CN105514972A - PSCAD modeling and simulation method for grid-connected inverter during unsymmetrical failure of power grid - Google Patents

Abstract

The invention relates to a PSCAD modeling and simulation method for a grid-connected inverter during an unsymmetrical failure of a power grid and belongs to the technical field of PSCAD modeling and simulation methods. The technical problem to be solved is providing a PSCAD modeling and simulation method for a grid-connected inverter under a positive-negative dual-sequence independent control strategy. The technical scheme comprises following steps: first, the positive-negative sequence voltage and current of the three-phase voltage and current of the AC side of the grid-connected inverter are separated; the three-phase positive and negative voltage and current are converted to a [alpha][beta] axis to obtain reference values of positive-negative sequence current of a dq axis; then the PI parameter of current inner loop and voltage outer loop is calculated. The method is suitable for electrical power systems.

Description

The PSCAD building model and simulation method of grid-connected converter during unbalanced grid faults

Technical field

During unbalanced grid faults of the present invention, the PSCAD building model and simulation method of grid-connected converter, belongs to PSCAD building model and simulation method and technology field.

Background technology

All will being realized by PWM converter of the element of the non-power frequency of current numerous output frequency is grid-connected, such as photovoltaic generating system, straight drive blower etc., when electrical network generation unbalanced fault, according to instantaneous power theory, meritorious and the reactive power of the AC of grid-connected converter contains second harmonic component, also can there is secondary ripple wave in DC side busbar voltage simultaneously, when ripple is enough large, the normal operation of current transformer will be affected, even damage current transformer, in order to limit the ripple of current transformer DC bus-bar voltage, the double-current ring control strategy that a kind of positive-negative sequence independently controls is widely applied, in order to analyze the characteristic of grid-connected converter under this kind of control strategy better, be necessary to study its modeling method.

Summary of the invention

The present invention overcomes the deficiency that prior art exists, and technical problem to be solved is: provide a kind of and realize the PSCAD building model and simulation method of grid-connected converter under positive and negative pair of sequence independence control strategy.

In order to solve the problems of the technologies described above, the technical solution used in the present invention is: the PSCAD building model and simulation method of grid-connected converter during unbalanced grid faults, comprises the following steps:

The three-phase voltage current of grid-connected converter AC is carried out positive-negative sequence separation;

Positive sequence and negative sequence voltage electric current are transformed to α β axle by Clark, is pinned the phase place of positive-negative sequence voltage by three-phase phase-locked loop, then converted the voltage current transformation of α β axle to dq rotatable coordinate axis by park;

According to the control objectives of restriction DC side busbar voltage ripple, in conjunction with the requirement that current transformer unit power is run, try to achieve the reference value of positive-negative sequence current dq axle;

By trying to achieve the transfer function of outer voltage and current inner loop, provide the parameter tuning method of 5 pi regulators; By the overvoltage of discharging circuit restriction DC bus; The positive-negative sequence finally realizing grid-connected converter independently controls and limits DC bus-bar voltage ripple.

Further, specifically comprise the following steps:

Step S1: the separation three-phase voltage current of grid-connected converter AC being realized positive-negative sequence electric current and voltage by phase shift:

$\left[\begin{array}{c}{F}_a^P\\ F_b^P\\ F_c^P\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& \mathrm{\α}& {\mathrm{\α}}^2\\ {\mathrm{\α}}^2& 1& \mathrm{\α}\\ \mathrm{\α}& {\mathrm{\α}}^2& 1\end{array}\right]\left[\begin{array}{c}{F}_a\\ F_b\\ F_c\end{array}\right],\left[\begin{array}{c}{F}_a^N\\ F_b^N\\ F_c^N\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& {\mathrm{\α}}^2& \mathrm{\α}\\ \mathrm{\α}& 1& {\mathrm{\α}}^2\\ {\mathrm{\α}}^2& \mathrm{\α}& 1\end{array}\right]\left[\begin{array}{c}{F}_a\\ F_b\\ F_c\end{array}\right]---\left(1\right)$

