Electrical Power System – APSEEE

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Generation of Electrical Energy

Generation of Electrical Energy

An electrical energy is produced or generated by generator. The generator is coupled with prime mover prime mover is a mechanical rotating device that rotates the generated. Prime mover takes energy from different kinds of sources. The name of generating station is depends upon types of sources used.

generation of electrical energy

The different generating stations are

  • Diesel power generating station
  • Thermal power generating station
  • Hydro electric power generating station
  • Nuclear power generating station

Diesel Power Generating Station

In such type of power stations diesel engine is used as the prime mover for the generation of electrical energy. These power stations are used where other sources of energy is not available (such as, coal water etc). Diesel power stations are finding favour at places where demand of electric power is less.

Advantages of Diesel Power Station

  1. The design is quite simple.
  2. It occupies less space.
  3. It can be located at any place.
  4. It can be started quickly and pick up load in short time.
  5. Its cost is very small as comparatively other stations.

Disadvantages

  1. The running cost of this plant is very high.
  2. The maintenance charges are generally high.

Thermal Power Generating Station

In such power station coal is used as fuel in boiler and steam is produced which is used to rotate the turbines. An alternator is coupled with turbines or prime mover which also rotates and electric power is generated.

Advantages

  1. The fuel used is cheap in cost.
  2. It requires less space as compared to hydroelectric power station.
  3. The cost of generation of electric energy is less as compare to diesel power station.

Disadvantages

  1. It pollutes the atmosphere.
  2. Running cost is high as compare to hydroelectric power station.

Hydroelectric Power Generating Station

A generating station in which potential energy of water is used to run trubines is called hydroelectric power plants. These plants are constructed where water is available in abundance. These plants are located in hilly areas.

Advantages of Hydroelectric Power Generating Station

  1. The useful life of this plant is around 50 year.
  2. Running cost is low.
  3. There is no stand by loss.
  4. These plants are free from air pollution

Disadvantages

  1. High initial cost
  2. Generation of electrical energy is used to generation of electrical energy is called nuclear power generating station.

Nuclear power generating station

In this type of plant, nuclear fuel such as uranium is used as fuel to generate electrical energy.

Advantages

  1. The amount of fuel required is small.
  2. A nuclear power plant requires less space.
  3. The operating cost is low.
  4. It can be located near the load centre.

Disadvantages

  1. Skilled personnel are required.
  2. The fuel used is expensive.
  3. The disposal of radioactive material is big problem.

 

 

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Underground Cables

Underground Cables

Underground Cables are used for transmission and electric poser in congested areas (Such as in cities, towns etc) at comparatively low at medium voltages. Usually, the underground cables used in congested cities or towns, substations, railway crossings and where safety is very important. However, recent improvements in the design and manufacture have led to the development of cables suitable for use at high voltage. In this article, we will discuss about definition of underground cable and construction of underground cables.

Define Cables

A conductor covered with a suitable insulation and protecting layer is called cable.

Requirement of Underground Cables

  • The conductor used in underground cable should be standard in order to provide flexibility. The conductor should have high conductivity so that current flows easily through them.
  • The area of cross section should be such that the cable carries the desired load current without overheating.
  • The voltage drop in conductor should be within permissible limits.
  • The cable must have proper thickness of insulation.
  • The cable must be provided with suitable mechanical protection.
  • The cable do not react with chemicals.

Construction of Cables

The cable have various parts. There are given below.

Conductors

The cable consists of one or more than on core. It depends upon the typ0es of service for which it is intented. The conductors are made of tinned copper or aluminium . The stranded conductor are used in order to increase flexibility.

underground cables

Insulation

Each conductor or core covered with proper thickness of insulation. Basically thickness of insulation depends upon the voltage level. The commonly used insulating materials are impregnated paper, varnished cambric etc.

Metallic Sheath

A metallic sheath is provided over the insulation in order to protect the cable from moisture, other liquids that are present in soil and atmosphere. The commonly used material for metallic sheath is lead or aluminium.

Bedding

Over the metallic sheath bedding layer is provided which consists of a fibrous metarial like jute or hessian tape. Bedding protects the metallic sheath against corrosion and from mechanical injury due to armovring.

