Control method and device for high-voltage DC transformer of all-DC system

Abstract

The present invention discloses a control method and device for a high-voltage DC transformer in a fully DC system. The control method of the present invention is 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 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 voltage on the inductor is obtained by subtracting the secondary-side modulation wave voltage from the primary-side modulation wave voltage. The voltage on the inductor is fed back to the output of the current inner loop controller as a superimposed term 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. The present invention feeds back the voltage on the inductor to the current inner loop through a state observer to improve the current inner loop, enhances the DC-side damping by introducing a virtual resistance on the DC side, realizes the coordinated control of the current inner loop and the voltage outer loop, improves the dynamic characteristics and stability of the current inner loop, and solves the oscillation problem.

Description

Technical Field

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

[0002] With the development of power electronics technology and flexible DC transmission technology, a new - energy all - DC collection and transmission solution has emerged as the times require. The new - energy all - DC technology based on DC collection - DC transmission only needs to maintain the stability of the DC voltage to achieve system stability, and 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 solution has a long collection and transmission distance, and there are no voltage / reactive power problems and synchronous stability problems, and it is very competitive in the scenario of large - scale new - energy wide - area collection and long - distance transmission. The DC transformer with the characteristics of high voltage, high gain, large capacity, unidirectional power flow and isolation is 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 devices has characteristics such as wide - band dynamics, low inertia, weak anti - interference ability and low overload capacity, which not only have a significant impact on traditional stability, but also cause stability problems such as resonance.

[0003] If the control system cannot provide sufficient damping, there will be a large medium - low - frequency oscillation component in the DC current. It is easy to cause the monitoring equipment to alarm, and in severe cases, it may lead to the locking and shutdown of the converter station. With the commissioning of multiple flexible DC projects, due to the insufficient damping of the converter in the sub - synchronous frequency range, multiple DC medium - high - frequency oscillation events have occurred. 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 flexible DC projects 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 oscillation.

[0004] To solve the above problems, 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, resulting in 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 MMC link delay has high requirements for system hardware and low feasibility. By adding measures such as delay compensation and additional damping control, the occurrence of high - frequency resonance within a certain range can be suppressed, and the stability margin of the system can be improved to a certain extent, but the effect is limited. By adding a filter in the voltage feed - forward link, the stability of the system can be improved, but the oscillation risk existing in the system cannot be completely eliminated. Summary of the Invention

[0005] In view of the problems of low inertia and weak anti-interference ability of the control system in the all-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 an all-DC system. By using a state observer to feedback the voltage on the inductor to the current inner loop to improve the current inner loop, and the voltage outer loop enhances the DC-side damping by introducing a virtual resistance on the DC side, realizing the 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] For this reason, the present invention adopts the following technical solutions.

[0007] In the first aspect, the present invention provides a control method for a high-voltage DC transformer in an all-DC system, the content of which is as follows:

[0008] Based on the traditional double-loop control method, the voltage on the inductor is feedback 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 difference between the secondary-side modulation wave voltage and the primary-side modulation wave voltage is taken to obtain the voltage on the inductor. The voltage on the inductor is feedback to the output of the current inner loop controller as a superimposed term for generating the dq-axis reference voltage;

[0009] 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, which is equivalent to connecting a resistor in parallel across the DC-side capacitor, realizing the coordinated control of the current inner loop and the voltage outer loop.

[0010] Furthermore, since state feedback requires the detection of all state variables of the controlled object, which requires more sensors and higher costs; through analysis, it is found that the secondary-side modulation wave voltage of the high-voltage DC transformer can be obtained by using the output AC-side current and the input control quantity bridge arm voltage according to the state observation equation, without the need to set up special sensors. Therefore, this kind of observation will not increase the cost of the all-DC system.

[0011] Furthermore, 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. The two MMC modules are connected by a DC / DC AC transformer. The DC / DC AC transformer plays functions such as boosting and electrical isolation. The modular configuration enables the high-voltage DC transformer to be conveniently expanded to higher voltage and larger capacity levels.

[0012] Furthermore, the first MMC module operates as a current source using constant DC voltage control. By introducing a virtual resistor on the DC side, the DC side damping is enhanced, and the reference current of the current inner loop is corrected. This is equivalent to connecting a resistor in parallel across the DC side capacitor, which improves the overall damping performance of the entire DC system without increasing the losses of the entire DC system. The second MMC module provides the frequency reference and the reference of the AC voltage effective value 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 to be inserted in each bridge of the MMC based on the reference value of the arm voltage through sub-module capacitor voltage balance control and the nearest level approximation modulation, and finally generate the pulse signal to drive the MMC.

