Western North American Power System Research Articles

Feedback Control Strategy for Transient Stability Application

Power systems are subjected to a wide range of disturbances during daily operations. Severe disturbances, such as a loss of a large generator, a three-phase bolted fault on a generator bus, or a loss of a transmission line, can lead to the loss of synchronism of a generator or group of generators. The ability of a power system to maintain synchronism during the few seconds after being subjected to a severe disturbance is known as transient stability. Most of the modern methods of controlling transient stability involve special protection schemes or remedial action schemes. These special protection schemes sense predetermined system conditions and take corrective actions, such as generator tripping or generation re-dispatch, in real time to maintain transient stability. Another method is the use of a real-time feedback control system to modulate the output of an actuator in response to a signal. This paper provides a fundamental evaluation of the use of feedback control strategies to improve transient stability in a power system. An optimal feedback control strategy that modulates the real power injected and absorbed by distributed energy-storage devices is proposed. Its performance is evaluated on a four-machine power system and on a 34-machine reduced-order model of the Western North American Power System. The result shows that the feedback control strategy can increase the critical fault clearing time by 60%, thereby improving the transient stability of the power system.

Variable Loop Gain Using Excessive Regeneration Detection for a Delayed Wide-Area Control System

Excessive time delay in the implementation of a wide-area damping controller may lead to poor time-domain response or instability of an otherwise stable closed loop system. Careful monitoring of the controller output reveals the relative phase margin as it tends toward a vanishingly small value. Design and analysis of the excessive regeneration detector for system stability migration is presented. Details of degradation are followed by the description of a variable loop gain controller to ensure stability in the presence of significant delay, with nominal controller design developed using loop shaping theory. Linear and nonlinear stability of the system is assessed with the variable loop gain controller in place. Simulations of multiple disturbance environments and time delay conditions validate the developed theory using a detailed nonlinear model of the western North American power system. Finally, a GPS spoofing algorithm is employed to demonstrate the efficacy of the approach to a malicious phasor measurement unit attack.

Multivariable loop-shaping control design for stability augmentation and oscillation rejection in wide-area damping using HVDC

Two high-voltage DC (HVDC) transmission lines known as the Pacific DC Intertie and Intermountain DC line operate in the western North American Power System. These HVDC lines have the potential to be capable actuators in both single-input, single-output (SISO) and multivariable wide-area damping controllers. Implementation of a multivariable design is investigated and compared to two SISO controllers implemented simultaneously using these actuators. Control synthesis is executed using a robust multivariable auto-synthesis methods with modifications based on closed-loop behavior. Sensitivity weighting functions which result in controllers that emphasize feedback at troublesome frequencies while meeting prescribed bandwidth restrictions are applied. Performance criteria include transient responses to large contingency events and disturbance rejection. A forced oscillation is used to perturb the system at an eigenfrequency of interest to determine the effectiveness of each controller in rejecting oscillatory disturbances system-wide.

Simultaneous Estimation of Electromechanical Modes and Forced Oscillations

Over the past several years, great strides have been made in the effort to monitor the small-signal stability of power systems. These efforts focus on estimating electromechanical modes, which are a property of the system that dictate how generators in different parts of the system exchange energy. Though the algorithms designed for this task are powerful and important for reliable operation of the power system, they are susceptible to severe bias when forced oscillations are present in the system. Forced oscillations are fundamentally different from electromechanical oscillations in that they are the result of a rogue input to the system, rather than a property of the system itself. To address the presence of forced oscillations, the frequently used AutoRegressive Moving Average (ARMA) model is adapted to include sinusoidal inputs, resulting in the AutoRegressive Moving Average plus Sinusoid (ARMA+S) model. From this model, a new Two-Stage Least Squares algorithm is derived to incorporate the forced oscillations, thereby enabling the simultaneous estimation of the electromechanical modes and the amplitude and phase of the forced oscillations. The method is validated using simulated power system data as well as data obtained from the western North American power system and Eastern Interconnection.

Design and Implementation of a Wide-Area Damping Controller Using High Voltage DC Modulation and Synchrophasor Feedback

This paper describes the design strategy and testing results of a control system to improve damping of inter-area oscillations in the western North American Power System (wNAPS) in order to maintain dynamic stability of the grid. Extensive simulation studies and actual test results on the wNAPS demonstrate significant improvements in damping of inter-area oscillations of most concern without reducing damping of peripheral oscillations. The design strategy of the control system features three novel attributes: (1) The feedback law for the control system is constructed using real-time measurements acquired from Phasor Measurement Units (PMUs) located throughout the power grid. (2) Control actuation is delivered by the modulation of real power flow through a High Voltage Direct Current (HVDC) transmission line. (3) A supervisory system, integrated into the control system is in charge of determining damping effectiveness, maintaining failsafe operation, and ensuring that no harm is done to the grid.

Loop Shaping of a Wide-Area Damping Controller Using HVDC

Insufficiently damped inter-area oscillatory behavior in large power systems may be mitigated by the application of a feedback control system. Thorough investigation of the resonant features unique to the western North American power system suggests that the modulation of active power using high-voltage dc has great impact on retaining system stability when actuation is provided by the geographically expansive pacific dc intertie transmission line. The controller is limited in bandwidth by the influences of time delay, sensor and actuator dynamics, and specific plant characteristics. While proportional feedback of the scaled difference of two disparate bus frequencies has shown to be a valid method for controlling this particular system in previous work, the authors submit that a shaped loop transmission function provides performance and noise response improvement with guaranteed stability in saturation. Results of simulations using a high fidelity model of the system show the efficacy of the approach.

