CN113258590B - Method and system for controlling voltage of AC/DC system in stages under high wind power permeability - Google Patents
Method and system for controlling voltage of AC/DC system in stages under high wind power permeability Download PDFInfo
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Abstract
The invention belongs to the field of smart grids, and provides a method and a system for controlling an AC/DC system in stages under high wind power permeability. In an emergency control stage, controlling to determine a next regional voltage control variable through prediction and inter-regional communication; the voltage control is then performed by model predictive control with minimum voltage deviation and control cost as objective functions. In the preventive voltage control stage, reactive replacement is carried out on the quick response equipment by using the generator set and the parallel capacitor in the area, so that the standby margin of the quick response equipment is increased, and the capability of the system for coping with sudden accidents is improved. Simulation results show that the strategy can improve the calculation efficiency of long-term voltage control and optimize the reactive power reserve of the system.
Description
Technical Field
The invention belongs to the field of smart grids, and particularly relates to a method and a system for controlling voltage of an alternating current-direct current system in stages under high wind power permeability.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
Along with the construction and operation of large-scale offshore wind farms in China, how to reasonably accommodate offshore wind power becomes one of the problems to be solved in the power system. The connection mode of the wind power plant and the alternating current system is an important factor influencing the stability of alternating current voltage. The existing connection modes are divided into alternating current access, traditional direct current (LCC-HVDC) access and multi-terminal flexible direct current access (VSC-MTDC). The VSC-MTDC has the advantages of active and reactive independent control, frequency decoupling operation characteristics of a wind farm and a land power grid, capability of forming a multi-terminal direct current power grid and the like, and becomes the optimal choice of the existing wind power grid connection.
As the proportion of the installed capacity of wind power in the power generation of the power grid is larger and larger, the influence area of wind power on the voltage of the power grid is enlarged from a wind power feed point to the whole alternating current system. When wind power is fed into an alternating current-direct current power grid through VSC-MTDC, voltage fluctuation of the alternating current system is even out of limit due to intermittence and fluctuation of the wind power. And wind power is fed into an alternating current power grid through different access points, the running characteristics of alternating current systems of the feed points are different, and the traditional centralized control is difficult to meet the requirements of multi-area and multi-target control. In order to improve the calculation efficiency of long-term voltage control, optimize the reactive power reserve of the system and ensure the voltage stability of the system, the method has important significance in researching the distributed voltage control method of the alternating current-direct current system under high wind power permeability.
The inventor knows that the following problems exist in the current distributed voltage control method of the alternating current-direct current system under high wind power permeability: (1) At present, in order to cope with uncertainty of renewable energy source output, internal voltage regulation, feed-in power grid voltage control and reactive power optimization of a wind power plant are mainly researched, but long-term voltage control research of an alternating current/direct current power grid under high wind power permeability is less. (2) The existing research only considers emergency voltage control after faults, and does not consider optimizing reactive power reserve of the system after the voltage enters a safety range. The quick response reactive standby is obviously reduced after the emergency voltage control, if reactive replacement is not carried out, the system has insufficient quick response reactive standby in the next control, and the voltage stability of the system is reduced. (3) In the existing long-term voltage instability control scheme, the control characteristic of flexible direct current cannot be fully exerted for the scene that the wind power plant is connected into an alternating current power grid through VSC-MTDC, and the long-term voltage stability of the system is reduced.
Disclosure of Invention
In order to solve the problems, the invention provides a staged voltage control method of an AC/DC system under high wind power permeability, which comprises the steps of firstly partitioning a power grid by taking a wind power feed point as a clustering center, and dividing control into one-stage emergency voltage control and two-stage preventive voltage control according to voltage deviation of the AC power grid. The method can effectively avoid voltage out-of-limit caused by wind power fluctuation, improve the calculation speed and ensure the long-term voltage stability of the system.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
in a first aspect, a method for controlling voltage of an ac/dc system in stages under high wind power permeability is disclosed, comprising:
using wind power prediction output data, and using wind power access points as clustering centers to perform voltage sensitivity-based partitioning;
the method for obtaining the voltage information of the lowest node and the highest node in each area and carrying out staged voltage control comprises the following steps: an emergency control stage, wherein an emergency control mode is adopted, and a preventive voltage control stage is adopted;
in an emergency control stage, controlling to determine the voltage control quantity of the area through prediction and inter-area communication; then, performing voltage control by using model predictive control with minimum voltage deviation and control cost as objective functions; and in the preventive voltage control stage, reactive replacement is carried out on the quick response equipment by using the generator set and the parallel capacitor in the area, so that the standby margin of the quick response equipment is increased.
And in the emergency control mode, judging whether reactive power and flexible direct current control quantity in the area can meet the voltage control requirement, if so, executing emergency voltage control in the area, if not, adding reactive power control variable with the maximum voltage sensitivity among systems into a control sequence, executing inter-area voltage coordination control, and if not, starting load shedding control.
According to a further technical scheme, when partitioning based on voltage sensitivity is carried out, firstly, the voltage sensitivity of a wind power access point to PQ nodes is solved, and the PQ nodes are clustered to form a preliminary partition;
then solving the average voltage sensitivity of the PV node to the partition after the PQ node is clustered;
on the premise of ensuring connectivity, the PV nodes are clustered into the area with the maximum average sensitivity.
