Method and device for controlling high-voltage direct-current transformer of full-direct-current system

Abstract

The invention discloses a control method and device for a high-voltage direct-current transformer of a full-direct-current system. The control method comprises the following steps: on the basis of a traditional double-closed-loop control method, the voltage on an inductor is fed back to a current inner loop through a state observer to influence the generation of dq-axis reference voltage, that is, the secondary side modulation wave voltage is obtained through state observation; subtracting the secondary side modulation wave voltage from the primary side modulation wave voltage to obtain a voltage on an inductor, and feeding back the voltage on the inductor to the output of a current inner loop controller as a superposition term for generating a dq-axis reference voltage; the voltage outer ring enhances DC side damping by introducing a DC side virtual resistor, and corrects the reference current of the current inner ring. The voltage on the inductor is fed back to the current inner ring through the state observer to improve the current inner ring, the direct current side damping is enhanced by introducing the direct current side virtual resistor, cooperative control of the current inner ring and the voltage outer ring is achieved, the dynamic characteristic and stability of the current inner ring are improved, and the oscillation problem is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of full DC system, and in particular to a control method and device for a high-voltage DC transformer of a full DC system. Background Art

[0002] With the development of power electronics technology and flexible DC transmission technology, the all-DC collection and transmission scheme for new energy has emerged. The all-DC technology for new energy based on DC collection and DC transmission only needs to maintain the stability of the DC voltage to achieve system stability. It has the advantages of single control target, fast response speed, high efficiency and power density. Compared with the AC collection method, the all-DC collection and transmission scheme has a long collection and transmission distance, and there are no voltage / reactive power problems and synchronous stability problems. It is very competitive in the scenario of large-scale new energy wide-area collection and long-distance transmission. DC transformers with high voltage, high gain, large capacity, unidirectional power flow and isolation are the core equipment in the DC transmission system. The power system with a high proportion of renewable energy generation and a high proportion of power electronic equipment has the characteristics of wide-band dynamics, low inertia, weak anti-interference, and low overload capacity, which not only has a significant impact on traditional stability, but also causes stability problems such as resonance.

[0003] If the control system cannot provide sufficiently large damping, the DC current will continue to have large medium and low frequency oscillation components. This can easily cause alarms in monitoring equipment, and in severe cases may cause the converter station to be locked and shut down. With the commissioning of multiple flexible DC projects, a number of DC medium and high frequency oscillation events have occurred due to insufficient damping of the converter in the subsynchronous frequency range. With the continuous deepening of the construction of flexible DC projects, high-frequency resonance phenomena have also emerged. Subsequent research on the oscillation phenomenon found that the high-frequency resonance phenomenon in the flexible DC project is closely related to the bottom control link of the MMC and the control link delay. To solve the above problems, it is necessary to improve the damping characteristics and dynamic characteristics of the system in order to achieve the effect of suppressing oscillations.

[0004] In order to solve the above problems and improve the damping characteristics and dynamic characteristics of the system in order to achieve the effect of suppressing oscillation, active damping and passive damping methods are mainly used. The passive damping method adds passive devices, additional losses, and has disadvantages such as high power consumption and low conversion efficiency. Currently, active suppression measures are mainly used to improve the stability of the system. Shortening the delay of the MMC link has high requirements for system hardware and low feasibility. The occurrence of high-frequency resonance within a certain range can be suppressed by adding delay compensation and additional damping control measures, which can improve the stability margin of the system to a certain extent, but the effect is limited. The stability of the system can be improved by adding a filter in the voltage feedforward link, but the oscillation risk of the system cannot be completely eliminated. Summary of the invention

[0005] In view of the problems of low inertia and weak anti-interference of the control system in a full DC system, there is a risk of oscillation when the DC side damping is insufficient. The present invention provides a control method and device for a high-voltage DC transformer in a full DC system, which improves the current inner loop by feeding back the voltage on the inductor to the current inner loop through a state observer, and enhances the DC side damping by introducing a DC side virtual resistor in the voltage outer loop, thereby achieving coordinated control of the current inner loop and the voltage outer loop, so as to improve the dynamic characteristics and stability of the current inner loop and solve the oscillation problem.

[0006] To this end, the present invention adopts the following technical solution.

