LU500835B1 - Grid frequency regulation method based on multi-terminal flexible direct current transmission system - Google Patents

Grid frequency regulation method based on multi-terminal flexible direct current transmission system Download PDF

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Publication number
LU500835B1
LU500835B1 LU500835A LU500835A LU500835B1 LU 500835 B1 LU500835 B1 LU 500835B1 LU 500835 A LU500835 A LU 500835A LU 500835 A LU500835 A LU 500835A LU 500835 B1 LU500835 B1 LU 500835B1
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alternating current
converter station
power
frequency regulation
grid frequency
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LU500835A
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French (fr)
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Zhou Li
Yazhou Li
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Liyang Research Institute Of Southeast Univ
Univ Southeast
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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
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H02J3/241—The oscillation concerning frequency
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • 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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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Supply And Distribution Of Alternating Current (AREA)

Abstract

The present invention provides a grid frequency regulation method based on a multi-terminal flexible direct current transmission system. A converter station may automatically select, according to a frequency deviation of a connected alternating current system, whether the alternating current system receives grid frequency regulation, or may select, according to an instruction of a scheduling system, whether the connected alternating current system receives grid frequency regulation. In addition, the scheduling system may select, by using an instruction, an alternating current system connected to one or more converter stations to participate in grid frequency regulation. In the grid frequency regulation method, a control strategy of a converter station is changed, and in combination with a plurality of power balance converter stations using an active power balance technology, joint frequency regulation among a plurality of alternating current systems is implemented.

Description

GRID FREQUENCY REGULATION METHOD BASED ON MULTI-TERMINAL FLEXIBLE DIRECT CURRENT TRANSMISSION SYSTEM
TECHNICAL FIELD The present invention belongs to the technical field of electric power systems, and relates to safety and stability analysis technologies for electric power systems, and specifically, to a grid frequency regulation method based on a multi-terminal flexible direct current transmission system.
BACKGROUND A voltage source converter based high voltage direct current (VSC-HVDC) transmission technology, also referred to as a flexible direct current transmission technology, has advantages such as that active and reactive power decoupling control can be implemented, power can be supplied to a passive network, a commutation failure 1s prevented, no communication is required between converter stations, and it is easy to form a multi-terminal direct current system, is one of the major technologies for constructing a smart grid, and will be widely applied. With the development of the flexible direct current transmission technology, a flexible direct current transmission system is developing toward a higher voltage level and a larger power transmission capacity, and the topology of a direct current grid will become increasingly complex. Compared with a conventional alternating current grid, the flexible direct current transmission system 1s constructed based on a lot of electrical and electronic equipment, has a capability of rapidly adjusting a transmission power, and may be applied to frequency regulation between interconnected alternating current grids, thereby implementing the sharing of rotating backup between the interconnected grids and improving the frequency stability of the interconnected alternating current grids. For a future direct current grid with a complex structure, to implement a frequency balance between interconnected grids, in the most direct method, a converter station directly adjusts an actual value of a transmission power of the converter station according to an unbalanced power of an alternating current system. However, when an unbalanced power of an alternating current grid 1s directly introduced into the direct current grid, the voltage stability of the direct current grid 1s affected.
