GB1595650A

GB1595650A - Converter control arrangements

Info

Publication number: GB1595650A
Application number: GB23123/78A
Authority: GB
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-08-12

Description

(54) IMPROVEMENTS IN OR RELATING TO CONVERTER CONTROL ARRANGEMENTS (71) We, THE ENGLISH ELECTRIC COMPANY LIMITED, of I Stanhope Gate London WIA IEH, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particlarly described in and by the following statement: This invention relates to converter control arrangements and particularly to such arrangements employing phase controlled rectifiers, typically thyristors, in bridge circuits.

The control of such circuits by phase control of firing pulses applied to gate electrodes of the rectifiers is well known.

With an A.C. supply connected across the rectifier, as the firing angle is retarded from zero (i.e. the 'natural' commutation point) rectification occurs with a decreasing D.C.

output until at 90 the output is zero. With further retardation the circuit works in an inversion mode with energy being passed from the load to the A.C. supply. Any degree of rectification or inversion can therefore be obtained by selection of the appropriate firing angle. In a three-phase thyristor bridge circuit, the thyristors are fired in sequence, each thyristor receiving a firing pulse at the same point on the associated A.C. waveform.

One application of such converter control arrangements is the energisation of a D.C.

field winding on the rotor of a brushless generator. One such arrangement is shown in Figure I of the accompanying drawings. The main generator has a three phase stator I energised by a rotating D.C.

field winding 2. The field winding 2 is in turn energised from an A.C. exciter having a three-phase 'stator' 3 and a stationary D.C.

field winding 4. A thyristor bridge converter 5 rectifies the A.C. exciter output to supply the main field winding 2 with D.C. The A.C.

exciter 3/4 is fed from a pilot exciter 6 having a permanent magnet rotor 7, by way of a diode rectifier bridge 8.

The whole of the rotating circuitry is shown within a broken line 9. It then remains to supply control signals between circuitry 10 and the rotating converter 5.

Previous proposals have included radio transmission, light transmission, and rotating transformers. Commonly it has been necessary to provide quite complex transmission paths between the 'ground' and the rotor to accommodate up to twelve firing/control signals for the converter.

It is therefore an object of the present invention to provide relatively simple control for a converter arrangement and particularly in an application of a controlled converter to the provision of a rotating D.C.

field for a brushless generator.

According to one aspect of the invention, in a converter control arrangement employing phase-controlled rectifiers arranged to be fired at equal intervals throughout a cycle, different operational conditions, selected from rectification, zero voltage output, and inversion, are obtainable by control of the distribution of firing pulses among the phase-controlled rectifiers without adjustment of the absolute timing of the firing pulses.

In such a converter control arrangement for connection between a D.C. Ioad and a 3phase A.C. supply, for each phase controlled rectifier an "associated" firing pulse is produced in operation at a firing angle of 150 , each firing pulse being applied to the associated rectifier or additionally to another rectifier according to the required operational condition.

Inversion, zero voltage output, and rectification may be respectively obtained by (I) application of the firing pulses to the "associated" rectifiers, (2) application of firing pulses to their "associated" rectifiers in the case of rectifiers of one output polarity and to rectifiers of the phase succeeding the "associated" phase in the case of rectifiers of the other output polarity, and (3) application of all firing pulses to rectifiers of the same output polarity as the "associated" rectifiers and of phase succeeding the "associated" phase.

According to another aspect of the invention, a brushless generator employing a rotating D.C. field winding is fed from an A.C. exciter by means of a rotating converter control arrangement as aforesaid and includes means for transmitting a control signal to the rotating converter arrangement to select one of said operational conditions in dependence upon the terminal conditions of the generator.

The brushless generator may include means for selecting a rectification condition alternately with a zero voltage output condition, the rectification condition producing an excessive D.C. field current and the two conditions being switched at predetermined upper and lower limits of the generator output voltage.

There may also be included means responsive to a generator output voltage in excess of the upper limit to establish an inversion condition whereby to de-energise the generator field winding more rapidly than by natural decay.

According to a further aspect of the invention in a method of controlling a brushless generator having a rotating circuit comprising a D.C. field winding fed by a controllable converter from an A.C. exciter, the converter comprises a phase-controlled rectifier bridge circuit which is switched at each of two predetermined values of D.C.

output to a respective firing condition in which the D.C. output tends towards the other of said two values, the switching being effected by altering the distribution of firing pulses of fixed absolute timing amongst the rectifiers of the bridge circuit.

A controlled converter arrangement in accordance with the invention and as employed in a brushless generator, will now be described by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of an arrangement known in its broad outline; Figure 2 is a circuit diagram of the converter 5 of Figure 1; Figures 3(a) and (b) show waveforms of the converter operation in rectifying, zero output and inverting modes; Figure 4 shows a graph of the generator field current in a typical operation; and Figure 5 shows firing angle patterns for the three modes of operation; Figure 6 shows a block diagram of firing pulse circuitry connected between the A.C.

exciter and the thyristor bridge.

