WO2003038989A1

WO2003038989A1 - Method and apparatus for magnetizing a synchronous generator

Info

Publication number: WO2003038989A1
Application number: PCT/FI2002/000793
Authority: WO
Inventors: Eino Silander
Original assignee: Abb Oy
Priority date: 2001-10-09
Filing date: 2002-10-09
Publication date: 2003-05-08
Also published as: FI20011964A; FI20011964A0; FI116107B

Abstract

The invention relates to a method and to an arrangement for exciting a synchronous generator (10) in case when the mains voltage drops under a predetermined limit value, the synchronous generator being excited via an excitation winding. The mains voltage of the synchronous generator is measured and compared to a reference value. According to the invention at least one current transformer (2) is arranged to a mains conductor (R,S,T), the primary of the current transformer being in series with the mains conductor and the secondary of the current transformer is coupled to a regulating circuit (8). The current of the excitation circuit (6) of the synchronous generator is controlled by the regulating circuit when the mains voltage drops under a reference value.

Description

METHOD AND APPARATUS FOR MAGNETIZING A SYNCHRONOUS GENERATOR

The invention relates to a method for exciting a synchronous generator as stated in the preamble part of the claim 1 as well as to an arrangement for exciting a synchronous generator as stated in the preamble ofthe claim 8.

The excitation of a synchronous machine, especially of a synchronous generator, has at present been arranged brushlessly, whereby the controlled exciting current is supplied into the exciting winding via an exciting machine. The exciting current is generated e.g. by a voltage transformer, which is coupled to the mains and the secondary voltage of the transformer is controlled by a voltage regulator. During exceptional occasions the exciting current generated hereby is not sufficient, especially when a short circuit is caused in the mains, because the mains voltage drops essentially.

Under the short circuit situation the excitation has been conventionally secured e.g. by a separate current transformer, which has been rated so large that a sufficient excitation level is ensured. This solution has a drawback that there is no possibility to regulate the excitation current but the excitation level is defined by the rating of the current transformer. Case by case a suitable transformer has had to be designed and produced for every application, which is not in most case a competitive solution.

The object of the present invention is to solve the problem described above and to achieve an advantageous and a cost-effective excitation of the synchronous generator for the situations, whereby the mains voltage has essentially dropped. The method according to the invention is characterised by the features as detailed in the characterising part of the claim 1. Correspondingly, the arrangement according to the invention is known from the features as defined in the characterising part of the claim 8. Other advantageous realisations have features of the dependent claims.

With the solution according to the invention a proper level of excitation is reliably achieved under the short circuit situation. The solution according the invention is simple and profitable to realise. The same realisation is easily applicable for a large power range, which lowers planning and manufacturing costs. In that case a limited group of standardised short current transformers can be used, which has a remarkable effect for the efficiency of the production chain. It is possible to attach the solution according to the invention also to the generators in use, because the arrangement is attachable to the component that are easily on hand. As well it is possible to tune the former installations function in a better operating point, if the current transformer has been rated erroneously too large. Except in the short circuit situation, the invention is applicable also in the other exceptional occasions, like when large motors are started, whereby the voltage of the generator tends to descend under the permitted limit value.

According to an advantageous embodiment the regulator connected to the current transformer is coupled parallel to the voltage regulator, whereby these regulators together generate the exciting current.

The invention will be described below with reference to the attached drawings wherein

Figure 1 discloses a prior art solution for excitation of the synchronous generator during a short- circuit,

Figure 2 describes the characteristic curve for the short-circuit current of the synchronous generator as function of excitation current according to prior art, - Figure 3 discloses a solution according to the invention,

Figure 4 describes the characteristic curve for the short-circuit current of the synchronous generator as function of excitation current according to the invention.

Figure 1 describes an excitation circuit for a synchronous generator, which generates the excitation during the normal use and according to the prior art technique during the short- circuit situation. The synchronous generator 10 is magnetised by the excitation current I_m conducted into the excitation winding 6. A three-phase transformer TI is coupled to the phase conductors of the generator and the secondary winding 1 of the transformer TI is arranged to supply a voltage regulator 3. Another secondary winding 7 of the transformer TI creates a three phase measured value for the regulator 3. A current transformer T2 coupled to the phase S of the generator measures the current value, which is also input to the voltage regulator. In the normal mode the excitation current I_m of the generator is lead via a diode 12 to the winding 6.

