Showing posts with label Power Systems. Show all posts
Showing posts with label Power Systems. Show all posts

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.

Line to Line Fault Definition, Fault Current Calculation

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.

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and efficient compared to all poly phase systems.Single phase system has some limitations and drawbacks so it's been replaced by three-phase system.Here we listed out major reasons why 3 phase/poly-phase system is better than single phase system.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.
disadvantages of three phase system
advantages of 3 phase system pdf
disadvantages of three phase supply
why is 3 phase power more efficient
difference between single phase and three phase supply pdf
advantages and disadvantages of single phase transformer
necessity of three phase system

Electrical bus -bar and types of bus - bar arrangements and its advantages and disadvantages

Electrical Bus - Bar And Types Of Bus - Bar Arrangements And Its Advantages And Disadvantages

What is a bus bar?

A bus - bar is a conductor or group of conductors and it collects electric energy from incoming feeders and distributes them to outgoing feeders. So bus - bar is a junction where all incoming and out going currents meet. Bus - bar is generally made up of aluminium but not with copper because aluminium has special characters like higher conductivity, lower cost, excellent corrosion resistance, etc.

Criteria For Choosing Type Of Bus - Bar Arrangement:

We have different types of bus - bar arrangements we need to choose the required arrangement. This depends on various factors such as

1. System voltage. 
2. Position of a substation in the system.
3. Reliability of supply.
4. Flexibility. 
5. cost.
6. Availability of alternative arrangements if outage of any of the apparatus happens.
7. Bus - bar arrangement should be simple and easy to maintain.
8. In case of load - growth there must be possibility to extend the system to meet the load requirements. 
9. The installation should be as economical as possible, keeping in view about the needs and continuity of supply.

Types Of Bus - Bar Arrangements:

1. Single bus-bar arrangement.

2. Single bus-bar arrangement with bus sectionalized.

3.Double bus arrangement.

4. Double bus double breaker arrangement.

5. One and a half Breaker arrangement.

6. Main and transfer bus arrangement.

7. Double bus system with bypass isolators.

8. Ring main arrangement.

9. Mesh arrangement.

Let us see each of bus - bar arrangements in detail now.

Note: Through outgoing feeders power is supplied to loads.

Single bus - bar arrangement:

This single bus - bar arrangement consists of only one bus - bar and all the incoming feeders and outgoing distributors are connected to this bus - bar only. All the fuses, circuit breakers, generators and transformers are connected to this as shown in the following figure.

Advantages of single bus - bar arrangement:

1. It is easy in operation.

2. Initial cost s less.

3. Requires less maintenance.

Disadvantages of single bus - bar arrangement:

1. When damage occurs then there will be whole interruption of power supply.

2. Flexibility and immunity are very less.

Single bus - bar arrangement with bus sectionalized:

In  single bus-bar arrangement with bus sectionalized we divide a single bus - bar into two sections with the help of a circuit breaker and isolator switches and load is distributed equally among both sections as shown in the following figure:

Advantages of sectionalized single bus - bar arrangement:

1. As we are using circuit breaker to divide a bus - bar into two sections fault on one section will not interrupt power supply on the other section only few loads will have lac of power supply.

2. The fault level can be reduced by adding a current limiting reactor.

Disadvantages of sectionalized single bus - bar arrangement:

1. We are using extra isolators and circuit breakers so that the cost will be high.

Double bus arrangement:

Double bus arrangement has two bus bars and the incoming feeders and outgoing feeders are connected in parallel to both buses with the help of isolators. By closing the isolator switch we can connect the feeders either to bus - bar 1 or to bus - bar 2 . We can divide the load among two buses with the help of isolator switches by closing the isolator switch that is connected to bus - bar 1 and feeder the load can be connected to bus - bar 1 and by closing the isolator switch connected to bus - bar 2 and feeder the load gets connected to bus - bar 2.  This can be shown in following figure:

We have a bus coupler breaker which is used for bus transfer operation. When we need to transfer load from one bus to other bus we need to close the bus coupler first and then close isolators of the associated bus to which load is to be connected and open the isolator switch coupled to fault bus and then open the bus coupler breaker.

