A full DC collection system for offshore multi-energy power generation systems

Abstract

The present invention discloses a full DC collection system for an offshore multi-energy power generation system, comprising: a submodule collection bridge arm and an onshore converter station; the onshore converter station is directly connected to or connected to the submodule collection bridge arm through a DC transformer; the submodule collection bridge arm is used to control the capacitor voltage value of the collection submodule according to the DC voltage, the onshore converter station and the DC transformer are used to adjust the DC voltage on both sides of the submodule collection bridge arm when it is determined that the output current of the submodule collection bridge arm is less than the charging current of the collection submodule capacitor; the submodule collection bridge arm, the onshore converter station, and the DC transformer are also used to control the collection submodule capacitor and energy storage, control the flexible DC converter capacitor voltage, and control the DC transformer submodule capacitor voltage when determining the frequency change of the receiving end. Therefore, the present invention provides a full DC collection system for an offshore multi-energy power generation system, which meets the current demand for large-scale DC networking and same-field collection and transmission of offshore multi-energy systems.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy grid connection, and in particular to a full DC collection system for an offshore multi-energy power generation system. Background Art

[0002] The development of offshore energy effectively alleviates the limitations of onshore energy development and contributes to the goal of achieving carbon peak and carbon neutrality. Offshore wind power has experienced rapid growth, with global installed capacity exceeding 60GW. Offshore photovoltaics have also seen some demonstration projects. Furthermore, wave power has also seen some development, with the "Nankun" wave energy device, currently under development by the Southern Power Grid. These wind, photovoltaic, and wave power generation technologies all exhibit significant intermittent and uncertain characteristics. Furthermore, deploying a single wind, photovoltaic, or wave power generation system in a single ocean area fails to fully utilize the available space. Current technologies do not allow for the simultaneous deployment of wind, photovoltaic, and wave power generation in the same area, thereby achieving high power density within a single area. Furthermore, given the increasing capacity of offshore wind turbines and the expansion of the collection area, DC collection and transmission is a major trend.

[0003] However, there is currently no DC aggregation and networking solution for offshore multi-energy sources. Current technology is limited to full DC networking control technology for offshore wind power, or full DC networking systems for onshore photovoltaic systems. There is no DC aggregation solution for offshore multi-energy systems. Summary of the Invention

[0004] An embodiment of the present invention provides a full DC collection system for an offshore multi-energy power generation system, which provides a full DC collection system for large-scale DC networking and transmission for the offshore multi-energy power generation system, meeting the current demand for large-scale DC networking and same-field collection and transmission of offshore multi-energy systems.

[0005] An embodiment of the present invention provides a full DC collection system for an offshore multi-energy power generation system, comprising: a submodule collection bridge arm formed by connecting collection submodules in series, and an onshore converter station;

[0006] The fixed DC voltage control of the onshore converter station is achieved by connecting the submodules to the bridge arms via the DC line, or by connecting the submodules to the bridge arms after stepping down the voltage via the DC transformer;

[0007] The submodule aggregation bridge arm is used to control the capacitor voltage value of the aggregation submodule according to the DC voltage, combined with the capacitor voltage sequence of each aggregation submodule and the PI controller to adjust the number of aggregation submodules put into use; and

[0008] When the receiving end frequency changes, frequency support is achieved by controlling the capacitance and energy storage of the aggregation submodule;

[0009] The onshore converter station is configured to adjust the DC voltage on both sides of the submodule aggregating bridge arm through a flexible DC converter when it is determined that the output current of the submodule aggregating bridge arm is less than the charging current of the aggregating submodule capacitor, so that the output current of the submodule aggregating bridge arm is greater than the charging current of the aggregating submodule capacitor, thereby maintaining control of the aggregating submodule capacitor voltage; and

[0010] When the receiving-end frequency changes, frequency support is achieved by controlling the capacitor voltage of the flexible DC converter;

[0011] The DC transformer is used to adjust the DC voltage on both sides of the submodule aggregating bridge arm when it is determined that the output current of the submodule aggregating bridge arm is less than the aggregating submodule capacitor charging current, so that the output current of the submodule aggregating bridge arm is greater than the aggregating submodule capacitor charging current, thereby maintaining the control of the capacitor voltage; and

[0012] When the receiving-end frequency change is determined, frequency support is achieved by controlling the capacitor voltage of the DC transformer submodule.

[0013] Furthermore, the collection submodule adopts a half-bridge submodule structure, and the DC wind turbine, wave power generation, photovoltaic, and / or energy storage of the offshore multi-energy power generation system are connected in parallel on the capacitor side to achieve multi-energy collection;

[0014] The capacitor side of the collection submodule is also connected in parallel to limit the capacitor voltage and to cut off the energy consumption system and bypass switch of the collection submodule in a fault state;

[0015] Among them, the DC wind turbine and the wave power generation both use AC / DC converters and DC-DC converters to access the collection sub-module; the photovoltaic system uses a DC-DC converter to access the collection sub-module; the DC wind turbine, wave power generation, and photovoltaic system all use maximum power tracking control mode, and are equipped with fault isolation switches that cut off specific new energy sources in the event of a fault.

[0016] Furthermore, the submodule aggregation bridge arm is composed of a plurality of aggregation submodules and an inductor connected in series;

[0017] One or more submodules are connected in parallel to form an offshore submodule to collect DC current, which is then connected to the onshore converter station, thereby realizing the collection of the offshore DC system; or

[0018] One or more sub-module aggregation bridge arms are connected in parallel to form an offshore sub-module aggregation DC current, which is then connected to a DC transformer and transmitted to an onshore converter station to meet the voltage insulation requirements of the sub-module aggregation bridge arm.

[0019] Furthermore, the submodule aggregation bridge arm adjusts the number of aggregation submodules according to the DC voltage, the capacitor voltage of each aggregation submodule and the PI controller to control the capacitor voltage value of the aggregation submodule, including:

[0020] The capacitor voltage of the aggregation submodule is stepped down and sorted by balancing control. When energy storage is used, the state of charge of the energy storage of the aggregation submodule is sorted, as shown below:

[0021] ASOC csm =sort(SOC csm ,descent);

[0022] Au csm =sort(u csm ,descent);

[0023] Where, ASOC csm is the state of charge sequence of the energy storage of the aggregated submodules, Au csm is the capacitor voltage sorting sequence of the collection submodule, u csm, is the capacitance voltage series of the unsorted aggregation submodule, SOC csm, is the state of charge sequence of the energy storage of the unsorted aggregation submodules.

