Thursday, 19 December 2019


                                                 OIL CIRCUIT BREAKER

Mineral oil better insulating properties than air. it is this property of oil which prompted steinmetz to break current under oil. he immersed an ordinary knife switch in oil and investigated the breaking capacity of the arrangement. Reliability, simplicity of construction and relative cheapness are particular virtues of oil, however, has the following disadvantages:

           1. It is inflammable and may cause fire hazards.
           2. There is a possibility of its forming an explosive mixture with air.
           3. because of the production of carbon particles in the oil due to heating, periodical reconditioning or replacement is required.

            The following are the categories of oil circuit breakers

           1. The plain-break oil c.b.
           2. The controlled break oil c.b. or bulk oil c.b.
           3. minimum oil c.b.

               The first and second categories of breakers are also known as the dead tank construction   because the tank is held at earth potential whereas the third category is known as live tank as the   tank containing oil is insulated from the ground.
                                        THE PLAIN-BREAK OIL CIRCUIT BREAKER

    The circuit breaker consists of a metal tank containing oil and encloses two or more contacts
. since large energies are to be dissipated within the tank, a large gaseous pressure is developed. to withstand such a large pressure the tank has to be a strong one. it is usual to make the tank and the top plates either of welded sheet steel or boiler plates. the distance between phases and the clearances between the live metal and the earthed metal are a function of the operating voltage.

principle of operation:the plain-break principle involves the simple process of separating the current carrying contacts under oil with on special control over the resulting arc other than the increase in length caused by the moving contact. the final arc extinction is obtained when a certain critical gap between the contacts is reached, the length of which depends upon the arc current and the recovery voltage.
                       At the instant of contact separation an arc is established between them. Initially, the separation is very small and a high voltage gradient between the contacts ionizes the oil. the gas obtained from the oil is mainly hydrogen which cannot remain in molecular form and is dissociated in its atomic form releasing lots of heat. with this, the arc core attains a temperature of 5000'k. the mixture of gases occupies a volume about one thousand times that of the oil decomposed. the oil is, therefore, pushed away form the arc and an expanding gas bubble surrounds the arc region based on energy balance principle, final extinction of arc takes place at a current zero when the power input to the arc is less then that dissipated between the contacts.

     form practical point of view the speed of the break should be as high as possible because a certain break distance has to be reached before interruption is likely to occur and the sooner this is achieved the smaller the energy released in the breaker and the less mechanically strong a breaker will have to be designed.
                       The double break arrangement as shown in Fig. 1.1 is perhaps the most familiar of all oil circuit breakers. the two breaks in series give rapid arc lengthening without the need for a specially fast contact speed, and the total gap distance at the end of stroke can conveniently be made ample. he vertical break also permits the use of a cylindrical oil tank requiring relatively low floor area.

Sunday, 17 November 2019

Hydroelectric power plant Basic questions and answers | MCQs on Hydroelectric power plant 05

1).Consider the following statements regarding pumped storage plants.
1)    A pumped storage plant is a base load plant.
2)    The starting time of a pumped storage plant is very short.
3)    Reversible turbines and pumps are highly suitable for pumped storage plants.
    which of the statements given above are correct.
(a)1 and 2. (b)1 and 3.
(c)2 and 3. (d)1.2 and 3.

2).In pump storage hydropower plant,the electrical machine is mode
to work alternately as generator and motor. The efficiency of the generator working at the same electric power level is.
a)    greater than that as motor.
b)    equal to that as motor.
c)    greater or less than that as motor depending on the type of the machine.
d)    les then that as motor.

3).A 'pumped storage hydroelectric plant' consists of.
a)    A synchronous machine and a multistage centrifugal pump in one shaft.
b)    A synchronous machine,a reaaction turbine,and a multistage centrifugal pump all in one shaft.
c)    An induction generator, a synchronous machine and a reaction turbine, all in one shaft.
d)    An induction generator, a synchronous machine,and a multistage centrifugal pump,all in one shaft.

4).Consider the following statements:
pumped storage plants when operated in interconnected power systems serve to
a)    increase load factor of steam plant.
b)    provide added capacity to meet peak loads.
c)    decrease load factor of steam plant.
d)    provide added capacity to meet base loads.
which of the above statements are correct.
(a)1,2,3 and 4. (b)1 and 3 only.
(c)1 and 2 only. (d)3 and 4 only.

5).Consider the following statements regarding the pumped storage plants.
1)    A pumped storage plant is a peak load plant.
2)    The starting time of a pumped storage plant.
3)    Reversible turbines and pumps are very suitable for pumped storage plants.
    Pumped storage plants can be used for load frequency control which of the above statements is/are correct.
(a)1 only. (b)1 and 2 only.
(c)1,3 and 4. (d)3 and 4 only.

6).In India, pumped storage power plant.
a)    does not exist at all.
b)    exists is Rajasthan alone.
c)    exists in kadampurai.
d)    exists in haryana.

