A low-voltage treatment method for a power distribution network based on direct-current bus interconnection

Abstract

This invention discloses a low-voltage management method for distribution networks based on DC bus interconnection. On a DC bus with a generator unit, an inverter unit is connected in series with the grid side of a set of AC buses and at least one set of AC outgoing lines. A rectifier unit is connected in parallel with the load side of the AC buses. A voltage source control strategy based on the difference between the power supply voltage and the ideal load voltage is set to control the inverter unit. If a low-voltage phenomenon occurs on the load side of an AC bus or an AC outgoing line, the inverter unit, the generator unit, and the rectifier unit are controlled to provide voltage compensation to the corresponding AC bus or AC outgoing line through the generator unit and / or the grid. This invention eliminates the need for a separate rectifier unit connected to the AC outgoing lines, and the generator unit itself can continuously supply power, thus addressing the phenomenon of low voltage on multiple buses for extended periods while saving equipment investment costs.

Description

Technical Field

[0001] This invention relates to the field of power distribution technology, and in particular to a method for managing low voltage in power distribution networks based on DC bus interconnection. Background Technology

[0002] Distribution networks of 10 kV and above have low expansion capacity and limited distribution, and are located over long distances. For remote towns and villages, the power supply radius of distribution network lines is large and the wire diameter is small, which can lead to low voltage at the end of the line, seriously affecting the normal power supply of users, and causing economic losses and waste of resources.

[0003] Currently, low-voltage management of distribution networks proposes using dynamic voltage restorers (DVRs) for reactive power compensation. When a voltage dip occurs in the system, the DVR injects an AC voltage with the same frequency as the system to offset the voltage change, injecting reactive power into sensitive loads or the distribution network to regulate the voltage. However, DC power supply is usually provided by energy storage devices, so it cannot provide power for extended periods to manage low-voltage problems at the end of the distribution network.

[0004] Compared to DVR, Unified Power Quality Conditioner (UPQC) can address the problem of prolonged low voltage; however, in actual distribution networks, adjacent AC buses may simultaneously experience low voltage at their terminals; when multiple lines require compensation at the same time, using UPQC for voltage compensation requires multiple parallel rectifier units, resulting in equipment redundancy and increasing investment and operation and maintenance costs. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-voltage management method for distribution networks based on DC bus interconnection, which can cope with the phenomenon of low voltage on multiple buses for a long time, while saving equipment investment costs.

[0006] A method for managing low-voltage distribution networks based on DC bus interconnection includes the following steps:

[0007] S1. On the DC bus with the power generation unit, an inverter unit is connected in series with the grid side of a set of AC buses and at least one set of AC output lines, and a rectifier unit is connected in parallel with the load side of the AC bus.

[0008] S2. Set a voltage source control strategy based on the difference between the power supply voltage and the ideal load voltage to control the inverter unit;

[0009] S3. If a low voltage occurs on the load side of the AC bus or the AC output line, the inverter unit, the power generation unit, and the rectifier unit are controlled to provide voltage compensation to the corresponding AC bus or the AC output line through the power generation unit and / or the power grid.

[0010] The beneficial effects of this invention are as follows: It provides a low-voltage management method for distribution networks based on DC bus interconnection. By connecting inverter units in series, multiple bus groups can share a single DC bus and the rectifier and generator units on the DC bus. During operation, the power grid and / or generator units, in conjunction with the corresponding inverter units, compensate for the voltage of AC buses with low voltage. For AC outgoing lines with low voltage, the power grid can compensate for the voltage through the rectifier units and / or generator units through the corresponding inverter units, eliminating the need to connect separate rectifier units to the AC outgoing lines. Furthermore, the generator units themselves can continuously supply power, thus addressing the phenomenon of low voltage on multiple buses for extended periods while saving on equipment investment costs. Attached Figure Description

[0011] Figure 1 This is a schematic diagram illustrating the steps of a low-voltage management method for distribution networks based on DC bus interconnection according to the present invention.

