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Introduction of Thermal Loading and Cooling Types of Transformers

Introduction of Thermal Loading and Cooling Types of Transformers
Transformer overloads can occur during contingency conditions that are the product of one, two, or various system elements being isolated from the power the system. They can also realize when transformers are already at 80%-90% of their full nameplate rating and extra capacity is needed, especially during hot summers. Depending on a utility’s criteria, transformers may be allowed to be overloaded, while still maintaining transformer integrity, to keep continuity of the load for economical or reliability reasons. The no-load and load-losses created by the transformer core and windings will cause high temperatures that, if not controlled in a timely manner, can damage the dielectric properties of the insulation. During normal operating conditions, the temperature thermal process is controlled by the cooling system that keeps the transformer in a thermal stabilization. Transformer manufacturers guarantee the longetivity of their product as long as it is operated under the temperature specifications of IEEE or IEC standards. What if transformers need to be loaded beyond normal conditions?
The IEEE Guide of Loading of Mineral Oil-Immerse Transformers C57.91-1995 aids us in calculating the effect of aging insulation and its exposure to high temperatures. The guide also leads us through the calculation of the winding hottest-spot temperature, which is the driving factor for limiting temperature overloads. Knowing how to calculate the loss of life and the winding hottest-spot temperature is the foundation for the development of dynamic ratings for power transformers. Such ratings can be used by system operators during contingency conditions, which will allow them to overload transformers for a predetermined time.
We must understand the causes for heat, the normal operation limitations, the insulation’s loss of life and the hottest-spot temperature; we must also develop philosophies and criteria for transformer dynamics ratings.
What Affects Thermal Loading
A transformer is a voltage changing device composed of a primary and secondary winding connected by a magnetic core. A three phase power transformer used in transmission and distribution systems shares the same principle. However, its core structure is bigger to accommodate the three phase primary and secondary windings. Additionally, insulation in the form of oil or paper is required to isolate the difference in potential between phases. Three phase transformer losses will generate enough heat so that external cooling systems must be added. A closer look at these characteristics is necessary to better understand the thermal aspects of power transformers.
The Core and Windings
When one thinks of a transformer core, one usually visualizes it as a piece of solid metal. On the contrary, the core is built up by horizontally or vertically stacking thin iron laminations or sheets, which eventually form the core’s leg and yoke, as seen in Figure 1.The primary function of the transformer core is to provide a low reluctance path for the flux that links the primary and secondary windings. Ideally, we would like a zero reluctance flux path between the two windings. However, due to the iron laminations that form the core, the transformer core experiences losses that eventually produce heat. These core losses can be classified as hysteresis and eddy current losses.
Figure 1. Three Phase Transformer Core.

Figure 1. Three Phase Transformer Core.

