FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
THE PATENTS RULES 2003
COMPLETE SPECIFICATION
(see section 10; rule 13)
TITLE OF THE INVENTION
COMPLETELY REDUNDANT EARTH FAULT PROTECTION SYSTEM FOR STATOR WINDINGS OF GENERATOR
APPLICANTS
TATA CONSULTING ENGINEERS LIMITED,
an Indian company having its registered office at
Matulya Centre 'A', 1st Floor,
249, Senapati Bapat Marg,
Lower Parel (West),
Mumbai - 400013, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF INVENTION
The present invention relates to a system for protecting an electrical power generator and, more particularly, to provide protection against short circuit faults to earth which may occur in stator windings of the electrical power generator.
BACKGROUND OF INVENTION
The electrical power generator also known as alternator should be protected against internal and external short circuit faults, and against abnormal operating conditions. One important protection is the protection against short circuit faults to earth also known as short circuit faults to ground, which occur inside the alternator when the insulation between the stator windings of the alternator and the stator slots which house them, fails. The heat energy of the electric arc which depends on the magnitude of the fault current and its duration, strikes at the instant of breakdown of insulation, destroying the insulation at the location of the fault and damaging the steel laminations, of which the stator core and slots are made. Measures are taken to minimize the arc energy by inserting a resistance between the neutral of the alternator and earth, so that the fault current is limited in magnitude to a value which causes little or no damage to the stator core, provided that the alternator is disconnected from the system quickly enough on occurrence of fault. This task is accomplished by the stator earth fault protection.
It is a common practice to provide two independent and fully redundant groups of protections, each group comprising all the necessary protections required to be provided for the alternators. But in the present state of art, redundancy in implementation of 100% stator earth fault protection is not achieved due to the fact that only one AC signal injection based protection, operating on a single fixed frequency with the detector tuned to this frequency can be implemented as shown in figures-100, 200a and 200b. Signal generator is a separate unit while the detector is integral to the numerical multifunction generator protection relay. The signal is injected either through the neutral earthing transformer or through the main voltage transformer as illustrated in figures-100, 200a and 200b. Two signal injection based protections, operating on same frequency cannot function simultaneously, without interfering with each other. This limitation prevents the

provision of two independent 100 percent stator earth fault protective functions, one in each group. If the single injection based protection fails to operate during an earth fault, a second fault can occur in course of time in another phase of the alternator, resulting in a phase-to-phase-to-earth (double phase to earth) fault creating short-circuit in the alternator windings. The resulting short-circuit current from the phase-to-phase-to-earth fault is of large magnitude which causes immediate damage to the generator and can prove costly in terms of repair and down time of the alternator.
US 2011/0085272 (Dl) discloses a system for detecting generator ground faults by injecting only one signal of fixed frequency at a time. In contrast to Dl, the present invention uses two distinct separate signals or two identical synchronized signals injected simultaneously to provide stator earth fault protection without any interference between the signals. Dl does not deal with the subject of providing redundancy in the stator earth fault protection of the alternator whereas the present invention provides a system for achieving redundancy with (a) two physically separate signal generators with corresponding detectors built into two physically separate numerical multifunction generator protection relays, (b) two physically separate numerical multifunction generator protection relays with built-in signal generators and detectors and (c) two physically separate stand alone devices, wherein the signal generator and the associated detector are both constituted into a single device known as discrete stator earth fault protection device. In order to address this shortcoming associated with conventional protection method of the generator stator windings, the inventors felt the need to develop the present earth fault protection system which provides two completely redundant and independent protections, one in each group of protections, to detect faults in the entire stator windings of the alternator and generate alarm or trip signals to disconnect the generator from the electric power system.
SUMMARY OF THE INVENTION
The present invention relates to a completely redundant earth fault protection system for stator windings of an electrical power generator; said earth fault protection system comprises a neutral earthing transformer connected to the power generator, one end of the transformer primary winding is connected to neutral point of said generator and the other

end connected to earth, a plurality of signal generators for generating voltage signals of predetermined waveform and magnitude connected to secondary winding of said neutral earthing transformer, said voltage signals are injected between said earth and the three-phase windings of the generator, a plurality of numerical multifunction generator protection relays for protection of said generator supplied with the voltage signals and current signals corresponding to the currents driven by the voltage signals on occurrence of an earth fault, which are processed therein, a plurality of detectors for detecting the amount of said signals corresponding to the respective currents and said voltage signals provided in said protection relays, said detectors are coupled with each of said signal generators, and a plurality of filters for filtering out unwanted frequency components of said signals corresponding to said current signals and voltage signals provided in said protection relays, said each of the coupled detector and signal generator are designed with filters and tuned to predetermined signals so that said detector responds only to the signal generated by said signal generator coupled with it. On occurrence of said earth fault in any of the three-phase windings between the neutral point and respective line ends of the windings, a fault current flows from the point of fault to the neutral end of the faulted winding, through earth and primary winding of said neutral earthing transformer, a current driven by the voltage signal also flows and is superposed over the fault current, the magnitude of said superposed current is determined by the magnitude of the voltage signal and impedance of the path of superposed current flow, each of said detectors are supplied with the voltage signals and signals corresponding to the respective superposed currents, said detectors are configured to determine the impedance of the fault by using the magnitude of said voltage signals and said extracted current signal and compare it with a preset value, if the measured value of impedance is less than the preset value, then the output is generated by the detector to disconnect the faulty generator from electric power system.
According to another embodiment of the present invention, said signal generators inject their respective voltage signals into the three-phase windings of said generator through the secondary windings of a main voltage transformer connected in an open-delta configuration.

