Part III - Operating renewable rich grids, some basic metrics have become so different!! technology, tools and market mechanisms hold the key...

Main graphic denotes resource mix as it is envisaged till 2050 globally. While it is one single graphic representing global scenario but it is going to be different for different synchronous areas. There will be synchronous areas that will see slope somewhat more steep than what is presented in the graphic and there will be control areas which will see less steep transition. Having spoken on transmission grid connected IBR's, challenges with that, DER's and challenges with that in distribution networks, now we move to third aspect which is types of machines that provide electricity, with inertia and without inertia (in fact discussions there are things like synthetic inertia but it is primarily programming IBR's to give a droop like performance), how tools and market mechanisms, way electricity is bought and sold, not just capacity and energy but "_______" holds the key (would look forward to suggestion on fill in the blanks here :)).

Inertia was and still remains one of the key grid stability metrics, in fact it's like first line of defense in the face of grid disturbances that cause demand-supply imbalance. There are intents to operate grids with zero inertia but more or less as of now it's just like outsourcing inertia to neighboring control areas (of course with associated reliability risks e.g. think when South Australia operates with very high solar but with interconnection to grid and if it has to operate in islanded mode with that generation mix!!).

In the earlier operational paradigm also, inertia used to keep on changing but variability was primarily because of load being served and broadly there was enough always. But as percentage of renewable generation in grids has changed, it has impacted the whole dynamism significantly e.g. on a perfectly sunny day it will need lesser of synchronous machines online and reduce inertia or on a highly windy night where even load is less it will again mean vey low inertia.

Impact?? Below graphs present a disturbance in UK grid with a generation loss of around 1GW and frequency drop of around 0.35Hz and event time is morning.

In contrast to this, there was another disturbance in Dec'23 in UK grid with interconnector lost and generation dip of around 1GW but it was during mid of the day and for same generation loss, frequency drop was almost 0.7Hz, probably resource mix was such that grid was operating with low inertia.

In both the occasions, a significant part of the recovery was supported by fast response services from storage (to be noted!!) and certainly there was no under frequency load shedding.

In comparison to above, below data is from a report published by India's grid controller wherein grid controller has analyzed some more than thirty events about challenges posed by ISTS (interstate transmission system) connected inverter based resources, mostly from last 2 years.

As we can see from this, frequency response is almost around 0.1 Hz per GW which is comparatively much higher as compared to what we saw for UK grid basic differentiation being grid inertia and reason for this is resource mix.

While above resource maps are for annual energy and instant generation mix may well be very different but these are roughly indicators. A key operational requirement as we get into grid with less and less inertia is faster services which even with higher RoCoF do not let the frequency get to such low levels that under frequency load shedding can get triggered.

This variety which is driven by market mechanism and trigger which is driven by technology deployment (these are complex systems with measurement and data exchange between multiple operational systems) and operational practices is the key to operate such low inertia grids. It includes assets which can respond, technology that can drive and market mechanisms and settlement norms which can drive investment in such assets and technologies.

Till the time this level of market maturity happens with parallel deployment of technology, business models which handle these transactions and we are confident that it can be managed, it makes sense and its safer to stick to systems with comparatively high inertia and move slowly because a stable grid is necessity for economic growth.

This is partly what Indian Power minister knows in background when he says that...

1. Coal is there to stay
2. Will not feel apologetic while adding thermal capacity

https://www.moneycontrol.com/news/business/budget/power-minister-qa-will-not-apologise-for-adding-thermal-capacity-says-rk-singh-12086611.html

Well aware of how market reform actions and stakeholders capacity building has been happening, at what pace (apart from little few like Grid India) and we have to have a strong and reliable grid to support highest growing economy.