Abstract:
A drive system for industrial plant sections, in particular for system sections in the basic industries, having at least one electric motor and at least one converter with a voltage link, through which the electric motor is connected to an AC-voltage power supply network, the converter regulating the power consumption or torque and rotation speed of the electric motor from the AC-voltage power supply network.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a drive system for industrial plants, in particular for systems in the basic industries, having at least one electric motor and at least one converter with a voltage link. 
     BACKGROUND INFORMATION 
     High-performance variable speed drives and their converters place a considerable load on the power supply lines due to current harmonics. Therefore, the use of such drives requires that their converters be equipped with expensive filter circuits in order to reduce these harmonics. It is known that harmonic components can be reduced by the use of self-commutated converters having gate turn-off power semiconductors. However, at a low pulse frequency, in particular at a pulse frequency as low as the fundamental oscillation of the AC-voltage power supply network powering the drive device, a self-commutated converter may not be able to reduce the harmonics to the desired degree. Therefore, even with this measure, the use of expensive filter circuits may be necessary. This problem occurs in particular with rolling stand drives. 
     W/O 92/15148 describes another option for compensating harmonics. According to this reference, residual degrees of freedom in a motor control are used for minimizing an interference index. The interference index is defiend as the sum of squares or the absolute values of the difference between setpoint current and actual current. 
     World Patent 92/15148 discloses another option for compensating harmonics. According to this document, residual degrees of freedom in a motor control are used for minimizing an interference index. The interference index is defined as the sum of squares or the absolute values of the difference between setpoint current and actual current. 
     The object of the present invention is to provide a drive system for which the harmonics injected in the power supply line are reduced. In doing so, it is desirable that the drive system be designed to be more cost effective compared to known drive systems having reduced harmonics. 
     This object is achieved according to the present invention through a drive system according to Claim  1 , according to which the link voltage of the converter is set so that the harmonics computes the link voltage as a function of the AC-voltage power supply network voltage. 
     The drive system according to the present invention is particularly advantageous in a power range of 1-20 MW, advantageously between 2-10 MW or, for impact load, in a power range of 2-30 MW, advantageously between 4-20 MW. 
     The drive system according to the present invention has been found to be particularly advantageous in connection with three-phase motors connected in tandem, i.e., in circuits where the three-phase motor has open windings, which are supplied by converters on both sides. 
     The drive system according to the present invention is particularly advantageous for driving rolling stands of a rolling mill train. 
     The converter of the drive system according to the present invention can also be designed as an air-cooled converter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a drive system according to the present invention, 
     FIG. 2 shows a link voltage computing system, 
     FIG. 3 shows a characteristic curve for determining an optimum control factor, 
     FIG. 4 shows a converter arrangement for supplying a three-phase motor with delta-connected converter sections on line and machine sides, 
     FIG. 5 shows a converter arrangement for supplying on both sides a three-phase motor having an open winding, with converter sections connected in delta. 
     FIG. 6 shows the use of a drive system according to the present invention in a rolling mill. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a drive system according to the present invention. A three-phase motor  6  is supplied with power through a converter and a transformer  2  or inductors via an AC-voltage power supply network  1 . The converter has a self-commutated rectifier  3  with gate turn-off power semiconductors, a DC voltage link  4 , whose link voltage U ZK  is set and regulated by rectifier  3 , and a self-commutated AC converter  5  for regulating three-phase motor  6 . Self-commutating rectifier  3  is regulated via a regulator  9  so that a link voltage U ZK  is established according to a predefined setpoint value U* ZK . Link setpoint voltage U* ZK  is computed using a link voltage computing device  8 , which determines link setpoint voltage U* ZK  as a function of the AC voltage of AC-voltage power supply network  1 . For this purpose, the drive system according to the present invention advantageously has a measurement device  7  for measuring the effective value U netz eff  of the line voltage of AC-voltage power supply network  1 . 
     FIG. 2 shows the details of a link voltage computing device  8 . The output signal of a control factor selector  11  for selecting an optimum control factor with regard to the total harmonic distortion, i.e., the harmonics content of input current I Netz  (see FIG. 1) is multiplied by the constant {square root over (6)}/n. The effective voltage U netz eff  of the AC-voltage power supply network is divided by this value. In an advantageous embodiment, the rate of change of effective voltage U netz eff  of the AC-voltage power supply network is, however, initially limited by a change limiter  10 . The minimum link voltage U ZKmin  is set so that self-commutating AC converter  5  (see FIG. 1) can provide three-phase motor  6  with a sufficiently high effective power. The maximum link voltage U ZKmax  is set so that the power semiconductor of self-commutating DC converter  3  and self-commutating AC converter  5  (see FIG. 1) are not damaged. The output signal of divider  13  is sent to a limiter  14 , whose limit values are the minimum link voltage U ZKmin  and the maximum link voltage U ZKmax . The output signal of limiter  14  is a setpoint link voltage U* ZK . 
