Open Loop Synchronization of Micro-grid to System Based on Impedance Pre-insertion

Transcription

1 First Version Open Loop Synchronization of Micro-grid to System Based on Impedance Pre-insertion First A. Author, Fellow, IEEE, Second B. Author, and Third C. Author, Jr., Member, IEEE Abstract This paper presents an open loop synchronization strategy for connecting an islanded Micro-grid to system. This strategy adopts the impedance pre-insertion method to reduce the synchronizing transients to be within an acceptable level, as long as both the Micro-grid side and system side are operating within the power quality requirements. To verify the feasibility of the method, it is applied to a sample case study. The results have shown that the size of impedance can be designed properly to effectively mitigate the transient phenomenon, and at the same time, the impacts to the stability of the generating units are within the normal limit. Index Terms microgrids, islands, open loop synchronization, impedance Pre-insertion D I. INTRODUCTION UE to electricity market deregulations and the need to reduce environmental footprints from centralized electric power plants, distributed generations (DG) and renewable resources have been gaining enormous support in both research and industrial applications. A Micro-grid is formed by an aggregation of DG units and local loads that is in complete autonomous operation (as an island) from the rest of the power grid []-[]. A fundamental component of the Micro-grid is the existence of a synchronizer, which allows the connection of the Micro-grid to the rest of the bulk grid. This process of connecting two operating electric systems is known as synchronization. Usually, there are 3 common synchronization scenarios to consider: - A single synchronous machine synchronizes to grid - Micro-grid (islanded system) synchronizes to grid - A grid synchronizes with another grid (During system restoration) Similar to a single synchronous machine to system synchronization, Micro-grids synchronization must be done carefully. If the synchronization criteria are not met at the moment when breaker closes, a high transient may occur, which will result in equipment damage and power quality concerns [9]-[0]. Faulty synchronization can: - Damage transformer and synchronous generator windings primarily due to the peak inrush current, which may also lead to misoperation of feeder relays [3]. Y. Zhou is with the Electrical Engineering Department in University of Alberta, Edmonton, T6G H9 Canada ( yaxiang@ualberta.ca). - Damage the generator due to the sudden mechanical stresses experienced by the generator shaft and prime mover []. - Cause generators to become unstable or unable to maintain in synchronism with the system primarily due to an initial out-of-step closing of breaker, which may lead to the relay tripping of generators [3]. From the synchronization concerns addressed above, the problem of synchronization is to reduce switching transients and ensure stability of all generators. The standard synchronization practice is to adjust the voltage difference V, angle difference θ, and frequency difference f into an acceptable range before the synchronism check relay sends the signal to close the breaker [5]. If the acceptable range is not met, the generators within one system are tuned based on remote sensing and feedback control of the frequency and voltage signals from the breaker location. Case studies of Micro-grid to system synchronizations based on feedback control strategies have been presented in literature [6], [7]. Active synchronization strategy that takes into account of voltage unbalances and harmonic distortions is presented in [4]. In order to implement such feedback control systems in reality, infrastructure support such as communication links must be built to carry the signals from remote sensing to local machines. The cost of this infrastructure support can be quite high especially when the Micro-grids are in remote areas, where it is located far away from the point of interconnection to the main grid. In addition, synchronizing a Micro-grid to system is more difficult than synchronizing a single generating unit to system due to the fact that the Micro-grid s voltage and frequency are determined by multiple DG units and local loads. In other words, for Micro-grid to system synchronization, multiple communication links would have to be built to allow the tuning of individual DG units. Therefore, synchronizing a Micro-grid to system by feedback control can not only drive up the cost of infrastructure support, but also may increase the time and effort to synchronize the two islands. Especially during system restoration process, the coordinated control of multiple generators within one system through feedback may delay the time required to synchronize any surviving islands [8]. Various transient mitigation methods have been implemented in practice in different industrial applications. The feedback control method used in generator synchronization to reduce the voltage difference across the

2 First Version breaker is only one of them. Other methods include: impedance pre-insertion has been widely used in industry for reducing the capacitor switching transients [4]; Point-onwave switching used in capacitor or transformer energization [5]; Silicon-Controlled Rectifier (SCR) based soft starter used in motor starting to reduce inrush current [6]; and Sequential phase energization for transformer [7]. This paper proposes an impedance pre-insertion based synchronization strategy that can connect the Micro-grid to system in an open loop manner. The main advantage of this strategy is that the synchronization can be performed without any feedback controls, i.e. reduces the cost and time required for Micro-grids synchronization. The rest of the paper is organized as follows. Section II presents the proposed method for open loop synchronization in detail. In Section III, the issues to consider for the proposed open loop scheme are addressed. Simulation results are shown in Section IV and the paper concludes in Section V. II. PROPOSED METHOD OPEN LOOP SYNCHRONIZATION The open loop synchronization scheme is to design the impedance pre-insertion value such that the peak transients are acceptable, when the two parties are operating within the power quality limits (PQ limits) as shown in Fig.. As shown, impedance is inserted in series with breaker (BRK). This impedance is purely an inductor. After breaker is closed and system has reached a new steady state, the impedance is bypassed by closing breaker (BRK). It is important to realize that both parties typically operate within their power quality limits (PQ limits) before the synchronization process. They are also the pre-established operating regions. The operating regions [9] (i.e. the power quality requirements) are established by the utility to ensure the grid operates safely and reliably during acceptable system disturbances. If both sides of the breaker are not operating within their normal operating range, the synchronization process is not permitted. Based on Fig., once BRK is closed, a transient current will flow through the impedance, which will propagate to the DG units within the Micro-grid. This transient current is composed of an AC and a DC component similar to the case of a three-phase fault at the generator terminal. The influence of the DC component on the peak transient current can be taken into account by applying a factor K to the AC component [8]. The peak current is reduced as the size of X insert increases as shown by (). The peak current I peak is proportional to: V V I peak " X X X X total insert d eq () where V is the rms voltage across breaker before it closes. X insert is an inductor inserted at the breaker location as shown in Fig.. X d and X eq are the synchronous machine s direct axis subtransient reactance and equivalent circuit impedance respectively. X eq includes any transformer leakage reactances, series line impedance (positive sequence) and system impedance. BRK Z insert BRK Circuit Breaker fg Generator, Microgrid ( Hz) 60. Bulk Grid Pre-established. 0.9 Pre-established Operating Region Vg ( pu) fs ( Hz). Operating Region V ( pu) Fig.. Power quality requirements for open loop synchronization The open loop synchronization procedure is developed in Fig.. As shown, all the measurements including voltage, angle and slip are performed at the circuit breaker location. In addition, if both parties are not operating within their respective PQ limit, then it implies that the power system potentially has a power quality issue, which should be resolved first before synchronization is performed. When both sides are naturally within PQ limits under the normal system condition, breaker can be closed at a small angle difference. This angle difference is explained in more detail in the Section III. Once the circuit breaker is closed, the impedance could be bypassed shortly by closing BRK after system stabilizes and reaches a new steady state operation. Fig.. Procedure for performing open loop synchronization In order to implement the open loop synchronization scheme, there are some fundamental issues, which have to be considered. These issues will be addressed in the next section. III. ISSUES TO SOLVE The issues that need to be solved in this section include: - What is the acceptable level of transients? - How to determine the proper impedance value? - Can the Micro-grid maintain stability with the designed impedance? - Are the impedance bypass transients acceptable? s

