A post-fault multi-circuit DC coordinated control method based on extended equal area law

Abstract

The present invention discloses a multi - circuit DC collaborative control method after a fault based on the extended equal - area criterion, including the following steps: S1. Divide the critical machine group G S and the remaining machine group G A ; S2. Calculate the support effect evaluation index of each DC converter station for all generators in the critical machine group G S to determine the optimal DC converter station; S3. If it is a three - phase short - circuit fault, reduce the power of the optimal DC converter station by f percentage points; if it is a DC fault, increase it by f percentage points; S4. If the sign of the difference between the two equivalent angular velocities switches, re - determine the optimal DC converter station. If the equivalent angular velocity of the critical machine group G S is greater than the equivalent angular velocity of the remaining machine group G A , increase the DC power of the optimal DC converter station by f percentage points; otherwise, reduce the DC power of the optimal DC converter station by f percentage points; S5. Repeat S1 - S4 until the time of DC power support reaches the short - time overload limit of the converter station t s .

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly to a multi-circuit DC cooperative control method after a fault based on the extended equal area criterion. Background Art

[0002] With the increasing impact of ten-million-kilowatt-level new energy access on the system security and stability, the DC sending-end system shows the characteristics of "double highs" (high proportion of new energy and high proportion of power electronic devices). Under the current condition of weak AC support, the stable control of the DC transmission system in high-altitude clean energy bases faces huge challenges.

[0003] New energy generating units such as photovoltaic and wind power do not have the same rotor dynamic characteristics as traditional thermal power units, resulting in significantly insufficient active support ability of the sending-end system compared with traditional power systems, and limited damping effect on fault impacts. When a DC blocking fault or an AC system fault occurs, a large amount of unbalanced power accumulates at the sending end. At the same time, the large-scale power flow transfer occurring in the system will cause the continuous swing of the power angles of each generator at the sending end, thereby posing a serious threat to the transient power angle stability of the system. Existing research mainly starts from the perspective of the receiving-end power grid, emphasizing that the range of DC power modulation must match the strength of the receiving-end power grid to prevent inappropriate DC power increase from inducing consecutive commutation failures of multi-circuit DCs. However, most of the multi-circuit DC transmission projects in China at present adopt hybrid DC transmission composed of conventional DC and flexible DC in parallel. The receiving-end station of the hybrid DC is generally a voltage-source converter station that will not experience commutation failures. There is also research focusing on the supporting role of DC power support in the AC-DC hybrid power grid for the system security and stability after an AC fault. Although this type of research comprehensively considers the security and stability of the sending and receiving-end power grids, most of its research objects are single-circuit DCs, and the scenarios of DC faults are not considered comprehensively enough.

[0004] Therefore, in a multi-circuit large-capacity hybrid DC transmission system, how to construct a set of multi-circuit DC cooperative control method for the transient power angle stability problem of the sending-end system is still a technical problem to be solved urgently. Summary of the Invention

[0005] In view of this, the present invention provides a multi-circuit DC cooperative control method after a fault based on the extended equal area criterion to at least solve the problem of unstable transient power angle existing in multi-circuit DC transmission projects in the prior art.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A multi-circuit DC cooperative control method after a fault based on the extended equal area criterion includes the following steps:

[0008] S1. Obtain the angular velocity monitoring data of all generators in the sending-end system after a fault, and divide the critical generator groups according to the angular velocity monitoring data G S and the remaining generator groups G A ;

[0009] S2. Calculate the support effect evaluation index of each DC converter station for all generators in the critical generator group G S to determine the DC converter station with the best support effect as the optimal DC converter station;

[0010] S3. If the fault type is a three-phase short-circuit fault, then reduce the power of the optimal DC converter station f by a certain percentage; if the fault type is a DC fault, then increase the power of the optimal DC converter station f by a certain percentage;

[0011] S4. Consider the dynamic process of the system and the relative magnitude change of the equivalent angular velocities of the critical generator group G S and the remaining generator groups G A during the dynamic process, that is, if the sign of the difference between the two equivalent angular velocities switches, then re-determine the optimal DC converter station; among them, if the equivalent angular velocity of the critical generator group G S is greater than the equivalent angular velocity of the remaining generator groups G A then increase the DC power of the current optimal DC converter station f by a certain percentage; otherwise, reduce the DC power of the current optimal DC converter station f by a certain percentage;

[0012] S5. Repeat S1 - S4 until the time for all increases or decreases in DC power reaches the short-term overload limit of the converter station t s .

