Fault Ride-through Requirements Research Articles

Investigation of condition monitoring system for grid connected photovoltaic (GCPV) system with power electronics converters using machine learning techniques

Power systems protection has become more vital in recent years to ensure stability, reliability, security, and power quality due to the exponential growth of grid-connected photovoltaic (GCPV) systems. As a result, several nations have set new grid codes for the grid integration of PV plant installations to overcome these concerns. Investigating Fault Ride Through (FRT) capacity is one of the primary criteria for grid codes. For 3-phase GCPV systems, fault detection and classification approaches are proposed in this study. Firstly, different faults that occurred in the GCPV system are categorized and compared, with the critical and analytical evaluation of grid codes, particularly FRT requirements such as Low Voltage Ride Through (LVRT) and High Voltage Ride Through (HVRT) for different nations. Further, a detailed classification and a comparison of the existing FRT techniques are presented for better control methods based on system complexity, detection accuracy, and other evolutionary criteria. To ensure smooth grid operation, accurate fault detection and condition monitoring of the systems are required. This paper discusses a machine learning (ML) based technique for detecting faults in 3-phase GCPV systems. Multi-peak phenomena caused by FRT capabilities and anti-islanding detection are the main problems experienced while integrating a PV system into the local grid. The fault classification technique is developed using weighted K-nearest neighbor (WKNN) and fine Gaussian support vector machine (FGSVM) based ML approaches utilizing Wavelet Transform. The proposed ML-based findings show that the fault detection algorithm-based classification accuracy has significantly improved.

PMSG-based wind farm high-voltage ride-through improvement for single-pole tripping of circuit breakers

For transient single phase-to-ground (SPG) faults in transmission lines, single-pole circuit breaker (SPCB) operation can improve system reliability. This issue is more critical in transmission lines emanating from wind farms (WFs). In the large-scale WFs, the grid codes necessitate that wind generation remains in operation during the transmission line faults. For this purpose, the grid codes focus on the development of fault-ride-through (FRT) requirements. In this paper, it is shown that after the SPCB operation, the terminal voltage of the permanent magnet synchronous generator (PMSG)-based wind turbine (WT) rapidly increases and may cross the high-voltage ride-through (HVRT) boundary. Therefore, the WT terminal overvoltage after the SPCB operation may result in undesirable tripping of the WT. The obtained results in this paper show that in most cases both the coupled and decoupled sequence controllers of the WT converter equipped with the FRT requirements are not able to improve the HVRT performance after the SPCB operation. To enhance the HVRT performance of PMSG-based wind turbines after the SPCB operation, a dc-link chopper controller is proposed. EMTP-RV simulation results on the IEEE 118-bus and 5-bus systems show the success and effectiveness of the proposed method.

A Communication-Assisted Distance Protection for AC Microgrids considering the Fault-Ride-Through Requirements of Distributed Generators

The conventional overcurrent protection is ineffective in isolating faults in microgrids due to the low fault current levels contributed by inverter-interfaced distributed generations (IIDGs). To extend the microgrid protection methods, the distance protection is considered as a common alternative to detect faults. However, the fault-ride-through (FRT) requirements and the high impedance fault (HIF) detection challenge the application of the conventional distance protection. To solve the two problems, a new inverse-time distance (ITD) protection for medium-voltage AC microgrids was proposed in this paper. The primary ITD protection was designed to meet the FRT requirements, and the backup protection was coordinated to isolate faults. The fault resistance and distributed generation infeed effect on the measured impedance were mitigated by a communication-assisted method proposed in this paper. Owing to the shorter communication channel time in distribution feeders and lower communication investments without a synchronous clock, the proposed method can be used to improve the ITD protection performance. An 11 kV AC microgrid was simulated with different fault types in the islanded mode and grid-connected mode. Time domain simulation verified the effectiveness of the proposed ITD protection.

