Power grid, power grid system and equivalent method and model optimization method thereof

Abstract

The present disclosure relates to the field of electric power, and discloses a power grid, a power grid system, and an equivalent method and a model optimization method thereof, including performing equivalent on converters in a system to obtain equivalent converters; when the number of buses in the system = the number of converters after equivalent, all converters connected to the buses are coherent converters; when the number of buses in the system > the number of converters after equivalent, let k = the number of buses in the system ‑ the number of converters after equivalent, and compare the impedance drop ratios between the equivalent converters between any buses respectively, the equivalent converters corresponding to the smallest k impedance drop ratios are coherent converters, and all converters on the buses connected to the two equivalent converters are coherent converters; so that the simulation time consumption can be effectively reduced, the accuracy of the equivalent model is improved, and a feasible technical solution is provided for realizing equivalent order reduction of phase-locked synchronous converters.

Description

Technical Field

[0001] The present disclosure relates to the field of electric power, and in particular to a power grid, a power grid system, and an equivalent method and a model optimization method thereof. Background Art

[0002] With the increasing problems of energy shortage and environmental pollution, a high proportion of new energy power generation units represented by wind power and photovoltaic power are connected to the power grid. The voltage source converter is the core part of the new energy power generation unit, and is often synchronized with the power grid through a phase-locked loop. In a weak power grid, the anti-disturbance capability of the phase-locked loop with the access point voltage as the input is weak, and transient synchronization instability is prone to occur, posing a serious threat to the safe and stable operation of the power grid. When multiple converters are operated in parallel, their transient response characteristics are more complex, and transient simulation research of multiple converters is required.

[0003] However, the switching frequency of the converter based on pulse width modulation is often high, and the converter often uses multiple control loops to achieve reliable power conversion, which makes the multi-converter power grid system model high-order and wide-response time scale, causing a large computational burden on the simulation software and resulting in a long simulation time. Summary of the invention

[0004] In a first aspect, the present disclosure proposes an equivalent method for a power grid system to improve the simulation efficiency of a multi-converter power grid system model.

[0005] In order to achieve the above-mentioned disclosure purpose, the technical solutions adopted in this disclosure are as follows:

[0006] An equivalent method for a power grid system is used to perform equivalent operations on the converters in the system and obtain the equivalent converters;

[0007] When the number of buses in the system = the number of converters after equalization, all converters connected to the buses are synchronous converters;

[0008] When the number of busbars in the system is greater than the number of converters after equalization, let k = the number of busbars in the system - the number of converters after equalization, and compare the impedance drop ratio between the converters after equalization between any busbars. The converters after equalization corresponding to the smallest k impedance drop ratios are synchronized converters, and all converters on the busbars connected to the two converters after equalization are synchronized converters.

[0009] In some disclosures, a converter-based multi-converter power grid system model is established, and the impedance voltage drop of the converters in the system is obtained according to the established multi-converter power grid system model.

[0010] In some disclosures, the coherent equivalence method is used to equivalence the converters in the system.

[0011] In a second aspect, the present disclosure proposes a power grid system model optimization method to improve the simulation efficiency of a multi-converter power grid system model.

[0012] A method for optimizing a power grid system model includes the equivalent method of the first aspect.

[0013] On the third aspect, the present disclosure proposes a power grid system to improve the simulation efficiency of a multi-converter power grid system model.

[0014] The power grid system comprises a processing module, wherein the processing module performs equalization on converters in the system to obtain equalized converters;

[0015] When the number of buses in the system = the number of converters after equalization, all converters connected to the buses are synchronous converters;

[0016] When the number of busbars in the system is greater than the number of converters after equalization, let k = the number of busbars in the system - the number of converters after equalization, and compare the impedance drop ratio between the converters after equalization between any busbars. The converters after equalization corresponding to the smallest k impedance drop ratios are synchronized converters, and all converters on the busbars connected to the two converters after equalization are synchronized converters.

[0017] In some disclosures, a multi-converter power grid system model module is included, which establishes a multi-converter power grid system model based on the converter, and obtains the impedance voltage drop of the converter in the system according to the established multi-converter power grid system model.

[0018] In some disclosures, an optimization module is included, which uses a coherent equivalence method to equivalence converters in the system.

[0019] In a fourth aspect, the present disclosure proposes a power grid to improve the simulation efficiency of a multi-converter power grid system model.

[0020] A power grid, comprising a storage medium, wherein the storage medium stores a program, and when the program is executed, the equivalent method of the first aspect or the power grid system model optimization method of the second aspect is implemented;

[0021] Or, a power grid, comprising a system module, wherein the system module comprises the power grid system of the third aspect.

