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Btech Aktu Power Quality and FACTS KEE-074 Short Question, Quantum Book Pdf

Explore the B.Tech AKTU Quantum Book Short Question Notes on Power Quality and FACTS. Learn about power conditioning concepts, voltage control, and how to improve power system stability.

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Unit-I: Introduction Power Quality and FACTS (Short Question)

Q1. Explain the term ‘good power quality.’ 

Ans. A stable supply voltage that stays within the prescribed range, a steady AC frequency near to the rated value, and a smooth voltage curve shape (similar to a sine wave) are all indicators of good power quality. 

Q2. Define the term ‘voltage sag’ and ‘voltage swell’.


Define the term voltage swell.


What is voltage sag ?


Difference between voltage sag and voltage swell. 


What is voltage swell ?  

Ans. A. Voltage sag : It is described as a 0.1 to 0.9 pu reduction in rms voltage or current at the power frequency over durations ranging from 0.5 cycle to 1 minute. 

B. Voltage swell : Voltage swell is described as an increase in rms voltage or current of 1.1 to 1.8 pu at the power frequency over durations ranging from 0.5 cycle to 1 minute. 

Q3. How are electrical transients generated ?  


What are the sources of electrical transients in power system.

Ans. The power system transients are generated due to: 

  • 1. Lightning surges
  • 2. Switching surges 
  • 3. Faults. 

Q4. Define the terms harmonics and ‘sub-harmonics’.  


What do you mean by harmonics ? 

Ans. A. Harmonics: A sinusoidal periodic wave component with a frequency that is an integer multiple of the fundamental frequency. 

B. Sub-harmonics: A specific subgroup of interharmonics with frequency values smaller than the fundamental frequency. 

Q5. Define the terms notching and harmonics

Ans. A. Harmonics: A sinusoidal periodic wave component with a frequency that is an integer multiple of the fundamental frequency. 

B. Notching : 

  • 1. Notching is a periodic voltage disturbance induced by power electronic device regular operation when current is commutated from one phase to another. 
  • 2. Because notching happens continually, the harmonic spectrum of the impacted voltage can be used to characterise it. 

Q6. Explain the different causes and effects of the power frequency variations. 


What are the causes of power frequency variations ? 

Ans. A. Causes :  

  • 1. Loss of generation
  • 2. Extreme loading conditions. 

B. Effects :

  • 1. Motors run slower  
  • 2. De-tuning of harmonic filters. 

Q7. Define Interruption. What are its causes ?

Ans. A. Interruption: An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min.

B. Causes :  

  • 1. Power system faults 
  • 2. Equipment failures.  

Q8. Explain the causes and effects of voltage fluctuations in power system.  

Ans. A. Causes : 

  • 1. AC motor drives 
  • 2. Inter-harmonic current components 
  • 3. Welding and arc furnaces. 

B. Effects : Flicker in fluorescent and incandescent lamps. 

Q9. Explain the term ‘phase angle imbalance’.

Ans. The angular gap between three-phase voltages is normally an integral multiple of 120 (120 or 240 degrees). Phase angle imbalance refers to the effects of unequal loading on displacement. 

Q10. What is the meaning of the term power quality. 

Ans. The ability of an electrical network or grid to provide “clean” and reliable power is measured as power quality. The stable voltage supply, where the frequency of the supply remains close to the rated value, is a hallmark of good electric power quality. 

Q11. What could be the impact of poor power quality on systems efficiency, reliability and operation ?

Ans. 1. Poor power quality can have a variety of detrimental effects for huge industrial complexes, data centres, sensitive medical apparatus, small businesses, and even households. 

2. It can slow down production in enterprises, damage sensitive equipment, and impair efficiency and increase expenses. 

3. Poor power quality leads to increased energy usage, which is bad for both homeowners and businesses. 

Q12. Explain how power quality issue voltage regulation’ affected by distributed generation. 

Ans. Distributed generation aids in the reduction of voltage sags on the local facility bus, hence improving voltage regulation. 

Q13. Define DC offset.  

Ans. The presence of a DC voltage or current in an AC power system is termed DC offset.

Q14. What is long duration voltage variation ? 

Ans. 1. Long duration fluctuations include RMS variances at power frequencies for more than 1 minute. 

2. Long duration variation can be either overvoltages or under-voltages.

Q15. Define Notches.   

Ans. A notch is a switching (or other) disruption of the usual power voltage waveform that lasts less than a half-cycle and is initially of opposing polarity. 

Q16. What is overvoltage and its cause ?

Ans. A. Overvoltage: An overvoltage is defined as a rise in the rms AC voltage of more than 110 percent at the power frequency for more than 1 minute. 

