৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২]

A fuse is designed to protect electrical circuits from excess current, which could potentially cause damage to the wiring, components, or even lead to fire hazards.
It works by melting or blowing when the current flowing through the circuit exceeds a certain value, thus breaking the circuit and stopping the flow of excess electricity.
So, the correct option is (ক) protective device. It does not limit or control current in a way other devices like circuit breakers or current-limiting resistors might. Instead, its primary role is to protect by disconnecting the circuit when a fault occurs.(Power System Protection and Switchgear (3rd Edition) by Badri Ram, D.N. Vishwakarma, and Soumya R. Mohanty)

A circuit breaker (CB) is an automatic electrical switch used to protect electrical circuits from damage caused by overload or short circuit. It is a crucial component in electrical distribution systems, designed to ensure the safe operation of electrical equipment by interrupting the flow of current in case of faults.

Under normal operating conditions, the contacts of the circuit breaker remain closed. This allows current to flow freely through the circuit. The breaker is designed to allow electrical power to pass without interruption under standard conditions, ensuring the proper functioning of the connected electrical system. When everything is working within the specified limits, the circuit breaker is in the closed state, meaning it is allowing electricity to flow through the system as intended.

However, if an overload or short circuit occurs, the circuit breaker detects the abnormal condition and automatically opens the contacts. This action cuts off the electrical supply, preventing damage to the wiring, components, or other electrical devices in the circuit. The breaker will typically remain open until it is manually reset or until the fault is cleared.

The operation of the circuit breaker is based on a simple mechanism. It uses either thermal or magnetic principles to detect faults:

1. In thermal breakers, a bimetallic strip bends when excessive current flows through it, causing the contacts to open.

2. In magnetic breakers, an electromagnet is used to pull the contacts open when the current exceeds a certain threshold.

Once the fault is cleared, the breaker can be manually reset to the closed position, restoring normal operation to the circuit. This design ensures both safety and reliability, making circuit breakers a vital part of electrical systems.(Power System Protection and Switchgear (3rd Edition) by Badri Ram, D.N. Vishwakarma, and Soumya R. Mohanty)

A relay is an electrical device used to detect faults and initiate the operation of other protective devices in a power system, such as a circuit breaker. When a fault, like an overload or short circuit, occurs in a circuit, the relay senses the abnormal condition by monitoring electrical parameters such as current, voltage, or frequency.

Once the relay detects a fault, it sends a signal to the circuit breaker or other protective devices, triggering them to act by disconnecting the faulty circuit. This prevents further damage to the system, ensuring the protection of the power system components.

To clarify the other options:

          * A circuit breaker does not detect faults on its own. It responds to signals sent by relays to interrupt the circuit.
          * An isolator is a switching device used to isolate a part of the system, ensuring that maintenance or repair work can be done safely, but it does not detect faults.
Thus, the relay is the device responsible for detecting faults and initiating the necessary protective actions in a power system.(Power System Protection with Artificial Intelligence Applications by Desai, Desai, and Nayak)

An arc is produced when the switch of a high-voltage and large-current circuit is opened.

When a high-voltage circuit is opened (i.e., when the switch is turned off), the current tries to continue flowing even though the circuit is being broken. This is because the current has inertia, and in high-current circuits, the energy can cause a spark or arc to form between the opening contacts. The arc occurs when the contacts of the switch start to separate. At high voltages and large currents, the potential difference across the contacts is enough to ionize the air between them, creating a conducting path for the current. This ionized path allows the current to continue flowing momentarily, forming an arc.
The arc can be damaging to the circuit and the switch, as it can cause excessive heating, wear, and even permanent damage to the components if not properly controlled. Arc extinguishing is an important feature in high-voltage switches and circuit breakers to ensure that once the circuit is opened, the arc is quickly extinguished.
Other options: What's Happening?

When the circuit is closed, there is no arc because the contacts are fully connected, allowing current to flow without any interruption.
Both opening and closing the switch at the right moment can cause arcing in some cases, but it's more common when opening high-voltage, high-current circuits.

Current chopping refers to the phenomenon where the current is interrupted before it naturally reaches zero, often resulting in a sudden interruption of current flow. This can cause a transient voltage spike, which may be damaging to the circuit. Current chopping primarily occurs in oil circuit breakers.In an oil circuit breaker, the current is interrupted by the arc that forms when the contacts of the breaker open. The oil serves to extinguish the arc by absorbing the heat and creating a barrier to the electrical current. However, in some cases, particularly under light load conditions, the breaker may open and interrupt the current before the arc naturally goes to zero, causing current chopping.

