Abstract:
A ground fault detector interrupter (GFDI) is described. The GFDI configured for use with a DC power supply and comprises the following elements. A ground current path is provided for coupling a ground terminal of the DC power supply and a system ground. A grounding switch is placed in the ground current path. A current detector is configured to detect a ground current in the ground current path. A controller is configured to compare the ground current with a predefined current set point and output a fault indication signal if the ground current exceeds the predefined current set point. The fault indication signal results in the grounding switch being open.

Description:
[0001]    The present disclosure relates generally to ground fault detector interrupters and specifically to such interrupters designed for alternative energy applications. 
       BACKGROUND 
       [0002]    Underwriters Laboratories Inc. (UL) is a well-known laboratory that develops standards and test procedures for materials, components, assemblies, tools, equipment and procedures, chiefly dealing with product safety and utility. 
         [0003]    UL 1741 is a standard that relates to inverters, converters, controllers and interconnection system equipment for use with distributed energy resources. UL 1741 was revised in November 2005 such that it requires all photovoltaic inverter systems to have a Ground Fault Detector Interrupter (GFDI). A GFDI is a solid-state electronic ground fault detector and interrupter designed to provide direct current (DC) fault protection on power conversion systems. 
         [0004]    Specifically, UL 1741 (Section 31.1) states that “inverters or chargers with direct photovoltaic inputs from a grounded photovoltaic array or arrays shall be provided with a ground-fault detector/interrupter (GFDI). The GFDI shall be capable of detecting a ground fault, providing an indication of the fault, interrupting the flow of the fault current, and either isolating the faulted array section or disabling the inverter to cease the export of power.” 
         [0005]    Typically, GFDI&#39;s operate by measuring a current balance between two conductors and opening a device&#39;s contacts if there is a difference in current between the conductors. However, since the photovoltaic array&#39;s positive or negative pole has to be grounded, such an arrangement cannot easily be implemented. 
         [0006]    Accordingly, although there are a number of commercially available GFDIs, they do not meet the current set points and timings required for this standard, where the photovoltaic array&#39;s positive or negative pole has to be grounded. 
       SUMMARY 
       [0007]    A GFDI is provided for DC fault protection on power conversion systems for alternative energy application where the photovoltaic array&#39;s positive or negative pole has to be grounded. The GFDI is designed to fulfill the requirements of section 31 of UL 1741. 
         [0008]    Therefore, in accordance with an aspect of the present invention there is provided a ground fault detector interrupter (GFDI) configured for use with a DC power supply, the GFDI comprising: a ground current path coupling a ground terminal of the DC power supply and a system ground; a grounding switch placed in the ground current path; a current detector configured to detect a ground current in the ground current path; and a controller configured to compare the ground current with a predefined current set point and output a fault indication signal if the ground current exceeds the predefined current set point, the fault indication signal resulting in the grounding switch being open. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    An embodiment of the present invention will now be described by way of example only with reference to the following drawings in which: 
           [0010]      FIG. 1  is a block diagram illustrating a Ground Fault Detector Interrupter in accordance with an embodiment of the present invention; and 
           [0011]      FIG. 2  is a flow chart illustrating the operation of the GFDI illustrated in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    For convenience, like numerals in the description refer to like structures in the drawings. Referring to  FIG. 1 , a block diagram illustrating a GFDI is shown generally by numeral  100 . The GFDI  100  includes a current sensor  102 , a bridge rectifier  104 , a polarity detector  106 , a signal conditioning module  108 , a sensor failure monitor  110 , a reset  112 , a relay and contact monitor  114 , a microcontroller  116 , relays  118 , and a grounding contactor  120 . 
         [0013]    The current sensor  102  senses a current flowing between a ground terminal  122  (negative or positive pole) of a photovoltaic DC source and a system ground  124 . It will be appreciated that the ground terminal refers to whichever one of the terminals is to be coupled to ground. In the present embodiment, the system ground is an enclosure ground, which is a ground terminal of an enclosure in which the GFDI is housed. However, it will be apparent to a person of ordinary skill in the art that another earth ground could be provided as the system ground. The grounding contactor  120  is coupled between the source ground  122  and the enclosure ground  124 . Output from the current sensor  102  is provided to the bridge rectifier  104  and the polarity detector  106 . Output from the rectifier bridge  104  is coupled to an input on the microcontroller  116  via the signal conditioning module  108 . Output from the polarity detector  106  is coupled to an input on the microcontroller  116 . Output from the microcontroller  116  is coupled to the relays  118 . Output from the relays  118  is coupled to the DC contactor  120  and the relay and contact monitor  114 . Output from the grounding contactor  120  is also coupled to the relay and contact monitor  114 . 
         [0014]    In the present embodiment, the current sensor  102  is a closed loop Hall effect, isolated transducer, which detects the current flowing between the ground and the photovoltaic array. The output of the current sensor  102  is a current that varies to mirror the value of the sensed current. 
