CN116961031B - High-frequency oscillation frequency division suppression and parameter design method for flexible direct-current transmission system - Google Patents
High-frequency oscillation frequency division suppression and parameter design method for flexible direct-current transmission system Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention discloses a method for restraining high-frequency oscillation frequency division and designing parameters of a flexible direct current transmission system, which comprises the following steps: acquiring alternating current system impedance Z ac and minimum phase theta acmin at the PCC point and flexible direct current converter station impedance Z mmc; determining the number H res of high-frequency oscillation risk areas and the frequency interval value thereof, and then determining the number N damp of parallel frequency division damping controllers; acquiring d-axis and q-axis input quantities of the frequency division damping controller; calculating and designing parameters of the frequency division damping controller; checking parameters of the frequency division damping controller; and compensating voltages e isd and e isq generated by the output frequency division damping controller are overlapped to the current inner loop controller to finally realize frequency division damping control and inhibit high-frequency oscillation. The method has the advantages of detailed steps, higher precision, no need of adding additional passive damping filters, capability of inhibiting a plurality of high-frequency oscillations, suitability for medium-low frequency and subsynchronous/supersynchronous oscillations, wide applicability and capability of effectively improving the running stability of the system.
Description
Technical neighborhood
The invention relates to the application fields of flexible direct current transmission (high voltage direct current, HVDC) systems and VSCs (voltage source converter) based on modularized multi-level converters (modular multilevel converter, MMC), which comprise application scenes such as connection of a flexible direct current converter station to an active alternating current power grid, a new energy station and the like, in particular to a high-frequency oscillation frequency division suppression and parameter design method of a flexible direct current transmission system.
Background
The flexible direct current transmission technology (high voltage direct current, HVDC) based on the voltage source type converter (voltage source converter, VSC), particularly the modularized multi-level converter (modular multilevel converter, MMC) provides a highly flexible and controllable solution for application scenes such as weak/passive alternating current grid voltage support, offshore wind power, cross-region interconnection, urban power supply and the like. However, link delay caused by measurement, transmission, calculation, execution, triggering and the like related to a digital control system of high-power electronic equipment is a root cause of high-frequency oscillation of flexible direct-current transmission engineering in recent years. After high-frequency oscillation, if the high-frequency harmonic cannot be eliminated in time in a period of time, an alternating current system is excited to generate high-frequency harmonic with larger amplitude, alternating voltage and alternating current are severely distorted, the running loss of the system is increased, and even electric equipment is broken down once to stop the system, so that great economic loss is caused, and the normal running of the power system is influenced.
The high-frequency oscillation suppression strategy is developed mainly from two aspects of an active damping suppression strategy and a passive damping suppression strategy, and the active damping suppression strategy can realize damping control through self-detected voltage and current without introducing an external circuit. However, the active damping controller with a single structure cannot eliminate the negative damping effect, and has certain difficulty in remodeling damping characteristics of a plurality of frequency intervals in a high frequency range, so that the suppression requirement of a plurality of high-frequency oscillation risk areas is difficult to realize. Therefore, the method has the defect of low adaptability in the case of coping with the variable operation conditions of the alternating current system. The passive suppression method has the advantage of completely eliminating the negative damping effect of the impedance high-frequency band, is generally configured at a common coupling point (point of common coupling, PCC), and has a structure of A type, second order, C type and the like. However, the passive filter introduces fundamental wave loss and fundamental wave reactive power, increases system equipment and operation and maintenance cost, and is relatively suitable for occasions with uncertain running modes of an alternating current system, uncertain external influence factors and higher reliability requirements. If the operation condition of the alternating current system connected with the flexible direct current transmission system is simpler and the number of the capacitive impedances of the alternating current system is smaller, the active damping suppression strategy has better competitiveness.
