Download - Epq Notes Module 2

8/10/2019 Epq Notes Module 2

1/9

MODULE 2: HARMONICS

In a modern power system, due to broader applications of nonlinear loads such as power

electronic equipment or arc furnaces, the degree of waveform distorted is increasingly serious

now. These loads may cause poor power factors, lead to voltage notch, or result in a high degree

of harmonics. Such cases have brought the power quality as an increasing concern Therefore;

efficient solutions for solving these pollution problems have become highly critical for both

utilities and customers.

HARMONICS

When a nonlinear load is supplied from a supply voltage of 60-Hz or 50-Hz frequency, it

draws currents at more than one frequency, resulting in a distorted current waveform. Fourier

analysis of this distorted current waveform resolves it into its fundamental component and

different harmonics.

Harmonics are sinusoidal voltages or currents having frequencies that are integer

multiples of the fundamental frequency (usually 60 Hz or 50 Hz in power systems).

Harmonic distortion is a growing concern for many customers and the utilities because of

increasing application of power electronics equipment.

Where do the harmonics come from:

When a non-linear load draws current, that current passes through all of the impedance

that is between the load and the system source. As a result of the current flow, harmonic voltages

are produced by impedance in the system.

These voltages sum and when added to the nominal voltage produce voltage distortion. The

magnitude of the voltage distortion depends on the source impedance and the harmonic

voltages produced.


2/9

If the source impedance is low then the voltage distortion will be low. If a significant portion

of the load becomes non-linear (harmonic currents increase) and/or (system impedance

increases), the voltage can increase dramatically.

Usually, the higher-order harmonics (above the range of the 25th to 50th, depending on the

system) are negligible for power system analysis. While they may cause interference with low-

power electronic devices, they are usually not damaging to the power system. It is also difficult

to collect sufficiently accurate data to model power systems at these frequencies. A common

exception to this occurs when there are system resonances in the range of frequencies. These

resonances can be excited by notching or switching transients in electronic power converters.

Causes for Harmonics

Different categories of harmonic-producing loads are supplied by the electric utilities, such as

1. Domestic loads such as fluorescent lamps, light dimmers, etc.

2. Ripple control systems for regulating hot-water loads

3. Medium-sized industrial loads such as several adjustable speed drives in

a cement mill, paper mill, etc.

4. Large loads such as high-voltage direct current (HVDC) converters, aluminum smelters, static

var compensators (SVCs), heavy single-phase ac traction loads for hauling coal trains, etc.

Common Sour ces of H armonics

Fluorescent lighting

Computer switch mode power supplies

Static VAR compensators

Variable frequency motor drives (VFD)

DC-DC converters

Inverters

Television power supplies

AC or DC motor drives

Single-phase non-linear loads are prevalent in modern office buildings, while three-

phase, non-linear loads are widespread in factories and Industrial plants.


3/9

Ef fects of Harmonics

The undesirable effects of the harmonics produced by the aforementioned loads are

1. Capacitors may draw excessive current and prematurely fail from increased dielectric loss and

heating.

2. Harmonics can interfere with telecommunication systems, especially noise on telephone lines.

3. Transformers, motors, and switchgear may experience increased losses.

4. Induction motors may refuse to start (cogging) or may run at subsynchronous speeds.

5. Circuit breakers may fail to interrupt currents due to improper operation of blowout coils.

6. The timecurrent characteristics of fuses can be altered, and protective relays may experience

erratic behavior. In particular, maloperation of the relays associated with ripple control systems

can occur.


4/9

7. Errors happen in induction kilowatt-hour meters.

8. Excitation problems cause generator failure.

9. Interference occurs with large motor controllers.

10. Overvoltages and excessive currents in the system happen due to resonances of

harmonics in the network and consequent dielectric instability of insulated cables.

