BACKGROUND Many methods are used or have been used or proposed for the detection of earth faults on syn- chronous generators. When choosing the detection method the layout of the power plant in which a generator is incorporated must be considered. Power plants exist which have their generator and a step-up transformer connected as one unit. The generator may or may not have a generator circuit breaker. Two or more generators may share a common step-up transformer. Several small generators may be connected to a common generator busbar. In such cases, the number of step- up transformers may be one or more. When choosing the method to be used for detection of earth faults also the system earthing must be considered. Wilheim and Waters have summarized the state of the art on system earthing, see the book R. Wilheim & M. Waters:"Neutral Grounding in High Voltage Transmission", Elsevier Publishing Co. , New York, 1956.

Small generators may, according to Tidestrom, S. H : son (editor) :"Ingenjorshandbolcen/Alhnan Elektroteknik", 3rd ed. , Nordisk Rotogravyr, Stockholm, 1959, have a solidly earthed neutral node.

In such generators, the short circuit protection system can also detect earth faults. A sensitive differential protection can, also according to the book by Tidestrom, also detect earth faults if the rated current of the generator is less than 500 A.

Many generators have a high-impedance earthed neutral node. This means that they have an isolated neutral, high-resistance earthed neutral node or a resonant earthed neutral node. Most generators have a high-resistance earthed neutral point. One can use a high voltage resistor and connect it directly to the neutral point of the generator. It is also possible to use a low voltage resistor and connect it on the secondary side of a single-phase distribution transformer. The neutral point of the generator is connected to the primary side of the distribution transformer.

The main task of the neutral point resistor is to limit the overvoltage on the windings and buswork of the generating units. There is a rule-of-thumb for the selection of the neutral point equipment: The effective resistance, RN [Q] seen from the neutral point of the generator should be equal to the capacitance to earth as in equation (1) : <BR> <BR> <BR> <BR> <BR> <BR> <BR> RN=1<BR> 3#(Cw + Cb + Ca + Ct) (1) where C,, is the zero sequence capacitance of the generator winding [F/phase], Cb is the zero sequence capacitance of the buswork [F/phase], Ca is the zero sequence capacitance of the auxiliary transformers [F/phase], and C, is the zero sequence capacitance of the step-up transformer [F/phase].

High earth fault currents may damage the iron core of the generator if the fault clearance time is too long. The risk for damage is small if the earth fault current is lower than 15 A when there is an earth fault on one generator phase terminal. According to C. W. Walker:"Relay Protection in Hydro-Electric Power Stations of the Snowy Mountains Authority", The Institution of Engineers, Australia, Electrical Engineering Transactions, Vol. EET-5, No. 2, pp. 311-316, September, 1969, the safe limit is 5 A.

In the following generators having a high-impedance earthed neutral point will be discussed.

Earth fault protections for generators and transformers connected to a unit will be described.

Earth fault protection systems for generators connected to a common busbar will also be described.

The task of the earth fault protection is to detect earth faults on the winding of the generator, on the associated buswork, on the primary winding of the auxiliary transformer and on the primary winding of the step-up transformer. A single phase-to-earth fault will cause an increase of the voltage on the other phases and on the neutral point. The voltage rise depends on the fault location and on the fault resistance. The healthy phases will assume full phase-to-phase voltage if an earth fault without fault resistance occurs at the terminal of one winding of the generator.

The neutral point will assume full phase-to-neutral voltage. The voltage rise will decrease when the fault resistance increases. The voltage rise will be negligible if the earth fault occurs on the phase winding close to the neutral point.

To detect an earth fault in the windings of a generating unit a neutral point overvoltage relay, a neutral point overcurrent relay, a zero sequence overvoltage relay or a residual differential protection circuit may be used. These protection schemes are simple and have served well during many years. However, at the very best, these simple schemes protect only 95% of the stator windings. They leave at least 5 % close to the neutral end without earth fault protection. Under unfavourable conditions the blind zone may extend up to 20 % from the neutral. There are several methods to detect an earth fault close to the neutral point. Fig. 1 illustrates some fundamental properties of some types of earth fault protection methods. The intention of the figure is to illustrate general methods and to define some classes of earth fault protections. The windings lA, 1B, 1C of a generator have their line ends connected to output terminals 3A, 3B, 3c and their neutral ends connected to a common node 5 or N, called the neutral or neutral point or node, that through some resistive component, here represented by an earth resistor R is connected to ground.

