CN109494785B - Phase self-tracking grid-connected presynchronization control method without phase-locked loop - Google Patents

Phase self-tracking grid-connected presynchronization control method without phase-locked loop Download PDF

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CN109494785B
CN109494785B CN201811266634.6A CN201811266634A CN109494785B CN 109494785 B CN109494785 B CN 109494785B CN 201811266634 A CN201811266634 A CN 201811266634A CN 109494785 B CN109494785 B CN 109494785B
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grid
voltage
inverter
amplitude
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CN109494785A (en
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李洁
张亮亮
任海鹏
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Xian University of Technology
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Xian University of Technology
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network

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Abstract

The invention discloses a phase self-tracking grid-connected presynchronization control method without a phase-locked loop, which comprises the following steps of: step 1) obtaining an electrical angle theta of a modulation wave through an active power loop in a virtual synchronous generator or droop control; and 2) compensating the electrical angle of the modulation wave, 3) adjusting the phase and amplitude of the output voltage of the grid-connected inverter to be the same as the phase and amplitude of the voltage of the power grid, 4) closing the three-phase grid-connected contactor, and 5) disconnecting the phase and amplitude to pre-synchronously control the adjustment of the grid-connected inverter, so that the grid-connected inverter is obtained. According to the method, the phase self-tracking grid-connected presynchronization control without a phase-locked loop is adopted, theta used by a dq-axis rotating coordinate system is used as a virtual synchronous generator or droop control active power loop output phase angle theta, so that the phase locking of the voltage of a power grid is omitted, and the algorithm is simplified.

Description

Phase self-tracking grid-connected presynchronization control method without phase-locked loop
Technical Field
The invention belongs to the technical field of grid-connected pre-synchronization control, and relates to a phase self-tracking grid-connected pre-synchronization control method without a phase-locked loop.
Background
In recent years, energy and environmental problems become a great challenge for the development of the human society, new energy is widely used, and more distributed power supplies start to be connected to a power grid. The new energy is connected to a power grid through a grid-connected inverter, and power can be stably transmitted to the power grid under the condition that the voltage of the power grid is normal. When the power grid fails or is in power failure, the inverter is disconnected from the power grid, the independent operation mode is continuously used for supplying power for nearby key loads uninterruptedly, and the grid-connected operation can be carried out again when the power grid is recovered. Therefore, the grid-connected smooth switching technology is always a hot research problem of the distributed grid-connected inverter, and the key point of the grid-connected smooth switching technology is phase pre-synchronization. Therefore, the phase pre-synchronization method is continuously improved, so that the grid-connected inverter can stably run, the reliability of the grid-connected inverter is ensured, and the method has very important significance.
In the 'virtual synchronous generator-based micro-grid operation mode seamless switching control strategy' published in "power system automation" of 2016, 6, a double-PLL pre-synchronization control strategy is proposed, wherein a phase-locked loop of two second-order generalized integrators is used to respectively obtain an output voltage of a grid-connected inverter and an electrical angle of the grid voltage, the electrical angle of the grid voltage is used as a reference value, the electrical angle of the output voltage of the grid-connected inverter is used as a feedback value, and then a compensation electrical angle of the grid-connected inverter is obtained through an integral regulator and is compensated to an output angular frequency of an active loop. The output voltage of the grid-connected inverter can be consistent with the electrical angle of the grid voltage by adjusting the electrical angle of the output voltage of the grid-connected inverter. The phase presynchronization control algorithm is complex, and the presynchronization time is long.
The presynchronization control method for self-tracking the voltage phase of the power grid by adopting the terminal voltage phase of the inverter is proposed in a text of presynchronization, multi-loop control and load unbalance control of a three-phase four-leg virtual synchronous generator, which is published by Binshi et al in the report of electrotechnical science in 7 month 2017. The control algorithm has short pre-synchronization time, but needs to carry out phase locking on the voltage of the power grid, and the complexity of the control algorithm is increased.
Referring to fig. 3, the pre-synchronization control method for the inverter terminal voltage phase to track the grid voltage phase automatically includes the specific control process of the grid voltage uga、ugb、ugcObtaining the electric angle theta of the power grid voltage through a phase-locked loop PLLgThen the inverter outputs a voltage ua、ub、ucElectric angle theta with network voltageg Simultaneous input 3s/2r transformation to uq，ud. Then to the output uqPI control is carried out to finally enable uqTo 0, the inverter voltage phase can be made the same as the grid voltage phase.
