A circuit, device and method for comprehensive management of power quality in a DC power distribution system

Abstract

The present invention discloses a circuit, a device and a method for comprehensive management of power quality in a DC power distribution system. The circuit includes: a dual active bridge (DAB), a DC active power filter (DC-APF), and a backup converter; the device includes a power quality management circuit for the DC power distribution system and a control module; the method includes: detecting in real time whether there are voltage deviations, DC ripples, and voltage sags; when a voltage deviation is detected, controlling the diagonal switching tubes in the first DC-APF and the second DC-APF to conduct; when a DC ripple is detected, controlling the first DC-APF and the second DC-APF to output a reverse compensation current to suppress the ripple; when a voltage sag is detected, controlling the diagonal switching tubes of the second DC-APF and the backup converter to conduct, and turning off all the controlled devices switches and the DC / AC converter in the first DC-APF; the present invention can achieve autonomous and collaborative regulation of voltage and harmonics, and has the capabilities of DC ripple management, voltage sag suppression, and voltage deviation suppression.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution networks, and particularly relates to a circuit, device and method for comprehensive management of power quality in a DC distribution system. Background Art

[0002] The progress of new energy power generation technology and power electronics technology has changed the ecology of the development of DC distribution networks. Compared with AC distribution networks, DC distribution networks have obvious advantages in adapting to DC loads and DC distributed power sources. Although the power quality of DC distribution networks is better than that of AC distribution networks and is easy to control, its disturbance characteristics have unique features. Steady-state events such as voltage deviation, voltage fluctuation, and ripple, as well as transient events such as voltage sag / interruption and voltage swell, still deeply affect many aspects of the planning, operation, protection, and control of DC distribution networks. Steady-state events in DC microgrids affect the power supply quality of the system and the safe operation of electrical equipment. Among transient power quality problems, voltage sag is one of the events with the highest occurrence frequency. Loads in DC distribution networks, such as frequency converters and data centers, are more sensitive to sudden changes in voltage. Therefore, stricter requirements are imposed on the voltage sag of the DC bus. Sensitive loads, such as DC motors, precision machine tools, programmable logic controllers (PLCs), etc., are very sensitive to voltage sag. The failure of a single device may cause the products of the entire production line to be scrapped, resulting in huge economic losses. Voltage sag has been considered as the main power quality problem affecting the normal and safe operation of many electrical equipment.

[0003] For the control of the above-mentioned power quality events in DC distribution networks, through the access of power quality management equipment, autonomous coordinated regulation of voltage and harmonics can be realized, so as to have the ability to control DC ripple, voltage sag, and voltage fluctuation.

[0004] Because the equivalent model of a DC distribution network is not an infinite power source, power fluctuations in distributed power sources, local load variations, and power exchange at the ports of power electronic devices such as converters and flexible switches can all cause voltage deviations and sags in the DC distribution network. Currently, most DC power quality management devices achieve active power-voltage balance through battery charging and discharging. Reducing reliance on batteries and reducing the configuration and investment of energy storage equipment is crucial. Some existing technologies deploy energy storage at sensitive loads and design a current inner loop feedforward control strategy to suppress DC voltage fluctuations caused by load variations. For grids with high photovoltaic penetration, some existing technologies propose coordinated control of local droop and distributed control to regulate the charging and discharging of battery energy storage and suppress voltage fluctuations. Other existing technologies, drawing on research experience with electrical springs in AC power grids, utilize controllable loads to suppress voltage variations in the DC grid. Due to the limited capacity of controllable loads, voltage fluctuations are subject to a maximum limit. When the capacity falls outside this limit, the voltage cannot be controlled to the rated value, limiting the ability to control the DC voltage. To address this issue, researchers have proposed a comprehensive control strategy based on controllable loads and battery energy storage. While utilizing controllable loads can reduce battery capacity requirements and improve economic efficiency, it still requires investment in energy storage equipment. DC voltage deviations require larger control devices. Currently, most research on voltage deviation control requires investment in energy storage equipment. However, considering the initial investment and floor space requirements, research on voltage deviation / sag control equipment that reduces or does not rely on energy storage equipment is very promising and necessary.