[0022] Beneficial effects of the present disclosure:

[0023] The present disclosure improves the simulation efficiency of a multi-converter power grid system model. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present disclosure or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is the control implementation diagram of the phase-locked synchronous converter;

[0026] Figure 2 It is the topological structure diagram of the multi-converter system;

[0027] Figure 3 It is the interactive coupling model diagram of the multi-converter system;

[0028] Figure 4 It is the transient equivalent swing model diagram of the multi-converter system;

[0029] Figure 5 A flowchart of the implementation steps of the disclosed method;

[0030] Figure 6 This is a simulation comparison chart of the coherent equivalent model based on impedance voltage drop and the original model. DETAILED DESCRIPTION

[0031] The present disclosure is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the embodiments are only used to illustrate the present disclosure and are not used to limit the scope of protection of the present disclosure. In addition, it should be understood that after reading the contents described in the present disclosure, those skilled in the art may make various changes or modifications to the present disclosure, and these equivalent forms also fall within the scope of protection defined by the present disclosure.

[0032] For ease of understanding, the present disclosure establishes the following conventional power system to illustrate the concept of the present disclosure. The converter hardware structure and control structure involved in the power system are as follows: Figure 1 As shown. The converter hardware structure includes a DC micro-source and a DC / AC converter for power conversion. The DC micro-source includes but is not limited to DC power sources such as energy storage batteries and photovoltaic panels. The DC input is converted into AC power by the DC / AC converter and connected to the common bus. The output end of the DC / AC converter is connected to the LC filter inductor and capacitor; the power system control structure mainly includes a phase-locked loop synchronization module, a current control module and a pulse width modulation / drive module. The control phase angle is obtained by collecting the converter port voltage input into the phase-locked loop synchronization module; the current control module controls the output current amplitude of the converter, and achieves zero-difference tracking through the PI control loop by giving a current reference value; the pulse width modulation and / or drive module generates a modulation signal to drive switching devices such as IGBT.

[0033] Step 1: The disclosed embodiment first obtains the number, topology and system parameters of the multi-converter system and establishes a multi-converter power grid system model. Figure 1 As shown in Figure 2, the topology of the multi-converter system is as follows: Figure 2 Before establishing the multi-converter power grid system model, considering that the transient response of the converter is mainly determined by the dynamics of the phase-locked loop, the current control loop can quickly reach a stable state, that is, the converter output current I dq Equal to the converter reference current value I dqref At this time, the phase-locked synchronous converter i multi-converter power grid system model is established as follows:

[0034]

[0035] Among them, Z si =Z ti +Z i +Z g =Z si ∠θ si is the total equivalent impedance between converter i and the grid, U ti is the terminal voltage of converter i. Φ represents the set of converters with the same bus, Ω represents the set of all converters outside the bus, and U g is the grid voltage vector, I is the converter output current vector, Z g is the grid impedance, Z ti is the line impedance between the converter and the busbar, Z i is the line impedance from different busbars to the common busbar.

[0036] Transform the above formula to the coordinate system of phase-locked loop i, and we get:

[0037]

[0038] Among them, U tqi It's U ti The q-axis component, θ si ,θ ti With θ zg Z si ,(Z i +Z g ) and Z g The impedance angle, I i and is the output current amplitude and output current phase angle of converter i. ji =θ pllj -θ plli is the phase angle difference between phase-locked loop i and phase-locked loop j on the same bus, δ ki =θ pllk -θ plliis the phase angle difference between phase-locked loop k and phase-locked loop j on different buses, δ i =θ plli -θ g is the phase angle difference between the phase-locked loop i and the grid voltage, and is defined as the synchronization angle between the converter i and the grid (hereinafter referred to as the synchronization angle). The first three terms in equation (2) are respectively denoted as U zqii , U zqji with U zqki .

[0039] The phase-locked loop motion equation is:

[0040] θ plli =∫(ω n +K pi U tqi +K ii ∫U tqi dt)dt (3)

[0041] Among them, K pi , K ii PLL i The proportional coefficient and integral coefficient, ω n is the rated angular velocity, θ plli is the phase-locked loop phase angle of the i converter.

[0042] Equations (2) and (3) constitute the transient model of the multi-converter system, as Figure 3 As shown. Figure 3 It can be seen that in the multi-converter system, the phase-locked loop input includes the impedance voltage drop term generated by the converter itself, other converters on the same bus, and converters on different buses. The three correspond to U zqii , U zqji with U zqki Among them, U zqji with U zqki It is mainly related to the phase angle difference of the output current phasor between the converters, that is, the phase angle of the current injected by the converter during the fault. Synchronous angle phase difference δ between converters ji , δ ki related.

