Filter capacitor degradation detection apparatus and method

Power conversion systems and methods are presented for detecting input filter capacitor degradation or approach of end of operational life based on filter capacitor current measurements using single and/or dual threshold comparisons for computed instantaneous sum of squares of filter currents or power values.

BACKGROUND

Motor drives and other power conversion systems operate using power from AC power sources, and typically include an input filter to reduce switching noise associated with operation of the power converter, particularly to control total harmonic distortion (THD) generated by high frequency operation of certain active front end (AFE) rectifiers. In particular, many power conversion systems utilize inductor-capacitor (LC) or inductance-capacitance-inductance (LCL) input filter circuitry associated with each AC input phase to control the harmonic content of a power grid. Such LC or LCL filter circuits are subject to damage or degradation of the filter capacitors. Failure of filter capacitors may be costly in terms of replacement component costs, labor for inspection and replacement, as well as downtime for the power conversion system and any associated machinery. Thusfar, however, assessing the performance and any degradation in the input filter capacitors has been difficult, and initial capacitor degradation may not be identifiable by visual inspection by service personnel.

SUMMARY

Various aspects of the present disclosure are now summarized to facilitate a basic understanding of the disclosure, wherein this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present various concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter. The present disclosure provides power converters and techniques for identifying suspected filter capacitor degradation based in whole or in part on measured filter capacitor currents.

Power conversion systems are provided, which include a filter circuit coupled between an AC input and a rectifier. The filter circuit includes a plurality of filter capacitors which may be connected in a delta configuration or in a Y configuration in various embodiments. A controller identifies suspected filter capacitor degradation at least partially according to currents flowing in the filter capacitors. In certain embodiments, a sum of squares value of the filter capacitor currents is compared with a threshold for identification of suspected filter capacitor degradation. The filter capacitor current values may be filtered in certain embodiments using a low pass filter with a cutoff frequency set between the second and third harmonics of a power source fundamental frequency, and the threshold value may be selectively adjusted according to various parameters including measured voltage balance conditions in the power converter. In some embodiments, an instantaneous active power and/or reactive power value is computed and compared with a threshold for selectively identifying suspected filter capacitor degradation. In certain embodiments, moreover, the individual filter capacitors are formed by interconnection of two or more component capacitors, and the threshold is at least partially based on the capacitance of the component capacitors and on the series and/or parallel interconnection configuration of the component capacitors. In certain embodiments, the controller selectively identifies suspected filter capacitor degradation if a computed sum of the squares value, instantaneous active power value, or an instantaneous reactive power value is greater than an upper threshold or less than a lower threshold. In certain implementations, moreover, the controller measures one or more power converter voltages and selectively adjusts the upper and/or lower threshold based on the voltage. For example, the controller in certain embodiments increases the threshold(s) if the voltage is greater than a nominal value and decreases the threshold(s) if the voltage is below the nominal value.

Methods and non-transitory computer readable mediums are provided with computer executable instructions for identifying suspected filter capacitor degradation in a power conversion system. The method and computer executable instructions provide for measuring currents associated with a plurality of filter capacitors of the power conversion system, and selectively identifying suspected filter capacitor degradation at least partially according to the filter capacitor currents. In certain embodiments, the method includes computing a sum of the squares of a plurality of filter capacitor currents, and selectively identifying suspected filter capacitor degradation if the computed sum exceeds a threshold. Certain embodiments, moreover, may include filtering the measured filter capacitor currents using a low pass filter, as well as adjusting the threshold at least partially according to a determined AC voltage balance condition. In certain embodiments, the method includes computing an instantaneous active and/or reactive power value and selectively identifying suspected filter capacitor degradation if the computed power value exceeds a threshold. In certain embodiments, moreover, suspected filter capacitor degradation is selectively identified if a computed sum of the squares value, instantaneous active power value, or an instantaneous reactive power value is greater than an upper threshold or less than a lower threshold. Certain embodiments of the method may further include measuring at least one power converter voltage and selectively adjusting one or both thresholds based on the voltage, such as by increasing the threshold(s) if the voltage is greater than a nominal value and decreasing the threshold(s) if the voltage is below the nominal value.

DETAILED DESCRIPTION

Referring now to the figures, several embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale.

