Bipolar direct current power distribution system architecture based on virtual voltage equalizer and operation method

Abstract

The application provides a bipolar direct current power distribution system architecture and an operation method based on a virtual voltage equalizer. Distributed power supply units and load units are regarded as virtual voltage equalizers in the bipolar direct current system, and the bipolar direct current power distribution system can realize positive and negative voltage equalization without additionally adding voltage equalizers. The virtual voltage equalizer can effectively realize voltage equalization of the positive and negative poles of the bipolar direct current system based on a three-port converter connected by the load and the distributed power supply, and compared with a traditional voltage equalization mode, the operation method based on the virtual voltage equalizer can effectively improve the operation efficiency of the system.

Description

Technical Field

[0001] This invention relates to the field of DC power distribution network and power electronic system control, specifically to a bipolar DC power distribution system architecture and operation method based on a virtual voltage equalizer. Background Technology

[0002] In DC distribution networks, the connection methods for sources, storage, and loads eliminate numerous AC conversion devices, resulting in higher efficiency compared to AC distribution networks and attracting widespread attention. Among these, bipolar DC distribution systems, compared to single-stage DC distribution systems, lower the voltage level to ground and offer more flexible connection methods for sources, storage, and loads, leading to higher system reliability. However, because distributed power sources and loads in bipolar DC distribution systems are often not symmetrically connected, voltage imbalance problems arise. When a bipolar DC system operates with voltage imbalance between the positive and negative poles, it causes problems on the DC load side and generates additional system losses. In severe cases of imbalance, it can even pose significant safety risks to system operation. Therefore, bipolar DC systems require a stable, reliable, and efficient voltage balancing method.

[0003] For voltage balancing in bipolar DC systems, voltage balancers have become a major research focus. For example... Figure 1 The diagram shows the architecture of a bipolar DC system based on a voltage equalizer. The voltage equalizer in the system is responsible for balancing power flow between the positive and negative poles and balancing the voltage between the positive and negative bus poles. It can be used to eliminate the voltage difference between the positive and negative bus poles generated in a bipolar DC system. Integrating one or more voltage equalizers into a bipolar DC system is currently the mainstream method for achieving voltage balancing in bipolar systems.

[0004] Li X, Guo L, Guo Z, et al. Coordinated Control of Multiple Voltage Balancers in a Bipolar DC Microgrid[J]. Transactions of China Electrotechnical Society, 2018. This paper proposes a control strategy for the parallel operation of multiple voltage balancers in a bipolar DC system, employing a droop method and a current feedforward based on a disturbance observer. However, this voltage balancing method relies on additional voltage balancers, fails to effectively utilize the active power distribution capabilities of distributed power sources and DC loads, and the voltage balancers in the bipolar DC system increase the system complexity and losses.

[0005] Li B, Fu Q, Mao S, et al. DC / DC Converter for Bipolar LVdc System With Integrated Voltage Balance Capability[J].IEEE Transactions on Power Electronics, 2020, PP(99):1-1. This paper proposes a DC / DC converter with integrated voltage balance capability by modifying a bipolar CLLC DC / DC converter. Voltage regulation is achieved through voltage equalization and energy storage integration. In contrast, the virtual voltage equalizer does not rely on the bipolar converter and achieves voltage balance by actively distributing power between the positive and negative poles of the load and power source, thus improving system efficiency and reliability. Furthermore, compared to using an additional voltage equalizer, this converter has fewer semiconductors, a smaller total footprint, and lower power loss. However, this paper only studies the converter level and lacks a system-level explanation of how the DC / DC converter with voltage balance function operates, as well as the system-level control strategy. It also lacks an efficiency comparison with a bipolar system based on a voltage equalizer. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this invention is to provide a bipolar DC power distribution system architecture and operation method based on a virtual voltage equalizer.

[0007] According to the present invention, a bipolar DC power distribution system architecture based on a virtual voltage equalizer is provided, comprising: treating distributed power supply units and load units as virtual voltage equalizers in a bipolar DC system, so that the bipolar DC power distribution system can achieve positive and negative voltage balance without the need to add an additional voltage equalizer.

[0008] Preferably, it further includes: an energy storage unit, which is connected to the PN bus via a converter and is responsible for supporting the voltage stability of the PN bus of the bipolar DC system and smoothing out the output fluctuations of the renewable power generation unit.

