FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
AND
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2005
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Time Variant System and Method for protecting transmission line
APPLICANT :
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR(S):
Mylavarapu Ramamoorty and Pasumarthi Umamahesh, both of Crompton Greaves Ltd, Electronic Design centre, CG Global R&D Center Centre, Crompton Greaves Limited, Kanjur Marg, Mumbai 400 042, Maharashtra, India; an Indian National.
PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

Field of the Invention:
This invention relates to the field of electric power system operation and control and power system protection, thereof.
Particularly, this invention relates to a time variant system and method for protecting transmission line.
Background of the Invention:
Transmission lines, specifically, electronic transmission lines refers to the transmission of bulk electric power from one location (typically, a power plant) to another (typically, a substation). These transmission lines are set up to cover long distances of transmission. High Voltage direct current technology is used for transmission in order to reduce the transmission losses in long distance transmission.
Transmitting electricity at high voltage reduces the fraction of energy lost to resistance. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor.
Operators of long transmission lines require reliable communications for control of the power grid and, often, associated generation and distribution facilities. Fault-sensing protective relays at each end of the line must communicate to monitor the flow of power into and out of the protected line section so that faulted conductors or equipment can be quickly de-energized and the balance of the system restored.

A 'fault' for the purposes of this specification relates to losses in electric supply along a transmission line, said losses resulting in los of power being transmitted.
In order to assure the safe power transfer over the transmission line There is a need for a system which protects the transmission line from transient and short circuit faults, in relation to the power being transmitted, at pre-defined intervals, and which is further adapted to take action within a defined time period (instantaneously) so as to provide uninterrupted and uncompromised transmission of power.
Prior Art:
US7714592 discloses a system and method for determining the impedance of a medium voltage power line.
GB364021 discloses improvements in or relating to electrical power transmission systems.
US6397156 discloses an impedance measurement system for power system transmission lines.
None of the above prior art documents disclose a faster fault detection and tripping system or the avoidance of a zonal system for fault detection in power transmission lines.

Objects of the Invention:
An object of the invention is to adopt a fast transient protection of transmission line for faults.
Another object of the invention is to provide tripping action depending on fault of monitored transmission line.
Yet another object of the invention is to provide a relatively faster tripping action.
Summary of the Invention:
According to this invention, there is provided a time variant system for protecting transmission line, said system comprises:
- reference impedance database means adapted to store reference impedances at pre-determined locations along a transmission line;
- voltage and current monitoring means at each of said pre-determined locations along said transmission line;
- calculation means adapted to calculate impedances based on said monitored voltages and currents;
- computation means adapted to compute a matrix graph of reference impedance versus time over the variation of calculated impedance versus time and further adapted to compare the monitored impedance value with a percentage value of reference impedance to obtain a time delay;
- fault detection means adapted to detect a fault, at any pre-defined location, after said time delay, based on said computation means; and

- actuation means adapted to actuate a trip relay after detection of said fault, at said pre-defined location.
Typically, said actuation means includes a timer means with a preset time adapted to actuate trip relay after expiration of said preset time.
Typically, said system includes a time measurement means adapted to measure time of transmission up to each of said pre-defined locations and to relay said measured time to said computation means.
Typically, said computation means includes a comparator means adapted to compare instantaneous calculated impedance value, with respect to time, with corresponding reference impedance value and further adapted to detect fault is instantaneous calculated impedance value decreases, with respect to time, decreases in relation to corresponding reference impedance value.
Brief Description of the Accompanying Drawings:
Figure 1 illustrates a schematic transmission line from a source (10) to sub-sources (12, 14, 16);
Figure 2 illustrates a schematic block diagram of the system for monitoring health of a transmission line, according to the prior art; and

Figure 3 illustrates a graph of conductance in relation to computed impedances Zl, Z2, Z3. A fault line on the graph depicts the time taken to detect the fault across the zones defined by Zl, Z2, and Z3.
The invention will now be described in relation to the accompanying drawings, in which:
Figures 4 and 5 illustrate a schematic of time variant protection system of a transmission line, according to this invention.
Figure 6 illustrates a graph of reference impedance versus time over the variation of calculated impedance versus time.
Detailed Description of the Accompanying Drawings:
Figures 1, 2, and 3 relate to the prior art system and technique of tripping m the event of fault detection along a transmission line.
Figure 1 illustrates a schematic transmission line from a source (10) to sub-sources (12,14, 16).
Reference numeral VI, II refers to the voltage and current, respectively, at sub-source 12. Hence, Zl refers to the impedance (= Vl/Il) at sub-source 12. Reference numeral V2, 12 refers to the voltage and current, respectively, at sub-source 14. Hence, Z2 refers to the impedance (= V2/I2) at sub-source 14. Reference numeral V3, 13 refers to the voltage and current, respectively, at sub-source 16. Hence, Z3 refers to the impedance (= V3/I3) at sub-source 16.

