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

Abstract

The present invention relates to a method for determining whether a double-fed fan can be connected to a weak receiving end system via VSC-HVDC, which is used to determine whether a double-fed fan can be connected to a weak receiving end system through VSC-HVDC. The method includes the following steps: 1) establishing a double-fed fan 2) Determine the control method of VSC; 3) Use the critical short-circuit ratio CSCR as the judgment index, and judge whether the DFIG can be connected to the weak receiving end through VSC-HVDC. receiving system. Compared with the prior art, the present invention has the advantages of comprehensive consideration, joint control, quantitative judgment and the like.

Description

A judging method for a doubly-fed fan connected to a weak receiving end system via VSC-HVDC

technical field

The invention relates to the incorporation of a VSC-HVDC doubly-fed wind farm into a weak AC system, in particular to a method for judging that a doubly-fed wind turbine is connected to a weak receiving end system via the VSC-HVDC.

Background technique

With the development of wind power generation technology, the grid connection of wind farms via DC transmission has received extensive attention. The commutation voltage of the traditional direct current transmission technology (LCC-HVDC) is provided by the receiving end system during the grid connection process, which is prone to commutation failure, and the system has a large demand for reactive power. The flexible direct current transmission technology (VSC-HVDC) based on voltage source converters does not require commutation voltage, can realize long-distance power transmission, and can be connected to weak receiving end systems or even passive networks.

The strength of the receiving end system has a great influence on the stability of VSC-HVDC. In the prior art, the short circuit ratio (SCR) is used as the judgment index of the strength of the receiving end AC system. When SCR>3 is a strong system, 2 <SCR<3 is a weak system, and SCR<2 is a very weak system. The SCR that divides the strong and weak systems is the boundary short-circuit ratio (BSCR), and the SCR that divides the weak and extremely weak systems is the critical short-circuit ratio (CSCR). .

When the SCR connected to the receiving end system is lower than the critical value, the current vector control system of the VSC converter cannot be decoupled normally, so that the VSC cannot maintain a stable operation state. "Study on the Stable Operation Region of VSC-HVDC Connected to Weak AC System" discusses the variation law of the parameters of the receiving end system with SCR under the two control methods of VSC, and then obtains the stable operation region of the VSC-HVDC connected weak receiving end system. However, the influence of the power generation end on the operating characteristics of the system is not considered. "Operating Characteristics of VSC-HVDC Connected to Weak AC System Based on Critical Short-Circuit Ratio" explores the influencing factors that restrict the connection of VSC-HVDC to weak AC system from the perspective of maximum transmission power, and points out that the connected AC system must meet the requirements of CSCR> 1.4, but its calculation process is more ideal. "Critical Operating Characteristics of Offshore Wind Farms Connected to Weak Receiving Terminals via VSC-HVDC" Based on the control methods of DFIG and VSC, the critical operating characteristics of systems connected to weak receiving terminals are studied, and new ideas for reactive power compensation measures are proposed. , but no examples are given.

At present, there have been researches on the operation characteristics of VSC (voltage source converter) connected to weak AC system based on short circuit ratio (SCR). Some studies divide the boundary short circuit ratio (BSCR) and critical short circuit ratio (CSCR) of strong AC system, weak AC system and very weak AC system, and point out that VSC-HVDC cannot be incorporated into very weak AC system below the critical short circuit ratio. Some studies have discussed the VSC control method connected to the weak AC system, and obtained the stable operation area and critical short-circuit ratio under different control methods. However, most of the existing studies do not give the specific value of the critical short-circuit ratio, and do not consider the situation when the DFIG is used as a power generation device.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a judging method for connecting a doubly-fed fan to a weak receiving end system via VSC-HVDC in order to overcome the above-mentioned defects in the prior art.

