Wind power plant voltage active support control method based on doubly-fed fan operation difference

Abstract

The invention discloses a wind power plant voltage active support control method based on doubly-fed fan operation difference, and belongs to the technical field of voltage safety and stability, and the method comprises the following steps: S1, calculating the reactive power demand of a system through the voltage drop degree of a grid-connected point of the system; s2, setting a wind power plant control strategy segmentation criterion; s3, reactive power distribution is carried out by considering the voltage supporting capability difference; and S4, carrying out coordination and secondary distribution on the reactive power. By adopting the wind power plant voltage active support control method based on the doubly-fed fan operation difference, the power grid voltage can be ensured to be within an allowable range when a power system is subjected to large disturbance, so that the reactive power capacity of each wind turbine generator in the wind power plant can be fully excavated and utilized; and reduction of active power output of the wind power plant during a fault period can be reduced, and safe and stable operation of the system is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of voltage safety and stability, and in particular to a method for actively supporting and controlling voltage in a wind farm based on the operational differences of a doubly-fed wind turbine. Background Art

[0002] With the large-scale grid connection of renewable energy power generation, wind power has become the main power source for building new power systems, and its installed capacity has steadily increased. At the same time, the problems of low inertia level and weak voltage support capacity of new power systems have become increasingly prominent, especially the low voltage ride through (LVRT) problem of wind farms under fault excitation is still one of the key issues that need to be solved urgently.

[0003] When the power grid is subjected to a large disturbance, wind farms that do not have strong low voltage ride-through capabilities may cause large-scale grid disconnection accidents due to continuous low voltage in the system, seriously threatening the safe and stable operation of the power system. The existing wind farm voltage control strategy formulation does not fully consider the voltage support capacity and constraints of wind turbines, and rarely considers the impact of active power constraints and reductions of wind turbines. There are problems such as insufficient exploration of the potential for reactive power regulation in wind farms and a single applicable scenario. Therefore, it is urgent to build an active voltage support strategy suitable for wind farms to ensure that wind farms have sufficient voltage support capabilities during faults, which is of great significance to the safe grid connection of large-scale wind power. Summary of the invention

[0004] The purpose of the present invention is to provide a method for actively supporting and controlling the voltage of a wind farm based on the operating differences of a doubly-fed wind turbine, which can ensure that the grid voltage is within an allowable range when the power system is subject to a large disturbance, can fully tap and utilize the reactive capacity of each wind turbine in the wind farm, and can also reduce the reduction in the active output of the wind farm during a fault, thereby ensuring the safe and stable operation of the system.

[0005] To achieve the above object, the present invention provides a method for actively supporting voltage control of a wind farm based on the operational differences of a doubly-fed wind turbine, comprising the following steps:

[0006] S1. Calculate the reactive power demand of the system by the voltage drop degree of the system grid connection point;

[0007] S2. Setting the segmentation criteria of wind farm control strategy;

[0008] S3. Consider the differences in voltage support capabilities for reactive power distribution;

[0009] S4. Coordinate and redistribute reactive power.

[0010] Preferably, in S1, when a three-phase short circuit fault occurs in the power grid, the wind farm has an external fault, and the fault voltage U of the wind farm connection point is calculated. ffor:

[0011]

[0012] Where, E is the infinite grid voltage, and its per-unit value is 1p.u., Z l is the line impedance, Z f is the equivalent impedance of the fault point grounding, a is the fault location; the target value of the grid connection point voltage is set to U ref , then the voltage value that the wind farm needs to raise is:

[0013] ΔU=U ref -U f ;

[0014] Ignoring the transverse component of voltage drop, the reactive power Q that the wind farm needs to provide to the grid is: WF_ref for:

[0015]

[0016] Preferably, in S2, stator flux directional vector control is adopted in the dq axis coordinate system, and the rotor current expression is derived as follows:

[0017]

[0018] In the formula, u s is the terminal voltage, i rd 、i rq are the rotor currents in the d and q axis directions respectively, i sd 、i sq are the stator currents in the d and q axis directions, P s , Q s are the active and reactive power output on the stator side, ψ s and ψ r are the stator and rotor flux, L m is the mutual inductance of the stator and rotor windings, ω 1 is the synchronous speed, ω slip is the slip rate;

[0019] Maximum allowable current of rotor-side converter I rmax The following constraints are met:

[0020] i rd 2 +i rq 2 =i r 2 ≤I rmax 2 ;

[0021] Get the reactive power limit Q on the stator side of the unit smaxThe expression is:

[0022]

[0023] The voltage support capacity of wind turbines is enhanced by reducing active power. The reactive power output limit of wind turbines after reduction is:

[0024]

[0025] In the formula, ΔP s is the active power reduction; ignoring the reactive power on the rotor side, the stator side reactive power limit Q smax Considered as the reactive power limit Q of a single unit imax ;

[0026] Different active power constraints are obtained according to the operating status of different wind turbines, and then the reactive power limit of the entire wind farm when a fault occurs is obtained by calculation and weighted summation according to the reactive power limit of a single unit. The total reactive power output limit Q of the wind farm when the power grid fails is WFmax for:

[0027]

[0028] In the formula, Q imax is the reactive power limit of unit i, and n is the total number of wind turbines;

[0029] After knowing the total reactive power limit of the wind farm, the voltage segment control criterion U min The calculation formula is:

[0030]

[0031] Preferably, the active power constraint of the unit in different operating states is obtained according to the operating state of the unit, specifically:

