Frequency response power allocation and energy balancing method of MMC hybrid energy storage topology

Abstract

The present invention discloses a frequency-responsive power distribution and energy balancing method for an MMC hybrid energy storage topology, comprising: performing converter modeling on the MMC hybrid energy storage system as a whole; calculating a power reference value for a supercapacitor to achieve power distribution between a battery and a supercapacitor; determining a reference value for the compensated DC circulating current of each phase to achieve energy balance between each phase of the MMC hybrid energy storage system; calculating a compensation amplitude for the AC circulating current of each phase to achieve energy balance between the upper and lower bridge arms of the same phase of the MMC hybrid energy storage system; and achieving a minimum level modulation method based on state of charge sorting to achieve voltage output of the bridge arm of the MMC hybrid energy storage system and achieve state of charge balance of submodules within the same bridge arm of the MMC hybrid energy storage system. This method can maintain energy balance between energy storage units in the MMC hybrid energy storage under various operating conditions, and can respond to external frequency changes in power, so that the high-frequency component of the power is borne by the supercapacitor and the low-frequency component by the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a frequency response power distribution and energy balancing method of an MMC hybrid energy storage topology. Background Art

[0002] With the integration of large-scale distributed renewable energy into power systems and the application of a large number of related power electronic devices, system inertia has decreased, and frequency stability issues have become more prominent. Energy storage can provide power response and inertia support for power systems and is seen as a solution to improve system frequency stability. Compared to single energy storage, hybrid energy storage can combine the advantages of different energy storage systems, has better economic efficiency, and is seen as a more promising solution. The more common hybrid energy storage used for grid power response consists of batteries and supercapacitors. Batteries are used for low-frequency, long-term power response, and supercapacitors are used for high-frequency, short-term power response.

[0003] Currently, hybrid energy storage topologies primarily include centralized configurations based on traditional two-level converters and distributed configurations based on multi-level (three-level and above) converters. Research on distributed configurations based on multi-level converters is still in its infancy, with a significant focus on hybrid energy storage systems based on MMCs (Multilevel Modular Converters). Energy storage can be configured within the MMC's submodules or on the MMC's DC side. Existing solutions include centralized placement of batteries on the MMC's DC side, with supercapacitors within the submodules; placement of batteries in the upper arm and supercapacitors in the lower arm; placement of both batteries and supercapacitors within the submodules; and placement of batteries within the submodules and supercapacitors on the DC side.

[0004] For distributed hybrid energy storage systems, the energy balancing strategies for the internal energy storage medium include interphase balancing and intersubmodule balancing. For the cascaded H-bridge topology, interphase energy balancing is achieved by zero-sequence voltage injection, and intersubmodule energy balancing is achieved by changing the modulation wave. For the MMC topology, energy balancing between bridge arms is achieved by controlling the AC and DC circulating currents, and energy balancing between submodules in the same bridge arm is also achieved by changing the modulation wave. Currently, there are many studies on energy balancing strategies for energy storage units in different distributed energy storage topologies, but the specific control strategies vary with the topology and require individual design. Among the existing distributed energy storage energy balancing strategies, control strategies for various MMC hybrid energy storage topologies have been studied. However, energy balancing strategies for MMC hybrid energy storage systems in which batteries are configured within submodules and supercapacitors are configured on the DC side are still lacking and need further research.

[0005] The power allocation strategy of hybrid energy storage is generally implemented based on frequency band decomposition, where low-frequency power is allocated to batteries and high-frequency power is allocated to supercapacitors. The power command can be decomposed into frequency bands using filters to achieve power distribution. For hybrid energy storage systems controlled by virtual synchronous machines, there are existing methods that use frequency differentials to obtain the inertia response part of the power as high-frequency power, and use frequency deviation values ​​to obtain the damping response part of the power as low-frequency power. In existing hybrid energy storage power allocation strategies, the filter-based frequency band decomposition method requires a certain power command. However, if a virtual synchronous machine is used as the control method for the hybrid energy storage system, its power loop cannot directly obtain the power command value. Using frequency differentials and frequency deviations to perform frequency band decomposition can achieve hybrid energy storage power distribution based on virtual synchronous machine control. However, the differential link is not easy to implement in actual controllers, which makes the controller design difficult.

[0006] In summary, it is necessary to propose a new hybrid energy storage topology and its corresponding control strategy to solve the problem of energy balance of battery energy storage units in the MMC hybrid energy storage topology where batteries are configured in submodules and supercapacitors are configured on the DC side, as well as the problem of power distribution based on virtual synchronous machine control. Summary of the Invention

[0007] In order to overcome the shortcomings of the above technologies, the present invention provides a frequency-responsive power distribution and energy balancing method for an MMC hybrid energy storage topology. This method can be used for internal energy balancing and power distribution of MMC hybrid energy storage, maintaining energy balance between energy storage units in various operating states of MMC hybrid energy storage, and being able to respond to external frequency changes in power, wherein the high-frequency component of the power is borne by the supercapacitor and the low-frequency component is borne by the battery.

