← Junior Engineer
Past Questions — 2024
The relay used for feeder protection is -
A)
Undervoltage relay
B)
overcurrent earth fault relay
C)
Thermal relay
D)
Buchholz relay
Step-by-step reasoning:
1. **Feeder Protection Requirements**: Feeders are the transmission lines that carry power from the substation to the distribution points. The most common faults occurring in feeders are short circuits (phase-to-phase) and earth faults (phase-to-ground). Therefore, the protection system must be capable of detecting overcurrents and leakage currents to ground.
2. **Analyzing Option (b)**: An 'Overcurrent Earth Fault' relay is specifically designed to trip the circuit breaker when the current exceeds a predefined threshold (overcurrent) or when current flows to the ground (earth fault). This is the standard protection scheme for feeders.
3. **Analyzing Option (a)**: Undervoltage relays are used to protect equipment from operating at low voltage levels or to signal a loss of power; they are not the primary protection for feeder faults.
4. **Analyzing Option (c)**: Thermal relays are primarily used for motor protection to prevent overheating due to prolonged overload; they are not suitable for the high-speed fault detection required for feeders.
5. **Analyzing Option (d)**: Buchholz relays are gas-actuated relays used specifically for the internal protection of oil-immersed transformers to detect incipient faults; they cannot be used on feeders.
At the secondary transmission substation in Mizoram, electrical power is transmitted at voltage level of-
A)
132kV
B)
66kV
C)
220kV
D)
33 kV
In the electrical power system hierarchy, power is categorized into Primary Transmission, Secondary Transmission, and Primary/Secondary Distribution.
1. Primary Transmission: High voltage levels (e.g., 220kV, 400kV, 765kV) are used to transport bulk power over long distances from generating stations to receiving stations.
2. Secondary Transmission: Power is stepped down from primary transmission levels to lower voltages for transport to regional substations. In the Indian power grid context, and specifically within the infrastructure of the Mizoram Power Department (MPD), the standard voltage level for secondary transmission is 66kV.
3. Primary Distribution: Power is further stepped down from 66kV to 33kV or 11kV for distribution to local areas.
Therefore, for the secondary transmission substation in Mizoram, the standard operating voltage is 66kV. Options a (132kV) and c (220kV) are typically associated with primary transmission, and option d (33kV) is associated with primary distribution.
Corona effect can be minimised by increasing -
A)
the length of the conductors
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 (potential gradient) at the surface of the conductor exceeds the dielectric strength of the surrounding air. The maximum electric field intensity (E) at the surface of a conductor is inversely proportional to the radius of the conductor and is influenced by the distance between conductors.
1. **Diameter of the conductor (Option c):** Increasing the diameter (or radius) of the conductor reduces the electric field intensity at its surface. A larger surface area spreads the charge, lowering the potential gradient and thereby increasing the 'Critical Disruptive Voltage' (CDV). This makes it harder for corona to initiate.
2. **Spacing between conductors (Option b):** Increasing the distance between the phases reduces the electric field intensity between them. This also increases the CDV, effectively minimizing the occurrence of corona.
3. **Length of the conductors (Option a):** Increasing the length of the conductor does not reduce the electric field intensity at any specific point; in fact, total corona loss is proportional to the length of the line. Therefore, increasing length would increase total losses, not minimize the effect.
Since both increasing the diameter and increasing the spacing serve to reduce the surface potential gradient and increase the critical disruptive voltage, option (d) is the most comprehensive and correct answer.
According to the IS code, the colour of earth wire is -
A)
Red
B)
Green
C)
Yellow
D)
Black
According to the Indian Standard (IS) codes for electrical installations (such as IS 732), specific colors are assigned to conductors to ensure safety and standardization during installation and maintenance.
1. **Earth Wire (Protective Conductor):** The standard color for the earth wire is Green (or Green-Yellow). Its primary purpose is to provide a low-resistance path to the ground to prevent electric shocks in case of a fault.
2. **Phase/Live Wire:** In traditional domestic wiring, Red is commonly used for the phase wire (though in newer 3-phase standards, Red, Yellow, and Blue are used for R, Y, and B phases respectively).
3. **Neutral Wire:** Black is the standard color used for the neutral conductor.
