High-proportion photovoltaic low-voltage transformer area flexible interconnection system and power coordination method

Abstract

The invention discloses a high-proportion photovoltaic low-voltage transformer area flexible interconnection system and a power coordination method. The method comprises the steps of reading related data information of each power distribution transformer area, generating real-time data, determining a master converter and a slave converter, and judging whether a heavy load area and a voltage out-of-limit condition exist or not; when the transformer area has a heavy load area, a power instruction of the flexible interconnection device is obtained according to the data information and the principle of bearing power distribution by own capacity, and power coordination is carried out; when a voltage out-of-limit condition exists in the transformer area, determining an optimal voltage regulation and control point according to an influence mechanism of active-reactive cooperative control on voltage, and distributing active power and reactive power according to the optimal voltage regulation and control point to perform power coordination; and obtaining the operation state of the flexible interconnection device in the transformer area. According to the invention, the power supply capability of the existing distribution transformer is fully released, and the functions of source-load space-time complementation, power interaction, global voltage regulation and control and maximization of new energy consumption in multiple distribution station intervals are realized.

Description

Technical Field

[0001] The present invention relates to the technical field of high-ratio photovoltaic low-voltage area flexible interconnection system and power coordination technology, and in particular to a high-ratio photovoltaic low-voltage area flexible interconnection system and a power coordination method. Background Art

[0002] A large number of distributed photovoltaic grid-connected power plants have brought adverse effects and huge challenges to the operation of low-voltage distribution stations. For example, photovoltaic power generation is easily affected by weather, temperature and other conditions, and its output is volatile and intermittent, which can easily cause frequent voltage fluctuations, seriously threatening the safe and stable operation of photovoltaic village / community distribution stations. At the same time, the multi-point and decentralized access of household distributed photovoltaic power plants has changed the operation mode and structural characteristics of traditional distribution stations, from radial passive networks to multi-connected active networks, changing the power flow direction of feeders and even causing reverse currents. On the one hand, the distribution transformer is limited by capacity and cannot complete the power transmission beyond the capacity, which may burn the transformer in severe cases; on the other hand, it causes the voltage at the photovoltaic grid-connected point to rise or even exceed the limit, affecting the power supply quality of power users in the area.

[0003] In order to improve the power supply quality of distribution stations in villages / communities with a high proportion of photovoltaic power and solve the problem of voltage over-limit, traditional voltage control methods include: on-load tap-changing transformers, parallel capacitors, residual capacity of photovoltaic inverters, static VAR compensators, energy storage systems, and installation of interconnection switches. However, these solutions have shortcomings such as slow voltage regulation response, limited regulation range, poor flexibility, and high cost. They are difficult to cope with the operation and regulation needs of distribution stations under high-proportion loads and distributed photovoltaic access. In addition, traditional voltage regulation methods have weak control capabilities for low-voltage distribution networks that are mainly resistive, and are difficult to cope with the risk of voltage over-limit in village / community distribution stations. In addition, traditional distribution stations lack flexibility and cannot achieve load transfer between stations and optimal load distribution in stations. Summary of the invention

[0004] The purpose of the present invention is to provide a high-proportion photovoltaic low-voltage area flexible interconnection system and a power coordination method, which not only fully utilizes the capacity resources of the distribution transformer to realize heavy-load transfer, but also utilizes the active-reactive collaborative control strategy to realize the effective management of the secondary side voltage of the distribution transformer, thereby playing a regulatory role in the overall distribution area, and can better cope with the wiring method combining "trunk type" and "chain type" in the low-voltage distribution area and the "self-generation and self-use, surplus power to the grid" operation mode.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A power coordination method for a high-proportion photovoltaic low-voltage area flexible interconnection system, comprising:

[0007] S1. Read the relevant data information of each distribution station area, generate real-time data, determine the master and slave converters, and judge whether there is a heavy load area and voltage over-limit situation.

[0008] S2. When there is a heavy-load area in the substation, according to the data information in step S1, the power command of the flexible interconnection device is obtained according to the principle of allocating power according to its own capacity, and power coordination is carried out; when there is a voltage over-limit situation in the substation, according to the influence mechanism of active-reactive coordinated control on voltage, the minimum capacity information of the flexible interconnection device is obtained, and the optimal voltage control point is determined in combination with the equivalent parameters of the feeder and transformer, and active and reactive power are allocated according to the optimal voltage control point to carry out power coordination.

