A high and low wear control method for grid-connected converter

Abstract

The present invention provides a high and low wear control method for a grid-connected converter, belonging to the technical field of converters. The states of the entire control process include: "system normal" state, "low wear entry" state, "low voltage ride-through" state, "low wear exit" state, "low wear recovery" state, "high wear entry" state and "high voltage ride-through" state; in the "system normal" state, the positive sequence voltage calculated by a frequency locked loop based on a second-order generalized integrator and the phase calculated by an iterative least squares method with covariance reset are detected. If the positive sequence voltage suddenly decreases and the phase changes suddenly, it is judged that the "low wear entry" state is entered; if the positive sequence voltage suddenly increases and the phase changes suddenly, it is judged that the "high wear entry" state is entered. The present invention enables the grid-connected converter to quickly detect and judge the occurrence of a fault from a normal working state when a temporary fault occurs in the power grid, thereby entering a high and low wear state.

Description

Technical Field

[0001] The invention belongs to the technical field of converters, and in particular relates to a high and low wear control method for a grid-connected converter. Background Art

[0002] With the rapid development of the new energy industry, more and more grid-connected converters are connected to the grid as energy conversion devices between the grid and new energy or energy storage batteries, such as photovoltaic inverters, wind power converters and energy storage converters. For details, please refer to the patent "module combined wind power converter" with announcement number "CN201444616U" and the patent "renewable energy feedback grid-connected circuit and its control device" with announcement number "CN100483917C". As a power electronic device with fast response and flexible control, the grid-connected converter needs to achieve high and low voltage ride-through when a temporary fault occurs in the grid, and provide reactive support for the grid. In the grid-connected converter, whether the grid synchronization algorithm adopts a frequency-locked loop or a phase-locked loop, its speed is limited by the bandwidth of the controller. During the high and low ride-through process, there will be a problem of slow response to disturbances.

[0003] Based on the above problems, it is a challenge for the grid-connected converter to quickly detect and determine the occurrence of a fault from a normal working state and enter a high-low wear state, and to safely and stably recover from the high-low wear state to the normal working state before the fault after the fault is recovered. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a high and low wear control method for a grid-connected converter to solve the above-mentioned technical problems.

[0005] The present invention provides a high and low wear control method for a grid-connected converter, comprising:

[0006] When a temporary fault occurs in the power grid, the grid-connected converter enters the high and low voltage ride-through state. The state machine is used to control the state of the converter control system. The states of the entire control process include: "system normal" state, "low voltage ride-through entry" state, "low voltage ride-through" state, "low voltage ride-through exit" state, "low voltage ride-through recovery" state, "high voltage ride-through entry" state and "high voltage ride-through" state;

[0007] In the "system normal" state, the positive sequence voltage calculated by the frequency locked loop based on the second-order generalized integrator and the phase calculated by the iterative least squares method are detected. If the positive sequence voltage suddenly decreases to the low wear-through voltage limit and the phase changes suddenly, it is judged that the system enters the "low wear-through entry" state; if the positive sequence voltage suddenly increases to the high wear-through voltage limit and the phase changes suddenly, it is judged that the system enters the "high wear-through entry" state;

[0008] After entering the "low voltage ride-through" state and the "high voltage ride-through" state, the timing starts. If the positive sequence voltage meets the low voltage ride-through limit in the first preset time period, the system enters the "low voltage ride-through" state. If the positive sequence voltage meets the high voltage ride-through limit in the second preset time period, the system enters the "high voltage ride-through" state. Otherwise, the system returns to the "normal system" state.

[0009] In the "low voltage ride-through state", the converter generates reactive current in accordance with the national standard requirements for reactive support during low voltage ride-through;

[0010] When the grid voltage recovers, the system will enter the "low voltage ride-through exit" state. If the detected voltage continues to meet the low voltage ride-through exit voltage condition within the third preset time, the system will enter the "low voltage ride-through recovery" state. Otherwise, it will be considered that the "low voltage ride-through exit" state is a misjudgment and the system will return to the "low voltage ride-through" state.

[0011] In the "Low Voltage Ride Through Recovery" state, the converter gradually recovers to the power value output before the low voltage ride through occurs according to the preset slope, and then the system returns to the "System Normal" state;

[0012] In the "high voltage ride through" state, the converter does not need to generate reactive current. When the grid voltage recovers and meets the voltage conditions for high voltage ride through exit, the system returns to the "system normal" state.

[0013] Furthermore: in the "low-through entry" state, the "low-through exit" state, the "high-voltage entry" state and the "high-through exit" state, the current of the converter is reduced to zero or directly blocked.