For the short-circuit situation three current transformers T3 are according to the prior art technique arranged to the phase conductors R-S-T of the synchronous generator. During the normal use the secondary windings are shorted out by a switch K2, which is controlled by the voltage regulator. Alternatively the switch may also be controlled by a contactor, which is coupled to the output of the voltage transformer TI. Hereby the contacts of the switch K2 open when the voltage falls so much that the contactor releases. The switch can also be controlled by a voltage relay. The structure of the transformers has structure of a simple toroidal transformer, whose primary circuit is, as well known in the art, formed by the phase conductor of the generator without any regulating means. The secondary circuits of the current transformers are connected to a rectifier 4, whose output is connected to supply the excitation winding 6 via a diode 14.The current transformer has been rated to generate for three seconds a short-circuit current, which is equal to the threefold rated current. The higher currents could destroy except the excitation arrangement but also the generator itself.

In Figure 2 there are described the characteristic curves of the prior art solution. The curve la illustrates the short-circuit current of the generator as function of the excitation current and the curve 2a illustrates the characteristic curve of the current transformer. The operating point of the generator settles to the intersection I ₂ ofthe generator curve and the current transformer curve. The curve 2b illustrates a characteristic curve for another current transformer, whereby the operating point has the value I i.

The figure 3 describes the solution according the invention. In the normal mode the design corresponds to the arrangement of the figure 1 and the corresponding parts are identified with the same reference numbers as in the figure 1. During the short-circuit situation the excitation current is generated by a new manner. Accordingly a regulating circuit 8 is coupled to the secondary windings of the current transformer 2 and the output of the regulating circuit is connected to the excitation winding 6 of the synchronous generator 10. As described in connection of the figure 1 the measured value is got from the secondary winding 7 of the voltage transformer TI, which is connected to the phase conductors R, S and T of the synchronous generator 10. The measured value of the voltage is compared to the predetermined reference value in the comparison circuit in the voltage regulator 3. When the voltage falls under the treshold value, the regulator opens the contacts K2, whereby the current transformers feed the excitation winding 6 via the regulating circuit 8.

The regulating circuit 8 essentially consists of three parts: the filter 22, the trigger circuits 42 and 44, and the thyristors 28, 30, 32, 34. The function of the filter 22 is to smooth the high over-voltage peaks, which may destroy the rectifying components or even the regulating circuit. The actual control circuit consists of two pairs of counter parallel- coupled thyristors. The thyristors 28 and 30 are connected between the output conductors 36 and 38 of the current transformer and respectively, the thyristors 32 and 34 are connected between the output conductors 38 and 40 of the current transformer. The trigger circuit 42 controls the thyristors 28 and 30 and respectively, the trigger circuit 44 controls the thyristors 32 and 34. The trigger circuits render the thyristors conductive, when the mains voltages ofthe current transformers exceed a pre-set value. The output conductors of the current transformers are further connected to the three-phase input of the rectifier 46. The dc output of the rectifier is coupled to the excitation winding 6 of the synchronous generator. The regulating circuit restricts the current fed by the current transformer to the desired level, whereby the standardised current transformers can be used.

The output of the regulating circuit 8 is connected parallel with the voltage regulator 3 to supply the excitation current to the excitation winding 6. The current of the current transformer does not thus flow through the voltage regulator and it cannot damage the voltage regulator in any situation. On the other hand the current transformers 2 and the regulating circuit 8 can generate the whole excitation current in case of urgency, if the voltage regulator is out of order or if the voltage regulator is cut out.

