Self-adaptive flexible direct-current transmission control method based on optimal power of power grid partitions

Abstract

The invention discloses a self-adaptive flexible direct-current transmission control method based on optimal power of power grid partitions, and relates to the technical field of direct-current transmission control. The method comprises the following steps: step 1, establishing a power grid partition flexible interconnection optimization control target in a steady state and a transient state operation state; step 2, according to the steady state control target established in the step 1, adopting a successive approximation method to quickly solve the flexible direct current optimal power; 3, designing a multi-terminal flexible direct current adaptive droop control strategy capable of realizing emergency power support between power grid partitions according to the transient control target established in the step 1; and 4, according to the step 2, the flexible DC optimal power is rapidly solved based on a successive approximation method, and according to the multi-terminal flexible DC adaptive droop control strategy designed in the step 3 for emergency power support between power grid partitions, the effectiveness and engineering practicability of the proposed method are verified through a simulation model.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission control, and in particular to an adaptive flexible direct current transmission control method based on optimal power of power grid partitions. Background Art

[0002] Flexible DC distribution system has the advantages of large transmission capacity and flexible control. It improves energy conversion efficiency while reducing the commutation link. It is an ideal way to realize the interconnection of centralized or distributed energy production, consumption, conversion and other units, and to solve the dense and diversified power supply needs of cities. As a key technology of the new generation of integrated energy distribution system, in recent years, a large number of institutions and relevant scholars at home and abroad have carried out theoretical research and application demonstration on its system architecture, application scenarios, voltage levels, key equipment, control strategies and protection systems. Among them, reasonable control strategy is the key link to improve power quality and ensure system reliability.

[0003] At present, the control strategies of flexible DC power grid mainly include constant DC voltage control, constant active power control and droop control. For flexible DC power grid, the control strategies of converter station and power grid greatly affect the power flow distribution of the system, and the control parameters of converter station are the key factors in the control strategy. They are inseparable from the power flow distribution of the system and should play an important role in the power flow optimization problem. However, most of the active power reference values ​​are currently designed based on experience, and the droop coefficient is allocated according to the capacity of the converter station, without considering the design of control parameters through the optimal power flow. In addition, as the proportion of new energy access to the power grid increases, the power of the converter station connected to it will fluctuate, resulting in an imbalance between the power setting value and the actual power, affecting the stable operation of the flexible DC power grid of the entire power grid and the power flow optimization effect. To this end, we propose an adaptive flexible DC transmission control method based on the optimal power of the power grid partition. Summary of the invention

[0004] The purpose of the present invention is to solve the problems mentioned in the above background technology. The present invention provides an adaptive flexible direct current transmission control method based on the optimal power of power grid partition.

[0005] In order to achieve the above-mentioned purpose, the present invention specifically adopts the following technical solutions:

[0006] The adaptive flexible direct current transmission control method based on the optimal power of power grid partition includes the following steps:

[0007] Step 1: Establish the optimization control objectives of flexible interconnection of power grid partitions under steady-state and transient operation conditions;

[0008] Step 2: According to the steady-state control target established in step 1, the optimal power of flexible DC is quickly solved by the stepwise approximation method;

[0009] Step 3: Based on the transient control target established in step 1, a multi-terminal flexible DC adaptive droop control strategy is designed to achieve emergency power support between power grid sections;

[0010] Step 4: According to step 2, the optimal power of flexible DC is quickly solved based on the stepwise approximation method, and the multi-terminal flexible DC adaptive droop control strategy for emergency power support between grid partitions designed in step 3 is designed. The effectiveness and engineering practicability of the proposed method are verified through simulation models.

[0011] Furthermore, the control objectives in step 1 include: steady-state power flow optimization and emergency power mutual assistance.

[0012] Furthermore, step 2 establishes a comprehensive evaluation index of weighted network loss, load balancing and voltage deviation based on the steady-state control target established in step 1, and its specific modeling is as follows:

[0013] The flexible DC power optimization strategy under steady-state operation evaluates the steady-state power flow distribution through comprehensive evaluation indicators to determine the optimal operating power; the comprehensive evaluation indicators are composed of three parts: load balancing indicator, network loss indicator and voltage deviation indicator;

[0014] (1) Comprehensive evaluation index By establishing specific evaluation indexes, the steady-state power flow distribution of urban sub-grids under different flexible DC transmission powers is evaluated, the optimal operating power is determined, and the comprehensive evaluation index is established as follows:

[0015] G=aG α +bG β +cG γ (1)

[0016] Among them, G is the comprehensive score of the urban sub-grid under a certain transmission power of flexible DC;

[0017] G α is the normalized load balancing indicator;

[0018] G β is the normalized network loss index;

[0019] G γ is the normalized line voltage deviation index;

[0020] a, b, and c are the weights of the three indicators, which are calculated using a method based on information entropy theory;

[0021] (2) Load balancing indicators

[0022] In steady state, the flexible DC is controlled to transfer power from the light-load partition to the heavy-load partition, balance the load rate of the main transformer in each partition, improve the utilization efficiency of the main transformer, and reduce the discrete degree of the load rate of the main transformer in each partition. In order to facilitate the analysis and comparison between different indicators, a normalized load balancing index G is established. α as follows:

[0023]

[0024] Among them, σ 0 and σ 1 They are the standard deviations of load factors of all 750kv main transformers in the urban power grid before and after the commissioning of flexible DC.

