Patent ID: 12218509

DETAILED DESCRIPTION

Electric codes, governing electrical standards of solar power systems, such as the National Electric Code (NEC), set forth conservative electrical standards for solar power systems in two key areas. First, the specification references voltage measurements taken as a voltage open circuit (VOC), which is very conservation theoretical measurement. A more real-world electrical measurement of PV string voltage is the “voltage maximum power point” (Vmp). In addition to the type of voltage measurement, the NEC guidelines are conservative by referencing, with respect to determining a maximum PV string voltage, the lowest historical ambient temperature for which the solar power system operates. However, in practice, solar power systems rarely operate at the lowest historical ambient temperature for which the system may be exposed, and thus it is not representative of the real-world operating conditions of photovoltaic panels. For example, the average difference between the ambient temperature and the lowest historical ambient temperature is 30° C. Thus, if the lowest historical ambient temperature is used to design the PV string voltage, then the solar power system will be designed to significantly underperform the potential of the modules on the PV string.

In National Electrical Code (NEC) (Section 690.7B DC-to-DC Converter Source and Output Circuits) in a dc-to-dc converter source and output circuit, the maximum voltage shall be calculated in accordance with 690.7(B)(1) (Single DC-to-DC Converter), which reads for circuits connected to the output of a single dc-to-dc converter, the maximum voltage shall be the maximum rated voltage output of the dc-to-dc converter. As is explained fully below, the present disclosure sets forth systems, methods and apparatus that realize maximum voltage efficiency by actively, as opposed to using a passive design, managing the voltage and current, thus maintaining safety of the solar power system.

The output voltage of a PV module is dependent upon both irradiance (i.e. the energy per unit of time incident on the surface of the module) and temperature of the module. The ambient temperature of the PV modules is a primary factor in determining the output voltage it produces.

FIG.1shows a graph that depicts PV string voltage as a function of occurrences in a solar PV system at an example site installation. Specifically, the graph depicts the distribution, measured over a period of time, of a number of occurrences versus the string voltage for a PV string containing 32 modules. The distribution graph ofFIG.1shows a PV string voltage of approximately 1096 volts, @ 34.7 Vmp.

FIG.2, similar toFIG.1, is also a graph depicting PV string voltage as a function of occurrences. While the graph ofFIG.1depicts string voltage for a PV string constructed of 32 modules, the graph ofFIG.2reveals a PV string voltage of approximately 1400 volts @ 34.7 Vmp.

FIG.3depicts a chart showing the average operating voltage based on ambient temperature. As shown inFIG.3, a PV string, which consists of 32 modules, has an average PV string voltage of 878 volts at 24° C., whereas it has an average PV string voltage of 1073 volts under an average ambient temperature of 14° C. A larger number of modules, of course, produce higher PV string voltages as seen in the graph ofFIG.3. Specifically, a 44 module solar power system has an average PV string voltage of 1207 volts, with an ambient temperature of 24° C., and an average PV string voltage of 1475 volts, under an ambient temperature of 14° C.

FIG.4is a block diagram illustrating one embodiment of a solar power system. The solar power system (300), as shown, includes a plurality of photovoltaic modules (312,313,314. . .316) coupled together to form a single PV string, which is coupled to a solar power system as shown by the home run connections318and319. The number of PV modules in the PV string is determined by the active design process such that the number of PV modules is configured based on the irradiance and temperature at the site of the solar power system that cause the PV string to exceed a maximum voltage specification of an electrical code, when operating at the lowest expected ambient temperature at the site.

As shown inFIG.4, PV string (310) is connected to string voltage control (320) and string voltage limiter (330). The string voltage control (320) is in communication with the string voltage limiter (330) such that the string voltage control (320) detects the PV string voltage and signals the string voltage limiter (330) to reduce the PV string voltage when the radiance and temperature at the site cause the PV string voltage to exceed the maximum voltage specification. When this occurs, the strength voltage limiter (330) reduces the power output from the PV string to the system via the combiner box350.

In some embodiments, when the site conditions result in a reduction of the PV string voltage below a level specified by the voltage specification, then the string voltage controller (320) may signal the string voltage limiter (330) to stop limiting power to the system. In some embodiments, the process to reactivate the PV string, and subsequent PV string power, may employ hysteresis to prevent the system from switching on and off too frequently.

The string voltage limiter (330) may either reduce the voltage, or it may eliminate the power from the PV string (310) altogether. Embodiments for reducing power output from a string are described more fully below.

