Zone Selective Interlocking Research Articles

Microprocessor-Based Protective Relay Configurations: Effective Documentation In Industrial Applications

The protective relays used in modern industrial installations are complex microprocessor-based devices. Some of them deserve to be called protection programmable logic controllers (PLCs) due to their complexity and flexibility. The impressive abilities of these relays require the careful design and documentation of the intended functions and philosophies underlying them. Interrelay communications for high-speed zone-selective interlocking and complex control interlocking and intertripping add further complexities, as does the common practice of having multiple design entities working on different portions of the same facility. This article considers documentation schemes developed for two recent projects that included complex protection equipment. Protection philosophies and narratives, communications scheme documentation, and programmable logic documentation are discussed in an effort to illustrate a complete approach that will be useful to both specialists and field personnel for understanding protection systems.

Preventing cascading failure of electric power protection systems in nuclear power plant

Cascading failure is the main cause of large blackouts in electrical power systems; this paper analyzes a cascading failure in Hanbit nuclear power plant unit two (2) caused by a circuit breaker (CB) operation failure. This malfunction has been expanded to the loss of offsite power (LOOP). In this study, current practices are reviewed and then the methodologies of how to prevent cascading failures in protection power systems are introduced. An overview on the implementation of IEC61850 GOOSE messaging-based zone selective interlocking (ZSI) scheme as key solution is proposed. In consideration of ZSI blocking time, all influencing factors such as circuit breaker opening time, relay I/O response time and messages travelling time in the communication network should be taken into account. The purpose of this paper is to elaborate on the effect of cascading failure in NPP electrical power protection system and propose preventive actions for this failures. Finally, the expected advantages and challenges are elaborated.

Cooling the Arc Flash Hot Spot

Maximum arc flash energy is seen at the secondary of transformers before the first protective device. Reducing transformer primary protection trip times to cool this “hot spot” starts with quicker tripping on the secondary. Zone selective interlocking (ZSI) is a proven means of avoiding the delays of traditional coordination time intervals but interlocking of all devices within a system can raise concerns about construction and testing complications. How much of an arc flash benefit would it be to interlock only main and tie breakers where interlock capable communication exists for automated transfer scheme functions. To determine benefits, arc flash hazards are first reduced to the extent possible with traditional measures. Communication elements are analyzed to determine those having the speed and reliability required for protection quality interlocking. ZSI is then applied between main and tie breakers to demonstrate how the hazards of the arc flash hot spot can be brought under control.

Application of Zone Selective Interlocking in Electrical Power Distribution System

The two major concerns in today’s electrical distribution system are the potential damage from fault stress and the costs associated with power outages. The optimal way to limit fault stress is to clear the fault in the shortest amount of time. Unfortunately, clearing the fault within the shortest amount of time might sacrifice coordination and lead to broader power outages. Zone Selective Interlocking Coordination assures the possible sustaining of faults for over currents and voltages with different faults. The circuit breaker operation and principle depend upon the open and close operation for the continuity of supply/service. To reduce the stress on the system, generated energy during fault conditions to be considered and its coordination to be checked.

Differential Protection in Low-Voltage Buses: An Exploration of Principles and Models

Current-differential principles are well known and commonly used for the protection of medium and large transformers, large motors, medium-voltage (MV) generators, MV and high-voltage buses, and any type of important power equipment with measurable input and output currents. However, is it practical to protect low-voltage (LV) distribution buses using differential protection? This article describes bus differential protection principles as well as interlocking principles for overcurrent protection. We discuss specific issues in applying differential protection in LV systems. Additionally, we present a concept of partial differential (PD) protection, which can be used in conjunction with zone-selective interlocking (ZSI) or as backup to traditional overcurrent protection to achieve high-speed and selective fault clearance. Additional concepts for the implementation of bus differential protection using networked data in LV systems are introduced.

