or Steam Power Station :

Thermal power generation plant or Steam Power Station is the most conventional source of electric power over the world.We can define thermal power station as "A generating station which converts heat energy of coal combustion into electrical energy through steam as intermediate energy form is known as a steam power station."

Theory of Thermal Power Station:- 

A steam power station basically works on the Rankine cycle. Steam is produced in the boiler by utilizing the heat of coal combustion. The steam is then expanded in the prime mover (i.e., steam turbine) and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.

Schematic Arrangement of Steam Power Station:-

Although steam power station simply involves the conversion of heat of coal combustion into electrical energy, yet it embraces many arrangements for proper working and efficiency.The bituminous coal is used as boiler fuel has volatile matter from 8 to 33 % and ash content 5 to 16 %. To increase the thermal efficiency, the coal is used in the boiler in powder form.

The thermal efficiency of a modern steam power station is about 30%. It means that if 100 calories of heat is supplied by coal combustion, then mechanical energy equivalent of 30 calories will be available at the turbine shaft and rest is lost. It may be important to note that more than 50% of total heat of combustion is lost in the condenser. The other heat losses occur in flue gases, radiation,ash etc.

Schematic Diagram Of Steam Power Station (Thermal Station):-

For Examination Purpose Draw This 

How Thermal Power Generation Plant Will Work?

1. Coal and ash handling plant(Numbers 1,2,8):- 

The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air. The pulverized coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal.

2.Boiler(Number 3):- 

The heat of combustion of coal in the boiler is utilised to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through superheater, economiser, air pre-heater and are finally exhausted to atmosphere through the chimney.

(ii) Superheater.

The steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated (i.e., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principal benefits.

Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the superheater is fed to steam turbine through the main valve.

(iii) Economiser.

An economiser is essentially a feed water heater and derives heat from the

 flue gases for this purpose. The feed water is fed to the economiser before supplying to the boiler. The economiser extracts a part of heat of flue gases to increase the feed water temperature.

(iv) Air preheater. 

An air preheater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are : increased thermal efficiency and increased steam capacity per square metre of boiler surface.

3.Steam turbine. 

The dry and superheated steam from the superheater is fed to the steam
turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation.

4. Alternator(Number 5). 

The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators.

5. Feed water.

The condensate from the condenser is used as feed water to the boiler. Some
water may be lost in the cycle which is suitably made up from external source.The feed water on its way to the boiler is heated by water heaters and economiser.This helps in raising the overall efficiency of the plant.

6. Cooling arrangement. 

In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser.

Choice of Site for Steam Power Stations or Thermal Power Generation Plant :-

(i) Supply of fuel.(should be near to mines to reduce transportation cost)

(ii) Availability of water.(high availability of water is required for boiling & cooling)

(iii) Transportation facilities.

(iv) Cost and type of land.

(vi) Distance from populated area.

(v) Nearness to load centre

Thermal efficiency, 

ηthermal =Heat equivalent of mech. energy transmitted to turbine shaft/Heat of coal combustion

The thermal efficiency of a modern steam power station is about.

Advantages & Disadvantages Of Thermal Plants:-

Advantages

(i) The fuel (i.e., coal) used is quite cheap.

(ii) Less initial cost as compared to other generating stations.

(iii) It can be installed at any place irrespective of the existence of coal. The coal can be transported

to the site of the plant by rail or road.

(iv) It requires less space as compared to the hydroelectric power station.

(v) The cost of generation is lesser than that of the diesel power station.

Disadvantages

(i) It pollutes the atmosphere due to the production of large amount of smoke and fumes.

(ii) It is costlier in running cost as compared to hydroelectric plant.

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Open Circuit(OC) And Short Circuit(SC) Test On Transformer

Open circuit and short circuit tests on single phase transformer:-

Why we need to conduct OC & SC test on transformers?

