SSC JE Electrical Conventional Paper Solved 2016-17

Ques5(a): How is the rating of circuit breakers decided? Explain in brief.

Ans: Circuit breakers are the mechanical device designed to close or open contacts members, thus closing or opening of an electrical circuit under normal or abnormal conditions.

The circuit breakers are rated in terms of

  • Maximum Voltage
  • Numbers of poles
  • Frequency
  • Maximum continuous current carrying capacity
  • Maximum interrupting capacity
  • Maximum momentary and 4-s capacity
  • Rated operating duty-(duty cycle)

The interrupting or rupturing capacity of a circuit breaker is the maximum value of current which can be interrupted by it without any damage. The circuit breakers are also rated in MVA which is the product of interrupting current, rated voltage and 10-6.

 

The main duty of the circuit breaker is

  1. .It should carry the full load current continuously without overheating or damage
  2. It should open and close the circuit on no load
  3. It makes and break the normal operating current
  4. It makes and breaks the short-circuit current of magnitude upto which it is designed for.
  5. It must be capable of opening the faulty circuit and breaking the fault current.
  6. It must be capable of carrying fault current for a short time while another circuit breaker is clearing the fault.

The Rating of circuit breaker Include.

  1. Rated Voltage
  2. Rated Current
  3. Rated Insulation Level
  4. Rated Frequency
  5. Rated Short-Time Current
  6. Rated short-circuit Breaking Current
  7. Rated short-circuit Making current
  8. Rated Peak Withstand Current

 

(1) Rated Voltage:- During normal operating conditions the voltage at any point of the power system is not constant. Due to this, the manufacturer guarantees perfect operation of the circuit breaker at the rated maximum voltage, which as a rule is higher than rated nominal voltage.Rated voltage of circuit breaker depends upon its insulation system. For below 400 KV systems, the circuit breaker is designed to withstand 10% above the normal system voltage. For above or equal 400 KV system the insulation of circuit breaker should be capable of withstanding 5% above the normal system voltage

The rated maximum voltage of a circuit breaker is the highest RMS voltage, above nominal system voltage, for which the circuit breaker is designed and is the upper limit for operation.

(2) Rated Current:- The rated circuit breaker current is the RMS value of the current that it can carry continuously without the temperature rise of its components exceeding the specified limits.

(3) Rated Insulation Level:- The rated insulation level of a circuit breaker is the withstand power frequency and impulse voltage of its insulation.The different circuit breakers connected in power system are subjected to power frequency overvoltages due to various effects such as regulation, Ferranti -effect etc. The circuit breaker must withstand this over-voltage. During some single phase to ground faults voltage of healthy line to earth increases. So higher values of insulation are suggested. The insulation is provided for each pole, between live parts and earth.

(4) Rated Frequency:- The rated frequency of the circuit breaker is the frequency at which it is designed to operate. Standard frequency is 50 Hz. Applications at other frequencies should receive special consideration.

The performance of circuit breaker is greatly influenced by frequency. The different characteristics, like breaking capacity are based on rated frequency. With an increase in frequency, eddy currents in the metallic parts will increase which will cause more heating and raise the temperature of current carrying parts.

Hence if a circuit breaker designed for one particular frequency is used at some another frequency then the temperature will not remain within specified limits. So the rating is to be changed accordingly. The breaking time decreases with increase in frequency.

(5) Rated Short-Time Current:- The rated short-time current is the maximum current (RMS current) that the circuit breaker can carry for 1s without damage to its conductors. insulation. operating mechanism. or tank.

(6) Rated short-circuit Breaking Current:- The rated short-circuit breaking current of a circuit breaker is the maximum short circuit current that the circuit breaker is capable of breaking under specified condition of transient recovery voltage and power frequency recovery voltage. 

When the breaker is closed and a fault occurs then it closes in the transient state(in transient state DC offset become zero) so the current capacity at this instant is only the RMS  value of current(symmetrical component)in the transient state which is also called breaking capacity of the breaker.i.e

Breaking capacity = RMS value of breaking current in the transient state.

