CN112003325B - Method for smoothly cutting off pre-synchronous controller for grid connection of inverter - Google Patents

Method for smoothly cutting off pre-synchronous controller for grid connection of inverter Download PDF

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CN112003325B
CN112003325B CN202010855843.5A CN202010855843A CN112003325B CN 112003325 B CN112003325 B CN 112003325B CN 202010855843 A CN202010855843 A CN 202010855843A CN 112003325 B CN112003325 B CN 112003325B
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grid
controller
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CN112003325A (en
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马俊鹏
吴子豪
王辰曦
唐露甜
李明
王顺亮
刘天琪
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Sichuan University
Electric Power Research Institute of State Grid Shanxi Electric Power Co Ltd
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Electric Power Research Institute of State Grid Shanxi Electric Power Co Ltd
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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
    • H02J3/381—Dispersed generators
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin

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  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

The invention discloses a method for smoothly cutting off a pre-synchronous controller by a grid-connected inverter, which comprises the following steps: the method comprises the following steps that an inverter works in an island operation state, a pre-synchronous controller is put into the inverter after a grid-connected instruction is received, and a grid-connected switch is switched on to realize grid connection when the phase and the frequency of the output voltage of the inverter are consistent with the voltage of a power grid; after the grid connection is finished, the grid connection condition judgment module compares the phase angle of the power grid with the phase angle of the inverter, if the phase angle is continuously smaller than xi and the difference value is smaller than xi in 5 switching cycles, the grid connection is considered to be successful, a presynchronization controller cutting signal is sent out, after a latch receives the signal, the current output signal is kept unchanged, and a gradient cutting controller reduces the input signal by 0.1 time successively and outputs the signal until the output is completely 0. The invention solves the problem of disturbance of inverter frequency and power when the pre-synchronous controller is cut off, and fills the blank of the prior engineering technology.

