Research on the interaction principle and system oscillation characteristics between DFIG and static var generators

A large number of asynchronous power sources and reactive power compensation devices based on power electronic equipment are connected to form a new power system, which makes the interaction between the cluster wind power and the power grid and with the adjacent power electronic drive equipment very complex. Changes may occur, which will bring many challenges to the safe and stable operation of the power grid. Through the theoretical analysis of the conduction process of the system disturbance in the DFIG and the static var generator, their dynamic response characteristics are given respectively, and the mutual excitation of the DFIG and the static var generator under the system disturbance is deduced. The form of expression, the interaction between the doubly-fed wind turbine and the static var generator is analyzed on the control parameters of the rotor-side converter and the static var generator of the doubly-fed wind turbine, and the grid connection of the doubly-fed wind power is given. Oscillation characteristics of the system. Finally, the correctness of the above theoretical analysis is verified by time domain simulation.

In the context of the rapid advancement of wind power generation technology and policy support, wind power has developed rapidly in recent years, and its proportion in the power system has continued to rise, making the interaction between wind power and power grids more and more complex.

In terms of wind power development, the DFIG can reduce the mechanical stress of the prime mover, greatly improve the energy conversion efficiency, realize the decoupling of the mechanical part and the electrical part, and the decoupling control of the active and reactive power, which improves the Therefore, it has become one of the main models of commercial operation at present.

At the same time, because static VARgenerators (SVG) have the advantages of rapid reactive power compensation and flexible control, they can effectively improve the voltage stability of wind power grid-connected systems, and have also been widely used in large-scale wind power grid-connected systems.

At present, the grid-connected guidelines all require wind turbines to operate without off-grid under certain frequency deviation, duration and voltage drop level, and the active and reactive characteristics of the units are explained.

However, the asynchronous power supply and reactive power compensation device based on power electronic technology have strong controllability on the one hand, and can adapt or change some basic operating characteristics of the original power system; on the other hand, the power electronic equipment has no inertia and overload capacity. Its control ability has a strong dependence on the external power grid. However, doubly-fed wind turbines or wind farms have multi-time-scale control links and fast control characteristics of power electronic drive equipment, which makes the operation control between cluster wind power and power grid and with adjacent power electronic drive equipment. The mutual coupling and influence are prominent.

Domestic and foreign scholars have carried out research on the interaction between machine and grid of wind power grid-connected system and the oscillation characteristics of grid-connected system. References [10-11] studied the stability of large-scale wind power connected to weak power grids. Based on the small disturbance stability analysis method, the linearized state equations of wind turbines and grid-connected systems were established. The influence of small disturbance stability and damping characteristics of the system; literature [12] studied the dynamic coupling interaction between the doubly-fed wind turbine and the power grid, and established a system including flux linkage, phase-locked loop, converter and its control system and wind turbines, etc. [13-15] studied the subsynchronous control interaction between the rotor-side converter and the series-compensated capacitor of the doubly-fed wind turbine; Reference [16] studied the dynamic interaction between the wind turbine and the synchronous machine According to the principle of wind power grid connection, the influence of wind power grid connection on the stability characteristics of power system with small disturbance angle; Reference [17] proposes a sequential optimization algorithm to design damping controller parameters for doubly-fed wind turbines. Active and reactive hybrid modulation damping controller; Reference [17] 18-19] studied the interaction between multi-FACTS controllers, and believed that when the feedback control gain increases, the pole of the closed-loop system will move to the open-loop zero, which may cause the closed-loop system to lose stability and cause the system to oscillate; otherwise, it will move. When it reaches the open-loop pole, the system is stable. Reference [20] discusses the reasons for frequent oscillations in a cluster wind farm in Xinjiang due to the unreasonable control of the reactive power compensation devices between the fields. The above studies are mostly based on the influence of wind power grid integration on the stability of traditional power systems, lack of understanding of the new problems of large-scale doubly-fed wind power grid integration, and the research on the interaction principle between doubly-fed wind turbines and other electrical equipment is in its infancy At this stage, a unified understanding has not been formed, and further in-depth research is needed.

Firstly, the response characteristics of DFIG and static var generator to system disturbance are studied respectively, the interaction principle between DFIG and static var generator is analyzed, and the current inner loop control parameters of the rotor-side converter of DFIG are analyzed. The influence law of the control parameters of the DFIG and the static var generator on the mutual excitation of the DFIG wind farm and the static var generator is given, and the oscillation characteristics of the DFIG grid-connected system are given. Finally, an actual wind power grid-connected system in Northwest China is established in the DIgSILENT/PowerFactory simulation software, and the correctness of the theoretical analysis is verified by simulation experiments.

At present, relevant literatures describe the functions and model structure of DFIG in detail, such as the structure of DFIG and its integrated control system. In steady-state operation mode, the speed controller provides the active power reference value to the rotor-side inverter power controller; for the reactive power control of the inverter, according to the requirements of the reactive power exchange degree at the grid-connected point, the rotor-side inverter is controlled.

