International Journal of Modern Engineering Researh (IJMER) www.ijmer.om Vol.2, Issue.3, MayJune 2012 pp11191124 ISSN: 22496645 Performane of Grid onneted DG Inverter System y Using Intelligent ontrollers K. MNOHR #1, M. VENU GOPL RO #2 1 M.Teh Student, Department of Eletrial and Eletronis Engineering, KL University, Guntur (P) India 2 Professor, Department of Eletrial and Eletronis Engineering, KL University, Guntur (P) India strat Reent development in the small sale power generation using distriuted energy resoures omined with appliation of power eletroni systems initiated the researhers to the onepts of future power generation tehnologies suh as mirogrid. The paper presented involves the ontrol tehniques required for mirogrid operation and implementation of a simple ontrol strategy in a mirogrid model realized with MTLB. To demonstrate the operation of a mirogrid in Grid onneted mode and intentional islanded mode, a simulink model has een designed with neessary parameters y onneting with the main grid allowing the sharing of different laods with referene to Grid onnetion and disonnetion. n islandingdetetion algorithm has een used to at as a swith etween the two ontrollers and this has minimised the effet of losses in the time of transistion. relosure algorithm has een used for the DG to resynhronize the inverter voltage with the grid. Index Terms: Distriuted Generation, Intentional Islanding, mirogrid, Grid tie inverter, lgorithm, PLL, synhronization ontroller, INRODUTION The reent trends in small sale power generation using the with the inreased onerns on environment and ost of energy, the power industry is experiening fundamental hanges with more renewale energy soures (RESs) or miro soures suh as photovoltai ells, small wind turines, and miroturines eing integrated into the power grid in the form of distriuted generation (DG). These RESased DG systems are normally interfaed to the grid through power eletronis and energy storage systems[1] One of the most ritial setions of the ontrol system for a distriuted generation (DG) unit s interonnetion to the utility grid lies within the gridonneted onverter s ontrol and protetion system; speifially the islanding detetion algorithms. Through this ontroller susetion, the system is ale to determine whether or not it is safe to remain onneted to the grid. These islanding detetion algorithms, whih are integrated into the ontrol system, are mainly present to prevent the undesirale feeding of loads during fault onditions and disonnetions from the grid, whether or not the disonnetion as intentional[2] This is required y standards sine the reation of suh power islands is foridden. Thus, in effet, standards require DG ontrol systems to sense islanding events and disonnet themselves from the grid. This rings into question the method of how to implement suh a detetion sheme.[ Islanding Detetion Using a oordinate Transformation Based PhaseLoked Loop] Islanding is a ondition in whih a mirogrid or a portion of the power grid, whih ontains oth load and distriuted generation (DG), is isolated from the remainder of the utility system and ontinues to operate Some distintions of islanding are: nonintentional islanding ours if after the fault it is not possile to disonnet the DG; nonintentional islands must then e deteted and eliminated as fast as possile; intentional islanding refers to the formation of islands of predetermined or variale extension; these islands have to e supplied from suitale soures ale to guarantee aeptale voltage support and frequeny, ontrollaility and quality of the supply, and may play a signifiant role in assisting the servie restoration proess mirogrids, seen as partiular types of intentional islands, asially operated in autonomous mode, not onneted to the supply system; the whole mirogrid an e seen from the distriution system as a single load nd has to e designed to satisfy the loal reliaility requirements, in addition to other tehnial harateristis onerning frequeny, voltage ontrol and quality of supply.[2] Fig 1. Shemati diagram of Grid onneted Inverter POWER MISMTHES The effets of power mismathes etween the DG and the loads have upon the system in terms of voltage and frequeny, the most rudimentary of sensed parameters, need to e known.[3] www.ijmer.om 1119 Page
