A method for evaluating the carrying capacity of UHV DC construction

Abstract

The invention belongs to the technical field of power system planning, and relates to a method for evaluating the carrying capacity of ultra-high voltage direct current construction, comprising: constructing an objective function and constraint conditions; inputting first-stage model parameters for solving; outputting the first-stage optimal solution; inputting the parameters required by the second-stage model and transforming the multi-objective problem by using a deviation satisfaction method; verifying the feasibility of the first-stage decision-making solution; inputting the second-stage model parameters and solving; cyclically obtaining the global optimal solution; and outputting the ultra-high voltage direct current construction carrying capacity considering the coordination of the transmission / receiving end power grid under the constraints of safety and stability. The invention pursues high cost-effectiveness in construction capacity, and takes into account the safety and stability of frequency and transient voltage. The invention can not only provide technical support for the decision-making of ultra-high voltage direct current construction site and capacity planning, but also effectively implement the management requirements of power grid companies for reducing costs and increasing efficiency, and also provide theoretical and practical basis for power companies to further improve the level of planning refinement and improve market competitiveness.

Description

Technical Field

[0001] The invention belongs to the technical field of power system planning and relates to a method for evaluating the carrying capacity of ultra-high voltage direct current construction. Background Art

[0002] Large-scale development and utilization of renewable energy has become an important measure for my country's energy structure transformation. Through the UHV DC grid, large-scale renewable energy can be transmitted to the load centers in East China and South China. However, with the continuous expansion and development of the AC grid in the load center, the receiving grid in the multi-feed DC environment faces severe challenges of DC commutation failure and bipolar blocking due to the high AC-DC coupling intensity and low system inertia level. In addition, the current flexibility configuration of large energy bases is still insufficient, and there is still power abandonment at some times. Existing methods for achieving coordinated optimization of the sending power supply and the receiving power grid, such as distributed energy coordination and virtual power plants. Distributed energy coordination achieves coordination between the sending power supply and the receiving power grid by optimizing its scheduling and control, but the control and coordination of distributed energy is relatively complex, and it is necessary to comprehensively consider the status of each distributed energy system; while the virtual power plant integrates distributed power sources, energy storage equipment and demand response resources to achieve the optimal scheduling of the overall power supply, which has high technical requirements and immature VPP market mechanisms.

[0003] Therefore, it is urgent to coordinate and optimize the sending-end power supply and the receiving-end power grid during the UHV DC line planning stage, and to clarify the optimal carrying capacity of UHV DC construction under the constraints of safety and stability. Summary of the invention

[0004] The technical solution adopted by the present invention to solve the technical problem is: a method for evaluating the carrying capacity of ultra-high voltage direct current construction, which optimizes and verifies the landing point and capacity of ultra-high voltage direct current by adopting a two-stage evaluation process, including the following steps:

[0005] Step 1: Construct a two-stage objective function and constraint conditions for the construction capacity of UHV DC considering the coordination of the transmission / receiving end power grid under the constraints of safety and stability; the two-stage objective function and constraint conditions include: the objective function and constraint conditions of the first stage, and the objective function and constraint conditions of the second stage; the objective function of the first stage is to maximize the carrying capacity of the UHV DC fed into the receiving end power grid; the objective function of the second stage is to achieve safe and stable operation of UHV DC with the lowest economic investment;

[0006] Step 2: inputting the parameters of the first stage into the objective function of the first stage under the constraint conditions of the first stage, so as to solve the objective function of the first stage by using an intelligent optimization algorithm to obtain the UHV DC line planning of the first stage;

[0007] Step 3: Output the five best solutions for the first phase of UHV DC line planning;

[0008] Step 4: Input the parameters of the second stage into the objective function of the second stage under the constraint conditions of the second stage, so as to transform the multi-objective problems of different dimensions into the objective function of the second stage by using the deviation satisfaction method to solve the second stage UHV DC line planning;

[0009] Step 5: Determine whether the second-stage UHV DC line planning is the optimal solution. When the second-stage UHV DC line planning is the optimal solution, output the second-stage UHV DC line planning result, and determine the UHV DC construction carrying capacity considering the transmission / receiving end power grid coordination under the safety and stability constraints based on the second-stage UHV DC line planning result.

[0010] Preferably, in step 5, determining the parameters of the second stage includes taking the output parameters of each line planned in the first stage of the ultra-high voltage direct current line as the parameters of the second stage, and the output parameters include capacity configuration, the maximum carrying capacity of the receiving-end power grid, and peak / valley load regulation margin requirements.

[0011] Preferably, the parameters of the second stage are determined, including that the UHV DC line planning of the first stage includes multiple schemes, and the parameters corresponding to the best scheme among the multiple schemes are used as the parameters of the second stage. When the UHV DC line planning of the second stage determined by the parameters corresponding to the best scheme of the UHV DC line planning of the first stage is not the optimal solution, the parameters corresponding to the suboptimal scheme among the multiple schemes are used as the parameters of the second stage to determine the UHV DC line planning of the second stage, until the UHV DC line planning of the second stage is the optimal solution.

