Patent Abstract:
A photovoltaic inverter for coupling a direct current photovoltaic source to an alternating current energy grid and performing a low voltage ride through. The inverter includes a power bridge to convert direct current voltage to alternating current voltage. A switching crowbar is coupled to the photovoltaic energy source and the power bridge. The crowbar has a switching device having a closed position causing the switching crowbar to dissipate energy from the photovoltaic energy source and an open position to allow direct output from the photovoltaic source to the power bridge. A voltage sensor detects a low voltage condition on the grid. A controller is coupled to the voltage sensor and controls the switching crowbar when a low voltage condition is detected. The switching device is placed in the closed position when the voltage from the photovoltaic energy source is higher than a predetermined threshold voltage and the switching device being placed in the open position when the voltage of the photovoltaic source reaches the voltage when the low voltage condition is detected.

Full Description:
FIELD OF THE INVENTION 
     The present disclosure relates generally to inverters for photovoltaic (“PV”) power plants and more particularly, to a crowbar control circuit in a photovoltaic inverter to perform low voltage ride through. 
     BACKGROUND 
     The drive for alternative energy has increased development of photo-voltaic (“PV”) power plants that may be connected to the electrical grid. A PV power plant has an inverter which converts direct current voltage from a PV source, such as solar panels, to alternating current voltage suitable for connection to the electrical grid. As more and more large-scale PV power plants are put into operation due to the increasing green energy requirements, utilities are applying more and more regulations on PV power plants, since the quality and stability of a power system may be affected by the installation of PV power plants. Presently, some European countries such as France and Germany are requiring that PV power plants which are connected to a medium voltage grid are capable of performing a low voltage ride through (“LVRT”) operation. The LVRT operation means once the grid voltage dips 10% to 95% of its nominal value, any individual PV inverter in a PV plant can still output the same amount of current as before the grid voltage dip for a specified period of time. 
     Recently, as the capacity of single PV power plants has increased rapidly, the emphasis on high PV voltage installations is getting higher because such installations can lower the installation cost. An open circuit voltage of about 1000V is currently desired from many PV installers. Facing this challenge, PV inverter manufacturers have two options: use high-voltage switching devices such as 1700V insulated gate bipolar transistors (“IGBT”) to build the power conversion bridge for PV inverters; or use regular voltage switching devices such as 1200V IGBTs in order to acquire high power conversion efficiency in inverters. Use of high-voltage switching devices is relatively expensive because of the higher voltage requirements. However, there are several challenges in use of regular voltage switching devices such as 1200V IGBTs for 1000V PV systems, one of which is the implementation of LVRT in PV inverters which may expose such switching devices to higher voltages than their operational design. 
     In general, running 1200V IGBTs above 850V at full load is not recommended for the safety of the IGBTs and the inverter. However 1200V IGBTs may be theoretically used for a 1000V PV installation since the Maximum Power Point Tracking (“MPPT”) voltage of this kind of PV installation will be less than 850V and usually less than 700V. However, as the grid voltage dip occurs, the DC voltage of the PV inverter may jump higher than 850V due to the instant energy build-up at the DC side. The IGBTs in the power bridge may fail in this situation and therefore the PV plant cannot perform LVRT. Thus, it would be desirable to have an inverter with relatively lower voltage components that can perform LVRT. 
     BRIEF SUMMARY 
     Aspects of the present disclosure include a method of regulating the voltage output of an inverter coupled between a photovoltaic source and a power grid to perform a low voltage ride through operation through operation. An occurrence of low voltage is sensed on the power grid. A crowbar circuit is controlled with a switching device in parallel with the photovoltaic source to create a closed circuit to dissipate power from the photovoltaic source at a pre-determined threshold voltage. The crowbar circuit opens the closed circuit when the voltage from the photovoltaic source is at substantially the operating voltage of the photovoltaic source just prior to the occurrence of the low voltage. 
     Another aspect of the present disclosure is a photovoltaic inverter for coupling a direct current photovoltaic source to an alternating current electrical grid and performing a low voltage ride through (LVRT) operation. The inverter includes a power bridge to convert direct current voltage to alternating current voltage. The power bridge is coupled to the electrical grid. A switching crowbar is coupled to the photovoltaic energy source and the power bridge. The crowbar has a switching device has a closed position causing the switching crowbar to dissipate energy from the photovoltaic energy source and an open position to allow direct output from the photovoltaic source to the power bridge. A voltage sensor detects a low voltage condition on the grid. A controller is coupled to the voltage sensor and controls the switching crowbar when a low voltage condition is detected. The switching device is placed in the closed position when the voltage from the photovoltaic energy source is higher than a predetermined threshold voltage and the switching device is placed in the open position when the voltage of the PV source reaches the voltage when the low voltage condition is detected. 