SUMMARY To resolve the foregoing problems, the present invention provides a grid frequency regulation method based on a multi-terminal flexible direct current transmission system, so that a transmission power of a converter station can be adjusted according to an unbalanced power of an alternating current system, and in addition, a direct current grid can have relatively high voltage stability. To resolve the foregoing technical problems, the present invention is implemented by using the following technical solutions: A grid frequency regulation method based on a multi-terminal flexible direct current transmission system includes the following steps: switching a converter station connected to an alternating current system that receives grid frequency regulation to an alternating current balance node control mode, and resetting an output integral value of a proportional integrator when the converter station switches a control strategy; setting a maximum value for a transmission power of a converter station that operates in the alternating current balance node control mode, detecting a transmission power of the converter station that receives frequency regulation, and when the transmission power exceeds the maximum value, changing control of the converter station to a limited power transmission control mode; selecting, by a scheduling system by using an instruction, an alternating current system connected to one or more converter stations to participate in grid frequency regulation; and detecting transmission powers of converter stations, calculating an unbalanced power of a direct current transmission system, and dynamically adjusting a reference value of a power of a power balance station according to the unbalanced power to implement power balance. Further, the method further includes the following prerequisite steps: automatically selecting, by a converter station when a frequency deviation of an alternating current system that is connected to the converter station exceeds an upper threshold, the alternating current system to receive grid frequency regulation, or selecting, according to an instruction of the scheduling system, whether the connected alternating current system receives grid frequency regulation. Further, the method further includes the following subsequent steps: after grid frequency regulation is received, when a frequency deviation of an alternating current system that is connected to a converter station and receives grid frequency regulation is less than a lower threshold, automatically switching the converter station that receives grid frequency regulation to a fixed active power control mode, or switching, by using an instruction of the scheduling system, the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation; and resetting the output integral value of the proportional integrator when the converter station switches a control strategy.
Further, the method specifically includes the following steps: step (1): switching a converter station to the alternating current balance node control mode, and controlling an amplitude and a phase angle of an alternating current side outlet voltage rather than a point of common coupling (PCC) of the converter station, to turn the converter station into a balance node, so that a power deficit of an alternating current system is imported into a direct current system without any delay and without any deviation; in a d-q coordinate system, the alternating current side outlet voltage of the converter station using the alternating current balance node control mode 1s: | Ups = Ruy + X dy EU if k,+ A | | AE Ni k, lu. = Roi, =X. i+ 0-U, )| | where S is a Laplacian operator, Usa and Uy, are respectively a d-axis component and a q-axis component of an alternating current voltage at the PCC, Uca and Uc are respectively a d-axis component and a q-axis component of the alternating current side outlet voltage of the converter station, isa and is, are respectively a d-axis component and a q-axis component of an alternating current side current of the converter station, k, and k; are respectively a proportional coefficient and an integral coefficient of the proportional integrator, and R. and X. are respectively an equivalent resistance and a converter reactance of the converter station; in a process of switching a control strategy, the output integral value of the proportional integrator is reset:
Faune =U cos| ares} | Xi ~Ri, 7 L crop PX, 4. .
AW BEN RF Gre Li © Se wg —_—_—_— a ei 1 ON (PXY i. | U _ = © | +] = © | \ % ER E A U J where Us and U. are respectively an effective value of the alternating current side outlet voltage of the converter station and an effective value of the alternating current voltage at the PCC, Vdreset and Vgreser are respectively an integrator reset value in d-axis control and an integrator reset value in q-axis control, and Ps and Os are respectively an active power and a reactive power injected at the PCC; step (2): setting maximum values F and PF, for a transmission power of a converter station that operates in the alternating current balance node control mode, detecting the transmission power of the converter station that receives frequency regulation, 1f the transmission power Ps of the converter station exceeds F;. changing control of the converter station to the limited power transmission control mode, and tuning the alternating current side outlet voltage of the converter station: in the coordinate axes d and q, when the transmission power Ps of the converter station exceeds Fo tuning the alternating current side outlet voltage of the converter station to: a . POP LU, 17 cos| arcs} | 7 L GU, J 10 La in the coordinate axes d and q, when the transmission power Ps of the converter station exceeds Po tuning the alternating current side outlet voltage of the converter station to: | 200 f PX) LU, =U cos arosin(--75) | ) ‘ | UU, 5 cg U where |#,_| and |# | are the maximum values of the transmission power of the converter station, Fi > 0, and |F{,,| represents a maximum value of the transmission power in a rectification direction, that is, represents that an active power is injected from an alternating current system into the converter station; and F_ <0, and |... represents a maximum value 5 of the transmission power in an inverter direction, that is, an active power is injected from the converter station into the alternating current system; step (3): selecting, by the scheduling system by using an instruction, one or more alternating current systems having a frequency regulation capability as a power balance station; and step (4): detecting the transmission powers of the converter stations, calculating an unbalanced power AP of the direct current transmission system by using an active power balance technology, and dynamically adjusting the reference value of the power of the power balance station according to AP; the unbalanced power AP of the direct current transmission system is: in the foregoing formula, n is a quantity of converter stations in the system, the first m converter stations are power balance stations, an (m+1)* converter station to an n'® converter station are actual values of the transmission powers of the converter stations using the alternating current balance node control mode or the fixed active power control mode; and dynamically adjusting the reference value of the power of the power balance station according to AP: where P, is an initial reference value of an active power of an i" converter station, Pas is an adjusted reference value of the active power of the i converter station, 1 <i < m, and K; is a droop coefficient of the power balance station.
