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.