A MODIFIED ISLANDING DETECTION METHOD FOR HYBRID AC/DC MICROGRIDS WITH REDUCED DETECTION TIME

Transcription

1 A MODIFIED ISLANDING DETECTION METHOD FOR HYBRID AC/DC MICROGRIDS WITH REDUCED DETECTION TIME Meam SADEGHI Tabriz Electric Power Dtribution Company Iran Shahid Beheshti University, A.C. Iran ABSTRACT Islanding detection a mandatory eature or gridconnected multi-inverter Microgrids and must be done within 2 seconds ater. However, the eectiveness o landing detection methods (IDMs) usually demonstrated by means o non-detection zones (NDZ). Among them, The Sandia requency-shit (SFS) considered as one o the eective landing detection methods that possess small NDZs. Furthermore, considering the hybrid AC/DC microgrids with both AC and DC subgrids which are connected through an interlinking converter, because o dierent types o parameters and charactertics o the mentioned subgrids, conlictions or mal-operation may occur in landing detection. Consequently, in th paper a new hybrid landing detection method, using SFS and Rate o Change o Frequency (ROCOF) relay, to eliminate NDZs has been proposed or landing detection in hybrid AC/DC microgrid. Furthermore, the landing detection method o the interlinking converter has been modiied and coordinated with converters o DC subgrid to improve its operation. Thus, the proposed method eectively combines the principles o active and passive methods that the landing can be detected aster than conventional methods. Th method tested under various operation conditions using PSCAD/EMTDC. Simulation results show that the proposed method correctly detects the landing operation and does not mal-operate in the other dturbances such as short circuit, load variations, capacitor switching in hybrid AC/DC microgrids. INTRODUCTION Islanding an undesired situation in active modern dtribution system with DGs because it could impair the saety o maintenance service workers and/or damage loads [1]. Thereore, a quick and accurate Islanding Detection Method (IDM) essential or active dtribution systems to avoid intentional landing or upgrade the control system o DGs to operate landed during grid outage. The main concept o most o the IDM techniques remains the same that some o the system parameters (voltage, requency, etc.) change considerably with landing but not much when the dtribution system grid connected [2]. Islanding detection techniques can broadly be divided into remote and local techniques. Mojtaba KHEDERZADEH Faculty o Electrical and Computer Engineering, Shahid Beheshti University, A.C. Iran Khederzadeh@pwut.ac.ir Local methods can be categorized into passive, active and hybrid methods [2]. In passive techniques, system parameters such as voltage, requency, harmonic dtortion, etc, are continuously monitored and compared with a predetermined threshold. Based on the system charactertics, one or more o these parameters may vary considerably when the microgrid landed. The diiculty o landing detection when the load and generation in the landed system closely match the main problem o the passive techniques. Active methods are based on the perturbation and observation concept. These methods inject intentionally deliberate dturbances in the DG circuit (via its electronic control scheme) to detect landing. Although, Active methods provide a cheaper approach or landing detection and can reduce the NDZ, the problems with these techniques are that they introduce perturbations in the system and detection time slow as a result o extra time needed to analyze the system response o the perturbations [3]. Considering the advantages and the problems o active and passive methods, a suitable integration o these methods can be used to improve the landing detection procedure and reducing the problems o these methods such as NDZs and long detection time. Consequently, Hybrid methods which are a combination o passive and active methods can be proposed. It should be mentioned that so many types o active [2-3], passive [4-6] and hybrid [7] IDMs has been proposed or single inverter or multi inverter microgrids, however, these methods are not considered to be applied in a AC/DC microgrid in literature. Considering the hybrid AC/DC microgrids, because o dierent types o parameters and charactertics o the mentioned subgrids, conlictions or mal-operation may occur in landing detection. Consequently, Th paper presents an integration o Sandia requency-shit (SFS) (active method) and Rate o Change o Frequency (ROCOF) relay (passive method), as a hybrid landing detection technique to overcome the short comings o both active and passive techniques in hybrid AC/DC microgrids. PROPOSED MODIFIED ISLANDING DETECTION METHOD FOR HYBRID AC/DC MICROGRID Among Various requency landing detection methods, CIRED 215 1/5

