Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a high-low penetration control method of a grid-connected converter, which aims to solve the technical problems.
The invention provides a high-low penetration control method of a grid-connected converter, which comprises the following steps:
The grid-connected converter enters a high-low penetration state when a power grid has temporary faults, a state machine is adopted to control the state of a control system of the converter, and the state of the whole control process comprises a system normal state, a low penetration state, a low voltage penetration state, a low penetration exit state, a low penetration recovery state, a high penetration state and a high voltage penetration state;
Detecting a positive sequence voltage calculated by a frequency locking loop based on a second-order generalized integrator and a phase calculated by an iterative least square method when the system is in a normal state, judging to enter a low penetration state if the positive sequence voltage suddenly drops to a low penetration voltage limit and the phase suddenly changes, and judging to enter a high penetration state if the positive sequence voltage suddenly rises to a high penetration voltage limit and the phase suddenly changes;
Starting timing after entering a low penetration state and a high penetration state, entering a low voltage penetration state if the positive sequence voltage always meets a low penetration voltage limit in a first preset time period, entering a high voltage penetration state if the positive sequence voltage always meets a high penetration voltage limit in a second preset time period, otherwise, returning the system to a normal state;
in the low voltage crossing state, the converter sends reactive current according to the national standard on reactive support during low crossing;
When the power grid voltage is recovered, the system enters a low-voltage-threading-out state, if the detected voltage always meets the low-threading-out voltage condition within a third preset time, the system enters the low-voltage-threading-out state, otherwise, the low-threading-out state is considered to be misjudgment, and the system returns to the low-voltage-threading-out state;
In the 'low-voltage penetration recovery' state, the converter gradually recovers to the power value output before the low-voltage penetration occurs according to the preset slope, and then the system returns to the 'system normal' state again;
in the high voltage crossing state, the converter does not need to send reactive current, and when the power grid voltage is recovered and the high crossing exit voltage condition is met, the system returns to the normal state of the system.
Further, in the low penetration state, the high voltage penetration state and the high penetration state, the current of the converter is reduced to zero or the current of the converter is directly sealed.
Further, the method comprises the step of executing the action of resetting the inner loop control parameter when the low-voltage crossing state is shifted from the low-voltage crossing state.
And further, when the system returns to the normal state from the low penetration state, the action of resetting the control parameters of the inner ring and the outer ring is executed.
Further, the method also comprises the step of executing the action of resetting the control parameters of the inner ring and the outer ring when the low-penetration exit state is transferred to the low-penetration recovery state, and limiting the recovery speed.
Further, the method also comprises the step of executing the action of normalizing the recovery speed when the state of low-pass recovery is transferred to the state of normal system.
Further, covariance reset is added in iterative least square method calculation, recorded phase information is deleted when abrupt voltage amplitude exceeds a certain value, reading and recording of the phase information are restarted, and when covariance reset occurs, high voltage crossing or low voltage crossing is judged.
And further, gradually recovering according to a preset slope when the converter recovers to the power value output before the low-voltage pass-through occurs. .
The high-low penetration control method for the grid-connected converter has the advantages that the grid-connected converter can rapidly detect and judge the occurrence of faults from a normal working state when the grid has temporary faults, so that the grid-connected converter enters the high-low penetration state, and emits specific reactive current, and in addition, the grid-connected converter can safely and stably recover to the normal working state before the faults from the high-low penetration state after the faults are recovered. In addition, in the grid-connected converter, no matter the frequency-locked loop or the phase-locked loop is adopted in the grid synchronization algorithm, the speed is limited by the bandwidth of the controller, and the problem of slower response speed to disturbance can be caused in the high-low threading process. Therefore, when the high-low pass-through enters and exits, the invention creatively uses the mixed detection method of calculating the positive sequence voltage mutation by using the frequency locking ring based on the second-order generalized integrator and calculating the phase mutation by using the iterative least square method with covariance reset to ensure that the system can respond quickly when the high-low pass-through occurs. In addition, the invention has reliable design principle, simple structure and very wide application prospect.
