Flexible DC power grid multi-current-limiting equipment parameter optimization method based on current-limiting contribution degree cost allocation

Abstract

The invention discloses a flexible DC power grid multi-current-limiting device parameter optimization method based on current-limiting contribution degree cost allocation, and the method comprises the steps: carrying out the equivalence of a flexible DC power grid according to the operation condition of an MMC before a fault, and obtaining a virtual far-end equivalent network; the FCL in the virtual far-end equivalent network is optimized; in order to optimize the current limiting equipment, the cost of FCL, CLR and DCCB is analyzed; constructing a target function of comprehensive cost effectiveness and fault current suppression of the current limiting device and the DCCB; and under the condition that constraints in the fault current limiting process are met, solving the target function to realize parameter optimization of the current limiting equipment. According to the invention, parameter optimization of the current limiting equipment can be carried out under the condition of ensuring economical efficiency optimization of the current limiting equipment and the DCCB, and the method is not limited by a topological structure of the current limiting equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of fault current limiting in flexible direct current grids, and in particular to a method for optimizing parameters of multiple current limiting devices in flexible direct current grids based on current limiting contribution cost sharing. Background Art

[0002] At present, a number of flexible DC transmission projects using MMC (Modular Multilevel Converter) as converters have been built. Flexible DC grids have fast decoupling power control performance and can connect different new energy sources, energy storage, and receiving-end power systems. They are an effective technical means to solve the problems of high-proportion renewable energy grid connection and absorption. Among them, the fault current of the flexible DC grid does not cross the zero point and rises quickly, and the grid equipment has poor overcurrent tolerance. If effective fault control is not carried out, it will seriously threaten the safe and stable operation of the flexible DC grid and even the AC system. In general, flexible DC grids have broad application prospects in the fields of receiving and transmitting clean energy, urban DC distribution, island and weak grid power supply.

[0003] When a short circuit fault occurs on the DC side of the half-bridge MMC flexible DC project, due to the small damping in the fault circuit, the submodule capacitor discharges quickly, causing the fault current to increase rapidly and the peak value is very high. In order to avoid the DC side fault causing the converter station to lock out, thereby affecting the reliability of the entire DC power grid, an efficient and reliable current limiting scheme is essential. In the network, current limiting devices can be divided into source-side current limiting devices and grid-side current limiting devices. Grid-side current limiting devices usually adopt impedance type, such as DC circuit breaker (DCCB), current limiting reactor (CLR), fault current limiter (FCL). The CLR current limiting mechanism is simple, which can well suppress the fault current and has little effect on the steady state of the flexible DC power grid. However, there is a current limiting limit. If the configuration parameter value is too large, the current limiting effect is not obvious, and the dynamic performance of the system is reduced and the grid voltage oscillates. It is usually necessary to cooperate with other current limiting devices to limit the current; there are two important parameters that affect the current limiting capability of FCL: the maximum voltage stress U that FCL can withstand. FCL and the time t of current limiting FCL Since FCL can maintain stable withstand voltage stress during the current limiting process by optimizing the current limiting impedance parameters, that is, its current limiting capability is mainly determined by its own parameters and is almost unaffected by the coupling of CLR, thus achieving full utilization of the current limiting capability of FCL.

[0004] Therefore, it is a problem that needs to be solved to fully analyze the contribution of each current limiting device and combine the coupling relationship of different converter stations to obtain a parameter optimization method for multiple current limiting devices to improve the overall stability of the system and reduce costs. Summary of the invention

[0005] In view of this, the present invention provides a method for optimizing parameters of multiple current limiting devices in a flexible DC grid based on current limiting contribution cost sharing.

[0006] The present invention discloses a method for optimizing parameters of multiple current limiting devices in a flexible direct current grid based on current limiting contribution cost sharing, which comprises:

[0007] Step 1: Based on the operating conditions before the MMC fault, the flexible DC grid is equivalent to obtain a virtual remote equivalent network;

[0008] Step 2: Optimize the FCL in the virtual remote equivalent network;

[0009] Step 3: Obtain the cost of FCL, CLR and DCCB to optimize the current limiting device;

[0010] Step 4: Construct the objective function of the comprehensive cost-effectiveness of the current limiting device, DCCB and fault current suppression; solve the objective function to achieve parameter optimization of the current limiting device while satisfying the constraints in the fault current limiting process.

