Abstract:
Apparatus for transferring power between an electricity network (U P ) operating on alternating-current electricity and a multiphase electric machine (M2, M3), which apparatus comprises low-voltage power cells (C S , C 11  . . . C N6 ) operating on a cascade principle, which power cells comprise a single-phase output connector (OUT), and at least one transformer (T A , T 1  . . . T N ), comprising for each power cell connected to it a single-phase or multiphase winding dedicated to the specific power cell, which transformer comprises at least one additional winding (W A , W B1  . . . W BN ) connected to the same magnetic circuit as the other windings for the purpose of at least one auxiliary circuit, which can be connected to the aforementioned additional winding.

Description:
FIELD OF TECHNOLOGY 
     The object of the present invention is a transfer apparatus for electric power for transferring power between two electricity networks or between an electricity network and a multiphase electric machine, which apparatus comprises low-voltage power cells and one or more transformers, to which the power cells are connected. 
     PRIOR ART AND DESCRIPTION OF PROBLEM 
     A medium-voltage network refers to an electricity distribution network of over 1 kV that is used in the transmission of electrical energy e.g. between a high-voltage (over 36 kV) main grid and a low-voltage (below 1 kV) consumer network. Electricity distribution networks normally operate with 50/60 Hz alternating-current electricity. 
     It is known in the art that it is advantageous to use medium-voltage in heavy-duty, such as over 1 MW, electrical devices owing to the smaller current and, as a consequence of this, smaller power losses. Often some kind of adjuster is needed between an electrical device, such as an electric machine, and an electricity distribution network, owing to the different frequencies, the different voltage levels or the need for galvanic isolation. It is known in the art that the adjustment needed can be arranged advantageously by means of a transformer and a power converter, e.g. a frequency converter. 
     The voltage endurance of the switch-type power semiconductor components generally used in frequency converters is, for reasons of manufacturing technology, so small that with medium-voltage a number of them must be connected in series. It is known in the art that to avoid the problems related to the serial connection of components, low-voltage frequency converter technology can be utilized by the aid of a so-called cascade circuit, wherein an adequate amount of low-voltage power cells are connected in series for achieving sufficient total voltage endurance. The circuit also comprises a transformer, which comprises its own winding per each power cell belonging to the power converter. Examples of cascade circuits are found e.g. in U.S. Pat. No. 5,625,545, in the solution according to which a transformer operates at the frequency of the supply network and all its windings are three-phase, and in patent publication US 2010/0327793, in the solution according to which a transformer operates at a high frequency, over 1 kHz, and all its windings are single-phase. 
     Disturbances, such as transient voltage outages, sometimes occur in electricity distribution networks. A process operating on electrical power loses its controllability in such a situation, owing to which it is normally desired to halt the process as quickly as possible. For example, in the case of a motor supplied with a frequency converter, this means that the speed of rotation is controlled towards zero, when the operating point of the motor can turn to the generator side, i.e. it starts to supply power towards the frequency converter. For preventing internal overvoltage in this type of situation, a frequency converter is often provided with a so-called brake chopper, by means of which the generator power can be supplied to a resistor. In a cascade-type frequency converter the implementation of dynamic braking is awkward and expensive for, inter alia, insulation reasons, because according to prior art each power cell must be provided with its own resistor brake. 
     The power cells comprised in a cascade circuit are known to operate on the so-called PWM principle, according to which they comprise a DC intermediate circuit provided with a high-capacitance filter capacitor. For preventing a large switching current surge, these types of capacitors must be charged to almost full voltage before connecting the device to the supply network. According to prior art e.g. a second circuit-breaker and charging resistors connected in parallel with the main breaker are used for this purpose, which raise the costs. In the so-called dual-cascade circuit according to patent publication US 2010/0327793, by means of a resistor it is possible to charge only the power cells disposed on the primary side of the transformer. 
