State of charge balancing method for energy storage MMC based on capacitor voltage correction

Abstract

Disclosed in the present invention is a state of charge (SOC) balancing method for an energy storage MMC based on capacitor voltage correction, comprising the following steps: acquiring important parameters such as the SOCs of energy storage units and current directions of bridge arms in an energy storage MMC; calculating SOC average values of phase units, of the bridge arms, and of three phases of energy storage units; calculating the inter-phase SOC imbalance degree, the inter-bridge arm SOC imbalance degree, and the inter-submodule SOC imbalance degree, obtaining an inter-phase power deviation, an inter-bridge arm power deviation, and an inter-submodule voltage deviation by means of a proportional controller, and calculating a submodule energy storage power reference value and submodule corrected capacitor voltages; calculating the duty cycle of a switching device in a bidirectional DC / DC converter; and sorting the submodule corrected capacitor voltages, and on the basis of the charging / discharging state of submodule capacitors, determining submodules to be connected. The present invention can realize SOC balancing of energy storage MMCs, and reduces the number of bidirectional DC / DC converter controllers, thereby improving the SOC balancing efficiency and the capacity utilization rate of energy storage units.

Description

Energy storage MMC state of charge balancing method based on capacitor voltage correction Technical Field

[0001] The present invention relates to the technical field of energy storage unit SOC balancing control, and in particular to an energy storage type MMC state of charge balancing method based on capacitor voltage correction. Background Art

[0002] The power system is facing the challenge of increasing renewable energy penetration. However, renewable energy generation is characterized by randomness and volatility, necessitating the integration of energy storage units to mitigate output uncertainty. Energy storage multi-module (MMC) systems have emerged as a response to this challenge. However, with a large number of energy storage submodules, variations in energy storage losses and initial values ​​inevitably lead to imbalanced SOCs, causing interphase circulating currents, overcharging and over-discharging of energy storage units, and impacting system stability. Therefore, balanced SOC control of the energy storage units in MMCs is essential.

[0003] Control strategies that use zero-sequence voltage and fundamental frequency current injection to achieve phase-to-phase SOC balancing and upper and lower bridge arm SOC balancing, respectively, are computationally complex and challenging to implement. Existing hierarchical SOC balancing control strategies require a large number of controllers to adjust the energy storage power of each submodule, resulting in low SOC balancing efficiency and limiting the utilization of energy storage unit capacity. Therefore, a fast SOC balancing method with simple calculations and control is urgently needed.

[0004] Summary of the Invention

[0005] The purpose of the present invention is to solve the defects of the existing technology of multiple SOC balancing controllers and low system balancing efficiency, and to provide a method for charging state balancing of energy storage type MMC based on capacitor voltage correction. This control method can utilize the linear relationship between the energy storage unit voltage and the sub-module capacitor voltage, and realize SOC balancing between sub-modules by correcting the capacitor voltage balancing of the sub-module, thereby improving the SOC balancing efficiency and thus improving the utilization rate of the energy storage unit capacity.

[0006] The purpose of the present invention can be achieved by taking the following technical solutions:

[0007] A method for equalizing the state of charge of an energy storage type MMC based on capacitor voltage correction, the method comprising the following steps:

[0008] S1. Obtain the bridge arm current direction and energy storage unit charge state in the energy storage MMC, obtain the phase unit modulation wave reference value and the submodule energy storage power setting value, obtain the actual values ​​of the submodule capacitor voltage, submodule energy storage power, and submodule energy storage current, and obtain the submodule capacitor voltage and submodule energy storage unit voltage rated value;

[0009] S2. Calculate the average SOC of the phase unit, bridge arm, and three-phase energy storage unit;

[0010] S3. Calculate the SOC imbalance between phases, bridge arms, and submodules, obtain the power deviation between phases and bridge arms and the voltage deviation between submodules through the proportional controller, and calculate the submodule energy storage power reference value and the submodule correction capacitor voltage;

[0011] S4. Calculate the duty cycle of the switching device in the bidirectional DC / DC converter;

[0012] S5. Sort the submodule correction capacitor voltages and determine the submodule to be put into operation according to the submodule capacitor charge and discharge status.

