Ashraf Khalil

Stability analysis of parallel-inverters in microgrid

Renewable Energy is one of the fastest growing technologies. Renewable energy sources such as PV, wind and fuel cells are usually connected through voltage-source inverters to form a Microgrid. In order to share the same loads these inverters are connected in parallel. With the advances in network technology the control signals of the parallel inverters are exchanged through shared network. The time delay and the data loss may destabilize the system. In order to stabilize the parallel inverters a stability analysis should be carried out without considering the time delay. In this paper the stability of three-phase parallel inverters is analyzed to indentify the most effective control parameters on the system stability. The stability analysis in this study can be used as a guide for designing the controller in the case of time delay between the inverters controllers.

Modeling and control of PV/Wind Microgrid

Microgrids are becoming the de facto for future power system. This paper presents the modeling and control of PV/Wind Microgrid. Renewable energy sources such as PV, wind and fuel cells are usually connected through voltage-source inverters. In order to share the same loads these inverters are connected in parallel to form a Microgrid. The system considered in the paper consists of two parallel inverters fed with PV array and wind turbine. The PV array and the wind turbine are variable nonlinear DC sources and hence the control system should achieve good power sharing even with this imperfections. The controller is designed in the dq reference frame where the Space Vector Pulse Width Modulation is used. The first inverter contains outer voltage control loop and inner current control loop while the second inverter contains only power control loop. The controller is tested through simulation where the PV array and the wind system produce different DC levels. The simulation results show that the power sharing is achieved.

Delay-dependent stability of LFC in Microgrid with varying time delays

Load frequency control systems used to rely on dedicated communication links. The deregulation of the electricity market and the emergence of the Microgrids and Smartgrids made the open communication infrastructure an exigent need for future power system. In this case the load frequency control signals will be exchanged through communication network. The communication network is characterized by variable time delay which may lead to system instability. This paper investigates the stability of load frequency control system in islanded micro-grid with the presence of the time delay. The stability analysis is based on solving a set of linear matrix inequalities. Also, the effects of the controller gains and time delay variation rate on the maximum allowable delay bound are investigated.

Delay-dependent stability of DC Microgrid with time-varying delay

In order to optimize the efficiency of renewable energy sources they are connected in a form of AC and DC Microgrids. Furthermore the increased number of DC loads has shifted the view to DC Microgrids. This paper presents the stability and maximum time delay calculation for a DC Microgrid implementing master-slave control strategy. The communication between the master and the slave is achieved through communication network which makes the DC Microgrid a time delay system. A stability criterion based on Lyapunov-Krasovskii functional and replacing the delay term by Newton-Leibnitz formula is used to derive a set of linear matrix inequalities. The binary iteration algorithm is used to solve the set of linear matrix inequalities and to calculate the maximum allowable delay bound. The effects of the voltage controller gains on the maximum allowable delay bound are also investigated. Further, the relation between the maximum allowable delay bound and the time delay variation rate is studied. The results are tested through simulation using Matlab/Simulink.

Networked control of parallel DC/DC buck converters

Microgrids and Hybrid Electric Vehicles rely on parallel DC/DC converters for power generation. DC/DC converters are connected in parallel to share the same load. The control signals are distributed through wires. The wiring increases the complexity and reduces the reliability. The recent advances in shared wireless networks made them viable and cheap alternative to direct wiring. Distributing the control signals over shared networks reduces the complexity in the wiring and enhances the reliability. Transmitting control signals through shared network induces time delay and data loss which may destabilize the system. These imperfections make the system stochastic and in many cases these imperfection can be modeled using Markov chains. This paper presents the stabilization of parallel DC/DC converters over stochastic nondeterministic network represented by Markov chains.

The benefits of the transition from fossil fuel to solar energy in Libya: A street lighting system case study