     SUMMARY OF THE INVENTION 
     The aim of this invention is to achieve a new kind of arrangement, with which the aforementioned drawbacks are avoided and with which both dynamic braking as well as initial charging of the filter capacitors of the power cells are advantageously enabled. In addition, the invention makes possible a test arrangement, in which the operability of medium-voltage circuits can be verified without a direct connection to a medium-voltage network, which can be useful e.g. when commissioning apparatus. 
     With the apparatus according to the invention electric power can be transferred between a first multiphase power circuit, e.g. a power circuit of a cascaded power converter, connected to a multiphase transformer and a second single-phase or multiphase power circuit (auxiliary circuit) connected to the same transformer by means of a separate additional winding. More particularly, the apparatus that is object of the invention is suited for use between a cascaded power converter operating in a medium-voltage environment and power circuits (auxiliary circuits) operating in a low-voltage environment. 
     The invention is suited for use between any two electricity networks fitted together by means of a cascade converter, such as e.g.:
         Between an alternating-current electricity network or direct-current electricity network and an electric motor operating on alternating-current electricity,   Between two alternating-current electricity networks of different voltages and/or of different frequencies,   Between an alternating-current electricity network and a direct-current electricity network.       

     In the solution according to the invention, a transformer comprised in a cascade-type converter comprises an additional winding, which can be low-voltage and dimensioned for a smaller power than the actual main windings comprised in the converter. In a transformer solution (U.S. Pat. No. 5,625,545) operating at the frequency of the supply network, the additional winding can be either single-phase or three-phase, and in a high-frequency transformer solution (US 2010/0327793) the additional winding is always single-phase. The additional winding is in the same magnetic circuit as also the windings connected to the power cells of the cascade converter, so that its voltage in relation to the number of winding turns is proportional to the voltages of the other windings. 
     According to one embodiment of the invention a brake circuit is connected to the additional winding, which brake circuit, comprises e.g. a rectifier, a filter capacitor for direct-current voltage, and also a brake chopper and a brake resistor. When the supply network drops, the power cells of the cascade converter can in this case supply their braking power via the additional winding to the brake circuit, which can be common to all the power cells. The brake circuit preferably operates in a low-voltage environment, in which case inexpensive components are available for it and the insulation of it is easier with regard to electrical safety than with medium-voltage. 
     According to one embodiment of the invention a power supply circuit is connected to the additional winding, which power supply circuit comprises e.g. a rectifier connected to a single-phase or three-phase low-voltage network, a filter capacitor for direct-current voltage, and also an inverter bridge implemented with controllable power semiconductor switches. The power supply circuit can in this case charge the filter capacitors of the power cells of the cascade converter to the desired voltage before connection of the converter to a medium-voltage supply network. Charging can take place by controlling on the PWM principle the magnitude of the voltage formed by the change-over switches to increase from zero towards final full voltage, following the rise of which voltage also the DC intermediate circuits of the power cells almost charge. The power supply circuit and the brake circuit can be connected to a common direct-current voltage. 
     According to one embodiment of the invention the circuits disposed on the primary side of the transformer are connected to the circuits disposed on the secondary side of the transformer e.g. via a filter. After the power supply circuit described above has charged, the voltages of the DC intermediate circuits of the power cells to the normal operating level, e.g. the cascade converter disposed on the secondary side of the transformer can be started to supply power to the circuits disposed on the primary side. In this case the power supply circuit remains to supply to the system the total dissipated power formed in it, in which case e.g. the medium-voltage primary circuits and secondary circuits can be tested at up to full power without a direct interface to the supply network. 