[0043] It is worth noting that the converter can also have different model forms, such as switch model and average model. The switch model keeps the switching dynamics of the converter and is described by discrete equations, with a higher model order; the average model ignores the switching dynamics of the converter and is described by differential algebraic equations, with a lower order. This technology can be applied to both the switch model and the average model of the converter, but both use the converter as the basic unit for aggregation and equivalence.

[0044] Step 2: From the multi-converter power grid system model established in step 1, it can be seen that the coherence characteristics of the converter are mainly determined by the impedance voltage drop generated by the converter injection current. Therefore, in this disclosure, the impedance voltage drop of the converter is used as a measure to distinguish the differences between converters. The impedance voltage drop ratio between any two converters is obtained as δ ij

[0045]

[0046] According to this characteristic, the impedance voltage drop ratio δ between any two converters in the multi-converter system can be calculated respectively: ij As shown in Table 1 below.

[0047] Table 1. Calculation results of homology identification

[0048]

[0049] Step 3: Perform a synchronous equivalence criterion based on the impedance drop ratio between any two converters obtained in step 2. The specific process is as follows: the number of buses in the system is n, and the number of converters after the system is equivalent is required to be m, where n is greater than or equal to m. When n=m, it can be seen from formula (3) that the converters on the same bus are synchronous converters, and the converters on different buses are not synchronous converters. When n>m, let k=nm. Compare the impedance drop ratio δ of the converter on any bus d and the converter on bus e respectively. ij , select the k smallest δ in the impedance drop ratio ij At this time, the smallest k δ ij The corresponding converters i and j are synchronous converters, and all converters on the bus to which the two converters are connected are synchronous converters. For example, if converter i belongs to bus d and converter j belongs to bus e, then the converters in bus d and bus e are synchronous converters. Otherwise, they are not synchronous converters.

[0050] In this example, the number of buses is 3, and the number of converters required after equalization is 2, 3>2. Bus 1 has converters 1, 2, and 3; bus 2 has converters 4 and 5; bus 3 has converters 6 and 7. The delta between converters 1-3 and 4-5 is ij The minimum value is 1.35; the delta between converter 1-3 and converter 6-7 ij The minimum value is 1.52; the delta between converter 4-5 and converter 6-7 ij The minimum value is 1.1. 1.1 is the minimum value, that is, converter 4-5 and converter 6-7 are synchronous converters, and converter 1-3 is a synchronous converter.

[0051] It is worth noting that the synchronization equivalence is a kind of dynamic equivalence method, which is an equivalence method to preserve the dynamic power angle of the converter as much as possible. This method has similarities with other methods in dynamic equivalence, but also has differences. Among them, the core difference is that the synchronization equivalence method can take into account the power angle characteristics to the greatest extent, so it is used as a synchronization criterion for the converter.

[0052] Step 4: According to the synchronization judgment result of step 3, the circuit equivalence and control parameter equivalence of the synchronous converter are performed. Substituting equation (2) into equation (3) and rewriting it in the form of synchronous generator swing equation, the motion equation of the multi-converter system is obtained as follows:

[0053]

[0054] in

[0055]

[0056] Among them, M i is the equivalent inertia constant, D ki , D gi Respectively with the angular velocity difference Δω between the converters ki , angular velocity difference between converter and grid Δω i It is related to the damping coefficient. n is the number of converters with the same busbar, L ∑i is the total equivalent inductance between converter i and the grid. tqi It is equivalent to the unbalanced active power of the synchronous generator, which is the unbalanced voltage input of the phase-locked loop and is the essential reason for the transient synchronization instability of the phase-locked loop. The block diagram of the equivalent swing equation of the multi-converter system is as follows: Figure 4 shown.

[0057] After the synchronous converter is identified, the aggregation and order reduction process should ensure that before and after the aggregation: 1) the output power remains unchanged; 2) the swing curve is approximate. Therefore, the aggregation calculation includes two parts: circuit parameter aggregation and control parameter aggregation, which are described in detail below:

[0058] (1) Circuit parameter aggregation

[0059] Equivalent impedance Z between converter and busbar ti The equivalent impedance Zi between the busbar and the PCC point is the same in terms of voltage drop expression, so Z is used below. ri To represent Z ti or Z i , then Z ri The upper pressure drop is ΔU ri =I i Z ri (i=1,2,…,n i ), the average impedance voltage drop is:

[0060]

[0061] where c i is the weight coefficient of converter i, which is equal to the ratio of the capacity of converter i to the total capacity of the coherence group.

