The report is starting with a short introduction of the Danish power supply system. In this part is has been decided to focus on the 150/60 kV station In Loldrup and especially have focus on transformer protection.
After this it is necessary to have a general knowledge about the station in Loldrup and all its components. Even more a better explanation about transformer theory is necessary to be managed. All this is done in Chapter 2.
When this knowledge is reached it is now possible to make a problem statement, which is sound like this; How can a 150/60 kV transformer like this in Loldrup be relay protected against high inrush current and short circuit current?
To solve this problem statement it is necessary to know more about sequence networks and short circuit in electrical power systems. This is done in Chapter 3 where also a knowledge about fault currents is represented.
To do some protection of a transformer it is also important to know something about different kind and relays and how they act. It is done in Chapter 4. It is realized that the best choice of relays is a differential relay, which is being more described in this Chapter. Even more is a theory about inrush current and short circuit current made in the same Chapter. It is realized that inrush current including a big part of second order harmonics. Even more is realized that the relay should be able to handle external short circuit current without tripping. But if the short circuit current is inside the transformer the relay has to give a trip.
To improve the theory there is made some models about switching transformers to the network in the computer program PSCAD. There is made three models which are; switching one transformer to the network, switching one transformer to the network parallel with another transformer, and short circuit on secondary side of the transformer. In all three models the results are agree with the theory.
From these models it is now possible to make some practical test in the laboratory to see if the differential relay is tripping or not with the signals from the models. Unfortunately the differential relay is being defect when doing the set up. That is why the test is only made hypothetical.

The conclusion of this report is that is possible to protect a 150/60 kV transformer with a differential relay, and sort out inrush current and external short circuit current, so the relay does not give a trip signal.