D-STATCOM additional damping control photovoltaic system subsynchronous / super-synchronous oscillation suppression method

Abstract

The invention discloses a D-STATCOM additional damping control method for suppressing subsynchronous / super-synchronous oscillation of a photovoltaic system, and belongs to the field of power system control. The method comprises the following steps: establishing a photovoltaic weak grid impedance model; according to the impedance characteristics of a photovoltaic grid-connected system, a D-STATCOM additional damping controller composed of a three-parameter wave trap is designed, and the D-STATCOM additional damping controller is added into a D-STATCOM controller; according to impedance analysis and simulation verification, the method can improve the positive damping of the photovoltaic system and eliminate the oscillation risk. The D-STATCOM additional damping controller designed by the invention is simple in structure and simple and convenient in parameter design, can adapt to flexible and changeable operation conditions of the photovoltaic power station on the basis of the basic function of D-STATCOM reactive compensation, and effectively inhibits subsynchronous / super-synchronous oscillation of the photovoltaic power station.

Description

Technical Field

[0001] The invention relates to the field of power system control, and in particular to a D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system. Background Art

[0002] Photovoltaic power generation has the advantages of being renewable and low-pollution, and has become an irreplaceable part of the modern power industry. Large-scale photovoltaic stations are mainly distributed in remote areas. The impedance of the transmission line cannot be ignored when transmitting electricity over long distances. In the case of weak power grids, the interaction between photovoltaic stations and the inductive weak power grid impedance will bring the risk of sub- / super-synchronous oscillation (SSO) to the system.

[0003] In the prior art, methods for suppressing subsynchronous / supersynchronous oscillation of photovoltaic systems are divided into two categories: one is to suppress SSO by using Flexible AC Transmission System (FACTS) equipment; the other is to suppress SSO by changing the control method inside the photovoltaic inverter.

[0004] In the use of FACTS equipment to suppress SSO: Static Var Generator (SVG) can provide reactive power compensation for the system. Existing inventions have added a supplementary sub-synchronous damping controller (SSDC) to make SVG have a positive resistance effect in the sub-synchronous frequency band, consuming the energy of sub-synchronous oscillation to achieve a suppression effect; Static Synchronous Compensator (STATCOM) can change the reactive power distribution of the system, and additional damping control can also be added to STATCOM to suppress SSO. However, the existing FACTS device additional damping control only considers the occurrence of sub-synchronous oscillations, and cannot cope with the situation where sub-synchronous and super-synchronous oscillations occur simultaneously in the power system. In addition, the structure and parameter design of the additional damping control are relatively complex and have poor applicability.

[0005] In changing the control method inside the photovoltaic inverter to suppress SSO: an existing invention has designed a subsynchronous oscillation suppression strategy for photovoltaic systems by drawing on the supplementary damping control (Supplementary Sub-synchronous Damping Controller, SSDC) in the high-voltage direct current transmission (HVDC) system. SSDC generates an additional damping signal to suppress SSO by filtering, phase shifting, and amplifying the input generator speed difference signal. However, the phase shifting link parameters in the SSDC need to be designed based on a certain specific operating condition. When the operating condition changes, the phase shifting parameters need to be recalculated, which brings great difficulties to the actual operation of the photovoltaic power station. Summary of the invention

[0006] In view of the defects of the prior art, the object of the present invention is to provide a D-STATCOM (Distributed Static Synchronous Compensation, D-STATCOM) additional damping control to suppress subsynchronous / supersynchronous oscillations of photovoltaic systems, aiming to further improve the existing suppression technology, simplify the structure of additional damping control, reduce the difficulty of parameter design, improve the damping of photovoltaic systems, enhance the stability of the system, and suppress subsynchronous / supersynchronous oscillations of photovoltaic systems.