3 First Version 3 A. Acceptable Transient Level Acceptable transient level is calculated based on the synchronization criteria from the standard IEEE C50. for salient-pole synchronous generators [0]. The standard specifies, Generators shall be designed to be fit for service without inspection or repair after synchronizing that is within the limits listed. The limits are: - Angle ±0 - Voltage 0 to 5% (of nominal) - Slip ±0.067Hz In other words, the current and torque transients experienced by the generator under the above conditions are considered acceptable. The IEEE limits established above can be illustrated by an acceptable transient region shown in Fig. 3. The worst acceptable transient is obtained based on examining the peak current incurred due to the conditions specified by the four corners of the region in Fig. 3. Based on the short circuit theory, higher voltage difference across breaker leads to higher transients, thus, only the right two corners are remained to examine. The worst case lies in the case V = +0.05p. u., f = Hz due to the fact that higher machine frequency than system immediately after the closing of breaker tends to advance the rotor angle and the transient torque. (-0.05pu, Hz) (-0.05pu, Hz) f (Hz) Acceptable Transient Region Fig. 3. Determine the worst acceptable transient level B. Impedance Value Design Worst Acceptable Transient (+0.05pu, Hz) V (pu) (+0.05pu, Hz) The method to effectively design the appropriate size of impedance to limit the worst transients under the open loop scheme is developed in this section. Firstly, possible worst case transients have been identified based on the PQ limits. Secondly, the peak current as a function of impedance is determined. Prior to synchronization, each party is expected to operate within their established power quality requirements, i.e. voltage and frequency are within their normal operating range as specified in Fig. 4. Although the angle requirement is 0 degrees based on IEEE standard, but an IEEE industrial survey reported that the maximum closing angle can reach up to 0 degrees when synchronized manually by an operator []. Therefore, for open loop synchronization, a worst case angle difference of 0 degrees is used throughout this paper. 0.9pu f 60.Hz Hz Micro-grid Side Operating Range 4.pu V 0.9pu 3 f 60.05Hz 59.95Hz System Side Operating Range Fig. 4. Worst case transients based on open loop synchronization It is intuitively clear based on Fig. 4 that the largest voltage and frequency mismatch will result in the largest synchronizing transients. As a result, there are four combinational cases to consider, which are case (, 3), case (4, ), case (3, ) and case (, 4). The first number refers to the operating point in the Micro-grid side, and the second number refers to the system side. Although the peak current due to closing of breaker can be obtained by repeated dynamic simulations, but the analytical expression has the advantage of simplifying the design process. Therefore, analytical expression for calculating the peak transient current as a function of the impedance value is derived. Analytically, the Micro-grid is modelled as a synchronous generator with a load connected to its terminal. Before the breaker is closed, the current through the impedance is zero. However, the stator current of the synchronous machine consists of a steady state flow of load current as shown in Fig. 5(a). At the instant of breaker switching, this switch can be represented by two opposing voltage sources in series, as shown in Fig. 5(b). By using the superposition theorem, the circuit shown in Fig. 5(b) can be split into two equivalent circuits, as shown in Fig. 5(c). SG SG jx insert I L jx insert I L V (a) jxsys V jxsys Vgrid Vgrid (c) SG '' jx d jx insert jx 4 V I L insert (b).pu V V jxsys V jxsys Vgrid Fig. 5. Equivalent circuit representation of closing breaker. (a) Circuit before breaker is closed. (b) Circuit after breaker is closed. (c) Equivalent circuits after breaker is closed. The circuit on the left side of Fig. 5(c) represents the same circuit before breaker is closed. The current in this circuit represents the steady state component. The right side circuit can be used to calculate the transient resulting from the switching. The steady state component of the stator current equals to the load current. Assuming f = 0, the analytical expression for the transient component based on Kirchhoff s Voltage Law (KVL) for the circuit is:

4 First Version 4 di() t L Ri( t) V sin( wt ) dt () where, L and R are the equivalent inductance and resistance seen by the breaker, respectively. w is the angular frequency at 60Hz. is the voltage difference phase across breaker. The solution to the differential equation () is: where, V i( t) sin( wt ) sin( ) e X R X eq eq Rt / L (3) includes the series impedances X d, X insert andx sys. equals tan (X eq /R) is the impedance angle. The peak transient stator current becomes the sum of load current and the synchronizing current computed analytically in time domain by (3). It is worth mentioning that this analytical expression is derived on the basis f = 0. However, the effects due to f from the PQ limits can be neglected in calculating the peak current. An intuitive explanation is realized by considering two voltage sources connected together with different frequencies. If one voltage source is taken as a reference running at angular frequency w, the instantaneous voltage difference when the breaker is closed at t=0 is dictated by: V V e e V e jwt j ( wt ) jwt (4) w represents the angular frequency difference between the two voltage phasors having magnitudes V and V, respectively. Based on the time window of sub-transient, the peak inrush current typically occurs within t < cycles from the initial start of synchronization. Therefore, the impact of f on V across the breaker is negligible since wt 0. C. Transient Stability Evaluation The section is to investigate the transient stability of the generator due to the impact of open loop synchronization. It is well known that the power transfer capability or the stability margin between a generator and the system decreases due to a fault on one of the parallel transmission lines. As the line trips, the equivalent impedance between the generators to the system are increased, thus the stability margin is decreased. A similar phenomenon occurs when the impedance is inserted at the synchronizing breaker location. Therefore, the goal is to ensure that the worst case maximum rotor angle reached does not go beyond the stability limit of a generator. Furthermore, the effect of high Micro-grid loading levels may aggravate the impacts on stability, so it should also be considered in the analysis. ) Generator to system synchronization The synchronization phenomenon from a transient stability point of view can be explained by a generator to system synchronization example in Fig. 6. The power angle curve (P-δ curve) is widely used for conducting transient stability analysis, e.g. application of equal area criterion based on power angle curves to find the critical fault clearing time to ensure generators remain in synchronism with the system. Similarly, power angle curves and equal area criterion can be applied accordingly to analyze the synchronization phenomenon after breaker is closed as shown in Fig. 7. E ' jxd ' jx t Synchronizing Breaker V V V f f f Zsys Infinite Bus Fig. 6. Generator to system synchronization example Before the closing of synchronizing breaker, the electric power transfer between the generator and system is zero. As the breaker is closed, the generator experiences a sudden loading condition primarily due to the relative phase difference to the grid. If the Micro-grid frequency is higher than the system prior to synchronization, the rotor angle is going to increase further until the synchronous frequency (i.e. the system frequency) is reached after breaker closes. This point where maximum rotor angle occurs is denoted by δ max in Fig. 7. The rotor during this time period is dissipating the kinetic energy offset stored initially. After δ max is reached, the rotor begins to swings in the opposite direction. Eventually, the electromechanical oscillation will stabilize due to the effects of damping. Pe Pm min o max o Power Angle (deg) o Pe max Kinetic energy dissipation due to f>f Initial angle difference o across breaker Fig. 7. Power angle curve for generator to system synchronization Based on the classical definition of the swing equation with damping ignored, the motion of the rotor dynamics are [], H d ( t) P m Pe wsyn dt (5) d () t w() t wsyn dt The electrical power transfer from the internal voltage of generator to the system after breaker is closed can be expressed as: ' EVs Pe sin( ) ' X X X d t sys (6) where, E represents the internal voltage behind direct axis transient reactance, Vs is the voltage magnitude of the system,

5 Electric Power (pu) Electric Power (pu) First Version 5 and δ is the power angle between E and V s. The first equation of (5) by taking integral can be expressed as: H w syn max 0 max d ( Pm Pe) d dt For generator synchronization, since the mechanical power P m 0, equation (7) can be integrated and expressed in a closed form to obtain the value δ max in the first swing: max cos HX cos( 0) [ ( ' )] T f f (8) EVs wsyn where, X T is the total series reactance, H is the machine inertia constant, and δ 0 = θ is the initial angle difference when breaker is closed. ) Micro-grid to system synchronization In the case of Micro-grid to system synchronization, an equivalent diagram for stability analysis is shown in Fig. 8(a). Before the breaker closes, the power is mainly consumed by the load as shown in Fig. 8(b). After the breaker closes in Fig. 8(c), the power produced by the generator is exchanged with the grid. E ' jxd ' jx L 0 Z insert jx sys Infinite Bus (7) machine terminal, which includes any local loads. and are the impedance angle corresponding to Y and Y, respectively. Similar to synchronization of generators, the theoretical δ max for the first swing when Micro-grid synchronizes to system can be obtained by substituting (9) into (7), which gives a non-linear equation: E ' V Y [sin( ) sin( )] s max H / W [ ( f f )] ( P P )( ) 0 syn m c max o (0) ' where, P E Y cos( ) represents the power dissipation in c the network from first part of (9). Due to the impact of loading, the initial power angle δ 0 in this case is the sum of the initial angle difference across breaker and the loading angle before synchronization. The impacts of load and impedance pre-insertion on the power-angle characteristic are shown in Fig. 0 and Fig., respectively. Both figures examine the case where Microgrid frequency is higher than the synchronous frequency. 5 4 Before Breaker Close After Breaker Close o E ' jxd ' Vt 0 Z L (a) E ' V V V f f f jxd ' V s jx eq Infinite Bus 3 Pm max o I L (b) Z L Fig. 8. Circuits for transient stability analysis (a) Micro-grid to system synchronization schematic diagram. (b) Before closing breaker (c) After breaker is closed. The load is modelled as constant impedances for transient stability studies. Y- transformation is used to eliminate the node V t of Fig. 8(c) such that the nodes retained are shown in Fig. 9, which are only the internal voltage E, the infinite bus, and the reference node to ground. Y E ' Z Z0 Z0 (c) Fig. 9. Equivalent circuit for one machine with load to an infinite bus From network theory, the real power at node of Fig. 9 is given by ReEI * or expressed as: P E Y cos( ) E V Y cos( ) ' ' e _ after s (9) where, Y Y0 Y, Y0 / Z0, and Y / Z. Z is the series impedance of the transmission network, including transformers, lines and the value of impedance pre-insertion. Z is the equivalent shunt impedance connected to the 0 Z L V s 0 - min o o Load Angle (deg) Fig. 0. Impact of load to Micro-grid synchronization Pm Before Breaker Close After (X Insert = 0) After (X Insert = 0. pu) After (X Insert = 0.3 pu) min 0 0 max 0 Pmax Load Angle (deg) Fig.. Impact of impedance pre-insertion to Micro-grid synchronization According to Fig. 0, increasing the loading level of Micro-grid reduces the stability margin, where the margin is defined as the ratio of the difference (P max P m ) to P max. Furthermore, by increasing the impedance size, the stability margin is decreased significantly. Therefore, it is true that both factors negatively impact the power transfer capability between the Micro-grid to the system. However, the maximum