Set to a value; the reactive power exchange of the grid-side inverter is set to 0.

The double-fed wind turbine controls the value of the external voltage of the rotor through double-loop decoupling, and then controls the active power and reactive power generated by the unit. In the dq rotating coordinate system, the voltage and flux linkage equations are

Figure 1 DFIG and its control system

In the formula: the subscripts s and r represent the stator and rotor components respectively;

is the voltage vector;

is the flux vector;

is the current vector;

is the winding resistance;

is the winding inductance;

is the mutual inductance between the stator and rotor windings;

is the synchronous speed;

is the slip rate; the subscripts d and q represent the reference coordinate system respectively.

The doubly-fed wind turbine adopts rotor current control. Based on the stator voltage qualitatively, the direction of the stator voltage vector is coincident with the d-axis of the synchronous rotation reference coordinate system. The electromagnetic transient process of the stator and the voltage drop on the stator resistance are ignored. According to equations (1)—(4) ) and Fig. 1 can obtain the response characteristics of the rotor-side converter control system to disturbance:

are the proportional coefficient and integral time of the power outer loop control, respectively;

are the proportional coefficient and integral time of the current inner loop control, respectively;

Assuming that the terminal voltage of the doubly-fed wind turbine is a three-phase symmetrical fundamental wave sine function, when the resonance angular frequency occurs, it is

When the disturbance signal is , its A-phase voltage and current are expressed as

are the initial phase of the fundamental voltage and current, respectively;

are the initial phase of the oscillating signal voltage and current, respectively;

are the rms value of the fundamental voltage and current, respectively;

are the rms value of the oscillation signal voltage and current, respectively.

Without considering the influence of oscillation on the system characteristics, the following relationship exists:

Without considering the influence of harmonics, the three-phase voltage and current shown in formula (6) can be transformed into the voltage and current components of synchronous rotation dq coordinates by park transformation, which can be expressed as:

Assuming that the controller can achieve accurate fundamental power, the instantaneous power of the unit is

Ignoring the high-order oscillating components, it can be obtained from equation (10) that the variation of the instantaneous power output of the unit only contains the oscillating signal components:

According to equations (1)-(5), the dq-axis incremental functions of rotor voltage and current can be obtained as

Assuming that the phase-locked loop can accurately change the system phase, then the equation (9) (11) (13) is put into the equation (5) to get:

Simultaneous equations (12) and (14) can obtain the first-order linear differential equation system of rotor current disturbance:

The dq-axis rotor oscillating current component generated by the solution can be obtained, and the dq-axis oscillating current component on the stator side generated by equation (13) can be obtained as:

are the phase deviation and amplitude gain of the stator winding d-axis current and the original d-axis disturbance current in the dq rotating coordinate system based on the output voltage disturbance of the rotor-side converter, and the difference between the stator winding q-axis current and the original q-axis disturbance current, respectively. Phase offset and amplitude gain.

The main control strategies of static var generators can be divided into two types: current indirect control and current direct control. Direct current control has the advantages of high accuracy, fast response, high stability and better harmonics, and is widely used in wind power grid-connected systems.

The AC voltage control loop is used to control the constant voltage of the controlled AC bus; the DC voltage control loop is used to control the constant voltage of the controlled DC bus;

respectively measure the AC bus voltage and the DC bus voltage;

is the voltage measurement time;

are the proportional coefficient and integral time of the outer loop control of the AC voltage, respectively;

are the proportional coefficient and integral time of the outer loop control of the DC voltage, respectively;

are the proportional coefficient and integral time of the current inner loop control, respectively;

They are the trigger pulse signal for AC voltage control and DC voltage control respectively.

The static var generator and the voltage and current double-loop control system model can obtain the response characteristics of the static var generator to disturbances:

Considering that the DC bus voltage of the static var generator is relatively stable, and the response of the DC voltage control loop to the system disturbance is weak, the influence of the DC voltage control loop is not considered. Assuming that the static var generator controls the bus voltage to be a three-phase symmetrical fundamental wave sine function and superimposes the resonant angular frequency as

The disturbance signal of , as shown in Eq. (6). Using the control and analysis method of coordinate change, combined with equations (8) and (9), the q-axis disturbance in the dq rotating coordinate system can be obtained as

Assuming that the controller can accurately measure the fundamental voltage, the simultaneous equations (17) and (18) can obtain the trigger pulse signal of the static var generator as:

The relationship between the reactive current sent by the static var generator and the trigger pulse signal can be expressed by the following formula:

Simultaneous equations (17)—(20) can obtain the incremental function of the reactive current emitted by the static var generator:

Then, the oscillating current component generated by the static var generator to the system disturbance is obtained:

are the phase deviation and amplitude gain of the output voltage disturbance oscillation current of the static var generator and the original disturbance current, respectively.

