Medium-voltage direct-current off-grid new energy microgrid and control method thereof

Abstract

The invention discloses a medium-voltage direct-current off-grid new energy microgrid which comprises a plurality of medium-voltage direct-current sub-grids and a medium-voltage direct-current main grid distribution board, in each medium-voltage direct-current sub-grid, a new energy station and an energy storage device are connected to a low-voltage direct-current distribution board, the low-voltage direct-current distribution board is connected to a medium-voltage direct-current sub-grid distribution board through a first DC / DC boost converter, and the medium-voltage direct-current sub-grid distribution board is connected to a medium-voltage direct-current main grid distribution board through a second DC / DC boost converter. The medium-voltage direct-current sub-network distribution board is connected to the medium-voltage direct-current main network distribution board, the medium-voltage direct-current main network distribution board is connected with a DC / DC buck converter through a plurality of direct-current fuses, and the DC / DC buck converter is connected with an adjustable load. The invention further discloses a medium-voltage direct-current off-grid new energy microgrid control method. According to the medium-voltage direct-current off-grid new energy micro-grid and the control method thereof, on the premise that the operation safety and stability of the system are guaranteed, maximum consumption of new energy output is achieved, the cooperative control difficulty of the system is reduced, the dependence of system source-load interaction cooperative control on a communication and energy management system is reduced, and the power supply reliability is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of off-grid direct current new energy microgrid power systems, and in particular to a medium-voltage off-grid direct current new energy microgrid and a control method thereof. Background Art

[0002] In order to make full use of new energy sources such as wind and solar power, they are generally connected to the large power grid through grid connection. The large power grid provides voltage support, and new energy sources such as wind and solar power are connected as power sources to transmit and utilize electric energy. However, this method of absorbing new energy output requires support from the large power grid, and pays grid capacity fees during operation, which increases the system operating costs.

[0003] At the same time, the uncertainty of renewable energy output will also cause varying degrees of impact on the local power grid, reducing the safety and stability of electricity use. The off-grid operation scheme of renewable energy can operate independently of the large power grid, without the need to purchase high-cost and large-capacity mains power supply, and only needs to consider the impact of the fluctuation of renewable energy such as wind and light on the off-grid system and its load. The output of fluctuating power sources such as wind and light can be effectively absorbed through adjustable loads, and the system loss is generally reduced through high-voltage DC transmission and distribution mode. This scheme provides a convenient and reliable way to effectively absorb the output of renewable energy, but the following problems still exist:

[0004] 1. The access of a high proportion of new energy sources such as wind and solar makes the system operation more volatile. As a volatile power source, the output of new energy sources such as wind and solar is greatly affected by natural conditions such as weather and region. Once there are extreme working conditions such as no wind and no light, the system needs to be able to respond quickly and ensure the stability of the main grid voltage;

[0005] 2. "Load follows source" collaborative control. The fluctuating output of new energy power stations needs to be absorbed by adjustable loads. The system collaborative control strategy needs to feed back the volatility of new energy output to the adjustable load end, so that the power of the adjustable load can be adaptively adjusted with the new energy output to achieve the goal of maximizing absorption;

[0006] 3. Power supply reliability. Generally, collaborative control between equipment in different regions or spaces is achieved by adding communication links. Once a communication failure occurs, it will directly affect the control stability of the system. Therefore, it is necessary to minimize the dependence of source-load interactive collaborative control on the communication system, minimize the participation of the energy management system in direct regulation, and respond to the reliability challenges that communication failures may bring to the new energy microgrid.

[0007] In order to solve the above practical problems faced by medium-voltage DC off-grid new energy microgrids for wind and solar power generation, it is necessary to design a medium-voltage DC off-grid new energy microgrid solution to maximize the consumption of new energy output while ensuring the safe and stable operation of the system, reduce the difficulty of system coordinated control, reduce the dependence of the system's source-load interactive coordinated control on the communication and energy management system, and improve power supply reliability. Summary of the invention

[0008] The purpose of the present invention is to address the deficiencies of the above-mentioned technologies and provide a medium-voltage DC off-grid new energy microgrid and a control method thereof, so as to maximize the consumption of new energy output while ensuring the safe and stable operation of the system, reduce the difficulty of system collaborative control, reduce the dependence of the system source-load interactive collaborative control on the communication and energy management system, and improve power supply reliability.