HARMONI C CURRENT

The rectifier, because it draws nonsinusoidal current from its source, along with notching

and ringing effects, introduces distortion to the voltage wave from the source. This is called

harmonic distortion. According to theories of waveform analysis, cyclical waveform is made up

of components consisting of fundamental sine wave plus other sine waves, called harmonics,

which are multiples of the fundamental frequency. Figure 3 shows separation of a distorted

waveform into its component parts. The nonlinear source, therefore, does not see distorted

current waveform as a single waveform, but as multiple, fundamental plus harmonic, waves.

Harmonics may have adverse effects upon the power source or other loads connected to the same

source. It is important to note loads drawing harmonic currents cause voltage distortion at the

source, the source does not produce the harmonic distortion.

I nterharmonics

Interharmonics are defined as voltages or currents having frequency components

that are not integer multiples of the frequency at which the supply system is designed to operate.

The causes include induction motors, static frequency converters and arcing devices. The effects

of interharmonics are not well known.

Tr iplen harmonics

As previously mentioned, triplen harmonics are the odd multiples of the third harmonic (h = 3, 9,

15, 21,). They deserve special consideration because the system response is often considerably


5/9

different for triplens than for the rest of the harmonics. Triplens become an important issue for

grounded-wye systems with current flowing on the neutral.

Two typical problems are overloading the neutral and telephone interference. One also hears

occasionally of devices that misoperate because the line-to-neutral voltage is badly distorted by

the triplen harmonic voltage drop in the neutral conductor.

For the system with perfectly balanced single-phase loads illustrated in Fig below an assumption

is made that fundamental and third-harmonic components are present. Summing the currents at

node N, the fundamental current components in the neutral are found to be zero, but the third-

harmonic components are 3 times those of the phase currents because they naturally coincide in

phase and time.

Transformer winding connections have a significant impact on the flow of triplen harmonic

currents from single-phase nonlinear loads.

When the currents are balanced, the triplen harmonic currents behave exactly as zero-sequence

currents, which is precisely what they are. This type of transformer connection is the most

common employed in utility distribution substations with the delta winding connected to the

transmission feed. Using grounded-wye windings on both sides of the transformer (bottom)

allows balanced triplens to flow from the low-voltage system to the high-voltage system

unimpeded. They will be present in equal proportion on both sides. Many loads in the United

States are served in this fashion.

.


6/9

Some important definitions:

Harmonic: A sinusoidal waveform with a frequency that is an integral multiple of the

fundamental 50/60 Hz frequency.

50/60 Hz fundamental

100/120 Hz 2nd harmonic

150/180 Hz 3rd harmonic

200/240 Hz 4th harmonic, etc

Triplen Harmonics: Odd multiple of the 3rd harmonic (3rd, 9th, 15th, 21st, etc.)

Harmonic Distortion: Non-linear distortion of a system characterized by the appearance in the

output of harmonic currents (voltages) when the input is sinusoidal.

Voltage Harmonic Distortion (VHD): Voltage harmonic distortion is distortion caused by

harmonic currents flowing through the system impedance. The utility power system has

relatively low system impedance, and the VHD is very low. VHD on the distribution power

system can be significant due to its relatively high system impedance.

Harmonic I ndices

The two most commonly used indices for measuring the harmonic content of a waveform are the

total harmonic distortionand the total demand distortion. Both are measures of the effective

value of a waveform and may be applied to either voltage or current.

Total harmonic distortion

The THD is a measure of the effective value of the harmonic components of a

distorted waveform. That is, it is the potential heating value of the harmonics

relative to the fundamental. This index can be calculated for either voltage orcurrent

whereMh is the rms value of harmonic component h of the quantity M. The rmsvalue of a distorted waveform is the square root of the sum of the squares as shown

in Eqs.The THD is related to the rms value of the waveform as follows


7/9

The THD index is most often used to describe voltage harmonic distortion.Harmonic voltages are almost always referenced to the fundamental value of the

waveform at the time of the sample. Because fundamental voltage varies by only afew percent, the voltage THD is nearly always a meaningful number.

