An AC / DC hybrid system energy storage control strategy and its evaluation method and system

Abstract

The present invention relates to the technical field of AC / DC hybrid systems, and discloses an energy storage control strategy for an AC / DC hybrid system and an evaluation method and system thereof, wherein the energy storage provided with the control strategy is connected to a converter station, and a DC power transfer impact factor is designed based on the cross-sectional flow distribution of the AC channel, the grid stability level, the short-circuit current level of relevant stations along the line, and the newly added DC transfer power of the AC channel line; the regulation capability of the energy storage is evaluated by calculating the magnitude of the DC power transfer impact factor. The present invention can evaluate the ability of energy storage to enhance the safe operation of the AC channel, so as to quickly view the regulation capability of the energy storage.

Description

Technical Field

[0001] The present invention relates to the technical field of AC / DC hybrid systems, and in particular to an AC / DC hybrid system energy storage control strategy and an evaluation method and system thereof. Background Art

[0002] In order to solve the problem of extremely unbalanced distribution of primary energy and power load in my country, State Grid and China Southern Power Grid have vigorously developed UHV DC transmission technology. As of the first half of 2022, my country has put into operation 18 UHV DC lines with a total rated capacity of 142.6GW. UHV DC is an important national energy strategic channel and an important part of the backbone power grid. It is of great significance to achieve the optimal allocation of national energy resources and build a unified national power market.

[0003] UHV DC has a large transmission power. Once a large disturbance such as pole blocking or commutation failure occurs, it will have a significant impact on the safe and stable operation of the power grids at the sending and receiving ends. For example, when the ±1100kV UHV Jiquan DC has a bipolar blocking under rated power operation, it is equivalent to the Xinjiang power grid at the sending end suddenly losing 12,000MW of "load" and the Anhui power grid at the receiving end losing 20 600MW thermal power units, which has a huge impact on the active and reactive power balance of the system. In order to eliminate the high power transfer of DC, the machine and load are cut to eliminate the over-limit power of the AC channel and improve the stability of the power grid.

[0004] However, the AC / DC hybrid system takes a long time to cut off the power supply and reduces the damping of the system, which may cause the system power angle to be unstable, reduce the system's ability to operate safely and stably, and take a long time to start the generator, which greatly increases the operating cost of the power supply. At the same time, the system also needs to cut off a certain amount of load when cutting off the power supply, which will further increase the operating cost of the power supply, and may lead to a decrease in power quality and affect the reliability of power supply. Energy storage has good dynamic active and reactive regulation capabilities, and can adapt to the operation control requirements of different time scales. The AC / DC hybrid system eliminates the power flow over-limit caused by the large DC power transfer of the AC / DC hybrid system through the configured energy storage, reduces the amount of machine cutting and load shedding after the system fault, improves the system operation flexibility and stability, and ensures the safe, reliable and efficient operation of the AC / DC hybrid system. Therefore, energy storage is connected to the AC / DC hybrid system. After a DC locking fault occurs, the active power is absorbed by the energy storage to eliminate the over-limit of the AC channel, reduce the amount of machine cutting and load shedding, and suppress power fluctuations and power angle oscillations.

[0005] Designing an energy storage control strategy that can better improve the safe operation capability of the AC channel in a hybrid system with strong DC and weak AC characteristics is a current technical problem. At the same time, there is currently little consideration of eliminating the power flow exceeding the limit caused by the high DC power transfer in the AC / DC hybrid system after a DC locking fault occurs by configuring energy storage, and there is no evaluation of the ability of energy storage to improve the safe operation capability of the AC channel. Summary of the invention

[0006] The present invention provides an energy storage control strategy for an AC / DC hybrid system and an evaluation method and system thereof, which can better improve the safe operation capability of an AC channel in a hybrid system with strong DC and weak AC characteristics. At the same time, it can analyze the energy storage near a converter station to improve the safe operation capability of an AC channel after a DC locking fault occurs in the AC / DC hybrid system.

[0007] The present invention is achieved through the following technical solutions:

[0008] An AC / DC hybrid system energy storage control strategy includes an upper control strategy and a lower control strategy, wherein the upper control strategy is a simulated synchronous power generation control strategy with PI control added, and the lower control strategy is a closed-loop PI control strategy, wherein the expression of the upper control strategy is:

[0009]

[0010] Among them, P D is the upper layer active power control instruction, P Dmax , P Dmin are the upper and lower limits of the upper layer active power control command, P′ D is the output active power without power limitation;

[0011] The expression of the lower control strategy is:

[0012]

[0013] Among them, I dm is the lower current control instruction, I dmax ,I dmin are the upper and lower limits of the lower current control command, I′ d is the output current without power limitation.

