Changes in Grid Inertia

This article was published in the March Newsletter of the Global Smart Energy Federation.

In the January-February 2022 GSEF Newsletter, I wrote about the changing facets of power systems reliability and resiliency in a grid of high renewable energy penetration. This month, I examine the changes in grid inertia in such a high penetration renewable energy scenario.

?Traditional grid inertia is the sum total of all rotating machine inertia operating in the electrical system (generators and motors together). This primary inertia is provided by large Fossil/Nuclear steam turbines and large industrial drives which act as a buffer (shock absorber) against rapid changes in generation and load. Since all machines are spinning at (or very close to) synchronous frequency, any rapid reduction in load causes the frequency to “speed up” (excessive generation) and vice-versa when generation is lost or load is added. The system inertia provides crucial time delay to allow control mechanism to restore the load-generation balance, thereby avoiding a system collapse. While an overall system inertia of 6-8 secs is good (for operational management), remote sites such as mines with larger drives may need up to 10 seconds.

Rotating inertia is a mechanical flywheel effect which smooths out the rate of change of frequency (ROCOF) over many seconds, giving ample time for the controls to restore generation-load balance (nominal frequency restoration). This slow change therefore ensures no abrupt over/under frequency excursions. Multiple rotating inertia (large generators) “tied-together” electrically at the transmission level, essentially produce a “single system inertia” with a vast influence over the entire grid (including MV and LV networks). Also, a slower ROCOF allows for staged corrective measures enabling timely actions in generation governor controls, demand-response initiations and finally selective load shedding. Typically, several “droop” and ROCOF settings are deployed before massive load shedding or blackout takes place. Smaller (or reduced) inertia introduces larger frequency deviations resulting in operations of vital frequency dependant (V/f, dV/df, df/dt) protection systems in generators, transformers and large motor loads, causing unwanted chain reactions.

High penetration of renewables (VRE) in the grid (PV, Wind, ESS) is displacing large thermal generation. This is resulting in rotating inertia being replaced by synthetic-inertia. Synthetic inertia is provided by power conversion systems (PCS) associated with Wind, PV and BESS, mimicking rotational inertia by fast response (boost or buck) in its internal power levels (typically 6-8% of nameplate rating). Ironically, even wind power provides only synthetic inertia as its rotating mass is effectively decoupled through a frequency converter. Also, VREs being smaller in unit size and more distributed/embedded in HV, MV and LV, they tend to influence a limited area around them. Fast response brings quicker system correction but the domino effect of uncoordinated, multiple, rapid frequency responses could cause its own runaway cascading trips. While synthetic inertia manages smaller localized perturbations effectively (in milli-seconds), it does not have the “extensive reach” to compensate for a far-away frequency excursion. Secondly, the boost/buck power levels are very limited to deal with a second (close-in) ROCOF incident.?Thirdly, the frequency recovery of such VRE systems post a significant ROCOF event is a “slower lift” back to normalcy.

Nevertheless, high VRE penetration with battery energy storage (BESS) is changing the grid inertia landscape globally. In Australia, BESS systems were successfully deployed to manage both inertia and reactive power. Examples include (a) BESS installations at Hornsdale Power Reserve (post Callide C incident May 2021); and (b) Dalrymple 30MW/8MWh BESS Station (radial transmission constraint) to manage 90 MW of Wind. Currently, sixteen large-scale BESS are under construction in Australia. Many utilities in Germany, ERCOT/USA and Hydro Quebec/Canada have changed their Grid Code requiring specific synthetic inertia capabilities. These requirements yield rapid (millisecond) response in power levels for such frequency excursions. Such designs generally come at the expense of maximization of VRE energy output for sale resulting in lower revenue and hence investment returns.

With falling VRE and BESS costs, this trend towards synthetic inertia will only grow. Utilities and system operators will need factor these crucial system inertia differences in their T&D planning. Without getting into computational details, the following would be key considerations:

1.????Overall Effectiveness:

The 8-10 second rotating system inertia (offered by large central generators) on interconnected transmission network, currently provides good overall T&D inertia support. Additionally, the typical “7% maximum power imbalance” contingency is rarely violated except for cascaded faults. However, with a lower synthetic inertia, it may require 7x-10x of VRE generation (without weather interdependencies) to provide the same equivalent system inertia of 8-10 seconds. A few thermal plants (at strategic locations) may still be needed to solely support system inertia.

2.????Locational Impact:

VREs are often dispersed on MV distribution grids with radial feeders. This lowers overall system inertia contributions particularly for far-away (upstream and downstream) disturbances, as synthetic inertia is essentially an electrical injection. Their varying degree of effectiveness is dependent on their location, unit-size, power availability and PCS capabilities. While large industrial motor-load inertia may offer leverage, such units need to be running and available all the time.?It is likely that distributed rotating inertia (flywheels, small-generators) will still be needed on high penetration VRE feeders.

?3.????Increasing synthetic inertia:

Increasing synthetic inertia beyond 6-8% of name plate, requires increasing internal power capability (or providing a unit derating). Alternatively, a group-mode control of several such VRE synthetic inertia could be a viable solution. With intermittency and variable nature of such VRE generation, this increased synthetic inertia requirement produces a decreasing rate of return on VRE investment. Increasing synthetic inertia beyond 15% name-plate may not be economical, even with falling VRE costs.

4.????Recovery:

With synthetic inertia, the post event frequency stabilization time period is substantially “stretched out” depending on the VRE type. For wind-power units, the power reduction can be significant and this recovery phase delays the grid’s frequency recovery. In Québec/Canada December 2015 transformer event, the system frequency was flat-lined at 59.4 Hz for several seconds, before additional power reserves could push it back to 60 Hz. In some cases, this could trigger a “double-dip” in system frequency, increasing the risk of blackouts.

5.????Black-start inertia:

Most VREs are “grid-following” and not “grid forming”. But new PCS designs are emerging that enable grid formation. However, their distributed and smaller unit sizes pose challenges in re-establishing the grid using synthetic inertia alone. The load increments have to be kept much smaller at 2% of the partial grid (often KW size) to ensure frequency stability when establishing network segments. Going forward, in the absence of large thermal plants, backup diesel generators in large buildings will provide critical stabilizing supply during such black-start restoration. The overall network re-build will also take much longer with interim disruptions due to unstable frequency conditions.

In closing, the trend towards high penetration VRE (and hence synthetic inertia) is here to stay with falling VRE and BESS costs. Notwithstanding its challenges (and differences) in providing synthetic inertia, it needs to be managed by careful T&D planning. The biggest challenge will be recognizing these crucial differences and making appropriate regulated T&D grid investments. ?