Swagata Das

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.

Distribution Fault-Locating Algorithms Using Current Only

Traditional impedance-based fault-locating methods implemented in modern overcurrent protection relays require voltage and current measurements to provide reasonable fault-location estimates. Although they capture voltage and current, depending on field condition or due to equipment failure, relays may record current measurements only. Voltage measurements are thus missing or unavailable. The objective of this paper is to develop practical impedance-based fault-locating algorithms with current data (magnitude or phasors) as the only input and demonstrate the efficacy of the algorithms with simulated and actual field data. These algorithms use the circuit model of the distribution feeder and Kirchhoffs circuit laws in estimating the fault voltage at the relay location and then use impedance-based methods for fault location. Based on the analysis conducted on actual fault data, error in estimation is generally less than 0.5 mi from the actual location of the fault.

Time-Domain Modeling of Tower Shadow and Wind Shear in Wind Turbines

Tower shadow and wind shear contribute to periodic fluctuations in electrical power output of a wind turbine generator. The frequency of the periodic fluctuations is 𝑛 times the blade rotational frequency 𝑝, where 𝑛 is the number of blades. For three-bladed wind turbines, this inherent characteristic is known as the 3𝑝 effect. In a weak-power system, it results in voltage fluctuation or flicker at the point of common coupling of the wind turbine to the grid. The phenomenon is important to model so as to evaluate the flicker magnitude at the design level. Hence, the paper aims to develop a detailed time-domain upwind fixed speed wind turbine model which includes the turbines aerodynamic, mechanical, electrical, as well as tower shadow and wind shear components. The model allows users to input factors such as terrain, tower height, and tower diameter to calculate the 3𝑝 oscillations. The model can be expanded to suit studies involving variable speed wind turbines. Six case studies demonstrate how the model can be used for studying wind turbine interconnection and voltage flicker analysis. Results indicate that the model performs as expected.

Effect of load current on fault location estimates of impedance-based methods

Impedance-based methods use simple load models to estimate fault location. However loads in a practical system do not conform to the simplified load models leading to an adverse impact on accuracy of estimation. The objective of this paper is to analyze the effect of load current on fault location estimates of the Takagi, positive-sequence reactance and loop reactance methods. The derivations of the three methods are presented paying special attention to load modeling. The methods are then used to conduct fault location analysis on a modified version of the IEEE 34-Node Test Feeder. The analysis is repeated using the long and lightly loaded original test feeder. When these feeders operate under no-load conditions, fault location estimates of all three methods are highly accurate. Increase in level of load current on both feeder conditions does not affect the accuracy of the Takagi and positive-sequencereactance methods severely. They can be used to locate faults under light-load and heavy-load conditions on short as well as long feeders. The loop reactance method gives highly erroneous estimates when load current magnitude is increased.

Fault location using impedance-based algorithms on non-homogeneous feeders

Impedance-based algorithms like the positive-sequence reactance and Takagi methods are derived assuming a homogenous line conductor. This assumption is violated in practical distribution feeder circuits. The objective of this paper is to demonstrate that these methods can still be effectively applied to non-homogenous feeders by using the line parameters of the most commonly occurring conductor type in the circuit. This approach is first tested on a modified IEEE Test Feeder by simulating two cases, namely, the reference case and the case with a homogenous feeder. For the positive-sequence reactance method the maximum absolute error in both cases is about 8%, while for the Takagi method it is 6.92% and 9.53% in the homogenous case and reference case, respectively. The proposed approach is then demonstrated using ten fault cases on utility distribution feeders. The average absolute error obtained for the positive-sequence reactance and Takagi estimates is 10.23% and 9.34%, respectively, which corresponds to a median error value of 0.16 miles and 0.15 miles in the location estimates.

Distribution fault location using short-circuit fault current profile approach

Impedance-based algorithms do not consider load current and non-uniform line impedance per unit, thus introducing errors in fault location estimates. To minimize these errors, this paper proposes a short-circuit fault current profile approach to complement impedance-based algorithms. In this approach, circuit model of the distribution feeder is used to place faults at every bus and the corresponding short-circuit fault current is plotted against reactance or distance to fault. When a fault occurs in the distribution feeder, fault current recorded by the relay is extrapolated on the current profile to get location estimates. Since the circuit model is directly used in building the current profile, this approach takes into account load and non-uniform line impedance. The approach is tested using modified IEEE 34 Node Test Feeder and validated against data provided by utilities. Location estimates are within 0.8 miles of the actual fault location when the circuit model closely represents the distribution feeder.

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.

Utilizing relay event reports to identify settings error and avoid relay misoperations

Event reports recorded by digital relays and other intelligent electronic devices provide a snapshot of the power system and contain a wealth of information. Analysis of event reports can uncover incorrect relay settings and help avoid relay misoperations. Momentary faults can be detected and repaired before they evolve into a system-wide blackout. Furthermore, system operators can use the knowledge gained from analysis of event reports to reconstruct the sequence of events, track down the location of a fault, gain insight into the root cause of the fault, and take action to prevent the same fault from occurring again. This paper analyzes a series of complex events collected from a distribution utility to emphasize the benefits of event report analysis in improving power system reliability.

Effects of distributed generators on impedance-based fault location algorithms

Impedance-based fault locating algorithms assume a radial distribution feeder when computing the distance to a fault. With increased penetration of distributed generators (DGs) to the distribution grid, this assumption is violated. Short-circuit current to a fault comes from two sources, the utility substation and DGs. Ignoring the latter term when the fault is located downstream from DGs will adversely affect the accuracy of location estimates. Therefore, this paper aims to evaluate the impact of DGs on the accuracy of existing impedance-based fault location algorithms. The goal is to understand how different factors such as DG technology, MVA size of the DG unit, DG interconnect transformer, tapped loads, location of the fault from the DG unit, and fault resistance affect fault location in the presence of distributed generators.

Estimating Zero-Sequence Line Impedance and Fault Resistance using Relay Data

Data recorded by intelligent electronic devices such as protective relays contain a wealth of valuable information that can be used beyond post-mortem analysis of fault events. The objective of this paper is to propose algorithms to estimate zero-sequence line impedance and fault resistance using protective relay data collected during short-circuit fault events. Protective relaying data may be available from one or both ends of the line. For the latter, they may be collected at different sampling rates with dissimilar fault time instants. Furthermore, they may be unsynchronized. This paper presents approaches which uses measurement data from one and both ends of the line. The efficacy of the proposed algorithms presented in this paper are demonstrated using a test case as well as field data. The error percentage of the zero-sequence line impedance magnitude estimated using the data from both ends of a two-terminal line was under 3% for all the test cases. The error percentage in estimating the fault resistance was less than 1% in the test cases presented.