IL263278B2 - A dc pullup system for optimizing solar string power generation systems - Google Patents

Description

VGDU-102-IL-18 - 1 - A DC PULLUP SYSTEM FOR OPTIMIZING SOLAR STRING POWER GENERATION SYSTEMS Technical Field The present invention relates to solar power generation systems. More particularly, the present invention relates to a method and apparatus for optimizing the performance of solar string power generation systems. Background As of today, Photovoltaic solar power generation systems, i.e. solar farms, are typically made up of many solar panels each comprising many photovoltaic "cells". Photovoltaic cells are semiconductor devices that convert light into energy. When light shines on a panel, a voltage develops across the panel, and when connected to a load, current flows. Typically, a number of solar panels are connected in series, referred to as a "string", to create an increased output voltage. As a general rule, the higher the voltage - the less the energy loss. This is especially significant in large scale systems which typically have high current densities and longer cables for carrying the produced power. Thus, it is desirable to connect as much panels in series as possible in a string in order to increase the voltage at the expense of the current. However, the permitted maximum output voltage, of a single string, is typically limited by standards and state rules, due to the hazardous nature of a very high voltage. Thus, the maximum number of solar panels in a string is restricted by the solar panels’ own open circuit voltage (Voc), meaning that the total voltage Voc of all the panels in a single string has to be under the permitted maximum output voltage. However, the Voc of the string is VGDU-102-IL-18 - 2 - typically 24% higher than the maximum power voltage (Vmp), which is what the string typically produces under operating conditions. To comply with the standards and state rules, photovoltaic power generation systems are typically comprised of many strings, each comprising as many panels as permitted, where the strings are connected in parallel. These parallel-connected strings are referred to as an "array". Since the Solar cells generate DC power, while the electricity grid is typically AC power, an "inverter" has to be connected. An inverter may be connected to an array of many parallel-connected strings, for converting their DC power to AC power, for feeding the electricity grid or local consumers. Many solar inverters contain Maximum Power Point Tracking (MPPT) circuitry for maximizing the power from the strings. These known-in-the-art MPPT circuits adjust the voltage (and the current) at which the arrays operate, measure their output power, and seek those voltage and current values at which the array’s power output is maximized. Thus, the MPPT of the array is typically done by the inverter. However, since the voltage of the array may vary greatly due to clouds, shading, filth, the amount of light shining on the panel, the angle of the sun light, the temperature of the panel, and other factors, the maximum power point Voltage (Vmpp) of the inverter may vary greatly. This variance, which may total to hundreds of Volts, may have a negative effect on the efficiency of the inverter, since the most efficient voltage entrance variance, for a typical inverter, is relatively narrow. In addition, when the voltage from the array drops - the total efficiency of the system drops as well, especially in large scale systems which have long electrical cables as described above.

VGDU-102-IL-18 - 3 - It is therefore desired to introduce a solution for enhancing the production of solar string power generation systems which is typically constrained by the solar panels open circuit voltage (Voc) and instable because of the varied environmental conditions. One approach to solve this problem is to design the inverters to work in a narrow operating mode at their most efficient voltage range, and equip each string with a boost circuit that boosts the panel voltage respectively. This approach has a disadvantage as it requires a special inverter that is designed to work at the appropriate narrow range voltage of the specific boosters. In addition, this approach only takes into account the inverter’s performance without considering the conduction losses, which are specific to each installation. Another approach to solve the problem is to equip each panel or two panels with a boost circuit, e.g. optimizer, which stabilized the string voltage level at fixed high voltage. This approach has disadvantages as it requires many boost circuits and a special inverter without MPPT mechanism that is designed to work at a fixed DC voltage. US 7,605,498 discloses a high efficiency photovoltaic DC-DC converter which achieves solar power conversion from high voltage, highly varying photovoltaic power sources. Voltage conversion circuits are described which have pairs of photovoltaic power interrupt switch elements and pairs of photovoltaic power shunt switch elements to first increase voltage and then decrease voltage as part of the desired photovoltaic DC-DC power conversion. Thus the Photovoltaic DC-DC converters can achieve efficiencies in conversion that are high compared to traditional through substantially power isomorphic VGDU-102-IL-18 - 4 - photovoltaic DC-DC power conversion capabilities. However, this approach has disadvantages as it requires an implementation for each panel. It would therefore be desired to propose a system void of these deficiencies. Summary It is an object of the present invention to provide a method for enhancing the efficiency of Photovoltaic power stations. It is another object of the present invention to provide an apparatus for controlling the DC voltage of the Photovoltaic arrays for reducing energy loss. It is still another object of the present invention to increase and stabilize the DC voltage level of the DC-Bus of an array to the maximum voltage allowed by the standards and state rules, regardless of the solar panels’ actual maximum power voltage (Vmp) level, which is restricted due to their open circuit voltage (Voc) level. It is still another object of the present invention to provide a method for enhancing the efficiency of a broad range of MPPT based inverters. It is still another object of the present invention to reduce DC energy conduction losses. It is still another object of the present invention to provide a method and apparatus for dynamically optimizing the efficiency, of a specific Photovoltaic power station, based on its specific conditions and based on its inverter’s characteristics.

