CN110429632B - Frequency consistency control method for asynchronous interconnected system containing double-loop flexible direct current two areas - Google Patents

Frequency consistency control method for asynchronous interconnected system containing double-loop flexible direct current two areas Download PDF

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CN110429632B
CN110429632B CN201910675654.7A CN201910675654A CN110429632B CN 110429632 B CN110429632 B CN 110429632B CN 201910675654 A CN201910675654 A CN 201910675654A CN 110429632 B CN110429632 B CN 110429632B
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direct current
frequency
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CN110429632A (en
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李鹏飞
郭力
李霞林
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Tianjin University
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24Arrangements for preventing or reducing oscillations of power in networks
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

The invention relates to a frequency consistency control method suitable for a double-loop flexible direct current two-region asynchronous interconnection system, which comprises a local control layer and an upper layer centralized control layer: control layer "upper layer centralization": the power control circuit consists of a direct-current voltage control law and an auxiliary power control loop, and is specifically realized in the following form: 1) a direct current voltage control law; 2) a secondary power control loop. "in-situ control" layer: a universal frequency-voltage auxiliary control structure is introduced into both the voltage end converter station and the power end converter station; in addition, a direct current voltage signal adjustment coefficient alpha is introduced, so that the voltage signal at the input end of the controller can be flexibly switched between the local voltage and the upper layer centralized control voltage reference, and even if the upper layer centralized controller fails, communication failures and other emergency working conditions occur, the double-loop direct current system can still provide certain frequency support for the disturbed alternating current power grid through local control and local measurement information.

Description

Frequency consistency control method for asynchronous interconnected system containing double-loop flexible direct current two areas
Technical Field
The invention belongs to the technical field of flexible direct current interconnection system control, and particularly relates to a frequency consistency control method suitable for a double-loop flexible direct current two-region asynchronous interconnection system.
Background
A Voltage Source Converter based High Voltage Direct Current Transmission (VSC-HVDC) technology is used as a new generation of Direct Current Transmission technology, can realize active and reactive fast decoupling, is flexible and controllable, and has the characteristics of low harmonic content of output Voltage and Current, High power factor, environmental friendliness and the like, so that the Voltage Source Converter based High Voltage Direct Current Transmission (VSC-HVDC) technology is widely concerned and developed in the world [1,2 ].
Along with the increase of the transmission capacity of an electric power system, the land of a transmission line is in tension day by day, and double-circuit direct current transmission can effectively improve the transmission capacity and the power supply reliability and flexibility of a unit corridor, reduce the construction cost and has wide application prospects [3,4 ]. The frequency quality of an electric power system is an important standard for judging the quality of electric energy, and when an alternating current system fails or power disturbance occurs, the frequency of the system deviates to influence the normal operation of power consumers, and the system frequency is broken down to cause large-range power failure in serious cases, so that the maintenance of the frequency stability of the electric power system is one of the main targets of the safe and stable operation of the electric power system [5,6 ].
According to the traditional two-region alternating current power grid, although emergency power support and rotary standby sharing of the two-region power grid can be realized and the dynamic and static characteristics of the frequency of the disturbed alternating current power grid are improved through alternating current interconnection, the fault cannot be effectively isolated, and the fault diffusion is prevented. And if the two-region alternating current power grids are asynchronously interconnected through a flexible direct current transmission technology, the two-region alternating current power grids have incomparable advantages compared with the traditional two-region alternating current power grid alternating current interconnection, the defects of an alternating current interconnection system can be overcome, and the safety and reliability of the asynchronous interconnection system are improved [7 ]. And when the alternating current power grid causes system frequency fluctuation due to power disturbance, the direct current system transmission power can be flexibly adjusted to participate in system frequency support, the frequency stability of the whole system can be improved, and inter-region resource complementation is realized [8,9 ].
For a flexible direct-current interconnection system of a two-area alternating-current power grid, the following conventional control strategy is generally adopted: one end of the converter station adopts constant voltage control to maintain the voltage stability of the direct current bus; and the converter station at one end adopts constant active power control. However, no matter which alternating current power grid generates power disturbance, the transmission power of the direct current system adopting conventional control is always maintained at a given value, emergency power support cannot be provided for a disturbed power grid, full-system rotating standby sharing is realized, and frequency fluctuation of the disturbed power grid is large.
Currently, the main method for the flexible dc interconnection system to participate in the frequency support of the ac system is additional frequency control [10-12 ]. Virtual synchronous generator control (VSG) [13,14], and the like. Document [10,11] introduces frequency-active power (f-P) slope control into voltage end converter station control, so that the voltage end converter station can respond to the frequency fluctuation of the voltage end access alternating current power grid, and participate in system frequency support control, but the power end converter station cannot respond to the system frequency and cannot provide frequency support for the system. In order to provide mutual support capability for ac systems on both sides of the VSC-HVDC system in case of an accident, document [12] proposes an additional frequency control strategy, which introduces a frequency-active power and dc voltage-active power slope characteristic into a fixed active power controller, and introduces a frequency-dc voltage slope characteristic into a fixed dc voltage controller, so as to realize that ac systems on both sides participate in mutual frequency adjustment through VSC-HVDC. Document [13] applies a frequency modulation control strategy adopting a virtual synchronous generator technology (VSG) to a receiving end converter station of a flexible direct current system, and introduces a virtual rotational inertia to enable the receiving end converter to have a dynamic inertial response of a synchronous generator; the primary frequency modulation function is realized by simulating the active-frequency drooping static characteristic of the synchronous generator. The frequency support and the rotating standby sharing of the receiving-end power grid by the transmitting-end power grid can be realized essentially, but the research on how to realize the frequency support of the transmitting-end power grid is lacked. Document [15] provides that frequency deviation caused by load is reasonably distributed by using frequency information of each alternating current power grid and dynamically adjusting active references to realize frequency consistency control of each alternating current power grid, but the control has high dependence on a communication system and cannot provide frequency support for the system when communication fails.
