Energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient

Abstract

The invention relates to an energy storage type MMC battery state-of-charge rapid equalization method. The method comprises the following steps: acquiring a power reference value, voltage and current parameters of the energy storage type MMC, and calculating the number of initial upper / lower bridge arm input sub-modules; sOC imbalance between phases and between bridge arms is calculated, and a circulating current instruction value is obtained through an adaptive equalization coefficient; obtaining an upper / lower bridge arm input sub-module quantity correction value through a circulation controller; virtual resistance of the in-arm energy storage sub-module is calculated through a virtual resistance controller; the sub-modules are divided into an input group and a cut-off group, and the number of the input sub-modules of the switching group is determined according to the initial number of the input sub-modules of the upper / lower bridge arm and the correction value of the number of the input sub-modules of the upper / lower bridge arm. According to the invention, the state of charge balance of the energy storage type MMC can be realized, and the SOC balance speed is improved.

Description

Technical Field

[0001] The invention relates to a method for equalizing the state of charge of an energy storage type MMC based on adaptive virtual impedance and equalization coefficient, which is applicable to the fields of power electronics technology and energy storage technology. Background Art

[0002] As the penetration rate of renewable energy power generation increases, distributed energy storage systems have become a reliable solution to the problem of renewable energy consumption due to their advantages in peak shaving, wave smoothing and improving energy utilization.

[0003] In the power conversion system of energy storage, the chain structure of modular multilevel converter (MMC) is conducive to the decentralized access of energy storage submodules (ESM), reducing the power level of energy storage devices and avoiding the series and parallel connection of a large number of single energy storage elements. At the same time, it can be used as an intermediate buffer link for AC and DC grid interconnection to achieve four-quadrant power compensation, which has received more and more attention.

[0004] Since the distributed energy storage system based on MMC is composed of a large number of ESMs connected in series, the overall energy utilization rate of the energy storage system is limited by the state of charge (SOC) of the energy storage components; in addition, the randomness and volatility of renewable energy generation also intensify the requirements of distributed energy storage systems for charge state balance. Therefore, how to improve the SOC balance speed and balance accuracy of ESM has become one of the difficulties in the stable operation of energy storage MMC. Summary of the invention

[0005] In order to achieve the above object, the present invention provides a method for balancing the state of charge of an energy storage type MMC based on adaptive virtual impedance and balancing coefficient, and the method includes the following numerical simulation steps:

[0006] The AC side current command value is obtained by the power outer loop control, and the three-phase voltage command value is obtained by the current inner loop control; the initial upper / lower bridge arm input sub-module number is obtained by the three-phase voltage command value, DC voltage and the upper / lower bridge arm sub-module average voltage:

[0007]

[0008] Where: k = a, b, c represent three phase units, n pk0 , n nk0 are the initial upper / lower bridge arm input submodule numbers of phase k; round is the rounding function; U Cavg is the average voltage of the submodule capacitor; V dc is the DC voltage; u diffk is the k-phase voltage command value.

[0009] Calculate the SOC imbalance between phases and bridge arms:

[0010]

[0011] Where: k = a, b, c represent three phase units, ΔSOC k is the phase-to-phase imbalance degree of phase k, ΔSOC km is the imbalance between the k-phase bridge arms, SOC a is the average SOC value of phase a, SOC b is the average SOC value of phase b, SOC c is the average SOC value of phase c, SOC k is the average SOC value of phase k, SOC pk is the average SOC value of the upper bridge arm of phase k, SOC nk is the average SOC value of the lower bridge arm of phase k.

[0012] Calculate the adaptive equalization coefficients:

[0013]

[0014] Among them: K1 is the phase-to-phase adaptive balancing coefficient, K2 is the bridge arm adaptive balancing coefficient, a1, a2, b1, b2 are constants.

[0015] Calculate the circulating current command value:

[0016]

[0017] Where: k = a, b, c represent three phase units, P ref is the active power command value, V dc is the DC voltage, K1 is the phase-to-phase adaptive balancing coefficient, K2 is the bridge arm adaptive balancing coefficient, ΔSOC k is the phase-to-phase imbalance degree of phase k, ΔSOC km is the imbalance between the k-phase bridge arms.