2 Sandia requency shit (SFS) one o the most eective methods in landing detection or active dtribution networks [6-8]. Furthermore,, among passive IDMs, the Rate o Change o Frequency (ROCOF) relay one o usual and general methods [8]. Consequently, in order to present the modiied hybrid IDM method that integrates SFS method with ROCOF relay, a brie description and most important requirements o these methods are presented as ollows: Sandia Frequency Shit (SFS) IDM Th method tries to ampliy small changes in requency. Consequently, or landed mode, the process produce a phase error in which continues until the requency exceeds the threshold [6]. Th method tries to ampliy small changes in requency. Consequently, or landed mode, the process produce a phase error in which continues until the requency exceeds the threshold [6]. For the SFS landing detection method, the inverter phase angle ( inv ) can be expressed as a unction o the land requency, requency o the grid prior to c and k [7]: landing,, and the SFS parameters g = π( c + k ( )) / 2 (1) inv g The load phase angle ( load ) a unction o land requency, the load resonant requency and the load quality actor Q [7] load = (2) 1 tan [ Q ( / / )] Considering, SFS method, the phase criterion can be extracted by equating the two phase angles in equations (1) and (2): + tan[ π ( c + k ( )) / 2] / Q = (3) 2 2 g From (3), it can be demonstrated that the NDZ depends on the SFS parameters ( c and k ) and load parameters ( and Q ) [6]. To eliminate the NDZ, it necessary to ensure that or all possible loading scenarios, the load and DG phase-angle inter-section point an unstable operating point. Th condition can be expressed as [6]: dϕ d load dϕ inv < (4) d Considering, equations (3) and (4), the value o k parameter can be selected to ensure that the requency will drit away rom the equilibrium point deined by the phase criterion. Dierentiating the two expressions or the phase angle in (1) and (2) with assuming = the value o k parameter can be expressed as [7]: 4Q k > (5) π Rate o Change o Frequency (ROCOF) Relay IDM ROCOF relay which categorized in passive IDM monitors the voltage waveorm and trips the breakers when the measured requency change rate exceeds a preset value or longer than a pre-set time period. The ROCOF relay settings are chosen such that the relay don t operates or luctuations governed by utility time constants [25]. ROCOF relay utilizes the generator swing equation to deine ROCOF as ollows [8-9]: t P = 2HG Where P the change in output power, the system requency, H the inertia constant o DG system and G the rated capacity o DG system. The block diagram which has been used to design the ROCOF relay illustrated in ig. 1. Fig. 1. Block diagram o ROCOF relay design. The proposed hybrid IDM method integrates SFS method with ROCOF relay. Consequently, requency drit will occurs immediately ater landing. With requency drit, ROCOF relay detects the change o requency. Thus, i the rate o change o requency exceeds the threshold more than 3ms, the ROCOF relay will change the control system mode rom grid-connected to landing and then the voltage and requency control o DGs are activated. It should be mentioned that, using proposed method or landing detection, the requency doesn t need to exceed the threshold. So, landing can be detected using the rate o change o requency, even or zero power mmatches, beore the requency exceed limitations o system. Furthermore, because o using the rate o change o requency instead o requency drit or landing detection in th proposed method, there will be no more need to select large values or SFS parameters. However, the parameters should be selected as a way that landing situation can be dtinguhed rom other normal events in grid-connected mode. For example, parameter k, which the positive eedback gain o SFS control block, may cause in instability o DG units, especially or small microgrids i its value increased in order to detect the landing aster. While, using the proposed method, the landing detection will be accelerated without need to (6) CIRED 215 2/5

