Power circulation suppression method for hybrid parallel system of network-constructed energy storage converter and network-following photovoltaic inverter

Abstract

This invention discloses a power circulating current suppression method for a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter. Based on the power outer loop and voltage / current inner loop control of the grid-connected energy storage converter and the grid-connected photovoltaic inverter's grid-connected current control, the method uses the state-space averaging method to model the impedance of a single grid-connected energy storage converter and a grid-connected photovoltaic inverter, thereby deriving the power circulating current expression for the hybrid parallel system. Combining the derived power circulating current expression, a differentiating notch filter current inner loop controller is added to the grid-connected energy storage converter to reduce the power circulating current within the hybrid parallel system. This invention, by employing an improved current controller in parallel connection of a grid-connected energy storage converter and a grid-connected photovoltaic inverter, reduces the power circulating current between the parallel systems, improves the operating efficiency of both the energy storage converter and the photovoltaic inverter, and extends the lifespan of the power switching transistors.

Description

Technical Field

[0001] This invention relates to a method for suppressing power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter, belonging to the field of converter control. Background Technology

[0002] As the proportion of new energy sources in the power grid increases, the short-circuit ratio at the common coupling point decreases significantly. Problems such as insufficient stability of grid-connected control converters under weak power grids and inability to provide voltage and frequency support for the system are becoming increasingly prominent.

[0003] Parallel converter systems suffer from circulating current problems. The power circulating current generated between converters can lead to a series of problems, including reduced converter efficiency, increased power consumption, shortened power transistor lifespan, and even system failure. When converters of the same control type are connected in parallel, the equivalent output voltage and equivalent impedance of each converter are not significantly different due to the generally similar main circuit and control circuit parameters, and the power circulating current between them can be ignored. However, when converters of different control types are connected in parallel, the significant differences in their equivalent output voltage and output impedance can generate substantial circulating currents in the hybrid parallel system. This reduces inverter efficiency, accelerates equipment aging, and can even cause equipment damage, seriously affecting the safe and stable operation of the system. Summary of the Invention

[0004] This invention addresses the shortcomings of existing technologies by proposing a power circulating current suppression method for a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter. The aim is to reduce the power circulating current within the hybrid parallel system, enabling the grid-connected energy storage converter to possess good steady-state accuracy and dynamic performance. This, in turn, improves the operating efficiency of both the energy storage converter and the photovoltaic inverter, and extends the lifespan of the power switching transistors.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0006] The present invention provides a method for suppressing power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter, characterized by comprising the following steps:

[0007] Step 1: Use the state-space averaging method to construct expressions for the equivalent output impedance and equivalent output voltage of the grid-type energy storage converter;

[0008] Step 2: Use the state-space averaging method to construct expressions for the equivalent output impedance and equivalent output voltage of the grid-connected photovoltaic inverter;

[0009] Step 3: Based on the expressions in Step 1 and Step 2, construct the power circulation expression for the hybrid parallel system of grid-connected energy storage converter and grid-connected photovoltaic inverter;

[0010] Step 4: Divide the power circulating current of the hybrid parallel system into differential pressure circulating current. and resistance circulation The differential pressure circulating current is generated by the difference in the equivalent output voltage of the parallel converter; the differential resistance circulating current is generated by the difference in the ratio of the equivalent output voltage to the equivalent output impedance of the parallel converter.

[0011] Step 5: Allow the differential resistance to circulate. With zero excitation source, and under the condition that the control structure and control parameters of the grid-connected photovoltaic inverter remain unchanged, a current inner loop controller for the grid-connected energy storage converter is constructed to suppress the power circulating current of the hybrid parallel system.

