Effectiveness of Anti-islanding Schemes Following A Faulty Recloser Operation

Parag Mitra, Vijay Vittal, Fellow, IEEE, Gerald T. Heydt, Life Fellow, IEEE, and Raja Ayyanar, Senior Member, IEEE Department of Electrical, Computer and Energy Engineering Arizona State University Tempe, AZ, U.S.A Email: {pmitra2}, {vijay.vittal}, {heydt}, {rayyanar} @asu.edu

Abstract—Islanding detection is one of the technical issues, Reclosers are frequently used in overhead distribution which has gained prominence with the increasing penetration of systems to interrupt faults or intentionally isolate portions of distributed photovoltaic generation. Various anti-islanding the feeder for maintenance [10]. A mechanical failure of a algorithms have been proposed in literature, which use either recloser to open all three phases can lead to an abnormal passive or active methods to detect an island formation. condition, where two phases or a single phase remains However, all the anti-islanding studies done so far, assume a connected to the system. Such a misoperation can lead to high proper breaker or recloser operation, which results in a voltage unbalance and high negative sequence currents at the complete disconnection from the grid. This paper studies the terminals of three phase equipments. Moreover, recloser effectiveness of two active anti-islanding schemes following a manufacturers are proposing voluntary single phase reclosing faulty recloser operation, when a single phase or two phases fail which can lead to a similar situation. Operating a grid tied to disconnect. Results and the findings of a simulation study performed on a realistic test distribution system are presented. inverter under such conditions may damage any sensitive equipment connected to the system or the inverter itself. It is Index Terms-- Grid tied inverters, photovoltaics, distribution desirable that the inverter anti-islanding controls detects such system, recloser, active anti-islanding mechanism, dq based a situation and disconnects the inverter from the grid. controller, distributed generation. This paper studies the effectiveness of two active anti- islanding methods, voltage based positive feedback [11] and I. INTRODUCTION frequency based positive feedback [11], following a faulty Despite sluggish (or negative) load growth in North America, recloser operation. A test system, representing an actual photovoltaic generation has been nonetheless implemented in distribution system with two utility scale inverters is simulated various jurisdictions to attain required renewable portfolio and the findings of the study are presented. standard levels [1], [2]. One of the prominent technical issues associated with grid interconnection of distributed II. INVERTER MODEL photovoltaic (PV) generation is that of islanding. Islanding The inverter is represented by a cycle-by-cycle average refers to off-grid operation of distributed generation, often model for this study. An average model takes less time to following a disturbance. Although PV generators can simulate and is ideal for small signal analysis [11]. The continue to supply loads in an island, operating under such switching network is represented by controlled voltage and conditions may lead to degradation of power quality [3-5]. current sources with averaged duty cycles and the controller Furthermore, energizing an isolated section of the feeder action is represented by continuous functions like proportional jeopardizes the safety of line maintenance personnel [3-5]. (P) or proportional integral (PI), instead of actual discrete The IEEE 1547 [5] standard requires a distributed generator functions. Fig. 1 shows the cycle-by-cycle average model of a (DG) to detect and disconnect from the system within 2 voltage source inverter (VSI) operating in a current control seconds of an island formation. The IEEE 929 standard further mode. A direct-quadrature (dq) axis control is implemented, documents islanding, as applied to PV systems [4]. References wherein the d-axis current controls the active power output [4-6] provide the minimum test procedure for non-islanding and the q-axis current controls the reactive power output of the PV inverters. To comply with the present standards, most grid inverter. A dual decoupled synchronous rotating frame connected PV inverters are equipped with anti-islanding (DDSRF) phase locked loop (PLL) [12] is used to track the controls. Over the past decade, several anti-islanding phase angle of the grid voltage. A DDSRF PLL ensures proper algorithms have been developed. These island detection tracking of the grid voltage phase angle when the grid voltage algorithms can be broadly classified under two major may be distorted due to unbalanced loads or unsymmetrical approaches, the passive methods and the active methods [7-9]. faults [12], [13]. Fig. 2 shows the block diagram of a current

The authors acknowledge the support of the U.S. Department of Energy under grant DE-EE0004679.

