Traditional power systems rely on conventional synchronous generators, whose output power is fully controllable, to satisfy the power demand at each time instant. With the increasing share of renewables in the grid, new stability and reliability problems arise in the power grid. The standard question is, what do we do if there is no sun and no wind? Indeed, energy cannot be stored in an efficient way, and most renewable sources cannot be fully regulated to accommodate for a time-varying demand. Less known by the general public is the problem of low inertia caused by non-synchronous generation. But what is actually inertia? The equation below describes the evolution of the frequency in the electric power
The parameter H denotes the so-called inertia of the system, and it represents the amount of kinetic energy of all rotating masses present in the grid. Due to electromechanical coupling, the rotating mass of a generator provides or absorbs kinetic energy to the grid if there is an unbalance between generation and demand. One can easily see that for very small values of H, any unbalance between generation and demand will cause large frequency excursions. Of course, primary and secondary control imply that generated power will react to changes in the grid frequency, but this change in power is not instantaneous. Let’s see what happens when a generator suddenly trips:
Clearly, if no inertial response is available, an unbalance will cause in this case the frequency to drop dramatically until primary response starts acting, causing machine tripping, load shedding and even blackouts.
While conventional generation such as gas turbines, steam turbines or hydro plants are connected to the grid via synchronous generators, several renewable sources such as wind turbines and solar photovoltaic are connected to the grid via power electronics, and add virtually no inertia. Low levels of inertia used to be a problem only in small, isolated power grids. As of today, most grid operators need to ensure the presence of a minimum amount of conventional, synchronous generation in the grid at all times, limiting the penetration of inexpensive, environmentally-friendly renewables sources.
Different strategies are envisioned in order to operate under low inertia grids:
- Control of conventional generators under high values of rate-of-change-of-frequency (RoCoF): sudden changes in frequency can cause serious issues in conventional generators, such as lean blow out. With current designs, turbines struggle to ride through sudden frequency changes, and can cause cascaded trips that lead to regional blackouts. New control laws and new mechanical designs for turbines might be needed to be able to operate in low inertia grids, where events associated with high RoCoF values might occur frequently. New grid codes in countries with high penetration of renewables (Denmark, Spain, Portugal) require all generators to ride through events with RoCoF values of 2 Hz/s.
- Synthetic inertia: can we control power converters in such a way to mimic the behaviour of mechanical inertia? That is, can we increase or decrease the power output according to frequency deviations? Indeed, new control laws could achieve this, but as all feedback laws, before acting it is necessary to first detect frequency deviations. And this detection is not as straightforward as it looks: signal filtering is always required to remove noise and transient faults, adding a certain delay in the loop. This delay, added to the computation times required to control the converters, implies that response is not as instantaneous as desired.
- Demand response: frequency deviations occur as generation does not match demand. While most techniques focus on controlling the generated power accordingly, another possibility lies in regulating the demand according to the frequency deviation.
- Inertia market: plant owners are paid for different services (primary, secondary response), but inertia is not considered to be a service in many grids. Some transmission system operators such as Hydro Québec realize the need for a minimum inertia in the grid and enforce all wind turbines to provide synthetic inertia. Other operators in Canada and Brazil are applying similar rules.
More research is indeed needed to provide an adequate substitute for synchronous generation in future grids.