[0009] S3. Obtain the operating status of the flexible interconnection device in the substation.

[0010] Furthermore, in step S1, the data information includes transformer capacity information, transmission line impedance information, real-time load conditions, photovoltaic output conditions and voltage information of each substation grid connection point.

[0011] The converter uses a voltage source converter, the main converter adopts constant DC voltage control, and the slave converter adopts constant power control.

[0012] Furthermore, the control link of the voltage source converter is composed of an outer loop and a current inner loop. The given values ​​of the outer loop of the main converter are voltage and reactive power, which form deviation signals with the actual values ​​and output current given values ​​through the PI regulator; the given values ​​of the outer loop of the slave converter are active and reactive power, which form deviation signals with the actual values ​​and output current given values ​​through the PI regulator.

[0013] Furthermore, in step S1, according to real-time data, when the load of the substation is greater than the capacity of the distribution transformer, it indicates that there is a heavy load area in the substation; when the photovoltaic power is greater than the load of the substation, it indicates that there is a voltage over-limit situation in the substation.

[0014] Furthermore, when there is a heavy load area in the substation, power coordination includes the following:

[0015] Each substation has load power, flexible interconnection device interaction power and transformer transmission power. The sum of load power and flexible interconnection device interaction power is transformer transmission power. The DC bus interconnects different substations and exchanges energy according to active power instructions. The transformer transmission power is:

[0016]

[0017] Among them, P acn represents the transmitted active power of the nth transformer, S n represents the capacity of the nth transformer, S i represents the capacity of the i-th transformer, PLi represents the load power of the i-th transformer, and n represents the total number of transformers.

[0018] According to the active power flow direction and power balance relationship, we can get:

[0019]

[0020] Among them, P aci represents the transmission power of the ith transformer, P ci represents the interaction power of the i-th flexible interconnection device, P cn represents the interaction power of the nth flexible interconnection device, P Ln Represents the load of the nth transformer.

[0021] Combining equations (10) and (11), we can obtain the power transmitted by the flexible interconnection device, that is, the power command, which is specifically:

[0022]

[0023] Furthermore, in step S2, when the voltage exceeds the limit in the substation, power coordination includes the following contents:

[0024] The vector form of the first-end voltage is:

[0025]

[0026] Among them, V n Indicates the voltage at the first end; Indicates V n The vector form of V cn Represents the terminal voltage; R n Represents the equivalent combined resistance of the feeder and transformer; X n Represents the equivalent synthetic reactance of the feeder and transformer; P cn =P VSCn +P Ln -P PVn , P VSCn represents the active power transmitted by the nth flexible interconnection device, P PVn Indicates the photovoltaic output power of the station area; Q cn It represents the reactive power flowing to the substation through the nth transformer, Q cn =Q VSCn +Q Ln , Q VSCn represents the reactive power transmitted by the nth flexible interconnection device, Q Ln Represents the reactive load of the nth substation; PCC n Indicates the photovoltaic grid-connected point in the nth area.

[0027] Ignoring the transverse component of the voltage drop, the terminal voltage expression is obtained by formula (13), which is:

[0028]

[0029] Set the maximum photovoltaic active power in the area to P PVnmax , the capacity of the flexible interconnection device is S VSCn , then the transmission power P of the flexible interconnection device VSCn , Q VSCn satisfy:

[0030] S VSCn 2 =P VSCn 2 +Q VSCn 2 (15).

[0031] Introducing the power factor to express active power and reactive power, combining equations (14) and (15) yields:

[0032]

[0033] in, represents the power factor angle,

[0034] when When taking different values, the head-end voltage, the maximum active power of photovoltaic power in the substation, the load data, the line parameters and the expected terminal voltage are substituted into formula (16) to obtain the minimum capacity information of the flexible interconnection device under different strategies that meet the regulation requirements.

[0035] Derivative of formula (16) yields:

[0036]

[0037] Let formula (17) equal to 0 to obtain:

[0038]

[0039] The minimum terminal voltage corresponds to It is the point of maximum voltage regulation capability, that is, the optimal voltage regulation point.

