Abstract:
An electrical power converter for converting power from a bipolar DC source to supply an AC load is disclosed. For one such embodiment the bipolar DC source is a photovoltaic array and the AC power is sourced into an electric power grid. The bipolar photovoltaic array has positive and negative voltage potentials with respect to earth ground. The converter is a utility interactive inverter which does not require an isolation transformer at the electric power grid interface. Embodiments of the invention include methods of detecting and interrupting DC ground faults in the photovoltaic array.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electrical power converters and, more specifically, to a photovoltaic bipolar-to-monopolar source converter used in conjunction with other equipment to supply AC loads. 
     BACKGROUND OF THE INVENTION 
     In the United States, two solar photovoltaic (PV) array configurations, grounded and ungrounded, are permitted by the National Electric Code (NEC), Section 690. The maximum voltage of a PV array is currently limited to 600 Vdc with respect to earth in grounded systems and 600 Vdc in ungrounded systems because of PV module insulation limitations. The NEC also requires that PV systems installed on dwellings have a means of detecting and interrupting fault currents from the PV array to earth ground. These faults are commonly caused by water intrusion into wiring junction boxes, degradation of the array wiring insulation, or a failure in the solar module insulating materials. Such faults can cause a low energy leakage path or a destructive direct current arc. The intent of the code, with respect to ground faults, is fire protection, not personnel protection. 
     SUMMARY 
     One embodiment provides an apparatus for selectively coupling and de-coupling at least two monopolar DC sources to and from an earth ground and positive and negative terminals of a monopolar load. A bipolar DC source that includes at least two monopolar DC sources is controllably coupled series aiding with a common connection point to earth ground, a positive connection to a positive monopolar load terminal, and a negative connection to a negative monopolar load terminal, with the stated polarities referenced to said earth ground. A frequency-selective network connected between the common connection point and the earth ground has a DC impedance that is lower than the AC impedance of the network at a preselected frequency, such as an integral multiple of the utility line frequency. 
     In one implementation, the DC impedance of the frequency-selective network is low enough to hold the common ground connection at substantially ground potential as required by the National Electric Code, and the AC impedance is low enough to prevent the common mode potential of the bipolar array from being raised above earth ground at lightning transient frequencies. 
     A DC-to-AC converter may be coupled to the bipolar DC source for converting a DC output of that source to an AC output, and the frequency-selective network may be a parallel RLC circuit having a resonant frequency that is about three times the frequency of the AC output. Specifically, the frequency-selective network may be a parallel RLC circuit in which the R, L and C values provide a low DC impedance, a maximum AC impedance at a resonant frequency that is about three times the frequency of the AC output, and a lower AC impedance at frequencies higher than the resonant frequency. The frequency-selective network preferably allows the common connection point to operate with an impressed common mode AC voltage with respect to earth ground, with an AC current to ground that is lower than the AC current in a direct connection of the common connection point to earth ground. 
     One embodiment includes a ground fault detector coupled to each of the monopolar DC sources to produce a ground fault signal when a ground fault occurs, and a controller responsive to the ground fault signal for de-coupling the faulted monopolar DC source from the common connection point. Any unfaulted monopolar DC source preferably floats with the highest voltage at the poles of any unfaulted monopolar DC source equal to ±½ the open circuit voltage of the unfaulted monopolar DC source with respect to the earth ground, when a faulted monopolar DC source is de-coupled from the common connection point. 
     Other features and advantages of embodiments of the present invention will be apparent from the accompanying drawings, and from the detailed description, that follows below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood from the following description of preferred embodiments together with reference to the accompanying drawings, in which: 
         FIG. 1  is an electrical schematic of a bipolar DC power source coupled to earth ground and to a utility grid via a DC-to-AC converter. 
         FIG. 2  is an electrical schematic of one embodiment of the system of  FIG. 1 , with the DC-to-AC converter modeled as a variable load. 
         FIG. 3  is an electrical schematic of another embodiment of the system of  FIG. 1 , with the DC-to-AC converter modeled as a variable load. 
     
    
    
     DETAILED DESCRIPTION 
     Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims. 