[0012] Preferably, in step 1, the objective function of the first stage is:

[0013] (1)

[0014] In formula (1), maxf A Indicates the maximum capacity of UHV DC feed-in that the receiving-end power grid can carry, Ω h represents the candidate set of pre-built UHV DC lines, Ph i R represents the rated capacity of the i-th UHV DC line in the candidate set.

[0015] More preferably, in step 1, the constraints of the first-stage objective function include: UHV DC line specification constraints, active power margin constraints, and UHV DC relative inertia constraints.

[0016] Preferably, the UHV DC line specification constraints include:

[0017] (2)

[0018] In formula (2), α i , β i , γ i represents a 0-1 integer variable, which is used to determine the specifications of the i-th UHV DC line to be constructed; Ph i 1 Ph i 2 Ph i 3 represents the rated capacity of the i-th UHV DC line constructed according to three standards, which are 6000MW, 4000MW and 2000MW respectively when the i-th UHV DC line is constructed according to ±1100kV, ±800kV and ±500kV standards;

[0019] Active power margin constraints include:

[0020] (3)

[0021] In formula (3), Pw j max , Pz k max , Pg m max Respectively represent the maximum technical output of wind turbine j, photovoltaic power station k, and thermal power unit m, Ω w ,Ω s ,Ω g Respectively represent the wind turbines, photovoltaic power stations, and thermal power units of large energy bases, Pl max It represents the maximum load of the receiving power grid, GI and RI represent the power generation and load margin coefficients;

[0022] UHV DC relative inertia constraints include:

[0023] (4)

[0024] In formula (4), H g represents the inertia time constant of thermal power unit m, J AC Represents the total inertia of the AC system, H UHV It represents the equivalent inertia time constant of UHV DC, H DC min It represents the minimum equivalent inertia time constant satisfied by UHV DC.

[0025] Preferably, in step 1, the objective function of the second stage is:

[0026] (7)

[0027] In formula (7), Ψ represents the deviation satisfaction, δ 1 , δ 2 They represent the deviation satisfaction weight values, f B1 、f B2 They represent the value of the objective function under the current single objective, f * B1 、f * B2 They represent the optimal values ​​of the objective function under a single objective.

[0028] Preferably, in step 1, f in formula (7) B1 satisfy:

[0029] (5)

[0030] In formula (5), Ω h represents the candidate set of pre-built UHV DC line i, Ω s represents the set of running scenarios s, Ω t represents the set of running times t, Ω d represents the set of operating DC faults d, r i Rh is the unit backup cost coefficient of UHV DC line i. up s,t,i and Rh down s,t,i represents the upper and lower reserve capacities of UHV DC line i at time t and scenario s, d t represents the load shedding penalty coefficient at time t, Pd s,t,d represents the load shedding amount when a DC fault d occurs at time t and scenario s, l i and p i represents the construction length and construction specification coefficient of the i-th UHV DC line, C v,i and C m,i represents the construction cost and maintenance cost per unit length;

[0031] f in formula (7) B2 satisfy:

[0032] (6)

[0033] In formula (6), S and T represent the total number of running scenarios s and running time t respectively, and Pw s,t,j , Pz s,t,k and Pg s,t,m Represents the output of wind turbine j, photovoltaic power station k, and thermal power unit m at time t and scenario s.

[0034] More preferably, in step 1, the constraints of the second stage include: unit output constraints, DC operation constraints, peak regulation margin constraints, transient voltage support constraints under commutation failure conditions, and frequency support constraints under DC locking conditions.

[0035] More preferably, the unit output constraints include:

[0036] (8)

[0037] In formula (8), Pw j min , Pz k min , Pg m min They represent the minimum technical output of wind turbine j, photovoltaic power station k, and thermal power unit m, respectively. j max , Pz k max , Pg m max They represent the maximum technical outputs of wind turbine j, photovoltaic power station k, and thermal power unit m respectively;

[0038] Peak load margin constraints include:

[0039] (15)

[0040] (16)

[0041] In formula (15) and (16), Pl s,t represents the receiving grid load at time t and scenario s, Pg up s,t,m , Pg d s,t,m represents the maximum landslide and climbing power of thermal power unit m at time t and scenario, PCP min 、TCP min Represents the minimum high and low peak capacity margins that need to be met for safe and stable operation;

[0042] The transient voltage support constraints under commutation failure conditions include:

[0043] (17)

[0044] In formula (17), MOESCR i represents the effective short-circuit ratio of the i-th UHV DC line; MOESCR min represents the effective short-circuit ratio threshold of the UHV DC line, Q i represents the reactive power required to restore the voltage under the commutation failure condition of the i-th UHV DC line, ηiq Q is a 0-1 variable, representing whether the reactive power support equipment at node q has a supporting effect on the voltage recovery of DC line i after commutation failure. iq represents the reactive power provided by the reactive support equipment of node q to DC line i, P iq represents the active power flowing into DC line i from node q, Z iq represents the impedance of node q to DC line i, Z qq represents the self-impedance of node q;

[0045] The frequency support constraints in the DC blocking state include:

[0046] (18)

[0047] In formula (18), ΔPd k s,t,i represents the load shedding amount at the time t in scenario s when the DC blocking fault k occurs; μ i represents the load shedding factor of the i-th UHV DC line; ΔPd max Represents the maximum allowable load shedding;

[0048] DC operation constraints include: operating power size constraints, power ramp constraints and adjustment constraints.