Variations in the THD over a period of time often follow a distinct pattern

representing nonlinear load activities in the system.

Total demand distortion

Current distortion levels can be characterized by a THD value, as has beendescribed, but this can often be misleading. A small current may have a high THD

but not be a significant threat to the system. For example, many adjustable-speed

drives will exhibit high THD values for the input current when they are operatingat very light loads. Some analysts have attempted to avoid this difficulty by

referring THD to the fundamental of the peak demand load current rather thanthe fundamental of the present sample. This is called total demand distortion

IL is the peak, or maximum, demand load current at the fundamental frequencycomponent measured at the point of common coupling (PCC). There are two ways

to measure IL. With a load already in the system, it can be calculated as theaverage of the maximum demand current for the preceding 12 months. The

calculation can simply be done by averaging the 12-month peak demand readings.For a new facility,IL has to be estimated based on the predicted load profiles.

Total harmonic distortion and Power Factor

The power factor PF for any non-sinusoidal quantities is defined by:


8/9

1111 cos

cos

S

S

SS

SS

I

I

IV

IVPF

1S

I is the rms value of the fundamental 60Hz component of the current. The displacement power

factor (DPF, which is the same as the power factor in linear circuits with pure sinusoidal voltage

and current) is defined as the cosine of the angle 1 (angle between the fundamental-frequency

(60Hz) current and voltage waveforms) which could be written as: 1cosDPF , therefore, the

power factor PF with a nonsinusoidal current is:

DPFI

IPF

S

S1

In terms of total harmonic current distortion iTHD)( , the PF and SI (the rms value of the total

current) could be written as:

DPF

THD

PF

i

21

1

Where 2

1 1 iSS THDII

From an examination of (4.1) and (4.2), we can conclude that the power factor value decreases

with any high current harmonic content or distortioni

THD)( . These definitions assume that the

source voltage is near sinusoidal of fundamental frequency (maximum allowableV

THD)( =5%).

Harmonics eff ect on induction motors:

Harmonics in the power system are generated due to the usage of non linear load. Some of the

effects of harmonics on the performance of induction machine are:

Core losses in the Induction machine increases

Torque of the Induction motor reduces

Increase in the Skin effect

(4.1)

(4.2)
http://electricalquestionsguide.blogspot.com/2011/06/harmoncs-distortion-effect-in-power.htmlhttp://electricalquestionsguide.blogspot.com/2011/06/harmoncs-distortion-effect-in-power.html


9/9

Damage of the induction motor insulation

Electromagnetic Interference

Deviation in the induction motor Torque-Speed curves

Increase in Core Loss:

Core loss in any machine machine constitutes both hysteresis loss and eddy current loss.

Hysteresis loss of the induction machine is proportional to the applied frequency and eddy

current loss is proportional to the frequency squared. Frequency of the harmonics will be in the

order of the multiples of the fundamental frequency. Therefore core loss is major concern at

higher frequencies due to harmonics. Also harmonic currents and voltages reduces the overall

efficiency of the machine

Increase in Skin effect:

Increase in the harmonics frequency results in increase in skin effect tending the current to flow

on the surface of the conductor. Skin effect contributes the loss in the form of resistance to the

flow of current

Electromagnetic Interference:

Harmonics increase the Electromagnetic Interference(EMI). This EMI does not affect the motor,

however it affects the operation of nearby electronics, control and communication circuits

Reduced Motor Torque:

Fundamental frequency of the induction motor produces forward operating torque. However it is

observed that harmonics frequency will generate torque in both forward and in reverse direction.

For example 5th7thand 11thharmonics produce torques in reverse direction and on the other hand

7th, 13thand 19thharmonics components produces torque in forward direction.5thharmonic

components will have higher magnitude compared to 7 th harmonics. Hence harmonics torque

will reduce the operating torque of the machine

Deviation from Torque-Speed Characteristics:

Losses which occur due to Harmonics will cause the deviation in the torque speed characteristics

of the motor from original and affects the performance of the motor