VGDU-102-IL-18 - 5 - Other objects and advantages of the invention will become apparent as the description proceeds. The present invention relates to an apparatus for optimizing the power from a solar string power generation system comprising: (a) at least one string of solar panels where said solar panels are connected in series in said string to form a string of solar panels; (b) at least one Injection Circuit (IC), connected at its input to said at least one string, comprising a DC/DC converter, for converting at least a part of the current of said connected string to voltage and having an MPPT mechanism, for finding the MPP of said connected string; (c) a DC bus, connected to the outputs of said at least one IC; and (d) an inverter, connected, at its input, to said DC bus, for converting the solar DC power, from said IC, to AC power, comprising an MPPT mechanism; wherein said IC controls its power output in order to control and stabilize the DC bus voltage, of said inverter MPPT mechanism, to a desired voltage. In one embodiment, the apparatus further comprises: (a) at least two strings of solar panels where said solar panels are connected in series in said strings, and wherein said strings are connected in parallel to form an array of strings of solar panels; and (b) at least two Injection Circuits (IC), each connected at their input to at least one of said strings, each comprising a DC/DC converter, for converting at least a part of the current, of said connected at least one string, to voltage, and each having an MPPT mechanism, for finding the MPP of said connected at least one string; wherein said ICs are communicately connected, and wherein at least one of said ICs is defined as master and the rest of said ICs are defined as slaves, and wherein said at least one master IC controls the operation of at least one of said slave IC in order to control and stabilize the input voltage of said inverter.