[0011] Furthermore, the step 1 comprises:

[0012] According to the operating conditions of the MMC before the fault, the flexible DC grid is transformed into a radial network after the open decoupling of the ring network; all remote networks in the radial network are equivalent to a virtual remote end; for a DC grid with m transmission lines, there are k most serious fault points on each line, then the DC grid is equivalent to km of virtual remote equivalent networks.

[0013] Furthermore, after all remote networks in the radial network are equivalent to a virtual remote end, the fault current originally fed by multiple remote MMCs is all fed by the virtual remote end, and the equivalent capacitor in the virtual remote end is changed to a constant voltage source to ensure that the fault current after equivalent is greater than or equal to the fault current before equivalent, ensuring that the parameter configuration of the current limiting device meets the current limiting requirements.

[0014] Furthermore, the step 2 comprises:

[0015] In each virtual remote equivalent network, CLR, FCL, DCCB and virtual remote are connected in series. When the negative pressure injected by FCL in the fault loop is equal to the withstand voltage limit U of the fault transfer branch, FCL When the difference between them is less than the preset value, the FCL reaches the theoretical maximum value of its current limiting contribution;

[0016] When FCL is put into current limiting, if the fault current is fully transferred to L f , then MOA recovers insulation, the characteristics of FCL change to be consistent with CLR, and its current limiting capability decreases. In the face of the most serious fault, in order to make full use of the current limiting capability of FCL, the reactor L in the current limiting impedance f It needs to be large enough to ensure that MOA does not restore insulation before the DCCB operates; the current is transferred from MOA to L f The speed is constant; U FCL As a variable represents FCL.

[0017] Furthermore, if the MOA starts to interrupt the fault current at the moment t2 when the DCCB starts to interrupt the fault current, the reactance L in the current limiting impedance f The value of satisfies:

[0018]

[0019] Among them, u FCL is the voltage across FCL, i dc is the fault current flowing through FCL, t FCL is the action delay of FCL, and t represents time.

[0020] Furthermore, the step 3 comprises:

[0021] The rated voltage of MOA in DCCB is U DCCB for:

[0022] U DCCB =U rate ×1.1×1.05×1.3=1.5U rate

[0023] Among them, U rate is the rated voltage;

[0024] Assume that the price of CLR is equal to the reactance value L of CLR dc If it is linear, the price curve of CLR is:

[0025] f CLR [L dc ]=26×L dc

[0026] f CLR is the price curve of CLR;

[0027] FCL only considers the reactor L f and the IGBT cost of the current transfer branch, then the FCL price satisfies:

[0028]

[0029] Among them, i t / 3 represents the number of parallel connection of power electronic units in the conversion branch, U N is the rated voltage of MOA, i t The current in the current transfer branch;

[0030] The cost of DCCB is mainly concentrated in the failover branch, while the price of MOA and UFD can be ignored. Its cost satisfies the following formula:

[0031]

[0032] f DCCB [i t ] is the cost of DCCB.

[0033] Furthermore, the step 4 comprises:

[0034] Step 41: Obtain the benefits after adding the current limiting devices, calculate the cost-effectiveness of each current limiting device based on the benefits after adding the current limiting devices, and construct a comprehensive cost-effectiveness target of the current limiting devices and DCCB according to the cost-effectiveness of each current limiting device;

[0035] Step 42: Obtain the fault current suppression target, combine the comprehensive cost-effectiveness targets of the current limiting device and the DCCB, and obtain the objective function corresponding to the DC grid.

[0036] Furthermore, the step 41 comprises:

[0037] The benefits after adding current limiting equipment satisfy the following formula:

[0038]

[0039] Where: i dc non (t2) and i dc (t2) are the fault current amplitudes of the DCCB disconnected without and after suppression, respectively, and f DCCB (i dc ) is the manufacturing cost of the corresponding DCCB; and are the fault current energy without suppression and after suppression, respectively. The cost of heat dissipation and cooling equipment required to dissipate the corresponding energy; C DCCB To save the DCCB manufacturing cost after suppressing the fault current, C H To save the cost of heat dissipation and cooling equipment after suppressing the fault current; combine the cost of the current limiting device and the fault current limiting contribution of each device to calculate the cost-effectiveness of each current limiting device:

[0040]

[0041] Among them, p i (t) and p E (t) are the fault current amplitude and energy contribution of each device; f FCLD is the manufacturing cost of each current limiting device; X FCLD Cost effectiveness of each current limiting device;

[0042] When FCL is put into operation, the speed at which the fault current is transferred from the MOA branch to the Lf branch is:

[0043]

[0044] Among them, C MOA After investing in FCL, the profit of MOA;

[0045] The comprehensive cost-effectiveness target of current limiting devices and DCCB satisfies the following formula:

[0046]

[0047] Where obj1 is the comprehensive cost-effectiveness target of the current limiting device and DCCB, and n is the number of devices.

