Self-adaptive fault ride-through method for maintaining stability of power angle of grid-forming type converter

Abstract

The invention discloses a self-adaptive fault ride-through method for maintaining the stability of the power angle of a grid-forming type converter. The method comprises the following steps: detecting whether the voltage of a power grid drops or not and the voltage drop depth; calculating a power angle stability coefficient according to system working conditions; acquiring the power grid voltage and current of a grid-connected point of the converter, the actual value and reference value of the active power output by the converter, and the reference value of the amplitude of the power grid voltage; calculating a phase reference value of the alternating current side of the converter; calculating a reference value of the current of the grid-connected point of the current converter under the dq rotating coordinate system according to a set voltage amplitude reference value of the grid-connected point of the current converter; and according to the phase reference value of the AC side of the converter and the reference value of the grid-connected point current in the dq rotating coordinate system, calculating the reference value of the modulation voltage at the AC outlet of the converter in the abc static coordinate system. According to the method, the power angle stability coefficient is adaptively adjusted when the power grid voltage drop is detected, the power angle is kept unchanged before and after the fault, the transient stability of the system is improved, and the short-circuit current is inhibited.

Description

Technical Field

[0001] The present invention belongs to the technical field of fault ride-through of network-forming converters, and particularly relates to an adaptive fault ride-through method for maintaining the power angle stability of network-forming converters. Background Art

[0002] With the continuous access of new energy represented by wind power and photovoltaic power to the power system to replace synchronous generators, the large-scale access of power electronic devices has led to a decrease in the equivalent inertia of the power system, the deterioration of the frequency dynamics of the power grid, and the decline of the stability margin of the power system. At present, most new energy power generation is connected to the power grid through voltage source converters (VSCs). One of the main reasons is that it can achieve bidirectional power flow and four-quadrant operation. There are two main control modes for VSCs: grid-following (GFL) control and grid-forming (GFM) control. GFL control usually adopts vector current control, which has a fast control response speed, but it cannot provide active support for the system and is prone to sub- / supra-synchronous oscillation problems in weak power grids. GFM control can establish voltage and frequency by itself, has less dependence on the power grid and can achieve off-grid operation, and can provide virtual inertia and damping for the system to improve system stability. Based on the above characteristics, GFM control can better adapt to the working conditions of weak grid strength and low physical inertia, and can better support the system operation in the scenario of large-scale replacement of traditional synchronous power sources by new energy. However, the power outer loop in GFM control has characteristics such as narrow bandwidth and slow response speed, resulting in a slow adjustment speed of reactive current. Therefore, it is difficult to quickly limit the short-circuit current during a fault. And the common virtual synchronous machine control in GFM control may also introduce power angle stability and synchronization stability problems in the transient state of synchronous generators while simulating the characteristics of synchronous generators, which further reduces its transient stability margin.

[0003] Therefore, in order to enable GFM control to suppress the fault current during a fault and maintain safe and stable operation, a fault ride-through method needs to be proposed so that the network-forming converter can maintain power angle stability and suppress short-circuit current when the grid voltage drops. Summary of the Invention

[0004] The main object of the present invention is to overcome the shortcomings and deficiencies of the prior art, and provide an adaptive fault ride-through method for maintaining the power angle stability of network-forming converters. The adaptive fault ride-through method adaptively adjusts the power angle stability coefficient according to the operating conditions of the power system, so that the power angle of the power system remains stable before and after the fault, improves the transient stability of the power system and achieves the effect of suppressing short-circuit current.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] An adaptive fault ride-through method for maintaining the power angle stability of a grid-forming converter, the adaptive ride-through method comprising the following steps:

[0007] S1. Detect whether the grid voltage of the power system drops and the depth of the voltage drop;

[0008] S2. Calculate the value of the power angle stability coefficient K of the controller in the power system according to the operating conditions of the power system h ;

[0009] S3. Obtain the grid voltage and current at the connection point of the converter in the power system, and perform dq decomposition on the voltage and current at the connection point of the converter to respectively obtain the d-axis component and q-axis component of the voltage and current at the connection point of the converter in the dq rotating coordinate system, obtain the active power output by the converter, and obtain the reference values of the active power output by the converter and the voltage amplitude at the connection point of the converter;

