Damping of Inrush Current in Low Voltage PFC Equipment

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Damping of Inrush Current in Low-Voltage PFC Equipment Low-Voltage PFC

Application Note 2001


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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


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Damping of Inrush Current in Low-Voltage PFC Equipment General

The 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 capacitors 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 bank

Inrush current (pulse) is a factor of:

a Remaining capacitor voltagedue to fast switching in automatic capacitor banks

a Shortcircuit power of supply transformer

a Output of capacitor switchedin parallel to others already energized

aFault level of supply network

a Output of capacitors already energized

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

KLK1709-W

2 5 k V A r

2 5 k V A r

2 5 k V A r

2 5 k V A r

2 5 k V A r

2 5 k V A r

1 2 . 5

k V A r

2 5 k V A r

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 ∞ © xc0 © î 200 * Ir



Inrush Current by ConnectingCapacitor in Parallel (Energization)

The risks of high inrush current

Connecting LV-PFC capacitors without 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 lifetime

a Welding or fast wearing out of the main contacts of contactors

a 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 tocomputer failure

a High cost of maintenance andproduction 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

C u r r e n t ( A )

Capacitor inrush current

ONOFF 5th capacitor connected

Peak current occurrence

i = 157* IN = 157*21 = 3300 A i

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

V o l t a g 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 shortcircuit Û ≤ 0 V results in wrong measurementscausing control failures

1

2

5EPCOS AG

Inrush Measurement of Capacitor Steps

Switching of power factor correction (PFC) capacitors is not only related to high currents but alsoto high voltage transients (ref.capacitor switching-on steps 1–6),causing degradation of power quality, if the negative influenceis not prevented by damping.

Capacitor sample,contact surface damagedby high inrush currents

High inrush current occurrencesdue to insufficient dampingcaused high electromechanicalforces within the capacitor. Espe- cially the contact area betweenelectrodes (windings) and themetal-spray layer was extremely stressed by high current forces.

The example shows that a fractionof metal-spray layer separatedfrom the windings. Even the MKK capacitor 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

Example

Metal-spray layer separated from thecapacitor windings



Inrush Current Calculation

KLK1706-7

G r i d

L 3

L

L 2

1 U N

Connecting a single capacitor

Circuit and formula

Terms

Result

Peak inrush current i A Transformer shortcircuit power Sk kVA Rated capacitor output Q kVAr Rated capacitor current IN A Rated voltage UN V

Ohmic resistance = X C Ω

3 *UN2

* (1/Q1+ 1/Q2)

Grid impedance = XI Ω

o*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 kVA a 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 kVA

2* 0.05 *72 A 50 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 kVAr a Q1 = 50 kVAr a Q2 = 300 kVAr a IN = 72 A ; V N = 400 V ; f = 50 Hz

a X C = 3 * U2N * ( + ) = 11.2 Ω

a L /phase = 0.4 µH (empirical)a X L = 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 V

11.2 Ω*0.125*10–3 Ω

1Q2

1Q1

7EPCOS AG

Various Solutionsfor Limiting Inrush Current

KLK1707-F

3C

1C

2C

C 1

1Q Q 2

C 2 3

C

Capacitor

Contactor

G r i d

L 3

L

L 2

1 U N

KLK1708-N

nL

K n K

L 2

2

1L

K 1

2L

L 1

L 3 G r i d

Q n Q 2 Q 1

U N

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 connected, 400 V/50 Hz, shortcircuit power 10.5 MVA,rated capacitor current 72 A: damping withapprox. 6 µH with turns.a X c = 11.2Ωa X L1 = 2 *π * f * L = 2 * π * 50 * 6 µH = 1.88 mΩ

a X L2 = 2 * π *f *L =0.125 mΩ

a X L total = 0.125 + 1.88 = 2 mΩ

a L /phase = 0.4 µH (empirical value)1)

i = = 3780 A

The inrush current is approximately 50 times the rated current. This means only about a quarter compared to a capacitor without damping (turns).

2*400 V

11.2 Ω*2 *10–3Ω

Typical inrush currents are

100–250 times rated current for single capacitors in parallel connectionto other capacitors in operation.

This example shows that some cable

turns in series with the capacitor contribute to reducing inrush current(to 50 times rated current).This improves capacitor life cycle.

i=2*UN

X c*X L

i=2*UN

X c*(X L1+ X L2)

^ ^

Eq 3 Eq 4

^^

1) For switch gear and connected cables



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

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

a On the other hand, cable turnsreduce the inrush current andincrease the life cycle of capacitors 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 capacitor) system.

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

Examples for detunedcapacitor banks (ref. page 2)

Selection table for connection cables

Detuned capacitor with series reactors

Various Solutions for Limiting Inrush Current

Capacitor Turns Approx. Cablerating diameter cross-section

5 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 /div horizontal: 0.625ms /div

Fig. 2: 25 kVAr (21A / 690 V)vertical: 200 A / div horizontal: 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 capacitor without damping



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 resistor wires 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 permanent losses at all. Due to the very short operation time (a few milliseconds

only) 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

Precontacts2...10 ms

on

Grid/ Mains

onoff

onoff

Functional diagram

i=C*dV

dt

^

Note:Due to pre-loading via aux.contacts the capacitor’svoltage difference will bereduced. Consequently also the capacitor current according to the formula:

Damping

resistor

Pre-switching

aux. contacts

Eq 5



With

damping resistors

Without

damping resistors

The following two diagrams show the difference between a capacitor’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 fifthof that of the standard contactor

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

a Superior switching behavior of contactor with seriesresistors 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/div horizontal: 0.5 ms/div

Fig.4 : 12.5 kVAr (18 A/400 V)


i = 1200 A

i = 260 A



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 andprecontacts of a “capacitor contactor”, the peak current

(Fig.7) is reduced to approx. 200 A

Fig. 5: 25 kVAr (21A /690 V)


Fig. 6: 25 kVAr (21 A/690 V)

vertical: 200 A/div horizontal: 10 ms/div

Fig. 7: 25 kVAr (21 A/690 V)

vertical: 200 A/div horizontal: 10 ms/div

i > 4000 A i = 500 A

i < 200 A



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Damping of Inrush Current in Low Voltage PFC Equipment

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