Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
  • G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
  • G01R15/18—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using inductive devices, e.g. transformers
  • G01R15/186—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using inductive devices, e.g. transformers using current transformers with a core consisting of two or more parts, e.g. clamp-on type
• • 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

Definitions

• the invention relates to a new embodiment of a measurement current transformer applicable especially for exact measuring of alternating currents within heavy current electrical engineering and electrical power engineering.
• the transformer will enable exact measurement of currents in the grid for power rating purposes.
• the proposed solution can be also suitably used to measure alternating currents of very low frequencies in the area of units to tens of Hz.
• a measurement current transformer 1 whose secondary winding is loaded with a shunt resistor 3, whose resistance value 3 ⁇ 4 should be as low as possible, is presented in Fig. 1.
• the voltage u ⁇ t) generated at the shunt 3 is the electrically measurable quantity carrying the information of the magnitude of the measured current.
• the figure indicates the selected positive orientation of the instantaneous voltages and currents on the input and output terminals of the measurement current transformer 1. The orientation of all four quantities was chosen in such a manner that these quantities correspond to reality. The orientation of the current is consistent with the assumption that the measurement current transformer 1, at its output, is loaded by passive resistance load consisting of a shunt 3.
• the thus connected measurement current transformer 1 exhibits principally irremovable measurement error which arises from the existence of the magnetization current. The occurrence of this error and its magnitude are apparent from the mathematical description provided below.
• the inductances Li, Lg are intrinsic inductances of the winding, M is the mutual inductance between the primary and the secondary winding, k is the coupling coefficient.
• the quantity j ⁇ o signifies an arbitrary integration constant and constitutes a part of the indefinite integral.
• the current transformer circuit model shown in Fig. 2 follows from the above-presented formulas (1-2a) and (1-2b); while the primary side of the circuit model is defined by the equation (1-2a), the secondary side is described by the formula (1-2b).
• magnetic coupling is represented by two controlled sources.
• the primary section thus includes a voltage source u ⁇ t) controlled by the secondary voltage u_L2£t ⁇ , and the secondary side comprises a current source i?j ⁇ (t) controlled by the primary current iji(t).
• the primary voltage converted from the secondary to the primary winding and induced into the measured primary circuit is expressed by the formula
• the input (or leakage) inductance of the transformer introduced into the measured primary circuit is the input (or leakage) inductance of the transformer introduced into the measured primary circuit.
• the state of the art involves solutions based on the inventions WO2007064487A and GB2388914A, in which the resistance R Cu 2 of the secondary winding is compensated by a synthetic resistor having a negative resistance and fabricated by means of an operational amplifier.
• the resistor is realized in such a manner that the low-ohm shunt carrying the measured current is connected invariably between the output and one of the inputs of the operational amplifier.
• the authors of the invention WO2007064487A utilize the inverting input, while the non-inverting input is utilized in GB2388914A.
• the output voltage of the op amp can be directly used as the information carrying quantity but only if compensation of the temperature change of the resistance Rc U 2 of the secondary winding is not required. If such compensation is required, the output voltage cannot be utilized, because the amplification of the amplifier changes due to the thermistor feedback connection, as described by the authors of WO2007064487A.
• the patent GB2388914A does not present any solution of temperature compensation whatsoever. Thus, if the said compensation requirement occurs, it is necessary in the above-described cases to obtain the carrier signal by other means, namely via voltage sensing directly from the low-ohm shunt.
• the first disadvantage of the approaches introduced above consists in that not even one of the terminals of the low-ohm shunt which carries the measured secondary current is connected to earth; such component is therefore referred to as a shunt potentially floating against ground
• a shunt potentially floating against ground In order to enable voltage sensing from floating shunts, we need to use another op amp, applying the very complex differential connection (shown in WO2007064487A).
• the patent GB2388914A does not comprise any solution of the sensing problem.
• the second disadvantage of the said approaches consists in that, at high overcurrents, the voltage on the floating shunts may reach high values, thus destroying the operational amplifier; such result stems from the fact that the shunt is invariably connected between the output and the input of the op amp.
