Cascaded Two-Level Inverter-Based Multilevel STATCOM for High-Power Applications

ABSTRACT

In this paper, a simple static var compensating scheme using a cascaded two-level inverter-based multilevel inverter is proposed. The topology consists of two standard two-level inverters connected in cascade through open-end windings of a three-phase transformer. The dc link voltages of the inverters are regulated at different levels to obtain four-level operation. The simulation study is carried out in MATLAB/SIMULINK to predict the performance of the proposed scheme under balanced and unbalanced supply-voltage conditions. A laboratory prototype is developed to validate the simulation results. The control scheme is implemented using the TMS320F28335 digital signal processor. Further, stability behavior of the topology is investigated. The dynamic model is developed and transfer functions are derived. The system behavior is analyzed for various operating conditions.

KEYWORDS

1.      DC-link voltage balance

2.      Multilevel inverter

3.      Power quality (PQ)

4.      Static compensator (STATCOM)

SOFTWARE:  MATLAB/SIMULINK

 BLOCK DIAGRAM

Fig. 1. Power system and the STATCOM model.

EXPECTED SIMULATION RESULTS

Fig. 2. Frequency response ∆V_dc1(s) /∆δ₁(s) at  i^’_q0 =1.02 p.u., δ₁=-0.9⁰,δ₂=178.9⁰,R₁=

80 p.u., R₂=60 p.u.

Fig. 3. Root locus of the transfer function  ∆V_dc1(s) /∆δ₁(s) at  i^’_q0 = - 0.75 p.u., δ₁=-0.57⁰,δ₂=179.6⁰,R₁=

80 p.u., R₂=60 p.u.

Fig. 4. Reactive power control. (a) Source voltage and inverter current.

(b) DC-link voltages of two inverters.

Fig. 5. Operation during fault. (a) Grid voltages on the LV side of the transformer. (b) -axis negative-sequence current component i^’_dn. (c) -axis negative- sequence current component i^’_qn.

CONCLUSION

DC-link voltage balance is one of the major issues in cascaded inverter-based STATCOMs. In this paper, a simple var

Fig. 6. Experimental result: Capacitive mode of operation. (a) Source voltage (50 V/div) and STATCOM current (5 A/div). (b) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 5 ms/div. (c) Harmonic spectrum of

current.

Fig. 7. Experimental result: Mode change from capacitive to inductive. (a) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 100 ms/div. (b) Source voltage (100 V/div) and STATCOM current (5 A/div) in steady state. Time scale: 100 ms/div.

compensating scheme is proposed for a cascaded two-level inverter- based multilevel inverter. The scheme ensures regulation of dc-link voltages of inverters at asymmetrical levels and reactive power compensation. The performance of the scheme is validated by simulation and experimentations under balanced and unbalanced voltage conditions. Further, the cause for instability when there is a change in reference current is investigated. The dynamic model is developed and transfer functions are derived. System behavior is analyzed for various operating conditions. From the analysis, it is inferred that the system is a non minimum phase type, that is, poles of the transfer function always lie on the left half of the -plane. However, zeros shift to the

right half of the -plane for certain operating conditions. For such a system, oscillatory instability for high controller gains exists.

REFERENCES

[1] N. G. Hingorani and L. Gyugyi, Understanding FACTS. Delhi, India: IEEE, 2001, Standard publishers distributors.

[2] B. Singh, R. Saha, A. Chandra, and K. Al-Haddad, “Static synchronous compensators (STATCOM): A review,” IET Power Electron., vol. 2, no. 4, pp. 297–324, 2009.

[3] H. Akagi, H. Fujita, S. Yonetani, and Y. Kondo, “A 6.6-kV transformerless STATCOM based on a five-level diode-clamped PWMconverter: System design and experimentation of a 200-V 10-kVA laboratory model,” IEEE Trans. Ind. Appl., vol. 44, no. 2, pp. 672–680, Mar./Apr. 2008.

[4] A. Shukla, A. Ghosh, and A. Joshi, “Hysteresis current control operation of flying capacitor multilevel inverter and its application in shunt compensation of distribution systems,” IEEE Trans. Power Del., vol. 22, no. 1, pp. 396–405, Jan. 2007.

