When a magnetic core reaches its limit in carrying the magnetic flux, it is called the core saturation in a transformer. This occurs when the transformer core cannot carry the flux beyond its rated capacity. This article describes the causes of core saturation and its negative impacts on transformers.
What is Core Saturation?
As you may know, transformers rely on magnetic materials to function correctly. By using a magnetic core, these materials can carry more flux than the air core, allowing for the construction of smaller transformers. The primary winding in a transformer generates the flux when it receives an AC supply. This flux then flows through the magnetic core and links to the secondary winding, producing a voltage in the secondary.
The magnetic core plays a crucial role in the transformer by carrying the magnetic flux. This flux, which measures the strength of a magnetic field, is essential for the transformer functioning. The capacity of a magnetic core to carry magnetic flux is measured by its rated flux density.
The rated flux density indicates the maximum amount of magnetic flux a particular core can carry without becoming saturated. A higher-rated flux density means the magnetic core can carry a stronger magnetic flux. The core’s rated flux density is expressed in Tesla or Weber/m2.
The core of a transformer is made of materials that can carry magnetic flux up to a specific limit. These materials are called ferromagnetic materials. The materials fail to carry the excess flux if the magnetic flux exceeds this capacity. This is known as core saturation.
If we try to increase the magnetic flux in the core by increasing the ampere-turns in the primary winding, the core may not be able to carry the increased flux. It is important to note that ferromagnetic materials cannot carry magnetic flux beyond their rated capacity.
For instance, a transformer core is limited to carrying a magnetic field of 1.7 Tesla. If we generate a flux density of 1.9 Tesla in the transformer primary, the core cannot carry the increased flux, resulting in a saturated transformer.
Reasons for Core Saturation in Transformer?
The excessive voltage, lower frequency, and DC offset current are the reasons for core saturation. These factors will be discussed in more detail in subsequent sections.
In electrical transformers, core saturation occurs when the magnetic flux in the core exceeds its capacity. To prevent this, it is important to consider the factors that influence the flux in the transformer. By understanding these factors, we can take measures to ensure the optimal performance of the transformer and prevent damage to the core due to excessive flux.
The transformer’s primary winding consists of coils with multiple turns. The image below shows the general arrangement of the winding and core.
When we apply AC voltage to the primary, the flux is produced in the primary and travels through the magnetic core.
The abovementioned equation indicates that the magnetic flux in the transformer core is directly proportional to the applied voltage and inversely proportional to the frequency. This means that with an increase in voltage, flux increases, and with a decrease in frequency, the flux decreases.
1. Increased Voltage above the Rated Voltage
When voltage is applied to the primary winding of a transformer, it creates a magnetic flux in the transformer’s core. The greater the voltage, the higher the flux becomes. However, if the voltage increases beyond the rated value while the frequency remains constant, the core flux will exceed the rated value.
This can lead to core saturation during the peak moments of the AC waveform, which can cause damage to the transformer. For instance, if a transformer rated at 400 volts and 60 Hz is operated at 440 volts and 60 Hz, the core flux will increase by around 10%. The image below illustrates the impact of increased voltage on the core flux.
Transformer core saturation can distort the flux, leading to a non-sinusoidal voltage in the secondary winding. This non-sinusoidal voltage generates harmonics that can adversely affect the electrical network.
Read More: Effects of Harmonics on Transformer
2. Lower Frequency
As per the above equation of flux in the transformer, it’s apparent that the flux in a transformer core is inversely proportional to the frequency. In simpler terms, when the frequency decreases below the rated value, the flux in the core surges up. For instance, operating a 60 Hz rated transformer at 50 Hz with the same voltage rating will make the core saturated due to the increased flux.
Look at the figure below, which clearly demonstrates how the flux increases with a decrease in frequency.
3. DC Offset Current
During an electrical fault, the current flowing through the network has a symmetrical AC component, and a DC offset current. The core flux in a fault condition is the sum of the magnetic flux produced by the AC and DC components. However, during normal conditions, only the AC component of the fault current generates the magnetic flux. It is worth noting that the DC offset current is zero under normal working conditions.
Under fault conditions, the DC offset current generates an extra flux that can push the core flux waveform closer to the saturation point. This is important to remember as it can impact the overall performance of the transformer.
Effects of Transformer Core Saturation
When the transformer experiences core saturation, it results in an increased level of thermal stress on the transformer. This is because the excitation current increases exponentially when the voltage exceeds 110% of the nominal voltage. The increased excitation current leads to the overheating of the core, which can eventually cause permanent damage to the transformer. Therefore, monitoring the voltage levels constantly is crucial to ensure that the transformer stays within the safe operating range.
When the amount of flux passing through the core exceeds its limit, the excess flux seeps into the other structural parts and causes a phenomenon known as eddy current heating. This heating occurs due to the resistance these parts offer to the flow of the leaking current, generating heat as a result.
When a transformer becomes saturated, it may produce harmonics in the electrical system that can cause increased copper loss in the electrical network. Specifically, a saturated transformer can produce 5th-order harmonics, leading to the differential protection relay operating and causing nuisance tripping. To address this issue, the differential relay contains a 5th harmonic restraint function that prevents the transformer from such nuisance tripping. This function helps ensure the electrical system’s reliable and efficient operation.
Conclusion
Maintaining the rated volts to Hertz ratio (V/f) is crucial in preventing core saturation in transformers. Any deviation from this rated V/f ratio leads to excessive heating and distortion of the sinusoidal voltage. Therefore, adhering to the rated V/f ratio is imperative to avoid such issues.