[0048] Further, the step 42 includes:

[0049] The fault current suppression objectives are:

[0050]

[0051] Where obj2 is the fault current suppression target, E dc (t2) represents the energy change of the fault current including all components, i non (t2) is the fault current in the scenario where it is suppressed by the corresponding element and not suppressed by the corresponding element at all, E non (t2) represents the unrestrained fault current energy change; when the DC power grid is equivalent to multiple virtual remote equivalent networks, the corresponding objective functions of all virtual remote equivalent networks are simultaneously calculated to be the minimum, that is, the objective function corresponding to the DC power grid is:

[0052]

[0053] Among them, the number of DC grids of the transmission line is m.

[0054] Furthermore, the constraints in the fault current limiting process include fault current peak constraint, MMC blocking current constraint and port minimum voltage constraint;

[0055] The fault current peak value is constrained as:

[0056] i peak ≤i DCCB_MAX

[0057] Among them, i peak is the fault current peak value; i DCCB_MAX is the theoretical maximum breaking current of DCCB;

[0058] The MMC blocking current constraint is:

[0059] i dc (t)≤i MMC_block

[0060] Among them, i dc (t) is the blocking current amplitude at time t, i MMC_block is the MMC over-current lockout critical value;

[0061] The minimum port voltage constraint is:

[0062] U dc (t)≥U MMC_restart

[0063] Among them, U dc (t) is the minimum port voltage at time t, U MMCrestart It is the MMC low voltage restart threshold.

[0064] Due to the adoption of the above technical solution, the present invention has the following advantages:

[0065] 1. The present invention analyzes the current limiting contribution of the remote converter station of the DC grid and combines the coupling relationship between different converter stations to propose a simplified modeling method for a flexible DC grid based on virtual remote equivalent.

[0066] 2. The present invention analyzes the cost structure of current limiting devices and DCCB, takes the cost-benefit balance of current limiting devices as the optimization goal, and proposes a method for optimizing the parameters of multiple current limiting devices. The method is not limited by current limiting devices and power grid topology, and can adapt to different current limiting device solutions.

[0067] 3. The present invention further improves the fault current suppression capability of the current limiting device through the effective coordination of various current limiting devices, and can ensure that there is a corresponding current limiting device parameter configuration scheme when the current limiting device and DCCB are economically optimized.

[0068] 4. The present invention proposes a simplified equivalent method for the power grid and FCL at each of the most serious fault points in the flexible DC grid. At the same time, by considering the DCCB cost reduction brought about by the suppression effect of the current limiting device in the two dimensions of fault current amplitude and energy, as well as the comprehensive cost-effectiveness of the equipment's own cost structure, parameter optimization with the economic efficiency of multiple current limiting devices and DCCB as the goal is achieved. This invention scheme can find the corresponding current limiting device parameter configuration scheme while ensuring the economic optimization of the current limiting device and DCCB.

[0069] 5. The present invention proposes a DC grid equivalent method based on fault point equivalence, taking into account different pre-fault working conditions, and further provides a simplified method for FCL in the optimization process. By analyzing the costs of FCL, CLR and DCCB, a multi-fault current limiting device parameter collaborative optimization model considering current limiting contribution sharing is proposed. This fault current limiting device optimization scheme can optimize parameters of different DC grids and single / multiple current limiting devices without being restricted by the topological structure of the current limiting device. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the embodiments of the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0071] Figure 1 A schematic flow chart of a method for optimizing parameters of multiple current-limiting devices in a flexible DC grid based on current-limiting contribution cost sharing according to an embodiment of the present invention;

[0072] Figure 2 It is the open type equivalent of the ring network in the embodiment of the present invention;

[0073] FIG3( a ) is a schematic diagram of a radial DC power grid in an embodiment of the present invention;

[0074] FIG3( b ) is a schematic diagram of a virtual remote equivalent in an embodiment of the present invention;

[0075] FIG3( c ) is a schematic diagram of a virtual remote equivalent in a multi-outlet scenario according to an embodiment of the present invention;

[0076] Figure 4 FIG. 4 is a topological diagram of the FCL in an embodiment of the present invention. DETAILED DESCRIPTION

[0077] The present invention is further described in conjunction with the accompanying drawings and embodiments, and the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field should fall within the scope of protection of the embodiments of the present invention.

