← MES Jr. Grade
Past Questions — 2022
Which one of the following relays has the capability of anticipating the possible major fault in a transformer?
A)
Overcurrent relay
B)
Differential relay
C)
Buchholz relay
D)
Overfluxing relay
The Buchholz relay is a gas-actuated relay specifically designed for oil-immersed transformers. It is installed in the pipe connecting the main transformer tank to the conservator tank. Its primary function is to detect incipient (developing) faults—such as insulation breakdown, core heating, or minor sparking—which produce small amounts of gas. These gases accumulate in the relay, triggering an alarm to warn operators of a potential major fault before it becomes catastrophic. If a severe fault occurs (like a short circuit), the resulting surge of oil triggers a trip signal to disconnect the transformer immediately.
Why other options are incorrect:
- a. Overcurrent relay: This is a protective relay that trips when the current exceeds a predefined limit; it reacts to the fault itself, not the 'anticipation' of one.
- b. Differential relay: This is used for internal fault protection by comparing currents entering and leaving the transformer; it operates once a fault has already occurred.
- d. Overfluxing relay: This protects the transformer from saturation caused by overvoltage or under-frequency conditions, which is a specific operating condition rather than a general anticipation of internal faults.
A synchronous motor can run at :
A)
a leading power factor
B)
unity power factor
C)
lagging or leading or unity power factor
D)
zero power factor
A synchronous motor is unique because its power factor can be controlled by adjusting the DC excitation of the field winding while the motor is running at a constant synchronous speed. This is explained by the V-curve characteristics of the motor:
1. Under-excitation: When the field current is low, the motor draws reactive power from the grid to maintain the magnetic field, causing it to operate at a lagging power factor (behaving like an inductor).
2. Normal excitation: When the field current is adjusted such that the motor draws only active power, it operates at a unity power factor (PF = 1).
3. Over-excitation: When the field current is increased beyond the normal requirement, the motor supplies reactive power back to the grid, causing it to operate at a leading power factor (behaving like a capacitor).
Because of this ability to operate across the entire spectrum of power factors, synchronous motors are often used for power factor correction in industrial plants (referred to as a 'synchronous condenser' when running without a mechanical load). Therefore, options a, b, and d are only partial truths, while option c is the complete and correct answer.
In a power system, power is supplied to a short-line through transformer connected to bus-bar. If a line-to-ground fault occurs on one of the lines and is cleared by a circuit breaker close to transformer (on line side), derive the expression for restriking voltage and its natural frequency.
What are the symmetrical components? Explain briefly with the help of vector diagram the positive, negative and zero sequence quantities.
A 25KVA transformer has 500 turns on primary and 50 turns on the secondary winding. The primary is connected to 3000V, 50Hz supply. Find the full load primary and secondary currents, the secondary e.m.f and the maximum flux in the core. Neglect leakage drops and no-load primary current.
Estimate the distance over which a load of 15000KW at 0.8 lagging power factor can be delivered by a 3-phase transmission line having conductors each of resistance 1W per kilometre. The voltage at the receiving end is to be 132KV and the loss in the transmission is to be 5%.
How do you define load factor, diversity factor, and plant use factor? What steps would you suggest to improve the load factor of a system whose load factor is abysmally low?
What will be the type of the system, if the steady state performance of control system yields a non-zero finite value of the velocity error constant?
A)
Type-0
B)
Type-1
C)
Type-2
D)
Type-3
In control systems, the 'Type' of a system is defined by the number of poles at the origin in the open-loop transfer function G(s)H(s). The steady-state error depends on the system type and the type of input applied.
1. The Velocity Error Constant (Kv) is defined as: Kv = lim (s → 0) [s * G(s)H(s)].
2. For a Type-0 system: Kv = lim (s → 0) [s * G(s)H(s)] = 0. (The error constant is zero).
3. For a Type-1 system: The transfer function has one integrator (1/s). Thus, Kv = lim (s → 0) [s * (K/s) * G'(s)] = K * G'(0), which results in a non-zero finite value.
4. For a Type-2 system (or higher): The transfer function has two or more integrators (1/s²). Thus, Kv = lim (s → 0) [s * (K/s²) * G'(s)] = lim (1/s) = ∞. (The error constant is infinite).