Circuits using line end earth fault protection can detect earth faults on almost the entire generator winding but have a blind zone close to the neutral point. The size of the blind zone may be 5- 20 %. The main task of circuits using neutral end earth fault protection is to detect earth faults close to the neutral point. Such protection circuits may cover 20-40 % of the winding.

Sometimes these protection circuits can detect earth faults close to the line terminals. To cover the entire winding, a circuit for combined earth fault protection that comprises line end earth fault protection and a neutral end earth fault protection can be used. There are finally circuits for total earth fault protection based on a method that makes it possible to detect earth faults anywhere along the entire generator winding.

Line End Earth Fault Protection Neutral point overvoltage protection methods, neutral point overcurrent protection methods, zero sequence overvoltage protection methods and residual differential protection methods are all various kinds of line end earth fault protection methods. Methods using neutral point overvoltage protection are applied for earth fault protection of unit-connected generators. In Fig. 2 such a circuit of overvoltage protection is shown. A single-phase auxiliary voltage transformer 7 that is connected to the generator neutral node 5, N in parallel to the earth resistor R provides signals to the overvoltage protection controller 9. Such a protection device detects earth faults on the generator windings, on the buswork and on the primary winding of the auxiliary transformer. It can also detect earth faults on the primary winding (medium voltage) of the step-up transformer in electric power generating units without generator breaker and while the generator breaker is closed. The blind spot near the neutral point may be as small as 5 %.

Methods using neutral point overcurrent protection have properties similar to those of methods of neutral point overvoltage protection. In methods using overcurrent protection earth faults on the generator windings, on the buswork, on the primary winding of the auxiliary transformer and on the primary winding of the step-up transformer are detected. In Fig. 3 a circuit diagram of a circuit for overcurrent protection is shown. A single-phase current transformer 11 has a primary winding that has a low resistance and is connected in the line connecting the neutral point N to earth. The primary current is thus equal to the current that flows from the neutral node 5 of the generator to earth. The second winding of the transformer 11 is connected to a protection circuit or controller 9'detecting a neutral point overcurrent state. In O. Evenson :"Relateknik for högspänningsanläggningar", Compendium, Lidingö, 1961, it is stated that the neutral point overcurrent protection method is inferior to methods using the neutral point overvoltage protection. The blind zone near the neutral point may be 20-30 %.

A method using zero sequence overvoltage protection can also detect faults in the generator system. In Fig. 4 a circuit for such zero sequence overvoltage protection is illustrated. Three single-phase voltage transformers 13A, 13B, 13c energize the zero sequence overvoltage protection. The primary winding 13'A, 13'B, 13'c of each voltage transformer has one end connected to a phase conductor such as the phase winding terminals 3A, 3B, 3c and the other end connected to ground. The secondary winding 13"A, 13"B, 13"c of each voltage transformer are connected in series with each other to form a broken delta or closed loop in which an overvoltage controller 9"is connected, a point of the loop at a terminal of the controller also connected to ground.

Each voltage transformer 13A, 13B, 13C provides over its secondary winding a voltage that has an implitude error and a phase error in relation to the voltage over the primary winding. This means that the secondary zero sequence voltage may not represent the primary zero sequence voltage exactly. To avoid unwanted operation, the zero sequence overvoltage setting must be higher that the neutral point overvoltage setting. Lohage and co-workers have documented practice for the earth fault protection in hydro power plants owned by the Swedish company Vattenfall, see B.

Andersson, H. Broman, P. -A. Eriksson, S. Fredriksson & L. Lohage:"Generatorskydd i Vatten- kraftstationer", Rapport, Vattenfall, November, 1982.

Generally, large hydropower units comprising generator breakers have one circuit for neutral point overvoltage protection and one circuit for zero sequence overvoltage protection. The circuit for neutral point overvoltage protection must cover at least 95 % of the stator windings of the generator. In new plants a combined circuit for earth fault protection that can detect earth faults anywhere along the generator winding replaces the circuit for neutral point overvoltage protection. The delay before this sensitive protection gives an alarm is usually 1.2 second. Often, a circuit for zero sequence overvoltage protection covers about 80 % of the generator winding.

Normally, the delay before alarm is 0.4 second.

Several Generators Connected to a Common Busbar Now a power plant comprising several generators connected to a common busbar is considered.

