Photovoltaic grid-connected system and control method thereof

Abstract

The application provides a photovoltaic grid-connected system and a control method thereof. The photovoltaic grid-connected system is connected to a grid by a synchronous machine pair system formed by two synchronous machines. The synchronous machine has strong voltage and current resistance and transient overload capacity, can continuously operate during grid failure, and has inherent mechanical rotational inertia. The photovoltaic grid-connected system greatly improves the stability and safety of the grid and reduces the impact of new energy intermittency on the grid. Moreover, the control method controls the photovoltaic inverter to operate in a current source mode after the photovoltaic grid-connected system is started. Meanwhile, the synchronous machine pair system is controlled to operate in a generator mode. The excitation current of the first synchronous machine is controlled to maintain the stability of the alternating current side voltage of the photovoltaic inverter. The maximum power point of the photovoltaic array can be tracked, power generation loss can be avoided, and a traditional photovoltaic inverter can be used without modification.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of photovoltaic grid-connected technology, in particular to a photovoltaic grid-connected system and a control method thereof. BACKGROUND

[0002] With the promotion of the transformation from fossil energy to renewable energy, the penetration rate of renewable energy dominated by photovoltaic and wind power is rapidly increasing worldwide, and the installed capacity proportion of conventional synchronous generators, especially coal-fired steam turbines, will decrease year by year. The decrease of the capacity proportion of synchronous generators will lead to the decrease of the inertia of the power system, the insufficient support of transient voltage and current, and thus the decrease of the stability of the system, which cannot guarantee the safety of the grid frequency in extreme weather or sudden failure; at the same time, a large number of new energy units are prone to be off the grid when a fault occurs, thereby causing regional large-scale power outages. SUMMARY

[0003] Therefore, the present application provides a photovoltaic grid-connected system and a control method thereof, which provides system inertia support for the power grid and improves the stability and safety of the power grid.

[0004] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0005] The first aspect of the present application provides a control method of a photovoltaic grid-connected system, the photovoltaic grid-connected system comprising, in series connection: a photovoltaic array, a photovoltaic inverter, a synchronous machine pair drag system and a transformer, the other side of the transformer being connected to a power grid; in the synchronous machine pair drag system, the rotor shaft of a first synchronous motor and the rotor shaft of a second synchronous motor are connected through a mechanical shaft; the control method comprising:

[0006] After the photovoltaic grid-connected system is started, the photovoltaic inverter is controlled to operate in a current source mode.

[0007] The synchronous machine pair drag system is controlled to operate in a generator mode, and the AC side voltage of the photovoltaic inverter is maintained stable by controlling the excitation current of the first synchronous motor.

[0008] Optionally, controlling the photovoltaic inverter to operate in a current source mode comprises:

[0009] Controlling the photovoltaic inverter to operate in a maximum power point tracking (MPPT) current source control mode.

[0010] Optionally, controlling the photovoltaic inverter to operate in a current source mode further comprises:

[0011] When receiving an active scheduling instruction and the active power instruction value is less than the maximum power of the photovoltaic array, the photovoltaic inverter is controlled to operate in a current source mode according to the active power instruction value.

[0012] Optionally, the control method further comprises:

[0013] According to the received reactive power instruction value in the reactive power instruction, the corresponding power factor is realized by controlling the excitation current of the second synchronous motor.

[0014] Optionally, the control method further comprises:

[0015] When the stator winding of the first synchronous motor has no input power, the synchronous motor system is controlled to work in the synchronous phase modifier mode.

[0016] Optionally, the photovoltaic grid-connected system further comprises a black start system, a first switch, a second switch and a third switch; the first switch is arranged between the AC side of the photovoltaic inverter and the stator winding of the first synchronous motor, the black start system is connected to the stator winding of the first synchronous motor through the third switch, and the second switch is arranged between the stator winding of the second synchronous motor and the transformer; before the photovoltaic inverter is controlled to operate in the current source mode, the control method further comprises:

[0017] The black start system, the first switch, the second switch and the third switch are controlled to act respectively, so that the photovoltaic grid-connected system is started.

[0018] Optionally, the control method further comprises:

[0019] The third switch is controlled to be closed, so that the black start system is connected to the stator winding of the first synchronous motor, and a black start process is performed;

[0020] When the stator rotating speed of the synchronous motor system reaches the grid synchronous rotating speed, the second switch is controlled to be closed, and the third switch is controlled to be opened, so that the synchronous motor system works in the synchronous phase modifier mode.

[0021] When the photovoltaic array meets the power generation condition, the stator winding terminal voltage of the first synchronous motor is adjusted to the grid-connected voltage operating range of the photovoltaic inverter by controlling the excitation current of the first synchronous motor.

