A) TECHNICAL FIELD

[0001] The present invention generally relates to a current measurement device and particularly to a current measurement device and method for measuring high current. The present invention is more particularly relates to current measurement device and method for measuring high current in devices like energy meter etc.

B) BACKGROUND OF THE PRESENT INVENTION

[0002] An energy meter is a device that measures the amount of electrical energy produced or supplied to residence, industries, business, machine etc. Most of the energy meters are capable of monitoring steady-state or continuous power quality conditions such as high current, voltage imbalance, under voltage or over voltage. The measurement of current with accuracy plays an important role in energy meters. Normally the current is measured using current transformers. By practice, it has been proved that the current transformer method is the best current measurement method in terms of cost and immunity to harmonic injection tests compared to Hall Effect and Shunt Resistor methods.

[0003] The Current Transformers (CT) are used to measure AC amperage in an electrical circuit. The CT is installed around an energized conductor and sense the magnetic field generated by the current flowing in the circuit. The output signal in the CT is proportional to the current flowing in the circuit. Generally, the current transformers comprise a primary winding which forms part of the main circuit carrying the current to be measured, and a secondary winding which is magnetically coupled to the primary winding and across which a measurement instrument is connected. The conventional transformer used for amplifying currents, however has the following disadvantages. When the primary current and hence the secondary current which is proportional thereto is large, or when the secondary side leakage impedance or the externally connected burden is large, the dimension of the core of the transformer should be large enough to avoid the saturation thereof. As a result, the size and the cost of the transformer become high.

[0004] However, when the value of load current becomes large, the size of current transformer too becomes large. This would demand larger space and another disadvantage posed is the associated magnetic field surrounding it. Also the harmonics and spurious noise are inserted to the amplifier input by using the current transformers.

[0005] Hence there is a need to measure the high current in energy meter easily efficiently and cheaply without increasing the size of the current transformers. Also there is a need to suppress the harmonics and the spurious noise from reaching the measurement device as well as the load.

[0006] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.

C) OBJECT OF THE INVENTION

[0007] The primary object of the present invention is to develop a current measurement method and device to measure high currents in energy meter easily, efficiently and cheaply.

[0008] Another object of the present invention is to develop a current measurement method and device to measure the high currents in a transformer with out increasing the size of the current transformer.

[0009] Yet another object of the present invention is to suppress the harmonics and the spurious noise from reaching the measurement device and the load.

[0010] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

D) SUMMARY OF THE INVENTION

[0011] The various embodiments of the present invention provide a current measurement method and device to measure the high currents in energy meter easily efficiently and cheaply without increasing the size of the current transformer. The method and device of the present invention uses the current amplification method to measure the high current in transformers using bipolar junction transistors.

[0012] According to one embodiment of the present invention, the system has a pair of bipolar transistors Ql 1 - Q21 and Q12 - Q22 arranged in a super – beta configuration for low drop. The transistors are connected top an input current to be measured from a source. The transistors are complementary current gain matched pairs. The pairs of the transistors Q11, Q12 and Q21, Q22 are used for operating in the positive and negative halves of the current waveforms respectively. A current transformer is connected to the two sets of transistors. A resistance is connected between the current transformer and the sets of transistors. A capacitor is connected to the resistance to form a filter circuit. A bleed resistor is connected across the current transformer. The bleeder resistor is calibrated to give a voltage which is proportional to a load current.

[0013] According to another embodiment of the present invention, pair of transistors Ql and Q2 are connected to a diode bridge rectifier including four diodes D1-D4. The base of the transistor Ql is connected to collector of the transistor Q2. The Collector of Ql and Emitter of Q2 are connected together and fed by the positive swing of rectified input from the diode bridge rectifier. A resistance Rl is connected across the base and collector of the transistor pair. A current transformer is connected across the base and the collector of the transistor Q2. A capacitor is connected with the resistance Rl to form a low pass filter which is used to remove the disturbing frequencies. A bleeder resistor R2 is connected across the current transformer. The current through the resistance Rl is equal to the inverse of the product of the gains of the transistors Ql, Q2. The bleeder resistor is calibrated to give a voltage which is proportional to a load current. The bridge rectifier makes the current through the transistor pair Q1Q2 to be always unidirectional. The bridge rectifier acts as follows: It keeps the power input source and power to load as AC (bidirectional) itself. For the measuring circuit formed by Q1-Q2 etc., it act as DC(unidirectional) link so that the transistor circuit behave uniformly irrespective of whether input voltage is positive swing or negative.

[0014] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.

E) BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:

[0016] FIG.l shows a block circuit diagram of a current measurement device according to one embodiment of the present invention.

[0017] FIG.2 shows a block circuit diagram of a current measurement device according to another embodiment of the present invention.

[0018] FIG. 3 shows a block diagram of the energy meter according to one embodiment of the present invention.

[0019] FIG. 4 illustrates a flow diagram to calculate energy in an energy meter according to an embodiment of the present invention.

[0020] Although specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the present invention.

F) DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0021] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.

[0022] The various embodiments of the present invention provide a current measurement method and device to measure the high currents in energy meter easily efficiently and cheaply without increasing the size of the current transformer. The method and devices of the present invention uses the current amplification method to measure the high current in transformers using bipolar junction transistors.

[0023] According to one embodiment of the present invention, the system has a pair of bipolar transistors Q11-Q21 and Q12-Q22 arranged in a super - beta configuration for low drop. The transistors are connected to an input current to be measured from a source. The transistors are complementary current gain matched pairs. The pairs of the transistors Ql 1, Q12 and Q21, Q22 are used for operating in the positive and negative halves of the current waveforms respectively. A current transformer is connected to the two sets of transistors. A resistance is connected between the current transformer and the sets of transistors. A capacitor is connected to the resistance to form a filter circuit. A bleed resistor is connected across the current transformer. The bleeder resistor is calibrated to give a voltage which is proportional to a load current.

[0024] According to another embodiment of the present invention, pair of transistors Ql and Q2 are connected to a diode bridge rectifier including four diodes. The base of the transistor Ql is connected to the emitter of the transistor Q2 while the collectors of the transistors are connected to each other. A resistance Rl is connected across the base and collector of the transistor pair. A current transformer is connected across the base and the collector of the transistor Q2. A capacitor CI is connected with the resistance Rl to form a low pass filter which is used to eject the disturbing frequencies. A bleeder resistor R2 is connected across the current transformer. The current through the resistance Rl is equal to the inverse of the product of the gains of the transistors Ql, Q2. The bleeder resistor is calibrated to give a voltage which is proportional to a load current. The bridge rectifier makes the current through the transistor pair Q1Q2 to be always unidirectional.

[0025] FIG. 1 shows a block circuit diagram of a current measurement device according to one embodiment of the present invention. The device has a pair of bipolar transistors Ql 1-Q21 and Q12-Q22 arranged in a super-beta configuration for low drop. The transistors are complementary current gain matched pairs. The positive half of the current waveform is conducted by Q11, Q12 complimentary pair. Transistor Q11 with NPN configuration is paired with Q12 with PNP configuration. Q11 sources the current when the input is positive. Ql2 provides base current to Ql1. The current gain of Ql 1 is denoted by pi and current gain of Q12 is denoted by (32.

[0026] The negative half of the current waveform is conducted by Q21, Q22 complimentary pair. Transistor Q21 with NPN configuration is paired with Q22 with PNP configuration. Q21 sources the current when input is negative. Q22 provides base current to Q21. The current gain of Q21 is denoted by pi and current gain of Q22 is denoted by p2.

[0027] A capacitor CI is connected to the emitters of Ql 1 and Q21. Capacitor CI stores the DC charge during flow of current in one of the two complimentary pair circuits. The capacitor CI together with the resistance Rl acts as a filter.

[0028] The resistor Rl is connected in series with filter capacitor CI. The current through the resistance Rl is equal to the product of load current and the inverse of the product of the gains of the transistors Ql, Q2 (1/ pi * P2). The resistor Rl and filter capacitor CI are connected in series to form an RC filter. The low pass RC filter is connected across the complimentary pairs Ql 1, Q12 and Q21, Q22. The filter is tuned to reject the disturbing frequencies.

[0029] A current transformer (CT) is connected across the complimentary pairs. The current transformer (CT) has single primary winding with primary current in The current transformer (CT) has plurality of windings with n of turns in the secondary
winding. The current in secondary winding is i2. The value of i2 can be calculated using the equation:

i2 = il /n

With reference to FIG. 1, the value of il can be calculated by using the equation:

i1 = (current to be measured)/ (P1 * P2)

[0030] A bleeder resistor R2 is connected in parallel to the current transformer (CT). The bleeder resistor R2 drains the DC charge stored in the filter capacitor CI. The voltage proportional to the load current (Im) may be calculated by the equation:

Proportional voltage = i2 x R2

The above equation can be written as:

Im = i2 x R2

[0031] The bleeder resistor R2 is calibrated to provide a voltage proportional to the load current. The value of R2 is selected in such a way that the voltage Im should be within the limits of sensing circuit for a range of possible load currents. Therefore, R2= Im/ i2.

[0032] FIG.2 shows a block circuit diagram of a current measurement device according to another embodiment of the present invention. With respect to FIG.2, pair of transistors Ql and Q2 are connected to a bridge rectifier including four diodes. The base of the transistor QI is connected to the collector of the transistor Q2 while the emitter of Q2 is connected to collector of the transistor Ql. A resistance Rl is connected across the base and collector of the transistor pair. A current transformer is connected across the base and the collector of the transistor Q2. A capacitor CI is connected with the resistance Rl to form a low pass filter which is used to eject the disturbing frequencies. A bleeder resistor R2 is connected across the current transformer (CT). The current through the resistance Rl is equal to the product of load current and the inverse of the product of the gains of the transistors Ql, Q2. The bleeder resistor is calibrated to give a voltage which is proportional to a load current. The bridge rectifier acts as follows. It keeps the power input source and power to load as AC (bidirectional) itself. For the measuring circuit formed by Q1-Q2 etc., it act as DC(unidirectional) link, so that the transistor circuit behave uniformly irrespective of whether the input voltage is in positive swing or negative. The bridge rectifier makes the current through the transistor pair Ql Q2 to be always unidirectional.