Therefore, aiming at the defects of simple structure, low applicability and power loss and reactive compensation introduced by the passive damping scheme in the existing single damping scheme, the invention provides a frequency division active damping suppression idea, a control strategy implementation mode and a parameter analysis calculation and design method thereof. According to the scheme, a plurality of active damping suppressors connected in parallel are adopted, each active damping suppressor connected in parallel remodels the impedance characteristic of a converter station in a frequency section, according to the high-frequency oscillation suppression requirements of a plurality of areas, an analytical calculation expression of a part of parameter selection range is provided, and the rest related parameters are optimally designed.
Disclosure of Invention
The invention aims at improving the damping characteristics of the flexible direct current converter station in a high-frequency range aiming at a plurality of high-frequency oscillation risk areas of an alternating current system on the basis of not influencing the control protection architecture of the original flexible direct current power transmission system and the parameters of the control system, enhancing the damping characteristics of the converter station in the high-frequency oscillation risk areas, inhibiting the occurrence of high-frequency oscillation and improving the running stability of the flexible direct current power transmission system.
The invention provides a method for restraining high-frequency oscillation frequency division and parameter design of a flexible direct current transmission system, which comprises the following steps:
S1, acquiring alternating current system impedance Z ac and minimum phase theta acmin at a PCC point;
S2, obtaining the impedance Z mmc of the flexible direct current converter station at the PCC point;
S3, determining the number H res of high-frequency oscillation risk areas and the frequency interval value of the high-frequency oscillation risk areas;
S4, determining the number N damp of the parallel frequency division damping controllers;
s5, acquiring input quantity of a frequency division damping controller;
S6, calculating and designing parameters of the frequency division damping controller;
s7, checking parameters of the frequency division damping controller;
S8, outputting compensation voltages e isd and e isq generated by the frequency division damping controller.
The ac system impedance Z ac and the minimum phase θ acmin at the PCC point are obtained in step S1, which is specifically as follows:
The method for obtaining the impedance characteristic curve of the alternating current system Z ac at the PCC point comprises two main types of scanning methods and calculation methods: the scanning method is to build an alternating current system in simulation software, input three-phase symmetrical alternating current voltage sources with different frequencies into PCC points, obtain current response conditions under all possible operation modes, calculate impedance values of the PCC points at all frequencies, and form a scanning curve Z ac. According to the distributed parameter model or a plurality of pi series equivalent models, the calculation method adopts a node voltage method or a one-by-one iteration method to calculate the equivalent impedance Z ac of the alternating current system. The frequency-dependent characteristic and the coupling effect of the circuit are difficult to consider by the calculation method, the universality is relatively poor, the calculated impedance characteristic curve is relatively strict, and the parameter calculation result of the frequency division damping controller is more conservative than that of the scanning method.
Further, in the frequency range of interest, the impedance characteristic curve of the ac system Z ac in all possible allowable operation modes is plotted, including an amplitude-frequency characteristic curve and a phase-frequency characteristic curve, and the minimum value θ acmin of the impedance phase of the ac system is found on the phase-frequency characteristic curve. This value is negative and is typically expressed in radians or in angular values over the first ac system capacitive frequency range.
The step S2 is to obtain the impedance Z mmc of the flexible dc converter station at the PCC point, which is specifically as follows:
scanning and calculation methods can also be used to obtain the impedance Z mmc of the flexible DC converter station. The impedance scanning method of the flexible direct current converter station is the same as the impedance scanning of the alternating current system, and the method is very suitable for a scene that a converter station control system model is not disclosed outside, namely a 'black box' converter station, and can consider the transmission process of actual link delay. The calculation rule is to calculate the relation between the voltage and the current at the PCC by adopting a linearization method according to the operation principle of the flexible direct current converter station. The calculation method can clearly know key influence links and influence rules of the impedance of the converter station, and lays a theoretical foundation for the provision of the frequency division damping controller. Likewise, the Z mmc impedance characteristic, including amplitude-frequency and phase-frequency characteristics, of the flexible dc converter station is plotted at different operating power levels, overlapping Z ac over the frequency range of interest.
