Damping of Inrush Current in Low-Voltage PFC Equipment

Transcription

1 Damping of Inrush Current in Low-Voltage PFC Equipment Low-Voltage PFC Application Note

2 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

3 Damping of Inrush Current in Low-Voltage PFC Equipment General The market trend to reduce losses in modern low-voltage powerfactor-correction capacitors (LV-PFCs) and the requirement for high output density result in reduced ohmic resistance in PFC capacitors. Especially the switching of capacitors in parallel to others of the bank, already energized, causes extremely high inrush current, up to 200 times the rated current, and limited only by the ohmic resistance of the capacitor itself. According to the formula (Eq1), such a capacitor s AC resistance is very low and thus contributes to high inrush current. x c = 1 2*π *f*c Eq1: Switching operation: f x c 0 î 200 * I r M 3~ 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 conductors Automatic capacitor bank with 6 capacitors in parallel EPCOS AG 3

4 Inrush Current by Connecting Capacitor in Parallel (Energization) Capacitor connection: I N = rated current = 21A Current (A) 4000 i Capacitor inrush current t (ms) ON OFF 5 th 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, adequate 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

5 Inrush Measurement of Capacitor Steps PFC capacitor cascade connection: High voltage transients occurrence due to no damping Voltage (V) 1500 û û û st step on 2nd step on 3rd step on 4th step on 5th step on 6th step on t (ms) û High peak voltage (transients) occurrence Voltage at 0.69 kv - busbar Û > U INS risk of shortcircuit 2 Û 0 V results in wrong measurements causing control failures 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. Especially the contact area between electrodes (windings) and the metal-spray layer was extremely stressed by high current forces. The example shows that a fraction 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. Example Metal-spray layer separated from the capacitor windings EPCOS AG 5

6 Inrush Current Calculation Connecting a single capacitor Circuit and formula ^i= 2*Sk *I N Q U N L L 2 1 L 3 Grid KLK Eq 2 Calculation example Terms Peak inrush current ^i A Transformer shortcircuit power Sk kva Rated capacitor output Q kvar Rated capacitor current I N A Rated voltage U N V Ohmic resistance = X C Ω 3*U N2 * (1/Q1+ 1/Q2) Grid impedance = XI Ω o*l (Ω) including contactor fuse busbars Given parameters: Grid connection of a single 50 kvar capacitor, no other capacitor connected: a Grid 400 V/50 Hz a Transformer shortcircuit voltage: 5% a Transformer output: 1600 kva a Capacitor Q = 50 kvar; I N = 72 A ^i = 1600 kva 2* kvar *72 A = 2575 A The inrush current is approximately 35 times the rated current. Result Typical inrush currents are times the rated current for single capacitors during connection. 6 EPCOS AG

7 Various Solutions for Limiting Inrush Current Parallel connecting of capacitor: Serial air coils ^i= 2*U N U N L 1 L 2 L 3 Contactor i= 2*U N X c *X L C 2 C 3 C C 2 3 Capacitor Grid ^ X c * (X L1 +X L2) K n L n U N K 2 L 2 K 1 L 1 L 1 L 2 L 3 Grid C 1 C 1 Q1 Q 2 KLK1707-F Q n Q 2 Q 1 KLK1708-N Eq 3 Eq 4 Given parameters: Connection of a 50 kvar capacitor, other 300 kvar capacitors are already connected: a Grid 400 V/50 Hz a Transformer shortcircuit voltage: 6% a Transformer output: 630 kvar a Q 1 = 50 kvar a Q 2 = 300 kvar a I N = 72 A ; V N = 400 V ; f = 50 Hz 1 1 a X C = 3 * U 2 N * ( + ) = 11.2 Ω a L/phase = 0.4 µh (empirical) a X L = o * L = 2 * π * f * L = mω ^i = Q 1 2*400 V 11.2 Ω*0.125*10 3 Ω Q 2 = A The inrush current is approximately 210 times the rated current. 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 with approx. 6 µh with turns. a X c =11.2Ω a X L1 =2*π *f*l=2*π* 50 * 6 µh = 1.88 mω a X L2 = 2* π *f*l=0.125mω a X L total = = 2 mω a L/phase = 0.4 µh (empirical value) 1) ^i = 2*400 V 11.2 Ω*2 *10 3 Ω = 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). Typical inrush currents are times rated current for single capacitors in parallel connection to 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. 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, diameters and cross sections. 1) For switch gear and connected cables EPCOS AG 7

