Abstract:
A method and arrangement for charging an intermediate circuit of a frequency converter, the intermediate circuit of the frequency converter comprising one or more capacitors, and the frequency converter having means for charging the intermediate circuit, the means comprising a rectifier bridge, a direct current choke, a direct-current transformer, a zero diode, and a semiconductor switch and a control circuit arranged to control the switch. The method comprises the steps of defining in advance high and low current limits, controlling the semi-conductor switch alternately to conductive and non-conductive states depending on the magnitude of a charging current, and stopping the controlling of the semiconductor switch when the charging current remains below the high current limit.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method and arrangement in a frequency converter as described in the preambles of independent claims  1  and  4 . In particular, the invention relates to the charging of a capacitor of an intermediate circuit of frequency converters with voltage intermediate circuits. 
   A frequency converter is a device used to generate an adjustable alternating voltage by using a constant-frequency voltage. The most typical application of frequency converters is controlling motors in such a manner that an adjustable voltage is generated from the alternating voltage of a supply network to control the motor. A frequency converter with a voltage intermediate circuit comprises a rectification unit, voltage intermediate circuit, and inverter unit. A task of the rectification unit is to provide direct voltage from the supplied alternating-current voltage to the voltage intermediate circuit, to which one or more capacitors are connected to store and equalize the direct voltage depending on the power of the device. The inverter unit connects the direct voltage of the voltage intermediate circuit to the load in such a manner that the direct voltage pulses generate the desired average alternating-current voltage or a corresponding adjustable property. 
   When switching a frequency converter on, the capacitor of the intermediate circuit should be charged before the actual use of the frequency converter. The charging requires a separate charging arrangement that limits the magnitude of the charging current. If the charging current were not limited, the current would increase in magnitude so much that the protective devices in the supply would switch the supply voltage off. Known solutions for charging the capacitor of an intermediate circuit include various charging resistance arrangements, in which the charging current is reduced by a series resistor that is bypassed with a switch after the charging of the intermediate circuit. Another known way is to use a half-controlled bridge circuit, in which the magnitude of the charging current can be controlled by altering the firing angle of the controlled components of the bridge circuit, which requires that the charging arrangement be synchronized with the supply voltage. 
   A drawback with both above-mentioned known solutions is that the charging arrangements should be dimensioned separately for each frequency converter power. In large MW-power frequency converters or system drives, the charging circuit also becomes quite large and expensive. 
   BRIEF DESCRIPTION OF THE INVENTION 
   It is an object of the present invention to provide a method and arrangement that avoid the above-mentioned drawbacks, and permit the charging of the frequency converter intermediate circuit with a similar arrangement regardless of the power range of the frequency converter. This object is achieved by the method and arrangement of the invention that are characterized by what is stated in the characterizing sections of the independent claims. 
   The invention is based on the fact that the voltage required for charging is generated with a separate rectifier bridge whose output current is directed to the frequency converter intermediate circuit to charge it. The charging current is controlled in such a manner that its value is kept within specific limit values. This way, the charging current is kept under control and does not cause any damage to the components of the frequency converter. 
   The method and arrangement of the invention provide the advantage that they can be applied to frequency converters of all powers. When applied to frequency converters having different powers and using the same current limits, the charging time varies depending on the magnitude of the capacitance of the frequency converter intermediate circuit. Especially when applied to high-power frequency converters, the method and arrangement of the invention provide significant space and cost savings. 

   
     BRIEF DESCRIPTION OF FIGURES 
     The invention will now be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which: 
       FIG. 1  shows an arrangement of the invention; 
       FIG. 2  shows the modulation principle of a method of the invention; and 
       FIG. 3  shows curve shapes of the charging current and intermediate circuit voltage provided by the method and arrangement of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a general description of the arrangement of the invention. A rectifier  12  made up of the diodes of the frequency converter rectifies a voltage to an intermediate circuit capacitor  13 . In normal operation, the rectifier  12  is supplied through chokes  10  and switching devices  11  from the network. When switching the frequency converter on, means  20  connected to the frequency converter in accordance with the invention are used to charge the intermediate circuit. 
   In the embodiment of  FIG. 1 , the means  20  for charging the intermediate circuit comprise a rectifier bridge  1 , which is specifically a 6-pulse diode bridge, and a capacitor  2  connected to the output of the bridge. A task of the capacitor is to equalize the output voltage of the rectifier bridge. A direct voltage is formed at the output of the diode bridge  1  having a direct-current transformer  7  connected to its positive pole. This direct-current transformer generates a signal proportional to the charging current to a control circuit  4 . The control circuit comprises a circuit for performing current measurement, control of the IGBT switch  5 , and powering from the intermediate circuit  2 . 
   A semiconductor switch, which in the embodiment of  FIG. 1  is an IGBT  5 , is connected to the negative pole of the diode bridge output. A DC choke  8  is in series with the switch, and, to a point between these two, an anode of a zero diode  6  is connected, with its cathode connected to the positive pole of the diode bridge output. 