[0008] Explanation of terms:

[0009] 1. MMC: Multilevel Modular Converter.

[0010] 2. SOC: State of charge.

[0011] The technical solution adopted by the present invention to overcome the technical problems is:

[0012] A frequency-responsive power distribution and energy balancing method for an MMC hybrid energy storage topology is applicable to an MMC hybrid energy storage system in which batteries are dispersedly configured in submodules and supercapacitors are configured on the DC side. The method comprises the following steps:

[0013] S1. Conduct converter overall modeling for the MMC hybrid energy storage system;

[0014] S2. Use a virtual synchronous machine control method including a voltage and current loop to control the overall external power of the MMC hybrid energy storage system. Calculate the power reference value of the supercapacitor based on the actual power feedforward value output by the MMC hybrid energy storage system, the dispatch instruction power value received by the MMC hybrid energy storage system, and the damping power in the virtual synchronous machine power loop to achieve power distribution between the battery and the supercapacitor.

[0015] S3. Calculate the total DC circulating current based on the power reference value of the supercapacitor, determine the compensation DC circulating current reference value of each phase according to the average energy storage state of charge of all phase batteries of the MMC hybrid energy storage system and the average energy storage state of charge of each phase battery of the three phases, and use a PI controller to implement DC circulating current control to achieve energy balance among the phases of the MMC hybrid energy storage system;

[0016] S4. Calculate the AC circulating current compensation amplitude of each phase based on the average value of the battery state of charge of the upper bridge arm and the lower bridge arm in the same phase of the MMC hybrid energy storage system, and use a PR controller to implement AC circulating current control to achieve energy balance between the upper and lower bridge arms in the same phase of the MMC hybrid energy storage system;

[0017] S5. A minimum level modulation method based on state of charge sorting is used to realize the voltage output of the bridge arm of the MMC hybrid energy storage system and to achieve charge state balancing of the submodules in the same bridge arm of the MMC hybrid energy storage system.

[0018] Furthermore, the MMC hybrid energy storage system in which the batteries are dispersedly configured in the submodules and the supercapacitors are configured on the DC side includes three phases, each phase includes a symmetrical upper bridge arm and a lower bridge arm, and the upper bridge arm or the lower bridge arm of each phase includes several half-bridge modules and several full-bridge modules, and the number of half-bridge modules is equal to that of full-bridge modules.

[0019] Furthermore, each half-bridge module includes a battery, a capacitor and two MOS transistors, and the two MOS transistors are connected in series and then in parallel with the battery and the capacitor.

[0020] Furthermore, each full-bridge module includes a battery, a capacitor and four MOS tubes. After every two MOS tubes are connected in series, they are connected in parallel with the other two MOS tubes connected in series and in parallel with the battery and the capacitor.

[0021] Furthermore, in step S1, the overall converter modeling of the MMC hybrid energy storage system specifically includes the following steps:

[0022] The power of each bridge arm is:

[0023]

[0024]

[0025] In the above formula, P px and Pnx They are the upper arm and lower arm power of each phase of MMC, x represents one of the three phases a, b, and c, U dc is the DC side voltage of MMC, e x is the AC side voltage of MMC, I ox is the AC current output by each phase of the MMC, I cirx and U unbx are the circulating current and unbalanced voltage in each phase respectively;

[0026] Let u xp and u xn are the upper arm voltage and lower arm voltage of each phase of MMC, I xp and I xn are the upper arm current and lower arm current of each phase of MMC, U xp and U xn is the upper arm voltage and lower arm voltage of each phase of the MMC; the relationship between the various physical quantities is:

[0027]

[0028] The sum of the power of the upper and lower bridge arms of a certain phase P x for:

[0029] P x =(U dc -2U unbx )I cirx +e x I ox (4)

[0030] In the above formula, e x I ox The output power of one of the three phases on the AC side is equal in the case of three-phase balance; the unbalanced voltage U unbx Compared to the DC voltage U dc can be ignored, then the difference in power between each phase is mainly caused by U dc I cirx A decision; due to U dc It is the DC component, which is mainly determined by the DC component of the three-phase circulating current;

[0031] The difference between the upper and lower bridge arm powers in a phase is:

[0032] ΔP npx =(4e x I cirx +U dc I ox -2U unbx I ox ) / 2 (5)

[0033] In the above formula, Udc I ox It is the AC quantity multiplied by the DC quantity, and its value is zero in one cycle; unbalanced voltage U unbx Compared to the AC voltage e x can be ignored, then the power difference between the upper and lower bridge arms of each phase is mainly composed of e x I cirx decision; due to e x is an AC quantity, so the power difference between the upper and lower bridge arms of each phase is given by I cirx It is determined by the fundamental component of

[0034] DC side power P dc for:

[0035]

[0036] The DC side power is determined by the sum of the DC components of the three-phase circulating current.

