Tony B. Nguyen

Dynamic security-constrained rescheduling of power systems using trajectory sensitivities

In the deregulated environment of power systems, the transmission networks are often operated close to their maximum capacity to achieve transfer of power. Besides, the operators must operate the system to satisfy its dynamic stability constraints under credible contingencies. This paper provides a method using trajectory sensitivity to reschedule power generation to ensure system stability for a set of credible contingencies while satisfying its economic goal. System modeling issue is not a limiting concern in this method, and hence, the technique can be used as a preventive control scheme for system operators in real time.

Energy Storage for Power Systems Applications: A Regional Assessment for the Northwest Power Pool (NWPP)

Wind production, which has expanded rapidly in recent years, could be an important element in the future efficient management of the electric power system; however, wind energy generation is uncontrollable and intermittent in nature. Thus, while wind power represents a significant opportunity to the Bonneville Power Administration (BPA), integrating high levels of wind resources into the power system will bring great challenges to generation scheduling and in the provision of ancillary services. This report addresses several key questions in the broader discussion on the integration of renewable energy resources in the Pacific Northwest power grid. More specifically, it addresses the following questions: a) how much total reserve or balancing requirements are necessary to accommodate the simulated expansion of intermittent renewable energy resources during the 2019 time horizon, and b) what are the most cost effective technological solutions for meeting load balancing requirements in the Northwest Power Pool (NWPP).

PMU-Based Wide-Area Security Assessment: Concept, Method, and Implementation

This paper presents a concept, method, and implementation of utilizing phasor measurement unit (PMU) information to monitor the wide-area security of a power system. The close dependency of major transmission paths requires an approach that takes that interaction into account while establishing operational transfer capability, and evaluates grid reliability and security on a system-wide basis. Thus, the concept of wide-area security region, which considers all essential constraints, including thermal, voltage stability, transient stability, and small signal stability, is proposed. This approach expands the idea of traditional transmission system nomograms to a multidimensional case, involving multiple system limits and parameters such as transmission path constraints, zonal generation or load, etc., considered concurrently. In this paper, the security region boundary is represented using piecewise approximation with the help of linear inequalities (so called hyperplanes) in a multidimensional space, consisting of system parameters that are critical for security analysis. The goal of this approximation is to find a minimum set of hyperplanes that describe the boundary with a given accuracy. Offline computer simulations are conducted to build the security region and the hyperplanes can be applied in real time with phasor information for on-line security assessment. Numerical simulations have been performed for the full size Western Electricity Coordinating Council (WECC) system model, which comprises 15 126 buses and 3034 generators. Simulation results demonstrated the feasibility and effectiveness of this approach, and proved that the proposed approach can significantly enhance the wide-area situation awareness for a bulk power system like WECC.

National Assessment of Energy Storage for Grid Balancing and Arbitrage: Phase 1, WECC

To examine the role that energy storage could play in mitigating the impacts of the stochastic variability of wind generation on regional grid operation, the Pacific Northwest National Laboratory (PNNL) examined a hypothetical 2020 grid scenario in which additional wind generation capacity is built to meet renewable portfolio standard targets in the Western Interconnection. PNNL developed a stochastic model for estimating the balancing requirements using historical wind statistics and forecasting error, a detailed engineering model to analyze the dispatch of energy storage and fast-ramping generation devices for estimating size requirements of energy storage and generation systems for meeting new balancing requirements, and financial models for estimating the life-cycle cost of storage and generation systems in addressing the future balancing requirements for sub-regions in the Western Interconnection. Evaluated technologies include combustion turbines, sodium sulfur (Na-S) batteries, lithium ion batteries, pumped-hydro energy storage, compressed air energy storage, flywheels, redox flow batteries, and demand response. Distinct power and energy capacity requirements were estimated for each technology option, and battery size was optimized to minimize costs. Modeling results indicate that in a future power grid with high-penetration of renewables, the most cost competitive technologies for meeting balancing requirements include Na-S batteries and flywheels.

Analysis of angle stability problems: a transmission protection systems perspective

Postfault rotor angle oscillations lead to power swings. Both unstable and stable swings can induce distance relay tripping. For unstable swings, a new computational procedure to locate all of the electrical centers is developed. It simplifies the work associated with visual screening of all the R-X plots. For stable swings, a generic three-tier hierarchy of stability-related norms defined by branch norm, fault norm, and system norm is proposed. Ranking by branch norm leads to ranking of power swings. Ranking by fault norm leads to ranking of faults or contingencies. Magnitude and rate of change of system norm can be used to detect an out-of-step condition. Results on a ten-machine system and a utility system with detailed models are also presented.