       FIG. 1  illustrates a system configured with a bipolar DC source that includes two monopolar PV subarrays  10  and  20  (e.g., maximum 600 volts each), and a three-phase DC-to-AC power converter  100  operating into a grounded-Wye AC utility service  300  (e.g., 480/277-volt, 60 Hz) that includes three phases  301 - 303  connected to earth ground  70  through a common neutral line  304 . The DC-to-AC converter  100  may be a conventional 6-pole bridge that includes six transistor/diode switches and three filter inductors connected to the three phases  301 - 303  of the utility service. Since the connection at the utility grid  300  is a four-wire, grounded-Wye configuration, and the DC source is ground referenced as well, each of the three phases operates independently. Control and regulation methodologies for utility grid interactive inverters are well known. 
     The positive terminal of the first PV subarray  10  and the negative terminal of the second PV subarray  20  are connected to the DC-to-AC converter  100 . The other terminals of the subarrays  10  and  20  are connected to earth ground  70  through a frequency-selective RLC network  8  for grounding the photovoltaic arrays through a network that provides a level of DC system protection equivalent to a solidly grounded system and also allows the PV arrays to move with common mode AC voltages. In the illustrative system, the RLC network  8  is formed by an inductor  8 A, a resistor  8 B and a capacitor  8 C connected in parallel. The parallel RLC network  8  has an AC impedance that is a maximum at a resonant frequency, and decreases at frequencies above the resonant frequency. The values of the components  8 A- 8 C are preferably selected to provide a resonant frequency that is about three times the line frequency of the AC power to be supplied to an AC load (e.g., a resonant frequency of 180 Hz for a 60-Hz line frequency) and a DC impedance that is lower than the AC impedance of the network at its resonant frequency. Specifically, the DC impedance of the parallel RLC network  8  is preferably low enough to hold the potential at the terminal  9  at substantially ground potential as required by the National Electric Code for bipolar photovoltaic arrays. The AC impedance at the resonant frequency is preferably low enough to prevent the common mode potential of the bipolar array from being raised above earth ground at lightning transient frequencies. 
     In one example using an inductor  8 A of 656 millihenries, a resistor  8 B of 371 ohms and a capacitor  8 C of 1.2 microfarads, the voltage between earth ground  70  and the RLC network  8  is about 37 Vac rms at 180 Hz, under nominal operating conditions when the power converter  100  is sourcing power into the utility grid  300 . The DC voltage component to ground is zero. The current flowing in the neutral conductor  304  is about 200 milliamperes at a frequency of 180 hertz. Power dissipation in the resistor  8 B is about 4 watts. 
     For the example shown in  FIG. 1 , the DC grounding resistance is effectively the DC resistance of the inductor  8 A, which can be less than one ohm. The AC grounding impedance is 186 ohms at 180 Hz and much lower at frequencies above 180 Hz to provide a low-impedance return path for lightning induced transients, which have frequencies substantially higher than 180 Hz. Thus, the RLC network  8  provides a frequency selective network that provides a level of system protection equivalent to that of a solidly grounded bipolar PV array during normal operation. 
       FIG. 2  is a more detailed schematic diagram of one implementation of the system of  FIG. 1 , but with the conventional DC-to-AC converter modeled as a variable load  90  and a parallel capacitor  80 . In normal operation, the monopolar PV arrays  10  and  20  are connected to earth ground  50  through a pair of indicating fuses  6 A and  7 A and a frequency-selective RLC network  8 . The negative pole of the subarray  10 , at terminal  12 , and the positive pole of the subarray  20 , at terminal  21 , are ground referenced in this way. The currents through the fuses  6 A and  7 A are effectively zero during normal operation. 
