A hybrid ac-ac flux switch machine

Abstract

The application discloses a hybrid AC-AC converter, comprising: an AC bus on the rectifier side, a converter transformer, a rectifier, an inverter, an AC bus on the inverter side, an AC filter and a DC filter; the rectifier is composed of a high-voltage valve group LCC and a low-voltage valve group MMC, the high-voltage valve group and the low-voltage valve group of the inverter are both in the form of half-bridge sub-module MMC, and a high-power diode valve D is connected in series at the DC outlet of the high-voltage valve group MMC; the rectifier side LCC adopts constant-current control based on the DC voltage of the rectifier side MMC, the rectifier side MMC adopts voltage amplitude-frequency control, and the inverter side MMC adopts constant DC voltage control and constant reactive power control. The application can realize low-frequency transmission of pure new energy power generation bases, well adapt to power fluctuation of the new energy power generation bases, fully utilize the technical maturity of the LCC and the MMC, and reduce engineering cost and power loss compared with a conventional AC-AC converter based on back-to-back MMC.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of converter, and particularly relates to a hybrid AC-AC converter. BACKGROUND

[0002] Low-frequency AC power transmission technology can realize pure new energy power generation base sending out, and its advantages mainly lie in two aspects: (1) compared with power frequency AC power transmission, reducing the transmission frequency can expand the distance of power transmission; (2) compared with DC power transmission, AC power transmission does not lack circuit breakers, and new energy power generation bases can be conveniently composed into an AC power grid. Therefore, for pure new energy power generation base sending out, using low-frequency AC power transmission technology is a competitive scheme.

[0003] The core device of low-frequency AC power transmission technology is an AC-AC converter. Existing documents mostly use a phase-controlled AC-AC converter based on thyristors, which has a simple structure and is convenient to control, but a large amount of reactive power needs to be consumed in the normal operation process, and a considerable amount of low-order harmonics and inter-harmonics are generated, which causes a series of power quality problems. Replacing the thyristors with fully controlled devices can improve the harmonic characteristics of the device, but the effect is not significant. In recent years, some research has proposed to use a matrix converter as an improved topology structure of the AC-AC converter, but its feasibility in the field of high-voltage and large-capacity power transmission needs to be verified. Another technical route is to use an AC-AC converter based on a back-to-back voltage source converter (VSC), and the VSC technology has high maturity, flexible control mode and good harmonic characteristics, but has defects of high device cost and large power loss.

[0004] In addition, in order to realize large-scale pure new energy sending out, the voltage support problem of the sending end AC system must be solved. The conventional method is to send out new energy together with hydropower, thermal power and the like, and there is a certain limit to the proportion of new energy, which deviates from the original intention of large-scale construction of supporting thermal power and development of new energy. In theory, the new energy base can be operated in a grid-forming control mode, but there is little relevant engineering experience at present. Therefore, the existing and future short-term new energy bases in China still mainly use the grid-following control mode, and the voltage support of the sending end AC system must be provided by the AC-AC converter.

[0005] So far, few documents have studied the AC-AC converter suitable for low-frequency sending out of pure new energy power generation bases, and it is necessary to study the topology and control strategy of the AC-AC converter suitable for low-frequency sending out of pure new energy power generation bases in order to further improve the economy and reliability of the AC-AC converter. SUMMARY

[0006] The present application provides a hybrid AC-AC converter, which solves the technical problems that the existing converter cannot well adapt to the power fluctuation of the new energy power generation base, has high engineering cost and large power loss.

[0007] Therefore, the first aspect of the present application provides a hybrid AC / AC converter, which comprises a rectifier module composed of a positive rectifier and a negative rectifier, and an inverter module composed of a positive inverter and a negative inverter;

[0008] The high-voltage end of the positive rectifier is connected to the high-voltage end of the positive inverter, the high-voltage end of the negative rectifier is connected to the high-voltage end of the negative inverter, the low-voltage end of the positive rectifier and the low-voltage end of the negative rectifier are connected and serve as the DC side neutral point of the rectifier module, the low-voltage end of the positive inverter and the low-voltage end of the negative inverter are connected and serve as the DC side neutral point of the inverter module, and the DC side neutral points of the rectifier module and the inverter module are connected to a grounding electrode, thereby forming a true bipolar structure.