[0009] Preferably, the distributed power supply unit is connected to the bipolar DC bus via a three-port converter to transmit power to the lower voltage stage in order to maintain the positive and negative voltage balance.

[0010] Preferably, the load unit is connected to the bipolar DC bus via a three-port converter to absorb power from the higher voltage stage in order to maintain the positive and negative voltage balance.

[0011] Preferably, when the bipolar DC power distribution system includes multiple virtual voltage equalizers, the multiple virtual voltage equalizers work together to maintain the voltage balance between the positive and negative poles; when some virtual voltage equalizers are taken out of operation, the remaining virtual voltage equalizers balance the voltage between the positive and negative poles.

[0012] According to the present invention, a method for operating a bipolar DC power distribution system based on a virtual voltage equalizer is provided. The following steps are performed using the aforementioned bipolar DC power distribution system based on a virtual voltage equalizer: the positive and negative voltages of the bipolar DC system are balanced by controlling the virtual voltage equalizer to coordinate the operation of the bipolar DC power distribution system.

[0013] Preferably, the method for controlling the virtual voltage equalizer to coordinate the operation of the bipolar DC power distribution system is as follows:

[0014] Step S1: The duty cycle is obtained through voltage and current closed-loop control based on the voltage difference between the positive and negative terminals, and a PWM waveform is generated after modulation.

[0015] Step S2: Control the neutral inductor current obtained by the three-level DC / DC converter based on the generated PWM waveform;

[0016] Step S3: Calculate and adjust the power difference between the distributed power supply unit and the load unit to output and absorb power to the positive and negative poles based on the control neutral inductor current;

[0017] Step S4: Based on adjusting the power difference between the positive and negative terminals output to and absorbed by the distributed power supply unit and the load unit, the distributed power supply unit or the load unit is used to maintain the voltage balance between the positive and negative terminals.

[0018] Preferably, when the bipolar DC power distribution system contains multiple virtual voltage equalizers, a droop control strategy is used to achieve coordinated operation.

[0019] The droop control strategy is based on assigning droop coefficients to multiple virtual voltage equalizers to achieve positive and negative voltage balance in a bipolar DC system.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The virtual voltage equalizer can effectively achieve voltage balance between the positive and negative poles of the bipolar DC system based on the load and the three-port converter of the distributed power source itself, without the need for an additional voltage equalizer.

[0022] 2. Integrating voltage balancing functionality into distributed power supply and load units simplifies the topology and overall size of bipolar DC systems and reduces system construction costs.

[0023] 3. Compared to bipolar DC distribution systems based on voltage equalizers, bipolar DC distribution systems based on virtual voltage equalizers have higher operating efficiency. Attached Figure Description

[0024] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0025] Figure 1 This is a schematic diagram of a traditional bipolar DC system architecture based on a voltage equalizer (VB).

[0026] Figure 2 This is a schematic diagram of a bipolar DC system architecture based on a virtual voltage equalizer (VVB).

[0027] Figure 3 This is a schematic diagram of the equivalent circuit of a traditional VB bipolar DC system.

[0028] Figure 4 This is a schematic diagram of the equivalent circuit of a VVB bipolar DC system.

[0029] Figure 5 This is a topology diagram of a three-level DC / DC converter on the photovoltaic side.

[0030] Figure 6 This is a control block diagram of a virtual voltage equalizer for a photovoltaic unit.

[0031] Figure 7 This is a topology diagram of a three-level DC / DC converter on the load side;

[0032] Figure 8 This is a block diagram of the virtual voltage equalizer control for the load unit.

[0033] Figure 9 A schematic diagram of the droop control curves for two virtual voltage equalizers;

[0034] Figure 10 This is a schematic diagram of the midline current waveform in a bipolar DC system photovoltaic unit based on a virtual voltage equalizer.

[0035] Figure 11 This is a schematic diagram of the line current waveform in the load unit of a bipolar DC system based on a virtual voltage equalizer.

[0036] Figure 12 This is a schematic diagram of the voltage equalization function test waveform for a bipolar DC system based on a virtual voltage equalizer.

[0037] Figure 13 This is a diagram illustrating the comparison of line losses.

[0038] Figure 14 This is a schematic diagram comparing converter losses.