[0022] When the photovoltaic inverter meets the starting condition, the AC side output of the photovoltaic inverter is adjusted to AC power synchronous with the stator winding terminal voltage of the first synchronous motor, and the first switch is controlled to be closed.

[0023] Optionally, the control method further comprises:

[0024] When the photovoltaic inverter meets a shutdown condition, the photovoltaic inverter is controlled to stop running, the first switch is turned off, and the stator winding of the first synchronous motor is not supplied with input power.

[0025] The second aspect of the present application provides a photovoltaic grid-connected system, comprising a photovoltaic array, at least one photovoltaic inverter, a synchronous motor pair system, a transformer, and a control system, wherein,

[0026] The synchronous motor pair system comprises a first synchronous motor and a second synchronous motor, and the rotors of the two synchronous motors are connected through a mechanical shaft.

[0027] The direct current side of the photovoltaic inverter is connected to a corresponding photovoltaic group string in the photovoltaic array.

[0028] The alternating current side of the photovoltaic inverter is connected to the stator winding of the first synchronous motor.

[0029] The stator winding of the second synchronous motor is connected to the power grid through the transformer.

[0030] The control system is used to perform the control method of the photovoltaic grid-connected system according to any one of the first aspect.

[0031] Optionally, the photovoltaic grid-connected system further comprises a black start system, a first switch, a second switch, and a third switch.

[0032] The first switch is arranged between the alternating current side of the photovoltaic inverter and the stator winding of the first synchronous motor.

[0033] The black start system is connected to the stator winding of the first synchronous motor through the third switch.

[0034] The second switch is arranged between the stator winding of the second synchronous motor and the transformer.

[0035] Optionally, the control system comprises:

[0036] A first controller integrated in the photovoltaic inverter is used to realize operation control of the photovoltaic inverter.

[0037] A second controller integrated in the first synchronous motor is used to realize control of excitation current of the first synchronous motor.

[0038] A third controller integrated in the second synchronous motor is used to realize control of excitation current of the second synchronous motor.

[0039] A fourth controller integrated in the black start system is used to control the black start system to perform black start.

[0040] The first controller is in communication connection with the second controller, the third controller and the fourth controller respectively.

[0041] The control method of the photovoltaic grid-connected system provided in the application increases a synchronous machine pair system composed of two synchronous machines in the photovoltaic grid-connected system to which the control method is applied, the rotors of the two synchronous machines are connected through a mechanical shaft; further, the photovoltaic inverter is connected to the grid through the synchronous machine pair system composed of two synchronous machines, compared with the traditional scheme that the photovoltaic array is directly connected to the grid through the photovoltaic inverter, the application has the characteristics of strong voltage and current resistance, transient overload capacity, continuous operation during power grid failure and inherent mechanical moment of inertia by virtue of the synchronous machine, which greatly improves the stability and safety of the power grid and reduces the impact of new energy intermittency on the power grid. Moreover, the control method controls the photovoltaic inverter to operate in a current source mode after the photovoltaic grid-connected system is started, at the same time, controls the synchronous machine pair system to operate in a generator mode, and maintains the AC side voltage of the photovoltaic inverter stable by controlling the excitation current of the first synchronous machine, thereby the maximum power point of the photovoltaic array can be tracked, the power generation loss can be avoided, and the traditional photovoltaic inverter can be used to realize the control method without modification. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of the provided drawings.

[0043] Figure 1 The structural schematic diagram of the photovoltaic grid-connected system provided in the embodiment of the present application;

[0044] Figure 2 The flowchart of the control method of the photovoltaic grid-connected system provided in the embodiment of the present application;

[0045] Figure 3 The waveform schematic diagram of the magnetization curve of the synchronous machine provided in the embodiment of the present application;

[0046] Figure 4 The control block diagram of the photovoltaic inverter provided in the embodiment of the present application;

[0047] Figure 5 Another flowchart of the control method of the photovoltaic grid-connected system provided in the embodiment of the present application;

[0048] Figure 6 The V-shaped curve diagram of the synchronous generator provided in the embodiment of the present application;

[0049] Figure 7 A V-shaped curve diagram of a synchronous condenser provided for an embodiment of the present application;

[0050] Figure 8 Another structural schematic diagram of a photovoltaic grid-connected system provided for an embodiment of the present application;

[0051] Figure 9 Another flowchart of a control method of a photovoltaic grid-connected system provided for an embodiment of the present application;

[0052] Figure 10a 、 Figure 10b and Figure 10c are structural schematic diagrams of a photovoltaic grid-connected system in different states respectively provided for an embodiment of the present application. DETAILED DESCRIPTION

[0053] 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, but not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present application.

