Arcing current faults (ACFs) occurring in a medium-low voltage $3\phi$ transformer represent a challenge for transformer protection. Such faults initiate currents with different features from those triggered by conventional faults, thus making it difficult for transformer protection to accurately detect and respond to arcing current faults. This paper proposes a method for accurate detection and identification of arcing current faults in MV-LV transformers. The presented method is based on extracting the magnitudes and phases of low frequency harmonics from the differential currents. Unlike magnetizing inrush and conventional fault currents, arcing current faults trigger currents that have harmonic components with non-stationary phases. These non-stationary phases can provide signature information of arcing current faults. Multi-channel filters can accurately extract harmonic components with complex time-frequency characteristics, including non-stationary phases. In this paper, a multi-channel filter bank that is used to extract harmonic components with non-stationary phases as a signature of ACFs on the low-voltage side of a medium-low voltage 3 phi transformer. The used filter bank is composed of digital bandpass finite impulse response filters, each of which has a linear phase over a wide frequency range. The non-stationary phase method is tested during various fault and non-fault events. Test results demonstrate protection responses with speed, accuracy, and reliability against ACFs. Observed response features are found to have minor sensitivity to the level of loading level and/or type of the ACFs.

The integration of renewable energy resources, a step forward for decarbonization, has posed trivial power system protection and reliability challenge for power supply to I&CPS facilities. Such switching surges and power system harmonics generated by the power electronics used by the renewable energy resources. Severe weather changes, extreme heat, across the insulators, lightning thunderstorms breaking overhead bare line conductors falling on ground to cause fire endangering safety of public and animals. Severe lightning and switching surges injecting additional harmonics to harmonics from renewable energy resources integrated into the grid, makes power system protection at the PCC challenging. This paper reviews application of power system surge arresters, insulators, surge limiters, protection of wood poles from fire by insulators flashover to poles metallic support structure. IEEE and IEC standards on surge arresters and insulators are included to establish difference of these devices' selection methods. This paper ends with practical application recommendations of surge protection devices based upon power system equipment configuration, insulation level, and surge propagation theory

The soft open point (SOP) is an emerging power electronics device in place of normally open points/tie switches in distribution systems. During a fault, service restoration can be effectively achieved by coordinating SOPs and distributed generation units (DGs). In this paper, a novel two-stage SOP-based service restoration method in distribution networks is proposed: In Stage 1, a dynamic load-shedding scheme is prepared/applied during a fault occurred at the upstream grid, and the power supply to priority loads is maintained through DGs; in Stage 2, DGs, SOPs and switches are coordinated and operated to realize restoration in the outage area with controllable/dispatchable distributed generation units (CDGs) dispatched to their maximum capacity limits. In both stages, real and reactive power of SOPs is regulated to maximize load restoration. A mixed-integer nonlinear programming (MINLP) model through AC power flow is developed to formulate the restoration problem mathematically. The modified IEEE 33-node test system is used to validate the proposed restoration method combined with centralized or decentralized optimization. The proposed method is also compared with an existing method, showing much improved restoration performance.

The integration of renewable energy resources, a step forward for decarbonization, has posed a power system protection and reliability challenge. Such challenge is switching surges and power system harmonics generated by the power electronics of renewable energy resources. Severe weather changes, switching surges, harmonics, and lightning surges cause electrical insulation failure which leads to unpredictable equipment hazards and electrical safety threats. This paper reviews application of power system surge arresters, insulators, surge limiters to mitigate power system insulation failure by diverting surge energy to earth. Application of ac and dc surge protection devices for industrial projects using grid power with renewable energy resources is included in this paper. Tables of current IEEE and IEC standards on surge arresters and insulators are included to establish difference of these devices’ selection methods. A design approach for appropriate selection and application of surge protection devices for electrical insulation protection, normally used equipment insulation coordination with margin of protection is included. Transient surge voltages and associated surge currents containing damaging transient surge energy obeys surge propagation laws using theory of surge reflection, surge refraction, surge impedance, requiring distributed inductance and capacitance parameters as if the surge is travelling long distance on overhead lines and underground cables within the facility. Surge propagation theory is known to industry long before the current software analysis of electromagnetic transients (EMT) used by engineers to model accurately power electronic devices and surge arresters for selection of adequately rated surge protection devices. This paper ends with practical application recommendations of surge protection devices based upon power system equipment configuration, insulation level, and surge propagation theory.

