Bidirectional adaptive inertia control method and device for interconnected converter

Abstract

The invention discloses a bidirectional self-adaptive inertia control method and device for an interconnected converter, and the method comprises a voltage source converter structure, a bidirectional inertia control method and self-adaptive inertia coefficient set.The bidirectional self-adaptive inertia control method and device are suitable for an AC-DC hybrid micro-grid so as to balance AC micro-grid frequency and DC micro-grid voltage dynamic response. Virtual synchronous motor control is adopted as a control outer ring to realize power adjustment and emergency control mode switching, an additional virtual capacitor control ring is designed to suppress the influence of high-frequency interference on a differential element, and the method can enable an IC to adjust a working mode to operate in a rectification or inversion state under a self-adaptive operation condition, so that the reliability of the IC is improved. The strategy enables the interconnection converter to smoothly adapt to three operation modes of grid connection, island power adjustment and island emergency control, thereby balancing the dynamic response characteristics of the two modes after disturbance, and achieving the improvement of the overall performance of the AC / DC micro-grid.

Description

Technical Field

[0001] The present invention relates to the technical field of interconnected converter control, and in particular to a bidirectional adaptive inertia control method and device for an interconnected converter. Background Art

[0002] The penetration rate of distributed generation DG (Distributed Generation) based on clean and renewable energy will continue to increase, bringing new challenges to the safe and stable operation and control of distribution networks. The topology of AC / DC hybrid microgrid can effectively combine the advantages of AC microgrid and DC microgrid, realize the coordinated operation between AC / DC microgrids and the efficient consumption of large-scale DG. Therefore, AC / DC hybrid microgrid will become the main form of DG access to distribution networks in the future.

[0003] Interlinking Converter (IC) is a key device for coupling AC / DC sub-microgrids. When there is a disturbance caused by large-capacity source-load switching in one subgrid, the adjacent subgrids can balance the transmission power between AC / DC microgrids through IC coupling, thereby regulating the frequency of the disturbed AC subgrid or the DC voltage of the DC subgrid. However, the coupling of IC will quickly transmit the disturbance to the adjacent subgrid, which will expand the scope of transient influence and thus have an adverse effect on the stable operation of the entire AC / DC microgrid. With the frequent fluctuations of electric vehicle loads and large-scale access to DG with high penetration, the above disturbances will have a great impact on the stability of AC / DC hybrid microgrids, especially when operating in an isolated island.

[0004] The coupling effect of AC / DC hybrid microgrid depends on the control strategy of bidirectional interconnected converters. According to the control target, the control strategy can be divided into two categories: power balanced distribution and frequency and voltage dynamic response regulation. The former realizes the optimal distribution of IC transmission power according to the capacity of the hybrid microgrid, and the latter realizes the optimization of the frequency and voltage dynamic response after the disturbance of the hybrid microgrid. Bidirectional droop control was first proposed to solve the power distribution problem. The imbalance of active power of AC / DC subgrids can be reflected by normalizing frequency and DC voltage; the relationship between frequency and DC voltage can be derived according to the AC / DC energy conservation relationship and the converter droop control method can be designed. However, due to the limitation of the dispatchable power regulation capacity of the microgrid, there is often a contradiction between AC frequency optimization and DC voltage optimization. When the penetration rate of distributed power sources gradually increases, the rotational inertia of the entire power grid decreases, which is not conducive to the stability of the dynamic characteristics of the system.

[0005] Inertial control is an effective method to improve the dynamic characteristics of the system. Inspired by synchronous generators, VSG control schemes are proposed in the prior art to optimize the dynamic characteristics of the voltage and frequency of inverter-type distributed power sources. Drawing on VSG, a DC virtual inertial control method is proposed in the prior art to optimize the dynamic characteristics of the DC microgrid voltage. The above research focuses on the inertia of AC or DC microgrids to improve the corresponding robustness. For the independent operation of AC / DC hybrid microgrids, the robustness of both AC and DC sides must be taken into account to ensure the dynamic response of frequency and DC voltage. In the prior art, dual ICs are used to connect AC / DC microgrids to achieve dynamic performance optimization on the AC / DC side, which depends to a certain extent on the structural design of dual DC capacitor charging and discharging; in the prior art, DG and IC coordinated control are used, which requires a greater degree of dispatching capability of the microgrid DG. After the microgrid is disturbed, the above research achieves simultaneous optimization of frequency and DC voltage by directly or indirectly increasing the dispatchable energy of the hybrid microgrid. However, when considering the limited system energy, the coupling effect of IC will sacrifice the dynamic performance of frequency or DC voltage.

[0006] The adjustability of the virtual inertia coefficient provides a basis for variable inertia control. According to the changes in DC voltage and frequency, the size of the inertia coefficient is adaptively adjusted to optimize the dynamic response of frequency or DC voltage at different stages after disturbance. The variable inertia coefficient can be realized through analytical formulas or intelligent algorithms. The former designs the quantitative analytical expression between the inertia coefficient and the frequency or DC voltage according to the response characteristics after the disturbance, such as stick control, trigonometric function control, exponential control, etc. The latter adaptively updates the inertia coefficient in the transient process through intelligent algorithms.