[0014] Further: including: when the "low voltage ride-through" state is transferred to the "low voltage ride-through" state, an action of resetting the inner loop control parameters is executed.

[0015] Further: when returning to the "system normal" state from the "low-penetration entry", the action of resetting the inner and outer loop control parameters is executed.

[0016] Furthermore, it also includes: when transferring from the "low-penetration exit" state to the "low-penetration recovery" state, resetting the inner loop and outer loop control parameters is executed, and the recovery speed is limited at the same time.

[0017] Furthermore, it also includes: when transferring from the "low-breakdown recovery" state to the "system normal" state, executing the action of normalizing the recovery speed.

[0018] Furthermore, it also includes: adding covariance reset to the iterative least squares method calculation, deleting the recorded phase information when the sudden voltage amplitude exceeds a certain value, and restarting to read and record the phase information; when the covariance reset occurs, it is determined whether high voltage ride-through or low voltage ride-through occurs.

[0019] Furthermore, the method further includes: when the converter recovers to the power value outputted before the low voltage ride-through occurs, the converter gradually recovers according to a preset slope.

[0020] The beneficial effect of the present invention is that the high and low wear control method of the grid-connected converter provided by the present invention can enable the grid-connected converter to quickly detect and judge the occurrence of the fault from the normal working state when a temporary fault occurs in the power grid, thereby entering the high and low wear state, and emitting a specific reactive current. In addition, after the fault is restored, it can be safely and stably restored from the high and low wear state to the normal working state before the fault. Moreover, in the grid-connected converter, whether the grid synchronization algorithm adopts a frequency-locked loop or a phase-locked loop, its speed is limited by the bandwidth of the controller. During the high and low wear process, there will be a problem of slow response speed to disturbances. Therefore, when entering and exiting the high and low wear, the present invention innovatively uses a hybrid detection method based on a second-order generalized integrator to calculate the positive sequence voltage mutation and an iterative least squares method with covariance reset to calculate the phase mutation to ensure that the system can respond quickly when high and low wear occurs. In addition, the design principle of the present invention is reliable, the structure is simple, and it has a very broad application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 is a schematic flow chart of a method according to an embodiment of the present invention.

[0023] Figure 2 This is the low-breakdown simulation result when the three-phase voltage balance drops to 5% under rated power according to the method provided by the present invention.

[0024] Figure 3 It is the low-breakdown simulation result when the two-phase voltage drops to 5% under rated power according to the method provided by the present invention.

[0025] Figure 4 The method provided by the present invention provides a low-voltage simulation result when the single-phase voltage drops to 5% under rated power.

[0026] Figure 5 This is the high penetration simulation result when the three-phase voltage increases to 130% under rated power of the method provided by the present invention. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0028] like Figure 1 As shown, an embodiment of the present invention provides a high and low wear control method for a grid-connected converter, which adopts a state machine for logic processing. The management in the state machine includes seven states: "system normal", "low wear entry", "low voltage ride through", "low wear exit", "low wear recovery", "high wear entry", and "high voltage ride through".

[0029] Among them, there are five states related to low voltage ride-through and three states related to high voltage ride-through. This is because the national standard requires that the grid-connected converter output reactive current to support the grid voltage during low voltage ride-through, so it is necessary to manage the transient process of entering and exiting low voltage ride-through more finely. If the granularity of the high voltage ride-through state is divided more finely with reference to the low voltage ride-through state, it also falls within the scope of protection of the present invention.

[0030] In the grid-connected converter, whether the grid synchronization algorithm adopts a frequency-locked loop or a phase-locked loop, its speed is limited by the bandwidth of the controller. During the high and low wear-through process, there will be a problem of slow response to disturbances. Therefore, when entering and exiting the high and low wear-through, the present invention innovatively uses a hybrid detection method based on a second-order generalized integrator to calculate the positive sequence voltage mutation and an iterative least squares method with covariance reset to calculate the phase mutation to ensure that the system can respond quickly when high and low wear-through occurs. The principle of this part is explained below.

[0031] When the grid voltage of the grid-connected converter is represented by a complex vector, the transformation matrix and inverse transformation matrix from the three-phase stationary coordinate system to the two-phase stationary coordinate system are respectively

[0032]

[0033] From the three-phase grid voltage and transformation matrix, the grid voltage expression in the two-phase stationary coordinate system can be obtained as follows:

[0034]

[0035] Since there are positive and negative sequence components in the voltage, it is divided into two parts: positive sequence component and negative sequence component, which are represented in the positive rotating coordinate system and the negative rotating coordinate system respectively.

