Dry Type Transformer

Dry type transformers are constructed using a variety of materials, each chosen for its electrical, mechanical, and thermal properties to ensure optimal performance and reliability. The primary materials used in dry type transformers include:

Steel laminations: The core of the transformer is typically made from silicon steel laminations. These are stacked to form a continuous path for the magnetic flux. The use of silicon steel reduces the hysteresis loss and provides better magnetic properties. Laminations are thin to minimize eddy current losses, which occur when alternating magnetic fields induce currents within the core material.

Winding material: The conductors used for the primary and secondary windings are usually made from copper or aluminum due to their excellent conductivity. Copper is more conductive but also more expensive than aluminum. Aluminum is lighter and cost-effective but has higher resistive losses.

Insulation material: Insulation is paramount in dry type transformers to prevent short circuits and ensure electrical isolation. Materials such as Nomex (a type of aramid paper), Mylar (biaxially oriented polyethylene terephthalate), and various types of polyester films are used for insulation between turns, between layers, and between the windings and the core. Epoxy resin is also commonly used for VPI (vacuum pressure impregnation) or encapsulation processes to provide a solid insulating matrix.

Breather and moisture absorber: In some designs, silica gel is used as a desiccant to absorb moisture that might enter the transformer through air movement. This helps to protect the internal components from humidity and moisture.

Cooling elements: Depending on the cooling method, either passive radiators or active cooling fans may be used. The radiators are usually made of aluminum or copper for efficient heat dissipation. Fans, if used, are typically made from plastics or metal alloys.

Structural materials: The case, tank, or frame that houses the transformer's core and windings is generally made from sheet metal, often galvanized steel or aluminum, to protect the transformer from environmental factors and to provide structural integrity.

Terminal hardware: The terminals used for connecting the transformer to the power grid are usually made from metals such as copper or brass for good electrical conductivity.

Overcurrent protection devices: These can include circuit breakers or fuses made from materials that melt or trip at predetermined current levels, such as tin-plated copper for the fuse element or bimetallic strips for thermal overload protection.

The design of a dry-type transformer depends on several factors that affect its performance, efficiency, and durability. Some of the important factors to consider when designing a dry-type transformer are:

Choice of insulation type: The insulation type determines the temperature rating, dielectric strength, mechanical strength, and thermal shock resistance of the transformer. Generally, F and H-class insulation materials are used for dry-type transformers because they can withstand high temperatures (up to 155°C and 180°C, respectively) and have good electrical and mechanical properties. Common insulation materials include varnish, epoxy resin, polyester resin, etc.

Selection of winding material: The winding material determines the conductivity, resistance, loss, and mechanical strength of the transformer. Generally, copper and aluminum are used as winding materials for dry-type transformers because they have high conductivity and low cost. Copper has better conductivity and mechanical strength than aluminum, but it is more expensive and heavier. For the same current rating, copper requires less cross-section area than aluminum.

Selection of core material with low hysteresis loss: The core material determines the magnetic flux density, permeability, hysteresis loss, and eddy current loss of the transformer. The core material should have high permeability and low hysteresis loss to reduce the no-load loss and improve the efficiency of the transformer. Common core materials include silicon steel, cold rolled grain oriented steel (CRGO), amorphous metal, etc.

Regulation: The regulation of a transformer is the ratio of the voltage drop at full load to the no-load voltage. The regulation indicates the ability of the transformer to maintain a constant output voltage under varying load conditions. The regulation depends on the impedance and resistance of the transformer. A low impedance and resistance result in low regulation and better voltage regulation. The leakage reactance of a dry-type transformer should be kept within 2% during design to achieve low regulation.

Life expectancy: The life expectancy of a transformer is the expected time that the transformer can operate without failure or degradation. The life expectancy depends on the breakdown of the winding insulation due to temperature rise, moisture, dust, corrosion, or other factors. The insulation class and quality of the dry-type transformer should be chosen to withstand high temperatures and harsh environments without degrading. The temperature rise of the transformer should not exceed the limit specified by the insulation class.

Losses: The losses of a transformer are the difference between the input power and the output power. The losses consist of no-load losses and load losses. The no-load losses are independent of the load and include core loss and eddy current loss. The load losses are proportional to the load and include copper loss and stray loss. The losses affect the efficiency, heating, and cooling of the transformer. The core material, winding material, insulation material, and design parameters should be selected to minimize the losses and maximize the efficiency of the dry-type transformer.

Overloading: The overloading of a transformer is the condition when the transformer operates beyond its rated capacity or temperature limit. The overloading causes overheating, insulation breakdown, short circuits, or fire in the transformer. The overloading can be caused by excessive load demand, harmonics, faults, or ambient temperature. The dry-type transformer should be designed with sufficient margin to handle overloads without damaging its components or performance. The dry-type transformer should also be equipped with a fan-cooling system or an air-conditioning system to dissipate the heat generated by overloads.

K-factor: The K-factor is a measure of the ability of a transformer to withstand the heat generated by non-sinusoidal currents in its windings. Non-sinusoidal currents are caused by various electronic devices that produce harmonics in the voltage and current waveforms. Harmonics increase the losses, heating, and distortion of the transformer. A high K-factor indicates that the transformer can handle higher levels of harmonics without overheating or degrading. The dry-type transformer should be designed with a high K-factor to provide long-lasting life and reliable performance in applications that involve non-sinusoidal currents.