[0012] Figure 2 This is a circuit connection diagram of a low-voltage management method for distribution networks based on DC bus interconnection according to the present invention.

[0013] Figures 3 to 13 This is a power flow path diagram for a low-voltage management method for distribution networks based on DC bus interconnection under different conditions;

[0014] Figure 14 This invention provides an inverter and its control block diagram for a low-voltage management method for distribution networks based on DC bus interconnection.

[0015] Figure 15 This invention relates to a parallel rectifier and its control block diagram for a low-voltage management method for distribution networks based on DC bus interconnection.

[0016] Figure 16 This invention relates to a photovoltaic DC / DC unit and its MPPT control block diagram for a low-voltage management method for distribution networks based on DC bus interconnection. Detailed Implementation

[0017] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0018] Please refer to Figures 1 to 16 A method for low-voltage management of distribution networks based on DC bus interconnection includes the following steps:

[0019] S1. On the DC bus with the power generation unit, an inverter unit is connected in series with the grid side of a set of AC buses and at least one set of AC output lines, and a rectifier unit is connected in parallel with the load side of the AC bus.

[0020] S2. Set a voltage source control strategy based on the difference between the power supply voltage and the ideal load voltage to control the inverter unit;

[0021] S3. If a low voltage occurs on the load side of the AC bus or the AC output line, the inverter unit, the power generation unit, and the rectifier unit are controlled to provide voltage compensation to the corresponding AC bus or the AC output line through the power generation unit and / or the power grid.

[0022] As can be seen from the above description, the beneficial effects of the present invention are as follows: It provides a low-voltage management method for distribution networks based on DC bus interconnection. By connecting inverter units in series, multiple groups of buses can share a single DC bus and rectifier units and generator units on the DC bus. During operation, the power grid and / or generator units, in conjunction with the corresponding inverter units, compensate for the voltage of AC buses with low voltage. For AC outgoing lines with low voltage, the power grid can compensate for the voltage through the rectifier units and / or generator units through the corresponding inverter units, eliminating the need to connect separate rectifier units to the AC outgoing lines. Furthermore, the generator units themselves can continuously supply power, thereby addressing the phenomenon of low voltage on multiple buses for extended periods while saving on equipment investment costs.

[0023] Furthermore, the procedure prior to step S1 includes:

[0024] S0. Connect a DC / DC converter and a photovoltaic array in series on the DC bus to form the power generation unit.

[0025] As can be seen from the above description, the power generation unit consists of a DC / DC converter and a photovoltaic array, which converts light energy into electrical energy used for voltage compensation. This not only improves the sustainable power supply capacity and reduces the power supply burden, but also saves energy and is environmentally friendly.

[0026] Furthermore, step S3 also includes:

[0027] While providing voltage compensation through the power generation unit, the output signal of the photovoltaic array is continuously detected, and the control signal of the DC / DC converter is adjusted according to the output signal to achieve maximum power point tracking of the photovoltaic array.

[0028] As can be seen from the above description, by setting control strategies and adjusting the output capacity of the power generation unit, the functions of comprehensive management of power quality and low-voltage compensation at the end of the power grid can be effectively realized.

[0029] Furthermore, step S3 also includes:

[0030] S31. Obtain and select, based on the current active power output capability of the power generation unit, whether voltage compensation is provided jointly by the power generation unit and the power grid or by the power generation unit alone.

[0031] Specifically, step S31 includes:

[0032] The system obtains and determines whether the current active power output capability of the power generation unit is greater than or equal to the power output capability of the power grid. If so, the power generation unit provides voltage compensation to the corresponding AC bus or AC outgoing line on its own. Otherwise, the power generation unit and the power grid jointly provide voltage compensation to the corresponding AC bus or AC outgoing line.

[0033] As can be seen from the above description, the voltage compensation method is selected based on the current active power output capacity of the power generation unit. This ensures that the problem of low AC line voltage can be solved, while rationally allocating the output of the power generation unit and the power grid, thereby maximizing the utilization of power generation resources such as solar energy.

