Bashar Zahawi

Assessment of Perturb and Observe MPPT Algorithm Implementation Techniques for PV Pumping Applications

The energy utilization efficiency of commercial photovoltaic (PV) pumping systems can be significantly improved by employing simple perturb and observe (P&O) maximum power point tracking algorithms. Two such P&O implementation techniques, reference voltage perturbation and direct duty ratio perturbation, are commonly utilized in the literature but no clear criteria for the suitable choice of method or algorithm parameters have been presented. This paper presents a detailed theoretical and experimental comparison of the two P&O implementation techniques on the basis of system stability, performance characteristics, and energy utilization for standalone PV pumping systems. The influence of algorithm parameters on system behavior is investigated and the various advantages and drawbacks of each technique are identified for different weather conditions. Practical results obtained using a 1080-Wp PV array connected to a 1-kW permanent magnet dc motor-centrifugal pump set show very good agreement with the theoretical analysis and numerical simulations.

Minimum-Threshold Crowbar for a Fault-Ride-Through Grid-Code-Compliant DFIG Wind Turbine

Doubly fed induction generator (DFIG) technology is the dominant technology in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially rated power converter. However, the DFIG is sensitive to dips in supply voltage and without specific protection to “ride-through” grid faults, a DFIG risks damage to its power converter due to overcurrent and/or overvoltage. Conventional converter protection via a sustained period of rotor-crowbar closed circuit leads to poor power output and sustained suppression of the stator voltages. A new minimum-threshold rotor-crowbar method is presented in this paper, improving fault response by reducing crowbar application periods to 11-16 ms, successfully diverting transient overcurrents, and restoring good power control within 45 ms of both fault initiation and clearance, thus enabling the DFIG to meet grid-code fault-ride-through requirements. The new method is experimentally verified and evaluated using a 7.5-kW test facility.

Assessment of the Incremental Conductance Maximum Power Point Tracking Algorithm

An efficient, cost-effective maximum power point tracking (MPPT) algorithm is required to improve the energy utilization efficiency of low power photovoltaic (PV) systems. This paper presents an experimental evaluation of the incremental conductance MPPT algorithm when employed by a standalone PV pumping system, using an experimental installation comprised of a 1080-Wp photovoltaic array connected to a 1-kW permanent magnet dc motor-centrifugal pump set. Particular focus is given to the evaluation of the two commonly utilized implementation techniques: reference voltage perturbation and direct duty ratio perturbation. The influence of algorithm parameters on system behavior is investigated and the energy utilization efficiency is calculated for different weather conditions. The performance of the incremental conductance algorithm is compared to that of the commonly used perturb and observe MPPT algorithm and the various advantages and drawbacks of each technique are identified.

Stability Analysis of the Continuous-Conduction-Mode Buck Converter Via Filippov's Method

To study the stability of a nominal cyclic steady state in power electronic converters, it is necessary to obtain a linearization around the periodic orbit. In many past studies, this was achieved by explicitly deriving the Poincare map that describes the evolution of the state from one clock instant to the next and then locally linearizing the map at the fixed point. However, in many converters, the map cannot be derived in closed form, and therefore this approach cannot directly be applied. Alternatively, the orbital stability can be worked out by studying the evolution of perturbations about a nominal periodic orbit, and some studies along this line have also been reported. In this paper, we show that Filippovs method - which has commonly been applied to mechanical switching systems - can be used fruitfully in power electronic circuits to achieve the same end by describing the behavior of the system during the switchings. By combining this and the Floquet theory, it is possible to describe the stability of power electronic converters. We demonstrate the method using the example of a voltage-mode-controlled buck converter operating in continuous conduction mode. We find that the stability of a converter is strongly dependent upon the so-called saltation matrix - the state transition matrix relating the state just after the switching to that just before. We show that the Filippov approach, especially the structure of the saltation matrix, offers some additional insights on issues related to the stability of the orbit, like the recent observation that coupling with spurious signals coming from the environment causes intermittent subharmonic windows. Based on this approach, we also propose a new controller that can significantly extend the parameter range for nominal period-1 operation.