[0037] Furthermore, step S2 specifically includes:

[0038] The power of the virtual synchronous machine is divided into three parts as shown in formula (7):

[0039]

[0040] In the above formula, P ref is the dispatching command power value received by the MMC hybrid energy storage system, ΔP D is the damping power in the virtual synchronous machine power loop, ΔP H is the inertia response power, w ref is the frequency reference value, w is the virtual synchronous and actual frequency, J and D are the inertia coefficient and damping coefficient of the virtual synchronous machine, respectively. In the control method based on the virtual synchronous machine, its output power is determined only by the voltage and current dual loops, and the internal power distribution method of the hybrid energy storage does not affect its external output characteristics.

[0041] The power control of the hybrid energy storage system based on virtual synchronous machine control and the power distribution between the energy storage media are as follows. The swing equation of the virtual synchronous machine is:

[0042]

[0043] According to formula (8), the power loop of the virtual synchronous machine is designed, taking into account the dispatch instruction power value P received by the MMC hybrid energy storage system. ref and the damping power ΔP in the virtual synchronous machine power loop D Relative to inertia response power ΔP H is a low-frequency signal, the battery reference power P batref The dispatching command power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loopD Composition, namely:

[0044] P batref =P ref +D(w ref -w) (9)

[0045] Since the voltage and current dual loops respond quickly, even in the transient process, it is considered that the external output power is consistent with the sum of the designed inertia response power, damping response power, and the reference power value of the dispatching side. Therefore, it is considered that the actual power feedforward value output by the MMC hybrid energy storage system minus the dispatching instruction power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loop D The value is the inertia response power of the hybrid energy storage system; the power reference value P of the supercapacitor SCref This is the inertia response power of the hybrid energy storage system, which is calculated as follows:

[0046] P SCref =P VSM -P batref (10)

[0047] In the above formula, the total output power P VSM The feedforward value minus the battery reference power P batref The value is the power reference value of the supercapacitor.

[0048] Furthermore, step S3 specifically includes:

[0049] The total DC circulating current is calculated based on the power reference value of the supercapacitor. The specific formula is:

[0050] I cirdc =P SCref / 3U dc (11)

[0051] Reference value of each phase compensation DC circulating current ΔP phasex The calculation formula is as follows;

[0052] ΔP phasex =k phase (SOC ave -SOC avex ) (12)

[0053] In the above formula, k phase is the phase compensation coefficient, SOC ave and SOC avex are the average energy storage state of charge of all phase batteries and the average energy storage state of charge of batteries in a certain phase respectively; the total amount of compensation DC circulating current of each phase is:

[0054]

[0055] From formula (13), it can be seen that the compensation process of the interphase DC circulating current does not affect the power distribution between the supercapacitor and the battery unit. It only generates a power difference between the phases to achieve energy balance. The reference value of the compensated DC circulating current of each phase is I cirdcxref Calculated as:

[0056] I cirdcxref =I cirdc +ΔP phasex / U dc (14)

[0057] After the reference value of each phase compensation DC circulating current is calculated, the unbalanced voltage DC component U of each phase is generated through PI control. unbdcx .

[0058] Furthermore, step S4 specifically includes:

[0059] Reference value of AC circulating current for compensation between bridge arms:

[0060]

[0061] In the above formula, k arm is the compensation coefficient of the upper and lower bridge arms, SOC xuave and SOC xpave are the average state of charge of the battery in the upper bridge arm and the lower bridge arm in a certain phase, θ VSG is the phase of the virtual synchronous machine;

[0062] Since the AC circulating current reference values ​​of the three-phase compensation are not necessarily the same, the AC circulating current will produce a zero-sequence component. The zero-sequence component of the AC circulating current will cause the power fluctuation on the DC side of the MMC. Therefore, the zero-sequence component needs to be eliminated. The zero-sequence component that needs to be eliminated is:

[0063]

[0064] After eliminating the zero-sequence component, the compensation amplitude of each phase AC circulating current generates the unbalanced voltage AC component U of each phase through the PR controller. unbacx .