[0039] Figure 15 This is a diagram showing the comparison of total power loss. Detailed Implementation

[0040] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0041] Example 1

[0042] This invention provides a bipolar DC power distribution system architecture based on a virtual voltage equalizer, comprising: distributed power sources and loads are both connected to a bipolar DC bus via a three-level DC / DC converter, which acts as a virtual voltage equalizer in the bipolar DC system. The three-level DC / DC converter performs functions including voltage level conversion and power transmission on both sides of the converter, as well as maintaining voltage balance between the positive and negative terminals of the DC bus. The three-level DC / DC converter for the distributed power source is as follows: Figure 5 As shown, the load's three-level DC / DC converter is as follows: Figure 7 As shown.

[0043] In this implementation example, the bipolar DC power distribution system operates as follows: (1) the bus voltage of the DC power distribution system remains stable; (2) the output current of the photovoltaic source remains stable at I. PV (3) The voltage across the load is stable at V R (4) The three-level Boost converter connected to the photovoltaic and load adopts the droop control strategy to achieve voltage balance between the positive and negative poles of the bipolar DC power distribution system.

[0044] The three-level DC / DC converters connected to the photovoltaic unit and the load unit have similar topologies, but they perform different functions. The three-level Boost converter on the photovoltaic side realizes the output current I of the photovoltaic source. PV The three-level boost converter on the DC load side stabilizes the voltage across the load at Vs, transferring more power to the stage with the lower voltage to maintain voltage balance between the positive and negative terminals. R It can also absorb more power to the higher voltage stage to maintain the voltage balance between the positive and negative electrodes.

[0045] This invention provides an operation method for a bipolar DC power distribution system architecture based on a virtual voltage equalizer, such as... Figure 6 and Figure 8 As shown, it includes:

[0046] Taking the load side as an example, the control method is explained. The control block diagram on the load side is as follows: Figure 8 As shown. Since the control quantity contains d and d... bWith two degrees of freedom, the control strategy comprises two voltage and current dual closed-loop controls. The control objective of the first voltage and current closed-loop control is the voltage V across the DC load. R The voltage v across the load remains constant. R With voltage reference value V ref The inner loop reference current I is generated by the PI controller. ref The inductor current i on the load side L With reference current I ref The difference is then used by the PI controller to generate d.

[0047] The control objective of the second voltage-current closed-loop control is to achieve control over v. p -v n The droop control achieves the function of a virtual voltage equalizer. The voltage difference v between the positive and negative buses... b Subtract the droop amount R*i b Then, the inner loop reference current I is generated through the PI controller. ref_b Neutral inductor current i b With reference current I ref_b The difference is then used by the PI controller to generate d. b d and d b The duty cycle is generated through addition and subtraction operations, and then modulated to generate a PWM waveform, which controls the on and off states of the switch. The droop control curves of multiple virtual voltage equalizers are shown below. Figure 9 As shown.

[0048] To verify the bipolar DC power distribution system and its operation method and system based on a virtual voltage equalizer proposed in this invention, a simulation model of the bipolar DC system including a virtual voltage equalizer was built based on the MATLAB-Simulink-PLECS Blocket environment.

[0049] This simulation model is a bipolar DC system containing two virtual voltage equalizers. The bipolar DC system includes an energy storage unit (used to maintain a constant voltage on the PN bus), a photovoltaic unit (virtual voltage equalizer 1), a DC load unit (virtual voltage equalizer 2), and several unbalanced loads.

[0050] In this simulation model, the photovoltaic unit and the DC load unit are connected to the DC bus through a three-level Boost converter, acting as two virtual voltage equalizers to jointly maintain the positive and negative voltage balance of the bipolar DC system.

[0051] The parameter design process is illustrated using a photovoltaic unit virtual voltage equalizer as an example. The DC bus voltage level of this bipolar DC distribution system is selected as ±375V, and the energy storage side output voltage V... in 240V, switching frequency f s 50kHz was selected.

[0052] For the selection of inductors and capacitors in the DC / DC converter on the energy storage side, the inductor current ripple ratio δ is selected as 10%, and the capacitor voltage ripple ratio γ is selected as 0.1%.

