Tap - changer ON - LOAD ON - LOAD Tap - Range (HV Side) +5% to -15% +5% to – 15% Tap - Step 1.25% 1.25% Insulation Levels Lightning Impulse / Power Freq. Withstand) Particular 2.5MVA 5MVA HV 170kVp/70kV (r.m.s) 170kVp/70kV (r.m.s) HVN 170kVp/70kV (r.m.s) 170kVp/70kV (r.m.s) LV 75kVp/28kV (r.m.s) 75kVP/28kV (r.m.s) LVN 75kVp/28kV (r.m.s) 75kVp/28kV (r.m.s) Temperature Rise (oil / wdg) (Ambient : 50°C) 50/55°C 50/55°C Connections - HV/LV Cable box/cable box Cable box/cable box Applicable Standard IEC-60076 IEC-60076
Distribution Planning Manual EPD-M1 Issue : 1 10-09-2017 Internal 16 126.96.36.199. Existing 33kV and 11kV System Neutral Earthing Philosophy As the main 66/11kV (and 132/33kV) transformers are 'delta' connected at the 11kV systems, are earthed through zig zag type earthing transformers, generally limiting the earth fault current to approximately 1.5kA (in some instance to about 4.5kA as in Doha Industrial - B substation). These values are taken from the relevant pages of short-circuit levels provided by KAHRAMAA for information. A. Neutral Earthing for the 33/11kV Package Substation 11kV System Considering single unit operation only and in line with the general philosophy of limiting the 11kV system earth - fault current value to a theoretical maximum value of 750A/main transformer, it could be considered that a 'Neutral Earthing Resister' (metal grid type) is used with a resistance value of about 8.5 ohms. This will limit the earth - fault current to a value of about 750A. The resister could be short-time rated for 30 sec. B. Typical Technical Particulars for Neutral Earthing Resister - Rated Voltage : 6.35kV - Earth fault current : 750 A - Earth fault duration : 30 sec. - Type : Metal Grid 1.4. Distribution substations The Distribution Substations for the 33kV network are the distribution substation having Low Voltage as secondary side voltage. 1.4.1. Type, layout and configuration The 33kV/LV substations are of outdoor type 188.8.131.52. 33kV/LV Ground Mounted Transformers The 33kV/LV Ground Mounted Transformer (GMT) shall be supplied in sizes of 500kVA, 1000kVA and 1600kVA. They are ground mounted on a prepared concrete pad and are totally enclosed units with cable box connections for high voltage and low voltage cable connections. The low voltage connections will be suitable for single core cable connections (2 per phase) for direct connection to a single consumer or 6 way feeder pillar The substation will be located in a secure area at the load centre for a group of consumers and also near to the 33kV overhead line. The substation when supplying an individual consumer will be constructed as near as possible to the consumer switchroom. The route of the low voltage single core cables from the substation to the consumer’s main switch should be as short as possible to avoid voltage drop. Normally the single core LV route should be of the order of 10-15 metres. Also, where possible, the 33kV/LV GMT Substation and the consumer’ s switchroom shall be located near to the 33kV OHL to avoid installing long lengths of 33kV cables.
Distribution Planning Manual EPD-M1 Issue : 1 10-09-2017 Internal 17
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