[0049] The present invention also discloses a UHV DC construction carrying capacity assessment device, comprising:

[0050] The first-stage evaluation process module is used to input the parameters of the first stage into the objective function of the first stage under the constraint conditions of the first stage, so as to solve the objective function of the first stage with an intelligent optimization algorithm to obtain the UHV DC line planning of the first stage;

[0051] The second-stage evaluation process module is used to input the parameters of the second stage into the objective function of the second stage under the constraints of the second stage, so as to convert the multi-objective problems of different dimensions into the objective function of the second stage by using the deviation satisfaction method to solve the second-stage UHV DC line planning;

[0052] The selection and determination module is used to determine whether a global optimal solution is obtained, and to select to execute a cyclic optimization operation or to output the optimal solution operation.

[0053] The present invention also discloses a computer-readable storage medium, in which at least one instruction is stored. The instruction is loaded and executed by a processor to implement the above-mentioned operation of evaluating the carrying capacity of ultra-high voltage direct current construction.

[0054] The beneficial effects of the present invention are:

[0055] 1. At present, the research on transient voltage stability in large energy bases is mostly based on the multi-feed short-circuit ratio as the theoretical basis, while UHV DC has the possibility of commutation failure and bipolar lockout failure, and lacks indicators for effectively evaluating the transient voltage boost of UHV DC by the reactive support equipment of the converter station bus. The present invention defines the effective short-circuit ratio (MOESCR) of the UHV DC line to characterize the transient reactive support capacity of the converter station bus, which is used to evaluate the transient voltage stability of the UHV DC line under commutation failure.

[0056] 2. Most of the current research on the optimal carrying capacity of UHV DC construction only evaluates the construction location and capacity from the perspective of the receiving-end power grid, and does not include the spare capacity of the sending-end power supply in the peak-shaving margin constraint. The present invention coordinates and optimizes the sending-end power supply and the receiving-end power grid, and considers the peak-shaving margin of the sending and receiving-end power grids, which can prevent the occurrence of load shedding to the greatest extent.

[0057] 3. Current research on frequency stability mostly constrains from the perspective of maximum frequency deviation rate and maximum frequency deviation, and does not constrain the relative inertia of UHV DC from the perspective of the inertia of the AC system of the sending-end power grid. The present invention sets a relative inertia constraint for UHV DC to prevent frequency instability.

[0058] 4. In the first stage of the present invention, the capacity of the UHV DC line is determined and evaluated with the goal of maximizing the carrying capacity of the UHV DC feed-in receiving-end power grid; in the second stage, the dynamic safety is verified in the basic form and fault form according to the results of the first stage, and the relative inertia constraint of the UHV DC, the peak regulation margin constraint, the transient voltage support constraint under the commutation failure condition and the frequency support constraint under the DC blocking state are innovatively added. Under this safety and stability constraint, the location and capacity of the UHV DC line are corrected with the goal of minimizing the economic cost and maximizing the grid utilization efficiency.

[0059] 5. The method of the present invention pursues high "cost-effectiveness" in capacity construction, and takes into account the safety and stability of frequency and transient voltage. The result can not only provide technical support for the construction location and capacity planning decision of UHV DC, but also effectively implement the "cost reduction and efficiency improvement" management requirements of power grid companies, and provide theoretical and practical basis for power companies to further improve the level of planning refinement and enhance market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Figure 1 It is a flow chart of a method for evaluating the carrying capacity of UHV DC construction according to the present invention;

[0061] Figure 2 is a network diagram of an embodiment of the present invention;

[0062] Figure 3 is a 24-hour load demand curve diagram of a typical day of an embodiment of the present invention;

[0063] Figure 4 It is a diagram of the construction result of an embodiment of the present invention;

[0064] Figure 5 It is a schematic diagram of the three-phase short circuit result of the node 8-9 line of the present invention;

[0065] Figure 6 It is a schematic diagram of the three-phase short circuit result of the node 6-10 line of the present invention;

[0066] Figure 7 It is a schematic diagram of the result of the DC bipolar locking frequency crossing caused by the three-phase short circuit of the node 6-10 line of the present invention. DETAILED DESCRIPTION

[0067] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the relevant technologies in the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0068] refer to Figure 1-7 As shown, in this implementation mode, the UHV DC construction carrying capacity assessment method includes a two-stage assessment method, wherein the first stage takes the maximum carrying capacity of the UHV DC feed-in receiving-end power grid as the goal, and makes decisions and assessments on the capacity of the UHV DC line under the constraints of UHV DC line specifications, active power margin constraints, and UHV DC relative inertia; the second stage verifies its dynamic safety under basic forms and fault forms based on the results of the first stage, and corrects the location and capacity of the UHV DC line with the goal of achieving safe and stable UHV DC operation with the lowest economic investment under the constraints of unit output constraints, UHV DC operation constraints, peak regulation margin constraints, transient voltage support constraints under commutation failure conditions during faults, and frequency support constraints under DC locking states.

