Copper Development Association Power Quality Partnership Harmonics in Power Systems.

Copper Development Association

Power Quality Partnership

Harmonics in Power Systems


Power Quality Partnership


Fluke (UK) Ltd

MGE UPS Systems Ltd

Rhopoint Systems Ltd



• Established 1933

• website - www.cda.org.uk

• Technical helpline 01727 731200

• IEE Endorsed Provider



Background to Harmonics, Problems, Solutions and Standards

David Chapman, Copper Development Association

Harmonic Measurement and Power Quality Surveys

Ken West, Fluke (UK) Ltd

Total Harmonic Management

Shri Karve, MGE UPS Systems Ltd

Applying Predictive Techniques to Power Quality

David Bradley, Rhopoint Systems Ltd


-1.100

0.000

1.100

0 90 180 270 360

Fundamental

Third harmonic

Fifth harmonic

Fundamental with third and fifth harmonics


-1.600

0.000

1.600

0 90 180 270 360

Composite waveform


Switched mode power supplies (SMPS)

Electronic fluorescent lighting ballasts

Variable speed drives

Un-interruptible power supplies (UPS)

These are all non-linear loads

Loads that generate harmonics


How harmonics are generated – linear load


How harmonics are generated – non-linear load


A Common non-linear load


Current waveform for a typical Personal Computer


Harmonic profile of a typical Personal Computer


Harmonic profile for electronic fluorescent ballast


Harmonic profile for magnetic fluorescent ballast


Six-pulse bridge


Typical harmonic profile - six-pulse bridge

Six pulse bridge - harmonic current

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Harmonic number

%


Twelve-pulse bridge


Typical harmonic profile - twelve-pulse bridge

Twelve pulse bridge - harmonic current

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Harmonic number

%


Why have harmonics become so important?

Harmonic generating equipment has been in use for decades

• Increase in the number of loads

• Change in the nature of loads

• Increase in those producing triple-Ns


Equivalent circuit of a harmonic generating load


Harmonic Diversity

Fund3rd

5th7th

9th11th 1

25

1020

410

10

20

30

40

50

60

70

80

% wrt RMS

HarmonicNo of Units (pairs)


THD

0

20

40

60

80

100

120

1 2 5 10 20 41

No of Units (pairs)

% w

rt F

un

dam

en

tal

Harmonic Diversity - THDI


Problems caused by harmonics

currents within the installation overloading of neutrals

overheating of transformers

nuisance tripping of circuit breakers

over-stressing of power factor correction capacitors

skin effect

voltages within the installation voltage distortion & zero-crossing noise

overheating of induction motors

currents in the supply


In balanced three phase systems the fundamental current cancels out

But triple-N harmonics add arithmetically!

Non triple-N harmonics cancel in the neutral

Overheating of neutrals


Harmonic neutral currents

-8 .0

-6 .0

-4 .0

-2 .0

0 .0

2 .0

4 .0

6 .0

8 .0

0 120 240 360 480 600 720

Phase 1 Phase 2 Phase 3

Phase 1 3rd harmonic



3rd harmonic neutral current


Neutral conductor sizing






Neutral currents can easily approach twice the phase currents - sometimes in a half-sized conductor.

IEEE 1100-1992 recommends that neutral busbars feeding non-linear loads should have a cross-sectional area not less than 173% that of the phase bars.

Neutral cables should have a cross-sectional area that is 200% that of the phases, e.g. by using twin single core cables.



Sizing the neutral conductor

BS 7671:2001 - From January 2002

473 - 03 - 04

where neutral current is expected to exceed phase current

473 - 03 - 05where neutral cross-section is less than phase cross section

- neutral overcurrent protection is required



For three phase circuits using single core cables:

• Use a neutral conductor sized for the actual neutral current

• If the neutral current is not known, use a double sized neutral cable

• Provide overcurrent protection

• But take account of the grouping factors!

