CN113725908B - Grid-connected static stability evaluation method based on converter limit transmission power - Google Patents
Grid-connected static stability evaluation method based on converter limit transmission power Download PDFInfo
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Abstract
A grid-connected static stability evaluation method based on converter limit transmission power comprises the following steps: the short-circuit capacity ratio SCR is used for representing the grid-connected power grid strength of the virtual synchronous converter system; establishing an equivalent circuit of converter power transmission, determining a converter power transmission vector relation, and calculating transmitted active power P; deriving a point P of maximum transmission power by differentiating the transmitted active power P max (ii) a Calculating to obtain SCR min And determining the required conditions of grid-connected static stability. Under the background of high power electronic penetration, the invention develops research aiming at the grid-connected stability mechanism of the power electronic converter under the condition of a weak power grid and provides theoretical support for grid-connected static stability evaluation based on the limit transmission power of the converter under the condition of the weak power grid. The method can be widely applied to the field of static stability evaluation of the power grid.
Description
Technical Field
The invention relates to the field of converter grid-connected static stability evaluation, in particular to a grid-connected static stability evaluation method based on converter limit transmission power.
Background
Under the drive of a double-carbon target, the source-load-storage direct-current characteristic is more obvious, and the traditional power distribution network is gradually developing to the advanced forms of high permeability, high access rate of renewable energy sources and alternating-current and direct-current hybrid connection of power electronic equipment. By adopting a networking mode mainly comprising a power electronic converter, the converter and the converter, and the converter and a power grid interact with each other, so that the internal mechanism of the stability of the power distribution network is changed, and a more complex oscillation phenomenon in a broadband range guided by the converter is presented.
In view of the fact that the internal mechanism of the stability of the alternating current-direct current hybrid power distribution network is changed under high power electronic penetration, the existing power frequency phasor method cannot meet the system analysis requirements, the stability problem of the alternating current-direct current hybrid power distribution network mainly based on a converter networking needs to be researched, the stability analysis is carried out on power electronic key networking equipment, a corresponding stability criterion is provided, and a theoretical basis is provided for the analysis, management and construction application of a future power electronic alternating current-direct current power distribution system.
According to the inherent consistency of a traditional synchronous motor and a power electronic converter on a synchronous mechanism, the stability problem of the power electronic converter can be divided into three categories of static stability, small disturbance stability and transient stability by utilizing the power angle stability theory of a traditional power system. Wherein, the static stability can be judged according to whether a stable static working point exists or not; the small disturbance stabilization refers to the problem that whether a system is unstable in small signals near a static working point, and a relatively mature small signal analysis method based on a linearization theory is formed at present; the transient instability problem of the system presents more complex nonlinear dynamic characteristics, and the stability analysis of the system is beyond the research scope of the linear analysis theory.
Whether a stable static operating point exists in the converter system is a necessary condition for the static stability of the system, and is also the basis for analyzing the small-disturbance stability of the system. Generally speaking, under the condition of strong power grid, the static stability condition can be met as long as the control design and various parameters of the converter system are set reasonably.
However, under the condition of weak grid, the impedance of a grid-side line is increased, the transmission power of a converter system is limited, the stable working area is greatly reduced, and the problem of static instability is easily caused. The problem of static instability of the system caused by the change of the impedance characteristic of the line is that non-periodic oscillation different from small disturbance instability appears on the characteristic outside the output of the system. Therefore, before studying the stability of the system in small disturbances, it is necessary to analyze the statically stable operating region of the system under weak grid conditions.
The reduction of the installed capacity of the traditional generator set and the access of a large number of power electronic devices influence the large power grid strength change of an alternating current and direct current power distribution system, and although the power electronic devices can provide certain inertia and damping support for a power grid through a virtual synchronous control technology, the trend that the power distribution network becomes weak is not enough changed. Under the background of high power electronic penetration, a converter system interacts with a power grid, and oscillation problems such as subsynchronization/supersynchronization, medium-frequency and high-frequency are easily caused. Under the condition of weak power grid, because the line impedance is not negligible, the interaction between the generator and the grid becomes stronger, the instability problem is more obvious, the generated energy and the stable operation thereof are influenced, and the service life of the generator set is reduced to a certain extent.
Meanwhile, the generation mechanism of the stability problems is different from that of the traditional power system, and the stability problems cannot be explained by using a traditional power system stability analysis method; if the traditional method is used for analysis, the accuracy is not enough.