International Journal of Modern Engineering Researh (IJMER) www.ijmer.om Vol.2, Issue.3, MayJune 2012 pp11191124 ISSN: 22496645 onsidering the generi system depited in Fig. 2, and that the DG an supply anywhere from partial to the full load demand, or even an exess of power to soure the grid. parallel RL load is used for this study s example; also that this is a loal load to the DG and there will not e a large reatane etween the DG and the P. s suh, there an exist a power demand mismath etween the DG and loads[9], whih the grid supplements; however when the grid is no longer supplying the remaining power demand of the loads, the system voltage and frequeny at the P will e affeted.. tive Power Mismath If the ative power portion of the load demand that is alulated is oming from the DG, the following is found. Plugging (3) into (4), we get the redued equation of (5), showing that an ative power mismath etween load and DG will ause voltage variations if the grid fails. B. Reative Power Mismath Now onsider the reative power mismath etween the DG and load during a grid fault. The demand required y the load is equated in (6). (4) (5) The resonant frequeny of the load is determined y the L relationship. Therefore we an rewrite (6) as (7). (6) Fig. 2: Generi Interonneted System Where Reff is the equivalent resistane seen y the DG for the amount of power it is supplying. If the grid fails and only the DG is left to supply the load at a onstant ative power, the voltage at the P would naturally hange, represented in (2). (1) s suh, if the grid stopped supplying its portion of the load s demand of reative power, the line frequeny would drift to the resonant frequeny to fore the mismath to eome zero. Therefore, let us write the resonant frequeny as a term of the line frequeny and the frequeny drift due to a mismath. (7) s suh, Reff an e written as a funtion of true load resistane, the voltage and hange of voltage that would our at the P, seen in (3) y equating and solving (1) and (2). Thus, to find the power mismath from the load demand and DG, we an write the following: (2) (3) www.ijmer.om When the max/min values of the voltage and frequeny deviations are plugged into (5) and (8), an NDZ range for power mismath an e alulated. s suh, it is seen that the standard OVP/UVP, OFP/UFP shemes are not enough to minimize the NDZs.[3] ISLNDING DETETION LGORITHM Islanding is the ondition where the DG remains operating in the distriution system with the utility disonneted. In the past years, several islanding detetion methods have een proposed and the detetion methods an e ategorized into two main groups: passive and ative methods. Passive methods depend on measuring system parameters and then thresholds are set to these parameters (8) 1120 Page
International Journal of Modern Engineering Researh (IJMER) www.ijmer.om Vol.2, Issue.3, MayJune 2012 pp11191124 ISSN: 22496645 to differentiate etween an islanding and a nonislanding ondition tive methods diretly interat with the power system operation y introduing perturations in the inverter output. The most ommonly used islanding detetion method is the Over/Under Voltage (OVP/UVP) and Over/Under Frequeny (OFP/UFP). [4],[8] Fig: 4 Voltageontroller when the Grid is Disonneted ondition II: WHEN GRID IS ONNETED Fig 3. Intentional Islanding lgorithm The DG interfae ontrol designed in this paper provides onstant DG output and maintains the voltage at the Point of ommon oupling (P) at 1 p.u. Maintaining oth the voltage and power onstant during an islanding ondition is not feasile for standalone operation of the DG sine oth depend on eah other, and the OVP/UVP and OFP/UFP ould e used to detet islanding[4]. This detetion method will operate effiiently for large mismathes etween load and DG apaity. Unfortunately, if the load losely mathes the DG apaity, the frequeny will reah the threshold value after a very long time. The DG was designed to supply 100 kw and the load onneted asors approximately 100 kw. The grid disonnets at t = 5 seonds and the frequeny at the P drifts away from the 60 Hz value. It an e seen that the time for the frequeny to reah the 59.3 Hz threshold is greater than 5 seonds..[8] ONTROLLERS: The system onsists of a mirosoure that is represented y the d soure. Under normal operation, eah DG inverter system in the mirogrid usually works in onstant urrent (or onstant power) ontrol mode in order to provide a preset power to the main grid. When the mirogrid is ut off from the main grid, eah DG inverter system must detet this islanding situation and swith to a voltage ontrol mode. In this mode, the mirogrid will provide a onstant voltage to the loal load[2]. ondition 1: WHEN GRID IS DISONNETED Fig 5: urrent ontroller when Grid is onneted For gridonneted operation, the ontroller shown in Fig 5. is designed to supply onstant urrent output. phase loked loop is used to determine the frequeny and angle referene of the Point of ommon oupling (P) voltage. To simplify the design and operation of the ontroller, the ontrol of the system is designed in a synhronous referene frame (SRF) [5]. Fig. 6 shows this ontrol topology employing synhronous frame urrent ontrol.[6] The inverter urrents are transformed into a synhronous frame y Park s transformation and regulated in dquantity orresponding to the urrent referenes Idqref. In the following stage, the voltage referenes in dquantities Vdq whih eing proessed y PI ontrollers are transformed into a stationary frame y the inverse of Park s transformation and utilized as ommand voltages for generating high frequeny pulse width modulated (PWM) voltage.[11] Fig 6: Blok diagram of urrent ontrolled Inverter www.ijmer.om 1121 Page
