Abstract:
A direct current (DC) power combiner operable to interconnect multiple interconnected photovoltaic strings is disclosed. The DC power combiner may include a device adapted for disconnecting at least one photovoltaic string from the multiple interconnected photovoltaic strings, each photovoltaic string connectible by a first and second DC power line. The device may include a differential current sensor adapted to measure differential current by comparing respective currents in the first and second DC power lines. A first switch is connected in series with the first DC power line. A control module is operatively attached to the differential current sensor and the first switch. The control module may be operable to open the first switch when the differential current sensor measures the differential current to be greater than a maximum allowed current differential, thereby disconnecting the photovoltaic string from the interconnected photovoltaic strings.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to patent application GB1020862.7, filed Dec. 9, 2010, in the United Kingdom Intellectual Property Office. Application GB1020862.7 is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates to multiple photovoltaic strings, which have direct current (DC) outputs, which are interconnected at an input of a power combiner junction box and specifically to a system and method to disconnect and or connect a photovoltaic string DC output from the input of the power combiner junction box. 
         [0004]    2. Description of Related Art 
         [0005]    In a photovoltaic distributed power harvesting system, a photovoltaic string includes a series connection of photovoltaic panels. Photovoltaic strings may be connected in parallel to give a parallel direct current (DC) power output. The parallel DC power output may connect to the input of a direct current (DC) to an alternating current (AC) inverter. The AC power output of the inverter connects across an AC load. The load may be an AC load such as an AC motor or may be an electrical power grid. 
         [0006]    A Residual-Current Circuit Breaker (RCCB) is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCCBs are designed to disconnect quickly enough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or short-circuit conditions. 
         [0007]    In the United States and Canada, a residual current device is most commonly known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In Australia they are sometimes known as “safety switches” or simply “RCD” and in the United Kingdom, along with circuit breakers, they can be referred to as “trips” or “trip switches.” 
         [0008]    Under some circumstances, such as in a photovoltaic distributed power harvesting system, a residual current may also represent a fire hazard. 
         [0009]    Thus, there is a need for and it would be advantageous to have a system and method for disconnection of a photovoltaic string carrying direct current power when residual current is detected in the photovoltaic string to prevent fire hazards in photovoltaic arrays. 
       BRIEF SUMMARY 
       [0010]    Various systems and methods are provided for ground fault protection in a photovoltaic power harvesting system. According to some aspects, ground fault protection is provided in a direct current (DC) power combiner, which combines photovoltaic strings to form a photovoltaic array. The detection of a ground fault or residual current in the string causes the string to be disconnected from the photovoltaic array. In some aspects, the possibility for arcing while disconnecting the photovoltaic string from the photovoltaic array is minimized. 
         [0011]    According to various aspects, there is provided a direct current (DC) power combiner operable to interconnect multiple interconnected photovoltaic strings. The DC power combiner may include a device adapted for disconnecting at least one photovoltaic string from the multiple interconnected photovoltaic strings, each photovoltaic string connectible by a first and second DC power line. The device may include a differential current sensor adapted to measure differential current by comparing respective currents in the first and second DC power lines. A first switch is connected in series with the first DC power line. A control module operatively is attached to the differential current sensor and the first switch. The control module may be operable to open the first switch when the differential current sensor measures the differential current to be greater than a maximum allowed current differential, thereby isolating the first DC power line from the DC power combiner and disconnecting the photovoltaic string from the interconnected photovoltaic strings. 
         [0012]    A second switch may be parallel connected to the first switch to form a first unit. The first unit may be connected in series with the first DC power line. When the first switch is closed, the differential current sensor measures the differential current to be greater than a maximum allowed current differential, then the control module may open the first switch and subsequently may open the second switch. The first switch may allow through substantially more of a current flowing in the first DC power line and the second switch may allow through substantially less of the current flowing in the first DC power line. 
         [0013]    A third switch and a fourth switch may be parallel-connected to form a second unit. The second unit may be connected in series with the second DC power line. The third switch may be closed and the fourth switch may be closed, when the differential current sensor measures the differential current to be greater than a maximum allowed current differential, then the control module opens the third switch and subsequently opens the fourth switch. The third switch may allow through substantially more of a current flowing in the second DC power line and the fourth switch may allow through substantially less of the current flowing in the second DC power line. 