[0078] See also Figure 1 The present invention provides an embodiment of a method for optimizing parameters of multiple current limiting devices in a flexible DC grid based on current limiting contribution cost sharing, which includes:

[0079] Step 1: Based on the operating conditions before the MMC fault, the flexible DC grid is equivalent to obtain a virtual remote equivalent network;

[0080] In one embodiment, the DC power grid is decoupled and equivalent. Before decoupling, after the ring network structure fails, MMC1 feeds fault current to the fault point from two directions, and the two currents together cause the MMC1 capacitor voltage to drop, thereby causing the coupling of fault currents at both ends. Since the MMC in the ring network is not directly connected to the fault line, the amplitudes of the two currents are not large, and the MMC1 capacitor voltage drop is limited. After decoupling, it can be considered that there are two MMC1s that feed current to the fault point from two directions of the fault. In the DC power grid after the above equivalent method, the currents at both ends of the fault line will become larger.

[0081] The open equivalent circuit of the ring network is as follows Figure 2 As shown. After the ring network is equivalent, the DC grid is transformed into a radial network, as shown in Figure 3(a). The entire remote network is equivalent to a virtual remote, as shown in Figure 3(b). For a DC grid with m transmission lines, each line has two most serious fault points, then the DC grid can be equivalent to 2m virtual remote equivalent networks, as shown in Figure 3(c).

[0082] After the ring network is openly equivalent, the DC grid is transformed into a radial network. Since the lower limit of the parameters of each CLR is that the MMC cannot be locked before the FCL is activated. Therefore, under the suppression of the fault current by the CLR and FCL on the fault line, the voltage of bus 1 has been supported to a certain extent, and the voltage of bus 2 will drop very limitedly under the further support of the CLR and line impedance on the adjacent line. Therefore, the entire remote network can be equivalent to a virtual remote end. After the equivalent, the fault current originally fed by multiple remote MMCs is all fed by the virtual remote end, which will inevitably cause the voltage of bus 2 to drop faster than the actual one after the equivalent, making the calculated fault current smaller. Therefore, it is necessary to change the equivalent capacitor in the virtual remote end to a constant voltage source to ensure that the fault current after the equivalent is greater than or equal to the fault current before the equivalent, and to ensure that the parameter configuration of the current limiting device meets the current limiting requirements.

[0083] If bus 1 has n outgoing lines, then n-1 non-fault outgoing lines can be constructed as n-1 virtual remote ends using the above method. It is considered that all fault currents on the outgoing line are fed by the virtual remote end, and the capacitor voltage of the virtual remote MMC is considered to be a constant voltage source, and its parameters are consistent with the MMC that is closest to the fault point in electrical distance. The virtual remote equivalent method in the multi-outgoing line scenario can be used to optimize the parameter configuration of the current limiting device on the fault line, and considers the coupled current limiting effect of the CLR on the adjacent current limiting, as well as the fault current injection of all remote MMCs. The network scale after equivalent is smaller than the original DC power grid, and the MMC simplified by the constant voltage source does not need to consider the equivalent capacitor voltage drop, so it can improve the parameter configuration optimization efficiency of the current limiting device.

[0084] Step 2: Optimize the FCL in the virtual remote equivalent network;

[0085] In one embodiment, when the negative pressure injected into the fault loop by the FCL approaches the withstand voltage limit U FCL When , the FCL reaches the theoretical maximum value of its current limiting contribution. Therefore, in the process of parameter optimization, when facing the most serious fault, the current limiting capability of the FCL is fully utilized as the constraint configuration parameter.

[0086] See also Figure 4 , in the FCL topology diagram, L f The reactance is the current limiting impedance, MOA (Metal Oxide Arrester) is the current limiting impedance, and the current transfer branch includes multiple insulated gate bipolar transistors (Insulated Gate Bipolar Transistor, IGBT).