[0013] Furthermore, the average SOC value of the bridge arm SOC rj , phase unit SOC average SOC j And the average SOC of the three-phase energy storage unit SOC ave The calculation formula is as follows:

[0014] Where N is the number of submodules in each bridge arm, j = a, b, c represents three phase units, r = p, n represents the upper and lower bridge arms of each phase unit, i = 1, 2, ..., N represents the i-th submodule in the bridge arm, i.e. SOC rji is the SOC of the i-th submodule in the j-phase r-bridge arm, SOC a is the average SOC value of phase unit a, SOC b is the average SOC value of phase unit b, SOC c is the average SOC value of phase unit a.

[0015] Furthermore, the inter-phase SOC imbalance ΔSOC j , SOC imbalance between bridge arms ΔSOC rj , SOC imbalance between submodules ΔSOC rji The calculation formula is as follows:

[0016] Among them, SOC ave is the average SOC value of the three-phase energy storage unit, SOC j is the average SOC value of phase j, SOC rj is the average SOC value of the bridge arm of phase j, SOC rji is the SOC of the i-th submodule in the j-phase r-bridge arm.

[0017] Furthermore, the phase unit power deviation ΔP sc_j , bridge arm power deviation ΔP sc_rj and submodule voltage deviation ΔU sc_rji The calculation formula is as follows:

[0018] Among them, k ph 、k arm 、k sm They are the proportional coefficients of the inter-phase SOC balance controller, the inter-bridge arm SOC balance controller, and the inter-submodule SOC balance controller, ΔSOC j , ΔSOC rj , ΔSOC rji They are inter-phase SOC imbalance, inter-bridge arm SOC imbalance, and inter-submodule SOC imbalance.

[0019] Furthermore, the submodule energy storage power reference value The calculation formula is as follows:

[0020] in, is the submodule energy storage power setting value, ΔP sc_j is the phase unit power deviation, ΔP sc_rj is the bridge arm power deviation.

[0021] Furthermore, the submodule corrects the capacitor voltage The calculation formula is as follows:

[0022] Among them, U sm_rji is the actual value of the capacitor voltage of the i-th submodule in the j-phase r-bridge arm, ΔU sc_rji is the voltage deviation of the corresponding submodule.

[0023] Furthermore, the reference value of the average value of the bridge arm energy storage current The calculation formula is as follows:

[0024] Among them, K PP and K IP are the proportional parameter and integral parameter of the energy storage power controller respectively, s is the Laplace operator, is the reference value of energy storage power of submodule, P sc_rj is the average energy storage power of the bridge arm neutron module, and its expression is as follows:

[0025] Among them, P sc_rji is the actual value of the energy storage power of the i-th submodule in the j-phase r-bridge arm. Through the above control rate, the energy storage power of the submodules can be quickly adjusted to make the energy storage power of the submodules in each bridge arm equal.

[0026] Furthermore, the bidirectional DC / DC converter adopts complementary PWM control, and the duty cycle D of the switching device is calculated as follows:

[0027] Among them, K PC and K IC are the proportional parameter and integral parameter of the energy storage current controller respectively, s is the Laplace operator, is the reference value of the average value of the bridge arm energy storage current, I L_rj is the average value of the bridge arm energy storage current, D0 is the duty cycle feedforward term, and the expression is as follows:

[0028] Among them, I L_rji is the actual value of the energy storage current of the i-th submodule in the j-phase r-bridge arm, is the rated voltage of the submodule energy storage unit, The above control ratio allows for rapid current regulation and limiting, accelerating SOC balancing. This allows for control of the switching devices in a bidirectional DC / DC converter in complementary PWM mode, achieving interphase SOC balancing and inter-arm SOC balancing.

[0029] Furthermore, when the nearest level approximation modulation is adopted, the number of submodules N required for the upper and lower bridge arms in each phase unit is pon 、N non The calculation formula is as follows:

[0030] Among them, N is the number of neutron modules in each bridge arm, u vj_ref is the reference value of the j-phase modulation wave, U c_sm is the rated value of the submodule capacitor voltage, and round(x) represents the integer closest to x.