[0007] The above method is mostly applied to AC microgrid or DC microgrid. In AC / DC hybrid microgrid, it is necessary to take into account the dynamic response optimization of AC frequency and DC voltage at the same time. Since IC coupling will expand the transient impact range, it is urgent to explore the coordination and mutual assistance function between AC / DC sub-microgrids under bidirectional adaptive inertial control, so as to improve the safe and stable operation capability of AC / DC hybrid microgrid under risks and disturbances. Therefore, the present invention provides a bidirectional adaptive inertial control method and device for interconnected converters to solve the problems raised in the above background technology. Summary of the invention

[0008] The object of the present invention is to provide a bidirectional adaptive inertia control method and device for an interconnected converter to solve the problems raised in the above background technology.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A bidirectional adaptive inertia control method and device for interconnected converters, a bidirectional adaptive inertia control device for interconnected converters, comprising a voltage source converter (VSC) structure, a Uabc 、i abc They are the three-phase voltage and current of the converter, E abc is the three-phase virtual potential, L ac is the total inductance of the filter and line between the converter and the AC bus, ignoring the parasitic resistance of the filter and assuming that the line resistance is much smaller than the line inductance, C dc is the DC filter capacitor, U dc and i dc are the voltage and current on the DC side of the converter, P dcin , P acin ,P acout They are the DC sub-microgrid input power, IC input and output power respectively. The following analysis takes the inverter direction as the positive power direction.

[0011] The voltage equation of the interconnected converter in the dq coordinate system is shown in equation (2), and the active power equation is approximated as equation (3).

[0012]

[0013] Where: U d , U q 、E d 、E q 、i d 、i q are the d-axis and q-axis components of the AC voltage, AC bus voltage and AC current of the interconnected converter, respectively. U and E represent the amplitude, and ω g is the common bus angular frequency, and δ is the phase difference between U and E.

[0014] The IC is responsible for the power exchange of the AC and DC subsystems and realizes the power balance between the subsystems. According to the actual possible working conditions of the IC, its operating mode is as follows Figure 3 As shown, IC can work in grid-connected and island operation modes at the same time, and when the ST on either side is abnormal or fails to operate, IC can seamlessly maintain the stable operation of the abnormal side and realize island emergency control. Taking the power flowing from the IC outlet to the AC grid as the positive direction, when the converter output power is positive, the converter works in the inverter mode, delivering active power to the AC grid and providing power to the DC grid, and vice versa, it is in the rectification mode. In the grid-connected mode, the large grid undertakes the voltage and frequency regulation of the AC / DC hybrid microgrid, and the voltage and frequency of the AC / DC hybrid microgrid in the island mode are more prone to fluctuations.

[0015] A bidirectional adaptive inertia control method for an interconnected converter includes a bidirectional inertia control method and adaptive inertia coefficient setting.

[0016] The AC inertia capability of the interconnected converter is realized by virtual synchronous motor control. The virtual inertia is introduced into the control algorithm of the converter, with the inverter direction as the positive direction and the internal loss of the converter not considered. The inertia link is shown in (4)

[0017]

[0018] Where: H vir is the virtual inertia coefficient, and ω is the virtual synchronous motor angular frequency. The virtual moment of inertia J reflects the system's ability to prevent frequency mutations, allowing the VSM to adjust the system's unbalanced power in sufficient time; while the virtual damping coefficient Dp reflects the change in system output power when the unit frequency changes. In a hybrid microgrid, when the load power or DG output fluctuations in the AC sub-microgrid cause the frequency to change, the DG unit of the DC sub-microgrid can be forced to transmit more supporting power to the AC sub-microgrid through the bidirectional IC to improve the dynamic performance of the AC frequency.

[0019] Following the effect of charging and discharging of DC capacitors, the DC inertia capability of the interconnected converter is realized through virtual capacitor control, as shown in formula (5):

[0020]

[0021] Where: U dc is the DC bus voltage, C dc is the DC capacitor, C vir It is a virtual capacitor. In a hybrid microgrid, when the load power or DG (power terminal) output fluctuation in the DC sub-microgrid causes the DC voltage to change, the DC side capacitor will charge and discharge to suppress the DC voltage fluctuation, but the suppression effect is limited due to the actual capacity. By introducing a virtual capacitor that is much larger than the actual capacitance, the DG unit of the AC sub-microgrid can be forced to transmit more supporting power to the AC sub-microgrid through the bidirectional IC to improve the dynamic performance of the DC voltage.

[0022] Combining virtual synchronous machine and virtual capacitor control, combining (3) and (4), and normalizing AC frequency and DC voltage, we can get the bidirectional inertial control as shown in equation (5):

[0023]

[0024] During normal operation, P dcin =P ref , the power reference value can be set by the upper system according to the power flow distribution. When any sub-microgrid is disturbed, the proposed strategy can suppress the sudden change of AC frequency and DC voltage. However, when the system reaches the steady state after the disturbance, the differential term is 0, and the IC output power is still P ref, the AC / DC sub-microgrid cannot participate in the primary frequency and voltage regulation of the opposite system through IC. In order to give full play to the power mutual assistance capability of the hybrid microgrid and expand the energy source, a bidirectional droop control is introduced, as shown in formula (6):

[0025] P ref -P acout =k1(ω ref -ω g )-k2(U dcref -U dc ) (6)

[0026] Where k1 is the active power-frequency droop coefficient, k2 is the active power-DC voltage droop coefficient, ω ref is the frequency reference value, U dcref It is the DC voltage reference value. During normal operation, the reference value is equal to the rated value.

[0027] Combining equations (5) and (6), we get the active loop control of the proposed bidirectional inertial control (7):

[0028]

[0029] When the IC is in grid-connected mode, the phase-locked loop phase-locks the grid power supply. g =ω ref ,When the IC is in island mode, the IC provides a phase-locked phase angle, at this time ω=ω g .