The object of the present invention can be realized through the following technical solutions:

A method for determining whether a doubly-fed wind turbine can be connected to a weak receiving end system via VSC-HVDC is used to determine whether a doubly-fed wind turbine can be connected to a weak receiving end system via VSC-HVDC, comprising the following steps:

1) Establish the structural model of the DFIG connected to the weak receiving end system via VSC-HVDC;

2) Determine the control mode of VSC;

3) The critical short-circuit ratio CSCR is used as the judgment index, and it is judged whether the DFIG can be connected to the weak receiving end system through VSC-HVDC.

In the step 2), the constant power control mode is adopted to keep the transmission power of the VSC unchanged.

In described step 3), the acquisition method of critical short-circuit ratio CSCR comprises the following steps:

31) Obtain the power flow equations of the VSC connected to the weak receiving end system, and use the simplified simultaneous power flow equations to have a real number solution as a constraint condition for stable operation after connecting to the weak receiving end system;

32) Set the safety constraints of the upper and lower limits of the weak receiving end system voltage;

33) Set the value range of SCR. For each SCR value, obtain its corresponding Q _r1 , Q _r2 , Q _h and Q _l values respectively, when min(Q _r1 , Q _r2 , Q _h )=Q _l , the corresponding SCR value at this time is taken as the critical short-circuit ratio CSCR.

In the described step 31), the expression of the power flow equation system of the VSC access weak receiving end system is:

U _s U _t cosδ=U _t ² -QX-PR

U _s U _t sinδ=PX-QR

After simplification, there are:

Among them, U _t is the AC bus voltage of the VSC, U _s is the ideal voltage source, X is the imaginary part of the equivalent impedance of the VSC connected to the receiving end system, Q is the reactive power transmitted by the VSC, P is the active power transmitted by the VSC, δ is the phase angle, and R is the real part of the equivalent impedance of the VSC connected to the receiving end system.

In the described step 32), the safety constraints of the upper and lower limits of the weak receiving terminal system voltage are:

U _tmax ≥U _t ≥U _tmin

Among them, U _tmax and U _tmin are the upper and lower limits of the weak receiving end system voltage, respectively.

In the described step 33), the Q _h and Q _l are respectively the reactive power values corresponding to the upper and lower limits of the weak receiving end system voltage;

The definition of the Q _r1 value is: with the reduction of the reactive power value Q, the power flow equation of the weak receiving end system tends to have no solution, and the corresponding Q value when the power flow equation has a critical solution is recorded as Q _r1 ;

The Q _r2 value is defined as: in the solution of the power flow equation of the weak receiving end system, the Q value corresponding to the minimum inflection point of the q-axis current i _sq is recorded as Q _r2 .

In the described step 33), the value range of the reactive power value Q is:

min(Q _r1 , Q _r2 , Q _h )≥Q≥Q _l .

In the step 3), when the short circuit ratio SCR value of the weak receiving end system is less than the critical short circuit ratio CSCR, it is determined that the DFIG cannot be connected to the weak receiving end system via the VSC-HVDC, and when the short circuit ratio of the weak receiving end system is When the SCR value is greater than the critical short-circuit ratio CSCR, it is determined that the DFIG can be connected to the weak receiving end system via VSC-HVDC.

Compared with the prior art, the present invention has the following advantages:

1. The present invention fully considers the joint control strategy of the DFIG fan and the VSC converter.

2. The present invention considers a variety of operational constraints, and obtains the critical short-circuit ratio of the VSC connected to the weak receiving end system.

3. The present invention considers the change in the strength of the receiving end system due to the operating characteristics of the fan, analyzes the reason, and provides the effective short-circuit ratio with reactive power compensation as the strength judging index under this working condition.

Description of drawings

Figure 1 shows the structure of the VSC access weak AC system.

Figure 2 shows the variation of U _t with Q when SCR takes different values.

Figure 3 shows the variation of i _ad with Q when SCR takes different values.

Figure 4 shows the variation of i _sq with Q when SCR takes different values.

Fig. 5 is the variation law of Q _r1 , Q _r2 , Q _h , Q _l and SCR respectively.

