Patent Publication Number: US-2022231615-A1

Title: Secondary magnetic excitation generator-motor device

Description:
FIELD 
     The present invention relates to a secondary magnetic excitation generator-motor device using a secondary magnetic excitation power converter. 
     BACKGROUND 
     An AC magnetic excitation generator motor using a secondary magnetic excitation power converter can control an output of reactive power in the same manner as a conventional fixed-speed synchronous machine, and additionally can achieve high-speed torque control or high-speed active power control within a rotational speed range around the synchronous speed. Due to this control, the AC magnetic excitation generator motor has an advantage of optimally operating a prime mover, such as a pump turbine system or a wind power system, under wider operational conditions as compared to a conventional fixed-speed generator motor. Further, the AC magnetic excitation generator motor has also an advantage of contributing to frequency stabilization in the power system by temporarily releasing and absorbing energy of a flywheel that is a rotational part to and from the power system. 
     In contrast, the secondary magnetic excitation power converter can reduce the capacity relative to the armature capacity of a generator; however, the secondary magnetic excitation power converter has not only a much larger capacity, but also a more complex circuit as compared to a magnetic excitation power converter of the conventional fixed-speed synchronous machine. Thus, it is economically difficult to ensure an overcurrent withstand capability and an excitation ceiling voltage equal to those of the magnetic excitation power converter of the conventional fixed-speed synchronous machine. 
     In view of the above circumstances, a method has been employed in general to operate a short circuit for an excitation winding overcurrent when an anomaly has occurred in the AC system to bypass the magnetic excitation power converter in order to reduce the overcurrent capacity. In particular, when an excitation winding is short-circuited, torque of the generator motor is suddenly varied to torque of a winding induction machine when the secondary resistance is short-circuited. In addition, the torque of the generator motor is varied significantly depending on the rotational speed before the excitation winding is short-circuited. This results in the drawback of greatly affecting the power system and causing instability of the power system, although the generator-motor device can run continuously. When the secondary resistance is short-circuited, reactive power is consumed, which further decreases the voltage in the AC system. This results in the drawback of inhibiting power supply to consumers. 
     In order to deal with this drawback, Non Patent Literature 1 discloses a method for a separately-excited secondary magnetic excitation power converter to which a thyristor converter is connected in inverse parallel, where the thyristor converter does not have a self-arc-extinguishing function. In this method, when the separately-excited secondary magnetic excitation power converter detects an excitation overvoltage, a power converter with reverse polarity to an excitation current command is ignited to continuously run. For another example, there is disclosed a 400MW-class secondary magnetic excitation generator-motor device that computes a loss generated on the basis of a current value of a power semiconductor, and regularly computes an element junction temperature on the basis of a coolant temperature to allow for a time limit to overcurrent protection. Due to this configuration, the 400MW-class secondary magnetic excitation generator-motor device has realized continued operation without interrupting an excitation control even during more than 250 times of system ground faults for a quarter of a century during which no short-circuit operation is performed at all. 
     Meanwhile, the technologies of self-arc-extinguishing semiconductor power elements have advanced remarkably in recent years. Development of larger-capacity and higher-voltage self-excited power converters has progressed, in which an IGBT (Insulated Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor), or other semiconductor elements are used. The self-excited power converters have advantages of having a function of regulating the converter power factor over separately-excited converters. In contrast, in a case where a secondary magnetic excitation generator-motor device is required to continuously run and continuously supply reactive power even when an anomaly has occurred in the power system, there is a problem that it is economically difficult for a converter using a self-arc-extinguishing semiconductor element to ensure a short-time overcurrent withstand capability. The reason for this is that the self-arc-extinguishing element has its current element rating that is determined by a momentary current interruption withstand capability, in contrast to the conventional element, such as a thyristor, that can have an overcurrent withstand capability in the order of 100 milliseconds to seconds up to the upper limit of the junction temperature in accordance with the heat capacity of the element itself. 
     In order to deal with this drawback, there is a method available, which focuses attention on the fact that a diode connected in inverse parallel to the self-arc-extinguishing element is a bipolar element having an overcurrent withstand capability like a thyristor. 
     There is the simplest method in which the gates to all the self-arc-extinguishing elements are switched off to perform a diode-bridge operation. According to this method, there is a problem that the degree of freedom of current flowing in an excitation circuit, which is normally “2”, is decreased to “1”, and it is thus difficult to return the degree of freedom of current to “2” rapidly in a stable manner, and to resume normal control. 