The busbar has one or more transformer bays. Usually, the busbar has no feeder bays. In such plants it is common practice that the generators have unearthed neutral points. Often, there is a requirement to limit the overvoltage on the busbar in the case where only one generator is in service. This case determines the maximum size of the resistor connected to the neutral point of the generator. When all generators having such resistors are in service, the total earth fault current may become too high.

Some busbars may have a bay for an earthing transformer with a neutral point resistor. In such cases, the system has a high-impedance earthed neutral. Some power utilities use a combined earthing transformer and station auxiliary transformer. Some plants may have only a step-up transformer with an Y-or Z-connected winding connected to the busbar. This neutral point can be used to connect a neutral point resistor. It is not necessary to install neutral point resistors at each generator if an earthing transformer is provided having a neutral point resistor if the step-up transformer has a neutral point resistor.

Neutral point overvoltage protection circuits, neutral point overcurrent protection circuits and zero sequence overvoltage protection circuits cannot select the faulty generator if several generators are connected to one common busbar.

Fig. 5 shows the stator windings of a generator having a circuit for residual differential protection that can select the faulty generator in the case where several generators are connected to a common busbar. Only one three-phase current transformer is required if the neutral point of the generator is unearthed. Unavoidable amplitude errors and phase errors of the current transformer limit the sensitivity of the earth fault protection. On external short-circuits, the fault current from the generator may be very high and may contain a substantial DC component. The fault currents may cause a false secondary zero sequence current. A risk exists that this false current will cause unwanted operation of the earth fault protection circuit. To avoid such unwant- ed operations, the short circuit protection circuit may block the earth fault protection circuit for external short-circuits. The closing of the generator breaker may cause transient residual cur- rents. These currents may limit the sensitivity of the residual differential protection circuit.

Assume now that each generator has a neutral point resistor R. To obtain selective clearance of earth faults, it is necessary to use a residual differential protection. Fig. 5 shows such a protection circuit including a controller 9"'that is connected to the three-phase current transformer 15: 15A, 15B, 15c and one neutral point current transformer 17 similar to the transformer 11 of the circuit illustrated in Fig. 3. The primary windings of the three-phase transformer 15 are thus portions of the phase windings 1 A, 1B, lc that have a low resistance. The secondary windings are connected in parallel to each other and to the controller 9"'.

Generally, it is necessary to clear earth faults on the buswork and on the primary winding of the step-up transformers. To detect such faults, a circuit using zero sequence overvoltage protection can be employed. Three single-phase voltage transformers connected to the primary winding of the step-up transformer may energize the zero sequence overvoltage protection.

Neutral End Earth Fault Protection Methods using overvoltage (or overcurrent) generator earth fault protection provide straightforward, secure and dependable earth-fault protection. However, they suffer from two disadvantages, see C. H. Griffin & J. W. Pope:"Generator Ground Fault Protection Using Over- current, Overvoltage, and Undervoltage Relays", IEEE Trans. on Power Apparatus and Systems, Vol. PAS-101, No. 12, pp. 4490-4501, December, 1982. First, they will not detect earth-faults near the generator neutral. Second, they are not self-monitoring. That is, an open circuit any- where in the relay or control circuit, primary or secondary of the voltage transformer (the current transformer) or an open neutral point resistor may not be detected before a fault occurs.

The induced EMF in many synchronous generators contains harmonics. It is possible to use the third harmonic to detect earth faults close to the neutral point and in the neutral point equipment. The induced third harmonic voltages cause a third harmonic current that flows through the neutral point resistor R.

An earth fault close to the neutral point will shunt the neutral point resistor and the third harmonic voltage over the neutral point resistor. According to R. L. Schlake, G. W. Bucldey & G. McPherson:"Performance of Third Harmonic Ground Fault Protection Schemes for Generator Stator Windings", IEEE Trans. on Power Apparatus and Systems, Vol. PAS-100, No. 7, pp. 3195- 3202, July, 1981, a circuit using such third harmonic protection can detect earth faults with a fault resistance less than 1 000 Q. It can detect such faults on 20 % of the generator winding near the neutral point.

Combined Earth Fault Protection When combined to form a protection system, each relay covers the blind zone of the other.

Therefore, a combined protection system will detect earth faults anywhere on the stator winding.

In the cited article by Griffin and Pope earth fault protection methods used by the Georgia Power Company are described. For over 30 years, the Georgia Power Company has grounded all system generators through a distribution transformer having a resistance-loaded secondary side.