[0053]

[0054] The parameter calculation processes for the photovoltaic (PV) side and the load side are similar. We will now explain using the calculation of the capacitor and inductor parameters on the PV side. The formulas for calculating the inductance and current ripple on the PV side are as follows:

[0055]

[0056] Based on calculations, the selected photovoltaic-side inductance and capacitance parameters are as follows:

[0057]

[0058] The complete simulation parameters of this bipolar DC simulation system are shown in the table below. The parameters for the converter loss model are a = 5Ω, b = 0.2kV, and c = 0.2kW.

[0059] Table 1 System Parameters of ±375V DC Power Distribution System

[0060]

[0061] Simulation operating conditions

[0062] This simulation demonstrates the DC bus voltage imbalance in a bipolar DC system after applying an unbalanced load and the DC bus voltage recovery after activating a virtual voltage equalizer.

[0063] In the first 0.3 seconds, the entire bipolar DC system operates under balanced load conditions, without activating the virtual voltage equalizers of the photovoltaic unit and the load unit. Its three-level DC / DC converter only performs voltage level conversion and power transmission functions and does not participate in the active power equalization between the positive and negative poles of the bipolar DC system.

[0064] At t = 0.3s, a constant resistive load is added to the Z / N bus of the bipolar DC system. At this time, the load in the bipolar DC system is asymmetrical on both sides of the bus.

[0065] At t=0.5s, the voltage balancing function of the photovoltaic unit and the load unit is activated, that is, the virtual voltage equalizer in the bipolar DC system is activated. The photovoltaic unit actively transmits more power to the low-voltage side bus, and the load actively absorbs more power to the high-voltage side, thereby realizing the voltage balancing of the bipolar DC system.

[0066] Control parameters

[0067] After designing the control parameters in the implementation plan and conducting experimental debugging, the PI parameters of each part are summarized as shown in Table 2.

[0068] Table 2 System Control Parameters

[0069]

[0070]

[0071] Simulation results

[0072] The simulation waveform of the bipolar DC system under operating conditions is as follows: Figure 10 , Figure 11 , Figure 12 As shown, the following are, in order: photovoltaic side neutral current, load side neutral current, P / Z bus voltage, Z / N bus voltage, and voltage difference between the positive and negative buses.

[0073] In the first 0.3 seconds, the bipolar DC system is connected as a symmetrical load. Simulated waveforms show that both the neutral current on the photovoltaic side and the neutral current on the load side are 0. Neither of the two virtual voltage equalizers has activated its voltage equalization function. The P / Z bus voltage is 374.8V, the Z / N bus voltage is 374.8V, and the voltage difference between the positive and negative terminals is 0V. This indicates that the bipolar DC system is in a state of positive and negative voltage balance, and the energy storage unit maintains the P / N bus voltage at the standard voltage of approximately 750V.

[0074] At 0.3s, an unbalanced load was added to the Z / N bus of the bipolar DC system. After a transient process of approximately 0.18s, a voltage difference of 8.0V was generated between the positive and negative buses. The P / Z bus voltage rose to 378V, while the Z / N bus voltage dropped to 370V, indicating that the added unbalanced load caused an imbalance in the positive and negative voltages. At this time, the neutral current on both the photovoltaic side and the load side was 0, indicating that the voltage equalizer in the bipolar DC system had not been activated.

[0075] At 0.5s, the virtual voltage equalizer is activated. The waveform change of the neutral current on the photovoltaic side shows that the photovoltaic unit no longer outputs equalized power to the positive and negative terminals, but instead actively outputs more power to the low-voltage side, assuming the role of a virtual voltage equalizer. Similarly, the waveform change of the neutral current on the load side shows that the load unit no longer absorbs equalized power from the positive and negative terminals, but instead actively absorbs more power to the high-voltage side, assuming the role of a virtual voltage equalizer. After a transient process of approximately 0.3s, the P / N bus voltage and Z / N bus voltage gradually recover to around 375V, and the voltage difference between the positive and negative terminals gradually decreases to 0V. Therefore, the voltage equalization function of the virtual voltage equalizer meets expectations.

[0076] like Figure 13As shown, this is a comparison of line losses between the VVB system and the VB system in the simulation model. Figure 14 As shown, this is a comparison of the converter losses between the VVB system and the VB system in the simulation model. Figure 15 As shown, the simulation model compares the total power loss of the VVB system and the VB system. It can be seen that the total power loss of the VVB system is less than that of the VB system, and the main power saving is due to converter losses. This verifies that the bipolar DC system based on a virtual voltage equalizer proposed in this patent has lower power loss and higher efficiency.