[0014] As an optimization, the upper layer actual active power P′ D Specifically:

[0015]

[0016] And Δf is specifically:

[0017]

[0018] Among them, f ref and f are the reference frequency and the actual frequency of the AC / DC hybrid system respectively; P ref , P are the reference power and the common bus power respectively; M is the virtual inertial damping coefficient, D is the virtual damping coefficient; s is the Laplace operator; Δf is the frequency deviation; are the upper and lower limits of the frequency deviation dead zone respectively; k f is the droop coefficient; k up , k ui They are the proportional coefficient and integral coefficient of the upper-level PI control respectively.

[0019] As an optimization, the output I′ under the condition of no power limitation d Specifically:

[0020]

[0021] Among them, k dp , k di are the proportional coefficient and integral coefficient of the lower PI control respectively; P D is the upper layer active power control instruction, and P is the common bus power.

[0022] The present invention also discloses an evaluation method for an AC / DC hybrid system energy storage control strategy, which is used to evaluate the above-mentioned AC / DC hybrid system energy storage control strategy, comprising:

[0023] S1. Connect the energy storage equipped with the control strategy to the converter station, and design the DC power transfer influencing factor based on the cross-sectional power flow distribution of the AC channel, the grid stability level, the short-circuit current level of the relevant stations along the line, and the newly added DC transfer power of the AC channel line;

[0024] S2. Evaluate the regulating capability of the energy storage by calculating the magnitude of the direct current power transfer impact factor.

[0025] As an optimization, the specific formula of the DC power transfer influencing factor is as follows:

[0026]

[0027] Among them, F DCPTIF is the DC power transfer impact factor; S ac P is the short-circuit capacity of the AC busbars at the stations on both sides of each AC channel; ac-max P is the static stability limit transmission power of each AC channel line; ac is the actual transmission power of each AC channel line; ΔP′ DCT，E is the amount of DC power transferred after connecting to the energy storage; c is the amplification factor, which is convenient for result identification.

[0028] As an optimization, the DC power transfer amount ΔP′ after connecting to the energy storage DCT，E The specific formula is:

[0029] ΔP′ DCT，E =P DCS -P E ;

[0030] Among them, P DCS is the total DC transfer, P E is the active power absorbed by the energy storage.

[0031] As an optimization, the evaluation process of S2 is:

[0032] When the DC power transfer impact factor is smaller, the energy storage has a better ability to eliminate the impact of DC power transfer on the AC system, and the oscillation suppression effect is also better, which can effectively improve the transient stability of the system;

[0033] When the DC power transfer impact factor is larger, the energy storage's ability to eliminate DC power transfer is worse, and the oscillation suppression effect is also poor, which is not conducive to the transient stability of the system.

[0034] The present invention also discloses an evaluation system for an AC / DC hybrid system energy storage control strategy, comprising:

[0035] The DC power transfer impact factor design unit is used to design the DC power transfer impact factor based on the cross-sectional power flow distribution of the AC channel, the grid stability level, the short-circuit current level of relevant stations along the line, and the size of the newly added DC transfer power of the AC channel line;

[0036] An evaluation unit is used to evaluate the regulating capability of the energy storage by calculating the magnitude of the direct current power transfer influence factor.

[0037] As an optimization, the evaluation process of the evaluation unit is:

[0038] The size of the DC power transfer impact factor is judged. When the DC power transfer impact factor is smaller, the energy storage has a better ability to eliminate the impact of DC power transfer on the AC system, and the oscillation suppression effect is also better, which can effectively improve the transient stability of the system; when the DC power transfer impact factor is larger, the energy storage has a worse ability to eliminate DC power transfer, and the oscillation suppression effect is also poor, which is not conducive to the transient stability of the system.