Detailed Description
In order to make the technical solution of the present invention better understood by those skilled in the art, the technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the present invention without making any inventive effort, shall fall within the scope of the present invention.
As shown in FIG. 1, the embodiment of the invention provides a high-low penetration control method for a grid-connected converter, which adopts a state machine to carry out logic processing, wherein the management of the state machine comprises seven states of system normal, low penetration, low voltage penetration, low penetration exit, low penetration recovery, high penetration and high voltage penetration.
The grid-connected converter is required to output reactive current to support the voltage of a power grid during low voltage ride through, so that the transient process of entering and exiting of the low voltage ride through needs to be managed more finely. If the low voltage crossing state is referred, the granularity of the high voltage crossing state is divided more finely, and the invention also belongs to the protection scope of the invention.
In the grid-connected converter, the speed of the grid synchronization algorithm is limited by the bandwidth of the controller no matter the frequency-locked loop or the phase-locked loop is adopted, and the problem of slower response speed to disturbance can be caused in the high-low threading process. Therefore, when the high-low penetration enters and exits, the invention creatively uses the mixed detection method of calculating the positive sequence voltage mutation by using the frequency locking ring based on the second-order generalized integrator and calculating the phase mutation by using the iterative least square method with covariance reset to ensure that the system can quickly respond when the high-low penetration occurs, and the principle of the part is explained below.
When the grid voltage of the grid-connected converter adopts a complex vector representation mode, a transformation matrix and an inverse transformation matrix which are respectively transformed from a three-phase static coordinate system to a two-phase static coordinate system are
The three-phase grid voltage and the transformation matrix can be used for obtaining the grid voltage expression under a two-phase static coordinate system:
since there are positive and negative sequence components in the voltage, it is divided into two parts, positive and negative sequence components, which are represented in a positive and negative rotation coordinate system, respectively.
The transformation matrix and the inverse transformation matrix transformed from the two-phase stationary coordinate system to the positive sequence rotating coordinate system are respectively:
The transformation matrix and the inverse transformation matrix which are transformed from the two-phase static coordinate system to the negative sequence rotation coordinate system are respectively as follows:
the grid voltage expression can be written according to the above transformation formula as:
Wherein the method comprises the steps ofIs an expression of the positive sequence component in the forward rotation coordinate system,Is an expression of the negative sequence component in the negative rotation coordinate system. Rewrites formula (8) in matrix form to
y(ti)＝H(ti)x(ti) (9)
Wherein:
y(ti)＝[vαs(ti) vβs(ti)]T
defining a cost function as:
where λ is a forgetting factor, the purpose of which is to attenuate the previous measurement gradually, the smaller λ is, the faster the attenuation speed, i.e. the more important the system is to the current sample value, the faster the tracking speed is, but also the more susceptible it is to interference.
And solving x (tj) for minimizing a cost equation by an iterative least square method, wherein the process is as follows:
R(ti)＝λI+H(ti)P(ti-1)H(ti)T (12)
K(ti)＝P(ti-1)H(ti)TR(ti)-1 (13)
P(ti)＝λ-1(P(ti-1)-P(ti-1)H(ti)TR(ti)-1H(ti)P(ti-1)) (14)
wherein the initial input of coefficients is solvedThe initial covariance coefficient P (t0)＝π0 I;
Finally, by
The system phase can be obtained.
In order to improve the response speed when the phase mutation occurs, covariance reset is added, namely when ||y (ti)-H(ti)x(ti) || > epsilon, P (ti-1) is reset to pi0 I, and when the voltage amplitude of the mutation exceeds a certain value epsilon, record information is deleted, and the current phase information is read and recorded again. In order to quickly judge whether the high-low penetration occurs, the system can possibly occur the high-low penetration when covariance reset occurs, and the detection of the occurrence of the high-low penetration can be finally completed through matching with a frequency locking ring based on a second-order generalized integrator.
