Virtual Synchronous Machine Coordinated Control Method for True Bipolar Flexible DC Transmission System

Abstract

The present invention discloses a coordinated control method for a virtual synchronous machine in a true bipolar flexible DC transmission system. Aiming at the deficiency that the existing control strategy of the true bipolar flexible DC transmission system cannot provide inertia support for the power grid during the grid fault stage, by simulating the mathematical model of a traditional synchronous machine, a coordinated virtual synchronous machine control strategy for the positive and negative poles applicable to the true bipolar system is proposed, which can provide inertia support for the AC power grid under the condition of ensuring that the system does not have overcurrent. Compared with the existing methods, this method can not only provide stable voltage and frequency for the new energy power grid, but also enhance the inertia stability of the power grid when a fault occurs, and is more friendly to the new power system containing a large-scale new energy power generation system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system. Background Art

[0002] With the gradual increase of the voltage level and transmission capacity of flexible DC transmission systems, true bipolar flexible DC transmission systems have received increasing attention due to their high flexibility and reliability. Flexible DC transmission technology based on the modular multilevel converter (MMC) topology has the advantages of low manufacturing difficulty, low system loss, and high waveform quality, and has very good application prospects in long-distance power energy collection and transmission. Due to the large-scale access of new energy power generation systems based on power electronic devices such as wind power and photovoltaic power, the inertia stability of the AC grid has been significantly reduced, the grid volatility and uncertainty have increased, and ultimately the safety risks of grid operation have increased significantly. Therefore, it is urgent to study the control strategy of flexible DC transmission systems with inertia support capabilities, so as to enhance their inertia support capabilities for the grid and improve grid stability.

[0003] When a true bipolar MMC converter station is connected to a passive AC grid or an island new energy grid, it is necessary to establish stable frequency and voltage for the AC grid. The traditional true bipolar flexible DC transmission system generally adopts a constant voltage and frequency control strategy, which has good control performance under steady-state operating conditions. However, when a grid fault occurs, it does not have the ability to provide inertia support for the grid like traditional synchronous generators, and is prone to system instability.

[0004] The virtual synchronous machine control strategy realizes the control goal of providing inertia support for the grid by simulating the mathematical model of a traditional synchronous machine, and has been widely used in converter control. However, for a true bipolar flexible DC transmission system, it is necessary to not only consider the coordination between the positive and negative poles, but also ensure that overcurrent does not occur during a fault, so as to ensure equipment safety. At present, there is still little research on the application of the virtual synchronous machine control strategy in a true bipolar flexible DC transmission system. It is urgent to propose a virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system, so as to provide inertia support for the AC grid and ensure the safe and stable operation of a new power system including a large-scale new energy power generation system. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiency that the existing control strategy of a true bipolar flexible DC transmission system cannot provide inertia support for the grid during the grid fault stage, and provide a virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system. By optimizing the control strategy, virtual inertia is introduced, and the coordinated control between the positive and negative poles of the true bipolar flexible DC transmission system is realized, so as to provide inertia support for the AC grid under the condition of ensuring that the system does not have overcurrent.

[0006] To achieve the above-mentioned invention object, the following technical solutions are adopted in this method:

[0007] A virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system. The control system adopted to implement the method includes: a virtual mechanical module, a virtual excitation module, a positive electrode sampling module and a negative electrode sampling module, a positive electrode coordinate transformation module and a negative electrode coordinate transformation module, a positive electrode power calculation module and a negative electrode power calculation module, a positive electrode voltage controller and a negative electrode voltage controller, a positive electrode current controller and a negative electrode current controller, a positive electrode internal circulating current controller and a negative electrode internal circulating current controller, a positive electrode bridge arm voltage calculation module and a negative electrode bridge arm voltage calculation module, a positive electrode modulation module and a negative electrode modulation module;

[0008] The virtual mechanical module controls the total active power of the positive and negative electrodes by simulating the mechanical equation of a synchronous generator. The output of the virtual mechanical module serves as the reference phase θ of the true bipolar flexible DC transmission system r ;

