CN110880778A - Improved nonlinear droop control method for multi-terminal flexible direct-current power transmission system - Google Patents

Improved nonlinear droop control method for multi-terminal flexible direct-current power transmission system Download PDF

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CN110880778A
CN110880778A CN201911061516.6A CN201911061516A CN110880778A CN 110880778 A CN110880778 A CN 110880778A CN 201911061516 A CN201911061516 A CN 201911061516A CN 110880778 A CN110880778 A CN 110880778A
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droop
gsvsc
characteristic curve
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朱介北
李峰
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Tianjin University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention discloses an improved nonlinear droop control method for a multi-terminal flexible direct-current transmission system, which comprises droop characteristic curve fuzzy search, real-time droop curve table look-up control and droop curve period updating control, wherein when the direct-current cable resistance or the tide of the multi-terminal flexible direct-current system changes, the droop characteristic curve of a converter station at the network side can be updated in real time, the accurate control of direct-current voltage and active power of the converter station is realized, and a control error caused by the droop control adopting a linear fixed droop coefficient is avoided. According to the method, the accurate nonlinear droop characteristic curve equation of the GSVSC is obtained by utilizing the fuzzy search technology, the accurate control of the direct-current voltage and the active power of the commutation station is realized, the active power control error caused by the adoption of linear droop control is avoided, and the real-time droop characteristic curve of the GSVSC is obtained in real time.

Description

Improved nonlinear droop control method for multi-terminal flexible direct-current power transmission system
Technical Field
The invention relates to the field of active power control and scheduling of a multi-terminal flexible direct current transmission system, in particular to an improved nonlinear droop control method of the multi-terminal flexible direct current transmission system based on fuzzy search.
Background
The maturity of the Voltage-Source Converter (VSC) technology promotes the development of a Multi-Terminal flexible high Voltage DC (VSC-MTDC) power transmission system, and promotes the extensive research on the related technology of the flexible DC power transmission system. Compared with a traditional high-voltage alternating-current transmission system, the VSC-MTDC system has the advantages of smaller transmission loss, high control flexibility of the VSC converter station, capability of providing reactive compensation for the converter station, capability of realizing cross-regional or cross-country asynchronous interconnection of power grids and the like, becomes a preferred scheme for remotely accessing a large offshore wind farm to an onshore alternating-current power grid, and is used for transmitting abundant wind power in the North sea to Germany, British, Norway and Belgium if the European North sea cross-country MTDC system transmits the abundant wind power in the North sea to Germany; china has built a south Australia +/-160 kV sea-crossing four-terminal flexible direct-current transmission demonstration project and a Zhoushan +/-200 kV five-terminal flexible direct-current transmission demonstration project, and will complete the construction of a Zhang-north +/-500 kV four-terminal renewable energy flexible direct-current power grid demonstration project in 2021, thereby realizing a truly super flexible direct-current transmission system.
Among many challenges faced by the VSC-MTDC power transmission technology, how to maintain the stability of the direct-current voltage and realize the accurate control and scheduling of the active power of a multi-terminal Grid-side converter station (GSVSC) is a key for determining the stable operation of the VSC-MTDC system. Aiming at the challenge, related scholars at home and abroad propose various active power-direct current voltage (P-U) applied to VSC-MTDC systemDC) According to the droop control method, the direct-current voltage of the multi-end GSVSC is controlled, and the distribution of active power among the GSVSCs is achieved.
Currently, P-U applied to VSC-MTDC systemDCThe droop control adopts linear droop control, namely P-UDCThe droop curve is a straight line, and the control method is based on the steady-state characteristic of the VSC-MTDC direct-current network and realizes the control of the GSVSC active power by controlling the direct-current voltage. However, in the VSC-MTDC system, the direct current network is equivalent to a pure resistance network in a steady state, and the active power P of the GSVSC is proportional to UDC 2P-U of the sameDCThe sag characteristic exhibits a characteristic non-linear characteristic. If linear P-U is adoptedDCDroop control adjusts the active power of the GSVSC, and inevitably causes control errors, so that the control and scheduling of the active power of the GSVSC are inaccurate. Furthermore, P-U due to GSVSCDCThe droop characteristic is related to the direct current network characteristic of the MTDC system, and when the resistance of a direct current cable in the MTDC system changes along with the change of external temperature and current, the P-U of the GSVSCDCThe droop characteristic is changed along with the change of the droop characteristic, and the droop control adopting the linear droop coefficient influences the accuracy of the GSVSC active power control. To sum upNon-linear P-U provided for GSVSC in VSC-MTDC systemDCSag characteristics and P-UDCDue to the characteristic that the droop characteristic changes along with the resistance of the direct current network, when the GSVSC in the existing VSC-MTDC system adopts droop control with a linear fixed droop coefficient, control errors inevitably exist, and the accuracy of active power control and scheduling of the VSC-MTDC system and the running performance of the system are seriously affected.
