Optical storage DC conversion device and control method thereof

Abstract

The invention discloses an optical storage direct-current conversion device and a control method thereof, and belongs to the technical field of multi-port direct-current converter control, and the optical storage direct-current conversion device comprises a multi-winding high-frequency transformer, a first direct-current branch, a second direct-current branch, a third direct-current branch and a controller. The controller is electrically connected with the first direct current branch, the second direct current branch and the third direct current branch. According to the invention, the power generation equipment, the energy storage equipment and the micro-grid are respectively connected to the multi-winding high-frequency transformer through the first direct-current branch, the second direct-current branch and the third direct-current branch; and the output power of the first direct current branch, the second direct current branch and the third direct current branch is changed through the sampling current and the sampling voltage of the input end of the first direct current branch and the sampling current and the sampling voltage of the output end of the third direct current branch.

Description

Technical Field

[0001] The invention relates to a photovoltaic storage direct current conversion device and a control method thereof, belonging to the technical field of multi-port direct current converter control. Background Art

[0002] As energy and environmental issues become increasingly prominent, photovoltaic power generation, as the best way to utilize solar energy resources, has the advantages of being clean, environmentally friendly, simple to build, and low-cost, and has developed rapidly in recent years. However, photovoltaic power generation is affected by external factors such as light intensity and ambient temperature, and the power generation is intermittent and random. Therefore, a certain capacity of energy storage device needs to be configured in the photovoltaic power generation system to form a photovoltaic-energy storage integrated system, which uses the charging and discharging of the energy storage device to buffer the fluctuation of the photovoltaic power output power and maintain the stability of the power generation system.

[0003] In order to achieve integrated photovoltaic-energy storage grid connection, it is usually necessary to use a DC converter to boost the photovoltaic power supply and energy storage device to achieve the grid-connected voltage level. The photovoltaic power supply and energy storage device have different output voltage, current and other characteristics, so multiple DC converters are needed to boost the voltage, but this will increase the system volume and construction cost, and there are multiple levels of DC conversion, which increases power loss. Summary of the invention

[0004] The purpose of the present invention is to overcome the deficiencies in the prior art and to provide a photovoltaic-storage DC conversion device and a control method thereof, so as to realize centralized control of the photovoltaic-storage integrated system and independent control of the transmission power of each port, thereby solving the problem that the current photovoltaic-storage equipment boosting system is complex and multi-stage DC conversion increases power loss.

[0005] In order to achieve the above purpose / solve the above technical problems, the present invention is implemented by adopting the following technical solutions: On one hand, the present invention provides a photovoltaic energy storage DC conversion device, comprising: a multi-winding high-frequency transformer, a first DC branch for connecting a power generation device to the multi-winding high-frequency transformer, a second DC branch for connecting an energy storage device to the multi-winding high-frequency transformer, a third DC branch for connecting a microgrid to the multi-winding high-frequency transformer, and a controller; The multi-winding high-frequency transformer includes a first transformer input terminal, a second transformer input terminal and a transformer output terminal; the first input terminal is electrically connected to the first DC branch, the second transformer input terminal is electrically connected to the second DC branch, and the transformer output terminal is electrically connected to the third DC branch; the transformation ratio of the multi-winding high-frequency transformer matches the output voltage of the power generation equipment, the output voltage of the energy storage equipment, and the voltage of the microgrid; The controller is electrically connected to the first DC branch, the second DC branch and the third DC branch respectively, and is used to change the output power of the first DC branch, the second DC branch and the third DC branch according to the sampled current and sampled voltage at the input end of the first DC branch and the sampled current and sampled voltage at the output end of the third DC branch.

[0006] Optionally, the multi-winding high-frequency transformer is a three-winding high-frequency transformer, and the transformation ratio of the three-winding high-frequency transformer is n 1 ：n 2:1, among which, n 1. n 2 represents the assumed parameters, power generation equipment output voltage: energy storage equipment output voltage: microgrid voltage = n 1 ：n 2:1.