• Take into account voltage drop



For multi-core cables :

• Multi-core cables are rated for only three loaded cores - applies to both 4 and 5 core cables

• When harmonics are present the neutral is also a current carrying conductor

• Cable rated for three units of current is carrying more - three phases plus the neutral current

• It must be de-rated to avoid overheating

• Neutral must have overcurrent protection

• Grouping factor must be taken into account


Sizing the neutral conductor - thermal

0.5

1.0

1.5

2.0

2.5

0 10 20 30 40 50 60 70

% third harmonic current in phase

Cab

le s

ize

mu

ltip

lier


Sizing the neutral conductor - IEC

0.5

1.0

1.5

2.0

2.5

0 10 20 30 40 50 60 70


Cab

le s

ize

mu

ltip

lier



0.5

1.0

1.5

2.0

2.5

0 10 20 30 40 50 60 70


Cab

le s

ize

mu

ltip

lier

Thermal

IEC


Neutral conductor protection

Neutral conductors should be appropriately

sized and provided with over-current protection.

The protective device must break all the phases,

but does not necessarily need to break the

neutral itself.

This implies a future need for 4 pole breakers

with double rated neutral poles.


Transformers supplying harmonic loads must be appropriately de-rated

Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings

Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity

Effect of harmonics on transformers


Increased Eddy current losses in transformers

2hh

1h

2hefeh hIPP

max

where: Peh is the total eddy current loss

Pef is the eddy current loss at fundamental frequencyh is the harmonic orderIh is the RMS current at harmonic h as a percentage of rated fundamental current

Increase in eddy current loss can be calculated by:


K-Rating of Transformers

Two rating or de-rating systems for transformers:-

• American system, established by UL and manufacturers, specifies harmonic capability of transformer - known as K factor.

• European system, developed by IEC, defines de-rating factor for standard transformers - known as factor K.


K-Rating of Transformers - US System

2hh

1h

2hhIK

max

where: h is the harmonic order Ih is the RMS current at h in per unit of rated load current

First, calculate the K factor of the load according to:


For this typical PC load, the K factor is 11.6

(See IEE 1100-1992 for a worked example)




Next, select a transformer with a higher K rating:

standard ratings are 4, 9, 13, 20, 30, 40 and 50.

NB - for non K-rated transformers:

The K factor describes the increase in eddy

current losses, not total losses.


K-Rating of Transformers - European System

In Europe, the transformer de-rating factor is calculated according to the formulae in BS 7821 Part 4. The factor K is given by:

5.0

2

2

1

2

1

11

Nn

n

nq

I

In

I

I

e

eK

e is ratio of eddy current loss (50 Hz) to resistive loss

n is the harmonic order

q is dependent on winding type and frequency, typically 1.5 to 1.7


K-Rating of Transformers - European System

For the same PC load, the de-rating factor is 78%


K Factor

The methods for rating transformers

are discussed in CDA Publication 144

In addition, calculation software is

available on our web site:

www.cda.org.uk


K-Rating - Calculation software


K-Rating - Calculation software


K Factor

0

2

4

6

8

10

12

14

16

1 2 5 10 20 41

No of Units (pairs)

K F

acto

r

Harmonic Diversity - K Factor


K-Rating or De-rating?

‘K-rated’ transformers are designed to supply harmonic

loads by :

• using stranded conductors to reduce eddy current

losses

• bringing secondary winding star point connections

out separately to provide a 300% neutral rating


‘De-rating’ a standard transformer has some disadvantages:-

primary over-current protection may be too high to protect the secondary and too low to survive the in-rush current

the neutral star point is likely to be rated at only 100% of the phase current

it is less efficient future increases in loading must take the de-rating

fully into account

K-Rating or De-rating?


Transformers supplying harmonic loads must be appropriately de-rated

Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings

Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity

Effect of harmonics on transformers


Effect of triple-n harmonics in transformers

Triple-n harmonic currents circulate in delta windings - they do not propagate back onto the supply network.

- but the transformer must be specified and rated to cope with the additional losses.


Alternating current tends to flow on the outer surface of a conductor - skin effect - and is more pronounced at high frequencies.

At the seventh harmonic and above, skin effect will become significant, causing additional loss and heating.

Where harmonic currents are present, cables should be de-rated accordingly. Multiple cable cores or laminated busbars can be used.

Skin effect


Skin effect - penetration depth

fd

51021

where:

d is the depth of penetration, mm

f is the frequency, Hz, and

is the resistivity of the conductor

At the fundamental, 50 Hz d = 9.32 mm

At the 11th harmonic, 550Hz d = 2.81 mm


Nuisance tripping can occur in the presence of harmonics for two reasons:

Residual current circuit breakers are electromechanical devices. They may not sum higher frequency components correctly and therefore trip erroneously.

