Indranil Saaki Paper On Improved Control Strategy For Fuel Cell and Photo Voltaic Inverters
Indranil Saaki Paper On Improved Control Strategy For Fuel Cell and Photo Voltaic Inverters
Indranil Saaki Paper On Improved Control Strategy For Fuel Cell and Photo Voltaic Inverters
Y.RAMESH REDDY1 , INDRANIL.SAAKI3, SRM UNIVERSITY,CHENNAI , UNIVERSITY,VIZAG, E-mail: yrreddy.1988@gmail.com indranil.saaki@gmail.com PRADEEP KUMAR.M 2, SRM UNIVERSITY,CHENNAI, E-mail: modani.nandan@gmail.com GITAM E-mail:
1 INTRODUCTION
A cluster of distributed generation (DG) units are usually connected to a distribution network to maintain the reliability of critical loads, especially when the grid supply is not available. However, an increase in the number of DG connections can give rise to several technical concerns in the operation of the power system. To overcome this problem, a microgrid is formed when multiple DG units in electrical proximity to each other are grouped together in a distribution system. Thus, the microgrid concept assumes a cluster of loads and DG units operating as a single controllable system that provides both power and heat to its local area [1]. From the grids point of view, a microgrid can be regarded as a controlled entity within a power system that can be operated as a single aggregated load and as a small source of power supporting the network. From a customer point of view, microgrids are considered to provide their thermal and electricity needs, enhance local reliability, reduce emissions, improve power quality by supporting voltage and reducing voltage sags and potentially reduce costs of energy supply. The DG units in a microgrid may take many forms of technologies, namely, diesel engines, micro turbines, fuel cells, photovoltaics and wind turbines. The capacity of the DG units usually varies from few kWs to 1-2 MWs. The form of the microgrid takes with the different types of DG units and loads will have a large
impact on the operating and control regime of the system. Therefore, coordinated operation and control of DG units together with storage devices, such as flywheels, energy capacitors, batteries, and controllable loads are central to the concept of microgrids. A microgrid must have the capability to operate either in grid connected mode or in islanding mode depending on factors like planned disconnection, grid outages or economical convenience [2,3]. Considering the operating modes, a series of technical challenges with regards to the operation and control of microgrids need to be addressed such as sharing of loads between DG units and smooth switching from grid-connected to island operation. An important consideration in managing microgrids is to control the parallel operation of inverters so that they can work efficiently to achieve high performance. In [4], a master/slave control scheme for operating the DG units both in grid-connected and islanding modes was developed. Here, two local controls schemes for master/slave operation were used. However, a drawback of this control method is that there is a need for having one unit to act as a master unit and if this unit fails the overall system will be down. A phaselocked loop control technique is commonly used in gridconnected converters to provide accurate estimation of phase angle for grid synchronization [5]. Another control scheme known as the voltage and frequency droop control method is proposed to obtain good sharing of the parallel inverters for different modes of operation [6]. The droop control method makes the parallel inverters share both active and reactive powers, which allows power management on power lines for far-away parallel inverters. This approach does not directly incorporate the load dynamics in the control loop, large or fast load changes and hence may result in either poor dynamic response or even voltage/frequency instability. Thus, it is very important for the microgrid to utilize the local measurements of the parallel inverters to achieve better active and reactive power sharing. In this paper, an improved control strategy is presented, which is able to better manage power sharing accuracy of the parallel connected inverters used in a microgrid. The developed control strategy combines the advantages of both power and voltage control schemes. Power control ensures stability of the whole conversion system, while voltage control allows a more accurate generation of the reference voltage necessary to apply in the pulse width modulation technique. The combined use of power and voltage control schemes allow the implement -tation of a simple and effective threephase
control scheme, particularly to deal with critical conditions that can occur in the microgrids. To test the performance of the proposed control strategy, simulations were carried out by considering two DG units, namely fuel cell and photovoltaic connected to the grid system.
The total power generated by the fuel cell is given by, The parameters of this model can be found in [8].
where Iph : Light generated current in a solar cell I0 : Reverse saturation current of diode Tc : Cell temperature in Kelvin A : Ideality factor K : Boltzman constant q : Electron charge : Current temperature coefficient G : Irradiance : Voltage temperature coefficient Ns : Number of modules connected in series Np : Number of modules connected in parallel
where V : Total stack voltage E0 : Standard reversible cell potential r : Internal resistance of stack I : Stack current N : Number of cells in stack R : Universal gas constant T : Stack temperature F : Faradays constant PH2 : Partial pressure of hydrogen
3 PROPOSED STRATEGY
INVERTER
CONTROL
Two DG units were considered in this study in which the interconnection between the two DG units with the main grid at the point of common coupling is shown in Figure 1. Each DG unit comprises of a dc source and a voltage source inverter (VSI). The configuration of the VSI used in each DG unit is shown in Figure 2. The inverter transfers energy produced by the DG and at the same time controls the power flow by using impedances with resistance R1, inductance L1 and parallel capacitive filter. These impedances provide a
path for some high-order harmonics generated at the switching frequency. For the inverter operating in voltage control mode, its controller generates three reference signals, Egrida, Egridb, Egridc, each of which is referred to the output voltage that is to be applied on each phase, so that the currents Ia, Ib, Ic, track its desired values corresponding to the power flows required between the dc and ac sides. The output voltages of the VSI are required to track the reference voltages by applying the pulse width modulation (PWM) technique.The proposed inverter control strategy is explained in terms of three different modes of operation, namely, i) grid connected mode (Switch close), ii) islanding mode (Switch open) and iii) transition mode (Switch open and close within 2 seconds). When the inverter operates in grid connected
Fig. 2 Inverter control system for each DG mode, it uses power control while in islanding mode, it uses voltage control.
