CN109412192B - Self-energy-storage multi-end back-to-back flexible straight device operation method - Google Patents

Abstract

The invention relates to an operation method of a self-energy storage multi-terminal back-to-back straightening device, comprising calculating whether the comprehensive power supply cost of a distribution network has a minimum value when the self-energy storage multi-terminal back-to-back straightening device performs power regulation; reading the self-energy storage multi-terminal back-to-back straightening device The state of charge of the energy storage unit in the device; the operation mode of the self-storage multi-terminal back-to-back flexible straightening device is switched according to the values of the above two steps; the operation mode of the self-storage multi-terminal back-to-back flexible straightening device includes: the energy storage unit does not operate, the self-storage The multi-terminal back-to-back flexible straightening device can perform power regulation; the energy storage unit is running, and the self-energy storage multi-terminal back-to-back flexible straightening device can perform power and energy sequence adjustment; . It can effectively reduce the configuration capacity of energy storage, improve the operating life of energy storage, effectively improve the absorption capacity of high-penetration clean energy, improve the reliability of power supply, improve the quality of power supply, and improve operating economy.

Description

Operation method of self-energy storage multi-end back-to-back flexible straightening device

technical field

The invention relates to the field of optimal operation of an active distribution network, in particular to a method and a device for operating a self-storage multi-terminal back-to-back flexible straightening device.

Background technique

High-reliability power supply and full-scale friendly consumption of high-penetration clean energy have put forward higher requirements for the construction and operation of my country's distribution network in the new era. The operation of the distribution network ring network is an important way to further improve the reliability of power supply. However, due to the difference in potential and short-circuit impedance on both sides of the feeder, the large impact/closed loop current endangers the safe operation of the power grid. The short-term peak power generation load of clean energy and the resulting high voltage problem are becoming more and more prominent. Adding/capacity grids and investing more reactive power compensation devices such as static var generators (SVG) will greatly increase the construction and operation of the distribution network. Cost, limiting the active power output and adjusting the reactive power output of the inverter can better solve the problem of voltage exceeding the limit, but it also causes the de facto "abandoning the wind and abandoning the light" in the distribution network.

To solve the above problems, new technical approaches need to be applied. Multi-terminal back-to-back VSC-HVDC (VSC-MTDC) is the latest developed power grid flexible control technology. AC decoupling and interconnection can realize long-term safe closed-loop operation of any feeder; PQ four-quadrant control can accurately control the power flow distribution of the power grid; omitting the DC line link reduces the cost and complexity of the control system, and is more suitable for the actual distribution network in my country.

The flexible power flow control technology represented by back-to-back flexibility and straightness is still essentially the regulation of the power level. In the energy level, the grid provides an "energy container" on the spatial axis, but when the adjustable capacity of the interconnected feeder is low, it will The optimal operation effect of the interconnected distribution network is reduced, and even the situation that the system security and power supply quality constraints cannot be met will still occur. Energy store system (ESS) technology, as the "energy container" of the time axis, essentially improves the simultaneity of electric energy production, transmission and consumption. For energy storage equipment invested by users or clean energy power stations, although the power grid can guide it to provide support for power grid operation through incentive measures such as demand response and auxiliary service pricing, the cost/effect of incentives is not fully controllable, which promotes the distribution network side. Configure energy storage needs.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a self-energy storage multi-terminal back-to-back flexible straightening device operation method and device for the above-mentioned defects of the prior art, which overcomes the inability of clean energy to be fully consumed, and the reliability and economy of power supply are poor. . When the total adjustment margin of each feeder in the system is insufficient, only relying on power adjustment cannot meet the operation requirements.

The technical solution adopted by the present invention to solve the technical problem is to provide a method for operating a self-energy-storage multi-terminal back-to-back straightening device, comprising: S1: calculating the configuration of the self-energy-storing multi-terminal back-to-back straightening device when performing power regulation. Whether the comprehensive power supply cost of the power grid has a minimum value; S2: Read the state of charge of the energy storage unit in the self-storage multi-terminal back-to-back flexible straightening device; S3: According to the self-storage multi-terminal back-to-back flexible straightening device performing power Whether the comprehensive power supply cost of the power distribution network has a minimum value during adjustment and whether the state of charge of the energy storage unit in the self-energy storage multi-terminal back-to-back flexible straightening device switches the operation mode of the self-storage multi-terminal back-to-back flexible straightening device The operation mode of the self-energy storage multi-terminal back-to-back straightening device includes: the energy storage unit does not operate, the self-energy storage multi-terminal back-to-back straightening device performs power regulation; the energy storage unit operates, the self-storage The multi-terminal back-to-back straightening device performs power energy time sequence adjustment; the energy storage unit performs state-of-charge recovery, and the self-storage multi-terminal back-to-back straightening device independently performs power adjustment.

The state of charge of the energy storage unit includes: a standby state, in which the energy storage unit is in a moderate state of charge, and can meet the optimal operation requirements of the distribution network in the next period at any time; in a normal non-standby state, the energy storage unit is in a more advanced state. In a wide state of charge, the adjustable capacity is small; in an abnormal state, the energy storage unit is in an emergency state of charge.

The standby state is that the charge value range of the energy storage unit is greater than or equal to 0.4 and less than or equal to 0.6; the normal non-standby state is that the charge value range of the energy storage unit is greater than or equal to 0.15 and less than or equal to 0.85; The abnormal state is that the range of the charge value of the energy storage unit is greater than or equal to 0 and less than 0.15 or greater than 0.85 and less than or equal to 1.

According to whether the comprehensive power supply cost of the self-energy storage multi-terminal back-to-back straightening device has a minimum value when performing power regulation and the state of charge of the energy storage unit, switching the operation mode of the self-storage multi-terminal back-to-back straightening device includes: : When the self-storage multi-terminal back-to-back flexible straightening device performs power regulation, the comprehensive power supply cost has a minimum value, and when the energy storage unit is in a standby state, the energy storage unit does not operate, and the self-storage multi-terminal The back-to-back straightening device performs power regulation; when the self-energy storage multi-terminal back-to-back straightening device is in the power regulation mode, the comprehensive power supply cost has the lowest value, and the energy storage unit is in a normal non-standby state or an abnormal state, The energy storage unit performs state-of-charge recovery, and the self-storage multi-terminal back-to-back flexible straightening device independently performs power adjustment; when the self-storage multi-terminal back-to-back flexible straightening device is in the power adjustment mode, the comprehensive power supply cost has no minimum value , and when the energy storage unit is in a standby state or a normal non-standby state, the energy storage unit is put into operation, and the self-storage multi-terminal back-to-back flexible straightening device performs power energy timing adjustment; When the direct device is in the power regulation mode, the comprehensive power supply cost has no minimum value, and when the energy storage unit is in an abnormal state, the energy storage unit performs state-of-charge recovery, and the self-storage multi-terminal back-to-back flexible straight device is independent Perform power regulation.

The restoring of the state of charge of the energy storage unit includes: restoring the current charge value of the energy storage unit to a minimum difference value from the intermediate value of the charge of the energy storage unit.

The objective function that the current charge value of the energy storage unit is restored to the smallest difference from the intermediate value of the energy storage charge is:

SOC(t) is the state of charge of the energy storage unit at time t;

P _ESS (t) is the output power of the energy storage unit in period t;

Δt is the duration of each time period;

S _ESS is the rated power of the energy storage unit;

SOC _mid is the middle value of the charge of the energy storage unit;

The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit;

The nuclear power state of the energy storage unit in the next period is calculated according to the charge value of the energy storage unit in the current period and the energy storage power released in the current period.

The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit as:

P _ESS (t)=uP _cd (t),u∈{-1,0,1};

The calculation formula of the nuclear power state of the energy storage unit in the next period according to the energy storage charge value of the current period and the energy storage power released in the current period is:

SOC(0)=SOC(T);

T is the number of time periods divided by the complete scheduling cycle;

Δt is the duration of each period;

P _ESS (t) is the output power of the energy storage unit in period t;

P _cd (t) is the charging and discharging power of the energy storage unit in the t period, which is always positive;

u is the charge and discharge mark of the energy storage unit;

P _ch,max and P _dis,max are the maximum charging and discharging power of the energy storage unit;

S _ESS is the rated power of the energy storage unit;

SOC(t) is the state of charge of the energy storage unit at time t;

SOC _max and SOC _min are respectively the upper and lower safe limits of the state of charge of the energy storage unit;

_ηch and _ηdis are the charge and discharge efficiencies of the energy storage unit, respectively.