     In Patent Literature 1, a secondary magnetic excitation generator transmission device provided with operational continuity performance and stability as high as those of a separately-excited secondary magnetic excitation power converter is realized by employing a control mode that is bidirectionally switchable to/from normal PWM control without operating the short circuit or without switching the gate off even when an overcurrent is generated at the occurrence of an accident in the system, that is, while maintaining the degree of freedom of current flowing in the excitation circuit at “2”. By employing this control mode, the secondary magnetic excitation generator transmission device allows all the excitation current to pass through the diode during the period during which an overcurrent is generated, and returns the control to the normal PWM control after the resumption from the overcurrent. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: Japanese Patent No. 5401213 
         Patent Literature 2: Japanese Patent No. 3222028 
       
    
     Non Patent Literature 
     
         
         Non Patent Literature 1: HITACHI REVIEW 1995 Vol. 44 
       
    
     SUMMARY 
     Technical Problem 
     The method in Patent Literature 1 is disclosed as a method to realize a self-excited secondary magnetic excitation power converter using a self-arc-extinguishing element. 
     In a case where the capacity of the secondary magnetic excitation power converter is increased in proportion to the increase in the capacity of the generator motor, the AC output voltage is required to increase to a relatively high level. However, when the secondary magnetic excitation power converter is a two-level converter, an AC-side output phase voltage relative to a DC capacitor voltage Vc is limited to two levels, (+Vc/2) and (−Vc/2). Therefore, an increase in the voltage of the secondary magnetic excitation power converter to a relatively high level is only achieved by increasing the DC capacitor voltage Vc. 
     However, as the DC capacitor voltage Vc is increased, the AC-side output phase voltage is also increased, and consequently a time rate of change dV/dt in the voltage to be applied to a rotor-side excitation winding coil is also increased. In the insulation specifications of coils, a rated AC voltage and a peak voltage value are defined in general. However, it is known that degradation of the coil insulation over time due to a dielectric loss rapidly progresses when the time rate of change in the voltage exceeds a predetermined value. Even though this predetermined value is assumed to be the same, when the polarity changes without a voltage-zero period as shown by an output from the two-level converter, it is assumed that “degradation of coil insulation over time is more likely to further progress as compared to the case where the voltage changes through the voltage-zero period”. Further, it is pointed out that as compared to a coil applied with varnish in a vacuum in an integrally assembled state in a factory, the allowable value of the time rate of change in the voltage, which is restricted due to the degradation of coil insulation over time, is substantially decreased in a case when a coil is assembled on site. 
     In view of the above circumstances, there is a problem that it is difficult to continuously use a two-level converter not only as a secondary magnetic excitation power converter to be used for a variable-speed pumped storage power plant, but also as a secondary magnetic excitation power converter for an offshore wind turbine with a higher capacity. 
     As a solution to the above problems, there is a possible method using a three-level NPC power converter. In a three-level converter, an AC output phase voltage relative to the voltage Vc of two DC capacitors connected in series has three levels, (+Vc), (0), and (−Vc). The three-level converter has an effect of ensuring an AC output voltage that is twice as high as that in the two-level converter even though the capacitor DC voltage Vc is equal between the three-level converter and the two-level converter. Consequently, the three-level converter has an effect of decreasing the time rate of change dV/dt in the voltage to be applied to the coil. Since the three-level converter has a voltage-zero period, there is also an effect of alleviating degradation of coil insulation over time due to dielectric loss even though the time rate of change dV/dt in the voltage to be applied to the coil is equal to that of the two-level converter. 
     However, the method in Patent Literature 1 is only applicable to two-level system power converters. 
     An object of the present invention is to provide a secondary magnetic excitation generator-motor device that employs a three-level NPC power converter, that ensures operational continuity at the occurrence of an accident in a system, and that contributes to system stabilization. 
     Solution to Problem 
     A three-level NPC power converter connected to a rotor-side excitation winding has a three-phase three-line circuit with the degree of freedom of current “2” similarly to the two-level converter disclosed in Patent Literature 1. Due to this configuration, unless a bypass circuit is operated, or gates of all elements are switched off, the magnitude relation between absolute values of three-phase currents IU, IV, and IW, and the signs of the three-phase currents IU, IV, and IW can be limited to 12 modes illustrated in  FIG. 9  not only during normal operation, but also when an overcurrent is generated at the occurrence of an accident in the system. 
       FIG. 10  illustrates a solution to the problems in the mode  1  in  FIG. 9  by describing the operation of the three-level NPC power converter. 
     When an absolute value of the V-phase current IV is largest, and the polarity of the V-phase current IV is negative, the gates of an element VPC and an element VP are fixed to the on-side, while the gates of an element VN and an element VNC are fixed to the off-side. Due to this operation, the current IV flows from a power converter V-phase into a positive-side DC capacitor CP to charge the positive-side DC capacitor CP with the current IV. 
