Abstract:
An apparatus for harvesting solar power includes a photovoltaic array for generating a DC voltage; a discharge circuit for causing the DC voltage to decay from a first value to a second value; and an inverter circuit for transforming an output voltage from the discharge circuit into an AC voltage.

Description:
FIELD OF INVENTION 
     This invention relates to photovoltaic inverter systems, and in particular, to reducing the voltage provided by a photovoltaic array to an inverter during the inverter start-up sequence. 
     BACKGROUND 
     A photovoltaic array outputs DC voltage. This voltage depends on such factors as solar irradiance, temperature, and the electrical load presented to the array. 
     To be more commercially useful, one provides this DC voltage to a utility class solar photovoltaic inverter. The inverter converts the DC photovoltaic array output into an AC voltage, which is then stepped up with a transformer and provided to a typical AC utility grid. In addition, the inverter maintains a maximum power point tracker (MPPT) for causing the photovoltaic array to operate at its maximum power point. 
     Occasionally, the inverter may fail and require maintenance. Or an alarm may trip, and the inverter protections will have to be reset. In either case, the inverter is disabled, and therefore no longer interacts with the photovoltaic array. Once this occurs, the inverter no longer controls the load. As a result, the photovoltaic array&#39;s output DC voltage increases. 
     Eventually, the inverter must be re-enabled so that the photovoltaic array can be put back into service. A difficulty can arise, however, if the output voltage of the array is higher than the maximum operating voltage of the inverter&#39;s switching elements. This is particularly likely at cold ambient temperatures. In such cases, re-enabling the inverter may expose the switching elements to excessively high voltage, and thus cause them to fail. 
     Another problem arises when, at the time the inverter is to be re-enabled, the ambient air temperature is at or near the minimum operating temperature of the switching elements. This arises because as the temperature falls, the breakdown voltage associated with the switching elements also falls. For example, at −40° C., the breakdown voltage of a switching element may be 5%-15% below its breakdown voltage at 25° C. 
     One solution is to reduce the array&#39;s DC voltage output. However, the output voltage depends on factors that are difficult to control, such as solar irradiance and outdoor temperature. It is difficult to temporarily shield the array from the sun, or to change the outdoor temperature so that the inverter can be re-enabled. 
     A known way to safely re-enable the inverter is to wait until dark, or until the temperature rises sufficiently. However, this is often inconvenient both because of the delay, the resulting lost revenue from electricity production, and the need to employ personnel after normal working hours. Moreover, in extreme latitudes, where days are long, it may be weeks or months until the sky is dark enough to reduce the voltage sufficiently to re-enable the inverter. And at those times when the sky is dark for extended periods, it may be weeks or months before the temperature rises enough to safely re-enable the inverter. 
     SUMMARY 
     In one aspect, the invention features an apparatus for harvesting solar power. Such an apparatus includes a photovoltaic array for generating a DC voltage; a discharge circuit for causing the DC voltage to decay from a first value to a second value; and an inverter circuit for transforming an output voltage from the discharge circuit into an AC voltage. 
     In one embodiment, the apparatus includes a discharge resistor, and a discharge switch in series with the discharge resistor, with the discharge circuit being in parallel with the inverter circuit. Among these embodiments are those in which the discharge switch includes an IGBT and those in which the discharge switch has a voltage rating equal to the voltage rating of the inverter circuit. Also included among these embodiments are those in which the discharge resistor has a discharge resistance selected to minimize energy absorbed by the discharge resistor subject to the constraint that transient current ratings of the discharge resistor and the discharge switch not be exceeded. In yet other embodiments, a clamping diode is in parallel with the discharge resistor. 
     Another embodiment further includes a discharge controller for controlling the discharge circuit. Such a discharge controller is programmed to control a rate at which the DC voltage decays from the first value to the second value. The discharge controller can control the rate manually, or automatically in response to receiving an instruction. 
     Among the embodiments of the apparatus are those in which the discharge circuit includes a capacitance in parallel with the discharge resistor and the discharge switch, and in which the discharge resistance is selected to discharge the bus capacitance quickly enough to prevent overheating at least one of the discharge resistor and the discharge switch when the discharge switch is closed. 
     Also included among the embodiments are those in which the inverter circuit includes parallel pairs of switching elements, each pair including a first switching element in series with a second switching element. 
     In some embodiments, the discharge circuit for causing the difference between the two values of DC voltage is sufficiently large to accommodate voltage transients resulting from activity of the inverter circuit. 
     In another aspect, the invention features an apparatus for reducing a voltage generated by a photovoltaic array that includes means for causing a DC voltage generated by a photovoltaic array to decay from a first value to a second value; and an inverter circuit for transforming an output voltage from the discharge circuit into an AC voltage. 