B. Cause of overvoltage : 

  • 1. Overvoltages are usually the result of load switching. 
  • 2. Overvoltages occur because the system is either too weak for the intended voltage regulation or the voltage controls are insufficient. 
  • 3. Incorrect transformer tap settings might potentially cause system overvoltages. 

Q17. What is undervoltage and its cause ? 

Ans. A. Undervoltage: An undervoltage is defined as a reduction in the rms AC voltage to less than 90% at the power frequency for more than 1 minute. 

B. Cause of undervoltage: A load turning on or a capacitor bank turning off can cause an undervoltage until the system’s voltage control equipment can bring the voltage back within limits. 

Q18. What is noise ?  

Ans. Unwanted electrical signals having a broadband spectral content of less than 200 kHz superimposed on power system voltage or current in phase conductors, or found on neutral conductors or signal lines, are referred to as noise. 

Q19. What are interharmonics ?  

Ans. Interharmonics are voltages or currents with frequency components that are not integer multiples of the frequency at which the supply system is meant to operate (e.g., 50 or 60 Hz).  

Q20. What do you mean by power quality monitoring ? 

Ans. It is the process of converting raw measurement data into meaningful information by obtaining, analysing, and interpreting it. 

Unit-II: Voltage Sag (Short Question)

Q1. Explain the sources of voltage sag. 

Ans. 1. Motor starting. 

2. Arc furnace. 

3. Fault.  

Q2. Give the block diagram of hybrid UPS. 


Give the block diagram of hybrid UPS. Power Quality and FACTS

Q3. Describe the block diagram of online UPS. 

Ans. The Fig. shows a typical configuration of an ON-line UPS.  

Describe the block diagram of online UPS. 
  • 1. In this design, the load is always fed through the UPS. 
  • 2. The incoming AC power is rectified into DC power, which charges a bank of batteries. This DC power is then inverted back into AC power, to feed the load. 
  • 3. If the incoming AC power fails, the inverter is fed from the batteries and continues to supply the load. 

Q4. Give the block UPS diagram of offline or standby UPS. 


Give the block UPS diagram of offline or standby UPS. 

Q5. What is rotary UPS ? 


Write a short note on rotary UPS. 

Ans. The term “rotary UPS” refers to the spinning components (such as a motor-generator) within the UPS that are utilised to deliver power to the load. 

Q6. What is active series compensator? 

Ans. These are new technologies that use power electronic components to enhance voltage by injecting a voltage in series with the residual voltage during a voltage sag scenario. 

Q7. What is voltage regulator ?

Ans. 1. A voltage regulator produces a fixed output voltage that remains constant regardless of input voltage or load conditions. 

2. It acts as a buffer for protecting components from damages. 

Q8. Give the application isolation transformer. 

Ans. 1. Isolation transformers are used for isolation in pulse circuits. 

2. Isolation transformers are used to provide electrical isolation in medical equipment. 

3. Isolation transformers are used for power supply of devices that are not at ground potential.

Q9. Describe the motor starting sag. 

Ans. Motors have the unfavourable characteristic of drawing several times their full load current while starting. By flowing across system impedances, this enormous current generates a voltage sag, which may dim lights, cause contactors to drop-out, and damage sensitive equipment. 

Q10. How to calculate sag severity during full voltage starting ?  

Ans. Sag voltage, in per unit of nominal system voltage, is 

How to calculate sag severity during full voltage starting ?  

Where, V(pu) = Actual system voltage, in per unit of nominal. 

kVALR = Motor locked rotor kVA. 

kVASC = System short-circuit kVA at motor. 

Unit-III: Electrical Transients (Short Question)

Q1. What are the sources of transient overvoltage ?


  • 1. Atmospheric phenomena (lightning, geomagnetic disturbances) 
  • 2. Switching loads ON or OFF 
  • 3. Interruption of fault currents 
  • 4. Switching of power lines 
  • 5. Switching of capacitor bank.  

Q2. Define crowbar devices. 

Ans. Gas tubes and semiconductor-based active crowbar protection circuits are examples of crowbar devices. When there is an issue with power flow, these devices and circuits can shunt a significant quantity of transitory energy. 

Q3. Explain clamping devices. 

Ans. Clamping devices apply a voltage clamp to safeguard delicate electronic equipment from transitory disturbances. 

Q4. What is neutral voltage swings? 

Ans. 1. Normally, neutral voltage should be within 0.5 V with respect to the ground. 

2. But the voltage variation at neutral more than specified value is known as neutral voltage swing. 

Q5. What is low pass filter ?

Ans. Low-pass filters are made up of parallel capacitors and series inductors. For certain resonant frequencies, this combination gives a low impedance connection to ground. 

What is low pass filter ?

Q6. Give the use of the Low Impedance Power Conditioner (LIPC). 

Ans. LIPCs are used primarily to interface with the switching mode power supplies found in electronic equipment. 

