Adaptive fault ride-through method for maintaining power angle stability of meshed converter

Abstract

The application discloses a kind of self-adapting fault ride-through methods for maintaining network construction type converter power angle stability, steps are as follows: detect whether grid voltage drop occurs and voltage drop depth;Power angle stability coefficient is calculated according to system operating condition;The grid voltage and current of converter grid-connected point, the actual value and reference value of converter output active power, the reference value of grid voltage amplitude are obtained;The phase reference value of converter ac side is calculated;The reference value of converter grid-connected point current under dq rotating coordinate system is calculated according to the reference value of converter grid-connected point voltage amplitude set;According to the phase reference value of converter ac side and the reference value of grid-connected point current under dq rotating coordinate system, the reference value of modulation voltage at converter ac outlet under abc stationary coordinate system is calculated.The method is adapted to adjust power angle stability coefficient when grid voltage drop occurs, maintains power angle unchanged before and after fault, improves system transient stability and inhibits short-circuit current.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of grid-forming converter fault ride-through, and particularly relates to a self-adaptive fault ride-through method for maintaining grid-forming converter power angle stability. BACKGROUND

[0002] With the continuous access of new energy represented by wind power and photovoltaic to the power system to replace synchronous generators, the large-scale access of power electronic equipment leads to the decrease of equivalent inertia of the power system, the deterioration of frequency dynamics of the power grid, and the decrease of stability margin of the power system. At present, new energy power generation is mostly connected to the power grid through voltage source converters (VSC), one of the main reasons for which is that it can realize bidirectional power flow and four-quadrant operation. There are two main control modes for VSC: grid-following (GFL) control and grid-forming (GFM) control. GFL control usually adopts vector current control, which has fast control response speed, but it cannot provide active support for the system and is prone to cause subsynchronous / ultrasub-synchronous oscillation problems under weak power grids. GFM control can self-establish voltage and frequency, has less dependence on the power grid and can realize 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 condition of weak grid strength and low physical inertia, and can better support 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 the characteristics of narrow bandwidth and slow response speed, which leads to slow adjustment speed of reactive current, so it is difficult to quickly limit short-circuit current during faults. And the virtual synchronous machine control commonly used in GFM control may also introduce power angle stability and synchronous stability problems in the transient state of synchronous generators while simulating the characteristics of synchronous generators, which further reduces the transient stability margin.

[0003] Therefore, in order to enable GFM control to complete the suppression of fault current during faults and maintain safe and stable operation, a fault ride-through method is needed, which enables the grid-forming converter to maintain power angle stability and suppress short-circuit current when the grid voltage drops. SUMMARY

[0004] The main purpose of the present application is to overcome the shortcomings and deficiencies of the prior art, and to provide a self-adaptive fault ride-through method for maintaining grid-forming converter power angle stability. The self-adaptive fault ride-through method adjusts the power angle stability coefficient according to the operating condition 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 purpose, the technical scheme adopted by the present application is as follows:

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

[0007] S1, detecting whether a grid voltage of a power system has dropped and a voltage drop depth;

[0008] S2, calculating a value of a controller power angle stability coefficient K h in the power system according to a running condition of the power system;

[0009] S3, obtaining a grid voltage and a current at a grid connection point of a converter in the power system, and performing dq decomposition on the voltage and the current at the grid connection point of the converter to obtain d-axis and q-axis components of the voltage and the current at the grid connection point of the converter in a dq rotating coordinate system, obtaining an active power output by the converter, and obtaining reference values of the active power output by the converter and a voltage amplitude at the grid connection point of the converter;

[0010] S4, calculating a phase reference value θ * of an alternating current side of the converter;

[0011] S5, calculating a d-axis current reference value i vd_ref and a q-axis current reference value i vq_ref of the grid connection point of the converter;

[0012] S6, calculating a d-axis voltage reference value u vd_ref and a q-axis voltage reference value u vq_ref at an alternating current outlet of the converter;

[0013] S7, calculating a-axis, b-axis and c-axis voltage reference values of a modulation voltage at the alternating current outlet of the converter in an abc stationary coordinate system;

[0014] S8, generating corresponding control pulses to realize control of the converter by using a pulse width modulation theory according to the reference values of the modulation voltage.