Figure 1 has already been described briefly as a preferred arrangement of the prior art.

It has the advantage over some other schemes that only one field time-constant is included in the control loop, and hence the response of the system to changes in load can be much faster than that in other schemes.

Figure 2 shows a three phase thyristor bridge having 'positive' thyristors Tl, T3 and T5 and 'negative' thyristors T4, T6 and T2, the 'positive' and 'negative' designations indicating the polarity of the D.C. output terminals 12 and 13 when the converter is rectifying. The three phase terminals R, Y and B are connected to the three-phase 'stator' 3 of the A.C. exciter.

In order to apply firing pulses to the gates of the thyristors Tl-T6 mounted on the shaft (9) of the generator from the operators control cabinet, there must be a suitable arrangement to transmit the necessary electrical signals from stator (i.e. 'ground') to rotor. If a fully controlled thyristor bridge were utilized then there could be as many as twelve electrical signals to be transmitted between stator and rotor, but owing to the problems associated with the linear expansion of the shaft due to heating, electrical noise and spurious spikes, the known methods for transmitting signals are complex with so many signals.

Furthermore, a large volume of space is required to transmit twelve signals between stator and rotor. These problems highlight the need for a reduction in the number of electrical signals to be transmitted between stator and rotor. Also, complexity in the control circuit makes it difficult to mount the control circuit on the shaft. The solution provided by the invention is to use a "bangbang" control (as will be described) on the thyristor bridge to obtain constant voltages at the generator terminals. In this case the number of electrical signals can be reduced to three. The process involved in this control is simply triggering the thyristor bridge in a maximum of three different modes of operation A) 'Rectification' at constant firing angle delay.

B) 'Zero volt' at constant firing angle delay.

C) 'Inversion' at constant firing angle delay.

Since only one mode is operative at a time, the three electrical signals do not require to be transmitted from stator to rotor simultaneously. Only one electrical link (11 in Figure 1) transmitting a signal having one of three different states selectively, is required between stator and rotor and the state of this signal will dictate the specific mode of operation of the thyristor bridge.

Referring now to Figure 3(a), this shows operation in the rectification mode, switched after one complete cycle to the zero-volt mode. The solid black line represents the potential of the positive output terminal 12 and the broken line the potential of the negative output terminal 13.

The output voltage is thus the difference between these two lines and its magnitude and polarity are represented by the lower, sawtooth graph related to the zero output line 15. In the rectification mode the firing angle is chosen as 30 for all of the thyristors, That is, each positive thyristor Tl, T3 and T5 is fired 30 after its anode voltage becomes the most positive and each negative thyristor T4, T6 and T2 is fired 300 after its cathode has become the most negative.

As will be explained, the firing pulses necessarv to achieve this rectification condition are the same firing pulses. or at least have the same absolute timing, as the firing pulses required for the other operational conditions. There are essentlally six to each cycle of the A.C. supply and they will be considered, for convenience of explanation only, to be "associated" with the thyristors and phases for which they are delayed by 1500. Thus the firing pulse occurring at the btue phase negative-going zero transition is "associated" with the blue phase and with thyristor T5 on that phase.

In this rectification condition, the resulting D.C. field current, smoothed by the inductance of the field winding 2,' exceeds the level that would maintain the generator output voltage at its desired value. An excessive rise in the generator output voltage is prevented by sensing the generator output voltage and switching the thyristor converter to a zero-volt mode when a predetermined upper limit is reached.

The zero-volt mode is shown in the second part of Figure 3(a). The negative thyristors T2, T4 and T6 are still fired at 300 but the positive thyristors Tl, T3 and T5 are fired at such an angle that the positive and negative output potentials are mirror images of each other in each half cycle, i.e.

an angle of 180"-30 or 1500. The two potentials therefore are identical, or balanced, within each half cycle and the average difference is zero.

It will be clear that no actual phaseshifting of the firing pulses is required, since the negative thyristors continue to receive firing pulses at 300, and the positive thyristors simply require a redistribution of the firing pulses to their associated thyristors, i.e. on the preceding phase. Thus, the pulse at a=3O0 applied to T3 on the yellow phase is re-routed to its associated thyristor Tl on the preceding (red) phase.

It would, of course, have been possible to fire all thyristors at 900 to obtain a zero (average) output voltage but this would be a very much less simple arrangement than that illustrated in which no regeneration of firing pulses is required.

Underneath the graphs is indicated the thyristors which are conducting at any time.

