A low-voltage flexible direct-current interconnection system and a control method thereof

Abstract

This invention discloses a low-voltage flexible DC interconnection system and its control method, comprising several distribution substations, DC buses, electrical loads, energy storage converters E, and newly added substations. Each distribution substation is equipped with an AC bus and a flexible converter, and each AC bus is connected to the DC bus via a flexible DC converter. The newly added substations include distribution transformers T. N+1 Meanwhile, the distribution transformer T N+1 Both ends are connected to the AC bus L respectively N+1 and AC circuit breaker S N+1 Connected, and the DC bus is connected to the photovoltaic converter D. N+1 With photovoltaic power generation device G N+1 Connected, and the DC bus is connected to the flexible DC converter C. N+1 With AC bus L N+1 The DC bus is connected to the energy storage module via the energy storage converter E, and is also connected to the electrical load. By setting up new distribution areas and DC buses, and relying on a reasonable design for the capacity ratio of new distribution areas and energy storage, this invention avoids the problem of wasted photovoltaic power when photovoltaic power generation devices are connected to the grid.

Description

Technical Field

[0001] This invention belongs to the field of flexible power transmission technology, specifically relating to a low-voltage flexible DC interconnection system and its control method. Background Technology

[0002] With abundant photovoltaic resources, the construction of photovoltaic projects will further develop and utilize clean energy, providing significant support for ecological environment governance and the improvement of farmers' living standards.

[0003] However, photovoltaic power generation is greatly affected by weather and has strong volatility. Rural electricity load is relatively light and seasonal. The substation equipment in rural distribution areas is also usually weak. Therefore, when a large number of photovoltaic systems are connected to the grid at the same time in rural areas, it is difficult for the photovoltaic system to be fully absorbed locally. This will lead to many problems such as long-term overload operation of distribution transformers and overvoltage at the end of the line. Currently, common solutions include: First, relying on upper-level control devices such as integrated terminals to limit the power of photovoltaic (PV) power within the system. When PV power is excessive, the integrated terminal issues a power-limiting command to the PV inverter, thereby achieving source-load balance. However, this results in a waste of PV power and negatively impacts farmers' economic income. Second, constructing electrochemical energy storage facilities to transfer energy by storing surplus PV power in batteries. Although energy storage has multiple functions such as frequency regulation and voltage regulation, and its response and adjustment speed is extremely fast, to absorb all the PV power in the system, it is necessary to construct energy storage equipment with a capacity matching that of the PV power in the system. Adding large-capacity energy storage will increase the investment cost of construction and bring huge challenges and risks to later operation, maintenance, and safe operation. Summary of the Invention

[0004] The purpose of this invention is to provide a low-voltage flexible DC interconnection system and its control method to solve the technical problem of wasted photovoltaic power when existing photovoltaic power generation devices are connected to the grid.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] Firstly, a low-voltage flexible DC interconnection system includes several distribution substations, DC buses, electrical loads, energy storage converters E, and newly added substations. Each distribution substation is equipped with an AC bus and a flexible converter. Each AC bus is connected to a DC bus via a flexible DC converter. The newly added substation includes a distribution transformer T. N+1 The distribution transformer T N+1 Both ends are connected to the AC bus L respectively N+1 and AC circuit breaker S N+1 The DC bus is connected via a photovoltaic converter D. N+1 With photovoltaic power generation device G N+1The DC bus is connected via a flexible DC converter C. N+1 With AC bus L N+1 The DC bus is connected to the energy storage module via the energy storage converter E, and the DC bus is also connected to the electrical load.

[0007] A further improvement of the present invention is that: the distribution substation includes substations 1, 2 to N, substation 1 includes an AC bus L1 and a flexible converter C1, a distribution transformer T1 is provided between the AC circuit breaker S1 and the AC bus L1, the AC bus L1 is connected to a photovoltaic power generation device G1 through a photovoltaic converter D1, and the AC bus L1 is connected to a DC bus through a flexible DC converter C1;

[0008] The No. 2 transformer area includes an AC bus L2 and a flexible converter C2. A distribution transformer T2 is provided between the AC circuit breaker S2 and the AC bus L2. The AC bus L2 is connected to the photovoltaic power generation device G2 through the photovoltaic converter D2. The AC bus L2 is connected to the DC bus through the flexible DC converter C2.