         [0014]    When the photovoltaic string begins to produce DC power, the first switch may be open and the second switch may be open, the control module closes the second switch and subsequently closes the first switch. The third switch may be open and the fourth switch may be open, the control module closes the fourth switch and subsequently closes the third switch. 
         [0015]    The second switch and the fourth switch respectively, may be an insulated gate bipolar transistor (IGBT), an IGBT with integral diode, a solid state switch, metal oxide semiconductor field effect transistor (MOSFET) or a field effect transistor (FET). The first switch and the third switch respectively may be a relay or a circuit breaker. 
         [0016]    According to various aspects, there is provided a method for providing ground fault protection. The method performed in a direct current (DC) power combiner operable to interconnect multiple photovoltaic strings, where each photovoltaic string may be connectible by a first and second DC power line. The method measures a differential current by comparing respective currents in the first and second DC power lines. When a control module measures the differential current to be greater than a maximum allowed current differential, a first switch is opened, thereby isolating the first DC power line from the DC power combiner and disconnecting the photovoltaic string from the interconnected photovoltaic strings. The first switch and a second switch may be parallel connected to form a first unit and the first unit may be connected in series with the first DC power line. A third switch and a fourth switch may be connected in parallel to form a second unit and the second unit may be connected in series with the second DC power line. The first unit may be operated simultaneously with the second unit. 
         [0017]    When the differential current is measured and found to be greater than a maximum allowed current differential, the third switch may be opened and subsequently the fourth switch opened which isolates the second DC power line from the DC power combiner. 
         [0018]    Prior to the comparing of respective currents in the first and second DC power lines, when the photovoltaic string begins to produce DC power with the first switch and the second switch both open. The second switch is closed and subsequently the first switch is closed, thereby connecting the first DC power line to the DC power combiner. 
         [0019]    Prior to the comparing of respective currents in the first and second DC power lines, when the photovoltaic string begins to produce DC power with the third switch and the fourth switch both open. The fourth switch is closed and subsequently the third switch is closed, thereby connecting the second DC power line to the DC power combiner. The first switch and the third switch may be operated simultaneously. The second switch and the fourth switch may also be operated simultaneously. 
         [0020]    The differential current may be indicative of current between the first DC power line and ground. The differential current may also be indicative of current between the second DC power line and ground. A failure of the string subsequent to the measuring of the differential current may be notified. 
         [0021]    A test of the measuring of the differential current may be initiated by injecting a current in the first DC power line prior to and during the measuring. A test of the measuring of the differential current may also be initiated by injecting a current in the second DC power line prior to and during the measuring. 
         [0022]    These, additional, and/or other aspects and/or advantages are set forth in the detailed description that follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Various aspects are herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0024]      FIG. 1  shows a power combiner box according to various aspects. 
           [0025]      FIG. 2  shows further details of an isolator and sensing unit according to various aspects. 
           [0026]      FIG. 3  shows more details of a digital controller according to various aspects. 
           [0027]      FIG. 4  shows a method for disconnecting a string from multiple parallel-connected strings, using an isolation and test unit, according to various aspects. 
       
    
    
       [0028]    The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying figures. 
       DETAILED DESCRIPTION 
       [0029]    Reference will now be made in detail to various aspects, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. 
         [0030]    Before explaining various aspects in detail, it is to be understood that embodiments are not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0031]    The term “string” or “photovoltaic string” as used herein is a series connection of multiple photovoltaic panels, which may be connected together in parallel to form a “photovoltaic array.” 
         [0032]    By way of introduction, various aspects may be directed to a device adapted for a disconnecting of at least one photovoltaic string from multiple interconnected photovoltaic strings. The disconnecting of at least one photovoltaic string may be required because a ground-fault has developed in the at least one photovoltaic string. Minimization of arcing may be performed whilst disconnecting or connecting a direct current (DC) string from multiple interconnected DC strings. Electric arcing can have detrimental effects on electric power distribution systems and electronic equipment. Arcing may occur in switches, circuit breakers, relay contacts, fuses, and poor cable terminations. When a circuit is switched off or a bad connection occurs in a connector, an arc discharge may form across the contacts of relay for example. An arc discharge is an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current flowing through a medium such as air, which may be normally non-conducting. At the beginning of a disconnection, the separation distance between the two contacts may be very small. As a result, the voltage across the air gap between the contacts produces a very large electrical field in terms of volts per millimeter. This large electrical field causes the ignition of an electrical arc between the two sides of the disconnection. If a circuit has enough current and voltage to sustain an arc, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire. The zero crossing of alternating current (AC) power systems may cause an arc not to reignite. However, a direct current system that has DC strings may be more prone to arcing than AC systems because of the absence of zero crossing in DC power systems. 
