APSEEE – Page 4

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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EMF Equation for Transformer

EMF Equation for Transformer

For drive an emf equation for transformer we will consider the case of an ideal transformer. An ideal transformer one which have no copper loss(I2R losses) and magnetic leakage flux. In other words, we can say that an ideal transformer consists of windings which have zero ohmic resistance and loss-free core.



In previous article, we have studied about basics of transformer such as working principle of transformer and necessity or need of transformer at generating power station, substations etc. In this article we will review the last article, by considering ideal case of transformer.

Consider an ideal transformer, whose secondary is open and primary is connected to alternating supply source. The alternating current flows in the primary winding. Since coils of primary winding is purely inductive and it draws magnetizing current only which is necessary to set up magnetic flux only. This magnetizing current is small in magnitude and lags primary voltage V1 by 900. The magnetic flux Φ sets up in primary circuit links with secondary circuit. According to faraday’s laws of electromagnetic induction induced emf is produced in secondary winding. If the secondary winding is closed current start flowing through the load.

Let the sinusoidal variation of flux Φ be expressed as

Φ = Φmax sinωt

emf equation for transformer

Φmax = maximum value of flux in webers

ω = angular frequency in rad/sec

The emf e1 induced in primary winding by the alternating flux Φ is given by

E1max  = N ω Φmax

Primary induced emf

E1max  = N1 ω Φmax

E1max = 2⊓ fN1 Φmax Volts.

According to above equation

I. Induced emf is directly proportional to no. of primary or secondary turns.

II. Induced emf is directly proportional to rate of change of flux linkage.

III. Induced emf is depends upon supply frequency.

Voltage Transformation Ration (K)

It is the ration of secondary voltage to the primary voltage.

  Or

Secondary induced emf to the primary induced emf.

Or

Secondary number of turns to the primary number of turns.

Or

Primary current to the secondary current





K is called transformation ratio of the transformer.

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Transformer on Load

Transformer on Load

When some load is connected across the secondary of the transformer, then it is said to be transformer on load. The current I2 flows through load and secondary winding. The magnitude of current I2 depends upon the terminal voltage V2 and impedance of load. The angle between current I2 and voltage V2 is depends upon the nature of load. Whether it resistive, inductive and capacitive. In this article, we will study about the transformer when it is loaded or certain load is connected to the secondary side. We will explain it step by step.

When certain load is connected to secondary side of the transformer. It draws no load current I0. The no load current I0 produces and mmf N1I0 which sets up magnetic flux in the core, shown in the figure.

transformer on load

The secondary current I2 sets up it own mmf (N2I2) and hence flux Φ2. This set up flux oppose the main primary flux Φ which is setup by no load current I0.

As secondary flux Φ2 oppose the primary flux, therefore the resultant flux also decreases and cause in reduction in self induced emf E1. This results the transformer draws additional current I1′  from supply main and flux in the core restored to its original value. So that V1 becomes equals to E1. The additional current draws by the primary winding is called counter balancing current. The additional current I1 produces and mmf N1I1 which sets up the flux Φ which is same in the direction of primary current and cancels the flux Φ2.

The flux produce secondary current is neutralized by flux produce by current I1. The flux produce secondary current is neutralized by flux produce by current I1. The total primary current I1 is the vector sum of current I0 and I1.

I1 = I0 + I1

 

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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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Silicon Controlled Rectifier

Silicon Controlled Rectifier  (SCR)

Silicon controlled rectifier belongs to thyristor family. SCR is used in industrial application. It is used for controlling high voltage in the order of 10KV and current in order of 3000A. It plays an important role in industries. In this topic we will discuss about what is an SCR, construction of SCR and working of SCR.

What is an SCR?

SCR (is called silicon controlled rectifier) is a three terminals (namely anode, cathode and gate), three junction (J1, J2, J3) and four layer semiconductor device used for rectification, inversion etc.

Construction

SCR consists of four-layer P-N-P-N semiconductor materials. Silicon material uses as the semiconductor to which the proper impurity is added. The junction may be diffused or alloyed.

It has three terminals.

SCR and Working of SCR

  • The terminal taken from outer P-type material is called anode.
  • The terminal taken from outer N-type material is called cathode.
  • The terminal taken from inner P-type material is called Gate.

The SCR is a unidirectional device like power diode. The difference between power diode and SCR. There is an extra terminal is used in SCR.

Working of SCR

Action of Anode Voltage ( Gate Being Open)

In this mode, anode made positive with respect to cathode, with gate circuit open. In this case junctions J1 and J3 become forward biased whereas junction J2 become reverse biased under such conditions small current flows through the device. This current flows due to minority carries. The device remains in blocking state. If the voltage at anode is continuously increased, the reverse biased junction J2 will have an avalanche breakdown and SCR starts conduct.

silicon controlled rectifier

The forward anode voltage at which SCR starts conducts called breakover voltage.

Action of Cathode Voltage (Gate Being Open)

In this case, cathode made positive with respect to anode, the junction J1 and J3 become reverse biased. If the voltage increases gradually at one stage. It may results in breaking of depletion region at junctions J1 and J3 and the current through it suddenly increases at a very high value. This may damage the device. This is called reverse breakdown.

Action of Gate Voltage

When gate is connected to the junction J2 , it becomes forward bias. A small gate voltage is applied to the junction J2 to turn on the SCR.

Applications of SCR

  • It is used as a rectifier in industries to control speed of dc motors
  • It is used to control large power.
  • it is used as a switching device.
  • It is used for over voltage protection.

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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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EMF Equation for DC Machine

EMF Equation for DC Machine

DC machine may be either works as a dc motor or dc generator. EMF equation is important for both. In case of DC generator an induced emf is called generated emf and in case of DC motors generated emf is called Back or Counter emf  The DC generator is rotate with the help of prime mover. Prime mover is directly coupled to the generator. When armature of dc generator rotates its conductor cuts by the magnetic flux that is produce in field winding and this results of an emf is induced in it. The induced emf depends upon the type of winding of dc generator whether it is wave or lap. In this article, we will drive the emf equation for dc machine.





Let,

Φ = flux per pole in weber

Z = total number of armature conductor

= number of slots * number of conductors per slot

P = number of parallel paths in armature

N = rotation speed of the armature in revolution per minute (r.p.m)

E = emf induced in any parallel path in armature

Eg = Generated emf

Flux cut by one conductor in one revolution = PΦ wb

Time to complete one revolution, t = 60/N seconds.

Average induced emf in one conductor,

emf equation for dc machine

The number of conductors connected in series in each parallel path = Z/A

The emf generated across terminals,

E = Average emf induced in one conductor * the number of conductors connected in series in each parallel path

Number of parallel paths in wave winding, A = 2

Number of parallel paths in lap winding, A = P

Conclusions from EMF Equation for DC Machine

  • In above equation, poles remain constant.
  • The emf induced in the armature is directly proportional to the flux per pole and speed.
  • The polarity of induced emf in armature is depends upon the connections of field winding and the direction of rotation. If reversed the connection of field winding and direction of rotation, the polarity induced emf will change.
  • The induced emf is fundamental phenomenon to all dc machines either it is operated as generator or motor.
  • When the machine is operating as a generator, this induced emf is called the generated emf Eg.
  • When the machine is operating as a motor, this induced emf is called back or counter emf, Eb. This emf plays an important role in dc motor. By using this equation we can find back emf.
  • Wave winding is used for high  voltage and low current applications and where as Lap winding used where large current at low voltage is generated.




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Induction Type Over Current Relay



Induction Type Over Current Relay

Induction type over current relay works on the induction principle. This relay operates when current in the circuit exceeds the predetermined value. These relays are used on ac circuits only.

Construction

This relay has two electromagnets.

  • Upper magnet
  • Lower magnet

The upper magnet has three limbs and primary and secondary windings are wound on central limb. This winding is connected to the CT of the line to be protected. The tapping is taken from this winding. These tappings are connected to a plug setting bridge.

The secondary is closed winding and wound on the central limb of the upper magnet and both the limbs of the lower magnet. The secondary winding is energized by induction from primary.