[0007] To achieve the above purpose, the method of suppressing subsynchronous / supersynchronous oscillation of photovoltaic system by using D-STATCOM in the present invention adopts the following technical means:

[0008] The impedance model of the photovoltaic system is established to determine whether SSO will occur when the PV power station is connected to the weak AC grid. The overall equivalent model includes the equivalent impedances on the inverter side and the grid side, that is,

[0009]

[0010] In the above formula, Z total (s) is the impedance model of the photovoltaic system; s is a complex variable; Z inv (s) is the equivalent impedance of the inverter side; Z g (s) is the equivalent impedance on the grid side.

[0011] The equivalent impedance model of the photovoltaic inverter side includes the current control link, the voltage control link, the control delay link and the LC filter, that is,

[0012]

[0013] In the above formula, s is a complex variable; G c (s)=k pi +k ii / s is the PI controller of the current control loop; G v(s)=k pv +k iv / s is the PI controller of the voltage control loop; G d (s) is the control delay; Z L and Z C are the LC filter inductor and capacitor impedances; Z g (s) is the grid-side equivalent impedance.

[0014] The grid-side impedance is equivalent to a series connection of resistance and inductance, that is,

[0015]

[0016] In the formula, R g is the equivalent resistance on the grid side; L g Grid side equivalent inductance.

[0017] Under certain conditions, the impedance of the PV inverter is capacitive in the subsynchronous and supersynchronous frequency domains, and the interaction with the impedance of the inductive weak grid transmission line is prone to subsynchronous and supersynchronous oscillations. The real part of the global admittance Re (Y total ) is less than zero, the system is in a negative damping state, and SSO is prone to occur when PV is connected to a weak grid system.

[0018] The invention suppresses subsynchronous oscillation and supersynchronous oscillation of photovoltaic and weak power grid systems, including a D-STATCOM additional damping control. The damping controller is added to the controller of the D-STATCOM and is composed of the following formula:

[0019]

[0020] In the formula, G SDC (s) is the transfer function of the additional damping control; G E (s) is the transfer function of the three-parameter notch filter; K is the gain factor of the gain link.

[0021] The impedance model of the D-STATCOM photovoltaic system with additional damping control is derived as follows:

[0022]

[0023] In the above formula, Z TOTAL (s) is the impedance model of the D-STATCOM photovoltaic system with additional damping control; Z M is the equivalent impedance of D-STATCOM with additional damping control.

[0024] In summary, compared with the prior art, the present invention can achieve the following beneficial effects:

[0025] (1) The present invention considers that a distributed PV power generation system can be equipped with a D-STATCOM to provide reactive power compensation for the system. The D-STATCOM can dynamically generate capacitive or absorb inductive reactive power according to the needs of the distribution system to improve the power factor and voltage stability of the system. However, there is currently no research that proposes the use of D-STATCOM to suppress the SSO problem. Therefore, the present invention innovatively proposes an additional damping control based on D-STATCOM to provide positive damping for the photovoltaic system, reshape the impedance of the photovoltaic system, and solve the SSO problem.

[0026] (2) The method proposed in the present invention simplifies the structure and parameter design of the additional damping control. Compared with the traditional SDC control which can only be designed for operation under a certain working condition, the additional damping control in the present invention uses a three-parameter notch filter for filtering and does not require a phase compensation link. The notch depth and bandwidth of this notch filter can be adjusted separately, which can effectively deal with the situation where subsynchronous components and supersynchronous components appear in the system at the same time. It can effectively suppress the SSO of the PV power station without adding hardware equipment, reducing costs and being easy to implement in engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is an equivalent model of the PV system with D-STATCOM access;

[0028] Figure 2 It is the overall control strategy of D-STATCOM to suppress SSO;

[0029] Figure 3 It is the SDC structure diagram;

[0030] Figure 4 It is the schematic diagram of D-STATCOM suppressing SSO;

[0031] Figure 5 is the impedance diagram of D-STATCOM at subsynchronous frequency;

[0032] Figure 6 It is the impedance diagram of D-STATCOM at super synchronous frequency;