• This condition is a factor limiting the use of both inventions within electrical power engineering, where it is necessary to measure also overcurrents exceeding ten times the value of the nominal current.
• neither invention is it possible to connect an electronic voltage limiter in parallel to the shunt, because even though such limiter would effectively protect the amplifier, it would also become the cause of the loss of information regarding the overcurrent magnitude.
• a measurement current transformer in which ballast resistance consisting of a shunt is connected to terminals of the secondary winding, based on the solution presented herein.
• the said solution consists in that, through its second terminal, a synthetic resistor having a negative resistance is connected in series to the first terminal of the shunt whose second terminal is connected to the second terminal of the secondary winding of the measurement current transformer.
• the first terminal of the synthetic resistor is connected to the first terminal of the secondary winding of the measurement current transformer.
• the resistance of the synthetic resistor is given by the formula
• the synthetic resistor with negative resistance has a first and a second) power supply terminals to enable the connection of a power source.
• a preferable embodiment of the synthetic resistor having a negative resistance is equipped with a control terminal of the control signal input; this terminal is coupled to the second terminal of the shunt.
• the synthetic negative resistance consists of an operational amplifier having a symmetrical power supply; the non-inverting input of the op amp is connected with the said control terminal, and the inverting input of the amplifier is coupled simultaneously to one end of the first feedback resistor, whose other end is connected with the second terminal of the synthetic resistor having a negative resistance, and to one end of the second feedback resistor, whose other end is connected with the output of the op amp.
• the output of the operational amplifier is connected with the first terminal of the synthetic resistor having a negative resistance; the first power supply terminal of the synthetic resistor having a negative resistance is connected with the positive power supply terminal of the operational amplifier, and the second power supply terminal of the synthetic resistor is connected with the negative power supply terminal of the op amp.
• a first source of direct supply voltage is connected between the positive supply terminal and the second terminal of the synthetic resistor having a negative resistance, and a second source of direct supply voltage is connected between the negative power supply terminal and the second terminal of the said resistor.
• the second terminal of the synthetic resistor having a negative resistance constitutes the common circuit earth.
• the input of a current amplifier is coupled to the output of the op amp; the output of the current amplifier is then coupled to the other end of the second feedback resistor, and the positive power supply terminal of the said amplifier is connected with the first supply terminal of the synthetic resistor having a negative resistance.
• the negative power supply terminal of the current amplifier is connected with the second power supply terminal of the synthetic resistor having a negative resistance.
• the two-way complementary emitter follower comprises a bipolar NPN transistor and a bipolar PNP transistor; the interconnected bases of the said transistors are the input of the current amplifier, while their interconnected emitters are the amplifier's output.
• the collector of the bipolar NPN transistor is then connected with the positive power supply terminal of the current amplifier, and the collector of the bipolar PNP transistor is coupled to the negative power supply terminal of the current amplifier.
• a major advantage of the presented measurement current transformer consists in that the device will exhibit only a very small measurement error, the reason being a very low field voltage on the secondary inductance and thus also a very low magnetization current, which is directly equal to the measurement error.
• the hysteresis and eddy losses in the transformer core will be almost zero thanks to the very low magnetic flux, and the reduction of these magnetic losses will facilitate further decrease of the measurement error of the transformer.
• the secondary current flows through the output stage of the operational amplifier, and therefore it has to be small. If this current is higher, such as within tens to hundreds of mA, the output stage of the operational amplifier will require furthter extension, for example by means of a two-way emitter follower.
• Fig. 1 contains the general diagram of the measurement current transformer loaded by shunt resistance
• Fig. 2 illustrates the circuit model of the measurement current transformer
• Fig. 3 shows the basic connection of the measurement current transformer according to the novel solution
• Fig. 4 indicates an example of concrete connection involving a synthetic resistor having a negative resistance and an operational amplifier
• Fig. 5 presents the solution from Fig. 4 completed with a current amplifier at the output of the op amp
• Fig. 6 represents one of the feasible versions of the current amplifier
• Fig. 1 contains the general diagram of the measurement current transformer loaded by shunt resistance
• Fig. 2 illustrates the circuit model of the measurement current transformer
• Fig. 3 shows the basic connection of the measurement current transformer according to the novel solution
• Fig. 4 indicates an example of concrete connection involving a synthetic resistor having a negative resistance and an operational amplifier
• Fig. 5 presents the solution from Fig. 4 completed with a current amplifier at the output of the op amp
• Fig. 6 represents one of the feasible versions
• Fig. 8 contains a graph describing three possible shapes of the secondary current time response to the unit step of the primary current.