[0010] S4. Calculate the phase reference value θ of the AC side of the converter * ;

[0011] S5. Calculate the reference value i of the d-axis current at the connection point of the converter vd_ref , the reference value i of the q-axis current vq_ref ;

[0012] S6. Calculate the reference value u of the d-axis voltage at the AC outlet of the converter vd_ref and the reference value u of the q-axis voltage vq_ref ;

[0013] S7. Calculate the reference values of the a-axis voltage, b-axis voltage, and c-axis voltage of the modulation voltage at the AC outlet of the converter in the abc stationary coordinate system;

[0014] S8. According to the reference values of the modulation voltage, use the pulse width modulation theory to generate corresponding control pulses to realize the control of the converter.

[0015] Further, in step S1, the magnitude of the grid voltage is detected, and a grid voltage threshold θ∈[0.8 p.u, 1 p.u] is set. If the grid voltage amplitude is lower than θ, it is determined that a grid fault has occurred, otherwise the grid is in a stable operating state.

[0016] Further, in step S2, the method for obtaining the value of the power angle stability coefficient K of the controller is as follows: h ;

[0017]

[0018] When the grid voltage is within the normal range, K h is K h0and remains unchanged, where K h0 is a constant with a value of 0.5, u g is the grid voltage of the power system, u sd and u sq are respectively the d-axis component and q-axis component of the grid-connected point voltage in the dq rotating coordinate system, u sf and u gf are respectively the grid-connected point voltage and the grid voltage during the fault, P* is the reference value of the active power output by the converter, D is the equivalent damping coefficient of the controller, m is the proportional coefficient of the controller, δ is the power angle of the controller, X g is the equivalent impedance of the line.

[0019] The specific derivation process of its value-taking method is as follows:

[0020] The calculation formula for the active power output by the converter before the fault is

[0021] Assume that the power angle remains unchanged after the fault. At this time, the power output by the converter is

[0022] It can be seen from the transient model of the control system in this paper that

[0023] It can be seen from the above formula that in order to keep the power angle unchanged before and after the fault, the active power after the fault should be the same as the calculation result of the above formula. It can be deduced that when the grid voltage drops and the power angle remains unchanged, the value-taking expression of K h is:

[0024]

[0025] Furthermore, in step S4, through the hybrid synchronous control used by the converter, the phase reference value θ of the AC side of the converter can be output * The calculation formula is as follows:

[0026]

[0027] where s is the Laplace operator, ω0 is the rated angular frequency of the power system, J is the equivalent moment of inertia of the controller, P and P* are respectively the actual value and reference value of the active power output by the converter, D is the equivalent damping coefficient of the controller, m is the proportional coefficient of the controller, u sq is the q-axis component of the grid-connected point voltage in the dq rotating coordinate system.

[0028] Furthermore, in step S5, through the virtual impedance control used by the voltage loop of the converter, the d-axis current reference value i vd_ref and the q-axis current reference value i vq_ref of the grid-connected point of the converter can be obtained. The calculation formula is as follows:

[0029]

[0030] Among them, L v and R v are virtual impedance control coefficients, u sd_ref , u sq_ref are respectively the reference values of the d-axis component and the q-axis component of the grid-connected point voltage in the dq rotating coordinate system, u sd , u sq are respectively the d-axis component and the q-axis component of the grid-connected point voltage in the dq rotating coordinate system.

[0031] Furthermore, in the step S6, the proportional-integral controller for the current inner-loop decoupling control of the converter can obtain the d-axis voltage reference value u vd_ref and the q-axis voltage reference value u vq_ref The calculation formulas are as follows:

[0032]

[0033] Among them, k p and k i are respectively the proportional coefficient and the integral coefficient of the current inner-loop controller, i vd and i vq are respectively the reference values of the d-axis current component and the q-axis current component of the converter grid-connected point, ω is the actual value of the power system angular frequency, L f is the filter inductor, u sd , u sq are respectively the d-axis component and the q-axis component of the grid-connected point voltage in the dq rotating coordinate system.