[0018] Calculate the correction value of the number of submodules put into use in the upper / lower bridge arm:

[0019]

[0020] Where: Δn pk is the correction value of the number of submodules put into use in the upper bridge arm, Δn nk U is the correction value of the number of submodules put into the lower bridge arm, Cavg is the average voltage of the submodule capacitor, i pk is the upper arm current, i nk is the lower arm current, K p and K i are the proportional parameter and the integral parameter, and s is the Laplace operator.

[0021] Calculate the number of submodules for upper / lower bridge arms:

[0022]

[0023] Where: k = a, b, c represent three phase units, n pk , n nk n is the number of submodules put into the upper / lower bridge arm respectively; pk0 , n nk0 The initial number of upper / lower bridge arm submodules respectively; Δn pk is the correction value of the number of submodules put into use in the upper bridge arm, Δn nk Correction value for the number of sub-modules invested in the lower bridge arm.

[0024] Calculate the virtual impedance value of each energy storage unit:

[0025]

[0026] Where: k = a, b, c represent three phase units, m = p, n represent upper and lower bridge arms, j = 1 to N represent N submodules, R vmkj and SOC mkj are the virtual resistance value of the jth submodule of the k-phase m-bridge arm and the SOC value of the energy storage unit, respectively. max is the upper limit of SOC of energy storage unit, SOC min is the lower limit of SOC of the energy storage unit, SOC mkm is the SOC of the dominant energy storage unit; I Lmax is the maximum current of the energy storage unit, SOC mkmax and SOC mkmin are the maximum SOC value and minimum SOC value of the ESM in the k-phase m bridge arm, V dmax To specify the maximum voltage deviation.

[0027] The DC / DC converter determines the voltage command value according to the virtual impedance value. Each bridge arm submodule is divided into an input group and a cut-off group. When the number of bridge arm SM inputs needs to be increased, the objective function J is selected from the cut-off group. cmkj The one with the smallest value is added to the input group; when the number of bridge arm SMs needs to be reduced, the objective function value J is selected from the input group cmkj The smallest ones were added to the resection group.

[0028]

[0029] Where: k = a, b, c represent three phase units, m = p, n represent upper and lower bridge arms, j = 1-N represent N submodules; i mk is the bridge arm current, U cmkj is the submodule capacitor voltage, V dc is a DC voltage. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1This is the topological structure diagram of the energy storage MMC;

[0031] Figure 2 It is the relationship diagram between SOC imbalance and circulating current command value;

[0032] Figure 3 is the circulation controller;

[0033] Figure 4 is the equivalent circuit of the submodule taking into account the virtual impedance;

[0034] Figure 5 It is the upper / lower bridge arm SOC diagram;

[0035] Figure 6 It is the phase SOC balance diagram;

[0036] Figure 7 It is the SOC balance diagram between phases, between bridge arms and within a bridge arm.

[0037] Figure 8 It is a control flow chart. DETAILED DESCRIPTION

[0038] The present invention is based on a Figure 1 The three-port voltage source converter with energy storage system shown in the figure. The upper / lower two bridge arms form the DC side of each phase, and the common DC bus is composed of the DC sides of the three phases in parallel; each bridge arm is composed of multiple energy storage units connected in series with the bridge arm inductor and the bridge arm equivalent resistance. The energy storage unit is composed of four IGBTs, submodule capacitors, energy storage inductors and energy storage elements. The switching state of the IGBT determines the three working states of the energy storage unit (on, off, locked) and the charging and discharging state of the energy storage.

[0039] The AC side current command value is obtained by the power outer loop control, and the three-phase voltage command value is obtained by the current inner loop control; the initial upper / lower bridge arm input sub-module number is obtained by the three-phase voltage command value, DC voltage and the upper / lower bridge arm sub-module average voltage:

[0040]

[0041] Where: k = a, b, c represent three phase units, n pk0 , n nk0 are the initial upper / lower bridge arm input submodule numbers of phase k; round is the rounding function; U Cavg is the average voltage of the submodule capacitor; V dc is the DC voltage; u diffk is the k-phase voltage command value.