In contrast, other types of circuit breakers like air-blast, SF6, and vacuum circuit breakers are designed to extinguish the arc more effectively and typically do not exhibit significant current chopping. For example:

##Air-blast circuit breakers use compressed air to extinguish the arc and are less prone to current chopping.

##SF6 circuit breakers use sulfur hexafluoride gas to quench the arc, providing effective interruption and minimal risk of current chopping.

##Vacuum circuit breakers operate in a vacuum, preventing the formation of an arc, and they generally experience less current chopping.

In a bulk oil circuit breaker, only a small portion of the total oil is actually used for arc extinction. Typically, around 10% of the oil is actively involved in the process of quenching the arc that forms when the circuit is interrupted.

When the contacts of the breaker open, an electric arc is formed. The oil serves to cool and extinguish the arc by absorbing the heat and creating a dielectric barrier.
However, the majority of the oil (about 90%) is used for other purposes, such as insulation and cooling the system, not directly for extinguishing the arc.
In bulk oil circuit breakers, the oil is kept in a large tank, and only a portion of the oil is directly involved in the arc-extinguishing process. The rest of the oil helps in maintaining the dielectric strength of the breaker and keeping the components cool and insulated.(Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

In this context, the rated normal current refers to the current that the circuit breaker can handle continuously under normal operating conditions.

The rated current is typically the current the breaker can carry without damage when the circuit is closed, which, in this case, is specified as 1500 A.

 1500/√3 A would be applicable if we were looking at the line-to-neutral current in a 3-phase system, but in this case, the question specifies the normal current, which refers to the line-to-line current in the system, not line-to-neutral.

A circuit breaker is designed to perform multiple essential functions in protecting electrical circuits. These functions include:

1. Opening the faulty circuit and breaking the fault current: When a fault, such as a short circuit or overload, occurs, the circuit breaker opens the circuit to isolate the fault and interrupts the fault current to prevent damage to the system.
2. Being closed onto a fault: In some cases, a circuit breaker must be capable of closing onto a fault condition. This happens during certain scenarios where the breaker is required to close the circuit to restore power or during fault detection.
3. Carrying fault current for a short time: Some circuit breakers are designed to carry fault current for a brief period (usually a few milliseconds to a few seconds) until they can open and interrupt the fault. This is especially important for fault detection and protection mechanisms to act swiftly.

Capacitive current breaking refers to the interruption of current in circuits that have a capacitive load, such as transmission lines or cables. When a circuit breaker interrupts a capacitive current, the energy stored in the capacitance can cause a voltage surge or transient across the contacts of the breaker.

Capacitive current behaves differently than inductive or resistive current. When the circuit is broken, the sudden disconnection of a capacitive load can result in a voltage surge due to the energy stored in the capacitor. This surge can cause potential damage to the breaker or other components in the circuit if not properly managed.

(Power System Protection (2nd Edition) by Paul M. Anderson)

The making current of a circuit breaker refers to the current that the breaker must be able to withstand when it closes onto a fault. This current is usually specified in terms of the peak value because the initial surge or peak of the fault current is typically much higher than the steady-state current. Peak value is used to account for the initial inrush of current when a circuit breaker closes onto a fault. This is the maximum instantaneous current value that can occur when the circuit breaker is closing onto a live fault. The r.m.s. (root mean square) value and average value are typically used to represent steady-state currents, but they are not suitable for describing the making current since the peak value represents the highest stress the breaker will experience when closing.(Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

The breaking capacity of an SF6 (sulfur hexafluoride) circuit breaker is not significantly affected by factors such as weather and moisture, pollution, or atmospheric pressure.

SF6 circuit breakers use sulfur hexafluoride gas as the arc-quenching medium. This gas has excellent insulating and arc-extinguishing properties, making it highly effective in maintaining the breaker’s performance under varying conditions.

Weather and moisture: SF6 gas is not sensitive to moisture or typical weather conditions, as the gas is highly stable and does not absorb moisture like air-based systems. It is contained within the circuit breaker, reducing its exposure to environmental elements.

Pollution: SF6 circuit breakers are also not significantly affected by pollution or contaminants in the air, as the gas inside the breaker provides the necessary insulation, and the breaker is sealed.