[0032] When the SDBR of the unit does not work, the minimum value of the unit output active power is set to P smin , the active power constraint of wind turbine is:

[0033] P s -ΔP s ≥P smin ;

[0034] When the SDBR of the unit is in operation, the DFIG rotor side outputs active power P r Must satisfy R SDBR The consumption is calculated according to the maximum rotor current I rmax Setting:

[0035]

[0036] The relationship between the active power output of the DFIG stator and rotor is:

[0037] P r =-sP s ;

[0038]

[0039] Where, s is the unit slip rate;

[0040] Then the constraint condition of active power is:

[0041]

[0042] Preferably, according to the operation of the power grid, the voltage segment control criterion U min Set as: U 1min , U 2min and U 3min , where U 1min , U 2min and U 3min is the limit voltage value that can meet the reactive power demand of the system under different control modes of the wind farm, and the relationship is:

[0043] U 1min >U 3min >U 2min ;

[0044] When U f ∈[U 1min ,U ref ], the reactive power of the wind farm meets the reactive power demand of the power grid, and there is no need to reduce the active power;

[0045] When U f ∈[U 2min ,U 1min ], the wind farm needs to support the grid voltage by reducing active power to expand the reactive capacity of the unit, and the unit SDBR does not operate;

[0046] When U f ∈[U 3min ,U 1min ], the wind farm needs to expand the reactive capacity of the unit by reducing active power to support the grid voltage, and the unit SDBR is activated.

[0047] Preferably, in S3, when the grid connection point voltage U f ∈[U 1min ,U ref ], the voltage support evaluation factor M is constructed according to the reactive capacity of the wind turbine. i :

[0048]

[0049] In the formula, Qimax is the reactive capacity of a single wind turbine, is the total reactive capacity of the wind farm;

[0050] According to the power transmission distance, establish the voltage support evaluation factor N i :

[0051]

[0052] In the formula, ΔU max is the voltage drop value of the unit with the most serious voltage disturbance, ΔU i is the terminal voltage drop value of unit i;

[0053] Calculate the voltage support coefficient λ of wind turbine i i for:

[0054] λ i =M i N i ;

[0055] In the formula, λ i The value range is 0 to 1. The power distribution is proportional to the voltage support evaluation coefficients of different wind turbines. The specific method is:

[0056] c=λ 1 +λ 2 +...+λ n ;

[0057]

[0058] Where c is the sum of the reactive power support capacity evaluation coefficients of the units in the field, Q iref is the reactive power command of unit i. The reactive power commands of different wind turbines are proportional to their voltage support coefficients.

[0059] Preferably, in S3, when the grid connection point voltage U f ∈[U 2min ,U 1min ], the wind turbine needs to reduce active power, the wind turbine SDBR does not operate, and the maximum reactive power Q is calculated based on the active power output of the wind farm. 0 , and further calculate the reactive power Q that needs to be increased by reducing the active power 1 :

[0060] Q 1 =Q WF_ref -Q 0 ;

[0061] A new voltage support evaluation factor H is constructed based on the initial active output of the wind turbine and its reactive power change rate. i :

[0062]

[0063] Where P i is the active power generated by unit i, α i is the average reactive power change rate of unit i, α i The method to find is:

[0064]

[0065] In the formula, b and a are the upper and lower limits of the active power of the wind turbine respectively;

[0066] Comprehensive voltage support evaluation factor N i , the voltage support coefficient w of wind turbine i in this scenario is obtained i :

[0067] w i =H i N i ;

[0068] Similarly, the reactive power is distributed in equal proportion according to the voltage support coefficient of each wind turbine:

[0069]

[0070] Where d is the sum of the coefficients of each unit, ΔQ iref The reactive power that needs to be generated by unit i by reducing its active power;

[0071] The reactive power command value of wind turbine i is:

[0072] Q iref =Q 0i +ΔQ iref ;

[0073] In the formula, Q 0i In order not to reduce the reactive capacity of group i during active operation.

[0074] Preferably, in S3, when the grid connection point voltage U f ∈[U 3min ,U 1min ], the wind turbine needs to reduce active power, and the wind turbine SDBR action, define a new voltage support evaluation factor G i :

[0075]

[0076] Where ΔQ imax is the maximum value of reactive power increase of unit i;

[0077] As above, the reactive power distribution method of each wind turbine is:

[0078] e i =G i N i H i ;

[0079]

[0080] In the formula, e i This is the reactive support coefficient of each unit in this scenario, and ε is the sum of the reactive support coefficients.

[0081] Preferably, in S4, after determining the fault scenario according to the voltage change at the wind farm grid connection point and selecting the corresponding reactive power distribution scheme, if the system detects that there are wind turbines that cannot issue the corresponding reactive power according to the command, the secondary distribution module of the reactive power command is started.

[0082] Preferably, the total remaining reactive capacity Q of the wind farm after the initial allocation is rest for:

[0083] Q rest =Σ(Q imax -Q iref )(Q imax >Q iref );

[0084] In the formula, Q imax >Q iref As a criterion for determining whether a wind turbine has reactive power margin or reactive power potential;

[0085] If some wind turbines fail to output reactive power according to the reactive power command, the system needs to coordinate the reactive power Q co for:

[0086] Q co =Σ(Q i -Q imax )(Q i ≥Q imax );

[0087] In the formula, Q i ≥Q imax Indicates that the reactive power command in the wind farm exceeds the limit, indicating that the reactive power control command of the units needs to be coordinated as a whole;

[0088] According to the current proportion of the remaining reactive capacity of each wind turbine, the reactive secondary generation of unit i ΔQ i 'for:

[0089]

[0090] Consider a serious power grid fault, that is, Q rest <Q co, indicating that the fault scenario has exceeded the voltage regulation range of the wind farm and that additional reactive compensation equipment is needed to support the voltage.