     When an absolute value of the U-phase current IU is the second largest value, and the polarity of the U-phase current IU is positive, the gates of an element UNC and an element UN are fixed to the on-side, while the gates of an element UP and an element UPC are fixed to the off-side. Due to this operation, the current IU flows out from a power converter U-phase into a negative-side DC capacitor CN to charge the negative-side DC capacitor CN with the current IU. 
     Since an absolute value of the remaining W-phase current IW is the third largest value (smallest), the gates of an element WPC and an element WNC are fixed to the on-side, while the gates of an element WP and an element WN are fixed to the off-side regardless of the current polarity. Due to this operation, the current IW flowing out from the power converter W-phase is supplied from the positive-side DC capacitor via the neutral point, and a positive-side clamp diode. 
     As a result of this, in the mode  1 , the absolute value of the current IV that charges the positive-side DC capacitor is larger than the absolute value of the current IU that charges the negative-side DC capacitor. Accordingly, the positive-side DC capacitor voltage is higher than the negative-side DC capacitor voltage, which becomes a factor of causing voltage imbalance. 
       FIG. 11  illustrates a solution to the problems in the mode  2  in  FIG. 9  by describing the operation of the three-level NPC power converter. 
     When an absolute value of the U-phase current IU is the largest, and the polarity of the U-phase current IU is positive, the gates of the element UNC and the element UN are fixed to the on-side, while the gates of the element UP and the element UPC are fixed to the off-side. Due to this operation, the current IU flows out from the power converter U-phase into the negative-side DC capacitor CN to charge the negative-side DC capacitor CN with the current IU. 
     When an absolute value of the V-phase current IV is the second largest value, and the polarity of the V-phase current IV is negative, the gates of the element VPC and the element VP are fixed to the on-side, while the gates of the element VN and the element VNC are fixed to the off-side. Due to this operation, the current IV flows from the power converter V-phase into the positive-side DC capacitor CP to charge the positive-side DC capacitor CP with the current IV. 
     Since an absolute value of the remaining W-phase current IW is the third largest value (smallest), the gates of the element WPC and the element WNC are fixed to the on-side, while the gates of the element WP and the element WN are fixed to the off-side regardless of the current polarity. Due to this operation, the current IW flows from the power converter W-phase via the neutral point, and a negative-side clamp diode into the negative-side DC capacitor to charge the negative-side DC capacitor with the current IW. 
     As a result, in the mode  2 , the absolute value of the current IU that charges the negative-side DC capacitor is larger than the absolute value of the current IV that charges the positive-side DC capacitor. Accordingly, the negative-side DC capacitor voltage is higher than the positive-side DC capacitor voltage, which becomes a factor of causing voltage imbalance. 
     Assuming that three-phase currents IU, IV, and IW are balanced as a precondition, each time the polarity of the third largest current changes, the positive-side DC capacitor and the negative-side DC capacitor are repeatedly overcharged and insufficiently charged alternately, and thus it is conceivable that there is a lower risk of causing significant voltage imbalance. However, in a case where an accident has occurred in a system that is required to run continuously even when an overcurrent is generated, the three-phase currents IU, IV, and IW significantly deviate from the precondition described above. Therefore, a method to maintain voltage balance is necessary. 
     As a solution to this necessity, a circuit configuration disclosed in Patent Literature 2 is used. That is, a positive-side DC capacitor and a negative-side DC capacitor are respectively connected to the DC-side of two units of two-level converters, and the AC-side thereof is connected to two sets of AC terminals insulated by an excitation voltage transformer. In this manner, the two units of two-level converters are configured to control their respective DC voltages independently from each other, and in addition to that, these two-level converters also control the positive-side and negative-side DC capacitors so as to maintain balance between the values of these DC capacitors. 
     The expected objectives can be attained by the device configuration and the control method described above. 
     Advantageous Effects of Invention 
     The secondary magnetic excitation generator-motor device of the present invention can achieve contribution to stable management of a power system by improving the operational continuity capability while minimizing torque fluctuations in the generator-motor particularly when a voltage decrease has occurred due to a ground fault or the like caused by lightning striking the power system, and simultaneously by promptly resuming the reactive-power supply capability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an embodiment of the present invention. 
         FIG. 2  is a circuit diagram illustrating another embodiment of the present invention. 
         FIG. 3  is a diagram illustrating an operation of a PWM modulation circuit  37  of a three-level NPC power converter. 
         FIG. 4  is a diagram illustrating an operation of a pulse generation circuit  45  of the three-level NPC power converter. 
         FIG. 5  is a configuration diagram of a second PWM modulation circuit  40  of the three-level NPC power converter. 
         FIG. 6  is a configuration diagram of a running-mode switching circuit  41  of the three-level NPC power converter. 
         FIG. 7  is a diagram illustrating an operation of the present invention. 
         FIG. 8  is a diagram illustrating an operation of the present invention. 