[0012] The power circulating current suppression method for a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter described in this invention is also characterized in that step 1 includes:

[0013] Step 1.1: Construct the expression for the equivalent output impedance of the grid-type energy storage converter using equation (1):

[0014] (1)

[0015] In equation (1), s is the frequency domain. The equivalent output impedance of a grid-type energy storage converter. This is the transfer function between the voltage reference in the forward path of the voltage control loop of a grid-type energy storage converter and the load current input point. Let be the transfer function between the load current input point and the output voltage in the forward path of the voltage control loop of the grid-connected energy storage converter, and we have:

[0016] (2)

[0017] (3)

[0018] In equations (2) and (3), The equivalent gain of the bridge arm of the grid-type energy storage converter. This is the transfer function of the voltage controller for a grid-type energy storage converter. This is the transfer function of the current controller for a grid-type energy storage converter. The filter inductance value of the grid-type energy storage converter. This refers to the filter capacitor value of a grid-type energy storage converter.

[0019] Step 1.2: Construct the expression for the equivalent output voltage of the grid-type energy storage converter using equation (4):

[0020] (4)

[0021] In equation (2), The equivalent output voltage of the grid-type energy storage converter in the frequency domain s. This is the voltage loop command value for the grid-type energy storage converter.

[0022] Furthermore, step 2 includes:

[0023] Step 2.1: Construct the expression for the equivalent output impedance of the grid-connected photovoltaic inverter using equation (6);

[0024] (6)

[0025] In equation (6), The equivalent output impedance of a grid-connected photovoltaic inverter. This refers to the transfer function between the current reference in the forward path of the current control loop of a grid-connected photovoltaic inverter and the grid voltage input point. Let the transfer function between the grid voltage input point and the output current in the forward path of the current control loop of the grid-connected photovoltaic inverter be:

[0026] (7)

[0027] (8)

[0028] In equations (7) and (8), To match the bridge arm equivalent gain of the grid-connected photovoltaic inverter, The transfer function of the controller for a grid-connected photovoltaic inverter. The filter inductance value is related to the grid-connected photovoltaic inverter. This is the value of the filter inductance on the grid side. The filter capacitor value is for a grid-connected photovoltaic inverter. The active damping coefficient for capacitor current feedback in grid-connected photovoltaic inverters.

[0029] Step 2.2: Construct the expression for the equivalent output voltage of the grid-connected photovoltaic inverter using equation (9);

[0030] (12)

[0031] In equation (9), This refers to the current command value for a grid-connected photovoltaic inverter. This refers to the equivalent output voltage of a grid-connected photovoltaic inverter. For the current gain of the grid-connected photovoltaic inverter; and:

[0032] (9).

[0033] Furthermore, in step 3, the power circulation expression of the hybrid parallel system is constructed using equation (19):

[0034] (19)

[0035] In equation (19), This represents the load impedance of a hybrid parallel system.

[0036] Furthermore, in step 4, the differential pressure circulation is obtained using equations (20) and (21) respectively. and resistance circulation :

[0037] (20)

[0038] (twenty one).

[0039] Furthermore, step 5 involves constructing the inner current loop controller for the grid-type energy storage converter using equation (24):

[0040] (twenty four)

[0041] In equation (24), The coefficients represent the current inner loop controller. Let be the transfer function of the notch filter, and we have:

[0042] (25)

[0043] (26)

[0044] In equation (25), Q is the resonant width of the notch filter;

[0045] In equation (26), This refers to the coefficient corresponding to the proportional term in the voltage loop PR controller.

[0046] The present invention provides an electronic device, including a memory and a processor, wherein the memory is used to store a program that supports the processor in executing a power circulating current suppression method, and the processor is configured to execute the program stored in the memory.

[0047] The present invention discloses a computer-readable storage medium on which a computer program is stored, wherein the computer program is executed by a processor to perform the steps of a power circulating current suppression method.

[0048] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0049] 1. In view of the problem of power circulating current in the above-mentioned hybrid parallel system of grid-type energy storage converter and grid-connected photovoltaic inverter, the present invention derives a specific expression of power circulating current in the hybrid parallel system of grid-type converter and grid-connected converter from the perspective of impedance and divides it into differential pressure circulating current and differential resistance circulating current, so as to more accurately characterize the mechanism of power circulating current generation.

