Yuan Liu

Co-simulation offers a time-efficient method for ac-curately analyzing the behavior of distribution systems dominated by inverter-based resources (IBRs). However, most co-simulation studies in the literature overlook distribution grids. Additionally, these studies do not discuss practical IBR models nor how to ini-tialize the interface model variables of co-simulation platforms. To obtain co-simulation results that are reasonably accurate for dis-tribution system-based planning and operation, it is pertinent to develop co-simulation platforms well-suited for studying the accu-rate behavior of IBRs coupled to unbalanced distribution grids. This paper introduces an initialization algorithm for initializing the interface model variables of co-simulation platforms. The ini-tialization algorithm is anchored on leveraging the fast execution speed of the per-phase phasor simulation to quickly pre-calculate initial values of interface model variables prior to starting co-sim-ulation. This paper further proposes the inclusion of rate limiters in the co-simulation platforms interface models to prevent the in-capability of the per-phase phasor simulator to clear faults at zero-crossing of currents from falsely impacting the control system of practical IBR models. The proposed methods are validated by benchmarking the results from a per-phase phasor/electromag-netic transient (EMT) co-simulation of an IBRs-dominated IEEE 34-Node test feeder against the results from a full EMT simulation.

Improving system-level resiliency of networked microgrids against adversarial cyber-attacks is an important aspect in the current regime of increased inverter-based resources (IBRs). To achieve that, this paper contributes in designing a hierarchical control layer, in conjunction with the existing control layers, resilient to adversarial attack signals. Considering model complexities, unknown dynamical behaviors of IBRs, and privacy issues regarding data sharing in multi-party-owned microgrids, designing such a control layer is non-trivial. Here, to tackle these issues, a novel federated reinforcement learning (Fed-RL) method is proposed. To grasp the interconnected dynamics of networked microgrids, the paper develops Federated Soft Actor-Critic (FedSAC) algorithm following the vertical structure of implementing Fed-RL. Next, utilizing the OpenAI Gym interface, we built a custom set-up in GridLAB-D/HELICS co-simulation platform, named Resilient RL Co-simulation (ResRLCoSIM), to train the RL agents with IEEE 123-bus benchmark comprising 3 interconnected microgrids. Finally, the learned policies in the simulation are transferred to the real-time hardware-in-the-loop (HIL) test-bed developed using the high-fidelity Hypersim platform. Experiments show that the simulator-trained RL controllers achieve desirable performance with the test-bed platform, validating the minimization of the sim-to-real gap.

This article describes a positive-sequence model to represent two widely reported droop-controlled grid-forming inverters for transmission system transient stability simulation studies. Methods of how to develop the equivalent voltage source behind impedance to represent inverters with and without inner control loops, modeling of P-f and Q-V droop controls, active and reactive power limiting modeling, and modeling of fault current limiting controls have been described in detail. The model was implemented in commercially available positive-sequence simulation tools and compared with detailed electromagnetic transient models in both a single-grid-forming-inverter infinite-bus system and a modified IEEE 39-bus system. Finally, the model was tested on the U.S. Western Interconnection system. The study results showed that the model has a good level of accuracy compared to the detailed EMT models, and at the same time also achieves a high computational efficiency, making it suitable for large-scale transmission system simulation studies.

The inverter-based resource (IBR) has been an important component of power generation within both transmission and distribution systems. The impacts of high IBR penetration on system transient stability have not been explored extensively due to the lack of high-fidelity power grid and inverter models. The aggregate IBR model in the transmission system is only suitable for representing bulk power system (BPS)-connected IBRs, and it cannot precisely reproduce the dynamics of distributed IBRs in the distribution system due to its oversimplified structure. In this study, we have developed a high-penetration fully-connected transmission and distribution (T&D) co-simulation platform that supports the dynamic co-simulation of a synthetic mini-WECC transmission system with multiple IEEE 8500-node distribution systems. The entire platform can accommodate 10,000+ dispersed and 40+ BPS-connected IBRs and achieve 100% power generation from IBRs. A novel iterative T&D power flow initialization technique was proposed to maintain stable transition from power flow to dynamic co-simulation mode with high penetration of IBR models. This study explores the impacts of grid-forming (GFM) and grid-following (GFL) inverter control on the dynamics of large-scale T&D network. Moreover, a co-simulation platform was developed and tested using high-performance computing (HPC) resources to facilitate parallel simulation.