[0009] To achieve the above-mentioned purpose, the medium-voltage DC off-grid new energy microgrid involved in the present invention includes several medium-voltage DC subgrids and a medium-voltage DC main network distribution board, each of the medium-voltage DC subgrids includes a new energy station, an energy storage device, a low-voltage DC distribution board and a medium-voltage DC subgrid distribution board, the new energy station and the energy storage device are connected to the low-voltage DC distribution board, the low-voltage DC distribution board is connected to the medium-voltage DC subgrid distribution board through a first DC / DC boost converter, the medium-voltage DC subgrid distribution boards are all connected to the medium-voltage DC main network distribution board through transmission cables and DC circuit breakers, the medium-voltage DC main network distribution board is connected to a DC / DC buck converter through several DC fuses, and the DC / DC buck converter is connected to an adjustable load.

[0010] Preferably, the new energy station includes a photovoltaic field and a wind field, the photovoltaic array in the photovoltaic field is connected to the low-voltage DC distribution board through a second DC / DC boost converter, the wind turbines in the wind field are connected to the low-voltage DC distribution board through an AC / DC rectifier, and the energy storage device is connected to the low-voltage DC distribution board through a bidirectional energy storage converter.

[0011] Preferably, the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field is a Boost boost converter; the AC / DC rectifier connected to the wind turbine in the wind farm is a three-phase two-level AC / DC rectifier; and the bidirectional energy storage converter connected to the energy storage device is a four-switch Buck-Boost converter.

[0012] Preferably, the first DC / DC boost converter adopts a topology structure with multiple branch inputs connected in parallel and outputs connected in series, a single branch adopts a resonant dual active bridge topology, the front stage is a dual active bridge converter topology, and the rear stage is a Buck circuit based on switched capacitors.

[0013] Preferably, the DC / DC buck converter includes a high-power DC / DC buck converter and a low-power DC / DC buck converter, the high-power DC / DC buck converter is connected to a high-power adjustable load, and the low-power DC / DC buck converter is connected to a plurality of parallel-connected low-power adjustable loads.

[0014] Preferably, the high-power DC / DC buck converter adopts a topology structure with multiple branch inputs connected in series and outputs connected in parallel, a single branch adopts a phase-shifted full-bridge DC buck converter topology based on a switched capacitor, the front stage is a Boost circuit based on a switched capacitor, the rear stage is a phase-shifted full-bridge DC converter topology, and the low-voltage side adopts a diode uncontrolled rectifier circuit structure;

[0015] The low-power DC / DC buck converter adopts a topology structure with multiple branch inputs connected in series and outputs connected in parallel, a single branch adopts a phase-shifted full-bridge DC buck converter topology based on a switched capacitor, the front stage is a Boost circuit based on a switched capacitor, and the rear stage is a phase-shifted full-bridge DC converter topology, wherein the isolation transformer is in a multi-winding form, and the low-voltage side of the transformer outputs multiple independent winding interfaces through a diode uncontrolled rectifier to achieve multi-channel independent output.

[0016] A control method for a medium-voltage DC off-grid new energy microgrid, wherein the new energy station adopts a constant power control mode to output the current maximum power in real time, and the energy storage device adopts a voltage and current double closed-loop control to provide voltage support for the low-voltage DC bus on the low-voltage DC distribution board;

[0017] The first DC / DC boost converter converts the low-voltage DC power output by the new energy station into medium-voltage DC power, controls the energy storage to not output power, and uses voltage rise control to feed back the output characteristics of the new energy station to the medium-voltage DC bus on the medium-voltage DC main grid distribution board, so that when the output of the new energy station is increased, the voltage of the medium-voltage DC bus is raised;

[0018] The DC / DC step-down converter feeds back the DC input voltage fluctuation to the output end, so that the power of the adjustable load can adaptively change with the medium voltage DC bus.

[0019] Preferably, in the new energy station, the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field and the AC / DC rectifier connected to the wind turbine in the wind farm adopt a constant power control mode;

[0020] In the constant power control of the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field, U PV1 and I PV1 is the output voltage and current of a single photovoltaic branch. The maximum power output capacity P of the current photovoltaic array is calculated by the maximum power tracking control. pvmax The corresponding output voltage reference value UPV_ref , output voltage reference value U PV_ref The output voltage U of the photovoltaic array PV1 The difference between the two is passed through the PI controller to obtain the modulation signal D of the second DC / DC boost converter. PV ;