[0009] The Nth transformer area includes AC bus L N and flexible converter C N The AC circuit breaker S N and AC bus L N A distribution transformer T is installed between them. N The AC bus L N Through photovoltaic converter D N With photovoltaic power generation device G N Connected, the AC bus L N Through flexible DC converter C N Connected to the DC bus.

[0010] A further improvement of the present invention is that it also includes a flexible DC-DC coordination controller, which is connected to all flexible DC-DC converters, all photovoltaic converters, all distribution transformers, energy storage converters E, and AC circuit breakers S. N+1 Signal connection.

[0011] A further improvement of the present invention is that: the photovoltaic converter D1, photovoltaic converter D2 and photovoltaic converter D... N The photovoltaic converter D is a converter employing a DCAC topology. N+1 The energy storage converter E is a converter using a DC-DC topology.

[0012] A further improvement of the present invention is that: the flexible DC converter C1 operates in DC bus voltage control mode, and the flexible DC converters C2 to C... N All operate in power control mode.

[0013] A further improvement of the present invention is that the voltage of the DC bus is 750V.

[0014] A further improvement of the present invention is that: the photovoltaic converters D1 to D2 are... N+1 All operate in MPPT (Maximum Power Point Tracking) mode.

[0015] Secondly, a low-voltage flexible DC interconnection control method includes a transformer area power backfeed mode and a transformer area power support mode. When the photovoltaic power generation is greater than the power load, control is carried out in the transformer area power backfeed mode, and when the photovoltaic power generation is less than the power load, control is carried out in the transformer area power support mode.

[0016] A further improvement of the present invention is that, when the power backfeed mode of the distribution area is controlled, if the energy storage module is not fully charged, the following steps are included:

[0017] The flexible DC-DC coordinating controller issues power command P E Send a 0 power command to energy storage converter E and then to flexible DC converters C1 through C2. N This allows the energy storage module to prioritize the storage of surplus photovoltaic power.

[0018] When the real-time power value of the energy storage converter E reaches the rated power, and the photovoltaic power generation devices G1 to G... N+1 When the total power generation exceeds the sum of the electrical load and the stored power of the energy storage module, the flexible DC-DC coordinating controller maintains the power command value of the energy storage converter E at the rated power, while simultaneously supplying power to the flexible DC-DC converters C1 to C2. N Issue total power command

[0019]

[0020] For flexible DC converters C1 to C1 N In the middle, the power command P of each flexible DC converter i for

[0021]

[0022] Where, k max k represents the upper limit of the load factor of the distribution transformer in the substation area. i Let N be the real-time load rate of the i-th transformer area, where 1 ≤ i ≤ N;

[0023] When the load rate k of transformer areas 1 to N i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to the energy storage converter E and the flexible DC-DC converters C1 to C2. NThe power command remains unchanged, and a closing command is simultaneously issued to the AC circuit breaker S of the N+1 distribution area. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 This makes distribution transformer T1 to distribution transformer T N The surplus photovoltaic power that cannot be absorbed even when the load factor limit is reached is distributed through the distribution transformer T of the N+1 transformer area. N+1 Grid connection, C N+1 Power command P of flexible DC converter N+1 :

[0024]

[0025] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power;

[0026] If the energy storage module reaches a fully charged state:

[0027] The flexible DC-DC coordinating controller sends a standby command to the energy storage converter E and a power command P to the energy storage converter E. E =0;

[0028] After the energy storage converter E goes into standby mode, the flexible DC-DC coordinating controller supplies power to flexible DC-DC converters C1 through C2. N Issue total power command

[0029]

[0030] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i ;

[0031] When the load rate k of the distribution transformers in areas 1 to N is i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to flexible DC-DC converters C1 to C1. N The power command remains unchanged, and a closing command is simultaneously issued to the AC circuit breaker S of the N+1 distribution area. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 Distribution transformers T1 to T1 N The surplus photovoltaic power that cannot be absorbed even when the load factor limit is reached is distributed through the distribution transformer T of the N+1 transformer area. N+1 Grid connection, C N+1 Power command P of flexible DC converter N+1 :

[0032]

[0033] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0034] A further improvement of the present invention is that, when the power support mode of the distribution area is controlled, if the energy storage module is not in a discharged state, the following steps are included:

[0035] The flexible DC-DC coordinating controller issues power command P E A zero-power command is issued to energy storage converter E and then to flexible DC converters C1 through C2. N This means that the photovoltaic power stored in the energy storage module is used to supply electricity to the load first;

[0036] When the real-time power value of the energy storage converter E reaches the rated power, and the photovoltaic power generation devices G1 to G... N+1 When the sum of the total power generation and the energy stored in the energy storage module is less than the user load, the flexible DC-DC coordinating controller maintains the power command value of the energy storage converter E at -P. E_rated Simultaneously supplying power to flexible DC converters C1 to C1. N Issue total power command

[0037]

[0038] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i The output of each flexible DC converter is dynamically adjusted to prevent positive overload of any distribution transformer in the area. This is achieved by adjusting the output of each flexible DC converter from C1 to C2. N In the middle, the power command P of each flexible DC converter i :

[0039]

[0040] When the load rate k of the distribution transformers in areas 1 to N is i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to the energy storage converter E and the flexible DC-DC converters C1 to C2. N The power command remains unchanged, meaning it will not worsen the load rate situation of distribution areas 1 to N. At the same time, a closing command is issued to the AC circuit breaker S of distribution area N+1. N+1 It also issues a start-up command and a power command P. N+1 Give C N+1 The No. 1 flexible DC converter will connect the distribution transformers T1 to T1. NThe power deficit that cannot be provided even when the load factor limit is reached is transmitted through the distribution transformer T of the N+1 transformer area. N+1 Provided by C N+1 Power command P of flexible DC converter N+1 :

[0041]

[0042] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0043] If the energy storage module reaches the venting state:

[0044] The flexible DC-DC coordinating controller sends a standby command to the energy storage converter E and a power command P to the energy storage converter E. E =0;

[0045] After the energy storage converter E goes into standby mode, the flexible DC-DC coordinating controller will remotely adjust the power deficit using power adjustment commands. The form of allocation is assigned to flexible DC converter C2 ~ flexible DC converter C N Execute power command

[0046]

[0047] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i ;

[0048] When the load rate k of the distribution transformers in areas 1 to N is i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to flexible DC-DC converters C1 to C1. N The power command remains unchanged, and a closing command is simultaneously issued to the AC circuit breaker S. N+1 It also issues a start-up command and a power command P. N+1 Give C N+1 The No. 1 flexible DC converter will connect distribution transformers T1 to T2. N The power shortfall that cannot be provided when the load factor limit is reached is supplied by transformer area N+1, C. N+1 The power command for the flexible DC converter is:

[0049]

[0050] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0051] Compared with the prior art, the present invention has at least the following beneficial effects:

[0052] 1. This invention avoids the problem of wasted photovoltaic power when photovoltaic power generation devices are connected to the grid by setting up new distribution areas and DC buses, and relying on a reasonable design of the capacity ratio of new distribution areas and energy storage.

[0053] 2. The flexible DC-DC coordinating controller distributes the system's power deficit to the flexible DC-DC converters C2 to C6 in the form of remote power adjustment commands. N When distributing power, the load rate of each distribution transformer in each area is considered, and the output of each flexible DC converter is dynamically adjusted to prevent any distribution transformer from being positively overloaded. The flexible DC coordination controller maintains the power supply to the flexible DC converter. N+1 The shutdown command;

[0054] 3. When all transformer substations from 1 to N reach their maximum load rate, the flexible DC-DC coordinating controller issues a closing command to AC circuit breaker S. N+1 Simultaneously, start-up and power commands are sent to C. N+1 The No. 1 flexible DC converter will connect the distribution transformers T1 to T1. N The power deficit that cannot be provided when the load rate limit is reached is provided by the N+1 transformer area, thus achieving economical operation of the system. Attached Figure Description

[0055] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0056] In the attached diagram:

[0057] Figure 1 This is a schematic diagram of the architecture of a low-voltage flexible DC interconnection system according to the present invention.

[0058] Figure 2 This is a flowchart of the power backfeed mode control in the low-voltage flexible DC interconnection system control method of the present invention;

[0059] Figure 3 This is a flowchart of the power support mode control in the low-voltage flexible DC interconnection system control method of the present invention. Detailed Implementation

[0060] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0061] The following detailed description is exemplary and intended to provide further detailed explanation of the invention. Unless otherwise specified, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used in this invention is for describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention.