A current transformer is then connected in series with the secondary resistor to supply current to one or more overcurrent relays or circuits. When properly set, these relays will provide sensitive protection for 90-95 % of the generator stator winding, and will not operate incorrectly for external faults. At the end of 1981, this system had been installed on 126 generating units, ranging in size from 15 to 900 MW. In the past 25 years, nearly 20 earth faults have been cleared with minimal equipment damage, and no incorrect operations have occurred.

In 1977, Georgia Power Company concluded that it would be prudent to protect all large generators using an additional earth fault protection system that was completely independent of the existing overcurrent scheme, would give reliable protection to 100% of the generator, and would continuously monitor the generator earthing system. Two types of systems have been installed. One type injects a current at a subharmonic current, and trips on an increase of current caused by the reduction in generator capacitance that results from a single phase-to-earth fault.

The other type employs two overlapping voltage relays-an overvoltage relay that protects the high voltage end of the machine, and an undervoltage relay, tuned to respond to the third harmonic, which protects the neutral. In the article by Griffin and Pope it is stated that both schemes have performed extremely well, and that the combined overvoltage/undervoltage scheme has already properly detected an earth fault. It should also be noted that in 1980, a Georgia Power Company generator on which a circuit giving 100 % earth fault protection had not yet been installed was badly damaged due to a ground fault that occurred very near the neutral and was not detected by the installed circuit providing 90 % protection, see the same article.

Total Earth Fault Protection In 1936, in W. Diesendorf & E. GroB : "Zur Theorie der Pohl'schen Nullpunlctsverlagerung fur vollstandigen GehauseschluBschutz", E und M, Vol. 54, No 22, pp. 253-256,31 May 1936, the need was pointed out for an earth-fault protection method that is capable of detecting earth faults on the entire stator winding. A method including that a power frequency voltage was injected at the neutral of the generator was analyzed.

Methods exist in which a subharmonic voltage is injected into the protected plant. An over- current relay monitors the subharmonic current that flows to the protected plant. An earth fault anywhere on the stator winding will increase the subharmonic current. Such methods scheme provide a total coverage of the entire stator winding. However, the cost of the implementation tends to be high due to the cost of the injection equipment.

Third-Harmonic Restricted Earth Fault Protection In the already installed Powerformer machines in Porjus, Porsi and Eskilstuna an earth fault protection circuit covering 100% of the stator winding has been implemented. The schematic circuit diagram of Fig. 6 shows a synchronous machine directly connected to a power network, such as is the case for said installed machines, and having a circuit for earth fault protection. A set of current transformers 21 A, 21 B, 21 c are connected to sense the current in the phase windings at a relative distance x from the neutral point 5 of the synchronous generator, the current transformers thus dividing the phase windings lA, 1B, 1C into two portions. The phase winding terminals 3A, 3B, 3c are through circuit breakers connected 25A, 25B, 25c to the rails 27A, 27B, 27c of the three-phase busbar to which other power generating units can be connected in the similar way.

The overcurrent relay or control circuit has nominal sensitivity for the third harmonic (150 or 180 Hz) of the nominal frequency (50 or 60 Hz) in the power system. The overcurrent relay has a very low sensitivity for the fundamental frequency.

The relative distance x from the neutral point 5 of the synchronous machine is defined by x= No (2) Error where No is the number of turns per phase from the neutral point to the location of the current transformer, and Ntot is the total number of turns per phase from the neutral point to the phase terminal of the synchronous machine.

The current transformers 21A, 21 B, 21 c are connected in parallel, in a Holmgren-connection, to excite the overcurrent relay 23 with the residual current. The protection circuit illustrated in Fig.

6 is based on the fact that the induced voltage of many generators and especially a Powerformer contains a sufficient amount of harmonics of order n, where n is defined by: n=3 k+3 (3) where k = 0, 27 2,.... The third harmonic is of practical importance but this circuit is not restricted to the use of third harmonic. In some applications it may be advantageous to use one or several harmonics of higher order possibly in combination with the third harmonic.

The third-harmonic restricted earth fault protection is used in some existing power plants, where the adjacent power network is effectively earthed. Analysis has shown that the third-harmonic restricted earth fault protection should work well in such effectively earthed power networks and measurements have also been performed confirming the analysis. It has not been proven that it will work properly in non-effectively earthed power networks.

Other directly connected high voltage generators (Powerformer) will be connected to non- effectively earthed power networks. In one typical case the capacitive earth fault current is in the range from 48 to 49 amperes, the Petersen coil, the arc suppression coil, has a rated current of 56 amperes and the neutral point resistor in the power network has a rated current of 30 amperes.

