Patent Application
20120215372
Photovoltaic panels (“PV”) provide electrical current when exposed to light. For a given level of insolation, the output voltage and current are a function of the load, and their product may be defined as the power delivered by the PV to its load. When the output is open circuit, voltage may be maximized and there may be no current. Likewise when the output terminals of a PV are shorted the current may be maximized but there may be very low voltage across the terminals. In both cases there may be no power delivered. The relationship between power and current is a nonlinear one which may be described by a characteristic current versus voltage curve (“IV curve”). The characteristic curve may be found by experimentation or by testing a panel when it completes manufacture. A complete characterization may be a family of curves, each curve corresponding to a specific value of insolation. It may be desirable to operate a given PV or collection of them at a condition that maximizes the power deliverable to a corresponding load.
An embodiment includes a method for controlling the operation of a solar panel by an electronic module in which the electronic module includes means for measuring a temperature of the solar panel and means for measuring a value of the voltage across the solar panel and means for configuring the electronic module to demand more or less current from the solar panel. The embodiment method may include determining a temperature of the solar panel, determining an expected output voltage of the solar panel as a function of the temperature of the solar panel, determining an instant value of the output voltage of the solar panel, comparing the instant value of the output voltage of the solar panel to the expected value of the output voltage of the solar panel, and configuring the electronic module so that a value of current drawn from the solar panel prevents the instant value of the output voltage from exceeding a negative difference value of the expected output voltage. An embodiment may further include determining a maximum power point for the solar panel at the temperature of the solar panel, determining a target voltage value corresponding to the maximum power point, and configuring the electronic module so that the a value of current drawn from the solar panel causes the instant value of the output voltage to approach the target voltage value without exceeding the negative difference value of the expected output voltage.
Another embodiment method for controlling the operation of a solar panel by an electronic controller may include determining an initial temperature of the solar panel, determining an instant voltage of the solar panel, determining an instant current of the solar panel, determining a current versus voltage characteristic curve (“IV curve”) for the solar panel based at least in part on the initial temperature, the instant voltage, the instant current, and a thermal model of the solar panel, determining an expected voltage of the solar panel as a function of the value of the initial temperature of the solar panel, determining a minimum current value, the minimum current value corresponding to the greatest value of current drawn from the solar panel which will result in the output value of the voltage of the solar panel exceeding a negative difference value of the expected output voltage of the solar panel, determining a target voltage value that maximizes the power generated by the solar panel for the determined IV curve, configuring the electronic controller to drive the output voltage of the solar panel toward the target voltage value, determining a new instant current of the solar panel, and configuring the electronic controller to increase the output voltage of the solar panel if the new instant current is below the minimum current value. An embodiment may further include determining a maximum current value, the maximum current value corresponding to safe current level for the solar panel, and configuring the electronic controller to reduce the current output of the solar panel if the new instant current is above the maximum current value. An embodiment may further include determining a maximum current value, the maximum current value corresponding to safe current level for the solar panel, and configuring the electronic controller to stop the generation of power by the solar panel if the new instant current is above the maximum current value. An embodiment may further include determining a new temperature of the solar panel, determining a new instant voltage of the solar panel, and determining whether the solar panel is uniformly illuminated or non-uniformly illuminated based at least in part on the new instant voltage and instant current of the solar panel. When it is determined that the solar panel is uniformly illuminated the method may include determining a new IV curve for the solar panel based at least in part on the new temperature, the new instant voltage, the new instant current, and the thermal model of the solar panel, determining a new expected voltage of the solar panel as a function of the value of the new temperature, determining a new minimum current value, the new minimum current value corresponding to the greatest value of current drawn from the solar panel which will result in the output value of the voltage of the solar panel exceeding a negative difference value of the new expected output voltage of the solar panel, determining a new target voltage value that maximizes the power generated by the solar panel for the new determined IV curve, and configuring the electronic controller to drive the output voltage of the solar panel toward the new target voltage value. When it is determined that the solar panel is non-uniformly illuminated the method may further include configuring the electronic controller to cause the output voltage of the solar panel to correspond to a safe operating voltage for the solar panel. In an embodiment, determining whether the solar panel is uniformly illuminated or non-uniformly illuminated may include configuring the electronic controller to increase the output voltage of the solar panel by a voltage increment, such as approximately 1 volt, comparing the output voltage of the solar panel to the target voltage value, determining the solar panel is uniformly illuminated if the output voltage is within the voltage increment of the target value, and determining the solar panel is non-uniformly illuminated if the output voltage is not within the voltage increment of the target value. In an embodiment, the electronic controller may be a pulse amplitude modulated current converter (“PAMCC”) which may be connected to direct electrical current output leads of the solar panel and comprises input terminals, first, second and third output terminals, and a controller. In an embodiment, the controller may be configured to perform operations including outputting a first pulse amplitude modulated current pulse at a first phase from the first output terminal, outputting a second pulse amplitude modulated current pulse from the second output terminal at a second phase 120 degrees out of phase with the first pulse; and outputting a third pulse amplitude modulated current pulse from the third output terminal at a third phase 120 degrees out of phase with the first pulse and the second pulse.
A further embodiment may include one or more solar panels coupled to a current converter configured to perform operations of the embodiment methods. A further embodiment may include a current converter configured to perform operations of the embodiment methods.
The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims.
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations.
Solar panels are expected by their makers to last at least twenty five years. However, the inverters used in today's installations require very large, high capacitance electrolytic capacitors. These capacitors suffer from temperature extremes, their lifetime particularly shortened by high temperature, such as that experienced on a roof. The liquid in these capacitors will eventually leak out of their canisters, and must be replaced in as little as five years by an experienced technician. This leads to an increased lifetime total cost of ownership. An example inverter circuit including an electrolytic capacitor 302 is illustrated in
Another of the many lifetime-limiting conditions to be dealt with to enable such a long lifetime is hot spots on the panel. Hot spots may limit lifetime by causing damage to the panel due to heat generated and/or longer term degradation of the panel cell material due to diffusion aging. Failure modes include melting solder joints, pin holes or open circuits in a cell, and damage to the panel case. Some causes of hot spots are manufacturing related, such as an assembly flaw, substandard materials, contamination of a solar cell, and the always-present manufacturing variations. Though a panel may have been manufactured with flaws, it may well be serviceable for an extended time, though less than expected. Other causes are beyond the control of the manufacturer or installer. For example, some cells in a panel may be exposed to more or less sunlight than other cells due to partial shade, dirt or bird droppings in a localized area, temperature variations across a panel, and non-uniform aging of the diffusion regions from cell to cell.
The destructive effects of hot-spot heating may be circumvented through the use of a bypass diode. A bypass diode may be connected in parallel, but with opposite polarity, to a solar cell. Under normal operation, each solar cell may be forward biased and therefore the bypass diode may be reverse biased and may effectively be an open circuit. However, if a solar cell is reverse biased due to a mismatch in short-circuit current between several series connected cells, then the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the external circuit rather than forward biasing each good cell. The maximum reverse bias across the poor cell may be reduced by the bypass diode to about a single diode drop, thus limiting the current and preventing hot-spot heating.