[0091] Therefore, the present invention adopts the above-mentioned wind farm voltage active support control method based on the operation differences of the doubly fed wind turbines, which can ensure that the grid voltage is within the allowable range when the power system is subjected to a large disturbance, and can fully tap and utilize the reactive capacity of each wind turbine in the wind farm. It can also reduce the reduction in the active output of the wind farm during a fault, thereby ensuring the safe and stable operation of the system.

[0092] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Figure 1 It is a control flow chart of a wind farm voltage active support control method based on the operation difference of a doubly-fed wind turbine according to the present invention;

[0094] Figure 2 This is a schematic diagram of external faults in a wind farm;

[0095] Figure 3 This is a schematic diagram of DFIG control mode switching;

[0096] Figure 4 Define a graph for scene partitioning;

[0097] Figure 5 Schematic diagram of active voltage support control in wind farms

[0098] Figure 6 is the relationship between the reactive output limit and active output of the wind turbine;

[0099] Figure 7 This is the topology diagram of a 24-machine wind farm;

[0100] Figure 8 The voltage comparison of the grid connection point of a 24-machine wind farm under scenario 1 is shown;

[0101] Fig. 9 The reactive power output comparison of the 24-machine wind farm under scenario 1;

[0102] Fig.10 The reactive power output of wind turbines in each control strategy of a 24-machine wind farm under scenario 1;

[0103] Fig.11 This is the voltage control effect diagram of a 24-machine wind farm under scenario 2;

[0104] Fig.12 The comparison of active power output of a 24-machine wind farm under scenario 2 is shown;

[0105] Fig.13Low voltage ride-through standard for wind farms;

[0106] Fig.14 The voltage comparison of the grid connection point of a 24-machine wind farm under scenario 3;

[0107] Fig.15 The reactive power output comparison of the 24-machine wind farm under scenario 3;

[0108] Fig.16 The reactive power output of each unit in a 24-unit wind farm under the control method proposed in scenario 3;

[0109] Fig.17 This is the reactive power output of each unit in a 24-unit wind farm under the average reactive power control method in scenario 3. DETAILED DESCRIPTION

[0110] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0111] Embodiment 1

[0112] like Figure 1 As shown, the present invention provides a method for active voltage support control of wind farms based on the operational differences of doubly-fed wind turbines, and establishes a mapping relationship between the action state of series dynamic braking resistors (SDBR), the active / reactive control mode switching of wind turbines, and the reactive power output capacity of wind farms; then, the voltage segmented control criterion is adjusted according to the reactive power output capacity of the wind farm to achieve refined control of voltage stability under different disturbance levels; then, the influence of the reactive adjustment amplitude of each wind turbine, the relationship between active reduction and reactive increase, and the difference in distance from the grid connection point are comprehensively considered, and the voltage support capacity of different wind turbines is quantitatively analyzed, and the reactive power output instructions are issued to different wind turbines with the help of the coefficient ratio allocation method, and the site-level voltage coordination and optimization control that takes into account the differences in the voltage support capacity of wind turbines is designed, thereby improving the voltage support capacity of the wind farm. Figure 1 As shown, the specific steps include:

[0113] S1. Calculate the system reactive power demand by the voltage drop degree at the system grid connection point.

[0114] When a three-phase short circuit occurs in the power grid, Figure 2 The wind farm external fault is shown, and the fault voltage U at the wind farm grid connection point is calculated. f for:

[0115]

[0116] Where, E is the infinite grid voltage, and its per-unit value is 1p.u., Z l is the line impedance, Z fis the equivalent impedance of the fault point grounding, and a is the fault location. The target value of the grid connection point voltage is set to U ref , then the voltage value that the wind farm needs to raise is:

[0117] ΔU=U ref -U f (2)

[0118] Ignoring the transverse component of voltage drop, the reactive power that the wind farm needs to provide to the grid can be obtained as:

[0119]

[0120] S2. Set the segmentation criteria for wind farm control strategy.

[0121] In the DFIG system, the reactive output on the stator side is mainly regulated by RSC. In order to achieve precise control of the active and reactive power output of wind power, stator flux directional vector control is usually adopted in the dq axis coordinate system. The rotor current expression can be derived as follows:

[0122]

[0123] In the formula, i rd 、i rq are the rotor currents in the d and q axis directions respectively, i sd 、i sq are the stator currents in the d and q axis directions, Q s is the stator side output reactive power, ψ s and ψ r are the stator and rotor flux, L m is the mutual inductance of the stator and rotor windings, ω 1 is the synchronous speed, ω slip is the slip rate.

[0124] The regulation range of reactive output on the stator side of a doubly-fed wind turbine is affected by many factors, among which the limiting effect of the maximum allowable current Irmax of the rotor-side converter is greater, and the following constraints need to be met:

[0125] i rd 2 +i rq 2 =i r 2 ≤I rmax 2 (5)

[0126] Combining equation (4) and equation (5) we can get the reactive power limit Q on the stator side of the unit: smax The expression is:

[0127]

[0128] From formula (6), we can see that the voltage support capacity of wind turbines can be enhanced by reducing active power. The reactive power output limit after reduction is:

[0129]

[0130] In the formula, ΔP s It is the amount of active power reduction.