Advantages of double bus system:

1. It has greater flexibility.

2. During fault conditions there is no interruption of power supply to load.

Disadvantages of double bus system:

1. We cannot transfer load from one bus to other bus without interruption of power supply for few minutes.

Double bus double breaker arrangement:

In double bus double  breaker arrangement we connect a feeder in parallel to both buses with the help of two circuit breakers and isolator switches instead of bus coupler as shown in the following figure:
Here we energize both the feeders and divide feeders among both the buses but we can connect desired feeder to desired bus at any time for this purpose we need to close the isolator and then circuit breaker associated with the required bus - bar and later open the circuit breaker and then isolator from which is has to be disconnected.

Advantages of double bus double breaker system:

1. During fault conditions load can be transferred to one bus so there will not be interruption in power supply. 

2. Here we are not using a bus coupler so there will not be much delay in power supply while closing circuit breaker to transfer load from one bus to other bus.

3. High flexibility.

Disadvantages of double bus double breaker system:

1. The number of circuit breakers used are high so cost is very high.

2. Maintenance cost will also be high.

So this type of arrangement is used very rare.

One and a half breaker arrangement:

Because of high cost of double bus double breaker arrangement we use one and a half breaker arrangement. Here the two feeders are fed through their corresponding bus - bars and these two feeders are coupled by a third circuit breaker called tie - breaker as shown in the following figure:
During normal conditions all the three circuit breakers are closed and the both circuits operate in parallel and power is fed to feeders from the two bus - bars. If fault occurs on one bus bar then  with the help of second  bus - bar feeder circuit breaker and tie breaker power is fed to feeders. This means each feeder breaker has to be rated to feed both feeders which are coupled by tie breaker.

Advantages of one and a half breaker system:

1. There will be no interruption of power in case of fault because all the feeders can be transferred to other healthy bus immediately.

2.  Additional circuits can be easily added to the system.

3. Cost is less compared to double bus double breaker arrangement.

Disadvantages of one and a half breaker system:

1. This arrangement is complicated because during fault two circuit breakers are to be opened. 

2. Maintenance cost is high.

Main and transfer bus arrangement:

In main and transfer bus arrangement we have two buses one is main bus and the other is transfer bus. With the help of isolator switches it is connected to the transfer bus which are called bypass isolators and with the help of circuit breakers and isolator switches it is connected to the main bus. There is also bus coupler as shown in the following figure:

In normal conditions the feeders are fed through main bus but during fault conditions load is transferred to the transfer bus. In order to transfer load from main bus to transfer bus we need to close the bus coupler first and then close by pass isolators of feeder to be connected to transfer bus and open the isolator switch of feeder coupled to main bus and then open the bus coupler breaker.

Advantages of main bus and transfer bus system:

1. No interruption of  power supply because in case of fault load can be shifted to transfer bus.

2. Load can be divided into two groups since they can be feed from either of the buses.

Disadvantages of main bus and transfer bus system:

1. Two bus - bars are used which increases the cost.

Double bus arrangement with by pass isolators:

Double bus system with bypass isolators is combination of double bus system and main and transfer bus system. Here the feeders are connected to both the buses with the help of isolators during fault conditions loads can be transferred to healthy bus by closing isolators of feeders associated to healthy bus and opening isolators of feeders associated to faulty bus. The connection diagram is given below:
A bus coupler is also provided so while transferring load to healthy bus close the bus coupler breaker first and close the isolator of feeder to which it has to be transferred and open the isolator switch of feeder coupled to fault bus and then open the bus coupler breaker.

Advantages of double breaker system with by pass isolators:

 1. No interruption of  power supply because in case of fault load can be shifted to transfer bus.

2. Load can be divided into two groups since they can be feed from either of the buses.

Disadvantages of double breaker system with by pass isolators:

1. cost is high as we are using two bus bars and extra isolator switches.

2. Complex in nature.

Ring main arrangement:

Ring main arrangement provides double feed to each feeder circuit. Here the end of the bus bars is returned upon themselves to form a ring. Hence it is called main ring arrangement. This arrangement is shown in the following figure:
Here if one circuit beaker is damaged it is opened and the feeder can be supplied from the other circuit breaker which is near to it.

