CN111711217A

CN111711217A - Direct-current voltage control method of multi-terminal flexible direct-current system facing alternating-current power fluctuation

Info

Publication number: CN111711217A
Application number: CN202010437819.XA
Authority: CN
Inventors: 陈继开; 李国庆; 李林; 王振浩; 辛业春; 金国彬; 李扬; 张轶珠; 刘座铭
Original assignee: STATE GRID JILINSHENG ELECTRIC POWER SUPPLY Co ELECTRIC POWER RESEARCH INSTITUTE; Northeast Dianli University
Current assignee: STATE GRID JILINSHENG ELECTRIC POWER SUPPLY Co ELECTRIC POWER RESEARCH INSTITUTE; Northeast Electric Power University
Priority date: 2020-05-21
Filing date: 2020-05-21
Publication date: 2020-09-25
Anticipated expiration: 2040-05-21
Also published as: CN111711217B

Abstract

The invention relates to a direct-current voltage control method of a multi-terminal flexible direct-current system facing alternating-current power fluctuation, and belongs to the technical field of flexible direct-current power transmission and distribution. The method comprises the steps of power deviation iterative calculation and droop coefficient correction considering the power deviation; the power deviation iterative computation is used for realizing the real-time power deviation iterative computation of different converter stations of the MMC-MTDC system and providing a data basis for droop coefficient correction; and the droop coefficient correction considering the power deviation is used for improving the power distribution capability of the MMC-MTDC system and stabilizing the direct-current voltage fluctuation of the system caused by alternating-current power fluctuation. The invention can carry out self-adaptive correction on the droop coefficient, not only improves the power distribution capability of the MMC-MTDC system, but also effectively stabilizes the direct-current voltage fluctuation of the MMC-MTDC system caused by alternating-current power fluctuation caused by station switching of a converter station or sudden change of wind speed of a wind field.

Description

Direct-current voltage control method of multi-terminal flexible direct-current system facing alternating-current power fluctuation

Technical Field

The invention relates to the technical field of flexible direct current power transmission and distribution, in particular to a direct current voltage regulation and control method of an MMC-MTDC converter station based on a self-adaptive droop control method, and particularly relates to a direct current voltage control method of a multi-terminal flexible direct current system facing alternating current power fluctuation.

Background

As an advanced direct current power transmission and distribution technology, a multi-terminal flexible direct current (MMC-MTDC) power transmission and distribution technology is being valued by china and the world electric power industry, and more MMC-MTDC projects are beginning to be built in china. As a main unit of a multi-terminal flexible direct current transmission and distribution system, an MMC-MTDC converter station bears the duty of alternating current-direct current power conversion and four-quadrant power scheduling control, so whether a control strategy of the converter station and an inter-station coordination control strategy are scientific, reasonable and directly related to whether an alternating current-direct current hybrid power system can stably and efficiently operate or not is directly determined.

At present, a large number of new energy power generation units such as wind power generation units and the like are merged into a direct current transmission system through an MMC-MTDC converter station, the power fluctuation characteristics are obvious, short-time power unbalance occurs in the direct current system, and then the voltage of the direct current system fluctuates; meanwhile, the power shock caused by locking of the MMC converter station and even switching of the MMC converter station due to various alternating current and direct current faults can also cause negative influence on the stability of direct current voltage of the alternating current and direct current hybrid system. In order to solve the above problems, a method for coordinating and controlling direct current voltage of an MMC-MTDC system is proposed by combining the changing characteristics of alternating current power and aiming at stabilizing the negative influence of power fluctuation on the direct current system, which is one of the problems to be solved in the current and future MMC-MTDC power transmission and distribution research fields.

Disclosure of Invention

The invention aims to provide a direct-current voltage control method of a multi-terminal flexible direct-current system facing alternating-current power fluctuation, which solves the problem of direct-current voltage mutation of an MMC-MTDC system caused by alternating-current power fluctuation, and provides a direct-current voltage control method of the MMC-MTDC system based on self-adaptive droop control by analyzing and improving a traditional droop control strategy, wherein a droop coefficient can be adaptively corrected according to power deviation so as to improve the direct-current voltage regulation and control capability of the MMC-MTDC system.