FIG.5Illustrates a solar power string in a solar power system configured in accordance with the teachings of the present disclosure. The example solar power system (400) includes a plurality of PV modules410(i.e., “1 to n” number of PV modules) that are configured on a single string of PV modules. The solar string (400) further includes bypass PV modules (420) (i.e., “1-m” bypass PV modules). The number of PV modules and bypass PV modules determine, under different operating conditions, the target string voltage for the string of PV and bypass PV modules (410and420). Designing a system to optimize the target string voltage by selecting a configuration of the PV modules (410) and the bypass PV modules (420) is referred to herein as the “active design.” Thus, the active design includes selection of a number of total PV modules, including PV modules (410) and bypass PV modules (420).

The string of PV modules (400) further includes a string control voltage module (440) and a string voltage bypass module (430). In general, the string control voltage module (440) monitors the “target string voltage”, and if the target string voltage exceeds a threshold voltage (i.e., a maximum voltage allowed by the regulations), then the string control voltage module (440) triggers the string voltage bypass module (430) to eliminate one or more bypass PV modules (420) from the PV string, so that the target string voltage does not exceed the maximum voltage.

FIG.5Aillustrates an example of a portion of a solar power system designed and configured in accordance with a “Passive System Design.” The PV module string (510), consisting of 32 PV modules, is designed to operate at a target string voltage of 1096 V, based on the lowest ambient temperature of the site. For this example, the passive system design system (500) includes, along with the 32 PV modules (510) connected in series, a junction box (520) connecting the PV module string through a home run bus (530).

In a situation where a combination of increased irradiance and decreased ambient temperature increases the voltage of each individual PV module, the PV string voltage, designed using the passive system design, will still operate below maximum operating voltage.

FIG.5Billustrates an example portion of a solar power system configured in accordance with an Active System Design. For this example, based on the site (i.e., location of the solar power system) and the configuration of the solar power system, a target string voltage of 1496 V is achieved when operating under favorable temperature and irradiance conditions. Using 1496 V as the target string voltage leads to an active design and configuration of a PV module string of 44 modules (i.e., 44 modules are coupled in series to create a PV module string). The string of PV modules is coupled to a junction box (570), via a home run bus (560).

In a situation where a combination of increased irradiance and decreased ambient temperature increases the voltage of each individual panel so as to exceed STC conditions, systems, configured in accordance with the teachings of the present disclosure, temporarily disconnect one or more PV modules (550), via module(s) bypass control560, to continue to operate below maximum operating voltage.

FIG.6illustrates an exemplary circuit for a solar power system configured in accordance with the present disclosure. Solar power system600includes a plurality of photovoltaic modules, configured into a serial PV string (610). The number of PV modules contained in PV string (610) is determined in accordance with the active system design methodology (as discussed above). In addition to the PV modules in PV string (610), the solar power system600further includes one or more bypass PV modules. For the example shown inFIG.6, the solar power system (600) contains 2 bypass PV modules; namely, bypass PV modules616and618. Although the example ofFIG.6illustrates two PV bypass modules, any number of bypass PV modules may be incorporated without deviating from the spirit or scope of the invention.

As shown inFIG.6, the PV string (610) incorporates the bypass PV modules (616and618), into a single string of PV modules (referred to herein as “total PV string”) through string voltage bypass circuits612and614. The power output of the PV string, at SV− on string voltage bypass circuit614, is input to inverter (620). The voltage at the SV− terminal on string voltage bypass circuit (614) constitutes the string voltage for both the PV string (610) and the bypass PV string (616and618).

For this example system, the maximum string voltage (from both the PV string and the bypass PV string) is 1500 vdc. If the string voltage input to inverter (620) does not exceed 1500 Vdc, then string voltage bypass circuits (612and614) connect bypass PV modules (616and618) through the PV+ and PV− connections, as shown inFIG.6. The string voltage bypass circuits (612and614) further include a bypass status LED, to indicate whether the associated bypass PV module has been eliminated from the total PV string.

The string voltage control (630) monitors the string voltage to determine whether the string voltage exceeds the maximum value (e.g., 1500 Vdc). As shown inFIG.6, the string voltage input to inverter620is also input to the Vstatinput on string voltage control (630). The string voltage control630provides status regarding such items as whether the maximum string voltage has been exceeded, whether one or more bypass circuits have been deactivated, etc. The string voltage control (630) also includes, for this example, a mechanical configuration switch to allow manual control of string voltage bypass circuits (612and614).

