Direct current collection system
By employing series capacitors and independently controlled converter modules in the photovoltaic DC collection system, the problem of decoupling control between bus voltage and neutral point voltage is solved, achieving high-precision and fast-response voltage stability and potential balance, thus improving the stability and coordination of the system.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- INST OF ELECTRICAL ENG CHINESE ACAD OF SCI
- Filing Date
- 2026-04-09
- Publication Date
- 2026-06-26
AI Technical Summary
In existing technologies, it is difficult to decouple the bus voltage stability and midpoint voltage balance of photovoltaic DC collection systems, resulting in decreased control accuracy and insufficient dynamic performance.
The system employs a first capacitor and a second capacitor connected in series, and independently controls the midpoint potential and bus voltage through the first converter module and the second converter module, respectively. It utilizes a PI regulator and a model predictive controller to achieve independent feedback targets, thus avoiding the problem of affecting the midpoint potential when adjusting the bus voltage in traditional methods.
This enables independent adjustment of the bus voltage and the capacitor midpoint potential, improving control accuracy and dynamic performance, and enhancing the stability and coordination of the system.
Smart Images
Figure CN121983940B_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of power electronics technology, specifically providing a DC collection system. Background Technology
[0002] With the rapid development of distributed new energy systems, photovoltaic (PV) power generation systems are widely used in smart grids and microgrids. To improve energy transmission efficiency, PV output is typically connected to a common DC bus via a DC-DC converter and further aggregated into a DC distribution network. However, due to the uncertainty of PV power supply and dynamic load changes, the DC bus voltage is prone to fluctuations, affecting the stable operation of the system. Furthermore, when using a voltage divider capacitor structure in the aforementioned PV aggregation process, midpoint drift is likely to occur, leading to uneven capacitor stress, voltage imbalance, and asymmetrical system operation.
[0003] Traditional capacitor midpoint potential balancing methods typically rely on introducing zero-sequence components into the modulation signal or on the coupling of multiple bridge arms in the topology to indirectly adjust the midpoint potential. These methods essentially treat midpoint potential adjustment as a byproduct of the adjustment process of the primary control objective (such as bus voltage or output current), thus limiting the adjustment speed and control response to the bandwidth of the primary control variable. They also struggle to independently adjust the midpoint offset under complex disturbances. Furthermore, coupling interference exists between the two objectives of bus voltage stability and midpoint voltage balance, making effective decoupling between them difficult, leading to decreased control accuracy or insufficient system dynamic performance. Summary of the Invention
[0004] This application aims to solve the aforementioned technical problem, namely, how to decouple the bus voltage stability and midpoint voltage balance of a photovoltaic DC collection system.
[0005] In a first aspect, this application provides a DC charging system, which includes a first capacitor, a second capacitor, a first converter module, and a second converter module;
[0006] The first capacitor and the second capacitor are connected in series between the DC bus on the input side;
[0007] The input terminal of the first converter module is coupled to both ends of either the first capacitor or the second capacitor, and is used to balance the midpoint potential between the first capacitor and the second capacitor.
[0008] The input terminal of the second converter module is coupled to the DC bus and is used to control the stable output of the DC bus.
[0009] In some embodiments, the first converter module includes a first DC-DC converter and a first regulation circuit;
[0010] The input terminal of the first DC-DC converter is coupled to both ends of either the first capacitor or the second capacitor;
[0011] The first adjustment circuit is used to generate a first control signal based at least on the measured midpoint potential, the target midpoint potential, and the first measured inductor current output by the first DC-DC converter, so as to drive and control the first DC-DC converter based on the first control signal; wherein, the second converter module includes a second DC-DC converter and a second adjustment circuit;
[0012] The input terminal of the second DC-DC converter is coupled to the DC bus;
[0013] The second adjustment circuit is used to generate a second control signal based on the measured bus output voltage, the reference bus output voltage and the second measured inductor current output by the second DC-DC converter, so as to drive and control the second DC-DC converter based on the second control signal.
