Instantaneous penetration level limits of non-synchronous devices in the British power system

The installed capacity of non-synchronous devices (NSD), including renewable energy generation and other converter-interfaced equipment is expected to increase and contribute a large proportion of total generation capacity in future power systems. Concerns have been expressed relating to operability and stability of such systems, since NSD are typically decoupled from the grid via power electronic devices and consequently reduce the natural' inertia, short-circuit levels and damping which are inherently provided by synchronous machines. This study establishes the instantaneous penetration level (IPL) limits of NSD connected to a model power system in terms of steady-state stability beyond which the system condition becomes unstable. The NSD used in this example will be a conventional dq-axis current injection (DQCI) convertor model. The study introduces a set of system viability' criteria relating to locking signal in converter phase-locked loop, frequency, rate of change of frequency and voltage magnitude, which are used to determine the IPL limits. Among many factors which can affect the IPL limits, the impact of frequency and voltage droop slopes and filter time-constants of DQCI converter is quantified. Finally, a frequency domain visualisation method referred here as network frequency perturbation' is introduced to provide additional insight into contributions of individual generators.