Therefore, among the given options, Green is the correct color for the earth wire.
Dam having very wide base as compared to its height is called
A)
buttress dam
B)
arch dam
C)
earth dam
D)
solid gravity dam
In dam engineering, the stability of a dam depends on its ability to resist the hydrostatic pressure of the stored water.
1. **Earth Dams (Embankment Dams):** These are constructed using compacted soil, sand, and gravel. Because the materials used have low shear strength compared to concrete, earth dams cannot be built with steep slopes. To ensure stability and prevent sliding or failure, they require very gentle slopes, which results in a base that is significantly wider than the height of the dam.
2. **Solid Gravity Dams:** These rely on their own weight to resist water pressure. While they have a wide base, the ratio of base-to-height is much smaller than that of an earth dam. They are typically triangular in cross-section.
3. **Arch Dams:** These are curved structures that transfer the water pressure to the canyon walls (abutments). They are thin and have a very narrow base relative to their height.
4. **Buttress Dams:** These consist of a thin upstream face supported by a series of supports (buttresses). While they use less material than gravity dams, their overall footprint is not as wide as an earth dam's base relative to its height.
Therefore, the earth dam is the structure characterized by having a very wide base compared to its height.
At what height a conductor may be string if the required ground clearance is 7.5m with a sag of 2.5m for a horizontal line?
A)
5.1m
B)
10m
C)
7.5m
D)
7.591 m
In transmission line engineering, the height of the conductor at the support (the stringing height) is the sum of the minimum ground clearance required at the lowest point of the conductor and the maximum sag of the conductor.
Given:
1. Ground Clearance (minimum height from ground to the lowest point of the conductor) = 7.5 m
2. Sag (the vertical distance between the support point and the lowest point of the conductor) = 2.5 m
Formula:
Height of support (Stringing height) = Ground Clearance + Sag
Height = 7.5 m + 2.5 m = 10 m
Therefore, the conductor must be strung at a height of 10m to ensure that even at its lowest point (the sag), it maintains the required clearance of 7.5m from the ground.
A substation which performs the switching operations of power transmission lines is called -
A)
Transformer substation
B)
Switching substation
C)
Industrial substation
D)
Intermediate substation
A substation is categorized based on its primary function within the power grid.
1. **Switching Substation**: This type of substation is designed specifically to perform switching operations. It allows operators to connect or disconnect different transmission lines or circuits to reroute power, isolate faults, or perform maintenance. Crucially, a switching substation does not contain transformers to change voltage levels; it only manages the routing of power.
2. **Transformer Substation**: These are designed to step up (increase) or step down (decrease) voltage levels using transformers. While they also perform switching, their primary purpose is voltage transformation.
3. **Industrial Substation**: These are dedicated substations located within industrial plants to step down high-voltage transmission power to usable levels for industrial machinery.
4. **Intermediate Substation**: This is a general term often referring to substations located between the generating station and the end consumer, which could be either switching or transformer substations depending on the design.
Since the question specifically asks for the substation that performs 'switching operations' (implying the primary function of routing/switching without necessarily changing voltage), the correct answer is the Switching Substation.
A converting substation changes quantity from -
A)
DC to AC
B)
AC to DC
C)
AC to AC
D)
DC to DC
In electrical power systems, a 'converting substation' (also known as a converter station) is specifically designed to change the form of electrical energy from Alternating Current (AC) to Direct Current (DC) or vice versa. However, in the context of power transmission (such as HVDC - High Voltage Direct Current systems), the primary function of the converting station at the sending end is to convert AC power from the grid into DC power for efficient long-distance transmission.
1. AC to DC: This process is called Rectification. A converting substation uses thyristors or IGBTs to perform this conversion.
2. DC to AC: This process is called Inversion. While converter stations do this at the receiving end, the term 'converting' in general power engineering contexts often refers to the rectification process for HVDC transmission.
3. AC to AC: This is handled by a standard transformer substation (changing voltage levels), not a converting substation.
4. DC to DC: This is handled by DC-DC converters (choppers), typically used in electronics, not at the substation level of a power grid.