The control strategy is only suitable for the single-loop flexible direct current interconnection system, and research on how to realize emergency support power distribution and flexible control of the direct current system in the double-loop flexible direct current two-region asynchronous interconnection system is lacked.
Reference to the literature
[1] Ma is Min, Wu Fang 21180, Yang Yiming, and the like, the current situation and the application prospect of the flexible direct-current transmission technology are analyzed by [ J ] high-voltage technology, 2014, 40(08) 2429-doped 2439.
[2] Xuzheng, Chenhai Rong, A voltage source converter type direct current transmission technology reviews [ J ]. high voltage technology, 2007, 33(1):1-10.
[3] Scheiwining, Lipoming, Xuxu, et al, same tower double-circuit HVDC engineering AC Filter design [ J ]. Power System Automation, 2010, 34(17):50-54.
[4] Wanjuanjuan, Xiaocleong, luozhihua, and the like, the function of double-circuit control of a co-station co-tower double-circuit high-voltage direct-current power transmission system and simulation thereof [ J ] the automation of the power system, 2014, 38(23): 119-.
[5] Xuntaishan, Xuezhifeng, quantitative analysis of transient frequency shift acceptability [ J ]. power system automation, 2002, 26(19):7-10.
[6] P KUNDUR. Power System stabilization and control [ M ] Beijing, Chinese Power Press, 2001.
[7] The middle Europe high voltage DC power grid technical forum reviews [ J ] power grid technology, 2017, 41(8): 2407-.
[8] HVDC auxiliary power/frequency control [ J ] power system automation, extra waves, shengra, china-east dongdui, 2005(01):77-82.
[9] The research on the frequency stability of an alternating current-direct current hybrid system is improved by a Zhuhongnu, Rolongfu and direct current modulation strategy [ J ]. Chinese Motor engineering report, 2012, 32(16):36-43.
[10]Chaudhuri N.R.,Majumder R.,Chaudhuri B.System Frequency Support through Multi-Terminal DC(MTDC)Grids[J].IEEE Transactions on Power Systems,2013,28(1):347-356.
[11] The method comprises the steps of DaizCan, Lixing source, stress, VSC-MTDC interconnection system frequency stability control strategy [ J ]. power grid technology, 2014, 38(10): 2729-.
[12] Zhuruike, Queen, Lixing, etc. additional frequency control strategies for VSC-HVDC interconnection systems [ J ] Power systems Automation, 2014, 38(16):81-87.
[13] The VSC-HVDC receiving end converter participates in the Vsg control of the grid frequency modulation and the improved algorithm [ J ] of China Motor engineering report, 2017, 37(2):525-533.
[14] Zhengtianwen, Chen-Yun, Chen-Tian-I, etc. the virtual synchronous generator technology and the prospect [ J ] of power system automation, 2015, 39(21):165-175.
[15]Kirakosyan Aram,El-Saadany Ehab Fahmy,Moursi Mohamed Shawky El,et al.DC Voltage Regulation and Frequency Support in Pilot Voltage Droop-Controlled Multiterminal HVDC Systems[J].IEEE Transactions on Power Delivery,2018,33(3):1153-1164.
The invention content is as follows:
the invention provides a frequency consistency control method suitable for a double-loop flexible direct current two-region asynchronous interconnection system. No matter which area alternating current power grid generates power disturbance, the direct current system responds to provide power support for the alternating current power grid, frequency dynamics of the disturbed alternating current power grid is improved, full-system emergency power support and rotary standby sharing are achieved, and emergency support power of the direct current system is reasonably borne by the double-circuit direct current line according to the standby capacity ratio. In addition, the invention adopts a universal control structure, has good portability and low dependence on centralized control and communication, and even if a communication system fails, the system can also respond to frequency fluctuation by utilizing local information and provide certain frequency support for a disturbed power grid. The technical scheme of the invention is as follows:
a frequency consistency control method suitable for a double-loop flexible direct current two-region asynchronous interconnection system comprises a local control layer and an upper layer centralized control layer, and is characterized in that:
first, the "upper concentrated" control layer: the power control circuit consists of a direct-current voltage control law and an auxiliary power control loop, and is specifically realized in the following form:
1) direct current voltage control law: the direct current system utilizes the local measured value of the direct current voltage at two ends of the double-loop direct current line to calculate the average value of the direct current voltage, and provides direct current voltage reference for an 'local control' layer, and finally realizes the frequency consistency control target of the double-loop flexible direct current two-region asynchronous interconnection system, and the direct current voltage control law is expressed as follows:
in the formula udc1And udc3The on-site direct current voltages of the converter stations at the voltage ends of the double-circuit direct current lines are respectively; u. ofdc2And udc4The local direct current voltages of the double-circuit direct current line power end converter stations are respectively; u. ofcom1And ucom2Respectively averaging two branch average voltage values of the double-circuit direct current line, and providing direct current voltage reference for a local control layer;
2) an auxiliary power control loop, which provides auxiliary power reference for the local control layer power end converter station by using auxiliary power reference information generated by the outer loop power control of the power end converter station and PI control, and finally realizes that the quick response power of the DC system after the AC power grid is disturbed is reasonably borne in the double-circuit DC line according to the spare capacity ratio thereof in the following specific realization mode
Padd=[m(Pref4+Padd)-(Pref2-Padd)](kp,PP+ki,PP/s)
In the formula, m represents a double-turn straight lineSpare capacity ratio of the stream line; pref2,Pref4And PaddRespectively representing auxiliary power references generated by a power end converter station VSC2, a VSC4 outer loop power control and an auxiliary power reference generated by an upper layer centralized control layer auxiliary power control loop; k is a radical ofp,PPAnd ki,PPRespectively representing the proportional coefficient and the integral coefficient of the auxiliary power control loop PI controller.
