Past Questions — 2024

Answer: Ampere-hours (Ah)

The capacity of a battery or a cell refers to the total amount of electric charge it can deliver to a load at a specific discharge rate before the voltage drops to a predetermined cutoff level. 1. Mathematically, Charge (Q) = Current (I) × Time (t). 2. The standard unit for electric current is the Ampere (A) and the standard unit for time is the Hour (h). 3. Therefore, the unit for capacity is Ampere-hours (Ah). For smaller cells (like those in smartphones or laptops), the capacity is often expressed in milliampere-hours (mAh), where 1 Ah = 1000 mAh. While energy is measured in Watt-hours (Wh), the 'capacity' of a cell specifically refers to its charge storage capability, which is measured in Ah.

Answer: Eddy current loss

In a transformer, the alternating magnetic flux induces an electromotive force (EMF) in the conductive core material. This EMF causes circulating currents to flow within the bulk of the core, known as 'Eddy Currents'. These currents result in power loss in the form of heat (I²R loss), which reduces the efficiency of the transformer. To minimize these losses, the core is not made from a single solid block of iron but is instead constructed from thin sheets called 'laminations'. Each lamination is coated with a thin layer of insulating varnish. This structure increases the electrical resistance of the path available for eddy currents, thereby restricting their flow and significantly reducing the total eddy current loss. Note: Lamination does NOT reduce hysteresis loss (which depends on the material's magnetic properties) or copper loss (which occurs in the windings).

Answer: Instrument Transformers

Current Transformers (CTs) and Potential Transformers (PTs) are collectively known as Instrument Transformers. They are specialized high-accuracy transformers used to scale down high currents and voltages to safe, standardized levels (typically 5A or 1A for CTs and 110V for PTs) so that they can be measured by standard instruments like ammeters, voltmeters, and wattmeters, or used for protective relaying. 1. Current Transformer (CT): A step-down transformer used to measure high alternating currents. It is connected in series with the line. 2. Potential Transformer (PT): A step-down transformer used to measure high voltages. It is connected in parallel across the line. Because their primary purpose is to provide a scaled-down version of the electrical parameters for 'instrumentation' purposes, they are categorized as Instrument Transformers.

The question refers to ACSR (Aluminium Conductor Steel Reinforced) conductors. 1. **Purpose of Steel Core**: Aluminium is an excellent conductor of electricity due to its low resistivity and light weight, but it has relatively low mechanical strength (tensile strength). In long-span transmission lines, the conductor must support its own weight and withstand external loads like wind and ice without excessive sagging or breaking. A steel core is placed at the center to provide the necessary mechanical strength (tensile strength) to support the aluminium strands. 2. **Why other options are incorrect**: - **Skin Effect (i)**: Skin effect is a phenomenon where current flows primarily on the outer surface of a conductor. It is managed by using stranded conductors or specific materials, not by adding a steel core. - **Proximity Effect (ii)**: This is the distribution of current in a conductor caused by the magnetic field of a nearby conductor. It is managed by proper spacing and bundling, not by a steel core. - **Line Inductance (iii)**: Adding a steel core (which has a higher permeability than aluminium) actually tends to slightly increase the inductance of the conductor, rather than reducing it.

Answer: AC Current

A Tong Tester, more commonly known as a Clamp Meter, is a specialized electrical tester that combines a basic digital multimeter with a current sensor. It is specifically used for measuring AC current flowing through a conductor without the need to disconnect the circuit or break the wire. It works on the principle of electromagnetic induction (using a current transformer) or the Hall Effect. The 'tongs' or clamps are placed around the conductor, and the magnetic field generated by the current flowing through the wire induces a current in the clamp's transformer, which is then calibrated and displayed as the current value. While some advanced models can measure DC current (using Hall Effect sensors), the primary and traditional use of a Tong Tester is for AC current measurement.

Transformer oil (mineral oil) serves two primary critical functions in a transformer: 1. Insulation: The oil acts as a dielectric medium. It provides electrical insulation between the windings and between the windings and the grounded tank, preventing electrical arcs and short circuits. 2. Cooling: As the transformer operates, the windings and core generate heat due to copper losses (I²R) and iron losses (hysteresis and eddy currents). The oil absorbs this heat and transfers it to the radiator or the tank walls via convection, where it is dissipated into the surrounding air or water. Since the oil performs both insulation and cooling roles, option (iii) is the correct choice.

The efficiency (η) of a transformer is defined as the ratio of output power to input power. The total losses in a transformer consist of two main components: 1. Iron Loss (Core Loss): This is constant regardless of the load (consisting of hysteresis and eddy current losses). 2. Copper Loss (I²R Loss): This varies with the square of the load current. Mathematically, Efficiency = Output / (Output + Constant Loss + Variable Loss). To find the maximum efficiency, we differentiate the efficiency expression with respect to the load current and set it to zero. This calculus derivation proves that the maximum efficiency occurs at the specific load where the variable losses (Copper losses) are exactly equal to the constant losses (Iron losses). Analysis of options: - Option i: Half load is not a universal rule; the load at which max efficiency occurs depends on the design of the transformer. - Option ii: Full load is where the transformer is rated to operate, but not necessarily where it is most efficient. - Option iii: Correct. This is the fundamental condition for maximum efficiency in any transformer. - Option iv: Overloading generally increases copper losses significantly, decreasing efficiency.

Answer: Volt-Ampere (VA)

In the context of Current Transformers (CTs), the 'burden' refers to the total load connected to the secondary winding of the CT. This load includes the resistance of the connecting leads and the internal resistance of the measuring instruments or relays connected to the secondary circuit. Since the burden is essentially an electrical load (impedance), it is expressed in terms of apparent power, which is measured in Volt-Ampere (VA). A CT is designed to maintain its accuracy only up to a specified rated burden; if the actual burden exceeds this rated value, the secondary current may not be a linear representation of the primary current due to saturation of the core.

An isolator (also known as a disconnecting switch) is a mechanical switch used to isolate a part of an electrical circuit for maintenance or repair. Unlike a circuit breaker, an isolator is NOT designed to interrupt the flow of current (it lacks arc-quenching capabilities). If an isolator is opened while current is flowing (under load), it will produce a severe electric arc that can damage the equipment and pose a fatal risk to the operator. Therefore, the standard operating procedure is to first open the circuit breaker (which is designed to extinguish the arc) to stop the current flow, and only then operate the isolator to provide a visible physical break in the circuit. Thus, isolators are operated only when there is no current in the line (no-load condition).

Answer: 6 months

According to the standard Supply Code followed by most State Electricity Boards and Distribution Companies (DISCOMs) in India, a temporary connection is provided for specific short-term requirements such as construction sites, festivals, or temporary exhibitions. The standard regulatory provision is that a temporary connection shall be granted for a maximum period of 6 months at a time. If the requirement extends beyond this period, the consumer is typically required to apply for an extension or convert the connection into a permanent one, subject to the approval of the licensee/utility company.

Answer: Low Head

A Kaplan turbine is a propeller-type water turbine with adjustable blades. It is specifically designed to operate under low-head conditions (typically below 30-40 meters) and high flow rates. To understand this, we compare it with other common turbines: 1. Pelton Wheel: High head, low flow rate (Impulse turbine). 2. Francis Turbine: Medium head, medium flow rate (Mixed flow turbine). 3. Kaplan Turbine: Low head, high flow rate (Axial flow turbine). Because the Kaplan turbine has adjustable blades, it can maintain high efficiency over a wide range of flow rates, making it the ideal choice for rivers or dams where the vertical drop (head) is small but the volume of water is large.

In a Hydro Electric Plant, the components serve the following functions: 1. **Spillway (Correct Answer):** A spillway is a structure used to provide the controlled release of water from a dam or reservoir into the downstream area. Its primary purpose is to discharge surplus water (floodwater) to prevent the dam from overtopping, which could lead to structural failure. 2. **Forebay:** This is a reservoir or basin located immediately upstream of the penstock. It acts as a settling basin for sediment and ensures a steady head of water is available for the turbines, but it is not used for discharging surplus water. 3. **Penstock:** This is a high-pressure steel pipe or conduit that carries water from the forebay or reservoir down to the turbine. Its purpose is to transport water for power generation, not to discharge excess water. Therefore, the spillway is the specific component designed for the discharge of surplus water.

Answer: The Central Electricity Regulatory Commission (CERC) or the State Electricity Regulatory Commission (SERC)

Under Section 2(1) of the Electricity Act, 2003, the term 'Authority' is defined to refer to the regulatory bodies established by the Act. Specifically, it means the Central Electricity Regulatory Commission (CERC) or a State Electricity Regulatory Commission (SERC), depending on the context of the provision being applied. These authorities are responsible for regulating the electricity market, determining tariffs, and ensuring the efficient functioning of the power sector in India.

Answer: Core loss (or Iron loss)

In a transformer, the no-load test (also known as the Open Circuit test) is performed by applying the rated voltage to one winding (usually the Low Voltage side) while keeping the other winding open-circuited. Under this condition, the current drawn is very small (no-load current), and the power consumed is primarily due to the magnetization of the core. This power consumption consists of Hysteresis loss and Eddy current loss, which together are termed as 'Core loss' or 'Iron loss'. Since these losses depend on the applied voltage and frequency (which are constant), they remain constant regardless of the load on the transformer. In contrast, the full-load test (Short Circuit test) is used to determine the copper loss (I²R loss).

Answer: Electrical Conductance

1. Definition: In electrical engineering, conductance (G) is the measure of how easily electricity flows through a conductor. It is the reciprocal of electrical resistance (R). 2. Formula: G = 1/R. 3. Unit Derivation: Since the SI unit of resistance is the Ohm (Ω), the SI unit of conductance is the reciprocal ohm (1/Ω), which is officially named the 'Siemens' (S) in honor of Werner von Siemens. 4. Relationship: 1 Siemens = 1 Ampere per Volt (A/V).