[0031] Furthermore, the submodule correction capacitor voltages are sorted from low to high, and the charge and discharge state of the submodule capacitor is determined according to the direction of the bridge arm current. When the submodule capacitor is in the charging state, the upper and lower bridge arms are respectively connected to N pon 、N non The submodule with the lowest voltage; when the submodule capacitor is in the discharge state, the upper and lower bridge arms are respectively put into N pon 、N non The above control rate can quickly achieve SOC balance between sub-modules and improve the capacity utilization of energy storage units.

[0032] The present invention has the following advantages and effects compared to the prior art:

[0033] Based on traditional hierarchical SOC balancing control, this invention leverages the linear relationship between the submodule capacitor voltage and the submodule energy storage unit voltage in an energy storage MMC to achieve SOC balancing between submodules, thereby achieving state-of-charge balancing in the energy storage MMC. This balancing control significantly reduces the number of bidirectional DC / DC converter controllers and effectively improves SOC balancing efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0035] FIG1 is a flow chart of a method for balancing the state of charge of an energy storage MMC based on capacitor voltage correction according to the present invention;

[0036] Figure 2 is a topological diagram of an energy storage MMC;

[0037] FIG3 is a control block diagram of a bidirectional DC / DC converter in an energy storage MMC;

[0038] FIG4 is a control block diagram based on corrected capacitor voltage balancing in an energy storage MMC;

[0039] 5 is a schematic diagram of the SOC simulation waveform of the upper bridge arm submodule of phase A in the energy storage MMC using the state of charge balancing method of the present invention;

[0040] 6 is a schematic diagram of the SOC simulation waveform of the lower bridge arm submodule of phase A in the energy storage MMC using the state of charge balancing method of the present invention;

[0041] 7 is a schematic diagram of the SOC simulation waveforms of the upper and lower bridge arms of phase A in the energy storage MMC using the state of charge balancing method of the present invention;

[0042] 8 is a schematic diagram of the SOC simulation waveforms of the upper and lower bridge arms of the B phase in the energy storage type MMC using the state of charge balancing method of the present invention;

[0043] FIG9 is a schematic diagram of the SOC simulation waveforms of the three phases A, B, and C in the energy storage MMC using the state of charge balancing method of the present invention. DETAILED DESCRIPTION

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0045] Example 1

[0046] As shown in FIG1 , this embodiment discloses a method for equalizing the state of charge of an energy storage MMC based on capacitor voltage correction, comprising the following steps:

[0047] S1. Obtain the bridge arm current direction and energy storage unit charge state in the energy storage MMC, obtain the phase unit modulation wave reference value and the submodule energy storage power setting value, obtain the actual values ​​of the submodule capacitor voltage, submodule energy storage power, and submodule energy storage current, and obtain the submodule capacitor voltage and submodule energy storage unit voltage rated value;

[0048] S2. Calculate the average SOC of the phase unit, bridge arm, and three-phase energy storage unit;

[0049] Among them, the average SOC value of the bridge arm is SOC rj , phase unit SOC average SOC j And the average SOC of the three-phase energy storage unit SOC ave The calculation formula is as follows:

[0050] Where N is the number of submodules in each bridge arm, j = a, b, c represents three phase units, r = p, n represents the upper and lower bridge arms of each phase unit, i = 1, 2, ..., N represents the i-th submodule in the bridge arm, i.e. SOC rji is the SOC of the i-th submodule in the j-phase r-bridge arm, SOC a is the average SOC value of phase unit a, SOC b is the average SOC value of phase unit b, SOC c is the average SOC of phase unit c.

