i Landing

Abstract - This paper investigates fault events that lead to is-landing of a distribution subsystem and formation of a micro-grid. The micro-grid includes two distributed generation (DG) units. The units used here are conventional rotating synchro-nous machine and DFIG unit. The studies shows that an appro-priate control strategy for the DG unit can ensure stability of the micro-grid during islanding transients. This are performed using the DIGSILENT power factory software package. Keywords: distributed generation, distributed resources, islanding, micro grid.

I. INTRODUCTION The significant potential of smaller DER (< 100 kW/unit) to meet customers’ and utilities’ needs can be best captured by organizing these resources into MicroGrids. MicroGrids are envisioned as clusters of generators, storage, and loads that are operated as single controllable systems. MicroGrids can operate both connected to and synchronized with the utility distribution grid and in isolation from the utility distribution grid (as an “island”). MicroGrids represent an entirely new approach to integrating DER, especially small generators, into utility distribution systems. To realize the emerging potential of distributed generation one must take a system approach which views generation and associated loads as a subsystem or a “microgrid”. During disturbances, the generation and corresponding loads can separate from the distribution system to isolate the mi-crogrid’s load from the disturbance (and thereby maintaining service) without harming the transmission grid’s integrity.[1] Formation of a micro-grid due to an islanding process can be due to disturbances, such as a fault and its subsequent switching incidents, or due to pre planned switching events. After islanding, the micro-grid is to remain operational in an autonomous mode and meet the corresponding load require-ments. The objective of this paper is to demonstrate the micro grid operation during fault event. To conduct the proposed stud-ies, a study system is defined and the corresponding DIg-SILENT power factory simulation model is developed. The study system is shown in Section II. Section III will discuss the islanding process to form microgrid. Study cases are dis-cussed in Section IV.

II. SYSTEM MODEL A single line diagram of system is shown in fig.(a) to investi-gate the operational scenario of microgrid due to fault event. The system configuration and parameters has been taken from the benchmark system of the IEEE Standard 399-1997,

with some modifications to allow for autonomous micro-grid operation. In this system a 13.8-kV, three-feeder distribution subsystem is shown which is connected to a large network through a 69-kV radial line and substation transformer. The 13.8-kV distribution substation is equipped with a three-phase 1.5-MVAr, fixed shunt-capacitor bank. The network at the end of the 69-kV line is represented by a 69-kV, 1000-MVA short-circuit capacity bus. A combination of linear and non linear loads (L1 to L5) is supplied through three radial feeders of the subsystem. The system also includes two DG units, i.e.DG1(5-MVA)and DG2 (2.5-MVA) on feeders F1 and F3 respectively. DG1 is synchronous machine of 5 MVA and DG2 is DFIG of 2.5 MVA. TABLE I represents the load parameters.

The component models used for the simulation are as fol-lows. The main grid is represented by a 69-kV three-phase voltage source with the short-circuit capacity of 1000 MVA and X/R ratio of 22.2. The substation 69/13.8-kV transform-er and the load transformers are represented as linear, three-phase transformers with the appropriate winding connec-tions, Fig. 1.DG1 is modeled as synchronous machine of 5 MVA and DG2 are modeled as DFIG unit of 2.5 MVA.

III. ISLANDING PROCESS LEADS TO MICROGRID FORMATION

The islanding process require that some of the generation, storage and corresponding loads need to separate from the distribution system to isolate sensitive loads from the dis-turbance (and thereby maintaining service) without harming the integrity of the remaining T&D system[2].

A micro-grid is expected to remain operational after separa-tion from the system. In Fig. 1, the 13.8-kV distribution sys-tem, including the loads and the two DG units, constitutes the micro-grid. The formation of a micro-grid due to islanding phenomenon can be due to either preplanned or unplanned switching events. In the case of a preplanned micro-grid formation, appropriate sharing of the micro-grid load amongst the DG units and the main grid may be scheduled prior to islanding.

Thus, the minimal transients may occur in islanding process and the micro-grid continues operation, as an autonomous system. In the context of this paper, by scheduled opening of the circuit breakers at both ends of the 69-kV line, a pre-planned islanding of the 13.8-kV system can happen e.g., for line maintenance.

