The accurate modelling of inertia in an islanded power system is crucial when attempting to integrate low or inertia-less generation into the network, particularly those from intermittent sources, e.g. solar PV, converter-fed wind turbines, etc.
In the context of synchronous generators, the term “inertia” generally refers to the kinetic energy in the rotating mass of a generator shaft. The inertia depends on the speed of rotation and mass of the shaft, i.e. the heavier the shaft and faster the rotational speed, the higher the inertia. Under normal operation, the instant a new load is applied to the system, rotational energy (inertia) is converted into electrical energy to supply the load. As energy is removed from the system to supply the load, the speed of rotation (or frequency) decreases. This is commonly referred to as the inertial response and occurs before primary frequency control actions (e.g. from governors) take place. As a result, inertia has a direct impact on transient frequency deviations resulting from sudden changes in generation and load.
Effects of varying inertia on system frequency
In large interconnected systems, these frequency deviations are minor since the instantaneous mismatches in generation and load are very small relative to the amount of synchronous generation dispatched. However, in small island systems, these frequency deviations are much more significant and can even lead to frequency collapse.
By virtue of their remoteness and lack of other resources, the small island power systems that are normally found in island nations and archipelagos typically have high penetration of diesel engine generation. As diesel engines generally have low inertia, the inertia constants selected during system modelling can have a large effect on frequency swings. For example, consider the load acceptance (25% load step event) of a 1.5MVA diesel generator with varying inertia constants:
The simulations show that maximum frequency swings can vary from 1.235Hz up to 3.548Hz depending on the inertia constant selected.
Small power systems are usually equipped with under-frequency load shedding (UFLS) systems to prevent network collapse during frequency swings and active power deficit events (e.g. trip of generator). Integrating inertia-less sources such as solar PV plants would displace synchronous generation and thus reduce the total inertia in the system. Grid simulation studies are performed to predict whether or not the UFLS system is at risk of operating during normal day-to-day fluctuations of the solar PV system, and this requires fairly accurate modelling of the system inertia.
Selecting appropriate inertia values
Generator inertia values are usually found in vendor / manufacturer data sheets and are often expressed as a moment of inertia quantity (e.g. in kg.m2 or slug.ft2). While some software packages can accept these quantities directly as inputs, other programs require that they are converted into per-unit inertia constants (H).
In some cases, particularly in older systems or networks with temporary generators, the inertia data is not available and must be estimated. Based on our database of actual equipment data, the following inertia values can be used as guidance: