Patent Publication Number: US-2007107767-A1

Title: DC power-generation system and integral control apparatus therefor

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to the field of direct-current power generation. More specifically, the present invention relates to the field of direct-current power-generation systems utilizing arrays of power-generation cells.  
     BACKGROUND OF THE INVENTION  
       FIG. 1  shows a prior-art direct-current (DC) solar power-generation system  10  in basic form. A solar generating station (not shown) may contain many such systems, effectively coupled in parallel, to produce the desired power.  
      The system is made up of a DC power-generation solar array  11  arranged as a plurality of strings  12 , with each string typically containing a multiplicity of series-connected DC power-generation solar cells (not shown). A given string is therefore a “string” of cells.  
      Each string has a positive string output  13  and a negative string output  14 . All positive string outputs electrically couple to a positive current summing bus  15 , and all negative string outputs electrically couple to a negative current summing bus  16 .  
      The solar cells making up a given string are electrically in series. Each string therefore has a string current that is substantially equal to a current through each solar cell in that string, and a string voltage that is substantially equal to a sum of the voltages of each of the solar cells in that string. The positive and negative summing buses place all strings in the array in parallel. The array, and the system, therefore has an array current that is substantially equal to a sum of all the string currents, and an array voltage substantially equal to an average of the string voltages. A positive array output  17  is taken from the positive summing bus, and a negative array output  18  is taken from the negative summing bus.  
      In some cases, it is desirable to include protection, monitoring, and/or connection control in the system. This may be accomplished through the insertion of several discrete components into the system. In the system of  FIG. 1 , for example a fuse  19 , a monitoring circuit  20 , and a switching circuit  21  have been inserted as discrete components and coupled between each string and the summing buses. The positive string output of each string is shown electrically coupled to the fuse by a first interconnect  22 . The fuse is shown electrically coupled to the monitor circuit by a second interconnect  23 . The monitor circuit is shown electrically coupled to the switching circuit by a third interconnect  24 . The negative string output is shown electrically coupled to the negative summing bus by a fifth interconnect  26 . In addition, a switching circuit typically requires a negative connection to the associated string. The switching circuit is therefore shown electrically coupled to the negative summing bus by a sixth interconnect  27 .  
      There are, however, several problems in the implementation of this system. One of these problems is the number of interconnects involved, which can fail in several ways.  
      Interconnects are cables or wires that must reliably carry a full string current, and that desirably have a low internal resistance to minimize power losses. In the system of  FIG. 1 , there are six such interconnects per string. In an array of fifteen strings, for example, there would be ninety interconnects that must be routed, installed, and maintained. Each interconnect has two connection points, one at each end, that each pose a risk of failure do to poor connections initially (installation problems) or over time due to thermal expansion and contraction, vibration, corrosion, etc. Each of these connection points is therefore a potential point of failure. In point of fact, these connection-point failures may be more likely in an average installation than is a failure of a solar cell within the array.  
      An interconnect connection may become disconnected. Should this occur, the relevant string would be electrically removed from the array. Besides the obvious potential loss of energy involved, the disconnected end of the interconnect may contact another component of the system, thereby establishing a short circuit. This short circuit may cause a failure of a string, of the solar array, or, in extreme cases, of the solar generating station itself. Such a short circuit may cause localized dissipation of high energy. This may lead to the production of excessive heat and potentially result in fire.  
      An interconnect connection may become intermittent. Such an intermittent connection may significantly affect the capacity of array, and may produce electrical noise that may adversely affect other components of the solar generation station, e.g., inverters, computers, controllers, etc.  
      An interconnect connection may become corroded or otherwise suffer an increase in the connection resistance. This may result in a decrease in the output of a string, with a corresponding decrease in the capacity of the array. Corrosion is pervasive. Where one connection has corroded, other connections are likely to be corroding. This pervasive nature of corrosion may lead to a failure in a surprisingly short time.  
