Power system planning for high renewables penetration: voltage stability and system frequency aspects

The installed capacity and the grid access request for renewables is anticipated to continue rising in the EU and US in the coming years. The scarcity of reactive power and synchronous inertia are two inherent consequences of high penetration of renewables in the power system. This raises voltage security concerns, and may endanger rotor angle and frequency stability. In order to foresee these challenges, it is essential to carry out extensive planning and operation studies. Thus, new methodologies and innovative metrics are required to ensure secure and reliable operation of power systems.In this thesis, a multi-operating condition AC voltage stability constraint optimal power flow framework has been presented for transmission system planning. This framework captures multiple wind and demand operating conditions within an optimal power flow tool. The voltage stability constrained optimal power flow was applied to wind capacity allocation. It was shown that the capacity allocation pattern affects steady state voltage stability and the total allocated wind capacity. This indicates that a well-chosen allocation of wind capacity is not only in line with the trend of renewables integration in power systems but also enables limiting the occurrence probability of insecure operating points that may require costly remedies.A procedure for wind capacity allocation has been presented based on the finding on the effects of the pattern of wind capacity allocation on voltage stability. This benefits from the potential of an optimal wind capacity allocation for enhancing the voltage stability margin. Unit commitment was employed to take into account the reduction of the available reactive power sources at each operating condition for wind capacity allocation. By setting the wind capacity target and tracking the loadability margin, it was shown how the risk for a reduction in loadability margin may increase with allocation of wind generation. This procedure showed that specific locations in the system are favored for capacity allocation. It also identified weak areas in the network that experience a reduction in the loadability margin as a result of the allocation of the wind capacity. The methodology can help system operators prioritize network access and investment in the network to enhance the integration of renewables.Further, this thesis focused on the power system frequency aspect of renewables integration. Synchronous inertia acts as a means of immediate frequency support in power imbalances. Renewables often inject power into the network through power electronic converters. As such, synchronizing torque and synchronous inertia are not available in the power from renewables. Reduced levels of synchronizing torque raise concerns on rotor speed behavior under power imbalance events. The interaction of generators has been investigated by deriving the synchronizing torque coefficient matrix from the multi-machine Heffron-Philips model. It was shown that the reactive power output of the generators can be used to control the elements of the synchronizing torque coefficient matrix. It was identified that the varying levels of synchronizing torque affect the rate of change rotor speed of generators following a loss of generation event. Furthermore, the effect of rotor speed deviation due to synchronizing torque was presented from the system frequency perspective. This provides a foundation for system operators to establish strategies that benefit from the synchronizing torque coefficient matrix characteristics for controlling the frequency behavior.

Appendix 1 has been removed by the author to preserve anonymity.