FIG. 1illustrates a power conversion system2including a precharge circuit10, an LCL or LC input filter circuit20, an active front end (AFE) rectifier30, a DC link circuit40, an inverter50, and a controller60. The power conversion system2receives multiphase AC input power from a power source4and provides AC output power to a single or multiphase load6, such as a motor. The power converter2includes an AC input3coupleable to receive AC input power from the power source4, and the inverter50provides an AC output52to drive the load6. In certain embodiments, the power conversion system is a current source converter (CSC) system having an LC filter circuit20and a DC link40with one or more inductances (e.g., such as a DC link choke) to accommodate DC link current provided by the rectifier30and used as input power by the inverter50. In other embodiments described herein, the converter2is a voltage source converter (VSC) type with an LCL filter circuit20, in which the DC link circuit40includes one or more DC link capacitances (e.g., C1and C2as seen inFIG. 5below). The power source4provides multiphase AC input power, where the illustrated examples show a three-phase implementation, although other multiphase implementations are possible having three or more input phases. Moreover, the inverter50can provide a single phase or multiphase output52, with the illustrated examples showing a three-phase inverter50driving a three-phase load6(e.g., a motor). The converter2, moreover, can be a motor drive although any form of power conversion system2may be implemented according to the present disclosure, whether driving a motor or a different form of single or multiphase load6.

The controller60can be implemented as any hardware, processor-executed software, processor-executed firmware, programmable logic and/or combinations thereof to implement the filter capacitor degradation detection functionality set forth herein including other functions associated with operation of the power conversion system2. In certain embodiments, the controller60may be implemented as a single processor-based circuit and/or may be implemented using multiple processor elements. For instance, certain filter capacitor degradation detection functions set forth herein may be implemented in a local controller60, such as a field programmable gate array (FPGA) implemented in the LCL input filter circuit20, and/or such features may be implemented using a centralized controller60in certain embodiments. In yet other possible implementations, hardware circuits may be used to implement one or more of the capacitor degradation detection features, alone or in combination with one or more processor components.

As seen inFIG. 2, the precharge circuit10includes a main circuit breaker12, a fused disconnect apparatus14, a precharge contactor16and precharge resistors18, and is operable in one of three modes. The precharge circuit10may be omitted in certain embodiments. As seen inFIG. 1A, alternate power converter embodiments can provide the main circuit breaker12between the AC input3and the input filter circuit20, with a precharge circuit10with precharge contactor16and precharge resistors18connected between the filter output22and the input of the rectifier30. In the illustrated example ofFIG. 2, the precharge circuitry10is operated by the controller60, which may be integrated with an overall power conversion system controller60and/or which may be a separate processor-based controller. In certain embodiments, the contacts of the fused disconnect14are typically closed and will be opened only upon occurrence of an excess current condition. In a normal operating mode, the controller60(e.g., a central controller or a local precharge I/O board or precharge controller) maintains the main circuit breaker12in the closed position to allow input power to flow from the power source4to precharge output terminals13, but maintains the precharge contactor16in an “open” (e.g., nonconductive) condition, whereby no current flows through the pre-charge resistors18. In a “precharge” mode (e.g., at startup or controlled reset of the power conversion system2), the controller60switches the main circuit breaker12into the “open” condition and closes the precharge contactor16, to allow current to flow from the AC source4through the precharge resistors18to the precharge output terminals13. This effectively inserts the precharge resistors18into the multiphase power circuitry during the “precharge” mode to control excessive current spikes to charge the capacitance of a DC bus in the DC link circuit40at the output of the rectifier30and/or at the input of the inverter50(e.g., capacitors C1and C2in the example ofFIG. 5below). In operation, the controller60may be provided with one or more feedback signals by which a DC link voltage can be monitored, and once the DC voltage exceeds a predetermined value, the controller60closes the main breaker12and opens the precharge contactor16to enter the normal mode of operation. The precharge circuitry10can also be operated in a “standby” mode, in which the controller60maintains both the main circuit breaker12and the precharge contactor16in the “open” condition, with auxiliary power being provided to various control circuits by a power supply19(FIG. 2). In certain embodiments, moreover, the precharge circuit10is operable by the controller60to selectively open both the main circuit breaker12and the precharge contactor16in response to indication of suspected filter capacitor degradation as described further below.

As seen inFIG. 1A, in other possible embodiments, the precharge circuit10may be located between filter circuit20and the rectifier30. In certain implementations, a main circuit breaker12may be connected between the AC input3and the filter circuit20to facilitate turning the power off, and the precharge circuit10will include a precharge contactor16and precharge resistors18connected in a bypass circuit around a precharge breaker, such as breaker12shown inFIG. 2.