[0051] S3. Calculate the SOC imbalance between phases, bridge arms, and submodules, obtain the power deviation between phases and bridge arms and the voltage deviation between submodules through the proportional controller, and calculate the submodule energy storage power reference value and the submodule correction capacitor voltage;

[0052] Among them, the inter-phase SOC imbalance ΔSOC j , SOC imbalance between bridge arms ΔSOC rj , SOC imbalance between submodules ΔSOC rji The calculation formula is as follows:

[0053] Among them, SOC ave is the average SOC value of the three-phase energy storage unit, SOC j is the average SOC value of phase j, SOC rj is the average SOC value of the bridge arm of phase j, SOC rjiis the SOC of the i-th submodule in the j-phase r-bridge arm.

[0054] In order to achieve SOC balance at all levels, it is necessary to calculate the phase unit power deviation ΔP sc_j , bridge arm power deviation ΔP sc_rj and submodule voltage deviation ΔU sc_rji , the calculation formula is as follows:

[0055] Among them, k ph 、k arm 、k sm They are the proportional coefficients of the inter-phase SOC balance controller, the inter-bridge arm SOC balance controller, and the inter-submodule SOC balance controller, ΔSOC j , ΔSOC rj , ΔSOC rji They are inter-phase SOC imbalance, inter-bridge arm SOC imbalance, and inter-submodule SOC imbalance.

[0056] In order to achieve inter-phase SOC balance and inter-arm SOC balance, it is necessary to add the phase unit power deviation and the bridge arm power deviation to the submodule energy storage power setting value to obtain the submodule energy storage power reference value. The calculation formula is as follows:

[0057] in, is the submodule energy storage power setting value, ΔP sc_j is the phase unit power deviation, ΔP sc_rj is the bridge arm power deviation.

[0058] In order to achieve SOC balance between submodules, it is necessary to add the voltage deviation between submodules to the submodule capacitor voltage to obtain the submodule corrected capacitor voltage. The calculation formula is as follows:

[0059] Among them, U sm_rji is the actual value of the capacitor voltage of the i-th submodule in the j-phase r-bridge arm, ΔU sc_rji is the voltage deviation of the corresponding submodule.

[0060] S4. Calculate the duty cycle of the switching device in the bidirectional DC / DC converter;

[0061] The bidirectional DC / DC converter adopts complementary PWM control. The control block diagram is shown in Figure 3. The duty cycle D of the switching device is calculated as follows:

[0062] Among them, K PC and K ICare the proportional parameter and integral parameter of the energy storage current controller respectively, s is the Laplace operator, is the reference value of the average value of the bridge arm energy storage current, I L_rj is the average value of the bridge arm energy storage current, D0 is the duty cycle feedforward term, and the expression is as follows:

[0063] Among them, I L_rji is the actual value of the energy storage current of the i-th submodule in the j-phase r-bridge arm, is the rated voltage of the submodule energy storage unit, is the submodule capacitor voltage rating.

[0064] Reference value of the average value of the bridge arm energy storage current The calculation formula is as follows:

[0065] Among them, K PP and K IP are the proportional parameter and integral parameter of the energy storage power controller respectively, s is the Laplace operator, is the reference value of energy storage power of submodule, P sc_rj is the average energy storage power of the bridge arm neutron module, and its expression is as follows:

[0066] Among them, P sc_rji is the actual energy storage power value of the i-th submodule in the j-phase r-bridge arm.

[0067] S5. Sort the submodule correction capacitor voltages and determine the submodule to be put into operation according to the submodule capacitor charge and discharge status.

[0068] The control block diagram based on the correction capacitor voltage balance is shown in Figure 4. When the submodule capacitor is in the charging state, the upper and lower bridge arms are respectively put into N pon 、N non The submodule with the lowest voltage; when the submodule capacitor is in the discharge state, the upper and lower bridge arms are respectively put into N pon 、N non The submodule with the highest voltage.

