Vishal Vekhande

Switching State Vector Selection Strategies for Paralleled Multilevel Current-Fed Inverter Under Unequal DC-Link Currents Condition

A paralleled multilevel inverter topology consists of n three-phase three-level current-fed inverters (CFIs) connected in parallel on ac side. AC currents with (2n+1) levels can be generated utilizing redundant switching states when all three-level inverters have equal dc-link currents. However, the multilevel space vector diagram gets modified, redundancy in the switching states is lost and the multilevel current pattern changes when the dc-link current of all inverters is not the same. This introduces low-frequency harmonics in output current, thereby deteriorating total harmonic distortion (THD). The presence of low-frequency components in the output current could be avoided by selecting suitable switching state vectors and ensuring proper time sharing among these vectors. Two methods to select such switching state vectors are proposed in this paper. In the first method, a reference current space vector is realized using the nearest switching state vectors. However, this method results in low-frequency pulsation in dc-link voltage of each inverter. In the second method, the switching state vectors are chosen to eliminate this low-frequency pulsation. Effectiveness of these methods is experimentally validated for a five-level CFI. Further, performance of these methods is compared based on efficiency, THD, and dc-link voltage ripple for various inequality ratios in dc-link currents.

Central multilevel current-fed inverter with module integrated DC-DC converters for grid-connected PV plant

This paper proposes a system of current-fed converters for the integration of a large photovoltaic (PV) plant with a utility ac grid. The system comprises a large number of PV modules with module integrated dc-dc converter. A dual inductor current-fed push-pull topology is proposed for this purpose. To meet the THD requirement along with low switching losses a central multilevel current-fed inverter (CFI) is proposed. It comprises two three-phase three-level CFI units connected in parallel on the ac side. The dc-dc converters inject continuous current into the dc-bus of the CFI units. Five-level output current waveform is obtained using space vector modulation (SVM) technique when dc-bus currents are equal. However, mismatch in module characteristics and non-uniform insolation levels cause unequal dc-bus currents leading to distortion of the output current waveform. This limitation is addressed by appropriate selection of the switching state vectors while employing SVM technique. Two methods for selection of the switching state vectors are presented. The performance of a 100 kW PV plant utilizing the proposed power converters system is studied using MATLAB/Simulink.

New direct torque control of DFIG under balanced and unbalanced grid voltage

This paper presents a direct torque control method for doubly-fed induction generator (DFIG) based wind power generation systems. The angle and magnitude of rotor voltage are controlled to achieve independent control of electromagnetic torque and reactive power respectively. Space vector modulation is used to address the limitations like, variable switching frequency and torque ripple, of hysteresis based schemes. Further, this paper presents a technique to reduce the torque pulsations of DFIG under unbalanced grid voltage condition. Under unbalanced grid voltage condition, the torque angle (δ) is controlled so that electromagnetic torque pulsations are reduced. To achieve this, a compensation method based on proportional-integral and resonant (PI+R) controller is explored. The proposed control method does not require rotating frame transformations and it maintains the simplicity of DTC. It has fast dynamic response, which is comparable to vector control. Simulation results for a 2 MW DFIG system demonstrates the effectiveness of the proposed control strategy with various loading conditions under both balanced and unbalanced grid voltage.

Bidirectional current-fed converter for integration of dc micro-grid with ac grid

Most of the loads found in various commercial installations require front end ac-dc conversion. Various storage systems and renewable sources interact with these loads and the ac grid through dc-ac converters. These multistage conversions reduce system efficiency. To avoid this, an ac distribution system can be replaced by a dc distribution system within a commercial facility. It results in improved system efficiency and reliability. This paper proposes a three-phase current-fed converter, interfacing a dc distribution system with a utility ac grid. It has bidirectional power flow capability. The converter injects or draws low ripple current from a dc micro-grid, while maintaining high power factor and low current distortion on ac grid side. It has improved reliability and size over conventional VSC. The steady state and dynamic performance of the converter is studied using MATLAB Simulink. The converter also has a limited capacity of supplying or absorbing reactive power. These limits are computed for both inversion and rectification modes of operation.

A new direct torque control of doubly-fed induction generator under unbalanced grid voltage

In this paper a new direct torque control method for doubly-fed induction generator (DFIG) based wind power generation system is presented. The angle and magnitude of rotor voltage are controlled to achieve independent control of electromagnetic torque and reactive power respectively. Space vector modulation is used to address the limitations, like variable switching frequency and torque ripple, of hysteresis based schemes. Further, this paper proposes the compensation of effects of unbalanced grid voltage. When stator of DFIG is connected to unbalanced grid, the stator flux consists of positive sequence component and negative sequence component. The scheme compensates the negative sequence stator flux by negative sequence rotor flux using rotor side converter. When negative sequence component of stator flux is compensated, all effects of stator unbalanced voltages such as second harmonic pulsation in torque, stator active power, stator reactive power, stator current and rotor current are reduced. Simulation results for a 3 kW DFIG system demonstrates the effectiveness of the proposed direct control method and scheme for compensation of effects of unbalanced grid voltage.

