David J. Hill

The University of Hong Kong | HKU · Department of Electrical and Electronic Engineering

Multimicrogrid (MMG) systems are inherently prone to instabilities, while the topological flexibility of their interconnection networks has potential to stabilize the system, which has not been explored. This paper proposes a novel emergency control approach for MMGs, i.e., topology control, which switches between different topologies of the interc...

This paper develops a new distributed attention-enabled multi-agent reinforcement learning method for frequency regulation of power systems. Specifically, the controller of each generator is modelled as an agent, and the reward and observation are designed based on the characteristics of power systems. All the agents learn their own control policie...

The impasse surface is an important concept in the differential-algebraic equation (DAE) model of power systems, which is associated with short-term voltage collapse. This paper establishes a necessary condition for a system trajectory hitting the impasse surface. The condition is in terms of admittance matrices regarding the power network, generat...

This paper focuses on the $N-k$ security-constrained optimal transmission switching (OTS) problem for variable renewable energy (VRE) penetrated power grids. A new three-stage stochastic and distributionally robust OTS model is proposed. The first stage has the primary purpose to schedule the power generation and network topology based on the for...

This paper focuses on the N-k security-constrained optimal transmission switching (OTS) problem for variable renewable energy (VRE) penetrated power grids. A new three-stage stochastic and distributionally robust OTS model is proposed. The first stage has the primary purpose to schedule the power generation and network topology based on the forecas...

Power system transient stability has been translated into a Lyapunov stability problem of the post-disturbance equilibrium for decades. Despite substantial results, conventional theories suffer from the stringent requirement of knowing the post-disturbance equilibrium a priori. In contrast, the wisdom from practice, which certificates stability by...

This paper proposes a robust transient stability constrained optimal power flow (OPF) problem that addresses the impact of renewable uncertainties by the coordination of power flow router (PFR) tuning and generation re-dispatch. PFR refers to a general type of network-side controller that enlarges the feasible region of OPF problem. The coordinatio...

The topology transition problem of transmission networks is becoming increasingly crucial with topological flexibility more widely leveraged to promote high renewable penetration. This paper proposes a novel methodology to address this problem. Aiming at achieving a bumpless topology transition regarding both static and dynamic performance, this me...

The topology transition problem of transmission networks is becoming increasingly crucial with topological flexibility more widely leveraged to promote high renewable penetration. This paper proposes a novel methodology to address this problem. Aiming at achieving a bumpless topology transition regarding both static and dynamic performance, this me...

Network topology has significant impacts on operational performance of power systems. While extensive research efforts have been devoted to optimization of network topology for improving various system performances, the problem of how to transition from the initial topology to the desired optimal topology requires study. To address this problem, we...

Network topology has significant impacts on operational performance of power systems. While extensive research efforts have been devoted to optimization of network topology for improving various system performances, the problem of how to transition from the initial topology to the desired optimal topology requires study. To address this problem, we...

High penetration of renewable generation poses challenges to power system operation due to its uncertain nature. In droop-controlled microgrids, the voltage volatility induced by renewable uncertainties is aggravated by the high droop gains. This paper proposes a chance-constrained optimal power flow (CC-OPF) problem with power flow routers (PFRs)...

Flexible transmission line impedances on one hand are a promising control resource for facilitating grid flexibility, but on the other hand add much complexity to the concerned optimization problems. This paper develops a convexification method for the AC optimal power flow with flexible line impedances. First, it is discovered that a flexible-impe...

This paper focuses on the issue of network connectedness (NC) in security-constrained optimal transmission switching problems, which is complicated by branch contingencies and corrective line switching. Two criteria are firstly proposed with the principle of preserving NC as much as possible within reasonable limits. By extending the electrical flo...

Power grids are critical infrastructure in modern society, and there are well-established theories for the stability and control of traditional power grids under a centralized paradigm. Driven by environmental and sustainability concerns, power grids are undergoing an unprecedented transition, with much more flexibility as well as uncertainty broug...

