Performance Analysis of 9, 11, 13, and 15 – Level Back-to-Back Connected Modular Multilevel Converters fed Doubly Fed Induction Machine

— In this paper, back-to-back Modular Multilevel Converters are used to feed the Doubly Fed Induction Machine. The MMC with 9, 11, 13, and 15 – Level output voltages are generated and fed to the DFIM and the performance of the DFIM is analyzed, when the machine is working as Doubly Fed Induction Generator (DFIG) and when the machine is working as Doubly Fed Induction Motor. The performance of the DFIG is analyzed in terms of power factor at Grid Side Converter (GSC) for different levels of operation of MMC. The results show that the power factor is maintained near to unity power factor at grid side. The performance of the Doubly Fed Induction Motor is analyzed in terms of variation of the load torque being applied to motor. The results show that there is clear variation of the speed for two different load conditions. The work is carried out by using Typhoon HIL Real-Time Simulation platform, i.e., both Software and Hardware.


I. INTRODUCTION
Nowadays, the role of transferring the energy generated from the renewable energy sources is taken care of by Power Electronic Converters (PECs), especially, two-level Voltage Source Converters (VSCs), Multilevel Converters MLCs, and presently Modular Multilevel Converters (MMCs) [1], [2].Academia and industries have adopted modular approach because of its benefits like maintenance and assembly of power converters is easy, achieving increased voltage levels and power ratings, and can be operated during failures, which may not be possible with other PECs [3].
Based on the submodule used, the MMC can be classified into two types: a) Half-Bridge MMC and b) Full Bridge or H-Bridge MMC [4].Depending on the number of submodules connected on the upper arm or lower arm of a high-power converter, the voltage level of the MMC can be decided.If 'n' represents the number of submodules in each arm (upper and lower arms) then the voltage level is given by (n+1) [5].Each submodule is connected to a capacitor which has initial voltage based on the applied DC voltage and it is maintained at its nominal value.
In literature, the authors mentioned mostly the implementation of the Doubly Fed Induction Machine with back-to-back converters consisting of 3-leg VSCs [6]- [10].With the application of 3-leg Voltage Source Converters, the problem of controlling the harmonic content in voltage and Submitted on February 05, 2023.Published on October 04, 2023.G. Venu Madhav, Anurag University, India.
current injected to the Point of Common Coupling (PCC) and maintaining the unity power factor at the grid side arises [11]- [13].Up to certain extent, the performance of the Doubly Fed Induction Machine can be improved by replacing the 3-leg Voltage Source Converters with Multilevel Converters [14]- [16].Few papers describe the performance of the DFIM fed by Modular Multilevel Converters (MMC).With MMC fed DFIM, the performance of the DFIM is improved in terms of harmonic content in the voltages and currents injected into the PCC [17]- [19].In this paper, the performance of the DFIM is shown for different levels of voltage generation by MMC with the nearest level control approach to maintain the Capacitor Voltages, which are used in the MMCs.With the control approach, the harmonic content present in the voltages and currents injected into the PCC is reduced along with the maintenance of the unity power factor towards the grid side.When the DFIM is made to work as Doubly Fed Induction Motor, the performance is analyzed for two different load conditions, i) when the load torque is 0.9 p.u. (21 N-m) and ii) when the load torque is 0.1 p.u. (2.4 N-m).
For validating any proposed model or controller, the authors have used different real-time simulation software or hardware in the literature [20], [21].In this paper, to run the proposed model, the Typhoon HIL Real-Time Simulation Software with Version 2022.4 along with Typhoon HIL 402 hardware is used and to check the results, a Digital Storage Oscilloscope (DSO) is connected to the Typhoon HIL 402 through the breakout board.
The back-to-back connected MMC based Voltage Source Converters fed Doubly Fed Induction Machine performance is discussed in very few papers in the literature as discussed previously [17]- [19].In these literature papers, there is not much discussion about maintaining the power factor at unity when the voltages and currents are fed to PCC of grid, which indicates that the harmonic content of the voltages and currents are reduced.If the voltages and currents generated by DFIM with back-to-back connected MMCs contains harmonic content and these voltages and currents are injected to the grid at PCC, then it may lead to destabilization of the grid.So, in this paper, by using 9, 11, 13, 15-level MMCs with Nearest Level Control algorithm for capacitor voltage balancing, the voltages and currents are generated without harmonic content and fed to the PCC, so that, the grid will not get disturbed.This is discussed by tabulating the fundamental component values of voltages and currents towards RSC with @ @ Performance Analysis of 9, 11, 13, and 15 -Level Back-to-Back Connected Modular Multilevel Converters fed Doubly Fed Induction Machine Gopala Venu Madhav and Anil Kumar T.
FFT (obtained by using Typhoon HIL Real-Time Software) of RSC Voltages and Currents and also GSC Voltages and Currents.Further, the power factor of GSC also discussed and it is obtained to be near unity.In this paper, the DFIM acting as motor is also analyzed for two different load conditions indicating the usage of the same machine for water pumping applications.