Further, the method further includes the following prerequisite steps: step 1, acquiring an actual value f of a frequency of an alternating current voltage at a PCC of an alternating current system;
step 2, calculating a deviation Af between the frequency f of the voltage of the alternating current system connected to the converter station and a reference frequency fer, and setting an upper threshold 4f7ax and one lower threshold Af... for a frequency deviation of the alternating current voltage; and step 3, when the frequency deviation Af of the alternating current system connected to the converter station does not exceed fra, automatically selecting, by the converter station, to perform frequency regulation by using an adjustment capability of the alternating current system, or selecting, by using an instruction of the scheduling system, the alternating current system connected to the converter station to receive grid frequency regulation; and when Af exceeds Afnax, automatically selecting, by the converter station, the connected alternating current system to receive grid frequency regulation. Further, the alternating current system includes an alternating current system connected to a converter station using the fixed active power control mode and a converter station using a fixed direct current voltage control mode.
Further, the method further includes the following subsequent steps: step 4, when the frequency deviation Af of the connected alternating current system that receives grid frequency regulation is less than 4fnin, automatically switching, by the scheduling system, the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation; or switching, by the scheduling system as required by using an instruction, the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation, where in a process of switching a control strategy, the output integral value of the proportional integrator is reset, and an integrator reset value in outer-loop control is: where Vireser and vgreser are respectively integrator reset values in d-axis outer-loop control and g-axis outer-loop control; and an integrator in inner-loop control is reset to:
Vener = Uy TV. cos! ATU J + Od j N v4, | Ves BU ET ln where Vareser and vgreser are respectively integrator reset values in d-axis inner-loop control and q-axis inner-loop control. Further, a reference value Pr of the active power is enabled to be equal to an actual value P of an active power of current transmission of the converter station.
Compared with the prior art, the present invention has the following advantages and beneficial effects: (1): A frequency regulation strategy provided in the present invention can automatically perform frequency regulation control according to a frequency deviation of an alternating current system or can perform frequency regulation control by using a scheduling instruction according to an actual requirement.
(2): When an alternating current system causes the frequency deviation due to a power deficit, an alternating current balance node control mode provided in the present invention turns a converter station into a balance node, so that the power deficit of the alternating current system can be directly imported into a direct current system without any delay and without any deviation.
(3): When the converter station switches a control strategy, a reset integrator value ensures status matching between parameters of a control system and electrical parameters of the direct current system, thereby reducing the impact and shock on a process of switching control and ensuring the stable operation of the system.
(4): In the limited power strategy provided in the present invention, an active power allowable to be injected into the direct current system 1s restricted according to an upper limit of a capacity of the converter station, thereby ensuring the safety of the direct current system.
(5): In the active power balance technology provided in the present invention, a scheduling system specifies one or more alternating current systems to participate in frequency regulation of the alternating current system, thereby improving the flexibility of frequency regulation control and dynamic stability of a power and a voltage.
(6): Compared with the prior art, in the present invention, only a control strategy of a local converter station needs to be changed, and a transmission power of the converter station is changed in real time according to an amount of an unbalanced power of an alternating current system, thereby improving the speed and accuracy of frequency regulation of the alternating current system.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a principle diagram of a system according to the present invention. FIG. 2 is a structural diagram of an alternating current balance node control mode. FIG. 3 is a structural diagram of active reactive decoupling control, where (a) is a structural diagram of outer-loop control of a fixed active power control mode, (b) is a structural diagram of outer-loop control of a fixed reactive power control mode, (c) is a structural diagram of inner-loop control of the fixed active power control mode, and (d) is a structural diagram of inner-loop control of a fixed reactive power control mode.
