CN112994072B - Two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation - Google Patents

Two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation Download PDF

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CN112994072B
CN112994072B CN202110460054.6A CN202110460054A CN112994072B CN 112994072 B CN112994072 B CN 112994072B CN 202110460054 A CN202110460054 A CN 202110460054A CN 112994072 B CN112994072 B CN 112994072B
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贾科
杨彬
毕天姝
武文强
郑黎明
侯来运
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North China Electric Power University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract

The invention discloses a two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation, and belongs to the technical field of power system analysis. Firstly, solving the transient state of the VSC active power at the fault side according to the drop degree of the AC fault voltage; then constructing a differential equation of the direct-current voltage fault component based on the instantaneous power relation of the direct-current capacitor branch, and solving the direct-current voltage transient component; calculating an alternating current output current analytic expression of fault influence transmitted to the non-fault side VSC according to a non-fault side VSC control equation; and finally, according to the output current-fault side voltage-to-voltage function of the back-to-back VSC non-fault side, constructing a two-port equivalent model of the back-to-back VSC system during the alternating current fault. The invention can realize analysis and analysis of the propagation characteristics of alternating current faults through back-to-back VSC, can express the VSC back-to-back system by a nonlinear two-port equivalent model, and has important guiding significance for fault propagation analysis and protection research of the power electronic distribution network.

