By Chen-Ching Liu, Jing Xie, and Ruoxi Zhu, Vikas Singhvi and Navin Bhatt
To increase power grid resiliency against catastrophic power outages caused by extreme events such as hurricanes, existing technology and available resources for power system recovery are lacking. The Puerto Rico power grid 2017 black-out, the largest one in the U.S. history in terms of customer hours, and the painfully slow recovery, is a powerful reminder of the urgent need for better engineering and planning with the necessary resources. One of the critical issues in power system restoration is the blackstart capability of the power grid. A sufficient level of blackstart capability enables the grid to restart large, non-blackstart generating units and pick up critical load and services. In power industry worldwide, this important task is primarily handled by power system restoration planners with a limited set of computational tools for power flow and system dynamics. In the last 30 years, research in the development of decision support tools for the optimal installation and use of blackstart capability has led to various optimization techniques. The available techniques, however, are primarily heuristic, e.g., various intelligent system methods. The other weakness is that these optimization tools are designed to determine the sequencing of restoration actions but they do not interact with existing power system dynamic simulation tools. Without thorough evaluation of the steady and dynamic states of blackstart restoration, the technical feasibility of the actions cannot be established.
Power system restoration planners face two major challenges: 1) Is there sufficient blackstart generation capability? 2) What are the optimal locations and sizes for additional blackstart units given the forecasted grid and load conditions and in light of the changing generation mix? Researchers from the Electric Power Research Institute (EPRI) and Virginia Tech have achieved a significant breakthrough in the innovation and implementation of an advanced decision support tool – Optimal Blackstart Capability (OBC). The OBC tool, available through EPRI, allows the system restoration planner to evaluate various options of the amount and location of blackstart capability. Based on an advanced integer optimization method, the OBC tool can identify the sequence of restoration actions to maximize the available generation capability of the power grid, while restoring the system following a blackout. An Optimal Power Flow (OPF) is used to determine whether operating constraints will be met and, if not, find the appropriate control settings to establish feasibility. The transformation of the formulation into a linear integer optimization problem ensures that the solution is globally optimal. This is a major step forward as previous system restoration planning algorithms tended to be heuristic without the guarantee of global optimality. Furthermore, for the OBC tool, a time-domain simulation module is developed for evaluating dynamic performance of the step-by-step facility energization sequence, utilizing the widely adopted power system simulation tool, Power System Simulation for Engineering (PSS®E). Both fixed and switched shunt devices (e.g., capacitors, reactors, static VAR compensators, and synchronous condensers) are included in the optimization objective function with priorities to energize buses with these MVar resources thus improving OPF convergence. As a practical application tool, OBC supports decision-making of power system restoration in an interactive manner. The OBC tool’s graphical user interface allows the user to input system data in PSS®E .raw file format, display the results graphically, and provide an animation to visually track restoration progress.
The generation mix is undergoing a major change, with retirement of fossil-fuel generators and penetration of renewable generation. Blackstart units (BSUs) may retire due to aging or regulatory requirements. New BSUs are needed to meet the load growth or increased reliability and resiliency requirements. The OBC tool as an advanced solution enables the industry to systematically evaluate the benefits of additional black start capabilities in terms of reduced restoration time and increased generation capability.
Over the last several years, there is a growing recognition of the need by government and industry to enhance the resiliency of the grid with respect to extreme events. To this end, the traditional concepts and tools are no longer appropriate. Power grids need to be planned and engineered with the capability to recover from catastrophic outages and serve critical load in a timely manner. Existing tools such as various probabilistic indices for loss of load are not tailored for rare and extreme events. For hypothesized power outage scenarios, an important metric for the grid resiliency is the ability and efficiency to maximize the generation capability, pick up critical load and services during system restoration. The OBC tool provides a powerful capability to quantify the proposed metric for planning and evaluation of new blackstart and control resources. To achieve a smarter power grid in the future, close coordination is necessary between transmission and distribution systems. The importance and urgency are increasing as more and more distributed resources, such as microgrids, energy storage, renewable energy, and distributed generation, are deployed in distribution systems.
EPRI’s collaborative research model provides a platform to work with the utility industry to develop innovative tools that meet the practical needs of the industry. The OBC tool has evolved through demonstrations performed on multiple members’ power systems that included realistic power system restoration scenarios. The demonstrations provided an opportunity to evaluate the effectiveness of the embedded algorithms, relatively to existing restoration plans and scenarios employed by utilities. The lessons learnt were used to improve the underlying tool approach and algorithms. Currently, EPRI members use the OBC tool to review the current restoration plans with regards to (1) verifying the adequacy of existing blackstart resources, and (2) verifying the cranking paths to start up all non-blackstart generators and to deliver safe shutdown power to the nuclear plants. Additionally, OBC can be used to identify alternate blackstart resources and alternate cranking paths, which provide assessment of system redundancy, a measure of power system resiliency.
As an example, the OBC tool was used to support the blackstart procurement process by an EPRI member, which has previously employed methods based on heuristic approaches. As a result, multiple new units were identified to be suitable to provide blackstart services, compared to the practice used previously.
The OBC tool is expected to fill the void in the area of tools available to verify the adequacy of available blackstart resources, to identify new suitable blackstart resources, and to assess system redundancy and resiliency from the viewpoints of having adequate alternate blackstart resources and cranking paths. These capabilities are essential in today’s power system environment that consists of rapidly changing generation mix due to retirement of fossil-fired generation and integration of renewable generation. While these benefits cannot be quantified in dollars, the strategic benefits are tremendous from the power system reliability perspective. Furthermore, improvements in restoration plans provided by the OBC tool could potentially reduce restoration time, in addition to suggesting optimum blackstart resources and cranking paths.
This article was edited by Mehrdad Rostami.
Dr. Chen-Ching Liu is currently American Electric Power Professor and Director, Power and Energy Center (PEC) at Virginia Tech. He is also Research Professor at Washington State University, and Visiting Professor at University College Dublin, Ireland..
Dr. Jing Xie received his Ph.D. from University College Dublin, Dublin, Ireland, in 2015. His research interests include distributed control of power systems, distribution system operation, and cyber-physical system testbed technologies
Ruoxi Zhu is pursuing a Ph.D. degree in electrical engineering at Virginia Tech, Blacksburg, VA, USA. Her research interests include power system stability and cyber security of power systems.
Vikas Singhvi is a senior technical leader in the Grid Operation and Planning Group of the Power Delivery & Utilization Sector at EPRI. His research interests include transmission system modeling & simulation, power system restoration, and bulk system integration of inverter-based generation.
Dr. Navin Bhatt has over 40 years of industry experience focused on advanced transmission technologies and power system dynamics. He was a member of the NERC technical team that investigated the August 14, 2003 blackout on behalf of the U.S. and Canadian governments.