The implementation method of the frequency locking ring based on the second-order generalized integrator is more general and will not be described in detail here.
In accordance with the principles described above, the process of state transition and action execution for low voltage ride through will be described in detail.
Firstly, a system is in a normal state at the beginning, and the voltage of a power grid can be detected in real time in the normal state;
And secondly, when the amplitude of the positive sequence component of the power grid voltage is suddenly dropped to reach a low-penetration voltage limit based on a frequency locking loop of the second-order generalized integrator, and the phase mutation is calculated by an iterative least square method with covariance reset, the system enters a low-penetration state, the current of the converter needs to be reduced to zero or directly sealed in the state, the first preset time is a reasonable time period from the normal state of the system to the low-voltage penetration state, if the low-penetration voltage condition is always met within the first preset time, namely, the low-penetration state is always met, the system enters the low-voltage penetration state, otherwise, the low-penetration state is considered to be misjudgment, and the system returns to the normal state of the system.
And thirdly, if the state is changed from the low-pass-in state to the low-voltage-ride-through state, the action of resetting the inner loop control parameters is required to be executed so as to prepare for reactive current generation during the low-pass period, and if the state is changed from the low-pass-in state to the normal state of the system again, the action of resetting the inner loop and the outer loop control parameters is required to be executed so that the converter can recover the running power state before the misjudgment occurs.
In the fourth step, in the "low voltage crossing state", the converter needs to send out specific reactive current according to the reactive support requirement in the national standard during the low crossing period, when the voltage of the power grid is recovered and the low crossing exit condition is met, including the voltage condition and the time condition, the system enters the "low crossing exit" state, in this state, the reactive current needs to be reduced to zero or directly sealed, the time is also needed until the low crossing recovery judgment time, if the low crossing exit condition is met all the time in this time period, including the voltage condition and the time condition, the system enters the "low crossing recovery" state, otherwise, the low crossing exit is considered to be misjudgment, and the system returns to the "low voltage crossing" state.
And fifthly, if the state of low-pass exit is changed to the state of low-pass recovery, the action of resetting the control parameters of the inner ring and the outer ring is required to be executed, and meanwhile, the recovery speed is limited, so that the converter can safely and stably recover to the power state before low-pass after the low-pass is finished.
And step six, in the 'low-voltage penetration recovery' state, the converter needs to gradually recover to the power value output before the low-voltage penetration occurs according to the preset slope, and then the system returns to the 'system normal' state again.
And seventh, when the state is changed from the low-power-consumption recovery state to the system normal state, the normal recovery speed is required to be executed, so that the original response speed of the converter to the power instruction is recovered after the low-power consumption is finished for a period of time.
The reactive current output by the energy storage converter during low voltage ride through in national standard GB/T34120-2017 has definite requirements that the dynamic reactive current injected into the power system by the energy storage converter should track the voltage change of the grid connection point in real time from the time of automatic reactive current response until the voltage is recovered to 0.85 (p.u.), and the requirements are as follows:
wherein UT is the per unit value of the grid-connected point voltage of the converter, and IN is the rated current of the energy storage converter.
The following describes a process for state transition and action execution of high voltage ride through.
Firstly, a system is in a normal state at the beginning, and the voltage of a power grid can be detected in real time in the normal state;
And secondly, when the amplitude of the positive sequence component of the power grid voltage is suddenly increased to reach a high-pass voltage limit based on the frequency locking loop of the second-order generalized integrator, and the phase mutation is calculated by an iterative least square method with covariance reset, the system enters a high-pass-in state, the current of the converter needs to be reduced to zero or directly sealed, the time is also needed to be counted until the high-pass-in judgment time, if the high-pass-in condition, including the voltage condition and the time condition, is always met in the time period, the system enters the high-voltage-pass state, otherwise, the high-pass is considered to be misjudged, and the system returns to the normal state.