According to another embodiment of the present invention, said signal generators inject their respective voltage signals into the three-phase windings of said generator through the secondary windings of an interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to another embodiment of the present invention, one of the said signal generators is connected to inject voltage signal through the secondary winding of the neutral earthing transformer, and the other signal generator is connected to inject voltage signal through the secondary windings of the main voltage transformer connected in open-delta configuration.
According to yet another embodiment of the present invention, one of the said signal generators is connected to inject voltage signal through the secondary winding of the neutral earthing transformer, and the other signal generator is connected to inject voltage signal through the secondary windings of the interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to further embodiment of the present invention, both the signal generators generate voltage signals identical in all respects, and are designed to stay in synchronism with each other with the aid of a synchronous link between the first and the second signal generator, so that the voltage signals generated by them have no phase difference between them, the generated voltage signals from the two signal generators are injected simultaneously into the three phase windings of the generator through the secondary winding of the neutral earthing transformer.
According to another embodiment of the present invention, both the signal generators inject identical and synchronized voltage signals into the three-phase windings of the generator through the secondary windings of the main voltage transformer connected in open-delta configuration.
According to another embodiment of the present invention, both the signal generators inject identical and synchronized voltage signals into the three-phase windings of the

generator through the secondary windings of an interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to another embodiment of the present invention, the first signal generator is connected to inject voltage signal through the secondary winding of the neutral earthing transformer, and the second signal generator is connected to inject voltage signal identical to and synchronous with the first voltage signal through the secondary windings of the main voltage transformer connected in open-delta configuration.
According to yet another embodiment of the present invention, the first signal generator is connected to inject voltage signal through the secondary winding of the neutral earthing transformer, and the second signal generator is connected to inject voltage signal identical to and synchronous with the first voltage signal through the secondary windings of the interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to further embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays and each of said numerical multifunction generator protection relays are connected to the secondary winding of the neutral earthing transformer.
According to another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays and each of said numerical multifunction generator protection relays are connected to the secondary windings of the main voltage transformer connected in open-delta configuration.
According to another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays and each of said numerical multifunction generator protection relays are connected to the secondary windings of the interposing voltage transformer connected in

open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, one of the said numerical multifunction generator protection relays is connected to the secondary winding of the neutral earthing transformer and other numerical multifunction generator protection relay is connected to the secondary windings of the main voltage transformer connected in open-delta configuration.
According to yet another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, one of the said numerical multifunction generator protection relays is connected to the secondary winding of the neutral earthing transformer and other numerical multifunction generator protection relay is connected to the secondary windings of the interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to further embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, each of said protection relays are connected to the secondary winding of the neutral earthing transformer and said first protection relay is connected to the second protection relay with the aid of a synchronous link between them, so that the identical voltage signals generated by the first and second protection relays are synchronized and have no phase difference between them.
According to another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, each of said protection relays are connected to the secondary windings of the main voltage transformer connected in open-delta configuration and said first protection relay is connected to the second protection relay with the aid of a synchronous

link between them, so that the identical voltage signals generated by the first and second protection relays are synchronized and have no phase difference between them.
According to another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, each of said protection relays are connected to the secondary windings of the interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer and said first protection relay is connected to the second protection relay with the aid of a synchronous link between them, so that the identical voltage signals generated by the first and second protection relays are synchronized and have no phase difference between them.
According to another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, said first protection relay is connected to the secondary winding of the neutral earthing transformer and the second protection relay is connected to the secondary windings of the main voltage transformer connected in open-delta configuration, and said first protection relay is connected to the second protection relay with the aid of a synchronous link between them, so that the identical voltage signals generated by the first and second protection relays are synchronized and have no phase difference between them.
According to yet another embodiment of the present invention, the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays, said first protection relay is connected to the secondary winding of the neutral earthing transformer and the second protection relay is connected to the secondary windings of the interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer and said first protection relay is connected to the second protection relay with the aid of a synchronous link between them, so that the identical voltage signals generated by the first and second protection relays are synchronized and have no phase difference between them.

According to further embodiment of the present invention, said signal generator and associated detector are constituted into a single discrete stator earth fault protection device which is separate from said numerical multifunction generator protection relay, two of said discrete stator earth fault protection devices are connected to the secondary winding of the neutral earthing transformer.
According to another embodiment of the present invention, said signal generator and associated detector are constituted into a single discrete stator earth fault protection device which is separate from said numerical multifunction generator protection relay, two of said discrete stator earth fault protection devices are connected to the secondary windings of the main voltage transformer which is connected in open-delta configuration.
According to another embodiment of the present invention, said signal generator and associated detector are constituted into a single discrete stator earth fault protection device which is separate from said numerical multifunction generator protection relay, two of said discrete stator earth fault protection devices are connected to the secondary windings of the interposing voltage transformer which is connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.
According to another embodiment of the present invention, one of the said discrete stator earth fault protection device is connected to the secondary winding of the neutral earth transformer and the other discrete stator earth fault protection device is connected to the secondary windings of the main voltage transformer which is connected in open-delta configuration.
According to yet another embodiment of the present invention, one of the said discrete stator earth fault protection device is connected to the secondary winding of the neutral earth transformer and the other discrete stator earth fault protection device is connected to secondary windings of the interposing voltage transformer which is connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer.