Disclosure of Invention
Aiming at solving the problems that the droop control of a linear fixed droop coefficient adopted by a GSVSC in a multi-terminal flexible direct current transmission system has inaccurate active power control, and the characteristic that the resistance of a direct current cable changes along with the temperature influences the active power-direct current voltage (P-U)DC) The invention provides an improved nonlinear droop control method of a multi-terminal flexible direct current transmission system, which is used for achieving accurate control of direct current voltage and active power of a commutation station by obtaining an accurate nonlinear droop characteristic curve equation of GSVSC (source voltage source converter) through a fuzzy search technology.
The invention discloses an improved nonlinear droop control method for a multi-terminal flexible direct current transmission system, which comprises four processes of droop characteristic curve fuzzy search, real-time droop curve table look-up control, droop curve period updating control and direct current outer ring-alternating current inner ring double closed-loop control of a GSVSC bottom layer, and comprises the following steps of:
step 1, generating a reference voltage of a direct-current voltage outer ring by utilizing a droop characteristic curve fuzzy search and a real-time droop curve table look-up control process, and specifically comprising the following steps of;
step (1-1), defining reference voltage U of a direct current voltage outer ring in the fuzzy search process of the droop characteristic curveDC-refThe expression of (a) is:
Figure BDA0002258082140000031
in the formula: u shapeDC-minAnd UDC-maxRespectively representing the minimum limit value and the maximum limit value of the GSVSC direct-current voltage operation, T representing the total time length of the droop characteristic curve fuzzy search, TSA sampling period representing a fuzzy search; nTSRepresenting the sampling period of the nth fuzzy search in the time length of T, and S representing the running state of the fuzzy search; u shapeDC-0Representing a direct current voltage reference value obtained by the table look-up control of the droop curve, and sending an active power scheduling command P by the far-end scheduling system to the droop curve table look-up controlrefThen obtaining the product through table look-up;
when S is equal to 1, starting fuzzy search, and obtaining a real-time droop characteristic curve of the converter station through scanning; reference value U of GSVSC direct-current voltage outer ring during fuzzy searchDC-refFrom the minimum limit UDC-minGradually increasing to a maximum limit value U after the total time T of the fuzzy searchDC-maxAt each sampling period TSActive power and direct current voltage data of GSVSC are collected, and the active power and direct current voltage data collected in the ith sampling period are expressed as (P)i,Ui)(i＝1,2,3…n)。
After the fuzzy search is completed within the total time T of the fuzzy search, the GSVSC nonlinear discrete operating point (P) is subjected to the least square methodi,Ui) (i is 1,2,3 … n), and obtaining a continuous nonlinear droop characteristic curve equation expression of the converter station as follows:
P＝kU2+bU
in the formula: k and b are both GSVSC converter station P-UDCThe coefficient of the droop characteristic.