[0065] Furthermore, step S5 specifically includes:

[0066] First, sort the submodules according to the state of charge;

[0067] Then determine the charging and discharging status of the bridge arm: when the bridge arm voltage and current are in the same direction, the submodule discharges; when the bridge arm voltage and current are in opposite directions, the submodule charges;

[0068] Finally, the charge states of the submodules within a single bridge arm are balanced: when the bridge arm is charging, the submodules with lower charge states are prioritized; when the bridge arm is discharging, the submodules with higher charge states are prioritized.

[0069] The beneficial effects of the present invention are:

[0070] 1. The present invention proposes an energy balancing and frequency-responsive power distribution method for an MMC hybrid energy storage topology in which batteries are configured in submodules and supercapacitors are configured on the DC side, which can ensure energy balance among the battery cells in this topology during operation.

[0071] 2. The present invention proposes a hybrid energy storage power distribution method based on power loop feedforward. Under the background of virtual synchronous machine control, it can achieve frequency band decomposition of the total power without using a phase-locked loop and a differential link, ensuring that low-frequency power is distributed to the battery and high-frequency power is distributed to the supercapacitor control target, simplifying the control link and reducing the difficulty of implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is a schematic diagram of the structure of the MMC hybrid energy storage topology targeted by an embodiment of the present invention.

[0073] Figure 2 Schematic diagram of the power distribution principle of a hybrid energy storage system based on power loop output feedforward according to an embodiment of the present invention.

[0074] Figure 3 This is a diagram showing the power distribution effect achieved by an embodiment of the present invention.

[0075] Figure 4 Schematic diagram of the DC unbalanced voltage generation principle according to an embodiment of the present invention.

[0076] Figure 5 Schematic diagram of the AC unbalanced voltage generation principle according to an embodiment of the present invention.

[0077] Figure 6 This is a diagram showing the charge state balancing effect between the three phases and between the upper and lower arms of the same phase in an embodiment of the present invention.

[0078] Figure 7 This is a diagram showing the charge state balancing effect of each submodule of the upper bridge arm of phase a in an embodiment of the present invention. DETAILED DESCRIPTION

[0079] In order to facilitate those skilled in the art to better understand the present invention, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. The following is only exemplary and does not limit the scope of protection of the present invention.

[0080] The present invention discloses a frequency response power distribution and energy balancing method of an MMC hybrid energy storage topology, which includes the following five steps.

[0081] Step S1: Conduct converter overall modeling for the MMC hybrid energy storage system.

[0082] like Figure 1 As shown, in the MMC hybrid energy storage system described in this embodiment, batteries are dispersed within submodules, and supercapacitors are configured on the DC side of the MMC. The MMC hybrid energy storage system includes three phases, each phase comprising a symmetrical upper and lower bridge arms. Each upper or lower bridge arm of each phase includes several half-bridge modules and several full-bridge modules, with the number of half-bridge modules equal to the number of full-bridge modules. Specifically, each half-bridge module includes a battery, a capacitor, and two MOS transistors, which are connected in series and in parallel with the battery and capacitor. Each full-bridge module includes a battery, a capacitor, and four MOS transistors, with each two MOS transistors connected in series and in parallel with the other two MOS transistors connected in series, and in parallel with the battery and capacitor. In this MMC hybrid energy storage topology, the number of batteries connected in series and parallel in a single submodule is relatively small, making the battery energy management system relatively simple. Furthermore, the batteries can be connected directly to the submodules without passing through a converter, avoiding the need for an additional DC-DC converter. This hybrid energy storage topology has a symmetrical structure with upper and lower bridge arms, which avoids the complex control associated with an asymmetric structure. The access of hybrid sub-modules can also cope with DC side faults, suppress DC fault current, and enhance resistance to DC side faults.

[0083] based on Figure 1 For the structure shown, the power of each bridge arm is:

[0084]

[0085]

[0086] In the above formula, P px and P nx They are the upper arm and lower arm power of each phase of MMC, x represents one of the three phases a, b, and c, U dc is the DC side voltage of MMC, e x is the AC side voltage of MMC, I ox is the AC current output by each phase of the MMC, I cirx and U unbx are the circulating current and unbalanced voltage in each phase respectively;

[0087] Let u xp and u xn are the upper arm voltage and lower arm voltage of each phase of MMC, I xp and I xn are the upper arm current and lower arm current of each phase of MMC, U xp and U xn is the upper arm voltage and lower arm voltage of each phase of the MMC; the relationship between the various physical quantities is:

[0088]

[0089] The sum of the power of the upper and lower bridge arms of a certain phase P x for:

[0090] P x =(U dc -2U unbx )I cirx +e x I ox (4)

[0091] In the above formula, e x I ox The output power of one of the three phases on the AC side is equal in the case of three-phase balance; the unbalanced voltage U unbx Compared to the DC voltage U dc can be ignored, then the difference in power between each phase is mainly caused by U dc I cirx A decision; due to U dc It is the DC component, which is mainly determined by the DC component of the three-phase circulating current;

