If these vouchers are no different than others, it depends what you mean by partial.
You can't decide to use only x % of the value of a voucher to pay for a ticket (e.g. you can't decide to use only 50€ of a 150€-voucher to pay for a 300€-ticket), but if the value of the voucher is higher than the cost of the ticket (e.g. a 300€-voucher to pay for a 150€-ticket), another voucher will be edited with the residual value (I am not sure whether this voucher has the same expiration date or if it expires 1 year after its edition).