[0131] Ignore the reactive power on the rotor side and set the reactive power limit Q on the stator side smax Considered as the reactive power limit Q of a single unit imax In practice, active power reduction is not unlimited and needs to be discussed based on the voltage drop situation.

[0132] The active power constraint of the unit in different operating states is obtained according to the operating state of the unit, and the reactive power limit of the entire wind farm when a fault occurs is obtained through weighted summation according to the reactive power limit of a single unit.

[0133] When the grid voltage drops slightly and the SDBR of the unit does not operate, in order to increase the reactive power output limit of the unit, the active power can be reduced to 0 in theory. However, considering that the unit may be disconnected from the grid due to overspeed, the minimum output active power of the unit is specified as P smin , the active power constraint of wind turbine is:

[0134] P s -ΔP s ≥P smin (8)

[0135] When the SDBR of the unit is in operation, the active power output on the DFIG rotor side must meet R SDBR The consumption is calculated according to the maximum rotor current I rmax Setting:

[0136] P r ≥I r 2 max R SDBR (9)

[0137] The relationship between the active power output of the DFIG stator and rotor is:

[0138] P r =-sP s (10)

[0139]

[0140] Then the constraint condition of active power is obtained as follows:

[0141]

[0142] The reactive output limit expressions and active power constraints of wind turbines under all operating conditions during the fault period are summarized as shown in Table 1. Based on this, the available reactive power and reactive power potential of the wind turbine under the current operating conditions can be quickly determined.

[0143] Table 1 Reactive power output limit of wind turbines under different working conditions

[0144]

[0145]

[0146] Therefore, if necessary, it is necessary to reduce the active power of the wind turbine to increase the reactive capacity of the wind turbine, that is, to switch its control mode. The specific switching process is as follows: Figure 3 As shown. Among them, U f is the grid voltage, U 1min is the limit voltage value that can meet the reactive power demand of the system without reducing the active power of the unit, ω r is the rotor angular velocity, P sopt is the active power obtained according to MPPT, P ref is the active command value of low-through control mode 1, is the active value when the rotor active current remains unchanged, To reduce the reactive power command value after the active power, ΔQ s is the reactive power shortage, Q iref is the reactive power instruction of wind turbine i. Figure 3 It can be seen that after a grid fault occurs, the wind turbine enters the low voltage ride-through state and selects and switches the control mode according to the grid connection point voltage. f Higher than U 1min When the reactive power capacity of the wind turbine is sufficient, there is no need to reduce the active power; low penetration control mode 2 is U f Lower than U 1min When the reactive capacity of wind turbines is insufficient, some active power needs to be reduced to meet the reactive power demand of the station.

[0147] like Figure 4 As shown, in this embodiment, the voltage segment control criterion is selected as U 1min , U 2min , U 3min and 0.2, where U 1min , U 2min and U 3min is the limit voltage value that can meet the reactive power demand of the system in different control modes of the wind farm. The relationship between the values ​​of the voltage segment control criteria is:

[0148] U 1min >U 3min >U 2min (13)

[0149] The detailed description of each value is as follows:

[0150] (1)When U f ∈[U 1min ,U ref ], that is, scenario 1, indicating that the wind farm can meet the reactive power demand of the power grid with available reactive power, and there is no need to reduce active power. After the wind turbine enters the low-through state, the low-through control mode 1 can be selected to ensure the active power output of the wind farm.

[0151] (2)When U f ∈[U 2min ,U 1min ], that is, scenario 2 or scenario 3, the wind farm needs to support the grid voltage by reducing active power to expand the reactive capacity of the unit, and the wind turbine unit switches to low penetration control mode 2. Among them, scenario 2 is U f ∈[U 2min ,U 1min ], and the unit SDBR does not act; Scenario 3 is U f ∈[U 3min ,U 1min ], and the unit SDBR is activated.

[0152] (3)When U f ∈[0.2,U 2min ), it indicates that the wind farm cannot meet the reactive power demand of the system and needs to call other reactive power equipment for compensation.

[0153] (4)When U f When it is less than 0.2, the voltage at the system grid connection point is too low and the wind farm is disconnected from the grid.

[0154] The voltage segment control criterion can be obtained according to Figure 2 The total reactive power output limit of the wind farm during a power grid failure is Q WFmax for:

[0155]

[0156] In the formula, Q imax is the reactive power output limit of unit i. Voltage segment control criterion U min for:

[0157]

[0158] S3. Reactive power distribution is carried out taking into account the differences in voltage support capabilities.

[0159] After determining the wind turbine control mode according to the total reactive power demand of the wind farm, the reactive power distribution of multiple wind turbines is carried out. The control method is as follows: Figure 5As shown in the figure, based on the analysis of differences in reactive capacity, geographical location, and active power reduction constraints of wind turbines, the voltage support capability of each unit is quickly and reasonably evaluated, and the reactive power distribution method of the wind turbines is adjusted according to the evaluation results. At the same time, a reactive power coordination mechanism for units within the field is introduced. Through the coordinated cooperation between different units, the overall voltage support capability of the wind farm is further improved.

