Anish Gaikwad

Impedance-Based Fault Location in Transmission Networks: Theory and Application

A number of impedance-based fault location algorithms have been developed for estimating the distance to faults in a transmission network. Each algorithm has specific input data requirements and makes certain assumptions that may or may not hold true in a particular fault location scenario. Without a detailed understanding of the principle of each fault-locating method, choosing the most suitable fault location algorithm can be a challenging task. This paper, therefore, presents the theory of one-ended (simple reactance, Takagi, modified Takagi, Eriksson, and Novosel et al.) and two-ended (synchronized, unsynchronized, and current-only) impedance-based fault location algorithms and demonstrates their application in locating real-world faults. The theory details the formulation and input data requirement of each fault-locating algorithm and evaluates the sensitivity of each to the following error sources: 1) load; 2) remote infeed; 3) fault resistance; 4) mutual coupling; 5) inaccurate line impedances; 6) DC offset and CT saturation; 7) three-terminal lines; and 8) tapped radial lines. From the theoretical analysis and field data testing, the following criteria are recommended for choosing the most suitable fault-locating algorithm: 1) data availability and 2) fault location application scenario. Another objective of this paper is to assess what additional information can be gleaned from waveforms recorded by intelligent electronic devices (IEDs) during a fault. Actual fault event data captured in utility networks is exploited to gain valuable feedback about the transmission network upstream from the IED device, and estimate the value of fault resistance.

Load model parameter derivation using an automated algorithm and measured data

This paper summaries some of the key results achieved in the second phase of a multi-year collaborative load modeling research project. After having identified suitable types of load monitoring devices, actual field data for load model development and validation were collected at appropriate locations for several months to more than a year in three different utilities. This data was post-processed using an automated methodology to filter out events suitable for load model parameter estimation. Two load model structures were then used with an automated parameter estimation algorithm to fit model parameters using the field data collected. The models thus developed were then validated using Siemens PTI PSS/ETM dynamic simulation program. This whole process resulted in some key insights and valuable conclusions for future load modeling research efforts.

Results of residential air conditioner testing in WECC

This paper summarizes the key results of testing work performed by three organizations (EPRI, SCE, and BPA) on a total of twenty seven air conditioning units in order to better understand and thus characterize their behavior for power system simulations. The diversity of the tested air conditioner units included sizes (tonnage), compressor technology (reciprocating and scroll), type of refrigerant (R-22 and R-410A), efficiencies (between 10 and 13 SEER), and vintage (new and old). A common test plan was developed by the three organizations. The tests were then performed independently by each of the three organizations. The EPRI work was sponsored by APS and SRP. This effort was part of the current load modeling effort going on in WECC under the load modeling task force. The key findings of this work are presented here together with a description of the testing methodology. All three organizations found very similar results despite testing a variety of different sizes and manufacturer units. The key results presented are associated with the stalling behavior of the units at different outdoor temperatures, the behavior of thermal overload tripping, contactor dropout, and the behavior of the units in response to different emulated types of system events.

Harmonic impacts of widespread use of CFL lamps on distribution systems

The widespread usage of more efficient but potentially harmonic rich residential loads such as CFL (Compact Fluorescent Light) lamps can be expected to contribute to increased harmonic levels in distribution systems. In some cases, additional distortion can push already high harmonic levels above the recommended limits. Also, the impact can be further exacerbated by the presence of reactive compensation in the form of capacitor banks on the feeders. This paper attempts to quantify the increase in voltage distortion at a distribution substation for the worst case scenario of CFL lamps replacing all the incandescent lamps. Detailed distribution system models have been developed for the purpose of computer simulations and the harmonic injection has been represented using harmonic spectrums obtained through laboratory tests carried out on actual off-the-shelf CFL lamps. One potential solution to mitigate anticipated harmonic levels has been studied and presented. The final goal of this effort is to develop guidelines that utility can follow in designing or modifying their distribution systems to handle increased harmonic injections that are expected in future.

