Distribution transformers for South Africa

Medium voltage by network owner

Eskom explains the dual-ratio unit as a way to upgrade an 11 kV network to 22 kV without replacing the transformer fleet; the voltage is selected with an external off-load switch on the primary.

Low voltage: no-load secondary voltage

• Eskom: 420 V no-load, three-phase
• City Power: 415 V no-load, at the 11 kV principal tap

What SANS 780 covers

SANS 780 is the South African standard for oil-immersed distribution transformers. Edition 5 (2019) replaced the 2009 edition with lower no-load and load losses, and covers single-phase and three-phase units from 16 kVA to 3.15 MVA. It applies to units up to 3,150 kVA with a highest system voltage of 36 kV, with de-energised tapping only, at 50 Hz.

Eskom, municipalities and electrical contractors are the main buyers of units built to it. Eskom’s standard is read together with SANS 780 and takes precedence where the two conflict.

What the big owners add

• Vector group: Dyn11 (City Power); Dyn11 or Yzn11 (Eskom)
• Tapping, off-load: −5 to +5 % in 2.5 % steps (Eskom); −6 to +6 % in 3 % steps (City Power). Eskom places the tap switch on the LV side of the tank.
• Insulation level: 95 kV BIL at 12 kV, 150 kV at 24 kV (both owners); Eskom also lists 200 kV at 36 kV
• Construction: hermetically sealed, ONAN. Eskom asks for mineral oil or natural ester oil and a 36-month warranty; bushing creepage 20 mm/kV inland and 31 mm/kV coastal.
• Windings: City Power accepts copper only
• Ratings: Eskom pole-mounted three-phase 25, 50, 100, 200, 315 and 500 kVA, ground-mounted up to 1,000 kVA; City Power pole-mounted 100, 200, 315 and 500 kVA
• Maximum losses (City Power pole-mounted): 100 kVA 190 W no-load and 1,500 W load; 200 kVA 320 W and 2,600 W; 315 kVA 450 W and 3,600 W; 500 kVA 630 W and 5,200 W. Both owners also rank tenders by capitalised cost of losses.

SABS mark and type-test documents

City Power requires every transformer to bear the SANS 780 mark of approval and asks for the SABS permit number in the data schedule. Eskom asks for type-test reports, quality-management certification and mark-scheme permits with the tender, and keeps the right to test production units.

NRCS compulsory specification VC 8055 is written for electrical and electronic apparatus rated up to 600 V and lists small power transformers under SANS 61558. Check with the network owner which approvals it requires for your purchase.

What our published ratings cover

• Oil-immersed S11: the 10/11 kV class lists HV 6 to 11 kV, LV 380, 400, 415 and 433 V, vector groups Dyn11, Yyn0 and Yzn11, tapping ±2 × 2.5 %, 50 Hz. That matches the 11 kV, 415 V and Dyn11 combination in City Power’s specification and the ±5 % tapping range of Eskom’s. Published ratings run from 30 to 4,500 kVA and include 50, 100, 200, 315, 500 and 1,000 kVA.
• 22 kV: the S11 20 kV class is published at 30–1,600 kVA; the YBW compact substation lists 22 kV on the high-voltage side. 22/0.4 kV oil-immersed units to a network owner’s specification are quoted on request.
• 420 V no-load secondary, ±6 % tapping, 25 kVA units, dual 11/22 kV ratio and SANS 780 or Eskom loss limits: quoted on request to the specification you send.
• The SABS mark permit, Eskom approval and the SANS 780 type-test reports are not part of the published ratings. Ask for them in writing on the quotation.

Send with your enquiry

• Network owner (Eskom, municipality or private) and the specification number and revision
• Rated power (kVA), HV (11 kV, 22 kV or dual ratio), no-load LV voltage, vector group, tapping range
• Pole-mounted, ground-mounted or compact substation; mineral or ester oil
• Inland or coastal site and the altitude
• Whether the SANS 780 mark and permit number are required, and the quantity and delivery date