Full-scale converter-interfaced renewable energy sources (CIRESs) can cause misoperation of distance relays installed in their vicinity. Such failure stems from the different fault behavior of CIRESs ...and synchronous generators (SGs), based on which available relays have been developed. Several measures have been devised to secure the performance of distance protection by modifying existing relays. This paper proposes a new approach to address misoperation of distance relays during asymmetrical faults by regulating the fault currents of CIRESs such that available distance relays function properly without requiring modification. The prime objective of this method is to mimic certain features of the symmetrical components of SGs' fault currents that affect distance relays. In the meantime, the proposed method satisfies the constraints of a converter, such as its limited phase current magnitude. As a result, correct operation of the distance relays close to CIRESs is ensured regardless of the fault conditions, including its type, resistance, and location. Some salient features of the proposed method are its simplicity, compatibility with off-the-shelf relays, and independence from the voltage and power rating of the CIRES. Moreover, the proposed method uses only local measurements, and so is cost-effective. PSCAD/EMTDC simulation studies verify the performance of this new method.
This paper proposes a sequential power flow algorithm for hybrid ac/dc microgrids operating in the islanded mode. Unlike in grid-connected systems, variable, rather than fixed, frequency and voltage ...are utilized for power coordination between the ac and dc microgrids, respectively. The main challenge is to solve the power flow problem in hybrid microgrids while considering the absence of a slack bus and the coupling between the frequency and dc voltage. In the proposed algorithm, the ac power flow is solved using the Newton-Raphson (NR) method, thereby updating the ac variables and accordingly utilizing these variables in a proposed interlinking converter model for the dc problem. This sequential algorithm is iterated until convergence. The proposed algorithm is generic and can include different operational modes not only for the distributed generation units (DGs), but also for the interlinking converters. Detailed time-domain simulations using PSCAD/EMTDC have validated the algorithm's accuracy. Its robustness and computational cost are contrasted to those of conventional algorithms.
The unique fault current signatures of inverter-interfaced renewable energy sources (IIRES) hinders the correct operation of the existing phase selection methods (PSMs) that operate based on the ...sequence currents. As a result, the system resilience during unbalanced faults is imperiled. In this paper, a new dual current controller is developed to regulate the relative angles of the IIRES sequence currents during unbalanced faults to enable accurate identification of the fault type by the existing PSMs. The proposed method consists of three design stages. In Stage I, the IIRES control system identifies the fault type using the sequence voltages at its terminal. Stage II aims at generating the appropriate reference angle for the IIRES negative-sequence current. In Stage III, the IIRES sequence currents are controlled in the double synchronous reference frame to track the reference angle. The proposed controller makes the existing PSMs immune against fault conditions and reactive current generation requirements imposed by grid codes. Consequently, the protection system can select and trip only the faulted phase(s), and thus, the system reliability is considerably enhanced. Time-domain simulations are used to confirm the successful operation of the proposed controller under different fault conditions and grid codes.
Directional overcurrent relays (DOCRs) depend on voltage and current measurements to initiate correct protection actions when faults occur. However, cyber-attacks can manipulate those measurements to ...make DOCRs output false tripping signals that may cause load shedding, power loss, and distribution lines isolation. Therefore, this paper employs Random Forest Decision (RFD), i.e., a machine learning algorithm, to detect and diminish false data injection attacks (FDIAs) and time synchronization attacks (TSAs) on inverse-time and time-current-voltage DOCRs. The employed RFD is trained to tackle grid faults and cyber-attack events as a classification problem. Thus, two classification types are considered: a binary classification that differentiates cyber-attacks from grid faults and a multi-class classification that identifies the attacked phase(s) and specifies the grid fault type. Furthermore, the training process for the binary and multi-class classification accounts for multiple noise levels that may affect the measured voltages and currents. The RFD is trained, validated, and tested on a Canadian benchmark distribution system. The results reveal the RFD's ability to produce high-accuracy predictions to identify cyber-attacks and verify grid fault conditions without being affected by the system's operating conditions and measurements' noise.
Directional, phase-selection, and distance elements of a relay are prone to misoperation in the presence of inverter-based resources (IBRs). To address these problems, modifications of relay elements ...as well as new control schemes for IBRs have been proposed in the literature. These methods focus mainly on addressing the misoperation of only one of the above relay elements. The available solutions that are based on a new control system focus only on the negative-sequence current loop. Thus, they work only for unbalanced faults. This paper proposes a comprehensive dual current control scheme for IBRs during all types of faults such that all of these relay elements operate correctly. The proposed method controls the IBR to operate as a voltage source behind a virtual impedance in the positive-sequence circuit, and a single impedance in the negative-sequence circuit, thereby emulating the phase angles of a synchronous generator's (SG's) fault current. The proposed method also maximizes the active, and reactive current generation of the IBR during faults, and emulates the incremental sequence current magnitudes of an SG. The proposed method does not require altering the relay elements. Moreover, correct operation of the relay elements is achieved regardless of the fault type, resistance, and location. The effectiveness of the proposed method is verified through not only simulation studies, but also hardware-in-the-loop testing of commercial relays.
