Test: Power Systems - 1 - Electrical Engineering (EE) MCQ

Correct Answer :- B

Explanation : The sag is reduced at low temperatures and is increases at higher temperatures.

Sag is directly proportional to the temperature.

Reactance relay used for the small transmission line.

Mho relay used in the long transmission line.

Impedance is non-directional and used in medium transmission lines.

Reactance relay is non directional and used in short transmission lines.

Correct Answer :- b

Explanation : Some advantages of AC over DC such as less loss in long transmission lines , the voltage can be changed easily ( step up / step down) using transformers.

Almost every electronic device that we use in our home and plug in to AC 230v outlets are working in DC . LED TV, home theater, disk players, electronic gadgets, laptop, computer , set top box etc all are working in DC. Though they are connecting to AC outlets, they are not working in AC, an AC to DC converter is used inside.

Usually electrical devices like air conditioner , refrigerator , mixer grinder, fan etc are designed to work in AC since we already have an AC connection in our home. Most of these devices can design in DC 12V/ 24V but it is not easy or good to design a computer motherboard or microcontroller circuit to work in 230v AC directly.

If the height of the transmission tower is increased, inductance won't change, that is a function of wire size. The increased height will decrease line to ground capacitance, but the line to line capacitance will be unchanged, unless you also change the spacing between lines.
 

VAR load is small which means reactive power is less or use of inductive loads are small. When reactive power is increases then power factor decreases. To maintain power factor as 1 , We use series capacitors in transmission lines. When the series capacitors are of little used in transmission line, VAR requirement is small.

STANDARD TRANSMISSION VOLTAGES 

Voltages adopted for transmission of bulk power have to conform to standard specifications formulated in all countries and internationally. They are necessary in view of import, export, and domestic manufacture and use. The following voltage levels are recognized in India as per IS-2026 for line-to-line voltages of 132 kV and higher. 

Nominal System 

Voltage kV 132 220 275 345 400 500 750 

Maximum Operating 

Voltage, kV 145 245 300 362 420 525 765

Explanation:

• AC lines require more copper than DC lines for the transmission of the same power. This is because AC lines experience skin effect, which causes the current to concentrate near the surface of the conductor, leading to higher resistance and more power loss. To compensate for this, AC lines need to have larger conductors with more copper.


• DC lines, on the other hand, do not experience skin effect and have lower resistance, resulting in less power loss. Therefore, DC lines require less copper compared to AC lines for transmitting the same amount of power.

 

For the transmission of same power AC line requires more copper than a DC line

Bedding - Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

Materials are also used to provide armoured protection to cables, including aluminium, but steel is the most common. SWA cable is made by winding solid steel wires of between 0.9mm to over 3mm diameter around the bedded cable in a spiral form.

1. Capacitance exists between two things with different electric charge and a dielectric in between.

2. The transmission line and the earth below it, have a charge difference and the air between them is dielectric. This constitutes a capacitance.

3. The longer the cable, the more part of it runs parallel to the earth and hence, more capacitance between them since the length of the cable and the capacitance of the cable are directly proportional to each other.

A hydroelectric power station consists of turbines that rely on a gravity flow of water from the dam to turn a turbine to generate electricity. The water can be either released to the river downstream of the dam or pumped back into the reservoir and reused.

In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight of the insulator become more. Handling and replacing bigger size single unit insulator are quite difficult task. For overcoming these difficulties, suspension insulator was developed.

In suspension insulator numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc like shape.

Porcelain in most commonly used material for over head insulator in present days. The aluminium silicate is mixed with plastic kaolin, feldspar and quartz to obtain final hard and glazed porcelain insulator material. The surface of the insulator should be glazed enough so that water should not be traced on it.

The principal insulating materials used in cables are rubber, vulcanised India rubber,impregnated paper, varnished cambric and polyvinyl chloride.

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected.