[0009] The virtual excitation module controls the total reactive power of the positive and negative electrodes by simulating the excitation equation of a synchronous generator. The output of the virtual excitation module serves as the d-axis voltage reference value U of the true bipolar flexible DC transmission system gdref ;

[0010] The positive electrode sampling module includes a positive electrode voltage sampling module and a positive electrode current sampling module, and the negative electrode sampling module includes a negative electrode voltage sampling module and a negative electrode current sampling module;

[0011] The positive electrode voltage sampling module and the negative electrode voltage sampling module sample the three-phase voltage U of the MMC AC power grid gabc ;

[0012] The positive electrode current sampling module and the negative electrode current sampling module respectively sample the three-phase currents I of the positive and negative MMC AC power grids gabc1 and I gabc2 , the internal circulating currents I of the positive and negative MMCs cabc1 and I cabc2 ;

[0013] The positive electrode coordinate transformation module includes a positive electrode Park transformation module and a positive electrode Park inverse transformation module, and the negative electrode coordinate transformation module includes a negative electrode Park transformation module and a negative electrode Park inverse transformation module;

[0014] The positive electrode Park transformation module and the negative electrode Park transformation module transform the three-phase voltage U of the MMC AC power grid gabc and the three-phase currents I of the positive and negative MMC AC power grids gabc1 and I gabc2Perform Park transformation to obtain the corresponding voltage vector U in the synchronous rotating d-q coordinate system gdq and current vector I gdq1 and I gdq2 , and the angle used for Park transformation is the reference phase θ r ;

[0015] The positive Park inverse transformation module and the negative Park inverse transformation module respectively perform Park inverse transformation on the positive and negative reference differential-mode voltages U difdq1 and U difdq2 to obtain the reference differential-mode voltages U difabc1 and U difabc2 in the positive and negative stationary three-phase coordinate systems, and the angle used for Park inverse transformation is the reference phase θ r ;

[0016] The positive power calculation module and the negative power calculation module respectively calculate the positive and negative active powers P gabc and the positive and negative reactive powers Q gabc1 and I gabc2 based on the three-phase voltages U g1 and P g2 of the MMC AC power grid and the three-phase currents I g1 and Q g2 ;

[0017] The positive voltage controller and the negative voltage controller control the d-axis and q-axis voltages U gdq through a PI controller to make them respectively follow the given reference values U gdref and U gqref , where U gqref is set to 0. After passing through the limiting link, the outputs of the positive voltage controller and the negative voltage controller are respectively used as the reference values I gdref1 and I gqref1 of the positive d-axis and q-axis currents, and the reference values I gdref2 and I gqref2 of the negative d-axis and q-axis currents;

[0018] The positive current controller and the negative current controller respectively use a PI controller to control the positive and negative d-axis and q-axis currents I gdq1 and I gdq2 to make them follow the reference values output by the positive voltage controller and the negative voltage controller. The outputs of the positive current controller and the negative current controller are used as the positive and negative MMC reference differential-mode voltages U difdq1 and U difdq2 ;

[0019] The positive - side internal - circulation controller and the negative - side internal - circulation controller respectively control the internal circulations I cabc1 and I cabc2 of the positive - side and negative - side MMCs by using a resonant controller, which can be achieved in the stationary coordinate system. The outputs of the positive - side internal - circulation controller and the negative - side internal - circulation controller are respectively used as the reference common - mode voltages U comabc1 and U comabc2 of the positive - side and negative - side MMCs;

[0020] The positive - side bridge - arm voltage calculation module and the negative - side bridge - arm voltage calculation module respectively use the positive - side reference differential - mode voltage and the negative - side reference differential - mode voltages U difabc1 and U difabc2 and the positive - side reference common - mode voltage and the negative - side reference common - mode voltages U comabc1 and U comabc2 , and after calculation, obtain the reference voltages U prefabc1 and U prefabc2 of the upper bridge - arms of the positive - side and negative - side MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower bridge - arms of the positive - side and negative - side MMCs;

[0021] The positive - side modulation module and the negative - side modulation module respectively control the positive - side and negative - side MMCs according to the reference voltages U prefabc1 and U prefabc2 of the upper bridge - arms of the positive - side and negative - side MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower bridge - arms of the positive - side and negative - side MMCs.