[0005] At present, the ripple suppression control methods are divided into active control and passive control. Active control is to improve the device that generates ripples, but it will complicate the system structure and control strategy, increase costs and reduce efficiency. Passive control is to eliminate ripples by installing active filtering devices or passive filters. In order to solve the above problems, different ripple extraction methods and current tracking control strategies can be used to implement DC-APF to achieve fast response tracking of the compensation current. The Bode diagram after the DC-APF is equivalent to impedance can also be used to analyze its good filtering effect on secondary power frequency ripple, but the filtering effect on high-frequency ripple is poor. The combination of DC-APF and LC filter can achieve the effect of suppressing each secondary ripple. Other invention patents propose an active filter that suppresses secondary ripples. It is composed of a series inductor on the AC side of a single-phase voltage source converter (VSC). When in use, its DC side is connected to the DC bus where the secondary ripple needs to be suppressed.

[0006] Most of the existing technologies focus on steady-state power quality problems, and there are few literatures proposing governance measures for DC transient power quality. The Dynamic Voltage Restorer (DVR) has become one of the most effective and comprehensive voltage regulation and protection schemes in AC systems, while the development of DVR in DC systems is still in its infancy. A series voltage regulator (SVR) is used for voltage dip and swell regulation, but the DC power electronic transformer adopted has disadvantages such as complex system, high operation cost, and complex control. There is also a transformerless series voltage restorer for DC load voltage protection based on SMES (SMES-tlsvr). Although it has the advantage of being transformerless, due to compensating for the limited ability of the transformerless structure DVR to compensate for voltage dips, the proposal to use superconducting technology has increased the upfront investment.

[0007] As can be seen from the above-mentioned existing technologies, most of the currently proposed power quality equipment can only address single power quality problems, such as voltage deviation, sag, and ripple. However, due to the zero-frequency characteristic of DC voltage and the structure of the DC distribution network itself, there are coupling characteristics between ripple and fluctuation, voltage deviation and steady-state voltage imbalance. How to provide a device that can simultaneously address voltage deviation, sag, and ripple is an urgent problem for those skilled in the art. Summary of the Invention

[0008] In view of this, the present invention provides a comprehensive power quality governance circuit, device, and method for a DC distribution system to at least solve the problem of the lack of integrated equipment for governing multiple power quality events in the existing technology.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] A comprehensive power quality governance circuit for a DC distribution system includes: a dual active bridge (DAB), a DC active power filter (DC-APF), and a backup converter; wherein,

[0011] Both sides of the dual active bridge (DAB) are respectively connected to two DC buses through the DC active power filter (DC-APF). The two DC buses are BUS1 and BUS2 respectively. The DC active power filter (DC-APF) connected to both ends of BUS1 is the first DC-APF, and the DC active power filter (DC-APF) connected to both ends of BUS2 is the second DC-APF. The first DC-APF is connected to the primary full bridge of the dual active bridge (DAB) through a voltage stabilizing capacitor C1, and the second DC-APF is connected to the secondary full bridge of the dual active bridge (DAB) through a voltage stabilizing capacitor C2;

[0012] The backup converter is connected to both ends of the voltage stabilizing capacitor C1 and is connected to the sensitive load on the BUS1 side.

[0013] A device for comprehensively improving the power quality of a DC power distribution system, based on a circuit for comprehensively improving the power quality of a DC power distribution system, is connected to a DC / AC converter on the DC side, and includes a circuit for improving the power quality of a DC power distribution system and a control module;

[0014] The control module includes a detection unit and a switch control unit; among them,

[0015] The detection unit is used to detect voltage deviation, DC ripple and voltage sag;

[0016] When the detection unit detects a voltage deviation, the switch control unit controls the diagonal switching tubes in the first DC-APF and the second DC-APF to conduct, controls the dual active bridge (DAB) converter to adopt a voltage control mode to achieve voltage balance through power mutual assistance, and the DAB converter filters out the high-frequency ripple components generated in the high-frequency switching state. The DC / AC converter takes power from BUS1 to supply power to sensitive loads;