[0007] Optionally, the first DC branch includes a first voltage stabilizing capacitor and a first full-bridge unit electrically connected to the first voltage stabilizing capacitor; The first transformer input terminal of the multi-winding high-frequency transformer is also electrically connected to a first auxiliary inductor, and the first auxiliary inductor is electrically connected to the first full-bridge unit; the current generated by the power generation equipment flows through the first voltage stabilizing capacitor and the first full-bridge unit respectively, and finally enters the first transformer input terminal.

[0008] Optionally, the second DC branch includes a second voltage stabilizing capacitor and a second full-bridge unit electrically connected to the second voltage stabilizing capacitor; The second transformer input terminal of the multi-winding high-frequency transformer is also electrically connected to a second auxiliary inductor, and the second auxiliary inductor is electrically connected to the second full-bridge unit; the current of the energy storage device flows through the second voltage stabilizing capacitor and the second full-bridge unit respectively, and finally enters the second transformer input terminal.

[0009] Optionally, the third DC branch includes a third voltage stabilizing capacitor and a third full-bridge unit electrically connected to the third voltage stabilizing capacitor; The transformer output end of the multi-winding high-frequency transformer is also electrically connected to a third auxiliary inductor, and the third auxiliary inductor is electrically connected to the third full-bridge unit; the current converted by the multi-winding high-frequency transformer flows from the transformer output end into the third full-bridge unit and then flows through the third voltage stabilizing capacitor and the microgrid respectively.

[0010] Optionally, the third voltage-stabilizing capacitor is also electrically connected to an auxiliary resistor.

[0011] Optionally, the third DC branch further includes a filter inductor, one end of the filter inductor is electrically connected to the auxiliary resistor and the third full-bridge unit, and the other end of the filter inductor is electrically connected to the microgrid.

[0012] Another aspect of the present invention provides a control method for a photovoltaic-storage DC conversion device, comprising: Sampling the first DC branch input terminal and the third DC branch output terminal respectively to obtain a first sampled current and a first sampled voltage of the first DC branch input terminal, and to obtain a second sampled current and a second sampled voltage of the third DC branch output terminal; Determine a first virtual variable based on the output voltage of the power generation equipment when it is operating at maximum power and the first sampled voltage; Determine a second virtual variable based on a given power of the microgrid, a second sampled current, and a second sampled voltage; Convert the first dummy variable and the second dummy variable into corresponding phase shift angle variables; The switch pulses of the full-bridge units are controlled according to the phase shift angle to adjust the output power of the first DC branch, the second DC branch and the third DC branch.

[0013] Further, the determining of the first virtual variable based on the output voltage and the first sampled voltage when the power generation equipment is operating at maximum power includes: Calculate the output voltage of the power generation equipment when it is operating at maximum power by using a disturbance observation method according to the first sampled current and the first sampled voltage; The output voltage of the power generation equipment when operating at maximum power is compared with the first sampled voltage and a first virtual variable is calculated.

[0014] Further, the determining of the second virtual variable based on the given power, the second sampled current, and the second sampled voltage of the microgrid includes: The sampling power is calculated according to the second sampling current and the second sampling voltage; The sampled power is compared with the given power to calculate the output current given value; The output current given value is compared with the second sampled current and a second virtual variable is obtained by calculation.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention connects the power generation equipment, the energy storage equipment and the microgrid to the multi-winding high-frequency transformer through the first DC branch, the second DC branch and the third DC branch respectively. While using the multi-winding high-frequency transformer for conversion, the output power of the first DC branch, the second DC branch and the third DC branch is changed by the sampling current and sampling voltage at the input end of the first DC branch and the sampling current and sampling voltage at the output end of the third DC branch, thereby realizing centralized control of the photovoltaic and energy storage integrated system, maximum power point tracking (MPPT) control of the power generation equipment and voltage stabilization control of the microgrid, solving the problem that the current photovoltaic and energy storage equipment boosting system is complex and multi-stage DC conversion increases power loss.