[0024] Furthermore, the submodule aggregation bridge arm adjusts the number of aggregation submodules according to the DC voltage, the capacitor voltage ranking of each aggregation submodule and the PI controller to control the capacitor voltage value of the aggregation submodule, and further includes:

[0025] According to the following formula, the input level is adjusted by the difference between the average submodule capacitor voltage and the reference value of the aggregation submodule capacitor voltage, or by the integral of the difference between the state of charge and the energy storage state of charge reference value, thereby performing carrier phase shift or nearest level approximation modulation:

[0026]

[0027]

[0028] Where u csm,ref The SOC is the reference value of the capacitor voltage of the aggregation submodule. csm,ref is the reference value of energy storage charge state, u csm,i is the capacitor voltage of the i-th collection submodule, N csm is the total number of aggregated submodules, SoC csm,i is the state of charge of the energy storage of the i-th aggregation submodule containing energy storage, PI vc is the capacitor voltage PI controller, PI soc is the PI controller of the state of charge, n0 is the initial number of submodules, NLM / CPS() is the nearest level approach / carrier phase shift modulation, g nc is the trigger pulse signal of the aggregation submodule when there is no energy storage in steady state, g nsoc It is the trigger pulse signal of the collection submodule when there is energy storage in steady state.

[0029] Furthermore, the onshore converter station, when determining that the output current of the submodule collecting bridge arm is less than the collecting submodule capacitor charging current, adjusts the DC voltage on both sides of the submodule collecting bridge arm through the flexible DC converter, so that the submodule collecting bridge arm current is greater than the collecting submodule capacitor charging current, thereby maintaining the control of the collecting submodule capacitor voltage, including:

[0030] By changing the number of submodules in the flexible DC converter station, the DC voltage at both ends of the submodule's bridge arm is reduced while the capacitor voltage remains unchanged, thereby increasing the submodule's bridge arm current while the power remains unchanged, ensuring that the submodule's bridge arm current is greater than the submodule current, thus achieving submodule energy balance.

[0031] According to the following formula, the DC modulation system is controlled to adjust the submodule input coefficient of the receiving-end flexible DC converter station:

[0032] m dc =PI idc (i dc -1.1Σi csm,imax );

[0033] Where i csm,imax For each submodule, the maximum current of the submodule in the bridge arm is collected, m dc is the submodule input coefficient, i dc is the DC current on the sea side, i.e. the sum of the bridge arm output currents collected by the submodules, PI idc is the DC current PI controller;

[0034] When the output current of the submodule aggregation bridge arm is greater than the aggregation submodule capacitor charging current, the system DC modulation coefficient is 1, and the proportional-integral controller outputs 0. When the aggregation submodule capacitor charging current is greater than the submodule aggregation bridge arm output current, the proportional-integral controller is enabled. When the sum of the submodule aggregation bridge arm output currents is greater than 1.2 times the sum of the maximum values ​​of the aggregation submodule currents in each submodule aggregation bridge arm, the energy balance between the aggregation submodules is determined. At this time, the system operates stably without changing the DC voltage on both sides of the submodule aggregation bridge arm to adjust the submodule aggregation bridge arm current.

[0035] The DC transformer, when determining that the output current of the submodule collecting bridge arm is less than the collecting submodule capacitor charging current, adjusts the DC voltage on both sides of the submodule collecting bridge arm, so that the submodule collecting bridge arm current is greater than the collecting submodule capacitor charging current, thereby maintaining the control of the capacitor voltage, including:

[0036] By changing the input number of the new energy side of the DC transformer, the DC voltage at both ends of the sub-module collection bridge arm is reduced while the capacitor voltage remains unchanged, thereby increasing the output current of the sub-module collection bridge arm while the power remains unchanged, ensuring that the output current of the sub-module collection bridge arm is greater than the charging current of the collection sub-module capacitor, thereby achieving sub-module energy balance.

[0037] Furthermore, the onshore converter station realizes frequency support by controlling the capacitance of the flexible DC converter, including:

[0038] The onshore converter station adopts a grid-following or grid-forming type to control the capacitor voltage;

[0039] The grid-type phase-locked loop is shown as follows:

[0040]

[0041] Where s is the Laplace operator, ω ref is the controller frequency reference value, PI pll is the phase-locked loop PI controller, u q is the q-axis voltage, θ gfl Phase of flexible DC and grid controller;

[0042] When grid-following control is adopted, the inner loop adopts a current loop with cross decoupling and feedforward control, and the outer loop adopts constant capacitance voltage control and constant reactive power / voltage amplitude control, as shown in the following formula:

[0043] i dref =PI d (u csm,avg -u csm,ref ),i qref =PI q (Q out -Q ref );

[0044] Where PI d is the d-axis outer loop PI controller, PI q is the q-axis outer loop PI controller, u csm,avg is the average value of the capacitor voltage of the collection submodule, u csm,ref is the reference value of the capacitor voltage of the collection submodule, Q out is the reactive current feedback value, Q ref is the reactive current reference value, i dref_gfl For the grid-type control of the flexible DC d-axis current reference, i qref_gfl It is the reference of the q-axis current of the flexible DC motor for grid-following control;

[0045] When using grid-type control, the constant capacitance voltage control is as follows:

[0046]

[0047] Where s is the Laplace operator, ω ref is the controller frequency reference value, u q is the q-axis voltage, G uq is the voltage synchronous controller, u csm,avg is the average value of the capacitor voltage of the collection submodule, u csm,ref is the reference value of the capacitor voltage of the aggregation submodule, k dc is the capacitor voltage synchronous steady-state gain, k dch is the high-pass gain, τ dc is the filtering time constant, θ gfm Generate phase for flexible DC network controller;

[0048] The inner loop can adopt voltage and current dual closed-loop control, or voltage admittance outer loop, or voltage single loop and open-loop control;

[0049] When using voltage and current dual closed-loop control, the voltage outer loop is as follows:

[0050] i dref =PI ac (u dref -u d ),i qref =PI ac (u qref -u q );

[0051] Where PI ac is the voltage outer loop PI controller, u dref is the reference value of the d-axis voltage amplitude, and u qref is the reference value of the q-axis voltage amplitude, u q is the q-axis voltage, u d is the d-axis voltage, i dref_gfm For the grid-type control of the flexible direct current d-axis current reference, i qref_gfm It is the q-axis current reference for the flexible DC grid control.

[0052] Furthermore, the onshore converter station, when determining the frequency change at the receiving end, controls the capacitor voltage of the flexible DC converter to achieve frequency support, including:

[0053] In the case of a collection mode without a DC transformer, when the receiving-end frequency changes, the receiving-end converter station is supported by adjusting the flexible DC capacitor voltage. The grid type is adjusted as follows:

[0054] u sm_ref =k uω Δω+u sm_0 ;

[0055] Among them, k uωis the DC voltage frequency droop coefficient; Δω is; u sm_ref for;u sm_0 is the initial design value of the voltage of the aggregation submodule;

[0056] For mesh control, frequency support is provided by the capacitor voltage of the flexible direct submodule;

[0057] The submodules aggregate the bridge arms, and when the receiving end frequency changes, frequency support is achieved by controlling the capacitors of the aggregated submodules and the connected energy storage, including:

[0058] The submodule bridge arm adjusts the capacitor voltage according to the DC voltage at both ends. The pulse signal expression is as follows:

[0059]

[0060] Among them, k uu k is the droop coefficient of capacitor voltage to DC voltage, socu is the droop coefficient of the state of charge to DC voltage, Δu dc is the DC voltage change value on both sides of the collection submodule, when the collection submodule is connected to the DC transformer, it is Δu dcm , g ec It is the trigger pulse signal of the submodule when there is no energy storage in the current control mode; g esoc It is used to collect submodule trigger pulse signals when there is no energy storage and when there is energy storage in the current control mode.

