Grid-connected stability control method suitable for grid-forming type flexible direct current system

Abstract

The invention provides a grid-connected stability control method suitable for a grid-forming type flexible direct current system, and the method comprises the steps: constructing a double-end MMC-HVDC power transmission system electromechanical transient simulation model, and setting main electrical parameters, control parameters and fault parameters; setting delay time TFS, and judging whether the duration t after the fault is cleared is greater than the delay time TFS or not and whether the system loses power angle stability or not; the fault is cleared and the TFS time lasts, an equipment level control strategy is adopted, GFM-MMC control parameters are adjusted, and whether the system recovers stable operation or not is judged; the method comprises the following steps: measuring voltage amplitude and voltage phase of nodes at two ends of each branch in a near area of the double-end MMC-HVDC system, calculating an sBTTC index of each branch, and defining the branch with the minimum sBTTC index value as a key branch; the change condition of the key branch is monitored, and the starting time Tsc of the system-level control strategy is determined; and at the starting moment Tsc, a system-level control strategy is adopted until the system is recovered to be stable. According to the invention, the transient power angle stability level of the double-end MMC-HVDC system containing network construction type control can be enhanced.

Description

Technical Field

[0001] The invention belongs to the technical field of large power grid power electronics applications, and in particular relates to a grid-connected stability control method suitable for a grid-forming flexible direct current system. Background Art

[0002] Grid-Forming (GFM) control converters do not require the use of a phase-locked loop. They can maintain synchronization with the power supply in the grid through self-synchronization, and can adjust the active and reactive power exchanged with the system at multiple time scales. They have inertia support and voltage support capabilities and are considered to be an important technology for realizing active support control of converters to ensure the safe and stable operation of power electronic power systems. Strategies for realizing grid-forming control include droop control, power synchronization control, virtual oscillation control, and virtual synchronous generator (VSG) control. Converters that use GFM control include two-level or three-level converters used in new energy sources such as doubly fed wind turbines, direct-drive wind turbines, and photovoltaics, as well as cascaded H-bridge converters and modular multilevel converters (MMC) used in energy storage and flexible DC.

[0003] In recent years, the MMC-HVDC technology of flexible direct current transmission based on MMC has become mature, and the voltage level and transmission capacity have been significantly improved. It has good application prospects in scenarios such as large-scale energy base networking and transmission, and long-distance feeding into the receiving load center, and will be an important part of the power electronic power system. The use of GFM control technology to improve the support performance of MMC has received increasing attention. Most of the existing research belongs to the category of electromagnetic transients, and the models and control structures are complex. Due to the limitations of simulation scale and efficiency, they cannot be used for transient stability analysis of large power systems. In addition, most of the existing research focuses on small disturbance stability, and the coupling effect of MMC-HVDC energy transmission at the sending and receiving ends is not paid attention to in the analysis of large disturbance stability. Summary of the invention

[0004] In view of this, the object of the present invention is to provide a grid-connected stability control method suitable for a grid-connected flexible direct current system, aiming to enhance the transient power angle stability of a double-terminal MMC-HVDC system with grid-connected control.

[0005] In a first aspect, an embodiment of the present invention provides a grid-connected stability control method applicable to a grid-connected flexible direct current system, comprising the following steps:

[0006] S1. Based on the power angle stability fault of the double-terminal flexible direct current system, an electromechanical transient simulation model of the double-terminal MMC-HVDC transmission system with grid-type control is constructed, and the main electrical parameters, control parameters, and fault parameters are set;

[0007] S2. Set the delay time T FS , determine whether the duration t after the fault is cleared is greater than the delay time T FS And whether the system loses power angle stability,

[0008] If the duration t after the fault is cleared is greater than the delay time T FS If the system loses power angle stability, execute S3;

[0009] If the duration t after the fault is cleared is less than the delay time T FS Or if the system does not experience power angle instability, then t=t+Δt and execute S2 again;

[0010] S3, based on fault clearing and lasting T FS Time, adopt equipment-level control strategy, adjust GFM-MMC control parameters, and determine whether the system has resumed stable operation.