[0062] Example 1

[0063] A low-voltage flexible DC interconnect system, such as Figure 1 As shown, the system includes several distribution areas and a DC bus, i.e., N+1 distribution areas. Distribution areas 1 to N have the same circuit structure and are distribution areas. The structure of the distribution area is a common structure in this field. Each distribution area includes a distribution transformer, an AC bus, an AC circuit breaker, a photovoltaic converter, a photovoltaic power generation device, and a flexible DC converter. In distribution areas 1 to N, a distribution transformer is provided between the AC bus and the AC circuit breaker. The photovoltaic power generation device is connected to the AC bus through the photovoltaic converter, and the AC bus is connected to the DC bus through the flexible DC converter. In this embodiment, the distribution areas include distribution area 1, distribution area 2, up to distribution area N+1. Distribution area N+1 is a newly added distribution area.

[0064] The distribution transformer T1 is located between the AC circuit breaker S1 and the AC bus L1. The photovoltaic power generation device G1 is connected to the AC bus L1 through the photovoltaic converter D1. The AC bus L1 is connected to the DC bus through the flexible DC converter C1.

[0065] The distribution transformer T2 is located between the AC circuit breaker S2 and the AC bus L2. The photovoltaic power generation device G2 is connected to the AC bus L2 through the photovoltaic converter D2. The AC bus L2 is connected to the DC bus through the flexible DC converter C2.

[0066] Distribution transformer T N Set in AC circuit breaker S N and AC bus L N Between, photovoltaic power generation device G N Through photovoltaic converter D N Connected to AC bus L N Connected, AC bus L N Through flexible DC converter C N Connected to the DC bus;

[0067] Distribution transformer T N+1 Set on AC bus L N+1 and AC circuit breaker S N+1 Between, photovoltaic power generation device G N+1 Through photovoltaic converter D N+1 Connected to the DC bus, AC bus L N+1 Through flexible DC converter CN+1 Connected to the DC bus;

[0068] As a preferred embodiment of the present invention, the AC circuit breaker S N+1 It is an AC circuit breaker with an electric operating mechanism, thereby enabling remote control.

[0069] The DC bus is connected to the energy storage module through the energy storage converter E, and the DC bus is also connected to the electrical load.

[0070] It also includes a flexible-DC coordination controller, which is connected to flexible-DC converter C1 and flexible-DC converter C2 respectively. 2…… Flexible DC converter C N Flexible DC converter C N+1； Photovoltaic converter D1, photovoltaic converter D2, photovoltaic converter D N Photovoltaic converter D N+1 Distribution transformer T1, distribution transformer T2, distribution transformer T N Distribution transformer T N+1 Energy storage converter E and AC circuit breaker S N+1 The signal connection, the flexible DC-DC coordinating controller is an embedded device architecture, used to obtain the signal connection of distribution transformer T1, distribution transformer T2, and distribution transformer T3. N and distribution transformer T N+1 Load rate;

[0071] Collect AC circuit breaker S N+1 It can determine the switch position and control the opening and closing; it uses fast Goose communication with all flexible DC converters, photovoltaic converters and energy storage converters in the system to monitor the operating status of each converter in real time and issue control commands.

[0072] In this embodiment, with the AC bus as the reference, the power direction of all converters is positive in the direction of flow towards the AC bus.

[0073] As a preferred embodiment of the present invention, photovoltaic converter D1, photovoltaic converter D2 and photovoltaic converter D... N Photovoltaic converter D is connected to the AC grid using a DCAC topology. N+1 The energy storage converter E is a converter using a DC-DC topology, which is directly connected to the DC bus for grid connection.

[0074] In a preferred embodiment of the present invention, the flexible DC converter C1 operates in DC bus voltage control mode to maintain stable DC bus voltage. In accordance with the provisions of the national standard GB / T35727 "Guidelines for Medium and Low Voltage DC Distribution Voltage", considering the safety of equipment operation and the maturity of technology, the DC bus voltage adopts a voltage level of 750V.

[0075] As a preferred embodiment of the present invention, flexible DC converter C2 to flexible DC converter C N Both operate in power control mode, and the power magnitude and direction can be flexibly controlled by the flexible DC-DC coordinating controller. The response speed when switching at full power in both positive and negative directions is less than 50ms.

