CN110518617B - MMC-MTDC droop control method considering operation state of alternating current side - Google Patents
MMC-MTDC droop control method considering operation state of alternating current side Download PDFInfo
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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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
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- 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
An MMC-MTDC droop control method considering an alternating current side operation state is characterized in that aiming at a multi-terminal flexible direct current transmission system, an improved droop control strategy is adopted to control a voltage source converter station in the system, a voltage droop coefficient is adjusted in real time according to the frequency and load rate change of the alternating current system, and the specific expression is as follows:
wherein
The control mode of the voltage source converter station is adjusted according to the change conditions of the frequency and the load factor of the alternating current system, so that the system can always run under a proper working condition, the direct current voltage deviation of a line can be reduced, the alternating current system can be prevented from being overloaded or the frequency can be prevented from exceeding an allowable range, and the stability of the interconnected system is improved.
Description
Technical Field
The invention relates to a power coordination control method of a direct current transmission system considering an alternating current side operation state, and belongs to the technical field of power transmission and distribution.
Background
Compared with the traditional alternating current transmission, the high-voltage direct current transmission has obvious advantages in the aspects of long-distance power supply, interconnection of systems with different frequencies, long-seabed cable transmission and the like. Compared with two-terminal high-voltage direct-current transmission, multi-terminal DC (MTDC) can realize power supply by multiple power sources, power receiving by multiple drop points and more flexible, more economical and more efficient power flow control. The Modular Multilevel Converter (MMC) has the advantages of flexible structure, convenience in expansion, capability of realizing high-voltage and high-power energy conversion, easiness in forming a multi-end system, capability of supplying power to a passive power grid and the like. Therefore, the MMC-MTDC power transmission system becomes an important direction for the development of future multi-terminal direct-current power transmission systems.
The control target of the multi-terminal direct-current transmission system is to achieve reasonable power distribution at a direct-current side and stable voltage of a direct-current bus. The main consideration of various coordination control strategies proposed at present is to realize power coordination distribution among all converter stations in the direct current side by using direct current side related information, and the consideration of the coordination distribution of the alternating current system operation state participating in the direct current side power is less. In an actual power grid, the unbalanced power distributed to each interconnected power grid needs to consider not only the power margin of the converter station, but also the operating state of the power grid on the ac side, such as the frequency and load rate change of the ac system, and some power grids have smaller rotating reserve capacity, so that even if the power margin of the converter station connected with the power grid is large, more unbalanced power cannot be distributed after the unbalanced power occurs in the dc power grid, otherwise, the ac power grid will have too large frequency fluctuation. Similarly, if the load rate of the ac system is not considered, it is easy to cause the individual ac system to be fully loaded and thus lose the ability to adjust the whole system.
The control modes of the current high-voltage direct-current transmission mainly comprise the following three types: master-slave control, direct current voltage margin control and voltage droop control. However, the current control strategy mainly has the following disadvantages:
(1) The master-slave control has high requirements on a communication system, and the stability of the system can face great threat when the communication is interrupted, so the development of the control mode is seriously restricted.
(2) The voltage margin control is easy to generate power oscillation in the switching process of the control modes of the converter stations, the selection of the voltage margin is relatively complex, and the priorities of several direct-current voltage converter stations are difficult to be matched with each other when the scale of the converter stations is large.
(3) When the converter station adopts the traditional droop control with fixed slope, the droop coefficient can not be flexibly adjusted according to the running state of the alternating current side, so that the full load and the frequency of an alternating current system are easily caused to exceed the allowable range, and the stability of an interconnected system is reduced.
In summary, the control method of the existing high voltage direct current transmission is not ideal, and needs to be improved.
Disclosure of Invention
The present invention is directed to provide an MMC-MTDC droop control method considering an operating state of an ac side to prevent full load and frequency of an ac system from exceeding an allowable range and improve stability of an interconnection system.
The problem of the invention is solved by the following technical scheme:
an MMC-MTDC droop control method considering an alternating current side operation state is characterized in that aiming at a multi-terminal flexible direct current transmission system, an improved droop control strategy is adopted to control a voltage source converter station in the system, a voltage droop coefficient is adjusted in real time according to the frequency and load rate change of the alternating current system, and the specific expression is as follows:
wherein
In the formula: k i The voltage droop coefficient of the converter station i is obtained; k is i 0 is an initial voltage droop coefficient set by the converter station i according to the rated capacity of the converter station i; Δ f thre Is a frequency threshold; eta is the load factor of the alternating current system; eta' is a set value of the load factor of the alternating current system; f is the measured value of the frequency of the alternating current system; f. of 0 The frequency initial value of the alternating current system is obtained; and deltaf' is a frequency fluctuation set value of the alternating current system.