[0092] The difference between the upper and lower bridge arm powers in a phase is:

[0093] ΔP npx =(4e x I cirx +U dc I ox -2U unbx I ox ) / 2 (5)

[0094] In the above formula, U dc I ox It is the AC quantity multiplied by the DC quantity, and its value is zero in one cycle; unbalanced voltage U unbx Compared to the AC voltage e x can be ignored, then the power difference between the upper and lower bridge arms of each phase is mainly composed of e x I cirx decision; due to e x is an AC quantity, so the power difference between the upper and lower bridge arms of each phase is given by I cirx It is determined by the fundamental component of

[0095] DC side power P dc for:

[0096]

[0097] The DC side power is determined by the sum of the DC components of the three-phase circulating current.

[0098] Step S2: Use a virtual synchronous machine control method including a voltage and current loop to control the overall external power of the MMC hybrid energy storage system. Calculate the power reference value of the supercapacitor based on the actual power feedforward value output by the MMC hybrid energy storage system, the dispatch instruction power value received by the MMC hybrid energy storage system, and the damping power in the virtual synchronous machine power loop to achieve power distribution between the battery and the supercapacitor.

[0099] The power of the virtual synchronous machine is divided into three parts as shown in formula (7):

[0100]

[0101] In the above formula, P ref is the dispatching command power value received by the MMC hybrid energy storage system, ΔP D is the damping power in the virtual synchronous machine power loop, ΔP H is the inertia response power, w ref is the frequency reference value, w is the virtual synchronous and actual frequency, J and D are the inertia coefficient and damping coefficient of the virtual synchronous machine, respectively. In the control method based on the virtual synchronous machine, its output power is determined only by the voltage and current dual loops, and the internal power distribution method of the hybrid energy storage does not affect its external output characteristics.

[0102] The power control of the hybrid energy storage system based on virtual synchronous machine control and the power distribution between the energy storage media are as follows. The swing equation of the virtual synchronous machine is:

[0103]

[0104] According to formula (8), the power loop of the virtual synchronous machine is designed as follows: Figure 2 As shown, θ VSG is the virtual synchronous machine phase. Considering the dispatching instruction power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loop D Relative to inertia response power ΔP H is a low-frequency signal, the battery reference power P batref The dispatching command power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loop D Composition, namely:

[0105] P batref =P ref +D(w ref -w) (9)

[0106] Since the voltage and current dual loops respond quickly, even in the transient process, it is considered that the external output power is consistent with the sum of the designed inertia response power, damping response power, and the reference power value of the dispatching side. Therefore, it is considered that the actual power feedforward value output by the MMC hybrid energy storage system minus the dispatching instruction power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loop D The value is the inertia response power of the hybrid energy storage system; the power reference value P of the supercapacitor SCref This is the inertia response power of the hybrid energy storage system, which is calculated as follows:

[0107] P SCref =P VSM -P batref (10)

[0108] In the above formula, the total output power P VSM The feedforward value minus the battery reference power P batref The value is the power reference value of the supercapacitor.

[0109] Through step S2, the calculation of the power reference value of the battery and the power reference value of the supercapacitor is completed, that is, the distribution of the power reference value has been realized. In the following steps, the actual power of the battery and supercapacitor is controlled to track the reference value to realize the power distribution between the two energy storage units.

[0110] The power distribution effect achieved according to step S2 is as follows: Figure 3 As shown, Figure 3 In the example, at t=2s, a sudden change in grid-side load power occurs, and the system frequency decreases. Figure 3 In, P grid is the total output power of the hybrid energy storage system, P SC is the supercapacitor output power, P bat is the battery output power. Figure 3 It can be seen that by adopting the power distribution method proposed in the method of the present invention, when the load power suddenly changes, the output power of the supercapacitor increases rapidly in a short period of time, realizing the rapid power response of the hybrid energy storage system to external frequency changes, and the output power of the battery remains unchanged. Then, as the system frequency gradually decreases, the battery power slowly increases, and accordingly, the supercapacitor power gradually decreases, and eventually all power is borne by the battery, and the supercapacitor does not release power to the outside in a steady state. Under the premise of avoiding the use of steps such as phase-locked loops and differential links, this control method achieves the control goal of the supercapacitor assuming high-frequency power and the battery assuming low-frequency power during the frequency response process.

[0111] Step S3: Calculate the total DC circulating current based on the power reference value of the supercapacitor, determine the compensation DC circulating current reference value of each phase according to the average energy storage state of charge of all phase batteries of the MMC hybrid energy storage system and the average energy storage state of charge of each phase battery of the three phases, and use a PI controller to implement DC circulating current control to achieve energy balance among the phases of the MMC hybrid energy storage system.

