Yuan Liao

Online estimation of power transmission line parameters, temperature and sag

This paper puts forward a framework for estimating transmission line parameters (including series resistance, reactance and shunt susceptance), line temperature and sag utilizing synchronous voltage and current phasors measured by the phasor measurement unit (PMU) in real time. Transmission line parameters are essential inputs to various power system analyses and applications such as power flow analyses and protective relaying applications. The estimated line temperature and sag can be utilized in dynamic thermal rating applications for increased power transfer. Different types of transmission line topology are classified and modeled to facilitate the development of corresponding algorithms. The proposed least square algorithms harness the nonlinear optimal estimation theory, and are capable of detecting and identifying bad measurement data, minimizing the impact of measurement errors and thus significantly improving the estimation accuracy. In addition, this paper studies an optimal approach for placing PMUs in order to determine the line parameters, temperature and sag. Numerical examples and preliminary results are presented.

Reactive-power control of photovoltaic inverters for mitigation of short-term distribution-system voltage variability

The output power variability of intermittent renewable sources can cause significant fluctuations in distribution system voltages. A local linear controller that exploits the capability of a photovoltaic inverter to provide both real and reactive power is described. This controller substitutes reactive power for real power when fluctuations in the output of the photovoltaic source are experienced. In this way, the inverter can help mitigate distribution system voltage fluctuations. The local linear controller has a control parameter called the substitution rate, and several methods of calculating this control parameter are described, including sensitivity minimization and violation optimization methods, which can be applied on a local or global basis and on a per-phase or phase-average basis. This controller is examined using an example distribution system, and it is found that the controller is effective at mitigating voltage violations. Furthermore, comparisons between the alternative methods of selecting the substitution rate are performed.

Voltage and var control to enable high penetration of distributed photovoltaic systems

Increasing utilization of renewable energy will reduce reliance on fossil-fuel-based generation and reduce greenhouse gas and pollutant emissions, benefiting the environment and economic growth. However, the intermittent nature of renewable energy sources such as photovoltaic (PV) and wind energy systems imposes great challenges on power system voltage and reactive power control. This paper presents a novel solution based on Cyber-Physical System (CPS) design principles that removes this technical barrier so that more renewable energy sources can be deployed and integrated into the power grid. This paper focuses on solar power integration and control, but the proposed solution is expected to be also applicable to integrating other intermittent renewable energy sources such as wind energy. Camera and weather stations will be installed to capture weather data. The solar generation in a future time horizon will be estimated. An intelligent algorithm is then applied to maintain the required voltage levels. Proper communication schemes will also be designed to ensure interoperability and security of the CPS. This paper presents the overall framework of the proposed solution, designs of the major components of the system, and preliminary results.

Fault Location Using Sparse Current Measurements

This paper proposes fault location algorithms by utilizing sparse current measurements. Bus impedance matrix techniques are utilized to find a direct solution and the need of iterative searching of the fault location is eliminated. Algorithms for scenarios where current phasors or current magnitude are available from one or two branches are developed.

State estimation in distribution system considering effects of AMI data

State Estimation (SE), being a key component of Energy Management Systems (EMS), plays an essential role in operating power distribution systems. The Weighted Least Squares (WLS) based methods, among many others, have been most widely used in conventional distribution system state estimation. The increasing deployment of Advanced Metering Infrastructure (AMI), which provides more frequent measurements from more points, enables improvement for the WLS state estimators. This paper aims at quantifying the impacts of AMI data on a typical WLS state estimator, through simulation on a 16-bus overhead radial system under various conditions. Beyond demonstrating the improvement in estimation accuracy, our evaluations quantitatively characterize the behavior of WLS estimator leveraged with AMI data.

Dual-layer Voltage and VAR Control Approach with Active Participation from Distributed Solar Generations

Abstract Power distribution systems are desired to operate at their optimal control settings. These settings typically mean operation at low energy consumption and minimized distribution losses. Renewable energy resources can be very important for efficient operation of the distribution systems. However, when the number of renewables in the distribution system increases, the complexity of the control also increases. Intermittent renewable resources, particularly wind and solar, can cause significant fluctuations in distribution feeder voltages. This article presents a dual-layer control approach for finding the optimal feeder control settings and maintaining the feeder voltages within specified limits between the optimization intervals. The load-tap changer transformers, voltage regulator, capacitor banks, and smart inverters participate in the global control, while the smart inverters deal with the fast changing transients at the local control level. Thus, this control scheme will take full advantage of power-electronics-based smart inverters to reduce or eliminate fast transients, and also reduce the wear and tear of mechanical controllers. Evaluation studies are performed based on the IEEE 37 node radial test feeder under varying load and generation conditions. The results demonstrate that the proposed approach is promising for voltage and VAR control of distribution feeders with intermittent resources.

Fault Location Using Sparse Voltage Measurements

This paper presents a general method for pinpointing location of fault on a transmission line by employing only voltage measurements on any bus for any type of fault and taking advantage of bus impedance matrix technique. The approach assumes that the network data is available. Possible scenarios where prefault measurements are not available, and scenarios where only voltage magnitude is available, are considered.

Implementation guidance of smart grid communication

Employing proper communication system is one of the most challenging tasks when implementing smart grid applications. Many utility personnel may lack expertise in communication engineering, and have insufficient understanding of strengths and weaknesses of different communication options. In addition, there is no formal decision process for utilities to choose the best communication option for specific applications. Due to these reasons, the current communication implementation process is time-consuming and inefficient. This paper studies the attributes of different communication types available to utilities, and provides detailed illustration of advantages and disadvantages of each communication type. Different smart grid applications may require communication options with different characteristics. Therefore, common smart grid applications along with their best communication types are also discussed. A possible decision process is proposed as a guidance for choosing communication options. Following this process, the implementation of communication technologies could be accelerated and improved.

Impacts of load levels and topology errors on WLS state estimation convergence

State estimation is important for real time power system monitoring and control. State estimation algorithms may suffer divergence under stressed system conditions. Understanding state estimation divergence characteristics is essential for designing more robust state estimation algorithm. This paper investigates impacts of variations of load levels and topology errors on the convergence property of the commonly used weighted least square (WLS) state estimator. The influence of topology errors on the condition number of the gain matrix in the state estimation is also analyzed. The effects of measurement standard deviations are studied. The IEEE 118-bus system is used as the test case and test results are reported.

New fault location technique for double-circuit transmission lines based on sparse current measurements

This paper presents algorithms to locate faults on a double-circuit transmission line by utilizing sparse current measurements from one or two branches. The branch current is derived as an analytical function of the unknown fault location and fault resistance, and then the fault location can be obtained based on the measured branch currents. The zero-sequence mutual coupling between the two branches of the parallel line is considered. The network data are assumed to be available and quite accurate results have been obtained based on simulation studies.