In Situ Diagnostics and Prognostics of Wire Bonding Faults in IGBT Modules for Electric Vehicle Drives

This paper presents a diagnostic and prognostic condition monitoring method for insulated-gate bipolar transistor (IGBT) power modules for use primarily in electric vehicle applications. The wire-bond-related failure, one of the most commonly observed packaging failures, is investigated by analytical and experimental methods using the on-state voltage drop as a failure indicator. A sophisticated test bench is developed to generate and apply the required current/power pulses to the device under test. The proposed method is capable of detecting small changes in the failure indicators of the IGBTs and freewheeling diodes and its effectiveness is validated experimentally. The novelty of the work lies in the accurate online testing capacity for diagnostics and prognostics of the power module with a focus on the wire bonding faults, by injecting external currents into the power unit during the idle time. Test results show that the IGBT may sustain a loss of half the bond wires before the impending fault becomes catastrophic. The measurement circuitry can be embedded in the IGBT drive circuits and the measurements can be performed in situ when the electric vehicle stops in stop-and-go, red light traffic conditions, or during routine servicing.

Comparison of Directly Connected and Constant Voltage Controlled Photovoltaic Pumping Systems

This paper presents a comparative investigation of the performance characteristics of a directly connected photovoltaic (PV) pumping system and a scheme utilizing a constant voltage maximum power point tracking algorithm. A simple and accurate model is developed for each individual component of the system based on its measured characteristics and the system is simulated numerically. System performance is analyzed and energy utilization efficiency is calculated for different weather conditions. A detailed comparison identifying the advantages and drawbacks of each technique is presented. Experimental results obtained using a 1080-Wp PV array connected to a 1-kW permanent magnet dc motor-centrifugal pump set show very good agreement with the numerical simulation of the systems.

Evaluation of the Performance of a DC-Link Brake Chopper as a DFIG Low-Voltage Fault-Ride-Through Device

The performance of the doubly fed induction generator (DFIG) during grid faults is attracting much interest due to the proliferation of wind turbines that employ this technology. International grid codes specify that the generator must exhibit a fault-ride-through (FRT) capability by remaining connected and contributing to network stability during a fault. Many DFIG systems employ a rotor circuit crowbar to protect the rotor converter during a fault. Although this works well to protect the generator, it does not provide favorable grid support behavior. This paper describes an experimental investigation of an alternative FRT approach using a brake chopper circuit across the converter dc link to ensure that the dc-link voltage remains under control during a fault. Two different approaches to chopper control are examined and the resulting FRT performance is compared with that of a conventional crowbar approach. The new chopper-based control methods are experimentally evaluated using a 7.5-kW DFIG test rig facility.

Analytical Study of Grid-Fault Response of Wind Turbine Doubly Fed Induction Generator

Grid-connected wind turbine doubly fed induction generators (DFIGs) are sensitive to dips in supply voltage, so-called “grid faults”. Fault ride through systems must be designed to manage the large and potentially dangerous fault currents in both stator and rotor circuits. However, DFIG fault response has been only partially or summarily treated in contemporary literature. This paper presents a detailed analytical analysis of wind turbine DFIG grid-fault response. The physical behavior of the machine is presented through a description of the flux-linkage response and later developed with analytical solutions of the generalized DFIG machine equations operating under fault conditions showing that the frequencies of the decay components of current deviate from pure dc and rotor speed due to magnetic drag effects. The analysis is experimentally verified and evaluated using a 7.5-kW test facility.

Control of Fast Scale Bifurcations in Power-Factor Correction Converters

This brief proposes a novel controller which greatly enhances the performance of a power-factor correction converter. This controller is optimally tuned to place the eigenvalues of the system well inside the unit circle and hence it guarantees stable operation over a wide range of input voltages. The design of the controller is based on the stability analysis of the system using the state transition matrix over a clock cycle. It is shown that the transition matrix across the switching manifold greatly influences the systems performance, allowing the system to be stabilized by periodically altering the manifold. The results are validated by analytical and numerical studies.

Complex Interaction Between Tori and Onset of Three-Frequency Quasi-Periodicity in a Current Mode Controlled Boost Converter

It is known that power electronic circuits like dc-dc converters are highly nonlinear systems, and that period doubling and Neimark-Sacker bifurcations are common sources of instability in such systems. It has also been shown that these two types of bifurcation may interact, giving rise to interesting dynamical phenomena. In this paper we show that in a current mode controlled dc-dc converter, periodic, quasi-periodic, and saturation behavior can coexist for the same parameter value, and there can be complex interactions between them. Furthermore, abrupt exit to saturation mode can be triggered by a torus-torus collision. Finally, we report the first observation of three-frequency quasi-periodicity in a power electronic system.