• a measurement current transformer ⁇ to whose secondary winding terminals is connected a load resistor comprising a shunt 3 is shown in Fig. 3.
• a synthetic resistor 2 having a negative resistance is connected in series to the first terminal 3J. of the shunt 3, whose second terminal 32 is connected to the second terminal 1 ⁇ 4 of the secondary winding Lg of the measurement current transformer
• the first terminal 2A_ of the synthetic resistor 2 having a negative resistance is connected to the first terminal 1,3 of the secondary winding Lg of the measurement current transformer 1.
• the resistance Rneg of the synthetic resistor 2 is given by the formula
• the synthetic resistor with negative resistance 2 has a first 2.4 and a second 2J> power supply terminals to enable the connection of a power source.
• the synthetic resistor 2 having a negative resistance is equipped with a control terminal .3 of the control signal input; this terminal is coupled to the second terminal 32 of the shunt 3.
• the synthetic resistor 2_having a negative resistance consists of an operational amplifier 6 having a symmetrical power supply; the non-inverting input 6 ⁇ of the op amp 6 is connected with the said control terminal 2 J, and the inverting input 62 of the operational amplifier 6 is coupled simultaneously to one end of the first feedback resistor 7, whose other end is connected with the second terminal 2,2 of the synthetic resistor 2_having a negative resistance, and to one end of the second feedback resistor 8, whose other end is interconnected with the output 6J3 of the op amp 6.
• This output is connected with the first terminal 2A_ of the synthetic resistor 2_having a negative resistance; the first power supply terminal ZA of the synthetic resistor 2 having a negative resistance is connected with the positive power supply terminal 6,4 of the operational amplifier 6, and the second power supply terminal 2J5 of the synthetic resistor 2 having a negative resistance is connected with the negative power supply terminal 6J5 of t ne op amp 6.
• a first source 4 of direct supply voltage is connected between the positive power supply terminal 2 ⁇ and the second terminal 2.2 of the synthetic resistor 2_having a negative resistance
• a second source 5 of direct supply voltage is connected between the negative power supply terminal 2J5 and the second terminal 22 of the synthetic resistor 2 having a negative resistance.
• the second terminal 22 of the said resistor 2 having a negative resistance constitutes the common circuit earth.
• Fig. 5 shows a connection similar to that indicated in Fig. 4.
• the input 9J . of a current amplifier 9 is coupled to the output 6J3 of the operational amplifier 6; the output 92 of the current amplifier 9 is then coupled to the other end of the second feedback resistor 8.
• the positive power supply terminal 9 ⁇ 3 of the current amplifier 9 is connected with the first power supply terminal 2 ⁇ of the synthetic resistor 2_having a negative resistance, and the negative supply terminal 9A of the said amplifier is connected with the second power supply terminal 2J> of the synthetic resistor 2_having a negative resistance.
• the current amplifier 9 can be composed of a two-way complementary emitter follower without bias at the base-emitter junctions, Fig. 6.
• the complementary emitter follower comprises an NPN transistor 10 and a PNP transistor 1_1 ; the interconnected bases are the input 9J . of the current amplifier 9, and the interconnected emitters are the output 92 of the said amplifier 9.
• the collector of the NPN transistor 10 is then connected with the positive power supply terminal SL3 of the current amplifier 9, and the collector of the PNP transistor 11 . is coupled to the negative power supply terminal 9 ⁇ 4 of the current amplifier 9,
• the principle of the invention consists in that the load resistance composed of a synthetic resistor 2_having a negative resistance and a shunt 3 connected in series is coupled to the first terminal 1J3 and the second terminal 1 ⁇ 4 of the secondary winding 1-2 of the measurement current transformer 1 (Fig. 3). Between its first terminal 2J . and second terminal 22, the said synthetic resistor 2_having a negative resistance exhibits a negative resistance Rneg.
