Harmonics in LV Installation & Solutions

Prepared by:
Er. Sadiq Hussain
Founder @ ELP, PMC, Electrical Designer, Trainer, IAAC, IAENG

References:

IEC 61000-2-2, 2-4

IEC 60831-1
IEEE 519
EN 50160
Eaton
Schneider electric

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 Topics to be covered
1. Introduction

2. Linear & Non-Linear load

3. Understanding the harmonics in the system

4. Concept of PCC

5. Origin of Harmonics

6. Harmonic Indicators

7. Effects of harmonics

8. Maximum permissible harmonic levels

9. Solutions

10. Q/A
 Introduction
Harmonics are the result of the always expanding number of power electronic
devices. In recent years, the use of power electronics are increased because of their
capabilities for precise process control and energy saving benefits.

Examples
Variable Speed Drives (VFD) in the Industry, and computers, computer hardware,
lighting electronic ballast in commercial and residential areas.

Note:
International standards have been published in order to help the designers of
equipment and installations. Harmonic emission limits have been set, so that no
unexpected and negative impact of harmonics should be encountered. In parallel
to a better understanding of effects, solutions have been developed by the Industry.
Harmonic consideration is now a full part of the design of electrical installations.

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 Introduction

Some of the effects the harmonic can cause, to the equipment, to the installation, or both,
are:
• Losses are increased in electrical installation and equipment
• Unexpected resonances
• Disturbances in electronic equipment/IT loads
• Unwanted overload (or need to oversize) for transformers, cables
• Malfunctions of motors and generators
• Unwanted Circuit Breakers tripping

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 Some of the main risks linked to harmonics in the power system
are:
a). Issues and risks related to the power system and equipment
b). Economic Impacts

a). Issues and risks related to the power system and equipment
• Losses are increased in electrical installation and equipment
• Unexpected resonances
• Disturbances in electronic equipment, communication lines, IT loads
• Overload (or need to oversize) for transformers, cables
• Overload, vibrations and pre-mature aging of motors and generators
• Overload and pre-mature aging of power capacitors
• Unwanted Circuit Breakers tripping

b). Economic impact:

• Premature ageing of equipment means less operation life.
• Overload on the distribution network means higher equipment rating and increased
power losses,

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 Linear Load
It is a load that draws instantaneously proportional current to the applied voltage, i.e., its
impedance is maintained constant along the whole alternating period.

For public electricity supply of 50 or 60 Hz sinusoidal voltage, this will mean a pure
sinusoidal current also.

Linear loads can be classified as

1. Resistive (electrical heaters, incandescence light bulbs)
2. Capacitive (capacitors usually found as part of systems or equipment)
3. Inductive (transformers, motors), or combinations of some of them.

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 Non-Linear Load
A non-linear load changes its impedance with instantaneous applied voltage, that will lead to
a non-sinusoidal current draw when the applied voltage it’s so.

This kind of load does not have a constant relation current vs. voltage along the alternating
period

Some examples : variable frequency drives (VFD), line-switched rectifiers, lighting

ballasts, devices drawing rectified current (TV, computers, printers, scanners, etc.)…

In other words, all the circuits contain semiconductor power devices such as diodes,
thyristors (SCR’s), transistors, and/or switching of loads or circuits.

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 Understand the harmonics in the system
Fourier theorem: all non-sinusoidal periodic functions can be represented as the sum of
terms (i.e. a series) made up of:
• A sinusoidal term at the fundamental frequency,
• Sinusoidal terms (harmonics) whose frequencies are whole multiples of the fundamental
frequency,
• A DC component, where applicable.

The harmonic of order h in a signal is the sinusoidal component with a frequency that is h
times the fundamental frequency.

Ex: Frequency of 5th harmonic = 5 * 50 Hz or 5*60 Hz

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 Understand the harmonics in the system

A periodic signal can be represented by

Note:
• For symmetrical waveforms, only “odd” harmonics may
appear (multiples 3rd, 5th, 7th, etc., of the
fundamental frequency)
• For asymmetrical waveforms, a part from “odd”, “even”
multiples of the fundamental may appear
(multiples 2nd, 4th, 6th, etc.).
Also DC components can appear in asymmetrical
waveforms, which are represented as 0Hz signals.

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 PCC (Point of Common Coupling)
As the current waveform to the load is distorted due to harmonics, the voltage waveform
will also be distorted at the load terminal. To understand the concept of voltage
distortion, know PCC:

The PCC for commercial

building may be a point where
utility line connected to the
end users’ main panel.

• As non-sine current drawn by the load, the

non-sine voltage will appear at the PCC due
to the drop at 𝐿𝑠

• Even non-sine waves = periodic waves

• Greater the current distortions, greater the

voltage harmonics. And the voltage
harmonics are depends on 𝐿𝑠

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 Origin of harmonics

Electrical equipment that consist power electronics circuits are typical non-linear loads
and generate harmonic currents.