Under the condition that large-scale doubly-fed wind power is connected to the power grid, the operation control between the cluster wind power and the static var generator is dynamically coupled and influenced by the voltage at the grid-connection point of the wind farm, which makes the stability of large-scale doubly-fed wind power connected to the power grid. Complex, interactive relationship between DFIG and static var generator is available

Under the system disturbance, it can be seen from the above analysis that firstly, because the stator of the DFIG is directly connected to the system, the disturbance amount exists in the unit stator, and secondly, the instantaneous power and current collected by the unit control system will change, resulting in the rotor-side converter. The output voltage and current change, and the coupling interaction between the stator and the rotor makes the stator output a changing voltage and current, which will be superimposed with the original disturbance; similarly, the instantaneous voltage collected by the static var generator control system changes, This leads to the change of the output trigger pulse signal of the AC voltage control loop, and causes the static var generator to generate a disturbance current, which will be superimposed with the original disturbance; finally, the currents of the DFIG and the static var generator will be superimposed with the original disturbance, and the combined current will be superimposed with the original disturbance. The vertical (16) (21) can be obtained under the combined action of the DFIG and the static var generator, the incremental function of the superimposed response to the disturbance signal is:

Then, the current component under the combined action of the doubly-fed wind turbine and the static var generator is obtained as

In the formula: π-Δ

are the phase deviation and amplitude gain of the current acting together with the DFIG and the static var generator and the original disturbance current, respectively, where:

Under the system disturbance, the currents from the DFIG and the static var generators are mutually excited. The current component shown in Equation (24) and the original disturbance current component shown in Equation (9) are superimposed to obtain:

When Equation (27) is satisfied, the amplitude of the disturbance current component at this frequency increases, and the original disturbance quantity is boosted and difficult to attenuate, which may cause the system to oscillate or collapse.

Equation (28) can be obtained from the solution of Equation (27), which can be used as the basis for judging whether the original disturbance is boosted by the mutual excitation of the current from the DFIG and the static var generator.

Combining equations (15) (17) (21)-(28), it can be seen that under the system disturbance, the mutual excitation effect of the current between the DFIG and the static var generator is related to the control parameters of the rotor-side converter of the DFIG. The control parameters of the static var generator are strongly correlated, and the current inner loop control parameters of the DFIG converter

The smaller the value, the static var generator control parameters

The smaller the value, the stronger the mutual incentive effect. Because of the strong interaction between the machine and the grid, the wind turbine operating under this condition makes the wind power grid-connected system more prone to instability.

In order to verify the accuracy of the analysis process and conclusions of this paper, an actual wind power grid-connected system in Northwest China is taken as an example to analyze, and the establishment of such as

In a wind power grid-connected system, 495MW of wind power is connected to the grid through the 220kV east wind power collection station, and 195MW of wind power is connected to the grid through the 220kV west wind power collection station. Taking Xihui Station and the four wind farms and reactive power compensation devices in the collection station as the research objects, considering wind farm 1 and static var generators (SVG configuration capacity ±12Mvar) in the field, wind farm 2 and static var generators in the field The reactive power compensation device (SVC configuration capacity - 24Mvar), wind farm 3 and static var generator (SVG configuration capacity ±11.5Mvar) are put into operation. The model and parameters of the power system in the study are calculated from the actual power grid dispatch model and parameters. Based on the grid-connected system, the effects of the control parameters of the rotor-side converter and the static var generator control parameters on the stability of the grid-connected system are considered separately.

The response characteristics of the wind power grid-connected system when the rotor-side converter of the doubly-fed wind turbine adopts different control parameters. The simulation event is that the initial transmission power of Xihui Station is 19.3MW, and the SVC in wind farm 2 suddenly quits operation at 0.1s, resulting in local voltage fluctuations in the grid-connected system. Under this disturbance, the response characteristics of the wind power grid-connected system under the current inner loop control parameters of the rotor-side converter of different units are as follows:

It can be seen that the increase of the current inner loop gain or the decrease of the integral time of the DFIG converter will enhance the interaction between the DFIG and the static var generator. When the parameters change to a certain value , this mutual excitation causes local voltage oscillation after the system is disturbed. Among them, the change of the inner loop gain has a greater influence on the oscillation, and the influence of the change of the inner loop integral time is slightly weaker. Therefore, the interaction between the doubly-fed wind farm and the static var generator has a greater effect on the current inner loop gain of the rotor-side converter.

The response characteristics of the wind power grid-connected system when the static var generator control system adopts different control parameters. imitation line, under this disturbance, the response characteristics of the wind power grid-connected system under different static var generator control parameters are as follows:

It can be seen that the increase of the control gain of the static var generator or the reduction of the integral time will enhance the interaction between the doubly-fed wind farm and the static var generator. When the parameters change to a certain value, the mutual excitation causes local voltage oscillation after the system is disturbed. The changes of the inner loop gain and the integral time of the inner loop have similar effects. The interaction between the doubly-fed wind farm and the static var generator is comparable to the control gain and the integration time of the static var generator.