The aluminum disc is placed in between the upper magnet and lower magnet. The disc is free to rotate about its axis. Spiral springs are provided on disc to get controlling torque. The spindle of disc carries a moving contact which bridges two fixed contacts when the disc rotates through a pre-set angle.

induction type over current relay

The pre-set angel can be adjusted to any value between 00 and 3600. By adjusting the angle, the relay can set for any desired value.

Operation

When current flows through primary winding, the flux is set up in primary winding. When this flux links with secondary winding, an emf is induced in it according to the laws of electromagnetic induction.

Since secondary is closed, a current flows through it.

The flux is produced by the current flowing through primary and secondary windings.

These fluxes interact each other because there is a phase difference between them, this produces a driving torque on the disc.

Driving torque on the disc opposed by controlling torque provided by spiral springs.

Under normal operating conditions, controlling or restraining torque is greater than the driving torque. Therefore aluminum disc remains stationary.

When fault occurs on the system, the value of current exceeds from pre-set value. Now, driving torque becomes greater than controlling torque.

Consequently, the disc rotates and moving contact bridges the fixed contacts and sends the signal to trip circuit.

 

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Parallel Operation of Alternators

Parallel Operation of Alternators

When the number of smaller units is connected in parallel instead of installing a large unit is called parallel operation of alternators. There are numbers of reasons, connecting alternator in parallel such as cost become less, maintenance and repair, efficiency and reliability of the power system. All the alternator of the system, work in parallel form a large capacity alternator. In this article, we will discuss about need for parallel operation of alternators

Need for parallel operation

In modern power systems alternators are operated in parallel to supply a common total load. Due to following reasons the alternators are connected in parallel.

  1. The demand of electrical power is huge, and it cannot be met by a single unit and it is difficult to build a large alternator, therefore to meet the demand of electrical power several alternators are connected in parallel.
  2. The parallel operation increases the reliability of the electric supply. If we use single large alternator in the event of fault on alternator or turbine whole the system is paralyzed. But with several alternators work in parallel maintain the continuity of supply rather than breakdown of one unit.
  3. Maintenance and repair of the alternator is more convenient if more number of small capacity alternators are installed at the power station. For repairing of one alternator there is no need to shut down the whole power plant.
  4. With increase the demand of electrical energy, we can install a alternator with existing plant.
  5. Transportation problems are faced with single large unit. But this problem can be eliminated by using small units.
  6. The load on power plant varies, usually having its peak value during the day and its minimum value during the night time.

Thus the number of units operating at a particular time can be varied depending upon the load at that time. If the alternators are connected in parallel the less efficient alternator can be shut down when the load requirement is less.

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Speed Control of DC Series Motor

Speed Control of DC Series Motor

The Speed control of dc series motor can be obtained by changing the field, called field control method and by changing the back emf is called armature control method.

Flux or Field Control Method

In this method , the speed varied by  changing the field or flux. The following methods are adopted in field control method

Field Diverters

In this method, a variable resistance is connected in parallel with series field winding.

The flow of current through field winding divided into two paths, in this way some value of current passes through variable resistance and it reduces the value of current through the field winding. In this way, flux is reduced and speed of the motor is increased.

speed control of dc series motor

The lowest speed is obtained by reducing the current through diverter to zero.

By this method, speed above normal only can be obtained.

Armature Diverters

In this method, a variable resistance is connected across the armature. The current flowing through the armature divided into two paths(I = Ia + ID).

The flow of current through the variable resistance or diverter depends on the value of resistance.

Divert current reduces the current passes through the armature. Due to reduction in armature current, the speed of the motor is decreased.

speed control method of dc series motor armature diverter

By this method, speeds below normal only can be obtained.

Tapped Field Control

In this method, the flux is reduced by decreasing the number of turns of the series field winding. The flux produces in the field winding depends on the current flows through field winding and number of turns.

Φ α Ise * N

When the number of turns is decreased that decrease the flux. Hence, speed is increased by this relation (N α 1/Φ).

By this method, speed higher than normal can be achieved.

Armature Resistance Control Method

In this method, a variable resistance is connected in series with series field winding and armature.

By increasing the value of resistance, the voltage across the armature terminals reduces and speed is also reduce.

This method has poor speed regulation and considerable power loss in it.

 

 

 

 

 

 

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