[0017] When the detection unit detects a DC ripple, the switch control unit controls the DAB converter to adopt a voltage control mode to suppress the voltage fluctuations of the DC side capacitors of the first DC-APF and the second DC-APF and maintain the stability of the DC side voltage. The first DC-APF and the second DC-APF control the output of reverse compensation current to suppress the ripple by tracking the ripple current on the DC bus side. The DC / AC converter takes power from BUS1 to supply power to sensitive loads;

[0018] When the detection unit detects a voltage sag, the switch control unit controls the diagonal switching tubes of the second DC-APF and the standby converter to conduct, and the DAB converter connects the second DC-APF and the standby converter through a voltage control mode. All the fully controlled device switches in the first DC-APF and the DC / AC converter are turned off, and the standby converter takes power from BUS2 to supply power to sensitive loads.

[0019] A method for comprehensively improving the power quality of a DC power distribution system, based on a device for comprehensively improving the power quality of a DC power distribution system, includes the following steps:

[0020] Realtime detect whether there are situations of voltage deviation, DC ripple and voltage sag;

[0021] When a voltage deviation is detected, control the diagonal switching tubes in the first DC-APF and the second DC-APF to conduct, control the DAB converter to adopt a voltage control mode to achieve voltage balance through power mutual assistance, and the DAB converter filters out the high-frequency ripple components generated in the high-frequency switching state. The DC / AC converter takes power from BUS1 to supply power to sensitive loads;

[0022] After detecting the DC ripple, the dual active bridge (DAB) converter is controlled to adopt the voltage control mode to suppress the voltage fluctuations of the DC-link capacitors of the first DC-APF and the second DC-APF and maintain the stability of the DC-link voltage. The first DC-APF and the second DC-APF control the output of the reverse compensation current to suppress the ripple by tracking the ripple current on the DC bus side. The DC / AC converter draws power from BUS1 to supply power to the sensitive load.

[0023] After detecting the voltage sag, the diagonal switching tubes and the standby converter of the second DC-APF are controlled to conduct, and the dual active bridge (DAB) converter connects the second DC-APF and the standby converter through the voltage control mode. All the controlled devices in the first DC-APF and the DC / AC converter are turned off, and the standby converter draws power from BUS2 to supply power to the sensitive load.

[0024] As can be seen from the above technical solutions, compared with the prior art, the present invention discloses a comprehensive power quality management circuit, device and method for a DC power distribution system, which has the following beneficial effects:

[0025] The present invention first adopts segmented DC bus power supply. The two segments of the bus are connected by a governance device capable of realizing bidirectional power flow. When a voltage deviation occurs at one segment of the bus, the other segment of the bus compensates for the active power through the device to achieve active power balance and maintain the DC bus voltage reference value. This solution realizes active power balance through the power mutual assistance between the segmented buses to achieve voltage balance, eliminating the investment in energy storage devices and greatly reducing the upfront capital investment. Here, the dual active bridge converter (DAB), which can realize bidirectional power flow and has been well studied, is first considered to be connected in parallel at both sides of the segmented buses.

[0026] Secondly, for the voltage sag event with a large voltage drop and a short duration, it is difficult to maintain the DC voltage by the normal bus assuming all the active power consumption of the loads at the fault bus through power mutual assistance. Therefore, at this time, only the voltage of the sensitive load on the fault bus side is considered first, that is, the sensitive load gives up taking power from the bus on the voltage sag side and is directly powered by the other segment of the DC bus through the governance device to maintain its normal operation. To ensure the normal power supply of the sensitive load on the fault bus side and the load on the normal bus side, the active power compensation and voltage stability of other loads on the voltage sag bus side are abandoned. Through the above analysis, when there is no voltage sag event, the sensitive load draws power from the bus. When a voltage sag occurs, the converter of the sensitive load at the bus is disconnected, and power is taken from the DC side of the device close to the fault bus, and the active power is prevented from flowing into the fault bus through the device.