Referring also toFIGS. 3, 3A, 4 and 4A, the precharge circuit outputs13are connected to an LCL or LC input filter circuit20. In certain embodiments, the precharge circuitry10may be omitted, and the LCL or LC filter circuit20is directly or indirectly coupled with the power converter AC inputs3. The filter circuit20inFIGS. 3 and 4includes an LCL circuit for each input phase, including a first (e.g., 3%) inductor L1(e.g., L1A, L1B and L1C) and a second (e.g., 9%) inductor L2(L2A, L2B and L2C) coupled in series with one another between the corresponding precharge circuit output13(or the corresponding AC input terminal3) and a corresponding phase output22of the filter circuit20. A plurality of filter capacitors CF are provided, with at least one of the filter capacitors CF connected to each of the phase lines at a center node between the corresponding first and second inductors L1and L2. In the example ofFIG. 3, the filter capacitors CF are connected in a delta configuration with a first capacitor CF connected between phases A and B, a second capacitor CF connected between phases B and C, and a third filter capacitor CF connected between phases C and A. Discharge resistors may be provided in certain embodiments as shown inFIG. 3, with each such resistor being connected between a corresponding one of the power phases and an internal node such as a neutral.FIG. 4illustrates another embodiment in which the filter capacitors CF and corresponding parallel-connected discharge resistors are connected in a “Y” configuration, with each filter capacitor CF being connected between a corresponding one of the power phases and a common node, which in turn may be connected to a system ground, a neutral of the input power source4, or which may be only connected to the filter capacitors CF in various embodiments.

As seen inFIGS. 3A and 4A, current source converter embodiments can include an LC filter with a plurality of filter capacitors CF connected downstream of corresponding inductors L1A, L1B and L1C connected in the corresponding power phases between the filter capacitor connection points and the AC input3. In these embodiments, moreover, discharge resistors may be connected in parallel with each of the filter capacitors CF as shown, or such discharge resistors may be omitted in other embodiments.FIG. 3Aillustrates a current source converter embodiment of the filter circuit20in which the filter capacitors CF are connected in a delta configuration with discharge resistors connected between the corresponding filter capacitor connections and a central node.FIG. 4Aillustrates another embodiment of an LC filter circuit20for a current source converter system2in which the filter capacitor CF are connected in a Y-configuration along with parallel-connected discharge resistors.

FIG. 5illustrates an active front end (AFE) rectifier circuit30in the power conversion system ofFIG. 1, as well as a DC link circuit40. In the illustrated example, the rectifier30includes switching devices Q1-Q6, such as insulated gate bipolar transistors (IGBTs) or other electrical switching devices. Q1-Q6are individually operable according to a corresponding rectifier switching control signal from the controller60to selectively couple a corresponding one of the phase lines A, B and C to one of two DC circuit nodes32or34to rectify input AC power to provide DC power to the DC link40, where the controller60may provide the switching control signals according to any suitable switching scheme such as pulse width modulation (PWM). The rectifier30may alternatively or in combination provide passive rectifier diodes D1-D6individually coupled between one of the AC nodes22at the filter circuit output and a corresponding one of the DC nodes32,34for passive rectification of AC input power to establish the DC link40. Certain embodiments of the rectifier30may provide regenerative operation (with or without the passive rectifier diodes D1-D6rectifying input power to charge the capacitors C1, C2of the DC link circuit40), in which the controller60selectively actuates the rectifier switches Q1-Q6via pulse width modulation or other suitable switching technique for selective connection of the DC nodes32,34with the input nodes22to allow conduction of regenerative current from the DC link40back towards the power source4.

The DC link circuit40(also shown inFIG. 5) includes one or more capacitances coupled between the DC circuit nodes32and34for voltage source converter implementations, whereFIG. 5illustrates one embodiment in which two capacitances C1and C2are connected in series with one another between the nodes32and34. The DC link capacitance may be constructed using any suitable number of capacitor devices connected in any suitable series, parallel or series/parallel configurations to provide a capacitance connected between the DC nodes32and34. Current source converter embodiments are possible in which the DC link circuit40includes one or more inductances (not shown) and the filter20can be an LC circuit as shown inFIGS. 3A and 4Aabove.