[0011] If yes, the equipment-level control strategy ends; if no, execute S4;

[0012] S4, measuring the voltage amplitude and voltage phase of the nodes at both ends of each branch in the near area of ​​the two-terminal MMC-HVDC system, calculating the sBTTC index of each branch, and defining the branch with the minimum sBTTC index value as the key branch;

[0013] S5. Based on the obtained sBTTC index, monitor the changes in key branches and determine the start time T of the system-level control strategy sc ;

[0014] S6, at the start time T sc , adopt a system-level control strategy to cut off some generator sets in the vicinity of the double-terminal flexible DC system until the system returns to stability.

[0015] Preferably, the sending-end converter station of the electromechanical transient simulation model of the double-terminal MMC-HVDC transmission system adopts grid-following GFL control, denoted as GFL-MMC; the receiving-end converter station adopts grid-forming GFM control based on the virtual synchronous machine VSG strategy, denoted as GFM-MMC.

[0016] Preferably, a low voltage power limiting link is added to the control system of GFM-MMC. sM When the power drops significantly, reduce the reference power value P cMref To lim P cMref , reduce its relative to the output electromagnetic power P cM The deviation between them limits the growth of the virtual power angle during the fault period.

[0017] Preferably, the fault parameters include fault type, fault location, fault start time, and fault end time.

[0018] The main electrical parameters include: voltage amplitude U at node m on branch i mi , the voltage amplitude U at node n on branch i ni , the voltage phase Δθ of node m on branch i mi , the voltage phase Δθ of node n on branch i ni , the voltage phase difference Δθ between the two end nodes of branch i i =|Δθ mi -Δθ ni |;

[0019] Control parameters include: delay time T FS 、GFM-MMC inertia time constant T jM 、GFM-MMC active reference value P cMref Duration T th , Start time T sc , power limit coefficient λ lim , Power limit duration T lim .

[0020] Preferably, after the fault is cleared and T FS After a certain time, increase the inertia time constant T of GFM-MMC jM Up to 2T jM , reduce the active reference value P cMref To 80%P cMref And last for 1s.

[0021] Preferably, the voltage phase difference Δθ i The calculation formula is,

[0022] Δθ i =Δθ mi -Δθ ni |

[0023] In the formula, Δθ mi is the voltage phase of node m on branch i; Δθ ni is the voltage phase at node n on branch i;

[0024] The calculation formula of sBTTC index is:

[0025]

[0026] Where U mi is the voltage amplitude of node m on branch i; U ni is the voltage amplitude at node n on branch i;

[0027] The calculation formula of the key branch k is:

[0028]

[0029] Preferably, when the sBTTC index value of the key branch meets the sBTTC k <ε th When the duration is equal to T th The time is set as the start time T of the system-level control strategy sc .

[0030] The embodiments of the present invention bring the following beneficial effects:

[0031] The method provided by the present invention is suitable for a multi-level emergency control strategy adopted by a two-terminal flexible direct current system in the case where a power angle instability occurs after the two-terminal flexible direct current system including a grid-following control technology is connected to the power grid. By establishing a two-terminal MMC-HVDC electromechanical transient simulation model in which a sending end adopts a grid-following (GFL) control and a receiving end adopts a GFM control, the transient power angle stability level of the two-terminal MMC-HVDC system including a grid-following control is enhanced.

[0032] Based on the sBTTC index, this paper proposes a key branch identification method that can quantitatively characterize the power angle stability of a double-terminal flexible DC system and an evaluation criterion for the start-up time of the system-level control strategy; for GFM-MMC, a VDPOL link is designed and added, which helps to improve the ability of GFM-MMC to restore stable operation under fault impact. For the actual power grid, simulation verifies the effectiveness of the proposed evaluation criteria and multi-level emergency control strategy.

[0033] Other features and advantages of the present invention will be described in the following description, and some features will become apparent from the description or understood by practicing the present invention. The purpose and other advantages of the present invention are realized and obtained by the structures particularly pointed out in the description, claims and drawings.