[0025] σ op is the minimum standard deviation of the load factor of the main transformer within the adjustable range of flexible DC power;

[0026] In order to measure the dispersion of the load factor of the main transformer, the standard deviation of the load factor of all 750kV main transformers within the urban power grid is calculated as follows:

[0027]

[0028] Where, σ is the standard deviation of the load factor of N 750kV main transformers;

[0029] P Ti , S Ti , η Ti are the power, capacity and load rate of the ith 500kV main transformer respectively;

[0030] is the arithmetic mean of the load factors of N main transformers;

[0031] (3) Network loss index

[0032] Under steady-state conditions, controlling the flexible DC transmission power can make the power flow between sub-zones evenly distributed, reduce line network losses and transformer losses, and improve the economic efficiency of system operation. In order to compare and analyze the network losses of urban sub-zone power grids before and after the commissioning of flexible DC, the normalized power loss index is used as follows:

[0033]

[0034] Among them, P 0 ,P 1 are the sum of network losses of all subareas of the urban power grid before and after the commissioning of the flexible DC power grid;

[0035] min(P 1 ) is the minimum value of network loss within the adjustable range of flexible DC power;

[0036] Network losses include 220kV line losses and 750 / 220kV transformer losses, as well as flexible DC converter and line losses. The calculation formula is as follows:

[0037] P 0,0 =P L,0 +P T,0 +P C,0 (7)

[0038] Among them, P 0,0 is the network loss of the urban power grid;

[0039] P L,0 , P T,0 They are line loss and transformer loss respectively;

[0040] P C,0 is the flexible DC converter loss;

[0041] (4) Voltage deviation index

[0042] In AC / DC hybrid systems, voltage is closely related to network power balance, so the voltage offset of all nodes is used as the performance indicator of the system:

[0043]

[0044] Among them, u 0 and u 1 They are the standard deviations of voltages of all transformers in the urban power grid before and after the commissioning of flexible DC power supply;

[0045] u op is the minimum voltage difference of the main transformer within the adjustable range of flexible DC power;

[0046]

[0047]

[0048] Where U is the voltage standard deviation of N 750kV main transformers;

[0049] U Ti , U T They are the actual voltage, rated voltage and voltage deviation rate of the i-th 750kV main transformer;

[0050] U T It is the arithmetic mean of the voltage deviation rates of N main transformers.

[0051] Furthermore, the optimal power is solved through the load balancing index, network loss index and voltage deviation index described in step 3; the load balancing index, network loss index and voltage deviation index have different characteristics. Under steady-state operation, as the flexible DC transmission power increases, the three-phase indicators first increase and then decrease, but the power corresponding to the turning point is different, and the load balancing effect is more obvious.

[0052] Further, step 2 includes the following steps:

[0053] Step 2-1: Online statistics of the power of each 750kV main transformer in the urban power grid zoning operation mode, and calculation of the standard deviation of the main transformer load factor σ before the flexible DC operation 0 、Network loss P 0,0 and line voltage deviation U;

[0054] Step 2-2: Determine the operating range of flexible DC power between zones based on the line rated power and main transformer capacity constraints [P min ,P max ];

[0055] Step 2-3: Set the flexible DC initial power P DC,0 , power growth direction d, power growth initial step s, convergence accuracy ε;

[0056] Step 2-4: Calculate the flexible DC transmission power P DC The standard deviation of the load factor of the main transformer in each partition is 1 、Network loss P 1 Substituting the voltage deviation U into equation (2), equation (6), and equation (1) to calculate G α ,G β ,G γ and comprehensive score G;

[0057] Step 2-5: If satisfied Then update the optimal score G = G max , while continuing to increase the flexible DC power P dc =P dc +ds, return to step 2-4; if G<G max , then update the step size s = s / 2, and reduce the flexible DC power P dc =P dc +ds, to accurately solve the optimal power, determine whether s is greater than ε, if so, go to step 2-4, otherwise you can determine G max The corresponding power is the flexible DC optimal power P dc,op .

[0058] Furthermore, the specific modeling of step 3 is as follows:

[0059] (1) Assessment of power supply capacity of different regions

[0060] The power supply capacity of the 220kV sub-grid in the power grid is a typical multi-dimensional and multi-constrained nonlinear programming problem in mathematics, and its objective function is:

[0061] P LSC =max(P load )=max(∑P T +ΣP G ) (12)

[0062] Among them, P LSC Assume the power supply capacity of 220kV sub-area grid;

[0063] P load For 220kV zone load;

[0064] ΣP T It is the sum of the power delivered by the 750kV main transformers in the zone;

[0065] ΣP is the sum of the outputs of the generator sets in the partition;

[0066] The constraints to be considered are as follows:

[0067] 750kV main transformer power constraints:

[0068] P Ti ≤P TiN ,i=1,2,3,...,l (13)

[0069] 220kV line transmission power constraints:

[0070] P Lj ≤P LjN ,j=1,2,3,...,m (14)

[0071] Voltage inequality constraints:

[0072] U Bkmin ≤U Bk ≤U Bkmax ,k=1,2,3,...,n (15)

[0073] In formulas (13)-(15): P Ti , P TiN is the actual power and rated value of 750kV main transformer i;

[0074] P Lj , P LjN Actual and rated values ​​of transmission power for 220 kV line j;

[0075] U Bkmin , UBk , U Bkmax The specified operating upper limit, voltage rating and specified operating lower limit of the voltage of the zone bus k;

[0076] (2) Adaptive droop control strategy based on converter station power margin

[0077] In order to solve the shortcomings of the traditional droop control, which is not flexible and economical, the droop coefficient is dynamically adjusted according to the power margin of the converter station;

[0078] The classic droop control characteristics are described as follows:

[0079] P dc =P dc +dsU dc -U dc,ref = k(P dc -P dc,ref ) (16)

[0080]

[0081] U dc ,U dc,ref , represent the actual value and reference value of DC voltage respectively;

[0082] P dc ,P dc,ref are the actual value and reference value of DC power respectively;

[0083] k is the droop coefficient;

[0084] U dc,H , U dc,L They are the maximum and minimum values ​​of DC voltage respectively:

[0085] P dc,max , P dc,min are the maximum and minimum values ​​of DC power respectively;

[0086] The adaptive droop control is described as follows:

[0087]

[0088] Where b′=sgn(P dc,ref -P dc )b represents the DC voltage variation range, and its value is generally (5% to 10%);

[0089] ξ can be used to describe the power margin of the converter station. The closer the |ξ| value is to 1, the smaller the power margin of the converter station is. When |ξ|=1, the converter station is running at full load.