[0050] 2. With the goal of suppressing the differential resistance circulating current to be always equal to zero, this invention derives a new current controller structure and parameters for grid-type converters, effectively avoiding the adverse effects of reduced bandwidth and slow response caused by changing the current controller of grid-type converters.

[0051] 3. This invention uses a differential notch filter as the current controller for the grid-type energy storage converter. While effectively suppressing the circulating current in the hybrid parallel system and reducing power loss, it also avoids voltage drop at the converter output, ensuring the stable operation of the parallel system. Attached Figure Description

[0052] Figure 1 This is a diagram showing the main circuit and control structure of the hybrid parallel system of energy storage converter and photovoltaic inverter in this invention;

[0053] Figure 2 This is a control block diagram of the grid-type energy storage converter in this invention;

[0054] Figure 3 This is a simplified control block diagram of the grid-type energy storage converter in this invention;

[0055] Figure 4 This is a control block diagram of the grid-connected energy storage converter in this invention;

[0056] Figure 5 This is a simplified control block diagram of the grid-connected energy storage converter in this invention;

[0057] Figure 6 This is an equivalent circuit model diagram of the hybrid parallel system of energy storage converter and photovoltaic inverter in this invention;

[0058] Figure 7 The power circulating waveforms corresponding to different control structures used in the grid-type energy storage converter in this invention;

[0059] Figure 8 The voltage waveform at PCC point and its THD analysis diagram for a traditional grid-type energy storage converter using single-voltage loop control.

[0060] Figure 9 The voltage waveform at PCC point and its THD analysis diagram for grid-type energy storage converters using voltage outer loop integral current inner loop control in traditional technology.

[0061] Figure 10This is a diagram showing the voltage waveform at point PCC and its THD analysis when the grid-type energy storage converter in this invention adopts differential current control with an outer voltage loop. Detailed Implementation

[0062] In this embodiment, to address the issue of power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter, a current inner-loop control method based on a differential notch filter of the grid-connected converter is proposed. This method is based on the equivalent circuit models of the grid-connected and grid-connected converters, combined with the specific expression of the power circulating current in the parallel system of the energy storage converter, which can significantly reduce the power circulating current between the parallel system of the energy storage converter and the photovoltaic inverter.

[0063] Impedance modeling was performed on the grid-connected energy storage converter and the grid-connected photovoltaic inverter before conducting the circulating current analysis. Figure 1 The diagram shows the main circuit and control circuit structure of a voltage-controlled energy storage converter. It can operate in off-grid mode, supplying power to local loads and providing necessary voltage and frequency support. Considering that the time constant of the power outer loop is much larger than that of the voltage-current inner loop, the influence of the power outer loop on the output impedance is ignored during modeling. The control block diagram of the voltage-controlled energy storage converter is as follows: Figure 2 As shown. According to the principle of block diagram equivalent transformation in automatic control theory, the control block diagram of the above system can be simplified to: Figure 3 As shown in the above block diagram. and They are respectively:

[0064] (1)

[0065] (2)

[0066] (3)

[0067] From equation (1) to equation (3), The equivalent gain of the bridge arm of the grid-type energy storage converter. This is the transfer function of the voltage controller for a grid-type energy storage converter. This is the transfer function of the current controller for a grid-type energy storage converter. The filter inductance value of the grid-type energy storage converter. This refers to the filter capacitor value of a grid-type energy storage converter.

[0068] Using equations (1) to (3), we derive the expressions for the output impedance and equivalent output voltage of the energy storage converter with grid-following control:

[0069] (4)

[0070] (5)

[0071] In equations (4) and (5), and These represent the output impedance and equivalent output voltage of the energy storage converter using grid-type control, respectively. This is the voltage loop command value for the grid-type energy storage converter.