In formula (1): P represents positive sequence, N represents negative phase-sequence; A, b, c represent A, B, B three-phase respectively; F representative voltage or electric current; α is rotor, wherein α=e ^{j2 π/3};

Step S2: by clark conversion by three-phase positive sequence and negative sequence voltage current transformation to α β axle, obtain and the phase place of positive-negative sequence voltage is pinned by phase-locked loop; Again by park conversion by the positive-negative sequence voltage transformation of α β axle to dq rotatable coordinate axis:

$\left[\begin{array}{c}{F}_d^P\\ F_q^P\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& -sin\mathrm{\θ}\\ sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{F}_{\mathrm{\α}}^P\\ F_{\mathrm{\β}}^P\end{array}\right],\left[\begin{array}{c}{F}_d^N\\ F_q^N\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -sin\mathrm{\θ}& cos\mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{F}_{\mathrm{\α}}^N\\ F_{\mathrm{\β}}^N\end{array}\right]---\left(2\right)$

Can realize thus: $\left\{\begin{array}{c}{u}_d^P=u_m^P\\ u_q^P=0\end{array}\right.,\left\{\begin{array}{c}{u}_d^N=u_m^N\\ u_d^N=0\end{array}\right.---\left(3\right)$

In formula (2), represent the voltage of α axle positive sequence or electric current, represent the voltage of β axle positive sequence or electric current, represent the voltage of α axle negative phase-sequence or electric current, represent voltage or the electric current of β axle negative phase-sequence; θ is the phase angle of the positive sequence voltage that phase-locked loop obtains; for AC positive sequence voltage amplitude, for AC negative sequence voltage amplitude;

In formula (3), be respectively d axle and the q axle component of positive-negative sequence voltage;

Step S3: the reference value obtaining positive-negative sequence current dq axle according to formula (4):

$\left[\begin{array}{c}{i}_d^{P*}\\ i_q^{P*}\\ i_d^{N*}\\ i_q^{N*}\end{array}\right]={\left[\begin{array}{cccc}{u}_d^p& u_q^p& u_d^N& u_q^N\\ u_q^p& -u_d^p& u_q^N& -u_d^N\\ u_q^N& -u_d^N& -u_q^p& u_d^p\\ u_d^N& u_q^N& u_d^p& u_q^p\end{array}\right]}^{-1}\left[\begin{array}{c}\frac{2}{3}{p}_0^{*}\\ 0\\ 0\\ 0\end{array}\right]=\frac{2p_0^{*}}{3D}\left[\begin{array}{c}{u}_d^p\\ u_q^p\\ -u_d^N\\ -u_q^N\end{array}\right]---\left(4\right)$

In formula (4), represent the reference value of positive and negative sequence electric current d axle and q axle component respectively; $D={\left(u_d^p\right)}^2-{\left(u_d^N\right)}^2;$

for the active power reference value that direct voltage outer shroud obtains, be specially:

$p_0^{*}=u_{dc}^{*}\[k_{vp}(u_{dc}^{*}-u_{dc})+k_{vi}\∫(u_{dc}^{*}-u_{dc})dt\]---\left(5\right)$

In formula (5), for DC bus-bar voltage reference value, u _dcfor DC bus-bar voltage, k _vpfor proportionality coefficient, the k of Voltage loop _vifor the integral coefficient of Voltage loop.

Step S4: obtain 4 PI parameters in current inner loop by formula (6), obtains 1 PI parameter in outer voltage by formula (7):

$\left\{\begin{array}{c}{k}_{ii}=L/\left(3K_{PWM}{T}_s\right)\\ k_{ip}=R/\left(3K_{PWM}{T}_s\right)\end{array}\right.---\left(6\right)$

$\left\{\begin{array}{c}{k}_{vi}=3C/528T_s^2\\ k_{vp}=60C/528T_s\end{array}\right.---\left(7\right)$

In formula (6), k _ipfor proportionality coefficient, the k of electric current loop _iifor the integral coefficient of electric current loop; R, L are equivalent electric resistance and the inductance of net side filter inductance; K _pWMfor the equivalent gain of PWM converter; T _sthe sampling period of simulation model;

In formula (7), C is the electric capacity of DC bus.

