In this blog post, we’ll examine the causes of blackouts—which can occur when the balance between power supply and demand is disrupted—the process of restoring the power grid, past instances of large-scale rolling blackouts in South Korea, and the preventive measures currently in use, such as energy storage systems and emergency power sources.
In the past, large-scale rolling blackouts in South Korea sparked significant concerns that a shortage of reserve power could lead to a nationwide blackout. At the time, analyses also suggested that restoring the power grid to normal operation could take a considerable amount of time in the event of a large-scale power outage. However, the actual situation that unfolded was not a blackout where the entire national power grid shut down, but rather rolling blackouts in which regional loads were forcibly cut off to prevent a power supply crisis. So, what exactly is “reserve capacity” in the context of the power grid? What constitutes a blackout? And why is the process of restoring power supply so complex when a large-scale power outage occurs?
First, we need to examine the characteristics of electrical energy. In the power grid, generation and consumption must be kept in balance almost in real time. In the past, it was extremely difficult to store large amounts of electrical energy economically; today, various storage technologies—including lithium-ion batteries, energy storage systems (ESS), and pumped-storage hydropower—are utilized in actual power grids. However, since it is still not feasible to store all electricity to the extent required to supply the nation’s entire power consumption over long periods, maintaining a real-time balance between generation and consumption remains a core principle of power grid operation. Korea Electric Power Corporation (KEPCO) currently utilizes ESS to stabilize the power grid—for example, for frequency regulation and to address the intermittency of renewable energy—and ESS is classified as a separate power resource in the Korea Power Exchange’s real-time power supply and demand data.
Therefore, power suppliers secure the necessary supply capacity by considering projected electricity demand and the status of generation facilities, and maintain reserve capacity to respond to sudden spikes in demand or generator failures. Here, supply reserve refers to the supply capacity secured in excess of electricity demand. Furthermore, in actual power grid operations, not only simple supply reserve but also operational reserve—which is essential for maintaining stable frequency and responding swiftly to unexpected situations such as generator failures—is crucial. Operational reserve includes frequency control reserve and multiple levels of reserve designed to restore frequency in the event of a failure. Therefore, it is inaccurate to view reserve power simply as “electricity that is generated but not used and is wasted.” It is more appropriate to understand it as a concept that encompasses the spare capacity secured within power generation facilities to increase output when needed, as well as resources capable of responding rapidly.
If electricity demand comes too close to supply capacity, or if large generators and transmission facilities suddenly trip, the frequency and voltage of the power system may become unstable. If an appropriate response is not taken in such situations, protective devices may isolate generators or transmission equipment from the grid; as a result, the load may become concentrated on other facilities, potentially triggering a chain reaction of additional equipment failures. When such incidents spread widely, leading to a disruption in power supply across a large area, the situation is generally referred to as a blackout. However, even if supply reserve capacity decreases, it does not mean that all generators will immediately shut down due to overload or that power plants nationwide will stop one after another. In actual power systems, the collapse of the entire grid is prevented through various protective measures, such as deploying reserve capacity, adjusting generator output, reducing demand, and shedding loads.
The process of restoring the power system after a blackout is also different from simply turning the power plants back on. Since large-scale power plants often require external power to restart on their own, some generating units must first be started using so-called “black start” power—which allows for startup without external power—and, based on this, the transmission grid must be restored in stages while other power plants and loads are reconnected sequentially. During this process, not only must the balance between generation and consumption be maintained, but frequency, voltage, and the condition of transmission lines must also be continuously monitored. For both thermal and nuclear power plants, the time required to restart varies significantly depending on the cause of the shutdown, the condition of the equipment, and the results of safety inspections. In particular, nuclear power plants require cooling to remove decay heat and maintenance of safety systems even after a shutdown; restarting is not simply a matter of “burning” the nuclear fuel again, but is carried out according to established procedures while verifying safety conditions. Therefore, it is difficult to make a blanket statement that all nuclear power plants must wait exactly one week after a blackout before resuming operations. The total restoration time for a large-scale power outage can also vary significantly depending on the scope and cause of the incident, the damaged equipment, and the state of the power grid.
