CN110336305A - An improved additional frequency control method suitable for doubly-fed wind turbines participating in system frequency regulation under short-circuit faults - Google Patents

Abstract

The invention discloses an improved additional frequency control method suitable for doubly-fed wind turbines participating in system frequency regulation under short-circuit faults, which belongs to the field of new energy technology. The conventional additional frequency control of DFIG units is only suitable for frequency fluctuations caused by sudden load changes, and is difficult To meet the limitation of frequency adjustment in the complete process of short-circuit faults from occurrence to fault removal, the invention adopts MPPT control for wind turbines in normal operation of the power grid, and when load fluctuations occur in the system, wind turbines adopt conventional additional frequency control to respond to system frequency changes and improve the system The transient stability of frequency, when the transmission line fails, according to the change law of the system frequency from the occurrence of the fault to the complete process of the fault removal, the parameters in the conventional additional frequency control of the DFIG unit are corrected, so that the output of the DFIG unit can be adjusted rapidly with the change of the system frequency , maintain the effective inertia damping characteristics of the DFIG unit in the whole process, and improve the system frequency dynamic response characteristics.

Description

An improvement suitable for doubly-fed wind turbines to participate in system frequency regulation under short-circuit faults Additional Frequency Control Method

technical field

The invention belongs to the technical field of new energy power systems and microgrids, and specifically relates to an improved additional frequency control method suitable for doubly-fed wind turbines participating in system frequency regulation under short-circuit faults.

Background technique

Northwest my country is rich in wind power resources. It has become the current development trend for large-scale wind farms to be connected to the grid for centralized power generation and to be transmitted to the load center through high-voltage transmission lines. The continuous increase in wind power penetration has brought new challenges to the safe and stable operation of the system. The doubly-fed induction generator (DFIG) has excellent characteristics such as high power generation efficiency, small inverter capacity, and decoupling control of active and reactive power, and has become the main model of large-scale wind farms. However, when the DFIG unit adopts the frequency converter control mode, the rotor speed of the unit is decoupled from the system frequency, which reduces the equivalent moment of inertia of the system. When the permeability increases to a certain level, it will greatly weaken the dynamic response capability of the system frequency. In fact, the operating range of the DFIG unit speed is 0.7pu ~ 1.2pu, and the rotor stores much more rotational kinetic energy than the synchronous machine. If the coupling between the DFIG unit rotor speed and the grid frequency can be realized through the control strategy, the grid frequency can be greatly increased Adjustment ability.

Controlling wind turbines to participate in system frequency modulation is usually achieved by controlling wind turbines to simulate the frequency modulation characteristics of synchronous machines. Common methods include virtual inertia control and droop control. Virtual inertia control and droop control are additional frequency control modules in the rotor side control system of wind turbines, which respectively introduce the system frequency change rate and change amount into the control system, and adjust the speed change through fast power control to release or absorb rotor kinetic energy to compensate or absorb System active power mutation amount. But the inertia control is at the cost of sacrificing the frequency overshoot and transition time; the droop control coefficient is not easy to determine, and the coefficient is too large to make the system difficult to reach a stable state. In addition to using additional frequency control, some methods deduce the relationship between the virtual inertia of DFIG units, speed regulation and grid frequency changes, and adjust the wind power tracking curve by detecting system frequency changes, but the response of this scheme is slow at the initial moment of frequency changes. Therefore, an improvement is made on the basis of this scheme, and the differential frequency control is introduced into the rotor side control system, so that the DFIG unit can provide effective inertial support during the whole process of frequency modulation. In addition, for the DFIG unit connected to the weak grid due to the PLL dynamic behavior affecting the system stability, some methods use active power control instead of the PLL technology to realize the synchronous operation of the DFIG unit and the grid and provide inertia support. It is also possible to change the traditional detection function of the PLL and use the PLL as a part of the control system. Without adding any additional control loops, the inertia of the DFIG unit can be controlled by controlling the PLL parameters to adjust the internal voltage. Wind turbines usually operate in Maximum Power Point Tracking (MPPT) mode, lacking reserve capacity, and increasing the virtual inertia of wind turbines can only participate in system frequency regulation for a short time. In order to expand the time scale for wind turbines to participate in frequency regulation, on the basis of additional frequency control, combined with the overspeed method or pitch method, the wind turbines can be operated under load reduction under normal conditions, and a certain reserve capacity can be obtained to participate in the primary frequency regulation of the system. Among them, the slow response speed of the pitch method and the existence of mechanical wear limit its engineering application, making the overspeed method the first choice under the condition of meeting the load reduction level. However, the reserved reserve capacity limits the active power output of wind turbines in normal operation, affecting their economy and practicability.