[0061] Furthermore, the DC transformer, when determining the frequency change at the receiving end, controls the DC transformer capacitor voltage to achieve frequency support, including:

[0062] The DC transformer can achieve frequency transmission and its own frequency support by changing the DC voltage on both sides of the bridge arm collected by the submodule and the capacitor voltage of its own submodule.

[0063] Furthermore, the DC transformer adopts a modular multi-level converter, or adopts an isolated or non-isolated type.

[0064] The following beneficial effects are achieved by implementing the present invention:

[0065] The present invention discloses a full DC collection system for an offshore multi-energy power generation system, comprising: a submodule collection bridge arm formed by connecting collection submodules in series, and an onshore converter station; the onshore converter station controls the DC voltage to connect the submodule collection bridge arm through a DC line, or connects the submodule collection bridge arm after stepping down the voltage through a DC transformer; the submodule collection bridge arm is used to adjust the number of collection submodules put into operation according to the DC voltage, in combination with the capacitor voltage sequence of each collection submodule and a PI controller, to control the capacitor voltage value of the collection submodule; and, when determining that the receiving end frequency changes, frequency support is achieved by controlling the collection submodule capacitor and energy storage; the onshore converter station is used to determine that the output current of the submodule collection bridge arm is less than the charging current of the collection submodule capacitor. When the submodule is flowing, the DC voltage on both sides of the submodule collection bridge arm is adjusted by the flexible DC converter, so that the output current of the submodule collection bridge arm is greater than the charging current of the collection submodule capacitor, thereby maintaining the control of the collection submodule capacitor voltage; and, when the receiving end frequency change is determined, the frequency support is achieved by controlling the flexible DC converter capacitor voltage; the DC transformer is used to adjust the DC voltage on both sides of the submodule collection bridge arm when it is determined that the output current of the submodule collection bridge arm is less than the charging current of the collection submodule capacitor, so that the output current of the submodule collection bridge arm is greater than the charging current of the collection submodule capacitor, thereby maintaining the control of the capacitor voltage; and, when the receiving end frequency change is determined, the frequency support is achieved by controlling the DC transformer submodule capacitor voltage. The present invention provides a full DC collection system for large-scale DC networking and transmission for the current offshore multi-energy power generation system, which meets the current demand for large-scale DC networking and same-field collection and transmission of offshore multi-energy systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 The diagram is a topological diagram of a full DC collection system of an offshore multi-energy power generation system provided by one embodiment of the present invention.

[0067] Figure 2 This is a schematic diagram of the energy balancing control strategy of the aggregation submodule provided in one embodiment of the present invention.

[0068] Figure 3 This is a schematic diagram of a grid-following control strategy and a grid-forming control strategy of a receiving-end flexible DC converter station provided by an embodiment of the present invention.

[0069] Figure 4 FIG. 4 is a schematic diagram of a DC current control strategy provided by an embodiment of the present invention.

[0070] Figure 5 This is an illustration of the frequency support strategy of the aggregation submodule provided in one embodiment of the present invention.

[0071] Figure 6 This is a waveform diagram of data changes using direct current control, shown by simulation experiment results provided by an embodiment of the present invention.

[0072] Figure 7 This is a waveform diagram of data changes using frequency support control, shown by simulation experiment results provided by one embodiment of the present invention. DETAILED DESCRIPTION

[0073] To make the objectives, technical solutions, and advantages of this application more clear, the technical solutions in this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are only part of the embodiments of this application, not all of them. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of this application.

[0074] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0075] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0076] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0077] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0078] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0079] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0080] See also Figure 1 , is a schematic diagram of the topological structure of a full DC collection system of an offshore multi-energy power generation system provided by one embodiment of the present invention, comprising: a sub-module collection bridge arm formed by series connection of collection sub-modules, and an onshore converter station;

[0081] The fixed DC voltage control of the onshore converter station is achieved by connecting the submodules to the bridge arms via the DC line, or by connecting the submodules to the bridge arms after stepping down the voltage via the DC transformer;

[0082] The submodule aggregation bridge arm is used to control the capacitor voltage value of the aggregation submodule according to the DC voltage, combined with the capacitor voltage sequence of each aggregation submodule and the PI controller to adjust the number of aggregation submodules put into use; and

[0083] When the receiving end frequency changes, frequency support is achieved by controlling the capacitance and energy storage of the aggregation submodule;

[0084] The onshore converter station is configured to adjust the DC voltage on both sides of the submodule aggregating bridge arm through a flexible DC converter when it is determined that the output current of the submodule aggregating bridge arm is less than the charging current of the aggregating submodule capacitor, so that the output current of the submodule aggregating bridge arm is greater than the charging current of the aggregating submodule capacitor, thereby maintaining control of the aggregating submodule capacitor voltage; and

[0085] When the receiving-end frequency changes, frequency support is achieved by controlling the capacitor voltage of the flexible DC converter;

[0086] The DC transformer is used to adjust the DC voltage on both sides of the submodule aggregating bridge arm when it is determined that the output current of the submodule aggregating bridge arm is less than the aggregating submodule capacitor charging current, so that the output current of the submodule aggregating bridge arm is greater than the aggregating submodule capacitor charging current, thereby maintaining the control of the capacitor voltage; and

[0087] When the receiving-end frequency change is determined, frequency support is achieved by controlling the capacitor voltage of the DC transformer submodule.

[0088] In a preferred embodiment of the present invention, Figure 1As shown, (a) is the topology diagram of the full DC collection system with energy storage and DC transformers, (b) is the topology diagram of the full DC collection system with DC transformers but without energy storage, (c) is the topology diagram of the full DC collection system with energy storage but without DC transformers, (d) is the topology diagram of the full DC collection system without energy storage and DC transformers, (g) is the topology diagram of the DC transformer, and (h) is the topology diagram of the converter carried by the onshore converter station. It should be noted that Figure 1 The various topological structures in the figure are only examples. The specific connection topologies of the converter station, DC transformer, photovoltaic power, wind power, wave power generation and energy storage are not fixed. The figure only provides typical cases. At the same time, the control strategy is also similar. The present invention can be expanded to different control methods as long as the above control objectives can be met.