[0034] Furthermore, in the step S7, according to the coordinate transformation principle of the converter, the a-axis voltage reference value u va_ref , the b-axis voltage reference value u vb_ref and the c-axis voltage reference value u vc_ref of the modulation voltage at the converter outlet in the abc stationary coordinate system can be calculated as follows:

[0035]

[0036] Among them, θ * is the phase reference value of the converter AC side, u vd_ref and u vq_ref are the d-axis voltage reference value and the q-axis voltage reference value of the modulation voltage at the converter outlet.

[0037] Further, in step S8, after obtaining the a-axis voltage reference value, b-axis voltage reference value, and c-axis voltage reference value of the modulation voltage in the abc stationary coordinate system, the control pulses corresponding to each IGBT of the converter can be generated through the common pulse width modulation theory, thereby controlling the grid-forming converter.

[0038] Among them, the specific process of generating the corresponding control pulses using the pulse width modulation theory is as follows:

[0039] S81. The a-axis reference value, b-axis reference value, and c-axis reference value of the modulation voltage at the converter outlet generated in step S7 are respectively compared with the triangular carrier wave to obtain the trigger signals Tg1r and Tg2r for the upper and lower bridge arms of the a-phase IGBT, obtain the trigger signals Tg3r and Tg4r for the upper and lower bridge arms of the b-phase IGBT, and obtain the trigger signals Tg5r and Tg6r for the upper and lower bridge arms of the c-phase IGBT.

[0040] S82. Use the trigger signals Tg1r, Tg2r, Tg3r, Tg4r, Tg5r, and Tg6r to control the on and off of the corresponding IGBTs, so that the converter outputs narrow rectangular pulses. Then, using the equal area principle, when the frequency of the triangular carrier wave is relatively large, the narrow rectangular pulses can approximate a sine wave.

[0041] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0042] (1) When a fault occurs on the grid side, resulting in a voltage dip of the grid voltage, the power angle curve of the converter moves downward. At this time, there is a difference between the actual value and the reference value of the active power of the converter. In order to make the actual value of the active power follow the reference value again and reach the stable operating point, the power angle of the converter increases. The increase in the power angle of the converter leads to an increase in the phasor difference between the grid-connected point voltage and the grid voltage, thereby resulting in a fault overcurrent. The power angle stability coefficient proposed by the present invention can adaptively change according to the depth of the grid voltage dip and other system operating parameters, so that the actual value of the active power of the converter is equal to the reference value at the moment after the fault occurs. Therefore, the converter reaches the stable operating point, and the power angle after the fault is equal to the value before the fault, thereby improving the transient stability of the converter and effectively suppressing the short-circuit current.

[0043] (2) Due to the hybrid synchronization control used in the present invention, while suppressing the change of the power angle through the power angle stability coefficient, it can also improve the damping of the converter, so that the converter can withstand a more severe voltage dip and still maintain stable operation and suppress the short-circuit current within a safe range. Finally, through theoretical analysis and simulation examples, it is verified that the converter using the fault ride-through method of the present invention can maintain power angle stability and effectively suppress the short-circuit current after a grid fault. Description of the Drawings

[0044] To more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the accompanying drawings required for the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0045] Figure 1 is a flowchart of the adaptive fault ride-through method for maintaining the power angle stability of a network-forming converter disclosed in the present invention;

[0046] Figure 2 is a simulation waveform diagram of the short-circuit current of the converter and the system power angle when the grid voltage drops to 0.6 p.u. without using the adaptive fault ride-through method for maintaining the power angle stability of the network-forming converter in Embodiment 2 of the present invention;

[0047] Figure 3 is a simulation waveform diagram of the short-circuit current of the converter and the system power angle when the grid voltage drops to 0.6 p.u. using the adaptive fault ride-through method for maintaining the power angle stability of the network-forming converter in Embodiment 3 of the present invention. Detailed implementation manners

[0048] In order to enable those skilled in the art of the present technology to better understand the solutions of the present application, the following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, rather than all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application.