Arcing current faults (ACFs) are undesired transient events that can occur in different power system equipment, including medium voltage-to-low voltage (MV-LV) power transformers. The main challenge in detecting, identifying, and responding to low-voltage (LV) side ACFs (in a MV-LV power transformer), is due to low magnitudes of MV side currents triggered by a LV side ACF. As a result, MV side protective devices fail to detect and respond to LV side ACFs. In many cases, the reduced ability to detect and respond to LV side ACFs prolongs the duration of LV side ACFs, and leads to a significant increase in the incident energy (may exceed acceptable limits). In this article, an analysis of MV side currents is developed to extract signature information to ensure accurate and fast detection and identification of LV side ACFs. The proposed signature of a LV side ACF is the high frequency components (with non-stationary phases) extracted from MV side currents. Desired frequency components can be extracted using a multi-channel filter bank that is composed of digital high pass filters with linear phase responses. Such digital filters are designed using phaselet functions to ensure a simplified implementation of the desired filter bank. The accuracy and response speed of the proposed approach are utilized for designing a new arc flash relay (AFR) for MV-LV power transformers. The phaselet-based AFR is implemented and tested for a 35 kVA transformer during several transient events including LV side ACFs. Performance results reveal accurate and reliable detection, identification, and response to LV side ACFs with negligible sensitivity to loading level and/or ACF type (series or parallel).

Historically, failures of distribution transformers due to transient overvoltage phenomena has led to the development of Resistor-Capacitor snubber circuits for the protection of the transformer and winding insulation. These transients are most often observed when dry-type transformers are close coupled to vacuum switching devices. Typically, snubber circuit design has been specific for the particular application, and complex engineering studies were required. However, the design of a snubber circuit itself may be performed without these detailed studies. If snubber studies are required, the procedures for system modeling and simulation are explained in a step by step manner.

This chapter examines some of the causes of circuit breaker failure. It also analyzes some of the factory testing procedures which help to ensure safe design and manufacture of circuit breakers. The most eminent reason for utilities to maintain or even reduce their fault current levels is to ensure proper functioning of circuit-interrupting devices such as circuit breakers and fuses. Circuit breakers in the United States are rated by the American National Standards Institute (ANSI) and the Institute of Electrical and Electronics Engineers (IEEE). While current-limiting fuses are enclosed in a sealed cylinder, and are usually contained in a metal-enclosed switchgear, limiting the danger even if their short-circuit rating is exceeded, expulsion fuse links are mounted in open tubes on utility poles. The chapter presents case studies that describe the replacement of air-magnetic circuit breakers whose interrupting ratings have been exceeded with new SF6 or vacuum interrupters.

This chapter reviews the literature on effects of high fault currents on protection and metering equipment. It discusses the capabilities and limitations of existing short-circuit protection devices. High fault currents are well known to cause saturation of iron core current transformers (CTs). This can adversely affect the performance of system protection devices. CTs are intended to deliver a secondary current that is directly proportional to the primary current with as little distortion as possible. During normal operation, the CT secondary winding induces a magnetic flux that opposes and nearly cancels the primary induced flux. CT saturation may cause overcurrent relays to misoperate or fail to operate, resulting in a failure of the protection system. Protective relaying is an integral part of any electrical power system. The fundamental objective of system protection is to quickly isolate a problem so that the unaffected portions of a system can continue to function.

Whenever possible, grounding, insulation, and physical distance should reduce or eliminate the hazards of electrical shock. During the design, the effect of contact with conductors can be evaluated using electrical circuit models of the human body. There will be many factors which affect such a model, and varying degrees of complexity of the model depending on the application and the level of potential risk which may be present. This chapter presents an introduction to this field, which is still evolving as new research is performed and more advanced mathematical modeling techniques are used. The circuit model of a human body can be constructed on basis of the skin model, the limbs, and the trunk. The chapter explains the steps involved in circuit reduction. Using the circuit model developed, the inductances may be added as elements in series with the resistances, and the effect of varying frequency can be found.