[0030] Since the differential link is very sensitive to high-frequency interference in the input signal, the signal is often obscured by the interference. Therefore, a virtual capacitor control is designed to dedifferentiate.

[0031] The inertia power provided by the virtual capacitor is:

[0032]

[0033] Integrating both sides simultaneously and expanding around the rated DC voltage, we get:

[0034]

[0035] Where: During normal operation, U dcref Rated DC voltage

[0036] The first-order inertia link is introduced, and the deviation of the DC voltage from the rated value is used as the input signal. The corresponding output signal lags behind the input signal by a time constant T in value. When the value of T is very small, the virtual inertia power released by the AC sub-microgrid through the converter applying additional control when the DC voltage fluctuates can be obtained:

[0037]

[0038] The reactive power control link of the virtual synchronous machine is shown in (11). It refers to the excitation control principle of the synchronous machine and adjusts the terminal voltage and reactive power by adjusting the VSG virtual potential E. The virtual potential E consists of three parts: no-load potential E0, reactive deviation adjustment amount E Q And voltage deviation adjustment E U .

[0039]

[0040] Where: Q ref is the reactive power reference value; Q is the reactive power measurement value, k q is the reactive-virtual potential droop coefficient, k v is the terminal voltage-virtual potential droop coefficient.

[0041] According to equations (7)-(11), the virtual angular frequency and virtual potential of the virtual synchronous machine can be obtained. The modulation signal is obtained through the inner loop control of the voltage and current double closed loop. In summary, the block diagram of the control strategy proposed in this paper is as follows: Figure 4 shown.

[0042] H vir and C vir The value of H has contradictory effects on the dynamic response of virtual angular frequency and DC voltage. It is necessary to design an adaptive coefficient to take into account the dynamic performance of both. vir At the same time, reduce C vir , thereby suppressing the change of virtual angular frequency, when the DC voltage U dc When the offset is large, increase H vir At the same time, reduce C vir , thereby suppressing DC voltage changes, the controller should be able to balance the above power and frequency regulation according to the operating conditions.

[0043] Proposed H vir and C vir The dual adaptive coefficients are shown in equations (14) and (15)

[0044]

[0045] In the formula, H virmax , H virmin They are the maximum and minimum values ​​of the inertia coefficient, C virmax , C virmin They are the maximum and minimum values ​​of virtual capacitance respectively. When in steady state operation, ω=ω ref ,U dc =U dcref , H vir =H virmin ,C vir =Cvirmin .

[0046] Virtual inertia coefficient H vir The definition of H vir =Jω 2 / S n , where J is the moment of inertia of the synchronous generator, S n is the rated capacity of the synchronous generator. The configuration of the inertia coefficient is constrained by the DC energy storage capacity. Considering the capacitor as an energy storage element, Jω 2 =C dc U dcref / 2, so H virmin The value of C dc U dcref / (2S nsys ) nearby, S nsys is the rated capacity of the system, H virmax The value of takes into account the infinite frequency offset in extreme cases. At this time, the energy storage element runs at maximum power and contributes all to the IC inertia power. At this time, H virmin The value of C dc U dcref / (2S nIC ) nearby, S nIC is the rated capacity of the interconnected converter.

[0047] Virtual capacitance coefficient C virmax It is related to the inertia capacity of the AC power supply, with the droop coefficient k ω1 Taking the frequency-droop controlled AC energy storage unit as an example, the maximum fluctuation allowed for AC frequency and DC voltage when the microgrid is connected to the grid is Δω max =1%ω n , ΔU dcmax =5%U dcn , then the maximum inertial power provided is k ω1 Δω max , in order to determine C virmin The value is determined according to the frequency of DC voltage fluctuations in the DC microgrid, thereby giving the DC sub-microgrid an initial DC inertia.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] (1) The traditional virtual synchronous generator control method cannot take into account the shortcomings of the DC side response, and the converter using the traditional method can only improve the AC frequency response characteristics. The present invention derives the proposed bidirectional inertia control method from the principle, and adds DC droop and DC inertia links on the basis of the traditional virtual synchronous generator control technology to make up for the shortcomings of the traditional virtual synchronous generator control method that cannot take into account the DC side response. The traditional virtual capacitor control cannot take into account the shortcomings of the AC side response, and the converter using the traditional method can only improve the DC voltage response characteristics. The present invention adds AC droop and AC inertia links on the basis of the traditional virtual capacitor control to make up for the shortcomings of the traditional virtual capacitor control that cannot take into account the AC side response, so that the converter can take into account the response performance of both AC and DC.

[0050] (2) The control strategies of AC / DC microgrids are mostly concentrated on bidirectional droop control. However, bidirectional droop control has no inertia and no damping. The lack of inertia and damping of a large number of power electronic devices can easily cause system instability. The present invention retains the power sharing feature of bidirectional droop control and proposes bidirectional inertia control on this basis, which increases the system's bidirectional stability maintenance and anti-interference capabilities, making the response characteristics of AC / DC frequency and DC voltage smoother, and achieving a balance between overshoot and recovery time.

[0051] (3) Traditional inertial control methods can only be applied unilaterally to small disturbances such as load switching or large disturbances such as faults. The proposed method can achieve bidirectional safe and stable operation of AC / DC systems in multiple scenarios, such as load switching on the AC / DC side, AC / DC side faults, etc., thus enhancing the safe and stable operation of AC / DC systems.

