A method for evaluating the maximum grid-connected capacity of a converter based on resonance stability

Abstract

The present invention relates to the field of power systems, and specifically to a method for evaluating the maximum grid-connected capacity of a converter based on resonance stability. A resonance stability analysis is performed for all converters to be evaluated when they are connected to the grid. If the system is unstable, the converter with the smallest capacity in the sequence is removed, and the resonance stability analysis is continued for the remaining units when they are connected to the grid until a stable combination is found. The obtained stable combination is regarded as a whole, and the removed converters are re-formed into a new sequence in order from large to small. The stable combination obtained last time is combined with the converters in the removed combination in turn, and the stability of the combination is judged until the last one in the removed converter combination is reached. The final combination is the desired maximum grid-connected capacity combination of the converter, thereby realizing the evaluation of the maximum grid-connected capacity of the converter. When the system needs to access or remove the converter, this method can reduce the impact on the system and enhance the stability of the system.

Description

Technical Field

[0001] The invention relates to the field of power systems, and in particular to a method for evaluating the maximum grid-connected capacity of a converter based on resonance stability. Background Art

[0002] Renewable energy generation represented by photovoltaic power generation, hydroelectric power generation and hydroelectric power generation is developing rapidly and will occupy an important position in the future energy composition. Renewable energy wind power and photovoltaic power generation are subject to factors such as weather and region, and have two obvious characteristics of dispersion and randomness. These two characteristics determine that renewable energy generation cannot be centralized like traditional large-scale thermal power generation. The most ideal choice at present is to generate electricity from new energy in a centralized manner, that is, to supply power to the outside in a centralized or decentralized manner in a cluster. In the actual application of new energy power generation, the distributed generation system can be used as an independent power source to supply power to the load, or it can be incorporated into the power grid to participate in the operation of the large power grid.

[0003] The concentrated grid connection of new energy sources leads to stability problems such as harmonic interaction, resonance, and oscillation, which may cause equipment failure and damage, threaten the lives of production personnel, and in severe cases may cause the grid voltage to drop, making it difficult for the grid to operate stably.

[0004] At present, in order to deal with the oscillations caused by the grid connection of large-scale converters, the method of cutting off new energy units or stations is usually adopted. For new energy stations operating in large power grids, their removal will have an impact on the frequency stability and voltage stability of the power grid. The most core method to reduce this impact is to reduce the capacity of the cut-off units as much as possible, that is, always maintain the maximum capacity access, which has a smaller impact on the power grid and has a stronger dynamic support capacity for the system. On the other hand, in the face of units waiting to be connected, it is also necessary to comprehensively consider the stability characteristics of the grid-connected units and the units to be connected to the grid under the premise of ensuring system stability, and select the maximum capacity combination among them to achieve effective and reasonable use of new energy. In summary, evaluating the maximum grid-connected capacity of the converter under the premise of the resonance stability that is prone to occur in the converter is the key to supporting the stable and reliable operation of new energy and its absorbing power grid. Summary of the invention

[0005] In order to solve the deficiencies mentioned in the above background technology, an object of the present invention is to provide a method for evaluating the maximum grid-connected capacity of a converter based on resonance stability.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A method for evaluating the maximum grid-connected capacity of a converter based on resonance stability, the method comprising the following steps:

[0008] S1. According to the current loop control of the converter, the converter is transformed into a Norton equivalent circuit;

[0009] S2. The converters to be analyzed are arranged in descending order of capacity to form a sequence S, where S is expressed as shown in formula (1):

[0010] S={C1,…C i ,…C n} (1)

[0011] Among them, C i is the rated capacity of i converters, and C1 ≥ C i ≥C n ;

[0012] Assume that the converter equivalent parallel admittance corresponding to sequence S is {Y1,…,Y i ,…,Y n}, the admittance corresponding to the sequence is the sum of the admittances corresponding to each converter, then the combined admittance corresponding to the sequence S is as shown in formula (2):

[0013]

[0014] Among them, Y s is the combined admittance corresponding to the sequence S;

[0015] Y s The Nyquist stability analysis is performed to draw the conclusion that the stable combination and the combination of the removed converter are expressed as formula (3);

[0016]

[0017] Among them, S j For a stable combination, S re The combination of the removed units is formed in descending order;

[0018] S3, sequence S j As a whole, S re The units in S j The Nyquist stability is determined based on the combined admittance.

