28 December 2012

Induction Motor Interview Questions Answers

What are the advantages and disadvantages of Induction Motors?
  • It is simple and rugged in construction
  • It is relatively cheap
  • Induction motors require less maintenance
  • Induction motor has high efficiency and reasonably good power factor
  • 3-phase induction machines are self starting
  • Induction motors are essentially a constant speed motor and its speed cannot be changed easily
  • Its starting torque is inferior to dc shunt motor

2. What is the condition for maximum torque in induction motor?
Answer: Starting torque will be maximum when the rotor resistance / phase is equal to standstill rotor reactance / phase

3. Slip ring induction motor advantages and disadvantages compared to squirrel cage motors?
  • High starting torque with low starting current
  • Smooth acceleration under heavy loads
  • No abnormal heating during starting
  • Good running characteristics after external rotor resistances are cut out
  • Adjustable speed
  • The initial and maintenance costs are greater than those of squirrel cage motors
  • The speed regulation is poor when run with resistance in the rotor circuit

4. Methods to control speed of Wound Rotor Motors?
Answer: The speed of wound rotor motors are changed by changing the slip of the motor. This can be achieved by:
  • Varying the stator line voltage
  • Varying the resistance in the rotor circuit
  • Inserting and varying a foreign voltage source in the rotor circuit

5. Explain how Torque-Slip Characteristics vary when adding resistance to rotor circuit?
Answer: The addition of resistance to the rotor circuit does not change the value of maximum torque but it only changes the value of the slip at which the maximum torque occurs

6. Disadvantages of Star-Delta Starting of Induction motor?
Answer: In Star-Delta starting induction motor stator is connected in star connection for starting after picking up speed it is connected to delta connection. When induction motor is connected in star connection stator phase voltage reduced by 1/(31/2 ) times the line voltage. This also results in reduced starting torque (1/3 times compared to delta connection). 

27 December 2012

Advantages of Star and Delta Connected Systems

In a 3-phase system the alternators or generators may be star connected or delta connected. Likewise 3-phase loads may be star connected or delta connected. Some of the advantages of star and delta cconnected systems are listed below

Star Connection:

  • In a star connection, phase voltage Vph = VL/ (3)1/2. Since the induced emf in the primary winding of an alternator is directly proportional to the number of turns, a star connected alternator will require less number of turns than a delta connected alternator for the same voltage.
  • For the same line voltage, a star connected alternator requires less insulation than a delta connected alternator. Due to the above reasons three phase alternators are generally star connected.
  • In star connection, we get 3-phase and 4-wire system. This permits the use of two voltages (phase voltages as well as line voltages). Single phase loads can be connected between any one lie and neutral wire while the 3-phase loads can be put across the three lines. Such a flexibility is not available in delta connection
  • In star connection, the neutral point can be earthed. Such a measure offers many advantages. For example, in case of line to earth fault (L-G fault), the insulators have to bear 1/31/2 (57.7%) times the line voltage. Earthing of neutral also permits the use of protective devices (relays) to protect the system in the case of ground faults

Delta Connection:

  • This type of connection is most suitable for rotatory conveyers
  • Most of the three phase loads are delta connected than star connected. One reason for this, atleast for the case of unbalanced load, is the flexibility with which loads may be added or removed on a single phase. This is difficult to do with star connected 3-wire load
  • Most of the 3-phase induction motors are delta connected

Why Sine Waveform Chosen for Alternating Voltage and Current

Commercial alternators produce sinusoidal alternating voltage. A sinusoidal alternating voltage is produced by rotating a coil with a constant angular velocity in a uniform magnetic field. Sinusoidal voltage always produce sinusoidal current unless the circuit is non linear.
Why Sine Wave is chosen rather than a simple curve such as a square or triangular wave. Some of the reasons are given below:
  • In alternating current (a.c) machines such as induction machines, synchronous machines, transformers.., sinusoidal voltages and currents respectively produce the least iron and copper losses for a given output. The efficiency of the machine therefore is better
  • Sinusoidal voltages and currents produce less interference (noise) on telephone lines
  • The sine waveform produces the least disturbance in the electrical circuit and is smoothest and efficient waveform
Due to above advantages, electrical supply is generate sinusoidal alternating voltage and currents. 