[0089] Preferably, the collection submodule adopts a half-bridge submodule structure, and the capacitor side is connected in parallel with the DC wind turbine, wave power generation, photovoltaic, and / or energy storage of the offshore multi-energy power generation system to achieve multi-energy collection;

[0090] The capacitor side of the collection submodule is also connected in parallel to limit the capacitor voltage and to cut off the energy consumption system and bypass switch of the collection submodule in a fault state;

[0091] Among them, the DC wind turbine and the wave power generation both use AC / DC converters and DC-DC converters to access the collection sub-module; the photovoltaic system uses a DC-DC converter to access the collection sub-module; the DC wind turbine, wave power generation, and photovoltaic system all use maximum power tracking control mode, and are equipped with fault isolation switches that cut off specific new energy sources in the event of a fault.

[0092] In a preferred embodiment of the present invention, Figure 1 As shown, Figure (f) shows the topology of the AC / DC converter, and Figure (e) shows the topology of the DC-DC converter. Connected in parallel with the DC-side capacitors of the collection submodule are distributed energy consumption, DC wind turbines, photovoltaics, and wave DC power generation units. Both wind turbines and wave power generation use permanent magnet synchronous generators, outputting three-phase AC power. This is converted to DC via AC / DC and then boosted via a DC-DC converter before being connected to the DC side of the submodule. Photovoltaics, on the other hand, are connected to the DC side of the submodule via a DC-DC converter (such as DAB). The wind turbine AC / DC uses maximum power tracking control, and the wave power generation is similar, so we will not elaborate on this here. Photovoltaics achieves maximum power tracking control via DAB. It should be noted that wind power, wave power, and photovoltaics do not participate in frequency support and are always in maximum power tracking, thus ensuring maximum power capture and the necessary frequency support functions.

[0093] Preferably, the submodule aggregation bridge arm is composed of a plurality of aggregation submodules and an inductor connected in series;

[0094] One or more submodules are connected in parallel to form an offshore submodule to collect DC current, which is then connected to the onshore converter station, thereby realizing the collection of the offshore DC system; or

[0095] One or more sub-module aggregation bridge arms are connected in parallel to form an offshore sub-module aggregation DC current, which is then connected to a DC transformer and transmitted to an onshore converter station to meet the voltage insulation requirements of the sub-module aggregation bridge arm.

[0096] In a preferred embodiment of the present invention, specifically, Figure 1 In the topology of the submodules, the submodules can be connected in series to boost the voltage, thus realizing the integration of wind turbines, photovoltaics and wave power generation. At the same time, considering the insulation requirements, it is possible to consider connecting the submodules in series to a DC step-up transformer to achieve voltage boost, thus avoiding the high insulation requirements of each submodule design. In general, according to whether energy storage is installed on the offshore side and whether a DC step-up transformer is used, it can be divided into Figure 1 The four types are shown, and the voltage on the medium and high voltage sides of the step-up transformer is denoted as u dcm with u dc For a step-up DC transformer, the sum of the currents of the parallel units is the sum of the medium voltage DC currents, as shown below:

[0097] i dcm =∑ k i dcm,k ;

[0098] Among them, i dcm is the sum of the currents of the parallel units in the scenario with a step-up DC transformer, i dcm,k In the scenario with a step-up DC transformer, the kth submodule aggregates the output current of the bridge arm.

[0099] For a scenario without a step-up DC transformer, the DC current is the sum of the currents of the parallel units, that is:

[0100] i dc =∑ k i dcm,k .

[0101] Among them, i dc is the sum of the currents of the parallel units in the scenario without a step-up DC transformer, i dcm,k In the scenario without a step-up DC transformer, the kth submodule aggregates the output current of the bridge arm.

[0102] Preferably, the submodule aggregation bridge arm adjusts the number of aggregation submodules according to the DC voltage, the capacitor voltage of each aggregation submodule and the PI controller to control the capacitor voltage value of the aggregation submodule, including:

[0103] The capacitor voltage of the aggregation submodule is stepped down and sorted by balancing control. When energy storage is used, the state of charge of the energy storage of the aggregation submodule is sorted, as shown below:

[0104] ASOC csm =sort(SOC csm ,descent);

[0105] Au csm =sort(u csm ,descent);

[0106] Where, ASOC csm is the state of charge sequence of the energy storage of the aggregated submodules, Au csm is the capacitor voltage sorting sequence of the collection submodule, u csm, is the capacitance voltage series of the unsorted aggregation submodule, SOC csm is the state of charge sequence of the energy storage of the unsorted aggregation submodules.

[0107] In a preferred embodiment of the present invention, a DC voltage is provided after the collection submodules are connected in series at an onshore converter station or a DC transformer, and the submodule collection bridge arm realizes balanced control of each energy through energy balanced input control, thereby realizing power output control.

[0108] Specifically, the aggregation submodule aggregates the energy of each submodule. It primarily uses energy balancing control to perform voltage reduction sequencing, switching on submodules with higher voltages. It also controls the number of submodules switched on to control the submodule capacitance. When energy storage is used, the port is measured based on the state of charge of the stored energy. This is specifically shown below:

[0109] ASOC csm =sort(SOC csm ,descent);

[0110] Au csm =sort(u csm ,descent);

[0111] Where, ASOC csm is the state of charge sequence of the energy storage of the aggregated submodules, Au csm is the capacitor voltage sorting sequence of the collection submodule, u csm, is the capacitance voltage series of the unsorted aggregation submodule, SOC csm is the state of charge sequence of the energy storage of the unsorted aggregation submodules.

[0112] Based on ASOC csm 、Av csm The index value of the collection submodule that should be invested can be obtained.

[0113] Preferably, the submodule aggregation bridge arm adjusts the number of aggregation submodules input according to the DC voltage, in combination with the capacitor voltage ranking of each aggregation submodule and the PI controller, to control the capacitor voltage value of the aggregation submodule, and further includes:

[0114] According to the following formula, the number of submodules put into operation is adjusted by the difference between the average submodule capacitor voltage and the reference value of the aggregate submodule capacitor voltage, or by the integral of the difference between the state of charge and the energy storage state of charge reference value, thereby performing carrier phase shift or nearest level approximation modulation:

[0115]

[0116] Where u csm,ref The SOC is the reference value of the capacitor voltage of the aggregation submodule. csm,ref is the reference value of energy storage charge state, u csm,i is the capacitor voltage of the i-th collection submodule, N csm is the total number of aggregated submodules, SoC csm,i is the state of charge of the energy storage of the i-th aggregation submodule containing energy storage, PI vc is the capacitor voltage PI controller, PI soc is the PI controller of the state of charge, n0 is the initial number of submodules, NLM / CPS() is the nearest level approach / carrier phase shift modulation, g nc is the trigger pulse signal of the aggregation submodule when there is no energy storage in steady state, g nsoc It is the trigger pulse signal of the collection submodule when there is energy storage in steady state.

[0117] In a preferred embodiment of the present invention, in order to know the number of submodules that need to be put into operation, a proportional integral controller (PI control) is generally used, as shown in the following example: Figure 2 As shown, the input level number is adjusted by integrating the difference between the average submodule capacitor voltage / state of charge and the reference capacitor voltage value / state of charge value, thereby performing carrier phase shift or nearest level approximation modulation.