[0027] Finally, in the current research on DC microgrid ripple control, the DC-APF can be used to compensate the AC component of the DC bus output to achieve the goal of suppressing current ripple. If the DC-APF is combined with the above-mentioned control equipment, the integration of voltage fluctuation, sag, and ripple control can be realized. When controlling voltage fluctuation, the DC-APF is not required to participate, that is, direct diagonal conduction can be used. When controlling voltage sag, the DC-APF acts as a switch to prevent active power from flowing into other loads on the faulty bus side. When controlling ripple, the DC-APF on the bus side outputs a compensation current to suppress ripple, and reactive power compensation is achieved through the DAB part to control the stability of the capacitor voltage in the DC-APF. Since both bus sections may need to suppress ripple, the DC-APF can be respectively connected in parallel to the bus and then connected in parallel with the above-mentioned DAB part.

[0028] By using the power mutual assistance of segmented buses to control voltage deviation, not only the problem of relying on energy storage equipment to achieve active power-voltage balance is solved, but also the economic benefits are improved. For sensitive loads on the voltage sag bus side, there is no need for a separate voltage restorer. Through the interconnection unit, the normal bus directly supplies power, and its control strategy is simpler and more direct. A new topology formed by efficiently combining modules for controlling different types of power quality problems can comprehensively control transient / steady-state power quality problems. Brief Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0030] Figure 1 The power quality control circuit of the DC distribution system provided by the present invention;

[0031] Figure 2 The schematic diagram of the operation mode for voltage deviation control provided by the embodiment of the present invention;

[0032] Figure 3 The schematic diagram of the operation mode for DC ripple suppression provided by the embodiment of the present invention;

[0033] Figure 4 The schematic diagram of the operation mode for voltage transient control provided by the embodiment of the present invention;

[0034] Figure 5 The experimental schematic diagram provided by the embodiment of the present invention;

[0035] Among them, 1 - the first DC-APF, 2 - the dual-active bridge converter DAB, 3 - the second DC-APF, 4 - the DC / AC converter, 5 - the standby converter. Specific implementation manners

[0036] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] The present invention provides a circuit for comprehensively improving the power quality of a DC power distribution system, as Figure 1 shown, including: the dual-active bridge converter DAB2, the DC active power filter DC-APF, and the standby converter 5; among them,

[0038] Both sides of the dual-active bridge converter DAB2 are respectively connected to two DC buses through the DC active power filter DC-APF. Among them, the two DC buses are BUS1 and BUS2 respectively. The DC active power filter DC-APF connected to both ends of BUS1 is the first DC-APF1, and the DC active power filter DC-APF connected to both ends of BUS2 is the second DC-APF3. The first DC-APF1 is connected to the primary full bridge of the dual-active bridge converter DAB2 through the voltage stabilizing capacitor C1, and the second DC-APF3 is connected to the secondary full bridge of the dual-active bridge converter DAB2 through the voltage stabilizing capacitor C2;

[0039] The standby converter 5 is connected to both ends of the voltage stabilizing capacitor C1 and is connected to the sensitive load on the BUS1 side.

[0040] A device for comprehensively improving the power quality of a DC power distribution system, based on a circuit for comprehensively improving the power quality of a DC power distribution system, is connected to the DC / AC converter on the DC side, and includes a circuit for comprehensively improving the power quality of the DC power distribution system and a control module;

[0041] The control module includes a detection unit and a switch control unit; among them,

[0042] The detection unit is used to detect voltage deviation, DC ripple, and voltage sag;

[0043] When the detection unit detects a voltage deviation, the switch control unit controls the diagonal switch tubes in the first DC-APF1 and the second DC-APF3 to conduct, controls the dual-active bridge converter DAB2 to adopt a voltage control mode to achieve voltage balance through power mutual assistance, and the dual-active bridge converter DAB2 filters out the high-frequency ripple components generated in the high-frequency switching state. The DC / AC converter 4 takes power from BUS1 to supply power to the sensitive load;

[0044] When the detection unit detects DC ripple, the switch control unit controls the dual active bridge converter DAB2 to adopt a voltage control mode to suppress the voltage fluctuations of the DC side capacitors of the first DC-APF1 and the second DC-APF3 and maintain the stability of the DC side voltage. The first DC-APF1 and the second DC-APF3 control the output of the reverse compensation current to suppress the ripple by tracking the ripple current on the DC bus side. The DC / AC converter 4 draws power from BUS1 to supply power to the sensitive load.