[0087] When FCL is put into current limiting, due to the reactor L f The current in the fault current cannot change suddenly. The fault current flows through MOA and then to L f If the fault current is fully transferred to L f , then MOA recovers insulation, the characteristics of FCL change to be consistent with CLR, and its current limiting capability decreases. Therefore, in the face of the most serious fault, in order to make full use of the current limiting capability of FCL, L f It needs to be large enough to ensure that MOA does not restore insulation before DCCB operates. Since the metal oxide arrester (MOA) is also called the current limiting impedance, the voltage U across FCL in the on state FCL Basically stable at 2 times the rated voltage U of MOA N , the current is transferred from MOA to L f Therefore, if MOA starts to interrupt the fault current at the moment t2, L f The value of should satisfy:

[0088]

[0089] Among them, u FCL is the voltage across FCL, i dc is the fault current flowing through FCL, t FCL is the action delay of FCL, and t represents time.

[0090] From the above formula, we can know that for the most serious fault, if the current limiting capability of FCL is required to be fully utilized, under the fixed action sequence, U N With L f The relationship is constant. FCL For U N Therefore, U can be used in the optimization process.FCL As a variable represents FCL.

[0091] Step 3: Obtain the cost of FCL, CLR and DCCB to optimize the current limiting device;

[0092] In one embodiment, the purpose of FCL is to limit the fault current rather than to cut off the fault current. That is, the maximum voltage stress it bears during the fault process must be less than the DC rated voltage. Otherwise, FCL can complete the fault current cutting. There is no essential difference between FCL and DCCB. That is, the voltage U across FCL is FCL It should be less than 500kV. In actual projects, cooling equipment is distributed in DCCB and resistive current limiting equipment. After the fault current energy is converted into heat energy, it needs to be dissipated through cooling equipment. This method focuses on the cost of cooling equipment.

[0093] U of MOA in DCCB N It is usually selected as 1-1.1 times the system voltage. In addition, considering that the power system voltage has a ±5% fluctuation and a reliability factor of 1.3 is used to improve the reliability and breaking speed of the DCCB. Therefore, the rated voltage of the MOA in the DCCB is U DCCB for:

[0094] U DCCB =U rate ×1.1×1.05×1.3=1.5U rate

[0095] Among them, U rate is the rated voltage; assuming that the price of CLR is the same as L dc The size is linear, so the price curve of CLR is as follows:

[0096] f CLR [L dc ]=26×L dc

[0097] f CLR This is the price curve of CLR.

[0098] Obviously, the main cost of FCL comes from the reactor L in the current limiting impedance. f The price of other components is several orders of magnitude lower than that of the IGBT of the current transfer branch. Therefore, FCL only considers the reactor L f and the cost of the insulated gate bipolar transistor IGBT, then the FCL price satisfies:

[0099]

[0100] Among them, U rate is the rated voltage, i t The current in the current transfer branch.

[0101] DCCB is mainly composed of a large number of IGBTs, ultra-high-speed mechanical switches and MOAs. The fault transfer branch in DCCB needs to be able to withstand overcurrent and overvoltage during the disconnection process, and the MOA in DCCB should be able to generate a voltage drop of 3 times the rated DC voltage. Therefore, the cost of DCCB is mainly concentrated in the fault transfer branch, and the price of MOA and UFD can be ignored, so its cost satisfies the following formula:

[0102]

[0103] Among them, f DCCB [i t ] is the cost of DCCB, i t / 3 represents the number of parallel connection of power electronic units in the conversion branch, U N is the rated voltage of MOA, i t The current in the current transfer branch.

[0104] Assuming that the price of cooling equipment is linear with the cooling capacity, its price is as follows:

[0105] f H [E] = 4 × 10 5 ×E dc

[0106] Among them, f H [E] is the price of the cooling equipment, E dc To suppress the post-fault current energy.

[0107] Step 4: Construct the objective function of the comprehensive cost-effectiveness of the current limiting device, DCCB and fault current suppression; solve the objective function to achieve parameter optimization of the current limiting device while satisfying the constraints in the fault current limiting process.