The two methods need phase locking on the grid voltage and the output voltage of the grid-connected inverter, so that the control algorithm is relatively complex, and the system instability factor is increased.
Disclosure of Invention
The invention aims to provide a phase self-tracking grid-connected pre-synchronization control method without a phase-locked loop, which solves the problems that when an inverter is switched from an off-grid working mode to a grid-connected working mode, the phase of the output voltage of the inverter is inconsistent with the phase of the voltage of a power grid, the traditional control algorithm is relatively complex, and the unstable factors of a system are increased.
The invention adopts the technical scheme that a phase self-tracking grid-connected presynchronization control method without a phase-locked loop is implemented according to the following steps:
step 1) obtaining an electrical angle theta of a modulation wave through an active power loop in a virtual synchronous generator or droop control;
step 2) compensating the electrical angle of the modulated wave;
step 3) adjusting the phase and amplitude of the output voltage of the grid-connected inverter to make the phase and amplitude of the output voltage of the grid-connected inverter the same as the phase and amplitude of the voltage of the power grid;
step 4), closing the three-phase grid-connected contactor;
and 5) disconnecting the phase and amplitude pre-synchronization control to regulate the grid-connected inverter, and thus obtaining the grid-connected inverter.
The invention has the advantages that when the grid-connected pre-synchronization is carried out, the smooth switching of the grid-connected inverter is realized by using the grid-connected pre-synchronization technology without a phase-locked loop, the phase pre-synchronization control algorithm is effectively simplified, and the control effect which is the same as that of the traditional phase self-tracking pre-synchronization control can be obtained.
Drawings
FIG. 1 is a schematic diagram of an embodiment of the hardware architecture on which the method of the present invention relies;
FIG. 2 is a grid-tie timing diagram of the inverter in the method of the present invention;
FIG. 3 is a block diagram of a conventional phase self-tracking grid-connected pre-synchronization control;
FIG. 4 is a block diagram of the phase self-tracking grid-connected presynchronization control based on the virtual synchronous generator to control the phase-locked loop;
FIG. 5 is a voltage amplitude value grid-connected presynchronization control block diagram of the virtual synchronous generator-based control phase-locked loop-free method of the invention;
FIG. 6 is a phase self-tracking grid-connected presynchronization process diagram without a phase-locked loop of the method of the present invention;
FIG. 7 is a graph of dual PLL presynchronization control inverter output voltage angle θ and grid voltage angle θg；
FIG. 8 is a diagram of the output voltage angle θ and the grid voltage angle θ of a conventional phase self-tracking pre-synchronous controlled inverterg；
FIG. 9 shows the phase self-tracking grid-connected presynchronization control of the inverter output voltage angle theta and the grid voltage angle theta without the phase-locked loop according to the method of the present inventiong；
FIG. 10 is a graph of dual PLL presynchronization control inverter output A-phase voltage uaPhase voltage u with the A phase of the power gridga；
FIG. 11 is a diagram of the output A-phase voltage u of a conventional phase self-tracking pre-synchronous controlled inverteraPhase voltage u with the A phase of the power gridga；
FIG. 12 shows the phase self-tracking grid-connected pre-synchronous control inverter outputting the A-phase voltage u without phase-locked loop according to the method of the present inventionaPhase voltage u with the A phase of the power gridga；
FIG. 13 is a diagram of dual PLL presynchronized control inverter output A-phase voltage uaPhase voltage u with the A phase of the power gridgaA difference value;
FIG. 14 is a diagram of the output A-phase voltage u of a conventional phase self-tracking pre-synchronous controlled inverteraPhase voltage u with the A phase of the power gridgaA difference value;
FIG. 15 shows the phase self-tracking grid-connected pre-synchronous control inverter outputting the A-phase voltage u without phase-locked loop according to the method of the present inventionaPhase voltage u with the A phase of the power gridgaA difference value;
FIG. 16 is a dual PLL presynchronization control inverter output current;
FIG. 17 is a conventional phase self-tracking pre-synchronous control inverter output current;
FIG. 18 is a phase self-tracking grid-connected presynchronization control inverter output current without a phase-locked loop of the method of the present invention;
FIG. 19 is a graph of the phase shift for a capacitance of 100uF with a change in inductance;
fig. 20 is a graph showing the phase shift when the inductance is 10mH and the capacitance is changed.
