Claims:We claim:

1. A modified voltage divider apparatus for voltage metering in a system having protection relays comprising a microprocessor based relay, wherein the apparatus comprises:
a means for an external supply of voltage;
a means for an internal supply of voltage;
a neutral ground reference serving as the reference potential for the microprocessor, wherein the said neutral connection is directly connected to the ground-reference of the other electrical and/or electronic components of the system which is on the relay side of the system;
one or more resistance components (R1, R2) as connected are provided in the neutral path to limit the current originating from the potential difference between external supply reference voltage and the internal supply reference voltage of the apparatus; and
wherein the microprocessor is configured to a means for measuring the voltage based on voltage divider, and
the one or more resistance components (R1, R2) are configured to modify the voltage divider circuit to limit the current.

2. The apparatus as claimed in claim 1, wherein the apparatus comprises an operational amplifier (Opamp), wherein the Opamp is configured to provide a fixed reference at the negative terminal of the Opamp.

3. The apparatus as claimed in claim 2, wherein the Opamp is configured to provide no direct link between the reference voltage of the system and the neutral reference.

4. The apparatus as claimed in claim 2, wherein the Opamp is configured to provide isolation to the apparatus thereby providing incomplete path for the flow of current to the microprocessor.

5. The apparatus as claimed in claim 1, wherein the one or more resistance values (R1, R2) are chosen from a Data Acquisition Module of the system.

6. The apparatus as claimed in claim 5, wherein the resistances are so chosen that the voltage divider satisfies the relation given as

Vo =(R2*V1)/(R1+R2)
where,
Vo is the stepped-down voltage that can be measured by the means for measuring the voltage, and
V1 is translated input voltage by using the values R1 and R2.

7. The apparatus as claimed in claim 1, wherein the voltage metering system is single phase voltage metering system or three phase voltage metering system.

8. The apparatus as claimed in claim 1, wherein multiple relays are fed through different generators keeping the neutral connection common for all the relays.

Dated this 29th day of March 2019

Abhishek Sen
Of S. Majumdar & Co.
(Applicant’s Agent)
Registration No. 980
, Description:
TECHNICAL FIELD OF THE INVENTION

The present subject matter described herein relates generally to voltage measurement or voltage metering, particularly, to an apparatus concerned with voltage metering in protection relays and, more particularly to an apparatus concerned with the protection of the relay in systems having protection relays.

BACKGROUND AND THE PRIOR ART

A protective relay is a relay device designed to trip a circuit breaker when a fault is detected. In conventional art, there exist microprocessor-based protection relays that provide different types of protection and has supervisory functions as well.

The Protection Relays which is microprocessor based relay are designed for feeder, motor, generator and transformer protection. Overvoltage and under voltage protection, over frequency and under frequency protection, breaker failure protection, directional current supervision, fault diagnostics, remote terminal unit (RTU), and other customizable logic functions are provided by it. This relay also provides phase, neutral, ground and negative sequence, instantaneous and time over current protection. The time over current function provides multiple curve shapes for optimum co-ordination.

In conventional voltage dividers, the neutral connection of the field side of the system having the protection relay is directly connected to the ground-reference for the electronics circuit which is on the relay side of the system. This same ground-reference serves as reference potential for the microprocessor and is directly connected to the microprocessor. Considering a situation where the relay is powered using an auxiliary supply source and its return path is common with neutral for both single phase voltage metering system and three phase voltage metering system. Multiple relays can be fed to this supply from a common generator source such that their neutral connection is common. Multiple relays can be fed through different generators keeping the neutral connection common for all the relays. Since the reference for different generators can be at different potentials with respect to earth for the whole station, even a slight voltage difference can damage the relay. This is so because the neutral does not see any current limiting component. As a result, even a slight voltage difference induces infinitely huge current in the path and damages all the components coming in the path. Since microprocessor is also connected in that path, the microprocessor will also get damaged leading to ultimate disaster in the absence of any decision making entity, thereby resulting in no protections and a situation of absolute blackout.

Reference is made to US4300182A that discloses a system for electronically metering an A.C. electrical power system to derive performance parameters such as real and reactive power useful in revenue metering and fault protection applications. It talks of filtering means for separately sampling of the voltage and current output signals at least once during each period of the power system to develop respective Walsh function correlation coefficient output signals for each of the voltage and current output signals and a combining means for combining the Walsh function correlation coefficient output signals to determine real and reactive power parameters of the power system.