Impact of inverter-based resources on transmission line relaying -part II: power swing protection

The power swing characteristic of transmission lines (TLs) can be affected by the large-scale integration of inverter-based resources (IBRs), resulting in the maloperation of the legacy power swing blocking (PSB) and out-of-step tripping (OST) functions. This paper presents a brief review of power swing phenomena and the impact on power swing protection functions. In this regard, the impact of IBR integration of type-III, and type-IV wind turbine generation (WTG) on legacy power swing protection functions has been scrutinized. To do so, the performance of impedance-based PSB and OST functions during the IBR integration has been investigated via comprehensive simulation studies. The results show that under a system contingency and high IBR penetration, depending on the IBR technology, the system experiences frequency oscillations and swinging impedance trajectories which are different from those from synchronous generators, such that the reliable operation of the legacy PSB and OST functions can be jeopardized. Moreover, during power swing phenomena, the simulation results have found that the security of distance protection cannot be guaranteed and the fault ride-through requirements cannot be maintained when a high share of IBRs have been integrated.

Grid-forming control VSC-based including current limitation and re-synchronization functions to deal with symmetrical and asymmetrical faults

Grid-forming converters operate as voltage sources behind impedance. This property makes them robust against Short-Circuit Ratio (SCR) variation, but vulnerable against large grid disturbances. As a precaution, grid-forming converters have to embed adequate control algorithms to ensure a stable system operation under various grid conditions, to deal with excessive overloadings mainly caused by faults, and to guarantee a stable re-synchronization after fault clearance. These expectations have been met in previous works considering balanced conditions. Nevertheless, the extension of the grid-forming control to deal with unbalanced grid conditions considering current limitation and angular stability is a point rarely discussed in the literature. To fill this research gap, this paper proposes an Extended Power Synchronization Method (EPSM) that allows the system to operate under balanced and unbalanced grid conditions while meeting the Fault Ride-Through requirements (FRT). The proposed method is a direct voltage control-based, which embeds a threshold current control loop, which is enabled only when a fault is detected. Additionally, the control is equipped with an algorithm that modifies the active power control during faults to aid the power converter to remain synchronized after fault clearance. The effectiveness of the proposed control has been demonstrated through time-domain simulations.

Enhancing Single-Phase Grid Integration Capability of PMSG-Based Wind Turbines to Support Grid Operation under Adverse Conditions

The proposed work delivers a robust control solution for a single-phase permanent magnet synchronous generator-based wind power conversion system (PMSG-WPCS) to enhance grid integration capability. The proposed control approach also offers an extended facility to fulfill low-voltage fault ride-through (LVRT) requirements under adverse grid conditions. Unlike the conventional observer-based PLL (O-PLL) approach, the proposed improved Lyapunov theory-based prefilter (ILP) is helpful in yielding a quadrature signal to solve the single-phase grid synchronization problem. Moreover, the proposed prefilter can leverage delayed signal operation, which improves the harmonic and the DC-offset component rejection abilities while eliminating the need for internal feedback-based submodule blocks for the case of an O-PLL. Consequently, the proposed ILP-PLL exhibits better dynamic behavior to rapidly synchronize a grid-tied power converter and can accurately track the fundamental amplitude information that is required for inverter control to meet the fault ride-through requirements. In addition, the suggested LVRT controller ensures smooth transition between the unity and non-unity power factor modes for superior converter control over reactive current injection into the grid to recover the grid from faults while maintaining a lower amount of total harmonic current distortions. The dynamic performance of the proposed control scheme is experimentally validated in view of the existing O-PLL approach for lower-rating wind-turbine-based PMSG-WPCS.