4 SIMULATION RESULTS
The performance of the proposed inverter control strategy is evaluated by simulation using the PSCAD/EMTDC simulation software. Simulations were carried out to investigate the effectiveness of the combined power and voltage control scheme in managing power sharing among the DG units in a microgrid. Simulation results are presented for the performance of the DGs in grid connected (Switch close), islanding mode (Switch open) and transition mode (Switch open and close within 2 seconds) of operations.
power generated is 0.35 MW. Figure 5 shows that load 1, 2 and 3 consume similar power of about 0.100MW, thus giving a total load of 0.3 MW.
generated by the DG units and the total power of load which is approximately 0.3 MW and 0.255 MW, respectively.
The next simulation results show the effect of increasing all the loads at t= 1s as shown in Figures 6 and 7. From Figure 6, it can be seen that the active power from the grid reduces slightly to 0.23 MW while the power from both DGs increases greatly to 0.5 MW. The increase in power is to provide sufficient power to the loads which is increased to 0.55 MW as shown in Figure 7.
5 CONCLUSIONS
Improved control strategy for SOFC and PV inverters used in a microgrid is established in this work. The control strategy is based on power control and voltage control schemes to control the SOFC and PV voltage source inverters. The effectiveness of the control strategy is evaluated by operating the microgrid in grid connected, islanding and transition modes of operation. Test results showed that the proposed control strategy is able to correctly manage power sharing among the DG units in a microgrid, regulate quickly the DG power outputs to meet the requirements of the loads, tolerate the rapid changes in various mode of operations and maintain voltage quality at the grid and loads.
[8] A A Salam, M A Hannan, A Mohamed, Dynamic modeling and simulation of solid oxide fuel cell system,2nd IEEE International Conference on Power and Energy PECON 2008, 1-3 December 2008, Johor Bahru, MALAYSIA. [9]S Moulahoum, O Touhami, A Rezzoug, L.Baghli, Rectified Self-Excited Induction Generator as Regulated DC Power Supply for Hybrid Renewable Energy Systems, WSEAS Transactions on Circuits and Systems, Vol.4, Issue 11, Nov. 2005, pp. 1457-1464. [10]A Kuperman, R Rabiniovici, Shunt Voltage Regulators for Autonomous Induction Generators, Part 1: Principles of Operation, WSEAS Transactions on Power Systems, Vol.1, Issue 1, Jan. 2006, pp. 221-226 [11]A Kuperman, R Rabiniovici, Shunt Voltage Regulators for Autonomous Induction Generators, Part 2: Circuits and Systems, WSEAS Transactions on Power Systems, Vol.1, Issue 1, Jan. 2006, pp.227-232.
6. REFERENCES
[1] R Lasseter, A Akhil, C Marnay, Integration of distributed energy resources, The CERTS Microgrid Concept, LBNL50829, Lawrence Berkeley National Laboratory, 2002 [2] T C Green and M. Prodanovic, Control of inverter-based microgrids, Electric Power Systems Research, Vol.77, Issue 9, 2007, pp.1204-1213. [3] F Katiraei, M R Iravani, P W Lehn, Micro-Grid Autonomous Operation During and Subsequent to Islanding Process, IEEE Trans. on Power Delivery, Vol.20, No.1, 2005, pp.248-257. [4] J. A. Peas Lopes, C. L. Moreira, and A. G. Madureira, Defining Control Strategies for MicroGrids Islanded Operation, IEEE Transactions on Power System, Vol 21, Issue 2, 2006, 916 -924. [5]S K Chung, Phase-locked loop for gridconnected threephase power conversion systems, Proc. Inst. Elect. Eng, Vol. 147, No. 3, 2000, pp. 213219. [6]K De Brabandere, B Bolsens, J Van den Keybus, A. Woyte, J. Driesen, and R. Belmans, A Voltage and Frequency Droop Control Method for Parallel Inverters, IEEE Transactions on Power Electronics, Vol.22, No.4, 2007, pp.1107-1115. [7] Y Zhu, K Tomsovic, Development of models for analyzing the load performance of microturbines and fuel cells, Electric Power System Research62, 2002, pp.1-11.