Load Frequency Control for Hybrid Microgrid using Battery Storage System

Transcription

1 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:14 No:03 26 Load Frequency Control for Hybrid Microgrid using Battery Storage System Engr. Naveed Khan, Prof. Muhammad Zahir Khan, Muhammad Iftikhar Khan, Engr. Fawad Ahmad, Engr. M.Aslam University of Engineering & Technology, Peshawar Pakistan Abstract-- This paper objective is to propose a LFC mechanism for BSS and Diesel units for a remote and isolated microgrid scheme. The microgrid scheme included a BSS, two Diesel units and two solar panels. The proposed LFC mechanism is implemented in a decentralized fashion. The proposed mechanism was tested in different operating conditions, variable power demand and emergency conditions when one or more of the generation unit is lost in every case. Results illustrate that under all situations the proposed system controls the frequency of the microgrid system efficiently. Simulink and SimPower environment is used to develop the microgrid model and the proposed control mechanism. Index Term-- BSS Battery Storage System, DG s Diesel Generators, ESS Energy Storage System, LFC Load Frequency Control generation units. The proposed LFC technique was to split the LFC signal between the ESS and thermal generation i.e. to dispatch the ESS and thermal generation to meet the system demand, and hence the balance between generation and demand achieved. The required rate from thermal generation and energy from ESS was obtained from the so-called ramp-rate-duration curve and load-duration curve. The load-duration curves and the so-called ramp-rateduration curves are based on historical system operation to expect how the system will operate over the next period of time. The authors did not show how the system is handled after an unexpected event, such as losing the ESS or a thermal unit. I. INTRODUCTION Microgrids are small power grids, there is no agreement on exact size or specific structure for a microgrid. Microgrid can be part of an electric distribution system or can be a small independent power grid of an island or a remote area where there is no access of primary grid power. Chowdhury, P. Crossle in [1] define microgrids as small- scale, Low Voltage Combined Heat and Power (LVCHP) supply networks designed for supplying electricity and heat load to a small community, such as a suburban locality or an academic institution or public community. As our microgrid system also contain two PV panels so the main advantage of the renewable energy resources is the reduction of environmental pollution and global warming since no gaseous emissions result from such generations. Figure 1 shows an example for microgrid system architecture. In the shown architecture, the microgrid encompasses different kinds of loads as well as different kinds of generations. All the time system should operate in such a way that there should be a balance between generation and consumption, if not so then there will be instability i.e. system frequency will deviate from nominal value (50Hz). Watson and Kimball in [2], and Pappu in [3] used solar power to propose a frequency regulation approach for a microgrid system. The proposed procedure is dependent upon tracking of MPP of the PV arrays. For purpose of regulation a power margin was reserved. But, it was not an effective use of solar power. Also, this method is not acceptable for LFC as it can vary any time suddenly. Therefore, all the available power should be taken when available o r s h o u l d b e stored or s e n d t o t h e transmission lines. Leitermann and Kirtley in [4] shed light on the option of providing LFC by sharing the system demand between the ESS and thermal Fig. 1. Microgrid system Architecture II. PROBLEM STATEMENT For stable operation of power system, it is very important to have a robust and automatic LFC mechanism for ensuring a balance between power generation and consumption under all circumstances; fluctuating power demands and emergency conditions when one or more of the generations unit is lost suddenly, especially for isolated systems when there is no way of getting help from large grids. For attaining matching between the generation and consumption for small and isolated power systems comprised of small DG units and renewable energy sources subject to such circumstances is an issue of great significance. The increasing integration of ESSs, however and their excellent capability of providing LFC will sooner or later necessitate the contribution of ESSs in LFC. This paper will focus on addressing the matching between