[0015] Further, in the step S1, a grid voltage threshold θ ∈ [0.8 p.u, 1 p.u] is set, and if the grid voltage amplitude is lower than θ, it is judged that the grid has a fault, otherwise, the grid is in a stable running state.

[0016] Further, in the step S2, the controller power angle stability coefficient K h is calculated as follows:

[0017]

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

[0019] The value determination method is specifically derived as follows:

[0020] The active power calculation formula of the converter before the fault is

[0021] It is assumed that the power angle keeps unchanged after the fault, and the converter output power at this time is

[0022] According to the transient model of the control system in the present application, it can be known that

[0023] According to the above formula, 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, the value expression of K h is as follows, under the condition that the power angle keeps unchanged:

[0024]

[0025] Further, in the step S4, the phase reference value θ * of the converter AC side can be output by using the hybrid synchronization control used by the converter.

[0026]

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

[0028] Further, in the step S5, the d-axis current reference value i vd_ref and the q-axis current reference value i vq_ref of the converter grid-connected point can be obtained by using the virtual impedance control of the converter voltage loop.

[0029]

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

[0031] Further, in the step S6, the proportional integral controller of 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 of the modulation voltage at the outlet of the converter, and the calculation formula is as follows:

[0032]

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

[0034] Further, 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 outlet of the converter in the abc stationary coordinate system can be calculated, and the calculation formula is as follows:

[0035]

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

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

[0038] The specific process of generating the corresponding control pulses through the pulse width modulation theory is as follows:

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

[0040] S82, the turn-on and turn-off of the corresponding IGBTs are controlled by the trigger signals Tg1r, Tg2r, Tg3r, Tg4r, Tg5r and Tg6r, so that the converter outputs narrow rectangular pulses, and the narrow rectangular pulses can approximate to the sine wave when the frequency of the triangular carrier is large by using the area equivalence principle.

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

[0042] (1) When a fault occurs on the grid side, the power angle curve of the converter moves downward, and 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 so as to reach the stable operating point, the power angle of the converter is increased. The increase of the power angle of the converter leads to the increase of the phasor difference between the grid voltage and the grid-connected point voltage, thereby causing the fault overcurrent. The power angle stability coefficient proposed in the present application can be adaptively changed according to the depth of the grid voltage drop 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 stable operating point of the converter is reached, and the value of 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) Since the hybrid synchronization control is used in the present application, the power angle change is suppressed through the power angle stability coefficient, and the damping of the converter is also improved, so that the converter can withstand more serious voltage drop 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 application can maintain power angle stability and effectively suppress the short-circuit current after the grid fault occurs. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to make the technical solutions in the embodiments of the present application clearer, the accompanying drawings needed in the embodiment description will be briefly introduced. Obviously, the accompanying drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

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

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

[0047] Figure 3 is a simulation waveform diagram of the short-circuit current of the converter and the power angle of the system when the grid voltage drops to 0.6 p.u. in the embodiment 3 of the present application using the adaptive fault ride-through method for maintaining the power angle stability of the grid-forming converter disclosed in the present application. DETAILED DESCRIPTION

[0048] In order to make the technical solutions in the embodiments of the present application clearer, the accompanying drawings needed in the embodiment description will be briefly introduced. Obviously, the accompanying drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort.

[0049] In the present application, the phrase "embodiment" means that the specific features, structures or characteristics described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of this phrase at various places in the specification does not necessarily mean the same embodiment, nor is it an independent or alternative embodiment to 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] Referring to Figure 1 The embodiment discloses an adaptive fault ride-through method for maintaining the power angle stability of the grid-forming converter, comprising the following steps:

[0052] S1, detect the grid voltage amplitude, set the grid voltage threshold θ ∈ [0.8p.u, 1p.u], if the grid voltage amplitude is lower than θ, it is judged that the grid has failed, otherwise the grid is in stable operation state. In the embodiment, θ takes the value of 0.9p.u.

