Dynamic Modeling of Microgrid for Grid Connected and Intentional Islanding Operation

Transcription

1 Dynamic Modeling of Microgrid for Grid Connected and Intentional Ilanding Operation Rinu J Vijayan, Subrahmanyam Ch, Ranjit Roy Abtract Microgrid i defined a the cluter of multiple ditributed generator (DG) uch a renewable energy ource that upply electrical energy. The connection of microgrid i in parallel with the main grid. When microgrid i iolated from remainder of the utility ytem, it i aid to be in intentional ilanding mode. In thi mode, DG inverter ytem operate in control mode to provide contant to the local load. During grid connected mode, the Microgrid operate in contant current control mode to upply preet power to the main grid. The main contribution of thi paper i ummarized a ) Deign of a network baed control cheme for inverter baed ource, which provide proper current control during grid connected mode and control during ilanding mode. 2) Development of an algorithm for intentional ilanding detection and ynchronization controller required during grid reconnection. 3) Dynamic modeling and imulation are conducted to how ytem behavior under propoed method uing SIMULINK. From the imulation reult uing Simulink dynamic model, it can be hown that thee controller provide the microgrid with a determinitic and reliable connection to the grid. Index Term Ditributed generation (DG), grid connected operation, intentional ilanding operation and ilanding detection, microgrid, ynchronization, ource converter (VSC) I. INTRODUCTION POWER SYSTEMS are experiencing tremendou growth in the field of ditributed generation becaue of economic benefit, environmental concern, reliability requirement etc. Mainly the DG include wind turbine, photovoltaic cell, micro turbine and fuel cell. The DG i a ource inverter with an output low pa filter upplying the load []. The microgrid configuration [2] with the control trategy i hown in Fig.. Rinu J Vijayan i currently doing MTech in Power electronic and Electrical drive in Electrical Engineering Dept in National Intitute of Technology, Surat, (Phone: rinu..vijayan@gmail.com). Subrahmanyam Ch i doing MTech in Power electronic and Electrical Drive in Electrical engineering Dept in National Intitute of Technology, Surat, Gujarat ( ubbu75@gmail.com). Dr. Ranjit Roy i working a Aociate Profeor with Electrical Engineering Dept, National Intitute of technology, Surat, Gujarat. ( rr@eed.vnit.ac.in). The flexibility and control needed by microgrid with the main grid i achieved with the help of power electronic interface [3]. During grid connected mode the controller ha to upply the preet power to the main grid. In thi mode the inverter ue the ignal of main grid a reference. Thu in grid connected mode the ytem operate in tiff ynchronization with main grid in current control mode. c Fig.. INVERTER LF Gate ignal CF Lf Circuit breaker Parallel RLC load Vabc_inverter Vabc_ grid Iabc_inverter CONTROLLER Utility grid Microgrid configuration with controller When the microgrid i iolated from the main grid i.e. intentional ilanding operation, the controller i deigned o a to upply contant to the local enitive load [4], [5]. In thi autonomou mode, the main grid reference i not available; therefore a new reference i required to continue good power quality generation. The new reference i found out by uing phae locked loop (DQPLL) and PI controller [6], [7]. In order to tranfer from current control to control mode, detection of tranition from grid connected to intentional ilanding mode i neceary. Thi i achieved by uing an intentional ilanding detection algorithm [4]. After ilanding operation, the DG are connected back to grid. At thi intant of grid reconnection, recloure algorithm ha to be etablihed to achieve ynchronization [4], [9]. II. CONTROL TECHNIQUES FOR INVERTER The and current control loop ha been implemented by uing PI controller working on the DQ ynchronou reference frame. AC quantitie are converted in to DC ynchronou reference frame by Park Tranformation. Correpondingly all reference quantitie become DC in nature, o that imple PI controller would be ufficient to yield zero teady tate error. A. DQ PLL tructure For unity power factor operation i.e. Grid current reference to be in phae with grid, etimation of phae angle i a AC /2/$3. 22 IEEE

