As @Anand said, you can do it via ir.sequence (Configuration / Sequences). There should be already default value for purchase order and you can try to modify it.

To make it more dynamic, I think you should add some code to detect required 'KODE CABANG''.

ok thanks ,

@Emipro Technologies, @Anand, @Ivan, @Ben Bernard  ... for your solutions, 

it works, with add method create, like :

def create(self, cr, uid, vals, context=None):
        seq = self.pool.get('res.partner').browse(cr, uid, vals['partner_id'])
        vals['name'] = self.pool.get('ir.sequence').get(cr, uid, seq.kd_sequence.code)
        return super(purchase_order, self).create(cr, uid, vals, context=context)

If your requirement is very specific to PO (not to other sequences), then I would advise that you don't touch the ir.sequence.  Make the sequence of PO to supply 2 DIGIT YEAR+2 DIGIT MONTH+5 DIGIT SEQUENCE.  Then in the code where PO name is applied (it is during create method in v7.0, see odoo/addons/purchase/purchase.py), put the "PO"+KODE CABANG as a prefix to the already provided string.

If you need the KODE CABANG to be applicable for more than one sequences, then maybe it is worth considering to inherit ir.sequence and enhance it's _next method (see odoo/openerp/addons/base/ir/ir_sequence.py).  The challange is how would you provide the information for KODE CABANG.  Some models actually called _next in their default_get, which is called even before the model instance is created.