New energy grid-connected equipment of duplex-winding asynchronous motor interface and control method of new energy grid-connected equipment

Abstract

The invention discloses new energy grid-connected equipment of a duplex winding asynchronous motor interface and a control method thereof, belongs to the technical field of new energy power generation, and solves the problems of broadband oscillation, inertia loss and short-circuit capacity reduction caused by a high-proportion grid-connected inverter to a power system. A double-winding asynchronous motor is adopted as a grid-connected interface, the alternating current side of a first three-phase inverter is directly connected with the head end of a control winding, a first set of switches of a change-over switch are connected between the tail end of the control winding and a second three-phase inverter, and a second set of switches of the change-over switch are connected between the tail ends of the control winding in parallel. The third group of switches are connected between the alternating current sides of the two three-phase inverters; a grid-connected winding of the double-winding asynchronous motor is connected with a power grid; the rotor of the double-winding asynchronous motor is in a rotating state during working, an LC / LCL filter and a phase-locked loop are prevented from being used, and the rotor inertia enhances the frequency stability; the double-winding asynchronous motor is high in overload capacity, the short-circuit current level is improved, and excess reactive current is output in a short time to enhance the voltage supporting capacity.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy power generation, and relates to a new energy grid-connected device with a dual-winding asynchronous motor interface and a control method thereof. Background Art

[0002] In recent years, the global energy structure has been accelerating its transformation towards clean and low-carbon, and the installed capacity of new energy power generation represented by wind power and photovoltaic has shown exponential growth. However, the large-scale access of new energy to the power grid not only brings about a reduction in carbon emissions, but also triggers a fundamental change in the stable operation mechanism of the power system. The stable framework of the traditional power system based on the rotational inertia and electromagnetic characteristics of synchronous generator sets is being subverted by the "static power system" composed of a large number of grid-connected inverters. The resulting problems such as broadband oscillation, inertia loss, and short-circuit capacity decline have become the key bottlenecks restricting new energy consumption and power system security.

[0003] The interaction between the grid-connected inverter and the power grid has triggered a series of new broadband oscillation problems, with the frequency band covering the sub-synchronous (10 - 100 Hz) to super-synchronous (100 - 1000 Hz) range, seriously threatening the safe and stable operation of the system. LC / LCL filters and phase-locked loops are key components of grid-connected inverters, but they are also the main factors causing broadband oscillation. After the inherent resonance frequency of the LC / LCL filter is coupled with the grid impedance, multiple resonance points are formed, which are easily excited to generate oscillation. The phase-locked loop not only affects the stability of the system with its own dynamic characteristics, but also the interaction between it and other control links is likely to cause oscillation in the system. In the traditional power system, the control of generators does not require LC / LCL filters and phase-locked loops, so the stability is relatively high and the possibility of generating broadband oscillation is small.

[0004] In the traditional power system, the rotating rotor of the generator set has mechanical inertia characteristics. When a power disturbance occurs in the system, the rotor can release or absorb energy on the time scale of dozens of seconds through the autonomous conversion of kinetic energy - electrical energy. This physical inertia provides a natural buffer for frequency stability. When new energy is grid-connected through a grid-connected inverter, its energy conversion is completely controlled by power electronic devices, losing the inertial response ability of the rotating mass. The dangerous situation where the frequency of the UK power grid dropped to 49.5 Hz in 2022 is a typical case of the lag in primary frequency modulation response caused by insufficient inertia reserve.

[0005] During a fault, the generator can provide a short-circuit current of 6 to 10 times the rated current, preventing the voltage at the fault point from collapsing immediately. This strong short-circuit capacity can provide reliable action signals for the relay protection device, ensuring rapid fault removal. The grid-connected inverter is restricted by the thermal limit of semiconductor devices and can usually only provide a short-circuit current of 1.2 to 1.5 times the rated current, with a duration not exceeding 100 ms. The decrease in the short-circuit current level may cause current differential protection, overcurrent protection, etc. to fail to operate reliably. In addition, insufficient short-circuit capacity leads to a decrease in voltage support ability during a fault and a slowdown in the voltage recovery speed, which may trigger a chain of new energy grid disconnection accidents.