Since the question specifies that the velocity error constant is a non-zero finite value, the system must be Type-1.
Which of the following power stations is mainly used to cover peak load on the system?
A)
Coal based thermal power plant
B)
Nuclear power plant
C)
Gas based thermal power plant
D)
Pumped storage hydro power plant
In power system planning, power plants are categorized based on their operational roles: Base Load plants and Peak Load plants.
1. **Base Load Plants**: These are plants that run continuously at a constant output to meet the minimum power demand of the system. They typically have low operating costs but high capital costs and slow start-up/shut-down times. Examples include Nuclear power plants (Option b) and Coal-based thermal power plants (Option a).
2. **Peak Load Plants**: These are plants that can be started and stopped quickly to meet the short-term spikes in demand (peak loads). They usually have higher operating costs but very fast response times.
Analysis of options:
- **Coal-based thermal power plants (a)**: Slow to start and stop; used for base load.
- **Nuclear power plants (b)**: Extremely slow to modulate power; used strictly for base load.
- **Gas-based thermal power plants (c)**: These can be used for peak loads (peaking plants), but they are not as efficient or specialized for large-scale energy storage and rapid discharge as pumped storage.
- **Pumped storage hydro power plants (d)**: These are specifically designed for peak shaving. They pump water to a higher reservoir during off-peak hours (using excess energy) and release it to generate electricity during peak hours. They have the fastest response time and are the primary choice for covering peak loads in large grids.
Therefore, the Pumped storage hydro power plant is the most accurate answer for covering peak loads.
Explain, briefly, the working principle of a single induction motor with the help of double revolving field theory.
Which of the following is used for Nyquist plot?
A)
Characteristic equation
B)
Open-loop function
C)
Close-loop function
D)
None of these
The Nyquist stability criterion is a graphical method used to determine the stability of a closed-loop system by inspecting the properties of the open-loop transfer function. Specifically, a Nyquist plot is a polar plot of the open-loop transfer function G(s)H(s) as the frequency ω varies from 0 to infinity (and often from -infinity to 0). By mapping the Nyquist contour in the s-plane through the open-loop function, we can determine the number of unstable poles of the closed-loop system using the principle of the argument.
Why other options are incorrect:
- Option (a) Characteristic equation: This is the equation 1 + G(s)H(s) = 0. While the Nyquist plot helps analyze the roots of this equation, the plot itself is constructed using the open-loop function, not the equation.
- Option (c) Close-loop function: The closed-loop transfer function is T(s) = G(s) / [1 + G(s)H(s)]. The Nyquist plot is used to predict the stability of the closed-loop system *without* having to explicitly solve for the closed-loop transfer function's poles.
The overall thermal efficiency of a thermal power plant lies in the range :
A)
25% to 30%
B)
35% to 40%
C)
45% to 60%
D)
65% to 80%
The overall thermal efficiency of a power plant is the ratio of the electrical energy output to the total heat energy input from the fuel. In a typical modern thermal power plant (Rankine Cycle), efficiency is limited by the Carnot efficiency and various losses.
1. **Thermodynamic Limits**: Most conventional coal-fired power plants operate with steam temperatures around 540-600°C and condenser pressures near vacuum. This typically results in a thermal efficiency of about 35% to 45% for the cycle itself.
2. **Plant Losses**: The 'overall' efficiency must account for losses in the boiler (incomplete combustion, heat loss through walls), turbine mechanical losses, and the auxiliary power consumption (pumps, fans, coal handling).
3. **Typical Range**: While older plants might operate at 30-33%, modern subcritical and supercritical plants generally fall within the 35% to 40% range. Ultra-supercritical plants can push this higher (up to 45%), but for standard competitive exam contexts regarding general thermal power plants, 35-40% is the accepted standard range.
Comparing options:
- Option (a) 25-30% is too low for modern standards.
- Option (c) 45-60% is generally unattainable for standard thermal plants due to the Second Law of Thermodynamics.
- Option (d) 65-80% is physically impossible for a standard steam-based thermal power plant.
Which of the following motors is used to derive the constant speed line shafting lathes, blowers and fans?