[0160] Scene 1:U f ∈[U 1min ,U ref ]

[0161] If the grid connection point voltage after the fault is, it can be seen from the above that the wind farm can meet the reactive power demand of the system. At this time, the first factor to be considered is the reactive capacity of the wind turbine. The larger the available reactive capacity, the stronger the voltage support capability. The voltage support evaluation factor M is constructed. i :

[0162]

[0163] Where the denominator is the total available reactive capacity of the wind farm.

[0164] In addition, the voltage support capacity of wind turbines is also affected by their geographical location. As the power transmission distance increases, the line loss gradually increases. The voltage support capacity of wind turbines closer to the grid connection point is stronger. The distance can be characterized by the terminal voltage of each wind turbine, and the voltage support evaluation factor N is established. i :

[0165]

[0166] Taking the above factors into consideration, the voltage support coefficient λ of wind turbine i is obtained: i for:

[0167] λ i =M i N i (19)

[0168] In the formula, λ i The value range is 0 to 1. The power distribution is proportional to the voltage support evaluation coefficients of different wind turbines. The specific method is:

[0169] c=λ 1 +λ 2 +...+λ n (20)

[0170]

[0171] Where c is the sum of the reactive power support capacity evaluation coefficients of the units in the field, Q irefis the reactive power command of unit i. The reactive power commands of different wind turbines are proportional to their voltage support coefficients.

[0172] Scene 2:U f ∈[U 2min ,U 1min ] and SDBR does not work

[0173] If the grid connection point voltage after the fault is, at this time, the wind turbine needs to reduce active power, the SDBR does not work, and all wind turbines operate in low-through control mode 2 during the fault. The maximum available reactive power Q can be calculated based on the active power output of the wind farm. 0 , and further calculate the reactive power Q that needs to be increased by reducing the active power 1 :

[0174] Q 1 =Q WF_ref -Q 0 (twenty two)

[0175] And because the voltage takes a fixed value, Q smax With P s relationship, such as Figure 6 As shown, U s is 1p.u.

[0176] Depend on Figure 6 It can be seen that under the condition of reducing the same active power, the greater the initial active power output of the wind turbine, the more sensitive the reactive power output limit is to the active power change. Therefore, a new voltage support evaluation factor H can be constructed based on the initial active power output of the wind turbine and its reactive power change rate. i :

[0177]

[0178] Where P i is the active power generated by unit i, α i is the average reactive power change rate of unit i, α i The method to find is:

[0179]

[0180] In the formula, b and a are the upper and lower limits of the active power of the wind turbine. smax with U s , P s Strong correlation, other parameters are fixed values, when Us is determined, Q can be obtained smax About P s The unary expression of , according to formula (24), can be used to obtain the average change rate of the reactive power output limit when the active power of the wind turbine changes.

[0181] Voltage support evaluation factor N proposed in comprehensive scenario 1i , the voltage support coefficient w of wind turbine i in this scenario can be obtained i :

[0182] w i =H i N i (25)

[0183] Similarly, the reactive power is distributed in equal proportion according to the voltage support coefficient of each wind turbine:

[0184]

[0185] Where d is the sum of the coefficients of each unit, ΔQ iref The reactive power that needs to be generated by unit i by reducing its active power;

[0186] The reactive power command value of wind turbine i is:

[0187] Q iref =Q 0i +ΔQ iref (28)

[0188] In the formula, Q 0i In order not to reduce the available reactive capacity of group i during active time.

[0189] Scene 3: U f ∈[U 3min ,U 1min ] and SDBR action

[0190] In the case of a serious fault, the wind turbine SDBR is activated. From S2, we can see that in order to ensure the stable operation of the wind turbine, the output active power must meet R SDBR The consumption of reactive power leads to the weakening of its reactive power regulation potential. Define a new voltage support evaluation factor G i :

[0191]

[0192] Where ΔQ imax It is the maximum reactive power increase of unit i, that is, the maximum reactive power that can be increased by reducing active power.

[0193] As above, the reactive power distribution method of each wind turbine is:

[0194] e i =G i N i H i (30)

[0195]

[0196]

[0197] Q iref =Q 0i +ΔQ iref (33)

[0198] In the formula, e i This is the reactive support coefficient of each unit in this scenario, and ε is the sum of the reactive support coefficients.

[0199] S4. Coordinate and redistribute reactive power.

[0200] Since the allocation method in S3 is not purely based on equal proportion of reactive capacity, in some special cases, some wind turbines may have reached their reactive output limit and it is difficult to output the corresponding reactive power according to the reactive power instruction, while other units still have underutilized reactive power, resulting in a waste of reactive regulation resources and unsatisfactory voltage support effect. To solve this problem, reactive power needs to be coordinated and secondary distributed.

[0201] After determining the fault scenario and selecting the corresponding reactive power distribution scheme based on the voltage change at the wind farm grid connection point, if the system detects that there are wind turbines that cannot issue the corresponding reactive power according to the command, the secondary distribution module of the reactive power command will be started. The specific method is as follows:

[0202] The total remaining reactive capacity Q of the wind farm after the initial allocation is rest for:

[0203] Q rest =Σ(Q imax -Q iref ) (Q imax >Q iref ) (34)

[0204] In the formula, Q imax >Q iref It is used as a criterion to determine whether a wind turbine has reactive power margin or reactive power potential.