Insulation
In an overhead three phase transmission line with bare conductors, no insulation is necessary between the conductors since air separation is used as an insulator, preventing the flow of current. However, in power transformers, distance between phase conductors is not an efficient way of separating the potential differences. As a result, paper is used as an insulator, allowing closer proximity between phases and thus maximizing space. By far, paper is the best insulating material used today because of its high dielectric strength properties. Paper insulation in a power transformer is installed between windings of the same phase, windings to ground, and windings from different phases. Other parts of the transformer also experience a difference in potential, such as the transformer tank wall with the windings, which also requires some form of insulation. Transformer manufacturers shorten the necessary distance by using insulating oil, which not only insulates, but also serves as a coolant within the transformer. Thus, transformer insulation is the heart of transformer design, and maximum transformer performance during loading depends on the insulation’s credibility. Transformer losses are one of the primary factors affecting this credibility, which is the focus of the next section.
Transformer Losses
Transformers are stable electrical machines therefore their efficiencies are clearly high when the comporasion with motors, generators. The losses in a power transformer can be classified as no-load losses and load losses.
No-Load Losses. With no load in the secondary windings, an energized transformer roles as a highly inductive element, similar to a shunt reactor. In order to keep this transformer energized, the alternating excitation current is drawn from the system, producing an alternating mutual flux in the primary winding. This mutual flux is taken by the core at a rate that depends on the system frequency. The energy requirements for this cyclic magnetization of the core results in two types of transformer losses: eddy and hysteresis losses. Induced voltage in the laminations produced by the alternating flux results in undesirable currents within the laminations. Such currents are called eddy currents, which do not contribute to power output, and their energy is lost to heat. The alternating magnetization of the core will cause the molecular composition of the iron core to align itself with the changing field. The energy lost from successive reversal of magnetization in the core is called hysteresis loss.
Load Losses. The load losses in a power transformer are due to the electric resistance of windings and stray losses. The resistive action of the winding conductor to the current flow will be lost in the form of heat and will be dissipated in the surrounding area inside the transformer. The magnitude of that loss increases by the square of the current . Stray losses occur due to the leakage field of winding and due to high currents seen in internal structural parts such as bus bars. Stray losses can affect the overall rating of the transformer because they can create hot spots when the current leads become excessive, affecting the overall life of the transformer.
Heat Transfer Effects
A load serving transformer not only experiences an electrical process but also goes through a thermal process that is driven by heat. The heat generated by the no-load and load losses is the main source of temperature rise in the transformer. However, the losses of the windings and stray losses seen from the structural parts are the main factors of heat generation within the transformer. The thermal energy produced by the windings is transferred to the winding insulation and consequently to the oil and transformer walls. This process will continue until an equilibrium state is reached when the heat generated by the windings equals the heat taken away by some form of coolant or cooling system. This heat transfer mechanism must not allow the core, windings, or any structural parts to reach critical temperatures that could possibly deteriorate the credibility of the winding insulation. The dielectric insulating properties of the insulation can be weakened if temperatures above the limiting values are permitted. As a result, the insulation ages more rapidly, reducing its normal life. According to the IEEE C57.91-1995 guide, the life of the insulation is the overall life of a transformer.
Due to the temperature requirements of the insulation, transformers utilize cooling systems to control the temperature rise. The best method of absorbing heat from the windings, core, and structural parts in larger power transformers is to use oil. As we will see in the next sections, the oil’s heat capacity and thermal conductivity affect the heat transfer process.
For smaller oil-field transformers, the tank surface is used to dissipate heat to the atmosphere. For larger transformers, heat exchangers, such as radiators, usually mounted beside the tank, are employed to cool the oil. The IEEE C57.12.00-2000 standard identifies the type of cooling system according to Table 1.
Cooling Class Definition
ONAN: Oil Natural-Air Natural
ONAF: Oil Natural-Air Force
OFAF: Oil Force-Air Force
ODAF: Oil Directed- Air Force
Table 1. Transformer Cooling Types.
 
ONAN refers to the dissipation of heat from the oil to the atmosphere. This is done by the natural circulation of the oil through the windings and cooling equipment, which is externally cooled by natural air. As the temperature of the oil rises due to the active parts of the transformer, the specific gravity of the oil decreases, causing the oil to travel upwards towards the inlet of the coolers. As the oil travels through the heat exchangers or coolers, its specific gravity increases, allowing it to flow downwards. See Figure (a).

(a) ONAN
The ONAF cooling designation maintains the natural circulation of oil through the windings and heat exchanger, except that air is now forced to the surface of the radiators. As load increases, the previous natural cooling process is no longer enough to dissipate the heat at a rate that can keep the temperature of the transformer in equilibrium. If fans are used to cool down the radiators, the heat transfer process will be increased, resulting in additional transformer capacity. A transformer rating of up to 133% of the base rating can be achieved by adding one stage of fans and up to 167% with two stages.

(b) OFAF
The OFAF cooling designation increases the heat transfer rate by forcing oil circulation with pumps. To achieve maximum heat dissipation under OFAF cooling, fans must continually blow air on the surface of the radiators . See Figure (b).

(c) ODAF
A better way to improve heat dissipation is to force oil through the winding as shown in Figure (c). When oil is forced to flow through the windings, it is indicated as Directed Flow and its designation is ODAF. When the oil is forced to flow freely inside the tank it is indicated to be Non-Directed flow. See Figure (b).
A transformer’s thermal process is controlled by keeping its temperature under the permitted values indicated by the manufacture. These limits are usually set by following IEEE or IEC industry standards for power transformers.
For the second part of this article, details of loading, calculation of aging of insulation, hottest-spot temperatures and formulas of temperature rises will be shared.
 
Source: Fundamental Principle of Transformer Thermal Loading and Protection Article.