[0090] (3) Design of multi-terminal flexible DC control strategy to support regional interconnection of urban power grids

[0091] According to the above method (1), the power supply capacity of each partition is evaluated to determine the maximum power supply capacity P of each partition. LSC :

[0092]

[0093] Among them, K P Provide power supply margin index for each zone;

[0094] P ASC The available power supply capacity for the zone;

[0095] P 0 is the total initial load of the partition.

[0096] Furthermore, the fault partition converter station control mode can be divided into the following two cases:

[0097] Case 1: Fixed active power control is adopted in the fault zone converter station

[0098] Calculate the overload of the fault zone line or main transformer as follows:

[0099] P OL =P AF -P R (twenty one)

[0100] Among them, P OL is the component overload;

[0101] P AF is the power after the fault;

[0102] P R is the line or transformer rated power;

[0103] In order to eliminate line or transformer overload, the active power reference value of the converter station in the fault zone is changed to:

[0104]

[0105] Among them, P ref,0 and P ref are respectively the active power reference values ​​before and after the converter station fault;

[0106] M is the total number of overload components;

[0107] P OL,i is the overload of the ith component;

[0108] Case 2: Adaptive droop control is adopted in the fault zone converter station

[0109] In order to eliminate component overload, it is necessary to accurately control the flexible DC transmission power. Therefore, the fault partition converter station is switched to fixed active power control, and the other non-fault partition converter stations adopt adaptive droop control.

[0110] The beneficial effects of the present invention are as follows:

[0111] 1. Aiming at the safety and economical operation requirements of the power grid, the present invention establishes steady-state and transient optimization control objectives. Secondly, it constructs a comprehensive evaluation index covering load balancing, network loss and voltage deviation. The successive approximation method is used to quickly solve the optimal power of flexible direct current, so as to realize the power flow optimization control of the urban power grid in steady state.

[0112] 2. The present invention designs a multi-terminal flexible DC adaptive droop control strategy based on the power margin of the converter station, reasonably allocates emergency power support to each partition after a fault, and reduces the frequency deviation of nodes with heavier loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0113] Figure 1 It is a work flow chart of the present invention. DETAILED DESCRIPTION

[0114] To make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0115] See also Figure 1 The present invention provides an adaptive flexible direct current transmission control method based on optimal power of power grid partition, comprising the following steps:

[0116] Step 1: Establish the optimization control objectives of flexible interconnection of power grid partitions under steady-state and transient operation conditions;

[0117] Step 2: According to the steady-state control target established in step 1, the optimal power of flexible DC is quickly solved by the stepwise approximation method;

[0118] Step 3: Based on the transient control target established in step 1, a multi-terminal flexible DC adaptive droop control strategy is designed to achieve emergency power support between power grid sections;

[0119] Step 4: According to step 2, the optimal power of flexible DC is quickly solved based on the stepwise approximation method, and the multi-terminal flexible DC adaptive droop control strategy for emergency power support between grid partitions designed in step 3 is designed. The effectiveness and engineering practicability of the proposed method are verified through simulation models.

[0120] In this embodiment, preferably, the control objectives in step 1 include: steady-state power flow optimization and emergency power mutual assistance.

[0121] In this embodiment, preferably, step 2 establishes a comprehensive evaluation index of weighted network loss, load balancing and voltage deviation based on the steady-state control target established in step 1, and its specific modeling is as follows:

[0122] The flexible DC power optimization strategy under steady-state operation evaluates the steady-state power flow distribution through comprehensive evaluation indicators to determine the optimal operating power; the comprehensive evaluation indicators are composed of three parts: load balancing indicator, network loss indicator and voltage deviation indicator;

[0123] (1) Comprehensive evaluation index By establishing specific evaluation indexes, the steady-state power flow distribution of urban sub-grids under different flexible DC transmission powers is evaluated, the optimal operating power is determined, and the comprehensive evaluation index is established as follows:

[0124] G=aG α +bG β +cG γ G=aG α +bG β +cG γ (1)

[0125] Among them, G is the comprehensive score of the urban sub-grid under a certain transmission power of flexible DC;

[0126] G α is the normalized load balancing indicator;

[0127] G β is the normalized network loss index;

[0128] G γ is the normalized line voltage deviation index;

[0129] a, b, and c are the weights of the three indicators, which are calculated using a method based on information entropy theory;

[0130] (2) Load balancing indicators

[0131] In steady state, the flexible DC is controlled to transfer power from the light-load partition to the heavy-load partition, balance the load rate of the main transformer in each partition, improve the utilization efficiency of the main transformer, and reduce the discrete degree of the load rate of the main transformer in each partition. In order to facilitate the analysis and comparison between different indicators, a normalized load balancing index G is established. α as follows:

[0132]

[0133] Among them, σ 0 and σ 1 They are the standard deviations of the load rates of all 750kv main transformers in the urban power grid before and after the commissioning of flexible DC.

