Off-grid MMC-BESS Energy Balancing Control Method for Three-Phase Load Unbalance

Abstract

An off-grid MMC-BESS energy balance control method for three-phase load imbalance, belonging to the field of electric energy operation management. It detects the negative-sequence current on the load side in real time, calculates the negative-sequence components of the three-phase load currents, calculates the positive-sequence components of the three-phase output terminal voltages, calculates the three-phase unbalanced power caused by the negative-sequence components in the three-phase load currents, sorts the absolute values of the three-phase unbalanced power according to their magnitudes, calculates the balance currents superimposed on the given values of the DC currents of each phase, and on the basis of the predetermined reference given values of the DC currents of each phase, superimposes the balance currents of the given values of the DC currents of each phase, thereby obtaining the adjusted new given values of the DC currents of each phase. It dynamically adjusts the distribution strategy of the three-phase DC balance currents according to the three-phase unbalanced power on the load side, reduces the influence on the battery charge and discharge power caused by the negative-sequence components, and reduces the influence on the battery charge and discharge power caused by the negative-sequence components by reasonably distributing the DC power of each phase.

Description

Technical Field

[0001] The present invention belongs to the field of electric energy storage systems, and particularly relates to a method for balancing control of three-phase unbalanced off-grid MMC-BESS energy. Background Art

[0002] The expansion of new energy power generation scale has promoted the research on battery energy storage systems (BESS).

[0003] At present, the research on cascaded topology battery energy storage systems has been relatively mature and there are already actual engineering applications; for example, a Chinese invention patent application with a publication date of January 28, 2022 and a publication number of CN 113991662 A discloses a "LCC-MMC based energy routing system and DC fault protection method", the topology of its system includes a grid phase-controlled converter LCC, a modular multilevel converter MMC, an energy storage device BESS, an improved hybrid DC circuit breaker DCCB and a DC bus; the energy regulation control method of the system includes fixed DC voltage control on the LCC side, AC side power decoupling control of the MMC and energy storage interface Buck / Boost control; the DC fault protection method of the system is to combine the energy storage unit power switching control with the improved hybrid DC circuit breaker DCCB to form a complete set of combined fault protection solutions. While quickly isolating the fault current, this technical solution can prevent the distortion of the AC current on the MMC side, greatly improve the power supply quality of the system, ensure the reliable and stable operation of the AC power grid, and is conducive to the rapid recovery of the system after a fault.

[0004] It can be seen that modular multilevel converter based battery energy storage systems (MMC-BESS) have received increasing attention due to their prominent modular characteristics. However, due to the absence of a DC bus in its topology, its application scenarios are limited to a certain extent.

[0005] In the actual operation process of MMC-BESS, the situation of three-phase load imbalance will inevitably occur. In a three-phase three-wire connection, it will cause the load current to contain negative sequence components and no zero sequence components. The negative sequence components will cause deviations in the originally balanced battery-side power, resulting in inconsistent charge and discharge powers of the batteries in each phase. Coupled with the original inconsistency and "short board effect" of the batteries, it may lead to overcharging, over-discharging, and over-current phenomena in the battery pack, which not only affects the life of the battery pack, but is more likely to cause safety problems such as fires. Therefore, the control of battery charge and discharge power is crucial.

[0006] At present, for the control of battery charge and discharge power, it is achieved by controlling the power on the DC side and AC side of the MMC-BESS, indirectly controlling the charge and discharge power of the battery. Therefore, most of the battery equalization of the MMC-BESS is also realized by controlling the AC and DC side power. The general method is to first detect relevant variables reflecting the battery state, such as the state of charge SOC (State of Charge, which refers to the ratio of the remaining capacity of the battery after using for a period of time to its fully charged capacity), and then perform equalization control according to these state variables. For example, the commonly used methods for the SOC equalization between phases are the DC current distribution method and the zero-sequence voltage injection method, which respectively use the DC power and AC power of the system to achieve SOC equalization; for the SOC equalization between arms, the fundamental frequency current injection method can be adopted. Utilizing the characteristic that the voltage phases of the upper and lower arms in the same phase are opposite, a fundamental frequency current with adjustable amplitude and the same or opposite phase as the AC output voltage of this phase is injected into this phase to achieve the SOC equalization between arms; the methods for the SOC equalization of sub-modules include the DC voltage distribution method, the AC voltage distribution method, etc. Similar to the SOC equalization between phases, DC deviation power or AC deviation power is generated according to the different SOCs of each sub-module to achieve SOC equalization.

