Zaka Ullah Zahid

Design and Analysis of an MPPT Technique for Small-Scale Wind Energy Conversion Systems

This paper proposes a maximum power point tracking (MPPT) algorithm for small-scale wind energy conversion systems. The proposed algorithm uses the dc current as the perturbing variable. The algorithm detects sudden wind speed changes indirectly through the dc-link voltage slope. The voltage slope is also used to enhance the tracking speed of the algorithm and to prevent the generator from stalling under rapid wind speed slowdown conditions. The proposed method uses two modes of operation: A perturb and observe (P&O) mode with adaptive step size under slow wind speed fluctuation conditions, and a prediction mode employed under fast wind speed change conditions. The dc-link capacitor voltage slope reflects the acceleration information of the generator, which is then used to predict the next step size and direction of the current command. The proposed algorithm shows enhanced stability and fast tracking capability under both high and low rate of change wind speed conditions and is verified using a 1.5 kW prototype hardware setup.

High-Efficiency Contactless Power Transfer System for Electric Vehicle Battery Charging Application

In this paper, a contactless charging system for an electric vehicle (EV) battery is proposed. The system consists of three parts: 1) a high-frequency power supply from a full-bridge inverter with frequency modulation; 2) a loosely coupled transformer that utilizes series resonant capacitors for both the primary and secondary windings; and 3) a rectification output circuit that uses a full-bridge diode rectifier. With carefully selected compensation network parameters, zero-voltage switching can be ensured for all the primary switches within the full range of an EV battery charging procedure, which allows the use of low ON-state resistance power MOSFETs to achieve high-frequency operation and system efficiency. The design of loosely coupled transformer is simulated and verified by finite element analysis software. For a 4-kW hardware prototype, the peak dc-dc efficiency reaches 98% and 96.6% under 4- and 8-cm air gap conditions, respectively. The prototype was tested with an electronic load and a home-modified EV to verify the performance of constant current and constant voltage control and their transitions.

A High-Efficiency MOSFET Transformerless Inverter for Nonisolated Microinverter Applications

State-of-the-art low-power-level metal-oxide-semiconductor field-effect transistor (MOSFET)-based transformerless photovoltaic (PV) inverters can achieve high efficiency by using latest super junction MOSFETs. However, these MOSFET-based inverter topologies suffer from one or more of these drawbacks: MOSFET failure risk from body diode reverse recovery, increased conduction losses due to more devices, or low magnetics utilization. By splitting the conventional MOSFET-based phase leg with an optimized inductor, this paper proposes a novel MOSFET-based phase leg configuration to minimize these drawbacks. Based on the proposed phase leg configuration, a high efficiency single-phase MOSFET transformerless inverter is presented for the PV microinverter applications. The pulsewidth modulation (PWM) modulation and circuit operation principle are then described. The common-mode and differential-mode voltage model is then presented and analyzed for circuit design. Experimental results of a 250 W hardware prototype are shown to demonstrate the merits of the proposed transformerless inverter on nonisolated two-stage PV microinverter application.

New Overall Control Strategy for Small-Scale WECS in MPPT and Stall Regions With Mode Transfer Control

This paper presents a new overall control strategy for small-scale wind energy conversion systems (WECS). The purpose of the proposed strategy is to utilize a maximum power point tracking control (MPPT) to maximize the captured energy when the wind speeds are below the rated speed. For high wind speed region; two stall controllers are developed: The constant speed stall controller which will limit the rotational speed of the generator to its rated value, and the constant power stall controller, which will regulate the captured power to be within the system rating. For the MPPT control, a modified perturb and observe (P&O) algorithm is utilized, where the dc side current is used as a perturbation variable and the dc-link voltage slope information is used to enhance the tracking speed and stability of the algorithm. For the speed and power regulation operation during high wind speeds, the system is controlled in the stall region to limit the rotational speed and the power of the generator. A stabilizing control loop is proposed to compensate for the stall region instability. A new mode transfer control strategy is developed to effectively control the transition between different modes of operation while ensuring the system stability without using any preknowledge of the system parameters. A 1 kW hardware prototype is developed and tested to validate the proposed new control strategy.

