WO2005112551A2

WO2005112551A2 - Method for compensating for partial shade in photovoltaic power system

Info

Publication number: WO2005112551A2
Application number: PCT/KR2005/000863
Authority: WO
Inventors: Hyeon Woo Lee; Kang Hoon Koh; Ki Young Suh; Hee Suk Koh
Original assignee: Hansung Engineering Co. Ltd
Priority date: 2004-05-21
Filing date: 2005-03-24
Publication date: 2005-12-01
Also published as: WO2005112551A3

Abstract

There is provided a method for compensating for partial shade using a photovoltaic power system, in which a plurality of serial-connected arrays of solar cell modules are connected in parallel, the arrays are connected to a converter, and a final power is outputted from the converter. The method includes the steps of: connecting an auxiliary power source to each solar cell module; if a low solar radiation is detected by an MPPT controller, supplying a necessary power through the auxiliary power source at a time, thereby maintaining constant voltage and preventing a temporarily shutdown; connecting in parallel the solar cell modules of the serial- connected arrays in multi-stages; and varying an operating voltage ratio of the solar cell module by controlling an off duty cycle of each chopper at the MPPT controller, thereby generating maximum output from the serial-connected array of each solar cell module.

Description

Description METHOD FOR COMPENSATING FOR PARTIAL SHADE IN PHOTOVOLTAIC POWER SYSTEM Technical Field

[1] The present invention relates to a method for compensating for a partial shade in a photovoltaic power system, and more particularly, to a method for compensating for a partial shade in a photovoltaic power system, in which a constant voltage can be maintained by supplying an insufficient voltage of a solar cell module in a low solar radiation, a temporary shutdown of the system can be prevented, and a maximum power can be generated at a serial-connected array of each solar cell module.

[2] Background Art

[3] Korea established an alternative energy resource development/usage/ distribution promotion policy for promoting an expanded distribution of new regeneration energy and supported its development of technology.

[4] Since a solar photovoltaic power generation utilizes a natural energy, many researches on it have been made for a long time. In recent years, home power generators are available.

[5] In such a photovoltaic power generation system, a plurality of solar cell modules are connected in serial/parallel fashion and are connected to a power conversion device. The reason is that since a solar cell module alone has a low output voltage of below 30 V, a serial connection of the module and a boost converter are required for boosting the low output voltage to an input voltage of a grid-tie inverter. A finial power is obtained by connecting the serial-connected arrays in parallel.

[6] In an output characteristic of each module, however, an operating point of the solar cell module array is changed depending on solar radiation, temperature, load variation, etc. Accordingly, in the solar cell, a maximum power point tracking (MPPT) control method capable of always providing a maximum output should be applied to the converter. Several MPPT control algorithms have been studied. There are an analog MPPT algorithm and a digital MPPT algorithm using a microprocessor. In view of a control object, the MPPT control algorithms are also classified into an MPPT control algorithm using a constant voltage control method, a P&O (Perturbation and Observation Method) MPPT control algorithm, an Inc Cond (Incremental Conductance) MPPT algorithm, and a current feedback algorithm. Although the conventional MPPT control algorithms operate well in a high solar radiation, the MPPT control does not operate well in a low solar radiation. Thus, an amount of power generation is reduced. Accordingly, a lot of improved algorithms have been studied.

[7] Also, in an installation of the photovoltaic power system, considering that the solar cells are used in places such as general home or places where the installation area is small, generation power is greatly reduced when the solar cells are connected in series, because solar radiation is different depending on the shade of the adjacent buildings or the installation place. In order to partially solve this problem, bypass diodes are connected to the solar cells. Also, in order to increase the generation efficiency, MPPT control algorithm is installed in each module, or a temporary voltage reduction is compensated using hardware.

[8] Disclosure of Invention Technical Problem

[9] In order to solve problems of an installation situation and temporary reduction of power generation, the present invention provides a system that can always supply maximum power by applying choppers to each module in multi-stage and controlling power generation of each serial-connected array. Also, the present invention provides an improved method using an auxiliary circuit to which a digital controller and a switching topology are applied. By configuring simulation and small-capacity circuit, efficiency of the compensation for partial shade and low solar radiation can be increased.

[10] An object of the present invention is to maintain constant voltage and prevent temporary shutdown by connecting an auxiliary power source to an output terminal of each solar cell module and supplying necessary power through the auxiliary power source when a low solar radiation is detected by an MPPT controller.

[11] Another object of the present invention is to provide generate maximum output from the serial-connected array of each solar cell module by connecting in parallel the solar cell modules of the serial-connected arrays in multi-stages and varying an operating voltage ratio of the solar cell module by controlling an off duty cycle of each chopper at the MPPT controller.

[12] Technical Solution

[13] According to an aspect of the present invention, there is provided a method for compensating for partial shade using a photovoltaic power system, in which a plurality of serial-connected arrays of solar cell modules are connected in parallel, the arrays are connected to a converter, and a final power is outputted from the converter, the method comprising the steps of: connecting an auxiliary power source to each solar cell module; if a low solar radiation is detected by an MPPT controller, supplying a necessary power through the auxiliary power source at a time, thereby maintaining constant voltage and preventing a temporarily shutdown; connecting in parallel the solar cell modules of the serial-connected arrays in multi-stages; andvarying an operating voltage ratio of the solar cell module by controlling an off duty cycle of each chopper at the MPPT controller, thereby generating maximum output from the serial- connected array of each solar cell module.