According to still further embodiment of the present invention, said each of the discrete stator earth fault protection devices are connected to the secondary winding of the neutral earthing transformer, and said first discrete stator earth fault protection device is connected to the second discrete stator earth fault protection device with the aid of a synchronous link between them, so that the identical voltage signals generated by the first and second protection devices are synchronized and have no phase difference between them.
According to another embodiment of the present invention, said each of the discrete stator earth fault protection devices are connected to the secondary windings of the main voltage transformer connected in open-delta configuration, and said first discrete stator earth fault protection device is connected to the second discrete stator earth fault protection device with the aid of the synchronous link between them, so that the identical voltage signals generated by the first and second protection devices are synchronized and have no phase difference between them.
According to another embodiment of the present invention, said each of the discrete stator earth fault protection devices are connected to the secondary windings of the interposing voltage transformer connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer, and said first discrete stator earth fault protection device is connected to the second discrete stator earth fault protection device with the aid of the synchronous link between them, so that the identical voltage signals generated by the first and second protection devices are synchronized and have no phase difference between them.
According to another embodiment of the present invention, said first discrete stator earth fault protection device is connected to the secondary winding of the neutral earthing transformer and said second discrete stator earth fault protection device is connected to the secondary windings of the main voltage transformer which is connected in open-delta configuration and said first discrete stator earth fault protection device is connected to the second discrete stator earth fault protection device with the aid of the synchronous link between them, so that the identical voltage signals generated by the first and second protection devices are synchronized and have no phase difference between them.

According to yet another embodiment of the present invention, said first discrete stator earth fault protection device is connected to the secondary winding of the neutral earthing transformer and said second discrete stator earth fault protection device is connected to the secondary windings of the interposing voltage transformer which is connected in open-delta configuration, the star connected primary windings of which are connected to the star connected secondary windings of the main voltage transformer, and said first discrete stator earth fault protection device is connected to the second discrete stator earth fault protection device with the aid of the synchronous link between them, so that the identical voltage signals generated by the first and second protection devices are synchronized and have no phase difference between them.
Preferably, said signal generator is a variable waveform signal generator.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, explain the invention. In the drawings,
Figure- 100 shows the conventional stator earth fault protection by signal injection through neutral earthing transformer;
Figure- 200a shows the conventional stator earth fault protection by signal injection method through main voltage transformer;
Figure- 200b shows the conventional stator earth fault protection by signal injection through main voltage transformer and interposing voltage transformers;
Figure- 300a illustrates the present invention of redundant stator earth fault protection by injection of two distinctly separate signals through the neutral earthing transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;

Figure- 300b illustrates one embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through main voltage transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;
Figure- 300c illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through interposing voltage transformer and main voltage transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;
Figure- 300d illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals one through the neutral earthing transformer and the other through main voltage transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;
Figure- 300e illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals one through the neutral earthing transformer and the other through interposing voltage transformer and main voltage transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;
Figure- 400a illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through neutral earthing transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;
Figure- 400b illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through main voltage transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;

Figure- 400c illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through interposing voltage transformer and main voltage transformer, wherein the signal generator is a separate device while the detector is built into the numerical multifunction generator protection relay;
Figure- 400d illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals one through the neutral earthing transformer and the other through main voltage transformer, wherein the signal generator is a separate device while the detector is built into numerical multifunction generator protection relay;
Figure- 400e illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals one through the neutral earthing transformer and the other through interposing voltage transformer and main voltage transformer, wherein the signal generator is a separate device while the detector is built into numerical multifunction generator protection relay;
Figure- 500a illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through neutral earthing transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 500b illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 500c illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through interposing voltage transformer and main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;

Figure- 500d illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals one through the neutral earthing transformer and the other through main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 500e illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals one through the neutral earthing transformer and the other through interposing voltage transformer and main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 600a illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through neutral earthing transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 600b illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 600c illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through interposing voltage transformer and main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 600d illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals one through the neutral earthing transformer and the other through main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;

Figure- 600e illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals one through the neutral earthing transformer and the other through interposing voltage transformer and main voltage transformer, wherein both the signal generator and the detector are integral to the numerical multifunction generator protection relay;
Figure- 700a illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through neutral earthing transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay;
Figure- 700b illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay;
Figure- 700c illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals through interposing voltage transformer and main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay;
Figure- 700d illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals one through the neutral earthing transformer and the other through main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay;
Figure- 700e illustrates another embodiment of the redundant stator earth fault protection by injection of two distinctly separate signals one through the neutral earthing transformer and the other through interposing voltage transformer and main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay;

Figure- 800a illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through neutral earthing transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay;
Figure- 800b illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single . discrete device separate from the numerical multifunction generator protection relay;
Figure- 800c illustrates another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals through interposing voltage transformer and main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay ;
Figure- 800d illustrates yet another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals one through the neutral earthing transformer and the other through main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay; and
Figure- 800e illustrates yet another embodiment of the redundant stator earth fault protection by injection of two identical synchronized signals one through the neutral earthing transformer and the other through interposing voltage transformer and main voltage transformer, wherein the signal generator and the associated detector are both constituted into a single discrete device separate from the numerical multifunction generator protection relay.
DETAILED DESCRIPTION OF THE INVENTION
Figure-100 illustrates a conventional system for providing stator winding earth fault protection of electrical power generators or alternators. The conventional system of