     The foregoing and additional aspects and implementations of the present disclosure will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the present disclosure will become apparent upon reading the following detailed description and upon reference to the drawings. 
         FIG. 1  is a functional block diagram of a PV grid tie inverter that may include a crowbar module to control power output for performing low voltage ride through (“LVRT”); 
         FIG. 2  is a timing diagram for the control signals switching the crowbar module in the inverter in  FIG. 1 ; 
         FIG. 3  is a graph showing the current reference used for vector current control of the inverter in  FIG. 1  to insure the previous current level maintained during LVRT; 
         FIG. 4A-4C  are simulation results showing grid voltage, voltage regulation and output current using the inverter in  FIG. 1  at a PV voltage of 800 volts during LVRT; 
         FIG. 5A-5C  are simulation results showing grid voltage, voltage regulation and output current using the inverter in  FIG. 1  at a PV voltage of 650 volts during LVRT; 
         FIG. 6A-6C  are simulation results showing grid voltage, voltage regulation and output current using the inverter in  FIG. 1  at a PV voltage of 500 volts during LVRT; 
         FIG. 7A-7C  are simulation results showing grid voltage based on low voltage on one phase of the grid, voltage regulation and output current using the inverter in  FIG. 1  at a PV voltage of 600 volts during LVRT; and 
         FIG. 8  is a flow diagram of a control algorithm for the inverter in  FIG. 1  showing operation during LVRT. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a functional block diagram of a photovoltaic (“PV”) power plant system  100  allowing LVRT operation with lower voltage bridge components. The power plant system  100  outputs power to an electrical grid  102 . In this example, the grid  102  has three phased voltage inputs coupled to a grid tie inverter  104 . The power plant system  100  includes the grid tie inverter  104  that is coupled between the electrical grid  102  and a photovoltaic (“PV”) power source  106 . A crowbar switching module  108  controls the voltage output from the PV power source  106 . The inverter  104  includes a power bridge  110  that converts direct current voltage from the PV power source  106  to alternating current suitable for the electrical grid  102 . The power bridge  110  in this example includes medium voltage IGBTs (not shown) which have a maximum breakdown voltage and corresponding diodes (not shown). Of course, different voltage IGBTs and other switching devices may be used for the power bridge  110 . The power bridge  110  uses a DC capacitor  112  to control the voltage of the PV source  106  and to convert direct current into alternating current. A DC voltage and current sensor  114  provides voltage and current measurements from the voltage generated from the PV power source  106 . 
     The alternating current output of the power bridge  110  is filtered via a line filter  116  which outputs the current into the three phased voltage inputs of the grid  102 . The AC output current is measured by a current sensor  118  and the output voltages are measured by a voltage sensor  120 . A controller  130  is coupled to the sensors  114 ,  118  and  120 . The controller  130  also controls the state of the crowbar module  108  and the power bridge  110 . 
     The crowbar module  108  includes a switching device  132  which in this example is an IGBT. The switching device  132  is coupled in series with a resistor  134  and a diode  136  which are coupled in parallel. As will be explained below, the resistor  134  absorbs excess power from the PV source  106  when the switching device  132  is closed. The diode  136  operates as a free wheel to allow current dissipation for extra energy from the PV source  106 . 
     Once a voltage dip occurs in the grid  102 , the extra energy produced by the PV source  106  will cause the increase of PV voltage due to the reduction of the output power at the inverter  104 . For example, the voltage of the grid  102  may drop to 5% of nominal value, which may cause the voltage from the PV source  106  to rise up close to the open voltage of the PV source  106 . Operating the inverter  104  at a higher DC voltage than permitted may damage the switching devices of the bridge  110  of the inverter  104  since such switching devices in this example are not capable of handling the higher voltage. 