Multi-Stage Sequential Network Energy Control for offshore AC asymmetric fault ride-through of MMC-HVDC system integrated offshore wind farms

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) is the preferred solution to the problem of long-distance transmission from large-scale offshore wind farms (OWFs). When an offshore AC asymmetric fault occurs, OWFs and MMC-HVDC will face the challenge of fault ride-through with power electronic converters on both sides of the fault location. In this paper, a Multi-stage Sequential Network Energy Control (MSNEC) is proposed for offshore AC asymmetric fault ride-through (FRT). Firstly, the constraint relationship among the sequence networks under AC asymmetric faults is revealed. Secondly, the energy control potential of WFMMC is fully explored from the perspective of energy transformation and transfer. Energy control reduces the fault phase current amplitude by putting in submodules at the fault phase. Additional sub-modules are put in the non-faulted phase to realize series voltage dividing effect to actively prevent non-faulted phase fault overvoltage. The MSNEC considers the FRT requirements for multiple stages of fault occurrence, fault steady state, and fault recovery. It combines sequential network control, energy control, and voltage support of the converter station. It realizes AC asymmetric FRT from four aspects: power equivalence, negative sequence suppression, energy control, and voltage stabilization. Finally, a simulation model of OWFs connected to the onshore grid via MMC-HVDC is established in PSCAD/EMTDC. Simulation results verify MSNEC not only improves the AC asymmetric FRT capability of the OWFs and WFMMC, but also promotes the efficient consumption of renewable energy and improves the economics of the OWFs grid-connected system.

Fault ride through constrained protection scheme for distribution networks with DFIG-based wind parks

• Investigate the effect of protection scheme on the FRT requirements of DFIGs. • Preventing disconnection of DFIGs by integrating FRT into overcurrent protection. • Determine least number of communication links for relays to prevent FRT violation. • Independent of DG control, applicable to different control modes during fault. The main protection of distribution networks is based on overcurrent relays (OCRs). Due to the slow operation of these relays, some distributed generations (DGs), e.g. wind parks, may be unable to meet the fault ride through (FRT) requirements, which leads to unnecessary generation loss during faults. This paper proposes a new protection scheme for distribution networks that considers the FRT requirements of DFIG-Based wind parks. This is achieved by considering both the protection coordination constraints and FRT requirements in a single protection scheme. Considering the FRT requirements, the new method determines whether each overcurrent relay operates fast enough. If not, the proposed scheme determines a suitable solution for each relay to facilitate a faster operation. The proposed method is tested on the IEEE 33 bus test network and compared with conventional methods. Its superior impact on improving the FRT requirements and hence preventing unnecessary disconnection of DFIG-Based wind parks during short circuit faults is demonstrated through simulation results, proving by this its applicability and efficacy.

A Control Method for Converter-Interfaced Sources to Improve Operation of Directional Protection Elements

The traditional fault control strategy of converter-interfaced renewable energy sources (CIRESs) may bring about a lower sensitivity level or misoperation of fault component-based directional elements. To overcome this problem, a new control scheme is proposed to adjust sequence impedance angles of CIRESs by computing suitable current references of the CIRES controller. Meanwhile, these current references are maximized by an iterative algorithm to make full use of the short-circuit capacity of CIRESs. The proposed control scheme is applicable to various faulty conditions such as different fault types, power factors, weak grids, and larger fault resistances. Compared with the new directional elements that need to update protection algorithms, the proposed control strategies can make CIRESs compatible with the existing directional elements whilst the necessary fault ride-through (FRT) requirements can still be satisfied. Furthermore, all the controller parameters are not required to be revised based on the detected fault type, even with only local measured data collected. The associated PSCAD simulations, real-time digital simulator (RTDS) testing and the downscale hardware experiment verify the proposed method.