The current flowing in the circuit will be higher than expected due to the presence of harmonic currents. Most portable measuring instruments do not read true RMS values.

Circuit breakers


Problems caused by harmonics

currents within the installation overloading of neutrals

over-heating of transformers

over-stressing of power factor correction capacitors

skin effect

nuisance tripping of circuit breakers

voltages within the installation voltage distortion & zero-crossing noise

over-heating of induction motors

currents in the supply


Voltage distortion


Reducing Voltage Distortion by Circuit Separation


Increased magnetic and copper losses

Each harmonic generates a field which may rotate forward (+), backward (-), or remain stationary (0)

1 2 3 4 5 6 7 8 9 10 11 12

+ - 0 + - 0 + - 0 + - 0

• Zero sequence harmonics produce a stationary field, causing over-heating and reduced efficiency

Effect of harmonics on induction motors


• The negative and positive sequence harmonics together cause torque pulsing, vibration and reduced service life

• Harmonics are induced in the rotor leading to overheating and torque pulsing

Stator harmonic 1 5 7 11 13 17

19

Rotor harmonic - 6 6 12 12 18

18

Harmonic rotation F B F B F B

F

Effect of harmonics on induction motors


Motor de-rating curve for harmonic voltages

0.7

0.75

0.8

0.85

0.9

0.95

1

0 2 4 6 8 10 12

Harmonic Voltage Factor (HVF)

De-

rati

ng

Fac

tor


Harmonic voltage factorThe Harmonic Voltage Factor (HVF) is defined as:

n

5n

2n

n

VHVF

where:

Vn is the RMS voltage at the nth harmonic as a percentage of the fundamental, and

n is the order of each odd harmonic, excluding triple-Ns


Harmonic currents cause harmonic voltage distortion on the supply that can affect other customers. This distortion can propagate onto the 11 kV grid and spread widely.

There are limits for harmonic voltage distortion - a supplier may refuse to supply power to a site that exceeds them.

Harmonic problems affecting the supply


Harmonic Standards

Electricity Association Engineering Recommendation G 5/4 (2001)

BS EN 61000

IEEE Std 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems

ISBN 1 - 55937 - 239 - 7


Why revise G5/3?

Levels at 132kV higher than Grid Code allows

Introduction of concept of Electromagnetic Compatibility

G5/3 didn’t include notching and burst harmonics

Introduction of the EU Compatibility Directive and developments in IEC and European Standards

Better information on network harmonic impedance (see ETR 112)


The Electromagnetic Compatibility concept

Satisfactory operation of supply systems and users’ equipment only when electromagnetic compatibility exists between them

Emission limits help fulfil this objective

G5/4 seeks to limit harmonic distortion levels on the network at the time of connection to below the immunity levels of equipment

Enforced via the Electricity Supply Regs, Grid & Distribution Codes, and connection agreements


Harmonic Compatibility

Disturbance Level

Total supply network disturbance

Pro

bab

ility

Den

sity

Compatibility Level

Susceptibility of local equipment

Immunity (test) levels

Planning levels

Emission limits for individual sources


Compatibility levels v Planning levels

Compatibility levels in IEC 61000-2-2 & 61000-2-12, for 400V and 6.6kV to 33kV systems are based on the immunity of capacitors

The margins between planning levels and the compatibility levels depend on voltage level and range from 3% at lv and 5% at mv to 0.5% at ehv

The margins are necessary to make allowance for system resonance and for loads connected where there is no consent required from the DNO


Stage 1

Applies only to lv connected loads

Requires reference to other IEC standards e.g. IEC 61000-3-2 emissions from lv connected

equipment <16A IEC 61000-3-4 ditto >16A (To be 61000-3-12)

Clarifies that levels may be modified by reference to relevant fault levels rather than the notional ones used to derive the table of emissions Table 7


Aggregate loads

G5/4 requires that aggregate non-linear loads be considered• An individual non-linear equipment complying

with 61000-3-2 can be connected without consideration

• Groups of non-linear equipment with aggregate rated current <16A and complying can be connected

• For >16A either 61000-3-4 or 61000-3-6 should be used to assess emissions using diversity rules from 61000-3-6 if necessary


Example of application - the problem

Connection of communication centre equipment– 15 off rectifier equipment type R2948-15– each equipment is rated at 12.37A– each equipment meets BS EN 61000-3-2– the connection will be at lv and single phase– future expansion expected to 30 units

Can they be connected?