Realized the time delay of rotor in step S1 by the delay cell in PSCAD, in step S2, phase-locked loop realizes with PLL element in PSCAD.

The present invention compared with prior art has following beneficial effect:

1, the PSCAD building model and simulation method of grid-connected converter during unbalanced grid faults of the present invention, in order to limit the impact of secondary ripple wave on current transformer, the present invention not only can limit the secondary ripple wave of active power and DC bus-bar voltage effectively, thus improve the operational efficiency of current transformer, and the low voltage ride-through capability of inverse distributed power can be improved.

2, the present invention, use PSCAD simulation software, PSCAD simulation software has intuitively easy-to-use graphical interfaces and accurately simulation efficiently, both AC and DC power system problem can be studied, power electronic device emulation and nonlinear Control can be completed again, the present invention is by PSCAD simulation software, give the two sequence of positive sequence independently control in the separation of positive-negative sequence and orientation, the calculating of positive-negative sequence voltage reference value, the implementation method of the important steps such as the adjustment of PI parameter, finally achieve the emulation under positive and negative pair of sequence independence control strategy of grid-connected converter, effectively inhibit the ripple of DC side busbar voltage and AC active power, method of the present invention is that the modeling and simulation of grid-connected power generation system under electrical network asymmetrical provides theoretical foundation.

Accompanying drawing explanation

Below in conjunction with accompanying drawing, the present invention will be further described in detail;

Fig. 1 is the topological structure schematic diagram of grid-connected converter;

Fig. 2 is the positive-negative sequence independence control strategy block diagram of grid-connected converter;

Fig. 3 is the superpotential additional control circuit schematic diagram of restriction DC bus;

Fig. 4 is AC three-phase positive sequence voltage schematic diagram;

Fig. 5 is AC three-phase negative/positive V diagram;

Fig. 6 is AC three-phase forward-order current schematic diagram;

Fig. 7 is AC three-phase negative/positive current diagram;

Fig. 8 is AC positive-negative sequence voltage dq axle component schematic diagram;

Fig. 9 is AC positive-negative sequence current dq axle component schematic diagram;

Figure 10 is AC active power and reactive power schematic diagram;

Figure 11 is DC side busbar voltage schematic diagram.

Embodiment

As shown in Figure 1 to 11, the PSCAD building model and simulation method of grid-connected converter during unbalanced grid faults, comprises the following steps:

First the three-phase voltage current of grid-connected converter AC is realized the separation of positive-negative sequence electric current and voltage by phase shift;

Positive sequence and negative sequence voltage electric current are transformed to α β axle by Clark, is pinned the phase place of positive-negative sequence voltage by three-phase phase-locked loop, then converted the voltage current transformation of α β axle to dq rotatable coordinate axis by park;

According to the control objectives of restriction DC side busbar voltage ripple, in conjunction with the requirement that current transformer unit power is run, try to achieve the reference value of positive-negative sequence current dq axle;

By trying to achieve the transfer function of outer voltage and current inner loop, provide the parameter tuning method of 5 pi regulators; By the overvoltage of discharging circuit restriction DC bus; The positive-negative sequence finally realizing grid-connected converter independently controls and limits DC bus-bar voltage ripple.

Specifically comprise the following steps:

Step S1: the separation three-phase voltage current of grid-connected converter AC being realized positive-negative sequence electric current and voltage by phase shift:

$\left[\begin{array}{c}{F}_a^P\\ F_b^P\\ F_c^P\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& \mathrm{\α}& {\mathrm{\α}}^2\\ {\mathrm{\α}}^2& 1& \mathrm{\α}\\ \mathrm{\α}& {\mathrm{\α}}^2& 1\end{array}\right]\left[\begin{array}{c}{F}_a\\ F_b\\ F_c\end{array}\right],\left[\begin{array}{c}{F}_a^N\\ F_b^N\\ F_c^N\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& {\mathrm{\α}}^2& \mathrm{\α}\\ \mathrm{\α}& 1& {\mathrm{\α}}^2\\ {\mathrm{\α}}^2& \mathrm{\α}& 1\end{array}\right]\left[\begin{array}{c}{F}_a\\ F_b\\ F_c\end{array}\right]---\left(1\right)$