As shown inFIG.6, string voltage control module (630) is coupled to string voltage bypass circuits (612and614) through a bypass control input. In operation, when the string voltage control module (630) senses that the string voltage approaches the maximum string voltage, the string voltage control module (630) signals, to the bypass control input on string voltage bypass circuits (612and614), to eliminate one or more of the bypass PV modules. For example, if the string voltage exceeds the maximum voltage allowed by a first predetermined amount, then the string voltage control (630) may signal the string voltage bypass circuit (614) to eliminate bypass PV module (618) from the total PV string output. If, however, the string voltage exceeds the maximum voltage allowed by a second and higher predetermined level, then the string voltage control (630) may signal the string voltage bypass circuit612to eliminate a second bypass PV module, bypass PV module616.

FIG.7illustrates an example string voltage control used in the solar power system of the present disclosure. As shown inFIG.7, the string voltage control (700) includes a mechanical configuration switch (740). The mechanical configuration switch (740) provides a means to program the string voltage control (700) to limit the total PV string to a predetermined voltage. For example, the string voltage control (700) may be set not to exceed a string voltage of a predetermined value, such as 1500 V or 1000 V. The string voltage control (700) may be programmed to set any maximum voltage value. Although the illustration inFIG.7shows programming the string voltage control (700) via a mechanical configuration switch (740), any means of programming the string voltage control (700) may be used without deviating from the spirit or scope of the invention. For example, string voltage control (700) may be controlled remotely, via a port on the string voltage control (700) configured to receive binary information, such as information transmitted by a wired connection, or even information transmitted by a wireless connection, controlled from a system control device.

String voltage control (700) receives a reference voltage, Vstat, for input to microcontroller unit (MCU)710. Also input to MCU710is the output of the mechanical configuration switch (740). In turn, the MCU710compares the reference voltage, Vstat(the total PV string output) to a voltage value set by the mechanical configuration switch (740). If the MCU (710) detects that the Vstatis above the voltage value set by mechanical configuration switch (740), then the MCU710programs the bypass generator (730) to signal the string voltage bypass circuits accordingly (e.g., string voltage bypass circuits612and614inFIG.6). As such, the bypass generator (730) may be set to bypass no bypass PV modules, or it may be set to bypass one or more bypass PV modules.

The string voltage control (700) includes, for this example, status LEDs (720), which provide status of the activation, or deactivation, of the bypass PV modules from the string. For example, the status LEDs (720) may indicate that one bypass PV module has been eliminated from the total PV string.

FIG.8illustrates a string voltage bypass circuit configured in accordance with one embodiment of the present disclosure. As shown inFIG.8, string voltage bypass circuit (900) serially couples two PV modules such that the power output of a PV module is input to the string voltage bypass circuit (900) through the PV+ and PV− terminals. Also, the string voltage bypass circuit (900) selectively couples the power from the PV+ and PV− terminals to a subsequent PV module on the string, via the SV+ and SV− terminals. Any type of bypass circuit, illustrated as bypass circuits (910) inFIG.8, which decouples the PV+ and PV− terminals from the SV+ and SV− terminals may be used. The string voltage bypass circuit (900) further includes a capacitor (940) and a diode (930) coupled, in parallel, across the PV+ and PV− terminals.

FIG.9illustrates one example of a bypass circuit, as used in the bypass control circuit depicted inFIG.8. Bypass circuit1000includes a “switch”, to couple, or decouple, the terminal voltage from a previous PV module to a subsequent PV module in the PV string. In some embodiments, the bypass circuit (1000) may include, as bypass control (1010), a field effect transistor (FET), or other semiconductor device, such as other types of transistors, that permit connecting and disconnecting the power between two PV modules, connected in series.

FIG.11illustrates one embodiment to reduce PV string voltage using a string voltage diverter. PV string (1012), which includes “1-n” photovoltaic panels, configured in accordance with the active design, is input to a string voltage diverter (1020) through home run returns1014and1015. The string voltage diverter (1020) is coupled to a string voltage control (1024) via a voltage trunk cable (1022). When the PV string voltage exceeds the maximum voltage specification, the string voltage control (1024) senses this condition, and activates the string voltage diverter (1020). The string voltage diverter (1020) shunts excess current to ground, so as to eliminate power from the PV string, and thereby lower PV string voltage. In some embodiments, the string voltage diverter (1020) uses a PV wire (1028) and a grounding rod (1030), buried within the earth, to dissipate power from the PV string.

