Description:TECHNICAL FIELD
[0001] The embodiments of the present disclosure generally relate to current transformer (CT) and utility meters. More particularly, the present disclosure relates to system and method to achieve class of accuracy during multi power factor DC immunity in current transformers.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Programmable electronic energy meters are rapidly replacing electro-mechanical meters due to the enhanced functionality achieved using programmable logic integrated into solid-state electronic meters. Some of these meters can be used to meter various different electrical services without hardware modification. Electronic energy meters are instruments that measure the flow of energy. Electronic energy meters typically do this by sensing the current and voltage. The power is derived from the sensed currents and voltages, and energy is defined as the measurement of power over time.
[0004] Voltage and current signals are primarily sinusoidal. Voltage and current sensors are used in a meter to convert the primary signals to a signal that can be processed. One type of current sensor commonly used in electronic meters is a current transformer. A current transformer is a device that is used to produce low ac current in the secondary winding which is proportional to the high ac current in the primary winding. It is used for both metering and protection system. So, with the help of CT, high value of current can be measured easily by using low value meters.
[0005] In an ideal current transformer, the secondary current is equal to the primary current divided by the turn’s ratio. In practice, current transformers are non-ideal, having losses in the burden, the copper wire in the windings, and the core itself. These characteristics result in amplitude and phase deviations as compared to an ideal current transformer.
[0006] The current transformer's phase shift is predominately determined by the inductance, the winding resistance, and the burden resistance. The current transformer essentially behaves as a high pass filter with the inductance and the sum of the winding and burden resistances setting the Cutt-Off frequency. In order to reduce this phase shift error, electronic energy meters typically use core materials having a very high relative permeability to obtain a high inductance.
[0007] In some markets, it is desired for meters in direct-connected applications to be accurate even in the presence of significant half-wave rectified currents. An example of this can be found in the IEC-1036 requirements. As a half-wave rectified waveform has significant DC content, it is necessary for current sensors in such meters to be sufficiently immune to DC in the primary current. High permeability cores become saturated quickly in the presence of DC current and hence have limited application with this requirement.
[0008] For current transformers, immunity from DC current can be improved by increasing core area, by selecting alternative core materials that have a higher saturation level, and by lowering the relative permeability of the core material. In general, increasing the core geometry is limited due to cost and space requirements. Examples of alternative core materials are nanocrystalline and amorphous materials. These materials have recently become economically feasible and reliable. Although such materials improve the DC immunity it is still necessary to lower the overall relative permeability to provide an appropriate solution. This DC immunity comes at a cost, however. As the permeability and inductance of the current sensors are reduced, the phase shift error is greater.
[0009] To summarize, the current transformers work on the principle of mutual inductance. The core of the CT should be ferro-magnetic material to achieve the flux coupling to the secondary of the CT. There is blending of Amorphous and nano crystalline materials (alloys) to achieve both mutual coupling as well as immunity to pulsating DC current. This blended CT is called as dual core CT.
[0010] As per the principle of magnetism, mutual inductance principle works on the alternating current, however during pulsating current or DC current, the CT tends to saturate and starts providing incorrect output after saturation point. During the positive half of the sinusoidal current, coercive path charges and during negative half, B-H curve (also known as the magnetization curve or hysteresis curve) takes the residual path of the Hysteresis. When the pulsating current is fed to the CT, then the only coercive path is active hence reaching near saturation of the BH curve as shown in FIG. 1. The error graph during 360-degree operation of voltage and current during pulsating DC input to the current transformer is shown in FIG. 2. CTs are having common tendency to have small leakage current flow when the flow of current is chopped as shown in FIG. 6. This behavior creates the issues during the different angles between the Voltage and current. For example, in case of different angle between voltage and current provided, the output goes beyond the expected result of error. In our case the expected error is +/- 3%.
[0011] Thus, a need exists to compensate phase shift due to multi power factor pulsating direct current (DC) output of the current sensors in an electronic energy meter.

SUMMARY
[0012] To overcome this observation, this invention is developed on the energy meters.
[0013] The behavior of the CT during the different power factor is captured and tabulated. Here the power factor is referred to the angle between the Incoming voltage and current to the energy meter as a system. These errors at different power factor (PF) are caused by the area under curve which is generated by the CT. The % errors are also tabulated below:

[0014] Aspects of the present invention relates to compensating for phase shift due to multi power factor pulsating direct current (DC) in an energy meter. The present invention dynamically corrects for phase shift in an electronic energy meter by sensing current provided to the energy meter, obtaining a waveform of the sensed current, determining an offset of the DC component (if DC offset is present), and thereby determining a difference between the detected offset and the obtained current from the input power supply. The determined difference and the detected offset are used to a new waveform for the current and feeding the new waveform current to the meter and thereby compensates for the phase shift due to multi power factor pulsating direct current (DC) in an energy meter.