In formula (1): P represents positive sequence, N represents negative phase-sequence; A, b, c represent A, B, B three-phase respectively; F representative voltage or electric current; α is rotor, wherein α=e ^{j2 π/3}, a vector is multiplied by rotor can think its time delay 120 °, and a vector is multiplied by α ²represent its time delay 240 °, specifically realize by the delay cell in PSCAD;

Step S2: by clark conversion by three-phase positive sequence and negative sequence voltage current transformation to α β axle, obtain and the phase place of positive-negative sequence voltage is pinned by phase-locked loop, phase-locked loop realizes with PLL element in PSCAD; Again by park conversion by the positive-negative sequence voltage transformation of α β axle to dq rotatable coordinate axis:

$\left[\begin{array}{c}{F}_d^P\\ F_q^P\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& -sin\mathrm{\θ}\\ \sin \mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{F}_{\mathrm{\α}}^P\\ F_{\mathrm{\β}}^P\end{array}\right],\left[\begin{array}{c}{F}_d^N\\ F_q^N\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -sin\mathrm{\θ}& cos\mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{F}_{\mathrm{\α}}^N\\ F_{\mathrm{\β}}^N\end{array}\right]---\left(2\right)$

Can realize thus: $\left\{\begin{array}{c}{u}_d^P=u_m^P\\ u_q^P=0\end{array}\right.,\left\{\begin{array}{c}{u}_d^N=u_m^N\\ u_d^N=0\end{array}\right.---\left(3\right)$

In formula (2), represent the voltage of α axle positive sequence or electric current, represent the voltage of β axle positive sequence or electric current, represent the voltage of α axle negative phase-sequence or electric current, represent voltage or the electric current of β axle negative phase-sequence; θ is the phase angle of the positive sequence voltage that phase-locked loop obtains; for AC positive sequence voltage amplitude, for AC negative sequence voltage amplitude;

In formula (3), be respectively d axle and the q axle component of positive-negative sequence voltage;

Step S3: the reference value obtaining positive-negative sequence current dq axle according to formula (4):

$\left[\begin{array}{c}{i}_d^{P*}\\ i_q^{P*}\\ i_d^{N*}\\ i_q^{N*}\end{array}\right]={\left[\begin{array}{cccc}{u}_d^p& u_q^p& u_d^N& u_q^N\\ u_q^p& -u_d^p& u_q^N& -u_d^N\\ u_q^N& -u_d^N& -u_q^p& u_d^p\\ u_d^N& u_q^N& u_d^p& u_q^p\end{array}\right]}^{-1}\left[\begin{array}{c}\frac{2}{3}{p}_0^{*}\\ 0\\ 0\\ 0\end{array}\right]=\frac{2p_0^{*}}{3D}\left[\begin{array}{c}{u}_d^p\\ u_q^p\\ -u_d^N\\ -u_q^N\end{array}\right]---\left(4\right)$

In formula (4), represent the reference value of positive and negative sequence electric current d axle and q axle component respectively; $D={\left(u_d^p\right)}^2-{\left(u_d^N\right)}^2;$

for the active power reference value that direct voltage outer shroud obtains, be specially:

$p_0^{*}=u_{dc}^{*}\[k_{vp}(u_{dc}^{*}-u_{dc})+k_{vi}\∫(u_{dc}^{*}-u_{dc})dt\]---\left(5\right)$

In formula (5), for DC bus-bar voltage reference value, u _dcfor DC bus-bar voltage, k _vpfor proportionality coefficient, the k of Voltage loop _vifor the integral coefficient of Voltage loop.