[0009] The AC side of the rectifier module is connected to one side of a rectifier side AC bus through a converter transformer, the DC side of the rectifier module is connected to the DC side of the inverter module, the AC side of the inverter module is connected to one side of an inverter side AC bus through a converter transformer, an AC filter is connected in parallel to the rectifier side AC bus, the high-voltage end of the positive rectifier and the high-voltage end of the negative rectifier are each connected in parallel to a DC filter, the other side of the rectifier side AC bus is connected to a pure new energy power generation base, and the other side of the inverter side AC bus is connected to a receiving end power grid.

[0010] Optionally, the positive rectifier and the negative rectifier each comprise a first high-voltage valve group and a first low-voltage valve group, and the first high-voltage valve group and the first low-voltage valve group are connected in series at the DC side and in parallel at the AC side.

[0011] The first high-voltage valve group is a line commutated converter (LCC), and the first low-voltage valve group is a modular multilevel converter (MMC) composed of half-bridge sub-modules.

[0012] Optionally, the LCC comprises two three-phase six-pulse rectifier bridges, the two three-phase six-pulse rectifier bridges are respectively connected to converter transformers adopting Y0 / Y and Y0 / △ connection modes, and the valve side phase difference of the two converter transformers is 30°.

[0013] The MMC has a three-phase six-bridge-arm structure, each bridge arm is composed of N half-bridge sub-modules connected in series and bridge-arm reactance connected in series, and the MMC is connected to a converter transformer adopting Y0 / △ connection mode.

[0014] Optionally, the positive inverter and the negative inverter each comprise a second high-voltage valve group and a second low-voltage valve group, and the second high-voltage valve group and the second low-voltage valve group are connected in series at the DC side and in parallel at the AC side.

[0015] The second high-voltage valve group and the second low-voltage valve group are both modular multilevel converters (MMCs) of half-bridge sub-modules. The DC outlet of the second high-voltage valve group is connected in series with a high-power diode valve D. The MMC is connected to a converter transformer using a Y0 / Δ connection method.

[0016] Optionally, the LCC employs constant current control based on MMC DC voltage;

[0017] The actual value of the MMC DC voltage on the rectifier side, U MMCrec After passing through the first-order inertial element, the command value U of the DC voltage of the rectifier-side MMC is... MMCrecref Subtract, then pass through PI control to output DC current command value I dcref The input of the constant current controller is I. dcref and the actual value of DC current I dc I dcref With I after passing through a first-order inertial element dc After subtraction, the PI control outputs the lead angle β. Subtracting β from π radians yields the lag angle α. The minimum lag angle α is then set... min =5°, take α and α min The maximum value is the trigger hysteresis angle α. R , which serves as the trigger signal for each switching device in the LCC on the rectifier side.

[0018] Optionally, the MMC on the rectifier side adopts voltage amplitude-frequency control, and the control system includes two control dimensions: d-axis and q-axis: outer loop controller, inner loop controller and triggering circuit;

[0019] Let the phase voltage amplitude U at the AC output of the MMC on the rectifier side be... m The d-axis voltage command value u dref Let the q-axis voltage command value u qref =0, the input of the outer loop controller is the d-axis component u of the rectifier-side MMC AC output voltage. d and q-axis component u q and u dref and u qref u dref and u qref respectively with u d and u q After subtraction, the d-axis current reference value i is output through PI control. dref1 and q-axis current reference value i qref1 The input to the inner loop controller is the d-axis component of the MMC AC output current on the rectifier side. d1 and q-axis component i q1 and i dref1 and i qref1 i dref1 and i qref1respectively, are subtracted from U d1 and U q1 respectively, and then PI control is performed to output d-axis voltage modulation wave U vdref1 and q-axis voltage modulation wave U vqref1 ; the input of the trigger link is U vdref1 and U vqref1 , and after dq / abc conversion and NLC modulation, the trigger signals of each switching device in the rectifier side MMC are output.

