Abstract:
A sensor-less technique is described for detecting the status of an AC contactor of a power inverter. In one embodiment, a method is provided of sensing an unexpected condition of an AC contactor used to couple to a power distribution system a power inverter having at least one power conductor. The method includes determining a voltage value for the conductor; determining a current value for the conductor; determining a phase difference using the voltage value and the current value; and monitoring successive values of the phase difference to produce a monitoring result. Depending on the monitoring result, a determination is made whether or not to issue an alert signal. Control routines embodying such technique may be stored on computer-readable media. A power inverter using such technique is described.

Description:
FIELD OF THE INVENTION 
     The present invention relates to power inverters, more particularly to the detection of abnormal conditions in power inverters. 
     BACKGROUND OF THE INVENTION 
     In distributed power generation systems, such as solar installations, DC power is generated and converted into AC power for delivery to the power grid. Conversion from DC to AC power is performed by a power inverter. A typical power inverter is computer controlled, using a microprocessor or other suitable system controller. The system controller transitions the inverter between several states such as shut-down, sleep, online and fault-handling states based on the AC and DC voltage and current conditions. AC and DC contactors are used to disconnect the inverter from the AC and DC sources. The AC and DC contactors are controlled by the system controller through Solid State Relays (SSRs). 
     A fault situation can occur when the AC contactor is unknowingly open but the system controller still commands the power matrix of the power inverter to switch. This situation will cause high and distorted magnetizing current to flow into an isolation transformer of the power inverter, eventually damaging the isolation transformer and the power inverter. 
     SUMMARY 
     A sensor-less technique is described for detecting the status of an AC contactor of a power inverter. In one embodiment, a method is provided of sensing an unexpected condition of an AC contactor used to couple to a power distribution system a power inverter having at least one power conductor. The method includes determining a voltage value for the conductor; determining a current value for the conductor; determining a phase difference using the voltage value and the current value; and monitoring successive values of the phase difference to produce a monitoring result. Depending on the monitoring result, a determination is made whether or not to issue an alert signal. Control routines embodying such technique may be stored on computer-readable media. A power inverter using such technique is described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a three-phase inverter. 
         FIG. 2  shows a block diagram of a model of the three-phase inverter during an operational state. 
         FIG. 3  shows a block diagram of a model of the three-phase inverter during another operational state. 
         FIG. 4  shows an illustration of a known α-β frame transformation. 
         FIG. 5  shows a process for computation of the phase displacement between the voltage and current of the three-phase inverter of  FIG. 1 . 
         FIG. 6  shows a plot of voltage, current and phase displacement during a normal condition. 
         FIG. 7  shows a plot of voltage, current and phase displacement during an abnormal condition. 
         FIG. 8  shows voltage and current waveform zero crossings in a detection method for single-phase inverters. 
         FIG. 9  shows a block diagram of a process for sensing an unexpected condition of an AC contactor used to couple to a power distribution system a power inverter having a power conductor; 
     
    
    