[0006] In addition, with the development of new energy, the new energy grid-connected power generation system guidelines have set high standards for the reactive current provided by new energy power stations during low-voltage ride-through. The "Technical Regulations for the Connection of Wind Farms to the Power System - Part 1: Onshore Wind Power" implemented in 2021 requires that for every 1% drop in the grid voltage, the wind farm needs to increase the injection of reactive current into the grid by 1.5% - 3%. The "Technical Regulations for the Connection of Photovoltaic Power Stations to the Power System" implemented in 2024 requires that for every 1% drop in the grid voltage, the photovoltaic power station needs to increase the injection of reactive current into the grid by 1.3% - 1.5%. It can be seen that in the extreme case where the grid voltage drops to zero, the new energy power station needs to provide reactive current exceeding the rated current to the grid. To meet the requirements of the new energy grid-connected power generation system guidelines, new energy power stations generally choose to overconfigure or specially equip reactive devices at present, which reduces the power generation efficiency and increases a large amount of costs.

[0007] The above analysis shows that problems such as broadband oscillation, inertia loss, and short-circuit capacity decline are caused by the replacement of a large number of generators by grid-connected inverters, weakening the motor characteristics of the power system. Therefore, to solve the above problems at the root, a grid-connected device is needed that can realize the new energy grid-connected function without weakening the motor characteristics of the power system. At the same time, the grid-connected device needs to have the ability to provide excess reactive current. Summary of the Invention

[0008] The technical solution of the present invention solves the problems of broadband oscillation, inertia loss, short-circuit capacity decline, and weak transient reactive current support ability brought by a high proportion of grid-connected inverters to the power system.

[0009] The present invention solves the above technical problems through the following technical solutions:

[0010] The present invention provides a new energy grid-connected device with a dual-winding asynchronous motor interface, including: two three-phase inverters and a switching switch;

[0011] The stator of the dual-winding asynchronous motor includes: a control winding and a grid-connected winding;

[0012] The three-phase output terminals on the AC side of the first three-phase inverter are directly connected to the starting ends of the control windings.

[0013] The switching switch includes three groups of switches. The first group of switches is connected between the ending end of the control winding and the second three-phase inverter. The second group of switches is connected in parallel between the ending ends of the control windings. The third group of switches is connected between the three-phase output terminals on the AC side of the two three-phase inverters.

[0014] The grid-connected winding of the dual-winding asynchronous motor is connected to the power grid.

[0015] The rotor of the dual-winding asynchronous motor is not connected to other devices and is in a rotating state during operation.

[0016] Further, the first group of switches includes: switch K 1a , switch K 1b and switch K 1c . The second group of switches includes: switch K 2ab and switch K 2bc . The third group of switches includes: switch K 3a , switch K 3b , switch K 3c . The three-phase output terminals on the AC side of the first three-phase inverter are connected to the three-phase output terminals on the AC side of the second three-phase inverter through switch K 3a , switch K 3b , switch K 3c correspondingly. The three-phase output terminals on the AC side of the second three-phase inverter are respectively connected to the a end, b end, c end of the control winding through switch K 1a , switch K 1b , switch K 1c . The two ends of switch K 2ab are connected in parallel between the a end and b end of the control winding. The two ends of switch K 2bc are connected in parallel between the b end and c end of the control winding.

[0017] Further, the new energy grid-connected device of the dual-winding asynchronous motor interface further includes: a grid-connected switch for controlling the disconnection and connection between the grid-connected winding and the power grid.

[0018] Further, the grid-connected switch includes: switch K 4u , switch K 4v , switch K 4w . The grid-connected winding is connected to the power grid through switch K 4u , switch K 4v , switch K 4w .

