CN110429632B  Frequency consistency control method for asynchronous interconnected system containing doubleloop flexible direct current two areas  Google Patents
Frequency consistency control method for asynchronous interconnected system containing doubleloop flexible direct current two areas Download PDFInfo
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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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 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

 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 hightension dc link

 Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSSSECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSSREFERENCE 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]
Abstract
The invention relates to a frequency consistency control method suitable for a doubleloop flexible direct current tworegion 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 directcurrent 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. "insitu control" layer: a universal frequencyvoltage 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 doubleloop direct current system can still provide certain frequency support for the disturbed alternating current power grid through local control and local measurement information.
Description
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 doubleloop flexible direct current tworegion asynchronous interconnection system.
Background
A Voltage Source Converter based High Voltage Direct Current Transmission (VSCHVDC) 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 (VSCHVDC) 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 doublecircuit 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 largerange 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 tworegion alternating current power grid, although emergency power support and rotary standby sharing of the tworegion 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 tworegion alternating current power grids are asynchronously interconnected through a flexible direct current transmission technology, the tworegion alternating current power grids have incomparable advantages compared with the traditional tworegion 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 interregion resource complementation is realized [8,9 ].
For a flexible directcurrent interconnection system of a twoarea alternatingcurrent 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, fullsystem 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 [1012 ]. Virtual synchronous generator control (VSG) [13,14], and the like. Document [10,11] introduces frequencyactive power (fP) 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 VSCHVDC system in case of an accident, document [12] proposes an additional frequency control strategy, which introduces a frequencyactive power and dc voltageactive power slope characteristic into a fixed active power controller, and introduces a frequencydc 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 VSCHVDC. 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 activefrequency drooping static characteristic of the synchronous generator. The frequency support and the rotating standby sharing of the receivingend power grid by the transmittingend power grid can be realized essentially, but the research on how to realize the frequency support of the transmittingend 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 singleloop 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 doubleloop flexible direct current tworegion 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 directcurrent transmission technology are analyzed by [ J ] highvoltage technology, 2014, 40(08) 2429doped 2439.
[2] Xuzheng, Chenhai Rong, A voltage source converter type direct current transmission technology reviews [ J ]. high voltage technology, 2007, 33(1):110.
[3] Scheiwining, Lipoming, Xuxu, et al, same tower doublecircuit HVDC engineering AC Filter design [ J ]. Power System Automation, 2010, 34(17):5054.
[4] Wanjuanjuan, Xiaocleong, luozhihua, and the like, the function of doublecircuit control of a costation cotower doublecircuit highvoltage directcurrent 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):710.
[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, chinaeast dongdui, 2005(01):7782.
[9] The research on the frequency stability of an alternating currentdirect current hybrid system is improved by a Zhuhongnu, Rolongfu and direct current modulation strategy [ J ]. Chinese Motor engineering report, 2012, 32(16):3643.
[10]Chaudhuri N.R.,Majumder R.,Chaudhuri B.System Frequency Support through MultiTerminal DC(MTDC)Grids[J].IEEE Transactions on Power Systems,2013,28(1):347356.
[11] The method comprises the steps of DaizCan, Lixing source, stress, VSCMTDC interconnection system frequency stability control strategy [ J ]. power grid technology, 2014, 38(10): 2729.
[12] Zhuruike, Queen, Lixing, etc. additional frequency control strategies for VSCHVDC interconnection systems [ J ] Power systems Automation, 2014, 38(16):8187.
[13] The VSCHVDC 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):525533.
[14] Zhengtianwen, ChenYun, ChenTianI, etc. the virtual synchronous generator technology and the prospect [ J ] of power system automation, 2015, 39(21):165175.
[15]Kirakosyan Aram，ElSaadany Ehab Fahmy，Moursi Mohamed Shawky El，et al.DC Voltage Regulation and Frequency Support in Pilot Voltage DroopControlled Multiterminal HVDC Systems[J].IEEE Transactions on Power Delivery，2018，33(3):11531164.
