CN110768286B - Multi-terminal flexible vertical control strategy based on common voltage reference point - Google Patents

Multi-terminal flexible vertical control strategy based on common voltage reference point Download PDF

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CN110768286B
CN110768286B CN201911174910.0A CN201911174910A CN110768286B CN 110768286 B CN110768286 B CN 110768286B CN 201911174910 A CN201911174910 A CN 201911174910A CN 110768286 B CN110768286 B CN 110768286B
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converter station
direct current
active power
land side
voltage
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CN110768286A (en
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汪震
钱一辉
饶宏
李岩
黄伟煌
陈怡静
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Zhejiang University ZJU
CSG Electric Power Research Institute
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Zhejiang University ZJU
CSG Electric Power Research Institute
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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
    • 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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  • Power Engineering (AREA)
  • Direct Current Feeding And Distribution (AREA)

Abstract

The invention discloses a multi-terminal flexible vertical control strategy based on a common voltage reference point. The strategy aims at the scene that an offshore wind farm supplies energy to a land side through a multi-terminal flexible direct current transmission system, the direct current voltage deviation and active power deviation signals at the public connection point of a star direct current power grid connected with a land side converter station are used for replacing the local direct current voltage deviation and active power deviation signals of each converter station in the original droop control, the influence of the resistance of a direct current transmission line is taken into consideration, and the prediction of the direct current voltage and the active power of a new operation point is realized under the working condition of disturbance.