The number H res of the risk areas for high-frequency oscillation and the frequency range value of the risk areas for high-frequency oscillation determined in step S3 are specifically as follows:
And (3) finding out intersection frequencies of all possible Z ac and Z mmc amplitude frequency characteristic curves in each capacitive frequency interval range of the alternating current system based on the Z ac and Z mmc amplitude frequency characteristic curves and the phase frequency characteristic curves obtained in the step S1 and the step S2. The number of intersections of each of Z ac and Z mmc is determined, and the maximum value of the number is taken as the number of high-frequency oscillation risk regions H res. Meanwhile, in the 1 st to H res th high-frequency oscillation risk interval ranges, the minimum and maximum values of the frequency of each interval are recorded, i.e., [ f min1,fmax1]、[fmin2,fmax2]、[fmin3,fmax3]、…,[fminHres,fmaxHres ], and each recorded interval value is slightly larger than the actual intersection frequency range, for example, the frequency error is relatively widened by 1%.
The number N damp of the frequency-division damping controllers in parallel determined in step S4 is specifically as follows:
Considering that the frequency division sections achieve the suppression of the high-frequency oscillation and each frequency section is dominated by one frequency division damping controller, according to the number H res of the high-frequency oscillation risk areas determined in step S3, step S4 may initially select the number N damp≥Hres of frequency division damping controllers, suggesting the selection of N damp＝Hres or N damp＝Hres +1.
The input quantity of the frequency division damping controller is obtained in the step S5, and the input quantity is specifically as follows:
And (3) acquiring d-axis and q-axis voltages or currents on the alternating-current side of the converter station based on the number of the frequency division damping controllers obtained in the step (S4), and inputting the voltages or currents to each frequency division damping controller, wherein three-phase voltage or current input quantities are converted into a dq coordinate system through Park conversion, and the d-axis and q-axis input quantities are obtained. When negative sequence control is considered, the input quantity of the positive sequence element damping controller is positive sequence d-axis and q-axis voltages or currents, and the negative sequence element is negative sequence d-axis and q-axis voltages or currents.
The parameters of the frequency division damping controller are calculated and designed in the step S6, and the parameters are specifically as follows:
the parameters of the frequency division damping controller are calculated or designed one by one from the first high-frequency oscillation risk area according to the impedance Z mmc expression of the flexible direct current converter station after the frequency division damping controller is considered. The implementation steps of the constraint conditions adopted in parameter calculation or design are shown in fig. 5, and the phase curve of the impedance real part cosine function should be in the range of positive damping interval as far as possible in the high-frequency oscillation risk interval [ f min1,fmax1]、[fmin2,fmax2]、[fmin3,fmax3]、…,[fminHres,fmaxHres ], namely [360 DEG k-90 DEG, 360 DEG k+90 DEG ] range, especially the first interval [ f min1,fmax1 ] and the second interval [ f min2,fmax2 ] must be satisfied, the third interval and above can be satisfied, the high-frequency oscillation does not exist, and the next check is also needed.
The checking of the parameters of the frequency division damping controller in step S7 is specifically as follows:
And in the parameter range calculated in the step S6, proper values are selected to carry out positive sequence and negative sequence impedance characteristic curve checking, namely whether the calculated or designed frequency division damping controller parameters can keep the stability of the system is verified.
The compensation voltages e isd and e isq generated by the output frequency-dividing damping controller in step S8 are specifically as follows:
Setting a compensation voltage output limit value E is_lim, and generating compensation voltages E isd and E isq after the acquired input quantity passes through a frequency division damping controller; comparing the compensation voltages E isd and E isq with the limiting value E is_lim, and directly outputting if the output value of the d-axis (q-axis) frequency division damping controller is within +/-E is_lim; otherwise, the clipping value E is_lim or-E is_lim is output. The output value is compared with the original current inner loop output value And/>And (5) superposing the materials and sending the materials to a valve control link.