generator earth fault protection described herein is related with the earthing of the generator neutral using neutral earthing transformer (012). A resistor of suitable value also known as loading resistor (014) is connected across the secondary winding of the neutral earthing transformer (012) for limiting stator earth fault current. The conventional system comprises signal injector (013), potential divider circuit (015) connected across the loading resistor (014), and numerical multifunction generator protection relay (016) having detector to detect the fault current on account of an internal earth fault in any of the windings of the generator and initiate the tripping of the generator. The protection is achieved by injection of a single alternating current (AC) signal Vi of frequency between 20Hz and 90Hz across the alternator windings (011 A, 01 IB, 011C) and earth through the neutral point (01 IN) and neutral earthing transformer (012). On occurrence of an earth fault between any of the alternator windings (011 A, 01 IB, 011C) anywhere between neutral point (01 IN) and line end, a current very similar in waveform to that of the injected voltage signal Vi will flow from the point of fault to the neutral end of the faulted winding, through earth and primary winding of neutral earthing transformer (012). This current is supplied by the signal generator (013). A current signal Ii corresponding to the current flowing in the faulted winding is derived inside the signal generator (013). Alternatively, the current signal Ii can be derived by means of a current transformer provided on the secondary side of neutral earthing transformer (012) as shown by dotted connections in figure-100. This arrangement is possible where voltage signal injection takes place through neutral earthing transformer. Numerical multifunction generator protection relay (016) is supplied with whole or part of the voltage signal kVi and current signal Ii which are processed therein. Both the signal generator (013) and the detector in the numerical relay (016) are fitted with filters which filter out unwanted frequency components of generated and received current and voltage signals. The numerical relay (016) will determine the impedance of the fault and compares it with a preset value. If the measured value of impedance is less than the preset value, an output is generated by the numerical relay (016) which is used to disconnect the alternator from the electric power system.
The voltage signal Vi can also be injected through the main voltage transformer (027) connected to the alternator output terminals as shown in figure-200a. In this case, the secondary windings of main voltage transformer (027) are connected in open-delta

configuration. In instances where open-delta configuration is not available in the main voltage transformer, an interposing voltage transformer (028) is used to derive the open-delta configuration as illustrated in figure-200b. In both cases, the protection function is similar to the case illustrated in figure-100. In instances where voltage signal injection takes place through the main voltage transformer (027) or through interposing voltage transformer (028) and main voltage transformer (027), current signal Ii cannot be derived by use of current transformer as shown in figure-100.
The present invention provides total redundancy in signal injection based 100 percent stator winding earth fault protection for electric power generators by providing two separate signal generators in each of the two protection groups employed for the protection of alternators, and arranging the detection of these signals in two detectors in the two groups of protections, both signal generators and detectors operating either with distinctly separate signals or identical synchronized signals, the signal generators and/or detectors being integral to the main numerical multifunction generator protection relays or constituting separate stand alone discrete devices.
According to the present invention, figure-300a illustrates a system or an arrangement to obtained 100 percent stator winding earth fault protection by injecting two distinct separate signals (Vil and Vi2) simultaneously using two different signal generators (003-1, 003-2) in each of the two group of protection. A loading resistor (034) is connected across the secondary winding of the neutral earthing transformer (032) for limiting stator earth fault current during short circuit. A potential divider circuit (035) is connected across the loading resistance (034). The output voltage signals (Vil and Vi2) of the two separate signal generators (033-1, 033-2) are connected across the secondary winding of the neutral earthing transformer (032). The two separate numerical multifunction generator protection relays (036-1, 036-2) are connected to the signal generators (033-1, 033-2) and the secondary winding of the neutral earthing transformer (032) whereby they receive voltage signals Vi1 and Vi2 and current signals In and Ii2 respectively. The protection relays (036-1, 036-2) are provided with detectors to detect the occurrence of fault by processing these voltage and current signals. Figure-300b shows that the two separate voltage signals (Vi1 and Vi2) can also be injected through the main voltage transformer (037) connected to the alternator outputs terminals (A, B, C). The secondary windings of main voltage

transformer (037) are connected in open-delta configuration. In instances where open-delta configuration is not available in the main voltage transformer, an interposing voltage transformer (038) is used to derive the open-delta configuration as illustrated in figure-300c.
The working of the 100 percent stator winding earth fault protection system is described herein. The injected voltage signals (Vil and Vi2) generated by the signal generators (033-1, 033-2) are continuously impressed on the stator conductors of the generator and no current corresponding to these voltage signals will flow under normal healthy condition of the generator. When earth fault occurs in the alternator windings, fault current driven by the generator voltage flows. Also, two more currents (m and Ii2) driven respectively by the injected voltage signals (Vil and Vi2) flow through the same path and are superposed over the actual fault current. These superposed currents are extracted by the filters in the detectors provided in the two numerical multifunction generator protection relays (036-1, 036-2). Each of the detectors measures the impedance by using the magnitudes of injected voltage signal and the extracted current signal. If the measured value of impedance is less than the preset value, an output is generated by the numerical relays (036-1, 036-2) which are used to disconnect the alternator from the electric power system.
The completely redundant and independent earth fault protection for the electrical generator is achieved in the following manner:
The complete stator winding earth fault protection is achieved by injecting two distinctly separate signals (ViI and Vi2) through (a) the secondary winding of neutral earthing transformer (032) as illustrated in figure-300a or (b) the secondary windings of main voltage transformer (037) as illustrated in figure-300b or (c) the secondary windings of interposing voltage transformer (038) and main voltage transformer (037) as illustrated in figure-300c or (d) injecting one of signals through the secondary winding of neutral earthing transformer (032) and the second signal through the secondary windings of main voltage transformer (037) as illustrated in figure-300d or (e) injecting one of signals through the secondary winding of neutral earthing transformer (032) and the second signal through the secondary windings of interposing voltage transformer (038) and main voltage transformer (037) as illustrated in figure-300e. In these embodiments shown in figures