The breaking capacity of three-phase circuit breaker is generally expressed in MVA and it is given as

Breaking capacity = √3 × rated voltage in kV × Rated current in kA.

 

(7) Rated short-circuit Making current:- It is the maximum r.m.s. current (which includes both ac and dc currents) against which the breaker is capable of closing and immediately opening Without welding of contacts (due to intense heat) or any other damage. The making current may also be expressed in terms of an instantaneous value of current in which case it is equal to the first peak of the short-circuit current wave.

The making current is measured at the first peak of the short-circuit current wave While the braking current is measured at the instant of separation of contacts.

when breaker is going to be closed and at that time fault occurs then it will close sub-transient state and the current carrying capacity at this instant contains RMS value of current (symmetrical component in transient state) and the DC offset current in sub-transient state which is also called Making capacity of the breaker at the sub-transient period amount of arc produce will be very high.Thus making capacity is very high.i.e

Making Capacity=DC offset current+rms value of breaking current in transient state

Making capacity used the peak value of current while breaking capacity work on the RMS value of current that’s why making capacity is more than breaking capacity.

Thus Making capacity > Breaking Capacity

(8).Rated Peak Withstand Current:- It is defined as the instantaneous value of short circuit current which circuit breaker can withstand safely in closed position. It is expressed in terms of kA. The value suggested for this current is equal to rated short-circuit making current.

(9)  Rated operating duty-(duty cycle):- The operating duty specifies the sequence of opening and closing, which the breaker can perform under specified conditions. The IEC recommendations for rated operating duty for breakers (excluding auto-reclosure breakers), there are two alternatives:

(i) Opening (O) ⇒ (time lapse t) ⇒ closing (C) followed by opening (O) ⇒ (time lapse t’) ⇒ closing followed by opening (CO)

(ii) Opening (O) ⇒ (time lapse t”) ⇒ closing followed by opening (CO).

There are breakers with an auto-reclosing provision. These breakers open to clear a fault and then automatically close the circuit after the certain time to find if the fault has disappeared from the system or not.

Note:- Explain any 5 of them including Breaking capacity and Making capacity.


Ques 5(b). Explain Merz-Price protection of generators with appropriate circuit diagram

Ans. The Merz-price protection scheme is also called biased differential protection and percentage differential protection. Differential circulating current protection scheme or Merz Price circulating current scheme is a most popular protection scheme for alternator stator protection. The basic idea is to compare the value of two currents i.e Incoming current and Outgoing current of the stator coil. In normal condition, the value of two current will be equal, during fault condition there will be some difference, and Merz price circulating current scheme works by detecting this difference or differential current.

Merz-Price Protection of Alternator for Star Connection

  • In this method, two sets of an identical current transformer are mounted on either side of stator phase winding.
  • The secondaries of the CT’s are connected in star and their ends are connected through pilot wires.

merz price protection 1

  • The differential relay gives protection against short circuit fault in the stator winding of a generator.
  • The C.T.s are connected in star and are provided on both, the outgoing side and machine winding connections to earth side.
  • The restraining coils are energized from the secondary connection of C.T.s in each phase, through pilot wires.
  • The operating coils are energized by the tappings from restraining coils and the C.T. neutral earthing connection.
  • At normal operating condition, the currents at the two ends of the protected section are same ( I1 = I2).
  • When the fault occurs the balance is disturbed and differential current ( I1 = I2) flow through the operating coil of the relay.
  • This trip the generator circuit breaker to isolate the faulty section.
  • The relay employed in this protection scheme is generally of electromagnetic type and are arranged for instantaneous operation as fault are expected to be cleared as quickly as possible.