VGDU-102-IL-18 - 6 - In one embodiment, the apparatus further comprises: (a) at least two strings of solar panels where said solar panels are connected in series in said strings, and wherein said strings are connected in parallel to form an array of strings of solar panels; and (b) at least two Injection Circuits (IC), each connected at their input to at least one of said strings, each comprising a DC/DC converter, for converting at least a part of the current of said connected at least one string to voltage and each having an MPPT mechanism, for finding the MPP of said connected at least one string; wherein each of said ICs individually controls and stabilizes the voltage if its output in order to reach the desired input voltage of said inverter. Preferably, the MPPT of the inverter works according to one or more of the following methods: Perturbation and Observation, Hill Climbing Method, Incremental Conductance, and/or Fuzzy logic control. In one embodiment, the desired voltage is pre-programmed. In one embodiment, the desired voltage is dynamically instructed. In one embodiment, the desired voltage is communicated from an outer source. The present invention also relates to a method for optimizing solar string power generation system comprising: (a) providing at least one string of solar cells where said solar cells are connected in series in said strings, and wherein said strings are connected in parallel to form an array of strings of solar cells; (b) providing at least one Injection Circuit (IC), connected at its input to said at least one string, comprising a DC/DC converter, for converting at least a part of the current of said connected string to voltage and having an MPPT VGDU-102-IL-18 - 7 - mechanism, for finding the MPP of said connected string; and (c) providing an inverter, for converting the solar DC power, from said IC, to AC power, comprising an MPPT mechanism; wherein said IC controls its power output in order to control the input voltage, of said inverter, to a desired voltage. In one embodiment, the method further comprises: (a) providing at least two strings of solar panels where said solar panels are connected in series in said strings, and wherein said strings are connected in parallel to form an array of strings of solar panels; and (b) providing at least two Injection Circuits (IC), each connected at their input to at least one of said strings, each comprising a DC/DC converter, for converting at least a part of the current, of said connected at least one string, to voltage, and each having an MPPT mechanism, for finding the MPP of said connected at least one string; wherein said ICs are communicately connected, and wherein at least one of said ICs is defined as master and the rest of said ICs are defined as slaves, and wherein said at least one master IC controls the operation of at least one of said slave IC in order to control and stabilize the input voltage of said inverter. In one embodiment, the method further comprises: (a) providing at least two strings of solar panels where said solar panels are connected in series in said strings, and wherein said strings are connected in parallel to form an array of strings of solar panels; and (b) providing at least two Injection Circuits (IC), each connected at their input to at least one of said strings, each comprising a DC/DC converter, for converting at least a part of the current of said connected at least one string to voltage and each having an MPPT mechanism, for finding the MPP of said connected at least one string; wherein each of said ICs individually controls and stabilizes the voltage if its output in order to reach the desired input voltage of said inverter.

