A novel voltage balancer with continuous input and output current and its control strategy

Abstract

The present invention proposes a new voltage balancer topology and control strategy suitable for bipolar DC microgrid architecture. The DC positive potential bus is connected to the midpoint of the bridge arm 2 of the full-bridge converter through the inductor L1, the DC negative potential bus is connected to the negative pole of the full-bridge converter through the inductor L3, the intermediate capacitor is connected in parallel with the full-bridge converter, and the inductor L2 is connected to the midpoint of the bridge arm 1 of the full-bridge converter to form a neutral line L. N , thus forming a bipolar bus distribution form; the action of the full-bridge converter bridge arm 2 can realize the control of the intermediate stage capacitor voltage, and the action of the full-bridge converter bridge arm 1 can realize the balance of the inter-pole unbalanced voltage. The two together with the inductance on the DC bus make the input and output currents of the system continuous, balancing the inter-pole unbalanced voltage while effectively reducing the ripple caused by the switching tube action to the system.

Description

Technical Field

[0001] The present invention relates to the fields of bipolar DC microgrid voltage balancing, power electronics technology application and power electronic converter control, and in particular to a novel voltage balancer topology and control strategy for constructing a neutral line and solving the problem of inter-pole voltage imbalance. Background Art

[0002] Among the mainstream architectures for DC microgrid systems, bipolar architectures offer advantages over unipolar architectures in terms of flexibility, reliability, and efficiency. However, these architectures often require the addition of a voltage balancer to stabilize the neutral voltage. Therefore, the design and research of voltage balancer solutions are particularly important in bipolar DC microgrids. A key challenge in voltage balancer design is the discontinuous current generated by switching transistors, which can affect DC bus power quality and increase grid voltage ripple. Currently, various solutions exist to balance inter-pole voltage imbalances, such as the traditional buck / boost voltage balancer. Furthermore, some researchers are considering adding voltage stabilization functionality to the voltage balancer, ensuring that the voltage balancer's balancing action does not affect the bus voltage. This has led to the development of voltage balancers based on three-level DC-DC converters. (For example, H. Kakigano, Y. Miura, T. Ise and R. Uchida, "DC Voltage Control of the DC Micro-grid for Super High Quality Distribution," 2007 Power Conversion Conference - Nagoya, Nagoya, Japan, 2007, pp. 518-525. (Conference paper)). Beyond this solution, some researchers have applied the three-port circuit conversion concept and multi-phase technology to voltage balancer design, resulting in a voltage balancer derived from a bidirectional DC-DC circuit and an interleaved parallel voltage balancer using multi-phase technology. (For example, F. Wang, Z. Lei, X. Xu and X. Shu, "Topology Deduction and Analysis of Voltage Balancers for DC Microgrid," in IEEE Journal of Emerging and Selected Topics in Power Electronics, vol. 5, no. 2, pp. 672-680, June 2017. (Journal paper)). Among them, the most typical bidirectional Cuk type voltage balancer utilizes the continuous input and output characteristics of the Cuk conversion circuit to make the P-pole and N-pole bus current states continuous. The staggered parallel type voltage balancer can theoretically achieve zero bus voltage ripple output by setting a suitable control scheme.Some scholars have also hybridized the basic DC-DC conversion circuit and obtained a new circuit structure through cross-reorganization of the structure or control method to achieve or less switching control (such as MB Ferrera, SP Litrán, E. Durán and JM Andújar, "A SEPIC-Cuk converter combination for bipolar DC microgrid applications," 2015 IEEE International Conference on Industrial Technology (ICIT), Seville, Spain, 2015, pp. 884-889 (conference paper)) or lower line loss (such as P. Prabhakaran and V. Agarwal, "Novel Boost-SEPIC Type Interleaved DC–DC Converter for Mitigation of Voltage Imbalance in a Low-Voltage Bipolar DC Microgrid," in IEEE Transactions on Industrial Electronics, vol. 67, no. 8, pp. 6494-6504, Aug. 2020 (journal paper)). Although existing voltage balancer designs acknowledge that the intermittent power flow generated by the switching tube operation will cause ripple in the system, and have taken certain measures to minimize ripple and optimize power quality, they do not fundamentally solve the problem of intermittent power flow. The intermittent current state on the DC bus still exists, and the risk of deteriorating DC system power quality still exists. Summary of the Invention