The basic principle of power-voltage (P-U) droop control is a control method for the droop characteristic of the dc terminal voltage of a voltage source type converter at a receiving terminal along with the change of power. The P-U droop control has the advantages that when the voltage of the power receiving end is increased, the power distributed to the converter station is correspondingly reduced, so that the trend of node power increase is relieved; when the voltage decreases, the power allocated to the converter stations increases accordingly, thereby mitigating the tendency of node power to decrease. Therefore, the relation between the voltage fluctuation and the power fluctuation is accurately calculated, the droop characteristic curve is reasonably designed, and the method plays an important role in realizing the voltage optimization control of the direct-current power transmission system.

The above object of the present invention is achieved by the following technical solutions:

the method for controlling the direct-current voltage of the multi-terminal flexible direct-current system facing the alternating-current power fluctuation comprises power deviation iterative calculation and droop coefficient correction considering the power deviation; wherein the content of the first and second substances,

the power deviation iterative computation is used for realizing the real-time power deviation iterative computation of different converter stations of the MMC-MTDC system and providing a data basis for the correction of a droop coefficient;

and the droop coefficient correction considering the power deviation is used for improving the power distribution capability of the MMC-MTDC system and stabilizing the direct-current voltage fluctuation of the system caused by alternating-current power fluctuation.

The method for iterative calculation of power deviation specifically comprises the following steps:

1) relation between transmission power variation deviation and receiving end voltage deviation

Firstly, carrying out power flow analysis on an equivalent direct current transmission network by utilizing a Newton-Raphson method, and selecting an unknown direct current voltage variable V and a specified power parameter P_spThen obtaining a nonlinear parameter function f (V); from the existing estimates of unknown voltages, the jacobian matrix J consists of the partial derivatives of the functions f (v):

from the inverse of the Jacobian matrix, the new set of voltage estimates is solved by:

V⁽ⁱ⁺¹⁾＝V⁽ⁱ⁾+J^-1·[P_sp-f(V⁽ⁱ⁾)](2)

wherein V⁽ⁱ⁾And V⁽ⁱ⁺¹⁾The ith estimation and the (i + 1) th estimation are respectively carried out; iteratively updating the estimated voltage until an acceptable tolerance is reached for a mismatch between the specified parameter and a parameter calculated using the estimated variable;

in an n-terminal dc system, the steady state relationship between dc voltage and current is expressed as:

I_dc＝YV_dc(3)

wherein I_dcIs the injection node current vector, V_dcIs a direct voltage vector, Y is the admittance matrix of the network;

assuming that the receiving end of the wind power plant is a power bus, the power injected into the MTDC system by the receiving end i is expressed as

Port i to port j branch power of

P_ij＝V_i·Y_ij(V_j-V_i) (5)

And (4) and (5) respectively obtaining receiving end and branch power partial derivatives (6):

obtaining a receiving end i power deviation by (10):

where (k) is the number of iterations of the voltage.

The droop coefficient correction considering the power deviation specifically includes:

according to the obtained △ P_i(i＝3,4,5)Introducing power distribution weights α, β, gamma, α△ P₃+β△P₄+γ△P₅＝△P,α+β+γ＝1, 0<α<1,0<β<1,0<γ<1; when large disturbance occurs in the system, because the capacities of all converter stations are different, part of the converter stations are easy to overload, and in order to improve the capability of the droop control station to adaptively respond to the tidal current change of the direct current network, specific numerical values of the power distribution weight are selected according to actual operation conditions, wherein the specific numerical values comprise the capacity, the power margin and the voltage margin of the converter stations, and the formula (8) is shown;

in the formula, P_r- | P | is a power margin, e (═ 5%) is a dc voltage deviation coefficient, typically 5% of the dc voltage reference value, and K is a constant, typically with an inner value [1,4 ]]Taking values;

with the converter station 3 as a reference converter station and the dc voltage as a variable, the droop coefficient piecewise function of the

converter stations

3, 4, 5 is:

wherein i is 4, 5.