FIG. 6illustrates an inverter circuit50including inverter switching devices Q7-Q12and corresponding parallel-connected rectifier diodes D7-D12, where the controller60provides inverter switching control signals to the devices Q7-Q12in order to selectively couple a corresponding DC terminal32,34with a corresponding one of the AC outputs52so as to convert DC link power to provide AC output power to drive the load6in a controlled manner. The controller60can provide the inverter switching control signals according to any suitable pulse width modulation or other switching technique in order to provide AC output power to drive the load6, which can be accomplished according to any suitable control technique, for instance, to regulate output frequency, output power, motor speed control, motor torque control, etc. or combinations thereof.

Referring now toFIGS. 3, 4 and 7, certain embodiments of the controller60include at least one processor (e.g., a microprocessor, microcontroller, field programmable gate array, programmable logic, etc.) programmed or otherwise configured to identify suspected degradation of one or more of the filter capacitors CF of the filter circuit20based at least in part on the filter capacitor currents Ic flowing in the filter capacitors CF (Ica, Icb and Icc in the three-phase example ofFIGS. 3 and 4). In certain embodiments, the controller60implements the filter capacitor degradation detection functionality using one or more processors of a general power conversion system controller. In other embodiments, one or more of these filter capacitor degradation identification functions is performed by a FPGA or other processor local to the LCL filter circuit20. In other embodiments, hardware circuitry can be used alone or in combination with one or more processor components to implement the filter capacitor degradation concepts disclosed herein.

As noted above, the filter capacitors CF can be connected in a delta configuration (e.g.,FIG. 3, alone or with optional discharge resistors as shown) or may be connected in a Y-configuration (e.g.,FIG. 4). The controller60is provided with signals or values indicating the levels of the filter capacitor currents Ica, Icb and Icc by any suitable means, such as by current sensors in the lines connecting the filter capacitors CF to the phase lines A, B and C as shown inFIGS. 3 and 4. In this regard, the capacitor currents in the delta-connected filter capacitor configuration ofFIG. 3can be sensed or measured using sensors configured in the lines connecting the delta configuration to the phase lines A, B and C as shown, or sensors can be connected in series with each of the individual delta-connected capacitors CF in other embodiments. It is noted that these filter capacitor currents Ica, Icb and Icc will be typically less than the phase currents iA, iBand iCflowing between the filter circuit inputs and outputs22. In the example ofFIG. 4, current sensors are provided in series with each of the filter capacitor CF in order to measure the corresponding filter capacitor current by virtue of the Y-connection. Moreover, in certain embodiments, the controller60may also be provided with signals or values indicating the AC voltages in the filter circuit, such as line-line voltages (e.g., VAB, VBCand VCA) and/or line-neutral voltages (VA, VBand VC) by suitable sensors or other means. In certain embodiments, the converter2includes voltage sensors to measure the converter voltages across the filter capacitors CF as seen inFIGS. 3 and 4, and other embodiments are possible in which the voltages are measured at the input side of the 3% inductors L1as shown in dashed lines inFIGS. 3 and 4.

As best seen inFIG. 3, in certain embodiments, the controller60identifies suspected filter capacitor degradation using an instantaneous sum of the squares of the filter capacitor current values Ica, Icb and Icc and one or more threshold values65. As mentioned, the controller60may be implemented using at least one processor, where one, some or all of the illustrated components61-65,67and68a-68ccan be implemented in hardware and/or as processor-executed components. In the illustrated implementation, the sensed filter capacitor current values Ica, Icb and Icc are low pass filtered using a filter61. In certain embodiments, the low pass filter61has a cutoff frequency FCUTOFFset to approximately 200 Hz, which is between third and fourth harmonics of the fundamental frequency of the AC input source4(e.g., for a source frequency of 50 Hz or 60 Hz). In other embodiments using input power of a different fundamental frequency, the low pass filter61may be preferably operated with a cutoff frequency set between the second and third harmonics of the source fundamental frequency. The filtered signals in certain embodiments are provided to a scaling component62with which the filtered signals or values are scaled according to any necessary scaling based on the calibration of the current sensors, the gain of the low pass filter circuit61, the scaling associated with computation and adjustment of the threshold value or values65, etc. In other embodiments, the scaling component62may be omitted.

The low pass filtered signals or values are then used to compute a sum of the squares of the low pass filtered filter capacitor currents Ica, Icb and Icc via an instantaneous measurement computation component63inFIG. 3. For example, the computation component63can compute an instantaneous measurement value I2TOTAL=Ica2+Icb2+Icc2and a comparison component64can selectively provide an alarm and/or initiate one or more remedial actions66if the instantaneous measurement value I2TOTAL(e.g., the sum of the squares of the filter capacitor currents) exceeds a threshold65or falls outside a range defined by upper and lower thresholds65.

