All about Harmonics in Electrical Power Systems: Origin of Harmonics Problems created by Harmonics Harmonic Mitigation Strategies

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2. The Basics of Harmonic Resonance
kVAr
kVAScc
Nres
)(

Vn
Harmonic
current
source (In)
Magnified In
kW
Loading
System
Impedance
PFC
Capacitors

3. Effects of Harmonic Resonance
Linear load = 0 kW
Linear load = 300 kW

4. Power System Impedance
• At low frequencies Power System Impedance is
determined by the impedance of the Transformer and
the Transmission lines. At Higher frequencies it is
determined by the impedance of the Power Factor
correction Capacitors.
• There is an intermediate range of frequencies where
the Capacitive and Inductive effects combine together
to give a very high impedance. A small Harmonic
Current within this Frequency range can give a very
high and undesirable Harmonic Voltage.
• This is the condition which is called Resonance

5. HARMONIC RESONANCE
• Harmonic resonance is generally caused by parallel
resonance between the Power Factor Correction capacitors
connected to a load and the transformer supplying that
load. When a number of harmonic current sources are
injecting currents into the supply and the frequency of one
of the harmonics coincides with the resonant frequency of
the supply transformer and Power Factor Correction
capacitor combination, the system resonates and a large
circulating harmonic current is excited between these
components. The result of this is that a large current at this
harmonic flows in the supply transformer, thus resulting in
a large harmonic voltage distortion being imposed upon
the load voltage.

6. HARMONIC RESONANCE CONTD---
• THE FORMULA TO DETERMINE THE ORDER OF
HARMONIC THAT MAY CAUSE RESONANCE IS
• Hr =
• SUPPOSE 20 MVAR CAPACITOR IS CONNECTED
ACROSS A SYSTEM OF 1000MVA SOURCE THERE
WILL BE CONDITION OF RESONSNCE AT 7TH
HARMONIC
• THE POSSIBILITY OF RESONANCE SHOULD BE
EXPLORED AND ELIMINATED DURING ANY
MODIFICATION OR ADDITION OF LOAD IN THE
POWER SYSTEM

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Resonance Frequency
• Resonance occurs at a frequency where system inductive
impedance equals capacitive impedance
• 2πftL =
• ft =
• In a typical system the order of Harmonic (Hr) where
resonance occurs can be calculated as
• Hr =
• For 1000 MVA source and 20 MVA capacitors
• Hr = = = 7.07

8. Power System Impedance
• Harmonic Order Nr where Resonance can occur is given by ‘Square Root’ of
• Short Circuit MVA of Source divided by MVAR of connected Capacitors.
•
• Nr =
• For 100 MVA Short Circuit Level and
800 KVAR Connected Capacitors
Nr =
• This is close to 11th Harmonic which may be present due to certain loads and will
cause Resonance at this undesirable point.
• If the Connected Capacitors are reduced to 500 KVAR then Nr increase to 14.1
and the risk of resonance at undesirable frequency close to some prevalent
Harmonics is eliminated.

9. HARMONIC RESONANCE CONTD
• SINGLE LARGEST CAUSE OF SEVERE
HARMONIC DISTORTION IS RESONANCE
• A NORMAL HARMONIC MAY BE AMPLIFIED 10
TO 25 TIMES IF RESONANCE OCCRSC AT OR
CLOSE TO CRITICAL FREQUENCIES
• IT OCCURS MAINLY DUE TO INDISCRIMINATE
USE OF PF CAPACITORS OR BECAUSE OF
INCORRECT APPLICATION OF FILTERS

10. LOAD ALLOCATION ON A
TRANSFORMER
• If a Transformer is loaded with mainly non
linear load the supply is highly corrupted as
there is no Damping of Harmonics
Amplification due to absence of linear load.
• Allocating combination of linear and non
linear loads on a Transformer is advisable as it
dampens the Harmonic Amplification and
minimizes Filtration needs

12. OVERCOMING RESONANCE
• SERIES COMBINATION OF REACTOR AND
CAPACITOR TO CONTROL SYSTEM IMPEDENCE
TO AVOID RESONANCE CONDITION WHICH
CAN AMPLIFY HARMONIC CURRENT.
• THE COMBINATION SHOULD BE INDUCTIVE AT
CRITICAL FREQUENCY BUT CAPACITIVE AT
FUNDAMENTAL FREQUENCY FOR THIS
TUNING FREQUENCY SHOULD BE BELOW
LOWEST ORDER HARMONIC 5TH (250 HZ)