[0007] In a first aspect, the present invention provides a control method for a high voltage DC transformer of a full DC system, the contents of which are as follows: On the basis of the traditional double closed-loop control method, the voltage on the inductor is fed back to the current inner loop through the state observer to affect the generation of the dq axis reference voltage, that is, the secondary side modulation wave voltage is obtained through state observation, and then the secondary side modulation wave voltage is subtracted from the primary side modulation wave voltage to obtain the voltage on the inductor, and the voltage on the inductor is fed back to the output of the current inner loop controller as a superposition item for generating the dq axis reference voltage; The voltage outer loop enhances the DC side damping by introducing a virtual resistor on the DC side and corrects the reference current of the current inner loop, which is equivalent to connecting a resistor in parallel across the DC side capacitor to achieve coordinated control of the current inner loop and the voltage outer loop.

[0008] Furthermore, since state feedback needs to detect all state quantities of the controlled object, this requires more sensors and is more expensive. Through analysis, it is found that the output AC side current and the input control variable bridge arm voltage can be used to obtain the secondary side modulation wave voltage of the high-voltage DC transformer according to the state observation equation. There is no need to set up special sensors, so this observation will not increase the cost of the full DC system.

[0009] Furthermore, the topological structure of the high-voltage DC transformer includes a first MMC module located on the primary side and a second MMC module located on the secondary side, and the two MMC modules are connected through a DC / DC AC transformer. The DC / DC AC transformer plays the role of boosting and electrical isolation, and the modular configuration enables the high-voltage DC transformer to be easily expanded to higher voltages and larger capacity levels.

[0010] Furthermore, the first MMC module operates as a current source using constant DC voltage control, enhances DC side damping by introducing a virtual resistor on the DC side, and corrects the reference current of the current inner loop, which is equivalent to connecting a resistor in parallel at both ends of the DC side capacitor, thereby improving the overall damping performance of the entire DC system without increasing the loss of the entire DC system; the second MMC module provides a frequency reference and an AC voltage effective value reference through open-loop control, and operates as a voltage source; the first MMC module and the second MMC module determine the number of sub-modules required for each MMC bridge based on the bridge arm voltage reference value through sub-module capacitor voltage balance control and nearest level approximation modulation, and finally generate a pulse signal to drive the MMC.

[0011] Furthermore, the current inner loop open-loop transfer function after the inductor voltage feedback is introduced is: , in, s represents the Laplace operator, , , In the formula, is the proportional coefficient of the current inner loop PI controller, is the integral coefficient of the current inner loop PI controller, is the delay time of the first MMC module, is the delay time of the second MMC module, and are the modulation ratios of the first MMC module and the second MMC module respectively, is the inductor voltage feedback coefficient, is the DC voltage at the output side of the high-voltage DC transformer, is the bridge arm impedance on the high voltage side, is the equivalent capacitance value of the high-voltage side submodule, is the AC transformer ratio, is the equivalent sum of the MMC bridge arm reactance and the transformer leakage reactance, is the d-axis component of the AC voltage.

[0012] Furthermore, the closed-loop transfer function of the inner current loop is for: .

[0013] Furthermore, in design k c When adjusting the parameters of the PI controller, the Routh stability criterion is used to obtain the stability conditions of the entire DC system. As long as the system stability conditions are met, the stability of the system can be guaranteed.

[0014] Furthermore, the voltage outer loop open-loop transfer function after damping is introduced for: , in, s represents the Laplace operator, is the d-axis component of the AC voltage, , are the proportional and integral coefficients of the voltage outer loop PI controller, is the DC voltage at the input side of the high-voltage DC transformer, is the equivalent capacitance value of the low-voltage side submodule, is the damping coefficient introduced into the voltage outer loop; based on the steady-state power balance formula, =1.5 / .

[0015] When modeling the voltage outer loop, the closed-loop control model of the current inner loop is treated as a first-order lag model, and the higher-order time constants are ignored, which can be simplified to the form of 1 / (T1s+1).

[0016] Furthermore, the The calculation method is as follows: .

[0017] In a second aspect, the present invention provides a control device for a high voltage DC transformer of a full DC system, comprising: Current inner loop control unit: Based on the traditional double closed-loop control method, the voltage on the inductor is fed back to the current inner loop through the state observer to affect the generation of the dq axis reference voltage. That is, the secondary side modulation wave voltage is obtained through state observation, and then the secondary side modulation wave voltage is subtracted from the primary side modulation wave voltage to obtain the voltage on the inductor. The voltage on the inductor is fed back to the output of the current inner loop controller as a superposition item for generating the dq axis reference voltage. Voltage outer loop control unit: The voltage outer loop enhances the DC side damping by introducing a virtual resistor on the DC side, corrects the reference current of the current inner loop, and realizes coordinated control of the current inner loop and the voltage outer loop.