300a to 300e, the signal generators (033-1, 033-2) along with filters are separate devices while the detectors are built into the numerical multifunction generator protection relays (036-1,036-2).
The complete stator winding earth fault protection can be achieved by injecting two identical synchronized signals Vil and Vi2 (Vil and Vi2 are identical to each other in all respects) through (a) the secondary winding of neutral earthing transformer (042) as illustrated in figure-400a or (b) the secondary windings of the main voltage transformer (047) as illustrated in figure-400b or (c) the secondary windings of interposing voltage transformer (048) and main voltage transformer (047) as illustrated in figure-400c or (d) injecting one of the signals through the secondary winding of neutral earthing transformer (042) and the second signal through the secondary windings of main voltage transformer (047) as illustrated in figure-400d or (e) injecting one of the signals through the secondary winding of neutral earthing transformer (042) and the second signal through the secondary windings of interposing voltage transformer (048) and main voltage transformer (047) as illustrated in figure-400e. The two signal generators (043-1, 043-2) generate identical signals without any phase shift with the aid of a synchronous link (040) between them. In these embodiments shown in figure 400a to 400e, the signal generators (043-1, 043-2) along with filters are separate devices while the detectors are built into the numerical multifunction generator protection relays (046-1, 046-2).
The complete stator winding earth fault protection can be achieved by injecting two distinctly separate signals (Vil and Vi2) through (a) the secondary winding of neutral earthing transformer (052) as illustrated in figure-500a or (b) the secondary windings of main voltage transformer (057) as illustrated in figure-500b or (c) the secondary windings of interposing voltage transformer (058) and main voltage transformer (057) as illustrated in figure-500c or (d) injecting one of signals through the secondary winding of neutral earthing transformer (052) and the second signal through the secondary windings of main voltage transformer (057) as illustrated in figure-500d or (e) injecting one of signals through the secondary winding of neutral earthing transformer (052) and the second signal through the secondary windings of interposing voltage transformer (058) and main voltage transformer (057) as illustrated in figure-500e. In these embodiments shown in figures

500a to 500e, the signal generators, detectors and filters are all built into the numerical multifunction generator protection relays (056-1, 056-2).
The complete stator winding earth fault protection can be achieved by injecting two identical synchronized signals Vil and Vi2 (Vil and Vi2 are identical to each other in all respects) through (a) the secondary winding of neutral earthing transformer (062) as illustrated in figure-600a or (b) the secondary windings of the main voltage transformer (067) as illustrated in figure-600b or (c) the secondary windings of interposing voltage transformer (068) and main voltage transformer (067) as illustrated in figure-600c or (d) injecting one of the signals through the secondary winding of neutral earthing transformer (062) and the second signal through the secondary windings of main voltage transformer (067) as illustrated in figure-600d or (e) injecting one of the signals through the secondary winding of neutral earthing transformer (062) and the second signal through the secondary windings of interposing voltage transformer (068) and main voltage transformer (067) as illustrated in figure-600e. The two signal generators (066-1, 066-2) generate identical signals without any phase shift with the aid of a synchronous link (060) between them. In these embodiments shown in figures 600a to 600e, both the signal generators, detectors and filters are all built into the numerical multifunction generator protection relays (066-1, 066-2).
The complete stator winding earth fault protection can be achieved by injecting two distinctly separate signals (Vil and Vi2) through (a) the secondary winding of neutral earthing transformer (072) as illustrated in figure-700a or (b) the secondary windings of main voltage transformer (077) as illustrated in figure-700b or (c) the secondary windings of interposing voltage transformer (078) and main voltage transformer (077) as illustrated in figure-700c or (d) injecting one of signals through the secondary winding of neutral earthing transformer (072) and the second signal through the secondary windings of main voltage transformer (077) as illustrated in figure-700d or (e) injecting one of signals through the secondary winding of neutral earthing transformer (072) and the second signal through the secondary windings of interposing voltage transformer (078) and main voltage transformer (077) as illustrated in figure-700e. In these embodiments shown in figures 700a to 700e, each signal generator, the associated detector and filters are all constituted into a single device known as Discrete Stator Earth Fault Protection Device which is

separate from the numerical multifunction generator protection relay. Two such discrete stator earth fault protection devices (079-1, 079-2) are provided for the protection of generator.
The complete stator winding earth fault protection can be achieved by injecting two identical synchronized signals Vil and Vi2 (Vil and Vi2 are identical to each other in all respects) through (a) the secondary winding of neutral earthing transformer (082) as illustrated in figure-800a or (b) the secondary windings of the main voltage transformer (087) as illustrated in figure-800b or (c) the secondary windings of interposing voltage transformer (088) and main voltage transformer (087) as illustrated in figure-800c or (d) injecting one of the signals through the secondary winding of neutral earthing transformer (082) and the second signal through the secondary windings of main voltage transformer (087) as illustrated in figure-800d or (e) injecting one of the signals through the secondary winding of neutral earthing transformer (082) and the second signal through the secondary windings of interposing voltage transformer (088) and main voltage transformer (087) as illustrated in figure-800e. In these embodiments shown in figures 800a to 800e, each signal generator, the associated detector and filters are all constituted into a single device known as Discrete Stator Earth Fault Protection Device which is separate from the numerical multifunction generator protection relay. Two such discrete stator earth fault protection devices (089-1, 089-2) are provided for the protection of generator. The two discrete stator earth fault protection devices (089-1, 089-2) generate identical signals without any phase shift with the aid of a synchronous link (080) between them.
In case of embodiments illustrated in figures- 300a, 300b, 300c, 300d, 300e, 500a, 500b, 500c, 500d, 500e, 700a, 700b, 700c, 700d and 700e, one signal generator and detector are provided in each of the two groups of protections (In illustrations 300a to 300e, the signal generator is a separate device while the detector is an integral part of numerical multifunction generator protection relay, in illustrations 500a to 500e, both the signal generator and the detector are integral part of numerical multifunction generator protection relay, in illustrations 700a to 700e, the signal generator and detector are together constituted into a separate stator earth fault protection device). The failure of any signal generator or failure of any detector or failure of any signal generator and its associated

