Protection of smart distribution systems with distributed energy resources and solid state transformers

A Solid-State Transformer as Enabling Technology for Microgrids for Integration into the Power Distribution System: An Overview

The dynamic behavior of microgrid compromises the security and selectivity in conventional protection schemes. Islanding mode of operation provides continuous power supply to customers. Mesh configuration of microgrid can further improves the reliability. However, protection of mesh microgrid is a challenging task especially the isolation of faulted part only with rest of the system in operation. This paper presents a communication assisted protection scheme using negative sequence based voltage and current information of a microgrid. Using the change in angle difference of negative sequence voltage and current from pre-fault to fault case, method identifies the fault is internal or external. A hardware setup is established in the laboratory and performance of the relay algorithm is tested. Arduino uno microcontroller board is used as relay processor and XBee module is used for wireless communication.

To design a perfect protection scheme is a big challenge for microgrid (MG) network implementation because it can operate either grid connected mode neither islanding mode. Due to distributed generation (DGs) resources the fault current always varied steadily hence the system parameters. This research focus on two vital properties of microgrid: peer-to-peer and plug-and-play to avoid sophisticated communication network for microgrid adaptive protection schemes. A microgrid model has been proposed by conventional protection scheme and addressing all possible drawbacks along with distributed energy resources. Whole microgrid is divided into different protection zone to identify perfect fault detection techniques by analyzing corresponding system parameters. Moreover, transformer protection also considered in new proposed protection scheme.

In recent years, the concept of solid-state transformers (SSTs) has evolved to be one of the front runners in the development and realization of a smart grid. The recent strides in the field of medium voltage wide bandgap semiconductor devices and the availability of 10 kV SiC MOSFETs and 15 kV SiC IGBTs have made it possible to build medium voltage SSTs using simple converter topologies. With the SST interfacing two grids, it forms a crucial juncture in the electrical distribution system which necessitates a high fault tolerance capacity of the SST during grid fault conditions. In addition, the grids need to be protected from any faults or abnormal operation in the SST which makes it imperative to implement different fault protection considerations on the SST. In this paper, these fault protection systems are studied in terms of their breaking time, breaking current/voltage, and fault coordination. The protection schemes are analyzed on a Gen3 10 kV SiC MOSFETs based Mobile Utility Support Equipment Solid-State Transformer (MUSE-SST) which is developed at FREEDM Systems Center aimed at interconnecting a 4.16 kV grid to a 480 V grid or load. Simulation results and are provided to evaluate the protection schemes. Initial experimental test results are also provided.

This paper presents a wind-solar hybrid system using solid state transformer (SST) for reactive power compensation to achieve power quality. The proposed hybrid system using SST can effectively suppress the voltage fluctuation. This work proposes a new modelling approach of a combined Photovoltaic and Wind power system. The controller has been designed with fuzzy logic and sliding mode along with SST. This approach enables the design and implementation of efficient controllers for distributed energy resource (DER) hybrid systems. The simulation of proposed work is carried out using MATLAB.

The increasing penetration of distributed energy resources (DERs) in distribution systems brings new challenges in protection of distribution systems. The bidirectional fault current flow caused by the presence of DERs and their characteristics of low fault current make the conventional relays deficient in protecting distribution systems with DERs. In this paper, we propose the use of Dynamic State Estimation-based protection(EBP) which is immune to bidirectional flows as well as low levels of fault currents to protect distribution systems with DERs. EBP identifies faults based on the goodness of fit between the measurements and the estimated state values from the protection zone, irrespecitively of fault current level and direction, or harmonic content that may be generated by DERs. Models of two real-world distribution systems with utility size PV arrays were built in WinIGS to test the performance of EBP. The results showed that EBP can effectively detect faults in distribution systems with substantial PV capacity and it is immune to the characteristics of active distribution systems.

DC distribution system has advantages of high power quality, large transmission capacity, high reliability, simple structure, economy and low energy consumption, and so forth. It has been a key part of smart grid nowadays. However, the development of DC distribution system is constrained by the lack of operational experience in DC system, the small interrupting capacity of DC circuit breaker (CB), and the lack of protection schemes for system itself. In this paper, protection for DC distribution system with distributed generator (DG) is fully investigated and verified. Firstly, the electromagnetic transient model of DC distribution system with DG is presented. Simulation based on the electromagnetic transient model is carried out. Both the step response and the steady-state performance verify the accuracy of the model. Then the fault characteristic mechanism is analyzed, and the protection principles and scheme are investigated in detail, including voltage mutation principle as protection starting component, differential current protection principle for DC bus, and two-section current protection for distribution lines. Finally, transient responses with protection scheme are analyzed during faults. The results present that the protection principles and scheme are feasible for DC distribution system with DG.