[0018] The beneficial effects of the present invention are as follows: under the traditional double closed-loop control method, the present invention improves the current inner loop by feeding back the voltage on the inductor to the current inner loop through a state observer, which can improve the dynamic characteristics and stability of the current inner loop and solve the oscillation problem; the voltage outer loop enhances the DC side damping by introducing a DC side virtual resistor, corrects the reference current of the current inner loop, realizes coordinated control of the current inner loop and the voltage outer loop, and improves the damping characteristics of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] 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 prior art descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 It is a flow chart of the control method of the high voltage DC transformer of the full DC system of the present invention; Figure 2 The topological structure diagram of the high voltage DC transformer of the full DC system of the present invention; Figure 3 It is the principle diagram of the traditional double closed-loop control method; Figure 4 It is a schematic diagram of the coordinated control of the current inner loop and the voltage outer loop of the present invention; Figure 5 This is the amplitude-frequency characteristic diagram of the system under traditional double closed-loop control; Figure 6 The figure is a graph of the amplitude-frequency characteristics of the current inner loop before and after the inductor voltage feedback is added to the current inner loop of the present invention; Figure 7 The amplitude-frequency characteristic diagram of the current outer loop before and after adding damping to the voltage outer loop of the present invention; Figure 8 The figure is a simulation result diagram before and after adding inductor voltage feedback to the current inner loop of the present invention; Fig. 9 The figure is a simulation result diagram before and after adding damping feedback to the voltage outer loop of the present invention; Figure 2 In the figure, MMC1 represents the first MMC module; MMC2 represents the second MMC module; SM represents the submodule; Lm represents the MMC bridge arm reactance value; I dc1 Indicates the DC current on the MMC1 side; I dc2 Indicates the DC side current of MMC2; NLM means nearest level approach modulation; Figure 3 middle, is the d-axis current reference value, are the q-axis current reference values, , are the d-axis and q-axis currents respectively, is the AC frequency; , are the d-axis and q-axis modulation wave voltages respectively; for Reference value of is the q-axis component of the AC voltage; Figure 4 middle, , Respectively represent the d-axis and q-axis modulation wave voltages of the first MMC module, , Respectively represent the d-axis and q-axis modulation wave voltages of the second MMC module; Figure 5 and Figure 7 middle, represents the cut-off frequency; P m Represents the phase margin. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution 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 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 are within the scope of protection of the present invention.

[0022] Example 1 This embodiment provides a control method for a high-voltage DC transformer in a full DC system. Figure 1 As shown, based on the traditional double closed-loop control method, the voltage on the inductor is fed back to the current inner loop through the state observer to affect the generation of the dq axis reference voltage, that is, the secondary side modulation wave voltage is obtained through state observation, and then the secondary side modulation wave voltage is subtracted from the primary side modulation wave voltage to obtain the voltage on the inductor, and the voltage on the inductor is fed back to the output of the current inner loop controller as a superposition item for generating the dq axis reference voltage; The voltage outer loop enhances the DC side damping by introducing a virtual resistor on the DC side and corrects the reference current of the current inner loop, which is equivalent to connecting a resistor in parallel across the DC side capacitor to achieve coordinated control of the current inner loop and the voltage outer loop.

[0023] Since state feedback needs to detect all state quantities of the controlled object, this requires more sensors and is more expensive. Through analysis, it is found that the output AC side current and the input control variable bridge arm voltage can be used to obtain the modulation wave voltage on the secondary side of the high-voltage DC transformer according to the state observation equation. There is no need to set up special sensors, so this observation will not increase the cost of the full DC system.

[0024] like Figure 2 As shown, the topological structure of the high-voltage DC transformer includes a first MMC module on the primary side and a second MMC module on the secondary side, and the two MMC modules are connected through a DC / DC AC transformer. The DC / DC AC transformer plays the role of boosting and electrical isolation, and the modular configuration enables the high-voltage DC transformer to be easily expanded to higher voltages and larger capacity levels.

[0025] The first MMC module operates as a current source using constant DC voltage control, and enhances the DC side damping by introducing a virtual resistor on the DC side, correcting the reference current of the current inner loop, which is equivalent to connecting a resistor in parallel at both ends of the DC side capacitor, thereby improving the overall damping performance of the entire DC system without increasing the loss of the entire DC system; the second MMC module provides a frequency reference and an AC voltage effective value reference through open-loop control, and operates as a voltage source; the first MMC module and the second MMC module determine the number of sub-modules required for each MMC bridge based on the bridge arm voltage reference value through sub-module capacitor voltage balance control and nearest level approximation modulation, and finally generate a pulse signal to drive the MMC.

