The invention relates to solutions for filtering the interference of a power bridge comprising controllable switches. Background of the invention

A power bridge, such as the network bridge or the motor bridge of a frequency converter, comprises controllable solid-state switches, such as IGBT transistors, by switching which the power to be supplied via the frequency converter is adjusted. Interference is produced by the operation of solid-state switches, which interference is transmitted into the environment as conducted and as radiated interference. Interference also connects to surrounding structures, causing leakage currents. Leakage currents can cause p.g. tripping of residual current protection.

The interference can be filtered with earthed filter circuits, via which interference is conducted to the ground plane. A problem when using these types of filter circuits is that the interference to be filtered further forms ground leakage currents, which cause tripping of residual cun-ent protectors. Also some electrical safety regulations limit the maximum permitted values for ground leakage current.

Aim of the invention

The aim of the invention is to disclose a solution for reducing the leakage currents produced by operation of a power bridge. To achieve this aim the invention discloses an arrangement according to claim 1, a frequency converter according to claim 8, and also an elevator system according to claim 11. The preferred embodiments of the invention are described in the dependent claims. Some inventive embodiments and also inventive combinations of the various embodiments are also presented in the descriptive section and in the drawings of the present application.

Summary of the invention The arrangement according to the invention comprises a power bridge connected between an AC voltage circuit and a DC voltage circuit, which power bridge comprises controllable switches for supplying electric power between the AC voltage circuit and the DC voltage circuit. The arrangement also comprises an unearthed common-mode filter circuit, which filter circuit is connected between the aforementioned AC voltage circuit and aforementioned DC voltage circuit, in connection with the power bridge. This means that the current path for the interference to be filtered connects from the AC voltage circuit back to the DC intermediate circuit, and consequently it is not necessary to conduct the interference to the ground plane. When interference is not conducted to the ground plane, ground leakage currents produced in the interference filtering dissipate or are at least essentially reduced. The term "unearthed common-mode filter circuit" means that the aforementioned filter circuit is not, in itself, connected to a ground potential from any point whatsoever.

In a preferred embodiment of the invention the power bridge is a bi-level inverter circuit, which comprises for each phase of the AC voltage circuit, connected in series, a high-side switch connected to the positive conductor of the DC voltage circuit and also a low-side switch connected to the negative conductor of the DC voltage circuit. A phase of the AC voltage circuit is connected to the connection point of the aforementioned high-side and low-side switches. In a preferred embodiment of the invention the high-side and the low-side switches are connected with pulse width modulation. A bi-level inverter circuit produces, particularly with pulse-width modulation, strong common-mode interference in the AC voltage circuit, which interference can be effectively filtered by means of the filter circuit according to the invention. In a preferred embodiment of the invention the aforementioned filter circuit is configured to form for the common-mode interference a current path from the AC voltage circuit to the DC voltage circuit. This means that the passage of common- mode interference via the AC voltage circuit to the load being supplied can be prevented by means of the filter circuit. By selecting the components of the filter circuit correctly, switching frequency interference, and the harmonics therefrom, produced by switching of the power semiconductors can be eliminated with the filter circuit according to the invention. In some preferred embodiments of the invention the switching frequency of the power semiconductors is between approx. 3 kilohertz - 10 kilohertz. In some other embodiments the aforementioned switching frequency is over 10 kilohertz but below 150 kilohertz.

In a preferred embodiment of the invention the filter circuit comprises a plurality of Filter capacitors, every other pole of which is connected to a different phase of the AC voltage circuit and every other pole is connected together with the other filter capacitors forming a neutral point. The solution enables a current path for common- mode interference from the AC voltage circuit via the neutral point of the filter capacitors onwards to the DC voltage circuit.

In one preferred embodiment of the invention the filter circuit comprises a resistor, which is connected between the aforementioned neutral point of the filter capacitors and the DC intermediate circuit. The solution enables a current path for common- mode interference via the neutral point of the filter capacitors and onwards via the resistor to the DC voltage circuit. This means that the current flowing via the filter circuit can be limited by aid of the resistor. By means of the resistor the resonances of the filter circuit can also be effectively damped.

In one preferred embodiment of the invention the filter circuit comprises a common- mode choke, which is fitted into the AC voltage circuit between the aforementioned connection point of the filter capacitors and the power bridge. This means that the common-mode choke forms a low-pass filter in the filter circuit, which low-pass filter also prevents e.g. overlarge current in the filter circuit caused by the third harmonic of the fundamental frequency of the AC voltage circuit.

