Bulk Power Grid Research Articles

Multichannel continuous wavelet transform approach to estimate electromechanical oscillation modes, mode shapes and coherent groups from synchrophasors in bulk power grids

Continuous Wavelet Transform (CWT) is a traditional single-channel method to estimate the dominant mode from measurements, but it is rarely applied to estimate mode shapes and coherent generators. On the other hand, estimation accuracy of traditional CWT is significantly affected by the observability of oscillations. This paper develops a multichannel CWT which is based on multichannel measurements and is less observability constrained so as to estimate not only dominant modes, but also mode shapes and coherent generators. First, wavelet power spectrum (WPS) is applied to wavelet coefficient matrices (WCMs) of multichannel measurements obtained by CWT to detect the critical scale ranges associated with the dominate modes. Then, the WCMs with the same scales in the detected ranges extracted from the raw WCMs are used to estimate the dominant modes and mode shapes. Meanwhile, the measurements that only contains the information of dominant modes are reformed by inverse CWT to detect the coherent groups of generators using direction cosines. The proposed approach is applied and evaluated with the simulation data from the 16-generator 68-bus test system and field measurements from Phasor Measurement Units (PMUs) in China Southern Power Grid (CSG). Results show that the proposed approach is accurate and efficient in estimating dominant modes, mode shapes and coherent groups of generators from synchrophasor measurements.

Distributed Real-Time Optimal Power Flow Control in Smart Grid

Conventionally, power system has a hierarchical control structure including primary, secondary, and tertiary controls. The drawbacks of this hierarchical scheme are manifest: 1) it lacks flexibility and scalability, which is against the trend toward an open-access power system; 2) load forecast as the basis of tertiary control could be inaccurate and infeasible, especially in microgrid for example; 3) as the penetration of renewable energy increases, the relatively long time-scales of secondary and tertiary controls cannot accommodate to more severe power fluctuation within the system. To avoid these drawbacks, a distributed real-time optimal power flow control strategy is introduced in this paper. With the aid of up-to-date smart grid technologies such as two-way communication and distributed sensor, the proposed approach can avoid the need of load forecast and achieve the same objective as hierarchical control with a feedback mechanism in real time, that is to recover the nominal system frequency and maintain the active power of the generators close to the optimal operational condition in the presence of any disturbance. Convergence of the proposed approach is analytically proved. Simulation results in a 34-bus islanded microgrid and the IEEE 118-bus bulk power grid validate the effectiveness and efficiency of the proposed approach.

Gini Coefficient for Voltage Security Assessment of Multi-stage Transmission Expansion Planning

Due to the voltage security uncertainty caused by construction sequence in multi-stage transmission expansion planning (MTEP) in bulk power grids, it is necessary to appropriately arrange the construction sequence of the MTEP schemes to reduce voltage security risk. The Gini coefficient is widely used to weigh the gap of residents' income distribution. In this paper, the voltage security Gini coefficient is defined as the voltage security risk index of the MTEP scheme, and is presented to quantitatively evaluate the entire voltage security of one MTEP scheme. Voltage security evaluation model of the MTEP is established based on voltage security Gini coefficient. Compared to the stabilization cost, the proposed model directly reflects the voltage security of the MTEP scheme, and the local optimum is avoided. The presented method is applied on the IEEE 118-bus 54-generator test system. Results show the validity and effectiveness of the proposed model.

Are high penetrations of commercial cogeneration good for society?

Low natural gas prices, market reports and evidence from New York State suggest that the number of commercial combined heat and power (CHP) installations in the United States will increase by 2%–9% annually over the next decade. We investigate how increasing commercial CHP penetrations may affect net emissions, the distribution network, and total system energy costs. We constructed an integrated planning and operations model that maximizes owner profit through sizing and operation of CHP on a realistic distribution feeder in New York. We find that a greater penetration of CHP reduces both total system energy costs and network congestion. Commercial buildings often have low and inconsistent heat loads, which can cause low fuel utilization efficiencies, low CHP rates-of-return and diminishing avoided emissions as CHP penetration increases. In the northeast, without policy intervention, a 5% penetration of small commercially owned CHP would increase CO2 emissions by 2% relative to the bulk power grid. Low emission CHP installations can be encouraged with incentives that promote CHP operation only during times of high heat loads. Time-varying rates, such as time-of-day and seasonal rates, are one option and were shown to reduce customer emissions without reducing profits. In contrast, natural gas rate discounts, a common incentive for industrial CHP in some states, can encourage CHP operation during low heat loads and thus increase emissions.