Step S4: obtain 4 PI parameters in current inner loop by formula (6), obtains 1 PI parameter in outer voltage by formula (7):

$\left\{\begin{array}{c}{k}_{ii}=L/\left(3K_{PWM}{T}_s\right)\\ k_{ip}=R/\left(3K_{PWM}{T}_s\right)\end{array}\right.---\left(6\right)$

$\left\{\begin{array}{c}{k}_{vi}=3C/528T_s^2\\ k_{vp}=60C/528T_s\end{array}\right.---\left(7\right)$

In formula (6), k _ipfor proportionality coefficient, the k of electric current loop _iifor the integral coefficient of electric current loop; R, L are equivalent electric resistance and the inductance of net side filter inductance; K _pWMfor the equivalent gain of PWM converter; T _sthe sampling period of simulation model; In formula (7), C is the electric capacity of DC bus.

The present invention is by PSCAD simulation software, give the two sequence of positive sequence independently control in the implementation method of the important step such as the separation of positive-negative sequence and orientation, the calculating of positive-negative sequence voltage reference value, the adjustment of PI parameter, finally achieve the emulation under positive and negative pair of sequence independence control strategy of grid-connected converter, effectively inhibit the ripple of DC side busbar voltage and AC active power, method of the present invention is that the modeling and simulation of grid-connected power generation system under electrical network asymmetrical provides theoretical foundation; There is outstanding substantive distinguishing features and significant progress; By reference to the accompanying drawings embodiments of the invention are explained in detail above, but the present invention is not limited to above-described embodiment, in the ken that those of ordinary skill in the art possess, various change can also be made under the prerequisite not departing from present inventive concept.

Claims (4)

1. the PSCAD building model and simulation method of grid-connected converter during unbalanced grid faults, is characterized in that: comprise the following steps:

The three-phase voltage current of grid-connected converter AC is carried out positive-negative sequence separation;

Positive sequence and negative sequence voltage electric current are transformed to α β axle by Clark, is pinned the phase place of positive-negative sequence voltage by three-phase phase-locked loop, then converted the voltage current transformation of α β axle to dq rotatable coordinate axis by park;

According to the control objectives of restriction DC side busbar voltage ripple, in conjunction with the requirement that current transformer unit power is run, try to achieve the reference value of positive-negative sequence current dq axle;

By trying to achieve the transfer function of outer voltage and current inner loop, provide the parameter tuning method of 5 pi regulators; By the overvoltage of discharging circuit restriction DC bus; The positive-negative sequence finally realizing grid-connected converter independently controls and limits DC bus-bar voltage ripple.

2. according to claim 1 unbalanced grid faults time grid-connected converter PSCAD building model and simulation method, it is characterized in that: specifically comprise the following steps:

Step S1: the separation three-phase voltage current of grid-connected converter AC being realized positive-negative sequence electric current and voltage by phase shift:

$\left[\begin{array}{c}{F}_a^P\\ F_b^P\\ F_c^P\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& \mathrm{\α}& {\mathrm{\α}}^2\\ {\mathrm{\α}}^2& 1& \mathrm{\α}\\ \mathrm{\α}& {\mathrm{\α}}^2& 1\end{array}\right]\left[\begin{array}{c}{F}_a\\ F_b\\ F_c\end{array}\right],\left[\begin{array}{c}{F}_a^N\\ F_b^N\\ F_c^N\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& {\mathrm{\α}}^2& \mathrm{\α}\\ \mathrm{\α}& 1& {\mathrm{\α}}^2\\ {\mathrm{\α}}^2& \mathrm{\α}& 1\end{array}\right]\left[\begin{array}{c}{F}_a\\ F_b\\ F_c\end{array}\right]---\left(1\right)$

In formula (1): P represents positive sequence, N represents negative phase-sequence; A, b, c represent A, B, B three-phase respectively; F representative voltage or electric current; α is rotor, wherein α=e ^{j2 π/3};

Step S2: by clark conversion by three-phase positive sequence and negative sequence voltage current transformation to α β axle, obtain and the phase place of positive-negative sequence voltage is pinned by phase-locked loop; Again by park conversion by the positive-negative sequence voltage transformation of α β axle to dq rotatable coordinate axis:

$\left[\begin{array}{c}{F}_d^P\\ F_q^P\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& -sin\mathrm{\θ}\\ sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{F}_{\mathrm{\α}}^P\\ F_{\mathrm{\β}}^P\end{array}\right],\left[\begin{array}{c}{F}_d^N\\ F_q^N\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{F}_{\mathrm{\α}}^N\\ F_{\mathrm{\β}}^N\end{array}\right]---\left(2\right)$

Can realize thus: $\left\{\begin{array}{c}{u}_d^P=u_m^P\\ u_q^P=0\end{array}\right.,\left\{\begin{array}{c}{u}_d^N=u_m^N\\ u_d^N=0\end{array}\right.---\left(3\right)$

In formula (2), represent the voltage of α axle positive sequence or electric current, represent the voltage of β axle positive sequence or electric current, represent the voltage of α axle negative phase-sequence or electric current, represent voltage or the electric current of β axle negative phase-sequence; θ is the phase angle of the positive sequence voltage that phase-locked loop obtains; for AC positive sequence voltage amplitude, for AC negative sequence voltage amplitude;

In formula (3), be respectively d axle and the q axle component of positive-negative sequence voltage;

Step S3: the reference value obtaining positive-negative sequence current dq axle according to formula (4):

$\left[\begin{array}{c}{i}_d^{P*}\\ i_q^{P*}\\ i_d^{N*}\\ i_q^{N*}\end{array}\right]={\left[\begin{array}{cccc}{u}_d^p& u_q^p& u_d^N& u_q^N\\ u_q^p& -u_d^p& u_q^N& -u_d^N\\ u_q^N& -u_d^N& -u_q^p& u_d^p\\ u_d^N& u_q^N& u_d^p& u_q^p\end{array}\right]}^{-1}\left[\begin{array}{c}\frac{2}{3}{p}_0^{*}\\ 0\\ 0\\ 0\end{array}\right]=\frac{2p_0^{*}}{3D}\left[\begin{array}{c}{u}_d^p\\ u_q^p\\ -u_d^N\\ -u_q^N\end{array}\right]---\left(4\right)$

In formula (4), represent the reference value of positive and negative sequence electric current d axle and q axle component respectively; $D={\left(u_d^p\right)}^2-{\left(u_d^N\right)}^2;$

for the active power reference value that direct voltage outer shroud obtains, be specially:

$p_0^{*}=u_{dc}^{*}\[k_{vp}(u_{dc}^{*}-u_{dc})+k_{vi}\∫(u_{dc}^{*}-u_{dc})dt\]---\left(5\right)$

In formula (5), for DC bus-bar voltage reference value, u _dcfor DC bus-bar voltage, k _vpfor proportionality coefficient, the k of Voltage loop _vifor the integral coefficient of Voltage loop.

Step S4: obtain 4 PI parameters in current inner loop by formula (6), obtains 1 PI parameter in outer voltage by formula (7):

$\left\{\begin{array}{c}{k}_{ii}=L/\left(3K_{PWM}{T}_s\right)\\ k_{ip}=R/\left(3K_{PWM}{T}_s\right)\end{array}\right.---\left(6\right)$

$\left\{\begin{array}{c}{k}_{vi}=3C/528T_s^2\\ k_{vp}=60C/528T_s\end{array}\right.---\left(7\right)$

In formula (6), k _ipfor proportionality coefficient, the k of electric current loop _iifor the integral coefficient of electric current loop; R, L are equivalent electric resistance and the inductance of net side filter inductance; K _pWMfor the equivalent gain of PWM converter; T _sthe sampling period of simulation model;

In formula (7), C is the electric capacity of DC bus.

3. according to claim 2 unbalanced grid faults time grid-connected converter PSCAD building model and simulation method, it is characterized in that: the time delay being realized rotor in step S1 by the delay cell in PSCAD.

4. according to claim 2 unbalanced grid faults time grid-connected converter PSCAD building model and simulation method, it is characterized in that: in step S2, phase-locked loop realizes with PLL element in PSCAD.