3 increase the k. For th method, the threshold value o SFS method will be selected as the base o normal events and the threshold value o ROCOF relay will be set considering the selected base. A threshold setting o.3hz/sec has been ound to be the optimum value, with.3s.7s operating time or the proposed method considering the mentioned rules. The main lowchart o proposed IDM which has been applied in interlinking converter illustrated in ig. 2. bidirectional converter. Each subgrid contains two DGs operating in parallel at both grid-connected and landed modes. The voltage source converter (VSC) used as an interlinking converter in order to connect AC and DC subgrids. urthermore, the control system o the DGs manage the output power o DGs. Considering the gridconnected and landing modes, two control strategies should be applied in control system o all VSCs. coordinated with interlinking conv. with proposed IDM coordinated with interlinking conv. Fig. 3. Hybrid AC//DC system with proposed IDM Fig. 2. Flowchart o the proposed methodology. The Studied Hybrid AC/DC Microgrid with Interlinking Converter Throughout the last decade, a number o studies have proposed the DC-AC hybrid MG system structure and control strategies to save energy, thereby reducing cost and increasing reliability. Dcrimination o the DC power collection and consumption eeder rom the conventional AC power dtribution line provides several beneits. Firstly, each DC dtributed generator does not have to install AC integration equipment to cope with AC integration problems, such as AC conversion and line synchronization. Secondly, DC loads can be directly ed rom DC sources without any power quality dorders which may aect the main AC dtribution system. Thus, it reduces power electronic equipment and maintenance costs or individuals, and promotes joining small scale domestic or even large scale urban systems to microgrid utilization [11-12]. Considering the utures o hybrid AC/DC microgrids, the main test system in which the proposed hybrid methodology tested, has been proposed in Fig. 3. The studied hybrid AC/DC microgrid consts o two AC and DC subgrids which are connected through an interlinking In grid connected mode, the two DG systems o both AC and DC microgrids adopt Constant Current Controller to provide local power and the interlinking converter manages the power transer between subgrids. Th coniguration uses dq space rame and reduces the burden o generation and delivery o power directly rom the utility grid and enhances the immunity o critical loads to system dturbances in the utility grid [1]. In landed mode, one o DG units must be used as a reerence source to control the voltage and requency o microgrid. For th test system, DG1 has been chosen or th purpose. Consequently, right ater landing detection, the control strategy o DG1 inverter must be switched rom Constant Current Control mode to voltage and requency control mode. Furthermore, th control strategy should supply reactive power o loads using DG1. The proposed control o microgrid or th microgrid Reverse Droop control. Th control system uses two P- Droop and Q-V Droop schemes as the ollowing equations [1]: Pn P = n (7) m Q V = V (8) n CIRED 215 3/5

4 The applied voltage and requency control using Droop control concept shown in ig. 4. in Figs 5 and 6, the requency and voltage o AC subgrid are almost constant during landed mode with zero power mmatch. Fig. 4. Block diagram o voltage and requency control using Droop control. SIMULATION RESULTS To veriy the eectiveness o the proposed hybrid IDM, a hybrid AC/DC microgrid consting o two DGs operating in parallel at both subgrids are implemented on PSCAD/EMTDC as shown in igure 3. During landed mode o multi inverter AC/DC microgrid, the AC subgrid system control method called master-slave operation, which means that one o DGs acts as the master and the others as ailiates. While, in grid-connected mode all DGs adopt Constant Current Control. Consequently, in landed mode the master DG (1) turns to Droop control to provide voltage and requency reerence to the other DGs and interlinking converter. For simulated scenario, the landing event occurs at t=1sec and the electrical parameters o system and loads are shown in Table I. Table I. The electrical parameters o simulated system Parameters Voltage 22V AC, 4V DC requency 6 Hz PI: k i =1, k p =.2 AC DG 1 1/n=15*1-4, 1/m=7*1-5 AC DG 2 PI: k i =1, k p =.2 Interlinking PI: k i =11, k p =.25 converter AC Load R=2.24, X=.1 DC Load R=2.34, X= In th scenario the AC/DC microgrid has been simulated considering zero power mmatch between DGs generation and loads consumption which can be supposed as the worst condition or landing detection. As shown Fig. 5. Frequency o microgrid with zero power mmatch o system without using