   The second pole of the DC choke  8  is connected through a switch element  9  to the intermediate circuit of the frequency converter. Similarly, after the direct-current transformer, the positive pole of the diode bridge is connected through a switch element  19  to the intermediate circuit of the frequency converter. The embodiment of  FIG. 1  shows how the charging arrangement supplies the capacitor  13  of the frequency converter intermediate circuit through the rectifier  12  of the frequency converter. However, it is clear that the charging unit can be directly connected to the intermediate circuit of the frequency converter. 
   According to the method of the invention, when charging the intermediate circuit of the frequency converter, a high current limit I hi  and a low current limit I lo  are defined in advance, and the charging current is maintained within the current range between the two limits during the charging of the capacitor of the intermediate circuit. Further, according to the invention, the semiconductor switch  5  is made conductive to connect the output voltage of the diode bridge to the intermediate circuit of the frequency converter. After the means for charging the intermediate circuit are switched on, i.e. after the switch elements  3  are made conductive, the diode bridge  1  generates in its output a direct-current voltage from the alternating-current voltage of its input, which is typically the voltage of the supply network. The control circuit  4  detects this voltage.  FIG. 1  shows how the control circuit is connected to both the positive and negative pole of the diode bridge output to detect this voltage and to generate its own operating voltage. 
   When the output of the diode bridge is live and the switch element  5  is made conductive, the charging means  20  are connected to the intermediate circuit of the frequency converter through said switch  5 , DC choke  8 , closed switch  9 , and the diode bridge of the frequency converter. Because the intermediate circuit of the frequency converter is dead, the current begins to run from the charging means to the intermediate circuit along the current path that has the direct-current transformer  7 , a diode of the frequency converter diode bridge  12 , the frequency converter intermediate circuit capacitor  13 , a second diode of the frequency converter diode bridge  12 , the DC choke  8 , and the semiconductor switch  5 . As the current runs to the intermediate circuit capacitor  13 , the magnitude of this voltage increases. 
   According to the method of the invention, the direct-current transformer continuously monitors the magnitude of the charging current. Because the frequency converter intermediate circuit capacitor is, in practice, dead when the charging begins, the magnitude of the current increases rapidly, i.e. di/dt is high. When the magnitude of the current defined by the direct-current transformer reaches the high current limit I hi , the semiconductor switch is made non-conductive, i.e. the current path described above from the charging unit to the intermediate circuit is disconnected. When the current path is disconnected, the current transfers, due to the DC choke  8 , to the zero diode  6 , and the current runs along a route that has the DC choke  8 , zero diode  6 , direct-current transformer  7  and the frequency converter intermediate circuit capacitor  13  through the diodes  12  of the rectifier. While this is the current path, the magnitude of the current begins to decrease. When the current reaches the low current limit, the control circuit  4  controls the switch  5  again to be conductive. 
     FIG. 2  shows by way of example the behaviour of the charging current I L  depending on the ON/OFF state of the switch component.  FIG. 2  shows that when the switch component is conductive, the magnitude of the current increases. When the current reaches the limit I hi , the switch component is made non-conductive, whereby the charging current I L  decreases. When the charging current reaches the low limit value I lo , the switch component is made conductive again, whereby the magnitude of the charging current increases. 
   When the voltage of the intermediate circuit has increased to the peak value of the supply voltage, the charging current decreases and the hysteresis control no longer controls the switch component. When the current measured with the control circuit remains below the high limit value, even though IGBT is conductive or when the maximum charging time (e.g. 2.5 s) set for the control circuit  4  is reached, the IGBT controls are stopped and information is transmitted to an upper control system that the charging is finished. This information is transmitted as a digital signal, for instance. The upper control system then decides on closing the main switch device  11 . When the switch elements  11  are closed, the intermediate circuit of the frequency converter receives its supply along the route used during operation. As the switch elements  11  are closed, the switch elements  3 ,  9 ,  19  can also be opened, whereby the charging means are completely disconnected from both the supply network and the frequency converter. The embodiment of  FIG. 1  does not separately show that the same voltage, which typically is the supply network voltage, supplies the charging means  20  and the frequency converter. 
     FIG. 3  shows the curve shapes of the charging current I L  and the voltage of the intermediate circuit capacitor Uc produced by the method of the invention during charging.  FIG. 3  is based on charging with a 690-V supply voltage, in which the capacitance of the intermediate circuit being charged is 10 mF.  FIG. 3  shows how the charging current varies between 100 A and 140 A, which are the low and high limit values. If the capacitance of the intermediate circuit were higher, the charging would correspondingly take more time. The only variable in the charging is thus the time it takes. If the charging were made with conventional resistance charging, the charging current surge would be over 1000 A. This would considerably stress the charging contactor, and the wiring and protective fuses should be made very large. In addition, the charging time would increase in a conventional implementation, and the charging would necessarily not achieve a full charging level. If the intermediate circuit short-circuits during charging, for instance due to the failure of one of the power semiconductors, the method of the invention limits the short circuit current to the limit value set for the control circuit  4 . In addition, the charging is automatically interrupted after the set maximum charging time. 
   It is apparent to a person skilled in the art that the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above, but may vary within the scope of the claims.