[0020] Optionally, the MMC on the inverter side adopts fixed DC voltage control and fixed reactive power control, and the control system comprises a DC side control loop, and an outer loop controller, an inner loop controller and a trigger link comprising two control dimensions of d-axis and q-axis;

[0021] The d-axis instruction value U MMCinvref of the outer loop controller is generated by the DC side control loop, and the q-axis instruction value Q sref of the outer loop controller is set to 0, the input of the outer loop controller is the actual value U MMCinv of the DC voltage of the inverter side MMC and the reactive power Qs of the AC outlet of the inverter side MMC, and U MMCinvref and Q sref , U MMCinv and Qs are subtracted from U MMCinvref and Q sref respectively, and then PI control is performed to output the d-axis current reference value i dref2 and the q-axis current reference value i qref2 ; the input of the inner loop controller is the d-axis component i d2 and the q-axis component i q2 of the AC outlet current of the inverter side MMC, and i dref2 and i qref2 , i dref2 and i qref2 are subtracted from i d2 and i q2 respectively, and then PI control is performed to output the d-axis voltage modulation wave u vdref2 and the q-axis voltage modulation wave u vqref2 ; the input of the trigger link is u vdref2 and u vqref2 , and after dq / abc conversion and NLC modulation, the trigger signals of each switching device in the inverter side MMC are output.

[0022] Optionally, the DC side control loop adopts backup fixed current control.

[0023] The input of the backup fixed current control is the actual value I dcinv of the DC current on the inverter side and the DC current instruction value I dcref generated by the rectifier side LCC, I dcref is multiplied by 0.9 and then subtracted from Idcinv Subtracting, and then outputting the DC voltage instruction value U dciref , taking the minimum value of U dciref and the given instruction value U dcsteady as the outer loop controller d-axis instruction value U MMCinvref ;

[0024] When the system is normally running, U MMCinvref is determined by U dcsteady ; when the AC fault occurs in the power generation base or the receiving end power grid, U MMCinvref is determined by U dciref , and the DC voltage of the inverter side MMC is actively reduced.

[0025] Optionally, the DC line fault control strategy of the DC side control loop comprises:

[0026] S1, when the DC current reaches 1.5 p.u., it is determined that the DC line fault occurs;

[0027] S2, the rectifier side MMC is locked and the LCC is forced to phase shift, and the forced phase shift step is to first set alpha R to 110°, and then to linearly increase alpha R to 135° after the DC current is reduced to below 1.0 p.u.;

[0028] S3, when the DC fault is cleared, step S2 is maintained for 0.2s to complete the fault point de-ionization;

[0029] S4, restart the system, unlock the fault pole MMC, and linearly reduce alpha R from 45° to 15°, and the DC voltage instruction value of the inverter side MMC is first reduced to 0.75 p.u., and then linearly increased to the steady state value after the DC current of the fault pole is restored to 1.0 p.u.

[0030] Optionally, the pure new energy power generation base specifically comprises: a wind power generation base and a photovoltaic power generation base adopting grid-connected control.

[0031] From the above technical solutions, the present application has the following advantages:

[0032] 1. The present application proposes a new topology of LCC-MMC and D-MMC hybrid AC-AC converter, which can greatly reduce the engineering cost and power loss compared with the conventional AC-AC converter based on back-to-back VSC, and has great application value in actual engineering.