The invention has the beneficial effects that: aiming at the problem that a large amount of wind power is merged into a direct current transmission system through an MMC converter station at present, the short-time power imbalance occurs in the direct current system due to the severe fluctuation of wind power of a wind field, and then the voltage of the direct current system fluctuates; meanwhile, a series of problems such as negative influence on the stability of the direct-current voltage of the system can be caused by locking of the MMC converter station even power shock caused by station switching caused by various alternating-current and direct-current faults, in order to enhance the stabilizing capability of the converter station in the MMC-MTDC system to the voltage fluctuation under various power fluctuation conditions, the traditional droop control strategy is analyzed and improved, the invention provides the converter station direct-current voltage control method suitable for the MMC-MTDC system, the droop coefficient is adaptively corrected by comprehensively considering the dynamic power deviation of the MMC converter station at the receiving end according to the conditions such as the inherent transmission limit and the power/voltage margin of the converter station, and the control effect of the direct-current voltage is improved through reasonable power distribution among the stations. The direct-current voltage control method of the multi-terminal flexible direct-current system can adaptively correct the droop coefficient, not only improves the power distribution capability of the MMC-MTDC system, but also effectively stabilizes direct-current voltage fluctuation of the MMC-MTDC system caused by alternating-current power fluctuation caused by station switching of a converter station or sudden change of wind speed of a wind field. The direct-current voltage regulation and control capability of the MMC-MTDC system is improved.

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 application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention.

FIG. 1 is a multi-terminal flexible DC transmission engineering model of the present invention;

FIG. 2 is a multi-terminal DC power transmission system network topology of the present invention;

fig. 3 is a graph of the power deviation droop characteristics of the

converter stations

3, 4, 5 according to the invention;

FIG. 4 is an MMC converter station controller of the present invention;

FIG. 5 is a simulation diagram of wind power fluctuation of a converter station MMC2 of the invention;

fig. 6 is a simulation diagram of the fan side converter station MMC1 fault exit operation of the present invention.

Detailed Description

The details of the present invention and its embodiments are further described below with reference to the accompanying drawings.

Referring to fig. 1 to 6, the method for controlling the dc voltage of the multi-terminal flexible dc system facing the ac power fluctuation of the present invention includes iterative calculation of power deviation and correction of droop coefficient considering the power deviation; the power deviation iterative calculation method is used for realizing the real-time power deviation iterative calculation of different converter stations of the MMC-MTDC system and providing a data basis for droop coefficient correction; and the droop coefficient correction considering the power deviation is used for improving the power distribution capability of the MMC-MTDC system and stabilizing the direct-current voltage fluctuation of the system caused by alternating-current power fluctuation.

Referring to fig. 1, a multi-terminal flexible direct current parallel ring network system model is constructed. In fig. 1, the multi-terminal flexible dc parallel ring network system mainly includes: the method comprises five parts of a cluster wind power plant, a wind power plant side converter station, a power grid side converter station, an alternating current line (AC line), a direct current network (DC grid) and the like.

Setting △ P power fluctuation of ith converter station at sending end_i＝P_refi-P_iThe total power at the delivery end variation △ P is expressed as:

the multi-terminal flexible direct-current parallel ring network system shown in fig. 1 is equivalent to a direct-current transmission network, as shown in fig. 2. In FIG. 2, MMC₁，MMC₂For sending-end converter station (power input end), constant active power control, MMC₃，MMC₄And MMC₅P-U droop control is adopted for the receiving end converter station. The direct current power-voltage droop control can know that the receiving end converter station meets the following requirements:

in the formula of U_dc3、U_dc4And U_dc5Is the converter station dc side outlet voltage (dc voltage measurement); p₃、P₄And P₅Absorbing the dc power (power measurement) for the converter station: k_droop3、K_droop4And K_droop5For the droop factor, R, of the

converter station

3, 4, 5_ijIs a DC network line impedance, R_n(n is 1, 2, 3, 4, 5) is the equivalent impedance of the converter station on the dc side. The

nodes

3, 4, 5 are connected by a power transfer line, the voltage of the nodes being approximately equal, i.e. U, due to the shorter power transfer line₃≈U₄≈U₅And P ═ UI, as exemplified by 3 and 4 (I)₃₄＝λI_dc3，0<λ<1) Can satisfy the following conditions:

the formula (2) and (3) can be simultaneously solved:

as can be seen from equation (4), the active power distributed by the

receiving terminals

3, 4, and 5 converter stations is related to the droop coefficient and the line impedance, while the impedance of the determined dc transmission system is generally regarded as a fixed value, and the active power and the dc voltage distributed by the

receiving terminals

3, 4, and 5 converter stations are closely related to the droop coefficient.

