En examinant la facture d'électricité et le dossier de fiabilité de production, l'ingénieur des installations est piégé dans une situation où une usine fonctionne 24 heures sur 24, tirant un pic d'énergie de 1,8 MW et subissant trois pertes de production (en raison de déclenchements sous tension) au cours de cet exercice fiscal jusqu'à présent. Une entreprise de services publics propose un approvisionnement dédié d'un réseau de 33 kV qui nécessitera une installation de transformateur à l'entrée. L'un des représentants commerciaux a affirmé qu'un transformateur de 2 000 kVA devrait convenir aux besoins de l'usine. L'ingénieur est conscient que “devrait” ne signifie pas dimensionner correctement un transformateur et une consommation de kVA incorrecte entraînerait des dépenses à long terme et un rapport de tension erroné neutraliserait tout avantage de l'installation de transformateur proposée. Ainsi, elle nécessite un système pour sélectionner un transformateur rempli d'huile correct.
Ce guide d'achat sert de système requis. Il guide à travers une série des huit décisions qui définissent le processus de sélection du transformateur, telles que la sélection de kVA, la détermination du rapport de tension, les paramètres et caractéristiques d'un transformateur, la plage de prises, le niveau de pertes, et bien d'autres avec tous les chiffres et normes respectifs.
En bref, lors de la sélection d'un transformateur rempli d'huile, huit variables importantes entrent en jeu : (1) la taille de kVA doit tenir compte des pics maximaux plus une marge de 15 à 25 pour cent ; (2) les rapports de tension doivent être alignés avec ceux du fournisseur de services et de la ligne du planteur ; (3) l'impédance est fréquemment dans la fourchette de 4,5 à 8 pour cent en fonction de la classe et de la coordination du niveau de défaut ; (4) les groupes vectoriels doivent être synchronisés avec la mise à la terre du système et le degré de phase ; (5) la plage de prises est fixée à ±2 fois 2,5 pour cent hors circuit pour des réseaux stables ou ±8 fois 1,25 pour cent en circuit pour des réseaux faibles ; (6) la classe de pertes du transformateur est établie conformément à la norme GB 20052 ou à l'Ecodesign de l'UE en capitalisant sur les pertes ; (7) le refroidissement doit être choisi entre ONAN ou ONAF en fonction du facteur de charge ; et (8) l'équipement optionnel comprend un relais Buchholz, un PRD, un moniteur de température et un vibrothermomètre.
Table des matières
- Décision 1 : Dimensionnement de la puissance kVA
- Décision 2 : Rapport de tension et prises
- Décision 3 : Impédance
- Décision 4 : Groupe vectoriel
- Décision 5 : Choix du changeur de prises
- Décision 6 : Classe de pertes et efficacité
- Décision 7 : Mode de refroidissement
- Décision 8 : Accessoires et protection
- La méthode de sélection en six étapes
- Marques et conseils de prix
- Questions Fréquemment Posées
- Références
- Conclusion
Décision 1 : Dimensionnement de la puissance kVA
La décision qui guide toutes les autres est le dimensionnement, car chaque décision ultérieure se prend dans le contexte d'une enveloppe de puissance. La bonne méthode :
- Déterminer ou évaluer la demande de pointe. Utilisez les données de mesure sur un an (par exemple, étude de charge) ou, dans le cas d'un site nouveau, déterminez la charge connectée et appliquez un facteur de diversité (475 pour commercial et 0,5-0,8 pour la charge industrielle).
- Convertir en kVA. kVA = kW/facteur de puissance. Pour un facteur de puissance égal à 0,9, la demande de pointe de 1,8 MW se convertit en 2 000 kVA ; avec un facteur de puissance de 0,8 – en 2 250 kVA.
- Utilisez toujours le facteur de puissance réel ou prédit, pas 1,0.
- Introduisez la marge de croissance. Ajoutez 15-25% pour la croissance de la charge sur l'horizon de planification de 5-10 ans. Par exemple, 2 000 kVA devient 2 300-2 500 kVA.
- Arrondissez à la puissance nominale standard. Les incréments standard de kVA dans cette zone sont 1600/2000/2500 et 3150.
| kVA calculé (étude de charge) | Puissance nominale standard à commander | Application typique |
|---|---|---|
| Jusqu'à 400 kVA | 400 / 500 kVA | Petits commerces, villages |
| 400–800 kVA | 630 / 800 / 1 000 kVA | Usines, centres commerciaux |
| 800–1 300 kVA | 1 000 / 1 250 / 1 600 kVA | Industrie moyenne |
| 1 300–2 200 kVA | 2 000 / 2 500 kVA | Grandes usines, campus |
| Au-dessus de 2 200 kVA | 3 150 kVA et plus | Sous-stations, industrie lourde |
Choisissez la puissance nominale appropriée la plus proche qui dépasse la valeur calculée. Dans ce cas, la valeur déduite était de 2 500 kVA.