Transitions of power networks demand more powerful solution approaches for optimal transmission switching (OTS) and distribution network reconfiguration (DNR) to tackle complicated cases with high computational efficiency. In this paper, we propose a learning-based solution approach for OTS and DNR, which is established based on an identified commo...

p>This paper focuses on the issue of network connectedness (NC) in security-constrained optimal transmission switching problems, which is complicated by branch contingencies and corrective line switching. Two criteria are firstly proposed with the principle of preserving NC as much as possible within reasonable limits. By extending the electrical...

In Internet of Things (IoT)-enabled modern power grids, advanced IoT devices, e.g., synchronous phasor measurement units (PMUs), have been widely deployed to closely monitor the grids’ states and dynamics. In practice, however, PMU measurements are often contaminated with anomalous (low-quality) data, e.g., data spikes, unchanged data, data losses/...

Short-term voltage stability (STVS) is closely related to load dynamics. Recently, load models have been periodically tracked through ambient signal-based identification, through which the role of load dynamics in STVS can be elaborated in more detail. In this paper, a load dynamic stability index (LDSI) is proposed, which can be applied in STVS as...

Distribution systems are becoming more active and provide more power support for transmission systems via their connection points at the transmission-distribution (T-D) interfaces. This paper proposes a chance constrained optimization model to estimate the flexibility area at a T-D interface. In this model, network flexibility is for the first time...

Since the publication of the original paper on power system stability definitions in 2004, the dynamic behavior of power systems has gradually changed due to the increasing penetration of converter interfaced generation technologies, loads, and transmission devices. In recognition of this change, a Task Force was established in 2016 to re-examine a...

To achieve a better frequency control performance for multi-area power systems, this paper proposes a novel decentralized event-triggered control (ETC) algorithm for automatic generation control (AGC) to replace the traditional periodic sampling/communication mechanism. For each control area, decentralized event-triggering conditions that only depe...

Network connectedness is indispensable for the normal operation of transmission networks. However, there still remains a lack of efficient constraints that can be directly added to the problem formulation of optimal transmission switching (OTS) to ensure network connectedness strictly. To fill this gap, this paper proposes a set of linear connected...

This paper proposes a novel distributed model predictive control (DMPC) scheme for frequency regulation of multi-area power systems with substantial renewable power sources and different types of controllable units including synchronous generators, flexible loads and energy storage devices. The frequency regulation task is firstly formulated as a m...

This paper investigates the potential of wind turbine generators (WTGs) and load aggregators (LAs) to provide supplementary damping control services for low frequency inter-area oscillations (LFOs) through the additional distributed damping control units (DCUs) proposed in their controllers. In order to provide a scalable methodology for the increa...

This paper establishes a general framework for network optimization in DC systems, namely the optimal admittance matrix (OAM) problem, which finds the optimal admittance matrix that minimizes power loss given the power injection information. The OAM problem offers a complementary viewpoint to the optimal power flow which finds the optimal power inj...

Rotor angle stability refers to the ability of synchronous machines in a power system to remain in synchronism after a disturbance. It is one of the basic requirements for secure operation of electric power systems. Traditional analysis methods for rotor angle stability are oriented to node dynamics, especially the impact of generator modeling and...

1)About the type of of RES-DG (wind -based or solar -based) used in case study and the efficacy of the proposed methodology under time -varying scenarios.In addition, we agree with the discusser that the proposed methodology aims to obtain a steady-state voltage profile in a single operating scenario that has lower risk of voltage violation in case...

High penetration of renewable generation poses great challenge to power system operation due to its uncertain nature. In droop-controlled microgrids, the voltage volatility induced by renewable uncertainties is aggravated by the high droop gains. This paper proposes a chance-constrained optimal power flow (CC-OPF) problem with power flow routers (P...

In modern smart grids deployed with various advanced sensors, e.g., phasor measurement units (PMUs), bad (anomalous) measurements are always inevitable in practice. Considering the imperative need for filtering out potential bad data, this paper develops a novel online bad PMU data detection (BPDD) approach for regional phasor data concentrators (P...

Load modelling is significant to ensure the accuracy of power system simulation. In previous research on load modelling, various optimisation algorithms have been widely applied. However, the achievement of the global optimal solution depends on the quality of the initial feasible solutions (IFSs). In this study, an enhanced measurement‐based load...