Importance or Significance of Hysteresis Loop or B-H Loop

Hysteresis Loss:

When a magnetic material is subjected to a cycle of magnetization (magnetized first in one direction and later magnetized in opposite direction in a cyclic manner), an energy loss takes place. This energy loss is due to molecular friction in the material. That is, the domains (or molecular magnets) of the material being turned first in one direction and then the other. Energy is thus expended in the material in overcoming this opposition. This loss is in the form of heat and is called hysteresis loss.
Hysteresis loss is present in all electrical machines whose iron parts are subjected to cycle of magnetization like transformers, induction motors and other machines operated on ac supply. When an alternating supply is provided to the electrical machines the flux in the iron of these machines change in both direction and value alternatively. During this process energy is lost and this loss constitutes the core loss of the machine. The obvious effect of hysteresis loss is the rise of temperature of the machine
  • Transformers and most electric machines operate on alternating current. In such devices, the flux in the iron changes continuously, both in magnitude and in direction. Hence hysteresis loss occurs in these machines
  • Hysteresis loss also occurs when iron parts rotates in constant magnetic field eg- dc machines


Hysteresis Loop or B-H Loop area:

Hysteresis loop is obtained by plotting B-H (B: flux density, H:magnetizing force) of iron for one cycle of magnetization. 
  When the core of the machine such as transformer, induction motors is magnetized with the magnetizing force (H) by applying voltage, magnetic flux density (B) increases as shown in the figure (dotted lines) and saturates (first quadrant). Saturation region in one in which increase in the magnetization force (H) will not further appreciably increase the magnetic flux density (B) in the material. 
When the applied voltage reaches back to the zero position from the peak (first half cycle), magnetizing force will also becomes zero. However the magnetic flux density in the core will not reach zero but have some finite value. This property of magnetic material to withhold some flux (B) when magnetizing strength becomes zero is called Retentivity.
During the next half cycle all the domains in the magnetic core tries to align in opposite direction as the applied voltage polarity changes and hence the magnetizing force applied. With increase in the magnetizing force (H) a point is reached where the magnetic flux density(B) becomes zero. This point is called coercivity. Beyond this point, magnetic flux will be in opposite direction with increase in field strength and reaches saturation as shown in figure (third quadrant).
Further applied magnetic field will starts decreasing from peak and reaches zero. This decrease in the magnetic flux with decrease in field strength can be seen in figure (third quadrant) and reaches back when the voltage polarity changes.

This forms the B-H loop of hysteresis loop of the magnetic material of the machines  


Importance or Significance of B-H Loop:

The shape and size of the hysteresis loop largely depends on the nature of the magnetic material. The choice of a magnetic material required for a particular application often depends on the shape and size of the hysteresis loop. 
  • The smaller the hysteresis loop area of a magnetic material, the less is the hysteresis loss. For example, the hysteresis loop area for silicon steel has very small, for this reason silicon steel is widely used for manufacturing of transformer cores and rotating machines which are subjected to rapid reversals of magnetism
  • The hysteresis loop for Hard Steel (large hysteresis loop area) indicates that the material has high retentivity and coercivity. Therefore hard steel is quite useful in making permanent magnets. But due to large area hysteresis loss is quite high. This is the reason hard steel is not used for construction of electrical machines


Methods to reduce Hysteresis Loop or B-H Loop:

Hysteresis loop can by using soft magnetic materials (like CRGO core magnetic material in transformers) having smaller loop to reduce the hysteresis loss.