[0021] In the constant power control of AC / DC rectifiers connected to wind turbines in wind farms, P wt is the active power output of the wind turbine, U wt is the effective value of the three-phase voltage output of the wind turbine, and the reference value of the active power output of the wind turbine is P wtref , U wtref is the reference value of the three-phase voltage effective value of the wind turbine output, and the current maximum power output P of the wind turbine wtmax is calculated by maximum power tracking, as P wtref The upper limit, P wt With P wtmax , U wt with U wtref The output result after the difference of the PI controller is i wtdref and i wtqref ,i wtdref and i wtqref are the d-axis current reference value and q-axis current reference value of the AC / DC rectifier, i wtd and i wtq are the d-axis component and q-axis component of the output current of the AC / DC rectifier, respectively, i wtd and i wtdref After the difference passes through the PI controller, the output result is subtracted from ω s L md i wtq +U wtrd Get the modulation voltage signal U of the AC / DC rectifier on the d-axis wtd_ref ,i wtq and i wtqref After the difference passes through the PI controller, the output result is added with ω s L mq i wtd +U wtrq , get the modulation voltage signal U of the AC / DC rectifier on the q axis wtq_ref , where U wtrd and U wtrq are the d-axis voltage reference value and q-axis voltage reference value of the AC / DC rectifier, L md and L mq is the inductance of the stator in the d-axis and q-axis;

[0022] The bidirectional energy storage converter connected to the energy storage device adopts voltage and current double closed-loop control, and the voltage outer loop of the double closed-loop control adopts PI control. Hes with U ref The difference between the output current and the reference value I after PI control ref , U Hes and U ref are the output voltage measurement value and output voltage reference value of the bidirectional energy storage converter, I es1 with I ref The difference is controlled by PI to obtain the modulation wave signal D of SVPWM control. es , I es1 is the output current measurement value of the bidirectional energy storage converter.

[0023] Preferably, the first DC / DC boost converter adopts a voltage rise control strategy based on secondary voltage conversion, the primary voltage conversion controls the output capacitor voltage through a voltage single loop, the secondary voltage conversion is controlled by a voltage and current double closed loop, and a voltage rise control is introduced in the voltage outer loop of the secondary voltage conversion, the voltage rise means that the output current of the energy storage device is output to 0 after PI control, I es with I esref The difference is output after PI control pf , I es is the output current measurement value of the energy storage device, I esref is the reference value of the output current measurement value of the energy storage device. In order to control the output power of the energy storage device to 0, I esref Set to 0, U pf As the feedback of the voltage outer loop of the first DC / DC boost converter, the given value of the voltage outer loop is U ref The actual output voltage of the first DC / DC boost converter is U out , U out with U ref The difference plus the feedback value U pf The reference value of the output current after PI control is I ref , the actual output current value of the first DC / DC boost converter is I1, I ref The difference between I1 and I2 is controlled by PI to obtain the modulation wave signal D of SVPWM control. M .

[0024] Preferably, the DC / DC buck converter adopts a constant input DC voltage + constant output current control strategy based on secondary voltage conversion. The primary voltage conversion controls the output capacitor voltage through a voltage single loop, and the secondary voltage conversion is controlled by input DC voltage + output current double closed loop, and a droop control loop is introduced in the voltage outer loop of the double closed loop control. The outer loop of the double closed loop control adopts voltage PI control to stabilize the capacitor voltage at the DC input end. The voltage outer loop controls the capacitor voltage value U at the DC input end. dc , with a given voltage reference value U dc_ref The difference is compared and the PI controller outputs the reference command value I of the DC current. ref The inner loop of the dual closed-loop control uses current PI control to stabilize the output current of the DC / DC buck converter. The current inner loop collects the output DC current I of the DC / DC buck converter. L with I ref After comparison, the difference is used as the control signal of SVPWM after passing through the PI controller.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1. Under the premise of ensuring the safe and stable operation of the system, the maximum consumption of new energy output can be achieved, the difficulty of system coordinated control can be reduced, the dependence of the system source-load interactive coordinated control on the communication and energy management system can be reduced, and the power supply reliability can be improved;

[0027] 2. Strong system scalability: New energy stations can be independently networked with the voltage stabilization support of their own energy storage devices, and can be plug-and-play. They can be put into or removed at any time according to actual needs, which enhances the scalability of the system;

[0028] 3. Single-point failure is not likely to cause power outage in the main network: New energy stations are distributedly connected to the DC main network and can be independently networked and operated. When a single DC subnet fails, other subnets in the system can still operate normally without causing power outage in the main network;

[0029] 3. High utilization rate of new energy: The system "source-load interaction" collaborative control strategy is designed, so that the adjustable load in the system can adaptively adjust its consumption power as the output of new energy changes, and basically achieve 100% consumption of new energy;

[0030] 4. System control does not rely on communication and energy management systems: The interactive collaborative control of the source and load of the entire system is completely implemented locally on the device, which reduces the system's dependence on energy management and communication, and avoids the risk of system instability caused by system communication failures or energy management system failures. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of a medium voltage DC off-grid new energy microgrid of the present invention;