In the MMC-MTDC droop control method considering the operation state of the ac side, the frequency fluctuation set value Δ f' of the ac system is set to 0.2Hz; setting the load rate set value eta' of the alternating current system to be 0.85; frequency threshold value deltaf thre Set to 0.02Hz.
The control mode of the voltage source converter station is adjusted according to the change conditions of the frequency and the load factor of the alternating current system, so that the system can always run under a proper working condition, the direct current voltage deviation of a line can be reduced, the alternating current system can be prevented from being overloaded or the frequency can be prevented from exceeding an allowable range, and the stability of the interconnected system is improved.
Drawings
The present invention will be described in further detail with reference to the accompanying drawings.
FIG. 1 is a block diagram of a three-terminal flexible DC power transmission system;
FIG. 2 illustrates a conventional droop control and improved droop control U dc -P dc A characteristic curve comparison graph;
FIG. 3 is a DC voltage simulation waveform after a small unbalanced power appears on the DC side;
FIG. 4 is a simulated waveform of the output power at the AC side after a large unbalanced power appears at the DC side;
the symbols used in the figures or text are: HVDC is high voltage DC transmission, MMC-MTDC is multi-terminal flexible DC transmission, K i For the voltage droop coefficient, Δ f, of the converter station i thre Is a frequency threshold value, delta f 'is a set value of frequency fluctuation of the alternating current system, eta is a load rate of the alternating current system, eta' is a set value of the load rate of the alternating current system, P dc Is the measured value of the active power on the DC side, f is the measured value of the frequency, f 0 Is an initial value of frequency, U dc Is the actual value of DC voltage, P is the actual measured value of active power emitted by AC system, P N Rated active power value, K, for an AC system i 0 And setting an initial voltage droop coefficient for the converter station i according to the rated capacity of the converter station i.
Detailed Description
The invention provides an MMC-MTDC droop control method considering the running state of an alternating current side, which aims to improve the voltage quality and the stability of an interconnection system.
Aiming at a voltage source converter station in a multi-terminal flexible direct current transmission system, the invention adopts an improved droop control strategy for control, and the expression of a voltage droop coefficient is as follows:
wherein
In the formula: k i 0 Setting an initial voltage droop coefficient for the converter station i according to the rated capacity of the converter station i; Δ f thre Is a frequency threshold; eta is the load factor of the alternating current system; eta' is a set value of the load factor of the alternating current system; f is the measured value of the frequency of the alternating current system; f. of 0 The frequency initial value of the alternating current system; and deltaf' is a frequency fluctuation set value of the alternating current system.
Considering that the frequency fluctuation range of the long-term stable operation of the alternating current system does not exceed [ -0.2,0.2] Hz, the delta f' is set to be 0.2Hz in the invention; setting the load rate set value eta' of the alternating current system to be 0.85; in order to prevent the frequency change caused by the small fluctuation of the alternating current frequency in the steady state from causing the frequent change of the slope coefficient value of the corresponding converter station, the frequency threshold value is set for the frequency change in the improved droop control, the frequency threshold value set by the invention is 0.02Hz, and the droop coefficient can be changed only when the frequency is lower than the lower limit of 49.98Hz or exceeds the upper limit of 50.02 Hz.
After the improved droop control strategy is adopted according to different running states of an alternating current system, the converter station can have the following 3 control modes:
(1) Control mode 1: each alternating current side system stably operates, no power disturbance or small power fluctuation exists in the direct current network, frequency fluctuation of the alternating current side is smaller than a frequency threshold value, and the converter station normally operates according to a fixed slope.
(2) Control mode 2: when the direct current side has small unbalanced power, for example, when a certain alternating current system has small accidents, and other alternating current systems participate in the unbalanced power regulation of the direct current side, the frequency fluctuation per se exceeds a frequency threshold, but the load rate and the frequency variation of the alternating current systems do not exceed respective set values, the droop coefficient of each converter station is reduced on the basis of the initial voltage droop coefficient, at the moment, the slope of the voltage droop curve is reduced, the voltage rigidity is good, the system operates in a state of emphasizing the stable direct current voltage, and the direct current voltage of the system operation is closer to the reference operation voltage in the power regulation process.