[0069] Example 2

[0070] Based on the energy storage type MMC state of charge balancing method based on capacitor voltage correction disclosed in Example 1, this embodiment uses the energy storage type MMC topology structure shown in Figure 2 for simulation verification, and builds a two-terminal transmission system based on a 5-level energy storage type MMC. The rated capacity of the converter stations at both ends is 1.5MW, and the rated capacity of the energy storage system is 0.36MW. The specific parameters are shown in Table 1. The energy storage unit SOC adopts the balancing method involved in the present invention. In Figure 2, each bridge arm of the energy storage type MMC is composed of a plurality of energy storage submodules in cascade, and the energy storage unit in each energy storage submodule is connected in parallel at both ends of the submodule capacitor through a bidirectional DC / DC converter. Among them, T1, T2 and C sm Forming a half-bridge submodule, T3, T4 and L sc A bidirectional DC / DC converter is formed, and the supercapacitor C sc Access submodule.

[0071] Table 1. Simulation parameters

[0072] Furthermore, at the initial moment, the SOCs of the upper arm submodules of Phase A were 54.0%, 52.0%, 50.0%, and 48.0%, respectively. The SOCs of the lower arm submodules of Phase A were 42.0%, 40.0%, 36.0%, and 44.0%, respectively. The SOCs of the upper arm submodules of Phase B were balanced at 46%, and the SOCs of the lower arm submodules of Phase B were also balanced at 40%. The SOCs of the submodules of Phase C were balanced at 48%. At time t = 1s, the energy storage unit step-changes from emitting 0.6 pu to outputting 0.8 pu. The SOC simulation waveforms are shown in Figures 5-9.

[0073] Figure 5 shows the SOC waveforms of each submodule in the upper bridge arm of phase A; Figure 6 shows the SOC waveforms of each submodule in the lower bridge arm of phase A; Figure 7 shows the SOC waveforms of the upper and lower bridge arms of phase A; Figure 8 shows the SOC waveforms of the upper and lower bridge arms of phase B; and Figure 9 shows the SOC waveforms of phases A, B, and C. The simulation results show that the SOCs between phases, between the bridge arms of phases A and B, and between the submodules of phase A have all reached equilibrium. The SOC balancing method for energy storage MMCs based on capacitor voltage correction is not only simple in structure, requires fewer controllers, and is easy to implement, but also has a fast balancing speed, further improving the capacity utilization of the energy storage unit and enhancing the stability of the energy storage MMC.

[0074] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for equalizing the state of charge of an energy storage type MMC based on capacitor voltage correction, characterized in that: The energy storage type MMC charge state equalization method comprises the following steps: S1. Obtain the current direction of the bridge arm and the charge state of the energy storage unit in the energy storage type MMC, obtain the reference value of the phase unit modulation wave and the set value of the submodule energy storage power, obtain the actual values ​​of the submodule capacitor voltage, submodule energy storage power, and submodule energy storage current, and obtain the submodule capacitor voltage and the submodule energy storage unit voltage rated value; S2, calculating the average SOC of the phase unit, bridge arm and three-phase energy storage unit; S3, calculate the SOC imbalance between phases, bridge arms, and sub-modules, obtain the power deviation between phases and bridge arms and the voltage deviation between sub-modules through the proportional controller, calculate the sub-module energy storage power reference value and the sub-module correction capacitor voltage; S4. Calculate the duty cycle of the switching device in the bidirectional DC / DC converter. The bidirectional DC / DC converter adopts complementary PWM control, and the duty cycle D of the switching device is calculated as follows: Among them, K PC and K IC are the proportional parameter and integral parameter of the energy storage current controller, s is the Laplace operator, is the reference value of the average value of the bridge arm energy storage current, I L_rj is the average value of the bridge arm energy storage current, D0 is the duty cycle feedforward term, and the expression is as follows: Wherein, j=a, b, c represent three phase units, r=p, n represent the upper and lower bridge arms of each phase unit, i=1, 2, ..., N represents the i-th submodule in the bridge arm, i.e., I L_rji is the actual value of the energy storage current of the i-th submodule in the j-phase r-bridge arm, is the rated voltage of the submodule energy storage unit, is the submodule capacitor voltage rating; The reference value of the average value of the bridge arm energy storage current The calculation formula is as follows: Among them, K PP and K IP are the proportional parameter and integral parameter of the energy storage power controller respectively, s is the Laplace operator, is the reference value of the energy storage power of the submodule, P sc_rj is the average energy storage power of the bridge arm neutron module, and the expression is as follows: Among them, P sc_rji is the actual value of energy storage power of the i-th submodule in the j-phase r-bridge arm; S5. Sort the submodule correction capacitor voltages, and determine the submodule to be put into use according to the submodule capacitor charge and discharge status.