The invention also provides a self-storage multi-terminal back-to-back flexible straightening device, comprising at least two AC/DC converters and at least one energy storage unit, the DC side of the AC/DC converter is connected in parallel with the DC bus, the The AC side of the AC/DC converter is connected to the feeder of the distribution network; one side of the energy storage unit is connected to the DC/DC converter, and the other side of the DC/DC converter is connected to the DC bus in parallel The self-energy storage multi-terminal back-to-back straightening device further includes an operation mode adjustment module, and the operation mode adjusting module is based on whether the comprehensive power supply cost of the power distribution network when the self-energy-storage multi-terminal back-to-back straightening device performs power adjustment. In the case of having the lowest value and the state of charge of the energy storage unit, switch the operation mode of the self-energy storage multi-terminal back-to-back straightening device; the operating mode of the self-energy back-to-back straightening device includes: the energy storage unit does not In operation, the self-energy storage multi-terminal back-to-back straightening device performs power regulation; the energy storage unit operates, and the self-storage multi-terminal back-to-back straightening device performs power energy timing adjustment; the energy storage unit performs state-of-charge recovery, The self-energy storage multi-terminal back-to-back flexible straight device independently performs power regulation; when the energy storage unit operates or performs state-of-charge recovery, the DC/DC converter controls the charging and discharging of the energy storage unit.

The state of charge of the energy storage unit includes: a standby state, in which the energy storage unit is in a moderate state of charge, and can meet the optimal operation requirements of the distribution network in the next period at any time; in a normal non-standby state, the energy storage unit is in a more advanced state. In a wide state of charge, the adjustable capacity is small; in an abnormal state, the energy storage unit is in an emergency state of charge.

The standby state is that the charge value range of the energy storage unit is greater than or equal to 0.4 and less than or equal to 0.6; the normal non-standby state is that the charge value range of the energy storage unit is greater than or equal to 0.15 and less than or equal to 0.85; The abnormal state is that the range of the charge value of the energy storage unit is greater than or equal to 0 and less than 0.15 or greater than 0.85 and less than or equal to 1.

The operating mode adjustment module includes a processor, and the processor switches according to whether the comprehensive power supply cost has a minimum value when the self-energy storage multi-terminal back-to-back flexible straight device performs power adjustment and the state of charge of the energy storage unit. The operating mode of the self-storage multi-end back-to-back flexible straightening device.

The operation mode adjustment module switches the self-storage multi-terminal back-to-back flexible straight device according to whether the comprehensive power supply cost has a minimum value when the self-storage multi-terminal back-to-back flexible straightening device performs power adjustment and the state of charge of the energy storage unit. The operation mode of the direct device includes: when the self-energy storage multi-terminal back-to-back flexible direct device performs power regulation, the comprehensive power supply cost has a minimum value, and when the energy storage unit is in a standby state, the operation mode adjustment module controls the The energy storage unit does not operate, and controls the self-energy storage multi-terminal back-to-back straightening device to perform power regulation; when the self-energy storage multi-terminal back-to-back straightening device executes the power regulation mode, the comprehensive power supply cost has a minimum value, and all When the energy storage unit is in a normal non-standby state or an abnormal state, the operation mode adjustment module controls the energy storage unit to perform state-of-charge recovery, and controls the self-energy storage multi-terminal back-to-back flexible straight device to independently perform power regulation; When the self-energy storage multi-terminal back-to-back flexible straightening device performs power regulation, the comprehensive power supply cost has no minimum value, and the energy storage unit is in a standby state or a normal non-standby state, the operation mode adjustment module controls the storage The energy unit is put into operation, and the self-storage multi-terminal back-to-back flexible straightening device is controlled to perform power energy time sequence adjustment; when the self-storage multi-terminal back-to-back flexible straightening device executes the power adjustment mode, the comprehensive power supply cost has no minimum value, and all When the energy storage unit is in an abnormal state, the operation mode adjustment module controls the energy storage unit to perform state-of-charge recovery, and controls the self-energy storage multi-terminal back-to-back flexible straightening device to perform power regulation independently.

The restoring of the state of charge of the energy storage unit includes: restoring the current charge value of the energy storage unit to a minimum difference value from the intermediate value of the charge of the energy storage unit.

The objective function that the current charge value of the energy storage unit is restored to the smallest difference from the intermediate value of the energy storage charge is:

SOC(t) is the state of charge of the energy storage unit at time t;

P _ESS (t) is the output power of the energy storage unit in period t;

Δt is the duration of each time period;

S _ESS is the rated power of the energy storage unit;

SOC _mid is the middle value of the charge of the energy storage unit;

The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit;

The nuclear power state of the energy storage unit in the next period is calculated according to the charge value of the energy storage unit in the current period and the energy storage power released in the current period.

The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit as:

P _ESS (t)=uP _cd (t),u∈{-1,0,1};

The calculation formula of the nuclear power state of the energy storage unit in the next period according to the energy storage charge value of the current period and the energy storage power released in the current period is:

SOC(0)=SOC(T);

T is the number of time periods divided by the complete scheduling cycle;

Δt is the duration of each period;

P _ESS (t) is the output power of the energy storage unit in period t;

P _cd (t) is the charging and discharging power of the energy storage unit in the t period, which is always positive;

u is the charge and discharge mark of the energy storage unit;

P _ch,max and P _dis,max are the maximum charging and discharging power of the energy storage unit;

S _ESS is the rated power of the energy storage unit;

SOC(t) is the state of charge of the energy storage unit at time t;

SOC _max and SOC _min are respectively the upper and lower safe limits of the state of charge of the energy storage unit;

_ηch and _ηdis are the charge and discharge efficiencies of the energy storage unit, respectively.

The present invention also provides a flexible interconnected power distribution network, using the self-energy storage multi-terminal back-to-back flexible straightening device of the present invention, the self-storage multi-terminal back-to-back straightening device is based on the comprehensive power supply cost of the power distribution network in the power adjustment mode Whether there is a minimum value and the state of charge of the energy storage unit in the self-energy storage back-to-back flexible straightening device switch the operation mode of the self-storage back-to-back flexible straightening device.

The invention aims at the self-storage multi-terminal back-to-back flexible straightening device, from the perspective of improving the clean energy consumption capacity and power supply reliability of the flexible interconnected distribution network, while effectively reducing the configuration capacity of the energy storage unit and improving the operating life of the energy storage unit. A composite control method for a self-energy storage multi-terminal back-to-back flexible straightening device is proposed. The composite control method of the present invention can effectively reduce the configuration capacity of the energy storage, improve the operating life of the energy storage, effectively improve the high-penetration clean energy consumption capacity, and improve the reliability of power supply. performance, improve power supply quality, and improve operating economy.

The self-storage multi-terminal back-to-back flexible straightening device of the present invention enables the energy storage unit to be in a reasonable state of charge at each time period in one operation cycle, and participates in the necessary energy sequence optimization adjustment, which can avoid overshoot or overdischarge of the energy storage unit, and prevent the energy storage unit from overshooting or overdischarging. Ensure that the energy storage unit has the ability to adjust for the next period. Compared with the multi-terminal back-to-back flexible straightening device (VSC-MTDC) without an energy storage unit, the operation method of the self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) proposed by the present invention can adapt to more complex high-permeability clean energy sources. Network operation scenarios and control requirements.

Description of drawings

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, in which:

1 is a flowchart of a method for operating a self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) according to an embodiment of the present invention;

Fig. 2 is the control flow chart of step 30 of the operation method of the self-energy storage multi-end back-to-back straightening device in Fig. 1;

3 is a schematic block diagram of a self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) according to an embodiment of the present invention;

4 is a schematic diagram of a 33-node computing example system according to an embodiment of the present invention;

Fig. 5 is the A feeder load and DG curve in Fig. 4;

Fig. 6 is the B feeder load and DG curve in Fig. 4;

Fig. 7 is the C feeder load and DG curve in Fig. 4;

Fig. 8 is the D feeder load and DG curve in Fig. 4;

Fig. 9 is the output curve of each feeder when the system of Fig. 4 is in open-loop operation;

Fig. 10 is the output curve of each feeder under the power regulation of the system of Fig. 4;

Fig. 11 is the output curve of each feeder under the power-energy timing adjustment of the system of Fig. 4;

Fig. 12 is a comparison curve of comprehensive power supply cost in each period when the system of Fig. 4 is in open-loop operation, power adjustment and power-energy timing adjustment;

Fig. 13 is the output curve of each inverter in Fig. 4;

Fig. 14 is the change curve of ESS power during the energy storage working period in Fig. 4;

Fig. 15 is the charge and discharge power curve of the ESS during the energy storage working period in Fig. 4;

Fig. 16 is the electricity change curve of the ESS in the whole operation cycle of Fig. 4;

FIG. 17 is the charge-discharge power curve of the ESS in the whole operation cycle of FIG. 4 .

Detailed ways

In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like parts throughout. Hereinafter, exemplary embodiments are described in order to explain the present invention with reference to the figures.