1. Iterative calculation of power offset

Firstly, carrying out power flow analysis on an equivalent direct current transmission network by utilizing a Newton-Raphson (NR) method, and selecting an unknown direct current voltage variable V and a specified power parameter P_spThen, a nonlinear parameter function f (v) is obtained. From the existing estimates of unknown voltages, the jacobian matrix J consists of the partial derivatives of the functions f (v):

V⁽ⁱ⁺¹⁾＝V⁽ⁱ⁾+J^-1·[P_sp-f(V⁽ⁱ⁾)](6)

wherein V⁽ⁱ⁾And V⁽ⁱ⁺¹⁾The ith estimate and the (i + 1) th estimate, respectively. Iteratively updating the estimated voltage until the specified parameter is addressedThe mismatch between the number and the parameter calculated using the estimated variables reaches an acceptable tolerance.

I_dc＝YV_dc(7)

wherein I_dcIs the injection node current vector, V_dcIs the dc voltage vector and Y is the admittance matrix of the network.

Assuming that the receiving end of the wind farm is a power bus, the power injected into the MTDC system by the receiving end i can be expressed as

Port i to port j branch power of

P_ij＝V_i·Y_ij(V_j-V_i) (9)

And (8) and (9) respectively obtaining receiving end and branch power partial derivatives (10):

obtaining a receiving end i power deviation by (10):

where (k) is the number of iterations of the voltage.

2. Droop coefficient correction taking power offset into account

△ P obtained according to 1)_i(i＝3,4,5)Introducing power distribution weights α, β, gamma, α△ P₃+β△P₄+γ△P₅＝△P,α+β+γ＝1, 0<α<1,0<β<1,0<γ<1. When the disturbance in the system is large, because the capacity of each converter station is different, part of the converter stations are easy to overload, and in order to improve the capability of the droop control station to adaptively respond to the tidal current change of the direct current network, the specific numerical value of the power distribution weight is according to the actualThe operation conditions (converter station capacity, power margin, voltage margin) are selected as shown in formula (12).

In the formula, P_r- | P | is a power margin, e (═ 5%) is a dc voltage deviation coefficient, typically 5% of the dc voltage reference value, and K is a constant, typically with an inner value [1,4 ]]And (4) taking values.

The power deviation droop characteristic of the

converter station

3, 4, 5 with the converter station 3 as a reference station is shown in fig. 3. In the figure P_MinAnd P_MaxIs the active power limit value, P, of the converter station_HAnd P_LThe maximum value and the minimum value of the active power margin of the converter station are obtained; u shape_dcMaxAnd U_dcMinFor converter station DC voltage limit, U_refIs a voltage reference value, P_refIs a power reference value; k_droop3，K_droop4,1，K_droop4,2And K_droop5,1，K_droop5,2The droop coefficients of the

converter stations

3, 4 and 5 respectively, point a' in the droop curve of the converter station 3 translates the curve to the right under the condition that the droop coefficient (curve slope) is not changed, the voltage reference value is not changed at the moment, and the power reference value is changed from the original P_3refIncrease α△ P_iIs then changed into P'_3ref(ii) a Similarly, B' in the droop curve of the converter station 4 is obtained by translating the droop curve from B without changing the slope of the droop curve, and the point B "is obtained by changing the slope of the droop curve according to the actual situation and obtaining a new voltage reference value U again"_dc4refReference value of power P'_4ref(ii) a The converter station 5 is identical to the converter station 4.

As can be taken from fig. 3, the droop coefficient piecewise function of the

converter stations

3, 4, 5 with the dc voltage as a variable is:

wherein i is 4, 5.

In order to verify the correctness of the theory and derivation formula provided by the invention, the method is based onAn RT-LAB5600 simulation platform builds a multi-terminal flexible direct current parallel ring network system model according to the graph 1, and the structure of a controller is shown in the graph 4. MMC of converter station at wind field side₁And MMC₂Constant power control or constant alternating voltage control is adopted; MMC₃，MMC₄And MMC₅Is a power receiving end; wherein L is₁₃＝100km，L₁₂＝100km，L₁₅＝150km，L₂₅＝100km，L₃₄＝15km，L₄₅20 km. The main parameters of the simulation platform are shown in table 1.

TABLE 1

1) Fluctuation of wind power

The system is in steady state operation in the initial stage, and the MMC is operated when t is 1s₂The wind speed in the connected wind field rises, the output of a wind field fan is increased from 2000MW to 3000MW, the system is stable after about 0.1s, the wind speed is reduced when t is 2s, the output of the wind field fan is reduced from 3000MW to 2200MW, and the system is stable after about 0.1 s. The simulation results of the dc voltage control using the adaptive droop and the conventional droop respectively are shown in fig. 5.