FIG. 4 is a structural diagram of a simulated model of a six-terminal flexible direct current transmission system.
FIG. 5 is a diagram of a simulated waveform, where FIG. 5(a) shows frequencies of alternating current voltages, FIG. 5(b) shows active powers of transmission at terminals, FIG. 5(c) shows direct current voltages at the terminals, and the absence of frequency regulation control is represented by a dash line.
DETAILED DESCRIPTION OF THE EMBODIMENTS The technical solutions provided in the present invention are described below in detail with reference to specific embodiments. It should be understood that the following specific embodiments are merely used for describing the present invention, but are not used to limit the scope of the present invention.
In a grid frequency regulation method based on a multi-terminal flexible direct current transmission system provided in the present invention, a converter station may automatically select, according to a frequency deviation of a connected alternating current system, whether the alternating current system receives grid frequency regulation, or may select, according to an instruction of a scheduling system, whether the connected alternating current system receives grid frequency regulation. In addition, the scheduling system may select, by using an instruction, an alternating current system connected to one or more converter stations to participate in grid frequency regulation. A control strategy of the converter station is changed, and joint frequency regulation among a plurality of alternating current systems is implemented. An electric power system for implementing a control method in the present invention includes a plurality of power balance converter stations that can use an active power balance technology, a plurality of converter stations that can use a fixed active power control mode, and a plurality of converter stations that can use an alternating current balance node control mode. The control method in the present invention includes the following steps: Step 1: Acquire an actual value f of a frequency of an alternating current voltage at a PCC of an alternating current system. The alternating current system includes an alternating current system connected to a converter station using the fixed active power control mode and a converter station using a fixed direct current voltage control mode.
Step 2: Calculate a deviation Af between the frequency f of the voltage of the alternating current system connected to the converter station and a reference frequency fe, and set an upper threshold 4f7ax and one lower threshold Af... for a frequency deviation of the alternating current voltage.
Step 3: When the frequency deviation Af of the alternating current system connected to the converter station does not exceed fnac, the converter station automatically selects to perform frequency regulation by using an adjustment capability of the alternating current system, or selects, by using an instruction of the scheduling system, the alternating current system connected to the converter station to receive grid frequency regulation; and when Af exceeds Afmac, the converter station automatically selects the connected alternating current system to receive grid frequency regulation. A condition for receiving grid frequency regulation may further be set to another condition as required.
Specifically, when the alternating current system connected to the converter station needs to receive grid frequency regulation, the method includes the following steps: Step (1): Switch a converter station to the alternating current balance node control mode, a principle of which is shown in FIG. 2, and control an amplitude and a phase angle of an alternating current side outlet voltage rather than a PCC of the converter station, to turn the converter station into a balance node, so that a power deficit of an alternating current system is imported into a direct current system without any delay and without any deviation.
In a d-q coordinate system, the alternating current side outlet voltage of the converter station using the alternating current balance node control mode 1s: ; , N i U wd = Rig +X chy + { BL ad } k, Lu | } % A i Ua = Ri, X ods + {0-U, ) k, + | ’ ) oe L os where S is a Laplacian operator, Usa and Usg are respectively a d-axis component and a q-axis component of an alternating current voltage at the PCC, Uca and Uc are respectively a d-axis component and a q-axis component of the alternating current side outlet voltage of the converter station, isa and is, are respectively a d-axis component and a q-axis component of an alternating current side current of the converter station, k, and k; are respectively a proportional coefficient and an integral coefficient of the proportional integrator, and R. and X. are respectively an equivalent resistance and a converter reactance of the converter station.
