CN109787267B - Judgment method for accessing doubly-fed wind turbine to weak receiving end system through VSC-HVDC - Google Patents

Judgment method for accessing doubly-fed wind turbine to weak receiving end system through VSC-HVDC Download PDF

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CN109787267B
CN109787267B CN201910214607.2A CN201910214607A CN109787267B CN 109787267 B CN109787267 B CN 109787267B CN 201910214607 A CN201910214607 A CN 201910214607A CN 109787267 B CN109787267 B CN 109787267B
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vsc
receiving end
end system
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边晓燕
杜中浩
邹黎明
李东东
周歧斌
林顺富
赵耀
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Shanghai University of Electric Power
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Abstract

The invention relates to a method for judging whether a double-fed fan is accessed to a weak receiving end system through VSC-HVDC (voltage source converter-high voltage direct current), which is used for judging whether the double-fed fan can be accessed to the weak receiving end system through VSC-HVDC or not, and comprises the following steps: 1) establishing a structural model of accessing the doubly-fed wind turbine into the weak receiving end system through VSC-HVDC; 2) determining a control mode of the VSC; 3) and taking the critical short circuit ratio CSCR as a judgment index, and judging whether the doubly-fed wind turbine can be accessed into the weak receiving end system through VSC-HVDC or not. Compared with the prior art, the method has the advantages of comprehensive consideration, combined control, quantitative judgment and the like.

Description

Judgment method for accessing doubly-fed wind turbine to weak receiving end system through VSC-HVDC
Technical Field
The invention relates to a method for judging whether a double-fed wind power plant of VSC-HVDC is incorporated into a weak alternating current system, in particular to a method for judging whether a double-fed fan is accessed into a weak receiving end system through VSC-HVDC.
Background
With the development of wind power generation technology, the grid connection problem of wind power plants through direct current transmission is widely concerned. The commutation voltage of the traditional direct current transmission technology (LCC-HVDC) in the grid connection process is provided by a receiving end system, commutation failure is easy to occur, and the system has great demand on reactive power. The flexible direct current transmission technology (VSC-HVDC) based on the voltage source type converter does not need phase-change voltage, can realize long-distance power transmission, and can be accessed to a weak receiving end system or even a passive network.
The strong and weak degree of a receiving end system has great influence on the stability of VSC-HVDC, and in the prior art, a Short Circuit Ratio (SCR) is used as a judgment index of the strong and weak degree of a receiving end alternating current system, when the SCR >3 is a strong system, 2< SCR <3 is a weak system, and SCR <2 is an extremely weak system. The SCR for dividing the strong and weak system is the Boundary Short Circuit Ratio (BSCR), the SCR for dividing the weak and weak system is the Critical Short Circuit Ratio (CSCR), the control mode of the converter is limited, and the VSC-HVDC can not be connected with the weak receiving end system.
When the SCR connected to the receiving end system is lower than a critical value, a current vector control system of the VSC converter cannot be normally decoupled, so that the VSC cannot keep a stable operation state. The VSC-HVDC stable operation area research which is connected with the weak alternating current system discusses that parameters of a receiving end system change along with SCR in two control modes, so that a stable operation area of the VSC-HVDC connected with the weak receiving end system is obtained, but the influence of a power generation end on the operation characteristics of the system is not considered. The operating characteristics of VSC-HVDC access weak alternating current system based on critical short-circuit ratio explores influencing factors restricting VSC-HVDC connection to the weak alternating current system from the perspective of maximum transmission power, and indicates that the accessed alternating current system needs to meet CSCR >1.4, but the calculation process is ideal. Critical operating characteristics of a remote sea wind power plant connected with a weak receiving end system through VSC-HVDC are researched based on a control mode of DFIG and VSC, a new thought capable of adopting reactive compensation measures is provided, and an example is not given.
