DE102007007287A1 - Electrical power supply unit for three-phase welding assembly has has three mains power input chokes and output capacitors - Google Patents

Abstract

An electrical power supply unit for e.g. a welding assembly has a three-phase mains power supply of known inductivity L2 at a known frequency fn. The power supply has a rectifier with three mains inputs and two direct current outputs. The power supply unit further has three mains power input chokes whose inductivity L1 together with the inductivity L2 produces a combined inductivity L. The direct current output is via an inverter at a target frequency fas, whose outputs are linked by capacitors of capacity C, which together with the combined inductivity L have a resonant frequency f0 close to that of the target working frequency fas. The inverter actual working frequency far is a whole number multiple N of the mains power supply frequency fn. The actual working frequency far is approximately the same as that of the target working frequency far.

Description

The invention relates to a three-phase, connectable to a three-phase power network power electronics device, wherein the voltage network has a known inductance L ₂ and network frequency f _n with a power output to which a consumer, z. B. a welding machine can be connected. Such power electronic devices have a network-side rectifier, an output-side inverter and an intermediate DC link capacitor.

Of the DC link capacitor allows the functionality the voltage injected inverter, because the DC link capacitors a voltage source for the inverter, ie Forming DC / AC converters. The voltage source, so The DC link capacitor is usually a capacitor bank executed, which from the network side over the rectifier is charged and discharged from the output side. The inverter is needed This also reactive power, the periodic to the consumer, so handed over to the load and taken over by this will and back to DC link led becomes. The DC link capacitor is thus constantly charged and discharged.

Around When a capacitor to reach a certain life, the need to Charge or discharge currents, So the capacitor AC currents per Capacitor size limited become. Execution As a capacitor bank here is a solution, since several capacitors are connected in parallel and here by the DC link alternating current several capacitor AC currents distributed. One consequence is that because of the lifespan condition of the DC link capacitor the capacitance values are relatively large. In the case of electrolytic capacitors z. B. about 4 mF required per 10 kVA output power

such DC link capacitors must first be summoned to then connect to the network. Would be the capacitor not pre-charged, charge currents of the order of magnitude arise of short-circuit currents, which is not allowed.

in the Operation, ie if the DC link capacitor is connected to the mains is, are the mains currents pulse shape with high, not allowed harmonic distortion (total harmonic distortion - THD). The network reactions of the power electronic device are too high and not allowed. Here is also the netzeffektive Current value much higher as if the currents sinusoidal would, the crest factor is higher.

to Troubleshooting There are several possibilities. For example, it is known additional Inductors on the network side or other capacitors in the DC link to place. Both solutions cause additional Cost, space requirements and volume in the device. According to DIN standard, the additionally necessary inductances on the net side so chosen be that their short-circuit voltage a maximum of 4% of the mains voltage is, why very big Chokes are necessary. A huge capacitor limits to one the device lifetime and poses an explosion hazard to the device. By the explosion and the energy stored in the capacitor is in case of failure in usually the power electronics device and the semiconductor devices completely destroyed.

In a power electronics device known today z. B. electrolysis DC link capacitors with a capacity of 40 mF per 100 kVA output power needed. A single capacitor has a capacitance of 500 μF at 450 V maximum voltage and 1 A maximum alternating current. The DC link alternating current is 160 A. Two capacitors each are connected in series with one another capacity of 250 μF. Of these, 160 series circuits, so a total of 320 capacitors needed.

aim in the development of power electronics devices it is an improved Network feedback to achieve, so to achieve a smaller harmonic distortion, almost sinusoidal net flows to pull out of the power grid. In addition, a cost reduction and sought a volume and weight savings. The above Precharging the capacitors before switching on the device requires a Vorladeschaltung, which possible is to be avoided. In case of failure should as possible Low energy storage exists to reduce the risk of explosion to diminish.

task The present invention is to provide an improved power electronic device.

The Invention is based on the knowledge, fewer capacitors in the DC link provided. Capacitors with smaller capacities should be suitable for higher alternating currents be. Such capacitors are z. B. film capacitors or Polypropylene capacitors.