Further, in the step voltage control, whether the deviation between the minimum and maximum node voltage information in each region and the set value is larger than a first set value is judged, if so, the emergency control mode is entered, if not larger than the first set value, whether the deviation between the maximum or minimum voltage and the set value is larger than a second set value is judged, if so, the preventive voltage control mode is entered, and if not larger than the normal operation mode is regarded as not performing control.
According to a further technical scheme, in the emergency control mode, the emergency control mode is started after the emergency control mode is greatly disturbed, the controller judges whether reactive power reserve of the area can meet control requirements, if the reactive power reserve of the area cannot meet the control requirements, a request is sent to an adjacent area, reactive power equipment is increased according to the voltage sensitivity until the reactive power equipment meets the control requirements, and then the quick response reactive power equipment and the multi-terminal flexible direct current access are used for voltage control, so that voltage deviation and control cost are minimized.
According to the further technical scheme, in the preventive voltage control mode, the standby margin of the reactive power equipment is increased through reactive power replacement while the system voltage is ensured to be within a safety range.
According to a further technical scheme, the emergency voltage control step in the emergency control mode comprises the following steps:
step 1: when the deviation between the node voltage of the alternating current system and the set value is larger than a first set value, starting emergency voltage control;
step 2: solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity;
step 3: judging whether the reactive power control quantity and the soft and straight active power control quantity in the area can meet the requirement of voltage control, if the reactive power control quantity and the soft and straight active power control quantity cannot meet the requirement, increasing the reactive power control quantity of the adjacent area, and increasing the installation sensitivity in sequence until the reactive power control quantity and the soft and straight active power control quantity meet the requirement, and if the reactive power control quantity and the soft and straight active power control quantity still cannot meet the requirement, starting load shedding control;
step 4: solving an emergency voltage control objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system;
step 5: and (3) rolling control: applying an optimal control sequence at the initial moment of the next period of control sampling, detecting the system voltage, and ending calculation if the system voltage meets the requirement; otherwise, returning to the step 2.
According to a further technical scheme, the emergency voltage control objective function is minimized in terms of voltage deviation and control cost.
According to a further technical scheme, the step of preventing voltage control in the preventive voltage control mode comprises the following steps:
step 1: when the deviation between the alternating current system voltage and the set value is smaller than a second set value, starting preventive control;
step 2: solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity;
step 3: solving an objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system;
step 4: at the beginning of the next cycle of control sampling, an optimal control sequence is applied. Detecting the system voltage, and ending the calculation if the system voltage meets the requirement; otherwise, returning to the step 2.
In a second aspect, a staged voltage control system for an ac-dc system with high wind power permeability is disclosed, comprising:
a partitioning module configured to: using wind power prediction output data, and using wind power access points as clustering centers to perform voltage sensitivity-based partitioning;
a phased voltage control module configured to: the method for obtaining the voltage information of the lowest node and the highest node in each area and carrying out staged voltage control comprises the following steps: an emergency control stage, wherein an emergency control mode is adopted, and a preventive voltage control stage is adopted;
In an emergency control stage, controlling to determine the voltage control quantity of the area through prediction and inter-area communication; then, performing voltage control by using model predictive control with minimum voltage deviation and control cost as objective functions; and in the preventive voltage control stage, reactive replacement is carried out on the quick response equipment by using the generator set and the parallel capacitor in the area, so that the standby margin of the quick response equipment is increased.
And in the emergency control mode, judging whether reactive power and flexible direct current control quantity in the area can meet the voltage control requirement, if so, executing emergency voltage control in the area, if not, adding reactive power control variable with the maximum voltage sensitivity among systems into a control sequence, executing inter-area voltage coordination control, and if not, starting load shedding control.
In order to solve the above-described problems, a third aspect of the present invention provides a computer-readable storage medium.
A computer-readable storage medium, on which a computer program is stored, which program, when being executed by a processor, carries out the steps in a control method as described above.
In order to solve the above-described problems, a fourth aspect of the present invention provides a terminal device.
A terminal device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, said processor implementing the steps in a control method as described above when said program is executed.
The beneficial effects of the invention are as follows:
according to the method, wind power uncertainty and the influence of the flexible direct current on the voltage stability of the alternating current system are considered, the flexibility of the flexible direct current system in power modulation is fully utilized, a two-stage voltage control method of the alternating current-direct current system under high wind power permeability is designed, and the long-term voltage stability of the system is improved.
The Gao Fengdian permeability refers to the fact that the wind power installation capacity is larger than the regional power grid installation capacity. In this case, the fluctuation and randomness of wind power can cause the voltage of the power grid to obviously fluctuate or even exceed the limit, and endanger the voltage stability of the power grid.
The invention provides a two-stage voltage control strategy, wherein in the first stage, rapid reactive equipment is used for emergency voltage control, so that the voltage stability of the system is ensured; reactive replacement is performed in the second-stage prevention voltage control, so that the quick response reactive standby margin of the system is increased, and the coping capacity of the system after being disturbed again is improved.
According to the invention, voltage control is performed through a distributed algorithm, each region can be controlled by only communicating with the adjacent region, the dependence of the system on communication is reduced, and the solving speed of the voltage optimization problem is improved.
The invention coordinates and optimizes the AC/DC control quantity by solving the quadratic programming problem of model predictive control, thereby improving the long-term voltage stability of the system.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.
FIG. 1 is a flow chart of a two-stage voltage control framework of an AC/DC system under high wind power permeability provided by an embodiment of the invention;
FIG. 2 is a flowchart of emergency voltage control of an AC/DC system under high wind power permeability provided by an embodiment of the invention;
FIG. 3 is a flowchart for controlling voltage prevention of an AC/DC system under high wind power permeability provided by the embodiment of the invention;
FIG. 4 is a diagram of partitioning results provided by an embodiment of the present invention;
FIG. 5 is a predicted output curve for a wind farm.
FIG. 6 is a graph showing the voltage of a weak bus when the wind power output fluctuation example system provided by the embodiment of the invention is controlled and not controlled.
Detailed Description
The invention will be further described with reference to the drawings and examples.
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
Example 1
As shown in fig. 1, the method for controlling the voltage of the ac/dc system in stages under the condition of high wind power permeability comprises the following steps:
step 1: acquiring wind power prediction capacity and real-time running state information of a power grid, and partitioning based on voltage sensitivity according to the acquired information;
step 2: acquiring voltage information of all nodes in each region, and judging whether the maximum deviation between the voltage and a set value is larger than delta V emer If the signal is larger than the preset threshold value, the system enters an emergency control mode, and if the signal is not larger than the preset threshold value, the system enters step 4;
step 3: and judging whether reactive power and flexible direct current control quantity in the area can meet the voltage control requirement. If so, in-zone emergency voltage control is performed. If the voltage between the two control sequences is not met, adding a reactive control variable with the maximum voltage sensitivity between the systems into the control sequence to perform inter-region voltage coordination control, and if the voltage between the two control sequences is still not met, starting load shedding control. Returning to the step 2 after the control is completed;
step 4: judging whether the maximum deviation between the voltages of all nodes of the system and the set value is larger than delta V prvt If the voltage is greater than the preset threshold, the system enters a preventive voltage control mode, and if the voltage is not greater than the preset threshold, the system is regarded as a normal operation mode and is not controlled.
Note that Δv emer Greater than DeltaV prvt That is, emergency voltage control is used when the voltage deviation is large; less, prophylactic control is employed.
The embodiment considers the influence of large-scale wind power access on the voltage stability of an alternating current system, fully utilizes the flexible controllability of VSC-MTDC, and improves the effectiveness and rapidity of long-term voltage control. In the first control phase, the system enters an emergency operation state after being subjected to a large disturbance. First, the controller determines whether the reactive reserve of the area can meet the control requirements. If not, sending a request to the adjacent area, and increasing the reactive equipment according to the voltage sensitivity until the control requirement is met. The voltage control is then performed by the MPC using fast response reactive devices and VSC-MTDC with the aim of minimizing voltage deviation and control costs. The second control stage is preventive voltage control, and when the system voltage is ensured to be within a safety range, the standby margin of the reactive power equipment is increased through reactive power replacement, so that the coping capacity of the system to emergency is improved.
In the step 1, wind power prediction output and power grid real-time running state information are obtained based on wide area measurement, and voltage sensitivity-based partitioning is performed according to the obtained information;
specifically, considering the scale of the ac/dc hybrid grid, the centralized control cannot meet the control requirement in terms of calculation time and multi-region independent control. It is necessary to divide the grid into a plurality of control areas according to the voltage sensitivity. Since wind power is fed into the ac system at different bus bars through VSC-MTDC, the control mode and parameters of each VSC converter will have different effects on the bus bars at different locations. In order to determine the influence area of each converter station, wind power prediction output data is used, wind power access points are used as clustering centers, and voltage sensitivity-based partitioning is performed, as shown in fig. 4. The clustering is divided into two steps, namely, the voltage sensitivity of the wind power access point to the PQ nodes is solved, and the PQ nodes are clustered to form a primary partition. The voltage sensitivity is obtained by adopting a jacobian matrix inverse matrix method:
wherein ΔP, ΔQ, Δθ, ΔV are the deviations of the active, reactive, phase angle, voltage, respectively, J Jacobian matrix, J pθ 、J pv 、J qθ 、J qv Is a submatrix of the jacobian matrix.
In order to accurately take into account the effect of active power on voltage, complete decoupling of the PQ is not considered here. Let Δp=0, the formula can be reduced to:
then, according to the sensitivity matrix S, the voltage sensitivity alpha between the nodes i and j is obtained ij The PQ nodes are clustered into respective regions with highest voltage sensitivity to the wind access point.
In order to increase the effectiveness of the control, it is necessary to determine which control variables are most effective for each control region. And then solving the average voltage sensitivity of the PV node to the partition after the PQ node clustering:
wherein S is r,g Average voltage sensitivity, n, for PV node g to nodes within PQ partition r r For the number of PQ nodes in region r, α kg The voltage sensitivity of the PV node g to the PQ node k can be obtained by a perturbation method. On the premise of ensuring connectivity, the PV nodes are clustered into the area with the maximum average sensitivity. After partitioning, the optimization variables and the control dimension are obviously reduced compared with the original network, so that the solving time of the control optimization problem can be greatly reduced.
In steps 2 and 4, the power system is classified into normal, alarm and emergency states according to the maximum value of the area voltage deviated from the reference value of 1.0p.u. The operation state of the regional power grid is obtained based on wide area measurement, corresponding control is carried out, and the operation state is classified as follows:
1) Normal operating state: the voltage deviation reference value is smaller than the preventive voltage control deviation DeltaV prvt No control is performed.