And thirdly, in the high voltage crossing state, the converter does not need to send reactive current, and when the voltage of the power grid is recovered and the conditions of high crossing exit are met, the system returns to the normal state of the system again.
The practical use effect of the method is verified in a simulation manner.
A converter with rated power of 1.725MW, rated voltage of 690V at the alternating-current side and rated current of 1443A is built in MATLAB, and the high-low penetration control method and the practical application effect of the system of the grid-connected converter are simulated. In the simulation, four worst high and low penetration conditions, namely, three-phase voltage balance drop to 5%, two-phase voltage unbalance drop to 5%, single-phase voltage unbalance drop to 5% and three-phase voltage rise to 130%, are selected.
Fig. 2 shows that when the power output of the converter is 1.725MW, the power grid fails, so that the power grid voltage is three-phase balanced and falls to 5% of rated voltage, at this time, the converter needs to safely pass through the low-voltage process, and reactive current of 104% of rated current is sent out according to national standard requirements in the process to support the power grid. As can be seen from fig. 2, after the grid voltage drops in 0.2 seconds, the active current of the converter is reduced to 0 first after a transient process is experienced, then the current starts to emit 104% rated reactive current, and when the grid returns to normal in 0.4 seconds, the current converter gradually returns to the working state before low-pass with a certain slope, and the output power is 1.725MW.
Fig. 3 shows that when the power output of the converter is 1.725MW, the power grid fails, so that the voltage of the power grid falls to 5% of rated voltage, at this time, the converter needs to safely pass through the low-voltage process, and a specific reactive current is sent out according to national standard requirements in the process to support the power grid. As can be seen from fig. 3, after the voltage of the power grid drops in 0.2 seconds, the active current of the converter is reduced to 0 first after a transient process is experienced, then a specific reactive current starts to be sent out, and after the power grid returns to normal in 0.4 seconds, the converter gradually returns to the working state before low-pass with a certain slope, and the output power is 1.725MW.
Fig. 4 shows that when the power output of the converter is 1.725MW, the power grid fails to cause the voltage of the power grid to drop to 5% of rated voltage, at this time, the converter needs to safely pass through the low-voltage process, and a specific reactive current is sent out according to national standard requirements in the process to support the power grid. As can be seen from fig. 4, after the voltage of the power grid drops in 0.2 seconds, the active current of the converter is reduced to 0 first after a transient process is experienced, then a specific reactive current starts to be sent out, and when the power grid returns to normal in 0.4 seconds, the converter gradually returns to the working state before low-pass with a certain slope, and the output power is 1.725MW.
Fig. 5 shows that when the power output of the converter is 1.725MW, the power grid voltage suddenly rises to 130% of the rated voltage, and the converter needs to safely pass through the high voltage process, and in the high-pass process, no reactive current is required. As can be seen from fig. 5, after the grid voltage drops in 0.2 seconds, the active current is reduced to 0 by the converter after a transient process, and after the grid returns to normal in 0.4 seconds, the converter gradually returns to the working state before high-pass with a certain slope, and the output power is 1.725MW.
In summary, the control method of the invention can enable the grid-connected converter to rapidly detect and judge the occurrence of faults from a normal working state when the power grid has temporary faults, so as to enter a high-low penetration state, send out specific reactive current, and safely and stably recover from the high-low penetration state to the normal working state before the faults after the faults are recovered.
Although the present invention has been described in detail by way of preferred embodiments with reference to the accompanying drawings, the present invention is not limited thereto. Various equivalent modifications and substitutions may be made in the embodiments of the present invention by those skilled in the art without departing from the spirit and substance of the present invention, and it is intended that the present invention encompass all such modifications and substitutions as would be within the scope of the present invention as defined by the appended claims. Therefore, the protection scope of the invention is subject to the protection scope of the claims.