The present invention is described below with reference to flowcharts of methods according to exemplary embodiments of the present invention. It will be understood that each block of the flowchart illustrations, and combinations of blocks in the flowchart illustrations, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a suitable computer, special purpose computer, or other programmable data processing apparatus to produce a machine, whereby the instructions are executed by the processor of the computer or other programmable data processing apparatus to implement the process blocks or the specified function in the box combination. These computer program instructions may also be stored in a computer-usable or computer-readable memory, which may instruct a computer or other programmable data processing apparatus to operate in a particular manner, whereby the instructions stored in the computer-usable or computer-readable memory An article of manufacture is created to perform the specified function in a process block or combination of blocks. Computer program instructions can also be loaded on a computer or other programmable data processing device to cause a series of operational steps to be performed on the computer or other programmable device to produce a computer-implemented process for execution on the computer or other programmable device The instructions provide for implementing the specified functions in a block or combination of blocks of the flowchart.

In addition, each block of the flowcharts may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions shown in the blocks may occur abnormally. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved.

As shown in FIG. 1, it is an operation method of the self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) according to an embodiment of the present invention. Step 10 calculates the distribution network of the self-storage multi-terminal back-to-back flexible straightening device in the power regulation mode. Whether there is a minimum value of the comprehensive power supply cost of Whether the comprehensive power supply cost of the network has a minimum value and the state of charge of the energy storage unit (ESS) in the self-storage multi-terminal back-to-back flexible straight device switches the operation mode of the self-storage multi-terminal back-to-back flexible straight device; The operation modes of the straight device include: Mode I: the energy storage unit does not operate, the self-energy storage multi-terminal back-to-back flexible straight device performs power regulation, Mode II: the energy storage unit operates, and the self-energy storage multi-terminal back-to-back flexible straight device performs power energy timing adjustment, Mode III: The energy storage unit performs state-of-charge recovery, and the self-energy storage multi-terminal back-to-back flexible straight device performs independent power regulation mode.

The power and energy sequence adjustment in the present invention refers to not only the power adjustment of the multi-terminal back-to-back flexible straightening device itself, but also the energy sequence adjustment of the energy storage unit, that is, the energy storage unit also participates in the charge and discharge control of the distribution network. . The operating modes of the self-energy storage multi-terminal back-to-back flexible straightening device include: power regulation mode, power energy sequence regulation mode and state-of-charge recovery mode. According to whether the comprehensive power supply cost of the distribution network has a minimum value in the simple power regulation mode and the storage capacity The state of charge of the energy unit is selected, and the corresponding control mode is selected.

Considering the cost and operating life requirements of the energy storage unit, the configuration capacity and charging and discharging times of the ESS should be minimized. Therefore, it is required only when the adjustable capacity of the interconnected feeder is not enough to meet the system power supply reliability and high-permeability clean energy full consumption. , ESS seamlessly cuts into operation according to the control strategy. The general idea of the SES-VSC-MTDC composite control strategy is mainly based on power regulation, and the energy storage is put into operation only when it cannot meet the constraints of system safety and power supply quality. It can avoid overshoot or overdischarge of the energy storage unit, and ensure that the energy storage unit has the ability to adjust in the next period. Compared with the multi-terminal back-to-back flexible straightening device (VSC-MTDC) without an energy storage unit, the operation method of the self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) proposed by the present invention can adapt to more complex high-permeability clean energy sources. Network operation scenarios and control requirements.

The invention aims at the self-storage multi-terminal back-to-back flexible straightening device, from the perspective of improving the clean energy consumption capacity and power supply reliability of the flexible interconnected distribution network, while effectively reducing the configuration capacity of the energy storage unit and improving the operating life of the energy storage unit. A composite control method of self-energy storage multi-terminal back-to-back flexible straightening device is proposed. The simulation example shows that the composite control method of the present invention can effectively reduce the configuration capacity of energy storage, improve the operating life of energy storage, and effectively improve the consumption of high-permeability clean energy. capacity, improve power supply reliability, improve power supply quality, and improve operating economy.

In one embodiment, when the self-storage multi-terminal back-to-back flexible direct device is in the power regulation operation mode, under normal conditions of the distribution network, the SES-VSC-MTDC main converter can adopt constant U _dc Q control, and the slave converter can Adopt fixed PQ control. The main converter balances the system active power to maintain the DC bus voltage. In the case of distribution network failure, when the feeder (and the upper-level power grid) connected to the slave converter fails, the SES-VSC-MTDC main converter can still use constant U _dc Q control, and the fault side slave converter can be switched to Fixed U _ac f control, other slave inverters can still be controlled by fixed PQ. When the feeder (and the upper-level power grid) connected to the main converter fails, any of the slave converters can be switched to the fixed U _dc Q control to become the new main converter, and the original main converter can be switched to the slave converter. Fixed U _ac f control, other slave converters can still use fixed PQ control.

Under the normal operation of the distribution network, each port realizes the flexible exchange of active power and independent control of reactive power between feeders according to the optimal operation scheduling instructions; when a feeder (including the upper-level power grid) fails, multiple groups of converters can be realized. The fast switching of control modes ensures real-time transfer of loads in non-faulty areas and improves power supply reliability.

When the self-storage multi-terminal back-to-back flexible straightening device is in the power energy sequential adjustment operation mode, under the normal working conditions of the distribution network, the SES-VSC-MTDC main converter can adopt constant U _dc Q control, the ESS converter and other slave converters All devices can be controlled by fixed PQ. In the case of distribution network failure, the control mode of each converter in the power regulation operation mode can be the same, and will not be repeated here.

Considering the power balance constraints of each port of SES-VSC-MTDC, active output constraints, converter capacity constraints, and DC bus voltage constraints, etc., the SES-VSC-MTDC power regulation and power energy timing regulation under normal/faulty operation of the distribution network are established. Mathematical model of the operating modes.

According to the normal/fault conditions of the distribution network, the mathematical models of the power regulation operation mode and the power energy timing regulation mode of the self-storage multi-terminal back-to-back flexible direct device are established.

Power regulation mode: Under normal working conditions of the distribution network, the SES-VSC-MTDC master inverter adopts constant U _dc Q control, and other slave inverters adopt constant PQ control. The main converter balances the system active power to maintain the DC bus voltage. In order to realize the decoupling control of active/reactive power, in the dq synchronous rotation coordinate system, the d-axis voltage of the converter is located in the direction of the grid voltage vector through the phase-locked loop, so U _sq =0, U _sd =U _s , Current active power and reactive power can be expressed as:

In the formula: id, i _q are the components of the d-axis and q-axis of the current vector on the distribution grid side of the converter, respectively, and U _sd is the d-axis component of the voltage vector on the grid side of the converter.

It can be known from equations (1) and (2) that active power and reactive power can be independently controlled by controlling the dq-axis components of the converter current. The power at both ends of the AC and DC converters is equal, we can get:

In the formula: N _VSC is the total number of converters of SES-VSC-MTDC; C is the DC side capacitance, U _dc is the DC bus voltage, and P _loss is the converter loss.

Due to the isolation of the DC link, the reactive powers output by the converters do not affect each other, so only the capacity constraints of the respective converters need to be met. Therefore, the mathematical model is as follows:

为第k个换流器的有功功率上限；S_k为第k个换流器的额定容量；U_dc为直流母线电压；U_dc,ref为直流母线指令电压。In the formula: P _k (t) and Q _k (t) are the active and reactive power of the k-th converter at time t, respectively, and the power flowing into the DC bus is positive; A _k is the loss of the k-th converter coefficient;

is the upper limit of the active power of the kth converter; _Sk is the rated capacity of the kth converter; U _dc is the DC bus voltage; U _dc,ref is the DC bus command voltage.

Under the condition of distribution network failure, when the feeder (and the upper-level power grid) connected to the slave converter fails, the SES-VSC-MTDC main converter still adopts the constant U _dc Q control, and the fault end is switched from the converter to the constant U _ac f control, other slave inverters still use fixed PQ control. When the feeder (and the upper-level power grid) connected to the main converter fails, any of the slave converters can be switched to the fixed U _dc Q control to become the new main converter, and the original main converter can be switched to the slave converter. Fixed U _ac f control, other slave converters still use fixed PQ control. When the power grid fails, the control requirements of the main converter remain unchanged. At this time, the secondary converter on the fault side is equivalent to supplying power to the passive network. The control objects are the AC voltage amplitude and frequency. The AC voltage amplitude control can be directly solved by dq. The coupling is controlled, and the frequency can be realized by modifying the period register in the controller. The control variables of SES-VSC-MTDC are the reactive power of the main converter and the active and reactive power of the converters at the fault-free side. The mathematical model adds the following formula on the basis of formula (4):

In the formula: U _ac is the AC voltage of the converter at the fault end; U _ac,ref is the rated voltage of the grid.

The following will focus on the mathematical model in the power energy timing adjustment mode.