[0054] In the present application, the terms “comprising”, “containing” or any other variants thereof are intended to cover non-exclusive containing, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment. Without more limitations, the element defined by the sentence “including a…” does not exclude the presence of other identical elements in the process, method, article or equipment including the element.

[0055] At present, due to the high proportion of wind power and photovoltaic power generation, the system inertia is low, so that the power grid frequency safety cannot be guaranteed in extreme weather or sudden failure, and a large number of new energy units are off the grid when the fault occurs, thereby causing regional large-scale power outages.

[0056] In the traditional photovoltaic grid-connected scheme, photovoltaic components output direct current, which is converted into alternating current by a power electronic converter and then connected to the grid. In such a system, there is a lack of rotating mechanical inertia like the rotor of a synchronous generator, which cannot provide natural inertia support for the system. Therefore, the present application provides a control method of a photovoltaic grid-connected system, which increases a synchronous machine pair drag system composed of two synchronous machines in the photovoltaic grid-connected system to provide system inertia support for the power grid and improve the stability and safety of the power grid.

[0057] As Figure 1As shown, the photovoltaic grid-connected system includes, in series: a photovoltaic array 101, a photovoltaic inverter 102, a synchronous machine pair drag system 103, and a transformer 104, the other side of the transformer 104 being connected to a power grid; in the synchronous machine pair drag system 103, the rotor shaft of a first synchronous machine M1 and the rotor shaft of a second synchronous machine M2 are connected through a mechanical shaft, so that the two always maintain the same speed.

[0058] As shown, Figure 2 The control method includes the following steps performed after the photovoltaic grid-connected system is started:

[0059] S101, control the photovoltaic inverter to operate in current source mode.

[0060] After the system is started, the photovoltaic inverter 102 is controlled to operate in current source mode, such as the commonly used MPPT (Maximum Power Point Tracking) current source control mode, so that it can track the maximum power point of the photovoltaic array 101 and all the power generated on the photovoltaic side can be connected to the grid; compared with the power conversion device controlled by the voltage source mode through the frequency droop control strategy, step S101 can avoid the loss of power generation.

[0061] Since the output power of the photovoltaic inverter 102 will change with the change of irradiance, at this time the stator current of the first synchronous machine M1 will also change, in order to ensure that the stator winding voltage of the first synchronous machine M1 is constant, step S102 needs to be performed.

[0062] S102, control the synchronous machine pair drag system to operate in generator mode, and maintain the stability of the alternating current side voltage of the photovoltaic inverter by controlling the field current of the first synchronous machine.

[0063] The synchronous machine stator voltage equation is:

[0064] U=E0-IR a -jI d X d -jI q X q (1)

[0065] Wherein, U is the effective value of the synchronous machine stator winding voltage, E0 is the field electromotive force, I is the effective value of the stator current, R a is the stator armature resistance, I d , I q are the quadrature axis and direct axis components of the stator current, X d , X q are the quadrature axis and direct axis reactance of the stator winding.

[0066] It can be seen from the calculation formula (1) that the stator winding terminal voltage is related to the excitation electromotive force E0 and the stator current I. In order to ensure that the stator winding terminal voltage remains unchanged, when the stator current I changes, the excitation electromotive force E0 can be adjusted to ensure that the stator terminal voltage remains unchanged. Here:

[0067] E0 = 4.44fNk N Φ0 (2)

[0068] Wherein, f is the main magnetic flux cutting the stator winding to induce three-phase fundamental electromotive force, N is the number of turns of each phase of the stator winding in series, k N is the winding coefficient, and Φ0 is the air gap main magnetic flux. At the rated frequency, f, N, and k N are all constants, so as shown in Figure 3 , changing the excitation current I f can obtain different air gap main magnetic fluxes Φ0, thereby obtaining different excitation electromotive forces E0, and then realizing the stability of the stator voltage of the first synchronous motor M1 in the synchronous motor-diesel system 103, that is, the stability of the alternating current side voltage of the photovoltaic inverter 102. Figure 3 Ff in the formula is the frequency.

[0069] Compared with the traditional scheme of directly connecting the photovoltaic array to the grid through the photovoltaic inverter, the photovoltaic inverter in the present application is connected to the grid through a synchronous motor-diesel system composed of two synchronous motors, and then the characteristics of the synchronous motor, such as strong voltage and current resistance, transient overload capacity, continuous operation during grid failure, and inherent mechanical rotational inertia, can greatly improve the stability and safety of the grid and reduce the impact of new energy intermittency on the grid.

