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SJVN JFE & FE study materials
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SJVN JFE & FE practice questions

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Question 1Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a hydroelectric power station, the function of the surge tank in the water conductor system is to:

  1. Convert pressure energy into kinetic energy at the runner
  2. Increase the gross head available at the turbine inlet
  3. Absorb pressure rise and supply water during load rejection and sudden demand respectively Correct answer.
  4. Remove suspended silt before the water enters the penstock

Explanation

A surge tank is placed between the reservoir and the turbine to dampen water-hammer pressure surges when load is suddenly rejected and to feed the turbine quickly when load increases. It protects the penstock from excessive pressure rise and limits negative pressure, so it does not add head or treat silt.

Question 2Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The primary purpose of the economiser in a thermal power plant boiler is to:

  1. Supply combustion air to the furnace
  2. Superheat the steam before it reaches the turbine
  3. Reduce the flue gas temperature below the acid dew point
  4. Heat the feed water using flue gases before it enters the drum Correct answer.

Explanation

An economiser recovers sensible heat from the exiting flue gases and transfers it to the boiler feed water, raising cycle efficiency. It is placed in the flue path ahead of the air preheater and does not superheat steam or supply air.

Question 3Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The synchronous speed of a 4-pole generator connected to a 50 Hz grid is:

  1. 1000 rpm
  2. 3000 rpm
  3. 750 rpm
  4. 1500 rpm Correct answer.

Explanation

Synchronous speed follows Ns = 120f/P. Substituting f = 50 Hz and P = 4 gives Ns = 120 × 50 / 4 = 1500 rpm. This relationship fixes the speed at which the machine must be driven to stay synchronised with the grid.

Question 4Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a Pelton turbine installation, the purpose of the spear valve in the nozzle is to:

  1. Convert the jet velocity into pressure energy
  2. Direct the jet tangentially onto the bucket splitter
  3. Prevent the runner from over-speeding during load rejection
  4. Regulate the flow of water by varying the nozzle opening Correct answer.

Explanation

The spear valve moves axially inside the nozzle to change the annular opening, thereby controlling the jet discharge and hence the power output. It is the main governing element for a Pelton wheel and is actuated by the governor.

Question 5Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 3-phase, 11 kV/433 V distribution transformer is to be protected against internal faults. Which protection scheme responds specifically to faults inside the transformer tank?

  1. Overcurrent relay on the LV side
  2. Under-voltage relay
  3. Buchholz relay Correct answer.
  4. Earth fault relay on the HV side

Explanation

A Buchholz relay senses gas accumulation and oil surge caused by incipient internal faults such as turn-to-turn insulation failure in oil-immersed transformers. It gives an alarm for slow gas generation and trips for severe oil flow. Overcurrent and earth fault relays respond mainly to external faults.

Question 6Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The critical speed of a hydraulic turbine shaft is the speed at which:

  1. The turbine attains runaway speed with the guide vanes fully open
  2. Cavitation begins at the runner outlet
  3. The generator first synchronises with the grid
  4. The shaft experiences resonant vibration due to matching of natural and running frequency Correct answer.

Explanation

Critical speed occurs when the rotational frequency coincides with a natural frequency of the shaft-rotor assembly, causing excessive vibration and possible damage. Units are designed so that normal operating speed stays well clear of critical speeds.

Question 7Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a Francis turbine, cavitation damage is most likely to occur at the:

  1. Main inlet valve seat
  2. Guide vane pivot pin
  3. Runner blade outlet near the draft tube Correct answer.
  4. Spiral casing inlet

Explanation

Cavitation occurs where local static pressure falls below vapour pressure, which in a Francis turbine is typically at the runner outlet near the draft tube. The resulting vapour bubbles collapse and erode the blade surfaces. Raising the setting or increasing suction head reduces the risk.

Question 8Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The main reason for using a draft tube in a reaction turbine is to:

  1. Prevent water from entering the generator hall
  2. Increase the head of water at the runner inlet
  3. Recover the kinetic energy of the water leaving the runner Correct answer.
  4. Support the weight of the runner assembly

Explanation

A draft tube decelerates the exit flow and converts its kinetic energy into a pressure rise, allowing the turbine to be set above tailrace level without losing that energy. This improves overall efficiency and permits easier inspection and maintenance.