Past large-scale rolling blackouts in South Korea occurred as the imbalance between electricity demand forecasts and the available supply capacity at the time worsened. Generally, during spring and fall—when electricity demand is relatively low—preventive maintenance on some power generation facilities tends to be concentrated. At that time, however, while several power generation facilities were undergoing maintenance, an unusual late-summer heat wave caused electricity demand for air conditioning to rise far beyond expectations. As supply capacity plummeted, measures were implemented to sequentially cut off power to certain regions to prevent a larger-scale accident involving the collapse of the entire power grid. Therefore, it is more accurate to view this incident as a forced rolling blackout implemented during a severe power supply crisis rather than a “blackout” in which all power plants nationwide were shut down. Since the blackouts were carried out without sufficient advance notice, they caused significant inconvenience and damage to households, stores, and industrial facilities, and the importance of a stable power supply and securing emergency power sources was strongly emphasized in South Korean society.
While power outages are a risk that is difficult to perceive in everyday life, even a brief outage can lead to significant damage at facilities that rely on the power supply. Refrigeration and freezing equipment in restaurants and distribution centers, oxygen supply systems in fish farms and aquariums, medical equipment in hospitals, data centers and telecommunications facilities, elevators, and various automated control systems can all experience immediate problems when power is cut off. Therefore, it is essential for critical facilities to be equipped with uninterruptible power supplies (UPS), emergency generators, and battery-based emergency power systems, and to regularly inspect their operational status. This is because even if emergency power is installed, if the automatic transfer switch fails to operate properly during an actual power outage or if fuel and battery management are not properly maintained, the system may not function as intended when needed.
Methods for preventing blackouts have also become far more diverse than in the past. While load shedding—such as rolling blackouts—was once viewed as a critical last resort in situations of severe power shortages, today’s approach involves the comprehensive utilization of stable operation of power generation facilities and transmission grids, accurate demand forecasting, securing operational reserves, demand response resources, energy storage systems, pumped-storage hydropower, grid protection devices, and automatic control technologies. The Korea Power Exchange manages supply capacity, current demand, supply reserves, and the supply reserve ratio in real time, and also operates a system of operational reserves to maintain frequency and respond to outages.
In particular, large-capacity battery-based energy storage systems—which were once expected to take a considerable amount of time to become commercially viable—are now being utilized in actual power grids. Energy can be stored when electricity usage is low or there is excess supply, and then discharged during periods of high demand. These systems can be used not only to manage peak demand but also for frequency regulation and to address output fluctuations in renewable energy. Korea Electric Power Corporation (KEPCO) is applying technology that diagnoses battery status in real time to ESS units used for grid stabilization and is also developing various grid stabilization technologies, such as systems that combine supercapacitors and batteries. However, since ESS systems also require consideration of factors such as storage capacity, cost, battery lifespan, fire safety, and charge/discharge efficiency, no single technology can solve all power supply and demand issues on its own.
Ultimately, preparing for blackouts is not a problem that can be solved simply by increasing power generation. It is necessary to ensure the stability of power generation sources and the transmission and distribution networks, maintain sufficient reserve capacity, and establish a system capable of accurately forecasting demand and adjusting it swiftly when necessary. This requires an appropriate combination of storage resources—such as ESS and pumped-storage hydropower—demand response, automated grid protection systems, emergency generators, and UPS units. Households and businesses also need to identify in advance which equipment is vulnerable to power outages and prepare the necessary emergency power sources, while power suppliers must continuously strengthen systems capable of providing rapid and accurate information when a power supply crisis is anticipated. In today’s environment, where power systems are becoming increasingly complex and renewable and distributed energy resources are expanding, managing generation, consumption, storage, and transmission and distribution networks as a single integrated system is a key precautionary measure for preventing blackouts and minimizing damage should a power outage occur.