VGDU-102-IL-18 - 8 - Brief Description of the Drawings The accompanying drawings, and specific references to their details, are herein used, by way of example only, to illustratively describe some of the embodiments of the invention. In the drawings: - Fig. 1 is a schematic diagram depicting strings of solar panels connected in an array, with injection circuits, according to an embodiment. - Fig. 2 is a schematic diagram depicting strings of solar panels connected with injection circuits, according to another embodiment. - Fig. 3 is a flowchart depicting the method for enhancing the efficiency of a Photovoltaic power station having an inverter using a Perturbation & Observation or Hill Climbing MPPT type, according to an embodiment. - Fig. 4 is a flowchart depicting the method for enhancing the efficiency of a Photovoltaic power station at system turn-on time using a Distributive Mode for all MPPT types, according to an embodiment. - Fig. 5 is a flowchart depicting the method for enhancing the efficiency of a Photovoltaic power station having an inverter using an Incremental Conductance type, according to an embodiment. Detailed Description When an inverter is connected to an array of strings of Photovoltaic panels, the inverter usually has a Maximum Power Point Tracking (MPPT) circuitry for maximizing the power from the strings. These known-in-the-art MPPT circuits adjust the voltage (and the current) at which the arrays operate, measure their output power, and seek those voltage and current values at which power output VGDU-102-IL-18 - 9 - is maximized. Thus, the MPPT of the array is typically done by the inverter of the array. However, the voltage of the array is determined by the number of solar panels and may vary greatly due to clouds, shading, filth, the amount of light shining on the panel, the angle of the sun light, the temperature of the panel, and other factors. Thus, in prior art systems, the voltage from the array, that is fed to the inverter, is dictated by the solar panels and may vary, due to their environmental conditions. Furthermore, it is dictated by the aggregate Maximum Point Voltage (Vmp) values of the solar panels connected in the string which is typically 24% lower than the voltage allowed by the standards and state rules. This is due, inter alia, to the fact that the measured voltage is derived from the aggregate maximum Open Circuit Voltage (Voc) of the solar panels which is typically higher that the Vmp. This voltage variance, of the array, which may total to hundreds of Volts, may have a negative effect on the efficiency of the inverter, since the most efficient voltage input variance, for a typical inverter, is relatively narrow and typically higher than the power feed of an average array. In addition, when the voltage from the array drops - the total efficiency of the system drops as well especially in large scale systems which have long electrical cables. It is therefore desired to control the output Voltage of the array, in order to best fit the efficient voltage input of the inverter, and to minimize the conduction losses, or otherwise, to enable the use of cheaper cables. Fig. 1 is a schematic diagram depicting strings of solar panels connected in an array, with Injection Circuits (IC), according to an embodiment. As depicted, a number of strings of solar panels, such as string 200, are connected in parallel to form an array of strings of solar panels. A solar inverter 600, is connected, at its input, to the DC bus lines 500-501, for converting the solar DC power, from the array of strings, to AC power. The Injection Circuit (IC) 100, is VGDU-102-IL-18 - 10 - connected, at its input, to the string 200, wherein its output is connected to the DC bus lines 500-501. Thus, the IC 100 may be connected, by the bus lines 500-501, to the other ICs of the strings of the array, such as IC 110, and connected to the input of the inverter 600. The IC 100 may also comprise a DC/DC converter, for converting at least a part of the power of its connected string 200. According to an embodiment, the converter of IC 100 may be a DC/DC booster converter or may have any other relevant topology. Thus, for example, the IC 100 can increase its output voltage, on the expense of the current, for increasing the DC voltage on bus lines 500-501 and therefore increase the input voltage to the inverter. The purpose of the ICs is to boost the voltage, of their connected string, using power conversion. By using well known power conversion techniques, part of the current of the connected string can be used to increase the voltage level of the DC bus lines 500-501. Thus, the IC 100 may be used to convert the voltage of the string 200 as desired, where each of the other ICs may be used to convert the current of their interconnected strings as well in order to control the voltage of the DC bus lines 500-501. In one embodiment, the ICs, in the array, are communicately connected and can communicate with one another. The communication may be a wired communication, a Power Line Communication over the DC bus lines, and/or wireless communication, and/or any other known communication method. In one embodiment, for example, an inverter using a Perturbation & Observation method or Hill Climbing method MPPT type, one of the ICs is defined as master and the rest of said ICs, in the array, are defined as slaves. Thus, the master IC can control the power injected by part or all the other ICs and thus control the voltage of the array. For example, when the output of the array, in a given time, is less that the desired Voltage for the DC bus lines, the master IC decides which slave ICs will participate, based on the voltage gap VGDU-102-IL-18 - 11 - and the energy required to close the gap. The master IC may then instruct the participants slave ICs at which power level they should start and by how much they should increment their power output at any stage. This procedure continues until the DC bus voltage reaches the desired Voltage, as will be discussed in relations to Fig. 3. In another embodiment, for all MPPT type inverters, at the beginning of the day, or after a system shut down, when the array starts at a low voltage output, the ICs can start working in distributive mode, boosting their output power. The boosting may be performed by each IC individually, without any command or master involved. The boosting may be done by the ICs where the ICs reduce their power output, in proportion to the ratio of the present voltage of the DC bus relative to the desired Voltage, so as to cause the inverter’s MPPT to increase accordingly, thus pulling the DC bus lines voltage to the desired voltage, which is the efficient range. As a result, each time the MPTT of the inverter increases the DC bus voltage, the ICs may increment their power output proportionally to the remaining gap between the new voltage level and the desired Voltage level. This procedure continues until the DC bus voltage reaches the desired Voltage, as will be discussed in relations to Fig. 4. In one embodiment, when the inverter MPPT is an Incremental Conductance type, the master IC may control the MPPT of the inverter alone - by boosting its own output power at the time when the MPPT of the inverter measures the MPP, as will be discussed in relations to Fig. 5. Thus, by boosting the power when measured, and by lowering the power in between measurements, the master IC can buff the inverter’s MPPT and boost the DC bus lines voltage to the desired value all alone. In other embodiments when the MPPT of the inverter belongs to a Fuzzy logic control method, then the Master IC may use one or more slave ICs for boosting their power output in order to cause the inverter’s MPPT to increase accordingly for pulling the DC bus lines to a desired voltage level and to keep it stabilized, as will be discussed in relations to Fig. 3.