Vulnerability of Large Steam Turbine Generators to Torsional Interactions During Electrical Grid Disturbances

This paper presents a formalized and systematic approach for identifying the zone of vulnerability of a power plant to potentially damaging torsional stress. That is the region of the electrical network surrounding a plant where electrical disturbance may result in significant loss of life at critical rotor locations on the turbine generator. The approach is based on comparing the peak transient torques observed for various network faults and disturbances to the peak transient torque for a machine terminal fault. This is shown to be a useful methodology in the absence of detailed shaft fatigue data. The results of the study provide significant insight into the various factors that influence the level of torsional stress and how one would be able to assess when detailed torsional interaction studies are necessary. Finally, some key general conclusions are drawn with respect to system events that may result in turbine-generator shaft fatigue loss-of-life.

Impact of grounded shield wire assumption on impedance-based fault location algorithms

Although shield wires are typically grounded through a finite tower footing resistance, positive- and zero-sequence line impedances are calculated assuming no tower footing resistance. Since impedance-based fault location algorithms require sequence line impedances to compute the distance to fault, this paper evaluates the impact of the grounded shield wire assumption on the accuracy of fault locating algorithms. A simple test case, which consists of a 3.73 long transmission line with two shield wires and a tower footing resistance every 0.19 miles, was setup to evaluate the assumption. The analysis shows that tower footing resistance affects only the zero-sequence line impedance. This leads to a marginal increase in error from one-ended impedance-based methods in locating single line-to-ground or double line-to-ground faults. On the other hand, fault location estimates from two-ended methods are not affected as they do not make use of the zero-sequence line impedance in fault location computation.

Load Sensitivity Studies in Power Systems With Non-Smooth Load Behavior

One of the most important aspects of time-domain simulations for power system planning studies is load modeling. For a realistic representation of the load, the Western Electricity Coordinating Council (WECC) model validation and working group developed the composite load model. The composite load model represents the aggregation of different types of loads at the substation level. However, there exists some uncertainty in determining the load parameters and the percentage composition of the different components. Trajectory sensitivity (TS) analysis provides a systematic approach to study the impact of parameter uncertainty on power system response to disturbances. The non-smooth nature of the composite load model may present some additional challenges to sensitivity analysis in a realistic power system. This paper presents an application of TS analysis to study the impact of load parameter uncertainty on the system response. The impact of the non-smooth nature of load models on the sensitivity analysis is also addressed. This paper further suggests a method to determine the perturbation size limit for which accurate linear approximations can be made. The study was performed using the WECC system model.

Probabilistic transmission planning at ERCOT

Traditional transmission planning for electric power systems involves analyzing certain system conditions for a number of contingency events based on deterministic criteria. This paper discusses the motivations for ERCOT to explore probabilistic approaches in transmission planning and presents the potential applications of probabilistic planning methods that ERCOT is currently investigating. It also presents a case study that ERCOT and EPRI have worked on as part of a joint effort and the potential challenges for implementing probabilistic planning methods.

Implementation of the WECC Composite Load Model for utilities using the component-based modeling approach

This paper presents an approach for developing dynamic load model data records for use in planning studies. The load model structure used is the so-called WECC Composite Load Model (CLM). The approach is based on using information about customer classes, various load components in each customer class, and electrical characteristics of each load component. The process is implemented in a tool that can be customized for an individual utility based on its geographical location within the US. Also, the tool can be refined over time due to changing load mix or as more accurate information becomes available. The output of the tool is a dynamic data record for the CLM which can be readily used in dynamic simulations. The main objective of these efforts is to provide an easy-to-use tool for transmission planners to develop load composition for CLM that can be used in time-domain dynamic simulations.

Effect of earth current return model on transmission line fault location - a case study

Impedance-based fault location algorithms require positive- and zero-sequence line impedance to determine the distance to fault. When solving for these line constants in a multi-grounded transmission system, power system analysis programs use Carsons model, modified Carsons model or Deri model to account for earth current return. The study is motivated by the fact that line constants calculated using each of these different approaches are not exactly identical. Since transmission lines traverse long distances, it was suspected that these seemingly minor differences can affect accuracy of fault location estimates. Therefore this paper aims to study the impact of line constants calculated using different earth return models on impedance-based fault location algorithms. Analysis was conducted on four actual fault events. For a fault located 14.37 miles from substation, variation between location estimates using each of the three earth models was observed to be within 1%. The paper therefore recommends using any of the three earth models for fault location without significant variation in accuracy.