978-1-4799-1303-9/13/$31.00 ©2013 IEEE controller modeled in a dq reference frame. A detailed voltage across the RLC load and ω is the system frequency in overview of control structure can be found in [13]. rad/s. Based on this load representation, the two anti-islanding algorithms are briefly discussed as follows. III. ANALYZED ANTI-ISLANDING ALGORITHMS Passive anti-islanding methods, detect an island formation A. Voltage based positive feedback by measuring the changes in voltage and current at the point Fig. 3 shows the dq implementation of the voltage based of common coupling (PCC) on the inverter side [7], [9]. positive feedback scheme [11] .When a voltage rise is sensed However, under certain operating conditions, when the local at the inverter terminals, a positive feedback signal is generation matches the load closely, these methods may fail to generated, which increases the inverter active power output. detect an islanding condition [8], [9]. Due to the load characteristic given by (1), increased active power causes the voltage to rise further. The positive feedback eventually drives the voltage beyond the nominal range, leading to island detection. A similar destabilization occurs in the opposite direction if an initial voltage dip is detected. In a dq control frame, for a VSI operating in a current control mode, the positive feedback signal is fed to the d-axis current, which controls the active power output.

Fig. 1. Average model of inverter

Fig. 3. Voltage based positive feedback in dq frame

B. Frequency based positive feedback Fig. 4 shows the dq implementation of the frequency based positive feedback scheme [11]. When the inverter controls senses an increase in frequency at its terminals, a positive feedback is generated which increases the reactive power absorbed by the inverter. Due to the load characteristics given by (2), an increase in reactive power absorbed, causes the frequency to increase further. The positive feedback eventually drives the frequency out of the nominal limits resulting in island detection. An initial frequency dip would result in a similar destabilization in an opposite direction. In the dq control frame, for a VSI operating in a current control mode, the positive feedback is fed to the q-axis current, which controls the reactive power output.

Fig. 2. Block diagram of current controller Active anti-islanding methods, which introduce a controlled disturbance at the PCC to create a voltage or Fig. 4. Frequency based positive feedback in dq frame frequency excursion, have relatively narrow non detection zones (NDZs), and are thus superior to the passive methods IV. SYSTEM DESCRIPTION AND ANALYSIS APPROACH [8],[9]. The active anti-islanding schemes discussed in this Fig. 5 shows the test distribution system modeled in paper are based on positive feedback and dq implementation PSCAD®, used for this study. The test system is modeled [11]. based on actual load data, transmission line parameters and For an anti-islanding study, it is a standard practice to transformer data of an urban distribution feeder located in represent the local load by a parallel combination of resistance Arizona. The utility grid is represented by a voltage source (R), inductance (L) and capacitance (C) connected at the supplying a line-to-line voltage of 12.47 kV at 60 Hz. Opening inverter terminals, as shown in [4], [14] and [15]. The the recloser REC leads to the formation of an island as shown relationships between active/reactive power and in Fig. 5. PV inverters, INV 1 and INV 2 are connected to the voltage/frequency for a parallel-connected RLC load, is given test system through 0.48 kV/ 12.47 kV transformers, T1 and by (1) and (2) respectively. T2 respectively. The inverter INV1 is rated at 500 kW and INV 2 is rated at 400 kW. The local loads and shunt capacitors ௏మ are represented as aggregated parallel connected R, L and C. ܲൌ (1) ோ As per the IEEE 1547 standard [5], the inverters are operated ܳൌܸଶሺ߱ܥ െ ሺ߱ܮሻିଵሻ (2) at unity power factor and supply only active power. where, P is the active power supplied by the inverter, Q With a high penetration of PV generation in this system, a represents the reactive power flowing into the inverter, V is the worst-case scenario for islanding detection is encountered