[0070] The control method of the photovoltaic grid-connected system provided in the embodiment can track the maximum power point of the photovoltaic array through the above principle, and avoid loss of power generation. Moreover, the photovoltaic inverter in the embodiment can be realized by using a traditional photovoltaic inverter without modification.

[0071] It is worth noting that there is also a virtual synchronous machine control technology in the prior art to make power electronic devices simulate the inertia, damping and other characteristics of synchronous motors, but due to problems such as multi-machine parallel operation, transient overcurrent capacity, and voltage recovery after low voltage ride through, the technology has not yet been maturely applied. Even if the virtual synchronous machine control technology is mature, its inertia support speed is still delayed compared with the natural mechanical inertia of the synchronous motor, and it still cannot meet the requirements of system frequency stability under the limit condition of nearly 100% new energy penetration.

[0072] The photovoltaic grid-connected system to which the control method of the photovoltaic grid-connected system provided in the embodiment is applied has inherent mechanical rotational inertia of the synchronous motor, and its inertia support speed is fast, meeting the requirements of system frequency stability.

[0073] Based on the previous embodiment, step S101 may optionally be: controlling the photovoltaic inverter to operate in MPPT current source control mode; or, if an active power dispatch command is received and the active power command value is less than the maximum power of the photovoltaic array, step S101 may also be: controlling the photovoltaic inverter to operate in current source mode according to the active power command value.

[0074] The control strategy for this photovoltaic inverter can be to adopt... Figure 4 The conventional voltage and current dual-loop control shown is the same as the existing technology, and will not be described in detail here. Wherein, v dc This refers to the actual voltage on the DC side of the photovoltaic inverter 102, i dc This refers to the actual current on the DC side of the photovoltaic inverter 102, v* dc This refers to the DC voltage reference value, i d This refers to the d-axis component of the AC side current of the photovoltaic inverter 102, i d * refers to the reference value of the d-axis component of the AC side current of photovoltaic inverter 102, i q This refers to the q-axis component of the AC side current of the photovoltaic inverter 102, i q * refers to the reference value of the q-axis component of the AC side current of photovoltaic inverter 102, e d e q These refer to the d-axis and q-axis components of the grid voltage, respectively. d v q These refer to the d-axis and q-axis components of the voltage command value, respectively. a S b S c These refer to the three-phase pulse width modulation waves A, B, and C, respectively. a e b e c This refers to the AC voltage of the three-phase windings A, B, and C of the stator of the synchronous machine. a i b i c This refers to the three-phase current on the AC side of photovoltaic inverter 102, and θ refers to the phase angle of the voltage of the stator winding of the first synchronous motor. In practical applications, the photovoltaic inverter can be controlled using either MPPT mode or power command mode; different modes result in different system states, specifically:

[0075] State 1: the photovoltaic power station generates electricity, the photovoltaic inverter 102 works in the MPPT mode, and the synchronous machine-drag system 103 operates in the generator mode. At this time, the magnetic field generated by the stator three-phase current of the first synchronous machine M1 in the synchronous machine-drag system 103 leads the rotor angle θrad of the first synchronous machine M1, the first synchronous machine M1 works in the motor mode, and serves as a prime mover to convert the electrical power input by the photovoltaic inverter 102 into mechanical power and transmit the mechanical power to the second synchronous machine M2 through the rotating shaft. At this time, the rotor angle of the second synchronous machine M2 leads the stator magnetic field angle, and the second synchronous machine M2 is in the generator state to convert the mechanical power transmitted by the first synchronous machine M1 into electrical power and deliver the electrical power to the power grid.

[0076] Compared with the photovoltaic inverter directly connected to the power grid, the photovoltaic grid-connected system has the characteristics of large system inertia and large short-circuit capacity in this state. Since the rotors of the first synchronous machine M1 and the second synchronous machine M2 naturally have mechanical rotational inertia, the system inertia constant H is 2-4 s. When the output power of the photovoltaic array 101 changes, the rotor speed of the second synchronous machine M2 cannot change abruptly due to inertia, thereby slowing down the system frequency change rate (RoCoF), providing more time for primary frequency modulation of the system, and increasing the stability of the grid frequency. The damping characteristics of the synchronous machine reduce the risk of grid resonance.

[0077] State 2: the photovoltaic power station generates electricity, the photovoltaic inverter 102 works in the power scheduling mode, and the synchronous machine-drag system 103 operates in the generator mode. When receiving an active scheduling instruction and the active power instruction value P is less than the maximum power of the photovoltaic array, the active power of the photovoltaic inverter 102 can be maintained at the instruction value P. At this time, the power angle of the first synchronous machine M1 and the second synchronous machine M2 remains constant, the second synchronous machine M2 outputs constant active power and is connected to the power grid.

