Lorsque l'ingénieur de sous-station examine le document pour un appel d'offres concernant la mise à niveau de transmission de 132 kV, l'une des premières fiches de spécifications examinées est normalement le tableau de classification des transformateurs de puissance, car un mauvais choix peut coûter jusqu'à des centaines de milliers ainsi que provoquer des mois de retard dans le processus de mise en service. Chacun des types de classification des transformateurs de puissance est créé pour répondre à un problème particulier en termes de tension, de charge et de fiabilité, que ce soit une unité de distribution de 10 MVA fonctionnant dans un bloc d'usines ou un transformateur élévateur de générateur de 400 MVA installé dans une centrale à charbon.
L'article fourni décrit toutes les méthodes possibles pour classer les transformateurs de puissance, qui peuvent être par fonction de puissance, par niveau de tension, par configuration de bobinage, par type de refroidissement et par type de matériau d'isolation, etc.
En résumé, les transformateurs de puissance peuvent être catégorisés en fonction de leur fonction, tels que transformateur élévateur, transformateur abaisseur, transformateur de distribution, transformateur d'isolement et auto-transformateur ; en fonction de leur classe de tension, tels que transformateur de générateur, transformateur de transmission et transformateur de distribution ; en fonction de leur configuration de bobinage, tels que transformateur monophasé vs transformateur triphasé, transformateur à deux enroulements vs transformateur à trois enroulements ; en fonction de leur approche de refroidissement, tels que transformateur ONAN, transformateur ONAF, transformateur OFAF et transformateur ODAF ; et en fonction de leur méthode d'isolation, tels que transformateur immergé dans l'huile et transformateur à sec.
Qu'est-ce qu'un transformateur de puissance ? (Définition et fonction principale)
Un transformateur de puissance est un instrument électromagnétique statique qui transmet de l'énergie électrique à travers deux circuits ou plus via le processus d'induction électromagnétique avec une modification des niveaux de tension tout en maintenant la fréquence et la puissance apparente constantes (avec toutes pertes). Alors que les transformateurs de distribution effectuent principalement la chute de tension finale pour la consommation, les transformateurs de puissance se trouvent dans les sous-stations, les centrales de production et les grandes industries avec des puissances supérieures à 200 kVA et sont capables de traiter des tensions allant de 3,3 kV à 1 150 kV.
Un transformateur de puissance peut être caractérisé par trois valeurs : la puissance apparente nominale en kVA ou MVA, le rapport de tension nominal et l'impédance en cas de court-circuit (qui est généralement entre 5% et 15% selon la norme internationale IEC 60076-5). L'efficacité d'un transformateur à pleine capacité de fonctionnement est généralement entre 98% et 99,7%. C'est pourquoi les services publics sont prêts à dépenser beaucoup d'argent pour des conceptions à faible perte à cœur amorphe ainsi que pour de l'acier au silicium orienté grain de haute qualité afin de minimiser les pertes à vide.
Comment fonctionne un transformateur de puissance : le principe électromagnétique
Le principe de fonctionnement fondamental est basé sur la loi de Faraday de l'induction électromagnétique. Le courant alternatif dans l'enroulement primaire produit un flux magnétique fluctuant dans le noyau en acier au silicium, reliant l'enroulement secondaire et induisant une force électromotrice (fem) proportionnelle au nombre de tours selon la formule classique :
Vprimaire / Vsecondaire = Nprimaire / Nsecondaire (ceci est connu sous le nom de rapport de transformation), de sorte que Iprimaire × Vprimaire ≈ Isecondaire × Vsecondaire si d'autres conditions sont idéales.
Un transformateur fonctionnant en mode élévateur a plus de tours dans l'enroulement secondaire que dans le primaire. Un transformateur fonctionnant en mode abaisseur a plus de tours dans la bobine primaire. Cependant, les transformateurs de puissance réels fonctionnent avec des pertes d'environ 0,3% à 2%, dues aux pertes en cuivre (pertes I²R dans les fils en cuivre) et aux pertes en fer (en raison de l'hystérésis et des courants de Foucault). Par conséquent, les caractéristiques techniques de tout transformateur mentionnent le niveau d'efficacité, qu'il soit de 50%, 75% ou 100%.