The Libyan economy is dominated by the oil and the gas industry which are considered as the primary energy sources for the generating power plants. With the increased energy demands in the near future, Libya will be forced to burn more oil and gas. This, in turn will result in reducing the country revenue, threatening the economy and increasing the CO2 emission. This triggers the alarm for Libya to an urgent plan to diversify the energy sources through using sustainable energy. The sun showers Libya every day by a huge amount of sunshine, especially during the peaks in the summer days. Recently, the country has been struggling to satisfy its escalating energy demands. The residential and street lighting loads constitute more than 50% of the electricity demands in Libya. Street lighting consumes more than 3.996 TW h, which is around one fifth of the energy demands in Libya. Energy conservation and transition from fossil fuel to renewable energy could have significant profit on the energy sector in Libya. For example, Libya is still relying on the old-fashioned, inefficient and unsustainable street lighting systems. Replacing the old technology lighting systems with up-to-date solar powered lighting system can achieve energy saving and sustainability. In this paper, improving the energy situation in Libya through replacing the high pressure sodium street lighting systems with solar powered LED street lighting systems is investigated. A four km road is chosen as a case study. Four alternatives are analyzed; grid-powered high pressure sodium lamp street lighting system, grid-powered LED lamp street lighting system, stand-alone solar powered LED street lighting system and grid-connected solar powered LED street lighting system. The four options are compared in terms of the capital cost, maintenance cost, total cost, fuel cost and the CO2 emission. Replacing the high pressure sodium lamp system with LED lamp system saves 75% of energy and reduces the CO2 emission by 75%. The stand-alone solar powered LED lighting system cuts the CO2 emission, saves the fuel and is economically feasible. Furthermore, improvement is attained if the solar powered lighting system is connected to the grid where the excess energy is fed to the grid. The two solar powered options are economically feasible and sustainable.

Stability of parallel DC/DC converters with time varying delay

The rapid development in communication networks made them very attractive to replace conventional wiring in distributed power electronic systems. Parallel DC/DC converters are one of the important distributed power electronic systems. Conventionally, the controllers exchange the control signals through wires. This reduces the reliability and increases the system cost and complexity. Replacing them with communication network will introduce a time delay that may lead to system instability. The paper addresses the stability of parallel DC converters controlled over communication networks. The network is represented as variable time delay. A theorem based on solving a set of linear matrix inequalities is used to investigate the delay-dependent stability of the system. The effects of the voltage controller parameters on the maximum allowable delay bound are investigated. Furthermore the effect of the time delay variation rate on the maximum allowable delay bound is discussed.

Evaluating the value of the excitation capacitance of a wind turbine/induction generator system using genetic algorithms

Due to their robustness, low cost, and low maintenance, induction generators are good candidates for wind energy systems. In addition, when the wind turbine is isolated from the grid, the induction generator can operate within a wide speed range which utilize maximum power extraction. To ensure that voltage buildup will take place between the machine terminals, the reactive power required by the machine must be supplied. To achieve that, a three phase capacitor bank is connected to the generator terminals. Evaluating the adequate value of the capacitance is very crucial since it affects the value of the generated voltage. In this paper, the desired operating voltage and the corresponding value of the magnetizing inductance are defined from the magnetizing curve. By using genetic algorithms (GAs), the corresponding values of the excitation capacitance and the rotor slip are evaluated. Matlab/Simulink model is then used for validation. It is noticed that there is a good agreement between the values predicted by the GAs and the ones obtained from the simulation.

Load frequency control system with smart meter and controllable loads

The advances in the network technology and the emergence of the renewable energy as primary sources have changed the current view of the power system control. Smart meters will become a very important component in future power systems. In the event of an emergency the demand will play vital role in the stability of the power system. Conventionally, when a sudden generation loss occurs the generators have the responsibility to restore the system frequency. This process is expensive, comparably lengthy and increases the CO2 emissions. The smart meters can control the loads and hence secure the system stability, faster than the conventional generators. One of the important issues is to understand the operation of these emerging devices. In this paper, we present a Matlab/Simulink model for load frequency control system with demand-side control through the smart meter. From the simulation the demand-side control can improve the performance of the load frequency control system.

Modeling and control of master-slave microgrid with communication delay

Microgrids gathered a lot of attention in the last decade and are believed to be the future power systems. The renewable energy sources can be easily integrated into the Microgrid. Renewable energy sources such as PV, wind and fuel cells are usually connected through voltage-source inverters in the Microgrid. In order to share the same loads, these inverters are connected in parallel and to achieve the load sharing the control strategy is a must. In this paper, the master-slave control strategy in the dq frame is presented. The reference current signals are sent from the master to the slave converters. A model for master-slave communication-based Microgrid is presented and the system is modeled as a general time delay system. The maximum time delay that guarantees the stability of the system is calculated using a Lyapunov-Krasovskii based linear matrix inequalities method. The communication delay mimics the scenario when the wireless network or the Controller Area Network are used as a communication medium between the inverters.