A variable speed generator-motor apparatus

Abstract: There is disclosed a generator-motor apparatus with economical protection measures for use in a variable speed pumping-up electric power plant, which comprises a main transformer (12), a secondary winding of which is coupled to an electric power system (10), a power system breaker (14) connected to a primary winding of the main transformer (12), a synchronizing breaker (16) connected in series with the power system break (14), a generator-motor (18), a rotor of which is mechanically coupled to a pump-turbine and an armature winding of which is connected to the power system (10) through the synchronizing breaker (16), the power system breaker (14) and the main transformer (12), and an excitation circuit, including a frequency converter (34) for supplying a field winding (22) of the generator-motor (18) with AC voltage of the variable frequency and a converter transformer (30) for feeding the converter (34), which is branched from a junction (28) of the power system breaker (14) and the synchronizing breaker (16).

Description: Field of the Invention The present invention relates to an improvement of a variable speed generator-motor apparatus, and more particularly to an variable speed generator-motor apparatus with economical protection measures against an earthing or short-circuiting fault occurring therein. Description of the Related Art A generator-motor apparatus of the alternating current (AC) excitation type, which is often used in a variable speed pumping-up electric power plant, comprises a generator-motor having an armature winding and a field winding provided on a stator and a rotor thereof, respectively, a main transformer connected to an electric power system, a synchronizing breaker provided between the main transformer and the armature winding of the generator-motor, a frequency converter, such as a cycloconverter, producing AC voltage of a low frequency for excitation of the field winding, a converter transformer for feeding the frequency converter, and an excitation breaker provided between the converter transformer and a junction of the main transformer and the synchronizing breaker. Incidentally, in the following description, when only a generator-motor is referred to, distinguished from the whole of a generator-motor apparatus, it will be called an electric rotating machine or simply a rotating machine. The rotor of the electric rotating machine is mechanically coupled to a water pump-turbine capable of operating selectively as a prime mover or a load. When driven by the pump-turbine, which operates as a water turbine, the rotating machine functions as a generator and supplies electric power to the power system through a main circuit of the generator-motor apparatus composed of the main transformer and the synchronizing breaker. On the contrary, when the rotating machine is supplied by the power system with electric power, it operates as a motor and drives the pump-turbine functioning as a water pump. In the generator-motor apparatus as mentioned above, by exciting the field winding with AC voltage of the frequency depending on the difference between a synchronous speed determined by the frequency of the power system and a rotational speed of the rotor, the frequency of an output voltage of the electric rotating machine functioning as a generator can be maintained constant at the frequency of the power system free from fluctuation in a rotational speed of the pump-turbine functioning as a water turbine. Similarly, in the case where the rotating machine operates as a motor, a rotational speed of the pump-turbine operating as a pump can be arbitrarily controlled irrespective of the fixed frequency of voltage applied to the rotating machine from the power system. As a result, the pump-turbine can be most efficiently operated at the adaptive speed both in the motor operation and in the generator operation. As a generator-motor apparatus applied to a variable speed pumping-up electric power plant, a prior art as disclosed, for example, in the earlier European Application EP-A-243 937 entitled "Variable-speed pumped-storage power generating system". According thereto, a main circuit of a generator-motor apparatus includes a main transformer and a synchronizing breaker connected in series thereto. Further, the main circuit is formed between an armature winding of an electric rotating machine and an electric power system and communicates electric power therebetween. An excitation circuit for a field winding of the rotating machine includes a cycloconverter and a converter transformer. The cycloconverter is fed by the converter transformer and supplies the field winding with AC voltage of the variable frequency. The converter transformer is connected to a junction of the main transformer and the synchronizing breaker through an excitation breaker. As will be apparent from the description above, therefore, the synchronizing breaker and the excitation breaker are provided in parallel with each other with respect to the power system. With the disposition of the excitation breaker as mentioned