Deux erreurs courantes de dimensionnement sont coûteuses. Le surdimensionnement (par exemple, utiliser une unité de 3 150 kVA pour un besoin de 2 000 kVA) entraîne des dépenses inutiles avec des pertes à vide encourues tout au long de la vie de l'unité – la perte à vide d'une unité de 3 150 kVA (d'environ 4,4) est égale à $3100 par an à $0,08 par kWh. Le sous-dimensionnement provoque un fonctionnement en surcharge, entraîne un vieillissement des isolateurs et provoque des déclenchements lors des pics. La bonne taille est définie par l'étude de charge, pas par le chiffre rond du revendeur.
Décision 2 : Rapport de tension et prises
Le rapport de tension doit être égal aux deux extrémités du système : la tension du réseau et la tension dans le bus. Les paires les plus courantes sont données ci-dessous :
| Tension d'alimentation | Bus côté charge | Unité typique |
|---|---|---|
| 33 kV | 11 kV | Distribution 33/11 kV |
| 35 kV | 10,5 kV | Puissance 35/10,5 kV |
| 22 kV | 11 kV | Distribution 22/11 kV |
| 11 kV | 4 kV | Montée sur poteau ou socle 11/0,4 kV |
| 6.6 / 6.3 kV | 4 kV | 6.3/0.4 kV industrial |
The range of the taps is determined on the high-voltage side and compensates for both the change of utility’s voltage and the voltage drop from the transformer. The case when voltage supply is constant with an accuracy of ±2.5% can be performed using off-circuit taps with position ±2 × 2.5%. In situations when supply is weak and/or load changes, an on-load tap-changer with regulation of ±8 × 1.25% maintains needed control. Economically speaking, OLTC increases the cost of the transformer by 15-30%, which is reasonable only when there are actually changes in voltage.
Décision 3 : Impédance
The factor of impedance (short-circuit voltage, UK%) determines two factors which can be in conflict with each other: voltage regulatory features and short-circuit current. The lower impairment is associated with stronger regulation, but at the same time it gives rise to bigger short-circuit current. On the other hand, the higher the impedance is the smaller the short-circuit current will be, but the voltage drop will be bigger. The general rule is to implement the transformer that is compatible with the protection and switching equipment already in use:
- Distribution transformers (≤1,000 kVA): 4 – 4.5% is the norm;
- Midsize transformers (1,6 – 10 MVA): 6% – 8% is the average; 7.5% is a common specification;
- Heavy transformers (>10 MVA): 8% – 14%, the higher the power the higher the impedance.
The improper impedance can be a hazard of coordination. If the impedance chosen is too small, the transformer will provide the short-circuit current that will be beyond the ability of the circuit breaker to clear.
Décision 4 : Groupe vectoriel
The vector group definition encompasses the winding configuration and phase displacement of the transformer as well as its earthing configuration and parallel operation requirements. Two of the most common vector groups used in oil-immersed transformers are:
- Dyn11: Delta configuration on the high side and star configuration on the low side (with the neutral lead-out from the transformer). This vector group designation is normally used for distribution transformers (e.g., 11/0.4 kV) operating in either single-phase or three-phase circuits.
- YNd11: Star configuration on the high side (with the neutral lead-out from the transformer) and delta configuration on the low side. This vector group designation is common for power transformers (e.g., 35/10.5 kV) feeding the grounded side through the neutral of the transformer.
The importance of the vector group becomes evident when paralleling two or more units. Two transformers connected to the same bus must have the same angle of phase displacement as well as matching impedances equal to ±7.5%. If the vector groups are different, then circulating currents are established causing overheating of transformers, which is a costly problem that can be avoided by checking the nameplate.
Décision 5 : Choix du changeur de prises
According to experts, tap changer is the sole moving part of transformer. The decision of selecting taps is related about reliability.
| Taper | Typical range | Fonctionnement | Cost impact | Maintenance |
|---|---|---|---|---|
| Off-circuit (DETC) | ±2 × 2.5% | De-energized only | Référence | None beyond inspection |
| On-load (OLTC), oil-immersed resistor | ±8 × 1.25% or ±6 × 1.67% | Under load | +15–30% | 50,000–100,000 ops |
| OLTC, vacuum type | ±8 × 1.25% | Under load | +20–35% | 150,000–200,000 ops |
If the power supply is stable and load is constant, off-circuit taps can be employed. If on-load regulation is required, the power supply must vary more than ±2.5% in case the load is unstable or the process cannot function due to undervoltage events. The difference in price ($6,000-12,000) of 10 MVA unit seems insignificant compared to the loss caused by the prolonged case of undervoltage.