[0014] In some embodiments, the first regulation circuit includes a subtractor, a first voltage loop PI regulator, and a first current loop PI regulator;
[0015] The subtractor is used to determine the midpoint potential offset based on the measured midpoint potential and the target midpoint potential.
[0016] The first voltage loop PI regulator is used to determine the first reference inductor current based on the midpoint potential offset;
[0017] The first current loop PI regulator is used to generate the first control signal based on the offset between the first reference inductor current and the first measured inductor current.
[0018] In some embodiments, the first adjustment circuit employs a model predictive controller based on state-space modeling. The model predictive controller takes the measured midpoint potential, the first measured inductor current, the target midpoint potential, and the sampling time duty cycle as inputs and outputs a first control signal to balance the midpoint potential.
[0019] In some embodiments, the second regulation circuit includes a second voltage loop PI regulator and a second current loop PI regulator;
[0020] The second voltage loop PI regulator is used to determine the second reference inductor current based on the offset between the measured bus output voltage and the reference bus output voltage;
[0021] The second current loop PI regulator is used to generate the second control signal based on the offset between the second reference inductor current and the second measured inductor current.
[0022] In some embodiments, a first converter module and a second converter module are used as a group of dual converter modules. The DC collection system may be provided with multiple groups of dual converter modules, which are connected in parallel to the output side of the DC bus.
[0023] In some embodiments, each group of dual converter modules is pre-configured with a priority; the first converter module in each group of dual converter modules is further configured to determine whether the master control converter module needs to take over based on the offset between the measured bus output voltage and the reference bus output voltage and the rate of change of the bus current; if so, a takeover request is sent to the master control converter module; wherein, the master control converter module is the dual converter module with the highest priority among the multiple groups of dual converter modules.
[0024] In some embodiments, the dual converter module with the second highest priority among the multiple dual converter modules is also used to automatically upgrade when it does not receive a heartbeat frame sent by the master converter module within a preset time.
[0025] In some embodiments, among the multiple sets of dual converter modules, the reference bus output voltage corresponding to the master converter module is greater than the reference bus output voltage corresponding to the other converter modules.
[0026] By adopting the above technical solution, this application can provide a DC collection system. By separately setting a first converter module coupled to both ends of either the first capacitor or the second capacitor, it is used to balance the midpoint potential between the first capacitor and the second capacitor. At the same time, the second converter module controls the stable output of the DC bus. This realizes that the bus voltage and the capacitor midpoint potential can be adjusted by different control channels, so that the bus voltage and the capacitor midpoint potential correspond to independent feedback targets. This avoids the problem in traditional systems where adjusting the bus voltage disturbs the midpoint potential at the same time, or affects the power output path when adjusting the midpoint. It is beneficial to achieve decoupled control between bus voltage stability and midpoint voltage balance. Attached Figure Description
[0027] The preferred embodiments of this application are described below with reference to the accompanying drawings, in which:
[0028] Figure 1 This is a schematic diagram of a DC collection system provided in an embodiment of this application;
[0029] Figure 2 This is a schematic diagram of a DC collection system provided in another embodiment of this application;
[0030] Figure 3 This is a schematic diagram of the first adjustment circuit provided in an embodiment of this application;
[0031] Figure 4 This is a schematic diagram of the second adjustment circuit provided in the embodiments of this application. Detailed Implementation
[0032] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.
[0033] Unless otherwise defined, the technical or scientific terms used in this disclosure shall have the ordinary meaning understood by one of ordinary skill in the art to which this disclosure pertains. The terms “first,” “second,” and similar terms used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as “comprising” or “including” mean that an element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as “coupled” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect.
[0034] This application provides a DC charging system, which includes a first capacitor, a second capacitor, a first converter module, and a second converter module.
[0035] The first capacitor and the second capacitor are connected in series between the DC bus on the input side; the input terminal of the first converter module is coupled to both ends of either the first capacitor or the second capacitor to balance the midpoint potential between the first capacitor and the second capacitor; the input terminal of the second converter module is coupled to the DC bus to control the stable output of the DC bus.