-To obtain the equivalent circuit parameters from Open circuit and short circuit tests , and to estimate efficiency & regulation at various loads.
-To understand the basic working principle of a transformer.
This method also called as indirect loading method of transformers.

Open Circuit Test (O.C. Test):-

This test mainly to find out 'no load loss (core loss)' and 'no load current I₀'.it is also called no load test because we kept high voltage (HV) winding is kept open and the low voltage (LV) winding is connected to its power supply.to measure voltage,current,power wattmeter (W), ammeter (A) and voltmeter (V) are connected to the LV winding as shown in the above figure. 

1.Apply voltage to the LV side in steps up to the rated voltage and record primary current and power drawn from the source when voltage reached to rated value of the LV winding.
2.The ammeter reading gives the no load current I₀. As I₀ itself is very small, the voltage drops due to this current can be neglected.
3.The total power sending to the transformer is calculated by wattmeter (W),this reading is also sum of core losses and copper losses,beacuse no load is appiled on transformer so output is zero.
4.Hence, this input power only consists of core losses and copper losses.

Sometimes, a high resistance voltmeter is connected across the HV winding. Though, a voltmeter is connected, HV winding can be treated as open circuit as the current through the voltmeter is negligibly small. This helps in to find voltage transformation ratio (K).

The two components of no load current can be given as,

I_μ = I₀sinΦ₀   and    I_w = I₀cosΦ₀.

cosΦ₀ (no load power factor) = W / (V₁I₀). ... (W = wattmeter reading)

From this, shunt parameters of equivalent circuit parameters of equivalent circuit of transformer (X₀ and R₀) can be calculated as

X₀ = V₁/I_μ  and  R₀ = V₁/I_w.

Short Circuit(SC)Or Impedance Test :-

The LV side of transformer is short circuited and wattmeter (W), voltmere (V) and ammeter (A) are connected on the HV side of the transformer.we short circuit only on LV side because to get high current,if we short circuit on HV side we get low current.

1.  As secondary is shorted, its resistance is very very small and on rated voltage it may draw very large current. Such large current can cause overheating and burning of the transformer. To limit this short circuit current, primary is supplied with low voltage which is just enough to cause rated current to flow through primary which can be observed on an ammeter. The low voltage can be adjusted with the help of variac. Hence this test is also called low voltage test or reduced voltage test. 

2.Voltage is applied to the HV side and increased from the zero until the ammeter reading equals the rated current.

Thus, the wattmeter reading can be taken as copper loss in the transformer.

Therefore, W = I_sc²R_eq....... (where R_eq is the equivalent resistance of transformer)

 Z_eq = V_sc/I_sc.

Therefore, equivalent reactance of transformer can be calculated from the formula  Z_eq² = R_eq² + X_eq².

These, values are referred to the HV side of the transformer.

Hence, it is seen that the short circuit test gives copper losses of transformer and approximate equivalent resistance and reactance of the transformer.

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Types Of Transformers : Different Types of Transformers

Different Types of Transformers

Transformers can be classified on different basis, like types of construction, types of cooling etc.

(A) On the basis of construction, transformers can be classified into two types as; 
(i) Core type transformer and (ii) Shell type transformer, which are described below.

(I) Core Type Transformer

In core type transformer, windings are cylindrical former wound, mounted on  the core limbs as shown in the figure above. The cylindrical coils have different layers and each layer is insulated from each other. Materials like paper, cloth or mica can be used for insulation. Low voltage windings are placed nearer to the core, as they are easier to insulate.

(Ii) Shell Type Transformer

The coils are former wound and mounted in layers stacked with insulation between them. A shell type transformer may have simple rectangular form (as shown in above fig), or  it may have a distributed form.

(B) On the basis of their purpose

Step up transformer: Voltage increases (with subsequent decrease in current) at secondary.
Step down transformer: Voltage decreases (with subsequent increase in current) at secondary.

