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Solutions to harmonic problems

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Solutions to harmonic problems

n Structural modificationsn Isolate harmonic sensitive loads from harmonic producing

n Use high pulse number drive configurations

n Use special transformer couplings

à Often only possible in design phase of the installation

à Not always possible for all non-linear loads

à May turn out to be very expensive with high space requirement (life cycle assessment)

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Solutions to harmonic problems

n Passive filters: reactor and capacitor in seriesn Offers ‘low’ impedance path

for harmonic component(s)

à Filtering efficiency depends on network parameters, hence filtering performance cannot be guaranteed

à Danger for overloadingà Difficult to extendà Danger for resonanceà Multiple branches required for filtering more than one harmonicà Large space requirementà Provides always capacitive power

à AC drives do not require capacitive powerà Generators may not cope with leading power factor

Frequencyω0

Serie

s im

peda

nce

R

Capacitive behaviour

Inductivebehaviour

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Variable speed motors: DPF and PF

0 5 10 15 20 25 30 35 40 45 500

100

200

300

400

500

600

PF

DPF

TimeSec0 5 10 15 20 25 30 35 40 45 50

0

0.2

0.4

0.6

0.8

1

Example: Acceleration of a motor controlled by an AC drive

Displacement power factor Power factor

Active power

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Solution to harmonic problems

n Series passive and active devicesAttempt to prevent the harmonics from flowing by imposing high series impedance at the harmonic frequencies

à Device is in series with the load: high quality bypass requiredà Affects the supply voltage value of the loadà Needs to be sized for the load rating (fundamental + harmonics)à Difficult to extend to higher powersà May lead to very high voltage distortion (‘current has to flow’)

LoadSupply Series device

idistortion

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Solution to harmonic problems

n ‘Low pollution’ AC drives:

U1V1W1

U1V1W1

L1

L2

L3

Supply

DC linkDiode bridge

rectifier

V1 V3 V5

V2V6V4

C

L +

-

Ud

L1

L2

L3

InverterSupply

V1 V3 V5

V2V6V4

C

L +

-

Ud

IGBT bridge

Inputfilter

Lor

LCL

Inverter

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Low pollution drive application area

n Low pollution drives can be used to control AC motors

n Low pollution drives are not used for applications that do not require the control of AC motors, e.g.n In 4-wire office applications (‘PQFK’-application area)n Control of DC motors (very high installed base)n …

M 3

Motor

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Low pollution drive application area

n Good low pollution drives (ABB):

‘THDI(%) < 5% of nominal drive rating’à At lower than nominal rating THDI may be (is) higher

à Beware of ‘bad’ clean drivesE.g.: High frequency ripple destroys components of the istallation

Current

Voltage

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Low pollution drives: conclusions

n Good low pollution drives do a good job in having a low harmonicsignature at nominal load

n However:- They do not solve the problems of the complete installation

- They are expensive when applied to all the loads

n Therefore:- Use them possibly for single large loads

- Filter the rest of the installation with an active filter

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Active filters:

Supply Load

Harmonics injection

Proposed solutions to overcome poor PQ in LV

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Active filter principles

n Parallel topology: basic conceptPut a switching inverter in parallel with the load that is injecting compensation currents at the harmonic frequencies

LoadSupply

Active Filter

idistortion

Couplingsystem

icompensation

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The ultimate solution to poor Power Quality

n ABB premium class Active Filters:

PQFK PQFM PQFI

Leading by excellence!

PQFS

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Modern Active Filter technology: Flexible answer to all problemsè Harmonic filtration up to high order