      In addition, connections that suffer increased resistance may produce a localized energy dissipation, resulting in excessive heat and a marked risk of fire.  
      The issue of expense in connection with the conversion of energy from solar and other renewal energy resources is worthy of attention. There is a strong need to make solar power generation as cost effective as possible. While the ongoing costs of solar and other renewable-resource DC power-generation stations can be lower than for other forms of power generation, the up-front costs are typically so great that solar and other forms of DC power generation from so-called renewable resources have yet to become a viable alternative. Accordingly, system architectures, construction techniques, and materials that contribute to the excessive up-front costs of such generation systems are particularly troublesome and in need of improvement so that up-front costs may be lowered and renewable energy sources may become more competitive with non-renewable energy sources.  
      But the interconnection schema of conventional solar power generation arrays contributes to the excessive up-front costs. This is especially true if electronic monitoring and/or connection control is desired. During the assembly of the system, components and interconnects are conventionally mounted and all connections securely and correctly made at the installation site. This represents a significant expenditure of time, and a significant expense. Following assembly, the system must be thoroughly checked and tested for possible assembly error prior to being placed on line. The use of discrete components often results in complex and convoluted interconnect routing paths. The greater the number of interconnects and the more convoluted the routing paths, the greater the likelihood of error, and the greater the time, complexity, and expense of the final pre-activation check.  
      In addition to the undesirably high up-front costs, the conventional solar power generation interconnection schema also increases on-going costs. During routine maintenance and servicing, each connection point in the system should be inspected and serviced as required. The greater the number of interconnects, the greater the likelihood that a problem will develop, and the more complex such inspections become. This increase in complexity is reflected in a proportionate expenditure of time and money, in addition to a significant increase in risk to the inspecting personnel.  
      The diagnosis and correction of interconnect failures in a timely manner is therefore important to the proper operation of the system. This has been conventionally performed using a hands-on procedure, typically involving visual inspection of all components, the measurement of voltage drops across all connections, and the physical tightening of those connections. Because a solar generating station may contain hundred or even thousands of such systems, and because an interconnect failure may provide no overt evidence, such as a blown fuse, many hundreds or even thousands of such procedures must be performed on a routine basis in order to find and diagnose a single failure. Such diagnosis is time consuming and expensive. Because string voltages may be significant, even lethally so, such hands-on procedures are also inherently dangerous.  
      The fuse  19  (or other protective device) is desirably placed in series with each string to protect the system in the event of a short circuit, overload, or other failure.  
      The monitor circuit  20  may be placed in series with each string to more easily determine string currents. The monitor circuit may be implemented as a simple device to indicate when the string current is zero, or may be implemented as a device to indicate when the string current is outside of a predetermined range. This more sophisticated monitor circuit may be used to detect and diagnose multiple types of failure.  
      The switching circuit  21  may be placed in series with each string to control connection of that string. The switching circuit may be realized as a simple switch or relay to electrically disconnect a given string from the array. When that string is electrically removed, the string current falls to zero, and the potentially damaging effects of certain string failures are converted into those of a less endangering open-string failure.  
      The monitoring and switching circuits conventionally require signal interconnections (not shown). These interconnections, while desirably of lower voltages and currents than the higher-voltage strings, may significantly increase the complexity of overall assembly and maintenance of the system, thereby exacerbating the problems discussed.  
      What is needed, therefore, is a means of integrating monitoring and switching circuitry for a DC power-generation system. This means should desirably reduce the number of system interconnects and other wiring, and allow the assembly, testing, and diagnosis of the system in a manner that significantly reduces the time, costs, and dangers involved.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an advantage of the present invention that a DC power-generation system and integral control apparatus therefor is provided.  
      It is an advantage of a preferred embodiment of the present invention a DC power-generation system having a reduced assembly time is provided.  
      It is an advantage of a preferred embodiment of the present invention that a system having a minimum of discrete components is provided.  