detector will not deprive the protection to the alternator against earth faults, but generates an alarm in any of the aforesaid cases.
In case of embodiments illustrated in figures- 400a, 400b, 400c, 400d, 400e, 600a, 600b, 600c, 600d, 600e, 800a, 800b, 800c, 800d and 800e, one signal generator and detector are provided in each of the two groups of protections (In illustrations 400a to 400e, the signal generator is a separate device while the detector is an integral part of numerical multifunction generator protection relay, in illustrations 600a to 600e, both the signal generator and the detector are integral part of numerical multifunction generator protection relay, in illustrations 800a to 800e, the signal generator and detector are together constituted into a separate stator earth fault protection device). The failure of any signal generator or failure of any detector or failure of any signal generator and its associated detector will not deprive the protection to the alternator against earth faults, but generates an alarm in any of the aforesaid cases. However, in these embodiments, it is essential to monitor the integrity of the synchronous links between the signal generators. In the event of failure of the synchronous link, one of the signal generators should be turned off to allow the other signal generator and detectors to continue to provide protection to the generator.
On occurrence of the earth fault, the resulting currents (1il and Ii2) corresponding to two distinctly separate voltage signals (Vil and Vi2) injected from the two signal generators, or the two identical current signals Iil and 1i2 corresponding to the identical synchronized voltage signals Vil and Vi2 injected from the two signal generators, are detected independently by the two detectors. The two detectors are tuned to detect the respective signals in cases where two distinctly separate voltage signals are used or to detect identical current signals in cases where identical synchronized voltage signals are used.
The power of the signal generators is selected such that (a) they inject signals of sufficient strength through the loop impedances of the path of signal flow which includes the leakage impedance of the neutral earthing transformer or of the main voltage transformer or combined impedance of the main and interposing voltage transformers through which injection takes place, and the impedance of the conductors on both sides of these

transformers and (b) the frequency or the harmonics of the generated signal are not close to the operating power frequency or its harmonics particularly of the lower order.
Site tunable signal generator and detector aid in selecting those signals at site which result in good signal strength and least interference between each other and between the protection relays and the primary power system apart from complying with the requirements of electromagnetic compatibility as laid down in relevant standards.
The signal generators and detectors are connected to the secondary winding of neutral earthing transformer or the secondary windings of main voltage transformer or the secondary windings of interposing voltage transformer through screened signal carrying cables.
The foregoing description is specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

WE CLAIM
1. A completely redundant earth fault protection system for stator windings of an electrical power generator comprising;
- a neutral earthing transformer (032) connected to said power generator, one end of primary winding of said transformer (032) connected to neutral point (03 IN) of said power generator and the other end connected to earth;
- a plurality of signal generators (033-1, 033-2) for generating voltage signals (Vil, Vi2) of predetermined waveform and magnitude connected to secondary winding of said neutral earthing transformer (032), said voltage signals (Vil, Vi2) are injected between said earth and three-phase windings (031 A, 031B, 031C) of said power generator;
- a plurality of numerical multifunction generator protection relays (036-1, 036-2) for protection of said power generator connected to said signal generators (033-1, 033-2) and secondary winding of said neutral earthing transformer (032), said protection relays (036-1, 036-2) are supplied with said voltage signals (Vil, Vi2) and current signals (Iil, Ii2) corresponding to the currents driven by the voltage signals, or a predetermined fraction or multiple of these voltage and current signals, on occurrence of an earth fault, which are processed therein;
- a plurality of detectors for detecting the amount of said current signals (Iil, Ii2) and said voltage signals (Vil, Vi2) provided in said protection relays (036-1, 036-2), said detectors are coupled with each of said signal generators (033-1, 033-2); and
a plurality of filters for filtering out unwanted frequency components of said current signals (Iil, Ii2) and voltage signals (Vil, Vi2) provided in said protection relays (036-1, 036-2), each of said coupled detectors and said signal generators are designed with filters and tuned to predetermined signals so that said detectors respond only to the signals generated by said signal generators coupled with respective detectors;

wherein on occurrence of said earth fault in any of said three-phases windings (031A, 031B, 031C) between said neutral point (031N) and respective line ends of the windings (031 A, 03 IB, 031C), a fault current flows from the point of fault to neutral end of the faulted winding through earth and primary winding of said neutral earthing transformer (032), currents driven by the voltage signals (Vil, Vi2) also flow and are superposed over said fault current, the magnitudes of said superposed currents are determined by the magnitudes of said voltage signals (Vil, Vi2) and impedance of the path of flow of currents, said detectors are supplied with said voltage signals (Vil, Vi2) and signals corresponding to the respective superposed currents, said superimposed currents are extracted by said filters in the detectors, said detectors are configured to determine the impedance of the fault by using the magnitude of said voltage signals (Vil, Vi2) and said extracted current signals and compare them with a preset value, if the measured value of impedances are less than said preset value, then outputs are generated by said detectors to disconnect the faulty generator from electric power system.
2. The completely redundant earth fault protection system as claimed in claim 1, wherein said signal generators (033-1, 033-2) inject their respective voltage signals (Vil, Vi2) into the three-phase windings (031A, 031B, 031C) of said generator through the secondary windings of main voltage transformer (037) connected in an open-delta configuration.
3. The completely redundant earth fault protection system as claimed in claim 1, wherein said signal generators (033-1, 033-2) inject their respective voltage signals (Vil, Vi2) into the three-phase windings (031 A, 031B, 031C) of said generator through the secondary windings of an interposing voltage transformer (038) connected in open-delta configuration and said main voltage transformer (037).
4. The completely redundant earth fault protection system as claimed in claim 1, wherein one of said signal generators (033-1) is connected to inject voltage signal (Vil) through the secondary winding of the neutral earthing transformer (032), and the other signal generator (033-2) is connected to inject voltage signal (Vi2)