[0072] One of the control objectives of grid-connected inverters is to ensure that the grid-connected current meets the grid-connection requirements and that the harmonic content is as low as possible, and directly controlling the grid-connected current is the simplest way to achieve this. Figure 1 The photovoltaic inverter control structure shown is a typical dual closed-loop current control, in which the outer loop feeds back the grid current, while the inner loop feeds back the capacitor current through a proportional element. The main purpose of the latter is to increase the damping at the resonant frequency to avoid resonance. Figure 4 The control block diagram for a grid-connected photovoltaic inverter is shown below. The grid-connected current reference value and grid voltage are the system inputs, while the grid-connected current is the output. Therefore, the study of a grid-connected system can be transformed into the study of the characteristics of this dual-input, single-output system. Based on the principle of equivalent block diagram transformation in automatic control theory, the control block diagram of the above system can be simplified as follows: Figure 5 As shown in the above block diagram. and They are respectively:

[0073] (6)

[0074] (7)

[0075] (8)

[0076] In equation (8), To match the current gain of the grid-connected converter; To obtain the equivalent admittance of the grid-type converter, we have:

[0077] (9)

[0078] (10)

[0079] From equation (6) to equation (10), To match the equivalent gain of the bridge arm of the grid converter, Here is the transfer function of the current controller. This is the value of the filter inductance on the converter side. This is the value of the filter inductance on the grid side. This refers to the value of the converter filter capacitor. To match the active damping coefficient of the grid-type converter, capacitor current feedback is used. This is the grid-connected current of the converter. This is the grid voltage. This refers to the transfer function between the current reference in the forward path of the current control loop of a grid-connected inverter and the grid voltage input point. This is the transfer function between the grid voltage input point and the output current in the forward path of the current control loop of a grid-connected inverter.

[0080] Using equations (6) to (8), we derive the expressions for the output impedance and equivalent output voltage of the energy storage converter with grid-following control:

[0081] (11)

[0082] (12)

[0083] In equations (11) and (12), and These represent the output impedance and equivalent output voltage of the energy storage converter using grid-connected control, respectively.

[0084] The equivalent output voltage and output impedance of the grid-connected energy storage converter and the grid-connected photovoltaic inverter, obtained from equations (4)-(5) and (11)-(12), can be used to obtain... Figure 6 The equivalent circuit model diagram of the hybrid parallel system of energy storage converter and photovoltaic inverter is shown. Based on the definition of circulating current, the following KCL and KVL equations are written:

[0085] (13)

[0086] (14)

[0087] (15)

[0088] (16)

[0089] (17)

[0090] (18)

[0091] From equation (13) to equation (18), This represents the equivalent output impedance of a grid converter. This represents the equivalent output impedance of a grid-type converter. Indicates the load impedance. This represents the equivalent output voltage of a grid-type converter. This represents the equivalent output voltage of the grid-connected converter. This represents the voltage across the load impedance. This indicates the output current of the grid-connected converter. This indicates the output current of the grid-connected converter. This represents the current flowing through the load. This indicates the circulating current between inverters.

[0092] Combining equations (13) to (18), we obtain the specific expression for the power circulation:

[0093] (19)

[0094] Observing the specific expression of the power circulation in equation (19), it can be divided into two categories:

[0095] (20)

[0096] (twenty one)

[0097] In equations (20) and (21), This is called differential voltage circulating current, which is generated by the difference in equivalent output voltage of each energy storage converter; It is called differential current circulation, which is generated by the difference between the ratio of the equivalent output voltage to the equivalent output impedance of each energy storage converter.

[0098] Because the equivalent output impedance of the grid converter tends to infinity, and the control target of the grid converter is the output voltage, therefore, in equation (19) It's difficult to make the value zero. To minimize the power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter, one can start with the second type of power circulating current. Furthermore, the control objective of a grid-connected converter is to control the filter inductor current, so single-loop current control is generally used (using an LCL filter adds an active damping inner loop). To quickly track changes in the current command, the current loop must adjust rapidly when the current loop power command or grid voltage fluctuations occur, ensuring the output current reaches the desired value within a short time. This requires the current loop to meet high bandwidth requirements in its design. Adding other control loops within the current loop significantly reduces its bandwidth, following the principle of decreasing bandwidth from the inside out, making it impossible to achieve good control performance.

