Construction network type energy storage converter and control method thereof, and construction network following integration system

Abstract

The invention discloses a constructed network type energy storage converter and a control method thereof, and a constructed network integration system. The constructed network integration system comprises a transformer which is provided with a first connecting end, a second connecting end and a third connecting end; the direct current side of the first inverter is connected with the new energy generator, and the alternating current side of the first inverter is connected with the first connecting end, one end of the first switch and one end of the second switch; the direct current side of the second inverter is connected with the energy storage unit, and the alternating current side of the second inverter is connected with one end of the third switch and the other end of the second switch; the other end of the third switch is connected with the second connecting end; the third connecting end is connected with the other end of the first switch and the power grid; and the controller is used for controlling on or off of the first switch, the second switch and the third switch, and is also used for switching a control strategy of the second inverter. The weak network adaptability of the network-following converter and the fault ride-through capability of network-following integration can be effectively improved.

Description

Technical Field

[0001] The present application relates to the field of power grid technology, and in particular to a grid-connected energy storage converter and a control method thereof, and a grid-connected fusion system. Background Art

[0002] As the proportion of clean energy such as wind and solar power in the power system continues to increase, modern power systems are developing towards a high proportion of renewable energy. Traditional converters use grid-following control, which has good current control capabilities, but also has poor voltage and frequency support capabilities, and is prone to instability under weak power grids. For this reason, a grid-forming control that simulates the dynamics of synchronous generators has been proposed, which has good voltage and frequency support capabilities and can still maintain stable operation under weak power grids. However, when a grid fault occurs, the grid-forming converter is prone to overcurrent and transient loss of synchronization during fault ride-through.

[0003] At present, the fault ride-through for grid-following converters is quite mature, but due to the difference with grid-building converters, it cannot be directly applied to grid-building converters. In addition, for the fault ride-through of grid-building converters, such as virtual impedance method, current saturation method, grid-building cutting grid, etc., these methods are mostly aimed at symmetrical faults and focus on a single grid-building converter. Therefore, how to improve the fault ride-through capability of grid-building fusion still needs further research. Summary of the invention

[0004] In view of this, the purpose of the present application is to provide a grid-type energy storage converter and a control method thereof, as well as a grid-connected fusion system, so as to effectively improve the weak grid adaptability of the grid-connected converter and the fault ride-through capability of the grid-connected fusion system.

[0005] The technical solutions adopted by this application to solve the above technical problems are as follows:

[0006] On one hand, the present application provides a grid-connected fusion system, which includes a first inverter, a second inverter, a first switch, a second switch, a third switch, a transformer, and a controller, wherein the transformer has a first connection end, a second connection end, and a third connection end;

[0007] The DC side of the first inverter is connected to the new energy generator, and the AC side of the first inverter is connected to the first connection end, one end of the first switch, and one end of the second switch; the DC side of the second inverter is connected to the energy storage unit, and the AC side of the second inverter is connected to one end of the third switch and the other end of the second switch; the other end of the third switch is connected to the second connection end, and the third connection end is connected to the other end of the first switch and the power grid;

[0008] The controller is used to control the on or off of the first switch, the second switch and the third switch, and is also used to switch the control strategy of the second inverter.

[0009] On the other hand, the present application provides a grid-type energy storage converter, the grid-type energy storage converter comprising a second inverter and a controller;

[0010] The DC side of the second inverter is connected to the energy storage unit, the AC side of the second inverter is connected to one end of the third switch and the other end of the second switch; the other end of the third switch is connected to the second connection end of the transformer, one end of the second switch is connected to the AC side of the first inverter and the first connection end of the transformer, one end of the first switch, and the DC side of the first inverter is connected to the new energy generator; the other end of the first switch is connected to the third connection end of the transformer and the power grid;

[0011] The controller is used to control the on or off of the first switch, the second switch and the third switch, and is also used to switch the control strategy of the second inverter.

[0012] On the other hand, the present application also provides a control method for a grid-type energy storage converter, wherein the grid-type energy storage converter includes a second inverter; the DC side of the second inverter is connected to the energy storage unit, the AC side of the second inverter is connected to one end of the third switch and the other end of the second switch; the other end of the third switch is connected to the second connection end of the transformer, one end of the second switch is connected to the AC side of the first inverter and the first connection end of the transformer, one end of the first switch, and the DC side of the first inverter is connected to the new energy generator; the other end of the first switch is connected to the third connection end of the transformer and the grid;

[0013] Control methods include:

[0014] When the power grid does not have a low voltage ride-through fault, the first switch is controlled to be turned on, the second switch is controlled to be turned on, and the third switch is controlled to be turned off, and a grid-type control strategy is adopted to control the operation of the second inverter;

[0015] When a low voltage ride-through fault occurs in the power grid, the first switch is controlled to be disconnected, the second switch is controlled to be disconnected, and the third switch is controlled to be turned on, and a transient voltage control strategy is adopted to control the operation of the second inverter.