Figure 2 illustrates a schematic block diagram of the time variant system for protection of a transmission line, according to the prior art.
VI, II (and hence, Zl), V2, 12 (and hence, Z2), V3, 13 (and hence, Z3) were monitored at discrete pre-defined time intervals at the sub-sources 12, 14, 16 (of Figure 1). Reference impedances Zlref, Z2ref, and Z3ref are fed to a individual comparators (22, 24, 26). The comparators (CI, C2, C3) receive a secondary input from the monitored and calculated impedances Zl, Z2, and Z3 respectively. Trip 1 relates to an instantaneous trip actuated by this system when Zl is not within predefined limits of Zlref. Trip 2 relates to a time-delayed trip actuated by this system when Z2 is not within pre-defined limits of Z2ref. Trip 3 relates to an IDMT (Inverse Definite Minimum Time) trip actuated by this system when Z3 is not within pre-defined limits of Z3ref.
Figure 3 illustrates a graph of conductance in relation to computed impedances Zl.
Z2, Z3. A fault line on the graph depicts the time taken to detect the fault across
the zones defined by Zl, Z2, and Z3.
As shown in the block diagram of Figure 3 of the accompanying drawings, existing
distance relays perform in different manners in all different zones.
Zonel (smallest circle): critical Zone or the nearest zone to the incoming bus;
operates instantaneously with the fault.
Zone 2 (medium circle): next zone to the critical zone a bit away from the incoming bus; operates with a constant time delay with fault.

Zone 3 (largest circle): this is a far-away zone from incoming bus and critical zone for other side bus, so for incoming bus it operates with a time variant IDMT curve characteristics with fault.
But, for all these zones, the decision making time is high where computing impedance from the input parameters and fault settling time varies as per the type of fault, where the decision time goes up to 50 power cycles. When the impedance value falls very closer to zone 1, but computes in zone 2 still it's a critical area; but system takes decision according to zone 2 which is a false decision as with the new improved technologies.
According to this invention, there is provided a time variant system and method for protecting transmission line from transient faults.
Figures 4 and 5 illustrate a schematic of system (100) for fast transient protection of a transmission line, according to this invention.
Figure 6 illustrates a graph of reference impedance versus time over the variation of calculated impedance versus time.
In accordance with an embodiment of this invention, there is provided a reference impedance database means (D) adapted to store reference impedances (Zlref, Z2ref, Z3ref) with respect to pre-determined locations (12, 14, 16 - as shown in Figure 1) along a transmission line. The reference impedances refer to an ideal impedance value at that location.

In accordance with another embodiment of this invention, there is provided a voltage and current monitoring means (VCM) at each of said pre-determined locations along said transmission line. There is also provided a calculation means (CM) adapted to calculate impedances at based on said monitored voltages and currents.
In accordance with yet another embodiment of this invention, there is provided a computation means (PM) adapted to compute a matrix graph of reference impedance versus time over the variation of calculated impedance versus time.
As shown in block diagram of Figure 6, a new relay operates irrespective of zone; the input value will be compared with a percentage value of Z reference to set a time. The hardware with embedded software draws the matrix graph for impedance versus time, and over the variation of impedance value, time also varies.
In accordance with still another embodiment of this invention, there is provided a fault detection means (FDM) adapted to detect a fault based on computation means.
When any fault occurs in any zone the impedance depends upon the distance of the fault from incoming bus bar. As the relation is governed by the equation, Impedance Z = Voltage / Current = V/I
When fault occurs, current increases suddenly up to 20 time of nominal current, with respect to a constant voltage the impedance value decreases suddenly by which the protection system detects the fault.

As the impedance is a property of material, the value varies with respect to the length of the line, over the increase in line length the impedance increases
So, for the total length of line the reference impedance will be computed and stored in a lookup table.
When fault occurs, with respect to increase in current, impedance decreases which will be compared with the percentage of reference impedance value and based on the impedance value time for trip will be set. The increase in impedance value is towards zone 3 to zone 1; so increase in impedance decreases the trip time. With this new system and methodology, the system, typically, takes only 10 power cycles for the decision and operates exactly on the time computed with respect to impedance value.
In accordance with an additional embodiment of this invention, there is provided an actuation means (TRIP) adapted to actuate a trip relay after detection of said fault, at said pre-defined location

We claim,
1. A time variant system for protecting transmission line , said system comprising:
- reference impedance database means adapted to store reference impedances at pre-determined locations along a transmission line;
- voltage and current monitoring means at each of said pre-determined locations along said transmission line;
- calculation means adapted to calculate impedances at based on said monitored voltages and currents;
- computation means adapted to compute a matrix graph of reference impedance versus time over the variation of calculated impedance versus time and further adapted to compare the monitored impedance value with a percentage value of reference impedance to obtain a time delay;
- fault detection means adapted to detect a fault, at any pre-defined location, after said time delay, based on said computation means; and
- actuation means adapted to actuate a trip relay after detection of said fault, at said pre-defmed location.

2. A system as claimed in claim 1 wherein, said actuation means includes a timer means with a preset time adapted to actuate trip relay after expiration of said preset time.
3. A system as claimed in claim 1 wherein, said system includes a time measurement means adapted to measure time of transmission up to each of said pre-defmed locations and to relay said measured time to said computation means.

4. A system as claimed in claim 1 wherein, said computation means includes a comparator means adapted to compare instantaneous calculated impedance value, with respect to time, with corresponding reference impedance value and further adapted to detect fault is instantaneous calculated impedance value decreases, with respect to time, decreases in relation to corresponding reference impedance value.