If the surrounding atmosphere is contaminated with impurities like fumes, dust and metal corroding then the indoor substations are usually preferred for a voltage level up to -
A)
33kV
B)
66kV
C)
132kV
D)
220kV
Substations are classified as either indoor or outdoor based on the voltage level and environmental conditions.
1. **Indoor Substations**: These are preferred when the surrounding atmosphere is contaminated with fumes, dust, or corrosive chemicals, as the equipment is enclosed in a building, protecting it from environmental degradation. However, indoor substations require significant space for insulation and safety clearances as voltage increases. Therefore, they are typically used for lower voltage levels, generally up to 33kV.
2. **Outdoor Substations**: For higher voltage levels (such as 66kV, 132kV, and 220kV), the required clearance distances between conductors and between conductors and ground become very large. Building a structure to house such equipment would be prohibitively expensive and impractical. Thus, outdoor substations are used for these higher voltage levels.
In the given scenario, because the atmosphere is contaminated, an indoor substation is preferred to protect the equipment, but the practical upper limit for such an installation is typically 33kV.
While a transformer is energized due to inrush current, the differentially connected relay may -
A)
mal operate
B)
not mal operate
C)
mal operate for lagging loads
D)
never mal operate
When a transformer is first energized, a phenomenon known as 'magnetizing inrush current' occurs. This happens because the core may be saturated depending on the point on the voltage wave where the transformer is switched on. This inrush current flows only into the primary side of the transformer, creating a significant imbalance between the primary and secondary currents. Since a differential relay operates based on the difference between these currents (I_primary - I_secondary), it perceives this imbalance as an internal fault, leading to a 'mal-operation' (false trip). To prevent this, 'harmonic restraint' (specifically 2nd harmonic restraint) is typically used in industrial differential relays, as inrush currents are rich in second harmonics, whereas actual fault currents are not.
A transducer is classified into how many number -
A)
2
B)
3
C)
4
D)
5
In instrumentation and measurement, transducers are fundamentally classified into two primary categories based on the method of energy conversion:
1. Active Transducers (Self-generating): These transducers generate an electrical signal (voltage or current) directly from the physical quantity being measured without requiring an external power source. Examples include Piezoelectric crystals, Thermocouples, and Photovoltaic cells.
2. Passive Transducers (Externally powered): These transducers require an external power source to operate. They work by changing a passive electrical property (such as resistance, capacitance, or inductance) in response to the measured physical quantity. Examples include RTDs (Resistance Temperature Detectors), LVDTs (Linear Variable Differential Transformers), and Strain Gauges.
While transducers can be further sub-classified by the type of output (Analog vs. Digital) or the physical quantity measured, the primary classification used in technical examinations for 'number of types' refers to the Active and Passive distinction.
Different types of insulators used for transmission lines are -
A)
pin type only
B)
suspension type only
C)
strain type only
D)
pin type, suspension type and shackle type
In electrical transmission and distribution, insulators are used to prevent the leakage of current from the conductors to the supporting poles or towers. Several types of insulators are used depending on the voltage level and the mechanical requirements of the line:
1. Pin Insulators: Used for lower voltage levels (typically up to 33kV). They are fixed to the cross-arm of the pole.
2. Suspension Insulators: Used for high voltage transmission lines (above 33kV). They consist of a string of porcelain discs suspended from the cross-arm.
3. Strain Insulators: Used at dead-ends, sharp curves, or river crossings to sustain the mechanical tension of the conductor.
4. Shackle Insulators: Used for low voltage distribution lines, typically at the end of a line or at turns.
Options (a), (b), and (c) are incorrect because they list only one type of insulator, whereas transmission and distribution systems utilize a variety of these types depending on the specific application. Option (d) correctly identifies a comprehensive list of insulators used in these systems.
