Patent Application
20150094973
The present invention relates to testing magnetic assemblies, such as transformers, and more specifically to a device and method to measure the excitation characteristics of magnetic assemblies using reversible direct current and a method for converting the direct current (DC) excitation data to alternating current (AC) excitation data at any power frequency.
According to the ANSI/IEEE C57.13.1 Standard, entitled “Guide for Field Testing of Relaying Current Transformers”, incorporated herein by reference in its entirety, the following items need to be verified on current transformers 100 destined for relaying applications:
1. Ratio
2. Polarity
3. Insulation resistance
4. Winding resistance
5. Excitation characteristics.
The most difficult test of the five listed above is the “excitation test.” The excitation test requires the application of voltage and current (kVA) well above the typical operating conditions of the current transformer 100.
Currently available conventional test equipment that can be used for the ANSI/IEEE tests listed above would typically consist of line operated and adjustable voltage/current sources, analogue or digital voltmeters and ammeters and, in some cases, specialized testers. The conventional equipment can be used for generating voltage and current that would appear on the secondary windings 104 of instrument current transformers 100 under normal operating and fault conditions, and thereby are suitable for conducting the excitation test in a non-operational unit. To test most typical current transformer 100 installations, the voltage and current sources need to have a low impedance output of up to about 10 kVA. However, sources of this capacity are typically heavy and bulky. So currently used devices have smaller voltage and current sources with high source impedance and less than the desired kVA perform the tests. Another disadvantage of current testing devices is the lack of adequate power at a test site. The lack of adequate power prevents the devices from supplying adequate power to conduct the required tests accurately.
In addition to being bulky, heavy and difficult to operate, conventional test equipment does not properly control all the variables affecting current transformer performance. This results in an overall inaccurate performance.
Therefore there is a need for a device to measure the excitation characteristics of magnetic assemblies using reversible direct current and a method for converting the direct current excitation data to alternating current excitation data at any power frequency.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying figures where:
The present invention overcomes the limitations of the prior art by providing a device to measure the excitation characteristics of a current transformer using reversible direct current and a method for converting the direct current excitation data to alternating current excitation data at any power frequency. The present invention provides a device and a method for testing magnetic assemblies, especially current transformers eliminating the problems in the prior art and providing reliable tests of current transformer performance even with the testing device consuming considerably less power.
The testing of magnetic assemblies, such as transformers, especially current transformers, requires the application of appropriate power frequency excitation that simulates normal as well as fault operating conditions. These signal generators require considerable power to operate, are heavy and voluminous. To avoid the need for large power sources, such magnetic assemblies can be tested by direct current that is periodically reversed. Magnetic testing using DC produces results that are different from those using a typical power frequency test. The present invention provides both a device and a method that allows the magnetic assembly, such as, for example, a current transformer, to be conveniently tested using DC excitation and then converts the tests results obtained from DC tests to AC test results at the power frequency of the transformer being tested.
As will be understood by those with skill in the art with reference to this disclosure, a magnetic assembly like a current transformer 100 does not need the application of high voltage and high current in order for a device to test the performance of the current transformer 100 accurately. Magnetic theory can be used to determine the performance of not only current transformers 100, but all magnetic assemblies like power transformers, generators, motors, voltage transformers and reactors. It is the magnetic field, namely the peak magnetizing current and the integral of the applied voltage that determine the performance of the test sample. The peak magnetizing current cannot be reduced. However, the applied voltage can be substantially reduced, drastically reducing the power requirement of the test set up. As the integral of the applied voltage depends on the period, or frequency of the voltage, maximum advantage can be attained by reducing the test frequency to zero or, in other words, using DC excitation. This allows for a maximum reduction in test kVA and allows the test equipment to be smaller, portable and consume little power, and in fact the testing device can be powered from a typical service outlet.
All dimensions specified in this disclosure are by way of example only and are not intended to be limiting. Further, the proportions shown in these Figures are not necessarily to scale. As will be understood by those with skill in the art with reference to this disclosure, the actual dimensions and proportions of any system, any device or part of a system or device disclosed in this disclosure will be determined by its intended use.
Methods and devices that implement the embodiments of the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. Reference in the specification to “one embodiment” or “an embodiment” is intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an embodiment of the invention. The appearances of the phrase “in one embodiment” or “an embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. In addition, the first digit of each reference number indicates the figure where the element first appears.
As used in this disclosure, except where the context requires otherwise, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised” are not intended to exclude other additives, components, integers or steps.