Armouring

Armouring is provided over the bedding which consists of one or two layers of galvanized steel wire. It protect the cable from mechanical injury.

Serving

In order to protect armouring from atmospheric conditions, a layer of fibrous material provided over the armouring. This layer is called serving.

 

 

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Buchholz Relay

Buchholz Relay

Buchholz relay is a protective and gas actuated relay . It is used in that transformer whose winding is  immersed in oil and rating having more than 500KVA. Buchholz relay is not used in small transformer because it increase the cost. It is used to protect transformer against internal or incipient faults only. Basically, it is used for detection of winding faults.

Buchholz relay is invented by Buchholz in 1921.

Buchholz Relay Construction

Buchholz relay is installed in the pipe connecting the conservator to the main tank. There are two mercury switches used in the relay. One of the mercury switches is attached to the upper float. This switch send signal to the alarm circuit when relay operates. Whereas the other switch is mounted on lower hinged type flap. This switch send signal to the trip circuit. A release cock is mounted top of the chamber.

Buchholz Relay

Diagram

Operation or Working of Buchholz Relay

When a minor fault occurs in a transformer, heat is produced, oil gets heat up. Hydrogen gas is produce in transformer. The gases being light, try to go into conservator tank but gas is trapped in Buchholz-Relay which is connected between main tank and conservator tank. It is collected in top of the chamber while passing to the conservator tank. When the predetermined amount of gas is accumulate in the upper part of the relay, oil level falls due to gas pressure. This tilt the upper float downward and closes the alarm circuit contacts through mercury switch attached to the float arm. The alarm rings bell and gives the warning signal.

In case of series fault within the transformer, large amount of gas in evolved in the main tank. In this time, lower mercury switch of the buchholz relay operates and sends the signal to the trip circuit. This completes the trip coil circuit of the circuit breaker. Thereafter, the transformer is disconnected automatically.

A release cock is provided at the top of the relay chamber so that after operation, gas pressure is released through this cock.

‘Buchholz Relay’ indicates internal faults or incipient faults in the transformer, such as inter turns fault, insulation failure. It does not respond to external faults.

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Bus-Bar Arrangements

Bus-Bar Arrangements

When a number generators or feeders have same voltage then there is necessity to connect all unit electrically, bus-bars are used as the common electrical component. In this article, we will discuss about different bus-bar arrangements.

What is Bus-Bar?

Bus-Bar are thick copper rods which is operate at constant voltage and carrying an electric current to which many co9nnections may be made.

The Bus-bar is arranged in different manner. The different bus-bar arrangements are given below.

  1. Single Bus-Bar Arrangement

It is a simplest form of arrangement of bus-bar. It is used in power stations and small outdoor substations having small number of incoming and outgoing feeders. Each generator and feeder is controlled by a circuit breaker.

single bus-bar arrangements

Advantages

It has low initial cost.

It required less maintenance.

Simple operation.

Disadvantages

If fault occurs on bus-bar, whole supply is affected.

For repair and maintenance of the bus-bar, whole of the system has need to be de-energized.

Single Bus-Bar Arrangement with Sectionalization

In large power generating stations, where several numbers of generators and feeders are required to be connected to the bus-bar. In that cases, Single bus-bar arrangement with sectionalization is employed. Normally the number of sections of a bus-bar is 2 to 3 in generating station and substation.

Advantages

  1. In case, when fault occurs on any section faulty section can be disconnected without affecting other section.
  2. This arrangement is more reliable that single bus-bar arrangement.
  3. The repair and maintenance of any section of the bus-bar can be carried out be disconnecting that section only.
  4. Future extension is possible in this arrangement.

Disadvantages

In this arrangement additional circuit breakers and isolators are required for sectionalisation. Hence, cost is increased.

Ring Bus-Bar Arrangement

In this arrangement, each feeder is supplied from two paths. This is an extension of the sectionalized arrangement.

Advantages

This arrangement provides greater flexibility. In case, fault occurs , it does not affects the other section.

The number of circuit breaker required in this arrangement almost same as in a single phase bus-bar system. It reduces the initial cost.