[0052] (4) The inertia coefficient of the traditional inertia control method adopts a fixed value and cannot be flexibly adjusted according to the operating conditions, thus affecting the control effect under different operating conditions. The proposed adaptive inertia coefficient method combines the operating conditions in different scenarios and can adaptively optimize the virtual inertia and capacitance values ​​according to the operating conditions. This method makes up for the shortcomings of fixed inertia, so that the converter can adaptively adjust the inertia value according to the operating conditions;

[0053] (5) When the traditional inertial control method is applied to the converter, the converter can only adopt the rectification mode or the inverter mode unilaterally, which limits the flexibility of the converter to connect to the AC and DC power grids. The converter using the proposed method can adaptively adjust to the rectification mode or the inverter mode according to the operating conditions without manual adjustment and communication, which greatly increases the flexibility of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 This is a schematic diagram of the structure of a typical AC / DC hybrid microgrid.

[0055] Figure 2 It is a structural schematic diagram of a bidirectional interconnected converter device.

[0056] Figure 3 Table of IC operation modes.

[0057] Figure 4 Schematic diagram of a bidirectional adaptive inertia control strategy for interconnected converters.

[0058] Figure 5 The simulation results of DC load changes (the upper left figure is the DC voltage graph, the upper right figure is the frequency graph, and the lower middle figure is the IC transmission power graph).

[0059] Figure 6 The following are simulation results of AC load changes (the upper left diagram is the DC voltage diagram, the upper right diagram is the frequency diagram, and the lower middle diagram is the IC transmission power diagram).

[0060] Figure 7 The following are simulation results of the simultaneous changes in loads on both sides (the upper left is the DC voltage diagram, the upper right is the frequency diagram, and the lower middle is the IC transmission power diagram).

[0061] Figure 8 This is the simulation result diagram of AST exiting operation (the upper left is the DC voltage diagram, the upper right is the frequency diagram, and the lower middle is the IC transmission power diagram).

[0062] Fig. 9 The following are the simulation results of DST exit operation (the upper left is the DC voltage graph, the upper right is the frequency graph, and the lower middle is the IC transmission power graph). DETAILED DESCRIPTION

[0063] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0064] The present invention is applicable to AC / DC hybrid microgrids to balance the dynamic response of AC microgrid frequency and DC microgrid voltage. In view of the insufficiency of the traditional virtual synchronous motor control method or virtual capacitor method in not being able to take into account the response of the AC and DC sides, the control methods of the two are combined, and the virtual synchronous motor control is used as the control outer loop to realize power regulation and emergency control mode switching, and an additional virtual capacitor control loop is designed to suppress the influence of high-frequency interference on the differential link. The proposed method can enable the IC to adjust the working mode to operate in the rectification or inverter state under adaptive operating conditions. This strategy enables the interconnected converter to smoothly adapt to the three operating modes of grid connection, island power regulation, and island emergency control. It has the steady-state characteristics of droop control and can improve the dynamic response characteristics of angular frequency and DC bus voltage. At the same time, in view of the deficiency that the traditional fixed coefficient method cannot meet the requirements of optimized operation under different working conditions, an adaptive virtual inertia coefficient and virtual capacitance coefficient are proposed. The coefficient size is adaptively adjusted according to the frequency and voltage offset of the AC and DC sides. When the AC frequency offset is large, the virtual inertia coefficient is increased, and when the DC voltage offset is large, the virtual capacitance coefficient is increased, so as to balance the dynamic response characteristics of the two after the disturbance and improve the overall performance of the AC and DC microgrid.

[0065] Figure 1 The figure shows a typical topology of an AC / DC hybrid microgrid. The AC microgrid and the DC microgrid are flexibly interconnected through an interconnected converter (IC). Power can flow bidirectionally between the AC and DC sub-microgrids. The closing or closing of the PCC switch determines the grid-connected or islanded operation mode of the hybrid microgrid.

[0066] The AC / DC hybrid microgrid adopts a peer-to-peer control mode, which does not require communication and can meet the plug-and-play requirements of DG and loads. The AC slack terminal (AST) in the AC microgrid is a unit with frequency regulation capability (such as ACESS). According to Pf and QU in formula (1), ac The droop relationship maintains the stability of the bus frequency and voltage; the DC slack terminal (DST) in the DC microgrid, that is, the unit with voltage regulation capability (such as DC ESS), according to the PU of formula (1) dc Droop relationship to maintain the stability of the DC bus voltage.

[0067]

[0068] Wherein, subscript i represents the number of AST, subscript j represents the number of DST, P is the active power output by ST, P0 is the active power output by ST under rated working conditions, f0 is the power frequency, f is the current operating frequency of AC microgrid, K f is the frequency droop coefficient, Q is the ST output reactive power, Q 0i is the output reactive power under rated working condition of ST, U ac0is the rated AC voltage, U ac is the current voltage of the AC microgrid, K q is the AC voltage droop coefficient, U dc0 is the rated DC voltage, U dc is the current operating voltage of the DC microgrid, K u is the DC voltage droop coefficient.

[0069] See also Figures 1 to 9 In an embodiment of the present invention, a bidirectional adaptive inertia control device for an interconnected converter includes a voltage source converter (VSC) structure, U abc 、i abc They are the three-phase voltage and current of the converter, E abc is the three-phase virtual potential, L ac is the total inductance of the filter and line between the converter and the AC bus, ignoring the parasitic resistance of the filter and assuming that the line resistance is much smaller than the line inductance, C dc is the DC filter capacitor, U dc and i dc are the voltage and current on the DC side of the converter, P dcin , P acin ,P acout They are the DC sub-microgrid input power, IC input and output power respectively. The following analysis takes the inverter direction as the positive power direction.