Advantages of ring main system:

1. Since each feeder is fed from two circuit breakers even if fault occurs in one system the feeder can be fed from other system so there will be no interruption of power supply under fault conditions.

2.The effect of fault is localized to that section alone and the rest of the section continues to operate normally.

Disadvantages of ring main system:

1. There will be difficulty to add  any new circuit in the ring.

2. Over loading problems may occur.

Mesh arrangement:

In mesh arrangement between the mesh formed by bus bars circuit breakers are installed as shown in the following figure:
From the node point of mesh circuit is tapped. We need to open two circuit breakers when the fault occurs so that protection can be obtained but switching is not possible.

Advantages of mesh system:

1. Provides protection against fault.

2. For substations having large number of circuits this arrangement is suitable.

Disadvantages of mesh system:

1. It doesn't provide switching facility.

2. Not suitable for all type of substations.

These are the different sub station layouts or bus bar arrangements.

In this post we have learnt about electrical bus -bar and types of bus - bar arrangements and its advantages and disadvantages.

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HRC Fuse Operation,Types And Characteristics

Operation,Types And Characteristics Of HRC Fuse

Fuse is a common switch gear device which we see even at our homes. It is used to protect a device or circuit from over currents.  For household purpose usage of HRC fuse is more economical than usage of circuit breakers. Let see a clear picture about what is fuse? how a fuse works?

What Is Meant By HRC Fuse?

HRC fuse means high rupturing capacity fuse which is a modern type fuse. It interrupts current flow and protects from over current.  It is used to provide protection from short circuit damages in low voltages and medium level voltages.

Construction Of HRC Fuse:

The body of HRC fuse is made of ceramic which is highly heat resistant. It has two end caps to this caps a silver current carrying element is welded. The internal space of fuse is filled with powder material such as plaster of paris, marble, chalk, cooling media etc. as shown in the following diagram.
The above diagram shows the cross-sectional diagram of a HRC fuse.

Working Of HRC Fuse:

The fuse wire inside the HRC fuse conducts the short circuit current for a period of time safely. During this time if the fault is removed the fuse doesn't blow off. But if the fault is not removed the fuse will melt and isolates the circuit from the electrical supply as shown in the below figure.
This is how a fuse works

Controlling Of Arc In HRC Fuse:

The inner portion of HRC fuse consists of cooling medium which helps to carry normal current by the fuse wire without heating. When over current flows then heat is produced in the current carrying fuse wire because of high I2Rf loss. This heat vaporizes the silver metal element and a chemical reaction takes place between this silver metal and filling powder which produces a high resistance substance. This high resistance substance helps in arc quenching.

Characteristics Of HRC Fuse:

As discussed for normal current the fuse wire doesn't melt but when over current flows due to high I2Rf loss the fuse wire melts.So fuse wire melts faster for higher fault currents and takes more time to melt for lower fault currents. This gives the time-current characteristics of HRC fuse. This is shown in the following figure.

Types Of HRC Fuse:

We have different types of HRC fuse. They are:

1.Semi- enclosed or rewireble type.

2.Totally enclosed or cartridge type.  

3. Current limiting fuse link. 

4. Drop-out fuse.

5. Explosion fuse.

6. Striker fuse.

7. Switch fuse.

Now let us see each of them in detail.

1. Semi- enclosed Or Rewireble Type Fuse:

In this type of fuse the carrier of fuse can be pulled out and the melted fuse wire can be replaced with the new wire. Carrier has to be replaced in the fuse base. This type of fuse is used in our houses. Its diagram is shown below.

2. Totally Enclosed Or Cartridge Type Fuse:

In this type we have a fuse wire inside a totally closed container and it has metal contacts on either sides. This is further divided into two types.

1. Bolted type 2. D-type.
The above diagram is cartridge type fuse. 

3. Current Limiting Fuse Link:

This fuse link brings the current to a value lower than a prospective value.

