Generator Stability Study for High ROCOF Events in a Low Inertia Grid
(Room Rheinsaal 1)
28 Jun 17
11:00 AM
-
12:30 PM
Tracks:
Track D - Integration & Energy Storage
The increasing penetration of inverter connected energy sources, such as wind and solar, which do not provide rotational inertia, is causing lower grid system inertia, resulting in higher rates of change of frequency (RoCoF). Higher RoCoF values introduce new challenges to power system stability and operation. This has first emerged in islands with a high share of renewable generation and without a synchronous connection to a larger grid comprising many synchronous generators. The Irish grid is an example of this due to its ever growing share of wind power. According to studies by the Irish grid operator, RoCoF values can exceed their conventional maximum standard value of 0.5 Hz/s: up to 1 Hz/s could be experienced, during times of high (>50%) wind generation following the single largest credible contingency. For this reason, a modification to the Irish Grid Code was proposed, which requires a generator to be capable of riding through a 1 Hz/s RoCoF event. Since the impact of such higher RoCoF events is uncertain, technical studies are required for each generator to assess compliance with the new RoCoF standard. This is a new type of compliance study which involves simulation of a power plant subjected to RoCoF events defined by the grid operator as grid frequency traces varying in time. This paper presents results of such a study for a CCGT plant. The model includes the generator and its controls (AVR, PSS, governor), protection functions, auxiliary systems, equivalent grid model and mechanical multi-mass model to assess torsional stress on the shaft line. The combustion process was also assessed. Based on the study methodology and assumptions, it was concluded that this particular CCGT can comply with the new RoCoF standard. This conclusion is however not universal and bespoke studies are required for an individual power plant.