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Damping of Inrush Currentin Low-Voltage PFC EquipmentLow-Voltage PFC

Application Note 2001

2 EPCOS AG

Power Quality

Contents

General 3

The risks of high inrush current 4

Single capacitor connection, inrush current calculation 6

Parallel capacitor connection, inrush current calculation 7

Various solutions for limiting inrush current serial aircoils 7

Detuning reactors, connection cable selection 8

Capacitor contactors with damping resistorsFunctionality / comparison 9

Comparison 10

Capacitor bank switching under various conditions 11

3EPCOS AG

Damping of Inrush Currentin Low-Voltage PFC EquipmentGeneralThe market trend to reduce lossesin modern low-voltage power-factor-correction capacitors (LV-PFCs)and the requirement for highoutput density result in reducedohmic resistance in PFC capacitors.Especially the switching of capa-citors in parallel to others of thebank, already energized, causesextremely high inrush current, up to 200 times the rated current,and limited only by the ohmicresistance of the capacitor itself.According to the formula (Eq1),such a capacitor’s AC resistance isvery low and thus contributes tohigh inrush current.

LV-PFC capacitor bankInrush current (pulse) is a factor of:

a Remaining capacitor voltagedue to fast switching in auto-matic capacitor banks

a Shortcircuit power of supplytransformer

a Output of capacitor switched in parallel to others alreadyenergized

a Fault level of supply network

a Output of capacitors alreadyenergized

a Ohmic resistance of capacitoritself and distribution switchgear, connection cables or con-ductors

KLK1709-W

25 k

VAr

25 k

VAr

25 k

VAr

25 k

VAr

25 k

VAr

25 k

VAr

12.5

kVA

r

25 k

VAr

M3~

187,5 kVAr

Automatic capacitor bank with 6 capacitors in parallel

High inrush current for grid, high balancing currents for capacitors

xc =1

2*π*f*c

Eq1:Switching operation: f ∞ xc 0 î 200 * Ir

4 EPCOS AG

Inrush Current by ConnectingCapacitor in Parallel (Energization)

The risks of high inrush currentConnecting LV-PFC capacitors with-out damping to an AC grid stressesthe capacitor like a shortcircuit. To avoid negative effects and toimprove a capacitor’s life time, ade-quate damping of inrush current ishighly recommended.

Influence of high inrush currentand resulting distortion:a High stress on the capacitor

reduced lifetimea Welding or fast wearing out of

the main contacts of contactorsa Negative effects on power

quality (eg. voltage transients)a Overvoltage:

– insulation problems– defects of electronic equipment– production stop

a Undervoltage/voltage zerocrossing– measurement failure– problems with numerical

control equipment– production stop due to

computer failurea High cost of maintenance and

production standstill

73.2 73.8 74.5 75.1 75.7 76.3 77.0 77.6 t (ms)

4000

3000

2000

1000

0

-1000

-2000

-3000

Cu

rren

t (A

)

Capacitor inrush current

ONOFF 5th capacitor connected

Peak current occurrence

i = 157 * IN = 157 * 21 = 3300 Ai

i

Capacitor connection:IN = rated current = 21A

0 10 20 30 40 50 60 70 80 90 100t (ms)

1500

1000

500

0

-500

-1000

-1500

Vo

ltag

e (V

)

1st step on 2nd step on 3rd step on 4th step on 5th step on 6th step on

1

1

High peak voltage (transients) occurrenceû

û

û

û

1 12

2

Û > UINS risk of shortcircuitÛ ≤ 0 V results in wrong measurementscausing control failures

1

2

5EPCOS AG

Inrush Measurement of Capacitor Steps

Switching of power factor correc-tion (PFC) capacitors is not onlyrelated to high currents but also to high voltage transients (ref.capacitor switching-on steps 1–6),causing degradation of powerquality, if the negative influence is not prevented by damping.

