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 such as energy storage, bi-directional transfer links, electric vehicles, etc., 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 systems with high penetrations of NSD, since NSD are typically decoupled from the grid via power electronic devices and consequently reduce the “natural” inertia, short-circuit levels and damping effects which are inherently provided by synchronous machines. It is therefore crucial to ensure secure and stable operation of power systems with high penetrations of NSD.
This paper will show and quantify the instantaneous penetration level (IPL) limits of NSD connected to a simple example power system in terms of steady-state stability beyond which the system can become unstable or unacceptable, defined as “unviable”. The NSD used in this example will be a conventional dq-axis current injection (DQCI) convertor model. The paper will introduce a set of criteria relating to locking signal in converter phase-locked loop, frequency, rate of change of frequency and voltage magnitude, which will be used to determine the system viability and the IPL limit. It will also be shown that there are several factors that can potentially affect the IPL limits. Frequency and voltage droop slopes and filter time-constant for DQCI converter are varied and it is shown how these settings influence the IPL limits. Finally, to provide additional insight into network viability under high penetrations of NSD, a visualisation method referred here as “network frequency perturbation” is introduced to investigate responses of individual generators to a change in network frequency.