[0049] Referring to "embodiment" in the present application means that the specific features, structures or characteristics described in combination with the embodiment can be included in at least one embodiment of the present application. The phrase appears at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments.

[0050] Embodiment 1

[0051] Refer to Figure 1 , this embodiment discloses an adaptive fault ride-through method for maintaining the power angle stability of a network-forming converter, including the following steps:

[0052] S1. Detect the magnitude of the grid voltage, set the grid voltage threshold θ ∈ [0.8 p.u, 1 p.u]. If the grid voltage magnitude is lower than θ, it is determined that a grid fault has occurred; otherwise, the grid is in a stable operating state. In the embodiment, θ is taken as 0.9 p.u.

[0053] S2. The power angle stability coefficient K of the controller h is obtained as follows:

[0054]

[0055] When the grid voltage is within the normal range, K h is K h0 and remains unchanged, where K h0 is the constant 0.5, u g is the grid voltage of the power system, u sd , u sq are respectively the d-axis component and q-axis component of the grid-connected point voltage in the dq rotating coordinate system, u sf , u gf are respectively the grid-connected point voltage and grid voltage during the fault, P* is the reference value of the active power output by the converter, D is the equivalent damping coefficient of the controller, m is the proportional coefficient of the controller, δ is the power angle of the controller, and X g is the equivalent impedance of the line. In the embodiment, u g is taken as 1 p.u, and P* is taken as 1 p.u.

[0056] When it is detected that u g is less than 0.9 p.u, the adaptive power angle stability coefficient K h is adaptively adjusted according to the operating conditions and state variables of the power system, so that the converter can quickly return to the stable operating point after the fault, and the power angle remains unchanged before and after the fault.

[0057] S4. Calculate the phase reference value of the AC side of the converter;

[0058] S4. Calculate the phase reference value of the AC side of the converter;

[0059] Among them, the phase reference value θ * of the AC side of the converter is calculated as follows:

[0060]

[0061] where s is the Laplace operator, ω0 is the rated angular frequency of the power system, J is the equivalent moment of inertia of the controller, P and P* are the actual and reference values of the active power output by the converter respectively, D is the equivalent damping coefficient of the controller, m is the proportional coefficient of the controller, and u sq is the q-axis component of the grid-connected point voltage in the dq rotating coordinate system. In the embodiment, ω0 is taken as 1 p.u., i.e., 314 rad / s, and P* is taken as 1 p.u.

[0062] From the phase reference value θ of the AC side of the converter * The calculation formula shows that the adaptive power angle stability coefficient K h can change the phase reference value of the converter, so the change of the power angle during the fault can be suppressed by adjusting K h .

[0063] S5. Calculate the reference value of the d-axis current and the reference value of the q-axis current at the grid-connected point of the converter;

[0064] where the reference value i vd_ref of the d-axis current and the reference value i vq_ref of the q-axis current at the grid-connected point of the converter are calculated as follows:

[0065]

[0066] where L v and R v are the virtual impedance control coefficients, u sd_ref , u sq_ref are the reference values of the d-axis component and the q-axis component of the grid-connected point voltage in the dq rotating coordinate system respectively, and u sd , u sq are the d-axis component and the q-axis component of the grid-connected point voltage in the dq rotating coordinate system respectively.

[0067] S6. Calculate the reference value of the d-axis voltage and the reference value of the q-axis voltage at the AC outlet of the converter;

[0068] where the reference value u vd_ref of the d-axis voltage and the reference value u vq_ref of the q-axis voltage of the modulation voltage at the outlet of the converter are calculated as follows:

[0069]

[0070] where k p and k i are the proportional coefficient and the integral coefficient of the current inner-loop controller respectively, i vd and i vq are the reference values of the d-axis current component and the q-axis current component at the grid-connected point of the converter respectively, ω is the actual value of the angular frequency of the power system, and L fis the filter inductor, u sd and u sq are the d-axis component and q-axis component of the grid connection point voltage in the dq rotating coordinate system respectively.