Figure 6 is a grid-connected structure diagram of a doubly-fed wind farm via VSC-HVDC.

Figure 7 shows the small-turbulence stability of wind speed of the receiving-end system with different short-circuit ratios.

Figure 8 shows the voltage changes of the PCC point busbars corresponding to different short-circuit ratios.

FIG. 9 is a control diagram of a rotor-side inverter.

FIG. 10 is a flow chart of the method of the present invention.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

Example

As shown in FIG. 10 , the present invention provides a method for determining that a doubly-fed fan is connected to a weak receiving end system via VSC-HVDC, including the following steps:

Step 1: Through the VSC in the constant power control mode, the expression of the relevant variables of the weakly affected system is connected, and the constraint condition criterion for ensuring the safe and stable operation of the system is proposed. The critical operating point of the stable operation of the system can be obtained, and then the reactive power How to find critical value and critical short-circuit ratio.

Step 2: Set up a mathematical model that uses DFIG as a power generation device to connect to the weak receiving end system via VSC-HVDC, select an appropriate control strategy, and point out that due to the operating characteristics of the DFIG, the critical short-circuit ratio of the connected receiving end system cannot be reached. The reason for the theoretical value.

Step 3: Using the effective critical short-circuit ratio as the judging index, according to the principle of reactive power compensation in situ, the fitting of the critical value and the theoretical value when DFIG is connected to the receiving end system as a power generation device is realized, and it also proves the critical value proposed above. The solution method still holds in this case.

Specific steps are as follows:

1. First, obtain the power flow equations of the VSC connected to the weak AC system:

U _s U _t cosδ=U _t ² -QX-PR

U _s U _t sinδ=PX-QR

2. In the actual receiver system, the frequent switching of power will cause loss and system instability. Combined with the control of the double-fed fan converter, the constant power control method is selected to ensure that the transmission power of the VSC is as constant as possible. In order to simplify the analysis process, only consider the case where the resistance of the AC system is ignored, that is, when the impedance angle φ=90°, there are:

U _s U _t cosδ=U _t ² -QX

U _s U _t sinδ=PX

3. In order to ensure the existence of the solution of the power flow equations after the VSC is connected to the weak AC system, the following conditions and constraints are satisfied:

Δ=U _s ⁴ +4U _s ² QX-4P ² X ² ≥0

The safety constraints of the upper and lower limits of the receiving end system voltage need to be met:

U _tmin ≤U _t ≤U _tmax

Usually U _tmin takes 0.95pu, U _tmax takes 1.05pu.

4. The VSC is connected to the weak AC system structure, and the short-circuit ratio is used as an indicator to measure the strength of the AC system:

When the system model parameters are set as S _c =1.00pu, U _s =1.00pu, X=1.00pu, by controlling the active power output by the VSC, the SCR of the receiving end system is continuously changed.

When SCR is a fixed value, U _t is proportional to Q, and when Q is a fixed value, i _sd is inversely proportional to SCR, that is, proportional to P.

As the value of Q decreases, the power flow equations also tend to have no solution, and the Q when the critical power flow has a solution is Q _r1 .

Under the condition of ensuring the safe operation of the upper and lower limits of the full voltage of the system voltage U _t , the range of the adjustable Q of the VSC also decreases with the decrease of the SCR. The upper and lower limits of the reactive power corresponding to U _max and U _min are set as Q _h and Q _l respectively. . When SCR=1, U _t >1.05 is constant, that is, the system does not meet the safe operation constraints and has nothing to do with the value of Q; when SCR=1.5, Q _h = 0.29pu, Q _l = 0.23pu; when SCR=2, Q _h = 0.18pu, Q _l = 0.09pu;

In Fig. 4, when SCR=1, i _sq has a minimum value at Q=0.77pu when di _sq /dQ=0, and the Q value corresponding to this inflection point is assumed to be Q _r2 . When the SCR is large, it can be guaranteed that di _sq /dQ is greater than zero, and there is no Q _r2 .

min(Q _r1 , Q _r2 , Q _h )≥Q≥Q _l

In summary, the solution method of CSCR can be summarized as follows:

According to the change curve of U _t , _isd and i _sq with Q, different SCRs are calculated in the above four parts, and the change law of Q _r1 , Q _r2 , Q _h , Q _l and SCR can be obtained respectively. When min(Q _r1 , Q _r2 , Q _h )=Q _l , the obtained Q can be identified as Q _r , and the corresponding SCR is defined as CSCR.

