{"id":172,"date":"2026-08-22T02:34:22","date_gmt":"2026-08-21T18:34:22","guid":{"rendered":"https:\/\/q8bbzjnpm5.wpdns.site\/?p=172"},"modified":"2026-08-29T23:43:33","modified_gmt":"2026-08-29T15:43:33","slug":"35kv-oil-immersed-transformer-a-key-node-in-the-power-network","status":"publish","type":"post","link":"https:\/\/subian-electric.com\/pt\/news\/35kv-oil-immersed-transformer-a-key-node-in-the-power-network\/","title":{"rendered":"35KV Oil – Immersed Transformer: A Key Node in the Power Network"},"content":{"rendered":"

The distribution planner is looking at a map of the network of a region that is still growing. Even though the 110kV backbone is well developed, the problem is with the 10kV feeders that are located at the end of each 35kV spur: the farthest villages are receiving only 9.2kV during evening peak load, while at the busbar of the substation located next to the 110\/35kV station, one can read 10.8kV. There is no solution to the problem in the form of new feeders\u2014 the utility can\u2019t afford this investment in the current year, but one can get a better idea of positioning and sizing the 35kV oil-immersed transformer stations which are located between the two voltage levels, where they are stepping down, regulating, and distributing the electricity in all the outlets below.<\/p>\n

The article provides a description of the situation with the 35kV oil-immersed transformer and gives the idea of why this equipment is considered to be the crucial node of the power network, how it corresponds with the 110\/35\/10kV hierarchy, which specifications are significant on this level, and what their pricing is as well as how the customers from emerging economies should deal with it.<\/p>\n

A 35kV transformer refers to an oil-immersed transformer which helps to decrease high voltage typically rated at either 110kV or 66kV to a switchers level of around 10kV and even lower to the transformer rating of 400V. The transformer usually used has a rating somewhere between 3,150 kVA and 31,500 kVA while the efficiency of the unit is greater than 99%. The acquisition of the transformer should be based on the results of the tests, influence of a manufacturer on transformer losses as well as the level of electric power machinery service, instead of purchasing cost only.<\/p><\/blockquote>\n

\"35Kv<\/p>\n

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The Role of the 35kV Voltage Level<\/h2>\n

Power distribution takes place in layers because it is not cost-effective to transmit at low voltage over long distances. The 35 kV level plays an important role in many networks around the world, sitting between the 10-20 kV distribution and the 110-220 kV transmission levels. In China and in many regions of Asia, Africa, the Middle East, and parts of Europe, 35 kV is the standard sub-transmission voltage, enabling the collection of power from 110 kV substations and the supply of power to 10 kV feeders, manufacturing plants, and large power consumers.<\/p>\n

But why 35 kV and not higher or lower voltage? This is the level that is optimal for utility companies because they are capable of serving several hundred square kilometers using a practical number of substations and overhead lines. At 10 kV, electricity distribution from 110 kV substations would lead to about three times that of feeder currents, which would require significantly higher amounts of copper or aluminum used. Practically everywhere, increasing voltage to 110 kV would considerably hike the expenses related to substations.<\/p>\n

What Makes a 35kV Transformer a Key Node<\/h2>\n

Transformers are nodes where voltage, protection, and control change simultaneously. In the electrical substation, the 35kv oil-immersed transformer is the most costly unit as it is the one that supplies electricity to all secondary feeders. So, if it fails, the supply of electricity to all users of electricity will also be lost (up to 100 distribution substations or thousands of users). If the transformer is oversized, the utility will incur no-load loss costs for many years. If the transformer is undersized, hot spots will damage the insulation.<\/p>\n

Being a 35kV transformer, it is also a control node. This is due to the presence of on-load tap changers which stabilize the voltage at 10kV during load swings. If the transformer cannot maintain that, the voltage will fluctuate in accordance with the load curve the whole day. The 35kV transformer is also an economic node and the level of losses, the impedance and the tap position will influence the operating costs and reliability of the whole network.<\/p>\n