Micro-grid formation due to unplanned islanding is due to

Micro Grid Behaviour including DFIG for Different Fault Locations

1Anjali Jain, 2Nikita Sharma 1Assistant Professor, 2M.Tech Student (Power Systems)

Amity University, U.P., India e-mail: [email protected], [email protected]

Volume 2, Issue ICRASE12, November 2012, ISSN Online: 2277-2677 120

International Journal of Systems , Algorithms &

Applications IIII JJJJ SSSS AAAA AAAA

either a fault and its subsequent switching incidents or some other unexpected switching process. Prior to islanding, the operating conditions of micro-grid could be widely varied, e.g., the DG units can share load in various manners and the entire micro-grid portion of the network may be delivering or importing power from the main grid [3]. The severity of the transients experienced by the micro-grid, due to an unplanned islanding process, is highly dependent on [3]

i) The pre-islanding operating conditions, ii) The type and location of the fault that initiates the is-

landing process, iii) The islanding detection time interval, iv) The post-fault switching actions that are envisioned for

the system and v) The type of DG units within the micro-grid.

After islanding, the microgrid can be connected to the main grid only once restoration of the main system and the micro-grid is achieved i.e when system voltages and frequencies have returned to and been maintained in a normal range for a reasonable period of time. This reconnection must be carried out through proper synchronization of the micro-grid to the utility at the point of common coupling (PCC). Limits have been proposed for acceptable voltage magnitude error, fre-quency error and phase-angle error between the micro-grid and the main grid. For micro-grids with DG units in 1.5-MW to 10-MW range, reconnection is acceptable if voltage error is below 3%, frequency error below 0.1 Hz, and phase-angle error below 100 [4]. Adhering to these limits ensures reconnec-tion of the micro-grid may be achieved with minimal transi-ents in the overall system.

Table I. Feeder Loads and their Equivalent Impedances

IV. STUDY CASES Two case studies are conducted to examine the 13.8-kV sys-tem operation in grid-connected mode and during islanded mode. Case studies are chosen to illustrate both the steady state response to the changes in the system operating point, and the dynamic response when the system undergoes a tran-sient. Case (1): Grid-Connected Mode: In the grid-connected mode, real power outputs from DG1 and DG2 are set at pre-specified levels. Thus any change in power demand by loads L1 to L5 is accommodated by the main grid. Initially the total load is 3.2-MW and 1.77-MVAr of which 90% is supplied by DG1 and DG2 and the rest by the main grid .L4 is then energized at time t=1sec Fig. 2 illustrates the impact of L4 energization on the bus voltages and the power output of DG1, DG2 and the main grid .Fig. 2 shows that the sensitive load voltage, bus 3, is maintained within ±1%. Fig. 2 also indicates that the real power of L4 is fully supplied by the grid. DG2 only adjusts its injected reactive power to maintain the bus voltage. 81% of the load is supplied by DG units and the rest imported from the main grid

MAXIMUM POWER ( MW+MVAr)

EQUIVALENT IMPEDENCE

(Ω)

FEEDER 1 0.8MW, 0.47Mvar 1.5MW, 1Mvar

176.91+ j104.00 88.17+ j58.48

FEEDER 2 3.2 MW, 1.9Mvar 48.25+ j17.04

FEEDER 3 0.9 MW, 0.6Mvar 0.9 MW

119.02+ j104.95 368



Fig 1(a) System Model



Case (2a): Three Phase Fault at 69 KV Bus: This case investigates the transient behaviour of the micro-grid due to a islanding scenario caused by three phase fault at 69 KV bus. Prior to islanding, DG1 and DG2 supply 3.0-MW and 0.5-MW respectively to the local load, i.e., 88% of the total load, and the rest is imported from the main grid. At t=1sec a three phase fault occurs at 69-kV line and the real power set point of DG2 gets increased. DG1, through the ac-tion of its Governor, adjusts itself to supply the rest of the micro-grid real power demand according to its dynamic re-sponse time. DG2 also controls its injected reactive power to maintain the voltage at bus 3. Fig. 3 shows the system transi-ents due to the islanding. Fig. 3(a) and 3(b) shows the varia-tions of the voltage at bus1and bus3 respectively. From fig3(b), there is further drop at 1.25 sec in voltage due to delay in reactive power supply. Fig3(d) and 3(e) also shows power sharing of the DG units after islanding, , where the response times of the real/reactive power controllers are shown. The study results indicate that the micro-grid can maintain the de-sired power quality, even for the sensitive load, during the islanding transients. The CB’s of the 69-kV line employ triple-pole auto-reclosure and attempt to re-connect the micro-grid to the main grid after the fault clearing, i.e., . Since the fault is temporary and clears before reclosure, the reclosure is suc-cessful. Case (2b): Three Phase Fault at Bus3: When fault occurs at bus3 at t=1sec, voltage of bus 1and bus 3severely drops [Fig. 4(a) and (b)] and the reactive power control of DG2 reaches its limit [Fig. 4(e)]to inject the maxi-mum reactive power in the system. Upon clearing the fault at t=1.15, control actions in DG1 and DG2 eventually return the voltage to its normal range. However, due to transients, large voltage variations [Fig. 4(a) and (b)] cannot be prevented. Fig. 4(d) and (e) also show power sharing between DG1 and DG2 after the fault. Fig. 4 shows the system transients due to the