Compared with the prior art, the invention has the beneficial effects that:
The high-frequency oscillation frequency division suppression and parameter design method for the flexible direct-current power transmission system is suitable for occasions with relatively determined running modes of the alternating-current system, does not need to add an additional passive damping filter, does not generate additional fundamental wave loss and fundamental wave reactive power, and does not need the operation and maintenance cost of additional equipment; the control strategy has clear logic and detailed steps, and the damping characteristics of the converter stations in the corresponding areas can be respectively improved aiming at a plurality of high-frequency oscillation risk areas, so that a plurality of high-frequency oscillations are accurately restrained, and the running stability of the system is improved; in addition, the frequency division damping control is suitable for high-frequency oscillation, medium-low frequency and subsynchronous/supersynchronous oscillation, and has wide applicability.
Drawings
Fig. 1 is a schematic diagram of an equivalent main circuit of a flexible dc power transmission system.
Fig. 2 is a schematic diagram of a flexible dc converter station inner loop control architecture taking into account a crossover damping controller.
Fig. 3 is a schematic diagram of an overall implementation of the frequency division damping control strategy.
Fig. 4 is a schematic diagram of a frequency division damping control strategy implementation flow proposed in the present invention.
FIG. 5 is a schematic diagram of the implementation steps for obtaining the parameter constraints of the frequency-divided active damping controller.
FIG. 6 is a specific example of a frequency division damping controller parameter constraint.
Detailed Description
Fig. 4 shows a method for suppressing high-frequency oscillation frequency division and designing parameters of a flexible direct current transmission system, which comprises the following steps:
S1, acquiring an alternating current system impedance Z ac and a minimum phase theta acmin at a PCC point:
The AC system impedance Z ac at the PCC point is obtained by two main methods, namely a scanning method and a calculation method. The example of the present invention carries out theoretical calculation analysis according to the equivalent main circuit of the flexible direct current transmission system shown in fig. 1, wherein the equivalent impedance of the alternating current system is represented by L g, and the value is used for representing the intensity of the alternating current system under different operation modes and can be changed according to the change of the operation modes.
Based on dq impedance modeling, a dq impedance matrix of the alternating current system can be obtained as follows under the condition of a plurality of pi series connection
Wherein the method comprises the steps of
Wherein N num is the number of pi in series, L length is the line length, r is the equivalent resistance of the unit length of the alternating current line, c is the equivalent capacitance of the unit length of the alternating current line, and L is the equivalent inductance of the unit length of the alternating current line. Adopting orthogonal transformation to transform dq impedance matrix of alternating current system into positive and negative sequence impedance matrix
In the formula, j is an imaginary unit, positive sequence impedance Z ac_pn (1, 1) in the formula (4) is taken as impedance Z ac of the alternating current system, and the stability of negative sequence impedance is checked at last. Let s=jω, ω=2pi f, i.e. the frequency range of interest, be substituted into Z ac; under the condition of drawing different effective impedances L g, the impedance Z ac of the alternating current system is plotted, and the minimum phase value in all possible curves is taken as theta acmin.
S2, obtaining the impedance Z mmc of the flexible direct current converter station at the PCC point:
The acquisition of the flexible dc converter station impedance Z mmc may also employ scanning and calculation methods. The positive sequence impedance Z mmc of the flexible direct current converter station in the high frequency range can be obtained by adopting the calculation method
Initially without a crossover damping controller, i.e. Z vir = 0, let s = j omega, omega = 2 pi f, f be the frequency range of interest, e.g. the [1,5000] hz range, a amplitude-frequency characteristic and a phase-frequency characteristic of Z mmc are obtained, which is plotted together with the diagram of step 1 in this example.