     As will be described, the controller  130  runs an algorithm to protect the switching devices of the bridge  110  while the inverter  104  performs LVRT. The controller  130  controls the crowbar module  108  across the PV source  106  in order to dissipate the extra energy stored in the PV source  106  and therefore maintain a proper PV voltage to operate the inverter  104  in LVRT. The controller  130  also controls the current output of the power bridge  110  via switching the IGBTs in the power bridge  110 . The controller  130  detects a voltage dip from the grid  102  via the voltage sensor  120  which requires a LVRT. The controller  130  starts the on and off control of crowbar module  108  across the PV by using the switching device  132 . The controller  130  turns the crowbar module  108  on by closing the switching device  132  thereby dissipating extra energy from the PV source  106  via the resistor  134  and the diode  136  when the PV voltage of the PV source  106  exceeds a threshold voltage. In this example, the threshold voltage is ΔV higher than the PV voltage prior to the low voltage leading to the LVRT operation. The crowbar module  108  is turned off by opening the switching device  132  once the voltage from the PV source  106  reaches the PV voltage level just before the initiation of the LVRT operation. 
     The sequence of the bang-bang control is shown in  FIG. 2  which is a timing diagram of the state of the crowbar module  108 .  FIG. 2  includes a control signal trace  200  of the on and off state of the switching device  132  of the crowbar module  108  in  FIG. 1 .  FIG. 2  includes a voltage trace  202  of the output voltage from the PV source  106 . A bottom dashed line  204  represents the operating voltage level output from the PV source  106  just before the low voltage condition that requires LVRT operation. The operating voltage level is well below the maximum operating voltage output from the PV source  106 . A top dashed line  206  represents the voltage level of the PV source  106  just before the low voltage condition with a ΔV value added. Once the controller  130  senses a low voltage level at the grid  102 , the controller  130  determines the voltage level of the PV source  106  and stores the value. The controller  130  monitors the voltage level  202  of the PV source  106  which begins to ramp up over the operating voltage level  204  prior to the grid voltage dip. When the voltage level  202  reaches the threshold level  206  (the voltage level when the LVRT is initiated with the ΔV value added), the crowbar module  108  is switched on by closing the switching device  132  as shown by the trace  200  at a point  212 . The resistor  134  and diode  136  are placed in parallel with the PV source  106  by closing the switching device  132 . The power from the PV source  106  is dissipated over the resistor  134  and diode  136  free wheels to dissipate current. The voltage output from the PV source  106  falls as shown by the trace  202 . When the voltage falls to the voltage level  204  just before the grid voltage dip causing the LVRT operation as shown in the trace  202 , the crowbar module  108  is switched off by opening the switching device  132  as shown by the signal trace  200  at a point  214 . The voltage from the PV source  106  thus begins to rise again as shown by the trace  202 . This process allows the inverter  104  to maintain current output to the grid  102  at a level substantially the same as prior to the grid voltage dip causing the LVRT operation. The crowbar module  108  protects the switching devices in the power bridge  110  from high voltages of the PV source  106 . 
     The relationship between PV power from the power source  106  and the PV voltage is a group of curves that vary according to irradiance and temperature. However, the PV voltage is relatively constant over a wide range of irradiance and temperature. The crowbar module  108  in this example is able to handle the maximum PV power (P pv-max ) from the PV source  106  for a specified period of time (T). Since the PV voltage is usually regulated at around the maximum power point by the inverter  104  and also considering the ΔV for band control, the resistance value (R) of the resistor  134  in the crowbar module  108  is chosen by
 
 R =( V   pv-max   +ΔV ) 2   /P   pv-max   (1)
 
In this equation V pv-max  is the maximum voltage at the maximum power point of the PV source  106 . Thus, the resistance value may be obtained for specific maximum voltages and power. For example, if V pv-max  is 700V and P pv-max is  575 kW then R is 0.978 Ohm.
 
     The ΔV component of the threshold voltage may be varied to optimize the performance of the crowbar module  108 . In this example, ΔV is set as 10% of minimum PV operation voltage. Therefore if the minimum PV operation voltage is 500V then the ΔV value is selected as 50V. 
     Based on the selection of the crowbar module  108 , the maximum upper limit of the bang-bang control is selected as the voltage at maximum power point of the PV source  106  plus 50V. This selection can guarantee that the crowbar handle the maximum PV power even though the inverter output power is down to zero. 
     Once the grid voltage dips significantly and the inverter  104  still needs to output large amount of current due to the LVRT requirements, the traditional synchronization method such as phase lock loop (“PLL”) or filter related techniques may not work properly due to the high voltage pollution on the small grid voltage by the inverter  104 . If the grid voltage dip is unsymmetrical, it is difficult for a conventional inverter to still generate the symmetrical current when it is in LVRT operation. The inverter  104  in  FIG. 1  uses a replica of the current reference frame just before the LVRT operation. The controller  130  uses the replica to control the current output by the power bridge  110  to the level just before the voltage dip triggering the LVRT operation.  FIG. 3  is a graph of the simulation of a voltage vector  300  when the voltage dip appears at time=0. The voltage vector  300  is the combination of the voltage of the three phased inputs of the grid  102 . The location of the current reference vector is obtained by the location of voltage vector just before the LVRT in the α-β reference frame.