In a process of switching a control strategy, an output integral value of the proportional integrator should be reset: Px 4. | Ya FL, cos! BECSIM(-— T5) | A a RT a, U & U © 4 ’ PX, . ; yoo ESA à SRI PIES i oon cos Foxy (PXY U = UE | BE k IN U s 7 x U sd where Us and U. are respectively an effective value of the alternating current side outlet voltage of the converter station and an effective value of the alternating current voltage at the PCC, Vdreset and Vgrese are respectively an integrator reset value in d-axis control and an integrator reset value in q-axis control, and Ps and Os are respectively an active power and a reactive power injected at the PCC.
The resetting of the integrator ensures status matching between parameters of a control system and electrical parameters of the direct current system, thereby minimizing the impact caused by the switching of a control strategy, reducing a shock on the system, and ensuring the safety and stability of the system.
Without initialization, the initial value of the integrator is 0. In this case, an output of a controller does not match the status of electrical parameters of the direct current system, leading to a severe shock or even instability in the system.
Step (2): Set maximum values PF; and PP“... for a transmission power of a converter station that operates in the alternating current balance node control mode, detect a transmission power of a converter station that receives frequency regulation, and if the transmission power Ps; of the converter station exceeds F__ change control of the converter station to a limited power transmission control mode.
In the coordinate axes d and q, when the transmission power P; of the converter station exceeds PP. the alternating current side outlet voltage of the converter station is tuned to: jv. wif cos| arsine) | x Se + U.
In the coordinate axes d and q, when the transmission power Ps of the converter station exceeds Pr. the alternating current side outlet voltage of the converter station 1s tuned to: a =U cos cs Pas, © : | UL) “ Li A case in which the transmission power Ps of the converter station exceeds #__ is used as an example in FIG. 1. In this step, according to an upper limit of a transmission capacity of a converter station, a difference between phase angles of a PCC and the alternating current side outlet voltage of the converter station is calculated, to adjust the phase angle of the alternating current side outlet voltage of the converter station, thereby ensuring synchronous changes of the phase angles of the two, to implement power limitation, so that an active power injected from a converter station into the direct current system can be constrained, to ensure the safety of the converter station.
In addition, the capacity of a converter can be utilized to the greatest extent, thereby improving the safety and stability of the direct current system.
Step (3): The scheduling system selects, by using an instruction, one or more alternating current systems having a frequency regulation capability as a power balance station.
Step (4): Detect the transmission powers of the converter stations, calculate an unbalanced power
AP of the direct current transmission system by using an active power balance technology, and dynamically adjust the reference value of the power of the power balance station according to AP. The unbalanced power AP of the direct current transmission system 1s: ; AP= 3 (Pau Purses Pan Bui Bae FB) In the foregoing formula, n is a quantity of converter stations in the system, the first m converter stations are power balance stations, an (m+1)" converter station to an n converter station are actual values of the transmission powers of the converter stations using the alternating current balance node control mode or the fixed active power control mode. The reference value of the power of the power balance station according to AP is dynamically adjusted: KY —
HK where Pr. is an initial reference value of an active power of an i (1 <i < m) converter station, Fis is an adjusted reference value of the active power of the i (1 <i < m) converter station, and Ki; is a droop coefficient of the power balance station. In this step, an unbalanced power in the direct current system can be calculated in real time, thereby ensuring the accuracy of frequency regulation. In addition, the scheduling system specifies one or more alternating current systems to participate in the frequency regulation of the alternating current system, thereby improving the flexibility of frequency regulation control and the dynamic stability of a power and a voltage. Step 4: When the frequency deviation Af of the connected alternating current system that receives grid frequency regulation is less than Afnin, the scheduling system automatically switches the converter station that receives grid frequency regulation to a fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation. A principle is shown in FIG. 3. The scheduling system may flexibly set as required by using an instruction to switch the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation.
To implement stable switching of a control strategy of the converter station, a reference value Pr of the active power is enabled to be equal to an actual value P of an active power of current transmission of the converter station.
In a process of switching a control strategy, the output integral value of the proportional integrator should be reset, and an integrator reset value in outer-loop control is: a Ps where Vireser and Vareser are respectively integrator reset values in d-axis outer-loop control and q-axis outer-loop control.