13. OVERCOMING RESONANCE CONTD---
• HARMONIC RESONANCE WAS NOT A
CONCERN EARLIER WHEN NON LINEAR LOADS
WERE NOT SIGNIFICANT

14. Harmonic Generation by Nonlinear
Loads
• UPS
• DC DRIVES
• VFD
• THYRISTOR CONTROLLED HEATING
• INDUCTION FURNACES
• ELECTRONIC CHOKES FOR FUORESCENT LAMPS
• BATTERY CHARGER
• WELDING CONTROLS AND RECTIFIERS
• SMPS SUPPLIES

15. ADVERSE EFFECTS OF HARMONICS
• INCREASED HEATING DUE TO IRON AND
COPPER LOSSES AT HARMONIC FREQUENCIES
• CHANGE IN PERFORMANCE CHARACTERISTICS
OF ELECTRICAL AND ELECTRONIC
EQUIPMENTS
• OVERLODING OF NEUTRAL
• CAPACITOR FAILURES
• SPURIOUS TRIPPINGS
• ERATIC OPERATION OF CONTROLS

16. ADVERSE EFFECTS OF HARMONICS
• INTERFERENCE WITH COMMUNICATIONS
• MEASUREMENT ERRORS IN METERING
DEVICES

17. RELEVANCE OF HARMONICS TODAY
• HIGH NON LINEAR LOADS
• VISIBILITY OF PROBLEM BECAUSE THE
MEASURES TO TACKLE HARMONICS ARE NOT
IN PLACE
• INCEASED USE OF CAPACITORS FOR PF
IMPROVEMENT LEADING TO AMPLIFICATION
OF HARMONICS

18. Series Resonance
At tuning frequency ft
XL = XC
• 2πftL =
• ft =
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19. • Capacitive Reactance decreases with frequency and inductive
reactance increases with frequency
• The total reactance of the combination takes minimum value at
resonance frequency
• The total reactance of the combination takes minimum value at
resonance frequency
• For 7% resonance frequency = 189Hz
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20. Series resonance
• Tuning frequency for 7% detuned reactor in LC series
circuit
• Detuning ratio p = 0.07
• Fundamental frequency = fn
• Tuning frequency = ft
• L = Inductance
• C = Capacitance
• 2πfnL = x 0.07
• fn
2 = x 0.07

21. Series Resonance –Tuning Frequency
• ft
2 =
•

22. HARMONIC FILTERS
• DETUNED FILTERS
• TUNED FILTERS
• ACTIVE FILTERS

23. DETUNED FILTERS
• DETUNED FILTER IS A SERIES LC CIRCUIT
TUNED TO AVOID RESONANCE CONDITION. IT
OFFERS LOW IMPEDENCE PATH TO SELECTED
HARMONIC FREQUENCIES DETERMINED BY
THE TUNING FREQUECY. IT PROVIDES
CAPACITIVE KVAR TO THE SYSTEM AND A
NOMINAL REDUCTION OF HARMONICS

24. DETUND FILTERS CONTD---
• VOLTAGE RISE ACROSS CAPACITOR
Vrise % = ( )2 x 100
Fn is Network Frequency
Ft is Tuning Frequency of LC combination
• 5.67% DETUNED FILTER– 210 HZ TUNING
FREQUENCY
• 7% DETUNED FILTER – 189 HZ TUNING
FREQUENCY
• 14% DETUNED FILTER – 134 HZ TUNING
FREQUENCY

25. Typical Solutions Applied to Resonant
Power Systems – Detuning Reactors
• This solution involves connecting a reactor in series with the Power
Factor Correction capacitor such that the tuning frequency of the L-
C combination is 189Hz (50 Hz system). This tuning frequency is
chosen in order to ensure that this capacitor branch appears
inductive for all of the major harmonics on the system, thus
eliminating the possibility of a resonance occurring. Any resonance
involving the 3rd harmonic is ignored as Power Factor Correction
capacitors are generally connected in a Delta configuration, this
preventing any 3rd harmonic (zero sequence) current flow. The
tuning frequency is chosen to be far enough below the 5th harmonic
to avoid the Power Factor Correction capacitor filtering the 5th
harmonic current but not so low that an excessive voltage rise is
produced across the capacitor at the fundamental frequency.