2 neceity. The phae angle and frequency at point of common coupling (PCC) i determined by uing a DQPLL tructure hown in Fig. 2. The PLL tructure analogy comprie of a controlled ocillator, an integrator and a phae detector [6], [7]. Phae etimation i achieved by ynchronization of the ocillating waveform generated by ocillator with the meaured waveform. Vq= Vq Vb Va Vc Kp Ki Fig. 2. Abc To αβ Vα αβ To dq Vβ ωf DQPLL tructure The DQPLL tructure conit of a Clarke tranformation, Park tranformation, PI regulator and an integrator. The expreion for the d axi component and the qaxi component fed to the PI controller i given a ^ Ѳ ia ib ic IDref IQref abc to dqo Fig. 3. ID IQ id vdref iq vqref CURRENT REGULATOR Synchronization controller Ѳ old vq DQ PLL Block diagram of current controller Finally the DC reference quantitie added with terminal are tranformed back to tationary frame by Invere Park Tranformation. Thereafter it i ued to generate the gate pule by SPWM technique [4]. C. Voltage control In thi control the microgrid will provide contant to the load []. Thi control cheme make ue of both current regulator a well a regulator. The control work on the principle of regulation through current compenation [2], [3]. Fig. 4 repreent the control cheme. The converter output i controlled by a ynchronou reference frame cloed loop controller. It output i tranferred in to a cloed loop current regulator which i further tranformed in to tationary frame, and then the pace vector PWM generate the gating ignal of the IGBT. va vb vc The realization of lock in PLL relie on regulating the quadrature component (3) of rotating reference frame to zero uing the PI controller. When generated by ocillator approximate, the PLL will be locked. The value of a hown in (4) will become V i.e. At the intant, when lock i realized, the direct axi component give the magnitude of the [8]. B. Current control In grid connected mode the magnitude and frequency of the microgrid terminal are impoed by the grid. Current controller i deigned to provide contant current output during grid connected operation [2], [3]. Control Sytem hown in Fig. 3 i ued to accomplih current control. In the trategy propoed here, the VSC line current i made controllable by a dedicated cheme and through the control of VSC terminal. The inverter AC output current i tranformed in to DC quantity in ynchronou rotating frame by Park tranformation. The direct and quadrature component are compared with the reference quantitie and the error ignal i paed to the PI controller to generate the reference []. The inverter terminal i conidered a a diturbance and hence fed forward to compenate it []. va vb vc Ѳ old DQ PLL SYNCHRONIZATION CONTROLLER Fig. 4. Vq ref Iqref Ia abc to dqo Ib Ic Iq ref Vqref Block diagram of controller A hown in Fig. 4, the filter output line i tranformed in to dc quantitie by DQPLL tructure. The direct and quadrature quantitie are forced to follow their correponding reference value by uing regulator. The current reference generated by regulator are compared with the dc value of load current and the error i compenated by current regulator. The output of current regulator provide the reference ignal. D. Intentional ilanding detection algorithm Parameter which are ued to determine the tate of grid i and frequency. Fig. 5 indicate the algorithm developed to accomplih the detection of intentional ilanding. Under deficient grid condition, the main witch i turned off and diconnect the main grid from the utility. Thi witching caue tranient in and frequency []. Therefore tracking of ytem magnitude and frequency indicate the tranition witching between grid connected and ilanding mode, and vice vera.

