Suleiman M. Sharkh

Energy and Battery Management of a Plug-In Series Hybrid Electric Vehicle Using Fuzzy Logic

Fuzzy logic is used to define a new quantity called the battery working state (BWS), which is based on both battery terminal voltage and state of charge (SOC), to overcome the problem of battery over-discharge and associated damage resulting from inaccurate estimates of the SOC. The BWS is used by a fuzzy logic energy-management system of a plug-in series hybrid electric vehicle (HEV) to make a decision on the power split between the battery and the engine, based on the BWS and vehicle power demand, while controlling the engine to work in its fuel economic region. The fuzzy logic management system was tested in real time using an HEV simulation test bench with a real battery in the loop. Simulation results are presented to demonstrate the performance of the proposed fuzzy logic energy-management system under different driving conditions and battery SOCs. The results indicate that the fuzzy logic energy-management system using the BWS was effective in ensuring that the engine operates in the vicinity of its maximum fuel efficiency region while preventing the battery from over-discharging.

Line-Interactive UPS for Microgrids

Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy, and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can be also exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line-interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the UPS when reconnecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60-kW line-interactive UPS systems are provided to validate the design.

Nearest-Vector Modulation Strategies With Minimum Amplitude of Low-Frequency Neutral-Point Voltage Oscillations for the Neutral-Point-Clamped Converter

This paper investigates the problem of low-frequency voltage oscillations that appear at the neutral point (NP) of a three-level neutral-point-clamped (NPC) converter. Starting with a detailed analysis of their origin, the paper derives the minimum amplitude of these oscillations that can be achieved by nearest-vector (NV) modulation strategies. It then proves that the criterion of the direction of dc-link capacitor imbalance, which is commonly adopted by NV strategies for performing the task of capacitor balancing, poses a barrier in achieving this minimum. A new criterion is proposed instead, together with an algorithm that incorporates it into existing NV strategies. For the case of NPC inverters operating as motor drives, the resulting reduction in the amplitude of NP voltage oscillations ranges from 30% to 50%. The approach has the advantage of avoiding the significant increment in switching losses and output voltage harmonic distortion, caused by other methods. Simulations in MATLAB-Simulink are used to illustrate its operation and verify that it offers the claimed benefit.

Design and Control of a Grid-Connected Interleaved Inverter

This paper is concerned with the design and control of a three-phase voltage source grid-connected interleaved inverter. This topology enables the use of low-current devices capable of switching at high frequency, which together with the ripple cancelation feature reduces the size of the output filter and the inverter considerably compared to an equivalent classical two-level voltage source inverter with an LCL output filter using high-current devices with considerably lower switching frequency. Due to its higher switching frequency and low-filter component values, the interleaved inverter also has a much higher bandwidth than the classical inverter, which improves grid voltage harmonics disturbance rejection and increases the speed of response of the inverter and its capability to ride through grid disturbance (e.g., voltage sags and swells). The paper discusses the selection of the number of channels and the filter component values of the interleaved inverter. The design of the digital control system is then discussed in detail. Simulation and practical results are presented to validate the design and demonstrate its capabilities.

Impact studies of distributed generation on power quality and protection setup of an existing distribution network

The increasing amount of distributed generation (DG) in distribution networks (DNs) is giving rise to power quality and protection coordination problems. Issues like voltage regulation, flicker, harmonics and loss of coordination between circuit breaker and fuse need to be addressed for integration of DG into DN. This paper discusses these issues with a special emphasis on protection coordination problems. A typical DN with DG is modeled and simulation results for impact of DG on protection system coordination are presented and discussed here. Some solutions are proposed to cope with these problems.

Electrical contact phenomena during impact

The authors outline the theory of impact as it applies to electrical contacts. The concept of the coefficient of restitution is evaluated as a means of modeling the events at impact. The events occurring during the impact of electrical contacts are vital to the long-term reliability of the contacts; so design parameters are considered in terms of their influence upon the dynamics of impact and bounce. Experimental studies are presented which include the measurement of impact forces, impact time, and both current and voltage characteristics. The influence of preimpact arcing is evaluated in the medium current range, and is shown to have an effect on the events occurring during the first impact. A mathematical model is proposed for electrical contact bounce, but it is shown that reducing the arcing may not always reduce contact wear.<<ETX>>

An Adaptive Relaying Scheme for Fuse Saving in Distribution Networks With Distributed Generation

In some situations, utilities may try to “save” the fuse of a circuit following temporary faults by de-energizing the line with the fast operation of an upstream recloser before the fuse is damaged. This fuse-saving practice is accomplished through proper time coordination between a recloser and a fuse. However, the installation of distributed generation (DG) into distribution networks may affect this coordination due to additional fault current contributions from the distributed resources. This phenomenon of recloser-fuse miscoordination is investigated in this paper with the help of a typical network that employs fuse saving. The limitations of a recloser equipped with time and instantaneous overcurrent elements with respect to fuse savings, in the presence of DG, are discussed. An adaptive relaying strategy is proposed to ensure fuse savings in the new scenario even in the worst fault conditions. The simulation results obtained by adaptively changing relay settings in response to changing DG configurations confirm that the settings selected theoretically in accordance with the proposed strategy hold well in operation.

Control of Transient Power During Unintentional Islanding of Microgrids

In inverter-based microgrids, the paralleled inverters need to work in grid-connected mode and stand-alone mode and to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using the droop control. Inverters can have different power set points during grid-connected mode, but in stand-alone mode, they all need their power set points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set points to inverters with low power set points, which can raise the dc-link voltage of the inverters causing them to shut down. This paper investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the dc-link voltage and adjusts the power set point in proportion to the rise in the voltage. A small-signal model of an islanded microgrid is developed and used to design the controller. Simulation and experimental results are presented to validate the design.

An Optimal Charging Method for Li-Ion Batteries Using a Fuzzy-Control Approach Based on Polarization Properties

Battery charging is growing in importance as it has direct influence on battery performance and safety. This paper develops a generalized online estimation method for a charge-polarization-voltage-based resistance-capacitance ( RC) circuit model to simulate the charging behavior of Li-ion batteries. The effects of charging current, initial state of charge (SOC), initial polarization state, and aging on the charge polarization voltage are quantitatively analyzed in both time and SOC domains. It is demonstrated that the charge polarization voltage is nonlinearly related to these impact factors. In the SOC domain, the change in the charge polarization voltage is also analyzed with the gradient analytical method, and the relations between current, polarization voltage amplitude at the inflection point, and SOC are quantitatively established. This can be used to estimate battery SOC and polarization voltage accurately. A constant-polarization-based fuzzy-control charging method is proposed to adapt charging current acceptance with battery SOC stages. Experimental results demonstrate that the proposed charging method significantly shortens charging time with no obvious temperature rise compared with the traditional constant-current-constant-voltage charging method.

Hybrid Modulation Strategies for Eliminating Low-Frequency Neutral-Point Voltage Oscillations in the Neutral-Point-Clamped Converter

Nearest vector (NV) modulation strategies for the neutral-point-clamped converter are known to generate low-frequency neutral point (NP) voltage oscillations. Non-NV strategies can eliminate these oscillations, but at the expense of higher switching losses and output voltage harmonic distortion. This letter proposes a simple way of creating hybrid strategies, as combinations of NV and non-NV strategies, which are also able to eliminate NP voltage oscillations. The approach minimizes the participation of non-NV strategies and hence their drawbacks, while it can be applied to any type of load (nonlinear and/or unbalanced). Simulations in MATLAB-Simulink are used to illustrate its operation.