| NSF Org: |
ECCS Division of Electrical, Communications and Cyber Systems |
| Recipient: |
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| Initial Amendment Date: | May 8, 2017 |
| Latest Amendment Date: | May 8, 2017 |
| Award Number: | 1711432 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Aranya Chakrabortty
ECCS Division of Electrical, Communications and Cyber Systems ENG Directorate for Engineering |
| Start Date: | May 15, 2017 |
| End Date: | April 30, 2021 (Estimated) |
| Total Intended Award Amount: | $358,152.00 |
| Total Awarded Amount to Date: | $358,152.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
201 ANDY HOLT TOWER KNOXVILLE TN US 37996-0001 (865)974-3466 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
TN US 37996-0003 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | EPCN-Energy-Power-Ctrl-Netwrks |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.041 |
ABSTRACT
In recent years, electric power systems have seen a rapid introduction of both new types of generation and loads. Many of these new components interface to the grid through power electronic converters that are controlled by embedded digital signal processors. Such converters have a very different behavior than the electromechanical machines and passive devices that have been predominant in the traditional grid. Power electronic interfaced devices have greater overall controllability because of their fast regulating time and the flexible programming of digital signal processors. Depending on the program settings, they can operate under a variety of different dynamic modes. Moreover, a variety of new complex hybrid dynamics have arisen in the power grid whose mechanisms are poorly understood. This proposal seeks to analyze these new dynamics and provide engineering design guidelines that can provide system-wide performance guarantees.
Due to a rapid increase of power electronic converters interfaced renewable resources and loads interconnected with the more traditional devices in the power grid, new complex hybrid dynamics have arisen. In this project, a set-theoretic framework is introduced and a feature set concept is proposed to explain switching behavior, label discrete events and synthesize modes for modern power systems. Under this principle, a passivity-based barrier certificate framework is formulated as a mathematical program to compute the feature sets of corresponding modes such that the supervisory level synthesis can be established. Moreover by taking advantage of converter controllability, a control Lyapunov-barrier function is employed to design a supplementary control to reduce the conservatism of traditional feedback controllers, enlarge the feature sets and provide system-wide performance guarantees. The mode synthesis and control objective is to ensure states operate within a safe range, i.e., within operational constraints. The main research tasks include: (a) establishing the proper mathematical foundation to estimate safe regions with desired properties, namely, estimate the errors and algorithm convergence; (b) design property-preserving feedback controllers to meet system requirements; (c) integrate designed controllers and existing controllers with nonlinearities (primarily, deadband and saturation) to form a comprehensive hybrid closed-loop system design; (d) synthesize multiple modes among different sources under a centralized, decentralized or distributed structure for a connected network; and (e) address scalability issues through alternative program realization of positivity certificates required in the framework of finite and discrete approximations of infinite dimensional continuous space and model reductions.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
- A theoretical framework was developed for switched multi-mode controllers for safe response. A convex optimization formula was proposed to calculate the switching conditions. By assuming a polynomial structure, the problem can be solved by sum of squares programs. A feasible result will generate a polynomial, the zero-sublevel set of which represents the region of safety (ROS) and is employed as the safety supervisor. A decentralized communication architecture is proposed for small-scale systems. The proposed controller is first verified on a single-machine three-phase nonlinear microgrid, and then implemented on the IEEE 39-bus system.
- Safety analysis of power systems is concerned with the system's ability to have an adequate frequency response within certain limits following a disturbance. Today renewable sources have been sharing a portion of total generation in power systems. This results in a decrease in system inertia and inadequacy of frequency response. Many converters in renewable sources have the capability for certain grid supportive functions, such as inertia emulation or primary frequency support. In these controllers, the input signal is the rate of change of the grid frequency. As simple as they are, they cannot guarantee any response characteristics. The high controllability of power converters has not been fully utilized under such simple control structures. As the improvement of actuators’ controllability and performance, control system verification and design are required. A safety feedback controller has been designed to force the system states to stay within certain sets, as a safety objective, when a fault occurs.
- Most wind turbines and other renewables connected to the grid through converters, cause a reduction in the natural inertial response to grid frequency changes. The doubly fed induction generator (DFIG) can then be controlled to compensate for this reduction and provide faster response than traditional synchronous machines. An observer based output feedback linear quadratic regulator (LQR) with H-infinity control laws based on an inertia emulation strategy to deliver fast frequency support have been developed.
- With the development of power grid interconnection, the power grid is becoming larger and more complex. The general configuration of a modern power system is that power sources and loads are widely dispersed, and the growing trend towards restructuring the power industry and the ever-increasing demand for power exchange calls for employing wide-area measurement systems (WAMS) for almost real-time measurements that could be used to maintain and improve the stability and performance of the system. A WAMS consists of a remote measurement device, e.g., phasor measurement units (PMUs), and a communication network that transmits the measurements to power system controllers. The involvement of the communication network in We investigated the stability problem of wide-area damping controllers with intermittent information transmission such that, the local measurement is exchanged between remote measurements and damping controller, or between the damping controller and damping actuators, over some disconnected time intervals due to unreliability of communication or limitations of sensing ability.
- A secondary voltage control method that can handle temporal logic specifications (TLSs) has been introduced. The control objective is to schedule a control signal for an energy storage system (ESS) such that the voltage variation of a defined critical bus satisfies TLSs. The TLS allows definitions of set and timing constraints at the same time, such as a finite-time restoration. A numerical optimal control (NOC)-based control synthesis approach is proposed to schedule a controller for frequency support to satisfy the TLSs.
- To comply with grid codes and avoid unnecessary relay protection actions, a model predictive control (MPC)-based control strategy employing temporal logic specifications (TLSs) has been introduced, such that, the TLSs strategy is introduced as a formalism to control the voltage variation at a critical load bus against operational bounds over time. The control objective is to schedule optimal control input signals from the available supportive energy storage systems, which provide reactive power injections, leading to satisfying the specified finite-time restoration described by the TLSs with minimal control efforts. The simulation results display that the TLSs strategy for power systems’ voltage control synthesis is extremely significant.
- New conditions for stabilizing dwell time and average dwell time of switched systems which evolve on non-uniform time domain using time scale theory have been derived. The considered system switch between continuous-time subsystems with variable interval length and discrete-time subsystems with variable discrete steps. The stability conditions have been derived using the general solution of the switched system computed by introducing Gronwall’s inequalities, and minimum dwell time conditions have been derived by introducing the Time Scales Lyapunov Function (TSLF). Cases of presence of unstable modes has been considered.
- Stability of this class of switched systems (continuous/discrete) on non-uniform time domain has been studied using Time Scale Multiple Lyapunov Function (TSMLF). The stability conditions have been implemented in the problem of consensus for multi-agent systems with intermittent information transmission.
Last Modified: 05/17/2021
Modified by: Kevin L Tomsovic
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