Contents of the invention

Most of the control strategies proposed in the above studies are aimed at frequency changes caused by system load disturbances. In addition, common disturbances that cause large changes in system frequency include line short-circuit faults: when a short-circuit fault occurs on a high-voltage transmission line, the sending end will Short-term power surplus occurs, and when the fault is removed and the line resumes normal operation, the system's demand for power at the sending end increases instantaneously. The power imbalance during this period will cause short-term large fluctuations in the frequency of the sending-end system. In view of this situation, the present invention deeply studies the inertia damping characteristics of the DFIG unit adopting conventional additional frequency control in the whole process of transmission line short-circuit fault occurrence, development and fault removal, thus analyzes the limitations of conventional additional frequency control, and based on this An improved additional frequency control strategy for DFIG unit and system frequency regulation is proposed above. When a fault occurs on the transmission line, this strategy corrects the parameters in the conventional additional frequency control of the DFIG unit according to the change law of the system frequency during the complete process from the occurrence of the fault to the removal of the fault, so that the output of the DFIG unit can be quickly adjusted with the change of the system frequency, and the system transient state can be improved. stability.

The present invention mainly includes two parts, the first part is the limitation analysis of conventional additional frequency control under short-circuit fault. In the conventional additional frequency control mode, on the basis of the inertia damping characteristics of the DFIG unit changing with frequency in the whole process of fault occurrence, development and fault removal, the limitations of this strategy for frequency adjustment in the whole fault process are analyzed. Frequency control is suitable for frequency fluctuations caused by sudden load changes, but it is difficult to meet the frequency adjustment of the complete process of short-circuit faults from occurrence to fault removal.

The second part is to propose an improved additional frequency control method suitable for doubly-fed wind turbines to participate in system frequency regulation under short-circuit faults. Aiming at the limitations of conventional additional frequency control in the whole process of line short-circuit fault frequency adjustment, an improved additional frequency control strategy in which wind farms participate in system frequency adjustment is proposed. When the power grid is operating normally, the wind turbines adopt MPPT control; The conventional additional frequency control is adopted to respond to system frequency changes and improve the transient stability of the system frequency; when a short-circuit fault occurs on the transmission line, the improved additional frequency control strategy is adopted to correct the DFIG unit according to the system frequency change law in the whole process from the line fault to the restoration of normal operation The parameters in the conventional additional frequency control maintain the effective inertia damping characteristics of the DFIG unit in the whole process, quickly adjust the output as the system frequency changes, and improve the dynamic response characteristics of the system frequency.

The beneficial effects of the present invention are:

First, the frequency change analysis at the sending end: when the DFIG unit has no additional frequency control, the system adopts MPPT control, and the frequency fluctuation range is the largest. The additional frequency control suppresses the maximum frequency deviation to a certain extent, the maximum frequency amplitude decreases during the fault period, the maximum frequency deviation decreases during the frequency rising period, the minimum frequency amplitude improves after the fault is removed, and the maximum frequency deviation decreases during the frequency falling period. The DFIG unit plays an obvious inertial support role in the process of frequency dynamic change; after adopting the improved additional frequency control, the DFIG unit adjusts the speed in time to release the kinetic energy stored in the rotor after the fault removal line resumes normal operation, increases the active power output, and makes the frequency The offset is effectively weakened. Compared with the conventional additional frequency control, the frequency offset is greatly reduced. Compared with the previous two methods, this method obviously weakens the frequency oscillation trend in the late stage of frequency modulation, making the system quickly stabilized.

Second, the analysis of the additional power change: when there is no additional frequency control, the additional power is 0; in the case of conventional additional frequency control, and in the case of improved additional frequency control, after the short-circuit fault is removed, the frequency deviation begins to decrease from the maximum positive value Until it is 0, during this period the additional power ΔP ₁ generated by the droop control changes from positive to negative, so that the total additional power rises from the negative value of the conventional additional control to a positive value at the moment of fault removal, and the increase of the active reference value makes the DFIG unit Adjust the speed in time to release kinetic energy and increase the active power output of the unit.

