Overcurrent control method for the sending-end converter of flexible DC grid-connected system

Abstract

The present invention provides an overcurrent control method for a sending-end converter of a flexible DC grid-connected system, comprising: obtaining the RMS value of the AC voltage, three-phase AC voltage and three-phase AC current of a common connection point; obtaining the amplitude and phase of the AC internal potential of the sending-end converter; obtaining the d-axis voltage component, q-axis voltage component, d-axis current component and q-axis current component of the common connection point according to the phase and the three-phase AC voltage and the three-phase AC voltage; obtaining the first reference current of the d-axis and the first reference current of the q-axis respectively through a virtual admittance method according to the difference between the d-axis voltage component and the amplitude of the AC internal potential and the q-axis voltage component; obtaining the second reference current of the d-axis and the second reference current of the q-axis respectively through a circular current limiting method; and then using the two to obtain a control signal; and controlling the on or off of the sending-end converter through the control signal. The present invention can enable the sending-end converter to quickly limit the output current and prevent the occurrence of overcurrent problems.

Description

Technical Field

[0001] The present invention relates to the technical field of power grids, and in particular to an overcurrent control method for a sending-end converter of a flexible direct current grid-connected system. Background Art

[0002] The wind farm-flexible direct current grid-connected system includes offshore wind farms, wind farm step-up transformers, sending-end converters, AC / DC submarine cables, receiving-end converters and matching transformers. Currently, both the receiving-end converters and the sending-end converters adopt modular multi-level topology structures. The receiving-end converter controls the DC voltage and grid-connected reactive power, the sending-end converter controls the AC voltage amplitude and frequency of the wind farm, and the wind turbines work in the maximum power tracking mode to control their output active and reactive power.

[0003] When a short circuit occurs in the output circuit of a wind farm, the sending-end converter may be unable to quickly limit the output current, causing an overcurrent problem. Summary of the invention

[0004] The purpose of the present invention is to provide an overcurrent control method for a sending-end converter of a flexible DC grid-connected system, which can enable the sending-end converter to quickly limit the output current and prevent the occurrence of overcurrent problems when a short-circuit fault occurs in the output circuit of a wind farm.

[0005] In order to achieve the above object, the present invention provides an overcurrent control method for a sending-end converter of a flexible DC grid-connected system, comprising:

[0006] Obtain the RMS value of the AC voltage, three-phase AC voltage and three-phase AC current at the common connection point;

[0007] Acquire the amplitude of the AC internal potential of the sending-end converter according to the AC voltage RMS value, and acquire the phase of the AC internal potential according to the rated frequency of the sending-end converter;

[0008] Acquire the d-axis voltage component and the q-axis voltage component of the common connection point according to the phases of the three-phase AC voltage and the AC internal potential, and acquire the d-axis current component and the q-axis current component of the common connection point according to the phases of the three-phase AC current and the AC internal potential;

[0009] Obtaining a d-axis first reference current and a q-axis first reference current respectively by a virtual admittance method according to the difference between the amplitude of the AC internal potential and the d-axis voltage component and the q-axis voltage component;

[0010] The d-axis first reference current and the q-axis first reference current are respectively obtained by a circular current limiting method to obtain a d-axis second reference current and a q-axis second reference current;

[0011] Obtain a d-axis control signal according to the d-axis current component and the d-axis second reference current, and obtain a q-axis control signal according to the q-axis current component and the q-axis second reference current;

[0012] Obtaining a control signal according to the d-axis control signal and the q-axis control signal; and

[0013] The control signal is used to control the sending-end converter to be turned on or off, so as to prevent the sending-end converter from having an overcurrent condition.

[0014] Optionally, in the overcurrent control method, the method for obtaining the amplitude of the AC internal potential of the sending-end converter includes:

[0015] The AC voltage root mean square value and the set voltage are used to obtain the amplitude of the AC internal potential of the sending-end converter through a proportional-integral controller.

[0016] Optionally, in the overcurrent control method, the method for obtaining the phase of the AC internal potential of the sending-end converter includes:

[0017] The phase of the AC internal potential is obtained by integrating the rated frequency of the sending-end converter.

[0018] Optionally, in the overcurrent control method, the method of obtaining the d-axis voltage component and the q-axis voltage component of the common connection point according to the phase of the three-phase AC voltage and the AC internal potential includes:

[0019] The three-phase AC voltage is parked at the phase of the AC internal potential to obtain a d-axis voltage component and a q-axis voltage component.