Research on the Automatically Independent Controlling Algorithm of the Grid-Connected PV Inverter Jointly Considering Dynamic Load Features and Active LVRT Capability

Due to environmental and energy policies in recent years, some countries will install many PV generations into power systems in the future. Under this condition, it is worrisome that the disconnection of the PV occurs because of a voltage drop triggered via a system fault, which impacts the stability of the PV(photovoltaic).A technique of FRT (Fault Ride Through) is an expected solution for PVs. However, just the FRT requirements for PVs do not inevitably increase the stability of the power system. Solving the problem of the instability of the inverter control in a grid-connected PV needs reasonable consideration. This paper proposed an automatically independent controlling algorithm of the PV inverter considering dynamic load features and Active LVRT (low voltage ride through) capability to increase the robustness and performance of the grid-connected PV system, and both simulation and experimental results showed that the condition of a range from an IM rate = 20[%] to an IM rate=40[%] could best meet the requirements for the PV Outputting recovery and the average tracking error of the input current reduced by nearly 10%, also the average output current tracking error reduced by about 4%, which shows that the tracking performance has significantly improved.

Fault Ride Through of Inverter-Interfaced Renewable Energy Sources for Enhanced Resiliency and Grid Code Compliance

Inverter-interfaced renewable energy sources (IIRESs) are typically controlled during fault conditions to meet fault ride-through (FRT) requirements, e.g., reactive current generation (RCG) requirements specified by grid codes (GCs). However, fault currents generated by inverters are different from the traditional sources, i.e., synchronous generators. Consequently, phase selection methods (PSMs) used by protection relays could suffer from erroneous fault type classification. This paper develops a dual current controller (DCC) that regulates the inverter's negative- and positive-sequence currents to simultaneously meet phase selection and RCG requirements. First, the negative-sequence-current angle is obtained based on the angles of both zero- and positive-sequence currents to enable a correct operation for phase selection. Then, the positive-sequence current angle is adjusted to reach a trade-off between RCG requirements and phase selection achieved by the negative-sequence current. Lastly, the reference currents of the IIRES are generated in the stationary frame without violating the inverter's current limits. The proposed DCC supports the grid voltage by meeting the RCG requirements and enhancing the grid reliability and resilience by enabling correct phase selection. Comprehensive time-domain and real-time simulation verify the precise operation of the proposed DCC under various fault conditions and GCs.

Comprehensive Review on Fault Ride-Through Requirements of Renewable Hybrid Microgrids

The world is interested in applying grid codes to increase the reliability of power systems through a micro-grid (MG). In a common practice, the MG comprises a wind farm, and/or photovoltaic (PV) arrays that are integrated with diesel generators and energy storage devices. Fault ride-through (FRT) capability is an important requirement of grid codes. FRT means that the MG is still connected to the grid during numerous disturbances such as faults. This is required to ensure that there is no loss of power generated due to grid faults. Reactive currents must be injected into the grid to increase the power system stability and restore voltage. To enhance FRT for doubly fed induction generator (DFIG) based WT installation, internal control modifications of rotor-side converters and grid-side converters are applied. The solutions that depend on these modifications are traditional and advanced control techniques. Advanced control techniques are needed due to the non-linear nature and less robustness of traditional ones. External hardware devices are also added to improve the FRT of DFIG which are classified into protection devices, reactive power injection devices, and energy storage devices. A comprehensive review of FRT enhancements of DFIG-based WTs, PV systems, and MGs using hardware and software methods is presented in this effort. A classification of FRT of PV systems is characterized plus various inverter control techniques are indicated. Several FRT methods for hybrid PV-WT are presented, with full comparisons. The overall operation and the schematic diagrams of the DFIG-WT with FRT methods are discussed and highlighted. Many Robust control methods for controlling grid connected AC, DC and hybrid AC/DC MGs in power systems are addressed. A total of 210 reported articles were review, including the most up-to-date papers published in the literature. This review may be used as the basis to improve system reliability for those interested in FRT methods. Various traditional and advanced control techniques to improve the FRT abilities are summarized and discussed, including protection devices, reactive power injection devices, and energy storage. In addition, the classifications of FRT hardware methods for DFIG are presented, including grid code requirements.