[0013] Preferably, the specific content of S1 includes:

[0014] Monitor the angular velocities of all generators in the sending-end system after a fault and obtain the generator power angle curves accordingly;

[0015] Calculate the angular velocity of the center of inertia of the entire sending-end system according to the generator angular velocities , and obtain the center-of-inertia power angle curve accordingly;

[0016] If the generator power angle curve of any generator in the sending-end system deviates from the center-of-inertia power angle curve, then determine that the current generator is severely disturbed and classify it into the critical generator group G S; Otherwise, classify it into the remaining fleet G A 。

[0017] Preferably, the calculation method of the angular velocity of the center of inertia is: :

[0018] ;

[0019] Wherein, M T is the sum of the inertia time constants of all generators in the sending-end system; and are respectively the inertia time constant and angular velocity of the i-th generator in the sending-end; n is the number of generators in the sending-end system.

[0020] Preferably, the support effect evaluation index in S2 is the modulation coefficient, and the calculation method of the modulation coefficient is:

[0021] ; ;

[0022] Wherein, is the modulation coefficient of any DC converter station in the multi-circuit DC sending-end system ; x is the number of generators included in the critical fleet; is the synchronous power coefficient of the converter station for any generator in the critical fleet; is the inertia time constant of the generator in the sending-end system; and are respectively the internal potential of the generator and the voltage amplitude of the converter bus of the converter station ; represents and the phase difference between; is the mutual admittance between the bus of the generator and the converter bus of the converter station ; is the reduction of the generator acceleration power after the DC emergency power support; and are respectively the mechanical power and electromagnetic power of the generator ; is the power value increased or decreased by the DC converter station.

[0023] Preferably, the specific content of S4 includes:

[0024] Classify the critical fleet GS and the remaining fleet G A The equivalent angular velocities are denoted as and ;

[0025] Considering the dynamic process of the system and the relative magnitude change of the angular velocities of the critical fleet G S and the remaining fleet G A Adjust the power of the current optimal HVDC converter station during the transient process within m seconds:

[0026] When the equivalent system at the sending end is in the forward acceleration and forward deceleration stages, increase the DC power of the current optimal HVDC converter station to reduce the forward swing acceleration area and increase the forward swing deceleration area;

[0027] When the equivalent system at the sending end is in the reverse acceleration and reverse deceleration stages, reduce the DC power of the current optimal HVDC converter station to increase the back swing deceleration area and reduce the back swing acceleration area.

[0028] Preferably, the calculation method of the equivalent angular velocities G S of the critical fleet G A and the remaining fleet and is as follows:

[0029] ;

[0030] where M GS and M GA are the sums of the inertia time constants of all generators belonging to the critical fleet G S and the remaining fleet G A respectively; M k and are the inertia time constant and angular velocity of the k th generator belonging to the critical fleet; M t and are the inertia time constant and angular velocity of the t th generator belonging to the remaining fleet; x and y are the numbers of generators included in the critical fleet and the remaining fleet respectively.

[0031] Preferably, the specific content of S5 includes:

[0032] Repeat S1 - S4 until the time of DC power support reaches the short - term overload time limit of the converter station t s ;

[0033] If it reaches t s , for all fault types, the power of the non - faulty DC converter station is increased to the long - term overload factor of the DC converter station L c , until the power - angle curve oscillation after the fault decays to stability.

[0034] As can be seen from the above - mentioned technical solutions, compared with the prior art, the present invention discloses a multi - circuit DC collaborative control method after a fault based on the extended equal - area criterion, which has the following beneficial effects:

[0035] The present invention first proposes a method for identifying severely disturbed generator groups after a fault based on the idea of equivalent machine groups in the extended equal - area criterion, which is beneficial to locating the severely disturbed generator groups after a fault; secondly, based on this identification method, by using the adjustable power characteristic of the DC converter station, through the collaborative modulation of multi - circuit DC power and the reasonable scheduling of the internal power of each area of the sending - end system, the unbalanced power generated by the fault is absorbed, the equivalent accelerating area of the system is reduced, and the equivalent decelerating area of the system is increased, ultimately achieving the effect of maintaining the power - angle stability of the sending - end system after a fault. By connecting the scheduling of DC power with the power - angle stability of the entire sending - end system through the extended equal - area criterion, the gaps in the applicability of the prior art in the sending - end system and the application of multi - circuit DC are filled. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0037] Figure 1 is a flowchart of a multi - circuit DC collaborative control method after a fault based on the extended equal - area criterion provided by the present invention;