[0034] Furthermore, it also includes:

[0035] S4. If the voltage of the AC bus and all the AC outgoing lines is normal, determine whether the power generation unit has active power output. If so, supply power to the load connected to the AC bus through the power generation unit.

[0036] As can be seen from the above description, when voltage compensation is not required, the stored electrical energy of the power generation unit can be used to supply power to the load of the bus, reducing the power consumption of the power grid and making full use of the economic benefits generated by the power generation unit.

[0037] Further, step S2 specifically includes:

[0038] S21. Based on the difference between the power supply voltage and the ideal load voltage, establish the expression for the inverter unit in the dq coordinate system:

[0039]

[0040] Where ω is the synchronous rotation angular frequency, u ld and u lq For the load voltage, i lq and i ld For the load current, u sq and u sd The value is obtained after the power supply voltage has undergone Parker transformation, N is the turns ratio of the AC side transformer, and L1 is the inductive component.

[0041] S22. Taking voltage control as the outer loop, and using the load voltage as the three-phase balanced fundamental voltage that is in phase with the positive sequence of the power supply voltage, the reference value for the inverter's output voltage is:

[0042]

[0043] S23. Using current control as the inner loop and employing a proportional controller, the current inner loop control expression is obtained:

[0044]

[0045] Among them, u * ld and u * lq i is the reference value for the load voltage. * ld and i * lq u is the reference value for the inverter's output current. * d and u * q This is the reference value for the output voltage.

[0046] As can be seen from the above description, the inverter adopts a dual closed-loop control strategy with an outer voltage loop and an inner current loop. Its control source is a voltage source, which provides the difference between the ideal load voltage and the power supply voltage, thereby cooperating with the rectifier and the generator unit to maintain the stability of the AC line voltage.

[0047] Furthermore, step S2 also includes:

[0048] S24. Establish the expression of the rectifier unit in the dq coordinate system:

[0049]

[0050] Where L2 is the inductance component, R is the resistance value, and u d and u q This refers to the output voltage.

[0051] S25. Substitute the following equation into the expression of the rectifier in the dq coordinate system:

[0052]

[0053] Where, k pi The inner loop current ratio is denoted by kii, and the integral parameter is i. d and i q This refers to the output current of the rectifier.

[0054] get:

[0055]

[0056] Among them, i * d and i * q This is the active reference current.

[0057] As can be seen from the above description, the rectifier adopts a dual closed-loop PI control strategy, which can not only improve the voltage compensation capability of the inverter, but also achieve compensation for reactive power, current harmonics and imbalance on the load side, thereby improving voltage quality.

[0058] Please refer to Figures 1 to 15 The first embodiment of this application is as follows:

[0059] A method for managing low-voltage distribution networks based on DC bus interconnection includes the following steps:

[0060] S0. Connect a DC / DC converter and a photovoltaic array in series on the DC bus to form a power generation unit.

[0061] In this embodiment, the power generation unit can not only use photovoltaic power generation, but also other forms of power generation such as wind power generation and hydropower generation, depending on the specific application scenario. The purpose is to provide a long-term power supply and ensure the sustainability of the voltage compensation function.

[0062] S1. On the DC bus with the power generation unit, an inverter unit is connected in series with the grid side of a set of AC buses and at least one set of AC output lines, and a rectifier unit is connected in parallel with the load side of the AC bus.

[0063] In this embodiment, as Figure 2 and Figure 14 As shown, the inverter unit consists of a transformer and an inverter (AC / DC2 or 3). The transformer connects the AC bus or AC output line to the AC side of the inverter, and then the DC side of the inverter is connected to the DC bus. During compensation, the inverter converts the compensation voltage provided by the power generation unit into AC power with the same frequency and amplitude as the power grid, thus enabling grid-connected operation.