An Efficient Approach for Fast and Accurate Voltage Stability Margin Computation in Large Power Grids

This paper proposes an efficient approach for the computation of voltage stability margin (VSM) in a large-scale power grid. The objective is to accurately and rapidly determine the load power margin which corresponds to voltage collapse phenomena. The proposed approach is based on the impedance match-based technique and the model-based technique. It combines the Thevenin equivalent (TE) network method with cubic spline extrapolation technique and the continuation technique to achieve fast and accurate VSM computation for a bulk power grid. Moreover, the generator Q limits are taken into account for practical applications. Extensive case studies carried out on Institute of Electrical and Electronics Engineers (IEEE) benchmark systems and the Taiwan Power Company (Taipower, Taipei, Taiwan) system are used to demonstrate the effectiveness of the proposed approach.

Projection Pursuit: A General Methodology of Wide-Area Coherency Detection in Bulk Power Grid

This paper presents a general approach for coherency detection in bulk power systems using the projection pursuit (PP) theory. Supported by the concept of center of inertia (COI) in power systems, the PP theory is employed to model the wide-area coherency detection as an optimization problem. In the proposed method, the optimal projection direction in high dimensional orthogonal space is explored in order to detect the coherent groups via the data from synchronous phasor measurement units (PMUs). Two quantitative indices constructed with projection assessment index (PI), the objective of the optimization model, are then defined in order to determine the critical coherent group and the dominant coherent groups. The coherency detection criterion and the implementation framework for the proposed approach are also presented. Simulation data from the 16-machine 68-bus test system and China Southern power Grid (CSG), along with actual field-measurement data retrieved from WAMS database in the CSG, are employed to demonstrate the effectiveness and applicability of the proposed algorithm under different disturbances. It is shown that the proposed methodology successfully detects the dominant coherent groups of generators and buses in bulk power system via the wide-area field-measurement data.

Estimating inter-area dominant oscillation mode in bulk power grid using multi-channel continuous wavelet transform

This paper proposes a novel continuous wavelet transform (CWT) based approach to holistically estimate the dominant oscillation using measurement data from multiple channels. CWT has been demonstrated to be effective in estimating power system oscillation modes. Using singular value decomposition (SVD) technique, the original huge phasor measurement unit (PMU) datasets are compressed to finite useful measurement data which contain critical dominant oscillation information. Further, CWT is performed on the constructed measurement signals to form wavelet coefficient matrix (WCM) at the same dilation. Then, SVD is employed to decompose the WCMs to obtain the maximum singular value and its right eigenvector. A singular value vector with the entire dilation is constructed through the maximum singular values. The right eigenvector corresponding to the maximum singular value in the singular-value vector is adopted as the input of CWT to estimate the dominant modes. Finally, the proposed approach is evaluated using the simulation data from China Southern Power Grid (CSG) as well as the actual field-measurement data retrieved from the PMUs of CSG. The simulation results demonstrate that the proposed approach performs well to holistically estimate the dominant oscillation modes in bulk power systems.

Synchrophasor Technology and the DOE: Exciting Opportunities Lie Ahead in Development and Deployment

synchrophasor technology has grown from a handful of phasor measurement units (PMUs) in the U.S. Pacific Northwest to a continental network of almost 2,000 PMUs in the past decade, which has helped to improve reliability of the North American electric bulk power grid. This progress is due in large part to research and deployment investments that the U.S. Department of Energy (DOE) has been making since the late 1990s to accelerate the development, adoption, and maturity of this valuable technology. The DOE?s early vision was that better real-time tools could improve grid reliability and prevent blackouts; today, the entire industry recognizes that synchrophasor technology can improve grid planning and investment decisions, enhance operational reliability, facilitate renewables integration, and reduce many operating and asset costs.

