Patent Publication Number: US-2023163695-A1

Title: System and method for detection of switching device failure conditions in a multi-level power converter

Description:
FIELD 
     The present disclosure relates generally to electrical power systems, and more particularly to a system and method for detection of switching device failure conditions in a multi-level power converter in an electrical power system. 
     BACKGROUND 
     Wind turbines have received increased attention as a renewable energy source. Wind turbines use the wind to generate electricity. The wind turns multiple blades connected to a rotor. The spin of the blades caused by the wind spins a shaft of the rotor, which connects to a generator that generates electricity. Certain wind turbines include a doubly-fed induction generator (DFIG) to convert wind energy into electrical power suitable for output to an electrical grid. DFIGs are typically connected to a converter that regulates the flow of electrical power between the DFIG and the grid. More particularly, the converter allows the wind turbine to output electrical power at the grid frequency regardless of the rotational speed of the wind turbine blades. 
     A typical DFIG system includes a wind driven DFIG having a rotor and a stator. The stator of the DFIG is coupled to the electrical grid through a stator bus. A power converter is used to couple the rotor of the DFIG to the electrical grid. The power converter can be a two-stage power converter including both a rotor side converter and a line side converter. The rotor side converter can receive alternating current (AC) power from the rotor via a rotor bus and can convert the AC power to a DC power. The line side converter can then convert the DC power to AC power having a suitable output frequency, such as the grid frequency. The AC power is provided to the electrical grid via a line bus. An auxiliary power feed can be coupled to the line bus to provide power for components used in the wind turbine system, such as fans, pumps, motors, and other components of the wind turbine system. 
     Multi-level converters (such as neutral point clamped converters) are generally used in high power industrial applications such as variable speed drive (VSD) systems or in energy conversion applications such as a solar (or photovoltaic) power generation systems, wind turbine generators or marine and hydrokinetic power generation systems. The general function of the multi-level converter is to synthesize a sinusoidal voltage by several levels of voltages, typically obtained from capacitor voltage sources. A three level converter includes two capacitor voltages in series with the center tap as the neutral. Each phase leg of the three-level converter has two pairs of switching devices in series. The switching device is bidirectional in current, often realized as anti-parallel connection of a unidirectional electronic switching device (e.g., IGBT, IGCT) and a diode (free-wheeling diode). 
     The switching devices in a multi-level converter receive high electrical and thermal stress during short-circuit conditions or during turn-off switching of a clamped inductive load. For example, if there is large power loss within the switching device due to electrical stress or overcurrent, the switching device overheats and it may lead to thermal breakdown of the switching device. Similarly, when an inductive load or inductive current is switched off, the switching device may observe overvoltage resulting into voltage breakdown of the switching device. Once a switching device is broken down or destroyed due to overheat or overvoltage, without protective measures, a chain reaction may occur resulting into destruction of the entire multi-level converter bridge. 
     Thus, it is important to detect failures in switching devices in multi-level converters. One example of a measurement used for detecting switching device failure in IGBT converters is a desaturation circuit. However, methods using the desaturation circuit detect a failure condition only when the corresponding switching device is turned ON and do not provide any signal when the gate drive signal to the switching device is turned OFF. Thus, the desaturation circuit does not detect all possible short-circuit failure conditions. 
     Accordingly, it is desirable to provide an improved system and method for detection of switching device failure conditions in a multi-level power converter in an electrical power system that addresses the foregoing issues. 
     BRIEF DESCRIPTION 
     Aspects and advantages of the disclosure will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the disclosure. 
     In one aspect, the present disclosure is directed to a computer-implemented method for operating a multi-level bridge power converter of an electrical power system connected to a power grid. The power converter includes a first converter and a second converter coupled via a DC link. The method includes arranging a plurality of switching devices having at least four inner switching devices and at least two outer switching devices in an active neutral point clamped topology. The method also includes determining whether any of the plurality of switching devices is experiencing a failure condition by implementing a failure detection algorithm. The failure detection algorithm includes generating a blocking state logic signal by comparing a switching device voltage and a threshold reference voltage for each of the plurality of switching devices, determining an expected voltage blocking state for each of the plurality of switching devices based on gate drive signals of the plurality of switching devices and an output current direction, and detecting whether a failure condition is present in any of the plurality of switching devices based on the blocking state logic signals and the expected voltage blocking states of the plurality of switching devices. 
     In an embodiment, the plurality of switching devices may be a plurality of insulated gate bipolar transistors (IGBTs). Thus, in such embodiments, the plurality of IGBTs may include a first IGBT, a second IGBT, a third IGBT, a fourth IGBT, a fifth IGBT, and a sixth IGBT, whereas the second IGBT, the third IGBT, the fifth IGBT, and the sixth IGBT are the four inner switching devices and the first IGBT and the fourth IGBT are the two outer switching devices. 
     In another embodiment, determining the expected voltage blocking state for each of the plurality of switching devices based on the gate drive signals of the plurality of switching devices and the output current direction further comprises determining the expected output voltage at output terminals of the multi-level bridge power converter. 
     In further embodiments, the expected output voltage is a zero voltage when the gate drive signals for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal of the second IGBT is high, the output current direction is negative, and the gate drive signals for the fifth IGBT and the sixth IGBT are high or when the gate drive signals for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal of the third IGBT is high, the output current direction is positive, and the gate drive signals for the fifth IGBT and the sixth IGBT are high or when the gate drive signals for the second IGBT or the third IGBT are high and the gate drive signals for both the first IGBT and the fourth IGBT are low. 