[0016] 2. The filter inductor and the third voltage-stabilizing capacitor in the third DC branch of the present invention can form a CL filter, which can suppress the output current ripple. At the same time, due to the existence of the auxiliary resistor, the current resonance caused by the CL filter can be suppressed, while improving the power quality of the transformer output end and ensuring the stable operation of the circuit. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of a photovoltaic-storage-DC conversion device and a control method thereof according to an embodiment of the present invention; Figure 2 It is a schematic diagram of a control method for a photovoltaic-storage-DC conversion device and an embodiment of a control method for a photovoltaic-storage-DC conversion device according to the present invention.

[0018] In the figure: 1. multi-winding high-frequency transformer; 201. first voltage-stabilizing capacitor; 202. first full-bridge unit; 301. second voltage-stabilizing capacitor; 302. second full-bridge unit; 401. third voltage-stabilizing capacitor; 402. third full-bridge unit; 403. filter inductor; 404. auxiliary resistor; 5. first auxiliary inductor; 6. second auxiliary inductor; 7. third auxiliary inductor; 8. controller; 9. drive circuit; 10. power generation equipment; 11. energy storage equipment; 12. microgrid. DETAILED DESCRIPTION

[0019] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention. Example

[0020] like Figure 1 As shown, a photovoltaic energy storage DC conversion device comprises: a multi-winding high-frequency transformer 1, a first DC branch, a second DC branch, a third DC branch and a controller 8, wherein the multi-winding high-frequency transformer 1 comprises a first transformer input terminal, a second transformer input terminal and a transformer output terminal; The first DC branch is used to connect the power generation device 10 to the multi-winding high-frequency transformer 1. In this example, the power generation device 10 is a photovoltaic cell array; The first DC branch includes a first voltage-stabilizing capacitor 201 and a first full-bridge unit 202 electrically connected to the first voltage-stabilizing capacitor 201; the first transformer input terminal of the multi-winding high-frequency transformer 1 is also electrically connected to a first auxiliary inductor 5, and the first auxiliary inductor 5 is electrically connected to the first full-bridge unit 202; the current generated by the power generation device 10 flows through the first voltage-stabilizing capacitor 201 and the first full-bridge unit 202 respectively, and finally enters the first transformer input terminal; The second DC branch is used to connect the energy storage device 11 to the multi-winding high-frequency transformer 1. In this example, the energy storage device 11 is a battery. The second DC branch includes a second voltage-stabilizing capacitor 301 and a second full-bridge unit 302 electrically connected to the second voltage-stabilizing capacitor 301; the second transformer input terminal of the multi-winding high-frequency transformer 1 is also electrically connected to a second auxiliary inductor 6, and the second auxiliary inductor 6 is electrically connected to the second full-bridge unit 302; the current of the energy storage device 11 flows through the second voltage-stabilizing capacitor 301 and the second full-bridge unit 302 respectively, and finally enters the second transformer input terminal; The third DC branch is used to connect the microgrid 12 to the multi-winding high-frequency transformer 1. In this example, the microgrid 12 is a DC microgrid 12. The third DC branch includes a third voltage stabilizing capacitor 401 and a third full-bridge unit 402 electrically connected to the third voltage stabilizing capacitor 401; The transformer output end of the multi-winding high-frequency transformer 1 is also electrically connected to a third auxiliary inductor 7, and the third auxiliary inductor 7 is electrically connected to the third full-bridge unit 402; the current converted by the multi-winding high-frequency transformer 1 flows from the transformer output end into the third full-bridge unit 402 and then flows through the third voltage stabilizing capacitor 401 and the microgrid 12 respectively; In this example, the first full-bridge unit 202, the second full-bridge unit 302 and the third full-bridge unit 402 have the same structure. Taking the first full-bridge unit 202 as an example, the first full-bridge includes four IGBT switch tubes, and the four IGBT switch tubes are connected in a bridge rectifier circuit structure. The transformation ratio of the multi-winding high-frequency transformer 1 matches the output voltage of the power generation device 10, the output voltage of the energy storage device 11, and the voltage of the microgrid 12. In this embodiment, the multi-winding high-frequency transformer 1 is a three-winding high-frequency transformer; wherein the transformation ratio of the three-winding high-frequency transformer is n 1 ：n 2:1, n 1. n 2 represents the assumed parameters, the output voltage of the power generation device 10 is : Energy storage device 11 output voltage :Microgrid 12 voltage = n 1 ：n 2:1; The controller 8 is used to change the output power of the first DC branch, the second DC branch and the third DC branch according to the sampled current and sampled voltage at the input end of the first DC branch and the sampled current and sampled voltage at the output end of the third DC branch. Specifically: The controller 8 is electrically connected to the first full-bridge unit 202 of the first DC branch, the second full-bridge unit 302 of the second DC branch, and the third full-bridge unit 402 of the third DC branch through the driving circuit 9, and uses switching pulses to control the IGBT switch tubes of each full-bridge unit, thereby realizing the regulation of the output power of the first DC branch, the second DC branch and the third DC branch.