Disadvantages

Extension is not possible in this arrangements. Separately protection system is required for each circuit.

Main and Transfer Bus-Bar Arrangements

Such a arrangement consists of two bus-bars a “main bus-bar” and “an auxiliary bus-bar. This arrangement is adopted where continuity of supply. Each generator and feeder may be connected to either bus-bar with the help of bus coupler which consists of a circuit breaker and isolating switches.

Advantages

If fault occurs on the bus-bar, supply can be maintained by transferring it to the healthy.

The other feeders can be connected to the bus-bars without disturbing the existing system.

 

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Advantages of HVDC Transmission

Advantages of HVDC Transmission

The electric energy can be transmitted either by mode of ac or dc. Now in these days, ac supply is transmitted. But high voltage dc system is more superior to ac transmission system. In this article, we will discuss about advantages of HVDC transmission or high voltage dc transmission . These advantages are as following.

Cheaper Cost
In case of dc transmission only two conductors are required, unlike ac transmission two lines required lesser in case of dc system. Tower design is simpler and the size is smaller . For the same operating voltage, less insulation is required in dc system. Hence insulation cost in less than ac system  . This makes the HVDC system cheaper than ac system.

Less Corona Loss
In case of dc system, frequency is zero, corona loss is proportional to (f+25). Hence, in HVDC system, corona is lesser than ac system for the same conductor diameter and supply voltage.

No Skin Effect
There is no skin effect in a dc system. The current is uniformly distributed over the surface of conductor  in case of HVDC system. Thus, there is complete utilization of conductor in dc system.

No Stability Problem
In dc system, there are no stability problems.

Lesser Dielectric Loss
The insulation required in HVDC lines is considerably low. Therefore, less dielectric loss in dc system. Due to less dielectric loss, current carrying capacity is higher in case of dc system. This improves the overall efficiency.

Surge Impedance Loading
In high voltage ac system, the transmission line is loaded up to less than 80% of natural load. In case of dc system problem is not exists.

Surge Impedance Loading ZC is given as

advantages of hvdc transmission Voltage Regulation
Due to absence of inductance in dc system, the voltage drop in this system is less than the ac transmission system. Therefore, voltage regulation in dc system is better than dc system.

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Vacuum Circuit Breakers (VCBs)

Vacuum Circuit Breakers (VCBs)

Since vacuum offers highest insulating strength, so it can use as arc quenching medium. The circuit breakers that use vacuum as a arc quenching medium, known as vacuum circuit breakers (VCBs).

Construction of Vacuum Circuit Breakers

It consists of two contact one is fixed and other is movable and these contacts placed in arc shield vacuum chamber. The movable contact is connected to the control mechanism. A glass vessel or ceramic vessel is used as the body of vacuum circuit breakers.

The arc shield is provides inside surface of the insulating cover which prevent the deterioration of the internal dielectric strength.

vacuum circuit breakers (vcbs))

The pressure below 10-3mm of mercury is considered to be high vacuum. In such a low pressure mean free path of the electrons is of the order of few metres and thus when charged particles move from one electrode to other. They do not collide with residual gas molecules.

Hence, the dielectric strength of vacuum relatively higher than other medium.

Working

When faults occurs on any part of the system breaker operates and the moving contact separates from the fixed contact. When these contacts  separate from each other, arc is struck between them. This results, hot spots are created at the contacts surface and metal vapour shoot off constituting plasma. The amount of vapour in plasma depends on vapour emission from electrodes and arc current. The arc is extinguished in vacuum because the vapours and ions produced during arc are differed in a short time and seized by the surface of moving and fixed contacts and shields. The contacts are so designed that the temperature at one point on the contact does not reach a very high value.

Applications of vacuum circuit breakers(VCBs)

  • The use of these circuit breakers found in transformer switching, capacitor bank switching etc, where high voltage and small current has interrupted.
  • These circuit breakers use in substations.

Advantages

  • These circuit breakers are light in weight.
  • These are very small in size.
  • These circuit breakers have very long life.

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Valve type Lightning Arrester

Valve type Lightning Arrester

Valve type lightning arrester is extensively used on system operating at high voltage. It consists of two parts.