Q7. Name the various strategies for utilities used to decrease the impact of lighting. 


  • 1. Shielding 
  • 2. Line arrester 
  • 3. Low side surges
  • 4. Cable protection.  

Q8. Define any two types of devices used for over voltage protection. 

Ans. i. Isolation transformer: High frequency sounds and transients are attenuated as they attempt to travel from one side to the other through an isolation transformer.

ii. Low-pass filters: Low-pass filters are made up of parallel capacitors and series inductors. For certain resonant frequencies, this combination gives a low impedance connection to ground. 

Q9. What are the advantages of capacitor bank in power system ? 

Ans. 1. It can work in normal conditions (i.e. ordinary atmospheric conditions). 

2. Do not require a foundation for installation. 

3. They are light weight so it is can be easy to installed. 

Q10. Give the disadvantage of capacitor bank in power system. 

Ans. 1. Once the capacitors spoiled, then repairing is costly. 

2. The age of static capacitor bank is less (8-10 years). 

3. With changing load, we have to ON or OFF the capacitor bank, which causes switching surges on the system. 

Q11. Explain the phenomena of lightning in transient overvoltage. 

Ans. 1. Lightning is a powerful source of transients. 

2. Some of the potential locations for lightning strikes, resulting in lightning currents being transmitted from the power system into loads. 

3. The most visible conduction path happens during a direct strike to a phase wire on the transformer’s primary or secondary side. This can result in extremely large overvoltages. 

Unit-IV: FACTS Systems (Short Question)

Q1. Explain FACTS and give its objectives,. 

Ans. A. FACTS: It can accept changes in the electric transmission system or operational conditions while still providing adequate steady and transient analysis.  

B. Objectives: 

  • i. Power flow control
  • ii. Effective load sharing 
  • iii. Voltage regulation. 
  • iv. Enhancement of transient stability. 

Q2. What does SVC stand for ? Define SVC and give its applications in power system. 

Ans. 1. SVC stands for Static VAr compensator. 

2. SVC is defined by the IEEE as “A shunt connected static VAr generator or absorber whose output is adjusted to exchange capacitive or inductive current in order to maintain or control specific parameters of the electric power system.” 

Applications :

  • 1. These compensators provide the required reactive power and damp out sub-harmonic oscillations in (HVDC) application. 
  • 2. They also reduce unbalanced voltage and current. 
  • 3. They regulate voltage and improve stability. 

Q3. Explain STATCOM. 

Ans. 1. STATCOM stands for Static Synchronous Compensator.

2. According to IEEE definition STATCOM is: “A static synchronous generator operated as a shunt connected static VAr compensator whose capacitive or inductive output current can be controlled independent of the AC system voltage.” 

Q4. Define TCSC. 

Ans. 1. TCSC stands for Thyristor-Controlled Series Capacitor. 

2. IEEE defines TCSC as: “A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor controlled reactor in order to provide a smoothly variable series capacitive reactance”. 

Q5. What makes the difference between SSSC and STATCOM ? 

Ans. A series linked controller is SSSC. It is nearly identical to STATCOM, with the exception that its output voltage is in series with the line. 

Q6. What do you mean by Thyristor Switched Capacitor? Explain with diagram. 

Ans. Thyristor-Switched Capacitor (TSC) is a shunt-connected capacitor. The thyristor valve’s full or zero-conduction action changes the effective reactance gradually. 

What do you mean by Thyristor Switched Capacitor? Explain with diagram. 

Q7. Give the advantage of FACT controller.

Ans. Mechanical circuit breakers, such as relays and contactors, are not very reliable and hence cannot correct power loss owing to reactive power of transmission networks, which can be performed by FACTS Controllers. 

Q8. Discuss the disadvantage of FACTs controller. 

Ans. FACTS is a very expensive mix of typical power system components (such as transformers, reactors, switches, and capacitors). 

Q9. Give the block diagram of series controller. 


Give the block diagram of series controller. 

Q10. Give the name of various FACTS controller.


  • i. Series 
  • ii. Shunt 
  • iii. Combined series  
  • iv. Combined series-shunt controller. 

Q11. Explain the advantage of SVC. 

Ans. 1. SVCs control voltage directly. 

2. SVCs control temporary overvoltages rapidly.  

Q12. Give the disadvantage of SVCs. 

Ans. 1. SVCs have limited overload capability. 

2. SVCs are expensive. 

Q13. Give the advantage of STATCOM over SVC. 

Ans. 1. STATCOM is implemented to help power networks with inadequate power factor and voltage regulation. 

2. STATCOM’s response time is faster than that of an SVC. This is mostly owing to the quick switching times given by the IGBTS of the voltage source converter. 