A typical circuit model of a solar panel is shown in
Four cases related to solar cells that are equal and unequal in power capacity may occur, each case in open and short circuit configurations. In a short circuit condition and with matched cells the voltage across both the solar cells and the bypass diodes may be zero; the bypass diodes may have no effect. When open circuit (also with matched cells) the short current from each cell may forward bias the cell. The bypass diodes may be reverse biased, and again, may have no effect on the circuit.
Assume now that cell 1604 may be shaded, and thus may have less power providing capacity than that of cell 1602. For the short circuit condition, some current may flow from cell 1602, forward biasing the cell 1602. The bypass diode 1610 may again be reverse biased and may have no effect. The voltage of the good cell 1602 may forward bias the bypass diode 1612 of the weak cell 1604, causing it to conduct current. The shaded cell 1604 itself may be reverse biased with approximately a diode drop of about −0.5 volts. For the fourth condition, that is a weak cell 1604 and an open circuit, the shaded cell 1604 may have a reduced voltage. The bypass diodes 1610, 1612 may be reverse biased and have no effect.
In practice, however, one bypass diode per solar cell may generally be too expensive and instead bypass diodes may usually be placed across groups of solar cells. The voltage across the shaded or low current solar cell may be equal to the forward bias voltage of the other series cells which share the same bypass diode plus the voltage of the bypass diode. The voltage across the unshaded solar cells may depend on the degree of shading on the low current cell. For example, if the cell is completely shaded, then the unshaded solar cells may be forward biased by their short circuit current and the voltage may be about 0.6V. If the poor cell is only partially shaded, then some of the current from the good cells may flow through the circuit, and the remainder may be used to forward bias each solar cell junction, causing a lower forward bias voltage across each cell. The maximum power dissipation in the shaded cell may be approximately equal to the generating capability of all cells in the group. The maximum group size per diode, without causing damage, may be about 15 cells/bypass diode, for silicon cells. For a normal 36 cell module, therefore, 2 bypass diodes may be used to ensure the module will not be vulnerable to “hot-spot” damage.
Consider now a typical solar panel configuration and response to partial shading. A set of 25 modules connected in series form a nominal Vmpp of 467.5 V at 11.23 A or 5,250 W. Assume each module is constructed of three strings of 38 cells (mpp @ 492 mV, 3.743 A) each and the top middle and bottom of each string are connected. Between the middle of top and middle to bottom are bypass diodes (Vf 410 mV). If one cell became shaded or soiled to the extent that its current dropped by 374 mA or more (10%) then two candidate operating points would be found by an MPPT scan for the string:
Since the portion of the module with the shaded cell only produces 10.853 A, its bypass diode may be forced into conduction forcing the bypass diode's 410 mV and the 9.350 V of the 19 bypassed cells to be subtracted from that modules voltage (total loss of 9.760V from the string of modules). Within the bypassed 19 cells the sum of the voltage across the good 18 cells plus the voltage across the shaded cell must equal −410 mV (the voltage across the bypass diode) at the current of the shaded cell (because all 19 cells are in series).
The solution is approximately 8.856V across the 18 good cells and −9.266V across the shaded cell @3.369 A or 31.2 W of power dissipation in the shaded cell. Note that a similar situation exists with the other two sets of 19 cells because they too are forced to sum to the −410 mV of the bypass diode.
The bypass diode has the difference of module string current minus the bypassed sections. The module is producing 97.026 W for a loss of 54% and dissipating an additional 100 W as heat. A string monitoring means, for example an ADC, would record a 10 V drop in nominal Vmp for the string. A technician dispatched to investigate would find a module operating at 9V when he expected 18V, no change in power when he cast a shadow across half of the module and that some cells in the module were abnormally hot (all standard trouble shooting observations). The technician may conclude that the module is below the 80% limit and assert that it has failed. However at the factory, this module may flash test as only 3.4% below nominal at 18.7V and 10.853 A or 203 W, although it may show a current step of 374 mA (3.3%) at about 8.940V.
The result of the reversal of one or more cells varies for differing solar cell technologies. For cells of a mono-crystalline type, there may be no lasting damage but a loss of efficiency. For cells of a thin-film construction, reversal of a voltage on a given cell is immediately catastrophic. As is seen, then, bypass diodes are a necessary and effective method for diminishing hot spots caused by partial shading or other causes for a weak cell. However, looking to
It may be desirable to operate a given PV or collection of PVs at a condition that maximizes the power deliverable to a corresponding load. The maximum power point (“MPPT”) may be considered to be the maximum area under the characteristic IV curve for a given level of insolation. A PV may be controlled by setting its output voltage, thus the current available may be a function of the illumination level. The power available from the PV is then a function of the controlled output voltage and the current generated by the panel. The voltage at which MPPT is attained may differ for different levels of illumination.
The light level experienced by a PV changes during a day as the sun rises and falls. In addition, passing clouds, birds, wind turbine blades, and aircraft may change the light incident on a PV at any instant. Longer term, a PV may experience a change of net light received due to dust and soiling accumulating on the panel, tree growth or nearby construction casting shadows on the panel, and cleaning which may remove dust and soiling. Thus, a control mechanism is needed to keep the PV operating at or near an ideal output voltage for producing power regardless of the light available at any given moment. Due to the possibility of a short duration change in light level, for example due to passing wind turbine blades, it is desirable for the control system to rapidly respond to short-duration changes.
In many instances PVs are used to generate electrical power for a grid system in which the power generated is three phase electricity. A power conversion apparatus, for example an array converter as described herein, may convert the direct current provided by a PV or collection of PVs into the desired three phase power. When the multiphase power is connected to a grid power system, noise and other errors in the grid power may affect the efficiency of power delivery to the grid by the power generation system. It may be beneficial for a power conversion control system to diminish any noise or mismatch between the power generator and a grid to which it is connected.
The amount of power derived from a PV may be substantial, enough to damage connected electronics or even the PV itself. It may be desirable to monitor actual and target operational conditions and override instant or anticipated operation outside of a safe operational envelope.
A DC to pulse amplitude modulated (“PAM”) current converter, denominated a “PAMCC” may be connected to an individual solar panel (“PV”). A solar panel typically may be comprised of a plurality, commonly seventy-two, individual solar cells connected in series, wherein each cell may provide approximately 0.5 volt at some current, the current being a function of the intensity of light flux impinging upon the panel. The PAMCC may receive direct current (“DC”) from a PV and may provide pulse amplitude modulated current at its output. The pulse amplitude modulated current pulses may typically be discontinuous or close to discontinuous with each pulse going from near zero current to the modulated current and returning to near zero between each pulse. The pulses may be produced at a high frequency relative to the signal modulated on a sequence of pulses. The signal modulated onto a sequence of pulses may represent portions of a lower frequency sine wave or other lower frequency waveform, including DC. When the PAMCC's output is connected in parallel with the outputs of similar PAMCCs an array of PAMCCs may be formed, wherein the output pulses of the PAMCCs may be out of phase with respect to each other. An array of PAMCCs may be constructed to form a distributed multiphase inverter whose combined output may be the demodulated sum of the current pulse amplitude modulated by each PAMCC. If the signal modulated onto the series of discontinuous or near discontinuous pulses produced by each PAMCC is an AC current sine wave, then a demodulated, continuous AC current waveform may be produced by the array of PAMCCs. This AC current waveform may be suitable for use by both the “load”, meaning the premises that may be powered or partially powered by the system, and may be suitable for connection to a grid. For example, in some embodiments an array of a plurality of PV-plus-PAMCC modules may be connected together to nominally provide split-phase, Edison system 60 cps 240 volt AC to a home.
