standards / manuals / guidelines for small hydro development - AHEC
standards / manuals / guidelines for small hydro development - AHEC
standards / manuals / guidelines for small hydro development - AHEC
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8.6 Flux densityThe maximum flux density in any part of the core and yokes, of each trans<strong>for</strong>mer atnormal voltage and frequency should be such that the flux density in following overvoltage conditions does not exceeds 1.9 tesla/19000 lines per cm 2 .i) The above flux density has been specified to meet with theover fluxing of the core due to temporary over voltage ofthe order of 31% <strong>for</strong> 1 min., 44% <strong>for</strong> 5 sec. That mayappear in abnormal conditions such as those followingsudden loss of large loads/ tripping of Generator breaker.ii) Yoke bolt area and flitch plate areas shall not be counted inthe net core area, if these are provided <strong>for</strong> fastening of core.iii) The design of limb and yoke shall be so coordinated thatthere is no cross fluxing at the joints.The flux density at normal voltage frequency and ratio should not exceed 1.57 tesla.8.7 Current DensityThe trans<strong>for</strong>mer shall be so designed that the current density of HV windings at thelowest tap should not exceed 250 A/cm 2 . And that of LV winding should not exceed 200A/cm 2 .8.8 Short Circuit StrengthTrans<strong>for</strong>mers shall be designed and constructed to withstand without damage the thermaland dynamic effects on external short circuits <strong>for</strong> 5 seconds under condition specified inIS: 2026 (Part-I)- 1977.The trans<strong>for</strong>mers shall be provided with separate tapping coil to limit the short circuit<strong>for</strong>ces.The position of the tapping coil shall be so arranged that at extreme negative tap, thepercentage regulation is less than at normal tap.The bidders shall submit test certificates of the short circuit test, if already done on theoffered design and rating. However, the thermal and dynamic ability to withstand shortcircuit<strong>for</strong>ces shall be demonstrated by calculations.Manufacturers shall supply calculation <strong>for</strong> Thermal & Dynamic withstand capacity of thetrans<strong>for</strong>mer as per their design alongwith the tenders.8.9 Frequency and System VoltageThe trans<strong>for</strong>mers shall be suitable <strong>for</strong> continuous operation with a frequency variation of3% from normal 50 cycles per second without exceeding the specified temperature rise.3-7
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