[0045] When the detection unit detects a voltage sag, the switch control unit controls the diagonal switching tubes and the standby converter 5 of the second DC-APF3 to conduct, and the dual active bridge converter DAB2 connects the second DC-APF3 and the standby converter 5 through the voltage control mode. All the fully controlled device switches in the first DC-APF1 and the DC / AC converter 4 are turned off. The standby converter 5 draws power from BUS2 to supply power to the sensitive load.

[0046] It should be noted that:

[0047] Since the traditional DC power quality control device realizes bidirectional power flow through the H-bridge to control the grid voltage balance, and the energy storage device adjusts the active power balance - voltage balance through the H-bridge, this increases the losses and costs of the system and is not conducive to its popularization. The present invention designs a topology based on replacing the energy storage device with segmented bus power mutual assistance. The dual active bridge converter DAB2 can realize bidirectional power flow. By connecting two segments of buses in parallel at both ends of the dual active bridge converter DAB2, it is used to maintain the voltage balance for correcting the voltage deviation.

[0048] The DC microgrid contains a large amount of second harmonic components due to the non-ideal characteristics of the AC side. The concave characteristic in the amplitude-frequency characteristic diagram of the active power filter DC-APF is just used to filter out the second harmonic. To suppress the ripple current of the two segments of buses, a DC-APF is connected in parallel on each segment of the bus side, and it is realized by outputting a compensation current with a phase opposite to the ripple current. When correcting the voltage deviation, the diagonal switches of the first DC-APF1 and the second DC-APF3 are turned on. Relative to the bus being directly connected to the dual active bridge converter DAB2, the voltage balance is achieved through power mutual assistance in the voltage control mode of the dual active bridge converter DAB2, and the high-frequency ripple components generated in the high-frequency switching state are filtered by the L and C at the output port of the dual active bridge converter DAB2. When dealing with DC ripple, the dual active bridge converter DAB2 adopts a voltage control mode to suppress the voltage fluctuations of the DC side capacitors of the DC-APF and maintain the stability of the DC side voltage. The first DC-APF1 and the second DC-APF3 control the output of the reverse compensation current to suppress the ripple by quickly and accurately tracking the ripple current on the DC bus side.

[0049] When the voltage sag exceeds a certain limit, it is difficult to maintain the rated value of the voltage of BUS1 only through power mutual assistance. Considering that even a slightly lower voltage than the rated value of the sensitive load will cause serious safety consequences and economic losses, the maintenance of the voltage of BUS1 is abandoned to ensure the power supply quality of the sensitive load through power mutual assistance of the equipment. The DC side capacitor DC terminal of the first DC-APF1 acts as BUS1 that only supplies the sensitive load. As Figure 1 shown, the diagonal switch of the second DC-APF3 is turned on, and all the fully controlled device switches in the first DC-APF1 are turned off. Due to the voltage sag, the bus voltage drops so low that the diodes cannot conduct, blocking the energy output from the dual active bridge converter DAB2 to bus 1. At this time, the sensitive load gives up taking power from bus 1 and directly obtains electric energy from bus 2 through the dual active bridge converter DAB2.

[0050] The operation mode of the topology structure disclosed by the present invention for governing the steady-state power quality is as follows:

[0051] Dual active bridge converter DAB2 → governs the voltage deviation on a long time scale. As Figure 2 shown, the diagonal switches of the first DC-APF1 and the second DC-APF3 are turned on, which is equivalent to directly connecting the dual active bridge converter DAB2 in parallel to the bus terminal. Since the energy of part of the topology of the dual active bridge converter DAB2 flows bidirectionally, power mutual assistance between each bus is realized, reducing the configuration and investment of energy storage; the sensitive load takes power from BUS1 through the DC / AC converter 4; the converter standby converter 5 is in standby;