[0070] The voltage equation of the interconnected converter in the dq coordinate system is shown in equation (2), and the active power equation is approximated as equation (3).

[0071]

[0072] Where: U d , U q 、E d 、E q 、i d 、i q are the d-axis and q-axis components of the AC voltage, AC bus voltage and AC current of the interconnected converter, respectively. U and E represent the amplitude, and ω g is the common bus angular frequency, and δ is the phase difference between U and E.

[0073] The IC is responsible for the power exchange of the AC and DC subsystems and realizes the power balance between the subsystems. According to the actual possible working conditions of the IC, its operating mode is as follows Figure 3As shown, IC can work in grid-connected and island operation modes at the same time, and when the ST on either side is abnormal or fails to operate, IC can seamlessly maintain the stable operation of the abnormal side and realize island emergency control. Taking the power flowing from the IC outlet to the AC grid as the positive direction, when the converter output power is positive, the converter works in the inverter mode, delivering active power to the AC grid and providing power to the DC grid, and vice versa, it is in the rectification mode. In the grid-connected mode, the large grid undertakes the voltage and frequency regulation of the AC / DC hybrid microgrid, and the voltage and frequency of the AC / DC hybrid microgrid in the island mode are more prone to fluctuations.

[0074] A bidirectional adaptive inertia control method for an interconnected converter includes a bidirectional inertia control method and adaptive inertia coefficient setting.

[0075] The AC inertia capability of the interconnected converter is realized by virtual synchronous motor control. The virtual inertia is introduced into the control algorithm of the converter, with the inverter direction as the positive direction and the internal loss of the converter not considered. The inertia link is shown in (4)

[0076]

[0077] Where: H vir is the virtual inertia coefficient, and ω is the virtual synchronous motor angular frequency. The virtual moment of inertia J reflects the system's ability to prevent frequency mutations, allowing the VSM to adjust the system's unbalanced power in sufficient time; while the virtual damping coefficient Dp reflects the change in system output power when the unit frequency changes. In a hybrid microgrid, when the load power or DG output fluctuations in the AC sub-microgrid cause the frequency to change, the DG unit of the DC sub-microgrid can be forced to transmit more supporting power to the AC sub-microgrid through the bidirectional IC to improve the dynamic performance of the AC frequency.

[0078] Following the effect of charging and discharging of DC capacitors, the DC inertia capability of the interconnected converter is realized through virtual capacitor control, as shown in formula (5):

[0079]

[0080] Where: U dc is the DC bus voltage, C dc is the DC capacitor, C vir It is a virtual capacitor. In a hybrid microgrid, when the load power or DG (power terminal) output fluctuation in the DC sub-microgrid causes the DC voltage to change, the DC side capacitor will charge and discharge to suppress the DC voltage fluctuation, but the suppression effect is limited due to the actual capacity. By introducing a virtual capacitor that is much larger than the actual capacitance, the DG unit of the AC sub-microgrid can be forced to transmit more supporting power to the AC sub-microgrid through the bidirectional IC to improve the dynamic performance of the DC voltage.

[0081] Combining virtual synchronous machine and virtual capacitor control, combining (3) and (4), and normalizing AC frequency and DC voltage, we can get the bidirectional inertial control as shown in equation (5):

[0082]

[0083] During normal operation, P dcin =P ref , the power reference value can be set by the upper system according to the power flow distribution. When any sub-microgrid is disturbed, the proposed strategy can suppress the sudden change of AC frequency and DC voltage. However, when the system reaches the steady state after the disturbance, the differential term is 0, and the IC output power is still P ref , the AC / DC sub-microgrid cannot participate in the primary frequency and voltage regulation of the opposite system through IC. In order to give full play to the power mutual assistance capability of the hybrid microgrid and expand the energy source, a bidirectional droop control is introduced, as shown in formula (6):

[0084] P ref -P acout =k1(ω ref -ω g )-k2(U dcref -U dc ) (6)

[0085] Where k1 is the active power-frequency droop coefficient, k2 is the active power-DC voltage droop coefficient, ω ref is the frequency reference value, U dcref It is the DC voltage reference value. During normal operation, the reference value is equal to the rated value.

[0086] Combining equations (5) and (6), we get the active loop control of the proposed bidirectional inertial control (7):

[0087]

[0088] When the IC is in grid-connected mode, the phase-locked loop phase-locks the grid power supply. g =ω ref ,When the IC is in island mode, the IC provides a phase-locked phase angle, at this time ω=ω g .

[0089] Since the differential link is very sensitive to high-frequency interference in the input signal, the signal is often obscured by the interference. Therefore, a virtual capacitor control is designed to dedifferentiate.

[0090] The inertia power provided by the virtual capacitor is:

[0091]

[0092] Integrating both sides simultaneously and expanding around the rated DC voltage, we get:

[0093]

[0094] Where: During normal operation, U dcref Rated DC voltage

[0095] The first-order inertia link is introduced, and the deviation of the DC voltage from the rated value is used as the input signal. The corresponding output signal lags behind the input signal by a time constant T in value. When the value of T is very small, the virtual inertia power released by the AC sub-microgrid through the converter applying additional control when the DC voltage fluctuates can be obtained:

[0096]

[0097] The reactive power control link of the virtual synchronous machine is shown in (11). It refers to the excitation control principle of the synchronous machine and adjusts the terminal voltage and reactive power by adjusting the VSG virtual potential E. The virtual potential E consists of three parts: no-load potential E0, reactive deviation adjustment amount E Q And voltage deviation adjustment E U .

