Method for suppressing subsynchronous oscillation when direct-driven wind power plant is connected into weak power grid

Abstract

The invention discloses a method for suppressing subsynchronous oscillation when a direct-driven wind power plant is connected to a weak power grid, mainly comprising a subsynchronous oscillation suppression device and an improved virtual synchronous machine control method thereof, which are used for suppressing subsynchronous oscillation when the direct-driven wind power plant is connected to the weak power grid. The subsynchronous oscillation suppression device comprises a static synchronous compensator (STATCOM) and a battery energy storage system, an energy storage battery is connected to the direct current side of the STATCOM, and the control method is improved virtual synchronous machine control. According to the subsynchronous oscillation suppression device and the control method thereof, subsynchronous oscillation caused by interaction of the direct-driven wind power plant and the weak power grid can be well suppressed, and a new solution is provided for suppression of subsynchronous oscillation when the direct-driven wind power plant is connected to the weak power grid.

Description

Technical Field

[0001] The present invention relates to the field of subsynchronous oscillation suppression of a direct-driven wind farm connected to a grid, and in particular to a method for suppressing subsynchronous oscillation of a direct-driven wind farm connected to a weak power grid. Background Art

[0002] With the development of the times, the proportion of renewable energy generation is increasing, among which wind power generation has been widely used. Direct-drive wind turbines are also more efficient at low wind speeds, and have many advantages such as low noise, long life, small unit size, and low operation and maintenance costs. Therefore, there are more and more direct-drive wind farms. Since most wind farms are far away from the power load, direct-drive wind farms are mostly connected to weak power grids. In 2015, a large direct-drive wind farm in Hami, Xinjiang, experienced continuous subsynchronous oscillations. Subsequently, reports on subsynchronous oscillations of direct-drive wind farms connected to weak power grids gradually increased. The subsynchronous oscillation of direct-drive wind farms connected to weak power grids seriously affects the absorption of large-scale grid connection of new energy power generation and the reliability of new energy power generation and power supply systems. Therefore, the subsynchronous oscillation problem of direct-drive wind farms connected to weak power grids has received extensive attention and research.

[0003] Since the output power of wind power generation is greatly affected by wind speed, in order to smooth the power output of wind farms, battery energy storage systems are usually equipped to reduce peaks and fill valleys and suppress power oscillations of wind power generation. The STATCOM device is a reactive power compensation device and an important equipment for reactive power regulation in wind power plants. Some wind power plants integrate battery energy storage systems with STATCOM to form a STATCOM device with energy storage, which can not only perform reactive power compensation but also smooth the power output of wind turbines. Virtual synchronous machine control simulates the characteristics of traditional synchronous generators and has primary voltage regulation and primary frequency regulation functions. It is increasingly used in renewable energy power generation. The impedance of the grid-connected inverter controlled by the virtual synchronous machine is inductive in the full frequency band. In the case of weak grid connection, the grid impedance can be increased and the system can still operate stably. Traditional virtual synchronous machine control is mainly based on outputting active power, and it cannot be directly applied to STATCOM devices that mainly send reactive power. The original controller and control parameter design method need to be improved. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for suppressing subsynchronous oscillation of a direct-drive wind farm connected to a weak power grid in view of the deficiencies in the prior art.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A method for suppressing subsynchronous oscillation when a direct-drive wind farm is connected to a weak power grid comprises the following steps:

[0007] Step 1: Connect the energy storage battery group to the DC side of the STATCOM to form a STATCOM device with battery energy storage;

[0008] Step 2: Adding advance correction to the damping feedback branch of the traditional virtual synchronous machine active control loop to obtain an improved virtual synchronous machine controlled STATCOM device with battery energy storage;

[0009] Step 3: Connect the improved virtual synchronous machine controlled STATCOM device with battery energy storage in parallel with the direct drive wind turbine;

[0010] Step 4: Collect the current i of the output filter inductor of the STATCOM device abc And the grid voltage v abc , calculate the grid voltage v abc The effective value V n According to the instantaneous power theory, the active power P generated by the STATCOM device with battery energy storage controlled by the improved virtual synchronous machine is calculated. e ;

[0011] Step 5: Set the grid voltage V n , active power P e Substitute it into the improved virtual synchronous machine controller to obtain the modulation signal and realize the control of the STATCOM device with battery energy storage;

[0012] In step 2), the design of the advance correction parameters T1 and T2 enables the STATCOM device to increase the phase margin in the sub-synchronous and super-synchronous frequency bands, thereby improving the system stability.