[0033] 2, The application provides a control strategy of a hybrid AC-AC converter of LCC-MMC and D-MMC, can realize 100% pure new energy power generation base sending out, and adapts to the power fluctuation of the new energy power generation base well, fully utilizes the technical maturity of LCC and MMC, and plays a certain guiding role for future engineering design. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 A structure schematic diagram of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0035] Figure 2 A topology structure schematic diagram of an MMC of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0036] Figure 3 A control structure schematic diagram of an LCC of a rectifier side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0037] Figure 4 A control structure schematic diagram of an LCC of a rectifier side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0038] Figure 5 A control structure schematic diagram of an MMC of a rectifier side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0039] Figure 6 A control structure schematic diagram of an MMC of an inverter side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0040] Figure 7a An AC voltage simulation waveform schematic diagram of an AC bus of a rectifier side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0041] Figure 7b An AC current simulation waveform schematic diagram of an AC bus of a rectifier side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0042] Figure 7c An active power simulation waveform schematic diagram of an AC bus of a rectifier side of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0043] Figure 8a A simulation waveform schematic diagram of a DC voltage of a positive rectifier side LCC and a positive rectifier side MMC of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0044] Figure 8b A simulation waveform schematic diagram of a positive DC current of a hybrid AC-AC converter provided in the embodiment of the application is provided.

[0045] Figure 9a An AC voltage simulation waveform schematic diagram of an AC bus on an inverter side of a hybrid AC-AC converter provided in an embodiment of the present application;

[0046] Figure 9b An AC current simulation waveform schematic diagram of an AC bus on an inverter side of a hybrid AC-AC converter provided in an embodiment of the present application;

[0047] Figure 9c An active power simulation waveform schematic diagram of an AC bus on an inverter side of a hybrid AC-AC converter provided in an embodiment of the present application. DETAILED DESCRIPTION

[0048] In order to enable persons skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by persons skilled in the art without creative work fall within the scope of protection of the present application.

[0049] Please refer to Figure 1 , a hybrid AC-AC converter provided in an embodiment of the present application, comprising: a rectifier module composed of a positive rectifier and a negative rectifier, and an inverter module composed of a positive inverter and a negative inverter;

[0050] Wherein, the high-voltage end of the positive rectifier is connected with the high-voltage end of the positive inverter, the high-voltage end of the negative rectifier is connected with the high-voltage end of the negative inverter, the low-voltage end of the positive rectifier and the low-voltage end of the negative rectifier are connected and serve as a DC side neutral point of the rectifier module, the low-voltage end of the positive inverter and the low-voltage end of the negative inverter are connected and serve as a DC side neutral point of the inverter module, and the DC side neutral points of the rectifier module and the inverter module are connected to a grounding pole, thereby forming a true bipolar structure, as shown in Figure 1 ;

[0051] The AC side of the rectifier module is connected with one side of a rectifier side AC bus through a converter transformer, the DC side of the rectifier module is connected with the DC side of the inverter module, the AC side of the inverter module is connected with one side of an inverter side AC bus through a converter transformer, an AC filter is connected in parallel to the rectifier side AC bus, the high-voltage end of the positive rectifier and the high-voltage end of the negative rectifier are both connected in parallel with a DC filter, the other side of the rectifier side AC bus is connected to a pure new energy power generation base, and the other side of the inverter side AC bus is connected to a receiving end power grid.

[0052] It should be noted that the rated frequency of the pure new energy power generation base, the rectifier side converter transformer and the rectifier is selected as 8-20 Hz.

[0053] Further, the rectifier of the embodiment is composed of a high-voltage valve group LCC and a low-voltage valve group MMC, which are connected in series at the DC side and in parallel at the AC side; the LCC contains two three-phase six-pulse rectifier bridges, which are connected with two converter transformers adopting Y0 / Y and Y0 / △ connection modes respectively, and the two converter transformers have a phase difference of 30° at the valve side; the MMC is connected with a converter transformer adopting Y0 / △ connection mode; the high and low voltage valve groups of the inverter both adopt half-bridge sub-module type MMC, the DC outlet of the high-voltage valve group MMC is connected in series with a high-power diode valve D, and the MMC is connected with a converter transformer adopting Y0 / △ connection mode; the structure of the three-phase six-pulse rectifier bridge in the LCC is shown in Figure 2 , and the structure of the MMC is shown in Figure 3 .