Question 9Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a thermal power plant, the air preheater is placed in the flue gas path after the economiser primarily to:

  1. Increase the oxygen content of the combustion air
  2. Reduce the speed of the induced draft fan
  3. Recover additional heat from the flue gases and improve boiler efficiency Correct answer.
  4. Cool the flue gases below the dew point to condense water vapour

Explanation

After the economiser has extracted heat, the air preheater recovers further sensible heat from the flue gases and transfers it to the incoming combustion air. This raises furnace temperature, improves combustion and reduces stack losses, increasing boiler efficiency.

Question 10Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A generating unit operating at 0.85 power factor lagging supplies 100 MW. The approximate reactive power delivered is:

  1. 85 MVAr
  2. 100 MVAr
  3. 53 MVAr
  4. 62 MVAr Correct answer.

Explanation

For P = 100 MW at cos φ = 0.85, the angle gives tan φ ≈ 0.6197, so Q = P tan φ ≈ 62 MVAr. Alternatively S = 100/0.85 ≈ 117.6 MVA and Q = √(S² − P²) ≈ 62 MVAr.

Question 11Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The function of the governor in a hydroelectric generating unit is to:

  1. Maintain the generator terminal voltage constant
  2. Lubricate the turbine guide bearing
  3. Control guide vane or needle opening to maintain frequency under load changes Correct answer.
  4. Limit the stator current during short circuits

Explanation

The speed governor senses frequency deviation and adjusts the water admission through guide vanes or the spear valve to match generation to load. This keeps the unit frequency within the permissible band and provides droop-based load sharing. Voltage control is a separate excitation function.

Question 12Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

Which relay characteristic is typically used for the distance protection of a transmission line feeding a power station switchyard?

  1. Differential characteristic of a transformer
  2. Buchholz characteristic
  3. Instantaneous overcurrent characteristic
  4. Mho characteristic Correct answer.

Explanation

Mho relays measure impedance with a circular characteristic that is inherently directional, making them suitable for distance protection of transmission lines. Their reach is set in ohms to cover a defined percentage of the line.

Question 13Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A penstock of internal diameter 2 m carries water at 6 m/s. The approximate discharge is:

  1. 9.4 m³/s
  2. 37.7 m³/s
  3. 75.4 m³/s
  4. 18.8 m³/s Correct answer.

Explanation

Discharge Q = A × V, where A = πd²/4 = π × 4 / 4 = 3.1416 m². Multiplying by 6 m/s gives Q ≈ 18.85 m³/s.

Question 14Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a thermal power plant, the purpose of the deaerator in the feed water cycle is to:

  1. Increase the pH of the boiler water
  2. Reduce the feed water temperature before the economiser
  3. Remove dissolved oxygen and other gases from the feed water Correct answer.
  4. Filter suspended solids from the condensate

Explanation

The deaerator heats feed water to saturation and strips dissolved oxygen and carbon dioxide, which would otherwise cause corrosion in the boiler and feed system. It also serves as a feed water storage and suction point for the boiler feed pump.

Question 15Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The runaway speed of a Pelton turbine is approximately what fraction of its normal speed?

  1. 2.5 to 3.0 times
  2. 1.8 to 2.0 times Correct answer.
  3. 1.1 to 1.2 times
  4. Equal to normal speed

Explanation

With the jet fully deflected and no load, a Pelton wheel can reach about 1.8 to 2.0 times its rated speed because the jet speed sets an upper limit on bucket speed. The generator and shaft are designed to withstand this runaway condition.

Question 16Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The stator core of a large generator is laminated to reduce:

  1. Windage losses
  2. Copper losses in the stator winding
  3. Eddy current losses Correct answer.
  4. Hysteresis losses in the rotor

Explanation

Laminating the stator core interrupts the paths of induced eddy currents, greatly reducing eddy current loss in the iron. Thin insulated laminations are stacked and clamped to form the core.

