Voltage Control By External Equipment MCQ Quiz - Objective Question with Answer for Voltage Control By External Equipment - Download Free PDF

Explanation:

High Transmission Voltage in AC Systems

Definition: High transmission voltage in alternating current (AC) systems refers to the practice of transmitting electrical power at high voltage levels, typically in the range of hundreds of kilovolts (kV) or more. This approach is widely used in power transmission networks to efficiently transport electricity over long distances.

Working Principle: The transmission of electrical power involves the generation, transmission, and distribution of electricity. High voltage transmission is employed to reduce the current flowing through the transmission lines. According to the power transmission formula, P = VI (where P is power, V is voltage, and I is current), increasing the voltage (V) allows for a reduction in current (I) for the same amount of power (P). This reduction in current minimizes energy losses due to the resistance of the transmission lines.

Advantages of High Transmission Voltage:

• Reduced Power Losses: By transmitting power at higher voltages, the current through the transmission lines is reduced, leading to lower resistive losses (I²R losses). This improves the overall efficiency of the power transmission system.
• Improved Transmission Efficiency: Higher voltage levels enable the transmission of larger amounts of electrical power over long distances with greater efficiency.
• Reduced Conductor Size: Lower current levels allow for the use of smaller conductor sizes, which can reduce the material and installation costs of transmission lines.
• Enhanced Load Capacity: High voltage transmission lines can carry more electrical power, allowing for the support of larger and more widespread electrical grids.

Disadvantages of High Transmission Voltage:

• Increased Cost of Insulation: The most significant limitation of high transmission voltage in AC systems is the increased cost associated with insulating the conductors. High voltage levels require better and more expensive insulation materials to prevent electrical breakdown and ensure the safety and reliability of the transmission system.
• Complexity in Design: High voltage transmission systems require careful design and engineering to manage issues such as corona discharge, electromagnetic interference, and insulation coordination.
• Higher Equipment Costs: Transformers, circuit breakers, and other high voltage equipment are more expensive and require specialized designs to handle the increased voltage levels.

Correct Option Analysis:

The correct option is:

Option 4: The cost of the insulating conductor.

This option correctly identifies the primary limitation of using high transmission voltage in AC systems. The cost of insulating conductors increases significantly with higher voltage levels. This is because the insulation must be capable of withstanding higher electrical stresses to prevent breakdown and ensure reliable operation. The need for advanced insulation materials and techniques drives up the overall cost of the transmission system.

Additional Information

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

Option 1: The increased efficiency.

While it is true that high transmission voltage can lead to increased efficiency by reducing resistive losses, this is not a limitation but rather a benefit of using high transmission voltage.

Option 2: The increased power.

High transmission voltage allows for the transmission of larger amounts of power over long distances, but this is also a benefit rather than a limitation. The ability to transmit more power is one of the reasons for using high voltage in transmission systems.

Option 3: The increased line conductor.

This option is incorrect because high transmission voltage actually allows for the use of smaller conductor sizes, as the current is reduced. Therefore, it does not represent a limitation of high transmission voltage.

Conclusion:

Understanding the advantages and limitations of high transmission voltage in AC systems is crucial for designing efficient and reliable power transmission networks. While high voltage transmission offers several benefits, including reduced power losses and improved efficiency, the increased cost of insulating conductors remains a significant challenge. Addressing this limitation requires careful consideration of insulation materials and techniques to ensure the safe and economical operation of high voltage transmission systems.

Series compensation:

Series compensation is the method of improving the system voltage by connecting a capacitor in series with the transmission line.

The series compensation will be considered if X/R ratio is very high.

Advantages of series compensation:

• Increases the power transfer capability of the line.
• It improves the stability of the system.
• Improves load division among parallel lines.
• Improves voltage regulation of the line.

Disadvantages:

• Fault level in the system increases
• There is a subsynchronous resonance occurs in the system such that the shaft of the alternator will be damaged. 
• During the fault, a large voltage appears across a series capacitor such that the capacitor may be damaged.
• So the series capacitor will be protected by using a sparking gap.