Therefore, research needs to be carried out on the grid-connected stability mechanism of the power electronic converter under the weak grid condition.
Disclosure of Invention
Aiming at the problems, the invention provides a grid-connected static stability evaluation method based on converter limit transmission power, which improves the accuracy of analyzing a steady-state working area along with the change of line impedance characteristics under a weak power grid.
In order to achieve the purpose, the technical scheme of the invention is as follows: the method comprises the following steps:
s1, representing grid-connected power grid strength of a virtual synchronous converter system by using a short-circuit capacity ratio SCR;
s2, establishing an equivalent circuit of converter power transmission, and then determining a converter power transmission vector relation; secondly, calculating the transmitted active power P;
s3, deriving a maximum transmission power point P by differentiating the transmitted active power P max ；
S4, calculating the minimum SCR of the short-circuit capacity ratio SCR min And determining the required conditions of grid-connected static stability.
In the step S1, the grid strength of the virtual synchronous converter system grid connection is represented by a short-circuit capacity ratio as follows:
wherein SCR is short-circuit capacity ratio, P sc For short-circuit capacity, P, of the AC network sc ＝3U g 2 /Z g Wherein U is g To the effective value of the grid voltage, Z g Is the line impedance modulus;
P converter for rated transmission power, P, of the converter set The rated transmission power of the virtual synchronous converter is obtained.
In the step S2, the equivalent circuit includes a grid side and a converter side, the converter side includes a converter equivalent voltage source, and the grid side includes a line impedance.
According to the equivalent circuit, the vector relation of each variable in the converter power transmission equivalent circuit is established as follows:
in the formula of U c Is an equivalent voltage source of a current transformer, U g Is the effective value of the voltage of the power grid, I is the current, X g Is reactance, R g Is a resistance.
Calculating the transmitted active power P as:
in step S3, the reactance X in the active power P transmitted in step S2 is first set g Resistance R g By means of the line impedance modulus Z g And impedance angle
Is represented as follows:
then, the differentiation is performed to obtain:
the maximum transmission power point P max At the point of maximum extreme value of current I pole_max And (4) solving the value as:
in step S4, under the condition that the virtual synchronous converter system is statically stable, the maximum transmission power point should satisfy the following inequality:
SCR min calculated by the following formula:
meanwhile, the grid connection needs to meet the following conditions:
according to the method, based on the line impedance characteristics, the static stable working condition of the virtual synchronous converter system under the weak grid condition is calculated by deducing the maximum transmission power point and the minimum grid short-circuit capacity ratio of the converter, so that the analysis accuracy is improved, and the stable operation of the grid is ensured.
Meanwhile, the method provides theoretical support for grid-connected static stability evaluation based on the converter limit transmission power under the condition of weak power grid, lays a foundation for the follow-up small signal stability and transient stability research, and has certain significance.
Drawings
FIG. 1 is a flow chart of a converter grid-connected static stability evaluation method provided by the invention;
FIG. 2 is an equivalent circuit diagram of the power transmission of the converter provided by the present invention;
FIG. 3 is a power transmission vector diagram of a converter provided by the present invention;
FIG. 4 is a P-I curve for different line impedance characteristics provided by the present invention;
FIG. 5 is P provided by the present invention max And Z g 、
A graph of (a);
FIG. 6 is a schematic representation of the present invention
A curve;
FIG. 7 is a waveform diagram of the variation of transmission power provided by the present invention;
fig. 8 is a graph of (a) an output current waveform and (b) an output voltage waveform at a transmission power of 10kW provided by the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments of the present invention. It should be apparent that the described embodiments are only some of the embodiments of the present invention, and not all of them. All other embodiments, which can be derived by a person skilled in the art from the description of the embodiments of the invention given above, are within the scope of protection of the invention.
Therefore, the invention provides a grid-connected static stability evaluation method based on the limit transmission power of a converter. According to the invention, expressions of the maximum transmission power of the converter grid connection and the minimum power grid short-circuit capacity ratio are deduced according to the virtual synchronous converter system equivalent power transmission model, and researches are performed based on the change of line impedance characteristics under weak power grids and mainly aiming at the change conditions of the system static stability working area under different power grid strengths. Finally, the effectiveness of theoretical analysis is verified by simulation.