[0099] Using equation (19) The value is always equal to zero, that is... Numerically equal to Combining equations (11) and (13), we get:

[0100] (twenty two)

[0101] Since the converter is controlled entirely in the abc stationary coordinate system, Employing a PR controller and , For sinusoidal quantities of the same frequency, equation (22) is rearranged as follows:

[0102] (twenty three)

[0103] Furthermore, since the PR controller exhibits resonant characteristics with gain approaching infinity within its resonant gain bandwidth frequency range, and displays proportional characteristics in other frequency bands, and these proportional characteristics are determined by its proportionality coefficient, then... Designed as follows:

[0104] (twenty four)

[0105] In equation (24), Let be the transfer function of the notch filter, and we have:

[0106] (25)

[0107] (26)

[0108] In equation (25), Q is the resonant width of the notch filter;

[0109] In equation (26), This refers to the coefficient corresponding to the proportional term in the voltage loop PR controller.

[0110] Observing equation (24), it can be seen that the newly designed grid-type converter current controller has a gain of It consists of a differentiator connected in series with a notch filter with a resonant frequency of 50Hz.

[0111] Table 1 shows the main parameters in this embodiment:

[0112]

[0113] To verify the power circulating current suppression strategy of the grid-connected parallel system proposed in this invention, the algorithm was verified on the Matlab / Simulink simulation platform. The simulation main circuit parameters are shown in Table 1. The simulation compares the proposed grid-connected converter control structure using a voltage outer loop plus a differentiating notch filter-type current inner loop with the traditional control structures using a single voltage loop, a voltage outer loop, and an integral current inner loop. The advantages of the proposed method are demonstrated by the THD value of the PCC point voltage and the magnitude of the power circulating current.

[0114] Figure 7 The circulating current of a hybrid parallel system is calculated when different control structures are used for grid-type energy storage converters. Figure 8 , Figure 9 and Figure 10 The table shows the PCC point voltage waveforms and THD values ​​for different control structures used in a grid-connected energy storage converter. It can be seen that when the grid-connected converter uses single-voltage-loop control, the corresponding power circulating current amplitude is approximately 6A and the PCC point voltage THD is 1.19%, which is the maximum among the three control methods. When the grid-connected converter uses voltage outer-loop integral-current inner-loop control, the corresponding power circulating current amplitude is approximately 5A and the PCC point voltage THD is 1.09%, showing some improvement compared to the former. When the grid-connected converter uses voltage outer-loop differential-current inner-loop control, the corresponding power circulating current is approximately 4A and the PCC point voltage THD is 0.33%, which is the minimum among the three control methods, showing a significant improvement in output power quality compared to the first two control methods.

[0115] In this embodiment, an electronic device includes a memory and a processor. The memory stores a program that supports the processor in executing the above-described method, and the processor is configured to execute the program stored in the memory.

[0116] In this embodiment, a computer-readable storage medium stores a computer program, which is executed by a processor to perform the steps of the above method.

Claims

1. A method for suppressing power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter, characterized in that, Includes the following steps: Step 1: Use the state-space averaging method to construct expressions for the equivalent output impedance and equivalent output voltage of the grid-type energy storage converter; Step 2: Use the state-space averaging method to construct expressions for the equivalent output impedance and equivalent output voltage of the grid-connected photovoltaic inverter; Step 3: Based on the expressions in Step 1 and Step 2, construct the power circulation expression for the hybrid parallel system of grid-connected energy storage converter and grid-connected photovoltaic inverter; Step 4: Divide the power circulating current of the hybrid parallel system into differential pressure circulating current. and resistance circulation The differential pressure circulating current is generated by the difference in the equivalent output voltage of the parallel converter; the differential resistance circulating current is generated by the difference in the ratio of the equivalent output voltage to the equivalent output impedance of the parallel converter. Step 5: Allow the differential resistance to circulate. With zero excitation source, and under the condition that the control structure and control parameters of the grid-connected photovoltaic inverter remain unchanged, a current inner loop controller for the grid-connected energy storage converter is constructed to suppress the power circulating current of the hybrid parallel system.