      It is an advantage of a preferred embodiment of the present invention that a DC power-generation system having a minimum number of interconnects is provided.  
      It is an advantage of a preferred embodiment of the present invention that a DC power-generation system having automatic string disconnection is optionally provided.  
      It is an advantage of a preferred embodiment of the present invention that a DC power-generation system having optional local and optional remote monitoring and operational and diagnostic control is provided.  
      The above and other advantages of the present invention are carried out in one form by a DC power-generation array system. The system includes a DC power-generation array comprising N strings comprising M DC power-generation cells each, where N is an integer greater than 1, and where M is a positive integer, and an integral control apparatus having a common substrate. The integral control apparatus includes a summing bus, N string units, and a process unit, all to the common substrate. Each of the N string units is coupled between one of the N strings and the summing bus, configured to measure a string current through the one string, and configured to effect electrical connection of the one string to the summing bus. The process unit is coupled to each of the N string units, configured to evaluate the string current through the one string, and configured to control electrical connection of the one string by the string unit.  
      The above and other advantages of the present invention are carried out in another one form by an integral control apparatus for a direct-current (DC) power-generation array formed of N strings, where N is an integer greater than 1. The apparatus includes a common substrate, a string unit affixed to the common substrate, coupled to one of the N strings, configured to measure a string current through the one string, and configured to electrically switch the one string into and out of the array, and a process unit affixed to the common substrate, coupled to the string unit, and configured to cause the string unit to electrically switch the one string into and out of the array. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, wherein like reference numbers refer to similar items throughout the Figures, and:  
       FIG. 1  shows a prior art direct-current power-generation array system;  
       FIG. 2  shows a direct-current power-generation array for use with a preferred embodiment of the present invention;  
       FIG. 3  shows a direct-current power-generation system for the array of  FIG. 2  and incorporating an integral control apparatus in accordance with a preferred embodiment of the present invention; and  
       FIG. 4  shows a direct-current power-generation system for the array of  FIG. 2  wherein the integral control apparatus incorporates a common dynamic load in accordance with an alternative preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      This discussion presumes the use of a solar photovoltaic array, where the array consists of a plurality of strings of photovoltaic cells in series. It will be appreciated by those skilled in the art, however, that arrays of other “cellular” electrical generation components may be used. For example, in a thermovoltaic array, a “cell” may be a single thermocouple or thermophotovoltaic device, and a “string” may be a multitude of such devices in series, e.g., a thermopile. Alternatively, a cell might be a single voltaic cell, a single wind turbine, or the like.  
       FIG. 2  shows a direct-current (DC) power-generation array  32  in accordance with a preferred embodiment of the present invention.  FIG. 3  shows a DC power-generation system  30  incorporating array  32  and an integral control apparatus  34  therefor in accordance with a preferred embodiment of the present invention.  
      Array  32  is an array of N×M DC power-generation cells  36  arranged as N strings  38  of M cells  36  each, where N is an integer greater than 1, and M is a positive integer. Each cell  36  is configured by design to generate a cell current I C(X,Y)  at a cell voltage V C(X.Y) , where X is a string designator having an integral value of 1, 2, . . . , N indicating in which of the N strings  38  that cell  36  is located, and Y is a cell designator having an integral value of 1, 2, . . . , M indicating the position of that cell  36  within that string  38 .  
      The M cells  36  making up a given string  38  are electrically in series. It will be appreciated, however, that in some embodiments (not shown), a given string  38  may consist of a single cell  36  (i.e., a series of one).  