23 December 2012

TCP/IP and OSI Architecture Similarities & Difference

ISO-OSI Architecture has following layers:
  1. Physical Layer
  2. Data Link Layer
  3. Network Layer
  4. Transport Layer
  5. Session Layer
  6. Presentation Layer
  7. Application Layer
TCP/IP Architecture has following layers:
  1. Host to Network Layer
  2. Internet Layer
  3. Transport Layer
  4. Application Layer

Similarities between TCP/IP & OSI Architecture:

  • Both models are based on concept of "Stack of Independent Protocols"
  • All the layers from bottom till Transport Layer provides End to End Transport Service
  • All the layers above the Transport Layer are application oriented and use the Transport Service

Difference between TCP/IP & OSI Architecture:

  • The protocols in ISO are better hidden and can be replaced relatively easily as technology changes
  • OSI supports both connectionless and connection oriented communication in network layer but only connection oriented communication in the transport layer. TCP/IP supports only connectionless communication in the network layer and both connectionless and connection oriented communication in the transport layer
  • The ISO model has 7 layers whereas the TCP/IP model is composed of 4 layers
  • The TCP/IP the protocol cam first and the model was made based on the protocol. ISO-OSI the model was devised before the protocols were inverted

22 December 2012

Difference between LAN WAN and MAN Networks

A Computer network that spans relatively small area is called Local Area Network (LAN). Most LANs are confined to small building or group of buildings. However one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs can be cconnected in this way is called Wide Area Network (WAN)
Most LANs can connect workstations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it is also can able to access data and devices anywhere in the LAN. This means that many users can able to share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mails or engaging in chat sessions or can able to play games together
Metropolitan Area Network or MANs are large computer networks usually spanning a campus or a city. They usually use wireless infrastructure or optical fiber communications to their sites
For instance a university or college may have MAN that joins together many of the local area networks (LANs) situated around site of a fraction of square kilometer. Then from their MAN they could have several Wide Area Network (WAN) links to other universities or the internet. Specially this type of MAN is known as campus area networks

R R-C Firing Circuits Disadvantages

Firing Circuits:

Gate triggering is the most commonly used turn-on method employed to switch on the thyristors. Triggering circuits is also called firing circuits. There are various firing circuits available. R-Firing circuits is simple but suffer from limited firing circuits. Firing angle is limited between 0o to 90o. In actual practice firing angle can be varied between 3o to 90o. Limitation of the firing angle range of R-Firing circuit is eliminated by introducing a capacitor and a diode. Thus R-C firing circuits can increase the firing angle limitation range. Theoretically firing angle can be varied from 0o to 180o. However due to low voltage at 0o and 180o thyristor cannot be turn-on. Hence practically the range of firing angle is between 3o and 177o
Both R and R-C firing circuits suffer from following disadvantage:
  • They can be employed in power circuits having only one thyristor
  • They are capable of open loop control only
  • Due to lower voltages near 0o to 180o, gate current is small. Especially in R-C firing circuit, near 180o gate current is minimum due to maximum value of R. This will increase the turn on time, especially for R-L load, leading to higher turn on loss
  • Higher frequency gate signal is desirable for reliable turn on. Both the circuits are not capable of providing the same
  • There is no electrical isolation between control circuit and power circuit
However the circuits are simple and cheap. R-C firing circuits is widely used in low power thyristor controllers, such as solid state ac regulators for speed control of fans and blowers. R-C firing circuits can also acts as snubber circuits

Crowbar fault protection circuit

Crowbar circuit is used to protect circuit or equipment under fault condition

Over Current Causes:

  1. Over current flow through thyristors in a circuit due to:
  2. Flow of short circuit current on the load
  3. Simultaneous triggering two thyristors in series in a circuit
  4. Sustained over load
  5. Stalling of an electrical motor

A Conventional fuse will take quite some time to clear the fault an is not suitable for protection of the device. Fast acting semiconductor fuses are connected in series with each thyristor

Crowbar Circuit:

A Crowbar circuit is used to protect circuit or equipment under the fault condition. It consists of thyristor of a rating well in higher than the rating of the devices that are to be protected. On detection of the fault, crowbar thyristor is fired consequently fuse blows. Thus the devices are protected against over current. Crowbar protection is recommended for MOSFET circuits.