[0019] Further, the new - energy grid - connection device of the dual - winding asynchronous motor interface further includes: two new - energy power generation units, and the DC sides of the two inverters are respectively connected to the two new - energy power generation units.

[0020] Further, the new - energy power generation unit is one of wind power, photovoltaic power, or energy storage power station, or a combination of two or three of the above new - energy power generation forms.

[0021] Further, the spatial distribution offset angle between the control winding and the grid - connection winding is 30 degrees or 60 degrees.

[0022] Further, the capacity of the three - phase inverter is half of the rated power of the dual - winding asynchronous motor, the rated voltage of the three - phase inverter is half of the rated voltage of the dual - winding asynchronous motor, and the rated current of the three - phase inverter is the same as that of the dual - winding asynchronous motor.

[0023] The present invention also provides a control method for the new - energy grid - connection device applied to the above - mentioned dual - winding asynchronous motor interface. In the steady state, the control winding of the dual - winding asynchronous motor is in the form of an open winding. The head end of the control winding is connected to the first three - phase inverter, and the tail end of the control winding is connected to the second three - phase inverter. In the transient state, the switching switch converts the control winding into a star connection. The head end of the control winding is connected to two parallel three - phase inverters, and the tail ends of the control winding are short - circuited together. At this time, the connection form of the two three - phase inverters is converted from each connecting a three - phase inverter at the head and tail ends of the control winding to two three - phase inverters being connected in parallel at the head end of the control winding.

[0024] Further, the method for the switching switch to convert the control winding into a star connection is: all the first - group switches are opened, all the second - group switches are closed, and all the third - group switches are closed.

[0025] The beneficial effects of the present invention are as follows:

[0026] The present invention innovatively uses a dual - winding asynchronous motor as the grid - connection interface and introduces the dual - winding asynchronous motor into the new - energy power generation system. It can avoid using LC / LCL filters and phase - locked loops, and can effectively suppress the occurrence of broadband oscillations. The rotor inertia of the dual - winding asynchronous motor enhances the frequency stability. The dual - winding asynchronous motor has a strong overload capacity, can improve the short - circuit current level, increase the short - circuit capacity, and enhance the stable operation ability of the high - proportion new - energy power system. In the transient state, the switching switch switches the two three - phase inverters to be in parallel, which can output excess reactive current in a short time and enhance the voltage support ability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the structural diagram of the new - energy grid - connection device of the dual - winding asynchronous motor interface;

[0028] Figure 2It is the working state diagram of the new energy grid-connected device with a dual-winding asynchronous motor interface in the steady state stage;

[0029] Figure 3 It is the working state diagram of the new energy grid-connected device with a dual-winding asynchronous motor interface in the transient state stage. Specific implementation manners

[0030] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0031] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings of the specification and specific embodiments:

[0032] Embodiment 1

[0033] As Figure 1 shown, a new energy grid-connected device with a dual-winding asynchronous motor interface according to an embodiment of the present invention includes: two new energy power generation units, two three-phase inverters, a dual-winding asynchronous motor, a switching switch, and a grid-connected switch; the switching switch includes: switch K 1a 、switch K 1b 、switch K 1c 、switch K 2ab 、switch K 2bc 、switch K 3a 、switch K 3b 、switch K 3c ; the grid-connected switch includes: switch K 4u 、switch K 4v 、switch K 4w .

[0034] The dual-winding asynchronous motor has two sets of three-phase stator windings, namely a control winding and a grid-connected winding. There is no direct electrical contact between the control winding and the grid-connected winding, and energy is transferred through electromagnetic induction; the spatial distribution offset angle between the control winding and the grid-connected winding can be 30 degrees or 60 degrees; the rotor of the dual-winding asynchronous motor is not connected to other devices and does not output torque. When working, it rotates at a synchronous speed following the grid frequency. The inertia of the rotor can suppress the grid frequency fluctuation, and the grid frequency information can be obtained by measuring the speed through an encoder, avoiding the use of a phase-locked loop.