      A string current I S(X)  generated by a given string  38  passes through each cell  36  in that string  38 , therefore each string  38  has a string current I S(X) , where: 
 
 I   S(X)   =I   C(X,1)   =I   C(X,2)   = . . . =I   C(X,M) .   (1) 
 
 Each string  38  generates string current I S(X)  at a string voltage V S(X) . Since the cells  36  within a string  38  are in series, the string voltage V S(X)  is the sum of the cell voltages V C(X,Y)  within that string: 
 
 V   S(X)   =V   C(X,1)     30  V   C(X,2)   + . . . +V   C(X,M) .   (2) 
 
      Within array  32 , each string  38  has a pair of string outputs  40  and  42 , one for each polarity of string current I S(X)  generated by that string  38 . All string outputs  40  of a first polarity are electrically coupled to a current summing bus  44  of that first polarity, and all string outputs  42  of a second polarity are electrically coupled to a current summing bus  46  of that second polarity. Therefore, the N strings  38  making up array  32  are electrically in parallel.  
      A first apparatus output  48  is taken from summing bus  44  for the first polarity, and a second apparatus output  50  is taken from summing bus  46  for the second polarity. Since the N strings  38  making up array  32  are in parallel, array  32  generates an array current I A  that is the sum of the string current I S(X)  for each of the N strings  38 : 
 
 I   A   =I   S(1)   +I   S(2)   + . . . +I   S(N) .   (3) 
 
 Array  32  generates array current I A  at an array voltage V A . Array voltage V A  is impressed across each of the N strings  38 . Therefore, array voltage V A  is substantially equal to the string voltage V S(X)  across each of the N strings  38 : 
 
 V   A   =V   S(1)   =V   S(2)   = . . . =V   S(N) .   (4) 
 
      For the sake of convention, this document shall hereinafter presume that the first polarity is positive and the second polarity is negative. It will be appreciated by those skilled in the art that this is not a requirement of the present invention. In any given embodiment, the positive and negative polarities may be reversed without departing from the spirit of the present invention.  
      Integral control apparatus  34  is made up of N string units  52 , summing buses  44  and  46 , and a process unit  54 , wherein one string unit  52  is coupled between each of the N strings  38  of array  32  and summing buses  44  and  46 .  
      In the preferred embodiment, a positive interconnect  56  electrically couples a positive string output  40  of each string  38  to a positive apparatus input  58  of integral control apparatus  34 . Similarly, a negative interconnect  60  electrically couples a negative string output  42  of each string  38  to a negative apparatus input  62  of integral control apparatus  34 .  
      String current I C(X)  for a string X  38  passes from positive string output  40 , through positive interconnect  56 , through positive apparatus input  58 , though an associated string unit  52 , and to positive summing bus  44 . In positive summing bus  44 , string currents I C(1) , . . . , I C(N)  from all N strings  38  are summed to become array current I A . Array current I A  passes from positive summing bus  44  to positive apparatus output  48 , and thence from system  30 . Similarly, array current I A  returns into system  30  at negative apparatus output  50  and is passed to negative summing bus  46 . In negative summing bus  46 , returning array current I A  is divided into string currents I S(1) , . . . , I S(N)  for each of the N strings  38 . For string X  38 , string current I S(X)  is passed from negative summing bus  46  to negative apparatus input  62 , through negative interconnect  60 , and thence to negative string output  42  of string X  38 .  
      Integral control apparatus  34  is integral. That is, all components of integral control apparatus  34  are mounted to and/or upon a common substrate  35 . Common substrate  35 , in the preferred embodiment, is desirably a printed wiring board formed in the preferred embodiment of a non-conductive base substrate upon which are formed a multiplicity of conductive traces using printing and etching techniques well known to those skilled in the art. The conductive traces serve, in lieu of wires, to electrically couple components of string units  52  and process unit  54 , summing buses  44  and  46 , and any other electrically coupled components affixed to common substrate  35 .  
      By being integral, integral control apparatus  34  is desirably prefabricated prior to installation in system  30 . This greatly decreases the possibility of assembly error within integral control apparatus  34  itself, and also significantly decreases the assembly costs of integral control assembly  34 .  