     In additional embodiment, the expected output voltage is a positive voltage when the gate drive signals for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal for the second IGBT is high the output current direction is negative, and the gate drive signals for the fifth IGBT or the sixth IGBT are low or when the gate drive signals for both the first IGBT and the second IGBT are high or when the output current direction is negative and the gate drive signals for the first IGBT, the second IGBT, the third IGBT, and the fourth IGBT are low. 
     In several embodiments, the expected output voltage is a negative voltage when the gate drive signals for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal of the third IGBT is high, the output current direction is positive, and the gate drive signals for the fifth IGBT or the sixth IGBT are low or when the gate drive signals for both the third IGBT and the fourth IGBT are high or when the output current direction is positive and the gate drive signals for the first IGBT, the second IGBT, the third IGBT, and the fourth IGBT are low. 
     In additional embodiments, the failure detection algorithm detects the failure condition in the first IGBT when the expected output voltage is not positive and the blocking state logic signal for the first IGBT is low. 
     In another embodiment, the failure detection algorithm detects the failure condition in the second IGBT when the expected output voltage is negative and the blocking state logic signal for the second IGBT is low. 
     In further embodiments, the failure detection algorithm detects the failure condition in the third IGBT when the expected output voltage is positive and the blocking state logic signal for the third IGBT is low. 
     In an embodiments, the failure detection algorithm detects the failure condition in the fourth IGBT when the expected output voltage is positive and the blocking state logic signal for the fourth IGBT is low. 
     In yet another embodiment, the failure detection algorithm detects the failure condition in the fifth IGBT when the expected output voltage is positive and the blocking state logic signal for the fifth IGBT is low. 
     In still further embodiments, the failure detection algorithm detects the failure condition in the sixth IGBT when the expected output voltage is negative and the blocking state logic signal for the sixth IGBT is low. 
     In another aspect, the present disclosure is directed to a multi-level bridge power converter. The multi-level bridge power converter a plurality of phase legs each including a plurality of switching devices. The plurality of switching devices includes at least four inner switching devices and at least two outer switching devices arranged in an active neutral point clamped topology. The multi-level bridge power converter further includes a switching device failure detection circuit. The switching device failure detection circuit includes a logic module for each of the plurality of switching devices to generate a blocking state logic signal by comparing a switching device voltage of each of the plurality of switching devices and a threshold reference voltage, a voltage calculation module to determine an expected voltage blocking state for each of the plurality of switching devices based on gate drive signals of the plurality of switching devices and an output current direction, and a failure detection algorithm to detect a failure condition in any of the plurality of switching devices based on the blocking state logic signals and the expected voltage blocking states of the plurality of switching devices. It should be understood that the multi-level bridge power converter may further include any of the additional steps and/or features described herein. 
     These and other features, aspects and advantages of the present disclosure will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present disclosure, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG.  1    illustrates a perspective view of one embodiment of a wind turbine according to the present disclosure; 
         FIG.  2    illustrates a schematic diagram of one embodiment of a DFIG wind turbine system according to the present disclosure; 
         FIG.  3    illustrates a circuit diagram of one leg of a conventional neutral point clamped multi-level converter and its output waveform; 
         FIG.  4    illustrates a representation of fault condition in a leg of a three phase three level active neutral point clamped (ANPC) converter; 
         FIG.  5    illustrates a switching device failure detection circuit, in accordance with an embodiment of the present disclosure; 
         FIG.  6    illustrates a diagrammatical representation of a voltage calculation module, in accordance with an embodiment of the present disclosure; 
         FIG.  7    illustrates a flow diagram of an embodiment of a failure detection algorithm in accordance with an embodiment of the present disclosure; and 
         FIG.  8    illustrates a flow diagram of an embodiment of a method for operating a multi-level bridge power converter of an electrical power system connected to a power grid according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Three-level neutral point clamped (NPC) converters must take less voltage stress because of the series connection arrangement compared to two-level converters. However, due to large commutation loops, inner switching devices of the power converter experience higher voltage spikes and demands to have lossy snubbers in place. To avoid snubbers, active neutral point clamped (ANPC) converters with active clamped switches offers shorter commutation loops compared to NPC. Further, double dead time PWM techniques applied to ANPC offers lesser voltage stress on the inner switching devices. In line with ANPC converters, it is important to detect device failure quickly and assign a failure response mode instantly to protect the DC link and avoid chain reaction failure on other devices. Thus, the present disclosure is directed to a system and method for controlling a multi-level power converter that detects any semiconductor failure conditions in the ANPC converter. 
     Referring now to the drawings,  FIG.  1    illustrates a perspective view of one embodiment of a wind turbine  10 . As shown, the wind turbine  10  generally includes a tower  12  extending from a support surface  14 , a nacelle  16  mounted on the tower  12 , and a rotor  18  coupled to the nacelle  16 . The rotor  18  includes a rotatable hub  20  and at least one rotor blade  22  coupled to and extending outwardly from the hub  10 . For example, in the illustrated embodiment, the rotor  18  includes three rotor blades  22 . However, in an alternative embodiment, the rotor  18  may include more or less than three rotor blades  22 . Each rotor blade  22  may be spaced about the hub  20  to facilitate rotating the rotor  18  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, as will be described below, the rotor  18  may be rotatably coupled to an electric generator  24  ( FIG.  2   ) to permit electrical energy to be produced. 