[0020] Optionally, in the overcurrent control method, the method of obtaining the d-axis current component and the q-axis current component of the common connection point according to the phase of the three-phase AC voltage and the AC internal potential includes:

[0021] The three-phase AC voltage is parked at the phase of the AC internal potential to obtain a d-axis current component and a q-axis current component.

[0022] Optionally, in the overcurrent control method, the virtual admittance method includes: using a virtual resistor to respectively obtain the DC component of the difference between the amplitude of the AC internal potential and the d-axis voltage component and the DC component of the AC q-axis voltage component, as the d-axis first reference current and the q-axis first reference current, respectively.

[0023] Optionally, in the overcurrent control method, a method of obtaining a d-axis second reference current and a q-axis second reference current respectively by using a circular current limiting method for the d-axis first reference current and the q-axis first reference current includes:

[0024] A sum of the square of the d-axis first reference current and the square of the q-axis first reference current;

[0025] calculating the square root of the sum;

[0026] If the square root is less than or equal to the current limiting current, the d-axis second reference current is the d-axis first reference current, and the q-axis second reference current is the q-axis first reference current; if the square root is greater than the current limiting current, the d-axis second reference current and the q-axis second reference current are respectively calculated based on the d-axis first reference current, the q-axis first reference current and the current limiting current.

[0027] Optionally, in the overcurrent control method, a method for obtaining a d-axis control signal according to the d-axis current component and the d-axis second reference current, and a method for obtaining a q-axis control signal according to the q-axis current component and the q-axis second reference current includes:

[0028] Passing the d-axis current component and the d-axis second reference current through a proportional-integral controller to obtain a d-axis control signal;

[0029] The q-axis current component and the q-axis second reference current are passed through a proportional-integral controller to obtain a q-axis control signal.

[0030] Optionally, in the overcurrent control method, a method for obtaining a control signal according to the d-axis control signal and the q-axis control signal includes:

[0031] The d-axis control signal and the q-axis control signal are transformed by inverse park to obtain a control signal.

[0032] Optionally, the overcurrent control method further includes: if an overcurrent occurs in the sending-end converter, increasing the resistance of the virtual resistor.

[0033] Optionally, in the overcurrent control method, the resistance of the virtual resistor increases, and the DC component passing through the virtual resistor decreases, and when it decreases to a certain value, the sending-end converter is shut down.

[0034] In the overcurrent control method of the sending-end converter of the flexible DC grid-connected system provided by the present invention, a control signal for controlling the opening or closing of the sending-end converter is obtained through a virtual admittance method and a circular current limiting method. When a short circuit fault occurs in the output circuit of the wind farm, the sending-end converter can quickly limit the output current to prevent the occurrence of overcurrent problems. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is a schematic diagram of a flexible DC grid-connected system according to an embodiment of the present invention;

[0036] Figure 2is a flow chart of an overcurrent control method for a sending-end converter of a flexible DC grid-connected system according to an embodiment of the present invention;

[0037] Figure 3 It is a simulation diagram of the voltage at the common connection point;

[0038] Figure 4 It is a simulation diagram of the input current of the wind farm input common connection point;

[0039] Figure 5 It is a simulation diagram of the input power of the wind farm into the common connection point;

[0040] In the figure: 110-wind farm, 120-public connection point, 130-connection transformer, 140-sending end converter, 150-receiving end. DETAILED DESCRIPTION

[0041] The specific implementation of the present invention will be described in more detail below in conjunction with the schematic diagram. The advantages and features of the present invention will become clearer based on the following description. It should be noted that the drawings are all in a very simplified form and are not in exact proportions, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0042] Hereinafter, the terms "first", "second", etc. are used to distinguish between similar elements and are not necessarily used to describe a particular order or chronological sequence. It is to be understood that these terms used in this manner are interchangeable where appropriate. Similarly, if the method described herein includes a series of steps, the order of these steps presented herein is not necessarily the only order in which these steps can be performed, and some of the steps described may be omitted and / or some other steps not described herein may be added to the method.

[0043] Please refer to Figure 1 The flexible DC grid-connected system of the embodiment of the present invention includes: the wind farm 110 generates electricity, which passes through the connecting transformer 130 and the sending-end converter 140 in sequence to reach the receiving end 150. In order to control the sending-end converter to prevent overcurrent, a control signal is calculated from the voltage of the common connection point 120 between the wind farm 110 and the connecting transformer 130.

[0044] Please refer to Figure 2 The present invention provides an overcurrent control method for a sending-end converter of a flexible DC grid-connected system, comprising:

[0045] S1: Obtain the RMS value of the AC voltage, three-phase AC voltage and three-phase AC current at the common connection point;

[0046] S2: obtaining the amplitude of the AC internal potential of the sending-end converter according to the RMS value of the AC voltage, and obtaining the phase of the AC internal potential according to the rated frequency of the sending-end converter;

[0047] S3: obtaining a d-axis voltage component and a q-axis voltage component of a common connection point according to the phases of the three-phase AC voltage and the AC internal potential, and obtaining a d-axis current component and a q-axis current component of a common connection point according to the phases of the three-phase AC current and the AC internal potential;

[0048] S4: obtaining a d-axis first reference current and a q-axis first reference current respectively according to the difference between the amplitude of the AC internal potential and the d-axis voltage component and the q-axis voltage component by a virtual admittance method;

[0049] S5: obtaining the d-axis first reference current and the q-axis first reference current respectively through a circular current limiting method;

[0050] S6: obtaining a d-axis control signal according to the d-axis current component and the d-axis second reference current, and obtaining a q-axis control signal according to the q-axis current component and the q-axis second reference current;

[0051] S7: obtaining a control signal according to the d-axis control signal and the q-axis control signal; and

[0052] S8: Control the sending-end converter to be turned on or off through a control signal to prevent the sending-end converter from having an overcurrent condition.

[0053] Preferably, the method for obtaining the amplitude of the AC internal potential of the sending-end converter includes: obtaining the amplitude of the AC internal potential of the sending-end converter by using the AC voltage root mean square value and the set voltage through a proportional-integral controller. The specific calculation formula is as follows:

[0054]

[0055] Among them, kpE is the proportional coefficient of AC voltage control, kiE is the integral coefficient of AC voltage control, s is the Laplace operator, u ref is the set voltage, u ref is the set value, u rms is the three-phase AC voltage, and E is the amplitude of the AC internal potential of the sending-end converter.

[0056] Preferably, the method for obtaining the phase of the AC internal potential of the sending-end converter includes: obtaining the phase of the AC internal potential by integrating the rated frequency of the sending-end converter. The rated frequency of the sending-end converter in the embodiment of the present invention is 50 Hz. The specific calculation formula is as follows:

[0057]

[0058] Among them, θ MMC is the phase of the AC internal potential, s is the Laplace operator, ω nom is the rated frequency.

[0059] Preferably, the method for obtaining the d-axis voltage component and the q-axis voltage component of the common connection point according to the phase of the three-phase AC voltage and the AC internal potential includes: park the three-phase AC voltage under the phase of the AC internal potential to obtain the d-axis voltage component and the q-axis voltage component. Similarly, the method for obtaining the d-axis current component and the q-axis current component of the common connection point according to the phase of the three-phase AC voltage and the AC internal potential includes: park the three-phase AC voltage under the phase of the AC internal potential to obtain the d-axis current component and the q-axis current component. The method for obtaining the root mean square value of the AC voltage, the three-phase AC voltage and the three-phase AC current is prior art and will not be described in detail here. The method for obtaining the d-axis voltage component and the q-axis voltage component is also prior art and will not be described in detail here.

[0060] Preferably, the virtual admittance method includes: the virtual admittance method includes: using a virtual resistor to obtain the DC component of the difference between the amplitude of the AC internal potential and the d-axis voltage component and the DC component of the AC q-axis voltage component, respectively, as the d-axis first reference current and the q-axis first reference current. The specific calculation formula is as follows:

[0061]

[0062] Where E is the amplitude of the AC internal potential, u d is the d-axis voltage component, u q is the q-axis voltage component, Lv is the virtual reactance, Rv is the virtual resistance, s is the Laplace operator, ω nom is the rated frequency, and They are the d-axis first reference current and the q-axis first reference current respectively.