This paper will discuss wind farm electrical power system issues in grid codes. Grid codes are published by utilities and system operators to define the requirements for interconnection of generation and other facilities to the grid. The purpose of the grid code requirements is to ensure reliability, stability, power quality, protection of equipment and worker safety. These requirements are given at the point of interconnection (POI) with the transmission or distribution system, and will be different for different sizes and types of generation. Grid codes can cover a broad range of requirements, from continuous operational constraints to behavior during contingencies such as faults. Grid codes vary considerably based upon the issuer, their country or region and the type of transmission or distribution system. Manufacturer specifications for wind turbines are generally given on a per-turbine basis at the collector system voltage. When a wind farm is proposed, an initial assessment is made of the suitability of the wind turbines to the system. Studies are performed of the grid and wind farm(s) being considered which evaluate system performance and conformance with the grid code. Finally, tests and measurements during and after commissIOning verify conformance of the wind farm to the applicable grid code.

The effects of electric currents flowing through a human body vary from little or no perceptible effect, to the sensation of shock, to severe injury or death by electrocution. This chapter provides considerable detail on how currents can pass though different parts of the body and each parameter that can impede or limit that flow of current. Human sensitivity to electric current is classified in an increasing scale dependent on current magnitude and duration. The impedance of the human body can be broken down into the impedances of the various body parts, resulting in an equivalent circuit for the electrical path through the body. Resistance of footwear has a considerable impact on total body resistance. The pathways electric current takes through the body are extremely dependent on the points of contact. Further investigation of electrical shock effects would help to more precisely characterize risks and improve electrical safety.

Equipment grounding and bonding is important for electrical safety, in protecting against unwanted voltage and current, which can cause injury. The configurations of grounding and bonding conductors can be broken down into a number of typical arrangements. Inductance is an important factor in calculating the magnitude of ground fault currents. This paper examines and compares inductance formulas from the literature for a variety of grounding conductor configurations and return paths. It is found that the use of more accurate formulas than are normally used can present a significant improvement in the calculation of these inductances.

This article discusses windfarm electrical power system issues in grid codes. Grid codes are published by utilities and system operators to define the requirements for the interconnection of generation and other facilities to the grid. The purpose of the grid code requirements is to ensure reliability, stability, power quality, protection of equipment, and worker safety. These requirements are given at the point of interconnection (POI) with the transmission or distribution system and are different depending on the sizes and types of generation. Grid codes can cover a broad range of requirements, from continuous operational constraints to behavior during contingencies such as faults, and they vary considerably based on the issuer, their country or region, and the type of transmission or distribution system. Manufacturer specifications for wind turbines are generally given on a per-turbine basis at the collector system voltage. When a wind farm is proposed, an initial assessment is made of the suitability of the wind turbines to the system. Studies that evaluate system performance and conformance with the grid code are performed of the grid and wind farm(s) being considered. Finally, tests and measurements during and after commissioning verify conformance of the wind farm to the applicable grid code.

Transformer failures have in recent years led to the development of Resistor-Capacitor snubber circuits for the protection of the transformer and winding insulation from the damaging effects of highvoltage high-frequency transients. Transformer insulation may be damaged if the Basic Insulation Level (BIL) is exceeded, turn-to-turn insulation when there is excessive rate of change of voltage with time (dv/dt), and to switching devices by restrikes when the Transient Recovery Voltage (TRV) is exceeded. These transients are most often observed when dry-type transformers are close coupled to vacuum switching devices. Some manufacturers are now including snubbers in their transformer designs. This paper provides a thorough review of the causes of the transients, methods of analysis, and mitigation of the effects of these transients. An example is provided of transformers to be installed in the basement of an urban high rise office building, where the space is limited and the available fault current is high, where the transformer enclosure includes built-in snubber circuits. The strengths and weaknesses of current methods are examined. Recommendations are made for improvements in snubber circuit design and analysis.

The purpose of this paper is to bring engineers up to date and give them new tools for solving harmonic problems. Variable Frequency Drive (VFD) technologies previously used only in high power applications, such as Pulse Width Modulation (PWM) Drives, Active Front Ends, and 18-pulse rectifiers have moved to lower voltage and power levels. They are now widely used in industrial and commercial power systems. The techniques used in conventional harmonic analysis for the 6-pulse SCR drive no longer apply in many cases. New harmonic limitation devices, such as broadband and active filters are replacing the conventional notch filter.

This paper provides an electrical equivalent circuit of a high-resistance grounded (HRG) power supply transformer to discuss damage at a fault location so long as the fault remains a line-ground fault. Fault resistance and fault current that have an effect on damage are included in the equivalent circuit. Three-line diagrams showing fault current flows along with their vector diagrams are also included. Part II of this paper provides a similar equivalent circuit for an HRG grounded generator. This paper provides guidance to update the current edition of IEEE STD. 142 with respect to HRG systems.