[0077] Example 2

[0078] Example 2 is a preferred example of Example 1.

[0079] This invention addresses the shortcomings of existing voltage equalization control strategies for bipolar DC systems by innovatively proposing a novel bipolar DC distribution system architecture and operation mode based on a virtual voltage equalizer. This invention introduces the concept of a "virtual transformer" and elaborates on the converter-level and system-level control strategies of the virtual voltage equalizer, enabling the bipolar DC system to maintain voltage balance between the positive and negative poles. The bipolar DC distribution system architecture and operation mode based on the virtual voltage equalizer can achieve the following functions:

[0080] To achieve voltage balance between the positive and negative poles of a bipolar DC system;

[0081] Integrating voltage balancing functionality into distributed power supply units and load units improves overall system operating efficiency while saving system construction costs;

[0082] Multiple virtual voltage equalizers can work together to ensure the reliability of voltage control in a bipolar system.

[0083] by Figure 2 The patent illustrates a typical application scenario of a bipolar DC system based on a virtual voltage equalizer. It explains the operating scheme and control strategy, compares the system power loss of the new scheme with that of a bipolar DC system based on a voltage equalizer, and verifies the effectiveness of the virtual voltage equalizer and the high efficiency of the new scheme through a simulation implementation case.

[0084] To address the need for voltage balancing in bipolar DC power distribution systems, a novel virtual voltage equalizer operation mode is proposed. This patent aims to solve the following problems:

[0085] The novel concept of a "virtual voltage equalizer" operating mode was proposed and elaborated, promoting the development of related theories;

[0086] By comparing the system power loss of a bipolar DC system based on a virtual voltage equalizer with that based on a voltage equalizer, the high-efficiency operation characteristics of the bipolar DC system based on a virtual voltage equalizer are verified.

[0087] This invention provides a bipolar DC power distribution system architecture and operation mode based on a virtual voltage equalizer. This scheme employs an operation control strategy based on the novel concept of a "virtual voltage equalizer," where distributed power sources and loads actively participate in positive and negative power distribution to maintain voltage balance between the positive and negative poles. The following sections will elaborate on this... Figure 2 The diagram illustrates the operation mode and high-efficiency operation characteristics of a bipolar DC power distribution system based on a virtual voltage equalizer.

[0088] Bipolar DC power distribution system architecture and operation mode

[0089] Bipolar DC power distribution system architecture as follows Figure 2 As shown. Without loss of generality, assume the system consists of an energy storage unit, a photovoltaic unit, a DC load unit, and several asymmetrical loads. Energy storage, loads, and renewable energy sources located elsewhere have similar equivalent connections due to their parallel connections. The energy storage unit is connected to the PN bus via a two-level DC / DC converter, responsible for maintaining voltage stability on the PN bus of the bipolar DC system. The photovoltaic unit is connected to the bipolar DC bus via a three-level DC / DC converter, injecting power into the bipolar system at the maximum power output of its photovoltaic cells. The DC load unit is connected to the bipolar DC bus via a three-level DC / DC converter. The bipolar DC system includes several asymmetrical loads, which are connected to the DC bus via DC / DC converters.

[0090] The generation of voltage imbalance and traditional voltage equalizer solutions

[0091] A bipolar DC system has PZ, ZN, and PN buses for connecting distributed power sources and loads to the system. Considering practical realities, the distributed power sources and loads connected to the positive and negative buses are not perfectly symmetrical. This power imbalance between the positive and negative poles leads to voltage imbalance. The bus with insufficient power will experience a significant voltage drop. This voltage imbalance in a bipolar DC system results in degraded power quality, increased system losses, and reduced reliability.

[0092] The traditional method for resolving voltage imbalance in bipolar DC systems is to connect one or more voltage equalizers. The voltage equalizers are responsible for transferring power from the higher-voltage bus to the lower-voltage bus in the bipolar DC system, thereby enabling power flow between the positive and negative poles and eliminating the voltage imbalance between the positive and negative poles.

[0093] Explanation of the concept of virtual voltage equalizer

[0094] In bipolar DC power distribution systems, distributed power sources and loads maintain the balance of positive and negative voltages by actively distributing power between the positive and negative poles. Because its control effect is similar to that of a voltage equalizer, active power sources and active loads operating in this mode are called "virtual voltage equalizers" (VVB).