Description

Multi-terminal flexible vertical control strategy based on common voltage reference point
Technical Field
The invention relates to a multi-terminal flexible vertical control strategy based on a common voltage reference point, and belongs to the field of system-level control strategy research in the field of multi-terminal flexible direct-current power transmission.
Background
At present, the multi-terminal flexible direct current system level control strategies mainly include the following 3 types: a master-slave control strategy, a direct current voltage surplus control strategy and a direct current voltage downward slope control strategy. The core of the master-slave control strategy is that one converter station determines the voltage of the whole multi-terminal flexible direct-current transmission system, and the other converter stations are controlled according to respective power requirements, but the master-slave control strategy has strong dependence on the system. The direct current voltage allowance control strategy is similar to a master-slave control strategy, when a main station exits due to faults or runs in full load, a standby converter station can be switched from constant power control to constant voltage control after detecting direct current voltage deviation, communication is not needed in the whole process, and the direct current voltage allowance control strategy is only suitable for small direct current systems with few ends and large converter station capacity difference. The direct-current voltage droop control strategy utilizes the similarity of direct-current power grid voltage and alternating-current power grid frequency in the aspect of representing energy, completes the tasks of voltage control and power distribution by setting a reasonable droop coefficient, has no requirement on communication, and is suitable for being applied to a multi-terminal flexible direct-current power transmission system accessed by offshore wind power.
For an ac grid, the frequency is a global quantity, and the frequencies in the ac grid are equal everywhere. However, for a dc network, the dc voltage is a local quantity, and the dc voltage in the dc network is generally not equal everywhere. The existing droop control strategy, or the coordinated distribution of power among all converter stations is realized through droop coefficients or communication, and the problems that the voltage deviation and the power distribution under the disturbance condition cannot be reasonably predicted or the normal operation cannot be carried out when the communication delay is serious exist.
Disclosure of Invention
In order to solve the above problems, the present invention provides a multi-terminal flexible vertical control strategy based on a common voltage reference point. The direct-current voltage deviation and active power deviation signals at the public connection point of the star-shaped direct-current power grid connected with the land side converter station are used for replacing the local direct-current voltage deviation and active power deviation signals of each converter station in the original droop control, the resistance of the direct-current transmission line is taken into account, and the direct-current voltage and active power of a new operation point are predicted under the working condition of disturbance.
The invention provides a multi-terminal flexible vertical control strategy based on a common voltage reference point, which is characterized by comprising the following steps:
1) searching a DC voltage common point in a multi-terminal flexible DC transmission network;
2) modifying an outer ring control structure of the land side converter station to control the voltage at a direct current voltage common point;
3) and determining the droop coefficient according to the N-1 principle.
The specific method for searching a DC voltage common point in the multi-terminal flexible DC transmission network in the step 1) is to search or search a central point of a star-shaped DC network connected with each land side converter station after circuit conversion.
The step 2) modifies an outer ring control structure of the land side converter station to make the land side converter station control the voltage at the common point of the direct current voltage by a specific method comprising the following steps: and controlling the direct current voltage at the common voltage reference point to be stabilized at a rated value for each land side converter station. For the control of active power, the reasonable distribution of the active power is realized by setting different droop coefficients for each converter station.
The specific method for determining the droop coefficient in the step 3) according to the N-1 principle comprises the following steps: at most consider the case of a land side converter station tripping. And (4) considering the influence of complete interruption on the change of the active power transmitted by the wind power. To simplify the analysis, we define the power split ratio CiThe following were used:
Figure BDA0002289712120000021
Figure BDA0002289712120000022
wherein SGSVSCRepresenting a set of land-side converter stations, KiRepresenting the droop coefficient of the converter station numbered i, j representing the cut-off converter station number, KjRepresenting the droop coefficient of the converter station numbered j,
Figure BDA0002289712120000023
representing the sum of the droop coefficients of all land side converter stations.
We define the maximum power offset considering the N-1 principle as:
Figure BDA0002289712120000024
wherein, PsendThe active power delivered to the common voltage reference point for the offshore wind farm,
Figure BDA0002289712120000025
the active power of the converter station numbered j.
The maximum voltage deviation amount of the public voltage reference point allowed by the actual power grid is assumed to be
Figure BDA0002289712120000026
Obtaining:
Figure BDA0002289712120000031
thereby determining the droop coefficient for each converter station:
Figure BDA0002289712120000032
the invention has the beneficial effects that:
the droop control strategy provided by the invention can accurately predict the direct current voltage of the direct current voltage common point voltage and the active power of each converter station under the condition that the land side converter station is tripped off or the active power transmitted by the wind power plant fluctuates. Through estimation, a proper droop coefficient can be determined to ensure reasonable distribution of power, and faults such as power overload and the like of a certain converter station are prevented.
Drawings
Fig. 1 a topology of a multi-terminal flexible dc power transmission system;
fig. 2 converter station main circuit;
fig. 3 is an equivalent circuit diagram of a land side of a multi-terminal flexible direct current transmission system;
fig. 4 control structure of a land side converter station.
Detailed Description
The invention is described in further detail below with reference to the figures and the specific examples.
The method comprises the following steps:
1) searching a DC voltage common point in a multi-terminal flexible DC transmission network;
2) modifying an outer ring control structure of the land side converter station to control the voltage at a direct current voltage common point;
3) and determining the droop coefficient according to the N-1 principle.
The step 1) of searching a DC voltage common point in the multi-terminal flexible DC transmission network is as follows:
for the topology structure of the multi-terminal flexible direct-current transmission system shown in fig. 1, a direct-current voltage common reference point PCDC in the figure is taken. The main circuit of the converter station is shown in fig. 2 and the equivalent circuit diagram on the land side is shown in fig. 3.
Step 2) modifying an outer ring control structure of the land side converter station to control the voltage at the common point of the direct current voltage as follows:
the control block diagram of the multi-terminal flexible vertical control strategy based on the common voltage reference point proposed by the present invention is shown in fig. 4. And controlling the direct current voltage at the common voltage reference point to be stabilized at a rated value for each land side converter station. Using the converter station DC voltage V numbered idc,iMinus the direct current Idc,iAnd a DC line resistor RiThe product of (a) and (b) can obtain the direct voltage at the common voltage. For active power control, different droop coefficients K are set for each converter stationiThe reasonable distribution of active power is realized.
The step 3) determines the droop coefficient according to the N-1 principle as follows:
for the determination of the droop coefficient we use the N-1 principle, i.e. consider at most the case of a tripping of one land side converter station. And (4) considering the influence of complete interruption on the change of the active power transmitted by the wind power. To simplify the analysis, we define the power split ratio CiThe following were used:
Figure BDA0002289712120000041
from formula (1), we can obtain:
Figure BDA0002289712120000042
then, the direct current voltage and the active power after the tripping of a certain converter station j can be estimated as formula (3) and formula (4), respectively, and the direct current voltage and the active power after the power transmission of the wind farm is interrupted can be estimated as formula (5) and formula (6), respectively.
Figure BDA0002289712120000043
Figure BDA0002289712120000044
Figure BDA0002289712120000045
Figure BDA0002289712120000046
Through the simplified analysis, we can define the maximum power deviation after considering the N-1 principle as:
Figure BDA0002289712120000047
assuming that the maximum voltage deviation allowed at the common point of the DC voltages is
Figure BDA0002289712120000051
We can get:
Figure BDA0002289712120000052
from the definition of equation (1), the droop coefficient for each converter station can be determined:
Figure BDA0002289712120000053
the foregoing detailed description is intended to illustrate and not limit the invention, which is intended to be within the spirit and scope of the appended claims, and any changes and modifications that fall within the true spirit and scope of the invention are intended to be covered by the following claims.