Question 17Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A hydroelectric unit has a rated head of 100 m and a discharge of 20 m³/s. Taking water density as 1000 kg/m³ and g as 9.81 m/s², the hydraulic power available is approximately:

  1. 19.6 MW Correct answer.
  2. 2.0 MW
  3. 39.2 MW
  4. 9.8 MW

Explanation

Hydraulic power equals ρgQH = 1000 × 9.81 × 20 × 100 ≈ 19.6 MW. This is the theoretical power before applying turbine and generator efficiencies.

Question 18Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a thermal power plant, the condenser is maintained at a low pressure primarily to:

  1. Increase the boiler drum pressure
  2. Reduce the cooling water requirement to zero
  3. Increase the enthalpy drop available across the turbine Correct answer.
  4. Prevent corrosion in the turbine blades

Explanation

Lowering condenser pressure reduces the turbine exhaust enthalpy, thereby increasing the useful enthalpy drop and the cycle efficiency. The vacuum is maintained by continuous steam condensation and air extraction.

Question 19Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The main purpose of the spiral casing in a reaction turbine is to:

  1. House the thrust bearing assembly
  2. Distribute water uniformly around the runner at nearly constant velocity Correct answer.
  3. Separate the generator from the water passage
  4. Increase the pressure of water before the runner

Explanation

The spiral casing, or volute, has a decreasing cross-section that keeps the flow velocity roughly constant while distributing water uniformly to all guide vanes around the runner. Uniform admission is essential to avoid unbalanced hydraulic forces.

Question 20Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A station service transformer is rated 5 MVA, 11 kV/415 V. Its rated LV line current is approximately:

  1. 2780 A
  2. 12000 A
  3. 6950 A Correct answer.
  4. 260 A

Explanation

For a three-phase transformer, I = S / (√3 × V). With S = 5 × 10⁶ VA and V = 415 V, I = 5 × 10⁶ / (1.732 × 415) ≈ 6950 A.

Question 21Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The function of the turbine head cover and shaft seal in a vertical Francis unit is to:

  1. Prevent water from leaking along the shaft into the generator area while permitting rotation Correct answer.
  2. Convert hydraulic energy into mechanical energy
  3. Support the entire weight of the rotor assembly
  4. Regulate the flow of water to the runner

Explanation

The head cover forms the upper boundary of the runner chamber, and the shaft seal limits leakage along the rotating shaft so water does not reach the generator. It also provides access for maintenance of guide apparatus components.

Question 22Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a power transformer, the tap changer is provided on the high-voltage winding mainly because:

  1. The LV winding cannot be earthed
  2. The HV winding carries lower current, allowing a lighter tap changer Correct answer.
  3. The HV winding has fewer turns and is easier to tap
  4. The HV winding is always star connected

Explanation

Tap changers are usually placed on the HV winding because it carries lower current for the same MVA rating, which reduces contact size and arcing duty in the diverter switch. This makes on-load tap changing more compact and reliable.

Question 23Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The primary function of the trash rack at the intake of a hydroelectric plant is to:

  1. Reduce the velocity of water before the turbine
  2. Provide access for maintenance personnel
  3. Prevent debris from entering the water conductor system and damaging the turbine Correct answer.
  4. Measure the discharge entering the penstock

Explanation

A trash rack is a screen of bars at the intake that stops floating debris and large solids from entering the penstock and damaging the turbine runner and seals. The head loss across it is monitored and cleaned periodically.

Question 24Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

During a sudden loss of load on a hydro unit, the governor closes the guide vanes rapidly. The resulting water hammer pressure rise in the penstock is limited by:

  1. Raising the tailrace level
  2. Increasing the penstock diameter at the turbine end only
  3. Providing a surge tank or pressure relief arrangement near the powerhouse Correct answer.
  4. Reducing the trash rack spacing

Explanation

Rapid valve closure converts kinetic energy into a pressure surge, and a surge tank or relief valve absorbs this transient. The surge tank provides a free surface that reflects the pressure wave and shortens the water column affected.

Question 25Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In an induction generator used for small hydro, the machine draws reactive power from the grid because:

  1. Its rotor resistance is too high for self-excitation
  2. Its stator winding is open-circuited
  3. Its slip is always negative
  4. It has no independent excitation and needs magnetising current from the supply Correct answer.

Explanation

An induction generator has no separate field winding, so it must draw magnetising current from the grid or a capacitor bank to establish the air-gap flux. It delivers active power while absorbing reactive power.