Merz-Price Protection of Alternator for Delta Connection

Diffrential protection

  • The C.T.s on the delta connected machine winding sides are connected in delta while the C.T.s at outgoing ends are connected in star.
  • The restraining coils are placed in each phase, energized by the secondary connections of C.T.s while the operating coils are energized from the restraining coil tappings and the C.T. neutral earthing.
  • If there is a fault due to a short circuit in the protected zone of the windings, it produces a difference between the currents in the primary windings of C.T.s on both sides of the generator winding of the same phase. This results in a difference between the secondary currents of the two current transformers.
  • Thus, under fault conditions, a differential current flows through the operating coils which are responsible to trip the relay and open the circuit breaker. The differential relay operation depends on the relation between the current in the operating coil and that in the restraining coil.
  • When differential relaying is used for the protection, C.T.s at both the ends of the generator must be of equal ratio and equal accuracy otherwise wrong operation of the relay may result.
  • The causes of unequal currents on both the sides of C.T.s without any fault are ratio errors, unequal lengths of the leads, unequal secondary burdens etc.
  • This scheme provides very fast protection to the stator winding against phase to phase faults and phase to ground faults. If the neutral is not grounded or grounded through resistance then additional sensitive earth fault relay should be provided.

Advantages of Merz Price Protection

  1. Very high-speed operation with operating time of about 15 msec.
  2. It allows low fault setting which ensures maximum protection of machine windings.
  3. It ensures complete stability during external faults.
  4. It does not require the current transformer with air gaps or special balancing features.

Disadvantages of Merz Price Protection

  1. This scheme does not give protection against external faults and overloading.
  2. Merz price scheme is insufficient to detect alternator interturns fault.
  3. While differential protection gives complete protection for phase-to-phase faults, for phase-to earth faults, the protection is 80% — 85% of the winding only. This is because the magnitude of the earth fault current depends upon the method of neutral grounding.

Ques 5(c). Define the following terms :

(i)Demand factor
(ii) Tariff
(iii) HRC Fuses
(iv) Diversity factor
(v) Derating factor of a cable

 Demand Factor:

The ratio of actual maximum demand on the system to the total rated load connected with the system is called demand factor.

[latex display=”true”]Demand{\text{ Factor = }}\dfrac{{{\text{Maximum Demand}}}}{{Co{\text{nnected Load}}}}[/latex]

Connected Load:- The sum of continuous rating of all the equipment which are connected to the supply system is called connected load.

Maximum demand:-  It is defined as the largest demand for the load on the generating station during a specified period. The load on the generating station is never constant but keeps on varying from time to time. The demand of load which is maximum of all, during a given period, forms maximum demand.

Maximum demand is normally less than the connected load because the connected load to the system of various consumers is not switched on at the same time. The installed capacity of the station is decided by the maximum demand. It is quite obvious that the power station must be able to supply the maximum demand.

Since the maximum demand is generally less than its connected load hence the demand factor is less than unity: typical value ranges from 0.30 – 0.90.

Tariff: 

The rate at which electrical energy is supplied to a consumer is known as the tariff.

Objectives of tariff :

  • Recovery of cost of producing electrical energy at the power station.
  • Recovery of cost on the capital investment in transmission and distribution systems.
  • Recovery of cost of operation and maintenance of the supply of electrical energy e.g.- metering equipment, billing etc.
  • A suitable profit on the capital investment.

The following points are to be considered for fixing a tariff:

  1. The total running and fixed charges or the cost of production of electricity
  2. The type of services rendered by the electricity.
  3. The ability of the consumer to pay.
  4. The tariff calculations should be simple.

Types of Tariff:

The Different Types of tariff are Listed below:

  1. Simple Tariff
  2. Flat Rate Tariff
  3. Block Rate Tariff
  4. Two-part tariff
  5. Three-part tariff
  6. Maximum demand Tariff
  7. Power factor Tariff

Simple Tariff:- When there is a fixed rate per unit of energy consumed, it is called a simple tariff or uniform rate tariff.