reclosure. The reactive power of DG2 is controlled to regu-late the voltage on the sensitive load bus. When fault occurs there is a high voltage drop at bus 3[ fig4(b)] and reactive power is supplied by DG2 to maintain the voltage level to its limit. The active and reactive power of the main grid is not much effected by the fault at bus 3 as shown in fig4(d) and fig4(e).

V. DISCUSSION Two assumptions are made in the given study model. First ,the study model of micro-grid is assumed to be balanced, i.e., no single-phase loads or unbalanced three-phase loads are con-sidered. Secondly, the two DG units within the micro-grid, due to their different time constants, do not exhibit any dy-namic interactions . Based on the case studies, following conclusions can be made: During grid connected mode, the micro-grid operation is de-termined by the load requirements and the available capacity of DG units. Normally, utility grid supply the variations in load demand primarily. DG units can be controlled to provide the voltage regulation or reactive power compensation of the sensitive load bus, where fast real power support is not the top concern. In case of pre-planned islanding, DG units set points are ad-justed for autonomous operation prior to islanding to mini-mize the transients. The study indicates that the micro-grid can maintain the desired power quality, even for the sensitive load, during the preplanned islanding transitions. Fault studies verify that successful transfer into autonomous micro-grid operation may be achieved under the investigated fault scenarios. Successful transfer does, however, require islanding detection and a subsequent change in control strate-gy of micro-grid DG units.



Fig2(a) Fig2(b) Fig2(c)

Fig2(d) Fig2(e)

Fig.(2) Modeling results for Load change




Fig3(d) Fig3(e)

Fig.(3) Modeling results for fault at 69KV bus


Fig4(d) Fig4(e)

Fig.(4) Modeling results for fault at Bus 3





Simulation results illustrate that reclosure after a temporary-fault is feasible. Although successful transfer to autonomous islanded operation of the micro-grid will eventually result, transiently the system undergoes severe disturbances due to the unsuccessful reclosure attempts. The reclosure attempts cause large currents to flow out of the micro-grid and into the fault, resulting in severely depressed micro-grid bus voltages. However, the adopted control strategy ensures angle stability and voltage recovery of the micro-grid and consequently en-sures continuity of supply for the loads within the micro-grid. For all scenarios, disturbances result in system frequency vari-ations and power swings within the micro-grid. The fast con-trol action of the converter interfaced DG unit can effectively damp such oscillations. In the fault scenarios, the fault has zero impedance and is im-posed on the system on the 69-kV line, in close proximity to the 13.8/69-kV transformer. Thus, the micro-grid is exposed to the most severe faults in terms of fault impedance and loca-tion. Consequently, a better performance can be expected un-der less severe transient conditions.

VI. CONCLUSION This paper investigates the operation of microgrid (13.8-KV distribution system) when connected to the grid and during

islanding due to fault event. The micro-grid is supplied by two distributed generation (DG) units, i.e., a synchronous machine and a DFIG unit. The former DG unit is equipped with excitation and governor controls . The simulation studies show that how transients occurs during fault event and how utility grid and DG units maintains the voltage quality at specified buses, mainly through its fast reac-tive power control. The studies also demonstrate that through the fast control of the DG units, the concept of micro-grid formation and its autonomous operation are technically viable.

REFERENCES [1] Lasseter, R.H., Madison, Paigi, P., “Microgrid: a conceptual

solution” in Power Electronics Specialists Conference, 2004. PESC 04. 2004 IEEE 35th Annua on 20-25 June Vol-ume: 6

[2] Extended CERTS Microgrid Robert H. Lasseter, University of Wisconsin-Madison Madison, Wisconsin

[3] Micro-Grid Autonomous Operation During and Subsequent to Islanding Process F. Katiraei, Student Member, IEEE, M. R. Iravani, Fellow, IEEE, and P. W. Lehn, Member, IEEE

[4] IEEE Draft Standard for Interconnecting Distributed Resources With Electric Power Systems, IEEEWGStd. IEEE Std. 1547-2003, July 2001.





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