[0108] In one embodiment, the comprehensive cost-effectiveness target of the fault current limiting device FCLD and DCCB is:

[0109] Enhancing the current limiting capability of current limiting devices will increase the construction cost of DC power grids, but it can reduce the fault current interrupted by DCCB and reduce the number of power electronic devices in its fault transfer branch. At the same time, due to the reduction of fault current energy, the cooling and heat dissipation requirements of DCCB and resistive current limiting devices are also reduced accordingly, which can reduce costs. Therefore, the benefits of adding current limiting devices meet the following formula:

[0110]

[0111] Where: i dc non (t2) and i dc (t2) are the fault current amplitudes of the DCCB disconnected without and after suppression, respectively, and f DCCB(i dc ) is the manufacturing cost of the corresponding DCCB; and are the fault current energy without suppression and after suppression, respectively. The cost of heat dissipation and cooling equipment required to dissipate the corresponding energy; C DCCB To save the DCCB manufacturing cost after suppressing the fault current, C H To save the cost of heat dissipation and cooling equipment after suppressing the fault current; combine the cost of the current limiting device and the fault current limiting contribution of each device to calculate the cost-effectiveness of each current limiting device:

[0112]

[0113] Among them, p i (·) and p E (·) are the contribution of each device to suppress the fault current amplitude and energy ssss respectively; f FCLD is the manufacturing cost of each current limiting device; X FCLD Cost effectiveness of individual current limiting devices.

[0114] Define the efficiency of the fault current limiting device as e = λ FCLD / f FCLD . Where λ FCLD It is the current limiting flux generated by the current limiting device in the current limiting process. CLR suppresses the rising rate of fault current. Its ability to suppress fault current decays rapidly as the current rising rate decreases. The price curve of CLR is CLR It changes linearly with the parameter, so the efficiency of CLR decays rapidly as the parameter increases. CLR It increases linearly with the increase of its inductance value. When the current limiting capability of CLR is improved by increasing inductance, the current limiting flux generated by the unit value decreases rapidly, and the corresponding CLR efficiency decreases.

[0115] When FCL is put into operation, the fault current is transferred from the MOA branch to the L f The speed of the branch is:

[0116]

[0117] Among them, it is the profit of MOA after FCL investment.

[0118] It should be noted that FIG3 is a topological diagram of FCL, that is, where there is FCL, there is MOA branch. Figure 4 It can be seen that the MOA branch and L f The branches are in parallel. Since UMOA is almost constant before MOA restores insulation, the speed of fault current transfer is also constant. With the improvement of FCL fault current suppression capability, i dc The ascent rate decreases, while UN Then it rises. f It needs to rise quickly to extend the time for MOA to restore insulation, and the efficiency of FCL will also decrease as its current limiting capability increases. The cost-effectiveness of FCLD will decrease as its current limiting capability increases. It is also known that FCL does not have internal dynamic changes, and its current limiting capability and cost are in a proportional linear relationship. Therefore, by considering the cost-effectiveness changes when multiple devices cooperate to limit current, the economy of current limiting devices and DCCB is improved. The comprehensive cost-effectiveness target of current limiting devices and DCCB satisfies the following formula:

[0119]

[0120] Where obj1 is the comprehensive cost-effectiveness target of the current limiting device and DCCB, and n is the number of devices.

[0121] It should be noted that when the CLR is located on the adjacent line of the fault line, it will still have a relatively obvious suppression effect on the fault power. Therefore, the benefits of the CLR on the suppression of the most serious fault current on the adjacent line should also be included in the denominator of the cost-effectiveness formula of each current limiting device.

[0122] Fault current suppression objectives:

[0123] Since the difficulty of suppressing and clearing the most serious fault current corresponding to each fault point is different, if the fault current size of the DCCB disconnection is directly used as the target, the current limiting effect of a few most serious fault points will inevitably be sacrificed by the algorithm to improve the economy of the final optimization result. Therefore, it is proposed to use the ratio of the suppressed DCCB breaking current to the unsuppressed DCCB breaking current as the fault current suppression target to incorporate the difficulty of suppressing and clearing fault currents at different fault points into the optimization. By considering the fault current amplitude and energy separately, and weighting them using the economic cost of clearing the fault current and dissipating the fault current energy, the normalization of the fault current limiting target is achieved. The fault current suppression target is as follows:

[0124]

[0125] Where obj2 is the fault current suppression target, E dc (t2) represents the energy change of the fault current including all components, i non (t2) is the fault current in the scenario where it is suppressed by the corresponding element and not suppressed by the corresponding element at all, E non (t2) represents the unrestrained fault current energy variation.

