Mitigation of Complex Non-Linear Dynamic Effects in Multiple Output Cascaded DC-DC Converters

ABSTRACT:

In the modern world of technology, the cascaded DC-DC converters with multiple output configurations are contributing a dominant part in the DC distribution systems and DC micro-grids. An individual DC-DC converter of any configuration exhibits complex non-linear dynamic behavior resulting in instability. This paper presents a cascaded system with one source boost converter and three load converters including buck, Cuk, and Single-Ended Primary Inductance Converter (SEPIC) that are analyzed for the complex non-linear bifurcation phenomena. An outer voltage feedback loop along with an inner current feedback loop control strategy is used for all the sub-converters in the cascaded system. To explain the complex non-linear dynamic behavior, a discrete mapping model is developed for the proposed cascaded system and the Jacobian matrix's eigenvalues are evaluated. For the simplification of the analysis, every load converter is regarded as a _xed power load (FPL) under reasonable assumptions such as _xed frequency and input voltage. The eigenvalues of period-1 and period-2 reveal that the source boost converter undergoes period-2 orbit and chaos whereas all the load converters operate in a stable period-1 orbit. The proposed configuration eliminates the period-2 and chaotic behavior from all the load converters and is also validated using simulation in MATLAB/Simulink and experimental results.

KEYWORDS:

1.      Bifurcation

2.       Chaos

3.      DC-DC power converters

4.      Non-linear dynamical systems

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

 

 

Figure 1. Block Diagram Of The Proposed Cascaded System.

EXPECTED SIMULATION RESULTS:

 

Figure 2. Inductor Current Ripple And Output Voltage Ripple Waveforms Of Stable Period-1 Operation For

A) Boost Converter At Vs D 35 V B) Buck Converter At Vs D 50 V C) Cuk Converter At Vs D 50 V D)Sepic

Converter At Vs D 50 V

Figure 3. Inductor Current Ripple And Output Voltage Ripple Waveforms Of Period-2 Operation For A) Boost Converter At Vs D 25 V B) Buck Converter At Vs D 36 V C) Cuk Converter At Vs D 24 V D) Sepic Converter At Vs D 40 V.

 

 

Figure 4. Inductor Current Waveforms Of All The Converters Of The Cascaded System At Vs D 35 V.

 

 

Figure 5. Inductor Current Waveforms Of All The Converters Of The Cascaded System At Vs D 25 V.

 

 

Figure 6. Inductor Current Waveform Of Source Boost Converter For Step Change In The Input Voltage Verifying Non-Linear Incident Effects.

 

CONCLUSION:

This paper presents a configuration of the cascaded multiple output DC-DC converters to eliminate complex non-linear dynamic behavior and improve the stability when subjected to varying source voltage. The proposed cascaded DC-DC converter system consists of one source boost converter, one load Buck converter, one load Cuk converter, and a SEPIC converter. All the converters in the proposed system are engaged with a current-mode controller with a compensation network technique in which an outer voltage feedback loop and an inner inductor current feedback loop are used along with an offset divided voltage protection circuit and an RS-latch. The simulation and experimental results reveal that the source boost converter undergoes period-2 orbit and ultimately chaos when the input voltage of the source boost converter is decreased. However, it is verified that all the converters that are acting as a load in the proposed system continue to operate in the stable period-1 orbit and the input voltage of the source boost converter does not affect their stability. The discrete mapping model is developed by considering all the load converters as FPLs because of their stable behavior which also generalizes it for other types of converters. The Jacobian matrix is developed using the data of the discrete mapping model and the eigenvalues are obtained which are close to 1. So, by decreasing the input source voltage, the eigenvalues move out of the unit circle which results in period-2 behavior of the system that severely affects the stability of the whole cascaded converter system. The proposed structure makes load converters in the system insensitive towards input voltage variation which has been demonstrated analytically and using experimental results.

REFERENCES:

[1] C. M. F. S. Reza and D. D.-C. Lu, ``Recent progress and future research direction of nonlinear dynamics and bifurcation analysis of grid-connected power converter circuits and systems,'' IEEE J. Emerg. Sel. Topics Power Electron., vol. 8, no. 4, pp. 3193_3203, Dec. 2020.