In another case the Petersen coil or arc suppression coil has a setting range from 169 to 304 amperes and a rated apparent power of 11 430 kVA whereas the neutral point resistor has a rated current of 200 amperes. The exact value of the capacitive earth fault current is not known but it can be assumed to be in the range of 200 to 300 amperes.

This means that there is a demand to find a dependable and secure 100 % earth fault protection that works even for synchronous machines connected to non-effectively earthed power networks.

Results from workshop test of a high voltage generator (Powerformer This section contains some experimental data supporting assumptions concerning the existence and the distribution of third harmonics in a Powerformer type high voltage generator. In Fig. 7 the magnitude (=-\/2-RMS Value) of the third harmonic of one third of the residual voltage at the terminal of such a Powerformer generator is shown as measured at a sudden earth fault from 100 % of rated voltage in the workshop test on 2000-06-15 (Case 4).

The magnitude decreases from 2.4 kV to 0.4 kV at the earth fault.

In Fig. 8 the magnitude of the third harmonic at the neutral of the Powerformer generator as measured at a sudden earth fault from 100% of rated voltage in the workshop test is shown. The magnitude increases creases from 2.0 kV to 4.8 kV at the earth fault.

In Fig. 9 the magnitude of the third harmonic of the difference of one third of the residual voltage at the phase terminals and the neutral point voltage of the considered Powerformer generator is shown. The magnitude remains fairly stable at 4.4 kV.

In Fig. 10 the magnitude of the positive sequence voltage of the Powerformer generator is shown as measured at a sudden earth fault from 100% of rated voltage in the workshop test on 2000-06- 15 (Case 4). The initial value of the magnitude of the positive sequence voltage is 110 lcV. The magnitude then decreases due to the removal of the excitation.

The magnitude of the induced third harmonic as estimated from Fig. 9 decreases when the induced positive sequence voltage (fundamental frequency) decreases, see Fig. 10. The decrease of the third harmonic is faster than proportional. Initially the relative third harmonic is equal to 0.04 but it decreases to 0.02 at the end of the recording.

Results from Commissioning Tests Lundblad has documented, see J. Lundblad :"Carhuaquero-Relaskydd", ASEA, 1981-08-21, results from measurements of generated harmonic voltages at seven operating states of a small hydro generator. In Fig. 11 the operating states are shown where the harmonic voltages have been measured.

The magnitude of the generated third harmonic voltage increases with armature current and decreases with field current. A straightforward application of the least squares method gives the following result: <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> U3=1.44+4.22#Ia-2.72#If (4)<BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> <BR> In Ifit Here U3 [%] is the measured third harmonic, Ia [A] is the armature current, In [A] is the rated armature current, If [A] is the calculated field current and If, [A] is the calculated field current at rated output power. The field current has been calculated using a linear model of the synchronous machine and a direct axis synchronous reactance of 114 percent.

In Fig. 13 the measured third harmonic voltage and the predicted third harmonic voltage are shown. The linear relation (4) has been used to estimate the third harmonic.

Published experimental results Engelhardt has published, see K. H. Engelhardt:"A Composite Ground Fault Detection Scheme for High Resistance Grounded Generator Stators", Paper 74-SP-140, Transactions of the Engineering and Operating Division, Canadian Electrical Association, Vol. 13, pt. 3, pp. 1-11, September 1974, results of measurement of the third harmonic voltage on three generators in Canada.

The measured third harmonic voltages range from 2 to 11 percent.

Marttila has published, see RJ. Marttila:"Design Principles of a New Generator Stator Ground Relay for 100% Coverage of the Stator Winding", IEEE Transactions on Power Delivery, Vol.

PWRD-1, No. 4, pp. 41-51, October 1986, results of measurements of the generated third harmonic on a number of two-pole and four-pole turbogenerator in Canada. In Fig. 15 some of the results are reproduced.

The maximum value of the induced third harmonic voltage ranges from 2 to 8 percent of the fundamental frequency voltage. These experimental results shows that there are operating states where the generated third harmonic falls to values below one percent.