     Referring now to  FIG.  2   , a schematic diagram of one embodiment of a DFIG wind turbine system  50  is illustrated in accordance with aspects of the present subject matter. It should be appreciated that the present subject matter will generally be described herein with reference to the system  50  shown in  FIG.  2   . However, those of ordinary skill in the art, using the disclosures provided herein, should understand that aspects of the present disclosure may also be applicable in other power generation systems. 
     As shown, the rotor  18  of the wind turbine  10  may, optionally, be coupled to a gear box  26 , which is, in turn, coupled to the generator  24 . In accordance with aspects of the present disclosure, the generator  24  is a doubly-fed induction generator (DFIG). For example, as shown, the DFIG  24  may be coupled to a stator bus  28  and a power converter  30  via a rotor bus  32 . The stator bus  28  may provide an output multiphase power (e.g., three-phase power) from a stator of the DFIG  24  and the rotor bus  32  may provide an output multiphase power (e.g., three-phase power) from a rotor of the DFIG  120 . As shown in  FIG.  2   , the power converter  30  includes a rotor side converter  34  and a line side converter  36 . The DFIG  24  may be coupled via the rotor bus  32  to the rotor side converter  34 . Additionally, the rotor side converter  34  may be coupled to the line side converter  36  which may, in turn, be coupled to a line side bus  38 . In particular embodiments, the power converter  30  may be a multi-level bridge power converter, such as a three-level bridge power converter, or higher. As described herein, three-level converters are converters that have three direct current (DC) poles. In addition to positive and negative DC poles, such converters also have a neutral DC pole. 
     In several embodiments, the rotor side converter  34  and the line side converter  36  may be configured for normal operating mode in a three-phase, pulse width modulation (PWM) arrangement using insulated gate bipolar transistor (IGBT) switching devices. The rotor side converter  34  and the line side converter  36  may be coupled via a DC link  40  across which is a DC link capacitor  42 . 
     In addition, the power converter  30  may be coupled to a controller  44  in order to control the operation of the rotor side converter  34  and the line side converter  36 . It should be noted that the controller  44  may, in several embodiments, be configured as an interface between the power converter  30  and a control system  46 . The controller  44  may include any number of control devices. In one embodiment, the controller  44  may include a processing device (e.g., microprocessor, microcontroller, etc.) executing computer-readable instructions stored in a computer-readable medium. The instructions when executed by the processing device may cause the processing device to perform operations, including providing control commands to the switching devices and/or the shorting devices of the power converter  30 . 
     In typical configurations, various line contactors and circuit breakers including, for example, a grid breaker  62  may also be included for isolating the various components as necessary for normal operation of the DFIG  24  during connection to and disconnection from the electrical grid  64 . For example, a system circuit breaker  58  may couple the system bus  56  to a transformer  60 , which may be coupled to the electrical grid  64  via the grid breaker  62 . In alternative embodiments, fuses may replace some or all of the circuit breakers. 
     In operation, alternating current power generated at the DFIG  24  by rotating the rotor  18  is provided via a dual path to the electrical grid  64 . The dual paths are defined by the stator bus  28  and the rotor bus  32 . On the rotor bus side  256 , sinusoidal multi-phase (e.g., three-phase) alternating current (AC) power is provided to the power converter  30 . The rotor side converter  34  converts the AC power provided from the rotor bus  32  into direct current (DC) power and provides the DC power to the DC link  40 . As is generally understood, switching devices (e.g., IGBTs) used in the bridge circuits of the rotor side converter  34  may be modulated to convert the AC power provided from the rotor bus  32  into DC power suitable for the DC link  40 . 
     In addition, the line side converter  36  converts the DC power on the DC link  126  into AC output power suitable for the electrical grid  124 . In particular, switching devices (e.g., IGBTs) used in bridge circuits of the line side power converter  36  can be modulated to convert the DC power on the DC link  40  into AC power on the line side bus  38 . The AC power from the power converter  30  can be combined with the power from the stator of DFIG  24  to provide multi-phase power (e.g., three-phase power) having a frequency maintained substantially at the frequency of the electrical grid  64  (e.g., 50 Hz or 60 Hz). 
     Additionally, various circuit breakers and switches, such as grid breaker  62 , system breaker  58 , stator sync switch  54 , converter breaker  52 , and line contactor  48  may be included in the system  50  to connect or disconnect corresponding buses, for example, when current flow is excessive and may damage components of the wind turbine  10  or for other operational considerations. Additional protection components may also be included in the DFIG wind turbine system  50 , such as the crowbar circuit described below. 
     Moreover, the power converter  30  may receive control signals from, for instance, the control system  46  via the controller  44 . The control signals may be based, among other things, on sensed conditions or operating characteristics of the DFIG wind turbine system  50 . Typically, the control signals provide for control of the operation of the power converter  30 . For example, feedback in the form of a sensed speed of the DFIG  24  may be used to control the conversion of the output power from the rotor bus  32  to maintain a proper and balanced multi-phase (e.g., three-phase) power supply. Other feedback from other sensors may also be used by the controller  44  to control the power converter  30 , including, for example, stator and rotor bus voltages and current feedbacks. Using the various forms of feedback information, switching control signals (e.g., gate timing commands for IGBTs), shorting control signals, stator synchronizing control signals, and circuit breaker signals may be generated. 