Question 26Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a hydroelectric station, the surge tank is installed on the penstock primarily to:

  1. Reduce the runaway speed of the turbine on full load loss
  2. Absorb pressure rise and supply water during load rejection and demand changes Correct answer.
  3. Remove suspended silt before the water enters the runner
  4. Increase the net head available at the turbine inlet

Explanation

A surge tank provides a free water surface close to the powerhouse so that rapid guide-vane movement produces only a short water column to accelerate or decelerate, limiting water hammer and allowing fast governor response. It does not add head or screen sediment; it moderates hydraulic transients.

Question 27Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

Which material is commonly used for the runner of a high-head Pelton turbine because of its resistance to silt erosion?

  1. Stainless steel Correct answer.
  2. Mild steel
  3. Cast iron
  4. Aluminium alloy

Explanation

Stainless steel, often 13/4 chromium-nickel grade, is standard for Pelton buckets and Francis runners because it combines high hardness with corrosion resistance, giving acceptable life under silt-laden high-velocity jets. Softer ferrous and light alloys erode rapidly.

Question 28Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The synchronous speed of a 50 Hz, 12-pole synchronous generator is:

  1. 600 rpm
  2. 500 rpm Correct answer.
  3. 1000 rpm
  4. 250 rpm

Explanation

Synchronous speed is N = 120f/P, so for f = 50 Hz and P = 12, N = 120 x 50 / 12 = 500 rpm. This relation links grid frequency to the number of poles chosen for the turbine speed.

Question 29Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a thermal power plant, the function of the superheater is to:

  1. Raise steam temperature above saturation before it enters the turbine Correct answer.
  2. Heat the incoming combustion air before the furnace
  3. Heat the condensate before it enters the boiler drum
  4. Dry the coal before it is pulverised in the mill

Explanation

The superheater transfers heat from flue gas to saturated steam, raising its temperature above the saturation value at drum pressure. This reduces moisture at the turbine exhaust and improves cycle efficiency.

Question 30Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The efficiency of a transformer at unity power factor is maximum when:

  1. Copper loss is twice the iron loss
  2. Copper loss equals iron loss Correct answer.
  3. Iron loss is twice the copper loss
  4. Copper loss is zero

Explanation

Transformer efficiency peaks at the load where variable copper loss equals constant iron loss, since at that point the derivative of efficiency with respect to load current is zero. The condition is independent of power factor for the loss balance itself.

Question 31Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The purpose of the governor oil pressure unit in a hydro generating station is to:

  1. Cool the thrust bearing during load rejection
  2. Supply high-pressure oil to operate the servomotors of the guide vanes Correct answer.
  3. Provide insulation oil for the generator step-up transformer
  4. Lubricate the turbine guide bearing during rotation

Explanation

The governor oil pressure unit maintains stored hydraulic energy so that servomotors can move the guide vanes and, where fitted, the runner blades quickly on speed or load changes. Bearing lubrication and transformer oil are separate systems.

Question 32Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 3-phase, 11 kV/415 V distribution transformer is star-star connected. The phase voltage on the HV side is approximately:

  1. 19.05 kV
  2. 3.18 kV
  3. 11 kV
  4. 6.35 kV Correct answer.

Explanation

In a star connection the phase voltage is the line voltage divided by root three, so 11000 / 1.732 is about 6350 V. The turns ratio per phase is based on this value, not on the line voltage.

Question 33Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The main reason for providing a concrete gravity dam with a drainage gallery near the upstream face is to:

  1. Provide access for the spillway gates
  2. Store water for the power plant cooling system
  3. Collect seepage water and reduce uplift pressure on the base Correct answer.
  4. Increase the weight of the dam for stability

Explanation

Drainage galleries and the drains drilled from them intercept seepage and relieve uplift pressure at the base, improving sliding stability. Grout curtains and drains work together to control seepage.

Question 34Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A penstock of 1.5 m internal diameter carries water at 4 m/s. The approximate discharge is:

  1. 1.8 m³/s
  2. 7.1 m³/s Correct answer.
  3. 4.7 m³/s
  4. 9.4 m³/s

Explanation

Discharge equals area times velocity; area is pi times diameter squared over four, giving about 1.767 m². Multiplying by 4 m/s gives approximately 7.07 m³/s.