[0013] Furthermore, the open-loop transfer function of the current inner loop after introducing the inductor voltage feedback is:

[0014] ,

[0015] where, s represents the Laplace operator,

[0016] ,

[0017] ,

[0018] 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 on the output side of the HVDC transformer, is the impedance of the high-voltage side arm, is the equivalent capacitance value of the high-voltage side sub-module, is the turns ratio of the AC transformer, is the equivalent sum of the MMC arm reactance and the transformer leakage reactance, is the d-axis component of the AC voltage.

[0019] Even further, the closed-loop transfer function of the current inner loop is:

[0020] .

[0021] Even further, in the design k cWhen determining the PI controller parameters, the stability conditions of the fully DC system are obtained by using the Routh stability criterion. As long as the system stability conditions are met, the stability of the system can be ensured.

[0022] Further, the open-loop transfer function of the voltage outer loop after introducing damping is as follows:

[0023] ,

[0024] where 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 respectively, is the DC voltage on the input side of the HVDC transformer, is the equivalent capacitance value of the sub-module on the low-voltage side, is the damping coefficient introduced by the voltage outer loop; based on the steady-state power balance equation, = 1.5 / .

[0025] When modeling the voltage outer loop, the closed-loop control model of the current inner loop is regarded as a first-order lag model, and the high-order time constants are ignored, which can be simplified to the form of 1 / (T1s + 1).

[0026] Even further, the calculation method of the is as follows:

[0027] .

[0028] In a second aspect, the present invention provides a control device for a high-voltage DC transformer of a fully DC system, which includes:

[0029] 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 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 difference between the secondary side modulation wave voltage and the primary side modulation wave voltage is used 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 superimposed term for generating the dq-axis reference voltage;

[0030] Voltage outer loop control unit: The voltage outer loop enhances the DC side damping by introducing a virtual resistance on the DC side, modifies the reference current of the current inner loop, and realizes the coordinated control of the current inner loop and the voltage outer loop.

[0031] 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 virtual resistor on the DC side, corrects the reference current of the current inner loop, and realizes the coordinated control of the current inner loop and the voltage outer loop, thereby improving the damping characteristics of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can also obtain other drawings based on these drawings without creative efforts.

[0033] Figure 1 is a flowchart of the control method for the high-voltage DC transformer in the all-DC system of the present invention;

[0034] Figure 2 is a topological structure diagram of the high-voltage DC transformer in the all-DC system of the present invention;

[0035] Figure 3 is a schematic diagram of the traditional double closed-loop control method;

[0036] Figure 4 is a schematic diagram of the coordinated control of the current inner loop and the voltage outer loop of the present invention;

[0037] Figure 5 is a system amplitude-frequency characteristic diagram under the traditional double closed-loop control;

[0038] Figure 6 is an amplitude-frequency characteristic diagram of the current inner loop before and after adding inductor voltage feedback in the present invention;

[0039] Figure 7 is an amplitude-frequency characteristic diagram of the current outer loop before and after adding damping in the voltage outer loop of the present invention;

[0040] Figure 8 is a simulation result diagram of the current inner loop before and after adding inductor voltage feedback in the present invention;

[0041] Figure 9 is a simulation result diagram of the voltage outer loop before and after adding damping feedback in the present invention;

[0042] Figure 2 wherein, MMC1 represents the first MMC module; MMC2 represents the second MMC module; SM represents the sub-module; Lm represents the MMC arm reactance value; I dc1 represents the DC current on the MMC1 side; I dc2represents the DC - side current of the MMC2; NLM represents the nearest - level modulation;

[0043] Figure 3 Among them, is the d - axis current reference value, are the q - axis current reference values respectively, and are the d - axis and q - axis currents respectively, is the AC frequency; and are the d - axis and q - axis modulated - wave voltages respectively; is 's reference value; is the q - axis component of the AC voltage;

[0044] Figure 4 Among them, and represent the d - axis and q - axis modulated - wave voltages of the first MMC module respectively, and represent the d - axis and q - axis modulated - wave voltages of the second MMC module respectively;

[0045] Figure 5 and Figure 7 Among them, represents the cut - off frequency; P m represents the phase - angle margin. Specific implementation manners

[0046] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0047] Embodiment 1

[0048] This embodiment provides a control method for a high - voltage DC transformer in a fully - DC system. As shown in Figure 1 , on the basis of the traditional double - closed - loop control method, through a state observer, the voltage on the inductor is fed back to the current inner loop to affect the generation of the dq - axis reference voltage, that is, the secondary - side modulated - wave voltage is obtained through state observation, then the difference between the secondary - side modulated - wave voltage and the primary - side modulated - wave voltage is used 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 term for generating the dq - axis reference voltage;

[0049] 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. It is equivalent to connecting a resistor in parallel across the DC-side capacitor, achieving the coordinated control of the current inner loop and the voltage outer loop.