The non-linear loads are increasingly frequent in all industrial, commercial and
residential installations and their percentage in overall electrical consumption is
growing steadily. (Important to understand the risks of harmonics and its solution)

Examples:
• Variable Speed Drives for AC or DC motors,
• UPS (in commercial and residential buildings),
• Office equipment (PCs, printers, servers, etc.),
• Household appliances (TV, microwave ovens, FL,
CFL, light dimmers)

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 Flow of harmonic currents in distribution system?

The non-linear loads push back harmonic currents into the distribution system,
towards the source. The harmonic currents generated by the different loads sum
up at the busbar level creating the harmonic distortion.

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 Harmonics Indicators
The following are the fundamental indicators used to evaluate the harmonic distortion
in current and voltage waveforms, namely:

• Power factor
• Crest factor
• Harmonic spectrum
• r.m.s. value

These indicators are very important in determining any necessary corrective action.

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 Power Factor

Power factor (𝑃𝐹1 ) without any harmonic:

𝑃𝐹1=Cos 𝜃

Power factor (𝑃𝐹2 ) with harmonic consideration:

𝑃𝐹1
𝑃𝐹2 =
1+ 𝑇𝐻𝐷2

Power factor 𝑃𝐹2 is used to determine the

rating for the different devices of the
installation.

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 Crest Factor
The crest factor is the ratio between the value of the peak current or voltage
and its r.m.s. value.

• For a sine wave, the crest factor is therefore equal to 2

• For a non-sine wave, the crest factor can be either greater than or less than 2

• The crest factor for the current drawn by non-linear loads is commonly much higher
than 2. It is generally between 1.5 and 2 and can even reach 5 in critical cases.

A high crest factor waveform high current peaks which, when detected by protection
devices, can cause nuisance tripping.

Example:
𝐼𝑟𝑚𝑠 = 0.16 A
𝐼𝑝𝑒𝑎𝑘 = 0.6A

𝐼𝑝𝑒𝑎𝑘
Crest Factor = 𝐼𝑟𝑚𝑠

Crest factor = 3.75

 Crest Factor
Crest factor is used to characterize the aptitude of a generator (or UPS) to supply
high instantaneous currents.
For example, computer equipment draws highly distorted current for which the crest factor
can reach 3 to 5.

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 Harmonic spectrum
The harmonic spectrum is the representation of the amplitude of each harmonic
order with respect to its frequency.

Each type of device causing harmonics draws a particular form of current, with
a particular harmonic content. This characteristic can be displayed by using the
harmonic spectrum.

Example:

𝐻 𝐼ℎ 2
𝑇𝐻𝐷𝑖 = σℎ=2
𝐼1

𝑇𝐻𝐷𝑖 = 0.152 + 0.122 + 0.092

𝑇𝐻𝐷𝑖 = 21.18%

Note: Harmonic spectrum provides a different representation of electrical signals and

can be used to evaluate their distortion.
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 Total Harmonic Distortion | THD
The Total Harmonic Distortion (THD) is an indicator of the distortion of a waveform.
It is widely used in Electrical Engineering and Harmonic management in particular.

THD is ratio of the r.m.s. value of all the harmonic components of the current/voltage
waveform, to the fundamental component.

For a current waveform “I”, the THD is defined as:

25 2
𝐼ℎ
෍
𝐼1
ℎ=2

Note:
- H is generally taken equal to 50, but can be limited in most cases to 25.
- THD can exceed 1 and is generally expressed as a percentage.

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 Total Harmonic Distortion | THD
Relation between 𝐼𝑟𝑚𝑠 and THD is
𝐼𝑟𝑚𝑠 = 𝐼1 1 + 𝑇𝐻𝐷2

If THD = 25%, then increase in the current value

𝐼𝑟𝑚𝑠 = 𝐼1 1 + (0.2)2

𝐼𝑟𝑚𝑠 = 𝐼1 ∗ 1.04

𝑰𝒓𝒎𝒔 = 𝒊𝒏𝒄. 𝒃𝒚 𝟎𝟒%

Where, 𝑰𝟏 = instant. Current at fundamental frequency

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 Total Harmonic Distortion | THD

For a current waveform “I”, the THD is defined as:

𝐻 2
𝐼ℎ
෍
𝐼1
ℎ=2

For a voltage waveform “V”, the THD is defined as:

𝐻 2
𝑉ℎ
෍
𝑉1
ℎ=2

Note:
- H is generally taken equal to 50, but can be limited in most cases to 25.
- THD can exceed 1 and is generally expressed as a percentage.

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 THD (Voltage) is an indicator of the distortion of the voltage wave. Below are given
indicative values of THD (Voltage) and the corresponding consequences in an
installation:
• ≤5%: normal situation, no risk of malfunctions,
• 5 to 8%: significant harmonic distortion, some malfunctions are possible,
• ≥8%: major harmonic distortion, malfunctions are probable. In-depth analysis
and the installation of mitigation devices are required.