19 December 2012

Transmission and Distribution Interview Questions

1. What for series and shunt compensation provided in EHV transmission lines?
Answer: Series capacitance is provided in EHV lines to artificially reduce the series reactance of the line so as to improve stability, voltage regulation and transmission efficiency
Shunt compensation is provided to artificially reduce the line susceptance so as to improve the voltage regulation under light load condition

2. What is the material used for overhead transmission lines?
Answer: ACSR conductors are employed.

3. What are the problems associated with EHV transmission?
Answer: The problems associated with EHV transmission are corona loss and radio interference, requirement of heavy supporting structures and insulation requirments

4. Why does surge impedance loading (SIL) increase with increase in voltage level?
Answer: SIL varies as the square of the operating voltage, so SIL inceases with increase in voltage level

5. What are the factors that limit the maximum power transfer capability in a transmission line?
Answer: Some of the factors which limits the maximum power transfer are:
  1. Electrical phase shift
  2. Voltage drop
  3. Thermal effects in the line

6. Explain some of the methods to improve the strength of transmission system?
  • Introducing or adding of new transmission lines in to the system to avoid the overloading of the existing lines
  • Application of devices such as series capacitors in the right location of power system which helps in increase in the power transfer capability
  • Up gradation of the existing transmission system
  • Adoption of multi-voltage levels an multi-circuits in the existing ac transmission system

7. Why phase shift is kept low for transmission of power for large distances?
                                              E1 X E2
        Power delivered P =      ---------   Sin δ
Generally  δ is kept low (around 30o) because any disturbance can affect the stability of the system if δ value is high.

18 December 2012

Shunt Capacitor Advantages in Power System

Normal loads on ac supply system are inductive in nature (eg: motors, power transformers, voltage regulators, induction furnace, choke coils, magnetic systems, discharge tubes etc) Inductive power requires reactive power in addition to active power (active power is required to do the true work). The reactive power increases the load imposed on the system.
When a capacitor is installed across an inductive load (i.e, parallel to the load), the load starts receiving reactive power from the capacitor and thus capacitor power neutralizes the reactive power requirement of the inductive load. Consequently the supply system is relieved from producing reactive power to the load and it can able to deliver greater active power. When the power factor improves towards unity, the transmission losses decreases due to reduced current and the system voltage improves. A healthy supply enables machines and protection equipment to give optimum output in terms of performance and operational life span. Some of the advantages due to installing shunt capacitors in the power system are explained below
  • For a particular active power (kW) the resultant demand (kVA) is subsequently reduced. So additional machines can be installed for given sanction of kVA load
  • Since voltage drop is minimized, motor torque capability ( Torque α Voltage2 ) improves, so starting time and the motor heating gets considerably reduced. Motor current requirement for the same output is lesser
  • Because of  less heating, the ageing of the insulation becomes slow and thus the life of the machine and the cables increases
  • Switchgear wear and tear is minimized because of lesser arcing energy dissipation (i.e, lesser acing time) at higher power factor
  • Reduced losses in the feeders lead to lesser voltage drop, hence greater voltage regulation

Cables Interview Questions Answers

1. What is the main reason for providing metallic sheath in underground cables?
Answer: The metallic sheath is provided around the insulation to protect it against the ingress of moisture, gas and other damaging liquids (acid or alkalies) from the soil and atmosphere

2. What are the main requirements of the insulation materials used for underground cables?
Answer: Some the properties that insulation material require to possess are high insulation resistivity, high dielectric strength, good mechanical properties, non-hygroscopic, non inflammable, immune to attacks by acids and alkalies

