FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITTE OF THE INVENTION
A System and method to detect excitation failures in electric generators
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
Inventors
Umamahesh Pasumarthi, Electronics Design Centre, C G Global R&D Centre, Crompton Greaves Ltd, Kanjur Marg, Mumbai, Maharashtra, India, Indian
Nationals
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 generators and health detection systems for electric generators.
Particularly, this invention relates to a system and method to detect excitation failures in electric generators.
Background of the Invention:
An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils in order to generate the magnetic field; otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.
Excitation in generators occurs in two ways; separate excitation and self excitation. Smaller generators are sometimes self-excited, which means the field coils are powered by the current produced by the generator itself. Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger.
Each type of excitation includes failure detection relays. Separate excitation failure detection relays are used to detect the status of excitation of the generator, which is very difficult to detect through these relays.
According to the prior art, separate excitation relays are used to protect the generator from excitation failure, which is less accurate with more time delay.

Also, according to the prior art, separate excitation failure detection relays are used to detect the status of excitation of the generator, which is very difficult and not accurate
Therefore there is a need to for a system and method for detecting excitation failures in generators, particularly in electric generators.
Objects of the Invention:
An object of the invention is to provide a faster and accurate detection technique to detect excitation failure of generators.
Another object of the invention is to provide directional impedance based technique to detect excitation failure of generators.
Yet another object of the invention to provide a method and system to detect excitation failures in generators, particularly in electric generators
Summary of the Invention:
According to this invention, there is provided a system to detect excitation failures in electric generators, said system comprises:
a) voltage reading means and current reading means adapted to read voltage parameters and current parameters, respectively, from said electric generator;
b) impedance computation means adapted to read instantaneous values of voltage and current to compute an instantaneous impedance value;

c) reference impedance means adapted to be plotted in a graphical manner to define a zone of reference impedance in a third quadrant where the excitation failure of electric generator is known to occur;
d) comparator means adapted to compare value of said computed impedance with said reference impedance in a continuous manner; and
e) tripping means adapted to trip said generator upon obtaining a positive comparison value of said computed impedance being in said reference impedance zone, thereby providing a directional impedance system to detect and prevent excitation failure.
Typically, said voltage reading means is continuous voltage reading means adapted to read said voltage parameters continuously in order to monitor excitation.
Typically, said current reading means is continuous current reading means adapted to read said current parameters continuously in order to monitor excitation.
Typically, said computation means is continuous current reading means adapted to compute said impedance continuously in order to monitor excitation.
According to this invention, there is also provided a method to detect excitation failures in electric generators, said method comprises the steps of: i. reading voltage parameters and current parameters, respectively, from said
electric generator; ii. reading instantaneous values of voltage and current and computing an instantaneous impedance value;

iii. plotting reference impedance in a graphical manner to define a zone of reference impedance in a third quadrant where the excitation failure of electric generator is known to occur;
iv. comparing value of said computed impedance with said reference impedance in a continuous manner; and
v. tripping said generator upon obtaining a positive comparison value of said computed impedance being in said reference impedance zone, thereby providing a directional impedance methodology to detect and prevent excitation failure.
Typically, said step of reading voltage is a continuous series of steps of reading voltage parameters in order to monitor excitation.
Typically, said step of reading current is a continuous series of steps of reading current parameters in order to monitor excitation.
Typically, said step of computing is a continuous series of steps of computing impedance in order to monitor excitation.
Brief Description of the Accompanying Drawings:
The invention will now be described in relation to the accompanying drawings, in which:
Figure 1 illustrates a schematic block diagram of the fault sensing technique i.e. the system to detect excitation failures in electric generators; and

Figure 2 illustrates a graphical plot of fall in current and entering a defined zone of impedance for purposes of detection.
Detailed Description of the Accompanying Drawings:
According to this invention, there is provided a system and method to detect excitation failures in electric generators.
Figure 1 illustrates a schematic block diagram of the fault sensing technique i.e. the system to detect excitation failures in electric generators.
In accordance with an embodiment of this invention, there is provided a voltage reading means (V) and a current reading means (I) adapted to read voltage parameters and current parameters from the electric generator. The reading means are adapted to read the parameters continuously in order to monitor excitation.
When there is an excitation failure in generator, the current drops suddenly due to which impedance comes into picture, but due to sudden fall of generator current.
In accordance with another embodiment of this invention, there is provided an impedance computation means (Z) adapted to read instantaneous parameters values of voltage and current to obtain an instantaneous impedance value. The computation means is adapted to read the parameters continuously in order to monitor excitation. Sudden fall is current results in fall of impedance. This condition needs to be monitored.

