A83- Special Transformers- Auto-transformers Part-3; 400 kV Autotransformers in India

Part 1 of the article in this series on autotransformers was published some time back https://www.dhirubhai.net/pulse/special-transformers-1-auto-transformers-part-1-p-ramachandran/?trackingId=WSjpRPJFS1%2BhgokioNjxVw%3D%3D

In India,400 kV Autotransformers are used for connecting the 400 kV grid to 220 kV and also to 132 KV. The majority of the application is for interconnecting 400 and 220 kV grids using 315 MVA, 500 MVA, and 750 MVA three-phase or bank autotransformers. Even though in earlier days, when the 400 kV grid was introduced in the late 1970s, 250 MVA 3-phase and 630 MVA banks were in use, today ratings are more or less standardized to the above three ratings.

400/132 kV auto-transformers are sparingly used at ratings of 200 or 315 MVA three-phase ratings. In earlier days 100 MVA and 200 MVA ratings were common. All 400/220 kV and 400/132 kV ratings are with on-load tap-changers to vary HV voltage by +- 10 %. These tap-changers are located at the IV-line end of series winding for HV voltage variation i.e., core flux will not change with tap-changing i.e., constant flux regulation.

When we compare 400/220 kV auto-transformers with 400/132 kV auto-transformers, the following salient features should be appreciated.

1. ONAN/ONAF cooling shall be preferred to ONAN/ONAF/OFAF cooling due to the economy and ease of maintenance. Due to the low winding current density used nowadays to get the high efficiency demanded, oil-forced and directed cooling may not result in relative benefits. ?

2. When the transformer has a three-limb core construction stabilizing tertiary winding is not required.

3. In case the system allows, eliminate tap-changers for price savings and higher reliability. Then restrict the rated core flux density to 1.65 T to take care of the voltage variations. ?When tap-changers are necessary, provide online filters for tap-changers in all 400/220 kV auto-transformers.

4. When using 400/132 KV class transformers, go for neutral end tap-changers for IV variation. Even though this will make Transformers variable flux regulation type (i.e. with tap changing, core flux density will vary) still this option will be cheaper than line end OLTC for HV variation or neutral end OLTC for HV variation. In other countries (e.g. the UK, and South Africa) 400/132 kV auto-transformers are with neutral end OLTC for IV variations.

5. For the same MVA rating, a 400/132 kV Autotransformer will be costlier, heavier and less efficient (i.e. more losses) than a 400/220 kV Autotransformer. The losses of autotransformers of the same MVA rating but of different voltage ratios will vary unlike in the case of two winding transformers. The size of the active part and quantities of core and copper also will increase when the voltage ratio increases i.e. the losses and the active part weight will be more for a 400/132 kV ratio Auto-transformer, compared to a 400/220 kV ratio Auto-transformer.

6. Compared to a two-winding transformer, in an autotransformer, only a fraction of the total kVA is transformed through core and windings, the rest flowing from primary terminals to secondary terminals directly through the series winding. Owing to this fact, it is seen that the percentage savings due to the use of autotransformers increases as the ratio of low and high line voltages approaches unity and decreases as the ratio differs from unity due to the increased requirements of core and copper.

Hence losses will be higher for a 400/132kV Autotransformer, compared to a 400/220kV Autotransformer of the same MVA rating.

7. Losses of a 400/132 kV transformer can be determined by extrapolating the losses of a similar autotransformer of the same voltage ratio but of a different MVA rating. We can find out the losses of a 315 MVA auto-transformer of 400/132 kV voltage ratio by extrapolating the losses of a 200 MVA 400/132 kV auto-transformer. When the MVA rating varies with the same voltage ratio and similar impedances, the losses and active part weights will vary not linearly but as MVA is raised to 0.75.

8. If the standardized losses for 200MVA 400/132 kV Autotransformer are Core loss = 70kW and copper losses = 400kW.

9. In India, the voltage regulating taps in 400 kV auto-transformers are provided in two ways- In North and East India, tap winding is provided in between tertiary and common windings to get constant ohmic impedance ( ie varying percentage impedance) at all taps. In South and West India, tap winding is provided in between series and common windings to get constant percentage impedance at all taps. These two types of transformers cannot be parallel-operated due to different % impedance at various taps. The constant ohmic impedance design was adopted by the influence of ASEA designers. ?The advantages claimed by ASEA were higher impedance with tertiary and hence better short circuit withstand capability for tertiary winding during HV or IV line faults. The tap winding will be away from the main leakage field and hence will be subjected to less heating from eddy losses. With modern design and materials, these advantages are not appreciable and some countries which were following constant ohmic impedance winding arrangement switched over to constant percentage impedance pattern. (e.g. South Africa)