(3) Control mode 3: when the direct current side has larger unbalanced power, such as a certain alternating current system has a larger accident or a certain converter station exits from operation, and other alternating current systems participate in the unbalanced power regulation of the direct current side, the frequency fluctuation per se exceeds a frequency threshold, and at least one of the load rate and the frequency variation of the alternating current system exceeds respective set values, the droop coefficient of each converter station is increased on the basis of the initial voltage droop coefficient, the slope of the droop curve is increased, the distributed power of the converter station is reduced, and the system operates in a state of emphasizing power distribution, so that the excessive power distributed by the alternating current system connected with the converter station is prevented, and the overload of the alternating current system or the frequency exceeding an allowable range is effectively avoided.
The method is described in detail below with reference to the embodiment, and referring to fig. 1, the three-terminal flexible dc power transmission system includes:
the alternating current system 1 is connected with the converter station 2;
the converter station 2 is connected with the alternating current system 1 and the bus 3, operates in a rectification state, and is used for converting alternating current of the alternating current system 1 into direct current to be transmitted to the direct current bus 3;
the direct current bus 3 is connected with the converter station 2, the converter station 9 and the direct current transmission line 4, and is used for collecting electric energy transmitted by the converter station 2 and the converter station 9 and transmitting the electric energy out through the direct current transmission line 4;
the direct current transmission line 4 is connected with the bus 3 and the bus 5 and is used for high-voltage direct current transmission;
an ac system 7 is connected to the converter station 6. The bus 5 is connected with the direct current transmission line 4 and the converter station 6, and is used for gathering electric energy of the direct current transmission line 4 and supplying power to the alternating current system 7 through the converter station 6;
the converter station 6 is connected with the bus 5 and the alternating current system 7, runs in an inversion state, and is used for converting direct current of the direct current bus 5 into alternating current and transmitting the alternating current to the alternating current system 7;
an alternating current system 8 connected with the converter station 9;
the converter station 9 is connected with the alternating current system 8 and the bus 3, operates in a rectification state, and is used for converting alternating current of the alternating current system 8 into direct current to be transmitted to the direct current bus 3;
the converter station 2 adopts constant active power control, the converter station 6 and the converter station 9 both adopt droop control, and the MMC-MTDC improved droop control strategy considering the running state of the alternating current side comprises the following specific steps:
the first step is as follows: detecting the frequency variation delta f and the load rate eta of the alternating current system, wherein the expression is as follows: Δ f = | f-f 0 |，η＝P/P N . Wherein f is an AC system frequency measurement; f. of 0 The frequency initial value of the alternating current system is obtained; p is an active power measured value sent by the alternating current system; p N And the rated active power value of the alternating current system.
The second step is that: threshold value delta f according to frequency variation thre The size determines the control mode of the converter station. Δ f thre When the frequency is less than or equal to 0.02Hz, the converter station is in accordance with a fixed droop coefficient
Normal operation is carried out; when Δ f thre The converter station is switched to the improved droop coefficient K 'when the frequency is more than 0.02 Hz' i And (5) operating.
The third step: and respectively calculating the ratio of the frequency variation delta f of the alternating current side to the frequency fluctuation set value delta f 'and the ratio of the load rate eta of the alternating current system to the load rate set value eta'. Where Δ f 'is set to 0.2Hz and η' is set to 0.85.
The fourth step: according to
The magnitude of the two quantities determines the control mode of the converter station under improved droop control. When in use
In the method, the improved droop coefficient is reduced according to the frequency change and the load factor change of the alternating current side, and the expression is as follows:
at the moment, the slope of the voltage sag curve becomes small, the voltage rigidity is good, and the system operates atEmphasizing the state of stable direct current voltage; when in use
Or
When the adaptive droop coefficient is changed according to the frequency change and the load factor change of the alternating current side, the adaptive droop coefficient is expressed as follows:
at this time, the slope of the droop curve becomes larger, the power distributed by the converter station is reduced, and the system operates in a state of emphasizing power distribution, so that the power distributed by the alternating current system connected with the system is prevented from being excessive, and the overload of the alternating current system or the frequency exceeding the allowable range is effectively avoided.
The improved droop control strategy provided by the invention can adjust whether the working state of the direct current side converter station is mainly focused on stabilizing direct current voltage or power distribution according to the frequency change of the alternating current system and the load rate change of the alternating current system, reduces the voltage deviation of the direct current side by adjusting the system to operate in a proper working condition, prevents the alternating current system from being fully loaded and the frequency from exceeding an allowable range, and improves the operation stability of an interconnected system.