[0019] Furthermore, the specific process in S1 is: the converter is equivalent to a controlled current source parallel admittance, and the admittance is expressed as a frequency domain analytical expression containing a Laplace operator s.

[0020] Furthermore, the stability conclusion in S2 is as follows:

[0021] If the converter combination formed by sequence S is stable, the current sequence S is the maximum grid-connected capacity;

[0022] If the converter combination formed by the sequence S is unstable, the converter with the smallest capacity is removed to form a new sequence, and the stability of the combined admittance corresponding to the new sequence is examined; if the new sequence is unstable, the converter with the smallest capacity in the current sequence is continued to be removed, and the stability of the remaining converters is examined until a stable combination is found. At the same time, the removed converter units are reconstructed into a sequence. The stable combination and the combination formed by the removed converters are expressed as shown in formula (3).

[0023] Furthermore, the stability determination method in S3 is as follows:

[0024] Pair combination j ,C j+1+p}(p=1,2,…,nj-1) to judge the stability. If the combination {S j ,C j+1+p} is stable, then the converter unit C j+1+p Add sequence S j ;

[0025] If the combination {S j ,C j+1+p}If it is unstable, remove unit C j+1+p , sequence S re After all the units in the system have completed the stability judgment, the sequence S j The converter combination with the maximum grid-connected capacity is the one with the largest grid-connected capacity.

[0026] Beneficial effects of the present invention:

[0027] The present invention solves the problem in the prior art that it is impossible to ensure that the grid-connected converter unit is a maximum capacity combination. By combining the capacity with the resonance stability judgment, a precise stability judgment is always made on the current maximum capacity. Under the premise of ensuring the correctness of the judgment, the scale of the stability judgment is reduced, the efficiency of the stability analysis is improved, and the stability of the large-scale new energy power grid is enhanced. It is ensured that the impact of the new energy converter on the grid or the disconnection thereof on the grid is minimized, thereby improving the overall safe operation level of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 It is a schematic diagram of the evaluation method of the present invention. DETAILED DESCRIPTION

[0030] 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.

[0031] like Figure 1 As shown, S n-1 The sequence is formed by removing the unit with the smallest capacity from all converter units. A method for evaluating the maximum grid-connected capacity of a converter based on resonance stability includes:

[0032] S1. According to the current loop control of the converter, the converter is transformed into a Norton equivalent circuit, that is, the converter is equivalent to a controlled current source parallel admittance, and the admittance is expressed as a frequency domain analytical expression containing the Laplace operator s;

[0033] S2. The converters to be analyzed are arranged in descending order of capacity to form a sequence S. S can be expressed as shown in formula (1):

[0034] S={C1,…C i ,…C n} (1)

[0035] Among them, C i is the rated capacity of i converters, and C1 ≥ C i ≥C n .

[0036] Assume that the converter equivalent parallel admittance corresponding to sequence S is {Y1,…,Y i ,…,Y n}, the admittance corresponding to the sequence is the sum of the admittances corresponding to each converter, then the combined admittance corresponding to the sequence S is as shown in formula (2):

[0037]

[0038] Among them, Y s is the combined admittance corresponding to the sequence S.

[0039] Y sPerform Nyquist stability analysis and draw conclusions about its stability. If the converter combination formed by sequence S is stable, the current sequence S is the maximum grid-connected capacity. If the converter combination formed by sequence S is unstable, remove the converter with the smallest capacity to form a new sequence, and examine the stability of the combined admittance corresponding to the new sequence; if the new sequence is unstable, continue to remove the converter with the smallest capacity in the current sequence, and examine the stability of the remaining converters until a stable combination is found. At the same time, the removed converter units are reconstructed into a sequence. The stable combination and the combination formed by the removed converters can be expressed as formula (3).