[0016] The grid-type energy storage inverter and its control method, as well as the grid-connected fusion system provided in the embodiments of the present application, effectively improve the weak-grid adaptability of the grid-connected inverter and the fault-crossing capability of the grid-connected fusion system by controlling the on or off of the first switch, the second switch, and the third switch, and switching the control strategy of the second inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of a network fusion system provided in an embodiment of the present application;

[0018] Figure 2 A specific schematic diagram of the network fusion system provided in the embodiment of the present application;

[0019] Figure 3 A schematic diagram of control strategy switching of a controller provided in an embodiment of the present application;

[0020] Figure 4 A transient voltage control schematic diagram provided in an embodiment of the present application;

[0021] Figure 5 Another transient voltage control schematic diagram provided in an embodiment of the present application;

[0022] Figure 6 A schematic diagram of the simulation effect provided in an embodiment of the present application.

[0023] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0024] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer and more understandable, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0025] like Figure 1 As shown, the grid-connected fusion system provided in the embodiment of the present application includes a first inverter Q1, a second inverter Q2, a first switch S1, a second switch S2, a third switch S3, a transformer T1 and a controller, and the transformer T1 has a first connection end, a second connection end and a third connection end.

[0026] The DC side of the first inverter Q1 is connected to the new energy generator, and a DC capacitor is connected in parallel on the DC path between the DC side of the first inverter Q1 and the new energy generator. The new energy generator can be a wind generator and / or a photovoltaic cell. Taking the wind generator as an example, the DC side of the first inverter Q1 can be connected to the wind generator through a DC bus and a machine-side converter; taking the photovoltaic cell as an example, the DC side of the first inverter Q1 can be connected to the photovoltaic cell through an MPPT control unit.

[0027] The AC side of the first inverter Q1 is connected to the first connection end of the transformer T1, one end of the first switch S1, and one end of the second switch S2; the DC side of the second inverter Q2 is connected to the energy storage unit, and the AC side of the second inverter Q2 is connected to one end of the third switch S3 and the other end of the second switch S2; the other end of the third switch S3 is connected to the second connection end of the transformer T1, and the third connection end of the transformer T1 is connected to the other end of the first switch S1 and the power grid;

[0028] The controller is used to control the on or off of the first switch S1 , the second switch S2 and the third switch S3 . The controller is also used to switch the control strategy of the second inverter Q2 .

[0029] The above grid-connected fusion system can effectively improve the weak grid adaptability of the grid-connected converter and the fault ride-through capability of the grid-connected fusion by controlling the on or off of the first switch S1, the second switch S2 and the third switch S3 and switching the control strategy of the second inverter Q2.

[0030] In one example, if Figure 2 As shown, the first inverter Q1 or the second inverter Q2 may be a two-level inverter, a three-level inverter or a multi-level inverter composed of switching devices, and the switching devices include but are not limited to IGBTs, MOS tubes and the like.

[0031] In one example, the grid-connected fusion system further includes a DC\DC converter Q3, the DC side of the second inverter Q2 is connected to the energy storage unit through the DC\DC converter Q3, and a DC capacitor is connected in parallel to the DC path between the DC side of the second inverter Q2 and the DC\DC converter Q3. Figure 2 As shown, the DC\DC converter Q3 is composed of a switching device and a diode.

[0032] In one example, the energy storage unit includes but is not limited to battery energy storage, supercapacitor energy storage, superconducting energy storage, etc.

[0033] In one example, the transformer T1 includes a step-up transformer, and the transformation ratio of the transformer is not limited. Other devices may be connected between the transformer T1 and the power grid, and the specific devices are not limited here.

[0034] In one example, the grid-connected fusion system further includes a filter, which is connected in series between the AC side of the second inverter Q2 and the second connection end of the transformer T1, for example, connected in series between the third switch S3 and the second connection end of the transformer T1.

[0035] The filter can be an L or LC filter. Through the filter, the high-order harmonics that may be generated by transient voltage control during a fault can be filtered out to prevent the high-order harmonics from polluting the power grid. The filter parameters are not specifically limited, and the cut-off frequency must be greater than the fundamental frequency.

