CN109474012B - Frequency coupling suppression method of three-phase grid-connected converter under weak grid condition - Google Patents

Frequency coupling suppression method of three-phase grid-connected converter under weak grid condition Download PDF

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CN109474012B
CN109474012B CN201811437103.9A CN201811437103A CN109474012B CN 109474012 B CN109474012 B CN 109474012B CN 201811437103 A CN201811437103 A CN 201811437103A CN 109474012 B CN109474012 B CN 109474012B
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张学广
易传卓
张思缘
夏丹妮
陈文佳
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Harbin Institute of Technology
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    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract

A frequency coupling suppression method of a three-phase grid-connected converter under the condition of weak power grid relates to the field of stability analysis of a power system grid-connected converter. The invention aims to solve the problem that the system stability is influenced by the impedance of a power grid, a phase-locked loop and the bandwidth of a voltage loop under the condition of a weak power grid. The frequency coupling suppression method of the three-phase grid-connected converter under the condition of weak power grid considers that the frequency coupling phenomenon is introduced by the coordinate transformation needed by the controller under a synchronous rotating coordinate system in the three-phase grid-connected converter, so that the problem of system stability caused by asymmetry of a phase-locked loop and a voltage loop in d and q axes under the condition of weak power grid is solved by using a compensation method, and the THD of grid side current is reduced when the grid voltage is disturbed.

