The North American Electric Reliability Corporation on Jan. 8 published a new incident review examining the risks and challenges posed by the increasing integration of voltage-sensitive large loads, such as data centers and cryptocurrency mining facilities, into the Bulk Electric System.
The review highlights the potential for significant load loss during normally cleared faults on the BES and emphasizes the growing presence of voltage-sensitive loads within the system, NERC said.
Summary of Incident
A 230 kV transmission line fault led to customer-initiated simultaneous loss of approximately 1,500 MW of voltage-sensitive load that was not anticipated by the BES operators.
“The electric grid has not historically experienced simultaneous load losses of this magnitude in response to a fault on the system, which has historically been planned for large generation losses but not for such significant simultaneous load losses,” NERC said.
It noted that simultaneous large load losses have two effects on the electric system.
First, frequency rises on the system as a result of the imbalance between load and generation and second, voltage rises rapidly because less power is flowing through the system.
In this incident, the frequency did not rise to a level high enough to cause concern. The voltage also did not rise to levels that posed a reliability risk, but operators did have to take action to reduce the voltage to within normal operating levels.
“However, as the potential for this type of load loss increases, the risk for frequency and voltage issues also increases. Operators and planners should be aware of this reliability risk and ensure that these load losses do not reach intolerable levels.”
Incident Details
At approximately 7:00 p.m. Eastern on July 10, 2024, a lightning arrestor failed on a 230 kV transmission line in the Eastern Interconnection, resulting in a permanent fault that eventually “locked out” the transmission line.
The auto-reclosing control on the transmission line was configured for three auto-reclose attempts staggered at each end of the line.
This configuration resulted in 6 successive system faults in an 82-second period. The protection system detected these faults and cleared them properly. The shortest fault duration was the initial fault at 42 milliseconds, and the longest fault duration was 66 milliseconds. The voltage magnitudes during the fault ranged from .25 to .40 per unit in the load-loss area.
Coincident with this six-fault disturbance, the same local area saw an approximate 1,500 MW of load reduction. None of this load was disconnected from the system by utility equipment; rather, the load was disconnected on the customer side by customer protection and controls. It was determined that the 1,500 MW of load reduction was exclusively data center-type load. The area where the disturbance occurred has a high concentration of data center loads.
Load Details
NERC said that discussions were held with data center owners to understand the specific cause of their load reductions.
It was determined that the data centers transferred their loads to their backup power systems in response to the disturbance.
Data center loads are sensitive to voltage disturbances. The data center protections and controls are designed to avoid equipment outages for voltage disturbances.
In addition to the computer equipment at these facilities, cooling equipment is also critical to the operation of the data center and sensitive to voltage disturbances, NERC noted.
To ride through voltage disturbances on the electric grid, data centers employ uninterruptible power supply systems that will instantaneously take over providing power to the data center equipment when a grid disturbance occurs.
The differing types and designs of these UPS systems cause differences in the characteristics of the data center responses to a voltage disturbance.
A centralized design uses UPS systems at the load-center level that are typically in the range of 2–5 MW, NERC noted.
“The UPS uses power electronics to switch the load to a battery bank connected to the UPS. These battery banks are not designed to supply the load for long periods of time but rather to power the load for the short time periods of disturbances or—in the case of a complete electric grid outage—long enough to start a backup generator that will then provide power to the UPS.”
The decentralized UPS design uses many smaller UPSs at the rack level.
These rack-mounted UPSs are typically in the range of 3–4 kW. The decentralized UPS systems operate similarly to the centralized UPS systems, just on a smaller scale.
“Another type of UPS is a dynamic/diesel rotary uninterruptible power supply. These systems use a flywheel to provide uninterruptible power and a clutch system to quickly start and connect a diesel engine upon a disturbance on the electric grid,” NERC said.
Future Considerations
“While this disturbance did not cause significant operating issues with the grid at this location and at this time, as data center loads continue to grow rapidly, the risk could quickly increase,” NERC said.
Actions that NERC said Transmission Operators and Transmission Planners should start taking to avoid significant issues in the future are listed below:
- Require dynamic response models of large loads in their facility interconnection requirements
- Perform studies to determine the potential magnitude of load loss for system disturbances (faults)
- Study the impact that these large load losses would have on the system
- Take into consideration the potential for voltage-sensitive load loss when configuring automatic reclosing schemes
- Actively monitor to detect load losses coincident with system faults
- Transmission Operators: Ensure that operating agreements with large loads include ramp rates when connecting/reconnecting large loads to the system
NERC said that critical questions that must be resolved are:
- Should large loads be a NERC registered entity?
- Should NERC Reliability Standard modifications be developed for large load interconnection requirements?
- What studies should Transmission Operators perform to “consider” the impacts of large load operation?
- What is the definition of a large load?
NERC said transmission Owners, Transmission Operators, Transmission Planners, and large-load owners will have to work collaboratively to identify and mitigate reliability risks posed by large load losses during system faults.
The NERC Large Load Task Force is one group where this type of collaboration can take place, it said.