[0078] It should be noted that the system may also receive a reactive scheduling instruction in the power scheduling mode, that is, in state 2, if the system also receives a reactive scheduling instruction, since the stator voltage of the first synchronous machine M1 is maintained by the excitation system of the first synchronous machine M1, the photovoltaic inverter 102 does not need to output reactive power; at this time, the reactive power instruction value will be transmitted to the excitation system of the second synchronous machine M2, and then the excitation current of the second synchronous machine M2 is adjusted to realize different power factors. That is, the control method can also include Figure 5 S103, according to the reactive power instruction value in the received reactive scheduling instruction, the corresponding power factor is realized by controlling the excitation current of the second synchronous machine. The relationship between the excitation current of the synchronous machine and the power factor is shown in Figure 6 . Figure 6 In the above formula, the vertical coordinate I is the armature current in the stator winding, the horizontal coordinate If is the excitation current in the rotor, and the power factor angle is the power factor angle. P0, P1, and P2 are the power factors. P0, P1, and P2 are the power of three different synchronous machines.

[0079] In practical applications, the system may also exist in another state, specifically:

[0080] State 3: The photovoltaic power station is not generating electricity. The photovoltaic inverter 102 is disconnected from the synchronous motor drive system 103, and the synchronous motor drive system 103 is operating in synchronous condenser mode.

[0081] The process of transitioning from state 1, 2 to state 3 is as follows:

[0082] (1) As the light intensity decreases, the mechanical power of the first synchronous motor M1 as the prime mover gradually decreases, the rotor decelerates, the power angle θ and electromagnetic power of the second synchronous motor M2 decrease, and when the power angle θ decreases to 0, the second synchronous motor M2 becomes unloaded, and its input power just offsets the unloaded loss.

[0083] (2) When the output power of the photovoltaic inverter 102 is further reduced, the power angle θ and the power of the second synchronous motor M2 become negative. The second synchronous motor M2 draws power from the grid to offset the no-load loss. The second synchronous motor M2 switches from generator mode to motor mode.

[0084] (3) When the sun sets completely, the photovoltaic power station does not generate electricity at night. At this time, the AC side of the photovoltaic inverter 102 no longer inputs power to the synchronous machine-to-motor system 103, and the synchronous machine-to-motor system 103 becomes a synchronous condenser that maintains synchronous speed idling.

[0085] Like synchronous generators, synchronous condensers can provide reactive power and voltage compensation to the power grid. Similar to synchronous motors in state 2, which achieve reactive power control through excitation current control, synchronous condensers also exhibit a similar V-shaped curve, such as... Figure 7 As shown, Figure 7 In the diagram, the vertical axis I represents the armature current in the stator winding, and the horizontal axis If represents the magnetizing current in the rotor. The power factor angle, P0, P1, and P2 are the power factors. P0, P1, and P2 are three different synchronous motor powers. Moreover, similar to synchronous generators, synchronous condensers can also provide strong voltage and current withstand capabilities, improve system inertia, optimize system frequency characteristics, and enhance system frequency stability and frequency support capabilities.

[0086] That is, in this control method, before and after any step, it may also include: when the stator winding of the first synchronous motor has no input power, controlling the synchronous motor to operate the tractor system in synchronous condenser mode.

[0087] Based on the above embodiments, in order to achieve the system's first startup, such as Figure 8As shown, the photovoltaic grid-connected system can further include: a black start system 105, a first switch S1, a second switch S2 and a third switch S3; wherein the first switch S1 is arranged between the AC side of the photovoltaic inverter 102 and the stator winding of the first synchronous motor M1, and can be a switch in the switch cabinet of the photovoltaic inverter 102; the black start system 105 is connected to the stator winding of the first synchronous motor M1 through the third switch S3; the second switch S2 is arranged between the stator winding of the second synchronous motor M2 and the transformer 104, and can be a grid-connected switch of the synchronous motor pair system 103.

[0088] At this time, the control method further includes, before step S101, as shown in the following: Figure 9 (For example, on the basis of the control method shown in Figure 5 ):

[0089] S100, control the black start system, the first switch, the second switch and the third switch to act respectively, so as to start the photovoltaic grid-connected system.

[0090] In the state that all switches are disconnected, that is, the state that the photovoltaic inverter 102 is disconnected from the synchronous motor pair system 103, and the synchronous motor pair system 103 is disconnected from the power grid, this step can realize the first start of the system, and the specific process includes:

[0091] (1) First, control the third switch S3 to close, so that the black start system 105 is connected to the stator winding of the first synchronous motor M1, and execute the black start process; the black start process is consistent with the conventional synchronous condenser start process in the prior art, and will not be described here.