Nonlinear Variable Resistor-Based FCL for Fault Ride-Through Performance Enhancement of DFIG-Based Wind Turbines

Fault ride-through (FRT) requirement is a matter of great concern for doubly fed induction generator (DFIG-) based wind turbines (WTs). This study presents a nonlinear variable resistor- (NVR-) based bridge-type fault current limiter (BFCL) to augment the FRT performance of DFIG-based WTs. First, the BFCL operation and nonlinear control design consideration of the proposed NVR-based BFCL are presented. Then, the NVR-BFCL performance is validated through simulation in PSCAD/EMTDC software. In addition, the NVR-based BFCL performance is compared with the fixed resistor- (FR-) based BFCL for a three-phase symmetrical short circuit fault at the grid side. Simulation results reveal that the NVR-based BFCL provides a smooth and effective FRT scheme and outperforms the FR-based BFCL.

Protecting Distribution Systems With Inverter-Interfaced PV Plants Using Q-Axis Components

Satisfying fault ride-through requirements, solar photovoltaic (PV) plant remains connected to the grid even during a fault. With current limiting feature, the plant current during a fault becomes comparable to its rated current. This imposes challenges to conventional local data-based protection methods employed in the relay on PV-side. In such a situation, the selectivity of a protection scheme is crucial for maintaining an uninterrupted supply to customers. In this article, a q-axis component-based protection scheme is proposed for the distribution system with a PV plant. The method extracts the second-order harmonic components of q-axis voltage and current at relaying point for fault detection and direction identification. Using this direction information at both ends of a line segment to be protected, internal and external faults are discriminated. The performance of the proposed method is tested on 14-bus distribution system for various fault cases. A comparative analysis with existing techniques shows the strength of the proposed method. Hardware-in-loop testing using OPAL-RT simulator as system and Arduino NANO microcontroller as relay validates the performance of the proposed method in real-time.

A DC Solid-State Transformer With DC Fault Ride-Through Capability

This article investigates a dc solid-state transformer (DCSST) with dc fault ride-through (FRT) capability for medium-voltage dc (MVDC) systems. The FRT requirement for DCSST in dc grid is investigated. Short-circuit fault clearance on the medium-voltage (MV) side is realized by introducing full-bridge converters with cascaded dc converter modules on the MV side of the DCSST. The FRT strategy under typical voltage sags is analyzed. Factors affecting the fault clearing and system recovery are considered, including the number of full-bridge converters, the values of dc inductance, and the dc-link bus voltage. The concept is verified with both simulations and experiments.

Positive Sequence Relaying Method for Solar Photovoltaic Integrated Distribution System

For fault ride-through requirement, solar photovoltaic (PV) plant remains connected to the grid during a fault. In such a situation, a PV plant operates in different modes and supplies current to comply with the requirements of the grid. The relative magnitude of phase current contributed by a PV plant during a fault is at par with the nominal current. Such a fault behavior restricts the application of available phase current based relaying approaches while protecting the distribution system with a PV plant. A protection method using local positive sequence voltage and current is proposed to protect a distribution system with PV plant. The method selects pickup current using proposed current-voltage curve for fault detection. A three-dimensional time-current-voltage characteristic is proposed for trip time calculation. Performance of the proposed method is tested for relays on both PV plant side and grid end using DigSilent PowerFactory simulation data for a CIGRE 14-bus distribution system with PV plant having different modes of operations. The method is found to be accurate and comparative assessment reveals its superiority.

