(I) Efficient use of natural convection using cool air inlet from bottom
(II) Preventing of shunting cooling air draft through other vents by use of baffle guides (III) Focusing thecooling air draft on coil structure.
HARMONIC DISTORATION IN OUTPUT WAVEFORM
i)The isolation transformer being a linear device, doesn’t add any harmonic distortion on linear load. ii)If the load current has got heavy harmonics such as say 60% hen the added output harmonic distortion is as below: iii)Added % harmonic distortion in output = (% harmonics in load current) * (% Impedance of transformer)
iv)The % impedance of our Isolation transformers are around 2%. v)Thus the max. added harmonic distortion in output = 60% * 2% = 1.2%
i)We do not manufacture Resin cast transformers.
ii)The data center environment is well controlled , unlike those in industries , chemical
plants etc. and hence does not require resin cast construction. iii)Vaccum impregnation is generally requred for smaller coils with multi layer winding where insulating varnish cannot penetrate well.
iv) PDU transformer coils are LV medium power coils with fewer turns and no. of layers and having large ducts between each layer which allows easy penetration of varnish. In our process ,the coils are preheated for removal of moisture and then immersed in H class insulating resin varnish for specified duration for thorough penetration.
i) Datsons transformers generally come with total 3 taps on primary one at nominal voltage (415 V) and other two approx +/- 5% of same (395 - 415 - 435 V). ii) In special cases 5 tappings at +2.5% difference can be provided in lower ratings up to 150 kva. iii)Being LT transformer, the voltage per turn itself is more than 2.5% in many cases and as such the tappings are approximate. iv)We do not advise more tappings as the supply voltages on most of the sites keep varying much more than the tapping difference during the day. v)We do not provide tappings of aluminium wound transformer above 300 KVA.
(i) K rated transformers have to be designed with low leakage: stray field.
(ii) The stray losses at 50Hz fundamental frequency have to be minimized as much as possible. (iii) The stray losses due to non linear loads are K factor times that of 50Hz stray losses. (for K13—13 times)
(iv) Such low leakage design results in very low leakage therefore very low leakage reactance and low % impedance. (v) % impedance in Datsons transformer vary in the range of 1.2% to 2.5% (vi) This improves the load regulation as well as % efficiency significantly. (vii) However this results in higher fault currents in the event of short circuit onload side. (viii) The selection of incomer MCCB or outgoing MCCB is made with larger KA rating to isolate such faults"
· The cooling system adopted by Datsons is natural air cooled for lower ratings and forced air cooled for larger ratings. · Higher ratings can also be provided in natural air cooled mode but with significant impact on cost. · Transformers with temp rise limited to 90 deg C have been manufactured and supplied. · Datsons adopts very unique and thermodynamically efficient cooling and ventilation arrangements wherein:
SHORT CIRCUIT WITHSTAND CAPACITY
(i) The short circuit impedances of K rated transformer manufactured by Datsons are in range of around 2 -3 %.
(ii) Datsons’ K rated designs are focused on reducing (stray ) magnetic field by minimizing leakage , for efficient working on non linear loads.
(iii)Despite this any short circuit on load side the clearing of SC faults are handled by using MCCB’s with higher fault ratings.
(iv)An internal fault in primary winding may have higher fault currents.
(v) We do not have short circuit withstand measurement facility in house.
(i) Datsons adopts low loss CRGO silicon steel for copper wound transformers.
(ii)The core assembly is rectangular joint for transformers up to around 30 kva.
(iii)Full MITRE step lap assembly for higher ratings.
(iv)For aluminium wound transformers use of special low loss CRNO grade laminationis used , but designed for flux densities 12- 15% to achieve low iron loss levels.
IMPULSE LEVEL TEST
· Applicable to HT transformer, connected directly to through overhead transmission lines subjected to lightning. · Not applicable to LT Isolation transformers in voltage class 1.1 kV- for indoor operations , preceded by and sourced from other distribution transformers.
· Not a routine test.