[0071] S7. Calculate the a-axis voltage reference value, b-axis voltage reference value and c-axis voltage reference value of the modulation voltage at the AC outlet of the converter in the abc stationary coordinate system;

[0072] Among them, the a-axis voltage reference value u va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc ref The calculation formulas are as follows:

[0073]

[0074] S8. According to the reference value of the modulation voltage, use the pulse width modulation theory to generate corresponding control pulses to realize the control of the converter.

[0075] Embodiment 2

[0076] Based on an adaptive fault ride-through method for maintaining the power angle stability of a network-forming converter disclosed in Embodiment 1, this embodiment uses a test system with a single network-forming converter connected to an infinite grid for simulation verification. Comparative simulations are set up to verify the effectiveness of the adaptive fault ride-through method proposed by the present invention. In this group of simulations, other control structures are the same as those disclosed in Embodiment 1, except that the adaptive power angle stability coefficient is taken as a constant 0.5, and the adaptive power angle stability coefficient is not allowed to adaptively change according to the system operating conditions. Before 20 s, the power system operates in the rated state, the grid voltage is 1.0 p.u., the converter outputs 1.0 p.u. active power, and the output current is also 1.0 p.u. At 20 s, a three-phase symmetrical short-circuit fault occurs on the grid side, the grid voltage drops to 0.6 p.u., and the fault is removed after 1.5 s, and the grid voltage returns to 1.0 p.u. It can be seen from Figure 2 the short-circuit current waveform diagram and power angle change waveform diagram that when the converter does not adopt the adaptive power angle stability coefficient, the fault current and power angle continue to increase during the fault, and the fault current has exceeded the safety threshold.

[0077] Embodiment 3

[0078] Based on an adaptive fault ride-through method for maintaining the power angle stability of a network-forming converter disclosed in Embodiment 1, in this embodiment, a test system with a single network-forming converter connected to an infinite power grid is used for simulation verification. Comparative simulations are set up to verify the effectiveness of the adaptive fault ride-through method proposed by the present invention. Before 20 s, the power system operates in the rated state, the grid voltage is 1.0 p.u., the converter outputs 1.0 p.u. active power, and the output current is also 1.0 p.u. At 20 s, a three-phase symmetrical short-circuit fault occurs on the grid side, the grid voltage drops to 0.6 p.u., and the fault is removed after 1.5 s, and the grid voltage recovers to 1.0 p.u. As can be seen from Figure 3 the short-circuit current waveform diagram and the power angle change waveform diagram. When the adaptive power angle stability coefficient is adopted, when the grid voltage drops, the adaptive power angle stability coefficient adaptively changes according to the operating conditions of the power system so that the steady-state component of the short-circuit current output by the converter is suppressed to 1.1 p.u., and the power angle of the converter is always controlled near the rated value of 1.0 p.u. before and after the fault occurs, verifying that this fault ride-through method can effectively suppress the short-circuit current and suppress the power angle change after the fault occurs, thereby greatly improving the transient stability and fault ride-through ability of the converter.

[0079] It should be noted that for the foregoing method embodiments, for the sake of simple description, they are all expressed as a series of action combinations. However, those skilled in the art should know that the present invention is not limited by the described action sequence, because according to the present invention, certain steps can be performed in other sequences or simultaneously.

[0080] The technical features of the above embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.