[0036] In one example, the controller may be integrated with the first inverter Q1; or, the controller may be integrated with the second inverter Q2, for example. Figure 1 As shown in the dotted box in ; alternatively, the controller is independent of the first inverter Q1 or the second inverter Q2.

[0037] In one example, the first inverter Q1 is configured to be controlled by a grid-following control strategy. Specifically, when a wind turbine is used for power generation, the power is usually controlled by the machine side, and the outer loop of the grid-side inverter usually adopts a grid-following control of DC voltage control; when photovoltaic power generation is used, the power is usually controlled by the grid side.

[0038] In one example, the controller is configured to control the first switch S1 to be turned on, the second switch S2 to be turned on, and the third switch S3 to be turned off when no low voltage ride-through fault occurs in the power grid, and adopt a grid-type control strategy to control the operation of the second inverter Q2; when a low voltage ride-through fault occurs in the power grid, the controller controls the first switch S1 to be turned off, the second switch S2 to be turned off, and the third switch S3 to be turned on, and adopts a transient voltage control strategy to control the operation of the second inverter Q2.

[0039] Among them, when the first switch S1 is turned on, the second switch S2 is turned on, and the third switch S3 is turned off, the AC side of the first inverter Q1 and the AC side of the second inverter Q2 are connected in parallel; and when the first switch S1 is turned off, the second switch S2 is turned off, and the third switch S3 is turned on, the AC side of the first inverter Q1 and the AC side of the second inverter Q2 are connected in series through a transformer. The use of transformer series connection can reduce the energy storage DC voltage level, improve the reliability of the device, reduce the withstand voltage level of the switching device, and save costs.

[0040] The grid-forming control strategy includes but is not limited to drooping grid-forming control, virtual synchronous grid-forming control, virtual oscillator grid-forming control, etc. The grid-forming control strategy is used to control the operation of the second inverter Q2, which can improve the voltage and frequency support capability of the grid-following converter and improve the weak grid adaptability of the system. Transient refers to the period of fault, and transient voltage control refers to supporting the voltage during the transient state to maintain the grid-connected point voltage at the voltage value before the fault, where the phase of transient voltage control can be obtained through a phase-locked loop. Figure 3 It is a schematic diagram of the control strategy switching of the controller.

[0041] Figure 4 Schematic diagram of transient voltage control provided by an embodiment of the present application. Figure 4 As shown in the figure, the grid voltage amplitude before the fault is U ref As a reference value for voltage amplitude open-loop control. Transient voltage feedforward control, based on the grid voltage amplitude U before the fault ref and the grid connection voltage amplitude U on the right side of the first switch S1 pcc The comparison result is used to control the output voltage of the second inverter Q2. Specifically, the grid connection voltage amplitude U before the fault is ref and the grid connection voltage amplitude U on the right side of the first switch S1 pccThe difference is used as the voltage regulation amount. After the voltage regulation amount passes through the inverter and the filter, the voltage is increased by the transformer to obtain the compensation voltage. The output voltage of the first inverter Q1 is compensated to obtain the pre-fault voltage, that is, the voltage value before the fault occurs is maintained.

[0042] Figure 5 is another transient voltage control schematic diagram provided by an embodiment of the present application. Figure 5 As shown in the figure, the grid voltage amplitude before the fault is U ref As a reference value for voltage amplitude open-loop control. Transient voltage feedback control, based on the grid voltage amplitude U before the fault ref and the grid connection voltage amplitude U on the left side of the first switch S1 pcc1 The comparison result is used to close the output voltage of the second inverter Q2. Specifically, the grid connection voltage amplitude U before the fault is ref and the grid connection voltage amplitude U on the left side of the first switch S1 pcc1 The difference is passed through a closed-loop regulator as the output voltage regulation amount. For example, the regulator can be a proportional integral algorithm, a proportional differential algorithm or a proportional algorithm. After the voltage regulation amount passes through the inverter and the filter, the voltage is increased by the transformer to obtain the compensation voltage. The output voltage of the first inverter Q1 is compensated to obtain the pre-fault voltage, that is, the voltage value before the fault occurs is maintained.

[0043] In one example, the controller is configured to obtain the grid connection point voltage; and determine whether a low voltage ride-through fault occurs in the power grid based on a comparison result of the grid connection point voltage amplitude with a preset voltage threshold.

[0044] Specifically, it can be done by Figure 2 The monitoring device in the circuit obtains the grid connection point voltage, such as the grid connection point voltage on the left or right side of the first switch S1. Then the grid connection point voltage amplitude is calculated by virtual three-phase method, peak voltage method, fundamental component method, etc. The grid connection point voltage amplitude is compared with the preset voltage threshold. If the grid connection point voltage amplitude is lower than the preset voltage threshold, it is determined that a low voltage ride-through fault occurs in the power grid; if the grid connection point voltage amplitude is not lower than the preset voltage threshold, it is determined that no low voltage ride-through fault occurs in the power grid.