[0039] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0040] 1. In order to improve the safe operation capability of the AC channel in the AC / DC hybrid system with strong DC and weak AC characteristics, and based on the control method of the actual energy storage power station, the present invention proposes a control method in which the upper layer is a simulated synchronous power generation control strategy and the lower layer is a closed-loop PI control strategy, which can eliminate the regulation error. The control strategy can effectively increase the damping of the AC / DC hybrid system, thereby suppressing the power oscillation of the hybrid system;

[0041] 2. The present invention combines the cross-sectional flow distribution of the AC channel, the stability level of the power grid, the short-circuit current level of relevant stations along the line, and the size of the newly added DC transfer power of the AC channel line, and proposes an indicator for evaluating the impact of DC power transfer on the AC channel power, namely the DC power transfer impact factor. Through the DC power transfer impact factor, the ability of energy storage to improve the safe operation of the AC channel can be evaluated, so as to quickly check the regulation capability of the energy storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0043] Figure 1 It is a structural schematic diagram of an AC / DC hybrid system energy storage control strategy according to the present invention;

[0044] Figure 2 It is the power variation curve of AC line after DC blocking fault;

[0045] Figure 3 This is the curve diagram of the generator power angle change after the DC blocking fault;

[0046] Figure 4 It is the power variation curve of AC line under different working conditions;

[0047] Figure 5 This is the power angle change curve of the generator under different working conditions. DETAILED DESCRIPTION

[0048] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0049] Example 1

[0050] like Figure 2As shown in Figure 1, after a DC lockout fault occurs in a DC converter station, the power of the AC line increases sharply and the power fluctuation is large. The power of the AC transmission line exceeds the thermal stability limit, which increases the risk of cascading failures in the system. Figure 3 It can be seen that after the DC locking fault occurs, the generator power angle oscillates violently, and the fluctuation amplitude of the generator power angle increases, which reduces the stability of the system.

[0051] Therefore, in order to improve the safe operation capability of the AC channel in a hybrid system with strong DC and weak AC characteristics, and based on the control method of an actual energy storage power station, the present invention proposes an AC / DC hybrid system energy storage control strategy, such as Figure 1 As shown, it includes an upper control strategy and a lower control strategy. The upper control strategy is a simulated synchronous power generation control strategy with PI control added, and the lower control strategy is a closed-loop PI control strategy. PI control is added to the upper layer of energy storage, which can speed up the adjustment, eliminate the steady-state error, and effectively reduce the error. The expression of the upper control strategy is:

[0052]

[0053]

[0054]

[0055] Among them, f ref and f are the reference frequency and the actual frequency of the AC / DC hybrid system respectively; P ref , P are the reference power and the common bus power respectively; M, D are the virtual inertial damping coefficient and the virtual damping coefficient respectively; s is the Laplace operator; Δf is the frequency deviation; are the upper and lower limits of the frequency deviation dead zone respectively; k f is the droop coefficient; k up , k ui are the proportional coefficient and integral coefficient of the upper PI control, P D is the upper layer active power control instruction, P Dmax , P Dmin are the upper and lower limits of the upper layer active power control command, P′ D is the output active power without power limitation.

[0056] In the lower closed-loop PI control, the upper active power control command is compared with the current actual active power, and the difference is passed through the PI controller to obtain the d-axis current control command I dm .

[0057] The expression of the closed-loop PI control of the energy storage lower layer is as follows:

[0058]

[0059]

[0060] Among them, I dm is the lower current control instruction, I dmax ,I dmin are the upper and lower limits of the lower current control command, I′ d is the output current without power limitation, k dp , k di They are the proportional coefficient and integral coefficient of the lower-level PI control respectively; PD is the upper-level active power control instruction, and P is the common bus power.

[0061] The energy storage system is usually used as a PQ node in the power system flow model to inject the calculated active power into the power grid. dm After that, the charging and discharging power P of the energy storage is obtained by multiplying it with the grid voltage. out , which is PE in the following formula (8). After a DC lockout fault occurs in a DC converter station, the power over-limit of the AC channel can be effectively eliminated by controlling the charging power, and the power angle oscillation and power angle oscillation can be effectively suppressed by the droop control of energy storage.

[0062] Example 2

[0063] In order to relatively accurately and effectively characterize the strong DC and weak AC characteristics of the AC / DC hybrid system and the impact of high-power flow transfer of the DC system on the AC system, the present invention combines the cross-sectional flow distribution of the AC channel, the grid stability level, the short-circuit current level of relevant stations along the line, and the size of the newly added DC transfer power of the AC channel line, and proposes an indicator for evaluating the impact of DC power transfer on the AC channel power, namely, the DC power transfer impact factor. The DC power transfer impact factor takes into account the temporary stable grid safety problem caused by the impact of DC power transfer on the AC system.

[0064] Specifically, an evaluation method of an AC / DC hybrid system energy storage control strategy of the present invention includes:

[0065] S1. Connect the energy storage equipped with the control strategy to the converter station, and design the DC power transfer influencing factor based on the cross-sectional power flow distribution of the AC channel, the grid stability level, the short-circuit current level of the relevant stations along the line, and the newly added DC transfer power of the AC channel line;

[0066] S2. Evaluate the regulating capability of the energy storage by calculating the magnitude of the direct current power transfer impact factor.