[0054] Further, the pure new energy power generation base of the embodiment includes a wind power generation base and a photovoltaic power generation base adopting grid-following type control, which are connected to the rectifier side AC bus of the LCC-MMC and D-MMC hybrid type AC-AC converter by a low-frequency AC transmission line.

[0055] In one embodiment, the rectifier side LCC adopts constant current control based on the DC voltage of the rectifier side MMC as shown in Figure 4 , specifically:

[0056] The actual value U MMCrec of the DC voltage of the rectifier side MMC is subtracted from the command value U MMCrecref of the DC voltage of the rectifier side MMC after passing through a first-order inertia link, and then the PI control is performed to output the DC current command value I dcref ; the input of the constant current controller is I dcref and the actual value I dc of the DC current, and I dcref is subtracted from I dc after passing through a first-order inertia link, and then the PI control is performed to output the trigger lead angle β, π radians is subtracted from β to obtain the trigger lag angle α, and the minimum trigger lag angle α min = 5°, the maximum value of α and α min is taken as the trigger lag angle α R , which is used as the trigger signal of each switching device in the rectifier side LCC.

[0057] The rectifier side MMC adopts voltage amplitude-frequency control as shown in Figure 5 , specifically:

[0058] The control system includes two control dimensions of d-axis and q-axis: outer loop controller, inner loop controller and trigger link;

[0059] Let the phase voltage amplitude U mThe d-axis voltage command value u dref Let the q-axis voltage command value u qref =0, the input of the outer loop controller is the d-axis component u of the rectifier-side MMC AC output voltage. d and q-axis component u q and u dref and u qref u dref and u qref respectively with u d and u q After subtraction, the d-axis current reference value i is output through PI control. dref1 and q-axis current reference value i qref1 The input to the inner loop controller is the d-axis component of the MMC AC output current on the rectifier side. d1 and q-axis component i q1 and i dref1 and i qref1 i dref1 and i qref1 respectively with i d1 and i q1 After subtraction, the output d-axis voltage modulation wave u is controlled by PI. vdref1 and q-axis voltage modulation wave u vqref1 The input to the triggering circuit is u. vdref1 and u vqref1 Furthermore, after dq / abc conversion and NLC modulation, the trigger signals of each switching device in the MMC on the output rectifier side are obtained.

[0060] The inverter-side MMC employs constant DC voltage control and constant reactive power control, such as... Figure 6 As shown, specifically:

[0061] The control system includes: a DC-side control loop, and two control dimensions including the d-axis and q-axis: an outer loop controller, an inner loop controller, and a triggering element;

[0062] Outer loop controller d-axis command value U MMCinvref Generated by the DC-side control loop, setting the outer loop controller's q-axis command value Q. sref =0, the input to the outer loop controller is: the actual value U of the inverter-side MMC DC voltage. MMCinv and the reactive power Qs and U at the AC output of the inverter-side MMC MMCinvref and Q sref U MMCinv Qs and U respectively MMCinvref and Q sref After subtraction, the d-axis current reference value i is output through PI control. dref2 and q-axis current reference value i qref2 The input to the inner loop controller is the d-axis component of the inverter-side MMC AC output current, i.d2 and q-axis component i q2 and i dref2 and i qref2 , i dref2 and i qref2 respectively minus i d2 and i q2 after PI control output d-axis voltage modulation wave u vdref2 and q-axis voltage modulation wave u vqref2 ; the input of the trigger link is u vdref2 and u vqref2 and after dq / abc conversion and NLC modulation, the trigger signal of each switching device in the inverter side MMC is output.