Capacitor sample, contact surface damaged by high inrush currentsHigh inrush current occurrencesdue to insufficient dampingcaused high electromechanicalforces within the capacitor. Espe-cially the contact area betweenelectrodes (windings) and themetal-spray layer was extremelystressed by high current forces.

The example shows that a fractionof metal-spray layer separated from the windings. Even the MKKcapacitor with excellent pulsecurrent capability and enhancedcontactability due to wavy cut andheavy edge design of the filmshows that extensive power cancause failures.

Voltage at 0.69 kV - busbar

PFC capacitor cascade connection: High voltage transients occurrence due to no damping

ExampleMetal-spray layer separated from the capacitor windings

6 EPCOS AG

Inrush Current Calculation

KLK1706-7

Grid

L 3

LL 2

1UN

Connecting a single capacitor

Circuit and formula

Terms

Result

Peak inrush current i ATransformer shortcircuit power Sk kVARated capacitor output Q kVArRated capacitor current IN ARated voltage UN V

Ohmic resistance = XC Ω3 * UN

2* (1/Q1 + 1/Q2)

Grid impedance = XI Ωo*L (Ω) including– contactor– fuse– busbars

Calculation example

Given parameters:

Grid connection of a single 50 kVAr capacitor, no other capacitor connected:a Grid 400 V/50 Hza Transformer shortcircuit voltage: 5%a Transformer output: 1600 kVAa Capacitor Q = 50 kVAr; IN = 72 A

i = = 2575 A

The inrush current is approximately 35 times the rated current.

Typical inrush currents are 10–40 times the rated current for single capacitors during connection.

1600 kVA2*0.05

*72 A50 kVAr

i=2*Sk

*INQ

^

Eq 2

^

^

Connection of a 50 kVAr capacitor, other 300 kVAr capacitors are already connected:a Grid 400 V/50 Hza Transformer shortcircuit voltage: 6%a Transformer output: 630 kVAra Q1 = 50 kVAra Q2 = 300 kVAra IN = 72 A ; VN = 400 V ; f = 50 Hz

a XC = 3 * U2N * ( + ) = 11.2 Ω

a L /phase = 0.4 µH (empirical)a XL = o * L = 2 * π * f * L = 0.125 mΩ

i = = 15118.6 A

The inrush current is approximately 210 times the rated current.

2*400 V11.2 Ω*0.125*10–3 Ω

1Q2

1Q1

7EPCOS AG

Various Solutionsfor Limiting Inrush Current

KLK1707-F

3C

1C

2C

C1

1Q Q2

C2 3C

Capacitor

Contactor

Grid

L 3

LL 2

1UN

KLK1708-N

nL

K n K

L 2

2

1L

K1

2LL 1

L 3 Grid

Qn Q2 Q1

UN

Parallel connecting of capacitor: Serial air coils

This example shows that cable turns in seriesbetween contactor and capacitor reduce theinrush current. Contactor suppliers recommendinductivity of 6–8 µH for damping inrush current.To achieve this inductivity, the following table pro-vides tips for selecting the required turns, diame-ters and cross sections.

Given parameters: Given parameters:

Parallel connection of a 50 kVAr capacitor with cable turns (serial aircoils) for damping,other 300 kVAr capacitors are already connect-ed, 400 V/50 Hz, shortcircuit power 10.5 MVA,rated capacitor current 72 A: damping withapprox. 6 µH with turns. a Xc = 11.2 Ωa XL1 = 2*π* f * L = 2* π* 50 * 6 µH = 1.88 mΩa XL2 = 2* π* f * L = 0.125 mΩa XL total = 0.125 + 1.88 = 2 mΩa L /phase = 0.4 µH (empirical value)1)

i = = 3780 A

The inrush current is approximately 50 times the rated current. This means onlyabout a quarter compared to a capacitorwithout damping (turns).

2*400 V11.2 Ω*2 *10–3 Ω

Typical inrush currents are100–250 times rated current for single capacitors in parallel connectionto other capacitors in operation.

