Method for stabilizing a power supply network against reactive load fluctuations, and a power factor compensation device
TECHNICAL FIELD 
     The invention proceeds from a method for stabilizing a power supply device or a power supply network against reactive load fluctuations of at least one electric device or installation of variable reactive load, and from a power factor compensation device. 
     PRIOR ART 
     U.S. Pat. No. 5,155,740 discloses a flicker compensation device for DC arc furnaces, in which reactive power control is performed only as a function of the detected reactive power of the arc furnace. 
     In the case of this compensation, no account is taken of currents in filter branches and auxiliary equipments. In the case of small electrode currents, the reactive power of the furnace can become too low. When there are permanently installed capacitor banks, this leads to overcompensation, which is frequently not accepted by power supply companies. 
     The printed publication No. CH-IT 123 090 D entitled Kompensationsanlagen fur die Industrie (Industrial Compensation Installations) from the Swiss company of BBC Aktiengesellschaft Brown, Boveri &amp; Cie. (now: Asea Brown Boveri AG), CH-5401 Baden/Switzerland, December 1983, pages 1 to 12 discloses the control, individually for each phase, of reactive load changes occurring during operation of arc furnaces by means of a 3-phase reactive element composed of 3 air-core inductors via thyristor AC controllers. The control is performed as a function of the supply voltage, the actual current value of the arc furnace and of the compensation installation, as well as of the desired value of the current through the reactive element. Connected in parallel with this reactive element are capacitors which act with their series inductors and, as the case may be, damping resistors as a reactive power source and simultaneously as a filter to suppress harmonic currents of different orders. The variable reactive load of the reactive element, and those of the consumers add together in this case in such a way that the sum of the two reactive loads is constant and, together with the constant capacitive reactive power of the fixed capacitor groups, yields a desired value of cos f. 
     A relatively long mean dead time of 5 ms is yielded for the system by measuring and analyzing the controlled variables. 
     EP-A1-0 498 239 discloses controlling an arc furnace by means of DC feed, having 2 control loops. A current controller ensures a constant current corresponding to a prescribed desired current value. An electrode control loop influences the position of the electrode, and thus the arc length. In the event of lengthening of the arc, the current controller must increase the voltage or drive the rectifier in such a way that the current remains constant. However, this is only possible as long as there is a voltage reserve. Controlling the electrode is performed by means of an adaptable DC voltage controller. The arc voltage, which is fed via a damping element to a comparator or summer, serves as the actual DC voltage value. The desired DC voltage value must be calculated in each case taking account of the transformer voltage stage and of the electrode current for each working point. Firstly, a limiter is used to limit in accordance with the transformer stage of the converter transformer, and thus with the possible voltage range of the power converter in such a way that the power converter is operated in a steady-state fashion at most just below the rectifier limit. The desired value is fed in a smooth fashion to the summer, in order in the event of sudden changes in the desired voltage values not to obtain any overshooting of the actual value, which could effect interrupting of the arc. Nothing is stated there about compensating reactive power fluctuations. 
     SUMMARY OF THE INVENTION 
     The invention achieves the object of further developing a method and a power factor compensation device for stabilizing a power supply device or a power supply network against reactive load fluctuations of at least one electric device or installation of variable reactive load of the type mentioned at the beginning in such a way that reactive load fluctuations are more effectively compensated. 
     An advantage of the invention is that the reactive powers of all the loads which are connected to the compensating installation are relatively quickly compensated by detecting the total current for the purpose of compensating the reactive load fluctuations. The result is a constant inductive reactive power for the entire installation, which can be compensated by means of permanently installed or connected capacitor banks or by means of harmonic filters. Installations of variable reactive load can also be connected to very weak power supply networks by means of this compensation of the reactive load fluctuations. It is thus possible to connect installations of over 50 MW to power supply networks whose short-circuit power is at least equal to 10 times the installation power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained below with the aid of an exemplary embodiment. 
     FIG. 1 shows a DC arc furnace having a current control loop, an electrode control loop and a reactive power control loop, 
     FIG. 2 shows a characteristic diagram for determining active power and reactive power of the arc furnace as a function of a striking angle actual value signal and a current actual value signal of the arc furnace in accordance with FIG. 1, and 
     FIG. 3 shows a characteristic for determining the striking angle for the reactive power control loop as a function of the total reactive power of an installation having the arc furnace in accordance with FIG. 1. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an arc furnace (8) having an electrode or cathode (7), which is connected to an AC power supply network (1) having an AC voltage of 22 kV via 2 reactive elements or inductors (6, 6&#39;), which are connected in series, in parallel branches respectively to a rectifier (5, 5&#39;), a furnace transformer (2, 2&#39;) having a plurality of switching stages, and to a current transformer (3, 3&#39;). A 2nd electrode or anode (12) arranged in the bottom region of the arc furnace (8) is connected to the positive terminal of the rectifier (5) (not represented). An arc (10) burns between the lower end of the cathode (7) and a material or scrap to be melted (not represented) as well as with the surface of a melt or of a molten bath (11). 