At present, the research on the operation characteristics of VSC (voltage source converter) connected to weak alternating current system based on Short Circuit Ratio (SCR) has been discussed. Some studies demarcate the Boundary Short Circuit Ratio (BSCR) and Critical Short Circuit Ratio (CSCR) of strong, weak and very weak ac systems and indicate that VSC-HVDC cannot incorporate very weak ac systems below the critical short circuit ratio. Some studies discuss VSC control modes of a weak alternating current system, and obtain stable operation regions and critical short circuit ratios under different control modes. However, most of the existing researches do not give specific values of the critical short-circuit ratio, and do not consider the situation when the double-fed wind turbine is used as a power generation device.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a method for judging that a double-fed wind turbine is accessed into a weak receiving end system through VSC-HVDC.
The purpose of the invention can be realized by the following technical scheme:
a method for judging whether a doubly-fed wind turbine is accessed to a weak receiving end system through VSC-HVDC (voltage source converter-high voltage direct current) is used for judging whether the doubly-fed wind turbine can be accessed to the weak receiving end system through VSC-HVDC or not, and comprises the following steps:
1) establishing a structural model of accessing the doubly-fed wind turbine into the weak receiving end system through VSC-HVDC;
2) determining a control mode of the VSC;
3) and taking the critical short circuit ratio CSCR as a judgment index, and judging whether the doubly-fed wind turbine can be accessed into the weak receiving end system through VSC-HVDC or not.
In the step 2), a constant power control mode is adopted to keep the transmission power of the VSC unchanged.
In the step 3), the method for acquiring the critical short-circuit ratio CSCR includes the following steps:
31) obtaining a power flow equation set of the VSC accessed to the weak receiving end system, and taking a real number solution of simplified and simultaneous power flow equations as a constraint condition capable of stably running after the VSC is accessed to the weak receiving end system;
32) setting safety constraint conditions of upper and lower voltage limits of a weak receiving end system;
33) setting the value range of the SCR, and respectively acquiring the corresponding Q for the value of each SCRr1，Qr2，QhAnd QlValue, when min (Q)r1，Qr2，Qh)＝QlIf so, the corresponding SCR value at this time is taken as the critical short-circuit ratio CSCR.
In the step 31), the expression of the flow equation set of the VSC accessing the weak receiving end system is as follows:
UsUtcos＝Ut 2-QX-PR
UsUtsin＝PX-QR
after simplification, the following steps are simultaneously carried out:
Figure GDA0002482620690000031
wherein, UtFor VSC AC bus voltage, UsThe VSC power supply is an ideal voltage source, X is an imaginary part of equivalent impedance of a VSC access receiving end system, Q is reactive power transmitted by the VSC, P is active power transmitted by the VSC and is a phase angle, and R is a real part of the equivalent impedance of the VSC access receiving end system.
In the step 32), the safety constraint conditions of the upper and lower voltage limits of the weak receiving end system are as follows:
Utmax≥Ut≥Utmin
wherein, Utmax、UtminRespectively the upper and lower limits of the weak receiving end system voltage.
In the step 33), QhAnd QlRespectively corresponding reactive power values of the upper limit and the lower limit of the voltage of the weak receiving end system;
said Qr1The values are defined as: with the reduction of the reactive power value Q, the power flow equation of the weak receiving end system tends to be unsolved, and when the critical solution of the power flow equation exists, the corresponding Q value is recorded as Qr1；
Said Qr2The values are defined as: in the solution of the power flow equation of the weak receiver system, the q-axis current isqThe Q value corresponding to the minimum inflection point is recorded as Qr2。
In the step 33), the value range of the reactive power value Q is as follows:
min(Qr1,Qr2,Qh)≥Q≥Ql。
in the step 3), when the short circuit ratio SCR value of the weak receiving end system is smaller than the critical short circuit ratio CSCR, it is determined that the doubly-fed wind turbine cannot be accessed to the weak receiving end system through the VSC-HVDC, and when the short circuit ratio SCR value of the weak receiving end system is larger than the critical short circuit ratio CSCR, it is determined that the doubly-fed wind turbine can be accessed to the weak receiving end system through the VSC-HVDC.