3. What are the advantages and disadvantages of oil filled cables?
Answer: Some of the advantages and disadvantages of oil filled cables are listed below:
  • Smaller overall size and smaller weight for given voltage and kVA rating due to reduction in the thickness of the dielectric required
  • No ionization, oxidation and formation of voids
  • More perfect impregnation
  • Smaller thermal resistance due to decrease in the dielectric thickness, so higher current rating
  • More maximum permissible stresses
  • Fault identification is easy 
  • Greater cost
  • Complicated laying of cables and maintenance

4. Advantages of XLPE cables compared to PVC cables?
  •  XLPE cable can withstand higher temperature (900C) compared to PVC (700C)
  • XLPE cables can have higher over-load capacity
  • XLPE cables are lighter in weight and smaller bend radius hence lower installation cost
  • XLPE cable has high short circuit rating
  • It has lower dielectric and constant power factor which can result in saving of power

5. What are different insulation materials used in cables for high voltage and medium voltage?
Medium Voltage:
  • Polyethylene (PE)
  • Cross linked polyethylene (XLPE)
  • Tree retardant cross linked polyethylene (TR-XLPE)
  • Ethylene-Propylene Elastomers (EPR)
  • PILC
High Voltage:
  • Cross linked polyethylene (XLPE)
  • Paper/Oil
  • Paper/Polypropylene
  • SF6 gas

03 November 2012

Neutral Grounding Practice in Power System

  • Generally on neutral grounding is provided at each voltage. There will be several voltage levels between the generation of the power and distribution of the power in the power system. Only one ground is provided for each voltage level of the power system
  • Grounding of the power system is provided at the source and not at the load end
  • Each of the major bus section in the system are grounded
  • For generator grounding, neutral of the generator is grounding through a resistance which limits the stator fault current. The value of the resistor employed for the grounding the generator decides the percentage of the generator windings left unprotected
  • Synchronous motors and synchronous capacitors are provided with reactance type of grounding. This reactance grounding provides additional reactance which provides additional lagging currents which nullifies the capacitive grounding currents

  • When several generators are connected to a common neutral bus, the bus is connected to the ground through a single grounding device. Disconnect switches are used to ground the desired generators to the neutral bus
  • When several generators are operating in parallel, only one generator neutral is earthed. This is to avoid the interference between the zero sequence currents
  • In generating stations there is a provision to ground neutral of at least two generators, though one at a time. The other generator neutral is grounded when the first generator is out of service
  • When there are one of the two supply sources, no switching equipment is used in the grounding circuit.
  • For the protection purpose, the neutral point of the star side of the power transformer is usually grounded
  • The star connected secondary sides of the protective CTs and PTs are grounded at one point. This ensures stable neutral, proper measurement of the voltages and currents, kWh and kVA on the secondary side measuring instruments and controls
  • For the circuits between 3 kV and 33 kV resistance or reactance grounding is used. But for low voltages less than 600V and high voltages above 33 kV solid or effective grounding is used. Effective grounding limits the voltages of healthy phases to line-to-neutral values in the events of ground faults and also eliminates the arcing grounds. The effective grounding causes the ground fault currents of very high magnitudes flow through the machine. But modern day protection systems are very sensitive and fast operating so that faults are cleared in very short time

02 November 2012

Outdoor Substation Advantages Disadvantages

Air Insulated Substation (AIS) or Outdoor Substations have all switchgear equipment, busbars and other switchyard equipment installed outside open to atmosphere. In earlier days for any voltage ratings AIS or outdoor substation is employed. Indoor Substation type is only employed in places where high pollution or saline environment exists. Indoor substations are of two types
  • Substation with conventional switchgear equipment enclosed in big building. Size of switchyard is similar to AIS Substation.
  • Substation with SF6 enclosed modules (Gas Insulated Substation) in building which takes about 10% of the total AIS substation space
Because of excellent properties of SF6 gas such as high dielectric strength, high electronegativity, for EHV substations more than 230kV now a days Indoor Gas Insulated Substations (GIS) are employed in place of AIS substations. However the cost of GIS indoor substation is higher compared to AIS substation but it has some benefits which includes high reliability, less space requirement and less maintenance. Some of the advantages and disadvantages of outdoor switchyard is discussed below.