2) Alert running state: voltage deviation greater than DeltaV prvt But still less than the emergency control start deviation Δv emer The system performs preventive voltage control to avoid voltage out-of-limit after being disturbed.
3) Emergency operation state: delta V when voltage deviation is larger than emer Firstly, emergency voltage control is adopted to enable the system to enter a warning running state, and then preventive voltage control is adopted to enable the system to enter a normal running state.
In step 3, based on the wide area measurement data, calculating the regional reactive margin, judging whether the regional reactive margin can meet the control requirement, if the regional reactive margin cannot meet the control requirement, adding a reactive control variable with the maximum voltage sensitivity between systems into a control sequence, carrying out regional voltage coordination control, and if the reactive control variable still cannot meet the control requirement, starting load shedding control.
Specifically, when the deviation of the lowest or highest node voltage from the reference value in the region is greater than Δv emer When the emergency voltage control is started. Before implementing the control, it is first calculated whether the reactive and flexible DC active control quantity in the area can meet the control requirement
In the formula, S i Sensitivity to control variables u i ,u i,max ,u i,min Respectively the current value, the maximum value and the minimum value of the control variable, N u For controlling the number of variables, θ is a margin coefficient, V is a voltage amplitude before control,
andV lim is the upper and lower limit of the emergency voltage control.
When the reactive margin requirement cannot be met, adding the control variable with the maximum sensitivity of the adjacent area into the control sequence, and calculating again until the requirement is met. And after all the reactive equipment in the adjacent area is added into the control sequence, starting the LCC arc extinguishing angle and active power control, and if the voltage control requirement still cannot be met, starting the load shedding control. It is noted that all LCC-HVDC control needs to be performed on the basis of ensuring that its commutation is successful, i.e. the arc extinction angle is larger than the critical arc extinction angle.
In one embodiment, as shown in FIG. 2, an AC/DC system emergency voltage control flow chart under high wind power permeability; the method comprises the following steps:
step 1: when the deviation between the voltage of the node of the alternating current system and the set value is larger than delta V emer When the emergency voltage control is started.
Step 2: and solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity.
Step 3: judging whether the reactive power control quantity and the soft and straight active power control quantity in the area can meet the requirement of voltage control, if the reactive power control quantity and the soft and straight active power control quantity cannot meet the requirement, increasing the reactive power control quantity of the adjacent area, and sequentially increasing the installation sensitivity until the requirement is met. If the load is still unsatisfied, starting load shedding control.
Step 4: and solving an emergency voltage control objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system.
Step 5: and (3) rolling control: at the beginning of the next cycle of control sampling, an optimal control sequence is applied. Detecting the system voltage, and ending the calculation if the system voltage meets the requirement; otherwise, returning to the step 2.
In the embodiment, the large-scale wind power access is greatly disturbed to the alternating current system to enter an emergency running state, and emergency voltage control is adopted to ensure the voltage safety of the system. And taking the voltage deviation and the control cost as objective functions, determining an optimization variable through information interaction among the areas, and applying control.
In step 1, the voltage level of each area after the partitioning is determined based on the wide area measurement data, and whether emergency voltage control is required is determined.
In the step 2, an implicit trapezoidal integration method and a Newton-Laportson method are applied to perform time domain simulation, and a system voltage output track is predicted. And obtaining the track sensitivity of each AC/DC control quantity of the AC/DC system to the load bus voltage through the jacobian matrix obtained in the time domain simulation.
Describing an AC/DC system model by using a differential-algebraic equation:
the power system model may be represented by a set of differential algebraic equations:
0＝g(x,y,λ)
Where x represents a state variable of the system, y is an algebraic variable, and λ is a parametric variable.
Traditional direct current system model:
wherein: v (V) acR 、V acI The voltage of the commutation bus of the rectifying and inverting station is respectively; v (V) d0r 、V d0i The direct current voltage is no-load direct current voltage of the rectifier and the inverter respectively; v (V) dr 、V di The direct current voltages of the rectifier and the inverter are respectively; i d Is a direct current line current; alpha and gamma are respectively the advanced trigger angle of the rectifier and the arc extinction angle of the inverter; n is the number of six pulse converter bridges; r is R d The resistor is a direct current line resistor; k (K) Tr 、K Ti The transformation ratio of the rectifying and inverting side converter transformers is respectively; x is X cr 、X ci The single bridge commutation reactance at the rectifying side and the inverting side respectively; p (P) dcr 、P dci Active power absorbed by the rectifier and active power output by the inverter respectively; q (Q) dcr 、Q dci Reactive power absorbed by the rectifier and the inverter respectively; p (P) order 、γ order The control value is the control value of the rectifying and inverting sides in a fixed power-fixed arc extinction angle control mode.
Flexible direct current system model:
let α=arctan (R/X L ),
R and X L And the equivalent resistance and the reactance of the converter transformer are respectively represented, and Y is the equivalent admittance of the converter transformer. The mathematical model of VSC-HVDC is then:
P c 、Q c for power absorbed by the converter station, P s 、Q s For power exchange with external ac system, U s Is the ac system voltage, delta is the PMW modulated wave phase angle. Assume that the DC voltage utilization rate of the PWM converter is 
M is the modulation value, then->
And calculating the track sensitivity of each busbar voltage and the voltage sensitivity of the conventional direct current feed busbar. On the side of an alternating current system, a traditional direct current system and a flexible direct current system are regarded as special loads, and a steady state tide equation of a bus l of the alternating current system and a bus k connected with an inverter is modified:
wherein: Δp and Δq are error train vectors of active power and reactive power in the power flow calculation process respectively; p (P) s And Q s Net injection of active and reactive power at the respective nodes for the generator and load, respectively; p (P) dc And Q dc Active power and reactive power are injected into direct current of a converter bus connected with the converter; v and delta are respectively the amplitude and phase angle of the alternating current bus voltage; g and B are the conductance and susceptance, respectively, of the corresponding element of the node admittance matrix.