Compared with the prior art, the invention has the following advantages:
firstly, the invention fully considers the combined control strategy of the DFIG fan and the VSC converter.
Secondly, the invention considers various operation constraint conditions to obtain the critical short circuit ratio of the VSC accessed to the weak receiving end system.
And thirdly, considering the change of the intensity degree of the system accessed to the receiving end caused by the operating characteristics of the fan, analyzing the reason, and giving an effective short circuit ratio added with reactive compensation as an intensity judgment index under the working condition.
Drawings
Fig. 1 shows a VSC access weak ac system architecture.
FIG. 2 shows U when SCR takes different valuestAs a function of Q.
FIG. 3 shows the values of i for SCR at different valuesadAs a function of Q.
FIG. 4 shows the values of i for SCR at different valuessqAs a function of Q.
FIG. 5 is Qr1，Qr2，Qh，QlRespectively with the variation law of the SCR.
FIG. 6 is a grid connection structure diagram of a doubly-fed wind power plant through VSC-HVDC.
FIG. 7 is the wind speed small disturbance stability of the receiving end system with different short circuit ratios.
Fig. 8 shows the voltage variation of the PCC point bus for different short circuit ratios.
Fig. 9 is a rotor side converter control diagram.
FIG. 10 is a flow chart of a method of the present invention.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
Examples
As shown in fig. 10, the present invention provides a method for determining that a doubly-fed wind turbine accesses a weak receiving end system through VSC-HVDC, including the following steps:
step 1: a VSC is connected with an expression of a weak system-related variable in a constant power control mode, a constraint condition criterion for ensuring the safe and stable operation of the system is provided, a critical operation point of the stable operation of the system can be obtained, and a method for solving a reactive critical value and a critical short-circuit ratio is summarized.
Step 2: a mathematical model that DFIG is used as a power generation device and is accessed into a weak receiving end system through VSC-HVDC is established, a proper control strategy is selected, and the reason that the critical short circuit ratio of the accessed receiving end system cannot reach a theoretical value due to the operation characteristics of a double-fed fan is pointed out.
And step 3: by means of the effective critical short-circuit ratio as a judgment index and according to the reactive local compensation principle, the matching between the critical value and the theoretical value of the DFIG connected to the receiving end system as the power generation device is realized, and meanwhile, the critical value solving method provided above is still established under the working condition.
The method comprises the following specific steps:
1. firstly, a power flow equation set of a VSC access weak alternating current system is obtained:
UsUtcos＝Ut 2-QX-PR
UsUtsin＝PX-QR
2. in an actual receiving end system, frequent switching of power can generate loss and the problem of system instability, and a fixed power control mode is selected to ensure that VSC transmission power is unchanged as far as possible by combining double-fed fan converter control. To simplify the analysis process, only ignoring the ac system resistance, i.e. when the impedance angle Φ is 90 °, we have:
UsUtcos＝Ut 2-QX
UsUtsin＝PX
3. in order to ensure that a flow equation block solution exists after the VSC is accessed into the weak alternating current system, the following condition constraints are met:
Δ＝Us 4+4Us 2QX-4P2X2≥0
Figure GDA0002482620690000051
Figure GDA0002482620690000052
Figure GDA0002482620690000053
the safety constraint conditions of the upper and lower limits of the voltage of the receiving end system need to be met:
Utmin≤Ut≤Utmax
usually UtminTake 0.95pu, Utmax1.05pu was taken.
The VSC is connected into the weak alternating current system structure, and the short circuit ratio is taken as a measure of the strength index of the alternating current system:
Figure GDA0002482620690000054
when the system model parameter is Sc＝1.00pu，UsAnd (4) continuously changing the SCR of the receiving end system by controlling the active power output by the VSC (voltage source converter) when X is 1.00 pu.
When SCR is constant, UtIs in direct proportion to Q, and when Q is a constant value, isdIn inverse proportion to SCR, i.e., in direct proportion to P.