[0007] However, the above battery equalization belongs to lag control. That is, after the battery has shown a certain degree of inconsistency, the charge and discharge power of the battery is controlled to make it balanced.

[0008] One of the reasons for the inconsistency of the battery is due to the unbalance of three-phase power. If the three-phase power balance control can be considered to intervene in advance, making the charge and discharge power of the battery tend to be consistent in advance, it can provide certain guarantee for the equalization of the battery. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide an off-grid MMC-BESS energy equalization control method for the case of three-phase load imbalance. When executing, determining or modifying the given value of the DC current of each phase, by detecting the negative sequence current on the load side in real time, calculating the three-phase unbalanced power generated thereby, dynamically determining the distribution strategy of the three-phase DC balance current, finally achieving the balance of three-phase power, and giving the calculation method of the three-phase DC balance current. By intervening in the equalization process of the battery SOC in advance during the three-phase power balance control, making the charge and discharge power of the batteries of each phase consistent, making the charge and discharge power of the battery tend to be consistent in advance, and further providing certain guarantee for the equalization of the battery performance, it can improve the working reliability of the system.

[0010] The technical solution of the present invention is: to provide an off-grid MMC-BESS energy equalization control method for the case of three-phase load imbalance, characterized in that the control method includes the following steps:

[0011] 1) Execute the predetermined DC current reference values for each phase;

[0012] 2) Detect the negative sequence current on the load side in real time;

[0013] 3) Calculate the negative sequence component of the load current in phase A;

[0014] 4) Based on the negative sequence component of the load current in phase A, shift the phase angles by 120° and -120° respectively to obtain the negative sequence components of the load currents in phases B and C;

[0015] 5) Using the voltage at the AC output terminal of phase A as the reference quantity, calculate the positive sequence, negative sequence, and zero sequence components of the voltage at the AC output terminal of phase A ;

[0016] 6) Based on the positive sequence component of the voltage at the output terminal of phase A, shift the phase angles by -120° and 120° respectively to obtain the positive sequence components of the output terminal voltages of phases B and C;

[0017] 7) Calculate the three-phase unbalanced power caused by the negative sequence components in the three-phase load currents;

[0018] 8) Sort the absolute values |P Δj | of the three-phase unbalanced powers of phases A, B, and C in order of magnitude, and calculate the balancing currents superimposed on the DC current set values of each phase;

[0019] 9) On the basis of the predetermined DC current reference values for each phase, superimpose the balancing currents of the DC current set values of each phase, and then obtain the new adjusted DC current set values for each phase;

[0020] 10) Execute the new adjusted DC current set values for each phase;

[0021] 11) Return to step 2), and continue to dynamically adjust the distribution strategy of the three-phase DC balancing currents according to the three-phase unbalanced power on the load side.

[0022] Specifically, in the described control method, by detecting the negative sequence current on the load side in real time, calculating the three-phase unbalanced power generated thereby, and determining the distribution strategy of the three-phase DC balancing currents.

[0023] Furthermore, in the described control method, during the three-phase power balance control, the equalization process of the battery SOC is intervened in advance, so that the charge and discharge powers of the batteries in each phase are kept consistent, the charge and discharge powers of the batteries tend to be consistent in advance, the influence on the charge and discharge powers of the batteries caused by the negative sequence components is reduced, and thus the balance of the battery performance is improved.

[0024] Specifically, assuming that the current phasor representations flowing through the three-phase loads are respectively and Taking the current of phase A as the reference phase, the load current of phase A can be decomposed into positive-sequence, negative-sequence, and zero-sequence currents according to the following formula:

[0025]

[0026] Wherein are respectively the positive-sequence, negative-sequence, and zero-sequence components of the load current of phase A.

[0027] Specifically, assuming that the voltages at the AC output terminals of the system are respectively and Taking the voltage at the AC output terminal of phase A as the reference quantity, the positive-sequence, negative-sequence, and zero-sequence components of can be calculated as follows:

[0028]

[0029] Furthermore, the three-phase power imbalance caused by the negative-sequence component in the three-phase load current is calculated according to the following formula:

[0030]

[0031] Wherein T in the formula is the power frequency period.

[0032] Furthermore, after sorting the absolute values of the unbalanced powers of phases A, B, and C, |P Δj | from large to small, the balancing current superimposed on the given value of the DC current of each phase can be calculated according to the following formula:

[0033]

[0034] Wherein Δi dcj_max —— the DC balancing current superimposed on the phase corresponding to max|P Δj |;

[0035] Δi dcj_min —— the DC balancing current superimposed on the phase corresponding to min|P Δj |;

[0036] Δi dcj_middle —— the DC balancing current superimposed on the remaining one phase except the above two phases.