In order to deal with issues caused by the increasing penetration of renewable resources in power systems, this paper proposes a novel distributed frequency control algorithm for each generating unit and controllable load in a transmission network to replace the conventional automatic generation control (AGC). The targets of the proposed control al...

The impasse surface is an important concept in the differential-algebraic equation (DAE) model of power systems, which is associated with short-term voltage collapse. This paper establishes a necessary condition for a system trajectory hitting the impasse surface. The condition is in terms of admittance matrices regarding the power network, generat...

Flexible transmission line impedances on one hand are a promising control resource for facilitating grid flexibility, but on the other hand add much complexity to the concerned optimization problems. This paper develops a convexification method for the AC optimal power flow with flexible line impedances. First, we discover that a line with flexible...

Network connectedness is indispensable for the normal operation of transmission networks. However, there still remains a lack of efficient, necessary and sufficient constraints that can be directly added to optimization models of optimal transmission switching (OTS) to ensure network connectedness. To fill this gap, this letter proposes a necessary...

It can be expected that the power systems of the future will be significantly different from today's, especially due to increasing renewable energy sources (RESs), storage systems, and price-responsive users leading to large uncertainty and complexity. The operation of these future grids (FGs) at levels of renewable energy approaching 100% will req...

This paper proposes a robust transient stability constrained optimal power flow problem that addresses renewable uncertainties by the coordination of generation re-dispatch and power flow router (PFR) tuning.PFR refers to a general type of network-side controller that enlarges the feasible region of the OPF problem. The coordination between network...

Widespread utilization of renewable energy sources (RESs) in subtransmission systems causes serious problems on power quality, such as voltage violations, leading to significant curtailment of renewables. This is due to the inherent variability of renewables and the high R/X ratio of the subtransmission system. To achieve full utilization of renewa...

A task force set up jointly by IEEE Power System Dynamic Performance (PSDP) Committee and CIGRE had addressed the issue of stability definition and classification in power systems from a fundamental viewpoint and had closely examined the practical ramifications. The relevant report published in 2004, primarily dealt with fairly slow, electromechani...

In modern smart grids deployed with various advanced sensors, e.g., phasor measurement units (PMUs), bad (anomalous) measurements are always inevitable in practice. Considering the imperative need for filtering out potential bad data, this paper develops a simple yet efficient online bad PMU data detection approach by exploring spatial-temporal cor...

In this paper, we study the vulnerability of the minimum cut-set (MCS) in power networks by assessing the effect of sequential attacks against the MCS and its corresponding effect on cascading failures triggering. Concepts of flow-networks and the DC power flow are integrated into the system model. Computational algorithms are developed to identify...

Due to the high flexibility and fast response capability, power electronic devices are versatile, and increasingly being considered to mitigate voltage violations and improve system efficiency in distribution networks with high penetration of distributed generation (DG). This paper proposes a novel voltage regulation framework in distribution syste...

In order to deal with issues caused by the increasing penetration of renewable resources in power systems, this paper proposes a novel distributed frequency control algorithm for each generating unit and controllable load in a transmission network to replace the conventional automatic generation control (AGC). The targets of the proposed control al...

In this article, we propose a learning-based fault diagnosis approach for a class of nonlinear sampled-data systems. First, the unmodeled sampled dynamics is acquired by the using deterministic learning method. The knowledge of the sampled dynamics of the normal and fault patterns is stored in the form of constant neural networks. Second, a fault d...

This paper investigates the utilization of transmission switching to improve synchronization performance of low-inertia grids. The synchronization performance of power girds is first measured by the H2 norm of linearized power systems. Laplacian-based bounds and a close-form formulation of the H2-norm synchronization performance metric are derived...

This paper develops a hierarchical deep learning machine (HDLM) to efficiently achieve both quantitative and qualitative online transient stability prediction (TSP). For the sake of improving its online efficiency, multiple generators' fault-on trajectories as well as the two closest data-points in pre-/post-fault stages are acquired by PMUs to for...

This paper proposes an optimal voltage control method for critical buses in an active distribution system using electric springs (ESs). The proposed optimization model relies on both an accurate ES and load model. The real-time load parameters are determined using the non-intrusive load monitoring (NILM) technique, which provides a flexible and con...

