CN106407494B - MMC-based bipolar short-circuit fault current calculation method for HVDC system - Google Patents
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- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
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- H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
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Abstract
The invention belongs to the technical field of direct-current power transmission of a power system, and particularly relates to a bipolar short-circuit fault current calculation method of an HVDC system based on MMC, which comprises the following steps: establishing a double-end MMC-HVDC operation system, and acquiring element parameters of a power transmission system; setting bipolar short-circuit faults of the direct-current system at different moments and fault points; according to the fault current component and the discharge current generation mechanism, the discharge process is equivalent, the equivalent parameters are introduced into the adjustment factor, and the discharge current calculation formula is simplified; based on an Lsqcurvefit function of MATLAB, an adjusting factor is introduced for the first time to fit a curve of the discharge current, and a final discharge current calculation formula is obtained. And the superiority of the fitting effect is described by the determinable coefficient, and the calculation formula can provide reference basis for equipment model selection, fault principle design and fault fixed value setting.
Description
Technical Field
The invention belongs to the technical field of direct current transmission of a power system, and particularly relates to a bipolar short-circuit fault current calculation method of an HVDC (high voltage direct current transmission) system based on an MMC (modular multilevel converter).
Background
The flexible direct current transmission is one of key technologies of the smart grid, can realize the rapid independent decoupling control of active power and reactive power by virtue of the flexible direct current transmission, reduces the harmonic content, has no problem of commutation failure, and is widely applied to the fields of distributed power generation grid connection, island power supply, asynchronous interconnection of alternating current systems, multi-terminal direct current transmission, urban distribution network capacity increase and the like. The Modular Multilevel Converter-type High Voltage Direct Current (MMC-HVDC) is used as a novel Multilevel Converter topology, and a method of serially connecting Sub-modules (Sub-modules SM) is used, so that Direct serial connection of a large number of switching devices is avoided, and the requirement on the consistency of the switching devices is lowered. Because the essence difference in structure, MMC compares with two levels of tradition, three level VSC transverter and has many characteristics: the high-voltage-output-voltage waveform has the advantages of high quality, small impact on the converter caused by the fault of the direct-current side, low requirement on switching devices, simple voltage-sharing of the devices, capability of working in a three-phase unbalanced state of an alternating-current system and the like.
A direct current bipolar short circuit fault is one of the faults within an MMC-HVDC converter station with serious consequences. Research shows that after a fault occurs, the current flowing through the submodule before the converter station is locked is the superposition of three-phase short-circuit current fed by an alternating current system and discharge current of a submodule capacitor; the flowing current in the submodule after locking is the superposition of the short-circuit current and the valve reactor follow current. Due to the late appearance of the MMC topological structure, the fault research of the MMC mainly focuses on the control strategy and simulation analysis during the fault of an alternating current system at present, the quantitative calculation of fault current is rarely researched, and the calculation accuracy of part of the researched fault current is not high.
Disclosure of Invention
In order to make up the defects of MMC-HVDC bipolar short-circuit fault current calculation at present, the invention provides a bipolar short-circuit fault current calculation method of an HVDC system based on MMC, which comprises the following steps of:
step 3, collecting operation parameters of the power transmission system, equating the discharge process according to the fault current component and the discharge current generation mechanism, and calculating corresponding equivalent parameters;
step 4, introducing an adjusting factor into the equivalent parameter to simplify a discharge current calculation formula;
step 5, carrying out a large number of simulation experiments according to the step 2, and extracting and storing simulation data;
and 6, determining a fitting function according to the target function, searching for an optimal regulating factor, and obtaining a final discharge current calculation formula.
The element parameters of the power transmission system include: the bridge arm equivalent resistance comprises the number of bridge arm sub-modules, a sub-module capacitance value, a bridge arm reactance value, a bridge arm equivalent resistance and a power transmission line parameter.
The operating parameters of the power transmission system include: the time of occurrence of the short-circuit fault, the dc outlet voltage of each converter station, the bridge arm current and its ac and dc components, and the position of the short-circuit fault point.
Step 4 introduces a tuning factor a, i.e. C, on the equivalent of the discharge capacitanceeq＝aC0and/N, the simplified calculation formula of the discharge current i is as follows:
wherein:
t is time, L0Is the reactance value of the bridge arm, C0Is the sub-module capacitance value, LeqIs equivalent to bridge arm reactance, CeqIs the equivalent of sub-module capacitor, N is the number of bridge arm sub-modules, R is the equivalent resistance of bridge arm, I0For the initial current at the moment of failure, Udcτ is a time constant and ω is a natural oscillation angular frequency for the converter station dc outlet voltage.