[0076] As a preferred embodiment of the present invention, the flexible DC converter C N+1 Normally, it is in a standby state and is only started up when the flexible DC-DC coordinating controller determines that the system needs to absorb photovoltaic or transfer power through the N+1 transformer area, so as to reduce the efficiency loss of the system caused by long-term light-load operation.

[0077] As a preferred embodiment of the present invention, photovoltaic converters D1 to D2 are used. N+1 All operate in MPPT (Maximum Power Point Tracking) mode, constantly outputting the current maximum power of the connected photovoltaic power generation device to ensure maximum benefit.

[0078] In a preferred embodiment of the present invention, the energy storage converter E operates in power control mode, and the flexible DC-DC coordinating controller issues charging, discharging, and standby commands to the energy storage converter according to the current power status of the energy storage module and system requirements.

[0079] Example 2

[0080] A low-voltage flexible DC interconnect control method, such as Figure 2-3 As shown, a low-voltage flexible DC interconnection system based on Embodiment 1 includes a transformer area power backfeed mode and a transformer area power support mode. The control methods for the two modes are as follows:

[0081] Photovoltaic power generation device G1 to photovoltaic power generation device G N+1 Total power generation P D Greater than the electrical load P load When using the transformer area power reverse feed mode for control, the following steps are included:

[0082] When the energy storage module is not fully charged:

[0083] S1, the flexible DC-DC coordinating controller issues a power command P. E For the energy storage converter E, i.e. P E =|P load |-P D (|P E |<P E_rated The zero-power command is issued to the flexible DC converters C1 to C2. N This allows the energy storage module to prioritize the storage of surplus photovoltaic power.

[0084] S2, When the real-time power value of the energy storage converter E reaches the rated P E_ratedAnd it cannot increase the output any further, i.e., |P E |=P E_rated And photovoltaic power generation device G1 to photovoltaic power generation device G N+1 When the total power generation exceeds the sum of the power load and the energy stored in the energy storage module, the flexible DC-DC coordinating controller maintains the power command value of the energy storage converter E at P. E_rated Simultaneously supplying power to flexible DC converters C1 to C1. N Issue total power command

[0085]

[0086] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i The output of each flexible DC-DC converter is dynamically adjusted to prevent reverse overload of any distribution transformer in the area. Specifically, this applies to flexible DC-DC converters C1 to C2. N Power command P for each flexible DC converter i for

[0087]

[0088] Where, k max k represents the upper limit of the load factor of the distribution transformer in the substation area. i This represents the real-time load factor of the i-th transformer area (1≤i≤N). Furthermore, since C1 in transformer area 1 operates in DC bus voltage mode, although the flexible DC coordinating controller will still allocate power command P1 to C1, the actual power of the flexible DC converter C1 is naturally determined by the system power flow, and its magnitude is the same as P1.

[0089] S3, when the load rate k of the distribution transformers in areas 1 to N is... i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to the energy storage converter E and the flexible DC-DC converters C1 to C2. N The power command remains unchanged, meaning it will not worsen the load rate situation of distribution areas 1 to N. At the same time, a closing command is issued to the AC circuit breaker S of distribution area N+1. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 Distribution transformers T1 to T1 N The surplus photovoltaic power that cannot be absorbed even when the load factor limit is reached is distributed through the distribution transformer T of the N+1 transformer area. N+1 Grid connection enables full absorption of photovoltaic power within the system. N+1 Power command P of flexible DC converter N+1 :

[0090]

[0091] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0092] When the energy storage module reaches a fully charged state:

[0093] A1. The flexible DC-DC coordinating controller sends a standby command to the energy storage converter E and a power command P to the energy storage converter E. E =0;

[0094] A2. After the energy storage converter E goes into standby mode, the flexible DC-DC coordinating controller supplies power to flexible DC-DC converters C1 through C2. N Issue total power command

[0095]

[0096] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i The allocation principle is the same as described above.

