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 !