     Referring now to  FIG.  3   , a schematic  100  of one leg  120  or one phase of a conventional neutral point clamped (NPC) or diode clamped three level converter and its output waveform  102  is illustrated. Accordingly, embodiments of the present disclosure enable a multi-level converter to convert DC power into an AC power with a switching device fault detection scheme. For example, in a three level NPC converter, a key failure mode exists when a switching device (such as any of switching elements  104 ,  106 ,  108 , or  110  of  FIG.  3   ) or a clamping diode (such as diodes  112  and  114  of  FIG.  3   ) fails short. Under this condition, one half of the DC link, V1 or V2 in  FIG.  3   , is charged to the peak line-to-line voltage of the machine or grid side voltage. This value is typically higher than the maximum allowed blocking voltage of the switching devices and the capacitors. In this case, other switching devices or clamping diodes may be stressed in terms of voltage or current beyond their capability. Hence, this will cause additional switching devices, in particular switching devices in phase legs connected to the same DC bus, to fail after the failure of the initial switching device. The switching device failure detection scheme of the present disclosure provides a suitable logic circuit to detect failures in switching devices and so prevents secondary damage of multi-level converters which may be used in applications such as a solar (or photovoltaic) power generation systems, wind turbine generators or marine and hydrokinetic power generation systems. 
     Further, as shown, one leg  120  of the three-level converter includes four switching devices  104 ,  106 ,  108 ,  110  and two clamping diodes  112 ,  114 . Input voltages V1 and V2 are controlled to each have a voltage equal to Vdc/2, where Vdc is the total DC link voltage. Voltage V3 is the phase A output voltage measured with respect to a center point  122  of DC link  124 . Switching device  104  is complementary to switching device  108  so that, when switching device  104  is gated on, switching device  108  is gated off and vice versa. Similarly, switching devices  106  and  110  are complementary. 
     In operation, each leg of the NPC three-level converter has three switching stages. In the first switching stage, switching devices  104 ,  106  are turned on and switching devices  108 ,  110  are turned off. Assuming a stable operation, V1=V2=Vdc/2, and V3 becomes Vdc/2. In the second switching stage, switching devices  106 ,  108  are turned on while switching devices  104 ,  110  are turned off. In this stage, V3 is equal to zero. In the third switching stage, switching devices  104 ,  106  are turned off whereas switching devices  108 ,  110  are turned on. This results in V3 becoming −Vdc/2 as shown in waveform  102 . Thus, it can be seen that the phase voltage V3 has three levels Vdc/2, −Vdc/2 and 0. When all three legs of the NPC three-phase converter are combined, then the resulting line to line voltages have five levels namely Vdc, Vdc/2, 0, −Vdc/2 and −Vdc. The three-level converter of  FIG.  3    may be increased to any level depending on the circuit topology and number of switching devices and diodes in the circuit. As the number of levels in the converter increases, the output waveform of the converter approaches a pure sine wave, resulting in lower harmonics in the output voltage. In general, the number of switching stages can be higher than three as switching devices may not be gated on if the corresponding free-wheeling diode is going to conduct current. This operation mode does not affect the number of levels of the output phase voltage. 
     Referring now to  FIG.  4   , a schematic  130  of an embodiment of one leg  131  or one phase of a three level active neutral point clamped (ANPC) converter is illustrated. In particular, as shown, an embodiment of a fault condition  132  in the leg  131  of the multi-level converter is illustrated. Further, as shown, each leg  131  of the converter includes two outer switching devices (e.g., switching devices  134 ,  140 ) and four inner switching devices (e.g., switching devices  136 ,  138 ,  142 ,  144 ). Thus, as described herein, the two outer switching devices (e.g., switching devices  134 ,  140 ) may also be referred to as a first IGBT and a fourth IGBT, whereas the four inner switching devices (e.g., switching devices  136 ,  138 ,  142 ,  144 ) may also be referred to as a second IGBT, a third IGBT, a fifth IGBT, and a sixth IGBT. 
     Moreover, as shown, leg  131  is connected to a split DC link  150  with a top capacitor  152  and a bottom capacitor  154 . Thus,  FIG.  4    shows outer switching device  134  of leg  131  failing short (represented by referral numeral  132 ). The condition may occur as a result of occurrences such as, for example, thermal breakdown of outer switching device  134 , the voltage breakdown of outer switching device  134 , failures due to cosmic rays, and failures due to weak manufacturing of the switching device. 
     Referring now to  FIG.  5   , a switching device failure detection circuit  160  in accordance with an embodiment of the present disclosure is illustrated. More particularly, as shown, the switching device failure detection circuit  160  includes logic modules or comparators  162 ,  164 ,  166 ,  168 ,  170 ,  172 , each corresponding to a respective one of the switching devices  134 ,  136 ,  138 ,  140 ,  142 ,  144  ( FIG.  4   ). Switching device failure detection circuit  160  further includes a voltage calculation module  174 , and a failure detection algorithm  176 . Thus, as shown, each comparator  162 ,  164 ,  166 ,  168 ,  170 ,  172  compares two inputs, i) a switching device voltage i.e., a voltage across a switching device Vsw and ii) a threshold reference voltage Vth and provides an output voltage referred as blocking state logic signal Vc. Moreover, the following relationships exist between switching device voltage Vsw ( FIG.  5   ) and channel Ch ( FIG.  4   , where the channel represents the channel of measurement, such as an oscilloscope): 
       Vsw1=Ch1−Ch2,
 
       Vsw2=Ch6−Ch5,
 
       Vsw3=Ch5, 
       Vsw4=Ch4, 
       Vsw5=Ch2, and 
       Vsw6=Ch3−Ch4,
 
       Where Ch1 and Ch3 are Vdctop and Vdcbot, 
       Ch7 is measuring pole voltage with DC Neg 
       Ch8 is measuring bridge current (where out of the bridge is positive). 
