Transmission expansion planning requires static and dynamic analyses. Planning criteria include the selection of contingencies consisting of faults and their clearing, performance standards, transmission system adjustments, and corrective action plan. Performance standards relating to power system dynamics mainly concern the voltage ride through and transient angle stability i.e. retaining synchronism after the occurrence of the contingencies mentioned. The selection of planning contingencies, for which performance standards must be complied with, is important in terms of capital expenditures in transmission network expansion and secure power system operation. This paper discusses planning contingencies accepted by various transmission network operators worldwide and describes a set of planning contingencies proposed by the authors.