[0098]

[0099] Where: Q ref is the reactive power reference value; Q is the reactive power measurement value, k q is the reactive-virtual potential droop coefficient, k v is the terminal voltage-virtual potential droop coefficient.

[0100] According to equations (7)-(11), the virtual angular frequency and virtual potential of the virtual synchronous machine can be obtained. The modulation signal is obtained through the inner loop control of the voltage and current double closed loop. In summary, the block diagram of the control strategy proposed in this paper is as follows: Figure 4 shown.

[0101] H vir and C vir The value of H has contradictory effects on the dynamic response of virtual angular frequency and DC voltage. It is necessary to design an adaptive coefficient to take into account the dynamic performance of both. vir At the same time, reduce C vir , thereby suppressing the change of virtual angular frequency, when the DC voltage U dc When the offset is large, increase H vir At the same time, reduce C vir , thereby suppressing DC voltage changes, the controller should be able to balance the above power and frequency regulation according to the operating conditions.

[0102] Proposed H vir and C virThe dual adaptive coefficients are shown in equations (14) and (15)

[0103]

[0104] In the formula, H virmax , H virmin They are the maximum and minimum values ​​of the inertia coefficient, C virmax , C virmin They are the maximum and minimum values ​​of virtual capacitance respectively. When in steady state operation, ω=ω ref ,U dc =U dcref , H vir =H virmin ,C vir =C virmin .

[0105] Virtual inertia coefficient H vir The definition of H vir =Jω 2 / S n , where J is the moment of inertia of the synchronous generator, S n is the rated capacity of the synchronous generator. The configuration of the inertia coefficient is constrained by the DC energy storage capacity. Considering the capacitor as an energy storage element, Jω 2 =C dc U dcref / 2, so H virmin The value of C dc U dcref / (2S nsys ) nearby, S nsys is the rated capacity of the system, H virmax The value of takes into account the infinite frequency offset in extreme cases. At this time, the energy storage element runs at maximum power and contributes all to the IC inertia power. At this time, H virmin The value of C dc U dcref / (2S nIC ) nearby, S nIC is the rated capacity of the interconnected converter.

[0106] Virtual capacitance coefficient C virmax It is related to the inertia capacity of the AC power supply, with the droop coefficient k ω1 Taking the frequency-droop controlled AC energy storage unit as an example, the maximum fluctuation allowed for AC frequency and DC voltage when the microgrid is connected to the grid is Δω max =1%ω n , ΔU dcmax =5%U dcn , then the maximum inertial power provided is k ω1 Δω max , in order to determine Cvirmin The value is determined according to the frequency of DC voltage fluctuations in the DC microgrid, thereby giving the DC sub-microgrid an initial DC inertia.

[0107] The working principle of the present invention is:

[0108] (1) The present invention proposes a bidirectional inertia control method for interconnected converters. The proposed bidirectional inertia control method is derived from the principle and combined with the traditional virtual synchronous generator control and virtual capacitor control. The method makes up for the shortcomings of the traditional virtual synchronous generator control method that cannot take into account the DC side response and the virtual capacitor control that cannot take into account the AC side response, so that the converter can suppress both the disturbance on the AC side and the fluctuation on the DC side. At the same time, the proposed scheme retains the power sharing characteristics of the bidirectional droop control and balances the dynamic response characteristics of the frequency and DC voltage, so as to solve the problem of over-limit caused by the sudden change of the corresponding electrical quantity after a disturbance or fault occurs on one side.

[0109] (2) The present invention proposes a multi-scenario application of bidirectional inertial control. Compared with the traditional virtual synchronous motor control method or virtual capacitor control unilaterally applied to small disturbances such as load switching or large disturbances such as faults, the proposed method can achieve safe and stable operation of AC and DC systems in multiple scenarios, such as grid-connected power regulation, island power regulation, island emergency control, etc., thereby enhancing the safe and stable operation of AC and DC systems.

[0110] (3) The present invention proposes an adaptive inertia coefficient method, which makes up for the deficiency of the traditional method of fixed virtual inertia or virtual capacitance coefficient, so that the converter can adaptively optimize the virtual inertia and virtual capacitance values ​​according to the operating conditions of the AC and DC sides. The proposed adaptive inertia coefficient method takes into account the dynamic response of the AC and DC sides. When the DC voltage fluctuates greatly, the virtual capacitance increases more than the virtual inertia, and when the frequency fluctuates greatly, the virtual inertia increases more than the virtual capacitance. The proposed method can adapt to the operating conditions, thereby balancing the dynamic response characteristics of the frequency and DC voltage, so that the electrical quantity on one side will not suddenly exceed the threshold due to disturbance, while reducing the dynamic recovery time of both.

[0111] In order to verify the effectiveness of the proposed bidirectional inertial control strategy, the following simulation model was built in PSCAD / EMTDC simulation software: Figure 1 The hybrid microgrid simulation model is shown in Figure 2. For the convenience of analysis, the hybrid microgrid structure is simplified. The AC microgrid contains an AST using Pf control, and the DC microgrid contains a PU dc Controlled DST, at the beginning of the simulation, the sources and loads inside the AC and DC microgrid are equal, and the transmission power of the interconnected converter is approximately 0. The IC parameters of the simulation system are shown below.