• the synthetic resistor 2 having negative resistance can be realized using an arbitrary physical principle.
• every resistor having a negative resistance principally behaves as a source, not a consumer. Energy must nevertheless be supplied to the synthetic resistor 2_having a negative resistance from an external source; the resistor 2 having a negative resistance is therefore equipped with the first power supply terminal 2 ⁇ and the second power supply terminal 2J>, which facilitate its connection to power sources.
• the information-carrying signal in the form of voltage carries the information of the magnitude of the measured current iijQl, and it can be invariably sensed between the first terminal 3J. and the second terminal 3.2 as a voltage drop u ⁇ (t) generated at the shunt 3.
• the said signal can be also sensed between the first terminal 2J. and the second terminal 22 as a voltage drop u (t) generated at the synthetic resistor 2 having a negative resistance; such operation, however is only possible if the value of the negative resistance Rneg is stable, or constant and not changing in time.
• the negative resistance Rneg must be large enough in order for the total equivalent resistance R E in the secondary circuit loop to be equal to zero: where R Cu 2 is the resistance of the secondary winding, R ⁇ is the resistance of the shunt 3.
• the formula can be rewritten to a form which will convey the necessary magnitude of the negative resistance: neg Cu2 + (2-2)
• the three resistances RCJ J 2, R S , Eaga are all connected in series; thus, their values will add up, and the sum (namely the resulting equivalent resistance RE) is equal to zero. At the zero resistance, zero field voltage will generate; this will result in zero magnetization current and principally also zero measurement error.
• the synthetic resistor 2 having a negative resistance is advantageously realized on an electronic basis, Fig. 4.
• Such electronic realization is supported by an operational amplifier 6, which requires information of the magnitude of the secondary current i?(t), meaning that it requires a control signal.
• the synthetic resistor 2_having a negative resistance is therefore equipped with a control terminal 2.3; this terminal functions as the input for leading in the control signal.
• the voltage drop obtained directly from the shunt 3 can be used as the control signal for the synthetic resistor 2 having a negative resistance, and this means that the control terminal 2 3 of the synthetic resistor 2_having a negative resistance is coupled to the second terminal 3,2 of the shunt 3.
• the output 6J3 of the operational amplifier 6 can be further expanded with any suitable current amplifier 9, Fig. 5; this step is carried out because the secondary current igii) of the measurement current transformer 1, namely the current which must flow out of the output of the operational amplifier 6, is usually higher than the maximum permissible current of the op amp 6.
• the current amplifier 9 can be realized, for example, according to Fig. 6.
• the device is a two-way complementary emitter follower comprising an NPN bipolar transistor 10 and a PNP bipolar transistor Y ⁇ _.
• the follower does not have to contain circuits for the generation of bias at the base-emitter junctions of both transistors, namely circuits eliminating junction distortion; the reason consists in that the secondary winding j_2 of the measurement current transformer 1 behaves towards the current amplifier 9 as a current source j ⁇ Ql, not as a voltage source. The output of the current amplifier 9 therefore operates in the current mode, in which junction distortion is not considered.
• the magnetization current t) is an integral from the secondary field voltage
• the current ⁇ ⁇ ⁇ signifies an arbitrary initial integration constant.
• the connection is non-inverting, the orientation of the voltages Ugjt) and LJ tl is selected in a homothetical manner towards earth.
• the voltage Un is the voltage of the dummy source representing the voltage asymmetry of the operational amplifier 6.
• condition (3.1 -12) can be satisfied if we use a very precise operational amplifier 6 exhibiting sufficiently low input voltage asymmetry U or apply an amplifier in which the input asymmetry can be compensated by an external compensation circuit, such as a resistor trimmer.
• the winding resistance Rcu2 is temperature- dependent; we can assume that the resistance of the shunt 3 and the resistance Ri of the first feedback resistor 7 are temperature-independent.