[0032] Figure 2 A schematic diagram of a preferred embodiment of a medium voltage DC off-grid new energy microgrid of the present invention;

[0033] Figure 3 is a circuit diagram of a second DC / DC boost converter in the present invention;

[0034] Figure 4 is a circuit diagram of an AC / DC rectifier in the present invention;

[0035] Figure 5 is a circuit diagram of a bidirectional energy storage converter in the present invention;

[0036] Figure 6 is a circuit diagram of a first DC / DC boost converter in the present invention;

[0037] Figure 7 It is a circuit diagram of a high-power DC / DC buck converter in the present invention;

[0038] Figure 8 It is a circuit diagram of a low-power DC / DC buck converter of the present invention;

[0039] Fig. 9 It is a strategy diagram of the maximum power tracking control adopted by the new energy station of the present invention;

[0040] Fig.10 It is a strategy diagram of voltage and current dual closed-loop control adopted by the bidirectional energy storage converter in the present invention;

[0041] Fig.11 is a strategy diagram of voltage rise control adopted by the first DC / DC boost converter in the present invention;

[0042] Fig.12 It is a strategy diagram of the DC / DC buck converter in the present invention using constant input DC voltage + constant output DC current control. DETAILED DESCRIPTION

[0043] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] like Figure 1 As shown, a medium voltage DC off-grid new energy microgrid includes m medium voltage DC subgrids and a medium voltage DC main grid distribution board, each medium voltage DC subgrid includes a new energy station G m , an energy storage device C m , a low voltage DC distribution board and a medium voltage DC subnet distribution board, new energy station G m and energy storage device C mThe low voltage DC distribution board is connected to the medium voltage DC sub-network distribution board through the first DC / DC boost converter. The medium voltage DC sub-network distribution board is connected to the medium voltage DC sub-network distribution board through the transmission cable and DC circuit breaker H m The medium voltage DC main grid distribution board is connected to the medium voltage DC main grid distribution board, which is connected to a DC / DC buck converter through a plurality of DC fuses, and the DC / DC buck converter is connected to an adjustable load.

[0045] The DC / DC buck converter includes k high-power DC / DC buck converters and r low-power DC / DC buck converters. The high-power DC / DC buck converter is connected to a high-power adjustable load, and the low-power DC / DC buck converter is connected to p parallel low-power adjustable loads.

[0046] Figure 2 As a preferred embodiment, the new energy station includes a photovoltaic power plant and a wind power plant, wherein WF1, WF2, ... WF n There are n wind farms, PV1, PV2, …PV m There are m photovoltaic fields, C1, C2, ...C m+n There are (m+n) energy storage devices, H1, H2, ...H m+n There are (m+n) DC circuit breakers, F1, F2, ... F k are k DC fuses suitable for high-power DC / DC step-down converters, F k+1 、F k+2 ,…F k+r The invention is a DC fuse suitable for a low-power DC / DC step-down converter.

[0047] In this embodiment, m+n medium-voltage DC subnets and a medium-voltage DC main grid distribution board are included. Each medium-voltage DC subnet includes a new energy station, an energy storage device C m , a low voltage DC distribution board and a medium voltage DC sub-grid distribution board. The new energy station includes a total of n wind farms (WF1, WF2, ... WF n ), m photovoltaic fields (PV1, PV2, ...PV m ), wind farms and photovoltaic farms use their own energy storage devices (C1, C2, ... C m+n ) is supported by the low-voltage DC distribution board, which is connected to the medium-voltage DC sub-network distribution board through the first DC / DC boost converter. The medium-voltage DC sub-network distribution board is connected through transmission cables and DC circuit breakers (H1, H2, ...H m+n ) is connected to the medium voltage DC main grid distribution board.

[0048] The medium voltage DC main grid distribution board is connected through k DC fuses (F1, F2, ... F k) is connected to k high-power DC / DC step-down converters to supply power to high-power adjustable loads, and the medium-voltage DC main grid distribution board is connected to k high-power DC / DC step-down converters to supply power to high-power adjustable loads. k+1 、F k+2 ,…F k+r ) is connected to r low-power DC / DC buck converters to supply power to a low-power adjustable load, wherein the high-power DC / DC buck converter is connected to one high-power adjustable load, and the low-power DC / DC buck converter is connected to three parallel-connected low-power adjustable loads.