In the figure, 10, a direct current power supply, 20, a power converter, 30, an inductor, 31, a capacitor, 40, a load, 50, an inductive current sensor, 51, a capacitive voltage sensor, 52, an output current sensor, 60, a three-phase grid-connected contactor, 61, a grid-connected pre-synchronous output voltage amplitude adjusting switch, 62, a grid-connected pre-synchronous output voltage phase adjusting switch, 70, grid line impedance, 71, a three-phase grid, 80, a first 3s/2s conversion module, 81, a second 3s/2s conversion module, 82, an instantaneous power calculation module, 83, a synthetic vector module, 90, a voltage amplitude calculation module, 91, a virtual speed regulator, 100, a virtual exciter, 101, a rotor mechanical equation module, 102, an LC filter phase compensation module, 110, synthetic three-phase voltage, 120, a stator electrical equation module, 130, an output voltage calculation module, 140. the third 3s/2s conversion module, 150 grid-connected inverter voltage loop module, 160 grid-connected inverter current loop module, 170.2s/3s conversion module, 180 SPWM modulation module, 190 grid-connected smooth switching control module.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
When the grid-connected inverter is switched from an off-grid working mode to a grid-connected working mode, the existing pre-synchronization control method needs to use a phase-locked loop for the grid voltage and the grid-connected inverter output voltage to obtain the inverter output voltage electrical angle and the grid voltage electrical angle, so that the design of phase pre-synchronization without the phase-locked loop has important significance for simplifying a control algorithm and increasing the stability factor of a system during grid-connected pre-synchronization. In order to solve the problem that phase-locked loops are needed in traditional grid-connected phase pre-synchronization, the invention provides a phase self-tracking grid-connected pre-synchronization control method without the phase-locked loops, wherein 3s in each conversion module represents a three-phase static coordinate system, 2s represents a two-phase static coordinate system, 2r represents a two-phase rotating coordinate system, and/' represents coordinate conversion.
Referring to fig. 1, a grid-connected inverter structure (i.e., hardware structure) based on virtual synchronous generator control, on which the method of the present invention relies, is such that a dc power source 10 is connected to an inductor 30 of a three-phase LC filter through a power converter 20 (the inductor 30 is a three-phase inductor L in fig. 1)f) (ii) a An inductor current sensor 50 of a three-phase LC filter is also connected between the inductor 30 and the power converter 20 (the inductor current sensor 50 includes three current sensors i in fig. 1La、iLb、iLc) (ii) a The inductor 30 is connected to the output current sensor 52 via the capacitor 31 of the three-phase LC filter (the capacitor 31 is the three-phase capacitor C in fig. 1)fThe output current sensor 52 includes three current sensors I in fig. 1a、Ib、Ic) Wherein the capacitor 31 of the three-phase LC filter adopts a star connection method, and the two ends of the capacitor 31 are connected with the capacitor voltage sensors 51 of the LC filter (the capacitor voltage sensors 51 comprise three voltage sensors in fig. 1)ua、ub、uc) (ii) a The output current sensor 52 is connected via a load 40 to a three-phase grid contactor 60 (the three-phase grid contactor 60 is the three-phase contactor PCC in fig. 1), and the three-phase grid contactor 60 is connected via a grid line impedance 70 to a three-phase grid 71 (the grid line impedance 70 includes the three-phase resistor R in fig. 1)gThree-phase reactance Lg)；
The output end of a capacitance voltage sensor 51 of the LC filter is simultaneously connected with the input end of an instantaneous power calculation module 82 output by the grid-connected inverter and the input end of a second 3s/2s conversion module 81, and the output end of an output current sensor 52 is simultaneously connected with the input end of the instantaneous power calculation module 82 and the input end of a synthetic vector module 83 of output current of the grid-connected inverter;
output signal P of instantaneous power calculation module 82eThe output signal Q of the instantaneous power calculation module 82 is connected to the power feedback input end of the virtual exciter 100;
two output signals u of the second 3s/2s conversion module 81α、uβConnected to the input of the voltage amplitude calculation module 90 of the inverter output, and two outputs u of the second 3s/2s conversion module 81α、uβThe signal is connected with the feedback end of the grid-connected inverter voltage loop module 150;
output signal u of voltage amplitude calculation module 90 output by invertermThe voltage amplitude feedback end of the virtual exciter 100 is connected, the grid-connected presynchronization output voltage amplitude adjusting switch 61 (namely KA in figure 1) is connected with the amplitude presynchronization compensation end of the virtual exciter 100, and the output signal of the synthetic vector module 83 of the output current of the grid-connected inverter