active IDM. Fig. 6. Voltage o microgrid with zero power mmatch o system without using active IDM. It should be mentioned that in th scenario, i just a ROCOF relay method used without any active IDM, the landing couldn t be recognized. Thus, using the proposed hybrid IDM, the microgrid landing has been detected even or zero power mmatch with no NDZ. Then, the microgrid landing has been recognized within.3sec and the ROCOF relay will operate. Consequently, the control system o DG1 can completely replace Constant Current mode with Droop control. inally, the Droop control compares the voltage and requency o landed microgrid with nominal values and tries to minimize their dierences by generating required active and reactive power o DG units. Th process has been completed at t=1.7sec, using the proposed hybrid IDM, as shown in ig. 7. While, th time or conventional SFS IDM at t=2.7sec which demonstrates 1sec delay compared with proposed hybrid method. Furthermore, the requency o microgrid varies so slightly in proposed method compared with conventional SFS method CIRED 215 4/5

5 Fig. 7. Frequency o microgrid with zero power mmatch o system. CONCLUSION In th paper, A hybrid IDM has been proposed to detect landing o an AC/DC microgrid system with multiple DG units operating at both grid-connected and landed modes. It integrates the SFS (active method) and ROCOF relay (passive method), to overcome the short comings o both active and passive techniques. Coordinated setting o ROCOF relay and SFS parameters which specialize the ROCOF relay operation just or SFS generated dturbances, decrease the values o positive gains and help the IDM system not to mal-operate in the other dturbances such as short circuit, load variations, capacitor switching. Furthermore, using proposed hybrid method, the landed mode has been detected aster than using conventional SFS method. Thus, the variation o requency and voltage domain with the duration o transients has been decreases, which could improve the stability o microgrid especially in landed mode. The proposed method very simple and Simulation results, considering a hybrid AC/DC microgrid, highlight the eectiveness o the proposed approach especially the advantages o the proposed method: NDZ elimination and aster landing detection compared with conventional method. REFERENCES comprehensive survey, Electric Power Systems Research79 (29) [4] G. Hung, C. Chang, and C. Chen, Automatic phase shit method or -landing detection o grid connected photovoltaic inverter, IEEE Trans. Energy Convers, vol. 18 (1) (23) [5] J. Yin, L. Chang, and C. Diduch, A new adaptive logic phase-shit algorithm or anti-landing protections in inverter-based DG systems, IEEE Power Electronics Specialts Con, (25) [6] H.H. Zeineldin, Conti, Sandia requency shit parameter selection or multi-inverter systems to eliminate non-detection zone, IET Renew. Power Gener. vol. 5 (2) (211) [7] H. H. Zeineldin, E. F. EI-Saadany, and M. M. A. Salama, Impact o DG interace control on Islanding detection and nondetection zones, IEEE Trans. Power Del. vol. 21 (3) (26) [8] M.A. Redern, O. Usta, G. Fielding, Protection against loss o utility grid supplyor a dpersed storage and generation unit, IEEE Trans. Power Delivery vol. 8 (3) (1993) [9] Freitas W, Wilsun X, Aonso CM, Zhenyu H. Comparative analys between ROCOF and vector surge relays or dtributed generation applications, IEEE Transactions on Power Delivery 25; vol. 2: [1] Liping Su, Guojie Li, Zhijian Jin, Modeling, Control and Testing o a Voltage Source Inverter Based Microgrid, IEEE 4th DRPT International Cone.(211) [11] J. M. Guerrero, J. C. Vasquez, J. Matas, L. G. de Vicuna, and M. Castilla, Hierarchical control o droop-controlled ac and dc microgrids A general approach toward standardization, IEEE Trans. Ind. Electron., Jan. 211, vol. 58, 1, [12] Poh Chiang Loh, Ding Li, Yi Kang Chai, and Frede Blaabjerg, Autonomous Control o Interlinking Converter WithEnergy Storage in Hybrid AC DC Microgrid; IEEE Transactions on Industry Applications, MAY/JUNE 213 vol. 49, [1] K. N. Edhura K. Ahmad, J. Selvaraj, N. Abd Rahim, A review o the landing detection methods in grid-connected PV inverters, Renewable and Sustainable Energy Reviews21 (213) [2] A.H. Mohammadzadeh Niaki, S. Asharnia, A new passive landing detection method and its perormance evaluation or multi-dg systems, Electric Power Systems Research 11 (214) [3] S.P. Chowdhury, S. Chowdhury, P.A. Crossley, Islanding protection o active dtribution networks with renewable dtributed generators: A CIRED 215 5/5