The 220 volt supply to your house goes above and below the 10 percent level How do you stabilize it?

You don't. It is the responsibility of the utility company that you purchase the power from. Request that they come out and check the voltage fluctuations. If you are on the end of a power line they can adjust the taps on the transformer. If not at least you are notifying them that there might be a potential problem with the system.

Under the UK relevant legislation, the nominal voltage of 230 V (not 220 V -there is no such nominal voltage within Europe) is allowed to vary between +10% and -6%. In other Words, supply voltages that vary between 253 V and 216.2 V is normal and acceptable, and all appliances are designed and manufactured to operate within this range.

As 220 V is well within this range, so there is no requirement for your electricity supplier to adjust the supply voltage to your property.

If, however, your supply voltage falls outside this range, then your supplier has a legal obligation to improve the situation.