In the various embodiments, a PAMCC may be controlled by a controller executing a control loop. The control loop may be embodied in a program stored in memory that is executed by a processor or by a state machine, or by programmable logic, such as a field programmable gate array (“FPGA”) that is part of the controller. In some embodiments the control loop may be partitioned between firmware and logic. In the various embodiments the control loop may comprise a fast “inner loop” portion that runs continuously, and a one or more slower “outer loops” which may require more time to complete. The outer loops may from time to time change various values that may be saved in shared memory, wherein the faster inner loop may use an instant value that has been most recently saved into shared memory, and the outer loops may update their various values asynchronously to the inner loop.
In the various embodiments, startup values may be prepositioned in memory for a given three-phase PAMCC. During startup the prepositioned values may be used by the control loop, then the values may be modified during ongoing operation. In the various embodiments a table of look up values may speed up calculation speeds, wherein the values may be modified by a scaling factor responsive to instant conditions. The scaling factors may be influenced primarily by temperature and instant insolation, and, optionally, by other conditions. Each of the three phases may be corrected against an ideal output curve, thereby also balancing the power.
Various embodiments may be suitable for power conversion from any direct current source to an arbitrary output signal configuration. Examples of suitable direct current sources include batteries, wind turbines, geothermal, chemical, tidal and piezoelectric power sources. Examples of output signal configurations include sinusoidal alternating current, direct current, trapezoidal, Gaussian, square wave, triangle wave, and adaptive signals. Adaptive signals may include, for example, modifying the output waveforms on a cycle-by-cycle or other time period basis to adapt to, modify, or cancel the effect of transient noise or other conditions. Such signals may also include symbols modulated or superimposed on the base (carrier) signal as a method for communicating between modules, subsystems, or out of systems modes.
To clearly describe the operation of the present invention, the operation of a similar two-phase system is described first. The operation of a three phase system according to the present invention then involves a different output stage.
In a single phase system, a PAMCC may be connected to an individual solar panel (PV). A solar panel typically may be comprised of a number, commonly seventy-two, of individual solar cells connected in series, wherein each cell may provide approximately 0.5 volt at some current. The current produced by individual solar cells is a function of the intensity of light flux impinging upon the panel. The PAMCC may receive direct current (DC) from a PV and may provide pulse amplitude modulated current at its output. The pulse amplitude modulated current pulses may typically be discontinuous or close to discontinuous with each pulse going from near zero current to the modulated current and returning to near zero between each pulse. The pulses may be produced at a high frequency relative to the signal modulated on a sequence of pulses. The signal modulated onto a sequence of pulses may represent portions of a lower frequency sine wave (e.g., a 60 Hz AC current waveform) or other lower frequency waveform, including DC.
When the PAMCCs output is connected in parallel with the outputs of similar PAMCCs, an array of PAMCCs may be formed in which the output pulses of the PAMCCs may be out of phase with THD (Total Harmonic Distortion) PCM (Pulse Code Modulation). PCM is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code. THD is a measure of noise on a signal with respect to each other. An array of PAMCCs form a distributed multiphase inverter whose combined output is the demodulated sum of the current pulse amplitude modulated by each PAMCC. If the signal modulated onto the series of discontinuous or near discontinuous pulses produced by each PAMCC was an AC current sine wave, then a demodulated, continuous AC current waveform may be produced by the array of PAMCCs. This AC current waveform may be suitable for use by both the “load”, meaning the premises that may be powered or partially power by the system, and suitable for connection to a grid. For example, in some embodiments an array of a plurality of PV-plus-PAMCC modules may be connected together to nominally provide a split-phase, Edison system 60 cps 240 volt AC to a home.
Before discussing an array comprising a plurality of PV-plus-PAMCC modules, an individual PAMCC is described. For example, referring to
In the embodiment for a three-phase system illustrated in
The controller 412 may comprise a plurality of output terminals, each operated independently. Controller 412 output terminals 415, 416, 417, and 418 may be connected to the control terminals of four triacs (CR11424; CR22423; CR12425; and CR21426 respectively) by four lines 419, 420, 421, and 422 respectively (inner-connections not shown). Each line, therefore each triac, may be independently controlled by control signals from the controller 412. The anode terminals of CR11424 and CR22423 may be connected in parallel to the positive output terminal of TI 407. The cathode terminals of triacs CR12425 and CR21426 may be connected in parallel to the negative output terminal of TI 407. The cathode terminal of triac CR11424 and the anode terminal of triac CR12425 may be connected in parallel to a coil L12430. The cathode terminal of triac CR22423 and the anode terminal of triac CR21426 may be connected in parallel to a coil L22431.
A terminal 434 from coil L12430 is arbitrarily designated as providing a “phase 1”(P1) output and a terminal 436 from coil L22431 is arbitrarily designated as providing a “phase 2” (P2) output. In some embodiments the coils L12430 and L22431 are embodied in a one-to-one transformer. In the embodiment illustrated in
Operation of the system may be implemented by control signals on lines 411 and 419 through 422. In particular the control signal sent to the control gate Q1G on line 411 and signals CR11T on line 419; CR22T on line 420; CR12T on line 421; and CR21T on line 422 may connect and disconnect the current provided by PV 401 in a sequence within the PAMCC 400 with a high-frequency period, for example 30 KHz, which provides a PCM signal which is modulated by a slower, 60 cycle pattern, thereby providing an output whose amplitude is a PAM signal approximating a sine wave.
Referring to
Circuit operation may begin with the solar panel 401 being exposed to sufficient light to produce significant current. The presence of the current may be observed as an increase in voltage across Q1404. At this point Q1404 may be initially turned on by applying a signal from controller 412 on line 411 between Q1G and Q1S. The interface between the controller 412 and the transistor Q1404 may be optically isolated, transformer coupled, or the controller 412 may be connected to Q1S. In this state L1406 and L2405 may begin to charge with current. When the voltage across PV 401 falls to a predetermined value, the time to charge the coils may be noted in order to calculate the current and standard operation may begin with the next grid zero crossing. In one embodiment this may be when the voltage at P1 crosses above P2 while P1 is going positive and P2 is going negative. At this point signals CR11T on line 419 and CR22T on line 421 may be asserted such that CR11424 and CR21426 may conduct when current may be applied to them.
CASE I: PWM modulation for positive half wave of the grid.