In accordance with yet another embodiment of this invention, there is provided a reference impedance means (Ze ref) adapted to be plotted in a graphical manner to define a zone of reference impedance in a third quadrant where the excitation failure of electric generator shall occur. I.e. if the instantaneous computed impedance is known to fall in this zone, there is bound to be failure, and subsequent actions should be taken. This zone can be seen in Figure 2 of the accompanying drawings. The circle
In accordance with yet another embodiment of this invention, there is provided a comparator means (C) adapted to compare value of computed impedance with reference impedance in a continuous manner. If the impedance matches, subsequent actions need to be taken to prevent failure.
Q1 refers to first quadrant. Q2 refers to second quadrant. Q3 refers to third quadrant. Q4 refers to fourth quadrant.
Due to sudden fall of generator current, the impedance follows a negative direction path from first quadrant to 3 rd quadrant in reverse direction, our system computes both impedance and direction, a mho circle is plotted in 3r quadrant, when impedance falls in that zone (as seen by the dark circle), trip action will be taken immediately. The dark curvilinear arrowed line from the first quadrant to the third quadrant through the fourth quadrant is the fault line as plotted by fall in current, and hence resulting in computation of fall in impedance of generator, and hence illustrating failure detection.

In accordance with still another embodiment of this invention, there is provided a tripping means (T) adapted to trip the generator upon identifying a breach of impedance.
The system of this invention specifies about a fast and accurate directional impedance methodology to detect excitation failure.
As shown in the block diagram of Figure 1, the excitation impedance Ze will be computed from input current and voltage, the computed Ze will be checked continuously for excitation failure. When the Ze falls in the range of Ze reference (which was the estimated Ze value upon excitation failure), generator is tripped.
While this detailed description has disclosed certain specific embodiments of the present invention for illustrative purposes, various modifications will be apparent to those skilled in the art which do not constitute departures from the spirit and scope of the invention as defined in the following claims, and it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We claim,
1. A system to detect excitation failures in electric generators, said system comprising:
a) voltage reading means and current reading means adapted to read voltage parameters and current parameters, respectively, from said electric generator;
b) impedance computation means adapted to read instantaneous values of voltage and current to compute an instantaneous impedance value;
c) reference impedance means adapted to be plotted in a graphical manner to define a zone of reference impedance in a third quadrant where the excitation failure of electric generator is known to occur;
d) comparator means adapted to compare value of said computed impedance with said reference impedance in a continuous manner; and
e) tripping means adapted to trip said generator upon obtaining a positive comparison value of said computed impedance being in said reference impedance zone, thereby providing a directional impedance system to detect and prevent excitation failure.

2. A system as claimed in claim 1 wherein, said voltage reading means is continuous voltage reading means adapted to read said voltage parameters continuously in order to monitor excitation.
3. A system as claimed in claim 1 wherein, said current reading means is continuous current reading means adapted to read said current parameters continuously in order to monitor excitation.

4. A system as claimed in claim 1 wherein, said computation means is continuous current reading means adapted to compute said impedance continuously in order to monitor excitation.
5. A method to detect excitation failures in electric generators, said method comprising the steps of:
i. reading voltage parameters and current parameters, respectively, from said
electric generator; ii. reading instantaneous values of voltage and current and computing an
instantaneous impedance value; iii. plotting reference impedance in a graphical manner to define a zone of
reference impedance in a third quadrant where the excitation failure of
electric generator is known to occur; iv. comparing value of said computed impedance with said reference impedance
in a continuous manner; and v. tripping said generator upon obtaining a positive comparison value of said
computed impedance being in said reference impedance zone, thereby
providing a directional impedance methodology to detect and prevent
excitation failure.
6. A method as claimed in claim 5 wherein, said step of reading voltage is a
continuous series of steps of reading voltage parameters in order to monitor
excitation.

7. A method as claimed in claim 5 wherein, said step of reading current is a continuous series of steps of reading current parameters in order to monitor excitation.
8. A method as claimed in claim 5 wherein, said step of computing is a continuous series of steps of computing impedance in order to monitor excitation.