Pin Insulator

Pin Insulator is earliest developed overhead insulator, but still popularly used in power network up to 33 KV system. Pin type insulator can be one part, two parts or three parts type, depending upon application voltage. In 11 KV system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass.

 

Strain and shackle insulators are used at dead ends in low voltage distribution lines (less than 11 kV).

The desired minimum depth of laying from ground surface to the top of cable is as follows:

1. High voltage cables, 3.3 kV to 11 kV rating = 0.9 m

2. High voltage cables, 22 kV, 33 kV rating= 1.05 m

3. Low voltage and control cables = 0.75 m

4. Cables at road crossings = 1.00 m

Rewirable Fuses

Rewirable or Kit – Kat Type Fuses are a type of Low Voltage (LV) Fuses. They are most commonly used in house wiring, small industries and other small current applications.

The fusing factor of HRC fuse is 1.1.
Explanation:
To understand the fusing factor of an HRC (High Rupturing Capacity) fuse, we need to know what it represents and how it is calculated.
1. What is the fusing factor?
- The fusing factor is a measure of the overload capacity of a fuse. It indicates the ratio between the minimum fusing current (the current at which the fuse will melt and open the circuit) and the rated current (the maximum continuous current that the fuse can carry).
2. How is the fusing factor calculated?
- The fusing factor is determined by dividing the minimum fusing current by the rated current of the fuse.
- Mathematically, the fusing factor (FF) is given by FF = Minimum fusing current / Rated current.
3. Applying the values for HRC fuse:
- For an HRC fuse, the fusing factor is typically 1.1.
- This means that the minimum fusing current is 1.1 times the rated current of the fuse.
Therefore, the correct answer is C: 1.1.

 

A fuse is nothing more than a short length of wire designed to melt and separate in the event of excessive current. Fuses are always connected in series with the component(s) to be protected from overcurrent, so that when the fuse blows (opens) it will open the entire circuit and stop current through the component(s). A fuse connected in one branch of a parallel circuit, of course, would not affect current through any of the other branches.

Time delay relay is used in thermal relay.
Explanation:
A time delay relay is a type of relay that is used to control the timing of an event. It is designed to introduce a time delay between the initiation of a signal and the occurrence of a specific action. In the case of a thermal relay, a time delay relay is used to control the timing of the trip or reset function.
Here is a detailed explanation of the different relay types mentioned in the question:
- Magnetic relay: A magnetic relay uses an electromagnet to control the switching of electrical circuits. It does not typically include a time delay function.
- Thermal relay: A thermal relay is a protective device that is used to protect electrical motors from overcurrent or overheating. It includes a time delay function to allow for temporary overloads without tripping the motor.
- Reed relay: A reed relay is a type of relay that uses a magnetic field to control the switching of electrical circuits. It does not typically include a time delay function.
- Latching relay: A latching relay is a type of relay that maintains its state even after the control signal is removed. It does not typically include a time delay function.
Therefore, the correct answer is B: Thermal relay.

To determine the correct voltage, we need to consider the standard voltage levels used in different countries. The standard voltage for residential homes varies around the world, but the most common voltages are 220 V, 230 V, 240 V, and 250 V.
To find the correct voltage, we can eliminate the options one by one based on the standard voltage levels:
1. Option A: 220 V - This is a commonly used voltage level in many countries, but it is not the correct answer as there are other options available.
2. Option B: 230 V - This is also a commonly used voltage level in many countries, especially in Europe. It is a valid answer and could be the correct voltage for the single bare copper wire.
3. Option C: 240 V - This voltage level is commonly used in countries like the United States and Canada. While it is a valid voltage level, it is not the correct answer based on the given options.
4. Option D: 250 V - This voltage level is less common and not widely used in residential settings. It is not the correct answer based on the given options.
Therefore, the correct answer is option B: 230 V.

Correct Answer :- b

Explanation : Two pin sockets should not be used in domestic wiring unless the appliance to be connected is double insulated.