[0040] Furthermore, in step S3, when the transfer power of the substation is positive, the flexible interconnection device of the substation is in a rectifying state; when the transfer power of the substation is negative, the flexible interconnection device of the substation is in an inverting state; when the transfer power of the substation is zero, the flexible interconnection device of the substation is in an idle state.

[0041] Furthermore, the present invention also proposes a high-proportion photovoltaic low-voltage area flexible interconnection system, comprising:

[0042] Traditional low-voltage power distribution system, flexible interconnection device and DC bus; Traditional low-voltage power distribution system includes distribution transformers, substation loads and rooftop photovoltaics.

[0043] The secondary side of the distribution transformer is connected to a flexible interconnection device, and DC ports of all the flexible interconnection devices are connected to a DC bus in parallel.

[0044] The traditional low-voltage distribution system is used to supply power to distribution stations through distribution transformers. When photovoltaic overflow occurs, power is sent up through distribution transformers.

[0045] The flexible interconnection device is used to connect all distribution substations, realize the mutual conversion between AC and DC, and is responsible for the stable operation within the distribution substation and the power interaction between substations.

[0046] The DC bus is used to connect different substations. Different substations absorb or release energy to the DC bus through flexible interconnection devices.

[0047] Furthermore, the flexible interconnection device uses an AC / DC converter, the AC end is connected to the secondary side of the distribution transformer, and the DC end is connected to the DC bus.

[0048] Furthermore, the DC bus is a 750V DC bus.

[0049] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0050] 1. The method proposed in the present invention can realize load transfer in heavy-load distribution station areas, make full use of the capacity of distribution transformers, and carry out active power mutual assistance through the constructed flexible interconnection channel, so as to realize the transfer of power supply in heavy-load stations and the temporal and spatial complementarity of source and load.

[0051] 2. The present invention absorbs photovoltaic resources to the greatest extent without replacing the distribution transformer. When there is excess photovoltaic power in the distribution station area, the power flow will flow back to the 10kV AC bus. Due to the limited capacity of the transformer, it is difficult to complete the active power transmission beyond the capacity. At this time, the photovoltaic power is transferred to other distribution stations through a flexible interconnection device to ensure the safe and stable operation of the excess station area.

[0052] 3. The device proposed in the present invention is connected to the secondary side of the distribution transformer, which can directly control the secondary side voltage of the transformer, and play an indirect and global regulatory role on the voltage of each feeder node in the substation. On the basis of traditional reactive voltage regulation, combined with active voltage regulation, "active-reactive" power coordinated control is proposed to give full play to the voltage regulation capability of the flexible interconnected device and improve the power quality of power users in the distribution substation area. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is an overall flow chart of the method of the present invention.

[0054] Figure 2 It is a control block diagram of the device of the present invention.

[0055] Figure 3 It is a schematic diagram of active power flow of the device of the present invention.

[0056] Figure 4 It is an equivalent circuit diagram of a photovoltaic village / community distribution station area according to an embodiment of the device of the present invention.

[0057] Figure 5 Schematic diagram of the voltage regulation capability of the device of the present invention.

[0058] Figure 6 It is the overall structural diagram of the device of the present invention. DETAILED DESCRIPTION

[0059] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0060] To achieve the above objectives, the present invention proposes a power coordination method for a high-proportion photovoltaic low-voltage area flexible interconnection system, such as Figure 1 As shown, specifically including:

[0061] S1. Read the relevant data information of each distribution substation, including transformer capacity information, transmission line impedance information, real-time load conditions, photovoltaic output conditions and voltage information of each substation grid connection point, generate real-time data, determine the master and slave converters, and determine whether there is a heavy load area and voltage over-limit situation. When the substation load is greater than the distribution transformer capacity, it indicates that the substation has a heavy load area; when the photovoltaic power is greater than the substation load, it indicates that the substation has a voltage over-limit situation.

[0062] In this embodiment, the converter is a voltage source converter, the main converter adopts constant DC voltage control to establish a stable DC voltage, and the slave converter adopts constant power control.

[0063] like Figure 2 As shown, the control link of the voltage source converter consists of an outer loop and a current inner loop, where: Figure 2 (a) is the control diagram of the main converter. The given values ​​of the main converter outer loop are voltage and reactive power, which form deviation signals with the actual values ​​and output current given values ​​through the PI regulator. Figure 2 (b) is the control diagram of the slave converter. The given value of the outer loop of the slave converter is active and reactive power, which forms a deviation signal with the actual value and outputs the current given value through the PI regulator. The inner loop is the current loop, which makes a difference between the output current given value of the outer loop and the actual value, and outputs the voltage modulation signal after PI control and decoupling.