Referring to
The resonate frequency for the reconstruction filters may be chosen to be about one half the switching frequency of Q1404 so that one half of a sine wave of current may be provided to P1434 and P2436 for each pulse width modulated current pulse delivered to them. Since the resonate frequency of each reconstruction filter may be independent of the pulse width of current applied to it, and the charge in the instant current pulse applied to the reconstruction filter may be equal to the charge in the half sine wave of current delivered out of the reconstruction filter to the grid. Changes in the pulse width of input current may be reflected as changes in the amplitude of the output of the reconstruction filters. As the current in the inductors in the reconstruction filters returns to zero, the next pulse of current may be delivered to the capacitors of the reconstruction filters because the frequency of the reconstruction filters may be one half the rate at which pulse width modulated current pulses are produced by Q1404.
The off time of Q1404 may be modulated such that the width of current pulses produced may be in the shape of the grid sine wave. The reconstruction filters may transform this sequence of pulse width modulated current pulses into a sequence of pulse amplitude modulated current pulses whose amplitude follows corresponding points of the shape of the grid sine wave.
So long as the grid half cycle remains positive at the terminal P1434 relative to the output of terminal P2436, further current pulses may be produced by repeating the process described hereinbefore, beginning at “CASE I: PWM modulation for positive half wave of the grid”.
The negative zero crossing of the grid voltage is defined as the condition wherein the voltage at terminal P1434 may be equal to the voltage at terminal P2436 after P1434 has been more positive than P2436. Prior to the negative zero crossing, Q1404 may be turned on, thereby removing current from CR11424 and CR21426. At this point the signals CR11T in line 419 and CR21T in line 421 may be de-asserted, preventing triacs CR11424 and CR21426 from conducting current during the grid negative half cycle. After the negative zero crossing, with the voltage of terminal P1434 more negative than the voltage of terminal P2436, the signals CR22T 420 and CR12T 421 may then be asserted, enabling CR22423 and CR12425 to conduct when current is applied to them.
CASE 2: PWM modulation for negative half wave of grid
Referring to
The reconstruction filters for Case 2 may be the same components as described in association with Case I; their design and operation are not repeated here.
The off time of Q1404 is modulated such that the width of current pulses produced may be in the shape of the grid sine wave. The reconstruction filters may transform this sequence of pulse width modulated current pulses into a sequence of pulse amplitude modulated current pulses whose amplitude follow corresponding points of the shape of the grid sine wave.
So long as the grid half cycle remains negative, with the voltage of terminal P1434 more negative than the voltage of terminal P2436, further current pulses may be produced by repeating the process described above, beginning at “CASE 2: PWM modulation for negative half wave of grid.”
The positive zero crossing of the grid voltage may be defined as the condition wherein the voltage at terminal P1434 is equal to P2436 after the voltage at terminal P1434 has been more negative than the voltage of terminal P2436. Prior to the positive zero crossing, Q1404 may be turned on, removing current from triacs CR12425 and CR22423. At this point the signals CR12T 421 and CR22T 420 may be de-asserted, preventing triacs CR12425 and CR22423 from conducting current during the grid positive half cycle. After the positive zero crossing with P1434 more positive than P2436, signals CR11T 419 and CR21T 421 may be asserted, enabling triacs CR11424 and CR21426 to conduct when current is applied to them.
With the grid again positive, the process may again return to the process described above, beginning with the section labeled CASE 1: PWM modulation for positive half wave of the grid.
In
Following on the summation of the currents of two PAMCC 400 outputs,
One of several ways to choose the phasing of the arrayed PAMCCs 400 may be for each PAMCC 400 to be pre-assigned a timing slot number, with the first slot being scheduled following a zero crossing and each PAMCC 400 firing its PAM signal in the predetermined (i.e., assigned) sequence. In some embodiments, especially where the number of PVs may be large, the phase relationship of individual PAMCCs may not be controlled, in that they will naturally be phase separated across a cycle without deterministic phase scheduling.
In an alternative embodiment, illustrated in
In some embodiments the system may be shut down for safety, maintenance, or other purposes. One example of a shut-down system is illustrated in
A solar panel may be controlled by an electronic module, the module including means for measuring the temperature of the panel cells, the voltage across the panel, and for controlling the power (current) provided by the panel. A solar panel may be expected to provide a certain output voltage under good operating conditions, as determined by specification, characterization data, or by the experience derived by accumulating performance data over time. The current available may be a function of the intensity of sunlight incident upon the panel, and the voltage a function of the temperature of the cells, assuming otherwise normal conditions for the cells. As described above, a weak cell, due to damage, deterioration, soil, or simply partial shading of the panel, may not provide the same power as may the other, unaffected cells. Because all cells in a string are electrically in series, the current must be in common. Therefore the weak cell may adjust for the instant lower power capacity by lowering the voltage for that cell. Again because the cells are electrically connected in series, the voltage across the string may be the sum of the voltages of all the cells in the string. When a cell in the string loses some voltage, the whole string may lose voltage as well.
An electronic module typically tests a panel periodically, for example once per hour, to determine the maximum power point (MPPT) operating condition. This may be accomplished by varying the current demanded from a panel, measuring the voltage across the panel, then determining the power for that condition as the product of voltage times current. By varying across a certain range of currents, a peak power point may be found. Previously, such MPPT testing was done without regard to whether the condition selected may drive a weak cell in a string into a forward bias condition, thereby causing the bypass diodes to be forward biased, as described hereinbefore.
In the various embodiments, the electronic module may first determine the temperature of the solar panel cells, determine expected panel voltage for the temperature found, and may not allow the current to cause the voltage to drop more than a predetermined amount below the expected voltage. For example, in one embodiment the maximum value below MPP to be allowed may be:
Wherein RT MPP may be the maximum power point condition for room temperature, “tolerance” may be a value provided by the solar panel manufacturer, and degradation(temp) may be the diode drop value that results from increasing temperature, for example −2.1 my/degree C. for a silicon solar cell. These values may be different for other solar cell chemistries.
As a result, if there were in fact bypass diodes the bypass diodes may never be forward biased, therefore the diodes may not be needed and a solar panel paired with the various electronic module embodiments may be made without bypass diodes, thereby saving the area that would be required for the interconnect of the bypass diodes.
The various embodiments avoid the condition of a hot spot without the use of an efficiency-lowering protection diode. The various embodiment methods assume an apparatus may be used to control the operating conditions of the panel, wherein the apparatus includes means for measuring the total voltage across the strings and means for changing the operating conditions of the panel. Bypass diodes may not be needed nor used, saving the area required for interconnect as required by previous methods. In the various embodiments, the instant voltage may be compared to the expected voltage for a measured operating temperature. If the voltage is less than expected by more than a certain amount, the power (current) demanded from the panel may be reduced such that the voltage may be less than a diode drop of the expected voltage, thereby avoiding a hot spot. In this manner, reducing the current may ensure that the voltage does not decrease below a determined safe limit. With hot spots (i.e., reverse biasing of a weak cell), avoided, bypass diodes are not needed.
As an example, a set of twenty-five modules may be connected in parallel form a total array of 5,250 W. Each panel may be controlled by an individual electronic module connected to the panel, for example an array converter as discussed previously, wherein the electronic module may include means for measuring the voltage across the strings and for controlling the current demanded from its associated module. As an example each module may be constructed of one string of 114 cells (mpp @492 mV, 3.743 A). If one cell became shaded or soiled to the extent that its current dropped by 374 mA (10%) then the power for that module only may be reduced by 10%. The array converter may only permit the MPP solution of approximately 56.088V*3.369 or 189 W (10% loss). This may be because any solution lower than 90% (a programmable limit) of nominal Vmp at the measured temperature may not be allowed as a MPPT solution. This may ensure that an array converter may not reverse a cell by more than 5.6V (half the amount of the bypass diode approach) even during an MPPT search.