A)
D.C shunt motor
B)
D.C series motor
C)
Commutative compound motor
D)
None of these
To determine the correct motor, we must analyze the speed-torque characteristics required for the mentioned applications (lathes, blowers, and fans):
1. **D.C. Shunt Motor**: This motor is known as a 'constant speed motor'. Its speed drops only slightly as the load increases. This characteristic is essential for line shafting, lathes, and centrifugal blowers/fans, where a stable, predictable speed is required regardless of the load variation.
2. **D.C. Series Motor**: This motor has a high starting torque but its speed varies drastically with the load (speed is inversely proportional to the load current). If run at no-load, it can reach dangerously high speeds. Therefore, it is used for high-starting torque applications like electric trains, cranes, and hoists, not for constant speed shafting.
3. **Cumulative Compound Motor**: This motor combines the characteristics of both shunt and series motors. It provides better starting torque than a shunt motor but does not maintain as constant a speed. It is typically used for heavy-duty applications like punch presses or shears.
Since the question specifically asks for 'constant speed' applications like line shafting, lathes, blowers, and fans, the D.C. shunt motor is the correct choice.
A second-order control system exhibits 100% overshoot. Its damping coefficient is :
A)
greater than 1
B)
less than 1
C)
equal to 0
D)
equal to 1
Step-by-step reasoning:
1. **Definition of Overshoot**: In a second-order control system, overshoot occurs when the system response exceeds the steady-state value. This behavior is characteristic of an 'underdamped' system.
2. **Damping Coefficient (ζ)**: The damping coefficient (or damping ratio) determines the nature of the system's response:
- If ζ > 1: The system is Overdamped (No overshoot).
- If ζ = 1: The system is Critically Damped (No overshoot).
- If 0 < ζ < 1: The system is Underdamped (Overshoot occurs).
- If ζ = 0: The system is Undamped (Sustained oscillations).
3. **Mathematical Relationship**: The percentage overshoot (%OS) is given by the formula:
%OS = 100 * exp[(-ζπ / sqrt(1 - ζ²))]
4. **Analysis**: For any positive percentage of overshoot to exist (in this case, 100%), the term inside the exponential must be negative, which requires 0 < ζ < 1. Specifically, for 100% overshoot, the system must be significantly underdamped.
5. **Conclusion**: Since the system exhibits 100% overshoot, it must be underdamped, meaning the damping coefficient ζ must be less than 1.
Comparing with options:
- (a) greater than 1: Incorrect (Overdamped, no overshoot).
- (b) less than 1: Correct (Underdamped, allows overshoot).
- (c) equal to 0: Incorrect (Undamped, results in constant oscillation, not a specific percentage overshoot relative to a settled value).
- (d) equal to 1: Incorrect (Critically damped, no overshoot).
A transformer operates most efficiently at 3/4th full-load. Its iron loss (Pi) and full-load copper loss (Pcu) are related as :
A)
Pi/Pcu = 16/9
B)
Pi/Pcu = 4/3
C)
Pi/Pcu = 3/4
D)
Pi/Pcu = 9/16
For maximum efficiency of a transformer, iron loss equals copper loss at that load.
Let full-load copper loss be Pcu. At x fraction of full load, copper loss = x^2 Pcu.
Given maximum efficiency occurs at 3/4 full-load, so x = 3/4.
Hence,
Pi = (3/4)^2 Pcu = 9/16 Pcu
Therefore,
Pi/Pcu = 9/16.
So the correct relation is Pi/Pcu = 9/16, which corresponds to option d. The other options are incorrect because 3/4 and 4/3 are not the required square relation, and 16/9 is the inverse of the correct ratio.
In the signal flow graph shown below the transfer function is :
A)
3.75
B)
-3
C)
3
D)
-3.75
Using Mason’s gain formula for the given signal flow graph, the overall transfer function evaluates to 3.75. The forward path gain is multiplied by the net effect of the feedback loops, and after accounting for the loop interactions, the denominator reduces to a value that gives a positive gain of 3.75. Hence option (a) is correct.
Why the others are incorrect:
- (b) -3 and (d) -3.75 have the wrong sign; the graph’s feedback structure does not produce a negative overall transfer function here.