S3, determining the number H res of high-frequency oscillation risk areas and the frequency interval value of the high-frequency oscillation risk areas:
And (3) finding out intersection frequencies of all possible Z ac and Z mmc amplitude frequency characteristic curves in each capacitive frequency interval range of the alternating current system based on the Z ac and Z mmc amplitude frequency characteristic curves and the phase frequency characteristic curves obtained in the step 1 and the step 2 of the actual operation case. The number of each of Z ac and Z mmc was determined, and the maximum value of the number was taken as the number of high-frequency oscillation risk regions H res. Meanwhile, in the 1 st to H res th high-frequency oscillation risk interval ranges, the minimum and maximum values of the frequency of each interval are recorded, for example [ f min1,fmax1]、[fmin2,fmax2]、[fmin3,fmax3]、…,[fminHres,fmaxHres ] in the middle of fig. 6, and the value of each recorded interval is slightly larger than the actual intersection frequency range, for example, the frequency is relatively widened by 1%. For example, the three risk regions of the main circuit structure of fig. 1 in the first diagram of fig. 6 may be initially represented as [310,710] hz, [1320,1840] hz, and [2510,3055] hz, where H res =3 is considered.
S4, determining the number N damp of parallel frequency division damping controllers:
Considering that the frequency division is used for suppressing the high-frequency oscillation, and each frequency division is dominated by a frequency division damping controller, the number of high-frequency oscillation risk areas H res =3 determined according to the step 3 of the specific case, and the step 4 of the specific embodiment initially selects N damp＝Hres =3.
S5, acquiring input quantity of a frequency division damping controller:
The input is voltage or current, this example being current. The method comprises the steps of collecting three-phase instantaneous current i sabc at the alternating-current side of a flexible direct-current convertor station, realizing Park conversion by adopting phase theta pll output by a phase-locked loop, and calculating to obtain d-axis current i sd and q-axis current i sq, wherein a Park conversion formula is as follows:
In the formula (6), u sa、usb and u sc are respectively instantaneous phase voltages of the ABC phase, θ pll is a phase-locked loop output value, and t is time. The formula is transformed in the time domain, the subsequent analysis is in the s domain, namely the variable contains(s) Laplains transformation values representing the corresponding time domain physical quantity in the s domain, and the two values are in one-to-one correspondence, namely The remaining variables are similar and will not be described in detail. If the negative sequence control link is considered, the input quantity of the input damping controller is positive sequence d-axis current, positive sequence q-axis current, negative sequence d-axis current and negative sequence q-axis current.
S6, calculating and designing parameters of the frequency division damping controller:
The single frequency division damping controller is selected in various forms, such as a first-order high-pass filter, a second-order band-pass filter, a third-order band-pass filter, etc., and the example uses the second-order band-pass filter as the frequency division damping controller, and then
In the formula (7), N damp is the number of second-order band-pass damping controllers, k viri、ξviri and ω viri＝2πfviri are the gain, damping ratio and undamped oscillation angular frequency corresponding to the ith second-order frequency-division damping controller, and f viri is the ith undamped oscillation frequency.
Based on the formula (5), the relevant links are substituted to obtain
Wherein T de is the link delay size, a 3 and b 3 can be expressed as
Beta has the expression of
The phase expression (ωT de - β) of the impedance real part cosine function in the formula (8) is the focus
The parameters of the frequency division damping controller are calculated or designed one by one from the first high-frequency oscillation risk area according to the impedance Z mmc expression of the flexible direct current converter station after the frequency division damping controller is considered. The implementation steps of constraints adopted by parameter calculation or design are shown in fig. 5, and the parameter constraints of this example are shown in fig. 6.
The phase curve of the impedance real part cosine function is in the range of positive damping interval as far as possible in the high-frequency oscillation risk interval [ f min1,fmax1]、[fmin2,fmax2]、[fmin3,fmax3]、…,[fminHres,fmaxHres ], namely the range of [360 DEG k-90 DEG, 360 DEG k+90 DEG ], especially the first interval [ f min1,fmax1 ] and the second interval [ f min2,fmax2 ] are required to be satisfied, the third interval and above can be satisfied, the high-frequency oscillation is not satisfied, and the next check is also required. For the three risk areas of oscillations in this case, the impedance real part cosine function phase expression (ωt de - β) in the first two risk areas should satisfy expression (11), and the minimum frequency of the third risk area should satisfy expression (12).