In hardware implementations, the threshold(s)65can be provided as one or more signals, and/or in programmable processor implementations, the threshold(s)65can be one or more values. In certain embodiments, the threshold65can be a predetermined value, and the controller60in certain embodiments selectively adjusts the threshold65based at least partially on voltage balance conditions in the power converter2. In such implementations, the controller60includes or otherwise implements a voltage unbalance component67that measures or otherwise receives signals or values indicating the line-line and/or line-neutral voltages associated with the phases A, B and C and determines a voltage balance condition (e.g., quantified by any suitable numeric techniques to indicate a degree of unbalance in the AC voltages associated with the phases A, B and C). In other embodiments as described below in connection withFIGS. 10 and 11, upper and lower threshold values65are provided, and one or both of these can be selectively adjusted by the controller60based on voltage unbalance conditions and/or on input voltage levels. For instance, upper and lower thresholds65can be used for detecting unbalanced capacitor and open capacitor conditions. Moreover, both thresholds65can be adjusted upward by the controller60if the AC input voltage received from the power source4is high and these can be adjusted downward if the AC input voltage is low relative to a nominal voltage value. The controller60may also be provisioned with a frame size68arating associated with the power conversion system2, a voltage class indicator or value68bassociated with the power converter2and/or capacitor tolerance data, value(s) or information68cindicating one or more tolerance values (e.g., maximum rated current values, etc.) associated with the filter capacitors CF. Based on the most recent voltage unbalance condition determination67, the controller60in certain embodiments selectively adjusts the threshold65based on the degree of unbalance in the converter AC voltages. In certain embodiments, the controller60selectively increases the threshold65if the AC voltages are unbalanced to facilitate detection of filter capacitor degradation as distinct from voltage unbalance conditions.

FIG. 7illustrates a graph70showing three-phase AC voltages for the phases A, B and C along with computed sum of squares filter capacitor current values I2TOTALand corresponding threshold THTfor balanced and unbalanced line voltage conditions, respectively, for good filter capacitors CF and degraded filter capacitors CF. The inventors have appreciated that the instantaneous measurement value I2TOTALwill generally have a fairly constant nonzero value during normal operation with balanced line voltages and good filter capacitors CF (section72in the graph70ofFIG. 7). The threshold THTin certain embodiments is determined according to the converter frame size68a, the voltage class68band/or capacitor tolerance information68c, and may be predetermined and stored in the controller60or elsewhere in electronic memory of the power conversion system2.

In certain embodiments, moreover, the individual filter capacitors CF are constructed using an interconnection of multiple component capacitors in series and/or parallel combinations. In such embodiments, the threshold THTis set at least partially according to the value(s) of the component capacitors forming the filter capacitance CF as well as according to the interconnection configuration of the component capacitors. For instance, if each filter capacitor CF is formed by a series connection of three component capacitors of equal capacitance value, the capacitance unbalance caused by failure of one of the component capacitors is about 25%, and the threshold THTmay be set according to the corresponding ripple resulting from such a capacitance imbalance. In contrast, embodiments in which the filter capacitor CF is formed by a series connection of two component capacitors, the resulting capacitance change is 50%, and the corresponding resulting ripple current effect is larger, whereby the threshold THTmay be set higher for such alternate implementations. Any series and/or parallel interconnection configuration of the component capacitors forming the individual filter capacitors CF can be accommodated by corresponding threshold values THT.

During operation, voltage unbalance conditions are verified periodically by certain embodiments of the controller60via the component67, and the threshold65may be selectively adjusted based on the amount of unbalance in the AC voltages to provide an adjusted threshold for unbalanced line voltage conditions. In certain embodiments, the threshold will then be reduced upon return to balanced AC voltages. In addition, the inventors have appreciated that the computed filter capacitor current sum of the squares value I2TOTALwill generally have an AC component generally at a frequency between the second and third harmonics of the power source4during unbalanced line voltage conditions as shown at74inFIG. 7. Accordingly, in certain embodiments, the controller low pass filters the sensed filter capacitor current signals or values (e.g., low pass filter61inFIG. 3) using a cutoff frequency FCUTOFFof about 200 Hz in one example for use with 50 Hz or 60 Hz power sources4before computing the value I2TOTAL. In certain embodiments, the controller60advantageously provides the threshold65at a level slightly above the normal operating level of the sum of the squares total I2TOTALof the sensed filter capacitor currents, and selectively increases the threshold65to the level above the maximum AC level of the instantaneous measurement value I2TOTALat74to differentiate between voltage unbalance effects and filter capacitor degradation.