      Also, by being integral, integrated control apparatus  34  may be mounted into system  30  as a single unit. This significantly decreases the assembly time of system  30  in the field, and the costs associated therewith. Being prefabricated and integrated, integral control apparatus  34  may benefit from economies of scale and may be manufactured in large numbers at a low per-unit cost. Moreover, integral control assembly  34  may benefit from conventional low-cost production-line quality assurance techniques. Such techniques nearly guarantee that integral control apparatus  34  will work reliability when initially installed in system  30  in the field. Also, by being integrated, in the rare situation where a given integral control apparatus  34  may suffer a failure, the entire integral control apparatus  34  may be quickly, easily, and reliably replaced at low cost and without great and expensive skill in troubleshooting. Moreover, the level of skill of the installer need not be as great as with conventional solutions, leading to still further savings.  
      Interconnects  56  and  60  are essentially wires or cables that connect string outputs  40  and  42  to apparatus inputs  58  and  62 . The connections of interconnects  56  and  60  are potential sources of failure. Therefore, the fewer interconnects  56  and  60  in system  30 , the less likelihood there is of connection failure. The embodiment of system  30 , as depicted in  FIG. 3  and incorporating integral control apparatus  34 , uses 2N interconnects  56  and  60  for an array  32  having N strings  38 , having one positive interconnect  56  and one negative interconnect  60  for each string  38 . This represents a significant reduction over the prior-art DC power-generation system  10  of comparable functionality depicted in  FIG. 1 , which has 6N interconnects  22 ,  23 ,  24 ,  25 ,  26 , and  27 . For example, for comparable preferred-embodiment and prior-art systems  30  and  10 , each having an array  32  and  11  of fifteen strings  38  and  12 , the preferred-embodiment system  30  has thirty interconnects  56  and  60 , while the prior-art system  10  has ninety interconnects  22 ,  23 ,  24 ,  25 ,  26 , and  27 .  
      All interconnects  56  and  60  are routed between appropriate string outputs  40  and  42  and apparatus inputs  58  and  62 . Desirably, all apparatus inputs  58  and  62  are mounted in an input terminal array  64 . Input terminal array  64  may then be positioned on common substrate  35  of integral control apparatus  34  so as to minimize the routing of interconnects  56  and  60 , thereby significantly decreasing the potential for short circuits and other problems in the event of a connection failure.  
      The use of input terminal array  64  also serves to reduce the assembly time of system  30 , thereby realizing a significant further reduction in assembly costs.  
      All component interconnections of integral control assembly  34  may be realized as traces upon common substrate  35  thereof. As traces, the possibilities of connection failure, potential short circuits, and other problems are minimized.  
      In order to accommodate array current I A , i.e., the sum of all string currents I S(1) , . . . , I S(N) , it is desirable that summing buses  44  and  46  be realized as physical bus bars  66 . Preferably, bus bars  66  are affixed to a trace on common substrate  35  of integral control assembly  34  by sweat soldering or similar technique. Those skilled in the art will appreciate, however, that this is not a requirement of the present invention. Other methods of affixing bus bars  66  to integral control assembly  34  may be used without departing from the spirit of the present invention.  
      Desirably, apparatus outputs  48  and  50  are mounted in an output terminal array  68 . Output terminal array  68  may then be positioned on common substrate  35  of integral control apparatus  34  so as to facilitate the routing of output cables (not shown) of system  30 .  
      Each string unit  52  is electrically affixed within integral control apparatus between apparatus inputs  58  and  62  and summing buses  44  and  46  for each of the N strings  38 . Each string unit  52  is made up of a fuse  70 , a monitor module  72 , and an optional switching module  74 . Those skilled in the art will appreciate that the relative positions of fuse  70 , monitor module  72 , and optional switching module  74  are not a requirement of the present invention, and that other relative positions remain within the spirit of the present invention. For example, in some embodiments, it may be desirable to place fuse  70  last in sequence, proximate summing busses  44  and  46 , as this would allow fuse  70  to be used as a “switch” to disconnect string unit  52  and associated string  38  from array for diagnostics and troubleshooting.  