Décision 6 : Classe de pertes et efficacité
Most lifetime costs are assessed in the loss category. Two loss measurements commonly used are no-load loss (P0, continuing all day) and load loss (Pk, dependent on the square of load factor). Follow recommendations:
- Stick to the applicable standards: Check GB 20052-2020 in China, EU Ecodesign Regulation 548/2014 in Europe and U.S. DOE 10 CFR Part 431 because they have a ceiling limit for losses which is highly useful.
- Value the difference: Each kW of P0 would be worth tariff × 8,760 h × project life. Each kW of Pk would be worth tariff × 8,760 h × (average load factor)² × project life.
- Select low-loss class in case of high load factor: With P0 values 60-70% lower with using amorphous-core transformer, an investment can be paid off within 6-12 years and would continue to give profit.
Some statistics on 1,000 kVA unit: Low-loss technology has approximately 1.1 kW lower P0 than S11, which is about $770/year at $0.08/kWh, $19,000 to pay during 25 years against $3,000-$6,000 to pay initially.
| Load factor | S11 no-load loss (1,000 kVA) | Amorphous no-load loss | Annual saving @ $0.08/kWh |
|---|---|---|---|
| 0.3 (light) | 1.7 kW | 0.6 kW | $770 |
| 0.6 (average) | 1.7 kW | 0.6 kW | $770 |
| 0.9 (heavy / 24/7) | 1.7 kW | 0.6 kW | $770 |
The no-load loss remains similar irrespective of the load factor. This means that savings stays constant. High operating hours ensure that the investment in unforgiving Premium gets quickly recovered.
Décision 7 : Mode de refroidissement
When putting a transformer in cooling mode, it will determine the thermal limit and the allowable overload. The following indicates the current standards and their interpretations:
- ONAN: Oil and air are used naturally to cool the transformer. This is the most basic type of cooling, the simplest, quiet and without any failure components.
- ONAF: Fans are added in addition to oil for cooling. This increases power by 20 to 30 percent with the use of the fans, which is only suitable for overloads or intermittent loads.
- OFAF/ODAF: The forced cooling method with the use of the fans for the biggest transformers – when the heat cannot be removed using the natural convection cooling.
If your load factor is smaller than about 0.8 and you have only few peaks – choose ONAN. If you expect to have short-term overloads often – choose ONAN/ONAF: in such a case your 1,000 kVA distribution transformer could reach 1,250 kVA with the help of fans. Make sure the overload map checks against IEC 60076-7.
| Cooling mode | Évaluation de base | Short-term overload headroom | Hardware |
|---|---|---|---|
| ONAN | 100% (nameplate) | Typically 120–130% for 1–2 h per IEC 60076-7 | Radiators only |
| ONAN/ONAF | 100% with fans off | +20–30% with fans on | Radiators + fans |
| OFAF / ODAF | Forced cooling rating | Limited by winding hot-spot | Pumps + fans |
Décision 8 : Accessoires et protection
The list of accessories is brief, it is compulsory in servicing and is frequently the most crucial point to tell the difference between the incident and catastrophe:
- Buchholz (gas) relay: Slow-gas alarming and fast-oil-surge trip for conservator-type devices of more than ~1 MVA. Wire it and do the tests.
- Pressure relief device: Releases the tank from internal pressure; size it according to the device.
- Oil and winding temperature indicators: With the alarms and trips set according to the 65K rise budget.
- Oil level gauge: Low level makes windings open; check it during the quarterly check-up.
- Silica gel breather: Dehumidifies the air that enters the conservator; change silica gel if it is already pink.
- Oil containing: Bund or a pit sized to accommodate the full volume of oil, as prescribed by IEC 61936-1 and local regulations.
None of these items costs much compared to the device and together, they convert the oil-immersed transformer from a fire threat to the managed asset. Budget 1–3% of the unit price for the entire protection set.
La méthode de sélection en six étapes
- Loading study: maximum kW, power factor, load factor, growth plan → potential kVA.
- Voltage coordination: supply voltage, bus voltage, earthing, interconnection requirements → ratio and phase group.
- Fault study: short-circuit capacity, switchgear ratings → impotency.
- Voltage quality decision: supply stability and load volatility → type and range of tap changers.
- Costing of losses: tariff and load factor → type of losses (S11, S13, amorphous, or regulated hallmark losses).
- Equipment and cooling system: overload plan, security, containment strategy → final configuration.
Make sure to perform all these steps in order and you’ll write the specification. The main shortcut — asking for pricing before actually applying steps 1-4 — is the reason of many unfortunate purchases because it gives all the power to those responding the letter first.