Figure BDA0001845028920000061
The input end of the stator electrical equation module 120 is accessed;
output signal P of virtual governor 91mThe power-giving end of the rotor mechanical equation module 101 is connected, and the frequency feedback end of the virtual speed regulator 91 is connected after the signal omega in the rotor mechanical equation module 101 is multiplied by 1/2 pi (namely f in fig. 1); output signal of rotor mechanical equation module 101The signal theta is connected to the input end of the LC filter phase compensation module 102;
output signal theta of LC filter phase compensation module 102IThe input end of the synthetic three-phase voltage 110 is connected with the output signal E of the virtual exciter 100 at the same time; synthesizing the output signals of the three-phase voltage 110 (i.e., E in FIG. 1)a、Eb、Ec) And the output signal of the stator electrical equation module 120 (i.e., Δ u in FIG. 1)a、Δub、Δuc) Meanwhile, the input end of the output voltage calculation module 130 of the virtual synchronous generator is accessed;
the output signal of the output voltage calculation module 130 (i.e., u in FIG. 1)a*、ub*、ucAccess to the input of the third 3s/2s transformation module 140; the output signal of the third 3s/2s transformation module 140 (i.e. u in fig. 1)α*、uβAccess to a given end of the grid-connected inverter voltage ring module 150;
output signal of grid-connected inverter voltage loop module 150 (i.e., i in fig. 1)Lα*、iLβAccess to the given end of the grid-connected inverter current loop module 160;
the inductive current sensor 50 is connected to an input terminal of a first 3s/2s conversion module 80, and an output signal of the first 3s/2s conversion module 80 (i.e. i in fig. 1)Lα、iLβ) A feedback end of the grid-connected inverter current loop module 160 is accessed;
output signal of grid-connected inverter current loop module 160 (i.e., u in fig. 1)α1、uβ1) The input end of the 2s/3s conversion module 170 is connected, the output signal (i.e. U in fig. 1) of the 2s/3s conversion module 170 is connected to the input end of the SPWM modulation module 180, and the output signal of the SPWM modulation module 180 is used as the driving signal of the power converter 20 to control the switching action of the power device so as to realize the electric energy conversion;
the three-phase grid-connected contactor 60 is further connected with a grid-connected smooth switching control module 190, an output signal θ of the rotor mechanical equation module 101 is also connected to the grid-connected smooth switching control module 190, and an output signal Δ ω of the grid-connected smooth switching control module 190 is connected to a feedback end of the rotor mechanical equation module 101 through a grid-connected pre-synchronization output voltage phase adjustment switch 62 (i.e., KB in fig. 1, referred to as switch 2 in fig. 2).
Referring to fig. 2, the phase self-tracking grid-connected pre-synchronization control method without a phase-locked loop is implemented based on the hardware structure of the virtual synchronous generator of the grid-connected inverter according to the following steps:
step 1) obtaining an electrical angle theta of a modulation wave through an active power loop in a virtual synchronous generator or droop control;
referring to fig. 1, a grid-connected pre-synchronous output voltage amplitude regulating switch 61(KA, referred to as switch 1 in fig. 2) and a grid-connected pre-synchronous output voltage phase regulating switch 62(KB, referred to as switch 2 in fig. 2) are both closed at 0, that is, neither the amplitude nor the phase of the grid-connected inverter output voltage is regulated, and a three-phase grid-connected contactor 60 (i.e., PCC in fig. 1) is in an off state; the grid-connected inverter operates in an island state, and a virtual synchronous generator or an active power loop controlled by droop obtains a voltage phase theta with the unit of degree;
step 2) compensating the electrical angle of the modulated wave,
due to the influence of the LC filter, the electrical angle of the output voltage of the inverter and the electrical angle theta of the modulated wave obtained by the virtual synchronous generator or the droop control through the active power loop have a fixed included angle delta thetaLC(ii) a Then the electrical angle theta and the electrical angle delta theta of the phase shift of the grid-connected inverter filter are carried outLCAdd to form θII.e. the actual modulated wave angle theta of the grid-connected inverterI。
As can be seen from FIGS. 19 and 20, Δ θ when the inductance changes by 50% at a nominal value of 10mH and the capacitance changes by 25% at a nominal value of 100 μ FLCThe variation is small, so that the fixed value is used for Delta thetaLCCompensating, wherein the electrical angle of the output voltage of the grid-connected inverter at the moment is theta and unit degree (degree);