[0015] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The accompanying drawings, which are incorporated herein, and constitute a part of this invention, illustrate exemplary embodiments of the disclosed methods and systems in which like reference numerals refer to the same parts throughout the different drawings. Components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Some drawings may indicate the components using block diagrams and may not represent the internal circuitry of each component. It will be appreciated by those skilled in the art that invention of such drawings includes the invention of electrical components, electronic components or circuitry commonly used to implement such components.
[0017] FIG. 1 illustrates hysteresis curve know in the prior-art.
[0018] FIG. 2 illustrates DC immunity test results before the invention is being implemented (prior-art).
[0019] FIG. 3 illustrates area under curve representation in unified power format (UPF) (prior-art).
[0020] FIG. 4 illustrates area under curve representation in 0.5 Lag (prior-art).
[0021] FIG. 5 illustrates area under curve representation in 0.5 Lead (prior-art).
[0022] FIG. 6 illustrates CT Raw waveform in pulsating DC application (prior-art).
[0023] FIG. 7 illustrates area under curve representation in UPF for pulsating DC, in according to embodiments of the present disclosure.
[0024] FIG. 8 illustrates area under curve representation in 0.5 Lag for Pulsating DC, in according to embodiments of the present disclosure.
[0025] FIG. 9 illustrates area under curve representation in 0.5 Lead for Pulsating DC, in according to embodiments of the present disclosure.
[0026] FIG. 10 illustrates wave correction in firmware, in according to embodiments of the present disclosure.
[0027] FIG. 11 illustrates a flowchart of a method for protecting a current transformer (CT) against a multi power factor pulsating direct current (DC), in according to embodiments of the present disclosure.
[0028] FIG. 12 illustrates a system to protect a current transformer (CT) against a multi power factor pulsating direct current (DC), in according to embodiments of the present disclosure.
[0029] The foregoing shall be more apparent from the following more detailed description of the invention.

DETAILED DESCRIPTION
[0030] Various example embodiments will now be described more fully with reference to the accompanying drawings in which only some example embodiments are shown. Specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. The present invention, however, may be embodied in many alternate forms and should not be construed as limited to only the example embodiments set forth herein.
[0031] Accordingly, while example embodiments of the invention are capable of various modifications and alternative forms, embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments of the present invention to the particular forms disclosed. On the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of the invention. Like numbers refer to like elements throughout the description of the figures.
[0032] It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or,” includes any and all combinations of one or more of the associated listed items.
[0033] It will be understood that when an element is referred to as being “connected,” or “coupled,” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected,” or “directly coupled,” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between,” versus “directly between,” “adjacent,” versus “directly adjacent,” etc.).
[0034] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the terms “and/or” and “at least one of” include any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
[0035] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0036] Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein are interpreted accordingly.
[0037] Following terms are used in the application, to explain the working of the invention:
[0038] UPF: Unity power factor.
[0039] DC: Pulsating or Direct current.
[0040] CT: Current transformer.
[0041] WS: wave shaped.
[0042] Lag: The condition where the current is delayed in time with respect to the voltage in an ac circuit.
[0043] Lead: The condition where the voltage is delayed in time with respect to the current in an ac circuit.
[0044] The present invention dynamically corrects phase shift in an electronic energy meter by sensing current provided to the energy meter, obtaining a waveform of the sensed current, determining an offset of the DC component (if DC offset is present), and thereby determining a difference between the detected offset and the obtained current from the input power supply. The determined difference and the detected offset are used to a new waveform for the current and feeding the new waveform current to the meter and thereby compensates for the phase shift due to multi power factor pulsating direct current (DC) in an energy meter.
[0045] To elaborate the invention, it is well known that the power in electrical system is a derived quantity from the voltage and current. This is the integration of voltage and current sampled over a time period (t). When an angle between the voltage and current increases, the error in DC wave increases. This is caused by the area under curve of the V and I. the additional angle introduced by the CT further shifts the power area (as shown in FIGs. 3, 4 and 5).
[0046] As shown in FIG. 8, during pulsating current, the errors during lag are in +VE side and increasing with the angle. The “power” graph before was little more compared to the after wave-shaped WS Power. This runtime wave shaping provides the proper amount of area under curve and thereby reducing the +ve errors during lag conditions.