     The characteristic features of the solution according to the invention are described in detail in the claims below. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       In the following, the invention will be described in more detail by the aid of some examples of its embodiments with reference to the attached drawings, wherein 
         FIG. 1  presents a prior-art cascade circuit between an electric machine and a medium-voltage network, 
         FIGS. 2A and 2B  present the power cells of a prior-art cascade circuit, 
         FIG. 3  presents a dynamic brake of a prior-art cascade circuit, 
         FIG. 4  presents a charging arrangement of a prior-art cascade circuit, 
         FIGS. 5A and 5B  present a dynamic brake of a cascade circuit according to the invention, 
         FIGS. 5C and 5D  present a second dynamic brake of a cascade circuit according to the invention, 
         FIG. 6  presents a power cell of a prior-art cascade circuit, 
         FIGS. 7A and 7B  present a charging arrangement of a cascade circuit according to the invention, 
         FIG. 8  presents a testing situation of a cascade circuit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  describes a basic circuit of so-called cascaded frequency converter known in the art, the operation of which type of circuit is known from e.g. U.S. Pat. No. 5,625,545. It normally comprises both medium voltages as well as the three-phase supply voltage U P , the frequency of which is normally 50 Hz or 60 Hz, and also three-phase output voltage U, V, W adjustable in frequency and magnitude for supplying an electric machine M 1 . The frequency converter comprises similar low-voltage power cells C S1  . . . C S9 , a number of which are connected in series in each output phase. The power cells are connected to a common transformer T P , the three-phase primary coil W P  of which is connected to the supplying medium-voltage network U P , and which transformer comprises a separate three-phase low-voltage secondary coil W S  per each power cell. Differing from the embodiment of the figure, the secondary windings can also be single-phase. The number of power cells connected in series depends as is known on the output voltage, in the embodiment of  FIG. 1  there are 3 power cells in series, in which case there are correspondingly 9 secondary windings. The more groups connected in series that are used, the more steps the pulse pattern of the output voltage can be comprised of, which has an advantageous effect on the harmonics content of the output voltage. 
       FIG. 2A  presents an example of a circuit of a power cell C S , which type of power cell C SQ2  it is known can be used when the direction of flow of the power is just from the supply network towards the motor. The three-phase secondary winding W S  of the transformer, which winding is connected to the supply connectors IN 2 , supplies the power cell. The power cell comprises a three-phase rectifying bridge REC comprising diodes, a filter capacitor C Q2  of direct-current voltage DC Q2  and also a single-phase inverting bridge H S , which comprises two so-called phase switches comprising controllable power semiconductor switches, e.g. IGBT, and diodes, which phase switches can be connected to either pole whatsoever of an output connector OUT 2  of either direct-current voltage pole whatsoever. 
     When power can flow in either direction whatsoever, it is known to use the possibility C SQ4  described in  FIG. 2B . In it an active network bridge AFE is used in place of a so-called passive network bridge REC, which active bridge comprises in this embodiment three similar phase switches, which are also used in the inverting bridge H S . A filter unit LFU can be connected between the secondary winding W S  to be supplied and the AFE bridge, for damping the harmonics of the current. By using an AFE bridge, it is known in the art that apart from being able to supply braking power to the network, also an almost sinusoidal waveform of the network current can be achieved. 
       FIG. 3  presents a possibility according to prior art to arrange dynamic braking, which is needed e.g. when the voltage of the supply network has disconnected owing to a disturbance and it is desired to stop a process as quickly as possible. The circuit comprises a controllable power switch V B , e.g. an IGBT, and a resistor R B , which are connected in parallel with the direct-current voltage filter capacitor (C Q ) of a power cell (C SQ2 , C SQ4 ). In a braking situation the power switch V B  is controlled to be conductive, in which case dissipation power braking the electric motor forms in the resistor according to the control of the direct-current voltage DC Q  and the power switch. Dynamic braking circuits are needed in each power cell, which is a problem from the viewpoint both of costs and of electrical insulation of the resistors. 