P-U Using least squaresDCFitting a drooping curve, and calculating an active power measured value P of the GSVSCiAnd the calculated active power (kU)i 2+bUi) Function f of minimum sum of squares of residualsLS(k, b), the expression is as follows:
Figure BDA0002258082140000032
k and b are droop coefficients corresponding to the GSVSC real-time droop characteristic curve;
by solving for fLS(k, b) obtaining coefficients k and b of a real-time droop characteristic curve equation of the converter station, thereby obtaining continuous and real-time P-U of the GSVSCDCA droop characteristic curve equation;
step (1-2), real-time droop curve table look-up control is carried out, namely after fuzzy search is finished, the obtained droop characteristic curve equation P is kU2+ bU is loaded to real-time droop curve look-up table control; switching S to 2, namely S is 2, and at the moment, the remote dispatching system sends an active power instruction P according to the dispatching requirement of the VSC-MTDC systemrefObtaining the power command P by looking up the tablerefReference value U of corresponding DC voltage outer loop controlDC-0Controlling the direct-current voltage of the GSVSC so as to realize the control of the active power of the converter station;
step 2, executing a process of sag curve period updating control, wherein the process comprises the following specific implementation steps:
step (2-1), collecting at t0Resistance value R of direct current cable constantly connected with GSVSC and between GSVSCm(0)(m is 1,2,3 …), wherein m is the number of the direct current cable;
step (2-2), starting fuzzy search of droop characteristic curve to obtain t0The expression of the initialized droop characteristic curve equation of the GSVSC at the moment is as follows:
P＝k(0)U2+b(0)U；
step (2-3), collecting the next time t1Resistance value R of time direct current cablem(1)(m-1, 2,3 …), and the direct voltage U of the GSVSC(1)And active power P(1)；
Step (2-4), judging | Rl(1)-Rl(0)If | is greater than 0, l is any one of m direct current cables: if | Rl(1)-Rl(0)If | 0 is true, it indicates t1Immediately executing the step (2-5) when the resistance of the direct current cable changes due to the change of the temperature or the system running state; if | Rl(1)-Rl(0)If the value of | is greater than 0, the resistance of all the direct current cables is not changed, but the running state of the system may be changed to cause the change of the GSVSC droop characteristic curve, and then the step (2-8) is executed;
step (2-5), starting fuzzy search of the droop characteristic curve to obtain t1Real-time droop characteristic curve equation P ═ k of time GSVSC(1)U2+b(1)U；
Step (2-6), after the droop characteristic curve fuzzy search is finished, t is carried out1Droop characteristic curve equation P ═ k of GSVSC at time(1)U2+b(1)Loading the U into a real-time droop curve look-up table;
step (2-7), updating the initial value, i.e. t1Droop characteristic equation of time and Rm(1)(m-1, 2,3 …) as the initial value for the next cycle, i.e. Rm(1)(m-1, 2,3 …) for Rm(0)(m＝1，2，3…)，k(1)Replacement of k(0)，b(1)Replacement b(0). Returning to execute the step (2-3) and carrying out the next circulation;
step (2-8) of judging | P6(1)-k(0)U6(1) 2-b(0)U6(1)If | < ε, then:
if | P6(1)-k(0)U6(1) 2-b(0)U6(1)If the | < epsilon is established, indicating that the change of the running state of the system does not cause the change of the GSVSC droop characteristic curve, returning to the step (2-3) for carrying out the next cycle;
if | P6(1)-k(0)U6(1) 2-b(0)U6(1)If the condition that the value is less than epsilon is not satisfied, the change of the system running state causes the change of the GSVSC droop characteristic curve, and the step (2-5) is returned to;
where ε represents a positive integer approaching 0 infinity.
The double closed-loop control of the direct-current voltage outer loop and the alternating-current inner loop is the bottom-layer control of the GSVSC, and the accurate tracking of a direct-current voltage reference value is realized.
Compared with the prior art, the invention has the advantages that:
(1) the fuzzy search technology is utilized to obtain an accurate nonlinear droop characteristic curve equation of the GSVSC, so that the accurate control of the direct-current voltage and the active power of the commutation station is realized, and the control error caused by the adoption of linear droop control is avoided;
(2) when the direct-current cable resistance of the flexible direct-current transmission system cannot be acquired, the running state of the system is detected, whether fuzzy search of a droop characteristic curve needs to be started or not can be judged, and a GSVSC real-time droop characteristic curve is acquired in real time.
Drawings
FIG. 1 is a four-terminal flexible DC power transmission system topology diagram;
fig. 2 is a block diagram of an implementation process of an improved nonlinear droop control method of a multi-terminal flexible direct-current power transmission system according to the present invention;
FIG. 3 is a flowchart of droop curve period update control;
fig. 4 is a graph comparing a droop curve of the GSVSC obtained by using the improved nonlinear droop control and the conventional linear droop control (the range of the dc voltage is 0.9985p.u. -1.0025 p.u.);
FIG. 5 is a graph comparing a droop curve of the GSVSC obtained by using the improved nonlinear droop control and the conventional linear droop control (the range of the DC voltage is 0.9985p.u. -0.99875 p.u.);
fig. 6 is a graph comparing step response of the GSVSC (GSVSC2 active power) when the improved nonlinear droop control and the linear droop control are adopted;
fig. 7 is a step response comparison graph (GSVSC2 dc voltage) of GSVSC using modified non-linear droop control and linear droop control.