This paper extends the definitions of effective resistance and effective conductance to characterize the overall relation (positive coupling or antagonism) between any two disjoint sets of nodes in a signed graph. It generalizes the traditional definitions that only apply to a pair of nodes. The monotonicity and convexity properties are preserved b...

This paper addresses the optimal allocation of battery energy storage systems (BESS) in radial distribution systems for bus voltage regulation and energy cost reduction. A hierarchical planning model is proposed to obtain the optimal configuration (number, locations, and sizes) of BESS. The three interacting levels in the proposed model are: (1) de...

With the expansion of power system scale, more and more network systems are interconnected to form a huge interdependent network. The most notable of these is the power-communication interdependent network. Modern power systems rely heavily on being controlled via communication networks; in reverse, power supply is the foundation of maintaining the...

This paper addresses the optimal allocation of the electric spring (ES) smart load device in radial distribution systems for voltage regulation. An ES can transform a connected noncritical load into a smart load providing voltage support and voltage suppression functions. A risk-limiting ES planning method is proposed to obtain the optimal ES confi...

Volatile voltage profiles in distribution systems caused by the fluctuating nature of renewable distributed generation (DG) are attracting growing concern. In this paper we develop a new formulation of network reconfiguration to mitigate voltage volatility. This formulation provides new insights into the voltage regulation problem with high renewab...

Prediction of vulnerable lines during cascading failures is an important issue for power networks. The identification of key lines may enable the application of targeted countermeasures and reduce cascading failures effects. Based on topological information and line power transmission capacity, we propose the use of cut-sets (CS) to quantify the li...

This paper extends the definitions of effective resistance and effective conductance to characterize the overall relation (positive coupling or antagonism) between any two disjoint sets of nodes in a signed graph. It generalizes the traditional definitions that only apply to a pair of nodes. The monotonicity and convexity properties are preserved b...

This paper investigates the potential of wind turbine generators (WTGs) and load aggregators (LAs) to provide supplementary damping control services for low frequency inter-area oscillations (LFOs) through the additional distributed damping control units (DCUs) proposed in their controllers. In order to provide a scalable methodology for the increa...

In static voltage stability analysis, participation factors have been used as an index that measures the contribution of the critical mode of power flow Jacobian in the system states. However, this index as usually defined is in a mode-in-state manner that cannot adequately reflect the impact of system states on voltage collapse. In this paper we t...

Data integrity of power system states is critical to modern power grid operation and control. Due to communication latency, state measurements are not immediately available at the control center, rendering slow responses of time-sensitive applications. In this paper, a new graph-based deep learning approach is proposed to recover and predict the st...

This paper presents a frequency control scheme for transmission networks with multiple control areas using load-side controllers in cooperation with automatic generation control (AGC) to regulate the system frequency and the tie-line powers between control areas. In transmission networks, a switched consensus-based distributed controller is propose...

High intermittency renewable generation reduces the system inertial and increase system uncertainties introducing new challenges for frequency regulation of future grid, including primary, secondary, and tertiary ancillary services. Residential air-conditioning contributes to electric utility critical peak load where the frequency regulation reserv...

This paper investigates the stabilization issue via event‐triggered controls (ETCs) for discrete‐time delayed systems (DDSs) and networks. Based on the recently proposed ETC scheme for discrete‐time systems without time delays, improved ETC (I‐ETC) and event‐triggered impulsive control (ETIC) are proposed for DDS. The algorithms for ETC, I‐ETC, and...

With the integration of more renewable energy, operational flexibility becomes a bottleneck for power system operation. Pumped storage units can enhance their operational flexibility by switching between different operation modes. They have limited capacity for adjustment when operated as generators and are non-adjustable when operated in the pump...

Transient stability assessment is critical for power system operation and control. Existing related research makes a strong assumption that the data transmission time for system variable measurements to arrive at the control center is negligible, which is unrealistic. In this paper, we focus on investigating the impact of data transmission latency...

This paper investigates small-disturbance angle stability of power systems with emphasis on the role of power network topology, which sheds new light on the instability mechanism. We introduce the concepts of active power flow graph and critical lines. It is shown that the inertia of the Laplacian matrix of this graph provides information on the st...