[0069] In the first stage, the goal is to maximize the carrying capacity of the UHV DC feed-in receiving grid, and make decisions and evaluations on the landing and capacity of the UHV DC line. The objective function is as follows:

[0070] (1)

[0071] In the formula, maxf A The maximum capacity of UHV DC feed-in that the receiving-end power grid can carry; Ω h Ph is the candidate set of pre-built UHV DC lines; i R is the rated capacity of the i-th UHV DC line.

[0072] The constraints in the first stage include UHV DC line specification constraints, active power margin constraints, and UHV DC relative inertia constraints.

[0073] (1) UHV DC line specification constraints

[0074] The UHV DC lines on the same planned path have different DC transmission capacity and voltage level specifications due to different power requirements of the receiving grid. Therefore, they need to be planned according to different voltage levels and capacity levels. The specification constraints of the UHV DC lines are as follows.

[0075] (2)

[0076] In the formula, the 0-1 integer variable α i , β i , γ i It is used to determine the specifications of the i-th UHV DC line to be built; the current UHV construction levels are ±1100kV, ±800kV, ±500kV and α i , β i , γ i Corresponding respectively. When α i =1,β i =0,γ i =0 means that the i-th UHV DC line is constructed according to the ±1100kV specification; when α i =0,β i =1,γ i =0 means that the i-th UHV DC line is constructed according to the ±800kV specification; when α i =0,β i =0,γ i =1 means that the i-th UHV DC line is constructed according to the ±500kV specification; when α i =0,β i =0,γ i =0 means that the requirements are not met and the i-th UHV DC line will not be put into construction. i 1 Ph i 2 Ph i 3 The rated capacities of the i-th UHV DC line constructed according to ±1100kV, ±800kV, and ±500kV standards are 6000MW, 4000MW, and 2000MW, respectively.

[0077] (2) Active power margin constraint

[0078] In order to ensure that the construction capacity of the UHV DC line can meet the active power transmission requirements of the sending power source and the receiving power grid, active power margin constraints are set as shown below.

[0079] (3)

[0080] Where Pw j max , Pz k max , Pg m max are the maximum technical outputs of wind turbine j, photovoltaic power station k, and thermal power unit m respectively; Ω w ,Ω s ,Ω g They are wind turbines, photovoltaic power stations, and thermal power units in large energy bases; Pl max is the maximum load of the receiving power grid; GI and RI represent the power generation and load margin coefficients, which are set to 0.3.

[0081] (3) UHV DC relative inertia constraints

[0082] As the proportion of UHV DC power supply gradually increases, the AC system's rotational inertia must meet extremely high requirements to ensure system frequency stability. However, unlike the rotor motion generation mechanism of traditional thermal power units, the output power of new energy units is only related to natural factors (wind speed, light intensity), and its grid connection uses a large number of power electronic components such as converters, and the active power is decoupled from the frequency change, and does not have rotational inertia. Therefore, the lack of rotational inertia of the AC system also limits the UHV DC transmission capacity. The relative inertia constraints of UHV DC are shown below.

[0083] (4)

[0084] In the formula, H g represents the inertia time constant of thermal power unit m, which is set to 2.8; J AC Indicates the total inertia of the AC system; H UHV H represents the UHV DC equivalent inertia time constant; DC min Indicates the minimum equivalent inertia time constant satisfied by UHV DC, which is set to 3.

[0085] In the second stage, the dynamic safety of the UHV DC line is verified under basic and fault conditions based on the results of the first stage. On the basis of safe and stable UHV DC operation, the location and capacity of the UHV DC line are corrected with the goal of minimizing investment and operating costs and maximizing grid operation efficiency.

[0086] In the second stage, the safe operation verification model is used to optimize the output of each type of unit and the allocation of system spinning reserves, so as to optimize the system investment and operation economy, and verify the feasibility of the first stage decision-making plan by traversing all fault conditions. Therefore, the second stage aims to minimize the investment and operation cost and maximize the grid operation efficiency, where the investment and operation cost includes the reserve cost, load shedding penalty cost, construction cost and operation and maintenance cost as shown below:

[0087] (5)

[0088] In the formula, Ω h is the candidate set of pre-built UHV DC line i; Ω s represents the set of operation scenarios s (including normal operation scenarios and fault operation scenarios); Ω t represents the set of running time t; Ω d represents the set of operating DC faults d; r i Rh represents the unit backup cost coefficient of UHV DC line i (set to 320 yuan / MW); up s,t,i and Rh down s,t,i represents the upper and lower reserve capacities of UHV DC line i in scenario s at time t; d t Pd represents the load shedding penalty coefficient at time t (set to 2000 yuan / MW); s,t,d represents the load shedding amount when a DC fault d occurs in scenario s at time t; l i and p i represents the construction length and construction specification coefficient of the i-th UHV DC line (set ±1100kV to 1.2, ±800kV to 1.0, and ±500kV to 0.8); C v,i and C m,i It represents the construction cost and maintenance cost per unit length (260 million, 170 million, 120 million yuan and 150 million, 130 million, 110 million yuan per kilometer respectively according to the three voltage levels).