6 First Version 6 rotor angle reach is mainly affected by the loading level of the Micro-grid and the phase difference across the breaker. Based on the open loop synchronization requirements and transient stability analysis, the worst case that should be examined in terms of maximum rotor angle advancement is based on the peak loading level and the designed impedance value. In addition, the voltage levels (Micro-grid and system sides) should be at the lowest of the PQ limits according to (9). The frequency difference should be positive (i.e. higher Micro-grid frequency than system) to result in higher rotor advancement within the first swing. As a result, the worst case open loop synchronization scenario from a transient stability s point of view is case (, 3) from Fig. 4. D. Impedance bypass consideration Similar to the first breaker switching, impedance bypass also produces a second transient effect on the system. Therefore, the peak current from the second transient must be evaluated to ensure an acceptable disturbance level. According to the principal of superposition, bypass switching can be represented by two opposing voltage sources as shown in Fig., which is equivalent to a single voltage source in the transient circuit. This circuit is quite similar to synchronization but with f = 0. The voltage difference across the impedance before the bypass depends on both the impedance value and the amount of current flow through the impedance. Micro-grid, Generators Superposition Impedance Insertion V Bulk Grid Impedance Insertion V V V V Fig.. Representing impedance bypass by equivalent voltage source In the impedance design section, an acceptable transient level is determined based on the IEEE C50.. Similar approach can be taken for impedance bypass. If the voltage across the impedance before bypass is lower than the acceptable bypass voltage, the bypass peak current will not be a concern. The acceptable voltage level for bypass according to IEEE C50. and Fig. 3 is computed to be p.u. The actual voltage across the impedance in steady state can be computed through load flow studies. The initializations to load flow study should consider the Automatic Voltage Regulation (AVR) and Governor Control settings because it will affect both the reactive and active power output from the synchronous machine. As a consequence, the machine controls can affect directly the power flow through the impedance, which may result in higher bypass voltage. Based on the open loop power quality limits in Fig., the voltage range for all busses in the system is typically between p.u. The AVR of synchronous generator generally maintains the terminal voltage of the machine at a preset level. After the impedance switching, since the grid voltage is stiff, the busses near the point of interconnection tend to follow the system voltage. However, due to the nature of open loop synchronization, the voltage set-point of AVR is not adjusted during the process of synchronization, which means it is regulating the terminal voltage to the level prior to synchronization. Therefore, there exists a reactive power (Q) flow at the impedance in steady state. In addition, Q can flow in either direction through the impedance. Normally in a Micro-grid, when there is more than one generator, the speed governors are operating in droop control mode to assure proper load sharing between the DG units. From synchronization point of view, before the impedance bypass in steady state, the Micro-grid frequency becomes the system frequency. The governor responds to the change in frequency by a percentage change in the mechanical torque or output power [] as in (). Therefore, real power (P) flow also exists through the impedance. f (%) fmicrogrid fsys Pm ( pu) () R f R where, R is the droop constant and f nom is the rated machine frequency. The larger the frequency and voltage differences prior to synchronization, the larger the power flowing through the impedance due to AVR and governor controls. Based on the open loop power quality limits, possible worst case bypass voltages can be identified as case (, 3), case (4, ), case (3, ) and case (, 4). nom IV. SIMULATION RESULTS The system under study consists of one 6.6 MVA synchronous generator connected in a 30 mile (5kV) feeder to the main substation. A peak loading of 6MW is evenly distributed along the feeder as shown in Fig. 3. The threephase short circuit level at the point of interconnection is 346 MVA. The system and synchronous generator data can be found in Appendix. The impedance pre-inserted at the circuit breaker location is purely an inductor. SUB 5kV 346MVA V,f Impedance Insertion CB V,f.0MW 0.65MVAr 5 miles 5 miles.0mw 0.65MVAr 5/4.6kV Yg/Yg.0MW 0.65MVAr SG 6.6MVA + AVR Fig. 3. Single line diagram of case study Determination of the acceptable transient level is illustrated through the case study presented in Fig. 3. Based on the EMTP simulations in MATLAB/Simulink, current and torque peaks under the operating point specified in Fig. 3 are found to be.5 and.7 p.u., respectively. Their time-domain simulation results are shown in Fig. 4.

7 Peak Current (pu) Reactive Power Flow (p.u.) Torque Te (pu) Stator Current (pu) First Version Acceptable current transient time (s) Fig. 7, it can be seen that increasing the inserted impedance does not have a significant effect on the maximum rotor angle. Based on the acceptable stability limit and the designed impedance value, a feasible impedance range is within 0.6p.u.-.p.u..5 Acceptable torque transient Breaker closes to begin synchronization time (s) Fig. 4. Time domain simulation to obtain acceptable transients Possible worst case synchronizing transients based on Fig. 4 will take place in the following 4 cases shown in Fig. 5, where the operating point of both parties deviate the most. The worst transient of all occurs in case (, 3). This is because prior to synchronization, the Micro-grid side voltage is maintained at. p.u. and the Micro-grid frequency is +0.5Hz higher than the system frequency. Fig. 7. Feasible impedance range based on short circuit and stability analysis In order to evaluate the severity of bypass transients, load flow study is used to find the worst bypass voltage among the four possible cases. Case (4, ) is the worst case as shown in Fig. 8 because it results in the largest complex power S. This S through the designed impedance (0.6p.u.) incurs a voltage difference of 0.78p.u. across the bypass breaker, which is lower than the acceptable level of 0.855p.u. Therefore, the bypass transient is acceptable Case (4, ) Case (, 3) S max Fig. 5. Worst case transients based on power quality limits Since the worst transients are identified in case (, 3), then the size of impedance can be chosen either by repetitive EMTP simulations or by the analytical expression shown in (3). A comparison of the two approaches is shown in Fig. 6. The analytical method provides an upper bound for the peak stator current. For this case study, the impedance value determined analytically is 0.6 p.u. or 56.8 Ω..5.5 Acceptable transient level Z Insertion = 0.6 pu EMTP Analytical Acceptable Impedance Size (pu) Fig. 6. Peak currents versus sizes of impedance pre-insertion for case (, 3) For transient stability analysis, the worst case occurs in case (, 3). The theoretical prediction of the maximum rotor angle reached is obtained by simplifying the circuit in Fig. 3 to Fig. 9 using repetitive Y- transformations. Therefore, the maximum swing angle can be calculated by using (0). From Case (3, ) Case (, 4) Real Power Flow (p.u.) Fig. 8. Worst possible power flows through the impedance computed by load flow studies V. CONCLUSION The purpose of this work is to propose an open loop synchronization strategy for a Micro-grid to system, which can be applied to practical Micro-grid implementations. An impedance pre-insertion method has been proposed to reduce synchronizing transients. Based on this idea, an open loop scheme to synchronize Micro-grids to grid has been developed. Methods to design the open loop scheme are established. Study results have shown that the proposed scheme is highly feasible. The potential uses of the proposed scheme include reducing cost of Micro-grid synchronization and support fast restoration of power systems. APPENDIX TABLE I THEVENIN EQUIVALENT (SUB) DATA Short circuit power (MVA) 346 Nominal voltage (kv) 5