[0038] Figure 2 is an equivalent two - machine system diagram provided by an embodiment of the present invention;

[0039] Figure 3 is a power - angle - active power curve diagram of an equivalent single - machine under two faults provided by an embodiment of the present invention, where Figure 3 (a) is a DC blocking fault,Figure 3 (b) is the SanYong N-1 fault. **Detailed implementation manners**

[0040] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] The present invention provides a coordinated control method for multiple HVDC after a fault based on the extended equal area criterion, as Figure 1 shown, including the following steps:

[0042] S1. Obtain the angular velocity monitoring data of all generators in the sending-end system after the fault, and divide the critical generator group G S and the remaining generator group G A ;

[0043] S2. Calculate the support effect evaluation index of each HVDC converter station for all generators in the critical generator group G S to determine the HVDC converter station with the best support effect as the optimal HVDC converter station;

[0044] S3. If the fault type is a three-phase short-circuit fault, then reduce the power of the optimal HVDC converter station by f percentage points; if the fault type is an HVDC fault, then increase the power of the optimal HVDC converter station by f percentage points;

[0045] S4. Consider the dynamic process of the system and the relative magnitude change of the equivalent angular velocities of the critical generator group G S and the remaining generator group G A during the dynamic process, that is, if the sign of the difference between the two equivalent angular velocities switches, then re-determine the optimal HVDC converter station; wherein, if the equivalent angular velocity of the critical generator group G S is greater than the equivalent angular velocity of the remaining generator group G A , then increase the DC power of the current optimal HVDC converter station by f percentage points; otherwise, reduce the DC power of the current optimal HVDC converter station by f percentage points;

[0046] S5. Repeat S1-S4 until the time for all increases or decreases in DC power reaches the short-term overload limit of the converter station ts 。

[0047] It should be noted that:

[0048] In this embodiment, an electromechanical transient simulation software is used to record the angular velocities of all generators in the sending-end system after a fault, and the angular velocity of the center of inertia of the entire sending-end system is calculated ; PSD-BPA is used as the time-domain simulation software in this embodiment.

[0049] In the present invention, the value obtained by increasing the DC power of the converter station by f a certain percentage is the DC power overload limit of the converter station. In this embodiment, f is taken as 30.

[0050] The following is an explanation of the content of the extended equal-area criterion in the present invention:

[0051] 1. Idea of equivalent machine groups:

[0052] The equivalent two-machine system diagram is as shown in Figure 2 ; After a fault, a large amount of unbalanced power accumulates at the sending end. All generators can be divided into two equivalent machine groups: the critically disturbed group with the power angle swinging out first is the critical machine group G S , and the less disturbed group is the remaining group G A . Further simplifying and equivalenting the rotor motion equations of the equivalent two machines, an equivalent single-machine infinite-bus system can be obtained, as shown in the following formula:

[0053] ;

[0054] Among them, M GS , M GA are the inertia time constants of the critical machine group and the remaining machine group respectively; P GS , P GA are the active powers generated by the critical machine group and the remaining machine group; U GS , U GA are the generator bus voltages of the critical machine group and the remaining machine group respectively; P dc is the DC power; X SA is the line reactance between the two equivalent machine groups; is the angular velocity of the equivalent single machine; is the power angle of the equivalent single machine; The equivalent mechanical power of the equivalent single machine is and the equivalent electromagnetic power is 。

[0055] 2. Extended equal - area criterion for equivalent single - machine:

[0056] The power - angle vs. active - power curves of the equivalent single - machine under two types of faults are as Figure 3 shown. When a DC - blocking fault occurs, the power characteristics of the equivalent single - machine remain unchanged. The DC power can be regarded as part of the equivalent mechanical power. Therefore, the impact of this fault can be regarded as the change of the system equivalent mechanical power from the straight line to . The accelerating area of the equivalent single - machine is S b’c’d’ , and the decelerating area of the equivalent single - machine is S c’d’ . Increasing the DC power is beneficial to reducing the accelerating area and increasing the decelerating area, thus avoiding the acceleration of the equivalent single - machine. After a three - phase short - circuit fault occurs, the faulty line is generally permanently removed to eliminate the fault. Therefore, it is called a three - permanent N - 1 fault. In this scenario, the change process of the power - characteristic curve of the equivalent single - machine (normal → fault → removal) is curve 1 → curve 3 → curve 2, and the equivalent mechanical power remains unchanged. At this time, the accelerating area of the equivalent single - machine is S abce , and the decelerating area is S def . Therefore, increasing the DC power is beneficial to reducing the accelerating area and increasing the decelerating area, avoiding the acceleration of the equivalent single - machine. The above analysis is only for the positive swing (the forward acceleration and deceleration process of the system). Reducing the DC power during the reverse swing (the reverse acceleration and deceleration process of the system) can achieve the same effect.