         [0033]    Reference is now made to  FIG. 1 , which shows a power combiner box  19 , according to various aspects. Power combiner box  19  includes multiple combiner circuit boards  12 , multiple digital controllers  10 , multiple isolation and test units  4 , multiple photovoltaic string inputs  8 , multiple bus bars  17  and  13 , user interface  16 , and power supply unit (PSU)  18 . 
         [0034]    Each combiner circuit board  12  has multiple units  4   a - 4   n  mechanically mounted to board  12 . Unit  4  typically receives a direct current (DC) output from a photovoltaic string  8  or other DC power sources such as a battery, electric fuel cell, or DC generator. Outputs  9  of units  4   a - 4   n  may be preferably connected in parallel using bus bar  17 . Alternatively, outputs  9  of units  4   a - 4   n  may first be connected to an input of a DC-to-DC converter. An output of the DC-to-DC converter may then be connected to bus bar  17 . Multiple bus bars  17  may be further connected in parallel using bus bar  13 . Bus bar  13 , therefore gives the combined DC power output of power combiner  19 . Power combiner  19  may be protected by lightning suppressor  192  (for example a varistor type) and may also be isolated using DC disconnect  194 . A digital controller  10  may be operatively attached to units  4   a - 4   n  via communication and control lines  11 . Communication and control lines  11  typically convey control signals to unit  4 , to switch on or off unit  4  for example, or to receive signals, which represent currents or voltages measured by sensors located in unit  4  for example. 
         [0035]    A user interface  16  may be operatively attached to digital controllers  10  via b-directional communication lines  3 . Communication lines  3  may typically be a dual RS-485 bus for example. User interface  16  may be supplied with a DC power from a power supply  18 , which converts a mains alternating current (AC) power into the DC power. The DC power may also be used to supply circuit boards  12 , controllers  10 , and unit circuits  4 . Alternatively, circuit boards  12 , controllers  10 , and unit circuits  4  may be supplied from DC to DC converters, which get an input from strings  8  or the DC from PSU  18 . 
         [0036]    Reference is now made to  FIG. 2 , which shows further details of unit  4  according to various aspects. A photovoltaic string  8  has a negative line connected to node X and a positive line connected to one end of a fuse  404 . The other end of fuse  404  connects to node A. Across nodes A and X may be a lightening suppressor  402 . Suppressor  402  may be additionally connected to electrical earth. 
         [0037]    Connected across nodes A and B may be a voltage sensor V 1  which provides an output  406 . Voltage sensor V 1  typically may measure the voltage at node A or node B or the voltage difference between nodes A and B. Output  406  may be operatively attached to controller  10  via control and communication line  11  (not shown). A collector of an insulated gate bipolar transistor (IGBT) Q 1  connects to node A. The emitter of Q 1  connects to an emitter of an IGBT Q 2 . The collector of Q 2  connects to node B. The emitters of Q 1  and Q 2  also connect to the anodes of two diodes Dl and D 2 . The cathode of D 1  connects to node A and the cathode of D 2  connects to node B. The base of Q 1  is connected to the base of Q 2 . One output of a drive circuit  400  connects to the bases of Q 1  and Q 2  and another output connects to the anodes of diodes D 1  and D 2 . One side of a relay RR 1  contact switch S 1  connects to node A. The other side of contact switch S 1  connects to one side of a contact switch S 2  of relay RR 2 . The other side of contact switch S 2  connects to node B. 