[0049] In this embodiment, the photovoltaic array in the photovoltaic field is connected to the low-voltage DC distribution board through the second DC / DC boost converter, the wind turbine in the wind farm is connected to the low-voltage DC distribution board through the AC / DC rectifier, and the energy storage device is connected to the low-voltage DC distribution board through the bidirectional energy storage inverter.

[0050] Specifically, Figure 3 As shown, the second DC / DC boost converter is a Boost boost converter, which realizes power conversion by controlling the switch tube S; Figure 4 As shown, the AC / DC rectifier is a three-phase two-level AC / DC rectifier, which realizes power conversion by controlling the switch tubes G1-G6. Figure 5 As shown, in this embodiment, the energy storage device is a lithium battery system, the bidirectional energy storage converter connected to the energy storage device is a four-switch Buck-Boost converter, the energy storage device is connected to the low-voltage DC distribution board through the bidirectional energy storage converter, and the power conversion is realized by controlling the on-off of the IGBT devices (S1-S4), that is, the energy flows bidirectionally between the low-voltage DC distribution board and the energy storage device, and the low-voltage DC distribution board is connected to the medium-voltage DC subnet distribution board through the first DC / DC boost converter. In the figure, U es and I es1 is the output voltage and output current of the energy storage device, L and C are the inductance and output capacitance of the bidirectional energy storage converter, respectively, U Hes is the output voltage of the high-voltage side of the bidirectional energy storage converter.

[0051] like Figure 6 As shown, the first DC / DC boost converter adopts a topology structure with multiple branches input in parallel and output in series, a single branch adopts a resonant dual active bridge topology, the front stage is a dual active bridge converter topology, and the back stage is a Buck circuit based on a switched capacitor. The boost of the first DC / DC boost converter is achieved by controlling the on-off of G1-G8, S1-S4, and T1-T2. Figure 6 In the figure, L1 and C1 are the filter inductance and capacitance of the circuit, C1-C 2n is the clamping capacitor of the circuit.

[0052] like Figure 7As shown in the figure, the high-power DC / DC buck converter adopts a topology structure with multiple branches input in series and output in parallel. A single branch adopts a phase-shifted full-bridge DC buck converter topology based on a switched capacitor. The front stage is a Boost circuit based on a switched capacitor, and the rear stage is a phase-shifted full-bridge DC converter topology. The low-voltage side adopts a diode uncontrolled rectifier circuit structure. By controlling G1-G4 and T1-T 2n The on-off of the Figure 7 In, L r1 -L rn , C1-C n is the filter inductance and capacitance of the circuit, C m1 -C mn It is the support capacitor at the input end of the high-power DC / DC step-down converter.

[0053] like Figure 8 As shown in the figure, the low-power DC / DC buck converter adopts a topology structure with multiple branches input in series and output in parallel. A single branch adopts a phase-shifted full-bridge DC buck converter topology based on a switched capacitor. The front stage is a Boost circuit based on a switched capacitor, and the back stage is a phase-shifted full-bridge DC converter topology. The isolation transformer is a four-winding form. The low-voltage side of the transformer outputs three independent winding interfaces through a diode uncontrolled rectifier to achieve three independent outputs. By controlling G1-G4 and T1-T 2n The on-off of the Figure 8 In the figure, L1 and C1 are the filter inductance and capacitance of the circuit, C m1 -C mn It is the support capacitor at the input end of the low-power DC / DC step-down converter.

[0054] In the control method of the low-voltage DC off-grid new energy microgrid in this embodiment, the new energy station adopts a constant power control mode to output the current maximum power in real time, and the energy storage device adopts a voltage and current double closed-loop control to provide voltage support for the low-voltage DC bus on the low-voltage DC distribution board;

[0055] The first DC / DC boost converter converts the low-voltage DC power output by the new energy station into medium-voltage DC power, controls the energy storage to not output power, and uses voltage rise control to feed back the output characteristics of the new energy station to the medium-voltage DC bus on the medium-voltage DC main grid distribution board, so that when the output of the new energy station is increased, the voltage of the medium-voltage DC bus is raised;

[0056] The DC / DC step-down converter feeds back the DC input voltage fluctuation to the output end, so that the power of the adjustable load can adaptively change with the medium voltage DC bus.