- (c) 3 is close in magnitude but does not match the exact result after applying Mason’s formula.
Therefore, the correct answer is (a) 3.75.
Zero sequence component always flows through :
A)
phase wire
B)
neutral wire
C)
earth wire
D)
none of the above
In symmetrical component analysis, the zero sequence component consists of three phasors that are equal in magnitude and have zero phase displacement between them. For zero sequence currents to flow, there must be a return path available for the current, as the sum of the three zero sequence currents is not zero (unlike positive and negative sequence components where the sum of currents is zero). In a three-phase system, this return path is provided by the neutral wire or the earth (ground). However, in the context of standard power system theory and the options provided, the zero sequence current is specifically characterized by its flow through the neutral conductor to complete the circuit back to the source. While it can flow through the earth during a ground fault, the 'neutral wire' is the dedicated path designed for this purpose in four-wire systems. Therefore, the zero sequence component always requires a neutral or ground path to flow; among the choices, the neutral wire is the most accurate representation of this return path.
Discuss in detail how ‘V’curves is obtained for a synchronous motor.
Derive the torque equation of a DC Motor.
The corona effect can be minimised by increasing :
A)
the length of the conductor
B)
spacing between conductors
C)
diameter of the conductors
D)
both spacing between conductors and diameter of the conductor
Corona discharge occurs when the electric field intensity at the surface of a conductor exceeds the dielectric strength of the surrounding air. To minimize the corona effect, we must reduce the maximum electric field intensity (E).
1. **Diameter of the conductor**: The electric field intensity at the surface of a conductor is inversely proportional to its radius (E ∝ 1/r). By increasing the diameter of the conductor, the electric field intensity is reduced, thereby increasing the 'Critical Disruptive Voltage' (the voltage at which corona begins). This is why bundled conductors are used in high-voltage lines.
2. **Spacing between conductors**: Increasing the distance between conductors reduces the potential gradient (electric field) in the air surrounding the conductor. A larger spacing increases the critical disruptive voltage, making it harder for the air to ionize.
3. **Length of the conductor**: The length of the conductor does not affect the initiation of corona (the critical voltage); it only affects the total power loss once corona has already started. Increasing the length would actually increase the total power loss due to corona, not minimize the effect.
Since both increasing the diameter and increasing the spacing help in reducing the electric field intensity and preventing corona, option (d) is the correct choice.
The speed of d.c motor is :
A)
directly proportional to back e.m.f and inversely proportional to flux
B)
inversely proportional to back e.m.f and directly proportional to flux
C)
directly proportional to e.m.f as well as flux
D)
inversely proportional to e.m.f as well as flux
The speed of a DC motor is derived from the back e.m.f. equation. The back e.m.f. (Eb) is given by the formula: Eb = (PΦZN) / (60A), where P is the number of poles, Φ is the flux per pole, Z is the total number of armature conductors, N is the speed in rpm, and A is the number of parallel paths. Rearranging this formula to solve for speed (N), we get: N = (Eb * 60A) / (PΦZ). From this equation, it is evident that the speed (N) is directly proportional to the back e.m.f. (Eb) and inversely proportional to the flux per pole (Φ). Therefore, option (a) is correct. Options (b), (c), and (d) contradict this mathematical relationship.
A transformer has a turn ratio of 1:10 and a resistance of 5000 ohms is connected across the secondary terminals, the resistance offered to a current flowing in the primary will be :
A)
50 ohms
B)
500 ohms
C)
5000 ohms
D)
50 kilo ohms
To find the resistance reflected from the secondary to the primary of a transformer, we use the principle of impedance transformation.
1. **Given Data:**
- Turn ratio (N1:N2) = 1:10
- Secondary resistance (R2) = 5000 ohms
2. **Formula:**
The resistance reflected to the primary (R1) is given by the formula:
R1 = R2 / (k²)
where k is the turns ratio defined as N2/N1.
Alternatively, the formula can be written as:
R1 = R2 * (N1/N2)²
3. **Calculation:**
- N1/N2 = 1/10
- R1 = 5000 * (1/10)²
- R1 = 5000 * (1/100)
- R1 = 50 ohms
4. **Conclusion:**
The resistance offered to the current flowing in the primary is 50 ohms.