[0063] The backup constant current control is adopted for the DC side control loop;

[0064] The input of the backup constant current control is the actual value I dcinv of the inverter side DC current and the DC current command value I dcref generated by the rectifier side LCC, I dcref multiplied by 0.9 and then subtracted from I dcinv , and then after PI control, the DC voltage command value U dciref is output, U dciref is taken and the minimum value in the given command value U dcsteady is taken as the d-axis command value U MMCinvref of the outer loop controller;

[0065] When the system is normally running, U MMCinvref is determined by U dcsteady ; when the AC fault occurs in the power generation base or the receiving end power grid, U MMCinvref is determined by U dciref , so as to realize the active reduction of the DC voltage of the inverter side MMC.

[0066] The DC line fault control strategy of the DC side control loop includes:

[0067] S1, when the DC current is detected to reach 1.5 p.u., it is determined that the DC line fault occurs;

[0068] S2, the rectifier side MMC is locked and the LCC is forced to phase shift, and the forced phase shift step is to first set α R to 110°, and then after the DC current is reduced to below 1.0 p.u., α R is ramped up to 135°;

[0069] S3, when the DC fault is cleared, step S2 control is maintained for 0.2 s to complete the fault point de-ionization;

[0070] S4, the system is restarted, the fault pole MMC is unlocked, and αR From 45° linear reduction to 15°, the DC voltage command value of the inverter side MMC is first reduced to 0.75 p.u., and then linearly raised to the steady-state value after the fault pole DC current is restored to 1.0 p.u.

[0071] The system parameters in the embodiment are shown in Table 1:

[0072] Table 1

[0073]

[0074] The corresponding simulation platform is built in the electromagnetic transient simulation software PSCAD / EMTDC, and the power fluctuation of the new energy power generation base is simulated. In the simulation, it is assumed that the active power output of the 2s new energy power generation base is stepped down from 5000MW to 3000MW, Figures 7a to 7c The simulation results of the key electrical quantities of the rectifier side AC bus are given, Figures 8a to 8b The simulation results of the DC voltage and DC current are given, Figures 9a to 9c The simulation results of the key electrical quantities of the inverter side AC bus are given, and the simulation results prove the effectiveness of the application.

[0075] The terms "first", "second", "third", "fourth" and the like in the specification of this application and in the above figures are used to distinguish similar objects, and do not necessarily have to be described in a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the application described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0076] It should be understood that in this application, "at least one" means one or more, and "multiple" means two or more. "And / or" is used to describe the association between the associated objects, which means that there can be three relationships, for example, "A and / or B" can represent three cases: only A, only B, and A and B at the same time, where A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects. "At least one of the following" or similar expressions means any combination of these items, including single or multiple combinations of any combination. For example, at least one of a, b or c, can represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", where a, b, and c can be single or multiple.

[0077] In several embodiments provided in the present application, it should be understood that the disclosed system, device and method can be implemented in other manners. For example, the described device embodiments are merely schematic. For example, the division of the units is only a logical function division. There can be another division manner for the actual implementation, for example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections between the units can be indirect couplings or communication connections through some interfaces, devices or units, and can be electrical, mechanical or in other forms.

[0078] The units described as separated components can or can not be physically separated, and the components displayed as units can or can not be physical units, i.e., can be located in one place, or can be distributed on a plurality of network units. In actual implementation, some or all of the units can be selected according to the actual needs to achieve the purposes of the embodiments of the present application.

[0079] In addition, each functional unit in the embodiments of the present application can be integrated in a processing unit, or each unit can exist physically as a separate unit, or two or more units can be integrated in one unit. The integrated unit can be implemented in the form of hardware, or in the form of a software functional unit.

[0080] When the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer readable storage medium. Based on such an understanding, the technical solutions of the present application essentially, or the part that contributes to the prior art, or all or a part of the technical solutions can be embodied in the form of a software product. The computer software product is stored in a storage medium, and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the embodiments of the present application. The foregoing storage medium includes: a U disk, a mobile hard disk, a read-only memory (English full name: Read-Only Memory, English abbreviation: ROM), a random access memory (English full name: Random Access Memory, English abbreviation: RAM), a magnetic disk or an optical disk, and various other media that can store program codes.