[0036] In this embodiment, balancing the midpoint potential can be achieved by making the voltage divisions of the first and second capacitors equal or close, and the intermediate potential between the first and second capacitors approaching or equaling the target midpoint potential, which can be equal to half of the bus output voltage. Controlling the stable output of the DC bus can be achieved by controlling the DC bus output voltage to approach or remain constant at the reference bus output voltage.
[0037] See Figure 1 As shown, Figure 1This is a schematic diagram of a DC-DC aggregation system provided in an embodiment of this application, with the input terminal of the first converter module 11 coupled to the two ends of the second capacitor C2 as an example.
[0038] In the embodiments of this application, such as Figure 1 As shown, this DC power collection system can use PV as an input power source for power supply. In other embodiments, the DC power collection system provided in this application is also applied to scenarios where other types of power sources are used for power supply.
[0039] The DC-DC converter system can also be equipped with an MPPT (Maximum Power Point Tracking) module. The MPPT module is positioned between the PV and the dual-capacitor structure (first capacitor C1 and second capacitor C2) to dynamically adjust the operating voltage, ensuring that the PV always operates at its maximum output power point. In some embodiments, such as... Figure 1 As shown, the output terminals of both the first converter module 11 and the second converter module 12 can be connected to a DC Grid.
[0040] In some embodiments, the first converter module 11 includes a first DC-DC converter (DCDC1) and a first adjustment circuit; the input terminal of the first DC-DC converter (DCDC1) is coupled to the two ends of either a first capacitor C1 or a second capacitor C2; the first adjustment circuit is used to generate a first control signal based at least on the measured midpoint potential, the target midpoint potential, and the first measured inductor current output by the first DC-DC converter (DCDC1), so as to drive and control the first DC-DC converter (DCDC1) based on the first control signal. See also Figure 2 As shown, Figure 2 This is a schematic diagram of a DC-DC converter system provided in another embodiment of this application, which takes the input terminal of the first DC-DC converter DCDC1 coupled to the two ends of the second capacitor C2 as an example.
[0041] In some embodiments, the first DC-DC converter DCDC1 may adopt a Buck topology, which may mainly include a switching transistor, an inductor, a freewheeling diode, and a filter capacitor.
[0042] In some embodiments, see Figure 3 As shown, Figure 3 This is a schematic diagram of the first adjustment circuit provided in an embodiment of this application. The first adjustment circuit includes a subtractor, a first voltage loop PI regulator, and a first current loop PI regulator. The subtractor is used to adjust the measured midpoint potential based on... and target midpoint potential Determine the midpoint potential offset The first voltage loop PI regulator is used based on the midpoint potential offset. Determine the first reference inductor current The first current loop PI regulator is used based on the first reference inductor current. and the first measured inductor current The offset generates the first control signal.
[0043] In some embodiments, the target midpoint potential It can be based on the measured bus output voltage Half of is determined, and its expression is as follows:
[0044]
[0045] Accordingly, the subtractor is based on the measured midpoint potential. and target midpoint potential Determine the midpoint potential offset It can be represented as:
[0046]
[0047] The midpoint potential offset can be used as a feedback variable and provided to the first voltage loop PI regulator, which is based on the midpoint potential offset. Determine the first reference inductor current It can be represented as:
[0048]
[0049] in, and The PI parameters represent the voltage loop. This represents an auxiliary time variable.
[0050] The first current loop PI regulator is based on the first reference inductor current. and the first measured inductor current The offset used to generate the first control signal can be expressed as:
[0051]
[0052] in, The first duty cycle represents the first control signal. and The PI parameter representing the current loop can drive and control the first DC-DC converter DCDC1 by generating a first control signal with a first duty cycle. Specifically, the first control signal with the first duty cycle can drive and control the switching transistors in the first DC-DC converter DCDC1 to achieve balanced control of the midpoint potential.
[0053] In other embodiments, the first regulating circuit may employ a model predictive controller based on state-space modeling, wherein the model predictive controller uses the measured midpoint potential. First measured inductor current Target midpoint potential and sampling time duty cycle As input, the first control signal is output to balance the midpoint potential.