above, the following problem is developed. Namely, an excitation breaker is required to have a current breaking capacity greater than that of a synchronizing breaker. For example, the synchronizing breaker is required to cut off a fault current incoming to a rotating machine from a power system, if an earthing or short-circuiting fault occurs on the side of the rotating machine with respect to the breaker, and cut off a fault current flowing toward the power system from the rotating machine, if such a fault occurs on the side of a main transformer or the power system with respect to the breaker. On the other hand, the excitation breaker must cut off a composite fault current from both sides of the power system and the rotating machine, in the case of occurrence of the earthing or short-circuiting fault in the excitation circuit. Therefore, the current to be cut off by the excitation breaker becomes substantially the double the current to be cut off by the synchronizing breaker. Accordingly, as the excitation breaker, there is required an expensive breaker with a large current breaking capacity, with the result that the protection measures for a generator-motor apparatus becomes expensive as a whole. SUMMARY OF THE INVENTION In a lecture held by H. Mühlöcker on the European Wind Energy Conference 1984 and published on pages 542 to 547 of the Proceedings of that Conference under the title Induction Generators Adapted to Variable Speed Operated Wind Turbines a generator-motor of the type defined in the preamble of claim 1 has been disclosed. With an arrangement combining a generator-motor, a power system and an AC excitation apparatus of the type captioned above one serious problem arises in connection with operating the breakers upon the occurrence of a fault in the system requiring a cutting off of a fault current. When the generator-motor and its AC excitation apparatus are simply separated from the power system a considerable amount of electrical energy remains stored therein. The energy in the armature winding will not become zero and may cause an overvoltage surge to occur in the generator-motor and the circuit coupled thereto. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a generator-motor apparatus which, with economical protection measures, is capable of reliably cutting off a fault current by a breaker which has a current breaking capacity well-matched with the capacity of the generator-motor apparatus and therefore is reasonable in cost. According to the present invention this object is achieved by a generator-motor apparatus as defined in the claim 1. Preferred further developments of the invention are indicated in the subclaims. Due to the operation sequence of the several breakers according to the present invention, upon opening of the synchronizing breaker electric energy stored in the inductance of the field winding during the normal operation of the generator-motor can be fed to the power system for regeneration through the still the still closed power system breaker. Neither the synchronizing breaker nor the power system breaker must have an unnecessarily large current breaking capacity. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a skeleton diagram schematically showing the arrangement of a generator-motor apparatus to be operated according to the present invention; Fig. 2 shows a detailed circuit arrangement of a frequency converter to be used in the generator-motor of Fig. 1; Fig. 3 shows a circuit for tripping the power system breaker and the synchronizing breaker in the generator-motor apparatus of Fig. 1 and Fig. 4 is a diagram for explaining the sequence operation of the circuit of Fig. 3. DESCRIPTION OF PREFERRED EMBODIMENTS In the following, the present invention will be explained in detail with reference to the accompanying drawings. Fig. 1 shows a generator-motor apparatus which may be applied to a variable speed pumping-up electric power plant. In the figure, reference numeral 10 denotes a line of an electric power system and reference numeral 12 denotes a main transformer, which is connected to the power system line 10. Incidentally, in the following description. let us call a winding of the transformer 12 on the side of a generator-motor a primary winding and a winding thereof on the side of the power system a secondary winding. As shown in the figure, the main transformer 12, which is coupled to the power system line 10 through its secondary winding, has plural primary windings (two windings in the example shown). Each primary winding thereof can be connected to a circuit as described below, which includes a particular generator-motor. A first breaker 14 is connected to one of the primary windings of the main transformer 12. Although in the figure the first breaker 14 is connected to the primary winding of the main transformer 12, it can be provided on the secondary side thereof, i.e., between the main transformer 12 and the power system line 10, in the case where a single generator-motor is associated. In this case, the required current breaking capacity of the first breaker 14 can be reduced. A second