Le responsable de l'établissement, qui a acheté un transformateur de distribution de 630 kVA pour $12,000 sur la base d'un devis reçu, a appris que six mois plus tard, il fonctionnait à une charge de 92% lors des journées chaudes, ce qui a entraîné l'absence de marge de surcharge, une augmentation des pertes et une réduction de la durée de vie de son appareil. Par conséquent, acheter le bon type de transformateur ne consiste pas à choisir le devis le moins cher, mais à faire correspondre les puissances motrices avec les pertes, les niveaux de tension, le type de système de refroidissement, ainsi que les exigences normatives.
Le guide complet sur la façon de choisir un transformateur couvre toutes les étapes que chaque client doit suivre, dans l'ordre dans lequel l'ingénieur les effectuera.
En termes simples, si vous souhaitez sélectionner le type approprié de transformateur à acheter, vous devez déterminer la charge maximale dont vous aurez besoin en kVA, puis choisir une puissance de transformateur appropriée de sorte que la charge de pointe soit d'environ 60% à 80% de la capacité du transformateur. De plus, assurez-vous de verrouiller le rapport de tension des transformateurs, le groupe vectoriel, l'impédance, la classe de refroidissement et le type d'isolation. Après cela, recherchez des devis de fournisseurs pour le transformateur en tenant compte des pertes sans charge et pendant la charge pendant 20 ans en utilisant la méthode des pertes capitalisées.
Étape 1 : Définir la charge et calculer la taille
Lors du choix d'un transformateur, il faut d'abord déterminer la charge. Ne totalisez pas la puissance nominale de chaque machine, sinon vous vous retrouverez avec un transformateur très surdimensionné. Au lieu de cela, suivez l'approche professionnelle :
| Étape | Calcul | Résultat |
|---|---|---|
| Charge connectée | Somme de toutes les puissances des équipements | 1 200 kVA |
| Demande maximale | Appliquer le facteur de demande 0,7 | 840 kVA |
| Correction du facteur de puissance | Compenser à 0,95 | 884 kVA |
| Marge de croissance | Ajouter 20% | 1 061 kVA |
| Puissance sélectionnée | Taille standard la plus proche | 1,000 ou 1,250 kVA |
La règle d'or stipule que la taille de l'unité doit être telle que la charge de pointe normale se situe entre 60% et 80% de sa capacité. Par exemple, une unité de 1000 kVA a une charge de pointe normale de 750 kVA, ce qui signifie que l'unité fonctionne avec une efficacité optimale avec une certaine marge de capacité pour le démarrage. Le surdimensionnement d'une unité entraîne des coûts inutiles ainsi que des pertes au repos, tandis que le sous-dimensionnement conduit à un cycle de vie rapide de l'unité ainsi qu'à un risque de panne par temps chaud.
Étape 2 : Sélectionner le rapport de tension et le groupe vectoriel
Le rapport de tension doit correspondre aux unités de générateur et de consommateur. En général, pour les applications industrielles et commerciales, le primaire du transformateur de tension est d'environ 11 ou 20 kV et le secondaire 400/230. Sur le site de la sous-station, les rapports de tension courants sont 110/20 kV, 33/11 kV et 220 kV/110 kV. Le groupe vectoriel aide à déterminer comment les phases sont connectées et si le transformateur est mis à la terre ou non.
| Groupe vectoriel | Configuration | Application typique |
|---|---|---|
| Dyn11 | Primaire delta, secondaire étoile avec neutre | Distribution, 11/0,4 kV — le plus courant |
| YNd11 | Primaire étoile avec neutre, secondaire delta | Transformateurs de puissance de transmission et de sous-transmission |
| Yyn0 | Étoile-étoile, les deux neutres | Petite distribution, réseaux spécifiques |
| YNyn0 | Étoile-étoile avec les deux neutres | Systèmes interconnectés avec neutres mis à la terre |
Lors du parallélisme des transformateurs, il est nécessaire de s'assurer de la compatibilité avec le groupe vectoriel, l'impédance et le rapport. Un transformateur Dyn11 ne peut pas fonctionner en tandem avec un YNd11. En cas de doute, un transformateur Dyn11 peut être demandé pour votre distribution et vous pouvez toujours vérifier auprès du fournisseur la compatibilité de vos configurations.
Étape 3 : Choisir l'impédance et la classe de refroidissement
Les caractéristiques de la tension d'impédance (qui est généralement de 4 % à 6 % pour les transformateurs de distribution et typiquement de 8 % à 12 % pour les transformateurs de puissance) définissent le courant de court-circuit et le partage de puissance entre les systèmes parallèles. Une faible impédance permet un courant de défaut plus élevé, mais offre une meilleure régulation de la tension ; une haute impédance protège l'équipement électrique, mais entraîne une mauvaise régulation de la tension. La plupart des entreprises de services publics et des concepteurs industriels ont calculé ce chiffre sur la base de l'analyse de leurs réseaux ; ne supposez pas que l'usine le connaît.