Series spark gaps

Non-linear resistor discs

The non-linear resistor is connected in series with the spark air gap. Both the assemblies are accommodated in tight porcelain container. It is also known as non-linear surge diverter.

The spark gap consists of a number of identical elements connected in series. Each gap consists of two electrodes with additional resistance element (grading resistor) having high ohmic value connected in parallel. Grading resistor is used to linearised the voltage distribution across the gap.

This construction is similar to that of a number of capacitors connected in series.

The spacing of the series gap is such that it will with stand the normal circuit voltage.

The non-linear resistance is usually made of silicon carbide disc. This material posses high resistance to how current and low resistance to high current. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice versa. The discs are 90mm in dia and 25mm thick. The characteristics of non-linear resistor is usually expressed as I = KVn, Where n is lie between 2 and 6 and K is constant.

Valve type Lightning ArresterValve type Lightning Arrester

Working

Under normal conditions, when system voltage is normal, there is no affect on the spark air gap at this time, air gap remains in non-conducting state.

On the occurrence of an over voltage or voltage surge, the break down of the series spark gap takes place and the surge current start flows through the non-linear resistor to earth. Since magnitude of surge current is very large, so that non-linear resistors will offer a very low resistance path to the passage of surge. When the surge is over, the value of non-linear resistance become high and stops the flow of current through them.

Advantages

  • The maintenance cost is low.
  • They do not required daily supervision.
  • The operation of this arrester is very fast.
  • The impulse ration is practically unity.
  • They provide effective protection against surges.

Disadvantages or Limitations

If moisture enters with enclosure, it affects the performance of arrester.

Applications or Uses

Power station operating on voltage upto 220KV.

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Advantages of High Transmission Voltage

Advantages of High Transmission Voltage

Electrical energy is generated at generating power station at voltage level 11KV. The generating power stations are for away from the consumer premises or load centre. The transmission lines are used to transmit electrical energy from generating station to load centre. If we transmit electric power at 11KV, then the line losses will be increased. So, we increased voltage level to reduce the line losses. In this article, we will discuss about the advantages of high transmission voltage.

The electric power delivered in a 3-phase system is given by the relation.

V= Line voltage in voltage

I = Line current in ampere

CosΦ = Power factor of the load

advantages of high transmission voltage

Volume of conductor material required for the line:

advantage

The following are the advantages of high transmission voltage.

Reduces Volume of Conductor Material

The volume of conductor required for transmission line is inversely proportional to the square of transmission voltage and power factor. In other words we can say that greater the transmission voltage, lesser conductor material required. This decreases the cost of transmission line.

This also reduces the cost of supporting structure. Because the towers with high voltage level has less tension or stress. There is also saving in supporting structure material.

Higher Transmission Efficiency

The current in the transmission line is inversely proportional to the transmission voltage. If the transmission voltage increases at same power, the current is decreases.

Greater the transmission voltage, smaller the current. Hence, power loss reduces and the transmission efficiency is improved.

In this way, we can say that the transmission line efficiency greatly depends on the transmission voltage.

Better Voltage Regulation

Transmission of electric power at high voltage, reduces the line current which result reduction in voltage drop in the transmission line. Hence, it improves the voltage regulation of the transmission line. Further, longer transmission lines can be used.

 

 

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Reactors

Reactors

The electrical energy or electricity has become a part and parcel of our life. Electricity plays an important role of our daily’s life. The demand of electrical energy is increasing day by day. To meet this demand, the power system expands and become more complex. In this way fault level is also increased. To protect the power system against fault current some equipment are used in power system (such as Reactors). In this article we will discuss about reactors, types of reactors and locations of reactors.

What is Reactor?

Reactor is a coil of number of turns having high inductive reactance as compared to its resistance. The reactors are connected in series with the system at suitable points to limit the short circuit fault currents. Iron core and air cored reactors are used. Air cored reactors are normally of two types (i) Oil immersed type (ii) Dry type.

Location of Reactors

The reactors are connected at suitable points in power system, short circuit current limiting.  Reactors may be connected in following locations.

  • Series with each generator called Generator Reactor.
  • Series with each feeder called Feeder Reactor.
  • In Bus-Bar.
  1. Generator Reactor

When the reactors are connected in series with each generator, such type of connections is known as generator reactor.