     In another aspect, the invention features a method for disabling and re-enabling an inverter circuit connected to a photovoltaic array. Such a method includes disabling an inverter circuit from interacting with the photovoltaic array; draining a voltage generated by the photovoltaic array from a first value to a second value; and after the voltage has reached the second value, re-enabling interaction between the inverter circuit and the photovoltaic array. 
     In some practices, draining a voltage comprises allowing current to flow through a resistance connected in parallel with the voltage generated by the photovoltaic array. Among these practices are those in which allowing current to flow comprises closing a switch in series with the resistance. Also among these practices are those that further include re-opening the switch, and those that further include clamping the voltage across the switch to a value equal to a concurrent voltage generated by the photovoltaic array. 
     In other practices, draining the voltage comprises causing a transition from the first value to the second value within a time interval selected to be short enough to avoid overheating circuit components. 
     Alternative practices also include receiving an instruction to re-enable the inverter circuit, and wherein draining the voltage is executed automatically in response to the instruction. 
     Practices also include those in which the second value is selected to be greater than an MPP operating voltage of the array, and those in which it is selected to be less than a maximum power point operating voltage of the photovoltaic array. Among those practices in which the second value is selected to be less than the maximum power point operating voltage are those that include detecting that the voltage has reached the second value, and waiting for a selected interval prior to enabling the interaction between the inverter circuit and the photovoltaic array. 
     These and other features of the invention will be apparent from the following detailed descriptions and the accompanying figures, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a diagram of a photovoltaic power plant; 
         FIG. 2  is a solar inverter for use in the power plant of  FIG. 1 ; and 
         FIGS. 3 and 4  show two methods for traversing the V/I characteristic of a photovoltaic array from the photovoltaic power plant of  FIG. 1   
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a typical photovoltaic power station  10  having one or more photovoltaic plants  12   a - c  that provide an AC voltage to a first transformer  14 . The first transformer  14  increases this AC voltage and places the resulting high voltage output on a transmission line  16  connected to the utility grid. 
     The first transformer  14  typically receives an AC voltage on the order of 34.5 kV, and steps it up to an AC voltage between about 115 kV and 345 kV. However, in some embodiments, the first transformer  14  steps the voltage up to as low as 69 kV, or a voltage in excess of 345 kV. 
     A typical photovoltaic plant  12   a  includes a photovoltaic array  20  that, in response to solar irradiation, generates a DC voltage on an input bus  22  of a solar inverter system  24 . The value of this voltage depends on the layout of individual solar panels, the amount of light, the electrical load, and the temperature. In typical utility scale applications under typical operating conditions, the output voltage is on the order of 750 volts. 
     The solar inverter system  24  converts the DC voltage into an AC voltage, which is then provided to a second transformer  26 . For cases in which power is to be provided to a utility grid, the solar inverter system  24  is a grid-tie inverter. 
     The output of the second transformer  26 , as well as outputs of corresponding transformers at other photovoltaic plants  12   b - c , is then provided to the first transformer  14 . 
     A typical solar inverter system  24 , shown in  FIG. 2 , includes a discharge circuit  34  and an inverter circuit  28 . The inverter circuit  28  includes a DC bus capacitor  25  in parallel with pairs of switching elements  30  that switch on and off in response to instructions from an operating controller  32 . Each pair of switching elements  30  can be used to generate an AC signal. Thus, the three pairs of switching elements  30  shown can be used to generate a three-phase AC signal. 
     The switching elements  30  in an inverter circuit  28  used in a photovoltaic plant  12   a  are expected to handle high current loads. Suitable switching elements  30  are insulated gate bipolar transistors, or IGBTs. 
     The DC voltage on the input bus  22  depends on a number of factors, including the amount of sunlight falling on the photovoltaic array  20 , the ambient temperature, the electrical load presented to the array  20 , and the age and condition of the array  20 . To some extent, the solar inverter system  24  controls the electrical load by adjusting the voltage across the bus  22 . This affects the power transferred from the photovoltaic array  20  to the solar inverter system  24 . One function of the operating controller  32  is to function as a maximum power point tracking device to maintain that voltage at a maximum power point. 
     In some cases, the switching elements  30  are held in an OFF, or open-circuit state, for example as a result of maintenance, in response to a fault, or any other time the inverter is disabled. When the solar inverter system  24  is disabled, interaction between the inverter circuit  28  and the photovoltaic array  20  ceases, and the voltage on the bus  22  can rise to a level in excess of the safe operating voltage associated with the inverter&#39;s switching elements  30 . 
