Medium power distributed energy resources (DERs) are commonly connected to medium voltage distribution systems via voltage source converters (VSCs). Several guidelines and standards have been developed to establish the needed criteria and requirements for DERs interconnections. In this respect, it is preferred to reinforce the VSC fault ride through (FRT) capability, which considerably minimizes the DG outage period and reconnection time and results in a resilient system against short circuits. Considering the significant number of asymmetrical faults in distribution systems, the VSC response in such conditions must be investigated, and consequently, its FRT capability must be reinforced. In this paper, first, a comprehensive review on the existing FRT methods has been presented and discussed. Accordingly, an adaptive virtual impedance-based voltage reference generation method is proposed, which enhances the VSC behavior under short circuits and increases the VSC FRT capability. Also, a fast sinusoidal current reference limiter is proposed to improve the performance. To evaluate the performance of the proposed scheme, state-space analysis is presented, and a complete set of simulations is performed in PSCAD/EMTDC environment. Also, a comparison with the conventional method is presented.

/pdf/reinforcing-fault-ride-through-capability-of-grid-forming-yiz8fwdkix.pdf

Reinforcing Fault Ride Through Capability of Grid Forming Voltage Source Converters Using an Enhanced Voltage Control Scheme

This paper proposes a decentralized multiagent system (MAS) approach to solve the service restoration problem considering the uncertainty of load demand and renewable distributed generators (RDGs). Service restoration is formulated as a multi-objective optimization problem and solved by implementing controlled DG islanding using the proposed MAS approach. First, the uncertainty of load demand and RDG generation is forecast by generating scenarios using Monte Carlo simulations. Next, the expected nodes to be restored in an island are determined using a heuristic rule-based technique. Finally, the maximum likelihood of the expected island ranges to be restored are estimated using the maximum likelihood estimation method to assist the utility in decision making. Extensive case studies are conducted on 38 bus and 119 bus distribution system to show the efficacy of the proposed restoration strategy. The use of electric vehicles and battery energy storage as flexible sources of energy to alleviate the uncertainties in the system is also highlighted.

A Decentralized Multi-Agent Approach for Service Restoration in Uncertain Environment

The deployments of inverter-interfaced distributed generators (IIDGs) in medium voltage distribution networks may cause undesirable trips or tripping failures of conventional two-terminal current differential protective relays. It is economically and technically unfeasible to reconfigure large numbers of time-synchronized sampling clocks and communication channels for each terminal inside the protected zone. In this paper, a more accurate IIDG mathematical equivalent model is established that considers the control strategies of IIDG, and the profile characteristics of the positive-sequence voltage at each point of common coupling are investigated. Then, a real-time estimation algorithm is introduced for the fault current contribution of grid-connected IIDGs. On this basis, this paper proposes a novel virtual multi-terminal current differential protection scheme. This scheme only necessitates exchanging the electrical quantities between the protective relays at both terminals via an existing two-terminal pilot channel, thus reducing the cost and requirements for communications. The feasibility of the proposed protection is demonstrated in both the PSCAD/EMTDC simulation platform and the RTDS hardware experimental laboratory. A comparison of the proposed protection alongside the conventional protection is also presented. Test results indicate that the proposed protection is widely applicable to various fault conditions, thereby improving the sensitivity and selectivity of conventional protection schemes.

A Virtual Multi-Terminal Current Differential Protection Scheme for Distribution Networks With Inverter-Interfaced Distributed Generators

The installation of photovoltaic (PV) systems is gaining great attention due to the matured PV technology and the lowered price of PV modules. With an increasing penetration of PV systems, the selectivity of distribution network protection may be affected which results in undesirable de-energisation of loads, damage of network equipment, and reduction of reliability. This study presents a method to preserve the protection coordination considering future PV systems installation with any penetration level and different locations along the distribution feeder. Depending on the accessibility of protection device settings or PV control parameters, the proposed method modifies the existing characteristic curve of overcurrent devices or limits the output current of PV sources, respectively. The proposed strategy does not change the structure of existing distribution network protection system and can also be implemented in the old and non-programmable relays. Meanwhile, it does not need the communication links. The merits of the proposed method are demonstrated through several case studies using the Isfahan distribution network.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2017.1229

Protection coordination scheme for distribution networks with high penetration of photovoltaic generators

Protection of a distribution network in the presence of distributed generators (DGs) using overcurrent relays is a challenging task due to the changes in fault current levels and reverse power flow. Specifically, in the presence of current limited converter interfaced DGs, overcurrent relays may fail to isolate the faulted section either in grid connected or islanded mode of operation. In this paper, a new inverse type relay is presented to protect a distribution network, which may have several DG connections. The new relay characteristic is designed based on the measured admittance of the protected line. The relay is capable of detecting faults under changing fault current levels. The relay performance is evaluated using PSCAD simulation and laboratory experiments.