      Fuse  70  is electrically coupled to the positive apparatus input  58  associated with a given string X  38 . A purpose of fuse  70  is to protect array  32 , system  30 , and any power generating station (not shown) of which system  30  is a part, from a failure of string X  38 . Another purpose of fuse  70  is to protect string X  38 , and the M cells  36  within string X  38 , from damage due to excessive string current I S(X) . Fuse  70  provides such protection by blowing or tripping in the event of overcurrent, thereby disconnecting that string  38  from array  32 . Those skilled in the art will appreciate that fuse  70  may be realized as a fuse, circuit breaker, or other like protective device without departing from the spirit of the present invention.  
      Monitor module  72  is electrically coupled to fuse  70 . It is a purpose of monitor module  72  to measure string current I S(X)  of string X  38 . In the preferred embodiment, monitor module  72  contains a predetermined monitor resistance  76  in series with fuse  70 . Monitor resistance  76  may be realized as a distinct resistor, as the resistance of a constriction within a trace on common substrate  35  of integral control apparatus  34 , or as the known resistance of a specified length of such a trace.  
      String current I S(X)  passes through monitor resistance  76 . In the preferred embodiment, monitor module  72  measures string current I S(X)  by ascertaining the voltage drop across monitor resistance  76 . A value of string current I S(X)  is passed to process unit  54  via a current data bus  78  (i.e., a collection of current-data conductors) extending along common substrate  35  of integral control apparatus  34 . Those skilled in the art will appreciate that monitor resistance  76  may be quite small to minimize power losses, and that other methodologies may be used to measure string current I S(X)  without departing from the spirit of the present invention.  
      In some embodiments, monitor module  72  may also be configured to measure string voltage V S(X)  of string X  38 , as depicted in  FIG. 3 . When string X  38  is disconnected from array  32  (discussed hereinafter), the measuring of string voltage V S(X)  becomes a valuable diagnostic too for the diagnosis of potential problems within string X  38 .  
      In the embodiment shown in  FIG. 3 , a value of string voltage V S(X)  is passed to process unit  54  via a voltage bus  80  (i.e., a collection of voltage-data conductors) extending along common substrate  35  of integral control apparatus  34 . Those skilled in the art will appreciate that other methodologies may be used to measure string voltage V S(X)  without departing from the spirit of the present invention.  
      Optionally, switching module  74  may be electrically coupled to monitor module  72  and positive summing bus  44 , as depicted in  FIG. 3 . Switching module  74  is configured to electrically switch string X  38  into and out of array  32 . Switching module  74  contains a switch  82  capable of electrically coupling and decoupling string  38  from array  32 , i.e., to effect connection and disconnection of string  38  from summing bus  44 .  
      In its simplest form, switch  82  may be a simple single-pole, single-throw switch or relay serving only to connect and disconnect string  38  from array  32 . In the embodiment of  FIG. 3 , switch  82  is realized as a single-pole, double-throw, center-off, switch or relay, and possesses an “on” position, depicted in “String Unit  1 ” in  FIG. 3 , an “off” position, depicted in “String Unit  2 ” in  FIG. 3 , and a “load” position, depicted in “String Unit N” in  FIG. 3 . In the “on” position, switch  82  electrically couples string  38  into array  32 . In the “off” position, switch  82  electrically decouples string  38  from array  32 , i.e., string  38  is turned off. In the “load” position, switch  82  decouples string  38  from array  32  and couples string  38  to a dynamic load  84 . Dynamic load  84  is configured to provide a load for string  38  that may vary string current I S(X)  from zero to a maximum allowable for string X  38 .  
      String voltage V S(X) , as measured by monitor module  72  while switch  82  is in the “load” position and string  38  is coupled to dynamic load  84 , is independent of array voltage A V . The use of dynamic load  84  allows string voltage V S(X)  to be determined for any string current I S(X)  from zero to a maximum value. String voltage V S(X)  then serves as a valuable diagnostic tool for string X  38 .  