Reference is made to US6504357B1 that discloses an apparatus for metering a plurality of types of electrical power and electronically communicating electrical power information. A further reference is made to US20110012603A1 that discloses a method of monitoring a protective relay system that talks of a controller electrically coupled to the sensor to receive the second signal, the controller having a processor responsive to executable computer instructions when executed on the processor for comparing the second signal to a database of reference signals and initiating a first alarm when the second signal substantially matches a signature in the database of reference signals.

Reference is made to EP2293401 A1 that discloses a protecting system for voltage transformers in which the switching element has a control input connected to an output of control means suitable for controlling a time sequence of activating and deactivating of the damping burden by controlling the conductive and nonconductive states of the switching element.

In view of the prior art, it is considered to be a good practice that every circuit is provided with the means for its isolation. Using a transformer is the ideal way of assuring Galvanic Isolation but it consumes lot of space and is a costly solution. Other means to assure isolation in the neutral path is by adding high impedance in the neutral path. However, in that case, it modifies the conventional resistive-based voltage divider formula such that it becomes more difficult to estimate the input voltages in 3-phase system in unbalanced condition.

Therefore hitherto, there is a dire need to provide an efficient, reliable, simple, and cost effective solution that isolates the circuit providing safety of the microprocessor in a system having microprocessor based protection relays.

SUMMARY OF THE INVENTION

The following disclosure presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.

An object of the present invention is to overcome the drawbacks of the prior art of the invention.

Another object of the present invention is to provide an apparatus to isolate the circuit providing safety of the microprocessor during over current in a system having microprocessor based protection relays.

Yet another object of the present invention is to provide an efficient, reliable, simple, and cost effective solution for protecting the microprocessor of a protection relay during over current.

A further object of the present invention is to provide isolation to stop the current rushing into microprocessor through the neutral line which offers zero impedance in a microprocessor based relay.

Briefly, the different aspects of the subject matter described herein are directed at a modified voltage divider apparatus for voltage metering in a system having protection relays comprising a microprocessor based relay, wherein the apparatus comprises a means for an external supply of voltage; a means for an internal supply of voltage; a neutral ground reference serving as the reference potential for the microprocessor, wherein this neutral connection is directly connected to the ground-reference of the other electrical and/or electronic components of the system which is on the relay side of the system; one or more resistance components as connected are provided in the neutral path to limit the current originating from the potential difference between external supply reference voltage and the internal supply reference voltage of the apparatus; and wherein the microprocessor is configured to a means for measuring the voltage based on voltage divider, and the one or more resistance components are configured to modify the voltage divider circuit to limit the current.

Other salient features and advantages of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which:

Figure 1 illustrates the protection system comprising microprocessor based protection relay in accordance with the conventional art.

Figure 2 illustrates the Voltage Divider Circuit (Single Phase) in accordance with the conventional art.

Figure 3 illustrates the Voltage Divider Circuit (Three Phase) in accordance with the conventional art.

Figure 4 illustrates the Output Waveforms of the voltage of the voltage divider circuit (Three Phase) in accordance with the conventional art.

Figure 5 illustrates the Voltage Divider Circuit (Three Phase) in accordance with the present invention (modification 1).

Figure 6 illustrates the Output Waveforms of the voltage of the voltage divider circuit (Three Phase) shown in figure 5, in accordance with the present invention.

Figure 7 illustrates the Voltage Divider Circuit (Three Phase) in accordance with the present invention (modification 2).

Figure 8 illustrates the Output Waveforms of the voltage of the voltage divider circuit (Three Phase) shown in figure 7, in accordance with the present invention.

Figure 9 illustrates an instance of the Voltage Divider Circuit (Three Phase) in accordance with an embodiment of the present invention (modification 2).

Figure 10 illustrates an instance of the Voltage Divider Circuit (Three Phase) in accordance with an embodiment of the present invention (modification 2) for deriving the mathematical relation.

Figure 11 illustrates the Voltage Divider Circuit (Three Phase) in accordance with an embodiment of the present invention (modification 3).

Figure 12 illustrates the implemented Voltage Divider Circuit (Three Phase) in accordance with an embodiment of the present invention (modification 3).

Figure 13 illustrates the implemented Output Waveforms of the voltage of the voltage divider circuit (Three Phase) shown in figure 12, in accordance with an embodiment of the present invention.

Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure. Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.

Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

The terms and words used in the following description are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

By the term “substantially” wherever used or will be used later it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.

Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.

It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

In one embodiment of the present invention is provided a modified voltage divider apparatus for voltage metering in a system having protection relays comprising a microprocessor based relay. The Protection Relay is a microprocessor based relay which can be designed for feeder, motor, generator and transformer protection. Overvoltage and under voltage protection, over frequency and under frequency protection, breaker failure protection, directional current supervision, fault diagnostics, remote terminal unit (RTU), and programmable logic functions are provided. This relay also provides phase, neutral, ground and negative sequence, instantaneous and time over current protection. The time over current function provides multiple curve shapes for optimum co-ordination.

Figure 1 shows a protection system having multiple protection relays in the conventional art.

The measurement of voltage in the system is based on voltage divider. Two main types of voltage dividers are existent in the art, namely the capacitive one and the resistive one. The output in both cases is a low-level voltage signal. The output is linear throughout the whole rated measurement range. A single phase conventional voltage divider circuit is shown in figure 2. In the following is given the conventional resistive-based voltage divider results into following formula:
Vo = (R2*V1)/(R1+R2) ……….(A)

where, Vo is the stepped-down voltage that can be measured by the microprocessor and can be translated into input voltage i.e. V1 by using the values resistance R1 and R2.

The same result is used and applied to a conventional 3-phase system as the one shown in figure 3, where each phase input is given by calculating potential across ‘R2’ connected in each phase. Thus, voltage divider results into following formula
Von= (R2*Vn)/(R1+R2) ,

where n = voltage at node marked as red, voltage at node marked as Yellow and voltage at node marked as Blue. Figure 4 illustrates the corresponding waveform of the voltage of the nodes marked as red, blue and yellow.

The neutral connection of the field side of the protection system comprising the microprocessor based relay is directly connected to the ground-reference for the electronics circuit which is on the relay side. This same ground-reference serves as reference potential for the microprocessor and is directly connected to the microprocessor. The relay may be powered by using an auxiliary supply source and its return path is common with the neutral for a single or a three phase voltage metering system. The multiple relays can be fed this supply from a common generator source such that their neutral connection is common.

In an aspect of the present invention is provided a modified voltage divider apparatus for voltage metering in a system having protection relays comprising a microprocessor based relay, wherein the apparatus comprises a means for an external supply of voltage and a means for an internal supply of voltage. The external supply of voltage may include but not limited to generator, while the internal supply of voltage may include the voltage from any other electronic component present in the section between the nodes of the circuit. Because of the common reference voltage, it is necessary to protect the microprocessor of the relay during over current situations. Accordingly, in the present invention, the following modifications of the circuit are suggested to protect the microprocessor.

Modification 1: A high impedance may be added in the neutral path to limit the current originating from the potential difference between external supply reference and internal reference of the circuit, as is seen in figure 5. However, with such modifications it was observed that the peak values of the waveform have changed with respect to the peak values observed in the conventional circuit. Figure 6 illustrates the waveform of the voltage metering done as per the modified 3 phase voltage divider circuit of figure 5.

Modification 2: To overcome problem associated with the aforementioned modification of the conventional voltage divider circuit, the circuit is modified by adding one more resistance as connected. The corresponding output waveforms of the voltage have been observed in this condition and the same is shown in figure 8.

In an embodiment of the present invention is provided an apparatus having modification in the conventional voltage divider circuit for single phase. The modified voltage divider circuit (single phase) of the said apparatus is shown in figure 9. The resistances (R1, R2) to be connected are chosen to meet the requirement of Data Acquisition Module of the system. These components are used to modify the conventional voltage divider circuit as shown in figure 9 to obtain the same voltage divider formula as applicable for conventional voltage divider circuit, i.e. equation (A).

In the circuit shown in figure 9, a nodal analysis has been applied to determine output Voltage at node ‘2’ with respect to the reference in terms of input voltage, V1 shown in the figure 9. Further, the voltages at node ‘4’, ‘1’ and ‘2’ are considered as X, (V1 + X) and Vo. Nodal analysis is applied at node ‘2’.