Step 6, implementing nonlinear fitting by using an Lsqcurvefit function of MATLAB, specifically including:
initial discharge current I at different fault moments0Next, a simulation value y is setiThe objective function i (t, a) is such that the sum of the squares of the deviations of the calculated value of the objective function at point t and the simulated value is minimized, i.e. the objective function i (t, a) satisfies the following formula:
where a is the adjustment factor to be determined, i is 1,2, … …, n, n is the simulation times, and
is the optimum adjustment factor, y, determined by the least squares methodiAs a simulated value, M is the sum of the squares of the deviations.
The invention has the beneficial effects that: according to the invention, from the angle of a mechanism of generating the fault current, a nonlinear fitting algorithm is combined, the adjustment factor is introduced, and the practical calculation method of the fault current is deduced, so that the error between a calculated value and a simulated value of the fault current can be reduced to be very small according to the change rule of the capacitor discharge current along with time after the fault is more accurately described according to circuit parameters and before the converter station is locked, and a reference basis is provided for the selection of equipment parameters and a protection strategy of the converter station in engineering.
Drawings
FIG. 1 is a capacitive current discharge path;
FIG. 2 is an equivalent discharge circuit;
FIG. 3 is a MMC-HVDC bipolar short circuit fault system diagram;
FIG. 4 is I0Fitting and comparing under three different equivalent modes when the equivalent value is 0.14 kA;
FIG. 5 is a graph showing the variation of M with a;
FIG. 6 is a graph showing the variation of coefficient and a;
Detailed Description
The embodiments are described in detail below with reference to the accompanying drawings. The invention provides a bipolar short-circuit fault current calculation method of an HVDC system based on MMC.
First, a bipolar short-circuit failure mechanism and an equivalent discharge circuit are analyzed.
(1) And when the bridge arm runs in a steady state, acquiring bridge arm current data, and determining the composition of alternating current side current and direct current when the bridge arm current is running.
(2) In the event of a fault, the sub-module capacitors discharge rapidly, similar to a three-phase short circuit occurring for an ac system. And (3) superposition of short-time current of bridge arm current and capacitor discharge current of the sub-module.
(3) According to the sub-module input and output rule, taking phase A as an example, the discharge circuit is shown in FIG. 1.
(4) The equivalent simplification processing is performed on fig. 1, and it can be seen that the discharge circuit is equivalent to fig. 2, and it can be seen that the discharge circuit is a second-order discharge circuit. Wherein L iseq＝2L0,Ceq＝C0/N
The capacitor voltage and discharge current can be calculated from fig. 2.
Wherein: p is a radical of1、p2Is a characteristic root
Time of failure, initial state of circuit
The following can be obtained:
wherein:
ω0is the resonant angular frequency.
Under normal circumstances
Therefore:
to accurately describe the discharge process, it is obviously necessary to know the magnitude of the discharge voltage and the magnitude of the discharge current at each moment of controller action. The whole discharging process is the continuous superposition of the discharging process when the controller acts once. Because the control action frequency is very high, the complete power generation process expression is a very huge piecewise function taking the action period T of the controller as a time period. Such as (7)
Considering that the discharging process is rapid and the frequency of the controller is high, the discharging process can be equivalent within the error tolerance range for the convenience of calculation. Equating Ceq to C0the/N is called Model1, the equivalent is 2C0the/N is called Model 2. In terms of discharge principle, only N capacitors are discharging at any one time, and the Model1 can be understood. However, due to the action of the capacitance-voltage balance control strategy, the controller puts the module with higher capacitance voltage into discharging every time the controller acts, that is, the discharging voltage is in shear, and the equivalent discharging process of Model1 is weaker than the actual discharging process. Model2 considers that the switching frequency is high, the switching-in and switching-out frequency of the sub-modules is high, and the sub-module groups adjacent to each other twice can be considered to be discharged simultaneously, so that the two groups of module groups are approximately considered to be discharged in parallel. The Model2 discharge is generally stronger than the actual discharge process. In view of the submodule activation rules, there may also be situations in which the actual discharge process is weaker. It can be said that Model1 and Model2 provide the lower and upper boundaries of the discharge process, respectively.
For more reasonable description of the discharge process, a regulatory factor a is introduced, i.e. Ceq is equivalent to aC0N, as Model 3. Determining the appropriate adjustment factor a is key to ensuring the accuracy of the fault current calculation.