[0047] Similarly, as shown in FIG. 9, during pulsating current, the errors during lead are in -VE side and decreasing with the angle. The power graph without wave shape provides the -VE errors. The wave-shaped WS power graph is having reduced area under curve compared to non-wave-shaped signal.
[0048] This runtime wave shaping provides the proper amount of area under curve and thereby reducing the -ve errors during lead conditions.
[0049] FIG. 10 shows the typical comparison of the normal waveform to modified waveform through the software implementation.
[0050] As sown in FIG. 11, the wave shaping is achieved by the following method steps:
[0051] Check for DC component in in the Current waveform.
[0052] Find the midpoint of the DC component. That is called as offset of DC component. DC component is the constant voltage added to a pure AC waveform. For example, the true average voltage a pure AC waveform would be zero. When the AC waveform has a DC component, the average voltage would be equal to the DC voltage instead, because opposite peaks cancel each other leaving only the DC component.
[0053] Check the sign of the DC offset by the region of axis. The DC offset can be either positive or negative, with the sign indicating the direction of the shift.
[0054] Get the new sample by:
X = Present sample – Dc offset
Present sample = Present sample – X.
[0055] To elaborate, FIG. 11 provides a flowchart for method of compensating for phase shift due to multi power factor pulsating direct current (DC) in an energy meter. In an embodiment, the method includes following steps:
[0056] Once a current transformer (CT) coupled to the to the energy meter senses current provided to the energy meter, and a waveform of the sensed current which describes a shape of at least one cycle of the current is obtained by the CT.
[0057] At step 1102, a processor determines if the DC component is present in the waveform or not.
[0058] At step 1104, the processor coupled to the CT detects an offset of the DC component if a DC component is present in the waveform.
[0059] At step 1106, the processor determines a difference between the detected offset and the obtained current from the input power supply.
[0060] At step 1108, the processor generates a new waveform for the current by adding the determined difference and the detected offset to thereby compensate for the phase shift due to multi power factor pulsating direct current (DC) in an energy meter.
[0061] In an exemplary embodiment, the current transformer (CT) is a dual core CT having a magnetic core and a conductor winding around the magnetic core.
[0062] In an exemplary embodiment, the offset of the DC component is a midpoint of the DC component.
[0063] In an exemplary embodiment, the difference is determined based on a sign of the DC offset by a region of an axis.
[0064] In an exemplary embodiment, the method achieves the accuracy under less than 3% when the pulsating DC input is received to the CT.
[0065] In an embodiment, a system (1200) compensating for phase shift due to multi power factor pulsating direct current (DC) in an energy meter is provided. The system includes a current transformer (CT) (1204) coupled to the to the energy meter, the CT configured to sense current provided to the energy meter, and obtain a waveform of the sensed current. The waveform describes a shape of at least one cycle of the current.
[0066] The system further includes a processor (1202) communicably coupled to the current transformer (1204). The processor is configured to determine a presence of a DC component in the waveform, detect if the DC component is present, an offset of the DC component, determine a difference between the detected offset and the obtained current from the input power supply, and generate a new waveform by adding the determined difference and the detected offset to thereby compensate for the phase shift due to multi power factor pulsating direct current (DC) in an energy meter.
[0067] In an exemplary embodiment, the current transformer (CT) is a dual core CT having a magnetic core and a conductor winding around the magnetic core.
[0068] In an exemplary embodiment, the offset of the DC component is a midpoint of the DC component.
[0069] In an exemplary embodiment, the difference is determined based on a sign of the DC offset by a region of an axis.
[0070] In an exemplary embodiment, the system achieves the accuracy under less than 3% when the pulsating DC input is received to the CT.
[0071] To summarize,
[0072] FIG. 1 illustrates hysteresis diagram that shows the Magnetic flux linking and relation to saturation by an ferromagnetic material.
[0073] FIG. 2 illustrates errors taken when DC component in signal is being processed by an CT at different angles.
[0074] FIG. 3 illustrates area of power quantity when both the V&I are sinusoidal and with 0 angle.
[0075] FIG. 4 illustrates area of power quantity when both the V&I are sinusoidal and with lagging angle.
[0076] FIG. 5 illustrates Shows area of power quantity when both the V&I are sinusoidal and with leading angle.
[0077] FIG. 6 illustrates the waveform of CT when the pulsating current is inputted.
[0078] FIG. 7 illustrates area of power quantity when Voltage is sinusoidal and current is Pulsating DC and angle between V&I is 0.