step 3) adjusting the phase and amplitude of the output voltage of the grid-connected inverter to make the phase and amplitude of the output voltage of the grid-connected inverter the same as the phase and amplitude of the output voltage of the grid-connected inverter,
in order to realize the smooth integration of the inverter into the power grid for operation, the output voltage of the inverter is required to be consistent with the frequency, the amplitude and the phase of the voltage of the power grid, so that the grid-connected smooth switching control comprises two parts, namely voltage phase pre-synchronous control and amplitude pre-synchronous control; when the inverter is ready for grid connection, the grid connection presynchronization output voltage amplitude regulation switch 61(KA, referred to as switch 1 in fig. 2) and the grid connection presynchronization output voltage phase regulation switch 62(KB, referred to as switch 2 in fig. 2) are closed at Δ u, Δ ω, respectively, that is, the grid connection smooth switching control is started,
3.1) adjusting the output voltage phase of the grid-connected inverter, specifically comprising the following steps,
referring to fig. 6, where the electrical angle output by the virtual synchronous generator or droop control active power loop is θ, rotating at an angular velocity ω in radians per second (rad/s);
at this time, the grid voltage ugIn V (volt) and in thetagUnit of degree (degrees) at an angular velocity ωgRotation, unit rad/s (radians/sec);
inverter modulated wave voltage uMIn V (volt) and in thetaMUnit ° (degrees), rotation at angular velocity ω, unit rad/s (radians/sec);
inverter output terminal voltage u, unit V (volt), phase θ, unit ° (degree), rotating at angular velocity ω, unit rad/s (radian/second);
network voltage ugThe q-axis component of the virtual synchronous generator or the droop control active power loop output theta rotation in the dq-axis coordinate system is ugqUnit V (volts);
network voltage ugD-axis component of the virtual synchronous generator or droop control active power loop output theta rotation in dq-axis coordinate system is ugdUnit V (volts);
referring to fig. 1, 4 and 6, when the grid voltage u isgWhen the angular velocity of the output end voltage u of the inverter is equal and kept constant, an initial phase difference theta always exists between the twoerror(ii) a When the inverter is ready to switch from the island mode to the grid-connected mode, the grid-connected presynchronization output voltage phase adjusting switch 62(KB, referred to as switch 2 in fig. 2) is closed at Δ ω, and the frequency phase presynchronization is started, so that the angular velocity ω of the inverter output voltage u can be adjusted, and the inverter output voltage u phase and the grid voltage u phase are enabled to be switched to operategPhase coincidence, i.e. inversionThe phase of the output voltage of the converter is consistent with the phase of the voltage of the power grid.
Will the network voltage ugIn the unit of V (volt), the expression is as follows when transforming to the dq axis coordinate system of the virtual synchronous generator or droop control active power loop output theta in the unit of degree (degree):
Figure BDA0001845028920000091
wherein theta iserrorThe unit degree is the initial phase difference between the grid voltage phase angle and the output phase angle of the virtual synchronous generator or the droop control active power loop.
The phase angle of the output voltage u of the inverter lags behind the phase angle delta theta of the modulation wave of the inverter due to the influence of the filter in the grid-connected inverterLCUnit degree, the phase self-tracking grid-connected presynchronization control method without the phase-locked loop compensates delta theta for theta output by the virtual synchronous generator or the droop control active power loopLCObtaining the modulation wave angle delta theta of the grid-connected inverterIThe phase angle of the inverter output voltage u is θ in degrees, which is the same as the output phase angle θ of the virtual synchronous generator or droop control active power loop, i.e., the d-axis phase angle of the dq axis, as shown in fig. 6.
The phase self-tracking grid-connected presynchronization control method without the phase-locked loop aims at adjusting the rotation angular frequency of a dq axis, so that the phase difference delta theta between the d axis of the dq axis and the grid voltage gradually tends to zero, wherein the d axis phase angle is the output phase angle theta of a virtual synchronous generator or a droop control active power loop and is equal to the phase angle of an inverter output voltage u; when Δ θ is zero, it can be seen from fig. 6 that the projection of the grid voltage on the q-axis component is zero, and the d-axis of the dq-axis is equal to the grid voltage ugThe phases are overlapped, and the phase angle of the output voltage u of the inverter at the moment is equal to the voltage u of the power gridgThe phase angles are uniform, so that u can be controlledgqTending to 0 to achieve synchronization of the phases of both.