As seen at76and78inFIG. 7, the amplitude of the AC component of the sum of the squares total I2TOTALincreases significantly upon the onset of filter capacitor degradation. Thus, using the threshold THT(at76inFIG. 7), the controller60uses the comparison component64to determine that the instantaneous measurement value I2TOTALexceeds the threshold65and may accordingly initiate an alarm and/or other remedial action via component66. The controller60in certain embodiments, moreover, may selectively modify the threshold based on voltage unbalance conditions, and can thus detect filter capacitor degradation (e.g., at78) while avoiding false alarms based on voltage unbalance conditions (e.g., at74inFIG. 7).

The inventors have further appreciated that the instantaneous measurement value I2TOTALadvantageously provides a suitable means for detecting filter capacitor degradation, wherein the nominal average value (e.g., 1 pu) for healthy filter capacitors is accompanied by only minimal peak ripple (e.g., about 0.02-0.2 pu in certain implementations) for voltage unbalance conditions. In contrast, the instantaneous sum of squares value I2TOTALincreases relatively significantly (e.g. to about 1.16-1.3 with a corresponding increase ripple of about 0.3-0.5 in certain implementations) when one or more filter capacitors degrade (e.g., for balanced line voltage conditions at76inFIG. 7) and therefore provides a significant change relative to the ripple associated with voltage unbalance conditions. Thus, this technique provides a fairly robust mechanism for distinguishing between voltage unbalance conditions and filter capacitor degradation affects in the power conversion system2.

FIG. 8illustrates a method200of identifying suspected filter capacitor degradation in a power conversion system (e.g., system2above) by threshold comparison of a sum of squares computation of filter capacitor current values. Although the exemplary method200ofFIG. 8and the method300ofFIG. 9below are hereinafter illustrated and described in the form of a series of acts or events, the various methods of the present disclosure are not limited by the illustrated ordering of such acts or events except as specifically set forth herein. In this regard, except as specifically provided in the claims, some acts or events may occur in different order and/or concurrently with other acts or events apart from those acts or events and ordering illustrated and described herein, and not all illustrated steps may be required to implement a process or method in accordance with the present disclosure. The disclosed methods, moreover, may be implemented in hardware, processor-executed software, programmable logic, etc., or combinations thereof, in order to provide the described functionality, wherein these methods can be practiced in the above described power conversion system2, such as in the controller60, although the presently disclosed methods are not limited to the specific applications and implementations illustrated and described herein. Moreover, the methods200and300may be embodied as a computer executable instructions stored on a non-transitory computer readable medium, such as a memory operatively associated with the controller60and/or with the power conversion system2.

A voltage balance condition is assessed at202inFIG. 8, such as by the controller60measuring one or more voltages (e.g., line-line and/or line-neutral voltages) associated with the converter2. For instance, as seen inFIGS. 3 and 4above, the controller60may receive signals and/or values indicating the voltages at the center nodes of the LCL filter circuit20. At204, the controller60sets or otherwise adjusts the threshold TH (e.g., THB, THUB) based at least partially on voltage unbalance, frame size, voltage class, and/or any capacitor tolerance information (e.g.,67and/or68a-68cinFIG. 3above). The filter capacitor currents (e.g., Ica, Icb and Icc) are measured or otherwise obtained at206inFIG. 8, and are filtered at208using a low pass filter (e.g., filter component61inFIG. 3above, with a cutoff frequency FCUTOFFset between the second and third harmonic). At210, the filtered signals or values may be scaled in certain embodiments, and an instantaneous measurement value is computed at212(e.g., I2TOTAL) as a sum of the squares of the measured (and filtered and optionally scaled) filter capacitor currents Ica, Icb and Icc.