[latex display=”true”]Simple{\text{ T}}ariff{\text{ = }}\dfrac{{{\text{Annual Running Charges + Annual Fix Charges}}}}{{Total{\text{ number of unit supplied to the consumers}}}}[/latex]

In this type of tariff, there is no differentiation between various types of consumers. All the consumers are sharing the equal burden of capital investment. The price charged per unit is constant i.e. independent of the number of units consumed.

Flat rate tariff:- When different types of consumers are charged at different uniform per unit rates, it is called a flat rate tariff. For example, for light and fan, the rate is low, but for other heavy loads, the rate is higher. Even in consumer premises, two meters are kept, one is for light and fan and the other is for power, e.g., heating.

Block rate tariff: When a given block of energy is charged at a specified rate and the succeeding blocks of energy are charged at progressively reduced rates, it is called a block rate tariff. The main
consideration here is that as the number of units generated increases, the cost of generation per unit decreases. Therefore the consumer having the large demand in terms of the number of units have to pay less as compared to the consumers with lower demand. The energy consumption is divided into blocks and each block is charged at the fixed rate.

Two-part tariff: In two-part tariff, the rate of electricity is based on the maximum demand of the consumer and the units consumed. The total charge is split into two components the fixed charges and running charges. The fixed charges depend on the maximum demand of the consumer while the running charges depend upon the number of units consumed by the consumer.

Total charge = Rs(a x Kw + b x kwh)

a = Charge per kW of maximum demand

b = Charge per kWh of maximum demand

The industrial consumers with appreciable maximum demand are charged with this type of tariff.

Advantages of this tariff are

  1. It is simple and can be easily understood by the consumers.
  2. The fixed charges which depend on the maximum demand but independent of units consumed are recovered.

Three-part tariff: When the total charge to be made from the consumer is split into three parts viz., fixed charge, semi-fixed charge and running charge, it is known as a three-part tariff. i.e.,

Total charge = Rs (a + b × kW + c × kWh)
where
a = fixed charge made during each billing period. It includes interest and depreciation on the cost of secondary distribution and labor cost of collecting revenues,
b = charge per kW of maximum demand,
c = charge per kWh of energy consumed.

Maximum demand tariff: It is similar to the two-part tariff with the only difference that the maximum demand is actually measured by installing maximum demand meter in the premises of the consumer.This type of tariff is applicable to big consumers but not suitable for a residential consumer as a separate maximum demand indicator is required.

Power factor tariff: The tariff in which power factor of the consumer’s load is taken into consideration is known as power factor tariff.The power factor is vital in case of ac systems. The low p.f. leads to large kVA rating of equipment required, greater conductor size required, larges losses and poor voltage regulation. Hence penalty is to be taken from the consumers having their equipment running at very low factor.


HRC Fuses

High rupturing capacity (HRC) Fuse links provides complete protection to cables, switchgear, control gear and other equipment by limiting the current, both in magnitude and in the time duration, that can pass through these devices in the circuit. The rapid operation in the event of a fault limits the let-through current and energy, thus minimizing the electromagnetic and thermal stresses on the electrical apparatus.

HRC Fuse

These type of fuses comprises a high-grade ceramic body within which fuse elements are placed and welded to the end plates. The assembly is filled with dry granular quartz sand. In the event of flow of high short circuit current, this quartz sand solidifies forming high resistance glass in the arc path to ensure effectively are quenching in optimum time. The fuse elements are the non-deteriorating type which means that they maintain their characteristics over the long service period.

The fuse element is either pure silver or bimetallic in nature.Silver plated tag contacts provide good contact with fuse base and keep the fuse temperature low. These type of fuses can be plugged in and out even when hot or in service with an insulated fuse puller.The fuse element in the form of a long cylindrical wire is not used because after melting, it will form a string of droplets and an arc will be struck between each of the droplets. The shape of the fuse element depends upon the characteristic desired.

When the fuse carries normal rated current, the heat energy generated is not sufficient to melt the fuse element. But when a fault occurs, the fuse element melts before the fault current reaches its first peak. As the element melts, it vaporizes and disperses.