Fig. 5. Test distribution system modeled in PSCAD when the local generation and the load match closely. In this B. Active feedback scheme enabled at both inverters condition, the active and reactive power flow through the Fig. 8 shows the system frequency, following a faulty recloser REC is close to zero. In the absence of any active recloser operation, when frequency based positive feedback is anti-islanding mechanism, opening the recloser REC does not enabled at both inverters. The positive feedback signal drives cause any appreciable change in the voltage or frequency at the system frequency upwards and pushes it beyond the upper the PCC, thus inhibiting island detection by passive methods. threshold of 60.5 Hz [5]. This over frequency condition is To simulate this scenario, the inverter active power outputs detected by an over/under frequency relay, which disconnects and the shunt capacitor reactive power outputs are adjusted to the inverter from the system. As shown in Fig. 9, this scheme match the active and reactive power consumption within the detects the islanding condition in 60 ms and disconnects the island. A summary of the load and generation in the island is inverter from the grid. given in Table I. The active and reactive power losses in the network account for the difference in load and generation Fig. 10 shows the line-to-neutral voltages at the terminals values. Two cases of recloser misoperation are considered in of inverter INV 1, following a faulty recloser operation, when this study. Case 1 refers to a condition when two phases of the voltage based positive feedback is enabled at both inverters. recloser opens and one phase fails to open. Case 2 refers to a The positive feedback signal pushes the terminal voltage of condition when one phase of the recloser opens and two the inverter beyond the upper threshold of 1.1 p.u. or 431.1 V phases fail to open. Simulation results for inverter INV 1 are (peak line-to-neutral) [5]. The overvoltage condition is presented here; inverter INV 2 has similar results. detected by an over/under voltage relay, which disconnects the inverter from the system. As shown in Fig. 11, this scheme TABLE I. LOAD AND GENERATION SUMMARY WITHIN THE ISLAND detects the islanding condition in 110 ms and disconnects the Active Reactive Reactive Active power inverter from the grid. power power power generated consumed consumed generated VI. CASE 2. ONE PHASE OF RECLOSER OPENS AND TWO INV 1 INV 2 PHASES FAIL TO OPEN 750 kW 306 kVAr 370 kVAr 460 kW 355 kW Consider the case that one phase of a recloser opens but V. CASE 1. TWO PHASES OF RECLOSER OPEN AND ONE the remaining two phases do not open. To simulate this case, PHASE FAILS TO OPEN after the initial transients settle down and a steady state is Consider the case that two phases of a recloser opens but reached, Phase A of the recloser REC is opened at time 10 one phase does not open. To simulate this case, after the initial seconds and phases B and C are deliberately kept closed. transients settle down and a steady state is reached, Phases A A. Active feedback scheme disabled at both inverters and B of the recloser REC are opened at time 10 seconds, and phase C is deliberately kept closed. In the absence of any anti-islanding scheme, the inverters fail to detect this condition as the voltage and frequency A. Active feedback scheme disabled at both inverters measured at the inverter terminals, remains within acceptable Fig. 6 and Fig. 7 show the line-to-neutral voltages at the limits. The inverters do not disconnect from the system and terminals of inverter INV1 and the system frequency, continues to energize the island. The line-to-neutral voltages at respectively, following a faulty recloser operation. With active the terminals of inverter INV 1 and system frequency, in this island detection scheme disabled, the inverter fails to detect case, are identical to those shown in Fig. 6 and Fig. 7 this condition as the voltage and frequency measured at the respectively. inverter terminals remain within the acceptable limits. The B. Active feedback scheme enabled at both inverters inverters thus continue to energize the island in this case. Fig. 12 shows the system frequency following a faulty recloser operation, when frequency based positive feedback is enabled at both inverters. The positive feedback signal destabilizes the system frequency pushing it beyond the upper limit of 60.5 Hz [5]. The over frequency condition is detected by an under/over frequency relay, which disconnects the inverter from the system. As shown in Fig. 13, this scheme detects the islanding condition in 450 ms and disconnects the inverter from the grid. The voltage based positive feedback scheme, however, fails to detect the island formation in this case. The line-to – neutral voltages are identical to those shown in Fig. 6, following a recloser misoperation, even when voltage based positive feedback is enabled at both inverters. The inverters remain connected to the system and continue to energize the island in this case.

Fig. 9 Line currents at the terminals of inverter INV 1 with frequency based positive feedback enabled

Fig. 6. Line-to-neutral voltages at the terminals of inverter INV 1 with active island detection disabled

Fig. 10. Line-to-neutral voltages at the terminals of inverter INV 1 with voltage based positive feedback enabled

Fig. 7. System frequency with active island detection disabled

Fig. 11. Line currents at the terminals of inverter INV 1 with voltage based Fig. 8. System frequency with frequency based positive feedback enabled positive feedback enabled could reduce the non-detection zone of the anti-islanding schemes. Further studies might be needed for a multi-inverter case where not all inverters have similar anti-islanding schemes.

