This article is for design, manufacture, performance and testing of metal enclosed, medium voltage switches. Main keywords for this articles are Medium Voltage Switch Design Requirements, SF6 Insulated Switches, Air Insulated Switches, Medium Voltage Switch Test, Medium Voltage Switches References.
Medium Voltage Switches References
American National Standards Institute (ANSI)
C37.71 Standard for Three- Phase, Manually Operated Subsurface Load-Interrupting Switches for Alternating Current System
C37.72 Manually Operated Dead-Front Padmounted Switchgear with Load Interrupting Switches and Separable Connectors for Alternating Current System
National Fire Protection Association (NFPA)
70 National Electrical Code (NEC)
The switches shall be designed for indoor or outdoor service, and shall use air or SF gas as the interrupting medium. The switches are intended for use as stand alone disconnects for transformers, capacitors and other isolated loads. The switches shall meet all applicable requirements of NFPA 70.
Each switch assembly should in general have one entrance way for the source and one or multiple switched ways to feed power to transformers.
Medium Voltage Switch Design Requirements
The switches shall comply with Table I.
In plants and refineries normally two types of switches are used as in medium voltage.
SF6 Insulated Switches
- Main Enclosure and Auxiliary Compartments
a. Switch components and common bus shall be housed in a welded steel enclosure. The enclosure shall be fabricated from stainless steel.
b. The tank shall be designed to withstand 103 kPa (15 psig) internal pressure without affecting performance of the switch.
c. The main enclosure shall be supported by a mounting frame. The mounting frame shall have the sides enclosed to form a dust-tight auxiliary compartment. The auxiliary compartment shall have an open bottom, and it shall rest on a support angle around the perimeter which shall serve to anchor the assembly to the foundation and seal out dust. Removable gasketed doors shall be provided on the front and rear of each auxiliary compartment to facilitate cable termination. - SF6 Switch Assembly
a. Each switched way shall be equipped with a rotary type operating mechanism capable of providing quick-make, quick-break operation in either switching direction. The mechanism shall be capable of delivering sufficient torque, and shall be provided with latches for each position, to ensure load interrupting, fault closing, and momentary ratings.
b. The contact system shall be an SF6 puffer type or epoxy enclosed/SF6, sealed-for-life, maintenance-free type, capable of the fault closing, momentary and load interrupting ratings indicated. The stationary contacts shall be supported independent of the cable entrance bushings, to avoid any misalignment due to deflection of tank walls.
c. Each switch position shall be clearly identified and padlockable. The operating mechanism shall be manually actuated from outside the switch tank by an external operating handle. The operating shaft shall be made of corrosion resistant material. As a minimum, a double ‘O’ ring type operating shaft seal shall be used for a leak resistant, long life seal without the requirement for moving bellows.
d. The contact material shall be an arc resistant copper alloy, which shall resist erosion of material during load switching. Contacts shall be designed to be self cleaning, and to increase contact pressure with increasing current flow. Temperature rise shall not exceed that specified in ANSI C37.71 for this type of device. Switch contact position shall be clearly visible through the viewing windows.
Main keywords for this articles are Medium Voltage Switch Design Requirements, SF6 Insulated Switches, Air Insulated Switches, Medium Voltage Switch Test, Medium Voltage Switches References.
Air Insulated Switches
Air insulated switches shall contain the following:
a. Compartments containing switch, bus, fuse mechanism and optional low voltage control
b. Steel enclosure
c. Epoxy insulators
d. Fuse/cable access panel interlocked with switch
e. Padlocking open or closed position
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Cable Terminations
- Switches shall be equipped with separable insulated type apparatus bushings.
- Auxiliary compartments shall have sufficient height to facilitate cable terminations and installation/removal of separable connectors.
- Grounded metal barriers shall be provided in the auxiliary compartment, to separate the cable terminations of adjacent switches.
Medium Voltage Switch Accessories
- The following accessories shall be furnished:
a. SF6 gas fill valve for SF6 insulated switches: For SF6 type switches, a gas density switch and pressure gage with gas density and pressure alarm contacts wired to terminal blocks to facilitate wiring to remote alarm panel.
Two alarm stages shall be provided.
b. Fixed style operating handles
c. Two welded lifting eyes
d. Cable support brackets shall be provided in the cable termination compartment to prevent stress on the cable terminations due to cable weight
e. Provisions for ground connections at diagonally opposite corners, and 70 mm2 to 150 mm2 brass ground lugs
f. Padlockable operating mechanism for each switch with position indication
g. Viewing windows for each switch way
h. A complete set of special tools required for installation, maintenance, and operation of all the electrical equipment, control devices, and instruments of the switch assembly
i. Two (2) barrel ‘Kirk Key’ interlock system shall be provided by vendor. The interlock system shall be mechanically interlocked with the switch, to make it inoperative with key(s) removed. The switch shall be in open position before the key can be removed. Two complete sets of keys shall be provided for each key interlock system.
Medium Voltage Switch Test and Inspection
Each switch assembly shall be tested in accordance with standard production tests defined in ANSI C37.71 and C37.72.
The following additional tests shall be performed:
- Bulk SF6 gas supply shall be tested for moisture content. Each individual switch shall undergo a mechanical operation check, leak detection test and a moisture content check. Switch shall be ac hi-pot tested phase-to-phase and across the open contacts. Circuit resistance shall be checked on all ways.
- A dc hi-pot test of bus work shall be performed on each switch assembly
Main keywords for this articles are Medium Voltage Switch Design Requirements, SF6 Insulated Switches, Air Insulated Switches, Medium Voltage Switch Test, Medium Voltage Switches References.