Detailed Description
The invention is further illustrated by the following detailed description in conjunction with the drawings in which:
the embodiment is mainly used for a multi-terminal (taking four terminals as an example) flexible direct-current transmission system, and is a topological diagram of the four-terminal flexible direct-current transmission system as shown in fig. 1. The four-end flexible direct current transmission system comprises 2 Wind power plant VSC converter stations (Wind-farm VSC, WFVSC) and two GSVSCs. The offshore wind farm is connected to a four-end flexible direct-current transmission system with a ring topology through WFVSC, wind power is transmitted through the flexible direct-current transmission system, and the GSVSC is connected to two asynchronous onshore alternating-current power grids.
Fig. 2 is a block diagram illustrating an implementation process of an improved nonlinear droop control method for a multi-terminal flexible dc power transmission system according to the present invention.
Step 1, fuzzy search of droop characteristic curves and real-time droop curve table look-up control are carried out to generate reference voltage of a direct-current voltage outer ring;
(1-1) fuzzy search process of droop characteristic curve, reference voltage U of outer ring of direct current voltageDC-refThe expression of (a) is:
Figure BDA0002258082140000061
in the formula: u shapeDC-minAnd UDC-maxRespectively representing the minimum limit value and the maximum limit value of the GSVSC direct-current voltage operation, T representing the total time length of the droop characteristic curve fuzzy search, TSA sampling period representing a fuzzy search; nTSRepresenting the sampling period of the nth fuzzy search in the time length of T, and S representing the running state of the fuzzy search; u shapeDC-0Representing a direct current voltage reference value obtained by the table look-up control of the droop curve, and sending an active power scheduling command P by the far-end scheduling system to the droop curve table look-up controlrefAnd then the data is obtained by looking up a table.
And when S is equal to 1, starting fuzzy search, and scanning to obtain a real-time droop characteristic curve of the converter station. Reference value U of GSVSC direct-current voltage outer ring during fuzzy searchDC-refFrom the minimum limit UDC-minGradually increasing to a maximum limit value U after the total time T of the fuzzy searchDC-maxAt each sampling period TSActive power and direct current voltage data of GSVSC are collected, and the active power and direct current voltage data collected in the ith sampling period are expressed as (P)i,Ui)(i＝1,2,3…n)。
After the fuzzy search is completed within the total time T of the fuzzy search, the GSVSC nonlinear discrete operating point (P) is subjected to the least square methodi,Ui) (i is 1,2,3 … n), and obtaining a continuous nonlinear droop characteristic curve equation expression of the converter station as follows:
P＝kU2+bU
in the formula: k and b are both GSVSC converter station P-UDCSag characteristic curveThe coefficient of (a).
P-U Using least squaresDCFitting a drooping curve, and calculating an active power measured value P of the GSVSCiAnd the calculated active power (kU)i 2+bUi) Function f of minimum sum of squares of residualsLS(k, b), namely:
Figure BDA0002258082140000071
at this time, k and b are droop coefficients corresponding to the GSVSC real-time droop characteristic curve.
By solving for fLS(k, b), namely obtaining coefficients k and b of a real-time droop characteristic curve equation of the converter station, thereby obtaining continuous and real-time P-U of the GSVSCDCSag characteristic curve equation.