The control variables of SES-VSC-MTDC in operating modes II and III of the self-storage multi-terminal back-to-back flexible straight device increase the energy storage power output on the basis of mode I, and the constraints increase the corresponding ESS charge and discharge power constraints and SOC constraints. SES-VSC-MTDC calculates the output range of ESS output active power P _ESS (t) according to the current SOC value of ESS and SES-VSC-MTDC operation mode, and uses this as a constraint to enter the distribution network optimization operation or energy storage SOC recovery model. Then calculate the SOC value of the ESS again to provide constraints for the decision of the next period.

In one embodiment, when the self-storage multi-terminal back-to-back flexible straightening device performs power energy timing adjustment, when the distribution network is fault-free, the input power and output power of the self-storage multi-terminal back-to-back flexible straightening device are equal. In the case of distribution network fault, the power-energy timing adjustment model is the same as the power adjustment model.

In one embodiment, the input power and output power of the self-storage multi-terminal back-to-back flexible straight device are equal to the output power of all inverters of the self-storage multi-terminal back-to-back flexible straight device, and all the inverters of the self-storage multi-terminal back-to-back flexible straight device. The sum of the power loss of and the output power of all energy storage units in the self-energy storage multi-terminal back-to-back flexible straightening device is 0.

As a preferred embodiment, when the self-energy storage multi-terminal back-to-back flexible direct device performs power energy timing adjustment, under the normal working conditions of the distribution network, the SES-VSC-MTDC main converter adopts constant U _dc Q control, and the ESS converter and other Both the flexible and direct-slave inverters adopt fixed PQ control. The power balance equation can be expressed as formula (6):

N _VSC is the total number of inverters of the self-energy storage multi-terminal back-to-back flexible straight device;

P _k (t) are respectively the output power of the kth converter at time t;

A _k is the loss coefficient of the kth converter;

P _ESS (t) is the output power of the energy storage unit in period t;

The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit; the nuclear power state of the energy storage unit in the next period is calculated according to the energy storage charge value of the current period and the energy storage power released in the current period.

As a preferred embodiment, the charging and discharging power of the energy storage unit can be calculated as follows according to the remaining power of the energy storage unit:

P _ESS (t)=uP _cd (t),u∈{-1,0,1} (7)

The nuclear power state of the energy storage unit in the next period can be calculated according to the energy storage charge value of the current period and the energy storage power released in the current period:

SOC(0)=SOC(T) (10)

N _VSC is the total number of inverters of the self-storage multi-terminal back-to-back flexible straightening device;

P _k (t) are respectively the output power of the kth converter at time t;

A _k is the loss coefficient of the kth converter;

T is the number of time periods divided by the complete scheduling cycle;

Δt is the duration of each period;

P _ESS (t) is the output power of the energy storage unit in period t;

P _cd (t) is the charging and discharging power of the energy storage unit in the t period, which is always positive;

u is the charge and discharge flag of the energy storage unit, and the values -1, 0 and 1 represent three states of charging, non-working and discharging respectively;

P _ch,max and P _dis,max are the maximum charging and discharging power of the energy storage unit;

S _ESS is the rated power of the energy storage unit;

SOC(t) is the state of charge of the energy storage unit at time t;

SOC _max and SOC _min are respectively the upper and lower safe limits of the state of charge of the energy storage unit;

_ηch and _ηdis are the charge and discharge efficiencies of the energy storage unit, respectively.

The present invention establishes the objective function with the smallest cost from the perspective of the comprehensive power supply cost of the electricity selling enterprise. The comprehensive power supply cost can include two parts, the power purchase cost and the distribution network loss cost, in which the power purchase cost can be shared with the on-grid electricity price of the unit and the high-voltage power transmission cost. The power purchase cost is reflected in the node electricity price of the busbar of the substation connected to the feeder. For the distribution ring network, the power price of the bus node of different feeders is usually different.

As a preferred embodiment, the minimum objective function of the comprehensive power supply cost can be as follows:

In the formula: n is the number of system nodes; f ₁ (t), f ₂ (t) are the power purchase cost and network loss cost in the t period; C _i (t), P _STi (t), P _DGi (t), P _Di (t) is the power price of the busbar node, the outlet power of the substation, the active output of the distributed power supply and the active power of the load at the node i in the t period; C _w (t) is the power price of the network loss cost in the t period, and C _i (t) is the general Take the electricity purchase price.

考虑的约束条件主要包括：系统潮流方程、变电站出口功率约束、电压约束、线路容量约束。On the basis of the constraints when the distribution network operates in the above mode, it is necessary to add substation output power P _STi (t), Q _STi (t), distributed power output P _DGi (t), Q _DGi (t), and SES- Converter output power of VSC-MTDC

The constraints considered mainly include: system power flow equation, substation outlet power constraints, voltage constraints, and line capacity constraints.

where U _i (t) and U _j (t) are the voltage amplitudes of node i and node j at time t; G _ij and B _ij are the mutual conductance and mutual susceptance between node i and node j, respectively; δ _ij (t) is the phase difference between node i and node j at time t; Q _Di (t) is the reactive power of the load at node i at time t. S _ij (t) is the line power between node i and node j at time t; the superscript "-" and subscript "_" of the variable represent the upper and lower limits of the variable.

In formula (14), the first two formulas are the system power flow equation, the third and fourth formulas are the substation outlet power constraints, the fifth formula is the voltage constraints of each node in the distribution network, and the sixth formula is the line capacity constraints. .

For the operating mode III of the self-storage multi-terminal back-to-back flexible straightening device, the ESS in the SES-VSC-MTDC performs state-of-charge (SOC) state recovery.

In one embodiment, considering the cost and operating life requirements of energy storage, the configuration capacity and the number of charging and discharging times of the ESS should be minimized. Therefore, the energy storage state of charge (SOC) is divided into three types: standby state, normal non-standby state and abnormal state. The standby state indicates that the ESS is in a moderate SOC range, which can meet the optimal operation requirements of the distribution network in the next period at any time; the normal non-standby state indicates that the ESS is in a relatively wider SOC range, and the adjustable capacity is relatively small due to the need to meet the safety margin; The abnormal state indicates that the ESS is in an urgent SOC interval. When the ESS is in an abnormal state, SOC recovery should be performed immediately. The configuration capacity and charging and discharging times of the ESS are reduced. When the adjustable capacity of the interconnected feeder is not enough to meet the system power supply reliability and the full consumption of clean energy with high permeability, the ESS can seamlessly switch to operation according to the control strategy to avoid overshoot/overdischarge And ensure that the energy storage has the ability to adjust in the next period.

As a preferred embodiment, the standby state may be that the range of the charge value of the energy storage unit is greater than or equal to 0.4 and less than or equal to 0.6; the normal non-standby state may be that the charge value range of the energy storage unit is greater than or equal to 0.15 and less than or equal to 0.85; abnormal The state can be the range of the charge value of the energy storage unit greater than or equal to 0 and less than 0.15 or greater than 0.85 and less than or equal to 1. This implementation ensures that the energy storage charge has strict continuity in time sequence, avoids overshoot/overdischarge and ensures that the energy storage has the ability to adjust for the next period, reasonably divides the energy storage operating state and controls its charge/discharge. Discharge, reducing the configuration capacity of the energy storage unit and the number of charging and discharging.

Based on the above analysis, the operation method of the self-storage multi-terminal back-to-back flexible straightening device is firstly preprocessed, that is, to determine whether the comprehensive power supply cost of the self-storage multi-terminal back-to-back flexible straightening device is the lowest when the self-energy storage multi-terminal back-to-back flexible straightening device is in operation mode I (ESS does not operate, simple power adjustment). Combined with the SOC value of the energy storage unit, the corresponding operation mode is executed, as shown in Table 1.

Table 1

In one embodiment, the control flow of step 30 of the operation method of the self-storage multi-terminal back-to-back flexible straightening device in FIG. 1 is shown in FIG. 2 . Step 301 determines that the ESS in the SES-VSC-MTDC is not running. Is there a solution to the comprehensive power supply cost objective function of the power grid, that is, is the comprehensive power supply cost the lowest? When the comprehensive power supply cost objective function has a solution, go to step 303 to determine whether the SOC value in the energy storage unit is in the standby state, when the SOC value in the energy storage unit is in the standby state, go to step 307, the ESS is not put into operation, and the SES-VSC -MTDC performs power regulation; when the SOC value in the energy storage unit is not in the standby state, step 309 is performed, the ESS is put into operation, the SOC state recovery is performed, and the SES-VSC-MTDC independently performs power regulation; when the comprehensive power supply cost objective function has no solution , go to step 305, determine whether the SOC value in the energy storage unit is in the standby state or the normal non-standby state, when the SOC value in the energy storage unit is in the standby state or the normal non-standby state, go to step 311, the ESS is put into operation, the SES-VSC -MTDC performs power energy timing adjustment; when the SOC value in the energy storage unit is not in the standby state or normal non-standby state, step 309 is performed, the ESS is put into operation, and the SOC state recovery is performed, and the SES-VSC-MTDC independently performs power adjustment.