[0057] Among them, in the new energy station, the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field and the AC / DC rectifier connected to the wind turbine in the wind farm adopt a constant power control mode;

[0058] like Fig. 9 As shown, in the constant power control of the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field, U PV1 and I PV1 is the output voltage and current of a single photovoltaic branch. The maximum power output capacity P of the current photovoltaic array is calculated by the maximum power tracking control. pvmax The corresponding output voltage reference value U PV_ref , output voltage reference value U PV_ref The output voltage U of the photovoltaic array PV1 The difference between the two is passed through the PI controller to obtain the modulation signal D of the second DC / DC boost converter. PV ;

[0059] In the constant power control of AC / DC rectifiers connected to wind turbines in wind farms, P wt is the active power output of the wind turbine, U wt is the effective value of the three-phase voltage output of the wind turbine, and the reference value of the active power output of the wind turbine is P wtref , U wtref is the reference value of the three-phase voltage effective value of the wind turbine output, and the current maximum power output P of the wind turbine wtmax is calculated by maximum power tracking, as P wtref The upper limit, P wt With P wtmax , U wt with U wtref The output result after the difference of the PI controller is i wtdref and i wtqref ,i wtdref and i wtqref are the d-axis current reference value and q-axis current reference value of the AC / DC rectifier, i wtd and i wtq are the d-axis component and q-axis component of the output current of the AC / DC rectifier, respectively, i wtd and i wtdref After the difference passes through the PI controller, the output result is subtracted from ω s L md i wtq +U wtrd Get the modulation voltage signal U of the AC / DC rectifier on the d-axis wtd_ref ,i wtq and i wtqref After the difference passes through the PI controller, the output result is added with ω sL mq i wtd +U wtrq , get the modulation voltage signal U of the AC / DC rectifier on the q axis wtq_ref , where U wtrd and U wtrq are the d-axis voltage reference value and q-axis voltage reference value of the AC / DC rectifier, L md and L mq is the inductance of the stator in the d-axis and q-axis.

[0060] In this embodiment, the wind turbine generator set in the wind farm may be a double-fed induction asynchronous generator or a permanent magnet direct-drive wind turbine generator.

[0061] like Fig.10 As shown, the bidirectional energy storage converter connected to the energy storage device adopts voltage and current double closed-loop control, and the voltage outer loop of the double closed-loop control adopts PI control. Hes with U ref The difference between the output current and the reference value I after PI control ref , U Hes and U ref are the output voltage measurement value and output voltage reference value of the bidirectional energy storage converter, I es1 with I ref The difference is controlled by PI to obtain the modulation wave signal D of SVPWM control. es , I es1 is the output current measurement value of the bidirectional energy storage converter.

[0062] like Fig.11 As shown, the first DC / DC boost converter adopts a voltage rise control strategy based on two-stage voltage conversion. The first-stage voltage conversion controls the output capacitor voltage through a voltage single loop, and the second-stage voltage conversion is controlled by a voltage and current double closed loop. In addition, a voltage rise control is introduced into the voltage outer loop of the second-stage voltage conversion. The voltage rise refers to the output current of the energy storage device being 0 after PI control. I es with I esref The difference is output after PI control pf , I es is the output current measurement value of the energy storage device, I esref is the reference value of the output current measurement value of the energy storage device. In order to control the output power of the energy storage device to 0, I esref Set to 0, U pf As the feedback of the voltage outer loop of the first DC / DC boost converter, the given value of the voltage outer loop is U ref The actual output voltage of the first DC / DC boost converter is U out , U out with U ref The difference plus the feedback value Upf The reference value of the output current after PI control is I ref , the actual output current value of the first DC / DC boost converter is I1, I ref The difference between I1 and I2 is controlled by PI to obtain the modulation wave signal D of SVPWM control. M . Among them, FLAG is the flag bit that indicates that the system regulation capability boundary is exceeded. If FLAG=0, it means that it is within the regulation capability range, and if FLAG=1, it means that it is outside the regulation capability range. When the microgrid system is within the automatic regulation capability range of "source-load interaction", the active power output of the energy storage system is controlled to zero through dispatching instructions to maximize the utilization of renewable energy. If the operation of the microgrid system exceeds the automatic regulation capability range of "source-load interaction", it is adjusted through energy management.

[0063] like Fig.12 As shown in FIG. 1 , the DC / DC buck converter adopts a constant input DC voltage + constant output current control strategy based on two-stage voltage conversion. The first-stage voltage conversion controls the output capacitor voltage through a single voltage loop, and the second-stage voltage conversion is controlled by a double closed-loop control of input DC voltage + output current. A droop control loop is introduced in the voltage outer loop of the double closed-loop control. The outer loop of the double closed-loop control adopts voltage PI control to stabilize the capacitor voltage at the DC input end. The voltage outer loop adjusts the capacitor voltage value U at the DC input end to the value of the capacitor voltage U. dc , with a given voltage reference value U dc_ref The difference is compared and the PI controller outputs the reference command value I of the DC current. ref The inner loop of the dual closed-loop control uses current PI control to stabilize the output current of the DC / DC buck converter. The current inner loop collects the output DC current I of the DC / DC buck converter. L with I ref After comparison, the difference is used as the control signal of SVPWM after passing through the PI controller.