[0081] The above-described embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacements for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A hybrid AC exchanger, characterized in that, include: A rectifier module consisting of a positive rectifier and a negative rectifier, and an inverter module consisting of a positive inverter and a negative inverter; In this configuration, the high-voltage terminal of the positive rectifier is connected to the high-voltage terminal of the positive inverter, the high-voltage terminal of the negative rectifier is connected to the high-voltage terminal of the negative inverter, the low-voltage terminal of the positive rectifier and the low-voltage terminal of the negative rectifier are connected and serve as the DC-side neutral point of the rectifier module, the low-voltage terminal of the positive inverter and the low-voltage terminal of the negative inverter are connected and serve as the DC-side neutral point of the inverter module, and the DC-side neutral points of both the rectifier module and the inverter module are connected to the ground electrode, thus forming a true bipolar structure. The AC side of the rectifier module is connected to one side of the rectifier side AC bus via a converter transformer. The DC side of the rectifier module is connected to the DC side of the inverter module. The AC side of the inverter module is connected to one side of the inverter side AC bus via a converter transformer. An AC filter is connected in parallel to the rectifier side AC bus. A DC filter is connected in parallel to the high-voltage ends of both the positive and negative rectifiers. The other side of the rectifier side AC bus is connected to the pure new energy power generation base, and the other side of the inverter side AC bus is connected to the receiving-end power grid. Both the positive rectifier and the negative rectifier are composed of a first high-voltage valve group (LCC) and a first low-voltage valve group (MMC); both the positive inverter and the negative inverter are composed of a second high-voltage valve group (MMC) and a second low-voltage valve group (MMC). The MMC on the inverter side adopts constant DC voltage control and constant reactive power control. The control system includes a DC side control loop, as well as two control dimensions including the d-axis and q-axis: an outer loop controller, an inner loop controller, and a triggering circuit. The DC-side control circuit adopts backup constant current control. The input for backup constant current control is the actual value I of the inverter-side DC current. dcinv The DC current command value I generated by the rectifier-side LCC dcref I dcref Multiply by 0.9 and then I dcinv Subtract, then pass through PI control to output DC voltage command value U dciref , take U dciref and given instruction value U dcsteady The minimum value in the range is used as the d-axis command value U of the outer loop controller. MMCinvref ; When the system is running normally, U MMCinvref byU dcsteady Decision; when an AC fault occurs at the power generation base or the receiving-end power grid, U MMCinvref byU dciref The decision was made to enable the inverter-side MMC to actively reduce the DC voltage; The DC line fault control strategy of the DC side control loop includes: S1. When the detected DC current reaches 1.5 pu, a DC line fault is determined to have occurred. S2. Block the MMC on the rectifier side and force phase shift the LCC. The forced phase shifting steps are as follows: First, ... α R Set it to 110°, and wait until the DC current drops below 1.0 pu before adjusting α. R The slope was raised to 135°; S3. After the DC fault is cleared, maintain the control of step S2 for 0.2s to complete the deionization of the fault point; S4. Restart the system to unlock the faulty MMC. R As the angle decreases linearly from 45° to 15°, the DC voltage command value of the inverter-side MMC first decreases to 0.75 pu, and then linearly increases to the steady-state value after the DC current of the fault electrode recovers to 1.0 pu.

2. The hybrid AC converter according to claim 1, characterized in that, The first high-pressure valve group and the first low-pressure valve group are connected in series on the DC side and in parallel on the AC side.

3. The hybrid AC converter according to claim 2, characterized in that, The LCC includes two three-phase six-pulse rectifier bridges, which are respectively connected to converter transformers using Y0 / Y and Y0 / Δ connection methods, and the valve-side phase difference between the two converter transformers is 30°. The MMC is a three-phase six-bridge structure. Each bridge arm is composed of N cascaded half-bridge sub-modules connected in series with the bridge arm reactor. The MMC is connected to a converter transformer using a Y0 / Δ connection.