[0112] PCC rated voltage U0 / V: 0.38

[0113] AC side equivalent inductance L T / pu:0.018

[0114] AC side equivalent resistance R T / pu:0.006

[0115] Rated capacity S / MVA: 2

[0116] DC link voltage U dc / V:700

[0117] PCC rated frequency f0 / Hz: 50

[0118] Voltage regulation droop coefficient k2 / kV(MW)-1:2

[0119] Frequency modulation droop coefficient k1 / Hz(MW)-1:1

[0120] (1) Scenario 1: Load change

[0121] In order to verify the effectiveness of the proposed bidirectional adaptive inertial control strategy in responding to AC and DC disturbances and maintaining the stability of frequency and DC voltage, different scenarios are set, such as unilateral load change and simultaneous load change on both sides. At the beginning of the simulation, both AC and DC STs are put into operation, the AC and DC loads are both 0.3MW, the internal source and load of the AC and DC microgrid are balanced, and the frequency and DC voltage are maintained near the steady-state value.

[0122] 1) Figure 5 :DC load changes. At 4s, the DC load suddenly increases from 0.3MW to 0.5MW. At the beginning, the AC frequency and DC voltage are both near the rated value. At 4s, the increase in DC load causes the DC voltage to drop. The interconnected converter works in the rectification mode, and the AC microgrid provides support to the DC microgrid.

[0123] 2) Figure 6 :AC load changes. At 4s, the AC microgrid load suddenly increases from 0.3MW to 0.5MW. Different virtual inertia and virtual capacitance values ​​are used to simulate and compare the effects of dual inertia on AC frequency and DC voltage under fixed coefficients and the proposed strategy. At the beginning, both the AC frequency and DC voltage are near the rated values. At 4s, the increase in AC load causes the AC frequency to drop. The interconnected converter works in inverter mode, and the DC microgrid provides support to the AC microgrid.

[0124] Different virtual inertia and virtual capacitance values ​​are used respectively. When the virtual capacitance increases, the DC voltage overshoot decreases, which is beneficial to the DC voltage recovery, but it will deteriorate the dynamic performance of the frequency; when the virtual inertia increases, the frequency overshoot decreases, which is beneficial to the frequency recovery, but it will deteriorate the dynamic performance of the DC voltage. The proposed strategy is adopted to adaptively adjust the DC voltage and frequency, taking into account the dynamic performance of both. When a unilateral load disturbance occurs on the AC and DC sides, the above results show that the proposed strategy can effectively respond to the changes in the AC and DC loads, and avoid the poor dynamic performance of the AC frequency or DC voltage through the rapid balance of the AC and DC power.

[0125] 3) Figure 7 :The loads on both sides change at the same time. At 4s, the AC load suddenly increases from 0.3MW to 0.5MW, and the DC load suddenly decreases from 0.3MW to 0.1MW. The increase and decrease of AC and DC loads form power complementarity. IC controls the power to provide support to the shortfall side. The size of virtual inertia and virtual capacitance affects the size and speed of power support. When the virtual inertia increases, the transmission power slows down. When the virtual capacitance increases, the transmission power increases, which in turn affects the dynamic performance of DC voltage and frequency. The proposed strategy is adopted to optimize DC voltage and frequency, so that the overall effect is improved.

[0126] 4) Scenario 2: See Figure 8 , Fig. 9 , the emergency control mode is switched, the AC load and DC load are both 0.2MW, and when a fault or anomaly causes AST and DST to exit operation respectively, the emergency control effect of the proposed strategy is verified. When DST / AST exits operation, the proposed strategy can effectively support DC voltage / frequency and realize the safe operation of AC / DC hybrid microgrid island emergency control.

[0127] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A bidirectional adaptive inertia control device for interconnected converters, characterized in that: Including voltage source converter structure, U abc 、i abc They are the three-phase voltage and current of the converter, E abc is the three-phase virtual potential, L ac is the total inductance of the filter and line between the converter and the AC bus, ignoring the parasitic resistance of the filter and assuming that the line resistance is much smaller than the line inductance, C dc is the DC filter capacitor, U dc and i dc are the voltage and current on the DC side of the converter, P dcin , P acin ,P acout are the DC sub-microgrid input power, IC input and output power respectively; The voltage equation of the interconnected converter in the dq coordinate system is as follows, and the active power equation will be approximated as follows: Where: U d , U q 、E d 、E q 、i d 、i q are the d-axis and q-axis components of the AC voltage, AC bus voltage and AC current of the interconnected converter, respectively. U and E represent the amplitude, and ω g is the common bus angular frequency, and δ is the phase difference between U and E.

2. A bidirectional adaptive inertia control method for interconnected converters, characterized in that: Including bidirectional inertia control method and adaptive inertia coefficient setting, the AC inertia capability of the interconnected converter is realized through virtual synchronous motor control. The virtual inertia is introduced into the control algorithm of the converter, with the inverter direction as the positive direction, and the internal loss of the converter is not considered. The inertia link is as follows: Where: H vir is the virtual inertia coefficient, ω is the virtual synchronous motor angular frequency; the virtual rotational inertia J reflects the system's ability to prevent frequency mutations, allowing the VSM to adjust the system's unbalanced power in sufficient time; and the virtual damping coefficient Dp reflects the change in system output power when the unit frequency changes.