[5] H. Akagi, S. Inoue, and T. Yoshii, “Control and performance of a transformerless cascaded PWM STATCOM with star configuration,” IEEE Trans. Ind. Appl., vol. 43, no. 4, pp. 1041–1049, Jul./Aug. 2007.

Dynamic Modeling of Electric Springs

ABSTRACT:

The use of ‘Electric Springs’ is a novel way of distributed voltage control while simultaneously achieving effective demand-side management through modulation of non-critical loads in response to the fluctuations in intermittent renewable energy sources (e.g. wind). The proof-of-concept has been successfully demonstrated on a simple 10 kVA test system hardware. However, to show the effectiveness of such electric springs when installed in large numbers across the power system, there is a need to develop simple and yet accurate simulation models for these electric springs which can be incorporated in large-scale power system simulation studies. This paper describes the dynamic simulation approach for electric springs which is appropriate for voltage and frequency control studies at the power system level. The proposed model is validated by comparing the simulation results against the experimental results. Close similarity between the simulation and experimental results gave us the confidence to use this electric spring model for investigating the effectiveness of their collective operation when distributed in large number across a power system. Effectiveness of an electric spring under unity and non-unity load power factors and different proportions of critical and non-critical loads is also demonstrated.

KEYWORDS:

1.      Demand side management

2.      Reactive power control

3.      Electric springs

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1: Block diagram of the test system with Electric Spring

EXPECTED SIMULATION RESULTS:

Fig. 2: Comparison between line voltages with ES operating in voltage support mode. Reactive power consumption of the intermittent source is increased from 467 to 1100 Var at t=2.0 s (a) simulated response and (b) experimental results.

Fig. 3: Comparison between voltages injected by the ES. Reactive power consumption of the intermittent source is increased from 467 to 1100 Var at t=2.0 s (a) simulated response and (b) experimental results

Fig. 4: Comparison between voltages across the non-critical load. Reactive power consumption of the intermittent source is increased from 467 to 1100 Var at t=2.0 s (a) simulated response and (b) experimental results

Fig. 5: Comparison between line voltages with ES operating in voltage suppression mode. Reactive power consumption of the intermittent source is reduced from 467 to 110 Var at t=2.0 s (a) simulated response and (b) experimental results

Fig. 6: Comparison between voltages injected by the ES. Reactive power consumption of the intermittent source is reduced from 467 to 110 Var at t=2.0 s (a) simulated response and (b) experimental results

Fig. 7: Comparison between voltages across the non-critical load. Reactive power consumption of the intermittent source is reduced from 467 to 110 Var at t=2.0 s (a) simulated response and (b) experimental results

Fig. 8: Variation of active power consumeb by the non-critical and critical loads under voltage support (subplot a) and suppression (subplot b) modes

Fig. 9: Simulated response following random variations in active power generation and reactive power consumption of the intermittent source. ES is activated at t = 720 s

Fig. 10: Experimental results following random variations in active power generation and reactive power consumption of the intermittent source. ES is activated at t = 720 s

Fig. 11: Simulated response for RL load (both critical load and non-critical load) with a power factor of 0.95 lagging following random variations in active power generation and reactive power consumption of the intermittent source. ES is activated at t = 720 s

Fig. 12: System response for different distribution of non-critical and critical loads (NC:C). Disturbance is increase in reactive power consumption of the intermittent source from 467 to 1100 VAr at t=1.0 s

CONCLUSION:

The electric spring is a new technology that has attractive features including dynamic voltage regulation, balancing power supply and demand [8,9], power quality improvement [14], distributed power compensation [15] and reducing energy storage requreiments for future smart grid [16]. In order to fully explore their full potentials in large-scale power system simulation, an averaged simulation model for the electric spring is proposed here for the smart grid research community. The simulation model is simple for inclusion into large-scale system simulation platforms and yet accurate enough to capture the dynamic behavior of interest in terms of studying voltage and frequency stability. The case studies reported in the previous section exhibited close match between simulation and experimental results confirming the accuracy. The discrepancies, wherever applicable have been accounted for. These are due to the fact that the DC link voltage control loop has been neglected. This results in attaining a different operation point in terms of ES voltage in order to obtain the same line voltage which is possible as suggested by the phasor diagrams in Fig. 3. It is possible, of course, to include the DC link voltage control loop in the model to eliminate these little discrepancies but only at the expense of significant increase in simulation time for large systems. The addition of DC link voltage control loop (with large number of ES distributed across the system) makes the simulation much more complex with little improvement in terms of accuracy.The use of ‘Electric Springs’ is a novel way of distributed voltage control while simultaneously achieving effective demand-side management through modulation of non-critical loads in response to the fluctuations in intermittent renewable energy sources (e.g. wind). This paper describes the simulation approach for electric springs which is appropriate for voltage and frequency control studies at the power system level. Close similarity between the simulation and experimental results gave us the confidence to use this electric spring model for investigating the effectiveness of their collective operation when distributed in large number across a power system. The proposed dynamic model is generic enough to study the performance of ESs under different load power factors and proportion of critical and non-critical loads. The effectiveness of an ES improves with the proportion of non-critical load.

REFERENCES:

[1] P. Palensky and D. Dietrich, "Demand Side Management: Demand Response, Intelligent Energy Systems, and Smart Loads," IEEE Transactions on Industrial Informatics, vol. 7, pp. 381-388, 2011.

[2] A. Brooks, E. Lu, D. Reicher, C. Spirakis, and B. Weihl, "Demand Dispatch," IEEE Power and Energy Magazine,, vol. 8, pp. 20-29, 2010.

[3] D. Westermann and A. John, "Demand Matching Wind Power Generation With Wide-Area Measurement and Demand-Side Management," IEEE Transactions on Energy Conversion, vol. 22, pp. 145-149, 2007.

[4] A. Mohsenian-Rad, "Autonomous Demand-Side Management Based on Game-Theoretic Energy Consumption Scheduling for the Future Smart Grid," IEEE Transactions on Smart Grid, vol. 1, pp. 320-331, 2010.

[5] S. C. Lee, "Demand Side Management With Air Conditioner Loads Based on the Queuing System Model," IEEE Transactions on Power Systems, vol. 26, pp. 661-668, 2011.

[6] M. Parvania and M. Fotuhi-Firuzabad, "Demand Response Scheduling by Stochastic SCUC," IEEE Transactions on Smart Grid, vol. 1, pp. 89-98, 2010.

Distributed Voltage Control with Electric Springs: Comparison with STATCOM

ABSTRACT:

 The concept of ‘Electric Spring (ES)’ has been proposed recently as an effective means of distributed voltage control. The idea is to regulate the voltage across the ‘critical loads’ while allowing the ‘non-critical’ impedance-type loads (e.g. water heaters) to vary their power consumption and thus contribute to demand-side response. In this paper a comparison is made between distributed voltage control using ES against the traditional single point control with STATCOM. For a given range of supply voltage variation, the total reactive capacity required for each option to produce the desired voltage regulation at the point of connection is compared. A simple case study with a single ES and STATCOM is presented first to show that the ES and STATCOM require comparable reactive power to achieve similar voltage regulation. Comparison between a STATCOM and ES is further substantiated through similar case studies on the IEEE 13-bus test feeder system and also on a part of the distribution network in Sha Lo Wan Bay, Hong Kong. In both cases, it turns out that a group of ESs achieves better total voltage regulation than STATCOM with less overall reactive power capacity. Dependence of the ES capability on proportion of critical and non-critical load is also shown.

KEYWORDS:

1.      Demand response

2.       Electric springs

3.       STATCOM

4.       Voltage control

5.       Voltage regulation

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

                                          

Fig. 1. Electric Spring set-up for Smart loads.

Fig. 2. Simulation set up with an intermittent source and an equivalent power grid.

EXPECTED SIMULATION RESULTS:

Fig. 3. System response following decrease in reactive power consumption of the intermittent source from 467 to 110 VAr

Fig. 4. System response following increase in reactive power consumption of the intermittent source from 1100 to 467 VAr.