Atmospheric pressure: SF6 circuit breakers are designed to operate effectively at a range of atmospheric pressures. Since the pressure inside the circuit breaker is controlled and maintained, changes in the external atmospheric pressure do not impact its breaking capacity.(Power System Protection and Switchgear (3rd Edition) by Badri Ram, D.N. Vishwakarma, and Soumya R. Mohanty)

Most of the relays used in electric power systems are electro-mechanical relays. These relays operate based on the principle of magnetic fields generated by the current flowing through the system. When the current exceeds a predetermined threshold, the electromagnetic force moves a contact, either opening or closing the circuit to protect the system from faults like overloads or short circuits.

Electro-mechanical relays have been traditionally used in power systems due to their reliability and robustness. They are widely implemented because they can handle high voltage and current, and they are simple in operation.

Electronic relays are becoming more common with the advancement of technology. These relays use electronic components like semiconductors to detect faults. They are more precise and faster than electro-mechanical relays but are not as widely used in traditional power systems due to cost and complexity in some cases.

Thermal relays operate based on the heating effect of current and are typically used in situations where protection against overload (but not short circuit) is needed. They are less commonly used than electro-mechanical relays in large-scale power systems.(Power System Protection and Switchgear (3rd Edition) by Badri Ram, D.N. Vishwakarma, and Soumya R. Mohanty)

To calculate the plug-setting multiplier (P.S.M.), you can use the following formula:

P.S.M.=Fault currentPick-up current/Pick-up current \
​Given:

Pick-up current = 7.5 A
Fault current = 30 A
Substitute the values into the formula:

P.S.M.=30/7.5=4

So, the plug-setting multiplier is 4.

In most power systems, current transformers (CTs) used for relays are rated with a secondary current of 5 A. This standard is widely adopted for protection and metering purposes. The 5 A secondary current allows for accurate current measurement and safe operation of relays, which typically require a 5 A input for correct operation.

5 A is the most commonly used standard secondary current for CTs, making it compatible with most protection relays, which are designed to operate with a 5 A input from the CT.(Power System Protection (2nd Edition) by Paul M. Anderson)

Back-up protection is typically used to provide additional protection in case the primary protection system fails to operate. It is mainly used for short-circuit faults, which are sudden and severe, and can cause significant damage if not interrupted quickly.

Back-up protection is designed to activate if the primary protection system fails to respond to a fault or if the fault occurs in a part of the system that is not adequately covered by the primary protection. This is more crucial in the case of short-circuit faults, as they require immediate interruption to prevent further damage.

Open-circuit faults, while they can disrupt the system, are generally easier to detect and don’t cause the same immediate threat as short-circuits, so they are less likely to need back-up protection.(Power System Protection (2nd Edition) by Paul M. Anderson)

Back-up protection serves several important roles in an electrical power system:

Second line of defense: It provides additional protection when the primary protection system fails to operate correctly. This ensures that there is a secondary method of fault detection and isolation in case the first line of defense (primary protection) does not respond.

Operates when primary protection fails: The primary protection system is designed to detect and isolate faults quickly, but in some cases, it may fail due to malfunctions, incorrect settings, or other issues. Back-up protection steps in and operates to isolate the fault if the primary protection doesn't function as expected.

Operation disconnects a large part of the system: While primary protection is typically more targeted, back-up protection might disconnect a larger portion of the system because it may need to act more broadly to clear the fault when the primary protection is unable to do so.

I²∝r³
Using the proportionality:

(I₁ / I₂)² = (r₁ / r₂)³

Substitute the given values:

(8 / 1)² = (0.8 / r₂)³

r₂ = 0.8 * (1/8)^2/3=  0.2 mm

The operating time of a circuit breaker is the time it takes from the moment a fault occurs to the moment the breaker interrupts the current and isolates the fault. In most cases, modern high-speed circuit breakers have very fast operating times to protect equipment and prevent damage from faults.

The typical operating time for a high-voltage circuit breaker is in the range of milliseconds, often around 0.001 seconds (1 millisecond) for quick operation in the event of a fault.

This fast response time is essential to minimize the damage caused by faults, such as short circuits, and to ensure the safety and reliability of the electrical system. Therefore, 0.001 sec is the correct answer for the typical operating time of a circuit breaker.(Power System Protection and Switchgear (3rd Edition) by Badri Ram, D.N.)

Fuses are designed to protect electrical systems from over-current conditions. Over-current occurs when the current flowing through a circuit exceeds the safe operating limit, which can result in overheating and potential damage to the circuit or components.

Here’s a breakdown of the options:

(ক) Under-current: Fuses are not used to protect against under-current, as this would mean there is too little current flowing, which typically does not pose a risk of damage to the system.

