Power circulating current suppression method for hybrid parallel system of network construction type energy storage converter and network following type photovoltaic inverter

Abstract

The invention discloses a power circulating current suppression method of a hybrid parallel system of a network construction type energy storage converter and a network following type photovoltaic inverter, which is based on control of a power outer loop and a voltage and current inner loop of the network construction type energy storage converter and control of network access current of the network following type photovoltaic inverter. A state space averaging method is used for carrying out impedance modeling on the single grid-constructing type energy storage converter and the grid-following type photovoltaic inverter, and then a power circulation expression of the grid-constructing type energy storage converter and grid-following type photovoltaic inverter hybrid parallel system is deduced; and in combination with the deduced power circulating current expression, a differential wave trap current inner loop controller is added to the network-forming type energy storage converter, so that the power circulating current in the hybrid parallel system is reduced. According to the invention, the grid-forming type energy storage converter of the improved current controller is connected in parallel with the grid-following type photovoltaic inverter, so that the power circulating current between parallel systems is reduced, the working efficiency of the energy storage converter and the photovoltaic inverter is improved, and the service life of a power switch tube is prolonged.

Description

Technical Field

[0001] The invention relates to a power circulation suppression method of a hybrid parallel system of a grid-building type energy storage converter and a grid-following type photovoltaic inverter, and belongs to the field of converter control. Background Art

[0002] As the proportion of renewable energy in the power grid increases, the short-circuit ratio at the common coupling point has dropped significantly. Problems such as the lack of stability of grid-following control converters in weak power grids and their inability to provide voltage and frequency support for the system have become increasingly prominent.

[0003] There are circulating current problems in the converter parallel system. The power circulating current generated between converters will reduce the converter working efficiency, increase power consumption, shorten the life of the power tube, and even paralyze the parallel system. When converters of the same control type are connected in parallel, since the main circuit and control circuit parameters of the general parallel converters are not much different, the equivalent output voltage and equivalent impedance of each converter are not significantly different, and the power circulating current between the parallel converters can be ignored. However, when converters of different control types are connected in parallel, due to the large difference in their equivalent output voltage and output impedance, a large circulating current will be generated between the hybrid parallel system, reducing the inverter efficiency, accelerating the aging of the equipment and even causing equipment damage, seriously affecting the safe and stable operation of the system. Summary of the invention

[0004] The present invention aims to solve the deficiencies of the above-mentioned prior art and proposes a power circulation suppression method for a hybrid parallel system of a grid-connected energy storage inverter and a grid-following photovoltaic inverter, in order to reduce the power circulation in the hybrid parallel system, so that the grid-connected energy storage inverter has good steady-state accuracy and dynamic performance, thereby improving the working efficiency of the energy storage inverter and the photovoltaic inverter, and extending the life of the power switch tube.

[0005] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme:

[0006] The power circulation suppression method of a hybrid parallel system of a grid-connected energy storage converter and a grid-following photovoltaic inverter of the present invention is characterized in that it comprises the following steps:

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

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

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

[0010] Step 4: Divide the power circulation of the hybrid parallel system into pressure difference circulation and resistance difference circulation , wherein the pressure difference circulating current is excited by the difference between the equivalent output voltages of the parallel converters; the resistance difference circulating current is excited by the difference between the ratio of the equivalent output voltage and the equivalent output impedance of the parallel converters;

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

[0012] The power circulation suppression method of the hybrid parallel system of the grid-connected energy storage converter and the grid-following photovoltaic inverter described in the present invention is also characterized in that the step 1 comprises:

[0013] Step 1.1: Use formula (1) to construct the expression of the equivalent output impedance of the grid-connected energy storage converter:

[0014] (1)

[0015] In formula (1), s is the frequency domain, Represents the equivalent output impedance of the grid-type energy storage converter, is the transfer function between the voltage reference and the load current input point in the forward channel of the voltage control loop of the grid-type energy storage converter. is the transfer function between the load current input point and the output voltage in the forward channel of the voltage control loop of the grid-type energy storage converter, and:

