Mohamed M. Aly

Supersession of large penetration photovoltaic power transients using storage batteries

This paper treats the impact of output fluctuations caused by cloud transients on the distribution power systems fed by large photovoltaic (PV) system by considering PHEVs to be main component of future smart grids. The PHEVs charging station is seen as bulk storage battery which acts as controllable loads and interacts with intermittent PV power generation. A control strategy for the PHEVs battery is developed in which the PV generation scheduled power is prepared based on clear sky assumption. In normal state, the storage batteries are set in charging mode. The results simulate the impact of 50% PV penetration on a 33-node radial distribution feeder during 24 hour period. PV generation plants were connected at buses 18 and 33 which display weakest voltages. The simulations are carried out for clear sky, full cloudy sky and cloud transient with different models of the PV interface inverter. The results show that the generating reactive power by PV inverter enhances the feeder voltage profiles. However, during cloud transient, application of PHEVs storage systems is required to alleviate the feeder voltage flicker. PHEVs charging stations were connected at the same buses of PV generation plants. The developed controllers can be integrated with charging station smart meter and can be extended for real time billing schemes in both charging and discharging modes.

Voltage stability assessment of photovoltaic energy systems with voltage control capabilities

This paper studies the impact of large-scale PV generation, up to 50% penetration level, on distribution system voltage regulation and voltage stability. The system voltage profiles are computed using power-flow calculations with load variation of a 24-hour time scale. The voltage stability is examined at different times of the day using a developed continuation power-flow method with demand as continuation parameter and up to the maximum loading conditions. The load-flow analysis implemented for both voltage regulation and voltage stability analysis is performed by using the forward/backward sweep method. The secant predictor technique is developed for predicting the node voltages which are then corrected using the load flow solver. Three models of the PV interface inverter are implemented in this study with full set of data representing environmental conditions. The voltage profiles are regulated using the PV interface inverters which support reactive power at unavailability of sun light. The available inverter capacity is utilized for regulating the system node voltages. The most possible scenarios of system voltage collapse are investigated at different times of the day. The developed methods and models are used to assess the performance of a 33-bus radial distribution feeder with high level of PV penetration. The results show that the PV interface inverters have to be designed to operate for reactive power support in order to improve voltage profile, secure power systems operation, and increase the lifetime of the online tap changing transformers.

Voltage stability modeling and analysis of unbalanced distribution systems with wind turbine energy systems

This paper presents a new method to asses voltage stability of unbalanced distribution systems with wind turbine generation systems (WTGSs). In the voltage tracing process, the Lagrange linear and quadratic interpolations have been applied to predict voltage magnitudes and phase angles. The predicted solutions are then corrected using the unbalanced three-phase forward/backward power flow solver. The solution process continues until the maximum loading conditions of the system under study are reached. The salient advantage of the method is avoiding construction of massive augmented three-phase Jacobian matrix of the classical Newton-Raphson method. The direct connected WTGSs have been modeled with sufficient details to account for distribution system unbalances and slip variations. Substation transformer tap-setting for voltage regulation is also considered. Voltage collapse scenarios are investigated for unbalanced systems using various unbalanced radial feeders. The results show the robustness of the proposed continuous power-flow method for voltage stability of power distribution networks. Assessment of the obtained results shows that WTGSs of synchronous generator types are more robust than those equipped with induction generators from the point of view of voltage stability.

Application of superconducting magnetic energy storage (SMES) for voltage sag/swell supression in distribution system with wind power penetration

This paper presents the impacts superconducting magnetic energy storage (SMES) in suppressing the voltage sag/swell in distribution systems with wind power penetration. Wind turbine used in this paper is of squirrel cage induction generator (SCIG) with shunt connected capacitor bank to improve the power factor. SMES system consists of step down transformer, power conditioning system, DC-DC chopper, and large inductance superconducting coil. Wind energy generation system (WEGS) and SMES system connected to the grid at the same bus to achieve high performance. Fuzzy logic controller (FLC) used for DC-DC chopper to control in power transfer between the grid and SMES coil. The FLC is designed so that the SMES can absorb/deliver active power from/to the distribution system. On the other hand, reactive power can be delivered/absorbed to/from the distribution system according to the voltage difference between the SMES voltage and DC link voltage. Two inputs were applied to the FLC; bus voltage and SMES current variations. This technique of two inputs was proved to enhance the control performance. Detailed simulation is carried out using Matlab/Simulink and Simpowersystem package.

Deployment and control of PHEVs in electrical power systems with wind power penetration

Large penetration of wind power in congested and weak power networks could lead to severe problems due to variation in wind speed. Hence, severe voltage and frequency fluctuations occur due to fast intermittent power generation. In this work, quasi-static models have been implemented to investigate the effect of wind power variations on classical power generation as well as network frequency. Probabilistic PHEVs models are deployed to absorb wind power fluctuations and improve system frequency response. The developed control strategy for PHEVs demand management is integrated with existing control infrastructure on both power plant and center control levels. The developed control reduces frequency fluctuations due to fast wind power transients and guarantees charging of the PHEVs plugged into the system by the end of their connection period. The developed quasi-static time-series (QSTS) simulation model accounts for primary control, optimized unit participation, and economic dispatch. The frequency is represented as state variable whereas the continuous power-flow is solved using Gauss-Seidel method. PHEVs are aggregated through the network based on probabilistic distribution of both traveling distance and parking time. The results calculated for the IEEE 30-bus shows that integration of PHEVs with wind power energy systems improves the system frequency response and provide fast and dynamic power supply in case of power shortcoming along the day.