[0064] Figure 2 In, V dcref , P ref , Q ref 、V dc , P, Q represent the voltage command value, active power command value, reactive power command value, voltage actual value, active power actual value, reactive power actual value of the DC bus, respectively. dref Indicates the d-axis current given value, I qref represents the q-axis current given value, αβ represents the two-phase stationary coordinate system, dq represents the two-phase rotating coordinate system, PWM represents pulse width modulation, i d 、i q Both represent the actual value of the current, and ωL represents the coupling term.

[0065] When there is no heavy load area and no voltage exceeding the limit in the substation, the flexible interconnection device is in idle mode, and the power instruction of the flexible interconnection device is 0. At this time, the power supply can be stable, and there is no need to consider the additional loss caused by the operation of the flexible interconnection device.

[0066] S2. When there is a heavy-load area in the substation, according to the data information in step S1, the power command of the flexible interconnection device is obtained according to the principle of allocating power according to its own capacity, and power coordination is carried out; when there is a voltage over-limit situation in the substation, according to the influence mechanism of active-reactive coordinated control on voltage, the minimum capacity information of the flexible interconnection device is obtained, and the optimal voltage control point is determined in combination with the equivalent parameters of the feeder and transformer, and active and reactive power are allocated according to the optimal voltage control point to carry out power coordination. The specific contents are:

[0067] When there is a heavy load area in the substation, power coordination includes the following:

[0068] like Figure 3 As shown in the figure, each substation has load power, flexible interconnection device interaction power and transformer transmission power. The sum of load power and flexible interconnection device interaction power is transformer transmission power. The DC bus interconnects different substations and exchanges energy according to active power instructions. The transformer transmission power is:

[0069]

[0070] Among them, P acn represents the transmitted active power of the nth transformer, S n represents the capacity of the nth transformer, S i represents the capacity of the i-th transformer, P Li represents the load power of the i-th transformer, and n represents the total number of transformers.

[0071] According to the active power flow direction and power balance relationship, we can get:

[0072]

[0073] Among them, P aci represents the transmission power of the ith transformer, P ci represents the interaction power of the i-th flexible interconnection device, P cn represents the interaction power of the nth flexible interconnection device, P Ln Represents the load of the nth transformer.

[0074] Combining equations (19) and (20), we can obtain the power transmitted by the flexible interconnection device, that is, the power command, which is specifically:

[0075]

[0076] Formula (21) is used to obtain the interactive power size except for the main converter. The main converter automatically transmits power under the constraint of power balance, thereby rationally utilizing the transformer capacity to achieve active power mutual assistance in multiple distribution station areas.

[0077] Figure 4 This is a simplified equivalent circuit of a single-area distribution network architecture based on a flexible interconnection system. The first end is a 10kV distribution network end, which supplies power to the area after passing through feeders and transformers. The area includes equivalent loads, equivalent photovoltaics, and flexible interconnection devices. n 、V cn They represent the 10kV distribution network head-end equivalent voltage and the distribution transformer secondary voltage, referred to as the head-end voltage and the terminal voltage respectively; P cn +jQ cn Indicates the power flowing to the substation through the transformer; P Ln +jQ Ln Indicates the load power of the substation; P PVn Indicates the photovoltaic output power of the station area; P VSCn +jQ VSCn Represents the transmission power of the flexible interconnection device; R n and X n They represent the equivalent combined resistance and reactance of the feeder and transformer respectively.

[0078] like Figure 4 As shown in the figure, when the voltage exceeds the limit in the substation area, power coordination includes the following:

[0079] when P cn , Q cn When known, is the reference vector, and the vector form of the first-end voltage is:

[0080]

[0081] in, Indicates Vn The vector form of P cn =P VSCn +P Ln -P PVn , P VSCn represents the active power transmitted by the nth flexible interconnection device; Q cn It represents the reactive power flowing to the substation through the nth transformer, Q cn =Q VSCn +Q Ln , Q VSCn represents the reactive power transmitted by the nth flexible interconnection device, Q Ln Represents the reactive load of the nth substation; PCC n Indicates the photovoltaic grid-connected point in the nth area.