[0021] In order to improve the power quality at the transformer output end and ensure stable operation of the circuit, the third DC branch also includes a filter inductor 403. The third voltage-stabilizing capacitor 401 is also electrically connected to an auxiliary resistor 404. One end of the filter inductor 403 is electrically connected to the auxiliary resistor 404 and the third full-bridge unit 402, and the other end is electrically connected to the microgrid 12. The filter inductor 403 and the third voltage-stabilizing capacitor 401 can form a CL filter, which can suppress the output current ripple. At the same time, due to the presence of the auxiliary resistor 404, the current resonance caused by the CL filter can be suppressed. Example

[0022] A control method for a photovoltaic-storage DC conversion device, comprising: like Figure 2 As shown, the first DC branch input terminal and the third DC branch output terminal are sampled respectively to obtain a first sampled current and a first sampled voltage of the first DC branch input terminal, and a second sampled current and a second sampled voltage of the third DC branch output terminal, wherein the first sampled voltage is the output voltage of the power generation device 10. , the second sampling voltage is the voltage of microgrid 12 ; The first virtual variable is determined based on the output voltage and the first sampled voltage when the power generation device 10 is running at maximum power, specifically: According to the first sampling current , the first sampling voltage , the perturbation observation method is used to calculate the output voltage of the power generation equipment 10 when it is running at maximum power ; The output voltage of the power generation device 10 when it is running at maximum power With the first sampling voltage Compare and calculate the first dummy variable ; Based on the given power, the second sampled current, and the second sampled voltage of the microgrid 12, a second virtual variable is determined, specifically: According to the second sampling current , the second sampling voltage Calculate the sampling power ; The sampling power With given power Compare and calculate to get the output current given value ; Set the output current to a given value With the second sampling current Compare and calculate the second dummy variable ; Calculate the first dummy variable by diagonal matrix decoupling , the second dummy variable The corresponding phase shift angle variables include: The decoupling matrix value is:

[0023] , , , represents the matrix elements of the diagonal decoupling matrix; represents the first auxiliary inductor 5, represents the second auxiliary inductor 6, represents a third auxiliary inductor 7; represents the equivalent inductance between the first transformer input terminal and the second transformer input terminal of the multi-winding high-frequency transformer 1, represents the equivalent inductance between the first transformer input terminal and the transformer output terminal of the multi-winding high-frequency transformer 1, represents the equivalent inductance between the second transformer input terminal and the transformer output terminal of the multi-winding high-frequency transformer 1; Indicates the switching angular frequency of the switch tube in the full-bridge unit; represents the output voltage of the power generation device 10, Indicates the output voltage of the energy storage device 11, represents the voltage of the microgrid 12; A certain steady-state operating point of the photovoltaic storage DC conversion device of the present invention , , represents the phase shift angle variable at the steady-state operating point; Indicates the first sampling current For the first dummy variable The partial derivative of Q at the steady-state operating point, Indicates the first sampling current For the second dummy variable The partial derivative of Q at the steady-state operating point, Indicates the second sampling current For the first dummy variable The partial derivative of Q at the steady-state operating point, Indicates the second sampling current For the second dummy variable The partial derivative of Q at the steady-state operating point; The phase shift angle variable between the switching tube pulses of the first full-bridge unit 202 and the second full-bridge unit 302 is calculated by the following formula : = + ; The phase shift angle variable between the switching tube pulses of the first full-bridge unit 202 and the third full-bridge unit 402 is calculated by the following formula : = + ; The switch pulses of the various full-bridge units are controlled according to the phase shift angle variable to adjust the output power of the first DC branch, the second DC branch and the third DC branch.