[14] The auxiliary power source includes: a switch for performing a switching operation according to a control of the MPPT controller; and a capacitor or small-capacity battery for discharging charged voltage according to the switching of the switch.

[15] The MPPT controller sets a reference voltage (Vref) as a lowest operating voltage of an MPPT control range of the serial-connected array, increases the reference voltage (Vref) and detects an output voltage (V(k)) of the serial-connected array. If the output voltage (V(k)) is greater than the reference voltage, the MPPT controller does not operate the switch. If the output voltage (V(k)) decreases due to an amount of the solar radiation or a change of load after the reference voltage (Vref) increases to the maximum, the MPPT controller decreases the reference voltage (Vref).

[16] The MPPT controller alternately switches each chopper.

[17] Advantageous Effects

[18] In the method for compensating for the partial shade in the photovoltaic power system, multi-stage choppers are applied to every solar cell module, and maximum power is always supplied at the serial-connected array by controlling power generation of the serial-connection array of each module. A maximum power point is obtained by connecting multi-stage choppers in parallel. Therefore, it is possible to compensate for the partial shade and obtaining constant output by controlling maximum power point even when the partial shade occurs.

[19] Brief Description of the Drawings

[20] FIG. 1 is a circuit diagram of a generation control circuit according to an embodiment of the present invention;

[21] FIG. 2 illustrates a switching sequence and a reactor current waveform in a generation control circuit according to an embodiment of the present invention;

[22] FIG. 3 is a flowchart illustrating a method for compensating for a partial shade in a photovoltaic power system according to an embodiment of the present invention;

[23] FIG. 4 is a circuit diagram of a power point control circuit according to an embodiment of the present invention;

[24] FIG. 5 is a current waveform of an inductor and each part in a power point control circuit according to an embodiment of the present invention;

[25] FIG. 6 is a circuit diagram of a photovoltaic power system to which a method for compensating for a partial shade is applied;

[26] FIG. 7 is an operation waveform of each part in a rapid change of a solar radiation in a photovoltaic power system according to an embodiment of the present invention;

[27] FIG. 8 is a test circuit of a photovoltaic power system according to an embodiment of the present invention; and

[28] FIG. 9 is a graph illustrating an output characteristic before and after a control of each solar cell in a photovoltaic power system according to an embodiment of the present invention.

[29] Best Mode for Carrying Out the Invention

[30] Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[31] In the embodiments of the present invention, multi-stage chopper is applied to each solar cell module, and a power generation in a serial-connected array of each module is controlled, such that a maximum power is always supplied from the serial-connected array. Also, using an MPPT controller, a maximum output voltage is always supplied. In this manner, by solving a problem that an output of an entire array is reduced in a low solar radiation or a partial shade is solved, a maximum power point can be obtained. Thus, a compensation for the partial shade is possible and a constant output can be obtained by the control of the maximum power point even at the partial shade. However, the present invention is not limited to the above-described system, but it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention.

[32] First, a shape-compensation MPPT technique will be described below.

[33] FIG. 1 is a circuit diagram of a generation control circuit in which two solar cells are connected in series. The generation control circuit is provided for solving a problem of a power-generation unbalanced state. FIG. 2 illustrates a switching sequence of switches SWl and SW2 and reactor current waveform. Here, the switching sequence controls to alternately operate the switches SWl and SW2, instead of turning off them at the same time.

[34] Meanwhile, an off duty cycle of the switches SWl and SW2 can be obtained by Equation 1 below.

[35] (Equation 1)

[36] [37] A relationship between the off duty cycle and an operating voltage of each solar cell can be expressed as Equations 2 to 6 below.

[38] (Equation 2)

[39]

[40] (Equation 3)

[41]

[42] (Equation 4)

[43] _τ , T₂QFF _ _τ/ TlOFF

[44] (Equation 5) [45] ■: V_lD₂= V₂D_i

[46] (Equation 6) [47] ₁: ₂ = 7 : /

[48] It can be seen from Equations 2 to 6 that a ratio (V1:V2) of the operating voltage in each solar cell can be freely set by changing the off duty cycle of each switch.

[49] Unlike the conventional method, when a low solar radiation is detected in a state that the MPPT control is not performed, an auxiliary power source configured with a switch and a capacitor is operated. A large-capacity super capacitor or a small-capacity battery can be used as the capacitor of the auxiliary power source. By charging as an input voltage using the voltage charged in the capacitor or the battery, an insufficient voltage is supplemented for a predetermined time, such that a temporary power generation is possible.

[50] At this point, after a reference voltage Vref is set as a lowest operating voltage of the MPPT control range and is increased, an output voltage V(k) is detected. When the output voltage V(k) is greater than the reference voltage Vref, the switches of the auxiliary power source do not operate. Instead, when the output voltage V(k) decreases due to an amount of the solar radiation or a change of load after the reference voltage Vref increases to the maximum, the reference voltage Vref is reduced.