[0118]

[0119] Where u csm,ref The SOC is the reference value of the capacitor voltage of the aggregation submodule. csm,ref is the reference value of energy storage charge state, u csm,i is the capacitor voltage of the i-th collection submodule, N csm is the total number of aggregated submodules, SoC csm,i is the state of charge of the energy storage of the i-th aggregation submodule containing energy storage, PI vc is the capacitor voltage PI controller, PI socis the PI controller of the state of charge, n0 is the initial number of submodules, NLM / CPS() is the nearest level approach / carrier phase shift modulation, g nc is the trigger pulse signal of the aggregation submodule when there is no energy storage in steady state, g nsoc It is the trigger pulse signal of the collection submodule when there is energy storage in steady state.

[0120] Preferably, the onshore converter station, when determining that the output current of the submodule collecting bridge arm is less than the collecting submodule capacitor charging current, adjusts the DC voltage on both sides of the submodule collecting bridge arm through the flexible DC converter, so that the submodule collecting bridge arm current is greater than the collecting submodule capacitor charging current, thereby maintaining the control of the capacitor voltage, including:

[0121] By changing the number of submodules in the flexible DC converter station, the DC voltage at both ends of the submodule's bridge arm is reduced while the capacitor voltage remains unchanged, thereby increasing the submodule's bridge arm current while the power remains unchanged, ensuring that the submodule's bridge arm output current is greater than the submodule current, thus achieving submodule energy balance.

[0122] According to the following formula, the DC modulation system is controlled to adjust the submodule input coefficient of the receiving-end flexible DC converter station:

[0123] m dc =PI idc (i dc -1.1Σi csm,imax );

[0124] Where i csm,imax For each submodule, the maximum current of the submodule in the bridge arm is collected, m dc is the submodule input coefficient, i dc is the DC current on the sea side, i.e. the sum of the bridge arm output currents collected by the submodules, PI idc is the DC current PI controller;

[0125] When the output current of the submodule aggregation bridge arm is greater than the aggregation submodule capacitor charging current, the system DC modulation coefficient is 1, and the proportional-integral controller outputs 0. When the aggregation submodule capacitor charging current is greater than the submodule aggregation bridge arm output current, the proportional-integral controller is enabled. When the sum of the submodule aggregation bridge arm output currents is greater than 1.2 times the sum of the maximum values ​​of the aggregation submodule currents in each submodule aggregation bridge arm, the energy balance between the aggregation submodules is determined. At this time, the system operates stably without changing the DC voltage on both sides of the submodule aggregation bridge arm to adjust the submodule aggregation bridge arm current.

[0126] The DC transformer, when determining that the output current of the submodule collecting bridge arm is less than the collecting submodule capacitor charging current, adjusts the DC voltage on both sides of the submodule collecting bridge arm, so that the submodule collecting bridge arm current is greater than the collecting submodule capacitor charging current, thereby maintaining the control of the capacitor voltage, including:

[0127] By changing the input number of the new energy side of the DC transformer, the DC voltage at both ends of the sub-module collection bridge arm is reduced while the capacitor voltage remains unchanged, thereby increasing the output current of the sub-module collection bridge arm while the power remains unchanged, ensuring that the output current of the sub-module collection bridge arm is greater than the charging current of the collection sub-module capacitor, thereby achieving sub-module energy balance.

[0128] In a preferred embodiment of the present invention, with the steady-state control of the above-mentioned collection submodule, the balanced distribution of internal energy is guaranteed to a certain extent. However, the above-mentioned control may exist to a certain extent, and the output current i of the collection submodule csm,i Greater than the overall DC current i dcm or i dc , resulting in voltage balancing failure. To overcome the above shortcomings, this embodiment adopts a constant DC current control mode. When the power output of each aggregation sub-module is uneven, and fully operational aggregation sub-modules still cannot fully output a specific high-power sub-module, energy storage can be used to smooth out some of the power in the short term. In the long term, a DC current control mode should be used to achieve energy balance control between the aggregation sub-modules.

[0129] Specifically, during normal operation, the onshore converter station only needs to provide DC voltage for the series-connected submodules. However, when the power imbalance between the system submodules is too large, resulting in the system DC current being less than the submodule DC current, the system will find it difficult to achieve voltage balancing. Therefore, additional current regulation on the DC side is required. At this time, DC current control needs to be introduced. The control strategy of the system is as follows: Figure 4 As shown in the figure, by changing the number of submodules in the flexible DC converter, the DC voltage is reduced while the capacitor voltage remains unchanged, thereby increasing the DC current while the power remains unchanged, thereby exceeding the DC current of the submodule and ensuring the energy balance of the submodule. The specific control is as follows:

[0130] m dc =PI idc (i dc -1.1Σi csm,imax );

[0131] Where i csm,imax For each submodule, the maximum current of the submodule in the bridge arm is collected, m dc is the submodule input coefficient, i dc is the DC current on the sea side, i.e. the sum of the bridge arm output currents collected by the submodules, PI idcis the DC current PI controller;

[0132] It's understandable that when the DC current exceeds the submodule current, the system's DC modulation coefficient is 1, and the proportional-integral controller output is 0. The proportional-integral controller is only enabled when the submodule current exceeds the DC current. When the DC current is greater than 1.2 times the sum of the currents, the wind turbines in each wind farm are assumed to be essentially identical in output. At this point, the system operates stably, and there's no need to change the DC voltage to achieve parallel aggregation of the series-connected submodules.

[0133] Preferably, the onshore converter station realizes frequency support by controlling the capacitor voltage of the flexible DC converter, including:

[0134] The onshore converter station adopts a grid-following or grid-forming type to control the capacitor voltage;

[0135] The grid-type phase-locked loop is shown as follows:

[0136]

[0137] Where s is the Laplace operator, ω ref is the controller frequency reference value, PI pll is the phase-locked loop PI controller, u q is the q-axis voltage, θ gfl Phase of flexible DC and grid controller;

[0138] When grid-following control is adopted, the inner loop adopts a current loop with cross decoupling and feedforward control, and the outer loop adopts constant capacitance voltage control and constant reactive power / voltage amplitude control, as shown in the following formula:

[0139] i dref_gfl =PI d (u csm,avg -u csm,ref ),i qref_gfl =PI q (Q out -Q ref );

[0140] Where PI d is the d-axis outer loop PI controller, PI q is the q-axis outer loop PI controller, u csm,avg is the average value of the capacitor voltage of the collection submodule, u csm,ref is the reference value of the capacitor voltage of the collection submodule, Q out is the reactive current feedback value, Q ref is the reactive current reference value, i dref_gfl For the grid-type control of the flexible DC d-axis current reference, i qref_gfl It is the reference of the q-axis current of the flexible DC motor for grid-following control;

[0141] When using grid-type control, the constant capacitance voltage control is as follows:

[0142]

[0143] Where s is the Laplace operator, ω ref is the controller frequency reference value, u q is the q-axis voltage, G uq is the voltage synchronous controller, u csm,avg is the average value of the capacitor voltage of the collection submodule, u csm,ref is the reference value of the capacitor voltage of the aggregation submodule, k dc is the capacitor voltage synchronous steady-state gain, k dch is the high-pass gain, τ dc is the filtering time constant, θ gfm Generate phase for flexible DC network controller;

[0144] The inner loop can adopt voltage and current dual closed-loop control, or voltage admittance outer loop, or voltage single loop and open-loop control;

[0145] When using voltage and current dual closed-loop control, the voltage outer loop is as follows:

[0146] i dref_gfm =PI ac (u dref -u d ),i qref_gfm =PI ac (u qref -u q );

[0147] Where PI ac is the voltage outer loop PI controller, u dref is the reference value of the d-axis voltage amplitude, and u qref is the reference value of the q-axis voltage amplitude, u q is the q-axis voltage, u d is the d-axis voltage, i dref_gfm For the grid-type control of the flexible direct current d-axis current reference, i qref_gfm It is the q-axis current reference for the flexible DC grid control.