The present invention, as shown in fig. 1-8, comprises the following steps:
s1, representing the grid-connected power grid strength of a virtual synchronous converter system by using a short-circuit capacity ratio SCR;
s2, establishing an equivalent circuit of converter power transmission, determining a converter power transmission vector relation, calculating transmitted active power P and analyzing P-I curves under different impedance characteristics;
s3, deriving a maximum transmission power point P by differentiating the transmitted active power P max (maximum extreme point I) pole_max )；
S4, analyzing the influence of the power grid intensity change on the steady-state working area of the system, and calculating to obtain the SCR min And determining the required conditions of grid-connected static stability.
According to the method, firstly, the power grid strength is established through the step S1, then, the step S2 is used for calculating the transmission power of the converter according to a specific network topological structure, the maximum transmission power point is deduced in the step S3 based on the result, and finally, the grid-connected static stability condition is given out in the step S4.
Through the derivation of the steps, the static stability criterion can be rapidly and conveniently derived according to the specific converter grid-connected topological structure, and the quantitative analysis and stabilization mechanism is realized.
In the step S1, according to the related definition of evaluating the power grid strength in the power system, the grid strength of the virtual synchronous converter system grid connection can be represented by a short-circuit capacity ratio:
in the formula, SCR is short-circuit capacity ratio, and the larger the value is, the higher the grid-connected strength is, P sc For short-circuit capacity of AC mains, P sc ＝3U g 2 /Z g Wherein U is g As effective value of the grid voltage, Z g Is the line impedance modulus, P converter The rated transmission power of the converter is P corresponding to the rated transmission power in the virtual synchronous converter set 。
In step S2, an equivalent circuit of the converter power transmission is established, and a vector relation graph of each variable in the converter power transmission equivalent circuit is established according to a circuit theorem and in combination with each variable relation in the equivalent circuit graph, so that a vector relation of each variable can be obtained:
in the formula of U c Is an equivalent voltage source of a current transformer, U g Is the effective value of the voltage of the power grid, I is the current, X g Is reactance, R g Is a resistor.
Calculating the active power P of transmission as follows:
analysing P-I curves for transmission at different impedance characteristicsThe active power P is influenced by the line impedance characteristics, including the line impedance modulus Z g And angle of impedance
Through the correlation analysis of the static stability, the maximum transmission power point is known to be the key influencing the static stability.
In step S3, the following arrangement is firstly made for the active power P formula transmitted in step S2:
i.e. reactance X g And a resistor R g By means of the line impedance modulus Z g And impedance angle
Represents;
then, the following is obtained by differentiating:
maximum transmission power point P max By setting dP/dI =0, the pole of the P (I) function can be obtained. Maximum transmission power point P max Should be obtained at the point where the current I assumes the maximum extreme value, and therefore the maximum extreme value I pole_max At the time corresponding to the maximum transmission power point P max The value can be solved as:
the above formula shows that the line impedance modulus value Z g And angle of impedance
The change of the voltage can directly influence the maximum transmission power point of the virtual synchronous converter system, and can be realized by establishing P max And Z g And
the graphs of (a) were further observed and analyzed.
In step S4, the analysis shows that the line impedance module value Z is reduced along with the reduction of the power grid strength g The maximum transmission power point P of the virtual synchronous converter system is increased continuously max And rapidly decreases. Under the condition of ensuring the static stability of the virtual synchronous converter system, the maximum transmission power point should satisfy the following inequality:
SCR min calculated by the following formula
Further, according to the analysis of the above formula, in order to ensure the static stability of the virtual synchronous converter, the grid connection needs to satisfy the following conditions:
in the step S2, an equivalent circuit of the converter power transmission is established as shown in fig. 2. In the figure, U c And U g Setting the power grid side as a reference phase for the effective values of the equivalent voltage source and the power grid voltage of the converter, wherein the phase at the converter side is the output power angle theta of the active loop of the converter, and the line impedance
Wherein j is an imaginary unit. In order to simplify analysis, the default system power factor is 1, and at the moment, the output current and the voltage of the corresponding converter are in phase and can be represented by I & lttheta & gt.
In application, the corresponding equivalent circuit is properly adjusted according to a specific network topology structure.
