An improved control method for high penetration of direct-drive wind turbines based on virtual synchronous technology

Abstract

The application is directed to the fact that the grid transient overvoltage is easy to cause the grid-side converter of the direct-drive wind turbine to reach the upper limit of the AC voltage regulation, thereby causing AC-DC power coupling oscillation, instability or overvoltage trip-out protection problem, and proposes an improved control scheme for increasing the controllability of GSC through dynamic adjustment of DC bus voltage in high penetration time. Among them, the active control is improved through the VSG technology, and the power compensation term is designed to increase the system frequency support outside and reduce the bus voltage fluctuation inside; the reactive power control is based on the industry standard, and the voltage recovery is supported by injecting reactive current into the grid. The improved control effectively improves the fault continuous penetration capability of the unit, improves the efficiency and power quality of the wind turbine generation, and improves the system stability.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed power generation micro-grid inverter control, in particular to an improved control method for high penetration of direct-drive wind turbines based on virtual synchronization technology. BACKGROUND

[0002] Effective consumption of large-scale wind power often requires high-voltage transmission systems. However, the sending system of typical large-capacity new energy transmission projects has a certain electrical distance from the main grid, and the capacity of the nearby supporting thermal power units is insufficient, making it a "weakly supported" grid.

[0003] When such a grid experiences various faults, it is prone to large fluctuations and oscillations in frequency and voltage, triggering wind turbine disconnection, and in severe cases, even causing cascading reactions and blackouts. The design of wind turbine fault ride-through strategies helps to enhance the ability of the unit to resist high and low voltage impacts. Although there are many research results on improving fault ride-through capability from the perspective of system stations, the actual feasibility is insufficient, so it is necessary to fully utilize and improve the reactive power support capability of the wind turbine itself.

[0004] Grid transient overvoltage can cause the grid-side converter to reach its upper limit of AC voltage regulation, resulting in AC-DC power coupling oscillation and instability or overvoltage generator tripping protection. For transient overvoltage, in addition to the hazards of breaking devices or triggering disconnection, it also affects the control ability of GSC on output active and reactive power, easily causing AC-DC coupled power oscillation. Due to the influence of modulation method and DC bus voltage, the AC side voltage amplitude of GSC has an upper limit When the transient overvoltage reaches , it will exceed the controllable range of GSC; if the GSC control strategy remains unchanged, on the one hand, the DC side U dc remains unchanged, and on the other hand, the AC side maintains unity power factor output, which easily causes AC-DC power coupling oscillation and then instability and disconnection.

[0005] The present patent proposes to increase the controllability of GSC by dynamically adjusting the DC bus voltage during high penetration. SUMMARY

[0006] The present application proposes an improved control method for high penetration of direct-drive wind turbines based on virtual synchronization technology. The specific technical solution is as follows.

[0007] A high-voltage fault recovery improvement control method for direct-drive wind turbines based on virtual synchronization technology is characterized by improving the controllability of the grid-controlled system (GSC) during high-voltage faults by dynamically adjusting the DC bus voltage, thereby enhancing the unit's continuous fault recovery capability. Specifically, active power control improves VSG technology by designing power compensation terms, increasing system frequency support externally, and reducing bus voltage fluctuations internally. Reactive power control, based on industry standards, supports voltage recovery by injecting reactive current into the grid. The detailed control principle is as follows:

[0008] 1) Obtain the reactive power loop control parameters according to the standard requirements:

[0009]

[0010] 2) Active power loop control parameters:

[0011]

[0012] 3) The DC bus voltage is stabilized through the GSC active power loop, but the active power loop control faces the problem of GSC runaway caused by transient overvoltage on the AC side. After reactive power support is implemented according to standard requirements, under the action of inductive current, I g Increase, V L As the inductance increases, the voltage-dividing effect of the inductance strengthens, V e The amplitude has decreased, and the vector imbalance has been improved, but it still cannot meet the maximum regulation capacity of the converter. Requirements: With reactive current support, the controllable ultimate fault voltage of the GSC is V3, which is obtained from the vector diagram:

[0013]

[0014] 4) By adjusting the bus voltage U dc Increase The upper limit of the amplitude is used to control its stability at the equilibrium point L. At point L, we have:

[0015]

[0016] If the power factor angle of inductor L is In the current vector triangle, we have:

[0017]

[0018] In the inductor voltage vector triangle, we have:

[0019]

[0020] In triangle Δ OLQ There is:

[0021]

[0022] Upper limit of amplitude At this time, the following should be met: The bus voltage that meets can be obtained as follows:

[0023]

[0024] When U T exceeds V3, the DC bus voltage reference value is set according to the above formula, and the GSC voltage vector control can be met.

