Inrush current is the high current that a transformer draws when it is first energized. This current is caused by the sudden change in magnetic flux in the transformer's core, and is proportional to the current flowing through the primary winding of the transformer.
When a transformer is first energized, there is no magnetic flux in the core, so a large current must flow through the primary winding to create the initial magnetic flux. This current can be several times the transformer's rated current, which means special precautions must be taken to manage it.
You can think of the transformer’s core like a large highway with many lanes for travel. Instead of cars filling the lanes, you have individual lines of a magnetic field, or polarized lines of flux moving through. Before voltage can be effectively induced in the secondary coil, these lines of flux must be present. When a transformer is first powered up, it has to do the initial work of filling up the empty sections of the core with these lines of magnetic flux. This initial work of filling up the core with magnetic flux is what we call inrush.
There is a certain amount of exciting current required to establish effective mutual induction between the primary and secondary coils.
The quality of the core and assembly and its construction influence how much excitation is needed during energization or startup. Imagine trying to roll a car with a dead battery down the street to a nearby parking lot. You would have to do some amount of work to get the car going, which would require a bit more heaving and grunting in the beginning; this extra heaving and grunting would merely be spent in getting the object out of its stationary state. In the same way, a poorly built (or damaged) core assembly will require more “heaving” and “grunting” when the transformer is energized. The additional work required at startup is what we refer to as inrush current. The excitation current and the associated no-load loss are the power that keeps the core energized during normal operation. The quality, orientation, and construction of the laminated core steel in a transformer will determine the exciting current.
Large transformers and reverse fed transformers are especially susceptible to large amounts of inrush current.
The larger the transformer, the higher the inrush current.
Inrush current in a transformer only lasts for a short period of time (less than a second), but the magnitude of the inrush current ultimately depends on the size and type of transformer. In general, the larger the transformer, the higher the inrush. Just as a 5-gallon bucket will require more time and energy to fill up than an 8 oz glass, a larger transformer will require more exciting current than a smaller one.
Inrush current is greater when reverse-feeding a transformer. Why is this the case? Since the secondary windings of a step-down transformer typically have lower impedance (being physically closer to the core itself), the expected inrush current can easily be doubled or tripled when reverse-feeding the transformer.
Inrush current calculations depend on several factors such as the saturation characteristics of the transformer core, the waveform of the supply voltage, and the specific transformer design.
Coordination software such as SKM will typically plot standard inrush curve values based on given transformer ratings.
To simplify inrush current calculations for distribution transformers, the following maximum values can be used as a function of the time of energization. These values are used for coordinating primary fusing for transformers, and they are generally accepted across the industry.
These limits are accepted in practice like maximum current values.
|Times In||Time (sec)|
Here is an example of the maximum inrush current values plotted out for a 300 kVA padmount transformer using the table above.
“All four points on a straight line TCC (time-current curve)”
Inrush is a normal occurrence when you first energize a transformer. However, in extreme cases, inrush can cause protection devices to trip, or even overload the power system or connected equipment.
If the inrush current is excessively high, it may cause circuit breakers or the transformer’s fuses to trip. These protective devices are designed to detect and respond to abnormal current levels to protect the system from damage. In cases of severe inrush current, false tripping of protective devices may occur, resulting in unnecessary power interruptions.
A fuse that accommodates the transformer’s initial inrush current should be selected. This is accomplished by selecting a fuse with a TCC (time current characteristic) curve which sits to the right of the transformer’s inrush curve.
Overcurrent protection devices must be coordinated to allow for transformer inrush current. If inrush current is not taken into account when sizing overcurrent protection for a transformer, nuisance tripping may result (breakers or fuses operating as a result of a transformer’s exciting current upon energization). Tables 450.3(A) and 450.3(B) of the NEC should be consulted when coordinating overcurrent protection for a transformer. If a particular application of a transformer (such as reverse feeding) yields an inrush current that requires a rating that would not allow these rules to be observed, a different transformer design may be required.
The initial surge of inrush current, which can be several times higher than the transformer's rated current, can overload the power sources (such as generators or other transformers) supplying power to the system. This can result in voltage instability, increased losses, and decreased efficiency of the power sources.
Though the best practices will vary somewhat depending on the transformer you are using, there are several steps that can be taken to manage inrush current and mitigate its effects. Here are some common practices:
If your project requires a step up application where the applied voltage is lower than the induced voltage, you may need a step-up transformer. Step up transformers are designed to reduce inrush current in the case where a typical reverse fed transformer would be used. As mentioned above. Reverse feeding a transformer will typically increase the initial inrush current several times the normal amount.
Soft-start mechanisms gradually apply voltage and/or current to inductive loads such as motors or transformers at the time of initial startup. With a soft-start, a transformer has time to build up the magnetic flux in the core gradually to reduce the sudden surge of starting current. Soft-start mechanisms utilize specialized control circuits and devices that gradually ramp up the supply voltage over a specific time period. Unlike a variable frequency drive (VFD) which controls the speed of a motor load continuously, a soft-start is designed to operate only at the point of startup. Once the reduction of larging starting currents is complete, a soft-start mechanism will be disengaged from the load.
Transformers can be designed with multiple winding taps on the primary side. By connecting the transformer to a tap with a higher voltage ratio during startup, the flux in the core can be built up more gradually, reducing the inrush current.
Transformer cores can be designed with low remanence and low saturation characteristics. Core materials with lower saturation levels can help reduce the magnitude of inrush current and minimize core saturation effects during startup.
Pre-magnetizing the transformer core by applying a small DC current before energizing it with AC can help reduce inrush current. This technique ensures that the core is at a certain magnetic flux level prior to energization, reducing the need for a large surge of current during startup.
Careful system planning is important to ensure that the transformer is adequately sized for the load and operating conditions. Oversizing a transformer can lead to higher inrush currents, since it takes a larger initial surge of current to energize the unit. By selecting a transformer that correctly matches the load requirements, the inrush current can be minimized.
Always consult with a qualified electrician to determine the best way of managing the inrush on your transformer—the best process will vary from project to project. If you’re currently looking for a distribution transformer for your project, Maddox stocks thousands of new and reconditioned transformers. We’re ready to help you get the power on for your project. Contact us today to receive a custom quote for your project!
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