• the temperature dependence of the resistance R CU 2 can be compensated using the resistance Ro_of the second feedback resistor 8. In this case, the resistance R 2 must exhibit the same relative temperature dependence as the resistance R ⁇ ; the assertion follows directly from the formula (3-14).
• the feedback loop closed from the output of the operational amplifier 6 through the secondary winding of the measurement current transformer 1 to the non- inverting input of the operational amplifier 6 creates positive feedback.
• the total voltage amplification Ku in the loop is given by the voltage transfer of the operational amplifier 6 and the voltage transfer of the resistor divider composed of the resistances Reu2, R ⁇ :
• the equivalent resistance RE can be positive, zero, or negative; the response (3-21) to the unit step of the primary current will then be realized in three different ways, Fig. 8: a) RE > 0, Ku ⁇ 1 , the stability condition:
• the current j ⁇ Xii will be constant at the magnitude of (I2,K-UN/RE); the time constant ⁇ 2 ⁇ .
• the voltage asymmetry U N must be, in its absolute value, as small as possible; otherwise, the fraction UN/RE may diverge, namely UN/RE ⁇ c) R E ⁇ 0, Ku > 1 , the non-stability condition:
• the current ⁇ will exponentially grow with the time constant x 2 from the initial value ( .K - UN/RE) to infinity.
• This is a non-stability mode (also referred to as the bistable mode) in which the output voltage of the OA saturates to the value of the positive or negative supply voltage.
• the operational amplifier 6 is supplied symmetrically (for example, ⁇ 15V).
• the resistance Reu2 of the secondary winding, the resistance R ⁇ of the shunt 3, the resistance of the first feedback resistor 7, and the resistance RQ of the second feedback resistor 8 must satisfy the elementary compensation condition (3-22). It is a well-known fact that the resistance RCu2 of the secondary winding is temperature-dependent, and the said dependence applies to copper and aluminium winding. If it is assumed that the resistance Rs of the shunt 3 and the resistance Ri of the first feedback resistor 7 are temperature- independent, the thermal dependence of the secondary winding resistance Rc U 2 can be compensated by means of the resistance R ⁇ of the second feedback resistor 8.
• the resistance Ro must exhibit relative temperature dependence identical with or similar to that found in the resistance Rcu2- The assertion follows directly from the condition (3-22). If the resistance Rg of the second feedback resistor 8 is constant and is not used for temperature compensation of the secondary winding resistance Rcu2 , then it is possible to collect the amplified output signal uo directly at the output of the operational amplifier 6. Conversely, if the resistance RQ is temperature- dependent because it provides for temperature compensation of the resistance RQ ⁇ then it is necessary to collect the non-amplified, output voltage signal u ⁇ directly from the shunt 3.
• the measurement current transformer based on the novel solution is well utilizable in the following branches of industry: electrical power engineering, where it can be advantageously used for exact measurement of alternating currents in power supply systems, especially when high measurement precision is required for power rating purposes; metrology, where the device will find application in the sector of very precise measurement current transformers designed for calibration or the realization of measurement standards; heavy current electrical engineering, where the requirement for the measurement of alternating currents at very low frequencies has to be satisfied (for example, in alternating controlled drive mechanisms with asynchronous motors, the level of the first harmonic component of stator currents changes between 1 Hz and approximately 200 Hz).
• the transformer can be also utilized in the measurement of electromagnetic compatibility (EMC); according to EMC standards, phase currents in power distribution systems have to be measured up to the 50 th harmonic component, including subharmonic components. Thus, the corresponding measurement should be performed in the frequency range of 1 Hz and 2500 Hz.
• EMC electromagnetic compatibility

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measurement Of Current Or Voltage (AREA)
• Measuring Instrument Details And Bridges, And Automatic Balancing Devices (AREA)

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• 2013
  • 2013-02-26 CZ CZ2013-142A patent/CZ304406B6/cs not_active IP Right Cessation
  • 2013-12-20 WO PCT/CZ2013/000175 patent/WO2014131378A1/en not_active Ceased

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