Inverter-interfaced Distributed Generations Research Articles

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.

Optimized Fault Location Identification in Power Distribution Systems With Inverter-Interfaced Distributed Generations

This article proposes a fault location identification method for radial power distribution systems with inverter-interfaced distributed generations (IIDGs). The proposed method is not restricted to any specific types of data. It can fully utilize all available data types in power distribution systems to enhance the accuracy of results. The dataset may include synchronized voltage and current measurements collected by micro-phasor measurement units (micro-PMUs) and/or unsynchronized measurements collected by legacy measuring devices, active and reactive power measurements, pseudo measurements of load values, and virtual measurements based on Kirchhoff's laws. The proposed method explicitly considers uncertainties in the dataset and is applicable to different load types. The article proposes a mathematical model for representing IIDGs in fault location identification applications that does not require any synchronized sampled data or during the fault period data collection. Simulation results on IEEE 34-node feeder demonstrate the accuracy of the proposed method.

Improvement of directional relays based on constant impedance angle control of inverter interfaced distributed generations

With a large number of Inverter Interfaced Distributed Generations (IIDGs) being connected to active distribution network, there is a serious problem that the sensitivity of directional relay declines. The basic reason has been found that the phase angle of IIDG equivalent impedance is no longer constant and fluctuates in a large range. Researchers have proposed some methods to improve the directional relay performance by means of controlling the characteristic of IIDG equivalent impedance after a fault. However, all existing methods ignore the mandatory requirement of low voltage ride through, resulting in less practical value. This paper proposes a novel IIDG control strategy of constant impedance angle considering the requirement of low voltage ride through and presents a coordination scheme of control and protection to improve the performance of directional relay. Firstly, this paper divides voltage drop degree represented by k into three intervals, which are k>0.9, 0.2⩽k⩽0.9 and k<0.2. In each interval, the calculation methods of inner loop reference currents for d-axis and q-axis are derived respectively to achieve the proposed control strategy. Then, the influence of fault current limitation on the adaptability of the proposed control strategy is analyzed in the interval of 0.2⩽k⩽0.9. In addition, the switching scheme of control strategies and the coordination scheme are also designed in detail. Finally, digital simulations verify that the proposed control strategy can achieve the control objective of constant impedance angle while satisfying the requirement of low voltage ride through, regardless of how deep the voltage drop degrees are. Afterwards, the proposed coordination scheme is compared with the scheme in which the pre-fault constant PQ control strategy continues to be used after a fault. The comparison results demonstrate that the coordination scheme can improve the performance of directional relays effectively.

Primary and Secondary Control of Grid Connected Micro Grids using ANFIS Controller

The microgrid has been defined to cope with the large penetration of distributed generations (DGs). The microgrid is capable of operating in grid-connected mode (GC) and islanded mode (IS). In microgrid, distributed generations (DGs) are always connected to power network via power inverters. In the Grid Connected mode, DGs commonly acts as current controlled sources to retain a high grid-current quality and a fast dynamic response in the photovoltaic and wind generation systems. However, this current controlled DGs cannot work alone without the voltage/frequency support from utility-grid. Islanding occurs when a portion of a micro-grid becomes electrically isolated, yet continues to be energized by DGs connected to the isolated subsystem. In islanding mode, the inverter-interfaced DGs work as voltage-controlled sources. As system voltage and frequency are not determined by utility-grid, DGs should take care of the voltage/frequency stability. Moreover, the power sharing should be guaranteed according to their individual ratings to avoid circulating currents among DGs. The operation and control of existing utility networks are becoming more and more complex due to the increased interconnection of a DG systems with diverse characteristics. So this paper intends to address the above problems and come out with a proper control strategy using ANFIS controller that could be implemented for the successful operation of a microgrid.

Faulted Line-Section Location in Distribution System With Inverter-Interfaced DGs Using Sparse Meters

This paper proposes a faulted line-section location method using sparse meters for the distribution networks (DNs) with high penetration of inverter interfaced distributed generations (IIDGs). By modeling the IIDGs as equivalent impedances instead of current sources, the fault component model of the DN with IIDGs can be transferred from a multi-current feeding system towards the sole-current feeding system. On this basis, the Bayesian Compressed Sensing (BCS) algorithm is applied to achieve the sparse solution of the fault current to indicate the faulted line-section. Moreover, to reduce the required meters for monitoring the IIDGs, a node partition scheme is designed based on the analysis of IIDG with different control strategies. The proposed fault location method requires only 15 meters for an IEEE 69-node distribution network with 20 IIDGs. Simulation results show that the location success rate is beyond 95% for various fault scenarios which is a 55% improvement over the existing BCS-based method.

Fault characteristics of Inverter-Interfaced Distributed Generation

Distributed generations can be divided into traditional synchronous generators and inverter interfaced distributed generations (IIDG) according to their different operation mode. While the fault characteristics of IIDG is different from traditional Distributed Generators, the in-depth analysis on output characteristics and fault characteristics of IIDG are the foundation of micro grid protection. The mathematical models under P/Q and V/F control strategy are discussed. After simplifying its model, the fault characteristics of IIDG under P/Q and V/F control strategy are studied using the simulation of PSCAD/EMTDC. The analysis on the fault characteristics of IIDG that lays the theoretical foundation for the micro-grid relay protection.