[0057] To further implement the above technical solution, the specific content of S1 includes:

[0058] Monitoring the angular velocity of all generators in the sending - end system after a fault and obtaining the generator power - angle curve accordingly;

[0059] Calculating the angular velocity of the center of inertia of the entire sending - end system based on the generator angular velocity , and obtaining the power - angle curve of the center of inertia accordingly;

[0060] If the power - angle curve of any generator in the sending - end system deviates from the power - angle curve of the center of inertia, it is determined that the current generator is severely disturbed and classified into the critical generator group G S ; otherwise, it is classified into the remaining generator group G A .

[0061] To further implement the above technical solution, the calculation method of the angular velocity of the center of inertia is as follows:

[0062] ;

[0063] Among them, M T is the sum of the inertia time constants of all generators in the sending-end system; M i and are respectively the inertia time constant and angular velocity of the i-th generator in the sending-end; n is the number of generators in the sending-end system.

[0064] To further implement the above technical solution, the support effect evaluation index in S2 is the modulation coefficient, and the calculation method of the modulation coefficient is:

[0065] ; ;

[0066] Among them, is the modulation coefficient of any DC converter station in the multi-circuit DC sending-end system; x is the number of generators included in the critical machine group; is the synchronous power coefficient of the converter station for any generator in the critical machine group; is the inertia time constant of the generator and are respectively the internal potential of the generator and the voltage amplitude of the converter bus of the converter station ; represents and the phase difference between; is the mutual admittance between the bus of the generator and the converter bus of the converter station ; is the reduction amount of the generator acceleration power after the DC emergency power support; and are respectively the mechanical power and electromagnetic power of the generator ; is the power value increased or decreased by the DC converter station.

[0067] To further implement the above technical solution, the specific content of S4 includes:

[0068] Denote the equivalent angular velocities of the critical machine group G S and the remaining machine group G A as and respectively;

[0069] Consider the dynamic process of the system and the critical machine group during the dynamic process G S and the remaining machine group G A Adjust the relative magnitude change of the equivalent angular velocity, and m the power of the current optimal DC converter station during the transient process within seconds:

[0070] When the sending-end equivalent system is in the forward acceleration and forward deceleration stages, increase the DC power of the current optimal DC converter station to reduce the positive swing acceleration area and increase the positive swing deceleration area;

[0071] When the sending-end equivalent system is in the reverse acceleration and reverse deceleration stages, reduce the DC power of the current optimal DC converter station to increase the back swing deceleration area and reduce the back swing acceleration area.

[0072] It should be noted that:

[0073] In this embodiment, m is taken as 3.

[0074] To further implement the above technical solution, the equivalent angular velocities of the critical machine group G S and the remaining machine group G A are calculated as follows: and are:

[0075] ;

[0076] where, M GS and M GA are the sums of the inertia time constants of all the generators belonging to the critical machine group G S and the remaining machine group G A respectively; M k and are the inertia time constant and angular velocity of the k th generator belonging to the critical machine group respectively; M t and are the inertia time constant and angular velocity of the t th generator belonging to the remaining machine group respectively; x and y are the numbers of generators included in the critical machine group and the remaining machine group respectively.

[0077] To further implement the above technical solution, the specific content of S5 includes:

[0078] Repeat S1 - S4 until the time of DC power support reaches the short-term overload limit of the converter station t s ;

[0079] If it reaches t s , for all fault types, the power of the non-faulty DC converter station is increased to the long-term overload factor of the DC converter station L c , until the power angle curve oscillation after the fault decays to stability, that is, the oscillation decay approaches 0.

[0080] It should be noted that:

[0081] In this embodiment, L c is the long-term overload factor of the DC converter station, generally 1.05 times the rated power of the converter station.