[0026] The current inner loop open-loop transfer function after the introduction of inductor voltage feedback is: , in, s represents the Laplace operator, , , In the formula, is the proportional coefficient of the current inner loop PI controller, is the integral coefficient of the current inner loop PI controller, is the delay time of the first MMC module, is the delay time of the second MMC module, and are the modulation ratios of the first MMC module and the second MMC module respectively, is the inductor voltage feedback coefficient, is the DC voltage at the output side of the high-voltage DC transformer, is the bridge arm impedance on the high voltage side, is the equivalent capacitance value of the high-voltage side submodule, is the AC transformer ratio, is the equivalent sum of the MMC bridge arm reactance and the transformer leakage reactance, is the d-axis component of the AC voltage.

[0027] The closed-loop transfer function of the inner current loop is: .

[0028] In design k c When adjusting the controller parameters, the Routh stability criterion is used to obtain the stability conditions of the entire DC system. As long as the system stability conditions are met, the stability of the system can be guaranteed.

[0029] The voltage outer loop open-loop transfer function after introducing damping for: , in, s represents the Laplace operator, is the d-axis component of the AC voltage, , are the proportional and integral coefficients of the voltage outer loop PI controller, is the DC voltage at the input side of the high-voltage DC transformer (i.e. the feedback value of the voltage outer loop), is the equivalent capacitance value of the low-voltage side submodule, is the damping coefficient introduced into the voltage outer loop; based on the steady-state power balance formula, =1.5 / .

[0030] When modeling the voltage outer loop, the closed-loop control model of the current inner loop is treated as a first-order lag model, and the higher-order time constants are ignored, which can be simplified to the form of 1 / (T1s+1).

[0031] Furthermore, the The calculation method is as follows: .

[0032] Figure 3 The schematic diagram of the traditional double closed-loop control method. The amplitude-frequency characteristics of the system under the traditional double closed-loop control method are as follows: Figure 5 As shown in the figure, the problems of traditional double closed-loop control are as follows: in the double closed-loop control system, the inner and outer loops affect each other, the voltage outer loop response speed is much slower than the current inner loop speed, and because the inner loop control bandwidth is usually much larger than the outer loop control bandwidth, the response speed is relatively slow, and the ability to suppress DC bus disturbance is poor. That is, traditional PI control cannot ensure that the system has good anti-disturbance while taking into account the good dynamic characteristics of the system.

[0033] Figure 4 The schematic diagram of the current inner loop and the voltage outer loop coordinated control of the present invention is as follows: Figure 6 This is the current loop amplitude-frequency characteristic diagram before and after the inductor voltage feedback is added to the current inner loop of the present invention. It can be seen that after the inductor voltage feedback is added, the system resonance peak gradually decreases to disappear, indicating the effectiveness of the inductor voltage feedback. In the mid-frequency band, after the inductor voltage feedback is introduced, the system mid-frequency bandwidth is narrower, indicating that its ability to resist disturbances within the bandwidth is stronger, and the system phase margin is also significantly improved. By introducing the inductor voltage feedback, the system damping can be increased and the system stability can be improved.

[0034] Figure 7This is the amplitude-frequency characteristic diagram of the voltage outer loop before and after the voltage outer loop of the present invention introduces damping. It can be seen that in the low frequency band, the system gain value after the introduction of damping is smaller than the system without the introduction of damping, but its phase lag is smaller and the stability is enhanced; it becomes narrower in the mid-frequency band, indicating that its ability to resist disturbances within the bandwidth is stronger and the system phase margin is significantly improved.

[0035] Figure 8 This is a simulation result diagram before and after the inductor voltage feedback is added to the current inner loop of the present invention. It can be seen that after the inductor voltage feedback is introduced, the overshoot of the current step response is significantly reduced, the response speed is accelerated, and the adjustment time is shortened.

[0036] Fig. 9 This is a simulation result diagram before and after the voltage outer loop of the present invention is introduced with damping. It can be seen that the voltage step response adjustment time is shortened after adding damping, and it can track the new steady-state operating condition more quickly, the response speed is also faster, and the overshoot is significantly reduced. It can be seen that the damping characteristics of the system are improved after adding damping optimization.

[0037] Example 2 This embodiment provides a control device for a high-voltage DC transformer of a full DC system, which is composed of a current inner loop control unit and a voltage outer loop control unit.

[0038] The current inner loop control unit described is based on the traditional dual closed-loop control method. The voltage on the inductor is fed back to the current inner loop through a state observer to affect the generation of the dq-axis reference voltage. That is, the secondary side modulation wave voltage is obtained through state observation, and then the secondary side modulation wave voltage is subtracted from the primary side modulation wave voltage to obtain the voltage on the inductor. The voltage on the inductor is fed back to the output of the current inner loop controller as a superposition item for generating the dq-axis reference voltage.