Sensitivity approaches for direct computation of critical parameters in a power system

In this paper we propose two techniques to estimate critical values of parameters of interest in a power system such as clearing time of circuit breakers, mechanical input power, etc. One is via the sensitivity of the transient energy function (TEF) and the other through computation of the norm of the trajectory sensitivities. Both these methods require some a-priori information about the range of the critical parameters. In real time operation, this information is generally available to the operator. A changing operating condition will result in new values of the critical parameters and the proposed technique can thus monitor them closely without the use of direct methods. The advantage of the first technique lies in not having to compute the unstable equilibrium point (uep) while in the second technique even the computation of stable equilibrium (sep) is not needed. However, there are additional computational costs involved in both the techniques, which can be addressed by faster algorithms for computing the sensitivities.

Impact of Wind Power Plants on Voltage and Transient Stability of Power Systems

With the scenario of wind power constituting up to 20% of the electric grid capacity in the future, the need for systematic studies of the impact of wind power on both voltage and transient stability of the grid has increased. A large number of parameters will affect such studies. For this paper, studies were conducted on a standard three-machine, nine-bus system augmented by a radially connected wind power plant (WPP), which contains 22 wind turbine generators (WTG). The studies include examining the voltage stability (P-V) curves of the system at the point of interconnection (POI), as well as in the radial system with and without the WPP. These voltage stability studies are done for the base case as well as for contingencies. This is followed by transient stability studies for three-phase faults, again at the POI as well on the radial system. The transient stability studies illustrate the capability of the WPP. The conclusions drawn from this study will be supported by an analytical study in the future.

Bounding uncertainty in power system dynamic simulations

Parameters of power system models can never be known exactly. Yet dynamic security assessment relies upon the simulations derived from those uncertain models. This paper proposes an approach to quantifying the uncertainty in simulations of power system dynamic behaviour. It is shown that trajectory sensitivities can be used to generate an accurate first order approximation of the trajectory corresponding to a perturbed parameter set. The computational cost of obtaining the sensitivities and perturbed trajectory is minimal. Therefore it is feasible to quickly generate many approximate trajectories from a single nominal case. To quantify the effect of parameter uncertainty on the nominal case, parameter sets are randomly generated according to their underlying statistical distribution. An approximate trajectory is obtained for each set. The collection of trajectories provides a bound within which the actual system dynamic behaviour should lie.

Energy storage for variable renewable energy resource integration — A regional assessment for the Northwest Power Pool (NWPP)

This paper addresses the following key questions in the discussion on the integration of renewable energy resources in the Pacific Northwest power grid: a) what will be the future balancing requirement to accommodate a simulated expansion of wind energy resources from 3.3 GW in 2008 to 14.4 GW in 2019 in the Northwest Power Pool (NWPP), and b) what are the most cost effective technological solutions for meeting the balancing requirements in the Northwest Power Pool (NWPP). A life-cycle analysis was performed to assess the least-cost technology option for meeting the new balancing requirement. The technologies considered in this study include conventional turbines (CT), sodium sulfur (NaS) batteries, lithium ion (Li-ion) batteries, pumped hydro energy storage (PH), and demand response (DR). Hybrid concepts that combine 2 or more of the technologies above are also evaluated. This analysis was performed with collaboration by the Bonneville Power Administration and funded by the Energy Storage Systems Program of the U.S. Department of Energy.

Use multi-dimensional ellipsoid to monitor dynamic behavior of power systems based on PMU measurement

The paper discusses a new idea of the multi-dimensional characteristic ellipsoid (CELL) approach to monitor dynamic behavior of an interconnected power system using phasor measurement unit (PMU) measurement. The multi-dimensional minimum volume enclosing ellipsoid (MVEE) is extracted based on phasor measurements to represent the feature of dynamic behavior of power systems. The orientation matrix of MVE describes the shape and orientation of the CELL. Extreme events can be identified by tracking the volume change of the CELL with respect to time. Some feature indexes that can be used to determine the disturbance are also proposed to interpret the dynamic behavior of the power system, and to determine the decision trees for extreme event identification procedure. This method will be a useful tool for providing wide-area situational awareness for grid operators, identification of system disturbances and detection of system stresses and their locations.