[0067] The calculation formula of the DC power transfer impact factor, which is a simple indicator for evaluating the impact of DC power transfer on AC channel power, is as follows:

[0068]

[0069] In the formula, F DCPTIF is the DC power transfer impact factor; ΔP DCT is the newly transferred DC power carried by each AC channel section. If the AC channel is directly connected to the converter station, then ΔP DCT =P DCS / n, where P DCS is the total DC transfer amount, n is the number of lines; S ac P is the short-circuit capacity of the AC busbars at the stations on both sides of each AC channel; ac-max P is the static stability limit transmission power of each AC channel line; ac is the actual transmitted power of each AC channel line; c is the amplification factor, which is convenient for result identification.

[0070] If a DC bipolar blocking or single machine blocking fault occurs in a DC transmission line, the large power in the AC grid on the sending end will cause the energy storage double-layer control strategy to be activated, and the energy storage will quickly absorb the excess power in the grid to ensure the safe operation of the AC channel. At this time, the DC power transfer impact factor is calculated as follows:

[0071]

[0072] ΔP′ DCT，E =P DCS -P E ; (8)

[0073] Among them, F DCPTIF is the DC power transfer impact factor; S ac P is the short-circuit capacity of the AC busbars at the stations on both sides of each AC channel; ac-max P is the static stability limit transmission power of each AC channel line; ac is the actual transmission power of each AC channel line; ΔP′ DCT，E is the DC power transfer amount after connecting to the energy storage; c is the amplification factor, which is convenient for result identification, P DCS is the total DC transfer, i.e. ΔP in the above formula (6) DCT .

[0074] The DC power transfer impact factor is used to evaluate the impact of energy storage on the AC line after eliminating DC blocking. The smaller the DC power transfer impact factor, the better the energy storage's ability to eliminate the impact of DC power transfer on the AC system, and the better the effect of suppressing oscillation, which can effectively improve the transient stability of the system. Conversely, the larger the DC power transfer impact factor, the worse the energy storage's ability to eliminate DC power transfer, and the effect of suppressing oscillation is also poor (the value of the DC power transfer impact factor is higher than 5, which indicates that the transfer ability is poor), which is not conducive to the transient stability of the system.

[0075] Example 3

[0076] The present invention also discloses an evaluation system for an AC / DC hybrid system energy storage control strategy, comprising:

[0077] The DC power transfer impact factor design unit is used to design the DC power transfer impact factor based on the cross-sectional power flow distribution of the AC channel, the grid stability level, the short-circuit current level of relevant stations along the line, and the size of the newly added DC transfer power of the AC channel line;

[0078] An evaluation unit is used to evaluate the regulating capability of the energy storage by calculating the magnitude of the direct current power transfer influence factor.

[0079] In this embodiment, the evaluation process of the evaluation unit is as follows:

[0080] The size of the DC power transfer impact factor is judged. When the DC power transfer impact factor is smaller, the energy storage has a better ability to eliminate the impact of DC power transfer on the AC / DC hybrid system, and the effect of suppressing oscillation is also better, which can effectively improve the transient stability of the AC / DC hybrid system; when the DC power transfer impact factor is larger, the energy storage has a worse ability to eliminate DC power transfer, and the effect of suppressing oscillation is also poor, which is not conducive to the transient stability of the system.

[0081] The comparative analysis is conducted through the following working conditions: 1) After a single machine locking fault occurs in the system, no protection action is performed; 2) After a single machine locking fault occurs in the system, the machine is disconnected; 3) After a single machine locking fault occurs in the system, the energy storage of the converter station accessories (using the control strategy of the present invention) eliminates the impact of DC power transfer on the AC system. Figure 4 and Figure 5 It can be seen that after a single-machine lockout fault occurs in the system, the power of the AC channel quickly stabilizes and the power increase is small after the energy storage is connected, while the line still has a large power increase after the power is cut off and exceeds the maximum value, exceeding the thermal stability limit; after the energy storage is connected, the generator power angle has a small oscillation, and the oscillation amplitude of the machine is about 20. It can be seen that connecting to energy storage can effectively improve the stability of the system.

