POWER QUALITY ENGINEERING QUESTION BANK

POWER QUALITY ENGINEERING QUESTION BANK

		UNIT#1- INTRODUCTION
2 MARK QUESTIONS
1.	Definition of Power Quality		(NOV/DEC 10)
	Power quality means supply of the power within the permitted variation of the voltage and frequency
	and without any deviation of sinusoidal waveform in balanced condition.
	Power quality is any deviation of the voltage or current waveform from its normal sinusoidal wave
	shape. These disturbances include, but are not limited to sag, overvoltage, interruption, swell and any
	other distortions to the sinusoidal waveform.
2.	Define voltage swell			(NOV/DEC 09)
	A swell is defined as an increase to between 1.1 pu and 1.8 pu in rms voltage or current at the power
frequency durations from 0.5 to 1 minute
3.	Define momentary interruption?			(NOV/DEC 09)
	Momentary interruption is said to occur when the RMS voltage decreases less than 0.1 per unit for time
	duration of 0.008333 second to 3 second.
4.	Define voltage sag			(APR/MAY 08)
	Sag (dip) a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for

durations of 0.5 cycles to 1 minute. Voltage sag is defined as a decrease in RMS voltage magnitude lasting from 0.5 to 30 cycles.

5. Define over voltage

Over voltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for duration longer than 1 min.

Over voltages is usually the result of load switching (e.g., switching off a large load or energizing a capacitor bank). The over voltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. Incorrect tap settings on transformers can also result in system over voltages.

6. Define under voltage

An under voltage is a decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min.

A load switching on or a capacitor bank switching off can cause an under voltage until voltage regulation equipment on the system can bring the voltage back to within tolerances. Overloaded circuits can result in under voltages also.

7. Define Sustained interruptions

When the supply voltage has been zero for a period of time in excess of 1 min, the long duration voltage variation is considered a Sustained interruptions.

Short interruptions (< 1min)

Disappearance of the supply voltage in all phases for a period of time in < 1min

Short interruptions:

IEEE = - ΔUn < 90% duration from 20 ms to 1 minute EN= - ΔUn < 99% duration from 20 ms to 3 minutes

Long interruptions (> 1min)

Disappearance of the supply voltage in all phases for a period of time in > 1min

Long interruptions:

IEEE = - ΔUn < 90% duration > 1 minute EN= - ΔUn < 99% duration > 3 minutes

8. Define 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.

POWER QUALITY ENGINEERING QUESTION BANK

9.Define Waveform Distortion

Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency

principally characterized by the spectral content of the deviation.

There are five primary types of waveform distortion:

1.DC offset

2.Harmonics

3.Inter harmonics

4.Notching

5.Noise

10.Define DC offset

The presence of a dc voltage or current in an ac power system is termed dc offset.

11. Define Harmonics.

Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency; usually 50 or 60 Hz).Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use a single quantity, the TOTAL HARMONIC DISTORTION (THD), as a measure of the effective value of harmonic distortion

12. Define Inter harmonics

Voltages or currents having frequency components that are not integer multiples of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called

inter harmonics.

13. Define Notching.

Notching is a periodic voltage disturbance caused by the normal operation of power electronic devices when current is commutated from one phase to another.

14. Why is power quality so important?

Power quality is an increasingly important issue for all businesses. Problems with powering and grounding can cause data and processing errors that affect production and service quality.

1.Lost production: Each time production is interrupted, your business loses the margin on the product that is not manufactured and sold.

2.Damaged product: Interruptions can damage a partially complete product, cause the items to be rerun or scrapped.

3.Maintenance: Reacting to a voltage disruption can involve restoring production, diagnosing and correcting the problem, clean up and repair, disposing of damaged products and, in some cases, environment costs.

4.Hidden costs: If the impact of voltage sag is a control error, a product defect may be discovered after customer delivery. The costs of losing repeat sales, product recalls and negative public relations can be significant and hard to quantify.

15.What causes power quality problems?

Most causes can be divided into two categories:

1.Internal causes: Approximately 80 percent of electrical disturbances originate within a business facility. Potential culprits may include large equipment start-up or shutdown, improper wiring and grounding, overloaded circuits or harmonics.

2.External causes: About 20 percent of power quality problems originate with the utility transmission and distribution system. The most common cause is a lightning strike; other possibilities include equipment failure, vehicle accidents, weather conditions, neighboring business, and even normal operation of utility equipment.