[0033] Figure 7 It is the dynamic response diagram of the active power of the PV grid-connected point when the series inductance L1 of the test system is 2.60H;

[0034] Figure 8 It is the dynamic response diagram of the active power of the PV grid-connected point when the series inductance L1 of the test system is 2.66H;

[0035] Fig. 9 The test system is tested at an input irradiance of 900W / m 2 The dynamic response waveform of the active power at the PV grid-connected point when ;

[0036] Fig.10 The test system is tested at an input irradiance of 950W / m 2 The dynamic response waveform of the active power at the PV grid-connected point when ; DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0038] The equivalent model of the PV system with D-STATCOM access in the specific embodiment of the present invention is as follows: Figure 1 As shown. The rated power of the PV power station is 10MW, R g is the equivalent resistance of the power grid, L g (0.1H) is the aggregate inductance of the transmission line and each level of step-up transformer (T1, T2) as seen from the grid connection point, and the series inductance L1 (2.60H, 2.66H) is used to simulate the change of the AC system access strength. f and C f are the inductance and capacitance of the LC filter respectively. D-STATCOM is boosted by a 1.25 / 25kV transformer T D The series inductor is short-circuited at the beginning of the simulation, and switch K is closed at 1s to trigger the SSO of the PV power plant.

[0039] D-STATCOM through step-up transformer T D , the coupling inductor L and the coupling capacitor C are connected in parallel to the line, and the overall control strategy includes the basic control strategy and the additional damping control strategy to suppress SSO, such as Figure 2 shown.

[0040] Figure 2 In the control strategy, the basic control strategy includes a phase-locked loop, two measurement systems, a voltage outer loop and a current inner loop control strategy. abc It is the three-phase electricity of the public grid connection point; abc is the three-phase current at the public grid connection point; u g is the grid voltage; θ is the phase angle of the phase-locked loop (PLL) output; i dg 、i qg are the d-axis and q-axis components of the current respectively; u dg 、u qg are the d-axis and q-axis components of the voltage at the D-STATCOM access point, respectively; i * dg and i * qg are the reference values ​​of the d-axis and q-axis components of the current respectively; U *dc is the DC side capacitor voltage reference value of the D-STATCOM inverter, U dc is the actual value; ωL is the decoupling term applied to the voltage and current of D-STATCOM; u dk and u qk are the d-axis and q-axis components of the modulation voltage generated by the dual-loop control; u ca is the modulation instruction of the input DC capacitor voltage controller; S abc It is the modulation instruction of D-STATCOM.

[0041] The input signal of the SDC control strategy is the grid-connected current i abc , generate the reference value u to suppress the SSO instruction * abc _ sub / super The suppression command is added to the modulation command of the DC capacitor voltage controller and does not affect the original control strategy of D-STATCOM. * abc In order to superimpose the modulation signal of the SSO suppression function, the signal passes through the SPWM link to form a modulation signal which is input into the D-STATCOM inverter unit.

[0042] The additional damping control strategy of D-STATCOM has the same three-phase structure. Taking the generation process of phase A command as an example, Figure 3 As shown, it includes three-parameter notch filter, gain, and limiting links. The three-parameter notch filter is used to separate the subsynchronous component and supersynchronous component in the grid-connected point current. The obtained components have no phase offset and there is no need to perform phase compensation on the extracted signal. The extracted signal passes through the gain and limiting links to obtain the A-phase reference command value u of the D-STATCOM to suppress SSO. * a_sub / super Similarly, the reference command values ​​u for phase B and phase C can be obtained. * b_sub / super and u * c_sub / super .