Types of 35kV Oil-Immersed Transformers<\/h2>\n\n\n\n\n\n\n\n\n\n
Tipo<\/th>\nClassifica\u00e7\u00e3o t\u00edpica<\/th>\nComutador de deriva\u00e7\u00e3o<\/th>\nUso t\u00edpico<\/td>\n<\/tr>\n<\/thead>\n
Distribution type (35\/0.4kV)<\/td>\n315\u20132,500 kVA<\/td>\nOff-circuit<\/td>\nVillage and small-town supply<\/td>\n<\/tr>\n
Power type (35\/10.5kV)<\/td>\n3,150\u201331,500 kVA<\/td>\nOn-load or off-circuit<\/td>\nSubstation step-down<\/td>\n<\/tr>\n
Step-up type (generator)<\/td>\n5,000\u201350,000 kVA<\/td>\nOff-circuit typically<\/td>\nRenewable and small thermal plants<\/td>\n<\/tr>\n
Three-winding type (35\/10\/6kV)<\/td>\n6,300\u201331,500 kVA<\/td>\nOn-load on HV<\/td>\nMixed-voltage substations<\/td>\n<\/tr>\n
Rural \/ single-phase<\/td>\n50\u2013200 kVA<\/td>\nNone<\/td>\nRemote single-phase distribution<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\"Types<\/p>\n

Most customers will be sold either small distribution systems (35\/0.4kV) responsible for supplying power to the local villages or larger power systems (35\/10.5kV or 35\/10kV) that take care of factories and substations. 6.3\u201320 MVA devices are the most widely used, large enough to provide good engineering quality and affordable enough regarding shipping and installation costs.<\/p>\n

Key Specifications and Standards<\/h2>\n

All 35kV transformers are built and tested to the IEC 60076 series (or the parallel IEEE C57.12.00 in the Americas). The numbers that matter at 35kV:<\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n
Par\u00e2metro<\/th>\nTypical value<\/th>\nObserva\u00e7\u00f5es<\/th>\n<\/tr>\n<\/thead>\n
Resist\u00eancia a impulso de raios<\/td>\n170 kVp<\/td>\nIEC 60076-3, 35kV class LI<\/td>\n<\/tr>\n
Power-frequency withstand<\/td>\n70 kV<\/td>\nAC applied test per IEC 60076-3<\/td>\n<\/tr>\n
Imped\u00e2ncia de curto-circuito<\/td>\n7\u20138%<\/td>\nIEC 60076-5<\/td>\n<\/tr>\n
Winding temp rise<\/td>\n65K average<\/td>\nAt 40\u00b0C ambient<\/td>\n<\/tr>\n
Grupo vetorial<\/td>\nYNd11 (35\/10.5kV)<\/td>\nOr Dyn11 for 35\/0.4kV<\/td>\n<\/tr>\n
Efici\u00eancia<\/td>\n99.0\u201399.5%<\/td>\nDepends on rating and loss level<\/td>\n<\/tr>\n
Resfriamento<\/td>\nONAN \/ ONAF<\/td>\nIEC 60076-2<\/td>\n<\/tr>\n
Loss levels<\/td>\nGB\/T 6451 or IEC class<\/td>\nGB 20052 efficiency grades apply<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

At 35 kV, it is important to consider impedance with care. Lower impedance (7%) reduces voltage drop but increases short-circuit current, which puts strain on the switchgear and on the windings of the transformer itself. Higher impedance (8-9%) reduces fault current but causes poorer voltage regulation. Sufficiently values are determined according to the fault level of the network and to the rating of the switchgear of the substation, so care should be taken and proper coordination of all schemes studied should be performed before changing impedance.<\/p>\n

Sizing and Load Characteristics<\/h2>\n

To size a 35kV transformer, there must be a careful consideration of 3 load parameters:<\/p>\n