Assessing the Risk of Cold-weather Generation Outages in ERCOT

With ERCOT reporting almost one-third (30GW) of non-renewable generation capacity offline as of November 12th, it’s clear that we’re in the height of fall outage season. Accordingly, let’s take a look at generation outage trends, patterns, and the risk that extreme cold this winter will force generators offline.

For this blog post we’re going to focus on outages of non-renewable generation capacity, which could also be thought of as controllable–or thermal–generation capacity. In other words, we’re not focusing on outages of wind and solar resources (which we’ll tackle in a separate piece).

The Seasonality and Variability of Generation Outages

Figure 1 plots maximum daily RTO load and average daily generation outages in ERCOT for each year since 2017.  Both are seasonal, with generation outages increasing during the Spring and Fall shoulder periods of lowest load. Spring generation outages tend to be higher than Fall outages, even though spring loads are slightly higher–-likely because generation owners want to ensure that maintenance and repairs are completed before the high-demand (and potentially high-priced) Summer season. 

Variability around the seasonal trend is also very apparent, with Spring and Fall outages varying between 20 and 30 GW and even Summer outages exhibiting a range (from the lowest observed outage levels to the highest observed levels) of at least 5 GW–more than enough to tip ERCOT pricing from moderate to extreme levels in a typical Summer season.

Beyond the normal variability, a couple of generation outage events stand out as outliers–in particular February 2021 (light purple) and August/September 2017 (dark purple). As winter is coming, let’s take a closer look at the February 2021 event and see if we can infer the likelihood of a similar event occurring again over the next few months. 

How Cold Does it Need to Get in Texas for Outages to Spike in Winter?

In short: quite cold for Texas. Excess February 2021 generation outages were caused by Winter Storm Uri, which in turn knocked out power for much of the state over a week-long period. Figure 2 plots the daily minimum ERCOT temperature (population-weighted) against average daily outages for December, January, and February.

We see essentially no correlation between temperature and generation outage levels–with the notable exception of extreme cold starting on February 15th, 2021 forcing approximately 10-15 GW of generation offline  (with respect to February 13th and 14th–right before the cold snap). 

Outages climbed significantly when temperatures bottomed out close to 10 degrees F and stayed elevated even after the daily lows increased, leading one to suspect the existence of a cold temperature threshold that’s likely to cause excess generation outages that may persist for days (or longer). Looking at February 2021, we estimate that this threshold is somewhere around 10 degrees F. 

Of course, while this analysis evaluates weather conditions across the entire ISO in order to infer an estimate of ISO-wide excess generation outages, the weather conditions that force generators offline are local. To make more precise cold-weather outage models, we’d want to repeat this analysis by grouping generators according to regional weather zones and then comparing local weather to regional outages.    

Will ERCOT Rule Changes Prevent Excess Cold-weather Outages From Happening Again?

We’re skeptical. The primary rule change that ERCOT instituted (at the behest of the Public Utility Commission) is a weatherization standard that will be “enforced”-without penalties. To be in compliance with the standard, generators are to:

1. Implement additional weather emergency preparation measures reasonably expected to ensure sustained operation at 95th percentile minimum average 72-hour wind chill value in the ERCOT weather study for facility’s weather zone
   
2. Create list of all cold weather critical components, review annually prior to beginning of winter, and update as necessary

(Source)

In other words, there’s still no economic penalty for an ERCOT generation resource that’s unavailable in a grid emergency. As a consequence, most thermal generators (e.g. natural gas plants) still do not have firm fuel supply arrangements with either pipelines or commodity suppliers. That means that on very cold winter days, a natural gas generator is not only subject to outages resulting from its own mechanical failures--but also from failures of its natural gas supply (pipeline capacity is maxed out, lower daily gas production due to natural gas wells freezing). 

How Can Sunairio Quantify the Likelihood of Extreme-Cold-Induced Outages this Winter?

We can use Sunairio weather simulations to find out the odds of breaching the 10 F daily low temperature threshold we identified above. We execute the following query to get simulations from Snowflake:

select
    convert_timezone('Etc/UTC', 'US/Central', sim_datetime) as sim_datetime,
    sim_number,
    sim_value*1.8 + 32 as sim_value -- Converting C to F
from
    ercot_weather_sims
where
    location = 'rto'
    and variable = 'temp_2m'
    and convert_timezone('Etc/UTC', 'US/Central', sim_datetime) 
        between '2023-12-01 00:00' and '2024-02-29 23:59'

By downloading this with the Snowflake connector in Python, you can make the Pandas DataFrame a timeseries of simulations with a simple pivot:

cur = conn.cursor()
cur.execute(query)
temp_sims_df = cur.fetch_pandas_all().pivot(index='SIM_DATETIME', columns='SIM_NUMBER', values='SIM_VALUE')
cur.close()

Then, with the resulting 1,000 hourly simulations of ERCOT population weighted hourly temperature for Dec 1, 2023 through Feb 29, 2024 we can calculate the probability distribution of minimum daily low temperatures for each simulation path (in python, for example):

temp_sims_df.resample('d').min().min().hist()

This gives us the histogram in Figure 3. The height of the bars in this histogram represent the number of paths in our 1,000-path simulation with the corresponding daily minimum temperature on the the x-axis. Most of the distribution is around 22 degrees. In other words, we’d expect the coldest daily low temperature this winter to be about 22 degrees.

So what’s the likelihood of having a daily low temperature 10 degrees or lower? We just count the number of paths < 10 and divide by the total number of paths (1,000).

In  [1]: (temp_sims_df.resample('d').min().min()<10).sum() / 1000
Out [1]: 0.055

We find a 5.5% chance (according to these assumptions) of experiencing extreme cold that could result in excessive non-renewable generation outages this winter. 

Sunairio Outage Simulations Natively Account for Cold-Weather Effects

At first glance, excess generation outages similar to those caused by Winter Storm Uri may seem like outliers. However, armed with a theory (cold temperatures beyond a certain threshold cause a cascade of multi-day gas supply and gas generation problems) and forward looking simulation data (1000 weather simulation paths) we can model the chance and effect of another similar event.

At Sunairio, we model generation outages (including the effect of extreme cold weather) as a percentage of thermal energy in parallel with weather simulations-ensuring that data and analyses are not left in the dark.

Conclusions

• Non-renewable generation outages in ERCOT follow a seasonal pattern and peak in Spring, though with significant variability.
  
• On an ISO population-weighted basis, daily low temperatures need to drop to about 10 degrees Fahrenheit to cause excess generation outages.
  
• New ERCOT rule changes require power plants to attest to winter weatherization but still don’t provide a strict penalty for noncompliance or unavailability.

• The odds of a cold-weather-induced generation outage event happening during Winter 2023-2024 is about 1 in 20.
  
• Sunairio non-renewable generation outage simulations for ERCOT incorporate these dynamics.