CN107425542B - Control method of DFIG reactive power compensation during low voltage fault ride-through - Google Patents

Abstract

The invention relates to the field of wind power generation, in particular to a control method for DFIG reactive power compensation in a low-voltage fault ride-through process, including grid voltage detection and DFIG control strategy switching. Through the real-time detection of the grid voltage, when judging that the voltage sag fault occurs, the unit power factor control strategy of the DFIG rotor-side converter is switched to the control strategy of reactive power output, so as to provide reactive power support for the grid voltage to help The grid voltage is restored. The method of the invention does not need to add an additional reactive power compensation device, and uses the switching of the unit's own control strategy to adjust the output support of the reactive power on the stator side during the fault period, so as to provide reactive current to the power grid to improve the voltage drop of the power grid. , to enhance the low-voltage fault ride-through capability of the entire wind power system.

Description

Control method of DFIG reactive power compensation during low voltage fault ride-through

technical field

The invention relates to the field of wind power generation, in particular to a control method for DFIG reactive power compensation in a low-voltage fault ride-through process.

technical background

In recent years, with the rapid growth of the global energy consumption rate, as a clean and infinite renewable energy, wind energy has received extensive attention. The double-fed generator (DFIG), as one of the mainstream models, has quickly become a research focus and development direction in the field of wind power due to its high utilization rate of wind energy and small required converter capacity.

However, with the increase of the installed capacity of wind turbines year by year, the integration of large-scale wind power into the power grid will bring great challenges to the power system. When the power grid fails and causes the voltage to drop, it will affect the normal operation of the doubly-fed wind power generation system, and even go offline. Therefore, the wind power generation system must have a certain low-voltage fault ride-through capability.

In the power flow of the power system, the flow of reactive power is related to the magnitude of the grid voltage. Therefore, after a low voltage sag fault occurs in the power grid, rationally injecting reactive power into the power grid will help to restore the faulted power grid voltage. At present, the main literature and research focus on the use of reactive power compensation devices to support the recovery of grid voltage in faults, such as SVC, STATCOM and shunt capacitors and other reactive power compensation devices. The application of the reactive power compensation device can quickly provide reactive power support for the wind farm, help the recovery of the grid voltage, improve the transient stability of the system, and improve its low-voltage fault ride-through capability. However, the additional reactive power compensation device will increase the manufacturing cost of the system and need to consider the installation operation of the device and its control under fault conditions.

SUMMARY OF THE INVENTION

The purpose of the present invention is mainly to use the reactive power output control capability of the DFIG itself to help restore the grid voltage by switching the control strategy in the voltage sag fault and adopting the control of DFIG output reactive power compensation.

The technical scheme adopted in the present invention is: a control method for DFIG reactive power compensation in a low-voltage fault ride-through process, comprising the following steps:

S1: Detect the grid voltage, and judge the detected grid voltage;

S2: When the grid voltage is greater than or equal to 0.9pu of the rated voltage and less than 1.0pu, it is judged that the grid voltage is normal. At this time, the reactive power reference value Q _ref1 = 0 is set, and the DFIG rotor-side converter keeps the unit in normal operation. Power factor operation;

S3: When the grid voltage is less than 0.9pu and greater than 0pu, it is judged as a voltage fault, the DFIG rotor-side converter outputs reactive power, and the reactive power reference value Q _ref2 is set. At this time, the reactive power reference value Q _ref2 is low according to DFIG The test requirements of the voltage fault and the reactive power limit setting are given, and the reactive power reference value Q _ref2 is calculated through the following steps:

S31: According to the test requirements, the reactive current provided by DFIG to the grid can be expressed as: I _q =1.5(0.9-U _g /U _N )I _qN 0.2≤U _g ≤0.9; U _g is the magnitude of the grid voltage, U g _N is the rated voltage of the grid, and I _qN is the reactive current that the stator side can send out in the rated operating state;

S32: During the fault, the reactive power required to output from the stator side of the DFIG can be expressed as: Among them, Q _N is the reactive power rating output by the DFIG stator side;

S33: Oriented to the grid voltage, the current dq component of the rotor converter is Among them, i _rd , i _rq are the current dq components of the rotor-converter respectively; P _s and Q _s are the active power and reactive power emitted by the DFIG stator side respectively; L _s and L _m are the stator winding inductance and the stator and rotor windings. The mutual inductance between , ω ₁ is the synchronous angular velocity;

S34: The active power P _s and reactive power Q _s from the stator side are limited by the rotor current and expressed as: Among them, i _rmax is the maximum fault current of the rotor during the fault; Wherein, X _s =ω ₁ L _s , X _m =ω ₁ L _m ;

S35: Under the condition of given wind speed, the active power P _s emitted from the stator side is constant, and the range of reactive power Q _s emitted from the stator side is: Q _smin ≤Q _s ≤Q _smax , where Q _smin is the reactive power that can be emitted from the stator side The minimum value of power, Q _smax is the maximum value of reactive power that can be emitted by the stator side, The maximum reactive power Q _smax corresponding to the maximum output reactive current on the stator side is:

S36: The reactive power limiter is set as: when I _q ≥I _sqmax , the reactive power reference value Q _ref2 is given as Q _smax ; when I _q <I _sqmax , the reactive power reference value Q _ref2 is given as Q _q ;

S4: After the given different reactive power Q _ref1 or Q _ref2 is acted by the PI regulator, the rotor voltage component ur _rq that can independently adjust the reactive power output on the stator side is obtained, and the effective control of the output reactive power on the stator side is completed. , so as to restore the grid voltage during the fault process.