REFERENCES [1] Arnulf Jäger-Waldau, "PV status report 2011," Report No. EUR 24807 EN, July 2011. [2] P. Brown, J. Whitney, "U.S renewable electricity generation: resources and challenges," Congressional Research Service report for Congress, CRS 7-5700, R41954, August 2011. Fig. 12. System frequency with frequency based positive feedback enabled [3] P.L. Villenueve, "Concerns generated by islanding," IEEE Power Energy Mag., vol. 2, no. 3, pp. 49-53, May/Jun. 2004. [4] IEEE recommended practice for utility interface of photovoltaic (PV) systems, IEEE Std. 929-2000, Piscataway NJ, 2000. [5] IEEE standard for interconnecting distributed resources with electric power systems, IEEE Std. 1547-2003, pp. 1-16, Piscataway NJ, 2003. [6] Standard for safety of inverters, converters and controllers for use in independent power systems, UL 1741, June 2002. [7] F. De Mango, M. Liserre, A. DellAquilla, A. Pigazo "Overview of anti-islanding algorithms for power systems. Part I: passive methods," Proc. of IEEE EPE-PEMC, August 2006, pp. 1878-1883. [8] F. De Mango, M. Liserre, A. DellAquilla, "Overview of anti-islanding algorithms for power systems. Part II: active methods," Proc. of IEEE EPE-PEMC, August 2006, pp. 1884-1889. [9] W. Bower, M. Ropp, "Evaluation of island detection methods for utility-interactive inverters in power systems," Sandia Report, SAND2002-3591, Nov. 2002. [10] R. C. Dorf, The electrical engineering handbook, Boca Raton FL: CRC Press, 1993, p. 1319. [11] Z. Ye, R. Walling, L. Garces, R. Zhou, L. Li, T. Wang, " Study and development of anti-islanding controls for grid-connected inverters," NREL Report NREL/SR-560-36243, May 2004. [12] P. Rodriguez, J. Pou, J. Bergas, J. I. Candela, R. P. Burgos, D. Bororyevich, "Double decoupled synchronous rotating frame PLL for power converters control," IEEE Trans. on Power Electronics, vol. 22, no. 2, pp. 584-592, Mar. 2007. [13] F. Blaabjerg, R. Teodorescu, M. Liserre, A.V. Timbus, "Overview of Control and Grid Synchronization for Distributed Power Generation Fig. 13. Line currents at the terminals of inverter INV 1 with frequency Systems," IEEE Transactions on Industrial Electronics, vol.53, no.5, based positive feedback enabled pp.1398-1409, Oct. 2006 [14] IEEE standard conformance test procedures for equipment VII. CONCLUSIONS interconnecting distributed resources with electric power systems, Active anti-islanding schemes have narrow non-detection IEEE Std. 1547.1-2005, Piscataway NJ, pp. 1-54, 2005. [15] Z. Ye, M. Dame, "Grid connected inverter anti-islanding test results zones and are effective in detecting island formation following for General Electric inverter based inverter technology," NREL a recloser misoperation. However, in high penetration Report, NREL/TP-560-37200, Jan. 2005. photovoltaic distribution systems, these anti-islanding schemes may fail to function satisfactorily following a BIOGRAPHIES recloser misoperation under certain load scenarios and grid Parag Mitra is currently pursuing an M.S degree, in Electrical Engineering topologies. It is possible that a voltage based positive feedback at Arizona State University, Tempe AZ. scheme will fail to disconnect inverters from the grid when a Vijay Vittal (S'78 F'97) is the director of the Power Systems Engineering faulty recloser operation results in two phases failing to open. Research Center (PSERC) and is the Ira A. Fulton Chair Professor in the The failure can be attributed to the close active power Department of Electrical Engineering at Arizona State University, Tempe. Gerald Thomas Heydt (StM ’62, M ’64, SM ’80, F ’91, LF ‘00) is the site matching within an island, and the fact that the two connected director of a power engineering center program at Arizona State University in phases will still hold the system voltages within nominal Tempe, AZ where he is a Regents’ Professor. ranges. Although increasing the gain of the positive feedback Raja Ayyanar (S’97–M’00–SM’07) received the M.S. degree from the loop may result in island detection, this reduces the selectivity Indian Institute of Science, Bangalore, and the Ph.D. degree from the of the scheme and may lead to nuisance tripping of inverters University of Minnesota, Minneapolis. Currently, he is an Associate Professor with Arizona State University, Tempe. during normal grid operation (e.g., a voltage sag or swell). A frequency based positive feedback scheme, however, operates satisfactorily under conditions of active power matching and a combination of both the voltage and frequency based schemes could be a better solution for island detection. Both the schemes can be implemented independently and this approach