I'm not sure of what it is you're trying to accomplish. Are you trying to prevent the main from tripping? If you are, you're on the wrong path.
Try disconnecting / turning off ALL of the secondary loads while the transformer is unpowered. Now energize the transformer - with NO load on it. You'll probably hear a big THUNK the moment you return power to the transformer, and you might trip the main anyway.
That 'thunk' is caused by the current inrush. Only after all the magnetic fields have stabilized does the current drawn by the transformer level off. This might take but a second or two, but that could well be enough to trip the breaker.
Your solutions are to use a bigger breaker, a breaker with a different trip curve, or change the 'instant trip' setting of the breaker (all depending on code rules and your specific equipment).
Apart from aging equipment, it's also likely that the transformer is fully loaded now - maybe even undersized. Those are other issues to examine.