A determination is made at214as to whether the sum of squares value I2TOTALexceeds a threshold (e.g., threshold THB, THUB, as set or adjusted at204). If not (NO at214), the process200repeats, returning to202-212as described above. If the threshold value is exceeded (YES at214), the controller60identifies or otherwise determines at216that one or more of the filter capacitors64is degraded/degrading, and may optionally report the suspected degradation and/or take one or more remedial actions at218. For instance, the controller may open the main circuit breaker12and the precharge contactor16in the precharge circuitry10ofFIG. 2above and/or may initiate other controlled shutdown and reporting operations, such as setting an alarm, indicating a capacitor degradation condition on a user interface of the power conversion system2, sending an error message to a supervisory controller associated with the power converter2, etc. In addition, or separately, the controller60may log a fault and reset the power converter2, such as by storing a value to a fault log in a nonvolatile memory of the power converter2(not shown), or the controller60may indicate a non-resettable fault to a human machine interface (HMI, not shown) for different levels of suspected degradation (e.g., as indicated by the relative comparison with the threshold65), and/or may only allow a fault to be reset upon password-protected input by service personnel after filter capacitor inspection.

Referring now toFIGS. 3, 7 and 9, the controller60in certain embodiments may measure a plurality of filter capacitor currents and AC voltages associated with a power converter2, and assess filter capacitor degradation based on a computed power value.FIG. 9illustrates an exemplary process300for identifying suspected power converter filter capacitor degradation using computed real and/or reactive power computations, which may be implemented using the controller60in certain embodiments. At302inFIG. 9, the controller60measures filter capacitor currents (e.g., Ica, Icb and Icc) and measures voltages at304associated with the power converter2. At306, the controller60computes a real or active power value (PACTIVE) and/or a reactive power value (PREACTIVE) based at least partially on the filter capacitor currents and the voltages obtained at302and304. For instance, the controller60in certain embodiments may compute an active power value as PACTIVE=Va×iA+Vb×iB+Vc×iCand/or compute reactive power value PREACTIVE(⅓1/2)(Vba×Ica+Vca×Icb+Vac×Icc). At308, the controller60compares the computed power value(s) (PACTIVEand/or PREACTIVE) with a threshold (e.g., active and reactive power thresholds THPAand THPR, respectively, inFIG. 7). If the threshold is not exceeded (NO at308), the process300returns to302-306as described above. However, if the threshold is exceeded (YES at308), the controller306identifies the suspected filter capacitor degradation at310and can report the degradation and/or take one or more remedial actions at312, for instance as described above in association with218ofFIG. 8.

As seen inFIG. 7, the inventors have appreciated that the active power (PACTIVEinFIG. 7) may have a small AC component for unbalanced line voltages, such as the situation (e.g., at74) where the filter capacitors CF are not degraded. Thus, the active threshold THPAis set in certain embodiments to be above this expected AC value (where the nominal active power is typically at 0). Moreover, the inventors have appreciated that filter capacitor degradation will result in a larger AC ripple component of both the active and reactive power values PACTIVEand PREACTIVE, and the controller60accordingly uses one or more of the threshold values THPAand THPRso as to be able to detect degraded filter capacitor conditions (e.g., at76and78inFIG. 7) as distinct from unbalanced line voltage conditions (e.g., at74inFIG. 7).

Referring now toFIGS. 10 and 11, in certain embodiments, multiple threshold values65can be employed by the controller60for detecting open and/or unbalanced filter capacitor conditions. These multiple threshold comparison techniques can be employed in association with instantaneous active and/or reactive power values (e.g., PACTIVEand PREACTIVE) and/or with at least one sum of squares value (e.g., I2TOTAL) or combinations thereof.

FIG. 10illustrates an implementation in the controller60using the instantaneous sum of squares value I2TOTALfrom the component63based on the filter capacitor currents (Ica, Icb, Icc) via the low pass filter61and the optional scaling component62as discussed above. The sum of squares value I2TOTALis provided to a comparison component64and is compared with an upper threshold THU65A and with a lower threshold THL65B. The controller60initiates an alarm and/or remedial action66if I2TOTALis greater than the upper threshold THU65A or lower than the lower threshold THL65B. The inventors have appreciated that certain filter capacitor configurations, such as three component capacitors connected in parallel to form one of the filter capacitances CF may be subject to open capacitor degradation effects, in which the instantaneous sum of squares value I2TOTALmay decrease. Accordingly, comparison of this value I2TOTALwith the lower threshold THL65B facilitates detection of such open capacitor type degradation. In this regard, the lower threshold THL65B in certain embodiments is determined according to the frame size68a, the voltage class rating68band/or any capacitor tolerance information68c.