La classe de refroidissement indique combien de charge l'unité peut accueillir et à quel prix :
| Code de refroidissement | Signification | Quand le choisir |
|---|---|---|
| ONAN | Huile naturelle, air naturel | Distribution, service extérieur simple |
| ONAN/ONAF | Ajoute des ventilateurs à air forcé | Substation and industrial units with peak/overload duty |
| OFAF / ODAF | Forced oil + air | Large units above ~50 MVA |
| Dry-type (AN/AF) | Air natural / forced | Installations intérieures sensibles au feu |
When the fans are activated, a transformer rated 20/28 MVA ONAN/ONAF provides 40% greater capacity, an economical approach to meet peak summer loads and other contingencies without purchasing a larger transformer.
Step 4: Oil-Immersed or Dry-Type?
This choice is driven mostly by location and fire codes:
| Facteur | Oil-Immersed | Dry-Type |
|---|---|---|
| Best location | Outdoor substations | Indoor, high-rise, marine |
| Rating range | 50 kVA–1,200 MVA | 100 VA–40 MVA |
| Relative cost (1 MVA) | Base ($18k–$45k) | +40%–60% ($28k–$70k) |
| Fire risk | Oil containment needed | Flame-retardant |
| Maintenance | Oil testing, DGA | Minimal, no oil |
| Bruit | Generally lower | Higher for cast resin |
If a transformer is outside or in an oil-filled substation, it must be oil-filled transformer. However, once it is inside a building, many codes insist on either dry-type units or ester-filled transformer.
Step 5: Tap Changer & Regulation Needs
Check how much your supply voltage changes. If it is stable within ±5%, then a simple off-circuit switch with 2 or 3 positions (±2.5%, ±5%) will work well. If daily fluctuations, peak and off-peak loads occur, the on-load switch (OLTC) should be used:
- Off-circuit switches can either be cheap (3-5 positions). However, they must de-energize the transformer to set the position, so they are used mainly on distribution transformers.
- OLTC can have various ratings from ±10% to ±16% with 13 to 17 iterations in automatic regulation mode, i.e it keeps the output voltage within ±2%; This is common for transmission or sub-transmission transformers.
- When you consider the cost of these systems, an OLTC presents an additional cost of about $15,000–$60,000 based on the transformer rating; therefore you must treat it as an investment for quality of power.
Step 6: Evaluate Losses & Efficiency
Make sure to make comparisons based on the total cost of ownership rather than only focusing on bid pricing. Transformers are in operation continuously which means losses are happening permanently. Request each supplier guaranteed values for both no-load and load losses so that you can complete a capitalized loss comparison:
| Supplier | No-Load Loss | Load Loss | Est. Annual Energy Loss* | 20-Year Loss Value** |
|---|---|---|---|---|
| A (standard) | 1,150 W | 10,500 W | ≈17 MWh | ≈$34,000 |
| B (low-loss) | 900 W | 9,500 W | ≈14 MWh | ≈$28,000 |
The initial cost of the low-loss apparatus is greater than normal, however, in most instances, the money saved over a period of time (3-6 years) pays for the unit. This is why stringent energy consumption standards exist.
Step 7: Standards, Certification & Testing
Your contract must mandate compliance with the IEC 60076-1 standard in general and with the specific IEC 60076 provisions relevant to your application and, additionally, the IEEE C57.12.00 standard for North America. Insist on receiving the following documents before payment:
- Type-Test Reports – for the temperature rise, lightning impulse, and short-circuit withstand tests conducted on a sample unit.
- Routine Test Certificates – for the ratio, impedance, losses, dielectric, and insulation resistance determined on your specific unit.
- Certifications – if necessary, CE mark, ISO 9001 quality certificate and IEC 60076-11 for dry-type transformers.
- Third-Party Inspection Report – usually by SGS or Bureau Veritas, or may be provided by your own expert supervising tests in the factory for relevant price orders.
It is also advisable to abandon suppliers that refuse independent inspection or do not possess type-test reports irrespective of their pricing policy as certificates and records of examinations are the proof of the quality of a product.
Step 8: Compare Brands & Prices
Both multinational companies and the Chinese industry adhere to IEC 60076 standards but differ in terms of price, lead time, and engineering services. The planning-level intervals for oil-immersed transformers are:
| Puissance | Chinese Factory (e.g. Subian) | European/US Brand |
|---|---|---|
| 100 kVA | $1,500–$8,000 | $4,000–$12,000 |
| 1 MVA | $18,000–$45,000 | $35,000–$70,000 |
| 2.5–5 MVA | $35,000–$90,000 | $70,000–$200,000 |
| 20 MVA / 110 kV | $160,000–$300,000 | $430,000–$700,000 |
Specifications, loss levels, and locality dictate prices. If an enterprise requires equipment that meets the IEC 60076 standards for a factory-direct price, Jiangsu Subian Electric Power is an excellent choice: this manufacturer of distribution and power transformers (50 kVA–220 kV classes, both oil-immersed and dry) also bears specifications in terms of voltage ratios, vector grouping, tap range, and some cooling methods. All the units are equipped with type test and routine test results, certification according to CE/ISO standards, and their prices are much lower than those of their foreign competitors (the price difference varies between 30% and 50%). Also, the factory is open for OEM/ODM projects and third-party inspections.