High-resolution modeling of the western North American power system demonstrates low-cost and low-carbon futures

Decarbonizing electricity production is central to reducing greenhouse gas emissions. Exploiting intermittent renewable energy resources demands power system planning models with high temporal and spatial resolution. We use a mixed-integer linear programming model – SWITCH – to analyze least-cost generation, storage, and transmission capacity expansion for western North America under various policy and cost scenarios. Current renewable portfolio standards are shown to be insufficient to meet emission reduction targets by 2030 without new policy. With stronger carbon policy consistent with a 450ppm climate stabilization scenario, power sector emissions can be reduced to 54% of 1990 levels by 2030 using different portfolios of existing generation technologies. Under a range of resource cost scenarios, most coal power plants would be replaced by solar, wind, gas, and/or nuclear generation, with intermittent renewable sources providing at least 17% and as much as 29% of total power by 2030. The carbon price to induce these deep carbon emission reductions is high, but, assuming carbon price revenues are reinvested in the power sector, the cost of power is found to increase by at most 20% relative to business-as-usual projections.

Estimating Electromechanical Mode Shape From Synchrophasor Measurements

A theoretical basis and signal-processing approach for estimating a power system's electromechanical mode-shape properties using time-synchronized phasor measurements are presented. The relationship between modal eigenvectors and measurable power system quantities are derived. Spectral correlation analysis is used to implement the approach with demonstrative examples. This includes simulation examples as well as measured data from the western North American power system.

Making Prony analysis more accurate using multiple signals

Prony analysis has proven to be a valuable tool in estimating the modal content of power oscillations from measured ringdowns. The accuracy of the mode estimates is limited by the noise content always found in field measured signals. Current Prony analysis methods assume the system to be single output, and individual signals are analyzed independently often resulting in conflicting frequency and damping estimates (due to noise effects). This paper considers a simple extension to Prony analysis that allows multiple signals to be analyzed simultaneously resulting in one set of mode estimates. Examples are used to show that this extension improves the accuracy of modal estimates and simplifies the analysis steps. The first example uses a Monte Carlo type simulation model and the second analyzes field measured data from the western North American power system.

Recording and analyzing the July 2 cascading outage [Western USA power system

On July 2, 1996, a short circuit on a 345-kV line in Wyoming, USA, started a chain of events leading to a breakup of the Western North American power system. Five islands formed with controlled and uncontrolled load shedding, uncontrolled generation tripping, and with a blackout in Southern Idaho. The Western power system is operated by power companies belonging to the Western Systems Coordinating Council, which had a peak 1996 summer load of around 118,000 MW. July 2 was the third and, until the August 10 large-scale power failure, the most disruptive of a series of Western system breakups. The first was caused by the January 17, 1994, Northridge California earthquake. The second breakup occurred in the early morning hours of December 14, 1994, and, like July 2, originated in Southern Idaho and Wyoming. Modern computer and communication technologies greatly facilitated dissemination of information, analysis, report writing and event simulation for the July 2 cascading outage. This article analyzes the breakup, relying heavily on recordings from portable power system monitors which are installed at key locations and serve as primary components of a wide-area measurement system project.

Evaluation of nonconventional HVDC converter controls in an AC/DC power system using Prony signal analysis

This paper presents an evaluation of various nonconventional high voltage DC (HVDC) converter controls in an AC/DC power system. A method based on Prony signal analysis is used to obtain reduced-order models of the system with different nonconventional HVDC converter control schemes. The linear models provide information on local and interarea modes of the system and the degree of damping that each control scheme provides for the system. A detailed swing program is used to simulate the system. The evaluation procedure is explained and results are presented. The system studied is a 42-bus, 17-machine test system which exhibits some steady-state and dynamic characteristics of the western North American power system.

An application of Prony methods in PSS design for multimachine systems

A method for obtaining transfer functions for power system stabilizer (PSS) design is presented. It results in an accurate reduced-order model of a generator connected to a large system. The method is based on Prony signal analysis and incorporates both local and interarea electromechanical oscillatory modes. Two PSS units are designed using the proposed method along with root-locus and sequential decentralized control techniques. The design procedure and results are illustrated using computer simulation of a 27 bus system with 16 generators. The test system was developed to exhibit interarea modes that are comparable to the western North American power system. This provides a new perspective on damping interarea oscillatory modes using PSS units. >

Experience with the R-RDOT out-of-Step Relay

A new out-of-step relaying concept has been previously reported. The concept involves augmaenting apparent resistance (R) measurement with rate-of-change of apparent resistance (Rdot) computation. The new R-Rdot relay thus has more intelligence for control decisions. This follow-up paper presents the following new information: large scale simulation results, including use of relay outputs for discrete supplemental control action; additional details on design and testing of the microprocessor based relay; and experience during an extensive monitoring period at Malin Substation on the Pacific AC Intertie. The relay is now energized for initiating controlled separations at several locations within the Western North American Power System.