The formula (7) is huge and has a large calculation amount, so that the formula has no great significance for engineering calculation. The invention simplifies the equation (7) by introducing a regulatory factor. According to the conduction rule of the sub-modules and the power generation loop, at the moment of each controller action, the discharge path is unchanged, the loop current is not changed suddenly, and only the capacitor voltage is changed suddenly. Therefore, the first and second electrodes are formed on the substrate,the invention introduces a regulating factor a, namely C, on the equivalent value of the discharge capacitoreq＝aC0N, then the discharge current calculation formula is transformed as the following formula (8)
In engineering practice, the converter station submodule is emergently locked after the bipolar short-circuit fault occurs, and then the alternating current circuit breaker is tripped. The fault current after the submodule is locked is in an attenuation state, and the research significance is not large, so the invention mainly researches the fault impact current before the submodule is locked.
Fig. 3 shows a double-ended MMC-HVDC system, which calculates the fault current of the bridge arm by the occurrence of a double-pole short-circuit fault at the outlet of the converter station. The main circuit parameters of a two-terminal system are as follows:
L0＝15mH,C0＝6000μF,N＝20,Udc＝40,R＝0.6Ω
Vac＝10kV,PT＝40MW,P0＝20MW,Q0＝0
the double-end system respectively adopts constant direct current voltage and constant power control. The converter is locked after 6ms is set in a simulation mode, so that the calculation formula of the discharge current before 6ms is researched.
(1) Setting bipolar short-circuit faults at different moments, extracting PSCAD simulation data to obtain a discharge current value within 6ms, and recording an initial current I at the moment of the fault0
(2) And (3) adopting an Lsqcurvefit function of MATLAB to realize nonlinear fitting, and setting an initial value of a to be 1 (or 2). Under a large number of simulation calculations, an optimal adjustment factor is found to minimize the error between the calculated value of equation (8) and the simulated value. For space reasons, the following 4 sets of comparative data are shown in Table 1 below:
TABLE 1 exemplary calculated values and simulated value error data
| I0 | Model 1 | Model2 | a | Model 3 |
| 0.14846 | 41.00 | 13.60 | 1.49 | 0.436 |
| 0.1441 | 22.81 | 28.15 | 1.33 | 0.489 |
| 0.1551 | 44.3061 | 11.7936 | 1.52 | 0.33 |
| 0.1355 | 33.405 | 18.1373 | 1.43 | 0.399 |
In the above table, Model1 and Model2 represent M values at a ═ 1 and a ═ 2, respectively, and Model3 represents the M value at the corresponding adjustment factor a. The initial value and the corresponding a value fluctuate slightly due to the failure of the ring current suppression to suppress the ring current completely. And selecting a proper value of a within the error allowable range to achieve a better fitting effect. Obviously, when the value of the adjustment factor a is 1.3-1.5, the M value is small, and the actual discharge process can be well fitted. Through verification, when a is 1.3-1.5, M is 0.3-0.5, and can meet the requirement within the error allowable range.
(3) Introduction of block coefficient to judge fitting effect
Coefficient of determinability
Wherein:
R2a value closer to 1 indicates a better fit.
(4) Taking the initial current value of 0.14kA as an example, Lsqcurvefit is used for fitting, and the calculation result shows that when a is 1.4, the fitting effect is best. FIG. 4 is a comparison graph of the Model3, Model1 and Model2 when a is 1.4, and it can be seen that the Model3 has better fitting effect. In order to show the variation trend of the fitting effect along with the a value, a variation trend graph of the M value along with the a value is drawn by adopting a step length fixing method, and is shown in FIG. 5.
(5) As can be seen from (2), when a is removed by 1.3-1.5, the fitting error is relatively small. For visual description of the fitting effect, the coefficient of determinability R is calculated by using the formula (9)2. Also, for example, the initial current value is 0.14kA, and the coefficient of a is calculated stepwise in the range of 1.3 to 1.5. The calculation results are shown in fig. 6. When a is 1.3-1.5, the coefficient of the decision is very close to 1, which shows that the fitting effect is good. When a takes 1.4, the coefficient is closest to 1, and the value of M is also minimal. Therefore, a is 1.4, which can be used as an adjustment coefficient of the equivalent capacitance in the case of a bipolar short circuit calculated in engineering. Within the allowable range of engineering calculation error, the formula (8) can be rewritten as
The invention researches the characteristics and the calculation method of the capacitor discharge current before the circulation station is locked when the MMC-HVDC has a bipolar short circuit. And the adjustment factor is introduced for the first time to achieve the purpose of more accurate calculation. The calculation of the discharge current can provide reference for the selection of equipment, and also provides basis for the design of a subsequent protection principle and the setting of a protection constant value.