Since the other 24 array converter modules may remain unaffected, the total power may be 5,228 W vs 5,140 W for the string inverter case. The single module with the single shaded cell may not dissipate any additional power.
While the preceding description of the pulse amplitude modulation current converter technology addressed a two-phase system, the technology may also be applied to three-phase electrical systems such as electric utility grids.
For a commercial power generator, the generation system may be connected to a low impedance three phase grid, wherein the power (therefore, the voltage-current product) may be kept the same. Accordingly, for a system according to the various embodiments the power in each of the three phases may be equal.
The various embodiments for controlling a three-phase PAMCC module may include an apparatus for controlling the operation of a PV. For example, the various PAMCCs in an array converter system, each connected to a corresponding PV and further connected in parallel with other PAMCCs, may be controlled to provide a three phase alternating current output. The output voltage may be controlled to match that of a connected grid power system. The current, and therefore power, available may be determined based on the radiance impinging on the PV, efficiency, and other factors. By controlling each PV to operate at or near its individual maximum power point condition, the power available from the system as a whole may be increased as well.
Referring to
The output terminals may be connected to a three phase load, for example a utility grid. The switches SWA 2102, SWB 2104, and SWC 2106 may be FETs with control gates driven by the controller 2116 with control signals on corresponding lines 2113, 2115, and 2117. A power controller 2168 may be provided. A digital power supply 2170 may also be provided.
The processor 2120 may include an ADC 2123 configured with three inputs or a single input with a MUX (not shown) to sense voltage at the output terminals 2108, 2110, 2112. The ADC 2123 may be configured to provide a digital representation of the sensed voltage to the processor 2120 firmware. In the various embodiments the ADC 2123 may be a discrete ADC which may provide its data output on a bus or via a serial link to the processor 2120. The ADC 2123 may include inputs corresponding to the positive 2122 and negative 2124 terminals of a solar panel, simplified in
A coil L12140 and a coil L22142 may provide energy storage. Unfolding circuits 2610, 2164, and 2166 may connect the reconstruction filters 2107, 2019, and 2111 to the controller 2116. A connection between coils L12140 and L22142 may be closed or opened by a switch ST 2114, whose control gate may be controlled by the controller 2116 via the main gate driver circuit 2162. By properly timing the opening and closing of the switch ST 2114 and two of the three output terminal switches, for example SWA 2102 and SWB 2106, a boost converter may be formed, thereby providing current through Phase A and Phase B to Phase C.
At the time of manufacture, a PAMCC 2100 register in NVM 2121 may be populated with values for TS1 and TS2 which have been predetermined for each switching time, for example 30 μSec, of the PAMCC 2100. Other switching times may be used. These values may be determined by simulation or by laboratory experimentation to determine typical values. In the various embodiments a table value may correspond to the number of clock ticks for a given pulse time. When the PAMCC 2100 begins actual operation these values may be used for initial conditions, then adapted to actual conditions. The initial TS1 and TS2 values may be saved for an entire 360 degree cycle, or values for sixty degrees saved, the index pointer starting over after each sixty degrees. The TS1 and TS2 initial values may be saved in tables TINIT1 and TINIT2. In the various embodiments TINIT1 and TINIT2 may be sixteen bit registers. Other bit widths may be used. Additionally, the PAMCC 2100 may store multiple temperature curves and thermal coefficients for use with the various embodiments.
Control Loop
As described below, in any sixty degree phase segment there may be two output terminals connected one at a time in a predetermined order to the third output terminal during a switching time T. It is not important which two phases are selected and in which order their switches are closed. The switch amongst SWA 2102, SWB 2104, and SWC 2106 that is connected first may be connected for a first time duration TS1. The switch that is connected second may be connected for a second time duration TS2. The switch ST 2114 may be opened at the beginning of the switching time, and then closed for the time duration TP. The timing of these switches may be determined by the control loop.
In the various embodiments, adjustments to the control loop may be segmented into those processes that can be executed quickly (fast enough to be accomplished within a single switching cycle time) and other processes that require more time to complete.
When a PAMCC 2100 is operated for the first time, the initial value of α, found previously, may be used. In succeeding loops α may be updated at block 2306, depending upon the instant value of another scale factor γ. If γ is not numerically equal to one, at block 2306 a new value for α may be found by
αNEW=αOLD*γ
The value of γ may be reset to be equal to a numerical value of one at block 2305.
Alternatively, some embodiments determine a new value for α at block 2306 by
αNEW=p*αOLD*γ+(1−p)*αOLD,
where p<1, for example p=0.5, then reset γ at block 2305 by
γ=αNEW/αOLD
In block 2308, the two tables TBL1 and TBL2 are populated by scaling the corresponding values of TINIT1 and TINIT2 per the formulas
T
S1TBL1
=T
INIT1*α and
T
S2TBL1
=T
INIT2*α.
In a similar fashion, tables TBLKO1 and TBLKO2 may be populated at block 2310. Values may be found for each switching time and the tables saved in registers. In the various embodiments the tables TBL1 and TBL2 may be ten bit registers. Using values expressed in fewer bits (e.g., 10 bits) shortens the time required for math operations.
During operation, the PAMCC 2100 may determine the voltage VRMS across a rolling window of several grid cycles, for example ten cycles. VRMS may then be evaluated as an ideal voltage VIDEAL at any instant in time by using a sine function. In this operation the ADC 2123 may read the instant output voltage of each of the two driven phases and compare the value read to the ideal voltage for that time period. Note that the phases may be offset from each other. Any difference may be denominated dVOUT1 and dVOUT2. Referring to
T
S1OUT
=T
S1TBL1
*γ−KO
1*(dVOUT1) and
T
S2OUT
=T
S2TBL2
*γ−KO
2*(dVOUT2),
where KOn (n=1,2) is the ratio (TsINITn/VIDEAL). In some embodiments the evaluation of terms may be made faster by populating a table of KOn values for a given sixty degree phase window, wherein KOn is evaluated for each switching time. The KOn tables TBLKO1 and TBLKO2 may be repopulated at block 2310 of each cycle of the loop 2300 illustrated in
The switching times may be determined during one switching cycle, and applied during a next succeeding switching cycle time, with these two processes running concurrently. Depending upon the conversion speed of the ADC 2123 and the conversion time available, conversion of a dVOUT value may lag by one or more switching cycles.
Still referring to
T
S1TEMP
=TBL1*γ and
T
S2TEMP
=TBL2*γ
dVOUT1(ΔVO1) may be determined in block 2506. TS1OUT may be calculated according to the equation below in step 2508.
T
S1OUT
=T
S1TEMP
−K
1
*dVOUT1
dVOUT2(ΔVO2) may be determined in step 2510. TS2OUT may be calculated according to the equation below in block 2512.