In the low-voltage fault ride-through process of the present invention, the control of the reactive power compensation of the DFIG itself is used, and the output support of the reactive power on the stator side is adjusted during the fault period through the switching of the unit control strategy, so as to provide reactive current to the grid to improve the grid voltage. The drop degree increases the low-voltage fault ride-through capability of the entire wind power system.

Description of drawings

FIG. 1 is a flow chart of the present invention.

FIG. 2 is a schematic diagram of control strategy switching according to the present invention.

Figure 3 is a waveform diagram of reactive power emitted from the stator side.

Figure 4 is a grid voltage waveform diagram.

The dotted line in the figure represents the relevant waveform when the control method of the present invention is not adopted, and the solid line represents the relevant waveform when the control method of the present invention is adopted.

Fig. 3 shows that in the fault process, the reactive power is basically kept at 0 when the control method of the present invention is not used; while the reactive power is output when the control method of the present invention is used.

Figure 4 shows that after the control method of the present invention is adopted, the voltage on the grid side can be recovered to a certain extent, and at the same time, it can be seen that, affected by its own parameters, the dropped voltage cannot be completely recovered to the normal value before the fault occurs.

Detailed ways

A control method for DFIG reactive power compensation in a low-voltage fault ride-through process is characterized by comprising the following steps:

S1: Detect the grid voltage, and judge the detected grid voltage;

S2: When the grid voltage is greater than or equal to 0.9pu of the rated voltage and less than 1.0pu, it is judged that the grid voltage is normal, the DFIG rotor-side converter maintains the unit power factor operation during normal operation, and the reactive power reference value Q _ref1 = 0;

S3: When the grid voltage is less than 0.9pu and greater than 0pu, it is judged as a voltage fault, the DFIG rotor-side converter outputs reactive power, and the reactive power reference value Q _ref2 is set _. The test requirements for voltage faults and reactive power limit settings are given. The reactive power reference value Q _ref2 is calculated by the following steps:

According to DFIG's reactive power compensation test requirements, at least 1.5% of reactive current should be provided for every 1% drop in grid voltage during a low voltage sag fault.

S31: The reactive current provided by DFIG to the power grid according to the test requirements can be expressed as: I _q =1.5(0.9-U _g /U _N )I _qN 0.2≤U _g ≤0.9(1); among them, I _q is according to the test requirements The reactive current provided by the DFIG to the grid, U _g is the amplitude of the grid voltage, U _N is the rated voltage of the grid, and I _qN is the reactive current that can be sent from the stator side in the rated operating state;

S32: During the fault, the reactive power required to output from the stator side of the DFIG can be expressed as: Among them, Q _N is the reactive power rating output by the DFIG stator side;

The reactive power limit setting is based on the current limit of the rotor-side converter.

S33: Oriented to the grid voltage, the current dq component of the rotor converter is

Among them, i _rd , i _rq are the dq components of the rotor current, respectively; P _s , Q _s are the active power and reactive power emitted by the DFIG stator side, respectively; L _s , L _m are the stator winding inductance and the mutual inductance between the stator and rotor windings , ω ₁ is the synchronous angular velocity;

S34: The active power P _s and reactive power Q _s emitted from the stator side are mainly limited by the rotor current and are expressed as:

Among them, i _rmax is the maximum fault current of the rotor during the fault, which is generally set to 2 times the rated current of the rotor; it can be sorted to get

Wherein, X _s =ω ₁ L _s , X _m =ω ₁ L _m ;

S35: When the wind speed is given, the active power P _s emitted by the stator side is constant. The range of reactive power Q _s emitted from the stator side is: Q _smin ≤Q _s ≤Q _smax , where Q _smin is the minimum value of reactive power that can be emitted by the stator side, and Q _smax is the maximum value of reactive power that can be emitted by the stator side, and: The maximum reactive power value corresponding to the maximum output reactive current on the stator side is:

S36: The reactive power limiter is set as: when I _q ≥ I _sqmax , the reactive power reference value Q _ref2 is given as Q _smax ; when I _q <I _sqmax , the reactive power reference value Q _ref2 is given as Q _q ;

S4: After the given different reactive power Q _ref1 or Q _ref2 is acted by the PI regulator, the rotor voltage component urq that can independently adjust the reactive power output on the stator side is obtained, and the effective control of the output reactive power on the stator side is completed. Thereby, the power grid voltage during the fault process can be adjusted recoverably.