4. Drop Out Fuse:

After fault current flows,fuse-carrier drops out. Now there will be isolation between the terminals. This diagram is shown below.

5. Explosion Fuse:

In this type of fuse the arc is quenched when produced arc produces heat which vaporizes the metal and chemical reaction is established between this vapors and powdered filling. This produces a high resistance substance which helps the produced arc to quench.

6. Striker Fuse:

This type of fuse consists of combination of a fuse and a mechanical device. This fuse releases a striker after fuse operation with a certain displacement and pressure. The following figure shows diagram of striker fuse before and after fuse operation.

7. Switch Fuse:

This is a fuse which contains both switch and fuse. This is shown in following figure.

In this post we have discussed about the operation, types and characteristics of HRC fuse.

Power Factor | It's Calculation, Power Factor Improvement Methods

Power Factor | Calculation and Power Factor Improvement Methods

What Is Electrical Power Factor ?

The cosine of angle between voltage and current in an a.c. circuit is known as power factor. In an a.c. circuit, there is generally a phase difference φ between voltage and current. The term cos φ is called the power factor of the circuit. If the circuit is inductive, the current lags behind the voltage and the power factor is referred to as lagging. However, in a capacitive circuit, current leads the voltage and power factor is said to be leading.

Consider an inductive circuit taking a lagging current I from supply voltage V; the angle of lag being φ. The phasor diagram of the circuit is shown in fig. The circuit current I can be resolved into two perpendicular components, namely ;

(a) I cos φ in phase with V
(b) I sin φ 90°  out of phase with V

The component I cos φ is known as active or wattful component, whereas component I sin φ is called the reactive or wattless component. The reactive component is a measure of the power factor. If the reactive component is small, the phase angle φ is small and hence power factor cos φ will be high. Therefore, a circuit having small reactive current (i.e., I sin φ) will have high power factor and vice-versa. It may be noted that value of power factor can never be more than unity.

(i) It is a usual practice to attach the word ‘lagging’ or ‘leading’ with the numerical value of power factor to signify whether the current lags or leads the voltage. Thus if the circuit has a p.f. of 0·5 and the current lags the voltage, we generally write p.f. as 0·5 lagging.

(ii) Sometimes power factor is expressed as a percentage. Thus 0·8 lagging power factor may be expressed as 80% lagging.

Power Triangle 

The analysis of power factor can also be made in terms of power drawn by the a.c. circuit. If each side of the current triangle oab of below figure is multiplied by voltage V, then we get the power triangle OAB shown in figure where
OA = VI cos φ and represents the active power in watts or kW
AB = VI sin φ and represents the reactive power in VAR or kVAR
OB = VI and represents the apparent power in VA or kVA
The following points may be noted form the power triangle :

(i) The apparent power in an a.c. circuit has two components viz., active and reactive power at right angles to each other.
OB² = OA² + AB²
or (apparent power)² = (active power)² + (reactive power)²
or (kVA)²= (kW)² + (kVAR)²

(ii) Power factor, cos φ = OA/OB = active power/apparent power = kW/kVA
Thus the power factor of a circuit may also be defined as the ratio of active power to the apparent power. This is a perfectly general definition and can be applied to all cases, whatever be the waveform.

(iii) The lagging reactive power is responsible for the low power factor. It is clear from the power triangle that smaller the reactive power component, the higher is the power factor of the circuit.
kVAR = kVA sin φ = (kW * sin φ)/cos φ
∴ kVAR = kW tan φ

(iv) For leading currents, the power triangle becomes reversed. This fact provides a key to the power factor improvement. If a device taking leading reactive power (e.g. capacitor) is connected in parallel with the load, then the lagging reactive power of the load will be partly neutralized, thus improving the power factor of the load.

(v) The power factor of a circuit can be defined in one of the following three ways :

(a) Power factor = cos φ = cosine of angle between V and I

(b) Power factor = R/Z= Resistance/Impedance

(c) Power factor = VI/(VI * cosφ) = Active power/Apparent Power

(vi) The reactive power is neither consumed in the circuit nor it does any useful work. It merely flows back and forth in both directions in the circuit. A wattmeter does not measure reactive power.