[0134] σ op is the minimum standard deviation of the load factor of the main transformer within the adjustable range of flexible DC power;

[0135] In order to measure the dispersion of the load factor of the main transformer, the standard deviation of the load factor of all 750kV main transformers within the urban power grid is calculated as follows:

[0136]

[0137]

[0138] Where, σ is the standard deviation of the load factor of N 750kV main transformers;

[0139] P Ti , S Ti , η Ti are the power, capacity and load rate of the ith 500kV main transformer respectively; is the arithmetic mean of the load factors of N main transformers;

[0140] (3) Network loss index

[0141] Under steady-state conditions, controlling the flexible DC transmission power can make the power flow between sub-zones evenly distributed, reduce line network losses and transformer losses, and improve the economic efficiency of system operation. In order to compare and analyze the network losses of urban sub-zone power grids before and after the commissioning of flexible DC, the normalized power loss index is used as follows:

[0142]

[0143] Among them, P 0 ,P 1 are the sum of network losses of all subareas of the urban power grid before and after the commissioning of the flexible DC power grid;

[0144] min(P 1 ) is the minimum value of network loss within the adjustable range of flexible DC power;

[0145] Network losses include 220kV line losses and 750 / 220kV transformer losses, as well as flexible DC converter and line losses. The calculation formula is as follows:

[0146] P 0,0 =P L,0 +P T,0 +P C,0 (7)

[0147] Among them, P 0,0 is the network loss of the urban power grid;

[0148] P L,0 , P T,0 They are line loss and transformer loss respectively;

[0149] P C,0 is the flexible DC converter loss;

[0150] (4) Voltage deviation index

[0151] In AC / DC hybrid systems, voltage is closely related to network power balance, so the voltage offset of all nodes is used as the performance indicator of the system:

[0152]

[0153] Among them, u 0 and u 1 They are the standard deviations of voltages of all transformers in the urban power grid before and after the commissioning of flexible DC power supply;

[0154] u op is the minimum voltage difference of the main transformer within the adjustable range of flexible DC power;

[0155]

[0156] Where U is the voltage standard deviation of N 750kV main transformers;

[0157] U Ti , U T They are the actual voltage, rated voltage and voltage deviation rate of the i-th 750kV main transformer;

[0158] U T It is the arithmetic mean of the voltage deviation rates of N main transformers.

[0159] In this embodiment, preferably, the optimal power solution is performed through the load balancing index, network loss index and voltage deviation index of step 3; the load balancing index, network loss index and voltage deviation index have different characteristics. Under steady-state operation, as the flexible DC transmission power increases, the three-phase indicators first increase and then decrease, but the power corresponding to the turning point is different, and the load balancing effect is more obvious.

[0160] In this embodiment, preferably, step 2 includes the following steps:

[0161] Step 2-1: Online statistics of the power of each 750kV main transformer in the urban power grid zoning operation mode, and calculation of the standard deviation of the main transformer load factor σ before the flexible DC operation 0 、Network loss P 0,0 and line voltage deviation U;

[0162] Step 2-2: Determine the operating range of flexible DC power between zones based on the line rated power and main transformer capacity constraints [P min,P max ];

[0163] Step 2-3: Set the flexible DC initial power P DC,0 , power growth direction d, power growth initial step s, convergence accuracy ε;

[0164] Step 2-4: Calculate the flexible DC transmission power P DC The standard deviation of the load factor of the main transformer in each partition is 1 、Network loss P 1 Substituting the voltage deviation U into equation (2), equation (6), and equation (1) to calculate G α ,G β ,G γ and comprehensive score G;

[0165] Step 2-5: If satisfied Then update the optimal score G = G max , while continuing to increase the flexible DC power P dc =P dc +ds, return to step 2-4; if G<G max , then update the step size s = s / 2, and reduce the flexible DC power P dc =P dc +ds, to accurately solve the optimal power, determine whether s is greater than ε, if so, go to step 2-4, if not, then you can determine G max The corresponding power is the flexible DC optimal power P dc,op .

[0166] In this embodiment, preferably, step 3 is specifically modeled as follows:

[0167] (1) Assessment of power supply capacity of different regions

[0168] The power supply capacity of the 220kV sub-grid in the power grid is a typical multi-dimensional and multi-constrained nonlinear programming problem in mathematics, and its objective function is:

[0169] P LSC =max(P load )=max(∑P T +∑P G ) (12)

[0170] Among them, P LSC Assume the power supply capacity of 220kV sub-area grid;

[0171] P load For 220kV zone load;

[0172] ΣP T It is the sum of the power delivered by the 750kV main transformers in the zone;

[0173] ∑P is the sum of the outputs of the generator sets in the partition;

[0174] The constraints to be considered are as follows:

[0175] 750kV main transformer power constraints:

[0176] P Ti ≤P TiN ,i=1,2,3,...,l (13)

[0177] 220kV line transmission power constraints:

[0178] P Lj ≤P LjN ,j=1,2,3,...,m (14)

[0179] Voltage inequality constraints:

[0180] U Bkmin ≤U Bk ≤U Bkmax ,k=1,2,3,...,n (15)

[0181] In formulas (13)-(15): P Ti , P TiN is the actual power and rated value of 750kV main transformer i;

[0182] P Lj , P LjN Actual and rated values ​​of transmission power for 220 kV line j;

[0183] U Bkmin , U Bk , U Bkmax The specified operating upper limit, voltage rating and specified operating lower limit of the voltage of the zone bus k;

[0184] (2) Adaptive droop control strategy based on converter station power margin

[0185] In order to solve the shortcomings of the traditional droop control, which is not flexible and economical, the droop coefficient is dynamically adjusted according to the power margin of the converter station;