Third, the analysis of the output active power change of the DFIG unit: During the frequency mutation process, the DFIG unit operates in the MPPT mode without additional frequency control, the output active power fluctuates in a small range, and has almost no response to the system frequency change. With the conventional additional frequency control, after the short circuit fault occurs, the DFIG unit uses the additional power signal to make the output power rapidly follow the active power reference value, and the output power decreases instantaneously. The output power decreases and the speed increases, which makes the fan deviate from the MPPT mode and run at an overspeed, increasing the rotor speed. Kinetic energy reserves, and a certain reserve capacity; after the fault is removed, the additional power signal makes the output power rise instantaneously, which is still lower than the mechanical power, which cannot meet the instantaneously increased active power demand of the system, and then the active output gradually increases until the electromagnetic power Greater than the captured mechanical power, the rotational speed begins to drop to release active power. When the improved additional frequency control is adopted, after the fault is removed, the additional power signal instantly increases the output power to be greater than the captured mechanical power, and the rotational speed drops instantaneously to release kinetic energy, replenishing active power for the system in time.

Fourth, the analysis of the speed change of the DFIG unit: When the DFIG unit has no additional frequency control, in order to maintain the optimal tip speed ratio, the speed is only adjusted with the change of the wind speed, which cannot respond to the change of the system frequency, and the speed is always maintained at 1pu. In the case of conventional additional frequency control, the fan can adjust the speed in response to changes in the system frequency. During the fault period, the speed will increase with the increase of the system frequency to absorb the short-term excess active power of the sending end system. However, after the fault removal line resumes normal operation, the frequency shows a downward trend, but is still higher than the rated value, and the additional power generated by droop control, which plays a dominant role, is related to the frequency deviation. The value is opposite to the frequency change rate, resulting in an upward trend in the fan speed, which is not conducive to the rapid stabilization of the system frequency. In view of the problems existing in the conventional additional frequency control, the improved additional frequency control can make the fan speed adjust quickly according to the change trend of the system frequency, and the improved additional frequency control can reduce the rotor speed in time after the frequency starts to drop, and release the kinetic energy to provide active support for the system.

Description of drawings

In order to illustrate the embodiments or technical solutions of the present invention more clearly, the following briefly introduces the accompanying drawings that are used in the description of the embodiments or technical solutions.

Fig. 1 is a conventional additional frequency control block diagram of an improved additional frequency control method suitable for doubly-fed wind turbines participating in system frequency regulation under short-circuit faults according to the present invention.

Fig. 2 is a block diagram of an improved additional frequency control method suitable for doubly-fed wind turbines participating in system frequency regulation under a short-circuit fault according to the present invention.

Fig. 3 is a flowchart of an improved droop control algorithm implementation of an improved additional frequency control method suitable for doubly-fed wind turbines participating in system frequency regulation under short-circuit faults according to the present invention.

Detailed ways

The invention will be described in further detail below in conjunction with the accompanying drawings.

The conventional additional frequency control method of the doubly-fed wind turbine is the basis of the present invention, as shown in Figure 1 conventional additional frequency control block diagram, ω _r is the rotor speed; P _ref is the active power reference value of the rotor side converter in the maximum power tracking mode ; Δf is the deviation between the system frequency f and the rated frequency f _N. The additional active power is It is transformed into a form similar to the equation of motion of the conventional synchronous generator rotor as The frequency modulation auxiliary power includes two parts: ΔP ₁ simulates the static characteristics of the active power of the synchronous generator set. When the system frequency deviates, the unit responds to the frequency change and increases the active power proportional to the frequency deviation item; ΔP ₂ simulates the inertial response of the synchronous generator set When the system frequency changes, the unit adjusts the speed change and injects or absorbs the active power proportional to the frequency differential term. The additional frequency control enables the DFIG unit to exhibit droop control characteristics, and at the same time have inertia similar to that of a synchronous machine rotor. When the differential control coefficient K _d >0, the rotational inertia similar to that of the synchronous machine is generated, and the differential control is also called inertia control; when the proportional control coefficient K _p >0, the damping coefficient can be increased to improve the frequency dynamic response capability, and the proportional control is also called Called droop control. Due to the different feedback signals, the inertia control and droop control adjustment processes are different. Inertial control is a transient process, and the frequency change rate is used as a feedback signal. It is mainly used to damp the rapid change of frequency. Therefore, it can provide a large active support at the initial moment of the disturbance. When the frequency change rate is close to zero near the frequency extreme point, the active support Weaker; in contrast, the droop control additional signal is related to the frequency deviation, and in most cases it is a steady-state process, mainly used to reduce the system frequency deviation, but in the process of transient frequency changes caused by various disturbances in the power grid, the droop The control is more of a damping effect, providing strong active support near the frequency maximum point, but the support effect is weak at the initial moment of the disturbance, making its control speed slower than inertial control. Effectively combining the quickness of inertia control and the continuity of droop control can make the system have good dynamic frequency characteristics in the whole disturbance process.