[0003] To overcome the shortcomings of existing voltage balancer solutions, the present invention provides a novel voltage balancer and control strategy with continuous input and output currents. To address the issue of voltage balancing between unbalanced poles, particularly the issue of intermittent power flow introduced into the system by switching transistors during the balancing process, the present invention incorporates an inductor on the DC bus and operates a full-bridge converter in parallel with capacitors. This allows the voltage balancer to balance unbalanced voltages between poles while also maintaining continuous input and output currents. This significantly reduces harmonics introduced by switching transistors, thereby optimizing power quality.

[0004] The technical solution adopted by the present invention to solve its technical problem is:

[0005] A novel voltage balancer topology structure suitable for bipolar DC microgrid architecture with continuous input and output currents. The DC positive potential bus is connected to the midpoint of the bridge arm 2 of the full-bridge converter, that is, the emitter of the switch tube S3, through the inductor L1. The DC negative potential bus is connected to the negative electrode of the full-bridge converter, that is, the emitters of the switch tubes S2 and S4, through the inductor L3. The positive electrode of the intermediate capacitor is connected to the collectors of the switch tubes S1 and S3, and the negative electrode of the capacitor is connected to the emitters of the switch tubes S2 and S4. The inductor L2 is connected to the midpoint of the bridge arm 1 of the full-bridge converter, that is, the emitter of the switch tube S1, to form a neutral line L. N , thus forming a bipolar bus distribution form; the load R1 and the filter capacitor C1 are connected between the positive potential bus and the neutral line L N The load R2 and filter capacitor C2 are connected to the neutral line L N Between the negative potential busbar.

[0006] The present invention also provides a control strategy for a novel voltage balancer system, the specific steps of which are as follows:

[0007] Step 1: The intermediate capacitor voltage is pumped up by the combined action of the total DC bus voltage, the L1 inductor current, and the full-bridge converter arm 2, so that the intermediate capacitor voltage is greater than the total DC bus voltage and is controlled to be approximately 120% to 180% of the total bus voltage:

[0008] When the voltage of the intermediate capacitor is lower than the control target, the full-bridge converter S4 acts as the main switch, S3 acts as the freewheeling diode, and the inductor L1 together form a boost circuit, so that the voltage of the intermediate capacitor can be pumped up;

[0009] When the voltage of the intermediate capacitor is higher than the control target, the full-bridge converter S3 acts as the main switch, S4 acts as a freewheeling diode, and the inductor L1 together form a step-down circuit, allowing the intermediate capacitor voltage to drop.

[0010] By controlling the voltage of the intermediate capacitor according to the above scheme, on the one hand, the upper pole current of the bipolar DC bus is always in a continuous state, and on the other hand, it prepares for balancing the unbalanced voltage of the system;

[0011] Step 2: The unbalanced voltage caused by the asymmetric load between the poles is balanced through the combined action of the intermediate capacitor voltage, the L2 inductor current, and the bridge arm 1 of the full-bridge converter:

[0012] When the upper voltage of the bipolar DC bus is greater than the lower voltage, the full-bridge converter S1 acts as the main switch and S2 acts as a freewheeling diode, injecting power into the lower voltage of the bipolar DC bus. This causes the lower voltage of the bipolar DC bus to rise, ultimately achieving the control goal of equalizing the upper and lower voltages of the bipolar DC bus.

[0013] When the upper voltage of the bipolar DC bus is lower than the lower voltage, the full-bridge converter S2 acts as the main switch and S1 acts as a freewheeling diode, drawing power from the lower voltage of the bipolar DC bus. This causes the lower voltage of the bipolar DC bus to drop, ultimately achieving the control goal of equalizing the upper and lower voltages of the bipolar DC bus.