[0022] Furthermore, in the virtual mechanical module, the reference phase θ r is calculated according to the following method:

[0023] θ r (k + 1)=∫ω r (k + 1)dt

[0024]

[0025] where θ r (k + 1) is the reference phase of the next sampling period, ω r (k + 1) is the angular frequency of the next sampling period, ω r (k) is the angular frequency of this sampling period, ω n is the rated angular frequency, P ref is the reference value of the active power, P g1 (k) and P g2 (k) are the active powers of the positive - side and negative - side at this sampling period, J is the moment of inertia of the virtual rotor, and D p is the active - damping coefficient.

[0026] Further, in the virtual excitation module, the d-axis voltage reference value U gdref is calculated according to the following method:

[0027]

[0028] where U gdref (k + 1) is the d-axis voltage reference value for the next sampling period, |U g (k)| is the voltage amplitude for the current sampling period, U ref is the voltage amplitude reference value, Q ref is the reactive power reference value, Q g1 (k) and Q g2 (k) are the positive and negative reactive powers for the current sampling period, K is the virtual excitation coefficient, and D q is the reactive power damping coefficient. Description of the Drawings

[0029] Figure 1 is a structural diagram of a specific example of a true bipolar flexible DC transmission system. Among them, u ga , u gb , u gc are the grid voltages; i ga , i gb , i gc are the grid currents; u pa , u pb , u pc are the upper arm voltages of the MMC; u na , u nb , u nc are the lower arm voltages of the MMC; i pa , i pb , i pc are the upper arm currents of the MMC; i na , i nb , i nc are the lower arm currents of the MMC; U dc is the DC bus voltage, i dc is the DC bus current, L0 is the arm inductor, SM (N) is the sub-module in the MMC; N is the sub-module serial number.

[0030] Figure 2 is a schematic diagram of a specific example system of the control method of the present invention. The names of each module are as follows:

[0031] 1 - Virtual mechanical module, 2 - Virtual excitation module, 3 - Positive - voltage sampling module, 4 - Positive - current sampling module, 5 - Positive - phase Park transformation module, 6 - Positive - power calculation module, 7 - Positive - voltage controller, 8 - Positive - current controller, 9 - Positive - phase Park inverse transformation module, 10 - Positive - internal circulating - current controller, 11 - Positive - arm - voltage calculation module, 12 - Positive - modulation module, 13 - Negative - voltage sampling module, 14 - Negative - current sampling module, 15 - Negative - phase Park transformation module, 16 - Negative - power calculation module, 17 - Negative - voltage controller, 18 - Negative - current controller, 19 - Negative - phase Park inverse transformation module, 20 - Negative - internal circulating - current controller, 21 - Negative - arm - voltage calculation module, 22 - Negative - modulation module.

[0032] Figure 3 is the specific control schematic diagram of the virtual mechanical module; where: θ r is the system reference phase, ω r is the angular frequency, ω n is the rated angular frequency, P ref is the active - power reference value, P g1 and P g2 are the positive - and negative - active powers, J is the virtual - rotor moment of inertia, D p is the active - damping coefficient.

[0033] Figure 4 is the specific control schematic diagram of the virtual flux - linkage module; where: U gdref is the d - axis voltage reference value, U g is the voltage amplitude, U ref is the voltage - amplitude reference value, Q ref is the reactive - power reference value, Q g1 and Q g2 are the positive - and negative - reactive powers, K is the virtual - excitation coefficient, D q is the reactive - damping coefficient. Specific implementation manner

[0034] To describe the present invention more specifically, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] The system implementation of the virtual - synchronous - machine coordinated control method for the true - bipolar flexible DC transmission system in the present invention is as Figure 2As shown in the figure, it includes a virtual mechanical module 1, a virtual excitation module 2, a positive voltage sampling module 3, a positive current sampling module 4, a positive Park transformation module 5, a positive power calculation module 6, a positive voltage controller 7, a positive current controller 8, a positive Park inverse transformation module 9, a positive internal circulating current controller 10, a positive arm voltage calculation module 11, a positive modulation module 12, a negative voltage sampling module 13, a negative current sampling module 14, a negative Park transformation module 15, a negative power calculation module 16, a negative voltage controller 17, a negative current controller 18, a negative Park inverse transformation module 19, a negative internal circulating current controller 20, a negative arm voltage calculation module 21, and a negative modulation module 22.