[0186] The classic droop control characteristics are described as follows:

[0187] P dc =P dc +dsU dc -U dc,ref = k(P dc -P dc,ref ) (16)

[0188]

[0189] U dc ,U dc,ref , represent the actual value and reference value of DC voltage respectively;

[0190] P dc ,P dc,ref are the actual value and reference value of DC power respectively;

[0191] k is the droop coefficient;

[0192] U dc,H , U dc,L They are the maximum and minimum values ​​of DC voltage respectively:

[0193] P dc,max , P dc,min are the maximum and minimum values ​​of DC power respectively;

[0194] The adaptive droop control is described as follows:

[0195]

[0196] Where b′=sgn(P dc,ref -P dc )b represents the DC voltage variation range, and its value is generally (5% to 10%);

[0197] ξ can be used to describe the power margin of the converter station. The closer the |ξ| value is to 1, the smaller the power margin of the converter station is. When |ξ|=1, the converter station is running at full load.

[0198] (3) Design of multi-terminal flexible DC control strategy to support regional interconnection of urban power grids

[0199] According to the above method (1), the power supply capacity of each partition is evaluated to determine the maximum power supply capacity P of each partition. LSC :

[0200]

[0201] Among them, K P Provide power supply margin index for each zone;

[0202] P ASC The available power supply capacity for the zone;

[0203] P 0 is the total initial load of the partition.

[0204] In this embodiment, preferably, the fault partition converter station control modes are divided into the following two situations:

[0205] Case 1: Fixed active power control is adopted in the fault zone converter station

[0206] Calculate the overload of the fault zone line or main transformer as follows:

[0207] P OL =P AF -P R (twenty one)

[0208] Among them, P OL is the component overload;

[0209] P AF is the post-fault power;

[0210] P R is the line or transformer rated power;

[0211] In order to eliminate line or transformer overload, the active power reference value of the converter station in the fault zone is changed to:

[0212]

[0213] Among them, P ref,0 and P ref are respectively the active power reference values ​​before and after the converter station fault;

[0214] M is the total number of overload components;

[0215] P OL,i is the overload of the ith component;

[0216] Case 2: Adaptive droop control is adopted in the fault zone converter station

[0217] In order to eliminate component overload, it is necessary to accurately control the flexible DC transmission power. Therefore, the fault partition converter station is switched to fixed active power control, and the other non-fault partition converter stations adopt adaptive droop control.

[0218] The working principle and use process of the present invention:

[0219] Step 1: Establish the optimization control objectives of grid partition flexible interconnection under steady-state and transient operation conditions; the control objectives include: steady-state power flow optimization and emergency power mutual assistance

[0220] Step 2: Based on the steady-state control target established in step 1, a comprehensive evaluation index of weighted network loss, load balancing and voltage deviation is established. The specific modeling is as follows:

[0221] The flexible DC power optimization strategy under steady-state operation evaluates the steady-state power flow distribution through comprehensive evaluation indicators to determine the optimal operating power; the comprehensive evaluation indicators are composed of three parts: load balancing indicator, network loss indicator and voltage deviation indicator;

[0222] (1) Comprehensive evaluation index By establishing specific evaluation indexes, the steady-state power flow distribution of urban sub-grids under different flexible DC transmission powers is evaluated, the optimal operating power is determined, and the comprehensive evaluation index is established as follows:

[0223] G=aG α +bG β +cG γ (1)

[0224] Among them, G is the comprehensive score of the urban sub-grid under a certain transmission power of flexible DC;

[0225] G α is the normalized load balancing indicator;

[0226] G β is the normalized network loss index;

[0227] G γ is the normalized line voltage deviation index;

[0228] a, b, and c are the weights of the three indicators, which are calculated using a method based on information entropy theory;

[0229] (2) Load balancing indicators

[0230] In steady state, the flexible DC is controlled to transfer power from the light-load partition to the heavy-load partition, balance the load rate of the main transformer in each partition, improve the utilization efficiency of the main transformer, and reduce the discrete degree of the load rate of the main transformer in each partition. In order to facilitate the analysis and comparison between different indicators, a normalized load balancing index G is established. α as follows:

[0231]

[0232] Among them, σ 0 and σ 1 They are the standard deviations of load factors of all 750kv main transformers in the urban power grid before and after the commissioning of flexible DC.

[0233] σ op is the minimum standard deviation of the load factor of the main transformer within the adjustable range of flexible DC power;

[0234] In order to measure the dispersion of the load factor of the main transformer, the standard deviation of the load factor of all 750kV main transformers within the urban power grid is calculated as follows:

[0235]

[0236] Where, σ is the standard deviation of the load factor of N 750kV main transformers;

[0237] PTi , S Ti , η Ti are the power, capacity and load rate of the ith 500kV main transformer respectively;

[0238] is the arithmetic mean of the load factors of N main transformers;

[0239] (3) Network loss index

[0240] Under steady-state conditions, controlling the flexible DC transmission power can make the power flow between sub-zones evenly distributed, reduce line network losses and transformer losses, and improve the economic efficiency of system operation. In order to compare and analyze the network losses of urban sub-zone power grids before and after the commissioning of flexible DC, the normalized power loss index is used as follows:

[0241]

[0242] Among them, P 0 ,P 1 are the sum of network losses of all subareas of the urban power grid before and after the commissioning of the flexible DC power grid;

[0243] min(P 1 ) is the minimum value of network loss within the adjustable range of flexible DC power;

[0244] Network losses include 220kV line losses and 750 / 220kV transformer losses, as well as flexible DC converter and line losses. The calculation formula is as follows:

[0245] P 0,0 =P L,0 +P T,0 +P C,0 (7)