Disadvantages of Low Power Factor

The power factor plays an importance role in a.c. circuits since power consumed depends upon this

P = VL IL cos φ

It is clear from above that for fixed power and voltage, the load current is inversely proportional to the power factor. Lower the power factor, higher is the load current and vice-versa. A power factor less than unity results in the following disadvantages :

(i) Large kVA rating of equipment. The electrical machinery (e.g., alternators, transformers, switchgear) is always rated in kVA.
Now, kVA = kW/cos φ
It is clear that kVA rating of the equipment is inversely proportional to power factor. The smaller the power factor, the larger is the kVA rating. Therefore, at low power factor, the kVA rating of the equipment has to be made more, making the equipment larger and expensive.

(ii) Greater conductor size. To transmit or distribute a fixed amount of power at constant voltage, the conductor will have to carry more current at low power factor. This necessitates large conductor size. 

(iii) Large copper losses. The large current at low power factor causes more I²R losses in all the elements of the supply system. This results in poor efficiency.

(iv) Poor voltage regulation. The large current at low lagging power factor causes greater voltage drops in alternators, transformers, transmission lines and distributors. This results in the decreased voltage available at the supply end, thus impairing the performance of utilization devices. In order to keep the receiving end voltage within permissible limits, extra equipment (i.e., voltage regulators) is required.

(v) Reduced handling capacity of system. The lagging power factor reduces the handling capacity of all the elements of the system. It is because the reactive component of current prevents the full utilisation of installed capacity. The above discussion leads to the conclusion that low power factor is an objectionable feature in the supply system.

Causes of Low Power Factor

Low power factor is undesirable from economic point of view. Normally, the power factor of the whole load on the supply system in lower than 0·8. The following are the causes of low power factor:

(i) Most of the a.c. motors are of induction type (1φ and 3φ induction motors) which have low lagging power factor. These motors work at a power factor which is extremely small on light load (0·2 to 0·3) and rises to 0·8 or 0·9 at full load.

(ii) Arc lamps, electric discharge lamps and industrial heating furnaces operate at low lagging power factor.

(iii) The load on the power system is varying ; being high during morning and evening and low at other times. During low load period, supply voltage is increased which increases the magnetization current. This results in the decreased power factor.

Power Factor Improvement Methods 

The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents. In order to improve the power factor, some device taking leading power should be connected in parallel with the load. One of such devices can be a capacitor. The capacitor draws a leading current and partly or completely neutralists the lagging reactive component of load current. This raises the power factor of the load.

Insulators | Types of Electrical Insulators in Overhead Lines

The overhead line insulators provide insulation between the overhead line conductors and the earthed cross-arm to which they are connected and also provide support to the conductors.An ideal insulator should not allow leakage of current from the line conductor to the earthed cross-arm besides providing adequate clearance between the line conductors and metal work.

Types of insulators in overhead Lines

Depending on the applications and the operating voltage levels ,the overhead line insulators are divided into three types.They are, 

(i) Pin type insulators 
(ii) Suspension type insulators and
(iii) Strain type insulators 

(i) Pin type insulators- Electrical Insulators 

Pin type insulators are used for supporting line conductors.For low voltages up to and including 11 KV one piece insulators can be used and two piece insulators are commonly used for 25 KV and for 44 KV up to and including 66 KV three and four piece insulators can be employed.The pin type insulator figure shown in fig.

Pin type insulator consists of single or multiple shells arranged in parallel and mounted on a cross-arm of the pole or tower.The designing of sheds or petticoats should be such that even when the outer surface is wet due to rain,the inner dry surface provides sufficient leakage resistance.On the top of the insulator the conductor bound into a groove,which is cemented on to a galvanized steel pin.A soft metal thimble is used to avoid the direct contact between the porcelain and the metal pin.The potential difference between the sheds can be increased by increasing distance between the sheds.

Advantages Of Pin type insulators :

1.Pin type insulators  are simple in construction and cheap in cost.
2.It provides economic and most efficient method of supporting conductors.
3.In many cases on pin type insulator is used instead of two suspension type insulators.