      Switch  82  and dynamic load  84  are under the control of process unit  54  (discussed hereinafter). Control signals are passed from process unit  54  to switching module  74  via a switching bus  86  (i.e., a collection of switching-data conductors) extending along common substrate  35  of integral control apparatus  34 .  
       FIG. 4  shows system  30  wherein integral control apparatus  34  incorporates a common dynamic load  85  in accordance with an alternative preferred embodiment of the present invention. The following discussion refers to  FIGS. 2, 3 , and  4 .  
      In  FIG. 3 , each switching module  74  incorporates a separate dynamic load  84 . In the  FIG. 3  embodiment, multiple strings  38  may be coupled to dynamic loads  84  substantially simultaneously. However, since each string  38  may pass a significant string current IS (X)  at a significant string voltage VS (X) , each dynamic load may be required to dissipate a considerable amount of power. This typically necessitates the use of heat sinks or other bulky devices. The physical inclusion of these devices, for each of the N strings  38 , may add considerably to the size, weight, and complexity of integral control apparatus  34 .  
      In the alternative embodiment of  FIG. 4 , the use of N dynamic loads  84  has been replaced by the use of a common dynamic load  85 . Common dynamic load  85  is “multiplexed” among switching modules  74  by process unit  54  (discussed hereinafter). Through the use of common dynamic load  85 , only one heat sink or other heat-dissipating device need be incorporated into integral control assembly  34 . Since common dynamic load  85  is multiplexed among switching modules  74 , the current and voltage through common dynamic load  85  never exceeds the current IS (X)  and voltage VS (X)  of a single string  38 . The use of the common-load or “multiplex” embodiment of  FIG. 4  therefore reduces the size, weight, and complexity of integral control apparatus  34  over the multiple-load embodiment of  FIG. 3 .  
      Those skilled in the art will appreciate that other methodologies may be used to control switch  82  and/or dynamic loads  84  and/or  85 , e.g., an embodiment (not shown) having multiple common dynamic loads  85 , without departing from the spirit of the present invention.  
      When a given string X  38  is coupled to dynamic load  84  ( FIG. 3 ) or  85  ( FIG. 4 ), it is desirable for string current I S(X)  to return to that string  38 , i.e., there needs be a complete circuit. This is accomplished by having a trace along common substrate  35  of integral control apparatus  34  electrically connect a return leg of each dynamic load  84  or  85  to negative apparatus input  62  of integral control apparatus  34 . Negative apparatus input  62  is coupled by negative interconnect  56  to negative string output  42  of string X  38 .  
      Integral control apparatus  34  also includes process unit  54 . Process unit  54  is coupled to each of the N string units  52 . For a given string unit  52 , process unit  54  is coupled to and receives a value of string current I S(X)  from monitor module  72  via current bus  78 . In the preferred embodiment, the value of string current I S(X)  through each string  38  is digitized by an analog-to-digital (A/D) converter  88  and passed to a processor  90 . Processor  90  may determine array current I A  as the sum of all string currents I S(1) , . . . , I S(N) . Processor  90  may then determine if a given string current I S(X)  is too low or too high, i.e., not within a predetermined range.  
      In a similar manner, for the embodiment depicted in  FIG. 3 , process unit  54  is coupled to and receives a value of string voltage V S(X)  from monitor module  72  via voltage bus  80  when switching module  74  has coupled string  38  to dynamic load  84 . In the preferred embodiment, the value of string voltage V S(X)  across that string  38  is digitized by A/D converter  88 , as is a value of array voltage A V  derived from summing buses  44  and  46 , and passed to a processor  90 . Processor  90  may determine if that string voltage V S(X)  is too low or too high relative to array voltage A V .  
      In the embodiment depicted in  FIG. 4 , voltage bus  80  is omitted but string voltage V S(X)  for any string switched to dynamic load  85  is routed to A/D converter  88  for measurement and subsequent processing in processor  90 .  
