In odoo9, 

     We use for this code this helpful for you,

currency_id = fields.Many2one('res.currency', string='Currency',default=lambda self: self.env['res.currency'].search([('name','=','USD')]))

Please Open this link /web#id=3&view_type=form&model=res.currency ,and

check that DTN is  available in res.currency or not ,

By default there is no as currency name DTN.

I am posting the code that i have used, it's work for me  .

    def _get_default_currency(self, cr, uid ,context=None):
        currency_obj = self.pool.get('res.currency')
        currency_id=None
        currency_ids = currency_obj.search(cr, uid, [('name','=','USD')],limit=1)   
        currency_res = currency_obj.read(cr,uid,currency_ids,['id'],context=context)    
        for v in currency_res : 
            currency_id = v['id']
        return currency_id