[0064] In the above implementation cases, the descriptions of various embodiments have different emphases. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0065] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the embodiments described above, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments can still be modified, or some of the technical features can be replaced by equivalents. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

Claims

1. A medium voltage DC off-grid new energy microgrid, characterized by: It includes several medium-voltage DC subnets and a medium-voltage DC main network distribution board, each of the medium-voltage DC subnets includes a new energy site, an energy storage device, a low-voltage DC distribution board and a medium-voltage DC subnet distribution board, the new energy site and the energy storage device are connected to the low-voltage DC distribution board, the low-voltage DC distribution board is connected to the medium-voltage DC subnet distribution board through a first DC / DC boost converter, the medium-voltage DC subnet distribution boards are connected to the medium-voltage DC main network distribution board through transmission cables and DC circuit breakers, the medium-voltage DC main network distribution board is connected to a DC / DC buck converter through several DC fuses, and the DC / DC buck converter is connected to an adjustable load.

2. The medium voltage DC off-grid new energy microgrid as claimed in claim 1, characterized in that: The new energy station includes a photovoltaic field and a wind field. The photovoltaic array in the photovoltaic field is connected to the low-voltage DC distribution board through a second DC / DC boost converter, the wind turbines in the wind field are connected to the low-voltage DC distribution board through an AC / DC rectifier, and the energy storage device is connected to the low-voltage DC distribution board through a bidirectional energy storage converter.

3. The medium voltage DC off-grid new energy microgrid as claimed in claim 2, characterized in that: The second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field is a Boost boost converter; the AC / DC rectifier connected to the wind turbine in the wind farm is a three-phase two-level AC / DC rectifier; The bidirectional energy storage converter connected to the energy storage device is a four-switch Buck-Boost converter.

4. The medium voltage DC off-grid new energy microgrid as claimed in claim 1, characterized in that: The first DC / DC boost converter adopts a topology structure with multiple branch inputs in parallel and outputs in series, a single branch adopts a resonant dual active bridge topology, the front stage is a dual active bridge converter topology, and the rear stage is a Buck circuit based on switched capacitors.

5. The medium voltage DC off-grid new energy microgrid as claimed in claim 1, characterized in that: The DC / DC buck converter comprises a high-power DC / DC buck converter and a low-power DC / DC buck converter. The high-power DC / DC buck converter is connected to a high-power adjustable load, and the low-power DC / DC buck converter is connected to a plurality of parallel-connected low-power adjustable loads.

6. The medium voltage DC off-grid new energy microgrid as claimed in claim 5, characterized in that: The high-power DC / DC buck converter adopts a topology structure with multiple branches connected in series and output in parallel, a single branch adopts a phase-shifted full-bridge DC buck converter topology based on a switched capacitor, the front stage is a Boost circuit based on a switched capacitor, the rear stage is a phase-shifted full-bridge DC converter topology, and the low-voltage side adopts a diode uncontrolled rectifier circuit structure; The low-power DC / DC buck converter adopts a topology structure with multiple branch inputs connected in series and outputs connected in parallel, a single branch adopts a phase-shifted full-bridge DC buck converter topology based on a switched capacitor, the front stage is a Boost circuit based on a switched capacitor, and the rear stage is a phase-shifted full-bridge DC converter topology, wherein the isolation transformer is in a multi-winding form, and the low-voltage side of the transformer outputs multiple independent winding interfaces through a diode uncontrolled rectifier to achieve multi-channel independent output.

7. A control method for a medium voltage DC off-grid new energy microgrid according to any one of claims 1 to 6, characterized in that: The new energy station adopts a constant power control mode to output the current maximum power in real time. The energy storage device adopts a voltage and current double closed-loop control to provide voltage support for the low-voltage DC bus on the low-voltage DC distribution board. The first DC / DC boost converter converts the low-voltage DC power output by the new energy station into medium-voltage DC power, controls the energy storage to not output power, and uses voltage rise control to feed back the output characteristics of the new energy station to the medium-voltage DC bus on the medium-voltage DC main grid distribution board, so that when the output of the new energy station is increased, the voltage of the medium-voltage DC bus is raised; The DC / DC step-down converter feeds back the DC input voltage fluctuation to the output end, so that the power of the adjustable load can adaptively change with the medium voltage DC bus.