[0095] The active power supply and active load operate in virtual voltage equalizer mode, which not only enables voltage level conversion and power transmission on both sides of the converter, but also effectively balances the positive and negative bus voltages of the two-stage DC distribution system. Therefore, the two-stage DC distribution system no longer requires an additional voltage equalizer to balance the positive and negative voltages. The virtual voltage equalizer-based bipolar DC distribution system improves system operating efficiency and reduces the number of devices. When the bipolar DC distribution system contains multiple virtual voltage equalizers, they can work together to maintain positive and negative voltage balance. Even if one virtual voltage equalizer fails due to a fault, the remaining virtual voltage equalizers in the system can still balance the positive and negative bus voltages. Therefore, the virtual voltage equalizer-based bipolar DC distribution system has high reliability.

[0096] System power loss comparison

[0097] Option 1: Traditional bipolar DC power distribution system based on voltage equalizer

[0098] Option 2: Bipolar DC power distribution system based on virtual voltage equalizer

[0099] The total power loss (Pl) in a bipolar DC system is composed of line losses (Pl). line ) and converter losses (Pl conv Composed of, i.e., Pl = Pl line +Pl conv The total line power loss of the system satisfies P1. line =∑i 2 The system includes two-level and three-level converters. The power loss of the two-level converter can be modeled as a quadratic function PL. conv =ai 2+bi+c, where i is the output current of the converter. Here, a corresponds to the converter's conduction loss coefficient, b corresponds to the converter's switching loss coefficient, and c corresponds to the converter's inherent loss. In this patent, the power loss of the three-level converter is modeled as a parallel model of two two-level converters. Assuming the three port currents of the three-level converter are i1, i2, and i3, where i3 is the neutral current, the converter loss model of the three-level converter is Pl. conv =ai1 2 +bi1+c+ai2 2 +bi2+c.

[0100] First, compare the line losses of Scheme 1 (bipolar DC distribution system based on voltage equalizer) and Scheme 2 (bipolar DC distribution system based on virtual voltage equalizer). The architecture of the bipolar DC distribution system based on voltage equalizer is as follows: Figure 3 As shown, a bipolar DC power distribution system based on a virtual voltage equalizer is as follows: Figure 4 As shown. For ease of comparison, assume the line resistance is r0≈r1≈r2≈r3≈r.

[0101] The essence of a virtual voltage equalizer is to integrate the voltage equalization function of a traditional voltage equalizer into the photovoltaic and load units. In both schemes, the bipolar system generates equal neutral currents i. b In Scheme 1, the neutral current i is handled by a voltage equalizer. b and satisfy

[0102]

[0103] Among them, i b0 i represents the current in the neutral line of the voltage equalizer in the equivalent circuit of Scheme 1. p0 i represents the current in the positive terminal connection of the voltage equalizer in the equivalent circuit of Scheme 1. n0 This represents the current in the negative terminal connection of the voltage equalizer in the equivalent circuit of Scheme 1.

[0104] For the photovoltaic unit, assuming the output power of the photovoltaic is the same in both schemes, we have

[0105]

[0106] Among them, i p1 i represents the current in the positive terminal connection line of the photovoltaic unit in the equivalent circuit of Scheme 2. n1 i represents the current in the negative terminal connection line of the photovoltaic unit in the equivalent circuit of Scheme 2. b1 i represents the current in the line of the photovoltaic unit in the equivalent circuit of Scheme 2. s This represents the output current of the photovoltaic unit in the equivalent circuit of Scheme 1.

[0107] For the load unit, assuming the power absorbed by the load is the same in both schemes, we have

[0108]

[0109] Among them, i p2 i represents the current in the positive terminal connection line of the load unit in the equivalent circuit of scheme 2. n2 i represents the current in the negative terminal connection line of the load unit in the equivalent circuit of scheme 2. b2 i represents the current in the line of the load cell in the equivalent circuit of scheme 2. l This represents the input current of the load unit in the equivalent circuit of Scheme 1.

[0110] In a bipolar DC system based on a virtual voltage equalizer, the photovoltaic (PV) unit and the load unit achieve voltage balancing through droop control. Therefore, the neutral current distribution of the PV unit and the load unit is related to their respective droop coefficients. Let the droop coefficients of the PV unit and the load unit be K1 and K2, respectively. Based on the droop relationship, we can obtain...