T
S2OUT
=T
S2TEMP
−K
2
*dVOUT2
The scaling factor γ may be updated in block 2514 using equation 1 (EQ1)
γNEW=γOLD−KS*(VIN−VTAR) [EQ1]
This may provide feedback to steer VIN towards the value desired for maximum power point delivery. The value of KS may be determined such that stability is maintained. For example, in some embodiments a value of KS may be found by equation 2 (EQ2):
K
S=1(50*VTAR) [EQ2]
γ is intended to be maintained at a value of approximately one. In the slow outer loop method 2300, α may be updated at block 2306 using the instant value of γ, then the stored value of γ is reset to one at block 2305.
The temperature of a PV 401, as measured at the back skin of the panel, plus measured values for voltage and electrical current at the output terminals, together with an electrical and thermal model of the panel, may analytically determine a voltage/current condition corresponding to the maximum power point (“MPP”). Temperature change may be relatively slow, and therefore dealt with by a slow outer control loop (method 2300,
Referring to
In a more general case, the MPP voltage for a range of irradiance values will follow a curve as a function of the MPP current. In the various embodiments the MPP controller, as described above, may operate using a table approximating the IV curve to be followed. If the temperature of a PV 401 (sensed via the back skin temperature sensor) changes, the new MPP curve and its approximation as a table may be calculated by the external loop using method 2300.
Sometimes a PV 401 may not be uniformly illuminated. For example, falling leaves, overhead wind turbine blades, bird droppings, soil, or passing clouds may cause some or as few as one cell in an array of cells of a full PV panel 401 to be shaded. The risk of a shaded cell entering reverse breakdown and dissipating a large amount of power is highest when there is strong illumination over the panel generally but a single cell is shaded. There are two cases of shading to be considered: shading in a cell that is increasing; and shading in a cell that is decreasing.
If a cell is shaded relative to the cells in the rest of an array, the IV curve of the panel changes shape because all other cells have the same characteristic, but the shaded cell (in series with the other cells of a common string) has a limited photocurrent. The characteristic curve may be very similar to that illustrated in
The voltage drop may be mainly on the shaded diode. So the voltage across the shaded diode may be the voltage difference between the nominal curve and the one with partial shading. For a given PV 401 operating condition, a higher voltage may always be safer than a lower voltage, in that a higher voltage may result in a lower current. So in the various embodiments the voltages corresponding to the MPP, Hot Spot Suppression (“HSS”), and safe operation may be compared and the higher of the three selected. Safe operation is a design consideration, wherein a PV 401 manufacturer may specify the maximum power that a PV cell may deliver without damage. A PV 401 manufacturer may desire to limit the voltage across a reversed cell and may therefore specify a limit voltage as a voltage corresponding to safe operation. Alternatively, a PV 401 manufacturer may limit the reverse power (PLim). From the PLim the manufacture may determine a voltage corresponding to safe operation that may be the limited reverse voltage (Vrev
V
rev
lim
=P
Lim
/I
Panel
The fast loop 2500 may follow the MPP and safety limit and effectively protect the shaded diodes from excessive reverse voltage. The slow loop 2300 may determine whether the inner loop 2500 is in a regular MPP condition or a hot spot protection condition. In the later case the slow loop 2300 may determine whether the limit was triggered by partial or uniform shading by observing the MPP position with respect to the regulation point.
Piece Wise Linear Method
In the various embodiments, control of a PV 401 may be effected by a piecewise linear (“PWL”) method. A PWL method may drive the operating point of a panel to be part of a piecewise linear curve with respect to a calculated I-V curve. In such a method the target voltage VTAR may be determined by equation 3 (EQ3):
V
TAR
=V
ZERO−(I−Ioffset)×KR, [EQ3]
where Vzero and KR are constant, Ioffset (in counts) may be based on the ADC result for zero coil current, and I may be the instant ADC value for the current measurement path. The loop may run once every switching cycle, while the current may be updated as fast as possible.
A piecewise linear method may be implemented in an MPP controller by a table of values for Vzero, KR and Ioffset appropriate for various regions corresponding to different desired control behaviors. The values on one line of the table may be applied if I (the instant coil current) is larger than Ioffset on a certain line and smaller than the Ioffset on the line above (the top line corresponds to a single ended comparison).
Referring to
A table may be constructed corresponding to
Consider an example wherein the instant current value I is 4000 mA. This value falls within the range corresponding to the middle row of Table 1, which corresponds to the regulation zone 3104. VTAR can be calculated using the EQ3 formula above, with the values for Ioffset, VZERO, and KR found in Table 2. Examination of Table 2 and EQ3 reveals that within the regulation zone 3104 the load line is a vertical line; that is, a constant voltage VZERO. Above the transition point 3116, corresponding to the top line of Table 1, KR is a negative number, thus VTAR increases. Current I may increase, but at a much slower rate than in the regulation zone.
Likewise for instant current I below transition point 3118, KR is a positive number and Ioffset is a negative number. KR being a positive number and Ioffset being a negative number causes the system to drive down the value of current I by increasing the voltage output of PV 401. VTAR may subsequently be used in EQ1 and EQ2 in the inner loop method 2500 and in turn in the outer loop method 2300.
It should be noted that the regions depicted in
V
TAR
n
=V
TAR
n-1
+K
S×(Vzero−(I−Ioffset)×KR−VTAR
where VTAR
In some embodiments dynamic resistance may be used to determine if a panel has experienced a change in overall insolation, or if one or more cells have become partially shaded. This is important if, for example, the operating point has been moved by the PWL protection limit to a new operating condition. The PWL protection limit may be provided by a fast loop for preventing damage due to a hot spot, as previously discussed. In various embodiments a recovery process may determine a new operating condition.
Upon a first measurement, a panel may be determined to be operating at its maximum power point 3706 on the I-V curve 3704. At a subsequent measurement the operating point may be determined to be at the condition denominated as 3708 by the fast loop of the PWL method. The condition 3708 corresponds to the point at which a current curve 3710 intersects the PWL protection curve 3106. However, the control loop 2300 (
At block 3815 Tp may be increased by an amount expected to increase the panel voltage V by approximately one volt. Increasing Tp may be beneficial because increasing the voltage of a panel may be safer than decreasing the voltage of a panel. At the new operating condition resulting from the increase in Tp, at block 3820 V and I for the panel may be read, and ΔI and ΔV may be calculated by taking the instant values of V and I read at this new operating point and subtracting the previous values for I and V determined before increasing Tp. At block 3825 dynamic impedance may then be found according to:
The manufacturer of the solar panel typically provides a value for the shunt resistance, Rp, of the panel, determined by testing at the time of manufacture. Using the shunt resistance Rp and the dynamic impedance Rdy found at block 3825, and the uniform dynamic resistance Rfy_unif calculated in block 3810, at determination block 3840 the dynamic impedance Rdy is compared to the result of the sum of the uniform dynamic resistance and one half the shunt resistance (Rdy_unif+Rp/2*Nbr of cells). The scale factor of (½) is an arbitrary selection; greater or lesser values may be used. What matters is that the discrimination level is spaced from the two impedances that need to be discerned (Rdy_unif and Rdy_unif+Rp/Nbr of cells). If the dynamic impedance Rdy is greater than the result of the sum of the uniform dynamic resistance and one half the shunt resistance (Rdy_unif+Rp/2*Nbr of cells), (determination block 3840=“TRUE”), the change in operating condition (for example, the determined new operating point 3708 illustrated in
With a new I-V curve established, the PWL tables may be updated at block 3855, and at block 3850 the operating point is driven to the Vmp point along the newly established I-V curve.