[0042] Calculate the SOC imbalance between phases and bridge arms:

[0043]

[0044] Where: k = a, b, c represent three phase units, ΔSOC k is the phase-to-phase imbalance degree of phase k, ΔSOC km is the imbalance between the k-phase bridge arms, SOC a is the average SOC value of phase a, SOC b is the average SOC value of phase b, SOC c is the average SOC value of phase c, SOC k is the average SOC value of phase k, SOC pk is the average SOC value of the upper bridge arm of phase k, SOC nk is the average SOC value of the lower bridge arm of phase k.

[0045] Calculate the adaptive equalization coefficients:

[0046]

[0047] Among them: K1 is the phase-to-phase adaptive balancing coefficient, K2 is the bridge arm adaptive balancing coefficient, a1, a2, b1, b2 are constants.

[0048] The SOC balance command value is defined as ξ, and the upper and lower limits of the circulation are i zmax and i zmin The upper limit of the circulation current is to ensure the safe operation of the system components, and the lower limit of the circulation current is to ensure the equilibrium speed of the SOC. zmin The relationship between SOC imbalance and circulating current command value is set as Figure 2 , according to ξ, i zmax and i zmin The values ​​of a1 and b1 can be determined; similarly, the values ​​of a2 and b2 can be obtained.

[0049] The calculation formula of circulating current command value is as follows:

[0050]

[0051] Where: k = a, b, c represent three phase units, P ref is the active power command value, V dc is the DC voltage, K1 is the phase-to-phase adaptive balancing coefficient, K2 is the bridge arm adaptive balancing coefficient, ΔSOC k is the phase-to-phase imbalance degree of phase k, ΔSOC km is the imbalance between the k-phase bridge arms.

[0052] Depend on Figure 3 Circulation controller, calculates the correction value of the number of submodules put into use in the upper / lower bridge arm:

[0053]

[0054] Where: Δn pk is the correction value of the number of submodules put into use in the upper bridge arm, Δn nk U is the correction value of the number of submodules put into the lower bridge arm, Cavg is the average voltage of the submodule capacitor, i pk is the upper arm current, i nk is the lower arm current, K p and K i are the proportional parameter and the integral parameter, and s is the Laplace operator.

[0055] Calculate the number of submodules for upper / lower bridge arms:

[0056]

[0057] Where: k = a, b, c represent three phase units, n pk , n nk n is the number of submodules put into the upper / lower bridge arm respectively; pk0 , n nk0 The initial number of upper / lower bridge arm submodules respectively; Δn pk is the correction value of the number of submodules put into use in the upper bridge arm, Δn nk is the correction value of the number of submodules put into the lower bridge arm. Since the analysis method of each bridge arm is the same, only one bridge arm is analyzed; R vmkj , I Lmkj are the virtual resistance and inductor current of the jth energy storage unit in the k-phase m-bridge arm, V ref is the submodule capacitor voltage reference value, V mkj is the voltage at the ESM outlet. By changing the virtual resistance, the voltage at the energy storage unit outlet can be adjusted. The equivalent circuit of the submodule taking into account the virtual impedance is as follows: Figure 4 shown.

[0058] Due to the existence of virtual resistance, the voltage deviation is:

[0059] V dmkj =I Lmkj R vmkj =V mkj -V ref ;

[0060] The SOC change of ESM is related to the virtual resistance. The smaller the virtual resistance, the greater the SOC change of ESM. Therefore, during charging, the ESM with a larger SOC value should have a larger virtual resistance value; during discharging, the ESM with a larger SOC value should have a smaller virtual resistance value. The larger the virtual resistance, the corresponding voltage deviation V dmkjThe larger the SOC, the larger the voltage. During the charging process, the ESM with a smaller SOC is selected as the dominant ESM; during the discharging process, the ESM with a larger SOC is selected as the dominant ESM. In order to ensure that the voltage deviation of each submodule is within a certain range, it is stipulated that:

[0061] max{V dmk1 |,|V dmk2 |,...,|V dmkN |}≤V dmax ;

[0062] Where: V dmax is the maximum allowable voltage deviation of the submodule.