Question 35Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a hydroelectric plant, the function of the draft tube in a Kaplan turbine is to:

  1. Allow the runner to be set above tailwater while recovering exit kinetic energy Correct answer.
  2. Increase the head available at the runner inlet
  3. Support the generator thrust bearing
  4. Guide water tangentially into the runner

Explanation

A draft tube connects the runner outlet to the tailrace, permitting the runner to be placed above tailwater while converting the high exit velocity into a pressure rise. This recovers energy that would otherwise be lost.

Question 36Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 100 MVA, 50 Hz generator has an inertia constant H of 4 MJ/MVA. The stored kinetic energy at rated speed is:

  1. 400 MJ Correct answer.
  2. 25 MJ
  3. 1600 MJ
  4. 100 MJ

Explanation

Stored energy equals H multiplied by the machine rating, so 4 MJ/MVA x 100 MVA = 400 MJ. This energy governs the initial rate of frequency change after a load imbalance.

Question 37Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a thermal power plant, the reheater between turbine stages is used mainly to:

  1. Reduce exhaust moisture and improve cycle efficiency Correct answer.
  2. Heat feed water before the economiser
  3. Remove dissolved gases from condensate
  4. Increase boiler drum pressure

Explanation

Reheating steam after partial expansion and returning it to the turbine raises the average temperature of heat addition and reduces the moisture at the low-pressure exhaust, protecting blades. It is not a pressure-raising device.

Question 38Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 3-phase induction motor draws 50 kW from the supply and delivers 45 kW at the shaft. Its efficiency is approximately:

  1. 85 percent
  2. 95 percent
  3. 111 percent
  4. 90 percent Correct answer.

Explanation

Efficiency is output over input, so 45/50 equals 0.90, or about 90 percent. The remaining 5 kW covers stator and rotor losses and windage.

Question 39Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

The purpose of the core-type construction with laminated silicon steel in a power transformer is to:

  1. Increase the leakage reactance deliberately
  2. Provide mechanical support for the bushings
  3. Reduce eddy current and hysteresis losses Correct answer.
  4. Improve the dielectric strength of the oil

Explanation

Thin laminations interrupt eddy current paths, and silicon steel has a narrow hysteresis loop, so both components of core loss are reduced. Core loss is present whenever the transformer is energised, regardless of load.

Question 40Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a 220 kV substation, a current transformer is specified with a 5P20 class. The 20 indicates that the composite error remains within limits up to:

  1. 20 times rated primary current Correct answer.
  2. 20 kA primary current
  3. 20 VA secondary burden
  4. 20 percent of rated current

Explanation

In the 5P20 designation, 5P denotes a protection class with 5 percent composite error and 20 is the accuracy limit factor, meaning twenty times rated primary current. This ensures faithful reproduction of fault currents for relays.

Question 41Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A transmission line of 100 km length is to be protected against faults with a scheme that does not depend on communication between the two ends. The most suitable choice is:

  1. Differential pilot protection
  2. Transfer trip over fibre
  3. Carrier-aided tripping scheme
  4. Distance protection with stepped zones Correct answer.

Explanation

Stepped distance protection uses local voltage and current to define zones and needs no channel between ends, making it suitable where communication is absent. Pilot and transfer-trip schemes depend on signalling.

Question 42Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a hydro turbine, the specific speed is defined for a geometrically similar machine that would develop:

  1. Unit power at unit head
  2. One kilowatt at one metre head Correct answer.
  3. Maximum power at runaway speed
  4. Rated power at rated head

Explanation

Specific speed is a similarity parameter referred to a homologous machine developing 1 kW under 1 m head, allowing comparison of runner types. It guides the choice among Pelton, Francis, and Kaplan runners for a given head and discharge.

Question 43Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A generator transformer is rated 315 MVA, 15.75 kV/400 kV. Its rated HV line current is approximately:

  1. 788 A
  2. 2730 A
  3. 455 A Correct answer.
  4. 1364 A

Explanation

For a three-phase transformer, line current equals MVA times 1000 divided by root three times line voltage, so 315000 / (1.732 x 400) is about 455 A. This current sets the HV bushing and conductor rating.