Advantages of Outdoor Substation (AIS):

  • This type of substation arrangement is best suited for low voltage rating substations (step down substations) and for those substations where there is ample amount of space available for commissioning the equipment of the substation
  • The construction work required is comparatively less to indoor switch yard and the cost of switchgear installation is also low
  • In future the extension of the substation installation is easier
  • The time required for the erection of air insulated substation is less compared to indoor substation
  • All the equipment in AIS switch yard is within view and therefore the fault location is easier and related repairing work is also easy
  • There is practically no danger of the fault which appears at one point being propagated to another point for the substation installation because the equipment of the adjoining connections can be spaced liberally without any appreciable increase in the cost

Disadvantages of Air Insulated Substation (AIS):

  • More space is required for outdoor substation when compared to indoor gas insulated substation (GIS)
  • Outdoor switch yards are more vulnerable to faults as it is located in outside atmosphere which has some influence from pollution, saline environment and other environmental factors. Deposition of saline particles on insulators can cause insulator failures. They are also vulnerable to direct lightning strikes and other external events such as heavy winds, rains and cyclones. Therefore reliability wise air insulated substation or outdoor substations are relatively low compared to indoor substation
  • Regular maintenance is required compared to indoor substations (Maintenance for Gas Insulated Substation is very minimal and reliability is very high) as they are exposed to outside environment

Electromagnetic Relays Advantages Disadvantages Applications

In Electromagnetic relays  operating current flows through the coil. When this operating current increases, coil energizes the electromagnet. When the operating current becomes large, the magnetic field produced by electromagnet is high such that this magnetic field pulls the armature or plunger making the trip circuit contacts to close. Some of the advantages, disadvantages and applications of electromagnetic relays are explained below

Advantages or merits:

  • Electromagnetic relays have fast operation and fast reset
  • They can be used for both ac and dc systems for protection of ac and dc equipments
  • Electromagnetic relays operating speeds which has the ability to operate in milliseconds are also can be possible
  • They have the properties such as simple, robust, compact and most reliable
  • These relays are almost instantaneous. Though instantaneous the operating time of the relay varies with the current. With extra arrangements like dashpot, copper rings etc. slow operating times and reset can be possible

Disadvantages or demerits:

  • High burden level instrument transformers are required (CTs and PTs of high burden is required for operating the electromagnetic relays compared to static relays)
  • The directional feature is absent in electromagnetic relays
  • Requires periodic maintenance and testing unlike static relays
  • Relay operation can be affected due to ageing of the components and dust, pollution resulting in spurious trips
  • Operation speed for an electromagnetic relays is limited by the mechanical inertia of the component


  • Electromagnetic relays are employed for the  protection of various ac and dc equipments
  • The over/under current and voltage protection of various ac and dc equipments
  • For differential protection
  • Used as auxiliary relays in the contact systems of protective relay schemes

01 November 2012

Synchronous motors Advantages Disadvantages

In industries induction motors are employed mostly because of less cost, rugged construction, good starting torques and very less maintenance. Synchronous motors are rarely used in industries for drive applications. They are generally used as power factor correction device. In industries they are employed to improve the power factor of the system. Some of the advantages and disadvantages related to synchronous motors are explained below: 

Advantage or Merits:

  • One of the major advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous motor can have leading power factor and can be operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor. 
  • In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes
  • Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable
  • In synchronous motors electro-magnetic power varies linearly with the voltage
  • Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors

  Disadvantages or Demerits:

  • Synchronous motors requires dc excitation which must be supplied from external sources
  • Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing
  • The cost per kW output is generally higher than that of induction motors
  • These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives)
  • Synchronous motors cannot be started on load. Its starting torque is zero
  • These motors have tendency to hunt
  • When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt 
  • Collector rings and brushes are required resulting in increase in maintenance
  • Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required

24 October 2012

Functions of Instrument Transformers (CTs and PTs)