[0126] In summary, by minimizing the two objective functions at the same time and optimizing the fault current limiting device parameters of the DC grid, the economic optimization of the DC grid and the DCCB is sought. When the DC grid is equivalent to multiple virtual remote equivalent networks, the corresponding objective functions and minimization of all virtual remote equivalent networks should be sought at the same time. Therefore, the objective function corresponding to the DC grid is:

[0127]

[0128] Among them, the number of DC grids of the transmission line is m.

[0129] In one embodiment, the constraints in the fault current limiting process include fault current peak constraint, MMC blocking current constraint and port minimum voltage constraint. In order to consider that the actual fault current cannot exceed the upper limit of the DCCB breaking capacity, the fault current peak constraint is added, that is,

[0130] i peak ≤i DCCB_MAX

[0131] Among them, i peak is the fault current peak value; i DCCB_MAX is the theoretical maximum breaking current of DCCB. Since the object of the present invention is a DC power grid, and multiple FCLDs are used to suppress fault current. Therefore, under any short-circuit fault, the MMC should be forced to lock due to overcurrent, and the MMC lockout current constraint is:

[0132] i dc (t)≤i MMC_block

[0133] Among them, i dc (t) is the blocking current amplitude at time t, i MMC_block is the MMC overcurrent lockout threshold. Similarly, the MMC should not be forced to restart due to low voltage at the short circuit fault terminal. Therefore, the minimum voltage constraint of the port is:

[0134] U dc (t)≥U MMC_restart

[0135] Among them, U dc (t) is the minimum port voltage at time t, U MMC_restart It is the MMC low voltage restart threshold.

[0136] The coordinated optimization of parameters of multiple fault current limiting devices in DC power grid is a complex hybrid nonlinear optimization problem. This method is based on the optimization model based on the current limiting contribution analysis, which fully considers the coordination and cooperation of multiple fault current limiting devices and DCCB in multiple dimensions.

[0137] In summary, the present invention reduces system cost, improves system optimization efficiency, and has high practical value for analyzing parameter configuration of multiple current limiting devices in a flexible DC power grid.

[0138] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for optimizing parameters of multiple current limiting devices in a flexible DC grid based on current limiting contribution cost sharing, characterized in that: include: Step 1: Based on the operating conditions before the MMC fault, the flexible DC grid is equivalent to obtain a virtual remote equivalent network; Step 2: Optimize the FCL in the virtual remote equivalent network; Step 3: Obtain the cost of FCL, CLR and DCCB to optimize the current limiting device; Step 4: Construct the objective function of the comprehensive cost-effectiveness of the current limiting device, DCCB and fault current suppression; solve the objective function to achieve parameter optimization of the current limiting device while satisfying the constraints in the fault current limiting process.

2. The method according to claim 1, characterized in that: The step 1 comprises: According to the operating conditions of the MMC before the fault, the flexible DC grid is transformed into a radial network after the open decoupling of the ring network; all remote networks in the radial network are equivalent to a virtual remote end; for a DC grid with m transmission lines, there are k most serious fault points on each line, then the DC grid is equivalent to km of virtual remote equivalent networks.

3. The method according to claim 2, characterized in that After all remote networks in the radial network are equivalent to a virtual remote end, the fault current originally fed by multiple remote MMCs is all fed by the virtual remote end, and the equivalent capacitor in the virtual remote end is changed to a constant voltage source to ensure that the fault current after equivalent is greater than or equal to the fault current before equivalent, ensuring that the parameter configuration of the current limiting device meets the current limiting requirements.

4. The method according to claim 1, characterized in that The step 2 comprises: In each virtual remote equivalent network, CLR, FCL, DCCB and virtual remote are connected in series. When the negative pressure injected by FCL in the fault loop is equal to the withstand voltage limit U of the fault transfer branch, FCL When the difference between them is less than the preset value, the FCL reaches the theoretical maximum value of its current limiting contribution; When FCL is put into current limiting, if the fault current is fully transferred to L f , then MOA recovers insulation, the characteristics of FCL change to be consistent with CLR, and its current limiting capability decreases. In the face of the most serious fault, in order to make full use of the current limiting capability of FCL, the reactor L in the current limiting impedance f It needs to be large enough to ensure that MOA does not restore insulation before the DCCB operates; the current is transferred from MOA to L f The speed is constant; U FCL As a variable represents FCL.