è Reactive power compensation

è Load imbalancecompensation

è Self-limiting

è Upgradeable

è Network monitor

è…PQF

-1 . 3

1 . 3

0 3 6 0

-1 .3

1 .3

0 3 60

-1 . 3

1 . 3

0 3 6 0

FeederFUNDAMENTAL ONLY

ONLY HARMONICS

The ultimate solution to poor Power Quality

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The ultimate solution to poor Power Quality

n ABB Active harmonic filtersn Filtering principle: cancellation of harmonics by equal and opposite

harmonic generation by an active filter device

-1 .3

1 .3

0 3 6 0

LOADCURRENT

-2 0

0

2 0

4 0

1 5 7 1 1 1 3 1 7 1 9

-1. 3

1.3

0 36 0

ACTIVE FILTERCURRENT

-2 0

0

2 0

4 0

1 5 7 1 1 1 3 1 7 1 9

-1 .3

1 .3

0 3 6 0

CLEANFEEDER

CURRENT

-2 0

0

2 0

4 0

1 5 7 1 1 1 3 1 7 1 9

Wav

efor

ms

+=

Har

mon

ics

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How does an active filter work (1)

PWM Inverter (IGBT-based)

Line reactor

PWM reactor

Output filter

Control system

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How does an active filter work (2)?

Measurements

State of the art digital DSP system

PhysicalSignals

Power invertercontrol signals

• High filtering efficiency and reliability• Modern digital technology using multi-DSP systems and MicroController

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Highest ← PRIORITY → Lowest

Mode 1 Filtering to curve

Maximum filtering

Reactive compensation

Mode 2 Filtering to curve

Reactive compensation

Maximum filtering

Mode 3 Filtering to curve

Reactive compensation

Different modes of operation

Highest ← PRIORITY → Lowest

Mode 1 Filtering to curve

Maximum filtering

Reactive compensation

Mode 2 Filtering to curve

Reactive compensation

Maximum filtering

Mode 3 Filtering to curve

Reactive compensation

Reactive compensation: - none- dynamic (cosφ target)- static (constant Q)- Load balancing combined with reactive compensation

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n Best results up to high frequencies are obtained by usingclosed loop control systems

è Directly measure and controlharmonic current flowing to network

è Correction for system inaccuraciesè Can verify harmonics according to

regulation directlyè Can be used for power factor

targettingè Simple CT connection è Easy for future harmonic load

extensionsè Appropriate for local & global

compensation

Active filter control approaches

Distortionsource

ActiveActivefilterfilter

open loop

Active Active filterfilter

closed loop Distortionsource

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Active filter control approaches

Open loop systems:§ In open loop control, the sequence of commands in the program is

carried out irrespective of the consequences. For example, a teacher may set a class some work to do (the instructions) then leave the room.

Closed loop systems:§ In closed loop control, the teacher would set a class work to do,

then monitor their progress to make sure that it is done. If pupils stop working or misbehave then she would take action. The teacher observes the class, obtains feedback and takes appropriate action if the target is not being met.

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Open & Closed loop control: examplesOpen Loop systems§ Switch microwave on to defrost for 2 minutes. The turntable will turn

and the food be microwaved for 2 minutes irrespective of whether it is thoroughly defrosted or not.§ Program a toy robot to walk in a certain direction. It will follow all

instructions even if there is an obstacle in the way.§ Switch a sprinkler system on to water the garden at set times. The

garden will continue to be watered at these times even if it is pouring with rain.

Closed Loop systems§ Central heating systems § Ovensè A closed loop system is one that involves feedback to

ensure that set conditions are met

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n Open loop step responseè Transient responseè Steady state errorè Process may not run

n Closed loop step responseè Transient responseè No steady state errorè Process runs smoothly

Picture source: ‘Automatic Control Systems – B.C. Kuo’

Open and Closed loop response example

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LOADCURRENT

-2 0

0

2 0

4 0

1 5 7 1 1 1 3 1 7 1 9

ACTIVE FILTERCURRENT

-2 0

0

2 0

4 0

1 5 7 1 1 1 3 1 7 1 9

CLEANFEEDER

CURRENT

-2 0

0

2 0

4 0

1 5 7 1 1 1 3 1 7 1 9

+=

Active filter control approachesn Best results up to high frequencies are obtained by using

frequency domain control approach

è Dedicated optimizing controllers for each harmonic up to high order

è Individual harmonic selection capability for optimal use of resources

è In combination with closed loop control, allows to set up user requirementsfor each harmonic (e.g. standard targetting etc.)

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DVD Demonstration