(খ) Over-current: This is the correct answer. Fuses are specifically designed to interrupt the current if it exceeds a predetermined value, thereby preventing damage to wiring and equipment due to excessive current. Once the current exceeds the fuse's rating, it blows and disconnects the circuit.

(গ) Resonance: Fuses do not protect against resonance, which is a condition where a circuit oscillates at its natural frequency. Protection against resonance is typically handled by other components like capacitors or circuit design.

(ঘ) Lightning surges: While lightning surges can cause over-voltage conditions, fuses are generally not used to protect specifically against lightning surges. Surge protectors or lightning arresters are designed for this purpose.

Sparking occurs when switching off a load in an electrical circuit, primarily due to inductive effects. Here's why:

When a circuit with an inductive load (like a motor or transformer) is switched off, the current does not immediately drop to zero. This is because inductance opposes changes in current. As a result, the collapsing magnetic field around the inductor generates a high voltage across the switch, which can cause a spark or arc to form between the contacts when they open. This is known as arc formation or arcing.

Inductive loads store energy in the magnetic field. When the circuit is suddenly opened, this stored energy can cause a high voltage spike (known as back emf), leading to sparking.

Skin effect: This refers to the tendency of alternating current (AC) to flow near the surface of conductors at high frequencies, but it does not directly cause sparking during switching off.

The operation of a fuse is based on the heating effect of electric current. A fuse is a safety device that protects electrical circuits from excessive current. It consists of a thin wire or strip that is designed to melt or blow when the current flowing through it exceeds a certain threshold. When the current is too high, the wire heats up due to the Joule heating effect (also known as the heating effect of electric current), causing it to melt and break the circuit. This interrupts the flow of current and prevents further damage to the circuit.(Power System Protection with Artificial Intelligence Applications by Desai, Desai, and Nayak)

Pros of HVDC over AC:
(a) Absence of capacitance: In an AC system, especially over long distances, the capacitance between conductors can cause significant issues, like reactive power loss. However, in HVDC systems, the transmission is purely DC, which means there is no capacitive reactance, making it more efficient for long-distance power transmission.
(b) Absence of inductance: While inductance does exist in both AC and DC systems (due to the conductors and transformers), in AC systems, inductive reactance can cause issues like voltage drops and energy losses. In HVDC systems, the absence of alternating current eliminates the inductive reactance issues associated with AC transmission lines.
(c) Absence of phase displacement: In AC transmission, phase displacement between different parts of the system can cause synchronization problems and complex grid management. HVDC avoids this because it is a unidirectional flow of power, so there are no phase angle issues, making it easier to control and manage the power flow between grids.

Statement: Corona is a phenomenon that occurs when the electric field around a conductor is strong enough to ionize the air surrounding it. This leads to violet glow, a hissing noise, and the formation of ozone. This happens when the voltage on the conductor exceeds a certain threshold, causing ionization of air molecules around the conductor.

The statement is correct, as it accurately describes the corona phenomenon.
Assertion: DC lines have no corona.

This assertion is wrong. While it's true that AC lines are more prone to corona due to the continuous change in the voltage, DC lines can also experience corona, especially at high voltages. The phenomenon can occur for both AC and DC transmission lines when the voltage is high enough to cause ionization of the air around the conductor.
Thus, the statement is correct (about corona) and the assertion is wrong (since DC lines can also experience corona). Therefore, the correct answer is (ক)."Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta"

An HVDC (High Voltage Direct Current) link consists of several components:

Rectifier: Converts AC to DC.

Inverter: Converts DC back to AC for transmission.

Transmission line: Carries the DC power between the two systems.

For HVDC systems, the converters (rectifier and inverter) interact with the AC system in the following ways:

1. Rectifier (AC to DC): The rectifier generally consumes reactive power from the AC system because it uses reactive power to produce the DC voltage required for transmission.

2. Inverter (DC to AC): The inverter, which converts the DC back into AC, also typically consumes reactive power to maintain its operation, although in some cases, it can be designed to supply reactive power under certain conditions.

Why (ঘ) is correct:

In HVDC systems, both the rectifier and inverter generally consume reactive power from the AC network. This is because the conversion processes (both AC to DC and DC to AC) require reactive power to maintain the voltage levels and control the power flow.

Rectifier: It consumes reactive power to support the conversion of AC to DC.

Inverter: It consumes reactive power to support the conversion of DC to AC.