Question 44Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a thermal power plant, the induced draft fan is located:

  1. Inside the boiler drum
  2. Between the forced draft fan and the air preheater
  3. Between the air preheater and the chimney Correct answer.
  4. At the furnace outlet before the superheater

Explanation

The induced draft fan handles hot flue gas downstream of the air preheater and maintains the furnace at slight negative pressure, drawing combustion products to the chimney. The forced draft fan handles cold air on the upstream side.

Question 45Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 500 m long penstock carries water at 5 m/s. Taking the pressure wave speed as 1000 m/s, the critical time for rapid closure of the turbine inlet valve is approximately:

  1. 1.0 s Correct answer.
  2. 5.0 s
  3. 2.0 s
  4. 0.5 s

Explanation

The critical time is 2L divided by the wave speed, giving 2 x 500 / 1000 = 1.0 s. Valve closure faster than this produces full water hammer pressure rise.

Question 46Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A hydro unit has a rated head of 80 m and a discharge of 30 m³/s. Taking water density as 1000 kg/m³, g as 9.81 m/s², and overall efficiency as 90 percent, the shaft power is approximately:

  1. 21.2 MW Correct answer.
  2. 18.9 MW
  3. 26.1 MW
  4. 23.5 MW

Explanation

Hydraulic power is density times g times head times discharge, about 23.5 MW, and the shaft output is 90 percent of this, approximately 21.2 MW. Efficiency is applied after the hydraulic power is computed.

Question 47Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a power station, the station service supply is arranged with a diesel generator as a standby source mainly to:

  1. Provide additional active power to the grid during peak load
  2. Supply reactive power for voltage support
  3. Supply essential auxiliaries during total loss of station AC power Correct answer.
  4. Replace the main transformer during maintenance

Explanation

The emergency diesel generator supplies critical loads such as bearing oil pumps, control and protection systems, and lighting when normal station service is lost. It is sized for essential loads, not for grid support.

Question 48Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 100 MVA generator with an inertia constant of 5 MJ/MVA is loaded at 0.8 pu power when a fault causes a sudden loss of 0.2 pu generation. The initial rate of change of frequency is approximately:

  1. 0.4 Hz/s Correct answer.
  2. 0.8 Hz/s
  3. 0.2 Hz/s
  4. 1.0 Hz/s

Explanation

The swing equation gives df/dt equal to the power imbalance times rated frequency divided by twice the stored energy in per unit, so 0.2 x 50 / (2 x 5) = 1.0 Hz/s for a base of the machine rating, and with the machine supplying 0.8 pu the deficit relative to its own base is 0.2 pu, giving about 1.0 Hz/s on a 100 MVA base; with the deficit of 0.2 pu on the system the rate is approximately 1 Hz/s, and using the per-unit inertia of 5 s the result is 0.2 x 50 / (2 x 5) = 1 Hz/s.

Question 49Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

In a hydroelectric plant, the spiral casing of a Francis turbine is designed with a gradually reducing cross-section because:

  1. The flow to the stay vanes must be distributed uniformly around the runner at nearly constant velocity Correct answer.
  2. The casing must act as a surge chamber for the penstock
  3. The velocity must increase towards the nose to reduce friction
  4. The water pressure must rise along the casing to assist the draft tube

Explanation

The decreasing area compensates for the water progressively leaving through the stay ring, keeping the circumferential velocity roughly constant and the admission uniform. Uniform distribution is essential for balanced hydraulic loading on the runner.

Question 50Technical (Electrical/Mechanical/Civil/Electronics discipline knowledge)

A 3-phase, 50 Hz, 4-pole induction generator is driven at 1530 rpm and connected to the grid. Its slip and approximate rotor frequency are:

  1. Slip minus 2 percent, rotor frequency 50 Hz
  2. Slip plus 2 percent, rotor frequency 25 Hz
  3. Slip minus 2 percent, rotor frequency 1 Hz Correct answer.
  4. Slip plus 2 percent, rotor frequency 1 Hz

Explanation

Synchronous speed is 1500 rpm, so slip is (1500 - 1530)/1500 = -0.02, that is minus 2 percent, indicating supersynchronous generation. Rotor frequency equals slip times supply frequency, giving about 1 Hz.