The specific control process is that referring to fig. 4, the power grid voltage is converted into u through 3s/2sgα、ugβUnit V (volt)) Then u isgα、ugβThe phase angle theta output by the virtual synchronous generator or the droop control active power loop is input at the same time to 2s/2r for conversion to obtain ugq、ugdUnit V (volt) and reference value UqComparing the two values to 0, performing PI regulation on the obtained difference value, and outputting delta omega output by the PI regulatorgAnd the unit rad/s compensates the virtual synchronous generator or droop control active power loop to output the angular speed omega, and the unit rad/s can adjust the phase and frequency of the output voltage of the inverter to gradually realize the synchronization of the output voltage of the inverter and the voltage phase of the power grid. The sampling points of theta and the compensated delta theta in fig. 4LCIs the innovation point of the invention.
Therefore, theta used by the dq axis rotating coordinate system in the method is the output phase angle theta of the virtual synchronous generator or the droop control active power loop, phase locking on the voltage of a power grid is omitted, and the algorithm is simplified.
3.2) adjusting the amplitude of the output voltage of the grid-connected inverter, wherein the specific process is that,
because the amplitude of the output voltage of the inverter during the off-grid operation may not be the same as the amplitude of the grid voltage, if the difference between the amplitudes of the two sides is large during the grid connection, too large current impact can be caused, so that the amplitude of the output voltage of the inverter must be adjusted during the grid connection.
When the inverter is ready to be switched to the grid-connected mode operation from the island mode, the grid-connected presynchronization output voltage amplitude regulating switch 61(KA, referred to as switch 1 in fig. 2) is closed at the position of delta u, the unit V (volt) and the starting voltage amplitude are presynchronized, that is, the output voltage amplitude u of the inverter can be regulatedmTo make the inverter output voltage amplitude umUnit V (volt) and grid voltage amplitude ugmThe unit V (volts), the same;
the specific control process is that, referring to FIG. 5, the network voltage ugaUnit V (volt), ugbUnit V (volt), ugcThe unit V (volt) is converted into u through 3s/2sgαUnit V (volt), ugβThe unit V (volt), then ugα、ugβU is obtained by an overvoltage amplitude value calculation modulegmThe unit V (volts). By the amplitude u of the mains voltagegmFor a given inverter output voltage amplitude umAnd for feedback, performing PI regulation on the obtained difference, compensating the output delta u of the PI regulator into the voltage of a virtual synchronous generator or a droop control reactive power loop by a unit V (volt), namely regulating the output voltage amplitude of the inverter, and finally enabling the output voltage amplitude of the inverter to track the voltage amplitude of the power grid.
Step 4) closing the three-phase grid-connected contactor,
when the frequency, the electrical angle and the amplitude of the grid voltage are consistent with those of the output voltage of the grid-connected inverter, closing a three-phase grid-connected contactor 60 (PCC);
step 5) the phase and amplitude pre-synchronous control is cut off to regulate the grid-connected inverter, namely,
when the inverter is successfully connected to the grid, a grid-connected presynchronization output voltage amplitude regulating switch 61(KA, referred to as switch 1 in fig. 2) and a grid-connected presynchronization output voltage phase regulating switch 62(KB, referred to as switch 2 in fig. 2) are closed at 0, that is, grid-connected presynchronization control is closed; and at this moment, the inverter performs grid-connected operation, namely.
Simulation verification
To illustrate the effectiveness of the method of the present invention, simulation verification was performed in MATLAB, and the verification results and data analysis were as follows:
the simulation working conditions are as follows: the rated active power of the inverter is 2500W, and the reactive power of the inverter is 0W. When the system is in grid-connected operation, the inverter is in off-grid load operation with 1000W, when the system is in off-grid operation, the presynchronization control is started when the system is in off-grid load operation, the grid-connected switch is closed when the system is in off-grid load operation, and when the system is in off-grid load operation, the grid-connected switch is opened when the system is in off-grid load operation 1.8 s; and when t is 2.5s, the simulation is ended.
Fig. 7 shows the dual PLL pre-synchronization control inverter voltage angle and the grid voltage angle, and it can be seen from fig. 7 that the inverter output voltage electrical angle is finally the same as the grid voltage electrical angle.
Fig. 8 shows the conventional phase self-tracking pre-synchronization control inverter voltage angle and the grid voltage angle, and it can be seen from fig. 8 that the inverter output voltage electrical angle is finally the same as the grid voltage electrical angle.
Fig. 9 shows that the phase self-tracking grid-connected pre-synchronization control inverter voltage angle without a phase-locked loop and the grid voltage angle of the method of the present invention are the same, and it can be seen from fig. 9 that the inverter output voltage electrical angle is the same as the grid voltage electrical angle.
It can be seen from fig. 7-9 that the synchronization time of the dual PLL pre-synchronization control is slightly longer than the phase self-tracking pre-synchronization control time, and the method of the present invention has the same performance as the conventional phase self-tracking pre-synchronization.
