Australian Journal of Basic and Applied Sciences, 6(9): 412-421, 2012 ISSN 1991-8178 Modeling and Simulation of Small Scale Microgrid System 1 Alias Khamis, 2 A. Mohamed, 3 H. Shareef, 4 A. Ayob 1 Department of Industrial Power, Faculty of Electrical Engineering, Universiti Teknikal Malaysia, Melaka, Malaysia 2,3,4 Department of Electrical, Electronics and System Engineering Faculty of Engineering and Built Environment Universiti Kebangsaan Malaysia Bangi, Malaysia Abstract: A microgrid systems is a new technology for improving reliability and providing alternative energy supplies to the grid system. Low voltage faults in the system are one of the critical issues that require distributed generating sources to disconnect from grid provide energy to the load. Therefore the techniques used in the microgrid system with microsoures can be important in reducing the problems in the grid system. In this paper two different microsources photovoltaic (PV) and wind turbine (WT) with battery storage for a small scale microgrid system are simulated. The aim is to observe the effect of microsources parameter on the outputs at the point of common coupling. Most of the results can be used for develop a small scale microgrid system for practical applications. Key words: Battery Storage, Inverter, Microgrid, Photovoltaic, PSCAD, Wind Turbine. INTRODUCTION A small scale microgrid system is a low voltage grid connected to the network that can improve the power system failure and power quality in the system. Low voltage single phase AC generating units in a small scale microgrid system become abundant in recent year. This system includes the combination of distribution generation or microsouces to connect to the load and grid system. The microsources such as PV and WT comprising of battery storage are designed to perform in both islanded and grid mode of operation (Yang, Z., et al., 2009; Georgakis, D., et al. 2004). A typical PV and WT need to optimize design with specific parameters. Effect of wind speed fluctuation and irradiance for both microsources effect the result from dynamic performance of microgrid (Rashad, M. et al., 2010). The stability and the performance hybrid WT, PV and battery system have to be studied for the variable voltage, frequency and loading effect (Li, W. and L. Tsung- Jen, 2007). Therefore the microgrid system need to be modelled and controlled in grid connected system to show the capability of - WT and PV hybrid generation system (Seul-Ki, K., et al., 2006). To ensure stable operation during network disturbances, maintaining stability and power quality in the islanding mode operation require the sophisticated control strategies for microgrid inverters in order to provide stable frequency and voltage in the presence of arbitrarily varying load (Kanellos, F.D., et al., 2005). Modelling and simulation of microgrid power system is an important first step to any sort of physical experimentation or field implementation. Models can be used to predict performance issues and simulate anomalous condition (Laurentiu Nastac, et al., 2009). Modeling of a commonly used microsources an a small scale microgrid system is studied using PSCAD (Hossienzadeh, H., et al., 2009). Therefore the objective of this paper is to model and simulation a small scale microgrid system included each of microsources PV, WT and battery storage with inverter are simulated by PSCAD. The simulation showed the grid connected and islanding mode performance varying by load and effect of PV and WT microsources with battery capacity storage. Ii. Small Scale Microgrid System: A. System Description:. Grid Connection System: A small scale microgrid system architecture is as shown in Figure 1. It is a single phase 60Hz, 240V AC system connected to grid. It comprises of PV, WT and battery storage microsources. Where the DC output voltage from PV is connected to inverter to change into AC voltage before connected to the grid system. While AC voltage output from WT and generator are connected to a bridge rectifier change to DC voltage before converting to AC again through an inverter. Finally the battery storage is connected to bidirectional inverter to maintain output in AC voltage charge and charging the battery. 2. The Load System: The load system are divided in three categories that the first load are connected with PV where when islanding mode this load are supplies from PV microsources. Then the second load are connected with WT also Corresponding Author: Alias Khamis, Department of Industrial Power, Faculty of Electrical Engineering, Universiti Teknikal Malaysia, Melaka, Malaysia E-mail: alias@utem.edu.my 412
when islanding happen this load are supplies from WT microsources. Finally the third load are connected to the system where if islanding happen this load are supplies from the microsources also battery storage. Fig. 1: Single diagram small scale microgrid system B. Distributed Generators: 1. The PV system: The PV microsource is shown in Figure 2. Solar radiation and cell temperature are input source to PV to convert in current and voltage by PV panel then connected with series diode and parallel capacitor. The function of the diode is to block output from reverse side and capacitor is used for charge the output from PV then generate to the load. Fig. 2: PV system electrical circuit For calculating current output from PV, the mathematical equation below is used [9]: I s = I ph I o [exp(v+i s R s )/N s V t ] (V+I s R s )/R sh (1) Where, I ph - the photo generated current (A) I o - the dark saturation current (A) R s - the panel series resistance (Ω) R sh - the panel parallel resistance (Ω) N s number of cell in the panel connected in series V t = AkT/q junction thermal voltage (A is diode ideality factor, k is Boltzmann s constant, T is temperature and q is charge of electron) 413
TABLE 1 show the PV module characteristic is used in this system. All of the parameter will be calculate by equation (Yang, Z., et al., 2009) to get the output current and voltage from PV module. Table1: PV module characteristic The outputs from PV in DC voltage need to change to AC voltage by inverter as shown in Figure 3. The circuit have four gates that convert DC voltage from PV to AC voltage and connected the system. This circuit are control by PWM circuit where the square wave graph injects to the gates in sine wave for AC voltage. AC voltages out from inverter are used for generation to the load and synchronies with the grid system. Fig. 3: DC/AC inverter circuit 2. The Wind Energy System: The microsources of WT are shown in Figure 4. The wind speed fluctuations is the wind sources input power to the WT. The WT has there blade that is attached to the rotor before connect to the induction generator. DC voltage output from WT turbine and the induction generator are used for changed to AC voltage. Fig. 4: WT system electrical ciruit 414