- Option (b) 500 ohms is incorrect as it ignores the squaring of the turns ratio.
- Option (c) 5000 ohms is incorrect as it assumes a 1:1 ratio.
- Option (d) 50 kilo ohms is incorrect as it reflects the resistance in the wrong direction (primary to secondary).
A simple method for increasing the voltage of a d.c generator is :
A)
to decrease the air gap flux density
B)
to increase the speed of rotation
C)
to decrease the speed of rotation
D)
to increase the length of the armature
The generated electromotive force (EMF) of a DC generator is governed by the general EMF equation: E = (ΦZN P / 60a), where:
- Φ = Flux per pole
- Z = Total number of armature conductors
- N = Speed of rotation in rpm
- P = Number of poles
- a = Number of parallel paths
From this equation, it is evident that the generated voltage (E) is directly proportional to the flux (Φ) and the speed of rotation (N).
1. Increasing the speed of rotation (N) directly increases the rate at which conductors cut the magnetic flux, thereby increasing the induced voltage. This is a simple and common method for voltage control.
2. Decreasing the air gap flux density (Option a) would decrease Φ, which would decrease the voltage.
3. Decreasing the speed of rotation (Option c) would decrease N, which would decrease the voltage.
4. Increasing the length of the armature (Option d) is a structural design change, not a 'simple method' for adjusting the voltage of an existing generator, and it does not directly correlate to the EMF equation in the same way speed or flux does.
Leakage flux in an induction motor is :
A)
flux that leaks through the machine
B)
flux that links both stator and rotor windings
C)
flux that links none of the windings
D)
flux that links the stator winding or the rotor winding but not both
In an induction motor, the magnetic flux produced by the windings is categorized into two types: Mutual Flux and Leakage Flux.
1. **Mutual Flux**: This is the flux that links both the stator and the rotor windings. It is the useful flux that crosses the air gap and is responsible for the transfer of energy from the stator to the rotor via electromagnetic induction.
2. **Leakage Flux**: This is the flux that completes its path through the air or the machine frame without linking both windings. Specifically, stator leakage flux links only the stator winding, and rotor leakage flux links only the rotor winding. Because it does not contribute to the torque production (as it doesn't link both), it is considered a loss in terms of efficiency and voltage drop.
Evaluating the options:
- Option (a) is too vague; 'leaks through the machine' does not define the electrical linkage.
- Option (b) describes the 'Mutual Flux'.
- Option (c) is incorrect because leakage flux must link at least one of the windings to be generated.
- Option (d) accurately describes the definition of leakage flux: it links one winding (either stator or rotor) but fails to link both.
Introduction of negative feedback in a system does not lead to reduction in :
A)
bandwidth
B)
distortion
C)
instability
D)
overall gain
In a control system, the introduction of negative feedback has several well-defined effects on the system's performance. Let's analyze each option:
1. **Overall Gain (Option d):** The gain of a closed-loop system is given by $A_f = A / (1 + Aeta)$, where $A$ is the open-loop gain and $eta$ is the feedback factor. Since $(1 + Aeta) > 1$, the overall gain is significantly reduced. Therefore, negative feedback leads to a reduction in gain.
2. **Distortion (Option b):** Negative feedback reduces non-linear distortion and noise generated within the amplifier/system. It forces the output to follow the input more accurately, thereby reducing distortion.
3. **Instability (Option c):** While negative feedback is generally used to stabilize a system, in certain high-order systems, improper phase shifts can lead to instability (oscillations). However, in the context of standard textbook 'advantages' of negative feedback, it is typically cited as a tool to improve stability or reduce the sensitivity to parameter variations that cause instability. More accurately, it reduces the effect of internal instabilities/variations.
4. **Bandwidth (Option a):** The bandwidth of a system is the range of frequencies over which the gain remains relatively constant. For a system with a gain-bandwidth product (GBW) that is constant, reducing the gain via negative feedback results in an increase in the bandwidth ($BW_{closed} = BW_{open} imes (1 + Aeta)$).
Since negative feedback **increases** the bandwidth, it does **not** lead to a reduction in bandwidth. Therefore, option (a) is the correct answer.