In the following description, specific details are given to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. Well-known circuits, structures and techniques may not be shown in detail in order not to obscure the embodiments. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail.
Also, it is noted that the embodiments may be described as a process that is depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be rearranged. A process is terminated when its operations are completed. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function.
Moreover, a storage may represent one or more devices for storing data, including read-only memory (ROM), random access memory (RAM), magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “machine readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data.
Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, or a combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine-readable medium such as a storage medium or other storage(s). One or more than one processor may perform the necessary tasks in series, distributed, concurrently or in parallel. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or a combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted through a suitable means including memory sharing, message passing, token passing, network transmission, etc.
In the following description, certain terminology is used to describe certain features of one or more embodiments of the invention.
The term “current transformer” refers to a transformer that produces a current in its secondary circuit that is in proportion to its primary current.
The term “knee point” refers to one of three selectable methods: 1) the point at which a line tangent to the log-log plot has an angle of 45 degrees, 2) the point at which a line tangent to the log-log plot has an angle of 30 degrees, 3) the point at which the rms current will in crease by 50% if the average-reading ac voltage is increased by 10%. Each of these three methods is called out in either an ANSI/IEEE or an IEC standard.
Various embodiments provide a device to measure the excitation characteristics of a current transformer using reversible direct current and a method for converting the direct current excitation data to alternating current excitation data at any power frequency. One embodiment of the present invention provides a device to measure the excitation characteristics of a current transformer using reversible direct current. In another embodiment, there is provided a method for converting the direct current excitation data to alternating current excitation data at any power frequency. The device and method will now be disclosed in detail.
Referring now to
Referring now to
Referring now to
As can be seen in the graphs 200 and 300, test objects, especially current transformers 100, can be tested by applying a DC test voltage to a winding 104 and measuring voltage, time and current. The volt-seconds from graph 300 are converted to a voltage 402 at the desired power frequency, and the rms value of the current 404, shown in the graph 200, is calculated. This voltage 402 and current 404 represent one single point on the excitation characteristics of the magnetic assembly. A method for calculating these values is now presented. First, a large number of such test points, typically 5 to 10 per decade, each representing a different excitation voltage 402 and current 404 represent the excitation characteristics of the magnetic assembly when tested with at a fixed DC voltage and reversed at different time periods. This characteristic is shown by the dotted line 408 on the graph 400.
Reversing polarity of the DC test voltage 202 can provide a complete full cycle of excitation tests. A calculation of the time integral of the applied voltage for each test point is proportional to the test voltage and is converted to the voltage 402 at the desired power frequency, and the rms value of the current 404 represents the excitation current for that test point. The test points are to be taken over the full range of excitation 408, namely from the fully saturated condition to the fully unsaturated condition. These test points can be shown plotted on log-log graph paper using Eq. 1 to calculate the point's rms current and Eq. 2 to calculate the point's average-reading voltage.
Next, a low level voltage at the desired power frequency is applied to the winding 104 of the magnetic assembly and the excitation current 404 is measured. This test is repeated over a range of voltages 402 so that a graph of this test can be plotted on log-log graph paper 400, where the plot is represented by line 412. As the measurements are gathered and plotted, the two graphs, one plotted using DC excitation 408 and the other using power frequency excitation 412 will cross at a point 410. This point 410 will be at a test condition where the period of the DC time reversal will be approximately equal to the period of the power frequency and the DC voltage and current 408 will be approximately equal to the power frequency voltage and current 412.
To determine the full excitation characteristics of the magnetic assembly at the desired power frequency from a measurement using DC excitation 408, the DC excitation characteristics need to be adjusted. This can be shown graphically, as shown by the arrows in the graph 400. Alternately, this can be done mathematically by a device comprising instructions for determining the adjustment and the equations for the DC excitation 408, the DC excitation characteristics and power frequency excitation 412.
The crossover 410 of the curves is determined and the adjustment in excitation current at any given voltage is calculated. Once the power frequency AC excitation characteristic is known, then other parameters, such as the ANSI/IEEE 45 and ANSI/IEEE 30 degree knee points can be determined.
Each point on the magnetic assembly excitation characteristics is obtained from a single hysteresis plot using DC excitation. The voltage 402 and current 404 for the point is calculated from a large number of measurements. Each measurement comprises a DC voltage, DC current and time. The rms value of the current and the average value of voltage are calculated from the following formulas:
Where: Ik is the rms current at point k, Uk—is the voltage at point k, k—number of the point, n—the number of samples for point k, i—number of the sample, Δt—the time interval between samples, f—the power frequency (50 or 60 Hz), Ii and Ui—the readings of DC current and DC voltage at each sample (i=1 to n).