[0035] The DC sides of the two three-phase inverters are respectively connected to the two new energy power generation units; the three-phase output terminals on the AC side of the first three-phase inverter pass through switch K 3a 、switch K3b and the switch K 3c is connected to the three-phase output terminals on the AC side of the second three-phase inverter; the three-phase output terminals on the AC side of the first three-phase inverter are directly connected to the A terminal, B terminal, and C terminal of the control winding respectively, and the three-phase output terminals on the AC side of the second three-phase inverter are respectively connected to the a terminal, b terminal, and c terminal of the control winding through the switch K 1a and the switch K 1b and the switch K 1c are connected to the a terminal, b terminal, and c terminal of the control winding; both ends of the switch K 2ab are connected in parallel between the a terminal and b terminal of the control winding; both ends of the switch K 2bc are connected in parallel between the b terminal and c terminal of the control winding; the grid-connected winding is connected to the power grid through the switch K 4u and the switch K 4v and the switch K 4w ; the switches K 4u and the switch K 4v and the switch K 4w are used to control the disconnection and connection between the grid-connected winding and the power grid.

[0036] The control winding connects two three-phase inverters. In the steady state, the control winding is in the open-winding form. The head end of the control winding is connected to the first three-phase inverter, and the A terminal, B terminal, and C terminal of the control winding are defined as the head end here; the tail end of the control winding is connected to the second three-phase inverter, and the a terminal, b terminal, and c terminal of the control winding are defined as the tail end here; in this way, the two three-phase inverters can jointly provide voltage for the control winding.

[0037] During the transient state, when the grid voltage drops, the control winding is converted to the star connection method by using the switching switch. The head end of the control winding is connected to two parallel three-phase inverters, and the tail ends of the control winding are shorted together. At this time, the connection form of the two three-phase inverters is changed from each of the head and tail ends of the control winding connecting one to two connected in parallel at the head end of the control winding, which can improve the ability of the double-winding asynchronous motor to deliver reactive current to the power grid.

[0038] The method of converting the control winding to the star connection method by using the switching switch is as follows: the switches K 1a and the switch K 1b and the switch K 1c are all disconnected, the switches K 2ab and the switch K 2bc are all closed, and the switches K 3a and the switch K 3b and the switch K 3c are all closed.

[0039] The new energy power generation unit can be one of wind power, photovoltaic, and energy storage power stations, or a combination of two or three of these new energy power generation forms. The two new energy power generation units respectively supply power to the DC sides of the two three-phase inverters.

[0040] The three-phase inverter is a three-phase inverter composed of fully controlled devices. The two three-phase inverters have the same capacity, which is half of the rated power of the double-winding asynchronous motor. The rated voltage of the three-phase inverter is half of the rated voltage of the double-winding asynchronous motor, and the rated current of the three-phase inverter and the double-winding asynchronous motor is the same. It is used to control the grid-connected power and the output characteristics of the double-winding asynchronous motor.

[0041] Embodiment 2

[0042] The present invention also proposes a control method for a new energy grid-connected device applied to the interface of a double-winding asynchronous motor, including a starting stage, a steady state stage, and a transient stage.

[0043] (1) When the new energy grid-connected device at the interface of the double-winding asynchronous motor is in the starting stage

[0044] At this time, switch K 1a , switch K 1b , switch K 1c are all closed, and switch K 2ab , switch K 2bc , switch K 3a , switch K 3b , switch K 3c are all open; two three-phase inverters are used to provide voltage to the control winding of the double-winding asynchronous motor to start the double-winding asynchronous motor. To avoid excessive starting current, the motor speed is gradually increased in a ramp manner to the rated speed; after the motor speed reaches the rated speed, the voltage frequency of the control winding is the rated frequency, that is, 50 Hz. Since the control winding and the grid-connected winding are in the same rotating magnetic field, the voltage induced in the grid-connected winding is also 50 Hz; the excitation current of the control winding is adjusted so that the voltage amplitude of the grid-connected winding is equal to the grid voltage amplitude; when the phase sequence, amplitude, frequency, and phase of the grid-connected winding voltage are the same as those of the grid voltage, close switch K 4u , switch K 4v , switch K 4w , and the double-winding asynchronous motor is connected to the grid and enters the steady-state operation stage.