Fig. 10 shows that the dual PLL presynchronization controls the inverter output a-phase voltage and the grid a-phase voltage, and it can be seen from fig. 10 that the inverter output a-phase voltage and the grid a-phase voltage are finally the same.
Fig. 11 shows that the conventional phase self-tracking pre-synchronization control inverter outputs the a-phase voltage and the a-phase voltage of the power grid, and it can be seen from fig. 11 that the a-phase voltage and the a-phase voltage of the power grid are finally the same.
Fig. 12 shows that the phase self-tracking grid-connected pre-synchronization control inverter without a phase-locked loop outputs the a-phase voltage and the grid a-phase voltage according to the method of the present invention, and it can be seen from fig. 12 that the inverter outputs the a-phase voltage and the grid a-phase voltage are finally the same.
It can be seen from fig. 10 to 12 that the three control methods can make the inverter output voltage coincide with the grid voltage. The double PLL pre-synchronization control has a slightly longer time than the phase self-tracking pre-synchronization control, and the traditional phase self-tracking pre-synchronization control has the same performance as the method of the invention.
Fig. 13 shows the difference between the output a-phase voltage of the dual PLL pre-synchronization controlled inverter and the a-phase voltage of the power grid, and it can be seen from fig. 13 that the difference between the output a-phase voltage of the inverter and the a-phase voltage of the power grid is finally 0.
Fig. 14 shows the difference between the output a-phase voltage of the conventional phase self-tracking pre-synchronization controlled inverter and the a-phase voltage of the power grid, and it can be seen from fig. 14 that the difference between the output a-phase voltage of the inverter and the a-phase voltage of the power grid is finally 0.
Fig. 15 shows the phase self-tracking grid-connected pre-synchronization control method without phase-locked loop according to the present invention, wherein the difference between the inverter output a-phase voltage and the grid a-phase voltage is finally 0.
As can be seen from fig. 13 to 15, the difference between the inverter output a-phase voltage and the grid a-phase voltage is almost 0 when the dual PLL pre-synchronization control is performed for 45.3 ms. Both the conventional phase self-tracking presynchronization control and the method of the present invention are 43.5 ms.
Fig. 16 shows the dual PLL pre-synchronization control inverter output current, and it can be seen from fig. 16 that the inverter output current has no current surge during grid connection and can be smoothly incorporated into the grid.
Fig. 17 shows the output current of the conventional phase self-tracking pre-synchronization controlled inverter, and it can be seen from fig. 17 that the output current of the inverter has no current surge during grid connection and can be smoothly incorporated into the power grid.
Fig. 18 shows the phase self-tracking grid-connection pre-synchronization control inverter output current without a phase-locked loop according to the method of the present invention, and it can be seen from fig. 18 that the inverter output current has no current impact during grid connection and can be smoothly incorporated into a power grid.
As can be seen from fig. 16 to 18, the inverters of the three pre-synchronization control methods have very small current impact at the moment of grid connection, and when the inverters are off-grid, the inverters can be well transited to off-grid operation.
Through the simulation verification and comparison, the control method can obtain the same control effect as the traditional phase self-tracking grid-connected pre-synchronization control method.