[0014] According to the above scheme, the voltage difference between the upper and lower poles is controlled to be 0, so that the neutral line current of the bipolar DC bus can always be in a continuous state. The system adds inductance to the lower polar DC bus of the bipolar DC bus, so the voltage balancer system can achieve continuous input and output currents.

[0015] The beneficial effect of the present invention is to solve the problem of voltage balancing between unbalanced poles, especially the problem of ripple introduced into the system by discontinuous power flow generated by the switching tube action during the balancing process. The present invention has the following advantages:

[0016] (1) The present invention solves the problem of ripple introduced into the system by discontinuous power flow generated by the switching tube operation at the circuit topology level. Existing voltage balancer solutions mostly suppress current ripple by using complex control algorithms, but fail to solve the ripple problem at the circuit level. This undoubtedly increases the control burden of the voltage balancer control system and is very likely to affect the feasibility of other complex functional algorithms.

[0017] (2) The control strategy adopted by the present invention can achieve voltage balancing action when the input and output currents are both in a continuous state. By coordinating the voltage of the intermediate capacitor and the unbalanced voltage between the poles, the system inter-pole voltage is dynamically balanced while the current on each bus is in a continuous state.

[0018] (3) The present invention realizes that the input and output currents of the voltage balancer are both in a continuous state, and the intermittent power flow generated by the operation of the switching tube at the bus end will not cause serious deterioration of the power energy of the power grid. At the load end, since the output current is continuous, there is no need to install additional filtering components, thereby avoiding waste of resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the topological structure of the novel voltage balancer of the present invention.

[0020] Figure 2 This is a schematic diagram of the working principle of the circuit in which the voltage of the intermediate capacitor is lower than the control target when the voltage balancing function step 1 is implemented in the present invention.

[0021] Figure 3 This is a schematic diagram of the working principle of the circuit in which the intermediate capacitor voltage is higher than the control target voltage when the voltage balancing function step 1 is implemented in the present invention.

[0022] Figure 4This is a schematic diagram of the working principle of the circuit when the upper pole voltage is greater than the lower pole voltage when the voltage balancing function step 2 is realized in the present invention.

[0023] Figure 5 This is a schematic diagram of the working principle of the circuit when the upper pole voltage is less than the lower pole voltage when the voltage balancing function step 2 is realized in the present invention. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to the accompanying drawings and examples.

[0025] A new voltage balancer topology suitable for bipolar DC microgrid architecture with continuous input and output currents. The DC positive potential bus is connected to the emitter of the switch tube S3 through the inductor L1, and the DC negative potential bus is connected to the emitters of the switch tubes S2 and S4 through the inductor L3. The positive electrode of the intermediate capacitor is connected to the collectors of the switch tubes S1 and S3, and the negative electrode of the capacitor is connected to the emitters of the switch tubes S2 and S4. The inductor L2 is connected to the emitter of the switch tube S1 to form a neutral line L. N , thus forming a bipolar bus distribution form; the load R1 and the filter capacitor C1 are connected between the positive potential bus and the neutral line L N The load R2 and filter capacitor C2 are connected to the neutral line L N Between the negative potential bus, the specific circuit topology is as follows Figure 1 shown.

[0026] A control strategy based on a new voltage balancer system, the specific steps are as follows:

[0027] A DC system with a total bus voltage of 200V is used as an example for explanation.

[0028] Step 1: The intermediate capacitor voltage is pumped up by the combined action of the total DC bus voltage, the L1 inductor current, and the full-bridge converter arm 2, so that the intermediate capacitor voltage is greater than the total DC bus voltage and is controlled to be approximately 240V to 360V.