breaker 16 is further connected in series with the series connection of the main transformer 12 and the first breaker 14. Reference numeral 18 denotes a generator-motor, which will be called, as already described, an electric rotating machine or simply as a rotating machine, distinguished from the whole of an generator-motor apparatus. A rotor of the electric rotating machine 18 is mechanically coupled to a pump-turbine (not shown) as a prime mover-load and rotates together therewith. The rotating machine 18 has an armature winding 20 provided on a stator thereof and a field winding 22 provided on the rotor. In the example shown, therefore, the rotating machine 18 is of a so called rotating field type, however the application of the present invention is never limited to this type of the rotating machine 18. A neutral point 24 of the armature winding 20 is grounded through an earthing resistor 26. Between a junction 28 of both the breakers 14 and 16 and the field winding 22, there exists an excitation circuit, which is composed of a converter transformer 30 fed with AC voltage through a line 32 from the junction 28 and a frequency converter 34 for converting the frequency of the voltage supplied by the transformer 30 and exciting the field winding 22. In the embodiment shown, a cycloconverter is used as the frequency converter 34. In order to obtain AC excitation voltage with the excellent waveform, there is used the excitation circuit, the detailed arrangement of which is shown in Fig. 2. Incidentally, in the following description, the first breaker 14 is called a power system breaker, because it is provided on the side of the electric power system with respect to the junction 28, whereas the second breaker 16 is called a synchronizing breaker similarly to the case of the prior art. Further, the junction 28 can also be regarded as a branching point of the excitation circuit from the main circuit. Moreover, there are cases in the following that, within the main circuit, a circuit on the side of the rotating machine 18 with respect to the junction 28 is especially referred to as an armature side circuit. In Fig. 2, the same reference numerals denote the same parts as in Fig. 1. Further, although the references of single numeral 30 or 34 have been given to the converter transformer and the cycloconverter in Fig. 1, respectively, they are actually composed of three transformers 301 to 303 for respective phases R, S, T of the three phase field winding 22 and three cycloconverters 341 to 343 connected to the corresponding transformers, as shown in Fig. 2. Output voltages of the three cycloconverters 341 to 343 are supplied to respective windings of the field winding 22 through corresponding slip ring 221 provided on a rotor axle of the rotating machine 18. Further, reference numeral 222 denotes a slip ring, which is also provided on the rotor axle and through which a neutral point of the cycloconverter 34 is connected to that of the field winding 22. In the case of Fig. 2, a twelve phase rectification system is adopted in order to suppress occurrence of harmonics. Therefore, each of the transformers 301 to 303 has two secondary windings, one of which is formed in the delta connection and the other in the star connection. Further, each of the three cycloconverters 341 to 343 is composed of a pair of unit converters. Further detailed construction of the cycloconverter 34 is not presented here because it is not essential with a view to the present invention. Returning to Fig. 1, description will be made of a control circuit for the generator-motor apparatus. Reference numeral 40 denotes a phase detector, which determines a phase  s (an integrated value of frequency) as a reference signal for controlling excitation of the field winding 22 by the cycloconverter 34. The phase detector 40 comprises voltage transformer 42, voltage phase calculator 44 and slip phase calculator 46. The transformer 42 and the calculator 44 are provided for detecting a phase  v of the voltage of the power system. The calculator 46 receives a rotating phase signal  r from an appropriate detector 48 such as a resolver synchro mechanically coupled to the rotor 22 of the rotating machine 18 as well as the phase signal  v obtained as above and calculates a slip phase  s on the basis of the difference of  v and  r . The thus obtained slip phase  s is used as the aforesaid reference signal in the excitation control of the field winding 22 of the rotating machine 18. Reference numeral 50 denotes an excitation controller, which receives the slip phase signal  s and produces a control instruction for adjusting an amplitude and a phase of AC current supplied for the field winding 22, which instruction rotates in synchronism with the slip phase signal  s . The control instruction is coupled to automatic pulse phase shifter 52, in which a gate pulse signal for the cycloconverter 34 is produced in accordance with the control instruction supplied. Further, the generator-motor apparatus includes a fault detection and protection system as described below. In Fig. 1, blocks 61 to 66 labeled by "OCR" or "RDfR" represent over current relays and ratio differential relays, respectively. The relays 61 to 66 are associated with particular current transformer or transformers 71 to 76, which are inserted in appropriate locations of the main and excitation circuits of the generator-motor apparatus. Details of the functions of the relays 61 to 66 are as follows. Incidentally, in the following description, every relay is referred to by a reference symbol labeled in a corresponding block and a current transformer by an abbreviation CT. OCR 61: a relay for protection against an over current caused by an earthing or short-circuiting fault occurring on the armature side circuit, on the basis of an output signal of CT 71; RDfR 62: a relay for protection against a fault occurring in a circuit inside an area covered by CTs 71 and 72, on the basis of output signals of those CTs (the detection of such a fault is called an interior fault detection); OCR63: a relay for protection against an over current caused by a fault, especially, between the neutral point 24 and the ground, on the basis of an output signal of CT 73; RDfR64: a relay for protection against an interior fault between CTs 71 and 73, especially in the vicinity of the neutral point 24, on the basis of output signals of CTs 71, 72 and 73; OCR 65: a relay for protection against an over current caused by a fault occurring in the excitation circuit, on the basis of an output signal of CT 74; and RDfR 66: a relay for protection against an interior fault occurring in a circuit inside an area covered by CTs 74, 75 and 76, on the basis of output signals of those CTs. Although, in addition to the relays listed above, various relays for protection against the abnormality or malfunction of the cycloconverter 34 may be further associated, they are omitted in this embodiment. If they are provided, output signals of those relays can be treated in the analogous manner to the output signals of the relays 61 to 66 as listed above. Now description will be given on the protection operation for the generator-motor apparatus referring to Fig. 3. When the electric rotating machine 18 is operating normally, it is connected to the power system line 10 through the closed breakers 14 and 16. If, however, a fault is detected by the relays 61 to 66 in the excitation circuit, the protection operation is started prioritizing the timing of trip of the breakers 14 and 16. If, for example a fault occurs in the armature side circuit and is detected by OCR 61, first the synchronizing breaker 16 is opened almost immediately and then the power system breaker 14 is opened after a predetermined time delay, whereby the energy stored in the inductance of the field winding 22 during the normal operation of the rotating machine 18 can be regenerated to the power system. The protection operation as mentioned above can be performed by a sequence circuit as described hereinafter. An example of such a sequence circuit is shown in Fig. 4 . In Fig. 4 reference numerals 14T and 16T denote trip coils of the power system breaker 14 and the synchronizing breaker 16, respectively, and reference numerals 142 and 162 denote auxiliary contacts (usually called pallet contacts) of the breakers 14 and 16, respectively, which are closed or opened together with closure and open of the related breaker 14 or 16. Further, reference numerals 82, 84, 88 and 90 denote auxiliary relays, among which the relay 88 has two relay coils A and B. Various contacts shown in the figure are as follows. 621, 641: normally open contacts of RDfRs 62 and 64, respectively; 821, 823: normally open contacts of the relay 82, which are closed instantaneously upon energization of the relay and opened with a predetermined time delay upon deenergization thereof; 822: a normally closed contact of the relay 82, which moves in synchronism with the contacts 821, 823, i.e., which is opened instantaneously upon energization of the relay and closed with the predetermined time delay upon deenergization thereof; 842: a normally closed contact of the relay 84, which is opened upon energization of the relay and maintains its state even upon deenergization thereof until it is manually closed; 861, 863: normally open contacts of the relay 86 (not shown) for the protection against a fault occurring in circuits other than an armature side circuit; 880: a transfer contact of the relay 88 with two fixed contacts a and b. When the coil A is energized, the contact 880 is closed on the side of the contact a, and when the coil B is energized, it is closed on the side of the contact b. Usually, the contact 880 is closed on the side of the contact b; 881, 883: contacts of the relay 88, which are opened when the contact 880 is closed on the side of the contact b and otherwise closed; 882: a contact of the relay 88, which is closed when the contact 880 is closed on the side of the contact b and otherwise opened; 901: a normally open contact of the relay 90, which is closed with a certain time delay upon energization of the relay and opened instantaneously upon deenergization thereof; and 163: an auxiliary contact of the breaker 