[0148] Preferably, the onshore converter station, when determining the frequency change at the receiving end, controls the capacitor voltage of the flexible DC converter to achieve frequency support, including:

[0149] In the case of a collection mode without a DC transformer, when the receiving-end frequency changes, the receiving-end converter station is supported by adjusting the flexible DC capacitor voltage. The grid type is adjusted as follows:

[0150] u sm_ref =k uωΔω+u sm_0 ;

[0151] Among them, k uω is the DC voltage frequency droop coefficient; Δω is the frequency deviation of the flexible DC controller; u sm_ref is the reference value of the flexible DC converter submodule capacitance; u sm_0 The initial design value of the submodule voltage at the receiving-end converter station;

[0152] For mesh control, frequency support is provided by the capacitor voltage of the flexible direct submodule;

[0153] The submodules aggregate the bridge arms, and when the receiving end frequency changes, frequency support is achieved by controlling the capacitors of the aggregated submodules and the connected energy storage, including:

[0154] The submodule bridge arm adjusts the capacitor voltage according to the DC voltage at both ends. The pulse signal expression is as follows:

[0155]

[0156] Among them, k uu k is the droop coefficient of capacitor voltage to DC voltage, socu is the droop coefficient of the state of charge to DC voltage, Δu dc is the DC voltage change value on both sides of the collection submodule, when the collection submodule is connected to the DC transformer, it is Δu dcm , g ec It is the trigger pulse signal of the submodule when there is no energy storage in the current control mode; g esoc It is used to collect submodule trigger pulse signals when there is no energy storage and when there is energy storage in the current control mode.

[0157] In a preferred embodiment of the present invention, the onshore converter station controls the capacitor voltage through grid-following and grid-forming control, and controls the capacitor voltage through the flexible DC converter. Specifically, Figure 3 As shown in the figure. s with i s They are the grid connection point voltage and current respectively, and the corresponding dq axis voltage and current are u d ,u q with i d ,i q , k f With k d is the feedforward coefficient and cross decoupling coefficient, u sm_0 is the initial design value of the submodule voltage, PI pll , PI d ,PI q , PI i Phase-locked loop, d-axis outer loop, q-axis outer loop, current inner loop PI controller, PI acis the voltage outer loop PI controller, s is the Laplace operator, u dref with u qref are the reference values ​​of the d-axis and q-axis voltage amplitudes respectively. ref and ω are the controller frequency reference value and actual value respectively, G uq is the voltage synchronous controller, k dc , k dch With τ dc are the capacitor voltage synchronous steady-state gain, high-pass gain and filter time constant respectively. ref Modulates the reference voltage value for the converter.

[0158] When the number of flexible DC upper and lower submodules and bridge arm submodules remains constant, DC voltage control can be achieved by controlling the capacitor voltage. The grid-type phase-locked loop is shown in the following formula:

[0159]

[0160] Where s is the Laplace operator, ω ref is the controller frequency reference value, PI pll is the phase-locked loop PI controller, u q is the q-axis voltage, θ gfl For the phase of flexible DC grid-following controller; for traditional grid-following control, the inner loop adopts a current loop with cross decoupling and feedforward control, and the outer loop adopts constant capacitance voltage control and constant reactive power / voltage amplitude control. Specifically, the outer loop can be expressed as

[0161] i dref_gfl =PI d (u csm,avg -u csm,ref ),i qref_gfl =PI q (Q out -Q ref );

[0162] Where PI d is the d-axis outer loop PI controller, PI q is the q-axis outer loop PI controller, u csm,avg is the average value of the capacitor voltage of the collection submodule, u csm,ref is the reference value of the capacitor voltage of the collection submodule, Q out is the reactive current feedback value, Q ref is the reactive current reference value, i dref_gfl For the grid-type control of the flexible DC d-axis current reference, i qref_gfl It is the reference of the q-axis current of the flexible DC motor for grid-following control;

[0163] For grid-type control, the constant capacitor voltage has the following expression:

[0164]

[0165] Where s is the Laplace operator, ω ref is the controller frequency reference value, u q is the q-axis voltage, G uq is the voltage synchronous controller, u csm,avg is the average value of the capacitor voltage of the collection submodule, u csm,ref is the reference value of the capacitor voltage of the aggregation submodule, k dc is the capacitor voltage synchronous steady-state gain, k dch is the high-pass gain, τ dc is the filtering time constant, θ gfm Generate phase for flexible DC network controller;

[0166] The inner loop can adopt voltage and current dual closed loop control, or voltage admittance outer loop, voltage single loop and open loop control. Taking voltage and current dual closed loop as an example, the voltage outer loop is:

[0167] i dref_gfm =PI ac (u dref -u d ),i qref_gfm =PI ac (u qref -u q );

[0168] Where PI ac is the voltage outer loop PI controller, u dref is the reference value of the d-axis voltage amplitude, and u qref is the reference value of the q-axis voltage amplitude, u q is the q-axis voltage, u d is the d-axis voltage, i dref_gfm For the grid-type control of the flexible direct current d-axis current reference, i qref_gfm It is the q-axis current reference for the flexible DC grid control.

[0169] Furthermore, the above analysis ensures the reliable integration of offshore multi-energy systems under steady-state operation. When the receiving grid frequency changes, the capacitor voltage can be further adjusted to support the receiving converter station. The grid type only needs to be adjusted as follows, which are specifically expressed as follows:

[0170] u sm_ref =k uω Δω+u sm_0 ;

[0171] Among them, k uω is the DC voltage frequency droop coefficient; Δω is the frequency deviation of the flexible DC controller; u sm_ref is the reference value of the flexible DC converter submodule capacitance; u sm_0The initial design value of the submodule voltage at the receiving-end converter station;

[0172] Based on this DC voltage, such as Figure 5 As shown, the collection submodule then adjusts the capacitor voltage according to the DC voltage. The specific expression is as follows:

[0173]

[0174] Among them, k uu k is the droop coefficient of capacitor voltage to DC voltage, socu is the droop coefficient of the state of charge to DC voltage, Δu dc is the DC voltage change value on both sides of the collection submodule, when the collection submodule is connected to the DC transformer, it is Δu dcm , g ec It is the trigger pulse signal of the submodule when there is no energy storage in the current control mode; g esoc It is used to collect submodule trigger pulse signals when there is no energy storage and when there is energy storage in the current control mode.