If the PV string voltage no longer exceeds the maximum specification (by a specified margin), then the string voltage control (1024) may signal the string voltage diverter (1440) to cease limiting the power.

FIG.12illustrates one embodiment to fully shut off a PV string when the PV string exceeds a maximum specification. The PV string, configured using the active design, comprises a plurality of PV modules, connected to a string voltage control module (1420) through home runs (1412and1413). In turn, the string voltage control (1420) is connected to a string voltage shut off module (1440) through a voltage trunk cable (1430). For this embodiment, when the PV string voltage exceeds a maximum voltage specification, as sensed by the string voltage control (1440), then the string voltage control (1420) signals the string voltage shut off module (1440) to shut off the power to the solar power system, through the combiner box (1450).

If the PV string voltage no longer exceeds the maximum specification (by a specified margin), then the string voltage control (1420) may signal the string voltage shut off module (1440) to reactivate the PV string into the solar power system via combiner box (1450).

FIG.13Aillustrates an example solar power system configured in accordance with a passive system design. For this example, solar power system (1100) includes four PV strings (1110,1120,1130and1140) input to a collection box (1150). With a passive system design, the solar power system is designed to include a maximum number of PV modules without exceeding the lowest ambient temperature at the site. As shown inFIG.10A, strings1through4each include 30 modules.

FIG.13Billustrates an example solar power system configured in accordance with the teachings of the active system design of the present disclosure. The solar power system (1135) contains 3 PV strings (1160,1170and1180), connected to a collection box (1195) through home run returns. In addition, the solar power system (1135) includes module(s) bypass control (1150) to eliminate modules from the PV string, as necessary. Using the active system design methodology, solar power system (1135) incorporates 40 PV modules per PV string, a 33% increase in the number of PV modules per string in a solar power system configured in accordance with the passive system design methodology.

Solar power systems, configured in accordance with the active system design, contain fewer PV strings in the solar power system. Fewer PV strings in the solar power system, including a significant reduction of PV strings in large solar power plants, results in a significant savings. For example, solar power systems that utilize the active system design of the present disclosure require less PV wire, connectors, combiner boxes, and trenching. For the example passive system design and active system design systems illustrated inFIGS.10A and10B, there are 25% fewer home runs, up to 20% less PV wire used, 25% less combiner boxes (connections), 25% less connectors, less trenching (site-specific), and 25% less labor hours for electricians. The active system design systems also require less labor to install the electrical balance of system.

FIGS.14A and14Billustrate a comparison between solar power systems using the active and passive system design when the solar power system use trackers. Solar power system (1200), configured in accordance with the passive system design methodology, includes two strings of 32 PV modules per string (1210and1220). As shown inFIG.11A, each string (1210and1220) is deployed on a single tracker. As is well known, trackers are used to optimize the irradiation on the surface of the PV panels as the sun moves across the sky. As shown, the solar power system (1200) includes a controller (1230), to control the tracking system and a motor (1240) to power the trackers for movement.

FIG.14Billustrates an example solar power system (1250) configured in accordance with the tracker system design methodology of the present disclosure. Using the active system design methodology, solar power system (1250) incorporates 44 PV modules per string, as shown. As such, 44 PV modules are configured on each tracker, as opposed to the 32 PV modules configured on each tracker in the solar power system (1200) configured in accordance with the passive system design methodology. Solar power system (1250), like solar power system (1200), includes infrastructure that consists of a controller (1280) and a motor (1290). Accordingly, with the same cost to implement a tracker system, the active system design (solar power system1250) implements 88 modules on a tracking system, whereas solar system1200, implemented with the passive system design, includes only 64 PV modules for the same tractor infrastructure. For the example passive system design and active system design system illustrated inFIGS.11A and11B, utilizing the same motor and control infrastructure, the active system design increases the power per tracker, thereby lowering the number of trackers in a PV plant, and correspondingly lowering the number of tracker motors and controllers by up to 20%, which, in turn, lowers overall PV plant costs. Therefore, the active system design provides efficiencies for solar power systems that implement trackers.

FIGS.15A and15Billustrate the ability to optimize the voltage for transmission when comparing solar power systems implemented with the passive system design versus the active system design. For this example, solar power system (1300), depicted inFIG.12A, includes 32 PV modules in a single PV string. Thus, the average voltage produced by a string is around 900 to 1100 V. In contrast, the solar power system (1330) depicted inFIG.12Bincorporates 44 PV modules per string, resulting in an average PV string voltage of 1300 to 1500 V. By increasing the average operating voltage by +/−300V, line resistance is lowered and more energy is harvested (i.e., approximately 0.3% power gain improvement).