[0089] The operating efficiency of the UHV DC line is expressed as the amount of electricity from wind turbines, photovoltaic power plants, and thermal power plants connected to the UHV line divided by the amount of electricity transmitted according to the rated capacity of the UHV line in various scenarios. Therefore, the maximum objective function of the grid operating efficiency is as follows:

[0090] (6)

[0091] Where S and T are the total number of running scenarios s and running time t respectively; Pw s,t,j , Pz s,t,k and Pg s,t,mRepresents the output of wind turbine j, photovoltaic power station k, and thermal power unit m in scene s at time t.

[0092] Since the two optimization objectives in the second stage have different dimensions, the present invention sets a deviation satisfaction function to find the global optimal solution with the goal of minimizing the deviation satisfaction.

[0093] (7)

[0094] In the formula, δ 1 , δ 2 are the deviation satisfaction weights, f * B1 、f * B2 are the optimal values ​​of the objective function under a single objective.

[0095] The constraints in the second stage include unit output constraints in steady state, DC operation constraints, peak regulation margin constraints, transient voltage support constraints under commutation failure conditions during faults, and frequency support constraints under DC blocking conditions.

[0096] (1) Unit output constraints

[0097] (8)

[0098] Where Pw j min , Pz k min , Pg m min They are the minimum technical outputs of wind turbine j, photovoltaic power station k, and thermal power unit m respectively.

[0099] (2) DC operation constraints

[0100] Due to the polarity change of the converter and the thermal stability limit of the conductor, the direction of the UHV DC power flow cannot change frequently, and its adjustment times are restricted. In addition, UHV DC is also subject to operating power constraints, power ramp constraints and adjustment constraints during steady-state operation.

[0101] (9)

[0102] Formula (9) represents the UHV DC operating power constraint, where Ph s,t,i represents the transmission power of UHV DC line i in scenario s at time t; Ph i min Represents the minimum transmission power of UHV DC line i (5000MW, 2000MW, 1000MW according to voltage level).

[0103] (10)

[0104] Formula (10) represents the power ramp and climbing constraints of UHV DC operation, Rh up i and Rh down ,i Indicates the maximum ramping and climbing powers specified for UHV DC line i (set to 300 and 200 MW / min); I up s,t,i and I down s,t,i It is a 0-1 variable, indicating whether the UHV DC line i in scene s at time t is adjusting its power upward or downward (0 means no action, 1 means action).

[0105] (11)

[0106] (12)

[0107] Formula (11) means that only power can be adjusted up or down at the same time, U s,t,i is a 0-1 variable, indicating whether the UHV DC line i in scenario s at time t is adjusted. Formula (12) indicates that opposite power adjustments cannot be made at adjacent times.

[0108] (13)

[0109] (14)

[0110] Formula (13) represents the minimum duration limit of constant power operation of UHV DC line i, T i,min represents the minimum constant power operation time of UHV DC line i (set to 1h). Formula (14) represents the adjustment limit of the number of power flows within the day, N h Represents the maximum number of adjustments of the daily tidal current frequency (10 times / day).

[0111] (3) Peak load margin constraints

[0112] The peak-to-valley difference of the receiving-end power grid load is increasing day by day. Since the thermal power units at the power generation end and the ultra-high voltage lines at the transmission end bear the backup demand, it is necessary to ensure that the power generation power and the transmission power must meet the peak-shaving margin requirements. The present invention uses high and low peak capacity margins (PCP, TCP) to characterize the peak-shaving margin constraints.

[0113] (15)

[0114] (16)

[0115] Formulas (15) and (16) represent the minimum peak-shaving margins required for thermal power units at the power generation end and UHV lines at the power transmission end during peak and valley periods of power consumption, respectively. s,t Represents the receiving grid load at time t and scenario s; Pg up s,t,m , Pg d s,t,m represents the maximum landslide and ramp power of thermal power unit m in the scenario at time t (set to 100 and 50 MW / min); PCP min 、TCP min Represents the minimum high and low peak capacity margins that need to be met for safe and stable operation (set to 1.8 and 1.5).

[0116] (4) Transient voltage support constraints under commutation failure conditions

[0117] When all faults occur and DC commutation fails, the busbars of the converter station are equipped with reactive support equipment such as phase regulators. In order to ensure that the UHV DC voltage can be quickly restored in the event of a fault, it is necessary to constrain the reactive support capacity. This paper defines the effective short-circuit ratio (MOESCR) of the UHV DC line to characterize the transient reactive support capacity of the converter station busbar.

[0118] (17)

[0119] Where, MOESCR i represents the effective short-circuit ratio of the i-th UHV DC line; MOESCR min represents the effective short-circuit ratio threshold of the UHV DC line, which is 2.6; Q i represents the reactive power required to restore the voltage under the commutation failure condition of the i-th UHV DC line; η iq Q is a 0-1 variable, representing whether the reactive power support equipment at node q has a supporting role in the voltage recovery of the DC line i after commutation failure (0 means no, 1 means yes); iq represents the reactive power provided by the reactive support equipment of node q to DC line i; P iq represents the active power flowing into DC line i from node q; Z iq represents the impedance of node q to DC line i; Z qq Represents the self-impedance of node q.

