An Essential Guide to Power System Restoration

Have you ever wondered how the power system is restored after a sudden blackout? It’s a complex process, but it’s more than just engineering. When we help restore power to a city or region after a blackout, we are not just fixing a technical issue. We are saving lives, keeping businesses running, and preventing major economic losses.

Power outages, especially weather-related ones, are becoming more frequent, causing significant disruptions and costs. The ability to quickly restore power after a blackout is crucial for maintaining a resilient power grid. Power system restoration is a highly intricate process, involving multiple steps, numerous operational decisions, and complex technical constraints, making it a challenging task.

Embark on a journey into the captivating realm of power system restoration, where we uncover the challenges, unveil strategic insights, and learn more about the remarkable efforts of those who ensure the lights stay on.

System Restoration Overview

Restoring a power system after a blackout involves returning it to normal operation, including power generation, transmission, and loads. The tasks for system restoration vary based on the system’s structure, characteristics, and outage scenarios. In regions with abundant hydropower, restoration is less challenging, as hydropower can serve as an initial power source. However, in areas with limited hydropower, restoring generation capacity becomes a critical task [1].

Power system restoration (PSR) is a complex process with multiple steps, including, system status determination, plant preparation, generation restoration, transmission path energization, load restoration, and system synchronization. Therefore, a well-designed and thoroughly practiced plan is essential for a successful restoration.

In the restoration phase, operators rely on primary power sources known as black-start (BS) generators. These generators can start independently without external grid power, they include hydro, thermal generators, and batteries equipped with backup fuel or power [2]. The primary task of these BS generators is to initiate the non-black-start (NBS) generators that require external power for starting, energize the transmission network, and gradually pick up loads.

The main principle of PSR is to recover power supply to customers as fast as possible to reduce the negative impacts of power outages [3]. From a problem-solving perspective, the objective is to minimize the start time of the last non-black-start (NBS) generator in the system [2].

Stepwise Power System Recovery

Restoring power after a blackout is an intricate process, involving a series of carefully planned steps that ensure the safe and efficient reestablishment of electricity service. This section delves into the step-by-step process of power system restoration.

Figure 1. Power system restoration flowchart (build-up process)

1. BS Capability

The black-start stage as part of the preparation period, is a stage in which a black-start generator provides cranking power to restart an NBS generator [4]. The post-blackout restoration process depends on initial power sources for energizing the transmission lines, cranking up generators, and supplying critical loads. These sources comprise tie lines connected to the active grid and the black-start generators. BS generators could be hydroelectric plants, batteries, diesel generators, gas turbines, and steam generators.

For most of the independent system operators (ISOs), black start is a mandatory service. The ISO identifies and contracts resources with black start capability, forming financial agreements to ensure they can provide this essential service [1].

2. Status determination and system preparation

The magnitude of outages and system status (e.g., restoration area boundaries, relay status, load shedding, generator status, and BS units’ readiness) must be determined before restoration starts [1]. When physically damaged equipment initiates or is a result of outages, the fault locations need to be identified. SCADA and phasor measurement unit (PMU) systems can be utilized to gather this information. In preparing the network for restoration, there are two strategies for breaker operation: the most used “all-open” strategy, which opens all breakers in the affected area to simplify status determination and restoration, and the “selective” strategy, which opens breakers only when needed and requires accurate knowledge of the breaker status [1].

At the same time, the restoration of fault facilities can introduce significant disturbances that are intolerable for the newly restored system. Thus, the availability of power equipment in outage areas should be comprehensively evaluated so that the restoration of vulnerable and risky facilities can be alleviated [3]. Facilities that have suffered from failures are considered more vulnerable and should not be prioritized during restoration. Modern fault diagnosis methodologies can identify the fault devices based on alarm information derived from an online database and predefined logics of protection relays.