[0037] Specifically, assuming that the original given value of the DC current of each phase is I dc_ref , the adjusted given value of the DC current of each phase can be obtained as:

[0038]

[0039] Compared with the prior art, the advantages of the present invention are:

[0040] 1. The technical solution of the present invention detects the negative-sequence current on the load side in real time, calculates the resulting three-phase unbalanced power, determines the distribution strategy of the three-phase DC balance current, finally achieves the balance of the three-phase power, and gives the calculation method of the three-phase DC balance current;

[0041] 2. The technical solution of the present invention intervenes in the battery SOC equalization process in advance, makes the battery charge and discharge power of each phase consistent, makes the battery charge and discharge power tend to be consistent in advance, and then provides a certain guarantee for the balance of battery performance, and can improve the working reliability of the system;

[0042] 3. The technical solution of the present invention reasonably distributes the DC power of each phase, reduces the influence on the battery charge and discharge power caused by the negative-sequence component, can play a role in protecting the battery, is beneficial to battery equalization, and prolongs the service life of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a schematic diagram of the topological structure of the off-grid MMC-BESS main circuit of the present invention;

[0044] Figure 2 is a block diagram of the energy equalization control method for each phase of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention will be further described below with reference to the drawings and embodiments.

[0046] Figure 1 As shown in, it is a schematic diagram of the topological structure of the MMC-BESS main circuit of the present invention.

[0047] The MMC-BESS main circuit therein includes three phase clusters, each phase cluster is divided into an upper arm and a lower arm, the arm contains a number of sub-modules, and the number of sub-modules contained in the upper and lower arms is the same.

[0048] In each phase cluster, the positive pole of the DC bus is connected to the positive output terminal of the first sub-module of the upper arm, and the negative output terminal is connected to the positive output terminal of the next sub-module, and so on; the negative output terminal of the last sub-module of the upper arm is connected to the arm inductor; the connection method of the lower arm is the same as that of the upper arm: after the arm inductor and each sub-module are connected in sequence, the negative output terminal of the last sub-module is connected to the negative pole of the DC bus.

[0049] The midpoints of the upper and lower arms are connected together to form the outlet of the AC grid voltage, and are led out through the AC inductor to form the AC side of the system, which is connected to the three-phase load, and the whole system is a three-phase three-wire system.

[0050] The system can be connected to the AC grid and the DC grid in parallel. The sub-module mainly consists of a switching device, a capacitor and a battery. Each sub-module has two terminals led out for cascading with other sub-modules.

[0051] Obviously, in Figure 1 the sub-module shown is a half-bridge structure, and the half-bridge circuit forms a parallel structure with the capacitor and the battery.

[0052] For Figure 1 the meanings of the various physical quantities appearing in

[0053] are explained in the following table.

[0054]

[0055]

[0056] It should be noted here that the reference point of the AC side output voltage u sj is the neutral point O of the AC side, and the reference point of the AC side grid-connected voltage u gj is the neutral point of the DC side, that is, the zero potential point O' of the system. It is stipulated that the reference direction of the DC bus current is from the positive end of the DC bus to the bridge arm; the reference direction of the upper bridge arm current is from the positive end of the DC bus to the bridge arm inductor; the reference direction of the lower bridge arm current is from the bridge arm inductor to the negative end of the DC bus; the reference direction of the phase current of the AC output is from the MMC AC outlet to the grid-connected inductor; the reference directions of the voltages in the system are all taken as the consistent reference direction.

[0057] On this basis, the technical solution of the present invention provides an off-grid MMC-BESS energy balance control method for the three-phase load unbalance situation, including the following steps:

[0058] (1) Detect the negative sequence current in real time. Let the current phasor representations flowing through the three-phase loads be and Taking the A-phase current as the reference phase, the A-phase load current can be decomposed into positive sequence, negative sequence and zero sequence currents according to the following formula:

[0059]

[0060] where are the positive sequence, negative sequence and zero sequence components of the A-phase load current respectively;

[0061] (2) Based on the negative sequence component of the A-phase load current, shift the phase angles of 120° and -120° respectively, and the negative sequence components of the B-phase and C-phase load currents can be obtained, denoted as and

[0062] (3) Let the voltages at the AC output terminals of the system be and Taking the voltage at the A-phase AC output terminal as the reference quantity, the positive-sequence, negative-sequence, and zero-sequence components can be calculated as follows:

[0063]

[0064] (4) Based on the positive-sequence component of the voltage at the A-phase output terminal, by shifting the phase angles by -120° and 120° respectively, the positive-sequence components of the voltages at the B-phase and C-phase output terminals can be obtained, denoted as and

[0065] (5) The three-phase power imbalance caused by the negative-sequence components in the three-phase load currents can be calculated as follows:

[0066]

[0067] where T is the power frequency period.

[0068] (6) Sort the absolute values of the unbalanced powers |P Δj | in descending order, and the balanced currents superimposed on the DC current set values of each phase can be calculated as follows:

[0069]

[0070] where Δi dcj_max —— the DC balanced current superimposed on the phase corresponding to max|P Δj |

[0071] Δi dcj_min —— the DC balanced current superimposed on the phase corresponding to min|P Δj |

[0072] Δi dcj_middle —— the DC balanced current superimposed on the remaining one phase other than the above two phases

[0073] (7) Let the original DC current set values of each phase be I dc_ref , and the adjusted DC current set values of each phase can be obtained as:

[0074]

[0075] Example:

[0076] The MMC-BESS shown in this example has a rated capacity of 50 kW, a DC side voltage of 750 V, includes 120 sub-modules, each arm includes 20 sub-modules, the arm inductance is 0.5 mH, and the AC side inductance is 0.5 mH.

[0077] In this embodiment, the resistance values of the three-phase load resistors are respectively:

[0078] R a = 2 Ω

[0079] R b = 5 Ω

[0080] R c = 8 Ω

[0081] In this embodiment, carrier phase-shifted modulation is adopted to realize the switching control of the sub-modules.

[0082] In this embodiment, the given value of the DC-side bus current is set to 90 A, and the given value of the DC current of each phase is 30 A; the amplitude of the AC-side current is 100 A.

[0083] In this embodiment, the amplitude of the AC terminal voltage output by the MMC-BESS is U m = 311 V. Taking the AC terminal voltage output from phase A as the reference quantity, the three-phase voltages at the AC output terminal of the MMC-BESS can be expressed as:

[0084] u a = U m sin(ωt)

[0085] u b = U m sin(ωt - 120°)

[0086] u c = U m sin(ωt + 120°)

[0087] Let the phase angle between the AC output current of phase A and the voltage of this phase be The expression of the AC current of each phase can be obtained:

[0088]

[0089]

[0090]

[0091] For the off-grid MMC-BESS energy balance control method for three-phase load imbalance, it includes the following steps:

[0092] (1) Detect the negative-sequence current in real time:

[0093] According to formula (1), calculate the negative-sequence component of the load current of phase A, and get:

[0094]

[0095] Where

[0096]

[0097]

[0098]

[0099] (2) Based on the negative-sequence component of the load current in phase A, by shifting the phase angles by 120° and -120° respectively, the negative-sequence components of the load currents in phases B and C can be obtained:

[0100]

[0101]

[0102] (3) Calculate the positive-sequence component of the voltage at the AC output end of the system:

[0103] In this embodiment, it can be considered that the three-phase AC output voltage only contains the positive-sequence component. Taking the voltage at the AC output end of phase A as the reference quantity, there is:

[0104] u a + = U m sin(ωt)

[0105] (4) Based on the positive-sequence component of the voltage at the output end of phase A, by shifting the phase angles by -120° and 120° respectively, the positive-sequence components of the voltages at the output ends of phases B and C can be obtained:

[0106] u b + = U m sin(ωt - 120°)

[0107] u c + = U m sin(ωt + 120°)

[0108] (5) Calculate the three-phase unbalanced power caused by the negative-sequence components in the three-phase load current:

[0109]

[0110]

[0111]

[0112] (6) Sort the absolute value |P Δj | of the unbalanced power in (5) according to the magnitude, and the balancing current superimposed on the given value of the DC current in each phase can be calculated as follows:

[0113]

[0114] Substituting the data of this embodiment, we can obtain:

[0115]

[0116] (7) The DC current set values of each phase after adjustment are:

[0117]

[0118] In the practical application of MMC - BESS, the situation of unbalanced three - phase load will inevitably occur, resulting in unbalanced charging and discharging power of the three - phase battery and affecting battery balance. For this situation, in the technical solution of the present invention, when setting and adjusting the DC current set values of each phase, the negative - sequence current on the load side is detected in real time, the three - phase unbalanced power generated thereby is calculated, the distribution strategy of the three - phase DC balance current is determined, the DC power of each phase is reasonably distributed, the influence on the battery charging and discharging power caused by the negative - sequence component is reduced. By intervening in advance in the battery SOC balancing process, the charging and discharging power of each phase of the battery is kept consistent, and the charging and discharging power of the battery tends to be consistent in advance. By reasonably distributing the DC power of each phase, the influence on the battery charging and discharging power caused by the negative - sequence component can be reduced, which can play a role in protecting the battery, is beneficial to battery balance, plays a role in protecting the battery, and further provides corresponding guarantee for the balance of battery performance, prolongs the service life of the system, and improves the working reliability of the system.

[0119] The present invention can be widely used in the operation and management field of electric energy storage systems.

Claims

1. An off-grid MMC-BESS energy balance control method for three-phase load imbalance conditions, characterized in that The described control method includes the following steps: 1) Execute the predetermined DC current reference given value for each phase; 2) Detect the negative sequence current on the load side in real time; 3) Calculate the negative sequence component of the load current of phase A; 4) Based on the negative sequence component of the load current of phase A, shift the phase angles by 120° and -120° respectively to obtain the negative sequence component of the load current of phase B and the negative sequence component of the load current of phase C; 5) Taking the voltage of the A-phase AC output terminal as the reference quantity, calculate the positive-sequence, negative-sequence, and zero-sequence components of the voltage of the A-phase AC output terminal ; 6) Based on the positive sequence component of the output terminal voltage of phase A, shift the phase angles by -120° and 120° respectively to obtain the positive sequence component of the output terminal voltage of phase B and the positive sequence component of the output terminal voltage of phase C; 7) Calculate the three-phase unbalanced power caused by the negative sequence components in the three-phase load current; 8) The absolute value of the unbalanced power of three phases A, B, and C, |P Δj | is sorted by magnitude, and the balanced current superimposed on the DC current set value of each phase is calculated; 9) On the basis of the predetermined DC current reference given value for each phase, superimpose the balanced current of the DC current given value for each phase, and then obtain the adjusted new DC current given value for each phase; 10) Execute the adjusted new DC current given value for each phase; 11) Return to step 2), and continue to dynamically adjust the distribution strategy of the three-phase DC balanced current according to the three-phase unbalanced power on the load side.

2. The off-grid MMC-BESS energy balancing control method for three-phase load imbalance according to claim 1, characterized in that The described control method determines the distribution strategy of the three-phase DC balanced current by detecting the negative sequence current on the load side in real time and calculating the resulting three-phase unbalanced power.

3. The off-grid MMC-BESS energy balancing control method for three-phase load imbalance according to claim 1, characterized in that The described control method intervenes in the battery SOC equalization process in advance during the three-phase power balance control, so that the battery charge and discharge power of each phase is kept consistent, the battery charge and discharge power tends to be consistent in advance, reducing the impact on the battery charge and discharge power caused by the negative sequence component, and thus improving the equalization of the battery performance.

4. The off-grid MMC-BESS energy balancing control method for three-phase load unbalance according to claim 1, characterized in that it is assumed that the current flowing through The current phasor representations of the three-phase loads are respectively and Taking the current of phase A as the reference phase, the load current of phase A can be decomposed into positive-sequence, negative-sequence and zero-sequence currents according to the following formula: In the formula are respectively the positive-sequence, negative-sequence and zero-sequence components of the load current of phase A.

5. The off-grid MMC-BESS energy balance control method for three-phase load imbalance according to claim 1 is characterized in that The system AC output terminal voltages are respectively and Taking the phase-A AC output terminal voltage as the reference quantity, the positive-sequence, negative-sequence, and zero-sequence components of 6. The off-grid MMC-BESS energy balancing control method for three-phase load imbalance according to claim 1, characterized in that The three-phase power imbalance caused by the negative sequence components in the three-phase load current is calculated according to the following formula: T in the formula is the power frequency period.

7. The off-grid MMC-BESS energy balance control method for three-phase load unbalance according to claim 1, characterized in that The absolute value of the unbalanced power of three phases A, B, and C, |P Δj After sorting by magnitude, the balancing current superimposed on the DC current set value of each phase can be calculated according to the following formula: where Δi dcj_max —— the DC balanced current superimposed on max|P Δj | corresponding to the phase Δi dcj_min —— superimposed on min|P Δj | the DC balance current of the corresponding phase; Δi dcj_middle —— Superimpose the DC balance current of the remaining one phase except the above two phases.

8. The off-grid MMC-BESS energy balancing control method for three-phase load unbalance according to claim 7, characterized in that each phase The original given value of the DC current is I dc_ref , and the given values of the DC currents of each phase after adjustment are as follows:

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