This article provides solutions to issues addressed in our prior article Renewable Energy Poses Challenges to Electric Power Quality. Previously, we have addressed issues with voltage inconsistencies.
Distributed renewable energy sources, such as solar photovoltaic (PV) panels or wind turbines, present an exciting opportunity for the electric utility industry to provide consumers with clean electricity. To prevent wide area blackouts from becoming more common as distributed renewables expand, electric utilities must embrace electric vehicles as a reserve source of grid support.
NERC Reliability Standards PRC-006-5: Automatic Underfrequency Load Shedding and PRC-024-2: Generator Frequency and Voltage Protective Relay Settings are intended to set parameters for preventing wide area blackouts when severe disturbances occur. These standards should be updated to include EV enhanced grid recovery methodology to maintain frequency consistency.
Before we explore how EVs can support the grid, let’s take a closer look at NERC reliability standards and traditional responses to underfrequency events.
NERC Sets Standards for Frequency Stability
The purpose of PRC-006 is to establish design and documentation requirements for automatic underfrequency load shedding programs that arrest declining frequency, assist recovery of frequency following underfrequency events, and provide last resort system preservation measures.
To comply with PRC-006, electric utilities prepare spreadsheets that list peak load, essential loads, and interruptible loads at every substation in their jurisdiction. In some areas, 10% of peak load will be shed if frequency decays to 59.2 Hz, an additional 10% will be shed if frequency decays to 58.8 Hz and an additional 10% will be shed if frequency decays to 58.2 Hz. In other areas, load is shed in 5% steps for comparable decays in frequency.
The purpose of PRC-024 is to ensure that owners set generator protective relays so that generating units remain connected during defined frequency excursions. Most large generators are expected to ride through underfrequency transients unless the power system frequency decays below 57.6 Hz for more than 30 seconds.
Challenges of Underfrequency Events
The traditional assumption is that imbalances of 10% will cause frequency dips to 59.5 Hz and that imbalances of 25% will cause frequency dips to 58 Hz. Imbalances can be caused by loss of energy sources, rapid load pickup, or loss of transfer capability across the electric power grid. Figure 1 illustrates a severe underfrequency decay from 60 Hz to 58 Hz in 6 seconds with stabilization in 60 seconds. Inherent in this representation is that 20% of connected load was interrupted within 10 seconds of the severe underfrequency event.
Before solar panels were placed on rooftops and electric vehicles (EVs) were common in driveways, the load cycle at every substation was predictable on an hourly basis. Peak load was between 3:00 and 7:00 PM, light load was from 1:00 to 5:00 AM. The difference between minimum load and maximum load at any substation was approximately 20% on any given day.
Power to critical facilities such as hospitals and police stations should not be interrupted during load shedding needed to stabilize an underfrequency event. After the winter storm in Texas in February 2021, more facilities were added to the list of critical loads, including water treatment plants, sewage treatment plants, and gas pressurizing stations. This further reduces the number of substations and distribution lines that can be deenergized to interrupt customer load, creating a challenge when load shedding is necessary.
Electric Vehicles Increase Recovery Options
Reliable power is necessary for business and residential consumers, in addition to critical facilities. However, increased distributed renewables and load from EVs makes it challenging for utilities to continue to provide reliable power. A hallmark of reliable power is when blackouts due to major load shedding events occur only every five years or more.
To support the grid during underfrequency events, and thereby prevent wide area blackouts, EVs can support the grid by providing energy stored in their batteries. When 10 million EVs are in service in the United States, there will be 600,000 MWH stored in EV batteries. This is equivalent to 3,000 large battery storage facilities like the Hornsdale Power Reserve in South Australia.
Let’s theorize that, in California, peak electric demand is 45,000 MW; at the same time, if there are 2 million registered EVs, then there is the potential for 120,000 MWH of stored energy.
With existing technology, if a 25% load imbalance occurred, 12,000 MW of customer load would be interrupted. EVs could reduce unbalances, first by transitioning from battery charge mode to standby mode when frequency drops to 59.95 Hz, then by transitioning to grid recovery mode when frequency drops below 59.9 Hz.
Figure 2 compares a 25% load imbalance in two scenarios; in both, there is a 12,000 MW discrepancy that needs to be resolved. In the first model, load is shed from non-critical facilities, such as homes, factories, and restaurants, which experience power interruptions. In the second model, EV enhanced grid recovery occurs, wherein 1 million battery chargers transition to standby mode at 59.95 Hz, and 1.5 million battery chargers transition to grid recovery mode for up to three hours. With EV enhanced grid recovery in place, no consumers will lose power because load shedding is not required.
Battery charges would be equipped with Monte Carlo simulators that would facilitate battery charger transitions on a somewhat random basis. Having all EV battery chargers transition in unison would present another challenge to the grid, which would be avoided with today’s technology.
EV Owner Compensation
During grid imbalances, electric energy is more valuable that during normal times. To compensate EV owners for participating in EV enhanced grid recovery initiatives, EV owners should receive credits after their EV batteries are called upon to support the grid. During standby intervals, EV owners should receive a credit equal to five times the amount they would have paid for energy. During a grid recovery interval, EV owners should receive a credit that equals ten times the amount they would normally pay for energy during that time.
Grid Recovery Standards
NERC reliability standards should be updated to include EV enhanced grid recovery as a method of maintaining frequency consistency. With EV batteries available to support the grid during underfrequency events, electric utilities will rely less on fossil fuels. This change is crucial at a time when transitioning to renewable energy sources must occur to prevent the worst impacts of climate change.
To take a closer look at the issues that these solutions are addressing, check out our article Renewable Energy Poses Challenges to Electric Power Quality. Check out our blog to learn more about Prescient’s innovative ideas for climate change mitigation strategies or next generation electric power system solutions. And contact us with your questions or ideas!
Comments