Specifically, in the present embodiment, according to the circuit theorem, a vector relationship diagram of each variable in the converter power transmission equivalent circuit is established in combination with the relationship of each variable in fig. 2, as shown in fig. 3. From this a specific expression of the transmitted active power P is derived. The active power P and the current I are subjected to per unit processing, and a P-I curve under different line impedance characteristics can be established according to a P formula, as shown in fig. 4, it can be seen that the total of the active power P with the increase of the current I shows a trend of increasing first and then decreasing, and a maximum value P exists max . Angle of impedance
Increase of (i.e. R) g /X g The value becomes smaller, the maximum value P thereof max Will also decrease. In other words, the maximum transmission power of the system will increase as the resistive component of the line impedance increases, all other things being equal.
According to the correlation analysis of the system static stability, in the virtual synchronous converter system, because the quasi-power control mode is adopted, the condition for ensuring the system static stability is considered that the reference value of the system output active power cannot be larger than the maximum transmission power value, namely P max ≥P set . From the image point of view, when P in FIG. 4 set When the curve does not intersect with the P-I curve, the virtual synchronous converter system does not have a steady-state working point, and the system is statically unstable at the moment.
In step S3, the pole of the P (I) function can be obtained by setting dP/dI = 0. As can be seen from the observation of the dP/dI formula, the number of poles of the function is 4, and two poles of the function are positive roots and negative roots. The negative root can be directly cut off according to the function independent variable constraint condition, and meanwhile, the maximum transmission power point P can be known by combining the curve of the figure 4 max Should be obtained at the point where the current I assumes the maximum extreme value, and therefore assumes the maximum extreme value I pole_max At this time, the maximum transmission power point P is corresponded max . From P max The equation shows that the module value Z of the line impedance g And angle of impedance
The variation of the voltage can directly influence the maximum transmission power of the virtual synchronous converter system. P is max And Z g 、
As shown in fig. 5.
In the step S4, as the power grid strength decreases, the line impedance modulus Z g The maximum transmission power P of the virtual synchronous converter system is increased continuously max And rapidly decreases. At P max During the falling process, the value of the power control signal will continuously approach the power command value P set Until P appears set >P max In order to avoid the system from static instability, the transmission power command value must be reduced, which results in a smaller steady-state operating region of the system. Although the line impedance angle
The increase of (b) can raise the maximum transmission power of the system to a certain extent, but from the graph, the situation with a larger raising only occurs
I.e. the moment when the line impedance mainly exhibits resistive characteristics, this characteristic is difficult to satisfy in practical situations, while under other resistive or inductive conditions, the line impedance angle has little effect on increasing the maximum transmission power of the system. Therefore, in general, as the strength of the power grid decreases, the steady-state operation area of the virtual synchronous converter system will be reduced continuously. The SCR minimum versus line impedance angle is shown in fig. 6.
Specifically, combining the above analysis, the following can be concluded:
1. the static stability of the virtual synchronous converter system can be controlled by the maximum transmission power P max Reflection, P max Is related to the impedance characteristic by the line impedance modulus, Z g The larger, P max Smaller, impedance angle
The larger, P max The smaller;
2. as the grid strength weakens, the line impedance increases, P max And the system becomes smaller, the steady-state working area of the system is correspondingly reduced, and static instability is easily caused. The resistive component of the line impedance is beneficial to improving the power transmission capability under the condition of weak power grid and improving the static stability, but the effect is very little in the actual scene;
3. to ensure the static stability of the system, the short-circuit capacity ratio SCR has a minimum value, which is determined by the line impedance angle
Determination of the impedance angle
Smaller, SCR min The smaller.
With Z g ＝10.05、
Taking the line impedance parameter of (1) as an example, simulation analysis is performed, and the other parameters are all consistent with the above parameters. According to the condition that the grid connection needs to be met, the corresponding system working condition at the moment should meet P set ≤P max Condition that the SCR is not less than 1.98 and not less than 7.2kW.
First, the maximum transmission power point of the system under the weak grid condition is found. Setting the virtual synchronous converter system in the simulation model to be connected to the grid at 1.6s, wherein the initial transmission power is 5kW, and increasing the active power at the speed of 1kW/s at the simulation time of 3.5s, and the corresponding waveform change diagram is shown in FIG. 7. As can be seen, the transmission power peaks at a simulation time of about 5.9s and then no longer increases following a given power command value, and it can be seen that the system enters a destabilizing state when the transmission power exceeds the maximum transmission power. Therefore, the maximum transmission power of the system under the condition of the line parameters is about 7.05kW. According to P max Formula, at this time P max The theoretical value of the model is 7.2kW, and the simulation result is basically consistent.