For traditional direct current, when two ends of a line are respectively connected with a converting bus i and a converting bus J, the transmission power of the direct current system is irrelevant to the voltage phase angle of the converting bus, so J in the tidal current jacobian matrix Pδ And J Qδ Remains unchanged for the corresponding element (J) PV ) m,n Sum (J) QV ) m,n And (3) modifying:
wherein: j (J) PV ' and J QV ' modified jacobian submatrix J respectively PV And J QV ；
And
sensitivity to dc power versus commutation bus voltage amplitude.
Under the control modes of active power determination at the rectifying side and arc extinguishing angle determination at the inverting side, the sensitivity of the voltage amplitude of the converter bus to the transmission power of the rectifying station is as follows:
similarly, the sensitivity of the voltage amplitude of the converter bus to the transmission power of the inversion station is as follows:
for flexible direct currents, the power inflow is defined as positive and the power outflow as negative. Because the flexible direct current active and reactive decoupling is adjustable, the sensitivity of the active and reactive control amounts to voltage is respectively analyzed, and the sensitivity is specifically as follows:
1) The active control mode is divided into a fixed active power control mode and a fixed direct current voltage control mode. In the case of a fault, the dc voltage is set to a predetermined value and is not used as a control variable for the ac/dc coordination control, so that only the voltage sensitivity of the active power control system is analyzed.
When the active power set point of the converter station is changed, the active power exchanged by the constant direct voltage converter station and the alternating current system is changed into the following active power balance due to the fact that the direct current system complies with the active power balance:
wherein: CV represents a constant DC voltage converter station; p (P) s,j The j-th fixed active power converter station set value is represented, and Δp represents the fixed active power converter station set value change amount. ΔP loss Is the loss of the dc network.
As can be seen from the above description, changing the active power set point of the converter station not only changes the active power of the converter station itself, but also affects the active power of the dc-dc converter station, and the two active powers change equally and in opposite directions, and the sensitivity of the two active powers can be obtained according to the jacobian matrix.
2) Reactive power control modes are divided into fixed reactive power and fixed alternating voltage. The voltage sensitivity of the output reactive power of each converter station to any alternating current bus can be obtained through a Jacobian matrix. For a fixed alternating voltage control mode, the difference value of reactive power output by different bus voltage set values can be obtained through a flexible direct current output power formula, and the difference value is multiplied by the sensitivity of the reactive power to voltage, namely:
S U,i ＝S Q,i *ΔQ
wherein: s is S U,i Indicating the voltage sensitivity of the set value of the fixed alternating current voltage to the alternating current bus i, S Q,i The voltage sensitivity of reactive power to the ac bus i is shown, and Δq is the amount of reactive power change corresponding to the change in the set value of the ac voltage.
And iteratively solving a differential-algebraic equation set of the AC-DC system model by adopting an implicit trapezoidal method and a Newton method which are commonly used in time domain simulation, so as to obtain a voltage prediction output track of the AC-DC system, and solving the sensitivity of the voltage track by a system jacobian matrix in the time domain simulation process. The linearization relationship of the system input and output under voltage stabilization control is expressed as:
wherein the subscript k denotes the corresponding variable at t k Taking value at moment;
predicting a track for the voltage;
A control amount change for input; / >
Is the voltage amplitude pair->
Trajectory sensitivity of (2).
In step 4, the objective function and constraints are as follows:
s.t.
u i,min ≤u k,i ≤u i,max k∈[1,N c ]
Δu i,min ≤Δu k,i ≤Δu i,max k∈[1,N c ]
in the formula, the objective function is divided into two parts: voltage deviation and control cost omega ac ,ω u The voltage deviation and the weight coefficient of the control variable respectively. k represents the kth control period. N (N) p ,N c The prediction period and the control period number, respectively.
Is the voltage reference at time k. V (V) k ,V′ k The voltage amplitudes before and after the k-th time control are respectively. Under normal conditions, V' k Taking the highest or lowest node voltage in the region, when the region contains LCC receiving end converter stations, V' k The highest or lowest node voltage in the area and the LCC feed-in terminal bus node voltage are taken.
Is the sensitivity of the ith control quantity to the control quantity u voltage at the kth time. Deltau k,i Is the variation of the ith control amount at the kth time. u (u) i,max ,u i,min Is the upper and lower limit values of the control variable. Deltau i,max ,Δu i,min Is the upper and lower limit values of a single change of the control variable. P (P) vsc ,Q vsc ,S vsc Active power, reactive power and capacity of the VSC converter station are indicated respectively.
The randomness of the wind power can affect the voltage control effect of the wind power feed-in regional power grid due to the deviation of the predicted data from the real-time data. The conventional deterministic control method does not consider the random characteristics of wind power, so that the control effect is not ideal when the prediction deviation is large. The processing method has robust control, opportunity constraint and the like. The opportunity constraint is selected here, because the wind power influence on the grid voltage is considered, and meanwhile, the economy is better, and the control is not limited to conservation. The constraint is defined as:
V p Is the current system voltage magnitude.