The present invention will be further described below with reference to specific embodiments. This embodiment provides a control method for DFIG reactive power compensation in the process of grid voltage symmetric sag fault. The specific application environment is based on the simulation software Matlab/Smulink to verify the control method described in the above invention, thereby helping to restore the grid in the fault. Voltage. In the simulation, the situation when the grid voltage drops symmetrically to 50% of the rated voltage at t=4.0s is selected. The specific implementation steps are as follows:

①Detect the grid voltage and judge the detected voltage. When the grid voltage is greater than or equal to 0.9pu of the rated voltage and less than 1.0pu, it is judged that the voltage is normal; when t=4.0s, the grid voltage is less than 0.9pu and greater than 0pu, and it is judged that the voltage is faulty.

② Switch the control strategy of the DFIG rotor-side converter according to the detection and judgment results. When it is judged to be normal, switch to a, the rotor-side converter (RSC) maintains the unit power factor operation during normal operation; when it is judged to be faulty, switch to b, RSC controls the output reactive power, and according to the current The test requirements and the requirements of the reactive power limit setting should provide a reactive current of I _q = 0.6pu, which is a non-unity power factor operation.

③ Meanwhile, different reactive power reference values Q _ref1 and Q _ref2 are given according to the result of DFIG rotor-side converter control strategy switching. Among them, Q _ref1 meets the requirements of unit power factor operation, and is given as 0; Q _ref2 is given as Q _q =0.45pu according to the test requirements of low-voltage faults of the unit and the requirements of reactive power limit settings.

(4) After the given different reactive powers are acted by the PI regulator, the rotor voltage component _urq that can independently adjust the reactive power output on the stator side is obtained, and the effective control of the output reactive power on the stator side is completed, so as to prevent the failure process. Restorative regulation of the grid voltage.

Claims (1)

1. The control method for DFIG reactive power compensation in the low-voltage fault ride-through process is characterized by comprising the following steps:

s1: detecting the voltage of a power grid, and judging the detected voltage of the power grid;

s2: when the power grid voltage is more than or equal to 0.9pu and less than 1.0pu of the rated voltage, the power grid voltage is judged to be normal, and a reactive power reference value Q is set at the moment_ref1When the DFIG rotor-side converter is equal to 0, the DFIG rotor-side converter keeps unit power factor operation in normal operation;

s3: when the voltage of the power grid is less than 0.9pu and largeWhen the voltage fault is judged to be 0pu, the DFIG rotor-side converter outputs reactive power, and a reactive power reference value Q is set_ref2At this time, the reactive power reference value Q_ref2Setting according to the testing requirement of the DFIG low-voltage fault and the reactive power amplitude limiting setting, and setting the reactive power reference value Q_ref2Calculated by the following steps:

s31: according to the test requirement, the reactive current provided by the DFIG to the power grid is expressed as: i is_q＝1.5(0.9-U_g/U_N)I_qN 0.2≤U_gLess than or equal to 0.9; wherein U is_gIs the amplitude, U, of the grid voltage_NFor the rated voltage of the grid, I_qNThe reactive current can be generated at the stator side in a rated operation state;

s32: during the fault, the reactive power required to be output by the stator side of the DFIG is expressed as:wherein Q is_NA rated value of reactive power output by the stator side of the DFIG;

s33: oriented by the network voltage, the current dq component of the rotor converter isWherein i_rd，i_rqRespectively the current dq components of the rotor converter; p_s、Q_sRespectively the active power and the reactive power emitted by the stator side of the DFIG; l is_s、L_mFor mutual inductance, omega, between stator winding inductance and stator-rotor winding₁Is the synchronous angular velocity;

s34: active power P emitted from stator side_sAnd reactive power Q_sThe limitation imposed by the rotor current is expressed as:wherein i_rmaxIs the maximum fault current of the rotor during the fault; is finished to obtainWherein,X_s＝ω₁L_s，X_m＝ω₁L_m；

s35: active power P emitted by the stator side under the condition of given wind speed_sConstant, reactive power Q from the stator side_sThe range is as follows: q_smin≤Q_s≤Q_smaxWherein Q is_sminFor the minimum value of the reactive power, Q, that can be emitted by the stator side_smaxThe maximum value of the reactive power that can be emitted on the stator side,maximum value Q of the reactive power_smaxThe maximum output reactive current of the corresponding stator side is as follows:

s36: the reactive clipping is set as: when I is_q≥I_sqmaxReference value of reactive power Q_ref2Given as Q_smax(ii) a When I is_q<I_sqmaxReference value of reactive power Q_ref2Given as Q_q；

S4: given different reactive power reference values Q_ref1Or Q_ref2After the action of the PI regulator, a rotor voltage component u capable of independently regulating the reactive power output of the stator side is obtained_rqAnd the effective control of the reactive power output by the stator side is completed, so that the recovery regulation is performed on the grid voltage in the fault process.