[0063] Preferably, the method of obtaining the d-axis second reference current and the q-axis second reference current respectively by using the circular current limiting method for the d-axis first reference current and the q-axis first reference current includes:

[0064] A sum of the square of the d-axis first reference current and the square of the q-axis first reference current;

[0065] Calculate the square root of the sum;

[0066] If the square root is less than or equal to the current limiting current, the d-axis second reference current is the d-axis first reference current, and the q-axis second reference current is the q-axis first reference current; if the square root is greater than the current limiting current, the d-axis second reference current and the q-axis second reference current are obtained according to the d-axis first reference current, the q-axis first reference current and the current limiting current. The specific calculation formula is as follows:

[0067]

[0068]

[0069] Among them, i dref is the second reference current of d axis, i qref is the second reference current of d axis, i lim is the current limiting current, and are the d-axis first reference current and the q-axis first reference current respectively

[0070] Preferably, the method of obtaining the d-axis control signal according to the d-axis current component and the d-axis second reference current, and obtaining the q-axis control signal according to the q-axis current component and the q-axis second reference current comprises:

[0071] Passing the d-axis current component and the d-axis second reference current through a proportional-integral controller to obtain a d-axis control signal;

[0072] The q-axis current component and the q-axis second reference current are passed through a proportional-integral controller to obtain a q-axis control signal.

[0073] Preferably, the method for obtaining the control signal according to the d-axis control signal and the q-axis control signal comprises: obtaining the control signal according to the d-axis control signal and the q-axis control signal through inverse park change.

[0074] Preferably, it also includes: if an overcurrent occurs in the sending-end converter, the resistance of the virtual resistor is increased. The resistance of the virtual resistor increases, and the DC component through the virtual resistor decreases faster. When it decreases to a certain value, the sending-end converter is shut down. In this virtual admittance control method, when a short circuit fault occurs in the wind farm transmission line, due to the existence of the virtual inductance, the fault current output by the sending-end converter cannot change suddenly, resulting in a DC component similar to that of a synchronous generator in the fault current. This DC component will increase the current stress borne by the power electronic devices in the sending-end converter, in order to reduce the size of the DC component. The embodiment of the present invention increases the size of the virtual resistor Rv after the fault occurs, accelerates the attenuation speed of the DC component in the fault current, reduces the DC component of the fault current, improves the operating stability of the sending-end converter and the ability to maintain the voltage of the wind farm, and reduces the oscillation of the sending-end converter when the fault is restored.

[0075] At the same time, the present invention adopts the above technology to carry out verification simulation. This paper uses PSCAD or EMTDC software based on Figure 1 The simulation model of the wind farm flexible direct current system was constructed, and the simulation diagram was obtained. Figure 3 , Figure 4 and Figure 5 The simulation system parameters are shown in Table 1, which shows the MMC parameters at the sending end.

[0076] Table 1

[0077]

[0078] From Table 1, we know that the initial output power of the wind farm is 800MW. A three-phase grounding fault occurs in the AC transmission line of the wind farm at t=3s, which may cause overcurrent in the converter at the sending end. The fault lasts for 100ms. The voltage of the common connection point before and after the fault occurs is as follows: Figure 3 , the input current of the wind farm input common connection point before and after the fault occurs is as follows Figure 4 ; The input power of the wind farm input common connection point before and after the fault occurs is as follows: Figure 5 In the figure, 2.8s~3s is the time before the fault occurs, the power grid is in a steady state, 3s~3.1s is the time when the fault occurs, and 3.1s~3.6s is the time after the fault occurs, the power grid is in a recovery state. Figure 3 and Figure 4 It can be seen that after adopting the overcurrent control method of the embodiment of the present invention, the sending-end converter can ensure that the output current does not exceed the limit value in the steady state and during the fault ride-through process, and the changes in the common connection point voltage and the common connection point input current are very smooth, and can complete the ride-through under extremely low voltage (10% rated value), and has good fault ride-through performance; after the fault is cleared, the input current recovery process of the common connection point is smooth, and there is no overcurrent problem during the recovery process, and the sending-end converter can quickly restore the common connection point voltage, and the recovery time is about 0.1s. Figure 5 It can be seen that the input power of the wind farm input common connection point, such as active power and reactive power, is also restored very smoothly. Therefore, the overcurrent control method of the embodiment of the present invention can well prevent the sending-end converter from overcurrent.

[0079] In summary, in the overcurrent control method of the sending-end converter of the flexible DC grid-connected system provided in an embodiment of the present invention, a control signal for controlling the opening or closing of the sending-end converter is obtained through a virtual admittance method and a circular current limiting method. When a short circuit fault occurs in the output circuit of the wind farm, the sending-end converter can quickly limit the output current to prevent the occurrence of overcurrent problems.

[0080] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any technician in the relevant technical field, without departing from the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, which does not depart from the content of the technical solution of the present invention and still falls within the protection scope of the present invention.