Advances in Protective Device Interlocking for Improved Protection and Selectivity

The challenge for protection engineers is to balance protection and system reliability (selectivity). Engineers may bias toward protection at the cost of system reliability or toward desired (or code required) selectivity at the cost of protection. The need for compromise is often accepted as an unalterable reality. Simultaneously, the need for greater personnel safety has added a desire for space protection in addition to the usual bus protection imperative. The industry has provided devices, techniques, and schemes to minimize, or even eliminate, this compromise. Tools include smarter instantaneous sensing algorithms, differential protection, zone-selective interlocking, and arc-flash relays. Zone-selective interlocking is a cost-effective solution for low-voltage (LV) and medium-voltage (MV) applications. Today, zone-selective interlocking spans different voltages between inverse protection characteristics, such as short-time slopes and fast instantaneous protection, in both LV and MV systems. However, traditional schemes must still nest pickup current thresholds to accommodate sensing tolerances. Arc-flash relays have emerged as excellent space protectors but present concerns in LV applications. This paper reviews recent improvements in zone-selective interlocking capabilities and presents schemes, which are developed by the authors, that eliminate the need for nesting pickup levels of multiple devices and for successfully mixing arc-flash and current sensing devices into one system to simultaneously provide fast, reliable, and sensitive protection.

A New Method of Early Short Circuit Detection

To reduce the pressure on contacts and circuit breaker and realize the zone selective interlocking (ZSI) function above the instantaneous protection threshold (e.g., >10In), the short circuit current needs to be early detected. The state-of–art of early short circuit detection (ESCD) method is reviewed. Based on the equivalent model of the short circuit, a new method based on the current and its integration is proposed. The prospective current value can be detected in the early stage of the short circuit. According to the evaluation result, the short circuit current can be early forecasted with the proposed method.

Protecting at the Speed of Light

One of the most critical aspects of reducing both personnel injury and equipment damage is through the reduction of the energy available to an arcing fault. The initiation of a trip and the clearing time (opening) of any upstream device is a critical component in the reduction of the resultant arc-flash incident energy. Combining technologies that detect and interrogate both the arc flash and associated arc current signatures in combination with arc-resistant switchgear or controlgear can provide a coordinated solution for controlling the level of incident energy at various points within the distribution network of a forest product facility. Hybrid systems of this type provide the highest level of personnel protection, along with comprehensive equipment protection. Detection and initiation of tripping is implemented faster than with conventional relaying techniques alone. The reduction of overall trip time can reduce equipment collateral damage, reconditioning time, and lost productivity resulting from downtime. These types of coordinated systems will also lower the personal protective equipment requirements based on incident energy. This article compares several systems including conventional overcurrent protection, zone-selective interlocking, and various light and current sensing systems.

Arc-Flash Incident Energy Reduction Using Zone Selective Interlocking

As a result of new requirements in the National Fire Protection Association (NFPA) 70E standard, many facilities are performing arc-flash hazard analyses to better understand how to protect personnel from the possibility of being injured in an arc-flash incident. In many cases in the petrochemical industry, it is unsafe or not practical to shut down electrical equipment to do work. This paper explores practicable options that a facility has to reduce incident energy levels in existing low-voltage systems where the arc-flash hazard analysis results in unacceptably high levels. It also explores a case study in the Gulf of Mexico, where an offshore oil production platform was able to cost effectively reduce incident energy levels from approximately 170 to under 15 by implementing a zone selective interlocking system into their low-voltage switchgear. The selection process logic involving equipment, personnel considerations, and commissioning are all addressed.

The Impact of Solid-State Technology on Molded Case Circuit Breakers

Molded case circuit breakers with solid-state trip units have been widely applied in industrial power systems for more than a decade. Today's Underwriters Laboratories (UL) listed molded case circuit breakers offer the application engineer many versatile features for systems selectivity such as high short-circuit interruption, high short-time ratings, integral ground fault protection, stored energy mechanisms, versatile mounting arrangements, manual or electrical operation, zone selective interlocking, and fault indicators. The development and application of solid-state technology in molded case circuit breakers are briefly traced, explaining the features and how they can be used.