Abstract:
The present invention relates to a method for injecting electrical energy into an electrical, three-phase grid, comprising the steps of: injecting current by means of an injection unit at a grid connection point, detecting an asymmetry in the grid, in particular a negative sequence component in the grid, injecting an asymmetrical current portion into the grid for, at least, partial compensation of the detected asymmetry, with injection of the asymmetrical current portion taking place such that the injection unit behaves like a consumer.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a method for injecting electrical energy and a device, in particular a wind power installation, for injecting the electrical energy. 
         [0003]    2. Description of the Related Art 
         [0004]    Nowadays, electric grids, which may be hereinafter also simply referred to as electric nets or electric networks, are increasingly supplied by regenerative sources of energy, such as wind power installations or wind farms exhibiting a different electrical behavior than common large-scale power plants, which employ at least one large-scale generator for injecting current. This means that such large-scale generators are being increasingly replaced with other injection units, such as cyclo-inverters. This is also referred to by experts as substitution. Countries like Germany, in particular, have a comparatively high degree of substitution, which means that comparatively many generators are replaced with other injection units. This may also have fundamental effects on the grid. The main concerns are that the possible balancing effects of the former injecting generators might be lost or at least weakened with an increasing degree of substitution. 
         [0005]    This is why the proposed European Network Directive ENTSO-E provides for network operators to be able to demand an asymmetrical current injection. Here, the notion of symmetry or asymmetry relates to the correlation of the three phases of a three-phase grid to each other. Especially in the case of an asymmetrical disturbance in the grid, for example a short circuit between two phases or a short circuit of one phase to ground, it is provided to inject the electrical energy in as compensatory a manner as possible. A disturbance is to be assumed, in particular, if the actual voltage in the grid of at least one phase departs by more than 10% from its set point value and/or its rated value. 
         [0006]    Initial objectives do exist, but they may not be reaching far enough. 
         [0007]    The German Patent and Trademark Office has researched the following prior art in the priority application: DE 10 2006 054 870 A1; U.S. Pat. No. 7,423,412 B2; ANDERSSON, G.: Elektrische Energiesysteme—Vorlesungsteil EnergieUbertragung, p. 127-147, EEH—Power Systems Laboratory, ETH Zurich, September 2009; Symmetrische Komponenten, in Wikipedia, Die freie Enzyklopadie (Wikipedia, the free encyclopedia), Version of 23 Apr. 2012, URL: http:de.wikipedia.org/w/index.php?title=Symmetrische_Komponenten&amp;oldid=102361863 [called up on 29 Jul. 2012]. 
       BRIEF SUMMARY 
       [0008]    One or more embodiments of the present invention are directed to improving grid quality or, at least, for making a contribution so that the grid quality does not become worse or significantly worse. It shall at least propose one alternative solution to already known concepts. 
         [0009]    Hence, electrical current is injected into the three-phase grid by means of an injection unit at a grid connection point. In addition, an asymmetry is recorded in the grid, which can be done, in particular, by recording a negative sequence component. An asymmetrical current portion is injected into the grid in answer thereto, in order to compensate for at least part of the recorded asymmetry. In this context, it is proposed to inject this asymmetrical current portion such that the injection unit behaves like a consumer in the area of the so-called negative sequence. The targeted injection of the asymmetrical current portion, i.e. the targeted asymmetrical injection, takes place by means of a corresponding definition of such consumer. This type of solution is based on the idea of seeing the injection unit&#39;s behavior as part of the grid and considering it in the overall behavior of the grid. 
         [0010]    The consumer is preferably referred to as impedance  Z   −  and defined by means of the following equation: 
         [0000]    
       
         
           
             
               
                 Z 
                 _ 
               
               - 
             
             = 
             
               
                 Z 
                 n 
               
                
               
                 
                   
                      
                     
                       jϕ 
                       - 
                     
                   
                   
                     k 
                     - 
                   
                 
                 . 
               
             
           
         
       
     
         [0011]    Impedance  Z   −  is thus defined by the value of rated impedance Z n , adjustment phase angle φ −  and scalar adjustment factor k − . 
         [0012]    The value of rated impedance Z n  can be defined through the following equation: 
         [0000]    
       
         
           
             
               Z 
               n 
             
             = 
             
               
                 
                   V 
                   n 
                   2 
                 
                 
                   S 
                   n 
                 
               
               . 
             
           
         
       
     
         [0013]    This value of impedance Z n  is thus calculated from line voltage V n , which here goes quadratically into the numerator, and from the injected apparent power S n , which here goes in the denominator of the quotient. Solely by way of precaution, it is pointed out that Z n  is referred to as the value of the rated impedance for the purpose of better illustration. For, in fact, the value of impedance  Z   −  does also depend on the adjustment factor k −  and on the adjustment phase angle φ − . 
         [0014]    The value of the negative impedance can thus be adjusted via the adjustment factor k −  and the adjustment phase angle φ − , and is hence presettable as needed. It is moreover proposed to preset the adjustment phase angle as needed. The idea is thus to go further and to not merely provide, for example, a reactance, i.e., an impedance with an adjustment phase angle of 90° or, respectively, −90°, whereby the angle—like the amplitude—is also set as needed. 
         [0015]    According to one embodiment, it is proposed to set the adjustment factor k −  and the adjustment phase angle φ −  of the impedance based on at least one net property. Thus, the specification or setting of such impedance is not only geared to current conditions within the grid, which is basically also referred to as a net to simplify matters, but it also takes into account net properties, i.e., properties of the grid. The voltage level in the grid, an existing asymmetry, or even a disturbance in the grid are examples of grid conditions. The grid reactance to resistance ratio, which is also referred to as the XR ratio, is an example of a grid property. This and other net properties must be seen in particular in relation to the grid connection point. Therefore, such grid properties regularly also depend on the geographical position of the grid connection point, at any rate in relation to the grid concerned. 
         [0016]    It is thus proposed to not only look at the current grid conditions but also at the grid properties. 
         [0017]    The adjustment phase angle φ is preferably set within a range of 0°-90°. The bigger the grid reactance to resistance ratio at the grid connection point—i.e., the bigger the XR ratio—the bigger such angle will be set. In the case of a large XR ratio, for example in a range of 10-15, the adjustment phase angle may be set close to 90°. If that ratio is smaller, for example having a value of 2, the aforementioned angle may be proposed to be set in a range of 50°-60°. Hence, this grid property, which may be also a net feature, can be considered in addition to the conditions within the grid. 
         [0018]    Preferably, an equivalent circuit diagram of the grid will be prepared for the injection point to serve as a basis for adjusting the consumer, in particular the impedance. In particular, the adjustment phase angle φ −  and/or the adjustment factor k −  will be adjusted based on the identified equivalent circuit diagram. Such equivalent circuit diagram, which in particular is supposed to reflect relevant grid properties, may even be prepared once or at least rarely at the grid connection point or in relation to the grid connection point. Such equivalent circuit diagram reflecting the grid properties thus is not subject to any, or is subject only to minor changes, like the described grid properties. In any event, the grid properties will basically change more rarely or slowly than the grid conditions. 
         [0019]    The asymmetry of the grid is preferably identified by identifying or determining a grid negative sequence component of the voltage within the grid. This means that the voltages of the three phases are identified and broken down into a positive and negative sequence according to the method of symmetrical components. For the sake of completeness, it is pointed out that the zero sequence, which is also included in the theory of the method of symmetrical components, is to be regularly disregarded. Asymmetry thus can be easily considered by looking at the negative sequence component. According to one embodiment, it is moreover or additionally proposed that the asymmetrical current portion be specified or injected as a negative sequence component. A negative sequence component is thus not only used for measuring, but also for concrete injection or at least preset for injection. 
         [0020]    An inverter is preferably used as an injection unit. At any rate, the injection unit comprises such an inverter and uses it to a significant degree for injecting the electrical energy. The use of such an injection unit allows for the injection of regeneratively generated electrical energy into the grid under consideration of grid requirements. By means of such an inverter, the current to be injected may be basically also adjusted dynamically according to value, frequency and phase. In this way, the inverter that is used as an injection unit may be set to exhibit the behavior of a consumer or to show impedance as a property, as desired. 
         [0021]    Preferably, the proposed method also includes checking the grid for an asymmetrical system incident. Asymmetrical injection, as described in at least one of the above embodiments, is proposed in the case that no asymmetrical system incident has been detected. This means that the injection unit is to behave like a consumer, in particular impedance, if there is no asymmetrical system incident. These methods, as described, are hence provided in particular to consider—and especially enhance—grid quality during normal operation of the electric grid. 
         [0022]    According to one embodiment, current is injected into a medium-voltage grid, and to this end the adjustment phase angle φ −  is set to a value in the range of 40°-70°, in particular 50°-60°. When it comes to medium-voltage grids, one must reckon with a comparatively small XR ratio, for example in the range of 2. It is thus proposed to set a corresponding impedance that, due to the aforementioned adjustment phase angle, is better adapted to the nature of such a medium-voltage grid than when using a different adjustment phase angle, especially a larger adjustment phase angle. 
         [0023]    What is further proposed is a wind power installation for injecting electrical energy generated from wind energy, which is prepared for the application of a method pursuant to at least one of the above-described embodiments. Such wind power installation for injecting current will, in particular, feature an inverter as an injection unit. 
         [0024]    With such inverter or other injection unit, a counter current component is injected and thus the impedance of the negative sequence is specified. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0025]    The invention is described in more detail below based on embodiments with reference to the accompanying figures. 
           [0026]      FIG. 1  shows a wind power installation in a perspective view. 
           [0027]      FIGS. 2   a  to  2   c  explain the concept of asymmetrical current injection. 
           [0028]      FIG. 3  explains the proposed injection method according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1  shows a wind power installation  100  with a tower  102  and a nacelle  104 . A rotor  106  with three rotor blades  108  and a spinner  110  is located on the nacelle  104 . When in operation, the rotor  106  is set into rotation by the wind and thereby drives a generator in the nacelle  104 . 
         [0030]    The following is explained with reference to  FIGS. 2   a ,  2   b  and  2   c.    
         [0031]    The fundamental frequency content of voltages (and currents) is represented via phasors in symmetrical components 
         [0000]        v   a =√{square root over (2)} V   a  cos(2π ft+φV   a )             V     a   =V   a   e   jφV     a    
 
         [0000]        v   b =√{square root over (2)} V   b  cos(2π ft+φV   b )             V     b   =V   b   e   jφV     b    
 
         [0000]        v   c =√{square root over (2)} V   c  cos(2π ft+φV   c )             V     c   =V   c   e   jφV     c    
 
         [0000]    and transformed as usual: 
         [0000]    
       
         
           
             
                
               
                 
                   
                     
                       
                         V 
                         _ 
                       
                       0 
                     
                   
                 
                 
                   
                     
                       
                         V 
                         _ 
                       
                       + 
                     
                   
                 
                 
                   
                     
                       
                         V 
                         _ 
                       
                       - 
                     
                   
                 
               
                
             
             = 
             
               
                 1 
                 3 
               
                
               
                  
                 
                   
                     
                       1 
                     
                     
                       1 
                     
                     
                       1 
                     
                   
                   
                     
                       1 
                     
                     
                       
                          
                         
                           j 
                            
                           
                             2 
                             3 
                           
                            
                           π 
                         
                       
                     
                     
                       
                          
                         
                           j 
                            
                           
                             4 
                             3 
                           
                            
                           π 
                         
                       
                     
                   
                   
                     
                       1 
                     
                     
                       
                          
                         
                           j 
                            
                           
                             4 
                             3 
                           
                            
                           π 
                         
                       
                     
                     
                       
                          
                         
                           j 
                            
                           
                             2 
                             3 
                           
                            
                           π 
                         
                       
                     
                   
                 
                  
               
                
               
                  
                 
                   
                     
                       
                         
                           V 
                           _ 
                         
                         a 
                       
                     
                   
                   
                     
                       
                         
                           V 
                           _ 
                         
                         b 
                       
                     
                   
                   
                     
                       
                         
                           V 
                           _ 
                         
                         c 
                       
                     
                   
                 
                  
               
             
           
         
       
     
         [0032]    The unbalancing level used as a metric for unbalancing is given by the ratio of the magnitudes of the negative respective zero and positive sequence phasor: 
         [0000]        V   —   /V   +  respectively  V   0   /V   + 
 
         [0033]    Grid connected inverters can be interpreted by typical (time and state dependent) equivalents with respect to fundamental frequency and (quasi-) steady state operation conditions. One option applicable for non-isolated operation conditions of the inverter is an impedance equivalent ( FIG. 2   a ). Due to the vector group of the transformer in the test power system a zero sequence equivalent is not of relevance for the inverter operated. The positive sequence impedance is determined by the standard power control layer of the inverter FACTS-control architecture, the negative sequence impedance is controlled by additional ACI-control ( FIG. 2   c ). 
         [0034]    Both sequence impedances influence the physical behavior simultaneously. They depend on actual terminal sequence voltages and the actual magnitude and reference of the currents of the inverters which are independently controlled for positive and negative sequence ( FIG. 2   b ). Negative real parts of the impedances indicate injection of active power in the grid, for reactive power negative imaginary parts respectively. Interpretation of this representation is limited to non-isolated operation conditions of the inverter. 
         [0035]    With respect to the magnitudes of the sequence-voltages the power exchange between inverter and grid during normal operation condition will be absolutely dominated by the positive sequence. Positive sequence impedance during normal operation condition therefore can be interpreted as consequence from actual total inverter-power and actual positive sequence terminal voltage. 
         [0036]    Negative sequence impedance specified from independent ACI-considerations will be achieved via negative sequence inverter-currents depend on actual negative sequence terminal voltage. This functionality provides an additional ACI control-module which belongs therefore to the power control layer of the architecture of inverter-control applied ( FIG. 2  right). Vector control generates the input signal for PWMcontrol as usual. 
         [0037]    The abbreviation ACI stands for “Asymmetrical Current Injection”. Solely by way of precaution, it is pointed out that FACTS stands for “Flexible AC Transmission System,” a term also commonly used in German language professional circles. 
         [0038]      FIG. 2   a  hence illustrates the layout of the control unit of an inverter  2  according to one embodiment such that it is broken down into the control and injection of portion  4  in the positive sequence and the control and thus injection of portion  6  in the negative sequence. This means that, for the positive sequence, an impedance  Z   +  is controlled, which has a real portion that is negative and which can be defined by the values  I   +   V   + . Accordingly, the negative sequence uses the impedance  Z   −  and thus the electrical values  I   −   V   − . 
         [0039]    The meaning of these two impedances  Z   +  and  Z   −  is shown on a complex level in the diagram of  FIG. 2   b.    
         [0040]      FIG. 2   c  shows by means of a wiring diagram, part of which is shown as a block diagram, how injection takes place according to one embodiment. 
         [0041]    At the three-phase grid  8 , which has phases marked with letters a, b and c, the voltage v(t) of all three phases is recorded at measuring point  10  and supplied to breakdown block  12 . Breakdown block  12  breaks down the thus recorded three-phase system into the positive sequence component of voltage v +  and the negative sequence component of voltage v − . The result, along with the positive and negative sequence components of the voltage, is delivered to injection default block  16  via yet another calculation block  14 , which determines required values, such as the reactive power Q. Injection default block  16  then determines the positive and negative sequence portions that are to be injected of the current that is to be injected, and to this end determines a d-portion and a q-portion each for the positive sequence current and for the negative sequence current. This may be also indicated in abbreviated form as d−, q−, d+ and q+. Information on the DC link voltage Vdc may also be delivered to injection default block  16 . Calculation block  14  and, in particular, injection default block  16  thus form power control block  18 . 
         [0042]    The values gathered from power control block  18 , in particular from injection default block  16 , are supplied to vector control block  20 , which in negative sequence block  22  or, respectively, in positive sequence block  24  determines the corresponding vectors for controlling the respective phase to be injected. In addition, negative sequence block  22  and positive sequence block  24  exchange information with breakdown block  12 . To this end, conversion block  26  converts the two vectors of the positive and negative sequence of the current to be injected into the concrete parameters of the phase currents to be injected and supplies this information to phase blocks  28   a ,  28   b  or, respectively,  28   c . To this end, block  26  determines the individual currents i aref , i bref  or, respectively, i cref  pursuant to the following calculation: i aref =i− aref +i+ aref +; i bref =i− bref +i+ bref  or, respectively, i cref =i− cref +i+ cref . These values are then delivered to tolerance band control blocks  30   a ,  30   b  or, respectively,  30   c  in inverter block  32 . Tolerance band control blocks  30   a ,  30   b  or, respectively,  30   c  then perform concrete actuation of the inverter bridges of inverter  34  via a known tolerance band control and may, in the process, consider the actual current i(t). 
         [0043]      FIG. 3  shows grid  15  as the starting point of control according to one embodiment. Grid  50  acts in particular through measurements onto a very general control, which is marked as grid control block  52 . In order to adjust an impedance  Z   − , such general grid control may specify values for the adjustment factor k −  or, respectively, k AB   −  and for the adjustment phase angle φ −  or, respectively, φ AB   − . Here, index AB means normal operation of grid  50 , i.e., operation without any system incidents. But there may be certain asymmetries. 
         [0044]      FIG. 3  also suggests that in the case of an asymmetrical disturbance, a constant value, such as 2, is set for adjustment factor k −  or, respectively, k VNSR   − . In such case, an absolute value of 90° is specified for adjustment phase angle φ −  or, respectively, φ VNSR   − . VNSR here means “Voltage Negative Sequence Reactance,” whereby for the negative sequence a reactance is specified in the case of a disturbance. In such case of an asymmetrical disturbance in the grid, no variable adjustment phase angle φ −  is used; instead, a pure reactance is applied as a consumer. 
         [0045]    Inverter control block  54  controls inverter  2  accordingly. Here, inverter  2  corresponds to that in  FIG. 2   a , and reference sign  54  for an inverter control block  54  has also been used in  FIG. 2   a . However,  FIG. 2   a  and  FIG. 3  are schematic illustrations and may differ in terms of their details. 
         [0046]    The controlling of inverter  2  by inverter control block  54 , as shown in 
         [0047]      FIG. 3 , comprises various control processes, and reference is therefore again made to the control process explained in  FIG. 2   c . However, when it comes to illustrating the aspect of how the impedance is specified,  FIG. 3  illustrates only the delivery or rather action of adjustment factor k −  and adjustment phase angle φ −  onto inverter  2 . But inverter control is not limited to only specifying such values. 
         [0048]    The dashed arrow also indicates a possible reaction of inverter  2  or of factors existing at inverter outlet  56  onto inverter control block  54  and thus onto the inverter control. Finally, inverter  2  releases a three-phase, asymmetrical current for injection at its inverter outlet  56  and injects it into grid  50  at grid connection point  60  via the illustrated transformer  58 .