Since in modern alternators, the reactance may as high as 2.0 P.V. So there is no need to connect reactors in series with generator.generator reactor

However, if some old machines are being used along with the modern alternators, these old machines need the reactors for limiting the short circuit current.

Disadvantages

  • There is a constant voltage drop during normal operation.
  • It protects only generator, if fault occurs at feeder or bus-bar, it may affects the performance.
  1. Feeder Reactors

When the reactors are connected in series with each feeder, such type of arrangement are called feeder reactors.

Mostly faults are occurs in feeder, so there are many reactors are required in this circuits.

Per unit value of reactance of a feeder based on its rating may be small but we compared with the rating of whole power system. Its value becomes quite large and hence a small reactor will be required to limit the short circuit current.

feeder reactors

Disadvantages

  • There is a constant voltage drop and constant power loss in the reactors even during normal operation.
  • If a short circuit occurs at the bus-bar, there is no protection is provided to the generator.

Bus-Bar Reactors

The generator reactors and bus-bar reactor suffer from disadvantages such as constant voltage drop and constant power loss. To overcome this disadvantages the reactor are locating in bus-bars. There are two methods of interconnecting the bus-bar through the rectors are namely

  • Ring Main System
  • Tie Bar System.

In ring main system, the bus-bar divided in two sections. This results in low power loss in the reactors.

In tie bar system, there are effectively two reactors in series between sections.

 

 

 

 

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Electrical Faults in Overhead Transmission Lines

Electrical Faults in Overhead Transmission Lines

The occurrence of faults  in overhead transmission line is common thing. In this article, we will discuss about, electrical faults in overhead transmission lines, types of electrical faults and reasons of electrical faults.

What is an electrical Fault?               

An electrical fault is any defect in the electrical circuit due to which electric current is diverted from the intented path.

Short Circuit Fault

Short circuit is a fault in which the value of current reaches upto infinity from its normal value. This type of faults results in damaging of power system equipment such as switchgears, transformers etc. If some protective steps are not taken with in time.

Open Circuit Fault

When the path of flow of current breaks, such type of fault is called open circuit faults. This fault is less dangerous than short circuit fault.

In three phase systems, a fault may involve one or more than one phases and ground. In case of earth fault current flows into earth.

In power systems, relays can detect fault and sent a signal to the circuit breakers.

Types of Electrical Faults

The faults are divided into two types.

  • Symmetrical fault.
  • Unsymmetrical fault.

Symmetrical Faults

In Symmetrical faults, when fault occurs in system, same fault current flows in all phases or three phase. In transmission line faults, 5% are symmetrical faults. The symmetrical faults are divided as follows.

All the Three Phase Short Circuited (L-L-L)

This may occur when all line conductors are short circuited due to failure of insulation.

electrical faults in overhead transmission lines

All Three Phases to Earth (L-L-L-G)

This may occur when insulators of all the three line conductors short circuited and fall into earth.

transmission line

Unsymmetrical Faults or Asymmetrical Faults

In case of unsymmetrical faults, different current flows in different phase when fault occurs. 95% faults are unsymmetrical or Asymmetrical type. These faults are divided as follows.

Single Line to Ground (L-G)

This may occur when insulation of one of the line conductor breaks and falls on the earth or ground wire.

faults

Phase to Phase (L-L)

This may occur when one of the line conductor breaks and falls on the other line conductor.

Electrical Faults in Transmission Line

Phase to Phase and Third Phase to Earth (L-L-G)

This may occur when two line conductors are short circuited and third line conductor breaks and falls on the ground or ground wire.

Electrical Faults in Transmission Line 3

Two Phases to Earth

This may occur when insulation between two line conductors fail or when two line conductors break and fall on the ground or ground wire.

Electrical Faults in Transmission Line overhead

Reasons of Faults

  • Insulation failure or breakdown between line conductors.
  • Lightning surge.
  • Over voltage.
  • Voltage drop.
  • Unbalance current.
  • Mechanical fault in transmission lines.
  • Reversal of power.
  • Under frequency.

 

 

 

 

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