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Harmonics a necessary evil in Rolling mills & its
management
By Amitabh Kumar Sinha.
FIE, B.Sc (Engg),DBM, FISLE
Ex VP-Projects, Jindal Stainless
Ex DGM (Elect- Mills) MECON
Copyright, 2015 © Amitabh Kumar Sinha.
Harmonics a necessary evil in Rolling mills & its management
I welcome you all to this presentation on “harmonics” its
effects & management
Introduction
Following topics will be discussed :
• What is Harmonics?
• Why it is a necessary evil in R.Mills?
• What is so bad about Harmonics?
• Do we interpret and specify IEEE-519
correctly?
• How to manage Harmonics.?
What is Harmonics?
• Harmonics are multiple frequency current
(or voltage) present in the power system,
e.g. if in a 50 Hz power system some 100
Hz current is flowing then it is 2nd
harmonic current. Harmonics have
frequencies as integral multiples of base
frequency (also called fundamental).
Hence 50 Hz system may have
100,150,200,250... Hz harmonics.
What is Harmonics?
Let us try to understand this with a common example of a full wave rectifier.
We may note that non-linear current such as rectangular current of rectifier may have lot of harmonics
What is Harmonics?• From the picture below is obvious that 1st, 3rd,
5th, 7th & 9th harmonics add up to make rectangular current of rectifier.
Why harmonics is a necessary evil?
• In modern Steel Rolling mills we need to install many
variable speed AC or DC drives, using converters.
Though these drives are source of harmonics, we
cannot do away with them. We need them for proper
functioning of our Rolling Mills. PC’s,Printers, copies,
fax machine, TV’s, tube lights.. All of them produce
harmonics. Hence term used “Harmonics a
necessary evil”.
• Power System harmonics is not a problem till it
becomes a problem. Like diabetes nobody notices its
presence in the power system till it hurts in the form of
inefficient power system, losses and penalties,
oversize equipment selection etc.
Why harmonics is a necessary evil?
Harmonic Caln for a TCM-Drive
Harmonics- How they look like?
Figure gives waveform of a linear load and a non-
linear load. If Fourier Analysis is done for the two
wave forms the first wave form shall have only
fundamental where as the non-linear wave form
will result in several components including
fundamental, 2nd, 3rd harmonic etc.
Harmonics- where & how they are generated ?
Statement: All non-linear loads generate harmonics.
Q: How & What is a non-linear load anyway?
A: One definition: A non-linear load is where current waveform is not exactly same as input voltage waveform (assumption input voltage waveform is perfectly sinusoidal). For example a half wave rectifier –where voltage wave forms swings into both sides of X-Axis, current wave form are only in the positive half since no current flows in negative half.
Harmonics- where & how they are generated ?
Q: How they are generated?
A: All non-linear loads generate a periodic non-sinusoidal wave forms.
• Some 100 yr. ago, Fourier, doing heat transfer work, demonstrated that any periodic signal can be viewed as a linear composition of sine (or cosine)waves- which were later termed as harmonics.
Harmonics-are generated, So what ?• Modern day technologies are incorporating sensitive
components, presenting new challenges for plant
managers and engineers. For example, widespread
use of thyristor or Diode Rectifiers used in the power
conversion section of Variable Frequency Drives
(VFDs), PC, copiers,solid state lighting ballast etc.,
have raised concerns regarding power quality and its
role in harmonic distortion. The problem multiplies
many fold when large steel rolling mill motors of
several Mega Watt are fed from variable speed drives
(I mean today’s AC-DC drives not Ward-Leonard
systems, which was free from harmonics). VFD/VSD
generates lot of harmonics and need to be managed
such that they don’t inject enough harmonics into utility
system. It is worthwhile to mention that even a traditional powerdistribution transformers when saturated contribute to harmonics
What is so bad about Harmonics?
• Harmonics is the non-useful component of load
current. It results in harmonic distortion of line
voltage and so affect other users on the same bus.
It results in overloading of equipment or else we
have to oversize the components. These may result
in following:
Equipment Failure and mal-operation.
Overheating/Failure (transformers, motors, cables/
neutral)
Nuisance Tripping / operation (fuse, breakers, PC-
ups)
Insulation failures.
PF Capacitor resonance
What is so bad about Harmonics?
• Economic Considerations
Harmonics results in extra expenditure in terms
of additional energy cost.
Extra investment due to over-sizing of
transformers, generators, neutrals, cables etc.
Losses/ Inefficiencies / Penalties
Understanding IEEE-519-1992
Some common guide lines were required to regulate Harmonics and IEEE Std 519 was first introduced in 1981 to provide direction on dealing with harmonics produced by static power converters and other nonlinear loads so that power quality problems could be averted. The standard was later revised in 1992.