THD (Current) is an indicator of the distortion of the current wave. The current
distortion can be different in the different parts of an installation. The origin of
possible disturbances can be detected by measuring the THD (Current) of different
circuits.

Below are given indicative values of THD (Current) and the corresponding phenomena
for a whole installation:
• ≤10%: normal situation, no risk of malfunctions,
• 10 to 50%: significant harmonic distortion with a risk of temperature rise and the
resulting need to oversize cables and sources,
• ≥50%: major harmonic distortion, malfunctions are probable. In-depth analysis
and the installation of mitigation devices are required.

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 Main effects of harmonics

1. Resonance

2. Inc. losses

3. Overloading

4. Economic impact

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 Resonance Effect
The simultaneous use of capacitive and inductive devices in
distribution system may result in parallel or series resonance.

Due to the resonance condition, Voltage distortion is maximum.

As the voltage distortion is max., the losses will increase in the

motors, and excessive heat. The net effect of these harmonics is
premature motor failure.

Solution: Use reactor in series with capacitor

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 Increase Losses
1. Losses Conductor:

𝐼𝑟𝑚𝑠 = 𝐼1 1 + 𝑇𝐻𝐷2

As THD inc. the current drawn by the load will also increase.

2. Losses in transformer

Harmonic currents flowing in transformers cause an increase in the “copper” losses

and increased “iron” losses due to eddy currents.

The harmonic voltages are responsible for “iron” losses due to hysteresis.

It is generally considered that losses in windings increase as the square of the THDi and that core
losses increase linearly with the THDv.

In Utility distribution transformers, where distortion levels are limited, losses increase between
10 and 15%.

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 Increase Losses

3. Losses in power capacitors:

The harmonic voltages applied to capacitors cause the flow of currents proportional
to the frequency of the harmonics. These currents cause additional losses in the
system.

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 Overloading

Generators
Generators supplying non-linear loads must be derated due to the additional losses
caused by harmonic currents.

The level of derating is approximately 10% for a generator where the overall load
is made up of 30% of non-linear loads. It is therefore necessary to oversize the
generator, in order to supply the same active power to loads.

Uninterruptible power systems (UPS)

The current drawn by computer systems has a very high crest factor. A UPS sized
taking into account exclusively the r.m.s. current may not be capable of supplying
the necessary peak current and may be overloaded.

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 Overloading

Transformers
The curve presented below shows the typical derating required for a transformer
supplying electronic loads

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 Overloading
Capacitors

The r.m.s. current flowing in the capacitors must not exceed 1.3 times
the rated current.

For example,
the fundamental voltage 𝑉1 ,
Harmonic voltages 𝑉5 = 8% (of 𝑉1 )
𝑉7 = 5%,
𝑉11 = 3%,
𝑉13 = 1%,

𝐼𝑟𝑚𝑠
THD (Voltage) equal to 10%, the result is = 1.19, at the rated voltage.
𝐼1

𝐼𝑟𝑚𝑠
For a voltage equal to 1.1 times the rated voltage, the current limit = 1.3 is reached
𝐼1
and it is necessary to resize the capacitors.

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 Overloading
Neutral Conductor

In case of linear load and balanced system, the current through the neutral conductor
will be very less. However, when the system is supplying to the linear load and non-
linear load simultaneously; the current in the neutral conductor will increase by
significant factor.

It is important to select the correct and adequate size of neutral conductor.

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 Economic Impact
1. Losses inc., efficiency dec.

2. Oversized equipment

3. Equipment life decreased

4. Additional devices are used to reduce the effect of harmonics

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 Maximum permissible harmonic levels

International studies have collected data resulting in an estimation of typical

harmonic contents often encountered in electrical distribution networks.

The levels shown in the table, in the opinion of many Utilities, should not be
exceeded.

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 Basic solutions
To limit the propagation of harmonics in the distribution network, different solutions
are available and should be taken into account particularly when designing a new
installation.

There are three types of filters:

• Passive
• Active
• Hybrid

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 Passive Filter
The passive filter is selected when:

1. Non-linear load ≥ 500 kVA (VFDs, UPSs, rectifiers, etc.)

2. Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
3. Installations where current distortion must be reduced to avoid overloads

An LC circuit, tuned to each harmonic order to be

filtered, is installed in parallel with the non-linear
load. This bypass circuit absorbs the harmonics,
thus avoiding their flow in the distribution
network.

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 Active Filter
The active filter is selected when:

1. Non-linear load ≤ 500 kVA (VFDs, UPSs, rectifiers, etc.)

2. Installations where current distortion must be reduced to avoid overloads

These systems, comprising electronic circuit and

installed in series or parallel with the non-linear load,
compensate the harmonic current or voltage drawn
by the load.
Fig. shows a parallel-connected active harmonic
conditioner (AHC) compensating the harmonic
current (Ihar = -Iact).
The AHC injects in opposite phase the harmonics
drawn by the non-linear load, such that the line
current Is remains sinusoidal.

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