Transmission Grid Topology Control Using Critical Switching Flow Based Preventive Stabilizing Redispatch
Authors: Lan Tian , Wang Wenzong , Huang Garng M. 
Published: June 2019

Transmission grid topology control (TC) is an emerging technology proposed in recent years to improve power system performance by using e xisting transmission lines more efficiently. Incorporated into the optimal power flow (OPF) with different objective functions, TC can be used for various purposes, such as operational cost reduction and corrective actions. As a leader of employing TC in the power industry, the Pennsylvania New Jersey Maryland (PJM) Interconnection has identified and published a list of switching solutions for the purpose of thermal limit violation and voltage control. Moreover, with significant penetration of wind and solar generations installed in the grid, the optimal system topology varies with time caused by the uncertainty of intermittent nature. TC has a promising future to track the optimal topology for each hour to reduce the operational cost and the line capacity expansion cost. However, these benefits are only possible if we can overcome the stability issues in operation, which is the focus of our research. A line switching action may introduce a large disturbance in the system and thus transient instability becomes a potential security concern. In previous literature, transient instability is observed in line switching actions in different systems. As a result, some beneficial switching plans are abandoned for the sake of security. Therefore, the key problem is how to enhance system stability to enable beneficial switching actions. To achieve that, a proper index that can be calculated offline and then used online to provide preventive stabilizing control guidelines is desired for TC applications. However, existing indices such as critical clearing time, transient stability index and transient energy function are unsuitable for preventive stabilizing control in TC applications. In addition, these indices are usually used for deterministic studies where the uncertainties in the grid are not considered. In our research work, a new quantitative index, critical switching flow (CSF), is proposed to assess transient stability in TC. CSF is the maximum real power flow allowed on a transmission line so that the system is stable when switching off the line. When a line flow is greater than CSF, the system has a risk of instability when switching off the line. The rigorous mathematical derivation of CSF is provided on a two-machine system and then extended to multi-machine systems. CSF establishes a direct connection between the line flow measurement of a switching target line and system transient stability in TC. Thus, CSF has a natural advantage of providing explicit control instructions in TC over the aforementioned indices. A preventive stabilizing redispatch scheme based on CSF is then proposed to enhance system transient stability in TC applications. First, the CSF values are calculated offline in the day-ahead unit commitment and scheduling stage using our probabilistic algorithm based on PSS/E with precise dynamic models from the industry. The proposed two-stage Monte Carlo simulation algorithm will consider the uncertainties of loads and generations and find the statistically precise CSF, which shows the boundary between stable and unstable switching actions based on the worst scenario. This offline learning process will generate sufficient knowledge for online TC applications. Then, the CSF based preventive stabilizing redispatch scheme will provide control guidelines to find a stable trajectory for an online TC application. Once a switching plan is found unstable in the online stability check, the proposed scheme is carried out first to decrease the line flow to a safe value below the CSF to ensure the stability of the subsequent switching action. The proposed algorithm and scheme are tested on the modified IEEE-118 bus system, which has large scale of wind power and solar power installed. In the numerical studies, the calculated CSF values are valid to avoid all unstable scenarios. Moreover, the proposed redispatch scheme is activated when there are sufficient resources to redispatch and decrease the flow on the switching target line to a value below the CSF. The proposed scheme is activated in 371 out of 800 unstable switching cases in total. All the 371 activated cases are stabilized using our proposed preventive stabilizing redispatch scheme. The proposed scheme will not be activated for the rest of unstable cases because of insufficient resources to redispatch. And details of system response in the proposed scheme are also shown for validation and illustration.