Why We Use Only 11KV / 22KV / 33KV / 66KV / 110KV ?

Why electricity is being transmitted in multiple of 11?


Have you ever wondered? why we use only 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV which are multiples of 1.11(we can say multiple of 11).We can generate this voltages efficiently because of form factor of sine wave is 1.11.So it is efficient to produce that range of voltages.Form factor is the ratio of RMS value to average value.For a pure sine wave it is around 1.11.
We can't have a combination of other then a multiple of 1.11.
So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).Also it provides us the best economic construction of step up and step down transformers.

Right Answer:The answer given above is taken standard electrical book.But that is wrong the actual answer is when we want to maintain 10KV at receiving end, transmission companies transfer 10% extra than receiving end voltage.i.e., 11KV simillary 10% of 20/30/60/100KV gives us 11KV / 22KV / 33KV / 66KV / 110KV respectively.
These questions sounds same :

why voltages are multiples of 11
why generation voltage is 11kv
why the substation voltages are multiple of 11kv
why transmission line 11kv or 33kv 66kv
why do we use ac and not dc system in india?
maximum generating voltage in world
what is 11kv electrical system
generation voltage of power plant
generating voltage levels
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Difference between Squirrel Cage and Slip Ring Induction Motor

Difference between Squirrel Cage and Slip Ring Induction Motor

On our electrical learning portal we have posted articles based on comparisons and differences.In this article we are going to discuss differences between squirrel cage and slip ring induction motor.Induction motors are classified based on the type of rotor they use.There are two types of rotors namely squirrel cage rotor and slip ring rotor.Lets have a comparison chart between slip ring and squirrel cage motor with key differences.
You can read here :

Types of induction motor at glance:


The squirrel cage induction motor is the most popular type of AC motor. It is very commonly used in industries because it is very cheap, robust, efficient, and reliable. The slip ring motor has very little application in industries. Rarely 5% – 10% slip ring motors are used in industries because it has several disadvantages like it required frequent maintenance, having a high copper loss, etc.

No.
Slip Ring / Wound Rotor Induction Motor
Squirrel Cage Induction Motor
1)     
   Rotor consists of a three phase winding similar to the stator winding.
Rotor consists of bars which are shorted at ends with help of end rings.
2)     
Construction is complicated.
Construction is simple.
3)     
Slip rings and brushes are present to  add external resistance.
 Slip rings and brushes are absent.
4)     
Construction is delicate,due to brushes frequent maintenance is necessary.Only
5% of motors in industry use slip ring rotors.
Construction is simple and robust maintenance is necessary.
95% of motors in industry use squirrel cage rotor.
5)     
High starting torque can be obtained.Rotor resistance starter can be used.
Low starting torque..Rotor resistance starter can not be used.
6)     
Carbon brushes, slip rings etc are provided in the rotor circuit.
No moving contacts in the rotor.
7)     
Comparatively less efficiency.
Squirrel cage and slip ring induction motor gives higher efficiency than other.
8)    
High starting torque. It can be obtained by adding external resistance in the rotor circuit.
Low starting torque. It has 1.5 time full load torque.
9)     
Speed control by rotor resistance is possible.
Speed control by rotor resistance is not possible.
10)  
Less starting current compared to squirrel cage Induction Motor.
Starting current is 5 to 7 times the full load current.
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How to Draw Potier Triangle/ZPF Characteristics ?

Regulation by Zero Power Factor ( ZPF) method  of Alternator:

During the operation of the alternator, resistance voltage drop IaRand armature leakage reactance drop IaXL are actually emf quantities and the armature reaction reactance is a mmf quantity. To determine the regulation of the alternator by this method OCC, SCC and ZPF test details and characteristics are required.

-As explained earlier OC and SC tests are conducted and OCC and SCC are drawn.

-ZPF test is conducted by connecting the alternator to ZPF load and exciting the alternator in such way that the alternator supplies the rated current at rated voltage running at rated speed.
-To plot ZPF characteristics only two points are required. One point is corresponding to the zero voltage and rated current that can be obtained from scc and the other at rated voltage and rated current under zpf load.
-This zero power factor curve appears like OCC but shifted by a factor IXL vertically and horizontally by armature reaction mmf as shown below in figure. 