[0045] (2) When the new energy grid-connected device at the interface of the double-winding asynchronous motor is in the steady state stage

[0046] The working state of the new energy grid-connected device at the interface of the double-winding asynchronous motor in the steady state stage is as Figure 2As shown, the control winding absorbs energy from the new energy power generation unit through a three-phase inverter, and the grid-connected winding transmits energy to the power grid. The three-phase inverter controls the type (active and reactive) and magnitude of the energy. The two three-phase inverters adopt a power sharing method, that is, the energy input to the dual-winding induction motor, whether it is reactive power or active power, is evenly divided by the two three-phase inverters, and each three-phase inverter provides half of the power. The voltage of the control winding is equal to the difference between the output voltages of the two three-phase inverters at its head and tail ends. Specifically:

[0047]

[0048] where u Aa 、u Bb 、u Cc are the phase voltages of the control winding, u A 、u B 、u C are the voltages output by the three-phase inverter connected to the head end of the control winding, and u a 、u b 、u c are the voltages output by the three-phase inverter connected to the tail end of the control winding.

[0049] The two three-phase inverters are controlled to output voltages in antiphase. The amplitude of the control winding voltage is the sum of the amplitudes of the voltages output by the two three-phase inverters. The amplitude of the control winding voltage can reach twice the amplitude of the voltage output by the three-phase inverter, making full use of the advantage that the open winding can improve the voltage output ability.

[0050] (3) When the new energy grid-connected device at the interface of the dual-winding induction motor is in the transient stage

[0051] When the grid voltage drops by more than 50% due to a fault, it enters the transient stage. After the grid voltage drops, the voltage required by the control winding from the three-phase inverter will also decrease. At this time, the topology structure is changed by using the changeover switch. First, switch K 1a , switch K 1b and switch K 1c are all opened to cut off the connection between the tail end of the control winding and the second three-phase inverter; then switch K 2ab and switch K 2bc are both closed to convert the control winding from the open winding form to the star connection; finally, switch K 3a , switch K 3b and switch K 3c are all closed to parallel the first three-phase inverter and the second three-phase inverter at the head end of the control winding.

[0052] After the switching is completed, the working state of the new energy grid-connected device at the interface of the dual-winding induction motor in the transient stage is as Figure 3As shown, two parallel three-phase inverters jointly provide reactive current for the control winding, specifically:

[0053]

[0054] where i Aa , i Bb , i Cc are the phase currents of the control winding, and i A , i B , i C and i a , i b , i c are the currents output by the two three-phase inverters respectively.

[0055] When two three-phase inverters are connected in parallel, the current output capacity of the control winding is the sum of the rated currents of the two three-phase inverters, reaching up to twice the rated current of the three-phase inverter, making full use of the overload capacity of the double-winding asynchronous motor to output reactive current greater than the rated current for a short time in the transient stage to support the grid voltage.

[0056] The new energy grid-connected device of the present invention does not use a traditional inverter as the grid-connected interface. Instead, it innovatively uses a double-winding asynchronous motor as the grid-connected interface, introducing the motor into the new energy power generation system. The stator winding of the double-winding asynchronous motor itself has a filtering function, eliminating the need for additional LC / LCL filters. Moreover, it can detect the rotor speed using an encoder to obtain frequency information, avoiding the use of a phase-locked loop, thus effectively suppressing the occurrence of broadband oscillations. The double-winding asynchronous motor is a rotating device that can increase the inertia in the power system and enhance frequency stability. The double-winding asynchronous motor has the same overload capacity as a traditional generator, capable of increasing the short-circuit current level and short-circuit capacity. Combined with the control method, it can switch the topology when the grid voltage drops, converting the two three-phase inverters into a parallel connection at the head end of the control winding, enhancing the reactive current output capacity, making full use of the overload capacity of the double-winding asynchronous motor to output reactive current greater than the rated current for a short time in the transient stage to support the grid voltage.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some 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 invention.

