When is transformer pre-magnetization recommended?
Some medium voltage drive systems feature a pre-magnetization unit. In certain VFDs the pre-magnetization might be a standard. Other VFDs offer such functionality as an option. And most VFDs probably do not have the pre-magnetization at all. No wonder that some end users may be confused. This short post aims to answer the question when is the transformer pre-magnetization recommended.
Transformer inrush current
When a transformer is directly connected to the main supply, the energization is accompanied by an inrush current. The phenomenon is linked with increased flux in the transformer and subsequent core saturation. That results in transient waveform of magnetization current characterized by a harmonic signal of power frequency with high initial peak and relatively fast decay. The magnitude of the first peak depends on the transformer design, actual grid voltage (higher grid voltages significantly increase the inrush current), grid strength and most importantly the instant (phase angle) of the grid voltage with respect to the remanent transformer voltage (worst case for closing the breaker close to a voltage zero crossing¹). It can reach 10-25 times the nominal current of the transformer. The calculation is performed for the worst case instant with respect to breaker closing.
The shape of an inrush current is an asymmetric waveform with dominant DC component and decaying pulses in just one polarity. It is characterized by the peak value and decay time constant (T50 .. time to reach 50% of peak value).
¹ In a three phase system one of the phases will be reasonably close to the zero crossing thus the worst case is probable to happen.
Consequences of inrush current
The inrush current brings few challenges:
- Transformer protection shall tolerate the inrush current – inrush is normally detected based on the current spectrum (rich in harmonics) and protection functions are blocked for a period of time
- Voltage drop (dip, sag) in the grid due to high transient current → impact on power quality and especially on neighboring consumers
- Thermal and mechanical stress for the transformer itself – thermal impact is less critical but mechanical stress can in long term affect the lifetime
Transformer pre-magnetization in variable speed drive systems
How do the drive systems deal with transformer inrush current? Some VFDs are able to pre-magnetize the transformer in order to to eliminate the inrush current phenomena. There are principally two concepts²:
1. Pre-magnetization using AFE
Active front end (AFE) type of VFDs principally have the possibility to magnetize the transformer from the rectifier and thus avoid the inrush current. Precondition is that the VFD has a precharging system (which most M VFDs have). The capacitor banks are pre-charged via low voltage auxiliary supply and dedicated charging step-up transformer. Afterwards the AFE can start to modulate and magnetize the input transformer. The AFE then synchronizes with the grid and closes the main circuit breaker. Such transition is smooth and the inrush current is eliminated. Moreover, this concept does not require any extra equipment.
2. Pre-magnetization using PreMag
The above concept obviously does not work for VFDs with diode rectifier. The solution here consists of a pre-magnetization system³ that magnetizes the transformer from the converter side winding. It is again powered from the auxiliary supply and uses a step-up transformer (e.g. 400 V/2’000 V if the secondary side of the main transformer is 2’000 V). When using the pre-magnetization system it must be ensured that the winding connection of the pre-mag transformer is selected properly. Dimensioning of the pre-magnetization unit depends on the magnetizing current of the main transformer. Typical PreMag system has a rating of tens of kVA. Naturally, the equipment utilizes a duty cycle as it normally operates just for few seconds. The remaining inrush current is mostly less than 1% of the original value without pre-magnetization.
AFE drives that for some reason cannot magnetize the main transformer by the rectifier can also use the concept of al pre-magnetization unit.
² Other methods to minimize the inrush current exist. One of them is e.g. synchronized switching. However, it results in a very expensive circuit breaker and the solution is not common at medium voltage.
³ Terms such as Pre-magnetization unit (PMU), PreMag or similar are used
When is transformer pre-magnetization recommended?
Most distribution and power transformers can deal with the inrush current. Similarly, many VFD input isolation transformers can work fine without any need of pre-magnetization. The decision whether a pre-magnetization is needed or not depends on several factors. One part is the impact on the transformer itself while the other part considers the whole system and its performance.
a) Weak grid connection
The weaker the grid the larger the voltage drop caused by the transformer inrush current. Inside the plant there can be large quantity on loads. Some of them might be sensitive to such voltage dip. The voltage sag can disturb the process or even cause a trip of certain consumers. In case of a utility grid the owner may restrict the maximum allowable transient current.
b) Exceptionally high inrush current
If the transformer design leads to a very high inrush current then any type of pre-magnetization sounds like a good idea. The information about maximum inrush current can be obtained from the transformer manufacturer during design phase. The calculation requires a detailed knowledge about the arrangement of the windings or coils, respectively.
c) Frequent energization of transformer
Power transformers are on-line practically all the time. Even though the loading fluctuates the transformer stays energized even in periods of light loading. In VFD applications the situation can be different. In some applications the transformer will be energized few times per week. Drives in marine propulsion will be de-energized while the ship is docking in the port. The transformers will then be energized when it is time to depart. In other cases, e.g. pumped storage plants, the transformer may be energized several times per day. Under such conditions the pre-magnetization unit is highly recommended.
d) Use of vacuum circuit breakers
Nowadays, most breakers in medium voltage range are of vacuum type. They are known to generate dangerous transient overvoltages that can harm the insulation system of the transformer. Obviously, the higher the number of switching operations the more significant the impact on the transformer and its lifetime.
Summary on transformer pre-magnetization
The transformer pre-magnetization unit effectively minimizes the inrush current of the main transformer. It is often an “option” thus the questions is whether such unit shall be installed or not. The decision depends on multiple factors. In case of high inrush current per design, unfavorable system parameters or frequent energizing of the transformer the pre-magnetization unit is recommended.
References
[1] Inrush current – Wikipedia, https://en.wikipedia.org/wiki/Inrush_current
[2] Protection of VFD transformers, https://mb-drive-services.com/protection_of_vfd_transformers/
[3] Li-Cheng Wu, Chih-Wen Liu, Shih-En Chien, Ching-Shan Chen, The effect of inrush current on transformer protection, IEEE, 2006
2 Comments
Mohamed-Seif Awadallah · April 8, 2023 at 12:17 pm
Perfectly explained in an easy way. Well done!
admin · April 12, 2023 at 9:20 pm
Thank you! Glad to hear that. Perfect motivation to continue 🙂
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