3. The method for bidirectional adaptive inertia control of interconnected converters according to claim 2, characterized in that: Following the effect of charging and discharging of DC capacitors, the DC inertia capability of the interconnected converter is realized through virtual capacitor control, as shown below: Where: U dc is the DC bus voltage, C dc is the DC capacitor, C vir is a virtual capacitor; Combining the virtual synchronous machine and virtual capacitor control, and normalizing the AC frequency and DC voltage, the bidirectional inertial control can be obtained as follows: During normal operation, P dcin =P ref , the power reference value can be set by the upper system according to the power flow distribution. When any sub-microgrid is disturbed, the proposed strategy can suppress the sudden change of AC frequency and DC voltage. However, when the system reaches the steady state after the disturbance, the differential term is 0, and the IC output power is still P ref , the AC / DC sub-microgrid cannot participate in the primary frequency and voltage regulation of the opposite system through the IC.

4. The method for bidirectional adaptive inertia control of interconnected converters according to claim 3 is characterized in that: In order to give full play to the power mutual assistance capability of hybrid microgrid and expand the energy source, bidirectional droop control is introduced as shown below: P ref -P acout =k1(ω ref -ω g )-k2(U dcref -IN dc ) Where k1 is the active power-frequency droop coefficient, k2 is the active power-DC voltage droop coefficient, ω ref is the frequency reference value, U dcref is the DC voltage reference value. During normal operation, the reference value is equal to the rated value, and the active loop control of the proposed bidirectional inertial control is obtained.

5. The method for bidirectional adaptive inertia control of interconnected converters according to claim 2, characterized in that: When the IC is in grid-connected mode, the phase-locked loop phase-locks the grid power supply. g =ω ref ,When the IC is in island mode, the IC provides a phase-locked phase angle, at this time ω=ω g ; Design virtual capacitor to control dedifferentiation. The inertial power provided by the virtual capacitor is: Integrating both sides simultaneously and expanding around the rated DC voltage, we get: Where: During normal operation, U dcref is the rated DC voltage; The first-order inertia link is introduced, and the deviation of the DC voltage from the rated value is used as the input signal. The corresponding output signal lags behind the input signal by a time constant T in value. When the value of T is very small, the virtual inertia power released by the AC sub-microgrid through the converter applying additional control when the DC voltage fluctuates can be obtained: The reactive power control link of the virtual synchronous machine is as follows. Drawing on the excitation control principle of the synchronous machine, the machine-end voltage and reactive power are adjusted by adjusting the VSG virtual potential E.

6. A bidirectional adaptive inertia control method for interconnected converters according to claim 5, characterized in that: The virtual potential E consists of three parts: no-load potential E0, reactive deviation adjustment E Q And voltage deviation adjustment E U Q ref is the reactive power reference value; Q is the reactive power measurement value, k q is the reactive-virtual potential droop coefficient, k v is the terminal voltage-virtual potential droop coefficient; H vir and C vir The value of H has contradictory effects on the dynamic response of virtual angular frequency and DC voltage. It is necessary to design an adaptive coefficient to take into account the dynamic performance of both. When the virtual angular frequency ω is greatly offset, increase H vir At the same time, reduce C vir , thereby suppressing the change of virtual angular frequency, when the DC voltage U dc When the offset is large, increase H vir At the same time, reduce C vir , thereby suppressing DC voltage changes, the controller should be able to balance the above power and frequency regulation according to the operating conditions.

7. A bidirectional adaptive inertia control method for interconnected converters according to claim 6, characterized in that: Proposed H vir and C vir The dual adaptive coefficients are as follows: In the formula, H virmax , H virmin They are the maximum and minimum values ​​of the inertia coefficient, C virmax , C virmin They are the maximum and minimum values ​​of virtual capacitance respectively. When in steady state operation, ω=ω ref ,U dc =U dcref , H vir =H virmin ,C vir =C virmin .

8. The method for bidirectional adaptive inertia control of interconnected converters according to claim 2, characterized in that: Virtual inertia coefficient H vir The definition of H vir =Jω 2 / S n , where J is the moment of inertia of the synchronous generator, S n is the rated capacity of the synchronous generator. The configuration of the inertia coefficient is constrained by the DC energy storage capacity. Considering the capacitor as an energy storage element, Jω 2 =C dc U dcref / 2, so H virmin The value of C dc U dcref / (2S nsys ) nearby, S nsys is the rated capacity of the system, H virmax The value of takes into account the infinite frequency offset in extreme cases. At this time, the energy storage element runs at maximum power and contributes all to the IC inertia power. At this time, H virmin The value of C dc U dcref / (2S nIC ) nearby, S nIC is the rated capacity of the interconnected converter.

9. The method for bidirectional adaptive inertia control of interconnected converters according to claim 3, characterized in that: Virtual capacitance coefficient C virmax It is related to the inertia capacity of the AC power supply, with the droop coefficient k ω1 Taking the frequency-droop controlled AC energy storage unit as an example, the maximum fluctuation allowed for AC frequency and DC voltage when the microgrid is connected to the grid is Δω max =1%ω n , ΔU dcmax =5%U dcn , then the maximum inertial power provided is k ω1 Δω max , in order to determine 10. A bidirectional adaptive inertia control method for interconnected converters according to claim 9, characterized in that: C virmin The value is determined according to the frequency of DC voltage fluctuations in the DC microgrid, thereby giving the DC sub-microgrid an initial DC inertia.

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