[0036] The transformation matrix and inverse transformation matrix from the two-phase stationary coordinate system to the positive sequence rotating coordinate system are:

[0037]

[0038]

[0039] The transformation matrix and inverse transformation matrix of the two-phase stationary coordinate system to the negative sequence rotating coordinate system are:

[0040]

[0041]

[0042] According to the above transformation formula, the grid voltage expression can be written as:

[0043]

[0044] in is the expression of the positive sequence component in the positive rotating coordinate system, is the expression of the negative sequence component in the negative rotating coordinate system. Rewriting equation (8) in matrix form gives

[0045] y(t i )=H(t i )x(t i ) (9)

[0046] in:

[0047] y(t i )=[v αs (t i ) βs (t i )] T

[0048]

[0049] Define a cost function as:

[0050]

[0051] Among them, λ is the forgetting coefficient, and its purpose is to make the previous measurement value gradually decay. The smaller λ is, the faster the decay rate is, that is, the more the system pays attention to the current sampling value, the faster the tracking speed is, but it is also more susceptible to interference.

[0052] Solve the x(t j ), the process is as follows:

[0053]

[0054] R(t i )=λI+H(t i )P(t i-1 )H(t i ) T (12)

[0055] K(t i )=P(t i-1 )H(t i ) T R(t i ) -1 (13)

[0056] P(t i )=λ -1 (P(t i-1 )-P(t i-1 )H(t i ) T R(t i ) -1 H(t i )P(t i-1 )) (14)

[0057] The initial input of the solution coefficients Initial covariance coefficient P(t0) = π0I;

[0058] Finally, by

[0059]

[0060] The system phase can be obtained.

[0061] In order to improve the response speed when a phase mutation occurs, covariance reset is added, that is, when ||y(t i )-H(t i )x(t i )||>ε, P(t i-1 ) is reset to π0I. The meaning of this judgment formula is that when the sudden voltage amplitude exceeds a certain value ε, the recorded information is deleted and the current phase information is read and recorded again. In order to quickly determine whether high-low wear occurs, it can be considered that when the covariance reset occurs, the system may have high-low wear. By coordinating with the frequency-locked loop based on the second-order generalized integrator, the detection of high-low wear can be finally completed.

[0062] The frequency-locked loop implementation method based on the second-order generalized integrator is relatively common and will not be described in detail here.

[0063] Based on the above principles, the state transition and action execution process of low voltage ride-through are described in detail.

[0064] In the first step, the system is initially in the "system normal" state, in which the grid voltage is detected in real time;

[0065] In the second step, when the amplitude of the positive sequence component of the grid voltage calculated by the frequency locked loop based on the second-order generalized integrator suddenly drops to the voltage limit of the low voltage ride-through, and the iterative least squares method with covariance reset calculates the phase mutation, the system will enter the "low voltage ride-through entry" state. In this state, the current of the converter needs to be reduced to zero or directly blocked. The first preset time is a reasonable time period between the system's "system normal" state and the "low voltage ride-through" state. If the low voltage ride-through entry condition is always met within the first preset time, that is, it is always in the "low voltage ride-through entry" state, then the system will enter the "low voltage ride-through" state, otherwise it will be considered that this low voltage ride-through is a misjudgment, and the system will return to the "system normal" state.

[0066] In the third step, if the state is transferred from the "low voltage entry" state to the "low voltage ride-through" state, it is necessary to reset the inner loop control parameters to prepare for the generation of reactive current during the low voltage ride-through period; if the state returns to the "system normal" state again from the "low voltage entry", it is necessary to reset the inner and outer loop control parameters so that the converter can restore the operating power state before the misjudgment occurred.

[0067] The fourth step is that in the "low voltage ride-through state", the converter needs to emit a specific reactive current in accordance with the requirements of the national standard for reactive support during low voltage ride-through. When the grid voltage recovers and meets the conditions for exiting the low voltage ride-through, including voltage conditions and time conditions, the system will enter the "low voltage ride-through exit" state. In this state, the reactive current needs to be reduced to zero or directly blocked, and timing is required until the low voltage ride-through recovery judgment time. If the exit conditions for the low voltage ride-through, including voltage conditions and time conditions, are always met during this time period, the system will enter the "low voltage ride-through recovery" state. Otherwise, it will be considered that this low voltage ride-through exit is a misjudgment, and the system will return to the "low voltage ride-through" state.

[0068] In the fifth step, if the state is transferred from the "low-breakthrough exit" state to the "low-breakthrough recovery" state, it is necessary to reset the inner and outer loop control parameters and limit the recovery speed to ensure that the converter can safely and stably recover to the power state before the low-breakthrough after the low-breakthrough ends.