Power stations and transmission substations in Mizoram are protected against direct lightning using -
A)
Rod gap arrester
B)
Overhead ground wires
C)
Earthing screen
D)
Horn gap arrester
Step-by-step reasoning: 1. The question asks for the method used to protect power stations and transmission substations against 'direct lightning'. 2. Direct lightning strikes occur when a lightning bolt hits the conductors or equipment directly. 3. Overhead ground wires (also known as shield wires or guard wires) are installed at the topmost position of transmission towers and substation structures. Their primary purpose is to intercept lightning strikes before they can hit the phase conductors, safely diverting the high-voltage surge to the ground via the tower earthing system. 4. Analyzing other options: (a) Rod gap arresters and (d) Horn gap arresters are types of surge arresters used to protect equipment from overvoltages (transients) by providing a path to ground once a certain voltage threshold is reached, but they do not prevent the 'direct strike' on the line/station in the way shield wires do. (c) Earthing screen is typically used for protecting underground cables or specific equipment foundations, not for protecting an entire substation/station from direct atmospheric lightning strikes. Therefore, overhead ground wires are the standard primary defense against direct lightning strikes in power systems.
Which of the following is the working principle of capacitive type transducer?
A)
Variable capacitance
B)
Constant capacitance
C)
Variable power
D)
Variable temperature
A capacitive transducer is a passive transducer that works on the principle of variable capacitance. The capacitance (C) of a capacitor is defined by the formula C = (εA) / d, where ε is the permittivity of the dielectric, A is the area of the overlapping plates, and d is the distance between the plates. A capacitive transducer converts a physical quantity (such as displacement, pressure, or humidity) into an electrical signal by varying one of these three parameters: 1) Changing the distance between the plates (d), 2) Changing the overlapping area of the plates (A), or 3) Changing the permittivity of the dielectric material (ε). Therefore, the working principle is based on 'Variable capacitance'. Options b, c, and d are incorrect because constant capacitance would result in no signal change, and power or temperature are not the fundamental operating principles of a capacitive sensor.
The transmission line which feed different substation represent -
A)
primary transmission
B)
secondary transmission
C)
primary distribution
D)
secondary distribution
In an electrical power system, the transmission and distribution network is divided into several stages based on voltage levels and function:
1. **Primary Transmission**: This refers to the transmission of power from the generating station to the receiving stations (main substations) at very high voltages (e.g., 132kV, 220kV, 400kV, 765kV). These lines feed the different substations that are located near load centers.
2. **Secondary Transmission**: This is the transmission of power from the main receiving stations to smaller substations (secondary substations) at lower voltages (e.g., 33kV or 66kV).
3. **Primary Distribution**: This is the stage where power is delivered from the secondary substation to the local distribution transformers at voltages like 11kV.
4. **Secondary Distribution**: This is the final stage where power is delivered from the distribution transformer to the end consumers at low voltages (e.g., 230V or 415V).
Since the question asks for the line that feeds 'different substations' (referring to the main receiving stations from the power plant), it describes the Primary Transmission stage.
Impulse turbines are used in -
A)
very low head plants
B)
high head plants
C)
medium head plants
D)
low head plants
Impulse turbines, such as the Pelton wheel, operate on the principle of converting the potential energy of water into kinetic energy through a nozzle before the water hits the turbine buckets. For this mechanism to be efficient and generate significant power, a high velocity of water is required, which is achieved by having a high head (vertical distance between the reservoir and the turbine).
1. High Head Plants: Impulse turbines (e.g., Pelton) are ideal here because they utilize the high pressure to create high-velocity jets.
2. Low to Medium Head Plants: Reaction turbines (e.g., Francis or Kaplan) are used. These turbines are completely submerged in water and utilize both the kinetic energy and the pressure energy of the water to rotate the runner, making them more efficient for lower heads where high-velocity jets cannot be easily produced.
Which substations are generally located in the power houses?
A)
Step-up
B)
Primary
C)
Secondary
D)
Distribution
In a power generation system, electricity is produced by generators (alternators) at a relatively low voltage (typically 11kV to 25kV). To minimize power losses (I²R losses) and reduce the size of conductors during long-distance transmission, the voltage must be increased significantly. This is achieved using a 'Step-up' substation, which houses the step-up transformers. Because these transformers must be located as close to the generators as possible to minimize losses at the generation voltage, these substations are generally located within the power house (generating station) premises.
Why other options are incorrect:
- Primary substations: These are typically located at the end of transmission lines to step down voltage for sub-transmission.
- Secondary substations: These are located closer to the load centers to step down voltage for distribution.
- Distribution substations: These are the final stage of the power system, stepping down voltage to the consumer level (e.g., 415V/230V), and are located within residential or industrial areas, far from the power house.