[0029] When the voltage of the intermediate capacitor is lower than the control target, the full-bridge converter S4 acts as the main switch tube, S3 acts as the freewheeling diode, and the inductor L1 together form a boost circuit, so that the voltage of the intermediate capacitor can be pumped up. The specific working method of the circuit is as follows: Figure 2 As shown;

[0030] When the voltage of the intermediate capacitor is higher than the control target, the full-bridge converter S3 acts as the main switch tube, S4 acts as the freewheeling diode, and the inductor L1 together forms a step-down circuit, so that the voltage of the intermediate capacitor can be reduced. The specific working method of the circuit is as follows: Figure 3 As shown;

[0031] Step 2: The unbalanced voltage caused by the asymmetric load between the poles is balanced through the combined action of the intermediate capacitor voltage, the L2 inductor current, and the bridge arm 1 of the full-bridge converter:

[0032] When the upper pole voltage of the bipolar DC bus is greater than the lower pole voltage, the full-bridge converter S1 acts as the main switch tube, and S2 acts as the freewheeling diode to inject power flow into the lower pole of the bipolar DC bus, causing the lower pole voltage of the bipolar DC bus to rise, and finally achieving the control target of equal upper and lower pole voltages of the bipolar DC bus. The specific working method of the circuit is as follows: Figure 4 As shown;

[0033] When the upper pole voltage of the bipolar DC bus is lower than the lower pole voltage, the full-bridge converter S2 acts as the main switch tube, and S1 acts as a freewheeling diode, drawing power flow from the lower pole of the bipolar DC bus, causing the lower pole voltage of the bipolar DC bus to drop, ultimately achieving the control goal of equal upper and lower pole voltages of the bipolar DC bus. The specific working method of the circuit is as follows: Figure 5 shown.

Claims

1. A novel voltage balancer topology suitable for a bipolar DC microgrid architecture with continuous input and output currents, characterized by: In the novel voltage balancer topology, the DC positive potential bus is connected to the midpoint of the bridge arm 2 of the full-bridge converter, i.e., the emitter of the switch tube S3, through the inductor L1; the DC negative potential bus is connected to the negative electrode of the full-bridge converter, i.e., the emitters of the switch tubes S2 and S4, through the inductor L3; the positive electrode of the intermediate capacitor is connected to the collectors of the switch tubes S1 and S3, the negative electrode of the capacitor is connected to the emitters of the switch tubes S2 and S4, and the inductor L2 is connected to the midpoint of the bridge arm 1 of the full-bridge converter, i.e., the emitter of the switch tube S1, to form a neutral line L. N , thus forming a bipolar bus distribution form; the load R1 and the filter capacitor C1 are connected between the positive potential bus and the neutral line L N The load R2 and filter capacitor C2 are connected to the neutral line L N Between the negative potential busbar.

2. A control strategy for a novel voltage balancer topology structure with continuous input and output currents applicable to the bipolar DC microgrid architecture of claim 1, characterized in that The steps include: Step 1: The intermediate capacitor voltage is pumped up by the combined action of the total DC bus voltage, the L1 inductor current, and the full-bridge converter arm 2, so that the intermediate capacitor voltage is greater than the total DC bus voltage and is controlled to be approximately 120% to 180% of the total bus voltage: When the voltage of the intermediate capacitor is lower than the control target, the full-bridge converter S4 acts as the main switch, S3 acts as the freewheeling diode, and the inductor L1 together form a boost circuit, so that the voltage of the intermediate capacitor can be pumped up. When the voltage of the intermediate capacitor is higher than the control target, the full-bridge converter S3 acts as the main switch, S4 acts as a freewheeling diode, and the inductor L1 together form a step-down circuit, allowing the intermediate capacitor voltage to drop. Step 2: The unbalanced voltage caused by the asymmetric load between the poles is balanced through the combined action of the intermediate capacitor voltage, the L2 inductor current, and the bridge arm 1 of the full-bridge converter: When the upper voltage of the bipolar DC bus is greater than the lower voltage, the full-bridge converter S1 acts as the main switch and S2 acts as a freewheeling diode, injecting power into the lower voltage of the bipolar DC bus. This causes the lower voltage of the bipolar DC bus to rise, ultimately achieving the control goal of equalizing the upper and lower voltages of the bipolar DC bus. When the upper pole voltage of the bipolar DC bus is lower than the lower pole voltage, the full-bridge converter S2 acts as the main switch tube and S1 acts as the freewheeling diode, drawing power flow from the lower pole of the bipolar DC bus, causing the lower pole voltage of the bipolar DC bus to drop, ultimately achieving the control target of equal upper and lower pole voltages of the bipolar DC bus.

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