The object is achieved by a three-phase power electronic device, with a power output to which a consumer can be connected, and which can be connected to a three-phase voltage network of known inductance L ₂ and line frequency f _n . The power electronic unit contains a rectifier with three mains inputs and two DC voltage outputs. At each of the power inputs is at its one end a throttle is on closed, which is connectable with their respective other end to the voltage network. The chokes are chosen so that their own inductance L ₁ together with the inductance L _{2 of} the voltage network results in a predefinable, ie desired total inductance L. An inverter with its DC voltage inputs is connected to the DC outputs of the rectifier. The inverter has a nominal operating frequency f _as , with which it can be operated as desired. Between the DC voltage outputs of the rectifier and thus the DC voltage inputs of the inverter, a capacitor is connected, the capacitance C is chosen so that it has a resonant frequency f ₀ together with the total inductance L, ie forms a corresponding resonant circuit, wherein the capacitance C is selected in this case in that the resonant frequency f _{0 is} at least approximately equal to the desired operating frequency f _as . Possibly deviating from the desired operating frequency, however, an actual operating frequency f _{ar is} selected for the inverter according to the invention, which represents an integer multiple of the network frequency f _{n of} the voltage network. So f _ar = N · f _n = f _as . The actual operating frequency f _ar or the integer multiple N is thus chosen so that at least approximately the actual operating frequency f _{ar is} equal to the desired working frequency f _as .

According to the invention, therefore, much smaller DC link capacitors than previously used in the power electronic device, which are, however, precisely determined in terms of their capacity or exactly in the remaining sizes of the power electronics device, in particular the actual operating frequency f _ar fit. The actual working frequency, ie the choice of the integer multiple N takes into account not only z. For example, the theoretical dimensioning values (f _as in the design of a corresponding power electronic device, but rather the values of the capacitances C actually used later in the production or resulting resonant frequencies f ₀ .

The higher the desired working frequency f _as and thus the actual operating frequency f _ar in the power electronic device, the easier it is to provide a smaller capacitor.

At operating frequencies in the range 1 kHz to 10 kHz and a network frequency of usually 50 Hz, there are several choices for the multiple N in order to achieve the approximate equality of working frequency f _ar and target working frequency f _as . N is therefore selectable here in certain areas z. B. ± 3 to 5 to still meet the conditions that f _ar and f _{as are} at least approximately equal.

However, the multiple N can also be chosen such that the distance between the actual operating frequency f _ar and the desired operating frequency f _{as is} minimal. The multiple N is then clearly defined.

Even during operation of the power device, capacities or other size may change. The actual operating frequency f _ar , ie the multiple N, can then also be variable during operation of the power electronic device in steps of the network frequency f _n . The value of the multiple N is thus changed in integer steps during operation in order to achieve an optimum adaptation of the actual operating frequency f _ar to the desired operating frequency f _as and thus the resonance frequency f ₀ . Thus, in addition to the production spread of components in the manufacture of the power electronic device and the change of components, eg. B. also in their replacement or aging, bill taken.

The Throttle can be selected be that the voltage dropping during operation at the throttle at most one percent of the phase voltage of the voltage network is. The in the power electronic device Converted apparent power then corresponds to only one percent of Output power of the device and is thus well below the z. In Germany (DIN) demanded four percent.

In operation, the intermediate circuit capacitor has a maximum nominal operating voltage U _B and the rectifier has a maximum nominal operating current I _B. The capacitor can then be selected such that its permissible maximum voltage U _max is sufficient to store the energy stored in the reactor at nominal operating current I _B. The intermediate circuit capacitor can then take over the energy stored in the network inductances without unduly high voltages occurring in the power electronic device. This case is z. B. necessary for stopping or switching off the device. In such a calculation, the energy of the load inductances or other energy storage in the power electronics unit and consumers can be included. The maximum voltage U _max must then be designed so that the energy stored in the chokes, the load inductance and other energy storage can be absorbed in the capacitor.

Of the In particular, a consumer may be a welding device. Especially with welding machines very high load currents and thus in particular the problems mentioned above, so that the power electronic device according to the invention here special Offers advantages.