[0050] Since state feedback requires the detection of all state variables of the controlled object, which requires more sensors and results in a higher cost. Through analysis, it is found that the modulation wave voltage on the secondary side of the HVDC transformer can be obtained by using the output AC-side current and the input control quantity bridge arm voltage according to the state observation equation, without the need to set up special sensors. Therefore, this kind of observation will not increase the cost of the all-DC system.

[0051] As Figure 2 shown, the topology of the HVDC transformer includes a first MMC module on the primary side and a second MMC module on the secondary side. The two MMC modules are connected by a DC / DC AC transformer. The DC / DC AC transformer functions as a step-up transformer and provides electrical isolation, etc. The modular configuration enables the HVDC transformer to be conveniently expanded to higher voltage and larger capacity levels.

[0052] The first MMC module operates as a current source using constant DC voltage control. It enhances the DC-side damping by introducing a virtual resistance on the DC side, and corrects the reference current of the current inner loop. It is equivalent to connecting a resistor in parallel across the DC-side capacitor, improving the overall damping performance of the all-DC system without increasing the loss of the all-DC system. The second MMC module provides a frequency reference and an AC voltage RMS 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 to be inserted in each bridge of the MMC according to the bridge arm voltage reference value through sub-module capacitor voltage balance control and the nearest level approximation modulation, and finally generate the pulse signal to drive the MMC.

[0053] The open-loop transfer function of the current inner loop after introducing the inductor voltage feedback is:

[0054] ,

[0055] where s represents the Laplace operator,

[0056] ,

[0057] ,

[0058] 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 inductance voltage feedback coefficient, is the DC voltage on the output side of the high-voltage DC transformer, is the impedance of the high-voltage side bridge arm, is the equivalent capacitance value of the high-voltage side sub-module, is the turns ratio of the AC transformer, is the equivalent sum of the MMC bridge arm reactance and the transformer leakage reactance, is the d-axis component of the AC voltage.

[0059] The closed-loop transfer function of the current inner loop is:

[0060] .

[0061] When designing k c and the controller parameters, the Routh stability criterion is used to obtain the stability conditions of the all-DC system. As long as the system stability conditions are met, the system stability can be guaranteed.

[0062] The open-loop transfer function of the voltage outer loop after introducing damping is:

[0063] ,

[0064] wherein, 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 respectively, is the DC voltage on 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 sub-module, is the damping coefficient introduced by the voltage outer loop; based on the steady-state power balance formula, = 1.5 / .

[0065] When modeling the voltage outer loop, the closed-loop control model of the current inner loop is regarded as a first-order lag model, and the high-order time constants are ignored, which can be simplified to the form of 1 / (T1s + 1).

[0066] Furthermore, the calculation method of the is as follows:

[0067] .

[0068] Figure 3is the schematic diagram of the traditional double - closed - loop control method. The amplitude - frequency characteristic of the system under the traditional double - closed - loop control method is as Figure 5 shown. The problems existing in the traditional double - closed - loop control are as follows: In the double - closed - loop control system, the inner and outer loops affect each other. The response speed of the voltage outer loop is much slower than that of the current inner loop. Since the control bandwidth of the inner loop is usually much larger than that of the outer loop, the response speed is relatively slow, and the ability to suppress the disturbance of the DC bus is poor. That is, the traditional PI control cannot ensure both good dynamic characteristics and good anti - disturbance ability of the system at the same time.

[0069] Figure 4 is the schematic diagram of the collaborative control of the current inner loop and the voltage outer loop of the present invention. The characteristic analysis of the collaborative control of the current inner loop and the voltage outer loop is as follows:

[0070] Figure 6 is the amplitude - frequency characteristic diagram of the current loop before and after adding the inductor voltage feedback of the present invention. It can be seen that after adding the inductor voltage feedback, the resonance peak of the system gradually decreases to disappear, indicating the effectiveness of the inductor voltage feedback. In the middle frequency band, after introducing the inductor voltage feedback, the middle - frequency bandwidth of the system is narrower, indicating that its ability to resist disturbances within the bandwidth is stronger, and the phase - margin of the system is also significantly improved. By introducing the inductor voltage feedback, the damping of the system can be increased, and the stability of the system can be improved.

[0071] Figure 7 is the amplitude - frequency characteristic diagram of the voltage outer loop before and after introducing damping of the present invention. It can be seen that in the low - frequency band, the gain value of the system after introducing damping is less than that of the system without introducing damping, but its phase lag is smaller and the stability is enhanced; in the middle - frequency band, it becomes narrower, indicating that its ability to resist disturbances within the bandwidth is stronger, and the phase - margin of the system is significantly improved.