Modeling and Control of Series–Series Compensated Inductive Power Transfer System

In this paper, a T-equivalent circuit for loosely coupled transformer, which does not involve magnetic coupling, is presented. A detailed dynamic analysis based on extended describing function technique is presented for a series-series compensated inductive power transfer system. The continuous-time large-signal model, the steady-state operating point, and the small-signal model are derived in an analytical closed-form. This model includes both the frequency and the phase-shift control. Simulation and experimental verification results of the derived models are presented to validate the presented analysis.

Design of Bidirectional DC–DC Resonant Converter for Vehicle-to-Grid (V2G) Applications

In this paper, a detailed design procedure is presented for a bidirectional CLLLC-type resonant converter for a battery charging application. This converter is similar to an LLC-type resonant converter with an extra inductor and capacitor in the secondary side. Soft-switching can be ensured in all switches without additional snubber or clamp circuitry. Because of soft-switching in all switches, very high-frequency operation is possible; thus, the size of the magnetics and the filter capacitors can be made small. To reduce the size and cost of the converter, a CLLC-type resonant network is derived from the original CLLLC-type resonant network. First, in this paper, an equivalent model for the bidirectional converter is derived for the steady-state analysis. Then, the design methodology is presented for the CLLLC-type resonant converter. Design of this converter includes determining the transformer turns ratio, design of the magnetizing inductance based on zero-voltage switching condition, design of the resonant inductances and capacitances. Then, the CLLCtype resonant network is derived from the CLLLC-type resonant network. To validate the design procedure, a 3.5-kW converter was designed following the guidelines in the proposed methodology. A prototype was built and tested in the laboratory. Experimental results verified the design procedure presented.

High efficiency contactless power transfer system for electric vehicle battery charging

In this paper, a contactless charging system for an electric vehicle (EV) battery is proposed. The system is composed of three parts, a high frequency power supply from a full-bridge inverter with frequency modulation, a loosely coupled transformer that utilizes series resonant capacitors for both the primary and secondary windings, and a rectification output circuit that uses a full-bridge diode rectifier. With carefully selected compensation network parameters, zero-voltage switching (ZVS) can be ensured for all the primary switches within the full range of an EV battery charging procedure, which allows the use of low on-state resistance power MOSFETs to achieve high frequency operation and system efficiency. The design of loosely coupled transformer is simulated and verified by finite element analysis (FEA) software. A peak efficiency of 96.6% is achieved with a 4 kW prototype with an 8 cm air gap transformer and 156 kHz switching frequency.

Analysis and parameters optimization of a contactless IPT system for EV charger

This paper discusses the characteristics of a series-series compensated inductive power transfer system (IPT) with theoretical analysis and experimental results. To maximize system efficiency, two-stage structure, which includes a PFC stage and a resonant DC-DC stage operating in ZVS region, is proposed. One of the major design challenges in implementation of a practical contactless EV charger is the variation of the coupling condition of the loosely coupled transformer. This combined with the battery chargers wide range load variation makes parameters design of the resonant DC-DC stage more complex. To optimize the parameter design for all coupling and load conditions, a parameter sweeping method is proposed. The design procedure searches parameters set to minimize the averaged primary current while keeping the voltage stress across the primary capacitor below the preset limit; both the high coupling and low coupling conditions are considered. To validate the analysis, a 4 kW resonant DC-DC prototype was built and tested. At 4 cm gap distance with 0.526 coupling coefficient, an efficiency of 97% for the DC-DC stage was achieved with output power range from 0.8 kW to 4 kW.

Damping impact on dynamic analysis of LLC resonant converter

In this paper, the small-signal model of LLC resonant converter is derived using extended describing function (EDF) methodology and the dynamics of the LLC resonant converter are studied based on the small signal model taking into account the circuit parasitic resistance. When designing the controller, this damping effect induced by the parasitic resistance is usually neglected. While it will be shown in this paper that for large transformer turns ratio, the secondary side parasitic resistance introduces significant damping in the resonant tank. A controller is designed for an example LLC circuit. It is shown that the control loop phase margin and the cross over frequency are highly dependent on the damping factor, which should be considered in the control loop design to have the desired transient performance. The experimental verification is provided as well.

Small-signal modeling of series-series compensated induction power transfer system

Based on Extended Describing Functions technique, small-signal modeling technique is applied to Series-Series compensated Induction Power Transfer (IPT) system. The derived model includes both frequency control and phase shift control. The derived model is in good agreement with the simulation results obtained from Simplis software.