Module integrated DC-DC converter for integration of photovoltaic source with DC micro-grid

A ac distribution system is very common in various commercial installations. Most of the loads require front end ac-dc converter to interact with the ac system. Also, various storage systems and renewable sources connect to the ac system through dc-ac converters. These multistage conversions in the power flow path from renewable sources and storage systems to the loads reduce system efficiency. In view of this, an ac distribution system can be replaced by a dc distribution system within a commercial facility to improve overall system efficiency. This paper proposes a module integrated dc-dc converter for integration of a photovoltaic (PV) source with a dc distribution system. Current-fed topology is suggested to avoid limitations associated with large energy storage capacitor. The converter injects very low ripple, continuous current into the dc microgrid. Effect of converter switching ripple at low insolation levels is analyzed. Use of L-C filter at the converter front end is proposed, to improve power extraction at low insolation levels. This modification also enables utilization of non-dissipative snubber circuits. The converter allows large voltage step-up ratio, suitable for integration of low voltage PV module with high voltage dc micro-grid. It has compact size and high reliability. The steady state and dynamic performance of the converter are studied through computer simulations and experimental tests.

A new direct torque control of doubly fed induction generator for wind power generation

This paper presents a new direct torque control (DTC) method for doubly-fed induction generator (DFIG) based wind power generation system. The angle and magnitude of rotor voltage are controlled to achieve independent control of electromagnetic torque and reactive power respectively. Space vector modulation (SVM) is used to address the limitations, of variable switching frequency and torque ripple, of hysteresis based schemes. The required voltage vector is generated in polar form. Hence, implementation of SVM for this vector is simpler compared to other methods reported in the literature. The proposed DTC is inherently a scalar control method. However, it has fast dynamic response, which is comparable to vector control. Simulation results for a DFIG system, during the variation of torque and reactive power, demonstrates the effectiveness of the proposed control strategy.

Modulation of Indirect Matrix Converter under unbalanced source voltage condition

This paper deals with the effects of unbalanced source voltages on Indirect Matrix Converter (IMC). A detailed theoretical analysis is presented for the harmonic content of input and output currents when unbalanced voltages are applied at IMC input. A control strategy is proposed, where the modulation index of inverter section of IMC is dynamically varied to eliminate the harmonic components of input currents caused due to unbalanced input voltages. A laboratory prototype is developed to verify the effectiveness of the control strategy. Experimental waveforms and harmonic spectrums show excellent agreement between theoretical analysis and simulation results.

Control of Three-Phase Bidirectional Current-Source Converter to Inject Balanced Three-Phase Currents Under Unbalanced Grid Voltage Condition

A single-stage, bidirectional, current-source converter (CSC) topology to interface a dc microgrid with an ac grid is reported in the literature. In this topology, under a balanced grid voltage condition, the dc-link inductor current can be regulated over a wide range-from zero to rated value-while the ac-side current has low harmonic distortion. However, unbalanced grid voltages result in second-harmonic pulsation in the current and power on the dc side of the converter. In addition, the ac-side currents will be unbalanced due to the presence of a negative-sequence component. This would result in undesired tripping of the converter if one of the phase currents exceeded its rated value. Various control loop structures for the operation of voltage-source converter under unbalanced grid voltage conditions are reported in the literature. However, use of similar control loop structures for CSC may lead to unstable operation. Therefore, a control scheme to inject balanced three-phase currents into the ac grid under an unbalanced grid voltage condition is proposed in this paper. The stability of the proposed control scheme is studied using a small-signal model of the converter. Performance of the proposed control scheme is studied using MATLAB/Simulink and is experimentally validated.

Central seven-level current-fed inverter with module-integrated DC-DC converters for grid-connected PV plant

This paper proposes a system of current-fed converters for the integration of a large photovoltaic (PV) plant with a utility AC grid. The system comprises of a central seven-level current-fed inverter (CFI) and a large number of PV modules with module-integrated DC-DC converter. The seven-level CFI consists of three three-phase, three-level CFI units connected in parallel on the AC-side. Dual inductor current-fed push-pull topology is used for DC-DC converters which inject continuous current into the DC-bus of the CFI units. Seven-level output current waveform is obtained using space vector modulation (SVM) technique when DC-bus currents are equal. However, mismatch in module characteristics and non-uniform insolation levels cause unequal DC-bus currents leading to distortion of the output current waveform. This limitation is addressed by appropriate selection of the switching state vectors while employing SVM technique. The performance of the proposed power converters system is studied using MATLAB/Simulink.