Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F27/00—Details of transformers or inductances, in general
  • H01F27/42—Circuits specially adapted for the purpose of modifying, or compensating for, electric characteristics of transformers, reactors, or choke coils
  • H01F27/422—Circuits specially adapted for the purpose of modifying, or compensating for, electric characteristics of transformers, reactors, or choke coils for instrument transformers
  • H01F27/427—Circuits specially adapted for the purpose of modifying, or compensating for, electric characteristics of transformers, reactors, or choke coils for instrument transformers for current transformers
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H1/00—Details of emergency protective circuit arrangements
  • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H1/00—Details of emergency protective circuit arrangements
  • H02H1/04—Arrangements for preventing response to transient abnormal conditions, e.g. to lightning or to short duration over voltage or oscillations; Damping the influence of dc component by short circuits in ac networks
  • H02H1/046—Arrangements for preventing response to transient abnormal conditions, e.g. to lightning or to short duration over voltage or oscillations; Damping the influence of dc component by short circuits in ac networks upon detecting saturation of current transformers

Definitions

• TECHNICAL FIELD Within largely all distribution of electric power, monitoring systems for detecting short circuits and other abnormal states are needed. Instrument transformers for measuring current and voltage therefore constitute an important and integrated part of control, monitoring and protective devices in most power distribution systems. It is, of course, very important that these measuring devices correctly reproduce the quantities they are designed to measure, both with regard to static and dynamic values. Since both control and monitoring devices and particularly protective devices are nowadays based on instantaneous value measurement, very high demands are placed on good dynamic conformity during the measurements.
• a typical example is a differential protection device which, when a current transformer saturation occurs, may lead to an external fault being incorrectly detected as an intersystem fault. It is therefore of great importance to be able to detect whether a current transformer has become subjected to such a current that it has become magnetically saturated.
• the present invention describes a method for reconstructing, during saturation, with the aid of the distorted secondary current, the primary current which causes the saturation. A reconstruction is also performed of the secondary current which would have been obtained unless the current transformer had become saturated.
• the invention also comprises a method for detecting saturation and detecting when the saturation ceases.
• the invention also comprises a device for carrying out the method.
• Figure 1 shows an equivalent diagram for a current transformer seen from the secondary side.
• FIG. 2 shows a flow diagram according to the invention.
• Equations (3), (4) and (5) describe the model in continuous time.
• x(k + 1) Ad(L ⁇ ) x(k) + B d (L ⁇ ) i p (k) (6)
• i s (k) C s x(k) (7)
• i ⁇ (k) C ⁇ x(k) (8)
• a d (L ⁇ ) e A c (L ⁇ )h (9)
• B d (L ⁇ ) e A c (L ⁇ )s ds B c (L ⁇ ) (10)
• i(k) i(t)
• t kh (12) where k is a running index.
• T means transposition
• ⁇ is a parameter vector
• ⁇ a regression vector
• K is a gain vector.
• One way of calculating "K” is shown in B Carlsson, "Digital Differentiating Filters and Model Based Fault Detection” , PhD thesis , Department of Technology, Uppsala University, Uppsala, Sweden, 1989 , pp . 72-76.
• LiTh-ISY-I 120 from the Department of Electrical Engineering, Linköping University, Linköping entitled “Estimation of the primary current in a saturated transformer” , 1991, presented by K W Chen and S T Glad, a method is described based on a Kalman filter applied to a signal model which is based on Taylor series expansion and connected to a current transformer model . Otherwise , continuous time is used here with a time-varying Kalmangain, which per se requires a high numerical capacity. To detect saturation, a simple detector is used, based on the variance of the estimated fault .
• the invention relates to a numerical method which, via discrete-time measurements of the secondary current i s (k) from a current transformer, reconstructs the primary current and the secondary current in case of saturation of current transformers and a method for detecting saturation as well as for detecting when saturation ceases.
• the reconstruction takes place with the aid of current transformer models of an unsaturated and a saturated current transformer as well as a signal model of the primary current, wherein the the current transformer models are fed from the signal model and saturation is decided by a decision logic unit when the output signal from the saturated current transformer model gives a better description of the secondary current than what the output signal from the unsaturated current transformer model gives.