Fault isolation in distributed generation connected distribution networks

This paper presents a novel fault current management (FCM) technique for radial distribution systems with embedded inverter-based distributed generators (IB-DGs). At the point of connection to a power system, many distributed generators (DGs) require power electronic (PE) interfaces, which are normally idle during faults. The proposed FCM method employs these PE interfaces for control of the fault current. For this purpose, operation of IB-DGs is modified to FCM mode at the moment of fault and new current references are applied. Of the two controllable parameters of the IB-DG output current-current magnitude and current phase angle-the current phase angle is chosen as the means of controlling the fault current magnitude. The reference current phase angle is calculated based on the relation between the fault current elements and their phase angles. As a result of this novel operation, IB-DGs with larger capacity can be connected at different locations of the system without affecting the fault current magnitude. Also, implementing this technique in smart grids is economically proven, since the asset of power system which have been designed for normal operation are employed to manage the fault current magnitude. Moreover, possibilities of synchronization problems are reduced by keeping IB-DGs connected to the system at all the time. The evaluation of the proposed FCM technique using the standard IEEE 33-bus distribution system demonstrates the effectiveness of the proposed method for managing the fault current magnitude.

Fault Current Management Using Inverter-Based Distributed Generators in Smart Grids

Of the numerous types of distributed generators (DGs), synchronous DGs represent the most substantial contribution to fault currents and, consequently, have the greatest effect on the operation of the protection system. On the other hand, many types of DGs require power electronic (PE) interfaces at the points where they connect to the grid, which are normally left idle during fault conditions. This paper presents a novel idea for employing the PE interfaces of inverter-based DGs (IBDGs) as a means of managing the fault current contribution of synchronous DGs through the modification of the IBDG current phase angle during fault conditions. This operation enables synchronous DGs and IBDGs to be kept connected to the system during fault conditions but with their contribution to the current having no effect on the magnitude of the fault current. Constraints related to DG locations and capacities are thus relieved. More interestingly, it is demonstrated that larger-capacity IBDGs are more effective for managing the fault current contribution of synchronous DGs, which means that for systems that include synchronous DGs, the availability of more numerous IBDGs would be beneficial.

Management of Fault Current Contribution of Synchronous DGs Using Inverter-Based DGs

This paper presents two analytical frameworks to study the fault current contribution from inverter based distributed generation (IB-DGs) in transient and steady state conditions. The developed models can be used to analyze the fault response of IB-DGs in active distribution networks. The steady state analysis of fault current contribution from IB-DGs is calculated using the forward-backward sweep method, whereas the transient model for the IB-DGs is developed using the IB-DG differential equations. The results of the both models are validated by PSCAD simulations.

Analysis of fault current contribution from inverter based distributed generation

This paper presents a version of the Newton-Raphson (NR) algorithm that has been modified to provide a means of facilitating the power flow analysis of loop systems in the presence of current sources that rely on current controlled inverters, such as distributed generators (DGs). The modifications entail the inclusion of the current elements of current sources in the Jacobian matrix of the NR algorithm. The effect is to enable current controlled sources to be modeled directly in the power flow algorithms without the need for converting their currents to the corresponding power components, which is the traditional practice in power flow algorithms. The proposed algorithm has been tested on an IEEE 14-bus system. For loop systems with current controlled sources, the test results show that using this algorithm for power flow analysis offers enhanced accuracy.

A novel Newton-Raphson algorithm for power flow analysis in the presence of constant current sources

This paper describes an approach to determine the impact zone and voltage sag metric for delivery points (DP) in large meshed transmission systems. The metric is to quantify the severity and frequency of voltage sag (VS) experienced at particular DPs. The network model of the system is developed and verified for voltage sag calculation using fault data captured by the power quality (PQ) meters installed across the system. The validated model is used to form the impact zone in terms of the circuit length causing the voltage sag event. Having the impact zones, stochastic fault data is used to create the voltage sag metric. To show the sensitivity of voltage sag metric to different treatment of stochastic fault data, the voltage sag metric is obtained with two sets of data: individual circuit outage frequency and zonal average outage frequency. The approach is implemented on Hydro One Ontario Transmission system. The results are examined to correlate the system behavior with network configuration, voltage level, and proximity to major stations with auto transformers. The results are validated using the data collected over 3 to 5 years by the PQ meters installed in the system. At utility-scale, the approach is used to proactively predict the impact of new planning projects, the retirement of big generation units, and major system reconfiguration on the voltage sag performance of major stations. Additionally, for customers connected on the low voltage (LV) side of transformer stations, the proposed metric can be used to differentiate the impact of transmission system from the events caused by faults on distribution feeders.

Nazila Rajaei

Papers

Fault Current Management Using Inverter-Based Distributed Generators in Smart Grids

Management of Fault Current Contribution of Synchronous DGs Using Inverter-Based DGs

Analysis of fault current contribution from inverter based distributed generation

A novel Newton-Raphson algorithm for power flow analysis in the presence of constant current sources

Development of Voltage Sag Metric for Large Meshed Transmission Systems and Stochastic Fault Data Sensitivity Analysis