         FIG. 9  is a diagram illustrating a conduction mode classification of the three-level NPC power converter. 
         FIG. 10  is a diagram illustrating a W-phase positive-side clamp circuit of the three-level NPC power converter at the time of conduction. 
         FIG. 11  is a diagram illustrating a W-phase negative-side clamp circuit of the three-level NPC power converter at the time of conduction. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a secondary magnetic excitation generator-motor device according to the present invention will be described in detail below with reference to the accompanying drawings. The present invention is not limited to the embodiments. 
     First Embodiment 
     With reference to  FIG. 1 , a device configuration according to a first embodiment of the present invention is described below. 
     A three-phase AC system  1  is connected to a stator-side armature winding  5  of a winding induction machine through a main voltage transformer  2 , a phase reverse disconnector (89GM)  3 , and a synchronous circuit breaker (52G)  4 . A rotor-side excitation winding  6  is connected to an AC-side of a three-level NPC power converter  7 . Between a DC-side positive terminal and a neutral point terminal of the three-level NPC power converter  7 , a positive-side DC capacitor (CP)  8  and a first DC output terminal (VDC 1 ) of a DC voltage source  71  are connected in parallel to each other. 
     In contrast, between a DC-side negative terminal and the neutral point terminal of the three-level NPC power converter  7 , a negative-side DC capacitor (CN)  12  and a second DC output terminal (VDC 2 ) of the DC voltage source  71  are connected in parallel to each other. 
     An AC terminal of the DC voltage source  71  is connected in parallel to a first terminal of an excitation circuit breaker (52E)  16 , while a second terminal of the excitation circuit breaker (52E)  16  is connected in parallel to a terminal of the main transformer  2  on the winding generator motor-side. 
     The DC voltage source  71  is constituted by an AC/DC power converter. For example, the DC voltage source  71  can be implemented by having a configuration in which the DC voltage source  71  is connected back to back to the DC-side positive terminal, the DC-side negative terminal, and the neutral point terminal of the three-level NPC power converter  7 . 
     Next, a configuration of the control system of the three-level NPC power converter  7  is described. 
     An automatic voltage regulator (AVR)  29  is provided to output a d-axis current command (I_Dref) such that the value of generator voltage VG becomes a set value. The value of generator voltage VG is computed on the basis of information from an instrument voltage transformer  28  at a stator-side armature terminal of the winding induction machine. An automatic voltage regulator (APR)  31  is provided to output a q-axis current command (I_Qref) such that the value of active power becomes a set value. The value of active power is computed on the basis of information from an instrument voltage transformer  3  and a main voltage-transformer terminal instrument current transformer  30 . 
     A phase sensor  32  is provided to sense a phase θs of slip frequency which is equal to the difference between an AC-system frequency and a rotational frequency of the winding induction machine on the basis of information from a rotational phase sensor (PLG)  55  and a main voltage-transformer terminal instrument voltage transformer  17 , so as to input two-phase current commands (I_Dref and I_Qref) to a two-phase/three-phase coordinates converter  33 , from which three-phase current commands (IU_ref, IV_ref, and IW_ref) with the slip frequency are output. 
     An excitation-current instrument current transformer  34  is provided between the three-level NPC power converter  7  and the terminals of the rotor-side excitation winding  6  of the winding induction machine to detect excitation current values (IU, IV, and IW), such that a three-phase/two-phase converter  35  computes two-phase current values (I_DfB and I_QfB) that are a steady-state DC amount. 
     An excitation current regulator  36  outputs modulation factor commands (αU, αV, and αW) such that the two-phase current values (I_DfB and I_QfB) correspond with the two-phase current commands (I_Dref and I_Qref), and such that the excitation current values (IU, IV, and IW) correspond with the three-phase current commands (IU_ref, IV_ref, and IW_ref). These modulation factor commands (αU, αV, and αW) are input respectively to three units of PWM modulation circuits  37 ,  38 , and  39  provided for respective phases, from which first modulation commands (MU 1 , MV 1 , and MW 1 ) for respective phases are output. 
     Meanwhile, the excitation current values (IU, IV, and IW) are input from the excitation-current instrument current transformer  34  to a second PWM modulation circuit  40 , from which second modulation commands (MU 2 , MV 2 , and MW 2 ) are output. 
     The excitation current values (IU, IV, and IW) are input from the excitation-current instrument current transformer  34  to a running-mode switching circuit  41 . The running-mode switching circuit  41  outputs a command value SW for simultaneously switching between the first modulation commands (MU 1 , MV 1 , and MW 1 ) and the second modulation commands (MU 2 , MV 2 , and MW 2 ). The running-mode switching circuit  41  also outputs a GB command for fixing ignition commands for all the self-arc-extinguishing elements to the off-side. 
