Power Systems MCQ Quiz - Objective Question with Answer for Power Systems - Download Free PDF

The term that defines the time taken by a protective relay to operate and initiate a trip after detecting a fault is operating time.

Explanation:

• Operating time: This is the time interval from the moment the fault current reaches the relay's pick-up value until the relay's output contacts close (or operate) to initiate a trip signal to the circuit breaker. This is the crucial time delay for the relay itself.

• Pick-up time: This refers to the time it takes for the relay's sensing element to respond and "pick up" or recognize that a fault condition exists, i.e., the fault current has exceeded a predefined threshold.

• Reset time: This is the time taken for the relay to return to its normal, pre-fault state after the fault has been cleared and the fault current has fallen below the reset value.

• Tripping time: This term can sometimes be used more broadly to refer to the total time from fault inception until the circuit breaker completely interrupts the fault current. It includes the relay's operating time and the circuit breaker's operating time. However, for the relay's action specifically, "operating time" is the precise term.

Explanation:

Operating Principle of a Thermal Relay

Definition: A thermal relay is an electrical protection device that operates based on the heat generated by the flow of current through the circuit. It is designed to protect electrical equipment, such as motors and transformers, from overheating due to prolonged overcurrent or short-circuit conditions.

Working Principle: The primary operating principle of a thermal relay relies on the thermal effect of electric current. When current flows through a conductor, it generates heat due to the resistance of the conductor. The amount of heat generated is proportional to the square of the current (I²R). If the current exceeds the normal operating range for an extended period, the heat buildup activates the thermal relay, causing it to trip and disconnect the circuit. This prevents damage to the equipment.

The thermal relay typically consists of a bimetallic strip or thermal sensing element. When the heat generated by the current flow causes the bimetallic strip to bend or deform, it triggers the relay mechanism, breaking the circuit. The relay is calibrated to operate at a specific temperature, ensuring protection against overheating.

Advantages:

• Simple and reliable operation, making it widely used in electrical protection systems.
• No requirement for external power supply, as the operating principle is based solely on the heat generated by the current flow.
• Cost-effective solution for protecting motors and other electrical equipment.

Disadvantages:

• Relatively slow response time compared to electronic relays, which may not be suitable for certain applications requiring rapid protection.
• Limited accuracy and sensitivity compared to advanced protection devices.

Applications: Thermal relays are commonly used in motor protection circuits, overload protection systems, and other applications where the thermal effect of current can be utilized to prevent equipment damage.

Correct Option Analysis:

The correct option is:

Option 4: It measures the amount of heat generated due to the current flow and operates when a preset temperature is reached.

This option correctly describes the operating principle of a thermal relay. The relay measures the heat generated by the current flow and trips the circuit when the temperature exceeds a predetermined threshold. This ensures that the electrical equipment is protected from overheating and potential damage.

Explanation:

Francis Turbine Components and Flow Regulation

Definition: A Francis turbine is a type of reaction turbine used for hydropower generation, designed to operate efficiently under a wide range of head and flow conditions. It converts the potential energy of water into mechanical energy, which is then used to generate electricity through a connected generator. A key feature of the Francis turbine is its ability to dynamically regulate flow and optimize power output under varying load conditions.

Correct Option Analysis:

The correct answer is:

Option 1: Guide Vanes

The guide vanes play a critical role in regulating the flow rate and optimizing the power output of a Francis turbine. They are adjustable components located just before the turbine runner. By dynamically altering their angle, guide vanes control the amount of water entering the runner and the direction of the flow. This ensures that the turbine operates at peak efficiency under varying load conditions.

Working Principle of Guide Vanes:

• Flow Regulation: The guide vanes adjust their position to regulate the flow of water entering the runner. When the load on the turbine decreases, the guide vanes close slightly to reduce the flow rate, and when the load increases, they open to allow more water to flow through.
• Flow Direction: The guide vanes direct the water flow at an optimal angle to the runner blades. This ensures that the kinetic energy of the water is effectively transferred to the runner, maximizing the turbine's efficiency.
• Dynamic Response: Modern Francis turbines are equipped with servo mechanisms that allow the guide vanes to respond quickly to changes in load or flow conditions, maintaining stable operation and consistent power output.

Advantages of Guide Vanes:

• Enable precise control over the turbine's power output by dynamically adjusting the water flow.
• Improve the overall efficiency of the turbine by ensuring optimal flow direction and velocity.
• Help in maintaining stable operation during fluctuating load conditions.
• Reduce the risk of cavitation and mechanical stress on the turbine components by controlling flow conditions.

Conclusion: The guide vanes are essential components in a Francis turbine that dynamically adjust to regulate the flow rate and optimize power output under varying load conditions. Their ability to control both the quantity and direction of water entering the runner ensures efficient and reliable operation of the turbine.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Spiral Casing

The spiral casing is the outer component of a Francis turbine that distributes the incoming water evenly to the guide vanes. It is designed to maintain a constant velocity of water around its circumference. However, it does not dynamically adjust to regulate the flow rate or optimize power output. Its primary function is to ensure uniform distribution of water, which is critical for the efficient operation of the turbine, but it does not have a direct role in dynamic regulation under varying load conditions.

Option 3: Draft Tube

The draft tube is a diverging passage located at the exit of the turbine runner. Its purpose is to recover the kinetic energy of the water leaving the runner and convert it into pressure energy, thereby increasing the overall efficiency of the turbine. While the draft tube plays a significant role in energy recovery, it does not dynamically adjust to regulate the flow rate or power output. Its function is passive rather than active in the context of flow regulation.

Option 4: Runner Blades

The runner blades are the primary moving components of a Francis turbine that convert the kinetic and potential energy of water into mechanical energy. While the shape and design of the runner blades are crucial for the turbine's efficiency, they do not dynamically adjust during operation. The runner blades are fixed in position and rely on the guide vanes to control the flow of water entering the runner.

Conclusion:

In a Francis turbine, the only component that dynamically adjusts to regulate the flow rate and optimize power output under varying load conditions is the guide vanes (Option 1). The spiral casing, draft tube, and runner blades, while essential for the turbine's operation, do not have the capability to dynamically adjust during operation. Understanding the distinct roles of these components is crucial for comprehending the operation and design of Francis turbines.

Explanation:

Solar Panels (Photovoltaic - PV) vs. Concentrating Solar Power (CSP):

Definition: Solar Panels (PV) and Concentrating Solar Power (CSP) are two prominent technologies used to harness solar energy. PV panels directly convert sunlight into electricity using semiconductor materials, while CSP systems use mirrors or lenses to concentrate sunlight onto a receiver to produce heat, which is then converted into electricity using a turbine or engine.

Correct Option Analysis:

The correct option is:

Option 2: They need costly batteries to store power for nighttime use.

This statement highlights one of the major challenges associated with Solar Panels (PV) compared to CSP systems. Solar Panels generate electricity during the daytime when sunlight is available, but they do not inherently have storage capabilities. To ensure a continuous power supply during nighttime or cloudy periods, it is necessary to pair PV systems with energy storage solutions, typically batteries.

While CSP systems often use thermal storage methods (e.g., molten salt) to store heat energy for later use, PV systems rely on batteries, which are expensive and can significantly increase the overall cost of the system. The integration of batteries into PV setups also presents challenges related to scalability, efficiency, and environmental concerns due to the mining and disposal of battery materials.

Detailed Explanation:

1. The Need for Energy Storage:

• Solar Panels (PV) produce electricity only when sunlight is available, meaning their output is intermittent and depends on the weather and time of day.
• To achieve a stable and reliable energy supply, PV systems often require batteries to store excess energy generated during the day for use during nighttime or cloudy periods.
• The cost of batteries, such as lithium-ion batteries, is a significant factor in the overall expense of a PV system. Additionally, battery lifespan and efficiency can impact the long-term viability of the system.

2. Cost Implications:

• Batteries are one of the most expensive components of a PV system. Their cost can rival or even exceed the cost of the solar panels themselves.
• The maintenance and replacement of batteries add to the operational costs, making PV systems less economically competitive compared to CSP in some cases.
• The environmental impact of battery production and disposal is another concern, as the extraction of materials like lithium and cobalt can cause ecological harm.

3. Comparison with CSP:

• CSP systems typically use thermal storage techniques, such as molten salt storage, which are more cost-effective and environmentally friendly compared to batteries.
• Thermal storage allows CSP systems to generate power even after sunset, providing a more consistent and reliable energy output.

4. Advances in Battery Technology:

• Research and development in battery technology, including solid-state batteries and flow batteries, aim to reduce costs and improve efficiency, which could make PV systems more competitive in the future.
• Despite these advancements, the current reliance on costly batteries remains a significant challenge for PV systems.

Conclusion:

Option 2 correctly identifies the major challenge of Solar Panels (PV) needing costly batteries for nighttime energy storage. This reliance on batteries increases the cost and complexity of PV systems compared to CSP systems, which often utilize more efficient and cost-effective thermal storage solutions.

Additional Information

Analysis of Other Options:

Option 1: They have lots of moving parts, making maintenance costly.

This statement is incorrect for Solar Panels (PV). PV systems have no moving parts, which is one of their advantages over CSP systems. CSP systems involve components like mirrors, tracking systems, and turbines, which require regular maintenance and have higher operational costs due to their mechanical complexity.