Due to the addition of energy storage units, SES-VSC-MTDC enables the flexible interconnected distribution network to have power flow transfer capabilities in both space and time dimensions. Load fluctuations, achieve full consumption of clean energy, improve power supply reliability, improve power supply quality and reduce comprehensive power supply costs, and ensure safe, economical and efficient operation of the power grid.

In one embodiment, the recovery of the state of charge of the energy storage unit includes restoring the current state of charge of the energy storage unit to a state with the smallest difference from the intermediate value of the energy storage charge, so as to ensure that the ESS can recover to a moderate value as soon as possible. state in order to deal with the regulation problems of energy storage in the future period.

As a preferred embodiment, the objective function for restoring the current charge value of the energy storage unit to the minimum difference from the intermediate value of the energy storage charge may be:

SOC(t) is the state of charge of the energy storage unit at time t;

P _ESS (t) is the output power of the energy storage unit in period t;

Δt is the duration of each time period;

S _ESS is the rated power of the energy storage unit;

SOC _mid is the middle value of the charge of the energy storage unit;

In the formula: SOC _mid is generally about 0.5, and the preferred value is 0.5. The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit; the nuclear power state of the energy storage unit in the next period is calculated according to the energy storage charge value of the current period and the energy storage power released in the current period.

As a preferred embodiment, the charging and discharging power of the energy storage unit can be calculated as follows according to the remaining power of the energy storage unit:

P _ESS (t)=uP _cd (t),u∈{-1,0,1};

The calculation formula of the nuclear power state of the energy storage unit in the next period according to the energy storage charge value of the current period and the energy storage power released in the current period can be:

SOC(0)=SOC(T);

T is the number of time periods divided by the complete scheduling cycle;

Δt is the duration of each period;

P _ESS (t) is the output power of the energy storage unit in period t;

P _cd (t) is the charging and discharging power of the energy storage unit in the t period, which is always positive;

u is the charge and discharge flag of the energy storage unit, and the values -1, 0 and 1 represent charging, non-working and discharging respectively;

P _ch,max and P _dis,max are the maximum charging and discharging power of the energy storage unit;

S _ESS is the rated power of the energy storage unit;

SOC(t) is the state of charge of the energy storage unit at time t;

SOC _max and SOC _min are respectively the upper and lower safe limits of the state of charge of the energy storage unit;

_ηch and _ηdis are the charge and discharge efficiencies of the energy storage unit, respectively.

In the specific embodiment, considering the total working time of the energy storage and the standby time period requirements, when the SOC(t) returns to the range of _0.9SOC mid≤SOC(t)≤1.1SOC _mid , the energy storage SOC state recovery operation mode is terminated. .

According to the SES-VSC-MTDC composite control method proposed in the present invention, there are three models to be solved, namely power regulation, power energy timing regulation and SOC state recovery. After the power energy timing adjustment and the SOC state recovery, after the output range of P _ESS (t) determined by the SOC value, the active power balance equation constraint becomes an inequality constraint, which is essentially a nonlinear optimization problem like the power regulation optimization. Solving by the dual interior point method.

3 is a schematic diagram of a self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) according to an embodiment of the present invention, comprising at least two AC/DC converters and at least one energy storage unit (ESS), AC/DC The DC side of the converter is connected in parallel with the common DC bus, and the AC side of the AC/DC converter is connected to the feeder of the distribution network to realize flexible interconnection between multiple feeders (AC-DC-AC decoupling); energy storage One side of the unit (ESS) is connected to the DC/DC converter, and the other side of the DC/DC converter is connected to the common DC bus in parallel. The DC/DC converter realizes the charge and discharge control of the energy storage unit, so that the SES-VSC - The MTDC system increases the timing adjustment capability of the energy and becomes a highly integrated integrated device. The self-storage multi-terminal back-to-back flexible straightening device also includes an operation mode adjustment module. The operation mode adjustment module is based on whether the comprehensive power supply cost of the distribution network has a minimum value and the storage capacity of the self-storage self-storage multi-terminal back-to-back flexible straightening device in the power adjustment mode. According to the state of charge of the energy unit, switch the operation mode of the self-storage and self-storage multi-terminal back-to-back flexible straightening device; the operating modes of the self-storage and self-energy back-to-back flexible straightening device include: Mode I: the energy storage unit does not operate and the self-storage and self-storage multi-terminal The back-to-back flexible straightening device executes the power regulation mode; Mode II: the energy storage unit operates and the self-storage multi-terminal back-to-back flexible straightening device executes the power energy timing adjustment mode; Mode III: The energy storage unit executes state-of-charge recovery and self-storage and self-storage multi-terminal Back-to-back flexures independently perform power regulation modes.

Under normal operation, each port realizes the flexible exchange of active power and independent control of reactive power between feeders according to the optimal operation scheduling command; when a feeder (including the upper-level power grid) fails, it can realize the control mode of multiple groups of converters. Fast switching ensures real-time transfer of loads in non-faulty areas and improves power supply reliability.

The self-storage multi-terminal back-to-back flexible straightening device of the present invention enables the energy storage unit to be in a reasonable state of charge at each time period in one operation cycle, and participates in the necessary energy sequence optimization adjustment, which can avoid overshoot or overdischarge of the energy storage unit, and prevent the energy storage unit from overshooting or overdischarging. Ensure that the energy storage unit has the ability to adjust for the next period. Compared with the multi-terminal back-to-back flexible straightening device (VSC-MTDC) without an energy storage unit, the operation method of the self-storage multi-terminal back-to-back flexible straightening device (SES-VSC-MTDC) proposed by the present invention can adapt to more complex high-permeability clean energy sources. Network operation scenarios and control requirements.

The invention aims at the self-storage multi-terminal back-to-back flexible straightening device, from the perspective of improving the clean energy consumption capacity and power supply reliability of the flexible interconnected distribution network, while effectively reducing the configuration capacity of the energy storage unit and improving the operating life of the energy storage unit. A self-storage multi-terminal back-to-back flexible straightening device is proposed, which can effectively reduce the configuration capacity of energy storage, improve the operating life of energy storage, effectively improve the absorption capacity of high-penetration clean energy, improve power supply reliability, improve power supply quality, and improve operating economy. .

In one embodiment, when the distribution network is fault-free and the self-storage multi-terminal back-to-back flexible straightening device performs power regulation, the main converter of the self-storage multi-terminal back-to-back flexible straightening device can adopt constant U _dc Q control, and the secondary inverter The inverter can be controlled by fixed PQ; when the distribution network is fault-free and the self-energy storage multi-terminal back-to-back flexible straight device performs power and energy sequence adjustment, the main converter of the self-storage multi-terminal back-to-back flexible straight device can be controlled by fixed U _dc Q. The inverter of the energy storage unit and the slave inverter of the self-storage multi-terminal back-to-back flexible straight device can be controlled by constant PQ; The fixed U _dc Q control can still be used, the slave inverter at the fault side can be switched to the fixed U _ac f control, and the other slave inverters can still be controlled by the fixed PQ; When the feeder fails, any slave inverter of the self-energy storage multi-terminal back-to-back flexible straight device can be switched to constant U _dc Q control, the original master inverter can be switched to constant U _ac f control, and other slave inverters can still use constant U ac f control. PQ control.

In one embodiment, when there is no fault in the distribution network and the self-storage multi-terminal back-to-back straightening device performs power and energy sequence adjustment, it is necessary to ensure the normal operation of the entire distribution network. The power is equal to the output power.

In a specific embodiment, the input power and output power of the self-energy storage multi-terminal back-to-back flexible straightening device may include: the output power of all AC/DC converters in FIG. 3 , the power loss of all AC/DC converters and all storage The sum of the output power of the energy units is 0. At this time, the power entering the SES-VSC-MTDC is guaranteed to be balanced with the output power, and the consumption balance of the distribution network is ensured.

As a preferred embodiment, the power balance equation in which the input power and the output power of the self-storage multi-terminal back-to-back flexible straightening device are equal is:

N _VSC is the total number of AC/DC converters of the self-energy storage multi-terminal back-to-back flexible straight device;

P _k (t) are the output powers of the k-th AC/DC converter at time t, respectively;

A _k is the loss coefficient of the kth AC/DC converter;

P _ESS (t) is the output power of the energy storage unit in period t;

The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit; the nuclear power state of the energy storage unit in the next period is calculated according to the energy storage charge value of the current period and the energy storage power released in the current period.

