Somasundaram Essakiappan

Multilevel Medium-Frequency Link Inverter for Utility Scale Photovoltaic Integration

A multilevel topology with medium-frequency ac link for medium-voltage grid integration of utility photovoltaic (PV) plants is discussed in this paper. A megawatt-scale PV plant is divided into many zones, each comprising of two series-connected arrays. Each zone employs a medium-frequency transformer with three secondaries, which interface with the three phases of the medium voltage grid. An insulated-gate bipolar transistor full-bridge inverter feeds the MF transformer. The voltages at the transformer secondaries are then converted to three-phase line frequency ac by three full-bridge ac-ac converters. Second line frequency harmonic power does not appear in the dc bus, thereby reducing the dc capacitor size. Cascading several such cells, a high-quality multilevel medium-voltage output is generated. A new control method is proposed for the cascaded multilevel converter during partial shading while minimizing the switch ratings. The proposed topology eliminates the need for line frequency transformer isolation and reduces the dc bus capacitor size, while improving the power factor and energy yield. This paper presents the analysis, design example, and operation of a 10-MW utility PV system with experimental results on a scaled-down laboratory prototype.

A new multilevel converter for Megawatt scale solar photovoltaic utility integration

This paper presents a new multi-level DC-AC-AC converter topology for medium voltage grid integration of Megawatt (MW) scale utility photovoltaic (PV) plants. It is envisioned that a large PV field is divided into many zones, each comprising of two PV arrays. The number of zones depends on the voltage of the grid with which it is interfaced. In the proposed approach, zonal power balancing is achieved by employing a current-sharing technique. The power conversion architecture consists of an IGBT based full-bridge inverter feeding a medium frequency (MF) transformer with three secondary windings. The voltages at the transformer secondaries are then converted to three phase line frequency AC by three, full-bridge AC-AC converters. This also eliminates the 2nd harmonic power from the DC bus, thereby reducing the capacitor size. By stacking several such modules in series, a high quality multilevel medium voltage output is generated. Further, the bulky line frequency utility interface transformer is eliminated. A new control method is proposed for the series connected modules during partial shading while minimizing the switch ratings. This paper presents the analysis, design example and simulation of a 10 MW PV system with preliminary experimental results on a laboratory prototype.

A Fault-Tolerant Three-Phase Adjustable Speed Drive Topology With Active Common-Mode Voltage Suppression

A fault-tolerant adjustable speed drive (ASD) topology is introduced in this paper. A conventional ASD topology is modified to address: 1) drive vulnerability to semiconductor device faults; 2) input voltage sags; 3) motor vulnerability to effects of long leads; and 4) minimization of common-mode (CM) voltage applied to the motor terminals. These objectives are attained by inclusion of an auxiliary IGBT inverter leg, three auxiliary diodes, and isolation-reconfiguration circuit. The design and operation of the proposed topology modifications are described for different modes: 1) fault mode, 2) active CM suppression mode, and 3) auxiliary sag compensation (ASC) mode. In case of fault and sag, the isolation and hardware reconfiguration are performed in a controlled manner using triacs/antiparallel thyristors. In normal operation, the auxiliary leg is controlled to actively suppress CM voltage. For inverter IGBT failures (short circuit and open circuit), the auxiliary leg is used as a redundant leg. During voltage sags, the auxiliary leg, along with auxiliary diodes, is operated as a boost converter. A current-shaping control strategy is proposed for the ASC mode. A detailed analysis of CM performance of the proposed topology is provided, and a new figure of merit, CM distortion ratio (CMDR), is introduced to compare the attenuation of CM voltage with that of a conventional ASD topology. The output filter design procedure is outlined. A design example is presented for an 80 kW ASD system, and simulation results validate the proposed auxiliary leg based fault-tolerant scheme. Experimental results from a scaled prototype rated at 1 hp are discussed in this paper.

Analysis and mitigation of common mode voltages in photovoltaic power systems

A typical photovoltaic (PV) power system is composed of multiple strings of PV arrays with one central inverter or a combination of PV modules with micro-inverters. Per-module DC-DC converter and series connected multilevel inverter topologies are possible candidates. In this paper, the analyses of common mode voltages and currents in the afore-mentioned PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. This paper analyzes the common mode voltages and current present in high and low power PV systems with different topologies. Mitigation strategies such as common mode filter and transformer shielding are proposed to minimize common mode voltages and currents. A practical rooftop PV system rated at 2.76 kW per string is used in the analysis.

A bidirectional series resonant matrix converter topology for electric vehicle DC fast charging

A series resonant matrix converter (MC) based topology for high power electric vehicle (EV) battery charging is presented. The system performs DC fast charging and is capable of bidirectional power flow, for V2G (vehicle-to-grid) applications. The proposed topology can be divided into three sections: (i) a front-end 3×1 matrix converter, (ii) LrCr series resonant tank and high frequency (HF) transformer, and (iii) a single phase PWM rectifier. The matrix converter takes a three phase line frequency voltage and produces a high frequency (14.94 kHz) AC output. The resonant tank frequency is set to 13.7 kHz and helps to achieve zero voltage switching (ZVS) turn ON and low turn OFF switching losses. The secondary of the transformer is then interfaced to the EV battery bank through a PWM rectifier. The advantages of such a system include high efficiency due to soft switching operation, low VA transformer ratings due to resonant operation, and high power density due to the absence of electrolytic capacitors. A design example rated 30 kW, which charges a 500 V battery system, is presented. Analysis and simulation results demonstrate the performance of the proposed bidirectional topology. Preliminary experimental results are provided for a scaled down prototype operating at 500 W using a 15 kHz ferrite transformer.