Description

Frequency coupling suppression method of three-phase grid-connected converter under weak grid condition
Technical Field
The invention belongs to the field of stability analysis of a grid-connected converter of a power system, and particularly relates to analysis of influence of a frequency coupling phenomenon on stability under a weak point grid condition.
Background
With the large number of applications of power electronic devices in power systems, the trend of electronization changes many characteristics of the conventional power systems, but a new stability problem inevitably occurs.
Currently, there are two main types of methods for analyzing the stability of a power system: the method can accurately analyze the resonance characteristics of the system through characteristic values, but needs to acquire detailed information of the system and is difficult to model a converter system containing multi-time scale control; the other is a frequency domain impedance analysis method, which converts the dynamic characteristics of the converter port into an impedance form through a frequency domain transfer function and further analyzes the system stability through a circuit theory.
Today, the most common method of analyzing the stability problem of the grid-connected converter is the impedance-based stability analysis method, which was first proposed by Middlebrook with the aim of designing the filter parameters of the DC/DC converter. The method has the advantages that the small signal stability of the system can be conveniently determined only by acquiring the output or input impedance of the converter or the power grid and applying the Nyquist stability criterion.
Under the condition of weak power grid, the system stability is influenced by the power grid impedance, the phase-locked loop and the voltage loop bandwidth, and the stability is also influenced by the frequency coupling phenomenon which is influenced by the factors. However, the existing stability improvement methods for these factors do not consider the influence of the frequency coupling phenomenon caused by these factors on the stability.
Disclosure of Invention
The invention aims to solve the problems that the system stability is influenced by the impedance of a power grid, the bandwidth of a phase-locked loop and the voltage loop under the condition of a weak power grid and the stability is influenced by the frequency coupling phenomenon which is influenced by the factors.
The frequency coupling suppression method of the three-phase grid-connected converter under the condition of weak power grid comprises the following steps:
the method comprises the following steps: respectively to three-phase voltage Ua、Ub、UcAnd three-phase current Ia、Ib、IcClark conversion and Park conversion are sequentially carried out to obtain d-axis and q-axis voltage components U under a synchronous rotating coordinate systemd、UqAnd d, q-axis current components Id、Iq，
Step two: are respectively paired with Ud、UqAnd Id、IqHigh-pass filtering is carried out to respectively extract Ud、Uq、IdAnd IqThe disturbance component of (a) is,
step three: according to the main circuit element parameters and the voltage ring PI parameters G of the three-phase grid-connected converterc_vdcObtaining the coefficientsk1According to Ud、Uq、IdAnd IqObtaining a vector of the disturbance component
Figure BDA0001884040190000021
Will be provided with
Figure BDA0001884040190000022
Real part and k of1The product of (a) and (b) is compensated into the d-axis given signal of the current loop
Figure BDA0001884040190000023
Imaginary part of and k1The product of (a) is compensated into the q-axis given signal of the current loop,
step four: obtaining a coefficient k from a phase-locked loop parameter2Will U isdAnd UqRespectively with a stable value I of the grid-connected point current0Multiply and then multiply Ud·I0And Uq·I0Are respectively connected with k2The multiplied product is compensated to the Park transformed grid-connected point current,
step five: will UdAnd UqRespectively with a stable value E of the grid voltage0Multiply and then multiply Ud·E0And Uq·E0Are respectively connected with k2And compensating the multiplied product into the grid voltage subjected to Park conversion to finish frequency coupling suppression.
The frequency coupling suppression method of the three-phase grid-connected converter under the condition of weak power grid considers that the coordinate transformation needed by the controller under a synchronous rotating coordinate system can introduce the frequency coupling phenomenon in the three-phase grid-connected converter, so that the system stability problem caused by asymmetry of phase-locked loops and voltage loops at d and q axes under the condition of weak power grid is solved by using a compensation method, and the THD (total harmonic distortion) of grid side current when the grid voltage has disturbance is reduced. The invention is completely realized in the controller, and other parameters of the main circuit do not need to be additionally measured, hardware does not need to be additionally added, and the cost is not increased.
Drawings
FIG. 1 is a schematic view ofThe structure of the three-phase grid-connected converter controller after frequency coupling suppression is added, and u in the figuredcRepresenting the voltage loop output voltage, EdAs d-axis component, E, of the grid voltageqThe q-axis component of the grid voltage, Acc for the current regulator and PWM for the pulse width modulation.
Detailed Description
The first embodiment is as follows: specifically describing the present embodiment with reference to fig. 1, the method for suppressing frequency coupling of a three-phase grid-connected converter in the present embodiment under weak grid conditions includes the following steps:
the method comprises the following steps: three-phase voltage U of grid-connected point by using voltage sensora、Ub、UcSampling, and utilizing current sensor to make grid-connected point three-phase current Ia、Ib、IcSampling is performed.
Step two: respectively carrying out Clark conversion (changing variables in an abc coordinate system with three phases being static and different from each other by 120 degrees into an alpha beta coordinate system with two phases being static and different from each other by 90 degrees) and Park conversion (converting into a synchronous rotating coordinate system) on three-phase voltage and three-phase current in sequence to obtain d-axis and q-axis voltage components U under the synchronous rotating coordinate systemd、UqAnd d, q-axis current components Id、Iq。
Step three: are respectively paired with Ud、UqAnd Id、IqHigh-pass filtering is carried out to eliminate fundamental wave components of all parameters, and then U is extracted respectivelyd、Uq、IdAnd IqThe disturbance component of (1).
Step four: according to the main circuit element parameters and the voltage ring PI parameters G of the three-phase grid-connected converterc_vdcObtaining the coefficient k1According to Ud、Uq、IdAnd IqObtaining a vector of the disturbance component
Figure BDA0001884040190000031
Will be provided with
Figure BDA0001884040190000032
Real part and k of1The product of the first and second currents is compensated to a d-axis given signal of the current loop to obtain a d-axis given signal I of the compensated current loopdref1：
Figure BDA0001884040190000033
Will be provided with
Figure BDA0001884040190000034
Imaginary part of and k1The product of the first and second currents is compensated to a q-axis given signal of the current loop to obtain a q-axis given signal I of the compensated current loopqref1：
Figure BDA0001884040190000035
In the above formula, IqrefA q-axis current reference value when no decoupling control is performed,
Figure BDA0001884040190000036
Is a transfer function of the outer ring of the DC voltage,
Figure BDA0001884040190000037
DC side impedance, U, in complex vector formdc,0The dc side voltage is in complex vector form.
Step five: obtaining a coefficient k from a phase-locked loop parameter2Will U isdAnd UqRespectively with a stable value I of the grid-connected point current0Multiply and then multiply Ud·I0And Uq·I0Are respectively connected with k2The multiplied product is compensated to the grid-connected point current after Park conversion, and the compensated grid-connected point current
Figure BDA0001884040190000038
(including d-axis current
Figure BDA0001884040190000039
And q-axis current
Figure BDA00018840401900000310
) Comprises the following steps:
Figure BDA00018840401900000311
in the above formula, IdqGrid connection point current G in complex vector form under synchronous rotating coordinate systemPLLFor the phase-locked loop transfer function, UdqThe grid-connected point voltage is in a complex vector form under a synchronous rotating coordinate system.
Step six: will UdAnd UqRespectively with a stable value E of the grid voltage0Multiply and then multiply Ud·E0And Uq·E0Are respectively connected with k2The multiplied product is compensated to the Park converted network voltage, the compensated network voltage
Figure BDA00018840401900000312
(including d-axis current
Figure BDA00018840401900000313
And q-axis current
Figure BDA00018840401900000314
) Comprises the following steps:
Figure BDA00018840401900000315
in the above formula, EdqGrid voltage G in complex vector form under synchronous rotating coordinate systemPLLFor the phase-locked loop transfer function, UdqThe grid-connected point voltage is in a complex vector form under a synchronous rotating coordinate system.
By respectively compensating the d-axis given signal and the q-axis given signal of the current loop, the grid-connected point current and the power grid voltage, the suppression of the frequency coupling phenomenon can be achieved, and the purpose of improving the stability of the converter under the condition of weak power grid is achieved.