Important point:

The power transfer capability of a transmission line is given by

\(P = \;\frac{{{V_s}{V_r}}}{{{X_l}}}\;sin\delta \)

Where

V_s is sending end phase voltage, V_r is receiving end voltage, X_l is series inductive reactance of the line, and \(\delta \) is load angle

From the above expression, we can observe that power transfer capability is inversely proportional to the reactance of the line.

If a capacitor having capacitive reactance X_c is connected in series with the line, then the reactance of the line is reduced from X_l to (X_l - X_c). As a consequence, the power transfer capability of the line increases.

Note:

Power factor improvement is done by the shunt capacitor.

Static VAR:

A static VAR compensator is a parallel combination of a controlled reactor and fixed shunt capacitor shown in the figure below. 

(i) The thyristor switch assembly in the SVC controls the reactor. 

(ii) The firing angle of the thyristor controls the voltage across the inductor and thus the current flowing through the inductor. 

(iii) In this way, the reactive power drawn by the inductor can be controlled.

Static Var Compensator can generate reactive power as to enhance voltage during heavy load condition and also it can absorb reactive power during light load condition so as to maintain or control specific parameters of the electrical power system (typically bus voltage).

Explanation:

Static VAR:

A static VAR compensator is a parallel combination of a controlled reactor and fixed shunt capacitor shown in the figure below. 

(i) The thyristor switch assembly in the SVC controls the reactor. 

(ii) The firing angle of the thyristor controls the voltage across the inductor and thus the current flowing through the inductor. 

(iii) In this way, the reactive power drawn by the inductor can be controlled.

Static Var Compensator can generate reactive power as to enhance voltage during heavy load condition and also it can absorb reactive power during light load condition so as to maintain or control specific parameters of the electrical power system (typically bus voltage).

Explanation:

Series compensation:

Series compensation is the method of improving the system voltage by connecting a capacitor in series with the transmission line.

The series compensation will be considered if X/R ratio is very high.

Advantages of series compensation:

• Increases the power transfer capability of the line.
• It improves the stability of the system.
• Improves load division among parallel lines.
• Improves voltage regulation of the line.

Disadvantages :

• Fault level in the system increases
• There is a subsynchronous resonance occurs in the system such that the shaft of the alternator will be damaged. 
• During the fault, a large voltage appears across a series capacitor such that the capacitor may be damaged.
• So the series capacitor will be protected by using a sparking gap.

Important point:

The power transfer capability of a transmission line is given by

\(P = \;\frac{{{V_s}{V_r}}}{{{X_l}}}\;sin\delta \)

Where

V_s is sending end phase voltage, V_r is receiving end voltage, X_l is series inductive reactance of the line, and \(\delta \) is load angle

From the above expression, we can observe that power transfer capability is inversely proportional to the reactance of the line.

If a capacitor having capacitive reactance X_c is connected in series with the line, then the reactance of the line is reduced from X_l to (X_l - X_c). As a consequence, the power transfer capability of the line increases.

• Power factor improvement is done by the shunt capacitor.
• Power system stability improvement is done by the series capacitor.
• Fault current in the power system is reduced by the series reactor.

Real power \({P_r} = \frac{{\left| {{V_s}} \right|\left| {{V_r}} \right|}}{{\left| x \right|}}\sin \delta\) 

\(\Rightarrow 1 = \frac{{1.0 \times 1.0}}{{0.5}}\sin \delta \Rightarrow \delta = 30^\circ\)

Reactive power \({\phi _r} = \frac{{\left| {{V_S}} \right|\left| {{V_r}} \right|}}{{\left| x \right|}}\cos \delta - \frac{{{V^2}}}{{\left| x \right|}}\)

\( = \frac{{\left( {1.0} \right)\left( {1.0} \right)}}{{0.5}}\cos 30 - \frac{{{{\left( {1.0} \right)}^2}}}{{0.5}}\)

= -0.268

Concept:

Under no-load conditions or light load conditions, medium and long transmission lines may operate at the leading power factor due to the capacitance effect.

So that receiving end voltage becomes greater than sending end voltage.

In this case, shunt reactors are needed to bring down receiving end voltage at light loads.

The leading power factor can be changed to a lagging power factor by using a shunt reactor. By using a shunt reactor, it will compensate for the effect of capacitance and changes the power factor.