Further,
(Vo-(Vin+X))/R1+(Vo- 0)/2R2 = 0
Vo{1/R1+1/2R2}– X/R1 = Vin/R1 ………….…(1)

At node ‘0’
(0-X)/R1 +(0-Vo)/2R2= 0 ………….… (2)
From equation (2),
X =-(R1*Vo)/2R2 ………….… (3)

Substituting the value of X in equation 1 from equation 3,
Vo{1/R1+1/2R2} +Vo/2R2 = Vin/R1
Vo/R1+Vo/2R2+Vo/2R2 = Vin/R1
Vo/R1+Vo/R2= Vin/R1
Vo{1/R1+1/R2}= Vin/R1
Vo {(R1+R2)/(R1*R2)} = Vin/R1
Vo = (R2*Vin)/(R1+R2)

This derived formula is same as the one for the conventional voltage divider formula.

In another embodiment of the present invention is provided an apparatus having modification in the conventional voltage divider circuit for three phase circuit. Figure 10 illustrates the modification in the circuit for 3-phase circuit as per the present invention. The result obtained in previous derivation for the former embodiment can be derived for the embodiment having 3-phase system also. For ease of calculation, V1 = V2 = V3 is considered. So, output potentials will also be equal, i.e. potential at node ‘2’, ‘3’ and ‘5’ will also be equal, (say, Vo).

The voltages at node ‘8’, ‘4’ and ‘1’ are considered equal to Y, X and (V1 + X). Now, by nodal analysis is applied at node ‘8’

3{(Y-Vo)/R2}+(Y- 0)/R2 = 0
Y= 3/4 Vo ………….… (1)

At node ‘0’
(0-X)/R1 +(0-Y)/R2= 0
X = - R1/R2 Y
X = - 3/4 R1/R2 Vo ………….… (2)

At node ‘2’
(Vo-Y)/R2 +(Vo-(V1+X))/R1= 0 ………….… (3)

Substituting the value of X and Y in equation 3 from equation 2 and 1 respectively,
Vo{1/R1+1/2R2}-3Vo/4R2 +3Vo/4R2= Vin/R1
Vo/R1+Vo/R2= Vin/R1
Vo =(R2*Vin)/(R1+R2)

Again, the above derived formula is same as the one for the conventional voltage divider formula.

Modification 3: With above mentioned ‘modification 2’, it was observed that the circuit is proven and tested for normal conditions and simulated for the field conditions in the laboratory. However, further to add robustness to the circuit an Operational Amplifier (Opamp) can be used. The Opamp and its property of high input impedance between its two input terminals of the Opamp can be used provide additional advantageous to the voltage divider. The use of the Opamp assures a fixed reference at its negative terminal which in turn is used as the reference for the calculations. The use of Opamp also assures that no direct link exists between reference of the system and the neutral reference of the field side. The thus modified circuit along with the Opamp is shown in figure 11.

In an implementation of the present invention, the apparatus having the modified circuit as detailed above is shown in figure 12 and the corresponding wave form of the output voltage is illustrated in figure 13. The complete circuit has been tested by simulating the field conditions and was found to work effectively.

For protecting the microprocessor of the microprocessor based relays, a need for isolation was identified from the field side to stop the current rushing into controller through neutral line which offers zero impedance. A transformer would have been the ideal solution but it was demanding much space and change in the design approach. Isolated supplies have been also identified but they are too costly.

The present invention explores the fact if current is to be limited in a path then limiting it with a resistor is feasible. But, it has been seen, that it becomes difficult to estimate the input current in 3-phase system since that resistor compensates the imbalance automatically and it becomes unpredictable to estimate it since it involves complex calculations involving phase angles of the input voltages. Since it was becoming difficult in 3-phase system, first calculations for only 1-phase have been analyzed. A mathematical formula has been derived with the addition of one more resistance which made the compensation more deterministic and, thus, the modified circuit as detailed herein above of the present invention has been obtained. The modification of the present invention involving the Opamp’s high input impedance is used for assuring the isolation and thereby assuring that there is no complete path of the flow of current to the microprocessor, eventually safe guarding the microprocessor from overcurrent.

Some of the noteworthy features of the present invention are mentioned below:
The present invention is simple to implement and works with single phase system and equally works well with three phase system.

The present invention is reliable and robust.

Some of the non-limiting advantages of the present invention over the prior art are mentioned below:

Issues have been reported from the sites where Voltage Module has been observed to fail right at the moment when it is powered on. It has then been observed that, a high current was flowing (captured by Power Analyzer) which was damaging the microprocessor, which in turn damaged the entire relay. With the present invention, such damages can be prevented, eventually providing longer life span to the microprocessor.

It does not take as much space as a transformer does to protect the microprocessor.

The solution provided by the present invention is not as costly as compared to its prior art.