(C) On the basis of type of supply
1.Single phase transformer
2.Three phase transformer

(D) On the basis of their use
1.Power transformer: Used in transmission network, high rating
2.Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.
3.Instrument transformer: Used in relay and protection purpose in different instruments in industries

 Current transformer (CT)

Potential transformer (PT)

(E) On the basis of cooling employed 
Oil-filled self cooled type
Oil-filled water cooled type
Air blast type (air cooled)

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Constructional features of Transformers

Constructional features of Transformers

Transformers used in practice are of extremely large variety depending upon the end use. In addition to the transformers used in power systems, in power transmission and distribution, a large number of special transformers are in use in applications like electronic supplies,rectification, furnaces, traction etc. Here the focus is on power transformers only.

The principle of operation of these transformers also is the same but the user requirements different .Here more common constructional aspects alone are discussed. Power transformers of smaller sizes could be air cooled while the larger ones are oil cooled. These machines are highly material intensive equipments and are designed to match the applications for best operating conditions. Hence they are `tailor made' to a job.This brings in a very large variety in their constructional features. Here more common constructional aspects alone are discussed.

These can be broadly divided into
1. Core construction
2. Winding arrangements
3. Cooling aspects

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Why Transformer Rating In kVA, Not in KW?

The transformer is usually rated in terms of its input and output voltages and apparent power that it is designed to safely deliver. 

What are the meanings of these transformer ratings?

The voltage ratio indicates that the transformer has two windings, the high-voltage winding is rated for 1100 Volts and the low-voltage winding for 110 volts.

These voltages are proportional to their respective number of turns. Therefore, the voltage ratio also represents the turns ratio a or k.

Why Transformer Rating In kVA, Not in KW?

Answer is very simple transformer is a static device  two type of losses in a transformer;
1. Copper Losses
2. Iron Losses or Core Losses or  Insulation Losses.

Iron Losses or Core Losses or  Insulation Losses depends on Voltage while Copper losses ( I²R) are variable losses varies with Current passing through transformer windings.at the time of manufacturing its not decided that which kind of load will be added to transformer.We can't predict the power factor while designing the machine, because power factor depends upon the load which varies time to time.without including power factor we have to give rating so That’s why the Transformer Rating may be expressed in kVA, Not in kW.

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What Is Transformer,Principle Of Working

Definition of Transformer:

Transformers are commonly used in applications which require the conversion of AC voltage from one voltage level to another (Step Up or Step Down),without changing frequency or constant frequency.
or

A transformer is a static electrical machine used for deliver power from one circuit to another without electrical coupling at constant frequency. 

For example, a farmer has a large, 480-V, 3-phase motor powering a well. The motor is in a building, and the farmer wants one 120-V circuit for a few lights and a receptacle outlet. A transformer is used to lower the voltage from 480 V to 120 V for the lighting circuit.

Working Principle Of Transformers:-

Transformers works on the principle of mutual induction.An important property of electricity is that a magnetic field is produced around a wire in which electrical current is flowing.The more current that flows, the stronger is the magnetic field. An even stronger magnetic field can be produced by winding the wire into a coil. Now the magnetic fields of adjacent wires add together to form one strong magnetic field. When a magnetic field moves across a wire, a voltage is induced into the wire.This is the most basic concept of the theory of transformer.

Here we take two coils named primary and secondary as shown in above figure,flux produces due to voltage in primary and voltage produce due to flux in core by following mutual induction.

Mutual Induction In Coils 

Mutual induction:-If the wire forms a complete circuit, current will flow in the wire. If a second coil of wire is placed in a moving magnetic field, then a voltage will be induced in this second coil. This phenomenon is called mutual induction.

Internal construction of transformers 

-Electrical energy is converted into magnetic field and then converted back into electrical energy in a second winding,with little or no loss of energy. 

Also Read:   Buchholz Relay - Construction, Working     OC & SC Test On Transformers

Main Constructional Parts of Transformer:

The three main parts of a transformer are,

1.Primary Winding of transformer - which produces magnetic flux when it is connected to electrical source.