A. Review Stage
The Doubly Fed Induction Machine is derived from the concept of Static Scherbius Drive.The word 'doubly' means feeding the stator of the machine with Grid, and feeding the rotor of the machine from the grid supply through back-toback converters called Grid Side Converter (GSC) and Rotor Side Converter (RSC).The DFIM model is chosen in Typhoon HIL Software platform such that the machine model equations are represented with currents as outputs and voltages as inputs. The

A. Modular Multilevel Converter
The 5-level Modular Multilevel Converter representation is shown in Fig. 1.The MMC consists of the upper arm and the lower arm, each arm may have atleast one half bridge converter (represented by a set of Switches [S1, S1'] and so on, with capacitor in shunt or full bridge converter.In this paper, half bridge type MMC is chosen.The number of half bridges connected in each arm determines the number of levels of the MMC.In the Figure above, two half bridges are connected in series in the upper arm and two half bridges are connected in series in the lower arm, which makes the MMC be 5-level converter, which leads to the formula of calculating the number of levels of MMC is equal to (2 multiplied by number of half bridges connected) + 1 or other way of calculating the number of levels is described previously.The switching table for the operation of 5-level MMC is shown in Table I below.The switching table itself is self explanatory representing all the switches/sub modules are in 'off' state meaning it is zero level voltage state and all the switches (S1, S2, S3, S4) are 'on' state meaning it is highest level of voltage.In this work, 9, 11, 13 and 15level backto-back connected MMCs are used to feed the DFIM, to know its performance when acting as generator and motor conditions.In the Table I, if same sequence is applied for lower arm of a 9-level MMC, the other four negative voltage levels are generated making the total to be 9-level (4Vc [where Vc is capacitor voltage], 3Vc, 2Vc, Vc, 0, -Vc, -2Vc, -3Vc, -4Vc); like wise, it can be extended for 11, 13 and 15level MMCs also.
In this paper, MMC leg switching function is chosen in Typhoon HIL Simulation Software wherein the number of levels of MMC can be given and it can be changed in the control for MMC leg.That means, the control of the MMC leg manages the MMC leg switching function.The parameter values of the MMC leg switching function are: Resistance of Converter = Rd = 0.25 Ω; Inductance of Converter = Ld = 5e3 H; Value of Capacitors in sub-modules = Cm = 105e-3 F. [22] As the name indicates 'Nearest Level Control (NLC)', the nearest voltage level is used, which can be generated by converting to the desired output voltage reference.The NLC method is also called 'round method'.Based on the process of independent comparisons, the 3-phases can be controlled separately.

B. Nearest Level Control
As illustrated in Fig. 2, the output voltage level (existing) is compared with the reference waveform to form the sampled waveform.From (1), the nearest output voltage level, nnearest level, can be determined by rounding the Voltage Reference,   , and dividing it by Sub-Module Capacitor Voltage,  − .By performing the round of function, the next step of determining how many Sub-Modules shall be switched 'ON' will be done.Taking an example, round (4.4) = 4, round (4.6) = 5, that means, the round function will return the nearest integer of the possible input number at that instant.The determined nearest integer (referring to (1)), is multiplied by

IV. RESULTS AND DISCUSSION
The results of the proposed work are carried out using the Typhoon HIL Real-Time Simulation Platform.The real-time simulation of the back-to-back MMCs fed DFIM is carried out in two ways, one is when the DFIM is acting as Doubly Fed Induction Generator with different levels of Converter output voltage levels i.e., 9, 11, 13, and 15.The performance of the DFIG is analyzed in terms of the power factor maintained at the GSC.The second one is when the DFIM is acting as Doubly Fed Induction Motor and the performance of the motor is analyzed with two different conditions of load torque.The parameters of the machine like speed, torque, GSC active power, GSC reactive power, RSC active power, RSC reactive power, DC-Link voltage, Grid voltages, Grid Currents, RSC voltages, RSC currents, mechanical angle, and power factor are discussed in this paper.