[0205] If some wind turbines fail to output reactive power according to the reactive power command, the system needs to coordinate the reactive power Q co for:

[0206] Q co =Σ(Q i -Q imax ) (Q i ≥Q imax ) (35)

[0207] In the formula, Q i ≥Q imax It indicates that the reactive power command in the wind farm exceeds the limit, indicating that the reactive power control command of the units needs to be coordinated as a whole.

[0208] If the wind turbine group satisfies equation (34), it is required to bear the reactive power shortage of the group shown in equation (35), and the reactive power secondary increase of group i is ΔQ according to the current remaining reactive power capacity of each wind turbine group. i 'for:

[0209]

[0210] In addition, consider the serious fault situation of the power grid, that is, Q rest <Q co , indicating that the fault scenario has exceeded the voltage regulation range of the wind farm and that additional reactive compensation equipment is needed to support the voltage.

[0211] In this embodiment, a DFIG wind farm grid-connected system model is built on the Simulink simulation platform. The wind farm contains 5 collector lines and 24 wind turbines. The actual wind farm wind speed is used to simulate the actual operation. Figure 7 Three simulation scenarios are set up, and the effectiveness of the proposed method is verified through comparative analysis of different voltage control schemes:

[0212] Scheme 1: Proposed voltage control strategy

[0213] Solution 2: Traditional voltage control strategy, that is, each unit outputs reactive current I during the fault period sq for:

[0214] I sq = k(0.9-U pcc ) 0.2pu≤U pcc ≤0.9pu (37)

[0215] In the formula, k=1.5.

[0216] Solution 3: Average reactive power control strategy

[0217]

[0218] Where n is the total number of wind turbines.

[0219] (1) Scenario 1: U f ∈[U 1min ,U ref ]

[0220] When the voltage at the connection point of the new energy station drops to 0.80pu due to the main grid fault, the fault start and clearing time are 4.2s and 4.5s respectively. ref is 0.9pu. At this time, the SDBR of the units in the field are not in action, and U 1min and U 2minThey are 0.785pu and 0.74pu respectively. 1min <0.8<0.9, it can be seen that the available reactive capacity of the wind farm can meet the reactive demand of the system.

[0221] Depend on Figure 8 As shown in FIG. 1 , after the fault occurs, the voltage control proposed in this embodiment raises the grid connection point voltage to 0.905 pu. Compared with the traditional voltage control and average reactive power control, the voltage raising amplitude increases by 0.053 pu and 0.009 pu respectively. Fig. 9 It can be seen that compared with the other two control schemes, the proposed voltage control reactive output increases by 0.299pu and 0.158pu respectively, which effectively compensates for the reactive power shortage of the system and is more conducive to the stable operation of wind power grid connection. The reasons for the above differences are further explored based on the specific reactive power output of the wind turbines.

[0222] In view of the large number of wind farm units, units 10, 11, 13 and 15 were selected for comparative analysis. Fig.10 The reactive output of the unit before fault removal is shown in Figure 2. It is found that under the proposed voltage control, the reactive output of DFIG10 and DFIG11 accounts for 96.7% and 87.1% of their remaining reactive capacity, respectively. In contrast, in the average reactive control, the reactive output of DFIG10 and DFIG11 only accounts for 60.78% and 69.97%, respectively. At the same time, due to the unreasonable distribution of the average reactive control command and the lack of coordination between the units, the reactive output of DFIG13 and DFIG15 is lower than their reactive command value, resulting in the total reactive output of the wind farm being difficult to meet the system requirements, and the voltage support effect is poor. It can be seen that the proposed voltage control can reasonably allocate reactive power and make full use of the reactive capacity of each unit, thereby tapping the voltage support potential of the entire wind farm.

[0223] (2) Scenario 2: U f ∈[U 2min ,U 1min ], and SDBR does not work

[0224] When the main grid fault causes the voltage at the grid connection point of the new energy station to drop to 0.76pu, the fault duration is 0.3s, and U ref At this time, the SDBR of the units in the field are not in action, because U 2min <0.76 1min , the current reactive power capacity of the wind farm alone cannot meet the reactive power demand of the system, and active power reduction control is required. Fig.11 It can be seen that under the proposed voltage control and average reactive power control, the grid connection point voltage has been significantly raised, while the voltage support effect under traditional voltage control is relatively poor, and the wind farm is at risk of disconnection from the grid when the fault lasts for a long time.​ Fig.12 It can be seen that the active power output of the proposed voltage control during the fault period is always higher than the other two controls, and the total active power reduction of the units in the field is less, which further verifies the effectiveness of the proposed voltage control method. Under the premise of ensuring the voltage support effect, compared with the average reactive power control, the voltage control proposed by the present invention saves 0.02pu (about 800kW) of active power reduction, which has better economic benefits. In addition, reducing active power reduction helps to avoid transient low-frequency problems caused by low voltage ride-through of wind turbines, and effectively improves the grid-connected stability of wind farms.

[0225] (3) Scenario 3: U f ∈[U 3min ,U 1min ], and SDBR action

[0226] When the main grid fault causes the voltage at the grid connection point of the new energy station to drop to 0.55pu, the fault duration is 1.6s. Fig.13 The wind farm low voltage ride-through standard shown in the figure shows that in the scenario where the fault is serious and lasts for a long time, according to my country's wind farm low voltage ride-through standard, when the grid connection point voltage is below the red line in the figure, that is, the shaded part, the wind farm will be disconnected from the grid. Because the fault duration is 1.6s, the grid connection point voltage must be raised to 0.7pu or above. Considering the limited reactive capacity of the wind farm, U ref Take it as 0.7pu.

