Abstract:
A signal is injected into a neutral point regulator of an inverter drive. The response to that injected signal is monitored as an indication of the capacitance of the inverter drive.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a system and method for monitoring DC link capacitance variance in three level inverters and more particularly to a system and method for injecting a signal into a neutral point regulator of a motor drive converter and measuring the response to the injected signal to monitor DC link capacitance. 
     Voltage source inverter drives for large AC motors incorporate large metalized film DC link capacitors. As these capacitors age, their capacitance decreases slowly. The gradual decreasing capacitance of these capacitors places an increasing burden on the neutral point clamped motor drive system and can lead to loss of function or to significant damage to the motor drive system. Additionally, when a catastrophic failure of a capacitor is experienced expensive and unexpected maintenance must be performed to replace the failed capacitor. Currently capacitors are replaced as part of a periodic maintenance program to avoid these problems. A more efficient system for avoiding failed capacitors is needed. 
     BRIEF SUMMARY OF THE INVENTION 
     Three level neutral point clamped inverters are able to produce current waveforms with low amounts of harmonic distortion at higher power levels than conventional two level inverters. The switching states available in a three level neutral point clamped inverter allow regulation of the neutral point voltage assuming the presence of DC link capacitors in the power circuit. The capacitance value of the DC link capacitors determines the magnitude of the neutral point error. The neutral point error is one measure of the successful operation of the drive system, and if the error exceeds preset limits, the drive will cease operation. 
     The inventors have discovered that it is possible to monitor the capacitance of DC link capacitors in an inverter by injecting a signal into a neutral point regulator and measuring the response to the injected signal. 
     An embodiment of the present invention injects a signal into a neutral point regulator such as that which is disclosed in co-assigned U.S. Pat. No. 5,910,892 (hereafter the “&#39;892 patent”) which is incorporated herein in its entirety. The &#39;892 patent discloses a high power motor drive converter having a three-level neutral point clamped output power conversion stage that includes control algorithms to balance the neutral voltage of a DC capacitor bank using space vector modulation and predictive charge calculations. The inventors of the present invention realized that the response to a signal injected into the neutral point regulator of the &#39;892 system can provide an indication of the DC link capacitance in the DC capacitor bank of the motor drive converter. The three-level inverter regulates the neutral potential by selecting switch states to regulate the neutral voltage to near zero. The invention modulates the neutral at a specified frequency and measures the neutral voltage and charge to determine the capacitance. 
     The present invention does not compromise the operation of the drive under load and does not require manual operator intervention to accomplish its task. No additional power supplies, PC boards, connectors, cables, etc. are required in the system so that costs, complexity and reliability are minimally impacted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments in this invention will be described in detail, with reference to the following figures, wherein: 
     FIGS. 1A and 1B include a schematic block diagram of one embodiment of the DC link capacitance determination system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B show a schematic block diagram of one embodiment of the capacitance determination system in accordance with the present invention. A signal generator  10  generates an injection signal and inputs the amplitude of the injection signal Ainj into multiplier  12  and also inputs the phase of the injection signal Θinj into processing components  14  and  16 . Processing component  14  determines the cosine of the phase of the injection signal and inputs the cosine of the signal into the multiplier  12 . The output of multiplier  12  is the neutral voltage command Vn*. 
     As shown in FIG. 1A, the neutral voltage command Vn* is input into comparator  18  that also receives the measured neutral voltage Vn to determine the neutral voltage error signal Vnerr as input into multiplier  20 . Multiplier  20  determines the neutral charge error signal Qerr by multiplying the neutral voltage error Vnerr by the nominal capacitance Cnom. The nominal capacitance Cnom is the capacitance of the DC link capacitors in a perfect condition. The neutral charge error signal Qerr is forwarded to comparator  22  that also receives a neutral charge signal Qn that is determined by the modulator algorithm as disclosed in the &#39;892 patent. The comparator  22  outputs the neutral charge command Qn* and forwards the signal to divider  24 . Divider  24  also receives a neutral current contribution Ino for dwell interval t 0  that is determined based upon the modulation algorithm disclosed in the &#39;892 patent. The divider then outputs the required stated dwell time (t 0 _req.) and forwards that signal to the stated dwell time calculator  26 . The stated dwell time calculator  26  outputs dwell times t 0  and t 3  and forwards these signals to limiter  28 , which in turn, limits these signals between a t minimum and t maximum signal. 
     Meanwhile, as shown in FIG. 1B, the output of processors  14  and  16  is provided to a bank of multipliers  30 ,  32 ,  34  and  36 . Upper bank multipliers  30  and  32  also receive the neutral charge Qn, from the modulation algorithm disclosed in the &#39;892 patent, to calculate each phase component of the neutral charge at the injection frequency and input them into low pass filters  38  and  40 . Low pass filters  38  and  40  filter out the frequency components that are unrelated to the injected frequency and forward their output to magnitude determination processor  46 . Magnitude determination processor  46  determines the absolute magnitude of the neutral charge |Qnn| at the injection frequency using the following equation: 
     
       
         | Qnn|={square root over (Qnα 2   +Qnβ   2 +L )}   (1) 
       
     
     and forwards that signal to divider  50 . 
     Meanwhile multipliers  34  and  36  receive the measured neutral voltage Vn and generate the neutral voltage at each injection frequency and forwards these signals to low pass filters  42  and  44 . Low pass filters  42  and  44  remove frequency components that are unrelated to the injected frequency and forward their outputs to magnitude determination processor  48 . The magnitude determination processor  48  determines the absolute magnitude of the injected neutral voltage Vni with using the following equation: 
     
       
         | Vni|={square root over (Vnα 2   +Vnβ   2 +L )}   (2) 
       
     
     and forwards that signal to divider  50 . Divider  50  determines the DC link capacitance by dividing the magnitude of the neutral charge by the magnitude of the neutral              C   =          Qnn             Vni                  (   3   )                                
     voltage. In accordance with the following equation: 
     If the DC link capacitance is outside of a predetermined range the system may provide an alarm to indicate that the capacitor should be replaced. 
     In one embodiment of the present invention, the controller is an entirely software based system as executed in a computer with interface circuits with voltage and current feedback data acquisition systems. However, one of ordinary skill in the art understands that any other analogous embodiment of the present invention would work equally well. 
     One of ordinary skill in the art understands that while this invention was described in relation to the converter system disclosed in the &#39;892 patent that any other type of neutral point control system may work with the present invention that actively controls the neutral point voltage. 
     While this invention has been described in conjunction with the specific embodiment outlined above, it is evident that many alternatives, modifications and variations are apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative and not limiting. Various changes may be made without departing from the spirit and scope of the invention.