8 First Version 8 X/R Ratio 7 TABLE II SYNCHRONOUS GENERATOR DATA Nominal Power (MVA) 6.6 Nominal Voltage (kv) 4.6 Pair of poles Inertia constant (s).5 Stator resistance (pu) 0.00 X d (pu).6 X d (pu) 0.6 X d (pu) 0.8 X q (pu).4 X q (pu) 0. T do (pu).4446 T do (pu) 0.08 (pu) T qo TABLE III EXCITATION SYSTEM DATA AVR IEEE Type T r 5ms K a 300 T a 50ms K E.0 T E 0.65s K f T f 0.95s E fmin -5 E fmax 8 [] W. M. Strang, C. J. Mozina, B. Beckwith, T. R. Beckwith, S. Chhak, E. C. Fennell, E. W. Kalkstein, K. C. Kozminski, A. C. Pierce, P. W. Powell, D. W. Smaha, J. T. Uchiyama, S. M. Usman, and W. P. Waudby, Generator synchronizing, industry survey results, IEEE Trans. Power Del., vol., no., pp , Jan [] J. V. Mitsche and P. A. Rusche, Shaft torsional stress due to asynchronous faulty synchronization, IEEE Trans. Power App. Syst., vol. PAS-99, no. 5, pp Sep [3] K. Malmedal, P. K. Sen, and J. P. Nelson, Application of out-of-step relaying for small generators in distributed generation, IEEE Trans. Ind. Appl., vol. 4, no. 6, pp , Nov./Dec [4] R. P. O leary, R. H. Harner, Evaluation of Methods for Controlling the Overvoltages Produced by the Energization of a Shunt Capacitor Bank, International Conference on Large High Voltage Electric Systems, Paris, Aug [5] R. W. Alexander, Synchronous Closing Control For Shunt Capacitors, IEEE Trans. Power App. Syst., vol. PAS-04, no. 9, Sep [6] A. Nied, J. Oliverira, R. F. Campos, R. P. Dias, and L. C. S. Marques, Soft Starting of Induction Motor With Torque Control, IEEE Trans. Ind. Appl., vol. 46, no. 3, May/June [7] Y. Cui, S. G. Abdulsalam, S. Chen, and W. Xu, A Sequential Phase Energization Technique for Transformer Inrush Current Reduction Part I: Simulation and Experimental Results, IEEE Trans. Power Del., vol. 0, no., Jan [8] A. J. Rodolakis, A comparison of North American (ANSI) and European (IEC) fault calculation guidelines. IEEE Trans. Ind. Appl., vol. 9, no. 3, pp. 55-5, May/Jun [9] Technical Requirements for Connecting to the Alberta Interconnected Electric System (IES) Transmission System, ESBI, Alberta, 999. [0] IEEE standard for Salient-Pole 50 Hz and 60 Hz Synchronous Generators and Generator/Motors for Hydraulic Turbine Applications Rated 5 MVA and Above, IEEE Standard C [] P. Kundur, Power System Stability and Control, in McGraw-Hill, 994, pp REFERENCES [] F. Katiraei, M. R. Iravani, and P. W. Lehn, Micro-grid Autonomous Operation During and Subsequent to Islanding Process, IEEE Trans. Power Del., vol. 0, no., Jan [] W. Kohn, Z. B. Zabinsky, and A. Nerode, A Micro-grid Distributed Intelligent Control and Management System, IEEE Trans. Smart Grid, Aug. 05. [3] H. Laaksonen, D. Ishchenko, and A. Oudalov, Adaptive Protection and Micro-grid Control Design for Hailuoto Island, IEEE Trans. Smart Grid, vol. 5, no. 3, May. 04 [4] F. Tang, J. M. Guerrero, J. C. Vasquez, D. Wu, and L. Meng, Distributed Active Synchronization Strategy for Micro-grid Seamless Reconnection to the Grid Under Unbalance and Harmonic Distortion, IEEE Trans. Smart Grid, Mar. 05 [5] M. J. Thompson, Fundamentals and Advancements in Generator Synchronizing Systems, 65 th Annual Conference for Protective Relay Engineers, pp. 03-4, Apr. 0. [6] T. M. L. Assis, G. N. Taranto, Automatic Reconnection From Intentional Islanding Based on Remote Sensing of Voltage and Frequency Signals, IEEE Trans. Smart Grid, vol. 3, no. 4, Dec. 0. [7] C. Cho, J. Jeon, J. Kim, S. Kwon, K. Park, and S. Kim, Active Synchronizing Control of a Microgrid, IEEE Trans. Power Electron, vol. 6, no., Dec. 0. [8] Ontario Power System Restoration Plan, IESO, Ontario, 05 [9] L. C. Gross, L. S. Anderson, and R. C. Young, Avoid Generator and System Damage Due to a Slow Synchronizing Breaker, proceedings of the 4 th Annual Western protective Relay Conference, Spokane, WA, Oct [0] IEEE Guide for AC Generator Protection, IEEE standard C

9 Final Version 9 A Method to Enable Open-Loop Synchronization of a Microgrid to Main Grid Yaxiang Zhou, Student Member, IEEE, Wilsun Xu, Fellow, IEEE, Moosa Moghimi Haji, Student Member, IEEE, Jing Yong, Member, IEEE Abstract This paper proposes a novel method to enable openloop synchronization of a microgrid to the main grid. The idea is to pre-insert an impedance to reduce the synchronization transients and then bypass it after the initial transients are over. With this method, infrastructure cost and complexity of synchronization can be reduced significantly since the communication link between the breaker and the microgrid generators is no longer required. In addition, the extra effort required for generator adjustments, especially for multiple units, can be avoided. Technical considerations and design method for the selection of pre-insertion impedance size is presented. A simulation study is conducted to evaluate the performance of the method. The results prove that the transient levels can be effectively reduced and open-loop synchronization is indeed achievable. Index Terms Impedance insertion, islanding, microgrid, open-loop synchronization. M VI. INTRODUCTION ICROGRIDS are becoming an integral feature of the power systems due to their environmental and economical benefits. A microgrid includes a variety of distributed energy resource (DER) units (including distributed generation (DG) and energy storage devices and different types of load. Improved reliability and sustainability are some of the desired characteristics affecting the distribution level provided by the implementations of microgrids []. Microgrids are able to operate in both grid-connected and islanded modes. For effective operation in islanded mode, several methods have been proposed for controlling the microgrid [-4]. Among the various types of distributed generators proposed for microgrids, the synchronous generators (SG) are actually the most common type. Examples are combined heat and power (CHP), internal combustion engine, and small hydro [6]. Each time the microgrid switches from islanded to gridconnected operation mode, the grid and its SGs need to be synchronized with the main grid. Precaution is necessary to make sure that synchronization criteria are met to avoid equipment damage and power quality concerns [7], [8]. In the traditional synchronizing practices, voltage across the The authors are with Electrical and Computer Engineering Department of University of Alberta, Edmonton, AB T6G H9, Canada ( wxu@ualberta.ca). breaker at the substation location is monitored and sent to the operator at the generation site. The operator then uses these measurements to adjust governor and exciter settings to meet the synchronization criteria [9]. Automatic synchronization methods based on feedback control strategies have been presented in literature [30], [3]. Recently, an active synchronization strategy that takes into account voltage unbalances and harmonic distortions is presented in [3]. In order to implement such feedback control systems in practice, infrastructure support such as communication links are necessary to carry the signals from the breaker location (substation) to the local machines as shown in Fig.. The problem is that the microgrid might be located in a rural or forestry area far away from the point of connection to the main grid. For example, in [30] it is stated that these rural microgrids are becoming common in Brazil. In such cases, the cost of communication infrastructure can be prohibitively high. Additional difficulties arise when there are multiple DG units in the microgrid. In this case, multiple communication links are necessary to allow the tuning of individual DG units. Therefore, synchronizing by feedback control can drive up the cost of infrastructure support and increase synchronization effort. Main Grid CB Synchronizing Controls Microgrid Communication Link Fig.. Schematic of the synchronization methods with feedback control ~ Machine Controls In response to the above challenges, a novel open-loop synchronization method is proposed in this paper. As the name implies, the method does not require a communication link between the DG units and the breaker location. The proposed method is inspired by the practices of controlling switching transients, since synchronization is an event where switching transients is the main concern. The idea here is to pre-insert an impedance to reduce the synchronization transients. With this method, infrastructure cost and complexity of synchronization