 
 i   α   =I  cos(ω t +θ)
 
 i   β   =I  sin(ω t +θ)  (2)
         where θ=tan −1 (V β /V α ), ω=2πf, f is the grid frequency before the LVRT       

       FIG. 4A  is a series of voltage graphs including voltage traces  400 ,  402  and  404  against time of the voltage from each of the three phased inputs of the grid  102 . In  FIG. 4A , the voltage on the grid  102  drops to 5% of the nominal at 0.5 seconds in the voltage traces  400 ,  402  and  404 . The power bridge  110  is put on line by the controller  130  at 0.3 seconds. The PV source  106  is then placed on line to the grid  102  at 0.4 seconds. The simulation in  FIG. 4A  uses a PV source such as the PV source  106  in  FIG. 1  with an open voltage of 996V, a short circuit current at 673 A and maximum power at 502 kW at 800V.  FIGS. 4A-C  show the entire process of the operation of the inverter  104  in  FIG. 1  under these conditions. 
     The voltage regulation of the inverter  104  via the control of the crowbar module  108  is shown in the voltage trace  410  in  FIG. 4B  which is voltage from the PV source  106  from the crowbar module  108  to the DC input of the bridge  110 . As shown in  FIG. 4B , the voltage output increases to 800 volts at 0.3 seconds when the PV source  106  is placed on line. The voltage output is maintained at 800 volts between 0.3 seconds and 0.4 seconds. The low voltage dip occurs at the grid  102  at 0.5 seconds and the voltage of the PV source  106  output to the power bridge  110  begins to increase. At 850 volts, the crowbar module  108  is activated and the voltage drops. The voltage output of the PV source  106  to the bridge  110  fluctuates between 800V and 850V thereafter due to the switching of the crowbar module  108  after the low voltage condition occurs at 0.5 seconds resulting in LVRT operation. 
       FIG. 4C  shows current traces  420 ,  422  and  424  of each of the three phase outputs of the bridge  110  coupled to the grid  102 . The current reference vector will be set as same as before the LVRT operation by the controller  130  controlling the switching devices of the bridge  110  as shown in the output current traces  420 ,  422  and  424  in  FIG. 4C . The output current traces  420 ,  422  and  424  show the current output of three phases after the LVRT operation occurs at 0.5 seconds. The location of the current reference vector is obtained by the location of voltage vector just before the LVRT in the α-β reference frame as explained above when the inverter  104  is put on line at 0.4 seconds. The current is replicated after the LVRT operation at 0.5 seconds as shown in the current traces  420 ,  422  and  424  in  FIG. 4C . 
       FIG. 5A  is a series of voltage graphs with voltage traces  500 ,  502  and  504  against time of the voltage from each of the three phased voltage inputs of the grid  102 . In  FIG. 5A , the voltage on the grid  102  drops to 5% of the nominal at 0.5 s seconds in the traces  500 ,  502  and  504 . The bridge  110  is put on line by the controller  130  at 0.3 seconds. The PV source  106  is then placed on line to the grid  102  at 0.4 seconds. The simulation in  FIG. 5A  uses a PV source such as the PV source  106  in  FIG. 1  with an open voltage of 996V, a short circuit current at 673 A and maximum power at 502 kW at 800V as in  FIGS. 4A-4C .  FIGS. 5A-C  show the entire process of the operation of the inverter  104  in  FIG. 1  under these conditions. 
     The voltage regulation of the inverter  104  via the control of the crowbar module  108  is shown in the voltage trace  510  in  FIG. 5B  which is voltage from the PV source  106  from the crowbar module  108  to the DC input of the bridge  110 . As shown in  FIG. 5B , the voltage output increases to 650 volts at 0.3 seconds when the PV source  106  is placed on line. The voltage output is maintained at 650 volts between 0.3 seconds and 0.4 seconds. The low voltage condition occurs at the grid  102  at 0.5 seconds and the voltage of the PV source  106  output to the bridge  110  begins to increase. The voltage output of the PV source  106  to the bridge  110  fluctuates between 650V and 700V due to the switching of the crowbar module  108  after the low voltage condition occurs at 0.5 seconds resulting in LVRT operation. The voltage output increases at a slower rate than the voltage output shown in  FIG. 4B  due to the lower operating voltage. 