[0043] When SSO occurs in the PV system, the rated frequency of the system is set to f0 and the oscillation frequency to f i At this time, the FFT analysis of the grid connection point voltage after the oscillation occurs shows that there is a sub-synchronous component f0-f in the frequency band symmetrical to the power frequency component. i and a supersynchronous component f0+f i , the difference between the two components is small, so in order to accurately extract the oscillating current component and simplify the structure and parameter design difficulty of the filtering link, the filter uses a three-parameter notch filter, and its transfer function and parameter relationship are as follows:

[0044] (1)

[0045] (2)

[0046] (3)

[0047] (4)

[0048] Among them, w n is the central angular frequency of the three-parameter notch filter, ξ1 and ξ2 are notch factors, depth is the notch depth, f ωb For bandwidth.

[0049] By using the simulation test method, the minimum K value that can achieve the suppression effect under the worst working conditions is selected as the final setting value of the gain.

[0050] based on Figure 4 The principle of D-STATCOM in the present invention to suppress SSO of photovoltaic system is explained. T1 and x T2 is the transformer; r g is the grid-side equivalent resistance; x g is the grid-side equivalent inductance; u g is the grid voltage. After being connected to D-STATCOM, the grid-connected current i pcc The subsynchronous current and supersynchronous current i are extracted through the three-parameter notch filter. sub / super ,i sub / super Then, the current reference command u that suppresses SSO is obtained through the gain and limiter links. * sub / super After this instruction is superimposed on the modulation wave of D-STATCOM, it can output the same as i sub / super Subsynchronous voltage and supersynchronous voltage with close phase u sub / super , so that D-STATCOM is equivalent to the positive impedance of the parallel access system, providing positive damping for the system and reducing the risk of system oscillation. Therefore, when the real and imaginary parts of the impedance of D-STATCOM at the subsynchronous and supersynchronous frequencies are positive, D-STATCOM can provide positive damping for the system, reshape the PV system damping, make the damping of the overall system positive, and thus suppress SSO.

[0051] In order to further analyze the impedance characteristics of D-STATCOM in the subsynchronous frequency domain and the supersynchronous frequency domain, the present invention adopts the frequency sweeping method to inject the harmonic voltage with continuously changing frequency at the grid connection point, measure the current of the D-STATCOM grid connection point, and thus obtain the impedance value of D-STATCOM at each frequency point, such as Figure 5 and Figure 6 shown. Figure 5 and Figure 6The impedance of D-STATCOM in some frequency bands (46~52Hz and 66~72Hz) is displayed. In the risk areas of 46~52Hz and 66~72Hz, D-STATCOM exhibits impedance with positive resistance and positive reactance, which verifies that the D-STATCOM with additional damping control can provide positive damping for the system and reduce the risk of oscillation.

[0052] The suppression effect of this embodiment is verified, and the simulation results are as follows: Figure 7 , 8 , 9, and 10.

[0053] The dynamic response of the active power of the public grid connection point before and after the D-STATCOM with additional damping control is analyzed when the system access strength changes. The access strength of the PV system can be reflected by the series inductance of the infinite power source. The series inductance L1 is set to 2.60H and 2.66H respectively, so that the grid access inductance L g =L g +L1 is 2.70H and 2.76H respectively. Figure 7 and Figure 8 In order to maintain the PV output at 10MW, the system series inductance L1 is set to 2.60H and 2.66H respectively, and the response characteristics of the active power are measured at the grid connection point before and after the D-STATCOM with additional damping control is put into operation.

[0054] Depend on Figure 7 and Figure 8 It can be seen that the system was in steady-state operation 1 second ago. After the system access strength changed at 1 second, the PV power station experienced different degrees of SSO. When the D-STATCOM with additional damping control was not added, the system access strength weakened with the increase of series inductance L1, and the oscillation amplitude of active power increased. After the D-STATCOM with additional damping control was added, the oscillation of active power was suppressed.

[0055] The dynamic response of the grid-connected active power before and after the D-STATCOM with additional damping control is put into operation when the PV input irradiance changes. Fig. 9 and Fig.10 The series inductance L1 is 2.62H and the PV input irradiance is 900W / m 2 、950W / m 2 The response characteristics of active power measured at the grid connection point before and after the D-STATCOM with additional damping control is put into operation.