VGDU-102-IL-18 - 12 - In one embodiment the master and/or salve ICs may be pre-programmed to achieve a certain desired voltage, when the master IC and the slave ICs are embedded in an array of a solar string power generation system. In another embodiment the master and/or salve ICs may be dynamically instructed to achieve certain desired voltages, depending on environmental, system condition or any other dynamic factors. In one embodiment the master and/or salve ICs may be communicately connected to an outer source, such as a monitoring system, and may receive its/their instructions from the outer source. In one embodiment, the outer source may communicate the instructions periodically or continuously. In one embodiment, the controlling of the DC bus lines voltage of the array may be done based on the environmental variants or may be set as a constant per array. In one embodiment, the communication between the ICs may enable the master IC to control the operation of the slave ICs, as described above, as well as to enable each of the ICs to send status and event information to a monitoring system or any other outer source. Fig. 2 is a schematic diagram depicting strings of solar panels connected with ICs, according to another embodiment. Similar to the described in relation to Fig. 1, this diagram depicts strings of solar panels connected in an array, with ICs. However, in this diagram, the string 200 is connected only to the IC 101, which regulates its voltage, and supplies the bus lines 503-504 directly with the desired voltage. As known in the art, there can be many electrical implementations for the IC described above, for example the IC may comprise a phase shift full bridge circuit. Thus, if a string produces only 700V with a current of 8A, in its MPP, then the voltage may be boosted by the IC, using power conversion methods, VGDU-102-IL-18 - 13 - for example. In this example, if a string produces 8A of current and 700V, the IC may convert the output power into 7A of current and 800V. Thus, the Power of the string stays 5600W, however instead of producing 700V*8A, the string produces now 800V*7A. Thus, by using ICs, all the outputs of the strings can be controlled in order to lead the MPPT of the inverter to the desired voltage value, e.g. to the voltage that is the most efficient for the system. Fig. 3 is a flowchart depicting the method for enhancing the efficiency of a Photovoltaic power station having a Perturbation & Observation MPPT type inverter during operation, according to an embodiment. In this embodiment, the master IC is first synched with the inverter’s steps. Then the DC bus voltage is measured periodically for checking if the DC bus Voltage corresponds to the desired voltage. If the measured DC bus Voltage does not correspond to the desired voltage, then the energy needed to close the gap is calculated and the number of IC slaves needed to participate for closing the gap is calculated as well. At this stage the master IC communicates to each participant IC slave the power level that it needs to increase/decrease, as needed, and then the master IC communicates to each participant IC slave to make the power change as needed. At this point the DC bus voltage is measured again for checking if the DC bus Voltage now corresponds to the desired voltage. If the measured DC bus Voltage does not correspond again to the desired voltage, then the master IC communicates to each participate IC slave the power level that it needs to increase/decrease, as needed, and then the master IC communicates to each participate IC slave to make the power change again, as needed. This loop may continue until the measured DC bus Voltage corresponds to the desired voltage. This method may be used for Hill Climbing methods and/or Fuzzy logic as well.