8. The control method of the medium voltage DC off-grid new energy microgrid as claimed in claim 7, characterized in that: In the new energy station, the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field and the AC / DC rectifier connected to the wind turbine in the wind farm adopt a constant power control mode; In the constant power control of the second DC / DC boost converter connected to the photovoltaic array in the photovoltaic field, U PV1 and I PV1 is the output voltage and current of a single photovoltaic branch. The maximum power output capacity P of the current photovoltaic array is calculated by the maximum power tracking control. pvmax The corresponding output voltage reference value U PVref , output voltage reference value U PVref The output voltage U of the photovoltaic array PV1 The difference between the two is passed through the PI controller to obtain the modulation signal D of the second DC / DC boost converter. PV ; In the constant power control of AC / DC rectifiers connected to wind turbines in wind farms, P wt is the active power output of the wind turbine, U wt is the effective value of the three-phase voltage output of the wind turbine, and the reference value of the active power output of the wind turbine is P wtref , U wtref is the reference value of the three-phase voltage effective value of the wind turbine output, and the current maximum power output P of the wind turbine wtmax is calculated by maximum power tracking, as P wtref The upper limit, P wt With P wtmax , U wt with U wtref The output result after the difference of the PI controller is i wtdref and i wtqref ,i wtdref and i wtqref are the d-axis current reference value and q-axis current reference value of the AC / DC rectifier, i wtd and i wtq are the d-axis component and q-axis component of the output current of the AC / DC rectifier, respectively, i wtd and i wtdref After the difference passes through the PI controller, the output result is subtracted from ω s L md i wtq +U wtrd Get the modulation voltage signal U of the AC / DC rectifier on the d-axis wtd_ref ,i wtq and i wtqref After the difference passes through the PI controller, the output result is added with ω s L mq i wtd +U wtrq , get the modulation voltage signal U of the AC / DC rectifier on the q axis wtq_ref , where U wtrd and U wtrq are the d-axis voltage reference value and q-axis voltage reference value of the AC / DC rectifier, L md and L mq is the inductance of the stator in the d-axis and q-axis; The bidirectional energy storage converter connected to the energy storage device adopts voltage and current double closed-loop control, and the voltage outer loop of the double closed-loop control adopts PI control. Hes with U ref The difference between the output current and the reference value I after PI control ref , U Hes and U ref are the output voltage measurement value and output voltage reference value of the bidirectional energy storage converter, I es1 with I ref The difference is controlled by PI to obtain the modulation wave signal D of SVPWM control. es , I es1 is the output current measurement value of the bidirectional energy storage converter.

9. The control method of the medium voltage DC off-grid new energy microgrid as claimed in claim 7, characterized in that: The first DC / DC boost converter adopts a voltage rise control strategy based on secondary voltage conversion. The primary voltage conversion controls the output capacitor voltage through a voltage single loop, and the secondary voltage conversion is controlled by a voltage and current double closed loop. In addition, a voltage rise control is introduced into the voltage outer loop of the secondary voltage conversion. The voltage rise refers to the output current of the energy storage device being 0 after PI control. es with I esref The difference is output after PI control pf , I es is the output current measurement value of the energy storage device, I esref is the reference value of the output current measurement value of the energy storage device. In order to control the output power of the energy storage device to 0, I esref Set to 0, U pf As the feedback of the voltage outer loop of the first DC / DC boost converter, the given value of the voltage outer loop is U ref The actual output voltage of the first DC / DC boost converter is U out , U out with U ref The difference plus the feedback value U pf The reference value of the output current after PI control is I ref , the actual output current value of the first DC / DC boost converter is I1, I ref The difference between I1 and I2 is controlled by PI to obtain the modulation wave signal D of SVPWM control. M .

10. The medium voltage DC off-grid new energy microgrid control method according to claim 7, characterized in that: The DC / DC step-down converter adopts a constant input DC voltage + constant output current control strategy based on two-stage voltage conversion. The first-stage voltage conversion controls the output capacitor voltage through a single voltage loop, and the second-stage voltage conversion is controlled by a double closed-loop control of input DC voltage + output current. A droop control loop is introduced into the voltage outer loop of the double closed-loop control. The outer loop of the double closed-loop control adopts voltage PI control to stabilize the capacitor voltage at the DC input end. The voltage outer loop adjusts the capacitor voltage value U at the DC input end. dc , with a given voltage reference value U dc_ref The difference is compared and the PI controller outputs the reference command value I of the DC current. ref The inner loop of the dual closed-loop control uses current PI control to stabilize the output current of the DC / DC buck converter. The current inner loop collects the output DC current I of the DC / DC buck converter. L with I ref After comparison, the difference is used as the control signal of SVPWM after passing through the PI controller.

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