As the Q value is reduced, the power flow equation set also tends to be unsolved, and the Q when the power flow critical is solved is made to be Qr1。
In ensuring the system voltage UtUnder the full-voltage upper and lower limit safe operation condition, the range of the VSC adjustable Q is also reduced along with the reduction of the SCR, and U is addedmaxAnd UminThe corresponding upper and lower reactive power limits are set as QhAnd Ql. When SCR is 1, Ut>1.05, the system does not meet the safe operation constraint and is irrelevant to the Q value; when SCR is 1.5, Qh＝0.29pu，Ql0.23 pu; when SCR is 2, Qh＝0.18pu，Ql＝0.09pu；
In fig. 4, when SCR is 1, isqDi is present at Q ═ 0.77pusqThe minimum value when/dQ is 0, the Q value corresponding to the inflection point is Qr2. When the SCR is larger, di can be ensuredsqthe/dQ is greater than zero, and Q is absentr2。
min(Qr1，Qr2，Qh)≥Q≥Ql
In summary, it can be summarized that the CSCR solution method:
according to Ut、isdAnd isqAlong with the variation curve of Q, different SCR are calculated by the 4 parts to obtain Qr1，Qr2，Qh，QlRespectively with the variation law of the SCR. When min (Q)r1，Qr2，Qh)＝QlWhen Q is obtained, Q can be regarded as QrThe corresponding SCR is defined as CSCR.
Thus, 4 reactive powers can be obtainedCritical operating conditions under beam conditions. The VSC-HVDC access U controlled by the double-fed fan through the fixed active power and the fixed reactive power can be obtained more intuitively by combining the graph and the tablesTheoretical calculation of critical point data Q for weak receiver systems of 1.00pu and 1.00pu Xr＝0.45，CSCR＝1.21。
Impact of doubly-fed wind turbine on CSCR:
a simulation model that DFIG is connected into a weak receiving end system through VSC-HVDC is built in DIgSILENT/PowerFactory, and wind speed small-disturbance stability simulation of different short circuit ratio receiving end systems is observed while the formula stable operation constraint condition is met, as shown in FIG. 7.
Three-phase grounding short circuit is arranged on a bus on the side of the rectifying converter, the fault is removed after 0.15s, and the voltage change of the PCC point bus corresponding to different short circuit ratios is shown in FIG. 8.
Neglecting transient state change of a direct current system, namely direct current bus voltage is a fixed value, a Rotor Side Converter (RSC) adopts stator flux linkage directional control based on a power control method, the aim of decoupling active power and reactive power can be achieved, a double closed-loop control mode is selected, and the outer ring is active power PsSum voltage UsAnd controlling, wherein the inner ring is controlled by vector current. The simple control block diagram is shown in FIG. 9, in which xαα ═ 1,2,3, 4;, k, the converter state variablepα、kiαRefers to the corresponding proportional integral constant; i.e. idr、iqrAnd Udr、UqrCorresponding to the dq-axis component of the RSC current voltage.
The control structure of a Grid Side Converter (GSC) is similar to that of a rotor side converter, an outer ring controls direct-current voltage, an inner ring controls rotor side induced current, and finally the output power expression of the DFIG is as follows:
Figure GDA0002482620690000061
when the voltage direction of the receiving end system is the same as the d-axis direction, U in the formula (2)d＝UN，UqWhen the value is 0, then:
Figure GDA0002482620690000062
combining a short-circuit ratio definition formula, a VSC power flow equation and a double-fed fan output power expression formula, and obtaining the following results in a simultaneous manner:
Figure GDA0002482620690000063
Figure GDA0002482620690000064
the invention introduces the concept of effective short-circuit ratio, and implements reactive local compensation on the receiving end system, so that the CSCR reaches the theoretical value. The generating capacity of the fan is used as a controllable variable of a simulation experiment, and the CSCR value which enables the system to meet safe and stable operation conditions is obtained by adjusting the impedance value of a receiving end system.

