What Is a Transformer?
A transformer (trafo / transformatör) is a static electrical machine that transfers electrical energy from one voltage level to another by electromagnetic induction without changing the frequency. It contains a magnetic core with two windings: primary (input) and secondary (output). The high voltage from the grid (for example 34.5 kV) is applied to the transformer’s primary winding, a magnetic flux is created, and electricity is obtained at the desired voltage level on the secondary winding (typically 0.4 kV / 400 V). Transformers are indispensable equipment for delivering electricity from the distribution network to the end user, supplying industrial facilities, and power transmission. In Türkiye, establishing a transformer substation for medium-voltage users is legally mandatory with TEDAŞ/BEDAŞ approval.
Transformer Operating Principle
A transformer is based on Faraday’s law of electromagnetic induction. When alternating current is applied to the primary winding, a varying magnetic flux is formed in the core. This flux also passes through the secondary winding and induces an electromotive force (EMF) there.
Transformation ratio: N1/N2 = V1/V2 = I2/I1
- N1, N2: Number of primary and secondary turns
- V1, V2: Primary and secondary voltage
- I1, I2: Primary and secondary current
If N1 > N2, it is called a step-down transformer; if N1 < N2, it is called a step-up transformer.
Types of Transformers
1. Oil-Type (Oil-Cooled) Transformer
- Cooling: With mineral or vegetable oil (ONAN, ONAF, OFAF types)
- Advantage: More economical, long service life (35+ years), 50 kVA – 100 MVA range
- Disadvantage: Fire risk, oil spill risk, requires periodic oil analysis
- Application: Outdoor-type transformer substations, outdoor use, industrial facilities
2. Dry-Type Transformer
- Cooling: By air (AN, AF) or resin-coated winding (cast resin)
- Advantage: Fire-safe (F1 class), no oil leakage, indoor-friendly
- Disadvantage: More expensive, higher acoustic noise, narrower capacity range
- Application: Shopping malls, hospitals, hotels, office buildings, underground transformer substations
3. Hermetically Sealed Oil Transformer
The oil tank is sealed and contains no air cushion inside. Oil oxidation is prevented and maintenance is reduced.
4. Special Transformers
- Isolation transformer: Galvanic isolation of sensitive loads
- Autotransformer: Single winding, small voltage change
- Instrument transformer (CT, VT): Supply for meters and relays
- Grounding transformer (zigzag): Creating a neutral point
Selecting Transformer Capacity (kVA)
Transformer capacity is selected by considering the facility’s total installed power, simultaneity factor (0.6–0.8), and future load increase (1.3 safety factor).
| Facility Type | Typical Capacity |
|---|---|
| Small workshop / villa | 50–160 kVA |
| Apartment building / commercial building | 250–400 kVA |
| Small factory | 400–630 kVA |
| Medium-scale factory | 800–1250 kVA |
| Large factory / shopping mall | 1600–2500 kVA |
| Data center / industrial zone | Above 2x2500 kVA |
Selecting an excessively large transformer increases no-load losses; a small transformer overheats and shortens its service life.
Transformer Substation Installation Process
- Project design: Single-line diagram, protection coordination, grounding calculation drawing by an engineer authorized by EMO.
- TEDAŞ/BEDAŞ approval: The project is submitted for approval to the regional distribution company with the required documents.
- Transformer building / switchgear kiosk: Construction of a concrete transformer kiosk (compact) or an indoor panel room.
- Procurement of transformer, MV switchgear cell, and LV main switchboard: TEDAŞ-approved brands (ABB, BEST, ELİN, BALIKESİR ELEKTROMEKANİK).
- Installation and cable pulling: MV cable (CU 95–630 mm²), busbar, protection relays, grounding electrodes.
- Testing and commissioning: Insulation, polarity, turns ratio, short circuit, oil analysis, protection relay test.
- Temporary-final acceptance: Power is energized with the approval of the TEDAŞ technical team.
Typical duration: 8–16 weeks (design + procurement + installation + approval).
Periodic Transformer Maintenance
- Oil transformer: 1 oil sample analysis per year (DGA, BDV, acidity), 1 full overhaul every 5 years.
- Dry transformer: Thermographic inspection every 6 months, cleaning once a year.
- All transformers: Protection relay test once a year, grounding resistance measurement (≤1 Ω).
DOA Enerji Transformer and Transformer Substation Service
As a TEDAŞ/BEDAŞ-approved MV systems specialist, DOA Enerji provides oil-type and dry-type transformer supply, switchgear cell installation, turnkey transformer substation installation, and commissioning services. BEST, ELİN, ABB, Schneider transformers are delivered with original invoices and test reports. In case of emergency failures, 24/7 service support is available.
Frequently Asked Questions (FAQ)
Should I choose an oil-type or dry-type transformer?
In indoor locations (inside buildings, basements) and facilities with fire sensitivity (hospital, shopping mall, hotel), a dry-type transformer is mandatory. For outdoor transformer kiosks and industrial facilities, an oil-type transformer is more economical. Oil-type has ~40 years, dry-type has ~25 years service life.
How is transformer capacity calculated?
Total installed power (kW) × simultaneity (~0.7) ÷ power factor (cosφ 0.9) × safety (1.25) = transformer kVA value. Example: 500 kW × 0.7 ÷ 0.9 × 1.25 = ~486 kVA → 630 kVA is selected.
How long does transformer substation installation take?
Project design 2 weeks, TEDAŞ approval 4–8 weeks, equipment procurement 6–10 weeks, installation and commissioning 2–3 weeks. Total is around 12–20 weeks.
What are the most common causes of transformer failures?
Loss of oil insulation (moisture ingress), overloading, lightning surges, misadjusted protection relays, grounding faults, and winding short circuits. Periodic maintenance prevents 80% of failures.
Which transformer brands are TEDAŞ-approved?
In Türkiye, brands such as BEST Transformatör, ELİN Trafo, BALIKESİR ELEKTROMEKANİK, ABB, Schneider Electric Türkiye, Astor, ELG, ULAŞ TRAFO are on the TEDAŞ-approved list. Products outside this list cannot be connected to the grid.