[0082] Ignoring the transverse component of the voltage drop, the terminal voltage expression is obtained by formula (22), which is:

[0083]

[0084] From formula (23), we can see that when the head-end voltage is determined, the load power is unchanged, and the equivalent parameters are determined, the grid connection point voltage depends on P PVn and P VSCn +jQ VSCn And from formula (23), it can be seen that the terminal voltage is positively correlated with the photovoltaic output power. When the photovoltaic output is too large, it will cause the flow to be reversed, causing the terminal voltage to be higher than the head voltage. In severe cases, it may even exceed the limit. This usually happens at noon. The terminal voltage is positively correlated with P VSCn , Q VSCn is inversely proportional to the terminal voltage, and the degree of its influence depends on the parameter R n With X n .

[0085] Formula (23) shows that after the flexible interconnection device is connected to the terminal, active power transfer can be performed on the basis of traditional reactive power-voltage regulation to achieve effective regulation of the terminal voltage, thereby ensuring that it is within a reasonable range, that is, ±7% of the nominal voltage.

[0086] In order to verify the voltage regulation capabilities of different strategies, the maximum photovoltaic active power in the area is set to P PVnmax , the capacity of the flexible interconnection device is S VSCn , then the transmission power P of the flexible interconnection device VSCn , Q VSCn satisfy:

[0087] S VSCn 2 =P VSCn 2 +Q VSCn2 (twenty four).

[0088] In order to uniformly analyze the regulation capability of active power and reactive power on voltage, the power factor is introduced to express active power and reactive power. By combining formulas (23) and (24), we can obtain:

[0089]

[0090] in, represents the power factor angle,

[0091] when When taking different values, the head-end voltage, the maximum active power of photovoltaic power in the substation, the load data, the line parameters and the expected terminal voltage are substituted into formula (25) to obtain the minimum capacity information of the flexible interconnection device under different strategies that meet the regulation requirements.

[0092] Derivative of formula (25) yields:

[0093]

[0094] Let formula (26) equal to 0 to obtain:

[0095]

[0096] Formula (27) shows that when the terminal voltage is adjusted using a flexible interconnection device of the same capacity, there is a maximum voltage drop, that is, the maximum voltage regulation capability. In addition, formula (25) is visualized, as Figure 5 As shown in the figure is the maximum voltage regulation capability point, satisfying formula (27), that is: It can be clearly observed from the figure that as the proportion of active power increases, the terminal voltage decreases first and then increases, and there is a minimum voltage. It is the maximum voltage regulation capability point of the active-reactive coordinated strategy, that is, the optimal voltage regulation point.

[0097] S3. Obtain the operating state of the flexible interconnection device in the substation. When the transfer power of the substation is positive, the flexible interconnection device in the substation is in a rectifying state; when the transfer power of the substation is negative, the flexible interconnection device in the substation is in an inverting state; when the transfer power of the substation is zero, the flexible interconnection device in the substation is in an idle state.

[0098] The present invention also proposes a high-proportion photovoltaic low-voltage area flexible interconnection system, such as Figure 6 As shown, including:

[0099] Traditional low-voltage power distribution system, flexible interconnection device and DC busbar; The traditional low-voltage power distribution system includes distribution transformers, substation loads and rooftop photovoltaics. The secondary side of the distribution transformer is connected to the flexible interconnection device, and the DC ports of all flexible interconnection devices are connected to the DC busbar in parallel.

[0100] The traditional low-voltage power distribution system is used to supply power to the distribution substations through the distribution transformer. When the photovoltaic power overflows, the power is sent up through the distribution transformer. The flexible interconnection device is used to connect all the distribution substations to realize the mutual conversion between AC and DC. It is responsible for the stable operation within the distribution substation and the power interaction between substations. The flow direction of the flexible interconnection device is bidirectional and depends on the instructions of the upper power control. The DC bus is used to connect different substations. Different substations absorb or release energy to the DC bus through the flexible interconnection device.