The Complete Selection Checklist
- Determine the peak load with the help of the demand factor and growth margin; ensure that the rating is chosen such that the peak load is between 60% and 80%. Verify the voltage values for supply and load and decide on both the ratio and vector group.
- The impedance must be acquired from the network study (5%-10% on average).
- Choose a cooling type: ONAN when working under standard loads, ONAN/ONAF if overload is expected.
- Choose between oil and dry-type based on the place of installation and fire code requirements.
- Choose the tap changer: off-circuit if the supply voltage is stable, OLTC if more precise voltage stability is required.
- Compare offers based on guaranteed losses using a capitalized loss study for 20-30 years.
- Request the type test reports according to IEC 60076 and third-party inspection.
- Budget for 20%-40% of costs for protection, installation, testing, and construction works.
- Check the lead time, warranty (which usually lasts from 12 to 24 months), and service after the sale.
Questions Fréquemment Posées
How do I calculate the right transformer size for my building or factory?
Determine the connected load, multiply with demand factor of 0.6-0.85, correct power factor to 0.9-0.95, add anything from 15%-25% for growth to that. Then select the nearest standard rating so that the normal peak load is equal to somewhere between 60%-80% of the capacity. For example, if connected load equals 1200 kVA, demand factor is 0.7, and growth is 20%, the result will be something close to 1060 kVA. In this case, a unit with rating equal to either 1000 or 1250 kVA will do.
What is the difference between a 11/0.4 kV and a 20/0.4 kV transformer?
Both units have the same function but operate at different voltages in this instance, where the 11-kV unit is suitable for 11 kV incoming medium voltage lines and the 20-kV unit operates in 20 kV systems. While the kVA ratings can be the same, the high-voltage unit has higher BIL insulation as well as different construction, and in general costs slightly more.
Should I buy an oil-immersed or a dry-type transformer?
In some cases, dry-type transformers are used in occupied buildings and outdoor locations. The dry-type transformers are very high rated and maintenance free. The cost of dry-type transformer in range of $28000-$70000 and compared to oil immersed transformer in a range of $18000-$45000.
Why are transformers with the same rating priced so differently?
Loss guarantees, main steel quality, winding material (copper or aluminium), brand of tap changer, cooling class, history of type testing, and brand premium. Always do a capitalized-loss comparison: a unit that has low losses and costs an additional 5% to 10% is likely to save more in power costs during 20 years than its entire price difference.
What paperwork should I request before buying a transformer?
Ask for the type test report (temperature climbing, lightning impulse, short circuit), routine testing certification for your unit, ISO 9001 quality certification, CE marking if applicable, and the possibility of third party inspection. According to IEC 60076-1, ratio tolerance is ±0.5%, therefore verify the test results with your specification prior to accepting delivery.
Références
- IEC 60076-1: Power Transformers — General — The base standard for ratings, tolerances, and tests.
- IEC 60076-7: Loading Guide for Oil-Immersed Power Transformers — Overload capability and loading recommendations for sizing.
- IEEE C57.12.00: General Requirements for Liquid-Immersed Transformers — North American selection and application standard.
- US DOE: Transformer Efficiency Standards — Efficiency regulations that shape loss specifications.
- NFPA 70: National Electrical Code — Installation requirements for transformer selection and placement.
- Maschinenfabrik Reinhausen (MR) — Reference for tap changer selection and voltage regulation.
Conclusion
The choice of the appropriate transformer is not simply one of price but consists of a process of eight specific engineering decisions. This means that proper sizing with reference to the actual load profile, choice of the right voltage (with the right vector group), type of cooling and the insulation suitable for specific locality, assessment of losses within the life cycle of the asset as well as strict insistence on the test evidence that is supported by standards are the factors making a good purchase rather than a costly mistake.
- The most general sizing of transformers means so-called normal peak = 60%−80% of the transformer rating with a 15%−25% margin for growth to be considered.
- The ratio, vector group, impedance, and cooling should be predetermined before the supplier’s selection.
- The comparison should be made based on the capitalized losses that may be incurred during 20−30 years as opposed to the first cost.
- Ensure the receipt of the type test reports as well as third-party inspection references.
- Prices are estimated in a range of $1.5k−$8k (for 100 kVA transformers), $18k−$45k (for 1 MVA transformers), $160k−$300k (for 20 MVA transformers) when purchased from the manufacturer directly.