[0227] Due to the serious fault of the main grid, the SDBRs of all units are activated. From S2, it can be seen that the wind turbine has a minimum active output constraint:

[0228]

[0229]

[0230] Combining equations (39) and (41), we can get the unit active current constraint as:

[0231]

[0232] At this time, the wind farm adopts the reactive power distribution method in scenario 3. The simulation results are as follows: Fig.14 As shown. Fig.14It can be seen that due to the low voltage at the wind farm grid connection point, and considering the limited number of units and the restrictions on active power reduction, the proposed voltage control strategy has little advantage in average reactive power control. However, taking into account special circumstances, at 5.8s, the proposed voltage control can raise the grid connection point voltage to above 0.7pu, so that the wind farm can maintain grid connection operation; while under the other two control strategies, the wind farm grid connection point voltage is lower than 0.7pu, causing the wind farm to be disconnected from the grid, seriously threatening the safe and stable operation of the power system and causing huge economic losses.

[0233] Depend on Fig.15 It can be seen that the proposed voltage control can fully tap the reactive power potential of wind turbines, making the reactive output of wind farms increase by 0.012pu and 0.099pu respectively compared with the average reactive power control and traditional voltage control. In order to analyze the specific reasons for the difference in reactive power output, DFIG units numbered 1 to 8 were selected to analyze the reactive power instructions of each unit and the actual reactive power output before fault removal. Fig.16 It can be seen that although some units (such as units numbered 1, 4, 5, and 6) are unable to output corresponding reactive power according to their reactive power instructions due to the limitation of unit capacity, through reactive power coordinated control, the units with reactive power capacity (such as units numbered 2, 7, and 8) can compensate for the reactive power shortage. Fig.17 The wind farm shown in the figure adopts average reactive power control. Due to the active power reduction restrictions of units 1, 2, 6, and 7, the corresponding reactive power cannot be issued according to its reactive power command value. In addition, the wind farm lacks reactive power coordination control, which ultimately leads to a small total reactive power output and a weakened voltage support effect.

[0234] Therefore, the present invention adopts the above-mentioned wind farm voltage active support control method based on the operation differences of the doubly fed wind turbines, which can ensure that the grid voltage is within the allowable range when the power system is subjected to a large disturbance, and can fully tap and utilize the reactive capacity of each wind turbine in the wind farm. It can also reduce the reduction in the active output of the wind farm during a fault, thereby ensuring the safe and stable operation of the system.

[0235] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A wind farm voltage active support control method based on the operation difference of double-fed wind turbines, characterized in that: The following steps are involved: S1. Calculate the reactive power demand of the system by the voltage drop degree of the system grid connection point; S2. Setting the segmentation criteria of wind farm control strategy; S3. Consider the differences in voltage support capabilities to distribute reactive power; S4. Coordinate and redistribute reactive power.

2. The method for actively supporting voltage of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 1 is characterized in that: In S1, when a three-phase short circuit fault occurs in the power grid, the wind farm has an external fault, and the fault voltage U at the wind farm connection point is calculated. f for: Where, E is the infinite grid voltage, and its per-unit value is 1p.u., Z l is the line impedance, Z f is the equivalent impedance of the fault point grounding, a is the fault location; the target value of the grid connection point voltage is set to U ref , then the voltage value ΔU that the wind farm needs to raise is: ΔU=U ref -U f ; Ignoring the transverse component of voltage drop, the reactive power Q that the wind farm needs to provide to the grid is: WF_ref for:

3. The method for actively supporting voltage of a wind farm based on the operation difference of a double-fed wind turbine according to claim 1 is characterized in that: In S2, stator flux oriented vector control is adopted in the dq axis coordinate system, and the rotor current expression is derived as follows: In the formula, u s is the terminal voltage, i rd 、i rq are the rotor currents in the d and q axis directions respectively, i sd 、i sq are the stator currents in the d and q axis directions, Q s , P s are the reactive and active power output on the stator side, ψ s and ψ r are the stator and rotor flux, L m is the mutual inductance of the stator and rotor windings, ω1 is the synchronous speed, ω slip is the slip rate; Maximum allowable current of rotor-side converter I rmax The following constraints are met: i rd 2 +i rq 2 =i r 2 ≤I rmax 2 ; Get the reactive power limit Q on the stator side of the unit smax The expression is: The voltage support capacity of wind turbines is enhanced by reducing active power. The reactive power output limit of wind turbines after reduction is: In the formula, ΔP s is the active power reduction; ignoring the reactive power on the rotor side, the stator side reactive power limit Q smax Considered as the reactive power limit Q of a single unit imax ; Different active power constraints are obtained according to the operating status of different wind turbines, and then the reactive power limit of the entire wind farm when a fault occurs is obtained by calculation and weighted summation according to the reactive power limit of a single unit. The total reactive power output limit Q of the wind farm when the power grid fails is WFmax for: In the formula, Q imax is the reactive power limit of unit i, and n is the total number of wind turbines; After knowing the total reactive power limit of the wind farm, the voltage segment control criterion U min The calculation formula is:

4. The method for actively supporting voltage of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 3 is characterized in that: According to the operating status of the unit, the active power constraints of the unit in different operating states are obtained, specifically: When the SDBR of the unit does not work, the minimum value of the unit output active power is set to P smin , the active power constraint of wind turbine is: P s -ΔP s ≥P smin ; When the SDBR of the unit is in operation, the DFIG rotor side outputs active power P r Must satisfy R SDBR The consumption is calculated according to the maximum rotor current I rmax Setting: The relationship between the active power output of the DFIG stator and rotor is: P r =-sP s ; Where, s is the unit slip rate; Then the constraint condition of active power is:

5. The method for actively supporting voltage of a wind farm based on the operation difference of a double-fed wind turbine according to claim 3 is characterized in that: According to the operation of the power grid, the voltage segment control criterion U min Set as: U 1min , U 2min and U 3min , where U 1min , U 2min and U 3min is the limit voltage value that can meet the reactive power demand of the system under different control modes of the wind farm, and the relationship is: IN 1min >In 3min >In 2min ; When U f ∈[U 1min ,U ref ], the reactive power of the wind farm meets the reactive power demand of the power grid, and there is no need to reduce the active power; When U f ∈[U 2min ,U 1min ], the wind farm needs to support the grid voltage by reducing active power to expand the reactive capacity of the unit, and the unit SDBR does not operate; When U f ∈[U 3min ,U 1min ], the wind farm needs to expand the reactive capacity of the unit by reducing active power to support the grid voltage, and the unit SDBR is activated.

6. The method for active voltage support control of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 5 is characterized in that: In S3, when the grid connection point voltage U f ∈[U 1min ,U ref ], the voltage support evaluation factor M is constructed according to the reactive capacity of the wind turbine. i : In the formula, Q imax is the reactive capacity of a single wind turbine, is the total reactive capacity of the wind farm; According to the power transmission distance, establish the voltage support evaluation factor N i : In the formula, ΔU max is the voltage drop value of the unit with the most serious voltage disturbance, ΔU i is the terminal voltage drop value of unit i; Calculate the voltage support coefficient λ of wind turbine i i for: l i =M i N i ; In the formula, λ i The value range is from 0 to 1. The power distribution is proportional to the voltage support evaluation coefficients of different wind turbines. The specific method is: c=λ1+λ2+...+λ n ; Where c is the sum of the reactive power support capacity evaluation coefficients of the units in the field, Q iref is the reactive power command of unit i. The reactive power commands of different wind turbines are proportional to their voltage support coefficients.

7. The method for actively supporting voltage of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 5 is characterized in that: In S3, when the grid connection point voltage U f ∈[U 2min ,U 1min ], the wind turbine needs to reduce active power, the wind turbine SDBR does not operate, and the maximum total reactive power Q0 is calculated according to the active power output of the wind farm, and the reactive power Q1 required to be increased by reducing active power is further calculated: Q1=Q WF_ref -Q0; A new voltage support evaluation factor H is constructed based on the initial active output of the wind turbine and its reactive power change rate. i : Where P i is the active power generated by unit i, α i is the average reactive power change rate of unit i, α i The method to find is: In the formula, b and a are the upper and lower limits of the active power of the wind turbine respectively; Comprehensive voltage support evaluation factor N i , the voltage support coefficient w of wind turbine i in this scenario is obtained i : w i =H i N i ; Similarly, the reactive power is distributed in equal proportion according to the voltage support coefficient of each wind turbine: Where d is the sum of the coefficients of each unit, ΔQ iref The reactive power that needs to be generated by unit i by reducing its active power; The reactive power command value of wind turbine i is: Q iref =Q 0i +ΔQ iref ; In the formula, Q 0i In order not to reduce the reactive capacity of group i during active operation.

8. The method for actively supporting voltage of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 5 is characterized in that: In S3, when the grid connection point voltage U f ∈[U 3min ,U 1min ], the wind turbine needs to reduce active power, and the wind turbine SDBR action, define a new voltage support evaluation factor G i : Where ΔQ imax is the maximum value of reactive power increase of unit i; As above, the reactive power distribution method of each wind turbine is: e i =G i N i H i ; Q iref =Q 0i +ΔQ iref ; In the formula, e i This is the reactive support coefficient of each unit in this scenario, and ε is the sum of the reactive support coefficients.

9. The method for active voltage support control of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 3 is characterized in that: In S4, after determining the fault scenario and selecting the corresponding reactive power distribution scheme according to the voltage change at the wind farm grid connection point, if the system detects that there are wind turbines that cannot issue the corresponding reactive power according to the command, the secondary distribution module of the reactive power command is started.

10. The method for active voltage support control of a wind farm based on the operation difference of a doubly-fed wind turbine according to claim 9, characterized in that: The total remaining reactive capacity Q of the wind farm after the initial allocation is rest for: Q rest =∑(Q imax -Q iref )(Q imax >Q iref ); In the formula, Q imax >Q iref As a criterion for determining whether a wind turbine has reactive power margin or reactive power potential; If some wind turbines fail to output reactive power according to the reactive power command, the system needs to coordinate the reactive power Q co for: Q co =∑(Q i -Q imax )(Q i ≥Q imax ); In the formula, Q i ≥Q imax Indicates that the reactive power command in the wind farm exceeds the limit, indicating that the reactive power control command of the units needs to be coordinated as a whole; According to the current proportion of the remaining reactive capacity of each wind turbine, the reactive secondary generation of unit i ΔQ i 'for: Consider a serious power grid fault, that is, Q rest <Q co , indicating that the fault scenario has exceeded the voltage regulation range of the wind farm and that additional reactive compensation equipment is needed to support the voltage.

Images