As a preferred embodiment, the charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit:

P _ESS (t)=uP _cd (t),u∈{-1,0,1};

The nuclear power state of the energy storage unit in the next period is calculated according to the energy storage charge value of the current period and the energy storage power released in the current period:

SOC(0)=SOC(T);

N _VSC is the total number of AC/DC converters of the self-energy storage multi-terminal back-to-back flexible straight device;

P _k (t) are the output powers of the k-th AC/DC converter at time t, respectively;

A _k is the loss coefficient of the kth AC/DC converter;

T is the number of time periods divided by the complete scheduling cycle;

Δt is the duration of each period;

P _ESS (t) is the output power of the energy storage unit in period t;

P _cd (t) is the charge and discharge power of the energy storage unit in the t period, which is always positive;

u is the charge and discharge flag of the energy storage unit, the values -1, 0 and 1 represent three states of charging, non-working and discharging respectively;

P _ch,max , P _dis,max are the maximum charge and discharge power of the energy storage unit;

S _ESS is the rated power of the energy storage unit;

SOC(t) is the state of charge of the energy storage unit at time t;

SOC _max and SOC _min are the upper and lower safe limits of the state of charge of the energy storage unit, respectively;

_ηch and _ηdis are the charging and discharging efficiencies of the energy storage unit, respectively.

In one embodiment, the state of charge of the energy storage unit of the self-energy energy back-to-back flexible straightening device may include a standby state, a normal non-standby state, and an abnormal state. The standby state indicates that the energy storage unit is in a moderate state of charge, which can meet the optimal operation requirements of the distribution network in the next period at any time; the normal non-standby state indicates that the energy storage unit is in a relatively wider state of charge, and the adjustable capacity is relatively small; The normal state indicates that the energy storage unit is in an emergency state of charge. When the ESS is in an abnormal state, SOC recovery should be performed immediately. Reasonably dividing the energy storage operating state and controlling its charge/discharge can avoid overshoot/overdischarge and ensure that the energy storage has the ability to adjust in the next period.

As a preferred embodiment, the standby state can be that the range of the charge value of the energy storage unit is greater than or equal to 0.4 and less than or equal to 0.6; the normal non-standby state can be that the charge value range of the energy storage unit is greater than or equal to 0.15 and less than or equal to 0.85; The abnormal state can be that the range of the charge value of the energy storage unit is greater than or equal to 0 and less than 0.15 or greater than 0.85 and less than or equal to 1, which can reduce the configuration capacity of the ESS and the number of charging and discharging times.

In one embodiment, the operation mode adjustment module of the self-storage multi-terminal back-to-back flexible straightening device may include a processor, and the processor can determine whether the comprehensive power supply cost has a minimum value when the self-storage multi-terminal back-to-back flexible straightening device is performing power adjustment and whether the The state of charge of the energy storage unit switches the operation mode of the self-storage multi-terminal back-to-back flexible straight device. It improves the clean energy consumption capacity and power supply reliability of the flexible interconnected distribution network, while effectively reducing the configuration capacity of energy storage and improving the operating life of energy storage.

As a preferred embodiment, the operation mode adjustment module switches the self-storage multi-terminal back-to-back straightening device according to whether the comprehensive power supply cost has a minimum value when the self-storage multi-terminal back-to-back straightening device performs power regulation and the state of charge of the energy storage unit The operation mode can include: when the self-storage multi-terminal back-to-back flexible straight device performs power regulation, the comprehensive power supply cost has the lowest value, and when the energy storage unit is in the standby state, the operation mode adjustment module controls the energy storage unit to not operate, and controls the self-storage unit. The multi-terminal back-to-back flexible straightening device can perform power regulation; when the self-energy storage multi-terminal back-to-back flexible straightening device executes the power regulation mode, the comprehensive power supply cost has the lowest value, and the energy storage unit is in a normal non-standby state or abnormal state, the operating mode The regulation module controls the energy storage unit to perform state-of-charge recovery, and controls the self-storage multi-terminal back-to-back flexible straight device to perform power regulation independently; when the self-storage multi-terminal back-to-back flexible straight device performs power regulation, the comprehensive power supply cost has no minimum value, and the energy storage When the unit is in the standby state or in the normal non-standby state, the operation mode adjustment module controls the energy storage unit to be put into operation, and controls the self-energy storage multi-terminal back-to-back straightening device to perform power and energy sequence adjustment; In the power regulation mode, the comprehensive power supply cost has no minimum value, and when the energy storage unit is in an abnormal state, the operation mode regulation module controls the energy storage unit to perform state-of-charge recovery, and controls the self-energy storage multi-terminal back-to-back flexible straight device to perform power regulation independently.

The composite control strategy of SES-VSC-MTDC can effectively reduce the configuration capacity of energy storage and improve the operation life of energy storage. The optimized operation strategy of self-storage flexible interconnected distribution network can effectively improve the absorption capacity of high-penetration clean energy and improve the power supply. reliability, improve power supply quality, and improve operating economy.

In one embodiment, the recovery of the state of charge of the energy storage unit includes restoring the current state of charge of the energy storage unit to a state with the smallest difference from the intermediate value of the energy storage charge, so as to ensure that the ESS can recover to a moderate value as soon as possible. state in order to deal with the regulation problems of energy storage in the future period.

As a preferred embodiment, the objective function for restoring the current charge value of the energy storage unit to the minimum difference from the intermediate value of the energy storage charge may be:

SOC(t) is the state of charge of the energy storage unit at time t;

P _ESS (t) is the output power of the energy storage unit in period t;

Δt is the duration of each time period;

S _ESS is the rated power of the energy storage unit;

SOC _mid is the middle value of the charge of the energy storage unit;

In the formula: SOC _mid is generally about 0.5, and the preferred value is 0.5. The charging and discharging power of the energy storage unit is calculated according to the remaining power of the energy storage unit; the nuclear power state of the energy storage unit in the next period is calculated according to the energy storage charge value of the current period and the energy storage power released in the current period.

As a preferred embodiment, the charging and discharging power of the energy storage unit can be calculated as follows according to the remaining power of the energy storage unit:

P _ESS (t)=uP _cd (t),u∈{-1,0,1};

The calculation formula of the nuclear power state of the energy storage unit in the next period according to the energy storage charge value of the current period and the energy storage power released in the current period can be:

SOC(0)=SOC(T);

T is the number of time periods divided by the complete scheduling cycle;

Δt is the duration of each period;

P _ESS (t) is the output power of the energy storage unit in period t;

P _cd (t) is the charging and discharging power of the energy storage unit in the t period, which is always positive;

u is the charge and discharge flag of the energy storage unit, and the values -1, 0 and 1 represent charging, non-working and discharging respectively;

P _ch,max and P _dis,max are the maximum charging and discharging power of the energy storage unit;

S _ESS is the rated power of the energy storage unit;

SOC(t) is the state of charge of the energy storage unit at time t;

SOC _max and SOC _min are respectively the upper and lower safe limits of the state of charge of the energy storage unit;

_ηch and _ηdis are the charge and discharge efficiencies of the energy storage unit, respectively.

In the specific embodiment, considering the total working time of the energy storage and the standby time period requirements, when the SOC(t) returns to the range of _0.9SOC mid≤SOC(t)≤1.1SOC _mid , the energy storage SOC state recovery operation mode is terminated. .

The present invention also provides a flexible interconnected power distribution network, which adopts the self-energy storage multi-terminal back-to-back flexible straightening device according to the present invention, and the self-storage multi-terminal back-to-back straightening device is based on the comprehensive power supply cost of the power distribution network in the power adjustment mode. Whether there is a minimum value and the state of charge of the energy storage unit in the self-storage multi-terminal back-to-back straightening device switches the operation mode of the self-storage multi-terminal back-to-back straightening device. The optimized operation strategy of the self-storage flexible interconnected distribution network can effectively improve the high-penetration clean energy consumption capacity, improve the reliability of power supply, improve the quality of power supply, and improve the operation economy.

In order to verify the feasibility and effectiveness of the optimized operation strategy proposed in the present invention, simulation and verification are carried out on the 33-node calculation example system shown in Fig. Connected flexible interconnected distribution network. The rated voltage of the system is 10kV, and the line adopts YJV22-3*400 cable, which is mainly used in urban distribution networks in my country. See Table 2 and Table 3 for cable parameters and system topology parameters. 5 groups of photovoltaics and 4 groups of wind power are connected to the system, and the configuration parameters are shown in Table 4. See Table 5 for the peak electricity price (07:00-19:00) and valley electricity price (19:00-07:00) of each bus. The converters of SES-VSC-MTDC have a rated capacity of 4.5MVA and a loss factor of 0.02. The configured ESS is 0.5MW/1MW·h, the initial SOC value is 50%, the upper and lower SOC safety limits are 100% and 10%, respectively, and the charge-discharge efficiency is 90%. The output power range of the substation is 0MW ~ 8MW (power reversal is not allowed), the line capacity is 8MVA, and the value range of each node voltage is [0.93, 1.07]. The three scenarios of system open-loop operation, power regulation flexible interconnection operation, and power-energy timing regulation flexible interconnection operation are compared and analyzed. A complete operation cycle is set to 24h, and a time period is divided into a period of 15 minutes, totaling 96 time periods. It is considered that the output and load of each DG remain unchanged within 15 minutes. The DG output and load curves of each feeder are shown in Figures 5-8. Assume that at 14:30, a fault occurs between nodes 20 and 21 on the C feeder, the switches on both sides are disconnected, and the line fault is cleared at 16:00.