[0069] Step 6. In the “low voltage ride-through recovery” state, the converter needs to gradually recover to the power value output before the low voltage ride-through occurs according to the preset slope, and then the system returns to the “system normal” state.

[0070] The seventh step is to perform a normalization recovery speed action when transferring from the "low-breakthrough recovery" state to the "system normal" state, so that the converter can restore the original response speed to the power command after a period of time after the low-breakthrough ends.

[0071] The national standard GB / T 34120-2017 clearly stipulates the reactive current output by the converter during low voltage ride-through: from the start of the dynamic reactive current response until the voltage recovers to 0.85 (pu), the dynamic reactive current injected into the power system by the energy storage converter should track the voltage change at the grid connection point in real time and meet the following requirements:

[0072]

[0073] Where U T is the per unit voltage of the converter grid connection point; I N is the rated current of the energy storage converter.

[0074] The following describes the state transition and action execution process of high voltage ride-through.

[0075] In the first step, the system is initially in the "system normal" state, in which the grid voltage is detected in real time;

[0076] In the second step, when the amplitude of the positive-sequence component of the grid voltage calculated by the frequency-locked loop based on the second-order generalized integrator suddenly increases and reaches the voltage limit of high penetration, and the iterative least squares method with covariance reset calculates the phase mutation, the system will enter the "high penetration entry" state. In this state, the current of the converter needs to be reduced to zero or directly blocked, and the timing is required until the high penetration entry judgment time. If the high penetration entry conditions, including voltage conditions and time conditions, are always met during this time period, then the system will enter the "high voltage ride-through" state, otherwise it will be considered that this high penetration is a misjudgment, and the system will return to the "system normal" state.

[0077] The third step is that in the "high voltage ride-through state", the converter does not need to generate reactive current. When the grid voltage recovers and the conditions for high voltage ride-through exit are met, including voltage conditions and time conditions, the system returns to the "system normal" state.

[0078] The actual use effect of this method is verified by simulation below.

[0079] A converter with a rated power of 1.725MW, an AC side rated voltage of 690V, and a rated current of 1443A was built in MATLAB to simulate the high and low wear control method of the grid-connected converter and the actual application effect of the system. The four worst high and low wear conditions were selected in the simulation: three-phase voltage balance dropped to 5%, two-phase voltage unbalance dropped to 5%, single-phase voltage unbalance dropped to 5%, and three-phase voltage rose to 130%.

[0080] Figure 2When the converter is working at a power output of 1.725MW, a grid fault occurs, causing the grid voltage to drop in three-phase balance to 5% of the rated voltage. At this time, the converter needs to safely pass through this low voltage process and generate a reactive current of 104% of the rated current to support the grid in accordance with national standards. Figure 2 It can be seen that after the grid voltage drops for 0.2 seconds, after a short transient process, the converter first reduces the active current to 0, and then starts to emit 104% of the rated reactive current. When the grid returns to normal after 0.4 seconds, the converter gradually returns to the working state before the low-voltage drop with a certain slope, and the output power is 1.725MW.

[0081] Figure 3 When the converter is working at a power output of 1.725MW, a fault occurs in the grid, causing the grid voltage to drop unbalanced in two phases, dropping to 5% of the rated voltage. At this time, the converter needs to safely pass through this low voltage process and generate a specific reactive current to support the grid in accordance with national standards. Figure 3 It can be seen that after the grid voltage drops for 0.2 seconds, after a short transient process, the converter first reduces the active current to 0, and then starts to generate a specific reactive current. When the grid returns to normal after 0.4 seconds, the converter gradually returns to the working state before the low-voltage breakout with a certain slope, and the output power is 1.725MW.

[0082] Figure 4 When the converter is working at a power output of 1.725MW, a fault occurs in the grid, causing a single-phase unbalanced drop in the grid voltage to 5% of the rated voltage. At this time, the converter needs to safely pass through this low voltage process and generate a specific reactive current to support the grid in accordance with national standards. Figure 4 It can be seen that after the grid voltage drops for 0.2 seconds, after a short transient process, the converter first reduces the active current to 0, and then starts to generate a specific reactive current. When the grid returns to normal after 0.4 seconds, the converter gradually returns to the working state before the low-voltage breakout with a certain slope, and the output power is 1.725MW.

[0083] Figure 5 When the converter is working at a power output of 1.725MW, the grid voltage suddenly rises to 130% of the rated voltage. At this time, the converter needs to safely pass through this high voltage process. During the high voltage process, there is no requirement for reactive current. Figure 5 It can be seen that after the grid voltage drops for 0.2 seconds, after a short transient process, the converter reduces the active current to 0. When the grid returns to normal after 0.4 seconds, the converter gradually returns to the working state before the high-voltage breakout with a certain slope, and the output power is 1.725MW.