Real-time balance between power generation and power consumption in the power system. When the system frequency changes greatly, the synchronous generator set speed is closely coupled with the system frequency, which can respond in time, release or absorb the kinetic energy of the rotor to damp the system frequency change, especially at the initial stage of disturbance, the generator inertia It directly affects the system frequency change rate and even system stability. The rotor motion equation of synchronous generator is: H is the inertia constant of the generating set; Δω=ω-ω ₀ , ω is the actual electrical angular velocity, ω ₀ is the rated electrical angular velocity; P _M is the mechanical power; P _E is the electromagnetic power; D is the damping coefficient. DFIG units usually operate in MPPT mode and do not have frequency response capability. When the penetration rate is high, in order to improve the system frequency dynamic characteristics, additional frequency control links such as virtual inertia control are usually added to the DFIG unit to increase the system inertia. The specific implementation method of conventional additional frequency control is to add frequency modulation auxiliary power on the basis of wind turbine MPPT control, and the additional power comes from the kinetic energy released or absorbed by the rotor speed.

The DFIG units are centrally connected to the grid and transmitted to the load center through the high-voltage transmission line. When a three-phase short-circuit fault occurs in one circuit of the high-voltage transmission line, and the fault lasts for a certain period of time, it will be released and resume normal operation. During this process, the conventional additional frequency control is adopted. When the short-circuit fault occurs on the transmission line and the fault is resolved, the terminal voltage of the DFIG unit drops, and the system frequency at the sending end rises sharply during the fault period, and then drops rapidly after the fault is resolved. Decline or rise, the short-circuit fault will continuously experience two processes of frequency rise and fall from occurrence to release.

During the frequency rise period, the additional power ΔP ₁ and ΔP ₂ generated by droop control and inertia control are both negative, which makes the reference value of active power on the rotor side smaller, thereby controlling the acceleration of the rotor, absorbing excess active power, and reducing the output power of the fan to dampen The system frequency goes up. Inertial control plays a leading role at the initial moment of fault and provides strong power support, while droop control plays a weaker supporting role at the initial moment of fault. As the frequency deviation increases, the effect of droop control gradually increases. After the fault is removed, the frequency rises to the maximum and then begins to drop. _The system’s active power demand for the generator set at the sending end increases instantaneously, requiring the generator set to reissue the shortfall active power in time. A positive value helps to increase the power reference on the rotor side, but the inertia support is weak. At this time, the frequency deviation is large, and the power ΔP ₁ generated by the droop control has a strong effect, but its value is negative, so the total additional power is negative, resulting in a low power reference value on the rotor side, and the electromagnetic power transmitted by the fan is less than the captured For mechanical power, the rotor maintains an acceleration trend and cannot release kinetic energy to meet the instantaneous surge in active power demand of the system. As the frequency gradually decreases, the additional power finally becomes a positive value, and the rotor starts to decelerate at the same time.

An improved additional frequency control method suitable for double-fed wind turbines to participate in system frequency regulation under short-circuit faults. After the fault is removed and the line resumes normal operation, the instantaneously increased active power demand causes the system frequency to drop sharply. If the DFIG unit can be used at this time With the advantages of quick response to speed adjustment, timely down-regulation of DFIG unit speed to release kinetic energy, make up for the active power deficit of the system, and relieve the frequency regulation pressure of synchronous units will help the system quickly restore stable operation. To lower the speed of the DFIG unit, the reference value of active power on the rotor side should be increased first, so that the output electromagnetic power of the fan is greater than the captured mechanical power. After the fault removal frequency starts to drop, the frequency deviation starts to drop from the positive maximum value. At this time, the droop control that plays a leading role in the additional power is still negative, resulting in the output electromagnetic power being lower than the captured mechanical power, and the fan rotor is still accelerating. This trend not only hinders the rapid release of kinetic energy of wind turbines, but also further aggravates the rapid drop of system frequency.