[0092] (2) When the stator speed of the synchronous motor pair system 103 reaches the synchronous speed of the power grid, control the second switch S2 to close and the third switch S3 to open; at this time, without considering the self-loss, the synchronous motor pair system 103 has no mechanical power input and output, and no electric power input and output, the power angle θ of the first synchronous motor M1 and the second synchronous motor M2 is 0 degree, and the synchronous motor pair system 103 works in the synchronous condenser mode.

[0093] (3) When the intensity of the photovoltaic array 101 and the direct current voltage meet the power generation conditions, the stator winding end voltage of the first synchronous motor M1 is adjusted by controlling the field current of the first synchronous motor M1, so as to adjust the stator winding end voltage of the first synchronous motor M1 to the grid-connected voltage working range of the photovoltaic inverter 102;

[0094] (4) When the photovoltaic inverter 102 detects that its DC and AC voltages meet the start-up conditions, it starts up. The photovoltaic inverter 102 obtains the amplitude, frequency and phase of the stator voltage of the first synchronous motor M1 on the AC side through the phase-locked loop (PLL), and then converts the DC power on its own DC side into AC power that is synchronized with the stator winding terminal voltage of the first synchronous motor M1 through PWM (Pulse Width Modulation). In other words, it adjusts its own AC side output to AC power that is synchronized with the stator winding terminal voltage of the first synchronous motor M1, and then controls the first switch S1 to close, and the whole system completes grid connection.

[0095] Compared to existing technologies where the motor needs to be disconnected from the grid and stop when the photovoltaic system is not generating electricity, such as during rain, snow, or at night, and the system needs to be restarted when power generation is restored, the solution provided in this embodiment does not require disconnection from the grid or reconnection after the synchronous motor-driven system 103 is successfully connected to the grid for the first time, unless there is a power outage or fault repair. The startup process is simple and fast, and it can also provide inertia and reactive power support to the grid at night.

[0096] Furthermore, in states 1 and 2, the first switch S1 and the second switch S2 are both closed, while the third switch S3 is open. However, in state 1, if... Figure 10a As shown, its photovoltaic inverter 102 operates by constantly tracking the maximum power point Pm of the photovoltaic array 101, and its power / voltage curve is as follows. Figure 10a The waveform diagram above the photovoltaic inverter 102 shows its daytime operating power as follows: Figure 10a The waveform diagram below the photovoltaic inverter 102 is shown. In state 2, the active power of the photovoltaic inverter 102 remains at the active power command value P, as shown below. Figure 10b The waveform diagram below the photovoltaic inverter 102 shows that the reactive power command value Q in the reactive power dispatch command will be transmitted to the excitation system of the second synchronous motor M2, and the second synchronous motor M2 will adjust its own excitation current to achieve different power factors. In state 3, the first switch S1 and the third switch S3 are both open, while the second switch S2 is closed, as shown below. Figure 10c As shown.

[0097] Before entering state 3, that is, after entering state 1 or state 2 through step S101, as the light intensity decreases, the control method also includes... Figure 9 As shown in S104, when the photovoltaic inverter meets the shutdown conditions, the photovoltaic inverter is controlled to stop running and the first switch is opened, so that the stator winding of the first synchronous motor has no input power. This causes the system to enter state 3.

[0098] The control method provided in this embodiment can be directly applied to existing photovoltaic inverter products without modification, and supports multiple inverter operating modes such as MPPT and power dispatch. At the same time, it can generate electricity during the daytime and, when there is no photovoltaic power generation at night, operate the synchronous condenser system through the synchronous machine to drive the system in synchronous condenser mode, so that the photovoltaic grid-connected system can still continuously and automatically share the grid disturbances in a timely manner.

[0099] Another embodiment of the present invention also provides a photovoltaic grid-connected system, which, as shown in the figure... Figure 1 As shown, it includes: a photovoltaic array 101, at least one photovoltaic inverter 102, a synchronous motor drive system 103, a transformer 104, and a control system (not shown in the figure); wherein,

[0100] The synchronous motor-driven system 103 includes a first synchronous motor M1 and a second synchronous motor M2. The rotor shafts of the two synchronous motors are connected by a mechanical shaft, so that the two always maintain the same speed.

[0101] The DC side of the photovoltaic inverter 102 is connected to the corresponding photovoltaic string in the photovoltaic array 101, and the AC side of the photovoltaic inverter 102 is connected to the stator winding of the first synchronous motor M1. The stator winding of the second synchronous motor M2 is connected to the power grid through the transformer 104. Furthermore, the AC power generated by the photovoltaic array 101 is converted into three-phase AC power by the photovoltaic inverter 102 and connected to the three-phase stator input of one of the synchronous motors M1 in the synchronous motor-coupled system 103. The output side of the synchronous motor-coupled system 103 is the three-phase stator winding port of the other synchronous motor M2, which is then stepped up by the transformer 104 and connected to the power grid.