HVDC (High Voltage Direct Current) transmission systems are often preferred for bulk power transmission over long distances due to the following reasons:

(ক) Low cost of HVDC Transmission: While HVDC transmission is beneficial for long-distance transmission, the initial cost of setting up HVDC systems (including rectifiers, inverters, and other equipment) is usually higher than AC systems. So, cost is not the primary advantage here.
(খ) No Harmonics Problem: HVDC systems are less susceptible to harmonics compared to AC systems, but they can still generate harmonics during the conversion process (rectification and inversion). Therefore, harmonics are still a consideration, though they are typically mitigated with filters.
(গ) Minimum line power losses: This is the primary reason why HVDC is preferred for long-distance transmission. HVDC lines have lower transmission losses compared to AC lines, especially over long distances. AC transmission suffers from reactive power losses and voltage drops, while HVDC systems do not experience these losses, making HVDC ideal for long-distance power transmission.
(ঘ) Simple protection: While HVDC systems have simpler protection mechanisms compared to AC systems, this is not the main reason for their preference in long-distance transmission. HVDC systems do require complex control systems and protection mechanisms, though simpler than AC systems for fault isolation.

HVDC (High Voltage Direct Current) transmission is often preferred over EHVAC (Extra High Voltage Alternating Current) transmission for several reasons, particularly in long-distance and interconnection scenarios:

(ক) HVDC terminal equipment are inexpensive: This is not correct. HVDC terminal equipment (such as converters, inverters, and transformers) tend to be more expensive than the equipment required for EHVAC systems. While the transmission lines themselves may have lower losses, the terminal equipment cost is a significant factor that makes HVDC more costly initially.

(খ) VAR compensation is not required in HVDC systems: This is misleading. While HVDC systems do not require VAR (Volt-Ampere Reactive) compensation in the same way as AC systems, it doesn't mean that there is no reactive power control or management. HVDC systems do require reactive power control at the converters, but it doesn't involve the same kind of complex VAR compensation needed in EHVAC systems.

(গ) System stability can be improved: This is true. One of the key advantages of HVDC systems is that they can significantly improve the stability of the overall power system, especially in multi-area power systems. HVDC lines can connect grids with different frequencies or areas that may be operating asynchronously, providing better control and stability. This is especially valuable in cases where synchronous AC connections might have stability or oscillation issues.

(ঘ) Harmonics problem is avoided: This is not accurate. HVDC systems can still generate harmonics due to the conversion processes (rectification and inversion), and special filters are used to mitigate these harmonics. While EHVAC systems can have harmonic issues related to capacitive effects and reactive power, HVDC systems still need careful management of harmonics.

Therefore, the most significant advantage of HVDC transmission over EHVAC is its ability to improve system stability, especially in large, interconnected grids with different operational characteristics. Thus, (গ) System stability can be improved is the correct answer.

The operation of an HVDC converter (either rectifier or inverter) involves both active power (real power) and reactive power consumption.

(ক) Does not consume reactive power: This is incorrect. HVDC converters do consume reactive power during their operation, especially during the conversion of AC to DC (rectification) and DC back to AC (inversion). The amount of reactive power required depends on the operating conditions of the system.
(খ) Consumes as much reactive power as real power: This is the correct answer. In an HVDC converter, especially during the rectification process, it typically consumes reactive power equal to or similar to the real power. This is because the converters need reactive power to maintain their operation, especially to support the conversion of AC to DC and vice versa. The ratio of reactive power to real power can vary depending on the control and design of the system, but in many cases, it is close to a 1:1 ratio.
(গ) Consumes 50% of the real power: This is incorrect. The amount of reactive power consumed is not fixed at 50% of the real power. While the amount of reactive power can vary, it can be more or less than 50% of the real power depending on system conditions and converter settings.
Thus, HVDC converters typically consume as much reactive power as real power during their operation, making (b) the correct answer.

In an HVDC (High Voltage Direct Current) system, the fault clearing process is primarily handled through the current control of converters. When a fault occurs on the DC side of the link, the converters (rectifier or inverter) are responsible for managing and controlling the current flow, especially under fault conditions. The converters are equipped with sophisticated control systems that can detect abnormal conditions, such as a short circuit or other disturbances, and reduce or stop the current to protect the system.

How Current Control Works:
Current control involves adjusting the firing angles of the thyristors in the converter to regulate the amount of current being supplied to the system. In the case of a fault, the converter’s control system can instantly reduce the output current by modifying the firing angles or by completely shutting down the power flow through the DC link.
This instantaneous response to the fault prevents damage to the system by stopping the current from continuing to flow through the faulty section. Once the fault is cleared, the system can resume operation with normal current control settings.
This method is preferred because it provides rapid fault isolation and minimizes damage to the system components, unlike relying on breakers that may not be as quick in responding or may cause other operational challenges.