[0053] S2, the controller power angle stability coefficient K h The value method is as follows:

[0054]

[0055] When the grid voltage is in the normal range, K h is constant, where K h0 is the constant 0.5, u h0 is the grid voltage of the power system, u g is the grid voltage of the power system, u sd , u sq are the d-axis component and q-axis component of the grid voltage at the grid-connected point in the dq rotating coordinate system, u sf , u gf are the grid voltage and the grid voltage during the fault, P* is the output active power reference value of 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. In the embodiment, u g takes the value of 1p.u, and P* takes the value of 1p.u.

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

[0057] S3, the grid voltage and current at the grid-connected point of the converter are obtained, and the voltage and current at the grid-connected point of the converter are dq decomposed to obtain the d-axis component and q-axis component of the voltage at the grid-connected point of the converter in the dq rotating coordinate system, the active power output by the converter is obtained, and the active power output reference value of the converter and the reference value of the voltage amplitude at the grid-connected point of the converter are obtained.

[0058] S4, the phase reference value of the AC side of the converter is calculated.

[0059] The phase reference value of the AC side of the converter θ * The calculation formula is as follows:

[0060]

[0061] where s is Laplace operator, ω0 is the rated angular frequency of power system, J is the equivalent moment of inertia of the controller, P and P* are the actual value and reference value of the active power output of the converter respectively, 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 dq rotating coordinate system. In the embodiment, ω0 is 1 p.u. (i.e. 314 rad / s) and P* is 1 p.u.

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

[0063] S5, calculating the d-axis current reference value and the q-axis current reference value of the grid-connected point of the converter;

[0064] where 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 are calculated as follows:

[0065]

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

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

[0068] where the d-axis voltage reference value u vd_ref and the q-axis voltage reference value u vq_ref of the modulated 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, i vd and i vq are the d-axis current component reference value and the q-axis current component reference value of the grid-connected point of the converter, and ω is the actual value of the angular frequency of the power system, L fFor filter inductance, u sd , u sq are the d-axis component and q-axis component of grid-connected point voltage in 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] Wherein, 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 The calculation formula is as follows:

[0073]

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

[0075] Embodiment 2

[0076] Based on the adaptive fault ride-through method for maintaining the power angle stability of the grid-forming converter disclosed in embodiment 1, this embodiment uses a single grid-forming converter to access the infinite grid test system for simulation verification. A comparative simulation is set up to verify the effectiveness of the adaptive fault ride-through method proposed in the present application. In this simulation, the other control structure is the same as that disclosed in embodiment 1, except that the adaptive power angle stability coefficient is constant 0.5, and the adaptive power angle stability coefficient is not allowed to change adaptively according to the system operating condition. 20s ago, the power system was operating at rated state, the grid voltage was 1.0p.u, the converter output 1.0p.u active power, and the output current was also 1.0p.u. At 20s, a three-phase symmetrical short-circuit fault occurred on the grid side, the grid voltage dropped to 0.6p.u, and 1.5s later the fault was removed, and the grid voltage recovered to 1.0p.u. From Figure 2 the short-circuit current waveform graph and the power angle change waveform graph, it can be seen that when the converter does not use the adaptive power angle stability coefficient, the fault current and the power angle continue to increase during the fault, and the fault current has exceeded the safety threshold.

[0077] Embodiment 3

[0078] Based on the adaptive fault ride-through method for maintaining the power angle stability of the network-forming converter disclosed in embodiment 1, this embodiment adopts a single network-forming converter to access the test system of the infinite grid for simulation verification. A comparative simulation is set to verify the effectiveness of the adaptive fault ride-through method proposed in the application. 20s ago, the power system was operating in the rated state, the grid voltage was 1.0p.u, the converter output 1.0p.u active power, and the output current was also 1.0p.u. At 20s, a three-phase symmetrical short-circuit fault occurred on the grid side, the grid voltage dropped to 0.6p.u, and the fault was removed after 1.5s, and the grid voltage recovered to 1.0p.u. From the short-circuit current waveform diagram and the power angle change waveform diagram, it can be seen that when the adaptive power angle stability coefficient is adopted, when the grid voltage drops, the adaptive power angle stability coefficient changes adaptively according to the operating condition of the power system to make the steady-state component of the short-circuit current output by the converter be suppressed to 1.1p.u, and the power angle of the converter is always controlled near the rated value 1.0p.u before and after the fault occurs, verifying that the fault ride-through method can effectively suppress the short-circuit current and the power angle change after the fault occurs, thereby greatly improving the transient stability and fault ride-through capability of the converter. Figure 3

[0079] It should be noted that for the foregoing method embodiments, in order to simplify the description, they are all described as a series of action combinations, but those skilled in the art should know that the present application is not limited by the order of the described actions, because according to the present application, certain steps can be performed in other order or simultaneously.