[0064] like Figure 2 and Figure 15 As shown, the rectifier unit consists of a rectifier, a transformer, and an inverter (AC / DC1). The transformer connects the AC bus or AC output line to the AC side of the rectifier, and then the DC side of the rectifier is connected to the DC bus. The AC bus supplies power to the inverter through a parallel rectifier. The inverter outputs a specific voltage, which is filtered and superimposed on the AC bus by a series inverter. This voltage can maintain the voltage stability at the end of the line for a long time, ensuring that the load is basically unaffected.

[0065] S2. Set a voltage source control strategy based on the difference between the power supply voltage and the ideal load voltage to control the inverter unit;

[0066] In this embodiment, combined with Figure 14 As shown, s represents the system-side component, l represents the load-side component, L1 represents the inductive component, N is the series transformer turns ratio, and uabc It is the output voltage of the series-side compensator. It is the command value of the output voltage of the series-side compensator, and ε is the DC voltage regulation coefficient, which is used to eliminate the influence of DC side voltage fluctuations on the output voltage of the compensator.

[0067] An isolated series-parallel AC / DC converter. The series inverter is controlled as a voltage source, providing the difference between the ideal load voltage and the supply voltage. The parallel rectifier is controlled as a current source, used to compensate for harmonic currents in the load, and simultaneously obtains energy directly from the system, i.e., continuously outputting energy through the rectifier circuit to the DC side of the series inverter to maintain the stability of the AC bus voltage. Therefore, the AC bus voltage compensation stage is designed, which includes the following processes:

[0068] S21. Based on the difference between the power supply voltage and the ideal load voltage, establish the expression for the inverter unit in the dq coordinate system:

[0069]

[0070] Where ω is the synchronous rotation angular frequency, u ld and u lq For the load voltage, i lq and i ld For the load current, u sq and u sd The value is obtained after the power supply voltage has been transformed by Parker transformer, N is the turns ratio of the AC side transformer, and L1 is the inductive component.

[0071] S22. Taking voltage control as the outer loop, and using the load voltage as the three-phase balanced fundamental voltage that is in phase with the positive sequence of the power supply voltage, the reference value for the inverter's output voltage is:

[0072]

[0073] S23. Using current control as the inner loop and employing a proportional controller, the current inner loop control expression is obtained:

[0074]

[0075] Among them, u * ld and u * lq i is the reference value for the load voltage. * ld and i * lq u is the reference value for the inverter's output current. * d and u * q This is the reference value for the output voltage.

[0076] S24. Establish the expression of the rectifier unit in the dq coordinate system:

[0077]

[0078] Where L2 is the inductance component, R is the resistance value, and u d and u q This refers to the output voltage.

[0079] S25. Substitute the following equation into the expression for the rectifier in the dq coordinate system to perform feedforward decoupling:

[0080]

[0081] Where, k pi The inner loop current ratio is denoted by kii, and the integral parameter is i. d and i q This refers to the output current of the rectifier.

[0082] get:

[0083]

[0084] Among them, i * d and i * q This is the active reference current. Therefore, i d with i q Complete decoupling and complete symmetry are achieved. The inner current loop in the dq coordinate system can be designed independently and is exactly the same.

[0085] Combination Figure 15 As shown, u abc i abc U represents the three-phase components. dc This refers to the DC-side voltage of the parallel compensator. The outer loop of the dual-closed-loop PI control strategy functions to stabilize the DC voltage. Therefore, the input DC voltage U... dc DC voltage reference value U * dc The comparison value is fed into the PI regulator to provide the active reference current i for the inner current loop. * d Typically, the reactive component of the reference current is set as i. * q =0. Thus, the overall dual-closed-loop PI control strategy structure design is complete.

[0086] S3. If a low voltage occurs on the load side of an AC bus or AC outgoing line, control the inverter unit, generator unit, and rectifier unit to provide voltage compensation to the corresponding AC bus or AC outgoing line through the generator unit and / or the power grid.