[0033] FIG. 3 shows a block diagram of the energy meter according to one embodiment of the present invention. The energy meter 30 is fed with signals proportional to input voltage and load current. The signal proportional to the input line voltage, Vm is measured by a voltage transformer or voltage divider. The signal processing module 31 conditions the analog signal according to the requirement of the analog to digital converter (ADC). The conditioned' analog signal is calibrated to reduce distortion. The ADC converts the calibrated analog signal to digital signal. The signal proportional to load current Im is fed to the signal processing module 32. The signal processing module 32 conditions the analog signal according to the requirement of the analog to digital converter (ADC). The conditioned analog signal is calibrated to reduce distortion. The ADC converts the calibrated analog signal to digital signal.

[0034] The processing unit 33 includes multiplier and accumulator circuits to compute the value of energy E and memory unit to store the value of E. The multiplier 34 calculates the value of energy using the equation:

E = Vm x Im.

[0035] The accumulator 35 queries the Memory 36 for stored value of E. T and adds the calculated value, Vm x Im, to the stored value of E. The Memory 36 stores the accumulated value of E which is displayed on the display panel 37.

[0036] FIG. 4 illustrates a flow diagram to calculate energy in an energy meter according to one embodiment of the present invention. The signal processor in the energy meter checks input voltage (41). When input voltage is present the energy meter gets the values of input voltage Vm and load current Im (42). The Vm and Im signals are processed and converted to digital signals. The multiplier multiplies the digital values of Vm and Im to calculate the value of energy, E (43). The multiplier sends the computed value of E to accumulator. The accumulator fetches the stored value of energy E from memory (44). The accumulator adds the computed value of E received from the multiplier to the stored value of E received from memory (45). The new accumulated value of E is updated in the memory (46). The energy meter checks the presence of input voltage signal until it is detected (41). G)

ADVANTAGES OF THE INVENTION

[0037] Thus the various embodiments of the present invention provide a current measurement method and device to measure the high currents in energy meter easily efficiently and cheaply without increasing the size of the current transformer. The method and devices of the present invention uses the current amplification method to measure the high current in transformers using bipolar junction transistors. The devices and method of the present invention suppresses the harmonics and the spurious noises from reaching the measurement instrument and load.

[0038] Although the invention is described with various specific embodiments, it will be obvious for a person skilled in the art to practice the invention with modifications. However, all such modifications are deemed to be within the scope of the invention.

[0039] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the present invention described herein and all the statements of the scope of the invention which as a matter of language might be
said to fall there between.

CLAIMS
What is claimed is:

1. A current measurement system for high current in energy meter comprising:

At least two sets of transistors connected in pair to an input current to measured from a source;

A current transformer connected to the two sets of transformer;

A resistance connected between the current transformer and the sets of transistors;

A capacitor connected to the resistance; and

A bleed resistor connected across the current transformer.

2. The system according to claim 1, wherein the each set of transistor conduct in positive swing and in the negative swing of the input current respectively.

3. The system according to claim 1, wherein the each set of transistor includes a pair of transistors.

4. The system according to claim 1, wherein the set of transistors are arranged in super beta configuration for low drop out.

5. The system according to claim 1, wherein each pair of transistors is complementary current gain matched pairs of transistors.

6. The system according to claim 1, wherein the resistance connected between the two sets of transistors.

7. The system according to claim 1, wherein the capacitor is connected to the resistance to form a filter circuit to remove the harmonics.

8. The system according to claim l, wherein the bleeder resistor is calibrated to give a voltage which is proportional to a load current.

9. A current measurement system for high current in energy meter comprising:

A diode bridge rectifier connected to an input current to be measured from a
power source;

A pair of transistors connected to the diode rectifier;

a current transformer connected to the pair of transistors;

a resistance connected to the pair of transistors;

a capacitor is connected to the resistance; and

a bleed resistor connected across the current transformer.

10. The system according to claim 9, wherein the diode bridge rectifier includes plurality of diodes.

11. The system according to claim 9, wherein the diode bridge rectifier and the pair of transistors provide two different paths for conducting current through the positive and the negative swing of the input current.

12. The system according to claim 9, wherein the pair of transistors are complementary pairs to provide a unidirectional flow of current during both the positive swing and the negative swing of the input current.

13. The system according to claim 9, wherein the resistance is connected across the base and collector of the transistor pair.

14. The system according to claim 9, wherein the capacitor is connected to the transistor to form a filter circuit to remove the unwanted harmonics.

15. The system according to claim 9, wherein the bleeder resistor is calibrated to
give a voltage which is proportional to a load current.