2 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:14 No:03 27 generation and consumption in an isolated small power system using a Battery Storage System. The problem is to design robust control mechanism for the Battery unit so that it will track frequency deviations and regulate system s frequency under the above mentioned operation conditions. The simulation model is developed in the Simulink and Simpowersystems environment. III. CONTROL DESIGN AND SIMULATION RESULTS: The goal of this LFC mechanism is to drive frequency error ( f = fs - f) of system to zero under different operation conditions. Whereas fs is the schedule frequency of 50Hz while f is the measured frequency. The goal is to minimize f in all regular operational conditions, when the electric power demand varies, and also contingency conditions, when suddenly any of the system generation unit is lost. Therefore, 3 control loop are recommended; one for charging or discharging of Battery Storage Unit for regulation of frequency error and other two control loops on every one Diesel Generators (i.e. on both of the DG s). a) Diesel Engine system: For simulating DG s full dynamics, a higher order as well as complex model is required, but from speed dynamics and control system perspective it s enough to have a lower order model. The model of DG shows a picture of fuel consumption rate as it depends on the speed as well as on the mechanical power at diesel engine s output, also normally first order model is used to model it associated with fuel consumption to the mechanical power of engine [5] and [6]. The diesel engine model is basically composed of three parts; the governor system, the diesel combustion engine, and the flywheel. Figure 2 shows the diesel engine system block diagram. Fig. 2. Diesel engine system block diagram. b) Battery Storage System: Several BSS models are present for the simulation of charging and discharging performance of Battery Storage System (BSS) [7], [8], and [9]. Normally, set of nonlinear equation are used to model the battery which represents current of battery as function of interior capacitances (C), internal resistance (R) of battery and State of Charge (SOC). The Battery unit in this work is selected to be LI because of its wide applications and environment friendly characters. Figure 3 is a block diagram that illustrates the structure of battery inverter control system for LFC purposes. The input to PI controller is the frequency error signal which outputs Idref.. As directly implementing t h e controls on the sinusoidal signals isn t an easy task, s o it is necessary to c h a n g e t h e sinusoidal currents from three phase to direct-quadrature (dqo) structure. The active Power which is absorbed or produced by the battery system is regulated by driving Id controller whic h drives Id to Idref. The inverter reactive power is regulated by Iq controller. For unity p.f, Iq is set to zero. The voltages of microgrid (3 phase) are calculated in per unit, after that it is fed in PLL controller, that outputs required synchronization signal. By using Frequency Response Estimation (FRE) procedure Id and Iq controllers are designed [10] and [11]. Fig. 3. Block diagram of battery system inverter controls. IV. CIRCUMSTANCES AND SIMULATION RESULTS a) Step load change: In operation of Power System, fluctuations are common in power demand, and the goal to change step load is for the checking of the proposed controller s behavior under normal operational conditions. At time t = 5 seconds there is a change i. e. sudden increase in the demand by about 4.4 KW. Figure 4 illustrates the current reference signal Idref and the BSS power. Figure 5 shows the power generated by the DGs. Figure 5 illustrates changes in the power generated from each of the DGs. Figure 6 shows total changes in DGs generations and BSS power as well as the power demand change. Figure 7 shows microgrid frequency error f.

3 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:14 No:03 28 Fig. 7. Microgrid frequency error ( f). b) Loss of 20KVA Diesel Generator: Fig. 4. BSS Power Fig. 5. Power generated from diesel generators. The 20KVA DG is disconnected at time 10.2 sec from the microgrid. The goal is to check the response of the battery unit when 20KVA Diesel Generator is gone. Figure 8 illustrate power produce and generated from BSS and DG s respectively. From figure 8 it is apparent that power from the BSS remains at zero until to the fault o c c u r s, while power generated from 100 KVA a n d 2 0 K V A generators was approximately 100 KW and 18KW. Figure shows that after the loss of 20 KVA generator, the power of battery raised from none to 18KW.This shows that invertor control system to battery reacted frequency error signal which increase battery power to required amount which was supposed to be produced by the generator which is lost. Figure 9 shows that the PV systems are connected both at time 20 sec and 25 sec to the microgid. Thus, the power generated from 100KVA was lessened in the equivalent time instant. Figure 10 shows that the frequency error of microgrid w a s restored to the desired values (50 Hz frequency and zero frequency error) at every stage. Fig. 6. Power demand change and total generation changes. Fig. 8. Power produced by the diesel generators and battery system.