Classification par Fonction : Élévateur, Abaisseur, Distribution, Auto
La fonction est le moyen le plus pragmatique de classer les transformateurs de puissance car elle indique l'emplacement de l'unité dans le transfert d'énergie. Le tableau qui suit résume quatre familles fonctionnelles différentes de transformateurs couramment utilisées par les ingénieurs dans le réseau.
| Fonction | Changement de Tension Typique | Évaluation typique | Où Vous Le Trouvez |
|---|---|---|---|
| Transformateur élévateur de générateur (GSU) | 11–27 kV → 110–765 kV | 100–1 200 MVA | Centrales électriques, parcs éoliens et solaires |
| Transformateur abaisseur de transmission | 220–765 kV → 66–132 kV | 50–400 MVA | Sous-stations HV |
| Sous-transmission / distribution | 33–132 kV → 6,6–35 kV | 1–20 MVA | Sous-stations primaires, alimentations industrielles |
| Auto-transformateur | par ex. 220 kV → 110 kV | 100–1 000 MVA | Interconnexion entre niveaux de réseau |
Le transformateur GSU (Transformateur Élévateur de Générateur) est situé à l'extrémité génératrice du spectre, ce qui en fait l'équipement le plus précieux au sein d'une centrale électrique. En fait, un transformateur GSU de 600 MVA dans une centrale thermique peut être évalué entre $800,000 et $2.5 millions. Bien que les auto-transformateurs tendent à être plus petits et moins chers que le transformateur à deux enroulements traditionnel, cela est possible puisque certaines de leurs enroulements sont partagés. Par conséquent, les entreprises de services publics d'interconnexion choisiraient d'utiliser des auto-transformateurs, en particulier pour des applications de tension à faible rapport telles que 220/110 kV ou 400/220 kV.
Classification par Niveau de Tension dans le Réseau
La classe de tension influence la conception de l'isolation et la conception des équipements énergétiques, y compris les techniques d'isolation. Le prix des équipements énergétiques est affecté par la classe de tension. Les normes IEC 60076-1 et IEEE C57.12.00 déterminent toutes deux les techniques d'isolation.
| Niveau de Réseau | Tension Nominale | BIL (Résistance à l'Impulsion de Foudre) | Rôle Typique |
|---|---|---|---|
| Tension de générateur | 3–33 kV | 40–200 kV | Côté d'entrée GSU, alimentations auxiliaires |
| Haute tension (HT) | 6–35 kV | 75–250 kV | Sous-stations de distribution, industrielles |
| Très haute tension (THT) | 66–220 kV | 325–950 kV | Regional transmission |
| Extra high voltage (EHV) | 330–765 kV | 1,175–1,950 kV | National transmission backbones |
About 70% of all transformers in place globally are at the distribution stage, yet they account for only approximately 30% of the cost. A high-voltage unit with a 500-kV electrical system and a BIL value of 1,550 fills the requirements for thick insulation, wide neutral zones, and special test procedures (partial discharge tests and impulse tests according to IEC 60076-3), so installation of a powerful transformer may require spending $1.5–$3 million.
Classification by Phase and Winding Configuration
Power transformers can be classified based on their construction into either single-phase or three-phase transformers. This is based not just on their types but also on the number of their windings i.e. as two-winding transformers, three-winding transformers or auto-configured transformers. Single-phase transformers are widely accepted since they are cheaper and easier to transport, which is particularly helpful when it comes to highest EHV ratings (e.g. the formation of 3×333 MVA bank consisting of three single-phase transformers operating at 500 kV), while for 300 MVA or smaller variances three-phase transformers would be more suitable due to their advantage over single-phase variants in terms of losses, cost-effective solutions, and efficiency.
Three-winding transformers come with a tertiary winding and can thus be used not only for standard electrical loads but for harmonic filters, and to derive the third voltage (e.g. 220/110/35 kV). The vector group can be defined with the help of special designations, such as Dyn11, YNyn0, YNd11 etc., which show the transformer configuration and point of the voltage.
Classification by Cooling Method (IEC 60076-2)
The cooling system is what dictates the maximum amount of electrical energy that a transformer is able to handle safely. The acronym used in line with IEC 60076-2 refers to the type of cooling medium and method of its circulation.
| Code de refroidissement | Signification | Évaluation typique | Load Capability vs. ONAN |
|---|---|---|---|
| ONAN | Huile naturelle, air naturel | 50 kVA–60 MVA | Baseline (100%) |
| ONAF | Huile naturelle, air forcé (ventilateurs) | 1–150 MVA | ≈120–135% |
| OFAF | Huile forcée, air forcé | 10–300 MVA | ≈140–160% |
| ODAF | Oil directed, air forced | 100–1 200 MVA | ≈150–170% |
A transformer designed to operate in two cooling modes, ONAN and ONAF, means that a power utility can run it under normal circumstances in natural cooling mode while using fans in case of sudden overloads. A transformer with a capacity of 40 MVA, indicating its ratio as “40/56 MVA ONAN/ONAF” is a viable example. The necessary device components such as fans or oil pumps are not simple to maintain thus, smaller substations prefer ONAN designs only.