26. Typical Solutions Applied to Resonant
Power Systems – Partial Filtration
• This solution involves connecting a reactor in
series with the Power Factor Correction capacitor
such that the tuning frequency is approximately
210Hz (50 Hz system). This configuration is series
resonant quite close to the 5th harmonic (250Hz)
and as such affords a degree of harmonic
filtration. Using this configuration will remove
typically 50% of the 5th harmonic current and
30% of the 7th harmonic current. This
configuration would be used where the existing
harmonics are higher than the ideal level but not
so high that a full filtration system is required.

27. Typical Solutions Applied to Resonant
Power Systems – Full Filtration
• This configuration involves a number of
capacitor arms being tuned very close to
particular harmonic frequencies with the
express purpose of removing these
harmonics. In the example shown the 5th and
7th harmonics are being targeted.
• TUNING FREQUENCIES ARE 240 HZ AND
334HZ

28. TUNED FILTERS
• TUNED FILTER IS AN LC CIRCUIT THAT IS
TUNED TO PROVIDE BOTH A HARMONIC
CURRENT REDUCTION WHILE AVOIDING A
RESONANCE CONDITION. IT IS TUNED CLOSE
TO THE FREQUENCY OF A SPECIFIC HARMONIC
CURRENT WITH THE GOAL OF PROVIDING A
LOW IMPEDANCE PATH TO THAT HARMONIC
CURRENT THUS REDUCING THE AMOUNT OF
THAT HARMONIC CURRENT FROM BEING
INJECTED INTO THE DISTRIBUTION NETWORK

29. Comparison of Harmonic Resonance
Solutions

30. Comparison of Harmonic Resonance
Solutions
0
2
4
6
8
10
12
5 7 11 13
Harmonic
Vn(%)
Vn Plain Capacitors
Vn Detuned Capacitors
Vn Partial Filtration
Vn Full Filtration

31. SYSTEM HARMONIC IMPEDANCE
• This graph shows how the system harmonic impedance
varies with 400 KVAr connected in the various
configurations detailed in the previous slides. Clearly,
the plain capacitor solution results in the highest peak
harmonic impedance at the 7th harmonic. The full
filtration has two peaks at the 4th and 6th harmonics.
However, these harmonics do not naturally occur and
the impedance at the significant harmonics (5th and 7th)
is very low. The other systems provide a moderately
low impedance throughout the harmonic range but
most importantly, avoid any resonant peaks.

32. COMPARISON OF HARMONIC
RESONANCE SOLUTIONS
• This slide shows the harmonic voltage
distortion resulting from the implementation
of the four capacitor configurations
mentioned earlier. The voltage distortion
resulting from the connection of plain
capacitor banks is not acceptable whereas
that resulting from all the other solutions is.

33. Objectives of Harmonic Mitigation
• To have normal operation of all electrical
equipments and to get their normal expected
life
• To obtain operational efficiency and energy
efficiency
• To maintain power quality
• To meet the harmonic current and voltage
distortion limits prescribed by international
standards or by utility supply companies.

34. Why to use Segmented Film MPP
Capacitors
• Inherently safe and Environment friendly construction
• Can safely clear an internal fault or a voltage transient without
isolating the capacitor
• Can isolate the capacitor safely at the end of service life or in
extreme loading conditions
• Fine manufacturing tolerances
• Absence of any trapped moisture and impurities
• Better suitability for handling high frequency harmonics due to their
low ESR values
• High power density leading to compact low space construction
• Excellent quality uniform metal layer end connection with metal
spray
• Lower CO2 emission requirements

35. Capacitors with High Current Carrying
Capacities
• The IEC standards require capacitors to be capable of
continuously handling 130% of rated current but in
harmonic environment this value can be exceeded and
values higher than 200% can only be considered safe.
This is now becoming possible with the advent of
segmented MPP type low ESR capacitors which have
excellent over current handling capabilities. ESR value
assumes higher significance in view of high frequency
harmonics being handled by capacitors. ESR value had
a limited significance when fundamental frequency
current was the only consideration.