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 A flow chart of a grid-connected stability control method applicable to a grid-connected flexible direct current system provided in an embodiment of the present invention;

[0036] Figure 2 A schematic diagram of a two-terminal MMC-HVDC power transmission system simulation model provided by an embodiment of the present invention;

[0037] Figure 3 A logic block diagram of the VDPOL link action provided by an embodiment of the present invention;

[0038] FIG4( a ) is a comparison diagram of transient responses of GFM-MMC with and without a VDPOL link provided by an embodiment of the present invention - a diagram of changes in output active power;

[0039] FIG4( b ) is a comparison diagram of transient responses of GFM-MMC with and without a VDPOL link provided by an embodiment of the present invention - a phase trajectory diagram;

[0040] Figure 5 A schematic diagram of electrical quantities selected for calculating the transient power transmission capacity of a branch provided in an embodiment of the present invention;

[0041] Figure 6 A diagram of an AC / DC hybrid system including a GFM-MMC provided in an embodiment of the present invention;

[0042] FIG. 7( a ) is a transient response curve diagram of an AC / DC hybrid system under a fault condition provided by an embodiment of the present invention - a diagram showing a change in the power angle difference of a generator;

[0043] FIG7( b ) is a transient response curve diagram of an AC / DC hybrid system under a fault condition provided by an embodiment of the present invention - a node voltage variation diagram;

[0044] FIG7( c ) is a transient response curve diagram of an AC / DC hybrid system under a fault condition provided by an embodiment of the present invention - a GFM-MMC power variation diagram;

[0045] Figure 8 A schematic diagram of the effect of the equipment-level control method provided by an embodiment of the present invention;

[0046] Fig. 9 A schematic diagram of the sBTTC index of each branch provided in an embodiment of the present invention;

[0047] FIG10( a ) is a schematic diagram of the effect of the system-level control strategy provided by an embodiment of the present invention - a sBTTC index diagram of the entire network branch;

[0048] FIG10( b ) is a schematic diagram of the effect of the system-level control strategy provided by an embodiment of the present invention - a generator power angle difference diagram. DETAILED DESCRIPTION

[0049] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] To facilitate understanding of this embodiment, firstly, a grid-connected stability control method applicable to a grid-connected flexible direct current system disclosed in an embodiment of the present invention is described in detail. Figure 1 As shown, the following steps are included:

[0051] S1. Based on the power angle stability fault of the double-terminal flexible direct current system, an electromechanical transient simulation model of the double-terminal MMC-HVDC transmission system with grid-type control is constructed, and the main electrical parameters, control parameters, and fault parameters are set. The fault parameters include fault type, fault location, fault start time, and fault end time;

[0052] like Figure 2 As shown in the figure, the sending-end converter station of the electromechanical transient simulation model of the two-terminal MMC-HVDC transmission system adopts GFL control, denoted as GFL-MMC; the receiving-end converter station adopts GFM control based on the virtual synchronous machine VSG strategy, denoted as GFM-MMC. The subscripts "L" and "M" in the figure correspond to the relevant quantities of GFL-MMC and GFM-MMC respectively. That is, the parameters include the grid common connection point voltage amplitude U c With phase θ c , the amplitude E of the converter output potential and the power angle δ, the equivalent reactance x of the bridge arm c , the active power P exchanged between the converter and the AC grid c and reactive power Q c , AC system equivalent reactance Z s And the grid voltage U s .

[0053] like Figure 3 As shown in the figure, a low voltage power limit VDPOL link is added to the control system of GFM-MMC. When the grid-connected node voltage U sM When it drops to 0.8pu, reduce the reference power value P cMref To lim P cMref , that is, reduce the GFM-MMC active reference value P cMref to 80% of the initial value and last for 1s, reducing its value and the output electromagnetic power P cM The deviation between them limits the increase of virtual power angle during fault. At this time, the comparison of active power and phase trajectory of GFM-MMC with and without VDPOL function is as follows: Figure 4(a)-Figure 4(b) As shown in Figure 2, the effectiveness of the designed VDPOL function in restoring the power angle stability of GFM-MMC is verified. The parameters include the starting voltage threshold U lim , power limit coefficient λ lim , Power limit duration T lim , where the power limitation factor λ lim The value range is 0 to 1.0.

[0054] S2. Set the delay time T FS , determine whether the duration t after the fault is cleared is greater than the delay time T FS And whether the system loses power angle stability,

[0055] If the duration t after the fault is cleared is greater than the delay time T FS If the system loses power angle stability, execute S3;

[0056] If the duration t after the fault is cleared is less than the delay time T FS Or if the system does not experience power angle instability, t=t+Δt and S2 is executed again.