[0016] (2)

[0017] (3)

[0018] In formula (2) and formula (3), is the equivalent gain of the bridge arm of the grid-type energy storage converter, is the transfer function of the voltage controller of the grid-type energy storage converter, is the transfer function of the current controller of the grid-type energy storage converter, is the filter inductance value of the grid-type energy storage converter, is the filter capacitance value of the grid-type energy storage converter;

[0019] Step 1.2: Use formula (4) to construct the expression of the equivalent output voltage of the grid-connected energy storage converter:

[0020] (4)

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

[0022] Furthermore, the step 2 comprises:

[0023] Step 2.1, use formula (6) to construct the expression of the equivalent output impedance of the grid-connected photovoltaic inverter;

[0024] (6)

[0025] In formula (6), is the equivalent output impedance of the grid-following photovoltaic inverter, is the transfer function between the current reference in the forward channel of the current control loop of the grid-following photovoltaic inverter and the grid voltage input point, is the transfer function from the grid voltage input point to the output current in the forward channel of the current control loop of the grid-following photovoltaic inverter, and:

[0026] (7)

[0027] (8)

[0028] In formula (7) and formula (8), is the equivalent gain of the bridge arm of the grid-following photovoltaic inverter, is the transfer function of the controller of the grid-following photovoltaic inverter, is the filter inductance value of the grid-following photovoltaic inverter, is the filter inductance value on the grid side, is the filter capacitor value of the grid-following photovoltaic inverter, is the active damping coefficient of the capacitor current feedback of the grid-following photovoltaic inverter,

[0029] Step 2.2, use formula (9) to construct the expression of the equivalent output voltage of the grid-connected photovoltaic inverter;

[0030] (12)

[0031] In formula (9), is the current command value of the grid-following photovoltaic inverter, is the equivalent output voltage of the grid-following photovoltaic inverter, is the current gain of the grid-following 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 formula (19), Represents the load impedance of the hybrid parallel system.

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

[0037] (20)

[0038] (twenty one).

[0039] Furthermore, the step 5 is to construct a current inner loop controller of the grid-type energy storage converter using equation (24):

[0040] (twenty four)

[0041] In formula (24), represents the coefficient of the current inner loop controller, is the transfer function of the notch filter, and we have:

[0042] (25)

[0043] (26)

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

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

[0046] An electronic device of the present invention includes a memory and a processor, wherein the memory is used to store a program that supports the processor to execute a power circulation suppression method, and the processor is configured to execute the program stored in the memory.

[0047] The present invention provides a computer-readable storage medium, wherein a computer program is stored on the computer-readable storage medium, and the computer program executes the steps of the power circulation suppression method when the computer program is executed by a processor.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] 1. In view of the problem of power circulation in the hybrid parallel system of the above-mentioned grid-forming energy storage inverter and the grid-following photovoltaic inverter, the present invention derives a specific expression of the power circulation of the hybrid parallel system of the grid-forming inverter and the grid-following inverter from the impedance perspective and divides it into pressure difference circulation and resistance difference circulation, which more accurately characterizes the mechanism of power circulation generation.

[0050] 2. The present invention aims to suppress the impedance difference circulating current to be always equal to zero, and derives a new current controller structure and parameters of the grid-type converter, which effectively avoids the adverse effects of reduced bandwidth and slow response caused by changing the current controller of the grid-type converter.