16.Define total harmonic distortion(THD).

Total harmonic distortion is the term used to describe the net deviation of a non linear waveform from ideal sine wave characteristics. THD is the ratio between the RMS value of the harmonics and the RMS value of the fundamental.

POWER QUALITY ENGINEERING QUESTION BANK

17. Define total demand distortion(TDD).

The total demand distortion is defined as the square root of the sum of the squares of the RMS value of the currents from 2nd to the highest harmonic divided by peak demand load current and is expressed a s a percent.

18.What are the various power quality issues?

1.Power frequency disturbance

2.Power factor

3.Power system transients

4.Grounding and bonding

5.Electro magnetic interference

6.Electro static discharge

7.Power system harmonics

19.How is power quality problems diagnosed?

Sophisticated monitoring and testing are usually required to isolate the specific type of disturbance and pinpoint the cause. A thorough power quality investigation might include:

§On-site monitoring

§Grounding studies

§Power factor correction

§Infrared scans

20.How is power quality problems solved?

The solutions depend on the nature of the problem, but may include the installation of devices like uninterruptible power supplies, transient voltage surge suppression or isolation transformers. Other potential fixes include facility rewiring or replacement of equipment.

21. Why is power quality such a widespread problem?

Today's electrical power is much more reliable that it was just ten years ago. While it may appear that you're experience more troubles, the issue is that modern high-tech equipment and control units are much more sensitive to power disturbances.

As a simple example, a momentary power "blink" won't affect an analog clock, but it will disrupt a digital clock.

22. Differentiate between sag and swell.

Voltage sag is an event in which the RMS voltage decreases between 0.1 and 0.9 pu at the power frequency. It lasts for duration of cycles to 1 min

Swell is an event in which the RMS voltage increases between 1.1 and 1.8 pu at the power frequency. It lasts for duration of 0.5 cycles to 1 min

23.What are the causes of sags?

1.Voltage sags are usually associated with system faults but can also be caused by the switching of heavy loads.

2.Voltage sags are caused by motor starting.

24.What are the causes of swells?

1.Swells are associated with system fault conditions but they are much less common than voltage sags. For example, faults on one line cause voltage rise on other phases.

2.Swells can be caused by switching off a large load and by switching on a large capacitor bank.

25.What are the causes of interruptions?

1.Voltage interruptions are caused by the operation of protective devices such as breakers and fuses.

2.Voltage interruptions longer than 1 minute are often permanent in nature and require manual intervention and restoration. Sometimes system maintenance can require voltage interruption in certain sections of power systems.

26.Define voltage quality?

Voltage quality is concerned with the deviations of the voltage from a normal sinusoidal waveform.  

POWER QUALITY ENGINEERING QUESTION BANK

27.Define power frequency variations?

Power frequency variations are a deviation from the nominal supply frequency. The supply frequency is a

function of the rotational speed of the generators used to produce the electrical energy.

28.List any four standards available in power quality.

IEEE power quality standards: Institute Of Electrical and Electronics Engineer. IEEE power quality standards: International Electro Technical Commission.

IEEE power quality standards: Semiconductor Equipment and Material International. IEEE power quality standards: The International Union for Electricity Applications.

29.Name any four IEC standards that define power quality.

1.	Definitions and methodology	61000-1-X
2.	Environment	61000-2-X
3.	Limits	61000-3-X
4.	Tests and measurements	61000-4-X

5.	Installation and mitigation	61000-5-X
6.	Generic immunity and emissions 61000-6-X
30. Define voltage fluctuation.		(NOV/DEC10)

Voltage fluctuation are rapid changes in voltage with the allowable limits of voltage magnitude of 0.95 to

1.05 of nominal voltage.
31. Define voltage imbalance.	(NOV/DEC10)

Voltage imbalance or Voltage unbalance is the deviation of each phase from the average voltage of all three phases.

		PART-B
1.	Explain The Long Duration Voltage Variation
2.	Explain The Short Duration Voltage Variation			(APR/MAY 08)
3.	Explain The Different Types Of Waveform Distortion
4.	Explain The Transients?			(APR/MAY 09, NOV/DEC 10.)

5.Explain the power quality issues?

6.Explain IEEE and IEC standards

7.Explain (i). Voltage imbalance, (ii) Voltage fluctuation (iii) power frequency variation