[51] In other words, the power P(k) and P(k-l) detected by the algorithm are compared with each other. If the two powers are identical to each other, the MPPT is not executed. When two powers are different, the power is reduced while following an array voltage, and an operation of reducing the reference voltage is executed. During this process, the reference voltage is maintained for a predetermined time, but the meaning of the increase/decrease value of the reference voltage Vref is different, although it is similar to the conventional P&O MPPT.

[52] FIG. 3 is a flowchart illustrating a method for compensating for a partial shade in a photovoltaic power system according to an embodiment of the present invention.

[53] FIG. 4 is a circuit diagram of a power point control circuit using PSIM according to an embodiment of the present invention. In FIG. 4, ms_user6.dll is a solar cell model to which a characteristic of a solar cell is applied, ms_user7.dll is applied for making an environment where solar radiation varied with time, and ms_user8.dll is an algorithm program.

[54] FIG. 5 is a current waveform of an inductor and each part in the power point control circuit according to an embodiment of the present invention. It can be seen that characteristic identical to ideal waveform is obtained.

[55] FIG. 6 is a circuit diagram of a photovoltaic power system to which a method for compensating for a partial shade is applied. In FIG. 6, there is shown a circuit for applying an improved IncCon control method.

[56] FIG. 7 is an operation waveform of each part in a rapid change of a solar radiation in a photovoltaic power system according to an embodiment of the present invention. Compared with the conventional control method, an error in a fast following speed, an actual power and a generated power is reduced.

[57] FIG. 8 is a test circuit of the photovoltaic power system according to an embodiment of the present invention. A solar cell used in the test is GMG01531, a characteristic of the solar cell is a rated maximum output of 53 W, an open circuit voltage of 21.7 V, a short circuit voltage of 3.25 V, a maximum operating voltage of 17.4 V, and a maximum operating current of 3.05 A.

[58] FIG. 9 is a graph illustrating an output characteristic before and after a control of each solar cell in a photovoltaic power system according to an embodiment of the present invention. Due to a partial shade at PV2 before the control, a current flows through bypass diode, causing the power generation to be reduced. However, due to the power point control operation, the amounts of power generation PVl and PV2 are identical to each other.

[59] In order to solve the problem of the conventional MPPT control algorithm in that power generation is impossible at a low solar radiation and a response speed is slow at a rapid change of solar radiation, an improved MPPT control algorithm is proposed. This method is implemented with a simple structure. A low solar radiation is detected and the switches of the auxiliary power source connected to the output terminal of the solar cell is operated. In this manner, an insufficient power is temporarily supplied, thereby maintaining a constant voltage and preventing a temporary shutdown of the system.

[60] Also, in order to solve a problem in that an output of an entire array is greatly lowered when a partial shade occurs in the serial/parallel connection of the system, a maximum power point is obtained by connecting multi-stage choppers in parallel. Therefore, it is possible to compensate for the partial shade and obtaining constant output by controlling maximum power point even when the partial shade occurs.

[61] It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

[62] Industrial Applicability

[63] In the method for compensating for the partial shade in the photovoltaic power system, multi-stage choppers are applied to every solar cell module, and maximum power is always supplied at the serial-connected array by controlling power generation of the serial-connection array of each module. A maximum power point is obtained by connecting multi-stage choppers in parallel. Therefore, it is possible to compensate for the partial shade and obtaining constant output by controlling maximum power point even when the partial shade occurs.

Claims

[1] A method for compensating for partial shade using a photovoltaic power system, in which a plurality of serial-connected arrays of solar cell modules are connected in parallel, the arrays are connected to a converter, and a final power is outputted from the converter, the method comprising the steps of: connecting an auxiliary power source to each solar cell module; if a low solar radiation is detected by an MPPT controller, supplying a necessary power through the auxiliary power source at a time, thereby maintaining constant voltage and preventing a temporarily shutdown; connecting in parallel the solar cell modules of the serial-connected arrays in multi- stages; and varying an operating voltage ratio of the solar cell module by controlling an off duty cycle of each chopper at the MPPT controller, thereby generating maximum output from the serial-connected array of each solar cell module.

[2] The method of claim 1, wherein the auxiliary power source includes: a switch for performing a switching operation according to a control of the MPPT controller; and a capacitor or small-capacity battery for discharging charged voltage according to the switching of the switch.

[3] The method of claim 1, wherein the MPPT controller sets a reference voltage (Vref) as a lowest operating voltage of an MPPT control range of the serial- connected array, increases the reference voltage (Vref) and detects an output voltage (V(k)) of the serial-connected array, if the output voltage (V(k)) is greater than the reference voltage, the MPPT controller does not operate the switch, and if the output voltage (V(k)) decreases due to an amount of the solar radiation or a change of load after the reference voltage (Vref) increases to the maximum, the MPPT controller decreases the reference voltage (Vref).

[4] The method of claim 1, wherein the MPPT controller alternately switches each chopper.