Frequency Response by Wind Farms in Islanded Power Systems with High Wind Power Penetration

Licentiatavhandling, 2015

The integration of variable speed wind turbines (VSWT) in power systems keep increasing in order to reduce the emission of green house gases. This increase of power electronic converter interfaced generation causes a decrease of power system inertia and issues previously existing in power systems tend to become more complicated and new solutions should be evaluated. This thesis investigates the impacts on frequency stability that can be caused by a decreased inertia in weak power systems. This by firstly investigating the frequency behavior and the roots and causes of the decreasing frequency quality in the Nordic power system (NPS). This by means of phasor measurement units (PMU) from different locations in the NPS. In particular the focus has been on large power imbalances, the rate of change of frequency (ROCOF) and the impact of location of disconnected generation, frequency quality and the evaluation of a new automatic frequency restoration reserve (FRR-A) service that was introduced in the NPS in 2013. Furthermore, the concept of inertia emulation to handle large power imbalances is developed further with the adaptation to handle variable wind. Two strategies of utilize the rotating mass of the VSWT in order to balance 20 and 50 % VSWT instantaneous wind penetration ratios (WPR) in a islanded power system based on a hydro unit. The temporary frequency drop for a wind penetration of 50 % was improved from 47.33 Hz, for the uncontrolled case, to 49.10 Hz utilizing the suggested adaption of inertia support responindg to a disturbance of 0.1 pu. Lastly, the capability of a VSWT to provide temporary primary frequency support in islanded power systems with 50 % through an alternating generation mix based on hydro, reheat or thermal units in charge of automatic generation control (AGC) considering delays, dead band settings and a combined pitch and droop controlled control strategy. The combined strategy showed clear improvements to the span of 49.9 Hz ≤ f < 50.1 Hz of 28 percentage points for a hydro based power system, this while only reducing the energy produced by 6 %.