     DETAILED DESCRIPTION 
     A schematic diagram of a three-phase grid-tie voltage source inverter  100  is shown in  FIG. 1 . The inverter  100  transforms DC power from photovoltaic (PV) panels or other DC sources to three-phase AC power through a power electronics switching module  101 , also called a power matrix. DC power is coupled to the power matrix  101  through a DC contactor K 2  having an associated coil and relay combination  103  having an operating voltage applied thereto. When the DC contactor K 2  is closed and the power matrix  101  is operating, a DC voltage Vdc is applied to terminals of the power matrix  101 , and a DC current idc flows into the power matrix  101 . 
     By a switching action of the power matrix  101 , an AC current, such as a three-phase AC current comprising currents ia, ib and ic is produced. The AC current is then filtered through filter inductors L 1 , L 2 , L 3  and capacitors  105  and isolated by an isolation transformer T 1 . The three-phase AC current is finally fed to a power distribution grid through an AC contactor K 1  and a circuit breaker CB 1 . The AC contactor K 1  has an associated coil and relay combination  107 . A system controller  110  inside the inverter performs two major functions: one is the current control and the other is inverter states control and fault protection. In the illustrated embodiment, the system controller  110  receives signals including signals representing the DC voltage Vdc, a signal representing the DC current idc, signals representing the primary-side currents ia and ic, and signals Va, Vb and Vc representing the three-phase AC voltage. In the same embodiment, the system controller  110  produces signals including pulse-width-modulation (PWM) switching signals for the power matrix  101 . 
     The current control function generates current commands, regulates the current through current feedback control, and synchronizes the inverter&#39;s current with the grid. (In the illustrated embodiment, only two phase currents ia and ic are needed for the feedback control in a three-phase system without neutral, since the sum of ia, ib and ic is always zero.) A PWM technique is then used to generate switching pulses to control the power matrix  101  to transform the DC power to AC power. 
     The inverter states control and fault protection function controls the inverter to transition the inverter into various states such as shut-down, sleep, online and fault-handling states based on the AC and DC voltage and current conditions. Controlled by the system controller  110  through the solid state relay (SSR) circuits  103  and  107 , the AC and DC contactors K 1  and K 2  are used to disconnect the inverter from the AC grid and the DC source. A fault situation can occur when the AC contactor K 1  is open but the system controller  110  unknowingly still commands the power matrix  101  to switch. This situation will cause high and distorted magnetizing current to flow into the isolation transformer T 1 , eventually damaging the isolation transformer T 1  and the inverter  100 . 
     A sensor-less technique of detecting the status of the AC contactor K 1  is now described, as applied for purposes of example to this type of three-phase inverter. The technique applies to inverters of various descriptions, including three-phase grid-tie voltage-source inverters having an isolation transformer inside the inverter, single-phase grid-tie voltage-source inverters, etc. 
     A simplified model of a voltage source inverter with a simple P controller (proportional controller) is shown in  FIG. 2 . The filter capacitors  105  are ignored because of their smaller impact on the system. 
     In  FIG. 2 , the reference voltage Vref is taken from the grid phase voltage and the reference current Iref is synchronized with the Vref or the grid phase voltage. The gain of the P controller is K p . K 0  represents the scale factor between the reference voltage Vref and the reference current Iref. The filter inductance is L. The closed-loop system transfer function can be written as: 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       Vref 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           
                             K 
                             0 
                           
                           ⁢ 
                           
                             K 
                             P 
                           
                         
                         - 
                         1 
                       
                       
                         SL 
                         + 
                         
                           K 
                           p 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The phase displacement between the fundamental elements of grid phase voltage Vref(s) and the inverter&#39;s output current I(s) is dependent on K p  and L. The absolute phase displacement at f (Hz) can be expressed in (2). The absolute phase displacement is tiny because of the very small value of L. 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                      
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             fL 
                           
                           
                             K 
                             p 
                           
                         
                         ) 
                       
                     
                      
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     However, when the AC contactor K 1  is unknowingly opened and the inverter  100  continuously powers the isolation transformer T 1 , the system model changes to that shown in  FIG. 3 . The closed-loop transfer function of this system can be written as: 
                       I   ⁡     (   s   )         Vref   ⁡     (   s   )         =         K   0     ⁢     K   P           S   ⁡     (     L   +     L   m       )       +     K   p                 (   3   )               
where, L m  is the magnetizing inductance of the isolation transformer.
 
     The phase displacement between the fundamental elements of grid phase voltage and the inverter&#39;s output current is dependent of K p  and L+L m . The absolute value of the phase displacement now can be expressed as: 
                   θ   =            tan     -   1       ⁡     (       2   ⁢           ⁢   π   ⁢           ⁢     f   ⁡     (     L   +     L   m       )           K   p       )                    (   4   )               
A significant absolute value increase in the phase displacement can be expected with the open condition of the AC contactor since the magnetizing inductance of the transformer, L m , is far greater than the filter inductance, L.
 
