Low penetration improvement control method for direct-drive wind turbine based on virtual synchronous technology

Abstract

The application aims at the active power sudden drop of GSC output of permanent magnet direct drive unit when grid-connected point voltage is reduced, if RSC output is unchanged, unbalanced power will be accumulated on the DC side, causing abnormal lifting of bus voltage, endangering safety of devices and even triggering off-grid problem, and proposes an improved control of low penetration of direct drive wind turbine based on virtual synchronization technology. During low voltage fault, through variable power tracking methods such as overspeed power limiting and emergency variable pitch, active power flow is quickly and accurately balanced; through improved VSG technology, a power compensation term is designed to reduce bus voltage fluctuation; through the coordinated cooperation of these strategies, long-term and large-scale sustainable fault voltage penetration is finally realized.

Description

TECHNICAL FIELD

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

[0002] In high-voltage alternating current transmission, short circuit and other faults are easy to cause low voltage penetration. For transient low voltage, when the grid-connected point voltage decreases, the GSC output active power of the unit suddenly decreases, if the RSC output remains unchanged, the unbalanced power will accumulate on the DC side, causing abnormal lifting of the bus voltage, endangering the safety of the device and even triggering off-grid. Once the machine terminal voltage appears sustained high and low oscillation, the uncontrollable of this converter and the adverse effects of power imbalance fluctuation superimposed together will cause the unit and the supporting reactive device to repeatedly adjust and expand the instability trend, increasing the difficulty of fault penetration.

[0003] Summarize the reasons for the large-scale off-grid and power failure of wind turbines: the response lag of the reactive power compensation device of the wind farm causes large fluctuations in reactive power, leading to sustained voltage oscillation, which is the external cause of the off-grid of a large number of wind turbines.

[0004] And the internal reason is that the wind turbine does not have the continuous penetration capability to cope with long-time, high-frequency and large-scale voltage faults, and the weak active support capability during the fault will further worsen the electrical operating environment, leading to a power failure.

[0005] It can be seen that under the new challenges brought by the "double high system", it is crucial to improve the continuous penetration capability of the wind turbine and make it have the same strong fault disturbance resistance as the synchronous unit, by maintaining the active and reactive power balance during the fault, so as to fully play the role of the wind turbine as the core power supply in the power grid support. SUMMARY

[0006] The present application proposes a direct-drive wind turbine low penetration improved control method based on virtual synchronization technology. The specific technical solution is as follows.

[0007] A direct-drive wind turbine high penetration improved control based on virtual synchronization technology, characterized in that the strategy quickly and accurately balances the active flow during low voltage fault through over-speed power limiting, emergency pitch and other variable power tracking methods; wherein the active control improves the VSG technology, designs a power compensation term, increases the system frequency support externally and reduces the bus voltage fluctuation internally; the reactive power control is based on industry standards, and supports voltage recovery by injecting reactive current into the grid; the detailed control principle is:

[0008] 1) Get the reactive power ring control parameters according to the standard requirements:

[0009]

[0010] 2) Active ring control parameters:

[0011] Due to the limitation of converter capacity, the active current needs to be limited:

[0012] 3) As the voltage decreases, in order to maintain the same active output, the active current will increase until it reaches the converter capacity limit, at this time, the adjustable limit voltage of GSC is V1, corresponding to the solution formula as follows:

[0013]

[0014] 4) When the terminal voltage continues to drop below V1, unbalanced power will accumulate on the DC side, causing the bus voltage to rise. In order to enhance the ability of the unit to cope with long-term continuous low voltage fault, a sustainable low voltage ride through method based on RSC variable power tracking (VPPT) is designed; RSC always runs in maximum power tracking mode (MPPT) in normal state, and its electromagnetic power control formula is:

[0015]

[0016] When the terminal voltage drops suddenly and the GSC power output is limited, the RSC output can be reduced to make P m = P e , to suppress power fluctuations, and the variable power tracking electromagnetic power control P VPPT expression is as follows:

[0017]

[0018] During this period, if the speed reaches the upper limit, the speed protection will be triggered, and then the electromagnetic power is adjusted to maintain the speed stable at the maximum value ω max , at this time, the speed regulation limit is reached, at this time, the voltage drop limit that the fan can withstand is V2, according to the wind power capture function of the unit itself, the mechanical power P m1 at this time is solved:

[0019]

[0020] Let P VPPT = P m1 , get

[0021]

[0022] The electromagnetic power control equation under constant speed is:

[0023]

[0024] When the voltage continues to drop beyond V2, the RSC output power needs to be further reduced. This can only be achieved by adjusting the pitch angle β while maintaining constant speed control, keeping the maximum speed and tip speed ratio λ constant. p An improved emergency pitch control method is adopted, which directly calculates and generates the target pitch angle β based on the fault voltage. x To achieve rapid and precise control, at this time there is

[0025] P m =C p (β x ,λ max )kv n 3 =P VPPT

[0026] Solving the above equation yields

[0027]

[0028] After obtaining the pitch angle, the pitch controller directs the pitch actuator to quickly track the target value. Analogous to the active power droop formula, substituting the power compensation term into the droop equation to replace the frequency feedback term, we obtain:

[0029]

[0030] The beneficial effects of this invention are: by optimizing and modifying the active frequency loop, relying on the compensation current, it can quickly reach the adjustment target value under fault conditions, stabilize the power output, and effectively mitigate the transient fluctuation transition process. Attached Figure Description

[0031] Figure 1 This is a block diagram of the variable pitch control system. Detailed Implementation

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

[0033] The key of fan fault ride-through is to maintain the balance of active flow and provide reactive power to the grid to support voltage recovery. However, the existing standards are mainly for quantitative requirements of reactive current, and less attention is paid to active control during fault. Since the traditional synchronous machine has high control capability in both active and reactive during fault, it is necessary to improve the requirement of active output of wind turbine. The application of virtual synchronous generator (VSG) technology to fan fault ride-through is an effective method to solve this problem. VSG technology better simulates the rotor inertia characteristics, frequency and voltage regulation characteristics of synchronous generators. When the above control strategy is applied to fan fault ride-through, it can effectively stabilize the active output, improve the system transient power frequency characteristics, and improve the transient fluctuation of the DC bus voltage of the unit.

[0034]

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

[0036]

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

[0038]

[0039] K ω is the active frequency regulation coefficient, K u is the reactive voltage regulation coefficient. The above is the classical control equation set of VSG, but this model is a steady-state model designed based on small disturbance of the system. This patent applies it to voltage ride-through working condition under large disturbance, and needs to make corresponding changes based on this to adapt to fault transient state. For the P-ω active loop, since the correlation between active imbalance and system frequency change during fault is limited, 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 suddenly changes, the current remains unchanged for a short time based on feedback regulation, causing sudden change of active output and causing transient fluctuation, so the imbalance mutation should be offset by setting a power compensation term. The specific method is: real-time detection of output active power before fault and current At the moment of fault, the fault voltage U is detected T , the current value maintaining the active power unchanged after fault is calculated The current difference value is calculated The power compensation term is obtained

[0041] The above power compensation term is substituted into the analog active droop formula to replace the frequency feedback term, and the following formula is obtained

[0042]

[0043] Through optimization and modification of the active frequency loop, the compensation current is relied on 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 fault, and meet the following formula:

[0044] ΔI T = K1 × (0.8 - U T ) × I N , (0.2 ≤ U T ≤ 0.8)

[0045] K1 is the dynamic reactive current proportion coefficient of the wind farm. The above standard requirement is substituted into the VSG droop equation to obtain the reactive loop control parameter:

[0046]

[0047] And in the VSG active loop:

[0048] Due to the limitation of the converter capacity, the active current needs to be limited:

[0049] With the decrease of voltage, in order to maintain the active output unchanged, the active current will increase until it reaches the converter capacity limit. At this time, the limit voltage adjustable by the GSC is V1, and the corresponding solving formula is as follows:

[0050]

[0051] When the terminal voltage continues to drop below V1, unbalanced power will accumulate on the DC side, causing the bus voltage to rise. In order to enhance the ability of the unit to cope with long-time continuous low-voltage fault, this patent proposes a sustainable low-voltage ride-through method based on RSC variable power tracking (VPPT). The principle is introduced as follows: under normal working conditions, the mechanical power P m captured by the wind turbine from the wind can be expressed as:

[0052] P m=C p (β,λ)kv 3

[0053] where k=0.5ρπR 2 , v is wind speed, ρ is air density (kg / m3), R is wind turbine radius (m), C p is wind energy utilization coefficient, C p is affected by pitch angle β and tip speed ratio λ:

[0054]

[0055] λ is affected by wind turbine speed ω r and wind speed v. RSC always runs in maximum power point tracking (MPPT) mode in normal state, and its electromagnetic power control formula is:

[0056]

[0057] K opt is optimal torque coefficient. By changing RSC output electromagnetic power, changing speed, affecting tip speed ratio, and then changing wind energy utilization coefficient, the wind turbine is stabilized at the maximum power point at all times, and the maximum power is output. When the terminal voltage drops suddenly, the GSC power output is limited, and the RSC output can be reduced to make P m =P e , to suppress power fluctuations. Based on the above considerations, the expression of variable power tracking electromagnetic power control P VPPT is as follows:

[0058]

[0059] After the electromagnetic power is switched, the electromagnetic torque decreases, and under the action of unbalanced torque, the rotor speed rises, causing the mechanical power to decrease, providing stable regulating output, so as to achieve active power balance. During this period, if the speed reaches the upper limit, the speed protection will be triggered, and then the electromagnetic power is adjusted to maintain the speed stable at the maximum value ω max , at this time, the variable speed regulation limit is reached. In view of the fact that wind turbine off-grid accidents are mostly large wind power generation, the voltage drop limit V2 that the wind turbine can withstand under rated working condition can be determined by using RSC variable speed limit power control. Assuming that the wind speed at this time is the rated value v n , according to the wind power capture function of the unit itself, the mechanical power P m1 at this time is solved:

[0060]

[0061] Let P VPPT =P m1 , get

[0062]

[0063] V2 is solved by the above formula. The electromagnetic power control equation under constant speed is:

[0064]

[0065] When the voltage continues to drop beyond V2, the RSC output power also needs to be reduced, and only on the basis of maintaining constant speed control, maintaining maximum speed and tip speed ratio λ unchanged, the pitch angle β is changed to adjust C p . The conventional pitch control is mostly based on power and speed feedback, and the PI regulation is used to generate pitch command. For example:

[0066]

[0067] But this conventional pitch has disadvantages: one is slow adjustment, and the other is overshoot oscillation, which is not conducive to stability, so it is not suitable for rapid power reduction during fault. This patent designs an improved emergency pitch control, which directly calculates the target value of the pitch angle according to the fault voltage, realizes rapid and accurate control, and the principle is as follows: according to the variable power tracking power P VPPT , combined with the unit wind energy utilization function expression, the accurate pitch angle β x under the corresponding wind speed and speed is solved, at this time

[0068] P m = C p (β x ,λ max )kv n 3 =P VPPT

[0069] Solving the above formula gives

[0070]

[0071] After the pitch angle is obtained by the pitch controller, the pitch actuator is commanded to act, and the target value is quickly tracked, as shown in Figure 1 . In theory, with the adjustment of the pitch angle, the mechanical power can be dynamically adjustable from P m1 to zero, and combined with the variable speed limited power control to realize long-term and large-range sustainable low voltage ride through.