An integrator in inner-loop control 1s reset to: ae = UU 08] Brest) + OL asc j L vu, ; PX. v= SIE apg l'a TT caves where Vireser and Vareser are respectively integrator reset values in d-axis inner-loop control and q-axis inner-loop control.
A six-terminal flexible direct current transmission system shown in FIG. 4 is used as an example to describe a coordinated control strategy in the present invention in detail.
Converter stations VSC2, VSC3, VSC4, and VSCS use direct current voltage droop control.
VSC1 is connected to a wind farm, and VSC6 is connected to a passive grid.
Both VSC1 and VSC6 use amplitude phase control.
A direction of injecting an active power from the alternating current system to the direct current system is used as a positive direction.
A range of the transmission powers of the converter stations is —750 MW to 750 MW.
A conventional master-slave control strategy is compared with a novel coordinated control strategy in the present invention.
In the master-slave control strategy, the station VSC3 is used as a master station and uses a fixed direct current voltage control mode.
The stations VSC2, VSC4, and VSCS are used as slave control stations, use the fixed active power control mode, and sequentially bear a voltage control operation after the master station exits.
VSCI and VSC6 use amplitude phase control.
A voltage level of a multi-terminal flexible direct current transmission system is £500 kV.
Case: The load of an alternating current system connected to the station VSCS surges. In the simulated scenario, at an initial moment, transmission powers of VSCI, VSC2, VSC3, VSC4, VSCS, and VSC6 are respectively 700 MW, 600 MW, 230 MW, —400 MW, —450 MW, and —600 MW. At the 6% s, the load of the alternating current system connected to the station VSC5 surges to 200 MW. At the 16% s, an alternating current grid shreds the load of 200 MW. It is set that a reference value of a power of a control system is 750 WVA, a reference value of an alternating current voltage is 500 kV, a reference value of a direct current voltage is 500 kV, and a per-unit value of a converter reactance is 0.15.
For a novel frequency regulation control strategy provided in the present invention: At the 6% s, the frequency of an alternating current grid connected to the station VSCS changes. After a change in the frequency exceeds a threshold Afmax = 0.05 Hz, a control strategy of the station VSCS 1s switched to the alternating current balance node control mode. In a process of switching control, reset values of the d axis voltage and the q axis voltage are Värese = 0.996, and Vareset = 0.
At the 16" s, the load of the alternating current grid connected to the station VSCS is adjusted, and the active load of 200 MW is shredded. A power deficit of the alternating current grid is less than a lower limit Pmin = —750 MW of a transmission power of VSC3. The control strategy of the station VSCS is still the alternating current balance node control mode.
At the 26" s, the system is stable. It is known that after the load is shredded, the power of transmission of VSCS required for the system is 450 MW. In this case, the system is switched to the fixed active power control mode and a fixed reactive power control mode. A reference value of an active power is —450 MW, and a reference value of a reactive power is 0 MVA. In a process of switching control, reset values of the integrators are: igreser = —0.6, Tareset = 0, Vdreset = 0.006, and Vareset = 0.012.
The simulated waveform is shown in FIG. 5. FIG. 5(a) shows frequencies of alternating current voltages. FIG. 5(b) shows active powers of transmission at terminals, and FIG. 5(c) shows direct current voltages at the terminals.
As can be seen from FIG. 5, in the novel frequency regulation control strategy, an unbalanced power of an alternating current system (an AC grid 5) in which a power disturbance occurs directly flows into a direct current grid via a converter station. In the rapid power adjustment strategy of the direct current grid, the unbalanced power is rapidly imported into an appropriate alternating current system to complete supplementation, and the response speed of frequency regulation is fast. The frequency has a minimum value of 49.78 Hz and has a smaller deviation compared with 49.09 Hz in conventional frequency control. In addition, the active power balance technology ensures the dynamic stability of a direct current voltage.