At the transition between rectification and zero-volt operation thyristor tl is not fired at its normal 300 but is delayed for a further 1200. Consequently thyristor T5 remains conducting throughout the transition by virtue of the highly inductive nature of the field circuit. Thyristor T2 is then triggered at its normal time and since the bridge output voltage is negative just before the triggering of T2, i.e. the load e.m.f. is driving the field current, both thyristors T2 and T5 conduct together. This situation is repeated throughout the zero-volt mode, pairs of thyristors T2 and T5, T4 and Tl, T6 and T3, providing a freewheel path for the field current periodically.

Figure 3(b) shows a transition from the zero-volt mode to inversion and then back to rectification.

In the period 'L' the thyristors T3 andT6 are conducting. At the start of period 'm', thyristor T5 is triggered while thyristor T6 remains in conduction. In the inverting mode, all of the thyristors are fired at a=150". This shift of the firing angle Is achieved in similar manner to that for the zero-volt operation by applying only the firing pulses at a 1500 associated with each thyristor. As in zero-volt operation, the stored energy in the inductive circuit is utilized to effect conduction of the thyristors. Inversion operation is followed by rectification in this figure as discussed earlier.

The time periods of the rectification, zero volt and inversion operations are determined by the various time constants of the generator and by its terminal voltage.

The thyristor bridge would normally be operated in rectification and zero volt modes alternately and the third mode inversion would be applied to the field of the generator in the event of overvoltage, instability or short circuit where fast deenergization of the field is necessary. Figure 4 shows the synchronous generator field current waveforms. When the thyristor bridge is working in the rectification mode, the field current of the synchronous generator will increase as shown in this diagram. The zero-volt mode is applied when the terminal voltage exceeds the rated value. This causes the field current to decay according to the natural time constant of the field winding. Thus the mean value of the field current I, (mean) is as shown in Figure 4. When the thyristor bridge operates in the inversion mode the field current decay is much faster than in zero volt operation. The field current decay in the inverting mode is shown in dotted line.

It can be seen from Figure 3 that the firing angles in all three operating modes occur when the phase voltages for each pair of phases are equal and opposite. These points can be identified as shown in Figure 5, by inverting the negative half cycles and marking the transitions between the voltage peaks. If the point at which the red phase voltage is positive and equals the blue phase voltage is taken as an overall reference then the firing angle patterns can be seen to be as shown in Figure 5.

The control circuitry is shown in block diagram form in Figure 6.

Input signals are provided at terminals 16 and 17 indicating a short-circuit condition of the generator and the magnitude of the generator terminal voltage respectively.

These are applied to a voltage control unit 18 together with a reference signal for comparison with the terminal voltage signal.

Output signals from the voltage control unit are shown on three separate lines for purposes of illustration but in practice can be three different signal states on a single path. When a short circuit is indicated the rectification mode is required to increase the excitation voltage and avoid pole slipping, in the case of a synchronous generator. An output signal on the R line is therefore produced.

When the terminal voltage reaches the upper limit of its rated value the control unit 18 produces a signal on the zero-volt line Z and, when the lower limit is reached the signal reverts to the rectification line R.

In the case of an overvoltage at the generator terminals the field has to be discharged as quickly as possible and the control unit produces a signal on the I line to cause operation in the inversion mode.

These three signals are transmitted to the control circuitry on the rotor of the generator by rotating transformers in known fashion.

The rotating control circuitry comprises a squaring circuit 19 to which the A.C.

exciter 3-phase output is applied. This has the effect of full wave rectifying the voltage waveforms and enabling the different phase magnitudes to be compared. The 60 crossover points shown in Figure 5 can thus be identified and a pulse train having pulses at 60 intervals produced by a pulse generator circuit 20. These pulses have to be distributed amongst the thyristors in accordance with the firing angle patterns of Figure 5. The patterns have to be synchronised with a phase marker (such as the red/blue marker mentioned above) and this is done by a synchronising circuit 21 supplied with a phase reference from the red and blue phases. Selection of the patterns and distribution to the appropriate lines is effected by logic circuitry 22, following which, the pulses are used in a firing circuit 23 to generate firing pulses for direct transmission to the thyristor gates.

By the above method of controlling the generator the number of signals that have to be transmitted across the stator rotor gap is much reduced, and in addition the control circuitry is sufficiently simple that it can be mounted and duplicated if necessary (for security) on the rotor.

WHAT WE CLAIM IS: 1. A converter control arrangement employing phase-controlled rectifiers arranged to be fired at equal intervals throughout a cycle, different operational conditions, selected from rectification, zero voltage output, and inversion, being obtainable by control of the distribution of firing pulses among the phase-controlled rectifiers without adjustment of the absolute timing of the firing pulses.