Oil-Immersed vs. Dry-Type Power Transformers
The insulation medium separates the market into two distinct categories, with both differences in pricing, safety, and use.
| Paramètre | Immergé dans l'huile | Type sec (résine coulée / VPI) |
|---|---|---|
| Insulation medium | Mineral oil or ester | Epoxy resin / air |
| Puissance typique | 50 kVA–1,200 MVA | 100 VA–40 MVA |
| Price (1 MVA class) | $18,000–$45,000 | $28,000–$70,000 |
| Fire / environmental risk | Higher (oil containment needed) | Low, flame-retardant |
| Best locations | Outdoor substations | Indoor, high-rise, offshore |
There are oil-immersed transformers that dominate in the zero2 MVA range due to the efficient dielectric properties of oil. Dry transformers perform better in places like office buildings, hospitals, and on-offshore oil rigs and platforms where there are laws, such as NFPA 70, that prevent the use of oil-filled transformers due to fire dangers. Cast resin transformers usually cost between 40%-60% more than comparable oil models, but they do not require oil-saving pits and fire extinguishing systems.
Key Specification Table for Common Transformer Classes
In order to make a fair comparison among transformer models, engineers will choose the necessary fundamental parameters for each of the options in question. One of the charts below shows nameplate data of three typical units.
| Paramètre | Distribution Unit | Substation Power Unit | GSU Unit |
|---|---|---|---|
| Puissance nominale | 1 000 kVA | 20 MVA | 250 MVA |
| Rapport de tension | 11/0,4 kV | 110/20 kV | 18/400 kV |
| Groupe vectoriel | Dyn11 | YNd11 | YNd11 |
| Tension d'impédance | 6% | 10% | 14% |
| Pertes à vide | ≈1,150 W | ≈14 kW | ≈90 kW |
| Pertes en charge | ≈10,500 W | ≈98 kW | ≈620 kW |
| Refroidissement | ONAN | ONAN/ONAF | ONAF/ODAF |
| Reference standard | IEC 60076-1 | IEC 60076-1 | IEC 60076-1 |
Loss assurance is important for people dealing with business: the loss value can improve or hinder the price of a transformer and can represent a 5%–10% difference as to the price of a transformer because a 1 kW difference in no-load losses over thirty years may cause the loss of tens of thousands of dollars.
The Unique Role of Each Transformer Class in the Power Grid
Each class is placed in its position in the energy chain, and none can be replaced with another one:
- GSU transformers — change the voltage produced by generators to the voltage for transmission at the generating plants and alternative sources site.
- Transmission step-down transformers function to interconnect high voltage and low voltage levels at major substations where they also regulate the voltage using on-load tap changers.
- Distribution transformers — are the last transformer which sets the voltage for consumption to 400/230 V and therefore constitutes the largest number of transformers in the world.
- Auto-transformer — interconnects neighboring voltage levels like 220 kV and 110 kV but should be installed only when the grounding is available due to its design.
- Phase-shifting transformer — an important tool in dealing with the issue of power flow in a parallel corridors and used more and more in mesh high voltage networks.
Distribution network designers say that up to 90 percent of losses in energy due to transformers happen in the distribution networks, which causes many countries (e.g. China and EU by virtue of GB 20052) to promote the use of amorphous core transformers with lower energy losses among the distribution transformers.
Top Brands & Realistic Price Ranges
Choosing a brand of power transformer comes down to balancing the initial purchase price, efficiency, lead time, and after-sales services that manufacturers provide. Well-known brands in the industry define the standard, whereas the oldest Chinese players provide approximately the same products that are compliant to IEC 60076 standards, but at 30%–50% lower prices.
| Marque | Headquarters | Force | Indicative Price (20 MVA, 110 kV) |
|---|---|---|---|
| ABB / Hitachi Energy | Switzerland / Japan | HVDC, EHV expertise | $450,000–$700,000 |
| Siemens Energy | Allemagne | EHV and digital substations | $430,000–$680,000 |
| Schneider Electric | France | Distribution, EcoStruxure | $280,000–$450,000 |
| GE Vernova / Prolec GE | USA / Mexico | Utility-scale GSU | $420,000–$650,000 |
| TBEA | Chine | Large MVA, global EPC projects | $180,000–$320,000 |
| Jiangsu Subian Electric Power | Chine | Custom 50 kVA–220 kV class, OEM/ODM | $160,000–$300,000 |
The pricing is not exact; it’s going to be variable depending on specification, loss levels and location. Thus, all these values given below should be understood as guides rather than fixed prices. Jiangsu Subian Electric Power offers transformers ranging from 50 kVA to 220 kV class with multiple options including dry-type and oil-filled transformers. The company provides a lot of additional advantages; first, they offer prices that are 30-50% lower compared to Europe; secondly, their distributors keep deadlines and deliver to over 60 countries of the world.