     Partial current actual value signals (i act1 , i act2 ) are detected by means of the current transformers (3, 3&#39;) in the AC supply leads to the rectifiers (5, 5&#39;) and fed to non-negating inputs of a summer (23) as well as to two negating inputs of a comparator or summer (13). A prescribable current desired value signal (i des ) is fed, for example from a potentiometer (not represented), to a non-negating input of this summer (13). On the output side, the summer (13) is connected to a current controller (14) having a proportional-integral characteristic, which on the output side supplies a rectifier manipulated variable signal (α act ) in a fashion corresponding to a striking angle to an ignition pulse transformer (15) which controls the rectifiers (5, 5&#39;) on the output side. 
     For the purpose of signal matching, limit value monitoring and suppressing undesired frequencies, the rectifier manipulated variable signal (α act ) is connected via a damping element (16&#39;) or a bandpass filter (16) to a negating input of a summer (17) whose non-negating input is fed a prescribable electrode controller reference variable signal (α des ) in accordance with a striking angle desired value in the range of 15°-50°, preferably in the range of 25°-35°. On the output side, the summer (17) is connected to an electrode controller (18) which has a proportional characteristic and acts on the output side via a amplifier (19) on a switch (e.g., valve such as a thyristor, transistor or the like) (20) of an electrode adjusting device (21). The electrode adjusting device (21), for example a hydraulic pump having an adjusting mechanism and an electrode speed controller, is mechanically coupled to the cathode (7) and permits the level thereof to be adjusted; it acts as a 1st order delay element. 
     The electrode control works approximately 10 times slower than the current control. The level adjustment of the cathode (7) is performed in such a way that the rectifier (5) works on average with a drive level of, for example, 25°el., irrespective of the secondary voltage of the furnace transformer (2) and of the current desired value (i des ) set. For the sake of simplicity, values and signals assigned to them are denoted identically. 
     The frequencies to be suppressed by the bandpass filter (16) comprise frequencies in the range of 0.5 Hz-20 Hz. 
     By controlling at a constant drive level at the rectifier (5), a constant mean power factor is achieved in the feeding AC power supply network (1). The power of a working point is determined very simply by selecting a voltage stage of the furnace transformer (2) and prescribing the direct current. 
     If the aim is to use a voltage stage of the furnace transformer (2) to operate different working points or a variable power, the current desired value (i des ) is prescribed accordingly. It is true that lower powers are obtained in the case when the current is reduced but the drive level at the rectifier (5) still remains the same. However, the arc (10) becomes longer, as a consequence of smaller voltage losses in the AC power supply network (1). The furnace process, however, also requires shorter arcs (10) in the case of lower powers. In order to achieve this, it is possible in the case of a change in the current desired value (i des ) simultaneously to prescribe as well a corresponding new desired value for the drive level of the rectifier (5). Provision is made for this purpose of a function generator (22) which is described in more detail in EP-A1-0 498 239 and which prescribes the electrode controller reference variable signal (α des ) as a function of the current desired value (i des ), as indicated by dashes in FIG. 1. It is thus also possible to enlarge the power range. 
     In order to compensate the variable reactive power of the arc furnace (8), provision is made of a 3-phase power factor compensator (31) which has per AC phase an AC power controller (28) in series with an inductor (29) and a switch (30) which is connected to the AC power supply network (1). The 3 AC power controllers (28) are interconnected in a delta connection, and are controlled in each case by an ignition pulse transformer (27). 
     Furthermore, via a current transformer (33) at which it is possible to tap a total current intensity (i 33 ) of the entire installation by means of auxiliaries and filter branches (4, 4&#39;) (which are not represented) for capacitive reactive power, the AC power supply network (1) is connected via a high-voltage transformer and 2 switches to a 3-phase high-voltage network (32) having an AC voltage of 220 kV. The total current intensity (i 33 ), a network voltage (U 1 ), detected by means of a voltage detector (34) of the AC power supply network (1) and a prescribable desired phase angle (φ des ) are fed to the input of a phase-angle controller (35) which supplies on the output side a desired reactive power signal (Q des8 ) for the arc furnace (8) to a negating input of a summer (25). A further negating input of the summer (25) is fed a reactive power actual value signal (Q 8 ) from the output of a function generator (24) which on the input side receives a current actual value signal (i act ), with i act  =i act1  +i act2 , from the summer (23), and the rectifier manipulated variable signal (α act ) from the output of the current controller (14). A filter reactive power signal (Q F ) is fed, in a fashion corresponding to the constant capacitive reactive power of the filter branches (4, 4&#39;), to a non-negating input of the summer (25). On the output side, the summer (25) supplies a total reactive power signal (Q G ), in a fashion corresponding to Q G  =Q F  -Q 8  -Q des8 , to a function generator (26), which on the output side supplies a stabilization striking angle signal (α st ) to the ignition pulse transformer (27). 
     FIG. 2 shows a characteristics field of the function realized by means of the function generator (24): 
     