     In one implementation, detection of the open condition is achieved using phase detection in a well-known α-β frame; in particular, the voltage-current phase angle, which as shown above exhibits a strong dependence on the open or closed state of the AC contractor, is detected. A well-known stationary frame transformation (“α-β frame transformation”) is used to transform the variables in the three-phase frame to the α-β frame, as expressed mathematically for example in (5): 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             V 
                             α 
                           
                         
                       
                       
                         
                           
                             V 
                             β 
                           
                         
                       
                       
                         
                           
                             V 
                             0 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         
                           2 
                           3 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 1 
                               
                               
                                 
                                   
                                     - 
                                     1 
                                   
                                   / 
                                   2 
                                 
                               
                               
                                 
                                   
                                     - 
                                     1 
                                   
                                   / 
                                   2 
                                 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                   
                                     3 
                                   
                                   / 
                                   2 
                                 
                               
                               
                                 
                                   
                                     - 
                                     
                                       3 
                                     
                                   
                                   / 
                                   2 
                                 
                               
                             
                             
                               
                                 
                                   1 
                                   / 
                                   2 
                                 
                               
                               
                                 
                                   1 
                                   / 
                                   2 
                                 
                               
                               
                                 
                                   1 
                                   / 
                                   2 
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               
                                 V 
                                 a 
                               
                             
                           
                           
                             
                               
                                 V 
                                 b 
                               
                             
                           
                           
                             
                               
                                 V 
                                 c 
                               
                             
                           
                         
                         ] 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In a symmetrical three-phase system, V 0  is zero. Therefore the three-phase variables such as voltage (Va, Vb and Vc) and current (ia, ib and ic) can be expressed by vectors V and I respectively in the α-β frame as shown in  FIG. 4 . The angle between vectors V and I is also the angle θ between the phase voltage and phase current in a symmetrical three-phase system. 
     Since the angle between vector V and I is the angle of the product of the voltage vector V and the conjugate of the current vector I* in the complex plane, there results: 
     
       
         
           
             
               
                 
                   
                     VI 
                     * 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           V 
                           α 
                         
                         + 
                         
                           j 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             V 
                             β 
                           
                         
                       
                       ) 
                     
                     · 
                     
                       
                         ( 
                         
                           
                             I 
                             α 
                           
                           - 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               I 
                               β 
                             
                           
                         
                         ) 
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The absolute value of the angle of the VI* can be expressed as in (7): 
     
       
         
           
             
               
                 
                   θ 
                   = 
                   
                     
                        
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ⁢ 
                         
                           { 
                           
                             
                               ( 
                               
                                 
                                   
                                     V 
                                     β 
                                   
                                   ⁢ 
                                   
                                     I 
                                     α 
                                   
                                 
                                 - 
                                 
                                   
                                     V 
                                     α 
                                   
                                   ⁢ 
                                   
                                     I 
                                     β 
                                   
                                 
                               
                               ) 
                             
                             / 
                             
                               ( 
                               
                                 
                                   
                                     V 
                                     α 
                                   
                                   ⁢ 
                                   
                                     I 
                                     α 
                                   
                                 
                                 + 
                                 
                                   
                                     V 
                                     β 
                                   
                                   ⁢ 
                                   
                                     I 
                                     β 
                                   
                                 
                               
                               ) 
                             
                           
                           } 
                         
                       
                        