[0036] As Figure 2 shown, the virtual synchronous machine coordinated control method for the true bipolar flexible DC transmission system in the present invention includes the following steps:

[0037] The positive (negative) voltage sampling module 3 (13) is used to collect the three-phase voltage U of the MMC AC power grid gabc , and the positive (negative) current sampling module 4 (14) is used to collect the three-phase current I of the positive (negative) MMC AC power grid gabc1 (I gabc2 ), and the internal circulating current I of the positive (negative) MMC cabc1 (I cabc2 ).

[0038] The positive (negative) power calculation module 6 (16) is used to calculate the active and reactive powers P gabc and Q gabc1 (I gabc2 ) of the positive (negative) pole according to the three-phase voltage U of the MMC AC power grid and the three-phase current I of the positive (negative) MMC AC power grid g1 and Q g1 (P g2 and Q g2 );

[0039] In the virtual mechanical module 1, the reference phase θ is calculated according to the following method r :

[0040] θ r (k + 1) = ∫ω r (k + 1)dt

[0041]

[0042] where θ r (k + 1) is the reference phase of the next sampling period, ω r (k + 1) is the angular frequency of the next sampling period, ω r (k) is the angular frequency of the current sampling period, ωn is the rated angular frequency, P ref is the reference value of active power, P g1 (k) and P g2 (k) are the positive and negative active powers in this sampling period, J is the virtual rotor inertia, D p is the active damping coefficient.

[0043] Using the virtual excitation module 2, calculate the d-axis voltage reference value U according to the following method gdref :

[0044]

[0045] where, U gdref (k + 1) is the d-axis voltage reference value of the next sampling period, |U g (k)| is the voltage amplitude of this sampling period, U ref is the voltage amplitude reference value, Q ref is the reference value of reactive power, Q g1 (k) and Q g2 (k) are the positive and negative reactive powers in this sampling period, K is the virtual excitation coefficient, D q is the reactive damping coefficient.

[0046] Using the positive (negative) pole Park transformation module 5 (15), transform the three-phase voltage U of the MMC AC power grid gabc and the three-phase current I of the positive (negative) pole MMC AC power grid gabc1 (I gabc2 ) by Park transformation to obtain the corresponding voltage vector U gdq and current vector I gdq1 (I gdq2 ) in the synchronous rotating d-q coordinate system. The angle used for Park transformation is the reference phase θ r ;

[0047] Using the positive (negative) pole voltage controller 7 (17) to control the d and q axis voltages U gdq through a PI controller, so that they respectively follow the given reference values U gdref and U gqref , where, U gqref is set to 0. After passing through the limiter, the outputs of the positive (negative) pole voltage controller are respectively used as the reference values I gdref1 and I gqref1 of the positive (negative) pole d and q axis currents (I gdref2 and I gqref2 ); The implementation method of the positive pole voltage controller 7 is as follows:

[0048]

[0049] Among them: F PI1 (s) is the transfer function of the positive - pole PI controller, k p1 is the proportional coefficient, k i1 is the integral coefficient, I gdref1 , I gqref1 corresponds to the d - axis and q - axis components of the current vector I gdqref1 .

[0050] The implementation method of the negative - pole voltage controller 17 is as follows:

[0051]

[0052] Among them: F PI2 (s) is the transfer function of the negative - pole PI controller, k p2 is the proportional coefficient, k i2 is the integral coefficient, I gdref2 , I gqref2 corresponds to the d - axis and q - axis components of the current vector I gdqref2 .

