Transformer protection
Transformer protection should take into account the power, voltage, vector group and the importance of the unit within a particular system. Depending on these factors, the transformers can be assigned to one of the two general groups.
HV/MV/LV Transformer Protection SchemesLet’s see these two groups in details:
MV/LV transformers
The protection of these units should include overcurrent protection for both the MV and LV windings, plus devices such as overpressure protection (e.g. Buchholz surge), and thermal protection, as indicated in Figure 1 below. Typical ratios for inter- busbar MV/LV transformers at substations are 33/11, 34.5/13.2 and 13.2/4.16 kV.
Go back to Transformer Groups ↑
HV/MV/LV transformers and autotransformers
In addition to the protection listed for the MV/LV transformers, the protection for transformers in this group should include overall differential protection, which is essential because of its reliability and high speed of operation. In this case, shown in Figure 2, since the transformer has three windings, a three terminal type of differential protection is required.
The diagram also includes the differential busbar protection, which is usually installed on large transformers connected to HV busbars. As the transformer has an LV winding, overcurrent protection for this winding has been included as well.
Typical ratios for HV/MV transformers are: 132/33, 145/ 11, 132/11, 115/34.5 and 115/13.2 kV.
It is common to use autotransformers where large powers are involved and the voltage ratio is around 2:1. Typical ratios for autotransformers are 275/132 kV and 230/115 kV.
The protection for this type of transformer is essentially the same as that quoted in the previous paragraph, taking into account the modifications to the overcurrent relay connections (see Figure 2).
Go back to Transformer Groups ↑
Reference // Protection of Electricity Distribution Networks – Juan M. Gers and Edward J. Holmes
Copyright Notice
This technical article is protected by U.S. and international copyright laws. Reproduction and distribution of PDF version of this technical article to websites such as Linkedin, Scribd, Facebook and others without written permission of the sponsor is illegal and strictly prohibited.© EEP-Electrical Engineering Portal.
Related electrical guides & articles
Edvard Csanyi
Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.
Profile: Edvard Csanyi
Thank you,I am electrical and computer engineer still I don’t have job I don’t still applied what I am studying so help me.
You have helped a lot of electrical engineer congratulations
Thank you so much sir for your website. It helps me a lot. I will suggest this site to my college
Thanks Edvard, your information which gave to us is very useful, I am very appreciated.
I want to ask a few questions about capacitor bank, what is different between parallel capacitor bank and series I need just aa brief answer. Thanks
Please let me know the difference between REF and SEF protection scheme of Transformers
your article is very good and very useful for me.
Please specify what the number means in the drawin4
please help me about document of HV/MV/LV Transformer Protection Schemes
hi your article is very good and very useful for me and other members