      When optional switching modules  74  are used, process unit  54  is coupled to each switching module  74  via switching bus  86 . Processor  90  sends instructions to switching modules  74  controlling the throw of switch  82  and determining the value of dynamic load  85 . These instructions may but need not be issued automatically. For example, under control of a suitable program, processor  90  may determine string currents I S(1) , . . . , I S(N)  for each of the N strings  38 , determine array current I A , then instruct a given switching module  74  to disconnect the associated string  38  from array  32  when string current I S(X)  for that string  38  is outside a given range relative to array current I A .  
      In a similar scenario, processor  90  may, under control of a suitable program, determine array voltage V A , cyclically instruct switching modules  74  to switch each string  38  in turn to dynamic load  84  and set dynamic load  84  to an appropriate value, and determine string voltage V S(X)  for each string  38 . Strings  38  having a string voltage V S(X)  not within a predefined range relative to array voltage V A  may be kept disconnected.  
      Those skilled in the art will appreciate that integral control apparatus  34  may also monitor and base decisions upon other parameters not depicted herein, e.g., temperature. The monitoring and utilization of these other parameters does not depart from the spirit of the present invention.  
      Integral control apparatus  34  may also comprise an optional interface unit  92  configured to allow electronic access to processor  90 , and hence to integral control apparatus  34 , for an operator in the field. Interface unit  92  includes a display module  94  and a selector module  96 . Display module  94  is configured to display the status of system  30  to the operator. Selector module  96  allows the operator to select what is to be displayed upon display module  94 , and to control operation of processor  90  and switching modules  74 .  
      For example, through the use of interface unit  92 , the operator may take control of processor  90  and perform diagnostic checks of system  30  without the necessity of physically probing into the circuitry. This provides for a significant lessening of the time require for field diagnostics, thereby lowering service costs and increasing efficiency. This also significantly decreases the danger of accidental damage to the equipment or injury to the operator.  
      Desirably, process unit  54  also includes an optional data input/output (I/O) module  98 . Data I/O module  98  is configured to be connected to a remote location via RS-232 or other data link well known to those of ordinary skill in the art. Through the use of data I/O module  98 , all the functionality of interface unit  92  may be realized remotely. This minimizes the number of physical trips into the field that must be taken for diagnostics, thereby further reducing costs.  
      In addition, the use of data I/O module  98  allows processor  90  to report the status of all strings  38  automatically or upon demand, and provides for notification of string disconnection. This allows diagnosis and repair to occur in a timely manner and increases the overall efficiency of system  30 .  
      Those skilled in the art will appreciate that neither interface unit  92  nor data I/O module  98  is a requirement of the present invention. Embodiments lacking either interface unit  92  or data I/O module  98  may be realized without departing from the spirit of the present invention.  
      Desirably, data I/O module is electrically isolated from the voltages and currents present in integral control apparatus by galvanic isolator  97 . Galvanic isolator  97  serves to protect equipment at the remote location and any intervening locations from damage by voltages that may be propagated due to a failure of or damage to integral control apparatus  34 . More importantly, galvanic isolator  97  protects personnel from injury or death that may occur with exposure to such voltages. Those skilled in the art will appreciate that, while highly desirable, the inclusion of galvanic isolator  97  is not a requirement of the present invention. Exclusion of galvanic isolator  97  does not depart from the spirit of the present invention.  
      In summary, the present invention provides a DC power-generation system  30  and integral control apparatus  34  therefor. System  30  has a minimum number of discrete components and a minimum number of inter-component interconnects  56  and  60 , resulting in reduced assembly and diagnostic times and costs, and a marked increase in operator safety. System  30  provides for optional automatic disconnection of defective strings  38  within a DC power-generation array  32 , and both optional local and optional remote monitoring and control of system diagnostics and operation.  
      Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.