The customer says that no data on emissions is available


The solution

Data must be available - cannot claim BS EN 61000-3-2 compliance otherwise!

Data was obtained simply by e-mailing the manufacturer in New Zealand

Simplified calculations were carried out on a spreadsheet to check compliance


Product data sheet

0.0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Harmonic (f1 = 50Hz)

IP C

urre

nty(

A)

EUT

EN61000-3-2

Product TestHarmonic Emissions

Product: R2948-15Serial #: 1040171Date tested: 07 October 1999

Test ParametersInput Voltage: 230v 50HzOutput Voltage: 54v at no loadOutput Current: 52AAmbient temperature: +20°C

Product Compliance Group

12.37A


The calculations

As a first estimate the current emissions are

multiplied by the number of units, and the result

compared with the values in Table 7 of G5/4.

This shows that there is no problem

The spreadsheet calculations would show that

the future increase to 30 units would give values

of emissions greater than the limits for triple-Ns

above 21st


Table 7: Stage 1 Max Harmonic RMS Current Emissions for aggregate loads and equipment rated >16A per phase

Harmonicorder ‘h’

Emissioncurrent Ih


Emissioncurrent Ih


Emissioncurrent Ih


Emissioncurrent Ih

2 28.9 15 1.4 28 1.0 41 1.8

3 48.1 16 1.8 29 3.1 42 0.3

4 9.0 17 13.6 30 0.5 43 1.6

5 28.9 18 0.8 31 2.8 44 0.7

6 3.0 19 9.1 32 0.9 45 0.3

7 41.2 20 1.4 33 0.4 46 0.6

8 7.2 21 0.7 34 0.8 47 1.4

9 9.6 22 1.3 35 2.3 48 0.3

10 5.8 23 7.5 36 0.4 49 1.3

11 39.4 24 0.6 37 2.1 50 0.6

12 1.2 25 4.0 38 0.8

13 27.8 26 1.1 39 0.4

14 2.1 27 0.5 40 0.7


Sample spreadsheet

Harmonicnumber

Emissionfrom EUT

Emissions15 units

Table 7emissions

Emissions30 units

3 0.42 6.3 48.1 12.6

5 0.21 3.1 28.9 6.2

7 0.16 2.3 41.2 4.7

9 0.11 1.65 9.6 3.3

15 0.03 0.43 1.4 0.8

21 0.035 0.525 0.7 1.05

Emissions in Amps (RMS)


0

Example flow chart for lv connection

Less than 16A

Complies with 61000-3-2

Complies with 6.2


N


Y

Y

Y

N

N

N

N

Y

Y

YY

Mitigation required Connect to network

Complies with 6.3.1

3 phase <5 kVA

Complies with Table 6

NComplies

with Table 7

N N

N

Y

Y

Go to Stage 2

START


Stage 2

This applies only to:

a load or aggregate load that doesn’t meet IEC 61000-3-2 and 61000-3-6, or Table 7 current emissions, i.e. Stage 1

PCC less than 33kV i.e. at 6.6, 11 or 22kV

Current emissions can be less than Table 12, or a simplified voltage assessment can be used based on the harmonic impedance just described


Harmonic Measurements


Assessment of the connection of new non-linear equipment under Stage 2

a) measure voltage distortion present at PCC

b) assess the voltage distortion which will be caused by the new equipment, and

c) predict the possible effect on harmonic voltage levels by an addition of the results of (a) and (b)


Assessment of the connection of new non-linear equipment under Stage 2

If the results of (c) are less than

• the harmonic voltage planning levels for the

5th harmonic and

• the THD planning level

connection of the equipment is acceptable


Combination rules

for harmonics up to and including the 5th and for all triple-Ns, the measured and calculated values of voltage distortion are assumed to peak at the same time and to be in phase - linear addition

for the other harmonics, an average phase difference of 90 is assumed at the time of maximum THD - rms addition

the THD is then given by the rms addition of all combined harmonics up to the 50th