[0246] Among them, P 0,0 is the network loss of the urban power grid;

[0247] P L,0 , P T,0 They are line loss and transformer loss respectively;

[0248] P C,0 is the flexible DC converter loss;

[0249] (4) Voltage deviation index

[0250] In AC / DC hybrid systems, voltage is closely related to network power balance, so the voltage offset of all nodes is used as the performance indicator of the system:

[0251]

[0252] Among them, u 0and u 1 They are the standard deviations of voltages of all transformers in the urban power grid before and after the commissioning of flexible DC power supply;

[0253] u op is the minimum voltage difference of the main transformer within the adjustable range of flexible DC power;

[0254]

[0255] Where U is the voltage standard deviation of N 750kV main transformers;

[0256] U Ti , U T They are the actual voltage, rated voltage and voltage deviation rate of the i-th 750kV main transformer;

[0257] U T It is the arithmetic mean of the voltage deviation rates of N main transformers.

[0258] The following steps are involved:

[0259] Step 2-1: Online statistics of the power of each 750kV main transformer in the urban power grid zoning operation mode, and calculation of the standard deviation of the main transformer load factor σ before the flexible DC operation 0 、Network loss P 0,0 and line voltage deviation U;

[0260] Step 2-2: Determine the operating range of flexible DC power between zones based on the line rated power and main transformer capacity constraints [P min ,P max ];

[0261] Step 2-3: Set the flexible DC initial power P DC,0 , power growth direction d, power growth initial step s, convergence accuracy ε;

[0262] Step 2-4: Calculate the flexible DC transmission power P DC The standard deviation of the load factor of the main transformer in each partition is 1 、Network loss P 1 Substituting the voltage deviation U into equation (2), equation (6), and equation (1) to calculate G α ,G β ,G γ and comprehensive score G;

[0263] Step 2-5: If satisfied Then update the optimal score G = G max , while continuing to increase the flexible DC power P dc =P dc +ds, return to step 2-4; if G<Gmax , then update the step size s = s / 2, and reduce the flexible DC power P dc =P dc +ds, to accurately solve the optimal power, determine whether s is greater than ε, if so, go to step 2-4, otherwise you can determine G max The corresponding power is the flexible DC optimal power P dc,op

[0264] Step 3: According to the transient control target established in step 1, design a multi-terminal flexible DC adaptive droop control strategy that can achieve emergency power support between power grid partitions; solve the optimal power through the load balancing index, network loss index and voltage deviation index in step 3; the load balancing index, network loss index and voltage deviation index have different characteristics. Under steady-state operation, as the flexible DC transmission power increases, the three-phase indexes increase first and then decrease, but the power corresponding to the turning point is different, and the load balancing effect is more obvious. The specific modeling is as follows:

[0265] (1) Assessment of power supply capacity of different regions

[0266] The power supply capacity of the 220kV sub-grid in the power grid is a typical multi-dimensional and multi-constrained nonlinear programming problem in mathematics, and its objective function is:

[0267] P LSC =max(P load )=max(∑P T +∑P G ) (12)

[0268] Among them, P LSC Assume the power supply capacity of 220kV sub-area grid;

[0269] P load For 220kV zone load;

[0270] ∑P T It is the sum of the power delivered by the 750kV main transformers in the zone;

[0271] ∑P is the sum of the outputs of the generator sets in the partition;

[0272] The constraints to be considered are as follows:

[0273] 750kV main transformer power constraints:

[0274] P Ti ≤P TiN ,i=1,2,3,...,l (13)

[0275] 220kV line transmission power constraints:

[0276] PLj ≤P LjN ,j=1,2,3,...,m (14)

[0277] Voltage inequality constraints:

[0278] U Bkmin ≤U Bk ≤U Bkmax ,k=1,2,3,...,n (15)

[0279] In formula (13)-15), P Ti , P TiN is the actual power and rated value of 750kV main transformer i;

[0280] P Lj , P LjN Actual and rated values ​​of transmission power for 220 kV line j;

[0281] U Bkmin , U Bk , U Bkmax The specified operating upper limit, voltage rating and specified operating lower limit of the voltage of the zone bus k;

[0282] (2) Adaptive droop control strategy based on converter station power margin

[0283] In order to solve the shortcomings of the traditional droop control, which is not flexible and economical, the droop coefficient is dynamically adjusted according to the power margin of the converter station;

[0284] The classic droop control characteristics are described as follows:

[0285] P dc =P dc +dsU dc -U dc,ref = k(P dc -P dc,ref ) (16)

[0286]

[0287] U dc ,U dc,ref , represent the actual value and reference value of DC voltage respectively;

[0288] P dc ,P dc,ref are the actual value and reference value of DC power respectively;

[0289] k is the droop coefficient;

[0290] U dc,H , U dc,L They are the maximum and minimum values ​​of DC voltage respectively:

[0291] P dc,max , P dc,min are the maximum and minimum values ​​of DC power respectively;

[0292] The adaptive droop control is described as follows:

[0293]

[0294] Where b′=sgn(P dc,ref -P dc )b represents the DC voltage variation range, and its value is generally (5% to 10%);

[0295] ξ can be used to describe the power margin of the converter station. The closer the |ξ| value is to 1, the smaller the power margin of the converter station is. When |ξ|=1, the converter station is running at full load.

[0296] (3) Design of multi-terminal flexible DC control strategy to support regional interconnection of urban power grids

[0297] According to the above method (1), the power supply capacity of each partition is evaluated to determine the maximum power supply capacity P of each partition. LSC :

[0298]

[0299] Among them, K P Provide power supply margin index for each zone;

[0300] P ASC The available power supply capacity for the zone;

[0301] P 0 is the total initial load of the partition.