(1-2) real-time droop curve look-up table control:
after the fuzzy search is finished, the obtained droop characteristic curve equation P is kU2+ bU is loaded into the real-time droop curve look-up table control. In order to control the GSVSC by using the droop characteristic loaded in the real-time droop curve look-up table control, S is switched to 2, namely S is 2. At the moment, the remote dispatching system sends an active power instruction P to the real-time droop curve table look-up control module according to the dispatching requirement of the VSC-MTDC systemref(ii) a Real-time droop control curve look-up table control is controlled at received PrefAfter the instruction, the power instruction P is obtained by table look-uprefCorresponding DC voltage reference value UDC-0；UDC-0And as a reference value for the outer loop control of the direct-current voltage, controlling the direct-current voltage of the GSVSC so as to realize the control of the active power of the converter station. The far-end scheduling system sends out any active power instruction P according to the scheduling requirement of the VSC-MTDC systemref。
And 2, updating and controlling the period of the droop curve:
when the resistance of a direct current cable of the VSC-MTDC system changes along with the change of the external environment temperature or the running state of a direct current network fluctuates randomly due to a wind power plant and the like, the P-U of the GSVSCDCThe droop characteristic will change accordingly, at this momentObtaining real-time P-U of GSVSC through sag curve period updating controlDCDroop characteristics. Fig. 3 shows a flowchart of the droop curve period update control. Whether a droop characteristic curve is updated or not is judged by detecting the resistance of a direct current cable and the running state of a system, and the implementation steps of the droop curve period updating control process are as follows:
(2-1) Collection at t0Resistance value R of direct current cable constantly connected with GSVSC and between GSVSCm(0)(m is 1,2,3 …), wherein m is the number of the direct current cable;
(2-2) starting fuzzy search of the droop characteristic curve to obtain t0The expression of the initialized droop characteristic curve equation of the GSVSC at the moment is as follows:
P＝k(0)U2+b(0)U；
(2-3) collecting the next time t1Resistance value R of time direct current cablem(1)(m-1, 2,3 …), and the direct voltage U of the GSVSC(1)And active power P(1)；
(2-4) judgment of | Rl(1)-Rl(0)If the l is more than 0, and l is any one of the m direct current cables. If R isl(1)-Rl(0)If | 0 is true, it indicates t1Immediately executing the step (2-5) when the resistance of the direct current cable changes due to the change of the temperature or the system running state; if | Rl(1)-Rl(0)If the value of | is greater than 0, the resistance of all the direct current cables is not changed, but the running state of the system may be changed to cause the change of the GSVSC droop characteristic curve, and then the step (2-8) is executed;
(2-5) starting fuzzy search of the droop characteristic curve to obtain t1Real-time droop characteristic curve equation P ═ k of time GSVSC(1)U2+b(1)U；
(2-6) after the droop characteristic curve fuzzy search is finished, t is added1Droop characteristic curve equation P ═ k of GSVSC at time(1)U2+b(1)Loading the U to a real-time droop curve look-up module;
(2-7) updating the initial value, i.e., t1Droop characteristic equation of time and Rm(1)(m-1, 2,3 …) as the initial value for the next cycle, i.e. Rm(1)(m-1, 2,3 …) for Rm(0)(m＝1，2，3…)，k(1)Replacement of k(0)，b(1)Replacement b(0)(ii) a Returning to execute the step (2-3) and carrying out the next circulation;
(2-8) judgment of | P6(1)-k(0)U6(1) 2-b(0)U6(1)Whether or not | < ε holds:
if | P6(1)-k(0)U6(1) 2-b(0)U6(1)If the | < epsilon is established, indicating that the change of the running state of the system does not cause the change of the GSVSC droop characteristic curve, returning to the step (2-3) for carrying out the next cycle;
if | P6(1)-k(0)U6(1) 2-b(0)U6(1)If | < epsilon is not true, it indicates that the change of the system operation state causes the change of the GSVSC droop characteristic curve, and the step (2-5) is returned to be executed (wherein epsilon represents a positive integer approaching 0 infinitely, and epsilon → 0).
The invention discloses an improved nonlinear droop control method for a multi-terminal flexible direct current transmission system, which comprises the following steps: the droop characteristic curve fuzzy search and real-time droop curve table look-up control process generates a reference voltage of a direct current voltage outer ring, the reference voltage is used as the input of the direct current voltage outer ring, the reference voltage is compared with the direct current voltage of the converter station obtained through actual measurement, and a reference current value of an alternating current inner ring is generated after the reference voltage passes through a proportional-integral (PI) regulator; and comparing the reference current of the alternating current inner ring with the alternating current of the converter station obtained through actual measurement, obtaining the alternating current reference voltage required by the switching device of the converter station after the reference current passes through a PI regulator, and producing a pulse signal for controlling the switching device of the converter station after the reference voltage is modulated by SPWM (sinusoidal pulse width modulation) to complete the control of the converter station.