[0045] When a low voltage ride-through fault occurs in the power grid, the first switch S1 is controlled to be disconnected, the second switch S2 is controlled to be disconnected, and the third switch S3 is controlled to be turned on, and a transient voltage control strategy is adopted to control the operation of the second inverter Q2 to maintain the grid connection point voltage at the voltage value before the fault.

[0046] When there is no low voltage ride-through fault in the power grid, the first switch S1 is controlled to be turned on, the second switch S2 is controlled to be turned on, and the third switch S3 is turned off, and a grid-type control strategy is adopted to control the operation of the second inverter Q2, thereby improving the voltage and frequency support capability of the grid-type converter and improving the weak grid adaptability of the system.

[0047] The monitoring device can be integrated into the controller or independent of the controller.

[0048] Figure 6 Schematic diagram of the simulation effect provided by the embodiment of the present application. Figure 6 As shown, the front stage of the grid-following control is connected to the photovoltaic power plant, the grid-following converter adopts PQ control, and the virtual synchronous grid-forming control is adopted before the energy storage inverter fails, and switches to transient voltage control when a fault occurs.

[0049] After the system is operating normally, a grid voltage drop fault is set at 4s. The monitoring device collects the grid connection point voltage and finds that the voltage drops and is lower than the preset voltage threshold. The grid configuration control of the energy storage converter is switched to transient voltage control, and the grid connection point voltage is compensated to maintain it at the voltage value before the fault. From the simulation effect diagram, it can be found that after switching to transient voltage control, the grid connection point voltage can be well maintained at the pre-fault value, and the active and reactive power also recover to the pre-fault value.

[0050] Another embodiment of the present application provides a grid-type energy storage converter, the grid-type energy storage converter includes a second inverter Q2 and a controller;

[0051] The DC side of the second inverter Q2 is connected to the energy storage unit, the AC side of the second inverter Q2 is connected to one end of the third switch S3 and the other end of the second switch S2; the other end of the third switch S3 is connected to the second connection end of the transformer T1, one end of the second switch S2 is connected to the AC side of the first inverter Q1 and the first connection end of the transformer T1, and one end of the first switch S1, and the DC side of the first inverter Q1 is connected to the wind turbine and / or the photovoltaic cell; the other end of the first switch S1 is connected to the third connection end of the transformer T1 and the power grid;

[0052] The controller is used to control the on or off of the first switch S1 , the second switch S2 and the third switch S3 . The controller is also used to switch the control strategy of the second inverter Q2 .

[0053] Another embodiment of the present application provides a control method for a grid-type energy storage converter, wherein the grid-type energy storage converter includes a second inverter Q2; the DC side of the second inverter Q2 is connected to the energy storage unit, and the AC side of the second inverter Q2 is connected to one end of a third switch S3 and the other end of the second switch S2; the other end of the third switch S3 is connected to the second connection end of the transformer T1, one end of the second switch S2 is connected to the AC side of the first inverter Q1 and the first connection end of the transformer T1, and one end of the first switch S1, and the DC side of the first inverter Q1 is connected to a wind turbine and / or a photovoltaic cell; the other end of the first switch S1 is connected to the third connection end of the transformer T1 and the power grid;

[0054] Control methods include:

[0055] When the grid does not have a low voltage ride-through fault, the first switch S1 is controlled to be turned on, the second switch S2 is controlled to be turned on, and the third switch S3 is turned off, and a grid-type control strategy is adopted to control the operation of the second inverter Q2;

[0056] When a low voltage ride-through fault occurs in the power grid, the first switch S1 is controlled to be disconnected, the second switch S2 is controlled to be disconnected, and the third switch S3 is controlled to be turned on, and a transient voltage control strategy is adopted to control the operation of the second inverter Q2.

[0057] In one example, the control method further includes:

[0058] Obtain the grid connection point voltage; and determine whether a low voltage ride-through fault occurs in the power grid based on a comparison result of the grid connection point voltage amplitude with a preset voltage threshold.

[0059] In one example, the control method further includes:

[0060] When a low voltage ride-through fault occurs in the power grid, the grid connection point voltage is compensated so that the grid connection point voltage is maintained at the voltage value before the fault occurs.

[0061] The preferred embodiments of the present application are described above with reference to the accompanying drawings, but the scope of the present application is not limited thereby. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present application shall be within the scope of the present application.