For a further explanation of the invention is based on the embodiments of the drawings directed. They show, in each case in a schematic outline sketch:

1 a welder with integrated power electronic device at the power supply,

2 Current and voltage curves of the power electronic device 1 in operation at unfavorably selected operating frequency,

3 Current / voltage curves of the power electronics unit off 1 in accordance with the invention selected operating frequency, during operation and during a switch-off.

1 shows a welding system 2 connected to a three-phase power supply 4 the mains frequency f _n and self-inductance L ₁ and mains voltage of 3 × 380 V-700 V via three mains connections 6a -C is connected. The welding system 2 includes a power electronic device 8th and a welder 10 , The power electronic device 8th here has three network connections 6a -C and two output terminals 12a , b on, where the welding machine 10 connected. During operation, the welding device prevails 10 a voltage U of 10 V, whereby an alternating current I of 10 kA flows, so that at a welding head 14 a power of 100 kW is delivered.

The power electronic device 8th includes a rectifier 16 with three AC voltage inputs 18a -C and two DC outputs 20a , b, one between the DC outputs 20a , b connected DC link capacitor 22 and an inverter 24 that with its DC inputs 26a , b at the DC voltage outputs 20a , b of the rectifier 16 is connected and the output terminals 12a , b for delivering an AC voltage. The output AC voltage here has the working frequency f _ar .

The inverter 24 is with respect to its DC inputs 26a , b and output terminals 12a , b as H-bridge with a total of four IGBTs 28 executed.

The AC voltage inputs 18a -C of the rectifier 16 are with the power outlets 6a -C via one throttle each 30 connected. In the example, for 100 kW output power at 450 V mains voltage, the inductance L _{1 of} the chokes 30 so chosen, together with the self-inductance L _{2 of} the voltage network 4 to achieve a total inductance L of 100 μH, so that the short-circuit voltage according to DIN standard is less than 4%. The value of 4% would be reached with L = 200 μH.

2 shows the current / voltage conditions in the power electronics unit 8th when the capacitance C of the DC link capacitor 22 unfavorable, namely to 40 mF at an operating frequency f _ar of 1 kHz is selected. The welding machine power is 100 kW, ie the welding current I = 10 kA and the welding voltage U = 10V. The input chokes L ₁ are dimensioned with 200 μH (4%). All diagrams in 2 are plotted over time t.

The top diagram in 2 shows the three mains currents I _r , I _s and I _t through the chokes 30 , It can be seen that these vibrate vigorously, namely by 400 A, which is unfavorable for the network load. The second diagram shows the intermediate circuit voltage or capacitor voltage U _C of the DC link capacitor 22 which also vibrates. The third diagram shows the H-bridge output current I _A and the rectifier current, ie the current into the capacitor 22 , The fourth diagram shows the welding current of 10 kA. The fifth diagram in 2 it can be seen that the harmonic distortion is 0.37%. At time t = 240 ms, the welding machine is switched off.

3 shows the current / voltage diagrams in the power electronic device 8th in the inventive choice of its characteristics. The capacitor now has C = 250 μF, the inductance L ₁ is 100 μH (2%) The THD is now 29%, the welding current again 10 kA, the welding power so 100 kW.

The starting point here was the selected or predetermined total inductance L of 100 μH. Since the operating frequencies f _{ar of} the inverter should be about 1 kHz, so the desired working frequency f _{as is} equal to 1 kHz, the capacitance C of the DC link capacitor 22 With C = 1 / (4π 2 f 2 as L) = 254 μF determined. As a real capacitor 22 Here, one of the capacitance C was chosen equal to 250 uF, which is why the actual working frequency f 0 = 1 / (2π√ LC ) as 1007 Hz. Finally, an actual resonant frequency f _{ar was} chosen which is an integer multiple N of the line frequency f _n of 50 Hz and which is closest to the theoretically determined frequency f ₀ of 1007 Hz. With N = 20 this results in f _ar = 1000 Hz. Hereby applies | f _ar - f _as | is minimal.

In the event that the capacity of the real capacitor 22 scatters, z. B. 240 μF, results from the above equation, a resonant frequency of about 1027 Hz, so that in this case N = 21 is selected and an actual operating frequency f _ar of 1050 Hz is selected. This is also true once again | f _ar - f _as | is minimal.