This means that a simple undervoltage relay energized by the third harmonic voltage at the neutral is more prone to unwanted operation than a simple overvoltage relay energized by the third harmonic voltage at the phase terminal is. The dependability of a simple overvoltage relay energized by the third harmonic voltage may be lower than a simple undervoltage relay energized by the third harmonic voltage at the neutral. One can increase the reliability of a 100% earth fault protection by relating the harmonic voltage at the neutral and the harmonic to the total generated harmonic voltage. This is possible if there is a voltage transformer at the neutral of the synchronous machine. A voltage transformer connected at the neutral point of a high voltage generator of the Powerformer type is very costly and required significant space. Thus, there is a need for a 100% earth fault protection circuit for high voltage generators that does not require a voltage transformer at the neutral point.

Zielichowski and Fulczyk have published, see M. Zielichowski & M. Fulczyk:"Influence of load on operating conditions of third harmonic ground-fault protection system of unit connected generators", IEEE Proceedings, Generation Transmission and Distribution, Vol. 146, No. 3, pp.

241-, May 1999, results of measurements on a turbogenerator at various values of the active output power. In Fig. 16 the generated third harmonic voltage, third harmonic voltage at the neutral and third harmonic voltage at the phase terminal as a function of the active output power are shown.

In Fig. 17 the ratio of the third harmonic voltage at the neutral and the third harmonic voltage at the phase terminal are shown.

The induced harmonic voltage seems to vary with the excitation of the synchronous machine. No simple explanation of the variation of the ratio of the third harmonic at the neutral and the third harmonic at the phase terminal can be found.

SUMMARY OF THE INVENTION It is an object of the invention to provide a protection scheme and a protection circuit for a synchronous electric generator that can detect earth faults occurring at any location in the phase windings.

It is another object of the invention to provide a protection scheme and protection circuit for a synchronous electric generator that can efficiently detect earth faults for a high voltage electric generator such as a Powerformer generator.

The problem solved by the invention is how to protect an electric synchronous generator from earth faults occurring in the phase windings, in particular how to protect an electric synchronous generator having a neutral point that is not connected to earth or is connected to earth through a high or very high impedance.

Thus generally, three elements can be combined to form a phase-segregated 100% earth fault protection scheme and circuit; namely (1) a directional element, (2) a fault detector element and (3) a range control element.

A synchronous electric generator has in the common way phase windings in which voltages are induced. The phase windings are connected at one end to each other at a neutral point. They have at an opposite end phase terminals on which the generated voltages are supplied. First extracting means such as current transformers and fourier filters are provided for extracting the third or higher order harmonic currents in the phase windings at locations at the neutral point.

Furthermore, there are first calculating means for calculating, from the extracted third or higher order harmonic currents, a directional criterion for each phase winding. Such a directional criterion indicates the relative magnitude of the third or higher order harmonic current for the considered phase winding reduced by the third or higher order harmonic currents of the other phase windings. First comparing means compare the directional criteria to a normal value valid for correctly operating phase windings. A signal is provided, in the case where at least one of the directional criteria deviates too much from the normal value, to a control device for issuing an alarm signal and/or for disconnecting or disabling the phase windings. The criterion that most deviates from the normal value can indicate the phase, in which the fault has occurred.

A fault detection criterion can also be calculated that distinguishes between the cases where the extracted third or higher order harmonic currents totally have or have not a sufficient magnitude, the control device being prevented from taking any action if said currents totally have not the sufficient magnitude.

Furthermore, a range control detection criterion can be calculated that distinguishes between the cases where the sum of the third or higher order harmonic voltages on the generator terminals have or have not a sufficient magnitude, the control device being prevented from taking any action if said sum has not the sufficient magnitude. The voltages can be obtained from second extracting means extracting the voltages at the phase winding terminals, the second extracting means e. g. comprising voltage transformers and fourier filters.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of non-limiting embodiments with reference to the accompanying drawings, in which: - Fig. 1 is a schematic circuit diagram of three-phase windings for which the range of different types of earth fault protection is illustrated, - Fig. 2 is a circuit diagram of three-phase windings having neutral point overvoltage protection, - Fig. 3 is a circuit diagram of three-phase windings having neutral point overcurrent protection, - Fig. 4 is a circuit diagram of three-phase windings having zero sequence overvoltage pro- tection, - Fig. 5 is a circuit diagram of three-phase windings having residual differential protection, - Fig. 6 is a circuit diagram of three-phase windings having third-harmonic restricted earth fault protection, - Fig. 7 is a diagram of the magnitude of the third harmonic as a function of time at the phase terminal of a Powerformer generator, - Fig. 8 is a diagram of the magnitude of the third harmonic as a function of time at the neutral point of a Powerformer generator, - Fig. 9 is a diagram of the magnitude of the third harmonic of the difference of the (residual voltage) /3 at the phase terminals and the neutral point voltage as a function of time of a Powerformer generator, - Fig. 10 is a diagram of the magnitude of the positive sequence voltage as a function of time of a Powerformer generator, - Fig. 11 is a diagram of reactive power plotted as a function of active power for a Powerformer generator showing operating states where harmonic voltages have been measured, - Fig. 12 is a diagram illustrating harmonic voltages derived from measurements at the operating states of Fig. 11, - Fig. 13 is a diagram of measured and predicted third order harmonic voltages at the operating states of Fig. 11, - Fig. 14 is a diagram of measured maximum and minimum third harmonic voltages, - Fig. 15 is a diagram of generated third harmonic voltages as measured by Marttila, - Fig. 16 is a diagram of measured generated third order harmonic, third order harmonic at the neutral and third order harmonic at the phase terminal as a function of active output power, - Fig. 17 is a diagram of measured ratio of the third order harmonic at the neutral point and the third harmonic at the phase terminal as a function of active output power, - Fig. 18 is a schematic of a synchronous electric machine or generator having a circuit for phase selective, 100% earth fault protection.