Each of the points has from n=200 to n=800 measurements of current and voltage.
Each point on the excitation curve is the result of calculations based on one hysteresis loop. To accomplish the 20-40 hysteresis loops (20-40 points) 200 to 800 readings of excitation voltage and current are taken on each of the loops.
This raw data 408 must be corrected. The corrected AC readings are determined by applying a correction method to the raw data 408. A correction method is described: First, a sine wave voltage of the power frequency is applied to secondary winding of the magnetic assembly. Then, the resulting excitation current (rms) at several selected voltages is measured. In one embodiment, the selected voltages comprise a range of 1-60 volts. However, as will be appreciated by those with skill in the art with reference to this disclosure, other voltage ranges are possible. The selected voltage range is an exemplar and not intended to be limiting. Next the log-log plot of the AC U/I characteristic curve 412 is examined to determine the angle of the (typically) straight line 412:
Next the log-log plot of the raw data U/I curve 408 is examined in the typically straight-line section of low voltage 402 and current 404 to determine the angle of the curve in this (typically) straight-line area:
Finally, a corrected value for current at each measurement point is determined using the formula:
where Ii is a reading of current (not corrected) at some measurement point, Ui is a corresponding voltage at the same measurement point; φI is an angle of Volts/Amps of the raw data determined in eq. 3A; φ is the angle of the power-frequency AC characteristic data 412; and U1 and I1 are the point of intersection of the log-log plot of the raw data 408 and the power-frequency plot 412.
Referring now to
The device 500 can conduct the DC excitation test on a magnetic assembly, like a magnetic assembly 100 and can be built very light weight so that it can be conveniently carried to a test site instead of the equipment to be tested having to be brought to the test equipment. The power required by the device 500 is very low, so that the device 500 can be powered from any convenient power outlet. The device will now be discussed in detail.
The main power module 502 supplies the required voltage and current necessary to test the magnetic assembly, such as a current transformer 100. Depending upon the application and the equipment to be tested, the device can use batteries, household outlets or higher current and voltage outlets without the need for specialized power to be supplied for the testing device as is prevalent in the current art.
The voltage and current regulator 504 provides accurate and consistent voltages and currents to an H bridge inverter 506. The H bridge inverter 506 comprises an electronic circuit that enables a voltage to be applied across a load in either direction. This provides the necessary reversal of DC voltage to conduct testing of the magnetic assembly. A relay steering circuit 508 receives both DC pulse signals from the H bridge inverter 506 and AC test signals from a transformer 512. The transformer 512 also receives 50 Hz to 240 Hz AC input signals from the H bridge inverter 506 to produce the correct AC test signals. The relay steering circuit 508 can transmit both the DC pulse signals and the AC test signals to the equipment being tested. The relay steering circuit 508 also receives and routes voltages and currents received from the equipment under test and can transmit the received voltages and currents to a voltage and current measurement module 514. The voltage and current measurement module 514 can comprise one or more than one measurement circuits depending upon the application and the equipment that the device 500 is designed to test.
The device 500 and all of the testing is controlled by one or more than one microprocessor 510. The one or more than one microprocessor 510 comprises instructions stored in a memory to control the testing parameters and to receive inputs from the voltage and current measurement module 514 and calculate the adjustment needed to complete tests compliant with the ANSI/IEEE C57.13.1 Standard noted above using the method previously discussed.
What has been described is a new and improved device to measure the excitation characteristics of magnetic assemblies using reversible direct current and a method for converting the direct current (DC) excitation data to alternating current (AC) excitation data at any power frequency, overcoming the limitations and disadvantages inherent in the related art.
Although the present invention has been described with a degree of particularity, it is understood that the present disclosure has been made by way of example and that other versions are possible. As various changes could be made in the above description without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be illustrative and not used in a limiting sense. The spirit and scope of the appended claims should not be limited to the description of the preferred versions contained in this disclosure.
All features disclosed in the specification, including the claims, abstracts, and drawings, and all the steps in any method or process disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Each feature disclosed in the specification, including the claims, abstract, and drawings, can be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
Any element in a claim that does not explicitly state “means” for performing a specified function or “step” for performing a specified function should not be interpreted as a “means” or “step” clause as specified in 35 U.S.C. §112.