36. Problems with APP Capacitors
• Absence of inherent safety features
• High dissipation factor
• High manufacturing tolerances
• Poor quality of bimetallic end connections
• Abnormally high operating temperatures
• Loose winding with trapped moisture and
impurities
• Low power density leading to higher space
requirements
• Higher CO2 emission requirements

37. Inherent Capacitor Safeties
• The absence of any inbuilt safety mechanism to
clear internal faults leads to continued operation
under abnormal conditions of over temperature,
over pressure and degradation of oil, thus leading
ultimately to complete failure under unsafe
conditions. The undesirable mode of failure
under heavy loads, or at the end of service life
has made APP capacitors unusable worldwide for
LT applications. The ongoing research for
development of HT capacitors with a superior
and safer technology may soon phase out APP
capacitors for HT applications too

38. Ventilation and Cooling Requirements
• Capacitor panels need adequate ventilation and
cooling. Present day practice where reactors and
thyristor switching are increasingly becoming part
of capacitor panels, the cooling needs assume
greater significance. Compartmentalized
construction is unsuitable and needs to be
discontinued. Similarly these reactive power
systems should not be clubbed with MCC and
PCC panels as the design, performance and
operating requirements are more complex and
need specialized attention.

39. Where to apply Harmonic Mitigation
Scheme
• The technical benefits of applying harmonic
solutions are available upwards of the point
• of application. Most solutions today are applied
by users close to the point of utility company’s
supply to meet mainly statutory requirements.
The harmonic solutions need to be shifted
downwards closer to the harmonic generating
loads in order to pass on the benefits to both the
utility company and the user.

40. Reactors
• Reactors should have Class F or Class H insulation level, less than 10
watts per Kvar power loss, continuous current handling capacity of
150% of rated value or higher, Linearity of 175%. In a changed
scenario the utility companies are moving towards imposing
significant penal charges for not maintaining harmonic levels below
specified values. At times the specified maximum limits are more
stringent than those recommended as per IEEE519. This calls for
following an elaborate and accurate designing process for reactor
selection. A hitherto common practice of using a 7% reactor which
was mainly meant to provide some protection to capacitors may
have to be done away with in favor of reactors which can provide
filtration to achieve desired harmonic levels. Reactors also dampen
effect of transients during capacitor switching.

41. Step configuration
• Step configuration involving sizing of individual steps and
number of total steps needs to be carefully selected to
avoid resonance occurrence at switching of any step. It may
be necessary to skip a particular step to achieve this
purpose. Most designers will have specific software
developed for the purpose. Indiscriminate selection of step
sizes and number of steps may lead to some undesirable
combination resulting in resonance occurrence and
resultant pitfalls thereof. Here the clients and consultants
who often inadvertently suggest particular step
combinations need to be warned of the possible dangers. It
is best to leave this exercise to the designers after giving
them the broad guidelines about the least count of power
factor correction over the normal operating range.

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IEEE 519 1992
 Total Harmonic Distortion (Current)
• Where I1 is fundamental component
 Total Demand Distortion (Current)
• Where IL is the maximum demand load current of the
facility within 15 or 30 minutes demand window.

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Detuning Frequencies
• For series LC Detuned Filters the tuning frequency (Ft) is set
below the lowest Harmonic with remarkable amplitude which
is normally the fifth Harmonic
Where f = fundamental frequency P = Detuning Ratio in
percentage
• For 7% Detuning Ratio

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• For 5.67% Detuning Ratio
• For 14 % Detuning Ratio

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Power Factor Correction
Case 1
• KVA Capacity of source required is 100 KVA, Active power is
80 KW and Reactive Power is 60 KVAR
• Reactive power is supplied by the source

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Power factor correction in the
presence of Harmonic Generating Loads
• Capacitors provide low impedance path to harmonics
• If the resonance caused by addition of capacitors occurs near
any of the generated harmonics then the harmonics get
amplified
• Capacitors get stressed due to flow of harmonics through them

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Reactive Power Requirement
• Initial power factor Cos ᴓ1
• Desired power factor Cosᴓ2
• Initial reactive Power Q1 =
Ptanᴓ1
• Final reactive Power Q2 = P
tanᴓ2
• Required reactive Power
= ᴓ1 - ᴓ2
= P tanᴓ1 - P tanᴓ2
= P (tanᴓ1 - tanᴓ2)
= P (Multiplying factor)

