Home | | Power Quality | Important short questions and answers: Harmonics

# Important short questions and answers: Harmonics

Power Quality - Harmonics - Important short questions and answers: Harmonics

Harmonics

1.     Define harmonics?

Harmonics is a sinusoidal components having a frequency that is an integral multiple of the fundamental frequency. Thus, a pure voltage or current sine wave has no distortion and no harmonics, and a non-sinusoidal wave has distortion and harmonics. To quantify the distortion, the term total harmonic distortion (THD) is used. The term expresses the distortion as a percentage of the fundamental (pure sine) of voltage and current waveforms.

2.  Give at least two IEC standards for EMC.

IEC 61000-2-2 (1993): Electromagnetic compatibility. Part 2: environment. Section 2: compatability levels for low frequency conducted disturbances and signaling in public low voltage power supply systems.

IEC 61000-3-2 (2000): Electromagnetic compatibility part3: limits section 2: limits for harmonic current.

3.  Define harmonic indices?

The power quality industry has developed certain index values that help us assess the quality of service as it relates to distortion caused by the presence of harmonics. These values or harmonic indices serve as a useful metric of system performance. The two most commonly indices are

1.     Total Demand Distortion

2. Total Harmonic Distortion

4. Mention the devices for controlling harmonic distortion?

1.     Series reactors

2.     Zigzag transformers

3.     Specially connected transformers

5.Give the IEC standard to define harmonics.

The International Electro technical Commission (IEC) has defined a category of electromagnetic compatibility standards that deal with power quality issues. The term electromagnetic compatibility includes considerations for both radiated and conducted interference with customer equipment.

6.What is crest factor?

The crest factor of any waveform is the ratio of the peak value to the RMS value. In a perfect sine wave, the crest factor is 1.414. Crest factors different than 1.414 indicate distortion in the waveform. Typically distorted current waveforms have crest factors higher than 1.414, and distorted voltage waveforms have crest factors lower than 1.414. Distorted voltage waveforms with crest factors lower than 1.414 are called “flat top” voltage waveforms.

7.What kind of equipment is needed to measure distorted waveforms?

A digital oscilloscope is needed to measure the wave shape, THD and amplitude of each harmonic. However, if we simply want to measure the RMS value of the waveform, a “True- RMS” multi meter will suffice. The term “True-RMS” is used because not all instruments give correct readings when measuring distorted waveforms.

8. Define TDD

Total Demand Distortion (TDD) The ratio of the root mean square of the harmonic current to the rms value of the rated or maximum demand fundamental Current, expressed as a percent.

9 .Define THD

Total Harmonic Distortion (THD) The ratio of the root mean square of the harmonic content to the rms value of the fundamental quantity, expressed as a percent of the fundamental.

11. What is the reason for existence of harmonic distortion?

Harmonic distortion exists due to the harmonic characteristics of the devices and loads on the power system. These devices act as current sources that inject harmonic currents into the power system.

12.            What is voltage and current distortion?

1.  Voltage distortion is any deviation from the nominal sine waveform of the ac Line voltage

2.  Current distortion is any deviation from the nominal sine waveform of the ac line current

13.            Mention the applications of cyclo converter.

1.     Steel rolling mill end tables

2.     Cement mill furnaces

3.     Mine hoists

4.     Ship propulsion drives

14.            List the some dynamic correction of power quality events.

a.     Resonance preventive

b.     Power factor correction

c.      Dynamic VAR compensation

15.            What is current source inverter?

The current source inverter receives DC power from an adjustable current source and adjusts the frequency and current.

16. 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 is called inter harmonics.

17. Give at least two IEEE standards for harmonics.

IEEE Std 519-1992, IEEE Recommended practices and requirements for Harmonic control in Electric power systems.

IEEE Std 1159-1995, IEEE Recommended practices for monitoring electrical power quality.

18. What is the classification of active harmonic conditioner?

Active harmonic conditioners can be broadly classified into three categories

a.                 Series conditioners

b.                 Parallel conditioners

c.                  Hybrid conditioners

19.            Mention the harmonic sources from industrial loads

Three phase converter with adjustable speed drives(DC drives and AC drives)

Arcing devices (Arc furnaces, welders, discharge lamps etc)

Saturable devices (transformer, electromagnetic devices with steel core)

20. State the principles of controlling harmonics.

There are many number of devices available to control harmonic distortion. They can be as simple as a capacitor bank or a line reactor or as complex as an active filter.

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail
Power Quality : Harmonics : Important short questions and answers: Harmonics |