[0081] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. An adaptive fault ride-through method for maintaining power angle stability of a grid-type converter, characterized in that: The adaptive fault ride-through method comprises the following steps: S1. Detect whether the grid voltage of the power system drops and the depth of the voltage drop; S2. Calculate the power angle stability coefficient K of the controller in the power system according to the operating conditions of the power system. h The value of S3, obtaining the grid voltage and current of the converter grid connection point in the power system, and performing dq decomposition on the voltage and current of the converter grid connection point, respectively obtaining the d-axis component and q-axis component of the voltage and current of the converter grid connection point in the dq rotating coordinate system, obtaining the active power output by the converter, and obtaining reference values ​​of the converter output active power and the voltage amplitude of the converter grid connection point; S4. Calculate the phase reference value θ on the AC side of the converter * ; S5. Calculate the d-axis current reference value i of the inverter grid connection point vd_ref , q-axis current reference value i vq_ref ; S6. Calculate the d-axis voltage reference value u at the AC outlet of the converter vd_ref and q-axis voltage reference value u vq_ref ; S7, calculating the a-axis voltage reference value, the b-axis voltage reference value and the c-axis voltage reference value of the modulation voltage at the AC outlet of the converter in the abc stationary coordinate system; S8. Based on the reference value of the modulation voltage and using pulse width modulation theory, a corresponding control pulse is generated to control the converter.

2. The adaptive fault ride-through method for maintaining power angle stability of a grid-type converter according to claim 1, characterized in that: In the step S1, the grid voltage amplitude is detected, and the grid voltage threshold θ∈[0.8pu,1p.u] is set. If the grid voltage amplitude is lower than θ, it is judged that the grid fails, otherwise the grid is in a stable operating state.

3. The adaptive fault ride-through method for maintaining power angle stability of a grid-type converter according to claim 1, characterized in that: In step S2, the controller power angle stability coefficient K h The value is obtained as follows: When the grid voltage is within the normal range, K h K h0 and remain unchanged, where K h0 is a constant, u g is the power grid voltage, u sd 、u sq are the d-axis component and q-axis component of the grid-connected point voltage in the dq rotating coordinate system, u sf 、u gf are the grid connection point voltage and grid voltage during the fault period, P* is the reference value of the converter output active power, D is the controller equivalent damping coefficient, m is the controller proportional coefficient, and δ is the controller power angle.

4. The adaptive fault ride-through method for maintaining power angle stability of a grid-type converter according to claim 1, characterized in that: In step S4, the phase reference value θ on the AC side of the converter * The calculation formula is as follows: Where s is the Laplace operator, ω0 is the rated angular frequency of the power system, J is the equivalent moment of inertia of the controller, P and P* are the actual value and reference value of the converter output active power, D is the equivalent damping coefficient of the controller, m is the proportional coefficient of the controller, and u sq is the q-axis component of the grid-connected point voltage in the dq rotating coordinate system.

5. The adaptive fault ride-through method for maintaining power angle stability of a grid-type converter according to claim 1, characterized in that: In step S5, the d-axis current reference value i of the converter grid connection point is vd_ref and q-axis current reference value i vq_ref The calculation formula is as follows: Among them, L v With R v is the virtual impedance control coefficient, u sd_ref 、u sq_ref are the reference values ​​of the d-axis component and q-axis component of the grid-connected point voltage in the dq rotating coordinate system, u sd 、u sq They are respectively the d-axis component and q-axis component of the grid-connected point voltage in the dq rotating coordinate system.

6. The adaptive fault ride-through method for maintaining power angle stability of a grid-type converter according to claim 1, characterized in that: In step S6, the d-axis voltage reference value u of the modulation voltage at the converter outlet vd_ref and q-axis voltage reference value u vq_ref The calculation formula is as follows: Among them, k p With k i are the proportional coefficient and integral coefficient of the current inner loop controller, i vd with i vq are the reference values ​​of the d-axis current component and the q-axis current component of the converter grid connection point, ω is the actual value of the angular frequency of the power system, L f is the filter inductance, u sd 、u sq They are respectively the d-axis component and q-axis component of the grid-connected point voltage in the dq rotating coordinate system.

7. The adaptive fault ride-through method for maintaining power angle stability of a grid-type converter according to claim 1, characterized in that: In step S7, the modulation voltage at the converter outlet is the a-axis voltage reference value u in the abc stationary coordinate system. va_ref , b-axis voltage reference value u vb_ref and c-axis voltage reference value u vc_ref The calculation formula is as follows: Among them, θ * is the phase reference value of the AC side of the converter, u vd_ref and u vq_ref are the d-axis voltage reference value and q-axis voltage reference value of the modulation voltage at the converter outlet.

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