As shown in fig. 4 and 5, a comparison of droop curves of the VSC converter station obtained by using the modified non-linear droop control and the linear droop control is shown. The obtained GS when an improved nonlinear droop control method based on fuzzy search and a droop control method adopting a traditional linear droop coefficient are respectively adopted are compared in the figureThe droop characteristic curves of the VSC2 are compared with the actual operating point of the GSVSC according to the droop characteristic curves obtained by the two droop control methods. In fig. 4, the dc voltage ranges from 0.9985p.u. to 1.0025p.u. GSVSC2 actual P-U when the DC voltage changes from 0.99875p.u. to 1.0025p.uDCThe droop characteristic is approximately linear, and droop characteristic curves of the GSVSC2 obtained by adopting an improved nonlinear droop control method based on fuzzy search and a droop control method adopting a traditional linear droop coefficient coincide, so that the active power control by adopting the two droop control methods is accurate. In fig. 5, when the dc voltage is changed from 0.9985p.u. to 0.99875p.u., the actual droop characteristic of the GSVSC exhibits an obvious nonlinear characteristic, and a droop curve obtained by using a droop control method with a conventional linear droop coefficient has an obvious deviation from an actual operating point, which is not favorable for accurate control of active power; and the droop curve obtained by adopting the improved nonlinear droop control method based on the fuzzy search is matched with the actual operating point of the GSVSC, which shows that the droop control method is more accurate in control compared with the droop control of the traditional linear fixed droop coefficient.
As shown in fig. 6 and 7, the active power command P is a comparison graph of the step response of the GSVSC when the improved nonlinear droop control and the linear droop control are adoptedrefInitial value 0.9 p.u.; when t is 5s, PrefChange to 0.92 p.u.; when t is 10s, PrefChange to 0.9 p.u.; when t is 15s, PrefChange to 0.94 p.u.. In fig. 6 and 7, when the conventional linear droop control is adopted, there is a significant deviation between the active power and the dc voltage of the GSVSC actually operating, and the active power command and the corresponding dc voltage reference value, and P isrefThe closer to 1p.u., the greater the deviation between the two; when the improved nonlinear droop control is adopted, the active power and the direct-current voltage of the actual operation of the GSVSC are consistent with the active power instruction and the corresponding direct-current voltage reference value basically, and the active power and the direct-current voltage of the GSVSC can be accurately controlled through the improved droop control.

Claims (1)

1. An improved nonlinear droop control method for a multi-terminal flexible direct-current transmission system is characterized by comprising the following steps:
step 1, generating a reference voltage of a direct-current voltage outer ring by utilizing a droop characteristic curve fuzzy search and a real-time droop curve table look-up control process, and specifically comprising the following steps of;
step (1-1), defining reference voltage U of a direct current voltage outer ring in the fuzzy search process of the droop characteristic curveDC-refThe expression of (a) is:
Figure FDA0002258082130000011
in the formula: u shapeDC-minAnd UDC-maxRespectively representing the minimum limit value and the maximum limit value of the GSVSC direct-current voltage operation, T representing the total time length of the droop characteristic curve fuzzy search, TSA sampling period representing a fuzzy search; nTSRepresenting the sampling period of the nth fuzzy search in the time length of T, and S representing the running state of the fuzzy search; u shapeDC-0Representing a direct current voltage reference value obtained by the table look-up control of the droop curve, and sending an active power scheduling command P by the far-end scheduling system to the droop curve table look-up controlrefThen obtaining the product through table look-up;
when S is equal to 1, starting fuzzy search, and obtaining a real-time droop characteristic curve of the converter station through scanning; reference value U of GSVSC direct-current voltage outer ring during fuzzy searchDC-refFrom the minimum limit UDC-minGradually increasing to a maximum limit value U after the total time T of the fuzzy searchDC-maxAt each sampling period TSActive power and direct current voltage data of GSVSC are collected, and the active power and direct current voltage data collected in the ith sampling period are expressed as (P)i,Ui)(i＝1,2,3…n)。
After the fuzzy search is completed within the total time T of the fuzzy search, the GSVSC nonlinear discrete operating point (P) is subjected to the least square methodi,Ui) (i is 1,2,3 … n), and obtaining a continuous nonlinear droop characteristic curve equation expression of the converter station as follows:
P＝kU2+bU
in the formula: k and b are both GSVSC converter station P-UDCThe coefficient of the droop characteristic.