[0051] 3. The present invention adopts a differential notch filter as the current controller of the grid-type energy storage converter, which effectively suppresses the circulating current of the hybrid parallel system and reduces the power loss, while avoiding the voltage drop at the output end of the converter and ensuring the stable operation of the parallel system. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0055] Figure 4 It is a control block diagram of the grid-type energy storage converter in the present invention;

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

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

[0058] Figure 7 The power circulation waveform corresponding to different control structures of the grid-type energy storage converter in the present invention;

[0059] Figure 8 The voltage waveform and THD analysis diagram of the PCC point when the grid-type energy storage converter adopts single voltage loop control in the traditional technology;

[0060] Fig. 9 It is the voltage waveform and THD analysis diagram of the PCC point when the grid-type energy storage converter adopts the voltage outer loop integral current inner loop control in the traditional technology;

[0061] Fig.10It is the voltage waveform at the PCC point and its THD analysis diagram when the grid-type energy storage converter in the present invention adopts voltage outer loop differential current inner loop control. DETAILED DESCRIPTION

[0062] In this embodiment, in order to solve the problem of power circulation in the hybrid parallel system of grid-type energy storage inverter and grid-following photovoltaic inverter, a current inner loop control method based on differential notch filter of grid-type inverter is proposed. This method is based on the equivalent circuit model of grid-following inverter and grid-type inverter, combined with the specific expression of power circulation of energy storage inverter parallel system, and can significantly reduce the power circulation between energy storage inverter and photovoltaic inverter parallel system.

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

[0064] (1)

[0065] (2)

[0066] (3)

[0067] In formula (1) to formula (3), is the equivalent gain of the bridge arm of the grid-type energy storage converter, is the transfer function of the voltage controller of the grid-type energy storage converter, is the transfer function of the current controller of the grid-type energy storage converter, is the filter inductance value of the grid-type energy storage converter, is the filter capacitance value of the grid-type energy storage converter;

[0068] Using equations (1) to (3), the expressions of output impedance and equivalent output voltage of the energy storage converter using grid-following control are derived:

[0069] (4)

[0070] (5)

[0071] In formula (4) and (5), and They represent the output impedance and equivalent output voltage of the energy storage converter using grid-type control, It is the voltage loop command value of the grid-type energy storage converter.

[0072] One of the control objectives of the grid-connected inverter is to ensure that the grid-connected current meets the grid-connected requirements and 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 in is a typical double closed-loop current control, in which the outer loop feeds back the grid-connected current, and the inner loop feeds back the capacitor current through a proportional link. The main purpose of the latter is to increase the damping at the resonant frequency to avoid resonance. Figure 4 This is the control block diagram of the grid-connected photovoltaic inverter. The grid-connected current reference value and the grid voltage are the input of the system, and the grid-connected current is the output. That is, the study of the grid-connected system can be converted into the study of the characteristics of the dual-input single-output system. According to the block diagram equivalent transformation principle in automatic control theory, the control block diagram of the above system can be simplified to Figure 5 As shown in the above block diagram and They are:

[0073] (6)

[0074] (7)

[0075] (8)

[0076] In formula (8), is the current gain of the grid-following converter; is the equivalent admittance of the grid-type converter, and:

[0077] (9)

[0078] (10)

[0079] In formula (6) to formula (10), is the equivalent gain of the bridge arm of the grid-type converter, is the transfer function of the current controller, is the filter inductance value on the converter side, is the grid-side filter inductance value, is the converter filter capacitor value, For grid-following converters, the active damping coefficient of capacitor current feedback is adopted. is the grid-connected current of the converter, is the grid voltage, is the transfer function between the current reference and the grid voltage input point in the forward channel of the current control loop of the grid-following inverter, It is the transfer function from the grid voltage input point to the output current in the forward channel of the current control loop of the grid-following inverter.

[0080] Using equations (6) to (8), the expressions of output impedance and equivalent output voltage of the energy storage converter using grid-following control are derived:

[0081] (11)

[0082] (12)

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

[0084] The equivalent output voltage and output impedance of the grid-connected energy storage converter and the grid-following photovoltaic inverter obtained by equations (4)-(5) and (11)-(12) can be obtained as Figure 6 The equivalent circuit model diagram of the hybrid parallel system of energy storage converter and photovoltaic inverter is shown in the figure. Combined with 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] In formula (13) to formula (18), Indicates the equivalent output impedance of the grid-type converter, represents the equivalent output impedance of the grid-type converter, represents the load impedance, Indicates the equivalent output voltage of the grid-type converter, represents the equivalent output voltage of the grid-type converter, It represents the voltage across the load impedance. Indicates the output current of the grid-following converter. represents the output current of the grid-type converter, represents the current flowing through the load, Indicates the circulating current between inverters.