Second section, "in-place control" layer: a universal frequency-voltage auxiliary control structure is introduced into both the voltage end converter station and the power end converter station; in addition, a direct current voltage signal adjustment coefficient alpha is introduced, so that the voltage signal at the input end of the controller can be flexibly switched between the local voltage and the upper layer centralized control voltage reference, and even if the upper layer centralized controller fails, communication failures and other emergency working conditions occur, the double-loop direct current system can still provide certain frequency support for the disturbed alternating current power grid through local control and local measurement information.
The specific implementation mode of the local control strategy of the layer voltage end converter station and the power end converter station is as follows:
1) voltage end converter station in-situ control
The control strategies of the voltage end converter stations VSC1 and VSC3 are improved in two places as follows: adding a universal frequency-voltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha.
The generalized frequency-voltage auxiliary control structure is realized in the following specific form
In the formula, kp,UUAnd ki,UURespectively representing a proportional coefficient and an integral coefficient of an outer ring frequency-voltage auxiliary control ring PI controller; u. ofref1And uref3Respectively representing direct-current voltage auxiliary references generated by generalized frequency-voltage auxiliary control; omega1Is the ac frequency of the first ac power grid; and alpha represents a direct current voltage signal adjustment coefficient.
Adjusting coefficient alpha of direct current voltage signal: the purpose of introducing the adjusting coefficient is to realize the flexible switching of the voltage signal at the input end of the controller between the local voltage and the upper layer centralized control voltage reference.
2) Power end converter station in-situ control
The control strategies of the VSC2 and the VSC4 of the power end converter station are improved in two places as follows: adding a universal frequency-voltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha.
The generalized frequency-voltage auxiliary control structure is specifically realized as follows
In the formula, kp,PUAnd ki,PURespectively representing a proportional coefficient and an integral coefficient of an outer ring frequency-voltage auxiliary control ring PI controller; omega2Is the ac frequency of the second ac power network.
The frequency consistency control method suitable for the asynchronous interconnection system with the double-loop flexible direct current and the two areas, provided by the invention, comprises the following steps: 1) the flexible and direct interconnected two-area alternating current power networks form an organic whole, and the frequency fluctuation caused by power disturbance is reasonably born together, so that emergency power support and rotary standby sharing of the two-area alternating current power networks are realized; 2) when power disturbance occurs, the double-circuit direct current line reasonably bears emergency support power according to the spare capacity ratio of the double-circuit direct current line, and the utilization rate of the double-circuit direct current line is effectively improved; 3) no matter which area alternating current power grid generates power disturbance, the direct current system responds to provide power support for the area alternating current power grid, and the frequency dynamic and static characteristics of the disturbed alternating current power grid are improved; 4) when emergency working conditions such as faults of the upper integrated controller, communication faults and the like occur, the double-loop direct current system can still provide certain frequency support for a disturbed alternating current power grid through local control and local measurement information; 5) and a universal control structure is adopted, so that the method has good transportability and is convenient for the reconstruction and implantation of the existing engineering control structure.
Description of the drawings:
FIG. 1 contains a two-circuit flexible DC two-zone asynchronous interconnect system;
FIG. 2 illustrates a conventional control strategy;
FIG. 3 frequency consistency control strategy;
FIG. 4 shows working conditions 1, frequency and direct current transmission power dynamics in power disturbance of power end grid 2
FIG. 5 shows working conditions 1, namely frequency and direct current transmission power dynamics during power disturbance of a voltage end power grid 1
FIG. 6 shows working conditions 2, namely frequency and direct-current transmission power dynamics during power disturbance of power end grid 2
FIG. 7 shows the working conditions 2, namely the dynamic state of frequency and direct current transmission power when the power of the voltage end power grid 1 is disturbed
The specific implementation mode is as follows:
the invention relates to a double-loop flexible direct current two-region asynchronous interconnection system which is shown in a figure 1. Two alternating current power grids interconnected by a direct current system adopt equivalent units and load simulation. The converter stations VSC1 and VSC3 interconnected with the alternating current power grid 1 are voltage control ends and are respectively used for maintaining the stability of direct current voltage of the double-circuit direct current line; the converter stations VSC2 and VSC4 interconnected with the ac grid 2 are power control terminals, and are respectively used for controlling the transmission power of the double-circuit dc line. The VSC of the converter stations at the two ends takes the power flow to the direct current side as the positive direction.
The equivalent frequency characteristic of the ac power grid i (i is 1,2) considering the equivalent unit rotor characteristic, the governor characteristic, and the steam turbine unit characteristic can be expressed as
In the formula, delta omegai、ΔPm,i、ΔPL,iAnd Δ PVSC,iAnd respectively representing the frequency increment, the mechanical power increment, the load increment of the equivalent unit and the total transmission power increment of the direct current system flowing to the alternating current power grid i. HiAnd DiRespectively representing equivalent inertia coefficients and damping coefficients of an equivalent unit of an alternating current power grid i; 1/RiRepresenting the primary frequency modulation coefficient of the equivalent unit; gM(s) is used for simulating the comprehensive dynamic characteristic of the equivalent unit speed regulator and the turbine, and the steady state value of the equivalent unit speed regulator and the turbine is 1. It is stated that all variables in the present invention are based on per unit value systems.
Is composed of(1) It can be known that when the ac power grid i (i ═ 1,2) has power disturbance Δ PL,iIn this case, if a certain method is adopted to control the dc transmission power Δ P of the dc system flowing to the ac power grid i (i is 1,2)VSC,iTherefore, rapid power support can be provided for the disturbed AC power grid, and the DC system can participate in the frequency control of the AC power grid. And in steady state, the frequency variation delta omega of the disturbed AC power gridiIs composed of
In the formula betaiSatisfies the equivalent rigidity coefficient of the AC power grid ii=Di+1/Ri
The converter station with the double-loop flexible direct current two-zone asynchronous interconnection system generally adopts a conventional control strategy based on a dq coordinate system as shown in fig. 2.
1) The voltage end converter stations VSC1 and VSC3 adopt constant dc voltage control to maintain the double-circuit dc bus voltage stable, as shown in fig. 2 (a). useti and udci respectively represent a set value and an actual value of the direct current voltage; k is a radical ofp,UAnd ki,URespectively representing a proportional coefficient and an integral system of the direct-current voltage PI controller. D-axis current reference i generated by direct current voltage PI control loopdi,refWith q-axis current reference i generated by a reactive control loopqi,refThe reference current inputs are used together, and control signals of the converter stations VSCi (i is 1 and 3) are generated through inner loop current control, so that corresponding control targets are achieved.
2) The VSC2 and the VSC4 of the power end converter stations adopt constant power control to realize that the transmission power of the direct current system is at a given value, as shown in fig. 2 (b). PsetiRepresenting VSCi (i-2, 4) transmission power set point as d-axis current reference idi,refWith q-axis current reference i generated by a reactive control loopqi,refThe reference current is input together, and a control signal of the converter station VSCi (i is 2,4) is generated through inner loop current control, so that a corresponding control target is realized.
As can be seen from FIG. 3, when the conventional control strategy is adopted, the transmission power of the DC system is a given value, and the AC power grid generates powerWhen power disturbance occurs, the DC system does not respond to the disturbance, i.e. delta PVSC,i0. In the steady-state situation, the frequency variation of the disturbed AC power grid i is
Δωi=-ΔPL,ii (3)
Therefore, the direct current system adopts a conventional control strategy, when the alternating current power grid generates power disturbance, the direct current system cannot respond to the power disturbance, the transmission power of the direct current system is still at a given value, and frequency fluctuation caused by the power disturbance is borne by a generator set in the disturbed alternating current power grid, so that the frequency deviation of the disturbed alternating current power grid is large. Therefore, the dc system adopting the conventional control strategy cannot realize the full-system rotation standby sharing and frequency support.
In order to realize the rotating standby sharing and the frequency supporting of the double-loop flexible direct current two-area asynchronous interconnection system, the invention provides a frequency consistency control strategy, and the control strategy comprises a local control layer and an upper layer centralized control layer, as shown in fig. 3.
Control layer "upper layer centralization": the emergency power sharing system is composed of a direct current voltage control law and an auxiliary power control loop, and has the main functions of processing acquired local voltage and power information, providing direct current voltage reference and auxiliary power reference for a local control layer through a corresponding controller design, and finally realizing mutual supporting and rotary standby sharing of emergency power of two-area alternating current power networks and reasonable bearing of the emergency supporting power of a double-circuit direct current circuit according to the standby capacity ratio. The specific implementation form of the upper layer centralized control layer is as follows:
1) direct current voltage control law: the direct current system utilizes the local measured value of the direct current voltage at two ends of the double-loop direct current line to calculate the average value of the direct current voltage, provides direct current voltage reference for an 'local control' layer and finally realizes the frequency consistency control target of the double-loop flexible direct current two-area asynchronous interconnection system. The dc voltage control law is expressed as follows:
in the formula ucom1And ucom2The two branch average voltage values of the double-circuit direct current line are respectively provided for providing direct current voltage reference for the local control layer.
2) The core control target of the auxiliary power control loop is to provide auxiliary power reference for the local control layer power end converter station by using auxiliary power reference information generated by the outer loop power control of the power end converter station and through PI control, finally realize that the quick response power of the direct current system after the disturbance of the alternating current power grid is reasonably borne in the double-circuit direct current line according to the spare capacity ratio, and the control expression is as follows
Padd=[m(Pref4+Padd)-(Pref2-Padd)](kp,PP+ki,PP/s) (5)
In the formula, m represents the spare capacity ratio of the double-circuit direct current line; pref2,Pref4And PaddRespectively representing auxiliary power references generated by a power end converter station VSC2, a VSC4 outer loop power control and an auxiliary power reference generated by an upper layer centralized control layer auxiliary power control loop; k is a radical ofp,PPAnd ki,PPRespectively representing the proportional coefficient and the integral coefficient of the auxiliary power control loop PI controller.
"in-situ control" layer: on the basis of a conventional control strategy, a universal frequency-voltage auxiliary control structure is introduced into a voltage end converter station and a power end converter station, the frequency fluctuation of an alternating current power grid accessed nearby can be responded, emergency power support is provided for the alternating current power grid, a direct current system can respond no matter which area of the alternating current power grid generates power disturbance, power support is provided for the alternating current power grid, and the dynamic and static characteristics of the frequency of the disturbed alternating current power grid are improved; in addition, a direct current voltage signal adjustment coefficient alpha is introduced, so that the voltage signal at the input end of the controller can be flexibly switched between the local voltage and the upper layer centralized control voltage reference, and even if the upper layer centralized controller fails, communication failures and other emergency working conditions occur, the double-loop direct current system can still provide certain frequency support for the disturbed alternating current power grid through local control and local measurement information.