A control system in which the control action is dependent on the output is known as -
A)
open loop system
B)
semiclosed system
C)
closed loop system
D)
no loop system
In control systems engineering, systems are categorized based on how the control action is determined:
1. **Closed-Loop System (Feedback System):** In this system, the control action is dependent on the output. The actual output is measured via a sensor and fed back to the input (feedback loop), where it is compared with the desired reference value. The difference (error signal) is then used to adjust the control action to minimize the error. Example: An air conditioner with a thermostat.
2. **Open-Loop System:** In this system, the control action is independent of the output. The system follows a preset instruction regardless of the actual result achieved. There is no feedback mechanism. Example: An electric toaster or a simple washing machine timer.
3. **Semiclosed/No Loop System:** These are not standard technical terms used to describe the fundamental architecture of control systems in this context.
Since the question specifies that the control action is 'dependent on the output', it describes a feedback mechanism, which is the defining characteristic of a closed-loop system.
Consumer with more than 500 kVA and upto 2000 kVA demand is supplied with -
A)
66kV
B)
33kV
C)
11kV
D)
400 V
In electrical power distribution systems, consumers are categorized based on their load demand to determine the appropriate voltage level for supply. This minimizes transmission losses and optimizes infrastructure costs.
1. Low Voltage (LV) Supply (400V/230V): Typically used for domestic consumers and small commercial loads (usually up to 50 kVA or 100 kVA depending on the utility).
2. High Tension (HT) Supply (11kV): This is the standard distribution voltage for medium-scale industrial and commercial consumers. Generally, consumers with a demand exceeding 50 kVA but up to 2000 kVA (or sometimes up to 5 MVA depending on the regional grid code) are supplied at 11kV. The consumer is then responsible for installing their own step-down transformer to bring the voltage down to usable levels.
3. Extra High Tension (EHT) Supply (33kV, 66kV, and above): These voltages are reserved for very large industrial consumers (Bulk Power Consumers) with demands typically exceeding 2000 kVA or 5 MVA to reduce current flow and line losses over longer distances.
Therefore, for a demand between 500 kVA and 2000 kVA, the standard supply voltage is 11kV.
A Current Transformer measures -
A)
voltage only
B)
voltage and current
C)
current only
D)
current and reactive power
A Current Transformer (CT) is an instrument transformer designed specifically to step down high primary currents to a lower, standardized secondary current (typically 1A or 5A) that can be safely measured by standard ammeters or used by protective relays.
1. **Function**: It works on the principle of electromagnetic induction. The primary winding carries the full load current, and the secondary winding produces a current proportional to the primary current based on the turns ratio.
2. **Why other options are incorrect**:
- **Option a (voltage only)**: Voltage is measured by a Potential Transformer (PT) or Voltage Transformer (VT).
- **Option b (voltage and current)**: While a system might use both CTs and PTs together to measure power, a single Current Transformer is dedicated solely to current measurement.
- **Option d (current and reactive power)**: Reactive power measurement requires both current (from a CT) and voltage (from a PT) inputs to a power analyzer or wattmeter; the CT alone cannot measure power.
Which of the following factors can affect the response time of a pressure sensor?
A)
The size of diaphragm
B)
The thickness of the diaphragm
C)
The type of fluid being used
D)
The operating temperature of the sensor
The response time of a pressure sensor is strongly influenced by the diaphragm’s mechanical properties, especially its thickness. A thicker diaphragm has greater mass and stiffness, so it deflects more slowly under pressure changes, increasing response time. A thinner diaphragm responds faster.
Why the others are incorrect:
- a) Size of diaphragm: diaphragm size mainly affects sensitivity and pressure range, not the primary response time factor.
- c) Type of fluid being used: the fluid can affect damping in some cases, but it is not the main direct factor asked here.
- d) Operating temperature of the sensor: temperature can affect calibration and accuracy, but the standard key factor for response time in this context is diaphragm thickness.
Which of the following additional devices is required in order to measured pressure using LVDT?