             

Fig. 5. System response for different distribution of non-critical and critical loads (NC:C). Disturbance is increase in reactive power consumption of the intermittent source from 467 to 1100 VAr.

CONCLUSION:

In this paper a comparison is made between distributed voltage control using ES against the traditional single point control with STATCOM. For a given range of supply voltage variation, the total voltage regulation and the total reactive capacity required for each option to produce the desired voltage regulation at the point of connection are compared. A simple case study with a single ES and STATCOM is presented first to show that the ES and STATCOM require comparable reactive power to achieve similar voltage regulation. Comparison between a STATCOM and ES is further substantiated through similar case studies on the IEEE 13-bus test feeder system and also on a part of the distribution network in Sha Lo Wan Bay, Hong Kong. In both cases, it turns out that the ESs requires less overall reactive power capacity than STATCOM and yields better total voltage regulation. This makes electric springs (ESs) a promising technology for future smart grids where selective voltage regulation for sensitive loads would be necessary alongside demand side response.

REFERENCES:

[1] N. G. Hingorani and L. Gyugyi, Understanding FACTS : concepts and technology of flexible AC transmission systems. New York: IEEE Press, 2000.

[2] S. Y. Hui, C. K. Lee, and F. F. Wu, "Electric Springs: A New Smart Grid Technology," Smart Grid, IEEE Transactions on, vol. 3, pp. 1552-1561, 2012.

[3] A. Brooks, E. Lu, D. Reicher, C. Spirakis, and B. Weihl, "Demand Dispatch," IEEE Power and Energy Magazine,, vol. 8, pp. 20-29, 2010.

[4] D. Westermann and A. John, "Demand Matching Wind Power Generation With Wide-Area Measurement and Demand-Side Management," IEEE Transactions on Energy Conversion, vol. 22, pp. 145-149, 2007.

[5] C. K. Lee and S. Y. Hui, "Reduction of Energy Storage Requirements in Future Smart Grid Using Electric Springs," Smart Grid, IEEE Transactions on, vol. PP, pp. 1-7, 2013.

H6-type Single Phase Full-Bridge PV Grid-Tied Transformerless Inverters

ABSTRACT:

Photovoltaic (PV) generation systems are widely employed in transformer less inverters, in order to achieve the benefits of high efficiency and low cost. Safety requirements of leakage currents are met by proposing the various transformers less inverter topologies. In this paper, three transformer less inverter topologies are illustrated such as a family of H6 transformer less inverter topologies with low leakage currents is proposed, and the intrinsic relationship between H5 topology, highly efficient and reliable inverter concept (HERIC) topology. The proposed H6 topology has been discussed as well. For a detailed analysis with operation modes and modulation strategy one of the proposed H6 inverter topologies is taken as an example. Comparison among the HERIC, the H5, and the proposed H6 topologies is been done for the power device costs and power losses. For evaluating their performances in terms of power efficiency and leakage currents characteristics, a universal prototype is built for these three topologies mentioned. Simulation results show that the proposed HERIC topology and the H6 topology achieve similar performance in leakage currents, which is slightly worse than that of the H5 topology, but it features higher efficiency than that of H5 topology.

KEYWORDS:

1.      Common-mode voltage

2.       Grid-tied inverter

3.       Leakage current

4.       Photovoltaic (PV) generation system

5.       Transformerless inverter

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Leakage current path for transformerless PV inverters

EXPECTED SIMULATION RESULTS:

Fig. 2. CM voltage and leakage current in H6 topology. (a) CM voltage. (b) Leakage current.

Fig. 3. Drain–source voltages in H6 topology. (a) Voltage stress on S5 and S6 . (b) Detailed waveforms.

Fig. 4. DM characteristic of H6 topology.

Fig. 5. Efficiency comparison of H5, HERIC and H6 topologies.