[0120] (5) Frequency support constraints in DC blocking state

[0121] When the commutation failure condition continues to deteriorate, it will cause bipolar blocking of the UHV DC line. At this time, to ensure the stability of the grid frequency and power flow, load shedding measures are allowed. Therefore, the following constraints need to be added to the safety verification model:

[0122] (18)

[0123] Where ΔPd k s,t,i represents the load shedding amount at the time t in scenario s when the DC blocking fault k occurs; μ i represents the load shedding factor of the i-th UHV DC line, which is 0.3; ΔPd max Represents the maximum allowable load shedding (set to 500MW).

[0124] (19)

[0125] Formula (19) represents the unbalanced power after the primary frequency regulation reserve is greater than the load shedding action. Ω ds represents the node set with automatic load shedding device; ΔP k s,t Represents the unbalanced power generated by the DC blocking fault in scenario s at time t.

[0126] (20)

[0127] Formula (20) represents that after load shedding, under the remaining unbalanced power, the system frequency change does not exceed the limit. max Represents the maximum frequency change rate of the system; f 0 Represents the rated frequency (50Hz).

[0128] (twenty one)

[0129] Formula (21) means that after load shedding, the system frequency change does not exceed the maximum deviation and will not cause instability. max Represents the maximum frequency deviation of the system; D() represents the load droop function (the relationship curve between load change and frequency change Δf=-0.002Pd+0.0035).

[0130] The two-stage evaluation method for UHV DC construction carrying capacity considering transmission / receiving grid coordination under safety and stability constraints is as follows: Figure 1 shown.

[0131] 1) Firstly, the objective function and constraint conditions of the two-stage evaluation method for UHV DC construction carrying capacity considering the coordination of the transmission / receiving power grid under the constraints of safety and stability are proposed;

[0132] 2) Input the parameters required for the first stage model and solve it using the intelligent optimization algorithm;

[0133] 3) Output the five best plans for the first phase of UHV DC line planning;

[0134] 4) Input the parameters required for the second stage model and transform the multi-objective problems of different dimensions using the deviation satisfaction method;

[0135] 5) Verify the feasibility of the first-stage decision-making plan of step 3);

[0136] 6) Input the parameters required for the second stage model and solve it using the intelligent optimization algorithm;

[0137] 7) If there is a global optimal solution, go to step 9) otherwise go to step 8);

[0138] 8) Select the suboptimal solution of the first stage for verification and go to step 5);

[0139] 9) The carrying capacity of UHV DC construction considering the coordination of the transmitting / receiving power grids under the constraints of output safety and stability.

[0140] Example

[0141] The wind power renewable energy power generation base in a province in western China is used as the sending end, and the power grid in a province is used as the receiving end to analyze and verify the method proposed in the present invention. Figure 2 As shown. Since the "±1100kV" type has a larger transmission capacity and a higher construction cost, it is not applicable in the calculation example of the present invention. Therefore, the DC transmission lines to be constructed are divided into "±800kV" and "±500kV" types according to the voltage level. The maximum allowable frequency change rate is 0.7Hz / s, the maximum frequency deviation is 0.1Hz, the total inertia of the AC system is 16.5, and the maximum and minimum technical outputs of the wind turbines are 2500MW and 1000MW respectively. The maximum and minimum technical outputs of thermal power units are 3500MW and 2500MW respectively. The 24-hour load demand curve for a typical day is shown in Figure 3 shown.

[0142] An example analysis is conducted on the two-stage UHV DC construction carrying capacity assessment model proposed in this paper, and the optimization results of the first and second stages on a typical day are shown in Table 1.

[0143]

[0144] From the comparison in Table 1, it can be found that the first phase aims to maximize the carrying capacity of the receiving-end power grid with UHV DC feed-in, and the optimal solution has a receiving-end power grid carrying capacity of 12,000MW. After the second phase of dynamic safety verification and correction based on economic investment and utilization efficiency, the UHV DC line at point 15 was downgraded (from ±800kV to ±500kV). After the modification, a total of 5.14 billion yuan in construction and maintenance costs were saved, and the grid efficiency was improved by 9%. Although the load shedding penalty cost was 78 million yuan higher, it still greatly improved the economy and practicality of the construction. The final construction results are as follows: Figure 4shown.

[0145] In order to demonstrate the effectiveness of the two-stage UHV DC construction carrying capacity assessment model proposed in the present invention, the present invention sets three scenarios for comparison. The comparison results are shown in Table 2.

[0146] Scenario 1: Only consider the N-1 fault (not considering the occurrence of DC commutation failure, DC blocking, etc.).

[0147] Scenario 2: Multiple types of faults are considered, but only steady-state safety constraints are considered. Transient voltage support constraints under commutation failure conditions and frequency support constraints under DC blocking conditions are not considered.

[0148] Scenario 3: The two-stage UHV DC construction carrying capacity assessment model proposed in this invention (based on Scenario 2, considering the transient voltage support constraints under commutation failure conditions and the frequency support constraints under the DC blocking state).