2. The energy storage type MMC charge state equalization method based on capacitor voltage correction according to claim 1 is characterized in that: In step S2, the bridge arm SOC average value SOC rj , phase unit SOC average SOC j And the average SOC of the three-phase energy storage unit SOC ave The calculation formula is as follows: Where N is the number of submodules in each bridge arm, j = a, b, c represents three phase units, r = p, n represents the upper and lower bridge arms of each phase unit, i = 1, 2, ..., N represents the i-th submodule in the bridge arm, i.e. SOC rji is the SOC of the i-th submodule in the j-phase r-bridge arm, SOC a is the average SOC value of phase unit a, SOC b is the average SOC of phase unit b, SOC c is the average SOC value of phase unit c.

3. The energy storage type MMC charge state equalization method based on capacitor voltage correction according to claim 1 is characterized in that: In step S3, the inter-phase SOC imbalance ΔSOC j , SOC imbalance between bridge arms ΔSOC rj , SOC imbalance between submodules ΔSOC rji The calculation formula is as follows: Among them, SOC ave is the average SOC value of the three-phase energy storage unit, j = a, b, c represents the three phase units, r = p, n represents the upper and lower bridge arms of each phase unit, i = 1, 2, ..., N represents the i-th submodule in the bridge arm, that is, SOC j is the average SOC value of phase j, SOC rj is the average SOC value of the bridge arm of phase j, SOC rji is the SOC of the i-th submodule in the j-phase r-bridge arm; Calculate the phase unit power deviation ΔP sc_j , bridge arm power deviation ΔP sc_rj and submodule voltage deviation ΔU sc_rji , the calculation formula is as follows: Among them, k ph , k arm , k sm They are the proportional coefficients of the inter-phase SOC balancing controller, the inter-bridge SOC balancing controller, and the inter-submodule SOC balancing controller, ΔSOC j , ΔSOC rj , ΔSOC rji They are inter-phase SOC imbalance, inter-bridge arm SOC imbalance, and inter-submodule SOC imbalance; Add the phase unit power deviation and bridge arm power deviation to the submodule energy storage power setting value to obtain the submodule energy storage power reference value The calculation formula is as follows: in, is the submodule energy storage power setting value, ΔP sc_j is the phase unit power deviation, ΔP sc_rj is the bridge arm power deviation; The voltage deviation between submodules is added to the submodule capacitor voltage to obtain the submodule corrected capacitor voltage. The calculation formula is as follows: Among them, U sm_rji is the actual value of the capacitor voltage of the i-th submodule in the j-phase r-bridge arm, ΔU sc_rji The corresponding submodule voltage bias Difference.

4. The energy storage type MMC charge state equalization method based on capacitor voltage correction according to claim 1 is characterized in that: In step S5, the submodule correction capacitor voltages are sorted from low to high, and the charge and discharge state of the submodule capacitor is determined according to the direction of the bridge arm current. When the submodule capacitor is in the charging state, the upper and lower bridge arms are respectively put into N pon 、N non The submodule with the lowest voltage; when the submodule capacitor is in the discharge state, the upper and lower bridge arms are respectively put into N pon 、N non The submodule with the highest voltage.

5. The energy storage type MMC charge state equalization method based on capacitor voltage correction according to claim 4 is characterized in that: When the nearest level approximation modulation is adopted, the number of submodules N required for the upper and lower bridge arms in each phase unit is pon 、N non The calculation formula is as follows: Where N is the number of submodules in each bridge arm, u vj_ref is the reference value of the j-phase modulation wave, j = a, b, c represents three phase units, U c_sm is the rated value of the submodule capacitor voltage, and round(x) means taking the integer closest to x.

Images