[0054] The model predictive controller can be based on a discretized state-space module, and its state vector can be based on the first measured inductor current, the measured midpoint potential, and the duty cycle at the sampling time at time k. The definition can be expressed as:
[0055]
[0056] in, Represents the state vector. Representing the control input, the state transition equation can be expressed as:
[0057]
[0058] The output function can be represented as:
[0059]
[0060] in, , , The discretized state matrix representing the circuit parameters can be extracted using modeling tools such as MATLAB or PLECS in some embodiments:
[0061]
[0062] in, This represents the equivalent resistance of an inductor in series. Let L represent the sampling period, L represent the inductance value, and C represent the capacitance value. In this embodiment, the model objective can be set to predict the midpoint potential trajectory over the next N control periods, and the corresponding quadratic cost function J is constructed as follows:
[0063]
[0064] Where λ is the control smoothness weight, and the constraint term can be used to limit the duty cycle from changing too quickly. This quadratic cost function measures the degree to which the predicted midpoint potential voltage trajectory deviates from the expected value, i.e., the target midpoint potential. The optimization problem of minimizing the quadratic cost function is transformed into a quadratic programming problem (QP) and solved. Rolling optimization is performed in each control cycle, and the first duty cycle can be output based on the optimization result, achieving balanced control of the midpoint potential. The optimization problem of minimizing the quadratic cost function, after being transformed into a quadratic programming problem, can be expressed as:
[0065]
[0066] Where U is the duty cycle vector of the control variable for the next N time steps, and the optimization constraints may include: PWM (Pulse Width Modulation) signal resolution constraint 0≤D1(k)≤0.95; current slope constraint. , H represents the maximum permissible change in inductor current during one switching cycle, and H represents the Hessian matrix. This represents the transpose of the coefficient vector f.
[0067] The first regulation circuit adopts a model predictive controller based on state-space modeling, which is superior to the control based on PI regulator in terms of bandwidth and anti-interference, and is also beneficial for achieving rapid suppression of large fluctuations in the midpoint potential.
[0068] In some embodiments, see Figure 2 As shown, the second converter module 12 includes a second DC-DC converter (DCDC2) and a second regulation circuit; the input terminal of the second DC-DC converter (DCDC2) is coupled to the DC bus; the second regulation circuit is used to adjust the output voltage based on the measured bus voltage. Reference bus output voltage And the second measured inductor current output from the second DC-DC converter DCDC2 A second control signal is generated to drive and control the second DC-DC converter DCDC2 based on the second control signal.
[0069] In some embodiments, the second converter module 12 may adopt a synchronous buck topology, with the main circuit consisting of the main switch and the synchronous rectifier forming the upper and lower bridge arms, and the output terminal connected in series with an energy storage inductor and in parallel with a capacitor bank to filter out high-frequency ripple.
[0070] In some embodiments, see Figure 4 As shown, Figure 4 This is a schematic diagram of the second regulation circuit provided in an embodiment of this application. The second regulation circuit includes a second voltage loop PI regulator and a second current loop PI regulator; the second voltage loop PI regulator is used to adjust the measured bus output voltage. and reference bus output voltage The offset determines the second reference inductor current. The second current loop PI regulator is used based on the second reference inductor current. Second measured inductor current The offset generates a second control signal.
[0071] The second voltage loop PI regulator is based on the measured bus output voltage. and reference bus output voltage The offset determines the second reference inductor current. It can be represented as:
[0072]
[0073] in, and The PI parameter represents the voltage loop.
[0074] The second current loop PI regulator is based on the second reference inductor current. Second measured inductor current The offset used to generate the second control signal can be expressed as:
[0075]
[0076] in The second duty cycle of the second control signal. and The PI parameter represents the current loop. The second DC-DC converter (DCDC2) can be driven and controlled by generating a second control signal with a second duty cycle. Specifically, the switching transistors in the second DC-DC converter (DCDC2) can be driven and controlled by the second control signal with a second duty cycle to achieve stable control of the bus output voltage.