[0062] Similarly, the equivalent impedance Z between the equivalent converter and the busbar is teq Or the equivalent impedance Z' between the busbar and the PCC point after the equivalent eq Use Z req If Z req The upper pressure drop is:

[0063] ΔU eq =I eq Z req (8)

[0064] In order to ensure that the output power remains unchanged before and after polymerization, the equations (7) and (8) are combined with I eq =∑I i =∑c i , the equivalent line impedance can be obtained:

[0065]

[0066] (2) Control parameter aggregation

[0067] Taking the weighted average of the converter swing equation, equation (5), we get:

[0068]

[0069] In order to reflect the collective motion of the synchronous converter group, with the help of the concept of center of mass, the center of inertia (COI) is used here to define the synchronization angle and angular velocity difference of the equivalent converter, as follows:

[0070]

[0071] Therefore, formula (10) can be rewritten as:

[0072]

[0073] The equivalent converter swing equation should be as follows:

[0074]

[0075] Since we only need to solve the equivalent phase-locked loop PI parameter K peq , K ieq , so using the equality of the first and third corresponding coefficients in the above formula, and combining with formula (6), we can get:

[0076]

[0077] Figure 6 The comparison of the power angle curve simulation results of the embodiment before and after the equivalent value of the multi-converter system when the grid voltage drops to 30% in 1s and the fault is removed in 1.1s is given. It can be observed from the figure that the synchronization characteristics of the two-converter model after the equivalent value are consistent with the original seven-converter model. The results verify the effectiveness and accuracy of the proposed synchronization equivalent method based on impedance voltage drop.

[0078] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "illustrative embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0079] Although embodiments of the present disclosure have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions and alterations may be made to the embodiments without departing from the principles and spirit of the present disclosure, the scope of which is defined by the claims and their equivalents.

Claims

1. An equivalent method for a power grid system, comprising a plurality of converters, characterized in that: Performing equalization on the current transformers in the system to obtain the equalized current transformers; When the number of buses in the system = the number of converters after equalization, all converters connected to the same bus are synchronized converters; When the number of buses in the system is greater than the number of converters after equalization, let k = the number of buses in the system - the number of converters after equalization, and compare the impedance voltage drop ratios between the converters after equalization. The converters after equalization corresponding to the smallest k impedance voltage drop ratios are synchronous converters, and all converters on the buses connected to the two converters after equalization are synchronous converters. According to the synchronization judgment result, the circuit equivalence and control parameter equivalence of the synchronous converter are carried out.

2. The equivalent method according to claim 1, characterized in that: A converter-based multi-converter power grid system model is established, and the impedance voltage drop of the converter in the system is obtained according to the established multi-converter power grid system model.

3. The equivalent method according to claim 2, characterized in that: The converters in the system are equivalent using the coherent equivalent method.

4. A method for optimizing a power grid system model, characterized in that: Including the equivalent method described in any one of claims 1-3.

5. A power grid system, characterized in that: It includes a processing module, wherein the processing module performs equalization on the converters in the system to obtain the equalized converters; When the number of buses in the system = the number of converters after equalization, all converters connected to the same bus are synchronized converters; When the number of buses in the system is greater than the number of converters after equalization, let k = the number of buses in the system - the number of converters after equalization, and compare the impedance voltage drop ratios between the converters after equalization. The converters after equalization corresponding to the smallest k impedance voltage drop ratios are synchronous converters, and all converters on the buses connected to the two converters after equalization are synchronous converters. According to the synchronization judgment result, the circuit equivalence and control parameter equivalence of the synchronous converter are carried out.

6. The power grid system according to claim 5, characterized in that: It comprises a multi-converter power grid system model module, which establishes a multi-converter power grid system model based on the converter, and obtains the impedance voltage drop of the converter in the system according to the established multi-converter power grid system model.

7. The power grid system according to claim 6, characterized in that: The invention comprises an optimization module, wherein the optimization module uses a coherent equivalence method to equivalence the converters in the system.

8. A power grid, characterized in that: It includes a storage medium storing a program, and when the program is executed, it implements the equivalent method described in any one of claims 1 to 3, or the power grid system model optimization method described in claim 4.

9. A power grid, characterized in that: It comprises a system module, and the system module comprises the power grid system described in any one of claims 5-7.

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