[0101] In this embodiment, there are n village / community distribution stations, each with several power users and rooftop photovoltaic resources. Each station is equipped with a set of high-proportion photovoltaic low-voltage station flexible interconnection devices, a 750V DC bus is constructed, and the DC port of the flexible interconnection device is connected to the DC bus. Through the flexible interconnection channel, the active power of each station is mutually beneficial, and the voltage regulation capability of the flexible interconnection device is combined with the capacity of the transformer to achieve active power transfer, heavy load transfer, power quality management and photovoltaic consumption. The flexible interconnection device uses a flexible AC / DC converter, the AC end is connected to the secondary side of the distribution transformer, and the DC end is connected to the DC bus.

[0102] In order to clearly demonstrate the voltage regulation effect of the active-reactive coordination strategy, the capacity of the flexible interconnection device required for voltage regulation using different strategies is calculated. It is reactive voltage regulation; When is active voltage regulation; When the voltage is adjusted, the active-reactive coordinated voltage is adjusted. The equivalent impedance of the transformer and feeder is 0.33+j0.48Ω, and the maximum overflow active power of the photovoltaic power distribution area is 100kW. The terminal voltage is controlled to the lower limit value, that is, -7% of the nominal voltage, so as to deal with the voltage over-limit risk caused by photovoltaic grid connection to the greatest extent, and thus the minimum capacity table corresponding to different voltage regulation strategies is obtained, as shown in Table 1.

[0103] Table 1 Minimum capacity of flexible interconnection devices with different voltage regulation strategies in photovoltaic village / community distribution areas

[0104]

[0105]

[0106] The results show that when the voltage is adjusted to 0.93pu, the minimum capacity of the flexible interconnection device required for active voltage regulation, reactive voltage regulation and active-reactive coordinated voltage regulation is 128.64kVA, 88.44kVA and 73.19kVA respectively; the active and reactive power are allocated according to the optimal voltage control point, which are 41.72kW and 60.02kVar respectively. Among them, active-reactive coordinated voltage regulation has obvious advantages, saving 43% of capacity compared with active voltage regulation; and saving 17% of capacity compared with reactive voltage regulation.

[0107] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A power coordination method for a high-proportion photovoltaic low-voltage area flexible interconnection system, characterized in that: include: S1, read the relevant data information of each distribution station area, generate real-time data, determine the master and slave converters, and judge whether there is a heavy load area and voltage over-limit situation; S2. When there is a heavy-load area in the substation, according to the data information in step S1, the power command of the flexible interconnection device is obtained according to the principle of allocating power according to its own capacity, and power coordination is performed; when there is a voltage over-limit situation in the substation, according to the influence mechanism of active-reactive coordinated control on voltage, the minimum capacity information of the flexible interconnection device is obtained, and the optimal voltage control point is determined in combination with the equivalent parameters of the feeder and transformer, and active and reactive power are allocated according to the optimal voltage control point to perform power coordination; S3. Obtain the operating status of the flexible interconnection device in the substation.

2. The power coordination method of the high-proportion photovoltaic low-voltage area flexible interconnection system according to claim 1 is characterized in that: In step S1, the data information includes transformer capacity information, transmission line impedance information, real-time load conditions, photovoltaic output conditions and voltage information of each grid connection point; The converter uses a voltage source converter, the main converter adopts constant DC voltage control, and the slave converter adopts constant power control.

3. The power coordination method of the high-proportion photovoltaic low-voltage area flexible interconnection system according to claim 2 is characterized in that: The control link of the voltage source converter is composed of an outer loop and a current inner loop. The given value of the outer loop of the main converter is voltage and reactive power, which form a deviation signal with the actual value and output the current given value through the PI regulator; The given value from the outer loop of the converter is the active and reactive power, which forms a deviation signal with the actual value, and outputs the current given value through the PI regulator.

4. The power coordination method of the high-proportion photovoltaic low-voltage area flexible interconnection system according to claim 1 is characterized in that: In step S1, according to real-time data, when the load of the substation is greater than the capacity of the distribution transformer, it indicates that there is a heavy load area in the substation; when the photovoltaic power is greater than the load of the substation, it indicates that there is a voltage over-limit situation in the substation.