     FIG. 3 shows the curve of line current total harmonic distortion factor  16  plotted against control factor  15  of the self-commutating DC converter. The control factor of a self-commutating DC converter is the ratio between the fundamental frequency peak of the voltage generated by DC converter  3  on the secondary side of transformer  2  and the maximum fundamental frequency peak of this voltage that can be generated by rectifier  3 . For different pulse patterns for triggering the semiconductors of self-commutating DC converter  3 , there is an optimum control factor A opt , for which line voltage total harmonic distortion  16  is close to minimum. Control factor and link voltage U ZK  have a certain relationship to one another, which depends on the effective value of the line voltage. Link voltage U ZK  is therefore set so that an optimum control factor A opt  is achieved, for which the respective available line voltage effective value is attained. 
     FIG. 4 shows a converter arrangement for powering a three-phase machine with line-side converter section  33  and motor-side converter section  34  designed with delta connected identical GTOs. The main circuit of each phase module is illustrated with its respective protective circuit  40 ,  41 . Converter sections  33  and  34  each have three such phase modules with their respective protective circuits. P-side link capacitor  37  forms, together with N-side link capacitor  39 , the DC voltage link circuit, through which both converter sections are connected. P-side protective charge-reversal resistor  36  and N-side protective charge-reversal resistor  38  are connected to the respective sides of protective circuits  40  and  41 . 
     The input of line-side converter section  33  is connected to line  30  via transformer  31  and circuit breaker  32 . The output of machine-side converter section  34  is connected to three-phase motor  35 . 
     In the arrangement of FIG. 5, the outputs of a first converter  74  and a second converter  75  are connected to one side  71  and  72 , respectively, of an open three-phase winding of three-phase motor  73 . In addition to twice the power, particularly advantageous operating characteristics are achieved using this arrangement, since, if the pulse procedures are appropriately tuned, a largely sinusoidal current with low harmonics is achieved in the motor even at a low switching frequency of the GTO thyristors. 
     On the line side, first converter  74  is connected to the power supply line via an optional line-side additional inductance  63  and a first transformer  61 , connected, for example, in star/delta with power supply line  60 . Second converter  75  is connected to power supply line  60  (for example, in star/delta) via an optional line-side additional inductance  64  and a second transformer  62 , preferably electrically offset by 30° with respect to first transformer  61 . This arrangement results in particularly advantageous line reactions, in particular when, as in the present example, the converters are composed of converter sections connected in delta. Even in the case when the self-commutated line current converter is clocked at the fundamental frequency, the resulting current is sinusoidal with very low harmonics content. 
     The two converters  74  and  75  have line-side converter sections  66  and  65 , respectively and machine-side converter sections  69  and  70 , respectively, which are connected via a DC voltage link  67  and  68 , respectively. The two DC voltage links  67  and  68  are electrically insulated from one another. All converter sections  66 ,  65 ,  69 ,  70  are connected in delta, preferably with RC-GTOs. 
     FIG. 6 shows the use of the drive systems according to the present invention in a rolling mill. Rolled stock  103  is rolled in roll stands  104 ,  105 ,  106 ,  107 , which are driven by electric motors  99 ,  100 ,  101 ,  102 . Motors  99 ,  100 ,  101 ,  102  are powered by a power supply line  90  via transformers  91 ,  92 ,  93 ,  94  and converters  95 ,  96 ,  97 ,  98 . Converters  95 ,  96 ,  97 ,  98  have a self-commutated DC converter  3  with regulator  9 , a voltage link  4 , a self-commutated AC converter  5 , a link voltage computing unit  8 , and optionally a measuring device  7 . In an alternative embodiment, one measuring device  7  whose measured values are supplied to all drive systems is used. 
     For higher horsepowers, the circuit according to FIG. 5, having two transformers, two converters and open motor windings, is used for the above-mentioned transformers  91 ,  92 ,  93 ,  94 , converters  95 ,  96 ,  97 ,  98  and motors  99 ,  100 ,  101 ,  102 .