The invention content is as follows:
the invention provides a frequency consistency control method suitable for a doubleloop flexible direct current tworegion 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, fullsystem emergency power support and rotary standby sharing are achieved, and emergency support power of the direct current system is reasonably borne by the doublecircuit 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 doubleloop flexible direct current tworegion 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 directcurrent 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 doubleloop 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 doubleloop flexible direct current tworegion asynchronous interconnection system, and the direct current voltage control law is expressed as follows:
in the formula u_{dc1}And u_{dc3}The onsite direct current voltages of the converter stations at the voltage ends of the doublecircuit direct current lines are respectively; u. of_{dc2}And u_{dc4}The local direct current voltages of the doublecircuit direct current line power end converter stations are respectively; u. of_{com1}And u_{com2}Respectively averaging two branch average voltage values of the doublecircuit 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 doublecircuit DC line according to the spare capacity ratio thereof in the following specific realization mode
P_{add}＝[m(P_{ref4}+P_{add})(P_{ref2}P_{add})](k_{p,PP}+k_{i,PP}/s)
In the formula, m represents a doubleturn straight lineSpare capacity ratio of the stream line; p_{ref2}，P_{ref4}And P_{add}Respectively 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 of_{p,PP}And k_{i,PP}Respectively representing the proportional coefficient and the integral coefficient of the auxiliary power control loop PI controller.
Second section, "inplace control" layer: a universal frequencyvoltage 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 doubleloop 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 insitu control
The control strategies of the voltage end converter stations VSC1 and VSC3 are improved in two places as follows: adding a universal frequencyvoltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha.
The generalized frequencyvoltage auxiliary control structure is realized in the following specific form
In the formula, k_{p,UU}And k_{i,UU}Respectively representing a proportional coefficient and an integral coefficient of an outer ring frequencyvoltage auxiliary control ring PI controller; u. of_{ref1}And u_{ref3}Respectively representing directcurrent voltage auxiliary references generated by generalized frequencyvoltage auxiliary control; omega_{1}Is 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 insitu 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 frequencyvoltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha.
The generalized frequencyvoltage auxiliary control structure is specifically realized as follows
In the formula, k_{p,PU}And k_{i,PU}Respectively representing a proportional coefficient and an integral coefficient of an outer ring frequencyvoltage auxiliary control ring PI controller; omega_{2}Is the ac frequency of the second ac power network.
The frequency consistency control method suitable for the asynchronous interconnection system with the doubleloop flexible direct current and the two areas, provided by the invention, comprises the following steps: 1) the flexible and direct interconnected twoarea 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 twoarea alternating current power networks are realized; 2) when power disturbance occurs, the doublecircuit direct current line reasonably bears emergency support power according to the spare capacity ratio of the doublecircuit direct current line, and the utilization rate of the doublecircuit 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 doubleloop 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 twocircuit flexible DC twozone 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 directcurrent 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 doubleloop flexible direct current tworegion 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 doublecircuit 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 doublecircuit 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 omega_{i}、ΔP_{m,i}、ΔP_{L,i}And Δ P_{VSC,i}And 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. H_{i}And D_{i}Respectively representing equivalent inertia coefficients and damping coefficients of an equivalent unit of an alternating current power grid i; 1/R_{i}Representing the primary frequency modulation coefficient of the equivalent unit; g_{M}(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 P_{L,i}In 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,i}Therefore, 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 grid_{i}Is composed of
In the formula beta_{i}Satisfies the equivalent rigidity coefficient of the AC power grid i_{i}＝D_{i}+1/R_{i}。
The converter station with the doubleloop flexible direct current twozone 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 doublecircuit 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 of_{p},_{U}And k_{i,U}Respectively representing a proportional coefficient and an integral system of the directcurrent voltage PI controller. Daxis current reference i generated by direct current voltage PI control loop_{di,ref}With qaxis current reference i generated by a reactive control loop_{qi,ref}The 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). P_{seti}Representing VSCi (i2, 4) transmission power set point as daxis current reference i_{di,ref}With qaxis current reference i generated by a reactive control loop_{qi,ref}The 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 P_{VSC,i}0. In the steadystate situation, the frequency variation of the disturbed AC power grid i is
Δω_{i}＝ΔP_{L,i}/β_{i} (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 fullsystem rotation standby sharing and frequency support.
In order to realize the rotating standby sharing and the frequency supporting of the doubleloop flexible direct current twoarea 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 twoarea alternating current power networks and reasonable bearing of the emergency supporting power of a doublecircuit 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 doubleloop 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 doubleloop flexible direct current twoarea asynchronous interconnection system. The dc voltage control law is expressed as follows:
in the formula u_{com1}And u_{com2}The two branch average voltage values of the doublecircuit 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 doublecircuit direct current line according to the spare capacity ratio, and the control expression is as follows
P_{add}＝[m(P_{ref4}+P_{add})(P_{ref2}P_{add})](k_{p,PP}+k_{i,PP}/s) (5)
In the formula, m represents the spare capacity ratio of the doublecircuit direct current line; p_{ref2}，P_{ref4}And P_{add}Respectively 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 of_{p},_{PP}And k_{i,PP}Respectively representing the proportional coefficient and the integral coefficient of the auxiliary power control loop PI controller.