• Terms used in the Standard:
Short Circuit Ratio (ISC/IL): The ratio of the shortcircuit current (ISC) available at the point of commoncoupling (PCC) to the maximum load current (IL)
Terms used in IEEE-519-1992
Maximum Load Current (IL): Average current of the
maximum demand for the preceding 12 months. (This
is not known at the design stage and is inherently
ambiguous)
• (Unfortunately, this value is inherently ambiguous)
Voltage THD: Total Harmonic Distortion of the voltage
waveform. The ratio of the root-sum-square value of
the Harmonic content of the voltage to the root-mean-
square value of the fundamental voltage.___________________________
• V TH D = \ (V 2 2 + V 3 2 + V 4 2 + V 5 2 +.... )x100%
V 1
Terms used in IEEE-519-1992
Current THD: Total Harmonic Distortion of the
current waveform. The ratio of the root-sum-square
value of the Harmonic content of the current to the
root-mean-square value of the fundamental current.
______________________
• I TH D = \/ I 22 + I 3
2 + I 42 + I 5
2 +.... x 100%
I 1
Terms used in IEEE-519-1992
Current TDD: Total Demand Distortion of the
current waveform. The ratio of the root-sum-square
value of the harmonic current to the maximum
demand load current
______________________
• I TD D = \/ I 22 + I 3
2 + I 42 + I 5
2 +.... x 100%
IL
Harmonic limits in IEEE-519-1992
• VOLTAGE HARMONIC LIMITS IN IEEE STD 519
• IEEE Std 519 specifies harmonic limits on voltage
as 5% for total harmonic distortion and 3% of the
fundamental voltage for any single harmonic. The
justifies this limits by stating that Computers and
allied equipment, such as programmable controllers,
frequently require ac sources that have no more
than a 5% harmonic voltage distortion factor, with
the largest single harmonic being no more than 3%
of the fundamental voltage. Higher levels of
harmonics result in erratic, sometimes subtle,
malfunctions of the equipment that can, in some
cases, have serious consequences.
Harmonic limits in IEEE-519-1992
• CURRENT HARMONIC LIMITS IN IEEE STD 519
The level of harmonic voltage distortion on a system that can be attributed to an electricity consumer will be the function of the harmonic current drawn by that consumer and the impedance of the system at the various harmonic frequencies. A system’s impedance can be represented by the short circuit capacity of that system. Therefore, the short circuit capacity can be used to define the size and influence of a particular consumer on a power system. It can be used to reflect the level of voltage distortion that current harmonics produced by that consumer would contribute to the overall distortion of the power system to which it is connected
Harmonic limits in IEEE-519-1992
• The Standard very wisely uses a ratio of Isc/IL To
define current distortion limits. This establishes a
customer’s size and potential influence on the
voltage distortion of the system. The short circuit
ratio (ISC/IL) is the ratio of short circuit current
(ISC) at the point of common coupling with the
utility, to the customer’s maximum load or demand
current (IL). Lower ratios or higher impedance
systems hence will have lower current distortion
limits to keep voltage distortion at reasonable levels
and vice versa.
• It may be noted that IEEE-519 puts a limit on ITDD
Harmonic limits in IEEE-519-1992
IEEE 519 is widely misunderstood and misapplied in the industry
• “Specification-man-ship” – is quite prevalent among
many Steel Plant customer and consultants who
specify every electrical equipment and sundry shall
meet IEEE-519 for harmonics, without giving due
respect to the aim of this standards. One
consultant while specifying a VFD states VFD
shall be “Suitable for variable torque or
constant torque applications requiring
harmonic control as defined by IEEE 519-1992”
• Clearly the Standard has not been understood.
Following are the main points to be taken care and
understood.
IEEE 519 is widely misunderstood
• IEEE-519 is designed to limit utility harmonics as well as
customer’s harmonic contribution to the utility grid.
• Standard ONLY applies to the Point of Common Coupling
(PCC)
• The concept of PCC has been used in the Standard. The
definition used in the standard clarifies about PCC, as
following:
• The PCC is ‘the closest point on the utility side of the
customer's service where another utility customer is or could
be supplied’. It also points out that the ownership of any
supply transformer is irrelevant. That is, if a supply
transformer connected to the public power grid supplies only
one customer, the PCC will be located at the primary of that
transformer, rather than the secondary, regardless of whether
the transformer is owned by that customer or the utility.
IEEE 519 is widely misunderstood
PCC may be @ point 1,2 or 3 depending upon where other consumers can get connected to the utility.
How to manage harmonics?
• Now we know it is a great idea to limit harmonics for operational & economic reasons as well as to meet IEEE-519
• Many methods are available. Let us discuss them.