         [0038]    Connected across nodes X and Y may be a voltage sensor V 2 , which provides an output  408 . Voltage sensor V 2  typically may measure the voltage at node X or node Y or the voltage difference between nodes X and Y. Output  408  may be operatively attached to controller  10  via control and communication line  11 . A collector of an insulated gate bipolar transistor (IGBT) Q 3  connects to node X. The emitter of Q 3  connects to an emitter of an IGBT Q 4 . The collector of Q 4  connects to node Y. The emitters of Q 3  and Q 4  also connect to the anodes of two diodes D 3  and D 4 . The cathode of D 3  connects to node X and the cathode of D 4  connects to node Y. The base of Q 3  is connected to the base of Q 4 . One output of a drive circuit  400  connects to the bases of Q 3  and Q 4  and another output connects to the anodes of diodes D 3  and D 4 . One side of a relay RR 3  contact switch S 3  connects to node X. The other side of contact switch S 3  connects to one side of a contact switch S 4  of relay RR 4 . The other side of contact switch S 4  connects to node Y. Relays RR 1 , RR 2 , RR 3  and RR 4  may be typically rated with a breakdown DC voltage of 700 volts for switch contacts S 1 , S 2 , S 3 , and S 4 . Relays RR 1 , RR 2 , RR 3 , and RR 4  may be typically AC relays or DC relays rated at around 1000 volts. During normal operation of strings  8  and combiner box  19 , relays RR 1 , RR 2 , RR 3  and RR 4  may be on, i.e. switch contacts S 1 , S 2 , S 3 , S 4  may be closed and IGBTs Q 1 , Q 2 , Q 3 , Q 4  may be on also. The typical collector emitter voltage (VCE) for an IGBT may be around 2 volts compared to the substantially zero voltage across switch contacts S 1 , S 2 , S 3 , and S 4 . Therefore, the bulk of the string current (Istring) flows through switch contacts S 1  and S 2  (in the positive line) and through switch contacts S 3  and S 4  (in the negative line). 
         [0039]    Node B connects to the positive input of residual current device RCD 1  and node Y connects to the negative input of residual current device RCD 1 . Residual current device RCD 1  provides a positive line output and a negative line output via a serially connected current sensor R 2  in the negative line output. Alternatively residual current device RCD 1  may be disposed between the positive and negative outputs of string  8  and nodes A and X. Residual current device RCD 1  typically may be Hall Effect residual current device (RCD). Operatively attached to RCD 1  may be a test circuit  414  via inductor L 2 . RCD 1  may be operatively attached to test circuit  414  via Hall Effect. Inductor L 2  may be connected in series with a battery B 1  or a DC power supply from PSU  18 , resistor R 1  and switch Q 5 . The gate of switch Q 5  may be operatively attached to controller  10  via control and communication line  11 . Residual current device RCD 1  provides a measure of a differential current between the currents in a positive line of DC output  9  and a negative line of DC output  9 . The differential current threshold may be optionally around 20 milliamperes. Additionally residual current device RCD 1  provides a measure of a differential voltage between the negative line and positive line of DC output  9 . The differential current and the differential voltage may be used to calculate the power of a string  8 . The measure of the differential current may be provided to controller  10  via output  410 . Output  410  may be operatively attached to controller  10  via control and communication line  11 . Output  410  may be provided from the output of an amplifier A 1  that has an inductor L 1  across an input of amplifier A 1 . Inductor L 1  operatively attaches amplifier A 1  to RCD 1 . A threshold of the differential current to indicate a current leakage may be optionally around a value of 20 milliamperes. The differential current above a value of 20 milliamperes, typically may indicate a current leakage in a photovoltaic string  8 . Typically, both a positive and a negative of string  8  may be isolated from electrical earth. The current leakage may be either between the negative and electrical earth or between the positive and electrical earth. The differential current also above  20  milliamperes occurs when for example, an additional current may be imposed onto positive at node B and/or node Y using test circuit  414 . 
         [0040]    A measure of string  8  current may be also provided from the output  412  of amplifier A 2 . Output  412  may be operatively attached to controller  10  via control and communication line  11 . Output  412  along with voltage sensors  408  and  406  may provide a measure of the power generated by a string  8 . Current sensor R 2  may be connected to the input of amplifier A 2  via series resistors R 3  and R 4 . Current sensor R 2  may be located in the positive DC power line or the negative powers line. 