[0057] Among them, T FS The delay time is set to avoid the influence of electrical quantity fluctuation in the initial stage after the fault is cleared.

[0058] S3, based on fault clearing and lasting T FS Time, adopt equipment-level control strategy, adjust GFM-MMC control parameters, and determine whether the system has resumed stable operation.

[0059] If yes, the equipment-level control strategy ends; if no, execute S4;

[0060] The fault is cleared and lasts for T FS After a certain time, increase the inertia time constant T of GFM-MMC jM Up to 2T jM , reduce the active reference value P cMref To 80%P cMref .

[0061] S4, measuring the voltage amplitude and voltage phase of the nodes at both ends of each branch in the near area of ​​the two-terminal MMC-HVDC system, calculating the sBTTC index of each branch, sorting them, and defining the branch with the smallest sBTTC index value as the key branch;

[0062] like Figure 5 As shown, the subscripts m and n represent the positions of two measurement points on a line, usually at the two ends of the line.

[0063] Voltage phase difference Δθ i The calculation formula is,

[0064] Δθ i =|Δθ mi -Δθ ni |

[0065] In the formula, Δθ mi is the voltage phase of node m on branch i; Δθ ni is the voltage phase at node n on branch i;

[0066] The calculation formula of sBTTC index is:

[0067]

[0068] Where U mi is the voltage amplitude of node m on branch i; U ni is the voltage amplitude at node n on branch i;

[0069] The calculation formula of the key branch k is:

[0070]

[0071] S5. Based on the obtained sBTTC index, monitor the changes in key branches and determine the start time T of the system-level control strategy sc ;

[0072] When the sBTTC index value of the key branch and the index threshold value ε th The relationship meets sBTTC k <ε th When the duration is equal to T th The moment is set as the start time of the system-level control strategy.

[0073] Among them, satisfying sBTTC k <ε th The duration is equal to T th , it indicates that the system stability has deteriorated significantly and power angle instability has occurred.

[0074] S6, at the start time T sc , adopt a system-level control strategy to cut off some generator sets in the vicinity of the double-terminal flexible DC system until the system returns to stability.

[0075] In this embodiment, if Figure 6 The AC / DC hybrid system shown in the figure includes 9 synchronous generators, 39 AC nodes, and a double-terminal MMC-HVDC. The sending end is connected to a strong AC system with equal value, and the receiving end is connected to the AC node B. 35 Connect to the AC grid, set the GFM-HVDC operating power to 1000MW, and the reference power value P cMref For 1000MW, the inertia time constant T jM The value is 2s.

[0076] like Figure 7(a)-Figure 7(c) As shown, set B 16 -B 17 Branch B 16A three-phase metallic ground short circuit occurred at 0.2s on the node side, and the branch was disconnected after 0.24s. After the fault, the virtual synchronous power angle of GFM-MMC increased rapidly, and the power angles of G3, G4, and G6 also increased synchronously. Affected by the voltage drop at the node near the oscillation center in the network, G2 and G1 also accelerated successively due to the obstruction of electromagnetic power output, and the system power angle became unstable and showed multi-group characteristics. After the power angle of the generator group became unstable, GFM-MMC could not operate stably due to the large fluctuation of network voltage, and its output active and reactive power both oscillated greatly. Therefore, it is necessary to adopt a control strategy from the GFM-MMC equipment level or system level to restore the system to stable operation as soon as possible after the fault occurs.

[0077] like Figure 8 As shown in the figure, 0.1s after the fault is cleared, that is, at the moment of 0.54s, the control parameter T of GFM-MMC is jM From 2.0s to 4.0s, and the active reference value P cMref Down to 80% P cMref And it lasts for 1s. After taking the above control measures, the system can restore the power angle stability and the MMC-HVDC can quickly resume stable operation after the fault disturbance, which verifies the effectiveness of the equipment-level control strategy.