· The principle objective of K rated transformer designs is to handle and feed non linear harmonic loads without over heating beyond the temperature class. · However use of primary delta and secondary zigzag configuration Dz0 , mitigates all triplen harmonic currents and prevents them to be reflected on the supply side. · Therefore Dz0 is a preferred and advised configuration for K rated transformer. · Even Delta star configuration mitigates triplen harmonic currents as the delta primary provides circulation of triplen harmonic currents inside delta network and do not get reflected on source. · However this requires additional current handling capacity for primary delta winding. · There are other schemes of mitigating 5th and 7th harmonic currents which use dual transformer having o/p with 30 deg phase shift which help to mitigate 5th and 7th harmonics by cancellation.
TEMPERATURE RISE AND INSULATION CLASS
· The insulation class system more commonly adopted and available in India is class H (180 deg c). We adopt this insulationclass for all our K rated transformers. All our designs are for temperature rise of 115 deg C max with hot winding temp limited to 115 + 40 = 155 deg C. · The balance 25 deg C margin (180-155 deg C) is left for hot spots in the winding. Refer IS:11171 – 1985 clause 10 – temp rise limits: table 4.
· The transformer impedance is measured in short circuit test IS 2026 part 1 clause 16.4.
· Winding resistance are measures using DC micro ohm meter.
· Reactance X=sqrt (Z^2 - R^2) ohm.
· Ratio can be calculated from R and X.
· Crest factor is the ratio of peak voltage to RMS voltage. In a true sine wave curve it is 1.414. Due to harmonics and non linear currents typically at the input of rectifier capacitor type power supplies the currents are drawn only during part of the power cycle as short duration and high magnitude pulses. Generally the crest factor observed in computer power supply is observed to be in range of 2.5 to 4. These can be handled by K13 rated transformers .
· High crest factor current loads result in higher distortion in voltage waveform. The distortion in output voltage waveform is (% impedance of transformer ) X ( % THD in load current) .
· Harmonic mitigating transformer (HMT) is more useful as it mitigates the triplen harmonic currents in load on the secondary side and has lower zero sequence impedance.
|PROPERTY||3PH DELTA STAR||3 PH DELTA ZIG-ZAG STAR|
|1||TRIPLEN MITIGATION||LESS EFFECTIVE||MORE EFFECTIVE|
CIRCULATING IN PRIMARY DELTA AS WELL AS SEC.
TRIPLEN CURRENTS IN SECO.ONLY, BUT IN OPPOSITION WITH TWO SECTIONS OF WINDING.
|3||TRIPLEN FLUX WITH ADDITIONAL CORE LOSS|
TRIPLEN FLUX IN CORE RESULTS IN
ADDITIONAL CORE LOSS AS WELL AS VOLTAGE DISTORTION.
TRIPLEN CURRENT IN TWO PARTS OF SEC. IN OPPOSITION CANCELLING TRIPLEN FLUXES & LOSSES.
|4||STRENGTHENING OF WINDINGS|
PRIMARY AND SECONDARY BOTH
STRENGTHENING IS REQUIRED.
|NO CIRCULATING TRIPLENS IN PRIMARY,ONLY SECONDARY NEED STRENGTHENED.|
|5||SECONDARY WINDING VOLTAGE|
SAME AS OUTPUT-NO LOSS DUE TO VECTORS.
|16 % MORE THAN OUTPUT DUE TO VOLTAGE LOSS OF VECTORS.|
|6||SECO. VA RATING|
SAME AS OUTPUT.
|16 % MORE|
|7||ZERO SEQUENCE IMPENDENCE||HIGH OF THE ORDER OF AROUND 2 %.||LOW-OF THE ORDER OF AROUND 1.2%.|
|8||SUPPLY VOLTAGE WAVE DISTORATION||MORE||MUCH LOWER-NO TRIPLEN DISTORTION|
|9||5TH & 7 TH HARMONIC MITIGATION||NO MITIGATION||NO MITIGATION|
|10||COST COMPARISON||SLIGHTLY ECONOMICAL||AROUND 8% MORE COSTY|