[0082] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A post-fault multi-circuit DC coordinated control method based on the extended equal area rule, characterized in that: The following steps are involved: S1. Obtain the angular velocity monitoring data of all generators in the sending-end system after the fault, and divide the critical group according to the angular velocity monitoring data G S and the remaining fleet G A ; S2. Calculate the critical group of each DC converter station G S The support effect evaluation index of all generators in the system is used to determine the DC converter station with the best support effect as the best DC converter station; S3. If the fault type is a three-phase short circuit fault, then reduce the optimal DC converter station power f percentage points; if the fault type is a DC fault, the optimal DC converter station power is increased f percentage points; S4. Consider the dynamic process of the system and the critical cluster in the dynamic process G S and the remaining fleet G A The relative size of the equivalent angular velocity changes, that is, if the sign of the difference between the two equivalent angular velocities changes, the optimal DC converter station is re-determined; among them, if the critical group G S The equivalent angular velocity is greater than that of the rest of the fleet G A The equivalent angular velocity of the current optimal DC converter station is increased by f percentage points; otherwise, the DC power of the current optimal DC converter station will be reduced. f percentage points; S5. Repeat S1-S4 until the time for all DC power increase or decrease in S3-S4 reaches the short-term overload time limit of the converter station. t s .

2. The method for coordinated control of multiple DC circuits after a fault based on the extended equal area rule according to claim 1, characterized in that: The specific contents of S1 include: Monitor the angular velocity of all generators in the sending-end system after the fault and obtain the corresponding generator power angle curve; Calculate the angular velocity of the center of inertia of the entire sending end system based on the angular velocity of the generator , and the corresponding inertia center power angle curve is obtained; If the power angle curve of any generator in the sending end system deviates from the power angle curve of the center of inertia, the current generator is judged to be seriously disturbed and is classified as a critical group. G S ; Otherwise, it is classified as the remaining cluster G A .

3. The method for coordinated control of multiple DC circuits after a fault based on the extended equal area rule according to claim 2 is characterized in that: Angular velocity of center of inertia The calculation method is: ； in, M T is the sum of the inertia time constants of all generators in the sending end system; and are the inertia time constant and angular velocity of the i-th generator in the sending end respectively; n is the number of generators in the sending-end system.

4. The method for coordinated control of multiple DC circuits after a fault based on the extended equal area rule according to claim 1 is characterized in that: The support effect evaluation index in S2 is the modulation coefficient, which is calculated as follows: ； ； in, Any DC converter station in a multi-circuit DC sending system The modulation coefficient of x is the number of generators included in the critical cluster; It is a converter station For any generator in the critical group The synchronous power factor; Generator in the sending end system The inertia time constant of and Generator Internal potential and converter station The voltage amplitude of the commutation bus; represent and The phase difference between For generator Busbar and converter station The mutual admittance between the commutation buses; The reduction in generator acceleration power after DC emergency power support; and Generator Mechanical power and electromagnetic power; The power value increased or decreased by the DC converter station.

5. The method for coordinated control of multiple DC circuits after a fault based on the extended equal area rule according to claim 1, characterized in that: The specific contents of S4 include: Critical Cluster and the remaining fleet The equivalent angular velocity is denoted as and ; Consider the dynamic process of the system and the critical cluster in the dynamic process G S and the remaining fleet G A The relative size of the equivalent angular velocity changes, adjust m The power of the current optimal DC converter station during the transient process within seconds: when When the equivalent system at the sending end is in the forward acceleration and forward deceleration stages, the DC power of the current optimal DC converter station is increased to reduce the positive sway acceleration area and increase the positive sway deceleration area; when When the sending-end equivalent system is in the reverse acceleration and reverse deceleration stage, the DC power of the current optimal DC converter station is reduced to increase the swing deceleration area and reduce the swing acceleration area.

6. The method for coordinated control of multiple DC circuits after a fault based on the extended equal area rule according to claim 5, characterized in that: Critical Cluster G S and the remaining fleet G A The equivalent angular velocity and The calculation method is: ； in, M GS and M GA They are respectively the critical clusters G S and the remaining fleet G A The sum of the inertia time constants of all generators; M k and They are the first k The inertia time constant and angular velocity of the generator; M t and For the remaining cluster t The inertia time constant and angular velocity of the generator; x and y are the number of generators in the critical cluster and the remaining cluster, respectively.

7. The method for coordinated control of multiple DC circuits after a fault based on the extended equal area rule according to claim 1, characterized in that: The specific contents of S5 include: Repeat S1-S4 until the DC power support time reaches the short-term overload time limit of the converter station. t s ; If reached t s For all fault types, the power of the non-faulty DC converter station is increased to the long-term overload factor of the DC converter station. L c , until the power angle curve oscillation after the fault decays to a stable state.

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