     In one embodiment, the threshold reference voltage Vth is set slightly higher than the rated voltage drop across the switching device when the switching device is conducting. In another embodiment, this signal is typically already available from de-saturation detection circuit, e.g., in IGBT converters. In yet another embodiment, the threshold reference voltage Vth may be set to an arbitrary suitable level between the voltage in the previous embodiment i.e., slightly higher than the rated voltage drop across the switching device and below the lower limit of Vdc/2. In another embodiment, the blocking state logic signal Vc is ‘high’ if Vsw is greater than Vth, else it is ‘low’. It should be noted that ‘high’ refers to the positive supply voltage of the comparator and ‘low’ refers to the negative supply voltage of the comparator. 
     Thus, as an example, comparator  162  compares Vsw 1  and Vth and outputs Vc 1 , where Vsw 1  refers to voltage across switching device  134 , comparator  164  compares voltage across switching device  136 , Vsw 2  and Vth and outputs Vc 2 . Similarly, comparator  166  compares Vsw 3 , which is voltage across switching device  138  and Vth, and outputs Vc 3 , comparator  168  compares voltage across switching device  140 , Vsw 4  and Vth and outputs Vc 4 , comparator  170  compares voltage across switching device  142 , Vsw 5  and Vth and outputs Vc 5 , and comparator  172  compares voltage across switching device  144 , Vsw 6  and Vth and outputs Vc 6 . 
     In an embodiment, the comparator(s)  162 ,  164 ,  166 ,  168 ,  170 ,  172  may be an open loop operational amplifier circuit or a dedicated voltage comparator integrated within a chip. In another embodiment, a portion of a desaturation circuit generally used in IGBT drivers, which provides a threshold voltage logic signal, may also be used as the comparator(s)  162 ,  164 ,  166 ,  168 ,  170 ,  172 . 
     Based on the gate signals of the switching devices  134 ,  136 ,  138 ,  140 ,  142 ,  144  and an output current direction (i.e., Io direction), the voltage calculation module  174  determines whether in the current switching stage, each switching device is supposed to block a voltage across its terminals or not. The output of the voltage calculation module  174  is the expected voltage (Vexp) for the switching devices that is input into switching device failure detection algorithm  176 . An expected voltage (Vexp) at the output terminal points of leg  131  of the multi-level converter during normal condition is calculated as an intermediate step in voltage calculation module  174 . 
     The expected voltage is determined based on the inputs such as gate drive signals G 1 , G 2 , G 3 , G 4 , G 5 , G 6  for switching devices  134 ,  136 ,  138 ,  140 ,  142 ,  144  and the corresponding output current direction, i.e., Io direction. The expected output voltage Vexp has three levels, positive, negative, and zero. Based on these levels, the gate drive signals, and the output current direction, the determination of whether a particular switching device should block the voltage or not is made and thus, the logic signals Vs 1 , Vs 2 , Vs 3 , Vs 4 , Vs 5 , and Vs 6  are implicitly generated and used by the failure detection algorithm  176  (as shown in  FIG.  7   ). If any of the logic signals is high, it indicates that the particular switching device should block the voltage, else it should not. Further, the switching device failure detection algorithm  176  can then compare for each switching devices the information whether a switching device is actually blocking or not (i.e., Vc 1 , Vc 2 , Vc 3 , Vc 4 , Vc 5 , Vc 6 ) with the information whether each device should be blocking or not (i.e., Vs 1 , Vs 2 , Vs 3 , Vs 4 , Vs 5 , and Vs 6 ) in the current switching stage. If actual blocking state and expected blocking state of any switching device do not match (for example if Vc 1 ≠Vs 1 ), a failure is detected. 
     The above switching device failure detection circuit  160  may be implemented in the analog domain, or the digital domain, or a combination thereof. Where digital circuitry is used for processing, the circuitry can generally include analog-to-digital conversion, although analog processing components may generally not require such conversion unless some processing is done in the digital domain. Examples of digital circuitry include digital components, such as a programmed microprocessor, field programmable gate array (FPGA), application specific digital signal processor (DSP) or the like. It should be noted that the particular order of processing as represented by the components shown in  FIG.  5    may be altered, and other components may be included in the overall circuitry, where desired. 
     Referring now to  FIG.  6   , an embodiment of an implementation of an expected voltage calculation module  180  for determining the expected voltage Vexp at the multi-level converter terminals is illustrated. In particular, as shown, the voltage calculation module  180  receives inputs such as gate drive signals G 1 , G 2 , G 3 , G 4 , G 5 , G 6  for switching devices  134 ,  136 ,  138 ,  140 ,  142 ,  144 , respectively, and also output current or Io direction. It should be noted the gate drive signal levels determine whether any switching device is ON or not. For example, if G 2  or G 3  is high, switching devices  136  and  138  are ON, respectively. Thus, for all the cases presented below, if the switching device is ON, the corresponding gate drive signal is referred to as high. 
     As shown at  181 ,  182 ,  183 , and  186 , if the gate drive signals (i.e., G 1 , G 2 , and G 4 ) for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal (G 2 ) for the second IGBT is high, the output current direction is negative, and the gate signals (i.e., G 5 , G 6 ) of the fifth IGBT and the sixth IGBT are high, then as shown at  186 , the expected output voltage is a zero voltage. 