[0092] Combining equations (13) to (18) gives the specific expression of power circulation:

[0093] (19)

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

[0095] (20)

[0096] (twenty one)

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

[0098] Because the equivalent output impedance of the grid-type converter tends to infinity, and the control target of the grid-type converter is the output voltage, so in formula (19) It is difficult to be equal to zero in value. If you want to reduce the power circulation between the grid-type energy storage converter and the grid-type photovoltaic inverter hybrid parallel system as much as possible, you can start from the second type of power circulation. In addition, the control target of the grid-type converter is to control the filter inductance current, so the current single loop control is generally used (the use of LCL filter will increase the active damping inner loop). In order to quickly track the changes in the current command, when the current loop power command, grid voltage fluctuations and other situations occur, the current loop must be adjusted quickly so that the output current can reach the desired value in a relatively short time. This requires the current loop to meet the high bandwidth requirements in design. Once other control loops are added to the current loop, according to the principle of decreasing the loop design bandwidth from the inside to the outside, the bandwidth of the current loop will be greatly reduced, and good control effects cannot be achieved.

[0099] Using formula (19) The value is always equal to zero, that is Numerically equal to , combining equation (11)-equation (13) to obtain:

[0100] (twenty two)

[0101] Since the converter is controlled in the abc stationary coordinate system, Using PR controller and , is a sinusoidal quantity with the same frequency, and formula (22) is arranged as follows:

[0102] (twenty three)

[0103] Since the PR controller exhibits a resonant characteristic gain that tends to infinity within the resonant gain bandwidth frequency range, and exhibits a proportional characteristic in other frequency bands, and the proportional characteristic is determined by its proportional coefficient, it can be Designed for:

[0104] (twenty four)

[0105] In formula (24), is the transfer function of the notch filter, and we have:

[0106] (25)

[0107] (26)

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

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

[0110] From equation (24), we can see that the current controller of the newly designed grid-connected converter has a gain of The differentiator is connected in series with a notch filter whose resonant frequency is 50Hz.

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

[0112]

[0113] In order to verify the power circulation suppression strategy of the grid-connected parallel system proposed in this invention, the algorithm is verified on the Matlab / simulink simulation platform, and the simulation main circuit parameters are shown in Table 1. The simulation compares the control structure of the grid-connected converter proposed in this paper using a voltage outer loop plus a differential notch filter type current inner loop with the traditional control structures of a single voltage loop and a voltage outer loop and an integral current inner loop. The advantages of the proposed method are shown by the two indicators of the PCC point voltage THD value and the power circulation amplitude.

[0114] Figure 7 To calculate the circulating current size of the hybrid parallel system when the grid-type energy storage converter adopts different control structures, Figure 8 , Fig. 9 and Fig.10 The PCC point voltage waveform and its THD value are shown when the grid-type energy storage converter adopts different control structures. It can be seen that when the grid-type converter adopts single voltage loop control, the corresponding power circulation amplitude is about 6A and the PCC point voltage THD is 1.19%, which is the maximum value of the power circulation and PCC point voltage THD corresponding to the three control methods. When the grid-type converter adopts voltage outer loop integral current inner loop control, the corresponding power circulation amplitude is about 5A and the PCC point voltage THD is 1.09%, which has a certain improvement compared with the former control effect. When the grid-type converter adopts voltage outer loop differential current inner loop control, the corresponding power circulation is about 4A and the PCC point voltage THD is 0.33%, which is the minimum value of the power circulation and PCC point voltage THD corresponding to the three control methods, and the output power quality is greatly improved compared with the first two control methods.