"insitu control" layer: on the basis of a conventional control strategy, a universal frequencyvoltage 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 doubleloop 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 insitu 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 frequencyvoltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequencyvoltage 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, k_{p},_{UU}And k_{i,UU}Respectively representing a proportional coefficient and an integral coefficient of an outer ring frequencyvoltage auxiliary control ring PI controller; u. of_{ref1}And u_{ref3}Respectively representing directcurrent voltage auxiliary references generated by generalized frequencyvoltage 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 control_{com1}And u_{com2}To 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 insitu 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 frequencyvoltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequencyvoltage 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, k_{p,PU}And k_{i,PU}Respectively representing the proportional coefficient and the integral coefficient of the outer loop frequencyvoltage 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 doublecircuit DC line_{com1}And u_{com2}At this time, the systemwide 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 1_{L,1}Resulting in an AC mains 1 frequency omega_{1}For example, the frequencyvoltage auxiliary control structure can be generalized by VSC on voltage terminalKnown as the DC voltage reference u_{com1}And u_{com2}Will follow the frequency omega_{1}Is increased by the voltage, thereby leading the auxiliary power reference P generated by the VSC universal frequencyvoltage auxiliary PI controller at the power end to be increased_{ref2}And P_{ref4}The 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 P_{L,1}Resulting in an AC mains 1 frequency omega_{1}When 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 disturbance_{1}＝ω_{2}1), 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 1_{L,1}In time, the steadystate frequencies of the voltageside ac grid 1 and the powerside ac grid 2 are equal (ω) due to the respective PI controllers acting_{1}＝ω_{2}＝u_{com1}＝u_{com2}) Namely, the frequency variation of the twoarea alternating current power grid is equal. The compound of the formula (2) can be obtained,
at steady state, Δ P_{VSC,1}＝ΔP_{VSC,2}The combination formula (8) can obtain that the steadystate 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 end_{L,1}When 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 twoarea 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 doublecircuit DC line_{dc1}And Δ P_{dc2}Are 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 doublecircuit direct current line in a steady state_{dc1}And Δ P_{dc2}Satisfy 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 doublecircuit 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 2_{L,2}Resulting in an AC network 2 frequency omega_{2}For example, the auxiliary power reference P generated by the controller is generated by the action of the frequencyvoltage auxiliary PI controller commonly used by the VSC at the power end_{ref2}And P_{ref4}The 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 P_{L,2}Resulting in an AC network 2 frequency omega_{2}When 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 disturbance_{1}＝ω_{2}1), 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 2_{L,2}In 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}＝ω_{2}＝u_{com1}＝u_{com2}) Namely, the frequency variation of the twoarea alternating current power grid is equal. The compound of the formula (2) can be obtained,
at steady state, Δ P_{VSC,1}＝ΔP_{VSC,2}The combination formula (12) can obtain that the steadystate frequency variation and the steadystate transmission power variation of the DC system are respectively
It follows that Δ P occurs when the powerside ac grid 2 is energized_{L,2}When 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 twoarea alternating current power grid according to the equivalent rigidity coefficient of the twoarea 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 doublecircuit direct current line in the steady state_{dc1}And Δ P_{dc2}The 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 doublecircuit direct current line.
In summary, the invention provides a method for controlling frequency consistency of a doubleloop flexible direct current tworegion asynchronous interconnection system, which comprises the following steps: 1) the flexible and direct interconnected twoarea 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 twoarea alternating current power networks are realized; 2) when power disturbance occurs, the doublecircuit direct current line reasonably bears emergency support power according to the spare capacity ratio of the doublecircuit direct current line, and the utilization rate of the doublecircuit 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 doubleloop 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 doubleloop flexible direct current tworegion asynchronous interconnection system shown in figure 1 is built in simulation software PSCAD/EMTDC. The main parameters of the doublecircuit flexible direct current tworegion 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 doublecircuit directcurrent line and the fault of the centralized controller.