[0079] FIG. 8 illustrates Shows area of power quantity when Voltage is sinusoidal and current is Pulsating DC and angle between V&I is lagging.
[0080] FIG. 9 illustrates area of power quantity when Voltage is sinusoidal and current is Pulsating DC and angle between V&I is leading.
[0081] FIG. 10 illustrates waveforms with before and after the firmware logic.
[0082] Working example: Energy meters in real world are subjected to injection of pulsating DC signals over transmission lines due to external noise signals or due to injection of DC signals on the load side by the load equipment. Also, as a part of tampering of energy meter in the field, some consumers may inject such DC signals on input or output of the meter. In all such scenarios the declared invention will take care of identifying such signals and eliminate those signals making measurement error free.
[0083] What are described above are merely preferred embodiments of the present invention, and are not to limit the present invention; any modification, equivalent replacement and improvement within the principle of the present invention should be included in the protection scope of the present invention.
[0084] The example embodiment or each example embodiment should not be understood as a restriction of the invention. Rather, numerous variations and modifications are possible in the context of the present disclosure, in particular those variants and combinations which can be inferred by the person skilled in the art with regard to achieving the object for example by combination or modification of individual features or elements or method steps that are described in connection with the general or specific part of the description and are contained in the claims and/or the drawings, and, by way of combinable features, lead to a new subject matter or to new method steps or sequences of method steps, including insofar as they concern production, testing and operating methods.
[0085] References back that are used in dependent claims indicate the further embodiment of the subject matter of the main claim by way of the features of the respective dependent claim; they should not be understood as dispensing with obtaining independent protection of the subject matter for the combinations of features in the referred-back dependent claims. Furthermore, with regard to interpreting the claims, where a feature is concretized in more specific detail in a subordinate claim, it should be assumed that such a restriction is not present in the respective preceding claims.
[0086] Since the subject matter of the dependent claims in relation to the prior art on the priority date may form separate and independent inventions, the applicant reserves the right to make them the subject matter of independent claims or divisional declarations. They may furthermore also contain independent inventions which have a configuration that is independent of the subject matters of the preceding dependent claims.
[0087] Further, elements and/or features of different example embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.
[0088] Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

, Claims:1. A method of compensating for phase shift due to multi power factor pulsating direct current (DC) in an energy meter, the method comprising
sensing, by a current transformer (CT) coupled to the to the energy meter, current provided to the energy meter;
obtaining, by the CT, a waveform of the sensed current, wherein the waveform describes a shape of at least one cycle of the current;
determining (1102), by a processor coupled to the CT, presence of a DC component in the waveform;
detecting (1104), by the processor, if the DC component is present, an offset of the DC component;
determining (1106), by the processor, a difference between the detected offset and the obtained current from the input power supply; and
generating (1108), by the processor, a new waveform for the current by adding the determined difference and the detected offset to thereby compensate for the phase shift due to multi power factor pulsating direct current (DC) in an energy meter.
2. The method as claimed in claim 1, wherein the current transformer (CT) is a dual core CT having a magnetic core and a conductor winding around the magnetic core.
3. The method as claimed in claim 1, wherein the offset of the DC component is a midpoint of the DC component.
4. The method as claimed in claim 1, wherein the difference is determined based on a sign of the DC offset by a region of an axis.
5. The method as claimed in claim 1, wherein the method achieves the accuracy under less than 3% when the pulsating DC input is received to the CT.
6. A system (1200) compensating for phase shift due to multi power factor pulsating direct current (DC) in an energy meter, the system comprising:
a current transformer (CT) (1204) coupled to the to the energy meter, the CT configured to sense current provided to the energy meter, and obtain a waveform of the sensed current, wherein the waveform describes a shape of at least one cycle of the current;
a processor (1202) communicably coupled to the current transformer (1204), wherein the processor is configured to:
determine presence of a DC component in the waveform;
detect if the DC component is present, an offset of the DC component;
determine a difference between the detected offset and the obtained current from the input power supply; and
generate a new waveform by adding the determined difference and the detected offset to thereby compensate for the phase shift due to multi power factor pulsating direct current (DC) in an energy meter.
7. The system as claimed in claim 6, wherein the current transformer (CT) is a dual core CT having a magnetic core and a conductor winding around the magnetic core.
8. The system as claimed in claim 6, wherein the offset of the DC component is a midpoint of the DC component.
9. The system as claimed in claim 6, wherein the difference is determined based on a sign of the DC offset by a region of an axis.
10. The system as claimed in claim 6, wherein the system achieves the accuracy under less than 3% when the pulsating DC input is received to the CT.