Under a fault condition, the fault current can easily exceed the capacity of the cable tape shield or concentric neutral ground; hence, a separate ground wire is necessary. Protective relay curves cannot be used in the same way as low-voltage circuit breaker curves or fuse curves. Magnetizing inrush currents to transformers. Short-circuit protection is still required. Furthermore, NEC Art 215.2(B) requires a separate ground to handle short circuit currents. Complete guide to protection of medium voltage networks, Electrical Engineering Books and Technical Guides, To provide the operator with an alarm indication under particular network or machine service conditions (for example protection against negative sequence for generators), To put the line or faulty machine out of commission within a short time, as defined in the selectivity study, To carry out automatisms under particular service conditions (for example, undervoltage protections which activate automatic changeover or automatic reclosing of the lines). protection of machines and plants in medium voltage networks. Feeder and transformer protection is defined by the applicable equipment standards. There are three fundamental objectives to overcurrent protection to include ground fault protection: 1. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility. Typically, service reliability tends to increase as service voltages increase. In industry practice, a feeder relay setting of 200% to 400% of full load is considered reasonable. Selectivity: Selectively requirements are intended to limit system fault or overload response to a specific area or zone of impact and limit disruption of service to the same. They should only be applied in situations where it will not be required to open small overcurrents. Medium-voltage surge arresters Surge arresters are used for protection of electrical equipment from all kind of overvoltages caused by lightening or switching operations. industrial substations. In cases involving redundant protective devices, nonselective breaker operation is of little or no concern. Furthermore, NEC Article 450.3(A) specifically covers transformers over 600 V to include MV transformers. The content is copyrighted to EEP and may not be reproduced on other websites. Fuse Types. Motor and generator curves are machine specific and are normally provided in the vendor data submittal packages. 13 Provide a full-size 600-volt insulated copper ground conductor in all conduits and 14 raceways with medium voltage cable. The norm, however, is that the utility will give several MV services that the engineer will need to integrate into an owner’s MV distribution system. An excessive number of protections may also be harmful, since, even if they operate correctly in the case of a fault, they can operate, Get access to premium HV/MV/LV technical articles, advanced electrical engineering guides, papers, and much more! Article 701.27: Legally Required Standby Systems Coordination. Auxiliary power is used only for communication and high sensitivity earth fault protection. The current will go to a high value when a fault occurs. Selection of the protection system and relays depends on and is correlated with the plant characteristics, type of industrial process and its service continuity requirements, with the status of the neutral, characteristics of the machines, levels and duration of the fault currents, etc. This paper covers general safe operations on Medium Voltage equipment and looks at the aspects of Switching, Isolating, Testing and Earthing, as well 7.6 Protection of Low-Voltage Capacitors. The focus of this article is on overcurrent protection requirements for MV transformers, and connecting transformers to common MV distribution systems. IEC – 61800-5 AC Drives Standard 6. It is important to underline at once that trip selectivity must always be looked for, but must only be looked for after having ensured protection of the network component. NEC Section II (Article 300.31 through 300.50) covers MV wiring methods. However, unless precautions are taken, this setting may be too low to prevent relay misoperation on inrush following an outage. If the 3-phase loads are unbalanced, then the ground relay should be set to pick up at about 50% of load current. 134. IEEE 141 divides system voltages into “voltage classes.” Voltages 600 V and below are referred to as “low voltage,” voltages of 600 V to 69 kV are referred to as “medium voltage,” voltages of 69 kV to 230 kV are referred to as “high voltage,” and voltages 230 kV to 1,100 kV are referred to as “extra high voltage” with 1,100 kV also referred to as “ultra-high voltage.”. Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers. The biggest difference is that medium voltage fuses are not intended to provide overload protection. Many of these complexes also have high-rise components with MV risers servicing unit substations at strategic locations on multiple levels. 148. Search Products And Discover New Innovations In Your Industry!! In distribution design, the three objectives still apply: For example, if the NEC requirements for transformer overcurrent protection are considered without reference to applicable standards and code requirements, the system may address protection of transformers, while other elements of the distribution system (such as the feeders connecting the transformer(s) to the distribution system) may not be protected in accordance with the code. For example, Southwire Co. has published tape shields fault current capacity to be 1893 amps at 12.5% tape overlap, and 2045 amps at 25% tape overlap. Ideally suited for use in Eaton's coordinated primary and secondary unit substations , the transformers are explosion-resistant, fire-resistant, non-polluting to the environment. Such overcurrent protection does not necessarily protect the primary or secondary conductors, or the equipment connected on the secondary side of the transformer. The RC-Snubber protects the primary windings of a medium voltage transformer from high Control of the network parameters to prevent false operations (for example, To activate network parameter recording to memorise the network disturbances (for example, the starting contacts of the overcurrent relay), To carry out protection of the interface with the external network (with contractual not protection settings). This form of protection employs relays with inverse time-current characteristics; that is, operating time of these relays becomes less as the current magnitude increases. Medium- Voltage Switching Devices and Switchgear. Selection of the protection system and relays depends on and is correlated with the plant characteristics, type of indus-trial process and its service continuity requirements, with the status of the neutral, characteristics of the machines, levels and duration of the fault currents, etc. Maintenance and installation costs also increase with higher service voltages. Sizing MV components such as motors, generators, transformers, wiring systems, the architecture of MV systems, or design of complicated protection schemes such as reclosers, zone interlocks, differential protection, etc., are all beyond the scope of this article. 147. and Terminating Guide Quality Workmanship that Lasts. He has more than 28 years of engineering and design and field experience that includes MV distribution systems for military, mining, tunneling, food manufacturing, power production facilities, high-rise facilities, and casino resort complexes. 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