Claims (4)

1. A phase self-tracking grid-connected presynchronization control method without a phase-locked loop is characterized by comprising the following steps of:
step 1) obtaining a voltage phase theta of a modulation wave through an active power loop in virtual synchronous generator or droop control, wherein the specific process is as follows:
the grid-connected presynchronous output voltage amplitude regulating switch (61) and the grid-connected presynchronous output voltage phase regulating switch (62) are both closed at 0, and the three-phase grid-connected contactor (60) is in a turn-off state; the grid-connected inverter operates in an island state, and a virtual synchronous generator or an active power loop controlled by droop obtains a voltage phase theta with the unit of degree;
step 2) compensating the voltage phase theta of the modulation wave;
step 3) adjusting the phase and amplitude of the output voltage of the grid-connected inverter to make the phase and amplitude of the output voltage of the grid-connected inverter the same as the phase and amplitude of the output voltage of the grid-connected inverter,
the grid-connected smooth switching control comprises two parts of voltage phase presynchronization control and amplitude presynchronization control; when the inverter is ready for grid connection, a grid connection presynchronization output voltage amplitude regulating switch (61) and a grid connection presynchronization output voltage phase regulating switch (62) are respectively closed at delta u and delta omega, namely grid connection smooth switching control is started,
3.1) adjusting the output voltage phase of the grid-connected inverter, specifically comprising the following steps,
the virtual synchronous generator or the droop control active power loop outputs voltage phase theta which rotates at an angular speed omega, and the unit is radian/second;
at this time, the grid voltage ugPhase of θgAt an angular velocity ωgRotating;
inverter modulated wave voltage uMPhase of θMRotating at an angular velocity ω;
the voltage u at the output end of the inverter rotates at an angular velocity omega with the phase theta;
network voltage ugThe q-axis component of the virtual synchronous generator or the droop control active power loop output theta rotation in the dq-axis coordinate system is ugq；
Network voltage ugD-axis component of the virtual synchronous generator or droop control active power loop output theta rotation in dq-axis coordinate system is ugd；
When the grid voltage ugWhen the angular velocity of the output end voltage u of the inverter is equal and kept constant, an initial phase difference theta always exists between the twoerror(ii) a When the inverter is ready to be switched to the grid-connected mode for operation from the island mode, a grid-connected presynchronization output voltage phase adjusting switch (62) is closed at delta omega, frequency phase presynchronization is started, the angular speed omega of the output end voltage u of the inverter is adjusted, and the phase of the output end voltage u of the inverter and the grid voltage u are enabled to be in phasegThe phases are overlapped to realize the phase consistency of the output voltage of the inverter and the voltage of the power grid,
will the network voltage ugChange to or from a virtual synchronous generatorThe vertical control active power loop output theta is expressed as follows under a rotating dq axis coordinate system:
Figure FDA0003015557860000021
wherein theta iserrorThe initial phase difference of the output phase angle of the active power loop is controlled for the grid voltage phase angle and the virtual synchronous generator or the droop,
adjusting the rotation angular frequency of the dq axis to enable the phase difference delta theta between the d axis of the dq axis and the grid voltage to gradually approach zero, wherein the d axis phase angle is the phase theta of the output voltage of the virtual synchronous generator or the droop control active power loop and is equal to the phase angle of the output voltage u of the inverter;
3.2) adjusting the amplitude of the output voltage of the grid-connected inverter, wherein the specific process is that,
when the inverter is ready to be switched to the grid-connected mode for operation from the island mode, a grid-connected presynchronization output voltage amplitude regulating switch (61) is closed at delta u, the voltage amplitude is started to presynchronize, and the output voltage amplitude u of the inverter is regulatedmTo make the inverter output voltage amplitude umWith the grid voltage amplitude ugmThe same;
the control method is that the voltage amplitude u of the power grid is usedgmFor a given inverter output voltage amplitude umFor feedback, performing PI regulation on the obtained difference, compensating the output delta u of the PI regulator to the voltage setting of a virtual synchronous generator or a droop control reactive power loop, regulating the output voltage amplitude of the inverter, and finally enabling the output voltage amplitude of the inverter to track the voltage amplitude of a power grid;
step 4), closing the three-phase grid-connected contactor;
and 5) cutting off the phase and amplitude pre-synchronization control to regulate the grid-connected inverter.
2. The phase self-tracking grid-connected presynchronization control method without the phase-locked loop according to claim 1, wherein in the step 2), the specific process is as follows:
voltage phase of inverter output voltage andthe voltage phase theta of the modulated wave obtained by the virtual synchronous generator or droop control through the active power loop has a fixed included angle delta thetaLC(ii) a Then the voltage phase theta and the grid-connected inverter filter phase are shifted backwards by an electrical angle delta thetaLCAdd to form θII.e. the actual modulated wave angle theta of the grid-connected inverterI(ii) a Using fixed value pairs for delta thetaLCAnd compensating, wherein the voltage phase of the output voltage of the grid-connected inverter at the moment is theta.
3. The phase self-tracking grid-connected presynchronization control method without the phase-locked loop according to claim 1, wherein in the step 4), the specific process is as follows: and when the frequency, the voltage phase and the amplitude of the grid voltage are consistent with the frequency, the voltage phase and the amplitude of the output voltage of the grid-connected inverter, closing the three-phase grid-connected contactor (60).
4. The phase self-tracking grid-connected presynchronization control method without the phase-locked loop according to claim 1, wherein in the step 5), the specific process is as follows: when the inverter is successfully connected to the grid, the grid-connected presynchronous output voltage amplitude regulating switch (61) and the grid-connected presynchronous output voltage phase regulating switch (62) are closed at 0, namely the grid-connected presynchronous control is closed; at this point, the inverter performs grid-connected operation.
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