If the additional power generated by the droop control becomes positive during the period from when the fault removal frequency starts to drop to zero, the power reference value on the rotor side will increase instantaneously, making the electromagnetic power higher than the mechanical power captured by the fan, thereby adjusting the fan rotor Run at reduced speed, releasing kinetic energy and damping system frequency drops. For the inertia damping characteristics analysis of the conventional additional frequency control strategy of DFIG units under fault conditions, the present invention proposes an improved additional frequency control strategy in view of its limitations. Regularly modify the droop control coefficient, so that the DFIG unit has effective inertia damping characteristics in the whole process, so that its active output can be adjusted in time with frequency changes. The implementation method of the improved additional frequency control strategy is shown in Figure 2. The specific implementation process of the core algorithm improved droop control algorithm is shown in Figure 3. e is the control signal. When the system is operating normally or the system frequency changes due to load disturbance, the value of e is 0, the wind turbine adopts the conventional additional frequency control strategy; when a short-circuit fault occurs in the system and the frequency changes, the control signal is triggered, and the value of e becomes 1, and the wind turbine adopts the improved additional frequency control strategy. The disturbances that cause a large increase in system frequency mainly include sudden load reduction and short-circuit faults. Load reduction will cause a short-term increase in machine terminal voltage, while short-circuit faults will cause a large drop in machine terminal voltage. In order to distinguish between the two disturbances, this paper introduces The terminal voltage and system frequency changes are used as the basis for judging whether a short-circuit fault occurs. As shown in Figure 3, when the amplitude of the terminal voltage is lower than 0.9pu and the system frequency is higher than 50.1Hz, it is determined that a short-circuit fault has occurred in the system. The value of e becomes 1. After the short-circuit fault is removed, the value of e remains 1. When the frequency starts to drop and the frequency is still higher than the rated value, the value of the additional power generated by the droop control remains unchanged, and the sign changes. Until the frequency is lower than the rated value, the value of e When the value becomes 0, the wind turbine resumes normal additional frequency control.

Claims (5)

1. a kind of suitable double-fed fan motor unit participates in the improvement additional frequency control method of system frequency adjusting under short trouble, It is characterized in that, elaborating the limitation of conventional additional frequency control, it is suitable only for frequency fluctuation caused by sudden load change, it is difficult to Meet frequency of the short trouble from occurring trouble shooting complete procedure to adjust, after failure removal, secondary power signal makes defeated Power moment rises out, is still below mechanical output, the active demand that the system that is unable to satisfy instantaneously increases, subsequent active power output is gradually It increases, until electromagnetic power is greater than the mechanical output of capture, it is active that revolving speed begins to decline release.

2. a kind of suitable double-fed fan motor unit according to claim 1 participates in changing for system frequency adjusting under short trouble Into additional frequency control method, which is characterized in that when power grid operates normally, Wind turbines are controlled using MPPT；System occurs When load fluctuation, for Wind turbines using conventional additional frequency control response system frequency variation, the transient state for improving system frequency is steady It is qualitative；When transmission line of electricity breaks down, the changing rule of the system frequency into trouble shooting complete procedure occurs according to failure, The parameter in the control of DFIG unit routine additional frequency is corrected, so that DFIG unit output is changed adjustment rapidly with system frequency, mentions High power system transient stability keeps the effective inertia damping characteristic of DFIG unit overall process, changes adjustment rapidly with system frequency Power output improves system frequency dynamic response characteristic.

3. a kind of suitable double-fed fan motor unit according to claim 2 participates in changing for system frequency adjusting under short trouble Into additional frequency control method, which is characterized in that DFIG unit is adjusted in time after failure removal route restores to operate normally to be turned Quick-release puts the kinetic energy stored in rotor, increases active output, weakens frequency offset effectively, obviously cuts in the frequency modulation later period Weak hunting of frequency trend, makes system quickly tend towards stability.

4. according to claim a kind of 2 suitable double-fed fan motor units stated participated under short trouble system frequency adjusting change Into additional frequency control method, which is characterized in that DFIG unit makes its output by secondary power signal after short trouble occurs Power follows active power reference value to change rapidly, and moment reduces, and output power reduction causes revolving speed to increase, and deviates blower MPPT mode overspeed increases rotor kinetic energy storage level, and there are certain spare capacities；After short trouble excision, frequency Rate deviation is landed by positive peak until being 0, during this secondary power caused by sagging control from just becoming negative, thus Total secondary power is increased to positive number by the negative of conventional additional control in the moment of failure removal, and the increase of active reference value makes DFIG unit adjusts revolving speed release kinetic energy in time, increases the active output of unit.

5. according to claim a kind of 2 suitable double-fed fan motor units stated participated under short trouble system frequency adjusting change Into additional frequency control method, which is characterized in that adjust rotation speed of fan rapidly according to the variation tendency of system frequency, age at failure Between revolving speed rise with system frequency and raise, absorb sending superfluous active power in short-term.The energy after frequency is begun to decline It is enough to reduce rotor speed in time, kinetic energy is discharged for system, and active support is provided.