Note:

• The shunt capacitor is used to improve the power factor.
•  A series reactor smoothens the wave shape.
• A Series capacitor reduces the net reactance in a line.
• The shunt inductor reduces the Ferranti effect by limiting overvoltages at the load side under lightly loaded conditions.

Before connecting shunt capacitor:

Maximum reactive power consumed by the load from the source.

Q₁ = 1 kVAR.

After connecting shunt capacitor:

Maximum reactive power consumed by the capacitor-load combination from the source. 

Q = 0.25 kVAR

Net reactive power supplied by the capacitor to the load is

Q_c = Q1 - Q = 0.75 kVAR

Net reactive power supplied from source to load changed from 1 kVAR to 0.25 kVAR, remaining 0.75 kVAR is supplied by the shunt capacitor.

\(P = \frac{{{V^2}}}{X}\)

Series capacitive will reduce the reactance. So power transmission will improve.

Explanation:

High Transmission Voltage in AC Systems

Definition: High transmission voltage in alternating current (AC) systems refers to the practice of transmitting electrical power at high voltage levels, typically in the range of hundreds of kilovolts (kV) or more. This approach is widely used in power transmission networks to efficiently transport electricity over long distances.

Working Principle: The transmission of electrical power involves the generation, transmission, and distribution of electricity. High voltage transmission is employed to reduce the current flowing through the transmission lines. According to the power transmission formula, P = VI (where P is power, V is voltage, and I is current), increasing the voltage (V) allows for a reduction in current (I) for the same amount of power (P). This reduction in current minimizes energy losses due to the resistance of the transmission lines.

Advantages of High Transmission Voltage:

• Reduced Power Losses: By transmitting power at higher voltages, the current through the transmission lines is reduced, leading to lower resistive losses (I²R losses). This improves the overall efficiency of the power transmission system.
• Improved Transmission Efficiency: Higher voltage levels enable the transmission of larger amounts of electrical power over long distances with greater efficiency.
• Reduced Conductor Size: Lower current levels allow for the use of smaller conductor sizes, which can reduce the material and installation costs of transmission lines.
• Enhanced Load Capacity: High voltage transmission lines can carry more electrical power, allowing for the support of larger and more widespread electrical grids.

Disadvantages of High Transmission Voltage:

• Increased Cost of Insulation: The most significant limitation of high transmission voltage in AC systems is the increased cost associated with insulating the conductors. High voltage levels require better and more expensive insulation materials to prevent electrical breakdown and ensure the safety and reliability of the transmission system.
• Complexity in Design: High voltage transmission systems require careful design and engineering to manage issues such as corona discharge, electromagnetic interference, and insulation coordination.
• Higher Equipment Costs: Transformers, circuit breakers, and other high voltage equipment are more expensive and require specialized designs to handle the increased voltage levels.

Correct Option Analysis:

The correct option is:

Option 4: The cost of the insulating conductor.

This option correctly identifies the primary limitation of using high transmission voltage in AC systems. The cost of insulating conductors increases significantly with higher voltage levels. This is because the insulation must be capable of withstanding higher electrical stresses to prevent breakdown and ensure reliable operation. The need for advanced insulation materials and techniques drives up the overall cost of the transmission system.

Additional Information

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

Option 1: The increased efficiency.

While it is true that high transmission voltage can lead to increased efficiency by reducing resistive losses, this is not a limitation but rather a benefit of using high transmission voltage.

Option 2: The increased power.

High transmission voltage allows for the transmission of larger amounts of power over long distances, but this is also a benefit rather than a limitation. The ability to transmit more power is one of the reasons for using high voltage in transmission systems.

Option 3: The increased line conductor.

This option is incorrect because high transmission voltage actually allows for the use of smaller conductor sizes, as the current is reduced. Therefore, it does not represent a limitation of high transmission voltage.

Conclusion:

Understanding the advantages and limitations of high transmission voltage in AC systems is crucial for designing efficient and reliable power transmission networks. While high voltage transmission offers several benefits, including reduced power losses and improved efficiency, the increased cost of insulating conductors remains a significant challenge. Addressing this limitation requires careful consideration of insulation materials and techniques to ensure the safe and economical operation of high voltage transmission systems.