[0097] A3. When the load rate k of the distribution transformers in areas 1 to N is... i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to flexible DC-DC converters C1 to C1. N The power command remains unchanged, and the load rate situation of distribution areas 1 to N will not be worsened. At the same time, a closing command is issued to the AC circuit breaker S of distribution area N+1. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 Distribution transformers T1 to T1 N The surplus photovoltaic power that cannot be absorbed even when the load factor limit is reached is distributed through the distribution transformer T of the N+1 transformer area. N+1 Grid connection enables full absorption of photovoltaic power within the system. N+1 Power command P of flexible DC converter N+1 :

[0098]

[0099] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0100] Photovoltaic power generation device G1 to photovoltaic power generation device G N+1 Total power generation P D Less than the included electrical load P loadWhen using the transformer area power support mode for control, the following steps are included:

[0101] When the energy storage module is not in the venting state:

[0102] B1. The flexible DC-DC coordinating controller issues a power command P. E Energy storage converter E, P E =|P load |-P D (P E ≤P E_rated The zero-power command is issued to the flexible DC converters C1 to C2. N This means that the photovoltaic power stored in the energy storage module is used to supply electricity to the load first;

[0103] B2. When the real-time power value of the energy storage converter E reaches its rated value and cannot be increased further, i.e., |P E |=P E_rated And photovoltaic power generation device G1 to photovoltaic power generation device G N+1 When the sum of the total power generation and the energy stored in the energy storage module is less than the user load, the flexible DC-DC coordinating controller maintains the power command value of the energy storage converter E at -P. E_rated Simultaneously supplying power to flexible DC converters C1 to C1. N Issue total power command

[0104]

[0105] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i The output of each flexible DC-DC converter is dynamically adjusted to prevent positive overload of any distribution transformer in the area. Specifically, this applies to flexible DC-DC converters C1 to C2. N Power command P for each flexible DC converter i :

[0106]

[0107] B3. When the load rate k of the distribution transformers in areas 1 to N is... i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to the energy storage converter E and the flexible DC-DC converters C1 to C2. N The power command remains unchanged, meaning it will not worsen the load rate situation of distribution areas 1 to N. At the same time, a closing command is issued to the AC circuit breaker S of distribution area N+1. N+1 It also issues a start-up command and a power command P. N+1 Give C N+1 The No. 1 flexible DC converter will connect the distribution transformers T1 to T1. NThe power deficit that cannot be provided even when the load factor limit is reached is transmitted through the distribution transformer T of the N+1 transformer area. N+1 This provides the necessary resources to achieve full grid integration of photovoltaic power within the system. N+1 Power command P of flexible DC converter N+1 :

[0108]

[0109] Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0110] When the energy storage module reaches the venting state:

[0111] C1. The flexible DC-DC coordinating controller sends a standby command to the energy storage converter E and a power command P to the energy storage converter E. E =0;

[0112] C2. After the energy storage converter E goes into standby mode, the flexible DC-DC coordinating controller will remotely adjust the power deficit of the system using power adjustment commands. The form of allocation is assigned to flexible DC converter C2 ~ flexible DC converter C N Execute power command

[0113]

[0114] When allocating power to each flexible DC converter, the load factor k of each distribution transformer in each substation should be considered. i The allocation principle is the same as described above.

[0115] C3. When the load rate k of the distribution transformers in areas 1 to N is... i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to flexible DC-DC converters C1 to C1. N The power command remains unchanged, and the load rate of distribution areas 1 to N will not be worsened. At the same time, a closing command is issued to AC circuit breaker S. N+1 It also issues a start-up command and a power command P. N+1 Give C N+1 The No. 1 flexible DC converter will connect distribution transformers T1 to T2. N The power shortfall that cannot be provided when the load factor limit is reached is supplied by transformer area N+1, thus achieving economical system operation. N+1 The power command for the flexible DC converter is:

[0116]

[0117] Among them, P i_realFor the flexible DC converters C1 to C in areas 1 to N N Actual real-time power.

