Wenn man die Stromrechnung und die Produktionszuverlässigkeitsaufzeichnungen betrachtet, ist der Anlageningenieur in einer Situation gefangen, in der eine Anlage rund um die Uhr betrieben wird, einen Energiepeak von 1,8 MW zieht und in diesem Geschäftsjahr bisher drei Produktionsausfälle (aufgrund von Unterspannungsabschaltungen) erlitten hat. Ein Versorgungsunternehmen schlägt eine dedizierte Versorgung eines 33 kV-Netzes vor, was eine Transformatorinstallation am Eingang erfordert. Einer der Vertriebsmitarbeiter hat behauptet, dass ein 2.000 kVA Transformator für die Bedürfnisse der Anlage geeignet sein sollte. Der Ingenieur ist sich bewusst, dass “sollte” nicht bedeutet, einen Transformator korrekt zu dimensionieren, und ein falscher kVA-Verbrauch zu langfristigen Kosten führen würde, während ein falsches Spannungsverhältnis jeden Vorteil der vorgeschlagenen Transformatorinstallation neutralisieren würde. Daher benötigt sie ein System zur Auswahl eines korrekten ölgefüllten Transformators.
Dieser Einkaufsführer dient als das erforderliche System. Er führt durch eine Reihe von acht Entscheidungen, die den Auswahlprozess für Transformatoren definieren, wie z.B. kVA-Auswahl, Bestimmung des Spannungsverhältnisses, Parameter und Eigenschaften eines Transformators, Abgriffbereich, Verlustniveau und viele andere mit allen entsprechenden Zahlen und Standards.
Kurz gesagt, bei der Auswahl eines ölgefüllten Transformators kommen acht wichtige Variablen ins Spiel: (1) Die kVA-Größe sollte die maximalen Spitzen plus eine Marge von 15–25 Prozent berücksichtigen; (2) Spannungsverhältnisse müssen mit der Linie des Dienstanbieters und des Planers abgestimmt sein; (3) Die Impedanz liegt häufig im Bereich von 4,5–8 Prozent, basierend auf Klasse und Koordination des Fehlerpegels; (4) Vektorgruppen müssen mit der Erdung des Systems und dem Grad der Phase synchronisiert werden; (5) Der Abgriffbereich ist auf ±2 mal 2,5 Prozent außerhalb des Stromkreises für stabile Netze oder ±8 mal 1,25 Prozent im Stromkreis für schwache Netze eingestellt; (6) Die Verlustklasse des Transformators wird gemäß GB 20052 oder EU-Ecodesign festgelegt, indem die Verluste kapitalisiert werden; (7) Die Kühlung sollte je nach Lastfaktor zwischen ONAN oder ONAF gewählt werden; und (8) Die optionale Ausrüstung umfasst Buchholz-Relais, PRD, Temperaturüberwachung und Vibrothermometer.
Inhaltsverzeichnis
- Entscheidung 1: Dimensionierung der kVA-Bewertung
- Entscheidung 2: Spannungsverhältnis und Abgriffe
- Entscheidung 3: Impedanz
- Entscheidung 4: Vektorguppe
- Entscheidung 5: Wahl des Abgriffwechslers
- Entscheidung 6: Verlustklasse und Effizienz
- Entscheidung 7: Kühlmodus
- Entscheidung 8: Zubehör und Schutz
- Die Auswahlmethode in sechs Schritten
- Marken und Preisrichtlinien
- Häufig gestellte Fragen
- Referenzen
- Fazit
Entscheidung 1: Dimensionierung der kVA-Bewertung
Die Entscheidung, die alle anderen leitet, ist die Dimensionierung, da jede spätere Entscheidung im Kontext eines Bewertungsrahmens erfolgt. Der richtige Weg:
- Bestimmen oder bewerten Sie die Spitzenlast. Verwenden Sie einjährige Messdaten (z.B. Laststudie) oder, falls der Standort neu ist, bestimmen Sie die angeschlossene Last und wenden Sie einen Diversitätsfaktor an (475 für kommerzielle und 0,5-0,8 für industrielle Last).
- Umrechnen in kVA. kVA = kW/Leistungsfaktor. Bei einem Leistungsfaktor von 0,9 wird die Spitzenlast von 1,8 MW in 2.000 kVA umgerechnet; bei einem Leistungsfaktor von 0,8 – in 2.250 kVA.
- Verwenden Sie immer den tatsächlichen oder vorhergesagten Leistungsfaktor, nicht 1,0.