DESCRIPTION OF A PREFERRED EMBODIMENT For detecting earth faults in a synchronous generator the directions of the third harmonic currents flowing in the three phase windings close to the neutral point can be used. A protection circuit using the directions of the third harmonic currents is illustrated in Fig. 18. The three phase windings 1A, 1B, lc of the generator have their line ends connected to output terminals 3A, 3B, 3c and their neutral ends connected to a common node 5 or N, the neutral point or node. The neutral point of the synchronous machine can be high-impedance earthed or high-reactance earthed but this is not necessary. Current transformers 31A, 31B, 31c are connected at the neutral ends of the windings and thus their primary windings have a low resistance and are actually portions of the phase windings lA, 1B, lc. The secondary windings are connected to one-cycle Fourier-filters 33A, 33s, 33c extracting values of the third order harmonic complex currents IQ3, Ib3 I3 from the total complex phase currents Ia, Ib, Ie detected by the current transformers and including load cur- rents, other harmonics and transient currents. For a faulty phase winding, say phase A, it appears that the third order harmonic current-7 flowing from the neutral point N into the faulty phase winding is equal to the sum of the third order harmonic currents IB3, Ic3 flowing from the healthy phase windings, such as 1B, lc towards the neutral point, the same relation also holding for the detected third order harmonic complex currents Ia3J Ib3, IC3.

For each phase a directional criterion Da, Db, De is calculated in a block 35 as defined by the following equations The directional criteria have several important properties. When all phase windings lA, 1B, lc are healthy, the values of the directional criteria Da, Db, Dc are all equal to 1/3. For a faulty phase winding the directional criterion becomes equal to 1 and is equal to 1/2 for the two healthy phases windings. This is a direct consequence of Kirchhoffs current law applied to the neutral point N. Assume that an earth fault has occurred in phase winding lA close to the neutral point and that the third harmonic current in phase winding 1B and the third harmonic current in phase winding lc both are equal to 3. Then This is a reasonable assumption as long as: (1) The induced third order harmonic voltage in phase winding 1B is equal to the induced third harmonic voltage in phase winding lc. (2) The third harmonic impedance of phase winding 1B is equal to the third harmonic impedance of phase winding 1B. (3) The third harmonic voltage on the generator terminals 3A, 3B, 3c is equal to zero.

It is now assumed that the third harmonic voltage VB3 on the terminal 3B of phase winding Is is equal to the third harmonic voltage Vc3 on the terminal 3c of phase winding lc. It is also assumed that the third harmonic impedance of phase winding 1B is equal to the third harmonic impedance of phase winding lc. It is further assumed that the induced third harmonic voltage in phase winding Is is equal to the induced third order harmonic voltage in phase winding lc. The third order harmonic voltage on said phase terminals will then cause a third order harmonic current A13 to flow in phase winding 1B and in phase winding lc. The directional criteria Da, Db, De now become: This means that the directional criteria should be independent of the third harmonic voltages on the generator terminals as long as they are equal in all three phases.