[0063] Calculate the virtual impedance value of each energy storage unit:

[0064]

[0065] Where: k = a, b, c represent three phase units, m = p, n represent upper and lower bridge arms, j = 1 to N represent N submodules, R vmkj and SOC mkj are the virtual resistance value of the jth submodule of the k-phase m-bridge arm and the SOC value of the energy storage unit, respectively. max is the upper limit of SOC of energy storage unit, SOC min is the lower limit of SOC of the energy storage unit, SOC mkm is the SOC of the dominant energy storage unit; I Lmax is the maximum current of the energy storage unit, SOC mkmax and SOC mkmin are the maximum SOC value and minimum SOC value of the ESM in the k-phase m bridge arm, V dmax To specify the maximum voltage deviation.

[0066] Due to the difference in virtual resistance, the submodule capacitor voltage U cmkj Depending on the SOC, a voltage gradient will be generated as shown below:

[0067] U cmkj =V ref -R vmkj I Lmkj ;

[0068] Under this voltage gradient, the ESM capacitor voltage with a larger SOC is higher, and the ESM capacitor voltage with a smaller SOC is lower; combined with the MMC following sorting strategy.

[0069] Each bridge arm submodule is divided into an investment group and a removal group. When the number of bridge arm SMs needs to be increased, the one with the smallest objective function Jcmkj value is selected from the removal group and added to the investment group; when the number of bridge arm SMs needs to be reduced, the one with the smallest objective function Jcmkj value is selected from the investment group and added to the removal group.

[0070]

[0071] Where: k = a, b, c represent three phase units, m = p, n represent upper and lower bridge arms, j = 1-N represent N submodules; i mk is the bridge arm current, U cmkj is the submodule capacitor voltage, V dc is a DC voltage.

[0072] For ease of understanding, in this embodiment, a three-phase eleven-level two-terminal MMC-DESS simulation model is built. The AC side is connected to the grid, and the DC side is connected to an ordinary MMC. Among them, the capacity of the energy storage module will not affect the control effect; due to the limitation of simulation time, the energy storage capacity is set to a lower value to verify the effectiveness of the control strategy in a short time. The SOC of 12 energy storage units selected from the 6 bridge arms differ by 0.1% in sequence, and the simulation runs for 8s. At t=0s, the SOC balancing control of this article is put into use, and the SOC simulation waveform is as follows Figure 5-Figure 7 shown.

[0073] like Figure 5 and Figure 6 As shown in Figure 1, after using the adaptive balancing coefficient, the system will maintain a high balancing speed, and gradually reduce the balancing speed after balancing the SOC to a relatively close level. Figure 7 As shown, the SOC balancing speed within the bridge arm is also improved; the SOC balancing speed between phases, between bridge arms and within the bridge arm is significantly improved. In addition, the maximum SOC difference is reduced to 18.75% of the three-level balancing control method. This shows that the energy storage MMC state of charge balancing method based on adaptive virtual impedance and balancing coefficient has greatly improved the SOC balancing speed and balancing accuracy. Rapid balancing of the SOC of energy storage units between phases, between bridge arms and within the bridge arm effectively improves the utilization rate of the energy storage system.

[0074] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for equalizing the state of charge of an energy storage type MMC based on adaptive virtual resistance and equalization coefficient, characterized in that: The AC side current command value is obtained by the power outer loop control, and the three-phase voltage command value is obtained by the current inner loop control; the initial upper / lower bridge arm input sub-module quantity is obtained by the three-phase voltage command value, DC voltage and the upper / lower bridge arm sub-module average voltage; the circulating current command value is calculated according to the SOC imbalance between phases and bridge arms according to the adaptive balancing coefficient; the upper / lower bridge arm input sub-module quantity correction value is calculated through the circulating current controller; The correction value of the number of upper / lower bridge arm submodules put into use is added to the initial number of upper / lower bridge arm submodules put into use to obtain the number of upper / lower bridge arm submodules put into use; The SOC value of the energy storage unit in the bridge arm is controlled by adaptive virtual resistance to obtain the virtual impedance value of each energy storage unit. The DC / DC converter determines the voltage command value according to the virtual impedance value. Each bridge arm submodule is divided into an input group and a removal group. When the number of bridge arm SM inputs needs to be increased, the objective function J is selected from the removal group. cmkj The one with the smallest value is added to the input group; when the number of bridge arm SMs needs to be reduced, select the objective function J from the input group cmkj The one with the smallest value was added to the resection group.

2. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1 is characterized in that: The initial upper / lower bridge arm input submodule quantity is calculated by the following formula: Where: k = a, b, c represent three phase units, n pk0 , n nk0 are the initial upper / lower bridge arm input submodule numbers of phase k; round is the rounding function; U Cavg is the average voltage of the submodule capacitor; V dc is the DC voltage; u diffk is the k-phase voltage command value.

3. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1 is characterized in that: The initial phase-to-phase and bridge-arm SOC imbalance calculation formula is: Where: k = a, b, c represent three phase units, ΔSOC k is the phase-to-phase imbalance degree of k, ΔSOC km is the imbalance between the k-phase bridge arms, SOC a is the average SOC value of phase a, SOC b is the average SOC value of phase b, SOC c is the average SOC value of phase c, SOC k is the average SOC value of phase k, SOC pk is the average SOC value of the upper bridge arm of phase k, SOC nk is the average SOC value of the lower bridge arm of phase k.

4. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1 is characterized in that: The adaptive equalization coefficient calculation formula is: Among them: K1 is the phase-to-phase adaptive balancing coefficient, K2 is the bridge arm adaptive balancing coefficient, a1, a2, b1, b2 are constants.

5. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1, characterized in that: The circulation controller comprises: The calculation formula of circulating current command value is: Where: k = a, b, c represent three phase units, P ref is the active power command value, V dc is the DC voltage, K1 is the phase-to-phase adaptive balancing coefficient, K2 is the bridge arm adaptive balancing coefficient, ΔSOC k is the phase-to-phase imbalance degree of phase k, ΔSOC km is the imbalance between the k-phase bridge arms. The calculation formula for the correction value of the number of upper / lower bridge arm submodules is: Where: Δn pk is the correction value of the number of submodules put into use in the upper bridge arm, Δn nk U is the correction value of the number of submodules put into the lower bridge arm, Cavg is the average voltage of the submodule capacitor, i pk is the upper arm current, i nk is the lower arm current, K p and K i are the proportional parameter and the integral parameter, and s is the Laplace operator.

6. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1 is characterized in that: The calculation formula for the number of upper / lower bridge arm input submodules is: Where: k = a, b, c represent three phase units, n pk , n nk n is the number of submodules put into the upper / lower bridge arm respectively; pk0 , n nk0 The initial number of upper / lower bridge arm submodules respectively; Δn pk is the correction value of the number of submodules put into use in the upper bridge arm, Δn nk Correction value for the number of sub-modules invested in the lower bridge arm.

7. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1, characterized in that: The adaptive virtual resistance calculation formula is: Where: k = a, b, c represent three phase units, m = p, n represent upper and lower bridge arms, j = 1 to N represent N submodules, R vmkj and SOC mkj are the virtual resistance value of the jth submodule of the k-phase m-bridge arm and the SOC value of the energy storage unit, respectively. max is the upper limit of SOC of energy storage unit, SOC min is the lower limit of SOC of the energy storage unit, SOC mkm is the SOC of the dominant energy storage unit; I Lmax is the maximum current of the energy storage unit, SOC mkmax and SOC mkmin are the maximum SOC value and minimum SOC value of the ESM in the k-phase m bridge arm, V dmax To specify the maximum voltage deviation.

8. The energy storage type MMC charge state equalization method based on adaptive virtual impedance and equalization coefficient according to claim 1, characterized in that: The objective function J cmkj for: Where: k = a, b, c represent three phase units, m = p, n represent upper and lower bridge arms, j = 1-N represent N submodules; i mk is the bridge arm current, U cmkj is the submodule capacitor voltage, V dc is a DC voltage.

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