Claims

1. A network fusion system, characterized in that: The grid-connected fusion system includes a first inverter, a second inverter, a first switch, a second switch, a third switch, and a transformer, wherein the transformer has a first connection end, a second connection end, and a third connection end; The DC side of the first inverter is connected to the new energy generator, and the AC side of the first inverter is connected to the first connection end, one end of the first switch, and one end of the second switch; the DC side of the second inverter is connected to the energy storage unit, and the AC side of the second inverter is connected to one end of the third switch and the other end of the second switch; the other end of the third switch is connected to the second connection end, and the third connection end is connected to the other end of the first switch and the power grid; The controller of the second inverter is used to control the on or off of the first switch, the second switch and the third switch, and the controller is also used to switch the control strategy of the second inverter.

2. The root-network fusion system according to claim 1 is characterized in that: The grid-connected fusion system further includes a DC\DC converter, and the DC side of the second inverter is connected to the energy storage unit via the DC\DC converter.

3. The root-network fusion system according to claim 1, characterized in that: The grid-connected fusion system further includes a filter, which is connected in series between the AC side of the second inverter and the second connection end.

4. The root-network fusion system according to claim 1, characterized in that: The controller is integrated with the first inverter; or, the controller is integrated with the second inverter; or, the controller is independent of the first inverter or the second inverter.

5. The root-network fusion system according to claim 1, characterized in that: The first inverter is configured to be controlled by adopting a grid-following control strategy.

6. The root-network fusion system according to claim 1, characterized in that: The controller is configured to control the first switch to be turned on, the second switch to be turned on, and the third switch to be turned off when no low voltage ride-through fault occurs in the power grid, and to control the operation of the second inverter using a grid-type control strategy; when a low voltage ride-through fault occurs in the power grid, the controller controls the first switch to be turned off, the second switch to be turned off, and the third switch to be turned on, and to control the operation of the second inverter using a transient voltage control strategy.

7. The root-network fusion system according to claim 6 is characterized in that: The controller is configured to obtain the grid connection point voltage; and determine whether a low voltage ride-through fault occurs in the power grid based on a comparison result of the amplitude of the grid connection point voltage and a preset voltage threshold.

8. The root-network fusion system according to claim 6, characterized in that: The controller is configured to compensate the grid connection point voltage so that the grid connection point voltage is maintained at the voltage value before the fault occurs when a low voltage ride-through fault occurs in the power grid.

9. A grid-type energy storage converter, characterized in that: The grid-type energy storage converter includes a second inverter and a controller; The DC side of the second inverter is connected to the energy storage unit, the AC side of the second inverter is connected to one end of the third switch and the other end of the second switch; the other end of the third switch is connected to the second connection end of the transformer, one end of the second switch is connected to the AC side of the first inverter and the first connection end of the transformer and one end of the first switch, the DC side of the first inverter is connected to the new energy generator; the other end of the first switch is connected to the third connection end of the transformer and the power grid; The controller is used to control the on or off of the first switch, the second switch and the third switch, and the controller is also used to switch the control strategy of the second inverter.

10. A control method for a grid-type energy storage converter, characterized in that: The grid-type energy storage converter includes a second inverter; the DC side of the second inverter is connected to the energy storage unit, the AC side of the second inverter is connected to one end of the third switch and the other end of the second switch; the other end of the third switch is connected to the second connection end of the transformer, one end of the second switch is connected to the AC side of the first inverter, the first connection end of the transformer, and one end of the first switch, and the DC side of the first inverter is connected to the new energy generator; The other end of the first switch is connected to the third connection end of the transformer and the power grid; The control method comprises: When the power grid does not have a low voltage ride-through fault, the first switch is controlled to be turned on, the second switch is controlled to be turned on, and the third switch is controlled to be turned off, and a grid-type control strategy is adopted to control the operation of the second inverter; When a low voltage ride-through fault occurs in the power grid, the first switch is controlled to be disconnected, the second switch is controlled to be disconnected, and the third switch is controlled to be turned on, and a transient voltage control strategy is adopted to control the operation of the second inverter.

11. The control method according to claim 10, characterized in that: Obtaining the grid connection point voltage; and determining whether a low voltage ride-through fault occurs in the power grid based on a comparison result of the grid connection point voltage amplitude with a preset voltage threshold.

12. The control method according to claim 10, characterized in that: When a low voltage ride-through fault occurs in the power grid, the grid connection point voltage is compensated so that the grid connection point voltage is maintained at the voltage value before the fault occurs.

Images