5. The power coordination method of the high-proportion photovoltaic low-voltage area flexible interconnection system according to claim 1 is characterized in that: In step S2, when there is a heavy load area in the substation, power coordination includes the following: Each substation has load power, flexible interconnection device interaction power and transformer transmission power. The sum of load power and flexible interconnection device interaction power is transformer transmission power. The DC bus interconnects different substations and exchanges energy according to active power instructions. The transformer transmission power is: Among them, P acn represents the transmitted active power of the nth transformer, S n represents the capacity of the nth transformer, S i represents the capacity of the i-th transformer, P Li represents the load power of the i-th transformer, and n represents the total number of transformers; According to the active power flow direction and power balance relationship, we can get: Among them, P aci represents the transmission power of the ith transformer, P ci represents the interaction power of the i-th flexible interconnection device, P cn represents the interaction power of the nth flexible interconnection device, P Ln represents the load of the nth transformer; Combining formulas (1) and (2), we can obtain the power transmitted by the flexible interconnection device, that is, the power command, which is specifically:

6. The power coordination method of the high-proportion photovoltaic low-voltage area flexible interconnection system according to claim 1 is characterized in that: In step S2, when the voltage exceeds the limit in the substation, power coordination includes the following: The vector form of the first-end voltage is: Among them, V n Indicates the voltage at the first end; Indicates V n The vector form of V cn Represents the terminal voltage; R n Represents the equivalent combined resistance of the feeder and transformer; X n Represents the equivalent synthetic reactance of the feeder and transformer; P cn represents the active power flowing to the substation through the nth transformer, P cn =P VSCn +P Ln -P PVn , P VSCn represents the active power transmitted by the nth flexible interconnection device, P Ln represents the load of the nth substation, P PVn Indicates the photovoltaic output power of the station area; Q cn It represents the reactive power flowing to the substation through the nth transformer, Q cn =Q VSCn +Q Ln , Q VSCn represents the reactive power transmitted by the nth flexible interconnection device, Q Ln Represents the reactive load of the nth substation; PCC n Indicates the photovoltaic grid-connected point in the nth area; Ignoring the transverse component of the voltage drop, the terminal voltage expression is obtained by formula (4), which is: Set the maximum photovoltaic active power in the area to P PVnmax , the capacity of the flexible interconnection device is S VSCn , then the transmission power P of the flexible interconnection device VSCn , Q VSCn satisfy: S VSCn 2 =P VSCn 2 +Q VSCn 2 (6); Introducing power factor to express active power and reactive power, combining equations (5) and (6) yields: in, represents the power factor angle, when When different values ​​are taken, the head-end voltage, the maximum active power of photovoltaic power in the substation, the load data, the line parameters and the expected terminal voltage are substituted into formula (7) to obtain the minimum capacity information of the flexible interconnection device under different strategies that meet the regulation requirements; Derivative of formula (7) yields: Let formula (8) equal to 0 to obtain: The minimum terminal voltage corresponds to It is the point of maximum voltage regulation capability, that is, the optimal voltage regulation point.

7. The power coordination method of the high-proportion photovoltaic low-voltage area flexible interconnection system according to claim 1 is characterized in that: In step S3, when the transfer power of the substation is positive, the flexible interconnection device of the substation is in a rectifying state; when the transfer power of the substation is negative, the flexible interconnection device of the substation is in an inverting state; when the transfer power of the substation is zero, the flexible interconnection device of the substation is in an idle state.

8. The high-ratio photovoltaic low-voltage area flexible interconnection system as described in claim 1 is characterized in that: include: Traditional low-voltage power distribution system, flexible interconnection device and DC bus; Traditional low-voltage power distribution system includes distribution transformer, area load and rooftop photovoltaic; The secondary side of the distribution transformer is connected to a flexible interconnection device, and the DC ports of all the flexible interconnection devices are connected to the DC bus in parallel; The traditional low-voltage power distribution system is used to supply power to the distribution area through the distribution transformer. When the photovoltaic overflow occurs, the power is sent up through the distribution transformer; Flexible interconnection device, used to connect all distribution substations, realize the mutual conversion between AC and DC, and is responsible for the stable operation within the distribution substation and the power interaction between substations The DC bus is used to connect different substations. Different substations absorb or release energy to the DC bus through flexible interconnection devices.

9. The high-ratio photovoltaic low-voltage area flexible interconnection system according to claim 8 is characterized in that: The flexible interconnection device uses an AC / DC converter, with the AC end connected to the secondary side of the distribution transformer and the DC end connected to the DC bus.

10. The high-ratio photovoltaic low-voltage area flexible interconnection system according to claim 8, characterized in that: The DC bus is a 750V DC bus.

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