2. The power circulating current suppression method for a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter according to claim 1, characterized in that, Step 1 includes: Step 1.1: Construct the expression for the equivalent output impedance of the grid-type energy storage converter using equation (1): (1) In equation (1), s is the frequency domain. The equivalent output impedance of a grid-type energy storage converter. This is the transfer function between the voltage reference in the forward path of the voltage control loop of a grid-type energy storage converter and the load current input point. Let be the transfer function between the load current input point and the output voltage in the forward path of the voltage control loop of the grid-connected energy storage converter, and we have: (2) (3) In equations (2) and (3), The equivalent gain of the bridge arm of the grid-type energy storage converter. This is the transfer function of the voltage controller for a grid-type energy storage converter. This is the transfer function of the current controller for a grid-type energy storage converter. The filter inductance value of the grid-type energy storage converter. This refers to the filter capacitor value of a grid-type energy storage converter. Step 1.2: Construct the expression for the equivalent output voltage of the grid-type energy storage converter using equation (4): (4) In equation (2), The equivalent output voltage of the grid-type energy storage converter in the frequency domain s. This is the voltage loop command value for the grid-type energy storage converter.

3. The power circulating current suppression method for a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter according to claim 2, characterized in that, Step 2 includes: Step 2.1: Construct the expression for the equivalent output impedance of the grid-connected photovoltaic inverter using equation (6): (6) In equation (6), The equivalent output impedance of a grid-connected photovoltaic inverter. This refers to the transfer function between the current reference in the forward path of the current control loop of a grid-connected photovoltaic inverter and the grid voltage input point. Let the transfer function between the grid voltage input point and the output current in the forward path of the current control loop of the grid-connected photovoltaic inverter be: (7) (8) In equations (7) and (8), To match the bridge arm equivalent gain of the grid-connected photovoltaic inverter, The transfer function of the controller for a grid-connected photovoltaic inverter. The filter inductance value is related to the grid-connected photovoltaic inverter. This is the value of the filter inductance on the grid side. The filter capacitor value is for a grid-connected photovoltaic inverter. The active damping coefficient for capacitor current feedback in grid-connected photovoltaic inverters. Step 2.2: Construct the expression for the equivalent output voltage of the grid-connected photovoltaic inverter using equation (9); (12) In equation (9), This refers to the current command value for a grid-connected photovoltaic inverter. This refers to the equivalent output voltage of a grid-connected photovoltaic inverter. For the current gain of the grid-connected photovoltaic inverter; and: (9)。 4. The power circulating current suppression method for a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter according to claim 3, characterized in that, In step 3, the power circulation expression of the hybrid parallel system is constructed using equation (19): (19) In equation (19), This represents the load impedance of a hybrid parallel system.

5. The method for suppressing power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter according to claim 4, characterized in that, In step 4, the differential pressure circulation is obtained using equations (20) and (21) respectively. and resistance circulation : (20) (21)。 6. The method for suppressing power circulating current in a hybrid parallel system of a grid-connected energy storage converter and a grid-connected photovoltaic inverter according to claim 5, characterized in that, Step 5 involves using equation (24) to construct the inner current loop controller for the grid-type energy storage converter: (24) In equation (24), The coefficients represent the current inner loop controller. Let be the transfer function of the notch filter, and we have: (25) (26) In equation (25), Q is the resonant width of the notch filter; In equation (26), This refers to the coefficient corresponding to the proportional term in the voltage loop PR controller.

7. An electronic device, comprising a memory and a processor, characterized in that, The memory is used to store a program that supports the processor in executing any of the power circulating current suppression methods of claims 1-6, and the processor is configured to execute the program stored in the memory.

8. A computer-readable storage medium storing a computer program thereon, characterized in that, When the computer program is run by the processor, it performs the steps of any of the power circulating current suppression methods in claims 1-6.

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