4 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:14 No:03 29 Fig. 11. Microgrid frequency error Fig. 9. Power produced from PV systems. Fig. 12. Microgrid frequency error with and without BSS Fig. 10. Microgrid frequency Error c) Loss of 100KVA Diesel Generator: If the microgrid could experience the most severe and difficult scenario is by losing its primary generation unit i.e. 100 KVA diesel generator. So at time 10.2 seconds the circuit breaker connected to the generator will trip. Also initially, the PV systems are n o t connected to the microgrid. To show the importance of BSS in LFC, frequency of microgrid is compared when there exists battery unit a n d wh e n i t is not existing. Figure 11 shows the frequency error ( f) of microgrid when battery system exist. Figure 12 shows importance of BSS unit involvement in the Load Frequency Control by relating the grid frequency when BSS exists and vice versa. Figure 13 illustrates power produce from battery unit and power generated from diesel generators. The figures shows, initially 100KVA DG is delivering its full generation capacity (100 KW) to microgrid. Load of 50KW is connected to the generator s bus and 50KW is supplied to the grid. After some time 10.2 sec, 50KW load and 100 KVA DG were disconnected from the grid. This impact of disconectivity on frequency clearly shown in figure 11. At the same instant, figure 13 shows that for this change in system frequency, the battery control system reacted and compensate the power by increasing the active power to 50KW injected by the battery unit, also microgrid frequency was restored to the scheduled value (50 Hz), and the frequency error to zero. Fig. 13. Power Generated by diesel generators and Battery system. The two PV systems was coupled to microgrid each at time =15 sec and 20 sec respectively. It s clear from the figure that by connecting first PV system the power generated by 20KVA reduced to 12KW and it is further reduced to 8KW when connected the second PV system. The power which is added to microgrid by the two PV systems are shown in Figure 14.

5 International Journal of Electrical & Computer Sciences IJECS-IJENS Vol:14 No:03 30 simulation APPEEC, [8] G. Rincon Mora and C. Min, "Accurate electrical battery model capable of predicting runtime and I-V performance" Energy Conversion, IEEE Transactions, [9] Rechargeable Battery, /battery.html [10] Mathwork. Simulink Control Design Documentation l [11] Mathwork. Documentation Design of PID Controller using Simulink. =/products/dem os/shipping/slcontrol/scdenginectrlpidpad.html. Fig. 14. Power produced from PV systems. From Fig 14 it is clear that powers which is produced by the PV systems aren t the same, which is due to setting of different irradiance level of PV systems i.e. 1000W/m2 of the first PV system and 200W/m2 for the second PV system. This is done in order to examine the effect of this on microgrid power by changing the irradiance level. As this particular study emphases on LFC which is examined in a time which ranges in seconds, so we assume constant irradiance level for the two PV systems. V. CONCLUSION This paper focused on implementing a LFC mechanism in an isolated microgrid system in order to regulate the system frequency. The control systems were tested under three different scenarios; the first scenario represented the normal operation of microgrid system where the demand is continually fluctuating, the second and third scenario each DG was lost. Results showed that the developed controls were successful in maintaining the system s frequency within acceptable limits under all circumstances. REFERENCES [1] UK: Institution of Engineering and Technology, "Dynamic Distribution Networks and microgrid". P. Crossley, S. Chowdhury, 2009 [2] J. Kimbal and L. Watson, "Microgrid frequency regulation using solar power," in APEC, 2011, IEEE 26 th, [3] R. Bhatt, V. Pappu and H. Chowdhury, "Implementation of frequency regulations ability in PV system," in NAPS, [4] L. Kirtley and Leitermann, "Energy storage for use in LFC," in CITRES conference, 2010 IEEE Conference, [5] H. F. Mohamed, "Online management and micro grid modeling University of Helsinki, Finland, [6] R. Majumder, "MODELING, STABILITY ANALYSIS AND CONTROL OF MICROGRID," Queensland University of Technology, Queensland, Australia, [7] Modeling of Lithium ion battery for Energy storage system