3 shows again in accordance with the illustration in 2 the choke currents, which now only a small residual ripple in contrast to 2 exhibit. The harmonic THD in this Case is only 29% so 0.29, so much better compared to 2 , The capacitor voltage also has significantly less ripple. Output current and output voltage U _A , I _A are opposite 2 unchanged. However, the grid inductance is twice smaller than in 2 and the capacitance of the capacitor 22 four hundred times smaller. C and L are thus in resonance and the resonance frequency is approximately equal to the switching frequency.

gewählt. So steigt die Spannung U_C am Zwischenkreiskondensator 22 nicht auf einen unzulässig hohen Wert.At time t ₀ is the power electronics device 8th off. The DC link capacitor 22 must therefore the whole, in the chokes 30 record stored energy, which is why its voltage U _C rises sharply. Starting from the operating voltage U _{B of} the capacitor 22 and the operating current I _B through the chokes 30 In normal operation, however, the dielectric strength of the capacitor is 22 with the minimum value

selected. Thus, the voltage U _C at the DC link capacitor increases 22 not to an inadmissibly high value.

2

welding system

4

voltage network

6a-c

mains connection

8th

Power electronics unit

10

welding machine

12a, b

output terminals

14

welding head

16

rectifier

18a-c

AC input

20a, b

DC output

22

Link capacitor

24

inverter

26a, b

DC input

28

IGBT

30

throttle

C

capacity

L _1.2 , L

inductance

f _n

power frequency

f _ar , f _as

Operating frequency

t

Time

I _{R, S, T}

inductor current

THD

THD

U _C

capacitor voltage

I _A

output current

U

tension

I, I _DC

electricity

Claims (6)

Three-phase, on a three-phase voltage network ( 4 ) known inductance (L ₂ ) and power frequency (f _n ) connectable power electronic device ( 8th ) with a power output ( 12a (b) where a consumer ( 10 ), - with a rectifier ( 16 ) with three network inputs ( 18a -C) and two DC voltage outputs ( 20a , b), - with three at each power inlet ( 18a -C) connected to the power supply ( 4 ) connectable throttles ( 30 ), the throttles ( 30 ) are selected such that their inductance (L ₁ ) together with the inductance (L ₂ ) of the voltage network results in a predefinable total inductance (L), - with one, with its DC voltage inputs ( 26a , b) at the DC voltage outputs ( 20a , b) connected, a nominal operating frequency (f _as ) and the power output ( 12a , b) having inverters ( 24 ), - with one between the DC outputs ( 20a , b) connected capacitor ( 22 ), whose capacitance (C) is chosen such that, together with the total inductance (L), it has a resonant frequency (f ₀ ) which is at least approximately equal to the desired operating frequency (f _as ), - where for the inverter ( 24 ) an actual operating frequency (f _ar ) is selected as integer multiple (N) of the line frequency (f _n ) so that the actual operating frequency (f _ar ) is at least approximately equal to the desired operating frequency (f _as ). Power electronics device ( 8th ) according to claim 1, wherein the multiple (N) is selected such that the distance between actual operating frequency (f _ar ) and target operating frequency (f _as ) is minimal. Power electronics device ( 8th ) according to claim 1 or 2, wherein the actual operating frequency (f _ar ) during operation of the power electronic device in steps of the mains frequency (f _n ) is variable. Power electronics device ( 8th ) according to one of the preceding claims, in which the throttle ( 30 ) is selected so that in operation at the throttle ( 30 ) falling voltage at most one percent of the phase voltage of the voltage network ( 4 ) is. Power electronics device ( 8th ) according to one of the preceding claims, wherein the capacitor ( 22 ) In operation, a maximum rated operating voltage (U _b ) and the rectifier ( 16 ) have a maximum nominal operating current (I _b ) at which the capacitor ( 22 ) is selected so that its permissible maximum voltage (U _max ) for storing the nominal operating current (I _b ) in the throttle ( 30 ) stored energy is sufficient. Power electronics device ( 8th ) according to one of the preceding claims, in which the consumer ( 10 ) is a welder.