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     In an exemplary embodiment, the system controller  110  performs an algorithm for real-time angle detection and AC contractor status detection based thereon. A diagram of this exemplary algorithm is shown in  FIG. 5 . To realize a real-time angle detection, voltages Va, Vb, Vc and currents ia, ib, ic are first transformed to the α-β frame ( 501 ,  503 ). Digital filtering is then applied to both the voltage ( 505 ) and current ( 507 ) in the α-β frame. In the case of voltage, voltage values Vα 0  and Vβ 0  are input to the digital filter  505 , which outputs filtered voltage values Vα and Vβ. In the case of current, current values Iα 0  and Iβ 0  are input to the digital filter  507 , which outputs filtered current values Iα and Iβ. The phase displacement θ between the voltage vector and current vector is then obtained through the conjugate complex computation ( 509 ) as described in (7). Based on the resulting phase displacement values, contactor status detection ( 511 ) is then performed. 
     In an exemplary embodiment, a dual second order Chebyshev filter is used as the filter  505  and as the filter  507 . Two second order Chebyshev digital filters are cascaded to obtain the desired fundamental components of voltage and current. The coefficients of the Chebyshev filter are shown in Appendix 1. Any of various other suitable filter arrangements may be used for this purpose. 
     In the embodiment of  FIG. 5 , to make the real time detection stable, the average of absolute phase displacement in one second is taken as the index for the detection of an AC contactor open condition in block  511 . If the number of abnormally high index values in twelve seconds is greater than seven, then a fault signal will be sent out to shut down the inverter. 
       FIG. 6  and  FIG. 7  illustrate application of the foregoing detection technique.  FIG. 6  shows three-phase voltage, three-phase current, and phase displacement of voltage and current in a three-phase grid-tie voltage-source inverter with the inverter was running normally. The average of the phase displacement of fundamental voltage and current in this normal situation is stable at about 0.07 radians. (Note: The average of the absolute phase displacement has been scaled (×100) in  FIGS. 6 and 7 .) 
     In the case of a single-phase inverter, to measure the phase angle between the AC voltage (v) and the AC current (i) after filtering, the time span (δt) between voltage and current zero-crossing points (usually crossing from negative to positive) can be detected as shown in  FIG. 8 . Theoretically, the phase angle can be calculated in radians as follows: 
                   θ   =     2   ⁢           ⁢   π   ⁢           ⁢   f   ⁢           ⁢   δ   ⁢           ⁢   t             (   8   )               
where, f is the frequency of voltage and current.
 
     With the phase angle being averaged over one second (60 AC cycles), sufficient stable and accurate phase angles for discerning the status of the AC contactor can be obtained, since the difference in phase angles is very large when the contactor is close or unknowingly open. 
       FIG. 7  shows the three-phase voltage, current and the phase displacement after the AC contactor has opened. The phase displacement of the fundamental voltage and current has increased to 1.05 radians and is stable at this value. In a field test, a threshold of 0.55 radians was set for the inverter to detect the fault. The inverter tripped the fault in about 20 sec as designed. 
       FIG. 9  shows a process  900  for sensing an unexpected condition of an AC contactor used to couple to a power distribution system a power inverter having a power conductor. The process  900  includes determining a voltage value for the conductor ( 902 ). The process  900  includes determining a current value for the conductor ( 904 ). The process  900  includes determining a phase difference using the voltage value and the current value ( 906 ). The process  900  includes monitoring successive values of the phase difference to produce a monitoring result ( 908 ). The process includes, depending on the monitoring result, determining whether or not to issue an alert signal or perform a remedial action ( 910 ). 
     While embodiments and applications have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts disclosed herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims. 
     APPENDIX 1 
     The second order Chebyshev digital filter 
     
       
         
           
             
               
                 
                   
                     H 
                     ⁡ 
                     
                       ( 
                       z 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         a 
                         0 
                       
                       + 
                       
                         
                           a 
                           1 
                         
                         ⁢ 
                         
                           z 
                           
                             - 
                             1 
                           
                         
                       
                       + 
                       
                         
                           a 
                           2 
                         
                         ⁢ 
                         
                           z 
                           
                             - 
                             2 
                           
                         
                       
                     
                     
                       
                         
                           b 
                           1 
                         
                         ⁢ 
                         
                           z 
                           
                             - 
                             1 
                           
                         
                       
                       + 
                       
                         
                           b 
                           2 
                         
                         ⁢ 
                         
                           z 
                           
                             - 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     a 0 =0.0004848 
     a 1 =0.0009696 
     a 2 =0.0004848 
     b 1 =−1.9576 
     b 2 =0.9603