     In an alternative path, as shown at  181 ,  182 ,  183 , and  184  if the gate drive signals (i.e., G 1 , G 3 , and G 4 ) for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal (G 2 ) of the second IGBT is high, the output current direction is negative, and the gate drivel signals (i.e., G 5 , G 6 ) for the fifth IGBT or the sixth IGBT are low, then as shown at  184 , the expected output voltage is a positive voltage. 
     As shown at  187 ,  188 ,  189 , and  191 , if the gate drive signals (i.e., G 1 , G 2 , and G 4 ) for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal (G 3 ) of the third IGBT is high, the output current direction is positive, and the gate signals (i.e., G 5 , G 6 ) of the fifth IGBT and the sixth IGBT are high, then as shown at  191 , the expected output voltage is a zero voltage. 
     In an alternative path, as shown at  187 ,  188 ,  189 , and  190 , if the gate drive signals (i.e., G 1 , G 2 , and G 4 ) for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal (G 3 ) of the third IGBT is high, the output current direction is positive, and the gate drivel signals (i.e., G 5 , G 6 ) for the fifth IGBT or the sixth IGBT are low, then as shown at  190 , the expected output voltage is a negative voltage. 
     Still referring to  FIG.  6   , as shown at  192 , it can be further determined whether any one of inner switching devices  136 ,  138  (e.g., second IGBT or third IGBT) are ON and whether both outer switching devices  134 ,  140  (e.g., first IGBT or fourth IGBT) are OFF. In such a situation, the expected voltage Vexp is equivalent to a zero voltage as represented by a block  193 . This condition is represented in step  192  by a Boolean equation, (G 2 +G 3 ).(G 1 +G 4 )=1, in which the period (.) is used to represent multiplication (logic-AND). 
     However, if this condition is not met, then as shown at  194 , it is determined whether both top switching devices  134 ,  136  (e.g., first IGBT and second IGBT) are ON i.e., whether G 1 .G 2 =1. If both top switching devices  134 ,  136  are ON, then the expected voltage Vexp at the output terminals should be a positive voltage i.e., V+, represented by a block  198 . If the condition checked at  194  is not met, then as shown at  195 , the algorithm  180  determines whether both bottom switching devices  138 ,  140  (e.g., third IGBT and fourth IGBT) are ON i.e., represented by G 3 .G 4 =1. Both bottom switching devices  138 ,  140  being ON indicates that expected voltage Vexp should be a negative voltage i.e., V− represented by a block  201 . If any of the conditions  192 ,  194 ,  195  are not met, then the output current (Io) direction helps in determining the expected voltage Vexp, as shown at  196  and  199 . 
     In particular, as shown at  196 , it is determined whether the output current Io is negative or not. If output current Io is negative, then as shown at  200 , the algorithm  190  determines whether the switching devices  134 ,  136 ,  138 ,  140  (e.g., first IGBT, second IGBT, third IGBT, and fourth IGBT) are OFF. This condition is represented as shown at  197  by a Boolean equation, NOT(G 1 +G 2 +G 3 +G 4 )=1, wherein the plus sign (+) is used to represent addition (logic-OR) . If these switching devices are OFF and the output current Io is negative, it indicates that freewheeling diodes of the top two switching devices are conducting. Thus, the expected voltage Vexp during this condition will be positive voltage V+, as represented by block  198 . If it is determined at  196 , that output current Io is not negative (i.e., positive or zero), then as shown at  199 , it is determined whether the output current Io is positive or not. If the output current Io is positive, as shown at  200 , it is again determined whether the switching devices  134 ,  136 ,  138 ,  140  are OFF. If the switching devices  134 ,  136 ,  138 ,  140  are OFF and the current direction is positive, it indicates that freewheeling diodes of the bottom two switching devices (e.g., third IGBT or fourth IGBT) are conducting and the expected voltage during this condition will be negative voltage V− (as shown at  201 ). 
     Referring now to  FIG.  7   , a failure detection algorithm  210  in accordance with an embodiment of the present disclosure is illustrated. In particular, as shown, the failure detection algorithm  210  determines whether any switching device that should block a DC voltage is actually not blocking DC voltage. In particular, as shown at  212 , it is determined whether expected voltage Vexp is positive (V+) or not. If Vexp is not equal to V+, i.e., if Vexp is equal to negative voltage V− or zero voltage, then as shown at  214 , it is checked whether blocking state logic signal Vc 1  of first switching device  134  is low or high. If Vc 1  is low then it indicates that switching device  134  is not blocking any voltage when actually it should block the voltage. The fault indication or the short circuit indication is then provided as shown in block  216 . Another condition that is checked if Vexp is not equal to V+is whether Vexp is negative voltage i.e., V−. If it is determined at  218  that Vexp is indeed V−, then as shown at  220 , it is checked whether blocking state logic signal Vc 2  for switching device  136  is low. If Vc 2  is low, then the second switching device  136  is considered to be faulty as shown in block  222 . If it is determined that Vexp is a positive voltage i.e., V+, then as shown at  224 , the blocking state logic signal Vc 3  for switching device  138  is compared with the low voltage. If it is found that Vc 3  is low, then it means the switching device  138  is faulty as shown in block  226 . 