         [0015]    A bridge rectifier  104  is an arrangement of four diodes, as is known in the art, that provides the same polarity of output current for any polarity of the input current. Persons of ordinary skill in the art would appreciate that the bridge rectifier converts an AC signal to a DC signal. In normal operating mode, the input current will be approximately zero and will deviate from this if a fault occurs. Using a bridge rectifier  104  provides two advantages. A first advantage of the bridge rectifier is avoiding the complication of implementing an absolute value calculator. 
         [0016]    A second advantage of the bridge rectifier is allowing the whole range of the microcontroller&#39;s analog input to be used to represent the current. Specifically, since the microcontroller  116  has a finite number of bits for analog to digital conversion, the smaller the analog range for conversion, the greater the resolution at which it can be represented. Accordingly, the use of a polarity detector allows a range of 0 to X to be used. Without use of the polarity detector, the range would be −X to X, which is twice as large and would result in a lower resolution. 
         [0017]    A signal conditioning block  108  is coupled to the output of the bridge rectifier. The signal conditioning block  108  converts the rectified current signal to a representative voltage signal, in a form that can be read by the microcontroller  116 . To ensure the voltage signal is reliable, the signal conditioning block  108  scales and filters the voltage signal. Although an op-amp is utilized in the present embodiment, will be understood that there are other ways to ensure signals are not distorted through interference. 
         [0018]    The polarity detector  106  is implemented using an op-amp, as is known in the art. The op-amp detects the polarity of the current sensed by the current sensor  102 . 
         [0019]    Accordingly, the signal conditioning block  108  provides an absolute value of the voltage signal to the microcontroller  116  and the polarity detector provides the polarity of the voltage signal. 
         [0020]    The sensor failure monitor  110  provides an additional level of safety in order to ensure that the current sensor  102  and the signal conditioning block  108  are operating properly. Accordingly, the sensor failure monitor  110  injects a predefined amount of current into the current sensor upon instruction from the microcontroller  116 . In the present embodiment, a 60 mA current is injected every second, and the microcontroller  116  monitors the input voltage for the expected change. If there are no changes for three consecutive current injections, the microcontroller  116  will trigger a fault, opening the grounding contactor  120 . 
         [0021]    The reset  112 , is an external input that allows a user to reset the microcontroller  116 , the relays  118  and the grounding contactor  120 . 
         [0022]    The relay and contact monitor  114  monitors feedback from the grounding contactor  120  and the relays  118  to ensure that the grounding contactor  120  is operating as it should. That is, when the relays  118  are active the grounding contactor  120  is closed, and vice versa. 
         [0023]    The microcontroller  116  has two inputs for receiving the voltage representing the ground current. Therefore, if one of inputs faults, for example is grounded or connected to a positive supply, the GFDI  100  will continue to function. Similarly, the microcontroller has two outputs which control two relays  124 . Although only one output is required, a second, redundant output provides additional protection if one output fails. When the outputs are active, the relays  124  are activated. 
         [0024]    In the present embodiment, the relays  124  have four outputs  125 ,  126 ,  127  and  128 . A first one of the outputs  125  controls whether the grounding contactor  120  is open or closed. In the present embodiment, the grounding contactor  120  is normally open. This default setting provides an additional safety feature since the grounding contactor  120  will be open, inhibiting the flow of current from the DC source  122 , in the even that part of the GFDI  100  fails. 
         [0025]    A second one of the outputs  128  provides a fault indicator to a main controller (not shown). A third one of the outputs  126  will controls an AC output contactor so the system will cease exporting power from the photovoltaic source. A fourth one of the outputs  127  transmits a status signal to the relay and contactor monitor  114 . 
         [0026]    The microcontroller  116  is configured to analyse input representing the current detected by the current sensor  102  and determine whether or not a ground fault is detected. The operation of the microcontroller  116  is described as follows. 
         [0027]    Referring to  FIG. 2 , a flowchart illustrating the operation of the GFDI is illustrated generally by numeral  200 . 
         [0028]    Initially, the grounding contactor  120  is open and there is no current flowing between the ground  122  and the enclosure ground  124 . At step  202 , a current set point and a delay set point are read by the microcontroller  116  and saved into its memory. The current set point represents a threshold for maximum expected current flowing between the ground  122  and the enclosure ground  124 , which will be described further on in the description. The delay set point represents a time period and will also be described further on in the description. Both the current set point and the delay set point can be established defaults or customised for a particular implementation. 
         [0029]    In step  204 , the microcontroller  116  reads the value and polarity of an offset current flowing through the current sensor  102 . This current is referred to as an offset current since there should theoretically be a current reading of zero due to the open grounding contactor  120 . Accordingly, the offset value represents the non-operating bias of the GFDI  100 . 
         [0030]    In step  206 , the absolute value of the offset current is compared with a predefined offset current threshold. In the present embodiment, the maximum acceptable offset current is 40 mA. If the offset current is greater than the offset current threshold the microcontroller  116  continues at step  208 . 
         [0031]    At step  208 , the microcontroller  116  outputs a fault signal to the relays  118 , which in turn open the grounding contactor  120 , open the AC output contactor and provide a fault indication to the main controller. The microcontroller waits for a reset command at step  209 , before returning to step  202 . 