Description

Method for smoothly cutting off pre-synchronous controller for grid connection of inverter
Technical Field
The invention belongs to the technical field of grid-connected pre-synchronous control, and particularly relates to a method for smoothly cutting off a pre-synchronous controller of an inverter during grid connection.
Background
The new energy has the advantages of high efficiency and sustainable development, and the installed capacity of the new energy in a power grid is rapidly increased, so that the new energy becomes a well-known energy development direction in the international society. The rotors of conventional Synchronous Generators (SG) have rotational inertia, so these generators are able to inject the kinetic energy stored in their rotors into the grid in case of sudden disturbances. Therefore, the system is very robust. However, most of the new energy power generation is usually connected to the power system or the microgrid through the inverter, and due to the application of the power electronics and the PWM control method, the total inertia is greatly reduced in the whole power system, especially the microgrid, so that the power system frequency becomes sensitive to the fluctuation of the load and the renewable energy, and as the number of the new energy inverters increases, the problem becomes more serious, and the stable operation of the power system becomes a great challenge.
Droop control is a common distributed power interface inverter control technique, and is controlled by using the principle that the output active power and frequency of a distributed power supply are in a linear relation, and the reactive power and the voltage amplitude are in a linear relation. This control method is generally used for the control of distributed power interface inverters in a peer-to-peer control strategy, as it has the potential to implement its control without requiring communication links between distributed power sources.
Because the grid-connected device is more fragile than the traditional generator, if the phase and frequency of the grid voltage are inconsistent with those of the inverter voltage, the inverter can generate impact current during grid connection, and even the inverter is damaged in serious conditions, so that the grid-connected pre-synchronization control plays an important role in the grid-connected inverter. In the new energy power generation, the inverter is connected with a large power grid, and energy is injected into the power grid. In summary, the grid-connected/off-grid switching technology is always a hot research problem of the distributed grid-connected inverter, wherein the pre-synchronization strategy is a key point of the grid-connected switching technology, but the pre-synchronization control strategy affects the original operation steady-state point after being put into use, so that the inverter needs to be cut off smoothly after the grid connection is completed.
Disclosure of Invention
The invention aims to solve the problem that when an inverter grid-connected pre-synchronous controller is cut off, the disturbance on a system is overlarge. Therefore, the invention provides a method for smoothly cutting off a pre-synchronous controller by an inverter grid connection.
The method for smoothly cutting off the pre-synchronous controller for the grid-connected inverter comprises the following steps.
Step 1: detecting inverter output voltage frequency and passing signals to a pre-synchronization controller:
after receiving a grid-connected signal, the isolated island operation inverter is put into a pre-synchronous controller to enable the phase and frequency of the output voltage of the inverter to be consistent with the voltage of a power grid, firstly, the voltage of the power grid is collected, and u is obtained through conversion of an abc-dq moduledAnd uqReference angle omega used by transformationtFor omega in droop controldroup(ii) a Will output uqAnd a reference value
Figure 100002_DEST_PATH_IMAGE002
After difference is made, the difference is led into a PI controller, and then omega is output through a latch and a gradient cutting modulesysWhen the latch is in an unlatched state, the output and input of the gradient cut-off module are completely consistent, and finally omega is setsysA droop control module is introduced to adjust the frequency and the phase of the output voltage of the inverter;
step 2: judging whether the grid connection process is finished or not, and sending a signal for cutting off the pre-synchronous controller when the grid connection process is finished:
after the output voltage phase and frequency of the inverter are judged to be consistent with the power grid, a grid-connected switch S is switched on, and after grid connection of the inverter is finished, a grid-connected condition judgment module of the pre-synchronous controller compares the phase angle theta of the power gridgridAnd inverter phase angle thetaoIf it is continuously less than
Figure 100002_DEST_PATH_IMAGE004
And can keep the difference value less than 5 switching periods
Figure 120648DEST_PATH_IMAGE004
If the grid connection is successful, sending a signal for cutting off the pre-synchronous controller;
and step 3: after receiving the signal, the pre-synchronous controller is cut off in a gradient mode, and the pre-synchronous controller is completely separated from the original inverter local controller:
after the latch and the gradient cutting module receive the signals, the latch keeps the current output signal omegasysAnd if not, the gradient cutting module reduces the input signal by 0.1 time successively and outputs the signal until the output is completely 0, and the presynchronization controller finishes smooth cutting.
Further, in step 1, the frequency and phase of the inverter output voltage are adjusted according to the following formula:
Figure 100002_DEST_PATH_IMAGE006
in the formula, ω*、P*And m and P respectively represent rated reference angular frequency, rated reference power, droop coefficient and actual inverter output power.
Compared with the prior art, the invention has the beneficial technical effects that:
the invention solves the problem of disturbance of the frequency and the power of the inverter when the pre-synchronous controller is cut off, and fills the blank of the prior engineering technology.
Drawings
FIG. 1 is a schematic diagram of inverter grid-connected control;
FIG. 2 is a diagram of a pre-synchronization controller architecture;
FIG. 3 is a waveform diagram comparing inverter voltage frequency with PCC point voltage frequency;
FIG. 4 is a waveform diagram comparing inverter output voltage with PCC point voltage phase;
FIG. 5 is a graph of a direct cut-away presynchronization controller and a cut-away inverter output voltage frequency waveform using a gradient method;
FIG. 6 is a waveform diagram of the difference between the output voltage frequency of the pre-synchronous controller and the voltage frequency of the PCC point cut by the gradient method.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
Fig. 1 shows a block diagram of a droop control structure of a grid-connected inverter according to the present invention. In FIG. 1u sabc 、u oabc 、i oabc 、i Labc Respectively representing three-phase grid voltage, three-phase output current and three-phase inductive current of the inverter.L f 、C f Respectively representing the filter inductor and the filter capacitor of the inverter.Z line Representing the line impedance. The control circuit comprises a power calculation module, a droop control module, a voltage outer ring control module, a current inner ring control module, a presynchronization module, a phase-locked loop module, an abc-dq conversion module from a three-phase static coordinate system to a two-phase rotating coordinate system and an SPWM modulation module.
The DC power supply UdcIs connected to the DC input side of the three-phase inverter, and the output of the inverter is connected to the three-phase filter inductor LfAnd a three-phase filter capacitor CfThe three-phase filter capacitor adopts a Y-shaped connection method, is connected with a grid-connected switch and is connected with a large power grid through line impedance.
The control circuit receives the filtered voltage and current signals as the input of the power calculation module, the active power P and the reactive power Q of the calculated signals are connected to the droop control module, the reference voltage and the frequency are obtained according to a droop formula and are used as the reference value of the voltage outer ring, the frequency can be integrated to obtain omega t which provides angle reference for all abc-dq of the inverter for local control, the output value of the voltage outer ring module is used as the current reference to the current inner ring module, and the u under the dq axis can be obtainedabAnd carrying out dq-abc inverse transformation on the signal to obtain a reference signal, and finally generating a PWM (pulse-width modulation) signal through an SPWM (sinusoidal pulse width modulation) module to drive a switching tube.
The method comprises the steps that an inverter works in an island operation state, a pre-synchronous controller does not work at the moment, the pre-synchronous controller is put into the pre-synchronous controller after a grid connection command is received, and a grid connection switch is switched on to realize grid connection when the phase and the frequency of the output voltage of the inverter are consistent with the voltage of a power grid. After the grid connection is finished, the pre-synchronization controller needs to smoothly exit the control link to avoid the influence of the pre-synchronization controller on the active power distribution of droop control, and at the moment, the grid connection condition judgment module compares the phase angle theta of the power gridgridAnd inverter phase angle thetaoIf it is continuously less than
Figure 693580DEST_PATH_IMAGE004
And can keep the difference value less than 5 switching periods
Figure 676580DEST_PATH_IMAGE004
The grid-connection is regarded as successful,sending out a pre-synchronous controller cut-off signal, after the latch receives the signal, keeping the current output signal omegasysAnd the gradient cutting module reduces the input signal by 0.1 times and outputs the input signal until the output is completely 0.
According to the circuit building simulation model verification method shown in FIG. 1, the parameters are as follows: u shapedc=800V，Lf=1.5mH，Cf=1.5mF，Pload=20kw，Qload=20kvar, sag factor: m =1e-5, n =3e-4, and the switching frequency fs =10 kHz. Wherein the presynchronization controller is shown in fig. 2.
Fig. 3 is a waveform diagram of the inverter voltage frequency after the pre-synchronization controller is put in, and it can be seen that after the pre-synchronization controller is put in at 0.5s, the output frequency of the inverter and the PCC point voltage frequency are consistent after 170ms, and the frequency value meets the grid-connected condition. Fig. 4 is a waveform diagram showing a comparison between phases of the inverter output voltage and the grid voltage after the pre-synchronization controller is put in, and it can be seen that after the pre-synchronization controller is put in at 0.5s, the phases of the inverter output voltage and the grid voltage are kept consistent and meet grid-connected conditions after 80 ms. Fig. 5 is a waveform diagram showing the frequency change of the output voltage after direct removal of the pre-synchronization controller after grid connection and removal of the pre-synchronization controller by using a gradient method, and it can be seen that the frequency fluctuates by 0.38Hz when the pre-synchronization controller is directly removed, and after the pre-synchronization controller is gradually removed by using the gradient method, the frequency is only disturbed by 0.14Hz, so that the pre-synchronization controller can be removed more smoothly. FIG. 6 is a waveform diagram of the frequency of the output voltage of the inverter of the pre-synchronous controller cut by the gradient method and the frequency of the voltage at the PCC point, and it can be seen that the frequency difference is only-0.12 Hz, and the method provided by the invention is effective.