A. DFIM Working as Doubly Fed Induction Generator with 9-level MMC Voltages
Fig. 3 shows the three-phase GSC voltages and currents, the waveforms show that voltages and currents at grid-side are sinusoidal in nature and are in phase, stating that the power factor is maintained near unity.Fig. 4 shows the DC-link voltage generated, which is around 500 V, and there are oscillations near that value, which is because of the oscillations generated in the active power of the GSC.Fig. 6 shows the three-phase DFIM mechanical angle, the electromagnetic torque which is 11.7 N-m (0.5 p.u. value) and the speed of the DFIG is 1492 rpm.Fig. 7 shows the active and reactive powers of the GSC and RSC.From Fig. 7 (a), (b), (c), and (d), the signs of the Pg, Qg, Pr, and Qr are negative, indicating that the mode of operation of the DFIM is Sub synchronous Generating mode [23].Further, it can be deduced that the power is flowing from the generator towards the grid through stator winding and the power is fed from the grid through the back-to-back mmcs towards the rotor of the DFIG.As the generator is operating in sub synchronous generating mode, the reactive power is negative and it is flowing from the grid to the generator, likewise, it is applicable to the rotor side reactive power.Fig. 8 shows the grid side converter power factor, the power factor is negative since the major part of the power is flowing from the DFIG towards the utility grid.Because of the NLC Strategy and the proper operation of the switches (igbts) in MMC, the power factor of the GSC or at utility grid is almost near to unity, from the it is -0.9989 and this value is for the 9-level mmcs.

B. DFIM Working as Doubly Fed Induction Generator with 11-level MMC Voltages
For the 11, 13, 15-Level MMCs, only the three-phase RSC Voltages and the GSC Power Factor are discussed here, since, all the other parameters mentioned in the previous case are almost the same.
Fig. 9 shows the 11-level three-phase RSC Voltages.From the above figure the 11-level voltages of the MMC can be clearly seen.The GSC power factor for this case is also maintained at -0.9989 (near to unity), which can be clearly seen from Fig. 10.For the case of 13-level output voltages from the MMCs, the power factor towards the grid is at -0.9989 with less number of oscillations as the converter output level of the voltages is increased, which is shown in Fig. 12.

D. DFIM Working as Doubly Fed Induction Generator with 15-level MMC Voltages
The 15-level output voltage generation from the rotor side MMC can be clearly seen from Fig. 13.From Fig. 14, it is again proved that the grid side the power factor is maintained near to unity, that is, -0.9989 with less oscillations.
Fig. 15 and tabulated fundamental component values of RSC Voltages and Currents shown in Table II indicates that the harmonic content of Voltages and Currents for 15-level is less compared to the 13, 11 and 9-level.There is clear indication of harmonic content other than fundamental value for 13, 11 and 9 output voltage levels.Likewise, the FFT of GSC Voltages and Currents are shown in Figure 16 for 15-level output voltage.From Fig. 16, it can be clearly seen that the fundamental component values of voltage and current are 980 V and 376 A and this value is approximately common for other voltage levels i.e., 9, 11, and 13-levels, because of the usage of filter, which is connected towards the PCC.Fig. 16 also shows that there is almost very little harmonic content in both voltage and current, this indicates that the voltages and currents injected into the PCC are almost pure sinusoidal in nature which further indicates that the power factor is almost near to unity as discussed and explained in previous sections, i.e., 0.9989.

E. DFIM Working as Doubly Fed Induction Motor for
Load Torque at 0.9 p.u Here, in this case, the back-to-back MMCs are considered generating 9-level output voltages.In the first case, the load torque is taken as 0.9 p.u. and in the second case, it is considered as 0.1 p.u. Fig. 17 shows the load torque which is put on the Doubly Fed Induction Motor, in the Typhoon HIL Real Time Simulation Software it is considered as 0.9 p.u. From the Fig. 17, the actual value of the load torque is shown as 21.024 Nm which is exactly equal to the 0.9 p.u. multiplied by the full load torque, which is 23.36 N-m.For the load torque of 0.9 p.u i.e., 21.024 N-m, the speed of the Doubly Fed Induction Motor is maintained at 1485.9 rpm, which can be clearly seen from the Fig. 18.Fig. 19 shows the active and reactive powers of the GSC and RSC.From 19(s) (a), (b), (c), and (d), the signs of the Pg, Qg, P,r and Qr are positive, indicating that the mode of operation of the DFIM is Sub synchronous motoring mode [18].Further, it can be deduced that the power is flowing from the grid towards the generator through stator winding and the power is taken from the grid through the back-to-back MMCs to the rotor of the machine.