Description

Two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation
Technical Field
The invention relates to the technical field of power system analysis, in particular to a two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation.
Background
With the development of high-proportion power electronics of power systems, essential structural changes of traditional power networks are occurring. On one hand, novel equipment such as a new energy power supply and a microgrid serving as terminal equipment and connected to the power grid through a power electronic converter interface breaks through the dominant power supply structure of an electromagnetic generator of a traditional power system; on the other hand, the structure that flexible direct current transmission, direct current back-to-back distribution stations and the like are connected in series into an input distribution network also fundamentally changes the electrical transmission rule of an alternating current line. Series connection of alternating current-direct current-alternating current structures such as a VSC back-to-back system changes the fault propagation characteristics of alternating current short circuit faults along voltage drop of a power transmission and distribution line, so that alternating current fault characteristics are weakened, difficulties are brought to fault analysis and protection research, and analysis of the propagation of the alternating current fault characteristics through an alternating current-direct current-alternating current system is urgently needed.
At present, methods based on time domain differential equations, frequency domain transfer function response and the like exist for analyzing faults of the VSC grid-connected inverter, and analysis and calculation for temporary and stable faults are accurate. However, the research on the dynamic mechanism of the fault influence transmitted to the opposite-side converter station of the VSC back-to-back system through the direct-current system is not yet developed, and the reason is that the misunderstanding of 'the back-to-back system blocks the transmission of the alternating-current fault' and the bottleneck of high-order difficulty of a back-to-back system mathematical model for a long time in the industry exist. Simulation results show that under the condition that the non-fault side of the back-to-back system is controlled by the fixed direct current voltage, alternating current faults dynamically influence the output of VSC alternating current of the non-fault side through the direct current voltage, and therefore fault propagation different from the traditional line voltage drop mode is achieved. In summary, in order to clarify the influence of the back-to-back system on the ac fault propagation, it is necessary to analyze the ac fault propagation for the VSC back-to-back system, so as to provide a theoretical basis for the back-to-back system coordination control and the ac outgoing line protection design.
Disclosure of Invention
The invention aims to provide a two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation, which is characterized by comprising the following steps of:
step 1, solving transient active current output I of fault side VSC according to the AC fault voltage drop degree df And transient output active power P f ；
Step 2, according to the transient output active power P of the fault side VSC obtained in the step 1 f Constructing a differential equation of a direct-current voltage fault component based on the instantaneous power relation of the direct-current capacitor branch, and solving a direct-current voltage transient component delta u according to the operation condition before the fault dc ；
Step 3, obtaining the transient component delta u of the direct current voltage according to the step 2 dc Calculating an alternating current output value i of the VSC with the fault influence transmitted to the non-fault side based on the non-fault side VSC control equation d ；
Step 4, according to the alternating current output value i of the non-fault side VSC obtained in the step 3 d And constructing two-port equivalent models of back-to-back VSC during the alternating current fault.
Transient active current output I of fault side VSC in the step 1 df The analytical formula (D) is as follows:
Figure BDA0003041922980000021
wherein the content of the first and second substances,
Figure BDA0003041922980000022
determining the value of the VSC active current instruction value for the fault back backrest according to the voltage amplitude after the fault and a low voltage ride through criterion; i is 1 、I 2 Is an initial value of the transient component of the transient current, r 1 、r 2 For the decay constants, respectively:
Figure BDA0003041922980000023
wherein k is ip 、k ii Proportional and integral coefficients of a back-to-back VSC current inner loop PI controller respectively, L is a back-to-back VSC alternating current outlet filter reactance, i d0 Is the active current value before the fault;
solving the transient output active power P of the VSC at the fault side by combining the descending degree of the alternating voltage at the VSC end of the backrest after the symmetrical fault f Is composed of
Figure BDA0003041922980000024
Wherein, U f Is the fault side ac voltage amplitude.
The differential equation of the fault component of the direct current voltage in the step 2 is as follows:
Figure BDA0003041922980000025
wherein C is a DC capacitance value u dc0 Steady state value before DC voltage fault, Δ u dc Is a DC voltage transient component, U is a non-fault side AC voltage amplitude, k up 、k ui Proportional coefficients and integral coefficients of a non-fault side VSC voltage outer ring PI controller are respectively set;
solving the formula (4) to obtain:
Figure BDA0003041922980000026
wherein the initial value D of each decay exponential term 1 、D 2 A, B and damping constant λ 1 、λ 2 Comprises the following steps:
Figure BDA0003041922980000031
wherein, Δ P 0+ The active power difference between the two ends of the back-to-back VSC is 0+ moment when the fault occurs.
The alternating current output value i of the non-fault-side VSC in the step 3 d Comprises the following steps:
Figure BDA0003041922980000032
wherein i d0 For the value of the VSC AC current on the non-fault side before the fault occurs, F (U) f ) AC output value i representing non-fault-side VSC d For fault side AC voltage amplitude U f Function of, parameter D 1 、D 2 A, B all contain fault side AC voltage amplitude U f 。
The two-port equivalent model in the step 4 is a nonlinear voltage-controlled current source equivalent model, and the fault side alternating voltage amplitude U is taken f To control the voltage, the non-fault side VSC is equivalent to a controlled current source.
The invention has the beneficial effects that:
the modeling method can accurately analyze the direct-current voltage transient of the VSC back-to-back system under the alternating-current fault, accurately depict the influence of the alternating-current symmetrical fault on the alternating-current output of the non-fault side VSC through the VSC back-to-back system, and provide a theoretical basis for back-to-back system coordination control and alternating-current outgoing line protection design.
Drawings
Fig. 1 is a flowchart of a two-port equivalent modeling method suitable for transient calculation of a VSC back-to-back system fault in the present invention;
FIG. 2 is a schematic diagram of an equivalent model of two ports of a VSC back-to-back system constructed by the invention;
FIG. 3 is a diagram of a VSC back-to-back simulation model structure and a partial control topology;
FIG. 4 shows DC voltage u in VSC back-to-back simulation model dc Comparing the simulated value and the analytic value of the waveform;
FIG. 5 shows non-fault side output i in VSC back-to-back simulation model d Comparing the simulated value and the analytic value of the waveform;
fig. 6 is a comparison graph of simulated value and analyzed value waveforms of non-fault-side VSC alternating current three-phase current.
Detailed Description
The invention provides a two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation, which is further explained by combining the attached drawings and specific embodiments.
Fig. 1 is a flowchart of a two-port equivalent modeling method suitable for transient calculation of a VSC back-to-back system fault according to the present invention. The method comprises the following steps:
step 1, solving fault side transient state active current output I according to alternating current fault voltage drop degree df And transient output active power P f ；
Step 2, according to the fault side VSC transient output active power P obtained in the step 1 f Transient state analytic expression, constructing differential equation of DC voltage fault component based on instantaneous power relation of DC capacitor branch, and solving transient state component delta u of DC voltage according to operation condition before fault dc ；
Step 3, calculating the alternating current output dynamic state of the fault influence propagated to the non-fault side VSC based on the non-fault side VSC control equation according to the direct current voltage transient component analytic expression solved in the step 2;
and 4, constructing two-port equivalent models of the VSC back-to-back system during the alternating current fault according to the output analytic expression of the non-fault side of the VSC back-to-back system deduced in the step 3.
Fig. 2 is a schematic diagram of equivalent models of two ports of a VSC back-to-back system constructed in the invention, and a nonlinear voltage-controlled current source equivalent model is constructed according to a non-fault-side VSC alternating current analytic formula (7) and VSC control and operation parameters, wherein the outlet voltage (amplitude) of a fault-side VSC filter is control voltage, and the non-fault-side VSC is equivalent to a controlled current source.
Fig. 3 is a VSC back-to-back simulation model structure and a partial control topological diagram, which is a VSC back-to-back system for verifying the effectiveness of the method of the present invention, wherein the dc voltage level is ± 100kV, the ac side connects with a filter, and the high voltage ac system is connected through 110/230kV "Y- Δ" coupling transformer, and the specific structure and control parameters of the VSC converter are as follows:
Figure BDA0003041922980000041
when the symmetrical alternating current faults occur on the VSC back-to-back right-side alternating current connecting lines, the method provided by the invention is adopted to analyze the VSC back-to-back system and the opposite-side VSC alternating current output during the fault.
FIG. 4 shows DC voltage u in VSC back-to-back simulation model dc Comparing the simulation value with the analysis value waveform, it can be seen that the step 2 of the invention can realize accurate analysis of the VSC back-to-back system direct current voltage transient under the alternating current fault.
FIG. 5 shows i of an AC current output by a non-fault side (left side) VSC of a back-to-back system of the VSC after an AC fault occurs d And comparing the simulation and the analytic calculation waveforms of the components, wherein a, b and c in fig. 6 are comparison graphs of simulation values and analytic values of the non-fault-side VSC alternating current three-phase current. Therefore, the analytic formula provided by the invention can realize the accuracy of the influence of the AC symmetric fault on the AC output of the non-fault-side VSC through the VSC back-to-back systemAnd (6) engraving.
The present invention is not limited to the above embodiments, and any changes or substitutions that can be easily made by those skilled in the art within the technical scope of the present invention are also within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (5)