             

CONCLUSION:

In this paper, based on the H5 topology, a new current path is formed by inserting a power device between the terminals of PV array and the midpoint of one of bridge legs. As a result, a family of single-phase transformerless full-bridge H6 inverter topologies with low leakage currents is derived. The proposed H6 topologies have the following advantages and evaluated by simulation results:

1) The conversion efficiency of the novel H6 topology is better than that of the H5 topology, and its thermal stress distribution is better than that of the H5 topology;

2) The leakage current is almost the same as HERIC topology, and meets the safety standard;

3) The excellent DM performance is achieved like the isolated full-bridge inverter with uniploar SPWM. Therefore, the proposed H6 topologies are good solutions for the single phase transformerless PV grid-tied inverters.

 REFERENCES:

 [1] S. B. Kjaer, J. K. Pederson, and F. Blaabjerg, “A review of single-phase grid-connected inverters for photovoltaic modules,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1292–1306, Sep/Oct. 2005.

[2] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.

[3] B. Sahan, A. N. Vergara, N. Henze, A. Engler, and P. Zacharias, “A single stage PVmodule integrated converter based on a low-power current source inverter,” IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2602–2609, Jul.2008.

[4] M. Calais, J. Myrzik, T. Spooner, and V. G. Agelidis, “Inverters for single phase grid connected photovoltaic systems—An overview,” in Proc. IEEE PESC, 2002, vol. 2, pp. 1995–2000.

[5] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.

A Unified Control Strategy for Three-Phase Inverter in Distributed Generation

ABSTRACT:

This paper presents a unified control strategy that enables both islanded and grid-tied operations of three-phase inverter in distributed generation, with no need for switching between two corresponding controllers or critical islanding detection. The proposed control strategy composes of an inner inductor current loop, and a novel voltage loop in the synchronous reference frame. The inverter is regulated as a current source just by the inner inductor current loop in grid-tied operation, and the voltage controller is automatically activated to regulate the load voltage upon the occurrence of islanding. Furthermore, the waveforms of the grid current in the grid-tied mode and the load voltage in the islanding mode are distorted under nonlinear local load with the conventional strategy. And this issue is addressed by proposing a unified load current feedforward in this paper. Additionally, this paper presents the detailed analysis and the parameter design of the control strategy. Finally, the effectiveness of the proposed control strategy is validated by the simulation results.

KEYWORDS:

1.      Distributed generation (DG)

2.      Islanding

3.      Load current

4.      Seamless transfer

5.      Three-phase inverter

6.       Unified control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Overall block diagram of the proposed unified control strategy.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation waveforms of load voltage v_{C a} , grid current i_ga, and inductor current i_La when DG is in the grid-tied mode under condition of the step down of the grid current reference from 9 A to 5 A with: (a) conventional

voltage mode control, and (b) proposed unified control strategy.

Fig. 3. Simulation waveforms of load voltage v_{C a} , grid current i_ga, and inductor current i_La when DG is transferred from the grid-tied mode to the islanded mode with: (a) conventional hybrid voltage and current mode control, and (b) proposed unified control strategy.

CONCLUSION:

A unified control strategy was proposed for three-phase inverter in DG to operate in both islanded and grid-tied modes, with no need for switching between two different control architectures or critical islanding detection. A novel voltage controller was presented. It is inactivated in the grid-tied mode, and the DG operates as a current source with fast dynamic performance. Upon the utility outage, the voltage controller can automatically be activated to regulate the load voltage. Moreover, a novel load current feed forward was proposed, and it can improve the waveform quality of both the grid current in the grid-tied mode and the load voltage in the islanded mode. The proposed unified control strategy was verified by the simulation results.

REFERENCES:

 [1] R. C. Dugan and T. E. McDermott, “Distributed generation,” IEEE Ind. Appl. Mag., vol. 8, no. 2, pp. 19–25, Mar./Apr. 2002.

[2] R. H. Lasseter, “Microgrids and distributed generation,” J. Energy Eng., vol. 133, no. 3, pp. 144–149, Sep. 2007.

[3] C. Mozina, “Impact of green power distributed generation,” IEEE Ind. Appl. Mag., vol. 16, no. 4, pp. 55–62, Jul./Aug. 2010.

[4] IEEE Recommended Practice for Utility Interface of Photovoltaic(PV) Systems, IEEE Standard 929-2000, 2000.

[5] IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard 1547-2003, 2003.