     Further, if the expected voltage Vexp is zero as determined at  218  or positive as determined at  212 , then as shown at  228 , it is checked whether the blocking state logic signal Vc 4  for the switching device  138  is low. If Vc 4  is low, then it is indication of the switching device  140  being faulty as shown in block  230 . Moreover, if it is determined that Vexp is a positive voltage i.e., V+, then as shown at  236 , the blocking state logic signal Vc 5  for switching device  142  is compared with the low voltage. If it is found that Vc 5  is low, then it means the switching device  142  is faulty as shown in block  238 . Further, if it is determined that Vexp is a negative voltage i.e., V−, then as shown at  232 , the blocking state logic signal Vc 6  for switching device  144  is compared with the low voltage. If it is found that Vc 6  is low, then it means the switching device  144  is faulty as shown in block  234 . Thus, in this way with various combinations, failure conditions of switching devices are determined. 
     It should be noted that the steps presented in  FIGS.  6  and  7    are not restricted by their sequence. For instance, in an embodiment, some of the steps can be performed in parallel or in another embodiment; the sequence of the steps can be interchanged as long as it does not affect the final outcome. 
     Referring now to  FIG.  8   , a flow diagram of one embodiment of a computer-implemented method  300  for operating a multi-level bridge power converter of an electrical power system connected to a power grid is illustrated in accordance with aspects of the present subject matter. In general, the method  300  will be described herein as being implemented using a wind turbine system, such as the DFIG wind turbine system  50  described above with reference to  FIGS.  1 - 7   . However, it should be appreciated that the disclosed method  300  may be implemented using any other suitable power generation system that is configured to supply power for application to a load. In addition, although  FIG.  8    depicts steps performed in a particular order for purposes of illustration and discussion, the methods described herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods can be omitted, rearranged, combined and/or adapted in various ways. 
     As shown at ( 302 ), the method  300  includes arranging a plurality of switching devices having at least four inner switching devices and at least two outer switching devices in an active neutral point clamped topology. As shown at ( 304 ), the method  300  includes determining whether any of the plurality of switching devices is experiencing a failure condition by implementing a failure detection algorithm. As shown at ( 306 ), the failure detection algorithm includes generating a blocking state logic signal by comparing a switching device voltage and a threshold reference voltage for each of the plurality of switching devices. As shown at ( 308 ), the failure detection algorithm includes determining an expected voltage blocking state for each of the plurality of switching devices based on gate drive signals of the plurality of switching devices and an output current direction. As shown at ( 310 ), the failure detection algorithm includes detecting whether a failure condition is present in any of the plurality of switching devices based on the blocking state logic signals and the expected voltage blocking states of the plurality of switching devices. 
     Further aspects of the disclosure are provided by the subject matter of the following clauses: 
     Clause 1. A computer-implemented method for operating a multi-level bridge power converter of an electrical power system connected to a power grid, the power converter comprising a first converter and a second converter coupled via a DC link, the method comprising: 
     arranging a plurality of switching devices having at least four inner switching devices and at least two outer switching devices in an active neutral point clamped topology; 
     determining whether any of the plurality of switching devices is experiencing a failure condition by implementing a failure detection algorithm, the failure detection algorithm comprising:
         generating a blocking state logic signal by comparing a switching device voltage and a threshold reference voltage for each of the plurality of switching devices;   determining an expected voltage blocking state for each of the plurality of switching devices based on gate drive signals of the plurality of switching devices and an output current direction; and   detecting whether a failure condition is present in any of the plurality of switching devices based on the blocking state logic signals and the expected voltage blocking states of the plurality of switching devices.       

     Clause 2. The method of clause 1, wherein the plurality of switching devices comprises, at least, a plurality of insulated gate bipolar transistors (IGBTs), the plurality of IGBTs comprising a first IGBT, a second IGBT, a third IGBT, a fourth IGBT, a fifth IGBT, and a sixth IGBT, and wherein the second IGBT, the third IGBT, the fifth IGBT, and the sixth IGBT are the four inner switching devices, and the first IGBT and the fourth IGBT are the two outer switching devices. 
     Clause 3. The method of clause 2, wherein determining the expected voltage blocking state for each of the plurality of switching devices based on the gate drive signals of the plurality of switching devices and the output current direction further comprises determining the expected output voltage at output terminals of the multi-level bridge power converter. 
     Clause 4. The method of clause 3, wherein the expected output voltage is a zero voltage when the gate drive signals for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal of the second IGBT is high, the output current direction is negative, and the gate drive signals for the fifth IGBT and the sixth IGBT are high or when the gate drive signals for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal of the third IGBT is high, the output current direction is positive, and the gate drive signals for the fifth IGBT and the sixth IGBT are high or when the gate drive signals for the second IGBT or the third IGBT are high and the gate drive signals for both the first IGBT and the fourth IGBT are low. 
     Clause 5. The method of clauses 3-4, wherein the expected output voltage is a positive voltage when the gate drive signals for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal of the second IGBT is high, the output current direction is negative, and the gate drive signals for the fifth IGBT or the sixth IGBT are low or when the gate drive signals for both the first IGBT and the second IGBT are high or when the output current direction is negative and the gate drive signals for the first IGBT, the second IGBT, the third IGBT, and the fourth IGBT are low. 
     Clause 6. The method of clauses 3-5, wherein the expected output voltage is a negative voltage when the gate drive signals for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal of the third IGBT is high, the output current direction is positive, and the gate drive signals for the fifth IGBT or the sixth IGBT are low or when the gate drive signals for both the third IGBT and the fourth IGBT are high or when the output current direction is positive and the gate drive signals for the first IGBT, the second IGBT, the third IGBT, and the fourth IGBT are low. 