[2] A. Kargarian, J. Mohammadi, J. Guo, S. Chakrabarti, M. Barati, G. Hug, S. Kar, and R. Baldick, ``Toward distributed/decentralized DC optimal power _ow implementation in future electric power systems,'' IEEE Trans. Smart Grid, vol. 9, no. 4, pp. 2574_2594, Jul. 2018.

[3] J. W.-T. Fan and H. S.-H. Chung, ``Bifurcation phenomena and stabilization with compensation ramp in converter with power semiconductor filter,'' IEEE Trans. Power Electron., vol. 32, no. 12, pp. 9424_9434, Dec. 2017.

[4] M. Schuck and R. C. N. Pilawa-Podgurski, ``Ripple minimization through harmonic elimination in asymmetric interleaved multiphase DC_DC converters,'' IEEE Trans. Power Electron., vol. 30, no. 12, pp. 7202_7214, Dec. 2015.

[5] A. Braitor, G. C. Konstantopoulos, and V. Kadirkamanathan, ``Stability analysis and nonlinear current-limiting control design for DC micro-grids with CPLs,'' IET Smart Grid, vol. 3, no. 3, pp. 355_366, Jun. 2020.

Multifunctional Cascade Control of Voltage-Source Converters Equipped With an LC Filter

ABSTRACT:

This paper proposes a multifunctional cascade controller structure for voltage-source converters. The proposed structure contains a decoupling loop between the outer voltage control loop and the inner current control loop, and operation in either voltage or current control mode is possible. In voltage control mode, the current controller can be made completely transparent. In the case of faults, the proposed structure enables inherent overcurrent protection by a seamless transition from voltage to current control mode, wherein the current controller is fully operational. Seamless transitions between the control modes can also be triggered with an external signal to adapt the converter to different operating conditions. The proposed structure allows for integration of simple, accurate, and flexible overcurrent protection to state-of-the-art single loop voltage controllers without affecting voltage control properties under normal operation. The properties of the proposed controller structure are validated experimentally on a 10-kVA converter system.

KEYWORDS:

1.      Ac-voltage control

2.      Cascade control

3.      Current control

4.      Overcurrent protection

5.      Voltage-source converters

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

 

 

Fig. 1. Block diagram of the experimental setup. CB stands for circuit breaker.

EXPECTED SIMULATION RESULTS:

 

Fig. 2. Experimental validation of the transparency of the current controller in the proposed cascade controller  structure. The application example controller presented in Section IV is compared with its single-loop counterpart based on the controller proposed in [14]: (left) reference tracking under no load (middle) reference tracking under 1 p.u. resistive and 0.45 p.u. inductive load and (right) disturbance rejection in the form of load change from no load to 1 p.u. resistive and 0.45 p.u. inductive load.

 

Fig. 3. Experimental transition between control modes with (a) 1 p.u. resistive and 0.45 p.u. inductive load (b) 0.08 p.u. resistive and 0.45 p.u. inductive load. Additionally, reference steps in both control modes are presented. VCM and CCM stand for voltage and current control mode, respectively.

 

Fig. 4. Experimental emulation of a load fault by connecting a low-resistance load in parallel with the steady-state load. Recovery from the fault, which is triggered by a circuit breaker, is also shown. The fault emulation is shown for the case where the converter is designed to trip in the event of overcurrent (left), for the reference current limitation method proposed in [24] (middle), and for the proposed structure (right).

CONCLUSION:

 This paper presented a multifunctional cascade controller  structure for VSCs. The proposed controller structure allows for operation in either voltage or current control mode. In voltage control mode and under linear operation, the current controller can be made completely transparent. Consequently, the properties of both control modes are purely determined by their corresponding control loops, which can be designed independently of each other. The transitions between control modes are seamless and occur either due to converter overloading, i.e., the controller inherently includes overcurrent protection, or by manually activating the current control mode of the controller. The properties of the proposed cascade controller structure are validated by means of experiments.

REFERENCES:

[1] R. Rosso, X. Wang, M. Liserre, X. Lu, and S. Engelken, “Grid-forming converters: an overview of control approaches and future trends,” in Proc. IEEE ECCE, Detroit, MI, USA, Oct. 2020, pp. 4292–4299.

[2] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodr´ıguez, “Control of power converters in AC microgrids,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4734–4749, Nov. 2012.

[3] Q. Lei, F. Z. Peng, and S. Yang, “Multiloop control method for high-performance microgrid inverter through load voltage and current decoupling with only output voltage feedback,” IEEE Trans. Power Electron., vol. 26, no. 3, pp. 953–960, Mar. 2011.