As is well known in the field of power transmission, to move electricity long distances, electrical voltages are increased dramatically to lower transmission “line losses.” High voltages are used in transmission systems because a higher voltage implies a lower current for a given power of transmission. With a lower current, less heat is generated in the transmission lines and so less energy is wasted. Since the active design system, as compared to the passive design system, increases the average operating voltage of a power plant, then less heat is generated in the transmission lines, and consequently, more energy is harvested. Consequently, as voltage in a PV power plant increases, the “line loses” of electricity decreases, and more electricity is harvested from the same PV power plant. Accordingly, the active system design improves voltage optimization for transmission of power, resulting in less loss of energy.

In some embodiments, a designer of a solar power system, which configures the system in accordance with the active system design methodology, may execute a process to determine an optimal number of PV modules per string. The basic premise for the active system design methodology is based on a regulatory voltage specification. In a DC PV source circuit, or output circuit, the maximum PV system voltage for that circuit shall be calculated as the maximum regulatory voltage divided by the rated maximum power point voltage (Vmp) of the PV module connected series, rounded down, and corrected for the lowest expected ambient temperature, using Conversion Table 1.

CONVERSION TABLE 1Correction factor for lowest ambient temperatureAmbientAmbientTemperatureCorrection:Temperature° F.Add Module° C.−11 to −26+1−24 to −313 to −10+2−16 to −2316 to 2+3−9 to −1528 to 17+4−2 to −841 to 29+55 to −152 to 42+611 to 6

As an example, a geographically location for a PV string has a lowest ambient temperature of −17 C, a 1,500V maximum regulatory voltage of a PV string, and a PV module with a Vmp of 34.7V using (STC) Standard Test Conditions (i.e., standard test conditions (STC) of irradiance of 1000 W/m2, spectrum AM 1.5 and cell temperature of 25° C.).

Step 1: Divide 1,500V by a PV module with a Vmp of 34.7V=43.227 modules

Step 2: Round Down=43 modules per string.

Step 3: Add correction factor from Conversion Table 1. −17° C.=+2 modules

Step 4: 43+2=45 modules per string

First, to specify a system with an active design, a maximum regulatory full page of a PV string is determined. As discussed above, the maximum regulatory voltage of a PV string is set by the governing authority. For example, the maximum regulatory voltage of a PV string may be 1500 volts.

To configure the system, a number of PV modules are selected for a PV string based on a relatively low occurrence that the site conditions (temperature and irradiance) will place the PV modules in an operating condition that would exceed the maximum regulatory voltage of a PV string.

FIG.16illustrates an example system configured with 44 modules per string for which operating conditions (temperature and irradiance) at a site result in either exceeding the maximum regulatory voltage of a PV string or staying within compliance of the maximum regulatory voltage. For the 44 PV module string example, as shown in pie chart1410, 96.74% of the time the atmospheric conditions result in PV string voltages that fall below the maximum regulatory voltage, and only 2.26% of the time the atmospheric conditions exceed the maximum regulatory voltage of a PV string. The chart (1410) ofFIG.13is based on 15 years of weather data, recorded in 15-minute intervals, with a minimum system output of 200 W/M2.

FIG.16further illustrates, in bar chart1420, the number of modules bypassed in order to maintain the string voltage below the maximum regulatory voltage for the atmospheric conditions depicted in chart (1410). As shown, for the 2.26% of time atmospheric conditions result in excessive regulatory string voltage, 64.37% of the time one module must be bypassed in order to maintain regulatory voltage; 29.53% of the time two modules must be bypassed to maintain regulatory voltage; and 5.99% of the time 3 modules must be bypassed to maintain regulatory voltage.

FIG.16further shows, in table1430, the total loss of energy from bypassing one or more PV modules. Specifically, if one PV module is bypassed, then the module percent of PV energy is decreased by 2.27%, and the weighted lost energy is equal to 0.03%. In the conditions where two PV modules are bypassed, 4.55% of module energy of a PV string is lost, resulting in a weighed loss energy of 0.03%. Under conditions for which three bypass modules are engaged, 6.82% of module energy of a PV string is lost, resulting in a weighted lost energy of 0.01%. Thus, given the atmospheric conditions example ofFIG.13, the total lost energy from deploying the active system (actively bypassing modules) is only 0.07%.

Those of skill in the art would understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

Those of skill would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments 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.