Disadvantages Of Pin type insulators :

For voltages below 50 KV,they give satisfactory performance ,beyond that,they become bulky and uneconomical.

(ii) Suspension type insulators- Electrical Insulators

At high voltages pin type insulators becomes more bulky,cumbersome and uneconomical.In this case,suspension type insulators are used to insulate the high voltage transmission lines.Normally,these are used up to and including 400 KV .The suspension type insulators shown in fig.

Suspension type insulators consists of porcelain discs arranged in series as shown fig.,by metal links to form a string.The conductor is suspected at its lower end  and it's other end hangs from cross-arm of the supporting structure tower.Thw whole arrangement of a suspension type insulators are called as string(String Efficiency).The number of insulators in a string depends upon the working voltage,weather condition,size of insulation etc.,and if necessary,the discs to that string can be added easily.

Advantages of Suspension type insulators:

1.If any one of the insulator in the string fails, only that insulator can be replaced instead of whole string. 
2.These are much cheaper in cost and used for operating voltages above 50 kV.
3. It gives more flexible operation and by this arrangement the mechanical stresses are reduced.
4. In suspension type insulator, additional insulation can be obtained by adding one or more discs to that string. 
5.When these are used in conjunction with the steel towers, provides protection against lightening. 

(iii) Strain type insulator- Electrical Insulators 

Strain type insulators are used when the line is subjected to greater tension such as dead ends,river crossings,sharp curves and when there is a change in the direction of line.The main function of this insulator is to reduce excessive tension on the line. It is basically an assembly of suspension insulator used in horizontal position as shown in figure.

A strain insulator are designed with considerable good strength and with necessary dielectric properties. In case, when the tension is exceedingly high, two or more strings of insulator can be used in parallel.For voltages up to 11 KV,Shackle type insulator can be used.But for high voltages,strain type insulators should be used.strain type insulators also called as tension insulators. 

In addition to that there are other two types of electrical insulator available mainly for low voltage application, e.i. Stay Insulator and Shackle Insulator.

Tags: what is an insulator of electricity,different types of electrical insulators pdf,types of insulators wikipedia

String Efficiency & It's Significance In Overhead Transmission Lines

String Efficiency & It's Significance In Overhead Transmission Lines 

Efficiency is a factor that defines the performance of any device or equipment. Generally, it is specified as the ratio of the output obtained to the input fed to a device. It suggests to what extent a device is utilized. 

In an overhead transmission line (OHTL) insulator, a number of insulator discs are arranged in a string in accordance with the operating voltage. The lowest disc or unit which is i near the power conductor experiences maximum stress or it is fully utilized. But, as we move towards the top most unit, the units are not heavily stressed and hence not fully utilized. Hence, the measure of utilization of the string gives string efficiency. Definition The string efficiency of an over head transmission line insulator is defined as the ratio of the voltage across the complete string to that of the product the number of units and the voltage across the unit nearest to the power conductor. 
It is given as, 

η=Voltage across the string/{ n x Voltage across the unit nearest to the conductor}

n= No.of units in the string.

 It can also be defined as the ratio of the spark over voltage of the complete string to that of the product of the number of discs and the spark over voltage of one disc. 

Significance of String efficiency 

String efficiency is an important factor as it suggests the overall potential distribution along the string of insulators. For obtaining uniform voltage distribution, the string efficiency should be as good as possible. if the distribution of voltage is uniform, then each disc will be fully and equally utilized. This also implies that, the discs in the string will experience equal stress so that the life of the string is enhanced.

In ideal case, the string efficiency is maximum and the voltage distribution across each disc will be exactly the same. But in real time situation, it is impossible to achieve 100% string efficiency. Efforts' can be made to improve string efficiency. If the value of 'V (i.e., ratio of mutual capacitance to self capacitance) is decreased then the potential distribution is more uniform and string efficiency is increased. The non-uniformity in voltage distribution increases with the increase in number of discs in the string. Therefore, shorter string has more efficiency than the larger one.

Tags:string efficiency in power system,string efficiency formula,string efficiency of suspension insulators,string efficiency in power system pdf,string efficiency transmission line improving methods