When operating a machine at higher output, the input power is obviously correspondingly increased. This gives rise to increased losses due to the resistance of the electricity supply. Mains line resistance is in fact the sum total of the resistance of all cables, terminals
etc. back to the power station. In normal domestic situations it may be ignored, but for x-ray units it becomes significant.
For instance, if the total mains resistance was just one ohm, the voltage drop across it would obviously become (that is, according to Ohm’s Law):-
For 1 Amp : 1 Volt
For 100 Amps : 100 Volts
For 250 Amps : 250 Volts
The mains resistance is normally just a fraction of an ohm, but when drawing heavy currents even that seemingly small value gives rise to significant voltage drops. And, of course, the higher the value of mains resistance (due, perhaps, to a poor quality mains supply), the more affect it will have on x-ray output.
How big is your unit (in kilowatts?) ... OK, so (as the Americans say) - "you do the math"! That is, how much current does that 230 volts supply get asked to cough up when the factors (kV, mA, time) are set to maximum?
The normal method of compensating for voltage losses due to mains resistance is to insert
extra resistance at the machine’s input to ensure that it operates from a supply whose mains resistance is a known value (within our control). This should be done during (before) initial calibration of the unit at site. In this way we can calculate the voltage loss at various tube currents and arrange that the off-load value is high by an amount equal to the predicted loss whilst on load.
Within the (typical) x-ray unit itself, other compensation circuits attempt to automatically compensate for variations in supply voltage. For instance, voltage stabilization of the filament circuit tries to ensure that supply voltage variations are not transmitted to the filament, causing unwanted changes in tube current.
Meanwhile, in your case I am wondering about the arrangement of the incoming power supply right at (immediately coming in to) the unit. Is it fed from a spur? How far from the distribution panel? Hopefully it will be right next to it. You might also like to make sure that the unit has a really good earth! It's all basic Ohm's Law, really (higher resistance, causing larger than necessary currents, leading to unwanted volt drops beyond the capabilities of compensation circuits to handle
etc., etc.).