[0084] To sum up, the control method of the present invention can enable the grid-connected converter to quickly detect and determine the occurrence of a temporary fault in the power grid from a normal working state, thereby entering a high-low wear-through state and emitting a specific reactive current. In addition, after the fault is restored, it can safely and stably recover from the high-low wear-through state to the normal working state before the fault.

[0085] Although the present invention has been described in detail by referring to the accompanying drawings and in combination with the preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, a person of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and these modifications or substitutions shall be within the scope of the present invention. Any person of ordinary skill in the art who is familiar with the present invention may easily think of changes or substitutions within the technical scope disclosed by the present invention, and these shall be within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A method for controlling high and low wear of a grid-connected converter, characterized in that: include: When a temporary fault occurs in the power grid, the grid-connected converter enters the high and low voltage ride-through state. The state machine is used to control the state of the converter control system. The states of the entire control process include: "system normal" state, "low voltage ride-through entry" state, "low voltage ride-through" state, "low voltage ride-through exit" state, "low voltage ride-through recovery" state, "high voltage ride-through entry" state and "high voltage ride-through" state; In the "system normal" state, the detection uses a frequency-locked loop based on a second-order generalized integrator to calculate the positive sequence voltage, and the iterative least squares method with covariance reset calculates the phase mutation. If the positive sequence voltage suddenly drops to the low voltage limit and the phase suddenly changes, it is judged that it enters the "low voltage entry" state; if the positive sequence voltage suddenly rises to the high voltage limit and the phase suddenly changes, it is judged that it enters the "high voltage entry" state; covariance reset is added to the iterative least squares calculation. When the voltage amplitude of the mutation exceeds a certain value, the recorded phase information is deleted, and the phase information is read and recorded again; when the covariance reset occurs, it is judged that high voltage ride-through or low voltage ride-through occurs; After entering the "low voltage ride-through" state and the "high voltage ride-through" state, the timing starts. If the positive sequence voltage meets the low voltage ride-through voltage limit in the first preset time period, the system enters the "low voltage ride-through" state. If the positive sequence voltage meets the high voltage ride-through voltage limit in the second preset time period, the system enters the "high voltage ride-through" state. Otherwise, the system returns to the "normal system" state. In the "low voltage ride-through state", the converter emits reactive current in accordance with the national standard requirements for reactive support during low voltage ride-through; the requirements for reactive current are: from the dynamic reactive current response until the voltage recovers to 0.85 (pu), the dynamic reactive current injected into the power system by the energy storage converter should track the voltage changes at the grid connection point in real time and meet the following requirements: In the formula is the per unit voltage of the converter grid connection point; is the rated current of the energy storage converter; When the grid voltage recovers, the system will enter the "low voltage ride-through exit" state. If the detected voltage continues to meet the low voltage ride-through exit voltage condition within the third preset time, the system will enter the "low voltage ride-through recovery" state. Otherwise, it will be considered that the "low voltage ride-through exit" state is a misjudgment and the system will return to the "low voltage ride-through" state. In the "Low Voltage Ride Through Recovery" state, the converter returns to the power value output before the low voltage ride through occurs, and the system returns to the "System Normal" state; In the "high voltage ride through" state, the converter does not need to generate reactive current. When the grid voltage recovers and meets the voltage conditions for high voltage ride through exit, the system returns to the "system normal" state.

2. The method according to claim 1, characterized in that Also includes: In the "low-through entry" state, the "low-through exit" state, the "high-voltage entry" state and the "high-through exit" state, the current of the converter is reduced to zero or directly blocked.

3. The method according to claim 1, characterized in that It also includes: when the state is transferred from the "low voltage ride-through" state to the "low voltage ride-through" state, an action of resetting the inner loop control parameters is executed.

4. The method according to claim 1, characterized in that: Also includes: When returning to the "system normal" state from the "low-through entry" state, the action of resetting the inner and outer loop control parameters is executed.

5. The method according to claim 1, characterized in that It also includes: when transferring from the "low-penetration exit" state to the "low-penetration recovery" state, resetting the inner and outer loop control parameters and limiting the recovery speed.

6. The method according to claim 1, characterized in that Also includes: When the system is transferred from the "low-break recovery" state to the "system normal" state, the recovery speed is normalized.

7. The method according to claim 1, characterized in that When the converter recovers to the power value outputted before the low voltage ride-through occurs, the converter recovers gradually according to a preset slope.

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