Selection Guide - S & C Electric

1 Selection Guide For Transformer Primary Fuses in Medium- and High-Voltage Utility and Industrial Substations S&C Power Fuses Types SMD-1A, SMD-2B, SMD-2C, SMD-3, and SMD-50. Outdoor Transmission ( kV through 138 kV). S&C Electric COMPANY. Table of Contents Section Page Number General.. 3. Application Principles Select the Primary Fuse Rating.. 5. Accommodate Expected Loading Levels.. 6. Withstand Inrush Currents.. 7. Protect Transformer Against Damaging Overcurrents.. 14. Coordinate with Other Protective Devices.. 25. Protect Downstream Conductors Against Damaging Overcurrents.. 39. The Fuse- Selection Tables Introduction to Fuse- Selection Tables.. 41. Basis for Listings in the Fuse- Selection Tables .. 42. How to Use the Fuse- Selection Tables.. 44. Fuse- Selection Tables .. 51. 2. General This information bulletin is a Guide for the Selection , former or when line-to-line or line-to-ground voltage exists application, and coordination of S&C Type SMD across the fuse. They also will detect or interrupt faults Power Fuses when applied on the primary side of small regardless of whether the transformer is adjacent to the to medium-sized transformers installed in utility and fuse or connected to it via a cable from a remote location, industrial substations.

2 For the purpose of this Guide , and regardless of transformer winding connection. SMD. transformers having primary voltage ratings between Power Fuses are capable of handling the full range of tran- kV and 138 kV, with medium-voltage ( kV through sient recovery voltages associated with these conditions. kV) secondaries, will be covered. They develop a positive internal gap of high dielectric High-voltage power fuses provide a reliable and strength after circuit interruption, thereby preventing de- economical means of protecting small to medium-sized structive re-ignitions when exposed to full system voltage. transformers installed in utility and industrial substa- The dropout action of these power fuses provides the tions. The considerable economies inherent in power-fuse additional benefit of visible air-gap isolation for the trans- protection are possible, first, because the fuse itself is former after fuse operation. much less costly than other types of protective equipment The close fusing necessary to provide superior protec- and, second, because there is no need for such auxiliary tion for secondary-side faults is possible with S&C Type equipment as station batteries, motor-driven switch op- SMD Power Fuses because: (1) they use silver or pre-ten- erators, and protective relays.

3 Further benefits of a com- sioned nickel-chrome fusible elements that are not dam- pact fuse-protection package are low installation cost and aged by surges that may heat the element nearly to the a space-saving design that will accommodate almost severing point; (2) they are available in a wide variety of any structure. In addition, unlike relay-actuated protec- ampere ratings and speed characteristics especially suited tive devices such as circuit breakers and reclosers, power to protecting transformers against very-low-magnitude fuses have maintenance-free time-current characteristics. fault currents; and (3) they possess substantial peak-load They require only minimal physical maintenance, such capabilities and surge capacities that are more than ad- as the periodic checking of the condition of the fuse-unit equate to withstand transformer magnetizing inrush cur- bore and occasional refinishing of fuse tubes exposed to rents and severe hot-load and cold-load pickup currents. severe weathering.

4 Close fusing with SMD Power Fuses, coupled with their The transformer primary fuse should be selected to pro- exceptional low-current fault-interrupting performance, vide both system and transformer protection. With respect ensures maximum protection for the transformer against to system protection, the primary fuse should detect a a broad range of secondary-side fault currents, thus mini- potentially damaging overcurrent condition and operate mizing the life-shortening thermal and mechanical stresses promptly to isolate only the faulted segment, thereby mini- associated with prolonged transformer through-faults. In mizing short-circuit stresses on the remainder of the sys- addition, the ability to fuse close to the full-load current tem and limiting the extent of the service interruption to of the transformer facilitates coordination with source- the smallest possible portion of the system. For transform- side protective devices by permitting the use of lower er protection, the primary fuse should operate promptly in ratings or settings for faster response.

5 Response to a bus or cable fault located between the trans- The Selection of transformer primary-side protective former and the nearest secondary-side overcurrent protec- devices and their ratings and settings has been a matter tive device. It should further provide backup protection of considerable complexity. This publication provides for the transformer in the event the secondary-side over- complete, simplified procedures for selecting the optimal current protective device either fails to operate because of transformer primary fuse, taking into consideration all of a malfunction, or operates too slowly because of incorrect the following factors associated with the application: (higher) ratings or settings. System voltage To best achieve these objectives, group protection of Available fault current transformers is not generally recommended each trans- Anticipated normal transformer loading schedule, former should be individually protected. The ampere rat- including daily or repetitive peak loads, and emergency ing of a primary fuse selected to accommodate the total peak loads loading requirements of two or more transformers would typically be so large that only a small degree of secondary Inrush currents, including the combined effects fault protection and almost no backup protection of transformer magnetizing-inrush current and the would be provided for each individual transformer.