1. Line reactors: It slows down rate of rise of current as well as offers higher impedance to higher harmonics.
Reactors are rated in %Z for the rated voltage system (i.e.3%, 5%, 8%, etc.) Line reactors greater than 5% are not recommended due to voltage drop.
Line reactors
• Advantages
• Lowest cost
• Moderate reduction in harmonics
• Provides increased protection for VFD
• Insensitive to system changes
• Disadvantages• Ineffective reduction in
prominent lower harmonics.
• May require larger enclosure / separate space.
• Possible voltage drop issues
• Produces heat
2. K-rated/Isolation transformers
• Isolation transformers are similar to Line reactors say 4-6% reactors.
• Same advantages and disadvantages as reactors. Additional advantage is isolation. Additional disadvantage is it is costlier.
3. Harmonic Mitigating Transformers/Phase Shifting
• When multi phase rectifiers are used certain lower harmonics are cancelled and not present on the primary side.
• Only harmonics which may be present are n*P+1 where n= positive integer, P= no of pulse. Thus a half wave 1 ph rectifier with P=1 will have 2nd, 3rd, 4th, 5th Harmonics. A full wave 1 phase rectifier with P=2 will have 3rd, 4th,5th,6th, 7th harmonics. 3phase bridge rectifier with P=6 will have 5th, 7th, 11th, 13th, 17th, 18th harmonics and so on.
• Multi phase / pulses can be obtained by phase
shifting through various vector groups.
3. Harmonic Mitigating Transformers/Phase Shifting
ROTATION THROUGH 15 DEG- 12 P SECONDARY.
3 TO 12 PH TRANSFORMATION MAY BE USED FOR 24 PULSE CONVERTER EFFECT.
Specifically wound transformer (Zig-zag, D/d0/Y11 etc.)
providing phase shift such a +150, -150 300 etc are used
to cancel targeted harmonics in primary side of the
transformer. Application depends on the targeted
harmonics.
3. Harmonic Mitigating Transformers/Phase Shifting
ZIG -ZAG CONNECTION
Exotic methods for phase shifting of mercury arc rectifier days are shown here. Most common method used today is D/d0/y11 type 3 wdgtransformer which provides 12 P effect if used with 2 bridge
rectifiers.
3. Harmonic Mitigating Transformers/Phase Shifting
3. Harmonic Mitigating Transformers/Phase Shifting
Advantages• Energy Savings• Heat reduction• Can provide additional
3th harmonic attenuation
• Cancels harmonics in• primary system• No deration• Additional shielding• Highly reliable (no
electronics)• No maintenance• Simple installation
Disadvantages• Engineering intensive
solution. • Multiple transformers
needed to target 5th, 7th, 17th, 19th etc.
• Load must be balanced• between transformer
pairs.• May need
supplemental harmonic reduction to meet
• IEEE 519
Multi Pulse Converters
4.Passive Parallel Tuned Filters
• Consists of LC combinations tuned to a specific
frequency (Typically the 5th or 7th) Act as a shunt (or
trap) for harmonics Applied close to harmonic
generating loads.
Advantages:
• Single filter for multiple drives, Can target specific
“trouble” harmonics, Can be designed to guarantee
compliance with IEEE 519. May need to tuning at odd
frequency such as 4.7th harmonics to avoid resonance.
Improves PF.
4.Passive Parallel Tuned Filters
Disadvantages:
Higher cost, Engineering intense solution, Separate mounting and
protection, May require multiple “steps” to meet IEEE 519, need
design to avoid overload, excessive voltage Rise, Interact with all
plant and utility non-linear loads, May require change as load
profile changes, may lead to resonance.
5. Passive Series / low pass Tuned Filters
Passive Series / low pass Tuned Filters
• Combination of reactors and capacitors to offer
higher impedance to certain targeted harmonics
such as 5th and 7th.
• Advantages and disadvantages same as
Parallel tuned filter.
6. Active VFD (AFE)
Active Rectifier & Regenerative VFD’s• Actively senses harmonics and Injects equal and
opposite currents to cancel harmonic currents
• Multiple units operate in parallel to get additional
capacity Can also use extra capacity to correct
power factor.
Advantages :
• Highly accurate control and monitoring, Flexible
harmonic control, system can grow as customer’s
needs change, size based on actual running loads
vs. provision, can be integral with drives or feeders.
Disadvantages :
• Typically more expensive than other methods, More
competitive where redundant VFD’s are
• Used, Size is larger, This is more complex
Conclusion
Conclusion
• While we have to live with harmonics as it is
generated by almost all electronics equipment
and even by tube lights, what to talk about
larger VFD’s for steel rolling mills, we also need
to manage the harmonics to save cost of
energy and for reasonable sizing of electrical
equipment. It is also needed to understand the
IEEE-519-92 standard so that it is applied
correctly.
We here by win over Harmonics !
THANK YOU !