[0072] As described above, the present application has been described in detail, and it is obvious that any modification which does not deviate from the essential point and effect of the present application and is obvious to those skilled in the art is also included in the protection scope of the present application.

Claims

1. A low penetration improvement control method for a direct-drive wind turbine based on virtual synchronous technology, characterized in that, During low voltage fault, the active power flow is quickly and accurately balanced by variable power tracking methods such as overspeed limiting power, emergency variable pitch, etc. Among them, the active control improves the VSG technology, designs a power compensation term, increases the system frequency support externally, and reduces the bus voltage fluctuation internally. The reactive power control is based on industry standards, and supports voltage recovery by injecting reactive current into the grid. The detailed control principle is as follows: 1) Get the reactive power ring control parameters according to the standard requirements: Wherein, K u is the reactive voltage regulation coefficient; K1 is the dynamic reactive current proportion coefficient of the wind farm; 2) Active ring control parameters: Limited by the capacity of the converter, the active current needs to be limited: 3) As the voltage decreases, in order to maintain the same active output, the active current will increase until it reaches the capacity limit of the converter. At this time, the adjustable limit voltage of the GSC is V1, and the corresponding solution formula is as follows: 4) When the terminal voltage continues to drop below V1, unbalanced power will accumulate on the DC side, causing the bus voltage to rise. In order to enhance the ability of the unit to respond to long-term continuous low voltage fault, a sustainable low voltage ride through method based on RSC variable power tracking VPPT of machine side converter is designed. RSC is always running in maximum power tracking mode MPPT under normal state, and its electromagnetic power control formula is: where K opt is the optimum torque coefficient; ω r is the wind turbine rotational speed; When the terminal voltage drops suddenly, the GSC power output is limited, and by reducing the RSC output, P m = P e , the power fluctuation is suppressed, and the power tracking electromagnetic power control P VPPT The expression is as follows: During this period, if the speed reaches the upper limit, the speed protection will be triggered, and then the electromagnetic power will be adjusted to maintain the speed stable at the maximum value ω max At this time, the speed regulation limit is reached, and at this time, the voltage drop limit of the fan is V2. According to the wind power capture function of the unit itself, the mechanical power P m1 : where P m1 is the mechanical power that the fan can capture when reaching the maximum rotational speed; C p is the wind energy utilization coefficient; R is the radius of the wind wheel, in meters; Let P VPPT = P m1 , we get The electromagnetic power control equation under constant speed is: When the voltage continues to drop beyond V2, the RSC output power also needs to be reduced, and only on the basis of maintaining constant speed control, maximum speed and constant tip speed ratio λ, the pitch angle β is changed to adjust C p An improved emergency pitch control is adopted, and the pitch angle target value β is directly calculated according to the fault voltage to realize fast and accurate control. At this time, there are x ​ P m = C p (β x , λ max ) k n 3 = P VPPT where P m is the mechanical power; β x is the precise pitch angle; Solving the above equation gives After the pitch angle is obtained by the pitch controller, the pitch actuator is instructed to move quickly to track the target value.

2. The improved control method for low penetration of direct drive wind turbines based on virtual synchronous technology according to claim 1, characterized in that, The principle based on virtual synchronous technology is: the 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; Considering the fault moment, voltage mutation, current based on feedback regulation short time remains unchanged, causing active output sudden change, causing transient fluctuations, so should be through the setting power compensation term to offset the imbalance mutation, the specific method is: real-time detection of active power before the output and current The moment of fault, detect fault voltage U T , calculate the current value of the active power after the fault Calculate the current difference Get power compensation term Analog active droop formula, the above power compensation term is substituted into the droop equation instead of the frequency feedback term, get: wherein, Ppre-fault is the pre-fault output active power; Ipre-fault is the pre-fault output current; Through optimization and modification of the active frequency ring, rely on compensation current to quickly reach the target value under fault.

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