[4] F. de Bosio, L. A. de Souza Ribeiro, F. D. Freijedo, M. Pastorelli, and J. M. Guerrero, “Effect of state feedback coupling and system delays on the transient performance of stand-alone VSI with LC output filter,” IEEE Trans. Ind. Electron., vol. 63, no. 8, pp. 4909–4918, Aug. 2016.

[5] P. C. Loh, M. Newman, D. Zmood, and D. Holmes, “A comparative analysis of multiloop voltage regulation strategies for single and threephase UPS systems,” IEEE Trans. Power Electron., vol. 18, no. 5, pp. 1176–1185, Sep. 2003.

Low-Voltage Ride Through Strategy for MMC With Y0/Y0 Arrangement Transformer Under Single-Line-to-Ground Fault

 ABSTRACT:

In the offshore wind farm high-voltage direct-current (HVDC) system, the power delivery capability of the onshore modular multilevel converter (MMC) decreases severely under grid fault, which makes the DC-bus voltage increase rapidly and threatens the safe operation of the system. This paper proposes a low-voltage ride through (LVRT) strategy for MMC with Y₀/Y₀ arrangement transformer under single-line-to-ground (SLG) fault. The influence of different transformer arrangements to the MMC under SLG fault is analyzed. On this basis, a power delivery capability enhanced method is proposed for MMC with Y₀/Y₀ arrangement transformer to take advantage of its control ability on zero sequence current. In addition, an optimized LVRT strategy based on resonant controller is proposed, which has simple control structure and can ride through the SLG fault without DC chopper. The offshore wind farm MMC-HVDC simulation system is established in PSCAD/EMTDC and simulation studies are conducted to validate the effectiveness of the proposed LVRT strategy.

KEYWORDS:

1.      Modular multilevel converter (MMC)

2.      Grid fault

3.      High-voltage direct-current (HVDC)

4.      Low-voltage ride through (LVRT)

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM:

 

 

Figure 1. Block Diagram Of The Conventional Control Strategy Of Mmc Under Slg Fault.

EXPECTED SIMULATION RESULTS:

Figure 2. Simulation Results Of Mmc With Y₀/1 Arrangement Transformer Using The Conventional Strategy (P D 935mw).

Figure 3. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Conventional Strategy (P D 935mw).

 

Figure 4. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Proposed Strategy (P D 935mw).

Figure 5. Simulation Results Of Mmc With Y₀/1 Arrangement Transformer Using The Conventional Strategy (P D

750mw).

Figure 6. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Conventional Strategy (P D 750mw).

 

Figure 7. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Proposed Strategy (P D 750mw).

 CONCLUSION:

In this paper, the influence of different transformer arrangements to MMC under SLG fault has been analyzed, and an LVRT strategy for MMC with Y₀/Y₀ arrangement transformer has been proposed. Comparative simulation studies have been conducted under SLG fault. The conclusions can be summarized as follow. (1) Compared with the Y₀/1 arrangement transformer, the grid-side zero sequence current can be restrained by using Y₀/Y₀ arrangement transformer, and the power delivery capability can be enhanced. However, the zero sequence current is transferred to the MMC side. (2) The proposed LVRT strategy can restrain the zero sequence current and enhance the power delivery capability for MMC with Y₀/Y₀ arrangement transformer effectively. The MMC can ride through SLG fault without DC chopper by using the proposed LVRT strategy when the wind farm works in the full-power mode. (3) The proposed LVRT strategy can work well under different power factors, which means the MMC using the proposed strategy can not only ride through the grid fault,but also provide reactive power support to the grid within its capability when the wind farm doesn't work in the full-power mode.

REFERENCES:

[1] S. M. Muyeen, R. Takahashi, and J. Tamura, ``Operation and control of HVDC-connected offshore wind farm,'' IEEE Trans. Sustain. Energy, vol. 1, no. 1, pp. 30_37, Apr. 2010.

[2] R. Shah, J. C. Sánchez, R. Preece, and M. Barnes, ``Stability and control of mixed AC-DC systems with VSC-HVDC: A review,'' IET Gener. Transm. Distrib., vol. 12, no. 10, pp. 2207_2219, 2018.