[0077] In this embodiment, the first converter module 11 uses the midpoint potential offset as input to construct a first adjustment circuit. Combined with a high-bandwidth current inner loop, the midpoint adjustment process can be completed independently of the main power channel, resulting in higher time-domain sensitivity and suppression capability. The second converter module 12 adopts a dual closed-loop controller structure combining voltage and current loops. It uses the reference bus output voltage as the main control target to construct a voltage-current cascaded adjustment mechanism, which features small steady-state error and fast dynamic response.
[0078] In some embodiments, a first converter module 11 and a second converter module 12 can be used as a set of dual converter modules. The DC collection system can be provided with multiple sets of dual converter modules, and the output terminals of the multiple sets of dual converter modules are connected in parallel to the DC bus.
[0079] In each dual-converter module, the output terminal of the second converter module 12 is coupled to the DC bus, and the input terminal of the first converter module 11 is connected in parallel between the first capacitor C1 and the negative terminal of the DC bus or in parallel between the second capacitor C2 and the positive terminal of the DC bus.
[0080] In some embodiments, each group of dual converter modules is pre-configured with a priority; the first converter module 11 in each group of dual converter modules is further configured to determine whether the master control converter module needs to take over based on the offset between the measured bus output voltage and the reference bus output voltage and the rate of change of the bus current; if so, a takeover request is sent to the master control converter module; wherein, the master control converter module is the dual converter module with the highest priority among the multiple groups of dual converter modules.
[0081] In some embodiments, the first converter module 11 in each dual-converter module group determines whether the main control converter module needs to take over based on the offset between the measured bus output voltage and the reference bus output voltage and the rate of change of the bus current, including:
[0082] The first converter module 11 in the current dual-converter module compares the bus voltage deviation with a first threshold and the bus current change rate with a second threshold. If both comparison results are greater than the threshold, it is determined that the main control converter module needs to take over. Specifically, it can be stated that the main control converter module needs to take over when the following conditions are met:
[0083]
[0084]
[0085] in, This represents the bus voltage deviation. Represents the first threshold. Represents bus current. Represents the rate of change of bus current. Represents the second threshold. This represents the first derivative of the bus current with respect to time t.
[0086] In some embodiments, the priority of each group of dual-converter modules can be expressed as follows: The following expression can be used to determine the dual-converter module with the highest priority as the master converter module from multiple dual-converter modules that are in monitoring status (i.e., health status is 1):
[0087]
[0088] Here, Master represents the main control converter module. This represents the health status of the i-th dual converter module.
[0089] In some embodiments, the dual converter module with the second highest priority among the multiple dual converter modules is also used to automatically upgrade when it does not receive a heartbeat frame sent by the master converter module within a preset time, that is, to use the dual converter module with the second highest priority as the master converter module.
[0090] The main control converter module and other dual converter modules can communicate via CAN (Controller Area Network) or SPI (Serial Peripheral Interface).
[0091] In some embodiments, among multiple sets of dual-converter modules, the reference bus output voltage corresponding to the master control converter module is greater than the reference bus output voltage corresponding to the other dual-converter modules. The reference bus output voltage is the preset target adjustment parameter corresponding to the second converter module 12. Accordingly, the reference bus output voltage corresponding to the second converter module 12 in the master control converter module is greater than the reference bus output voltage corresponding to the second converter module 12 in the other dual-converter modules. The reference bus output voltage corresponding to the second converter module 12 in the other dual-converter modules can be expressed as:
[0092]
[0093] in, This represents the reference bus output voltage corresponding to the second converter module 12 in the remaining dual converter modules. The voltage drop introduced can be flexibly set according to requirements; this is just an example. 2-3V can be selected. By setting the reference bus output voltage corresponding to the second converter module 12 in the other dual converter modules to be less than the reference bus output voltage corresponding to the second converter module 12 in the main control converter module, a subordinate, suppressed output soft control state is formed, which can prevent positive modulation overlap between multiple second converter modules 12 in multiple sets of dual converter modules.
[0094] The first converter module 11 of the multiple dual converter modules each operates independently, performing midpoint potential adjustment of their respective subarrays.