Consequently, the components of the filter circuit are disposed on the current path of common-mode interference in series between the AC voltage circuit and the DC voltage circuit. The series circuit of the common-mode choke, the filter capacitors and the resistor form a series circuit of low-pass and high-pass filters from the viewpoint of common-mode interference current as well as a low-pass filter from the viewpoint of common-mode voltage. The circuit also forms a low-pass filter from the viewpoint of the differential-mode voltage signal. In one preferred embodiment of the invention the AC voltage circuit comprises at the connection point of the aforementioned plurality of filter capacitors and the common- mode choke a connection for the load to be supplied with the power bridge. This means that the filter circuit according to the invention prevents common-mode interference from traveling to the load being supplied. The frequency converter according to the invention comprises a network bridge connected between the AC voltage input and the DC intermediate circuit, which network bridge comprises controllable solid-state switches for supplying electric power between an AC network to be connected to an AC voltage input and the DC intermediate circuit and also a motor bridge connected between the DC intermediate circuit and the AC voltage output, which network bridge comprises controllable electronic solid-state switches for supplying electric power between the DC intermediate circuit and an alternating current motor to be connected to the AC voltage input. The frequency converter comprises a filter arrangement according to the description configured in such a way that the network bridge and/or the motor bridge is a power bridge, in connection with which bridge bridges a filter circuit is connected. This means that the filter arrangement according to the description can be fitted either in connection with a network bridge or in connection with a motor bridge, or the frequency converter can comprise separate filter arrangements fitted in connection both with the network bridge and with the motor bridge. Consequently the common- mode interference connecting to the AC network, the common-mode interference connecting to the supply cables of a motor, or both, can be filtered by means of the filter arrangement. By means of the filtering solution according to the invention a frequency converter can also be used behind residual current protection without the residual current protection triggering as a result of common-mode interference. In one preferred embodiment of the invention a differential-mode LCL circuit is connected to the phases of the network bridge for filtering the current of the AC voltage network. In this case common-mode interference, which the differential-mode LCL circuit is unable to filter, is filtered with the filter circuit according to the description.

In a preferred embodiment of the invention the main circuit of the frequency converter is at a fixed voltage potential. Ground leakage currents, which are produced when connecting a frequency converter to a fixed voltage potential, can be filtered by means of the filtering solution according to the description. The filtering solution according to the description is particularly advantageous if the zero point of the supply network of the frequency converter is permanently connected to a ground potential.

The elevator system according to the invention comprises a hoisting machine, which is configured to drive an elevator car in response to elevator calls. The elevator system also comprises a frequency converter according to the description for driving the hoisting machine. This means that the leakage currents caused by operation of the frequency converter driving the hoisting machine of the elevator can be eliminated by means of the filtering solution according to the description. Likewise the electricity supply of the elevator can be connected behind residual current protection, which improves the electrical safety of the elevator. The arrangement according to the invention also reduces radiated EMC interference.

The preceding summary, as well as the additional features and additional advantages of the invention presented below, will be better understood by the aid of the following description of some embodiments, said description not limiting the scope of application of the invention. Brief explanation of the figures

Fig. 1 presents as a circuit diagram one frequency converter according to the invention. Fig. 2 presents the connection of common-mode interference as an equivalent circuit in the frequency converter of Fig. 1.

Figs. 3a, 3b present circuit diagrams of alternative filter circuits for the frequency converter of Fig. 1. More detailed description of preferred embodiments of the invention

Fig. 1 presents as a circuit diagram a frequency converter having regenerative braking to the network. This type of frequency converter can both take electric power from the electricity network and also return the electrical energy returning in connection with e.g. motor braking back to the electricity network 15. This type of frequency converter can be used e.g. for driving the hoisting machine of an elevator or also for driving the drive machinery of an escalator or travelator. The frequency converter is connected to the electricity network 15 and also to the supply cables of an electric motor 16 with connectors 14, 18.

The frequency converter comprises a network bridge 5, with which the voltage of the AC network 15 is rectified into DC voltage between the positive 2A and negative 2B busbar of the DC intermediate circuit 2 of the frequency converter. The DC voltage of the DC intermediate circuit 2 is further converted by the motor bridge 6 into the variable-amplitude and variable-frequency supply voltage of the electric motor 16.

The network bridge 5 and the motor bridge 6 are both bi-level inverter circuits. The network bridge 5 comprises for each phase LI, L2, L3 of the AC network 15 a high- side IGBT transistor 7A connected to the positive busbar 2A of the DC intermediate circuit as well as a low-side IGBT transistor 7B connected to the negative busbar 2B of the DC intermediate circuit. The phase LI, L2, L3 of the AC network 15 is connected to the connection point of the aforementioned high-side 7 A and low- side 7B IGBT transistors. A typical differential-mode LCL filter module 17, which filters the current of the AC network 15 and stabilizes the adjustment of the network current performed by the network bridge 5, is also connected to the phases LI, L2, L3 of the network bridge 5. The motor bridge 6 comprises for each phase R, S, T of the alternating-current motor 16 a high-side IGBT transistor 8A connected to the positive busbar 2A of the DC intermediate circuit as well as a low-side IGBT transistor 8B connected to the negative busbar 2B of the DC intermediate circuit. The phase R, S, T of the AC motor 16 is connected to the connection point of the aforementioned high-side 8A and low-side 8B IGBT transistors.

The IGBT transistors 7 A, 7B, 8 A, 8B of both the network bridge 5 and the motor bridge 6 are connected by producing with a control circuit, such as with a DSP processor, short, preferably PWM modulated, pulses in the gates of the IGBT transistors. By connecting alternately the IGBT transistors of the high-side 7A, 8 A and of the low-side 7B, 8B, a PWM modulated pulse pattern forms from the DC voltages of the positive busbar 2A and the negative busbar 2B in the AC outputs of the network bridge 5 and of the motor bridge 6, the frequency of the pulses of which pulse pattern is essentially greater than the frequency of the fundamental frequency of the AC voltage. The amplitude and frequency of the fundamental frequency of the output voltages can in this case be changed steplessly by adjusting the modulation index of the PWM modulation.

In the embodiment of Fig. 1 the neutral conductor of the three-phase AC network 15 is earthed in the supply distribution panel, in which case the main circuit of the frequency converter is in fixed contact with ground potential. In this case switching of, in particular, the IGBT transistors 7A, 7B of the network bridge 5 produces common- mode interference, which tries to travel into the AC network 15. The distributed capacitance between the motor windings and the frame of the motor, on the other hand, produces common-mode interference caused by the switching of the IGBT transistors 8A, 8B of the motor bridge 6, which interference travels via the distributed capacitance into the frame of the motor. The common-mode interference of the network bridge 5 also tries to travel into the motor 16.

An unearthed common-mode filter circuit is connected between the phases LI, L2, L3 of the network bridge 5 and the negative busbar 2B of the DC intennediate circuit. said filter circuit comprising, connected in series, a common-mode choke 9, star- connected filter capacitors 10 and also a resistor 12. The filter circuit 9, 10, 12 could also be connected to the positive intermediate circuit busbar 2A, instead of to the negative intermediate circuit busbar 2B, or, if instead of one intermediate circuit capacitor 19 the intermediate circuit has two intermediate circuit capacitors connected in series with each other between the positive 2A and the negative 2B intermediate circuit busbar, the filter circuit 9, 10, 12 could be connected to the connection point of the aforementioned capacitors. The values of the components of the filter circuit 9, 10, 12 are selected in such a way that the filter circuit forms a current path for common- mode interference from the phases LI, L2, L3 of the network bridge 5 to the negative busbar 2B of the DC voltage circuit, in which case the common-mode interference tries to convert into heat in the resistor 12 of the filter circuit.

A similar filter circuit 9, 10, 12 is also formed between the phases R. S, T of the motor bridge 6 and the negative intermediate circuit busbar 2B of the DC intermediate circuit. On the side of the motor bridge 6, however, the choke 9 must have, in addition to common-mode inductance, also differential-mode inductance owing to the capacitors 10 connected to the phases R, S, T of the motor bridge. Since the filter circuit 9, 10, 12 on the motor bridge 6 side is otherwise similar in its operation to the filter circuit of a network bridge 5, in the following only the operation of a filter circuit 9, 10, 12 of a network bridge 5 is described in more detail.

The single-phase equivalent circuit of the filter circuit 9, 10, 12 of the network bridge 5 (and thus also of the motor bridge 6) is presented in Fig. 2.

The following abbreviations are used for the components of the filter circuit: inductance of the common-mode choke 9: L capacitance of the filter capacitors 10: C resistor 12: R

Obtained as the transmission function of the filter circuit 9, 10, 12 is: where:

R

CR ml =

Obtained as the equation for the damping constant D of the transmission function is:

Correspondingly, when the poles of the band-pass filter are on the real axis, obtained as the equation of the transmission function is:

_ I/As) ω _{ι κ}

U _CM {s) {s + ^ )

So that damping of the common-mode interference can be perforaied by selecting for the filter the desired bandwidth eo _LR .

From the preceding equations, obtained as the selection criteria for the components of the filter circuit are: R = G R

L fi¾-L

Selected as the values of the components in the embodiment of Fig. 1 are: inductance of the common-mode choke 9: 1.5 mH values for the filter capacitors 10: lOuF value for resistor: 5Ω.

In this case with the filter circuit 9, 10, 12, damping of 16 decibel common-mode voltage is achieved at a frequency of 10 kilohertz. The filter circuit also damps a 10 kilohertz current with 30 decibel damping as well as a 150 hertz current with 30 decibel damping. Figs. 3a and 3b present some alternative circuit diagrams of the filter circuit, which are suited for use in the frequency converter of Fig. 1.

The filter circuit of Fig. 3a differs from the filter circuit of Fig. 1 in that the filter circuit has three resistors 12, every other pole of which resistors is connected to a filter capacitor 10 and every other pole is connected together with the other resistors 12 into a neutral point 13. The resistors 12 also damp the differential- mode resonances of the filter circuit. The values of the components are selected as follows: filter capacitor 10: 10 uF resistor 12: 15Ω common-mode choke 9: 1.5 mH in which case UCM damping of 16 decibel common-mode voltage is achieved with the circuit at a frequency of 10 kilohertz. The filter circuit also damps a current of 10 kilohertz frequency flowing in the filter circuit with 30 decibel damping as well as a current of 150 hertz frequency with 30 decibel damping. In Fig. 3b the filter circuit comprises three filter capacitors 10 star-connected with each other, every other pole of which filter capacitors is connected to a different phase LI, L2, L3 of the network bridge 5 and every other pole is connected together with the other filter capacitors 10 into a neutral point 13. The neutral point 13 is connected to the negative busbar 2B of the DC intermediate circuit. The filter circuit also comprises a common-mode choke 9, which is connected in series with the filter capacitors 10.

The resonance frequency of the filter circuits of Fig. 1 and Fig. 3a is adjusted to one kilohertz. The filter circuit of Fig. 3b differs from the filter circuits of Fig. 1 and Fig.

3a in that the filter circuit of Fig. 3b does not have a resistor 12 damping the resonances. Consequently the resonance frequency of the filter circuit of Fig. 3b must be selected in such a way that no resonance frequency excitations occur in the system.

The resonance frequency of the filter circuit of Fig. 3b can be selected e.g. to be significantly higher than the filter circuits of Fig. I and of Fig. 3a, e.g. to be approx.

150 kilohertz, in which case also the power losses occurring in the common-mode chokes damp the resonances. In this case the filter circuit of Fig. 3b is not, of course, suited to filtering switching-frequency interference of 10 kilohertz, but the filter circuit can be used for filtering higher- frequency EMC interference.

In this embodiment of the invention the switching frequency of the IGBT transistors 7A, 7B of the network bridge is 10 kilohertz, whereas the switching frequency of the IGBT transistors 8 A, 8B of the motor bridge is 3.5 kilohertz, so that when dimensioning the components 9, 10, 11, 12 of the filter circuit of a motor bridge, the values of the filter components must change the change in switching frequency.

It is obvious to the person skilled in the art that e.g. MOSFET transistors, bipolar transistors or corresponding components can be used, instead of IGBT transistors, as the controllable switches 7A, 7B, 8A, 8B of the network bridge 5 and of the motor bridge 6.

It is further obvious to the person skilled in the art that the network-bridge- side common-mode choke 9 of the filter circuit as well as the capacitors 10 could also be connected between the LCL circuit 17 and the network bridge 5, or between the chokes in the center of the LCL circuit.

The invention is described above by the aid of a few examples of its embodiment. It is obvious to the person skilled in the art that the invention is not only limited to the embodiments described above, but that many other applications are possible within the scope of the inventive concept defined by the claims.