[0078] like Fig. 9 As shown in the figure, since there is no clear standard for the optimization of control parameters of the equipment-level control strategy, it is set manually and has certain limitations. Under different working conditions, there may be situations where improper parameter selection leads to equipment-level control failure. Therefore, it is necessary to add system-level control as a backup. 0.1s after the fault is cleared, if the control parameter T of GFM-MMC jM or P cMref If the appropriate value is not adjusted, the MMC-HVDC will not resume stable operation after the parameter adjustment, and the system power angle stability will continue to deteriorate, and additional system-level control strategies are required. th and its duration T th The following are 0.4 and 0.2s respectively. From 2.14s to 2.34s, branch B 14 -B 15 With the smallest sBTTC index value, B14-B15 is the key branch in the system, and the duration of T is 2.34s. th Meet sBTTC k <ε th , so the system-level control strategy can be initiated.

[0079] like Figure 10(a) and 10(b) As shown in the figure, considering the 0.2s control delay, after the emergency control of cutting off the G3 unit is implemented for 2.54s, the system can restore the power angle stability, which verifies the effectiveness of the system-level control strategy.

[0080] It should be noted that, in this article, the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0081] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A grid-connected stability control method suitable for a grid-connected flexible direct current system, based on an equipment-level control strategy and a system-level control strategy, characterized in that: The following steps are involved: S1. Based on the power angle stability fault of the double-terminal flexible direct current system, an electromechanical transient simulation model of the double-terminal MMC-HVDC transmission system with grid-type control is constructed, and the main electrical parameters, control parameters, and fault parameters are set; S2. Set the delay time T FS , determine whether the duration t after the fault is cleared is greater than the delay time T FS And whether the system loses power angle stability, If the duration t after the fault is cleared is greater than the delay time T FS If the system loses power angle stability, execute S3; If the duration t after the fault is cleared is less than the delay time T FS Or if the system does not experience power angle instability, then t=t+Δt and execute S2 again; S3, based on fault clearing and lasting T FS Time, adopt equipment-level control strategy, adjust GFM-MMC control parameters, and determine whether the system has resumed stable operation. If yes, the equipment-level control strategy ends; if no, execute S4; S4, measuring the voltage amplitude and voltage phase of the nodes at both ends of each branch in the near area of ​​the two-terminal MMC-HVDC system, calculating the sBTTC index of each branch, and defining the branch with the minimum sBTTC index value as the key branch; S5. Based on the obtained sBTTC index, monitor the changes in key branches and determine the start time T of the system-level control strategy sc ; S6, at the start time T sc , adopt a system-level control strategy to cut off some generator sets in the vicinity of the double-terminal flexible DC system until the system returns to stability.

2. The grid-connected stability control method according to claim 1, characterized in that: The sending-end converter station of the double-terminal MMC-HVDC transmission system electromechanical transient simulation model adopts the grid-following GFL control, denoted as GFL-MMC; the receiving-end converter station adopts the grid-forming GFM control based on the virtual synchronous machine VSG strategy, denoted as GFM-MMC.

3. The grid-connected stability control method according to claim 2, characterized in that: In the control system of GFM-MMC, a low voltage power limit link is added. When the grid-connected node voltage U sM When the power drops significantly, reduce the reference power value P cMref To lim P cMref , reduce its relative to the output electromagnetic power P cM The deviation between them limits the growth of the virtual power angle during the fault period.

4. The grid-connected stability control method according to claim 1, characterized in that: The fault parameters include fault type, fault location, fault start time, and fault end time.

5. The grid-connected stability control method according to claim 1, characterized in that: The S3 further includes: After the fault is cleared and lasts for T FS After a certain time, increase the inertia time constant T of GFM-MMC jM Up to 2T jM , reduce the active reference value P cMref To 80%P cMref And last for 1S.

6. The grid-connected stability control method according to claim 1, characterized in that: In S4, Voltage phase difference Δθ i The calculation formula is, Dth i =|Δθ mi -Dth ni | In the formula, Δθ mi is the voltage phase of node m on branch i; Δθ ni is the voltage phase at node n on branch i; The calculation formula of sBTTC index is: Where U mi is the voltage amplitude of node m on branch i; U ni is the voltage amplitude at node n on branch i; The calculation formula of the key branch k is:

7. The grid-connected stability control method according to claim 1, characterized in that: The S5 further includes: When the sBTTC index value of the key branch and the index threshold value ε th The relationship meets sBTTC k <ε th When the duration is equal to T th The time is set as the start time T of the system-level control strategy sc .

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