How to Choose Between Dry-Type and Oil-Immersed Power Transformers?

Introduction

What are the differences between dry-type and oil-immersed power transformers? How to choose between them? This article elaborates on their core characteristics to help you make a suitable choice.

What are Dry-Type Transformers

A dry-type transformer refers to a transformer whose iron core and windings are not immersed in insulating oil. It has two cooling methods: Natural Air Cooling (AN) and Forced Air Cooling (PF). Structurally, it is divided into fixed insulation encapsulation (SCB type) and non-encapsulated winding structure. In terms of the relative position of high and low voltage windings, there are concentric type and overlapping type. The concentric type has a simple structure and is easy to manufacture, so most dry-type transformers adopt this structure; the overlapping type is mainly used for manufacturing special transformers.

Specifications of Dry-Type Transformers

Taking the SCB-11—1250KVA/10KV/0.4KV dry-type transformer as an example, the meaning of the model is explained as follows:

S: Three-phase power transformer

C: Transformer winding is a resin-cast solid (G at the position of C indicates that the external insulation medium of the winding is air; B indicates foil winding; R at the position of B indicates wound winding)

11: Series number

1250KVA: Rated capacity of the transformer

10KV: Rated voltage of the primary side of the transformer

0.4KV: Rated voltage of the secondary side of the transformer

Technical Parameters of Dry-Type Transformers

① Frequency: 50Hz

② No-load current: Required to be less than 4%

③ Low-voltage withstand strength: No breakdown at 2KV/min

④ Insulation resistance (low-voltage side): Not less than 2MΩ

⑤ Winding connection method: Y/yn0 and D/yn0

⑥ Allowable winding temperature rise: 100K

⑦ Heat dissipation method: Natural air cooling or temperature-controlled air cooling

⑧ Noise coefficient: Less than 30dB

Loss Parameters of Dry-Type Transformers (SCB Type) (Table 1)

Rated Capacity (KVA)	160	250	400	500	630	800	1000	1250
No-load Loss (W)	550	720	980	1160	1380	1380	1770	2090
Load Loss (W)	2130	2760	3990	4800	5960	6960	8130	9690

Operating Environment Requirements for Dry-Type Transformers

① Ambient temperature: -10°C to 45°C

② Relative air humidity: Daily average not greater than 95%, monthly average not greater than 90%

③ Altitude: Below 1600 meters (at rated capacity)

Advantages and Disadvantages of Dry-Type and Oil-Immersed Transformers

Aspect Dry-Type Transformers Oil-Immersed Transformers

Cost More expensive than oil-immersed transformers More economical

Capacity Smaller maximum capacity Larger maximum capacity

Application Scenarios Underground floors, floors, crowded spaces, people-intensive areas, box-type substations Independent substations, large spaces

Climate Adaptability Suitable for fire and explosion-proof occasions Suitable for humid areas

Load Capacity Poor; should operate at rated capacity Strong; allows short-term overload

Differences Between SCB Type and SGB Type Dry-Type Transformers

Regarding coil winding: SCB type dry-type transformers use foil winding for the low-voltage coil.

Winding structure: Single-layer winding with copper foil, the inter-layer material contains epoxy resin with latent curing agent and a lower-grade composite foil.

Winding material: High-conductivity oxygen-free copper is used, with a copper content of 99.99%. SGB type dry-type transformers use wire winding for the low-voltage coil.

Winding structure: Cylindrical coil, multiple strands of ordinary glass fiber-covered flat copper wire.

The short-circuit resistance of SGB type dry-type transformers is stronger than that of SCB type dry-type transformers.

Heat dissipation: SCB type dry-type transformers have better heat dissipation than SGB type transformers.

Load loss: SCB type dry-type transformers have lower load loss than SGB type dry-type transformers.

In terms of temperature rise, SCB type transformers have better heat dissipation than SGB type transformers.

Price Comparison of SGB, SCB, and S13 Type Power Transformers

Taking the 1250KVA power transformer as an example, quotations from the same type of manufacturers found online are compared as follows:

Model Price (CNY/Unit)

SGB11—1250KVA/10KV/0.4KV CNY 93,800

SCB11—1250KVA/10KV/0.4KV CNY 95,600

S13—1250KVA/10KV/0.4KV CNY 59,600

It can be seen that the prices of SCB type and SGB type are not much different, and the price of dry-type transformers is about 1.5 times that of oil-immersed transformers.

Loss Parameters of S13 Type Oil-Immersed Power Transformers (Table 2)

Rated Capacity (KVA)	160	250	400	500	630	800	1000	1250
No-load Loss (W)	500	290	410	480	570	700	830	970
Load Loss (W)	2130	3200	4520	5410	5410	7500	10300	12000

From Tables 1 and 2, it can be seen that dry-type transformers have higher no-load loss but lower load loss than oil-immersed transformers.

Guidelines for Selecting Power Transformers

When selecting a power transformer, refer to GB/T17468—2008 "Guidelines for the Selection of Power Transformers" and GB4208—2008 "Degrees of Protection Provided by Enclosures (IP Code)" to choose a transformer suitable for the site and environmental requirements.

General principles for selecting power transformers:

Based on the overall reliability of the transformer.

Comprehensively consider the advancement, rationality, and economy of technical parameters.

Combine the operation mode to propose technical and economic indicators.

Consider the possible impacts on system safe operation, environmental protection, material saving, transportation, and installation space.

Case Study of Power Transformer Selection

A newly built cold-rolled steel mill on an industrial platform originally designed to use three SCB type dry-type transformers (2 units of 1250KVA and 1 unit of 400KVA). The person in charge of the owner consulted the author about the price of these three transformers. Based on the site, environment, and load conditions of the factory, the author suggested that the cold-rolled steel mill switch to S13--M type sealed oil-immersed power transformers. The factory accepted the author's suggestion, which not only saved a lot of valuable funds but also met the production requirements economically and with high performance.

Oil-immersed power transformers have mature technology, natural air cooling, stable quality, strong short-circuit resistance, suitability for high humidity and high temperature environments, strong overload capacity, long service life, and their price is about two-thirds of that of dry-type transformers. Therefore, unless there are fire protection requirements that mandate the use of dry-type transformers, sealed oil-immersed power transformers should be preferred.