Option 3: They completely stop working on cloudy days.

This statement is misleading. While the efficiency of PV systems decreases on cloudy days due to reduced sunlight, they do not "completely stop working." Advanced PV panels can still generate some electricity under diffuse light conditions, although at a lower output. CSP systems, on the other hand, rely on direct sunlight for optimal performance and are more affected by cloudy weather.

Option 4: They need high-tech factories to be made.

This statement is partially correct but not unique to PV systems. Both PV and CSP technologies require specialized manufacturing facilities. PV panels involve semiconductor fabrication, which necessitates high-tech factories, but CSP systems also require precision engineering for mirrors, receivers, and tracking systems. Therefore, this challenge is not exclusive to PV systems.

Conclusion:

While all the options highlight challenges related to solar technologies, Option 2 accurately identifies the significant issue of costly batteries for energy storage in PV systems, making it the correct choice. Understanding these challenges is essential for selecting the appropriate solar technology based on specific needs and conditions.

Tarapur Atomic Power Station:

• Tarapur Atomic Power station is located in Tarapur, Maharashtra.
• It was the first commercial atomic power station of India commissioned on 28th October 1969.
• It was commissioned under 123 agreements signed between India, the United States and International Atomic Energy Agency. 
• The station is operated by the National power corporation of India.

About Tarapur Atomic Power Station:

• Tarapur Atomic Power station is located in Tarapur, Maharashtra.
• It was the first commercial atomic power station of India commissioned on 28th October 1969.
• It was commissioned under 123 agreements signed between India, the United States, and International Atomic Energy Agency. 
• The station is operated by the National power corporation of India.

​

The minimum clearance distance that equipment should be kept away from power lines of different voltage levels is shown in below table.

Transmission lines are classified based on three criteria.

a) Length of transmission line

b) Operating voltage

c) Effect of capacitance

The table below summarizes the classification of transmission lines.

Concept:

Load factor:

The load factor is the ratio of average energy consumed to maximum demand.

Load factor = average energy consumed / maximimum energy consumed

Calculation:

Given load factor = 0.5

Average energy consumed at 0.5 load factor = 600 kWh

Maximum energy consumed =  = 1200 kWh

Now maximum energy consumed is constant and load factor is increased to 0.8

Average energy consumed = load factor × maximum energy consumed

= 0.8 × 1200

= 960 kWh

Load factor 

Average demand = (50) (0.6) = 30 MW

Plant capacity factor 

Plant capacity 

Reserve capacity = Plant Capacity – Maximum Demand = 60 - 50 = 10 MW

CONCEPT:

Nuclear reactor:

• It is a device in which a nuclear reaction is initiated, maintained, and controlled.
• It works on the principle of controlled chain reaction and provides energy at a constant rate.

EXPLANATION:

• The moderator's function is to slow down the fast-moving secondary neutrons produced during the fission.
• The material of the moderator should be light and it should not absorb neutrons.
• Usually, heavy water, graphite, deuterium, and paraffin, etc. can act as moderators.
• These moderators are rich in protons. When fast-moving neutrons collide head-on with the protons of moderator substances, their energies are interchanged and thus the neutrons are slowed down.
• Such neutrons are called thermal neutrons which cause fission of U235 in the fuel. 

The following types of cables are generally used for 3-phase service:

1. Belted cables - up to 11 kV

2. Screened cables - from 22 kV to 66 kV

3. Pressure cables - beyond 66 kV

Belted cables:

• These cables are used for voltages up to 11 kV but in extraordinary cases, their use may be extended up to 22 kV
• The belted type construction is suitable only for low and medium voltages as the electrostatic stresses developed in the cables for these voltages are more or less radial i.e., across the insulation
• For high voltages (beyond 22 kV), the tangential stresses also become important
• These stresses act along the layers of paper insulation
• As the insulation resistance of paper is quite small along the layers, therefore, tangential stresses set up leakage current along the layers of paper insulation
• The leakage current causes local heating, resulting in the risk of breakdown of insulation at any moment

Recovery Voltage:

The RMS voltage that appears across the circuit breaker contacts after final arc interruption (when breaker opens) is called “recovery voltage”

Restriking Voltage:

It may be defined as the voltages that appears across the breaking contact at the instant of arc extinction

Active Recovery Voltage:

It may be defined as the instantaneous recovery voltage at the instant of arc extinction

Arc Voltage:

It may be defined as the voltages that appears across the contact during the arcing period, when the current flow is maintained in the form of an arc. It assumes low value except for the point at which the voltage rises rapidly to a peak value and current reaches to zero.