Figure 2. Flowchart of availability assessment module to identify faults and BS

3. Restoration strategies

The two most popular strategies are the build-down and the buildup strategies, depending on the network structure and generation portfolio. Additionally, a third hybrid strategy combines elements from both approaches. The build-down restoration strategy, as the name suggests, first reenergizes the entire transmission system and then recovers generation capacity and loads sequentially. The buildup strategy is favored in most power systems, where more than the initial power sources are needed to energize and stabilize the transmission network. The buildup strategy can be simplified into three stages: preparation, system restoration, and load restoration. In the preparation stage, the post-disturbance system status is evaluated, a target system for restoration is defined, and the system is sectionalized into a few subsystems, each of which has initial sources of power. Sectionalization of a system allows parallel restoration and thus reduces restoration time [1].

4. Sectionalization

The sectionalization problem partitions a large system into a set of smaller subsystems, each with an initial power source. Sectionalizing the system is beneficial for the following reasons: (1) Black-start resources are scarce. Partitioning the system into subsystems can better utilize the black-start resources and achieve a faster restoration with subsystems restored in parallel. (2) Generators lose synchronism in a large-scale blackout.

To avoid severe voltage fluctuations it is often practical to partition the generators into multiple smaller self-supporting subsystems or islands before re-synchronizing them. Within each subsystem or island, the initial power source (BS generator), is used to energize transmission paths, restart non-black-start generators, and restore generation capability. While only BS generators provide initial power, NBS generators can also provide power once they are online to assist in restarting other generators. Before they restart, however, each NBS generator consumes a certain amount of cranking power for a given duration of time to restart. This makes the resulting scheduling problem complicated [2].

In the buildup strategy after a complete blackout, a power system is often intentionally sectionalized into subsystems by the system operators. There are several considerations in sectionalization [1].

• Generation-load balance: In each subsystem, the difference between generation capacity and loads should be kept minimal, to avoid excessive over or under-frequency deviations.
• Reactive power balance: Voltage security is a critical issue at the early stage of the restoration process since the charging currents of a transmission line with a light load could incur an increase in voltage across the system.
• Generator cohesion: Generators with similar rotor angles, before they are offline, are said to be cohesive and should be grouped into the same subsystem so that reconnecting them to the grid is less challenging.
• Connectivity: Subsystems will eventually be connected and synchronized. The more contact points between subsystems, the more challenging the synchronization will be, so connectivity among subsystems is an important consideration.

How the system is sectionalized and where the boundaries of the sections depend on several factors such as capacities of BS units, voltage and frequency control requirements, and utilities’ jurisdictional boundaries. There are different criteria for the sectionalization of a power system during restoration. The network may be sectionalized in a way that the synchronous requirement and load-generation balance requirement are satisfied, or the grid is sectionalized based on the BS service cost. The system may also be sectionalized into a set of subsystems such that the number of intersystem transmission lines is minimal.

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In practice, the sections are formed by leaving certain breakers open as presented in Figure 3.

Figure 3. Sectionalization [1]

5. Transmission Path Energization

A transmission path is established to send power to restart NBS units or pick up loads. Transmission lines with lower voltages are preferred to avoid voltage violations because of their smaller charging currents. Since there are normally multiple paths in the meshed transmission network, operators need to select the most reliable and secured path. A transmission path is often selected as the shortest path between the energized network and a target bus regarding charging currents (or switching operations to connect all the lines on the path). Important transmission paths are often enhanced by utilities to secure their availability for restoration purposes [1]. As more transmission paths are energized, the energized block expands. To prevent circular power flows at the early stage of the restoration process, the transmission network is operated in a radial structure.

Figure 4. Initial power system restoring process [3]

Eventually, multiple paths may exist from the energized network to a generator to be restarted. A path that can be reliably energized and helps maintain voltage security should be selected. The charging current of lines can lead to serious steady-state voltage security problems and transformers may increase the likelihood of ferroresonance. Therefore, when determining a cranking path, it is essential to choose lines with minimal susceptance to reduce the impact of the charging current [1]. Furthermore, transformers should be avoided in a cranking path. The capacitors should also be switched off during the early stages of restoration.