     In such a manner that when the command value SW is 0, the first modulation commands (MU 1 , MV 1 , and MW 1 ) are selected, and when the command value SW is 1, the second modulation commands (MU 2 , MV 2 , and MW 2 ) are selected, three units of switches  42 ,  43 , and  44  for respective phases selectively output modulation commands (MU, MV, and MW). 
     The modulation commands (MU, MV, and MW) are input to three units of pulse generation circuits  45 ,  46 , and  47  for respective phases to execute on/off control on gate commands for the self-arc-extinguishing elements of the three-level NPC power converter  7 . 
     With reference to  FIG. 3 , an operation of a U-phase PWM modulation circuit  37  is described below. 
     The PWM regulation circuit  37  has a positive carrier between the neutral point (0) and the positive terminal (+1), and a negative carrier between the negative terminal (−1) and the neutral point (0). On the basis of the magnitude relation between these carriers and a modulation wave computed from an input-side modulation factor command αU, the PWM regulation circuit  37  outputs the modulation command MU 1  that is selected from among three values (+1, 0, and −1). 
     A V-phase PWM modulation circuit  38  and a W-phase PWM modulation circuit  39  operate in the same manner as the U-phase PWM modulation circuit  37 , and therefore descriptions of the operation thereof are omitted to avoid redundant explanations. 
       FIG. 4  illustrates an operation of the pulse generation circuit  45  in form of a table. 
     First, descriptions are made on an operation of the pulse generation circuit  45  when a GB command issued from the running-mode switching circuit  41  shows level 0. 
     When the modulation command MU is (+1), gate commands G_UP and G_UPC for self-arc-extinguishing elements UP and UPC are on, while the other gate commands are off. When the modulation command MU is (0), the gate command G_UPC for the self-arc-extinguishing element UPC and a gate command G_UNC for a self-arc-extinguishing element UNC are on, while the other gate commands are off. When the modulation command MU is (−1), a gate command G_UN for a self-arc-extinguishing element UN and the gate command G_UNC for the self-arc-extinguishing element UNC are on, while the other gate commands are off. In contrast, when the GB command shows level 1, the gate commands G_UP, G_UPC, G_UNC, and G_UN for the self-arc-extinguishing elements UP, UPC, UNC, and UN are off regardless of the value of the modulation command MU. 
       FIG. 5  illustrates a configuration of the second PWM modulation circuit  40 . 
     Absolute value computation devices  101 ,  102 , and  103  calculate absolute values |IU|, |IV|, and |IW| of the excitation current values (IU, IV, and IW) input from the instrument current transformer  34 , respectively. Then, subtractors  104 ,  105 , and  106  output a difference between the absolute values, and comparators  107 ,  108 , and  109  output a sign determination result selectively from two values (0 and 1). The outputs from the comparators  107 ,  108 , and  109  are input to a logic circuit  110 . The logic circuit  110  outputs a signal  111  that shows level 1 when the absolute value |IU| is smallest, or otherwise outputs the signal  111  that shows level 0. In the same manner as |IU|, the logic circuit  110  also outputs a signal  112  for |IV| and a signal  113  for |IW|. 
     Meanwhile, comparators  114 ,  115 , and  116  output the polarity of the excitation current values (IU, IV, and IW) selectively from two values (0 and 1). The comparator  114  outputs a signal  117  that shows level 1 when the excitation current value IU is positive, or otherwise outputs the signal  117  that shows level 0. In the same manner as IU, the comparators  115  and  116  also output a signal  118  for IV and a signal  119  for IW, respectively. 
     The signals  111 ,  112 ,  113 ,  117 ,  118 , and  119  are input to a logic circuit  120 . The logic circuit  120  outputs three values (+1, 0, and −1) for respective phases to three-value selection output circuits  121 ,  122 , and  123 . 
     A U-phase three-value selection output circuit  121  outputs “0” as the second modulation output MU 2  when the absolute value of IU is smallest in three phases, outputs “+1” as the second modulation output MU 2  when IU is positive, and outputs “−1” as the second modulation output MU 2  when IU is negative. A V-phase three-value selection output circuit  122  and a W-phase three-value selection output circuit  123  operate in the same manner as the U-phase three-value selection output circuit  121 , and therefore descriptions of the operation thereof are omitted to avoid redundant explanations. 
       FIG. 6  illustrates a configuration of the running-mode switching circuit  41 . Since like reference signs to those illustrated in  FIG. 5  denote like constituent elements, descriptions thereof are omitted to avoid redundant explanations. 