Table 2 YJV22-3*400 cable data

Table 3 System parameter data

Table 4 DG configuration parameters

Table 5 Electricity price parameters

During open-loop operation, the DG output of each feeder is greater than the load demand for several periods, and the distributed power cannot be fully absorbed, and 12.25 MW h of wind and light must be abandoned. The power supply quality is poor, the safe operation of the system is affected, and the wind and solar power needs to be abandoned at 3.23MW·h, as shown in Tables 6 and 7. During the fault period from 14:30 to 16:00, all loads in the non-fault area downstream of node 21 on the C feeder lose power supply. In addition, due to the natural distribution of power flow, the network loss and power purchase cost cannot be optimized, and the comprehensive power supply cost is relatively high (see Table 7). To sum up, open-loop operation cannot meet the friendly consumption of clean energy at full capacity, and the reliability and economy of power supply are poor.

Table 6 Time periods and reasons for system power flow without feasible solutions

Table 7 Comparison of distribution network operation results in three scenarios

When the power regulation is flexible and interconnected, the power flow of the system is optimally distributed, and the comprehensive power supply cost during the operation cycle is significantly reduced (see Table 7). In open-loop operation, the power reversal and voltage overruns caused by high output of clean energy in multiple time periods can be effectively solved by adjusting the power of interconnected feeders. However, there are still a few time periods (01:45-02:45, 11:30-12:30, 13:00-13:15), and there are no solutions due to constraints, as shown in Table 6. After analysis, during the period of 01:45～02:45 and 11:30～12:30, the total distributed energy output in the system is greater than the total load, which cannot satisfy the restriction of power not to be reversed; during the period of 13:00～13:15, The output of DG on feeder A is greater than the load, and the output of DG on feeders C and D is basically the same as the load, and the feedable power is small. Therefore, the unconsumed DG on feeder A needs to be transferred to feeder B, so that some nodes on feeder B need to be transferred. Voltage limit exceeded. Therefore, in the above three periods, it is still necessary to abandon wind and light.

During the fault period from 14:30 to 16:00, the flexible straight port of the C feeder changes the control mode to supply power to the passive network, and the power supply from node 21 to all downstream loads with power loss is guaranteed. However, there are some time periods (14:45-15:00, 15:15-15:30), and there are no solutions due to constraints. The analysis shows that during the period from 14:45 to 15:00, the feeders A, B, and D are heavily loaded, subject to the line capacity, and the adjustable power of the feeder is small, which cannot meet the demand of the power loss load on the C feeder; 15:15～ During the period of 15:30, because the C feeder is a passive network and does not have the ability to adjust, the D feeder DG output is basically the same as the load, and the excess DG output on the A feeder needs to be transferred to the B feeder, which makes the voltage of some of its nodes exceed the limit. Therefore, it is still necessary to abandon wind and light during the above period.

Based on the above analysis, in the scenario of flexible interconnection of power regulation, when the feeder's adjustable capacity is relatively sufficient, the power flow distribution can be effectively optimized to meet the constraints such as system power not reversed, node voltage and line capacity. However, in a few time periods, when the total adjustment margin of each feeder in the system is insufficient, only relying on power adjustment cannot meet the operation requirements, and energy storage needs to be introduced for time axis energy adjustment.

In the scenario of flexible interconnection of power-energy timing adjustment, in the above non-solution period, due to the participation of energy storage in the optimization, the system has a solution, no need to abandon wind and light, and the comprehensive power supply cost is further reduced, as shown in Table 7. In the three scenarios, the power curve of each feeder outlet is shown in Figures 9 to 11, and the comparison curve of the comprehensive power supply cost in each period is shown in Figure 12.

The output of each port of SES-VSC-MTDC is shown in Figure 13, which reflects that due to the addition of energy storage in SES-VSC-MTDC, the flexible interconnected distribution network has the power flow transfer capability in both spatial and temporal dimensions. The distribution network can be dynamically adjusted to change the operating state of the distribution network, quickly respond to the fluctuation of DG and load, realize the full consumption of clean energy, improve the reliability of power supply, improve the quality of power supply and reduce the cost of comprehensive power supply, and ensure the safe, economical and efficient operation of the power grid.

The power change curve and charge-discharge power curve of the energy storage are shown in Figure 14 and Figure 15 respectively (only the energy storage working period is selected, and the entire operation cycle of the energy storage is shown in Figure 16 and Figure 17). At the initial moment, the SOC is 0.5, and it is on standby state. During the period from 01:45 to 02:45, the preprocessing (judging whether the comprehensive power supply cost is the lowest when the self-energy storage multi-terminal back-to-back flexible straightening device is in operation mode I (ESS does not operate, simple power adjustment)) has no solution, and the energy storage is put into operation. Provides optimal adjustment of energy timing and absorbs excess clean energy. At 02:45, the SOC is in an abnormal state. In order to cope with the unsolved situation in the next preprocessing, during the period from 02:45 to 03:30, the energy storage operates in the SOC recovery mode and returns to the standby state. During the period from 11:30 to 12:30, the preprocessing calculation has no solution, and the energy storage is put into operation to absorb excess clean energy. If there is no SOC recovery mode, during the period from 11:30 to 12:30, the energy storage has no adjustment ability. During the period from 12:30 to 13:00, the energy storage operates in the SOC recovery mode; at 13:00, there is no solution to the preprocessing again, and the energy storage is in a normal non-standby state. In order to ensure the operation of the system, the energy storage stops the SOC recovery, and provides the energy sequence Optimized adjustment. During the period from 13:15 to 13:45, the energy storage continues to operate and the SOC recovers, returning to a moderate standby state. During the period from 14:45 to 15:00, the energy storage feeds electric energy into the system, compensating for the power shortage of the power-loss load of the C feeder. At 15:00, the SOC is in a non-standby state, and the running SOC resumes. 15:15～15:30 time period is the same as 13:00～13:15 time period.

It can be seen from the above analysis that the SES-VSC-MTDC composite control strategy proposed in this paper makes the energy storage unit in a reasonable state of charge at each time period in an operation cycle, and participates in the necessary energy sequence optimization adjustment. After calculation, if the composite control strategy proposed in this paper is not adopted, in order to achieve the optimal operation target of the distribution network, the energy storage configuration capacity needs to be increased to 2.3MW·h, and the number of charging and discharging of the energy storage in one operation cycle is increased by 6 times. Therefore, The composite control strategy in this paper can also effectively reduce the energy storage configuration capacity and improve the energy storage operating life.

The invention proposes a SES-VSC-MTDC composite control method from the perspective of improving the clean energy consumption capacity and power supply reliability of the flexible interconnected distribution network, while effectively reducing the configuration capacity of the energy storage and improving the operation life of the energy storage. The energy transfer in the two dimensions of time and space is effectively combined, and the two are integrated, complementary and coordinated. On the one hand, it can improve the level of high-penetration clean energy consumption, improve the reliability of power supply, and optimize the operation economy of the distribution network. At the same time, it can effectively Reduce the configuration capacity of energy storage. And the two operating modes of SES-VSC-MTDC power regulation and power-energy timing regulation are modeled and analyzed, and an optimal operation model of flexible interconnected distribution network based on SES-VSC-MTDC is established. The simulation example verifies the SES-VSC- The effectiveness of MTDC technology and its operational control strategies.

It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them. For those skilled in the art, the technical solutions recorded in the above embodiments can be modified, or some of the technical features can be modified. Equivalent replacements are made; and all such modifications and replacements should fall within the protection scope of the appended claims of the present invention.

Claims (10)

1. A self-energy-storage multi-end back-to-back flexible straight device operation method is characterized by comprising the following steps:

s1: calculating whether the comprehensive power supply cost of the power distribution network has a lowest value when the self-energy-storage multi-end back-to-back flexible-straight device executes a power regulation mode;

s2: reading the charge state of an energy storage unit in the self-energy-storage multi-end back-to-back flexible-straight device;

s3: switching the operation mode of the self-energy-storage multi-end back-to-back flexible-and-straight device according to the condition that whether the comprehensive power supply cost of the power distribution network has the lowest value or not when the self-energy-storage multi-end back-to-back flexible-and-straight device executes the power regulation mode and the charge state of an energy storage unit in the self-energy-storage multi-end back-to-back flexible-and-straight device;

when the self-energy-storage multi-end back-to-back flexible-straight device executes a power regulation mode, the comprehensive power supply cost has the lowest value, and when the energy storage unit is in a standby state, the energy storage unit does not run, and the self-energy-storage multi-end back-to-back flexible-straight device executes power regulation;

when the self-energy-storage multi-end back-to-back flexible-straight device is in a power regulation mode, the comprehensive power supply cost has the lowest value, and the energy storage unit is in a normal non-standby state or a non-normal state, the energy storage unit executes charge state recovery, and the self-energy-storage multi-end back-to-back flexible-straight device independently executes power regulation;

when the self-energy-storage multi-end back-to-back flexible-straight device is in a power regulation mode, the comprehensive power supply cost has no lowest value, and the energy storage unit is in a standby state or a normal non-standby state, the energy storage unit is put into operation, and the self-energy-storage multi-end back-to-back flexible-straight device executes power energy time sequence regulation;

when the self-energy-storage multi-end back-to-back flexible-straight device is in a power regulation mode, the comprehensive power supply cost has no lowest value, and when the energy storage unit is in an abnormal state, the energy storage unit executes charge state recovery, and the self-energy-storage multi-end back-to-back flexible-straight device independently executes power regulation;

the standby state is that the charge value range of the energy storage unit is more than or equal to 0.4 and less than or equal to 0.6; the normal non-standby state is that the charge value range of the energy storage unit is more than or equal to 0.15 and less than 0.4 or more than 0.6 and less than or equal to 0.85; the abnormal state is that the charge value range of the energy storage unit is more than or equal to 0 and less than 0.15 or more than 0.85 and less than or equal to 1.