International Journal of Modern Engineering Researh (IJMER) www.ijmer.om Vol.2, Issue.3, MayJune 2012 pp11191124 ISSN: 22496645 When using the urrent ontrol, the output urrent from the filter is fed ak and ompared with referene urrent Iref and the error is passed to the PWM to generate voltage referene for the inverter. In order to get a good dynami response V dq is fed forward. Fig. 6 shows the lok diagram of the DG interfae ontrol for gridonneted operation. For unity power fator operation, iqref is set to zero.[2] IntentionalIslanding Operation Mode The voltage losedloop ontrol for intentionalislanding operation is shown. The ontrol works as voltage regulation through urrent ompensation. The ontroller uses voltage ompensators to generate urrent referenes for urrent regulation. s shown, the load voltages (Vd and Vq) are fored to trak its referene y using a PI ompensator (voltage regulator). The outputs of this ompensator (IDref and IQref ) are ompared with the load urrent (ID and IQ), and the error is fed to a urrent regulator (PI ontroller). The output of the urrent ompensator ats as the voltage referene signal that is fed Fig 7: Syhronisation ontroller 2) In order to otain the information of θ, two sets of voltage values are used Using the variales k and g, sin(θ) an e found as (11) (10) Fig. Syn otrl shows how sin(θ) is used to otain the new phase angle for whih the grid and inverter voltages are synhronized. MODELLING OF IRUIT IN MTLB SIMULINK. Sope3 to the sinusoidal pulsewidth modulator to generate the high frequeny gating signals for driving the threephase voltage soure inverter. The urrent loop is inluded to stailize the system and to improve the system dynami response y rapidly ompensating for nearfuture variations in the load voltages. In order to get a good dynami response, VDQ is fed forward. This is done eause the terminal voltage of the inverter is treated as a isturane, and the feedforward is used to ompensate for it. R D Voltage Soure Pulses pwm Universal Bridge g B v Sope Vol Va Ia VI Measurement1 Sope1 Sope2 Breaker Sope4 Va Breaker1 5 1 Ia 6 14 Breaker2 7 15 VI Measurement 8 9 10 2 3 4 Sope6 11 12 13 VI Measurement2 Va Ia a B 1 2 3 B Synhronization for Grid Reonnetion When the griddisonnetion ause disappears, the transition from islanded to gridonneted mode an e started. To avoid hard transients in the reonnetion, the DG has to e synhronized with the grid voltage. The DG is operated in the synhronous island mode until oth systems are synhronized. One the voltage in the DG is synhronized with the utility voltage, the DG is reonneted to the grid, and the ontroller will pass from the voltage to the urrent ontrol mode. This synhronization is ahieved y implementing the following algorithm.[5] In1 In2 Out1 In3 PWM R Universal Bridge g D Voltage Soure B v Sope Vol Disrete, Ts = 5e005 s powergui Fig.8: Without any ontrollers Sope3 Sope1 Sope2 Sope6 Sope5 Sope7 Breaker Sope4 VI Measurement2 2 Va 1 Va Ia Va Breaker1 Ia 5 1 Ia 11 a B 2 1 6 14 12 VI Measurement1 Breaker2 7 15 VI Measurement 2 1 Step 13 8 9 10 2 3 4 V p.u. Va Freq Measurement Sope8 I_a Da VI_a D V pu VG_a D Trigger Freq urrent ontroller lgorithm I_a Da 1 2 3 0 Display VI_a D 1) ssume that the phase differene etween the grid and inverter voltages is given y VG_a D Voltage ontroller Sope9 Fig 9. With ontrollers (9) www.ijmer.om 1122 Page
International Journal of Modern Engineering Researh (IJMER) www.ijmer.om Vol.2, Issue.3, MayJune 2012 pp11191124 ISSN: 22496645 SIMULTION RESULTS The performane of the proposed ontrol strategies was evaluated Fig. shows the simulated system. This system was tested under the following onditions: 1) swithing frequeny fs: 10 khz; 2) output frequeny: 60 Hz; 3) filter indutor Li: 1 mh; 4) filter indutor LL: 0.5 mh; 5) filter apaitor f: 31 μf; 6) dlink voltage Vd: 400 V; 7) output phase voltage Vo1ϕ: 120 Vrms; 8) output apaity: 10 KW. The RL load was adjusted to e resonant at 60 Hz and to onsume 10 KW. The DG system was designed to supply 10 KW and zero reative power. The system was operated initially in gridonneted operation. The grid was disonneted at 0.3 s, and this event was deteted at 0.30155 s. fter 0.30155 