Series compensation:

Series compensation is the method of improving the system voltage by connecting a capacitor in series with the transmission line.

The series compensation will be considered if X/R ratio is very high.

Advantages of series compensation:

• Increases the power transfer capability of the line.
• It improves the stability of the system.
• Improves load division among parallel lines.
• Improves voltage regulation of the line.

Disadvantages :

• Fault level in the system increases
• There is a subsynchronous resonance occurs in the system such that the shaft of the alternator will be damaged. 
• During the fault, a large voltage appears across a series capacitor such that the capacitor may be damaged.
• So the series capacitor will be protected by using a sparking gap.

Important point:

The power transfer capability of a transmission line is given by

\(P = \;\frac{{{V_s}{V_r}}}{{{X_l}}}\;sin\delta \)

Where

V_s is sending end phase voltage, V_r is receiving end voltage, X_l is series inductive reactance of the line, and \(\delta \) is load angle

From the above expression, we can observe that power transfer capability is inversely proportional to the reactance of the line.

If a capacitor having capacitive reactance X_c is connected in series with the line, then the reactance of the line is reduced from X_l to (X_l - X_c). As a consequence, the power transfer capability of the line increases.

• Power factor improvement is done by the shunt capacitor.
• Power system stability improvement is done by the series capacitor.
• Fault current in the power system is reduced by the series reactor.

Real power \({P_r} = \frac{{\left| {{V_s}} \right|\left| {{V_r}} \right|}}{{\left| x \right|}}\sin \delta\) 

\(\Rightarrow 1 = \frac{{1.0 \times 1.0}}{{0.5}}\sin \delta \Rightarrow \delta = 30^\circ\)

Reactive power \({\phi _r} = \frac{{\left| {{V_S}} \right|\left| {{V_r}} \right|}}{{\left| x \right|}}\cos \delta - \frac{{{V^2}}}{{\left| x \right|}}\)

\( = \frac{{\left( {1.0} \right)\left( {1.0} \right)}}{{0.5}}\cos 30 - \frac{{{{\left( {1.0} \right)}^2}}}{{0.5}}\)

= -0.268

Concept:

Under no-load conditions or light load conditions, medium and long transmission lines may operate at the leading power factor due to the capacitance effect.

So that receiving end voltage becomes greater than sending end voltage.

In this case, shunt reactors are needed to bring down receiving end voltage at light loads.

The leading power factor can be changed to a lagging power factor by using a shunt reactor. By using a shunt reactor, it will compensate for the effect of capacitance and changes the power factor.

Note:

• The shunt capacitor is used to improve the power factor.
•  A series reactor smoothens the wave shape.
• A Series capacitor reduces the net reactance in a line.
• The shunt inductor reduces the Ferranti effect by limiting overvoltages at the load side under lightly loaded conditions.

• A shunt reactor is an absorber of reactive power, thus increasing the energy efficiency of the system. And as it is an absorber of reactive power under the no-load condition it can be used for reducing the Ferranti effect.
• Whenever an inductive load is connected to the transmission line, the power factor lags because of the lagging load current. To compensate this, a shunt capacitor is connected which draws current leading to the source voltage. The power factor can be improved.
• Series capacitors are used to compensate the inductance of the transmission line. They will increase the transmission capacity and the stability of the line. These are also used to share the load between parallel lines.
• Series reactors are used as current limiting reactors to increase the impedance of a system. They are also used to limit the starting currents of synchronous electric motors and to compensate reactive power in order to improve the transmission capacity of power lines. So these are used to reduce the current ripples.

Static VAR:

A static VAR compensator is a parallel combination of a controlled reactor and fixed shunt capacitor shown in the figure below. 

(i) The thyristor switch assembly in the SVC controls the reactor. 

(ii) The firing angle of the thyristor controls the voltage across the inductor and thus the current flowing through the inductor. 

(iii) In this way, the reactive power drawn by the inductor can be controlled.

Static Var Compensator can generate reactive power as to enhance voltage during heavy load condition and also it can absorb reactive power during light load condition so as to maintain or control specific parameters of the electrical power system (typically bus voltage).

Explanation: