Flywheel energy storage system topology integrated with voltage balance function and method

Abstract

The invention provides a topology and method for a flywheel energy storage system driven by a common-bus open-winding permanent magnet synchronous motor and integrated with a bipolar direct-current micro-grid voltage balancer, and relates to the field of power electronics. Under the condition of not increasing power devices, u +, v + and w + winding ends of the open-winding permanent magnet synchronous motor are respectively connected with a filter inductor through a connector, and the other end of the filter inductor is connected with a neutral point of the bipolar direct-current micro-grid. And a dual-function operation mode is determined to be a main energy storage operation mode or a main voltage balance operation mode by selectively conducting one connector or three connectors. A corresponding flywheel energy storage algorithm with a voltage balance function is designed in different operation modes, and the problem of path conflict when the two functions are achieved at the same time is solved. The q-axis current is set to be 0 or the zero-sequence current is set to be 0, so that a single-function operation mode of only voltage balance and only flywheel energy storage can be realized. Economic cost is reduced, space is saved, and coordination of a power grid is improved.

Description

Technical Field

[0001] The present invention relates to the field of power electronics, and to a topology and method for voltage balancing integration, and in particular to a topology and method for integrating a voltage balancing function into a flywheel energy storage system driven by an open-winding permanent magnet synchronous motor. Background Art

[0002] With the improvement of power efficiency and the stringent environmental requirements, multi-electrification and full electrification have gradually entered our lives. As the most advanced power supply system, bipolar DC microgrids have been widely used in the conception of future commercial, industrial and transportation industries. Compared with unipolar DC microgrids, bipolar DC microgrids only need to add one conductor to generate more voltage output levels and higher system reliability. However, bipolar DC microgrids have the common problems of DC microgrids, namely voltage instability, as well as their characteristic problems, bipolar voltage imbalance.

[0003] Voltage instability will have an impact on both electrical equipment and the large power grid, and may seriously cause equipment damage or affect the normal operation of the large power grid. The problem of voltage instability can be solved by adding energy storage equipment. There are many types of energy storage methods, and energy storage systems with different characteristics are suitable for different scenarios. In some occasions where a large amount of energy needs to be stored or released in a short time, such as nuclear energy storage, electromagnetic catapults, or devices that need to be started and stopped frequently, such as subways, it is more suitable to use flywheel energy storage systems with fast charging and discharging and high power density. The flywheel energy storage system can theoretically generate any amount of power, but due to the limited power of the power switch of the motor drive system, the energy absorption and release power of the flywheel system is restricted. In order to solve the problem of flywheel power limitation, in recent years, a new solution has been proposed for permanent magnet synchronous motors, which are also restricted by power devices, namely permanent magnet synchronous motors with open winding structures. By opening the neutral point of the permanent magnet synchronous motor, the two ends of the winding are connected to two sets of inverters respectively, which can halve the power of each inverter without reducing the power of the motor. The open-winding permanent magnet synchronous motor has the characteristics of both the open-winding structure and the permanent magnet synchronous motor. It can generate three levels to reduce the generation of harmonics while ensuring the high-power operation of the motor. Therefore, the open-winding permanent magnet synchronous motor drive system has a broad application prospect in the field of flywheel energy storage.

[0004] Unbalanced voltage may shorten the life of electrical equipment or cause irreversible damage. For the voltage imbalance problem, the main research methods can be divided into two types: one research idea combines the model of the power grid and macro-adjusts the power generation, energy storage equipment or load of the microgrid to achieve the purpose of balancing power. However, the initial investment of this method also increases significantly. Another method starts from a micro perspective and equips the grid with a dedicated voltage balancer to balance the power of the bipolar DC microgrid.

[0005] Although the open-winding motor drive system has many advantages, the flywheel system driven by the open-winding permanent magnet synchronous motor is equipped with two inverters, which increases the system cost. At the same time, considering that the power device is the most vulnerable device in the entire system, a redundant voltage balancer is required in the bipolar DC microgrid to avoid the risk of single-point failure. This increases the complexity of the system and undoubtedly increases the cost and volume of the system. In some cost-sensitive occasions, the promotion of this stable and reliable structure will be limited. Costs can be saved by using an integrated method. Reference 1 [Reference 1 is B.Li et al., "DC / DC Converter for Bipolar LVdc System With Integrated VoltageBalance Capability," IEEE Trans.Power Electron., vol.36, no.5, pp.5415-5424, May2021. (Journal article)] adds two inductors to the bidirectional CLLC DC / DC converter and integrates the voltage balancing function into the DC / DC converter. Reference 2 [Reference 2 is GVdBroeck, J.Beerten, MDVecchia, S.Ravyts, and J.Driesen, "Operation of the full-bridge three-level DC–DC converter in unbalanced bipolar DC microgrids," IET Power Electron., vol.12, no.9, pp.2256–2265, Aug.2019. (journal article)] designed a three-level DC-DC converter that integrates battery storage and voltage balancing functions. These methods avoid the use of dedicated voltage balancers, but these methods increase the cost, volume, and control complexity of the converter itself. In addition, these methods are all integrations of voltage balancers in bidirectional DC / DC controllers, which are not suitable for motor drive systems.Reference 3 [Reference 3 is W.Zhao, P.Zhao, D.Xu, Z.Chen and J.Zhu, "HybridModulation Fault-Tolerant Control of Open-End Windings Linear Vernier Permanent-Magnet Motor With Floating Capacitor Inverter," in IEEE Trans.PowerElectron., vol.34, no.3, pp.2563-2572, March 2019. (Journal article)] uses an open-winding permanent magnet synchronous motor flywheel energy storage system to achieve power transmission between two isolated power sources. However, this method can only achieve the flow of energy between independent power sources, and cannot achieve voltage balance in a bipolar DC microgrid.

[0006] In summary, if the voltage balancing function can be integrated into the flywheel energy storage system, and both flywheel energy storage and voltage balancing functions can be realized at the same time, the reliability of the bipolar DC microgrid can be guaranteed while reducing the cost and volume. The flywheel energy storage system with integrated voltage balancing function can coordinate the bipolar positive and negative voltages and the total voltage, further promoting the development of bipolar DC microgrids. Summary of the invention

[0007] In order to save volume and cost, in a bipolar DC microgrid, a voltage balancer is usually integrated into an energy storage system with a bidirectional DC / DC controller. However, if a bipolar DC microgrid needs to use a flywheel energy storage system for energy storage, the original method cannot complete the integration of the voltage balancing function. The present invention makes full use of the power density and power transmission capacity of the open-winding motor driver to complete the voltage balancing and stabilization tasks of the bipolar DC microgrid. The voltage balancing function is integrated into the flywheel energy storage system by utilizing the characteristics of the open-winding permanent magnet synchronous motor having a zero-sequence circuit. The method has two working modes, main flywheel energy storage and main voltage balancing, according to the number of branches reused for the voltage balancing function, and is flexibly applicable to different working requirements of the power grid. The two integrated functions are relatively independent, and the integrated system can perform flywheel energy storage and voltage balancing tasks at the same time, or it can complete one of the functions separately. From the perspective of the entire bipolar DC microgrid system, the present invention will not significantly increase the cost and floor space. On the contrary, by applying the present invention to reduce the power requirements of the voltage balancer, cost and volume will be saved to a certain extent.

[0008] The topology and technical solution adopted by the present invention to solve its technical problem are:

[0009] The present invention integrates the voltage balancing function into the open-winding permanent magnet synchronous motor flywheel energy storage system. The circuit topology is as follows Figure 1As shown, P, O, and N are the positive pole, neutral line, and negative pole of the bipolar DC microgrid, respectively; C1 and C2 are capacitors; S1, S2, S3, S4, S5, and S6 are six power switches of inverter 1, U1, V1, and W1 are three output interfaces of inverter 1, S1', S2', S3', S4', S5', and S6' are six power switches of inverter 2, and U2, V2, and W2 are three output interfaces of inverter 2; current sensors and voltage sensors are as shown in Figure 1 As shown, A 、i B 、i C The arrow direction represents the positive direction of the current. The U1, V1, and W1 ports of inverter 1 are connected to the u+, v+, and w+ windings of the open-winding motor, and are also connected to the open-winding motor through connector K. A , K B , K C It is connected to the inductor L, and the other end of L is connected to the neutral point O of the bipolar DC microgrid; the U2, V2, and W2 ports of the inverter 2 are connected to the u-, v-, and w-windings of the open-winding motor.

[0010] The specific steps of power migration are as follows:

[0011] Step 1: Select the number of voltage balancing branches

[0012] The number of branches reused for voltage balancing function is selected according to the actual needs of the power grid. If the power grid requires the integrated system to be mainly energy storage function, then the connector K is turned on. A At this time, only the A-phase winding is reused for the voltage balancing function, and the topology of the integrated system can be simplified to Figure 2 ,in Figure 2 (a) Simplified topology for flywheel energy storage function, Figure 2 (b) is a simplified topology for voltage balancing. If the grid requires the integrated system to focus on voltage balancing, then the connector K is turned on at the same time. A , K B , K C At this time, the three-phase windings are reused for voltage balancing, and the topology of the integrated system can be simplified to Figure 3 ,in Figure 3 (a) Simplified topology for flywheel energy storage function, Figure 3 (b) is a simplified topology of the voltage balancing function.

[0013] Step 2: Calculate the reference voltage vector V for vector control Ref_α , V Ref_β

[0014] If the integrated system is required to perform the flywheel energy storage task, the flywheel system needs to set the target voltage value to be stable, and the feedback value U DC1 +UDC2 The target voltage value is subtracted from the voltage given by the voltage outer loop, and after PI adjustment, the q-axis target current is obtained. If the integrated system is not required to perform the flywheel energy storage task, the q-axis target current is set to 0. Then the q-axis target current is subtracted from the feedback q-axis true value, and after PI adjustment, u is obtained. q The target current of the d-axis is 0, which is subtracted from the real value of the d-axis feedback, and after PI adjustment, u is obtained. d . Using u q 、u d And position feedback θ is used to transform the two-phase rotating coordinate system to the two-phase stationary coordinate system, and V Ref_α , V Ref_β .

[0015] Step 3: Calculate the zero-sequence reference voltage V for vector control zero

[0016] If the integrated system is required to perform voltage balancing tasks, the voltage feedback value U DC1 with U DC2 The difference is taken as the given value of the zero-sequence voltage outer loop, and after PI adjustment, the given value of the zero-sequence current is calculated. If the integrated system is not required to perform the voltage balancing task, the zero-sequence current given value is set to 0. Then, the given value is subtracted from the feedback value of the zero-sequence current, and after the modified PI and quasi-PR controller adjustment, its transfer function is shown in formula (1).

[0017]

[0018] Among them, K p , K i , K r1 and K r3 are the parameters of proportion, integration, first resonance and third resonance respectively, ω1 and ω3 are the fundamental frequency and triple frequency, ω 1c and ω 3c is set to 5% of ω1 and ω3.

[0019] Step 4: Sector Division for Vector Control

[0020] If only the A-phase winding is used for voltage balancing, then according to V Ref_α , V Ref_β and voltage offset Δu α The modulation area is divided into 6 sectors according to the value of . The sector division method is shown in formula (2), where Δu α =(U DC1 -U DC2 ) / 3,U DC =U DC1 +U DC2 .

[0021]

[0022] If the three-phase windings are reused for voltage balancing, the sector division method is shown in formula (3):

[0023]

[0024] Step 5: Selection of valid vectors

[0025] If only the A-phase winding is reused for the voltage balancing function, six voltage vectors are selected to synthesize the effective vectors. The six voltage vectors are: Voltage vector V1 is the voltage vector generated by inverter 1 N00 , inverter 2 generates V 011 '; Voltage vector V2 generates V for inverter 1 N10 , inverter 2 generates V 001 '; Voltage vector V3 generates V for inverter 1 N10 , inverter 2 generates V 101 '; Voltage vector V4 generates V for inverter 1 N11 , inverter 2 generates V 100 '; Voltage vector V5 generates V for inverter 1 N01 , inverter 2 generates V 110 '; Voltage vector V6 generates V for inverter 1 N01 , inverter 2 generates V 010 '; Wherein, subscript 1 represents that the upper power switch is on, subscript 0 represents that the lower power switch is on, and subscript x represents that both the upper and lower power switches are off. V1 and V2 are combined to form the vector of sector I, V2 and V3 are combined to form the vector of sector II, V3 and V4 are combined to form the vector of sector III, V4 and V5 are combined to form the vector of sector IV, V5 and V6 are combined to form the vector of sector V, and V6 and V1 are combined to form the vector of sector VI.

[0026] If the three-phase windings are reused for voltage balancing, the six effective vectors are: V1' is the voltage generated by inverter 2 011 ';V2' is the V generated by inverter 2 001 ';V3' generates V for inverter 2 101 ';V4' generates V for inverter 2 100 '; V5' generates V for inverter 2 110 ';V6' generates V for inverter 2 010 '. V1' and V2' are synthesized into sector I, V2' and V3' are synthesized into sector II, V3' and V4' are synthesized into sector III, V4' and V5' are synthesized into sector IV, V5' and V6' are synthesized into sector V, and V6' and V1' are synthesized into sector VI.

[0027] Step 6: Calculation of the effective vector action time

[0028] If only the A-phase winding is reused for the voltage balancing function, it is stipulated that the action time of each sector when synthesizing the vector subscript is an odd number is T v_odd , the action time of the even-numbered subscript is T v_even The action time of different sector voltage vectors is shown in Table 1, where T S To control the cycle.

[0029] Table 1 Effective vector action time when only phase A winding is reused for voltage balance

[0030]

[0031]

[0032] If the three-phase windings are reused for the voltage balancing function, the action time of the voltage vectors in different sectors is shown in Table 2.

[0033] Table 2 Effective vector action time when all three-phase windings are reused for voltage balance

[0034]

[0035] Step 7: Selection of Zero Voltage Vector

[0036] If only the A-phase winding is used for voltage balancing, four voltage vectors are selected to synthesize the zero voltage vector, which are: voltage vector V 0_1 Generates V for inverter 1 N00 , inverter 2 generates V 111 '; Voltage vector V 0_2 Generates V for inverter 1 N01 , inverter 2 generates V 001 '; Voltage vector V 7_1 Generates V for inverter 1 N11 , inverter 2 generates V 000 '; Voltage vector V 7_2 Generates V for inverter 1 N01 , inverter 2 generates V 101 '.

[0037] If the three-phase windings are reused for voltage balancing, the two zero voltage vectors are: Voltage vector V0' is the voltage vector generated by inverter 2 V 000 '; Voltage vector V7' generates V for inverter 2 111 '.

[0038] Step 8: Calculation of the action time of the zero voltage vector

[0039] The total action time of the zero voltage vector must satisfy formula (4):

[0040]

[0041] Among them, T zero It is the time of zero voltage vector synthesis.

[0042] If only the A-phase winding is used for voltage balancing, the zero vector action time T v0 and T v7 As shown in formula (5). When the zero-sequence voltage V Zero >0, introduce V 7_1 , adjust V 7_1 With V 7_2 The calculation method of voltage vector action time is shown in formula (6), where Δu = (U DC1 -U DC2 ) / 2U DC When the zero sequence voltage V Zero <0, introduce V 0_1 , adjust V 0_1 With V 0_2 The calculation method is shown in formula (7).

[0043]

[0044] If the three-phase windings are all used for voltage balancing, the calculation method is shown in formula (8).

[0045]

[0046] Step 9: PWM Wave Generation Method

[0047] If only the A-phase winding is used for voltage balancing, the voltage vector generation sequence is as follows: first generate the zero vector, V 7_2 In V 0_1 Then generate valid vectors in sequence, where odd vectors are generated before even vectors, and finally generate zero vectors, V 7_1 Before V 0_2 The operation time T1, T2, T3, T4, and T5 for generating PWM waves are shown in formula (9):

[0048]

[0049] If the three-phase windings are reused for the voltage balancing function, a symmetrical PWM wave is generated, with V0' on both sides and V7' in the middle. Then, according to the minimum switching principle, V1', V3', and V5' are on both sides of vector V7', and V2', V4', and V6' are adjacent to V0'.

[0050] Step 10: Sampling point setting and signal feedback

[0051] At the beginning of each control cycle, the voltage signal U DC1 , U DC2 , three-phase current signal i A 、i B 、i C The current signal and position signal θ are sampled, and the current signal and position signal are used to transform from the three-phase stationary coordinate system to the two-phase rotating coordinate system to generate feedback currents of the d, q, and 0 axes for feedback.

[0052] The beneficial effects of the present invention are:

[0053] (1) The present invention reduces economic costs and space requirements, and reduces the power requirements of the original voltage balancer: The strategy proposed by the present invention makes full use of the inverter of the open-winding motor drive system, and can achieve two functions without adding additional power switches, thereby improving the utilization rate of the original power devices of the system and reducing costs and space. The present invention integrates the voltage balancing function into the flywheel energy storage system, thereby sharing or directly replacing the original voltage balancer of the system.

[0054] (2) The present invention increases the coordination of the bipolar DC microgrid: Since the present invention can achieve voltage stability and balance with only one controller, the cost of coordination between different controllers is reduced, and the coordinated control of flywheel energy storage and voltage balance can be achieved at zero cost.

[0055] (3) The present invention can be flexibly adjusted according to the requirements of the system: the present invention is divided into two solutions, namely, main flywheel energy storage and main voltage balance, according to the number of branches reused for voltage balancing. The main flywheel energy storage solution has a larger modulation area, and the flywheel allows more energy to be stored; the main voltage balancing function can transmit more voltage balancing power.

[0056] (4) The two multiplexing functions of the present invention are relatively independent: the present invention can directly control the multiplexing system to realize only one of the functions of flywheel energy storage or voltage balance by modifying the given value of the current loop without changing the topology. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is the topological structure proposed by the present invention.

[0058] Figure 2 Simplify the topology of the integrated system where only the A-phase winding is reused for the voltage balancing function. Figure 2 (a) is the simplified topological structure of the flywheel system energy storage. Figure 2 (b) is a simplified topology for voltage balancing.

[0059] Figure 3 It is an integrated system with simplified topology structure where all three-phase windings are reused for voltage balancing function. Figure 3(a) is the simplified topological structure of the flywheel system energy storage. Figure 3 (b) is a simplified topology for voltage balancing.

[0060] In the figure, P, O, and N are the three-terminal interfaces of the bipolar DC microgrid; S1, S2, S3, S4, S5, and S6 are the power switches of inverter 1; S1', S2', S3', S4', S5', and S6' are the power switches of inverter 2; K A , K B , K C is a connector; U1, V1, W1 are the outlets of inverter 1, and U2, V2, W2 are the outlets of inverter 2; u+, v+, w+, u-, v-, w- are the interfaces of the motor windings respectively; i A 、i B 、i C are the currents of the three-phase windings of the motor, and the direction of the arrow represents the positive direction of the specified current; U DC1 , U DC2 They are the positive voltage of the bipolar DC microgrid and the negative voltage of the bipolar DC microgrid respectively; the symbols of the voltage sensor and the current sensor are marked in the figure; C1 and C2 are electrolytic capacitors; L is the filter inductor. DETAILED DESCRIPTION

[0061] The present invention is further described below in conjunction with embodiments.

[0062] The loads of the upper and lower levels of the bipolar DC microgrid system are not equal, that is, the upper pole carries a load of 200Ω, and the lower pole does not carry a load. The design target voltage is 300V, and the control period is T S =100us. According to the grid requirements, the main flywheel energy storage mode is selected. At this time, the power supply current is limited to 0.2A to simulate the situation where the bipolar DC microgrid power supply is insufficient. At this time, the measured grid voltage U DC1 =147.5V, U DC2 =142.5V, that is, using the method of the present invention, the specific implementation method is as follows:

[0063] (1) Perform step 1: The reuse system is already working in the main flywheel energy storage mode, only the A phase winding is reused for the voltage balancing function, and connector K A Conductivity.

[0064] (2) Perform step 2: According to the voltage sampling feedback value U DC1 and U DC2 , compared with the target voltage 300V, -10V is obtained, and the q-axis reference current is calculated through PI adjustment. The d and q-axis feedback values ​​are compared with the reference values, and then the reference voltage vector V of the vector control is obtained through PI adjustment and coordinate transformation. Ref_α =0.5, VRef_β =0.1.

[0065] (3) Perform step 3: Use the voltage feedback value U DC1 with U DC2 The zero-sequence voltage is given by 5V as the outer loop, and the zero-sequence current is given by PI control. Then, it is compared with the feedback of the zero-sequence current. After adjustment by PI and PR controllers, the zero-sequence current given value V is calculated. Zero =0.05.

[0066] (4) Perform step 4: Use formula (2) to divide the vector control sectors. At this time, Δu α =1.67V,

[0067] U DC =290V, the reference vector falls in sector I.

[0068] (5) Perform step 5: Only the A-phase winding is reused for the voltage balancing function, and the voltage vectors V1 and V2 are selected to synthesize sector I.

[0069] (6) Perform step 6: Use Table 1 to calculate the effective vector action time, T v_odd =0.474T S , T v_even =0.087T S .

[0070] (7) Perform step 7: Only the A-phase winding is reused for the voltage balance function, and four voltage vectors are selected to synthesize the zero voltage vector: V 0_1 、V 0_2 、V 7_1 and V 7_2 .

[0071] (8) Perform step 8: Use formula (4) to calculate T zero =0.439T S , and then use formula (5) to calculate T V0 =0.223T S , T V7 =0.216T S . Since V Zero >0, then Δu=0.0086, therefore, T is calculated using formula (6) v0_2 =0.223T S , T v7_1 =0.017T S , T v7_2 =0.199T S .

[0072] (9) Perform step 9: The order of generating voltage vector is as follows: V7_2 , V1, V2, V 7_1 、V 0_2 , using formula (9) to calculate the PWM wave action time points are: T1 = 0.199T S , T2=T1,T3=0.673T S ,

[0073] T4=0.760T S , T5=0.777T S At T1, V 7_2 At T3, it changes from V1 to V2, and at T4, it changes from V2 to V 7_1 , at T5, V 7_1 becomes V 0_2 .

[0074] (10) Perform step 10: At the beginning of each cycle, the voltage signal U DC1 , U DC2 , three-phase current signal i A 、i B 、i C The current signal and position signal θ are sampled, and the current signal and position signal are used to transform from the three-phase stationary coordinate system to the two-phase rotating coordinate system to generate feedback currents of the d, q, and 0 axes for feedback.

Claims

1. A flywheel energy storage system topology and method with integrated voltage balancing function, characterized in that: The flywheel energy storage system based on the open-winding motor drive is integrated with a voltage balancing function to change the topology of the common bus open-winding motor driver connected to the positive and negative poles of the bipolar DC bus: the u+, v+, and w+ terminals of the open-winding permanent magnet synchronous motor are connected to the connector K A , K B , K C Connect connector K A , K B , K C The other end of the filter inductor is connected to the same filter inductor, and the other end of the filter inductor is connected to the zero pole of the bipolar DC bus; voltage and current double closed-loop control is adopted, and only K can be selected according to the actual situation of the power grid. A Dual-function operation mode with closed main flywheel energy storage, or K A , K B , K C A dual-function operation mode of main voltage balance is closed at the same time; a single-function operation mode of voltage balance and flywheel energy storage can also be achieved by setting the q-axis current setting to 0 or the zero-sequence current setting to 0.

2. A flywheel energy storage system topology and method using an integrated voltage balancing function as claimed in claim 1, characterized in that The steps include: Step 1: Select the number of voltage balancing branches: Select the number of branches reused for voltage balancing function according to the actual needs of the power grid: If the power grid requires the integrated system to mainly use energy storage function, then connect connector K A At this time, only the A-phase winding is reused for the voltage balancing function. If the grid requires the integrated system to be mainly used for the voltage balancing function, the connector K is turned on at the same time. A , K B , K C ,At this time, the three-phase windings are reused for the voltage balancing function; Step 2: Calculate the reference voltage vector V for vector control Ref_α , V Ref_β : If the integrated system is required to perform the flywheel energy storage task, the flywheel system needs to set the target voltage value to be stable, and the feedback value U DC1 +U DC2 The target voltage value is subtracted from the voltage of the voltage outer loop, and after PI adjustment, the q-axis target current is obtained. If the integrated system does not need to perform the flywheel energy storage task, the q-axis target current is set to 0; then the q-axis target current is subtracted from the feedback q-axis true value, and after PI adjustment, u is obtained. q The target current of the d-axis is 0, and the difference between it and the real value of the d-axis feedback is obtained through PI adjustment. d , using u q 、u d And position feedback θ is used to transform the two-phase rotating coordinate system to the two-phase stationary coordinate system, and V Ref_α , V Ref_β ; Step 3: Calculate the zero-sequence reference voltage V for vector control zero : If the integrated system is required to perform voltage balancing tasks, the voltage feedback value U DC1 with U DC2 The difference is taken as the given value of the zero-sequence voltage outer loop. After PI adjustment, the given value of the zero-sequence current is calculated. If the integrated system does not need to perform the voltage balance task, the zero-sequence current given value is set to 0. Then, the given value is subtracted from the feedback value of the zero-sequence current. After the modified PI and quasi-PR controller adjustment, its transfer function is shown in formula (1), where K p , K i , K r1 and K r3 are the parameters of proportion, integration, first resonance and third resonance respectively, ω1 and ω3 are the fundamental frequency and triple frequency, ω 1c and ω 3c is set to 5% of ω1 and ω3; Step 4: Sector division for vector control: If only the A-phase winding is used for voltage balancing, then according to V Ref_α , V Ref_β and voltage offset Δu α The modulation area is divided into 6 sectors according to the value of. The sector division method is shown in formula (2), where: Δu α =(U DC1 -U DC2 ) / 3,U DC =U DC1 +U DC2 , If the three-phase windings are reused for the voltage balancing function, the sector division method is as follows; Step 5: Selection of valid vectors: If only the A-phase winding is reused for the voltage balancing function, six voltage vectors are selected to synthesize the effective vectors. The six voltage vectors are: Voltage vector V1 is the voltage vector generated by inverter 1 N00 , inverter 2 generates V 011 '; Voltage vector V2 generates V for inverter 1 N10 , inverter 2 generates V 001 '; Voltage vector V3 generates V for inverter 1 N10 , inverter 2 generates V 101 '; Voltage vector V4 generates V for inverter 1 N11 , inverter 2 generates V 100 '; Voltage vector V5 generates V for inverter 1 N01 , inverter 2 generates V 110 '; Voltage vector V6 generates V for inverter 1 N01 , inverter 2 generates V 010 '; Wherein, subscript 1 represents that the upper power switch is turned on, subscript 0 represents that the lower power switch is turned on, and subscript x represents that both the upper and lower power switches are not turned on; V1 and V2 are the vector of sector I, V2 and V3 are the vector of sector II, V3 and V4 are the vector of sector III, V4 and V5 are the vector of sector IV, V5 and V6 are the vector of sector V, and V6 and V1 are the vector of sector VI; If the three-phase windings are reused for voltage balancing, the six effective vectors are: V1' is the voltage generated by inverter 2 011 ';V2' is the V generated by inverter 2 001 ';V3' generates V for inverter 2 101 ';V4' generates V for inverter 2 100 '; V5' generates V for inverter 2 110 ';V6' generates V for inverter 2 010 '; V1', V2' synthesize sector I, V2', V3' synthesize sector II, V3', V4' synthesize sector III, V4', V5' synthesize sector IV, V5', V6' synthesize sector V, V6', V1' synthesize sector VI; Step 6: Calculation of the effective vector action time If only the A-phase winding is reused for the voltage balancing function, it is stipulated that the action time of each sector when synthesizing the vector subscript is an odd number is T v_odd , the action time of the even-numbered subscript is T v_even , the action time of different sector voltage vectors is shown in Table 1, where T S To control the cycle; Table 1 Effective vector action time when only phase A winding is reused for voltage balance If all three-phase windings are reused for the voltage balancing function, the action time of the voltage vectors in different sectors is shown in Table 2; Table 2 Effective vector action time when all three-phase windings are reused for voltage balance Step 7: Selection of Zero Voltage Vector If only the A-phase winding is used for voltage balancing, four voltage vectors are selected to synthesize the zero voltage vector, which are: voltage vector V 0_1 Generates V for inverter 1 N00 , inverter 2 generates V 111 '; Voltage vector V 0_2 Generates V for inverter 1 N01 , inverter 2 generates V 001 '; Voltage vector V 7_1 Generates V for inverter 1 N11 , inverter 2 generates V 000 '; Voltage vector V 7_2 Generates V for inverter 1 N01 , inverter 2 generates V 101 '; If the three-phase windings are reused for voltage balancing, the two zero voltage vectors are: Voltage vector V0' is the voltage vector generated by inverter 2 V 000 '; Voltage vector V7' generates V for inverter 2 111 '; Step 8: Calculation of the action time of zero voltage vector: The total action time of the zero voltage vector must satisfy formula (4), where T zero It is the time of zero voltage vector synthesis; If only the A-phase winding is used for voltage balancing, the zero vector action time T v0 and T v7 As shown in formula (5); when the zero-sequence voltage V Zero >0, introduce V 7_1 , adjust V 7_1 With V 7_2 The calculation method of voltage vector action time is shown in formula (6), where Δu = (U DC1 -U DC2 ) / 2U DC ; When the zero sequence voltage V Zero <0, introduce V 0_1 , adjust V 0_1 With V 0_2 The ratio of is calculated as shown in formula (7); If the three-phase windings are all used for voltage balancing, the calculation method is shown in formula (8); Step 9: PWM Wave Generation Method: If only the A-phase winding is used for voltage balancing, the voltage vector generation sequence is as follows: first generate the zero vector, V 7_2 In V 0_1 Then generate valid vectors in sequence, where odd vectors are generated before even vectors, and finally generate zero vectors, V 7_1 Before V 0_2 , the operation time T1, T2, T3, T4, T5 of generating PWM wave is as shown in formula (9); If the three-phase windings are reused for the voltage balancing function, a symmetrical PWM wave is generated, with V0' on both sides and V7' in the middle. Then, according to the minimum switching principle, V1', V3', and V5' are on both sides of vector V7', and V2', V4', and V6' are adjacent to V0'. Step 10: Sampling point setting and signal feedback: At the beginning of each control cycle, the voltage signal U DC1 , U DC2 , three-phase current signal i A 、i B 、i C The current signal and position signal θ are sampled, and the current signal and position signal are used to transform from the three-phase stationary coordinate system to the two-phase rotating coordinate system to generate feedback currents of the d, q, and 0 axes for feedback.

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