Claims

1. A new energy grid-connected device with a dual-winding asynchronous motor interface, characterized in that: include: Two three-phase inverters, transfer switches; The stator of the dual-winding asynchronous motor comprises: a control winding and a grid-connected winding; The three-phase output terminal of the AC side of the first three-phase inverter is directly connected to the first end of the control winding. The switching switch includes three groups of switches, the first group of switches is connected between the tail end of the control winding and the second three-phase inverter, the second group of switches is connected in parallel between the tail ends of the control winding, and the third group of switches is connected between the three-phase output ends of the AC side of the two three-phase inverters; The grid-connected winding of the dual-winding asynchronous motor is connected to the power grid; The rotor of the dual-winding asynchronous motor is not connected to other devices and is in a rotating state during operation.

2. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 1, characterized in that: The first group of switches includes: switch K 1a , switch K 1b and switch K 1c , the second set of switches includes: switch K 2ab and switch K 2bc , the third switch group includes: switch K 3a , switch K 3b , switch K 3c ; The three-phase output terminal of the first three-phase inverter on the AC side is connected through switch K 3a , switch K 3b , switch K 3c The three-phase output terminals of the second three-phase inverter on the AC side are connected to the three-phase output terminals of the second three-phase inverter on the AC side through switches K 1a , switch K 1b , switch K 1c Connect to the a, b and c terminals of the control winding, switch K 2ab The two ends of the control winding are connected in parallel at ends a and b, and the switch K 2bc The two ends are connected in parallel at the b and c ends of the control winding.

3. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 1, characterized in that: Also includes: The grid-connected switch is used to control the disconnection and connection of the grid-connected winding with the power grid.

4. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 3 is characterized in that: The grid-connected switch includes: switch K 4u , switch K 4v , switch K 4w , the grid-connected winding is connected to the grid through a switch K 4u , switch K 4v , switch K 4w connect.

5. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 1, characterized in that: Also includes: Two new energy power generation units, the DC sides of the two inverters are respectively connected to the two new energy power generation units.

6. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 5, characterized in that: The new energy power generation unit is one of wind power, photovoltaic or energy storage power station, or a combination of two or three of the new energy power generation forms.

7. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 1, characterized in that: The spatial distribution offset angle between the control winding and the grid-connected winding is 30 degrees or 60 degrees.

8. The new energy grid-connected device of the dual-winding asynchronous motor interface according to claim 1, characterized in that: The capacity of the three-phase inverter is half of the rated power of the dual-winding asynchronous motor, the rated voltage of the three-phase inverter is half of the rated voltage of the dual-winding asynchronous motor, and the rated current of the three-phase inverter and the dual-winding asynchronous motor are the same.

9. A control method for a new energy grid-connected device applied to a dual-winding asynchronous motor interface as claimed in any one of claims 1 to 8, characterized in that: In steady state, the control winding of the dual-winding asynchronous motor is in an open winding form, the first end of the control winding is connected to the first three-phase inverter, and the tail end of the control winding is connected to the second three-phase inverter; In transient state, the switching switch converts the control winding into a star connection, the head end of the control winding is connected to two three-phase inverters in parallel, and the tail end of the control winding is short-circuited together; at this time, the connection form of the two three-phase inverters is converted from connecting one three-phase inverter at each end of the control winding to two three-phase inverters in parallel at the head end of the control winding.

10. The control method according to claim 9, characterized in that: The method of converting the control winding into a star connection by the switching switch is as follows: the first group of switches are all disconnected, the second group of switches are all closed, and the third group of switches are all closed.

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