Independent control of series connected utility scale multilevel photovoltaic inverters

A new control strategy for megawatt scale series connected multilevel photovoltaic (PV) inverters is proposed. The proposed strategy ensures balanced operation under partial shading. Further under transient grid conditions such as low voltage ride through (LVRT) it is shown that the system remains connected and is fully controllable. In the proposed system, the PV arrays are grouped into zones and each zone is connected to a DC-AC-AC converter. The DC to AC inverters of multiple zones are connected in series to form the required medium voltage and transfer power to the grid. When all zones are uniformly illuminated, the output voltages magnitudes and phases of all the inverters are equal. When the insolation is different for some of the zones, the output voltage magnitude and phase angles of individual DC to AC inverters need to be adjusted in such a way that the real power from the maximum power point calculation is transferred to the grid and the power factor of the overall system is high. Also, when the grid experiences voltage sags, the system is shown to perform a low voltage ride-through (LVRT) algorithm to stay connected to the grid, without a surge in the output current. A control strategy is developed to operate the series connected multilevel inverter configuration under partial shading, by continuously monitoring the real and reactive powers supplied by each inverter. This does not employ a central controller and/or communications between the various DC to AC inverter blocks. The proposed control strategy is simulated for a 6.6 kV, three phase utility scale PV system rated at 5 MW.

State space analysis and duty cycle control of a switched reactance based center-point-clamped reactive power compensator

A new approach for static reactive power compensation has been presented in this paper. Conventional thyristor controlled Static VAr Compensators (SVC) have inherent disadvantages like slow response times and poor harmonic performance as these FACTS devices are based on slow switching power electronics devices. Alternately, Pulse Width Modulated (PWM) dc-ac inverters and direct ac-ac converter structures offer higher bandwidth and push the spectral content to higher switching frequencies that are easier to filter. However, the application space of this approach is limited by the low voltage blocking capability of power devices employed in these converters. A center-point-clamped ac-ac direct power converter has been reported recently in literature which operates on the principle of neutral-point-clamped dc-ac inverter. By clamping the grid voltage to its mid-point, the center-point-clamped converter structure reduces voltage stress on the bi-directional switches by 50%. Compared to the conventional two-level and multilevel dc-ac inverters, the proposed compensator based on direct ac-ac conversion has a simpler structure and control. The operating principle as well as dynamic analysis for the proposed VAr compensation approach has been presented in the paper. A feedback controller has been designed for closed loop control. Simulation results presented in the paper verify that proposed converter offers better control of reactive power, retrofit capability, and reduced voltage stress on the bi-directional switches. Furthermore, it has been shown that leading and lagging reactive compensation can be accomplished with a smooth control of the reactance through duty cycle modulation.

A new control strategy for megawatt scale multilevel photovoltaic inverters under partial shading

In this paper, a new control strategy for megawatt scale multilevel photovoltaic (PV) inverters under partial shading is proposed. In the proposed system, the photovoltaic arrays are divided into zones and each zone is connected to DC to AC inverter. The DC to AC inverters of multiple zones are connected in series to form the required medium voltage and transfer power to the grid. In such a system where all zones are uniformly illuminated, the output voltage vectors of all the inverters are of equal magnitude and in phase with each other. However, under partial shading, the output voltage magnitude and phase angles of individual DC to AC inverters need to be adjusted in such a way that the available real power is transferred to the grid and the power factor of the overall system is as close to unity as possible. A control strategy is developed to operate the series connected multilevel inverter configuration under partial shading, by continuously monitoring the real and reactive powers supplied by each inverter. The developed strategy does not require a central controller and/or communications between the various DC to AC inverter blocks. The proposed control strategy is simulated for a 6.6 kV, three phase utility scale PV system rated at 5 MW.

A framework for reliability evaluation of electric vehicle charging stations

The focus of this paper is to develop a framework to evaluate and analyze the reliability of the power electronic circuits in electric vehicle chargers, which are expected to be a major component of the utility load in the future. A general infrastructure of electric vehicle charging systems is described and different charging levels are discussed. To illustrate a numerical reliability analysis, two high power dc-dc topologies are selected and the mean time between failures of these converters that are widely used in electric vehicle charging stations are calculated and compared. The impact of each component in the power electronic circuit on overall system reliability is discussed and in particular the sensitivity to temperature, safety factor and capacitor type are studied. These factors have the highest impact on the system reliability. The framework developed in this paper has the potential to serve as a template for reliability evaluation and comparison of future on-board and off-board electric vehicle chargers.

Analysis and design of active inductor as DC-link reactor for lightweight adjustable speed drive systems

Size and weight considerations are very important in certain specialized industries as offshore oil drilling and marine/subsea systems that employ adjustable speed drives (ASD). Typical ASD topology consists of a three phase diode rectifier front-end. It is also common to specify a dc-link inductor and/or ac line reactors to reduce utility line current harmonics to acceptable levels. However, the required dc-link inductor/ac line reactors contribute to additional weight and volume. In this paper an active inductor approach is explored to emulate a large size dc-link inductor. Analysis and design of an active inductor for a 1 MW ASD is proposed and analyzed. The required inductance value is commanded and current control regulates the active inductor current to match inductor behavior. It is shown that the proposed active inductor topology is similar in weight/volume as the smaller inductor while emulating a higher per unit inductance. The proposed active inductor meets the required compact and lightweight specifications of ASDs in retrofit and special applications. Furthermore, the active inductor is tunable to achieve high performance operation during faults. Experimental results from a scaled down prototype are presented. A size, weight and loss analysis is detailed and a soft switching method to reduce losses is also explained.