A cutset is a concept of importance in both graph theory and many engineering problems. In this paper, cutset properties are studied and applied to transient stability problems in power systems. First, we reveal that cutsets are intrinsically linked to cycles, another important concept in graph theory. The main theoretical results include that any...

In this paper, the impact of load dynamics on the oscillatory stability of power systems is studied. The well-known composite load model is linearized to yield third order transfer functions for active and reactive power dynamics of loads. It is shown that the third order model exhibits a recovery response that is accompanied by damped oscillations...

To implement controllable loads for frequency regulation in transmission networks in a practical way, the control scheme needs to be granulated down at least to subtransmission networks since loads in transmission networks are usually the aggregation of lower voltage networks. However, not only frequency but also bus voltage will be affected by act...

This paper presents a novel delay aware synchrophasor recovery and prediction framework to address the problem of missing power system state variables due to the existence of communication latency. This capability is particularly essential for dynamic power system scenarios where fast remedial control actions are required due to system events or fa...

The increasing popularity of electric vehicles (EVs) has made electric transportation a popular research topic. The demand for EV charging resources has significantly reshaped the net demand profile of power distribution systems. This paper proposes an online optimal charging strategy for multiple EV charging stations in distribution systems with p...

This paper investigates the issue of stabilization for discrete-time dynamical systems (DDS) by event-triggered impulsive control (ETIC). Based on some relatively simple threshold constants, three levels of event conditions are set and thus the ETIC scheme is designed. Three cases for ETIC with and without time-delays and data dropouts are studied...

Wide area measurement system (WAMS) is one of the essential components in the future power system. To make WAMS construction plans, practical models of the power network observability, reliability, and underlying communication infrastructures need to be considered. To address this challenging problem, in this paper we propose a unified framework fo...

Recent advances in a power electronic device called an electric spring (ES) provide feasible solutions to meeting critical customers’ requirements for voltage quality. A new version of the ES was introduced based on a back-to-back converter (ESBC) configuration which extends the operating range and improves the voltage suppression performance to fa...

In this paper, a distributed framework is established to evaluate and improve the small-disturbance stability of a microgrid with inverter-based distributed generators (DGs). First, we propose a distributed protocol such that a selected pilot DG bus collects necessary data from finite steps of communication between physically neighbouring buses. Th...

The increasing penetration of non-synchronous renewable energy sources (NS-RES) and demand side-technologies alter the dynamic characteristics, and particularly, the frequency behavior of a power system. Given this, we propose a framework for assessing renewable integration limits concerning power system frequency performance using a time-series sc...

In this paper, a distributed control scheme is proposed for voltage control and power management of coupled microgrids (MGs). The control scheme is implemented in a two-level structure. In the primary level, a voltage-current droop controller is adopted in the dq reference frame to facilitate proportional load sharing among the distributed energy r...

This article studies KL-stability (the stability expressed by KL-class function) for a class of hybrid dynamical systems (HDS). The notions ofKLK-property andKL-stability are proposed for HDS with respect to the hybrid-event-Time. The KL-stability, which is based on K or L property of the continuous flow, the discrete jump, and the event in an HDS,...

This paper proposes a novel approach on controller design for a doubly fed wind generator based on Input-to-State stability (ISS) theory. In order to guarantee the stability of the nonlinear system with external disturbances, a systematic methodology for constructing locally input-to-state stabilizing control Lyapunov functions (LISS-CLF) and desig...

An electric spring (ES) is effective in enhancing the operational flexibility and facilitating renewable energy integration. An ES can determine the operation status of the connected noncritical loads (NCLs) by adjusting its output voltage. It is possible to use NCLs as operation resources for renewable integration. In this paper, a model predictiv...

The changing inertia profile caused by the increased penetration of inertia-less renewable energy sources has raised concerns about power system frequency control. The kinetic energy of the turbines in wind turbine generators (WTGs) can be utilized to support power system frequency during contingencies. In this paper, we investigate the frequency s...

As the power system is facing a transition towards a more intelligent, flexible and interactive system with higher penetration of renewable energy generation, load forecasting, especially short-term load forecasting for individual electric customers plays an increasingly essential role in the future grid planning and operation. Other than aggregate...