Power control of fluctuating wind/PV generations in an isolated Microgrid based on superconducting magnetic energy storage

An effective power control strategy of intermittent wind/PV generations in an isolated Microgrid is proposed to address frequency and voltage requirements. The studied Microgrid is a 33-bus system supplied by small PV unit and squirrel cage induction generator (SCIG)-based wind energy conversion system (WECS). Due to its fast response, high efficiency and long lifetime in comparison to other alternatives storage systems, superconducting magnetic energy storage (SMES) has been used as a storage system. The control strategy is based on fuzzy logic controller (FLC) to control the power exchange between SMES and Microgrid. The Microgrid, WECS, PV, and SMES are modelled with MATLAB/SIMULINK package and the simulated results show the effectiveness of the control strategy in smoothing the power transfer to the Microgrid during the wind/PV power fluctuations. The frequency is successfully maintained at 50Hz and the voltages are maintained constant at the desired values.

Efficient time series simulation of distribution systems with voltage regulation and PV penetration

This paper presents an approach for solving the time-series simulation (TSS) for three-phase unbalanced distribution systems with voltage regulator control and photovoltaic (PV) generation. Efficient time-series simulation approach based on Lagrange polynomial approximation is developed to decrease the execution time and enhance the convergence characteristics. The formulation is developed to find efficient initial values for voltage magnitudes and phase angles corresponding to load variations during the day. These initial values of voltage magnitudes and phase angles are calculated from the variations of reactive and active powers, respectively. The predicted values are updated using forward/backward sweep power-flow engine. A line drop compensator (LDC) is used to regulate the voltage by modelling the drop of transmission line from the transformer to the load center. In addition, the effect of PV penetration on tap setting, to regulate the voltage magnitude, is investigated. The developed TSS technique with voltage regulator and PV generation are validated using the standard three-phase unbalanced IEEE 123-node. The developed technique is compared with the traditional method that uses the previous voltage magnitudes and phase angles as initial values. The obtained results prove the effectiveness of the developed TSS in reducing both of the number of iterations and execution time compared to the traditional method.

Plug-in hybrid electric vehicles aggregation and real-time active power control simulation analysis in distribution systems

This paper presents a control strategy for plug-in hybrid electrical vehicles (PHEVs) demand in power distribution systems to control the peak load due to PHEVs charging. The stochastic nature for both instants of start charging time and the initial state of charge (SOC) are fully exploited to model the intermittent demand nature in future smart grids. The appropriate control methodology adopted in this work is the decentralized control. Each power conditioning unit (PCU) interface of PHEV charger extracts control variable from the grid and the PHEV internal state, then considers a proper smart action. The developed control method uses the real-time electricity tariff rate, maximum time permit of the vehicle plugged in, and the initial state of each vehicles battery as control variables. To achieve such control, a fuzzy logic controller (FLC) is designed to work with the PCU of each vehicle charger. The analysis of the developed control method is performed using accelerated quasi-static time-series (QSTS) power-flow method. The QSTS utilizes the Lagrange polynomial function as an accelerator for the forward/backward power-flow method. Extensive simulations of the unbalanced three-phase IEEE 123-node radial feeder are carried out with a combination of commercial, industrial, residential daily demands, and PHEVs penetration at different electricity rates. The results show the effectiveness of the developed control strategy in dynamic peak shaving at various scenarios as well as the superior performance of the developed accelerated quasi-static time-series power-flow method.

Development energy management strategy of SMES-based Microgrid for stable islanding transition

Seamless switching between grid-connected mode and islanding mode is important to Microgrid stable operation. The grid can be disconnected from the utility during large events (i.e., faults, voltage collapses, etc.). As a result, these events may affect the stable operation of Microgrid. This paper presents a control strategy to mitigate the frequency and voltage fluctuations during the islanding transition of Microgrids. The strategy depends on managing the energy stored of a superconducting magnetic energy storage (SMES). The studied Microgrid is a 33-bus system supplied by utility at normal operation. During the islanding transition, the Microgrid is supplied by two diesel synchronous generators. Wind energy conversion system of squirrel cage induction generator type is connected to the Microgrid with high penetration level of 30% at constant wind speed. SMES unit consists of superconducting coil, cryogenic refrigerator, step down transformer, power conditioning system, cryostat/vacuum vessel, and DC-DC chopper. The procedure of control is based on fuzzy logic controller. The results obtained show the effectiveness of the control strategy of SMES energy in seamless switching between grid-connected mode and islanding mode and mitigating the frequency and voltage fluctuations under switching states.