V p An upper limit and a lower limit, respectively, for acceptable system voltage magnitudes. S is S q ，S w The sensitivity of reactive power and wind power active power respectively. ΔQ, ΔP w Reactive control quantity and wind power active power change quantity are respectively adopted. Beta is the probability that needs to be satisfied.
Assume that the prediction error of the wind power output follows a normal distribution with a mean of 0 and a variance of σ2. P (P) f Is the wind power predicted value, P r Is the capacity of the total assembly machine of the wind power plant, and the probability expression is as shown in the formula
σ 2 ＝0.2P f +0.02P r
To facilitate solution, the opportunity constraint may be translated into an inequality constraint equivalent thereto. The specific method comprises the steps of obtaining wind power prediction output, probability distribution of wind power prediction deviation, and setting confidence coefficient beta of constraint conditions. According to a normal distribution formula, under a certain probability distribution, a corresponding wind power prediction deviation value under a certain confidence coefficient can be obtained, and the opportunity constraint is converted into a deterministic inequality constraint. Adding the constraint condition to the optimization control problem.
In an embodiment, as shown in fig. 3, the ac/dc system prevention voltage control flow chart under high wind power permeability; the method comprises the following steps:
step 1: when the deviation of the AC system voltage from the set value is smaller than DeltaV prvt At that time, preventive control is initiated.
Step 2: and solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity.
Step 3: solving a preventive voltage control objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system.
Step 4: and (3) rolling control: at the beginning of the next cycle of control sampling, an optimal control sequence is applied. Detecting the system voltage, and ending the calculation if the system voltage meets the requirement; otherwise, returning to the step 2.
In the embodiment, when the large-scale wind power access is greatly disturbed to enter an emergency running state and then is recovered to be in an alert range through emergency voltage control, the system voltage is in a safer state, and reactive standby unbalance of the system is caused if the reactive resource is still used. When the system is disturbed again, a quick response cannot be made. Therefore, the traditional generator and the parallel capacitor are required to replace the quick response reactive power resource, so that the reactive power of the whole system is more balanced, the system is ensured to have more quick reactive power standby, and the voltage stability of the system is improved.
In step 3, compared with the emergency control, the reactive standby term weight coefficient in the prevention control is larger, and the control cost and the voltage fluctuation term weight coefficient are smaller. The primary purpose is to increase the fast response device (VSC) reactive power reserve margin while reducing voltage fluctuations and control costs, with the voltage fluctuation term acting as a soft constraint for the voltage, in the case of preventive control.
ΔU min ≤|ΔU k |≤ΔU max k∈[1,N p ]
In the formula ω u To control the weight coefficient of the cost omega i Weight coefficient, ω, for the ith reactive margin ac Is the weight coefficient of voltage fluctuation, theta i,k For reactive margin of the ith control variable at time k, V ave To average voltage over time period, Q max Q is the maximum and current value of the reactive power output, respectively.
Fig. 4 is a hybrid multi-feed ac/dc system based on Nordic 32. The partitioning method can obtain the partitioning result as shown in fig. 4, and partitions the power grid into four areas. 4032-4044 ac line is changed to conventional dc power transmission (LCC-HVDC). The rated capacity of the offshore wind farm is 3000MW, and the flexible direct current converter stations VSC1, VSC2 and VSC3 are respectively corresponding to the flexible direct current converter stations VSC4 through multi-terminal flexible direct current (VSC-MTDC) feeding into an alternating current power grid bus4012,4044,4062. To maintain balance between power generation and load, the central area load L1-L5 (2750 MW) was increased by a factor of 1.2. The original system generator g10 (600 MVA) and generator g17 (530 MVA) are deleted. The converter stations VSC1, VSC2 are P-V droop control [69], VSC2 is constant active control, and VSC4 is constant frequency control. The specific control parameters of VSC-HVDC are:
VSC1 (bus 4012): P-V droop control, ps=600 mw, udc=1.0p.u., droop factor k=6, fixed reactive power control, qs=255 Mvar.
VSC2 (bus 4044): P-V droop control, ps=550 mw, udc=0.99 p.u., droop factor k=6, reactive power control, qs=0 Mvar.
3) VSC3 (bus 4063): active power control, ps=530 MW, ac voltage control, us=1.01 p.u.
3) VSC4 (wind farm): and (3) fixed frequency control: f=50 HZ, constant ac voltage control, us=1.01 p.u..
Conventional DC voltage + -500 kV, capacity 2000MVA. A filter and a parallel capacitance compensation device are respectively arranged on the rectifying inversion side. The rectification side adopts constant direct current voltage control, the inversion side adopts constant arc extinction angle control, and the initial arc extinction angle setting value is 18 degrees.