Advantages of HRC Fuses

  1. Capability of clearing high values of fault currents
  2. Fast operation
  3. Non-deterioration for long periods
  4. No maintenance needed
  5. Reliable discrimination
  6. Consistent in performance
  7. Cheaper than other circuit interrupting devices
  8. Current limitation by cut-off action
  9. Inverse time-current characteristic

Disadvantages of HRC Fuses

  1.  It requires replacement after each operation.
  2. Inter-locking is not possible.
  3. It produces overheating of the adjacent contacts.

Applications of HRC Fuses

  1. Protection of low voltage distribution systems against overloads and short-circuits.
  2. Protection of cables
  3. Protection of busbars
  4. Protection of motors
  5. Protection of semiconductor devices
  6. Back up protection to circuit breaker

Diversity Factor

The ratio of the sum of individual maximum demands to the maximum demand on power station is known as diversity factor i.e.

[latex display=”true”]{\text{Diversity factor = }}\dfrac{{{\text{Sum of individual peak demand}}}}{{{\text{Maximum demand on power station}}}}[/latex]

A power station supplies load to various types of consumers whose maximum demands generally do not occur at the same time. Therefore, the maximum demand on the power station is always less than the sum of individual maximum demands of the consumers. Obviously, diversity factor will always be greater than.

Diversity factor helps in computing peak demand thus improving load factor and economic operation of power plant.The diversity factor can be equal or greater than 1. If the value of the diversity factor is greater than 1, then it is a good diversity factor, and 1.0 represents a poor diversity factor.The greater the diversity factor, the lesser is the cost of generation of power.


Derating factor of a cable

The factor due to which electrical cable lose its current carrying capacity means it can not carry the amount of current for which it is designed.

Derating factor depends on ambient temperature and how you are laying the cable i.e. in Air-Duct, Buried. Normally the current carrying capacity of a particular cable is calculated at 40*c, but if u lay the cable in air then it includes the temperature around it, if it increases more the 40*c then the conductor gets heated up by increasing the resistance in it thus the current carrying capacity of a given cable decreases, so we will derate the cable.

Following are the reason for derating factor comes into action

  1. Ambient Temperature: If the temperature of the environment increases correspondingly cable starts getting derated due to resistance changes.
  2. Air laying/Underground laying: cable get derated if it laid underground rather than upper ground.
  3. Soil Thermal resistivity: Basically the max temp in the surrounding. 40*C is the standard.
  4. Conductor grouping: Due to the arrangement of multiple conductors there is a development of an electromagnetic field which opposes the flow and thus we need to consider this factor.
  5. Temperature Derating Factor: In this, the arrangement of cables in a conduit is of prime importance. It should be in such a way that all cables have a minimum space to dissipate heat
    into the surroundings. The cables can touch each other but anyone cable should not be fully surrounded by other cables.
  6. Burial Depth: Depends on the length below ground where the conduit is laid.

In order to deal with this, a Derating Factor is associated with cables to arrive at an actual value of current carrying capacity.

Actual Current Carrying Capacity = Derating Factor × Cable current carrying capacity under standard conditions

Thus for a 100 A cable with a derating factor of 0.8 the actual current carrying capacity would
be: 0.8 × 100 = 80 A


Ques 5(d):-What are the different methods of power factor improvement?

Ans:- Power factor is the ratio of real power P to apparent power S or the cosine of the angle between voltage and current in an AC circuit and is denoted by cosΦ. For all types of inductive loads, the angle between voltage V and current I is negative and the cosine of this angle is called the lagging power factor. Similarly, when the angle between V and I is positive it is called leading power factor (this occurs for capacitive loads).

