A 61 - Standard Power Transformer Ratings and Efficiency Levels (losses) for Indian Power Grid

This is a summary of the proposal for reducing the variety of Power transformers used in India. Such standardization will reduce costly special test charges (like short circuit withstand test charges in a high-power laboratory) by freezing transformer designs for a few years till there are technological improvements to be incorporated. Utilities will be benefitted by the simplified procurement process, reduced procurement time, inspection charges, shorter delivery time, interchangeability of different makes and less inventories. Data provided may be useful for planning engineers as reference.

1.????Ratings and energy losses for distribution Transformers (up to 2.5 MVA 33 kV) are standardized in India vide IS:1180-2014 “Outdoor Type Oil Immersed Distribution Transformers up to and including 2500 kVA 33 kV-Specifications”. Transformers above 2.5 MVA are considered here for proposing the ratings and efficiency levels. Of course, 100 % requirement of Power Transformers cannot be covered in such an exercise, but even if 80-90 % of requirements can be brought under this rationalization, it will be highly beneficial to users and manufacturers. The list does not cover special transformers like Generator Transformers for hydro-electric projects, industrial transformers (Furnace and Rectifier), transformers for renewable energy projects and static compensators (STATCOM, SVC), traction transformers and mining transformers.

?2.????Preferred Losses for transformers are arrived at on the following assumptions/requirements:

??2.1??????The MVA ratings are all of three phase units except where 3x ABC is shown. Such transformers are of three phase banks, formed from three single-phase transformers of rating ABC MVA. Preferred MVA ratings are generally from R 5 series ie multiples of fifth root of 10 ie. 1-1.6-2.5-4-6.3-10 series instead of R10 series (multiples of tenth root of 10- 1,1,25,1.6,2,2.5,3.15,4,5,6.3,8,10) used so far.

?2.2??????The losses (Wi= No-load Loss; Wc = Load Loss; Waux = Auxiliary or cooler loss) indicated are the maximum limits except for stray losses. Stray losses may vary plus or minus from the indicated values, but maximum I2R and maximum load loss limits shall not be exceeded and are treated as guaranteed figures. Loss capitalization is not applied during the tendering stages and no advantage is given for lower measured losses during testing. ?2.3????Maximum current density is limited to 2.5 A/mm2 at all taps in all active windings except stabilizing tertiary (unloaded) windings where maximum current density is to be limited to 4.5 A/mm2. Maximum flux density limit at rated voltage and frequency is 1,7 T for Generator Transformers and 1.65 T for all other transformers. High permeability (Hi B) or domain refined CRGO is considered for the no-load losses.?

?2.4??????Continuously transposed cable (CTC) is to be used for any winding current path above 400 A for transformer ratings of 10 MVA and above. Conductor strand thickness of CTC shall not be less than 1mm.

??2.5.?????Taps and Arrangement:

All two winding transformers - +- 5 % (de-energised tap changer) or +5 % to??-15 % (on-load Tap-changer) with body taps or separate tap winding on HV winding neutral end for HV variation. Winding arrangement - Core -Tertiary - LV-HV -Regulating

Generator Transformers - +- 5 % (de-energised tap-changer) or fixed ratio, without taps. Taps on HV neutral for HV variation. Body taps or LV-HV-Regulating winding arrangement.

Auto- transformers up to and including 420 kV:

At present in the country there are 220 kV class auto-transformers of different tapping arrangement

a)???Taps on HV (series winding) for HV variation by +- 10% with two options- constant ohmic or constant percentage impedance.

b)???Taps on MV line end for MV variation by +-10 % with two options- constant ohmic or constant percentage impedance.

As standard, HV (series winding) taps for HV variation with constant percentage impedance is selected.

With respect to 400 kV class auto-transformers, there are two voltage ratios- 400/ 220 kV (most common) and 400/132 kV. In case of 400/132 kV class transformers, OLTC is provided in HV (series) winding for HV variation by +-10 % with regulating winding positioned between tertiary and common so as to get constant ohmic impedance at all taps. But an economical solution used in most countries is to provide OLTC at neutral end of common winding for MV variation. This neutral end application of OLTC is selected as the standard.