4. The hybrid AC converter according to claim 3, characterized in that, The second high-pressure valve group and the second low-pressure valve group are connected in series on the DC side and in parallel on the AC side; The second high-voltage valve group has a high-power diode valve D connected in series at its DC outlet, and the MMC is connected to a converter transformer using a Y0 / Δ connection.

5. The hybrid AC converter according to claim 4, characterized in that, The LCC adopts constant current control based on MMC DC voltage; The actual value of the MMC DC voltage on the rectifier side, U MMCrec After passing through the first-order inertial element, the command value U of the DC voltage of the rectifier-side MMC is... MMCrecref Subtract, then pass through PI control to output DC current command value I dcref The input of the constant current controller is I. dcref and the actual value of DC current I dc I dcref With I after passing through a first-order inertial element dc After subtraction, the PI control outputs the lead angle β. Subtracting β from π radians yields the lag angle α. The minimum lag angle α is then set... min =5°, take α and α min The maximum value is the trigger hysteresis angle α. R , which serves as the trigger signal for each switching device in the LCC on the rectifier side.

6. The hybrid AC converter according to claim 4, characterized in that, The MMC on the rectifier side adopts voltage amplitude-frequency control. The control system includes two control dimensions: d-axis and q-axis: outer loop controller, inner loop controller, and triggering circuit. Let the phase voltage amplitude U at the AC output of the MMC on the rectifier side be... m The d-axis voltage command value u dref Let the q-axis voltage command value u qref =0, the input of the outer loop controller is the d-axis component u of the rectifier-side MMC AC output voltage. d and q-axis component u q and u dref and u qref u dref and u qref respectively with u d and u q After subtraction, the d-axis current reference value i is output through PI control. dref1 and q-axis current reference value i qref1 The input to the inner loop controller is the d-axis component of the MMC AC output current on the rectifier side. d1 and q-axis component i q1 and i dref1 and i qref1 i dref1 and i qref1 respectively with i d1 and i q1 After subtraction, the output d-axis voltage modulation wave u is controlled by PI. vdref1 and q-axis voltage modulation wave u vqref1 The input to the triggering circuit is u. vdref1 and u vqref1 Furthermore, after dq / abc conversion and NLC modulation, the trigger signals of each switching device in the MMC on the output rectifier side are obtained.

7. The hybrid AC converter according to claim 4, characterized in that, The inverter side includes two control dimensions: the d-axis and the q-axis. It consists of an outer loop controller, an inner loop controller, and a triggering circuit. Outer loop controller d-axis command value U MMCinvref Generated by the DC-side control loop, setting the outer loop controller's q-axis command value Q. sref =0, the input to the outer loop controller is: the actual value U of the inverter-side MMC DC voltage. MMCinv and the reactive power Qs and U at the AC output of the inverter-side MMC MMCinvref and Q sref U MMCinv Qs and U respectively MMCinvref and Q sref After subtraction, the d-axis current reference value i is output through PI control. dref2 and q-axis current reference value i qref2 The input to the inner loop controller is the d-axis component of the inverter-side MMC AC output current, i. d2 and q-axis component i q2 and i dref2 and i qref2 i dref2 and i qref2 respectively with i d2 and i q2 After subtraction, the output d-axis voltage modulation wave u is controlled by PI. vdref2 and q-axis voltage modulation wave u vqref2 The input to the triggering circuit is u. vdref2 and u vqref2 Furthermore, after dq / abc conversion and NLC modulation, the trigger signals of each switching device in the inverter-side MMC are output.

8. The hybrid AC converter according to claim 1, characterized in that, The pure new energy power generation base specifically includes: wind power generation base and photovoltaic power generation base that adopt grid-connected control.

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