3 Va Vb Vc 3 phae equence analyzer Voltage meaurement pu. pu.88 AND AND inverter i given by (5) Va Vb Vc 3 phae PLL frequency meaurement f 59.3 f 6.5 AND (7) Fig. 5. Propoed algorithm for intentional ilanding detection Voltage magnitude and frequency meaurement i achieved with the help of three phae equence analyzer and DQPLL [8]. According to thi algorithm, value of frequency and magnitude are contrained to particular limit. During tranition from grid connected to ilanding operation, the and frequency parameter get hifted from thi contrained range a hown in Fig. 3. Thi end a logical high output to the witch, which witche the inverter to the uitable control accordingly. E. Reynchronization controller algorithm Ilanded operation can change it operational mode to grid connected operation by reconnection to the grid, which i referred a ynchronization [9]. Synchronization i achieved by uing the phae difference between ilanded microgrid and utility grid inuring a tranient free operation. The microgrid continue to operate in the ilanding mode until both ytem are ynchronized. Thu ynchronization controller enure the microgrid to operate in tiff ynchronization with the utility grid [5]. Uing the variable and, can be found a () The control circuit in Fig. 6 depict how i determined from grid and inverter, which ha to be ynchronized. VG_abc f(u)= g f(u)= inѳ VI_abc f(u)= k Fig. 6. Control circuit to determine ( ) When paralleling microgrid with utility grid, it i neceary to have the ame phae angle for both of them. By cloing the breaker at PCC, the two individual ytem begin to have parallel operation. In order to achieve tiff ynchronization with the utility grid during grid reconnection, ynchronization controller i ued. The algorithm ued here [4] i to determine the new phae angle at which both the microgrid and utility grid have to operate. The algorithm i a follow III. DESIGN OF PI CONTROLLER A. Current control tranfer function Current control cheme i needed during grid connected operation. The block diagram of current controlled operation i hown in Fig. 7. The inverter i modeled with an ideal gain G I =. In order to obtain the tranfer function of the filter and the load block, the deigned LCL filter and RLC load circuit mut be taken in to conideration. Fig. 8 depict the circuit diagram of LCL filter and RLC load. The tranfer function of ) uppoe the phae difference between the grid and

4 the LCL filter and the RLC load remain the ame for control tranfer function alo. Iref Ierror KpKi PI CONTROLLER Gi= INVERTER LCL FILTER RLC LOAD Vref Verror KpKi Iref Ierror KpKi PI CONTROLLER Gi= INVERTER LCL FILTER RLC LOAD Fig. 7. Block diagram of current controlled inverter Correpondingly tranfer function of PI controller block i given a Fig. 8. Vin LF Lf ID CF RL LL CL LCL filter and parallel RLC load circuit The tep to obtain tranfer function [2] of thi tage are given below. ) For the parallel RLC load, the equivalent impedance i given a 2) By uing KVL to the mehe, the tranfer function i obtained a ( ( ) ) 3) Uing (5) and (6) the fourth order tranfer function of the filter and load cheme i obtained and i given in (). 4) The value of and of the PI controller are determined by uing ZieglerNichol tuning formula [5]. A MATLAB function Ziegler () exit to deign PI controller uing the Ziegler Nichol tuning formula. 5) The complete tranfer function of the block diagram i then obtained by block diagram reduction method and thereafter by ubtituting the correponding deigned value of the parameter. The tranfer function of the current control ytem i given in (2). B. Voltage control tranfer function Voltage control cheme i required during ilanding mode of operation. Fig. 9 repreent the block diagram of controlled operation. Fig. 9. Block diagram of controlled inverter Thi cheme conit of an inner current control loop and an outer control loop. The tranfer function of individual block, uch a PI controller, inverter, filter and load remain the ame a in current control tranfer function. The ame methodology o a to find tranfer function in current control cheme i followed here alo. The tranfer function of the control ytem hence obtained i given by (3). C. Current control and control tability The control method ued here ha two operating mode, current control and control correponding to grid connected and intentional ilanding operation of microgrid. The tability of the current and controller can be determined by uing their tranfer function [4]. Stability analyi i carried out by uing the conventional control theory. According to it, the bode plot of the controller tranfer function i plotted uing the SISO deign tool in MATLAB. The poitive Gain margin in the bode plot of both the tranfer function of correponding controller indicate that the ytem i table. IV. DYNAMIC MODELING AND SIMULATION A. Simulation reult To invetigate microgrid operational mode, the effect of deigned current controller, controller, propoed intentional ilanding detection algorithm and recloure algorithm, the MATLAB/SIMULINK i ued to develop a domain imulation model of the tudy ytem. Electrical power ytem component are imulated with a phyical modeling product called impower ytem upported by MATLAB. The imulation have been run with the dynamic model hown in Fig. to invetigate the behavior of grid connected and intentional ilanding mode of operation. Inide the current and regulator block, there exit the cheme hown in Fig. 3 and Fig. 4. Similarly inide ilanding detection algorithm and reynchronization controller block, there exit the circuit correponding to Fig. 5 and Fig. 6. For both the cae the parameter ued for imulation i given in the Table I [4]. In the dynamic model depicted here, the inverter i connected through a filter and a circuit breaker to the local load. The imulation are conducted here with the aumption that the irradiation variation are completely abent in thi ytem. Two cae tudie baed on the influence of ynchronization controller are conducted to examine the ytem performance during grid connected and intentional ilanding mode.