         [0041]    Reference is now made to  FIG. 3 , which shows more details of digital controller  10 , according to various aspects. Digital controller  10  includes multiplexors  106   a,    106   b  / 108   a,    108   b,  digital signal processors (DSP)  100   a / 100   b,  analogue to digital (AD) converters  104   a  / 104   b,  user interface  16 , power supply unit  18  and complex programmable logic device (CPLD)  102 . User interface  16  may be supplied with direct current (DC) power from PSU  18 . User interface  16  may be operatively attached to DSP  100   a  and DSP  100   b  using bi-directional buses  3 , Bus  3  may be typically a dual RS-485 bus. Using  16  photovoltaic strings  8  as an example; preferably DSP  100   a  and multiplexors  106   a  and  108   a  may be responsible for  8  of the photovoltaic strings  8  and DSP  100   b  and multiplexors  106   b  and  108   b  may be responsible for the remaining  8  photovoltaic strings  8 . Control line  120   a  may be supplied from DSP  100   a  to control multiplexor  106   a  and control line  120   b  may be supplied from DSP  100   b  to control multiplexor  106   b.  Using the example, multiplexor  106   a  receives outputs  406 ,  408  and  412  for 8 strings  8 . Multiplexor  106   a  may be controlled by DSP  100   a  via control line  120   a  to select which string  8  of the 8 strings  8 , may be used to provide outputs  406 ,  408  and  412  to analogue to digital (AD) converter  104   a.  Similarly (as multiplexor  106   a ) multiplexor  106   b  receives outputs  406 ,  408 , and  412  for the other 8 strings  8 . Multiplexor  108   a  receives output  410  for 8 strings  8 . Multiplexor  108   a  may be controlled by DSP  100   a  via control line  130   a  to select which string  8  of the 8 strings  8 , may be used to provide output  410  to analogue to digital (AD) converter  104   a.  Similarly (as multiplexor  108   a ) multiplexor  108   b  receives output  410  for the other 8 strings  8 . Synchronization between DSP  100   a  and DSP  100   b  may be by use of bi-directional synchronization control line  132 . Complex programmable logic device (CPLD)  102  provides outputs to control test circuit  414 , drive circuit  400  and relays RR 1 -RR 4  in each unit  4  used for each string  8 . The working operation of DSP  110   b  and DSP  100   a  may be also verified by CPLD  102  using watchdog bi-directional control lines WDb and WDa respectively. According to another aspect, just one DSP  100  and multiplexors  106  and  108  may be used to implement controller  10  for a number of photovoltaic strings  8 . 
         [0042]    Reference is now made  FIG. 4 , which shows a method  401  for disconnecting a string  8  from multiple parallel-connected strings  8  using isolation and test unit  4 , according to various aspects. Referring again to  FIG. 2 , an isolator may be formed (step  403 ) between nodes A and B, so as to place the isolator in series with the positive power line (step  405 ) of string  8 . Similarly, a second isolator may be formed between nodes X and Y, so as to place the second isolator in series with the negative DC power line of string  8 . The isolator and the second isolator may be identical circuits and may be typically activated simultaneously by drive circuits  400 . Typically, the negative power line of a string  8  may be not connected to electrical earth. Therefore, operation of both the isolator and the second isolator to disconnect string  8  in the event of a current leakage to earth due to a fault in string  8  prevents the current leakage to electrical earth from other parallel-connected strings  8 . 
         [0043]    Formation of an isolator in step  403  between nodes A and B includes a first switch and a second switch. The isolator connects and disconnects a string  8  from output  9 . The first switch may be connected in parallel with the second switch to form a parallel connection. The parallel connection may be then connected serially between nodes A and B. The first switch may be formed by connecting in series switches S 1  and S 2  of relays RR 1  and RR 2  respectively. The second switch includes a collector of Q 1  connected to node A and a collector of Q 2  connected to node B. Emitters of Q 1  and Q 2  may be connected together. Where the emitters of Q 1  and Q 2  may be connected together may be also connected the anodes of diodes D 1  and D 2 . The cathode of D 1  connects to node A and the cathode of D 2  connects to node B. The bases of Q 1  and Q 2  may be connected together and where the bases of Q 1  and Q 2  may be connected together, a connection to drive circuit  400  may be made. A second connection to drive circuit  400  may be made where the emitters of Q 1  and Q 2  may be connected together. 