16, which is opened and closed together with the closure and open of the breaker 16. In the following, the operation of the circuit of Fig. 4 will be explained, referring to Fig. 5, in which there is illustrated the change in the states of the relays, contacts, coils and breakers in the circuit of Fig. 4. Further, in Fig. 5, a state of closure of the contacts and breakers is indicated as ON, and a state of open thereof as OFF. Moreover, numerals shown on the lefthand side in each drawing of Fig. 5 correspond to the reference numerals of the contacts in Fig. 4. The state of each contact shown in Fig. 4 is a state in the case where the generator-motor apparatus continues to operate normally. Namely, when it operates normally, the power system breaker 14 and the synchronizing breaker 16 are both closed, so that their pallet contacts 142, 162 are closed, too, but the auxiliary contact 163 of the breaker 16 is opened. Under such a normal operation state of the generator-motor apparatus, all of relays 82, 84, 88, 90 and the trip coils 14T, 16T are under the deenergized state, and contacts 822, 842, 882 are closed with the rest contacts being open. Further, the transfer contact 880 is closed on the side of the contact b. In respect of the foregoing, see the state before time point t 0 in Fig. 5. First of all, let us assume that a fault occurs in the armature side circuit at time point to. The armature side circuit is, as already described, a circuit on the side of the rotating machine 18 with respect to the junction 28. In the case as described above, either RDfR 62 or 64 must make the interior fault detection, so that either the contact 621 or 641 is closed (cf. Fig. 5-(a)). In the example shown in Fig. 4, the contacts 621, 641 are connected in parallel with each other. The configuration of wiring of the contacts in this portion determines the condition of initiating the protection operation. For example, the contacts of OCRs 61, 63 can be added in series to the contacts 621, 641, respectively, if necessary. Various variations or modification of combination of the contacts in this portion will be considered. In any case, the configuration of wiring of contacts in this portion must be determined in accordance with the necessity of the protection operation to be carried out. When either the contact 621 or 641 is closed at time point t 1 , the relays 82, 84 are energized (cf. Figs. 5(b) and (e)). As a result, the contacts 821, 823 are closed quickly and the contact 822 is opened quickly, too, at time point t 2 (cf. Figs. 5(c) and (d)). The contact 842 is opened at time point t 3 and maintained open thereafter (cf. Fig. 5(f)). Further, it is to be noted that the time scale is somewhat exaggerated in these figures. By closure of the contact 823 at time point t 2 , the energization of the trip coil 16T starts, because the pallet contact 162 is still closed (cf. Figs. 5(g) and (j)). The synchronizing breaker 16 is tripped at time point t 5 (cf. Fig. 5(h)). Simultaneously therewith, the pallet contact 162 is opened and the contact 163 is closed (cf. Figs. 5(j) and (i)), whereby the energization of the trip coil 16T is stopped at time point t 5 (cf. Fig. 5(g)). In this manner, by trip of the synchronizing breaker 16, the armature winding 20 of the rotating machine 18 is electrically separated from the power system and therefore a fault current never flows into a fault point within the armature side circuit from the power system, i.e., a fault is removed, whereby RDfRs 62, 64 are released and accordingly the contacts 621, 641 thereof are opened again (cf. Fig. 5(a)). By the way, since the contact 821 is closed at time point t2 (cf. Fig. 5(c)), the coil A of the relay 88 is energized and the transfer contact 880 is changed over to the contact a at time point t 4 . At the same time, the contact 882 is opened and the contacts 881, 883 are closed (cf. Figs. 5(k) and (I)). By open of the contact 882, an energization circuit of the trip coil 14T is never established. That is to say, when a fault occurs in the armature side circuit, only the synchronizing breaker 16 is at first opened and the power system breaker 14 is for a while prevented from tripping. Further, since the contact 881 is closed at time point t 4 (cf. Fig. 5(I)), a gate shift instruction is given to the excitation controller 50 (cf. Fig. 5(m)). The gate shift instruction is available for modifying the control instruction generated in the controller 50 such that the gate pulse signal generated in the phase shifter 52 based on the modified control instruction makes the cycloconverter 34 operate as an inverter. As a result, the cycloconverter 34 functioning as an inverter can regenerate the energy stored in the inductance of the field winding 22 during the normal operation of the rotating machine 18 to the power system through the still closed power system breaker 14. Also the relay 90 is energized by closure of the contact 883 from time point t 4 (cf. Fig. 5(n)), and the contact 901 is closed after a predetermined time delay τ 2 at time point t 6 (cf. Fig. 5(o)). Therefore, the coil B of the relay 88 is