Two-level unit commitment and reserve level adjustment considering large-scale wind power integration

Summary Integrating wind power generation into bulk power grids imposes significant challenging to system operations due to the stochastic variation and limited predictability of wind power generation. One of the key issues is how to optimally schedule various units and determine a proper spinning reserve level to guarantee a reliable operation of the integrated system in a robust and economic manner. In this paper, we propose a two-level unit commitment (TLUC) for power systems facing large uncertainties in the context of adjustable robust optimization. The commitment decision acts as first-stage decisions that need to be robust against uncertainty, whereas the output decision acts as adjustable decisions that is decided after uncertainty is revealed. This decision mechanism naturally decomposes a conventional unit commitment (UC) problem into two levels, the upper level provides unit working status, whereas the lower level checks if there is at least one dispatch solution under uncertainty subject to the status. Unlike conventional UC, where the reserve level is fixed from operating experiences, in TLUC, the reserve level also acts as a decision variable in the upper level that controls the robustness of UC decisions. A heuristic algorithm is developed to solve this TLUC problem, providing the robust UC decision against uncertainty as well as the minimal spinning reserve level required to maintain reliability. Numerical experiments are carried out on the IEEE 39-bus system and large-scale realistic Guangdong power grid in China and demonstrate the effectiveness of our approach. Copyright © 2013 John Wiley & Sons, Ltd.

A Novel Method of Static Security Analysis Based on Multithread Technology

With the UHV (Ultra High Voltage) power grid construction and the interconnection of regional power grids, the scale of power grids in China is increasing rapidly. At the same time, significant uncertainty and variability is being introduced into power grid operation with the integration of large-scale renewable energy in power systems. All of these pose an enormous challenge to the operation control of power systems in China. For a long time, online static security analysis, as an important part of EMS (Energy Management System), has been an effective tool for power grid operation. However, it is increasingly difficult for traditional static security analysis in serial computing mode to be online applied in bulk power grids in China. A new practical parallel approach for online static security analysis is put forward in this paper. A multithread parallelism is introduced into contingency screening, detailed contingency evaluation and decision support for reducing the execution time. By employing the multithread technology, the hardware resources of multi-processor/multi-core computer can be fully used and the program can be speeded up effectively. The performance of the parallel static security analysis is demonstrated by tests on two large-scale power systems. The test results show that the proposed method can be online applied in real bulk power grids.

Bulk power risk analysis: Ranking infrastructure elements according to their risk significance

Disruptions in the bulk power grid can result in very diverse consequences that include economic, social, physical, and psychological impacts. In addition, power outages do not affect all end-users of the system in the same manner. For these reasons, a risk analysis of bulk power systems requires more than determining the likelihood and magnitude of power outages; it must also include the diverse impacts power outages have on the users of the system. We propose a methodology for performing a risk analysis on the bulk power system. A power flow simulation model is used to determine the likelihood and extent of power outages when components within the system fail to perform their designed function. The consequences associated with these failures are determined by looking at the type and number of customers affected. Stakeholder input is used to evaluate the relative importance of these consequences. The methodology culminates with a ranking of each system component by its risk significance to the stakeholders. The analysis is performed for failures of infrastructure elements due to both random causes and malevolent acts.

Power Quality Assessment of Large Motor Starting and Loading for the Integrated Steel-Making Cogeneration Facility

This paper presents the power quality assessment of large synchronous motor starting and loading in the integrated steel-making cogeneration facility. To execute the transient stability analysis, the proper mathematical models, and the accurate parameters of the cogeneration units, excitation systems, governor systems, load, and static var compensators (SVCs) are investigated in detail. Four case studies with or without considering the connection of the power grid, the installation of autotransformer (AT) starter, and SVC are performed to demonstrate the dynamic responses of system frequency, voltage, and cogeneration units due to motor starting and loading. Also, the voltage sag ride-through curve of sensitivity load has been included, and a power quality index (PQI) due to voltage variation in the assessment period has been proposed to find the impact of motor starting and loading on the power quality of the cogeneration system. It is concluded that the system dynamic responses and PQI values have better performance if the AT starter is applied with either the regulation of the SVC system or connection to the bulk power grid

個別VQC装置への目標電圧指令による中央VQC方式

The authors have developed a new central VQC (Voltage Q(reactive power) Control) method that cooperates with local voltage control devices AVQC (Automatic V-Q Controller). Each AVQC, located at principal substations or power stations, brings bus voltage close to its target voltage independently by controlling the on-load tap changers and shunt capacitors and reactors status. Each target bus voltage is optimally determined through on-line optimal power flow calculation to minimize active power transmission losses in the bulk power grid. Using this central VQC method, efficient voltage control is realized in comparison with the conventional VQC since the target voltage is updated continuously in accordance with the current power grid state. The voltage control function employing the described method has been already implemented into the energy management system at Tohoku EPCO and has been in operation from 2004.This paper presents the newly developed method for central VQC and its excellent performance through bus voltages behaviors at the real Tohoku EPCO power grid.