Claims (6)

1. A method for judging whether a doubly-fed wind turbine accesses a weak receiving end system through VSC-HVDC is used for judging whether the doubly-fed wind turbine can access the weak receiving end system through VSC-HVDC or not, and is characterized by comprising the following steps:
1) establishing a structural model of accessing the doubly-fed wind turbine into the weak receiving end system through VSC-HVDC;
2) determining a control mode of the VSC;
3) the critical short circuit ratio CSCR is used as a judgment index, and whether the doubly-fed wind turbine can be accessed into a weak receiving end system through VSC-HVDC or not is judged according to the judgment index, and the method for acquiring the critical short circuit ratio CSCR comprises the following steps:
31) obtaining a power flow equation set of the VSC accessed to the weak receiving end system, and taking a real number solution of simplified and simultaneous power flow equations as a constraint condition capable of stably running after the VSC is accessed to the weak receiving end system;
32) setting safety constraint conditions of upper and lower voltage limits of a weak receiving end system;
33) setting the value range of the SCR, and respectively acquiring the corresponding Q for the value of each SCRr1，Qr2，QhAnd QlValue, when min (Q)r1，Qr2，Qh)＝QlIf so, the corresponding SCR value at this time is taken as the critical short-circuit ratio CSCR,
said QhAnd QlRespectively corresponding reactive power values of the upper limit and the lower limit of the voltage of the weak receiving end system;
said Qr1The values are defined as: with the reduction of the reactive power value Q, the power flow equation of the weak receiving end system tends to be unsolved, and when the critical solution of the power flow equation exists, the corresponding Q value is recorded as Qr1；
Said Qr2The values are defined as: in the solution of the power flow equation of the weak receiver system, the q-axis current isqThe Q value corresponding to the minimum inflection point is recorded as Qr2。
2. A method for determining whether a doubly-fed wind turbine accesses a weak receiving system through VSC-HVDC according to claim 1, wherein in the step 2), the transmission power of the VSC is kept unchanged by using a constant power control mode.
3. The method for determining the access of the doubly fed wind turbine to the weak receiving end system through the VSC-HVDC system according to claim 1, wherein in the step 31), the expression of the power flow equation system of the VSC accessing the weak receiving end system is:
UsUtcos＝Ut 2-QX-PR
UsUtsin＝PX-QR
after simplification, the following steps are simultaneously carried out:
Figure FDA0002467782890000011
wherein, UtFor VSC AC bus voltage, UsThe VSC power supply is an ideal voltage source, X is an imaginary part of equivalent impedance of a VSC access receiving end system, Q is reactive power transmitted by the VSC, P is active power transmitted by the VSC and is a phase angle, and R is a real part of the equivalent impedance of the VSC access receiving end system.
4. A method for determining whether a doubly-fed wind turbine accesses a weak receiving end system through VSC-HVDC in accordance with claim 3, wherein in the step 32), the safety constraints of the upper and lower voltage limits of the weak receiving end system are:
Utmax≥Ut≥Utmin
wherein, Utmax、UtminRespectively the upper and lower limits of the weak receiving end system voltage.
5. The method for determining the access of the doubly-fed wind turbine to the weak receiving end system through the VSC-HVDC converter according to claim 1, wherein in the step 33), the value range of the reactive power value Q is:
min(Qr1,Qr2,Qh)≥Q≥Ql。
6. the method for determining whether the doubly-fed wind turbine accesses the weak receiving end system through the VSC-HVDC converter according to claim 1, wherein in the step 3), when the short circuit ratio SCR value of the weak receiving end system is smaller than the critical short circuit ratio CSCR, it is determined that the doubly-fed wind turbine cannot access the weak receiving end system through the VSC-HVDC converter, and when the short circuit ratio SCR value of the weak receiving end system is larger than the critical short circuit ratio CSCR, it is determined that the doubly-fed wind turbine can access the weak receiving end system through the VSC-HVDC converter.
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