F. DFIM Working as Doubly Fed Induction Motor for
Load Torque at 0.1 p.u Fig. 20 shows the load torque which is put on the Dobly Fed Induction Motor, in the Typhoon HIL Real Time Simulation Software it is considered as 0.1 p.u. From Fig. 18, the actual value of the load torque is shown as 2.4 N-m (appx.)which is exactly equal to the 0.1 p.u. multiplied by the full load torque, which is 23.36 N-m.For the load torque of 0.1 p.u i.e., 2.4 N-m (appx.), the speed of the Doubly Fed Induction Motor is maintained at 1498.5 rpm, which can be clearly seen from Fig. 21.
Fig. 22 shows the GSC and RSC active and reactive powers and from the Figure 22, it can be clearly seen that the GSC active and reactive powers are almost same as the previous case, but the RSC active and reactive powers are less compared to the previous case as the load put on the motor is less.

V. CONCLUSIONS
In this research work, the performance of the DFIM is analyzed when it is working as Generator and Motor.The performance of the DFIM when working as Generator is indicated by the power factor at grid-side, which is well maintained at -0.9989 (near to unity), for all the four cases, that is, 9, 11, 13, 15-level back-to-back MMC fed DFIG.When the machine is working as motor, it is analyzed for the load torques at 0.9 p.u. and 0.1 p.u. values and the parameters of the motor show its performance during different load conditions.The work is carried out by using the Typhoon HIL Real-Time Software and Typhoon HIL 402 Hardware Setup.Further this work can be extended by using the Voltage and Current Sensors, Power Amplifier and run the DFIM for various conditions, so that the experimental and the real-time simulation results can be compared and validated.

1 𝑉 8 . 4 .Fig. 2 .
− , which corresponds to the closed level to the reference that is generated by the inverter.The Fig. 2(a), shows a demonstration of the first quarter cycle of a sinusoidal reference for 9-level output voltage of a converter operation.The numbers of Sub-Modules required is n = 8 per arm.The Sub-Module capacitor voltage,  − = The approximation error is equal to   Fig. 2(b) shows the nearest voltage level generation implementation.With this modulation technique, the capacitor voltages of the Sub-Modules are maintained at equal values, which leads to a complete symmetrical regime.Nearest Level Control Method: (a) demonstration of method; (b) implementation of control.

Fig. 6 Fig. 6 .
Fig.6shows the three-phase DFIM mechanical angle, the electromagnetic torque which is 11.7 N-m (0.5 p.u. value) and the speed of the DFIG is 1492 rpm.

Fig. 11
clearly indicates the 13-level output voltage being generated form the RSC.

Fig. 23
Fig. 23 shows the actual model developed in Typhoon HIL Real-Time Simulation Software and the running the developed model in the software (which can be clearly seen from Fig. 23 (b) from down the right corner).

Fig. 24 .
Fig. 24.Typhoon HIL Real-Time Simulation Software and Hardware Set up running the Proposed Model.

Fig. 24
Fig. 24 shows the complete set up of the Real-Time Simulation of proposed model, which clears indicates the Typhoon HIL Real-Time Simulation Software running the proposed model in terms of SCADA Panel and the Laptop is connected to the Typhoon HIL 402 Hardware box, which is further interfaced with the breakout board from which the analog outputs are taken and connected through wires to probe of the Digital Storage Oscilloscope (DSO), The DSO is displaying the scaled down real-time waveform

TABLE I :
OPERATION OF SWITCHES TO GENERATE THE VOLTAGE LEVELS

TABLE II :
COMPARISON OF FUNDAMENTAL COMPONENT VALUES OF ROTOR SIDE CONVERTER VOLTAGES AND CURRENTS FOR 9, 11, 13, AND 15-LEVEL Fig. 16.FFT Analysis using Typhoon HIL Software for Grid Side Converter Voltages and Currents.