The behavior of anomalous dielectrics : H. Jordan. ( Deutsch. Phys. Gesell. Verh., xiv, 9, 451)

In line with global efforts to achieve 100% renewable energy targets, it is expected to see significantly higher ratio of inverter-based resources (IBRs) integrated into distribution systems. The impacts of high penetration of IBRs on distribution systems has become a critical consideration for utilities to further investigate. Currently, a large percentage of IBRs are distributed roof-top behind-the-meter PV installations which utilize single-phase PV inverters.The existing PV inverters connected to distribution feeders are a combination of legacy and smart inverters. Legacy inverters are the traditional inverter technology with settings and capabilities that follow RULE 21–2003 version. However, the latter generation as called Smart Inverters incorporates specific functions (following the latest version of Rule 21 (after September 2017)) that are intended to support grid and achieve higher penetration.This paper presents a summary of observations from a series of studies with focus on analyzing the impact of high penetration of single-phase smart and legacy PV systems on the existing feeder protection systems. A representative SCE’s distribution feeder was modeled in the PSCAD/EMTDC software and used as a benchmark to perform different scenarios. The remarkable increase in the penetration of solar PV systems, exceeding annual peak load of the feeder was shown to create several protection challenges due to bi-directional power flow and change in fault current level seen by protective elements. A comparative analysis of fault current seen by the feeder protection system with a mix of Distributed Energy Resources (DERs) at different penetration levels is done in this study. Key takeaways from various simulation studies are described.

The performance of conventional protections has deteriorated in active distribution networks (ADN) because of high penetration of distributed generator (DG) and solid state transformer (SST). In this paper, based on the features of short-circuit current fed by DG and SST, the current distribution characteristics of ADN are analyzed. Moreover, a networked protection scheme is proposed for ADN. This scheme compares the amplitudes of negative-sequence, zero-sequence and phase currents between different ports of grid component to realize fault identification. It can act rapidly and has the ability to cope with large transition resistance. Communication system only needs to transmit current amplitude, and synchronous sampling is not required. In addition, there is no requirement for voltage measurement, which will help to reduce cost. The effectiveness of the protection scheme is verified via simulation tests.

Summary In this paper a new framework is introduced to develop a coupled active and reactive market in distribution networks. Distributed energy resources (DERs) such as synchronous machine–based distributed generations and wind turbines offer their active and reactive powers to the proposed market. For the considered DERs, multicomponent reactive power bidding structures are introduced based on their capability curves. Also, the hourly speed variations of wind turbines are considered in the proposed model. A distribution company buys active and reactive powers from a wholesale market and sells them via this market. The demand side is active, and responsive loads or aggregators can participate in the market using a demand bidding/buyback program. The objective function of the proposed market is to minimize the active and reactive power costs of DERs and distribution companies, the penalty cost of CO2 emission, and the cost of running a demand bidding/buyback program. The effectiveness of the proposed method is examined on a 22-bus 20-kV radial distribution test system.

In recent years, the networked distribution system (NDS), which is normally connected to the distribution line (DL), was actively studied as the topology of the future distribution system for reasons such as improving supply reliability, improving line utilization, and increasing the capacity of distribution generators (DGs). However, the NDS creates new issues in terms of protection coordination because of its bidirectional power flow and fault current flow. The issues associated with conventional protection schemes in the NDS include malfunction of protective devices due to bi-directional fault currents and failure of protection coordination due to communication failures between protective devices. When applying a conventional protection method in the NDS, the protection schemes become complicated, and there is a risk of protection coordination failure due to communication failure between protective devices. To solve this problem, this paper proposes an effective and innovative non-communication-based protection algorithm for protection coordination in the NDS. The proposed protection algorithm utilizes fault impedance characteristics, which allow not only determination of whether a fault occurred, but also the ability to identify the exact fault point. Therefore, the proposed method is expected to be sustainably utilized and contribute to developing protection schemes and devices in various system topologies and scenarios in the future. Additionally, this paper addresses the overall concept of hardware-in-the-loop simulation (HILS) and directly verifies the proposed protection algorithm using HILS. Therefore, this study establishes a sustainable foundation for future research on protection coordination using HILS.