[0302] In this embodiment, preferably, the fault partition converter station control modes are divided into the following two situations:

[0303] Case 1: Fixed active power control is adopted in the fault zone converter station

[0304] Calculate the overload of the fault zone line or main transformer as follows:

[0305] P OL =P AF -P R (twenty one)

[0306] Among them, P OL is the component overload;

[0307] P AF is the power after the fault;

[0308] P R is the line or transformer rated power;

[0309] In order to eliminate line or transformer overload, the active power reference value of the converter station in the fault zone is changed to:

[0310]

[0311] Among them, P ref,0 and P ref are respectively the active power reference values ​​before and after the converter station fault;

[0312] M is the total number of overload components;

[0313] P OL,i is the overload of the ith component;

[0314] Case 2: Adaptive droop control is adopted in the fault zone converter station

[0315] In order to eliminate component overload, it is necessary to accurately control the flexible DC transmission power. Therefore, the fault partition converter station is switched to fixed active power control, and the other non-fault partition converter stations adopt adaptive droop control.

[0316] Step 4: According to step 2, the optimal power of flexible DC is quickly solved based on the stepwise approximation method, and the multi-terminal flexible DC adaptive droop control strategy for emergency power support between grid partitions designed in step 3 is designed. The effectiveness and engineering practicability of the proposed method are verified through simulation models.

[0317] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An adaptive flexible direct current transmission control method based on optimal power of power grid partitions, characterized in that: The steps include: Step 1: Establish the optimization control objectives of flexible interconnection of power grid partitions under steady-state and transient operation conditions; Step 2: According to the steady-state control target established in step 1, the optimal power of flexible DC is quickly solved by the stepwise approximation method; Step 3: Based on the transient control target established in step 1, a multi-terminal flexible DC adaptive droop control strategy is designed to achieve emergency power support between power grid sections; Step 4: According to step 2, the optimal power of flexible DC is quickly solved based on the stepwise approximation method, and the multi-terminal flexible DC adaptive droop control strategy for emergency power support between grid partitions designed in step 3 is designed. The effectiveness and engineering practicability of the proposed method are verified through simulation models.

2. The adaptive flexible direct current transmission control method based on optimal power of power grid partition according to claim 1 is characterized in that: The control objectives in step 1 include: steady-state power flow optimization and emergency power mutual assistance.

3. The adaptive flexible direct current transmission control method based on optimal power of power grid partition according to claim 1 is characterized in that: The step 2 establishes a comprehensive evaluation index of weighted network loss, load balancing and voltage deviation based on the steady-state control target established in step 1, and its specific modeling is as follows: The flexible DC power optimization strategy under steady-state operation evaluates the steady-state power flow distribution through comprehensive evaluation indicators to determine the optimal operating power; the comprehensive evaluation indicators are composed of three parts: load balancing indicator, network loss indicator and voltage deviation indicator; (1) Comprehensive evaluation index By establishing specific evaluation indexes, the steady-state power flow distribution of urban sub-grids under different flexible DC transmission powers is evaluated, the optimal operating power is determined, and the comprehensive evaluation index is established as follows: G=aG α +bG β +cG γ (1) Among them, G is the comprehensive score of the urban sub-grid under a certain transmission power of flexible DC; G α is the normalized load balancing indicator; G β is the normalized network loss index; G γ is the normalized line voltage deviation index; a, b, and c are the weights of the three indicators, which are calculated using a method based on information entropy theory; (2) Load balancing indicators In steady state, the flexible DC is controlled to transfer power from the light-load partition to the heavy-load partition, balance the load rate of the main transformer in each partition, improve the utilization efficiency of the main transformer, and reduce the discrete degree of the load rate of the main transformer in each partition. In order to facilitate the analysis and comparison between different indicators, a normalized load balancing index G is established. α as follows: Among them, σ0 and σ1 are the standard deviations of the load factors of all 750kv main transformers in the urban power grid before and after the commissioning of flexible DC. σ op is the minimum standard deviation of the load factor of the main transformer within the adjustable range of flexible DC power; In order to measure the dispersion of the load factor of the main transformer, the standard deviation of the load factor of all 750kV main transformers within the urban power grid is calculated as follows: Where, σ is the standard deviation of the load factor of N 750kV main transformers; P Ti , S Ti , η Ti are the power, capacity and load rate of the ith 500kV main transformer respectively; is the arithmetic mean of the load factors of N main transformers; (3) Network loss index Under steady-state conditions, controlling the flexible DC transmission power can make the power flow between sub-zones evenly distributed, reduce line network losses and transformer losses, and improve the economic efficiency of system operation. In order to compare and analyze the network losses of urban sub-zone power grids before and after the commissioning of flexible DC, the normalized power loss index is used as follows: Among them, P0 and P1 are the total network losses of all partitions of the urban power grid before and after the flexible DC operation respectively; min(P1) is the minimum value of network loss within the adjustable range of flexible DC power; Network losses include 220kV line losses and 750 / 220kV transformer losses, as well as flexible DC converter and line losses. The calculation formula is as follows: P 0,0 =P L,0 +P T,0 +P C,0 (7) Among them, P 0,0 is the network loss of the urban power grid; P L,0 , P T,0 They are line loss and transformer loss respectively; P C,0 is the flexible DC converter loss; (4) Voltage deviation index In AC / DC hybrid systems, voltage is closely related to network power balance, so the voltage offset of all nodes is used as the performance indicator of the system: Among them, u0 and u1 are the standard deviations of the voltages of all transformers in the urban power grid before and after the commissioning of flexible DC. op is the minimum voltage difference of the main transformer within the adjustable range of flexible DC power; Where U is the voltage standard deviation of N 750kV main transformers; U Ti , U T They are the actual voltage, rated voltage and voltage deviation rate of the i-th 750kV main transformer; U T It is the arithmetic mean of the voltage deviation rates of N main transformers.