- Führen Sie die Wachstumsreserve ein. Fügen Sie 15-25% für das Lastwachstum über den Planungszeitraum von 5-10 Jahren hinzu. Zum Beispiel wird aus 2.000 kVA 2.300-2.500 kVA.
- Runden Sie auf die Standardbewertung. Standard-kVA-Inkremente in diesem Bereich sind 1600/2000/2500 und 3150.
| Berechnete kVA (Laststudie) | Standardbewertung zur Bestellung | Typische Anwendung |
|---|---|---|
| Bis zu 400 kVA | 400 / 500 kVA | Kleine Gewerbe, Dörfer |
| 400–800 kVA | 630 / 800 / 1.000 kVA | Fabriken, Einkaufszentren |
| 800–1.300 kVA | 000 / 1.250 / 1.600 kVA | Mittlere Industrie |
| 300–2.200 kVA | 000 / 2.500 kVA | Große Anlagen, Campus |
| Über 2.200 kVA | 150 kVA und mehr | Umspannwerke, Schwerindustrie |
Wählen Sie die nächstgelegene geeignete Bewertung, die den berechneten Wert übersteigt. In diesem Fall war der abgeleitete Wert in unserem Fall 2.500 kVA.
Zwei häufige Größenfehler sind kostspielig. Überdimensionierung (z.B. Verwendung eines 3.150 kVA Geräts für benötigte 2.000 kVA) verursacht unnötige Kosten durch Leerlaufverluste, die während der Lebensdauer des Geräts anfallen – der Leerlaufverlust eines 3.150 kVA Geräts (von etwa 4,4) entspricht $3100 pro Jahr bei $0,08 pro kWh. Unterdimensionierung führt zu Überlastbetrieb, führt zur Alterung von Isolatoren und verursacht Auslösungen bei Spitzenlast. Die richtige Größe wird durch die Laststudie definiert, nicht durch die runde Zahl vom Händler.
Entscheidung 2: Spannungsverhältnis und Abgriffe
Das Spannungsverhältnis muss an beiden Enden des Systems gleich sein: die Spannung vom Versorgungsunternehmen und die Spannung im Bus. Die häufigsten Paare sind unten aufgeführt:
| Einspeisespannung | Lastseitiger Bus | Typisches Gerät |
|---|---|---|
| 33 kV | 11 kV | 33/11 kV Verteilung |
| 35 kV | 10,5 kV | 35/10,5 kV Leistung |
| 22 kV | 11 kV | 22/11 kV Verteilung |
| 11 kV | 4 kV | 11/0,4 kV Pfosten- oder Bodenmontage |
| 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.
Entscheidung 3: Impedanz
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.
Entscheidung 4: Vektorguppe
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.
Entscheidung 5: Wahl des Abgriffwechslers
According to experts, tap changer is the sole moving part of transformer. The decision of selecting taps is related about reliability.
| Typ | Typischer Bereich | Operation | Cost impact | Wartung |
|---|---|---|---|---|
| Off-circuit (DETC) | ±2 × 2.5% | De-energized only | Basislinie | 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.
Entscheidung 6: Verlustklasse und Effizienz
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.
Entscheidung 7: Kühlmodus
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 | Base rating | 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 |
Entscheidung 8: Zubehör und Schutz
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.
Die Auswahlmethode in sechs Schritten
- 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.
Marken und Preisrichtlinien
| Marke | Herkunft | 000 kVA | 500 kVA | 10 MVA |
|---|---|---|---|---|
| Hitachi Energie | Japan/Weltweit | $22,000–$34,000 | $38,000–$55,000 | $55.000–$82.000 |
| ABB | Schweiz/Weltweit | $20,000–$31,000 | $35,000–$52,000 | $52.000–$78.000 |
| Siemens Energie | Deutschland/Weltweit | $21,000–$32,000 | $36,000–$53,000 | $50.000–$75.000 |
| Schneider Electric | Frankreich/Weltweit | $19,000–$29,000 | $33,000–$48,000 | $48.000–$72.000 |
| Jiangsu Subian Electric Power | China | $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.
Häufig gestellte Fragen
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.
Referenzen
- IEC 60076-1: Leistungstransformatoren – Allgemeines — Ratings, taps, impedance tolerance, and temperature-rise rules for the selection decisions in this guide.
- IEC 60076-5: Fähigkeit, Kurzschluss zu widerstehen — 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: Flüssigkeitsgetauchte Verteilungs- und Leistungstransformatoren — North American requirements counterpart for selection comparison.
- Offizielle Website von Jiangsu Subian Electric Power — Oil-immersed transformer range, efficiency grades, and certification documentation.
Fazit
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.