through the secondary windings of the main voltage transformer (037) connected in open-delta configuration.
5. The completely redundant earth fault protection system as claimed in claim 1, wherein one of said signal generators (033-1) is connected to inject voltage signal (Vil) through the secondary winding of the neutral earthing transformer (032), and the other signal generator (033-2) is connected to inject voltage signal (Vi2) through the secondary windings of said interposing voltage transformer (038) connected in open-delta configuration and main voltage transformer (037).
6. The completely redundant earth fault protection system as claimed in claim 1, wherein both the signal generators (043-1, 043-2) generate voltage signals (Vil,
Vi2) which are identical in all respects to each other, and are designed to stay in synchronism with each other with the aid of a synchronous link (040) between the first and the second signal generator (043-1, 043-2), so that the voltage signals generated by them have no phase difference between them, the generated voltage signals from the two signal generators (043-1, 043-2) are injected simultaneously into the three-phase windings (041-A, 041B, 041C) of the generator through the secondary winding of said neutral earthing transformer (042).
7. The completely redundant earth fault protection system as claimed in claim 6, wherein both the signal generators (043-1, 043-2) inject identical and synchronized voltage signals (Vil, Vi2) into the three-phase windings (041-A, 041B, 041C) of the generator through the secondary windings of said main voltage transformer (047) connected in open-delta configuration.
8. The completely redundant earth fault protection system as claimed in claim 6, wherein both the signal generators (043-1, 043-2) inject identical and synchronized voltage signals (Vil, Vi2) into the three-phase windings (041-A, 041B, 041C) of the generator through the secondary windings of said interposing voltage transformer (048) connected in open-delta configuration and said main voltage transformer (047).

9. The completely redundant earth fault protection system as claimed in claim 6, wherein the first signal generator (043-1) is connected to inject said voltage signal (Vil) through the secondary winding of said neutral earthing transformer (042), and the second signal generator (043-2) is connected to inject said voltage signal (Vil, Vi2) identical to and synchronous with the first voltage signal (Vil) through the secondary windings of said main voltage transformer (047) connected in open-delta configuration.
10. The completely redundant earth fault protection system as claimed in claim 6, wherein the first signal generator (043-1) is connected to inject said voltage signal (Vil) through the secondary winding of said neutral earthing transformer (042), and the second signal generator (043-2) is connected to inject said voltage signal ( Vi2) identical to and synchronous with the first voltage signal (Vil) through the secondary windings of said interposing voltage transformer (048) connected in open-delta configuration and said main voltage transformer (047).
11. The completely redundant earth .fault protection system as claimed in claim 1, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (056-1, 056-2) and each of said numerical multifunction generator protection relays (056-1, 056-2) are connected to the secondary winding of said neutral earthing transformer (052).
12. The completely redundant earth fault protection system as claimed in claims 1 and 11, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (056-1, 056-2) and each of said numerical multifunction generator protection relays (056-1, 056-2) are connected to the secondary windings of said main voltage transformer (057) connected in open-delta configuration.
13. The completely redundant earth fault protection system as claimed in claims 1 and 11, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (056-1, 056-2) and each of said numerical multifunction generator protection relays (056-1, 056-2) are connected to the secondary windings of said interposing voltage transformer

(058) connected in open-delta configuration and said main voltage transformer (057).
14. The completely redundant earth fault protection system as claimed in claims 1 and 11, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (056-1, 056-2), said one of the numerical multifunction generator protection relay (056-1) is connected to the secondary winding of said neutral earthing transformer (052) and other numerical multifunction generator protection relay (056-2) is connected to the secondary windings of said main voltage transformer (057) connected in open-delta configuration.
15. The completely redundant earth fault protection system as claimed in claims 1 and 11, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (056-1, 056-2), said one of the numerical multifunction generator protection relay (056-1) is connected to the secondary winding of said neutral earthing transformer (052) and other numerical multifunction generator protection relay (056-2) is connected to the secondary windings of said interposing voltage transformer (058) connected in open-delta configuration and main voltage transformer (057).
16. The completely redundant earth fault protection system as claimed in claim 11, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (066-1, 066-2), each of said protection relays (066-1, 066-2) are connected to the secondary winding of said neutral earthing transformer (062) and said first protection relay (066-1) is connected to the second protection relay (066-2) with the aid of a synchronous link (060) between them, so that the identical and synchronized voltage signals generated by the first and second protection relays (066-1, 066-2) have no phase difference between them.
17. The completely redundant earth fault protection system as claimed in claim 16, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (066-1, 066-2), each of