10 can be reduced significantly since the communication link between the breaker and the microgrid generators is no longer required. The rest of the paper is organized as follows. The proposed scheme is presented in Section II. The impedance design method is explained in Section III. A case study is conducted in Section IV to evaluate the effectiveness of the proposed method. Finally, the conclusion is presented in Section V. Final Version 0 VII. PROPOSED OPEN-LOOP SYNCHRONIZATION METHOD The main concern on synchronizing two systems or a generator with a system is the transients produced when the synchronizing circuit breaker is closed. Excessive transients can lead to large inrush current and transient torque, damaging generator and other equipment [33], [34]. The current practice of reducing synchronization transients is to limit the voltage, angle, and frequency differences between the two parties. There are other ways to reduce switching transients [35]. A good example is the impedance insertion/bypass scheme used to limit capacitor switching transients. In this scheme, the inserted impedance increases the total circuit impedance and thus reduces the inrush current. The impedance is bypassed after the system has reached the steady-state. In view of the excellent performance of this scheme on reducing switching transients, one may wonder if it can be used to simplify the synchronization of a microgrid to a system. Before synchronization, the microgrid and the system are expected to operate within their respective power quality limits. This means that each party has a known operating region of voltage and frequency at the synchronization point. The proposed idea is to establish a value of the pre-insertion impedance such that the resulting transient is always within acceptable limit as long as the two parties are operating within their power quality limits at the time of synchronization. Consequently, feedback control is not needed to adjust the microgrid operating point. Fig. illustrates the proposed scheme. It is clear that a larger Z insert will result in more reduction of inrush currents. However, it may increase the transient when the impedance is bypassed. In addition, larger Z insert will weaken the synchronizing power between the microgrid and the system so instability may occur. Therefore, the following issues must be addressed for the proposed scheme: a) What is the acceptable synchronization transient level? b) What is the minimum value of Z insert which can lead to acceptable synchronization transients? c) What is the maximum value of Z insert that can maintain system stability and results in acceptable bypass transients? In the following section, the above issues are addressed one by one. A method to design the proposed scheme is developed accordingly. VIII. IMPEDANCE SIZE DESIGN Current surges and power oscillations should be taken into Fig.. The scheme of the proposed method and power quality limits of the both sides account for designing the impedance size. The first step is to find the maximum acceptable surge level and power oscillation. Maximum surge level could be expressed by maximum current and maximum torque. These values are calculated based on the synchronization criteria from the standard IEEE C50. for salient-pole synchronous generators [38]. The limits are: - Angle ±0 - Voltage 0 to 5% (of nominal) - Slip ±0.067Hz These limits are established to ensure an acceptable transient level experienced by the generator in terms of stator current and shaft torque. In order to find the worst transient, the limits have been transformed into an acceptable region shown in Fig. 3. As the synchronizing condition moves away from the origin, the transient level will increase due to an increase in the voltage across the breaker. As a result, the four corners of the region should be examined through simulations to determine the maximum acceptable current and torque, since they are farthest from the origin. Fig. 3. Acceptable transient region defined by standard The maximum power oscillation level is expressed by the maximum rotor angle which usually happens at the first swing. A limit is required to ensure that the synchronous generator of the DG unit will successfully be synchronized with the grid and will not become unstable. The industry practice is to consider 45 degrees as the maximum rotor angle for transient stability studies [39]. The same value is used in this paper as the maximum acceptable rotor angle. The current transients and transient stability of the