TABLE 1 Dualloop Flexible DC tworegion 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 doublecircuit 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 doublecircuit 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 doubleloop flexible direct current tworegion 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 P_{dci}Each of (i ═ 1,2) represents the doublecircuit dc line transmission power, and the forward direction is the direction from the voltageside grid 1 to the powerside 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 doublecircuit 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 steadystate 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 doublecircuit direct current line is respectively about 22.2MW and 44.4MW, the proportional relation that the standby capacity ratio of the doublecircuit 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 doublecircuit 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 fullsystem 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 doubleloop flexible direct current tworegion asynchronous interconnected system are respectively shown in fig. 5(a) and (b).
It can be known from the figure that, after the voltageside ac power grid 1 has a load disturbance, 1) when a conventional control strategy is adopted, the dc transmission power of the doublecircuit dc line maintains its rated transmission power of 400MW, the maximum frequency fluctuation variation of the voltageside power grid 1 is about 0.22Hz, the steadystate frequency variation is about 0.04Hz, the powerside power grid 2 is not affected by the load fluctuation of the voltageside 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 doublecircuit direct current line is respectively about11.1 MW and22.2 MW, the proportional relation that the standby capacity ratio of the doublecircuit direct current line is 0.5(m is 0.5) is met, the maximum variation and the steadystate 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 steadystate 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 doublecircuit 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 fullsystem 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 doubleloop 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 "insitu control" layer. Next, the auxiliary power reference value P of centralized control is obtained_{add}And (3) simulating the fault of the 'upper layer centralized' controller, and performing simulation verification when the directcurrent voltage signal adjustment coefficient alpha is 1, the standby capacity ratio of the doublecircuit directcurrent 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 dualloop flexible dc twozone 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 steadystate frequency deviation, and implement the fullsystem rotating standby sharing, but at this time, the doublecircuit 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 dualloop flexibledirect 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 voltageside 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 voltageside power grid, so as to provide an emergency power support for the disturbed voltageside power grid, improve the frequency dynamics of the disturbed voltageside power grid to a certain extent, reduce the steadystate frequency deviation of the disturbed power grid, and implement the fullsystem rotating standby sharing, but at this time, the transmission power increment of the doublecircuit dc line no longer satisfies the standby capacity ratio (m is 0.5).
Claims (1)
1. A frequency consistency control method suitable for a doubleloop flexible direct current tworegion 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 directcurrent 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 doubleloop 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 doubleloop flexible direct current tworegion asynchronous interconnection system, and the direct current voltage control law is expressed as follows:
in the formula u_{dc1}And u_{dc3}The onsite direct current voltages of the converter stations at the voltage ends of the doublecircuit direct current lines are respectively; u. of_{dc2}And u_{dc4}The local direct current voltages of the doublecircuit direct current line power end converter stations are respectively; u. of_{com1}And u_{com2}Respectively averaging two branch average voltage values of the doublecircuit 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 doublecircuit DC line according to the spare capacity ratio thereof in the following specific realization mode
P_{add}＝[m(P_{ref4}+P_{add})(P_{ref2}P_{add})](k_{p,PP}+k_{i,PP}/s)
In the formula, m represents the spare capacity ratio of the doublecircuit direct current line; p_{ref2}，P_{ref4}And P_{add}Respectively 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 of_{p,PP}And k_{i,PP}Respectively representing a proportional coefficient and an integral coefficient of an auxiliary power control loop PI controller;
second section, "inplace control" layer: a universal frequencyvoltage 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 doubleloop 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 insitu control
The control strategies of the voltage end converter stations VSC1 and VSC3 are improved in two places as follows: adding a universal frequencyvoltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequencyvoltage auxiliary control structure is realized in the following specific form
In the formula, k_{p,UU}And k_{i,UU}Respectively represent the outer loop frequencyProportional coefficient and integral coefficient of the voltage auxiliary control loop PI controller; u. of_{ref1}And u_{ref3}Respectively representing directcurrent voltage auxiliary references generated by generalized frequencyvoltage auxiliary control; omega_{1}Is 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 insitu 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 frequencyvoltage auxiliary control structure; introducing a direct current voltage signal adjustment coefficient alpha;
the generalized frequencyvoltage auxiliary control structure is specifically realized as follows
In the formula, k_{p,PU}And k_{i,PU}Respectively representing a proportional coefficient and an integral coefficient of an outer ring frequencyvoltage auxiliary control ring PI controller; omega_{2}Is the ac frequency of the second ac power network.
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