[0118] As is known from common technical knowledge, this invention can be implemented through other embodiments that do not depart from its spirit or essential characteristics. Therefore, the disclosed embodiments described above are merely illustrative in all respects and are not the only ones. All modifications within the scope of this invention or equivalent to the scope of this invention are included in this invention.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A low-voltage flexible DC interconnection control method, implemented based on a low-voltage flexible DC interconnection system, characterized in that, The low-voltage flexible DC interconnection system includes several distribution substations, DC buses, electrical loads, energy storage converter E, and substation N+1. Each distribution substation is equipped with an AC bus and a flexible DC converter. Each AC bus is connected to the DC bus via a flexible DC converter. The distribution substations include substations 1, 2 to N. Substation 1 includes AC bus L1 and flexible DC converter C1. A distribution transformer T1 is installed between AC circuit breaker S1 and AC bus L1. The photovoltaic converter D1 is connected to the photovoltaic power generation device G1, and the AC bus L1 is connected to the DC bus via the flexible DC converter C1; the second distribution area includes the AC bus L2 and the flexible DC converter C2, and a distribution transformer T2 is installed between the AC circuit breaker S2 and the AC bus L2. The AC bus L2 is connected to the photovoltaic power generation device G2 via the photovoltaic converter D2, and the AC bus L2 is connected to the DC bus via the flexible DC converter C2; the Nth distribution area includes the AC bus L... N Flexible DC converter C N AC circuit breaker S N and AC bus L N A distribution transformer T is installed between them. N The AC bus L N Through photovoltaic converter D N With photovoltaic power generation device G N Connected, the AC bus L N Through flexible DC converter C N Connected to the DC bus; The N+1 transformer area includes distribution transformer T N+1 The distribution transformer T N+1 Both ends are connected to the AC bus L respectively N+1 and AC circuit breaker S N+1 The DC bus is connected via a photovoltaic converter D. N+1 With photovoltaic power generation device G N+1 The DC bus is connected via a flexible DC converter C. N+1 With AC bus L N+1 The DC bus is connected to the energy storage module via the energy storage converter E, and the DC bus is also connected to the electrical load. Low-voltage flexible DC interconnection control methods include transformer area power backfeed mode and transformer area power support mode. When photovoltaic power generation device G1~photovoltaic power generation device G N+1 When the total power generation exceeds the power load, control is implemented in the power backfeed mode for the photovoltaic power generation device G1~PV power generation device G N+1 When the total power generation is less than the power load, control is carried out in the power support mode of the distribution area. When the power backfeed mode of the transformer area is controlled, if the energy storage module is not fully charged, the following steps are included: The flexible DC-DC coordinating controller issues power command P E Send a 0 power command to energy storage converter E and then to flexible DC converters C1 through C2. N This allows the energy storage module to prioritize the storage of surplus photovoltaic power. When the real-time power value of the energy storage converter E reaches the rated power, and the photovoltaic power generation devices G1 to G2... N+1 When the total power generation exceeds the sum of the power load and the energy stored in the energy storage module, the flexible DC-DC coordinating controller maintains the power command value of the energy storage converter E at the rated power P. E_rated At the same time, power is supplied to flexible DC converters C1 to C1. N Issue total power command : For flexible DC converter C1 ~ flexible DC converter C N In the middle, the power command P of each flexible DC converter i for Where, k max k represents the upper limit of the load factor of the distribution transformer in the substation area. i P represents the real-time load factor of the i-th transformer area, where 1 ≤ i ≤ N; D P represents the total power generation. load For electrical load; When the load rate k of transformer areas 1 to N i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to the energy storage converter E and the flexible DC-DC converters C1 to C2. N The power command remains unchanged, and a closing command is simultaneously issued to the AC circuit breaker S of the N+1 distribution area. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 This makes distribution transformer T1~distribution transformer T N The surplus photovoltaic power that cannot be absorbed even when the load factor limit is reached is distributed through the distribution transformer T of the N+1 transformer area. N+1 Grid connection, flexible DC converter C N+1 Power command P N+1 : Among them, P i_real For the flexible DC converters C1~C in areas 1~N N Actual real-time power; If the energy storage module reaches a fully charged state: The flexible DC-DC coordinating controller sends a standby command to the energy storage converter E and a power command P to the energy storage converter E. E =0; After the energy storage converter E goes into standby mode, the flexible DC-DC coordinating controller supplies power to flexible DC-DC converters C1 through C2. N Issue total power command : When allocating power to each flexible DC converter, the load rate k of each distribution transformer in the substation area should be considered. i ; When the load rate k of the distribution transformers in areas 1 to N is i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to flexible DC-DC converters C1 to C2. N The power command remains unchanged, and a closing command is simultaneously issued to the AC circuit breaker S of the N+1 distribution area. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 Distribution transformers T1 to T1 N The surplus photovoltaic power that cannot be absorbed even when the load factor limit is reached is distributed through the distribution transformer T of the N+1 transformer area. N+1 Grid connection, flexible DC converter C N+1 Power command P N+1 : Among them, P i_real For the flexible DC converters C1~C in areas 1~N N Actual real-time power; When the power support mode of the transformer area is controlled, if the energy storage module is not in the venting state, the following steps are included: The flexible DC-DC coordinating controller issues power command P E Send a 0 power command to energy storage converter E and then to flexible DC converters C1 through C2. N This means that the photovoltaic power stored in the energy storage module is used to supply electricity to the load first; When the real-time power value of the energy storage converter E reaches the rated power, and the photovoltaic power generation devices G1 to G2... N+1 When the sum of the total power generation and the energy stored in the energy storage module is less than the user load, the flexible DC-DC coordinating controller maintains the power command value of the energy storage converter E at -P. E_rated At the same time, power is supplied to flexible DC converters C1 to C1. N Issue total power command : When allocating power to each flexible DC converter, the load rate k of each distribution transformer in the substation area should be considered. i The output of each flexible DC converter is dynamically adjusted to prevent positive overload of any distribution transformer in the area. This is achieved by adjusting the output of each flexible DC converter from C1 to C2. N In the middle, the power command P of each flexible DC converter i : When the load rate k of the distribution transformers in areas 1 to N is i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to the energy storage converter E and the flexible DC-DC converters C1 to C2. N The power command remains unchanged, meaning it will not worsen the load rate situation of distribution areas 1 to N. At the same time, a closing command is issued to the AC circuit breaker S of distribution area N+1. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 The distribution transformers T1~T N The power deficit that cannot be provided even when the load factor limit is reached is transmitted through the distribution transformer T of the N+1 transformer area. N+1 Provides flexible DC converter C N+1 Power command P N+1 : Among them, P i_real For the flexible DC converters C1 to C in areas 1 to N N Actual real-time power; If the energy storage module reaches the venting state: The flexible DC-DC coordinating controller sends a standby command to the energy storage converter E and a power command P to the energy storage converter E. E =0; After the energy storage converter E goes into standby mode, the flexible DC-DC coordinating controller will remotely adjust the power deficit using power adjustment commands. The form of allocation is assigned to flexible DC converter C1~flexible DC converter C N Execute power command : When allocating power to each flexible DC converter, the load rate k of each distribution transformer in the substation area should be considered. i ; When the load rate k of the distribution transformers in areas 1 to N is i All have reached the upper limit k max At that time, the flexible DC-DC coordinating controller maintains the transmission of power to flexible DC-DC converters C1 to C2. N The power command remains unchanged, and a closing command is simultaneously issued to the AC circuit breaker S. N+1 It also issues a start-up command and a power command P. N+1 For flexible DC converter C N+1 Distribution transformers T1 to T1 N The power shortfall that cannot be provided when the load factor limit is reached is supplied by transformer area N+1, via flexible DC converter C. N+1 The power command is: Among them, P i_real For the flexible DC converters C1~C in areas 1~N N Actual real-time power.

2. The low-voltage flexible DC interconnection control method according to claim 1, characterized in that, The low-voltage flexible DC interconnection system also includes a flexible DC coordination controller, which is connected to all flexible DC converters, all photovoltaic converters, all distribution transformers, energy storage converter E, and AC circuit breaker S. N+1 Signal connection.

3. The low-voltage flexible DC interconnection control method according to claim 1, characterized in that, The photovoltaic converters D1, D2, and D... N The photovoltaic converter D is a converter employing a DCAC topology. N+1 The energy storage converter E is a converter using a DC-DC topology.

4. The low-voltage flexible DC interconnection control method according to claim 1, characterized in that, The flexible DC converter C1 operates in DC bus voltage control mode, and the flexible DC converters C2 to C... N All operate in power control mode.

5. The low-voltage flexible DC interconnection control method according to claim 1, characterized in that, The voltage of the DC bus is 750V.

6. The low-voltage flexible DC interconnection control method according to claim 1, characterized in that, The photovoltaic converters D1 to D2 are described. N+1 All operate in MPPT (Maximum Power Point Tracking) mode.

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