[0053] Using the positive (negative) - pole current controller 8 (18) to control the positive (negative) - pole d - axis and q - axis currents I gdq1 (I gdq2 ) with a PI controller to make it follow the reference value output by the positive (negative) - pole voltage controller. The output of the positive (negative) - pole current controller is used as the positive (negative) - pole MMC reference differential - mode voltage U difdq1 (U difdq2 );

[0054] The implementation method of the positive - pole current controller 8 is as follows:

[0055]

[0056] L is the equivalent inductance including the commutation transformer and the arm - reactor, U difd1 , U difq1 corresponds to the d - axis and q - axis components of the voltage vector U difdq1 .

[0057] The implementation method of the negative - pole current controller 18 is as follows:

[0058]

[0059] L is the equivalent inductance including the commutation transformer and the arm - reactor, U difd2 , U difq2 corresponds to the d - axis and q - axis components of the voltage vector U difdq2 .

[0060] Using the positive (negative) pole Park inverse transformation module 9 (19), perform Park inverse transformation on the positive (negative) pole reference differential mode voltage U difdq1 (U difdq2 ) to obtain the reference differential mode voltage U difabc1 (U difabc2 ) in the stationary three-phase coordinate system. The angle used for the Park inverse transformation is the reference phase θ r ;

[0061] Using the positive (negative) pole internal circulating current controller 10 (20), control the positive (negative) pole MMC internal circulating current I cabc1 (I cabc2 ) using a resonant controller, which can be achieved in the stationary coordinate system; the output of the positive (negative) pole internal circulating current controller serves as the positive (negative) pole MMC reference common mode voltage U comabc1 (U comabc2 );

[0062] Using the positive (negative) pole bridge arm voltage calculation module 11 (21), calculate the upper and lower bridge arm reference voltages U difabc1 (U difabc2 ) and the reference common mode voltage U comabc1 (U comabc2 ) of the positive (negative) pole MMC to obtain the upper and lower bridge arm reference voltages U prefabc1 and U nrefabc1 (U prefabc2 and U nrefabc2 );

[0063] Using the positive (negative) pole modulation module 12 (22), according to the upper and lower bridge arm reference voltages U prefabc1 and U nrefabc1 (U prefabc2 and U nrefabc2 ) of the positive (negative) pole MMC, realize the control of the positive (negative) pole MMC.