11 Final Version microgrid during the open-loop synchronization are analyzed in the next subsections. Then the impedance design method is presented. A. First switching surges As mentioned before, the method is meant to enable openloop synchronization of the micro grid at any normal operating point of the microgrid and the main grid shown in Fig. 4. To make sure that the current and torque transients are both lower than their acceptable limits, all of the operating points of the two systems should be examined. However, it is evident that the largest voltage and frequency mismatch will result in the largest synchronizing transients. Therefore, there are four combinational cases to consider: case (, 3), case (4, ), case (3, ) and case (, 4). Where, the first number refers to the operating point in the microgrid side, and the second number refers to the system side shown in Fig. 4. It should be mentioned that 0 degrees angle difference is used to find the transient limits according to IEEE standard [38]. In practice, according to an IEEE industrial survey [40], the maximum closing angle can reach up to 0 degrees when synchronization is done manually by an operator. Therefore, a worst case angle difference of 0 degrees is used for impedance design. The worst (highest) transient current and torque could be determined by simulating the four aforementioned cases. However, to have a better insight, an analytical expression is derived for the case of having only one synchronous generator. The microgrid is modelled as a synchronous generator with a load connected to its terminal. Before the breaker is closed, the current through the impedance is zero. However, the stator current of the synchronous machine consists of a steady state flow of load current as shown in Fig. 5(a). At the instant of the first breaker switching, this switch can be represented by two opposing voltage sources in series, as shown in Fig. 5(b). By applying the superposition theorem, the circuit shown in Fig. 5(b) can be split into two equivalent circuits shown in Fig. 5(c). The circuit on the left side of Fig. 5(c) represents the same circuit before breaker is closed. The current in this circuit represents the steady state component. The right side circuit can be used to calculate the transient resulting from the switching. Assuming f is equal to zero, the analytical expression for the transient component based on Kirchhoff s voltage law (KVL) for the circuit is di() t L Ri( t) V sin( wt ) () dt where, L and R are the total equivalent inductance and resistance, respectively. is the angular frequency and is the voltage phase difference across the breaker. The solution to the differential equation () is V i( t) sin( wt ) sin( ) e X R where, eq Rt / L () X includes the series impedances X eq d, X insert, and X sys. Fig. 4. Operating regions of the microgrid and main grid Fig. 5. Equivalent circuit representation of closing breaker. (a) Circuit before breaker is closed. (b) Circuit after breaker is closed. (c) Equivalent circuits after breaker is closed. is the impedance angle given by tan - (X eq /R). The peak stator current is the sum of load current and the synchronizing current, where the synchronizing current is given by (). It is worth mentioning that () is derived on the basis f=0 because the frequency difference effect on the transient current is negligible. An explanation is realized by considering two voltage sources connected together with different frequencies. If one voltage source is taken as a reference running at angular frequency, the instantaneous voltage difference when the breaker is closed at t=0 is given by V V e e V e (3) jwt j ( wt ) jwt where, ω represents the angular frequency difference between the two voltage phasors having magnitudes V and V, respectively. Based on the time window of subtransient, the peak inrush current typically occurs within t < cycles from the synchronization instant. Since wt is almost zero, the impact of f on V across the breaker will be negligible. B. Transient Stability Evaluation Every time the microgrid has to be synchronized with the main grid, transient stability is one of the main concerns. The effect of inserting the impedance on the transient stability will be evaluated in this section. Then, the worst case which causes the maximum rotor angle will be identified. The ultimate goal is to ensure that the maximum rotor angle in the worst case does not exceed the transient stability limit of the generator.

12 Final Version First, a simple case including a single generator shown in Fig. 6 is considered. The rotor dynamics can be expressed by the classical definition of the swing equation with damping ignored [4] H d ( t) P m Pe wsyn dt (4) d () t w() t wsyn dt where, P m and P e are mechanical and electrical power, respectively. H is the machine inertia constant. ω(t) and ω sys are the generator and system angular frequency, respectively. After breaker is closed, the electrical power transfer from the internal voltage of generator to the system can be expressed as ' EVs Pe sin( ) (5) ' X X X d t sys where, E' represents the internal voltage behind direct axis transient reactance, V s is the voltage magnitude of the system, and δ is the power angle between E' and V s. The following equation can be derived from the first equation of (4) H w syn max 0 max d ( Pm Pe) d dt (6) 0 For generator synchronization, since the mechanical power is close to zero, (6) can be integrated and expressed in a closed form to obtain the value of δ max in the first swing HXT max cos cos( 0) [ ( f ' f)] (7) EVs w syn where, X T is the total series reactance. In the case of microgrid to system synchronization, an equivalent diagram for stability analysis is shown in Fig. 7(a). Before the breaker is closed, the power is mainly consumed by the load as shown in Fig. 7(b). After the breaker is closed, the power is exchanged with the main grid, as shown in Fig. 7(c). By modeling the loads as constant impedances and applying Y- transformations, the circuit shown in Fig. 7(c) is transformed to the circuit shown in Fig. 8. From network theory, the real power at node of Fig. 8 is given by * Re EI or P E Y cos( ) E V Y cos( ) (8) ' ' e _ after s where, Y Y0 Y, Y0 / Z0, and Y / Z. Z is the series impedance of the transmission network, including transformers, lines and the inserted impedance. Z0 is the equivalent shunt impedance connected to the machine terminal, which includes any local loads. and are the impedance angle corresponding to Y and Y, respectively. Similar to synchronization of single generator, the theoretical δ max for the first swing when microgrid synchronizes to system can be obtained by substituting (8) into (6), which gives a non-linear equation Fig. 6. Generator to system synchronization example Fig. 7. Circuits for transient stability analysis (a) microgrid to system synchronization single line diagram. (b) Before closing breaker (c) After breaker is closed. Fig. 8. Simplified equivalent circuit of the microgrid after applying Y- transformations E ' V Y [sin( ) sin( )] s max H / W [ ( f f )] ( P P )( ) 0 syn m c max o ' where, P E Y cos( ) represents the power dissipation in c the network from first term of (8). To consider the transient stability in designing the impedance, first the worst case should be identified. Then stability of the system under the worst case should be ensured. The effect of increasing the loading level on the transient stability is shown in Fig. 9. As it can be seen, increasing the loading level of microgrid reduces the stability margin, where the margin is defined as (P max -P m )/P max. The reason is that the initial angle of the rotor will be higher as the load is increased. The effect of inserted impedance on the transient stability is shown in Fig. 0. As it was expected, the inserted impedance will reduce P max and consequently reduce the transient stability margin. As it will be seen in the case study, since the frequency difference between the two grids is small, the inserted impedance will only have a small impact on the maximum rotor angle. To sum up, the worst case that should be examined in terms of maximum rotor angle is based on the peak loading level and the designed impedance value. In addition, the voltage levels (microgrid and system sides) should be at the lowest of the o (9)

