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.

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