2. A converter control arrangement according to Claim 1 for connection between a D.C. load and a 3-phase A.C.

supply, wherein for each phase-controlled rectifier an "associated" firing pulse is produced in operation at a firing angle of 1500, each firing pulse being applied to the "associated" rectifier or to another rectifier according to the required operational condition.

3. A converter control arrangement according to Claim 2, wherein inversion, zero voltage output, and rectification are respectively obtained by (1) application of the firing pulses to the "associated" rectifiers, (2) application of firing pulses to their "associated" rectifiers in the case of rectifiers of one output polarity and to rectifiers of the phase succeeding the "associated" phase in the case of rectifiers of the other output polarity, and (3) application of all firing pulses to rectifiers of the same output polarity as the "associated" rectifiers and of phase succeeding the "associated" phase.

4. A brushless generator employing a rotating D.C. field winding fed from an A.C. exciter by means of a rotating converter control arrangement according to any preceding claim, including means for transmitting a control signal to the rotating converter arrangement to select one of said operational conditions in dependence upon the terminal conditions of said generator.

5. A brushless generator according to Claim 4, including means for selecting a rectification condition alternately with a zero voltage output condition, the rectification condition producing an excessive D.C. field current and the two conditions being switched at predetermined upper and lower limits of the generator output voltage.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. It can be seen from Figure 3 that the firing angles in all three operating modes occur when the phase voltages for each pair of phases are equal and opposite. These points can be identified as shown in Figure 5, by inverting the negative half cycles and marking the transitions between the voltage peaks. If the point at which the red phase voltage is positive and equals the blue phase voltage is taken as an overall reference then the firing angle patterns can be seen to be as shown in Figure 5. The control circuitry is shown in block diagram form in Figure 6. Input signals are provided at terminals 16 and 17 indicating a short-circuit condition of the generator and the magnitude of the generator terminal voltage respectively. These are applied to a voltage control unit 18 together with a reference signal for comparison with the terminal voltage signal. Output signals from the voltage control unit are shown on three separate lines for purposes of illustration but in practice can be three different signal states on a single path. When a short circuit is indicated the rectification mode is required to increase the excitation voltage and avoid pole slipping, in the case of a synchronous generator. An output signal on the R line is therefore produced. When the terminal voltage reaches the upper limit of its rated value the control unit 18 produces a signal on the zero-volt line Z and, when the lower limit is reached the signal reverts to the rectification line R. In the case of an overvoltage at the generator terminals the field has to be discharged as quickly as possible and the control unit produces a signal on the I line to cause operation in the inversion mode. These three signals are transmitted to the control circuitry on the rotor of the generator by rotating transformers in known fashion. The rotating control circuitry comprises a squaring circuit 19 to which the A.C. exciter 3-phase output is applied. This has the effect of full wave rectifying the voltage waveforms and enabling the different phase magnitudes to be compared. The 60 crossover points shown in Figure 5 can thus be identified and a pulse train having pulses at 60 intervals produced by a pulse generator circuit 20. These pulses have to be distributed amongst the thyristors in accordance with the firing angle patterns of Figure 5. The patterns have to be synchronised with a phase marker (such as the red/blue marker mentioned above) and this is done by a synchronising circuit 21 supplied with a phase reference from the red and blue phases. Selection of the patterns and distribution to the appropriate lines is effected by logic circuitry 22, following which, the pulses are used in a firing circuit 23 to generate firing pulses for direct transmission to the thyristor gates. By the above method of controlling the generator the number of signals that have to be transmitted across the stator rotor gap is much reduced, and in addition the control circuitry is sufficiently simple that it can be mounted and duplicated if necessary (for security) on the rotor. WHAT WE CLAIM IS:

1. A converter control arrangement employing phase-controlled rectifiers arranged to be fired at equal intervals throughout a cycle, different operational conditions, selected from rectification, zero voltage output, and inversion, being obtainable by control of the distribution of firing pulses among the phase-controlled rectifiers without adjustment of the absolute timing of the firing pulses.

6. A brushless generator according to

Claim 5 including means responsive to a generator output voltage in excess of said upper limit to establish an inversion condition whereby to de-energise the generator field winding more rapidly than by natural decay.

7. A method of controlling a brushless generator having a rotating circuit comprising a D.C. field winding fed by a controllable converter from an A.C.

exciter, wherein the converter comprises a phase-controlled rectifier bridge circuit which is switched at each of two predetermined values of D.C. output to a respectlve firing condition in which the D.C. outputs tends towards the other of said two values, the switching being effected by altering the distribution of firing pulses of fixed absolute timing amongst the rectifiers of the bridge circuit.

8. A converter control arrangement substantially as hereinbefore described with reference to the accompanying drawings.

9. A brushless generator substantially as hereinbefore described with reference to the accompanying drawing.