[0087] In this embodiment, step S3 includes:

[0088] S31. Obtain and, based on the current active power output capacity of the power generation unit, select whether voltage compensation is provided jointly by the power generation unit and the power grid, or solely by the power generation unit, and combine this with... Figures 3 to 13 This includes the following situations:

[0089] When both AC grid 1 (AC bus) and AC grid 2 (AC outgoing line) experience low-voltage issues, if there is insufficient sunlight and the active power provided by the photovoltaic array and DC / DC converter is lower than that of the grid, then the photovoltaic array, DC / DC converter, and grid simultaneously supply power to the series inverters AC / DC22 and AC / DC23 for low-voltage compensation. The power flow path is as follows: Figure 3 As shown;

[0090] If there is sufficient sunlight at this time, and the active power provided by the photovoltaic power supply unit is higher than that of the grid, then the photovoltaic array and the DC / DC converter provide low-voltage compensation through series inverters AC / DC2 and AC / DC23, while simultaneously providing active power to the grid through parallel inverter AC / DC1. The power flow path is as follows: Figure 4 As shown;

[0091] If it is nighttime and the photovoltaic array and DC / DC converter have no active power output, then the grid provides low-voltage compensation only through the parallel inverter AC / DC1. The power flow path is as follows: Figure 5 As shown.

[0092] When AC grid 1 experiences a low-voltage problem while AC grid 2 has normal voltage, under the same conditions, the photovoltaic array, DC / DC converter, and grid only need to supply power to inverter AC / DC2 to compensate for the low voltage of AC grid 1. The power flow path is as follows: Figures 6 to 8 As shown.

[0093] When AC grid 1 has normal voltage but AC grid 2 experiences a low-voltage problem, under the same conditions, the photovoltaic power supply unit and the grid only need to supply power to the series converter AC / DC 3 to compensate for the low voltage of AC grid 2. The power flow path is as follows: Figures 9 to 11 As shown.

[0094] S4. If the voltage of the AC bus and all AC outgoing lines is normal, determine whether the generator unit has active power output. If so, supply power to the load connected to the AC bus through the generator unit.

[0095] When the terminal voltages of both AC grid 1 and AC grid 2 are normal, the power flow only needs to consider whether the photovoltaic DC / DC unit is supplying power. During the daytime, when there is sunlight, the photovoltaic array and DC / DC converter provide active power to the grid load through the parallel converter AC / DC1. The power flow path is as follows: Figure 12 As shown; when there is no sunlight at night, the photovoltaic array and DC / DC converter have no active power output, and the power flow path is as follows. Figure 13 As shown.

[0096] Please refer to Figure 16 Embodiment two of this application is as follows:

[0097] A low-voltage management method for distribution networks based on DC bus interconnection, based on the above embodiment one, includes the following steps: Step S3 further includes:

[0098] When providing voltage compensation through the power generation unit, the output signal of the photovoltaic array is continuously monitored, and the control signal of the DC / DC converter is adjusted according to the output signal to achieve maximum power point tracking of the photovoltaic array.

[0099] In this embodiment, as Figure 16 As shown, the DC / DC converter uses a DC / DC Boost circuit. This Boost converter is mainly used to adjust the ratio of the output voltage to the input voltage, ensuring that the output voltage of the photovoltaic array matches the load voltage. Simultaneously, it changes the equivalent input resistance of the photovoltaic array (i.e., adjusts the duty cycle) to achieve maximum power point tracking (MPPT) control, thus fully utilizing solar energy. Connecting the photovoltaic power generation system to the DC bus effectively enables comprehensive power quality management and low-voltage compensation at the grid's end. The MPPT control strategy is as follows:

[0100] The output voltage and current U of the photovoltaic array are continuously monitored by the MPPT module. pv I pv The duty cycle of the PWM drive signal is adjusted according to its changes to obtain the control signal of the switching device S in the Boost converter. The photovoltaic maximum power point tracking is achieved by controlling the on and off of S.

[0101] In summary, the low-voltage management method for distribution networks based on DC bus interconnection provided by this invention has the following advantages:

[0102] 1. The AC bus supplies power to the series inverter through a parallel rectifier. The inverter outputs a specific voltage, which is filtered and superimposed on the AC bus by the series inverter. This can maintain the voltage stability at the end of the line for a long time, ensuring that the load is basically unaffected.