Thus, the critical operating state under the four reactive power constraints can be obtained. Combining the figures and tables, the theoretical calculation value of the critical point data of the doubly-fed fan can be intuitively obtained when the VSC-HVDC controlled by the constant active and constant reactive power is connected to the weak receiving end system of U _s =1.00pu and X = 1.00pu Qr=0.45, _CSCR =1.21.

The impact of double-fed fans on CSCR:

In DIgSILENT/PowerFactory, the simulation model of DFIG connected to the weak receiving end system via VSC-HVDC is built. While satisfying the stable operation constraints, the stability simulation of small wind speed disturbance of the receiving end system with different short-circuit ratios is observed, as shown in Figure 7. Show.

The three-phase grounding short circuit is set on the busbar on the rectifier-converter side, and the fault is removed after 0.15s. The voltage changes of the busbar at the PCC point corresponding to different short-circuit ratios are shown in Figure 8.

Ignore the transient changes of the DC system, that is, the DC bus voltage is a constant value, and the rotor-side converter (RSC) adopts the stator flux-linkage directional control based on the power control method, which can achieve the purpose of decoupling the active power and reactive power. Select DoubleClosed -loop control mode, the outer loop is active power P _s and voltage U _s control, and the inner loop is vector current control. The simple control block diagram is shown in Figure 9. In the figure, x _α is the state variable of the inverter, α=1, 2, 3, 4; k _pα and k _iα refer to the corresponding proportional integral constants; i _dr , i _qr and U _dr , U _qr Corresponds to the dq-axis components of the RSC current and voltage.

The control structure of the grid-side converter (GSC) is similar to that of the rotor-side converter. The outer loop controls the DC voltage and the inner loop controls the rotor-side induced current. Finally, the output power of the DFIG is expressed as:

When the voltage direction of the receiving end system is the same as the d-axis direction, U _d =U _N and U _q =0 in formula (2), then:

Combined with the definition of the short-circuit ratio, the power flow equation of the VSC, and the expression of the output power of the doubly-fed fan, the three can be obtained simultaneously and then:

The invention introduces the concept of effective short-circuit ratio, and performs on-site reactive power compensation for the receiving end system, so that the CSCR can reach the theoretical value. Taking the power generation of the wind turbine as the controllable variable of the simulation experiment, and then adjusting the impedance value of the receiving end system, the CSCR value that can make the system meet the safe and stable operation conditions is obtained.

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 SCR_r1，Q_r2，Q_hAnd Q_lValue, when min (Q)_r1，Q_r2，Q_h)＝Q_lIf so, the corresponding SCR value at this time is taken as the critical short-circuit ratio CSCR,

said Q_hAnd Q_lRespectively corresponding reactive power values of the upper limit and the lower limit of the voltage of the weak receiving end system;

said Q_r1The 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 Q_r1；

Said Q_r2The values are defined as: in the solution of the power flow equation of the weak receiver system, the q-axis current i_sqThe Q value corresponding to the minimum inflection point is recorded as Q_r2。

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:

U_sU_tcos＝U_t ²-QX-PR

U_sU_tsin＝PX-QR

after simplification, the following steps are simultaneously carried out:

wherein, U_tFor VSC AC bus voltage, U_sThe 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:

U_tmax≥U_t≥U_tmin

wherein, U_tmax、U_tminRespectively 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(Q_r1,Q_r2,Q_h)≥Q≥Q_l。

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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