Book Reference:
For more detailed information on fault management in HVDC systems, you can refer to "High Voltage Direct Current (HVDC) Power Transmission Systems Technology Review Paper" by S. K. Gupta and A. K. Jain, which discusses the fault protection mechanisms, including the role of current control in the HVDC link. This control method is frequently highlighted in "HVDC and FACTS Controllers" by K. R. Padiyar, which offers an in-depth explanation of HVDC system controls and operations during faults.

Thus, current control of converters is the primary method for removing faults on a two-terminal DC link in an HVDC system, as it allows for fast and effective isolation of faults.

Creepage distance refers to the shortest path along the surface of an insulating material between two conductive parts that are at different potentials. This distance is crucial for the insulation to withstand the electrical stress and prevent the risk of electrical breakdown or flashover.

Option (ক): "Shortest distance between two conducting parts along a stretched string" is incorrect because creepage distance pertains to the surface of the insulator, not along a string or conductor.
Option (খ): This is the correct definition. The creepage distance is measured along the surface of the insulating material and helps prevent the flow of current along the surface, which could otherwise lead to a breakdown of the insulation.
Option (c): "Distance between ground and the highest earthed point on the equipment" refers to clearance distance, not creepage distance.
Option (গ): "All of these" is incorrect because only option (b) correctly describes creepage distance.
In summary, creepage distance is a critical factor in the design and safety of electrical equipment, ensuring proper insulation and reducing the risk of electrical failure.

An HVDC (High Voltage Direct Current) link is a power transmission system that involves the transmission of electrical energy via direct current (DC) instead of alternating current (AC). HVDC systems are used for long-distance power transmission, interconnecting different grids, and providing control over the direction of power flow. An HVDC link typically includes the following components:

Two Earth Electrodes: HVDC systems often require two earth electrodes for the return path of the current when one of the terminals is connected to the ground. This is especially important for monopolar HVDC systems, where one terminal is grounded and the other is connected to the AC grid. The return current flows through the earth electrodes, ensuring the system's continuity and stability.

Converter Valves: The converter valves are a critical part of an HVDC system. They are used in both the rectifier (AC to DC) and inverter (DC to AC) sections of the system. The valves consist of semiconductor devices (such as thyristors) that control the flow of current by switching on and off at precise intervals. These valves are responsible for converting AC to DC at the rectifier and DC back to AC at the inverter.

Bipolar DC Line: In a bipolar HVDC system, there are two conductors carrying equal but opposite currents. A bipolar DC line helps in increasing the transmission capacity and ensures a more stable and reliable system. In case one pole (conductor) fails, the other pole can continue operation, providing redundancy and ensuring the system’s resilience. (Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

Load shedding refers to the intentional interruption of electricity supply to certain areas or consumers in order to prevent the entire power system from becoming overloaded or failing. It is typically implemented when there is not enough power generation to meet the demand.

Distribution level: Load shedding is generally carried out at the distribution level because this is where the power is delivered to the consumers. If there is a deficit in power generation or if there are transmission constraints, distribution companies may implement load shedding by cutting off supply to certain areas or customers in a controlled manner. This helps to balance the supply and demand within the distribution network and prevents the grid from becoming unstable.

Transmission level: Load shedding is not typically carried out at the transmission level, as transmission networks are mainly responsible for carrying electricity from power plants to the distribution systems. Load shedding at this level would involve cutting off supply to entire regions, which is generally avoided unless there is a widespread issue in the power grid.

Both (ক) and (খ): While load shedding could theoretically occur at both levels, it is predominantly carried out at the distribution level.

Depending upon the load: The decision to implement load shedding does depend on the load demand and generation, but the actual action is generally carried out at the distribution level to manage local supply and demand imbalances.

Given the following parameters for a single-phase AC switch in between a 230V source and a load of 2kW with a lagging power factor of 0.8, calculate the rms current rating required by the SCR. Use the safety factor of 2.
Given:
- Voltage (V) = 230 V (rms value)
- Power (P) = 2 kW = 2000 W
- Power factor (pf) = 0.8 (lagging)
- Safety factor = 2
Solution:
Step 1: Calculate the apparent power (S):
S = P / Power factor = 2000 / 0.8 = 2500 VA
Step 2: Calculate the rms current (I) without the safety factor:
I = S / V = 2500 / 230 = 10.87 A
Step 3: Apply the safety factor:
I_required = I * Safety factor = 10.87 * 2 = 21.74 A
Therefore, the required rms current rating for the SCR is 21.74 A.