Claims (2)

1. The frequency coupling suppression method of the three-phase grid-connected converter under the condition of weak power grid is characterized by comprising the following steps of:
the method comprises the following steps: respectively to three-phase voltage Ua、Ub、UcAnd three-phase current Ia、Ib、IcClark conversion and Park conversion are sequentially carried out to obtain d-axis and q-axis voltage components U under a synchronous rotating coordinate systemd、UqAnd d, q-axis current components Id、Iq，
Step two: are respectively paired with Ud、UqAnd Id、IqHigh-pass filtering is carried out to respectively extract Ud、Uq、IdAnd IqThe disturbance component of (a) is,
step three: according to the main circuit element parameters and the voltage ring PI parameters G of the three-phase grid-connected converterc_vdcObtaining the coefficient k1According to Ud、Uq、IdAnd IqObtaining a vector of the disturbance component
Figure FDF0000015742160000011
Will be provided with
Figure FDF0000015742160000012
Real part and k of1The product of (a) and (b) is compensated into the d-axis given signal of the current loop
Figure FDF0000015742160000013
Imaginary part of and k1The product of (a) is compensated into the q-axis given signal of the current loop,
step four: obtaining a coefficient k from a phase-locked loop parameter2Will U isdAnd UqRespectively with a stable value I of the grid-connected point current0Multiply and then multiply Ud·I0And Uq·I0Are respectively connected with k2The multiplied product is compensated to the Park transformed grid-connected point current,
step five: will UdAnd UqRespectively with a stable value E of the grid voltage0Multiply and then multiply Ud·E0And Uq·E0Are respectively connected with k2Compensating the multiplied product into the grid voltage subjected to Park conversion to finish frequency coupling suppression;
in the third step, the first step is that,
d-axis given signal I of compensated current loopdref1Comprises the following steps:
Figure FDF0000015742160000014
q-axis given signal I of compensated current loopqref1Comprises the following steps:
Figure FDF0000015742160000015
in the above formula, IqrefA q-axis current reference value when no decoupling control is performed,
Figure FDF0000015742160000016
Is a transfer function of the outer ring of the DC voltage,
Figure FDF0000015742160000017
DC side impedance, U, in complex vector formdc,0A dc side voltage in complex vector form;
in step four, the compensated grid-connected point current
Figure FDF0000015742160000018
Comprises the following steps:
Figure FDF0000015742160000019
in the above formula, IdqGrid connection point current G in complex vector form under synchronous rotating coordinate systemPLLFor the phase-locked loop transfer function, UdqThe grid-connected point voltage is in a complex vector form under a synchronous rotating coordinate system;
step (ii) ofFifthly, the compensated network voltage
Figure FDF0000015742160000021
Comprises the following steps:
Figure FDF0000015742160000022
in the above formula, EdqGrid voltage G in complex vector form under synchronous rotating coordinate systemPLLFor the phase-locked loop transfer function, UdqThe grid-connected point voltage is in a complex vector form under a synchronous rotating coordinate system.
2. The method for suppressing the frequency coupling of the three-phase grid-connected converter under the weak grid condition according to claim 1, wherein before the step one, a voltage sensor is used for comparing a grid-connected point three-phase voltage U with a voltage sensora、Ub、UcSampling, and utilizing current sensor to make grid-connected point three-phase current Ia、Ib、IcSampling is performed.
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