s, the ontrol mode was hanged from urrent to voltage ontrolled operation. Fig:12 Voltages Without Synhronisation lgorithm Fig 13: Voltages With Synhronisation lgorithm FFT nalysis for the Inverter. Fig 14. shows the voltages and urrents at the P efore and after grid disonnetion. The grid was reonneted at 0.3 s. Fig 10: FFT analysis of Inveter Using the aove analysis we an oserve that the inverter has operated with the THD of 5.6% The DG was operated in the synhronous island mode until oth systems were resynhronized. Fig. 14 shows the synhronization of the voltages at oth ends of the P when the synhronization algorithm starts to work in the intentionalislanding mode. s an e seen, the proposed algorithm suessfully fores the voltage at the DG to trak the voltage at the grid. One the synhronization was ompleted, the DG was reonneted to the grid, and the ontroller was swithed from the voltage to the urrent ontrol mode. Fig. 12,13 shows the phase voltage Va without and with the synhronization algorithm implemented. Notie that the algorithm avoids a hard transient in the reonnetion from intentionalislanding to gridonneted operation to keep the magnitude of the voltage in its normal operational range when there is a power mismath. Fig 11. Inveter Voltages when onneted to the Grid. www.ijmer.om ONLUSION: Here in this paper a ontroller is designed oth for grid onneted operation and the other for Intentional islanding operation. n algorithm for the detetion of islanding is presented whih was responsile for the swith 1123 Page
International Journal of Modern Engineering Researh (IJMER) www.ijmer.om Vol.2, Issue.3, MayJune 2012 pp11191124 ISSN: 22496645 etween the two ontrollers and also a relosure algorithm whih auses the DG to resynhronize itself with the grid is also designed. Thus the paper summarizes the traditional independent inverter and Gridonneted inverter ontrol strategy, omining the distriuted power and mirogrid inverter harateristis, a suitale mirogrid inveter ontrol strategy is put forward. Swithing etween Gridonneted mode and Griddisonneted mode for mirogrid inverter has een studied. On the Griddisonneted operation mirogrid inveter supplies the important loads that ensures load voltage and frequeny staility. Mirogrid inverter an smoothly swith etween Gridonneted operation and Griddisonneted operation, and swithing operation of the system has good performane. The system ontroller design is simple, pratial and effiient, easy to implement. The simulation results show that the proposed ontrol method is feasile and effetive. [9] IEEE Std. 1547, Standard for Interonneting Distriuted Resoures with Eletri Power Systems, 2003. [10] Wu hunsheng; Liao Hua; Wang YiBo; Peng Yanhang; Xu Hong Hua, Design of intelligent utilityinterative inverter with I detetion, Third International onferene on Eletri Utility Deregulation and Restruturing and Power Tehnologies, DRPT 2008, pages: 2012 2017, 69 pril 2008. [11] T.. Green; M. Prodanovi, ontrol of inverterased mirogrids, Eletri Power Systems Researh, Distriuted Generation: Volume 77, Issue 9, pages 12041213, July 2007. REFERENES [1] Yun Wei Li, Memer, IEEE, and hingnan Kao, n urate Power ontrol Strategy for Power EletronisInterfaed Distriuted Generation Units Operating in a LowVoltage Multius Mirogrid, IEEE transations on power eletronis, vol. 24, no. 12, deemer 2009 [2] Irvin J. Balaguer, Uthane Supatti, Qin Lei,NamSup hoi, Intelligent ontrol for Intentional Islanding Operation of Mirogrids ISET 2008 [3] Thaker, T.; Wang, F.; Burgos, R.; Boroyevih, D., Islanding Detetion Using a oordinate Transformation Based PhaseLoked Loop, Power Eletronis Speialists onferene, 2007, PES 2007, IEEE, pages: 1151 1156, 1721 June 2007 [4] L. Qin, F. Z. Peng, and I. J. Balaguer, Islanding ontrol of DG in mirogrids, in Pro. IEEE 6th IPEM, 2009, pp. 450 455. [5] Irvin J. Balaguer, Qin Lei, Shuitao Yang, Uthane Supatti, Fang Zheng Peng, ontrol for Grid onneted and Intentional Islanding Operations of Distriuted Power Generation, IEEE transations on industrial eletronis, vol. 58, no. 1, january 2011 [6] Jayaweera, D.; Galloway, S.; Burt, G.; MDonald, J.R., Sampling pproah for Intentional Islanding of Distriuted Generation, Power Systems, IEEE Transations on, Volume 22, Issue 2, pages: 514 521, May 2007 [7] arpaneto, E.; hio, G.; Prunotto,., Reliaility of reonfigurale distriution systems inluding distriuted generation, International onferene on Proailisti Methods pplied to Power Systems, pages:1 6, 1115 June 2006. [8] Zeineldin, H.; ElSaadany, E.F.; Salama, M.M.., Intentional islanding of distriuted generation, Power Engineering Soiety General Meeting, 2005. IEEE, Vol. 2, pages: 1496 1502, 1216 June 2005. www.ijmer.om 1124 Page