New protection scheme for internal fault of multi-microgrid

Multi-microgrids have many new characteristics, such as bi-directional power flow, flexible operation and variable fault current consisting of the different control strategy of inverter interfaced distributed generations (IIDGs), which all present challenges in multi-microgrid protection. In this paper, the current and voltage characteristics of different feeders are analyzed considering faults at different locations of the multi-microgrid. Based on the voltage and current distribution characteristics of the line parameters, a new protection scheme for the internal faults of multi-microgrids is proposed, which takes the change of phase difference and amplitude of measured bus admittances as the criterion. This proposed scheme has high sensitivity and reliability, is based on a simple principle, and can be easily adjusted. Simulation results using PSCAD/EMTDC verify the correctness and effectiveness of the protection scheme.

Calculating the maximum penetration capacity of distributed generation considering current protection

With significant increasing penetration of the inverter interfaced distributed generations(IIDG), the radial network turns into a multipower-supply network. Working as an additional source, IIDG may increase the total fault level and altering the magnitude and direction of the fault current, which compromise the correct operation of the overcurrent protection system. This paper presents a new method to calculate the optimal allocation of IIDG without any amendment of protection settings, which will make great convenience for operators to review the accessible capacity of IIDG reported by users. Under the consideration of low voltage ride through(LVRT), the method uses iterative correction to figure out the nonlinear relationship between the fault current of IIDG and the fault voltage of the point of common coupling(PCC) in order to get the optimal capacity which meets the constraints of the current protection’s selectivity and sensitivity.

Real-time Validation of a Robust LQG based Decentralized Supplementary Control Loop to Mitigate Instability in an Islanded AC Microgrid

Abstract: Microgrids comprising of inverter-interfaced distributed generations (IIDGs) can be subjected to high penetration levels of rectifier interfaced active loads (RIALs). The interaction between the control loops of these loads and the IIDGs can cause instability in these microgrids. To mitigate the instability, this letter proposes a robust decentralized linear quadratic gaussian (LQG) control loop around each IIDG unit. The LQG control combines the Kalman estimator with the linear quadratic regulator with prescribed degree of stability (LQRPDS). In order to obtain the optimal values of the diagonal weighting matrices of Kalman estimator and LQRPDS, a bi-objective optimization problem has been formulated and solved using a fast and elitist multi-objective non-dominated sorting genetic algorithm (NSGA-II). The performance of the proposed control loop in real time environment has been validated through simulations carried out in OPAL-RT eMegaSim® real-time digital simulator and is found to provide robust control under small load disturbances.

Toward intelligent transient stability enhancement in inverter-based microgrids

Nowadays, the concept of multiple inverter-interfaced distributed generations (IIDGs)-based MG is recognized as a renowned notion. Encountering unexpected transient situations, the fast inflexible response of IIDG may contribute in serious concerns over its successful operation. Contemplating the transient stability paradigm, first swing stability of the investigated system is the mostly pinpointed matter. In the state-of-the-art indices in transient analysis of IIDG-based technologies, the current index is referred as the frequently deployed one. However, this index is capped within the switches’ twice rated current to afford the inverter’s physical constraints. To tackle this requirement, the ongoing study aims at devising an efficient transient current control loop (TCCL) embedded as a part of main control procedure. In this practice, the well-known simple proportional–integral (PI) controller, as the most persuasive industrial choice, is regarded as the supplementary TCCL key unit. The main functionality of the founded TCCL is deemed as a talented transient current limiter in IIDGs during the versatile possible short-circuit situations. In spite of this, the conventional fixed tuning of gains in PI controller would depreciate its safe and reliable operation encountering different contingencies. To rehabilitate this matter, fuzzy logic and artificial neural network concepts are deployed for realizing an adaptive PI controller capable of handling both the connected and autonomous modes of operation. Precise numerical studies are carried out to interrogate the performance of the proposed approach. Results are analyzed in depth.

A fuzzy logic-based droop control for simultaneous voltage and frequency regulation in an AC microgrid

Modern power systems require increasing intelligence and flexibility in the control and optimization to ensure the capability of maintaining balance between generation and load under violent disturbances. There are several inverter interfaced distributed generations (IIDGs) with local and global control loops in a microgrid (MG). Voltage and frequency of MGs are strongly impressionable from the active and reactive load fluctuations. A change in load leads to imbalance between generation and consumption. The output voltage and frequency of the IIDGs are primarily controlled by the droop characteristics. But, in case of severe changes in load, the IIDGs may be failed and the MG is collapsed. In this paper, for optimally tuning of a generalized droop control (GDC) structure and also secondary voltage and frequency controllers, the fuzzy logic technique is utilized. It is shown that the proposed fuzzy logic controller exhibits high performance and desirable response for different scenarios of change in load.