     A maximum-value selection output device  202  selectively outputs the maximum value of the absolute values |IU|, |IV|, and |IW| of the excitation current values (IU, IV, and IW) input from the instrument current transformer  34 . A minimum-value selection output device  201  selectively outputs the minimum value of the absolute values |IU|, |IV|, and |IW|. When the maximum value exceeds a set value I 1 , a comparator  203  outputs level 1, or otherwise outputs level 0. When the minimum value is equal to or smaller than a set value I 2 , a comparator  204  outputs level 1, or otherwise outputs level 0. A flip-flop  205  receives a set signal output from the comparator  203 , and a reset signal output from the comparator  204 , and on the basis of these signals, outputs a command value SW. 
     When the command value SW shows level 0, the switches  42 ,  43 , and  44  select the first modulation commands (MU 1 , MV 1 , and MW 1 ). When the command value SW shows level 1, the switches  42 ,  43 , and  44  select the second modulation commands (MU 2 , MV 2 , and MW 2 ). 
     The set values I 1  and I 2  are defined as “I 1 &gt;I 2 ”. The value of I 1  is set with reference to the maximum interrupting current of the self-arc-extinguishing elements of the three-level NPC power converter  7  to such a value as not to exceed the maximum interrupting current. 
       FIGS. 7 and 8  illustrate an operation of the secondary magnetic excitation generator-motor device in  FIGS. 1 to 6  when a ground fault has occurred in the three-phase AC system  1  at a time t 0 , and then the accidental phase is removed to resume a normal operation at a time tCB. 
     During a period (from t 1  to t 2 ) and a period (from t 3  to t 4 ) in  FIG. 7 , and during a period (from t 5  to t 6 ) and a period (from t 7  to t 8 ) in  FIG. 8 , the command value SW shows level 1. Thus, the second modulation commands (MU 2 , MV 2 , and MW 2 ) are selected. 
     According to the configuration of the invention described in the present embodiment, the secondary magnetic excitation generator-motor device can be protected from an overcurrent without the need for bypassing the three-level NPC power converter  7  and without the need for blocking the gates. Therefore, the secondary magnetic excitation generator-motor device can precisely switch between two different modulation commands, and consequently can be protected from an overcurrent and can run continuously in a stable manner. 
     Second Embodiment 
     With reference to  FIG. 2 , a device configuration according to a second embodiment of the present invention is described below. 
     Between a DC-side positive terminal and the neutral point terminal of the three-level NPC power converter  7 , the positive-side DC capacitor (CP)  8  and a DC terminal of a first two-level power converter  9  are connected in parallel to each other. An AC terminal of the first two-level power converter  9  is connected to a first excitation voltage transformer  11  through a first harmonic suppression filter  10 . 
     In contrast, between the neutral point terminal and a DC-side negative terminal of the three-level NPC power converter  7 , the negative-side DC capacitor (CN)  12  and a DC terminal of a second two-level power converter  13  are connected in parallel to each other. An AC terminal of the second two-level power converter  13  is connected to a second excitation voltage transformer  15  through a second harmonic suppression filter  14 . 
     The first excitation voltage transformer  11  and a terminal of the second excitation voltage transformer  15  on the AC system-side is connected in parallel to a first terminal of the excitation circuit breaker (52E)  16 . A second terminal of the excitation circuit breaker (52E)  16  is connected in parallel to the terminal of the main voltage transformer  2  on the winding generator motor-side. 
     Next, a configuration and an operation of the control system of the first two-level power converter  9  is described. 
     A first power factor regulator (APFR 1 )  19  is provided to output a d-axis DC current command (IC 1 _Dref) such that the power factor becomes 1 on the basis of reactive power computed from information provided by the main voltage-transformer terminal instrument voltage transformer  17  provided at the terminal of the main voltage transformer  2  on the winding generator motor-side, and provided by a first instrument current transformer  18  provided between the first harmonic filter  10  and the AC terminal. A first DC-voltage regulator (ADCVR 1 )  21  is provided to output a q-axis DC current command (IC 1 _Qref) such that a first instrument DC voltage transformer  20  detects a voltage VDCP in the positive-side DC capacitor  8  to regulate the voltage VDCP to a set value. A first two-level converter current regulator  22  executes on/off control on the gates to self-arc-extinguishing elements (RP 1 , SP 1 , TP 1 , RN 1 , SN 1 , and TN 1 ) forming the first two-level converter  9 . 
     Similarly to the first two-level power converter  9 , a configuration of the control system of the second two-level power converter  13  is described below. 
     A second power factor regulator (APFR 2 )  24  is provided to output a d-axis DC current command (IC 2 _Dref) such that the power factor becomes 1 on the basis of reactive power computed from information provided by the main voltage-transformer terminal instrument voltage transformer  17 , and by a second instrument current transformer  23  provided between the second harmonic suppression filter  14  and the AC terminal. A second DC-voltage regulator (ADCVR 2 )  26  is provided to output a q-axis DC current command (IC 2 _Qref) such that a second instrument DC voltage transformer  25  detects a voltage VDCN in the negative-side DC capacitor  12  to regulate the voltage VDCN to a set value. A second two-level converter current regulator  27  executes on/off control on the gates to self-arc-extinguishing elements (RP 2 , SP 2 , TP 2 , RN 2 , SN 2 , and TN 2 ) forming the second two-level converter  13 . 
     According to the configuration of the first and second two-level power converters described above, two units of two-level converters (the first two-level converter  9  and the second two-level converter  13 ) connected to the AC power supply and insulated from each other by two units of excitation voltage transformers (the first excitation voltage transformer  11  and the second excitation voltage transformer  15 ) control the positive DC capacitor voltage VDCP and the negative DC capacitor voltage VDCN independently from each other. Therefore, even during a transition period such as at the occurrence of an accident in the system, balance between the voltages of two units of DC capacitors can still be maintained in a stable manner. 
     According to the configuration of the invention described in the present embodiment, the two units of two-level converters can control their respective DC voltages independently from each other, and therefore can control the positive-side and negative-side DC capacitors so as to maintain balance between the values of these DC capacitors. 
     Third Embodiment 
     With reference to  FIG. 2 , a device configuration according to a third embodiment of the present invention is described below. 
     A bypass circuit  48  is provided between the excitation-current instrument current transformer  34  and the terminals of the rotor-side excitation winding  6  of the winding induction machine. It is allowable that the bypass circuit  48  is formed of power semiconductor elements. However, as illustrated in the present embodiment, it is also allowable to use a vacuum circuit breaker having achieved a remarkable technological progress, such as ensuring that the vacuum circuit breaker can properly operate 150k times. This bypass circuit is closed in accordance with an 86E command issued from the running-mode switching circuit  41 . In accordance with a GB command, the pulse generation circuits  45 ,  46 , and  47  block the gates of the self-arc-extinguishing elements of the three-level NPC power converter  7  to stop the operation. 
       FIG. 6  illustrates a circuit to output an operation command (86E command) for the bypass circuit  48 . 
     A maximum-value selection circuit  206  selectively outputs the maximum value of the absolute values |IU|, |IV|, and |IW| of the excitation current values (IU, IV, and IW). When the maximum value exceeds a set value I 3 , a comparator  207  outputs a signal of level 1, or otherwise outputs a signal of level 0. The comparator  207  outputs a GB command and an 86E command via an OR circuit  208 . 
     The set value I 3  is set larger than the set value I 1  (I 3 &gt;I 1 ). The reason for this is that while the value of overcurrent generated due to a ground fault or the like in the three-phase AC system  1  is reduced by impedance of the main voltage transformer  2 , the value of overcurrent generated at an occurrence of a fault in the winding induction machine of the main voltage transformer  2  is not reduced. The set value I 3  is set with reference to the maximum current generated due to a ground fault in the three-phase AC system  1 . Due to this setting, a fault in the device is distinguished from a fault in the three-phase AC system, so that when a fault has occurred in the device, the device can stop running immediately to be protected from the fault. 
     Fourth Embodiment 
     With reference to  FIG. 6 , a device configuration and a circuit according to a fourth embodiment of the present invention is described below. 
     Operations of output switches  209 ,  210 , and  211  are described below. Hereinafter, an operation of a U-phase output switch  209  is described. A V-phase output switch  210  and a W-phase output switch  211  operate in the same manner as the U-phase output switch  209 , and therefore descriptions of the operations thereof are omitted to avoid redundant explanations. 
     When |IU| is smallest, the signal  111  shows level 1 or otherwise shows level 0. The output switch  209  outputs |IU| that is an absolute value of the third-phase current when the signal  111  shows level 1, while outputting “0” when the signal  111  shows level 0. 
     A maximum-value selection circuit  212  selectively outputs the maximum value of the absolute values |IU|, |IV|, and |IW| of the respective phase currents. When the maximum value selectively output by the maximum-value selection circuit  212  exceeds a set value I 4 , a comparator  213  outputs a signal of level 1, or otherwise outputs a signal of level 0. An on-delay circuit  214  outputs a signal of level 1 when the absolute value of the third-phase current is maintained for a set time or longer, and outputs a GB command and an 86E command via the OR circuit  208 . 
     The set value I 4  is set smaller than the set value I 1  (I 4 &lt;I 1 ). The reason for this is that the third-phase current flows via any of the self-arc-extinguishing elements (UPC, UNC, VPC, VNC, WPC, and WNC). In general, since a self-arc-extinguishing element has a higher conduction loss than that in a diode connected in inverse parallel, the self-arc-extinguishing element has a lower overcurrent withstand capability. Due to this setting, time-limited overcurrent protection for the third phase enables the device to be safely protected from overcurrent. 