[0115] In this embodiment, an electronic device includes a memory and a processor, wherein the memory is used to store a program that supports the processor to execute the above 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 on the computer-readable storage medium, and the computer program executes the steps of the above method when executed by a processor.

Claims

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

2. The power circulation suppression method of the hybrid parallel system of the grid-connected energy storage converter and the grid-following photovoltaic inverter according to claim 1 is characterized in that: The step 1 comprises: Step 1.1: Use formula (1) to construct the expression of the equivalent output impedance of the grid-connected energy storage converter: (1) In formula (1), s is the frequency domain, Represents the equivalent output impedance of the grid-type energy storage converter, is the transfer function between the voltage reference and the load current input point in the forward channel of the voltage control loop of the grid-type energy storage converter. is the transfer function between the load current input point and the output voltage in the forward channel of the voltage control loop of the grid-type energy storage converter, and: (2) (3) In formula (2) and formula (3), is the equivalent gain of the bridge arm of the grid-type energy storage converter, is the transfer function of the voltage controller of the grid-type energy storage converter, is the transfer function of the current controller of the grid-type energy storage converter, is the filter inductance value of the grid-type energy storage converter, is the filter capacitance value of the grid-type energy storage converter; Step 1.2: Use formula (4) to construct the expression of the equivalent output voltage of the grid-connected energy storage converter: (4) In formula (2), represents the equivalent output voltage of the grid-type energy storage converter in the frequency domain s, It is the voltage loop command value of the grid-type energy storage converter.

3. The power circulation suppression method of the hybrid parallel system of the grid-connected energy storage converter and the grid-following photovoltaic inverter according to claim 2 is characterized in that: The step 2 comprises: Step 2.1, use formula (6) to construct the expression of the equivalent output impedance of the grid-connected photovoltaic inverter; (6) In formula (6), is the equivalent output impedance of the grid-following photovoltaic inverter, is the transfer function between the current reference in the forward channel of the current control loop of the grid-following photovoltaic inverter and the grid voltage input point, is the transfer function from the grid voltage input point to the output current in the forward channel of the current control loop of the grid-following photovoltaic inverter, and: (7) (8) In formula (7) and formula (8), is the equivalent gain of the bridge arm of the grid-following photovoltaic inverter, is the transfer function of the controller of the grid-following photovoltaic inverter, is the filter inductance value of the grid-following photovoltaic inverter, is the filter inductance value on the grid side, is the filter capacitor value of the grid-following photovoltaic inverter, is the active damping coefficient of the capacitor current feedback of the grid-following photovoltaic inverter, Step 2.2, use formula (9) to construct the expression of the equivalent output voltage of the grid-connected photovoltaic inverter; (12) In formula (9), is the current command value of the grid-following photovoltaic inverter, is the equivalent output voltage of the grid-following photovoltaic inverter, is the current gain of the grid-following photovoltaic inverter; and: (9)。 4. The power circulation suppression method of the hybrid parallel system of the grid-connected energy storage converter and the grid-following photovoltaic inverter according to claim 3 is characterized in that: In step 3, the power circulation expression of the hybrid parallel system is constructed using equation (19): (19) In formula (19), Represents the load impedance of the hybrid parallel system.

5. The method for suppressing power circulation 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 pressure difference circulation is obtained by using equations (20) and (21) respectively. and resistance difference circulation : (20) (21)。 6. The method for suppressing power circulation 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: The step 5 is to construct the current inner loop controller of the grid-type energy storage converter using formula (24): (24) In formula (24), represents the coefficient of the current inner loop controller, is the transfer function of the notch filter, and we have: (25) (26) In formula (25), Q is the resonance width of the notch filter; In formula (26), is 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 to execute any power circulation current suppression method in claims 1-6, and the processor is configured to execute the program stored in the memory.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of any power circulating current suppression method in claims 1-6 are executed.

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