The specific implementation mode of the local control strategy of the layer voltage end converter station and the power end converter station is as follows:
1) voltage end converter station in-situ control
The voltage end converter station VSC1 and VSC3 control strategies are shown in fig. 3(a) and 3(c), respectively. Compared with a conventional control strategy, the main control structure of the voltage end converter station is the same, PI control is adopted, and the following two improvements are performed: adding a universal frequency-voltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequency-voltage auxiliary control structure comprises the following steps: the core control target is to provide auxiliary voltage reference for voltage control by using local frequency, voltage information and voltage reference provided by upper layer centralized control through PI control, so that the auxiliary voltage reference can respond to the frequency fluctuation of the voltage end accessed to the alternating current power grid and provide emergency power support for the voltage end accessed to the alternating current power grid. It is specifically described as follows
In the formula, kp,UUAnd ki,UURespectively representing a proportional coefficient and an integral coefficient of an outer ring frequency-voltage auxiliary control ring PI controller; u. ofref1And uref3Respectively representing direct-current voltage auxiliary references generated by generalized frequency-voltage auxiliary control; and alpha represents a direct current voltage signal adjustment coefficient.
Adjusting coefficient alpha of direct current voltage signal: the purpose of introducing the adjusting coefficient is to realize the flexible switching of the voltage signal at the input end of the controller between the local voltage and the upper layer centralized control voltage reference.
When the communication system works normally, alpha is 0, and the voltage signal input by the voltage end converter station is the DC voltage reference u provided by the upper layer centralized controlcom1And ucom2To achieve the frequency consistency control objective mentioned herein;
when the communication system has a fault, alpha is 1, at the moment, the input voltage signal of the voltage end converter station is local direct current voltage, and the local control layer can also utilize local information to maintain the stable operation of the system and provide certain frequency support for a disturbed alternating current power grid.
2) Power end converter station in-situ control
The power end converter station VSC2 and VSC4 control strategies are shown in fig. 3(b) and 3(d), respectively. Compared with a conventional control strategy, the main control structure of the power end converter station is the same at this time, and PI control is adopted, but the following two improvements are carried out: adding a universal frequency-voltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequency-voltage auxiliary control structure: the core control target is to provide auxiliary power reference for power control by using local frequency, voltage information and voltage reference provided by upper layer centralized control through PI control, so that the auxiliary power reference can respond to the frequency fluctuation of the power end accessed to the alternating current power grid and provide emergency power support for the power end accessed to the alternating current power grid. It is specifically described as follows
In the formula, kp,PUAnd ki,PURespectively representing the proportional coefficient and the integral coefficient of the outer loop frequency-voltage auxiliary control loop PI controller.
In addition, the function and meaning of the dc voltage signal adjustment coefficient α are described above, and are not described herein again.
When the adjustment coefficient alpha of the DC voltage signal is 0, the voltage references provided by the upper layer centralized control layer for the local control layer are respectively the average value u of the voltages at two ends of the double-circuit DC linecom1And ucom2At this time, the system-wide frequency consistency control target is realized. Next, the working principle of controlling the frequency consistency of the system will be explained when the voltage end is connected to the ac power grid and the power end is connected to the ac power grid to generate power disturbance.
1) Power disturbance of voltage end connected to AC power grid
Load sudden decrease delta P with voltage terminal connected to AC network 1L,1Resulting in an AC mains 1 frequency omega1For example, the frequency-voltage auxiliary control structure can be generalized by VSC on voltage terminalKnown as the DC voltage reference ucom1And ucom2Will follow the frequency omega1Is increased by the voltage, thereby leading the auxiliary power reference P generated by the VSC universal frequency-voltage auxiliary PI controller at the power end to be increasedref2And Pref4The voltage is negative, so that the transmission power of the direct current system injected into the voltage end alternating current power grid 1 is reduced, and the frequency of the voltage end alternating current power grid is restrained from being too high due to sudden load reduction. Similarly, when the voltage end is connected into the alternating current power grid 1, the load is increased steeply by delta PL,1Resulting in an AC mains 1 frequency omega1When the load drops, the transmission power injected into the sending end alternating current power grid 1 by the direct current system can be increased, so that the frequency of the sending end alternating current power grid is prevented from being too low due to the sudden load increase.
Assuming that the frequency of the AC power grid system in two areas is stabilized at the rated frequency (namely omega) before the load disturbance1=ω21), as can be seen from the frequency control strategy shown in fig. 3, load disturbance Δ P occurs when the voltage end is connected to the ac power grid 1L,1In time, the steady-state frequencies of the voltage-side ac grid 1 and the power-side ac grid 2 are equal (ω) due to the respective PI controllers acting1=ω2=ucom1=ucom2) Namely, the frequency variation of the two-area alternating current power grid is equal. The compound of the formula (2) can be obtained,
at steady state, Δ PVSC,1=-ΔPVSC,2The combination formula (8) can obtain that the steady-state frequency variation and the transmission power variation of the DC system are respectively
It can be seen that Δ P occurs in the ac network 1 at the voltage endL,1When power is disturbed, the consistency control strategy provided by the text is adopted, so that direct current transmission power can be effectively adjusted, emergency power support is provided for the alternating current power grid 1, and frequency deviation of the alternating current power grid 1 is reduced. And in steady state, the changed power increment will be in the two-area alternating current networkThe distribution is made according to its equivalent stiffness coefficient.
In addition, in steady state, the transmission power variation Δ P of the double-circuit DC linedc1And Δ Pdc2Are respectively as
Due to the effect of the auxiliary power PI controller of the upper layer centralized control layer, the transmission power variation delta P of the double-circuit direct current line in a steady statedc1And Δ Pdc2Satisfy the requirement of
According to the formula, the quick response power of the direct current system after the alternating current power grid is disturbed is reasonably distributed in the double-circuit direct current line according to the spare capacity ratio m.
2) Power disturbance of power terminal connected to AC power grid
Load steep increase delta P with power end connected into AC network 2L,2Resulting in an AC network 2 frequency omega2For example, the auxiliary power reference P generated by the controller is generated by the action of the frequency-voltage auxiliary PI controller commonly used by the VSC at the power endref2And Pref4The frequency is negative, so that the transmission power of the direct current system injected into the power end alternating current power grid 2 is increased, and the frequency of the power end alternating current power grid caused by the load steep increase is restrained from being too low. Similarly, when the power end is connected into the AC power grid 2, the load is suddenly reduced by delta PL,2Resulting in an AC network 2 frequency omega2When the frequency rises, the transmission power injected into the sending end alternating current power grid 2 by the direct current system is reduced, so that the overhigh frequency of the power end alternating current power grid caused by sudden load reduction is restrained.
Assuming that the frequency of the AC power grid system in two areas is stabilized at the rated frequency (namely omega) before the load disturbance1=ω21), as can be seen from the frequency control strategy shown in fig. 3, the load disturbance Δ P occurs when the power end is connected to the ac power grid 2L,2In time, the voltage end exchanges the power grid due to the action of respective PI controllers1 and power end AC network 2 steady state frequency equality (omega)1=ω2=ucom1=ucom2) Namely, the frequency variation of the two-area alternating current power grid is equal. The compound of the formula (2) can be obtained,
at steady state, Δ PVSC,1=-ΔPVSC,2The combination formula (12) can obtain that the steady-state frequency variation and the steady-state transmission power variation of the DC system are respectively
It follows that Δ P occurs when the power-side ac grid 2 is energizedL,2When power is disturbed, the consistency control strategy provided by the text is adopted, direct current transmission power can be effectively adjusted, emergency power support is provided for the power end alternating current power grid 2, and frequency deviation of the alternating current power grid 2 is reduced. And in a steady state, the power increment is distributed in the two-area alternating current power grid according to the equivalent rigidity coefficient of the two-area alternating current power grid.
In addition, in a steady state, due to the effect of the auxiliary power PI controller of the 'upper concentrated' control layer, the transmission power variation delta P of the double-circuit direct current line in the steady statedc1And Δ Pdc2The relation of the formula (11) is still satisfied, namely the quick response power of the direct current system after the disturbance of the alternating current power grid is reasonably distributed according to the spare capacity ratio m of the double-circuit direct current line.
In summary, the invention provides a method for controlling frequency consistency of a double-loop flexible direct current two-region asynchronous interconnection system, which comprises the following steps: 1) the flexible and direct interconnected two-area alternating current power networks form an organic whole, and the frequency fluctuation caused by power disturbance is reasonably born together, so that emergency power support and rotary standby sharing of the two-area alternating current power networks are realized; 2) when power disturbance occurs, the double-circuit direct current line reasonably bears emergency support power according to the spare capacity ratio of the double-circuit direct current line, and the utilization rate of the double-circuit direct current line is effectively improved; 3) no matter which area alternating current power grid generates power disturbance, the direct current system responds to provide power support for the area alternating current power grid, and the frequency dynamic and static characteristics of the disturbed alternating current power grid are improved; 4) when emergency working conditions such as faults of the upper integrated controller, communication faults and the like occur, the double-loop direct current system can still provide certain frequency support for a disturbed alternating current power grid through local control and local measurement information; 5) and a universal control structure is adopted, so that the method has good transportability and is convenient for the reconstruction and implantation of the existing engineering control structure.
In order to verify the effectiveness of the control strategy provided by the invention, a double-loop flexible direct current two-region asynchronous interconnection system shown in figure 1 is built in simulation software PSCAD/EMTDC. The main parameters of the double-circuit flexible direct current two-region asynchronous interconnection system and the main parameters of the converter station are shown in tables 1 and 2. And then, respectively carrying out simulation verification on two working conditions of taking 0.5 of the spare capacity ratio of the double-circuit direct-current line and the fault of the centralized controller.
TABLE 1 Dual-loop Flexible DC two-region asynchronous interconnection system main parameters
Table 2 basic main parameters of the converter station
Working condition one, the ratio of the spare capacity of the double-circuit direct current circuit is 0.5(m is 0.5)
This condition is used to verify the effectiveness of the frequency consistency control strategy provided by the present invention when the dc voltage signal adjustment coefficient α is 0 and the spare capacity ratio of the double-circuit dc line is 0.5(m is 0.5). And then, respectively carrying out simulation verification when the power end is connected into the power grid and the voltage end is connected into the power grid to generate power disturbance.
(1) Power end power grid power disturbance simulation verification
In order to verify the effectiveness of the control strategy provided by the invention when the power end alternating current power grid 2 generates power disturbance, 200MW (0.2pu) load is put into the power end power grid 2 at the 50 th moment. By using a common ruleThe power grid frequency and the direct current transmission power dynamics of the double-loop flexible direct current two-region asynchronous interconnected system are respectively shown in fig. 4(a) and (b) during the regulation control strategy and the control strategy provided by the invention. In the figure, the DC transmission power PdciEach of (i ═ 1,2) represents the double-circuit dc line transmission power, and the forward direction is the direction from the voltage-side grid 1 to the power-side grid 2.
As can be seen from the figure, after the load disturbance occurs to the power end ac grid 2, 1) when a conventional control strategy is adopted, the dc transmission power of the double-circuit dc line maintains its rated transmission power of 400MW, the maximum fluctuation variation of the frequency of the power end grid 2 is about 0.42Hz, the steady-state frequency variation is about 0.083Hz, the voltage end grid 1 is not affected by the load fluctuation of the power end grid 2, and the frequency is still maintained at the rated value of 50 Hz. Therefore, by adopting conventional control, when the power end power grid has load fluctuation, the direct current system does not respond to the power end power grid, the direct current transmission power is maintained at a rated value, and the load fluctuation is all borne by the power generator set 2 at the disturbed power end, so that the dynamic frequency response and the steady state deviation of the power end power grid are larger. 2) When the frequency consistency control strategy provided by the invention is adopted, the transmission power increment of the double-circuit direct current line is respectively about 22.2MW and 44.4MW, the proportional relation that the standby capacity ratio of the double-circuit direct current line is 0.5(m is 0.5) is satisfied, the maximum variation and the steady state variation of the power grid frequency fluctuation at the power end are respectively about 0.29Hz and 0.056Hz, and compared with the conventional control strategy, the maximum variation and the steady state variation of the frequency fluctuation are respectively reduced by about 0.13Hz and 0.027 Hz. Therefore, by adopting the control strategy provided by the invention, when the power end power grid generates load fluctuation, the direct current system can provide emergency power support for the disturbed power grid, the double-circuit direct current line reasonably bears the emergency support power of the direct current system according to the spare capacity ratio (m is 0.5), the alternating current power grids in two areas bear the load fluctuation increment together according to the equivalent rigidity coefficient, the frequency dynamic of the disturbed power end power grid is improved, the steady state frequency deviation of the disturbed power end power grid is reduced, and the full-system rotating standby sharing is realized.
(2) Simulation verification of power disturbance of voltage end power grid
In order to verify the effectiveness of the control strategy provided by the invention when the voltage end alternating current power grid 1 generates power disturbance, 50MW (0.05pu) load is put into the voltage end power grid 1 at the 50s th moment. When the conventional control strategy and the control strategy provided by the invention are adopted, the power grid frequency and the direct current transmission power dynamics of the double-loop flexible direct current two-region asynchronous interconnected system are respectively shown in fig. 5(a) and (b).
It can be known from the figure that, after the voltage-side ac power grid 1 has a load disturbance, 1) when a conventional control strategy is adopted, the dc transmission power of the double-circuit dc line maintains its rated transmission power of 400MW, the maximum frequency fluctuation variation of the voltage-side power grid 1 is about 0.22Hz, the steady-state frequency variation is about 0.04Hz, the power-side power grid 2 is not affected by the load fluctuation of the voltage-side power grid 1, and the frequency is still maintained at the rated value of 50 Hz. Therefore, by adopting conventional control, when the voltage end power grid 1 has load fluctuation, the direct current system does not respond to the voltage end power grid 1, the direct current transmission power is maintained at a rated value, and the load fluctuation is all borne by the disturbed voltage end power grid 1 generator set, so that the dynamic frequency response and the steady state deviation of the voltage end power grid are larger. 2) When the frequency consistency control strategy provided by the invention is adopted, the transmission power increment of the double-circuit direct current line is respectively about-11.1 MW and-22.2 MW, the proportional relation that the standby capacity ratio of the double-circuit direct current line is 0.5(m is 0.5) is met, the maximum variation and the steady-state variation of the power grid frequency fluctuation at the voltage end are respectively about 0.075Hz and 0.014Hz, and compared with the conventional control strategy, the maximum variation and the steady-state variation of the frequency fluctuation are respectively reduced by about 0.145Hz and 0.026 Hz. Therefore, by adopting the control strategy provided by the invention, when the load fluctuation occurs to the voltage end power grid, the direct current system can provide emergency power support for the disturbed power grid, the double-circuit direct current line reasonably bears the emergency support power of the direct current system according to the spare capacity ratio (m is 0.5), the alternating current power grids in two areas bear the load fluctuation increment together according to the equivalent rigidity coefficient, the frequency dynamic of the disturbed power end power grid is improved, the steady state frequency deviation of the disturbed power end power grid is reduced, and the full-system rotating standby sharing is realized.
In conclusion, no matter power disturbance occurs to the power end power grid or the voltage end power grid, the double-loop direct current system can provide frequency support for the disturbed power grid by adopting the frequency consistency control strategy, and the rotating standby sharing of the whole system is realized.
Working condition two, upper layer integrated controller fault
This condition is used to verify the effectiveness of the frequency consistency control strategy proposed by the present invention when the "upper layer centralized" controller fails and cannot provide a dc voltage reference and an auxiliary power reference for the "in-situ control" layer. Next, the auxiliary power reference value P of centralized control is obtainedaddAnd (3) simulating the fault of the 'upper layer centralized' controller, and performing simulation verification when the direct-current voltage signal adjustment coefficient alpha is 1, the standby capacity ratio of the double-circuit direct-current line is 0.5(m is 0.5) and power disturbance occurs to the power end power grid and the voltage end power grid respectively.