     To facilitate re-enabling the switching elements  30 , the inverter includes a discharge circuit  34  in parallel with the inverter circuit  28 . The discharge circuit  34  includes a discharge resistor  36  placed across the input bus  22 , and a discharge switch  38  that selectively connects the discharge resistor  36  to ground in response to control signals provided by a discharge controller  40 . 
     The start-up of the inverter circuit  28  is best understood in connection with  FIG. 3 , which shows the voltage/current characteristic of a typical photovoltaic array  20 . In this example, the switching elements  30  are rated at 1200 V. However, because the switching elements  30  switch at high frequency (on the order of 1-6 kHz in a 1 MW inverter) a voltage spike arises across any parasitic inductances in the inverter circuit  28 . To accommodate this voltage spike, the inverter circuit  28  is operated only when the bus voltage is less than 70%-75% of the switching element&#39;s rating. Thus, in the present example, the inverter circuit  28  should not be operated until the bus voltage falls to about 850 volts. 
     In the particular example shown in the figure, the open circuit bus voltage rises to 967 volts when the ambient temperature is 5 degrees Fahrenheit (−15 degrees Celsius). As a result, operation of the inverter circuit  28  should be delayed until the voltage is drained from 967 volts to approximately 850 volts. 
     When the inverter circuit is disabled, the open circuit voltage on the bus  22  may be too high to safely operate the inverter circuit  28 , as seen at point “A” in the figure. Before the inverter circuit  28  can be re-enabled, the discharge controller  40  first closes the discharge switch  38 . This causes the voltage to drop and the current to rise. When the voltage has dropped to a safe level (e.g. below 850V), shown as point “B” in  FIG. 3 , the inverter circuit  28  is re-enabled and the discharge controller  40  re-opens the discharge switch  38 . The operating controller  32  then causes the voltage to settle at the maximum power point value, which is at point “C” in  FIG. 3 . 
       FIG. 4  shows the same voltage/current characteristic of  FIG. 3  but with different operating points. These operating points are used to start the inverter circuit  28  during periods in which the ambient temperature is close to the minimum operating temperature of the switching elements  30  (for example, below −40 C). Under such conditions, the switching elements  30  should not be operated at the point “B” in  FIG. 3  because of the risk of exceeding the breakdown voltage of the switching elements  30 . 
     In the example of  FIG. 4 , the open circuit bus voltage has risen to 967 volts, as was the case in  FIG. 3 . However, in this case, operation of the inverter circuit  28  is delayed until the voltage is drained from 967 volts to about 700 volts (point “D”). This is done to avoid having voltage spikes within the switching elements  30  exceed the breakdown voltage of those elements  30 . 
     The procedure for draining the voltage, and thereby moving from point “A” to point “D” in  FIG. 4 , is the same as that discussed in connection with  FIG. 3 . The discharge controller  40  first closes the discharge switch  38 . This causes the voltage to drop and the current to rise. When the voltage has dropped to a safe level (e.g. to 700 V), shown as point “D” in  FIG. 4 , the inverter circuit  28  is re-enabled and the discharge controller  40  re-opens the discharge switch  38 . 
     Once the discharge switch is re-opened, there are two options that can be carried out by the operating controller  32 . 
     One option is to promptly cause the voltage to settle at the maximum power point value, which is at point “C” in  FIG. 3 . However, if the switching elements  30  have not yet warmed up enough, their breakdown voltage may be still be too low to accommodate voltage spikes. 
     A second option is to wait for a selected period to allow the switching elements  30  to warm up. During this warm up interval, the power output is reduced because the bus voltage is below the maximum power point voltage. After an interval that depends on the thermal design of the inverter circuit  28 , the junction temperature of the switching elements  30  will have risen sufficiently to raise their breakdown voltages. At that point, the operating controller  32  can bring the operating voltage up to the maximum power point voltage. 
     The selected time interval before allowing the operating controller  32  to bring the voltage up to the maximum power point voltage (i.e. to begin traversing from point “D” to point “C” in  FIG. 4 ) depends on such factors as the thermal design of the inverter circuit  28  and the ambient temperature. In some embodiments, a warm up interval between 10 and 90 seconds is necessary. In other embodiments, a temperature sensor can be provided to assist the operating controller  32  in determining when it is safe to begin raising the voltage. 
     A disadvantage of having to discharge all the way from point “A” to point “D” is that more energy is absorbed by the discharge resistor  36 . In addition, the time required to carry traverse the voltage/current curve from point “A” all the way to point “D” is longer than that require to traverse it only up to point “B”. As a result, the transient rating of all elements in the discharge circuit is necessarily higher. 