13 Final Version 3 power quality limits according to (6) and (8). The frequency difference should be positive (i.e. higher microgrid frequency than system) to result in higher rotor angle within the first swing. As a result, the worst case open loop synchronization scenario from a transient stability point of view is case (, 3) shown in Fig. 4. C. Second Switching Surges Similar to the first breaker switching, impedance bypass also produces a transient effect on the system which must be evaluated to ensure an acceptable disturbance level. According to the principal of superposition, bypass switching can be represented by two opposing voltage sources as shown in Fig., which is equivalent to a single voltage source in the transient circuit. This circuit is quite similar to the first switching circuit, but with f=0. The voltage difference across the impedance before the bypass depends on both the impedance value and the amount of current flowing through the impedance. The active and reactive power flowing through the impedance after closing the first breaker depend on the governor and AVR settings of the synchronous generator. The reason is that due to the nature of open loop synchronization, the voltage set-point of AVR is not adjusted during the process of synchronization. Therefore, the AVR is regulating the terminal voltage to the level prior to synchronization. As a result a reactive power flow is expected which can flow in either direction depending on the voltage levels of the synchronous generator and the main grid. Active power flow through the inserted impedance is also expected. The speed governors of the synchronous generators are operating in droop control mode to assure proper load sharing between the DG units. When the system reaches the steady-state before impedance bypass, the microgrid frequency becomes equal to the system frequency. The governor responds to the change in frequency by a percentage change in the mechanical torque or output power [4] as in (0). Therefore, a real power exchange between the microgrid and the main grid exists which can be in either direction depending on the pre-synchronization frequencies of the both sides. f (%) fmicrogrid fsys Pm ( pu) (0) R f R nom where, R is the droop constant and f nom is the rated machine frequency. The larger the frequency and voltage differences prior to synchronization, the larger the power flowing through the impedance. The worst case caused by the highest voltage difference should be identified to make sure that the worst possible transient is lower than the acceptable limit. Possible worst case bypass voltages can be identified as case (, 3), case (4, ), case (3, ) and case (, 4) shown in Fig. 4. The voltage across the impedance can be found through load flow studies. Then the highest voltage difference is compared to the acceptable voltage limit. If the maximum voltage is lower than the limit, the bypass is realized in one step. Otherwise, more Fig. 9. Impact of load to microgrid synchronization Fig. 0. Impact of impedance pre-insertion to microgrid synchronization Fig.. Representing impedance bypass by equivalent voltage source bypass steps are required. The voltage difference limit is p.u. based on standard IEEE C50. [38]. D. Impedance Design Summary The design method for the pre-insertion impedance size can be summarized as follows: - The acceptable transient current and torque limits are determined. - The four possible worst cases identified before are established to find the case with the worst transient. - The worst case is then used to determine the minimum impedance size through analytical approach or

14 Peak Stator Current and Torque (pu) Final Version 4 simulation. - Analytical approach or simulation is then used to find the maximum acceptable impedance to maintain the transient stability of the generators. - An acceptable impedance range is now developed according to transient current and transient stability limits. This impedance size is selected in the lower range in this paper to have the maximum transient stability margin. - Power flow studies are utilized to find the case causing the highest voltage across the inserted impedance among the four cases identified before. The highest V is then compared to the maximum acceptable value to find how many steps are required to bypass the inserted impedance. Fig.. Single line diagram of the case study IX. CASE STUDY To evaluate the effectiveness of the method, it is applied to a system shown in Fig.. The system consists of one 6.6 MVA synchronous generator connected through a 30 mile (5kV) feeder to the main substation. A total load of 6MW is distributed along the feeder as shown in the figure. The system and synchronous generator data can be found in Appendix. The impedance pre-inserted at the circuit breaker location is considered to be purely inductive. The system is simulated in MATLAB/Simulink. Based on EMTP simulations, maximum acceptable transient current and torque are found to be.5 p.u. and.7 p.u., respectively. The respective simulation results are shown in Fig. 3. A. Impedance Design The 4 possible worst cases for the first switching discussed in Section III are simulated. The Peak current and torque for these cases are presented in Fig. 4. As it can be seen, the worst transients occur in case (, 3). Now that the worst case is identified, the minimum size of impedance required to limit the transients can be found either by EMTP simulations or by the analytical expression given in (). A comparison of the two approaches is shown in Fig. 5. The analytical method provides an upper bound for the peak stator current. For this case study, the impedance value determined analytically is 0.6 p.u. or 56.8 Ω. For transient stability analysis, the worst case occurs in case (, 3). Similar to the previous step, either EMTP simulations or analytical approach could be used to determine the predicted maximum rotor angle. The theoretical prediction of the maximum rotor angle is obtained by simplifying the circuit in Fig. to Fig. 8 using Y- transformations. The maximum swing angle can then be calculated by (9). The maximum rotor angles found by simulation and analytical approaches for different impedance sizes are compared in Fig. 6. It can be seen that the results of the two methods are close to each other. In addition, increasing the inserted impedance size does not have a significant effect on the maximum rotor angle. According to the maximum acceptable transient level and stability limit, the feasible impedance range is within 0.5p.u.-.p.u. However, to ensure a high stability margin, 0.6pu is Fig. 3. Time domain simulation to obtain acceptable transient limits Worst Case transient Fig. 4. Worst case transients based on power quality limits..6 case (, 3) case (4, ) case (3, ) case (, 4) I_peak Te_max Fig. 5. Peak current versus impedance size for the worst case chosen as the required impedance size. The last design step is to evaluate the severity of bypass transients. Load flow studies are used to find the worst bypass voltage among the four possible cases mentioned in Section III and the results are presented in Fig. 7. As it can be seen, case

15 Final Version 5 Fig. 6. Feasible impedance range base d on short circuit and stability analysis Fig. 8. Stator current during synchronization with and without impedance insertion Fig. 7. Worst possible power flows through the impedance from load flow studies (4, ) has the highest complex power which leads to the highest voltage across the impedance. For the selected impedance size (0.6p.u.), this complex power incurs a voltage difference of 0.78 p.u., which is lower than the acceptable level of 0.855p.u. Therefore, the impedance can be bypassed in one step while the transient level is acceptable. B. Simulation Results To better show the performance of the proposed method, the whole synchronization process using the designed impedance is shown in Fig. 8 to Fig. 0. For these simulations it is assumed that the synchronization is done under case (, 3) which has the worst transient at the synchronization instance. The voltage angle difference across the breaker is assumed to be 0 degrees. For comparison, the synchronization under the same condition but without the inserted impedance is also shown in these figures. As it can be seen in Fig. 8 and Fig. 9, without the inserted impedance the current and torque could reach as high as p.u. and.34 p.u., respectively, which are higher than the acceptable limits. After inserting the impedance, the peak current for the first and second switching are.37 p.u. and.38 p.u. which are within the acceptable limit. Also the peak torque for the first and second switching are.54 p.u. and.36 p.u., respectively, which are below the maximum acceptable limit. Comparing the rotor angles in Fig. 0 shows that although inserting the impedance increases the oscillations, the oscillations will not causing the rotor to pass the maximum stability limit and the generator will be stable after the Fig. 9. Torque during synchronization with and without impedance insertion Fig. 0. Rotor angle during synchronization with and without impedance synchronization. X. CONCLUSION An impedance insertion method has been proposed in this paper to enable open-loop synchronization of a microgrid. Since the primary concern in synchronization is the switching transients, the idea is to pre-insert an impedance before the breaker closing to reduce the transients to an acceptable level. The impedance is then bypassed using a second breaker. The necessary requirements are considered in the impedance design to ensure acceptable transient level and transient stability. Using the open-loop scheme, both synchronization