If the dynamic impedance Rdy is less than the result of the sum of the uniform dynamic resistance and one half the shunt resistance (Rdy_unif+Rp/2*Nbr of cells), (determination block 3840=“FALSE”), the I-V curve determined at block 3805 actually is the instant I-V curve, and at block 3855 the PWL tables may be updated. At block 3860 the operating point is driven to the Vmp point along the I-V curve determined at block 3805.
State Diagram Method
In some embodiments control of a PV 401 may be effected by a parameterized model of the panel data that is instantiated in a programmable processor. An example of such a state diagram is illustrated in
Using the intrinsic model illustrated in
The locus of Vmp may be calculated as a function of photocurrent current at nominal temperature by using the panel model. The junction temperature may be calculated based upon the back skin temperature. The Vmp locus may be recalculated using a new Tj value. In some embodiments, the temperature model may not be updated unless there is a change of Tj larger than 5° C. (˜1.2 V).
The temperature needed to evaluate the panel electrical model may be the junction temperature Tj. Tj may be measured directly if the panel includes a temperature sensor that is integrated with the cells, or a spare cell may be used to measure the temperature at the level of the cells. Tj may be determined indirectly by measuring the temperature on the back skin of the panel.
Referring to the thermal model shown in
(Tj−Ta)/(Rf+Rcf)+(Tj−Ta)/(Rcb+Rb)=Pin
Assuming Rcf>Rb, then Pin≈((Tj−Ta)/(Rcb+Rb)=(Tj−Tb)/Rcb) in the static case.
Tj=Tb+Pin*Rcb
Pin=−Ii×Vi+Pth
where Pth is the thermal input power resulting from photon absorption. Pth may be extracted from measured electrical data and meteorological data for a site, the (−Ii×Vi) term is the power that is generated electrically and is transferred to the power modulator. For a clean panel, a value of Pth proportional to Iph current may be used due to the almost linear dependency of Iph to the illumination.
Tj=Tb+Pin*Rcb=Tb+(ζIph−Ii Vi)Rcb
where ζ is the model parameters that describes the Pin proportionality to Iph.
Initially, the system 3200 may start at in an initial state 3208 which assumes a non-uniform shading environment and needs to find Vmp. Regulation begins at block 3208 at Voc and transitions via pathway 3224 to state NI 3220.
System 3200 may be in (an assumed) state NI 3220 and may slowly walk VTAR towards Vmp of the uniform shaded case by dropping the voltage by a voltage increment, such as 1 volt direct current (VDC), such as approximately 1 VDC, such as 2 VDC to 1.75 VDC, 1.75 VDC to 1.5 VDC, 1.5 VDC to 1.25 VDC, 1.25 VDC to 1 VDC, 1 VDC to 0.75 VDC, 0.75 VDC to 0.5 VDC, 0.5 VDC to 0.25 VDC, 0.25 VDC to 0.01 VDC, such as less than 1 VDC, such as more than 1 VDC, such as approximately 0.5 VDC, or such as approximately 0.1 VDC. Additionally, the voltage increment may be a voltage selected to allow minimal power variation near a maximum power point of the photovoltaic panel, PV 401, such as a power variation such as 1 watt (W), such as 2 W, such as 10 W to 5 W, 5 W to 4 W, 4 W to 3 W, 3 W to 2 W, 2 W to 1 W, 1 W to 0.5 W, 0.5 W to 0.4 W, 0.4 W to 0.3 W, 0.3 W to 0.2 W, 0.2 W-0.1 W, 0.1 W to 0.5 W, 0.5 W to 0.4 W, 0.4 W to 0.3 W, 0.3 W to 0.2 W, 0.2 W to 0.1 W, or such as 0.01 W. System 3200 may slowly walk Vtar towards Vmp of the uniform shaded case by dropping the voltage by a single voltage increment at a time while calculating the input power, keeping track of the maximum value. If Vmp is approached by less than 1.0 VDC, a determination that the system 3200 is actually in the uniform shading case may be made, and the system 3200 performs operational controls 3238 to transition to the US state 3212.
In the NI state 3220, if power decreases by more than 1% of the actual maximum value so far, the maximum may be reached, and PV 401 voltage may be decreased by 0.5 V. The PAMCC 400 may set up a regulation table with current limit and voltage regulation at the determined value and a safety limit below flash data, for example 2.0 VDC, scaled for Tj, and the system 3200 may perform operational controls 3228 to transition to the NS state 3218.
In the NS state 3218, if there is a non-uniform shading condition, and power increases, the system 3200 may perform operational controls 3230 to transition to the NI state 3220.
In the NS state 3218, if there is a non-uniform shading condition, and power is decreasing, then the new Vmp may be tracked and the system 3200 may perform operational controls 3232 to transition to the ND state 3216. In the ND state 3216, the regulation voltage may be increased by 1.0 VDC.
In the NS state 3218, if power decreases operational controls 3226 may be performed to transition the system 3200 to the NI state 3220.
In the NS state, if power increases, voltage may be increased by 1.0 VDC at a time while keeping track of the maximum. When power decreases by more than 1% of the maximum value so far, voltage may be decreased by 1.5 VDC and the PAMCC 400 may set up a regulation table with current limit and voltage regulation at the determined values and safety limit at 2 V below flash data scaled for Tj. Operational controls 3234 may be performed to transition to the NS state 3218.
If the system is in a uniform shading environment, the PAMCC 400 may build the regulation table based on assumed current limit, voltage regulation at Vmp @ max irradiance, safety limit 20. VDC below nominal flash data, adjusted for Tj.
If the operating point results in the protection region, regardless of the instant state, operational controls 3257 may be performed to transition to the P state 3206. The protection region may be triggered (reached) anytime the panel current reduces. The panel current may be reduced due to uniform or non-uniform shading conditions. However, a fast loop method 2500 may not know the reason for the current drop, and to protect against unsafe conditions the P state 3206 may be entered and the uniform or non-uniform shading condition may be determined.
In the P state 3206, while increasing voltage by 1.0 VDC steps, if power increases then voltage may be increased by 0.5 V and operational controls 3222 performed to transition to the NI state 3220, then operational controls 3228 may be performed to transition to the NS state 3218. Then the PAMCC 400 may build a regulation table with current limit and voltage regulation at the determined values and may set a safety limit at 2 V below the flash data, adjusted for Tj. In an alternative embodiment (not shown) transition from the P state 3206 directly to the NS state 3218 may be performed.
In the P state 3206, if power decreases, then operational controls 3248 may be performed to transition to the UD state 3214, where the PAMCC 400 may use the original Vmp and may set up a regulation table with current limit and voltage regulation at the determined values and may set a safety limit at 2 VDC below the flash data, adjusted for Tj.