Multi-resistor BFCL for FRT capability improvement of DFIG-based wind farm

Grid connection of wind farms (WFs) results in increment of short circuit level and decrement of fault ride-through (FRT) capability under fault conditions. Application of bridge-type fault current limiter (BFCL) was recognised as a promising and cost effective solution to cope with these problems. However, full resistor insertion of the conventional BFCL provides high transient overvoltage at the terminal of wind generator under low voltage sag condition, which can lead to failure operation of wind generator in despite of FRT requirement. This study presents a voltage adaptive multi-resistor BFCL (MRBFCL) to solve these problems for wide voltage sag levels. According to the depth of voltage sag level at the grid coupling point voltage, the MRBFCL inserts an appropriate combinational of resistors to compensate voltage sag level at acceptable level. To verify the efficiency of the proposed MRBFCL, time domain simulations were performed in PSCAD/EMTDC software under three different voltage sag levels. Also, the MRBFCL performance is compared with the conventional single-resistor BFCL. Simulation results reveal that the MRBFCL effectively enhance the FRT performance of doubly-fed induction generator (DFIG)-based WFs under wide range of voltage sag level and alleviate over voltage for low voltage sag conditions.

Multi‐objective control scheme for operation of parallel inverter‐based microgrids during asymmetrical grid faults

The growing interest in connecting more distributed generation (DG) units to the utility grid, microgrids deal with the various challenges to satisfy a sufficient level of ancillary services such as active power oscillations (APOs), reactive power oscillations (RPOs), fault ride-through (FRT) capability, and overcurrent problem. Hence, for parallel operated grid-connected inverters (GCIs) based MG, this study presents a multi-objective control scheme that simultaneously ensures elimination of the collective APOs/RPOs at point of common coupling (PCC), overcurrent protection and reactive power injection. One of the significant parts of this study compared with similar existing studies is that provides reactive power support capability to fulfil the FRT requirements of the grid-connected multi-DG units and to remain grid-connected during asymmetrical grid faults. A current restraining control is also presented to ensure the safe operation of the MG system and to avoid overcurrent. The cancellation of the collective APOs and RPOs at the PCC for parallel operation of the GCIs has been achieved by using adjustable control coefficients and demonstrated with theoretical analyses in detail. Extensive case studies are presented and discussed to demonstrate the performance of the proposed ideas and to meet the shortcomings of the previous studies.

Fault Ride-Through Characteristics of Small Wind Turbines

There is significant potential for an increase in the use of kilowatt-class small wind turbines (SWTs) in Japan due to reduced limitations with respect to installation, despite their high cost. At this stage, the Japanese grid code has not been considered sufficiently with respect to grid-connected SWTs, and the addition of fault ride-through (FRT) requirements for SWTs has been requested. Moreover, the FRT of SWTs is challenging to achieve owing to the low inertia constants when compared with those of large-scale wind turbines, which result in significant acceleration of the rotor speed and an increase in the input voltage of the power conditioning system (PCS) during FRT operation. In this study, FRT field tests were conducted on SWT systems against a voltage dip with a duration of ~1 s, and it was confirmed that the SWT systems satisfied the FRT requirements for photovoltaic (PV) systems connected to low-voltage distribution lines in Japan. The behaviors of the rotational speed of the SWTs and the PCS input voltage in an FRT operation were then analyzed, and it was noted that the increase in the PCS input voltage with the overspeed of the turbine can reach the upper limit and make the PCS cease operation, which indicates failure of the FRT. The overvoltage, therefore, requires restriction using a method such as pitch control, furling, and electrical and/or mechanical brakes.

Protection of distribution network considering fault ride through requirements of wind parks

Fault ride through (FRT) requirements are imposed by grid codes in order to prevent from the loss of power generation due to the voltage drop caused by grid faults. To keep the wind turbine connected to the network the protection system should remove the fault, within the time specified by FRT requirements. This paper proposes a method for considering the FRT requirement characteristic of wind parks in over current relays protection coordination problem. For this purpose, a mathematical relation is presented between the fault current passing through the relay and the voltage drop at the point of common coupling (PCC) of wind turbine. Then, using this formulation, a new index is defined to evaluate the time coordination between FRT requirements and protection system. Also, a new objective function is proposed to determine the optimal setting of relays and the optimal impedance value of fault current limiter (FCL) in order to FRT capability enhancement. The proposed approaches are tested on the IEEE-30 bus and IEEE-33 bus test systems to verify high performance of them in improving the FRT capabilities of wind parks.