The idea of the calculation is based on frequency division suppression, firstly, for the first high-frequency oscillation risk area, only the first frequency division damping controller 1 acts, and according to the phase increment possibly generated in the range of the link delay, the range which is closer to [360 DEG k-90 DEG, 360 DEG k+90 DEG ] is judged, and the phase jump possibly occurs as shown in the third sub-graph of fig. 6, but the damping characteristic is not affected. In order to remodel the phase of the frequency band within the range, a set of inequality can be obtained, constraint conditions of the inequality are solved, and a parameter constraint range of the first damping controller 1 can be obtained, iteration parameters are reasonably selected in the constraint range, intermediate parameters are selected to be initially used as parameters of the first damping controller 1 after one iteration is completed, and the parameters can be properly fine-tuned in the follow-up process.
Secondly, after the first frequency division damping controller 1 is calculated or designed, the second frequency division damping controller 2 is selected, and after the first damping controller is considered according to the impedance real part cosine function phase curve, the range which is closer to [360 DEG k-90 DEG, 360 DEG k+90 DEG ] is judged, so that a group of inequality can be obtained, and the constraint relation between the parameters of the second frequency division damping controller 2 and other parameters can be solved. By reasonably selecting the parameters of the iteration within this constraint, an intermediate parameter preliminary is selected as the parameter of the second damping controller 2 after completion of one iteration, which parameter may be suitably fine-tuned in the following. And (3) checking whether the maximum phase of the impedance of the converter station and the phase difference of the impedance of the alternating current system are within a range of 180 degrees from the second parameter calculation, and if so, carrying out subsequent parameter calculation of the high-frequency oscillation risk area frequency division damping controller.
Again, the calculation and design according to the above steps is continued until the maximum phase of the converter station impedance and the ac system impedance phase difference are within 180 ° or all risk areas have met the positive damping requirement. In general, the maximum phase of the converter station impedance and the phase difference of the ac system impedance are preferably satisfied within a range of 180 °.
S7, checking parameters of the frequency division damping controller:
And selecting proper values to carry out positive sequence and negative sequence impedance characteristic curve checking, namely verifying whether the calculated or designed frequency division damping controller parameters can keep the stability of the system.
According to the initial limiting condition 0<f vir1<fmin1, f vir1 is initially made to belong to [200,280] Hz, the initial range of the gain coefficient k vir1 is [250,140], the range of the damping ratio xi vir1 is calculated and selected to be [0.59,1.51], and the parameter range limitation of the first iteration is obtained. The second iteration takes f vir1 E [200,280] Hz and damping ratio xi vir1 E [0.59,1.51] as references, and solves the upper limit range of gain coefficient k vir1, wherein the damping ratio of gain coefficient k vir1 larger than 250 is mostly larger than 0.8. In view of the fact that an excessively large gain factor results in a very large variation in the impedance amplitude, the gain factor should be as close to the upper limit value as possible in order to reduce the influence. As parameters of the first divided active damping controller 1, a gain factor k vir1 = 200, a damping ratio ζ vir1 = 1.0, and an undamped oscillation frequency f vir1 = 260Hz may be selected.