[0039] The voltage outer loop control unit: the voltage outer loop enhances the DC side damping by introducing a DC side virtual resistor and corrects the reference current of the current inner loop, which is equivalent to connecting a resistor in parallel at both ends of the DC side capacitor to achieve coordinated control of the current inner loop and the voltage outer loop.

[0040] For the specific definition of the control device of a high-voltage DC transformer of a full DC system, please refer to the definition of the control method of a high-voltage DC transformer of a full DC system in Example 1. The two have the same functions and effects and will not be repeated here.

[0041] It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A control method for a high voltage DC transformer of a full DC system, characterized in that: On the basis of the traditional double closed-loop control method, the voltage on the inductor is fed back to the current inner loop through the state observer to affect the generation of the dq axis reference voltage, that is, the secondary side modulation wave voltage is obtained through state observation, and then the secondary side modulation wave voltage is subtracted from the primary side modulation wave voltage to obtain the voltage on the inductor, and the voltage on the inductor is fed back to the output of the current inner loop controller as a superposition item for generating the dq axis reference voltage; The voltage outer loop enhances the DC side damping by introducing a virtual resistance on the DC side and corrects the reference current of the current inner loop.

2. The control method according to claim 1, characterized in that: The output AC side current and the input control variable bridge arm voltage are used to obtain the modulation wave voltage on the secondary side of the high-voltage DC transformer according to the state observation equation.

3. The control method according to claim 1, characterized in that: The topological structure of the high-voltage direct current transformer comprises a first MMC module located on the primary side and a second MMC module located on the secondary side, and the two MMC modules are connected via a DC / DC alternating current transformer.

4. The control method according to claim 3, characterized in that: The first MMC module operates as a current source using constant DC voltage control, and enhances the DC side damping by introducing a virtual resistor on the DC side to correct the reference current of the current inner loop, which is equivalent to connecting a resistor in parallel at both ends of the DC side capacitor; the second MMC module provides frequency reference and AC voltage effective value reference through open-loop control, and operates as a voltage source; the first MMC module and the second MMC module determine the number of sub-modules required for each MMC bridge based on the bridge arm voltage reference value through sub-module capacitor voltage balance control and nearest level approximation modulation, and finally generate a pulse signal to drive the MMC.

5. The control method according to claim 1, characterized in that: The current inner loop open-loop transfer function after introducing inductor voltage feedback for: ， in, s represents the Laplace operator, ， ， In the formula, is the proportional coefficient of the current inner loop PI controller, is the integral coefficient of the current inner loop PI controller, is the delay time of the first MMC module, is the delay time of the second MMC module, and are the modulation ratios of the first MMC module and the second MMC module respectively, is the inductor voltage feedback coefficient, is the DC voltage at the output side of the high-voltage DC transformer, is the bridge arm impedance on the high voltage side, is the equivalent capacitance value of the high-voltage side submodule, is the AC transformer ratio, is the equivalent sum of the MMC bridge arm reactance and the transformer leakage reactance, is the d-axis component of the AC voltage.

6. The control method according to claim 5, characterized in that: The closed-loop transfer function of the inner current loop is for: 。 7. The control method according to claim 1, characterized in that: In design k c When the parameters of the PI controller are determined, the Routh stability criterion is used to obtain the stability conditions of the entire DC system.

8. The control method according to claim 1, characterized in that: The voltage outer loop open-loop transfer function after introducing damping for: ， in, s represents the Laplace operator, is the d-axis component of the AC voltage, , are the proportional and integral coefficients of the voltage outer loop PI controller, is the DC voltage at the input side of the high-voltage DC transformer, is the equivalent capacitance value of the low-voltage side submodule, is the damping coefficient introduced into the voltage outer loop; based on the steady-state power balance formula, =1.5 / .

9. The control method according to claim 8, characterized in that: Said The calculation method is as follows: 。 10. A control device for a high voltage DC transformer of a full DC system, characterized in that: include: Current inner loop control unit: Based on the traditional double closed-loop control method, the voltage on the inductor is fed back to the current inner loop through the state observer to affect the generation of the dq axis reference voltage. That is, the secondary side modulation wave voltage is obtained through state observation, and then the secondary side modulation wave voltage is subtracted from the primary side modulation wave voltage to obtain the voltage on the inductor. The voltage on the inductor is fed back to the output of the current inner loop controller as a superposition item for generating the dq axis reference voltage. Voltage outer loop control unit: The voltage outer loop enhances the DC side damping by introducing a virtual resistor on the DC side and corrects the reference current of the current inner loop.

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