[0112] The total DC circulating current is calculated based on the power reference value of the supercapacitor. The specific formula is:

[0113] I cirdc =P SCref / 3U dc (11)

[0114] Reference value of each phase compensation DC circulating current ΔP phasex The calculation formula is as follows;

[0115] ΔP phasex =k phase (SOC ave -SOC avex ) (12)

[0116] In the above formula, k phase is the phase compensation coefficient, SOC ave and SOC avex are the average energy storage state of charge of all phase batteries and the average energy storage state of charge of batteries in a certain phase respectively; the total amount of compensation DC circulating current of each phase is:

[0117]

[0118] From formula (13), it can be seen that the compensation process of the interphase DC circulating current does not affect the power distribution between the supercapacitor and the battery unit. It only generates a power difference between the phases to achieve energy balance. The reference value of the compensated DC circulating current of each phase is I cirdcxref Calculated as:

[0119] I cirdcxref =I cirdc +ΔP phasex / U dc (14)

[0120] After the reference value of each phase compensation DC circulating current is calculated, the unbalanced voltage DC component U of each phase is generated through PI control. unbdcx The energy is balanced among the phases of the MMC hybrid energy storage system through the difference of the DC component of the three-phase unbalanced voltage.

[0121] Step S4: Calculate the AC circulating current compensation amplitude of each phase according to the average value of the energy storage charge state of the upper bridge arm and the lower bridge arm in the same phase of the MMC hybrid energy storage system, and use the PR controller to implement AC circulating current control to achieve energy balance between the upper and lower bridge arms in the same phase of the MMC hybrid energy storage system.

[0122] AC circulating current control is used to achieve the same-phase charge state balance of battery energy storage units, such as Figure 5 shown.

[0123] Reference value of AC circulating current for compensation between bridge arms:

[0124]

[0125] In the above formula, k arm is the compensation coefficient of the upper and lower bridge arms, SOC xuave and SOC xpave are the average state of charge of the battery in the upper bridge arm and the lower bridge arm in a certain phase, θ VSG is the phase of the virtual synchronous machine;

[0126] Since the AC circulating current reference values ​​of the three-phase compensation are not necessarily the same, the AC circulating current will produce a zero-sequence component. The zero-sequence component of the AC circulating current will cause the power fluctuation on the DC side of the MMC. Therefore, the zero-sequence component needs to be eliminated. The zero-sequence component that needs to be eliminated is:

[0127]

[0128] After eliminating the zero-sequence component, the compensation amplitude of each phase AC circulating current generates the unbalanced voltage AC component U of each phase through the PR controller. unbacx , and then generate the power difference between the upper and lower bridge arms of the same phase to achieve energy balance between the upper and lower bridge arms of the same phase in the MMC hybrid energy storage system.

[0129] Step S5: Based on the lowest level modulation method of the state of charge sorting, the voltage output of the bridge arm of the MMC hybrid energy storage system is realized, and the state of charge of the submodules in the same bridge arm of the MMC hybrid energy storage system is balanced.

[0130] Through steps S3 and S4, the charge state of each bridge arm is balanced. At this time, the charge state of each submodule within a single bridge arm also needs to be balanced. This embodiment uses the lowest level modulation method based on the charge state to modulate the MMC submodule, as follows.

[0131] First, sort the submodules according to the state of charge;

[0132] Then determine the charging and discharging status of the bridge arm: when the bridge arm voltage and current are in the same direction, the submodule discharges; when the bridge arm voltage and current are in opposite directions, the submodule charges;

[0133] Finally, the charge states of the submodules within a single bridge arm are balanced: when the bridge arm is charging, the submodules with lower charge states are prioritized; when the bridge arm is discharging, the submodules with higher charge states are prioritized.

[0134] The energy storage state of charge balancing strategy used in this embodiment has the following effects: Figure 6 and Figure 7 shown. Figure 6 In SOC xp Represents the average state of charge of a phase upper bridge arm (where SOC ap , SOC bp , SOC cp are the average state of charge of the upper bridge arm of phase a, phase b, and phase c respectively), SoC xn is the average state of charge of the bridge arm under a certain phase (where SOC an , SOC bn , SOC cn are the average state of charge of the lower bridge arm of phase a, the lower bridge arm of phase b, and the lower bridge arm of phase c respectively). In the initial state, the state of charge of each bridge arm is different, but after a sufficiently long time, the energy storage state of charge of each bridge arm tends to be consistent, achieving a state of charge balance between the three phases and between the upper and lower bridge arms of the same phase. Figure 7 In SOC n (n=) represents the state of charge of each submodule in the upper bridge arm of phase a. Similarly, after a certain period of time, the energy storage state of charge in each submodule tends to be consistent, achieving a balanced state of charge for each submodule within the bridge arm. In other words, the method described in this embodiment achieves balanced state of charge among the three phases, between the upper and lower bridge arms of the same phase, and within each submodule within the bridge arm.