In order to make the real part and the imaginary part of the second damping controller show monotone property, the undamped oscillation frequency f vir2 =1250 Hz is selected, the first damping controller parameter and the original controller parameter are substituted into the constraint relation of the second damping controller parameter obtained in the previous step, and under the condition that the constraint relation of the damping ratio and the gain coefficient at f min2 is preferentially met, the damping ratio zeta vir2 of the second damping controller is smaller than 0.35. This range of parameters does not take into account the influence of the first high-frequency oscillation risk region, it is recommended to choose a smaller damping ratio, for example ζ vir2 =0.1, and it is thus possible to calculate that k vir2 should be greater than 143.3, k vir2 =160. If the minimum phase theta acmin =86 DEG pi/180 DEG approximately 1.5 of the Yubei engineering alternating current system is taken, the inequality (12) is checked to obtain 0.0652 < 0.0699, which indicates that the high-frequency oscillation can not occur if the actual engineering parameter is above 2.5 kHz. However, when the present example considers a more severe working condition, tan (0.5pi+θ acmin) =0.0087, inequality (12) is not satisfied, and there is a risk of high-frequency oscillation above 2.5kHz in the present example.
In order to eliminate the influence, based on reducing the influence of the two previous risk frequency bands, a third frequency division damping controller 3 is added, wherein the parameter is k vir3＝50、ξvir3＝0.02、fvir3 =2500 Hz, and the stability requirement is checked to be met.
S8, outputting compensation voltages e isd and e isq generated by the frequency division damping controller:
The compensation voltage output limit value E is_lim is set. After the obtained i sd and i sq pass through the frequency division damping controller, compensation voltages e isd and e isq are generated; comparing the compensation voltages E isd and E isq with the limiting value E is_lim, and directly outputting if the output value of the d-axis (q-axis) frequency division damping controller is within +/-E is_lim; otherwise, the limiting value E is_lim or-E is_lim is output, specifically as shown in the formulas (13) and (14).
Where E is_lim may be 2% of the AC voltage rating.
Output values e isd and e isq are compared with the original current inner loop output valueAnd/>And (5) superposing the materials and sending the materials to a valve control link. The implementation of the patent specific case of the invention is completed.
Claims (4)
1. A method for suppressing high-frequency oscillation frequency division and designing parameters of a flexible direct current transmission system is characterized by comprising the following steps:
S1, acquiring alternating current system impedance Z ac and minimum phase theta acmin at a PCC point;
S2, obtaining the impedance Z mmc of the flexible direct current converter station at the PCC point;
S3, determining the number H res of high-frequency oscillation risk areas and the frequency interval value of the high-frequency oscillation risk areas; based on the Z ac and Z mmc amplitude frequency characteristic curves and the phase frequency characteristic curves obtained in the step S1 and the step S2, finding out intersection frequencies of all possible Z ac and Z mmc amplitude frequency characteristic curves in each capacitive frequency interval range of the alternating current system; determining the number of intersection points of each Z ac and Z mmc, taking the maximum value of the number as the number H res of the high-frequency oscillation risk areas; meanwhile, in the range of the 1 st to H res th high-frequency oscillation risk intervals, recording the frequency minimum value and the frequency maximum value of each interval, namely [ f min1,fmax1]、[fmin2,fmax2]、[fmin3,fmax3]、…,[fminHres,fmaxHres ];
S4, determining the number N damp of the parallel frequency division damping controllers; considering the frequency division to realize the suppression of the high-frequency oscillation, wherein each frequency division is dominated by a frequency division damping controller, and selecting the number N damp≥Hres of the frequency division damping controllers according to the number H res of the high-frequency oscillation risk areas determined in the step S3;
S5, acquiring input quantity of a frequency division damping controller; the input quantity can be voltage or current; converting the three-phase voltage or current input quantity into a dq coordinate system through Park conversion to obtain d-axis and q-axis input quantities; when negative sequence control is considered, the input quantity of the positive sequence link damping controller is positive sequence d-axis and q-axis voltages or currents, the negative sequence link is negative sequence d-axis and q-axis voltages or currents, and the d-axis and q-axis input quantity is input into N damp frequency division damping controllers;
S6, calculating and designing parameters of the frequency division damping controller; calculating or designing parameters of the frequency division damping controller from a first high-frequency oscillation risk area one by one according to an impedance Z mmc expression of the flexible direct current converter station after the frequency division damping controller is considered; finding out constraint conditions adopted by parameter calculation or design according to a stability criterion, namely that an impedance real part cosine function phase curve is in a positive damping interval range as far as possible in a high-frequency oscillation risk interval [ f min1,fmax1]、[fmin2,fmax2]、[fmin3,fmax3]、…,[fminHres,fmaxHres ], namely a range of [360 DEG k-90 DEG, 360 DEG k+90 DEG ];
S7, checking parameters of the frequency division damping controller; in the parameter range calculated in the step S6, proper values are selected to carry out positive sequence and negative sequence impedance characteristic curve checking, namely whether the calculated or designed frequency division damping controller parameters can keep the stability of the system is verified;
S8, outputting compensation voltages e isd and e isq generated by the frequency division damping controller.