[3] X. Zeng, T. Liu, S. Wang, Y. Dong, B. Li, and Z. Chen, ``Coordinated control of MMC-HVDC system with offshore wind farm for providing emulated inertia support,'' IET Renew. Power Gener., vol. 14, no. 5, pp. 673_683, Apr. 2020.

[4] J. Lyu, X. Cai, M. Amin, and M. Molinas, ``Sub-synchronous oscillation mechanism and its suppression in MMC-based HVDC connected wind farms,'' IET Gener. Transmiss. Distrib., vol. 12, no. 4, pp. 1021_1029, Feb. 2018.

[5] S. Xue, C. Gu, B. Liu, and B. Fan, ``Analysis and protection scheme of station internal AC grounding faults in a bipolar MMC-HVDC system,'' IEEE Access, vol. 8, pp. 26536_26548, 2020.

Improved DC-Link Voltage Regulation Strategy for Grid-Connected Converters

ABSTRACT:

In this paper, an improved dc-link voltage regulation strategy is proposed for grid-connected converters applied in dc microgrids. For the inner loop of the grid connected converter, a voltage modulated direct power control is employed to obtain two second-order linear time invariant systems, which guarantees that the closed-loop system is globally exponentially stable. For the outer loop, a sliding mode control strategy with a load current sensor is employed to maintain a constant dc-link voltage even in the presence of constant power loads at the dc-side, which adversely affect the system stability. Furthermore, an observer for the dc-link current is designed to remove the dc current sensor at the same time improving the reliability and decreasing the cost. From both simulation and experimental results obtained from a 15-kVA prototype setup, the proposed method is demonstrated to improve the transient performance of the system and has robustness properties to handle parameter mismatches compared with the inputoutput linearization method.

KEYWORDS:

1.      Dc microgrid

2.      Direct power control

3.      Grid connected converter

4.      Observer

5.      Sliding mode control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Block diagram of the proposed control method (SMC with observer) for a rectifier system in the dc microgrid.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation results when the dc load is changed from 460 W to 153 W. at 0.05 s and the reactive power is changed from 0 Var to 1 kVar at 0.75 s. (a) Real power; (b) reactive power; (c) is;c line current; (d) dc-link voltage.

 

Fig. 3. Simulation results when the dc load is changed from 460 W to 153 W at 0.05 s and vs;a has 10% sag. (a) Grid voltage; (b) is;c current; (c) dc-link voltage; (b) real and reactive power.

 

 

Fig. 4. Simulation results when the dc load is changed from 460 W to 153 W at 0.05 s and the THD of the grid voltage is 2.2%. (a) Grid voltage; (b) is;c current; (c) dc-link voltage; (b) real and reactive power.

 

CONCLUSION:

A three-phase PWM rectifier was controlled by the proposed control strategy, which has a dc-link current observer based SMC in the outer loop and a voltage modulated-DPC in the inner loop. The SMC was applied to generate the real power reference in the inner loop in order to make sure the dc link voltage to be within a certain level in the dc microgrids even there exist CPLs. Furthermore, an observer for the dc link current was designed in order to remove the need for a current sensor. Both simulation and experimental results show that the proposed method effectively reduces the overshoot of the dc-link voltage and is robust to parameter mismatch of the capacitance value in the dc-link.

REFERENCES:

[1] J. Liu, X. Lu, and J. Wang, “Resilience analysis of DC microgrids under denial of service threats,” IEEE Trans. Power Syst., vol. 34, no. 4, pp. 3199–3208, July 2019.

[2] F. Blaabjerg, M. Liserre, and K. Ma, “Power electronics converters for wind turbine systems,” IEEE Trans. Ind. Appl., vol. 48, no. 2, pp. 708– 719, 2012.

[3] B. Wei, Y. Gui, A. Marzabal, Trujillo, J. M. Guerrero, and J. C. Vasquez, “Distributed average secondary control for modular UPS systems based microgrids,” IEEE Trans. Power Electron., vol. 34, no. 7, pp. 6922–6936, July 2019.

[4] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, 2006.

[5] M. Kazmierkowski and L. Malesani, “Current control techniques for three-phase voltage-source PWM converters: a survey,” IEEE Trans. Ind. Electron., vol. 45, no. 5, pp. 691–703, Oct 1998.