[0175] Preferably, the DC transformer, when determining the frequency change at the receiving end, controls the capacitance of the DC transformer to achieve frequency support, including:

[0176] The DC transformer can achieve frequency transmission and its own frequency support by changing the DC voltage on both sides of the bridge arm collected by the submodule and the capacitor voltage of its own submodule.

[0177] Preferably, the DC transformer adopts a modular multi-level converter, or an isolated or non-isolated type.

[0178] In a preferred embodiment of the present invention, in order to verify the effectiveness of the above control strategy, two groups of simulations were carried out, using a sub-module aggregation method without energy storage and step-up transformers. Here, the output of new energy is simplified to a constant power source, the number of sub-modules in series is 40, the sub-module capacitance is 10mF, the rated power is 1.25MW, the rated power of the offshore multi-energy system DC network system is 50MW, the series sub-modules in series sub-modules have an aggregated bridge arm inductance of 100mH and a 0.5Ω resistance. The number of flexible DC converter sub-modules is 15, the capacitor 10mF sub-modules have an aggregated bridge arm inductance of 10mH, the sub-modules have an aggregated bridge arm resistance of 0.25Ω, and the rated DC voltage is 36kV. The DC line uses the equivalent value of π, 600 km line, 0.5Ω / 600km, 0.1mH / km, 0.376μF / km. The control parameters are shown in the following table:

[0179]

[0180] from Figure 6It can be seen that when the output is unbalanced, the total DC current may be smaller than the DC current of a certain sub-module, resulting in an increase in the capacitor voltage of the aggregated sub-module. After controlling the DC current to be greater than the overall DC current, the sub-module voltage begins to recover to the rated value, demonstrating the feasibility of the proposed capacitor voltage balancing control strategy under the coordinated control of the offshore multi-energy system.

[0181] from Figure 7 It can be seen that when the system frequency changes, the proposed control strategy can coordinate the capacitor energy voltage adjustment to provide corresponding frequency support, which shows the feasibility of frequency support of the proposed offshore multi-energy DC networking system.

[0182] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A full DC collection system for an offshore multi-energy power generation system, characterized in that: include: A submodule collection bridge arm formed by connecting collection submodules in series, and an onshore converter station; Onshore converter station fixed DC voltage control, connecting submodules to aggregate bridge arms through DC lines, or connecting submodules to aggregate bridge arms after voltage reduction through DC transformers; The submodule aggregation bridge arm is used to adjust the number of aggregation submodules input according to the DC voltage, combined with the capacitor voltage sequence of each aggregation submodule and the PI controller, to control the capacitor voltage value of the aggregation submodule; as well as, When the receiving-end frequency change is determined, frequency support is achieved by controlling the capacitor and energy storage of the collection submodule; wherein the collection submodule adopts a half-bridge submodule structure, and the capacitor side is connected in parallel with the DC wind turbine, wave power generation, photovoltaic, and / or energy storage of the offshore multi-energy power generation system to achieve multi-energy collection; The onshore converter station is configured to adjust the DC voltage on both sides of the submodule aggregating bridge arm through a flexible DC converter when it is determined that the output current of the submodule aggregating bridge arm is less than the charging current of the aggregating submodule capacitor, so that the output current of the submodule aggregating bridge arm is greater than the charging current of the aggregating submodule capacitor, thereby maintaining control of the aggregating submodule capacitor voltage; and When the receiving-end frequency changes, frequency support is achieved by controlling the capacitor voltage of the flexible DC converter; The DC transformer is used to adjust the DC voltage on both sides of the submodule aggregating bridge arm when it is determined that the output current of the submodule aggregating bridge arm is less than the aggregating submodule capacitor charging current, so that the output current of the submodule aggregating bridge arm is greater than the aggregating submodule capacitor charging current, thereby maintaining the control of the capacitor voltage; and When the receiving-end frequency change is determined, frequency support is achieved by controlling the capacitor voltage of the DC transformer submodule.

2. The full DC collection system of an offshore multi-energy power generation system according to claim 1, characterized in that: The capacitor side of the collection submodule is also connected in parallel to limit the capacitor voltage and to cut off the energy consumption system and bypass switch of the collection submodule in a fault state; Among them, the DC wind turbine and the wave power generation both use AC / DC converters and DC-DC converters to access the collection sub-module; the photovoltaic system uses a DC-DC converter to access the collection sub-module; the DC wind turbine, wave power generation, and photovoltaic system all use maximum power tracking control mode, and are equipped with fault isolation switches to cut off the faulty new energy in the event of a fault.

3. The full DC collection system of an offshore multi-energy power generation system according to claim 2, characterized in that: The submodule aggregation bridge arm is composed of multiple aggregation submodules and an inductor connected in series; One or more submodules are connected in parallel to form an offshore submodule to collect DC current, which is then connected to the onshore converter station, thereby realizing the collection of the offshore DC system; or One or more sub-module aggregation bridge arms are connected in parallel to form an offshore sub-module aggregation DC current, which is then connected to a DC transformer and transmitted to an onshore converter station to meet the voltage insulation requirements of the sub-module aggregation bridge arm.

4. The full DC collection system of the offshore multi-energy power generation system according to claim 3, characterized in that: The submodule aggregation bridge arm adjusts the number of aggregation submodules input according to the DC voltage, combined with the capacitor voltage sequence of each aggregation submodule and the PI controller, to control the capacitor voltage value of the aggregation submodule, including: The capacitor voltage of the aggregation submodule is stepped down and sorted by balancing control. When energy storage is used, the state of charge of the energy storage of the aggregation submodule is sorted, as shown below: ； ； Where, is the state of charge sequence of the energy storage of the aggregated submodules, is the capacitor voltage sorting sequence of the aggregation submodule, is the capacitor voltage series of the unsorted aggregation submodule, is the state of charge sequence of the energy storage of the unsorted aggregation submodules.

5. The full DC collection system of the offshore multi-energy power generation system according to claim 4, characterized in that: The submodule aggregation bridge arm adjusts the number of aggregation submodules according to the DC voltage, the capacitor voltage sequence of each aggregation submodule and the PI controller to control the capacitor voltage value of the aggregation submodule, and further includes: According to the following formula, the number of submodules put into operation is adjusted by the difference between the average submodule capacitor voltage and the reference value of the aggregate submodule capacitor voltage, or by the integral of the difference between the state of charge and the energy storage state of charge reference value, thereby performing carrier phase shift or nearest level approximation modulation: ； ； Where, is the reference value of the capacitor voltage of the aggregation submodule, is the reference value of energy storage charge state, is the capacitor voltage of the i-th collection submodule, is the total number of aggregated submodules, is the state of charge of the energy storage of the i-th aggregation submodule containing energy storage, is the capacitor voltage PI controller, is the PI controller for the state of charge, is the initial input number of the submodule, For the nearest level approach / carrier phase shift modulation, It is the trigger pulse signal of the collection submodule when there is no energy storage in steady state. It is the trigger pulse signal of the collection submodule when there is energy storage in steady state.