said protection relays (066-1, 066-2) are connected to the secondary windings of said main voltage transformer (067) connected in open-delta configuration and said first protection relay (066-1) is connected to the second protection relay (066-2) with the aid of said synchronous link (060) between them, so that the identical and synchronized voltage signals generated by the first and second protection relays (066-1, 066-2) have no phase difference between them.
18. The completely redundant earth fault protection system as claimed in claim 16, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (066-1, 066-2), each of said protection relays (066-1, 066-2) are connected to the secondary windings of said interposing voltage transformer (068) connected in open-delta configuration and said main voltage transformer (067) and said first protection relay (066-1) is connected to the second protection relay (066-2) with the aid of said synchronous link (060) between them, so that the identical and synchronized voltage signals generated by the first and second protection relays (066-1, 066-2) have no phase difference between them.
19. The completely redundant earth fault protection system as claimed in claim 16, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (066-1, 066-2), said first protection relay (066-1) is connected to the secondary winding of said neutral earthing transformer (062) and the second protection relay (066-2) is connected to the secondary windings of said main voltage transformer (067) connected in open-delta configuration, and said first protection relay (066-1) is connected to the second protection relay (066-2) with the aid of said synchronous link (060) between them, so that the identical and synchronized voltage signals generated by the first and second protection relays (066-1, 066-2) have no phase difference between them.
20. The completely redundant earth fault protection system as claimed in claim 16, wherein the signal generator and the associated detector are both integral to each of said numerical multifunction generator protection relays (066-1, 066-2), said first protection relay (066-1) is connected to the secondary winding of said neutral

earthing transformer (062) and the second protection relay (066-2) is connected to the secondary windings of interposing voltage transformer (068) connected in open-delta configuration and said main voltage transformer (067), and said first protection relay (066-1) is connected to the second protection relay (066-2) with the aid of said synchronous link (060) between them, so that the identical and synchronized voltage signals generated by the first and second protection relays (066-1,066-2) have no phase difference between them.
21. The completely redundant earth fault protection system as claimed in claim 1, wherein said signal generator and associated detector are constituted into single discrete stator earth fault protection device, which is separate from numerical multifunction generator protection relay, two such devices (079-1, 079-2) are connected to the secondary winding of said neutral earthing transformer (072).
22. The completely redundant earth fault protection system as claimed in claim 21, wherein the two said discrete stator earth fault protection devices (079-1, 079-2) are connected to the secondary windings of said main voltage transformer (077) which is connected in open-delta configuration.
23. The completely redundant earth fault protection system as claimed in claim 21, wherein the two said discrete stator earth fault protection devices (079-1, 079-2) are connected to the secondary windings of said interposing voltage transformer (078) which is connected in open-delta configuration and main voltage transformer (077).
24. The completely redundant earth fault protection system as claimed in claim 21, wherein one of the said discrete stator earth fault protection devices (079-1) is connected to the secondary winding of said neutral earth transformer (072) and said other discrete stator earth fault protection device (079-2) is connected to the secondary windings of said main voltage transformer (077) which is connected in open-delta configuration.
25. The completely redundant earth fault protection system as claimed in claim 21, wherein one of the said discrete stator earth fault protection devices (079-1) is

connected to the secondary winding of said neutral earth transformer (072) and said other discrete stator earth fault protection device (079-2) is connected to the secondary windings of said interposing voltage transformer (078) which is connected in open-delta configuration and said main voltage transformer.
26. The completely redundant earth fault protection system as claimed in claim 21, wherein said each of discrete stator earth fault protection devices (089-1, 089-2) are connected to the secondary winding of said neutral earth transformer (082), and said first discrete stator earth fault protection device (089-1) is connected to the second discrete stator earth fault protection device (089-2) with the aid of a synchronous link (080) between them, so that the identical and synchronized voltage signals (Vil, Vi2) generated by the first and second protection devices (089-1, 089-2) have no phase difference between them.
27. The completely redundant earth fault protection system as claimed in claim 26, wherein said each of discrete stator earth fault protection devices (089-1, 089-2) are connected to the secondary windings of said main voltage transformer (087) connected in open-delta configuration, and said first discrete stator earth fault protection device (089-1) is connected to the second discrete stator earth fault protection device (089-2) with the aid of said synchronous link (080) between them, so that the identical and synchronized voltage signals (Vil, Vi2) generated by the first and second protection devices (089-1, 089-2) have no phase difference between them.
28. The completely redundant earth fault protection system as claimed in claim 26, wherein said each of discrete stator earth fault protection devices (089-1, 089-2) are connected to the secondary windings of said interposing voltage transformer (088) connected in open-delta configuration and said main voltage transformer (087), and said first discrete stator earth fault protection device (089-1) is connected to the second discrete stator earth fault protection device (089-2) with the aid of said synchronous link (080) between them, so that the identical and synchronized voltage signals (Vil, Vi2) generated by the first and second protection devices (089-1, 089-2) have no phase difference between them.

29. The completely redundant earth fault protection system as claimed in claim 26, wherein said first discrete stator earth fault protection device (089-1) is connected to the secondary winding of said neutral earthing transformer (082) and said second discrete stator earth fault protection device (089-2) is connected to the secondary windings of said main voltage transformer (087) which is connected in open-delta configuration and said first discrete stator earth fault protection device (089-1) is connected to the second discrete stator earth fault protection device (089-2) with the aid of said synchronous link (080) between them, so that the identical and synchronized voltage signals (Vil, Vi2) generated by the first and second protection devices (089-1, 089-2) have no phase difference between them.
30. The completely redundant earth fault protection system as claimed in claim 26, wherein said first discrete stator earth fault protection device (089-1) is connected to the secondary winding of said neutral earthing transformer (082) and said second discrete stator earth fault protection device (089-2) is connected to the secondary windings of said interposing voltage transformer (088) which is connected in open-delta configuration and said main voltage transformer (087), and said first discrete stator earth fault protection device (089-1) is connected to the second discrete stator earth fault protection device (089-2) with the aid of said synchronous link (080) between them, so that the identical and synchronized voltage signals generated by the first and second protection devices (089-1, 089-2) have no phase difference between them.
31. The completely redundant earth fault protection system as claimed in any of the preceding claims, wherein said signal generators are variable waveform signal generators.