[0149]

[0150] Since the maximum load capacity in scenario 1 is higher than that in scenarios 2 and 3, but only the safety inspection constraint of N-1 faults is considered, and all possible faults are not traversed, the solution cannot meet the frequency stability and dynamic voltage support capabilities after the DC fault. As shown in the transient voltage of each scenario in Figure X, when a three-phase short circuit occurs in the node 8-9 line, the effective short circuit of the UHV DC line in scenario 1 is relatively low, and the voltage goes out of the line. In addition, the load shedding cost of scenario 1 (2.478 billion yuan) is significantly higher than that of scenarios 2 and 3, which also proves that it does not meet the safety and stability requirements. Scenario 2 does not consider the transient voltage support constraint under the condition of commutation failure and the frequency support constraint under the DC blocking state. Its MOESCR is 3.5, which is lower than 3.7 in scenario 3, because after a three-phase short circuit occurs in the node 6-10 AC line near the DC landing point 6, the corresponding DC system has a higher probability of commutation failure and is more likely to cause voltage out of the line and DC blocking. As shown in Figure X, scenario 2 did not restore the voltage in time during the transient stage, resulting in the occurrence of DC commutation failure. Finally, the frequency goes out of the line as shown in Figure X.

[0151] In summary, the present invention pursues high cost-effectiveness in capacity construction, and takes into account the safety and stability of frequency and transient voltage. The present invention can not only provide technical support for UHV DC construction location and capacity planning decisions, but also effectively implement the cost reduction and efficiency improvement management requirements of power grid companies, and provide theoretical and practical basis for power companies to further improve the level of planning refinement and enhance market competitiveness.

[0152] It should be emphasized that the above are only preferred embodiments of the present invention and do not limit the present invention in any form. Any simple modification made to the above embodiments based on the technical essence of the present invention also falls within the protection scope of the present invention. Other equivalent changes and modifications still fall within the scope of the technical solution of the present invention.

Claims

1. A method for evaluating the carrying capacity of ultra-high voltage direct current construction, characterized in that: The following steps are involved: Step 1: Construct a two-stage objective function and constraint conditions for the construction capacity of UHV DC considering the coordination of the transmission / receiving power grids under the constraints of safety and stability; The two-stage objective function and constraint conditions include: the objective function and constraint conditions of the first stage, and the objective function and constraint conditions of the second stage; the objective function of the first stage is to maximize the ultra-high voltage direct current feed-in capacity that the receiving-end power grid can bear; the objective function of the second stage is to achieve safe and stable operation of ultra-high voltage direct current with the lowest economic investment; Step 2: inputting the parameters of the first stage into the objective function of the first stage under the constraint conditions of the first stage, so as to solve the objective function of the first stage by using an intelligent optimization algorithm to obtain the UHV DC line planning of the first stage; Step 3: Output the first phase UHV DC line planning; Step 4: Input the parameters of the second stage into the objective function of the second stage under the constraint conditions of the second stage, so as to transform the multi-objective problems of different dimensions into the objective function of the second stage by using the deviation satisfaction method to solve the second stage UHV DC line planning; wherein the parameters of the second stage are determined according to the UHV DC line planning of the first stage; Step 5: Determine whether the second-stage UHV DC line planning is the optimal solution. When the second-stage UHV DC line planning is the optimal solution, output the second-stage UHV DC line planning result, and determine the UHV DC construction carrying capacity considering the transmission / receiving end power grid coordination under the safety and stability constraints according to the second-stage UHV DC line planning result; In step 1, the objective function of the second stage is: （7） In formula (7), Ψ represents the deviation satisfaction, δ1 and δ2 represent the deviation satisfaction weights, and f B1 、f B2 They represent the value of the objective function under the current single objective, f * B1 、f * B2 They represent the optimal values ​​of the objective function under a single objective; In step 1, f in formula (7) B1 satisfy: （5） In formula (5), Ω h represents the candidate set of pre-built UHV DC line i, Ω s represents the set of running scenarios s, Ω t represents the set of running times t, Ω d represents the set of operating DC faults d, r i Rh is the unit backup cost coefficient of UHV DC line i. up s,t,i and Rh down s,t,i represents the upper and lower reserve capacities of UHV DC line i at time t and scenario s, d t represents the load shedding penalty coefficient at time t, Pd s,t,d represents the load shedding amount when a DC fault d occurs at time t and scenario s, l i and p i represents the construction length and construction specification coefficient of the i-th UHV DC line, C v,i and C m,i represents the construction cost and maintenance cost per unit length; f in formula (7) B2 satisfy: （6） In formula (6), S and T represent the total number of running scenarios s and running time t respectively, and Pw s,t,j , Pz s,t,k and Pg s,t,m represents the output of wind turbine group j, photovoltaic power station k, and thermal power group m at time t and scenario s; In step 1, the constraints of the second stage include: unit output constraints, DC operation constraints, peak regulation margin constraints, transient voltage support constraints under commutation failure conditions, and frequency support constraints under DC blocking conditions.

2. The method for evaluating the carrying capacity of UHV DC construction according to claim 1, characterized in that: In step 5, the parameters of the second stage are determined, including using the output parameters of each line of the UHV DC line planning in the first stage as the parameters of the second stage, and the output parameters include capacity configuration, the maximum carrying capacity of the receiving end power grid, and peak / valley peak regulation margin requirements.