1. A two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation is characterized by comprising the following steps:
step 1, solving transient active current output I of fault side VSC according to the drop degree of alternating-current fault voltage df And transient output active power P f ；
Step 2, according to the transient output active power P of the fault side VSC obtained in the step 1 f Constructing a differential equation of a direct-current voltage fault component based on the instantaneous power relation of the direct-current capacitor branch, and solving a direct-current voltage transient component delta u according to the operation condition before the fault dc ；
Step 3, obtaining the transient component delta u of the direct current voltage according to the step 2 dc Calculating an alternating current output value i of the VSC with the fault influence transmitted to the non-fault side based on the non-fault side VSC control equation d ；
Step 4, according to the alternating current output value i of the non-fault side VSC obtained in the step 3 d And constructing two-port equivalent models of back-to-back VSC during the alternating current fault.
2. The two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation according to claim 1, wherein in step 1, the transient active current output I of the fault-side VSC is df The analytical formula (D) is as follows:
Figure FDA0003589319240000011
wherein the content of the first and second substances,
Figure FDA0003589319240000012
determining the value of the VSC active current instruction value for the back backrest of the fault according to the voltage amplitude value after the fault and the low-voltage ride-through criterion; i is 1 、I 2 Is an initial value of the transient component of the transient current, r 1 、r 2 For the decay constants, respectively:
Figure FDA0003589319240000013
wherein k is ip 、k ii Proportional and integral coefficients of a back-to-back VSC current inner loop PI controller respectively, L is a back-to-back VSC alternating current outlet filter reactance, i d0 Is the active current value before the fault;
solving the transient output active power P of the VSC at the fault side by combining the descending degree of the alternating voltage at the VSC end of the backrest after the symmetrical fault f Is composed of
Figure FDA0003589319240000014
Wherein, U f Is the fault side ac voltage amplitude.
3. The method for modeling a two-port equivalent suitable for back-to-back VSC fault transient calculation according to claim 1, wherein the differential equation of the dc voltage fault component in step 2 is:
Figure FDA0003589319240000021
wherein C is a DC capacitance value u dc0 Steady state value before DC voltage fault, Δ u dc Is the transient component of the DC voltage, U is the amplitude of the AC voltage at the non-fault side, k up 、k ui Proportional coefficients and integral coefficients of a non-fault side VSC voltage outer ring PI controller are respectively set; u shape f To the fault-side AC voltage amplitude, I 1 、I 2 Is an initial value of the transient component of the transient current, r 1 、r 2 Is the decay constant;
solving the formula (4) to obtain:
Figure FDA0003589319240000022
wherein the initial value D of each decay exponential term 1 、D 2 A, B and damping constant λ 1 、λ 2 Comprises the following steps:
Figure FDA0003589319240000023
wherein, Δ P 0+ The difference of active power at two ends of the back-to-back VSC is 0+ moment when the fault occurs.
4. The method for modeling a two-port equivalent suitable for back-to-back VSC fault transient calculation according to claim 1, wherein the output value i of the AC current of the non-fault VSC in step 3 is d Comprises the following steps:
Figure FDA0003589319240000024
wherein i d0 For the value of the VSC AC current on the non-fault side before the fault occurs, F (U) f ) AC output value i representing non-fault-side VSC d For fault side AC voltage amplitude U f Function of, parameter D 1 、D 2 A, B all contain fault side AC voltage amplitude U f ，D 1 、D 2 A, B are the initial values of the respective decay exponential terms, r 1 、r 2 、λ 1 、λ 2 As attenuation constant, k up 、k ui And the proportional and integral coefficients are respectively of the non-fault side VSC voltage outer ring PI controller.
5. The two-port equivalent modeling method suitable for back-to-back VSC fault transient calculation according to claim 1, wherein the two-port equivalent model in step 4 is a nonlinear voltage-controlled current source equivalent model, and the fault-side AC voltage amplitude U is taken f To control the voltage, the non-fault side VSC is equivalent to a controlled current source.
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