[0072] Figure 8 is the simulation result diagram before and after adding the inductor voltage feedback to the current inner loop of the present invention. It can be seen that the overshoot of the current step response after introducing the inductor voltage feedback is significantly reduced, the response speed is increased, and the adjustment time is shortened.

[0073] Figure 9 is the simulation result diagram before and after introducing damping to the voltage outer loop of the present invention. It can be seen that after adding damping, the adjustment time of the voltage step response is shortened, it can re - track the new steady - state operating condition faster, the response speed is also faster, and the overshoot is significantly reduced. It can be seen that the damping characteristic of the system is improved after adding damping optimization.

[0074] Embodiment 2

[0075] This embodiment provides a control device for a high - voltage DC transformer in a fully DC system, which is composed of a current inner - loop control unit and a voltage outer - loop control unit.

[0076] The described current inner-loop control unit: Based on the traditional double-loop control method, the voltage across 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 voltage across the inductor is obtained by subtracting the secondary-side modulation wave voltage from the primary-side modulation wave voltage. The voltage across the inductor is fed back to the output of the current inner-loop controller as an additive term for generating the dq-axis reference voltage.

[0077] The described voltage outer-loop control unit: 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, which is equivalent to connecting a resistor in parallel across the DC-side capacitor to achieve the coordinated control of the current inner loop and the voltage outer loop.

[0078] For the specific limitations of a control device for a high-voltage DC transformer in a fully DC system, refer to the limitations of a control method for a high-voltage DC transformer in a fully DC system in Embodiment 1. The two have the same functions and effects and will not be elaborated here.

[0079] Those skilled in the art can obviously make various modifications to the above embodiments easily and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art to the present invention according to 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 in a fully DC system. The topology of the high-voltage DC transformer includes a first MMC module on the primary side and a second MMC module on the secondary side. The two MMC modules are connected by a DC / DC AC transformer. It is characterized in that Based on the traditional double-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 voltage on the inductor is obtained by subtracting the secondary-side modulation wave voltage from the primary-side modulation wave voltage. The voltage on the inductor is fed back to the output of the current inner loop controller as a superimposed term 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. The open-loop transfer function of the inner current loop after introducing inductor voltage feedback is as follows: ， Among them, s represents the Laplace operator, ， ， Wherein, 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 inductance voltage feedback coefficient, is the DC voltage on the output side of the high-voltage DC transformer, is the impedance of the high-voltage side bridge arm, is the equivalent capacitance value of the high-voltage side sub-module, is the turns ratio of the AC transformer, is the equivalent sum of the MMC bridge arm reactance and the transformer leakage reactance, is the d-axis component of the AC voltage.

2. The control method according to claim 1, wherein The secondary-side modulation wave voltage of the high-voltage DC transformer is obtained according to the state observation equation by using the output AC-side current and the input control quantity bridge arm voltage.

3. The control method according to claim 1, wherein The first MMC module operates as a current source using constant DC voltage control. By introducing a virtual resistance on the DC side to enhance the DC-side damping and correct the reference current of the current inner loop, it is equivalent to connecting a resistor in parallel across the DC-side capacitor. The second MMC module provides a frequency reference and an AC voltage RMS 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 to be inserted in each bridge of the MMC according to the bridge arm voltage reference value through sub-module capacitor voltage balancing control and the nearest level approximation modulation, and finally generate pulse signals to drive the MMC.

4. The control method according to claim 1, characterized in that Closed-loop transfer function of the inner current loop is as follows: 。 5. The control method according to claim 1, characterized in that, In the design k c and PI controller parameters, the stability conditions of the fully DC system are obtained by using the Routh stability criterion.

6. The control method according to claim 1, characterized in that, The open-loop transfer function of the voltage outer loop after introducing damping is as follows: ， Among them, s represents the Laplacian operator, is the d-axis component of the AC voltage, , are the proportional and integral coefficients of the voltage outer-loop PI controller respectively, is the DC voltage on the input side of the HVDC transformer, is the equivalent capacitance value of the sub-module on the low-voltage side, is the damping coefficient introduced by the voltage outer loop; based on the steady-state power balance equation, = 1.5 / .

7. The control method according to claim 6, characterized in that The calculation method is as follows: 。 8. A control device for a high-voltage DC transformer in an all-DC system, which is used to implement the control method described in any one of claims 1-7, characterized in that, It includes: Current inner loop control unit: Based on the traditional double-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 voltage on the inductor is obtained by subtracting the secondary-side modulation wave voltage from the primary-side modulation wave voltage. The voltage on the inductor is fed back to the output of the current inner loop controller as a superimposed term 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 resistance on the DC side and corrects the reference current of the current inner loop.

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