     The power cells comprised in a cascade circuit are known to comprise a DC intermediate circuit provided with a high-capacitance filter capacitor. For preventing a large switching current surge these types of capacitors must be charged to almost full voltage before connection of the device to the supply network.  FIG. 4  contains an example of a prior-art charging arrangement, which comprises a charging contactor K 1  and charging resistors R 1 , R 2  connected to the primary circuit of the transformer. At first, when the capacitors C Q  of the power cells are de-energized, the contacts of the contactor K 1  are open. When the supply voltage U P  is connected to the input connectors L 1 , L 2 , L 3  and onwards to the contactor, the current supplied via the resistors R 1  and R 2 , the transformer T P  and also the diodes of the network bridges (REC, AFE) of the power cells C S  charges the filter capacitors C Q . When the capacitors have charged to almost full voltage, the contacts of the contactor K 1  can be closed, in which case normal operation can start. 
     The charging circuits K 1 , R 1 , R 2  and the control circuits of the contactor are medium-voltage, which is a problem from the viewpoint both of costs and of electrical insulation. 
       FIG. 5A  presents a solution according to the invention for arranging dynamic braking in a cascade converter according to  FIG. 1 . The power cells are connected to a common transformer T A , the three-phase primary winding W PA  of which is connected to the medium-voltage network U P  to be supplied, and which transformer comprises a separate three-phase low-voltage secondary winding W SA  per each power cell. The cascade transformer T A  according to the invention comprises an additional winding W A  to which a brake unit BR A  is connected. The additional winding W A  is, according to the figure, preferably three-phase, but it is possible for it to be single-phase. 
       FIG. 5B  contains a more detailed example of a brake unit BR A , in which the voltage of the additional winding is rectified in a diode bridge REC A1  into direct-current voltage DC A1 , which can be filtered with a capacitor C A1 . The actual brake circuit comprises a resistor R BA  and a controllable power switch V BA , which is controlled in the same way as is presented above in connection with  FIG. 3 . 
     In the arrangement according to the invention all the power cells can supply their braking energy to the same common brake unit. The brake circuit is preferably low-voltage, in which case correspondingly inexpensive components can be used in it as well as in the power cells, and the insulation of it is easier with regard to electrical safety than with medium-voltage. 
       FIG. 5C  presents a solution according to the invention for arranging dynamic braking in a so-called dual-cascade circuit, a type of which is presented in, inter alia, patent publication US 2010/0327793. The dual-cascade circuit comprises groups G 1  . . . G N  connected in series, each of which groups comprising its own transformers T 1  . . . T N , operating at a frequency of over 1 kHz, and cascaded similar power cells C 11  . . . C N6  on both sides of them. One example of the internal circuit of a power cell C 11  is presented in  FIG. 6 . According to the invention, each group-specific transformer (T 1  . . . T N ) comprises a single-phase additional winding (W S1  . . . W BN ), which can be connected either to a brake unit BR B  common to all the groups, according to the preferred embodiment of the figure, or to a brake unit specific to the individual group. 
     According to the embodiment of  FIG. 5D , the brake unit comprises for each group-specific additional winding its own rectifying bridge (REC B1  . . . REC BN ), a filter capacitor C B1  of direct-current voltage DC B1  and also a brake resistor R BB  and a power switch V BB , which is controlled in the same way as is presented above in connection with  FIG. 3 . The additional winding and the brake unit are low-voltage according to the preferred dimensioning. 
       FIG. 6  contains an example of a power cell C 11  of a dual-cascade circuit. It comprises a single-phase bridge circuit H 1  to be connected to a transformer, a filter capacitor C 1  of the DC intermediate circuit DC 1  and a single-phase inverter bridge H 2 . Both the so-called H-bridge circuits H 1  and H 2  are comprised of similar phase switches, which are presented above in connection with  FIG. 2A . As is obvious to a person skilled in the art, the power cell presented by  FIG. 6  can transfer power in either direction, from the input connectors IN 11  to the output connectors OUT 11 , or vice versa. 