How to Choose the Right Transformer Class for Your Project
- Define the duty — a GSU, transmission, or distribution role resolves the entire design envelope.
- Check the voltage levels and tap range — establish the nominal ratio, ±10% to ±16% on-load tap range, and vector group with respect to your network.
- Calculate the load profile — determine the kVA/MVA size so that peak load lies between 60%–80% of the rating for the highest efficiency and margin.
- Establish loss budgets — ask for the no-load and load loss guarantees and carry out capitalized-loss comparison for a span of 20–30 years.
- Select cooling and insulation — ONAN for regular outdoor duty; ONAN/ONAF for overload flexibility; dry-type within the building.
- Check standards — obtain IEC 60076 series type tests (dielectric, temperature rise, short-circuit) and factory test papers.
- Consider access and transport — a 40 MVA unit has a weight of 40–60 tons and may need special transportations and labor study.
Questions Fréquemment Posées
What is the difference between a power transformer and a distribution transformer?
Power transformers manage high capacities (generally over 200 kVA) at the transmission or sub-transmission level and are built for full-load operation with voltage regulation; a distribution transformer is the last step-down transformer (normally 5 kVA–2,500 kVA) that reaches end users and operates mostly at partial load. Distribution transformers greatly outnumber power transformers; there are millions of them in the world while power transformers make the most valuable part of the substation.
What does the turns ratio of a transformer mean in classification?
The turns ratio N1/N2 is equal to the primary-to-secondary voltage ratio at no-load conditions which provides a true indication as to whether the transformer is a step-up transformer (i.e. N2 > N1), a step-down transformer (i.e. N1 > N2) or if it is an isolation transformer operating at 1:1 ratio. For example, the transformer having 110 kV/20 kV rating will have a ratio of 5.5:1 whereas GSU transformer stepping voltage down from 18 kV to 400 kV will have a ratio of around 22:1.
Which cooling class is best for a 20 MVA substation transformer?
The majority of electricity providers opt for ONAN/ONAF models, as its normal load is supported by natural cooling and forced air cooling (usually in the range of 120%-135% of the ONAN rating) accommodates peak loads and temporary overloads. Complete OFAF/ODAF designs include a pump and the oil’s movement direction, increasing both the efficiency as well as its maintenance costs.
How much does a power transformer cost in 2025?
The majority of electricity providers opt for ONAN/ONAF models, as its normal load is supported by natural cooling and forced air cooling (usually in the range of 120%-135% of the ONAN rating) accommodates peak loads and temporary overloads. Complete OFAF/ODAF designs include a pump and the oil’s movement direction, increasing both the efficiency as well as its maintenance costs.
Do I need an oil-immersed or a dry-type transformer?
When it comes to outdoor substations with capacities greater than 2 MVA, the most cost-effective option is oil-immersed type. However, within the indoor or high-rise applications such as fire-sensitive locations and offshore buildings, dry-type varieties should be used since oil containment and fire suppression are thus difficult to achieve. So, while a 1 MVA cast resin unit may cost $28,000–$70,000, oil-immersed units are generally sold in the range of $ 18,000 to $45,000.
Références
- IEC 60076-1: Power transformers — General — The international standard defining ratings, tolerances, and test procedures for power transformers.
- IEC 60076-2: Temperature rise — Specifies temperature-rise limits and cooling classes used in transformer classification.
- IEC 60076-3: Insulation levels and dielectric tests — Defines BIL and dielectric test requirements per voltage class.
- IEEE C57.12.00: General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers — The North American companion standard to IEC 60076.
- NFPA 70: National Electrical Code — Governs transformer installation, clearances, and protection in the United States.
- Hitachi Energy Transformer Portfolio — Reference for modern EHV, HVDC, and generator transformer technology.
Conclusion
It is crucial to understand the importance of power transformer classification. This is the biggest factor that will determine whether one will buy correct equipment. The function of transformer, voltage class, winding configuration, cooling method, and insulation type will indicate the presence of the transformer in the grid, amount of load, and its price — starting from $1,500 for distribution transformers and ending with $2.5 million for EHV transformers.
- First of all, it is important to categorize equipment according to its function (GSU, transmission transformer, distribution transformer, and auto transformer), then according to its voltage, cooling method, and insulation type.
- Cooling class should be aligned to load profile of the transformer. Best flexibility-cost ratio for the transformer of up to 150 MVA is ONAN/ONAF.
- It is required to follow IEC 60076 in the contract for loss reserves, dielectric tests, and short circuit testing.
- There should be at least three suppliers compared according to the price for the MVA, capital loss estimation, and delivery period.
If your project needs distribution or power transformers from 50 kVA to 220 kV class with custom ratios and IEC type-test documentation, contact Jiangsu Subian Electric Power for a direct factory quotation and engineering consultation.