6. The full DC collection system of the offshore multi-energy power generation system according to claim 5, characterized in that: The onshore converter station, when determining that the output current of the submodule collecting bridge arm is less than the charging current of the collecting submodule capacitor, adjusts the DC voltage on both sides of the submodule collecting bridge arm through the flexible DC converter, so that the output current of the submodule collecting bridge arm is greater than the charging current of the collecting submodule capacitor, thereby maintaining the control of the collecting submodule capacitor voltage, including: By changing the number of submodules in the flexible DC converter station, the DC voltage at both ends of the submodule's bridge arm is reduced while the capacitor voltage remains unchanged, thereby increasing the submodule's bridge arm current while the power remains unchanged, ensuring that the submodule's bridge arm output current is greater than the submodule's capacitor charging current, thus achieving submodule energy balance. According to the following formula, the DC modulation system is controlled to adjust the submodule input coefficient of the receiving-end flexible DC converter station: ； Where, For each submodule, the maximum current of the submodule in the bridge arm is collected. is the DC modulation coefficient, is the DC current on the offshore side, i.e. the sum of the bridge arm output currents collected by the submodules. is the DC current PI controller; When the output current of the submodule aggregation bridge arm is greater than the aggregation submodule capacitor charging current, the system DC modulation coefficient is 1, and the proportional-integral controller outputs 0. When the aggregation submodule capacitor charging current is greater than the submodule aggregation bridge arm output current, the proportional-integral controller is enabled. When the sum of the submodule aggregation bridge arm output currents is greater than 1.2 times the sum of the maximum values ​​of the aggregation submodule currents in each submodule aggregation bridge arm, the energy balance between the aggregation submodules is determined. At this time, the system operates stably without changing the DC voltage on both sides of the submodule aggregation bridge arm to adjust the submodule aggregation bridge arm current. The DC transformer, when determining that the output current of the submodule collecting bridge arm is less than the collecting submodule capacitor charging current, adjusts the DC voltage on both sides of the submodule collecting bridge arm, so that the output current of the submodule collecting bridge arm is greater than the collecting submodule capacitor charging current, thereby maintaining the control of the capacitor voltage, including: By changing the input number of the new energy side of the DC transformer, the DC voltage at both ends of the sub-module collection bridge arm is reduced while the capacitor voltage remains unchanged, thereby increasing the output current of the sub-module collection bridge arm while the power remains unchanged, ensuring that the output current of the sub-module collection bridge arm is greater than the charging current of the collection sub-module capacitor, thereby achieving sub-module energy balance.

7. The full DC collection system of the offshore multi-energy power generation system according to claim 6, characterized in that: The onshore converter station realizes frequency support by controlling the capacitor voltage of the flexible DC converter, including: The onshore converter station adopts a grid-following or grid-forming type to control the capacitor voltage; The grid-type phase-locked loop is shown as follows: ； Where s is the Laplace operator, is the controller frequency reference value, is a phase-locked loop PI controller, is the q-axis voltage, Phase of flexible DC converter and grid controller; When grid-following control is adopted, the inner loop adopts a current loop with cross decoupling and feedforward control, and the outer loop adopts constant capacitance voltage control and constant reactive power / voltage amplitude control, as shown in the following formula: ； Where, is the d-axis outer loop PI controller, is the q-axis outer loop PI controller, is the average value of the capacitor voltage of the collection submodule, is the reference value of the capacitor voltage of the aggregation submodule, is the reactive current feedback value, is the reactive current reference value, To control the d-axis current reference of the flexible DC converter with grid-following control, It is the q-axis current reference of the flexible DC converter for grid-following control; When using grid-type control, the constant capacitance voltage control is as follows: ； Where s is the Laplace operator, is the controller frequency reference value, is the q-axis voltage, For voltage synchronous controller, is the average value of the capacitor voltage of the collection submodule, is the reference value of the capacitor voltage of the aggregation submodule, is the capacitor voltage synchronous steady-state gain, is the high-pass gain, is the filtering time constant, Generate phase for flexible DC converter grid controller; The inner loop adopts voltage and current dual closed-loop control, or adopts voltage admittance outer loop, or voltage single loop and open-loop control; When using voltage and current dual closed-loop control, the voltage outer loop is as follows: ； Where, is the voltage outer loop PI controller, is the d-axis voltage amplitude reference value, is the reference value of the q-axis voltage amplitude, is the q-axis voltage, is the d-axis voltage, For the grid-type control of flexible DC converter d-axis current reference, It is the q-axis current reference of the grid-controlled flexible DC converter.

8. The full DC collection system of the offshore multi-energy power generation system according to claim 7, characterized in that: The onshore converter station, when determining a frequency change at the receiving end, controls the capacitor voltage of the flexible DC converter to achieve frequency support, including: In the case of a collection mode without a DC transformer, when the receiving-end frequency changes, the flexible DC converter adjusts the capacitor voltage to achieve frequency support at the receiving-end converter station. The grid-following control is adjusted as follows: ； in, is the DC voltage frequency droop coefficient; is the frequency deviation of the flexible DC converter controller; is the reference value of the capacitance of the flexible DC converter submodule; The initial design value of the submodule voltage at the receiving-end converter station; For grid-type control, frequency support is provided by the capacitor voltage of the flexible DC converter submodule; The submodules aggregate the bridge arms, and when the receiving end frequency changes, frequency support is achieved by controlling the capacitors of the aggregated submodules and the connected energy storage, including: The submodule bridge arm adjusts the capacitor voltage according to the DC voltage at both ends. The pulse signal expression is as follows: ； ； in, is the droop coefficient of capacitor voltage to DC voltage, is the droop coefficient of the state of charge to DC voltage, is the DC voltage change value on both sides of the collection submodule. When the collection submodule is connected to the DC transformer, , To collect submodule trigger pulse signals when there is no energy storage in current control mode; It is used to collect submodule trigger pulse signals when there is no energy storage and when there is energy storage in the current control mode.

9. The full DC collection system of the offshore multi-energy power generation system according to claim 8, characterized in that: The DC transformer, when determining a frequency change at the receiving end, controls the DC transformer capacitor voltage to achieve frequency support, including: The DC transformer can achieve frequency transmission and its own frequency support by changing the DC voltage on both sides of the bridge arm collected by the submodule and the capacitor voltage of its own submodule.

10. The full DC collection system of the offshore multi-energy power generation system according to claim 9, characterized in that: The DC transformer adopts a modular multi-level converter and is of isolated or non-isolated type.

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