The foregoing simulation verifies that the coordinated control strategy in the present invention 1s better than a conventional control strategy. A dynamic response speed 1s fast. In a steady state, a direct current voltage in a system is at a reference operate voltage. Apart from a converter station under amplitude phase control, powers of converter stations are all reference values, and the steady-state control performance is adequate.
The technical means disclosed in the solutions of the present invention 1s not only limited to the technical means disclosed in the foregoing embodiments, and further includes technical solutions formed by arbitrarily combining the foregoing technical features. It should be noted that for those of ordinary skill in the art, several improvements and modifications may further be made without departing from the principle of the present invention. These improvements and modifications are also deemed as falling within the protection scope of the present invention.

Claims (8)

1. A grid frequency regulation method based on a multi-terminal flexible direct current transmission system, comprising the following steps: switching a converter station connected to an alternating current system that receives grid frequency regulation to an alternating current balance node control mode, and resetting an output integral value of a proportional integrator when the converter station switches a control strategy; setting a maximum value for a transmission power of a converter station that operates in the alternating current balance node control mode, detecting a transmission power of a converter station that receives frequency regulation, and when the transmission power exceeds the maximum value, changing control of the converter station to a limited power transmission control mode; selecting, by a scheduling system by using an instruction, an alternating current system connected to one or more converter stations to participate in grid frequency regulation; and detecting transmission powers of converter stations, calculating an unbalanced power of a direct current transmission system, and dynamically adjusting a reference value of a power of a power balance station according to the unbalanced power to implement power balance.
2. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 1, further comprising the following prerequisite steps: automatically selecting, by a converter station when a frequency deviation of an alternating current system that is connected to the converter station exceeds an upper threshold, the alternating current system to receive grid frequency regulation, or selecting, according to an instruction of the scheduling system, whether the connected alternating current system receives grid frequency regulation.
3. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 1 or 2, further comprising the following subsequent steps:
after grid frequency regulation 1s received, when a frequency deviation of an alternating current system that is connected to a converter station and receives grid frequency regulation is less than a lower threshold, automatically switching the converter station that receives grid frequency regulation to a fixed active power control mode, or switching, by using an instruction of the scheduling system, the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation; and resetting the output integral value of the proportional integrator when the converter station switches a control strategy.
4. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 1, specifically comprising the following steps: step (1): switching a converter station to the alternating current balance node control mode, and controlling an amplitude and a phase angle of an alternating current side outlet voltage rather than a point of common coupling (PCC) of the converter station, to turn the converter station into a balance node, so that a power deficit of an alternating current system is imported into a direct current system without any delay and without any deviation; in a d-q coordinate system, the alternating current side outlet voltage of the converter station using the alternating current balance node control mode is: Lo Cee SE k | Une = Rig + Xd, LU ) +) | Ua = Rah Aa {OU ) x A à LL... AT wherein S is a Laplacian operator, Uy and Uy, are respectively a d-axis component and a g-axis component of an alternating current voltage at the PCC, Uca and U,, are respectively a d-axis component and a q-axis component of the alternating current side outlet voltage of the converter station, 7s4 and iy, are respectively a d-axis component and a q-axis component of an alternating current side current of the converter station, k, and k; are respectively a proportional coefficient and an integral coefficient of the proportional integrator, and R. and X are respectively an equivalent resistance and a converter reactance of the converter station; in a process of switching a control strategy, the output integral value of the proportional integrator 1s reset: Vor SU, cos) arcsin(2) | Ki Rh \ fA wherein Us and U: are respectively an effective value of the alternating current side outlet voltage of the converter station and an effective value of the alternating current voltage at the PCC, Varese and Vgreset are respectively an integrator reset value in d-axis control and an integrator reset value in g-axis control, and Ps and Os are respectively an active power and a reactive power injected at the PCC;
