Evaluation of Criteria for Transmission Capacity Calculation

Voltage and angle stability along with the thermal loadability of the transmission lines are limiting the transmission capacity of the Nordic power system. Voltage stability after a dimensioning fault limits the transmission capacity of the Finnish power system when power is imported from Sweden to Finland. When power is exported from Finland to Sweden, the damping of the inter-area electromechanical oscillations is the transmission capacity limiting factor. The oscillations appear in many power system quantities, but they are usually referred to as the power oscillations. In transmission capacity calculation, limiting values for the operation of the power system are used as calculation criteria, which determine the allowed operating conditions. The effects of choosing the limits for voltage stability and damping of power oscillations on the transmission capacity and security margins of the Finnish power system are examined in this thesis. The transmission capacity calculation criteria are applied in various operating conditions, and it is examined how the transmission capacity and security margins would change if different criteria were used. The research methods include load flow- and dynamics calculation. In addition, the damping of the power oscillations is examined utilizing the Prony’s method. The research indicated that decreasing the limit for voltage stability would increase the transmission capacity but decrease the security margins. The transmission capacity was discovered to differ between various operating conditions considerably. On the other hand, smaller fluctuation was observed in the security margins between various operating conditions. Based on the studied operating conditions, the security of the Finnish power system does not appear to be endangered if the voltage stability calculation criterion is slightly decreased. An evident difference in the damping of the power oscillations was observed between the summer and winter operating conditions. The summer situations were observed to be more sensitive regarding power oscillations due to fewer generators equipped with power system stabilizers in operation compared with the winter situations. Especially, the impact of the largest generators on damp-ing emerged in the research. The security margins were considerably smaller in the summer situations than in the winter situations when the damping ratios were used as calculation criteria. The currently used calculation criterion for power oscillations was observed to produce more consistent security margins compared with the examined damping ratio based criteria