P-U Using least squaresDCFitting a drooping curve, and calculating an active power measured value P of the GSVSCiAnd the calculated active power (kU)i 2+bUi) Function f of minimum sum of squares of residualsLS(k, b), the expression is as follows:
Figure FDA0002258082130000021
k and b are droop coefficients corresponding to the GSVSC real-time droop characteristic curve;
by solving for fLS(k, b) obtaining coefficients k and b of a real-time droop characteristic curve equation of the converter station, thereby obtaining continuous and real-time P-U of the GSVSCDCA droop characteristic curve equation;
step (1-2), real-time droop curve table look-up control is carried out, namely after fuzzy search is finished, the obtained droop characteristic curve equation P is kU2+ bU is loaded to real-time droop curve look-up table control; switching S to 2, namely S is 2, and at the moment, the remote dispatching system sends an active power instruction P according to the dispatching requirement of the VSC-MTDC systemrefObtaining the power command P by looking up the tablerefReference value U of corresponding DC voltage outer loop controlDC-0Controlling the direct-current voltage of the GSVSC so as to realize the control of the active power of the converter station;
step 2, executing a process of sag curve period updating control, wherein the process comprises the following specific implementation steps:
step (2-1), collecting at t0Resistance value R of direct current cable constantly connected with GSVSC and between GSVSCm(0)(m is 1,2,3 …), wherein m is the number of the direct current cable;
step (2-2), starting fuzzy search of droop characteristic curve to obtain t0The expression of the initialized droop characteristic curve equation of the GSVSC at the moment is as follows:
P＝k(0)U2+b(0)U；
step (2-3), collectingA time t1Resistance value R of time direct current cablem(1)(m-1, 2,3 …), and the direct voltage U of the GSVSC(1)And active power P(1)；
Step (2-4), judging | Rl(1)-Rl(0)If | is greater than 0, l is any one of m direct current cables: if | Rl(1)-Rl(0)If | 0 is true, it indicates t1Immediately executing the step (2-5) when the resistance of the direct current cable changes due to the change of the temperature or the system running state; if | Rl(1)-Rl(0)If the value of | is greater than 0, the resistance of all the direct current cables is not changed, but the running state of the system may be changed to cause the change of the GSVSC droop characteristic curve, and then the step (2-8) is executed;
step (2-5), starting fuzzy search of the droop characteristic curve to obtain t1Real-time droop characteristic curve equation P ═ k of time GSVSC(1)U2+b(1)U；
Step (2-6), after the droop characteristic curve fuzzy search is finished, t is carried out1Droop characteristic curve equation P ═ k of GSVSC at time(1)U2+b(1)Loading the U into a real-time droop curve look-up table;
step (2-7), updating the initial value, i.e. t1Droop characteristic equation of time and Rm(1)(m-1, 2,3 …) as the initial value for the next cycle, i.e. Rm(1)(m-1, 2,3 …) for Rm(0)(m＝1，2，3…)，k(1)Replacement of k(0)，b(1)Replacement b(0). Returning to execute the step (2-3) and carrying out the next circulation;
step (2-8) of judging | P6(1)-k(0)U6(1) 2-b(0)U6(1)If | < ε, then:
if | P6(1)-k(0)U6(1) 2-b(0)U6(1)If the | < epsilon is established, indicating that the change of the running state of the system does not cause the change of the GSVSC droop characteristic curve, returning to the step (2-3) for carrying out the next cycle;
if | P6(1)-k(0)U6(1) 2-b(0)U6(1)If the condition that the value is less than epsilon is not satisfied, the change of the system running state causes the change of the GSVSC droop characteristic curve, and the step (2-5) is returned to;
where ε represents a positive integer approaching 0 infinity.
The double closed-loop control of the direct-current voltage outer loop and the alternating-current inner loop is the bottom-layer control of the GSVSC, and the accurate tracking of a direct-current voltage reference value is realized.
CN201911061516.6A 2019-11-01 2019-11-01 Improved nonlinear droop control method for multi-terminal flexible direct-current power transmission system Pending CN110880778A (en)

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