Claims

1. A method for controlling overcurrent of a sending-end converter of a flexible DC grid-connected system, characterized in that: include: Obtain the RMS value of the AC voltage, three-phase AC voltage and three-phase AC current at the common connection point; Acquire the amplitude of the AC internal potential of the sending-end converter according to the AC voltage RMS value, and acquire the phase of the AC internal potential according to the rated frequency of the sending-end converter; Acquire the d-axis voltage component and the q-axis voltage component of the common connection point according to the phases of the three-phase AC voltage and the AC internal potential, and acquire the d-axis current component and the q-axis current component of the common connection point according to the phases of the three-phase AC current and the AC internal potential; Obtaining a d-axis first reference current and a q-axis first reference current respectively by a virtual admittance method according to the difference between the amplitude of the AC internal potential and the d-axis voltage component and the q-axis voltage component; The d-axis first reference current and the q-axis first reference current are respectively obtained by a circular current limiting method to obtain a d-axis second reference current and a q-axis second reference current; Obtain a d-axis control signal according to the d-axis current component and the d-axis second reference current, and obtain a q-axis control signal according to the q-axis current component and the q-axis second reference current; Obtaining a control signal according to the d-axis control signal and the q-axis control signal; as well as Controlling the on or off of the sending-end converter by means of the control signal to prevent the sending-end converter from having an overcurrent condition; The method for obtaining the amplitude of the AC internal potential of the sending-end converter includes: The AC voltage root mean square value and the set voltage are used to obtain the amplitude of the AC internal potential of the sending-end converter through a proportional-integral controller.

2. The overcurrent control method according to claim 1, characterized in that: The method of obtaining the phase of the AC internal potential of the sending-end converter includes: The phase of the AC internal potential is obtained by integrating the rated frequency of the sending-end converter.

3. The overcurrent control method according to claim 1, characterized in that: The method of obtaining the d-axis voltage component and the q-axis voltage component of the common connection point according to the phase of the three-phase AC voltage and the AC internal potential includes: The three-phase AC voltage is parked at the phase of the AC internal potential to obtain a d-axis voltage component and a q-axis voltage component.

4. The overcurrent control method according to claim 1, characterized in that: The method of obtaining the d-axis current component and the q-axis current component of the common connection point according to the phases of the three-phase AC voltage and the AC internal potential includes: The three-phase AC voltage is parked at the phase of the AC internal potential to obtain a d-axis current component and a q-axis current component.

5. The overcurrent control method according to claim 1, characterized in that: The virtual admittance method includes: using virtual resistance to obtain the DC component of the difference between the amplitude of the AC internal potential and the d-axis voltage component and the DC component of the AC q-axis voltage component, respectively, as the d-axis first reference current and the q-axis first reference current.

6. The overcurrent control method according to claim 1, characterized in that: The method of obtaining the d-axis second reference current and the q-axis second reference current respectively by using the circular current limiting method comprises: A sum of the square of the d-axis first reference current and the square of the q-axis first reference current; calculating the square root of the sum; If the square root is less than or equal to the current limiting current, the d-axis second reference current is the d-axis first reference current, and the q-axis second reference current is the q-axis first reference current; if the square root is greater than the current limiting current, the d-axis second reference current and the q-axis second reference current are respectively calculated based on the d-axis first reference current, the q-axis first reference current and the current limiting current.

7. The overcurrent control method according to claim 1, characterized in that: A method for obtaining a d-axis control signal according to the d-axis current component and the d-axis second reference current, and obtaining a q-axis control signal according to the q-axis current component and the q-axis second reference current comprises: Passing the d-axis current component and the d-axis second reference current through a proportional-integral controller to obtain a d-axis control signal; The q-axis current component and the q-axis second reference current are passed through a proportional-integral controller to obtain a q-axis control signal.

8. The overcurrent control method according to claim 1, characterized in that: The method for obtaining a control signal according to the d-axis control signal and the q-axis control signal comprises: The d-axis control signal and the q-axis control signal are transformed by inverse park to obtain a control signal.

9. The overcurrent control method according to claim 5, characterized in that: Also includes: If an overcurrent occurs in the sending-end converter, the resistance of the virtual resistor is increased.

10. The overcurrent control method according to claim 9, characterized in that: The resistance of the virtual resistor increases, and the DC component passing through the virtual resistor decreases. When it decreases to a certain value, the sending-end converter is turned off.

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