At the US state 3212, if current increases operational controls 3242 may be performed to transition to the UI state 3210, and the PAMCC 400 may set up a regulation table with current limit and voltage regulation at the determined values and may set a safety limit at 2 VDC below the flash data, adjusted for Tj.
At the US state 3212, if current decreases, operational controls 3244 may be performed to transition to the UD state 3214. The controller may determine if the PV is in a uniform or partial shading condition (i.e., the operating condition should be for uniform or partial insolation) by increasing voltage by a voltage increment, such as approximately 1 VDC. such as 1 volt direct current (VDC), such as approximately 1 VDC, such as 2 VDC to 1.75 VDC, 1.75 VDC to 1.5 VDC, 1.5 VDC to 1.25 VDC, 1.25 VDC to 1 VDC, 1 VDC to 0.75 VDC, 0.75 VDC to 0.5 VDC, 0.5 VDC to 0.25 VDC, 0.25 VDC to 0.01 VDC, such as less than 1 VDC, such as more than 1 VDC, such as approximately 0.5 VDC, or such as approximately 0.1 VDC. Additionally, the voltage increment may be a voltage selected to allow minimal power variation near a maximum power point of the photovoltaic panel, PV 401, such as a power variation such as 1 watt (W), such as 2 W, such as 10 W to 5 W, 5 W to 4 W, 4 W to 3 W, 3 W to 2 W, 2 W to 1 W, 1 W to 0.5 W, 0.5 W to 0.4 W, 0.4 W to 0.3 W, 0.3 W to 0.2 W, 0.2 W-0.1 W, 0.1 W to 0.5 W, 0.5 W to 0.4 W, 0.4 W to 0.3 W, 0.3 W to 0.2 W, 0.2 W to 0.1 W, or such as 0.01 W.
At the UD state 3214, if a partial shading case is determined, the processor may perform operational controls 3236 to transition to the NS state 3218. If power increases then voltage may be increased by 0.5 VDC and the PAMCC 400 may set up a regulation table with current limit and voltage regulation at the determined values, and may set a safety limit at 2 V below the flash data, adjusted for Tj.
At the UD state 3214, if a uniform shading case is determined, operational controls 3246 may be performed to transition to the US state 3212. The original Vmp may be returned to and the PAMCC 400 controller may set up a regulation table with current limit and voltage regulation at the determined values and may set a safety limit at 2 V below the flash data, adjusted for Tj.
In the UD state, periodic testing may be performed to determine that the PV 401 is still in fully shaded (uniform) case, increasing voltage by a voltage increment such as 1 volt direct current (VDC), such as approximately 1 VDC, such as 2 VDC to 1.75 VDC, 1.75 VDC to 1.5 VDC, 1.5 VDC to 1.25 VDC, 1.25 VDC to 1 VDC, 1 VDC to 0.75 VDC, 0.75 VDC to 0.5 VDC, 0.5 VDC to 0.25 VDC, 0.25 VDC to 0.01 VDC, such as less than 1 VDC, such as more than 1 VDC, such as approximately 0.5 VDC, or such as approximately 0.1 VDC. Additionally, the voltage increment may be a voltage selected to allow minimal power variation near a maximum power point of the photovoltaic panel, PV 401, such as a power variation such as 1 watt (W), such as 2 W, such as 10 W to 5 W, 5 W to 4 W, 4 W to 3 W, 3 W to 2 W, 2 W to 1 W, 1 W to 0.5 W, 0.5 W to 0.4 W, 0.4 W to 0.3 W, 0.3 W to 0.2 W, 0.2 W-0.1 W, 0.1 W to 0.5 W, 0.5 W to 0.4 W, 0.4 W to 0.3 W, 0.3 W to 0.2 W, 0.2 W to 0.1 W, or such as 0.01 W. As the voltage is increased by the voltage increment, if power increases the voltage may increase by 0.5V and the PAMCC may set up a regulation table with current limit and voltage regulation at the determined values and may set a safety limit at 2 V below flash data, adjusted for Tj. The system transitions 3236 to the NS state 3218. If the power does not increase, the controller may return to the original Vmp and set up a regulation table with current limit and voltage regulation at the determined values and may set a safety limit at 2 V below flash data, adjusted for Tj. At this point the system transitions 3246 to the US state 3212.
If the current increases beyond the higher limit in the MPPT regulation segment the inner loop transitions in the current limiting segment, the outer loop observes this and transitions (arc 3545) to the CL 3204 state. In this state if current increases further the outer loop may transition to the STOP 3202 state via the 3250 transition arc. Other hardware errors or operating conditions like switching errors, input bound conditions, arithmetic exceptions, license expiration, could set the transition to STOP 3202 over 3252 arc. If current falls back then the outer loop will follow the inner loop to UD state.
While discussed in relation to a PAMCC the MPPT control, piecewise linear method, and state machine processes may be used in conjunction with any controller and are not dependent on the use of a PAMCC in a solar panel system. The various embodiments described herein may provide a combined method for controlling solar panel system operations using a calibrated panel model and panel back skin temperature based MPPT determination, identifying uniform versus non-uniform panel shading conditions, and setting protection limits as a form of hot spot suppression all in a unified control routine.
In an embodiment the controller 2116 illustrated in
The various embodiments described herein may be useful for controlling any source of direct current and converting the direct current to three phase alternating current. Examples of direct current sources include solar panel, wind turbine, battery, geothermal, tidal, hydroelectric, thermoelectric and piezoelectric power systems. For the purpose of discussion, the example of a solar system embodiment is used as an example for describing the functioning and capabilities of the various embodiments. However, one skilled in the art would recognize that the circuits and processes described herein may be applied to other direct current sources as well. Accordingly, the scope of the claims should not be limited to solar power applications except as expressly recited in the claims.
The foregoing method descriptions and the process flow diagrams are provided merely as illustrative examples and are not intended to require or imply that the steps of the various aspects must be performed in the order presented. As will be appreciated by one of skill in the art the order of steps in the foregoing aspects may be performed in any order. Words such as “thereafter,” “then,” “next,” etc. are not intended to limit the order of the steps; these words are simply used to guide the reader through the description of the methods. Further, any reference to claim elements in the singular, for example, using the articles “a,” “an” or “the” is not to be construed as limiting the element to the singular.
The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the aspects disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.
The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some steps or methods may be performed by circuitry that is specific to a given function.
In one or more exemplary aspects, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module executed which may reside on a tangible non-transitory computer-readable medium or processor-readable medium. Non-transitory computer-readable and processor-readable media may be any available media that may be accessed by a computer or processor. By way of example, and not limitation, such non-transitory computer-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to carry or store desired program code in the form of instructions or data structures and that may be accessed by a computer. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and/or instructions on a non-transitory processor-readable medium and/or computer-readable medium, which may be incorporated into a computer program product.
The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the following claims and the principles and novel features disclosed herein.
This application is a continuation in part of and claims priority to U.S. patent application Ser. No. 12/335,357 entitled “Detection and Prevention of Hot Spots in a Solar Panel” filed Dec. 15, 2008, the entire contents of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12335357 | Dec 2008 | US |
| Child | 13277907 | US |