[0040]

[0041] Among them, S j For a stable combination, S re The combination of the removed units is formed in order from large to small.

[0042] S3, sequence S j As a whole, S re The units in S j Combine them and make Nyquist stability judgment based on their combined admittance, that is, j ,C j+1+p}(p=1,2,…,nj-1) to judge the stability. If the combination {S j ,C j+1+p} is stable, then the converter unit C j+1+p Add sequence S j , if the combination {S j ,C j+1+p}If it is unstable, remove unit C j+1+p . Sequence S re After all the units in the system have completed the stability judgment, the sequence S j The converter combination with the maximum grid-connected capacity is the one with the largest grid-connected capacity.

[0043] The present invention constructs a Norton equivalent circuit of a grid-connected converter and calculates the frequency-domain analytical expression of the Norton equivalent circuit admittance. First, a resonance stability analysis is performed for all converters to be evaluated when they are connected to the grid. If the system is unstable, the converter with the smallest capacity in the sequence is removed, and the resonance stability analysis is continued for the remaining units when they are connected to the grid until a stable combination is found. The obtained stable combination is regarded as a whole, and the removed converters are re-formed into a new sequence in order from large to small. The stable combination obtained last time is combined with the converters in the removed combination in turn, and the stability of the combination is judged until the last one in the removed converter combination. The final combination is the desired converter maximum grid-connected capacity combination, thereby realizing the evaluation of the converter maximum grid-connected capacity. When the system needs to access or remove the converter, this method can reduce the impact on the system and enhance the stability of the system.

[0044] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A method for evaluating the maximum grid-connected capacity of a converter based on resonance stability. It is characterized in that The evaluation method comprises the following steps: S1. According to the current loop control of the converter, the converter is transformed into a Norton equivalent circuit; S2. The converters to be analyzed are arranged in descending order of capacity to form a sequence S, where S is expressed as shown in formula (1): S={C 1 ,…C i ,…C n } (1) Among them, C i is the rated capacity of i converters, and C 1 ≥C i ≥C n ; Assume that the converter equivalent parallel admittance corresponding to sequence S is {Y 1 ,…,Y i ,…,Y n }, the admittance corresponding to the sequence is the sum of the admittances corresponding to each converter, then the combined admittance corresponding to the sequence S is shown in formula (2): Among them, Y S is the combined admittance corresponding to the sequence S; Y S The Nyquist stability analysis is performed to draw the conclusion that the stable combination and the combination of the removed converter are expressed as formula (3); Among them, S j For a stable combination, S re The combination of the removed units is formed in descending order; S3, sequence S j As a whole, S re The units in S j Combine them and make Nyquist stability judgment based on their combined admittance. The sequence S re After all the units in the system have completed the stability judgment, the sequence S j The converter combination of is the combination with the maximum grid-connected capacity; The stability conclusion in S2 is as follows: If the converter combination formed by sequence S is stable, the current sequence S is the maximum grid-connected capacity; If the converter combination formed by the sequence S is unstable, the converter with the smallest capacity is removed to form a new sequence, and the stability of the combined admittance corresponding to the new sequence is examined; if the new sequence is unstable, the converter with the smallest capacity in the current sequence is continued to be removed, and the stability of the remaining converters is examined until a stable combination is found. At the same time, the removed converter units are reconstructed into a sequence. The stable combination and the combination formed by the removed converters are expressed as shown in formula (3).

2. A method for evaluating the maximum grid-connected capacity of a converter based on resonance stability according to claim 1, It is characterized in that The specific process in S1 is: the converter is equivalent to a controlled current source parallel admittance, and the admittance is expressed as a frequency domain analytical expression containing a Laplace operator s.

3. A method for evaluating the maximum grid-connected capacity of a converter based on resonance stability according to claim 1, It is characterized in that The stability determination method in S3 is as follows: Pair combination j ,C j+1+p }(p=1,2,…,nj-1) to judge the stability. If the combination {S j ,C j+1+p } is stable, then the converter unit C j+1+p Add sequence S j ; If the combination {S j ,C j+1+p }If it is unstable, remove unit C j+1+p .

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