Following are the steps to draw ZPF characteristics:

-By suitable tests plot OCC and SCC. Draw air gap line. Conduct ZPF test at full load for rated voltage and fix the point B.

-Draw the line BH with length equal to field current required to produce full load
current on short circuit..
-Draw HD parallel to the air gap line so as to cut the OCC. Draw DE perpendicular to HB or parallel to voltage axis.
-Now, DE represents voltage drop IXL and BE represents the field current required to overcome the effect of armature reaction.
-Triangle BDE is called Potier triangle and XL is the Potier reactance. Find E from V, IRa, IXL and .
-Use the expression E =√ (V cos  Ø+ IRa)² + (V sin ) + IXL)² to compute E. Find field current corresponding to E. Draw FG with magnitude equal to BE at angle (90+ ) from field current axis, where is the phase angle of current from voltage vector E (internal phase angle).
-The resultant field current is given by OG. Mark this length on field current axis. From OCC find the corresponding E0. Find the regulation.

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Auto Transformer Principle of Operation,Working & Applications

An auto transformer is a electrical transformer having only single winding which acts as both primary and secondary,so in input and output connected to shared single winding.

Why we need to go for Auto Transformer ?

We have some advantages of auto-transformer over normal two winding transformer.
1.Auto transformers usually smaller in size,because one winding is eliminated.
2.as size is small cost also low(so cheap in cost)
3.as the winding is same so leakage reactance will be less.
4.increased kVA rating.

Auto Transformer

Construction, Principle of Operation Of Auto Transformer: 

In Auto Transformer, one single winding is shared as primary winding as well as secondary winding.as in transformer copper wire wound on silicon steel.as shown in the figure input connected at fixed positions.but on other side we employ some tapping to get variable output voltages.Variable turns ratio at secondary can be obtained by changing the tappings of the winding.

Auto Transformer

Copper Savings in Auto Transformer:

If we compare conventional two winding transformer and  auto transformer the amount of copper needed for auto transformer is less.weight of copper of any winding depends upon its length and cross - sectional area.and length depends on no.of turns,cross - sectional area varies with rated current.

Auto Transformer

copper in the section AC proportional to, 

(N1-N2)I1

weight of copper in the section BC proportional to

(I1-I2)N2

Hence, total weight of copper in the winding of auto transformer proportional to,



In similar way it can be proved, the weight of copper in two winding transformer is proportional to,


N1I1+N2I2
=2N1I1(Since, in a transformer N1I1=N2I2)

Let's assume, Wa and Wtw are weight of copper in auto transformer and two winding transformer respectively,



∴ Saving of copper in auto transformer compared to two winding transformer,
∴ Wtw -Wa=kWtw 
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9 Advantages of Three Phase System Over Single Phase System

Three Phase System Advantages 

In modern power generation, transportation, distribution we use poly-phase system over single phase system.In poly-phase AC system it  might be two , three or more individual circuits, operate at same frequency, their voltages and currents are out of phase from one another.But when it comes to practical scenario three phase system considered as poly-phase system.It is most reliable and efficient compared to all poly phase systems.Single phase system has some limitations and drawbacks so it's been replaced by three-phase system.Here we listed out major reasons why 3 phase/poly-phase system is better than single phase system.
8 Advantages of Three Phase System Over Single Phase System

Advantages of polyphase systems over single phase systems are

1. A polyphase/3 phase transmission line requires less conductor material than a single phase line for transmitting the same amount power at the same voltage.

2. For a given frame size a polyphase/3 phase machine gives a higher output than a single-phase machine. For example output of a 3-phase motor is 1.5 times the output of single-phase motor of same size.

3. Polyphase/3 phase motors have a uniform torque where most of the single-phase motors have a pulsating torque.

4. Polyphase/3 phase induction motors are self-starting and are more efficient. On the other hand single-phase induction motors are not self-starting and are less efficient.

6. Per unit of output. the polyphase/3 phase machine is very much cheaper.

7. Power factor of a single-phase motor is lower than that of polyphase motor of the same rating.

8. Rotating field can be set up by passing polyphase current through stationary coils.

9. Parallel operation of polypahse alternators is simple as compared to that of single-phase alternators because of pulsating reaction in single-phase alternator.

It has been found that the above advantages are best realized in the case of three-phase systems.Consequently, the electric power is generated and transmitted in the form of three-phase system.