step (2): setting maximum values PE, and FF for a transmission power of a converter station that operates in the alternating current balance node control mode, detecting a transmission power of the converter station that receives frequency regulation, if the transmission power Ps of the converter station exceeds #__. changing control of the converter station to the limited power transmission control mode, and tuning the alternating current side outlet voltage of the converter station; in the coordinate axes d and q, when the transmission power Ps of the converter station exceeds FF. tuning the alternating current side outlet voltage of the converter station to:
lu, = U cos | arcsin( “BE | + X Seed = Ul in the coordinate axes d and q, when the transmission power Ps of the converter station exceeds
PF,» tuning the alternating current side outlet voltage of the converter station to: £ =U cos aresing Frs X y we © | vr) 05 [ wherein |P__| and |P,_| are the maximum values of the transmission power of the converter station, | ©. | > 0, 7 represents a maximum value of the transmission power in a rectification direction, that is, represents that an active power is injected from an alternating current system into the converter station; and |__| < 0, |F__| represents a maximum value of the transmission power in an inverter direction, that is, an active power is injected from the converter station into the alternating current system;
step (3): selecting, by the scheduling system by using an instruction, one or more alternating current systems having a frequency regulation capability as a power balance station; and step (4): detecting the transmission powers of the converter stations, calculating an unbalanced power AP of the direct current transmission system by using an active power balance technology,
and dynamically adjusting the reference value of the power of the power balance station according to AP; the unbalanced power AP of the direct current transmission system is:
AP= 3 (Po Bone Pa Bas }
in the foregoing formula, n is a quantity of converter stations in the system, the first m converter stations are power balance stations, an (m+1)® converter station to an n“ converter station are actual values of the transmission powers of the converter stations using the alternating current balance node control mode or the fixed active power control mode; and dynamically adjusting the reference value of the power of the power balance station according to
AP:
PS KS wherein P, is an initial reference value of an active power of an i" converter station, Post is an adjusted reference value of the active power of the i" converter station, 1 < i < m, and K; is a droop coefficient of the power balance station.
5. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 4, further comprising the following prerequisite steps: step 1, acquiring an actual value f of a frequency of an alternating current voltage at a PCC of an alternating current system; step 2, calculating a deviation Af between the frequency f of the voltage of the alternating current system connected to the converter station and a reference frequency fes, and setting an upper threshold Afnax and one lower threshold Af... for a frequency deviation of the alternating current voltage; and step 3, when the frequency deviation Af of the alternating current system connected to the converter station does not exceed fn, automatically selecting, by the converter station, to perform frequency regulation by using an adjustment capability of the alternating current system, or selecting, by using an instruction of the scheduling system, the alternating current system connected to the converter station to receive grid frequency regulation; and when Af exceeds Afmac, automatically selecting, by the converter station, the connected alternating current system to receive grid frequency regulation.
6. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 5, wherein the alternating current system comprises an alternating current system connected to a converter station using the fixed active power control mode and a converter station using a fixed direct current voltage control mode.
7. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 5 or 6, further comprising the following subsequent steps: step 4, when the frequency deviation Af of the connected alternating current system that receives grid frequency regulation is less than Afnin, automatically switching, by the scheduling system, the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation; or switching, by the scheduling system as required by using an instruction, the converter station that receives grid frequency regulation to the fixed active power control mode, to stop the connected alternating current system from receiving grid frequency regulation, wherein in a process of switching a control strategy, the output integral value of the proportional integrator is reset, and an integrator reset value in outer-loop control is: F Py | Loin 7 J * 3 Qu Flier FUI wherein Väreser and Vareser are respectively integrator reset values in d-axis outer-loop control and q-axis outer-loop control; an integrator in inner-loop control is reset to: { (RX Wine = EI cos] arosing 2540) | oli as i { Bu 105 | PX. a nk re PTE to wherein Väreser and Vareser are respectively integrator reset values in d-axis inner-loop control and q-axis inner-loop control.
8. The grid frequency regulation method based on a multi-terminal flexible direct current transmission system according to claim 7, wherein a reference value Pr of the active power is enabled to be equal to an actual value P of an active power of current transmission of the converter station.
LU500835A 2020-08-26 2020-09-23 Grid frequency regulation method based on multi-terminal flexible direct current transmission system LU500835B1 (en)

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