25-08-2017, 09:32 PM
WIND AND SOLAR INTEGRATED TO SMART GRID USING ISLANDING OPERATION
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ABSTRACT
Utility grid is disconnected for any reason; the distributed generation still supplies the required power to that section of local loads. This Phenomenon is called islanding operation. When an islanding occurs, the voltages and frequencies in the islanded area cannot be controlled by the grid system. This may lead to damage of electrical equipments and pose a danger to the working personnel. To avoid the occurrence of islanding phenomena, many control schemes have been proposed and devised to sense the islanding. A basic distribution system consists of Distributed Generators such as Photovoltaic panels, Wind turbines and other forms of renewable energy. As these renewable sources produce a Direct current, a DC to AC inverter is needed to convert the Direct current to an Alternating current with the right frequency and harmonics in relative to the AC coming out of the utility grid. A battery storage system can be inculcated into the system to store the excess energy. Due to big disturbance in a micro grid, voltage and frequency fluctuation occurs during transition from emergency mode to islanding mode. However, due to the power fluctuation from renewable energy sources, voltage and frequency deviations occur in islanded power systems. This work presents an islanding operation method of AC smart grid. The power system consists of photovoltaic, wind generators and controllable loads. In this work, the bus voltage and frequency fluctuations of AC grid are reduced by the photovoltaic, the wind generators and the controllable loads. Therefore, the AC bus voltage is maintained within the acceptable range by applying the power control of the photovoltaic and pitch angle control of wind turbine.
. INTRODUCTION
This paper presents about renewable energies such as photovoltaic’s and wind energy is important for greenhouse gas reduction and oil substitution. Renewable power resources are safe, clean, and abundant in nature. However, due to the power fluctuation of renewable energy sources, voltage and frequency deviations are occurred in island power systems whose ability to maintain stable supply-demand balance is low. Therefore, it is necessary to control the system frequency and voltage at the supply-side. At supply-side, installation of storage equipment and pitch angle control of wind generator has been proposed for control of distribution power system. However, the installation of storage equipment that needs large storage capacity and the cost of maintenance for battery degradation are not expected. Hence, in case of using the renewable energy plants connected to power system, the supply-side control has limitations. Therefore, the mutual cooperation control with the demand-side is required because it is difficult to maintain the power quality by only the supply-side control. Therefore, the study on the islanding operation of AC smart grid is important. In this paper, an islanding operation of AC smart grid is presented. The proposed AC smart grid consists of PVs, a wind turbine generator (WTG), a generator-side converter, and controllable loads. The AC bus voltage and frequency fluctuation due to the renewable power plants (WTG and PV) and loads is suppressed by the consumed power control of controllable loads based on droop characteristics and the power control of the renewable power plants. The renewable power plants are operated to suppress the AC bus voltage fluctuation by reducing the output power when the controllable loads reach at the rated power. By using the proposed method, stable power supply can be achieved even in the islanding operation. Besides, power companies can expect high quality power supply and can reduce the cost by cooperative control between supply side and demand side.
1 System Configuration
The configuration of the AC smart grid is shown in Fig. (1). The WTG is a gearless
2MW permanent magnet synchronous Generator (PMSG). PMSG has a simple structure and high efficiency and is expected to be installed in next generation WTG systems. The AC smart grid also consists of PV generators, a generator-side converter, a grid-side inverter, controllable loads (Batteries and EWHs) and variable load. The system is connected to a
10MVA diesel generator and variable AC load through the grid side inverter and the transformer. Wind power energy obtained from the windmill is sent to the PMSG. In order to generate maximum power, the rotational speed of the PMSG is controlled by the PWM converter. PMSG’s output power and PV’s output power are supplied to the AC load through the AC distribution line. And, the remaining power of the PMSG is supplied to the AC load throug
AC DISTRIBUTION VOLTAGE AND FREQUENCY CONTROL BY DROOP CHARACTERISTICS
This section describes the control of decentralized controllable loads according to droop characteristic. By using the droop control, the AC network needs no central control and no communication between the different elements of the network. In the AC grid, AC bus voltage fluctuations occur due to the output fluctuation produced by WTG, PV and loads. The suppression of this fluctuation is achieved by controllable loads connected to the AC grid. Determination of the power consumption command is needed for each controllable load which has different capacity. Therefore, the controllable loads are controlled according to the droop characteristics and load is shared according to the capacities of controllable loads.
The droop characteristics of EWHs for AC bus voltage are shown in Fig (4). When the AC bus voltage rises, the droop characteristics are configured such that the bigger the capacity. When the DC bus voltage rises, the droop characteristics of batteries are configured such that the bigger the capacity of battery is, the more the charging power of battery are. Additionally, when the DC bus voltage falls, the droop characteristics are also configured such that the bigger the capacity of battery is, the more the discharging power of battery are. The droop characteristics of EWH and battery are presented by the following equations:
. CONCLUSION
Due to big disturbance in a micro grid, voltage and frequency fluctuation occurs during transition from emergency mode to islanding mode. The AC bus voltage and frequency fluctuation is suppressed considerably by the use of renewable energy sources, such as wind turbine and PV array. Voltage and frequency fluctuations caused are reduced by applying power control of photovoltaic and varying pitch angle of wind generator. Stable power supply can be achieved even during islanding operation