400/220 kV class auto-transformers form bulk of the 400 kV transformer population and in these units, OLTC is provided in HV (series) winding for HV variation by +-10 %. In this application, there are two variants used in the country. In North and East India, regulating winding is positioned between tertiary and common winding, resulting in constant ohmic impedance at all taps. But in West and South India, regulating winding is placed between common and series winding, resulting in constant percentage impedance at all taps. Since the 400 kV grid in the country is strong and well interconnected, these tap-changers in 400/220 kV units are found to be ineffective and rarely used. Hence as standard, fixed ratio 400/220 kV auto-transformers without OLTC is selected as standard ?and where ever OLTC is essential, constant percentage impedance arrangement (Try-Common-Reg-Ser) is proposed as standard.

800 kV Auto-Transformers

When the first 3x500 MVA 765/400 kV auto-transformers were procured in 2005, these were double limb wound, with TRY-Reg-Common-Series winding arrangement and with OLTC on HV neutral end for HV variation by +-5 %. But transformers procured in later years were single limb wound type with tertiary and regulating windings on the side core limb. These were with very high tertiary impedances (HV-Try 195 % and MV-Try with 180 % instead of the earlier 67 % and 49 % respectively) and at present these high tertiary impedance transformers are the standard. Units from these two types of banks cannot be interchanged to form three phase banks, due to the difference in tertiary impedance. In future, at many locations fixed ratio units will be sufficient and where ever, OLTC is required, single limb Try-Reg-Common-Series winding arrangement is proposed.

2.6.????????Purchaser has the right to check the specific loadings (current density in winding and flux density in core) at various stages of procurement (tender stage, design review stage, manufacturing stage) against the values furnished in GTP

2.7.????????Stabilizing delta tertiary winding is not required for transformers with 3 phase 3 limbed core construction. At present, stabilizing delta tertiary winding is not provided in YNyn connected transformers up to a rating of 100 MVA. Since rating has no significance with stabilizing tertiary, core construction is considered as criteria for deciding the necessity of stabilizing delta winding.

?2.8?????????ONAN cooling will be used for ratings up to 31.5 MVA. Preferred cooling for ratings of 50MVA and above is ONAN/ONAF-1/ONAF-2(with 60:80:100 rating). ONAN/ONAF/ODAF or OFAF can be alternate to ONAN/ONAF-1/ONAF-2 cooling subject to purchaser’s approval. As very low current densities are ?used in windings (due to the high energy efficiency specified) there is very little advantage with forced oil cooling and the preferred cooling is ONAN/ONAF-1/ONAF-2. ???

?2.9??The Insulation levels for various voltage classes are as below.

Note 1- Generator Transformers are with one step higher test voltages- 630/1175/1425 kV

Note 2- Generator Transformers are with one step higher test voltages -/1700/2050 kV

MAXIMUM LOSSES FOR STANDARD TRANSFORMERS

Note:

1.????Taps on HV for HV variation – Constant percentage impedance- Try-Reg-Common- Series

2.?????Neutral end taps for MV Variation. Winding arrangement, Ter-Reg-Com-Ser

3.????Taps on HV for HV Variation; Constant Ohmic Impedance, Ter-Reg- Common- series

4.????Taps on HV for HV Variation; Constant Percentage Impedance -Ter-Com-Reg-Ser

5.????Without taps (Ter-com-ser)

6.????Taps on HV for HV variation- Constant Ohmic Impedance- Ter-Reg-Common-Series

7.????Taps on HV for HV variation; Constant Percentage Impedance -Ter-com-reg-Ser

8.????Without taps (Ter-Com-Ser)

9.????Taps on HV for HV Variation; Constant Ohmic Impedance – Ter- Reg- Common- Series

10.?Taps on HV for HV variation; Constant Percentage Impedance -Ter-com-reg-Ser

11.?Without taps (Ter-com-ser)

12.?Taps on HV for HV Variation- Constant Ohmic impedance – Ter-Reg- Common-Series

13.?Taps on HV for HV variation; Constant Percentage Impedance -Ter-Com-Reg-Ser

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Notes:

9.????Side limb regulation (Regulating winding and tertiary on side limb)

10.????Without tap changer (Ter-com-Ser)

11.????Single limb (Without side limb regulation) (Ter-Reg-Common-Series)

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