[0025] The principle based on the virtual synchronization technology is that the classic control steady-state droop equation of VSG is as follows:

[0026]

[0027] K ω is the active frequency modulation coefficient, K u is the reactive voltage modulation coefficient; for the P-ω active loop of the above formula, since the correlation between active imbalance and system frequency change during the fault is limited, it is mainly caused by voltage mutation, so the P-U relationship between active and fault voltage should be established; considering the voltage mutation at the fault moment, the current based on feedback regulation remains unchanged for a short time, causing the active output to change suddenly, causing transient fluctuations, so the imbalance mutation variable should be offset by setting a power compensation term, the specific method is as follows: real-time detection of the output active power and the current at the moment of fault, detection of the fault voltage U T , calculation of the current value that maintains the active power unchanged after the fault Calculation of the current difference value Get the power compensation term

[0028]

[0029] The beneficial effects of the present application are: by optimizing the active frequency ring, relying on the compensation current, the adjustment target value under the fault is quickly reached, the power output is stabilized, and the transition process of transient fluctuations is effectively slowed down. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a GSC vector diagram of a traditional control mode;

[0031] Figure 2 is an improved control principle diagram under continuous ride-through strategy. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application.

[0033] The VSG principle is to simulate the inertia, damping and primary frequency modulation characteristics of a traditional synchronous unit by adding a "swing equation", "droop control" and the like in power electronic device control. The control part mainly includes: active and frequency control loops, reactive and voltage control loops, virtual impedance control, current closed-loop control and PWM modulation. The active and frequency control loop is composed of an inertia damping model and a frequency regulator model. The differential equation is:

[0034]

[0035] P ref is an active power reference value, P e is the electromagnetic power output by the VSG, ω s is a system reference angular velocity, J is the rotational inertia of the VSG, and D is a damping coefficient. The differential equation of the reactive and voltage control is:

[0036]

[0037] Q ref is a reactive power reference value, Q e is the reactive power output by the VSG, E g is a system reference voltage. The VSG steady-state droop equation is:

[0038]

[0039] K ω is an active frequency modulation coefficient, K u is a reactive voltage modulation coefficient. The above is the classical control equation set of the VSG, but the model is a steady-state model designed based on small perturbations of the system. In this paper, it is applied to the voltage ride-through working condition under large disturbance, and corresponding changes need to be made on this basis to adapt to the fault transient state. For the P-ω active loop, since the correlation between active imbalance and system frequency change is limited during the fault, it is mainly caused by voltage mutation, so the P-U relationship between active and fault voltage should be established.

[0040] Considering that at the moment of fault, the voltage is suddenly changed, the current is maintained unchanged for a short time based on feedback regulation, causing a sudden change in active output and triggering transient fluctuations, so a power compensation term should be set to offset the imbalance mutation. The specific method is: real-time detection of the output active power before the fault and the current At the moment of fault, the fault voltage U T is detected, the current value maintaining the active power unchanged after the fault is calculated, and the current difference value Power compensation term

[0041] Substitute the power compensation term into the frequency feedback term in the active power droop formula to obtain

[0042]

[0043] By optimizing the active frequency loop, the compensation current is used to quickly reach the adjustment target value under fault, stabilize the power output, and effectively slow down the transient fluctuation transition process. According to the industry standard, the wind farm should inject dynamic reactive current into the power system during the fault, and meet the following formula:

[0044] ΔI T = K2 x (U T - 1.1) x I N , (1.1 ≤ U T ≤ 1.3)

[0045] K2 is the dynamic reactive current proportion coefficient of the wind farm. Substitute the above standard requirement into the VSG droop equation to obtain the reactive loop control parameter:

[0046]

[0047] Under this scheme design, the GSC active loop stabilizes the DC bus voltage, but the active loop control will face the problem of GSC out of control caused by transient overvoltage on the AC side, as shown in the principle Figure 1 When V pcc increases, in order to maintain the unit power factor operation under rated conditions, V e also needs to be increased to V e '. Due to the limitation of pulse width modulation, there is an upper limit for the regulation of the AC side voltage amplitude of GSC When

[0048]