3. The method for evaluating the carrying capacity of UHV DC construction according to claim 1, characterized in that: Determine the parameters of the second stage, including that the UHV DC line planning of the first stage includes multiple schemes, and use the parameters corresponding to the best scheme among the multiple schemes as the parameters of the second stage. When the UHV DC line planning of the second stage determined by the parameters corresponding to the best scheme of the UHV DC line planning of the first stage is not the optimal solution, use the parameters corresponding to the suboptimal scheme among the multiple schemes as the parameters of the second stage to determine the UHV DC line planning of the second stage, until the UHV DC line planning of the second stage is the optimal solution.

4. The method for evaluating the carrying capacity of UHV DC construction according to claim 1, characterized in that: In step 1, the objective function of the first stage is: （1） In formula (1), maxf A Indicates the maximum capacity of UHV DC feed-in that the receiving grid can carry, Ω h represents the candidate set of pre-built UHV DC lines, Ph i R represents the rated capacity of the i-th UHV DC line in the candidate set.

5. The method for evaluating the carrying capacity of UHV DC construction according to claim 4, characterized in that: In the step 1, the constraints of the first stage include: UHV DC line specification constraints, active power margin constraints, and UHV DC relative inertia constraints.

6. A method for evaluating the carrying capacity of UHV DC construction according to claim 5, characterized in that: The UHV DC line specification constraints include: （2） In formula (2), α i , β i , γ i Represents a 0-1 integer variable, Ph i 1 , Ph i 2 , Ph i 3 represents the rated capacity of the i-th UHV DC line under the three construction standards; The active power margin constraint includes: （3） In formula (3), Pw j max , Pz k max , Pg m max Respectively represent the maximum technical output of wind turbine j, photovoltaic power station k, and thermal power unit m, Ω w ,Ω s ,Ω g Respectively represent the wind turbines, photovoltaic power stations, and thermal power units of large energy bases, Pl max It represents the maximum load of the receiving power grid, GI and RI represent the power generation and load margin coefficients; The UHV DC relative inertia constraint includes: （4） In formula (4), H m represents the inertia time constant of thermal power unit m, J AC Represents the total inertia of the AC system, H UHV It represents the equivalent inertia time constant of UHV DC, H DC min It represents the minimum equivalent inertia time constant satisfied by UHV DC.

7. The method for evaluating the carrying capacity of UHV DC construction according to claim 1, characterized in that: The unit output constraints include: （8） In formula (8), Pw j min , Pz k min , Pg m min They represent the minimum technical output of wind turbine j, photovoltaic power station k, and thermal power unit m, respectively. j max , Pz k max , Pg m max They represent the maximum technical outputs of wind turbine j, photovoltaic power station k, and thermal power unit m respectively; The peak load margin constraint includes: （15） （16） In formula (15) and (16), Pl s,t represents the receiving grid load at time t and scenario s, , represents the maximum landslide and climbing power of thermal power unit m at time t and scenario, PCP min 、TCP min Represents the minimum high and low peak capacity margins that need to be met for safe and stable operation; The transient voltage support constraints under the commutation failure condition include: （17） In formula (17), MOESCR i represents the effective short-circuit ratio of the i-th UHV DC line; MOESCR min represents the effective short-circuit ratio threshold of the UHV DC line, Q i represents the reactive power required to restore the voltage under the commutation failure condition of the i-th UHV DC line, η iq is a 0-1 variable, representing whether the reactive power support equipment at node q has a supporting effect on the voltage recovery of DC line i after commutation failure. iq represents the reactive power provided by the reactive support equipment of node q to DC line i, P iq represents the active power flowing into DC line i from node q, Z iq represents the impedance of node q to DC line i, Z qq represents the self-impedance of node q; The frequency support constraints in the DC blocking state include: （18） In formula (18), ΔPd k s,t,i represents the load shedding amount at the time t in scenario s when the DC blocking fault k occurs; μ i represents the load shedding factor of the i-th UHV DC line; ΔPd max Represents the maximum allowable load shedding; The DC operation constraints include: operation power size constraints, power ramp constraints and adjustment constraints.

8. A device for evaluating the carrying capacity of ultra-high voltage direct current construction, characterized in that: The UHV DC construction carrying capacity assessment device adopts the UHV DC construction carrying capacity assessment method according to any one of claims 1 to 7, and the UHV DC construction carrying capacity assessment device comprises: The first-stage evaluation process module is used to input the parameters of the first stage into the objective function of the first stage under the constraint conditions of the first stage, so as to solve the objective function of the first stage with an intelligent optimization algorithm to obtain the UHV DC line planning of the first stage; The second-stage evaluation process module is used to input the parameters of the second stage into the objective function of the second stage under the constraints of the second stage, so as to convert the multi-objective problems of different dimensions into the objective function of the second stage by using the deviation satisfaction method to solve the second-stage UHV DC line planning; The selection and determination module is used to determine whether a global optimal solution is obtained, and to select to execute a cyclic optimization operation or to output the optimal solution operation.

9. A computer-readable storage medium, characterized in that: The storage medium stores at least one instruction, and the instruction is loaded by the processor and executed by the method for evaluating the carrying capacity of ultra-high voltage direct current construction according to any one of claims 1 to 7.

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