[0056] Depend on Fig. 9 and Fig.10It can be seen that when the D-STATCOM with additional damping control is not put into use, the active power of the PV system grid connection point will be unstable and oscillate under the two input irradiance conditions, and the oscillation amplitude will increase with the increase of input irradiance. After the D-STATCOM with additional damping control is put into use, the active oscillation will be suppressed to a certain extent, and the oscillation will converge significantly, indicating that the suppression effect of the D-STATCOM with additional damping control will not be affected by the change of photovoltaic input irradiance.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A D-STATCOM additional damping control to suppress subsynchronous / supersynchronous oscillation of photovoltaic system, characterized in that: The following steps are involved: Step 1: Based on the impedance model of the photovoltaic system, analyze the impedance characteristics of the photovoltaic system and the subsynchronous / supersynchronous oscillation mechanism; Step 2: Based on step 1, a D-STATCOM additional damping control is proposed to suppress the subsynchronous / supersynchronous oscillation of the photovoltaic system, reshape the impedance model of the photovoltaic system, eliminate the negative damping characteristics of the system at the subsynchronous / supersynchronous frequency, and eliminate the oscillation risk; Step 3: Based on step 2, an additional damping control link is added to the controller of the D-STATCOM to suppress subsynchronous / supersynchronous oscillations in the photovoltaic system.

2. A D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system according to claim 1, characterized in that: The impedance model of the photovoltaic system needs to include the current control link, voltage control link and total control delay link related to the subsynchronous / supersynchronous frequency characteristics, that is, In the above formula, Z total (s) is the impedance model of the photovoltaic system; s is a complex variable; G c (s)=k pi +k ii / s is the PI controller of the current control loop; G v (s)=k pv +k iv / s is the PI controller of the voltage control loop; G d (s) is the control delay; Z L and Z C is the inductor impedance and capacitor impedance of the LC filter; Z g (s) is the grid-side equivalent impedance.

3. A D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system according to claim 1, characterized in that: The impedance model of the photovoltaic system consists of the inverter side impedance and the grid side impedance, that is, In the above formula, Z inv is the equivalent impedance of the inverter side of the photovoltaic grid-connected system, Z g is the grid-side equivalent impedance of the photovoltaic grid-connected system; R g and L g is the resistance and inductance of the power grid; T s is the sampling period.

4. A D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system according to claim 2, characterized in that: In the impedance model of the photovoltaic system, the D-STATCOM is connected to the common coupling point of the photovoltaic system, which is equivalent to the impedance of the parallel connection system. By adding an additional damping control link in the D-STATCOM DC capacitor voltage control, the impedance of the D-STATCOM at the oscillation frequency is changed, which has the effect of eliminating the negative damping area and suppressing oscillation.

5. A D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system according to claim 1, characterized in that: The subsynchronous / supersynchronous oscillation suppression method of the photovoltaic system proposed in step 2 includes an additional damping controller based on a three-parameter notch filter, and an additional damping control link is introduced into the D-STATCOM DC capacitor voltage control link to generate a subsynchronous / supersynchronous voltage signal with the same phase as the subsynchronous / supersynchronous current of the grid-connected point.

6. A D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system according to claim 5, characterized in that: The additional damping controller includes a three-parameter notch filter, a gain link and a limiter. The additional damping controller is composed of the following formula: In the formula, G SDC (s) is the transfer function of the additional damping controller; G E (s) is the transfer function of the three-parameter notch filter; s is a complex variable; K is the gain link.

7. A D-STATCOM additional damping control for suppressing subsynchronous / supersynchronous oscillation of a photovoltaic system according to claim 1, characterized in that: After adopting the D-STATCOM additional damping control in step 3, the impedance model of the PV system is reshaped as: In the above formula, Z TOTAL (s) is the impedance model of the photovoltaic system after adding the D-STATCOM additional damping control link; Z M (s) is the D-STATCOM equivalent impedance; s is a complex variable.

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