CAUSES OF LOW POWER FACTOR

  1. The induction motors work at a low lagging power factor at light loads and improved power factor with increased loads.
  2. The transformers have a lagging power factor because they draw magnetizing current.
  3. Miscellaneous equipment like arc lamps, electric discharge lamps, welding equipment etc., operates at a low power factor.
  4. The industrial heating Fumaces operate at a low lagging power factor.
  5. The variation of load on the power system also causes low power factor.

Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower and in such cases In order to improve the power factor, some device taking leading power should be connected in parallel with the load. This can be achieved by the following equipment

  1. Static capacitors
  2. Synchronous condenser
  3. Phase advancers

Static Capacitor:-

  • The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor.
  • The capacitor (generally known as the static capacitor) draws a leading current and partly or completely neutralizes the lagging reactive component of load current.
  • This raises the power factor of the load. For three-phase loads, the capacitors can be connected in delta or star as shown in Fig. Static capacitors are invariably used for power factor improvement in factories.

Static capacitor

  • Static capacitors are connected across the mains at the load end.
  • This supplies a reactive component of the current to reduce the out-of-phase component of current required by an inductive load i.e., it modifies the characteristics of an inductive load by drawing a leading current which counteracts or opposes the lagging component of the inductive load current at the point of installation.
  • So the reactive VAr’s transmitted over the line is reduced, thereby the voltage across the load is maintained within the specified limits.
  • By the application of the shunt capacitor to a line the magnitude of source current can be reduced, the power factor can be improved and consequently the voltage drop between the sending and receiving ends is also reduced.
  • It is important to note that it does not affect the current or the power factor beyond their point of installation.

Advantages

  1. They have low losses.
  2. They require little maintenance as there are no rotating parts.
  3. They can be easily installed as they are light and require no foundation.
  4. They can work under ordinary atmospheric conditions.

Disadvantages

  1. They have short service life ranging from 8 to 10 years.
  2. They are easily damaged if the voltage exceeds the rated value.
  3. Once the capacitors are damaged, their repair is uneconomical.

Synchronous condenser

A synchronous condenser is a synchronous motor running without the mechanical load. The Synchronous motor takes a leading current when over-excited and therefore behave as a capacitor. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.

synchronous condenser

Advantages

  1. A synchronous condenser has an inherently sinusoidal waveform and the voltage does not exist.
  2. It can supply as well as absorb kVAr.
  3. The PF can be varied in smoothly.
  4. It allows the overloading for short periods.
  5. The high inertia of the synchronous condenser reduces the effect of sudden changes in the system load and improves the stability of the system.
  6. It reduces the switching surges due to sudden connection or disconnection of lines in the system.
  7. The motor windings have the high thermal stability to short-circuit currents.
  8. By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving stepless control of power factor.
  9. The faults can be removed easily.

Disadvantages

  1. There are considerable losses in the motor.
  2. The maintenance cost is high.
  3. It is not possible to add or take away the units and to alter the rating of the synchronous condenser.
  4. For small rating it is uneconomical.
  5. As a synchronous motor has no self-starting torque, therefore, an auxiliary equipment has to be provided for this purpose

Phase Advancer:

There are special commutator machines, which are used to improve the power factor of the induction motor. When the supply is given to the stator of an induction motor, it takes a lagging current. So, the induction motor has low lagging power factor. For compensating this lagging current, a phase advancer (mounted on the same shaft) is used. It supplies mmf to the rotor circuit at slip frequency.

Advantages

  1. The lagging kVAr drawn by the motor is reduced by compensating the stator lagging current at slip frequency.
  2. Where the use of synchronous motors is not suitable, phase advancer can be used. Generally, these compensators are not recommended for the economical operation of motors of low rating below 200 HP.

Advantages of power factor improvement.

  1. The kW capacity of the prime movers is better utilized due to decreased reactive power.
  2. This increases the kilowatt capacity of the alternators, transformers, and the lines.
  3. The efficiency of the system is increased.
  4. The cost per unit decreases.
  5. Improves the voltage regulation of the lines.
  6. Reduction in power losses in the system due to the reduction in load current.

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