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Voltage across Capacitor in a
series LC Filter
Vc = Voltage across capacitor
Vn = System voltage
P = Detuning Ratio in percentage
For 7% detuning ratio
• Plus 10 % Tolerance = 520 V
• Next available standard voltage is 525 V

49. • For 5.67% Detuning Ratio
Next available standard voltage is 525 V
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50. Capacitor inrush current
Isolated bank
• Isolated bank
•
• Ipk = inrush current
• Ft = inrush transient frequency
• Io = Steady State current value
• Fo = power frequency
• Ipk is 5 to 15 times the normal capacitor current
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51. Discharging of capacitors
• IEC 60831-1 Clause 22
• Each capacitor unit/bank is provided with a means for
discharging each unit in 3 min. to 75 volts or less
• From an initial peak voltage of times rated voltage Un
• In a single phase unit
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52. • R = Discharge Resistance in Ω
• t = Discharge time from Un to Ur in seconds
• Un = Rated voltage in volts
• Ur = Residual voltage in volts
• K = Coefficient depending on connection module of resistor to
capacitor (k=1 for R in parallel with C )
• C = Capacitance in Farads
• Indian standards
Discharge time = 1 min
Residual voltage = 50 V
•
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53. 6-pulse diode rectifier
• The most common rectifier circuit in 3-phase AC drives is a
6-pulse diode bridge. It consists of six uncontrollable
rectifiers or diodes and an inductor, which together with a
DC-capacitor forms a low-pass filter for smoothing the DC-
current. The inductor can be on the DC- or AC-side or it can
be left totally out. The 6-pulse rectifier is simple and cheap
but it generates a high amount of low order harmonics 5th
especially with small smoothing inductance.
• If the major part of the load consists of converters with a 6-
pulse rectifier, the supply transformer needs to be
oversized and meeting the requirements in standards may
be difficult. Often some harmonics filtering is needed.

54. 12-pulse diode rectifier
• The 12-pulse rectifier is formed by connecting
two 6-pulse rectifiers in parallel to feed a
common DC-bus. The input to the rectifiers is
provided with one three-winding transformer.
The transformer secondaries are in 30° phase
shift. The benefit with this arrangement is that
in the supply side some of the harmonics are
in opposite phase and thus eliminated. The
major drawbacks are special transformers and
a higher cost than with the 6-pulse rectifier

55. 18 pulse and 24-pulse diode rectifier
• 18 pulse converter uses three sets of 6 pulse
bridge rectifiers that are supplied from three
different power sources each of which are phase
shifted by 20 electrical degrees. This results in
cancellation of the 5th, 7th, 11th and 13th
harmonics
• 24-pulse rectifier has two 12-pulse rectifiers in
parallel with two three- winding transformers
having 15° phase shift. The benefit is that
practically all low frequency harmonics are
eliminated but the drawback is the high cost.

56. FORMULAE
• KVAR = 2 π f CV² (Capacitive KVAR 1 Ph)
• KVAR = √3 2 π f C V² (Capacitive KVAR 3 Ph)
• KVAR = V²∕ 2 π f L (Inductive KVAR 1 Ph)
• KVAR = √3 V²∕ 2 π f L (Inductive KVAR 3 Ph)

57. FAULT LEVEL CALCULATIONS
Transformer Fault MVA =
Transformer MVA Capacity/ Per Unit Impedance
For 10 MVA Transformer having 9 % Impedance
Fault MVA = 10/.09 = 111MVA

58. Conclusion
• There is increasing presence of non linear loads leading to
harmonic generation and their further amplification due to
resonance conditions induced by capacitors. To safeguard
their installation utility companies are rightly coming out
with penal tariff structures. Correctly designed effective
harmonic solutions designed by using ETAP or equivalent
software which can provide greater filtration volumes are
imperative. Precautions need to be taken while allocating
loads on transformers, ensuring avoidance of resonance
while adding capacitors steps and selecting only reliable
and technologically superior components. A casual
approach in selecting capacitor steps and reactor detuning
factor may lead to resonance and associated problems.

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