     The discharge controller  40  is configured to automatically carry out either one of the foregoing sequences in response to receiving an instruction to re-enable the inverter circuit  28 . Thus, maintenance personnel providing such instructions would not have to be aware that the foregoing re-enablement procedure was taking place at all. The discharge controller  40  thus renders the foregoing re-enablement procedure transparent to maintenance personnel. 
     The discharge controller  40  can be configured in hardware, or as an ASIC programmed to carry out the foregoing procedure. It can also be implemented as a general purpose controller having a processor programmed to carry out the foregoing procedure. A controller programmed to carry out the foregoing procedure is clearly structurally different from a controller that has not been so programmed. For example, its memory stores instructions that are different from those of a controller not so programmed. These differences are manifested in voltages present at each memory location in a programmed controller that are in some cases different from those in corresponding memory locations in a controller not so programmed. These differing voltages provide tangible physically measurable evidence of such structural differences. 
     In general, the discharge resistor&#39;s resistance (discharge resistance) is selected to be low enough to quickly discharge the discharge capacitor  25 , thereby reducing the bus voltage, while simultaneously absorbing the current flow from the PV array as the voltage drops. However, as a practical matter, the lower the discharge resistance becomes, the higher the discharge current will be. A high discharge current will mean that both the discharge resistor  36  and the discharge switch  38  must safely handle higher currents. This would increase component costs. 
     However, if the discharge time, i.e. the time required to reduce the voltage across the capacitor  25  to a safe level, is selected to be short enough, then the transient ratings of both the discharge resistor  36  and the discharge switch  38  can be relied upon to accommodate the high current that would momentarily flow through those elements. 
     Preferably, the discharge resistance is selected to minimize energy absorbed by the resistor  36  while still not exceeding the transient current ratings of the resistor  36  and the switch  38 . 
     Alternatively, the discharge resistance can be low enough to quickly drain the bus voltage by causing a high current pulse that is nevertheless still within the transient current ratings of both the discharge resistor  36  and the discharge switch  38 . 
     The voltage ratings of the discharge switch  38  and discharge resistor  36  can be the same as those of the switching elements  30 . This is because by the time the discharge switch  38  is opened, the bus voltage will already be low enough so that any transients generated by opening the discharge switch  38  will, when added to the bus voltage, not exceed the maximum voltage rating of the switching elements  30 . 
     In some cases, the discharge circuit  34  may have parasitic inductances that would generate additional voltage transients upon opening the discharge switch  38 . To accommodate such transients, it is useful to provide a clamping diode  42  to clamp the voltage to the bus voltage. 
     In the example shown in  FIG. 3 , the time it takes to traverse the V/I curve from point “A” to point “B” is on the order of 100 ms. In general, the time required to traverse the V/I curve is transparent to the operator who re-enables the inverter circuit  28 . Since the operator is not expected to disable and re-enable the inverter circuit  28  repeatedly in rapid succession, all components are expected to have sufficient time to cool down before the next re-enablement of the inverter circuit  28 . 
     The discharge circuit  34  can be implemented as single elements within the solar inverter system  24 . However, the discharge circuit  34  can also be implemented outside the solar inverter system  24 . This feature is useful for retrofitting existing photovoltaic inverters. 
     In some cases, an inverter circuit  28  may have multiple banks of switching elements  30  operating in parallel. In such cases, it may be useful to provide a discharge circuit  34  for selected subsets of those banks of switching elements  30 . 
     The discharge switch  38  can be implemented in a variety of ways. For example, the discharge switch  38  can be a junction transistor, an IGBT, an IGCT (integrated-gate commutated thyristor), a MOSFET (metal oxide silicon field effect transistor), or any similar device. 
     The foregoing devices and methods are particularly useful for photovoltaic arrays  20  since such arrays cannot easily be turned off. However, the devices and methods can be applied to any DC power source having an operating voltage that is lower than its open circuit voltage. The devices and methods are particularly advantageous when the gap between the operating voltage and the open circuit voltage is large, for example when the operating voltage is on the order of 76% of the open circuit voltage, and when the switching element voltage rating is only 10% to 20% above the open circuit voltage. 
     The foregoing methods and devices are also applicable in those cases in which a solar inverter system  24  having a particular rated voltage is to be used with a low power photovoltaic array  20  whose open circuit voltage is below the rated voltage by an amount this is smaller than the additional voltage arising from transients within the solar inverter system  24 . 
     As described above, the voltage rating of the discharge switch  36  is equal to that of the switching elements  30  in the inverter circuit  28 . In some applications, however, it may be desirable to have a discharge switch  38  with a voltage rating in excess of that of the switching elements  30 . This might arise, for example, if the discharge switch  38  is not rated to be closed at the open circuit voltage.