Implement a transient pulse on top of another signal in Simulink

Hello!

I have a circuit which consists of a sinusoidal source, a transformer, a load, some other impedances and so on. Now I want to implement a transient pulse representing a switching overvoltage but i can't get it working. I try to connect a "Pulse generator" into the circuit between the transformer and the load. The output of the "Pulse generator" is 0 for a while, then it goes straight up to a certain value and then back to 0. The problem seems to be that when the output is 0 the "Pulse generator" acts like some kind of ground so the total output of the two signals becomes really strange. I want the transient to just affect the circuit when the pulse is "on". I have tried some different things like the "merge function" but I have not found a working solution.

Best regards