     When a fault to ground occurs in either PV array  10  or  20  and produces a DC fault current large enough to clear either of the fuses  6 A,  7 A, the clearing of the fuse interrupts the ground fault current. At the same time, a blown-fuse indicator signal is sent to a controller  1  by the closing of the switch  6 B or  7 B associated with the cleared fuse, i.e., the indicating fuses serve as a ground fault detectors that produce ground fault signals when a ground fault occurs, in addition to interrupting the ground fault current. The blown-fuse indicator signal causes the controller  1  to de-energize a contactor coil  2 A to open contact  2 B, so that the faulted PV subarray is then connected to earth ground only through the ground fault impedance and one of the equal-valued resistor networks formed by respective resistor pairs  16 ,  17  and  26 ,  27 . During this fault mode of operation, any un-faulted subarrays will “float” with the highest voltages at the subarray poles equal to ±½ the subarray open circuit voltage with respect to the earth ground  70 . The resistor networks  16 ,  17  and  26 ,  27  provide a minimally dissipative common mode voltage reference and are used to bleed off subarray static charges. 
     From a cost standpoint, it is desirable to use PV modules, wiring and fuses just below the maximum voltage permitted for a given class of equipment. For an optimum bipolar array, therefore, the highest DC voltage with respect to ground at terminals  11  and  22 , under all conditions, is the rated DC equipment voltage. With soft or resistively grounded bipolar PV arrays, the occurrence of a hard (low impedance) DC ground fault at terminal  11 , for example, reduces the voltage at that terminal with respect to ground to zero, which means the voltage at terminal  22  with respect to ground is twice the allowable equipment voltage because the impedance of the fault may be much lower than the impedance of the resistive ground. To alleviate this problem, the illustrative system monitors the voltage, with respect to ground, on all the PV subarray terminals  11 ,  12  and  21 ,  22  with voltage sensors  18 ,  19  and  28 ,  29  across the respective resistors  16 ,  17  and  26 ,  27 . The controller  1  reads scaled voltage signals from the voltage sensors  18 ,  19  and  28 ,  29  and compares these values to preprogrammed overvoltage limits. If the limit is exceeded on any terminal, both PV subarrays  10  and  20  are disabled and “floated.” The disabling sequence works as follows: 
     1. The voltage with respect to ground on at least one of the terminals  11 ,  12 , and  21 ,  22  exceeds the preprogrammed limit for that terminal. 
     2. The controller  1  commands a load  100  (e.g., a DC-to-AC converter) to shut down, via an isolated serial link  101 , thereby effectively setting the resistive portion of the load  100  to an open circuit. 
     3. Concurrently with the load shutdown, a contactor coil  3 A is de-energized to open contacts  3 B and  3 C. 
     4. After a delay to ensure that the contacts are fully open, a pair of current sensors  4  and  5  are read to verify that the load current commutation is complete. 
     The PV subarrays  10  and  20  are also disabled if the ground current read by a current sensor  9  exceeds a preprogrammed limit. In either case, steps  2  through  4  of the disabling sequence are executed. 
       FIG. 3  is a schematic diagram of another implementation of the system of  FIG. 1 , again with the conventional DC-to-AC converter modeled as a variable load. This system is the same as that shown in  FIG. 2  except that the DC contactor  2  has two contacts  2 A and  2 B, which are connected in parallel with the two fuses  6 A and  7 A, respectively. As in the system of  FIG. 2 , when a fault to ground occurs in either PV array  10  or  20  and produces a fault current large enough to clear either of the fuses  6 A,  7 A, the clearing of the fuse interrupts the ground fault current. At the same time, a blown-fuse indicator signal is sent to a controller  1  by the closing of the switch  6 B or  7 B associated with the cleared fuse. This causes the controller  1  to de-energize a contactor coil  2 A to open both contacts  2 B and  2 C, so that the faulted PV subarray is then connected to earth ground only through the ground fault impedance and one of the equal-valued resistor networks formed by respective resistor pairs  16 ,  17  and  26 ,  27 . During this fault mode of operation, any un-faulted subarrays will “float” with the highest voltages at the subarray poles equal to ±½ the subarray open circuit voltage with respect to the earth ground  70 . The resistor networks  16 ,  17  and  26 ,  27  provide a minimally dissipative common mode voltage reference by bleeding off subarray static charges. The use of the dual contacts  2 A and  2 B in the system of  FIG. 3  changes the rating requirements for each contact, which can reduce the cost of the contactor. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.