The normal operation of the power grid is not failed, and the voltage of partial bus bars of the receiving end alternating current system is greatly fluctuated or even out of limit due to the fluctuation of wind power output, so that the voltage needs to be controlled. The wind output curve is shown in fig. 5, and the voltage curves before and after control are shown in fig. 6. No out-of-limit voltage of the control rear bus4044 occurs. And within 80-300s, the reactive power control of the VSC convertor station with the highest response speed is preferentially used for controlling the voltage, so that the voltage is stabilized between 0.95 and 1.05p.u, preventive reactive voltage control is started after 300s, the input quantity of the parallel capacitor and the generator is increased, the reactive power output of the VSC is reduced, the reactive power margin of the VSC convertor station is increased, and finally the voltage is stabilized between 1.01 and 1.03 p.u. When t=100 s, the voltage is out of limit, at this time, the control speed is the fastest, the reactive power of the VSC2 converter station with the greatest sensitivity is preferentially used, and when t=150 s, the voltage reaches the highest value, at this time, the reactive margin of the VSC converter station is the smallest, which is 0.343. In 300-900 s, the condition of voltage out-of-limit does not appear, but the voltage fluctuation is larger. At the moment, preventive voltage control is adopted, the investment of a generator and a parallel capacitor is gradually increased to replace the reactive power of the quick-response VSC converter station, the reactive margin of the converter station is increased, and enough quick-response reactive standby is reserved for emergency
Example two
The embodiment provides a two-stage voltage control system of an AC/DC system under high wind power permeability, which comprises:
a partitioning module configured to: using wind power prediction output data, and using wind power access points as clustering centers to perform voltage sensitivity-based partitioning;
a phased voltage control module configured to: the method for obtaining the voltage information of the lowest node and the highest node in each area and carrying out staged voltage control comprises the following steps: an emergency control stage, wherein an emergency control mode is adopted, and a preventive voltage control stage is adopted;
in an emergency control stage, controlling to determine the voltage control quantity of the area through prediction and inter-area communication; then, performing voltage control by using model predictive control with minimum voltage deviation and control cost as objective functions; and in the preventive voltage control stage, reactive replacement is carried out on the quick response equipment by using the generator set and the parallel capacitor in the area, so that the standby margin of the quick response equipment is increased.
The partition module comprises: a sensitivity calculation module for: based on initial value information of an AC/DC system, a DC feed-in bus and a DC system under high wind power permeability, track sensitivity of each bus voltage and voltage sensitivity of each control quantity to the DC feed-in bus are calculated by combining time domain simulation;
A phased voltage control module, comprising:
a control start judging module for: judging whether to start emergency voltage control and preventive voltage control according to the regional voltage deviation;
an inter-region coordination control amount determination module for: calculating reactive margin in the areas based on the wide area measurement data, and determining whether to increase inter-area participation coordination control quantity based on the reactive margin;
an optimal control sequence solving module for: and respectively carrying out optimization solving on the corresponding optimal control sequences of the voltage stability coordination control aiming at the respective control objective functions of the emergency voltage control and the prevention voltage control.
Example III
The present embodiment provides a computer-readable storage medium having stored thereon a computer program which, when executed by a processor, implements the steps of the control method as in the above-described embodiment one.
Example IV
The present embodiment provides a terminal device including a memory, a processor, and a computer program stored on the memory and executable on the processor, the processor implementing the steps in the control method as in the above embodiment example one when executing the program.
It will be appreciated by those skilled in the art that embodiments of the present invention may be provided as a method, system, or computer program product. Accordingly, the present invention may take the form of a hardware embodiment, a software embodiment, or an embodiment combining software and hardware aspects.
The present invention is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Those skilled in the art will appreciate that implementing all or part of the above-described methods in accordance with the embodiments may be accomplished by way of a computer program stored on a computer readable storage medium, which when executed may comprise the steps of the embodiments of the methods described above. The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. The method for controlling the voltage of the AC/DC system in a staged manner under the condition of high wind power permeability is characterized by comprising the following steps:
using wind power prediction output data, and using wind power access points as clustering centers to perform voltage sensitivity-based partitioning;
the method for obtaining the voltage information of the lowest node and the highest node in each area and carrying out staged voltage control comprises the following steps: an emergency control stage, wherein an emergency control mode is adopted, and a preventive voltage control stage is adopted;
in an emergency control stage, controlling to determine the voltage control quantity of the area through prediction and inter-area communication; then, performing voltage control by using model predictive control with minimum voltage deviation and control cost as objective functions; in the preventive voltage control stage, reactive replacement is carried out on the quick response equipment by utilizing the generator set and the parallel capacitor in the area, so that the standby margin of the quick response equipment is increased;
When the voltage control is carried out in stages, based on the voltage information of the lowest and highest nodes in each area, judging whether the deviation between the voltage information and a set value is larger than a first set value, if so, entering an emergency control mode, if not larger than the first set value, judging whether the deviation between the highest or lowest voltage and the set value is larger than a second set value, if so, entering a preventive voltage control mode, and if not larger than the first set value, considering a normal operation mode, and not carrying out control;
the first set value is larger than the second set value, namely emergency voltage control is used when the voltage deviation is larger; less, prophylactic control is employed;
in an emergency control mode, entering an emergency running state after being greatly disturbed, judging whether reactive power reserve of the area can meet control requirements by a controller, if not, sending a request to an adjacent area, adding reactive power equipment according to the voltage sensitivity until the control requirements are met, and then performing voltage control by using a quick response reactive power device and multi-terminal flexible direct current access to minimize voltage deviation and control cost;
the step of preventing voltage control in the preventive voltage control mode is as follows:
step 1: when the deviation between the alternating current system voltage and the set value is smaller than a second set value, starting preventive control;
Step 2: solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity;
step 3: solving an objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system;
step 4: applying an optimal control sequence at the initial moment of the next period of control sampling; detecting the system voltage, and ending the calculation if the system voltage meets the requirement; otherwise, returning to the step 2;
wherein the objective function of the preventive voltage control is:
ΔU min ≤|ΔU k |≤ΔU max k∈[1,N p ]
wherein omega u In order to control the weight coefficient of the cost,ω i weight coefficient, ω, for the ith reactive margin ac Is the weight coefficient of voltage fluctuation, N p 、N u Respectively, a prediction period and a control period number, deltau k,i Is the variation quantity of the ith control variable at the kth time, theta i,k For reactive margin of the ith control variable at time k, V' k Is the voltage amplitude after the k time control, V ave To average voltage over time period, Q max Q is the maximum and current value of the reactive power output, respectively.