[0049] After reactive support according to the standard requirement, as shown in Figure 2 , under the action of inductive current, I g increases, V L increases, and the voltage division effect of inductance is enhanced, the end of the voltage vector has moved to L point, and the amplitude of V e ' is reduced, and the vector imbalance is improved, but it still cannot meet the requirement of the maximum regulation capacity of the converter. With or without reactive current support, the controllable limit fault voltage of GSC is V3, which is obtained according to the vector diagram:

[0050]

[0051] To solve the problem, the patent proposes: by adjusting the bus voltage U dc Increase The upper limit of the amplitude, control its stable at the equilibrium point L, such as Figure 2 As shown; at L point:

[0052]

[0053] If the power factor angle of inductance L is In the current vector triangle:

[0054]

[0055] In the inductance voltage vector triangle:

[0056]

[0057] In triangle delta OLQ There is:

[0058]

[0059] The upper limit of the amplitude At this time, it should meet: The bus voltage that meets Can be obtained:

[0060]

[0061] When U T Exceeds V3, the DC bus voltage reference value set according to the above formula can meet the GSC voltage vector control. The continuous ride-through strategy designed above is theoretically not limited by fault time and number, and improves the grid-connected survival ability of the unit under severe working conditions

[0062] As described above, the present application has been described in detail, obviously, as long as the essence does not deviate from the inventive point and effect of the present application, and it is obvious to those skilled in the art that the deformation is also included in the protection scope of the present application.

Claims

1. An improved control method for high penetration of direct drive wind turbines based on virtual synchronous technology, characterized in that, During high voltage fault, the controllability of GSC is increased by dynamically adjusting the DC bus voltage, so as to improve the fault ride-through capability of the unit; wherein, the active control is improved by the VSG technology, and a power compensation term is designed to increase the system frequency support and reduce the bus voltage fluctuation; the reactive control is based on the industry standard, and the voltage recovery is supported by injecting reactive current into the power grid; the detailed control principle is: 1) The reactive ring control parameters are obtained according to the standard requirements: Wherein, K u is the reactive voltage regulation coefficient; K2 is the dynamic reactive current proportion coefficient of the wind farm; 2) The active ring control parameters: 3) The DC bus voltage is stabilized through the GSC active power loop, but the active power loop control faces the problem of GSC runaway caused by transient overvoltage on the AC side. After reactive power support is implemented according to standard requirements, under the action of inductive current, I g Increase, V L As the inductance increases, the voltage-dividing effect of the inductance strengthens, V e The amplitude has decreased, and the vector imbalance has been improved, but it still cannot meet the maximum regulation capacity of the converter. Requirements: With reactive current support, the controllable ultimate fault voltage of the GSC is V3, which is obtained from the vector diagram: 4) by adjusting the bus voltage U dc increasing the upper limit of the amplitude, controlling its stabilization at the equilibrium point L, at which point L there is: If the power factor angle of the inductance L is In the current vector triangle we have: In the inductance voltage vector triangle, there is: In triangle Δ OLQ has: upper limit of the amplitude At this time, the following should be satisfied: The bus voltage that satisfies can be obtained as When U T When V3 is exceeded, the DC bus voltage reference value is set according to the above formula, which can meet the GSC voltage vector control.

2. The improved control method for direct drive wind turbine with high penetration based on virtual synchronous technology according to claim 1, characterized in that, The principle based on the virtual synchronization technology is that the classic control steady-state droop equation of VSG is: where P ref is the active power reference value; Q ref is the reactive power reference value; ω s is the system reference angular velocity. K ω K is the active frequency regulation coefficient u K is the reactive voltage regulation coefficient; for the P-ω active loop of the above equation, given the limited correlation between active imbalance and system frequency variation during faults, it is mainly caused by voltage jumps, so a P-U link between active and fault voltage should be established; Considering the fault moment, voltage mutation, current based on feedback regulation short time remains unchanged, causing active output sudden change, causing transient fluctuations, so the power compensation term should be set to offset the imbalance mutation, the specific method is: real-time detection of active power before the fault And current The moment of fault, detect fault voltage U T , calculate the current value of maintaining active after fault Calculate the current difference Get power compensation term Analogous to the active droop formula, the above power compensation term is substituted into the droop equation to replace the frequency feedback term, and the following equation is obtained: Through the optimization of the active frequency ring, the compensation current is relied on to make it quickly reach the adjustment target value under fault, stabilize the power output, and effectively slow down the transition process of transient fluctuation.

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