[0103] 2. By using parallel rectifiers, not only can the voltage compensation capability of series inverters be improved, but also the reactive power, current harmonics and imbalance on the load side can be compensated, thereby improving voltage quality.

[0104] 3. Taking advantage of the vast and sparsely populated remote areas, photovoltaic power generation units are integrated for power supply. Voltage compensation can be achieved through series inverters, or the power can be fed into the bus through parallel converters, reducing the burden on the system power supply. It has the advantages of no pollution and low noise, and can achieve energy conservation and emission reduction.

[0105] 4. Other AC output lines are connected to the DC bus via a bus-connected transformer and inverter, and then connected to the DC bus via a shared bus-connected transformer, rectifier, and photovoltaic DC / DC unit. This not only provides continuous and effective voltage compensation but also reduces manufacturing and operating costs, thus improving the economic efficiency of the device.

[0106] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention's specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for low-voltage management of distribution networks based on DC bus interconnection, characterized in that, Includes the following steps: S0. Connect a DC / DC converter and a photovoltaic array connected in series on the DC bus to form a power generation unit; S1. On the DC bus with the power generation unit, an inverter unit is connected in series with the grid side of a set of AC buses and at least one set of AC output lines, and a rectifier unit is connected in parallel with the load side of the AC bus. S2. Set a voltage source control strategy based on the difference between the power supply voltage and the ideal load voltage to control the inverter unit, specifically including: S21. Based on the difference between the power supply voltage and the ideal load voltage, establish the expression for the inverter unit in the dq coordinate system: ； in, To synchronize the rotational angular frequency, and This is the load voltage. and For load current, and The voltage is obtained after Parker transformation of the power supply voltage, and N is the turns ratio of the AC-side transformer. This is the inductive component; S22. Taking voltage control as the outer loop, and using the load voltage as the three-phase balanced fundamental voltage that is in phase with the positive sequence of the power supply voltage, the reference value for the inverter's output voltage is: ； S23. Using current control as the inner loop and employing a proportional controller, the current inner loop control expression is obtained: ； in, and This is the reference value for the load voltage. and This is the reference value for the inverter's output current. and This is a reference value for the output voltage; S24. Establish the expression of the rectifier unit in the dq coordinate system: ； Where L2 is the inductive component and R is the resistance value. and This refers to the output voltage. S25. Substitute the following equation into the expression of the rectifier in the dq coordinate system: ； in, k represents the ratio of the inner current loop. ii For integration parameters, and This refers to the output current of the rectifier. get: ； in, and This is the active reference current; S3. If a low voltage occurs on the load side of the AC bus or the AC output line, the inverter unit, the power generation unit, and the rectifier unit are controlled to provide voltage compensation to the corresponding AC bus or the AC output line through the power generation unit and / or the power grid. Step S3 further includes: while providing voltage compensation through the power generation unit, continuously detecting the output signal of the photovoltaic array, and adjusting the control signal of the DC / DC converter according to the output signal to achieve maximum power point tracking of the photovoltaic array; S31. Obtain and, based on the current active power output capability of the power generation unit, select whether voltage compensation is provided jointly by the power generation unit and the power grid, or whether the power generation unit provides voltage compensation alone, specifically including: The system obtains and determines whether the current active power output capability of the power generation unit is greater than or equal to the output power of the power grid. If so, the power generation unit provides voltage compensation to the corresponding AC bus or AC outgoing line on its own. Otherwise, the power generation unit and the power grid jointly provide voltage compensation to the corresponding AC bus or AC outgoing line.

2. The method for low-voltage management of distribution networks based on DC bus interconnection according to claim 1, characterized in that, Also includes: S4. If the voltage of the AC bus and all the AC outgoing lines is normal, determine whether the power generation unit has active power output. If so, supply power to the load connected to the AC bus through the power generation unit.

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