A)
Strain gauge
B)
Pilot Tube
C)
Rotameter
D)
Bourdon Tube
An LVDT (Linear Variable Differential Transformer) is a transducer that converts linear displacement into an electrical signal. However, it cannot sense pressure directly because pressure is a scalar quantity, not a displacement. To measure pressure using an LVDT, a primary sensing element is required to convert the pressure into a mechanical displacement. A Bourdon Tube is a C-shaped tube that tends to straighten out as internal pressure increases; this movement (displacement) can be mechanically linked to the core of an LVDT. Thus, the Bourdon Tube acts as the pressure-to-displacement converter, and the LVDT acts as the displacement-to-voltage converter.
Why other options are incorrect:
- a. Strain gauge: This is an independent pressure/force sensing element that changes resistance; it does not provide the linear displacement required to operate an LVDT.
- b. Pilot Tube (Pitot Tube): This is used to measure fluid flow velocity, not static pressure for LVDT conversion.
- c. Rotameter: This is a device used for measuring the flow rate of liquids or gases, not pressure.
Isolators must be open only -
A)
after opening of circuit breaker
B)
after closing of circuit breaker
C)
under closed position of circuit breaker
D)
while on load
An isolator (or disconnecting switch) is a mechanical switch used to isolate a part of an electrical circuit for maintenance purposes. Unlike a circuit breaker, an isolator is NOT designed to interrupt current flow or quench the resulting electric arc.
1. **Operational Sequence**: To safely isolate a circuit, the load must first be interrupted using a circuit breaker (which is designed to handle arcs). Once the circuit breaker is open and the current flow is zero, the isolator is then opened to provide a visible physical gap in the circuit for safety.
2. **Danger of On-Load Operation**: If an isolator is opened while the circuit is under load (current is flowing), a massive electric arc will form between the contacts. Since isolators lack arc-quenching mechanisms, this arc can cause severe equipment damage, lead to phase-to-phase faults, and pose a lethal risk to the operator.
Therefore, the strict sequence is: Circuit Breaker Open $\rightarrow$ Isolator Open. This makes option (a) the only correct and safe procedure.
What is the ideal site of thermal power plant?
A)
In hilly regions
B)
On islands
C)
Near water resources
D)
In centre lands
A thermal power plant requires a massive and continuous supply of water for two primary reasons: 1. Condensation: The steam produced to drive the turbines must be condensed back into water using a condenser, which requires a large volume of cooling water (usually from a river, lake, or sea). 2. Boiler Feed: High-purity water is needed for the steam generation process in the boiler. Therefore, proximity to water resources is the most critical factor for site selection to minimize pumping costs and ensure operational stability.
Why other options are incorrect:
- a. Hilly regions: These are generally unsuitable due to high construction costs, difficulty in transporting heavy machinery (like turbines and boilers), and lack of flat land.
- b. On islands: While islands have water, the logistics of transporting massive amounts of fuel (coal) and connecting the plant to the main electrical grid via undersea cables make it economically unviable.
- d. In centre lands: While land availability is important, 'centre lands' (inland areas far from water) would require expensive piping systems or cooling towers that are less efficient than once-through cooling from a nearby water body.
Trash racks are built for -
A)
discharging the water freely from the turbine exit to tailrace
B)
preventing the turbine from ingress of floating and other materials
C)
creating artificial head to store sufficient potential energy of water
D)
controlling the opening of valves
Step-by-step reasoning:
1. **Definition of Trash Racks**: In a hydroelectric power plant, trash racks are screens or grids made of metal bars placed at the intake of the water conduit (penstock) or the forebay.
2. **Primary Function**: Their main purpose is to act as a filter to prevent debris, such as floating logs, leaves, plastic, and other foreign materials, from entering the penstock and reaching the turbine.
3. **Impact of Debris**: If such materials enter the turbine, they can cause severe mechanical damage to the runner blades, clog the wicket gates, or cause imbalances, leading to plant failure or costly repairs.
4. **Evaluating Options**:
- **Option (a)**: Discharging water from the turbine exit to the tailrace is the function of the **Draft Tube**.
- **Option (b)**: This correctly describes the function of the trash rack (preventing ingress of floating materials).
- **Option (c)**: Creating artificial head to store potential energy is the function of the **Dam or Reservoir**.
- **Option (d)**: Controlling the opening of valves is the function of **Actuators or Control Valves** (e.g., Guide Vanes/Wicket Gates).