In the recent years, there has been an increase in distributed inverter-based resources (D-IBRs) in the system that is impacting the grid performance. Dynamic transmission and distribution (T&D) co-simulation is an effective method to evaluate the impact of D-IBRs on the system response, however, implementation is a challenge. This article innovatively achieves accurate T&D model coordination with inclusion of dynamic models of power electronic controls. It also demonstrates that D-IBRs operating with grid-forming (GFM) controls with frequency support capability can aid in primary frequency regulation in a low-inertia power system. The article discusses the fundamentals of dynamic T&D co-simulation to analyze the primary frequency response on the IEEE 39-Bus transmission system coupled to IEEE 123-Bus distribution system with D-IBRs with GFM controls. To demonstrate scalability, a large system (286383 T&D buses) is co-simulated: IEEE 39-bus transmission system with all load buses coupled to a total of 2328 parallel IEEE 123-Bus distribution feeders containing a mix of D-IBRs is considered. The article also discusses the use of parallel computation used for distribution system solutions enabled in the scalable multi-timescale T&D co-simulation framework along with computation time comparisons between the large and small system simulations with GFM inverter dynamics.

Modular multilevel converters (MMCs) are widely used in the design of modern high-voltage direct current (HVdc) transmission system. High-fidelity dynamic models of MMCs-based HVdc system require small simulation time step and can be accurately modeled in electro-magnetic transient (EMT) simulation programs. The EMT program exhibits slow simulation speed and limitation on the size of the model and brings certain challenges to test the high-fidelity HVdc model in system-level simulations. This paper presents the design and implementation of a hybrid simulation framework, which enables the co-simulation of the EMT model of Atlanta-Orlando HVdc line and the transient stability (TS) model of the entire Eastern Interconnection system. This paper also introduces the implementation of two high-fidelity HVdc line models simulated at different time steps and discusses a dedicated method for sizing the buffer areas on both sides of the HVdc line. The simulation results of the two HVdc models with different sizes of buffer areas are presented and compared.

Protection equipment is used to prevent damage to induction motor loads by isolating them from power systems in the event of severe faults. Modeling the response of induction motor loads and their protection is vital for power system planning and operation, especially in understanding system's dynamic performance and stability after a fault occurs. Induction motors are usually equipped with several types of protection with different operation mechanisms, making it challenging to develop adequate yet not overly complex protection models and determine their parameters for aggregate induction motor models. This paper proposes an optimization-based nonlinear regression framework to determine protection model parameters for aggregate induction motor loads in commercial buildings. Using a mathematical abstraction, the task of determining a suitable set of parameters for the protection model in composite load models is formulated as a nonlinear regression problem. Numerical examples are provided to illustrate the application of the framework. Sensitivity studies are presented to demonstrate the impact of lack of available motor load information on the accuracy of the protection models.

Protection strategies for transmission and distribution systems have been extensively investigated to facilitate better coordination of physical protection devices. A diverse range of functional motors with dedicated protection schemes are being used more and more in commercial, residential, and industrial buildings. This paper focuses on simulating several of the most popular protection schemes using the ElectroMagnetic Transient Program (EMTP) model for three-phase and single-phase induction motors in existing commercial buildings connected to typical distribution feeders. To investigate the behaviors of single-phase motors stalling, the actions of motor protection and reconnections, and the impacts of device-level protection on system-level dynamics, we imposed voltage depressions at the head of a feeder fully loaded with functional induction motors. Several distribution feeders are represented in a standard IEEE 39-bus transmission system to simulate fault-induced delayed voltage recovery (FIDVR) and explore mitigation strategies by optimally configuring the building-level motor protection settings.