From fig. 5, it can be seen that when a converter station MMC₂When the output of the connecting wind field rises suddenly, the transmission power in the MTDC system rises along with the rising of the output of the connecting wind field, so that the voltage of the direct current bus rises temporarily; when the wind speed is reduced and the wind field force is returned, the voltage of the direct current bus is temporarily reduced. Wherein, taking MMC5 as an example for analysis, t_s5ADC＝0.05s,σ_5ADC％＝20％，t_s5TDC＝0.30s，σ_5TDC35%, the direct current voltage control based on the self-adaptive droop method provided by the patent is adopted, and the MMC is realized₃、MMC₄And MMC₅The power redistribution and the direct-current voltage control effect are better than those of the traditional control method (TDC).

2) Wind power plant side converter station quit operation

Wind field side converter station MMC with t being 1.5s₁As the fault exits the multi-terminal direct-current power transmission system, the simulation comparison result of respectively adopting the method and the traditional droop is shown in figure 6Shown in the figure.

As can be seen from part (a) of FIG. 6, MMC₁After the operation is quitted, because the sending end power is suddenly reduced to 1000MW, the sending end power is far less than the receiving end power, MMC₁The voltage of the direct current bus is reduced instantly, and the MMC of the receiving end converter station₃、MMC₄、 MMC₅The DC bus voltage does not return to the rated voltage under the action of the traditional droop control. As can be seen from part (b) of FIG. 6, t_s5TDC＝0.40s，σ_5TDC％＝35.2％，t_s5ADC＝0.04s,σ_5ADCAnd the percent is 20%, the droop coefficient of the converter station is adaptively changed by adopting an adaptive droop control system, reasonable power distribution is carried out on the receiving-end converter station, the voltage of a direct-current bus of the MTDC system is stabilized within a safety margin, and the transition time and the overshoot are superior to those of the traditional droop control.

As can be seen from fig. 5 and 6, no matter wind power fluctuates or the converter station exits from operation due to a fault, the direct-current voltage control method provided herein can be used to reasonably distribute power to the MTDC power transmission system, so that the voltage of the direct-current bus is constant, and the dynamic response is faster.

Simulation experiment results show that the direct-current voltage control method of the multi-end flexible direct-current system can perform self-adaptive correction on the droop coefficient, not only improves the power distribution capability of the MMC-MTDC system, but also effectively restrains direct-current voltage fluctuation of the MMC-MTDC system caused by alternating-current power fluctuation caused by station switching of a converter station or sudden change of wind speed of a wind field.

The above description is only a preferred example of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like of the present invention shall be included in the protection scope of the present invention.

Claims

1. A multi-terminal flexible direct current system direct current voltage control method facing alternating current power fluctuation is characterized in that: the method comprises the steps of power deviation iterative calculation and droop coefficient correction considering the power deviation; wherein the content of the first and second substances,

2. The method for controlling the direct-current voltage of the multi-terminal flexible direct-current system facing the alternating-current power fluctuation according to claim 1, wherein: the method for iterative calculation of power deviation specifically comprises the following steps:

V⁽ⁱ⁺¹⁾＝V⁽ⁱ⁾+J^-1·[P_sp-f(V⁽ⁱ⁾)](2)

I_dc＝YV_dc(3)

Port i to port j branch power of

P_ij＝V_i·Y_ij(V_j-V_i) (5)

obtaining a receiving end i power deviation by (10):

where (k) is the number of iterations of the voltage.

3. The method for controlling the direct-current voltage of the multi-terminal flexible direct-current system facing the alternating-current power fluctuation according to claim 1, wherein: the droop coefficient correction considering the power deviation specifically includes:

according to the obtained delta P_i(i＝3,4,5)Introducing power distribution weights α, β, gamma, α△ P₃+β△P₄+γ△P₅＝△P,α+β+γ＝1,0<α<1,0<β<1,0<γ<1; when large disturbance occurs in the system, because the capacities of all converter stations are different, part of the converter stations are easy to overload, and in order to improve the capability of the droop control station to adaptively respond to the tidal current change of the direct current network, specific numerical values of the power distribution weight are selected according to actual operation conditions, wherein the specific numerical values comprise the capacity, the power margin and the voltage margin of the converter stations, and the formula (8) is shown;

with the converter station 3 as a reference converter station and the dc voltage as a variable, the droop coefficient piecewise function of the converter stations 3, 4, 5 is:

wherein i is 4, 5.