Referring to fig. 2, in the conventional droop control, a droop coefficient is a fixed value, and when a power shortage occurs in a direct current system, power shared by each converter station is a fixed value, which easily causes large fluctuation of the frequency of an alternating current system with weak frequency modulation capability or causes an excessively high load rate of the alternating current system, thereby affecting the stability of the system.
In the improved droop control strategy provided by the invention, the expression of the voltage droop coefficient is as follows:
wherein
The voltage droop coefficient in the improved droop control is not a fixed value, but is automatically adjusted according to the load rate of the alternating current system and the frequency variation of the alternating current system.
In fig. 2, the solid line represents a conventional droop control strategy, because the droop coefficient is constant, and is represented as an inclined straight line in the figure. The dashed line represents the improved droop control strategy, at segment AB, when
The improved droop coefficient is reduced according to the frequency change and the load factor change of the alternating current side, and the expression is as follows:
at the moment, the slope of a voltage sag curve is reduced, the voltage rigidity is good, and the system operates in a state of emphasizing on stabilizing direct-current voltage; in the CD and EF sections, when
The improved droop coefficient is increased according to the frequency change and the load factor change of the alternating current side, and the expression is as follows:
at the moment, the slope of the droop curve becomes larger, the power allocated by the converter station is reduced, and the system operates in a state of emphasizing power distribution, so that the phenomenon that the alternating current system connected with the system distributes excessive power is effectively avoided, and the alternating current system is prevented from being overloaded or the frequency is prevented from exceeding the allowable range.
FIG. 3 is a DC voltage simulation waveform after a small unbalanced power appears on the DC side
As can be seen from fig. 3, when unbalanced power occurs on the dc side and each converter station performs power adjustment, the voltage variation of the converter station using improved droop control is significantly smaller than that of the converter station using conventional droop control, because when smaller unbalanced power occurs on the dc side and other ac systems participate in dc-side unbalanced power adjustment, the frequency fluctuation of the converter station itself exceeds the frequency threshold, but when the load rate of the ac system and the frequency variation of the ac side do not exceed their respective set values, the droop coefficient of each converter station decreases on the basis of the initial voltage droop coefficient, and at this time, the slope of the voltage droop curve decreases, the voltage rigidity is good, the system operates in a state that the system is heavier than a stable dc voltage, and the system operating dc voltage is closer to the reference operating voltage in the power adjustment process.
FIG. 4 is a simulation waveform of the output power of the AC side after the large unbalanced power appears on the DC side
As can be seen from fig. 4, after a large unbalanced power occurs on the dc side, the active power increase of the ac system output by the converter station with improved droop control is significantly lower than the active power increase of the ac system output by the converter station with conventional droop control, and full load of the ac system is avoided because when a large unbalanced power occurs on the dc side, the frequency fluctuation exceeds the frequency threshold, at least one of the ac system load rate and the ac side frequency variation exceeds the respective set value, at this time, the droop coefficient of each converter station increases on the basis of the initial voltage droop coefficient, the droop curve slope increases, the power allocated by the converter station decreases, and the system operates in a state of emphasizing power allocation, so that it is possible to effectively avoid excessive power allocation by the converter station, and prevent the ac system from being overloaded or the frequency from exceeding the allowable range.
Claims (2)
1. An MMC-MTDC droop control method considering an alternating current side operation state is characterized in that aiming at a multi-terminal flexible direct current transmission system, an improved droop control strategy is adopted to control a voltage source converter station in the system, a voltage droop coefficient is adjusted in real time according to the frequency and load rate change of the alternating current system, and the specific expression is as follows:
wherein
In the formula: k i For electricity of converter station iSag factor;
setting an initial voltage droop coefficient for the converter station i according to the rated capacity of the converter station i; Δ f thre Is a frequency threshold; eta is the load factor of the alternating current system; eta' is a set value of the load rate of the alternating current system; f is the measured value of the frequency of the alternating current system; f. of 0 The frequency initial value of the alternating current system; and deltaf' is the frequency fluctuation set value of the alternating current system.
2. The MMC-MTDC droop control method in consideration of ac-side operation state of claim 1, wherein the ac system frequency fluctuation set-point Δ f' is set to 0.2Hz; setting the load rate set value eta' of the alternating current system to be 0.85; frequency threshold value delta f thre Set to 0.02Hz.
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| CN112542849B (en) * | 2020-11-12 | 2023-07-14 | 华北电力大学（保定） | Adaptive virtual inertial frequency modulation control method for flexible direct current transmission system |
| CN112736921B (en) * | 2020-12-30 | 2023-11-17 | 丽水市普明电力建设工程有限公司 | Three-slope sagging control method with voltage threshold crossing early warning |
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