[0095] By setting priorities and using different reference bus output voltages as regulation targets, a master-slave coordination mechanism is established. Only the master converter module undertakes the bus voltage control task, while the remaining dual converter modules follow the voltage drop. This avoids modulation signal conflicts and power distribution chaos among multiple dual converter modules, enhancing the system's stability and coordination in multi-node environments. Furthermore, the second-highest priority dual converter module can perform heartbeat detection, priority judgment, and automatic master promotion functions, ensuring the system's self-healing capability in the event of master converter module failure, thus contributing to the system's stable operation.
[0096] The technical solutions of this application have been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of this application is obviously not limited to these specific embodiments. Without departing from the principles of this application, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of this application.
Claims
1. A DC collection system, characterized in that, It includes a first capacitor, a second capacitor, a first converter module, and a second converter module; The first capacitor and the second capacitor are connected in series between the DC bus on the input side; The input terminal of the first converter module is coupled to both ends of either the first capacitor or the second capacitor, and is used to balance the midpoint potential between the first capacitor and the second capacitor. The input terminal of the second converter module is coupled to the DC bus and is used to control the stable output of the DC bus; wherein, the second converter module includes a second DC-DC converter and a second regulation circuit; The input terminal of the second DC-DC converter is coupled to the DC bus; The second adjustment circuit is used to generate a second control signal based on the measured bus output voltage, the reference bus output voltage and the second measured inductor current output by the second DC-DC converter, so as to drive and control the second DC-DC converter based on the second control signal.
2. The DC collection system according to claim 1, characterized in that, The first converter module includes a first DC-DC converter and a first regulation circuit; The input terminal of the first DC-DC converter is coupled to both ends of either the first capacitor or the second capacitor; The first adjustment circuit is used to generate a first control signal based at least on the measured midpoint potential, the target midpoint potential, and the first measured inductor current output by the first DC-DC converter, so as to drive and control the first DC-DC converter based on the first control signal.
3. The DC collection system according to claim 2, characterized in that, The first regulation circuit includes a subtractor, a first voltage loop PI regulator, and a first current loop PI regulator; The subtractor is used to determine the midpoint potential offset based on the measured midpoint potential and the target midpoint potential. The first voltage loop PI regulator is used to determine the first reference inductor current based on the midpoint potential offset; The first current loop PI regulator is used to generate the first control signal based on the offset between the first reference inductor current and the first measured inductor current.
4. The DC collection system according to claim 2, characterized in that, The first adjustment circuit adopts a model predictive controller based on state-space modeling. The model predictive controller takes the measured midpoint potential, the first measured inductor current, the target midpoint potential, and the sampling time duty cycle as inputs and outputs a first control signal to balance the midpoint potential.
5. The DC collection system according to claim 1, characterized in that, The second regulation circuit includes a second voltage loop PI regulator and a second current loop PI regulator. The second voltage loop PI regulator is used to determine the second reference inductor current based on the offset between the measured bus output voltage and the reference bus output voltage; The second current loop PI regulator is used to generate the second control signal based on the offset between the second reference inductor current and the second measured inductor current.
6. The DC collection system according to claim 1, characterized in that, The DC collection system can be configured with multiple sets of dual converter modules, with one first converter module and one second converter module forming a group of dual converter modules, and the multiple sets of dual converter modules are connected in parallel to the output side of the DC bus.
7. The DC collection system according to claim 6, characterized in that, Each set of dual converter modules is pre-configured with a priority; the first converter module in each set of dual converter modules is also used to determine whether the master control converter module needs to take over based on the offset between the measured bus output voltage and the reference bus output voltage and the rate of change of the bus current; if so, a takeover request is sent to the master control converter module; wherein, the master control converter module is the dual converter module with the highest priority among the multiple sets of dual converter modules.
8. The DC collection system according to claim 7, characterized in that, The dual converter module with the second highest priority among the multiple dual converter modules is also used to automatically upgrade when it does not receive a heartbeat frame sent by the master converter module within a preset time.
9. The DC collection system according to claim 7, characterized in that, In the multiple sets of dual converter modules, the reference bus output voltage corresponding to the main control converter module is greater than the reference bus output voltage corresponding to the other converter modules.