2.Magnetic Core of transformer - the magnetic flux produced by the primary winding, that will pass through this low reluctance path linked with secondary winding and create a closed magnetic circuit. 


3.Secondary Winding of transformer - the flux, produced by primary winding, passes through the core, will link with the secondary winding. This winding also wounds on the same core and gives the desired output of the transformer.

Types Of Power Transformer:-

Step Up Transformer:- This type of transformers we can only at power generation places to step up,because we generate power with  low voltage if we transmit low power losses will be high,so to reduce transmission losses we step up the voltage.transformers used to step up the voltage are called Step Up Transformers.

Step Down Transformer:-Transformers used to step down the voltage are called Step Down Transformers. we find at streets,sub stations,industries.

Uses of Power Transformer:-

-Transformer can rise or lower the level of level of Voltage or Current ( when voltage increases, current decreases and vice virsa because  P =V x I, and Power is same )  in an AC Circuit.

-Used in reduction of capital cost of the system and it also improves the voltage regulation of the transmission system.

-Transformer can increase or decrease the value of capacitor, an inductor or resistance in an AC circuit. It can thus act as an impedance transferring device.

  

-Transformer can be used to prevent DC from passing from one circuit to the other. 

-Transformer can isolate two circuits electrically.

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Synchronizing of Alternators:Paralelle Operation of Alternators

Paralelle Operation of Alternators: 

Synchronizing:The operation of connecting two alternators in parallel is known as synchronizing.Certain conditions must be fulfilled before this can be effected. The incoming machine must have its voltage and frequency equal to that of the bus bars and, should be in same phase with bus bar voltage. The instruments or apparatus for determining when these conditions are fulfilled are called synchroscopes.

Synchronizing can be done with the help of
(i) dark lamp method or (ii) by using synchroscope.
Reasons for operating in parallel:
a) Handling larger loads.
b) Maintenance can be done without power disruption.
c) Increasing system reliability.

d) Increased efficiency.

Conditions required for Paralleling Operation of Alternators:

The figure below shows a synchronous generator G1 supplying power to a load, with another generator G2 about to be paralleled with G1 by closing switch S1.What conditions must be met before the switch can be closed and the 2 generators connected in parallel?

Paralleling 2 or more generators must be done carefully as to avoid generator or other system component damage. Conditions to be satisfied are as follows:

a) RMS line voltages must be equal.
b) The generators to be paralleled must have the same phase sequence.
c) The oncoming generator (the new generator) must have the same operating frequency as compared to the system frequency.

General Procedure for Paralleling Generators:

Consider the figure shown below. Suppose that generator G2 is to be connected to the running system as shown below:

1. Using Voltmeters, the field current of the oncoming generator should be adjusted until its terminal voltage is equal to the line voltage of the running system.

2. Check and verify phase sequence to be identical to the system phase sequence. There are 2 methods to do this:

i. One way is using the 3 lamp method, where the lamps are stretched across the open terminals of the switch connecting the generator to the system (as shown in the figure below). As the phase changes between the 2 systems, the lamps first get bright (large phase difference) and then get dim (small phase difference). If all 3 lamps get bright and dark together, then the systems have the same phase sequence. If the lamps brighten in succession, then the systems have the opposite phase sequence, and one of the sequences must be reversed.

ii. Using a Synchroscope – a meter that measures the difference in phase angles (it does not check phase sequences only phase angles).
3. Check and verify generator frequency is same as that of the system frequency. This is done by watching a frequency of brightening and dimming of the lamps until the frequencies are close by making them to change very slowly.
4. Once the frequencies are nearly equal, the voltages in the 2 systems will change phase with respect to each other very slowly. The phase changes are observed, and when the phase angles are equal, the switch connecting the 2 systems is closed.