2. The method for controlling the voltage of the alternating current and direct current system in the high wind power permeability according to claim 1, wherein when the partitioning based on the voltage sensitivity is carried out, the voltage sensitivity of a wind power access point to a PQ node is solved, and the PQ node is clustered to form a preliminary partition;
Then solving the average voltage sensitivity of the PV node to the partition after the PQ node is clustered;
on the premise of ensuring connectivity, the PV nodes are clustered into the area with the maximum average sensitivity.
3. The method for controlling the voltage of the alternating current and direct current system in the high wind power permeability according to claim 1, wherein the standby margin of the quick response reactive equipment is increased through reactive power replacement while the system voltage is ensured to be within a safe range in the preventive voltage control mode.
4. The method for controlling the voltage of the ac/dc system in the high wind power permeability according to claim 1, wherein the emergency voltage controlling step in the emergency control mode is as follows:
step 1: when the deviation between the node voltage of the alternating current system and the set value is larger than a first set value, starting emergency voltage control;
step 2: solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity;
step 3: judging whether the reactive power control quantity and the soft and straight active power control quantity in the area can meet the requirement of voltage control, if the reactive power control quantity and the soft and straight active power control quantity cannot meet the requirement, increasing the reactive power control quantity of the adjacent area, and increasing the installation sensitivity in sequence until the reactive power control quantity and the soft and straight active power control quantity meet the requirement, and if the reactive power control quantity and the soft and straight active power control quantity still cannot meet the requirement, starting load shedding control;
Step 4: solving an emergency voltage control objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system;
step 5: and (3) rolling control: applying an optimal control sequence at the initial moment of the next period of control sampling, detecting the system voltage, and ending calculation if the system voltage meets the requirement; otherwise, returning to the step 2.
5. The alternating current-direct current system staged voltage control system under high wind power permeability is characterized by comprising:
a partitioning module configured to: using wind power prediction output data, and using wind power access points as clustering centers to perform voltage sensitivity-based partitioning;
a phased voltage control module configured to: the method for obtaining the voltage information of the lowest node and the highest node in each area and carrying out staged voltage control comprises the following steps: an emergency control stage, wherein an emergency control mode is adopted, and a preventive voltage control stage is adopted;
in an emergency control stage, controlling to determine the voltage control quantity of the area through prediction and inter-area communication; then, performing voltage control by using model predictive control with minimum voltage deviation and control cost as objective functions; in the preventive voltage control stage, reactive replacement is carried out on the quick response equipment by utilizing the generator set and the parallel capacitor in the area, so that the standby margin of the quick response equipment is increased;
When the voltage control is carried out in stages, based on the voltage information of the lowest and highest nodes in each area, judging whether the deviation between the voltage information and a set value is larger than a first set value, if so, entering an emergency control mode, if not larger than the first set value, judging whether the deviation between the highest or lowest voltage and the set value is larger than a second set value, if so, entering a preventive voltage control mode, and if not larger than the first set value, considering a normal operation mode, and not carrying out control;
in an emergency control mode, entering an emergency running state after being greatly disturbed, judging whether reactive power reserve of the area can meet control requirements by a controller, if not, sending a request to an adjacent area, adding reactive power equipment according to the voltage sensitivity until the control requirements are met, and then performing voltage control by using a quick response reactive power device and multi-terminal flexible direct current access to minimize voltage deviation and control cost;
the step of preventing voltage control in the preventive voltage control mode is as follows:
step 1: when the deviation between the alternating current system voltage and the set value is smaller than a second set value, starting preventive control;
step 2: solving the predicted track of the system in the prediction time domain and the track sensitivity of each control quantity;
Step 3: solving an objective function to obtain an optimal control sequence of the voltage control optimization problem of the AC-DC system;
step 4: applying an optimal control sequence at the initial moment of the next period of control sampling; detecting the system voltage, and ending the calculation if the system voltage meets the requirement; otherwise, returning to the step 2;
wherein the objective function of the preventive voltage control is:
ΔU min ≤|ΔU k |≤ΔU max k∈[1,N p ]
wherein omega u To control the weight coefficient of the cost omega i Weight coefficient, ω, for the ith reactive margin ac Is the weight coefficient of voltage fluctuation, N p 、N u Respectively, a prediction period and a control period number, deltau k,i Is the variation quantity of the ith control variable at the kth time, theta i,k Is the firstReactive margin of i control variables at k time, V' k Is the voltage amplitude after the k time control, V ave To average voltage over time period, Q max Q is the maximum and current value of the reactive power output, respectively.
6. A computer-readable storage medium, on which a computer program is stored, characterized in that the program, when being executed by a processor, realizes the steps in the control method as claimed in any one of the preceding claims 1-4.
7. A terminal device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, characterized in that the processor implements the steps of the control method according to any of the preceding claims 1-4 when said program is executed.
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