Meanwhile, the operation condition that the system transmits 10kW active power is observed under the condition of the line impedance parameters, and theoretical analysis shows that the system is in a static instability state at the moment, and the simulation waveforms of the output voltage and the current are shown in figure 8. As can be seen from the figure, the output voltage and the current of the system show large-amplitude low-frequency (< 1 Hz) non-periodic oscillation, the oscillation characteristics are greatly different from small disturbance instability, and the system has the characteristics that the system can not be synchronized with an alternating current power grid all the time after the calculation of a controller, and the system does not have a stable working point.
Due to the adoption of the technical scheme, the invention has the following advantages:
1. on the basis of analyzing the instability type of the power electronic converter, the stability problem of static stability of a virtual synchronous converter system is researched based on an impedance analysis method. Under the power frequency, based on the line impedance characteristic, the static stable working condition of the virtual synchronous converter system under the weak grid condition is calculated by deducing the maximum transmission power point and the minimum grid short-circuit capacity ratio of the converter, and the correctness and the effectiveness of the provided index system are analyzed through examples.
2. The invention overcomes the defects of the prior art, the prior art only carries out power grid strength analysis aiming at the operating conditions that the traditional generating set has larger installed capacity occupation ratio and a small amount of power electronic equipment is accessed, however, under the background of high power electronic penetration, because the line impedance is not negligible, the interaction of the power grid is more intense, and the instability problem is more obvious. Because the generation mechanism of the stability problems is different from that of the traditional power system and cannot be explained by using the traditional power system stability analysis method, research needs to be carried out on the grid-connected stability mechanism of the power electronic converter under the weak grid condition. The method provides theoretical support for grid-connected static stability evaluation based on the converter limit transmission power under the condition of weak power grid, lays a foundation for the follow-up small signal stability and transient stability research, and has certain significance.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and so forth) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Claims (4)
1. A grid-connected static stability evaluation method based on converter limit transmission power is characterized by comprising the following steps:
s1, representing grid-connected power grid strength of a virtual synchronous converter system by using a short-circuit capacity ratio SCR;
s2, establishing an equivalent circuit of converter power transmission, and then determining a converter power transmission vector relation; secondly, calculating the transmitted active power P;
s3, deriving a maximum transmission power point P by differentiating the transmitted active power P max ；
S4, calculating the minimum SCR of the short-circuit capacity ratio SCR min Determining the required conditions of grid-connected static stability;
in the step S1, the grid strength of the virtual synchronous converter system grid connection is expressed by a short-circuit capacity ratio as follows:
wherein SCR is short-circuit capacity ratio, P sc For short-circuit capacity of AC mains, P sc ＝3U g 2 /Z g Wherein U is g As effective value of the grid voltage, Z g Is the line impedance modulus;
P converter for rated transmission power, P, of the converter set The rated transmission power of the virtual synchronous converter is set;
in the step S2, the equivalent circuit comprises a power grid side and a converter side, the converter side comprises a converter equivalent voltage source, and the power grid side comprises line impedance;
according to the equivalent circuit, establishing a vector relation of each variable in the converter power transmission equivalent circuit as follows:
in the formula of U c Is an equivalent voltage source of a converter, U g Is the effective value of the voltage of the power grid, I is the current, X g Is reactance, R g Is a resistance.
2. The grid-connected static stability assessment method based on converter limit transmission power as claimed in claim 1, wherein the active power P of the transmission is calculated as:
3. the grid-connected static stability assessment method based on converter limit transmission power as claimed in claim 2, wherein in step S3, the reactance X in the active power P transmitted in step S2 is first estimated g Resistance R g By means of the line impedance modulus Z g And impedance angle
Is represented as follows:
then, the following is obtained by differentiating:
the maximum transmission power point P max At the point of maximum extreme value I of current I pole_max The value is solved as:
4. the grid-connected static stability assessment method based on the converter limit transmission power as claimed in claim 1, wherein in step S4, under the condition of static stability of the virtual synchronous converter system, the maximum transmission power point should satisfy the following inequality:
SCR min calculated by the following formula:
meanwhile, the grid connection needs to meet the following conditions:
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