[0064] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art to the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system, characterized in that, The control system adopted to implement the above method includes: a virtual mechanical module, a virtual excitation module, a positive sampling module and a negative sampling module, a positive coordinate transformation module and a negative coordinate transformation module, a positive power calculation module and a negative power calculation module, a positive voltage controller and a negative voltage controller, a positive current controller and a negative current controller, a positive internal circulating current controller and a negative internal circulating current controller, a positive arm voltage calculation module and a negative arm voltage calculation module, a positive modulation module and a negative modulation module; The virtual mechanical module controls the positive and negative active powers by simulating the mechanical equations of a synchronous generator, and the output of the virtual mechanical module serves as the reference phase θ of the true bipolar flexible DC transmission system r ; The virtual excitation module controls the total positive and negative reactive power by simulating the excitation equation of the synchronous generator, and the output of the virtual excitation module serves as the d-axis voltage reference value U of the true bipolar flexible DC transmission system gdref ; The positive sampling module includes a positive voltage sampling module and a positive current sampling module, and the negative sampling module includes a negative voltage sampling module and a negative current sampling module; The positive voltage sampling module and the negative voltage sampling module sample the three-phase voltage U of the MMC AC power grid gabc for sampling; The positive current sampling module and the negative current sampling module respectively sample the three-phase currents I gabc1 and I gabc2 of the positive and negative MMC AC power grids, and the internal circulating currents I cabc1 and I cabc2 of the positive and negative MMCs; The positive coordinate transformation module includes a positive Park transformation module and a positive Park inverse transformation module, and the negative coordinate transformation module includes a negative Park transformation module and a negative Park inverse transformation module; The positive - pole Park transformation module and the negative - pole Park transformation module perform Park transformation on the three - phase voltage U of the MMC AC power grid gabc and the three - phase currents I of the positive - pole and negative - pole MMC AC power grids gabc1 and I gabc2 to obtain the corresponding voltage vectors U gdq and current vectors I gdq1 and I gdq2 in the synchronous rotating d - q coordinate system. The angle used for Park transformation is the reference phase θ r ; The positive - pole Park inverse transformation module and the negative - pole Park inverse transformation module respectively perform Park inverse transformation on the positive - pole and negative - pole reference differential - mode voltages U difdq1 and U difdq2 to obtain the reference differential - mode voltages U difabc1 and U difabc2 in the positive - pole and negative - pole stationary three - phase coordinate systems. The angle used for the Park inverse transformation is the reference phase θ r ; The positive - electrode power calculation module and the negative - electrode power calculation module respectively calculate, according to the three - phase voltage U of the MMC AC power grid gabc and the three - phase currents I gabc1 and I gabc2 of the positive - electrode and negative - electrode MMC AC power grids, the active powers P g1 and P g2 of the positive - electrode and negative - electrode, and the reactive powers Q g1 and Q g2 ; The positive - terminal voltage controller and the negative - terminal voltage controller control the d - axis and q - axis voltages U gdq through a PI controller to make them follow the given reference values U gdref and U gqref , where U gqref is set to 0. After passing through the limiting links, the outputs of the positive - terminal voltage controller and the negative - terminal voltage controller are respectively used as the reference values I gdref1 and I gqref1 of the positive - terminal d - axis and q - axis currents, and the reference values I gdref2 and I gqref2 ; The positive current controller and the negative current controller respectively control the positive and negative d-axis and q-axis currents I gdq1 and I gdq2 using PI controllers to make them follow the reference values output by the positive voltage controller and the negative voltage controller. The outputs of the positive current controller and the negative current controller serve as the positive and negative MMC reference differential-mode voltages U difdq1 and U difdq2 ; The positive internal circulating current controller and the negative internal circulating current controller respectively control the internal circulating currents I cabc1 and I cabc2 of the positive and negative MMCs by using a resonant controller, which can be achieved in the stationary coordinate system. The outputs of the positive internal circulating current controller and the negative internal circulating current controller are respectively used as the reference common-mode voltages U comabc1 and U comabc2 of the positive and negative MMCs; The positive bridge arm voltage calculation module and the negative bridge arm voltage calculation module respectively use the positive reference differential mode voltage and the negative reference differential mode voltage U difabc1 and U difabc2 and the positive reference common mode voltage and the negative reference common mode voltage U comabc1 and U comabc2 , and through calculation, obtain the reference voltages U prefabc1 and U prefabc2 of the upper bridge arms of the positive and negative MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower bridge arms of the positive and negative MMCs; The positive electrode modulation module and the negative electrode modulation module respectively implement the control of the positive electrode and negative electrode MMCs according to the reference voltages U prefabc1 and U prefabc2 of the upper bridge arms of the positive electrode and negative electrode MMCs and the reference voltages U nrefabc1 and U nrefabc2 of the lower bridge arms of the positive electrode and negative electrode MMCs.

2. The virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system according to claim 1, characterized in that: In the virtual mechanical module, the reference phase θ is calculated according to the following method r : θ r (k + 1) = ∫ω r (k + 1)dt Among them, θ r (k + 1) is the reference phase of the next sampling period, ω r (k + 1) is the angular frequency of the next sampling period, ω r (k) is the angular frequency of this sampling period, ω n is the rated angular frequency, P ref is the reference value of active power, P g1 (k) and P g2 (k) are the positive and negative active powers of this sampling period, J is the virtual rotor inertia, D p is the active damping coefficient.

3. The virtual synchronous machine coordinated control method for a true bipolar flexible DC transmission system according to claim 1, characterized in that: In the virtual excitation module, the d-axis voltage reference value U is calculated according to the following method gdref : Among them, U gdref (k + 1) is the d-axis voltage reference value for the next sampling period, |U g (k)| is the voltage amplitude for this sampling period, U ref is the voltage amplitude reference value, Q ref is the reactive power reference value, Q g1 (k) and Q g2 (k) are the positive and negative reactive powers for this sampling period, K is the virtual excitation coefficient, D q is the reactive power damping coefficient.

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