2. The method of claim 1, wherein performing state-of-charge recovery on the energy storage unit comprises: and recovering the current charge value of the energy storage unit to the minimum difference value with the intermediate value of the stored energy charge.

3. The method of claim 2, wherein the objective function of the energy storage unit to restore the current charge value to the minimum difference from the intermediate value of the energy storage charge is:

P_ESS(t)＝uP_cd(t),u∈{-1,0,1}

SOC (t) is the state of charge of the energy storage unit at time t;

P_ESS(t) is the period of tThe output power of the energy unit;

Δ t is the duration of each time segment;

S_ESSthe rated electric quantity of the energy storage unit is set;

SOC_midthe charge intermediate value of the energy storage unit is obtained;

P_cd(t) the charging and discharging power of the energy storage unit is constant positive in a period t;

u is a charging and discharging mark of the energy storage unit, wherein-1 is a charging state, 0 is an inoperative state, and 1 is a discharging state;

the charging and discharging power of the energy storage unit is calculated according to the residual electric quantity of the energy storage unit;

and the charge state of the energy storage unit in the next period is calculated according to the stored energy charge value in the current period and the stored energy power released in the current period.

4. The method according to claim 3, wherein the charge/discharge power of the energy storage unit is calculated according to the remaining capacity of the energy storage unit by the following formula:

the charge state of the energy storage unit in the next period is calculated according to the stored energy charge value in the current period and the stored energy power released in the current period by the following formula:

SOC(0)＝SOC(T)；

t is the number of time periods divided by the complete scheduling cycle;

Δ t is the duration of each time period;

P_ESS(t) the output power of the energy storage unit in a period t;

P_ch,max、P_dis,maxis the most important of the energy storage unitLarge charging and discharging power;

S_ESSthe rated electric quantity of the energy storage unit is set;

SOC (t) is the state of charge of the energy storage unit at time t;

SOC_max、SOC_minthe safe upper limit and the safe lower limit of the charge state of the energy storage unit are respectively set;

η_ch、η_disand respectively charging and discharging efficiencies of the energy storage unit.

5. A self-energy-storage multi-terminal back-to-back flexible direct current device comprises at least two AC/DC converters and at least one energy storage unit, wherein the direct current sides of the AC/DC converters are connected in parallel with a direct current bus, the alternating current sides of the AC/DC converters are connected with a feeder line of a power distribution network, one side of the energy storage unit is connected with a DC/DC converter, the other side of the DC/DC converter is connected in parallel with the direct current bus,

the self-energy-storage multi-end back-to-back flexible-straight device further comprises an operation mode adjusting module, and the operation mode adjusting module switches the operation mode of the self-energy-storage multi-end back-to-back flexible-straight device according to the condition that whether the comprehensive power supply cost of the power distribution network has the lowest value or not and the charge state of the energy storage unit when the self-energy-storage multi-end back-to-back flexible-straight device executes the power adjusting mode;

when the self-energy-storage multi-end back-to-back flexible-straight device executes a power regulation mode, the comprehensive power supply cost has the lowest value, and when the energy storage unit is in a standby state, the operation mode regulation module controls the energy storage unit not to operate and controls the self-energy-storage multi-end back-to-back flexible-straight device to execute power regulation;

when the self-energy-storage multi-end back-to-back flexible-straight device executes a power regulation mode, the comprehensive power supply cost has the lowest value, and when the energy storage unit is in a normal non-standby state or a non-normal state, the operation mode regulation module controls the energy storage unit to execute charge state recovery and controls the self-energy-storage multi-end back-to-back flexible-straight device to independently execute power regulation;

when the self-energy-storage multi-end back-to-back flexible straight device executes a power adjusting mode, the comprehensive power supply cost has no lowest value, and when the energy storage unit is in a standby state or a normal non-standby state, the operation mode adjusting module controls the energy storage unit to be operated and controls the self-energy-storage multi-end back-to-back flexible straight device to execute power energy time sequence adjustment;

when the self-energy-storage multi-end back-to-back flexible and straight device executes a power regulation mode, the comprehensive power supply cost has no lowest value, and when the energy storage unit is in an abnormal state, the operation mode regulation module controls the energy storage unit to execute charge state recovery and controls the self-energy-storage multi-end back-to-back flexible and straight device to independently execute power regulation;

the standby state is that the charge value range of the energy storage unit is more than or equal to 0.4 and less than or equal to 0.6; the normal non-standby state is that the charge value range of the energy storage unit is more than or equal to 0.15 and less than 0.4 or more than 0.6 and less than or equal to 0.85; the abnormal state is that the charge value range of the energy storage unit is more than or equal to 0 and less than 0.15 or more than 0.85 and less than or equal to 1.

6. The self-storing multi-ended back-to-back soft-straight device of claim 5, wherein the operation mode adjustment module comprises a processor,

and the processor switches the operation mode of the self-energy-storage multi-end back-to-back flexible-straight device according to the condition that whether the comprehensive power supply cost has the lowest value or not when the self-energy-storage multi-end back-to-back flexible-straight device executes a power regulation mode and the charge state of the energy storage unit.

7. The self-storing multi-ended back-to-back soft-straight device of claim 5, wherein the energy storage unit performing state-of-charge recovery comprises: and recovering the current charge value of the energy storage unit to the minimum difference value with the intermediate value of the stored energy charge.

8. The self-energy-storing multi-end back-to-back soft and straight device according to claim 7, wherein the objective function of the energy-storing unit for recovering the current charge value to the minimum difference value with the intermediate value of the energy-storing charge is as follows:

P_ESS(t)＝uP_cd(t),u∈{-1,0,1}

SOC (t) is the state of charge of the energy storage unit at time t;

P_ESS(t) the output power of the energy storage unit in a period t;

Δ t is the duration of each time segment;

S_ESSthe rated electric quantity of the energy storage unit is set;

SOC_midthe charge intermediate value of the energy storage unit is obtained;

P_cd(t) the charging and discharging power of the energy storage unit is constant positive in a period t;

u is a charging and discharging mark of the energy storage unit, wherein-1 is a charging state, 0 is an inoperative state, and 1 is a discharging state;

the charging and discharging power of the energy storage unit is calculated according to the residual electric quantity of the energy storage unit;

and the charge state of the energy storage unit in the next period is calculated according to the stored energy charge value in the current period and the stored energy power released in the current period.

9. The self-energy-storage multi-terminal back-to-back flexible-straight device as claimed in claim 8, wherein the charge and discharge power of the energy storage unit is calculated according to the remaining capacity of the energy storage unit by the following formula:

the charge state of the energy storage unit in the next period is calculated according to the stored energy charge value in the current period and the stored energy power released in the current period by the following formula:

SOC(0)＝SOC(T)；

t is the number of time periods divided by the complete scheduling cycle;

Δ t is the duration of each time period;

P_ESS(t) the output power of the energy storage unit in a period t;

P_ch,max、P_dis,maxthe maximum charging and discharging power of the energy storage unit is obtained;

S_ESSthe rated electric quantity of the energy storage unit is set;

SOC (t) is the state of charge of the energy storage unit at time t;

SOC_max、SOC_minthe safe upper limit and the safe lower limit of the charge state of the energy storage unit are respectively set;

η_ch、η_disand respectively charging and discharging efficiencies of the energy storage unit.

10. A flexible interconnected power distribution network comprising the self-energy-storing multi-terminal back-to-back flexible-straight device as claimed in any one of claims 5 to 9,

the self-energy-storage multi-end back-to-back flexible-straight device switches the operation mode of the self-energy-storage back-to-back flexible-straight device according to the condition that whether the comprehensive power supply cost of the power distribution network has the lowest value or not in the power regulation mode and the charge state of the energy storage unit in the self-energy-storage back-to-back flexible-straight device.

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