[0024] In this example, the power generation device 10 is a photovoltaic cell array, the energy storage device 11 is a battery, the microgrid 12 is a DC microgrid, and the controller 8 is a PI controller; About the first dummy variable The PI controller is:

[0025] Voltage The open-loop transfer function of the control loop is:

[0026] represents the equivalent negative resistance value of the power generation device 10, Indicates the sampling period, represents the capacitance of the first voltage stabilizing capacitor 201, Indicates voltage The open-loop transfer function, s represents the transfer function independent variable, Represents the proportionality factor, represents the differential coefficient; It should be noted that: Regarding the first dummy variable In the PI controller, s The value of , the output is ; According to the Nyquist stability criterion, the control loop regarding the output voltage of the power generation device 10 is stable.

[0027] Second dummy variable The PI controller is:

[0028] About current The open-loop transfer function of the control loop is:

[0029] represents the equivalent negative resistance value of the power generation device 10, Indicates the sampling period, represents the capacitance of the third voltage stabilizing capacitor 401, Indicates filter inductor 403, Indicates the current The open-loop transfer function, s represents the transfer function independent variable, Represents the proportionality factor, represents the differential coefficient; It should be noted that: Regarding the second dummy variable In the PI controller, s The value of , the output is ; In order to suppress the resonance of the CL filter and ensure the stable operation of the circuit, the PI controller parameter range is:

[0030] represents the resistance value of the auxiliary resistor 404; represents the angular frequency at which the phase of the open-loop transfer function crosses -180°, The value of is:

[0031] About the current given value of microgrid 12 The PI controller is:

[0032] Represents the proportionality factor, represents the differential coefficient; It should be noted that: Regarding the current given value of the microgrid 12 In the PI controller, s The value of , the output is ; Regarding the power control of the energy storage device 11: Because under the control of the above-mentioned single-phase shift switch mode, the input / output power of the energy storage device 11 port is equal to the power difference between the power generation device 10 port and the microgrid 12 port, that is, using the law of conservation of power, the energy storage device 11 will fill the imbalance between the power of the power generation device and the power of the microgrid end by automatic charging and discharging. While controlling the power of the power generation device 10 and the microgrid 12, the power of the energy storage device 11 is actually also indirectly controlled.

[0033] In the description of the present invention, it is to be understood that the terms "first", "second", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", etc. may explicitly or implicitly include one or more of the feature. In the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0034] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

[0035] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A photovoltaic DC conversion device, characterized in that: include: A multi-winding high-frequency transformer (1), a first DC branch for connecting a power generation device (10) to the multi-winding high-frequency transformer (1), a second DC branch for connecting an energy storage device (11) to the multi-winding high-frequency transformer (1), a third DC branch for connecting a microgrid (12) to the multi-winding high-frequency transformer (1), and a controller (8); The multi-winding high-frequency transformer (1) comprises a first transformer input terminal, a second transformer input terminal and a transformer output terminal; the first input terminal is electrically connected to a first DC branch, the second transformer input terminal is electrically connected to a second DC branch, and the transformer output terminal is electrically connected to a third DC branch; the transformer ratio of the multi-winding high-frequency transformer (1) matches the output voltage of the power generation equipment (10), the output voltage of the energy storage equipment (11), and the voltage of the microgrid (12); The controller (8) is electrically connected to the first DC branch, the second DC branch and the third DC branch respectively, and is used to change the output power of the first DC branch, the second DC branch and the third DC branch according to the sampled current and sampled voltage at the input end of the first DC branch and the sampled current and sampled voltage at the output end of the third DC branch.

2. The photovoltaic-to-DC conversion device according to claim 1 is characterized in that: The multi-winding high-frequency transformer (1) is a three-winding high-frequency transformer, and the transformation ratio of the three-winding high-frequency transformer is n 1 ：n 2:1, among which, n 1. n 2 represents the assumed parameters, the output voltage of the power generation device (10): the output voltage of the energy storage device (11): the voltage of the microgrid (12) = n 1 ：n 2:

1.