[0111]

[0112] Combining the above equations, the port current of the photovoltaic unit in Scheme 2 is:

[0113]

[0114] The port current of the load unit in Scheme 2 is

[0115]

[0116] First, compare the line losses of Scheme 1 and Scheme 2.

[0117] The line loss in Option 1 is

[0118]

[0119] Among them, Pl line_PV1 P1 represents the line loss of the photovoltaic unit circuit in Scheme 1. line_l1 P1 represents the line loss of the load line in Scheme 1. line_VB1 P1 represents the line loss of the voltage equalizer circuit in Scheme 1. line_ess1 i represents the line loss of the energy storage unit circuit in Scheme 1, and i3 represents the output current of the energy storage unit in the equivalent circuit of both schemes.

[0120] The line loss in Option 2 is

[0121]

[0122] The difference in line loss between the two schemes is

[0123]

[0124] Obviously ΔPl line Since the value is less than 0, it can be concluded that Scheme 2 (bipolar DC power distribution system based on virtual voltage equalizer) has lower line loss than Scheme 1 (bipolar DC power distribution system based on voltage equalizer).

[0125] Next, the total converter losses of Scheme 1 and Scheme 2 are compared.

[0126] The converter loss of Scheme 1 is

[0127]

[0128] Among them, Pl conv_PV1 P1 represents the converter loss of the photovoltaic unit in scheme 1. conv_l1 P1 represents the converter loss of the load unit in Scheme 1. conv_VB1 P1 represents the converter loss of the voltage equalizer in scheme 1. conv_ess1 This represents the converter loss of the energy storage unit in Scheme 1.

[0129] The converter loss in Scheme 2 is

[0130]

[0131] The difference in converter losses between the two schemes is

[0132]

[0133] Where 'a' corresponds to the converter's conduction loss coefficient, 'b' to the converter's switching loss coefficient, and 'c' to the converter's inherent loss. It can be seen that, compared to Scheme 1, Scheme 2 increases the converter's conduction loss, while reducing both the converter's switching loss and inherent loss. Analyzing the order of magnitude of the converter losses reveals a reduction in total converter losses. Distribution network voltage levels generally do not exceed 10kV, and power generally does not exceed 5MW, so the neutral current does not exceed 100A. The order of magnitude of 'a' is below 20Ω, 'b' is in the kV range, and 'c' is in the kW range. Therefore, it can be analyzed that Scheme 2 has lower losses. Therefore, the bipolar DC distribution system based on a virtual voltage equalizer proposed in this patent has higher operating efficiency.

[0134] System voltage balancing total capacity

[0135] For traditional bipolar DC systems based on voltage equalizers, the power imbalance capacity that the system can handle depends on the upper limit of the current stress that the switching devices in the voltage equalizer can withstand. For the bipolar DC distribution system based on a virtual voltage equalizer proposed in this patent, the power imbalance capacity that the system can handle is related to the total output power of the source-type VVB and the total absorbed power of the load-type VVB.

[0136] Let the total output power of the photovoltaic units in a bipolar DC power distribution system based on a virtual voltage equalizer be S. PV Let S1 be the power imbalance capacity handled by the photovoltaic unit acting as a virtual voltage equalizer. Let the capacity factor a be the ratio of the power imbalance capacity of the virtual voltage equalizer to the total output power. The capacity factor a satisfies the following formula:

[0137]

[0138] The capacity factor is related to the specific converter topology that implements the VVB function. Taking a three-level boost converter to implement the VVB function as an example, the capacity factor 'a' of the VVB depends on the magnitude of the neutral current of the three-level boost converter. Let I... b I is the neutral current of the three-level Boost converter. PV D is the output current of the photovoltaic panel. b The duty cycle difference between the upper and lower arms of the three-level Boost converter satisfies the following formula:

[0139] I b =2D b I PV

[0140] Let the standard voltage of a single pole of the bus be U. bus Then, the power imbalance capacity S1 undertaken by the photovoltaic unit as a virtual voltage equalizer can be expressed as:

[0141] S1=U bus I b

[0142] The above formulas can be used to obtain...