The inventors have further appreciated that certain filter capacitor architectures be subject to unbalanced capacitor degradation, for instance, where three capacitor components are connected in series to form one or more of the filter capacitances CF. In this situation, unbalanced capacitor degradation may increase the instantaneous sum of squares value I2TOTAL. Accordingly, use of the upper threshold THU65A and facilitates detection of such degradation conditions. Also, both upper and lower thresholds65may be used in certain embodiments, for example, where the filter capacitances CF include series and/or parallel-connected capacitor components or in other situations in which different forms of filter capacitor degradation may lead to increases and/or decreases in the instantaneous sum of squares value I2TOTAL. As noted above, moreover, such dual-threshold techniques may also be employed in association with real and/or reactive power signals or values (e.g., PACTIVEand PREACTIVE) computed or otherwise derived based at least in part on one or more filter capacitor currents in certain embodiments of the controller60.

FIG. 11provides a graph80showing examples of upper and lower thresholds THU65A and THL65B used in the controller60ofFIG. 10along with the instantaneous sum of squares value I2TOTALfor balanced line voltages at82and unbalanced line voltages at84, as well as for unbalanced filter capacitors connected in series at86(e.g., for balanced line voltage conditions) and open filter capacitor degradation for parallel connection at88(also for balanced line voltage conditions). As seen in these examples, the upper threshold THU65A is set to a level (e.g., 1.16 pu in one embodiment) sufficient to avoid false triggering based on purely unbalanced line voltage conditions at84, while triggering initiation of an alarm and/or remedial action based on unbalanced filter capacitor degradation conditions shown that86. Also, the lower threshold THL65B is less than 1 pu (e.g., 0.82 pu in one example) which will not cause an alarm for unbalanced line voltage conditions at84, but will initiate alarm or remedial action for an opened filter capacitor degradation situation as shown at88inFIG. 11.

In certain embodiments, moreover, one or both of the upper threshold THU65A and lower threshold THL65B can be adjusted upward by the controller60if the AC input voltage received from the power source4is high and these can be adjusted downward if the AC input voltage is low relative to a nominal voltage value. As seen inFIG. 10, a voltage detection component67can be provided in the controller60to monitor one or more of the power converter voltages (e.g., VA, VB and/or VC). The detection component67in certain embodiments selectively adjusts one or both of the thresholds THU65A and/or THLbased at least in part on one or more of the AC voltages associated with the power conversion system2. For instance, the voltage detection component67in certain embodiments increases one or both of the thresholds THU65A and/or THL65B if at least one AC voltage is greater than a nominal value (e.g., greater than 240 V AC in certain embodiments) and decreases one or both of the thresholds65A and/or65B if a system voltage is less than the nominal value. In this regard, the inventors have appreciated that high or low voltages provided by the power source4may cause respective increases or decreases in the per unit instantaneous sum of squares value I2TOTAL, and the same is true for power signals or values PACTIVEand/or PREACTIVE. Accordingly, the controller60in certain embodiments may selectively adjust one or both of the thresholds65A and/or65B accordingly.

In the example ofFIG. 10, moreover, I2TOTALis also provided to a peak ripple component63A, and the peak ripple component of the instantaneous sum of squares value is compared with a ripple peak threshold THRP65C using a comparison component64A. If the peak ripple component is above the threshold THRP65C, the comparison component64A in certain embodiments initiates an alarm and/or remedial action66. For instance, as seen inFIG. 11, the threshold THRP65C is set to a positive value (e.g., about 0.3 pu in one example) to avoid false tripping for unbalanced line voltage conditions at84, while initiating an alarm and/or remedial action66for unbalanced or opened filter capacitor degradation conditions at86and/or88. As seen inFIG. 10, moreover, the ripple peak threshold THRP65C in certain embodiments is set according to one or more of the power converter frame size68a, voltage class68band/or the capacitor tolerance information68c. Furthermore, the voltage detection component67in certain embodiments may selectively increase or decrease the ripple peak threshold THRP65C based at least partially on one or more voltages associated with the power conversion system2, for example by increasing the threshold THRP65C if the power converter voltage is above a nominal value, and decreasing the threshold THRP65C if the converter voltage is below the nominal value. The controller60in certain embodiments can provide for selective alarm and/or remedial action initiation66based on one, some, or all of the above threshold comparisons, such as triggering based on the instantaneous sum of squares value I2TOTALfalling outside of a range defined by the lower and upper thresholds65A and65B or triggering on the ripple peak value63A exceeding the ripple peak threshold THRP65C in one possible embodiment. Any other Boolean logic may be used to selectively initiate the alarm and/or remedial action based on one or more of the above described threshold comparisons.