[0052] The first DC-APF1 + the second DC-APF3 → govern the DC ripple on a long time scale. As Figure 3 shown, the first DC-APF1 governs the ripple voltage on BUS1, and the dual active bridge converter DAB2 governs the ripple voltage on BUS2. Through the charging and discharging of the capacitor, an output current with a phase opposite to the ripple current is generated, and the dual active bridge converter DAB2 adopts a power control mode to stabilize the capacitor voltage; the sensitive load takes power from BUS1 through the DC / AC converter 4; the converter standby converter 5 is in standby;

[0053] The operation mode of the topology structure disclosed by the present invention for governing the transient power quality is as follows:

[0054] As Figure 4 shown, the dual active bridge converter DAB2 + the standby converter 5 → govern the voltage sag on a short time scale. All the fully controlled devices of the first DC-APF1 are turned off. Due to the voltage sag, the voltage on BUS1 drops so low that the upper arm diodes of the first DC-APF1 cannot conduct, and the current cannot form a path to block the energy from flowing into BUS1; the converter DC / AC converter 4 is disconnected, the sensitive load gives up taking power from BUS1, the diagonal switch of the second DC-APF3 is turned on, and BUS2 supplies power to the sensitive load through the converter standby converter 5 by the dual active bridge converter DAB2.

[0055] A method for comprehensively improving the power quality of a DC distribution system, based on a device for comprehensively improving the power quality of a DC distribution system, includes the following steps:

[0056] Realtime detect whether there are voltage deviations, DC ripples and voltage sags;

[0057] When a voltage deviation is detected, control the diagonal switching tubes in the first DC-APF1 and the second DC-APF3 to conduct, control the dual-active bridge (DAB) converter DAB2 to adopt a voltage control mode to achieve voltage balance through power mutual assistance, and filter out the high-frequency ripple components generated in the high-frequency switching state by the DAB2. The DC / AC converter 4 draws power from BUS1 to supply power to sensitive loads;

[0058] When a DC ripple is detected, control the DAB2 to adopt a voltage control mode to suppress the voltage fluctuations of the DC-side capacitors of the first DC-APF1 and the second DC-APF3 and maintain the stability of the DC-side voltage. The first DC-APF1 and the second DC-APF3 control the output of reverse compensation current to suppress the ripple by tracking the ripple current on the DC bus side. The DC / AC converter 4 draws power from BUS1 to supply power to sensitive loads;

[0059] When a voltage sag is detected, control the diagonal switching tubes of the second DC-APF3 and the standby converter 5 to conduct, and connect the second DC-APF3 and the standby converter 5 through a voltage control mode by the DAB2. All the fully controlled devices switches in the first DC-APF1 and the DC / AC converter 4 are turned off, and the standby converter 5 draws power from BUS2 to supply power to sensitive loads.

[0060] The present invention will be further described through specific experiments below:

[0061] As Figure 5 shown, the voltage levels of the DC buses BUS1 and BUS2 are selected as 750V, the power of the DAB2 is 120kW, the single-bus power is 990kW, and the support capacity that can be provided when dealing with steady-state power quality problems is 240kW / 660kW>10%. Since the voltage levels of BUS1 and BUS2 are the same, the transformer ratio can be selected as 1:1.

[0062] Treat DC ripple: The switching tubes of the first DC-APF1 / second DC-APF3 bear 750V, the switching tubes of the DAB2 bear Uc, and the switching tubes of the standby converter 5 bear Uc / 2V;

[0063] Voltage deviation governance: The switching tubes of the dual-active bridge converter DAB2 bear 750V, the non-conducting arms of the first DC-APF1 / second DC-APF3 bear 750V, and the switching tubes of the standby converter 5 bear 750 / 2V;

[0064] Voltage sag governance: The switching tubes of the dual-active bridge converter DAB2 bear 750V, the switching tubes of the first DC-APF1 bear approximately 750 / 2V, the non-conducting arms of the second DC-APF3 bear 750V, and the switching tubes of the standby converter 5 bear 750V;

[0065] Considering factors such as the overload capacity of the switching tubes and voltage spikes, switching tubes with a breakdown voltage of 1200V (medium voltage level) need to be selected.

[0066] The square-wave signals on the primary and secondary sides of the transformer in the interconnected unit dual-active bridge converter DAB2 are equivalent to sine waves, and the voltage is referred to the primary side.

[0067]

[0068]

[0069] In the formula, φ is the external phase-shift angle; δ1 is the internal phase-shift angle of the H-bridge on the primary side of the transformer; δ2 is the internal phase-shift angle of the H-bridge on the secondary side of the transformer; U1 is the voltage of BUS1; U2 is the voltage of BUS2; w is the switching frequency of the interconnected part.

[0070] The power transmitted by the dual-active bridge converter DAB2 part.

[0071]

[0072]

[0073] The large-scale access of distributed energy, combined with the frequent switching of loads, causes voltage deviation of BUS1. The BUS2 compensates for the active power gap to stabilize the voltage of BUS1 to 750V. For the interconnected unit dual-active bridge converter DAB2, when inputting φ, δ1, and δ2, there is a coupling between the controlled output P and U1. If the output U 1ref is controlled to 750V, the dual-active bridge converter DAB2 outputs the corresponding active power to compensate for the energy gap of BUS1 to achieve bus voltage stabilization.

[0074] When the first DC-APF1 / second DC-APF3 outputs the compensating current, since it is in the high-frequency switching state, there may be some high-frequency ripple currents in the compensating current. Therefore, it is necessary to set a filter inductor on the AC side to suppress the ripple after the output compensating current passes through the filter inductor. When the selected filter inductor is too large, the ripple component in the compensating current will be eliminated, but the dynamic response speed of the system and the compensating current tracking performance will be reduced; the smaller the inductance value is selected, the ripple content in the output compensating current will not be completely eliminated, which is likely to cause system oscillation and affect the current tracking performance.

[0075] i L The rate of change of satisfies

[0076]

[0077] where f max is the maximum frequency of the ripple current (Hz); I P is the peak value of the ripple current; w x is the angular frequency.

[0078] According to the operating structure characteristics of the H-bridge type DC-APF, it can be obtained that,

[0079]

[0080] where U dc is the effective value of the DC bus voltage; U c is the effective value of the DC side capacitor voltage of the DC-APF.

[0081] U dc = 750V, U c = 1000V

[0082]

[0083] When the ripple can still be effectively compensated when the rate of change of the current is the smallest, and it is also necessary to satisfy that the current output by the filter can be reduced from the maximum value to 0, then there is the following formula

[0084]

[0085] I P needs to be determined through on-site research according to the ripple current to be treated on the DC side. In this embodiment, from the 750V and 990KW in the treatment scenario, the bus current can be obtained as 1320A, and the ripple current content can be taken as about 5%, that is, I P = 90A, f[[ID=?]] max = 100HZ, then the value of L is taken as 4mH.

[0086] It should be noted that there seems to be a problem with the numbering in the original text. The "? max " in the translation may need to be corrected according to the correct numbering in the original text.The DC-side capacitors of the first DC-APF1 / second DC-APF3 can play a role in suppressing voltage fluctuations and maintaining the stability of the DC-side voltage. Moreover, the capacitance of the capacitor is related to the voltage stabilization effect. The larger the capacitance, the better the voltage stabilization effect. However, as the capacitance increases, the volume of the device also increases. Therefore, when selecting the DC-side capacitor of the DC-APF, it is necessary to consider comprehensively. On the premise of ensuring the stable compensation current output of the DC-APF, try to select a capacitor with a smaller capacitance.

[0087] The DC-side capacitor is designed according to the energy flow:

[0088]

[0089] where Δu c is the capacitor voltage deviation; i 2m is the compensation current value on the DC-APF side.

[0090] After further simplification, it can be obtained that

[0091]

[0092]

[0093] Taking I 2m = 50 A, w = 628 rad / s, U dc = 750 V, Uc = 1000 V, Δu c = 10 V, C = 0.005971 F can be obtained and taken as 5 mF.