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Salient pole alternators and Blondel’s Two Reaction Theory

The details of synchronous generators developed so far is applicable to only round rotor or nonsalient pole alternators. In such machines the air gap is uniform through out and hence the effect of mmf will be same whether it acts along the pole axis or the inter polar axis. Hence reactance of the sator is same throughout and hence it is called synchronous reactance.

But in case salient pole machines the air gap is non uniform and it is smaller along pole axis and is larger along the inter polar axis. These axes are called direct axis or d-axis and quadrature axis or q-axis. Hence the effect of mmf when acting along direct axis will be different than that when it is acting along quadrature axis. Hence the reactance of the stator can not be same when the mmf is acting along d – axis and q- axis. As the length of the air gap is small along direct axis reluctance of the magnetic circuit is less and the air gap along the q –axis is larger and hence the along the quadrature axis will be comparatively higher. Hence along d-axis more flux is produced than q-axis. Therefore the reactance due to armature reaction will be different along d-axis and q-axis. 

These reactances are

Xad = direct axis reactance; Xaq = quadrature axis reactance.

Hence the effect of armature reaction in the case of a salient pole synchronous machine can be taken as two components - one acting along the direct axis (coinciding with the main field pole axis) and the other acting along the quadrature axis (inter-polar region or magnetic neutral axis) - and as such the mmf components of armature-reaction in a salient-pole machine cannot be considered as acting on the same magnetic circuit. Hence the effect of the armature reaction cannot be taken into account by considering only the synchronous reactance, in the case of a salient pole synchronous machine.

In fact, the direct-axis component Fad acts over a magnetic circuit identical with that of the main field system and produces a comparable effect while the quadrature-axis component Faq acts along the interpolar axis, resulting in an altogether smaller effect and, in addition, a flux distribution totally different from that of Fad or the main field m.m.f. This explains why the application of cylindrical rotor theory to salient-pole machines for predicting the performance gives results not conforming to the performance obtained from an actual test.

Blondel’s Two reaction Theory:

Blondel’s two-reaction theory considers the effects of the quadrature and direct-axis components of the armature reaction separately. Neglecting saturation, their different effects are considered by assigning to each an appropriate value of armature-reaction “reactance,” respectively xad and xaq . The effects of armature resistance and true leakage reactance (XL) may be treated separately, or may be added to the armature reaction coefficients on the assumption that they are the same, for either the direct-axis or quadrature-axis components of the armature current (which is almost true). Thus the combined reactance values can be expressed as : Xsd = xad + xi and Xsq = xaq + xi for the direct-and cross-reaction axes respectively.

In a salient-pole machine, xaq, the quadrature-axis reactance is smaller than xad, the direct-axis reactance, since the flux produced by a given current component in that axis is smaller as the reluctance of the magnetic path consists mostly of the interpolar spaces. It is essential to clearly note the difference between the quadrature and direct-axis components Iaq, and Iad of the armature current Ia, and the reactive and active components Iaa and Iar. Although both pairs are represented by phasors in phase quadrature, the former are related to the induced emf Et while the latter are referred to the terminal voltage V. These phasors are clearly indicated with reference to the phasor diagram of a

(salient pole) synchronous generator supplying a lagging power factor (pf) load, shown in above Fig.

Iaq = Ia cos(δ+Ø); Iad = Ia sin(δ+Ø); and Ia = √[(Iaq)² + (Iad)²]

Iaa = Ia cosØ; Iar = Ia sinØ; and Ia = √[(Iaa)² + (Iar)²]

where δ = torque or power angle and Ø = the p.f. angle of the load.

The phasor diagram below Fig. shows the two reactance voltage componentsIaq *Xsq and Iad * Xsd which are in quadrature with their respective components of the armature current. The resistance drop Ia x Ra is added in phase with Ia although we could take it as Iaq x Ra and Iad x Ra separately, which is unnecessary as Ia = Iad + jIaq.

 

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