Starting October 4, 2017, campaigns will be able to spend up to twice the average daily budget to help you reach your advertising goals, like clicks and conversions.
On days with high traffic, your costs could be up to 2 times your daily budget. This spending is balanced by days when your spend is below your daily budget. However, for campaigns where you pay for conversions, your daily spend may exceed your average daily budget by more than 2 times.
Keep in mind, you won’t be charged more than your monthly charging limit: the average number of days in a month (30.4) multiplied by your average daily budget.
Read more here – https://support.google.com/adwords/answer/2393018?hl=en