5 Fig.. Dynamic model of microgrid with controller. 2 logical value Fig.. Tranition from grid connected to ilanding mode. 5 5 Fig. 4(b) Line Voltage with controller 2 frequency magnitude (pu) Fig. 2. Variation in parameter during ilanding (a) Frequency (b) Magnitude Fig. 5. Phae waveform (a) without recloure controller (b) with recloure controller 25 current Fig. 3. Line Current without current controller Fig.4(a). Line Voltage without Voltage controller Fig. 6. Synchronization for grid reconnection (a) without recloure algorithm (b) with recloure algorithm

6 B. Without and with ynchronization controller At firt, imulation wa conducted when the microgrid i connected to the utility grid without any ynchronization controller. The grid wa diconnected by etting the tatu in three phae circuit breaker a cloed initially and tranition at.3. Fig. 3 and Fig. 4(a) how the inverter current and at the point of common coupling. The occurrence of tranient at.3 i een clearly. Occurrence of large tranient during the intant of grid reconnection i undeirable. Fig. 5(a) how the phae at PCC without the implementation of any reynchronization algorithm. Fig. 6(a) how per phae at both ide of PCC in the abence of reynchronization controller. It can be oberved that the ytem take longer duration to achieve the ynchronization, which i quite undeirable for the load. Grid current doen t get influenced under the effect of ynchronou controller, a there will not be any upply of current to the load from the utility grid during tandalone mode. The grid wa then reconnected at.6. The microgrid continuou to operate in ynchronou ilanding mode until both the utility ytem and the microgrid ytem i reynchronized. Fig. 6(b) how the ynchronization of at both ide of the PCC in the preence of reynchronization controller. On invetigating from the beginning of intentional ilanding mode, the ytem achieve ynchronization in much le comparing with that ytem without ynchronization controller. With the implementation of reynchronization controller algorithm, the DG i forced to track the at the grid. On completion of the ynchronization, the DG i reconnected to the grid, and the controller will be witched from the to the current control mode. Fig. 5(b) how the phae with the ynchronization algorithm implemented. Thi graph depict the effectivene of reynchronization algorithm in avoiding hard tranient during grid reconnection. TABLE I DESIGNED VALUES OF PARAMETERS USED FOR SIMULATION Symbol Value Decription mh Filter inductance 3µF Filter capacitance.5 mh Filter inductance.535mf Load capacitance 4.585mH Load inductance 4.33Ώ Load reitance V dc 4V Dc ource R grid. Ώ Grid reitance L grid.mh Grid inductance K p, K i.24,.2 Voltage loop PI contant value K p, K i.8, 5 Current loop PI contant value f 6hz Sytem frequency f Khz Switching frequency V. CONCLUSION Current and Control technique have been developed for grid connected and intentional ilanding mode of operation uing PI controller. An intentional ilanding detection algorithm reponible for witching between current control and control i developed uing logical operation and proved to be effective. The reconnection algorithm coupled with the ynchronization controller enabled the DG to ynchronize itelf with the grid during grid reconnection. 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