3. Because the reactive power of a STATCOM drops linearly with the AC voltage, it provides superior reactive power support at low AC voltage than SVC. 

Q14. What are the limitation of STATCOM?  

Ans. 1. Since STATCOM is created from DC capacitor, hence it has very little active power capability. 

2. STATCOM exhibits higher loss and considered to be more expensive.

Q15. What are the advantage and disadvantage of TCSC in power system network ? 

Ans. A. Advantages of TCSC: 

  • 1. Rapid and continuous control of transmission line reactance. 
  • 2. Dynamic control of power flow in selected network transmission lines to enable power flow circumstances. 

B. Disadvantages of TCSC: TCSC cannot reverse the power flow in a line, unlike HVDC controller and phase swifter. 

Q16. What are the basic parameters controlled by FACTs controller in power system network ?  


  • 1. FACTS devices can be used to improve system voltage profiles by coordinating FACTS controller operation in a multimachine power system. 
  • 2. FACTS controllers can be used to improve the efficiency and quality of power system operations. 
  • 3. It can be used to improve system security and system reliability. 
  • 4. Loop flow control can be improved to improve system stability and capacity. 

Q17. What is custom power ?  

Ans. 1. The usage of power electronics controllers for distribution systems is referred to as bespoke power. 

2. It improves the quality and dependability of power given to clients. 

Unit-V: Harmonics (Short Question)

Q1. What is harmonics ?  

Ans. Harmonics are sinusoidal voltages or currents with frequencies that are integer multiples of the supply system’s operating frequency. 

Q2. Define best possible method of harmonic elimination. 

Ans. 1. Isolation transformer 

2. Reactors 

3. Fitters. 

Q3. Explain harmonic analyzers. 

Ans. A harmonic analyzer is a device that measures the frequencies, amplitudes, phases, and other components of a non-sinusoidal waveform. Harmonics in electrical systems are checked using a harmonic analyzer. 

Q4. Define disturbance analyzers. 

Ans. 1. Disturbance analyzers monitor a subset of equipment designed exclusively for power quality measures. 

2. They are often capable of measuring a wide range of system disturbances, ranging from very short duration transient voltages to long-term outages or under voltages. 

Q5. What is power factor?  

Ans. 1. The power factor is defined as the ratio of the active power (P) and voltamperes (S).  

What is power factor?  

2. For sinusoidal waveforms the power factor is the cosine of the phase angle ɸ between voltage and current. 

Power factor = cos ɸ

Q6. Discuss the causes of poor load power factor.

Ans. 1. The majority of induction motors have a trailing power factor. The power factor of these motors decreases as the load decreases. 

2. Low power factor is used by arc lamps, electric discharge lamps, and some other electrical devices. 

3. Arc and induction furnaces have extremely low lag power factors. 

Q7. What are the causes of voltage and current harmonics ? 

Ans. A. Cause of current harmonics: Current harmonics are caused by non-linear loads. 

B. Cause of voltage harmonics: Voltage harmonics are mostly caused by current harmonics.

Q8. What do you mean by non-linear load ? 

Ans. 1. The impedance of non-linear loads is not constant, and the current is not proportional to or equal to the voltage. 

2. The current drawn by non-linear loads is periodic rather than sinusoidal, which means that the current wave looks the same from cycle to cycle. 

3. Because non-linear loads have low impedance, they can consume up to three times the peak current of a resistive load.   

Q9. Discuss the impact of Harmonic distortion on motor. 

Ans. 1. Electric motors suffer losses due to hysteresis and eddy currents that form in the motor’s iron core. These are proportional to the current’s frequency. 

2. Because harmonics have greater frequencies, they result in increased core losses in a motor. As a result, the motor core heats up more. 

Q10. Give the name of harmonic measurement technique. 

Ans. i. THD (Total Harmonic Distribution)

ii. TDD (Total Demand Distortion) 

Q11. Discuss the effect of harmonic on transformer. 

Ans. 1. The RMS current of the transformer is greater than its capacity due to harmonics. 

2. Harmonics in the transformer increase eddy current and core losses.

Q12. Explain complication introduced by power electronic switches. 

Ans. The power electronics switches are non-linear, resulting in significant odd ordered harmonics such as 3rd, 5th, 7th, and so on, which eventually pollute the power supply. When the power supply becomes polluted, it offers and impacts the end user, resulting in significant power loss. 

Q13. Give the name of harmonics current mitigation technique. 


  • 1. In line reactor or chokes Harmonics 
  • 2. Zig-Zag transformer 
  • 3. Passive filters 
  • 4. Active filter

Q14. What do you understand by linear load ? 

Ans. 1. The impedance of linear loads is constant, and the current is proportional to or equal to the voltage. 

2. In a power system, a linear element is one in which the current is proportional to the voltage. 

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