     Fifth Embodiment 
     With reference to  FIG. 2 , a device configuration according to a fifth embodiment of the present invention is described below. 
     When the positive-side DC capacitor voltage VDCP from the first instrument DC voltage transformer  20  exceeds a set value, a first overvoltage suppressor (OVP 1 )  49  executes on/off control on a switch circuit (CHV 1 )  51  formed of a self-arc-extinguishing element connected in series to a limiting resistance  50  such that the limiting resistance  50  consumes power so as to suppress an increase in the positive-side DC capacitor voltage VDCP. 
     Similarly, when the negative-side DC capacitor voltage VDCN from the second instrument DC voltage transformer  25  exceeds a set value, a second overvoltage suppressor (OVP 2 )  52  executes on/off control on a switch circuit (CHV 2 )  54  formed of a self-arc-extinguishing element connected in series to a limiting resistance  53  such that the limiting resistance  53  consumes power so as to suppress an increase in the negative-side DC capacitor voltage VDCN. 
     According to the configuration of the present invention, even when the second modulation commands (MU 2 , MV 2 , and MW 2 ) are selected, the charging operation is still ensured for both the positive-side DC capacitor (CP)  8  and the negative-side DC capacitor (CN)  12 . This eliminates the need for a solution to a decrease in the DC voltage. Accordingly, it is only necessary to include a unit to suppress an increase in the DC voltage. Due to this configuration in which only a simple voltage suppression circuit is added, even at an occurrence of an accident in the system, the secondary magnetic excitation generator-motor device can still run continuously in a stable manner. 
     However, in a case with the three-level NPC power converter, during a period for which the second modulation commands (MU 2 , MV 2 , and MW 2 ) are selected, charging/discharging with the third-phase current becomes a factor of causing imbalance between the positive-side DC capacitor (CP)  8  and the negative-side DC capacitor (CN)  12 . However, the configuration of the present invention can reduce the positive-side DC voltage and the negative-side DC voltage independently from each other, so that the secondary magnetic excitation generator-motor device can achieve continuous and stable running even at an occurrence of an accident in the system. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  three-phase AC system 
               2  main voltage transformer 
               3  phase reverse disconnector (89GM) 
               4  synchronous circuit breaker (52G) 
               5  stator-side armature winding 
               6  rotor-side excitation winding 
               7  three-level NPC power converter 
               8  positive-side DC capacitor (CP) 
               9  first two-level power converter 
               10  first harmonic suppression filter 
               11  first excitation voltage transformer 
               12  negative-side DC capacitor (CN) 
               13  second two-level power converter 
               14  second harmonic suppression filter 
               15  second excitation voltage transformer 
               16  excitation circuit breaker (52E) 
               17  main voltage-transformer terminal instrument voltage transformer 
               18  first instrument current transformer 
               19  first power factor regulator (APFR 1 ) 
               20  first instrument DC voltage transformer 
               21  first DC-voltage regulator (ADCVR 1 ) 
               22  first two-level converter current regulator 
               23  second instrument current transformer 
               24  second power factor regulator (APFR 2 ) 
               25  second instrument DC voltage transformer 
               26  second DC-voltage regulator (ADCVR 2 ) 
               27  second two-level converter current regulator 
               28  instrument voltage transformer 
               29  automatic voltage regulator (AVR) 
               30  main voltage-transformer terminal instrument current transformer 
               31  automatic voltage regulator (APR) 
               32  phase sensor 
               33  two-phase/three-phase coordinates converter 
               34  excitation-current instrument current transformer 
               35  three-phase/two-phase converter 
               36  excitation current regulator 
               37 ,  38 ,  39  PWM modulation circuit 
               40  second PWM modulation circuit 
               41  running-mode switching circuit 
               42 ,  43 ,  44  switch 
               45 ,  46 ,  47  pulse generation circuit 
               48  bypass circuit 
               49  first overvoltage suppressor 
               50 ,  53  limiting resistance 
               51  switch circuit (CHV 1 ) 
               52  second overvoltage suppressor 
               54  switch circuit (CHV 2 ) 
               55  rotational phase sensor (PLG) 
               71  DC voltage source 
               101 ,  102 ,  103  absolute value computation device 
               104 ,  105 ,  106  subtractor 
               107 ,  108 ,  109 ,  114 ,  115 ,  116  comparator 
               110 ,  120  logic circuit 
               111 ,  112 ,  113 ,  117 ,  118 ,  119  signal 
               121 ,  122 ,  123  three-value selection output circuit 
               201 ,  206 ,  212  maximum-value selection output device 
               202  minimum-value selection output device 
               203 ,  204  comparator 
               205  flip-flop 
               208  OR circuit 
               209 ,  210 ,  211  output switch 
               214  on-delay circuit