The Challenge

to keep the harmonic voltage distortion at the point of

common coupling below levels permitted by G5/4

to keep harmonic currents below levels

that cause equipment overload and damage within the

installation

that are permitted by G5/4


Steps to be taken to reduce voltage distortion on the supply include, for example:

Passive harmonic filters

Isolation transformers

Active harmonic conditioners

Harmonic solutions


Filters are useful when

the harmonic profile is well defined – such as motor controllers

the lowest harmonic is well above the fundamental frequency

- but filter design can be difficult and, especially for lower harmonics, the filters can be bulky and expensive

Passive harmonic filters


Passive harmonic filter


V I

0 2

IpIq

Power FactorPower Factor

Copper Development Association0 2

Ip

POWER



0 2

Iq

POWER


Copper Development Association0 2

V

I1

I5

I 7

LI



Mactivepower

reactive power

G



Mactivepower

reactive power

CAPACITOR

Power Factor CorrectionPower Factor Correction

G


Mactivepower

reactive power

CAPACITOR

G



M M M M

• Diversity• Self Excitation• Harmonics



M M M M

Control



M M M M

Control

• Transformer overloading

• Step voltage

• Bank Size

• Switch-fuse & Cable load ratings

• Load make/break rating of main isolator/switch-fuse



Power Factor Correction Bank Sizing

Required improvement in % wattess X kW Maximum Demand

equivalent to {tan(cos-1PFA) - tan(cos-1PFR)} X MD (kW)

or

kVArh (actual) - kVArh (required) running hours X load factor


• Capacitor Discharge time required for standard Power Factor banks (1 minute)

• Rapidly switching loads require Zero crossing Thyristor or IGBT switched steps

e.g. Spot WeldersLift motorsCranes



M

CONVERTOR

TO POWER SYSTEM

HARMONICS

LV

Harmonic ResonanceHarmonic Resonance

AMPLIFIED HARMONICS


Detuned or Blocking Banks

SOURCE IMPEDANCE WITH FILTER IN CIRCUIT

0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

100

112

124

136

148

160

172

184

196

208

220

232

244

256

268

280

292

304

316

328

340

352

364

376

388

400

412

424

436

448

460

472

484

496

508

520

532

544

556

568

580

592

Frequency

Y =

Ln (Z

+1)

Inductive

Capacitive

Fo = 189 to 204 Hz

5th 7th 11th



0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

Frequency

Y =

Ln (Z

+1)

Inductive

Capacitive

Fo = 235 to 245Hz7th

Filter Banks - 5th Harmonic


Filter Banks 5th & 7th Harmonic


0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

0.3500

0.4000

0.4500

0.5000

100

112

124

136

148

160

172

184

196

208

220

232

244

256

268

280

292

304

316

328

340

352

364

376

388

400

412

424

436

448

460

472

484

496

508

520

532

544

556

568

580

592

Frequency

Y =

Ln (Z

+1)

5th 7th


Third harmonic filtersThird harmonic filters

10 Amps

10 Amps

10 Amps

30 Amps

Load

R

N

S

T

E


Third harmonic filtersThird harmonic filters

10 Amps

10 Amps

10 Amps

30 Amps

Load

R

N

S

T

E

v

I3 = 0 Amps


R S T

N

R S T

N

Delta Interconnected-Star Transformer


Load

Interconnected Star Transformer sized for

harmonic currents only

I3

Harmonic reduction transformers


Delta-star isolating transformers reduce propagation of harmonic current into the supply.

Transformers should be adequately rated to cope with the harmonics

Although the transformer effectively establishes a new neutral, don’t use half-sized neutrals

Provide a well rated four wire feed so that the transformer can be isolated for service

Isolating transformers










Where the harmonic profile is unpredictable or contains a high level of lower harmonics, active filters are useful

Active harmonic conditioners operate by injecting a compensating current to cancel the harmonic current

Active filters


Keep circuit impedances low

Rate neutrals and multi-core cables generously - 1.73 to 2 times normal size

Always use true RMS meters

Provide a large number of separate circuits to isolate problem and sensitive loads

Take harmonics into account when rating transformers

Provide appropriate filtration where required

Harmonic solutions



www.cda.org.uk


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Copper Development Association Power Quality Partnership Harmonics in Power Systems.

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