Patent Publication Number: US-2011057631-A1

Title: Static exciter of an electric generator, method for retrofitting, and method for operating

Description:
This application claims priority under 35 U.S.C. §119 to European application no. 09169645.0, filed 7 Sep. 2009, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of Endeavor 
     The present invention relates to a static exciter of an electric generator, a method for retrofitting, and a method for operating the same. 
     2. Brief Description of the Related Art 
     Electric synchronous generators include a stator typically having an armature circuit and a rotor typically hosting a field circuit. 
     During operation, while the rotor rotates, the field circuit is energized such that electric voltage is induced in the armature circuit, due to the flux rate of change. 
     In order to energize the field circuit, usually a transformer is connected to the grid (or busbar) at its high voltage side and has its low voltage side connected to an AC/DC converter. 
     This AC/DC converter rectifies the AC voltage supplied from the transformer into DC voltage and supplies it to the field circuit of the generator. 
     Since the electric power for feeding the field circuit is directly supplied from the grid, in case a failure or disturbance at the grid occurs, causing the voltage at the grid to temporary fall, the stability of the generator/grid system may be compromised such that grid and generator may lose their synchronization. 
     Electric grids are often provided with automatic security and protection systems that intervene when such failures occur to restore the nominal grid voltage; nevertheless it usually takes up to some seconds to restore the grid voltage, thus the stability of the generator/grid system cannot be guaranteed if a failure occurs. 
     U.S. Patent Application Publication No. 2007/0296275 discloses providing a security circuit arranged to inject electric power into the field circuit in case a failure at the grid occurs, to guarantee the stability of the generator/grid system and to prevent losing synchronization. 
     This security circuit includes a capacitor bank, a switch to be activated in case of grid failure, and a diode to be directly inserted into the field circuit (i.e., this diode must be placed on the rotor). 
     In case a failure at the grid occurs, the switch is activated such that the capacitor bank supplies its energy into the field circuit. 
     Nevertheless, even if this technique proved to be very efficient, its implementation may be difficult, costly and time-consuming in some cases, since it requires the diode to be placed on the rotor. 
     In particular, retrofitting of existing electric generators is difficult because the generator casing must be opened and the diode must be installed on the rotor. 
     This operation is particularly time consuming and costly. 
     SUMMARY 
     One of numerous aspects of the present invention includes a static exciter, a method for retrofitting, and a method for operating a static exciter by which the aforementioned problems of the known art are addressed. 
     Another aspect of the present invention includes a static exciter and a method for operating the same that can be easily, cost-efficiently, and time-efficiently implemented. 
     Another aspect includes a retrofitting method that can be easily, cost-efficiently, and time-efficiently implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the invention will be more apparent from the description of a preferred but non-exclusive embodiment of the static exciter and methods according to the invention, illustrated by way of non-limiting example in the accompanying drawings, in which: 
         FIG. 1  is a schematic view of the static exciter in a first embodiment of the invention; 
         FIG. 2  illustrates a detail of a second embodiment of the static exciter of the invention; 
         FIG. 3  illustrates a third embodiment of the static exciter of the invention; and 
         FIG. 4  illustrates a detail of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     With reference to the figures, a static exciter  1  of an electric generator  2  is illustrated. 
     The electric generator  2  has a field circuit  3  and a busbar  4  (stator three-phase circuit) connected to an electric grid  5  via a transformer  6 . 
     The static exciter  1  includes a main transformer  8  having its high voltage side connected to the grid  5  (typically, as shown in the figures, the main transformer  8  is directly connected to the busbar  4  and is connected to the grid  5  via the transformer  6 ) and its low voltage side connected to a field circuit converter  9  (typically a rectifier). 
     The field circuit converter  9  converts the AC voltage from the main transformer  8  and feeds the generator field circuit  3  with a DC voltage. 
     The static exciter  1  further includes a control unit  11  arranged to detect the status of the grid  5  to cause at least a capacitor bank  13  to supply energy into the generator field circuit  3  in case the grid voltage falls below a prefixed voltage value. 
     The capacitor bank  13  is connected between the low voltage side of the main transformer  8  and the AC side of the field circuit converter  9 . 
     Advantageously, the static exciter also includes at least a coupling circuit  15  connecting the capacitor bank  13  to connection lines  16  between the low voltage side of the main transformer  8  and the AC side of the field circuit converter  9 . 
     In the following, different embodiments embodying principles of the present invention will de described in detail. 
     Embodiment 1 
     In a first embodiment of the static exciter  1  ( FIG. 1 ), the coupling circuit  15  includes an auxiliary three-phase converter  17  having a DC side connected to the capacitor bank  13  and an AC side coupled to the connection lines  16 . 
     Preferably the auxiliary converter  17  is a bidirectional converter, such that charging and discharging of the capacitor bank  13  is allowed. 
     For example, the auxiliary converter  17  is made of a power semiconductor element bridge (IGBT, thristors); in any case these converters are well known in the art and any suitable converter or power semiconductor element disposition may be used. 
     The AC side of the auxiliary converter  17  is inductively coupled to the connection lines  16  and, in this respect, the inductive coupling is realized via an auxiliary transformer  18 . 
     In this embodiment of the invention, the control unit  11  is connected to at least two auxiliary connection lines  19 . Naturally a connection also to all three auxiliary connection lines  19  may be provided and, in case of different configuration, for example more or less than three phases, more or less than two connections may be provided. 
     In addition, the control unit  11  is connected to the auxiliary converter  17  to drive it. 
     The control unit  11  detects the voltage of the auxiliary connection lines  19  between the auxiliary converter  17  and the auxiliary transformer  18  and drives the auxiliary converter  17  accordingly. 
     In particular, the control unit  11  drives the auxiliary converter  17  such that it lets the capacitor bank  13  be discharged or charged. 
     The control unit  11  is also connected to the capacitor bank  13  to control its charging level. 
     The operation of the static exciter in this embodiment is apparent from that described and illustrated and is substantially the following. 
     During normal operation (with the voltage grid at its nominal value) the control unit  11  drives the auxiliary converter  17  such that it lets the electric power pass through it from the auxiliary transformer  18  to the capacitor bank  13 . 
     In this situation the capacitor bank  13  is charged, such that in case of failure it is always charged and ready to discharge its energy into the field circuit  3 . 
     When at the grid  5  a failure occurs, the grid voltage falls and, thus, the busbar voltage also falls and the voltage at the low voltage side of the main transformer  8  falls; this causes also the voltage at the auxiliary connection lines  19  to fall. 
     When the control unit  11  detects that the voltage at the auxiliary connection lines  19  has fallen below a prefixed voltage value, it drives the auxiliary converter  17  such that it allows the electric power to pass through it, from the capacitor bank  13  to the auxiliary transformer  18 . 
     This causes the capacitor bank  13  to be discharged and its energy to be provided, through the auxiliary converter  17  and auxiliary transformer  18 , to the connection lines  16  and thus, through the field circuit converter  9 , to the field circuit  3 . 
     As known in the art, when a failure at the grid  5  occurs, a number of automatic protection and control systems intervene to restore the grid voltage; usually restoration of the grid voltage is carried out in few seconds. 
     Therefore, as soon as the grid voltage is restored (thus also the auxiliary connection line voltage is restored) the control unit  11  drives the auxiliary converter  17  such that it allows electric power to pass though it from the auxiliary transformer  18  to the capacitor bank  13  again. 
     Thus the capacitor bank  13  is charged to be ready to intervene in case of further grid failures. 
     Embodiment 2 
       FIG. 2  shows a second embodiment of the static exciter embodying principles of the present invention. In  FIG. 2  elements similar or the same as those already described with reference to the first embodiment have the same references. 
     The static exciter in the second embodiment has the same structure and features of that of the first embodiment. 
     In addition, this static exciter has three capacitor banks  13  each connected to an auxiliary single phase converter  17  connected to an auxiliary transformer  18  coupled to a connection line  16  between the main transformer  8  and the field circuit converter  9 ; moreover, a control unit  11  for each auxiliary converter  17  is provided or, alternatively, a single control unit  11  controlling independently each auxiliary converter  17  is provided (this embodiment is shown in  FIG. 4 ). 
     In other words, the static exciter has one electric power injection circuit for each phase (each electric power injection circuit including a capacitor bank  13 , an auxiliary converter  17 , and an auxiliary transformer  18 ), whereas the embodiment of  FIG. 1  has only one three-phase electric power injection circuit. 
     The operation of the static exciter in this embodiment is clear in particular in light of the operation of the static exciter of the first embodiment. 
     Embodiment 3 
       FIGS. 3 and 4  show a third embodiment of the static exciter  1  embodying principles of the present invention. Also in these figures, elements similar or the same as those already described have the same references. 
     In particular, in  FIG. 3  the main transformer  8  and the field circuit converter  9  are shown connected by connection lines  16 . 
     Each connection line  16  has a first branch  20  ( FIG. 4 ) with a first switch  21  and the coupling circuit  15  comprises a second branch  22  in parallel with the first branch  20 . 
     The second branch  22  has a second switch  23  in series with a one-way component  24 ; also the capacitor bank  13  is series with these components. 
     The control unit  11  (also in this case a single control unit  11  for each connection line  16  or a single control unit  11  that independently controls the switches  21 ,  23  of each connection line  16 ) is connected to the respective connection line  16  and, in addition, drives the switches  21  and  23 , i.e., it detects the voltage of the connection line  16  and operates the first and second switches  21 ,  23  accordingly. 
     The one-way component  24  can be a diode arranged to cause the capacitor bank  13  to supply energy to the field circuit  3  when it is discharged. 
     In addition, also a charging component  25  such as a battery or the like in parallel with the capacitor bank  13  may be provided. 
     It is anyhow clear that (thanks to the diode  24 ) charging of the capacitor bank  13  may also be carried out during operation via the branch  22  (i.e., with switch  21  open and switch  23  closed) in case there is no fault at the grid  5  and the voltage at the connection lines  16  has its nominal value. 
     The operation of the static exciter  1  in the third embodiment of the invention is apparent from what described and illustrated and is substantially the following. 
     During normal operation (i.e., the grid voltage is at its nominal value) the switch  21  is closed (dashed line in  FIG. 4 ) whereas the switch  23  is open (dashed line in  FIG. 4 ); therefore during normal operation the branch  20  is operative whereas the branch  22  is not operative. 
     When a failure occurs and the grid voltage falls, also the voltage at the low voltage side of the main transformer  8  falls and, thus, the voltage of one or more of the connection lines  16  falls. 
     With reference to only one connection line (the operation is the same for each connection line) when the control unit  11  detects that the voltage of the connection line  16  (that is indicative of the voltage of the grid  5 ) falls below a prefixed voltage value, it drives the switch  21  to open, and the switch  23  to close. 
     In this respect,  FIG. 4  shows in full line the configuration of the switch  21  and the switch  23  in case of failure with grid voltage drop; therefore, in case of failure the branch  20  is not operative and the branch  22  is operative. 
     When the switch  23  is closed, the capacitor bank  13  is discharged and the diode  24  forces the discharge direction, such that electric power is injected into the field circuit converter  9  and thus into the field circuit  3 ; the switch  21  (that is open) prevents the capacitor bank  13  from being short-circuited. 
     After a failure, the automatic security and protection systems restore the grid voltage in a short time. 
     When the control unit  11  detects that the voltage at the connection line  16  is above the prefixed voltage value, it drives the switch  21  to close and the switch  23  to open. 
     Closing of switch  21  and opening of switch  23  may also be carried out by the control unit  11  with a delay, such that the capacitor bank  13  is charged. 
     In addition or alternatively, the charging component  25  may charge the capacitor bank  13 . 
     Method for Retrofitting 
     The present invention also refers to methods for retrofitting a static exciter of an electric generator. 
     Exemplary methods include providing at least one capacitor bank  13  between the low voltage side of the main transformer  8  and the AC side of the field circuit converter  9 . 
     Advantageously, also providing a coupling circuit  15  connecting the capacitor bank  13  to connection lines  16  between the low voltage side of the main transformer  8  and the AC side of the field circuit converter  9  is provided. 
     This retrofitting method is particularly advantageous because there is no need of modifying the rotor and, thus, there is no need of opening the generator casing and installing any component aboard of the rotor. 
     Method for Operating a Static Exciter 
     The present invention also refers to methods for operating a static exciter of an electric generator. 
     According to exemplary methods, the capacitor bank  13  injects electric power between the low voltage side of the main transformer  8  and the AC side of the field circuit converter  9  when the grid voltage falls below a prefixed voltage value. 
     Naturally the features described may be independently provided from one another. 
     In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art. 
     REFERENCE NUMBERS 
     
         
         
           
               1  static exciter 
               2  electric generator 
               3  field circuit 
               4  busbar 
               5  electric grid 
               6  transformer 
               8  main transformer 
               9  field circuit converter 
               11  control unit 
               13  capacitor bank 
               15  coupling circuit 
               16  connection line 
               17  auxiliary converter 
               18  auxiliary transformer 
               19  auxiliary connection line 
               20  first branch 
               21  first switch 
               22  second branch 
               23  second switch 
               24  one-way component 
               25  charging component 
           
         
       
    
     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.