AC type protective relays are actuated by the current and voltage supplied by the current and potential (voltage) transformers which are generally classified as instrument transformers. Generally instrument transformers are used for mainly two purposes. For metering purpose which steps down and displays voltage and current levels from kV to (0-110 volts in case of PT) and few kilo amps to (0-5 amps in case of CT) . Second purose is to supply current and voltage magnitudes to the relays to carryout protection functions
The main functions of instrument transformers are:

  • Instrument transformers (current and potential transformers) provide insulation against the high voltages of the power circuit and to protect the apparatus and the operating personnel from contact with the high voltages of the power circuits
  • Instrument transformers (CTs and PTs) supply protective relays with current and voltages of magnitude proportional to those of the the power circuits. These current and voltage magnitudes supply by the instrument transformers are sufficiently reduced such that the relays can be made relatively small and inexpensive
  • Instrument transformers helps in attaining different types of secondary connections to obtain the required current and voltages
For proper applications of CTs and PTs required considerations are:
Mechanical construction , type of insulation (dry or liquid), ratio in terms of primary and secondary currents or voltages, continuous thermal rating, short time thermal and mechanical ratings, insulation class, impulse level, service conditions, accuracy and connections
For the safety purpose, the secondaries of the current and potential transformers (CTs and PTs) are grounded

23 October 2012

Advantages of Suspension Insulators and Pin type Insulators

With increase in the operating voltage the insulation requirement will also increases. Transmission system will have transmission voltages as high as 400kV and above voltage levels. At these voltages the pin insulators become bulky, cumbersome and costly.  Some of the advantages of suspension insulators over pin type insulators are explained below:

Advantages of suspension insulators:

  • Suspension insulators are cheaper  in cost compared to pin type insulators for operating voltage above 50kV
  • Each unit of suspension insulators (insulator disc) is designed for comparatively low voltage (11kV) and can be increase the insulation strength by connecting these insulator disc modules in series. The number of insulator discs require depends on the operating voltage
  • Suspension type insulators give more flexibility to the line and mechanical stresses due to wind and other factors are reduced in this suspension type insulator arrangement. The connection at the cross arm is such a way that the insulator string is free to swing in any direction and thus takes up a position where it experiences only a pure tensile stress
  • The suspension type insulators when used in conjunction with steel supporting structure has the advantage of rendering the conductor less liable to the affected by cross-arm thus enabling the tower to function as lightning rod
  • In case of rapid increase in the load on the transmission line the increase demand can be met by raising the line voltage than to provide another set of conductors. With suspension type insulators additional line insulation requirement can be obtained by simply adding one or more discs to the string
  • In case of long spans (river or valley crossings) where heavy conductor load is to be sustained, two disc insulators can be yoked. Such an arrangement is not possible in pin type insulators
  • The only disadvantage of suspension type insulators is that large spacing between the conductors are required than with the pin type insulators due to large amplitude of the swing of the conductors 

AC DC Power Transmission Advantages Disadvantages

AC and DC modes of transmission and distribution of power has both advantages and disadvantages: 
  • DC requires only two conductors for transmission and it is possible to transmit the power through only one conductor by using earth as return path. Hence much copper is saved
  • There will be no inductance, capacitance, phase displacement and surge problem in dc transmission
  • Because of the skin effect in ac system current conducts only through the surface of the conductor. On the the other hand dc system will not have skin effect. Hence all the conductor will be utilized for carrying conductor. Therefore conductor size reduces in dc for the same current carrying capacity when compared to ac system
  • The potential stress on the insulator in the case of dc system is 1/(21/2) times of that in ac system for the same working voltage. Hence for the same working voltage less insulation is required in dc compared to ac system
  • Charging currents which contributes to the continuous loss even on no load is eliminated in dc system compared to ac system
  • A DC line has less corona loss compared to ac system and reduced interference with the communication circuits