For calculating output power from WT, the WT model represents the mechanical mechanism as equation below (Shi, S.S., et al., 2009): P = 1/2ρC p Aυ 3 (2) Where ρ - the air density which around 1.25 kg/m 3 C p - the coefficient performance of the turbine A = πr 2 - the turbine swept area (R is the rotor blade radius) υ - the wind speed Then for the calculating of the mechanical torque of WT, the mathematical equation below is used (Bunlung Neammanee, 2007): T = 1/2ρC p Aυ 3 / ω (3) For calculating the tip speed ratio as the WT operating point for extracting maximum power the mathematical equation below is used (Engr, G., et al., 2008): λ = ωr/ υ (4) Where ω is the rotor angular speed in rad/sec. Generator output from WT in there phase AC voltage change to DC voltage by bridge rectifier before change to AC voltage again by inverter as shown in Figure 5. DC voltages from bridge rectifier are single phase input of inverter. Fig. 5: AC/DC bridge rectifier and DC/DC inverter circuit C. Energy Storage System: The battery storage is used in this microgrid system as shown in Figure 6. The electrical circuit show the battery source connected with series diode and parallel capacitor. The DC/DC converter is used to convert from small voltage to large voltage before change to AC voltage by inverter (Chong, M.Y., et al., 2010). Fig. 6: Battery storage and DC/DC converter circuit 415
The output from battery storage in DC voltage as a input for inverter as shown in Figure 7. This bidirectional inverter can comprise with AC voltage in microgrid system for backup when the operation system in grid connected or islanding mode. Fig. 7: DC/DC inverter circuit III. Simulation Result: A. Grid Connected Mode: The grid connected mode is the microgrid system with PV, WT and battery storage microsources are connected to grid AC system. The simulations result of the microgrid system on grid connected mode are shown in Figure 8. The voltage at the grid system showed 240V AC voltage while the current output from PV, WT and battery are Ia, Ib and Ic. Fig. 8: Voltage and current microgrid system The plots in Figure 9 shows the load power in three types for this microgrid system. P1 and Q1 is the active power and reactive power for load connected with PV system. While P2 and Q2 is the active power and reactive are load connected to the WT system. P3 and Q3 is the Active power and reactive power for the load in this system. 416
Fig. 9: Active power and reactive power microgrid load system PV system: Figure 10 shows the supply and load voltage and current from PV system. Both of the voltage and current from PV are variables that effect from solar irradiance and temperature from the source. While the voltage and current at the load or out from inverter is in the sinusoidal waveform. Fig. 10: Supply and load voltage and current PV system WT system: Figure 11 shows the supply and load voltage and current from WT system. Both of the voltage and current from WT also are variables that effect from speed fluctuation from the source. While the voltage and current at the load or out from inverter is in the sinusoidal waveform. Fig. 11: Supply and load Voltage and current WT system 417
3. Battery Storage: Figure 12 shows the supply and load voltage and current from PV system. Both of the voltage and current from PV is variable that effect from charge and discharge from the bidirectional source. While the voltage and current at the load or out from inverter is in the sinusoidal waveform. Fig. 12: Supply and load voltage and current battery storage system B. Islanding Mode: The islanding mode is the microgrid system with PV, WT and battery storage microsources are not connected to grid AC voltage system. The simulations result of the microgrid system on islanding mode are shown in Figure 13. The voltage at the grid system showed 240V AC voltage while the current out from PV, WT and battery are Ia, Ib and Ic. Fig. 13: Voltage and current microgrid system The polts in Figure 14 shows the load power in three types for this microgrid system. P1 and Q1 is the active power and reactive power for load connected with PV system. While P2 and Q2 is the active power and 418
reactive are load connected to the WT system. P3 and Q3 is the Active power and reactive power for the load in this system. Fig. 14: Active power and reactive power microgrid load system PV system: Figure 15 shows the supply and load voltage and current from PV system. Both of the voltage and current from PV are variables that effect from solar irradiance and temperature from the source. While the voltage and current at the load or out from inverter is in the sinusoidal waveform. Fig. 15: Supply and load voltage and current PV system WT system: Figure 16 shows the supply and load voltage and current from WT system. Both of the voltage and current from WT also are variables that effect from speed fluctuation from the source. While the voltage and current at the load or out from inverter is in the sinusoidal waveform. 419
Fig. 16: Supply and load Voltage and current WT system Battery Storage: Figure 17 shows the supply and load voltage and current from PV system. Both of the voltage and current from PV are variables that effect from charge and discharge from the bidirectional source. While the voltage and current at the load or out from inverter is in the sinusoidal waveform. Fig. 17: Supply and load voltage and current battery storage system Conclusions: In this paper two different microsources photovoltaic (PV) and wind turbine (WT) with battery storage for a small scale microgrid system are simulated. Simulation is focus on the parameter of the each microsources to consider the outputs and effectiveness of inverter. Most of the results can be used for develop a small scale microgrid system for practical applications. REFERENCES Bunlung Neammanee, 2007. Somporn Sirisumrannukul and Somchai Chatratana. Development of a Wind Turbine Simulator for Wind Generator Testing in International Energy Journal, 8: 21-28. 420
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