4. The adaptive flexible direct current transmission control method based on optimal power of power grid partition according to claim 1 is characterized in that: The optimal power is solved through the load balancing index, network loss index and voltage deviation index described in step 3; the load balancing index, network loss index and voltage deviation index have different characteristics. Under steady-state operation, as the flexible DC transmission power increases, the three-phase indexes first increase and then decrease, but the turning points correspond to different powers, and the load balancing effect is more obvious.

5. The adaptive flexible direct current transmission control method based on optimal power of power grid partition according to claim 1 is characterized in that: Step 2 includes the following steps: Step 2-1: Online statistics of the power of each 750kV main transformer in the urban power grid zoning operation mode, and calculation of the standard deviation of the main transformer load factor σ0 and network loss P before the flexible DC operation 0,0 and line voltage deviation U; Step 2-2: Determine the operating range of flexible DC power between zones based on the line rated power and main transformer capacity constraints [P min ,P max ]; Step 2-3: Set the flexible DC initial power P DC,0 , power growth direction d, power growth initial step s, convergence accuracy ε; Step 2-4: Calculate the flexible DC transmission power P DC Substitute the standard deviation of the load factor σ1, network loss P1 and voltage deviation U of each partition into equations (2), (6) and (1) to calculate G α ,G β ,G γ and comprehensive score G; Step 2-5: If satisfied Then update the optimal score G = G max , while continuing to increase the flexible DC power P dc =P dc +ds, return to step 2-4; if G<G max , then update the step size s = s / 2, and reduce the flexible DC power P dc =P dc +ds, to accurately solve the optimal power, determine whether s is greater than ε, if so, go to step 2-4, if not, then you can determine G max The corresponding power is the flexible DC optimal power P dc,op .

6. The adaptive flexible direct current transmission control method based on optimal power of power grid partition according to claim 1 is characterized in that: The specific modeling of step 3 is as follows: (1) Assessment of power supply capacity of different regions The power supply capacity of the 220kV sub-grid in the power grid is a typical multi-dimensional and multi-constrained nonlinear programming problem in mathematics, and its objective function is: P LSC =max(P load )=max(∑P T +∑P G ) (12) Among them, P LSC Assume the power supply capacity of 220kV sub-area grid; P load For 220kV zone load; ∑P T It is the sum of the power delivered by the 750kV main transformers in the zone; ∑P is the sum of the outputs of the generator sets in the partition; The constraints to be considered are as follows: 750kV main transformer power constraints: P Ti ≤P TiN ,i=1,2,3,...,l (13) 220kV line transmission power constraints: P Lj ≤P LjN ,j=1,2,3,...,m (14) Voltage inequality constraints: U Bkmin ≤U Bk ≤U Bkmax ,k=1,2,3,...,n (15) In formulas (13)-(15): P Ti , P TiN is the actual power and rated value of 750kV main transformer i; P Lj , P LjN Actual and rated values ​​of transmission power for 220 kV line j; U Bkmin , U Bk , U Bkmax The specified operating upper limit, voltage rating and specified operating lower limit of the voltage of the zone bus k; (2) Adaptive droop control strategy based on converter station power margin In order to solve the shortcomings of the traditional droop control, which is not flexible and economical, the droop coefficient is dynamically adjusted according to the power margin of the converter station; The classic droop control characteristics are described as follows: P dc =P dc +dsU dc -U dc,ref =k(P dc -P dc,ref ) (16) U dc ,U dc,ref , represent the actual value and reference value of DC voltage respectively; P dc ,P dc,ref are the actual value and reference value of DC power respectively; k is the droop coefficient; U dc,H , U dc,L They are the maximum and minimum values ​​of DC voltage respectively: P dc,max , P dc,min are the maximum and minimum values ​​of DC power respectively; The adaptive droop control is described as follows: Where b′=sgn(P dc,ref -P dc )b represents the DC voltage variation range, and its value is generally (5% to 10%); ξ can be used to describe the power margin of the converter station. The closer the |ξ| value is to 1, the smaller the power margin of the converter station is. When |ξ|=1, the converter station is running at full load. (3) Design of multi-terminal flexible DC control strategy to support regional interconnection of urban power grids According to the above method (1), the power supply capacity of each partition is evaluated to determine the maximum power supply capacity P of each partition. LSC : Among them, K P Provide power supply margin index for each zone; P ASC The available power supply capacity for the zone; P0 is the total initial load of the partition.

7. The adaptive flexible direct current transmission control method based on optimal power of power grid partition according to claim 6 is characterized in that: The fault partition converter station control mode can be divided into the following two cases: Case 1: Fixed active power control is adopted in the fault zone converter station Calculate the overload of the fault zone line or main transformer as follows: P OL =P AF -P R (21) Among them, P OL is the component overload; P AF is the power after the fault; P R is the line or transformer rated power; In order to eliminate line or transformer overload, the active power reference value of the converter station in the fault zone is changed to: Among them, P ref,0 and P ref are respectively the active power reference values ​​before and after the converter station fault; M is the total number of overload components; P OL,i is the overload of the ith component; Case 2: Adaptive droop control is adopted in the fault zone converter station In order to eliminate component overload, it is necessary to accurately control the flexible DC transmission power. Therefore, the fault partition converter station is switched to fixed active power control, and the other non-fault partition converter stations adopt adaptive droop control.

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