  • Since there is no inductance, the voltage drop in the dc transmission system line due to inductive reactance does not exist. Hence the same load and sending end voltage voltage regulation of the dc system is better compared to ac system
  • No stabilizer is required for transmission over long distances
  • Since the concept of power factor is absent in dc systems, no need of power factor correction equipment in the power system
  • The only difficulty in the dc system is to obtain the high voltage required for transmission as electrical power neither generated at high voltages nor the dc voltage cannot be stepped up where as ac system can be stepped up and can be stepped down based on the requirement
  •  Other advantage of ac system is that electrical power can be generated at high voltages easily and maintenance of ac substation is cheaper and easier
  • Distribution of ac system is undoubtedly superior to that of a dc system as in the ac system voltage control is easy by means of transformers

21 October 2012

Ungrounded or Isolated Neutral System Disadvantages

  • There will be no flow in the zero sequence currents
  • In ungrounded or isolated neutral system there will be little interference with the communication lines because of the absence of zero sequence currents
  • In case of single line to ground fault or one phase becoming earthed, the voltages of the remaining two phases to the earth raise from normal phase to neutral voltage to full line value ((3)1/2 times their normal value). This causes the stress on the insulation on all the machines and equipment connected to the system. The voltage rise of a phase above earth is sustained and thereby insulation failure is likely to occur on connected machines.
  • The capacitance currents in the two healthy phases increases to ((3)1/2) times their normal values

  • The capacitance currents in the faulty phase becomes 3 times the normal value
  • For operation of the protective device it is necessary that magnitude of current supplied should be adequate to operate them. In the case of earth fault of an isolated neutral or ungrounded systems, the fault current may be too small to actuate the protective devices. Thus in ungrounded system the earth fault relaying is more complicated
  • The over voltages due to induced static charges are not discharged to the ground in an isolated neutral system. The voltage due to lightning surges do not find path to earth
  • The danger to the equipment on the occurrence of line to line ground fault is appreciable and danger to the life in the proximity of the fault is often prolonged
  • A capacitive fault current flows into the earth. Such a current if exceeds 4-5 amperes is sufficient to maintain an arc in the ionized path of the fault, even though the medium causing the fault has cleared itself. The persistence of  the arc due to the flow of capacitance currents gives rise to condition known as "Arcing Grounds" in which cyclic charging and discharging of the system capacity through the fault results in high frequency oscillations superimposed on the whole system and build up of very high voltage can occur. This results in phase voltage to rise to 5 to 6 times of normal voltage. The buildup of high voltage may result in insulation breakdown

Advantages Disadvantages of Oil Circuit Breakers

Oil circuit breakers are the oldest type of circuit breakers. The separating contacts of the oil circuit breakers are made to separate within the insulating oil which has better insulating properties than air. On the occurrence of the fault as the breaker contacts open under the oil, an arc is struck between the breaker contacts and the heat of the arc evaporates the surrounding oil and dissociates into a substantial volume of gaseous hydrogen (hydrogen gas with a small percentage of methane, ethylene and acetylene) at high pressure. 
Oil Circuit breakers have the virtues of reliability, simplicity and relative cheapness. In order to determine the advantages of oil circuit breakers, advantages and disadvantages of oil as a arc quenching medium should be understood


  • Arc energy is absorbed in decomposing of oil
  • The gas formed which is mainly hydrogen, has high diffusion rate and high heat absorption in changing from diatomic to mono-atomic and thus provides good cooling properties
  • The oil has high dielectric strength and provides insulation between the contacts after the arc has been finally extinguished and there has been time for the oil to flow into the gap between contacts
  • Cooling oil presents the cooling surface in close proximity to the arc
  • The oil used (such as transformer oil) is a very good insulator and allows smaller clearance between live conductors and earth components


  • Oil may be flammable and can cause fire hazards, if a defective oil circuit breaker should fail under pressure and cause an explosion
  • There is a risk of formation of explosive mixture with the air
  • Due to the decomposition of the oil in the arc, the oil becomes polluted by carbon particles, which reduces its dielectric strength. Hence periodical maintenance and replacements are required