[0080] The technical features of the above embodiments can be combined in any way, and in order to make the description concise, not all possible combinations of the technical features in the above embodiments are described, however, as long as the combination of the technical features does not exist contradictory, it should be considered as the scope of the present application.

[0081] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods, and are all included in the protection scope of the present application.​

Claims

1. An adaptive fault ride-through method for maintaining the power angle stability of a grid-type converter, characterized in that, The adaptive fault-crossing method includes the following steps: S1. Detect whether the grid voltage of the power system has dropped and the depth of the voltage drop; S2. Calculate the power angle stability coefficient K of the controller in the power system based on the power system operating conditions. h The value of ; where, the controller power angle stability coefficient K h The method for obtaining the value is as follows: When the grid voltage is within the normal range, K h For K h0 And remain unchanged, where ϴ is the grid voltage threshold, K h0 u is a constant g For the power grid voltage, u sd u sq These are the d-axis and q-axis components of the grid connection point voltage in the dq rotating coordinate system, respectively. sf u gf S1 represents the grid connection point voltage and grid voltage during the fault period, P* represents the reference value of the converter output active power, D represents the controller equivalent damping coefficient, m represents the controller proportional coefficient, and δ represents the controller power angle; S2 represents the grid voltage and current at the converter connection point in the power system, and dq decomposition is performed on the voltage and current at the converter connection point to obtain the d-axis and q-axis components of the voltage and current at the converter connection point in the dq rotating coordinate system, respectively, to obtain the active power output of the converter, and to obtain the reference values ​​of the converter output active power and the voltage amplitude at the converter connection point; S3 represents the phase reference value of the AC side of the converter. S5. Calculate the reference value i of the d-axis current at the converter grid connection point. vd_ref q-axis current reference value i vq_ref ; S6. Calculate the reference value u of the d-axis voltage at the AC output of the converter. vd_ref and q-axis voltage reference value u vq_ref ; S7. Calculate the reference values ​​of the modulation voltage at the AC output of the converter in the a-axis, b-axis, and c-axis coordinate system. S8. Based on the reference value of the modulation voltage, the corresponding control pulses are generated using pulse width modulation theory to control the converter.

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

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

4. The adaptive fault ride-through method for maintaining power angle stability in a grid-type converter according to claim 1, characterized in that, In step S5, the d-axis current reference value i at 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 u is the virtual impedance control coefficient. sd_ref u sq_ref These are the reference values ​​for the d-axis and q-axis components of the grid connection point voltage in the dq rotating coordinate system, respectively. sd u sq These are the d-axis and q-axis components of the grid connection point voltage in the dq rotating coordinate system, respectively.

5. The adaptive fault ride-through method for maintaining power angle stability in 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 is... vd_ref and q-axis voltage reference value u vq_ref The calculation formula is as follows: Where, k p With k i These are the proportional and integral coefficients of the current inner loop controller, i vd with i vq These are the reference values ​​for the d-axis current component and the q-axis current component at the converter grid connection point, respectively. L is the actual value of the power system angular frequency. f For the filter inductance, u sd u sq These are the d-axis and q-axis components of the grid connection point voltage in the dq rotating coordinate system, respectively.

6. The adaptive fault ride-through method for maintaining power angle stability in a grid-type converter according to claim 1, characterized in that, In step S7, the reference value u of the modulation voltage at the converter outlet along the a-axis in the ac stationary coordinate system is... 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: in, u is the phase reference value on the AC side of the converter. vd_ref and u vq_ref These are the d-axis and q-axis voltage reference values ​​for the modulation voltage at the converter outlet.

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