energized, whereby the transfer contact 880 is changed over from the contain a to the contact b at time point t 7 . At the same time, the contact 882 is closed and the contacts 881, 883 are opened (cf. Figs. 5(k) and (I)). Although the contact 882 is closed, the trip coil 14T is not yet energized, since the contact 822 of the time delay reset type is still opened (cf. Fig. 5-(d)). Further, by open of the contacts 881 and 883, the gate shift instruction disappears and the relay 90 is released, at time point t 7 (cf. Figs. 5(m) and (n)). A time duration, in which the cycloconverter 34 is operated as an inverter, is almost determined by the time constant τ 2 of the contact 901 of the time delay operating type. When time point t 8 is reached, the time delay reset contact 822 is closed again (cf. Fig. 5(d)). Although the contacts 821, 823 of the same kind are opened, too (cf. Fig. 5(c)), there occurs no influence thereby. When the contact 822 is closed, the trip coil 14T is energized (cf. Fig. 5(p)), because the contacts 163, 882 and the pallet contact 142 included in an energization circuit of the trip coil 14T are all closed (cf. Figs. 5(i), (k) and (r)). As a result, the power system breaker 14 is tripped at time point t 9 (cf. Fig. 5(q)). Simultaneously therewith, the pallet contact 142 is opened and thereafter the energization of the trip coil 14 is stopped (cf. Figs. 5(r) and (p)). By the trip of the power system breaker 14, the excitation circuit of the rotating machine 18 is separated from the power system. As will be understood from the foregoing description, a time interval or delay T d between the trip of the synchronizing breaker 16 and that of the power system breaker 14 is substantially adjusted by a time constant τ 1 of the time delay reset contact 822. Next, the explanation will be given of the protection operation against a fault occurring in circuits other than the armature side circuit, i.e., in the excitation circuit of the rotating machine 18 or in a circuit on the side of the power system with respect to the junction 28. In this case, OCR 65 operates and/Or RDfR 66 makes the interior fault detection. The relay 86 (not shown) operates in response to the operation of those relays 65, 66. As a result, the contacts 861, 863 are closed and therefore the trip coils 14T, 16T are energized almost simultaneously. Therefore, against the fault in this case, the synchronizing breaker 16 and the power system breaker 14 are opened almost simultaneously with each other. With the aforesaid disposition of a power system breaker and a synchronizing breaker, the former, that is used in place of an excitation breaker in the prior art, is not necessary to have the so large current breaking capacity as in the prior art excitation breaker. Accordingly, there can be realized a generator-motor apparatus with the economical protection measures.

Claims: A generator-motor of the alternating current excitation type apparatus, having: a generator-motor (18) with an armature winding and a field winding, a rotor of which is mechanically coupled with a prime-mover load; a main transformer (12) provided between one of the windings of said generator-motor (18) and an electric power system (10); a synchronizing breaker (16) for connecting the one winding of said generator-motor (18) with said electric power system (10) through said main transformer (12); a power system breaker (14), which is connected in series with said main transformer (12), between said electric power system (10) and said synchronizing breaker (16) and an excitation circuit which is branched from a junction (28) between said synchronizing breaker (16) and said series connection of said main transformer (12) and said power system breaker (14) and which includes a frequency converter (34) for exciting the other winding of said generator-motor (18) and a converter transformer (30) connected to said frequency converter (34) but is not provided with an excitation breaker, characterised in that when there is detected a fault occurring on the side of said generator-motor (18) with respect to said branching junction (28) of said excitation circuit, said synchronizing breaker (16) is tripped almost instantaneously and then said power system breaker (14) is tripped with a certain delay. An apparatus as defined in claim 1, characterised in that said power system breaker (14) is connected to said main transformer (12) on the side of said generator-motor (18) with respect to said main transformer (12). An apparatus as defined in claim 1, characterised in that said power system breaker (14) is connected to said main transformer (12) on the side of said electric power system (10) with respect to said main transformer (12).

Inventors: BANDO AKIRA, MIYASHITA IKURO, NAGAI HASHIME, NAGURA OSAMU, NAKAGAWA HIROTO, OONO YASUTERU, SAITO KEIJI, BANDO, AKIRA, MIYASHITA, IKURO, NAGAI, HASHIME, NAGURA, OSAMU, NAKAGAWA, HIROTO, OONO, YASUTERU, SAITO, KEIJI
Applicants: HITACHI LTD, KANSAI ELECTRIC POWER CO, HITACHI, LTD., THE KANSAI ELECTRIC POWER CO., INC.
Application year: 1988
Main classification: H02H7
IPC codes: H02P9, H02H7