         [0044]    An input of residual current device RCD 1  may be connected across nodes B and Y. RCD 1  provides a measure (step  407 ) of a level of differential current between current flowing in the positive line of string  8  and the current flowing in the negative line of string  8 . A differential current, which may be greater than a predetermined value, may be typically indicative of leakage current to electrical earth owing to fault in a string  8  or power lines connected to string  8 . Also, a differential current, which may be greater than a predetermined value, may be provided by test circuit  414  so as to ensure that measurement step  407  may be functioning correctly as part of periodic test function. The level of differential current may be measured when the isolation between nodes A and B/X and Y may be ON, in a normal mode of operation. During the normal mode of operation, current in the positive line may be the sum of current flowing through switch contacts S 1 , S 2  and current flowing through Q 1  and D 1  and Q 2  and D 2 . During the normal mode of operation, current in the negative line may be the sum of current flowing through switch contacts S 3 , S 4  and current flowing through Q 3  and D 3  and Q 4  and D 4 . The lower ON resistance of switches S 1 -S 4  means that the current going through switches S 1 -S 4  may be much greater than the current going through IGBTs Q 1 -Q 2  and diodes D 1 -D 2 . During the normal mode of operation, the level of differential current between current flowing in the positive line of string  8  and the current flowing in the negative line of string  8  may be substantially zero and/or less than 20 milliamperes. In decision box  409  if the modulus of the level of differential current may be substantially zero and/or less than 20 milliamperes, monitoring of the differential current continues with step  407 . In decision box  409  if the modulus of the level of differential current may be greater than a predetermined value (typically greater than 20 milliamperes.), relays RR 1 -RR 4  may be switched OFF (step  411 ) thereby opening switches S 1 -S 4 . The opening of switches S 1 -S 4  substantially increases the current IGBTs Q 1 -Q 4  and diodes D 1 -D 4 , which may be still ON. Substantial increase in the current through IGBTs Q 1 -Q 4  and diodes D 1 -D 4  means that the opening of switches S 1 -S 4  in step  411  allows for minimized arcing of switches S 1 -S 4 . After switches S 1 -S 4  may be opened, IGBTs Q 1 -Q 4  may be turned OFF (step  413 ). 
         [0045]    In decision  415 , a check may be made to see if IGBTs Q 1 -Q 4  and switches S 1 -S 4  may be indeed turned OFF by measuring the voltages across nodes A, B and nodes X, Y. The voltages across nodes A, B and nodes X, Y may be measured by voltage sensor V 1  and voltage sensor V 2  respectively. Voltage sensor V 1  provides output  406  and voltage sensor V 2  provides output  408 . Additionally current and voltage sensing from outputs  410 ,  412  and RCD 1  may be used to see if IGBTs Q 1 -Q 4  and switches S 1 -S 4  may be indeed turned OFF. If IGBTs Q 1 -Q 4  and switches S 1 -S 4  may be indeed turned OFF a disconnected status for string may be initiated (step  419 ), otherwise an alarm or indication of a fault may be made with step  417 . 
         [0046]    Connection of a string  8 , for example when multiple strings  8  begin to generate DC power, has IGBTs Q 1 -Q 4  and switches S 1 -S 4  initially turned OFF. First, IGBTs Q 1 -Q 4  may be turned on, followed by switches S 1 -S 4  being closed. Turning on IGBTs Q 1 -Q 4  first before switches S 1 -S 4  being closed prevents arcing of switches S 1 -S 4 . During a normal operation of a string  8 , the lower ON resistance of switches S 1 -S 4  means that the current going through switches S 1 -S 4  may be much greater than the current going through IGBTs Q 1 -Q 2  and diodes D 1 -D 2 . 
         [0047]    The term “comprising” as used herein, refers to an open group of elements for example, comprising an element A and an element B means including one or more of element A and one or more of element B and other elements other than element A and element B may be included. 
         [0048]    The terms “sensing” and “measuring” are used herein interchangeably. 
         [0049]    The indefinite articles “a”, “an” is used herein, such as “a string”, “a switch” have the meaning of “one or more” that is “one or more strings or “one or more switches.” 
         [0050]    Examples of various features/components/operations have been provided to facilitate understanding of various embodiments. In addition, various preferences have been discussed to facilitate understanding of the disclosed aspects. It is to be understood that all examples and preferences disclosed herein are intended to be non-limiting. 
         [0051]    Although selected aspects have been shown and described individually, it is to be understood that at least aspects of the described aspects may be combined. Also, although selected aspects have been shown and described, it is to be understood that other embodiments are not so limited. Instead, it is to be appreciated that changes may be made to these aspects without departing from the principles and spirit of the disclosure.