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 bipolar short-circuit fault current calculation method of an HVDC system based on MMC is characterized by comprising the following steps:
step 1, establishing a 21-level double-end MMC-HVDC operation system to obtain element parameters of a power transmission system;
step 2, setting bipolar short-circuit faults of the direct-current system at different moments and fault points;
step 3, collecting operation parameters of the power transmission system, equating the discharge process according to the fault current component and the discharge current generation mechanism, and calculating corresponding equivalent parameters;
step 4, introducing an adjusting factor into the equivalent parameter to simplify a discharge current calculation formula;
step 5, carrying out a large number of simulation experiments according to the step 2, and extracting and storing simulation data;
and 6, determining a fitting function according to the target function, searching for an optimal regulating factor, and obtaining a final discharge current calculation formula.
2. The method according to claim 1, characterized in that the component parameters of the power transmission system comprise: the bridge arm equivalent resistance comprises the number of bridge arm sub-modules, a sub-module capacitance value, a bridge arm reactance value, a bridge arm equivalent resistance and a power transmission line parameter.
3. The method of claim 1, wherein the operating parameters of the power transmission system comprise: the time of occurrence of the short-circuit fault, the dc outlet voltage of each converter station, the bridge arm current and its ac and dc components, and the position of the short-circuit fault point.
4. Method according to claim 1, characterized in that step 4 introduces a conditioning factor a, C, on the equivalent of the discharge capacitanceeq＝aC0and/N, the simplified calculation formula of the discharge current i is as follows:
wherein:
t is time, L0Is the reactance value of the bridge arm, C0Is the sub-module capacitance value, LeqIs equivalent to bridge arm reactance, CeqIs the equivalent of sub-module capacitor, N is the number of bridge arm sub-modules, R is the equivalent resistance of bridge arm, I0For the initial current at the moment of failure, Udcτ is a time constant and ω is a natural oscillation angular frequency for the converter station dc outlet voltage.
5. The method according to claim 1, wherein the step 6 of implementing the non-linear fitting by using an Lsqcurvefit function of MATLAB specifically comprises:
initial generation current I at different fault moments0Next, a simulation value y is setiThe objective function i (t, a) is such that the sum of the squares of the deviations of the calculated value of the objective function at point t and the simulated value is minimized, i.e. the objective function i (t, a) satisfies the following formula:
where a is the adjustment factor to be determined, i is 1,2, … …, n, n is the simulation times, and
is the optimum adjustment factor, y, determined by the least squares methodiAs a simulated value, M is the sum of the squares of the deviations.
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| CN107069795B (en) * | 2017-04-27 | 2020-06-12 | 国家电网公司 | Multi-terminal MMC-HVDC bipolar short-circuit fault current calculation method |
| CN107677931B (en) * | 2017-08-22 | 2020-06-02 | 华北电力大学 | Fault rapid identification method based on DC waveform curvature change difference |
| CN107359605A (en) * | 2017-09-12 | 2017-11-17 | 东北电力大学 | A kind of suppression module multilevel converter DC side fault overcurrent method |
| CN108933445A (en) * | 2018-06-28 | 2018-12-04 | 东北电力大学 | DC grid short-circuit current calculation method based on the loop method of analysis |
| CN109117567B (en) * | 2018-08-24 | 2023-04-21 | 武汉大学 | MMC discrete modeling-based DC side short-circuit current calculation method |
| CN111856322B (en) * | 2020-05-09 | 2021-04-23 | 上海交通大学 | Direct-current distribution network bipolar short-circuit fault accurate positioning method and device based on MMC |
| CN112134266B (en) * | 2020-09-07 | 2021-06-15 | 四川大学 | Method for suppressing interelectrode short-circuit fault current of true bipolar direct-current power grid based on topology optimization |
| CN113922408B (en) * | 2021-09-30 | 2023-12-08 | 合肥工业大学 | MMC-HVDC power grid bipolar short-circuit fault current calculation method based on parameter inversion |
| CN113824338B (en) * | 2021-11-09 | 2024-03-12 | 哈尔滨工业大学（深圳） | Method and device for calculating asymmetric MMC bridge arm current of submodule during low-voltage ride through |
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| CN104977485A (en) * | 2015-06-11 | 2015-10-14 | 中国西电电气股份有限公司 | MMC-HVDC flexible DC power module short-circuit current test device and method |
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