       FIG. 5C  shows current traces  520 ,  522  and  524  of each of the three phase outputs of the bridge  110  coupled to the grid  102 . The current reference vector will be set as same as before the low voltage dip causing LVRT operation by the controller  130  controlling the switching devices of the bridge  110  as shown in the output current traces  520 ,  522  and  524  in  FIG. 5C  thereby resulting in the same current outputs before and after the LVRT operation occurring at 0.5 seconds. 
       FIG. 6A  is a series of voltage graphs including voltage traces  600 ,  602  and  604  against time of the voltage from each of the three phased inputs of the grid  102 . In  FIG. 6A , the voltage on the grid  102  drops to 5% of the nominal at 0.5 seconds in the voltage traces  600 ,  602  and  604 . The bridge  110  is put on line by the controller  130  at 0.3 seconds. The PV source  106  is then placed on line to the grid  102  at 0.4 seconds. The simulation in  FIG. 6A  uses a PV source such as the PV source  106  in  FIG. 1  with an open voltage of 996V at the time of the low voltage condition, a short circuit current at 673 A and maximum power at 502 kW at 800V as in  FIGS. 4A-4C .  FIGS. 6A-C  show the entire process of the operation of the inverter  104  in  FIG. 1  under these conditions. 
     The voltage regulation of the inverter  104  via the control of the crowbar module  108  is shown in the voltage trace  610  in  FIG. 6B  which is voltage from the PV source  106  from the crowbar module  108  to the DC input of the bridge  110 . As shown in  FIG. 6B , the voltage output increases to 500 volts at 0.3 seconds when the PV source  106  is placed on line. The voltage output is maintained at 500 volts between 0.3 seconds and 0.4 seconds. The low voltage condition occurs at the grid  102  at 0.5 seconds and the voltage of the PV source  106  output to the bridge  110  begins to increase. The voltage output of the PV source  106  to the bridge  110  gradually increases to 640V and stay at this value after 0.5 seconds due to the turn-on of the crowbar module  108  for LVRT operation. Since the resistance of the crowbar  134  shall be selected to sustain the maximum power of the PV source  106  and to maintain the PV voltage at permitted level, for example 850V based on Equation (1) above, the crowbar module  108  is kept on during this LVRT operation. 
       FIG. 6C  shows current traces  620 ,  622  and  624  of each of the three phase outputs of the power bridge  110  coupled to the grid  102 . The current reference vector will be set as same as before the low voltage dip causing LVRT operation by the controller  130  controlling the switching devices of the bridge  110  as shown in the output current traces  620 ,  622  and  624  in  FIG. 6C  thereby resulting in the same current outputs before and after the LVRT operation occurring at 0.5 seconds. 
       FIG. 7A  is a series of voltage graphs with voltage traces  700 ,  702  and  704  against time of the voltage from each of the three phase outputs of the grid  102 . In  FIG. 7A , the voltage on the grid  102  drops to 5% of the nominal at 0.5 seconds in one of the phase outputs of the grid as shown in the trace  700 . The voltage output of the other phase inputs remains the same as shown in traces  702  and  704 . The bridge  110  is put on line by the controller  130  at 0.3 seconds. The PV source  106  is then placed on line to the grid  102  at 0.4 seconds. 
     The voltage regulation of the inverter  104  via the control of the crowbar module  108  is shown in the voltage trace  710  in  FIG. 7B  which is voltage from the PV source  106  from the crowbar module  108  to the DC input of the bridge  110 . As shown in  FIG. 7B , the voltage output increases to 650 volts at 0.3 seconds when the PV source  106  is placed on line. The voltage output is maintained at 800 volts between 0.3 seconds and 0.4 seconds. The low voltage condition occurs at the grid  102  at 0.5 seconds and the voltage of the PV source  106  output to the bridge  110  begins to increase. The voltage output of the PV source  106  to the bridge  110  fluctuates between 650V and 700V due to the switching of the crowbar module  108  after the low voltage condition occurs at 0.5 seconds resulting in LVRT operation. 
       FIG. 7C  shows current traces  720 ,  722  and  724  of each of the three phase outputs of the bridge  110  coupled to the grid  102 . The current reference vector will be set as the same as that before the voltage dip causing the LVRT operation by the controller  130  controlling the switching devices of the bridge  110  as shown in the output current traces  720 ,  722  and  724  in  FIG. 7C  thereby resulting in the same current outputs before and after the LVRT operation occurring at 0.5 seconds. 