Solid State Relays (SSRs) are electronic switching devices that control the flow of current without the use of any mechanical parts. They function by using semiconductor components (such as triacs, thyristors, or phototransistors) to perform the switching action. Since SSRs do not rely on mechanical components or moving parts, they offer several advantages:

No moving parts: This results in faster switching, longer lifespan, and higher reliability compared to electromechanical relays.
Advantages: SSRs are more durable, silent, and capable of handling higher switching frequencies. They are also less susceptible to wear and tear since they don't rely on physical contacts.(Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

In HVDC (High Voltage Direct Current) transmission systems, smaller conductor sizes are typically required compared to HVAC (High Voltage Alternating Current) transmission systems. This is because, in HVDC systems, the power losses are lower, and there is no reactive power involved, resulting in a more efficient transmission of power over long distances. As a result, the conductor size can be smaller, and HVDC transmission can achieve higher power transfer capacity with less transmission loss.

Smaller transformer size: HVDC systems generally require larger converter stations and associated equipment, not necessarily smaller transformer sizes.
Higher corona loss: HVDC transmission systems generally suffer less corona loss than HVAC systems, especially at high voltages.
Smaller power transfer capabilities: HVDC systems can transmit larger amounts of power over long distances compared to HVAC systems, making them more suitable for long-distance, high-capacity transmission.

In HVDC converters, thyristors are connected in series to achieve the required voltage rating. The individual thyristors, each having a lower voltage rating, are connected in series to withstand the higher voltage levels needed for HVDC operation.

                      Turned on at the same instant: All the thyristors in series are typically triggered (turned on) at the same instant to allow current to flow through them at the same time, ensuring synchronized operation for proper current control and power conversion.

This setup is crucial to provide the voltage rating for high-voltage operations.

                    To provide the required current rating, and they are turned on at different instants: This is not true; the thyristors are turned on simultaneously to ensure current flows properly across all thyristors connected in series.

A Static Var Compensator (SVC) is an essential component of a Flexible AC Transmission System (FACTS) that primarily functions to control reactive power.

Reactive power control: The SVC is used to regulate the reactive power in the system by either injecting or absorbing it, depending on the system’s requirements. This helps to maintain voltage stability and improve the overall power quality of the transmission network.
Voltage regulation: By controlling the reactive power, the SVC can help maintain voltage at desired levels, reducing the risk of voltage instability or voltage fluctuations in the system.
Breakdown of other options:
(ক) Regulates active power: SVC does not directly regulate active power. Active power (or real power) is controlled by generation and load balance, not by reactive power compensation devices.
(গ) Adjusts frequency: Frequency regulation is typically handled by other components in the power system, such as governors and load control systems.
(ঘ) Monitors voltage stability: While the SVC indirectly supports voltage stability by controlling reactive power, it does not directly monitor voltage stability. Monitoring is usually done by dedicated control systems.
Thus, the primary function of an SVC is to control reactive power, making (খ) the correct answer.

 UPFC is an interconnected FACTS device, consisting of STATCOM and SSSC. Hence it constitutes of two voltage sourced converters. These back-to-back connected converters are operated through a common dc link.(Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

The Static Synchronous Series Compensator (SSSC) is a FACTS (Flexible AC Transmission System) device that primarily functions to dampen power system oscillations and enhance transient stability. It does this by providing series compensation to the transmission system, which helps in controlling power flow and improving the system’s ability to withstand disturbances. Specifically, the SSSC can inject or absorb reactive power into the system, which helps to dampen oscillations and improve the overall stability of the grid, especially during transient conditions or faults.

Breakdown of other options:
(ক) Interline Power Flow Controller (IPFC): The IPFC is used to manage the power flow in multiple transmission lines, but its primary purpose is not specifically to dampen oscillations or improve transient stability.
(খ) Thyristor-Controlled Series Reactor (TCSR): The TCSR provides series compensation by controlling the inductive reactance in the transmission line, but it is not primarily designed to dampen oscillations or enhance transient stability.
(ঘ) Static Compensated Series Reactor (SCSR): The SCSR provides series compensation and is used to control power flow in the transmission network, but it doesn't specifically target damping power system oscillations or enhancing transient stability.

 On-load tap changer can be used to accomplish fast and unrestricted thyristor control in FACTS. The regulating transformer employed in this concept can do the insertion of voltage from its appropriate taps. Also converters can be aptly configured to offer the desired voltage injection to compensate for the required unit. Thus voltage and phase angle regulation can be achieved in FACTS.(Elements of Power System Analysis by William D. Stevenson)

: TCBR consists of a resistor which is thyristor switched. TCVL contains a variable resistor (varistor) made of metal-oxide. Transformer is present in both TCVR and TCPST.