[0102] Preferred, such as Figure 8 As shown, the photovoltaic grid-connected system also includes: a black-start system 105, a first switch S1, a second switch S2, and a third switch S3; wherein:

[0103] The first switch S1 is located between the AC side of the photovoltaic inverter 102 and the stator winding of the first synchronous motor M1.

[0104] The black start system 103 is connected to the stator winding of the first synchronous motor M1 via the third switch S3.

[0105] The second switch S2 is located between the stator winding of the second synchronous motor M2 and the transformer 104.

[0106] This control system is used to execute the control method for the photovoltaic grid-connected system as described in any of the above embodiments. The specific process and principle of this control method can be found in the above embodiments and will not be repeated here.

[0107] The photovoltaic inverter 102 described above can be a conventional photovoltaic DC / AC converter currently commonly used, which does not need to be additionally modified and has flexible control modes. The photovoltaic inverter 102 can adopt a popular MPPT current source control mode to track the maximum power point of the photovoltaic array 101, and can also adopt a power instruction mode to respond to active and reactive power instructions below the maximum power of the photovoltaic array 101.

[0108] Moreover, the synchronous machine-drag system 103 always operates at the synchronous speed of the power grid after completing the first black start. When the photovoltaic power station has power input (during the day), the synchronous machine-drag system 103 operates in the generator mode. When the photovoltaic system has no power output (at night), the synchronous machine-drag system 103 operates in the synchronous condenser mode. Thus, the synchronous machine-drag system 103 can continuously provide inertia support and reactive power support for the power grid for 24 hours without interruption, thereby improving the stability of the frequency and voltage of the power grid.

[0109] In actual applications, the control system can specifically include controllers arranged in the interiors of the respective devices, and specifically includes: a first controller integrated in the photovoltaic inverter 102, which is used to implement operation control of the photovoltaic inverter 102; a second controller integrated in the first synchronous machine M1, which is used to implement control of the field current of the first synchronous machine M1; a third controller integrated in the second synchronous machine M2, which is used to implement control of the field current of the second synchronous machine M2; and a fourth controller integrated in the black start system 105, which is used to control the black start system 105 to perform black start.

[0110] In actual applications, the first controller can be responsible for communication with the outside world to receive active power instruction, reactive power instruction, and the like. Moreover, the first controller is in communication connection with the second controller, the third controller, and the fourth controller, and thus can implement control of the field current of the corresponding synchronous machine and the like. However, the control system can also be provided with an additional system controller responsible for communication with the outside world and in communication connection with the first controller, the second controller, the third controller, and the fourth controller. The specific application environment determines the system controller, which is within the protection scope of the present application.

[0111] The same parts and features of each of the embodiments described in the specification can be referred to, and each of the embodiments focuses on the differences from other embodiments. In particular, for the system or system embodiments, since they are basically similar to the method embodiments, they are described more simply, and the relevant parts can be referred to the part of the method embodiments. The above-described system and system embodiments are only illustrative, wherein the units described as separate components can or can not be physically separated, and the components shown as units can or can not be physical units, i.e. they can be located in one place or distributed on multiple network units. Part or all of the modules can be selected according to actual needs to achieve the purpose of the present embodiment. Those skilled in the art can understand and implement without creative labor.

[0112] The skilled person can further realize that the units and algorithm steps of each example described in combination with the embodiments disclosed herein can be realized in electronic hardware, computer software or a combination of both. In order to clearly illustrate the interchangeability of hardware and software, the components and steps of each example have been described in the above description in general terms. Whether the functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. The skilled person can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0113] The above description of the disclosed embodiments, the features recorded in each embodiment of the specification can be replaced or combined with each other, so that the skilled person in the art can implement or use the present application. Various modifications of the embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A control method of a photovoltaic grid-connected system, characterized by, The photovoltaic grid-connected system comprises, in series, a photovoltaic array, a photovoltaic inverter, a synchronous machine pair drag system and a transformer, the other side of the transformer being connected to a power grid; in the synchronous machine pair drag system, a rotor rotating shaft of a first synchronous machine and a rotor rotating shaft of a second synchronous machine are connected through a mechanical shaft; the control method comprises: after starting the photovoltaic grid-connected system, controlling the photovoltaic inverter to operate in a current source mode; controlling the synchronous machine pair drag system to operate in a generator mode, and maintaining the alternating current side voltage of the photovoltaic inverter stable through controlling the excitation current of the first synchronous machine.