5. The method according to claim 4, characterized in that If MOA starts to interrupt the fault current at the moment t2 when DCCB starts to interrupt the fault current, the reactance L in the current limiting impedance f The value of satisfies: Among them, u FCL is the voltage across FCL, i dc is the fault current flowing through FCL, t FCL is the action delay of FCL, and t represents time.

6. The method according to claim 1, characterized in that The step 3 comprises: The rated voltage of MOA in DCCB is U DCCB for: IN DCCB =U rate ×1.1×1.05×1.3=1.5U rate Among them, U rate is the rated voltage; Assume that the price of CLR is equal to the reactance value L of CLR dc If it is linear, the price curve of CLR is: in CLR [L dc ]=26×L dc f CLR is the price curve of CLR; FCL only considers the reactor L f and the IGBT cost of the current transfer branch, then the FCL price satisfies: Among them, i t / 3 represents the number of parallel connection of power electronic units in the conversion branch, U N is the rated voltage of MOA, i t The current in the current transfer branch; The cost of DCCB is mainly concentrated in the failover branch, while the price of MOA and UFD can be ignored. Its cost satisfies the following formula: f DCCB [i t ] is the cost of DCCB.

7. The method according to claim 1, characterized in that The step 4 comprises: Step 41: Obtain the benefits after adding the current limiting devices, calculate the cost-effectiveness of each current limiting device based on the benefits after adding the current limiting devices, and construct a comprehensive cost-effectiveness target of the current limiting devices and DCCB according to the cost-effectiveness of each current limiting device; Step 42: Obtain the fault current suppression target, combine the comprehensive cost-effectiveness targets of the current limiting device and the DCCB, and obtain the objective function corresponding to the DC grid.

8. The method according to claim 7, characterized in that The step 41 comprises: The benefits after adding current limiting equipment satisfy the following formula: Where: i dc non (t2) and i dc (t2) are the fault current amplitudes of the DCCB disconnected without and after suppression, respectively, and f DCCB (i dc ) is the manufacturing cost of the corresponding DCCB; and are the fault current energy without suppression and after suppression, respectively. The cost of heat dissipation and cooling equipment required to dissipate the corresponding energy; C DCCB To save the DCCB manufacturing cost after suppressing the fault current, C H To save the cost of heat dissipation and cooling equipment after suppressing the fault current; combine the cost of the current limiting device and the fault current limiting contribution of each device to calculate the cost-effectiveness of each current limiting device: Among them, p i (t) and p E (t) are the fault current amplitude and energy contribution of each device; f FCLD is the manufacturing cost of each current limiting device; X FCLD The cost effectiveness of individual current limiting devices; When FCL is put into operation, the fault current is transferred from the MOA branch to the L f The speed of the branch is: Among them, C MOA After investing in FCL, the profit of MOA; The comprehensive cost-effectiveness target of current limiting devices and DCCB satisfies the following formula: Where obj1 is the comprehensive cost-effectiveness target of the current limiting device and DCCB, and n is the number of devices.

9. The method according to claim 7, characterized in that: The step 42 comprises: The fault current suppression objectives are: Where obj2 is the fault current suppression target, E dc (t2) represents the energy change of the fault current including all components, i non (t2) is the fault current in the scenario where it is suppressed by the corresponding element and not suppressed by the corresponding element at all, E non (t2) represents the unrestrained fault current energy change; when the DC power grid is equivalent to multiple virtual remote equivalent networks, the corresponding objective functions of all virtual remote equivalent networks are simultaneously calculated to be the minimum, that is, the objective function corresponding to the DC power grid is: Among them, the number of DC grids of the transmission line is m.

10. The method according to claim 1, characterized in that The constraints in the fault current limiting process include fault current peak constraint, MMC blocking current constraint and port minimum voltage constraint; The fault current peak value is constrained as: i peak ≤i DCCB_MAX Among them, i peak is the fault current peak value; i DCCB_MAX is the theoretical maximum breaking current of DCCB; The MMC blocking current constraint is: i dc (t)≤i MMC_block Among them, i dc (t) is the blocking current amplitude at time t, i MMC_block is the MMC over-current lockout critical value; The minimum port voltage constraint is: U dc (t)≥U MMC_restart Among them, U dc (t) is the minimum port voltage at time t, U MMC_restart It is the MMC low voltage restart threshold.

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