[0094] [[ID=

Claims

1. A comprehensive power quality management circuit for a DC power distribution system, characterized in that Comprising: Dual Active Bridge (DAB), DC Active Power Filter (DC-APF), and a standby converter; wherein, Both sides of the Dual Active Bridge (DAB) are respectively connected to two DC buses through the DC Active Power Filter (DC-APF). The two DC buses are BUS1 and BUS2 respectively. The DC Active Power Filter (DC-APF) connected to both ends of BUS1 is the first DC-APF, and the DC Active Power Filter (DC-APF) connected to both ends of BUS2 is the second DC-APF. The first DC-APF is connected to the primary full-bridge of the Dual Active Bridge (DAB) through a voltage stabilizing capacitor C1, and the second DC-APF is connected to the secondary full-bridge of the Dual Active Bridge (DAB) through a voltage stabilizing capacitor C2; The standby converter is connected to both ends of the voltage stabilizing capacitor C1 and is connected to the sensitive load on the BUS1 side.

2. A device for comprehensively improving the power quality of a DC power distribution system, which is based on the circuit for comprehensively improving the power quality of a DC power distribution system described in claim 1 and is connected to the DC / AC converter on the DC side, and is characterized in that, Comprising a comprehensive power quality improvement circuit for the DC power distribution system and a control module; The control module includes a detection unit and a switch control unit; wherein, The detection unit is used to detect voltage deviation, DC ripple, and voltage sag; When the detection unit detects a voltage deviation, the switch control unit controls the diagonal switching tubes in the first DC-APF and the second DC-APF to conduct, controls the Dual Active Bridge (DAB) to adopt a voltage control mode to achieve voltage balance through power mutual assistance, and the Dual Active Bridge (DAB) filters out the high-frequency ripple components generated in the high-frequency switching state. The DC / AC converter draws power from BUS1 to supply power to the sensitive load; When the detection unit detects DC ripple, the switch control unit controls the Dual Active Bridge (DAB) to adopt a voltage control mode to suppress the voltage fluctuations of the DC side capacitors of the first DC-APF and the second DC-APF and maintain the stability of the DC side voltage. The first DC-APF and the second DC-APF control the output of reverse compensation current to suppress ripple by tracking the ripple current on the DC bus side. The DC / AC converter draws power from BUS1 to supply power to the sensitive load; When the detection unit detects a voltage sag, the switch control unit controls the diagonal switching tubes of the second DC-APF and the standby converter to conduct, and the Dual Active Bridge (DAB) connects the second DC-APF and the standby converter through a voltage control mode. The fully controlled device switches in the first DC-APF and the DC / AC converter are both turned off. The standby converter draws power from BUS2 to supply power to the sensitive load.

3. A method for comprehensively managing the power quality of a DC power distribution system, based on the device for comprehensively managing the power quality of a DC power distribution system described in claim 2, characterized in that, Including the following steps: Real-time detection of whether there are situations of voltage deviation, DC ripple, and voltage sag; When a voltage deviation is detected, control the diagonal switching tubes in the first DC-APF and the second DC-APF to conduct, control the Dual Active Bridge (DAB) to adopt a voltage control mode to achieve voltage balance through power mutual assistance, and the Dual Active Bridge (DAB) filters out the high-frequency ripple components generated in the high-frequency switching state. The DC / AC converter draws power from BUS1 to supply power to the sensitive load; When the DC ripple is detected, the dual active bridge (DAB) converter is controlled to adopt a voltage control mode to suppress the voltage fluctuations of the DC-side capacitors of the first DC-APF and the second DC-APF and maintain the stability of the DC-side voltage. The first DC-APF and the second DC-APF control the output of the reverse compensation current to suppress the ripple by tracking the ripple current on the DC bus side. The DC / AC converter draws power from BUS1 to supply power to the sensitive load. When a voltage sag is detected, the diagonal switching tubes and the standby converter of the second DC-APF are turned on, and the dual active bridge (DAB) converter connects the second DC-APF and the standby converter through the voltage control mode. All the controlled devices in the first DC-APF and the DC / AC converter are turned off, and the standby converter draws power from BUS2 to supply power to the sensitive load.

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