6 In ad- energizing-inrush currents associated with connected dition, with group protection of transformers, the degree loads particularly following a loss-of-source voltage of service continuity is significantly reduced because a (momentary or extended). fault associated with any one transformer will result in the The degree of protection provided to the transformer loss of service to all transformers protected by the fuse. against damaging overcurrents S&C Type SMD Power Fuses provide full fault-spectrum Coordination with secondary-side and other primary side protection for transformers. These fuses will detect and overcurrent protective devices interrupt all faults large, medium, and small (even down Protection of the downstream conductors against dam- to their minimum melting currents), regardless of whether aging overcurrents the fault is on the primary or secondary side of the trans- Information Bulletin 210-110. 3. General General continued .. characteristics, along with the information necessary to confirm coordination of a given fuse with a variety of These factors are discussed in detail in the next section, secondary-side protective devices.

7 The tables also feature entitled Application Principles. This discussion refers a specially designed Transformer Protection Index, . to Selection tables located on pages 51 to 122 of this which indicates the degree of transformer protection document. provided by the primary fuse, as well as listings of the These tables are designed specifically to simplify the se- loading capabilities of the fuses when used with each lection of the optimal transformer primary fuse for your of the transformers shown. You need only refer to these particular application. tables to obtain the information required to make your The fuse Selection tables list for each transformer Selection . a variety of fuse-unit ampere ratings and speed 4. Application Principles Select the Primary Fuse Rating .. The interrupting rating of the transformer primary fuse should be chosen with sufficient margin to accommodate A transformer primary fuse must be selected for the voltage anticipated increases in the interrupting duty due to sys- rating, the available fault current, and the continuous tem growth.

8 Again, because fuses are available with a current-carrying requirement of the transformer on wide variety of interrupting ratings, you should choose a which it is to be applied. Because there are a multitude of primary fuse having an interrupting rating only as large as voltage, short-circuit interrupting, and maximum ampere necessary to meet your present and future requirements. ratings available, you should choose the most economical primary fuse that will meet both your present and your future requirements. In addition, from the wide variety of Ampere rating and speed characteristic. The am- ampere ratings and speeds available, you should select pere rating and speed characteristic of the transformer the primary fuse providing the optimum protection for the primary fuse should be selected to: transformer against secondary-side faults. Accommodate the anticipated normal transformer Voltage rating. The maximum voltage rating of the loading schedule, including daily or repetitive peak transformer primary fuse should equal or exceed the loads, and emergency peak loads maximum phase-to-phase operating voltage level of the Withstand the magnetizing-inrush current associated system.

9 S&C Type SMD Power Fuses are not voltage with the energizing of an unloaded transformer, as critical, so they may be applied at any system operating well as the combined magnetizing- and load-inrush voltage equal to or less than the maximum voltage rating of currents associated with the re-energization of a the fuse. Moreover, these fuses operate without producing loaded transformer following either a momentary or overvoltages that can cause spurious operation of surge extended loss of source voltage arresters or damage transformer insulation. Protect the transformer against damaging overcurrents Short-circuit interrupting rating. The symmetrical Coordinate with secondary-side as well as other short-circuit interrupting rating of the transformer primary primary-side overcurrent protective devices fuse should equal or exceed the maximum available Protect downstream conductors against damaging fault current at the fuse location. When determining overcurrents. These principles, which are examined the interrupting rating of the primary fuse, you should in greater detail in the following sections, provide consider the X/R ratio of the system at the fuse location the basic foundation of transformer primary fuse because power fuses may have higher-than-nominal Selection symmetrical interrupting ratings for those applications where the X/R ratio is less than the value of 15 specified by the applicable standard.

10 As a result, you may be able to use a less expensive primary fuse having a lower nominal symmetrical interrupting rating. Refer to your local S&C. Sales Office for these higher symmetrical short-circuit interrupting ratings. IEEE Standard , Specifications for Power Fuses and Fuse Disconnecting Switches.. Information Bulletin 210-110. 5. Application Principles Accommodate Expected Loading Levels .. time. Where emergency peak loads are contemplated, the transformer primary fuse ultimately selected should have In general, the transformer primary fuse should be selected an emergency peak-load capability at least equal to the based on the anticipated normal transformer loading magnitude and duration of the emergency peak load. Refer schedule, including daily or repetitive peak loads. The to S&C Information Bulletin 210-190 for emergency peak- primary fuse ultimately selected should have a continuous load capability values. loading capability, as differentiated from its ampere rating, equal to or greater than this highest anticipated loading It is important to remember that a transformer primary level.