Final Points :
Single Phase SupplyThree Phase supply
power delivered is pulsatingPower delivered is constant
Single Phase induction motors are not self starting as it does not have starting torque.Three phase induction motors are self starting.
Parallel operation is not easy.Parallel Operation is easy.
Efficiency of single phase motor is lesser.High efficiency.
Single phase motors have pulsating torque.Three phase motors have uniform torque.
Single phase motors have lower power factor.Three phase motors have higher power factor.
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Current Differential Relay Working || Types of Differential Relays

Current Differential Relay Working || Types of Differential Relays

What is differential relay?

In the over-current relays, a current is sensed but such relays are not very sensitive as these relays cannot distinguish between heavy loads and minor fault conditions. in such castes. differential relays can be need.


A differential relay in defined an the relay that operates when the phasor drill-me of two or more similar electrical quantities exceeds a predetermined value,

Thus a current differential relay operates on the result of comparison between the phase angle and magnitudes of the currents entering and leaving the system to be protected. Under normal conditions. the two currents are equal in phase and magnitude hence relay is inoperative. But under fault conditions. this condition no longer exist.
The relay is connected in such a manner that the difference between current entering and current leaving flows through the operating coil.If this differential current exceeds a preset value then the relay operates and opens the circuit breaker.

Almost any type of relay when connected in a certain way can be made to operate as a differential relay.

Types of differential relays:

1. Current differential relay.
2. Biased beam relay or percentage differential relay.
3. Voltage balance differential relay.

Current differential relay working:

Most of the differential relays are at current differential type. Consider an over current relay connected in the circuit so as to operate as the current differential relay. This it shown in the figure.
Two current transformers are used having same ratio are connected on the either  side of the section to be protected.The secondaries of current transformers are  connected in series, so they carry induced currents in the same direction. Let current I is flowing through the primary of current  transformers towards the external fault. As the current transformers are identical, the secondaries of current transformers will carry equal currents. Due to the connection of relay, no current will flow through the operating coil for the relay, Hence relay will remain Inoperative. So relay cannot operate if there is an external fault.

The current flows through the fault from both sides. The two secondary currents through C.Ts are not equal. The current flowing through the relay coil  is now i1+i2. This high current causes the relay to operate.
It should be noted that the fault current need not always flow to the fault from both sides. A flow on one side only or even some current flowing out of one side while a large current entering  the other side can cause differential relay to operate. Thus the amount of current flowing through a relay coil depends upon the way the fault is being fed‘

Disadvantages of current differential relay:

1. The current transformers are connected through cables called pilot cables. The impedance of such pilot cables generally causes a slight difference between the currents at the ends of the section to be protected, A sensitive relay can operate to a very small difference in the two currents, though there is no fault existing.
2. The relay is likely to operate inaccurately with heavy through current flows.This is because the assumed identical current transformers may not have identical secondary currents due to the constructional errors and pilot cable impedance.
3. Under severe through fault conditions,. the current transformers may saturate and cause unequal secondary currents. The difference between the currents may approach the pick value to cause the inaccurate operation for the relay.
4. Under heavy current flows. pilot cable capacitance may cause inaccurate operation of the relay.
All these disadvantages are overcome in biased beam relay.
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What is Creeping in Energy Meters ?

Creeping in Energy Meters: 


In some energy meters a slow but continuous rotation of the disc is obtained when the pressure coils are energized and there is no load current passing through the current coil i.e. current coil is not energized.The main reason for creeping is over-compensation the aluminium disk to over come the static friction of disk and another reason is over voltage across the shunt magnet.Due to this creeping consumers suffers from high tariff. This may be due to incorrect friction compensation to vibration, to stray magnetic field or to the fact that the supply voltage is in excess of the normal voltage. This unwanted effect is called as "creeping in energy meter".

Read Here : How Energy Meter Works?


How to reduce creeping error in energy meter?

To prevent such creeping of the meter two holes or slots are cut in the disc on opposite sides of the spindle. The disc tends to remain stationary when one of the holes comes under one of the poles of the shunt magnet. In some cases a small piece of iron wire is attached to the edge of the disc. The force of attraction of the brake magnet upon this iron wire is sufficient to prevent the creeping of the disc under no load condition.
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