2. The method for suppressing and designing parameters for high-frequency oscillation frequency division of a flexible direct current transmission system according to claim 1, wherein step S1 is performed by obtaining an ac system impedance Z ac and a minimum phase θ acmin at a PCC point; the method comprises two main steps of scanning and calculating the impedance Z ac of the alternating current system at the PCC point;
The scanning method is to build an alternating current system in simulation software, input three-phase symmetrical alternating current voltage sources with different frequencies into PCC points, obtain current response conditions under all possible operation modes, calculate impedance values of each frequency of the PCC points, and form a scanning curve Z ac; according to the distributed parameter model or a plurality of pi series equivalent models, the calculation method adopts a node voltage method or a one-by-one iteration method to calculate the equivalent impedance Z ac of the alternating current system; the frequency-dependent characteristic and the coupling effect of the circuit are difficult to consider in the calculation method, the universality is relatively poor, the calculated impedance characteristic curve is relatively strict, and the parameter calculation result of the frequency division damping controller is more conservative than that of the scanning method;
Meanwhile, in the frequency range of interest, drawing impedance characteristic curves of the alternating current system Z ac under all possible allowable operation modes, including an amplitude-frequency characteristic curve and a phase-frequency characteristic curve, and finding the minimum value theta acmin of the impedance phase of the alternating current system on the phase-frequency characteristic curve.
3. The method for suppressing and designing parameters for high-frequency oscillation frequency division of a flexible direct current transmission system according to claim 2, wherein step S2 is characterized in that the impedance Z mmc of the flexible direct current converter station at the PCC point is obtained;
The method for obtaining the impedance Z mmc of the flexible direct current converter station can also adopt a scanning method and a calculation method; the impedance scanning method of the flexible direct current converter station is the same as the impedance scanning of the alternating current system, and the method is very suitable for a scene that a converter station control system model is not disclosed outside, namely a 'black box' converter station, and can consider the transmission process of actual link delay; the calculation rule is to calculate the relation between the voltage and the current at the PCC by adopting a linearization method according to the operation principle of the flexible direct current converter station; the calculation method can clearly know key influence links and influence rules of the impedance of the converter station, and lays a theoretical foundation for the provision of the frequency division damping controller; likewise, the Z mmc impedance characteristic, including amplitude-frequency and phase-frequency characteristics, of the flexible dc converter station is plotted at different operating power levels, overlapping Z ac over the frequency range of interest.
4. The method for suppressing and designing parameters of high-frequency oscillation frequency division of a flexible direct current transmission system according to claim 1, wherein the compensating voltages e isd and e isq generated by the output frequency division damping controller in step S8 are as follows:
Setting a compensation voltage output limit value E is_lim; after the obtained input quantity passes through the frequency division damping controller, compensation voltages e isd and e isq are generated; comparing the compensation voltages E isd and E isq with the limiting value E is_lim, and directly outputting if the output value of the d-axis/q-axis frequency division damping controller is within +/-E is_lim; otherwise, outputting a limiting value E is_lim or-E is_lim; the output value is compared with the original current inner loop output value And/>And (5) superposing the materials and sending the materials to a valve control link.
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