8 Various Solutions for Limiting Inrush Current Damping as described is a possible simple solution, but this method deals with two contradicting effects: a Longer (or additional) cables cause electrical losses higher losses cause higher inherent temperature within the capacitor. a On the other hand, cable turns reduce the inrush current and increase the life cycle of capacitors and contactors. Plus, you must make sure that the capacitor works below its maximum operating temperature. Selection table for connection cables Capacitor Turns Approx. Cable rating diameter cross-section 5 kvar mm 2.5 mm 2 10 kvar mm 4 mm kvar mm 4 mm kvar mm 6 mm mm 10 mm 2 33 kvar mm 25 mm 2 50 kvar mm 35 mm 2 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 (series anti-harmonic reactors) In detuned capacitor banks the inductivity of filter circuit reactors provides an excellent damping effect for limiting inrush current. The following diagrams show the connection of a detuned and non-detuned (reactor and capacitor) system. The peak current of a conventional capacitor is higher than 4000 A. The peak current of detuned capacitors is only approx. 500 A. The purpose of filter circuit reactors is of course not the damping of inrush current, but this example shows that in the case of detuned capacitors no additional damping measures are required. Conventional capacitor without damping î > 4000 A î = 190 * I N Fig. 1: (21A /690 V) vertical: 2000 A /div horizontal: ms /div Because of the high inductance in the circuit, the breaking quality of the contactor is important to avoid restriking during switch-off. Especially large contactors (oversized motor contactors) may be too slow and are therefore critical. Detuned capacitor with series reactors i = 500 A î = 24 * I N Fig. 2: (21A / 690 V) vertical: 200 A / div horizontal: 10 ms / div Examples for detuned capacitor banks (ref. page 2) 8 EPCOS AG

9 Capacitor contactors with damping resistors Damping resistor Pre-switching aux. contacts How does it work? The series damping resistors are switched by socalled precontacts or auxiliary contacts. The precontact closes before main contacts and preloads the capacitor. a Reduced voltage differences. a The peak current is limited. a The resistor is temporarily in the 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 resistors is only a matter of construction. a During operation (main contacts are closed) the resistor wires are disconnected or shorted out, and do not cause any permanent losses at all. Due to the very short operation time (a few milliseconds only) during switch-on of the contactor, a long life cycle of the damping resistors is ensured. Grid/ Mains Functional diagram Main contacts Precontacts on off on off Note: Due to pre-loading via aux. contacts the capacitor s voltage difference will be reduced. Consequently also the capacitor current according to the formula: on ms Eq 5 Auxiliary switched contact with serial resistor (precontacts) Capacitor contactor (main contacts) Capacitor ^i=c* d V d t EPCOS AG 9

10 Comparison The following two diagrams show the difference between a capacitor s inrush current without and with damping series resistors when a capacitor is switched in parallel to an already energized capacitor bank/unit: Without damping resistors i = 1200 A With damping resistors i = 260 A 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) vertical: 250 A/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

11 Comparison Capacitor bank switching under various conditions Without precontacts (non-detuned capacitor) Without precontacts (detuned capacitor) With precontacts (detuned capacitor) i > 4000 A i = 500 A i < 200 A Fig. 5: (21A/690 V) vertical: 2000 A/div horizontal: ms/div Fig. 6: (21 A/690 V) vertical: 200 A/div horizontal: 10 ms/div Fig. 7: (21 A/690 V) vertical: 200 A/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 EPCOS AG 11

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