Claims (1)

1. A method for smoothly cutting off a pre-synchronous controller during grid connection of an inverter is characterized by comprising the following steps:
step 1: detecting inverter output voltage frequency and passing signals to a pre-synchronization controller:
after receiving the grid-connected signal, the isolated island operation inverter puts into a pre-synchronous controller to enable the inverter to output powerThe voltage phase and frequency are consistent with the voltage of the power grid, the voltage of the power grid is collected firstly, and u is obtained through conversion of an abc-dq moduledAnd uqReference angle omega used by transformationtFor omega in droop controldroup(ii) a Will output uqAnd a reference value
Figure DEST_PATH_IMAGE002
After difference is made, the difference is led into a PI controller, and then omega is output through a latch and a gradient cutting modulesysWhen the latch is in an unlatched state, the output and input of the gradient cut-off module are completely consistent, and finally omega is setsysAnd (2) introducing a droop control module, adjusting the frequency and the phase of the output voltage of the inverter, specifically, adjusting the frequency and the phase of the output voltage of the inverter according to the following formula:
Figure DEST_PATH_IMAGE004
；
in the formula, ω*、P*M and P respectively represent rated reference angular frequency, rated reference power, droop coefficient and actual inverter output power;
step 2: judging whether the grid connection process is finished or not, and sending a signal for cutting off the pre-synchronous controller when the grid connection process is finished:
after the output voltage phase and frequency of the inverter are judged to be consistent with the power grid, a grid-connected switch S is switched on, and after grid connection of the inverter is finished, a grid-connected condition judgment module of the pre-synchronous controller compares the phase angle theta of the power gridgridAnd inverter phase angle thetaoIf it is continuously less than
Figure DEST_PATH_IMAGE006
And can keep the difference value less than 5 switching periods
Figure 854199DEST_PATH_IMAGE006
If the grid connection is successful, sending a signal for cutting off the pre-synchronous controller;
and step 3: after receiving the signal, the pre-synchronous controller is cut off in a gradient mode, and the pre-synchronous controller is completely separated from the original inverter local controller:
after the latch and the gradient cutting module receive the signals, the latch keeps the current output signal omegasysAnd if not, the gradient cutting module reduces the input signal by 0.1 time successively and outputs the signal until the output is completely 0, and the presynchronization controller finishes smooth cutting.
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