Of the given options, SVC has the highest influence on voltage control. TCPAR shows an average influence on voltage control. Both TCSC and SSSC have the least influence on voltage control.(Elements of Power System Analysis by William D. Stevenson)

 IPFC is basically a unified series-series FACTS controller. Here the series controllers are separately connected to each line of a multiline transmission network and they are connected via the power dc link. Hence the ‘combined series-series compensation’ is employed by an IPFC.(Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

In the transmission arena of electrical power system, application of power electronics comprises of High-Voltage Direct Current (HVDC) power transmission and Flexible AC Transmission System (FACTS). HVDC involves conversion of ac to dc at one end and conversion of dc to ac at the other end; in the range of a hundred megawatts to a few thousand megawatts. ‘FACTS’ employs conversion and/or switching power electronics, in the range of a few tens to a few hundred megawatts.(Power System Analysis by Hadi Saadat)

 To enhance the system stability of an ac transmission system of mesh network, a FACT controller can be a thyristor-contolled series inductor or a series capacitor or a phase angle –regulator. A series capacitor added to a line in a mesh network helps to remove overload from the other overloaded line. A series inductor added to a line in a mesh network helps to remove overload from the same overloaded line to which it is connected.(Principles of Power System (4th Edition) by V.K. Mehta & Rohit Mehta)

 TCSC (Thyristor-Controlled Series Capacitor): The TCSC is a series compensation device that is used to control the reactive power in a series configuration by varying the reactance in the transmission line, not a shunt device.
 SSSC (Static Synchronous Series Compensator): The SSSC is a series compensation device that provides series reactive compensation to control the power flow in transmission lines, so it is not a shunt device.
UPFC (Unified Power Flow Controller): The UPFC is a series and shunt compensation device, but its primary function is to provide both series compensation (via SSSC) and shunt compensation (via a Static Synchronous Compensator or STATCOM). While it has shunt compensation capabilities, it is more complex and not a purely shunt compensation device.
 SVC (Static Var Compensator): The SVC is a shunt compensation device. It provides reactive power compensation by injecting or absorbing reactive power at the point of connection. The SVC helps in voltage regulation and stability by controlling the reactive power in the system.

The Unified Power Flow Controller (UPFC) is a FACTS (Flexible AC Transmission System) device that combines two important components:

1. STATCOM (Static Synchronous Compensator): The STATCOM provides shunt compensation by injecting or absorbing reactive power to regulate voltage and improve system stability.
2. SSSC (Static Synchronous Series Compensator): The SSSC provides series compensation by controlling the reactive power flow in the transmission line, thus allowing control of power flow and enhancing system stability.
The UPFC combines the capabilities of both the STATCOM and SSSC to provide both shunt and series compensation in the same device. This allows the UPFC to control both voltage and power flow in the transmission system, offering superior control over the power grid.

 STATCOM and TCSC: This would combine a shunt compensation device (STATCOM) with a series compensation device (TCSC). However, the UPFC specifically uses SSSC and not TCSC.
SSSC and TSC: This combination is not correct for UPFC as it does not involve TSC.
TSSC and TCR: This is not related to the UPFC configuration.

In bypassed mode, the thyristors are made to fully conduct, essentially acting as a short circuit or bypass for the load. This is achieved by allowing the thyristors to conduct for the full half-cycle of the AC waveform, which is 180 degrees.

Bypassed mode means that the thyristors are triggered in such a way that they conduct for the entire duration of the positive or negative half of the AC cycle (depending on the design). This results in a conduction angle of 180 degrees, allowing the current to flow freely through the device for that entire period.(Power System Analysis and Design by J. Duncan Glover, Thomas Overbye, and Mulukutla S. Sarma)

The TSSC (Thyristor-Controlled Series Capacitor) is a FACTS device used to provide series compensation in a power system. It can control power flow and dampen power oscillations, but its speed of response is typically moderate.

Power flow control: The TSSC adjusts the reactive power in the system by controlling the series capacitive reactance. This allows it to regulate the power flow in the transmission lines.
Damping power oscillations: The TSSC helps to dampen low-frequency oscillations in the system, improving stability. The speed at which it responds to such oscillations is important, but it typically responds at a moderate speed that is suitable for power system control, without the need for ultra-fast reactions.(Power System Analysis by Hadi Saadat)