• the output signals i pr (k) and i sr (k) in case of a decision about saturated current transformer consist of a reconstructed primary current i pn (k) and a reconstructed secondary current i sn (k) and, in case of a decision about unsaturated current transformer, of a reconstructed primary current i pn (k) and an actually measured secondary current i s (k).
• the invention also relates to a device for carrying out the above-mentioned methods.
• the invention is based on the current transformer model which is described by equations (1) and (2) and the general state-space model described by equations (3), (4) and (5).
• the invention is based on discretization according to equations (9) and (10), a sampled signal model according to equations (13) ... (16), and on the above-described combined signal and current transformer model which is described by equations (17) ... (25).
• two values of the inductance L ⁇ are used, which is denoted
• a d (L ⁇ s ) e A c (L ⁇ s ) h
• the numerical method according to the invention completes a number of steps for each sampled measurement of the secondary current.
• the different steps comprise reconstruction from an unsaturated model of a current transformer, reconstruction from a saturated model, detection of saturation, as well as decision about returning from a saturated to an unsaturated state.
• i pn (k) is a reconstruction of the primary current
• i sn (k) is a reconstruction of the secondary current
• the invention is further based on the fact that the effect of i s (k) on the state reconstruction (k) is shut off as soon as a saturation has occurred. This is done with the aid of the gain parameter "g" which, the first time a saturated state is detected, is changed from 1 to 0. This means that during saturation, the currents will be reconstructed based on measurements of the secondary current prior to saturation.
• the invention calculates and uses a state reconstruction (k) for the saturated model which is determined by the state reconstruction (k-d-1), that is, a state reconstruction obtained d+1 samples earlier.
• the counter for the variable d shows that equation (40) is true for a number of consecutive samples d > f according to block B5, a decision about saturation is made, which takes place in the block B6 where also the variable M is set equal to 0.
• block B8 will deliver a value of the primary current i pr (k) which is equal to the reconstructed primary current i pn (k) and a value of the reconstructed secondary current i sr (k) which is equal to the measured secondary current i s (k).
• the output signal i pr (k) from block B8 will then be represented by the reconstructed primary current i pn (k) and the output signal i sr (k) by the reconstructed secondary current i sn (k).
• a preferred embodiment with two current transformer models is used, one representing an unsaturated model and the other a saturated model.
• several current transformer models based on fixed magnetizing inductances with a value which lies between those corresponding to a nominal and a saturated value, may be used.
• the signal model may also consist of harmonics according to equation (11).
• a variant of an embodiment with regard to detection of saturation may be replaced by another detection principle, for example a method described in SE 9100917-5.
• the reconstructed magnetizing current i ⁇ when saturation has been determined, may be stored. During saturation, the magnetizing current will be practically equal to the reconstructed primary current i p (k). This means that the magnetizing current increases drastically during saturation. Information to the effect that saturation has ceased may be obtained when the magnetizing current decreases below the stored value. It is, of course, also possible to use i s (k) directly to make a decision about restoration to an unsaturated state.
• a device for carrying out the method according to the invention preferably forms an integral part of a protective relay where current measurement takes place with the aid of current transformers. From the design point of view, the device may be arranged in the form of a number of blocks according to Figure 2 or constitute an integrated device.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Measurement Of Current Or Voltage (AREA)
• Measuring Instrument Details And Bridges, And Automatic Balancing Devices (AREA)
• Transformers For Measuring Instruments (AREA)

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• 1992
  • 1992-01-03 SE SE9200011A patent/SE469676B/sv not_active IP Right Cessation
  • 1992-12-14 CA CA 2126867 patent/CA2126867A1/en not_active Abandoned
  • 1992-12-14 WO PCT/SE1992/000863 patent/WO1993013581A1/en not_active Application Discontinuation
  • 1992-12-14 EP EP93901453A patent/EP0619922A1/en not_active Withdrawn

Non-Patent Citations (1)

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