[0082] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for controlling energy storage in an AC / DC hybrid system. It is characterized in that It includes an upper control strategy and a lower control strategy, wherein the upper control strategy is a simulated synchronous power generation control strategy with PI control added, and the lower control strategy is a closed-loop PI control strategy, wherein the expression of the upper control strategy is: Among them, P D is the upper layer active power control instruction, P Dmax , P Dmin are the upper and lower limits of the upper layer active power control command, P′ D is the output active power without power limitation; The expression of the lower control strategy is: Among them, I dm is the lower current control instruction, I dmax ,I dmin are the upper and lower limits of the lower current control command, I′ d is the output current without power limitation; The output active power P′ under the condition of no power limitation D Specifically: And Δf is specifically: Among them, f ref and f are the reference frequency and the actual frequency of the AC / DC hybrid system respectively; P ref , P are the reference power and the common bus power respectively; M is the virtual inertial damping coefficient, D is the virtual damping coefficient; s is the Laplace operator; Δf is the frequency deviation; are the upper and lower limits of the frequency deviation dead zone respectively; k f is the droop coefficient; k up , k ui They are the proportional coefficient and integral coefficient of the upper-level PI control respectively.

2. The AC / DC hybrid system energy storage control method according to claim 1, It is characterized in that The output current I' under the condition of no power limitation d Specifically: Among them, k dp , k di are the proportional coefficient and integral coefficient of the lower PI control respectively; P D is the upper layer active power control instruction, and P is the common bus power.

3. An evaluation method for an AC / DC hybrid system energy storage control method, used to evaluate an AC / DC hybrid system energy storage control method according to any one of claims 1-2, It is characterized in that include: S1. Connect the energy storage equipped with the control method to the converter station and design the DC power transfer influence factor; S2. Evaluate the regulating capability of the energy storage by calculating the magnitude of the direct current power transfer impact factor.

4. The evaluation method for the energy storage control method of an AC / DC hybrid system according to claim 3, It is characterized in that The DC power transfer influencing factor is designed based on the cross-sectional power flow distribution of the AC channel, the grid stability level, the short-circuit current level of the stations along the line related to the converter station, and the influencing factors of the newly added DC transfer power of the AC channel line.

5. The evaluation method for the energy storage control method of an AC / DC hybrid system according to claim 4, It is characterized in that The specific formula of the DC power transfer influencing factor is as follows: Among them, F DCPTIF is the DC power transfer impact factor; S ac P is the short-circuit capacity of the AC busbars at the stations on both sides of each AC channel; ac-max P is the static stability limit transmission power of each AC channel line; ac is the actual transmission power of each AC channel line; ΔP′ DCT,E is the amount of DC power transferred after connecting to energy storage; c is the amplification factor.

6. The evaluation method of the AC / DC hybrid system energy storage control method according to claim 5, It is characterized in that The DC power transfer amount ΔP′ after connecting to the energy storage DCT,E The specific formula is: ΔP′ DCT,E =P DCS -P E ; Among them, P DCS is the total DC transfer, P E is the active power absorbed by the energy storage.

7. The evaluation method for an AC / DC hybrid system energy storage control method according to claim 3, It is characterized in that The evaluation process of S2 is: When the DC power transfer impact factor is smaller, the energy storage has a better ability to eliminate the impact of DC power transfer on the AC system, and the oscillation suppression effect is also better, which can effectively improve the transient stability of the system; When the DC power transfer impact factor is larger, the energy storage's ability to eliminate DC power transfer is worse, and the oscillation suppression effect is also poor, which is not conducive to the transient stability of the system.

8. An evaluation system for an AC / DC hybrid system energy storage control method, used for evaluating an AC / DC hybrid system energy storage control method according to claim 1 or 2, It is characterized in that include: The DC power transfer impact factor design unit is used to design the DC power transfer impact factor based on the cross-sectional power flow distribution of the AC channel, the grid stability level, the short-circuit current level of relevant stations along the line, and the size of the newly added DC transfer power of the AC channel line; An evaluation unit is used to evaluate the regulating capability of the energy storage by calculating the magnitude of the direct current power transfer influence factor.

9. An evaluation system for an AC / DC hybrid system energy storage control method according to claim 8, It is characterized in that The evaluation process of the evaluation unit is: Determine the size of the DC power transfer impact factor. When the DC power transfer impact factor is smaller, the energy storage has a better ability to eliminate the impact of DC power transfer on the AC system, and the oscillation suppression effect is also better, which can effectively improve the transient stability of the AC / DC hybrid system; When the DC power transfer impact factor is larger, the energy storage's ability to eliminate DC power transfer is worse, and the oscillation suppression effect is also poor, which is not conducive to the transient stability of the system.

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