Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1999-02-24
2001-07-03
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S058000, C702S060000, C324S512000, C324S525000
Reexamination Certificate
active
06256592
ABSTRACT:
DESCRIPTION
1. Technical Field
This invention relates generally to fault location systems for electric power systems, and more particularly concerns double-ended or even multi-ended fault location systems.
2. Background of the Invention
Accurate calculated fault location information is important to the proper operation of an electric power system. The information can be used by the automatic protection system, as well as by system operators and the power system protection engineers. One significant aspect of power system operation affected by fault location concerns the time necessary to physically find the actual fault point along the power line. Another aspect involves reclosing performance after a circuit breaker has opened due to a fault.
Single-ended fault location, meaning fault location calculated from information provided by a single relay on a power line, has become a standard feature in most current microprocessor-based protective relays. Fault location capability is also offered as an option in many digital fault recorder systems as well. Single-ended fault location techniques, which typically use power line impedance-based calculations, are typically simple and fast and do not require communication with other terminal devices, such as a downstream relay, on the protected line.
However, it is well known that large errors in fault location results can occur with single-ended systems. The circumstances where such errors can occur include situations such as where there is strong zero sequence mutual coupling between adjacent lines, where the power system includes multiple remote terminals, including specifically three-terminal line situations, where there are large angle differences between the power sources feeding the line and the line itself, or where the power lines are non-transposed.
More specifically, there are typically two types of single-ended fault location systems, one using a reactance-based calculation method and the other using what is known as a Takagi-based calculation method. In the reactance method, the fault location system first measures the apparent impedance of the line and then ratios the reactance (the imaginary part) of the apparent impedance to the known reactance of the entire line. This ratio is in fact proportional to the distance to the fault from the single-ended terminal. The reactance method works reasonably well for homogeneous power systems where the fault does not involve significant fault resistance or load flow. The reactance calculations do not take into account or attempt to compensate for the errors which result in those situations involving significant fault resistance or load flow.
The Takagi method, as described in an article by T. Takagi et al, entitled “Development of a New Fault Locator Using the One Terminal Voltage and Current Data”, IEEE Transaction on PAS, Vol. PAS-s101, No. 8, August 1982, pp. 2842-2898, does attempt to compensate for those specific sources of errors. The Takagi method, however, typically requires knowledge of pre-fault phase current for the faulted phase portion of the protected line. If the fault location calculation system does not have accurate pre-fault data, the fault location result is severely affected. Further, non-homogeneous power systems still result in errors in the fault location information. The amount of error depends upon the difference in system angle on either side of the fault, the magnitude of the fault resistance, and the direction of load flow. The error is most pronounced at high values of fault resistance. A comprehensive review of impedance-based fault location methods can be found in a paper by Edmund O. Schweitzer III, entitled “A Review of Impedance-Based Fault Locating Experience”, presented at the Fourteenth Annual Iowa-Nebraska System Protection Seminar on October 16, 1990.
While compensation attempts for non-homogeneous system errors have been made, including “tilting” of the top line reactance characteristic calculation by a selected factor e
jT
, where T is a setting equal to the angle of shift required for a particular fault location, this and other compensation techniques have not been very effective. The reactance tilting method, for instance, is valid for only one point on the protected line.
The basic difficulty with single-ended fault location methods is that they must compensate for not knowing the total current which flows through the fault resistance. The basic solution to that particular difficulty, however, is to have all of the relays which protect a particular line communicate with each other and play a part in the fault location calculation. For instance, on a single protected line having two relays at both ends of the line, information about the power signal on the line can be obtained from both relays and then used to make fault location determinations. This is referred to generally as double-ended fault location. This method significantly improves the accuracy of the fault location information relative to single-ended (single relay) methods, but requires both the communication of a significant amount of data from the remote relay and phase alignment of the data (referred to as data sets) between the local and the remote relays (both ends of the line).
The communication of data adds a significant amount of time to fault location, such that it cannot be done in real time, a significant operational disadvantage. Aligning the data can be accomplished by several methods, including aligning (synchronizing) the sampling clocks in each relay with a single time source (such as from a satellite) or using one relay as a reference and synchronizing the other relay thereto, but such methods add time and expense to the overall system. Furthermore, maintaining the synchronization over a period of time is typically difficult and the resulting alignment is still often not completely accurate.
Unfortunately, however, conventional double-ended fault locating systems also have significant disadvantages. One disadvantage which affects single-ended calculations, but also affects double-ended systems using phase quantities, involves the effect of zero sequence mutual coupling in parallel line applications. If the calculations include phase voltage values, which include zero sequence quantities, mutual coupling of the zero sequence quantity between the faulted line and a parallel line will result in inaccuracies in the resulting fault location determination. As the distance increases from the relay making the calculation to the fault, the error becomes more significant. Attempts to compensate for zero sequence mutual coupling typically add significant complexity to the relay design and to the overall protection approach.
Another significant disadvantage to conventional double-ended systems, mentioned briefly above, involves the communication of the data from a remote terminal (relay) to the local terminal making the fault location determination. Typically, a large amount of data must be transmitted from the remote terminal, usually involving a complete event report record following the initial recognition of a fault. An event report is a conventional term meaning a complete set of voltage, current and other data for the power signal, covering a time span of shortly before the fault to the end of the power interruption.
The information in the event report is quite substantial and comprises the data set which must be transmitted to the local terminal and then aligned with the local data set. Typically, this data set will include voltage and current information for all three phases of the power signal. This is a significant amount of data and requires a substantial amount of time to transmit, resulting in a significant delay for the fault location determination to be made by the local relay following initiation of the fault. This delay can have a significant effect on the overall desired operation of the power system.
Once the remote data is received, it must be aligned, as indicated above. If the alignment includes pre-fault data, that pre-fault data must be valid. If the a
Benmouyal Gabriel
Roberts Jeffrey B.
Tziouvaras Demetrios
Bui Bryan
Schweitzer Engineering Laboratories Inc.
Shah Kamini
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