         [0032]    If the offset current is less than the offset current threshold the microcontroller  116  continues at step  210 . At step  210 , the microcontroller  116  outputs a go-ahead signal to the relays  118 , which in turn close the grounding contactor  120  and close the AC output contactor. 
         [0033]    At step  212 , feedback from the relays  118  and the grounding contactor  120  are read by the microcontroller  116  via the relay and contact monitor  114 . At this step, the microcontroller  116  is aware that the relays  118  should be active and that the grounding contactor  120  should be closed. If the relays  118  and the grounding contactor  120  are not operating as expected, the microcontroller continues at step  208 . If the relays  118  and the grounding contactor  120  are operating as expected, the microcontroller continues at step  214 . 
         [0034]    At step  214 , the microcontroller  116  reads the value of the current flowing between the ground  122  and the enclosure ground  124 , also referred to as the ground current. In the present embodiment the microcontroller  116  also combines the offset current with the ground current to get a more accurate representation of the actual ground current. Whether or not the offset current and the ground current are added or subtracted from each other depends on their respective polarities. 
         [0035]    For example, for a ground current with a positive polarity and an offset current with a negative polarity, the absolute values of the currents are summed. For a ground current with a positive polarity and an offset current with a positive polarity, the absolute value of the offset current is subtracted from the absolute value of the ground current. For a ground current with a negative polarity and an offset current with a positive polarity, the absolute values of the currents are summed. For a ground current with a negative polarity and an offset current with a negative polarity, the absolute value of the offset current is subtracted from the absolute value of the ground current. 
         [0036]    At step  216 , it is determined whether or not the ground current is less than the current set point. As previously described, the current set point is the maximum expected ground current that is considered acceptable by the GFDI. In the present embodiment, the current set point can range between 0.5 A and 6 A, although other values may be acceptable, as will be appreciated by a person of ordinary skill in the art. 
         [0037]    If the ground current is less than the current set point, the microcontroller  116  continues to step  218 . At step  218 , the reset  112  is checked to determine whether or not an operator has request a system reset. If a reset has not been requested, the microcontroller  116  returns to step  212  and reads the feedback from the relays  118  and the grounding contactor  120 . 
         [0038]    If a reset has been requested the microcontroller  116  continues to step  220  and outputs a stop signal to the relays  118 , which in turn opens the grounding contactor  120  and opens the AC output contactor. The microcontroller  116  then returns to step  202 . 
         [0039]    Returning to step  216 , if it is determined that the ground current is greater than the current set point, the microcontroller  116  continues at step  221 . At step  221 , the microcontroller determines whether it is more likely that the ground current exceeds the current set point due to a glitch or spike in the ground current as compared to a true fault. 
         [0040]    Specifically, at step  222 , it is determined whether or not the ground current is within a first range in excess of the current set point. In the present embodiment, the first range is 115% of the current set point. If the ground current is within the first range, at step  224  the microcontroller  116  waits for a first predefined period of time. In the present embodiment, the first period of time is three times the delay set point. In the present embodiment, the delay set point can range between 0.25 seconds and 3 seconds. 
         [0041]    After the delay, the microcontroller  116  continues at step  234 , at which the ground current is compared against the current set point once again. If the ground current is still greater than the current set point, the microcontroller  116  continues at step  208  as described above. Otherwise, the microcontroller returns to step  218 . 
         [0042]    At step  226 , it is determined whether or not the ground current is within a second range in excess of the current set point. In the present embodiment, the second range is between 115% and 150% of the current set point. If the ground current is within the second range, at step  228  the microcontroller  116  waits for a second predefined period of time. In the present embodiment, the second predefined period of time is twice the delay set point. After the delay, the microcontroller  116  continues at step  234  as described above. 
         [0043]    At step  230 , it is determined whether or not the ground current is within a third range in excess of the current set point. In the present embodiment, the third range is between 150 and 250% of the current set point. If the ground current is within the third range, at step  232  the microcontroller  116  waits for a third predefined period of time. In the present embodiment, the third predefined period of time is equal to the delay set point. After the delay, the microcontroller  116  continues at step  234  as described above. 
         [0044]    In an alternate embodiment, the microcontroller  116  determines whether the ground current exceeds the current set point due to a glitch or spike as follows. If the ground current is greater than the set current point, a timer is started. In the present embodiment, the duration of the time is set in accordance with the difference between the ground current and the set current point, similar to the previous embodiment. The ground current is continuously monitored. If the ground current falls below the current set point before the timer expires, the microcontroller  116  returns to normal operation. If, however, the ground current does not fall below the current set point before the timer expires, the microcontroller  116  opens the grounding contactor  120 , indicating a fault. 
         [0045]    It will be apparent to a person of ordinary skill in the art that the examples given above are provided for illustrative purposes only and are in no way intended to limit the scope of the description. For example, the threshold and set points are merely exemplary. Further, although the description above refers to a microcontroller  116 , other types of controllers, either analog or digital, can be implemented to achieve the same function, as will be appreciated by a person of ordinary skill in the art. Yet further, although the use of bridge rectifier provides the advantages previously discussed, embodiments can be implemented in which it is not required. 
         [0046]    Accordingly, although preferred embodiments of the invention have been described herein, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.