     The controller  130  in  FIG. 1  may be conveniently implemented using one or more general purpose computer systems, microprocessors, digital signal processors, micro-controllers, application specific integrated circuits (ASIC), programmable logic devices (PLD), field programmable logic devices (FPLD), field programmable gate arrays (FPGA) and the like, programmed according to the teachings as described and illustrated herein, as will be appreciated by those skilled in the computer, software and networking arts. 
     In addition, two or more computing systems or devices may be substituted for any one of the controllers described herein. Accordingly, principles and advantages of distributed processing, such as redundancy, replication, and the like, also can be implemented, as desired, to increase the robustness and performance of controllers described herein. The controllers may also be implemented on a computer system or systems that extend across any network environment using any suitable interface mechanisms and communications technologies including, for example telecommunications in any suitable form (e.g., voice, modem, and the like), Public Switched Telephone Network (PSTNs), Packet Data Networks (PDNs), the Internet, intranets, a combination thereof, and the like. 
     The operation of the example regulation of a power inverter output in LVRT operation to protect components from high voltage, will now be described with reference to  FIGS. 1-3  in conjunction with the flow diagram shown in  FIG. 8 . The flow diagram in  FIG. 8  is representative of example machine readable instructions for regulation of a power inverter output in LVRT operation. In this example, the machine readable instructions comprise an algorithm for execution by: (a) a processor, (b) a controller, and/or (c) one or more other suitable processing device(s). The algorithm may be embodied in software stored on tangible media such as, for example, a flash memory, a CD-ROM, a floppy disk, a hard drive, a digital video (versatile) disk (DVD), or other memory devices, but persons of ordinary skill in the art will readily appreciate that the entire algorithm and/or parts thereof could alternatively be executed by a device other than a processor and/or embodied in firmware or dedicated hardware in a well known manner (e.g., it may be implemented by an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable logic device (FPLD), a field programmable gate array (FPGA), discrete logic, etc.). For example, any or all of the components of the regulation of a power inverter output in LVRT operation could be implemented by software, hardware, and/or firmware. Also, some or all of the machine readable instructions represented by the flowchart of  FIG. 8  may be implemented manually. Further, although the example algorithm is described with reference to the flowchart illustrated in  FIG. 8 , persons of ordinary skill in the art will readily appreciate that many other methods of implementing the example machine readable instructions may alternatively be used. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, or combined. 
     The controller  130  first reads the grid voltage via the voltage sensors  120  and voltage from the PV source  106  via the voltage sensor  114  ( 800 ). The controller  130  determines whether the voltage from the grid  102  is lower than a value indicating a voltage dip requiring LVRT operation ( 802 ). If the voltage does not constitute a voltage dip, the controller  130  continues to read the grid voltage ( 800 ). 
     If the voltage of the grid  102  dips to a level requiring LVRT operation ( 802 ), the controller  130  determines the current level via the current sensors  118  ( 804 ). The controller  130  controls the power bridge  110  to regulate the current level from the power bridge  110  to that at the point of the voltage dip ( 806 ). The controller  130  reads the voltage level from the PV source  106  ( 808 ). The controller  130  determines whether the voltage level from the PV source exceeds the threshold voltage which is the voltage of the PV source  106  at the time of the voltage dip plus ΔV ( 810 ). If the voltage of the PV source  106  does not exceed the threshold voltage, the controller continues to measure the voltage of the PV source  106  ( 808 ). If the voltage of the PV source exceeds the threshold voltage, the controller  108  turns on the crowbar module  108  thereby closing the switching device  132  and allowing energy from the PV source to dissipate n the resistor  134  ( 812 ). The voltage of the PV source  106  to the power bridge  110  begins to fall when the crowbar module  108  is turned on. The controller  130  continues to read the voltage of the PV source  106  to the power bridge  110  ( 814 ). The controller  130  determines whether the PV voltage is less than the voltage of the PV source  106  at the time of the voltage dip ( 816 ). If the PV voltage is not less than the voltage of the PV source  106  at the time of the voltage dip, the crowbar module  108  stays on. If the PV voltage is less than the voltage of the PV source  106  at the time of the voltage dip, the crowbar module  108  is turned off ( 818 ), and the voltage from the PV source  106  to the power bridge  110  rises. The controller  130  then continues to read voltage of the PV source  106  ( 808 ). 
     While particular implementations and applications of the present disclosure have been illustrated and described, it is to be understood that the present disclosure is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.

Technology Classification (CPC): 8