Zakaria Ziadi

Optimal Power Scheduling for Smart Grids Considering Controllable Loads and High Penetration of Photovoltaic Generation

The distributed generator (DG) has a huge economical and environmental potential, especially if it is based on renewable energy sources (RESs). It is very important for the future development of smart grids. However, high penetration of DGs into distribution systems can cause voltage deviations beyond the statutory range, and reverse power flow toward the substation transformer. Consequently, it can increase the distribution system losses if it is not well supervised. Thus, in order to meet smart grid objectives, DGs have to be controlled in coordination with other power resources existing in the distribution system. Controllable loads (CLs) can also help in promoting smart grids through demand response (DR) application. Therefore, this paper proposes a decision technique of an optimal reference schedule for DGs, battery energy storage system (BESS), CLs, and tap changing transformers. The main objective of the proposed optimization problem is to achieve loss reduction in the distribution system. However, other aims such as voltage control and power flow smoothing have been achieved. The optimization is performed based on predicted values of load demand and DG generation. Simulations are conducted for one operation day to illustrate the optimality of the proposed scheduling method and to assess the impact of CLs in smart grids.

Optimal Voltage Control Using Inverters Interfaced With PV Systems Considering Forecast Error in a Distribution System

In recent years, distributed generation (DG) and renewable energy sources (RESs) have been attracting special attention in distribution systems. RESs such as photovoltaic (PV) systems are used as a source of green energy. However, a large amount of DGs causes voltage deviations beyond the statutory range in the distribution systems. This paper proposes a methodology for voltage control using the PV interfaced inverters and tap changing transformers. The proposed method uses a one-day schedule of voltage references for the control devices, which is determined by an optimization technique based on the forecast values of load demand and PV power generation. However, since the forecast value includes a forecast error, there is a possibility that the voltage control performance is affected. Thus, this paper introduces a replanning of the control reference schedule in order to reduce the forecast error impact on the control performance and its objectives.

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.

Coordinated charging of plug-in hybrid electric vehicle for voltage profile enhancement of distribution systems

Because of increasing in the number of plug-in hybrid electric vehicles (PHEV) is expected in the next years, so it will have impact on the power system performance, stability, voltage profile and system loses. Consequently, it is necessary to study the strategy and control methods of the PHEV charging strategies. In this paper, a new technique is used for charging the batteries of PHEVs in real time. The objective of the developed control strategy is to keep the system voltage in secure operation irrespective of the number of vehicles and their place along the distribution feeder. The strategy adopts the steady state voltage profile of the system that is easy to compute using the smart-grid load flow program implemented in the distribution management systems. The developed control method uses fuzzy logic controller. The developed strategy uses real-time Network voltage and the PHEV state of charge as the main inputs of the fuzzy controller. Based on the controller output, the bi-directional converters of each PHEVs converter decide the desired level of charging. This ensures secure operation of distribution systems during charging whatever the number of connected PHEVs to the grid. Besides, the control strategy decides the level of charging for each PHEV according to its state of charge. The Newton-Raphson method is used as the continuous power-flow solver in the distributed management system for network voltage calculation. Lithium-Ion battery is used for each PHEV is used to test out the developed control strategy.

Optimal scheduling and real time voltage control method for unbalanced distribution systems

This paper presents an optimal scheduling and real time voltage control of tap changing transformers and converters interfaced with Distributed Generators (DGs) in unbalanced three-phase distribution systems. The DGs considered in this paper are supposed to be Photovoltaic (PV) generators, due to the growing demand of renewable energies. However, high penetration of DGs may cause considerable voltage fluctuations and deviations from the statutory limits. Thus, based on predicted values of load demand and PV generation, a set of voltage references of tap transformers and DGs is optimized, using Genetic Algorithms (GA) optimization method, to reduce the losses while keeping the distribution voltage within an acceptable range. For that, the three phase power flow equations, which must be satisfied, are solved using Newton-Raphson method. The voltage control references are optimized for every preset period of time. However, in the meanwhile the voltage may fluctuate due to the PV power nature. Thus, a real time voltage control is applied on the DGs inverters exploiting their available reactive power based on the voltage drop. According to local voltage measurements at each node containing DG, fuzzy logic controllers adjust the voltage references of the optimal schedule. Twenty-four-hour data are used to simulate a 14-bus distribution system with unbalanced three-phase loads in 6 nodes to verify the effectiveness of the method.

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.

Smoothing of Wind Power Fluctuations for Permanent Magnet Synchronous Generator-Based Wind Energy Conversion System and Fault Ride-through Consideration

Abstract Due to the increment of penetration level of wind power generation, output power fluctuation is one of the most important issues that can destabilize the power system operation. This article mainly deals with the smoothing of the output power fluctuations of a wind energy conversion system based permanent magnet synchronous generator and fault ride-through enhancement during a grid fault. The concerned wind energy conversion system based permanent magnet synchronous generator adopts an AC-DC-AC converter system. The proposed control method limits the wind energy conversion system output power by adjusting the pitch angle of the wind turbine blades when wind speed is above the rated wind speed. In the grid-side converter, a fuzzy logic controller is used to determine the torque reference for which the kinetic energy stored by the inertia of wind turbine can smooth the output power fluctuations of the permanent magnet synchronous generator. Also, the DC-link voltage, controlled by the grid-side inverter, is adjusted in accordance with the output power fluctuations of the permanent magnet synchronous generator using a voltage smoothing index. Moreover, in this aticle, the proposed method ensures that the wind turbine stays operational during grid faults and provides fast restoration once the fault is cleared. To show the effectiveness of the proposed method, simulations under different conditions have been performed by using MATLAB/Simulink® (The Math Works, Natick, MA, USA).

Optimal scheduling method in distribution system considering controllable loads

In recent years, distributed generation (DG) and renewable energies source (RES) are attracting special attention to distribution systems. Renewable energies, such as photovoltaic and wind turbine generator systems, are used as green energy. However, the large amount of distributed generation causes voltage deviation beyond the statutory range in distribution systems. Furthermore, power flow at interconnection point is fluctuated substantially. So distribution systems need a solution for voltage control and power flow control method. This paper proposes a decision technique of optimal reference scheduling of the battery energy storage system(BESS), controllable loads(CL), interfaced inverters with DG, and the existing voltage control devices for optimal distribution system operation. In this proposed method, the formulated control objective is solved by optimization algorithm. The proposed method achieves the following a tunes: The node voltage is maintained in the acceptable range, the fluctuations of active and reactive power flows are reduced at the interconnection point in the distribution system. The effectiveness of the proposed method is verified by numerical simulations using MATLAB®.

Load balancing of active distribution systems with high Photovoltaic power penetration

This paper treats the problem of voltage control and balancing of the unbalanced three-phase loads in distribution systems with Distributed Generators (DG) and tap change transformers. The DGs considered in this paper are supposed to be Photovoltaic (PV) generators, due to the growing demand of renewable energies. Unbalanced three-phase loads operation can have undesirable effects on the distribution system components, beside making the voltage control more difficult. For that reason, the proposed method aims to balance the unbalanced three-phase loads and control the voltage using the available reactive power generated from the DG through the inverters interfaced with PV modules. The Newton Raphson method is used solve the unbalanced three phase power flow equations of the distribution system, while the three phase elements models are considered. Twenty-four-hour data are used to simulate a 14-bus distribution system with unbalanced three-phase loads in 7 nodes to verify the effectiveness of the method. The simulation results show that the node voltages are balanced and kept within the statutory range.