3. The photovoltaic-to-DC conversion device according to claim 1 is characterized in that: The first DC branch comprises a first voltage-stabilizing capacitor (201) and a first full-bridge unit (202) electrically connected to the first voltage-stabilizing capacitor (201); The first transformer input terminal of the multi-winding high-frequency transformer (1) is also electrically connected to a first auxiliary inductor (5), and the first auxiliary inductor (5) is electrically connected to the first full-bridge unit (202); the current generated by the power generation device (10) flows through the first voltage-stabilizing capacitor (201) and the first full-bridge unit (202) respectively, and finally enters the first transformer input terminal.

4. The photovoltaic-to-DC converter according to claim 1, characterized in that: The second DC branch comprises a second voltage-stabilizing capacitor (301) and a second full-bridge unit (302) electrically connected to the second voltage-stabilizing capacitor (301); The second transformer input terminal of the multi-winding high-frequency transformer (1) is also electrically connected to a second auxiliary inductor (6), and the second auxiliary inductor (6) is electrically connected to the second full-bridge unit (302); the current of the energy storage device (11) flows through the second voltage-stabilizing capacitor (301) and the second full-bridge unit (302) respectively, and finally enters the second transformer input terminal.

5. The photovoltaic-to-DC converter according to claim 1, characterized in that: The third DC branch comprises a third voltage-stabilizing capacitor (401) and a third full-bridge unit (402) electrically connected to the third voltage-stabilizing capacitor (401); The transformer output end of the multi-winding high-frequency transformer (1) is also electrically connected to a third auxiliary inductor (7), and the third auxiliary inductor (7) is electrically connected to the third full-bridge unit (402); the current converted by the multi-winding high-frequency transformer (1) flows from the transformer output end into the third full-bridge unit (402) and then respectively flows through the third voltage stabilizing capacitor (401) and the microgrid (12).

6. The photovoltaic-to-DC converter according to claim 5 is characterized in that: The third voltage-stabilizing capacitor (401) is also electrically connected to an auxiliary resistor (404).

7. The photovoltaic-to-DC converter according to claim 6 is characterized in that: The third DC branch further comprises a filter inductor (403), one end of the filter inductor (403) being electrically connected to the auxiliary resistor (404) and the third full-bridge unit (402), and the other end of the filter inductor (403) being electrically connected to the microgrid (12).

8. A control method for the photovoltaic-to-DC converter according to any one of claims 1 to 7, characterized in that: include: Sampling the first DC branch input terminal and the third DC branch output terminal respectively to obtain a first sampled current and a first sampled voltage of the first DC branch input terminal, and to obtain a second sampled current and a second sampled voltage of the third DC branch output terminal; Determining a first virtual variable based on the output voltage of the power generation equipment (10) when it is operating at maximum power and the first sampled voltage; Determining a second virtual variable based on a given power, a second sampled current, and a second sampled voltage of the microgrid (12); Convert the first dummy variable and the second dummy variable into corresponding phase shift angle variables; The switch pulses of the full-bridge units are controlled according to the phase shift angle to adjust the output power of the first DC branch, the second DC branch and the third DC branch.

9. The control method according to claim 8, characterized in that: The step of determining the first virtual variable based on the output voltage of the power generation equipment (10) when operating at maximum power and the first sampled voltage comprises: Calculating the output voltage of the power generation equipment (10) when it is operating at maximum power using a perturbation observation method according to the first sampled current and the first sampled voltage; The output voltage of the power generation equipment (10) when operating at maximum power is compared with the first sampled voltage and a first virtual variable is calculated.

10. The control method according to claim 8, characterized in that: The method of determining a second virtual variable based on a given power, a second sampled current, and a second sampled voltage of the microgrid (12) comprises: The sampling power is calculated according to the second sampling current and the second sampling voltage; The sampled power is compared with the given power to calculate the output current given value; The output current given value is compared with the second sampled current and a second virtual variable is obtained by calculation.

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