Here is the general circuit for using an autotransformer as a voltage booster. Notice that one side of the primary and secondary are common, so this circuit cannot be used for every application.
Here is a link to a 120/240 volt to 12/24 volt transformer, that can be connected as autotransformer booster to produce 24 volts more than primary. So if your relay voltage is as low as 216, you still get 240 out. (216 + 24 = 240 volts out). One effect caused by using transformer is that the relay will not operate immediately when voltage is applied to other end of the wire. There will be a time delay while transformer coils "charge", could be 2 or 3 seconds. If that is a problem for your control circuits, then you should use larger wire. Even then, due to the 1000 meter distance, there may be a delay in relay operation. You can pre-charge the wire, applying less than enough voltage to energize the relay, but enough to keep wire charged, then when circuit is energized, voltage is switched to boost mode, so that relay on other end energizes.