Claims (3)

1. A multi-terminal flexible vertical control strategy based on a common voltage reference point is characterized in that aiming at a scene that an offshore wind farm supplies energy to a land side through a multi-terminal flexible direct current transmission system, a direct current voltage deviation and an active power deviation signal at a common connection point of a star-shaped direct current power grid connected with a land side converter station are utilized to replace a local direct current voltage deviation and an active power deviation signal of each converter station in vertical control, the influence of the resistance of a direct current transmission line is considered, and the direct current voltage and active power distribution of a new balance state after disturbance is predicted;
the multi-terminal flexible vertical control strategy based on the common voltage reference point comprises the following steps:
1) searching a central point of a star-shaped direct-current power grid connected with each land side converter station in the multi-terminal flexible direct-current power transmission network, namely a direct-current voltage common point;
2) modifying an outer ring control structure of the land side converter station to control the voltage at a direct current voltage common point;
3) and determining the droop coefficient according to the N-1 principle.
2. The common voltage reference point based multi-terminal flexible vertical drop control strategy of claim 1, wherein: the specific method for modifying the outer ring control structure of the land side converter station in the step 2) to control the voltage at the common point of the direct current voltage comprises the following steps: for each land side converter station, controlling the direct current voltage at the common voltage reference point to be stabilized at a rated value; for the control of active power, the reasonable distribution of the active power is realized by setting different droop coefficients for each converter station.
3. The common voltage reference point based multi-terminal flexible vertical drop control strategy of claim 1, wherein: the specific method for determining the droop coefficient in the step 3) according to the N-1 principle comprises the following steps: at most, considering the condition of cutting off one land side converter station, and considering the influence of complete interruption on the change of active power transmitted by wind power; defining a power allocation ratio CiThe following were used:
Figure FDA0003061068060000011
Figure FDA0003061068060000012
wherein SGSVSCRepresenting a set of land-side converter stations, KiRepresenting the droop coefficient of the converter station numbered i, j representing the cut-off converter station number, KjRepresenting the droop coefficient of the converter station numbered j,
Figure FDA0003061068060000021
represents the sum of the droop coefficients of all land side converter stations;
based on the N-1 principle, defining the maximum power deviation:
Figure FDA0003061068060000022
wherein, PsendThe active power delivered to the common voltage reference point for the offshore wind farm,
Figure FDA0003061068060000023
the active power of the converter station with the number j;
assuming a common voltage reference allowed by the actual gridThe maximum voltage deviation of the point is
Figure FDA0003061068060000024
Obtaining:
Figure FDA0003061068060000025
thereby determining the droop coefficient for each converter station:
Figure FDA0003061068060000026
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CN103606945A (en) * 2013-11-19 2014-02-26 国家电网公司 Control system and control method for multi-terminal flexible direct current power transmission system

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CN103606945A (en) * 2013-11-19 2014-02-26 国家电网公司 Control system and control method for multi-terminal flexible direct current power transmission system

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"An Improved Droop Control Method for VSC- MTDC Considering Power Mismatch Allocation";Yihui Qian等;《2019 4th IEEE Workshop on the Electronic Grid》;20191114;第1-6页 *

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