2. The control method of a photovoltaic grid-connected system according to claim 1, characterized by, controlling the photovoltaic inverter to operate in a current source mode comprises: controlling the photovoltaic inverter to operate in a maximum power point tracking (MPPT) current source control mode.

3. The control method of a photovoltaic grid-connected system according to claim 2, characterized by, controlling the photovoltaic inverter to operate in a current source mode further comprises: when receiving an active power dispatch instruction and the active power instruction value is less than the maximum power of the photovoltaic array, controlling the photovoltaic inverter to operate in a current source mode according to the active power instruction value.

4. The control method of a photovoltaic grid-connected system according to claim 1, characterized by, controlling the synchronous machine pair drag system to operate in a generator mode further comprises: according to the reactive power instruction value in the received reactive power dispatch instruction, realizing a corresponding power factor through controlling the excitation current of the second synchronous machine.

5. The control method of a photovoltaic grid-connected system according to claim 1, characterized by, before or after any step, further comprising: when there is no input power in the stator winding of the first synchronous machine, controlling the synchronous machine pair drag system to operate in a synchronous phase modifier mode.

6. The control method of a photovoltaic grid-connected system according to any one of claims 1 to 5, characterized in that, The photovoltaic grid-connected system further comprises a black start system, a first switch, a second switch and a third switch; the first switch is arranged between the alternating current side of the photovoltaic inverter and the stator winding of the first synchronous machine, the black start system is connected to the stator winding of the first synchronous machine through the third switch, and the second switch is arranged between the stator winding of the second synchronous machine and the transformer; in the control method, before controlling the photovoltaic inverter to operate in a current source mode, further comprising: controlling the black start system, the first switch, the second switch and the third switch to act respectively, so that the photovoltaic grid-connected system starts.

7. The control method of a photovoltaic grid-connected system according to claim 6, characterized in that, controlling the black start system, the first switch, the second switch and the third switch to act respectively, so that the photovoltaic grid-connected system starts, comprises: controlling the third switch to close, so that the black start system is connected to the stator winding of the first synchronous machine, and a black start process is performed; when the stator rotating speed of the synchronous machine pair drag system reaches a power grid synchronous rotating speed, controlling the second switch to close and the third switch to open, so that the synchronous machine pair drag system operates in a synchronous phase modifier mode; when the photovoltaic array meets a power generation condition, adjusting the stator winding end voltage of the first synchronous machine to the grid-connected voltage operating range of the photovoltaic inverter through controlling the excitation current of the first synchronous machine; when the photovoltaic inverter meets a starting condition, adjusting the alternating current output of the alternating current side to alternating current synchronized with the stator winding end voltage of the first synchronous machine, and controlling the first switch to close.

8. The control method of a photovoltaic grid-connected system according to claim 6, characterized by, after controlling the photovoltaic inverter to operate in a current source mode, further comprising: When the photovoltaic inverter meets the shutdown condition, the photovoltaic inverter is controlled to stop running, the first switch is turned off, and the stator winding of the first synchronous motor is not supplied with input power.

9. A photovoltaic grid-connected system, characterized by, The photovoltaic grid-connected system comprises: a photovoltaic array, at least one photovoltaic inverter, a synchronous motor pair drag system, a transformer, and a control system; wherein, the synchronous motor pair drag system comprises a first synchronous motor and a second synchronous motor, and the rotors of the two synchronous motors are connected through a mechanical shaft; the direct current side of the photovoltaic inverter is connected to a corresponding photovoltaic group string in the photovoltaic array; the alternating current side of the photovoltaic inverter is connected to the stator winding of the first synchronous motor; the stator winding of the second synchronous motor is connected to the power grid through the transformer; the control system is used to perform the control method of the photovoltaic grid-connected system as claimed in any one of claims 1 to 8.

10. The photovoltaic grid-tie system of claim 9, wherein, Further comprising: a black start system, a first switch, a second switch, and a third switch; the first switch is arranged between the alternating current side of the photovoltaic inverter and the stator winding of the first synchronous motor; the black start system is connected to the stator winding of the first synchronous motor through the third switch; the second switch is arranged between the stator winding of the second synchronous motor and the transformer.

11. The photovoltaic grid-tie system of claim 10, wherein, The control system comprises: a first controller integrated in the photovoltaic inverter, which is used to realize the operation control of the photovoltaic inverter; a second controller integrated in the first synchronous motor, which is used to realize the control of the excitation current of the first synchronous motor; a third controller integrated in the second synchronous motor, which is used to realize the control of the excitation current of the second synchronous motor; a fourth controller integrated in the black start system, which is used to control the black start system to perform black start; the first controller is communicatively connected to the second controller, the third controller, and the fourth controller, respectively.

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