     Clause 7. The method of clauses 3-6, wherein the failure detection algorithm detects the failure condition in the first IGBT when the expected output voltage is not positive and the blocking state logic signal for the first IGBT is low. 
     Clause 8. The method of clauses 3-7, wherein the failure detection algorithm detects the failure condition in the second IGBT when the expected output voltage is negative and the blocking state logic signal for the second IGBT is low. 
     Clause 9. The method of clauses 3-8, wherein the failure detection algorithm detects the failure condition in the third IGBT when the expected output voltage is positive and the blocking state logic signal for the third IGBT is low. 
     Clause 10. The method of clauses 3-9, wherein the failure detection algorithm detects the failure condition in the fourth IGBT when the expected output voltage is positive or zero and the blocking state logic signal for the fourth IGBT is low. 
     Clause 11. The method of clauses 3-10, wherein the failure detection algorithm detects the failure condition in the fifth IGBT when the expected output voltage is positive and the blocking state logic signal for the fifth IGBT is low. 
     Clause 12. The method of clauses 3-11, wherein the failure detection algorithm detects the failure condition in the sixth IGBT when the expected output voltage is negative and the blocking state logic signal for the sixth IGBT is low. 
     Clause 13. A multi-level bridge power converter, comprising: 
     a plurality of phase legs each comprising a plurality of switching devices, the plurality of switching devices comprising at least four inner switching devices and at least two outer switching devices arranged in an active neutral point clamped topology; and 
     a switching device failure detection circuit, comprising:
         a logic module for each of the plurality of switching devices to generate a blocking state logic signal by comparing a switching device voltage of each of the plurality of switching devices and a threshold reference voltage;   a voltage calculation module to determine an expected voltage blocking state for each of the plurality of switching devices based on gate drive signals of the plurality of switching devices and an output current direction; and   a failure detection algorithm to detect a failure condition in any of the plurality of switching devices based on the blocking state logic signals and the expected voltage blocking states of the plurality of switching devices.       

     Clause 14. The multi-level bridge power converter of clause 13, wherein the plurality of switching devices comprises, at least, a plurality of insulated gate bipolar transistors (IGBTs), the plurality of IGBTs comprising a first IGBT, a second IGBT, a third IGBT, a fourth IGBT, a fifth IGBT, and a sixth IGBT, and wherein the second IGBT, the third IGBT, the fifth IGBT, and the sixth IGBT are the four inner switching devices, and the first IGBT and the fourth IGBT are the two outer switching devices. 
     Clause 15. The multi-level bridge power converter of clause 14, wherein determining the expected voltage blocking state for each of the plurality of switching devices based on the gate drive signals of the plurality of switching devices and the output current direction further comprises determining the expected output voltage at output terminals of the multi-level bridge power converter. 
     Clause 16. The multi-level bridge power converter of clause 15, wherein the expected output voltage is a zero voltage when the gate drive signals for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal of the second IGBT is high, the output current direction is negative, and the gate drive signals for the fifth IGBT and the sixth IGBT are high or when the gate drive signals for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal of the third IGBT is high, the output current direction is positive, and the gate drive signals for the fifth IGBT and the sixth IGBT are high or when the gate drive signals for the second IGBT or the third IGBT are high and the gate drive signals for both the first IGBT and the fourth IGBT are low. 
     Clause 17. The multi-level bridge power converter of clauses 15-16, wherein the expected output voltage is a positive voltage when the gate drive signals for the first IGBT, the third IGBT, and the fourth IGBT are low, the gate drive signal of the second IGBT is high, the output current direction is negative, and the gate drive signals for the fifth IGBT or the sixth IGBT are low or when the gate drive signals for both the first IGBT and the second IGBT are high or when the output current direction is negative and the gate drive signals for the first IGBT, the second IGBT, the third IGBT, and the fourth IGBT are low. 
     Clause 18. The multi-level bridge power converter of clauses 15-17, wherein the expected output voltage is a negative voltage when the gate drive signals for the first IGBT, the second IGBT, and the fourth IGBT are low, the gate drive signal of the third IGBT is high, the output current direction is positive, and the gate drive signals for the fifth IGBT or the sixth IGBT are low or when the gate drive signals for both the third IGBT and the fourth IGBT are high or when the output current direction is positive and the gate drive signals for the first IGBT, the second IGBT, the third IGBT, and the fourth IGBT are low. 
     Clause 19. The multi-level bridge power converter of clauses 15-18, wherein the failure detection algorithm detects the failure condition in the first IGBT when the expected output voltage is not positive and the blocking state logic signal for the first IGBT is low, wherein the failure detection algorithm detects the failure condition in the second IGBT when the expected output voltage is negative and the blocking state logic signal for the second IGBT is low, wherein the failure detection algorithm detects the failure condition in the third IGBT when the expected output voltage is positive and the blocking state logic signal for the third IGBT is low, and wherein the failure detection algorithm detects the failure condition in the fourth IGBT when the expected output voltage is positive or zero and the blocking state logic signal for the fourth IGBT is low. 
     Clause 20. The multi-level bridge power converter of clauses 15-19, wherein the failure detection algorithm detects the failure condition in the fifth IGBT when the expected output voltage is positive and the blocking state logic signal for the fifth IGBT is low, and wherein the failure detection algorithm detects the failure condition in the sixth IGBT when the expected output voltage is negative and the blocking state logic signal for the sixth IGBT is low. 
     This written description uses examples to disclose the disclosure, including the best mode, and to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.