This example shows that some cableturns in series with the capacitorcontribute to reducing inrush current (to 50 times rated current). This improves capacitor life cycle.

i= 2*UN

Xc*XLi= 2*UN

Xc*(XL1+XL2)

^ ^

Eq 3 Eq 4

^^

1) For switch gear and connected cables

8 EPCOS AG

Damping as described is a possiblesimple solution, but this methoddeals with two contradicting effects:

a Longer (or additional) cablescause electrical losses – higherlosses cause higher inherenttemperature within the capacitor.

a On the other hand, cable turnsreduce the inrush current andincrease the life cycle of capaci-tors and contactors.

Plus, you must make sure that thecapacitor works below its maximumoperating temperature.

Detuning reactors (series anti-harmonic reactors)In detuned capacitor banks theinductivity of filter circuit reactorsprovides an excellent dampingeffect for limiting inrush current.The following diagrams show the connection of a detuned andnon-detuned (reactor and capac-itor) system.

The peak current of a conventionalcapacitor is higher than 4000 A.The peak current of detuned capac-itors is only approx. 500 A. Thepurpose of filter circuit reactors isof course not the damping ofinrush current, but this exampleshows that in the case of detunedcapacitors no additional damp-ing measures are required.

Examples for detuned capacitor banks (ref. page 2)

Selection table for connection cables

Detuned capacitorwith series reactors

Various Solutions for Limiting Inrush Current

Capacitor Turns Approx. Cable rating diameter cross-section5 kVAr 10 100 mm 2.5 mm2

10 kVAr 10 100 mm 4 mm2

12.5 kVAr 10 100 mm 4 mm2

16.7 kVAr 7 100 mm 6 mm2

25 kVAr 7 100 mm 10 mm2

33 kVAr 7 100 mm 25 mm2

50 kVAr 7 100 mm 35 mm2

Fig. 1: 25 kVAr (21A /690 V)vertical: 2000 A /divhorizontal: 0.625 ms /div

Fig. 2: 25 kVAr (21A / 690 V)vertical: 200 A / divhorizontal: 10 ms / div

Because of the high inductance inthe circuit, the breaking quality of the contactor is important toavoid restriking during switch-off.Especially large contactors (over-sized motor contactors) may betoo slow and are therefore critical.

î > 4000 A

î = 190 * IN

i = 500 A

î = 24 * IN

This table should help to find the appropriate cable and required turns.Our PFC-CDROM (available upon request) contains calculation softwarewhich enhances precise calculation of the application (capacitors andswitch gear).

Conventional capacitorwithout damping

9EPCOS AG

Capacitor contactors with damping resistors

How does it work?The series damping resistors areswitched by socalled precontactsor auxiliary contacts. The precon-tact closes before main contactsand preloads the capacitor.

a Reduced voltage differences.

a The peak current is limited.

a The resistor is temporarily in thecircuit and has no thermal losses.

a The total resistance of the resistorwires is mainly ohmic in nature,its inductance can be neglected.The coiling up of the dampingresistors is only a matter of con-struction.

a During operation (main contactsare closed) the resistor wires aredisconnected or shorted out, anddo not cause any permanentlosses at all. Due to the very shortoperation time (a few millisecondsonly) during switch-on of thecontactor, a long life cycle of thedamping resistors is ensured.

Auxiliary switchedcontact with serialresistor (precontacts)

Capacitor contactor(main contacts)

Capacitor

Main contacts

Precontacts 2...10 ms

on

Grid/Mains

onoff

onoff

Functional diagram

i =C*dVdt

^

Note: Due to pre-loading via aux.contacts the capacitor’s voltage difference will bereduced. Consequently also the capacitor currentaccording to the formula:

Damping resistor

Pre-switching aux. contacts

Eq 5

10 EPCOS AG

With damping resistors

Without damping resistors

The following two diagrams showthe difference between a capa-citor’s inrush current without andwith damping series resistorswhen a capacitor is switched inparallel to an already energizedcapacitor bank/unit:

Comparison

Facts and conclusion

a Rated current of a 12.5 kVar/400 V capacitor is 18 A

a Peak inrush current without series resistors is 1200 A

a Peak inrush current with series resistors is only 260 A

a 1200 A is equal to 66 times the rated current

a Inrush current with series resistors is only one fifth of that of the standard contactor

a Substantial difference also in terms of power (integrated area)

a Superior switching behavior of contactor with series resistors compared with a standard contactor, results inextended life cycle of contactors as well as of capacitors

a Improved power quality ensures trouble-free and safeoperation of the PFC system and application

Fig. 3: 12.5 kVAr (18 A /400 V)vertical: 250 A/divhorizontal: 0.5 ms/div

Fig. 4: 12.5 kVAr (18 A /400 V)vertical: 250 A/divhorizontal: 0.5 ms/div

i = 1200 A

i = 260 A

11EPCOS AG

Without precontacts(detuned capacitor)

With precontacts(detuned capacitor)

Without precontacts(non-detuned capacitor)

ComparisonCapacitor bank switching under various conditions

Facts and conclusion

The peak current during switching without using precontacts (Fig. 5) exceeds 4000 A

If capacitors are detuned (Fig. 6) the peak is only 500 A

The latter case shows the influence of inductivity and precontacts of a “capacitor contactor”, the peak current (Fig. 7) is reduced to approx. 200 A

Fig. 5: 25 kVAr (21A/690 V)vertical: 2000 A/divhorizontal: 0.625 ms/div

Fig. 6: 25 kVAr (21 A/690 V)vertical: 200 A/divhorizontal: 10 ms/div

Fig. 7: 25 kVAr (21 A/690 V)vertical: 200 A/divhorizontal: 10 ms/div

i > 4000 A i = 500 A

i < 200 A

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(089) 636-09, FAX (089) 636-2 2689© EPCOS AG 2000. Alle Rechte vorbehalten. Vervielfältigung, Veröffentlichung, Verbreitung und Verwertung dieser Broschüre und ihres Inhalts ohne ausdrückliche Genehmigung der EPCOS AG nicht gestattet. Mit den Angaben in dieser Broschüre werden die Bauelemente spezifiziert, keine Eigen-schaften zugesichert. Bestellungen unterliegen den vom ZVEI empfohlenen Allgemeinen Lieferbedingungen für Erzeugnisse und Leistungen der Elektroindustrie, soweit nichts anderes vereinbart wird. Diese Broschüre ersetzt die vorige Ausgabe. Fragen über Technik, Preise und Liefermöglichkeiten richten Sie bitte an den Ihnen nächstgelegenen Vertrieb der EPCOS AG oder an unsere Vertriebsgesellschaften im Ausland. Bauelemente können aufgrund technischer Erfordernisse Gefahrstoffe enthalten. Auskünfte darüber bitten wir unter Angabe des betreffenden Typs ebenfalls über die zuständige Vertriebsgesellschaft einzuholen.

Published by EPCOS AG, Marketing CommunicationsP.O.B. 801709, 81617 Munich, GERMANY

++49 89 636-09, FAX (089) 636-2 2689© EPCOS AG 2000. All Rights Reserved. Reproduction, publication and dissemination of this brochure and the information contained therein without EPCOS’ prior express consent is prohibited. The information contained in this brochure describes the type of component and shall not be considered as guaranteed characteristics. Purchase orders are subject to the General Conditions for the Supply of Products and Services of the Electrical and Electronics Industry recommended by the ZVEI (German Electrical and Electronic Manufacturers’Association), unless otherwise agreed. This brochure replaces the previous edition. For questions on technology, prices and delivery please contact the Sales Offices of EPCOS AG or the international Representatives. Due to technical requirements components may contain dangerous substances. For information on the type in question please also contact one of our Sales Offices.

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