VGDU-102-IL-18 - 14 - Fig. 4 is a flowchart depicting the method for enhancing the efficiency of a Photovoltaic power station for all MPPT type inverters, at the beginning of the day or after a system shut down, according to an embodiment. In this embodiment, when the system is turned on, each IC is first synched with the inverter’s steps individually and independently. Then, the DC bus voltage is measured for checking if the DC bus Voltage corresponds to the desired voltage. If the measured DC bus Voltage does not correspond to the desired voltage, then the energy is increased or decreased in order to close the gap between the measured DC bus Voltage and the desired voltage. At this point the DC bus voltage may be measured again for checking if the DC bus Voltage now corresponds to the desired voltage. If the measured DC bus Voltage does not correspond again to the desired voltage, then the energy is increased or decreased again in order to close the gap between the measured DC bus Voltage and the desired voltage. This loop may continue until the gap between the measured DC bus Voltage and the desired voltage is closed. In one embodiment, during regular operation, when the gap between the actual and the desired voltage is relatively small, a minimum number of ICs is used, in order to minimize energy losses when the stabilization process is taking place. However, during the initial operation, when the voltage gap is relatively high and energy production is still low, all the ICs may be used, in the distributed methodology, for speeding up the stabilization process. Fig. 5 is a flowchart depicting the method for enhancing the efficiency of a Photovoltaic power station having an Incremental Conductance type MPPT inverter, according to an embodiment. In this embodiment, the master IC may control the MPPT of the inverter alone. The DC bus voltage is measured periodically for checking if the DC bus Voltage corresponds to the desired voltage. If the measured DC bus Voltage does not correspond to the desired voltage, then the energy is increased or decreased and backward increased or VGDU-102-IL-18 - 15 - decreased accordingly between measurements, in order to close the gap between the measured DC bus Voltage and the desired voltage. At this point the DC bus voltage may be measured again for checking if the DC bus Voltage now corresponds to the desired voltage. If the measured DC bus Voltage does not correspond again to the desired voltage, then the energy is increased or decreased and backward increased or decreased accordingly again in order to close the gap between the measured DC bus Voltage and the desired voltage. This loop may continue until the gap between the measured DC bus Voltage and the desired voltage is closed. In this example the master IC may control the MPPT of the inverter all alone. While the above description discloses many embodiments and specifications of the invention, these were described by way of illustration and should not be construed as limitations on the scope of the invention. The described invention may be carried into practice with many modifications which are within the scope of the appended claims.

Claims (12)

1.VGDU-102-IL-18 - 16 - 263278/ Claims 1. An apparatus for optimizing the power from a solar string power generation system comprising: a number of strings of solar panels where said solar panels are connected in series in said strings to form an array of strings of solar panels; a master Injection Circuit (IC), connected at its input to at least one string of said strings, comprising a DC/DC converter, for converting at least a part of the current of said connected string to voltage and having a Maximum Power Point Tracking (MPPT) mechanism, for finding the Maximum Power Point (MPP) of said connected string; at least one slave IC, connected at its input to at least one string of said strings, comprising a DC/DC converter, for converting at least a part of the current of said connected string to voltage and having an MPPT mechanism, for finding the MPP of said connected string; a DC bus, connected to the outputs of said master IC and slave IC; an inverter, connected, at its input, to said DC bus, for converting the solar DC power, to AC power, comprising an MPPT mechanism; and wherein said master IC controls its power output, and instructs said slave IC, to increase their output voltages, on the expense of their currents, in order to lead said MPPT mechanism of said inverter to the desired voltage value.

2. The apparatus according to claim 1, wherein the master IC and slave IC are communicately connected.

3. The apparatus according to claim 1, where the MPPT of the inverter works according to one or more of the following methods: Perturbation VGDU-102-IL-18 - 17 - 263278/ and Observation, Hill Climbing Method, Incremental Conductance, and/or Fuzzy logic control.

4. The apparatus of claim 1, where the desired voltage is pre-programmed.

5. The apparatus of claim 1, where the desired voltage is dynamically instructed.

6. The apparatus of claim 1, where the desired voltage is communicated from an outer source.

7. A method for optimizing solar string power generation system comprising: providing an array of strings of solar cells; providing a master IC, connected at its input to at least one string of said strings, comprising a DC/DC converter, for converting at least a part of the current of said connected string to voltage and having an MPPT mechanism, for finding the MPP of said connected string; providing at least one slave IC, connected at its input to at least one string of said strings; providing an inverter, for converting the solar DC power to AC power, comprising an MPPT mechanism; and wherein said master IC controls its power output, and said slave IC’s output, to increase their output voltages, on the expense of their currents, in order to lead said MPPT of said inverter to the desired voltage value.

8. The method according to claim 7, wherein the master IC and the slave IC are communicately connected.

9. The method according to claim 7, where the MPPT of the inverter works according to one or more of the following methods: Perturbation and VGDU-102-IL-18 - 18 - 263278/ Observation, Hill Climbing Method, Incremental Conductance, and/or Fuzzy logic control.

10. The method of claim 7, where the desired voltage is pre-programmed.

11. The method of claim 7, where the desired voltage is dynamically instructed.

12. The method of claim 7, where the desired voltage is communicated from an outer source.