         Q.sub.8 =k1·i.sub.act ·[1-(cos α.sub.act -k2·i.sub.act).sup.2 ].sup.0.5, 
    
     k1 and k2 being installation-specific factors with 0.1 ≦k1≦1, preferably with 0.3≦k1≦0.6 and 0.7·10 -3  ≦k2≦1.3·10 -3 , preferably with 0.9·10 -3  ≦k2≦1.1 ·10 -3 . In this case, the rectifier manipulated variable signal (α act ) is specified in degrees, the current actual value signal (i act ) in kA, the active power (P) in MW on the ordinate, and the reactive power (Q) in MVar on the abscissa. The result for an active power value (P) of 50 MW and a rectifier manipulated variable signal (α act ) of 35° is, for example, a reactive power (Q 8 ) of 46 MVar. 
     FIG. 3 shows a characteristic (36) of the function realized by means of the function generator (26): 
     
         Q.sub.G ≈α.sub.st °/90°-2-(sin 2·α.sub.st)/π, 
    
     (α st ) being plotted in degrees on the ordinate, and (Q G ) being plotted in % on the abscissa. In practice, the ideal curve (36) can be approximated by segments (37) indicated by dashes, and can be used to determine the value of the stabilization striking angle (α st ). 
     The example represented relates to an installation having an active power (P) of 60 MW in conjunction with a direct current of 100 kA and an installed reactive or compensation power of 30 MVar. The installation is designed so as to yield a power factor cos φ=0.9 given 100 kA and a drive level of the rectifiers (5, 5&#39;) with a rectifier manipulated variable signal (α act ) of 35° at the feed AC power supply network (1). 
     
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LIST OF REFERENCE SYMBOLS                                                 
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1           AC power supply network                                       
2, 2&#39;       Furnace transformers having a plurality                       
            of switching stages                                           
3, 3&#39;, 33   Current transformers                                          
4, 4&#39;       Filter branches for capacitive reactive                       
            power                                                         
5, 5&#39;       Rectifiers                                                    
6, 6&#39;, 29   Inductors, reactive elements                                  
7           Cathode, electrode                                            
8           Arc furnace                                                   
10          Arc                                                           
11          Melt, Molten bath                                             
12          Anode, 2nd electrode                                          
13, 17, 23, 25                                                            
            Summers                                                       
14          Current controller                                            
15, 27      Ignition pulse transformers                                   
16          Bandpass filter                                               
16&#39;         Damping element                                               
18          Electrode controller                                          
19          Valve amplifier                                               
20          Valve                                                         
21          Electrode adjusting device                                    
22, 24, 26  Function generators                                           
28          AC power controller                                           
30          Switch                                                        
31          Power factor compensator                                      
32          High-voltage network                                          
34          Voltage detector                                              
35          Phase-angle controller                                        
36          Characteristic                                                
37          Segment                                                       
cos φ   Power factor                                                  
i.sub.33    Total current intensity                                       
i.sub.act   Current actual value signal                                   
i.sub.act1, i.sub.act2                                                    
            Partial current actual value signals                          
i.sub.des   Current desired value signal                                  
Q.sub.F     Filter reactive power signal from 4, 4&#39;                       
Q.sub.G     Total reactive power signal                                   
Q.sub.8     Reactive power actual value signal from                       
            8                                                             
Q.sub.des8  Desired reactive power signal from 8                          
U.sub.1     Supply voltage of 1                                           
α.sub.act                                                           
            Rectifier manipulated variable signal,                        
            striking angle actual value                                   
α.sub.des                                                           
            Electrode controller reference variable                       
            signal, striking angle desired value                          
α.sub.st                                                            
            Striking angle for 27, stabilization                          
            striking angle signal                                         
φ des   Desired phase angle                                           
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