Improved Controller and Design Method for Grid-Connected Three-Phase Differential SEPIC Inverter

ABSTRACT:

Single-ended primary-inductor converter (SEPIC) based differential inverters (SEPIC-BDI) have received wide concerns in renewable energy applications due to their modularity, galvanic isolation, decreased power stages, continuous input current, and step up/down capability. However, its design still has several challenges related to component design, the existence of complex right half plane (RHP) zeros, and increased sensitivity to component mismatches. In this context, this paper presents an improved control and enhanced design method for the three-phase SEPIC-BDI for grid-tied applications. A generalized static linearization approach (SLA) is proposed to mitigate the low-order harmonics. It practically simplifies the control complexity and decreases the required control loops and sensor circuits. The mismatch between the SEPIC converters in each phase is highly mitigated due to the independent operation of the SLA in each phase and the output dc offset currents are reduced. The proposed enhanced design methodology modifies the SEPIC open-loop transfer function by moving the complex RHP zeros to the left half-plane (LHP). Therefore, a simple proportional-integral (PI) controller effectively maintains converter stability without adding higher-order compensators in the literature. Moreover, a straightforward integrator in the control loop eliminates the negative sequence harmonic component (NSHC) and provides a low computational burden. Simulations and experimental results based on 200V, 1.6 kW, 50 kHz prototype with silicon carbide (SiC) devices are provided to validate the effectiveness of the proposed work. The results show that the proposed controller and design method achieve pure output current waveforms at various operating points of the inverter and dc voltage variations.

 KEYWORDS:

1.      Differential inverter

2.      Renewable energy applications

3.      Negative sequence harmonic component

4.      Power converters

5.      Power losses

6.      Single-ended primary-inductor converter (SEPIC)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Figure 1. Circuit Schematic Of The Isolated Sepic-Bdi.

 EXPECTED SIMULATION RESULTS:

Figure 2. Simulation Results Of The Output Voltage And Grid Voltage Of One Sepic At Sepic-Bdi Using Cms.

 

Figure 3. Simulation Results Of The Output Voltage And Grid Voltage Of One Phase Leg Of Sepic-Bdi Using Proposed Mcms.

 CONCLUSION:

An enhanced design methodology and improved controller for three-phase SEPIC-BDI inverter have been proposed for grid-connected renewable energy applications. Additionally, this paper presented a generalized method based on the static linearization approach (SLA) for mitigating the low-order harmonic components, which are usually inherent by differential inverters. The superiority and effectiveness of the proposed controller and SEPIC-BDI inverter system are validated using simulation and experimental results at voltage range (100-120 V) and power range (0.2-1.6 kW). By using the proposed SLA method with the SEPIC-BDI system, the mismatch effects between the different SEPIC converters are alleviated and the DC offset components in the output currents are eliminated. Moreover, by selecting the converter parameters based on the proposed enhanced design methodology, more stable operation can be obtained by moving the complex RHP zeros to the LHP. Therefore, a simple PI controller is needed to maintain converter stability compared to the required nonlinear controllers and high order compensator types in the existing methods in the literature.

REFERENCES:

[1] S. Kouro, J. I. Leon, D. Vinnikov, and L. G. Franquelo, ``Grid-connected photovoltaic systems: An overview of recent research and emerging PV converter technology,'' IEEE Ind. Electron. Mag., vol. 9, no. 1, pp. 47_61, Mar. 2015.

[2] E. M. Ahmed, E. A. Mohamed, A. Elmelegi, M. Aly, and O. Elbaksawi, ``Optimum modiFIed fractional order controller for future electric vehicles and renewable energy-based interconnected power systems,'' IEEE Access, vol. 9, pp. 29993_30010, 2021.

[3] M. M. Alhaider, E. M. Ahmed, M. Aly, H. A. Serhan, E. A. Mohamed, and Z. M. Ali, ``New temperature-compensated multi-step constant-current charging method for reliable operation of battery energy storage systems,'' IEEE Access, vol. 8, pp. 27961_27972, 2020.

[4] M. Aly and H. Rezk, ``A differential evolution-based optimized fuzzy logic MPPT method for enhancing the maximum power extraction of proton exchange membrane fuel cells,'' IEEE Access, vol. 8, pp. 172219_172232, 2020.

[5] H. D. Paulino, P. J. M. Menegaz, and D. S. L. Simonetti, ``A review of the main inverter topologies applied on the integration of renewable energy resources to the grid,'' in Proc. XI Brazilian Power Electron. Conf.,Sep. 2011, pp. 963_969.