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

Application Note 2001

http://www.epcos.com 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 resistors Functionality/comparison 9 Comparison 10 Capacitor bank switching under various conditions 11

2 EPCOS AG Damping of Inrush Current in Low-Voltage PFC Equipment

General The market trend to reduce losses in modern low-voltage power- factor-correction capacitors (LV-PFCs) and the requirement for high output density result in reduced 1 ohmic resistance in PFC capacitors. xc = Especially the switching of capa- π citors in parallel to others of the 2* *f*c bank, already energized, causes extremely high inrush current, up to 200 times the rated current, Eq1: and limited only by the ohmic Switching operation: f ➝∞ x ➝0 î ➝200 I resistance of the capacitor itself. c * r According to the formula (Eq1), such a capacitor’s AC resistance is very low and thus contributes to high inrush current.

M 3~ 25 kVAr 25 kVAr 25 kVAr 25 kVAr 25 kVAr 25 kVAr 25 kVAr 12.5 kVAr

187,5 kVAr KLK1709-W

High inrush current for grid, high balancing currents for capacitors

LV-PFC capacitor bank Inrush current (pulse) is a factor of: a Remaining capacitor voltage due to fast switching in automatic capacitor banks a Shortcircuit power of supply transformer a Output of capacitor switched in parallel to others already energized a Fault level of supply network a Output of capacitors already energized a Ohmic resistance of capacitor itself and distribution switch gear, connection cables or con- ductors

Automatic capacitor bank with 6 capacitors in parallel

EPCOS AG 3 Inrush Current by Connecting Capacitor in Parallel (Energization)

Capacitor connection: IN =rated current = 21A

4000 i 3000 Current (A) Current 2000 Capacitor inrush current 1000

-1000

-2000

-3000 73.2 73.8 74.5 75.1 75.7 76.3 77.0 77.6 t (ms) ON OFF 5th capacitor connected i Peak current occurrence i = 157 * I N = 157 * 21 = 3300 A

The risks of high inrush current Connecting LV-PFC capacitors without damping to an AC grid stresses the capacitor like a shortcircuit. To avoid negative effects and to improve a capacitor’s life time, ade- quate damping of inrush current is highly recommended.

Influence of high inrush current and 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 zero crossing – measurement failure – problems with numerical control equipment – production stop due to computer failure a High cost of maintenance and production standstill 4 EPCOS AG Inrush Measurement of Capacitor Steps

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

1500 û 1 1000 Voltage (V) Voltage

500

0 û 2

-500

-1000 û 1 1st step on 2nd step on 3rd step on 4th step on 5th step on 6th step on -1500 0102030405060708090100 t (ms) û High peak voltage (transients) occurrence 1 1 1 Û > UINS risk of shortcircuit 2 2 Û ≤ 0 V results in wrong measurements causing control failures Voltage at 0.69 kV - busbar

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

Capacitor sample, contact surface damaged by high inrush currents High inrush current occurrences due to insufficient damping caused high electromechanical forces within the capacitor. Espe- cially the contact area between electrodes (windings) and the metal-spray layer was extremely Example stressed by high current forces. Metal-spray layer separated from the The example shows that a fraction capacitor windings of metal-spray layer separated from the windings. Even the MKK capacitor with excellent pulse current capability and enhanced contactability due to wavy cut and heavy edge design of the film shows that extensive power can cause failures.

EPCOS AG 5 Inrush Current Calculation

Connecting a single capacitor Circuit and formula N

U L 1 L 2 ^ 2*Sk L 3 Grid i= *IN Q

KLK1706-7

Eq 2

Calculation example

Terms Given parameters: Grid connection of a single 50 kVAr Peak inrush current^ i A capacitor, no other capacitor connected: Transformer shortcircuit power Sk kVA a Grid 400 V/50 Hz Rated capacitor output Q kVAr a Transformer shortcircuit voltage: 5% Rated capacitor current IN A a Transformer output: 1600 kVA Rated voltage U V N a Capacitor Q = 50 kVAr; IN = 72 A Ω Ohmic resistance = XC 1600 kVA 2 2* 3*UN * (1/Q1+ 1/Q2) ^i = 0.05 = 2575 A *72 A 50 kVAr Grid impedance = XI Ω Ω o*L ( ) including The inrush current is approximately – contactor 35 times the rated current. – fuse – busbars

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

6 EPCOS AG Various Solutions for Limiting Inrush Current

Parallel connecting of capacitor: Serial air coils N N L U 1 U L 1 L L 2 2

L Grid

Grid 3 L 3 ^^2*UN 2*UN K n K 2 K 1 i= Contactor i= C C C C (XL1+ XL2) Xc*XL 2 3 2 3 Xc* L n L 2 L 1 Capacitor