Marques et conseils de prix
| Marque | Origine | 1 000 kVA | 2,500 kVA | 10 MVA |
|---|---|---|---|---|
| Hitachi Energy | Japan/Global | $22,000–$34,000 | $38,000–$55,000 | $55,000–$82,000 |
| ABB | Switzerland/Global | $20,000–$31,000 | $35,000–$52,000 | $52,000–$78,000 |
| Siemens Energy | Germany/Global | $21,000–$32,000 | $36,000–$53,000 | $50,000–$75,000 |
| Schneider Electric | France/Global | $19,000–$29,000 | $33,000–$48,000 | $48,000–$72,000 |
| Jiangsu Subian Electric Power | Chine | $14,000–$22,000 | $22,000–$34,000 | $28,000–$48,000 |
Global corporations provide solutions with specialized engineering, field-testing experience, and global service capabilities, while offshore producers provide IEC-compliant products at very competitive rates. After assessing risks, the maintenance of the expense is reliable for important items, while the cost-saving alternative for mass production is usually feasible.
Jiangsu Subian Electric Co. Ltd is a Chinese manufacturer of oil pumps producing distribution transformers and power transformers with voltage ratings of 35–110 kV in line with IEC 60076 specifications. Every unit’s technical reports, compliance with the GB 20052 standards, and no-load voltage regulation capability are part of the experience acquired by the company. Product details and certificates are at subian-electric.com.
Questions Fréquemment Posées
How do I calculate the kVA I need?
kVA = maximum kW / power factor, plus 15 to 25 percent growth margin rounded to a standard rating. For instance, 1.8 MW at 90 percent power factor equals 2,000 kVA; with 25 percent growth margin equals 2,500 kVA. Whenever possible, use measured peak data; for new sites, add up the connected load and use a diversity factor that varies from 0.3 to 0.8, depending on the type of facility.
What voltage ratio should I choose?
Ensure your bus operating voltage at the low voltage side matches with the utility voltages on the high voltage side e.g., 33/11 kV or 35/10.5 kV or 22/11 kV or 11/0.4 kV. Get a confirmation of the operating voltage in writing from the utility company before placing the order; note that changing from the 33 kV voltage system to the 35 kV voltage system is not a straightforward task and requires redesign.
Do I need an on-load tap changer?
It only applies if your voltage is really fluctuating. If the utility is holding steady within ± 2.5 percent, and your load is constant, off-circuit taps at ± 2 by 2.5 percent are adequate. On the other hand, if the grid is weak, load fluctuations are rather large, or your process is sensitive in terms of undervoltage, the extra cost of an OLTC of 15–30 percent is made back by a single avoided lost production.
How much does the right transformer cost?
In general terms, quoting from a supplier, prices are FOB $6,000 to $16,000 for the 500 kVA transformer, $14,000 to $34,000 for the 1,000 kVA transformer, $22,000 to $55,000 for the 2,500 kVA transformer, and finally $28,000 to $82,000 for 10 MVA, depending on the type, manufacturer, and size of the transformer. Transport, installation and civil work fees range between 15 to 40%, depending on the locality.
What is the difference between Dyn11 and YNd11?
The standard combination for distribution units that provide single- and three-phase loads with neutral is Dyn11. For 35/10.5 kV power transformers, the YNd11 combination applies as the high voltage side is connected to the ground via the neutral of the transformer. The choice of connection type depends on the earthing arrangement and requirements for parallel operation.
Références
- IEC 60076-1: Power transformers – General — Ratings, taps, impedance tolerance, and temperature-rise rules for the selection decisions in this guide.
- IEC 60076-5: Ability to withstand short circuit — Short-circuit coordination basis for impedance selection.
- IEC 60076-3: Insulation levels and dielectric tests — Impulse levels by voltage class referenced in specification.
- EU Ecodesign Regulation 548/2014 (amended 2019/1783) — Loss benchmarks for European buyers.
- U.S. DOE distribution transformer efficiency standards — Minimum efficiency baselines for U.S. buyers.
- IEEE C57.12.00: Liquid-Immersed Distribution and Power Transformers — North American requirements counterpart for selection comparison.
- Jiangsu Subian Electric Power official site — Oil-immersed transformer range, efficiency grades, and certification documentation.
Conclusion
An oil-immersed transformer requires the input of eight different input requirements, instead of just a single input in terms of cost. If sized, matched with other components, and specified properly, your transformer will become and remain a silent asset for about 25 years. If these steps are neglected and haste is shown in this process, the transformer becomes a constant headache.The following are the key points that you must take into account when specifying your oil-immersed transformer:
- Start with an appropriate load study and leave 15-25% margin; transformer kVA size calculator is only a first step in finding out the required transformer kVA.
- Make sure you know the ratio, impedance and vector group characteristics of you transformer before inviting offers.
- You should only buy an OLTC for maintaining a varying voltage level; OLTC increases the cost of the transformer by 15-30%.
- Real cost should be calculated for power loss in transformers; usually, a low-loss transformer pays off over the years.