## Line to Line Fault Definition, Fault Current Analysis, Calculation

#### Line to Line Fault Definition, Fault Current Calculation

What are the points to be covered?
• What is line to line fault?
• How line to line fault occurs?
• Fault current in line to line fault.
• Calculate  line to line fault current.

#### What is Meant by Line to Line Fault ?

When two conductors of a 3 phase system are short circuited line to line fault or unsymmetrical fault occurs.This fault is severe compared to symmetrical faults in power system.In order to safeguard power system network analysis of unsymmetrical faults (in this case line to line fault) must be done.

#### Fault Current Calculation in Line to Line Fault

Consider a 3-phase system with an earthed neutral.Assume a line-to-line fault between the blue (B) and yellow (Y) lines as shown in figure The conditions created by this fault lead to :
Again taking R-phase as the reference, we have,
Take above result as equation (i)

Take above result as equation (ii)

Note :: From above steps it is clear that the line-to-line fault the zero sequence component of current I0 is equal to zero in line to line fault.

Fault Current in Line-to-Line Fault: Examination of exp. (i) and exp (ii) reveals that sequence impedance should be connected as shown in above figure.It is clear from the figure that :
Phase Voltages. Since the generated e.m.f. system is of positive phase sequence only, the sequence
components of e.m.f. in R-phase are :
Summary: For line-to-line fault (Blue and Yellow lines) :

Above what is line to line fault and how to find line to line current fault current are discussed.

## 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 :

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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.

#### 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.

## 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.

## 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.

### 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.

#### 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.

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

## 9 Advantages of Three Phase System Over Single Phase System

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.

### 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 Supply Three Phase supply power delivered is pulsating Power 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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