       FIG. 7A  presents a solution according to the invention for arranging initial charging in a cascade converter according to  FIG. 1 . The cascade transformer T A  according to the invention comprises an additional winding W A  to which a power supply unit PS A  is connected. The additional winding W A  is, according to the figure, preferably three-phase, but it is possible for it to be single-phase. The supply voltage U LV  of the power supply unit PS A , which voltage is preferably low-voltage 50/60 Hz distribution voltage, is rectified in a diode bridge REC AP  into direct-current voltage DC A2 , which can be filtered with a capacitor C A2 . According to the embodiment of the figure, the unit also comprises a three-phase inverter bridge INU A , which comprises three phase switches comprising controllable power semiconductor switches and diodes. The output connectors IN A  of the phase switches can be connected, either directly or via a filter (such as LFU in  FIG. 2B ), to the additional winding W A . The initial charging of the filter capacitors of the power cells, e.g. C S1 , of the cascade converter occurs such that the bridge INU A  forms on the prior-art PWM principle a three-phase rising voltage, which via the transformer T A  and rectified by the diodes (REC or AFE,  FIGS. 2A ,  2 B) of the network bridges of the power cells C S  charges the filter capacitors. 
       FIG. 7B  presents how initial charging according to the invention can be arranged in a dual-cascade converter. According to the invention, each group-specific transformer (T 1  . . . T N ) comprises a single-phase additional winding (W B1  . . . W BN ), which can be connected either to a power supply unit PS B  common to all the groups, according to the preferred embodiment of the figure, or to a power supply unit specific to the individual group. The supply voltage U LV  of the power supply unit PS B , which voltage is preferably low-voltage 50/60 Hz distribution voltage, is rectified in a diode bridge REC BL  into direct-current voltage DC B2 , which can be filtered with a capacitor C B2 . In the embodiment of the figure, the power supply unit comprises for each group-specific additional winding its own inverter bridge (H B1  . . . H BN ) comprising two change-over switches. The initial charging of the filter capacitors of the power cells, e.g. C 11 , of the cascade converter occurs such that the inverter bridges H B  form on the prior-art PWM principle single-phase rising voltages specific for each group, which voltages via the transformers T 1  . . . T N  and rectified by the diodes of the network bridges (such as H 1  in the embodiment of  FIG. 6 ) of the power cells charge the filter capacitors. 
       FIG. 8  presents how the invention can be applied in the testing of a dual-cascade converter. According to the figure the input connectors (L 1 , L 2 , L 3 ) of the converter are connected with the output connectors (U 2 , V 2 , W 2 ) via a filter unit FILT. When it is assumed that the internal DC circuits of the power cells C 11  . . . C N6  are fully charged, the cascaded power cells can form the nominal voltages for both the input side and the output side on the PWM principle. By adjusting the phase shift between the voltages, it is possible to adjust the magnitude of the current flowing through the filter unit, i.e. the load current of the converter. In this way the converter can be tested at even up to rated current without a connection to the actual supply network. 
     According to the invention, via the additional windings (W B1  . . . W BN ) of the group-specific transformers (T 1  . . . T N ) in this type of test arrangement, only the dissipation power consumed by the system is supplied to the system, by the action of the power supply unit presented above in connection with  FIG. 7B . As a result of the invention a medium-voltage power converter can be tested with full voltage and current without a direct connection to a medium-voltage network, which is a great advantage in, inter alia, field conditions. 
     As the person skilled in the art will note, the power supply units PS A , PS B  presented in  FIGS. 7A and 7B  can also handle the rectifying functions (REC A1 , REC B ) required by dynamic braking, when the power semiconductor switches controlling the inverter bridges (INU A , H B ) are left uncontrolled and the dynamic braking circuit presented in  FIG. 3  is connected to the DC intermediate circuits DC A2 , DC B2  of them. A person skilled in the art will also note that the circuit diagram of  FIG. 7A  presents a normal PWM frequency converter, the type of which can be used according to the invention connected to the additional winding to handle both the dynamic braking functions and the power supply functions. 
     It is obvious to the person skilled in the art that the different embodiments of the invention are not limited solely to the examples described above, but that they may be varied within the scope of the claims presented below.