7. Generator restart and load pick-up

Generator restart is the most complicated operational decision during power system restoration. The goal at this stage is to restore the generation capacity as fast as possible. At the same time, critical loads must be served within their time limits and the system stability must be maintained. As shown in Fig. 5, an NBS generator first consumes a certain amount of cranking power for a certain amount of time, and then it starts to produce power. Cranking power is the power supply required to prepare a power plant before it can produce power. The capacity curve P(t) of an NBS generator i with Ci MW of cranking power, a period of cranking time and a period of ramping time, and full capacity at Pi.

Figure 5. Power generation curve of an NBS generator

Note that each generator has its ramping rate (the rate generator increases its power output) and maximal capacity.

To maintain system stability (voltage and frequency) as generation capacity increases, a certain amount of load may be served, the amount served at one time being limited by the prime mover of the system. In general, the load should not exceed a certain percentage of the total online generation capacity of the subsystem to prevent frequency issues [1]. Some loads are categorized as critical loads, such as nuclear plant shutdown, traffic lights, hospitals, and police stations. The critical loads must be restored within certain time limits. Load pickup must be closely coordinated with generation restoration.

7. System Synchronization

After restoring the majority of generation and loads and stabilizing subsystems, the next step involves reconnecting and synchronizing these subsystems. Transmission operators play a key role in coordinating the resynchronization process, ensuring that voltage, frequency, and phase angle align. Specifically, the voltage magnitudes at both endpoints of the connecting line should be closely matched, frequencies must be nearly identical, and the voltage phase angle within allowable tolerance levels, as defined by system engineers for the specific area of synchronization [1].

Major Challenges in Power System Restoration

the power system restoration stage involves tackling a multitude of challenges, each critical to the process. As the system is meticulously built up, numerous operations such as breaker switching and topology changes occur, potentially leading to severe disturbances and security or stability issues. Here are some key challenges faced during this intricate stage:

• Voltage control
• Frequency control
• Distinct generator characteristics
• Loss of substations during the outage
• Fault location
• Automatic protective relays under restoration conditions
• Renewable energy source (RES) integration
• RES forecast accuracy for generator scheduling

Conclusions

The restoration process involves a series of steps, from identifying the cause of the outage to prioritizing the restoration of critical loads. Restoration teams meticulously assess the grid’s condition, isolating damaged components and rerouting power to affected areas. This complex operation requires collaboration between grid operators, utilities, and emergency personnel, who work tirelessly to bring power back to communities as quickly as possible.

We’ve delved into the intricate steps, from black-start capabilities to transmission path energization, understanding how these pivotal actions bring life back to our grids. Exploring strategies and challenges, we’ve witnessed the complex problems of rebuilding power networks, emphasizing the critical need for resilience in our systems. If you want to go more into detail, I fully recommend checking the references 🔎

This exploration of power system restoration is just the beginning. Let’s continue learning together. Follow me on Medium for more insights, discussions, and a front-row seat to the dynamic landscape of power systems and their restoration. Together, we shape the future of energy resilience. Follow now and stay electrified! ⚡👏🏻👏🏻

References

1. F. Qiu and P. Li, An Integrated Approach for Power System Restoration Planning, in Proceedings of the IEEE, vol. 105, no. 7, pp. 1234–1252, July 2017, doi: 10.1109/JPROC.2017.2696564
2. S. Chopra, F. Qiu, and S. Shim (2022) Parallel Power System Restoration. INFORMS Journal on Computing 35(1):233–247. doi: 10.1287/ijoc.2022.1258
3. W. Liu, J. Zhan, C. Y. Chung and L. Sun, Availability Assessment Based Case-Sensitive Power System Restoration Strategy, in IEEE Transactions on Power Systems, vol. 35, no. 2, pp. 1432–1445, March 2020, doi: 10.1109/TPWRS.2019.2940379.
4. Y. Liu, R. Fan, and V. Terzija, Power system restoration: a literature review from 2006 to 2016, in Journal of Modern Power Systems and Clean Energy, vol. 4, no. 3, pp. 332–341, July 2016, doi: 10.1007/s40565–016–0219–2.