Power converter and method for controlling a power converter The present invention relates to a power converter and a method for controlling a power converter, especially

controlling an electronic smoothing inductor in a power converter . Power converters connected to a three-phase AC mains power supply typically use a rectifier, for example a three-phase diode bridge rectifier, to transform the mains AC voltage into a DC voltage. Such power converters connected to a three-phase network are required to comply with the IEC61000- 3-12 standard relating to Electromagnetic compatibility

(EMC) , which defines maximum values for THDi (Total Harmonic Distortion of Current) and PWHD (Partial Weighted Harmonic Distortion) , as well as values for the harmonics of order 5, 7, 11 and 13 of the fundamental input frequency of the input current. Traditionally, in order to reduce harmonic currents drawn from the mains power supply by a load connected to the DC link of the power converter, a smoothing inductor is arranged at the DC side of the rectifier. More recently, however, a smaller inductor with a

electronically controlled variable impedance, a so called Electronic Smoothing Inductor (ESI), is being used instead of a rather large and expensive inductor which would otherwise be required to reduce the harmonic distortions below the values defined by the above standard. The ESI is able to control the output current of the rectifier bridge and makes it possible to reduce not only mains current harmonics but also output voltage ripple. Examples of three-phase

rectifiers employing such an ESI are disclosed in references [1] , [2] , [3] and [4] .

The examples given in references [1], [2] and [3] aim at a perfect tuning of the electronic smoothing inductor such that it draws perfectly smooth DC current from the bridge

rectifier. Such perfectly smooth DC current is not desirable since the mains input currents are three step square waves with sharp edges which cause large higher order harmonic currents. Reference [4] discloses a power converter in which specifically proportions of harmonics of orders 6 and 12 of the fundamental input frequency are added to reduce the total harmonic distortion of the input current waveform. This approach, however, requires additional computation by for example a digital controller.

It is therefore an object of the present invention to control the shape of the current in an electronic smoothing inductor such that higher order harmonic currents are reduced, thereby allowing realising compliance with standard requirements.

According to a first aspect of the invention, a power

converter is provided which comprises a rectifier bridge connected to an AC mains power supply, a DC link inductor realised as an electronic smoothing inductor circuit, a controller for controlling the electronic smoothing inductor circuit, and a DC link capacitor, wherein the controller uses a ripple on the voltage across a capacitor of the circuit to control the shape of the current drawn from the AC mains power supply.

According to a second aspect of the invention, a method for controlling a power converter is provided, wherein the power converter comprising a rectifier bridge connected to an AC mains power supply, a DC link inductor realised as an

electronic smoothing inductor circuit, a controller for controlling the electronic smoothing inductor circuit, and a DC link capacitor, and wherein the method comprises the step of using, by the controller, a ripple on the voltage across a capacitor of the circuit for controlling the shape of the current drawn from the AC mains power supply. According to a third aspect of the invention, a variable speed drive is provided which uses a power converter

according to the first aspect of the invention. Exemplary embodiments of the invention as well as related advantages will now be described with reference to the following figures, where

FIG 1 shows an exemplary power converter circuit, and FIG 2 shows an exemplary control block diagram for the

ESI of the power converter circuit.

FIG 1 shows an exemplary converter circuit comprising of a rectifier bridge RB connected to an AC mains power supply, a electronic smoothing inductor ESI circuit, an ESI controller EC for controlling the electronic switches Tl, T2 of the ESI, and a smoothing (DC link) capacitor Cdc, and connected at the DC side to a load. Such circuit is well known from references [1] to [4], for example fig. 5 of reference [1] or fig. 1 of references [2], [3] and [4], respectively, and described in more details in these documents.

FIG 2 shows an exemplary block diagram of a control loop realised in the control means CM, and explained in the following with reference to FIG 1.

The voltage Vc across the capacitor C in the ESI inherently has a ripple. This ripple voltage Vc is a function of the operation of the ESI circuit. Energy is taken out of the ESI capacitor C when the voltage across the rectifier bridge VIn+ is less than the voltage Vdc+ across the smoothing capacitor Cdc, and energy is stored into the capacitor C when VIn+ is greater than Vdc+ . This charging and discharging of capacitor C causes the ripple voltage while the shape of the ripple voltage is dependent on the ripple voltage seen at VIn+ and the amount of required Vdc+ ripple voltage. The ripple voltage Vc increases as the load current Iload drawn from the ESI increases and thus increases as the input current drawn from the rectifier bridge RB increases.

According to the invention, the ripple voltage Vc which is inherently present across the capacitor C in the ESI is fed into the voltage loop of the ESI controller EC. This results in the ripple voltage being superimposed on the controller' s current set point and, as a consequence, being present in the ESI inductor L and hence in the currents of the AC mains power supply input phases.

In the exemplary block diagram of FIG 2, the voltage Vc measured across the ESI capacitor C is compared with the capacitor's set point voltage. The difference, which

comprises the ripple voltage, is then subjected to a variable gain K.

The adjusted difference is compared with the resulting difference of a comparison of the measured DC link voltage Vdc+ across the DC link capacitor Cdc with the measured rectified mains voltage Vln+, the latter being additionally filtered by a low-pass filter LPF.

Subsequently, the difference is introduced into a

proportional regulator P and an integral regulators I arranged in parallel, the sum of which determines the current set point.

The current set point is compared with the measured load current Iload feedback from the ESI inductor L. The resulting difference of this comparison is then introduced into to a high pass filter HPF, a proportional regulator P and an integral regulators I, all arranged in parallel. The

resulting signal is fed into a pulse width modulator PWM which controls the electronic switches Tl, T2 of the ESI.

The gain stage K determines the extent of the presence of the ripple in the input phase currents. If, by increasing the gain, more ripple is applied the lower order harmonics will increase while the higher order harmonics and the partial weighted current will decrease. The gain stage K can thus be used to tune the harmonic currents to the desired result.

The ESI controller according to the invention uses a signal inherently present within the ESI circuit to cause the ESI controller EC to demand a shaped current waveform. This has the advantage that no additional circuitry or complexity is required to achieve input phase currents which comply with the above mentioned IEC61000-3-12 standard.

References

[1] H. Ertl, J.W. Kolar, F.C. Zach, A constant output current three-phase diode bridge rectifier employing a novel

"Electronic Smoothing Inductor", IEEE Transactions on

Industrial Electronics, vol. 52, no. 2, pp. 454-461, Apr.

2005

[2] K. Mino, M.L. Heldwein, J.W. Kolar, Ultra Compact Three- Phase Rectifier with Electronic Smoothing Inductor, Applied Power Electronics Conference and Expo, vol. 1, pp. 522-528, Mar. 2005

[3] Y.V. Singh, P.O. Rasmussen, T.O. Andersen, H. Shaker, Modelling and control of three phase rectifier with

electronic smoothing inductor, IECON 2011, 37 ^th Annual

Conference on IEEE Industrial Electronics Society, pp. 1450- 1455, Nov. 2011

[4] US 8, 964, 425 B2