(1) Power end power grid power disturbance simulation verification
In order to verify the effectiveness of the control strategy provided by the invention when the power end alternating current power grid 2 generates power disturbance, 200MW (0.2pu) load is put into the power end power grid 2 at the 50 th moment. The frequency and dc transmission power dynamics of the dual-loop flexible dc two-zone asynchronous interconnected system when the control strategy proposed herein is applied are shown in fig. 6(a) and (b), respectively. As can be seen from the figure, when the power end ac power grid 2 has a load disturbance, although the "upper layer centralized" controller fails, it is not possible to provide a dc voltage reference and an auxiliary power reference for the "local control" layer, the dc system can still use the local measurement information to respond to the power end grid frequency fluctuation, so as to provide an emergency power support for the disturbed power end grid, improve the disturbed power end grid frequency dynamics to a certain extent, reduce the disturbed grid steady-state frequency deviation, and implement the full-system rotating standby sharing, but at this time, the double-circuit dc line transmission power increment no longer satisfies the standby capacity ratio (m is 0.5).
(2) Simulation verification of power disturbance of voltage end power grid
In order to verify the effectiveness of the control strategy provided by the invention when the voltage end alternating current power grid 1 generates power disturbance, 50MW (0.05pu) load is put into the voltage end power grid 1 at the 50s th moment. The power grid frequency and the direct current transmission power dynamics of the dual-loop flexible-direct system when the control strategy provided by the invention is adopted are respectively shown in fig. 7(a) and (b).
It can be known from the figure that, when the voltage-side ac power grid 1 has a load disturbance, although the "upper layer centralized" controller fails, it is not possible to provide a dc voltage reference and an auxiliary power reference for the "local control" layer, the dc system can still use the local measurement information to respond to the frequency fluctuation of the voltage-side power grid, so as to provide an emergency power support for the disturbed voltage-side power grid, improve the frequency dynamics of the disturbed voltage-side power grid to a certain extent, reduce the steady-state frequency deviation of the disturbed power grid, and implement the full-system rotating standby sharing, but at this time, the transmission power increment of the double-circuit dc line no longer satisfies the standby capacity ratio (m is 0.5).

Claims (1)

1. A frequency consistency control method suitable for a double-loop flexible direct current two-region asynchronous interconnection system comprises a local control layer and an upper layer centralized control layer, and is characterized in that:
first, the "upper concentrated" control layer: the power control circuit consists of a direct-current voltage control law and an auxiliary power control loop, and is specifically realized in the following form:
1) direct current voltage control law: the direct current system utilizes the local measured value of the direct current voltage at two ends of the double-loop direct current line to calculate the average value of the direct current voltage, and provides direct current voltage reference for an 'local control' layer, and finally realizes the frequency consistency control target of the double-loop flexible direct current two-region asynchronous interconnection system, and the direct current voltage control law is expressed as follows:
in the formula udc1And udc3The on-site direct current voltages of the converter stations at the voltage ends of the double-circuit direct current lines are respectively; u. ofdc2And udc4The local direct current voltages of the double-circuit direct current line power end converter stations are respectively; u. ofcom1And ucom2Respectively averaging two branch average voltage values of the double-circuit direct current line, and providing direct current voltage reference for a local control layer;
2) an auxiliary power control loop, which provides auxiliary power reference for the local control layer power end converter station by using auxiliary power reference information generated by the outer loop power control of the power end converter station and PI control, and finally realizes that the quick response power of the DC system after the AC power grid is disturbed is reasonably borne in the double-circuit DC line according to the spare capacity ratio thereof in the following specific realization mode
Padd=[m(Pref4+Padd)-(Pref2-Padd)](kp,PP+ki,PP/s)
In the formula, m represents the spare capacity ratio of the double-circuit direct current line; pref2,Pref4And PaddRespectively representing auxiliary power references generated by a power end converter station VSC2, a VSC4 outer loop power control and an auxiliary power reference generated by an upper layer centralized control layer auxiliary power control loop; k is a radical ofp,PPAnd ki,PPRespectively representing a proportional coefficient and an integral coefficient of an auxiliary power control loop PI controller;
second section, "in-place control" layer: a universal frequency-voltage auxiliary control structure is introduced into both the voltage end converter station and the power end converter station; in addition, a direct current voltage signal adjustment coefficient alpha is introduced, so that the voltage signal at the input end of the controller can be flexibly switched between the local voltage and the upper layer centralized control voltage reference, and even under the emergency working condition of the fault and the communication fault of the upper layer centralized controller, the double-loop direct current system can still provide a certain frequency support for the disturbed alternating current power grid through local control and local measurement information;
the specific implementation mode of the local control strategy of the layer voltage end converter station and the power end converter station is as follows:
1) voltage end converter station in-situ control
The control strategies of the voltage end converter stations VSC1 and VSC3 are improved in two places as follows: adding a universal frequency-voltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequency-voltage auxiliary control structure is realized in the following specific form
In the formula, kp,UUAnd ki,UURespectively represent the outer loop frequency-Proportional coefficient and integral coefficient of the voltage auxiliary control loop PI controller; u. ofref1And uref3Respectively representing direct-current voltage auxiliary references generated by generalized frequency-voltage auxiliary control; omega1Is the ac frequency of the first ac power grid; alpha represents a direct current voltage signal adjustment coefficient;
adjusting coefficient alpha of direct current voltage signal: the voltage signal at the input end of the controller is flexibly switched between the local voltage and the upper layer centralized control voltage reference;
2) power end converter station in-situ control
The control strategies of the VSC2 and the VSC4 of the power end converter station are improved in two places as follows: adding a universal frequency-voltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequency-voltage auxiliary control structure is specifically realized as follows
In the formula, kp,PUAnd ki,PURespectively representing a proportional coefficient and an integral coefficient of an outer ring frequency-voltage auxiliary control ring PI controller; omega2Is the ac frequency of the second ac power network.
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