Types of Losses in Transformers — Why They Happen & How to Reduce Them

We all know the significance of transformers in power systems. They can transmit electricity between different circuits, alter the voltage level, and allow electricity to be transmitted over long distances. But, just as with any other kind of electrical machine, these aren’t flawless and have their own share of energy losses.

To know about the different kinds of losses that occur in a transformer, understanding what causes these losses and how can we reduce them will help us to improve the performance and reliability of a transformer.

What Are Transformer Losses

Transformer losses refer to the energy that is lost in the form of heat, or some other forms rather than being transferred to the load. The losses take place in both the core and the windings. And they have effect on the transformer’s work and are important part for deciding how efficient is the transformer.

Transformer has many kinds of loss, from large losses such as copper loss and core loss to small ones such as dielectric loss and stray loss. It’s necessary to understand all of them when designing and operating.

Two Major Types of Transformer Losses

Generally speaking, transformer losses can be divided into two major categories of losses in a transformer:

No-load losses – this happens even if the transformer is powered up but isn’t giving out any load.
Load Losses – this happens when there’s current flowing through the windings to give out power.

They are often referred to as two kinds of energy loss in a transformer. Each one has their own subcategories and causes.

No-Load Losses

Core losses (iron losses) of transformers

Core loss, also known as iron loss, occurs within the transformer’s magnetic core. They mainly consist of hysteresis loss and eddy losses.

Hysteresis losses in transformers

The core of a transformer is like a big block of magnetic material. When electricity runs through the transformer’s coils, it creates a magnetic field that “magnetizes” this core.

We know that transformers only use alternating current (AC), which means the direction of the current changes for dozens of times per second. This forces the magnetic “tiny bits” inside the core to change their direction over and over again.

Every time these magnetic bits switch direction, they have to overcome a kind of internal resistance in the core material. That extra energy wasted is called hysteresis loss.

How to reduce it: Good quality core material, such as cold rolled grain oriented steel, can lower hysteresis loss.

Eddy current losses in transformers

Change of magnetic flux induces circulating currents within the conductive core, which are called eddy currents. And these currents make heat and waste energy.

How to reduce them: Laminated cores are used with insulating layers between the sheets to minimize eddy current losses.

The above two are called types of core losses in transformer, which account for most of the no-load loss of most power transformer.

Dielectric losses in transformers

The coils inside a transformer are always wrapped with insulating paper and paint, in case there will be electric leakage or short circuits. These materials are called insulating dielectrics, they’re like protective suits worn to keep electricity “in line”.

Transformers use alternating current, so the voltage goes up and down constantly. This fluctuation makes the molecules move inside the insulating material. The friction between these molecules generates heat. This heat is completely a waste of power since it doesn’t help transfer electricity. that’s what we call dielectric losses.

How to reduce it: Use the good insulating material and keep the oil or gas insulation system intact so that it won’t be affected by moisture or dirt.

Load Losses

Copper Losses in Transformers

Copper loss has several names. You can also called it I²R loss, ohmic loss, or winding loss. It happens due to the current which flows through the transformer windings. The copper wire resists the flow of electricity so some of the energy is changed to heat. Copper loss is proportional to square of the load current, it will be more under heavy load.
How to reduce it: Use conductors with a larger cross-sectional area. Use high purity copper or aluminum with a lower resistance. Tight connections also help avoid overheating.

Stray Losses in Transformers

There are coils of wire inside a transformer. When electricity runs through them, they create a magnetic field. Under ideal conditions, this magnetic field should only stay inside the transformer’s iron core, help with energy transmission.

But some of this magnetic field leaks out of the iron core and ends up hitting the transformer’s metal parts, like the oil tank, the metal clips and even the bolts.
When that stray magnetic field touches these metal parts, it creates tiny, useless currents inside them. This extra energy will turn into heat, and that’s where stray losses come from. It is the wasted energy that only heats up the transformer for nothing.

We can reduce it by using proper magnetic shielding and improving the transformer’s design to reduce stray flux and minimize such losses.

How to Reduce Transformer Losses

– Choose high efficiency transformer that uses low loss core material.
– Make sure it is properly maintained, check if there is any overheating, loose connections, and oil quality deterioration.
– Avoid overloading because it raises copper losses and temperature.
– Temperature needs to be checked and shouldn’t get too hot. Temperature should always be within the designed range.
– Do routine checks and watch carefully for any rise in losses.

How to measure the efficiency of transformers?

Transformer efficiency refers to the degree of effectiveness with which a transformer can convert input power into useful output power. It is expressed in this way:
Efficiency(%) = Output Power / Input Power × 100
Since these Transformers are made for better performance, efficiency is usually between 95-99%, depending on the size/design and loading.

Two common methods to find out how efficient something is

Direct Method

Input and output powers are measured directly with wattmeters when loaded. This is simple but its not really accurate for big transformers as the losses are much less than the total power.

Indirect Method (Loss Method)

Measure the core loss (no-load loss) and copper loss (load loss) separately, and then calculate:
Efficiency = Output Power / (Output Power + Total Losses)
It’s a popular way to test transformers and it can provide better results.

Tip:
The maximum efficiency comes in when the core loss=copper loss. Maintain balanced loading condition and proper cooling so as to operate transformer near this optimum point.

Conclusion

Transformer losses will always happen, but knowing why they occur helps us reduce them better. With the help of good quality material, proper working condition, efficient cooling and monitoring system, we can make our transformer perform well and save much energy.

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