Abstract:
In an excitation device for stators of generators for high-induction measurement, the greatest possible compensation of the reactive power requirement is achieved. Depending on the size and energy requirement of the generator, a number of individual excitation modules with a corresponding number of individual excitation windings are provided, wherein a minimum necessary total excitation current can be supplied by superimposing the individual currents of the excitation windings.

Description:
RELATED APPLICATIONS 
     The present application hereby claims priority under 35 U.S.C. Section 119 to European Patent application number 11171988.6, filed Jun. 29, 2011, the entire contents of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present disclosure relates to an excitation device for stators and generators for high-induction measurement. 
     BACKGROUND 
     Excitation devices of this kind are used, for example, in the so-called high-induction testing of stators of large power station generators. For this type of testing, the stator is excited at mains frequency to a magnetic induction of approximately 1.5 Tesla. 
     The technical situation relating to high-induction stator measurement on power station generators in particular will be discussed here. 
     By this measuring process, the stator of a generator is magnetically excited by means of excitation coils and an alternating electrical current flowing through these coils. A conventional device is shown in  FIG. 1 . The figure shows the annular stator of a hydro generator in schematic form. The rotor is removed for this test. The windings of an excitation coil are arranged around this stator ring. For large stator diameters, individual coil segments are typically distributed symmetrically along the stator circumference for this purpose. 
     The excitation coil is operated with alternating voltage at mains frequency. With large generators, considerable voltages and currents and a large inductive reactive power are required in order to achieve the required nominal induction of 1.5 Tesla. Typical voltage values are several kV at a current strength of several kA, which means a reactive power requirement of several MVAr (energy content approx. 450 J/m 3  and reactive power 2*50*450 Ws/s=45 kVAr/m 3 ). This high reactive power requirement is one of the main problems when carrying out this measurement. 
     The electrical energy source (mains connection or generator) is often unable to supply this high unbalanced load. Furthermore, the high voltages used are basically hazardous to life, which means a significant outlay for safety measures. For these reasons, the measurement is very laborious. However, it is a compulsory requirement by many customers, especially in the acceptance testing of new generators after fabrication. 
     The reactive power requirement can be reduced by compensation by means of capacitors. However, there is then still the problem of high voltages, as the winding does not usually consist of one turn but has to be distributed around the circumference of the stator by means of a plurality of turns in order to effect a uniform field strength distribution. 
     A further solution consists in using an electronic power converter for supplying the excitation winding. The reactive power requirement of the excitation coil can be compensated by means of a suitably sized link circuit for the supply of energy. An advantage of this solution is that all load cases up to a maximum reactive power requirement can be covered. However, with this solution there is the problem that the converter has to be rated for the highest possible load case, which gives rise to relatively high costs. This rating relates in particular to the link circuit, which must absorb the reactive energy, and also to the electronic output components, which must withstand high voltages and currents. 
     Moreover, high voltages and currents which can endanger personnel are again present in the excitation coil. 
     SUMMARY 
     The present disclosure is directed to an excitation device for stators and generators for high-induction measurement with the greatest possible compensation of the reactive power requirement. Depending on the size and energy requirement of the generator, a plurality of individual excitation modules with a corresponding plurality of individual excitation windings are provided. The necessary total excitation current is provided by superimposing individual currents of the excitation windings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in more detail below with reference to the figures. All elements which are not important for the immediate understanding of the invention have been omitted. Identical elements in the different figures are given the same references. In the drawings: 
         FIG. 1  shows a device belonging to the prior art; 
         FIG. 2  shows an excitation device having four excitation modules; 
         FIG. 3  shows an individual excitation module; and 
         FIG. 4  shows an individual excitation module with two coil outputs. 
     
    
    
     The physical particulars and quantities in  FIGS. 1 to 4  are shown with the internationally applicable and usual designations for better understanding by the person skilled in the art. The introduction of references for the individual physical particulars and quantities would make the description unnecessarily difficult to understand. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS INTRODUCTION 
     The invention, as described in the appended claims, is based on the object of proposing an improved device for the excitation which requires little reactive power from the energy source, which manages with lower supply voltages and supply currents, which produces lower voltages and currents at the outputs and is therefore less dangerous, and which is flexible in application, i.e. suitable for a multiplicity of load cases. 
     The device according to the invention provides a cost-effective, flexible and far less dangerous excitation device for stators of generators for high-induction measurement, and also substantially compensates the reactive power requirement. 
     The solution of the problem according to the invention is achieved by the use of a modular design of the overall excitation system. Depending on the size and excitation requirement of the generator, a number of individual excitation modules, hereinafter also referred to as individual modules for short, are used together with a corresponding number of individual excitation windings, which in totality supply the necessary excitation current by superimposing the individual currents. 
     Each of the individual modules supplies part of the total excitation current required via its own excitation winding, which consists of a number of dedicated excitation turns. The current feeds to the individual modules are controlled and synchronized with one another so that the required total current flows with the required time characteristic by superimposing the individual currents. 
     Advantageously, the individual single modules are each equipped with individual energy-supplying electrical link circuits so that the individual modules mainly draw only active power from the supplying energy source. In a non-limiting embodiment, the individual link circuits are equipped with energy-storing device. 
     Furthermore, advantageously, the individual excitation coils of the individual modules are electrically connected or isolated from one another in such a way that no excessively high voltages can occur between the windings. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a conventional device  1 . This figure therefore in particular shows the annular stator  2  of a hydro generator in schematic form. The rotor is removed for this test. The windings of an excitation coil are arranged around this stator ring. For large stator diameters, individual coil segments are typically distributed symmetrically along the stator circumference for this purpose. 
     The excitation coil is operated with alternating voltage at mains frequency. With large generators, considerable voltages and currents and a large inductive reactive power are required in order to achieve the required nominal induction of 1.5 Tesla. Typical voltage values are several kV at a current strength of several kA, which means a reactive power requirement of several MVAr (energy content approx. 450 J/m3 and reactive power 2*50*450 Ws/s=45 kVAr/m 3 ). This high reactive power requirement is one of the main problems when carrying out this measurement. 
       FIG. 2  shows an excitation device  10  having four individual single excitation modules  30   a - 30   d . All individual modules  30   a - 30   d  are supplied from the same 400 Vrms energy source  40 . The individual modules  30   a - 30   d  generate the electrical currents i 1 , i 2 , i 3 , i 4 , which flow through the associated individual excitation windings  45   a - 45   d . The total excitation current is given by superimposing the individual currents. 
     The individual excitation windings  45   a - 45   d  themselves are not electrically connected to one another. However, there can be an electrical connection of the individual windings via the electronics of the excitation modules  30   a - 30   d  or via their connections to the common energy source. However, this connection is devised in such a way that there is no electrical series connection of the individual windings  45   a - 45   d , so that the individual winding voltages do not add together and therefore no excessively high voltages can occur between the windings  45   a - 45   d . The use of heavy medium-voltage cables is therefore unnecessary. 
     An important characteristic of the device  10  is to be seen in that the individual excitation modules  30   a - 30   d  have an output current controller  80 . They therefore act as current sources and supply the required output current regardless of the load situations occurring at any one time. This is important to implement a uniform current distribution of the total excitation current between the modules. Without an effective current controller, the output currents would be highly dependent on the different impedances of the individual windings, the different voltage levels of the link circuits, the different switch characteristics, etc. 
     In order to simplify the current control, the individual modules advantageously have an output filter which to some extent dynamically isolates the current control circuit of the individual module from the overall excitation circuit. Typically, this filter comprises at least a series inductor in the output circuit or, in an improved embodiment, R, L, C filters with series arrangements of inductors and parallel arrangements of capacitors. 
     Furthermore, a monitoring circuit, which feeds back the load conditions, typically the voltage across the respective individual excitation coils which is driven by the individual excitation module, to the individual controller, can be arranged for improved control. 
     Each of the individual modules  30   a - 30   d  has its own energy store, which in each case can absorb part of the total reactive energy which occurs when the laminated core is excited. 
     In a particular embodiment, an individual module can drive a plurality of individual excitation windings via individual current controllers in each case, wherein a common intermediate energy store is used. In a non-limiting embodiment, the excitation wingings of the individual excitation moduldes have an electric circuit which minimizes a voltage between the excitation windings. 
     The current setpoints of the individual excitation modules are typically obtained from a central control module  100  ( FIG. 3 ) which outputs a sinusoidal current setpoint, for example, to the individual modules  30   a - 30   d . Advantageously, all sub-modules have the same current demand. Furthermore, the central control module  100  can be connected to sensors, by means of which the magnetic state of the laminated core can be measured, e.g. the magnetic flux in the stator core. 
     However, the individual modules can also be designed so that they themselves generate an alternating current with specified frequency and specifiable time characteristics or amplitude. In this case, a simple time synchronization device, which effects a synchronous current output of the individual modules, is sufficient. Among other things, this requires that the phase angle of the current output of the individual modules can be manipulated. 
     The individual excitation modules  30   a - 30   d  according to  FIG. 3  typically include the following function blocks which can be seen qualitatively from  FIG. 4 : 
     The input AC/DC converter  50 : this converts the alternating input voltage into a DC quantity, typically a DC voltage. Advantageously, the input circuit and the output circuit of this converter can be galvanically isolated from one another in order to guarantee higher personnel safety against electric shock on the load side. The input converter  50  can be supplied from two or more phases. 
     Energy storage  60 : This function block is used for storing the reactive energy which occurs transiently. Typically, the energy is stored in capacitors. 
     The output DC/AC converter  70 : this converts the direct voltage of the link circuit into an alternating quantity, typically an output voltage. The time characteristic of this alternating quantity can be manipulated by means of a control input. 
     The current controller  80 : the controller  80  controls the output converter in such a way that the required output current i(t) is generated in the form of a control variable according to the applied setpoint. The setpoint is fed in via an appropriate input. 
     The output filter  90 : this can optionally be arranged to dynamically decouple the control variable i(t) to a certain extent from the voltage which appears across the excitation coil, and thus ease the control task. 
     In the meantime, another version of the individual module  30  is shown in  FIG. 4 . This variant can drive two excitation coils independently of one another, wherein, however, only a single intermediate energy store  60  and a single input converter  50  are used. Output converter  70 , controller  80  and filter  90  are duplicated. The number of these output blocks can be increased so that further coils can be operated independently of one another. 
     This variant can advantageously be used when a large-scale distribution of the excitation coils is not required. This version is therefore intrinsically well suited for a turbogenerator. The use of a plurality of these multiple output modules can also provide advantages for large-scale distributed arrangements, as it is possible to work with further reduced supply voltages and winding voltages. 
     In a further advantageous version, the individual converter has a galvanically isolated input stage (galvanically isolated AC/DC input converter) and one or more output stages which act in a switchable manner as controllable, regulated current sources or controllable, regulated voltage sources. This enables a current source output to be connected in series with one or more voltage source outputs in order to achieve higher voltage levels. 
     Calculation Examples 
     Generator 
     An excitation of 20,000 Ampere turns is required in order to achieve a core induction of 1.5 Tesla. At the same time, a voltage of 470 V per turn again occurs (1 m 2  core area). 
     Excitation Device a):
     One (1) winding and a single converter with the following data:   Winding: 10 turns   Converter:   Output current: 2000 Ampere   Output voltage: 4700 V   

     Excitation Device b):
     Four (4) converters with individual outputs and four (4) individual windings with the following data:   Individual winding: 5 turns   Converter:   Output current: 1000 Ampere   Output voltage: 2350 V   

     Excitation Device c):
     Four (4) converters with triple outputs; 12 individual windings with the following data:   Individual winding: two (2) turns   Converter:   Output current per individual winding: 833.3 Ampere   Total output current per converter: 3×833.3 Ampere   Output voltage: 940 V   

     Excitation for an Implemented System:
     Core approx. 5 m 2  (7 m×0.7 m) Cross-section: . . . 2.5 kV/turn   Mean radius: 1.3 m . . . d=2.6 m 8 m circumference . . . approx. 3500 Ampere turns   

     The devices are distinguished as follows: 
     The overall excitation device comprises a plurality of sub-modules, which each supply part of the excitation current, and a plurality of sub-windings which are connected to these sub-modules. 
     Each of the sub-modules is designed as a complete converter which converts the input voltage, has a link circuit for energy storage, and supplies the required output current. 
     On the output side, the sub-modules act as current sources which supply the partial excitation current regardless of the load voltage occurring at any one time. 
     The time characteristic of the output current of the sub-module is controlled via a control input. 
     The current outputs of the sub-modules of the excitation device are controlled so that the required total excitation current flows. 
     In a preferred version, the output stages of the individual module can act as current sources and also, alternatively, as voltage sources, wherein the method of operation can be selected by means of a switch. 
     In a further preferred embodiment, the outputs are galvanically isolated from ground and from the voltage inputs. 
     The device can also be used for other excitation tasks. 
     LIST OF REFERENCE NUMERALS 
     
         
           1 ,  10  Device 
           2 ,  20  Stator 
           30   a - d  Excitation modules 
           40  400 Vrms energy source 
           45   a - d  Individual excitation windings 
           50  DC/AC input converter 
           60  Energy storage 
           70  DC/AC output converter 
           80  Current controller 
           90  Output filter 
           100  Central control module