It is now assumed that the three phase-windings all are healthy. It is further assumed that an earth fault has occurred at the neutral point N beyond the current transformers 31A, 31B, 31C at the neutral end of the three phase-windings. It is also assumed that the third harmonic impedance of the three phase-windings all are equal. Furthermore, it is assumed that the three induced third harmonic voltages in the phase windings all are equal. It is finally assumed that the three third harmonic voltages on the generator terminals all are equal. Then the three third order harmonic currents flowing in the phase-windings all will be equal 13,. The directional criteria Da, Db, De now obtain the values: These properties justify the use of the terms directional criterion and directional criteria.

In the circuit of Fig. 18, the directional criteria Da, Db, De are evaluated in a block 37 and it is first determined whether they are all sufficiently equal to 1/3. Then it may e. g. be tested whether the directional criteria deviates only by some small quantity £ from the target or normal value 1/3, i. e. whether 1/3-£ < Dj< 1/3-£ (9) is true for all j = a, b, c. If this is not true, the circuit breakers 25A, 25B, 25c can be activated and an alarm signal triggered as indicated in the decision block 39. It can also then be tested whether only one of the directional criteria is sufficiently large, i. e. has a value larger than some suitable threshold value, e. g. equal to 3/4. In the case where this is true it can also be signalled to the decision block or control device 39 that the phase winding associated with the directional criterion having the large value probably is the phase winding in which an earth fault has occurred. The circuit breakers 25A, 25B, 25c are connected between the output terminals or line terminals 3A, 3B, 3c of the phase winding and the rails 27A, 27B, 27c of a three-phase busbar to which other power generating units can be connected in the similar way.

However, the directional criteria Da, Db, De may assume erratic values during normal operation when the (complex) third order harmonic currents are small and not have the same phase (are not aligned). Therefore it is necessary to combine the results obtained by analyzing the directional criteria with a signal from a fault detector. For this purpose the following quantity can also be calculated in the block 35: <BR> <BR> <BR> <BR> <BR> <BR> <BR> ID3=#Ia3#+#Ib3#+#Ic3# (10) This quantity is compared to a threshold value TFD in a block 41 to provide a fault detection criterion FD according to FD = 1 l-TRUE) if ID3 = IIa3)' I rb3l + I rc3l > TH. D 11 0 (= FALSE) otherwise [0 (= FALSE) otherwise The fault detection criterion is thus operative or active if the total amount of third harmonic currents exceeds a threshold. The threshold value TFD is set so that the result of the comparison in the block 37 of the direction criteria can be considered reliable in the case where the fault detection criterion FD has a high value, i. e. is equal to 1. Then, if the result of the comparison in block 37 indicates a fault, the respective measures should be taken. The fault detection criterion FD can have a low value, i. e. be equal to 0, if the third harmonic voltage source at or close to the neutral point N is short-circuited.

The combination of the directional criteria and the fault detection criterion constitutes a non-unit protection, which may start and perhaps operate also at external faults, i. e. faults not located within the considered synchronous generator. Time grading is normal practice for earth fault protections in non-effectively earthed power networks. The reason is that the earth fault current depends mainly on the shunt elements in the network and not on the series elements. This means that the distance to the fault has a minor influence on the fault current and polarizing voltages. It is, however, an advantage to define a range control element that limits the forward reach of the directional criteria.

Prior art indicates that the third order harmonic content of the residual voltage measured at the terminals of the generator may be a suitable quantity for range control. Several experimental investigations show that the third order harmonic content of one third of the residual voltage measured at a terminal of a synchronous machine is close to one half of the induced third order harmonic voltage in one phase winding. The use of this quantity is that it may be used as an earth fault detector when the generator is disconnected from the power network.

A range control criterion RC can then be defined according to: RC = 1 (= TRUE) if "3 + 3b3 + ys TRC (12) 0 (= FALSE) otherwise where V3A, V3B, V3c are the third order harmonic voltages on the generator terminals 3A, 3B, 3c and TRC is a suitably chosen threshold value. The voltages are extracted by voltage transformers 43A, 43B, 43C having their primary windings connected to the respective phase windings at the generator terminals and having their secondary windings connected to Fourier filters 45A, 45B, 45c. The third harmonic voltages extracted in the filters are provided to a block 47 in which the quantity (Va3 + Vb3 + Fe)/3 is calculated. This quantity is in a block 49 compared to the threshold TRC and the range control criterion RC is obtained. This criterion is directly supplied to the control block 39 to stop the issuing of signals to the circuit breakers in the case where the criterion RC has a low value, i. e. is equal to 0.

Criteria similar to the range control criterion have been proposed and used for the detection of earth faults at or close to the neutral point N.