[0135] The above only describes the basic principles and preferred embodiments of the present invention. Those skilled in the art may make many changes and improvements based on the above description, and these changes and improvements should fall within the scope of protection of the present invention.

Claims

1. A frequency response power distribution and energy balancing method for MMC hybrid energy storage topology, characterized in that: Applicable to MMC hybrid energy storage systems where batteries are distributed in submodules and supercapacitors are configured on the DC side, including the following steps: S1. Conduct converter overall modeling for the MMC hybrid energy storage system; S2. Use a virtual synchronous machine control method including a voltage and current loop to control the overall external power of the MMC hybrid energy storage system. Calculate the power reference value of the supercapacitor based on the actual power feedforward value output by the MMC hybrid energy storage system, the dispatch instruction power value received by the MMC hybrid energy storage system, and the damping power in the virtual synchronous machine power loop to achieve power distribution between the battery and the supercapacitor. S3. Calculate the total DC circulating current based on the power reference value of the supercapacitor, determine the compensation DC circulating current reference value of each phase according to the average energy storage state of charge of all phase batteries of the MMC hybrid energy storage system and the average energy storage state of charge of each phase battery of the three phases, and use a PI controller to implement DC circulating current control to achieve energy balance among the phases of the MMC hybrid energy storage system; S4. Calculate the AC circulating current compensation amplitude of each phase based on the average value of the battery state of charge of the upper bridge arm and the lower bridge arm in the same phase of the MMC hybrid energy storage system, and use a PR controller to implement AC circulating current control to achieve energy balance between the upper and lower bridge arms in the same phase of the MMC hybrid energy storage system; S5. A minimum level modulation method based on state of charge sorting is used to realize the voltage output of the bridge arm of the MMC hybrid energy storage system and to achieve charge state balancing of the submodules in the same bridge arm of the MMC hybrid energy storage system.

2. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 1 is characterized in that: The MMC hybrid energy storage system in which batteries are dispersedly configured in submodules and supercapacitors are configured on the DC side includes three phases, each phase includes a symmetrical upper bridge arm and a lower bridge arm, and the upper bridge arm or the lower bridge arm of each phase includes several half-bridge modules and several full-bridge modules, and the number of half-bridge modules is equal to that of full-bridge modules.

3. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 2 is characterized in that: Each half-bridge module includes a battery, a capacitor and two MOS tubes. The two MOS tubes are connected in series and then in parallel with the battery and the capacitor.

4. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 2 is characterized in that: Each full-bridge module includes a battery, a capacitor and four MOS tubes. After every two MOS tubes are connected in series, they are connected in parallel with the other two MOS tubes connected in series and in parallel with the battery and the capacitor.

5. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 2 is characterized in that: In step S1, the overall converter modeling of the MMC hybrid energy storage system specifically includes the following steps: The power of each bridge arm is: In the above formula, P px and P nx They are the upper arm and lower arm power of each phase of MMC, x represents one of the three phases a, b, and c, U dc is the DC side voltage of MMC, e x is the AC side voltage of MMC, I ox is the AC current output by each phase of the MMC, I cirx and U unbx are the circulating current and unbalanced voltage in each phase respectively; Let u xp and u xn are the upper arm voltage and lower arm voltage of each phase of MMC, I xp and I xn are the upper arm current and lower arm current of each phase of MMC, U xp and U xn is the upper arm voltage and lower arm voltage of each phase of the MMC; the relationship between the physical quantities is: The sum of the power of the upper and lower bridge arms of a certain phase P x for: P x =(U dc -2U unbx )I cirx +e x I ox (4) In the above formula, e x I ox The output power of one of the three phases on the AC side is equal in the case of three-phase balance; the unbalanced voltage U unbx Compared to the DC voltage U dc can be ignored, then the difference in power between each phase is mainly caused by U dc I cirx A decision; due to U dc It is the DC component, which is mainly determined by the DC component of the three-phase circulating current; The difference between the upper and lower bridge arm powers in a phase is: ΔP npx =(4e x AND cirx +U dc AND ox -2U unbx AND ox ) / 2 (5) In the above formula, U dc I ox It is the AC quantity multiplied by the DC quantity, and its value is zero in one cycle; the unbalanced voltage U unbx Compared to the AC voltage e x can be ignored, then the power difference between the upper and lower bridge arms of each phase is mainly composed of e x I cirx decision; due to e x is an AC quantity, so the power difference between the upper and lower bridge arms of each phase is given by I cirx It is determined by the fundamental component of DC side power P dc for: The DC side power is determined by the sum of the DC components of the three-phase circulating current.

6. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 1 is characterized in that: Step S2 specifically includes: The power of the virtual synchronous machine is divided into three parts as shown in formula (7): In the above formula, P ref is the dispatching command power value received by the MMC hybrid energy storage system, ΔP D is the damping power in the virtual synchronous machine power loop, ΔP H is the inertia response power, w ref is the frequency reference value, w is the virtual synchronous and actual frequency, J and D are the inertia coefficient and damping coefficient of the virtual synchronous machine, respectively. In the control method based on the virtual synchronous machine, its output power is determined only by the voltage and current dual loops, and the internal power distribution method of the hybrid energy storage does not affect its external output characteristics. The power control of the hybrid energy storage system based on virtual synchronous machine control and the power distribution between the energy storage media are as follows. The swing equation of the virtual synchronous machine is: According to formula (8), the power loop of the virtual synchronous machine is designed, taking into account the dispatch instruction power value P received by the MMC hybrid energy storage system. ref and the damping power ΔP in the virtual synchronous machine power loop D Relative to inertia response power ΔP H is a low-frequency signal, the battery reference power P batref The dispatching command power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loop D Composition, namely: P batref =P ref +D(w ref -w) (9) Since the voltage and current dual loops respond quickly, even in the transient process, it is considered that the external output power is consistent with the sum of the designed inertia response power, damping response power, and the reference power value of the dispatching side. Therefore, it is considered that the actual power feedforward value output by the MMC hybrid energy storage system minus the dispatching instruction power value P received by the MMC hybrid energy storage system ref and the damping power ΔP in the virtual synchronous machine power loop D The value is the inertia response power of the hybrid energy storage system; the power reference value P of the supercapacitor SCref This is the inertia response power of the hybrid energy storage system, which is calculated as follows: P SCref =P VSM -P batref (10) In the above formula, the total output power P VSM The feedforward value minus the battery reference power P batref The value is the power reference value of the supercapacitor.

7. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 6 is characterized in that: Step S3 specifically includes: The total DC circulating current is calculated based on the power reference value of the supercapacitor, and the specific formula is: I cirdc =P SCref / 3U dc (11) Reference value of each phase compensation DC circulating current ΔP phasex The calculation formula is as follows; ΔP phasex =k phase (SOC ave -SOC avex ) (12) In the above formula, k phase is the phase compensation coefficient, SOC ave and SOC avex are the average energy storage state of charge of all phase batteries and the average energy storage state of charge of batteries in a certain phase respectively; the total amount of compensation DC circulating current of each phase is: From formula (13), it can be seen that the compensation process of the interphase DC circulating current does not affect the power distribution between the supercapacitor and the battery unit. It only generates a power difference between the phases to achieve energy balance. The reference value of the compensated DC circulating current of each phase is I cirdcxref Calculated as: I cirdcxref =I cirdc +ΔP phasex / U dc (14) After the reference value of each phase compensation DC circulating current is calculated, the unbalanced voltage DC component U of each phase is generated through PI control. unbdcx .

8. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 1 is characterized in that: Step S4 specifically includes: Reference value of AC circulating current for compensation between bridge arms: In the above formula, k arm is the compensation coefficient of the upper and lower bridge arms, SOC xuave and SOC xpave are the average state of charge of the battery in the upper bridge arm and the lower bridge arm in a certain phase, θ VSG is the virtual synchronous machine phase; Since the AC circulating current reference values ​​of the three-phase compensation are not necessarily the same, the AC circulating current will produce a zero-sequence component. The zero-sequence component of the AC circulating current will cause the power fluctuation on the DC side of the MMC. Therefore, the zero-sequence component needs to be eliminated. The zero-sequence component that needs to be eliminated is: After eliminating the zero-sequence component, the compensation amplitude of each phase AC circulating current generates the unbalanced voltage AC component U of each phase through the PR controller. unbacx .

9. The frequency response power distribution and energy balancing method of the MMC hybrid energy storage topology according to claim 1 is characterized in that: Step S5 specifically includes: First, sort the submodules according to the state of charge; Then determine the charging and discharging status of the bridge arm: when the bridge arm voltage and current are in the same direction, the submodule discharges; when the bridge arm voltage and current are in opposite directions, the submodule charges; Finally, the charge states of the submodules within a single bridge arm are balanced: when the bridge arm is charging, the submodules with lower charge states are prioritized; when the bridge arm is discharging, the submodules with higher charge states are prioritized.

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