7).The pumped storage scheme is employed to supply.
a)    during the peak hours.
b)    during the off-peak hours.
c)    system base load.
d)    all of the above.

8).In pumped storage scheme the generator is also used as.
a)    induction generator ar synchronous condenser.
b)    induction generator or synchronous motor.
c)    synchronous generator or induction generator.
d)    synchronous motor or synchronous condenser.

9).pump storage schemer are used to improve.
a)    the power factor. (b)the diversity factor.
c)    the load factor. (d)the quality of voltage.
e)    the reactive power capacity.
f)    the plant capacity factor as well as the load factor of the power system.

10).A hydel power plant of run-off-river type should be provided with pondage so that the.
a)    film capacity of the plant is increased.
b)    operating head is controlled.
c)    pressure inside the turbine casing remains constant.
d)    kinetic energy of the running water is fully utilized.

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Saturday, 16 November 2019

Hydroelectric power plant basic question and answers | Mcqs on hydroelectric power plant 04

1).The "specific speed" of a water turbine is the speed at which the turbine develops.
a)    maximum horse power.
b)    unit horse power at all heads.
c)    unit horse power at unit head.
d)    minimum horse power.

2).The specific speeds of kaplan, francis and pelton turbines are in.
a)    The increasing order.
b)    The decreasing order.
c)    neither increasing nor decreasing order.
d)    none of all.

3).An impulse turbine.
a)    is most suited for low head and high discharge power plants.
b)    operates by initial complete conversion to kinetic energy.
c)    makes use of a draft tube.
d)    always operates submerged.

4).Turbines installed at Bhakra Nangal are.
a)    pelton wheels.
b)    Francis turbines.
c)    Kaplan turbines.
d)    propeller turbines.

5).In water turbines the runaway speed of pelton wheel is.
a)    1.8 times rated speed.
b)    2-22 times rated speed.
c)    25-3 times rated speed.
d)    full load speed.

6).The number of buckets (Z) for a pelton wheel in terms of jet rati(m) is given by empirical relation.
a)    Z=15 m+0.5.
b)    Z=0.5 m+15.
c)    Z=2 m+15.
d)    Z=2+15 m.

7).In reaction turbine, the function of draft tube is to.
a)    reduce water hammer.
b)    increase flow rate of water.
c)    convert kinetic energy of water to potential energy by a gradual expansion in divergent pan.
d)    none of the above.

8).In Francis turbine of blades is usually of the order of.
a)    16-24.
b)    8-10.
c)    6-8.
d)    3-6.

9).The maximum head of a kaplan turbine is limited to.
a)    25 m.
b)    70 m.
c)    125 m.
d)    185 m.

10).which of the following statements is/are correct, In pumped storage power plants.
a)    water is recirculated through water turbines.
b)    reversible turbines are used which operate as turbines for
power generation during peak load hours and as pumps for
pumping water during peak-off hours.

c)    plain Francis turbines are used.
d)    both (a) and (c) above are employed.

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Monday, 11 November 2019

Hydroelectric power plant basic questions and answers | MCQS on hydroelectric power plant 03

1).The function of a surge tank is to.
a)    supply water at constant pressure.
b)    relieve water hammer pressures in the penstock pipe.
c)    produce surge in the pipeline.
d)    none of the above.

2).water hammer occurs in.
a)    surge tank.
b)    penstock.
c)    turbine casing.
d)    draft tube.

3).In a medium or high head hydroelectric power station,a surge tank is provided to.
a)    reduce the length of the penstock pipes.
b)    augment water at the forebay.
c)    control the pressure variations in the pennstock pipes due to sudden load changes.
d)    control the water flow through the turbine.

4).where is the draft tube of a hydropower station that is an airtight pipe located.
a)    Near the surge tank.
b)    In between the penstock and the runner.
c)    In between the runner exhaust and the tailrace.
d)    At the beginning of pennstock.

5).The draft tube is provided to.
a)    raise the water surface of the stream to create an artificial head.
b)    reduce the effect of water hammer.
c)    increase the acting head on the water wheel.
d)    none of the above.

6).For variable heads of near above but less than 30 metres which type of turbines is used in hydropower stations.
a)    pelton.
b)    kaplan.
c)    Francis.
d)    None of the above.

7).For harnessing lower variable water heads,the suitable hydraulic turbine with high percentage of reaction and runner adjustable vanes is.
a)    kaplan.
b)    Francis.
c)    pelton.
d)    Impeller.

8).For harnessing low variable water heads the suitable hydroulic turbine with reaction and adjustable vanes is.
a)    Francis.
b)    Impeller.
c)    kaplan.
d)    pelton.

9).For high head and low discharge the water turbine used is.
a)    pelton. wheel.
b)    kaplan turbine.
c)    Francis turbine.
d)    propeller turbine.

10).The specific speed (Ns) of a turbine is given by the equation.
a)    Ns=N√P/H0.75
b)    Ns=√N.P/H3/2
c)    Ns=N√P/H1.25
d)    Ns=N√P/H2/3

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