[0143]

[0144] Because of I PV ∝S PV And the single-pole standard voltage U of the bus bus If we treat it as a constant, we can obtain a∝D b In other words, the capacity factor of the virtual voltage equalizer is proportional to the difference in duty cycle between the upper and lower arms of the converter. When the converter injects all the photovoltaic output power into a single pole, the voltage equalization capacity of the photovoltaic unit VVB reaches its maximum. The voltage balancing capacity of the load unit VVB is analyzed in the same way as that of the photovoltaic unit VVB. When the load unit converter absorbs all power from a certain pole, the voltage balancing capacity of the load unit VVB reaches its maximum.

[0145] The above analysis shows that the power balancing capacity of a source-type VVB is proportional to the total output power, while the power balancing capacity of a load-type VVB is proportional to the total absorbed power. In the bipolar DC system based on a virtual voltage equalizer proposed in this patent, when the imbalance power between the positive and negative poles does not exceed the maximum power balancing capacity of the VVBs in the system, this scheme can effectively utilize the active balancing capabilities of the source and load, eliminating the need for a voltage equalizer in traditional schemes, thereby reducing the construction cost of the bipolar DC system.

[0146] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0147] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A bipolar DC power distribution system architecture based on a virtual voltage equalizer, characterized in that, include: The distributed power supply unit actively adjusts the power difference between its output to the positive and negative poles, and the load unit actively adjusts the power difference between its absorption to the positive and negative poles, thereby maintaining voltage balance between the positive and negative poles in a bipolar system without a voltage equalizer. The distributed power supply unit and the load unit that play the role of voltage equalization act as virtual voltage equalizers. The distributed power supply unit that plays the role of voltage equalization is a power supply type virtual voltage equalizer, and the load unit that plays the role of voltage equalization is a load type virtual voltage equalizer. When a bipolar DC power distribution system contains multiple virtual voltage equalizers, the multiple virtual voltage equalizers work together to maintain the voltage balance between the positive and negative poles; when some virtual voltage equalizers are taken out of operation, the remaining virtual voltage equalizers balance the voltage between the positive and negative poles. When a bipolar DC power distribution system contains multiple virtual voltage equalizers, a droop control strategy is used to achieve coordinated operation. The droop control strategy is based on assigning droop coefficients to multiple virtual voltage equalizers to achieve positive and negative voltage balance in a bipolar DC system.

2. The bipolar DC power distribution system architecture based on a virtual voltage equalizer according to claim 1, characterized in that, Also includes: The energy storage unit is connected to the PN bus via a converter and is responsible for supporting the voltage of the PN bus of the bipolar DC system and smoothing out the output fluctuations of the renewable power generation unit.

3. The bipolar DC power distribution system architecture based on a virtual voltage equalizer according to claim 1, characterized in that, The distributed power unit is connected to the bipolar DC bus via a three-port converter and maintains voltage balance between the positive and negative poles by actively adjusting the power difference output to the positive and negative poles.

4. The bipolar DC power distribution system architecture based on a virtual voltage equalizer according to claim 1, characterized in that, The load unit is connected to the bipolar DC bus via a three-port converter and maintains voltage balance between the positive and negative poles by actively adjusting the power difference absorbed to the positive and negative poles.

5. An operation method for a bipolar DC power distribution system based on a virtual voltage equalizer, characterized in that, The bipolar DC power distribution system architecture based on a virtual voltage equalizer, as described in any one of claims 1 to 4, achieves positive and negative voltage balance of the bipolar DC system by controlling the virtual voltage equalizer to coordinate the operation of the bipolar DC power distribution system.

6. The operation method of the bipolar DC power distribution system based on a virtual voltage equalizer according to claim 5, characterized in that, The control virtual voltage equalizer coordinates the operation of the bipolar DC power distribution system using the following method: Step S1: The duty cycle is obtained through voltage and current closed-loop control based on the voltage difference between the positive and negative terminals, and a PWM waveform is generated after modulation. Step S2: Control the neutral inductor current of the three-level DC / DC converter based on the generated PWM waveform. ; Step S3: Based on the control neutral inductor current Adjust the power difference between the positive and negative terminals output by the distributed power supply unit and the load unit; Step S4: Based on the power difference between the output and absorption of the distributed power supply unit and the load unit to the positive and negative terminals, maintain the voltage balance of the positive and negative terminals using the distributed power supply unit or the load unit.

Images