Synchronverters are inverters that provide damping and inertia, similar to synchronous generators. Like traditional inverters, synchronverters are adaptable to the microgrid's operation modes. ...However, synchronverters may generate oscillatory active and reactive powers during faults and produce phase currents with excessive magnitudes. Therefore, this paper develops two fault ride-through (FRT) strategies for synchronverters: average power control and enhanced power control. Both strategies aim to reduce power oscillations, limit current generation, and ensure compliance with grid codes during faults, regardless of the fault conditions and the grid's operation mode. Additionally, synchronverters with the proposed FRT strategies are incorporated into an optimal protection coordination (OPC) program for microgrid protection. The OPC program obtains the settings of time-current-voltage overcurrent relays (TIV-OCRs) and demonstrates the effect of the proposed FRT strategies on microgrid protection. The obtained settings are suitable for both the microgrid's grid-connected and islanded modes during balanced and unbalanced faults. Evaluation results demonstrate the capability of the proposed strategies to reduce power oscillations, limit current generation, and comply with grid codes. Furthermore, the OPC results reveal the most appropriate strategy to minimize TIV-OCRs' operating times in a coordinated manner, regardless of the fault type and microgrid operating mode.
Inverter-interfaced distributed generators (IIDGs) have fault current signatures that could jeopardize protective relaying. This paper unveils the failure of phase selection methods (PSMs) utilized ...by commercial relays in the presence of four-wire IIDGs, which adversely impacts the grid resiliency and reliability. A triple current controller (TCC) is proposed to regulate the inverter's sequence currents during unbalanced faults to ensure accurate fault type identification. The negative- and positive-sequence components are extracted using a decoupled double synchronous reference frame (DDSRF)-based phase-locked loop (PLL). Further, a second-order generalized integrator (SOGI)-based PLL is employed for zero-sequence component synchronization. The negative- and zero-sequence reference currents are generated to force the angles of IIDG sequence currents to behave like those from synchronous generators (SGs) and abide by the inverter's current limits. Consequently, commercial PSMs can correctly identify the fault type. The proposed TCC scheme pertains to four-wire as well as transformer-less IIDGs. A performance evaluation using time-domain simulations is conducted on a CIGRE benchmark system to confirm the success of the proposed control scheme under different fault conditions.
Optimal protection coordination is usually solved for the original network topology with all lines, loads, and generation intact. However, power grids may experience contingencies due to transient ...events, e.g., generation or line outages. Low fault currents of inverter-interfaced distributed generators (IIDGs) necessitate a sensitive and reliable protection scheme. This paper proposes a protection scheme for islanded microgrids powered by droop-based IIDGs. The protection scheme utilizes virtual impedance-fault current limiters to limit IIDGs fault currents and achieve protection coordination. A two-stage method for optimal protection coordination (OPC) of directional overcurrent relays (DOCRs) is devised. In Stage I, relays short-circuit currents are calculated. Then, constraints on the operation times of primary and backup DOCRs are formulated for the islanded topology and each possible topology following an N-1 contingency. Lastly, in Stage II, the OPC problem is formulated as a constrained nonlinear programming problem and solved to obtain the optimal DOCRs settings. A radial test microgrid that is part of a Canadian urban distribution system is used to ensure the success of the proposed OPC method.
Outages may impact power grids, e.g., lines or generation forced out of service. Thus, solving the optimal protection coordination (OPC) problem for the main network topology results in protection ...miscoordination under contingencies. Further, the limited fault currents of inverter-interfaced distributed generators (IIDGs) require a highly sensitive and reliable protection scheme. This paper proposes an overcurrent protection scheme for islanded microgrids powered by droop-based IIDGs. The inverter controller is modified to include a virtual impedance-fault current limiter (VI-FCL) to protect inverter switches from overcurrent and limit IIDG fault currents. The VI-FCL is designed such that it adapts to fault severity. OPC of directional overcurrent relays (DOCRs) is achieved using a two-stage optimization algorithm. The first Stage calculates the short-circuit currents involving various fault resistances. Next, constraints on the DOCRs operation times are formulated for each topology resulting from an N-1 contingency and the main topology. The OPC problem is formulated as a constrained nonlinear programming problem. Finally, the second Stage is dedicated to obtaining the DOCRs optimal settings. The OPC method is tested on a radial microgrid that is part of a Canadian urban distribution system.
Renewable energy sources (RESs) possess different fault signatures as compared to the traditional synchronous generators, thus introducing new challenges for microgrid protection. Among these ...challenges is phase selection, which is a prerequisite for selective phase tripping (SPT). This paper analyzes the root causes behind the failure of current-angle-based phase selection methods (PSMs) used by commercial relays in microgrids with inverter-interfaced RESs (IIRESs). Besides, the operation of the recently-developed voltage-angle-based PSM is investigated to unveil its points of strength and weakness. Then, adaptive zoning for the current-angle-based PSM is proposed to cope with the unconventional IIRES fault current signatures and enable precise SPT. As a result, microgrid reliability and resiliency during faults are enhanced. The proposed PSM depends on local measurements to update the PSM zones online by estimating the angles between the sequence impedances behind a relay. Comprehensive time-domain simulations verify the effectiveness of the proposed PSM at various fault resistances, fault locations, and microgrid topologies.