[0013] The virtual synchronous machine control after adding the advance correction is as follows:

[0014]

[0015] Among them, P set are the rated active power of STATCOM, D(s) is the improved active droop control, D p is the damping coefficient, T1 and T2 are the lead correction coefficients, T e is the electromagnetic moment of inertia, T set is the rated moment of inertia, J is the active inertia coefficient, ω n and ω are the rated angular frequency and the actual angular frequency respectively, and θ is the STATCOM power angle.

[0016] The calculation formula of the modulation signal is as follows:

[0017]

[0018] Among them, V dcis the DC side voltage of STATCOM, V n is the effective value of STATCOM rated voltage.

[0019] Compared with the existing technology, this technology has the following beneficial effects:

[0020] The present invention aims at the problem of subsynchronous oscillation caused by the interaction between a direct-driven wind farm and a weak power grid, and proposes a method for suppressing subsynchronous oscillation of a direct-driven wind farm connected to a weak power grid. A STATCOM device with an energy storage battery is connected in parallel to the direct-driven wind farm, and its control method is an improved virtual synchronous machine control method with added lead correction. The proposed method for suppressing subsynchronous oscillation of a direct-driven wind farm connected to a weak power grid can well solve the problem of subsynchronous oscillation of a direct-driven wind farm connected to a weak power grid. The proposed subsynchronous oscillation suppression device and its control method are simple, reliable and highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Simplified circuit diagram for the weak grid connection system of direct-drive wind farm;

[0022] Figure 2 The impedance amplitude-frequency characteristic diagram of the direct-drive wind farm grid-connected system;

[0023] Figure 3 It is the control structure diagram of STATCOM based on virtual synchronous control;

[0024] Figure 4 Bode diagram of active control link of energy storage STATCOM before and after correction;

[0025] Figure 5 To add the impedance amplitude-frequency characteristic diagram of the energy storage STATCOM with improved virtual synchronous machine control before and after the direct-drive wind farm is connected to the grid;

[0026] Figure 6 To increase the voltage and current waveforms of the energy storage STATCOM device before and after the direct-drive wind farm weak grid connection with improved virtual synchronous machine control;

[0027] Figure 7 This is a flow chart of a method for suppressing subsynchronous oscillation when a direct-drive wind farm is connected to a weak power grid according to the present invention; DETAILED DESCRIPTION

[0028] In order to describe the present invention in more detail, the present invention is further illustrated below with reference to the accompanying drawings and specific implementation examples.

[0029] Simplified circuit of direct-drive wind farm grid-connected system Figure 1 As shown. The grid-connected inverter is connected via L f After filtering, the inductor L g and resistor R gconstitutes the grid impedance.

[0030] The positive and negative sequence impedance expressions of direct drive fans are:

[0031]

[0032] Among them, L f , R f , C f They are the inductance, resistance and capacitance of the filter circuit respectively, K m is the modulated signal gain, K dq is the dq axis decoupling coefficient, H d (s) = K dp +K di / s is the current inner loop transfer function, H q (s) = K qp +K qi / s is the voltage outer loop transfer function, U dc is the DC side voltage, C dc For DC capacitance measurement, U1 is the grid voltage amplitude, I1 and are the amplitude and initial phase angle of the grid-connected current, ω is the fundamental angular frequency, G(s) is the phase-locked loop transfer function, and the specific formula is:

[0033]

[0034] In the formula,

[0035] Among them, K pp is the proportional gain of the phase-locked loop, K pi is the phase-locked loop integral gain.

[0036] When the three-phase AC power grid is completely symmetrical, the positive and negative sequence impedance Z gp (s) = Z gn (s) = Z g (s), the equivalent impedance of the power grid is

[0037] Z g (s)=sL g +R g (3)

[0038] According to equations (1), (2) and (3), the impedance amplitude-frequency characteristics of the direct-drive wind farm connected to the weak grid are obtained as follows: Figure 2 As shown in Figure 1, the phase difference at the intersection of the wind turbine impedance and the grid impedance is greater than 180°, which makes sub- / super-synchronous oscillations more likely to occur.

[0039] The virtual synchronous machine control after adding the advance correction is as follows:

[0040]

[0041] According to the instantaneous power theory, the STATCOM with battery energy storage outputs active power P e The expression is

[0042] P e =1.5v d i d +v q i q (5)

[0043] Among them, v d and v q is the output voltage of the STATCOM with battery energy storage in the dq coordinate system; i d and i q is the value of the grid-connected current in the dq coordinate system.

[0044] According to formula (4), the modulation signal expression of STATCOM with battery energy storage can be obtained:

[0045]

[0046] Depend on Figure 3 The open-loop transfer function expression of the active control link of the energy storage type STATCOM before correction can be obtained:

[0047]

[0048] Similarly, the transfer function expression after correction can be obtained as

[0049]

[0050] Among them, E n is the rated voltage of the grid-connected power grid, X r is the STATCOM filter inductor L f impedance.