C C Q Q Q 1 1 KLK1707-F n 2 1 KLK1708-N QQ1 2

Eq 3 Eq 4

Given parameters: Given parameters: Connection of a 50 kVAr capacitor, other Parallel connection of a 50 kVAr capacitor 300 kVAr capacitors are already connected: with cable turns (serial aircoils) for damping, a Grid 400 V/50 Hz other 300 kVAr capacitors are already connect- a Transformer shortcircuit voltage: 6% ed, 400 V/50 Hz, shortcircuit power 10.5 MVA, a Transformer output: 630 kVAr rated capacitor current 72 A: damping with a Q1 = 50 kVAr approx. 6 µH with turns. a Q2 = 300 kVAr a Xc =11.2Ω a IN = 72 A ; VN = 400 V ; f = 50 Hz a XL1 =2*π *f*L=2*π * 50 * 6 µH = 1.88 mΩ 1 1 a XL2 = 2* π *f*L=0.125mΩ a XC = 3 * U2N * ( + ) = 11.2 Ω Q1 Q2 a XL total = 0.125 + 1.88 = 2 mΩ a L/phase = 0.4 µH (empirical) a L/phase = 0.4 µH (empirical value)1) a XL = o * L = 2 * π * f * L = 0.125 mΩ ^i = 2*400 V = 3780 A ^i = 2*400 V = 15118.6 A 11. 2 Ω*2 *10–3 Ω Ω –3 Ω 11. 2 *0.125*10 The inrush current is approximately The inrush current is approximately 50 times the rated current. This means only 210 times the rated current. about a quarter compared to a capacitor without damping (turns).

Typical inrush currents are This example shows that some cable 100–250 times rated current for turns in series with the capacitor single capacitors in parallel connection contribute to reducing inrush current to other capacitors in operation. (to 50 times rated current). This improves capacitor life cycle.

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

1) For switch gear and connected cables

EPCOS AG 7 Various Solutions for Limiting Inrush Current

Damping as described is a possible Selection table for connection cables simple solution, but this method deals with two contradicting effects: Capacitor Turns Approx. Cable a rating diameter cross-section Longer (or additional) cables 2 cause electrical losses – higher 5 kVAr 10 100 mm 2.5 mm losses cause higher inherent 10 kVAr 10 100 mm 4 mm2 temperature within the capacitor. 12.5 kVAr 10 100 mm 4 mm2 a On the other hand, cable turns 16.7 kVAr 7 100 mm 6 mm2 reduce the inrush current and 25 kVAr 7 100 mm 10 mm2 increase the life cycle of capaci- 2 tors and contactors. 33 kVAr 7 100 mm 25 mm 2 Plus, you must make sure that the 50 kVAr 7 100 mm 35 mm capacitor works below its maximum operating temperature. This table should help to find the appropriate cable and required turns. Our PFC-CDROM (available upon request) contains calculation software which enhances precise calculation of the application (capacitors and switch gear).

Detuning reactors Conventional capacitor Detuned capacitor (series anti-harmonic reactors) without damping with series reactors In detuned capacitor banks the inductivity of filter circuit reactors provides an excellent damping i = 500 A effect for limiting inrush current. î > 4000 A The following diagrams show the connection of a detuned and non-detuned (reactor and capacitor) system. î = 190 * IN î = 24 * IN The peak current of a conventional capacitor is higher than 4000 A. The peak current of detuned capac- Fig. 1: 25 kVAr (21A /690 V) Fig. 2: 25 kVAr (21A / 690 V) itors is only approx. 500 A. The vertical: 2000 A /div vertical: 200 A / div purpose of filter circuit reactors is horizontal: 0.625 ms /div horizontal: 10 ms / div of course not the damping of inrush current, but this example Because of the high inductance in Examples for detuned shows that in the case of detuned the circuit, the breaking quality capacitor banks (ref. page 2) capacitors no additional damp- of the contactor is important to ing measures are required. avoid restriking during switch-off. Especially large contactors (over- sized motor contactors) may be too slow and are therefore critical.

8 EPCOS AG Capacitor contactors with damping resistors

Damping resistor

Pre-switching aux. contacts

How does it work? Auxiliary switched The series damping resistors are contact with serial switched by socalled precontacts Grid/ resistor (precontacts) or auxiliary contacts. The precon- Mains Capacitor contactor tact closes before main contacts (main contacts) and preloads the capacitor.

a Reduced voltage differences. a The peak current is limited. a The resistor is temporarily in the Capacitor circuit 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 damping Functional diagram resistors is only a matter of con- struction. on off a During operation (main contacts Main contacts are closed) the resistor wires are on off on disconnected or shorted out, and Precontacts do not cause any permanent 2...10 ms losses at all. Due to the very short operation time (a few milliseconds Note: only) during switch-on of the Due to pre-loading via aux. contactor, a long life cycle of the contacts the capacitor’s voltage difference will be ^ dV damping resistors is ensured. reduced. Consequently i=C* also the capacitor current dt according to the formula:

Eq 5

EPCOS AG 9 Comparison

The following two diagrams show Without With the difference between a capa- damping resistors damping resistors citor’s inrush current without and with damping series resistors when a capacitor is switched in i = 1200 A parallel to an already energized capacitor bank/unit: i = 260 A

Fig. 3: 12.5 kVAr (18 A/400 V) Fig. 4: 12.5 kVAr (18 A/400 V) vertical: 250 A/div vertical: 250 A/div horizontal: 0.5 ms/div horizontal: 0.5 ms/div

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 in extended life cycle of contactors as well as of capacitors a Improved power quality ensures trouble-free and safe operation of the PFC system and application

10 EPCOS AG Comparison Capacitor bank switching under various conditions

Without precontacts Without precontacts With precontacts (non-detuned capacitor) (detuned capacitor) (detuned capacitor)

i > 4000 A i = 500 A

i < 200 A

Fig. 5: 25 kVAr (21A/690 V) Fig. 6: 25 kVAr (21 A/690 V) Fig. 7: 25 kVAr (21 A/690 V) vertical: 2000 A/div vertical: 200 A/div vertical: 200 A/div horizontal: 0.625 ms/div horizontal: 10 ms/div horizontal: 10 ms/div

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

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