Under the short-circuit situation the excitation current must on the other hand be high enough in order that a suitable current relay operation is achieved. The excitation current, however, must not increase too high that the excitation circuit or the generator would not be damaged. Typically, the operation is designed such that the short-circuit current (Fig. 4) of the generator is about three times the rate value I_n. The characteristic curve of the current transformer (Fig. 4, curve 2c) gives a short-circuit current value I^_b, whereas the short-circuit current can be restricted to the desired value I_ka, when the current of the current transformer is restricted to the desired value I_mh. According to the invention the maximum value of the excitation current can be regulated (Fig. 4, line 3a) by choosing suitably the triggering voltages of the trigger circuit. In practice this can be done e.g. by jumpers. Pre-set values for the excitation voltage and for the settings of the regulator are defined from the design estimates and before generator is implemented a short-circuit test is carried out, whereby the suitable value for the excitation voltage is adjusted. After this the jumpers of the trigger circuits are set to the positions respective to the voltage level in order to achieve the correct short-circuit excitation. Because it is question of a device- specific characteristic, there is no need to change the settings in use. When applying the invention the excitation of the generator can be realised except by the brushless excitation used nowadays commonly but also in connection of excitation with brushes, especially in case of small generators. The realisation of the excitation in normal mode has no essential effect when applying the present invention. Though in the short- circuit situation the normal excitation is not usable especially in the modification described above, the invention can also be utilised in situation, where the voltage falls temporarily, like when a large motor is started. In this case the normal excitation is in use, but the additional excitation generated by the current transformer and controlled by the regulator will be added thereto as required by the load.

The invention has been described above with one embodiment. The description is not to be seen to limit the scope of the patent, but the modifications of the invention are feasible within the scope ofthe appended claims.

Claims

1. A method for exciting a synchronous generator in case when the mains voltage drops under a predetermined limit value, the synchronous generator being excited via an excitation winding, whereby according to the method the mains voltage is measured and the measured mains voltage is compared to a reference value, characterised in that at least one current transformer (2) is arranged to a mains conductor (R,S,T), the primary of the current transformer being in series with the mains conductor and the secondary of the current transformer is coupled to a regulating circuit (8), and that the current of the excitation circuit of the synchronous generator is controlled by the regulating circuit (8) when the mains voltage drops under a reference value.

2. A method according to claim 1 , characterised in that the current (I_m) controlled by the regulating circuit (8) ofthe excitation circuit is proportional to the mains current.

3. A method according to claim 1, characterised in that the excitation current generated by the current transformer is added to the current of a normal excitation circuit.

4. A method according to claim 1, characterised in that the excitation level is controlled by changing settings of the regulating circuit in order to achieve a sufficient excitation level.

5. A method according to claim 1, characterised in that the operating point of the regulating circuit is controlled on the basis of the rated excitation voltage of the synchronous generator.

6. A method according to claim 1, characterised in that the synchronous generator is excited by the regulating circuit (8) coupled to the secondary of the current transformer (2) and by a voltage regulator (3) coupled parallel to the regulating circuit (8).

7. A method according to claim 1, characterised in that the mains voltage is measured by a three-phase voltage transformer (TI ,7).

8. An arrangement for exciting a synchronous generator when the mains voltage of the synchronous generator drops under a predetermined limit value, the arrangement comprising a measuring device of the mains voltage and a comparison device, with which the measured value of the mains voltage is comparable to the limit value, characterised in that in series with the mains conductor of the synchronous generator there is arranged at least one primary of a current transformer (2), and that a secondary of the current transformer (2) is attached to a regulator circuit (8), which is arranged to generate a required excitation power when the mains voltage drops under the limit value.

9. An arrangement according to the claim 8, characterised in that the short circuit excitation is connectable parallel to normal excitation, whereby the total excitation of the synchronous generator is their combination.

10. An arrangement according to the claim 8, characterised in that the regulating circuit comprises regulating components, by which the amount of the short circuit excitation is controllable as a function ofthe excitation voltage ofthe synchronous generator.

11. An arrangement according to the claim 10, characterised in that the regulating components are realised as jumpers, by which the level of the excitation voltage is controllable.

12. An arrangement according to any of the claims 8 - 11, characterised in that the excitation is controllable by regulating the secondary voltage ofthe current transformer.

13. An arrangement according to any of the claims 8 - 12, characterised in that the arrangement comprises means (Tl,7) for measuring the three-phase voltage.