[0051] According to equations (7) and (8), the Bode diagram of the active control loop transfer function of the energy storage STATCOM before and after correction is as follows: Figure 4 As shown. Figure 4 It can be seen that after adding the lead correction, the amplitude-frequency curve is reduced by 20log(b)dB in the sub- and super-synchronous frequency bands, which enhances the system's anti-interference ability, improves the phase margin, and increases the system stability.

[0052] according to Figure 3 By modeling the sequence impedance of the STATCOM device with battery energy storage controlled by a virtual synchronous machine, the expression of its positive and negative sequence impedance can be obtained:

[0053]

[0054] Among them, Vr and I r are the amplitudes of the fundamental voltage and fundamental current, respectively, and f r is the fundamental frequency, is the initial phase of fundamental current, Sampling delay T s is the switching period, ω v and ω i are the cut-off frequencies of the voltage and current sampling low-pass filters, L r is the filter inductor of STATCOM.

[0055] Figure 5 To add the impedance amplitude-frequency characteristic diagram of the energy storage STATCOM with improved virtual synchronous machine control before and after the direct-drive wind farm is connected to the grid, where Z g ||Z vp The impedance Z g and Z vp The parallel equivalent value of Figure 5 It can be seen that when the energy storage STATCOM with improved virtual synchronous machine control is added to the direct-drive wind farm grid-connected system, the system is unstable before it is added, and the system becomes stable after the addition.

[0056] Figure 6 The voltage and current experimental waveforms of the direct-drive wind farm before and after the energy storage STATCOM with improved virtual synchronous machine control are shown. The system entered stability at 2s, and the weak grid was connected at 2.5s, and subsynchronous oscillation occurred. At 2.6s, the STATCOM was connected to the grid, suppressing the subsynchronous oscillation, and the system regained stability.

[0057] The results of the actual example show the effectiveness of the proposed method for suppressing subsynchronous oscillations of direct-drive wind farms connected to weak power grids. Figure 7 shown.

[0058] Finally, it should be noted that the above implementation cases are only used to illustrate the technical solution of the present invention rather than to limit it, but the protection scope of the present invention is not limited thereto, and any technician in the relevant field can still modify or make equivalent substitutions for the specific implementation modes of the present invention, which should all be covered within the protection scope of the claims of the present invention.

Claims

1. A method for suppressing subsynchronous oscillation when a direct-drive wind farm is connected to a weak power grid, characterized in that: The following steps are involved: Step 1: Connect the energy storage battery group to the DC side of the STATCOM device to form a STATCOM device with battery energy storage; Step 2: Perform advance correction in the damping feedback branch of the traditional virtual synchronous machine active control loop, and its transfer function is: STATCOM device with battery energy storage for improved virtual synchronous machine control; Step 3: Connect the improved virtual synchronous machine controlled STATCOM device with battery energy storage in parallel with the direct-drive wind farm; Step 4: Collect the current i of the output filter inductor of the STATCOM device abc And the grid voltage v abc , calculate the grid voltage v abc The effective value V n According to the instantaneous power theory, the active power P generated by the STATCOM device with battery energy storage controlled by the improved virtual synchronous machine is calculated. e ; Step 5: Set the grid voltage V n , active power P e Substitute it into the improved virtual synchronous machine controller to obtain the modulation signal, and realize the control of the STATCOM device with battery energy storage.

2. The control method of the weak grid-connected subsynchronous oscillation suppression device for a direct-drive wind farm according to claim 1 is characterized in that: In step 2), the design of the advance correction parameters T1 and T2 enables the STATCOM device to increase the phase margin in the sub-synchronous and super-synchronous frequency bands, thereby improving the system stability.

3. The method for suppressing subsynchronous oscillation of a direct-drive wind farm connected to a weak power grid according to claim 1, characterized in that: The virtual synchronous machine control after adding the advance correction is as follows: Among them, P set are the rated active power of STATCOM, D(s) is the improved active droop control, D p is the damping coefficient, T1 and T2 are the lead correction coefficients, T e is the electromagnetic moment of inertia, T set is the rated moment of inertia, J is the active inertia coefficient, ω n and ω are the rated angular frequency and the actual angular frequency respectively, and θ is the STATCOM power angle.

4. The method for suppressing subsynchronous oscillation when a direct-drive wind farm is connected to a weak power grid according to claim 3, characterized in that: The calculation formula of the modulation signal is as follows: Among them, V dc is the DC side voltage of STATCOM, V n is the effective value of STATCOM rated voltage.

Images