The AGGREGATE project team successfully developed and validated various modules for outage management. Brief summaries of each module are provided to showcase their strength for outage management and restoration for a distribution system with a high penetration of connected distribution energy resources (DERs).In recent years, inverter-based DERs have been widely deployed in distribution system. A most of behind-the-meter (BTM) solar power generation is not visible to the utility. The data-driven DER and load estimation modules are using machine learning (ML) and artificial intelligence (AI) to manage this issue, which provides an opportunity for distribution system operators (DSOs) to operate systems and make decisions in real-time for a distribution system with a high penetration of DERs deployed. Also, the estimated DER and true load can be further leveraged in network aggregation and cold-load pick up estimation for reducing the computing complexity and providing for fast restoration. After load demand and DER power generations have been estimated, the information will support topology and state estimation(SE). The topology estimation module demonstrated the viability of mixed integer linear programming (MILP) formulation to estimate the most likely operational radial topology and outage sections using power flow measurements, historical/estimated load and DERs data and smart meter ping measurements. Formulation includes continuous (power flow, load and DERs data) and binary measurements (smart meter ping measurements) in a single formulation. Errors in continuous data and binary data are modeled as normal distribution and Bernoulli distribution, respectively. In the future distribution grid, the power injection from controllable DERs will be essential for efficient and resilient grid operation. However, determining the optimal DER injections and restoration actions is dependent on knowledge of the system states. State estimation (SE), already the cornerstone of transmission energy management systems, will become commonplace in distribution management systems as more measurements become available from deployment of automated metering infrastructure (AMI). Observability analysis is the first step in SE, as it determines the sufficiency of the available measurements for accurately estimating the current system states. A new type of pseudo-measurement called a Correlational Measurement (CM) is introduced in this module, to enhance the observability of the system to enable more accurate SE. CMs encapsulate knowledge of correlation between demand patterns for similar classes of loads as well as injection patterns for same-technology renewable DERs. During grid contingency scenarios, DERs have been traditionally disconnected, without any fault ride-through capabilities. However, with new regulations and better technology, it is feasible for these resources to contribute to the grid’s restoration after an adverse event and hence enhance resilience. The controllability module proposes a two-step restoration scheme for the power system restoration process by leveraging additional degrees of freedom in power electronics interfaced DERs for mitigating voltage problems. In a resilience mode without the utility system, the distribution grid relies on DERs to serve critical load. In such a severe event with multiple faults on the distribution feeders, actuation of various protective devices (PDs) divides the distribution system into electrical islands. The undetected actuated PDs due to fault current contributions from DERs can delay the restoration process, thereby reducing the system resilience. The Advanced Outage Management (AOM) and the Advanced Feeder Restoration (AFR) modules developed in this project provide improved system resilience with multiple DERs. AOM identifies the faulted sections and actuated PDs in a distribution system with DERs by incorporating smart meter data. The most credible outage scenario including fault locations, PD actuations, and fault indicator (FI) failures is identified by a set of binary integer linear programming incorporating hypotheses. The AFR module serves to restore a distribution system with available energy resources taking into consideration the availability of utility sources and DERs. By partitioning the system into islands, critical load will be served with the available generation resources within islands based on the solution of a MILP. When the utility systems become available, the optimal path will be determined by a spanning tree search algorithm that reconnects these islands back to substations and restores the remaining load. The transmission and distribution (T&D) co-simulation module was used to validate the effect of a control action performed on the distribution side assets as it propagates to the transmission side. This ensures that the control action performed results in a feasible operating point on both the transmission and the distribution system. In addition to validation, the team used the T&D co-simulation module to demonstrate how distribution system assets can be used to mitigate issues on the transmission system. Specifically, the team demonstrated that appropriate switching operations on the distribution side can alleviate the line overload condition on the transmission side without causing new operational constraint violations.

The dependence of modern societies on electric energy is ever increasing by the emergence of smart cities and electric vehicles. This is while unprecedented number of cyber-physical hazards are threatening the integrity and availability of the power grid on a daily basis. On one hand, physical integrity of power systems is under threat by more frequent natural disasters and intentional attacks. On the other hand, the cyber vulnerability of power grids is on the rise by the emergence of smart grid technologies. This underlines an imminent need for the modeling and examination of power grid vulnerabilities to cyber-physical attacks. This paper examines the vulnerability of the communication-assisted protection schemes like permissive overreaching transfer trip to cyberattacks using a co-simulation platform. The simulation results show that the transient angle stability of power systems can be jeopardized by cyberattacks on the communication-assisted protection schemes. To address this vulnerability, two physical solutions including the deployment of communication channel redundancy, and a more advanced communicated-assisted protection scheme, i.e., directional comparison unblocking scheme (DCUB), are considered and tested. The proposed solutions address the vulnerability of the communication-assisted protection schemes to distributed denial of service attack to some extent. Yet, the simulation results show the vulnerability of the proposed solutions to sophisticated cyberattacks like false data injection attacks. This highlights the need for the development of cyber-based solutions for communication channel monitoring.

Implementation of micro grid concept creates several issues to the existing protection scheme in the radial distribution network. Depending on the location and capacity of the distributed generators, the short-circuit current from the main grid and distributed generations causes variations in the fault current levels inside the micro grid. The changes cause effect of sympathetic tripping, blinding of protection, relay over-reaching, relay under-reaching and loss of selectivity of the overcurrent protection scheme. Such issues must be managed carefully to ensure full benefit from micro grid adaptation. Conventional protection schemes are unable to detect fault current levels common in micro grid. Several methods have been proposed in various research paper. Micro grid protection is substantial to provide reliable and continuous power supply to the customers. The main aim of proposed work is to detect the fault for fast restoration of the system back to the normal operation. The proposed protection scheme for micro grid uses intelligent electronics device for detecting the fault